Economic Optimization of the Reflux Ratio of Two Components Stage Distillation Columns

Environmental Energy and Economic Research 2019 3(1): 11-21 DOI 10.22097/eeer.2019.145503.1040 Economic Optimization of the Reflux Ratio of Two Components Stage Distillation Columns Bahador Abolpour*, Shahram Nasiri, Ehsan Khosravi Sirjan University of Technology, Sirjan, Iran Received: 17 May 2018 /Accepted: 1 December 2018 Abstract Distillation columns are complex processes for modeling and controlling. These columns are significant parts of most chemical industries for separation of components. Control of this process is essential for achieving certain purity for products with a minimum cost. However, nonlinearities, multivariable interaction, non-stationary behavior and severity of disturbances inside the column made this process too complex for controlling. In this study a graphical method is applied to model steady state continues tow components distillation column. First, a MATLAB code was developed to solve the mathematical model of the column. Then, the column was simulated using HYSYS software. Finally, the reflux ratio of this column was optimized to minimize the operating cost. A formula is presented to calculate the optimum value of this reflux ratio as an exponential function of a certain economic parameter of energy prices and depreciation costs. It is resulted that at low energy prices or high equipment depreciation costs, the optimum reflux factor is high. Keyword: Two Components Mixture; Distillation Column; Optimized Reflux Ratio; Operating Costs Introduction Distillation columns are complex processes for modeling and controlling that are significant parts of most chemical industries for separation of components (Luyben, 1990; Balchen and Mumme, 1998; Luyben, 1992; Shinskey, 1984; Enagandula and Riggs, 2006). Control of this process is essential for achieving certain purity for products with a minimum cost. But, nonlinearities, multivariable interaction, non stationary behavior and severity of disturbances inside the column made this process too complex for controlling (Hurowitz, 2003). The basis of this process is thermodynamic vapor-liquid equilibrium (VLE). Distillation causes the separation of chemical components and provides different concentrations in both phases. Diwekar et al. (1985) optimized the multicomponent batch distillation columns and formulated the optimization problem for the single-fraction and multi-fraction batch distillation columns * Corresponding author E-mail: 12 Abolpour et al. operating under variable and constant reflux conditions. Ren et al. (2010) presented a mathematical model for operation profits with reflux ratio of a stage distillation column. They solved the nonlinear objective function with an improved particle swarm algorithm to optimize the reflux ratio. Mauhar et al. optimized the distillation process of Propylene-Propane using Aspen Plus simulation engine, based on the real operating data taken from the factory (Mauhar et al. 2004). They optimized the right combination of pressure and reflux rate to minimize the energy consumption in the reboiler and to obtain the required product purity. Fazlali et al. (2009) optimized operating conditions of an atmospheric distillation unit of a petroleum refinery using a simulator for earning more overhead products. They presented a net economical balance between the increments of the overhead products and the energy consumption for an energy saving in the refinery. Chen and Lin (2001) obtained the optimum reflux ratio of distillation towers in petroleum refining processes (propylene splitter and debutanizer plants) using an optimization software. In the present study, the enthalpy-concentration method (Treybal, 1981) is used to simulate steady state continues two components distillation columns. This study accommodates the experience of the authors in both fields as they modeled binary distillation columns via general coding language, MATLAB as well as industrial dynamic simulation package, HYSYS. Method and material Distillation uses the degradation of heat to drive a chemical separation (Henley and Seader, 1981; King, 1971; Robinson and Gilliland, 1950). To achieve separation by distillation or other equilibrium countercurrent exchange processes a minimum quantity of internally circulating fluid is required. In distillation the vapor-liquid countercurrent flows are typically established by a continuous reflux. A flow of liquid (L) maintained at its boiling point and of vapor (G) are circulated between stages in order to purify a quantity of feed per unit time (F) into products. Figure 1 depicts the purification of the feed flow where the distillate (D) is being enriched with the more volatile component and bottoms (W) with the less volatile. In this Figure, the trays are numbered top to down, and subscripts indicate the tray from which a stream originates. The bar sign over a variable indicates that it applies to the section of the column below the point of introduction of the feed. x, y and z are mole fractions of the more volatile component in liquid, vapor and a mixture of the two phases, respectively. Also, QC and QB are the heats of condenser and reboiler, respectively. In distillation heat is added below the feed point, that is, to the stripping section of the column, to drive off volatile materials into the descending liquid stream. Similarly, heat is removed above the feed point, that is, to the rectifying section of the column, to condense less volatile materials out of the ascending vapor stream. This Figure introduces the variables used and points out the six valves available to control the column. No matter which valve is used for composition control or how it is used, fundamentally there are three factors that could be manipulated: The feed split, the fractionation, and the reflux ratio. An overall material balance for the column indicates that the distillate plus the bottoms should equal the feed flow. The feed split is simply the ratio of the distillate to bottoms. The other fundamental variable is the fractionation which is the amount of separation that occurs per stage. The overall fractionation in a column depends on the number of stages, the energy input, and the difficulty of the separation. Valves number 2 and 3 controls the reflux ratio in the distillation operation. In a distillation column the liquid and vapor flows are almost unchanged in a subsection containing no heat exchangers or side-stream inputs or outputs. By assuming constant flows and considering mass balance, one can obtain linear relations, known as operating lines, for the components concentrations in the flow streams passing between adjacent stages. Equilibrium curves limit the Environmental Energy and Economic Research 2019 3(1): 11-21 13 concentrations of flow streams leaving a stage. Thus, concentration differences in the flow streams at any point in the column are bounded by the operating lines and the equilibrium curve. Figure 1. Schematic of distillation column. (...truncated)