DE4039177A1 - Quenching of acrylonitrile - using catalytic cracking heavy cycle oil or residual oil as polymerisation inhibitor - Google Patents

Abstract

Gases contg. acrylonitrile (ACN) produced by ammoxidation of propylene and/or propane are quenched in the presence of a (polymerisation) inhibitor (I) comprising a heavy cycle oil or residual oil from the catalytic cracking of high-boiling petroleum distillates. (I) is added to the bottoms stream from the distn. of the ACN-contg. quench water before this stream is recycled to the quenching process. The amt. added is 0.001-0.15% of the ACN content of the quench water. ADVANTAGE - (I) dissolves readily in the bottoms stream and is distributed uniformly throughout the system when the bottom stream is recycle

Description

The invention relates to a method for quenching acrylnitrilhaltiger Gases in the acrylonitrile process at a decrease the acrylonitrile losses by reducing the proportion the organic waste products in the processing stages of a after the ammoxidation of propene and / or propane Acrylnitrilsynthese.

For workup, the hot reactor gases, in addition to Acrylonitrile and hydrogen cyanide also other organic Reaction products and unreacted starting components contained, cooled in the heat exchanger and then in directed the Quenchkolonnen.

In countercurrent cold with sulfuric acid added Water on the still hot reaction gases.

Ammonia components are neutralized and washed out removed from the reaction mixture.

The overhead exiting from the quench columns reaction mixture then passes into the absorber. One as strong water designated product stream leaves the absorber column in the lower floor area. This product stream contains all Products of the process (water, acrylonitrile, hydrocyanic acid, organic by-products). In this predominantly aqueous Medium, the acrylonitrile is about 5 percent in mass fractions.

The strong water is after leaving the absorber column over Head fed to the recovery column. In this Column are the target products of the process (acrylonitrile and hydrocyanic acid) from the water and from the higher-boiling ones separated by organic by-products. The top product of Recovery column contains about 80 percent acrylonitrile and 15 percent hydrogen cyanide. The rest is water.

In the subsequent work-up and purification steps in the main and drying column and in the fine distillation on the one hand, hydrocyanic acid is completely eliminated from the Acrylonitrile separated and on the other hand, the water content of Acrylonitrile to <0.5 volume percentages.  

This work-up and purification technology works with losses, since the acrylonitrile after leaving the Reactor, especially during quenching, thermally and is heavily loaded chemically. This is also testified by the relatively high waste accumulation in the bottom of the quench columns, their organic content mainly on oxidation and Attributed to polymerization of the acrylonitrile is. These burden the acrylonitrile yield or raw material supply ongoing radical-initiated reactions in acrylonitrile-saturated hydrous product streams could not be cost-effective with synthetic inhibitors be suppressed.

DD-AP 2 83 728 also disclosed the use of heavy cycle oil of catalytic cracking as an inhibitor in the fine distillation stage of acrylonitrile production. In contrast to the product streams of quenching and the Starkwasserkreislaufs is the fine distillation water-saturated product streams of acrylonitrile, in which o. g. heavy circulatory and residual oils the catalytic cracking of petroleum residues effortlessly can be solved and distributed.

The inhibition was thus directed to parts of the plant Acrylonitrile cleaning in which the acrylonitrile content is high the majority of organic by-products are already present is separated and the water content in the product streams only a few percent.

Those caused by radically initiated reactions Acrylonitrile losses in mainly aqueous work-up and purification media from the quenching of the reaction gases until the separation of the organic by-products and the water in the recovery column, on the other hand with the means of the known prior art can not be suppressed inexpensively.

An immediate addition of cycle oil to product cycles of quench and strong water is for reasons  the water insolubility of the circulatory oil without success remained. Under the conditions mentioned is an inhibitor distribution in the water-containing cycles of Not to realize quenching. They form on the cold parts of the heat exchangers insulating mineral oil layers, which disturb the heat exchange process and to Can cause blockages.

