Treatment method of MDI production tail gas

Abstract

The invention provides a treatment method of MDI production tail gas. The method specifically comprises the following steps: condensingof MDI production tail gas in a rectifying tower through a two-stage cryogenic heat exchanger before the MDI production tail gas enters a vacuum unit is carried out; wherein the condensation temperature in a first-stage cryogenic heat exchanger is 0 to 15 DEG C, and the first-stage cryogenic heat exchanger is used for cooling most MDI into solid; and the condensation temperature in a second-stage cryogenic heat exchanger is -45 to -25 DEG C, and the second-stage cryogenic heat exchanger is used for cooling most PI and a part of chlorobenzene into solids. After cryogenic treatment, the content of MDI, PI and other light components in the tail gas can be reduced to an acceptable range for normal operation of a vacuum unit, and MDI with very high purity can be recovered after first-stage cryogenic treatment and is directly doped into an isocyanate product.In addition, PI and chlorobenzene can be reused in the front-stage MDI preparation process after being treated, resource comprehensive utilization maximization is achieved, and three wastes are prevented from being generated to pollute the environment.

Description

technical field [0001] The invention relates to a method for treating tail gas, in particular to a method for treating tail gas produced by MDI. Background technique [0002] At present, the MDI (diphenylmethane diisocyanate) production process mainly adopts phosgenation reaction. First, polyamine (DAM) is generated through condensation reaction of aniline and formaldehyde under the action of an acid catalyst. Then, DAM is reacted with phosgene in a solvent. The reaction generates crude MDI. At the same time, the crude MDI is prepared by rectification to obtain pure MDI and polymerized MDI. Finally, pure MDI is obtained according to a certain recovery ratio to obtain different MDI isomers (including 4,4'-MDI, 2, 4'-MDI and 2,2'-MDI and their mixture). The MDI reaction mechanism in the industrial production process can be expressed as follows: [0003] Step 1: Salt Formation Reaction [0004] Aniline + hydrochloric acid → aniline hydrochloride [0005] The second step: co...

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Problems solved by technology

However, the condenser medium in the prior art is usually warm water at 30°C. Since the boiling points of MDI and PI decrease with the decrease of pressure, in the high vacuum environment inside the rectification tower, the condensation temperature of 30°C cannot make all the light components When all the components are condensed, it can only cool most of the MDI and a very small amount of PI, and the residual MDI and PI in the exhaust gas will still have a great impact on the vacuum unit, causing the unit to frequently block and lock.

In addition, chlorobenzene containing MDI and PI will form chlorobenzene waste liquid after condensation at the outlet of the vacuum unit, which is difficult to treat as waste liquid. At present, there is no comprehensive recovery and utilization method for MDI, PI, and chlorobenzene in the tail gas.

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