Continuously stirred tank reactor absorber and flash tank stripper system

Abstract

The invention relates to a process for separation or purification of gaseous streams by removal of acid gases using a liquid amine solution. The process involves the steps of contacting the gaseous stream with liquid lean amine solution in at least one continuous flow stirred-tank reactor (10; 10a, 10b; 10c); removing a sweetened gaseous flow from said continuous flow stirred-tank reactor (10); removing rich amine from said continuous flow stirred-tank reactor (10; 10a, 10b; 10c) for regeneration; passing rich amine solution through at least one flash tank stripper 20; 20a, 20b; 20c; removing acid gases and vapor from said flash tank stripper 20; 20a, 20b; 20c; removing lean amine from said flash tank stripper for recirculation to said continuous flow stirred-tank reactor (10; 10a, 10b; 10c).

Claims

1. A process for separation or purification of gaseous streams by removal of acid gases using a liquid amine solution, the process comprising the steps of: contacting the gaseous stream with liquid lean amine solution in at least one continuous flow stirred-tank reactor; removing a sweetened gaseous flow from said continuous flow stirred-tank reactor; removing rich amine solution from said continuous flow stirred-tank reactor for regeneration; passing rich amine solution through at least one flash tank stripper; removing acid gases and vapor from said flash tank stripper; and removing lean amine solution from said flash tank stripper for recirculation to said continuous flow stirred-tank reactor; wherein the gaseous stream is contacted with liquid lean amine solution in a plug flow reactor downstream of said continuous flow stirred-tank reactor.

2. The process according to claim 1, wherein the gaseous stream is contacted with liquid lean amine solution in multiple continuous flow stirred-tank reactors arranged in series.

3. The process according to claim 1, wherein the gaseous stream is contacted with liquid lean amine solution in at least one continuous flow stirred-tank reactor and a column absorber arranged in series.

4. The process according to claim 1, wherein the gaseous stream is contacted with liquid lean amine solution in multiple continuous flow stirred-tank reactors arranged in parallel.

5. The process according to claim 1, wherein the gaseous stream is contacted with liquid lean amine solution by supplying the gaseous stream at the bottom of the continuous flow stirred-tank reactor and removing said gaseous stream at the top of said reactor.

6. The process according to claim 1, wherein rich amine solution is passed through two or more flash tank strippers arranged in series.

7. The process according to claim 1, wherein rich amine solution is passed through two or more flash tank strippers arranged in parallel.

8. The process according to claim 1, wherein rich amine solution is passed through at least one flash tank stripper and a column stripper arranged in series.

9. The process according to claim 1, further comprising passing acid gases and vapor from said flash tank stripper through a condenser and removing lean amine and condensate for recirculation.

10. The process according to claim 1, further comprising using an amine solution including one or more of monoethanol amine (MEA), diethanol amine (DEA), or methyl diethanol amine (MDEA).

11. The process according to claim 1, wherein the process is performed to remove CO.sub.2 and/or H.sub.2S from the gaseous stream.

12. The process according to claim 1, wherein the process is performed at a predetermined pressure dependent on a supply pressure of the gaseous stream.

13. The process according to claim 12, wherein the process is performed at a gaseous stream supply pressure of 60-70 bar.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) 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.

(2) FIG. 1 shows a schematic combined CFSTR absorber and flash tank stripper system according to the invention;

(3) FIG. 2A shows a first alternative combined CFSTR absorber and flash tank stripper system according to the invention;

(4) FIG. 2B shows a second alternative combined CFSTR absorber and flash tank stripper system according to the invention; and

(5) FIG. 3 shows a schematic illustration of a CFSTR as used in the invention.

