Membrane Gas Separation Principles

Abstract

Some industrial processes require the separation of gas or vapor mixtures. Methods for separating the mixtures vary from separation by diffusion to separation by distillation. Many of the methods, such as distillation, are energy intensive. Membranes can reduce the energy required to produce a desired separation. Because of their corrosion resistance and high temperature applications, engineered inorganic membranes can significantly increase the efficiency of many of these processes. The magnitude of the separation factor, available operating conditions, enrichment, yield, and cost of the membranes play a large role in determining whether membranes can be more economical than other methods of separation. These factors have to be evaluated on a case-by-case basis.Martin Marietta Energy Systems' Office of Technology Transfer conducted a preliminary market survey with the assistance of the University of Tennessee and commercial marketing experts in inorganic membranes. The survey assumed that membranes could be made with permeabilities a factor of 3 larger and with cost per unit area a factor of 3 smaller than is currently available. The results indicated that active implementation of such technology could expect to achieve the following results:• $2 billion dollar per year sales market,• $16.6 billion increase in the national GDP,• $2 billion improvement in the balance of trade, and• 6 quads per year decrease in energy use.

Advanced Membrane Separation Technologies For Energy Recovery From Industrial Process Streams

“…Heat flux due to convection was modeled using an enhanced value of 20 W/m 2 K for the heat transfer coefficient, to reflect the contact with flue gas containing water vapor. Heat input due to condensation was computed based on Nusselt's theory, which provides a correlation for the heat flux based on the temperature difference at the tube surface: Q cond = f * (t gas -t surface ) 0.75 (6) The factor f in the above equation depends on the properties of the vapor and the condensate. Effect of changing the thermal conductivity of the tube material was examined using the heat transfer model of the single tube.…”

“…Non-porous membranes usually display high separation ratios however their transport fluxes are relatively low [4,5]. Porous membranes typically depend on some combination of molecular sieving, diffusivity, and surface effects to manage the segregation of gaseous species [6]. Porous membranes, depending on pore size can achieve higher transport fluxes than nonporous membranes but the separation ratio is usually much lower [4,7,8].…”

Advanced Energy and Water Recovery Technology from Low Grade Waste Heat

“…Non-porous membranes usually display high separation ratios however their transport fluxes are relatively low [4,5]. Porous membranes typically depend on some combination of molecular sieving, diffusivity, and surface effects to manage the segregation of gaseous species [6]. Porous membranes, depending on pore size can achieve higher transport fluxes than nonporous membranes but the separation ratio is usually much lower [4,7,8].…”

Membranes for solubility-based gas separation applications

“…Separation selectivities with this mechanism are proportional to the ratio of the inverse square root of the molecular weights. This mechanism is often predominant in macroporous and mesoporous membranes [4][5][6][7]. (c) Surface diffusion: Surface diffusion occurs when the permeating species exhibit a strong affinity for the membrane surface and adsorb along the pore walls.…”

“…In this mechanism, separation occurs due to differences in the amount of adsorption of the permeating species. Surface diffusion often occurs in parallel with other transport mechanisms such as Knudsen diffusion [5,6]. (d) Capillary condensation: Capillary condensation is one form of surface flow where one of the gases is a condensable gas.…”