LIQUIDS"

DESCRIPTION

The present invention relates to a regeneration process of dehydrating liquids used in the oil industry and in the natural gas production, said process allowing to obtain said liquids in the essentially pure state suitable to reduce the water content of the gas to less than 1 ppm volume. Said invention is particularly useful in the off-shore treatment of natural gas before being introduced into the pipelines or being fed to the cryogenic fractionation or liquefaction plants.

The natural or associated pressurized gas coming from the separation steps or from washing treatment with aqueous solvents is water saturated. The gas pressure drop in the following pipes system transportation with the consequent temperature drop and/or its cooling because of the thermal exchange with the surrounding environment when it crosses regions having severe environmental conditions, causes the condensation of part of said water. Liquid formation into the pipes is source of corrosion, erosion and an anomalous mechanical stress in pipe fittings caused by the liquid collision with said fittings and by the sudden momentum variation caused for example by bends. Moreover the temperature drop can lead to the so called hydrates formation: hard and abrasive solids comprising water molecules surrounding methane or carbon dioxide molecules. Hydrates seriously damage pipes and their fittings due both to the erosion and the occlusion of the block and control systems and sometimes of the pipeline itself as well.

In those cases where the natural or associated gas must undergo a cryogenic treatment for some of its fraction recovery or must be liquefied, the water contained in the gas would give rise to serious operation problems in case of freezing.

Therefore before being directed to the pipeline or sent to the cryogenic plants the gas must be appropriately dried. Normally for pipeline transportation it is required that gas has a residual water content comprised between 1 and 7 lb/MMSCF, corresponding approximately at 20-120 ppm volume.

Prior art reports methods commonly used for water natural gas removal; these consist in its dehydration through hygroscopic liquids such as glycols, among which diethylene glycol (DEG) and triethylene glycol (TEG), or through molecular sieves. The last ones operate discontinuously, require the adsorption bed substitution every 2-3 years and must be regenerated through heating. The water desorption energy from the sieves is about 1000 kcal/kg of water against a latent heat of 540 kcal/kg of water at atmospheric pressure. Then in addition to the plant extra cost, the molecular sieves have high operation cost. By consequence on the economic and energetic level it is really expedient removing the highest water amount upstream the cryogenic plants so that the water load on the molecular sieves would be reduced to the minimum.

The adsorption process using hygroscopic liquids (glycols) is economically convenient compared with the molecular sieves (or equivalents) but it allows a limited water capacity removal usually sufficient only for the levels normally required in pipe transportation.

The glycol treatment is based on the traditional regeneration absorption scheme in which gas, normally at pressures sometimes even higher than 120 bar g, is put in contact with the glycol solution (solvent) in a drying column where the two fluids move, typically counter-current, and where the gas dehydrates (dewaters) while the solvent (glycol) gets diluted by water absorption. The diluted solvent is collected at the bottom of the absorption column and is subsequently regenerated through a distillation process at a pressure slightly higher than the atmospheric one. The only distillation allows obtaining a glycol having a purity degree between 98.5% and 99% by weight corresponding to water content in the gas between 250 and 330 ppm volume (referred to atmospheric pressure). In order to obtain a greater purity degree and then to further reduce the natural (associated) gas water content, the hot distillate residue is submitted to a stripping operation at a basically atmospheric pressure, the natural gas itself or another flue gas being the stripping fluid. Then the stripping column effluent is a low pressure wet fuel gas which is usually discharged to flare since it cannot be recovered, by consequence having production loss and carbon dioxide emission into the atmosphere.

Italian patent n.1223516 describes a process which enables to reduce said discharge to flare using as stripping agent hydrocarbon mixtures having a boiling point lower than 35°C or a fraction of the gas to be treated. From the stripping column, said gas is sent to the still column, cooled down in its head condenser, separated from the condensed phase and recycled to the stripper, through an appropriate compressor. In order to obtain a high purity solvent the process of said patent provides the stripping gas drying in two stages. To that purpose on the recycling compressor suction, a dehumidification column is inserted where the stripping gas gets in contact, in counter-current, with the rich glycol to be regenerated (through the mass removal of the humidity contained in it) and in a second stage it then gets in contact with a regenerated small stream for further dehydration to the desired level. The recycled gas so de-humidified is subsequently heated at the expense of the regenerated glycol collected at the bottom of the stripping column ad used as stripping agent.

Italian patent n.1274031 is a variant of the first one and provides extreme glycol regeneration in order to minimize its circulation rate so that to limit the aromatic quantity contained in the gas to be treated and co-absorbed into the solvent.

