Quenching the vapour with cold air in the chamber may increase the rate of heat removal
although excessive nucleation is likely and the product crystalswill be very small. Condenser
walls may be kept free of solid by using internal scrapers, brushes, and other devices, and
all vapour lines in sublimation units should be of large diameter, be adequately insulated,
and if necessary, be provided with supplementary heating to minimise blockage due to
the buildup of sublimate. One of the main hazards of air-entrainment sublimation is the
risk of explosion since many solids that are considered safe in their normal state can form
explosive mixtures with air. All electrical equipment should therefore be flame-proof, and
all parts of the plant should be efficiently earthed to avoid build-up of static electricity.
The method of calculating the density of deposited layers of sublimate and of other
variables and the optimisation of sublimate condenser design, has been discussed by
WINTERMANTEL et al.(113). It is generally assumed that the growth rate of sublimate layers is
governed mainly by heat and mass transfer. The model which is based on conditions in the
diffusion boundary layer takes account of factors such as growth rate, mass transfer, and
concentrations in the gas. The model shows a reasonably good fit to experimental data.
In a variant of the large-chamber de-sublimation condenser, the crystallisation chamber
may be fitted with gas-permeable walls as described by VITOVEC et al.(101). The vapour and
the entrainer gas are cooled by evaporation of water dispersed in the pores of the walls, and
an inert gas passes through the porous walls into the cooling space and protects the internal
walls from solid deposits. Crystallisation takes place in the bulk vapour–gas mixture as
a result of direct contact with the dispersed water. This arrangement has been used, for
example, for the partial separation of a mixture of phthalic anhydride and naphthalene by
using nitrogen as the entrainer. Although fluidised-bed condensers have been considered for
large-scale application, most of the published reports are concerned with laboratory-scale
investigations