TECHNICAL FIELD The present invention relates to a method and device for removal of contaminant from natural gas by means of an absorbent medium, such as glycol.

BACKGROUND ART

Removal of acid gas, such as carbon dioxide (CO ₂ ) and/or hydrogen sulphide (H2S) from natural gas, commonly termed "gas sweetening" is a well known technology. There are several commercial technologies available for this purpose such as absorbent mediums or solvents (i.e. amines, glycol), physical solvents, membranes, cold processes, etc. Such an absorbent medium can also be referred to as a lean absorbent, prior to the absorption of acid gas from the natural gas, or a rich absorbent, after the absorption of acid gas from the natural gas.

Chemical solvents such as amines are widely used and extensive operating experience has been gained. Examples of applicable absorbents comprise amine based absorbents such as primary, secondary and tertiary amines; one well known example of applicable amines is mono ethanol amine (MEA). The liquid diluent is selected among diluents that have a suitable boiling point, are stable and inert towards the absorbent in the suitable temperature and pressure interval. An example of an applicable diluent is water. Examples of suitable amines for use with a diluents such as water are aqueous solutions of monoethanolamine (MEA), methylaminopropylamine (MAPA), piperazine, diethanolamine (DEA), triethanolamine (TEA), diethylethanolamine (DEEA), diisopropylamine (DIPA), aminoethoxyethanol (AEE), dimethylaminopropanol (DIMAP) and methyldiethanolamine (MDEA), methyldiisopropanolamine (MDIPA), 2-amino-1 -butanol (2-AB) or mixtures thereof. The removal process comprises a closed circulation loop containing the solvent. In an amine based absorption process the C0 ₂ /H ₂ S reacts with the amine in an absorber unit and is bound strongly to the solvent. The reaction between the amine and the acid gas is strongly exothermic. The solvent can be regenerated, often by combining flash regeneration by pressure reduction and thermal regeneration by supplying heat in a stripper, where the C0 ₂ /H ₂ S is released from the solvent. The regenerated solvent is passed through suitable compressor and heat exchange units for pressurization and temperature adjustment, and is returned to the absorber unit. A typical, conventional amine plant using an absorber column is schematically shown in Figure 9.

In the prior art arrangement shown in Figure 9, there is provided an acid gas removal process wherein a sour gas stream containing undesirable hydrogen sulphide (H ₂ S) and carbon dioxide (C0 ₂ ) is introduced to a contact absorber A through line 901 . As the sour gas flows upward through absorber A the sour gas contacts downward flowing mixture of normal lean amine which is introduced to the absorber through line 902. The process gas has most of the acid gases removed by the time it leaves the absorber after contacting the lean amine from line 902. A product gas (sweet gas) having a substantially reduced content of the hydrogen sulphide and carbon dioxide is withdrawn from the top of the absorber via line 903.

A stream of rich amine solution containing absorbed hydrogen sulphide and carbon dioxide as salts of amine is removed from the absorber through line 904. The pressure of the solution is reduced and it then goes to a rich amine flash tank C. The flash gases exit through line 905 and the rich amine solution exits through line 906. The rich amine stream passes through a lean/rich heat exchanger D to recover sensible heat from hot lean amine and is then introduced to an amine regenerator stripping column B through line 907. Internal stripping steam is generated by reboiling the amine solution in a stripper reboiler, or a heat exchanger E, using a suitable heat medium 908. The lean amine temperature can vary from about 100 °C to 140 °C, depending on the type of amine, its concentration and its pressure. The steam generated from the reboiled amine is introduced near the bottom of stripping column B through line 91 1 and passes upward through the amine solution providing heat to decompose the hydrogen sulphide and carbon dioxide amine salts and the stripping vapour to sweep the acid gas away from the amine solution and out of the stripping column. The mixture of steam, hydrogen sulphide, and carbon dioxide exits the stripper overhead through line 910.

A hot lean amine stream 909 exits the bottom of the stripper B, is passed through the lean/rich heat exchanger D and through a cooler F where the lean amine solution is cooled to a temperature of about 35 °C to 55 °C. The cooled lean amine stream in line 912 continues through line 902 to the top of the absorber A.

Current process equipment as used at e.g. at the Sleipner T installation is very large. In this case, the absorber column inner diameter is almost 4 m and the total height is almost 18 m. The footprint and weight of the absorber column are thus significant. Depending on the applied amine and amount of acid gas to be removed the circulation rates on the solvent will also be significant. This requires a significant amount of power for pumping, heating and cooling of the circulating amine solution. Amine solutions are also known to be corrosive, especially in the C0 ₂ rich parts of the process. Depending on operating conditions and impurities accumulated in the solution, the amine is susceptible to degradation and contamination. Equipment using amines may also experience various types of failure caused, for instance, by foaming or by insufficient contact between gas and liquid. Foaming of the gas and liquid, caused by e.g. condensation of hydrocarbon or solids suspended in the gas after insufficient pre-filtration, is also a known issue in conventional absorbers. Carry-over of amine droplets in the sweet gas from the absorber to downstream equipment is another cause of foaming. A further problem with the absorber column relates to the importance of maintaining good contact between the natural gas and the liquid amine and to provide a good liquid distribution in order to achieve an effective removal of acid gas.

In addition to separating oil and some condensate from the wet gas stream, it is necessary to remove most of the associated water. Most of the liquid, free water associated with extracted natural gas is removed by simple separation methods at or near the wellhead. However, the removal of the water vapor that exists in solution in natural gas requires a more complex treatment. This treatment consists of 'dehydrating' the natural gas, which usually involves one of two processes: either absorption, or adsorption. Absorption occurs when the water vapor is taken out by a dehydrating agent. Adsorption occurs when the water vapor is condensed and collected on the surface. An example of absorption dehydration is known as Glycol Dehydration. In this process, a liquid desiccant dehydrator serves to absorb water vapour from the gas stream. Glycol, the principal agent in this process, has a chemical affinity for water. This means that, when in contact with a stream of natural gas that contains water, glycol will serve to take the water out of the gas stream. Essentially, glycol dehydration involves using glycol or a glycol solution, usually either diethylene glycol (DEG) or triethylene glycol (TEG), which is brought into contact with the wet gas stream The purpose of glycol dehydration unit is to remove water from natural gas. When produced from a reservoir, natural gas usually contains large amounts of water and is typically saturated or at the water dew point. This water can cause several problems for downstream processes und equipment. At low temperatures, the water can either freeze in piping, or as is more commonly the case, form hydrates with C0 ₂ und hydrocarbons. Depending on composition, these hydrates can form at relatively high temperatures, plugging equipment and piping. Glycol dehydration units depress the hydrate formation point of the gas trough water removal. Without dehydration, a free water phase (liquid water) could also drop out of the natural gas as it is either cooled or the pressure is lowered through the equipment and piping.

The process described in connection with Figure 9 above can also be applied to a dehydration process using, for instance, glycol. Lean, water free glycol (purity >99%) is fed to the top of an absorber column where it is contacted with the wet natural gas stream. The glycol removes water from the natural gas by physical absorption and is removed at the bottom of the column. Upon exiting the absorber the glycol stream is often referred to as "rich glycol". The dry natural gas leaves the top of the absorption column and is fed either to a pipeline system or to a gas plant. Glycol absorbers can be either tray columns or packed columns.

After leaving the absorber, the rich glycol is fed to a flash vessel where hydrocarbon vapours are removed and any liquid hydrocarbons are skimmed from the glycol. This step is necessary as the absorber is typically operated at high pressure and the pressure must be reduced before the regeneration step. Due to the composition of the rich glycol, a vapour phase having high hydrocarbon content will form when the pressure is lowered.

After leaving the flash vessel, the rich glycol is heated in a cross-exchanger and is fed to the stripper (also known as a regenerator). The glycol stripper consists of a column, an overhead condenser, and a reboiler. The glycol is thermally regenerated to remove excess water and regain the high glycol purity.

The hot, lean glycol is cooled by cross-exchange with rich glycol entering the stripper. The lean glycol is then fed to a lean pump where its pressure is elevated to that used in the glycol absorber. The lean solvent is cooled again with a trim cooler before being fed back into the absorber. The term trim cooler is used to indicate a cooler designed to remove a small heat duty to satisfy a temperature specification of a process. The trim cooler can either be a cross-exchanger with the dry gas leaving the absorber, a surface condenser or an aerial type cooler. Aerial coolers are often used to cool a hot fluid to near ambient temperature.

A problem common to both the above processes is that the stripper which requires a significant amount of heat. A conventional means of supplying heat is to provide a steam generator that produces superheated steam for use in the stripping process. In the same way as the process equipment described in Figure 9, a steam generator installation is relatively large and contributes to the footprint and total weight of the processing equipment. The object of the invention is to solve the above problems by providing an improved process for the removal of acid gas from natural gas by means of an absorbent medium, and for the regeneration of the absorbent medium by the removal of acid gas from the absorbent.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a novel process for removal of a contaminant, such as carbon dioxide, hydrogen sulphide or water, from natural gas by means of an absorbent medium and subsequent removal of said contaminant from the absorbent medium. The object is achieved by means of a process and a processing plant as described in the appended claims.

In the subsequent description, the terms "absorbent" or "absorbent medium" are used to denote a fluid such as an amine solution for carbon dioxide removal, an organic scrubbing agent for hydrogen sulphide removal, or a dehydrating medium in the form of glycol or a suitable glycol solution. The invention relates to a continuous process for removal of a contaminant from a fluid stream, preferably natural gas comprising mainly hydrocarbons, by means of an absorbent medium. Subsequently, the contaminant is desorbed from the absorbent medium to allow the absorbent medium to be recirculated and the contaminant is removed or processed further. This process can be carried out continuously, as the absorption medium is reused. The process comprises the steps of;

- supplying a pressurized fluid stream and a lean absorption medium to an absorber;

- passing the fluid stream, which is in contact with the lean absorption medium within a rotating absorption zone in the absorber, wherein the contaminant is removed from the fluid stream by the lean absorption medium when passing through the rotating absorption zone;

- passing rich absorption medium containing said contaminant into a first rotating desorption zone within a desorber, wherein a first portion of the contaminant is removed from the rich absorption medium by means of a heated vapour to obtain a partially regenerated absorption medium,

- passing the partially regenerated absorption medium into a second rotating desorption zone within the desorber; wherein the remaining portion of the contaminant is removed from the partially regenerated absorption medium by heating the absorption medium to evaporate said contaminant and allowing liquid lean absorption medium to be removed from the desorber;

- passing said evaporated contaminant through said first rotating desorption zone; and

- passing said evaporated contaminant to a rotating condenser within the desorber, wherein contaminant is condensed and removed.

