The invention relates to a process for regenerating an absorbent solution containing one or more normally gaseous compounds capable of being released by heating and/or stripping. It relates in particular to the regeneration of an absorbent solution, and in particular an absorbent aqueous solution containing acid gaseous compounds such as H.sub.2 S and/or CO.sub.2, said solution coming for example from a stage of absorption in the course of which the absorbent solution, which is fresh or which has just been subjected to a regeneration, is placed in contact with a gas containing one or more acid compounds such as H.sub.2 S and CO.sub.2. It also relates to an installation for carrying out said process.
The acid compounds, particularly H.sub.2 S and/or CO.sub.2 contained in gases are generally eliminated by scrubbing said gases with an absorbent solution which retains the acid compounds by simple physical dissolution or/and by dissolution after formation of a salt or of a thermally unstable complex, by reaction of said acid compounds with a basic compound present in the absorbent solution. In practice, the gas to be treated, containing the acid compounds to be eliminated, is placed in contact, in an absorption zone, with the chosen absorbent under conditions of pressure and temperature such that the absorbent solution fixes virtually all the acid compounds. The purified gas emerges at the top of the absorption zone and, if necessary, it is then directed towards a scrubber employing sodium hydroxide, in which the last traces of acid compounds are removed. At the bottom of the absorption zone, the absorbent solution containing acid compounds is drawn off and subjected to a regeneration treatment, i.e. to free it of the fixed acid compounds and thus restore its absorbent power vis-a-vis said acid compounds. To effect this regeneration, the absorbent solution to be regenerated, i.e. the laden absorbent solution drawn off from the absorption zone, is introduced into the upper half of a regeneration zone and the absorbent solution to be regenerated is maintained at its boiling point under pressure in this zone. The heat necessary for maintaining the boiling is furnished by reboiling the absorbent solution contained in the regeneration zone, i.e. by indirect heat exchange between part of the solution to be regenerated located in the lower half of the regeneration zone and a hot fluid at appropriate temperature, generally saturated water vapour. In the course of regeneration, the acid compounds contained in the absorbent solution to be regenerated maintained at its boiling point are released and stripped by the vapors of the absorbent solution. Said acid gaseous compounds emerge at the top of the regeneration zone and are passed through a condenser system which returns to the regeneration zone the liquid phase resulting from the condensation of the vapors of the absorbent solution which pass out of the regeneration zone with the gaseous acid compounds. At the bottom of the regeneration zone, the hot regenerated absorbent solution is drawn off and said regenerated solution is recycled to the absorption zone after having used part of the heat content of said solution to heat, by indirect heat exchange, the solution to be regenerated, before its introduction into the regeneration zone.
The indirect heat exchange between the absorbent solution to be regenerated and the hot regenerated solution makes it possible to recover part of the sensible heat of this latter solution, but the energy saving resulting from such a recovery is not consistent. In fact, a considerable part of the energy recovered in the course of said indirect heat exchange serves to vaporize a fraction of the absorbent solution to be regenerated, which emerges directly at the top of the regeneration zone with the acid gaseous compounds released when the reheated solution to be regenerated is introduced into the regeneration zone. Consequently, said vaporized fraction does not participate in the stripping of the acid gaseous compounds in the regeneration zone, but it requires a consumption of cooling fluid for condensation thereof in the condenser system through which the released acid compounds pass. Moreover, the indirect heat exchange between the hot regenerated absorbent solution and the absorbent solution to be regenerated makes it possible to recover only a relatively small part of the sensible heat of the hot regenerated absorbent solution. It is necessary, after this heat exchange, to subject the regenerated absorbent solution to a subsequent indirect cooling by means of a cold fluid such as for example water, to take it to the appropriate temperature for contact, in the absorption zone, with the gas to be purified containing the acid compounds to be eliminated. This additional stage of indirect cooling also consumes an appreciable quantity of cooling fluid.
