Liquid mixtures separation and heat consumption in the process of distillation

Objectives. The aim of this study is to investigate different distillation modes of a binary ideal mixture and determine how various factors affect heat consumption in the column boilers. In addition, it intends to assess the difficulty of separating mixtures. Our research is based on analyzing the characteristics of vapor-liquid equilibrium.Methods. To conduct our study, we used a graphic-analytical tool to calculate the distillation process of a binary mixture and mathematical models based on the Aspen Plus software package along with DSTWU, RadFrac, and the Sensitivity module. We also used the Peng-Robinson equation (PENG-ROB) to determine the liquid-vapor equilibrium.Results. We employed the graphical method and mathematical models to obtain the operation parameters of two column variants for the distillation of binary ideal benzene-toluene mixtures. In each variant the initial mixture contained the same amount of the low- and high-boiling component. The number of plates in the column sections, reflux ratio, energy consumption, and indicators of internal energy saving were determined.Conclusions. Study results show that using the coefficient of the component distribution between the vapor and liquid phases is a promising method for preliminary assessments of the separation difficulty and measurements of the expected heat consumption in the boilers of columns. Comparison studies showed that the heat consumption in the boiler decreases as the internal energy saving in the columns increases.

COMPARISON OF ENERGY SAVING METHODS FOR DISTILLATION OF LIQUID MIXTURES

The analysis and review of the scientific literature on methods and approaches to energy saving in distillation, as one of the most energy-intensive processes for the separation of binary and multicomponent mixtures was carried out. Directions of the scientific literature are highlighted, showing the relevance of the thermodynamic method assessment of separation processes. A comparative analysis of various methods of energy conservation in the separation of liquid binary mixtures by the distillation method is performed (a binary mixture of benzene-toluene was chosen as an example). A conventional distillation column as well as a column with a heat pump and a column with heat integration were considered. As a result of the calculation experiment for each option performed using the Aspen Plus software package the optimal column parameters were determined - the total number of stages and the position of feed stage. The heat consumption in the column boiler was taken as the objective function. It is also shown that with the same characteristics of the columns, the best way to organize the process for separating the selected mixture is to compress the steam stream from rectifying profile of the column with its subsequent use in the boiler of the stripping profile according to the principle of a heat pump. It was established by a calculation experiment that heat integration by compressing steam from rectifying profile of the column and supplying it to the stripping profile gives significantly less energy saving. The calculation of internal energy saving by distillation column was carried out and it was shown that the distributed heat removal from the plates of the rectifying profile of the column and the supply of this heat to the stripping profile plates reduces internal energy saving and leads to an additional increase in heat consumption in the boiler.

Comprehensive Utilization of Ammonia-containing Exhaust Gas in Soda Ash Chemical Plants

Industrial chemical plants of soda ash mainly adopted the ammonia-soda production process and the combined process (Hou's process for soda manufacture), during which the mother liquor was recycled and the exhaust gas purified and recovered, realizing continuous manufacturing. However, in practical operation, there was still a small amount of fugitive ammonia emitted during storage and transport of materials. To improve the environment, an idea of using ammonia-containing exhaust gas for process improvement was introduced: collecting the ammonia-containing exhaust gas, extract some of the thin liquid from the circulating mother liquor for preparation of pure water, from which the concentrate return to the mother liquor circulation system and the pure water for absorption of the ammonia-containing gas; dilute ammonia water that took in the ammonia-containing gas and the feedstock ammonia were again used for preparation of ammonia water for industrial use, which, at the service of the boiler desulfurizing installations of the enterprise, helped achieve internal energy saving and emission reduction.

SELECTING THE OPTIMUM SCHEME OF THE SEPARATION OF HYDROCARBON GASES BY DISTILLATION

The heat costs in distillation columns were determined for the purpose of separating a four-component gas mixture in gas fractionators with various component selection sequences. A method for selecting the optimal scheme was developed. It is based on the concept of “internal energy saving” upon rectification. Internal energy saving means multiple steam operation on the plates of a distillation column, namely, its condensation on each plate with the release of the heat of condensation, which is used for the evaporation of the liquid entering the plate to obtain a new steam composition. It was shown that when binary and three-component mixtures are separated and products of equal purity are obtained, the heat costs are related to the internal energy saving. The calculation of the internal energy saving in a three-column system for the separation of a four-component mixture was carried out. The calculation confirms that, as the internal energy saving increases and, accordingly, the average internal energy saving in the system of columns increases, the total heat consumption is reduced. The results of calculating three separation schemes of the four-component mixture were compared using the program Aspen Plus with different methods for describing the phase equilibrium (NRTL, Peng-Robinson, Chao-Seader) was carried out. It was shown that the calculations with the use of Peng-Robinson and Chao-Seader methods match.

THEORETICAL RATIONALE FOR THE SELECTION OF THE OPTIMAL SCHEME OF SEPARATION OF THE THREE-COMPONENT MIXTURE

It is theoretically justified that for the scheme of separation of three-component mixtures with the release of the high-boiling component in the first column there is a region of initial compositions for which the heat costs are the least. An expression describing the boundary of the optimality regions of different separation schemes is given. The calculation of the heat costs for both rectification schemes of the three-component mixtures performed using the Aspen Plus software package confirmed the theoretical justification for the existence of optimality regions for each of them. With the help of the concept of internal energy saving, the advantage of one or another scheme of separation of a three-component mixture is explained.

INTERNAL ENERGY SAVING IN DISTILLATION COLUMNS WITH REAL PLATES AND IN FILLED DISTILLATION COLUMNS

Perspective ways of power saving in the distillation of liquid mixtures are considered. The phenomenon of internal energy saving on theoretical plates is considered in detail. As shown in this article, the phenomenon depends on the number of theoretical plates in the rectifying and stripping sections of the column and also on the aggregative state of the feed mixture. The processes of heat and mass transfer on real plates with various values of efficiency factor are considered. Blocks of real plates equivalent to one theoretical stage, that is, blocks where the leaving flows of the liquid and vapor are in equilibrium, are isolated. Analogically, heat and mass transfer in apparatuses with a continuous phase contact having a layer height providing one theoretical plate is considered. It is proved that internal energy saving in filled rectifying columns with real plates also depends on the number of theoretical plates and the aggregative state of the initial mixture.

DEPENDENCE OF EXTERNAL AND INTERNAL ENERGY-SAVING FOR RECTIFICATION OF BINARY MIXTURES

The comparison of energy consumption for rectification of binary mixtures in one-column and twocolumn apparatuses was carried out with the use of vapor flow energy from the first column for heating the second column. It is shown that the expected two-fold reduction of the flow of heat energy is impossible due to the decrease of the internal energy-saving.