Газ қоспаларын бөлуге арналған жаңа мембраналық технологиялар

Technology for splitting specific gases out of mixtures
Gas mixtures can be effectively separated by synthetic membranes made from polymers such as polyamide or cellulose acetate, or from ceramic materials. This limit affects polymeric membrane use for CO2 separation from flue gas streams, since mass transport becomes limiting and CO2 separation becomes very expensive due to low permeabilities. Membrane materials have expanded into the realm of silica, zeolites, metal organic frameworks, and perovskites due to their strong thermal and chemical resistance as well as high tunability (ability to be modified and functionalized), leading to increased permeability and selectivity. Membranes can be used for separating gas mixtures where they act as a permeable barrier through which different compounds move across at different rates or not move at all. The membranes can be nanoporous, polymer, etc. and the gas molecules penetrate according to their size, diffusivity, or solubility.

The material of the membrane plays an important role in its ability to provide the desired performance characteristics. It is optimal to have a membrane with a high permeability and sufficient selectivity and it is also important to match the membrane properties to that of the system operating conditions (for example pressures and gas composition). Synthetic membranes are made from a variety of polymers including polyethylene, polyamides, polyimides, cellulose acetate, polysulphone and polydimethylsiloxane.

Polymeric membranes are a common option for use in the capture of CO2 from flue gas because of the maturity of the technology in a variety of industries, namely petrochemicals. The ideal polymer membrane has both a high selectivity and permeability. Polymer membranes are examples of systems that are dominated by the solution diffusion mechanism. The membrane is considered to have holes which the gas can dissolve (solubility) and the molecules can move from one cavity to the other (diffusion). There are two methods that researchers are using to break the Robeson limit, one of these is the use of glassy polymers whose phase transition and changes in mechanical properties make it appear that the material is absorbing molecules and thus surpasses the upper limit. The second method of pushing the boundaries of the Robeson limit is by the facilitated transport method. As previously stated, the solubility of polymers is typically fairly constant but the facilitated transport method uses a chemical reaction to enhance the permeability of one component without changing the selectivity.

Nanoporous membranes are fundamentally different from polymer based membranes in that their chemistry is different and that they do not follow the Robeson limit for a variety of reasons. The simplified figure of a nanoporous membrane shows a small portion of an example membrane structure with cavities and windows. The white portion represents the area where the molecule can move and the blue shaded areas represent the walls of the structure. In the engineering of these membranes, the size of the cavity (Lcy x Lcz) and window region (Lwy x Lwz) can be modified so that the desired permeation is achieved. It has been shown that the permeability of a membrane is the production of adsorption and diffusion. In low loading conditions, the adsorption can be computed by the Henry coefficient. While previously, silica membranes were impractical due to their technical scalability and cost (they are very difficult to produce in an economical manner on a large scale), there have been demonstrations of a simple method of producing silica membranes on hollow polymeric supports. These demonstrations indicate that economical materials and methods can effectively separate CO2 and N2. Ordered mesoporous silica membranes have shown considerable potential for surface modification that allows for ease of CO2 separation. Surface functionalization with amines leads to the reversible formation of carbamates (during CO2 flow), increasing CO2 selectivity significantly. It is generally accepted that CO2 has the largest adsorption energy because it has the largest quadrupole moment, thereby increasing its affinity for charged or polar zeolite pores. At low temperatures, zeolite adsorption capacity is large and the high concentration of adsorbed CO2 molecules blocks the flow of other gases. Therefore, at lower temperatures, CO2 selectively permeates through zeolite pores. Several recent research efforts have focused on developing new zeolite membranes that maximize the CO2 selectivity by taking advantage of the low temperature blocking phenomena. Researchers have synthesized Y type (Si:Al>3) zeolite membranes which achieve room temperature separation factors of 100 and 21 for CO2/N2 and CO2/CH4 mixtures respectively. DDR type and SAPO 34 membranes have also shown promise in separating CO2 and CH4 at a variety of pressures and feed compositions. The SAPO 34 membranes, being nitrogen selective, are also strong contender for natural gas sweetening process. Researchers have also made an effort to utilize zeolite membranes for the separation of H2 from hydrocarbons. Hydrogen can be separated from larger hydrocarbons such as C4H10 with high selectivity. This is due to the molecular sieving effect since zeolites have pores much larger than H2, but smaller than these large hydrocarbons. Smaller hydrocarbons such as CH4, C2H6, and C3H8 are small enough to not be separated by molecular sieving. Researchers achieved a higher selectivity of hydrogen when performing the separation at high temperatures, likely as a result of a decrease in the competitive adsorption effect.