U.S. Pat. No. 3,823,222 concerns a process for separation of CO.sub.2 and H.sub.2 S gases contained in hot gases also containing steam. Said gases are scrubbed with the aid of an alkaline absorbent solution, particularly an aqueous solution of potassium carbonate, in an absorption zone to fix the acid compounds. The alkaline absorbent solution containing the acid compounds is regenerated by steam-stripping in a regeneration zone in which the solution in the course of regeneration is heated by reboiling with steam. The regenerated absorbent solution is recycled to the absorption zone. A process has been proposed to improve recovery of the sensible heat contained on the one hand in the gases to be purified of their acid gas content and on the other hand in the regenerated absorbent solution at the regeneration phase. In the process, the hot gases to be purified first pass in indirect heat exchange with boiling water, with production of a first stream of steam under pressure and partial cooling of said gases. The partially cooled gases are then brought into indirect heat exchange with a part of the absorbent solution contained in the lower half of the regeneration zone, with production of a second stream of steam having a lower pressure than that of the first stream and further cooling of the gases to be purified. The gases having undergone this further cooling are then directed towards the absorption zone whilst the second stream of steam is introduced into the regeneration zone as stripping steam. Finally, the regenerated absorbent solution drawn off from the regeneration zone is directed to an expansion zone maintained under reduced pressure by a steam ejector, so as to form a certain quantity of steam under said reduced pressure and to partially cool the regenerated absorbent solution. Said partially cooled solution is then directed to the absorption zone after supplementary cooling to take it to the appropriate temperature for contact thereof with the gases to be purified. The first stream of steam under pressure is used as motive steam in the steam ejector to reduce the pressure in the expansion zone and compress the steam under reduced pressure formed in this zone, and the mixture of motive steam and of steam issuing from expansion, which leaves the ejector at a pressure close to the pressure at the bottom of the regeneration zone, is introduced into said zone as additional stripping steam.
Although it improves heat recovery, such a process presents certain drawbacks. Firstly, it is applied to gases to be purified, of which the temperature is relatively high, for example 150.degree. to 200.degree. C., which is not the general case. The use of a steam ejector to maintain the expansion zone under reduced pressure and to compress the steam formed in the expansion zone then requires, for driving this ejector, a quantity of motive steam greater than the quantity of steam produced in the course of expansion of the regenerated absorbent solution, this obliging, in the case of the motive steam not being produced directly by cooling of the gas to be purified, a source of steam to be provided in addition to the source of water vapor used for reboiling the absorbent solution in the course of regeneration. Finally, the quantity of motive steam used for operating the ejector must be permanently eliminated from the regeneration zone to maintain the water balance of the absorbent solution. This excess water vapor gives, after condensation, water polluted by the acid gases, and particularly by H.sub.2 S, whose disposal raises problems.
The invention is a process for regenerating an absorbent solution containing one or more normally gaseous compounds, particularly H.sub.2 S and CO.sub.2, capable of being released by reboiling of the absorbent solution. In the process an expansion of the regenerated absorbent solution is effected with formation of a vapor phase which is reinjected into the regeneration zone. The regeneration process is generally applicable and overcomes the drawbacks of the prior art process mentioned hereinabove which uses an expansion of the regenerated absorbent solution.
In a process for scrubbing gases containing acid gaseous compounds such as H.sub.2 S and/or CO.sub.2 and comprising a stage of absorption in the course of which the gas to be purified is scrubbed by an absorbent solution retaining the acid gaseous compounds, a stage for regeneration of the absorbent solution containing the acid compounds, and a recycling of the regenerated absorbent solution to the absorption stage, the regeneration as proposed by the invention makes it possible to make an overall saving of energy which is appreciable with respect to regeneration carried out in conventional manner or according to the technique employing a steam ejector as taught by the U.S. Patent mentioned hereinabove.
The process according to the invention for the regeneration of an absorbent solution containing one or more normally gaseous compounds which are capable of being released by heating and/or stripping, is of the type in which the solution to be regenerated is injecting into a regeneration zone, the solution to be regenerated is maintained in said zone under conditions ensuring an absolute pressure at the bottom of the zone greater than 1.2 bars and preferably between 1.3 and 5 bars, and allowing a release and/or a stripping of the absorbed gaseous compounds, the released gaseous compounds are removed at the top of the regeneration zone and the regenerated solution is drawn off at the bottom of said zone. A series of operations is carried out on the drawn off regenerated solution comprising reducing the pressure of the regenerated solution with release of a vapor, compressing the released vapor to a pressure substantially equal to the pressure at the bottom of the regeneration zone, and introducing the compressed vapor obtained into the lower half of the regeneration zone. The process is characterised in that the rate of flow pertaining to mass of the recompressed vapor introduced into the lower half of the regeneration zone is substantially equal to the rate of flow pertaining to mass of the vapor released in the course of the reduction in the pressure of the regenerated solution drawn off from the regeneration zone.
