Figure 5.Flowsheet for the process simulation model used to simulate the MCD
system.
For all model runs, the feed stream (stream 1 in Figure 5) was
configured with a temperature of 25°C, a pressure of 1 atm, and a molar
fractional composition representative of common air
(0.78 N2, 0.21 O2, 0.01 Ar). This stream
was also configured to enter the column at stage 1. The temperature of
the nitrogen-rich outlet stream from the heat exchanger (stream 5) was
set to ‑2°C. This value was based on measurements from the experiments
and indicated a 27°C difference between the hot side inlet and cold side
outlet for the exchanger.
To ensure the model results were not dependent on any single simulator,
the model was executed in three different software packages: CHEMCAD
(v7.1.6), Aspen HYSYS V11, and DWSIM (v6.7.1). The Peng-Robinson
equation of state (EOS) was used for all thermodynamic calculations.
To model the separation efficiency (which, again, was focused on
comparing the separation efficiency of the three different columns), the
standard volumetric flow rates of the feed (stream 1) and bottoms liquid
product (stream 3) were set to 1.0 and 0.01 SLM, respectively, and the
condenser duty was fixed at ‑16 W. The number of stages in the model
column was adjusted from 2 to 20, and the composition of the bottoms
liquid product was recorded for each stage. The number of theoretical
stages of separation achieved by each MCD column was then determined by
matching the composition measured in the experiments to that of the
model.
To model the maximum oxygen production the standard vapor volumetric
flow of the feed (stream 1) was increased to 5 SLM, the number of column
stages was fixed at 8, the condenser duty was initially kept at ‑16 W,
and the standard vapor volumetric flow of the bottoms liquid product
(stream 5) was incremented from 0.1 SLM until the oxygen mole fraction
in this stream fell below 0.90.