The absorption process outlined above can comprise supplying the lean absorption medium to an inner portion of at least one annular rotating absorber unit within the absorber, causing the lean absorption medium to flow radially outwards through the rotating absorption zone; and supplying the pressurized fluid stream to an outer portion of said annular rotating absorber unit, causing the fluid stream to flow radially inwards in order to create a radial cross-flow in the absorption zone. In this way, evaporated contaminant is absorbed from the fluid stream by the lean absorption medium during the resulting radial cross-flow through the rotating absorption zone. The cross-flow is created by the pressurized fluid stream being forced radially inwards and the absorption medium being forced radially outwards by centrifugal forces caused by the rotation of the absorption zone.

The pressurized fluid stream flows radially inwards and is removed from an inner perimeter of the rotating absorption zone. At this stage the content of evaporated contaminant in the fluid stream has been reduced to a predetermined level. At the same time, the absorption medium flows radially inwards and is removed from an outer perimeter of the rotating absorption zone. The rich absorption medium with its content of absorbed water is transferred to a desorber for regeneration.

As stated above, the absorber comprises at least one annular rotating absorber unit. Preferably the lean absorption medium is supplied to a pair of identical and mirrored annular rotating absorber units rotating about a common axis within the absorber. By arranging the absorber units in this way it is possible to avoid problems with, for instance, balancing of the rotating components during high speed rotation of the absorber units. This will be described in further detail below. The desorption process outlined above can comprise supplying the rich absorption medium to an inner portion of at least one annular rotating desorber unit, causing the rich absorption medium to flow radially outwards through the first rotating desorption zone.

Heat is supplied to an outer perimeter of the first rotating desorption zone by combusting natural gas in a combustion chamber in or adjacent the at least one annular rotating desorber unit. The exhaust gas from the combustion process is used for transferring heat to heat pipes extending through at least the first rotating desorption zone. The natural gas used for the combustion can be taken from an external source, but is preferably taken from the stream of processed natural gas leaving the absorber.

A heat pipe consists of a sealed aluminum or copper container whose inner surfaces have a capillary wicking material. A heat pipe has the ability to transport heat against gravity by an evaporation-condensation cycle, optionally with the help of porous capillaries that form a "wick". The wick provides the capillary driving force to return the condensate to the evaporator. The quality and type of wick usually determines the performance of the heat pipe, for this is the heart of the product. Different types of wicks are used depending on the application for which the heat pipe is being used.

Inside the walls of the pipe, the optional wick structure exerts a capillary pressure on the liquid phase of the working fluid. The wick can comprise a sintered metal powder or a series of grooves parallel to the pipe axis, but it may be any material capable of exerting capillary pressure on the condensed liquid to wick it back to the heated end. The capillary material can also be a porous structure made of materials like steel, aluminium, nickel or copper in various ranges of pore sizes. They can be fabricated using metal foams or felts, the latter being more frequently used. By varying the pressure on the felt during assembly, various pore sizes can be produced. By incorporating removable metal mandrels, an arterial structure can also be molded in the felt. The heat pipe may not need a wick structure if gravity or some other source of acceleration is sufficient to overcome surface tension and cause the condensed liquid to flow back to the heated end.

For the current invention the capillary action can be used. Alternatively, or in combination, the heat pipes in the rotating desorber zone can be placed at an angle relative to the central axis through the desorber for returning the condensed liquid to the heated end.

The prime purpose of the wick is to generate capillary pressure to transport the working fluid from the condenser to the evaporator. It must also be able to distribute the liquid around the evaporator section to any area where heat is likely to be received by the heat pipe. Often these two functions require wicks of different forms. The selection of the wick for a heat pipe depends on many factors, several of which are linked to the properties of the working fluid.

The maximum capillary head generated by a wick increases with decrease in pore size. The wick permeability increases with increasing pore size. Another feature of the wick, which must be optimized, is its thickness. The heat transport capability of the heat pipe is raised by increasing the wick thickness. The overall thermal resistance at the evaporator also depends on the conductivity of the working fluid in the wick. Other necessary properties of the wick are compatibility with the working fluid and wettability.

Inside the container is a liquid under its own pressure, that enters the pores of the capillary material, wetting all internal surfaces. Applying heat at any point along the surface of the heat pipe causes the liquid at that point to boil and enter a vapor state. When that happens, the liquid picks up the latent heat of vaporization. The gas, which then has a higher pressure, moves inside the sealed container to a colder location where it condenses. Thus, the gas gives up the latent heat of vaporization and moves heat from the input to the output end of the heat pipe. In the case of the invention, heat is given off to the rich absorbent medium, causing it to evaporate.

The selection of the working fluid must also be based on thermodynamic considerations which are concerned with the various limitations to heat flow occurring within the heat pipe. The heat pipe can be designed to maintain a constant temperature or temperature range, which is determined by the working fluid.

In heat pipe design, a high value of surface tension is desirable in order to enable the heat pipe to operate against gravity and to generate a high capillary driving force. In addition to high surface tension, it is necessary for the working fluid to wet the wick and the container material i.e. contact angle should be zero or very small. The vapor pressure over the operating temperature range must be sufficiently great to avoid high vapor velocities, which tend to setup large temperature gradient and cause flow instabilities. A high latent heat of vaporization is desirable in order to transfer large amounts of heat with minimum fluid flow, and hence to maintain low pressure drops within the heat pipe. The thermal conductivity of the working fluid should preferably be high in order to minimize the radial temperature gradient and to reduce the possibility of nucleate boiling at the wick or wall surface. The resistance to fluid flow will be minimized by choosing fluids with low values of vapor and liquid viscosities. The table shown below indicates a few mediums with their useful ranges of temperature.

MEDIUM MELTING PT. BOILING PT. AT ATM. RANGE

(° C ) PRESSURE (° C) (° C)

Methanol - 98 64 10 - 130

I Flutec PP2 | - 50 76 10 - 160

I Ethanol - 1 2 78 0 - 130

Water o 100 30 - 200

I Toluene - 95 1 10 50 - 200

Mercury - 39 361 250 - 650 Sodium 98 892 600 - 1200 Lithium 179 1340 1000 - 1800

I Silver 960 2212 1800 - 2300 Heat pipes have an effective thermal conductivity many thousands of times that of copper. The heat transfer or transport capacity of a heat pipe is specified by its "Axial Power Rating (APC)". It is the energy moving axially along the pipe. The larger the heat pipe diameter, greater is the APR. Similarly, longer the heat pipe lesser is the APR. Heat pipes can be built in almost any size and shape

A typical heat pipe consists of a sealed pipe or tube made of a material with high thermal conductivity such as copper or aluminium at both hot and cold ends. A vacuum pump is used to remove all air from the empty heat pipe, and then the pipe is filled with a fraction of a percent by volume of working fluid chosen to match the operating temperature. Examples of such fluids include water, ethanol, acetone, sodium, or mercury, as listed above. Due to the partial vacuum that is near or below the vapour pressure of the fluid, some of the fluid will be in the liquid phase and some will be in the gas phase. The use of a vacuum eliminates the need for the working gas to diffuse through any other gas and so the bulk transfer of the vapour to the cold end of the heat pipe is at the speed of the moving molecules. In this sense, the only practical limit to the rate of heat transfer is the speed with which the gas can be condensed to a liquid at the cold end. Heat pipes contain no mechanical moving parts and typically require no maintenance.

The materials chosen depend on the temperature conditions in which the heat pipe must operate, with coolants ranging from liquid helium for extremely low temperature applications (2^1 K) to mercury (523-923 K) & sodium (873-1473 K) and even indium (2000-3000 K) for extremely high temperatures. The vast majority of heat pipes for low temperature applications use some combination of ammonia (213-373 K), alcohol (methanol (283-403 K) or ethanol (273^103 K)) or water (303-473 K) as working fluid. Since the heat pipe contains a vacuum, the working fluid will boil and hence take up latent heat at well below its boiling point at atmospheric pressure. Water, for instance, will boil at just above 273 K (0 ° C) and so can start to effectively transfer latent heat at this low temperature.

Active control of heat flux can be effected by adding a variable volume liquid reservoir to the evaporator section. Variable conductance heat pipes employ a large reservoir of inert immiscible gas attached to the condensing section. Varying the gas reservoir pressure changes the volume of gas charged to the condenser which in turn limits the area available for vapour condensation. Thus a wider range of heat fluxes and temperature gradients can be accommodated with a single design. Thin planar heat pipes (heat spreaders) have the same primary components as tubular heat pipes. These components are a hermetically sealed hollow vessel, a working fluid, and a closed-loop capillary recirculation system.

In operation heat pipes employ evaporative cooling to transfer thermal energy from one point to another by the evaporation and condensation of a working fluid or coolant. Heat pipes rely on a temperature difference between the ends of the pipe, and cannot lower temperatures at either end beyond the ambient temperature.

When one end of the heat pipe is heated the working fluid inside the pipe at that end evaporates and increases the vapour pressure inside the cavity of the heat pipe. The latent heat of evaporation absorbed by the vaporisation of the working fluid reduces the temperature at the hot end of the pipe.