EMBODIMENTS OF THE INVENTION

(6) FIG. 1 shows a schematic combined CFSTR absorber and flash tank stripper system according to the invention. Only the main operational units are shown. A continuous flow stirred-tank reactor or CFSTR 10 is supplied with lean amine solution through a first inlet conduit 11. The lean amine solution contacts a gaseous stream containing CO.sub.2 supplied to the CFSTR 10 through a second inlet conduit 12 at the bottom of the CFSTR 10. The CFSTR 10 shown in this example is provided with a stirring device 13 to enhance mixing of lean amine and gas bubbles within the liquid volume 14. Such a stirring device can be used to assist the mixing in the tank. Rich amine with absorbed CO.sub.2 leaves the CFSTR 10 through a first outlet 15 for regeneration in a stripping process. Optionally, the liquid outlet 15 can act as a plugged flow reactor or PFR 5 if the liquid in 15 contains remaining dissolved CO.sub.2 gas. Part of liquid flow 15 may be recycled to liquid volume 14 if desired. This optional PFR 5 is schematically indicated with dashed lines in FIG. 1. In its simplest form, the PFR 5 comprises a pipe with a predetermined cross-section leaving the CSTR 10, which pipe can have any suitable shape along its length, such as straight or spiral. Remaining lean amine and any dissolved CO.sub.2 from the CFSTR 10 will move along the PFR 5, wherein the remaining dissolved CO.sub.2 in the outlet liquid flow will react with the solvent. A sweetened gaseous stream leaves the CFSTR 10 through a second outlet 16 for immediate use of for suitable further processing. In this example, the gaseous stream is natural gas with a predetermined CO2 content. The CFSTR 10 will be described in further detail in connection with FIG. 3.

(7) Rich amine with absorbed CO.sub.2 flows through a heat exchanger 17, in which the relatively cool rich amine is used for cooling relatively hot lean amine leaving the stripping process. A pump 18 is provided for pressurizing the rich amine which is subsequently heated to a predetermined temperature in a heater 19.

(8) The rich amine solution is passed to a flash tank stripper 20. The rich amine stream undergoes a pressure reduction by passing through a flash valve 21 at an inlet conduit 23 at the entry into the flash tank 20. The flash valve 21 is used to control pressure and temperature at the inlet conduit 23 to the flash tank 20 in order to maintain optimum flash conditions within the flash tank 20.

(9) A part of the liquid amine solution immediately flashes into vapor to release CO.sub.2. Both the vapour and the residual liquid are cooled to the saturation temperature of the liquid at the reduced pressure. The remaining liquid amine solution is heated by steam flowing through steam pipes 24 in a lower part of the flash tank 20 to release additional CO.sub.2. The lower part of the flash tank 20 is also provided with a liquid controller 25 for maintaining the liquid level in the lower part of the flash tank 20. Lean amine solution leaves the lower part of the flash tank 20 through a first outlet conduit 26 and is returned to the absorber process. The residence time in the flash tank 20 is relatively short to avoid degradation.

(10) Flashed and released CO.sub.2 will leave the flash tank 20 through a second outlet conduit 27 at the upper part of the flash tank 20. In addition to CO.sub.2, an amount of vapour containing amine and water diluent will leave the upper part of the flash tank 20.

(11) Acid gas and vapour containing amine and water is passed from said flash tank 20 to a first inlet conduit 31 of a condenser 30. A solution comprising lean amine and condensate diluent water is removed through a first outlet conduit 32 for recirculation. This lean amine solution is merged with the lean amine from the first outlet conduit 26 of the flash tank 20 and is pumped back via a pump 33 to said heat exchanger 17, where it is cooled by the rich amine solution from the absorber process before being supplied to the CFSTR 10. In the condenser 30, pure CO.sub.2 is removed through a second outlet conduit 34 for further processing or storage.

(12) Pressure controllers and other utility equipment installed for control of the process are not shown in FIG. 1. A more detailed description of the CFSTR absorber is shown in FIG. 3.

