Gas Separation

The process The study of gas permeation through membranes has a long history dating back to the work of Thomas Graham in the mid-nineteenth century. However, the first systematic studies with polymers of the type used today did not begin until 100 years later.

The mechanism of gas permeation developed in the 1950s and 1960s was the solution-diffusion model. In this model, the rate of diffusion through the polymer membrane is governed by Fick's law of diffusion. For simple gases, it can be shown that Fick's law leads to the expression

J"

DkAp

where J is the membrane flux (cm3(STP)/cm2 s), k is the Henry's law sorption coefficient linking the concentration of gas in the membrane material to the pressure of the adjacent gas (cm3(STP)/cm Hg), Ap is the partial difference across the membrane, l is the membrane thickness (cm), and D is the permeant diffusion coefficient (cm2s_1), a measure of the permeant's mobility in the membrane. This expression can be further simplified to

J"

where P is a permeability, equal to the product Dk, and is a measure of the rate at which a particular gas moves through the membrane of a standard thickness (1 cm) under a standard driving pressure (1 cm Hg). The permeability unit, 1x10~10cm3 (STP) cm/ cm2 s cm Hg, is called a Barrer, after R.M. Barrer, a pioneer in membrane permeation studies.

A measure of the ability of a membrane to separate two gases (1) and (2) is the ratio of their permeabilities, called the membrane selectivity, a:

a1'2"P2"D2xk2

Figure 16 Flow scheme of a typical electrodialysis process used in a seawater salt concentration plant.

The factors that determine membrane permeability can best be understood by considering the component terms D and k. For simple gases, the diffusion coefficient tends to decrease with increasing per-meant diameter, because large molecules interact with more segments of the polymer chains and are thus less mobile. On the other hand, the sorption coefficient of gases increases with the condensability of the gas. Normally, the sorption coefficient also correlates with molecular diameter, larger molecules being more condensable than smaller molecules, and the Henry's law sorption coefficient increases with increasing permeant diameter. Thus, the effect of increasing permeant size on permeability is a balance between the opposing effects of diffusion coefficient, which decreases with increasing size, and solubility, which increases with increasing size. This balance determines the selectivity of a membrane for any pair of gases and is a function of the membrane material.

In glassy, rigid polymers such as polysulfone or polyimides, permeant diffusion coefficients are most important. Therefore, these polymers preferentially permeate the small, noncondensable gases, hydrogen, nitrogen and methane, over the larger, condensable gases, propane and butane. On the other hand, in rubbery polymer such as silicone rubber (polydimethylsiloxane), permeant solubility coefficients are most important. Therefore, these polymers preferentially permeate the larger, more condensable gases, propane and butane, over the smaller, noncondensable gases, hydrogen, nitrogen and methane.

Applications The principal developed gas separation processes are listed in Table 2. The first large-scale commercial application of gas separation was the separation of hydrogen from nitrogen in ammonia purge gas streams. The process, launched in 1980 by Permea, then a Division of Monsanto, was followed by a number of similar applications, such as hydrogen/methane separation in refinery offgases and hydrogen/carbon monoxide adjustment in oxo-chemical synthetic plants.

Following Permea's success, several US companies produced membrane systems to treat natural gas streams, particularly to remove carbon dioxide. The goal is to produce a stream containing less than 2% carbon dioxide to meet the national pipeline specifications and a permeate enriched in carbon dioxide

Figure 17 Flow scheme of (A) a one-stage and (B) a two-stage membrane gas separation system for the separation of carbon dioxide from natural gas.

to be flared or reinjected into the ground. Currently, cellulose acetate is the most widely used membrane material for this separation, but because the carbon dioxide/methane selectivity of cellulose acetate is only 15-20, two-stage systems are often required to achieve a sufficient separation. More selective polyimide membranes are beginning to replace cellulose acetate membranes in this application. Flow schemes for a one-stage (A) and a two-stage (B) cellulose acetate membrane system for carbon dioxide/natural gas separations are shown in Figure 17. The single-stage system has a low capital cost, but 12.7% of the methane in the gas is lost with the carbon dioxide. This loss becomes unacceptable for large systems, so a two-stage unit is used. The methane loss is reduced to less than 2% but at the expense of more membrane area and a large compressor. The membrane process is generally best suited to relatively small streams in the 5-20 MMscfd range, but the economics of the process have slowly improved over

Table 2 Membrane gas separation applications

Separation

Status

H2/N2, CO, CH4, etc.

&500 units installed. Various hydrogen recovery applications in refineries, petrochemical and

ammonia plants

CO2/CH4

&200 units installed, some very large (5000-50 000 scfm) to separate carbon dioxide from

natural gas

N2/air

&5000 units installed, most small in the 50-500 scfm range (98-99.5% nitrogen)

Organic solvent vapour/air, N2

&100 units installed. Diverse applications include gasoline vapour recovery at oil terminals,

recovery of monomers from reactor vents

H2O/air

Many thousands of small modules sold for drying compressed air

the years and more than 200 natural gas treatment plants have now been installed - some quite large.

By far the largest gas separation process in current use is the production of nitrogen from air. The first membranes used for this process were based on poly-sulfone, poly(trimethylpentane) and ethyl cellulose. These polymer materials had oxygen/nitrogen selec-tivities of 4 to 5, and the economics of the process were marginal. The second-generation materials now used have selectivities in the range 6 to 7. With these membranes, the economics of nitrogen production from air are very favourable, especially for small plants producing 50-500 scfm of nitrogen; 5000 of these small systems are now in operation. In this range, membranes are the low-cost process, and most new nitrogen plants use membrane systems.

A growing application of membrane systems is the removal of condensable organic vapours from air and other streams. Unlike the process described above, organic vapour separation uses rubbery membranes, which are more permeable to the organic vapour. More than 100 organic vapour recovery plants have been installed. In Europe, most of the plants recover gasoline vapours from air vented during transfer operations; in the USA, most plants recover chlorinated and fluorinated hydrocarbons from refrigeration or chemical processing streams. Separation of propylene from nitrogen in polyolefin plants is an emerging application worldwide.

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