The Application Study Of Recovering Toluene From Waste By Vacuum Batch Distillation

In many industries, such as petroleum, chemical, machinery, coating, fiber, grease, rubber, medicine and electronic, toluene is widely used as an organic solvent and it may eventually form a large amount of liquid waste. Toluene is volatile, irritant, toxic, and can be dangerous to both human health and the ecological environment. But, on the other hand, toluene has great economic value. Currently, the treatment of high concentrations of toluene liquid waste is mainly done by the incineration method. However, carbon monoxide, nitrogen oxides, sulfur oxides, dioxins and fly ash produced in the process of burning such waste have aroused widespread concern by scientists as well as the society. Therefore, developing new technologies for the treatment of high concentrations of toluene in liquid waste is of significant research and practical interest. This research studied the vacuum batch distillation technology to recover high purity toluene from its liquor waste, changing waste resource into renewable organic solvent for reuse.This research aimed at the toluene liquid waste produced by preparing functional materials in our laboratory. The liquid waste was first pretreated through suction filtration and then by supersaturated salt water, and then was analyzed by chromatography-mass spectrometry(GC-MS). The results showed that the mass fractions of toluene, ethanol and water in the toluene liquid waste were 89.8%,8.2% and 1.0%, respectively. On the basis of the characteristics of the liquid waste composition, this research identified vacuum batch distillation technology as a potential method to recover high concentration of toluene from the liquor waste for possible reuse. We designed, installed and tested the experimental setup for the vacuum batch distillation process. The theoretical plate number of packing column was determined to be 11, and the column pressure drop of the packing column tower to be 400 Pa. The influence of the operation parameters, including operation pressure, operation time, reflux ratio and composition of liquid waste on the vacuum batch distillation process were examined. We found that:(1) with the decrease of the pressure, the purity of the product increased gradually and the toluene yield proved decreased correspondingly. For the desired purity of the product, the maximum toluene yield was obtained at the operation pressure of 30.4 kPa. Therefore, in the following experiments, operating pressure was controlled at 30.4 kPa; (2)in the process of the whole distillation experiments, the temperatures of the tower kettle changed slightly, while the tower top temperature changed with the compositions of the tower top stream. We estimated the tower top compositions by observing and recording the tower top temperatures; (3) with the increase of reflux ratio, the product purity of toluene and the toluene yield increased. In consideration of the product yield and energy consumption,we set the reflux ratios of the cut fraction and product segment as 5 and 7, respectively; (4) the final operating conditions were obtained as the operating pressure was 30.4 kPa; the cut fraction lasted 120 minutes and the reflux ratio was 5; the product segment lasted 120 minutes; the reflux ratio was 7 and the mass fraction of the recovered toluene was 99.7%, while the first and second recovery yields were at 70.5% and 92.2%, respectively; (5) toluene yield increased with the mass ratio of ethanol/water in the liquid waste. Therefore, measures must be taken to reduce the water content and to make sure the ethanol/water mass ratio was in a reasonable range; (6) the experiment could not be carried out or when the mass ratio of toluene/ethanol was less than 1(water fraction was 2%). Low or high toluene yield would be obtained when the mass ratio of toluene/ethanol was less than 4 or greater then 9, respectively. Measures also must be taken to reduce the ethanol content. Then the recovered products were tested according to the relevant national standards and we found that they can reach the level of analytical pure. Finally, the whole vacuum batch distillation processes were simulated by the ChemCAD software The simulated data and experimental data fitted well, laying a good foundation for possible industrialization of the process. The economic efficiency of the system was also analyzed, showing obvious economical benefit.