Membrane absorption process for CO2 capture

Processes and systems for the capture of CO2 from a CO2-containing gas stream are provided. The CO2-containing gas stream is passed to a membrane contactor absorber wherein the CO2-containing gas contacts or passes a first side of a membrane element while a CO2 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 CO2 permeates through the hollow fiber membrane pores and is chemically absorbed into the solvent.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates generally to the separation and/or capture of CO2such as resulting from power plant operation from and, more particularly, to such CO2separation and/or capture via membrane contactor absorption.

Description of Related Art

Existing technology, products, systems and methods previously used or currently available to address CO2produced or resulting from power plant operation and such as may be present in power plant flue gases include:

Amine absorption—Amine absorption is the current DOE and industry benchmark technology for separation and/or capture of CO2from power plant flue gases and natural gases.

DOE/NETL systems analysis studies estimate that using chemical absorption with an aqueous monoethanolamine system to capture 90% of the CO2from flue gas will require an increase in the levelized cost of energy (LCOE) services of 75-85%. These values are well above the 2020 DOE NETL Sequestration Program post-combustion capture goal of 90% capture in existing plants with less than 35% increase in LCOE. Therefore, it is important to develop new advanced CO2capture technologies in order to maintain the cost-effectiveness of U.S. coal-fired power generation.

Conventional Gas Separation Membrane Process—Membrane Technology and Research (MTR) has developed a gas separation membrane, Polaris™, which has exhibited pure-gas CO2permeance of about 1,650 GPU at 50° C. with an ideal selectivity (ratio of single gas permeances) of about 50 for CO2/N2.

Ho et al. at Ohio State University (OSU) have disclosed zeolite/polymer composite membranes containing amine cover layer for CO2capture. The scaled membranes showed selectivities of 140 for binary CO2/N2mixtures.

Hybrid Solvent/Membrane Process—University of Kentucky is developing an absorption solvent/membrane hybrid process. The absorption cycle uses aqueous ammonium and some typical alkyl amines. A T-type hydrophilic zeolite membrane is placed between the absorption and regeneration steps to reject water allowing a more concentrated solution to be sent to the regenerator.

The University of Texas at Austin is developing a hybrid piperazine solvent/Polaris™ membranes process for CO2capture. They reported that a hybrid series configuration requires a minimum of 70% removal by amine process, whereas a hybrid parallel configuration requires a minimum of 53% to 65% of the flue gas directed to the amine absorber.

Other Membrane Contactor Systems—Membrane contactor systems have been reported for CO2capture from flue gas. However, technical hurdles in long-term stability had to be overcome, as well as cost. Liquid side concentration polarization is one the issues that affect the long-term stability.

The CO2concentration profile in the membrane contactor is shown inFIG. 1. Cg, Cgm, Cml, Clmand Clare the CO2concentration in the gas phase, at the gas-membrane interface, at the membrane-liquid interface, at the liquid-membrane interface and in the liquid, respectively.

The CO2mass transfer coefficient for a gas-liquid membrane absorption process can be expressed as follows:

1K=1kg+1km+HadimE·kl(1)
where K is the overall mass transfer coefficient [cm/s], kgthe mass transfer coefficient in the gas phase [cm/s], kmthe mass transfer coefficient in the membrane [cm/s], klthe mass transfer coefficient in the liquid phase [cm/s], Hadimthe non-dimensional Henry's constant, and E the enhancement factor due to chemical reaction. The overall resistance to CO2transport and the overall mass transfer coefficient have an inverse relationship. Equation 1 shows the overall resistance comes from three parts: in the gas phase, in the membrane, and in the liquid phase.

SUMMARY OF THE INVENTION

One aspect of the subject invention regards a membrane absorption process for CO2capture. In accordance with one embodiment, a process for CO2capture involves passing a CO2-containing gas stream to a membrane contactor absorber wherein the CO2-containing gas contacts or passes a first side of a membrane element while a CO2selective 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 CO2permeates through the hollow fiber membrane pores and is chemically absorbed into the solvent. The CO2-rich solvent can be desirably regenerated and returned or recycled to the membrane contactor absorber.

Another aspect of the invention regards a method for improving separation of CO2from a CO2-containing gas stream via membrane processing is provided. A method for improving separation of CO2from a CO2-containing gas stream via membrane processing in accordance with one embodiment of the invention wherein the CO2-containing gas stream contacts a first side of a membrane element and a CO2selective 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 and wherein CO2permeates through the membrane element and is chemically absorbed into the CO2selective solvent, the improvement involves adjusting the CO2selective solvent to have a viscosity in a range between 0.2 and 7 cP.

