System for the removal of heat stable amine salts from an amine absorbent

Abstract

A system for the removal of heat stable amine salts from an amine absorbent used in a carbon dioxide (CO.sub.2) capture process.

Claims

1. A carbon dioxide (CO.sub.2) capture system using an amine absorbent for absorption of CO.sub.2 from a gas stream, the CO.sub.2 capture system comprising a subsystem for removal of heat stable amine salts from a slipstream comprising the amine absorbent, the subsystem comprising: a residual CO.sub.2 removal unit in liquid connection with, and configured to receive, the slipstream containing heat stable amine salts from a CO.sub.2-lean amine absorbent stream of the CO.sub.2 capture system, and operative for separating residual CO.sub.2 from the CO.sub.2-lean amine absorbent stream; and an amine reclaimer in liquid connection with, and configured to receive, the slipstream containing heat stable amine salts and having a reduced concentration of CO.sub.2 from the residual CO.sub.2 removal unit, operative for separating heat stable amine salts from the slipstream, and configured to return the slipstream separated from heat stable amine salts to the CO.sub.2-lean amine absorbent stream.

2. The CO.sub.2 capture system according to claim 1, wherein the amine reclaimer comprises an electrodialysis unit or an ion exchange unit.

3. The CO.sub.2 capture system according to claim 1, wherein the subsystem further comprises an amine absorbent cooler operationally arranged between the residual CO.sub.2 removal unit and the reclaimer and operative for cooling the slipstream from the residual CO.sub.2 removal unit before the slipstream enters the reclaimer.

4. The CO.sub.2 capture system according to claim 1, wherein the subsystem further comprises an indirect heat exchanger operative for subjecting the slipstream from the residual CO.sub.2 removal unit to indirect heat exchange with the slipstream coming from the reclaimer.

5. The CO.sub.2 capture system according to claim 1, wherein the residual CO.sub.2 removal unit comprises a stripper and/or a flash drum.

6. The CO.sub.2 capture system according to claim 5, wherein the residual CO.sub.2 removal unit comprises the stripper.

7. The CO.sub.2 capture system according to claim 5, wherein the residual CO.sub.2 removal unit comprises the flash drum.

8. The CO.sub.2 capture system according to claim 5, wherein the residual CO.sub.2 removal unit comprises the stripper and the flash drum arranged in series.

9. The CO.sub.2 capture system according to claim 1 further comprising: a CO.sub.2 absorber operative for scrubbing the gas stream comprising CO.sub.2 with the amine absorbent such that a CO.sub.2-rich amine absorbent stream is formed; and a regenerator operative for regenerating the CO.sub.2-rich amine absorbent stream by heating the CO.sub.2-rich amine absorbent stream to separate CO.sub.2 from the amine absorbent, thereby forming the CO.sub.2-lean amine absorbent stream.

10. The CO.sub.2 capture system according to claim 9, wherein the slipstream is taken from the CO.sub.2-lean amine absorbent stream from the regenerator.

11. The CO.sub.2 capture system according to claim 9 further comprising an amine absorbent heat exchanger between the CO.sub.2 absorber and the regenerator.

12. The CO.sub.2 capture system according to claim 11, wherein the slipstream is taken from the CO.sub.2-lean amine absorbent stream from the amine absorbent heat exchanger.

13. The CO.sub.2 capture system according to claim 11, wherein the slipstream is taken from between the regenerator and the amine absorbent heat exchanger.

14. The CO.sub.2 capture system according to claim 11, wherein the slipstream is taken from between the amine absorbent heat exchanger and the CO.sub.2 absorber.

15. The CO.sub.2 capture system according to claim 11, wherein the amine reclaimer comprises an electrodialysis unit.

16. The CO.sub.2 capture system according to claim 11, wherein the amine reclaimer comprises an ion exchange unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Referring now to the figures, which are exemplary embodiments, and wherein the like elements are numbered alike:

(2) FIG. 1 is a diagram generally depicting an amine based gas purification system comprising an amine absorbent reclaimer circuit.

