Acetonitrile-water azeotrope

The resin is extracted with 200 mL of refluxing solvent for at least 12 h. A 1 1 mixture of methanol and dichloromethane is an excellent eluent. However, the high concentration of organically bound chlorine makes it a poor choice, if the resin is to be used for analyses of chlorinated hydrocarbons. In that case the acetonitrile-water azeotrope (16.3% water) should be used despite lower recoveries of aliphatic hydrocarbons measured with surrogate standard additions. After cooling, extraction is resumed with fresh solvent for another 12 h period. 

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. 

Axial flow pumps, 134, 136, 140 applicafion range, 150 Azeotrope separation, 387,388,420-426 Azeotropic distillation, 420-426 acetonitrile/water separation, 422 commercial examples, 421-424 design method, 424 ethanol/water/benzene process, 424 n-heptane/toluene/MEK process, 424 vapor-liquid equilibrium data, 421, 423, 425,426... 

The selection of acetonitrile versus methyl alcohol has several considerations even though their flammabilities and toxicities are relatively close. The advantages of using acetonitrile over methanol are (1) its lower UV absorbance cut-off. (2) its lower viscosity, (3) and its smaller viscosity dependence on temperature. The advantages of using methanol over acetonitrile are (1) its lower cost and lower cost fluctuation in the market place, and (2) its ease to recycle with water as a co-solvent (e.g. in reversed-phase chromatography) since methanol-water does not have an azeotrope as does acetonitrile-water. At a boiling point of 76.5°C, the azeotrope composition of acetonitrile-water is 83.7 16.3 [78]. 

The separation of the azeotrope acetonitrile/water can be accomplished without an entrainer as shown in Figure 13.26(a). Two columns are required, operating at two different pressures. A single column suffices if trichloroethylene is added as an entrainer, shown in Figure 13.26(b). 

In specific cases when the vapor-liquid equilibrium favors distillation at the side of the organic component of an azeotrope and a high purity of the organic component is specified the membrane system may be used just to split the azeotrope. For the separation of the system acetonitrile-water a hybrid system as shown in Fig. 3.19 may be economically advantageous. Here the membrane system is used to cross the azeotropic point the partially dehydrated vapor enters the second column in which final dehydration is effected. Again it is necessary to determine the economical optimum between the size of both columns, the energy consumption of the first one and the volume of the recycle stream from the second column at one side, and the size of the membrane system and its outlet concentration on the other side. 

The two-pressure system is also used for the separation of acetonitrile-water, tetrahydrofuran-water, methanol-MEK, and methanol-acetone tFrank. 19971. In the latter application the second column is at 200 torr. Realize that these applications are rare. For most azeotropic systems the shift in the azeotrope with pressure is small, and use of the system shown in Figure 8-6 will involve a very large recycle stream This causes the first column to be rather large, and costs become excessive. 

The vapor, enriched with the more volatile component, flows upwards, whereas the condensed fluid enriched with the less volatile component flows downwards. The efficiency of the column is increased by increasing number of plates. By fractional distillation (Figure 2), it is possible to recover solvents of chromatographic grade, such as acetonitrile or methanol obtained from an azeotropic mixture of methanol-water or acetonitrile-water. 

Several studies have been published on the use of special stationary phases (using packed GC columns) or special types of deactivated retention gaps [11,12]. Among these, a series of papers by Goosens et al. [13-15] on the use of a carbo-wax-deactivated retention gap to transfer acetonitrile-water eluents from the LC to the GC part of the system, merit attention. When using an on-column interface and a solvent vapor exit (SVE), up to 200 Xl of aqueous-organic eluent could be introduced, provided that the water content of the eluent did not exceed that of the azeotropic mixture (16 vol%). Otherwise, water will be left in the retention gap after evaporation of the azeotropic mixture and will mar the analysis. In order... 

Isoprene [78-79-5] (2-methyl-1,3-butadiene) is a colorless, volatile Hquid that is soluble in most hydrocarbons but is practically insoluble in water. Isoprene forms binary azeotropes with water, methanol, methylamine, acetonitrile, methyl formate, bromoethane, ethyl alcohol, methyl sulfide, acetone, propylene oxide, ethyl formate, isopropyl nitrate, methyla1 (dimethoxymethane), ethyl ether, and / -pentane. Ternary azeotropes form with water—acetone, water—acetonitrile, and methyl formate—ethyl bromide (8). Typical properties of isoprene are Hsted in Table 1. 

Dilution of urine with acetonitrile, azeotropic distillation for water removal, evaporation of solvent, redissolution in acetone and derivatization using pentafluorobenzyl bromide. 

Ref 3a). Miscible with w, ale eth. Can be prepd by dehydration of acetamide or by other methods. Used as a solvent for many org compdsfamong them RDX, HMX, etc) and as a starting material for the prepo of some org compds. Its toxicity and fire hazard are discussed in Ref 6. The expl hazard is great when acetonitrile is exposed to heat, flame or cnem reactions with oxidizers. It forms an azeotrope with water... 

Samples were eluted in the reverse direction by using the Milton-Roy pump with the pulse dampener removed. The eluant flow (50-75 mL/min at 200-300 lb/in.2) was monitored at 254 nm by using an Altex 153 detector with a biochemical flow cell. Elution with each solvent was continued until the detector response returned to base line. All columns were eluted with acetonitrile this solvent was preceded by 4.5 M NaCl/0.04 M HC1 and 0.04 M HC1 elutions on the MP-1 column and by 4.5 M NaCl and distilled water elutions on the MP-50 column. The aqueous column effluents were adjusted to pH 2 (MP-1) or pH 11 (MP-50) and then extracted three times with dichloromethane. The acetonitrile column effluents were saturated with NaCl to separate the water, which was extracted twice more with acetonitrile. Fifty percent aliquots of the processed organic solvents from each respective column were concentrated in Kudema-Danish evaporators to a final volume of about 10 mL (any remaining water was removed as the low-boiling azeotrope in the process) to give 25,000 1... 

The extract is pumped from the bottom of D-l to a stripper D-2 with 35 trays. The stripped solvent is cooled with water and returned to D-l. An isoprene-acetonitrile azeotrope goes overhead, condenses, and is partly returned as top tray reflux. The net overhead proceeds to an extract wash column D-3 with 20 trays where the solvent is recovered by countercurrent washing with water. The overhead from D-3 is the finished product isoprene. The bottoms is combined with the bottoms from the raffinate wash column D-4 (20 trays) and sent to the solvent recovery column D-5 with 15 trays. 

Two feasible methods for removal of as much water as desired from the azeotrope are depicted on Figure 13.27. The dual pressure process takes advantage of the fact that the azeotropic composition is shifted by change of pressure operations at 100 and 760Torr result in the desired concentration of the mixture. In the other method, trichlorethylene serves as an entrainer for the water. A ternary azeotrope is formed that separates into two phases upon condensation. The aqueous layer is rejected, and the solvent layer is recycled to the tower. For economic reasons, some processing beyond that shown will be necessary since the aqueous layer contains some acetonitrile that is worth recovering or may be regarded as a pollutant. 

Figure 13.27. Separation of the azeotropic mixture of acetonitrile and water which contains approximately 69 mol % or 79.3 wt % of acetonitrile. (Pratt, Countercurrent Separation Processes, Elsevier, New York, 1967, pp. 194, 497). (a) A dual pressure process with the first column at 100 Torr and the second at 760 Torr. (b) Process employing trichlorethylene as entrainer which carries over the water in a ternary azeotrope that in turn separates into two phases upon condensation.

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