Azeotropic distillation, recycling acetic acid

Crude vinyl acetate is separated from acetic acid and water in an azeotropic distillation system. Acetic acid is recycled to the acetic acid vaporizer and the vinyl acetate product is separated from other by-products in a two-column recovery section. Light ends are removed in the first column followed by a heavy ends in the final column. The light ends, primarily methyl acetate, and the heavy ends, mostly ethyl acetate and acetaldehyde, are incinerated. The vinyl acetate product from the overhead of the heavy ends column is cooled and sent to storage. 

Most of the energy is consumed in the distillation section, namely for VAM recovery and purification. The reboiler duty for the azeotropic distillation of VAM is particularly high, of about 30 MW. It can be observed that this is due to the large recycle of VAM necessary to carry out the water formed by reaction (3 mole VAM per mol water). Thus, any measure is welcome that can reduce the water content in the crude VAM/acetic acid mixture. Figure 10.8 shows an ingenious method known as gas dehydration [1]. The reactor outlet, cooled up to the dew point,... 

Step 1. For this process we must be able to set the production rate of vinyl acetate while minimizing yield losses to carbon dioxide. During the lifetime of the catalyst charge, catalyst activity decreases and the control system must operate under these different conditions. To maintain safe operating conditions, the oxygen concentration in the gas loop must remain outside the explosivity region for ethylene. The azeotropic distillation column must produce an overhead product with essentially no acetic acid and a bottoms product with no vinyl acetate. The absorber must recover essentially all of the vinyl acetate, water, and acetic acid from the gas recycle loop to prevent yield losses in the CCf removal system and purge,... 

Step 5. The azeotropic distillation column does not produce the final salable vinyl acetate product. Its primary role is to recover and recycle unreacted acetic acid and to remove from the process all of the vinyl acetate and water produced. So we want little acetic acid in the overhead because this represents a yield loss. Also, the bottoms stream should contain no vinyl acetate since it polymerizes and fouls the heat-exchange equipment at the elevated temperatures of the column base and the vaporizer. Hence we have two control objectives base vinyl acetate and top acetic acid compositions. And we have two manipula-... 

Donald F. Othmer while at Eastman Kodak during the 1920 s experimented using salts to concentrate acetic acid (14). He also developed an industrial process for distilling acetone from its azeotrope with methanol by passing a concentrated calcium chloride brine down the rectification column (15). Pure acetone was condensed overhead, and acetone-free methanol was recovered in a separate still from the brine which was then recycled. The improved Othmer recirculation still (16) has been the apparatus generally favored by investigators who have studied the effects of salts on vapor-liquid equilibrium. 

Acetic acid separation and purification. Azeotropic distillation serves to separate and recycle the unconverted butyl acetate and the other intermediate products of the reaction. The formic and acetic adds are then isolated. The first is sent to the burner, and the second purified and partly recycled to esterification. 

The azeotropic mixture is subjected to an extractive distillation (extraction medium wastewater from the methanol distillation). The still bottoms so obtained, a water - methanol mixture, are fed to the methanol distillation. Methyl acetate taken from the top of the column is fed to a reactor where it is hydrolyzed to methanol and acetic acid with the aid of an ion-exchange resin. In the subsequent distillation, acetic acid is removed, and the mixture of methanol and unreacted methyl acetate from the extractive distillation is recycled (Fig. 17). Wastewater from this recovery process is treated in a biological wastewater treatment plant. 

DMAC reacts with water to form acetic acid. The previous massive incursion of water had led to the formation of two separate layers in a process vessel. This resulted in some of the water not being removed because the pump line was connected to the lower part. The upper water layer was pumped into the distillation column during the later high temperature stages of the distillation when the operators thought that all the water had been removed. Acetic acid was formed, distilled off as an azeotrope with DMAC and recycled to the reactor, reacting with DCNB and initiating the explosion as described. 

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