Membrane Induced Nucleation And Automatic Control For Cooling Crystallization Of Thiourea

Cooling crystallization is an important purification technology.It is environmentally friendly,which has low energy consumption.The traditional cooling crystallization technology is mainly to manually add seed crystals.The manual selection of seed crystals in the early stage and the timing of addition in the later stage need to be accurately controlled to avoid the occurrence of crystal quality degradation caused by nucleation and secondary nucleation.At the same time,the process cannot be automatically controlled,which severely limits large-scale continuous production.Aiming at the problems of traditional cooling crystallization,this paper used thiourea aqueous solution as the system to establish a crystallization process that used hollow fiber membrane modules to induce nucleation to achieve automatic control,which fundamentally changed the way of seed crystal addition.The process could be easily operated,the loss was small and it could be used for industrial large-scale crystallizer control.This paper first studies two typical hollow fiber membranes,namely polytetrafluoroethylene membrane(PTFE for short)and polyethersulfone ester membrane(PES for short),and discusses the interface properties and thermal conductivity of the two.It is found that the PTFE membrane has a larger contact angle and a smaller roughness.According to the classical nucleation theory model,it can be known that PTFE-induced nucleation rate is slow.An improved fractal theory is proposed to calculate the thermal conductivity.Compared with the thermal conductivity measuring instrument,the error is very small.Through the experiment of membrane permeability and the experimental observation of the movement behavior of crystal particles on the membrane,it is confirmed that the crystal will automatically fall off when it grows to a certain scale on the membrane.The fallen crystal is transported to the crystallizer along with the fluid,and the membrane induced nuclear is verified the feasibility.Further,based on the basic data research of the crystallization of thiourea aqueous solution,the solubility data of thiourea in water was measured,and the formula was fitted;the width of the metastable zone at 39? saturated thiourea solution at different cooling rates and different speeds is measured,and the formula was fitted,and the error is about 5%.By studying the induction period of the same initial concentration of thiourea solution at differentinitial supercooling temperatures,it can be known that the lower the cooling water temperature and the higher the rotation speed,the larger the membrane area and the shorter the nucleation induction period.By testing the nucleation induction period of thiourea under the conditions of different initial temperatures and the same end temperature,it can be known that the surface tension of PES is smaller than that of PTFE,the nucleation energy barrier is smaller,and the nucleation rate is greater(Under the test conditions,the nucleation rate of the PES membrane interface is 3.6 times that of the PTFE membrane interface).Finally,the cooling crystallization induced by the two membranes is compared with the cooling crystallization with different amounts of seed crystals and the ordinary cooling crystallization and rapid cooling crystallization without membrane.Both the temperature change curve and the online microscopic observation images indicate that the nucleation rate has been successfully controlled.Both of the cooling crystals produced different degrees of temperature rise,that is,secondary nucleation occurred;both of the cooling crystals without membrane produced explosive nucleation.Comparing the final product,it can be found that the product obtained by cooling crystallization of PTFE hollow fiber membrane has the highest purity(> 99.5%),the most complete crystal morphology,the largest crystal particle size(> 1.35 mm),and the most concentrated distribution(C.V.=7.8).In summary,the research in this paper provides a new idea for the precise control of cooling crystallization,which has important practical significance for automatic control of the process and design optimization.