Process for freezing and crystallizing triformol continuouslly by sleeved crystallizer

Abstract

A paraformaldehyde continuous freezing and crystallization process using a casing type crystallizer, the process flow is: the fresh paraformaldehyde solution from the front system enters the circulating pump I, the post crystallizer and the connecting pipeline to form a circulation loop and trimerization The formaldehyde suspension is mixed, and a part of the paraformaldehyde suspension in the rear crystallizer is pumped to the centrifuge to separate the paraformaldehyde crystals from the mother liquor. Part of the mother liquor is returned to the rear crystallizer, and the other part of the mother liquor is returned to the front system, the rear crystallizer A part of the paraformaldehyde suspension inside is transported by the circulation pump II to the casing type crystallizer for heat exchange with chilled water to form paraformaldehyde crystal nuclei, and the paraformaldehyde suspension after heat exchange returns to the rear crystallizer. Due to the use of the casing type crystallizer, the heat exchange capacity of this process is greatly improved, and the power consumption and freezing water consumption are greatly reduced. The content of paraformaldehyde in the product obtained by centrifugal separation is increased, and the formaldehyde content is lower than that of the existing process. about half.

Description

technical field [0001] The invention belongs to the field of paraformaldehyde preparation, in particular to a freezing and crystallization process of paraformaldehyde. Background technique [0002] Trioxane is an important monomer in the production of polyoxymethylene. For the polymerization of polyoxymethylene, the total content of impurities in the monomer must not exceed 100PPM. Therefore, paraldehyde needs to be refined to polymer grade purity. The refining process of paraformaldehyde mainly includes: extraction, crystallization, pressure swing rectification, critical extraction and membrane separation, among which extraction separation and crystallization separation are more widely used. [0003] It is understood that the continuous freezing and crystallization process of paraformaldehyde imported from Poland in my country is as follows: figure 1 As shown, the fresh paraformaldehyde solution from the front system enters the circulation loop formed by the circulation p...

AI Technical Summary

Problems solved by technology

Because this kind of process adopts the trough crystallizer as the front crystallizer, it mainly has the following disadvantages: 1. The heat transfer effect is not good, and the temperature difference of the chilled water system is very low. , power consumption increases

2. The crystallizer is cleaned frequently, and it needs to be cleaned every two to three days during operation

Frequent cleaning will not only increase power consumption, chilled water consumption and manpower consumption, but also cause fluctuations in system operation and be detrimental to the environment

3. The volume of the crystallizer is large, the circulation volume of the paraformaldehyde suspension is relatively small, and the residence time in the crystallizer is long. When the surface of the heat exchanger is clean at the beginning of the start-up, the paraformaldehyde suspension in the crystallizer will be too high. The coldness is high, and a large number of fine crystals are formed

The presence of a large number of fine crystals will aggregate into blocks in the paraformaldehyde suspension, causing pipeline blockage and even paralyzing the entire system; the more fine crystals of the same quality, the larger the specific surface area of ​​the crystals. Its adsorption effect will increase the adsorption of formaldehyde and water and other impurities on the crystal surface during the centrifugal separation process, thus increasing the difficulty of subsequent separation; the high content of fine crystals will also make the centrifugal separation system run poorly

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