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Title:
METHOD OF ABSORBING CARBON DIOXIDE FROM INDUSTRIAL GASEOUS MIXTURES
Document Type and Number:
WIPO Patent Application WO/2010/110687
Kind Code:
A1
Abstract:
Method of absorbing CO2, tipically with K2CO3, from an industrial gas mixture (1). The method comprises regeneration (10) of the absorbent by stripping. A semi-lean absorbent stream (12) is taken from the regeneration column (10) and expanded in a drum (13). The liquid phase (22) of the expansion drum is compressed (14) and sent into the regenerator (10) as a stripping gas (17) under the lowest packing of said regenerator.

Inventors:
CWALINA JANUSZ (PL)
MOZENSKI CEZARY (PL)
MOZENSKA BOZENA (PL)
Application Number:
PCT/PL2009/050042
Publication Date:
September 30, 2010
Filing Date:
December 31, 2009
Export Citation:
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Assignee:
INST NAWOZOW SZTUCZNYCH (PL)
International Classes:
B01D53/14; C01B3/52; C10K1/12
Foreign References:
US4035166A1977-07-12
US5145658A1992-09-08
US4409191A1983-10-11
US3563695A1971-02-16
US3823222A1974-07-09
US3823222A1974-07-09
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Claims:
Patent Claims

1. Absorption method of carbon dioxide removal from industrial gaseous mixtures in cyclic absorption process, in which aqueous solution of chemisorbent circulates between the absorber, where CO2 chemical bonding with an active agent of chemisorbent solution takes place at a higher pressure and the regenerator, where CO2 desorption from chemisorbent solution to stripping steam flowing through the regenerator under the pressure close to the atmospheric pressure and semi-lean solution directed from the middle part of the regenerator and directed into the middle part of the absorber flows through the expander, in which it is cooled as the result of a solution rapid evaporation under the pressure lower than the pressure of the middle part of the regenerator essentially consisting of that semi-lean solution discharged from the middle part of the regenerator and directed to the middle part of the absorber is fed to a one-chamber expander over the solution surface and the low pressure steam generated by rapid solution evaporation in the expander is first compressed and then fed under the bottom packing layer in the regenerator.

2. Absorption method of carbon dioxide removal according to claim 1. essentially consisting of that semi-lean solution is discharged from the middle part of the regenerator where it reaches carbonization [KHCO3]/[(2[K2CO3] + [KHCO3] degree of 0.40-0.50.

3. Absorption method of carbon dioxide removal according to claim 1. essentially consisting of that such solution is discharged from the regenerator in boiling state at the pressure of 0.14-0.16 MPa and the temperature of 114- 118°C. Boiling solution is fed to the expander where the pressure is reduced to 0.10-0.12 MPa. Absorption method of carbon dioxide removal according to claim 1. essentially consisting of that low pressure steam generated as the result of rapid evaporation of hot solution in expander is compressed to the pressure of 0.15-0.20 MPa with the use of a higher pressure steam of 0.4-1. OMpa and is directed with stripping steam used for compression under the bottom packing of regenerator.

Description:
METHOD OF ABSORBING CARBON DIOXIDE FROM INDUSTRIAL GASEOUS MIXTURES

The absorption method of carbon dioxide removal from industrial gaseous mixtures in water solutions of chemisorbents is a subject matter of the invention.

CO 2 removal from industrial gaseous mixtures is a very important process applied in many technological processes connected with a large scale production of synthesis gases by autothermal reforming or natural gas higher hydrocarbons steam conversion, or a gas prepared by coal gasification. For example, in the production process of a two-component H 2 -N 2 synthesis mixture for ammonia synthesis by autothermal reforming or natural gas steam conversion, the gaseous mixture obtained after CO to CO 2 shift conversion contains 17-25 vol. % of carbon dioxide which must be removed in a subsequent step of synthesis gas preparation. CO 2 absorption with alkaline aqueous solution of a chemisorbent which reacts with CO 2 in a reversible reaction is the most commonly applied method of carbon dioxide removal from industrial gaseous mixtures.

