Azeotropic distillation acrylonitrile

In a process for the production of acrylic fibres by the emulsion polymerisation of acrylonitrile, the unreacted monomer is recovered from water by distillation. Acrylonitrile forms an azeotrope with water and the overhead product from the column contain around 5 mol per cent water. The overheads are condensed and the recovered acrylonitrile separated from the water in a decanter. The decanter operating temperature will be 20 °C. 

An important property for process design is the limited reciprocal solubility of acrylonitrile in water. Table 11.3 shows the dependence against temperature. The solubility of AN in water is around 7% w/w, while water in AN about 3% at 20 °C. Therefore, liquid-liquid separation by decantation can be combined advantageously with azeotropic distillation for acrylonitrile purification. 

CaH2 treatment of MeCN has been recommended [338], but it does not remove traces of aromatic hydrocarbons and is a surprisingly ineffective drying agent in MeCN [355]. For removal of acrylonitrile, which boils 4 C lower than MeCN, treatment with NaH [357] or azeotropic distillation with ethanol [358] has been recommended. 

Today the most cost-effective processes are those based on propylene as the starting material. There are three major variations of propylene processes, the Distillers process [21-23], the Sohio process [24], and the DuPont process [25,26]. All three processes are based on the ammonoxidation of propylene. The Distillers process is carried out in two stages. In the first, propylene is oxidized in air to form acrolein and water. These intermediate products are allowed to react in the second stage with ammonia in the presence of molybdenum oxide and air to form crude acrylonitrile. The pure monomer is recovered by a series of azeotropic distillations. The Sohio process is carried out in just one stage. Ammonoxidation of propylene takes place in air at 2-3 atmospheric pressure and 425-510°C. With catalysts, such as concentrated bismuth phosphomolybdate or other oxides of molybdenum and cobalt, the reaction takes place with over 50% yield in a reaction time of only about 15 s. In the DuPont version of this process, the ammonoxidation is brought about with nitric oxide at 500°C using silver on silica catalyst. The chemistry of acrylonitrile monomer has been reviewed by a number of authors [27-30]. 

For dilute solutions, both gas chromatography (66) and polarography (67) are rapid, sensitive, and precise. Small amounts of acrylonitrile can be separated from other components by azeotropic distillation with alcohols, followed hy polaro-graphic (67,68) or chromatographic (69,70) analysis. 

The applicability of the azeotropic-distillation procedure described by Daues and Hamner [2] for separating acrylonitrile monomer from interfering impurities before polarography was examined. In this procedure the aqueous sample is distilled in the presence of a mixture of methanol and aqueous sulfuric acid. The methanol-acrylonitrile azeotrope, boiling at 61.4 °C, distils first from this mixture and thus the acrylonitrile is recovered in the initial distillate. 

The azeotropic-distillation procedure was tested by using a synthetic solution of acrylonitrile in water that had been shown by direct polarographic analysis to contain 0.75 ppm of acrylonitrile. A measured volume (500 ml) of this solution, i.e., 0.375 mg of acrylonitrile, together with 5 ml of concentrated sulfuric acid, 25 ml of methanol and 0.1 g of 2,4-dinitrophenyl-hydrazine (to destroy carbonyl compounds)... 

The methanol azeotropic-distillation procedure was also applied to a synthetic solution of acrylonitrile in 6% hydrochloric acid extractant. Polarographic analysis of the methanol-acrylonitrile azeotrope was not possible, however, owing to the presence of an appreciable amount of free acid originating from the hydrochloric acid extractant, in the distillates, which interfered in the polarography of acrylonitrile. In a further experiment, a 6% hydrochloric acid solution of 47.3 ppm of acrylonitrile was neutralised by the addition of a small excess of solid calcium oxide. Methanol and sulfuric acid were added and the azeotropic distillation continued as before. It can be seen from Table 11.1 (sample B) that under these conditions more than 90% of the added amount of acrylonitrile was recovered in the first 8 ml of methanol distillate. A preliminary neutralisation with lime was incorporated, therefore, into the procedure for determining acrylonitrile in 6% hydrochloric acid extraction liquids. This procedure should also be applicable to the determination of acrylonitrile in the 3% aqueous acetic acid extractant recommended by the Food and Drug Administration (FDA) [3]. 

