Effect of operating pressure on design and control of extractive distillation process separating DMC-MeOH azeotropic mixture

Abstract

Number of industrial facilities increase rapidly which leads to rise in their negative impact on their environmental destruction. In order to reduce this negative impact, there is a noteworthy increase in the usage of environmentally friendly raw materials and chemical processes. Dimethyl carbonate (DMC), due to its favorable properties such as having low ecotoxicity and being biodegradable, stands out as an environmentally friendly "green chemical". Since DMC has no harmful impact on environment, it is commonly used as substitute material of dimethyl sulphate and phosgene in methylation and carbonylation reactions. In addition to this, it is used as a co-solvent for non-aqueous electrolytes for lithium rechargeable batteries and adequate side material for internal combustion engine fuels. Transesterification of propylene carbonate and methanol is a preferable path for DMC synthesis. However, reaction with excess methanol (MeOH) being fed to the system results in azeotropic mixtures with DMC which eventually leads to difficulties in separation of DMC-MeOH mixture. Extractive distillation which is one of the most preferred methods for separation of azeotropic mixtures is known as an expensive process due to requirement of regeneration of extractive agent. Nevertheless, since DMC-MeOH azeotropic mixture is sensitive to pressure changes, a reduction in extractive agent requirement is considered achievable by operating the extractive column at higher pressures. For that reason, it is aimed to design an increased-pressure extractive distillation process which provides DMC with 99.8% purity and MeOH with 99.99% purity. The proposed process consists of two columns. The purpose of extractive distillation column is to separate methanol from azeotropic mixture using methyl isobutyl ketone (MIBK) as an extractive agent, while the recovery column is used to purify DMC and recycle the regenerated extractive agent MIBK back to the extractive column. The thesis consists of two stages. In the first stage, it is aimed to simulate alternate DMC-MeOH separation process configurations using Aspen Plus where the extractive distillation column operates at different pressures such as 1 bar, 5 bar, 7.5 bar and 10 bar. All increased-pressure extractive distillation process options and the base case where both columns operate at atmospheric pressure are optimized based on the total annual cost (TAC). As per the simulation results, significant amount of reduction in entrainer requirement is observed by increasing the operating pressure of extractive distillation column. By operating extractive distillation column at 10 bar pressure, a 34.1% decrease in total annual cost and 29.8% reduction in carbon dioxide emissions are observed compared to the base case where extractive distillation column operates at atmospheric pressure. Among the design studies of 1 bar, 5 bar, 7.5 bar and 10 bar, control structures are implemented for the case with 10 bar operating pressure which results in the lowest TAC and CO2 emissions. Prior to exporting steady-state design from Aspen Plus to Aspen Dynamics, equipment sizing for reflux drums and column sumps are completed. As steady-state simulation is exported to Aspen Dynamics, necessary controllers such as flow, level, pressure and temperature controllers are implemented to the system. ATV test is used to tune the temperature controllers. After that, seven different control structures which are distillate to reflux ratio control, feed to reflux ratio control, feed to reboiler duty ratio, feed to reflux & reboiler duty ratio controller, combined ratio control of distillate to reflux, feed to reflux and feed to reboiler duty, and feed to entrainer flow ratio are designed, and two different types of disturbances such as ± 20% change in the feed flowrate and ± 3% change in the feed composition are introduced to the system. Each of the simulations is conducted for 50 hours in which first two hours are operates in steady-state conditions. At the end of each simulation, generated data is exported to MATLAB to produce graphical results. According to outcomes of this study, it is seen that in case of change in the feed flowrate, solely feed to reflux ratio, combination of feed to reflux and feed to reboiler duty ratio and combination of distillate to reflux ratio control of extractive column, feed to reflux ratio control of recovery column and feed to reboiler duty ratio control in both of the column gives the best results for product purities by taking into account the lowered offset values and oscillations in addition to quicker response time. In more detailed observation, although there are very small differences between the control strategies yielding good results, combination of distillate to reflux ratio control of extractive column, feed to reflux ratio control of recovery column and feed to reboiler duty ratio control in both of the column gives better results with respect to other two control structures. Two results can be reached from this observation. First, addition of feed to reboiler duty ratio control improves response time of the process against flow rate disturbances although single application of feed to reboiler duty ratio alone does not provide satisfactory results. Secondly, although distillate to reflux ratio alone gives bad results, addition of feed to reflux ratio improves the control structure response to a reasonable level. On the other hand, each of the simulated scenarios except for the scenario seven have converged to its new steady state value in around 20-25 hours and stabilized for any kind of disturbances. It is aimed to control the entrainer make-up flow via a certain ratio from feed flow in 7th control structure, however, proper control of the system was not achieved. As the disturbanced are intoruduced to the system, it is not possible to conduct column operations decently.