In the known methods of carbon dioxide removal from industrial gaseous mixtures the aqueous solution of potassium carbonate activated with suitable promoters accelerating the process of carbon dioxide chemical bond, is continuously recirculated between the absorber, where carbon dioxide is dissolved and chemically bonded with the active component of chemisorbent solution at a higher pressure of 2-3 MPa and a regenerator where carbon dioxide is desorbed in the stripping steam flowing counter currently through the regenerator packing layers as a result of pressure reduction to almost atmospheric value and temperature increase to the solution boiling point.

The cyclic absorption process of CO 2 from industrial gaseous mixtures with alkaline solution of a chemisorbent in the absorber operating at elevated pressure of 2-3 MPa and CO 2 desorption from the solution to the stripping steam flowing through the regenerator at the pressure of almost atmospheric value takes place usually in quasi- isothermal conditions, i.e. at only small temperature difference between the absorber and the regenerator which provides for reduction of heat demand for the process to minimum. Carbon dioxide contained in the process gas fed to the absorber at a higher pressure, most favourably in the range of 2-3 MPa, at temperature 70-100 0 C is absorbed in a counter-current stream of regenerated alkaline solution fed to the absorber at the temperature 70-110 0 C. The solution saturated with carbon dioxide is directed from the bottom part of the absorber to the top of the regenerator operating at the pressure of almost atmospheric value where it flows down through packing layers and is heated to the temperature 110-120 0 C with stripping steam flowing counter-currently and regenerated with liberation of gaseous CO 2 which is discharged from the regenerator with stripping steam.

In the known and widely used methods of carbon dioxide removal from industrial gaseous mixtures which consist in CO 2 absorption with alkaline chemisorbent solution circulating between the absorber and the regenerator the basic heat source necessary for the solution regeneration is hot process gas, containing substantial excess of process steam fed to the CO 2 removal plant at the temperature 220-280 0 C.

Almost all heat of the hot process gas fed to the CO 2 removal unit is recovered in heat exchangers located upstream the absorber where steam is generated there it is used for the solution regeneration.

In many known industrial solutions there is deficiency of heat necessary for generation of sufficient stripping steam necessary for the provision of required solution regeneration degree. The deficiency is usually supplemented with heat from external sources. Such heat is fed to the regenerator as a low pressure steam in a direct and indirect way through an additional heat exchanger. However, such solutions lead to higher heat consumption coefficient.

Heat recovery from hot solution, which leaves the regenerator combined with generation of an additional low pressure steam directed to the regenerator is another method of providing for deficiency of heat indispensable for generation of sufficient steam necessary for providing the required solution regeneration degree.

In a method based on US patent No. 3 823 222, which is widely applied in commercial practice, a stream of semi-lean solution is discharged at the temperature of 115-120 0 C from the middle plate of the regenerator, at the pressure of 0.14-0.16 MPa and flows through a series of four expander chambers where reduced pressure in the range of 0.103-0.105 MPa is maintained with steam ejectors fixed over each expander chamber. The solution, cooled in the expander to the temperature of 105-110 0 C by evaporation of low pressure steam, is fed to the middle part of an absorber. Low pressure steam generated as a result of boiling of hot solution flowing through successive expander chambers is compressed to the pressure of the middle part of the regenerator and then directed to the regenerator together with steam applied for driving steam ejectors and it increases the amount of stripping steam used for the solution regeneration.

However, the method of carbon dioxide removal from industrial gaseous mixtures described in US patent No. 3 823 222 has an important drawback, i.e. high content, defined by absorption equilibrium state, of carbon dioxide in the low pressure steam generated by evaporation of solution flowing through a multi-chamber expander. Semi-lean solution discharged from the middle part of the regenerator in thermal equilibrium state is fed to the first expander chamber and then it flows through successive chambers separated with weir where the pressure is lowered with the use of steam injectors fixed over gas space of each chamber. Reduction of steam pressure over the surface of solution flowing to each chamber results in bubble boiling of the bulk solution volume flowing through the expander.