(13) The process in FIG. 1 can be enhanced further by contacting the gaseous stream with the liquid lean amine solution in a plug flow reactor 5 (PFR; indicated in dashed lines) downstream of said continuous flow stirred-tank reactor 10. Lean amine and dissolved CO.sub.2 will move with the same velocity along the PFR, wherein CO.sub.2 is absorbed into the lean amine during the mixing. If a PFR is used, the subsequent pump must be able to handle two-phase flow.

(14) FIG. 2A shows a first alternative combined CFSTR absorber and flash tank stripper system according to the invention. The system in FIG. 2A differs from that of FIG. 1 in that the single CFSTR 10 has been replaced by a first and a second CFSTR 10a, 10b respectively. Similarly, the single flash tank 20 has been replaced by a first and a second flash tank 20a, 20b, respectively, in series. The remaining components are the same as in FIG. 1 and are provided with the same reference numbers.

(15) Hence, in the system in FIG. 2A a first continuous flow stirred-tank reactor or CFSTR 10a is supplied with lean amine solution through a first inlet conduit 11a. The lean amine solution contacts a gaseous stream containing CO.sub.2 supplied from a second CFSTR 10b to the first CFSTR 10a through a second inlet conduit 12a at the bottom of the first CFSTR 10a. In this example the first CFSTR 10a is provided with a first stirring device 13a to enhance mixing of lean amine solution and gas bubbles within the first liquid volume 14a. Such a stirring device is used to assist the mixing in the tank if deemed necessary. Partially enriched amine solution with absorbed CO.sub.2 leaves the first CFSTR 10a through a first outlet conduit 15a and is supplied to the second CFSTR 10b. A sweetened gaseous stream leaves the first CFSTR 10a through a second outlet conduit 16a for further processing.

(16) Partially enriched amine solution enters the second CFSTR 10b through a further amine solution inlet conduit 11b and contacts a gaseous stream containing CO.sub.2 supplied to the second CFSTR 10b through a further gas inlet conduit 12b at the bottom of the second CFSTR 10b. The further gas inlet conduit 12b is connected to a source of gas, such as flue gas or natural gas. The second CFSTR 10b is provided with a further stirring device 13b, in the same way as the first CFSTR 10a, to enhance mixing of amine solution and gas bubbles within the further liquid volume 14b. Rich amine solution with absorbed CO.sub.2 leaves the second CFSTR 10b through a further amine solution outlet conduit 15a for regeneration in a stripping process. A partially sweetened gaseous stream leaves second CFSTR 10b through a further gas outlet conduit 16b and is supplied to the first CFSTR 10a.

(17) As in described in FIG. 1, the rich amine solution is subsequently passed from the absorbing process to the stripping process.

(18) In this case, the rich amine solution is passed to a first and a second flash tank 20a, 20b. The rich amine stream undergoes a pressure reduction by passing through a first flash valve 21a at a first inlet conduit 23a at the entry into the first flash tank 20a. The first flash valve 21a is used to control pressure and temperature at the first inlet conduit 23a to the first flash tank 20a in order to maintain optimum flash conditions within the first flash tank 20a. A part of the liquid amine solution immediately flashes into vapor to release CO.sub.2. The remaining liquid amine solution is heated by steam flowing through first steam pipes 24a in a lower part of the first flash tank 20a to release additional CO.sub.2. The lower part of the first flash tank 20a is also provided with a first liquid controller 25a for maintaining the liquid level in the lower part of the flash tank 20a. Lean amine solution leaves the lower part of the first flash tank 20a through a first outlet conduit 26a and is supplied to the second flash tank 20b.

(19) The above stripping process is then repeated in the second flash tank 20b. Hence, the partially stripped amine stream undergoes a further pressure reduction by passing through a second flash valve 21b at a further inlet 23b at the entry into the second flash tank 20b. The second flash valve 21b controls pressure and temperature at the further inlet 23b to the second flash tank 20b in order to maintain optimum flash conditions within the first flash tank 20b. A further part of the partially stripped liquid amine solution immediately flashes into vapor to release CO.sub.2. The remaining liquid amine solution is heated by steam flowing through second steam pipes 24b in a lower part of the second flash tank 20b to release additional CO.sub.2. The lower part of the second flash tank 20b is also provided with a second liquid controller 25b for maintaining the liquid level in the lower part of the second flash tank 20b. Lean amine solution leaves the lower part of the second flash tank 20b through a further outlet conduit 26b and is supplied to the absorber process.

