DD140879A1 - METHOD FOR SEPARATING AND CLEANING UNSATURATED NITRILE - Google Patents

Abstract

The process relates to the separation and purification of the unsaturated nitriles, in particular of acrylonitrile, methacrylonitrile and butenitrile, which are formed in the ammoxidation of propylene and isobutylene. The invention serves to improve the process with the aim of achieving a higher than the previously customary purity for the end products and, in addition, the isolation of such by-products as the cis-buteno-tetrasilver while largely exploiting the raw materials and intermediates. The essence of the invention consists in particular in the decomposition of the bottom stream of the last fractional distillation in the workup of the reaction products of said ammoxidation by distillation with the exclusion of water or water vapor in two fractions, of which only the Hauptproduktreichere first is returned to the work-up process. From the second, by distillation, e.g. cis-butenitrile obtainable. Furthermore, the entire top product of the column, with the help of which for the last time in the work-up process low-boiling impurities are separated from the main product, subjected to a fractional condensation.

Description

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Process for the separation and purification of unsaturated nitriles ; ; _

IPK: C 07 C 121 - 32

Areas of application of the invention

The process relates to the separation and purification of the unsaturated nitriles, in particular acrylonitrile, methacrylonitrile and butenonitrile (crotonitrile), which are produced as main or by-products of the ammoxidation of propylene and isobutylene.

It is known that these are known n technical solutions

According to the state of the art, z _o B _{0 is} the separation of the acrylonitrile from the higher-boiling impurities and by-products in the work-up of the reaction products of ammoxidation by multistage extractive and fractional distillation, details of which are described in the patent literature or Z ₀ BV "Ulimanns Enc.clopädie der technischen Chemie "(Verlag Weinheim, 1974, Volume 7, page 96) V

According to these data, the bottom of the last column for fractional distillation of the acrylonitrile contains cyanohydrins, higher-boiling impurities, inhibitors and polymers. Moreover, it must inevitably contain a considerable proportion of acrylonitrile ¹

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Because of its content of acrylonitrile, acids and inhibitors, the recycling of this bottoms stream into the work-up process, ie z _o Bv in one of the columns for the purification of the crude Akryl- nitrils, nah'V Die. Repatriation of this. Swamp stream to the quench system at the beginning of the work-up r process HoCV Wu describes in DE OS 25 52 233.

In principle, however, any recirculation and circulation of contaminated product streams in these work-up procedures is associated with the risk of enrichment of by-products and deterioration of end-product quality. can lead to difficulties in such further processing processes that require highly pure starting materials, such as the fiber production from acrylonitrile V The same applies after the ammoxidation of the isobutene for the purification of Methakrylnitrils and its further processing.

The same applies to the last, in the distillation of the main products acrylonitrile or methacrylonitrile separated via the top of the column and contaminated by low-boiling substances Kopfproduktstromö '

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Object of the invention

Accordingly, the aim of the invention is, in the interest of maximum utilization of raw materials, the reduction of material losses and to ensure high quality of the final products, the process for separating and purifying the unsaturated nitriles in the workup of the reaction products of the ammoxidation of propylene and isobutylene to improve and keep the necessary effort as low as possible. "The ITachteile of the known methods should be avoided ·

Explanation of the nature of the invention

The invention is based i.a. on the following unexpected findings of the prior art which are not to be expected:

Despite multiple-step separation and purification operations by extractive and fractional distillation nitrile even with a boiling point = 20 ° above the acrylonitrile are not sufficiently separated from the acrylonitrile in the usual production process for acrylonitrile, so that when recycling the bottom stream of the last column for fractional distillation in the Aufarbeitungsproζeß a highly undesirable accumulation of these substances in the process and thus also a reduction in the quality of the final product acrylonitrile takes place;

- Depending on the composition of the bottom stream of the last fractional distillation, some of the higher-boiling nitriles with justifiable

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The relative expense of recovering it in pure formV The content of the bottom stream of cyanohydrins mentioned in the literature, but unexpectedly low, does not interfere with this; ,

although it is state of the art to separate zY BV acrylonitrile from higher boiling by-products by extractive distillation with water, the separation of the acrylonitrile from the aforementioned nitriles requires the most extensive exclusion of water.

