GB1275833A - Process and apparatus for drying substances in vacuum drying installations to a predetermined moisture content - Google Patents

Abstract

1275833 Controlled drying process FARBENFABRIKEN BAYER AG 12 June 1970 [13 June 1969] 28493/70 Heading F4G [Also in Division G1] A process for drying a material in a vacuum drying chamber relies on #p/#t, the rate of change of equilibrium vapour pressure of the solvent with time, as an indication of the completeness of the drying operation. In one form of the process (Fig. 2) the chamber 2 is evacuated from ambient pressure through p w to p w - #p, both pressures being able to be pre-set; p w is the desired final equilibrium vapour pressure of the solvent and #p is a small increment. The values are chosen using the relationship between p and the moisture content of the material. When the pressure has reached the lower level, valve 3 is opened allowing the pressure p to be measured at a position 7 while the temperature T is measured by a thermometer 6. The measurements are then relayed to converters 7a and 6a respectively which feed a circuit 10 which calculates a value # proportional to the relative humidity in the chamber. The signal then passes to a controller 11 which can be set to the two values # w and # w - ## such that the controller closes valve 3 for ### w -## and opens it for #># w . The valve 3 allows the chamber 2 to be evacuated by the pump 5 via a condenser 4. At first the rise in pressure such as a to b is rapid because the material is wet but later the rise is slower; when the rise-time exceeds a chosen value #t max the drying process is deemed to be complete. An alternative scheme (Fig. 3, not shown) allows the valve to operate at fixed, equal intervals of time, the process finishing when the rise of pressure between the opening and closing of the value is not greater than a certain #p min .