Rapid Swing Adsorption Processes represent the extreme limit of pressure swing adsorption in which the cycle time is
dramatically reduced in an effort to shrink the size of the equipment for a given production.
First introduced in 1971, RPSA is suitable when large quantities of relatively
low purity gas such as oxygen-enriched air are required to improve oxidation
processes. When high gas throughput is
used in an adsorption process, several limitations manifest themselves, in particular the kinetics of gas
adsorption and the pressure drop in the bed. This
project was started in 1999 to investigate how mass transfer kinetics and
pressure drop intrudes on RPSA performance for air separation. A combination of
simulation and experimental work is needed to understand the mechanisms of mass transfer in zeolite
systems. Simple models such as the Linear Driving Force model for mass transfer are not
satisfactory for simulating RPSA processes and more sophisticated approaches such as the Dusty Gas Model are needed.
In addition, it is important to understand the nature of pressure drop in
adsorbing systems particularly when rapid changes in gas momentum occur.
The experimental apparatus built to address these and other questions is of a
pilot scale size to enable scale-up. The 2-bed unit is skid mounted and is
equipped with rapid acting Valqua-valves allowing investigation of cycle times
down to 3s. Feed and product tanks buffer the extreme pressure
changes. The apparatus is fully instrumented with measurement of bed
temperature, pressure, and composition profiles as well as all flows to and from
the beds and tanks. The air feed to the RPSA is first filtered and dried
in an upstream PSA drier to ensure that water and CO2 contamination
do not obscure our data.
Control of the position and switch valves and access of the instrumentation
is done through a PLC/SCADA system.
Since the gas velocity in the beds is very large, a suitable hold-down device
must be used to prevent sieve fluidisation. Considerable attention has
been spent on our apparatus to this feature as shown below.
A
large number of cycle configurations are possible with PSA systems but RPSA
requires fairly simple cycles in order to reduce cycle time as much as
possible. We have used the simple 2 bed cycle shown at the right and
lately, even one bed cycles, to investigate the effect of cycle time on RPSA
performance. The graph below shows the overall performance as cycle time is
reduced keeping the pressure ratio constant.
More
rapid cycles (12s) lead to higher production (Tonnes per day) but not
proportionally so since recovery decreases due to intrusion of mass transfer and
pressure drop. By measuring mass transfer parameters and pressure drop
parameters for our sieve in independent ways (using our KTU and LUB equipment),
and using our numerical models, we are able to "untangle" the effect
of pressure drop and mass transfer and determine which is more important in
affecting process performance.
For more information on this project see the
publications
or contact Paul Webley