Abstract

The permeances of several gaseous species (He, H2, CO2, N2, CH4, and C2H4) were measured through silica hollow fiber membranes over a temperature range of 298 to 473 K at a feed pressure of 20 atm. Permeances ranged from 10 to 2.3 * 105 Barrer/cm at 298 K and were inversely proportional to the kinetic diameter of the penetrant. The permeance of gaseous species were found to decrease with decreasing differential pressure driving force at ?P<5 bar. No pressure dependency was found at ?P>5 bar. Mass transfer through the silica hollow fiber membrane was found to be an activated process. Activation energies for diffusion through the silica membrane calculated from the slopes of Arrhenius plots of the permeation data ranged from 4.61 to 14.0 kcal/mole and correlate well with the kinetic diameter of the penetrants. Large separation factors were obtained for the penetrants. The separation factors decreased with increasing temperature. The CH4/CO2 mixed separation factors were higher than the values calculated from pure gases at temperatures below 373 K. This behavior was observed after the membrane had been heated to at least 398 K and then cooled in an inert gas flow. The differences between the mixture and ideal separation factors is attributed to a competitive adsorption effect in which the more strongly interacting gases saturate the surface and block the transport of the weakly interacting gases.

Introduction

Membrane separations are important in chemical and related industries for product purification and the removal of toxins prior to the release of waste streams into the environment. A membrane is a barrier between two phases, and can be used to separate gas and liquid mixtures if one component of the mixture moves through the membrane faster than the others. Membrane processes have several advantages over other conventional separation technologies each as distillation, absorption, and extraction [1] including:

  • low capital investment

  • ease of operation

  • low energy consumption

  • cost effectiveness even at low gas volumes

  • good weight and space efficiency

In addition to applications such as macromolecule separation and water desalination, membrane technology has been applied in industrial gas separations such as removal of sulfur and nitrogen oxides from combustion gases, removal of acid gases from natural gas, production of nitrogen from air, and hydrogen recovery in petrochemical production.

There is an increasing interest to develop membrane technology with the objective of not only getting high permeability and separation factor when using gas separations but also to be able to perform at severe conditions (high temperature, and pressure). Materials such as silica, ceramics, and carbon are potential membrane materials for gas separation because these materials can perform at high temperatures and in aggressive chemical environments [2], and they also can exhibit high separation factors.

One of the potential membranes is hollow fiber microporous glass membrane produced by PPG Industries [3]. The objective of this study is the investigation of the gas transport mechanism(s) through the microporous silica hollow fiber membrane.

Transport Mechanisms in Microporous Membrane

Transport of gases through microporous materials consists of four different mechanisms as shown in Figure 1 [4]. These mechanisms are Knudsen diffusion, capillary condensation, surface diffusion, and activated diffusion which is also referred to as molecular sieving [5,6].

Transport Mechanisms in Microporous Membrane

Transport of gases through microporous materials consists of four different mechanisms as shown in Figure 1 [4]. These mechanisms are Knudsen diffusion, capillary condensation, surface diffusion, and activated diffusion which is also referred to as molecular sieving [5,6].

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