Pressure swing adsorption (PSA) is employed to separate certain gases from a gas mixture under pressure as determined by the molecular characteristics of the gases and their affinity for an adsorbent material. Typically, gases tend to be adsorbed onto solid surfaces at high pressures. When the pressure is reduced, these adsorbed gases are desorbed, or released back into the environment.
Pressure swing adsorption involves two primary vessels, one held at a high pressure to adsorb the gas, and another at held at a lower pressure to release the adsorbed gas. In general, this method is preferred for industrial applications involving bulk separation of gases. Large containers are usually employed in this process.
How Pressure Swing Adsorption Works
The principle of adsorption serves as the foundation of PSA. In the adsorption process, molecules of a gas or liquid adhere to the surface of an adsorbent. Unlike absorption, which involves the entire absorbent volume, adsorption is a function of the active surface area of the adsorbent.
Therefore, adsorbent solids are usually porous materials that provide a large surface area per unit mass. The gas or liquid forms a film over the adsorbent surface. As the pressure increases, the amount adsorbed also increases, and this surface phenomenon reverses as pressure is reduced. As explained earlier, PSA requires two main zones, a high-pressure zone to facilitate adsorption and a low-pressure zone to facilitate desorption. In reality, however, the conventional process actually involves four to 16 intricately connected vessels.
Incredibly versatile, PSA can be employed in a wide range of applications, such as solvent vapor recovery, gas drying, air fractionation, and hydrogen production (from steam methane reformation and alcohol dehydration, among other methods).
Pressure Swing Adsorption for Fuel Ethanol Production
While the most common industrial PSA applications involve hydrogen drying, many other applications are now making use of this adsorption process. One such application is fuel ethanol production. Ethanol is usually produced by distilling starch. The outlet of the distilling column is high-ethanol vapor, which can be supplied to a PSA unit in order to filter out ethanol. This process is much more energy-efficient than the conventional ethanol dewatering processes.
The process involves four steps:
- adsorption, in which moisture is adsorbed into a suitable material at high pressure
- the blowdown or depressurizing stage, in which moisture is removed
- a purge, to remove the gaseous mixture being dried, and
- a re-pressurizing step, after which the same vapor is again passed through the same cycle.
Ethanol dehydration can be performed with high efficiency using PSA methods. In fact, ethanol of up to 99% purity can be achieved using the PSA separation process. It should be noted that the adsorbent materials regenerated during the depressurizing phase use a very low-pressure vacuum to remove adsorbed moisture.
Choosing the Right PSA Unit
Versatile and reliable, the PSA method of industrial gas separation finds applications in a large range of fields. The drying or dehydration of various gases, for instance — which was initially used to dry hydrogen — is now being used to filter ethanol after distillation.
It’s well-understood that pressure swing adsorption is ideally suited for ethanol production, but keep in mind that designing and installing a PSA unit requires a high degree of technical and industrial expertise. Given the capital investment involved, great care should be taken in finding a suitable partner for obtaining and installing a pressure swing adsorption unit.