Developed surface of steam bubbles formed in the bulk volume of boiling solution favours CO 2 desorption from the solution to gaseous phase. As the result of development of high interfacial surface CO 2 content in the stream of water vapour discharged from the expander is close to an equilibrium value determined by absorption equilibrium state over the solution of defined KHCO3/K2CO3 ratio.

Low pressure steam, generated with the heat of semi-lean hot solution in a way described in US patent 3 823 222, contains substantial CO 2 amount, close to an equilibrium value, can be fed only under the middle regenerator plate and used only for regeneration of solution at the top of the regenerator. Steam generated in this way cannot be used for final regeneration of solution flowing through the bottom packing layer of the regenerator because of substantial CO 2 content and it limits its application efficiency for regeneration of the whole solution.

The method of this invention eliminates drawbacks of the above described method of CO 2 removal from industrial gaseous mixtures coupled with low pressure steam generation used for solution regeneration out of heat of hot semi-lean solution and provides for steam generation form hot partly regenerated solution with CO 2 content much lower than the equilibrium value. The essence of our invention consists in that semi-lean solution discharged from the middle part of the regenerator and directed to the middle part of the absorber is fed to a one-chamber expander over the solution surface and the low pressure steam generated by rapid solution evaporation in the expander is first compressed and then fed under the bottom packing layer in the regenerator.

Semi-lean solution is discharged from the middle part of the regenerator where it reaches carbonization [KHCO 3 ]/[(2[K 2 CO 3 ] + [KHCO 3 ] degree of 0.40-0.50. Such solution is discharged from the regenerator in boiling state at the pressure of 0.14-0.16 MPa and the temperature of 114-118°C. Boiling solution is fed to the expander where pressure is reduced to 0.10-0.12 MPa.

In the expander conditions, the solution discharged from the middle part of the regenerator in boiling state becomes superheated solution which tends to restoration thermal equilibrium rapidly. Thermal equilibrium is restored by rapid evaporation of a part of the stripping steam and solution temperature reduction to the equilibrium value corresponding to reduced pressure in the expander.

Low pressure steam generated as the result of rapid evaporation of hot solution in the expander is compressed to the pressure of 0.15-0.20 MPa with the use of a higher pressure steam of 0.4-l.OMpa and is directed with steam used for compression under the bottom packing of regenerator. As the result of this, low pressure steam generated in the expander out of heat of hot solution discharged from the middle part of the regenerator increases the whole stream of stripping steam directed to the regenerator.

Unexpectedly, the content of CO 2 in the low pressure steam generated by evaporation from hot solution of potassium carbonate turned out to be strongly dependant of the process method of steam evaporation from the solution. Kinetics investigation of CO 2 desorption from potassium carbonate hot solution of various KHCO 3 /K 2 CO 3 ratio and the speed of steam evaporation from overheated solution, which flows to the expander where the pressure is lower than the pressure determined by equilibrium state of boiling, showed that the method of feeding the solution to the expander and the time of its staying in the expander have a great impact on the content of steam phase, which is discharged from the surface of the solution. Restoring thermal equilibrium state between liquid and gaseous phase, which was disturbed by feeding hot potassium carbonate solution to the expander, in which the pressure is lower than the equilibrium pressure over boiling solution is an instantaneous process, which consists in rapid evaporation of an adequate amount of water, which causes solution temperature decrease up to the temperature determined as a new thermal equilibrium state. Small interphase surface developed during solution evaporation directed over the surface in expander causes small CO 2 desorption during evaporation, the result of which CO 2 partial pressure in stripping steam directed out of the expander is considerably lower than CO 2 equilibrium pressure over the solution.

The process of rapid evaporation of water in the expander and cooling the solution up to the temperature of a new thermal equilibrium state is accompanied with a relatively slow CO 2 desorption and restoring adsorption equilibrium between liquid and gaseous state, which was disturbed by generation of additional stripping steam, which changed the content of gaseous phase. Thus, as the result of conducted in a special way evaporation process from overheated potassium carbonate solution it is possible even at high ratio of KHCO 3 /K 2 CO 3 to obtain stripping steam, which is slightly contaminated with CO 2 .

