The importance of incorporating a vacuum pump performance
curve
into a mathematical model that describes the regeneration steps in
a vacuum swing adsorption (VSA) cycle was assessed. This was accomplished
by evaluating the relationship between the pressure and flow rate
at the exit of a 13X zeolite bed, initially and uniformly saturated
with 70 vol % CO2 in N2 at 333.15 K and 1 atm,
which then underwent only countercurrent depressurization and light
reflux (LR) steps, that is, the regeneration steps in a VSA cycle.
Overall, this study revealed that it is imperative to use a realistic
vacuum pump model in cyclic adsorption process simulation, especially
for estimating the ideal isentropic energy consumed during the regeneration
steps in a VSA cycle, and it revealed the possible inaccurate performance
predictions that might result when using a valve equation alone to
describe these steps. The study on the effect of valve size (c
v) without the vacuum pump present to mimic
the behavior of the vacuum pump performance curve showed that ideal
isentropic energies from the valve equation can be overestimated significantly,
in this case ranging from 1 to 6 kJ/mol CO2 removed. The
study on the effect of LR flow rate only with the valve or only with
the vacuum pump present showed marked differences in performance when
using a valve equation to mimic a vacuum pump performance curve during
the LR step. As secondary outcomes, the study on the effect of bed
size when operating with a Pfeiffer ACP 40 vacuum pump alone showed
that it was easier for it to regenerate beds of a smaller size; it
showed a power consumption benefit during depressurization, a feature
of VSA processes when the vacuum pump is not restricted by a valve;
and it showed how to determine an approximate bed size. The study
on the effect of valve size (c
v) when
placed before the suction side of the vacuum pump showed that a valve
should never be placed there to avoid choking of the vacuum pump and
expending more energy; it showed that if a valve is required for flow
rate control, then its c
v should be chosen
carefully; and it showed how to determine an approximate valve size.