(20) The residence time in the flash tanks 20a, 20b is relatively short to avoid degradation of the amine.

(21) Flashed and released CO.sub.2 will leave the first and second flash tanks 20a, 20b through outlet conduits 27a, 27b at the upper part of the respective flash tank 20a, 20b. In addition to CO.sub.2, an amount of vapour containing amine and water diluent will leave the upper parts of said flash tanks 20a, 20b.

(22) Acid gas and vapour containing amine and water is passed from said flash tanks 20a, 20b to a first inlet conduit 31 of a condenser 30. A solution comprising lean amine and condensate diluent water is removed through a first outlet conduit 32 for recirculation. This lean amine solution is merged with the lean amine from the outlet conduit 26b of the second flash tank 20b and is pumped back via a pump 33 to said heat exchanger 17, where it is cooled by the rich amine solution from the absorber process before being supplied to the CFSTR 10. In the condenser 30, pure CO.sub.2 is removed through a second outlet conduit 34 for further processing or storage.

(23) FIG. 2B shows a second alternative combined CFSTR absorber and flash tank stripper system according to the invention. The system in FIG. 2B differs from that of FIG. 2A in that the initial CFSTR 10 has been replaced by a column absorber 40. Similarly, the second flash tank 20b has been replaced by a column stripper 60. The remaining components are the same as in FIG. 1 and are provided with the same reference numbers.

(24) Hence, in the system in FIG. 2B an initial column absorber 40 is supplied with lean amine solution through a first inlet conduit 41. The lean amine solution contacts a gaseous stream containing CO.sub.2 supplied from a CFSTR 10c through a second inlet conduit 42 at the bottom of the column absorber 40. The column absorber 40 is a conventional absorber which will not be described in detail here. Partially enriched amine solution with absorbed CO.sub.2 leaves the column absorber 40 through a first outlet conduit 45 and is supplied to the CFSTR 10c. A sweetened gaseous stream leaves the column absorber 40 through a second outlet conduit 46 for further processing.

(25) Partially enriched amine solution enters the CFSTR 10c through a first inlet conduit 11c and contacts a gaseous stream containing CO.sub.2 supplied to the CFSTR 10c through a second inlet conduit 12c at the bottom of the CFSTR 10c. The second gas inlet conduit 12c is connected to a source of natural gas. The CFSTR 10c is provided with a stirring device 13c to enhance mixing of amine solution and gas bubbles within the liquid volume 14c in the tank. Rich amine solution with absorbed CO.sub.2 leaves the CFSTR 10c through a first outlet conduit 15c for regeneration in a stripping process. As described in connection with FIG. 1, an optional plugged flow reactor (nor shown) can be provided at this stage. A partially sweetened gaseous stream leaves the CFSTR 10c through a second outlet conduit 16. As in described in FIG. 2A, the rich amine solution is subsequently passed from the absorbing process to the stripping process.

(26) In the stripping process, the rich amine solution is passed to a flash tank 20c. The rich amine stream undergoes a pressure reduction by passing through a first flash valve 21c at a first inlet conduit 23c at the entry into the flash tank 20c. The first flash valve 21c is used to control pressure and temperature at the first inlet conduit 23c to the flash tank 20c in order to maintain optimum flash conditions within the flash tank 20c. A part of the liquid amine solution immediately flashes into vapor to release CO.sub.2. The remaining liquid amine solution is heated by steam flowing through steam pipes 24c in a lower part of the flash tank 20c to release additional CO.sub.2. The lower part of the flash tank 20c is also provided with a liquid controller 25c for maintaining the liquid level in the lower part of the flash tank 20c. Partially stripped amine solution leaves the lower part of the first flash tank 20a through a first outlet conduit 26c and is supplied to a subsequent column stripper 60.

