Purification Of P-Dichlorobenzene By Direct Contact Cooling Solidification

Melt crystallization is one kind of green separation technology. It need not solvent and is operated at low temperature. There are two kinds of melt crystallization technology applied in industrial production: layer and suspension melt crystallization. Layer melt crystallization is famous for getting high purity crystals, but it is only operated intermittently. Suspension crystallization is operated continuously, but the crystal grow rate is low and the size of crystals is dissimilar, which is difficult for latter purification. Pastilles purification technology which has high grow rate and is operated continuously in which solidification is operated on cooling band. The equipment of solidification is complicated, which is difficult for production and maintenance. This paper provided a new kind of solidification technology: the melt materials was injected into cooling solvent and solidified. The method of solidification is simple and the shape and size of pastilles are uniform.At 45℃, the pastilles produced in different process were sweated and the sweating was separated from the pastilles by vacuum pump, which studied the influence of the composition of melt, the temperature of cooling medium and the diameter of pastille on the pastille purification. The results showed that the purity of pastilles in different compositions tended towards the same at the same sweating temperature, which indicated the purity of pastilles were determined by sweating temperature. The structure of pastilles was more compact and the pastille purification rate was slower at lower solidification temperature, however the yield of pastilles was higher. The purification rate and the content of p-dichlorobenzene were higher with lesser diameter of the pastilles. Experimental results showed that after 100min sweating the purity of p-dichlorobenzene pastilles could be enhanced up to 99.6%, its purification rate coefficient can be 1.02×10-3 under the conditions of melt composition of p-dichlorobenzene 85%, cool medium temperature 0℃ and diameter of pastille 3.1mm.Improving sweating temperature in different raw material concentration, solidificaton temperature and pastille diameter, the influence on the purification and yiled of pastille was different. In the condition of higher composition of melt, higher solidification temperature and smaller diameter of pastille, the influence on the purification of pastille was small. Contrary, the influence was big. In the condition of higher composition of melt, lower solidification temperature and smaller diameter of pastille, the influence on the yield was small. Contrary, the influence was great. The coarse crystal which prepared by suspension crystallization was sent into washing column to study the influence of the operation conditions of washing column on crystal purification. The results showed that the time of contact of the crystals with the reflux influenced crystals purification. So the proper agitation and higher contact time of crystals with reflux and higher materials concentration were favorable for crystal purification. The pastilles were purified in washing column to study the influence of solidification conditions on pastilles purification. The results showed that the concentration of pastilles and the diameter of pastilles greatly influence the pastille purification in washing column, the temperature of cooling medium has less influence on the pastille purification. The purity of pastilles increased with higher pastilles concentration and smaller diameter of pastilles.