Membrane gas separation

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.

Basic process

Gas separation across a membrane is a pressure-driven process, where the driving force is the difference in pressure between inlet of raw material and outlet of product. The membrane used in the process is a generally non-porous layer, so there will not be a severe leakage of gas through the membrane. The performance of the membrane depends on permeability and selectivity. Permeability is affected by the penetrant size. Larger gas molecules have a lower diffusion coefficient. The polymer chain flexibility and free volume in the polymer of the membrane material influence the diffusion coefficient, as the space within the permeable membrane must be large enough for the gas molecules to diffuse across. The solubility is expressed as the ratio of the concentration of the gas in the polymer to the pressure of the gas in contact with it. Permeability is the ability of the membrane to allow the permeating gas to diffuse through the material of the membrane as a consequence of the pressure difference over the membrane, and can be measured in terms of the permeate flow rate, membrane thickness and area and the pressure difference across the membrane. The selectivity of a membrane is a measure of the ratio of permeability of the relevant gases for the membrane. It can be calculated as the ratio of permeability of two gases in binary separation. The second (c), molecular sieving, is the case where the pores of the membrane are too small to let one component pass, a process which is typically not practical in gas applications, as the molecules are too small to design relevant pores. In these cases the movement of molecules is best described by pressure-driven convective flow through capillaries, which is quantified by Darcy's law. However, the more general model in gas applications is the solution-diffusion (d) where particles are first dissolved onto the membrane and then diffuse through it both at different rates. This model is employed when the pores in the polymer membrane appear and disappear faster relative to the movement of the particles.

In a typical membrane system the incoming feed stream is separated into two components: permeant and retentate. Permeant is the gas that travels across the membrane and the retentate is what is left of the feed. On both sides of the membrane, a gradient of chemical potential is maintained by a pressure difference which is the driving force for the gas molecules to pass through. The ease of transport of each species is quantified by the permeability, P_i. With the assumptions of ideal mixing on both sides of the membrane, ideal gas law, constant diffusion coefficient and Henry's law, the flux of a species can be related to the pressure difference by Fick's law:

The solution to the above quadratic expression can be expressed as:

:<math> n_i = \frac{-(\phi+\phi(\alpha-1)n_i'+\alpha-1)\pm \sqrt{\phi+\phi(\alpha-1)n_i'+\alpha-1)^2+4(1-\alpha)\alpha\phi n_i'{2(1-\alpha)} </math>

Finally, an expression for the permeant concentration is obtained by the following:

:<math> n_i(\phi \alpha n_i')=\frac{\phi}{2}\left(n_i'+\frac{1}{\phi}+\frac{1}{\alpha-1}-\sqrt{\left(n_i'+\frac{1}{\phi}+\frac{1}{\alpha-1}\right)^2-\frac{4\alpha n_i'}{(\alpha-1)\phi \right)</math>

Along the separation unit, the feed concentration decays with the diffusion across the membrane causing the concentration at the membrane to drop accordingly. As a result, the total permeant flow (q"_out) results from the integration of the diffusion flow across the membrane from the feed inlet (q'_in) to feed outlet (q'_out). A mass balance across a differential length of the separation unit is therefore:

:<math> q'(x)=q'(x+dx)+\int_{x}^{x+dx} q(x)dx</math>

where:

:<math>q(x)=J_i(x)+J_j(x)</math>

Because of the binary nature of the mixture, only one species needs to be evaluated. Prescribing a function n'_i=n'_i(x), the species balance can be rewritten as:

:<math> q'(x)n'_i(x)=q'(x+\Delta x)n'_i(x+\Delta x) +\int_{x}^{x+dx}q(x)dx \bar{n_i} </math>

Where:

:<math> \int_{x}^{x+dx} q(x)dx= \delta q, \quad \bar{n_i}=\frac{n_i(x)+n_i(x+\Delta x)}{2} </math>

:<math> \delta q= \frac{n'_i(x)-n'_i(x+\Delta x)}{\bar{n_i}-n'_i(x+\Delta x)} q'(x) </math>

Lastly, the area required per unit membrane length can be obtained by the following expression:

:<math> A=\frac{\delta q}{J_i+J_j}</math>

Membrane materials for carbon capture in flue gas streams

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.

Polymer membranes

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

thumb|upright=2|right| Microscopic model of a nanoporous membrane. The white open area represents the area the molecule can pass through and the dark blue areas represent the membrane walls. The membrane channels consists of cavities and windows. The energy of the molecules in the cavity is U_c and the energy of a particle in the window is U_w.

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 (L_cy x L_cz) and window region (L_wy x L_wz) 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.

Metal-organic framework (MOF) membranes

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.

thumb|Structure of a perovskite. A membrane would consist of a thin layer of the perovskite structure.

Perovskite membranes

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&nbsp;°C to 500&nbsp;°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.

Other membrane technologies

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.

Construction

Membranes are typically contained in one of three modules:

• The separation of nitrogen or oxygen from air (generally only up to 99.5%)
• Separation of hydrogen from gases like nitrogen and methane
• Recovery of hydrogen from product streams of ammonia plants
• Recovery of hydrogen in oil refinery processes
• Separation of methane from the other components of biogas
• Enrichment of air by oxygen for medical or metallurgical purposes. One of the methods used for commercial production of nitrox breathing gas for underwater diving.
• Enrichment of ullage by nitrogen in inerting systems designed to prevent fuel tank explosions
• Removal of water vapor from natural gas and other gases
• Removal of SO₂, CO₂ and H₂S from natural gas (polyamide membranes)
• Removal of volatile organic liquids (VOL) from air of exhaust streams

Air separation

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.

Current status of CO₂ capture with membranes

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.