Another aspect of the invention regards a system for capturing CO2from a CO2-containing gas stream. In accordance with one embodiment, such a system includes a membrane contactor absorber as well as a desorber. The membrane contactor absorber contains a membrane element. The CO2-containing gas stream contacts a first side of the membrane element and a CO2selective solvent is in flow communication with a second side of the membrane element. The second side of the membrane element is opposite or opposed to the first side. The CO2selective solvent desirably has a viscosity between 0.2 and 7 cP. The CO2permeates through the membrane element and is chemically absorbed into the CO2selective solvent to form a CO2-rich solvent. The desorber accepts the CO2-rich solvent from the membrane contactor absorber to remove CO2from the CO2-rich solvent to form a regenerated solvent. The system further includes a return line to forward the regenerated solvent to the membrane contact absorber.

DETAILED DESCRIPTION

FIG. 2illustrates a system, generally designated by the reference numeral10, for the capture of CO2from a CO2-containing gas stream in accordance with one embodiment of the invention.

As shown, a CO2-containing gas stream12is passed to or introduced into an absorber such as a membrane contactor absorber14. While the broader practice of the invention is not necessarily limited by or to the source of the CO2-containing gas stream, the invention is believed to have particular applicability to or in practice with CO2-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.

A CO2selective solvent, such as in or as a stream16, is also passed to or introduced into the membrane contactor absorber14such as containing or including a membrane element18such as more specifically illustrated and described below making reference toFIG. 3.

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.

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 CO2separation and capture, CO2permeates through the membrane and reacts with the solvent, whereas N2does not react and has low solubility in the solvent. Thus, high CO2/N2selectivity can be achieved. Solvent liquid typically passes on the membrane side in direct contact with the super-hydrophobicity layer and the CO2-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 CO2-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 CO2-containing gas would desirably be passed on or through the bore side of the membrane.

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.

As further detailed below, CO2permeates through the membrane element and is chemically absorbed into the CO2selective solvent to form a CO2-rich solvent. A CO2-rich solvent stream20is passed from the membrane contactor absorber14and, such as after appropriate heat exchange processing in or via a heat exchanger22, forming a stream23, is forwarded to a desorber24for desorption of CO2and hence regeneration of the CO2selective solvent. For example, a desorbent such as steam can be introduced in or to the desorber24such as via a stream26. The desorbed CO2is shown as leaving the desorber24as a stream30for subsequent use or sequestration, as may be desired. The regenerated CO2selective solvent is shown as leaving the desorber24as a stream32.

The regenerated CO2selective solvent stream32is passed to the heat exchanger22for heat transfer communication with the CO2-rich solvent stream20passed from the membrane contactor absorber14and resulting in a regenerated CO2selective solvent stream34. The regenerated CO2selective solvent stream34can be passed to a CO2selective solvent storage tank36. CO2selective solvent can be passed from the CO2selective solvent storage tank36and introduced into the membrane contactor absorber14such as in the form of the stream16via a solvent pump42.

The CO2-depleted gases exiting from the membrane contactor absorber14, shown as the stream44, can be appropriately processed as may desired such as by being passed to a stack.

As shown in more specific detail inFIG. 3, CO2capture 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 CO2-containing flue gas to and through one side of a membrane, e.g., the bore side of the membrane, while a chosen CO2selective solvent, e.g., an absorption liquid, flows on the other side of the membrane, e.g., the permeate side of the membrane. The CO2permeates through the hollow fiber membrane pores and is chemically absorbed into the solvent. The CO2-rich solvent can be appropriately regenerated and returned or recycled to the membrane contactor absorber.

CO2selective 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, CO2selective 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.

We have found that liquid side concentration polarization occurs in membrane absorption. Concentration polarization refers to the unexpected high CO2concentration at the membrane-liquid interface, and thus gradually decreases the mass transfer rate of CO2.FIG. 4shows a typical solvent-side concentration polarization that we have observed in our testing.

In the subject testing, CO2capture was performed during the daytime hours (about 8 hours) and CO2capture 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. 4shows 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 CO2removal 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 CO2concentration (relative to the bulk flow stream) in the fluid boundary layer of the liquid side next to the membrane.

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.

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.

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.

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.

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

Continuous testing was performed using the 35 wt. % solvent (i.e., with lower viscosity). During such operation, the CO2concentration of the flue gas was between 11.7 vol. %-12.7 vol. % (dry basis). The CO2capture rate increased with decreasing flue gas flow rate as shown inFIG. 6for 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.

FIG. 7shows solvent saturation loading decreases with decreasing amine concentration. For treating same amount of CO2using membrane contactor absorber processing, although the CO2loading in weight percent of the rich solvent remained unchanged as amine concentration decreases, the CO2loading in terms of mole CO2absorbed/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.

While the invention has been described above making specific reference to the capture of CO2from 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 CO2from 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 NOxand SOx, for the separation of CO2from hydrogen in refinery streams, and for the separation of CO2from 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., O2, H2, etc.) from liquids or for the controlled dissolution of gases (e.g., O2, H2, etc.) in liquids.

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.

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