The results in Table 11.1 (samples C and D) show that above 80% of the added amount of acrylonitrile was recovered in the methanol distillate when the azeotropic-distillation procedure was applied to 5% sodium carbonate extractants containing up to 72.4 ppm of acrylonitrile monomer. The recoveries of acrylonitrile in these experiments are lower than those obtained for the distilled water and 6% hydrochloric acid extractants. This may be due to an increased amount of hydrolysis of acrylonitrile... 

Sample Composition of synthetic test solution used for azeotropic distillation Volume of test solution used for azeotropic distillation, ml Acrylonitrile added, pg (X) Weight of acrylonitrile in test solution, pg Recovery of acrylonitrile in first two fractions obtained by azeotropic distillation Y X 100/X, % w/v... 

The possibility of aqueous extraction of acrylonitrile from the liquid paraffin was examined. Various synthetic solutions of acrylonitrile in liquid paraffin were extracted with two 250 ml portions of distilled water. The distilled water extracts were filtered into a one litre flask and an azeotropic distillation with methanol made as described previously. The recoveries of acrylonitrile obtained by this procedure for liquid-paraffin extractants containing up to 538 ppm of acrylonitrile are shown in Table 11.2. In all these mixtures, the recovery of acrylonitrile in the first 8 ml of the methanol azeotrope was within 5% of the amount known to be present. Duplicate recoveries obtained in the azeotropic-distillation procedure are reasonably reproducible. 

Procedures, based on these principles, involving extraction with water and azeotropic distillation with methanol before polarography should also be applicable to the -heptane extractant recommended by the FDA [3] and also might be useful for the determination of acrylonitrile in the FDA vegetable oil extractants, provided that these can be successfully extracted with water. Trials have not been made on these particular extractants. 

Acrylonitrile content of the synthetic light liquid paraffin sample solution for extraction with water Weight of Ught liquid paraffin sample extracted with 2 x250 ml of water Weight of acrylonitrile present in light liquid paraffin test solution Weight of acrylonitrile recovered in fractions of azeotropic distillation, pg Mean recovery of acrylonitrile in first two fractions obtained by azeotropic distillation... 

Provided that a suitable sample size is taken for analysis, the azeotropic distillation-polarographic procedure can be used for determining acrylonitrile in extractants in concentrations down to 1 ppm or a little lower. Thus, it is seen from Table 11.1 that approximately 90% of the added amounts of acrylonitrile is recovered when the procedure is applied to 500 ml of a 0.75 ppm solution of acrylonitrile in the distilled-water extractant. The method can be used for achieving a similar level of sensitivity in the determination of acrylonitrile in the other aqueous alcoholic or oily extractants for plastics recommended by the British Plastics Federation [4] and the FDA [3]. This level of sensitivity is quite adequate for the examination of extractants that have been brought into contact with styrene-acrylonitrile copolymers under the British Plastics Federation and FDA extractability-test conditions. 

Most, if not all, of the acetonitrile that was produced commercially in the United States in 1995 was isolated as a by-product from the manufacture of acrylonitrile by propylene ammoxidation. The amount of acetonitrile produced in an acrylonitrile plant depends on the ammoxidation catalyst that is used, but the ratio of acetonitrile acrylonitrile usually is ca 2—3 100. The acetonitrile is recovered as the water azeotrope, dried, and purified by distillation (28). U.S. capacity (1994) is ca 23,000 t/yr. 

The bottom product from column (G) passes to the hydroextractive distillation column (H). The water feed rate to column (H) is five times that of the bottom product flow from column (G). It may be assumed that the acetonitrile and other by-products are discharged as bottom product from column (H) and discarded. The overhead product from column (H), consisting of the acrylonitrile water azeotrope, is condensed and passed to a separator. The lower aqueous layer is returned to column (H). 

Most, if not all, of the acetonitrile produced commercially in the United States recently was isolated as a by-product from the manufacture of acrylonitrile by propylene ammoxidation. The acetonitrile is recovered as the water azeotrope, dried, and purified by distillation. 

Table 11.2 presents fundamental physical properties for the key components implied in separations. The difference in the boiling points favors the separation, except of acrylonitrile and acetonitrile. The differences in the freezing point are also sensitive, but they did not justify the investment in a separation by crystallization. It remains that distillation-based separation methods should be tried in the first place. However, the formation of azeotropes of components with water will present difficulties. 

The water-free acrylonitrile is obtained as bottoms in the column C-4. Water leaves in top as a binary azeotrope, followed by decantation and reflux of organic phase. The water phase also removes some light impurities. The final product meets closely the specifications indicated in Table 11.1. Since heavy impurities inevitably appear, a final vacuum distillation of acrylonitrile is performed in practice before shipping. 