(3) FIG. 2 is a diagram generally depicting an embodiment of an amine based gas purification system comprising an amine absorbent reclaimer circuit.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(4) The term amine absorbent or simply absorbent, as used herein, refers to a liquid composition comprising at least one amine compound useful in absorption of CO.sub.2 from gas streams. Such compositions and suitable amine compounds are well known to a person skilled in the art. Examples of amine compounds commonly used in absorption of CO.sub.2 from gas streams include, but are not limited to, monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), diisopropylamine (DIPA) and aminoethoxyethanol (diglycolamine) (DGA). The most commonly used amines compounds in industrial plants are the alkanolamines MEA, DEA, and MDEA. The absorbent may comprise a single amine compound or a mixture of two or more amine compounds. In addition, the absorbent may comprise up to about 90% by volume of water, for example from about 50 to about 90% by volume of water. The absorbent may also comprise varying amounts of absorbed CO.sub.2. Absorbent containing none or only a low concentration of absorbed CO.sub.2, e.g. following regeneration, is referred to as CO.sub.2 lean or simply lean absorbent, whereas absorbent containing higher concentrations of absorbed CO.sub.2, e.g. following absorption, is referred to as CO.sub.2 rich or simply rich absorbent.

(5) FIG. 1 is a schematic representation of an amine based carbon dioxide (CO.sub.2) capture system (100). The system comprises an absorption unit (101) arranged to allow contact between a gas stream to be purified and one or more wash liquids. The absorption unit represented in FIG. 1 comprises a CO.sub.2 absorption section (102) and a water wash section (103). Flue gas, from which CO.sub.2 is to be removed, is fed to the absorption unit (101) via line (104). In the CO.sub.2 absorption section (102), the flue gas is contacted with a first wash liquid comprising an amine compound, e.g. by bubbling the flue gas through said first wash liquid or by spraying the first wash liquid into the flue gas. The first wash liquid is fed to the absorption unit (101) via line (105). In the CO.sub.2 absorption section (102) CO.sub.2 from the flue gas is absorbed in the first wash liquid. Flue gas depleted of CO.sub.2 in the CO.sub.2 absorption section then enters the water wash section (103) of the absorption unit. The water wash section (103) is arranged to allow contact between the flue gas depleted of CO.sub.2 from the CO.sub.2 absorption section (102) and a second wash liquid, which is generally water. The second wash liquid is fed to the absorption unit via line (106). In the water wash section, contaminants remaining in the flue gas when it leaves the CO.sub.2 absorption section are absorbed in the second wash liquid. Flue gas depleted of CO.sub.2 and contaminants leaves the absorption unit via line (107). The used first and second wash liquid containing absorbed CO.sub.2 and contaminants leave the absorption unit via line (108). The used first and second wash liquid may be recycled via a regenerator unit (109), wherein contaminants and CO.sub.2 are separated from the wash water. The separated CO.sub.2 leaves the system via line (110).

(6) The used first and second wash liquid to be regenerated enters the regenerator (109) via line (111). In the regenerator, the used wash liquids are heated, generally using steam, in a reboiler (112). The heating causes desorption of absorbed CO.sub.2 from the wash liquids. The desorbed CO.sub.2 then exits the regenerator via line (113) together with some water vapor also formed during heating. Regenerated wash liquid, containing a reduced concentration of CO.sub.2, leaves the regenerator (109) via line (114). The regenerated wash liquid is also referred to herein as CO.sub.2 lean amine absorbent or simply lean amine absorbent. The lean amine absorbent may also contain heat stable salts (HSS) formed as degradation products in the regenerator as a result of the exposure to high temperature and/or the presence of O.sub.2 (absorbed by the absorbent in the absorption unit). The lean amine absorbent leaving the regenerator may be directed to a lean absorbent/rich absorbent heat exchanger (123) where it is used for pre-heating rich amine absorbent from line (108) directed towards the regenerator (109).

