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.