The expansion or reduction of pressure of the regenerated absorbent solution is advantageously effected in a plurality of successive expansion stages. The stages comprise an initial stage, optionally one or more intermediate stages and a final stage. The compressed vapor, which is introduced into the lower half of the regeneration zone, is formed by gradual recompression of the vapor released in the course of the successive expansion stages. The vapor released in the course of any expansion stage following the initial expansion stage is compressed to at a pressure substantially equal to the pressure of the vapor released in the course of the expansion stage immediately preceding said expansion stage following the initial expansion stage, this recompression being effected either only on the vapor released in the course of the expansion stage following the initial expansion stage, if it is the final stage, or on a vapor obtained by combination, during said compression, of the vapor released in the course of said expansion stage following the initial expansion stage and of the vpaor of substantially equal pressure formed by gradually compressing the vapor released in the course of the expansion stages, following the expansion stage following the initial expansion stage, and the vapor phase released in the course of the initial expansion stage is combined with the vapor of substantially equal pressure constituted from all the vapor released in the course of the expansion stages following the initial expansion stage and the whole compressed to a pressure substantially equal to the pressure at the bottom of the regeneration zone, each recompression being effected with conservation of the rate of flow pertaining to mass between recompressed fluid and fluid to be compressed.
In particular, the expansion of the regenerated solution drawn off from the regeneration zone is effected in n successive expansion stages, namely an initial stages, (n-2) intermediate stages, and a final stage, n being a whole number of from 2 to 4.
In a preferred embodiment of the process of regeneration according to the invention, the expansion of the regenerated solution, drawn off from the regeneration zone, is effected in two stages, the pressure of the vapor released in the final expansion stage is raised to a value substantially equal to that of the pressure of the vapor released in the course of the initial expansion stage, said phases of substantially equal pressures are combined and the whole is compressed to a pressure substantially equal to the pressure at the bottom of the regeneration zone. The compressed vapor is introduced into the lower half of the regeneration zone. In the present invention each recompression is effected so that the rate of flow pertaining to mass of the fluid to be compressed is substantially equal to the rate of flow pertaining to mass of the compressed fluid resulting from this recompression.
The pressure of the vapour phase released in the course of the final expansion stage may reach values which, depending on the case, may be close to atmospheric pressure or even much lower than atmospheric pressure. Expansion of the regenerated absorbent solution, drawn off from the regeneration zone, is advantageously effected so that the pressure of the vapour phase released in the course of the final expansion stage has a value ranging from 0.5 to 2 bars absolute, this pressure being, of course, lower than the pressure at the bottom of the regeneration zone.
In the course of the successive expansion stages, the drop in pressure for each expansion stage can vary from one expansion stage to the other or may comprise a substantially constant value.
In the regeneration zone, the release and stripping away of the gaseous compounds absorbed by the absorbent solution to be regenerated may be effected by addition, to the solution to be regenerated contained in said zone, of the calories necessary for the release of the absorbed gaseous compounds and for the production, from this solution, of a gaseous stripping fluid. The absorbed gaseous compounds may also be released and stripped solely by injection of a gaseous stripping fluid in the regeneration zone, or by combining a heating of the solution to be regenerated with an injection of a gaseous stripping fluid.
The absorbent solution to be regenerated containing one or more gaseous compounds capable of being released by heating said solution and/or stripped from this solution, is in particular an absorbent solution which contains acid compounds such as CO.sub.2, H.sub.2 S and possibly COS.
Such an absorbent solution containing acid compounds is for example obtained, during the scrubbing of various gases, particularly natural gas, containing one or more acid gaseous compounds such as CO.sub.2, H.sub.2 S, COS, by bringing the gas to be purified into contact with the absorbent solution chosen by operating under appropriate conditions of pressure and temperature, for example under absolute pressures ranging from 1 to 120 bars and at temperatures of the order of 30 to 110.degree. C.
The absorbent solution used for fixing the absorbable gaseous compounds and in particular the acid gaseous compounds such as CO.sub.2, H.sub.2 S, COS, may be any one of the absorbent solutions known in the art to this end. This absorbent solution may in particular be constituted by a solvent of organic type, for example phosphoric ester or sulfolane, possibly containing additives such as amines. The absorbent solution comprises, most often, an aqueous solution of a basic compound which fixes the acid compounds to be absorbed, particularly CO and H.sub.2 S, in the form of complexes decomposable by heating. The basic aqueous solution being for example an aqueous solution of potassium phosphate or potassium carbonate, an aqueous solution of an aminoacid such as glycine, and in particular an aqueous solution of a primary, secondary or tertiary alkanolamine such as in particular monoethanolamine, diethanolamine, triethanolamine, methyldiethanolamine, diisopropanolamine.