The process described in the two above mentioned patents presents the drawback to be applied only in those cases where the cooling water is not available and/or in warm climate countries where the gas coming out from the still column cannot be cooled to temperatures lower than 35°C. In these situations said gas contains a significantly high quantity of saturation water (at least 4% by volume) that the glycol rich solution is still able to absorb in the first dehumidification stage of the recycled gas. If the gas will be cooled lower than 35°C, the residual humidity partial pressure of said saturated gas would be lower than the equilibrium vapor pressure in the diluted solvent which then could not absorb it. It follows that the matter transportation in the lower section of the dehydrating column set on the recycle line could not take place. On the other hand the only use of the completely regenerated solution to carry out the stripping fluid drying in absence of cooling water presents the double drawback of requiring a significantly higher flow rate of the solution circulating inside the regenerating unit so increasing the plant dimensions and requiring higher energy, by consequence increasing the carbon dioxide vented to atmosphere, since the same water quantity is continuously vaporized into the still column reboiler and reabsorbed by the solvent in the stripping gas dehumidification stage.

Another disadvantage of this method consists in the stripping fluid heating at the expense of the concentrated solution sensible heat. In fact the warming heat is taken away from the one available for the solution sent to the still column preheating, with a subsequent reboiler duty increase requiring then an higher heat quantity to compensate the lower intake temperature at the distillation inlet.

The current increase of hydrocarbons demand urges the "Oil Company" to look for fossil origins energy sources in remote places or in areas characterized by particularly severe environmental conditions as for example the northern seas and the areas near the arctic polar circle. In these cases, the hydrocarbons off-shore production on floating platforms subjected to wave motions and the natural (associated) gas transportation from the production to the destination markets require a control of the water content in the natural gas well lower than the current practice since the treatment plant must operate correctly even in condition of high departure from the vertical axis of the platform because of the wave motions and since the methane pipelines cross particularly cold areas. In addition, more and more often the natural (associated) gas is submitted to cryogenic treatments to recover commercially more valuable hydrocarbon cuts, for example gasoline or LPG. In these cases as well the maximum water content in the gas to the cryogenic treatment is often much lower than commonly specified to reduce to the minimum the economic impact in the gas preparation section before the cooling. The present invention has the purpose to overtake the prior art limitation and allows to obtain essentially pure regenerated solvents corresponding to a natural (associated) gas residual water concentration as low as 1 ppm volume.

According to this, the present invention relates to a process for the re-concentration of diluted dehydrating liquids, said dehydrating liquids once re-concentrated being used in the dehydration plants of gaseous mixtures, said process comprising a step related to the distillation of said diluted dehydrating liquids and a stripping step using an inert gas as stripping gas, said process comprising the following steps:

(a) cooling the gases coming out from said distillation (2) at a temperature lower than 35°C so producing a first condensed phase and a first gaseous phase in the condenser (4);

(b) a first separation in the separator (5) of the first gaseous phase obtained in step (a) from the first condensed phase produced in step (a);

(c) compressing in the compressor (6) the first gaseous phase obtained at the end of step (b);

(d) cooling the compressed first gaseous phase obtained in step (c) at a temperature lower than 35°C so producing a second gaseous phase and a second condensed phase;

(e) a second separation in the second separator (7) of the second gaseous phase obtained in step (d) from the second condensed phase produced in step (d);

(f) dehydration in the drier (8) of the second gaseous phase obtained in step (e) so obtaining a re- concentrated dehydrating liquid;

(g) stripping in stripper (3) said re-concentrated dehydrating liquid.

In the preferred embodiment the cooling phases (a) and (d) occurs at a temperature between 20°C and 34°C.

The numbers in brackets refer to figure 2, reporting the preferred embodiment of the present invention. The term "dehydrating liquid" refers to liquids able to dehydrate said gaseous mixtures, preferably consisting of natural gas. In the preferred embodiment said dehydrating liquids are selected in the group consisting of ethylene glycol, diethylene glycol, triethylene glycol and relating mixtures. The dehydrating liquid even more preferred is triethylene glycol. The term "rich" refers to the above mentioned dehydrating liquids diluted with water since they are coming from the natural gas dehydrating process. Usually they are present as aqueous solution with a dehydrating agent contents comprised between about 94% and 97%. The distillation of the above mentioned dehydrating diluted liquids takes place at atmospheric pressure at a temperature lower than the decomposition temperature of the glycol used as dehydrating fluid, for example around 200°C.

Figure 1 shows the process according to the prior art, while figure 2 shows the process according to the present invention.

