Membrane absorption process for CO.SUB.2 .capture

Abstract

Processes and systems for the capture of CO.sub.2 from a CO.sub.2-containing gas stream are provided. The CO.sub.2-containing gas stream is passed to a membrane contactor absorber wherein the CO.sub.2-containing gas contacts or passes a first side of a membrane element while a CO.sub.2 selective solvent with a viscosity between 0.2 and 7 cP contacts, passes or flows on second side of the membrane, opposed to the first side. The CO.sub.2 permeates through the hollow fiber membrane pores and is chemically absorbed into the solvent.

Claims

1. A process for CO.sub.2 capture, the process comprising: passing a CO.sub.2-containing flue gas stream to a membrane contactor absorber containing a membrane element, wherein the CO.sub.2-containing gas stream contacts a first side of the membrane element and wherein a CO.sub.2 selective solvent is in flow communication with a second side of the membrane element, the second side of the membrane element being opposed to the first side, wherein the CO.sub.2 selective solvent has a viscosity between 0.2 and 7 cP and wherein CO.sub.2 permeates through the membrane element and is chemically absorbed into the CO.sub.2 selective solvent, wherein the CO.sub.2 selective solvent has a concentration of less than 40 wt. %, and wherein the CO.sub.2-containing flue gas stream passes along the first side of the membrane element at a velocity in a range of 1-30 m/s and the CO.sub.2 selective solvent passes along the second side of the membrane element at a velocity in a range of 0.001-0.010 m/s.

2. The process of claim 1 wherein the CO.sub.2 chemically absorbed into the CO.sub.2 selective solvent forms a CO.sub.2-rich solvent and wherein said process additionally comprises: regenerating the solvent by removing CO.sub.2 from the CO.sub.2-rich solvent, and recycling the regenerated solvent to the membrane contactor absorber.

3. The process of claim 1 wherein the viscosity is between 1 and 5 cP.

4. The process of claim 1 wherein the membrane element comprises a hollow fiber membrane element having a bore side and an opposed outside.

5. The process of claim 4 wherein the CO.sub.2-containing gas stream is passed on the bore side of the hollow fiber membrane element and the CO.sub.2 selective solvent is passed on the outside of the hollow fiber membrane element.

6. In a process of operating of a power plant that produces the CO.sub.2-containing gas stream, the process of claim 1 wherein the CO.sub.2-containing gas stream passed to the membrane contactor absorber comprises the power plant produced CO.sub.2-containing gas stream.

7. The process of claim 6 wherein the power plant is a natural gas-fired power plant.

8. The process of claim 6 wherein the power plant is a coal powered power plant.

9. The process of claim 1 wherein the CO.sub.2 selective solvent is selected from the group consisting of amines and promoted carbonate solvents.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Objects and features of this invention will be better understood from the following description taken in conjunction with the drawings, wherein:

(2) FIG. 1 is a diagram presenting the CO.sub.2 concentration profile in a membrane contactor.

(3) FIG. 2 is a simplified flow diagram of a system for the capture CO.sub.2 from a CO.sub.2-containing gas stream in accordance with one embodiment of the invention.

(4) FIG. 3 is a schematic diagram of membrane absorption of CO.sub.2 in accordance with one embodiment of the invention.

(5) FIG. 4 is a graphical presentation of CO.sub.2 removal rate, flue gas flow rate, and CO.sub.2 inlet and outlet concentrations as functions of duration for a membrane absorption testing using a 50 wt. % aMDEA solvent.

(6) FIG. 5 is a graphical presentation of solvent viscosity as a function of temperature results obtained in testing herein described.

(7) FIG. 6 is a graphical presentation of the run chronology for a single 8-inch diameter module at NCCC for testing herein described.

(8) FIG. 7 is a graphical presentation of solvent saturation loading as a function of temperature for testing herein described.

DETAILED DESCRIPTION

(9) FIG. 2 illustrates a system, generally designated by the reference numeral 10, for the capture of CO.sub.2 from a CO.sub.2-containing gas stream in accordance with one embodiment of the invention.

(10) As shown, a CO.sub.2-containing gas stream 12 is passed to or introduced into an absorber such as a membrane contactor absorber 14. While the broader practice of the invention is not necessarily limited by or to the source of the CO.sub.2-containing gas stream, the invention is believed to have particular applicability to or in practice with CO.sub.2-containing flue gas streams such as produced or resulting in or from power plant operation such as natural gas-fired power plant operation or coal-fired power plant operation and in particular coal-fired power plant operation based on or utilizing pulverized coal.

(11) A CO.sub.2 selective solvent, such as in or as a stream 16, is also passed to or introduced into the membrane contactor absorber 14 such as containing or including a membrane element 18 such as more specifically illustrated and described below making reference to FIG. 3.

