TECHNICAL FIELD

The present invention relates to a method and device for removal of acid gas from natural gas by means of chemical absorbents or solvents.

BACKGROUND ART

Removal of acid gas, such as carbon dioxide (C0 ₂ ) and/or hydrogen sulphide (H ₂ S) from natural gas, commonly termed "gas sweetening" is a well known technology. There are several commercial technologies available for this purpose such as chemical absorbents or solvents (i.e. amines, glycol), physical solvents, membranes, cold processes, etc. Such a chemical absorbent 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. 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 6.

In the prior art arrangement shown in Figure 6, 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 601. 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 602. The process gas has most of the acid gases removed by the time it leaves the absorber after contacting the lean amine from line 602. A product gas (sweet gas) having a substantially reduced content of the hydrogen sulphide and carbon dioxide is withdrawn from the top of the absorber via line 603. A stream of rich amine solution containing absorbed hydrogen sulphide and carbon dioxide as salts of amine is removed from the absorber through line 604. The pressure of the solution is reduced and it then goes to a rich amine flash tank C. The flash gases exit through line 605 and the rich amine solution exits through line 606. 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 607. Internal stripping steam is generated by reboiling the amine solution in stripper reboiler, or heat exchanger E, using a suitable heat medium 608. 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 611 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 610.

A hot lean amine stream 609 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 612 continues through line 602 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.

The object of the invention is to solve the above problems by providing an improved method and absorber for the removal of acid gas from natural gas.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a novel arrangement for removal of acid gas from natural gas by means of a chemical absorbent. The object is achieved by means of a method and an apparatus as described in the appended claims.

The invention relates to a method for removal of acid gas from natural gas, comprising mainly hydrocarbons, by means of a chemical absorbent. The method comprises the steps of;

supplying pressurized natural gas containing acid gas to an outer perimeter of at least one annular absorber packing, wherein the natural gas is forced towards the inner perimeter of the annular absorber packing;

distributing the chemical absorbent by rotating a distribution means relative to the inner perimeter at a first rotational speed;

supplying a chemical absorbent to an inner perimeter of the annular absorber packing;

rotating the annular absorber packing about its longitudinal axis at a second rotational speed, subjecting the chemical absorbent 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;

wherein a cross flow for mass transfer of acid gas from the natural gas to the chemical absorbent occurs to produce sweet natural gas.

The acid gas can comprise C0 ₂ and/or H ₂ S, and the natural gas comprises mainly hydrocarbons. A suitable chemical absorbent for this purpose is an ammonia compound such as amine.

The at least one annular absorber packing has a substantially cylindrical shape with a predetermined extension along its longitudinal axis. 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 subsequent text refers to a number of different gas or liquid inlets and outlets. Any references to such inlets and outlets in determinate form should be interpreted as "at least one" inlet or outlet, unless otherwise specified.

The method involves rotating the distribution means at a rotational speed that differs from the rotational speed of the said annular absorber packing. The distribution means and the said annular absorber packing are preferably, but not necessarily rotated in opposite directions. Rotation of the distribution means can be achieved by using an external source of power or by using the momentum of the natural gas contacting and interacting with the distribution means as it flows past the inner perimeter towards the natural gas outlet.

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 chemical absorbent removes acid gas from the natural gas, leaving a sweet natural gas. The method comprises the further step of guiding the sweet natural gas 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 chemical absorbent is supplied to an inlet at an inner perimeter of an annular absorber packing through a rotor shaft supporting the absorber packing. From the inlet, nozzles are arranged at different locations along the inner perimeter to provide a substantially even distribution of chemical absorbent 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 chemical absorbent is forced towards the outer perimeter of the annular absorber packing by relatively high centrifugal force which, depending on the speed of rotation and the diameter, can be several hundred G. The chemical absorbent, 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.

The method can involve a further step of recovering kinetic energy from the sweet natural gas by means of radial discharge vanes arranged in the said open section. The radial discharge vanes are fixed to the at least one absorber packing and form a turbine or impeller wheel which is acted on by the pressurized sweet natural gas, whereby momentum is transferred from the flowing gas to the rotating absorber packing. This energy recovery allows the power consumption for driving the rotating absorber packing to be reduced.