(7) The amine based carbon dioxide (CO.sub.2) capture system (100) may further comprise an amine absorbent reclaimer circuit (115) operative for at least partial removal of HSS from the circulating amine absorbent, so as to prevent accumulation of HSS and the problems associated therewith. The amine absorbent reclaimer circuit (115) is generally configured to withdraw a slipstream of the main amine absorbent flow. The amine absorbent reclaimer circuit (115) may preferably be configured to withdraw the slipstream of lean amine absorbent from a point in the process where the amine absorbent has a low CO.sub.2 loading, i.e. lean amine absorbent. More particularly, the slipstream of amine absorbent may be withdrawn from the regenerator (109) or from the liquid conduit (114) between the regenerator (109) and a lean absorbent/rich absorbent heat exchanger (123). The lean amine absorbent from the regenerator generally has a temperature of 100 C. or higher, such as 120 C. or higher. This allows the thermal energy provided to the lean amine absorbent in the regenerator to be utilized in the stripping and/or flashing step. If necessary, the slipstream of lean amine absorbent containing HSS may also be withdrawn from the lean absorbent/rich absorbent heat exchanger (123) or from the liquid conduit (105) between the lean absorbent/rich absorbent heat exchanger (123) and the CO.sub.2 absorber (101) performing the scrubbing step. The slipstream may generally comprise in the range of 0.001-50% by volume of the main amine absorbent flow, such as in the range of 0.01-10% by volume of the main amine absorbent flow.

(8) FIG. 2 represents an amine based carbon dioxide (CO.sub.2) capture system according to the invention, comprising an amine absorbent reclaimer circuit (115). The amine absorbent reclaimer circuit (115) is connected to the regenerator side of an amine based carbon dioxide (CO.sub.2) capture system, e.g. as described above with reference to FIG. 1.

(9) The amine absorbent reclaimer circuit (115) comprises an amine reclaimer (116) for separating heat stable salts from the amine absorbent. In this embodiment, the amine reclaimer (116) is an electrodialysis (ED) unit.

(10) The ED unit is used to transport salt ions, e.g. HSS, from the amine absorbent through ion-exchange membranes to another solution under the influence of an applied electric potential difference. This is done in a configuration called an electrodialysis cell. The cell consists of a feed (diluate) compartment and a concentrate (e.g. brine) compartment formed by an anion exchange membrane and a cation exchange membrane placed between two electrodes. Multiple electrodialysis cells may be arranged into a configuration called an electrodialysis stack, with alternating anion and cation exchange membranes forming the multiple electrodialysis cells. The ED process results in a reduction of HSS in the amine absorbent as HSS ions are concentrated in the concentrate solution.

(11) In an alternative embodiment, the amine reclaimer (116) is an ion exchange unit comprising an ion exchange resin suitable for the removal of HSS ions from the amine absorbent.

(12) The amine absorbent reclaimer circuit (115) further comprises a residual CO.sub.2 removal unit (117) arranged upstream of the amine reclaimer (116) with reference to the lean amine absorbent stream. In the embodiment of FIG. 2 the residual CO.sub.2 removal unit (117) is a flash drum. Flash (or partial) evaporation is the partial vaporization that occurs when a saturated liquid stream undergoes a reduction in pressure by passing through a throttling valve or other throttling device. If the throttling valve or device is located at the entry into a pressure vessel so that the flash evaporation occurs within the vessel, then the vessel is often referred to as a flash drum.

(13) The flash drum (117) comprises a pressure vessel having a feed inlet, a gas outlet and a liquid outlet. The feed inlet is equipped with a throttling device configured to decrease the pressure of the feed stream before it enters the pressure vessel. The exact configuration of flash drums suitable for use in the system described herein will be readily recognized by a person skilled in the art.

(14) The lean amine absorbent enters the flash drum (117) via a feed line (118). The temperature and pressure of the lean amine absorbent is determined by the temperature and pressure of the lean amine absorbent in, or leaving, the regeneration unit (109). The pressure of the lean amine absorbent may optionally be decreased by means of a throttling valve or device arranged in the feed inlet of the flash drum. In the flash drum (117), the pressure is then reduced, such that more volatile components, e.g. residual CO.sub.2, at least partially evaporate, while less volatile components, e.g. amine absorbent and water, remain in liquid phase. The pressure inside of the flash drum may preferably be low, such as in the range of 0-2 bar gauge. Evaporated components, e.g. residual CO.sub.2, leave the flash drum (117) through a gas outlet via line (119), while liquid components, e.g. amine absorbent and water, leave the flash drum (117) through a liquid outlet via line (120).

