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.

[1] While polymeric membranes are economical and technologically useful, they are bounded by their performance, known as the Robeson limit (permeability must be sacrificed for selectivity and vice versa).

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.

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 first (b), Knudsen diffusion holds at very low pressures where lighter molecules can move across a membrane faster than heavy ones, in a material with reasonably large pores.

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.

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.

A mass balance across a differential length of the separation unit is therefore: where: Because of the binary nature of the mixture, only one species needs to be evaluated.

Synthetic membranes are made from a variety of polymers including polyethylene, polyamides, polyimides, cellulose acetate, polysulphone and polydimethylsiloxane.

[7] 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.

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.

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.

In the analysis, both the diffusion and Henry coefficients can be modified without affecting the permeability of the material which thus can exceed the upper limit for polymer membranes.

[12] Ordered mesoporous silica membranes have shown considerable potential for surface modification that allows for ease of CO2 separation.

The capacity to discriminate based on both molecular size and adsorption affinity makes zeolite membranes an attractive candidate for CO2 separation from N2, CH4, and H2.

At low temperatures, zeolite adsorption-capacity is large and the high concentration of adsorbed CO2 molecules blocks the flow of other gases.

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.

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.

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.

[25] 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.

In special cases other materials can be utilized; for example, palladium membranes permit transport solely of hydrogen.

Cryogenic distillation is the mature technology for commercial air separation for the production of large quantities of high purity oxygen and nitrogen.

[28][3] 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[when?]

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.

[4][10][29] Today, membranes are used for commercial separations involving: N2 from air, H2 from ammonia in the Haber-Bosch process, natural gas purification, and tertiary-level enhanced oil recovery supply.

Single-stage membranes devices are not feasible for obtaining a high concentration of separated material in the permeate stream.

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.

[10][35] 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.

Membrane cartridge
(a) Bulk flow through pores; (b) Knudsen diffusion through pores; (c) molecular sieving; (d) solution diffusion through dense membranes.
A simplified design schematic of a membrane separation process
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 .
A typical zeolite. Thin layers of this crystalline zeolite structure can act as a membrane, since CO 2 can adsorb inside of the pores.
Structure of a perovskite. A membrane would consist of a thin layer of the perovskite structure.