The vapour pressure over the hot liquid working fluid at the hot end of the pipe is higher than the equilibrium vapour pressure over condensing working fluid at the cooler end of the pipe, and this pressure difference drives a rapid mass transfer to the condensing end where the excess vapour condenses, releases its latent heat, and warms the cool end of the pipe. Non-condensing gases (caused by contamination for instance) in the vapour impede the gas flow and reduce the effectiveness of the heat pipe, particularly at low temperatures, where vapour pressures are low. The velocity of molecules in a gas is approximately the speed of sound and in the absence of non condensing gases; this is the upper velocity with which they could travel in the heat pipe. In practice, the speed of the vapour through the heat pipe is dependent on the rate of condensation at the cold end. The condensed working fluid then flows back to the hot end of the pipe. In the case of heat pipes containing wicks, the fluid is returned by capillary action.

When making heat pipes, there is no need to create a vacuum in the pipe. One simply boils the working fluid in the heat pipe until the resulting vapour has purged the non condensing gases from the pipe and then seals the end.

An interesting property of heat pipes is the temperature over which they are effective. Initially, it might be suspected that a water charged heat pipe would only work when the hot end reached the boiling point (100 °C) and steam was transferred to the cold end. However, the boiling point of water is dependent on absolute pressure inside the pipe. In an evacuated pipe, water will boil just slightly above its melting point (0 °C). The heat pipe will operate, therefore, when the hot end is just slightly warmer than the melting point of the working fluid. Similarly, a heat pipe with water as a working fluid can work well above the boiling point (100 °C), if the cold end is low enough in temperature to condense the fluid.

The main reason for the effectiveness of heat pipes is the evaporation and condensation of the working fluid. The heat of vaporization greatly exceeds the sensible heat capacity. Using water as an example, the energy needed to evaporate one gram of water is equivalent to the amount of energy needed to raise the temperature of that same gram of water by 540 °C (hypothetically, if the water was under extremely high pressure so it didn't vaporize or freeze over this temperature range). Almost all of that energy is rapidly transferred to the "cold" end when the fluid condenses there, making a very effective heat transfer system with no moving parts. Heat pipes must be tuned to its particular application. The choice of pipe material, size and coolant all have an effect on the optimal temperatures in which heat pipes work.

In order to reduce the amount of contaminants such as nitrous oxide and other combustion products, a portion of the exhaust gas can be used for exhaust gas recirculation (EGR). Dependent on the current combustion conditions and the quality of the natural gas used, the amount of recirculated exhaust gas can be varied between zero and 70 percent of the total injected volume of combustible gas mixture to be combusted. In addition to the use of exhaust gas recirculation for the reduction of combustion products such as soot and nitrous oxide, the exhaust gas can be used for pre-heating the natural gas prior to the injection into the combustion chamber in the desorber.

The resulting exhaust gas can be conducted to an exhaust treatment unit for carbon dioxide removal before being released to the atmosphere. A preferred exhaust treatment method is a channel integrated treatment (CIT), wherein exhaust gas is passed through a channel section having a constant cross- sectional area. Injection nozzles are arranged to inject atomized droplets of an absorbent medium, such as an amine, at high pressure into the exhaust stream. The injected absorbent has a higher velocity than the exhaust gas and causes both an increase of the exhaust gas flow velocity as well as a mixing enhancing turbulence. The atomized droplets of rich absorbent containing absorbed carbon dioxide are captured by demisters extending across the channel. By injecting the absorbent at a higher velocity, kinetic energy is transferred to the exhaust gas. In this way the channel injection treatment does not cause a pressure drop in the exhaust gas through the treatment section. The rich amine captured by the demisters can be passed to a rotating desorber of the same type as described here. If the process involves, for instance, carbon dioxide removal from natural gas, then the absorbed rich amine from the channel integrated treatment (CIT) can be supplied to the rotary desorber unit used by the process. Lean amine from the rotary desorber can then be returned to the channel integrated treatment unit.

The heating of the outer perimeter of the first rotating desorption zone causes the contaminant in the rich absorption medium to evaporate, or be stripped, from the absorption medium and to flow radially inwards in order to create a radial cross-flow in the first rotating desorption zone. During this radial cross- flow a first portion of contaminant is desorbed from the rich absorption medium by the vapour during said radial cross-flow. The cross-flow is created by contaminant vapour under pressure being forced radially inwards and the absorption medium being forced radially outwards by centrifugal forces caused by the rotation of the first desorption zone. The release of the first portion of contaminant is achieved by the said vapour heating the absorption medium to a predetermined temperature, which is dependent on the type of absorption medium used. The predetermined temperature required by the absorption medium determines the type of heat pipe and medium used in the heat pipes in the rotating desorber zone.

The partially regenerated absorption medium will be forced through the first rotating desorption zone and into the second rotating desorption zone, where the remaining contaminant is removed to produce a lean absorption medium. This is achieved by supplying heat to a heat exchanger in the second rotating desorption zone, wherein the remaining portion of the contaminant and the absorption medium containing glycol are heated to form contaminant vapour and liquid absorption medium. The rich absorption medium is heated to a temperature above the evaporation temperature of the contaminant, such as carbon dioxide, hydrogen sulphide or water, but below the evaporation temperature of the absorption medium. The evaporation causes an increase in pressure which forces the vapour radially inwards through both the second and the first rotating desorption zones. This flow of vapour through the first rotating desorption zone has been described above. Lean absorption medium is removed from an outer perimeter of the second rotating desorption zone and transferred back to an absorber.

The first and the second rotating desorption zones described above are preferably arranged as concentric annular desorption zones rotating about a common central axis. The first rotating desorption zone performs the same function as a conventional stripper unit, while the second rotating desorption zone performs the same function as a conventional reboiler unit.

Alternatively, desorption is performed in a single rotating desorption zone comprising an integrated annular stripper and reboiler unit rotatable about a common axis. The stripper and reboiler unit can be provided with a number of tubes for heat supply. A heating medium in the form of exhaust gas from natural gas combusted in a chamber adjacent the reboiler unit. The hot exhaust gas is supplied to an inlet and is used for heating the heat pipes, which can be arranged in parallel with, or at an angle to the axis of rotation. The heat pipes are thus heated by the exhaust gas and are used for heating the entire stripper and reboiler unit. The combustion chamber is connected to an outlet for removing the exhaust gas. The rich absorbent medium is introduced at an inner perimeter of the stripper and reboiler unit and the stripping will take place as the absorbent medium is heated while flowing radially outwards. Regenerated lean absorbent medium leaves the stripper and reboiler unit at the outer circumference thereof at a lean absorbent medium outlet. The contaminant in vapour phase leaves the stripper and reboiler unit near its inner perimeter. The contaminant vapour is then directed into the outer perimeter a condenser. Dependent on the type of absorbent medium used, this contaminant vapour may contain a carry-over of absorbent medium in vapour form. In a conventional stripper unit, the residence time of a rich absorption medium in contact with a heated vapour, such as steam, is determined by the temperature of the steam. If the temperature is too low, then the absorption medium may not be completely regenerated. If the temperature is increased then the contaminant are removed more effectively. However, if the temperature is too high or the residence time too long, this will cause degradation of the absorption medium. For instance, in the case of amine, the temperature can vary from about 100 °C to 140 °C, depending on the type of amine, its concentration and its pressure. The use of a rotating desorption unit will allow higher temperatures to be used, as the residence time is considerably shorter compared to a conventional stripper unit. In a process according to the invention, the absorption medium can pass the rotating desorption zones within 2-10 seconds, depending on the diameter of the desorption unit. As stated above, the vapour flows radially inwards through both the first and the second rotating desorption zones. Alternatively, the vapour flows radially inwards through a single rotating desorption zone. The vapour is then removed from an inner perimeter of the first or the single rotating desorption zone and passed to an outer portion of the rotating condenser. The alternative desorbers described above can be combined with either of the condensers described below.

In one example, the condenser is arranged to rotate about the same axis as the rotating desorption zones and is preferably provided with two separate sections placed side-by-side. The condenser sections can be supplied by a common source, or by individual sources, of coolant. According to one example, the condenser sections are supplied by a single source of coolant, which is supplied to an inner portion of the second condenser section. The coolant is then passed to the first condenser section at an outer portion thereof and is subsequently removed from an inner portion of the first condenser section. The heated vapour is supplied to the outer portion of a first section of the rotating condenser, causing the vapour to flow radially inwards through the first section of the rotating condenser. The flow of vapour is caused by the internal pressure forcing the vapour towards an outlet from the desorption unit. The flow can be assisted by an external pump unit for creating a reduced pressure in the desorber.

The remaining heated contaminant vapour is supplied to an inner portion of a second section of the rotating condenser, causing the remaining heated vapour to flow radially outwards through the second section of the rotating condenser. The contaminant is condensed and is made to flow radially outwards by centrifugal forces caused by the rotation of the condenser.

The desorbed contaminant is removed from the outer portion of the second section of the rotating condenser. The contaminant is removed from the desorber for further processing, while the condensed absorbent medium can be returned to the absorber.

This condenser arrangement is particularly useful for contaminants that can be removed in gaseous form, such as carbon dioxide, although it can be adapted for liquid contaminants such as water.

Alternatively, the condenser is provided with a single condenser section placed side-by-side with the desorber section. The condenser section is supplied by a source of coolant, wherein the coolant is introduced adjacent the inner portion of the condenser section and flows radially outwards to an outlet adjacent the outer portion of the condenser section. In this example, contaminant vapour free from absorption medium received from the desorber section is condensed in a single step. The heated contaminant vapour is supplied to the inner portion of a rotating condenser, forced to flow radially outwards through the condenser. The condensed contaminant is removed from the outer portion of the first section of the rotating condenser. This alternative condenser arrangement is particularly useful for contaminants that can be removed in liquid form, such as water, although it can be adapted for gaseous contaminants such as carbon dioxide.

In the above examples, the rich absorption medium is preferably supplied to a pair of identical and mirrored annular rotating desorber units rotating about a common axis within the desorber. A desorber unit used in the process described above can be defined as comprising an inner stripper section and outer reboiler section, arranged concentrically and rotated about the same axis. Alternatively, the desorber unit can be defined as comprising an integrated stripper and reboiler section, rotated about the same axis. When a pair of identical and mirrored annular rotating desorber units is used, a combustion chamber for supplying heat is provided at each end of the combined unit. The desorber further comprises a condenser unit with a single section or a double section arranged axially displaced from the stripper and reboiler sections and rotated at the same speed about the same axis.

