Figure 9.4 Schematic diagram of a
packed bed solid gas bioreactor
working at reduced pressure.
Although the system allows a better control of the molar flow of substrates compared
to the previous system described, care must be taken in order to insure a complete
vaporisation of the different molecules injected into the system. To this end, saturation
pressure curves are necessary for defining the minimal temperature that has to be used at
the flash level. As a result, temperature must be chosen with regard to maximising the
boiling point of the compound at working pressure. This point can be considered as a
bottleneck, since in some cases, the use of very high temperatures can represent a serious
constraint.
Nevertheless, this problem can be solved by performing solid/gas catalysis under
reduced pressure. This strategy offers multiple advantages for the use of longer chain
compounds or for the improvement of productivity. Moreover, according to the saturation
pressure curves it is possible to minimise the temperature of the flash operation, and this
allows an important enrichment of the gaseous phase in reactants while minimising
greatly the quantity of carrier gas needed.
As an example, one can compare the effect of pressure on a simple theoretical
transformation. The transformation of X to Y will be carried out at 100°C and ax=aw= 0.1.
The saturation pressure of X at 100°C is 0.2 atm and partial pressure of water is 1 atm.
The boiling point of X is 200°C at 1 atm. Ppx is set to 0.02 atm and PpH20 thus will be 0.1
atm in the inlet gas. The total molar flow will be 100 moles/h, and the reaction will be
carried out at two absolute pressures, 0.2 atm and 1 atm.
The characteristics of the two gases are summarised in Table 9.1.
Solid/gas systems, theory and applications 269