(27) Partially stripped amine solution is supplied to a first inlet conduit 61 at the top of the column stripper 60 and leaves as lean amine solution from a first outlet conduit 62 at the bottom of the column stripper 60. Heat for the stripping process is supplied through a second inlet conduit 63 from a reboiler 65 connected at the bottom of the column stripper 60. Acid gas and vapour containing amine and water is passed from a second outlet conduit 64 at the top of the column stripper 60 to a first inlet conduit 31 of a condenser 30. The counter-current columns 40, 60 for absorption and stripping shown in FIG. 2B are well known and are therefore not described further here.

(28) A solution comprising lean amine and condensate diluent water is removed through a first outlet conduit 32 of the condenser 30 for recirculation. A fraction of the output from the condenser 30 is returned to the reboiler 65 through a second inlet conduit 66. The remaining lean amine solution from the condenser 30 is merged with the lean amine from a second outlet conduit 67 from the reboiler 65 and is pumped back via a pump 33 to said heat exchanger 17, where it is cooled by the rich amine solution from the absorber process before being supplied to the CFSTR 10. In the condenser 30, pure CO.sub.2 is removed through a second outlet conduit 34 for further processing or storage.

(29) FIG. 3 shows a schematic illustration of a CFSTR 70 as used in the invention. The CFSTR 70 comprises a tank 75 having a gas volume V.sub.g and a liquid volume V.sub.l as shown in FIG. 3. The CFSTR 70 is supplied with lean amine solution through a first inlet conduit 71. The lean amine solution contacts a gaseous stream supplied to the CFSTR 70 through a second inlet conduit 72 and is injected directly into the liquid at the bottom of the CFSTR 70. The CFSTR 70 is provided with a stirring device 73 to enhance mixing of lean amine and gas bubbles within the liquid volume V.sub.g. Such a stirring device 73 is used to assist the mixing in the tank if deemed necessary. Rich amine with absorbed CO.sub.2 leaves the CFSTR 70 through a first outlet 75 for regeneration in a stripping process. An optional PFR can be used at this stage, as described above. A sweetened gaseous stream leaves the CFSTR 70 through a second outlet 76 for further processing. A mixture of CO.sub.2 (gas), N.sub.2 (gas), O.sub.2 (gas) and H.sub.2O (gas) flows into the liquid volume V.sub.l where the gas is mixed with a solvent. In the example presented here the solvent is mono ethanol amine (MEA) and H.sub.2O and the main reaction products are MEAH.sup.+ (MEA+H.sub.2O=MEAH.sup.++OH.sup.) and carbamate. Initially the liquid volume contains a mixture of MEA and H.sub.2O, MEAH.sup.+ and carbamate with an initial CO.sub.2 loading .sub.0.

(30) The gas volume V.sub.g in the tank is initially filled with N.sub.2, O.sub.2 and H.sub.2O vapour and possibly some CO.sub.2. The CO.sub.2 loading increases due to the chemical reaction of dissolved CO.sub.2 (gas) with MEA (liquid) in the liquid volume V.sub.l. Mixing of gas and liquid in volume V.sub.l can be achieved in alternative ways. After some time the CO.sub.2 loading will reach a steady-state level .sub.r. The steady-state loading is frequently called the rich loading .sub.r.

(31) The gas components N.sub.2 and O.sub.2 are assumed inert (does not take part in the chemical reactions). The solubility of N.sub.2 and O.sub.2 in a MEA and H.sub.2O liquid mixture is low compared to CO.sub.2. Hence, N.sub.2 and O.sub.2 are transferred from the liquid volume V.sub.l to the gas volume V.sub.g by mass transfer across the gas/liquid interface. A variable rate of CO.sub.2 (gas) will be transferred from the liquid volume to the gas volume depending on the rate of the chemical reaction. Gas transfer of a component from liquid to gas occurs as long as the partial pressure of the component in volume V.sub.g is less than the equilibrium pressure, which can be calculated.

