Use of a two-phase turbine in a gas treating process

The invention relates to the field of gas treating processes using a solvent. It proposes using a two-phase turbine to regenerate the solvent used for processing a gas.

Processing of a gas, for example natural gas, synthesis gas, combustion gas, gas from integrated combined cycles, generally involves removal of impurities such as nitrogen (N 2 ), ammonia (NH 3 ), and acid compounds such as carbon dioxide (CO 2 ), hydrogen sulfide (H 2 S), sulfur dioxide (SO 2 ), COS, CS 2 and mercaptans. These impurities are present in various proportions depending on the origin of the gas. In the case of natural gas, CO 2 and H 2 S can be present as traces, some ppm, but they can also represent a quite significant proportion of the raw gas, up to 70% by volume. Other impurities such as COS, CS 2 and mercaptans can also be encountered, generally a few thousand ppm.

According to the initial proportions of impurities in the gas to be processed, and also according to the specifications required for the processed gas, various scrubbing processes are used. The most commonly used processes use solvents. These solvents can be of physical or chemical nature. Solvents of physical nature are based on the preferred solubility of the impurities in the solvent, and are therefore favoured by the high partial pressures of the impurities in the gas to be processed. Solvents of chemical nature are ideally used to reach the strictest specifications for the processed gas, by chemical consumption of the species absorbed by reaction with an active agent contained in the absorption solvent. There are also solvents of hybrid nature consisting of a mixture of physical and chemical solvents so as to enjoy the advantages of these two solvents.

Whatever the origin of the gaseous effluent to be processed, the purification loop generally consists of an impurities collection stage using a solvent and of a solvent regeneration stage. The regeneration stage is conditioned by the nature of the solvent used. In the case of a solvent involving a chemical reaction, a thermal regeneration is generally used to obtain a sufficient solvent purity allowing the desired specifications to be reached. Thermal regeneration can be preceded by regeneration by expansion in order to limit the energy required for thermal regeneration. In the case of a solvent of hybrid or physical nature, regeneration is essentially carried out by expansion, possibly completed by a thermal regeneration stage.

The invention proposes improving the regeneration by expansion of a solvent by carrying out an expansion in a two-phase turbine.

In general terms, the present invention relates to a treating process for a gas comprising at last one of the impurities CO 2 , H 2 S, SO 2 , COS, CS 2 , mercaptans, N 2 and NH 3 . The following stages are carried out:

a) contacting the gas with a solvent absorbing the impurities so as to obtain an impurity-laden solvent and a scrubbed gas,

b) expanding the impurity-laden solvent through a two-phase turbine so as to release an amount of impurities in gaseous form and to obtain an impurity-depleted solvent,

c) separating said amount of impurities from said impurity-depleted solvent.

According to the invention, the two-phase turbine can be a rotodynamic turbine, for example, comprising at least one impeller and at least one diffuser.

The method according to the invention can comprise the following stages:

d) expanding said impurity-depleted solvent through a second two-phase turbine so as to release a second amount of impurities in gaseous form and to obtain an expanded solvent,

e) separating said second amount of impurities from said expanded solvent.

According to the invention, the two-phase turbine used in stage b) can comprise at least one impeller and at least one diffuser, the second two-phase turbine used in stage d) can comprise at least one impeller and at least one diffuser, and the impellers of said turbines can be mounted on a single shaft.

The method according to the invention can comprise the following stages:

f) distilling said impurity-depleted solvent so as to separate the impurities from the solvent,

g) recycling the distilled solvent to stage a).

According to the invention, stage a) can be carried out at a pressure ranging between 1 MPa abs. and 15 MPa abs., preferably between 3 MPa abs. and 10 MPa abs., and in stage b) the solvent can be expanded to a pressure ranging between 0.1 MPa abs. and 3 MPa abs.

The solvent used in the method according to the invention can be a solvent of physical nature.

Other features and advantages of the invention will be clear from reading the description hereafter of non limitative embodiment examples, with reference to the accompanying drawings wherein

FIG. 1 diagrammatically shows a method according to the invention,

FIG. 2 diagrammatically shows a variant of the method according to the invention,

FIGS. 3 and 4 show a two-phase turbine used in the method according to the invention,

FIG. 5 shows a second two-phase turbine used in the method according to the invention.

In FIG. 1 , the gas to be processed flowing in through line 10 comprises impurities such as CO 2 , H 2 S, SO 2 , COS, CS 2 , mercaptans, N 2 and NH 3 . This gas can be a natural gas comprising notably hydrocarbons, a synthesis gas comprising notably hydrogen and carbon monoxide, a combustion gas possibly resulting from combined cycles, comprising notably nitrogen and oxygen. The gas is possibly cooled in heat exchanger E 1 , then fed through line 11 into absorption column C 1 . The gas is then contacted with a liquid solvent fed through line 18 into the top of column C 1 . The solvent absorbs the impurities contained in the gas. The scrubbed gas, i.e. depleted in impurities, is discharged at the top of column C 1 through line 19 .

The solvent can be of physical or chemical nature. For example, the solvent can be an alcohol, a glycol, a heavy hydrocarbon such as propylene carbonate, a potassium carbonate, a morpholine, a polyethylene glycol dimethylether, an amine such as an alkanolamine or an alkylamine. The solvent can also be a mixture of two or more aforementioned solvents.

The liquid impurity-laden solvent is discharged from column C 1 through line 12 , then expanded in two-phase turbine T 1 . The expanded solvent is fed into separating drum B 1 . The impurities released in gaseous form upon expansion are discharged at the top of drum B 1 through line 20 .