According to one example, the energy recovery is achieved by guiding sweet natural gas through a radially open section arranged between two sections of annular absorber packings towards the outlet. In this example the outlet is separated from the inlet by a pair of radial walls which extend from the inner perimeter of the annular absorber packing towards the inner wall of the vessel. The respective radial walls are attached to facing annular end surfaces of the absorber packing sections and are mechanically connected to the radial vanes. A further radial wall can be provided between the said radial walls, extending from the rotor shaft to the outer perimeter of the absorber packings, in order to assist and guide the flow of natural gas towards the outlet.

According to a second example, the energy recovery is achieved by guiding sweet natural gas through radially open sections arranged at each end of the annular absorber packing, towards the outlet. As in the above example the outlet is separated from the inlet by a pair of radial walls which extend from the inner perimeter of the annular absorber packing towards the inner wall of the vessel. The respective radial walls are attached to opposing annular end surfaces of the absorber packing sections and are provided with radial vanes located on the side of the respective wall remote from the end of the absorber packing.

Depending on the longitudinal extension of the absorber packing, a combination of the examples described above is also possible.

The sweet natural gas discharge vanes have the additional function of separate chemical absorbent droplets from the gas flow. The latter function requires suitable droplet traps to be integrated in the design. The recovery of absorbent droplets from the sweet 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.

The invention also relates to an apparatus for removal of acid gas from natural gas by means of a chemical absorbent. The apparatus 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 apparatus further comprises an 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. An absorbent distribution means is arranged to be rotated relative to the inner perimeter about the said longitudinal axis, in order to distribute chemical absorbent axially and circumferentially over the inner perimeter. The apparatus 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 chemical absorbent 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 acid gas from the natural gas to the absorbent to produce sweet 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 ends of two absorber packings located end-to-end or 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 a 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 chemical absorbent removes acid gas from the natural gas, leaving a sweet natural gas. The sweet 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 chemical absorbent is supplied as a lean absorbent to an inlet at an inner perimeter of an annular absorber packing through a hollow rotor shaft extending through the absorber packing. The distribution means is mounted on the rotor shaft and is arranged to be rotated at a rotational speed that differs from the rotational speed of the said annular absorber packing. The distribution means and the said annular absorber packing are preferably, but not necessarily, arranged to be rotated in opposite directions.

Rotation of the distribution means can be achieved by means of an external power source, such as an electric motor or other suitable means. Alternatively the distribution means is arranged to be acted on and rotated by the momentum of the natural gas contacting the distribution means as it flows through the central portion of the absorber packing towards the natural gas outlet.

From the central inlet, chemical absorbent flows through the hollow shaft and into radially extending channels in the distribution means towards openings or nozzles arranged at regularly spaced locations along an outer limiting peripheral surface adjacent the inner perimeter of the absorber packing to provide an initial distribution of chemical absorbent around the inner cylindrical surface of the absorber packing.

The distribution means comprises at least one vane extending radially from the central axis. The at least one vane may be straight, curved or comprise multiple vanes or blades. According to a first example the distribution means comprises at least one helical vane, which can include one or more continuous helical vanes having substantially the same longitudinal extension as the absorber packing. A distribution means of this type can either be driven or be acted on and rotated by the momentum of the natural gas. The at least one helical vane comprises radial channels for chemical absorbent, which channels extend through an outer peripheral edge of each said vane. The outer peripheral edge of the at least one helical vane is arranged to distribute chemical absorbent axially and peripherally over the inner perimeter of the said absorber packing. According to one example, the flow of absorbent is controlled to provide the correct amount of absorbent for the current flow of natural gas through the absorber. Variations in the flow of gas or absorbent may result in a temporary and/or local excess of absorbent being fed through the nozzles, whereby the at least one vane will carry any excess absorbent along the inner perimeter to ensure an even distribution. Alternatively, the flow of absorbent is controlled to provide a predetermined constant excess of absorbent which is carried along by the peripheral edge to safeguard against any irregularities in the radial flow through the absorber packing.

According to an alternative first example, the at least one helical vane can comprise multiple angled blades arranged in a circumferentially spaced and axially overlapping relationship. The blades can have an airfoil cross-section and be mounted on the outer periphery of a cylindrical body and extend towards the inner periphery of the absorber packing in the same way as a bladed turbine rotor. Each radial vane comprises a radial channel for chemical absorbent, which channel extends through an outer peripheral edge of each said vane. The outer peripheral edge of the at least one vane is arranged to distribute chemical absorbent axially and peripherally over the inner perimeter of the said absorber packing, as described for the helical vane above.

