The benefit of a reduced absolute pressure is clearly established, since the gas at 0.2
atm is highly charged in X molecules, while the nitrogen consumption is divided by a
factor of 2.2 for a productivity increased by a factor of 5 compared to the system
functioning at atmospheric pressure. Flashing temperature is also lowered by the use of
reduced pressure, thus greatly minimising the power required for this operation.
Nevertheless, if one compares the two systems at the same productivity level, the
advantages of reduced pressure are still significant. While total volumetric flow and
molar flows for X and water will be the same in both cases, the required amount of
nitrogen will be now 5 times lower at reduced pressure. Thus, nitrogen consumption is
divided by a factor of 11 compared to a system at atmospheric pressure at the same
productivity.
Moreover, the use of solid/gas systems functioning at reduced pressure is of great
importance, considering the step following catalysis that allows recovery of the products.
Table 9.1 Compositions of two different gases for
the same reaction in term of thermodynamic
activity performed at two different absolute
pressures
Absolute working pressure 0.2 atm 1.0 atm
Qx (mole/h) 10 2
QH20 (mole/h) 50 10
QN2 (mole/h) 40 88
Minimal flash temp. (°C) 100 200
Condensable fraction of the gas 60% 10%
Qy total (m3/h) 15.3 3.06
If one considers now that after catalysis, the gas stream is injected onto a heat
exchanger for the recovery of the product Y at a temperature of 10°C and that saturation
pressure of Y at 10°C is 0.01 atm, then the system depicted in Figure 9.4 allows a more
complete recovery of Y, which it is impossible for the system depicted in Figure 9.3.
If all X is transformed into Y, then in both systems the gas at the outlet of the reactor
will have the following characteristics at the steady state: PpH20=0.1 atm, Ppx=0 atm,
PpY=0.02 atm.
Prior to the cooling operation, the gas on system described in Figure 9.4 will have an
absolute pressure at the outlet of the vacuum pump of 1 atm. In this case, the
characteristics prior the inlet of the condenser will be: PPH20=0.5 atm, Ppx=0 atm,
PpY=0.10 atm since Pa is multiplied by 5.
Once cooled at 10°C, condensation will take place, while the partial pressure of Y is
higher than the saturation pressure of Y at 10°C. Thus, the efficiency will be completely
different in the two cases. For the reactor working at atmospheric pressure, only 50% of
Multiphase bioreactor design 270