The liquid solvent recovered at the bottom of drum B 1 through line 13 is depleted in impurities. It can be recompressed by pump P 1 , then fed through line 18 to the top of column C 1 . Expansion in turbine T 1 has the advantage of cooling the solvent. However, if the regenerated solvent is not sufficiently cold, it can be supercooled by heat exchanger E 5 prior to being fed into column C 1 .

FIG. 2 shows a variant of the method described in connection with FIG. 1 . The stages carried out in exchanger E 1 , column C 1 , turbine T 1 and drum B 1 of FIG. 2 are identical to the stages carried out in the same elements of FIG. 1 .

In FIG. 2 , the liquid solvent coming from the bottom of drum B 1 through line 13 is expanded in two-phase turbine T 2 . The expanded solvent is fed into separating drum B 2 . The impurities released in form of gas upon expansion are discharged at the top of drum B 2 through line 21 . Expansion in turbine T 2 is optional, which means that, in the method described in connection with FIG. 2 , elements T 2 , B 2 and 21 can be removed and line 13 can be directly connected to line 14 .

The liquid solvent recovered at the bottom of drum B 2 through line 14 is depleted in impurities. It is cooled in heat exchanger E 4 , then fed into distillation column C 2 . The distillation column allows to carry out advanced regeneration of the solvent, i.e. to reduce the proportion of impurities in the solvent to a very low level, in any case to a lower level than that obtained by means of regeneration by expansion. Reboiler E 2 provides the heat required for distillation in column C 2 . The gas phase discharged at the top of column C 2 through line 15 mainly comprises impurities. This gas phase is partly condensed by cooling in heat exchanger E 3 , then fed into separating drum B 3 . A gas phase mainly comprising impurities is discharged from drum B 3 through line 17 . The condensates recovered at the bottom of drum B 3 are injected through line 16 to the top of column C 2 as reflux.

The regenerated solvent available at the bottom of column C 2 is pumped by pump P 1 , cooled in heat exchanger E 4 , then injected to the top of absorption column C 1 . Expansion in turbines T 1 and T 2 has the advantage of cooling the solvent. Thus, the solvent obtained after expansion constitutes a coolant for cooling the regenerated solvent intended to be fed into column C 1 . However, if the regenerated solvent is not sufficiently cold, it can be supercooled by heat exchanger E 5 prior to being fed into column C 1 .

In the methods described in connection with FIGS. 1 and 2 , expansion of the solvent by means of two-phase turbines T 1 and T 2 affords the following advantages.

The pressure decrease allows to release the impurities in gaseous form.

Expansion of the solvent by means of a turbine is accompanied by a temperature reduction of the solvent. In fact, expansion by means of a turbine is close to an isentropic expansion. This cold generation allows to compensate for the temperature increase in the absorption column during the absorption reaction, which is generally exothermic. Thus, the method according to the invention requires no exterior cooling source and it is therefore an autothermal process. For example, when the solvent is regenerated only by expansion, the solvent obtained after expansion requires no or only a low temperature reduction before injection of the solvent into the absorption column. Similarly, when the solvent is regenerated by expansion and by distillation, the solvent obtained at low temperature after expansion constitutes a coolant allowing to cool the high-temperature regenerated solvent from the distillation column.

The expansion energy can be recovered at the shaft of the two-phase turbine. This expansion energy can be re-used for compression of the solvent by pump P 1 and/or recompression of the impurities in gaseous form circulating in lines 20 and 21 at the outlet of drums B 1 and B 2 .

Furthermore, the two-phase turbines used in the method according to the invention accept gas freeing, in large amounts, of the impurities contained in the solvent to be regenerated.

In the present description, a two-phase turbine is a turbine designed to expand a fluid whose proportion by volume of gas can be above 5%. Two-phase turbines T 1 and T 2 can be rotodynamic type turbines comprising impellers and diffusers, for example a machine as described in one of the following patents: FR-2,333,139, FR-2,471,501 and FR-2,665,224.

FIG. 3 shows a two-phase turbine 30 which comprises two expansion stages arranged in housing 32 . The first stage consists of the diffuser D 1 and impeller I 1 pair, the second stage consists of the diffuser D 2 and impeller I 2 pair. Impellers I 1 and I 2 are mounted on rotor 31 which is mobile in rotation in housing 32 of the turbine. Diffusers D 1 and D 2 are secured to housing 32 . The fluid to be expanded is fed into the housing through opening 33 , it successively circulates between the blades of D 1 , I 1 , D 2 and I 2 , then the fluid expanded by turbine 30 is discharged through opening 34 . Diffusers D 1 and D 2 allow to convert the potential energy of the fluid to kinetic energy. The moment change exerted by the fluid allows an energy transfer from the fluid to the rotor by means of the impellers.

FIG. 4 is a developed view of impellers I 1 and I 2 and of diffusers D 1 and D 2 . Reference numbers 40 and 41 relate to the blades of diffusers D 1 and D 2 , reference numbers 42 and 43 relate to the blades of impellers I 1 and I 2 .

Turbines T 1 and T 2 can be integrated in a single machine 50 as shown in FIG. 5 . Turbines T 1 and T 2 are arranged in housing 52 . Turbine T 1 is made up of diffusers D 3 and D 4 , and impeller I 3 , and turbine T 2 is made up of diffusers D 5 and D 6 , and impeller I 5 . Impellers I 3 and I 5 are mounted on the same shaft 51 mounted mobile in rotation in housing 52 . Separation and sealing device 57 allows to insulate turbine T 1 from turbine T 2 . The fluid flows into turbine T 1 through orifice 53 , it successively circulates between the blades of D 3 , I 3 and D 4 , then it is discharged through orifice 54 . The fluid then enters turbine T 2 through orifice 55 , it successively circulates between the blades of D 5 , I 5 and D 6 , then it is discharged through orifice 56 .