The invention has the object of a method for quenching To create acrylonitrile-containing gases by the radically initiated reactions caused acrylonitrile losses during quenching in predominantly acrylonitrile-saturated aqueous Product flows, lower. In particular, those in the Swamps of Quenchkolonnen accumulating Abproduktmengen by the amount of polymerization and oxidation reactions of the acrylonitrile-derived products to reduce. The yield based on the starting products of acrylonitrile is to increase after working through.

According to the invention the object has been achieved in that the Swamp-backflow products of fine distillation with per se known heavy cycle oil of the catalytic cleavage doped by heavy petroleum distillates and in the processing stages the quenching be conducted. This will taking advantage of the outstanding solution-enabling Properties of the bottom products of fine distillation for Circulatory oil in predominantly aqueous product streams a even distribution of the cycle oil in the system parts Ensures quenching.

The achieved reduction of the waste product quantities correlates directly with the circulating oil quantity added to the bottom product. Based on the acrylonitrile content of the predominantly aqueous quenching product stream is the advantageous Dosage of circulatory oil 0.001-0.15 percent in Parts by mass wherein the acrylonitrile to be inhibited in the product flow approximately from the produced  Acrylonitrile is dependent.

This results in a significant reduction in the work-up and recovery losses in these parts of the plant.

Even if for technological reasons (external distillation the bottom product of fine cleaning) a substantial smaller but higher doped bottoms product fraction for the return to the Quenchwasserkreislauf available stands, is a complete distribution and effect guaranteed.

Another advantage arises from the fact that or residual oils outstanding on and dissolving properties towards agglomerates, polymers and others preferably in bottom reboiler resulting deposits has.

So could the effective running time of these plant parts since the use of circulatory and residual oils approximately be extended ten times.

It is also possible, for. B. by distillation or deasphalting the circulatory or residual oils lighter To produce fractions whose characteristics are included in the and residual oil are given limits.

These fractions, like the untreated circulatory and residual oils are also used successfully.

embodiment

The invention is based on examples and two Drawings are explained in more detail.

Fig. 1 shows a bar graph of the residue formation in acrylonitrile-saturated water,

Fig. 2 shows a bar graph of residue formation in water-saturated ACN.

Example 1

To radially initiated oxidation and polymerization reactions of acrylonitrile primarily in the aqueous Circuits to avoid quenching was Catalytic cracking residue oil from the vacuum distillate  with the following characteristics:

Start of boiling (K) 593 Viscosity (323 K) (mm² · sec ^-1 ) 215 Density (g / cm³) 1.13 Total aromatics content (mass fractions in%) 82 Share of 3 u. 4-ring aromatics (mass fractions in%) 86

introduced into the process. Before being fed into the acrylonitrile-saturated aqueous systems became the residual oils dissolved in acrylonitrile.

The cycle oil concentration was adjusted according to column 1 with 0, column 2 with 420 ppm, column 3 with 1250 ppm, column 4 with 1200 ppm and column 5 with 1125 ppm based on the amount of ACN in a test mixture. This test mixture consisting of 25 parts of ACN and 200 parts of H₂O was mixed with 0.2 parts destabilizer (H₂O₂, 25%) and boiled at reflux.

The test mixture corresponds in its composition the quench and strong water circuits.

Subsequently, the evaporation residue was determined.

It contains all substances <150 ° C (at atm. Pressure).

The experimental conditions (columns 1-4 ) were chosen (constant duration, same type of impurities) that forms a Auswiebare amount of high-boiling by-products without adding residual oil (column 1 ). For this purpose, acrylonitrile-saturated water which has been doped with a very small amount of hydrogen peroxide (columns 1-4 ) and, on the other hand, strongly hydrocyanic-containing acrylonitrile-saturated water are used (column 5 ).

The experiment was carried out in each case with and without residual oil doping carried out.

The resulting evaporation residues were compared.

As expected, the test mixture loaded without residual oil doping had the highest amount of evaporation residue (column 1 ).

Fig. 1 shows a comparison of the results of the evaporation residue determination.

The highest amount of evaporation residue was set equal to 100%.

