| Types of PSA Process |
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Pressure Swing adsorption has undergone rapid changes as the technology
of separation processes advanced with the passage of time. It is very
important to choose the correct Pressure swing adsorption system that
exactly caters to the specific requirqment in terms of production needs
like quality, etc. The following parameters play an important role in
determining as to which type of PSA system to opt for:
- Unit Capacity: For example a typical four Bed system will
be used when requirement is less than 5,000 Nm3/h of
feed rate. Medium to large-capacity PSA units are employed i.e.
greater than 50,000 Nm3/h of feed for maximum product
recovery.
- Unit Cost: As the the cost of a PSA unit increases with
the increase in pressure equalisations, smaller PSA units are
preferred that utlizes one or two equalisations at the expense of
little recovery loss. The investment costs needed to be combined
with the value of the product gas to find the exact gas recovery
system that incurrs the lowest overall cost.
The following table can be taken as a guideline to choose a PSA
system based on the ratio of feed and offgas pressures vis-a-vis the
feed gas flow rate. The Pressure Swing Adsorpotion units for feed
gas rate are in the range of 5,000 - 50,000 Nm3/h.
- Feed and offgas pressure: The adsorption pressure is
fixed by two ways-
- available feed pressure
- required product pressure
For example taking the case of Hydrogen recovery, the range of
feed pressure hovers between 10 and 40 bar. It is to be noted that a
high feed gas pressure is the choice when it comes for high hydrogen
recovery with low investment expenditure. Here, the pressure drop
from feed to product is comparatively small, also the product is
found available at a pressure of 0.5 bar lower than the feed gas.
Offgas pressures typically range between atmospheric -10 bar. This
is the most influencing parameter on hydrogen recovery. It has been
generally found that lower is the offgas pressure, morer the
hydrogen recovery a possibility, and with it obviously comes lower
the investment costs for a fixed production rate. A 4:1 minimum
ratio must be maintained by the absolute feed pressure with the
absolute waste pressure.
- Composition of the Feed Gas: The PSA process are geared
to handle feed gas with a large variety of impurities. Generally the
process performance changes with the adsorption-desorption
properties of a specific componenton the selected adsorbent. The
table below gives a description of the impurities in the PSA
process:
- Product Purity: Some advantages with respect to product
gas recovery and investment expenditure can be gained at the expense
of a less-strict purity specification.
Types of PSA Process:
