Abstract:
Falling film crystallization is an important way to separate and purify
p-xylene. Presently,
p-xylene is mainly produced by toluene disproportionation technology, toluene methanol alkylation technology, benzyl alcohol alkylation technology and so on. However, all the products produced by these processes are a mixture containing
o-xylene,
m-xylene and
p-xylene, with a mass fraction of
p-xylene of more than 70%. It is difficult to separate and get high purity
p-xylene by distillation because of their comparable melting points. Therefore, it is necessary to develop a low-cost and high-efficiency approach for
p-xylene separation. In this paper, falling film crystallization was used to separate and purify
p-xylene from mixed xylenes with high concentration of
p-xylene (mass fraction 75%). Based on the growth rate, recovery rate and purity of
p-xylene crystal layer, the effects of the hanging film, feed rate, precooling temperature of raw material, and crystallization temperature and cooling rate on the crystallization process were studied. Moreover, the effects of heating rate and terminal temperature of sweating on the sweating process were investigated as well. The key parameters of the falling film crystallization were provided for the industrial optimization. The experimental results suggested that the optimal crystallization conditions were hanging film, spray density of feed rate of 360.53 mL/(m·min), precooling temperature of raw material of 25 ℃, crystallization temperature of 15 ℃ and cooling rate of 0.15 ℃/min. And the optimal sweating conditions were heating rate of 1 ℃/min and end temperature of 10 ℃. As a result, the
p-xylene product with purity of higher than 99.50% was obtained. The results can be benefit for the fundamental studies of industrial scale-up and process design.