Introduction

In 1996 the worldwide industrial gas market was in excess of $29 billion (US). It continues to grow at an average rate of 4-5% per annum. Industrial gases account for some of the largest production volume chemicals (1998 US): nitrogen (843 bcf (billion cubic feet)), oxygen (698 bcf) and ammonia (19 700 million tons). Oxygen and nitrogen are separated from purified air. Ammonia is produced by the reaction of nitrogen and hydrogen. Certainly, the vast majority of industrial gases are purified using cryogenic distillation or adsorption technology. However, in the last 20 years there has been a growing interest in and an intense effort on the part of major gas producers to evaluate and develop membrane technology to produce or purify gases. By 1999 sales of gas separation membrane technology exceeded $100 million per year. This article will describe basic concepts along with various practical aspects of polymeric gas separation membranes including permeability measurement, membrane formation, module fabrication and applications.

A polymeric membrane is defined as a thin, semipermeable barrier between two gaseous phases. Gases will permeate the membrane if a difference in their chemical potential exists between the two gaseous phases. The chemical potential difference is most often a result of pressure differences across the membrane. Thus, gases will solubilize into the membrane at the high pressure interface, diffuse across the membrane in a concentration gradient to the low pressure interface and evolve into the low pressure gas phase (Figure 1). If a mixture of gases comprised of components i and j is brought into contact with the membrane, the permeate stream will be enriched in the more permeable gas i, leaving the retentate enriched in gas j.

The realization that gases permeate through polymers is not new. Every child knows that a balloon filled with air or helium deflates over time. Indeed, this phenomenon was observed by Mitchell in 1831. Balloons made of natural rubber filled at different rates depending on the gaseous atmosphere they were placed into. Carbon dioxide filled the balloon fastest, air slowest. Thirty-five years later Graham expanded on Mitchell's experiments and quantitatively measured the permeation rates of gases through natural rubber. He found that the permeation rate was not related to the known gaseous diffusion coefficients and so concluded that permeation does not proceed through microscopic pores in the rubber but must occur within the rubber itself. He also demonstrated that natural rubber could be used to produce from air a permeate which was enriched in oxygen to 46%.

A mathematical description of the permeation process was proposed by Fick. The relationship between permeation rate J, gas pressure P, membrane area A and membrane thickness l, known as Fick's first law, is governed by eqn [1], where AP is the pressure difference across the membrane:

The proportionality constant, Po, is termed the permeability:

The customary unit of permeability is the barrer where:

Permeability can also be written as the product of the gas solubility times its diffusivity, the so-called solution-diffusion mechanism (eqn [4]). The per-mselectivity (a) for two gases i and j is defined as the ratio of the permeabilities:

Thickness

Figure 1 Schematic representation of membrane permeation.

Thickness

Figure 1 Schematic representation of membrane permeation.

Permeation is an activated process. The effect of temperature on permeation is given by eqn [6], where Ep is the activation energy of permeation, R is the gas constant and T is temperature:

In typically encountered cases Ep is postive and the permeability increases exponentially with temperature. Additionally, Ep is related to penetrant size and therefore selectivity usually decreases with increasing temperature. This treatment is not true when dealing with gases below their critical temperature. The reader is referred to the Further Reading section for these special cases.

The above equations give a mathematical, phe-nomenological description of gas permeation through polymers but imply nothing of the molecular-level processes giving rise to permeation. While we speak of gas-separating polymers as being dense films, on a molecular level one must consider that the membrane is not 'solid'; that is, there are molecular-size gaps between the polymer chains. These gaps arise from packing defects in the solid state and also arise from the thermal motions of the polymer chains themselves. It is through pemanent and transient gaps that gas transport is believed to occur. Solution-diffusion behaviour has proven adequate to describe permeation through rubbery polymers - those whose glass transition temperature, Tg, is below the experimental temperature. As a family, rubbery materials are highly permeable but unselective for the same molecular-level rationalization. In the rubbery state polymer chains are highly mobile, generating a high frequency of transient gaps which the penetrant gases can easily diffuse through. However, these gaps are not very selective. From a practical perspective, the purity of the product is related to the membrane permselectivity. With some exceptions, such as the

Temperature

Figure 2 Schematic of free volume.

Pressure

Figure 3 Dual-mode sorption isotherm.

Pressure

Figure 3 Dual-mode sorption isotherm.

production of oxygen-enriched air for medical applications or the recovery of C4 hydrocarbons, low-selectivity membranes and hence rubbery polymers have found limited commercial utility in the purification of industrial gases.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment