Y will be condensed while 90% of Y will be
condensed for the system working at 0.2 atm.
Absolute pressure then appears a crucial parameter for optimising such processes.
Moreover, the use of carrier gas can be avoided by working at an absolute pressure equal
to the sum of the partial pressures of the different condensable molecules present in the
gas phase.
As a result of these examples, it appears clear that the use of systems derived from the
one presented in Figure 9.4 is more suitable for industrial purposes. But systems working
under reduced pressure have their own constraints. Since temperature is also a key
parameter, heat exchanges under partial vacuum are less effective, thus leading to
oversized thermal exchange surfaces.
Nevertheless, in all cases special care must be taken to very accurately control
physical parameters such as temperature and absolute pressure when one wants to work
at controlled thermodynamic conditions.
APPLICATION OF SOLID GAS CATALYSIS: REACTIONS USING
LIPOLYTIC ENZYMES
As described previously, what influences the activity and the stability of a gas/solid
biocatalytic system is the combination between the hydration level and the applied
temperature. When the water activity is too high, denaturation phenomena are more
important and when free solvent water is present in the system, serious diffusional
limitations exist since the system cannot be considered as solid/gas biphasic media but
rather as solid/liquid/gas system. Solid-gas biocatalysis exists only when the biocatalyst
has no free solvent surrounding the protein as mentioned previously. When one wants to
use solid/gas systems involving biocatalysts, one has to first study the sorption and
desorption isotherms of the catalytic preparation.
When using enzymes in low water media, the most important question is: what is the
minimal amount of water needed to obtain a stable and fully catalytic protein?
Experiments performed by Yang and Rupley (1979) related to the hydration of
proteins shed much light on the processes involved. The main result was to show that the
hydration of lysozyme was a sequential process, with a controlled distribution of water
molecules onto the protein.
In order to characterize the hydration state of solid phases, one needs to characterise
the isotherm sorption curve, the variation of water content versus the water activity.
These curves allow the determination of the different states of water (Drapron, 1985).
Different methods can be used for determination of the water content; gravimetric, or for
more sensitivity, iodometric and coulometric methods (Macleod, 1991; Bernetti et al.,
1984). Typically, isotherms present two characteristic break points (A and B)
corresponding to two important states of water as shown in Figure 9.5.
Solid/gas systems, theory and applications 271