Process swing adsorption

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30 Years of PSA Technology
for Hydrogen Purification
J. Stöcker and M. Whysall
Antwerp, Belgium
G.Q. Miller
Des Plaines, Illinois

Timetable of PSA Technology Development
This paper presents an overview of UOP’s PSA development work and also addresses the key role of the
PSA technology for hydrogen purification. The main development steps have been the extension of four
adsorber unitsinto systems with 10 and more adsorbers for unlimited plant capacity, the development of
high-performance adsorbents to achieve the highest hydrogen recoveries, and the development of the fully
automatic operation of PSA units for unattended operation.
The main PSA application has been recovery of hydrogen from a wide range of gas streams. Since its
commercialisation in 1966, more than 530 PSAunits have been installed worldwide. Between 1966 and
1976, the plant capacities did not exceed hydrogen product rates greater than 20,000 Nm3/h generally
provided by four-bed PSA systems. The hydrogen was mainly used in chemical and petrochemical plants.
The major breakthrough of the PSA technology was achieved in 1976 with the commercialisation of
POLYBED™ PSA units. Hydrogen recoveries exceeding90%, and the possibility of designing POLYBED
PSA units for capacities of 100,000 Nm3/h met the increasing hydrogen demand of refineries. The inherent
capability of POLYBED PSA units to maintain design capacity with a reduced number of adsorbers in
operation provided the on-stream reliability required for the downstream process units.
In the late 1980s, UOP added two important patented controlfeatures to the POLYBED PSA units:
improved repressurisation control and automatic purity control. The improved repressurisation control allows
the PSA units to be designed for “zero downtime operation.” The automatic purity control completely
removes any need for operator attention. The latest control development features self-tuning of the most-critical
Important progress has been madein the reduction of the total adsorbent volume required to purify a specific
feed gas rate. Today’s PSA units require less than 50% of the relative adsorbent volume used in PSA designs
of 1975, as a result of stepwise improvement in adsorbent and PSA cycles.


Process Definition
Basic Flow Diagram
The PSA process is based on the principle that adsorbents are capable of adsorbing more impuritiesat a
higher gas-phase partial pressure than at a lower partial pressure. The impurities are adsorbed in a fixed-bed
adsorber at high pressure and then rejected as the system pressure “swings” to a lower level. Hydrogen is
essentially not adsorbed. The ability to completely adsorb impurities allows the production of a hydrogen
product with very high (> 99.9 vol-%) purity.
The basic flow scheme ofthe POLYBED PSA process is shown in Figure 1. The process operates at ambient
temperature on a cyclic basis. The PSA process is a semibatch-type process that uses multiple adsorbers to
provide constant feed, product, and offgas (for fuel) flows. The high-purity hydrogen product leaves the
system close to the feed gas pressure. The offgas (impurities and the hydrogen losses) is available at lowpressure as fuel.

Figure 1

PSA Basic Flow Scheme

UOP 2730-1


Physical Description of a PSA Unit
A PSA installation consists of four major parts:

Adsorber vessels made from carbon steel and filled with adsorbent

Valve and piping skid, including all valves and instrumentation, prefabricated and tested in the

Control system, which is normally located in a remote control roomand contains the cycle controls

Mixing drum to minimise the composition variation of the offgas

A packaged system approach is used. The process valves and piping are shop mounted on a steel frame and
transported to the site as one or more pieces for quick and simple installation. An installed PSA unit is
shown on Figure 2.
Recently, UOP has developed a stacked skid design to reduce the plot...
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