The following relationships can be deduced from the results derived:

• - Despite ionic reaction conditions in strongly water-containing Systems use residual oil concentrations of 500-1000 ppm in quench cycles of ACN workup a strong reduction in residue formation reached.
• - With residual oil dopants of 1000 ppm, the residue formation due to polymerization and oxidation is reduced by an average of 2/3. However, in contrast to the organic system, the experimental results (according to columns 2-4 ) show a strong dispersion of the results.

example 2

The distribution of mineral oil fractions (residual oils) in predominantly aqueous cycles of the ACN work-up realized by solution mediation (ACN) must be evaluated. ( Figure 2) It is believed that the distribution of residual oil and thus also the inhibitor drug components correlates directly with the results of loss reduction (evaporation test) in ACN workup.

A test mixture consisting of 50 parts ACN and 10 Parts H₂O was with 0.2 parts destabilizer (H₂O₂, 25 %) and boiled at reflux.

This test mixture corresponds in its composition the product cycles of the water-saturated Rohacrylnitrils before fine cleaning.

Subsequently, the evaporation residue was determined again.  

The experimental conditions were realized analogously to Example 1. FIG. 2 shows the weighable amounts of high-boiling waste products from water-saturated crude acrylonitrile (columns 6-9 ) and from strongly hydrocyanic acid-containing crude acrylonitrile (column 10 ). The test mixtures doped with circulating oil in different concentrations (column 7 = 230 ppm, column 8 = 625 ppm, column 9 = 1300 ppm, column 10 = 775 ppm) were compared in undoped water-saturated crude acrylonitrile mixture (column 6 ). All concentration data relate to the acrylonitrile content of the test mixtures. The compositions of the test mixtures used in the example are typical of product streams after the recovery column in the work-up and purification process of ACN.

Based on the results shown in the example the following conclusions can be drawn:

• As shown by the bar graph of Figure 2, the results of reducing the residue formation in the water-saturated ACN can be transferred to systems consisting of acrylonitrile-saturated water.
• - The doped residual oil quantity correlates directly with the achieved reduction in the formation of residues, wherein high residual oil concentrations the polymerization and Suppress oxidation the strongest.

example 3

In a large-scale plant was the bottom product of Fine cleaning before its return to the circuits of the Quenching with 0.05 percent (in parts by weight) of residual oil doped based on the acrylonitrile content.

Without doping of recycled bottoms product shares the loss of ACN was in quenching process approximately from the difference in the reactor outlet contained ACN concentration and the actual produced Calculate ACN quantity after fine cleaning on average 8.16 percent.

After doping the bottom products with residual oil  at the specified height with subsequent reduction of the solvent-mediated ACN swamps into the circuits the quenching reduced the ACN loss average during the three-month observation period 3.5 percent.

The proportion of oxidizable and polymerizable matter was in the total of even higher-boiling by-products the ACN synthesis and ammonium sulfate existing Abproduktmengen drastically reduced.

All in all, the daily quantities of waste produced could be used be halved.

The ones obtained in this large-scale trial in a production plant Results are in good agreement with the statement (Example 1) agree that with sufficient doping and distribution of Active substance of the waste product from oxidizable and polymerizable compounds can be lowered to 2/3.

Claims (1)

Process for the quenching of acrylonitrile-containing gases in the acrylonitrile process by ammoxidation of propene and / or propane formed, acrylonitrile-containing reaction mixtures by inhibition with heavy cycle or residual oil of the catalytic cleavage of high-boiling petroleum distillates alone or in combination with known synthetic inhibitors, characterized in that the per se known inhibiting organic ingredients of the heavy cycle oils the acrylonitrile-containing bottom products of the fine cleaning before their return to the workup process in amounts of 0.001-0.15%, based on the acrylonitrile content of the predominantly aqueous product streams of quenching, the amount of approximately equivalent to the produced finely purified acrylonitrile added be, with a return of the treated with circulating or residual oil bottom product of fine cleaning in the workup stages of the quenching takes place.