(476d) Control of an Extractive Distillation Column with Sequential Heat Exchangers

Andrés Herrera Morales; Iván Darío Gil Chaves, Carlos Arturo Martínez Riascos, Gerardo Rodríguez Niño

Grupo de Procesos Químicos y Bioquímicos

Department of Chemical and Environmental Engineering

Universidad Nacional de Colombia – Sede Bogotá

Abstract

Control of an extractive distillation column with sequential heat exchangers for the purification of an azeotropic mixture of ethanol and water using ethylene glycol as entrainer was analyzed. In this research is pretended to analyze the energy distribution, control system and improvement of the column efficiency, in order to obtain a product that reaches the world consumers specifications.

Distillation column design was done in order to obtain the appropriate operating conditions and the configuration parameters of the internal heat exchangers that would be installed in some trays. This analysis allows entropy and exergy minimization, generating a process with a reduced energy consumption. A distillation column of twelve ideal stages with a total condenser and a kettle reboiler was studied.

Steady state simulation was performed in ASPEN Plus v8.4® using NRTL as thermodynamic model. The main objective was to obtain a high purity ethanol on the top of the column,  nearly to the 99.9%, in order to achieve the quality specifications for its use in mixtures with gasoline. The sequential heat exchangers configuration was selected using a sensitivity analysis founded on the minimum thermodynamic condition, where at minimun feeding ratios, the entropy generated is minimal, reducing the entropy that the reboiler yields. The sensitivity analysis shows that the configuration that generates the less entropy consists in three coils under the ethanol mixture feed stage, where is spread 75% of the energy used in the adiabatic column. Other important aspect in the column operation is related with entrainer feed temperature. This must be in accordance with the temperature profile of the column reducing the temperature difference in the mixing and, in consequence, entropy production. Besides, the reflux ratio must be fixed at the minimun value, nevertheless this variable is constrained by product specifications with 0.5 as optimum value.

Once the steady state simulation is done, the control strategy is proposed. In order to know how to control the system, three new degrees of freedom were generated, all related with the heat introduced by the sequential coils. These three new possible manipulated variables should be paired with three controlled variables allowing that the total heat introduced alongside the column is the same. One of the controlled variables selected was the temperature in the eleventh stage, as a result of the thermal analysis of the steady state operation, where at this point is more susceptible to changes. The other two control loops were related with the oil flow into the coils.

This strategy was compared with the commonly used in extractive distillation columns, finding more sensible responses after four different disturbances, feed flow of ethanol, temperature of the entrainer and set point of the 11^th stage temperature. Also this strategy allows quicker responses with less magnitude changes than the conventional ones, with almost 1 hour of the new steady state after each disturbance. Additionally of that, the dynamic responses shows that this control scheme tolerates greater changes of the manipulated variables than the conventional one, allowing more response time.