1. Pressure swing adsorption (PSA) is a technology used to separate some gas species
from a mixture of gases under pressure according to the species' molecular
characteristics and affinity for an adsorbent material. It operates at near-ambient
temperatures and differs significantly from cryogenic distillation techniques of gas
separation. Specific adsorptive materials (e.g., zeolites, activated carbon, molecular
sieves, etc.) are used as a trap, preferentially adsorbing the target gas species at high
pressure. The process then swings to low pressure to desorb the adsorbed material.
2. Process
Pressure swing adsorption processes utilize the fact that under high 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.
Adsorbents
Aside from their ability to discriminate between different gases, adsorbents for PSA
systems are usually very porous materials chosen because of their large specific
surface areas.
Typical adsorbents are activated carbon, silica
gel, alumina, resin and zeolite. Though the gas adsorbed on these surfaces may
consist of a layer only one or at most a few molecules thick, surface areas of several
hundred square meters per gram enable the adsorption of a significant portion of the
adsorbent's weight in gas. In addition to their selectivity for different gases, zeolites
and some types of activated carbon called carbon molecular sieves may utilize their
molecular sieve characteristics to exclude some gas molecules from their structure
based on the size of the molecules, thereby restricting the ability of the larger
molecules to be adsorbed.
3. Membrane separation processes operate without heating and therefore use less
energy than conventional thermal separation processes such
as distillation, sublimation or crystallization. The separation process is purely
physical and both fractions (permeate and retentate) can be used.
Pressure driven operations
Concentration driven operations
Operations in an electric potential gradient
Operations in a temperature gradient
5. Hydrogen recovery (cryogenic) The pre-purified purge gas from the ammonia recovery
plant is water saturated. As a first step, the purge gas is dried and freed from traces of
ammonia in an adsorber station. The hydrogen is separated in a coldbox. The drop in
temperature necessary for a stable process is achieved with a throttle valve using the
Joule-Thomson effect. The purge gas is cooled down in the coldbox heat exchanger
using the cooling power of some of the cryogenic hydrogen separated in the coldbox.
Thus the gas is partly liquefied. The gas phase is separated in the hydrogen separator,
warmed up, and the hydrogen is sent to the syngas compressor.
Hydrogen recovery (membrane) The pre-purified purge gas is warmed up to increase
the water saturation value of the purge gas before entering the membrane unit. The
membrane unit is usually set up in two stages. One part of the hydrogen recovered is
fed to the suction part of the syngas compressor and another part is fed back to an
intermediate pressure stage of the compressor.
Hydrogen, argon and nitrogen recovery (cryogenic) The separation of hydrogen, argon
and nitrogen takes place in a coldbox. First the pre-purified purge gas from ammonia
6. recovery needs to be dried and freed from traces of ammonia in an adsorber
station. The purge gas is cooled down in the coldbox heat exchanger. The
hydrogen product is separated in the hydrogen separator and warmed up. Some
of the hydrogen is then sent to the syngas compressor and some is used as a
cooling medium in the coldbox. The argon, nitrogen and fuel gas are separated in
cryogenic rectification columns. Recovery is based on an open nitrogen cycle,
which generates the refrigeration capacity required to liquefy the argon gas. If
required, the customer can also recover liquid nitrogen. With small modifications,
this cryogenic recovery solution for hydrogen, argon and nitrogen can also be
installed as a separate argon and nitrogen recovery solution downstream of an
existing hydrogen recovery unit
7. In thermodynamics, the Joule–Thomson effect (also known as the Joule–Kelvin
effect, Kelvin–Joule effect, or Joule–Thomson expansion) describes the temperature
change of a real gas or liquid (as differentiated from an ideal gas) when it is forced
through a valve or porous plug while kept insulated so that no heat is exchanged with the
environment.[1][2][3] This procedure is called a throttling process or Joule–Thomson
process.[4] At room temperature, all gases except hydrogen, helium and neon cool upon
expansion by the Joule–Thomson process; these three gases experience the same effect
but only at lower temperatures.