(12) While the broader practice of the invention is not necessarily limited by or to operation with specific solvent materials, useful solvents for use in the practice of the invention include amines and promoted carbonate solvents, such as known in the art.

(13) In the membrane contactor absorber, gas is on one side of the membrane element and liquid is on the opposed or other side of the membrane element. If desired or required, a super-hydrophobicity coating layer can be suitably applied on or to the surface of the membrane to ensure that the membrane does not wet out upon or when in contact with the liquid. For flue gas CO.sub.2 separation and capture, CO.sub.2 permeates through the membrane and reacts with the solvent, whereas N.sub.2 does not react and has low solubility in the solvent. Thus, high CO.sub.2/N.sub.2 selectivity can be achieved. Solvent liquid typically passes on the membrane side in direct contact with the super-hydrophobicity layer and the CO.sub.2-containing gas passes on the other side of the membrane. In other words, if the super-hydrophobicity coating layer is on the inside surface of a tubular membrane, then the solvent liquid would desirably be passed on the bore side of the membrane and the CO.sub.2-containing gas would desirably be passed on the shell side of the membrane; if the super-hydrophobicity coating layer is on the outside surface of a tubular membrane, then the solvent liquid would desirably be passed on the shell side of the membrane and the CO.sub.2-containing gas would desirably be passed on or through the bore side of the membrane.

(14) In such operation, the gas velocity in the fiber is desirably in a range of 1-30 m/s and the liquid velocity in the module unit is desirably in a range of 0.001-0.010 m/s.

(15) As further detailed below, CO.sub.2 permeates through the membrane element and is chemically absorbed into the CO.sub.2 selective solvent to form a CO.sub.2-rich solvent. A CO.sub.2-rich solvent stream 20 is passed from the membrane contactor absorber 14 and, such as after appropriate heat exchange processing in or via a heat exchanger 22, forming a stream 23, is forwarded to a desorber 24 for desorption of CO.sub.2 and hence regeneration of the CO.sub.2 selective solvent. For example, a desorbent such as steam can be introduced in or to the desorber 24 such as via a stream 26. The desorbed CO.sub.2 is shown as leaving the desorber 24 as a stream 30 for subsequent use or sequestration, as may be desired. The regenerated CO.sub.2 selective solvent is shown as leaving the desorber 24 as a stream 32.

(16) The regenerated CO.sub.2 selective solvent stream 32 is passed to the heat exchanger 22 for heat transfer communication with the CO.sub.2-rich solvent stream 20 passed from the membrane contactor absorber 14 and resulting in a regenerated CO.sub.2 selective solvent stream 34. The regenerated CO.sub.2 selective solvent stream 34 can be passed to a CO.sub.2 selective solvent storage tank 36. CO.sub.2 selective solvent can be passed from the CO.sub.2 selective solvent storage tank 36 and introduced into the membrane contactor absorber 14 such as in the form of the stream 16 via a solvent pump 42.

(17) The CO.sub.2-depleted gases exiting from the membrane contactor absorber 14, shown as the stream 44, can be appropriately processed as may desired such as by being passed to a stack.

(18) As shown in more specific detail in FIG. 3, CO.sub.2 capture from a gas, e.g., a flue gas, can effectively and efficiently be accomplished in accordance with one embodiment of the invention by passing a CO.sub.2-containing flue gas to and through one side of a membrane, e.g., the bore side of the membrane, while a chosen CO.sub.2 selective solvent, e.g., an absorption liquid, flows on the other side of the membrane, e.g., the permeate side of the membrane. The CO.sub.2 permeates through the hollow fiber membrane pores and is chemically absorbed into the solvent. The CO.sub.2-rich solvent can be appropriately regenerated and returned or recycled to the membrane contactor absorber.

(19) CO.sub.2 selective solvents for use in the practice of the invention desirably have a viscosity of at least about 0.2 cP and up to about 7 cP, e.g., in an inclusive range of between 0.2 and 7 cP. In accordance with certain preferred embodiments, CO.sub.2 selective solvents for use in the practice of the invention desirably have a viscosity of at least about 1 cP and up to about 5 cP, e.g., in a range of between 1 and 5 cP.

(20) We have found that liquid side concentration polarization occurs in membrane absorption. Concentration polarization refers to the unexpected high CO.sub.2 concentration at the membrane-liquid interface, and thus gradually decreases the mass transfer rate of CO.sub.2. FIG. 4 shows a typical solvent-side concentration polarization that we have observed in our testing.