The process according to the invention uses processing plant comprising an absorber for removal of contaminants from natural gas by means of an absorbent medium. The absorber comprises a vessel containing at least one annular absorber packing rotatable about its longitudinal axis within the vessel, which absorber packing has a predetermined axial extension with an inner radius and an outer radius. The absorber further comprises a lean absorbent inlet, arranged radially inside an inner perimeter of the absorber packing, and a rich absorbent outlet, arranged radially outside an outer perimeter of the absorber packing. The absorber also comprises a natural gas inlet for pressurized natural gas, arranged radially outside an outer perimeter of the absorber packing, and a natural gas outlet, arranged radially outside an outer perimeter of the absorber packing.

When the absorber packing is rotated around its longitudinal axis, a cross flow of absorbent and natural gas occurs. The rotation of the annular absorber packing is arranged to subject the absorbent medium to a centrifugal force sufficient to force the absorbent towards the outer perimeter of the annular absorber packing, in the opposite direction of the natural gas, in order to cause a cross flow for mass transfer of contaminant from the natural gas to the absorbent to produce dehydrated natural gas.

As described above, the at least one annular absorber packing has a substantially cylindrical shape with a predetermined extension along its longitudinal axis. An absorber packing assembly can comprise a single absorber packing or multiple absorber packings which are symmetrical on either side of a central plane at right angles to the rotational axis of the absorber packing. The central plane is, for instance, taken through a position located at the mid-point of a single absorber packing or through a position half way between the facing ends of two absorber packings located end-to- end with an axial separation along a common axis of rotation. Preferably, the at least one absorber packing and any component parts enclosed by or enclosing the absorber packing and rotated with the said absorber packing should be symmetrical or substantially symmetrical relative to the central plane. The said absorber packing has an outer perimeter in the form of a cylindrical surface at a first radius from the longitudinal axis and an inner perimeter in the form of a cylindrical surface at a second radius from the longitudinal axis. The thickness of the annular absorber packing is determined by the difference between the said first and second radii. The absorber packing is preferably filled with a material having a relatively high specific area, such as metal foam or a similar suitable alveolar material.

The pressurized natural gas is supplied to an inlet at the outer perimeter of the at least one annular absorber packing, from where it flows radially inwards towards the centre of the absorber packing. As the natural gas flows through the absorber packing, the cross-flow with the absorbent medium removes contaminant from the natural gas, leaving a dehydrated or a sweetened natural gas. As explained above, the condition of the gas is dependent on the type of absorbent medium used. The dehydrated or sweetened natural gas is guided from the inner perimeter of the annular absorber packing towards an outlet at the outer perimeter of said absorber packing through a radially open section in the annular absorber packing. The outlet is separated from the inlet by at least one radial wall that extends from the inner perimeter of the annular absorber packing to a gas tight seal at the inner wall of the vessel.

The absorbent medium is supplied as a lean absorbent to an inlet at an inner perimeter of an annular absorber packing through a hollow rotor shaft supporting the absorber packing. From the central inlet, the lean absorbent flows through the hollow shaft and into radially extending channels. The channels are connected to axially extended distribution tubes along the inner perimeter of the absorber packing. A number of openings or nozzles are arranged at regularly spaced locations along the distribution tubes adjacent the inner perimeter to provide a substantially even distribution of absorbent medium around the inner cylindrical surface of the absorber packing. The said nozzles may be kept stationary relative to the rotating absorber packing, or be rotated at a different speed than said absorber packing. As the annular absorber packing is rotated, the absorbent medium is forced from the inner perimeter towards the outer perimeter of the annular absorber packing by relatively high centrifugal force which, depending on the speed of rotation, can be several hundred G. The absorbent medium, which at this stage is a rich absorbent, will be thrown radially outwards onto the inner wall of the vessel upon leaving the absorber packing and will then flow downwards to an outlet at the lower portion of the vessel.

In the case of an absorber packing rotated about a horizontal axis, the natural gas outlet is axially spaced from both the absorbent outlet and the natural gas inlet. The absorbent outlet can be arranged radially outside an outer perimeter of the absorber packing in a lower section of the vessel. Also, the natural gas inlet can be arranged radially outside an outer perimeter of the absorber packing circumferentially spaced from the absorbent outlet, that is, above the lower section of the vessel. The annular absorber packing can comprise a radially open section extending from the inner perimeter to the outer perimeter of the annular absorber packing, wherein the open section is arranged to guide dehydrated natural gas from the inner perimeter towards an outlet at the outer perimeter of said absorber packing. As indicated above, the outlet from an open section is arranged separate from the inlet in the axial direction of the absorber packing. The annular absorber packing can be provided with an open section at either end of the absorber packing or between two substantially identical absorber sections making up the absorber packing. Kinetic energy can be recovered from the dehydrated natural gas by means of radial vanes arranged in the said open section. As described above, a radially open section can be arranged between two adjacent annular absorber packings, or at each end of the at least one annular absorber packing. The natural gas discharge vanes have the additional function of separate absorbent medium droplets from the gas flow. The latter function requires some sort of droplet traps to be integrated in the design. The recovery of absorbent droplets from the dehydrated natural gas can be achieved by means of droplet traps arranged at the outer perimeter of the annular absorber packing adjacent at least one side of the said open section or sections described above. The droplet traps may comprise a labyrinth or analogues type of seal to prevent the gas flow from taking a short cut from the inlet directly to the outlet, past the at least one radial wall.

According to the present invention, the traditional gravimetrical packed absorber column is replaced with a relatively high speed rotating annular column where a much more dense packing can be used in combination with more viscous absorbents. This is made possible because centrifugal forces of more than 400 G may be reached.

Depending on the longitudinal extension of the absorber packing, a combination of the examples described above is also possible. The desorption unit outlined above comprises a radially outer first rotating desorption zone. A combustion chamber is provided adjacent the outer radial surface of the first rotating desorption zone and is arranged to rotate with the desorber. Hot exhaust gas from combustion of natural gas is used for transferring heat to heat pipes extending through at least the first rotating desorption zone. The heat pipes supply heat to an outer perimeter of the first rotating desorption zone, causing evaporation of the contaminant in the rich absorbent medium flowing radially outwards. The evaporated contaminant flows radially inwards in order to create a radial cross-flow in the first rotating desorption zone. During this radial cross-flow a first portion of the contaminant is desorbed from the rich absorption medium by the vapour during said radial cross-flow. The cross-flow is created by vapour under pressure being forced radially inwards and the absorption medium being forced radially outwards by centrifugal forces caused by the rotation of the first desorption zone. The release of the first portion of acid gases is achieved by the said vapour heating the absorption medium to a predetermined temperature, which is dependent on the type of absorption medium used. Lean absorption medium is removed from an outer perimeter of the outer rotating desorption zone and transferred back to an absorber. The first and the second rotating desorption zones described above are preferably arranged as concentric annular desorption zones rotating about a common central axis. The first rotating desorption zone performs the same function as a conventional stripper unit, while the second rotating desorption zone performs the same function as a conventional reboiler unit. Alternatively, desorption is performed in a single rotating desorption zone comprising an integrated annular stripper and reboiler unit rotatable about a common axis. The stripper and reboiler unit can be provided with a number of tubes for heat supply. Heat is supplied to an outer perimeter of the first rotating desorption zone by heat pipes heated by combusting natural gas in a chamber adjacent the at least one annular rotating desorber unit. The exhaust gases from the combustion process heats heat pipes passing through a heat exchanging section for heating the entire stripper and reboiler unit. The combustion chamber is connected to an outlet for removing the exhaust gas. The rich absorbent medium is introduced at an inner perimeter of the stripper and reboiler unit and the stripping will take place as the absorbent medium is heated while flowing radially outwards. Regenerated lean absorbent medium leaves the stripper and reboiler unit at the outer circumference thereof at a lean absorbent medium outlet. The fluid stripped from the absorbent medium leaves the stripper and reboiler unit near its inner perimeter. In the case of C0 ₂ desorption, the fluid comprises C0 ₂ gas and an amount of absorbent medium comprising amine absorbent and a diluent such as water in vapour phase. The fluid stripped from the rich absorbent medium is then directed into the outer perimeter of a condenser.

Depending on the type of fluid stripped from the rich absorbent medium, said fluid is passed through a single or double condenser. If the fluid is a liquid, such as water stripped from glycol in a dehydration process, then the fluid is condensed while flowing radially outwards through a single condenser. If the fluid is a gas, such as C0 ₂ stripped from an amine in a C0 ₂ removing process, then the fluid is condensed in a first step while flowing radially inwards to remove any amine carry-over and then radially outwards to remove any water from the C0 ₂ gas.

As stated above, heat is supplied to an outer perimeter of the first rotating desorption zone by heat pipes, which pipes are heated by combusting natural gas in a chamber adjacent the at least one annular rotating desorber unit. The heat pipes are used for transferring heat for the evaporation process in at least the first rotating desorption zone. Providing the desorber with a combustion chamber adjacent at least each first desorption zone is advantageous in that the process can supply its own fuel and the combustion process can be instantaneously adjusted to the current heat requirement of the heat pipes in the desorber unit. Otherwise the desorber would have to be placed in the vicinity of an external source of heat, such as a gas turbine with a steam generator. In the case of the invention, the need for such a steam generator can be eliminated. The natural gas used for the combustion can be taken from an external source, but is preferably taken from the stream of processed natural gas leaving the absorber.

The use of exhaust gas recirculation (EGR) assists in reducing emissions from the process. Also, by placing exhaust gas purification components, such as a channel integrated treatment, adjacent to the source of exhaust gas as well as in conjunction with absorber and desorber means for the absorbent used in the exhaust treatment, the process plant can be made very compact.

