Study On Vapor-liquid Equilibrium In Low Alcohols-water Binary System

Lower alcohols generally refer to alcohols with shorter carbon chains.Among them,C2-C4 and other lower alcohols are widely used in industrial production and daily life.However,in the process of producing lower alcohols,the main components contain a large amount of water,which has many restrictions on the application of the product.For example,the water content of absolute ethanol is ?200 ppm,the water content of ultra-clean and high-purity isopropanol is ?100 ppm,and the water content of n-butanol as biofuel is ?300ppm.Therefore,the removal of water in lower alcohols is particularly important in production.Although the special distillation(salt addition,extraction,etc.)that is widely used nowadays can realize the preliminary separation of water,there are few studies and reports on the separation and distillation of trace water.Therefore,it is very meaningful to study the separation of trace water and lower alcohols,which can provide some technical guidance for the removal of trace water of lower alcohols.In this paper,ethanol,isopropanol,and n-butanol,which are representative of lower alcohols,are selected as the research objects.Using a self-made vapor-liquid equilibrium kettle,the vapor-liquid equilibrium data of binary systems such as ethanol/water,isopropanol/water,and n-butanol/water under normal pressure were measured,and the water content was analyzed.Under extremely low conditions,the vapor-liquid equilibrium data of these three alcohols.At the same time,the total reflux distillation experiment was used to supplement the research on the vapor-liquid equilibrium characteristics of the isopropanol/water system.The specific research content is as follows:Using the Herrington area test method and the point test method,the two sets of binary system vapor-liquid equilibrium data obtained from the experiment are tested for thermodynamic consistency.The test results show that the experimental data have good thermodynamic consistency.Further,the law of water equilibrium constant and activity coefficient with water content was studied,and it was found that the water equilibrium constant and activity coefficient of ethanol/water and n-butanol/water systems gradually approached with the decrease of water content.In constant,However,the equilibrium constant of water in the isopropanol system has abnormal characteristics: as the water content decreases,the equilibrium constant first tends to be constant,but decreases rapidly after dropping to a certain level,resulting in a sudden change in water volatility.The characteristics of these waters at very low concentrations provide a theoretical reference for the process design and optimization of the subsequent distillation and purification of ethanol,isopropanol,and n-butanol.Use the experimental data to evaluate the model parameters that come with the Aspen Plus simulation software.It is found that the model parameters that come with the software can predict the ethanol/trace water system very well;it can be very good when the water content in isopropanol is greater than130 ppm.The prediction results of less than 130 ppm are obviously larger;the prediction of the n-butanol/trace water system is biased.For this reason,according to the change law of vapor-liquid phase equilibrium,using Wilson,NRTL and UNIQUAC three activity coefficient models to carry out the vaporliquid phase equilibrium experimental data of the binary system of isopropanol/water and n-butanol/water After fitting,the binary interaction parameters between these components were obtained.Taking into account the change law of the isopropanol/water system,a piecewise fitting was carried out.The maximum absolute deviation,average absolute deviation,and root mean square deviation are used to test the fitting effects of these models,and it is found that the models can fit the experimental data well within a reasonable error range.Finally,using Aspen Plus software,using the NRTL model parameters regressed from experimental data,simulated purification studies of isopropanol and n-butanol were carried out.The influence of the number of theoretical plates,feed position,reflux ratio,etc.on the purity of the product was investigated.Through the comparison of conventional distillation and heat pump distillation simulation,it is concluded that the total cost of heat pump distillation is lower on the basis of ensuring product purity requirements,and it is more suitable for promotion in actual production.