In figure 1 ^•

(1) Is the first heat recovery,

(2) Is the still column,

(3) Is the stripper,

(4) Is the condenser,

(5) Is the separator,

(6) Is the first drying tower,

(7) Is the second drying tower,

(8) Is the compressor,

(9) Is the second heat recovery

In fig.1 the rich dehydrating liquid stream is sent to the drying tower (6) for the bulk removal of the water contained in the gas from the still column overhad system - stream j).-. The solvent further on diluted, current (b), is sent to the heat recovery section (1) prior being blended with the semi-rich current (o) coming from the second drying tower (7). From the heat recovery (1) the dehydrating liquid to be purified (d) is fed to the distillation (2); the residue, current (f), is fed to the stripper (3) while the purified glycol stream (h) is sent to the heat recovery section (1) after giving out part of its sensible heat to the stripping gas (s) in the exchanger (9). The vapors (g) coming out from the stripper are directed to the distillation (2). The greater part of the regenerated and cooled solvent (p) is sent to the natural gas dehydrating section, while the rest of it (n) is conveyed to the drying tower (7) for the deep removal of the water contained in the recycled gas (k) pre-dried in (6). The gas (e) coming out from the distillation column (2) is cooled in (4) and separated from the condensed phase in (5). The overhead gas (i) coming out from separator (5) is dried by two steps in (6) and (7) as previously described. The dried stripping gas (q) is retrieved from compressor (b) and recycled to stripper (3), current (r), upon heating in (9).

In figure 2:

(1) Is the heat recovery unit,

(2) Is a still column,

(3) Is the stripper,

(4) Is the condenser,

(5) Is the first separator,

(6) Is the compressor,

(7) Is the second cooler and second separator,

(8) Is the drying tower.

Unlike the process illustrated in fig.l, the present invention process represented in fig.2 requires cooling the gases coming out from distillation (2) at temperatures from 20°C to 34°C so producing a first condensed phase [streams (s) and (t)] and a first gaseous phase [stream (1)]. The above mentioned cooling can be carried out through cold air or cooling water, chilled water, cooling fluid, or through a thermal integration with the cryogenic plant or their combinations in order to separate in (5) most part of the water contained in current (j) coming out from distillation (2). The first condensed phase, consisting of streams (t) and (s), that is water and heavy hydrocarbons usually present in natural gas, is separated from the first gaseous phase in the first separator (5). The gaseous stream (1) coming out from the first separator (5) is compressed in compressor (6) and cooled again at a temperature from 20°C to 34°C so producing a second gaseous phase and a second condensed phase. These two phases are submitted to a second separation in the second separator (7), originating a liquid stream (o), essentially consisting of water, and a gaseous stream (p) that is fed to the drying tower (8). The so dried gas [stream (q)] is sent to the stripper (3).

Moreover, unlike the process in fig.l, by the present invention because of the low temperature, in the separator (5) in addition to water (t) even the hydrocarbon portion (s) initially contained in the gas to be dehydrated and dissolved in the glycol in the absorption process is separated, by consequence maximizing the hydrocarbon well oil production. Another advantage introduced by this scheme, compared to fig.l, is given by the lowest compression cost; in fact the gas that has to be compressed is colder, therefore easier to compres.

The gaseous stream (1) coming out from the present invention separator (5) is not partially dried through the diluted solution as in process in fig.l, but it is directly compressed in ^' (6) in order to raise the dew point temperature of the water still contained in it and to allow a further separation by condensation through cooling in the cooling and separation unit (7) as described above. Doing so, stream (p), equivalent to (j) n fig.l, has really nil water content and the complete removal of the water can be easily carried out using a minimum quantity of essentially pure solvent (h) in the drying tower (8).

The present invention process lowering drastically the recycled gas dew point temperature through appropriated combinations of cooling/compressing steps, allows to obtain an essentially pure glycol (concentration higher or equal to 99.99%) capable to reduce the natural gas water content until 1 ppm volume. Fig.2 shows the preferred embodiment of the present invention but it must not be considered a limitation of the present invention. In fact other process plans can result from fig.2.

The following example is reported for a better comprehension of the present invention.

Example

28 MSm ³ /d of natural gas at 74 bar g and 20°C must be dehydrated so that the residual humidity concentration will be less than 5 ppm volume corresponding to a real dew point temperature (referred to 74 bar g) of -45°C. The re-concentrated glycol plant must be installed on a ship placed in the Barents's sea where the surrounding temperature ranges from -38°C in winter to 15°C in summer. The wave motions cause the platform angular movement of 15° relative to the vertical. On the platform a refrigerant fluid consisting of a glycol water solution of 70% at 15°C is available. In order to dehydrate the natural gas, triethylene glycol - TEG - was selected in a recirculation amount of 13 t h. It has been estimated that to compensate the operation (distillation and stripping) unit efficiency loss caused by the platform movements, the TEG regeneration unit must be designed to remove gas humidity until 1 ppmv.

For this reason gases coming out from the distillation column and the compressor are cooled to 25°C. On the whole 96% of the water carried by the recycled gas is condensed and separated from the gas, The gas water content is reduced to about 700 ppm in volume in the drying column where 2 t/h of regenerated solvent equivalent to about the 11% of the circulating solvent are fed. The 10% internal circulation increase is the same order of magnitude of the machines and equipment overdesign that is normally applied in the plant design, then the use of the regenerated TEG small fraction to dry the recycled gas does not adversely affect the investment costs. The TEG regenerated by said stripping gas contains 150 ppm in weight of residual water and is suitable to reduce the natural gas water content from the initial 430 ppm to 1 ppmv.

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