The location of the process plant can be made more or less independent of other suitable source of heat, as the use of natural gas from the process can make the plant virtually self-sufficient for its supply of heat required by the process. This is a particular advantage of, for instance, an offshore facility. BRIEF DESCRIPTION OF DRAWINGS

The invention will be described in detail with reference to the attached figures. It is to be understood that the drawings are designed solely for the purpose of illustration and are not intended as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to schematically illustrate the structures and procedures described herein:

Figure 1 shows a schematic, partially cross-sectioned absorber for use in a process according to the invention;

Figure 2 shows a cross-section through a first alternative embodiment of an absorber for use in a process according to the invention;

Figure 3 shows a schematic diagram of the flow of absorbent and natural gas through the absorbers of Fig. 1 and Fig.2; Figure 4 shows a cross-section through a second alternative embodiment of an absorber for use in a process according to the invention;

Figure 5 shows a schematic diagram of the flow of absorbent and natural gas through the absorber of Fig.4;

Figure 6A shows a first embodiment of a direct heated desorber for use in a process according to the invention;

Figure 6B shows a schematic diagram of a heat pipe circuit used for heating the direct heated desorber in Figure 6A;

Figure 6C shows a schematic second embodiment of a direct heated desorber for use in a process according to the invention;

Figure 7 shows a second embodiment of a desorber for use in a process according to the invention; and

Figure 8 shows a process according to the invention, using a rotating absorber and a direct heated rotating desorber.

Figure 9 shows a conventional prior art process using absorber and desorber columns.

EMBODIMENTS OF THE INVENTION

The embodiments described below are intended for use in CO ₂ removal. However, the described devices and processes can also be adapted for use in H ₂ S removal using organic scrubbing fluids or for dehydration using glycol.

Figure 1 shows a schematic, partially cross-sectioned absorber for use in a process according to the invention. The absorber in Figure 1 comprises a vessel 101 in the form of a cylindrical outer stator shell containing a first and a second annular absorber packing 102a, 102b rotatable about a longitudinal axis X within the vessel 101 , which absorber packings have a predetermined axial extension with an inner radius r and an outer radius R (see Fig.3). The absorber further comprises an absorbent inlet 103, arranged for supplying lean absorbent to the absorber packing, and absorbent outlets 104a, 104b, arranged for removing rich absorbent on the vessel radially outside the outer perimeter 1 1 1 of the absorber packings at the lower section of the vessel 101 . Lean absorbent is supplied to the vessel 101 from a conduit (not shown) connected to a rotary joint 105 attached to an inlet shaft 106 in the form of a hollow idle shaft comprising a central pipe for transport of lean absorbent through the inlet shaft 106 to a number of radial channels 107a, 107b for transport of lean absorbent to a number of longitudinal distribution tubes 108a, 108b, which distribution tubes adjacent the inner perimeter 1 12 of the absorber packings 102a, 102b is provided with nozzles (not shown) for even distribution of lean absorbent tangentially and axially on the inner perimeter of the absorber packings. The absorber also comprises natural gas inlets 109a, 109b for pressurized natural gas, arranged radially outside an outer perimeter of the absorber packing, and natural gas outlets 1 10, arranged on the vessel radially outside the outer perimeter of the absorber packing. In this example, the natural gas outlets 1 10 are arranged axially separated from the natural gas inlets 109a, 109b, and aligned with a radially open section 1 13 separating the absorber packings 102a, 102b. The arrangement in Figure 1 has four natural gas outlets 1 10 arranged in a radial plane through the vessel, which outlets are located equidistant around the circumference of the vessel. The radially open section 1 13 is separated from the natural gas inlets 109a, 109b and the facing end surfaces of the absorber packings 102a, 102b by first and second radial walls 1 14a, 1 14b. Each radial wall 1 14a, 1 14b extends from the inner perimeter of the annular absorber packing to a gas tight labyrinth seal 1 15a, 1 15b at the inner wall of the vessel. A third radial wall 1 14c is located between the first and second radial walls 1 14a, 1 14b and extends from the inlet shaft 106 to the outer perimeter of the absorber packings 102a, 102b. The third radial wall 1 14c is provided to guide the flow of sweet natural gas from the inner perimeter of the absorber packings towards the natural gas outlets 1 10. The opposing end surfaces of the absorber packings 102a, 102b are sealed by a pair of rotor end plates 1 16a, 1 16b to form a an absorber packing assembly or rotor assembly. The rotor end plates 1 16a, 1 16b are each supported inside the vessel by a rotor shaft journalled at each end of the vessel. The rotor assembly is held together by means of multiple axial tension rods 1 17 (one shown) which extend through all the radial walls in the assembly outside the outer perimeter of each absorber packing and are bolted to the rotor end plates and the radial walls 1 14a, 1 14b adjacent the open section 1 13. The vessel 101 comprises a cylindrical outer stator shell having a pair of end domes, wherein each dome is provided with a gas tight seal 1 18a, 1 18b around the respective rotor shaft. The entire absorber assembly located between these rotor shafts is rotated as a unit by a driving torque T applied to a driven rotor shaft 1 19 located on the opposite side of the vessel relative to the absorbent inlet 103. The absorber packing assembly comprises two absorber packings which are symmetrical on either side of a central plane at right angles to the rotational axis of the absorber packing. The central plane is, in this case, taken through a position half way between the facing ends of two absorber packings located end-to-end or with an axial separation along a common axis of rotation.

For the embodiment described in Figure 1 , an example of a suitable size for the absorber arrangement is a pair of absorber packings each having an inner diameter of 1 m, an outer diameter of 2,5 m and a length of 2,2 m. Using a suitable metal foam having a surface area of 2500 m2/m3, theses dimensions give surface area of 223 m3 and a volume of 18 m3 of metal foam. Four natural gas inlets with a diameter of 200-250 mm will give a gas velocity of up to 20 m/s. A lean absorbent inlet with a diameter of 169 mm at the idle rotor shaft will give an absorber velocity of 10 m/s. In order to rotate the assembly at 450 rpm to achieve 400 G on the absorbent in the absorber packing, 1250 kW is required for transport of the lean absorbent alone. The power consumption for the gas will be lower because momentum is exchanged from the lean absorbent. Figure 2 shows a cross-section through a first alternative embodiment of an absorber for use in a process according to the invention. The absorber in Figure 2 differs from that of Figure 1 in that it comprises an absorber provided with means for recuperating energy from the gas flow through the absorber. The recuperating means is placed within the open section 1 13 between the absorber packings 102a, 102b, as shown in Figure 1 , and comprises a radial discharge fan 201 with curved radial vanes, as shown in Figure 2. In Figure 2, the radially open section 1 13 is separated from the natural gas inlets (not shown) and the facing end surfaces of the absorber packings 102a, 102b by first and second radial walls 1 14a, 1 14b. A third radial wall 1 14c is located between the first and second radial walls 1 14a, 1 14b and extends from the inlet shaft 106 to the outer perimeter of the absorber packings 102a, 102b. The third radial wall 1 14c is provided to guide the flow of sweet natural gas from the inner perimeter of the absorber packings towards the natural gas outlets (not shown).

The radial discharge fan 201 comprises a first and a second set of radial vanes 202a, 202b, wherein the first set of radial vanes 202a is attached between the first radial wall 1 14a and the third radial wall 1 14c. Similarly, the second set of radial vanes 202b is attached between the second radial wall 1 14b and the third radial wall 1 14c.

The radial vanes have several functions, such as acting as a mechanical, torque transmitting connection between the two absorber sections, assisting in transport of sweet gas from centre to periphery while recovering some of the momentum to rotational power, and assisting in separating rich absorbent droplets from the sweet natural gas. The latter function requires droplet traps to be integrated in the design, as described for the embodiment according to Figure 1 above.

The energy recovery is achieved by guiding sweet natural gas through the radial vanes 202a, 202b in the radially open section 1 13, whereby some of the momentum from the pressurized sweet natural gas flowing towards the outlet is transferred to the vanes of the discharge fan 201 . The recovered momentum causes a driving torque applied to the rotor shaft and assists in rotating the absorber assembly.

Figure 3 shows a schematic illustration of the flow patterns of fluids in the rotated absorber assembly in the rotational symmetric radial-axial plane of the units shown in Figures 1 and 2. Absorbent medium is supplied from an inlet a- _\ through a central rotor shaft and is distributed on the inner perimeter a ₂ of the absorber packing. The lean absorbent is distributed and transported a ₃ to the outer perimeter a ₄ by the high centrifugal forces (high G) in the rotating absorber assembly. The rich absorbent is then removed via an outlet a ₅ for processing and re-use. The sour gas is supplied from inlets A at the outer periphery of the absorber packing and is forced towards the centre of the assembly in the opposite direction A ₂ allowing efficient cross flow for mass transfer of C0 ₂ to the lean absorbent. The sweet gas is guided along the inner perimeter A ₃ and then outwards A ₄ through an open section from the axial centre of the assembly. This open section can have vanes allowing recovery of kinetic energy from the gas as well as droplet traps to remove absorbent droplets carried over in the gas. Finally, the sweet gas is removed from the assembly through an outlet A ₅ . Figure 4 shows a cross-section through a second alternative embodiment of an absorber for use in a process according to the invention. The absorber in Figure 4 differs from that of Figure 2 in that it comprises an absorber provided with means for recuperating energy from the gas flow through the absorber placed within a first and a second open section 413a, 413b at either end of an absorber packing 400. Each open section 413a, 413b comprises a radial discharge fan 401 a, 401 b with curved radial vanes, similar to the arrangement shown in Figure 2. In Figure 4, the radially open sections 413a, 413b are separated from the natural gas inlets (not shown) and the opposite end surfaces of the absorber packing 400 by first and second radial walls 414a, 414b. A third radial wall 414c is located in a position equidistant from the first and second radial walls 414a, 414b and extends from the inlet shaft 406 to the inner perimeter 412 of the absorber packing 400. The third radial wall 414c is provided to guide the flow of sweet natural gas from the inner perimeter of the absorber packing towards the natural gas outlets 410a, 410b. This arrangement also ensures that the flow of natural gas is distributed so that each radial discharge fan 401 a, 401 b will receive approximately the same gas flow.