According to a second example the distribution means comprises at least one straight vane, which can include one or more continuous straight vanes having substantially the same longitudinal extension as the absorber packing. A distribution means of this type can be driven by an external motor. The at least one straight vane comprises radial channels for chemical absorbent, which channels extend through an outer peripheral edge of each said vane. The outer peripheral edge of the at least one straight vane is arranged to distribute chemical absorbent axially and peripherally over the inner perimeter of the said absorber packing, as described for the helical vane above. As the annular absorber packing is rotated, the chemical absorbent 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 chemical absorbent, 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 sweet 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 sweet 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. Droplet traps, as indicated above, can be arranged at the outer perimeter of the annular absorber packing adjacent the said open section, wherein absorbent droplets are recovered from the sweet natural gas.

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. The annular column according to the invention is preferably, but not necessarily rotated about a horizontal axis. 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 apparatus for removal of acid gas from natural gas by means of a chemical absorbent according to the invention;

Figure 2 shows a cross-section through an apparatus according to a first

alternative embodiment of the invention;

Figure 3 shows a schematic diagram of the flow of absorbent and natural gas through the apparatus of Fig. 1 and Fig.2;

Figure 4 shows a cross-section through an apparatus according to a second

alternative embodiment of the invention;

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

Figure 6 shows a conventional prior art process using an absorber column.

EMBODIMENTS OF THE INVENTION

Figure 1 shows a schematic, partially cross-sectioned apparatus for removal of acid gas from natural gas by means of a chemical absorbent according to the invention. The apparatus 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 apparatus 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 111 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 an idle shaft comprising a central pipe for transport of lean absorbent through the inlet shaft 106 to a number of radial channels 107a, 107b extending through at least two radial vanes 108a, 108b for transport of lean absorbent to the inner perimeter of the absorber packings. The vanes 108a, 108b shown in Figure 1 are straight vanes arranged symmetrically about the longitudinal axis X and each have an outer edge extending parallel to and adjacent the inner perimeter 112 of the absorber packings 102a, 102b. The vanes can optionally be provided with nozzles (not shown) for assisting in spreading lean absorbent tangentially and axially onto the inner perimeter of the absorber packings. The vanes can be held stationary or be driven by an external motor whereby the outer edges of the vanes ensure an even distribution of lean absorbent along the entire inner perimeter of the absorber packings. The apparatus 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 110, arranged on the vessel radially outside the outer perimeter of the absorber packing. In this example, the natural gas outlets 110 are arranged axially separated from the natural gas inlets 109a, 109b, and aligned with a radially open section 113 separating the absorber packings 102a, 102b. The arrangement in Figure 1 has four natural gas outlets 110 arranged in a radial plane through the vessel, which outlets are located equidistant around the circumference of the vessel. The radially open section 113 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 114a, 114b. Each radial wall 114a, 114b extends from the inner perimeter of the annular absorber packing to a gas tight labyrinth seal 115a, 115b at the inner wall of the vessel. A third radial wall 114c is located between the first and second radial walls 114a, 114b and extends from the inlet shaft 106 to the outer perimeter of the absorber packings 102a, 102b. The third radial wall 114c is provided to guide the flow of sweet natural gas from the inner perimeter of the absorber packings towards the natural gas outlets 110.

The opposing end surfaces of the absorber packings 102a, 102b are sealed by a pair of rotor end plates 116a, 116b to form a an absorber packing assembly or rotor assembly. The rotor end plates 112a, 112b 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 117 (schematically indicated in Fig. l) 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 114a, 114b adjacent the open section 113. 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 around the respective rotor shaft. The entire absorber assembly located between these rotor shafts is rotated as a unit by a driving torque applied to the rotor shaft located opposite 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 an apparatus according to a first alternative embodiment of the invention in Figure 1. The apparatus in Figure 2 differs from that of Figure 1 in that it comprises an absorber provided with a distribution means for transport of lean absorbent through the inlet shaft 106 to a number of radial channels 207a, 207b (only two shown) extending through a pair of radially extending helical vanes 208a, 208b for transport of lean absorbent to the inner perimeter of the absorber packings. The vanes 208a, 208b shown in Figure 2 are helical vanes arranged about the longitudinal axis X and each have an outer edge extending parallel to and adjacent the inner perimeter 112 of the absorber packings 102a, 102b. The vanes can optionally be provided with nozzles (not shown) for assisting in spreading lean absorbent tangentially and axially onto the inner perimeter of the absorber packings. The helical vanes 208a, 208b are driven by the momentum of the natural gas contacting the distribution means as it flows towards a central open section between the absorber packings 102a, 102b. The recovered momentum causes a driving torque applied to the central shaft to rotate the vanes. If the vanes are connected to the rotor shaft, it can also assist in rotating the absorber assembly. The outer edges of the helical vanes ensure an even distribution of lean absorbent along the entire inner perimeter of the absorber packings. Optionally, the vanes can be held stationary or be driven by an external motor.