(32) The chemical reaction of CO.sub.2 with MEA in the liquid is exothermic and temperature dependent with known kinetics. No chemical reactions are assumed in the gas volume V.sub.g of the tank.

(33) Small amounts of CO.sub.2 gas will accumulate in the gas volume V.sub.g if the inlet flow rate of CO.sub.2 is comparable with the rate of CO.sub.2 consumed in the chemical reaction in the given liquid volume V.sub.l.

(34) Flue gas with CO.sub.2 is supplied to a mixture of liquid MEA/H.sub.2O in order to increase the CO.sub.2 loading in the tank and in the tank outlet due to the chemical reaction of CO.sub.2 with MEA. Most of the CO.sub.2 reacts rapidly with MEA in the liquid mixing tank. Mass and heat transfer of gas components from the liquid volume to the gas volume occur across the gas/liquid interface.

(35) A dynamic mass and energy balance for both liquid and gas in the CFSTR have been modelled. The model is programmed in Fortran 77 with name SORBER_CSTR. The chemical reaction with MEA and CO.sub.2 in the liquid is modelled in addition to the mass transfer between liquid and gas in the gas/liquid interface. The outlet CO.sub.2-loading , the gas temperature T.sub.g and the liquid temperature T.sub.l are calculated as function of time knowing the gas volume V.sub.g and the liquid volume V.sub.l. In addition, the initial gas and liquid temperature must be known together with the initial gas and liquid composition in the tank. The inlet gas temperature and the inlet liquid temperature must be specified, together with the inlet composition and the inlet flow rate of both gas and liquid. The program easily calculates the liquid residence time .sub.l for a specified input. From that information the time to steady-state appearance can be calculated. SORBER_CSTR is therefore a useful tool with respect to experiment planning and for understanding the CFSTR absorber behaviour in general, for both small and large scale absorbers.

(36) A flash tank stripper used in the process according to the invention is shown in FIG. 1. CO.sub.2, H.sub.2O and MEA are separated in the condenser and H.sub.2O and MEA from the condenser are mixed with the liquid lean amine outlet stream from the flash tank. The mixed stream is returned to the CFSTR absorber after heat exchange in the heat exchanger, as shown in FIG. 1. The optimal temperature T and pressure P conditions are tuned experimentally based on simulated start values. The flash valve in front of the flash tank is used to keep the fluid pressure and temperature at the inlet to the flash tank at optimum flash conditions. The liquid pressure at the flash valve inlet shall be sufficiently high in order to set the correct flash pressure in the flash valve. Pumping of liquids to moderately high pressures is inexpensive compared to gas compression. Between the pump and the flash valve a fluid heater may optionally be installed. An extra heat supply in the flash tank should also be considered in order to keep the best flash temperature. Pressure and temperature controlling means are not shown in the figures.

(37) The invention is not limited to the above embodiments, but may be varied freely within the scope of the appended claims. Consequently, the figures described above only show a limited, basic number of possible combinations of CFSTRs and flash tank strippers arranged in series. The CFSTRs and flash tank strippers can also be arranged in parallel or in both series and parallel. In addition, multiple CFSTRs and flash tank strippers arranged in series can be combined in series with additional column absorbers and/or column strippers.

(38) Also, the invention is not restricted for use in low pressure processes. In for example liquid natural gas (LNG) plants CO.sub.2 must be removed before natural gas processing can begin in order to avoid clogging. The inlet pressure is typically 60-70 bars in LNG plants and gas at this pressure would be supplied to a CFSTR in the process. The invention can also be used to capture multiple gases, for example to capture CO.sub.2 and H.sub.2S simultaneously.