The radial discharge fans 401 a, 401 b comprise a first and a second set of radial vanes 402a, 402b, wherein the first set of radial vanes 402a is attached between the first radial wall 416a and a first rotor end plate 412a. Similarly, the second set of radial vanes 402b is attached between the second radial wall 414b and a second rotor end plate 416b, in order to form a an absorber packing assembly or rotor assembly. The rotor assembly is held together by means of multiple axial tension rods 417 (schematically indicated in Fig.4) which extend through all radial walls in the assembly outside the outer perimeter 41 1 of the absorber packing and is bolted to the rotor end plates. The absorber packing assembly comprises a single absorber packing which is symmetrical on either side of a central plane at right angles to the rotational axis of the absorber packing. The central plane is, in this case, taken through a position located at the mid-point of the single absorber packing along the axis of rotation.

As stated above, the radial vanes have several functions, such as acting as a mechanical, torque transmitting connection between the two absorber sections, assisting in transport of sweet gas from centre to periphery while recovering some of the momentum to rotational power, and assisting in separating rich absorbent droplets from the sweet natural gas. The latter function requires droplet traps to be integrated in the design, as described for the embodiment according to Figure 1 above. In the embodiment of Figure 4, the radial walls 414a, 414b extends from the inner perimeter of the annular absorber packing to a gas tight labyrinth seal 415a, 415b at the inner wall of the vessel.

The energy recovery is achieved by guiding sweet natural gas through the radial vanes 402a, 402b in the radially open section 413a, 413b, whereby some of the momentum from the pressurized sweet natural gas flowing towards the outlet is transferred to the vanes of the discharge fans 401 a, 401 b. The recovered momentum causes a driving torque applied to the rotor shaft 1 19 and assists in rotating the absorber assembly.

Figure 5 shows a schematic illustration of the flow patterns of fluids in the rotated absorber assembly in the rotational symmetric radial-axial plane of the unit shown in Figure 4. Absorbent medium is supplied from an inlet a through a central rotor shaft and is distributed on the inner perimeter a ₂ of the absorber packing. The lean absorbent is distributed and transported a ₃ to the outer perimeter a ₄ by the high centrifugal forces (high G) in the rotating absorber assembly. The rich absorbent is then removed via an outlet a ₅ for processing and re-use. The sour gas is supplied from inlets A at the outer periphery of the absorber packing and is forced towards the centre of the assembly in the opposite direction A ₂ allowing efficient cross flow for mass transfer of C0 ₂ to the lean absorbent. The sweet gas is guided along the inner perimeter A ₃ and then outwards A' ₄ from the axial centre of the assembly through open sections at each end of the assembly. The open sections can have vanes allowing recovery of kinetic energy from the gas as well as droplet traps to remove absorbent droplets carried over in the gas. Finally, the sweet gas is removed from the assembly through outlets A' ₅ at each end of the assembly.

Figure 6A shows a first embodiment of a desorber for use in a process according to the invention. The axis of rotation of the desorber is horizontally aligned. Arrows in Figure 6A schematically illustrates the directions of flow through the different sections of the desorber. The cover enclosing the units making up the desorber is schematically indicated in the figure. Although only one rotating desorber wheel is described below, the desorber is preferably configured with two identical, mirrored desorber/condenser units on each side of a plane at right angles to the axis of rotation.

The desorber for the process is provided with a number of Inlets and outlets. A desorber unit comprises an annular stripper unit 618 rotatable about a hollow central rotor axle 629 having a rich absorbent inlet 601 . The rich absorbent is supplied through a static pipe mounted into the centre of the hollow rotor axle 629 by means of a fixture 625. The absorbent inlet 601 supplies rich absorption medium to an inner portion of a first rotating desorption zone 618, or stripper, causing the rich absorption medium to flow radially outwards through the first rotating desorption zone (see arrow 61 1 ). A number of absorption medium distribution pipes 617 are arranged to distribute the rich absorbent over the inner surface of the first rotating desorption zone 618. A packing with a relatively high specific surface area can be used in the first rotating desorption zone 618 due to the centrifugal forces created by the rotation. Heated vapour is supplied to an outer perimeter of the first rotating desorption zone 618, causing the vapour to flow radially inwards (see arrow 612) in order to create a radial cross-flow in the first rotating desorption zone 618. During this radial cross-flow a first portion of acid gases are desorbed from the rich absorption medium by the hot vapour during said radial cross-flow. The cross-flow is created by vapour under pressure being forced radially inwards and the rich absorption medium being forced radially outwards by centrifugal forces caused by the rotation of the first desorption zone 618. The release of the first portion of acid gases is achieved by the heated vapour heating the absorption medium to a predetermined temperature, which is dependent on the type of absorption medium used. If the contaminant and the absorption medium are both in liquid form, e.g. water and glycol, then they are heated to a temperature above the evaporation temperature of the contaminant, but below the evaporation temperature of the absorption medium. The partially regenerated absorption medium will be forced through the first rotating desorption zone 618 and into the second rotating desorption zone 619, or reboiler, where the remaining acid gases are removed to produce a lean absorption medium. The the second rotating desorption zone 619 comprises a heat exchanger made up of a large number of thin, closely stacked discs and a number of parallel heat pipes (not shown). Desorption is achieved by supplying a heat, in the form of exhaust gas, to the heat pipes which transfer the heat into the second rotating desorption zone 619, wherein a remaining portion of the acid gases and a portion of the absorption medium containing amine and liquid diluent are heated to form a vapour. Exhaust gas is supplied through an exhaust gas inlet 603 comprising a ring volume between a first end lid 633 and the rotating desorber unit. The exhaust gas inlet 603 is connected to one or more combustion chambers 641 (schematically indicated) arranged on an outer surface of the first end lid 633. The combustion chamber 641 is connected to a supply of natural gas 642, which is supplied to the combustion chamber together with ambient air 643 to form a combustible mixture.

Alternatively, a combustion chamber can be arranged between the first end lid and the rotating desorber unit. In this case, the combustible mixture is supplied through the first end lid into an annular combustion chamber (See Figure 6C) in direct contact with the heat pipes extending into the exchanger in the second rotating desorption zone. A schematic heat pipe circuit will be described in connection with Figure 6B.

Recirculated exhaust gas can be taken from the exhaust gas outlet 604 to be mixed with the ambient air 643, if desired. Dependent on the current combustion conditions and the quality of the natural gas used, the amount of recirculated exhaust gas can be varied between zero and 70 percent of the total injected volume of combustible gas mixture to be combusted.

The combustible mixture is ignited in the combustion chamber 641 and the exhaust gas is passed into contact with the heat pipes 609 extending through the second rotating desorption zone 619. A control unit (not shown) continuously controls the amount of natural gas, air and, if desired, recirculated exhaust gas to be combusted in order to control the amount of heat transferred to the heat pipes and thus the temperature in the second rotating desorption zone 619. The natural gas used for direct heating of the desorber can be taken from the gas being treated in the process. If the natural gas leaving the rotating absorber wheel has a suitable quality, the gas is preferably taken directly from the rotating absorber.

In this example, the first end lid 633 of the desorber faces the desorber unit, while the second end lid 636 faces the condenser unit. The ring volume is sealed by a labyrinth seal, as mechanical contact may not be used in view of the high tangential velocities. A number of heat pipes 609, which are in contact with the exhaust gas at one end, ensure a good axial distribution of heat through the desorber unit. The evaporation causes an increase in pressure which forces the hot vapour radially inwards through both the second and the first rotating desorption zones 619, 618. This flow of vapour through the first rotating desorption zone has been described above. The absorption medium is thrown outwards into a concentric collection trough 620 that is an integrated part of the rotor. The kinetic energy is recovered by a special Pitot tube arrangement. A series of pitot tubes will be mounted around the stator to obtain enough capacity to remove the large quantity of absorbent treated. Lean absorption medium is removed from an outlet 602 at the outer perimeter of the second rotating desorption zone 619 and transferred back to an absorber. The desorber unit is provided with an outlet 604 for exhaust gas from the combustion chamber. The exhaust gas outlet 604 comprises a ring volume between the first end lid 633 and the rotating desorber unit. The exhaust gas is in turn removed for treatment (see Fig. 8). The vapour containing diluent vapour, remaining absorbent medium in vapour form and acid gas, such as C0 ₂ is collected in a central flow channel 621 leading to a condenser unit (see arrow 613). The first and the second rotating desorption zones described above are preferably arranged as concentric annular desorption zones rotating about a common central axis. The first rotating desorption zone 618 performs the same function as a conventional stripper unit, while the second rotating desorption zone 619 performs the same function as a conventional reboiler unit.

The desorber in Figure 6A comprises a condenser unit provided with a single condenser section 622 placed side-by-side with the desorber unit. The condenser unit is supplied by a source of coolant, such as water, through a coolant inlet 605 comprising a ring volume between a first end lid and the rotating condenser unit. In this example, the first end lid of the condenser unit faces away from the desorber unit, while the second end lid faces the desorber unit. The ring volume is sealed by a labyrinth seal. The coolant is introduced adjacent the inner portion of the condenser section and flows radially outwards (see arrow 614) to a coolant outlet 606 adjacent the outer portion of the condenser section. The coolant outlet 606 comprises a further ring volume between the first end lid and the rotating condenser unit. This ring volume is also sealed by a labyrinth seal. In this example, the portion of the vapour containing absorption medium received from the desorber section is condensed in a single step. The condenser section 622 is made up of a number of closely stacked, thin coolant filled discs. The condensed liquid containing amine and diluent flows radially outwards (see arrow 616) and is removed from the outer portion of the first section of the rotating condenser. A conical collection trough 623 using pitot tubes is used for removing process condensate to a condensate outlet 607. The condensed liquid is returned to the inner perimeter of the first or the second rotating desorption zone. At the same time, desorbed acid gases from the outer portion of the condenser section are removed through a ring volume provided with labyrinth seals. The acid gases flow radially outwards (see arrow 615) and are passed to an outlet 608 and are removed from the desorber for further processing. The cover enclosing the desorber and condenser units comprises a stationary first end cover 633, a stationary second end lid cover 636 and a cylindrical stator part 634 connecting the end covers. The desorber and condenser units are supported by the rotor axle 629, which is mounted in a first rotor bearing 627 at the first end cover 633, and a driving axle 630, which is mounted in a second rotor bearing 631 at the second end cover 636. A first and a second seal 628, 632 are arranged between the rotor axle 629 and the first end cover 633, and the driving axle 630 and the second end cover 636, respectively. A further seal 626 is arranged around the outer surface of a central pipe making up the absorbent inlet 601 and the the rotor axle 629. The desorber and condenser units are connected by a hollow intermediate axle 635 connecting the second end lid of the desorber unit and the second end lid of the condenser unit.