The inventive method is that

- i: 'i, the marsh stream of the last fractionated. Distillation of the acrylonitrile or methacrylonitrile in the working up of the reaction products. Ammoxidation before recycling in the workup process by distillation and optionally additionally separated by fractional JBbden condensation is in a 1V fraction containing mainly acrylonitrile or _{e e} Methakrylnitril and a corresponding boiling point at normal pressure of ^. 90 ⁰ C for_Akrylnitril or ^ i03 ⁰ C for methacrylonitrile possesses, "and in at least one higher-boiling 2 _Ο ^; Fraction which predominantly contains the remaining components of said bottom stream, · wherein the access of water or water vapor is to be avoided and only the Ά · fraction is recirculated in vapor or liquid to a corresponding their degree of purity in the workup process;

 From the 2, fraction of the bottom stream by further fractional continuous or discontinuous distillation under a pressure between 0.3MPa

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and ΛΟ_ Pa "at atmospheric pressure in the range of 105 109 ⁰ C, boiling cis-butenitrile as the distillate, optionally also other of the nitriles contained obtained and removed from the process;

Volatile fractions of the 2 "fraction can also be recycled to the use of their carbon and nitrogen content in the Ammoxidations synthesis reactor or removed from the process and used as a solvent mixture;

the content of the end product acrylonitrile or Methakrylnitrils low-boiling impurities is further reduced in that the entire overhead stream of the column, with the help last low-boiling impurities are separated from the final product before use as reflux or before recycling in the workup process, either to reduce the content low-boiling impurities, in particular carbonyl compounds, subjected to suitable chemical treatment by known means, or by distillation or partial condensation of the vaporous product stream at 77 C, calculated for atmospheric pressure, into a first fraction, containing more of an acrylonitrile, and separated by separate complete condensation of the vapors leaving the first condensation stage into a second fraction enriched in low-boiling impurities " ^:

Only the first fraction enriched in acrylonitrile is used as the reflux for the column;

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the second fraction having an increased content of impurities is recycled to such a position of the work-up process which corresponds to its composition, or only this 2V fraction is subjected to the mentioned treatment to reduce the content of low-boiling impurities, thereafter this fraction is recycled to the workup process or also used as reflux;

If, in the case of removal of the 2V fraction or part of the overhead stream, the residual amount of overhead stream is no longer sufficient to meet the demand for the column to remove the low-boiling impurities, the required proportion of the final purified product will additionally be recycled used. In the case of distillation below or above the normal pressure in the range between 0, 3i $? A and 1CH Pa, the vapor pressure of the substances involved is correspondingly higher or lower than those mentioned above.

The invention is illustrated hereinafter by Examples, which den.Umfang the invention but do not limit ⁰ * ¹

example 1

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From the bottom stream of the last fractional distillation of the acrylonitrile in the workup of the products of propylene ammoxidation was by fractional distillation under a pressure of

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4 V 10 Pa, the majority of the acrylonitrile largely from the above. Separately, acrylonitrile boiling impurities were separated. "In the following, the composition of the bottom stream was referred to as 1V in Table 1 below." For comparison, this table also shows the composition of this bottom stream after recycling without prior separation of the higher-boiling impurities for two different catalysts:

Table 1:

Composition of the bottom stream of the last fractional distillation

component Content in mass% Recycling with boiling at catalyst 1 Better for catalyst 2 7 for example 1 92.8, .94,3 66 "... 77., 6 acrylonitrile 98,4.698,8 0.1 " ₀ ^: 1.6 0,14..O, O propionitrile 0,3.oO, 6 2,3oo4,3 20 ..27, ice-butenitrile 0.2 ,,. 0, I 0,5..I, O 0.6 .. 2, trans-butenitrile = Tracks = 4.7 i 13,26 Other = Tracks

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The table shows the extent to which the higher-boiling impurities can accumulate. Separation of these impurities prior to recycling, on the other hand, reduces the content of buteno-acid nitriles in the bottom stream to Z ₀ B " = 10% of the original value

