Page 114 - Principles of Catalyst Development
P. 114
102 CHAPTER 6
Promoters, if needed, may be added during the gelation period. Alterna-
tively, they may he deposited, using the same methods described later for
active components.
6.2.4. Washing and Filtering
The next two steps, washing and filtering, are sometimes reversed. A
satisfactory, but time-consuming, method is to wash by decantation. The
hydrogel is added to a large volume of distilled water in a conveniently
sized container and the suspension thoroughly mixed. Upon standing,
particles settle slowly, desorbing foreign ions as they fall. When a definite
interface is visible, water is removed by decantation and the process
repeated. The purified gel takes longer to settle with each washing, since
removal of counterions increases the zeta potential. The gel partly reverts
back to a sol, a process known as peptization. Care must be taken not to
wash too much or settling times become too great. A good method is to
check impurity levels in the decantation water during the operation. When
washing is complete, the hydrogel is filtered.
If filtering is done first, the filtrate is washed with successive amounts
of water until a satisfactory level of impurity in the wash water is found.
Again, care must be taken not to wash too much or the gel peptizes and
passes through the filter paper.
6.2.5. Drying
Drying is necessary IJ1 order to remove the large volume of water in
the hydrogel. Some collapse of the structure is to be expected, as shown in
Fig. 6.7, but care must be taken to properly control drying operations if
high porosity is desired.
Initially, drying occurs through evaporation of moisture from the
outside surface of the hydrogel. The rate of water loss is constant and the
mass transfer controlled by temperature, relative humidity, flow rate of air
over the surface, and size of the filtrate. This process continues until moisture
content drops to about 50%. The filtrate mass now begins to shrink as most
of the external water disappears. The material is now called a xerogel.
Continued moisture loss occurs with a declining rate, in which evapor-
ation is controlled by capillary forces. The saturation point decreases as
pores become smaller and evaporation slows until water is forced into larger
pores by concentration gradients. If evaporation occurs but removal of
moisture is blocked by smaller pores, then large internal pressures of steam