Gas permeation is based on the mass transfer principles of gas diffusion through a permeable membrane. In its most basic form, a membrane separation system consists of a vessel divided by a single flat membrane into a high- and a low-pressure section. Feed entering the high-pressure side selectively loses the fast-permeating components to the low-pressure side. Flat plate designs are not used commercially, as they do not have enough surface area. In the hollow-fiber design, the separation modules contain anywhere from 10,000 to 100,000 capillaries, each less than 1 mm diameter, bound to a tube sheet surrounded by a metal shell. Feed gas is introduced into either the shell or the tube side. Where gas permeability rates of the components are close, or where high product purity is required, the membrane modules can be arranged in series or streams recycled.
The driving force for the separation is differential pressure. CO2 tends to diffuse quickly through membranes and thus can be removed from the bulk gas stream. The low pressure side of the membrane that is rich in CO2 is normally operated at 10 to 20% of the feed pressure.
It is difficult to remove H2S to pipeline quality with a membrane system. Membrane systems have effectively been used as a first step to remove the CO2 and most of the H2S. An iron sponge or other H2S treating process is then used to remove the remainder of the H2S.
Membranes will also remove some of the water vapor. Depending upon the stream properties, a membrane designed to treat CO2 to pipeline specifications may also reduce water vapor to less than 7 Ib/MMscf. Often, however, it is necessary to dehydrate the gas downstream of the membrane to attain final pipeline water vapor requirements.
Membranes are a relatively new technology. They are an attractive economic alternative for treating CO2 from small streams (up to 10 MMscfd). With time they may become common on even larger streams.