Modified azeotropic distillation

Unlike the corresponding reactions of organotin compounds 42), the release of water from a trialkylgermoxane and imidazole or benzimidazole does not take place with reasonable speed at the boiling point of benzene. The reactions can be made to proceed by a modified azeotropic distillation, viz. by heating a mixture of the reactants at 150°—170° C and by adding from time to time a few drops of toluene. In this way the water produced is swept away by the solvent vapor and the reaction temperature can be maintained at the required level. 

In the example, the minimum reflux ratio and minimum number of theoretical plates decreased 14- to 33-fold, respectively, when the relative volatiHty increased from 1.1 to 4. Other distillation systems would have different specific reflux ratios and numbers of theoretical plates, but the trend would be the same. As the relative volatiHty approaches unity, distillation separations rapidly become more cosdy in terms of both capital and operating costs. The relative volatiHty can sometimes be improved through the use of an extraneous solvent that modifies the VLE. Binary azeotropic systems are impossible to separate into pure components in a single column, but the azeotrope can often be broken by an extraneous entrainer (see Distillation, A7EOTROPTC AND EXTRACTIVE). 

Several enhanceci distihation-based separation techniques have been developed for close-boihng or low-relative-volatihty systems, and for systems exhibiting azeotropic behavior. All of these special techniques are ultimately based on the same differences in the vapor and liquid compositions as ordinaiy distillation, but, in addition, they rely on some additional mechanism to further modify the vapor-hquid... 

In a 3-I. round-bottomed flask fitted with a 3-ft. indented column to which is attached a condenser set for downward distillation are placed 321 g. (3 moles) of methylaniline, 300 g. of formic acid (85-90 per cent), and 1800 cc. of toluene (Note 1). The solution is distilled slowly. As long as the azeotrope containing water is present, the temperature of the vapor is 87-88° when the water has been removed, the temperature rises to 108-110° (Note 2). The distillation is continued until approximately 1500 cc. of toluene has been collected (five to six hours). The residue is then transferred to a modified Claisen flask (Org. Syn. Coll. Vol. 1, 125) and distilled in vacuo, the portion boiling at 114-121° at 8 mm. being collected. This has a freezing point of 13.6-13.7° d i-5S3 I SSS- The yield is 380-393 g. (93-97 per cent of the theoretical amount). This product is satisfactory for the preparation of aldehydes (p. 11). Upon redistillation it boils at 117-121° at 8 mm., 130-132° at 22 mm. The freezing point and refractive index are unchanged. 

The extractive distillation profits from the capacity of an entrainer (solvent) to modify selectively the relative volatility of species. Normally, the entrainer is the highest boiler, while the component to be separated becomes heavier, being carried out in bottoms. For this reason, this operation may be regarded as an extractive absorption. Extractive distillation can be used for separating both zeo-tropic and azeotropic mixtures. The entrainer is fed near the top for a zeotropic mixture or a minimum-boiling azeotrope, or mixed with the feed for a maximumboiling azeotrope. The separation sequence normally has two columns, for extraction and solvent recovery [5]. 

Apparatus—Conhnued automatic water trap for azeotropic distillation, 23, 38, 25, 42, 46 bain marie ware, 25, 72, 97 Claisen flask modified with column, 22, 11... 

The Modified van Laar Equations can represent vapor-liquid and liquid-liquid equilibria. Accordingly, they can be used to predict three-phase equilibria when conditions allow two liquid phases as well as a vapor phase to exist. This might occur on the trays in the distillation column or at the condenser and accumulator for the overhead product from the azeotropic distillation column. 

Acetonylacetone, CHjCOCHiCHjCOCHi, is cyclized to 2,5-dimethyl-pyrrole by heating to 100° with ammonium carbonate. The yield is 86%. This reaction has been modified and extended to the syntheses of N-alkyl-2,5 dimethylpyrroles by the substitution of amines for ammonia and removal of water by azeotropic distillation with benzene ... 

Cason and Walba obtained the highest yields in the modified procedure when excess acid was used and the reaction conducted in refluxing CCI4., with removal of water by azeotropic distillation. They were able to obtain tridccyl bromide from myristic acid in 90% yield. 

Ci2-Cig olefins are prepared by a modified Ziegler a-olefin process based on ethylene and triethylaluminum. The process consists of five steps buildup, displacement, separation, alkylation, and recycle. The major chemical and economic problems encountered are wide molecular weight distribution of the products and incomplete recovery of triethylaluminum. Catalysts consisting of alkyl-aluminums and colloidal nickel are needed for the alkylation-displacement steps however, the omount of nickel has to be very low because in the other process steps, nickel favors side reactions. The yield of a-olefins in the C 2" i8 creased by using coordination compounds of triethylaluminum with a Lewis base followed by azeotropic distillation. In the Chlorex process, (bis-a-chloroethyl) ether is used because of easy availability and low cost. 

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