(15) In an alternative embodiment, the residual CO.sub.2 removal unit (117) is a stripper. The stripper may, for example, comprise a generally cylindrical steel vessel configured to operate within a pre-determined pressure range. The stripper is preferably equipped with one or more suitable mass transfer devices, such as valve trays, sieve trays, structured packing, random packing or other suitable packing materials, or a combination thereof. A heating system/device may be provided in the stripper for heating the amine absorbent. The stripper is preferably configured to provide sufficient heat to the amine absorbent so that low boiling point components, for example CO.sub.2, are transferred to a gas phase, while high boiling point components, for example water and amine, are collected in a liquid phase at the bottom of the stripper. The amine absorbent may be heated up appropriately via, for example, a reboiler. The reboiler may be heated using, for example, electrically generated heat or steam. The stripper is configured to discharge the gas phase, containing CO.sub.2, via a gas exit, and the liquid phase, containing water and amine, via a liquid exit.

(16) In yet another alternative embodiment, the residual CO.sub.2 removal unit (117) comprises a stripper and a flash drum arranged in series, such that a first portion of residual CO.sub.2 may be removed in the stripper, and a second portion of residual CO.sub.2 may be removed in the flash drum. The stripper and flash drum may be as described above. The lean amine absorbent first enters the stripper, where it is heated to a temperature sufficient to transfer low boiling point components, for example CO.sub.2, to a gas phase, while high boiling point components, for example water and amine, are collected in a liquid phase at the bottom of the stripper. The liquid phase is then forwarded to the flash drum, where the pressure is reduced so that more volatile components, e.g. residual CO.sub.2, at least partially evaporate, while less volatile components, e.g. amine absorbent and water, remain in liquid phase. The liquid components, e.g. amine absorbent and water, leave the flash drum through a liquid outlet and is forwarded to the reclaimer.

(17) Referring now to FIG. 2, the lean amine absorbent, from which residual CO.sub.2 has been at least partially removed, is forwarded via line (120) to the amine reclaimer (116), wherein heat stable salts are at least partially separated from the amine absorbent to produce a lean amine absorbent depleted in HSS.

(18) Optionally, the amine absorbent reclaimer circuit (115) further comprises a cooler (121) arranged between the residual CO.sub.2 removal unit (117) and amine reclaimer, and configured to adjust the temperature of the lean amine absorbent from the residual CO.sub.2 removal unit before it enters the amine reclaimer (116).

(19) Furthermore, an amine absorbent reclaimer circuit (115) comprising a cooler (121), may optionally further comprise an indirect heat exchanger (not shown) arranged between the residual CO.sub.2 removal unit (117) and the cooler (121) and configured to cool the lean amine absorbent from the residual CO.sub.2 removal unit (117) using the lean amine absorbent depleted in HSS leaving the amine reclaimer (116). The indirect heat exchanger may for example be a conventional plate or shell and tube type heat exchanger.

(20) The lean amine absorbent depleted in HSS leaves the amine reclaimer (116) and is forwarded via return line (122) back to the CO.sub.2 capture system (100). The lean amine absorbent depleted in HSS may, for example be reintroduced into the regenerator (109), absorber (101), or into a suitable liquid conduit connecting the regenerator (109) and absorber (101). The position for reintroduction of the lean amine absorbent from the amine reclaimer circuit may be selected depending on the specific temperature and pressure of the absorbent. One suitable position for reintroduction, as shown in FIG. 2, would be into line (114), either upstream or downstream of a lean absorbent/rich absorbent heat exchanger (123). The separated heat stable salts leave the amine reclaimer via line (124).

(21) While the invention has been described with reference to a number of preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another.

ExampleAmine Loss into the Waste Brine Stream of the Electrodialysis Unit

(22) Amine losses from a lean amine absorbent into the waste brine of a 3-loop ElectroSep electrodialysis unit (ElectroSep Inc., USA) was evaluated with various CO.sub.2 loadings in the lean amine absorbent. 1.2 and 1.4 wt % amine was observed in the waste brine stream when the lean amine absorbent had CO.sub.2 loadings of 2.2 and 2.9 wt % CO.sub.2, respectively. Only 0.3 wt % amine was observed in the waste brine stream with 0.01 wt % CO.sub.2 in the lean amine absorbent. This represents a 75-80% reduction in amine losses compared to the higher lean loadings. This example shows that a significant reduction of amine losses can be achieved by reduction of the CO.sub.2 loading of the lean amine absorbent, e.g. by stripping or flashing, prior to feeding it to a reclaimer unit, such as an electrodialysis unit.