Gas
Plants
Buyer's
Guide
Pressure swing adsorption is known to be one of the most economic and widespread commercial processes for hydrogen purification, air separation and small scale air driers. PSA was introduced in 1960s and today it has numerous other actual and potential uses such as the recovery of methane from landfill gas, the production of carbon dioxide and other large scale applications.
Pressure Swing Adsorption processes rely on the fact that under pressure gases tend to be attracted to solid surfaces, or adsorbed. The higher the pressure, the more gas is adsorbed, when the pressure is reduced, the gas is released, or desorbed. PSA processes can be used to separate gases in a mixture because different gases tend to be attracted to different solid surfaces more or less strongly.
A typical PSA system involves a cyclic process where
a number of connected vessels containing adsorbent material undergo
successive pressurization and depressurization steps in order to produce
a continuous stream of purified product gas. Adsorbent Materials
Typical adsorbents are activated carbon, silica gel, alumina and zeolite. The PSA units produce nitrogen or oxygen by passing air through a vessel containing one or more adsorbent materials. The most common adsorption materials:
- In oxygen PSA includes a variety of zeolite
molecular sieve which selectively adsorbs nitrogen, moisture and
carbon dioxide gas. This allows the oxygen molecules to pass through
the unit.
- In nitrogen PSA includes an activated carbon material to remove oxygen and other undesired components.
The name zeolite comes from a Greek name, meaning "boiling rock" and was given because of the mineral's ability to absorb water, which is readily given off when the mineral is heated. Appreciation of zeolite's several unique qualities led to the successful synthesis of several varieties in the 1950's, and a substantial industry has been built over the use of synthetic zeolites in several industrial uses.
The characteristic of molecular sieve crystals to differentially adsorb the gases comprising air make oxygen concentration possible in devices of small-scale and modest power consumption. The efficiency of the process is promoted under conditions of higher pressure, lower temperature, and higher concentrations of the gases to be adsorbed.
Components of PSA
The main components are:
- Two carbon or zeolite sieve containers
- Nitrogen or oxygen receiver
- Refrigerated dryer
- Feed air compressor
- Air receivers
- Air filters
- Oxygen enriched air
PSA Procedure
- The contaminated feedstock gas is pumped into a cylinder at
pressure.
- The cylinder contains beads of adsorbent material.
- The impurities in the feedstock gas, such as carbon dioxide, are
adsorbed onto the internal surfaces of the adsorbent beads.
- Hydrogen is left in the vessel, most of which is removed as
purified hydrogen product.
- Pressure in the cylinder is reduced, releasing the impurities from the adsorbent material.
- There is no need to provide a gas which has already been
previously enriched in certain components for the pressurizing step
in the last pressure-increasing stage
- The gas to be separated can be directly utilized for the pressure
increase.
- The full adsorption pressure is not even reached until toward the
end of the adsorption cycle.
- Low maintenance.
- Gasoline vapors
- Sweet crude oil vapors
- Certain chemicals including light alkanes and heavier, light
alkenes
- Ion exchange agents
- Desiccants
- Catalylists in petroleum refining
- Adsorbents for gas separations
- Carriers in detergents.