The apparatus in Figure 2 additionally comprises means for recuperating energy from the gas flow through the apparatus. The recuperating means is placed within the open section 113 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 113 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 114a, 114b. A third radial wall 114c is located between the first and second radial walls 114a, 114b and extends from the inlet shaft 106 to the outer perimeter of the absorber packings 102a, 102b. The third radial wall 114c 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 114a and the third radial wall 114c. Similarly, the second set of radial vanes 202b is attached between the second radial wall 114b and the third radial wall 114c.

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 215a, 215b 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 113, 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. The energy recovering vanes shown in the open central section of Figure 2 can also be applied to the embodiment of Figure 1.

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. Chemical absorbent is supplied from an inlet ai 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 a4 by the high centrifugal forces (high G) in the rotating absorber assembly. The rich absorbent is then removed via an outlet as for processing and re-use. The sour gas is supplied from inlets Ai 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 form the assembly through an outlet A ₅ .

Figure 4 shows a cross-section through an apparatus according to a second alternative embodiment of the invention. The apparatus in Figure 4 differs from that of Figure 2 in that it comprises an absorber provided with a distribution means for transport of lean absorbent through the inlet shaft 406 to a number of radial channels 407a, 407b (only two shown) extending through a pair of radially extending helical vanes 408a, 408b for transport of lean absorbent to the inner perimeter of the absorber packing 400. The vanes 408a, 408b shown in Figure 4 are helical vanes arranged about the longitudinal axis X and each have an outer edge extending parallel to and adjacent the inner perimeter 412 of the absorber packing 400. The vanes have the same axial extension and are separated by a radial wall 414c located at a position equidistant from the ends of the absorber packing. This wall will be described in further detail below. The vanes can optionally be provided with nozzles (not shown) for assisting in spreading lean absorbent tangentially and axially onto the inner perimeter of the absorber packing 400. The helical vanes 408a, 408b are driven by the momentum of the natural gas contacting the distribution means as it flows towards the open sections at each end of the absorber packing 400. The recovered momentum causes a driving torque applied to the central shaft to rotate the vanes. If the vanes are connected to the rotor shaft, it can also assist in rotating the absorber assembly. The outer edges of the helical vanes ensure an even distribution of lean absorbent along the entire inner perimeter of the absorber packing 400. Optionally, the vanes can be held stationary or be driven by an external motor.

The apparatus in Figure 4 additionally comprises means for recuperating energy from the gas flow through the apparatus 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 401a, 401b 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 annular 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 towards, but not into contact with, the inner perimeter of the absorber packing 400. The third radial wall 414c is provided to guide

substantially equal portions of the flow of sweet natural gas from the inner perimeter of the absorber packing, through central openings in the first and second annular radial walls 414a, 414b and towards the natural gas outlets (not shown). This arrangement also ensures that the flow of natural gas is distributed so that each radial discharge fan 401a, 401b will receive approximately the same gas flow.

The radial discharge fans 401a, 401b 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 414a. 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 the radial walls in the absorber assembly outside the outer perimeter 411 of the absorber packing and are bolted to the rotor end plates. The absorber packing assembly comprises a single absorber packing 400 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, 402. The recovered momentum causes a driving torque applied to the rotor shaft 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. Chemical absorbent is supplied from an inlet ai 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 as for processing and re-use. The sour gas is supplied from inlets Ai 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 form the assembly through outlets A' ₅ at each end of the assembly.

The rotating absorber assemblies described above are significantly smaller and more compact, as compared to traditional absorbers as shown in Figure 6. 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 solution, the viscosity of the absorbent solution will not be such a limiting factor, as compared to conventional solutions used in conventional columns, and thus higher concentration absorbent solutions can be applied. The higher concentration absorbent solutions will allow circulation rates to be significantly reduced. The higher concentration absorbent solutions will also reduce pumping duties and heating and cooling demands in the overall process. The regenerator, or stripper size could 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.