(21) In the subject testing, CO.sub.2 capture was performed during the daytime hours (about 8 hours) and CO.sub.2 capture rate was monitored. At the end of each day, the system was shut down by 1) shutting off the liquid pump, 2) shutting off the flue gas, and 3) draining out the shell side solvent. This on-off cycle was performed for a couple of days. FIG. 4 shows that for the first data point collected of a day was generally better than the last data point collected on the pervious day. Further, during the operation of each single day, the CO.sub.2 removal rate decreased with operation time when the flue gas flow rates were kept constant (as an example, please see data collected on Day 5). This is typically a liquid-side concentration polarization issue, where there is a higher CO.sub.2 concentration (relative to the bulk flow stream) in the fluid boundary layer of the liquid side next to the membrane.

(22) Concentration polarization strongly affects the performance of the separation process. First, concentration changes in the solution reduce the driving force within the membrane, hence, the useful flux/rate of separation. Secondly, concentration polarization increases probability of fouling issue, leading to a poor stability of the separation/capture system. Thus, the selectivity of separation and the membrane lifetime are deteriorated.

(23) Generally, to reduce the concentration polarization, flow rate of the solvent should be increased to promote the turbulence. This approach results in better mixing of the solution and in reducing the thickness of the diffusion boundary layer. However, an increase in solvent flow rate increases the L/G ratio of the operation. As a result, the pumping and regeneration costs for the liquid solvent would increase.

(24) Periodically removing the solvent contacted with the membrane surface and replacing with bulk solvent can be another approach to reduce the concentration polarization. However, this approach affects the continuity of the membrane operation.

(25) In the current invention, the viscosity of the solvent is desirably adjusted or otherwise controlled to minimize concentration polarization, and unexpectedly good stability performance was achieved.

(26) In a portion of our testing, solvent amine concentration was decreased from 50 wt. % to 35 wt. % with the remainder being water. As shown in FIG. 5, the solvent viscosity significantly deceased as the amine concentration decreased from 50 wt. % to 30 wt. %.

(27) Continuous testing was performed using the 35 wt. % solvent (i.e., with lower viscosity). During such operation, the CO.sub.2 concentration of the flue gas was between 11.7 vol. %-12.7 vol. % (dry basis). The CO.sub.2 capture rate increased with decreasing flue gas flow rate as shown in FIG. 6 for the first 83 hours. After 83 hours, the operation conditions including flue gas flow rate were kept fairly constant. Stable performance was achieved. This test confirmed the liquid side concentration polarization issue had been resolved when using low-viscosity solvent.

(28) FIG. 7 shows solvent saturation loading decreases with decreasing amine concentration. For treating same amount of CO.sub.2 using membrane contactor absorber processing, although the CO.sub.2 loading in weight percent of the rich solvent remained unchanged as amine concentration decreases, the CO.sub.2 loading in terms of mole CO.sub.2 absorbed/mole amine of the rich solvent increases. In other words, the amine strength decreased by about 20% (35 wt. % vs. 50 wt. %), but the concentration polarization was successfully resolved.

(29) While the invention has been described above making specific reference to the capture of CO.sub.2 from a flue gas such as generated, produced or otherwise resulting from the operation of a power plant, for example, those skilled in the art and guided by the teachings herein provided will understand and appreciate that the broader application and/or practice of the invention is not necessarily so limited. That is, while the invention may, for example, be applied or used for cost effective capture of CO.sub.2 from flue gases, the invention can also have applicability for the removal of other constituents from other materials or streams. For example, the invention can be suitably applied in alternative embodiments for the removal of numerous other gas pollutants such as NO.sub.x and SO.sub.x, for the separation of CO.sub.2 from hydrogen in refinery streams, and for the separation of CO.sub.2 from natural gas (natural gas sweetening). Further, the invention technology can be further utilized in, with or as a generic membrane contactor absorber for the removal of dissolved gases (e.g., O.sub.2, H.sub.2, etc.) from liquids or for the controlled dissolution of gases (e.g., O.sub.2, H.sub.2, etc.) in liquids.

(30) While in the foregoing detailed description this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purposes of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.

(31) Moreover, those skilled in the art and guided by the teachings herein identified, described or discussed will understand and appreciate that the subject development encompasses a variety of features and is thus capable of manifestation in a variety of specific forms or embodiments and is thus not to be construed as limited to the specific forms or embodiments herein identified or described.

(32) The claims are not intended to include, and should not be interpreted to include, means-plus- or step-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase(s) “means for” or “step for,” respectively.

(33) Further, although specific advantages have been enumerated above, various embodiments may include some, none, or all of the enumerated advantages.

(34) The invention illustratively disclosed herein suitably may be practiced in the absence of any element, part, step, component, or ingredient which is not specifically disclosed herein.