The separation of acetonitrile from acetonitrile by extractive distillation with water can be done in a more efficient two-column heat integrated setup. The separation of acrylonitrile from water, which is hindered by the existence of an azeotrope, can actually take advantage of the large immiscibility gap. Valuable byproducts, such as HCN and acetonitrile can be efficiently separated. Chemical conversion can solve the separation of difficult impurities, such as acroleine. 

Chapter 11 Acrylonitrile by Ammoxidation of Propene illustrates the synthesis of a flowsheet in which a difficult separation problem dominates. In addition, large energy consumption of both low- and high-temperature utilities is required. Various separation methods are involved from simple flash and gas absorption to extractive distillation for splitting azeotropic mixtures. The problem is tackled by an accurate thermodynamic analysis. Important energy saving can be detected. 

Vinyl acetate-ethyl acetate Propane-propylene Ethanol-isopropanol Hydrochloric acid-water Nitric acid-water Close-boiling Close-boihng Close-boihng Maximum-boiling azeotrope Maximum-boiling azeotrope Phenol, aromatics Acrylonitrile Methyl benzoate Sulfuric acid, calcium chloride for salt process Sulfuric acid, magnesium nitrate for salt process Alternative to simple distillation Alternative to simple distillation, adsorption Alternative to simple distillation Sulfuric acid process rehes heavily on boundary curvature Sulfuric acid process rehes heavily on boundary curvature... 

All Michael reactions are performed in water at a concentration of up to 50 wt% of amine, to which 2.5-4 moles of acrylonitrile per mole of primary amine are added. In the double cyanoethylation, the first acrylonitrile molecule reacts at room temperature, while the reaction temperature has to be raised to 80°C to accomplish the double Michael adduct. The reaction time required to achieve complete conversion increases with generation from 1 h for DAB-JenAfter completion of the reaction the excess acrylonitrile is removed by distillation, making use of the water-acrylonitrile azeotrope. 

In 1969 Yamada and coworkers reported the first in a series of investigations on the alkylation of chiral enamines, derived from L-proline esters, with methyl acrylate and acrylonitrile The enamines were prepared under the usual azeotropic conditions but were not distilled since this resulted in cyclization, by nucleophilic attack of the enamine on the ester function, or partial racemization ". Optical yields were found to be... 

Acetonitrile. CH3CN, b.p. 81.6° [1, 7, before a-Acetoxyacrylonitrile]. Aprotic. water-miscible Diels-Alder solvent (1, 239). Supplier of pure solvent. 1, 1110. For polara-graphic use. Moe1 notes that the main impurity is acrylonitrile, which differs from it in b.p. by only 4.2°, and recommends separation by distillation of the ternary and secondary azeotropes which the two liquids form with ethanol and with water. Ethanol (95%) is added to practical acetonitrile and the mixture is distilled through an H. Stedman column of 60-65 theoretical plates. The purified material is suitable also for U V spectroscopy. 

Water can be removed from methanol by a membrane of polyvinyl alcohol cross-linked with polyacrylic acid, with a separation factor of 465.204 A polymeric hydrazone of 2,6-pyridinedialdehyde has been used to dehydrate azeotropes of water with n- and /-propyl alcohol, s- and tort butyl alcohol, and tetrahydrofuran.205 The Clostridium acetobutylicum which is used to produce 1-butanol, is inhibited by it. Pervaporation through a poly(dimethyl-siloxane) membrane filled with cyclodextrins, zeolites, or oleyl alcohol kept the concentration in the broth lower than 1% and removed the inhibition.206 Acetic acid can be dehydrated with separation factors of 807 for poly(4-methyl-l-pentene) grafted with 4-vinylpyridine,207 150 for polyvinyl alcohol cross-linked with glutaraldehyde,208 more than 1300 for a doped polyaniline film (4.1 g/m2h),209 125 for a nylon-polyacrylic acid membrane (5400 g/m2h), and 72 for a polysulfone.210 Pyridine can be dehydrated with a membrane of a copolymer of acrylonitrile and 4-styrenesulfonic acid to give more than 99% pyridine.211 A hydrophobic silicone rubber membrane removes acetone selectively from water. A hydrophilic cross-linked polyvinyl alcohol membrane removes water selectively from acetone. Both are more selective than distillation.212... 

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