Example 2

A sample of the bottoms stream already mentioned in Example 1 was fully fractionally distilled to a head temperature of 116 G calculated for normal pressure, using a column with 25 theoretical plates and preventing direct access of water or water vapor to the product receive:

Table 2:

Fractions and constituents obtained from the bottom stream

boiling range ... main ingredients in mass proζ ent (converted to 760 Torr) in C in the distillate the fraction of the use product 75r5 to 76x6 = 99.8 52 76γ6 to 101 * 6 acrylonitrile = 37 3.5 101 _T 6 to 106.2 Methacrylonitrile = 97 8.8 106.2 to 108 ·. , cis-formic acid nitrile = 100 9.5 108 to 116 cis-butenitrile .. ^ 31 7 2 / trans-butene ) acid nitrile ^ 29 ') (Butyronitrile

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This example proves that both acrylonitrile and cis-butenitrile with relatively high purity and yield are separable from the bottom stream

Example 3

The entire overhead stream of the column of the workup ssystems, with the help last low-boiling impurities are separated from the final product acrylonitrile or Methykrylnitril is fractionally condensed using two heat exchangers, wherein at the output of the first heat exchanger, a temperature of 70 C (converted to atmospheric pressure) in the gas stream was set. The condensate of this heat exchanger was called T. ' Fraction removed All other condensed sierbaren ingredients were liquefied with dem.zweiten heat exchanger at 20 to 30 ⁰ C. In this way, the overhead product stream is decomposed to a more carbonylic and hydrocyanic content of carbonyl compounds and hydrogen cyanide · only the first fraction is reused together with the required amount of final grade acrylonitrile as the top back Fraction with an increased content of impurities is returned to a point in the work-up process which corresponds to their composition. The content of the final product acrylonitrile in the said low-boiling impurities could thus be reduced to 1/2 to 1/10 of the original amount

Claims (1)

~ 210 040 Invention application Process for the separation and purification of unsaturated nitriles, in particular of acrylonitrile and methacrylonitrile in the work-up of the reaction products of the ammoxidation of propylene and isobutylene, characterized in that the bottom stream of the last fractional distillation of the acrylonitrile or methacrylonitrile in the work-up by distillation and optionally additionally separated by fractional condensation is in a 1 "fraction containing mainly acrylonitrile or methacrylonitrile and a corresponding boiling. end at normal pressure of -i-90 C for acrylonitrile or ^ 103 ⁰ C for Methakrylnitril possesses, as well as in at least one higher-boiling 2 _O fraction containing predominantly the other constituents of the bottom stream, wherein the access of water or water vapor is to be avoided, only the 1V fraction is attributed to a stage of the workup process that corresponds to its degree of purity, and from the. 2, fraction of the bottom stream by further fractional distillation as distillate cis-butenitrile won: and "taken from the process is, and that the top, product stream of the column, with the help last in the work-up process low-boiling impurities from the end product acrylonitrile or Methacrylonitrile before, use as reflux or before recycling - AA- in the Aufarbextungsprozeß by chemical treatment largely freed of low-boiling impurities or by distillation or fractional condensation of the vapor overhead bei_ / 77 C, calculated for normal pressure j in a 1 · ', on acrylonitrile richer fraction and by separate complete condensation of the leaving the first condensation stage vapors is decomposed into a second fraction enriched in low-boiling impurities, with only the 1Ό 'fraction being used as reflux for the said column, while the 2 <> fraction has undergone the mentioned chemical treatment or returned to a position __ of the work-up process corresponding to its composition, with all the separation operations being performed in the pressure range between 0.3 Pa and ICr Pa. 2o 'method of item 1, characterized in that volatile constituents of the _O 2 fraction of Sumpströmes for the use of their carbon and nitrogen content of the. 2 "fraction is separated and recycled to the synthesis reactor, or used as a solvent mixture" 3V method according to item 1, characterized in that in the case of removal of the _O 2 fraction or a portion of the overhead product stream, if necessary, a portion of the purified final product additionally used as reflux for the column for the separation of the low-boiling impurities,