There have been advances in zeolitic imidazolate frameworks (ZIFs), a subclass of metal organic frameworks (MOFs), that have allowed them to be useful for carbon dioxide separation from flue gas streams. Extensive modeling has been performed to demonstrate the value of using MOFs as membranes. MOF materials are adsorption based, and thus can be tuned to achieve selectivity. The drawback to MOF systems is stability in water and other compounds present in flue gas streams. Select materials, such as ZIF 8, have demonstrated stability in water and benzene, contents often present in flue gas mixtures. ZIF 8 can be synthesized as a membrane on a porous alumina support and has proven to be effective at separating CO2 from flue gas streams. At similar CO2/CH4 selectivity to Y type zeolite membranes, ZIF 8 membranes achieve unprecedented CO2 permeance, two orders of magnitude above the previous standard.

Perovskite are mixed metal oxide with a well defined cubic structure and a general formula of ABO3, where A is an alkaline earth or lanthanide element and B is a transition metal. These materials are attractive for CO2 separation because of the tunability of the metal sites as well as their stabilities at elevated temperatures. The separation of CO2 from N2 was investigated with an α alumina membrane impregnated with BaTiO3. It was found that adsorption of CO2 was favorable at high temperatures due to an endothermic interaction between CO2 and the material, promoting mobile CO2 that enhanced CO2 adsorption desorption rate and surface diffusion. The experimental separation factor of CO2 to N2 was found to be 1.1 1.2 at 100 °C to 500 °C, which is higher than the separation factor limit of 0.8 predicted by Knudsen diffusion. Though the separation factor was low due to pinholes observed in the membrane, this demonstrates the potential of perovskite materials in their selective surface chemistry for CO2 separation.

In special cases other materials can be utilized; for example, palladium membranes permit transport solely of hydrogen. In addition to palladium membranes (which are typically palladium silver alloys to stop embrittlement of the alloy at lower temperature) there is also a significant research effort looking into finding non precious metal alternatives. Although slow kinetics of exchange on the surface of the membrane and tendency for the membranes to crack or disintegrate after a number of duty cycles or during cooling are problems yet to be fully solved.

Oxygen enriched air is in high demanded for a range of medical and industrial applications including chemical and combustion processes. Cryogenic distillation is the mature technology for commercial air separation for the production of large quantities of high purity oxygen and nitrogen. However, it is a complex process, is energy intensive, and is generally not suitable for small scale production. Pressure swing adsorption is also commonly used for air separation and can also produce high purity oxygen at medium production rates, but it still requires considerable space, high investment and high energy consumption. The membrane gas separation method is a relatively low environmental impact and sustainable process providing continuous production, simple operation, lower pressure/temperature requirements, and compact space requirements.

A great deal of research has been undertaken to utilize membranes instead of absorption or adsorption for carbon capture from flue gas streams, however, no current projects exist that utilize membranes. Process engineering along with new developments in materials have shown that membranes have the greatest potential for low energy penalty and cost compared to competing technologies. Single stage membrane operations involve a single membrane with one selectivity value. Single stage membranes were first used in natural gas purification, separating CO2 from methane. To combat the loss of product in the membrane permeate, engineers use “cascade processes” in which the permeate is recompressed and interfaced with additional, higher selectivity membranes. Typically, membranes are integrated into already existing processes such that they can be retrofitted into already existing carbon capture systems. MTR, Membrane Technology and Research Inc., and UT Austin have worked to create hybrid processes, utilizing both absorption and membranes, for CO2 capture. First, an absorption column using piperazine as a solvent absorbs about half the carbon dioxide in the flue gas, then the use of a membrane results in 90% capture. A parallel setup is also, with the membrane and absorption processes occurring simultaneously. Generally, these processes are most effective when the highest content of carbon dioxide enters the amine absorption column. Incorporating hybrid design processes allows for retrofitting into fossil fuel power plants. For example, hydrogen and carbon dioxide can be separated, first using cryogenic gas separation, whereby most of the carbon dioxide exits first, then using a membrane process to separate the remaining carbon dioxide, after which it is recycled for further attempts at cryogenic separation. Recent studies have demonstrated that multi stage CO2 capture/separation processes using membranes can be economically competitive with older and more common technologies such as amine based absorption. For carbon capture at an average 600 MW coal fired power plant, the cost of CO2 capture using amine based absorption is in the $40–100 per ton of CO2 range, while the cost of CO2 capture using current membrane technology (including current process design schemes) is about $23 per ton of CO2. Additionally, running an amine based absorption process at an average 600 MW coal fired power plant consumes about 30% of the energy generated by the power plant, while running a membrane process requires about 16% of the energy generated. CO2 transport (e. g. to geologic sequestration sites, or to be used for EOR) costs about $2–5 per ton of CO2. This cost is the same for all types of CO2 capture/separation processes such as membrane separation and absorption. In terms of dollars per ton of captured CO2, the least expensive membrane processes being studied at this time are multi step counter current flow/sweep processes.