In convenient conditions of solution evaporation in the expander with the use of this method it turned out that the content of CO 2 in steam generated by solution evaporation discharged over the solution, of the medium grade of carbonization [KHCO 3 ]/(2[K 2 CO 3 ] + [KHCO 3 ]) equaling to 0.45 is lower than the content of CO 2 in steam generated by evaporation from lean solution of low ratio of carbonization equaling to 0.25 in reboiler where water evaporation form the solution is accompanied with CO 2 desorption in the amount determined as an absorption equilibrium state.

Low CO 2 content in stream generated by evaporation of hot solution in the expander using the method of this invention and the possibility of using this stripping steam for regeneration of solution in bottom part of regenerator have an essential impact on the effectiveness of the use of an additional stripping steam generated out of hot solution directed from the middle part of the regenerator for regeneration of absorption solution. Feeding additional stripping steam generated in the expander under the bottom packing of the regenerator improves the conditions of mass transfer in both parts of the regenerator, which is the advantage of this invention in comparison to the method based on U.S. Pat. No. 3823 222.

The invention is explained in the example using the diagram below. 200000 Nm /h syngas stream containing 17.5 % volume CO 2 , directed to CO 2 removal unit through the pipe 1 at the temperature of 24O 0 C was directed into the absorber 2 under the pressure of 2.9 MPa, after cooling in medium pressure steam boiler 3, reboiler 4 and boiler feed water heater 5 up to the temperature of 80 0 C. 1600 t/h potassium carbonate solution stream at the temperature of 108°C, containing 30% wt. of 0.45 carbonization ratio was fed into the absorber over the bottom packing layer through the conduit 6 and 320t/h of potassium carbonate solution at the temperature of 80°C of 0.25 carbonization ratio was fed over the top part of packing layer of the regenerator through conduit 7. Purified gas, which contains CO 2 in the amount of up to 0.1% CO 2 was discharged from the absorber through conduit 8.

Approximately 2000 t/h of saturated potassium carbonate solution discharged from the bottom part of the absorber 2 through the conduit 9 was directed into the top of the regenerator 10. As the result of regeneration which takes place due to the pressure reduction at the top of regenerator up to 0.14 MPa and the contact with stripping steam flowing counter currently from the bottom to the top through all the packing layers, under the pressure close to the atmospheric pressure through conduit 11 approximately 40 000 Nm /h of carbon dioxide with the excess of stripping steam was discharged. 1600 t/h stream of semi-lean solution discharged at boiling state under the pressure of 0.15 MPa, at the temperature of 116 0 C from the middle part of regenerator after obtaining carbonization ratio of [KHCO 3 ]/(2[K 2 CO 3 ] + [KHCO 3 ]) equaling to 0.45 was fed through conduit 12 to one-chamber expander over the solution surface. In the expander the pressure was reduced to 0.105 with the use of stripping steam ejector 14 driven by 30 t/h steam with the pressure of 0.6 MPa fed from boiler 3 through conduit 15.

Solution cooled in the expander up to the temperature of 107-108°C as the result of evaporation of low pressure steam was directed under the bottom packing layer of the absorber 2 through conduit 6 with the use of pump 16. The remaining solution in the amount of 320t/h flowing through the bottom packing layer of the regenerator was directed through conduit 18 to the reboiler 4, where the solution was heated up to boiling with process gas stream and stripping steam was generated, which was directed with boiling liquid under the bottom of the regenerator through conduit 17. Lean solution of potassium carbonate with carbonization ratio of 0.25 discharged from the bottom of the regenerator through conduit 19, was cooled in the cooler 29 up to the temperature of 80°C and was directed to the top of the absorber 2 with the use of the pump 21.

Low pressure steam, which was generated as the result of solution rapid evaporation in the expander was directed through conduit 22 and after the compression in the steam ejector 14 up to the pressure of 0.18 MPa with the use of steam of the pressure of 0.6 MPa, which was directed through conduit 15 and under the bottom packing layer of the regenerator 10 through conduit 23 and 17, with steam used for its compression and stripping steam generated in reboiler.