In particular, the absorbent solution to be regenerated which is introduced into the top half of the regeneration zone, comes from an absorption zone in which a gas, for example natural gas, containing absorbable compounds, and in particular one or more acid gaseous compounds such as CO.sub.2, H.sub.2 S, COS, is scrubbed. The gas stream is preferably scrubbed counter currently by the regenerated absorbent solution issuing from the final expansion stage following the regeneration. The expanded regenerated solution being used, before being injected into the absorption zone, to reheat, by indirect exchange, the absorbant solution to be regenerated.
An installation for regeneration, according to the invention, of an absorbent solution containing one or more normally gaseous compounds capable of being released by heating and/or stripped away, comprises a regeneration column associated with a system for heating and/or injecting a stripping gas, said column being further provided with a draw-off conduit at the bottom of the column for the regenerated solution, a conduit for supplying a compressed vapor opening in its lower half, a conduit for supplying the solution to be regenerated, and with a conduit for removing the gases at the top of the column. The installation also includes, an assembly for expansion of the regenerated solution. The assembly for the expansion of the regenerated solution comprises an inlet connected to the draw-off conduit of the regeneration column and an outlet for the expanded regenerated solution constituting the outlet of the regeneration installation for the regenerated absorbent solution. The expansion assembly produces a vapor, and utilizes apparatus for compressing the vapor and for introducing the compressed vapor into the regeneration column. The expansion assembly comprises a plurality of expansion chambers each comprising an inlet and an outlet for the regenerated absorbent solution and an outlet for a vapor. The expansion chambers being connected in series so that the inlet of the first chamber of the series is connected to the draw-off conduit of the regeneration column. The inlet of each of the following chambers is connected to the outlet for the regenerated solution of the chamber immediately upstream, the outlet for the regenerated solution of the expansion chamber most downstream of the regeneration column constituting the outlet of the expansion assembly. The compression assembly comprises in a plurality of compressor stages of the type with conservation of the rate of flow pertaining to mass between fluid to be compressed and compressed fluid, each compressor stage being associated with an expansion chamber. The compressor stages are disposed in series so that each compressor stage has its suction connected to the outlet for the vapor of the corresponding expansion chamber and its outlet connected to the suction of the compressor stage corresponding to the expansion chamber located immediately upstream. The outlet of the compressor stage associated with the expansion chamber connected to the bottom of the regeneration column being connected to the conduit for conducting the compressed vapor to the regeneration column, thus constituting the vapor outlet of the recompression assembly.
As indicated above, the expansion assembly according to the invention comprises a plurality of expansion chambers, i.e. an initial expansion chamber, possibly one or more intermediate expansion chambers, and a final expansion chamber. The associated recompression assembly contains the same number of compressor stages, each compressor stage comprising one or more compressors. In particular, the installation contains n expansion chambers and n compressor stages associated therewith, n being a whole number of from 2 to 4.
The plurality of compressor stages can comprise by mechanical compressors driven independently or mounted on the same drive shaft, said compressors being, for example, centrifugal compressors, axial compressors, or rotary volumetric compressors.
The regeneration installation according to the invention may advantageously be substituted for the conventional regeneration installation of a process for purifying gases containing absorbable gaseous compounds. The process is particularly useful for removing acid gaseous compounds such as CO.sub.2, H.sub.2 S and COS, by scrubbing the gases with an appropriate absorbent solution. The scrubbing installations comprising, as is well known, an absorption column associated with a regeneration column. The regeneration installation according to the invention is associated with the conventional absorption column. The outlet for the expanded regenerated absorbent solution of the expansion chamber most downstream of the regeneration column, is connected, through one of the two fluid circulation circuits of an indirect heat exchanger, to one or more absorbent inlets of the absorption column. The outlet for the solution to be regenerated, containing absorbed gaseous compounds, located at the bottom of the absorption column is connected, through the other fluid circulation circuit of said heat exchanger, to the conduit supplying solution to be regenerated, provided on the regeneration column.
The absorption and regeneration columns of the above installations are for example columns with packing materials or columns with plates, as is well known in the art.