Although only one rotating desorber wheel is shown in Figure 6A, the desorber is preferably configured with two mirrored desorber/condenser units on each side of a plane at right angles to a common axis of rotation. In this case, the absorbent medium is supplied at both ends of the mirrored units, and the driving axle replaces one of the rotor axles.

Figure 6B shows a schematic diagram of a heat pipe circuit used for heating the direct heated desorber in Figure 6A. In operation a heat pipe 650 employs evaporative cooling to transfer thermal energy from a first end 651 to a second end 652 by the evaporation and condensation of a working fluid or coolant. Heat pipes rely on a temperature difference between the ends of the pipe, and cannot lower temperatures at either end beyond the ambient temperature.

According to the invention, the first end 651 of the heat pipe 650 comprises an evaporator wick 653 which is heated by exhaust gas 654 from a combustion chamber (see Figure 6A). The evaporator section, or wick 653, is heated so that the working fluid inside the pipe at that end evaporates and increases the vapour pressure inside the cavity of the heat pipe. The latent heat of evaporation absorbed by the vaporisation of the working fluid reduces the temperature at the hot end of the pipe. The vapour flows through a vapour transport line 655 into a condenser section 656, which comprises the first rotating desorption zone 618 in Figure 6A. Heat 657 is given off at the second end 652 of the heat pipe 650 and the vapour will condense.

The condensed working fluid then flows back to the hot first end 651 of the heat pipe through a liquid return line 658. In the case of heat pipes containing wicks, the fluid is returned by capillary action. Active control of heat flux can be effected by adding a variable volume liquid reservoir 659 to the evaporator section 653.

Figure 6C shows a schematic second embodiment of a direct heated desorber for use in a process according to the invention. The desorber for the process is provided with a number of Inlets and outlets. A desorber unit 660 comprises two identical symmetrically arranged annular stripper and reboiler units 661 , on either side of a central condenser unit 681 . The stripper and reboiler units 661 form a desorption zone rotatable about a hollow central rotor axle 662 and having a rich absorbent inlet 663 at each end. In the subsequent text, only one such stripper and reboiler unit 661 will be referred to. The rich absorbent is supplied through a static pipe mounted into the centre of the hollow rotor axle 662, as indicated in Figure 6A. The absorbent inlet 663 supplies rich absorption medium to an inner portion 664 of the rotating stripper and reboiler unit 661 , causing the rich absorption medium to flow radially outwards through the stripper and reboiler unit 661 . A number of absorption medium distribution pipes (not shown) are arranged to distribute the rich absorbent over the inner surface of the stripper and reboiler units 661 . A packing with a relatively high specific surface area can be used in the first rotating desorption zone 663 due to the centrifugal forces created by the rotation. Heat is supplied to the rich absorption medium by means of heat pipes 665 extending through the stripper and reboiler unit 661 in parallel with the central axle 662. A schematic heat pipe circuit is shown in Figure 6B. This causes the contaminant in the rich absorption medium to evaporate, whereafter the vapour will flow radially inwards to create a radial cross-flow with the absorption medium in the rotating desorption zone 661 . During this radial cross-flow hot vapour from the outer perimeter, or stripper, will assist in heating and evaporating the rich absorption medium entering the inner perimeter, or reboiler, of the desorption zone 661 . The cross-flow is created by vapour under pressure being forced radially inwards (see arrow 670) and the rich absorption medium being forced radially outwards (see arrow 671 ) by centrifugal forces caused by the rotation of the first desorption zone 661 . Lean absorption medium is removed from the outer perimeter of the rotating desorption zone 661 through an absorption medium outlet 680 The release of the first portion of contaminant (e.g. acid gas) is achieved by the heated vapour heating the rich absorption medium to a predetermined temperature, which is dependent on the type of absorption medium used. If the contaminant and the absorption medium are both in liquid form, e.g. water and glycol, then they are heated to a temperature above the evaporation temperature of the contaminant, but below the evaporation temperature of the absorption medium.

The rotating desorption zone 661 comprises a heat exchanger made up of a large number of thin, closely stacked discs (not shown) and a number of parallel heat pipes 665. Desorption is achieved by supplying a heat, in the form of combustion heat and exhaust gas, to the heat pipes 665 which transfer the heat into the rotating desorption zone 661 . A combustible mixture of natural gas and air is supplied through a combustion gas inlet 666 into a volume forming a combustion chamber 667 between the desorber casing 668 and the rotating desorption zone 661 . The heat generated by the combustion process and the hot exhaust gas in the combustion chamber 667 is in direct contact with the ends of the heat pipes 665 extending from the rotating desorption zone 661 into said combustion chamber 667. A control unit (not shown) continuously controls the amount of natural gas, air and, if desired, recirculated exhaust gas to be combusted in order to control the amount of heat transferred to the heat pipes and thus the temperature in the rotating desorption zone 661 . The natural gas used for direct heating of the desorber can be taken from the gas being treated in the process.

The desorber unit is provided with an outlet 668 for exhaust gas from the combustion chamber. The exhaust gas is in turn removed for treatment (see Fig. 8). The vapour containing diluent vapour, remaining absorbent medium in vapour form and acid gas, such as C0 ₂ is collected in a central flow channel 669 leading to a condenser unit 681 (see arrow 682).

The desorber in Figure 6C comprises a condenser unit 681 provided with a single condenser section placed between the desorber units 661 and receives vapour from both desorber units 661 . The condenser unit 681 is supplied by a source of coolant, such as water, through a coolant inlet 683 in the hollow central rotor axle 662. In this example, a first end of the hollow rotor axle has a conduit for rich absorbent medium, while a second end has concentric conduits for coolant and rich absorbent medium. The rotor axle can be driven at either end. The coolant is introduced through a central conduit 683 and is supplied adjacent an inner portion 684 of the condenser section and flows through channels (not shown) radially outwards (see arrow 685) to a coolant outlet 686 adjacent the outer portion of the condenser section. In this example, the portion of the vapour containing absorption medium received from the desorber section is condensed in a single step. The condenser section 681 is made up of a number of closely stacked, thin coolant filled discs (not shown). Any condensed liquid containing absorption medium carry-over, such as amine and diluent, flows radially outwards (see arrow 687) and is removed as condensate from a condensate outlet 688 at the outer portion of the first section of the rotating condenser. The condensed liquid is returned from the outlet 688 to the inner perimeter of the rotating desorption zone 661 . At the same time, desorbed acid gases are removed from the outer perimeter of the condenser section 681 . The acid gases flow radially outwards (see arrow 689) towards an outlet 690 and are removed from the desorber for further processing.

Figure 7 shows a second embodiment of a desorber for use in a process according to the invention where the axis of rotation is horizontally aligned. The embodiment has a number of similarities with the embodiment shown in Figure 6A. The main difference between the embodiments is the condenser arrangement. Figure 7 schematically illustrates the directions of flow through the alternative desorber. The cover enclosing the units making up the desorber is not shown in this schematic figure. Figure 7 shows an integrated annular stripper and stripper and reboiler unit 717 unit rotatable about an axis X. In the illustrated embodiment the stripper and reboiler unit 717 is designed with a number of small diameter tubes for heat supply. Exhaust gas is supplied trough conduit 704 and transfers heat to heat pipes passing through the rotating absorption zone. The heat pipes can be parallel with the axis of rotation, or placed at an angle relative to said axis. The exhaust gas is in communication with a conduit 706 for removing the said gas. For the purpose of illustration, only a few heat pipes are indicated parallel to the axis of rotation. However, the stripper and reboiler unit may comprise any number of heat pipes. In this embodiment the stripper is integrated in the reboiler. The C0 ₂ rich absorbent medium is introduced via conduit 702 and the stripping will take place when the absorbent medium is introduced at an inner perimeter of the stripper and reboiler unit 717. Regenerated lean absorbent medium leaves the stripper and reboiler unit 717 at the outer circumference thereof at a lean absorbent medium outlet 718. The C0 ₂ and an amount of absorbent medium comprising absorbent and diluent in vapour phase leaves the stripper and reboiler unit 717 near its inner perimeter and is passed into conduit 720. The C0 ₂ and absorbent medium vapour is then directed into the outer perimeter a first condenser section 716. In order to create additional surface area for the mass transfer, it is proposed in one aspect of the invention to include layers of thin metal mesh between the rows of heat pipes. Other dimensions and configurations may of course equally well be used. A fine metal mesh with wire diameter 0.5-1 mm diameter gives specific surface areas over 1000 m ² /m ³ , depending on mesh spacing. The heat pipes can be fixed to the end plates using conventional techniques. In this embodiment it is proposed to use horizontal tubes in the stripper and reboiler unit instead of sloping heat pipes. This is mainly because of design and manufacturing considerations. In one aspect of the present invention sieve trays or perforated plates are included between the rows of heat pipes for heat supply instead of thin metal mesh. The sieve trays/perforated plates will increase the area of liquid gas contact and also contribute to enhanced distribution of the liquid phase.

In another aspect of the present invention small spherical elements can be included between the rows of heat pipes.

Due to design considerations it is preferred to use a design with CO ₂ gas flow towards the rotation centre. The regenerated absorbent medium will flow towards the periphery under the influence of centrifugal forces generated by the rotation of the stripper and reboiler unit 717. Subsequently, the gas stripped from the absorbent medium is guided from the central portion of the stripper and reboiler unit 717 to the outer perimeter the first condenser section 716. This can be achieved by including radial flow channels with rigid steel plates.

The embodiment illustrated in Figure 7 comprises a two stage condenser unit comprising a first condenser section 716 and a second condenser section 746 mounted on the same axis of rotation X as the stripper and reboiler unit 717. Cooling liquid is entered at the centre of the second condenser section 746 through conduit 708 and flows radially outwards through the second condenser section 746 to an outer portion thereof. The cooling liquid is then supplied to an outer portion of the first condenser section 716 and flows radially inwards before being removed through conduit 710 arranged at the centre of the first condenser section 716.

In the first condenser section 716 diluent and absorbent is condensed and will due to the rotation of the condenser unit be transported to the outer perimeter where it leaves the condenser section 716 as a liquid stream at an outlet 722. The outlet 722 can be connected to the absorbent medium inlet 702 at the stripper and reboiler unit 717 to return the condensed absorbent medium as reflux. The reflux of condensed vapour towards the outlet 722 creates a cross-flow over the gas mixture in the first condenser section716. This reflux contributes to the elimination of absorbent vapour from the desorbed C0 ₂ . The remaining vapour comprising C0 ₂ and diluents is passed from the first to the second condenser section 746 through a conduit 741 at the inner periphery of the respective condenser sections. In the second condenser section 746 a diluent free of absorbent is condensed and leaves the second condenser unit as a liquid stream at an outlet 742 at the outer periphery of the second condenser section. If water is used as diluent the obtained water stream from the second condenser section 746 may in one aspect of the present invention be utilized as washing liquid in the absorption process to remove traces of the absorbent from the regenerated C0 ₂ gas stream. The gas stream leaving the outlet 724 at the outer periphery of the second condenser unit will contain the desorbed C0 ₂ fit for drying and compression if needed for sequestration.

Although only one rotating desorber wheel is shown in Figure 7, the desorber is preferably configured with two mirrored desorber/condenser units on each side of a plane at right angles to a common axis of rotation. This arrangement solves a number of mechanical challenges. In the above example, the plane is located between two identical condensers. The axial load on the desorber caused by exhaust gas under pressure supplied for heating of the process can be significant. The symmetry implies that the load on each desorber is eliminated by the load of the opposite desorber. Another advantage is that the mass and energy flow to each part is reduced by 50% which makes the inflow and outflow of liquids/gases easier to handle.

Splitting the reboiler in two sections makes it possible to handle twice the volume of absorbent medium. The process according to the invention can regenerate more than 250 litres per second, which is considered to be a very large volume in comparison with conventional processes.

Yet another advantage is that the desorber section is a compact part of the rotor with respect to the mass of steel per unit volume. Splitting the reboiler in two sections and installing them as close as possible to the main bearings of the shaft reduces the mechanical loads of the rotating equipment significantly.

Still another advantage of providing symmetry according to the present invention is that the rotating desorber easily can handle varying volumes of absorbent mediums. A natural gas production plant, a gas power plant or a coal power plant does not operate at 100% all the time and the gas volume that needs to be cleaned for C0 ₂ will vary. The volume of liquid absorbent medium will thus vary. Since the liquid absorbent medium is equally distributed to the two reboiler sections, the problems with weight balance are not an issue.

Figure 8 shows a process according to the invention, using rotating absorber and desorber units. A sour gas stream containing undesirable hydrogen sulphide (H ₂ S) and carbon dioxide (C0 ₂ ) is introduced to a rotating absorber wheel RAW through line 801 . As the sour gas flows through the rotating absorber wheel RAW the sour gas contacts a cross-flow of normal lean amine which is introduced to the rotating absorber wheel RAW through line 802. The process gas has most of the acid gases removed by the time it leaves the rotating absorber wheel RAW after contacting the lean amine from line 802. A product gas (sweet gas) having a substantially reduced content of the hydrogen sulphide and carbon dioxide is withdrawn from the top of the rotating absorber wheel RAW via line 803.

A stream of rich amine solution containing absorbed hydrogen sulphide and carbon dioxide as salts of amine is removed from the rotating absorber wheel RAW through line 804. The pressure of the solution is reduced and it then flows to a rich amine flash tank C. The flash gases exit through line 805 and the rich amine solution exits through line 806. The rich amine stream passes through the line 806 to a lean/rich absorbent heat exchanger D and is then introduced to a rotating desorber wheel RDW through line 807. Stripping of the rich amine solution is carried out in a combined rotating desorber and condenser unit within the rotating desorber wheel RDW. The rich amine is first stripped in a rotating desorber unit, using a heat medium 808 in the form of exhaust gas contacting heat pipes arranged in the rotating desorber unit. The exhaust gas is subsequently removed at an exhaust gas outlet 812. Exhaust gas 808 is supplied from a combustion process in at least one combustion chamber (not shown) arranged adjacent or within the desorber unit of the rotating desorber wheel RDW. The combustion chamber is connected to a supply of natural gas, which is supplied to the combustion chamber together with ambient air to form a combustible mixture. This combustible mixture is then ignited in the combustion chamber. The exhaust gas from the combustion process is used for transferring heat to heat pipes extending through at least the first rotating desorption zone of the rotating desorber wheel RDW. A control unit (not shown) controls the amount of natural gas and air to be combusted in order to control the temperature in the desorber unit. The natural gas used for direct heating of the desorber unit is preferably taken directly from the outlet 803 from the rotating absorber wheel RAW.

In order to reduce the amount of contaminants such as nitrous oxide and other combustion products, a portion of the exhaust gas can be used for exhaust gas recirculation (EGR). This is achieved by removing an amount of exhaust gas from the exhaust outlet conduit 812 through an EGR conduit 821 and returning it to the combustion chamber (not shown) where natural gas and ambient air is combusted. Dependent on the current combustion conditions and the quality of the natural gas used, the amount of recirculated exhaust gas can be varied between zero and 70 percent of the total injected volume of combustible gas mixture to be combusted. In addition to the use of exhaust gas recirculation for the reduction of combustion products such as soot and nitrous oxide, the exhaust gas can be used for pre-heating the natural gas prior to the injection into the combustion chamber in the desorber.

The exhaust gas from the outlet 812 is conducted to an exhaust treatment unit CIT for carbon dioxide removal before being released to the atmosphere. A preferred exhaust treatment method is a channel integrated treatment, wherein exhaust gas is passed through a channel section 815 having a constant cross-sectional area. As schematically indicated in Figure 8, the cross-sectional area of the exhaust treatment unit CIT is the same as for the exhaust conduit entering and leaving the exhaust treatment unit. Injection nozzles 816 are arranged to inject atomized droplets of an absorbent medium, such as an amine, at high pressure into the exhaust stream. The amine can be supplied through a line 817 connected to the lean amine line 802 connecting the rotating absorber wheel RAW to the rotating wheel desorber RDW. The injected absorbent has a higher velocity than the exhaust gas and causes both an increase of the exhaust gas flow velocity as well as a mixing enhancing turbulence. The atomized droplets of rich absorbent containing absorbed carbon dioxide are captured by demisters 818 extending across the channel section 815 downstream of the nozzles 816. By injecting the absorbent at a higher velocity, kinetic energy is transferred to the exhaust gas. In this way the channel injection treatment does not cause a pressure drop in the exhaust gas through the exhaust treatment unit CIT. The rich amine captured by the demisters can be passed through a line 819 to the supply line of the rotating desorber RDW. Purified exhaust leaves the exhaust treatment unit CIT through an outlet conduit and is released to the atmosphere or is removed for further treatment.

The stripped lean amine temperature can vary from about 100 °C to 140 °C, depending on the type of amine, its concentration and its pressure. The hot lean amine stream 809 exits the outer periphery of the rotating desorber wheel RDW, is passed through the lean/rich absorbent heat exchanger D and through a cooler F where the lean amine solution is cooled to a temperature of about 35 °C to 55 °C. The cooled lean amine stream continues through line 802 to the top of the rotating absorber wheel RAW. The mixture of diluents vapour from the amine stream, hydrogen sulphide, and carbon dioxide exits the rotating desorber unit and flows through a rotating condenser unit. A coolant, such as water, is supplied to the rotating condenser unit through line 813 and is removed through line 814. The condensate from the rotating condenser unit is returned to the rotating desorber unit, while the acid gases are removed through line 810 for further processing or storage.

The rotating absorber/desorber assemblies described above are significantly smaller and more compact, as compared to traditional absorbers/desorbers as shown in Figure 9. The process according to the present invention saves footprint and weight, which can be critical in offshore installations. Since the absorber is rotated the liquid distribution for the gas/liquid contact is improved. As the absorber rotates the absorbent medium, the viscosity of the absorbent medium will not be such a limiting factor, as compared to conventional solutions used in conventional columns, and thus higher concentration absorbent mediums can be applied. In this context 60-100%, preferably 70-90%, is considered a high concentration of absorbent medium. The higher concentration absorbent mediums will allow circulation rates to be significantly reduced. The higher concentration absorbent mediums will also reduce pumping duties and heating and cooling demands in the overall process. The desorber, or stripper, size can therefore also be reduced. One example of a chemical solvent/absorbent to remove C0 ₂ /H ₂ S from the natural gas is amines, but the invention is not limited to amines. Other suitable absorbents can be used with the same result.

The rotating absorber/desorber assemblies described above are significantly smaller and more compact, as compared to traditional absorbers/desorbers as shown in Figure 9. The process according to the present invention saves footprint and weight, which can be critical in offshore installations. Since the absorber is rotated the liquid distribution for the gas/liquid contact is improved. Since the absorber rotates the absorbent medium, the viscosity of the absorbent medium will not be such a limiting factor, as compared to conventional solutions used in conventional columns, and thus higher concentration absorbent mediums can be applied. In the context of CO ₂ removal, 60-100%, preferably 70-90%, is considered a high concentration of an absorbent medium such as amine. The higher concentration absorbent mediums will allow circulation rates to be significantly reduced. The higher concentration absorbent mediums will also reduce pumping duties and heating and cooling demands in the overall process.

In the process according to the invention, any one or more of the described absorber embodiments can be combined with any one or more of the described desorber embodiments.