Design and Control of a Complete Ethyl Acetate Reactive Distillation Process

• Main Authors: Yeong Tarng Tang, 湯永堂
• Other Authors: 黃孝平
• Format: Others
• Language: zh-TW
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/27631231897179833349

博士 === 國立臺灣大學 === 化學工程學研究所 === 91 === In this research, design and control of a complete process for the production of ethyl acetate (EtAc) via esterification of acetic acid (HAc) with ethanol (EtOH) using reactive distillation (RD) will be studied. The complete overall ethyl acetate process designed in this study includes a RD column with overhead decanter separating into organic phase and aqueous phase. The aqueous phase is drawn out of the process as waste water. Part of the organic phase is refluxed back into the RD column; the rest of the organic phase is fed into the second stripping column for further purification. The bottom product stream of the RD column is designed to be rich in acetic acid so that it can be recycled and mixed with fresh make-up acid stream to serve as acid feed to the RD column. In order to achieve very high ethyl acetate product purity at stripping column bottom, an additional water flow is needed to be injected into the decanter. The final bottom product of the additional stripping column can produce ethyl acetate purity of over 99.5wt%. The top draw of the second column will be recycled back into the decanter. The ethanol feed flow rate which is used as the throughput manipulator of the overall process. The basic regulatory control loops of the overall process included four level loops (reboiler level at RD column, organic phase and aqueous phase levels at decanter, and reboiler level at stripping column) and pressure loop for each column. The control strategy of the reboiler level at RD column is unconventional to manipulate the reboiler heat duty instead of the usual column bottom flow to prevent snowball effect via column bottom recycle stream. After the basic regulatory control loops have been decided, there are four additional manipulated variables to be used for composition control. These four manipulated variables are: flow ratio of the fresh make-up acetic acid feed to ethanol feed, organic reflux ratio, fresh water addition into decanter, and the reboiler duty at stripping column. For the composition control, only conventional control strategy of using inferred tray temperature measurements will be considered. Sensitivity analysis has been performed to determine the temperature control points of the process. Two tray temperature at RD column and one tray temperature at stripping column are selected as feedback variables for control purpose. The final control pairings use organic reflux ratio to control temperature at tray 4, ratio of two feeds to control temperature at tray 8 of the RD column and reboiler duty to control temperature at tray 6 of the stripping column. The feed flow rate of water injected into the decanter is kept at a constant ratio to ethanol feed flow rate. The constant ratio is fixed at constant value for economical consideration. Feed composition as disturbances at both acid and alcohol feed streams, and, throughput changes via the set point changes of the fresh ethanol feed flow rate have been added into the process to test the closed-loop control performance of the proposed overall control strategy. The acetic acid and ethanol impurity in the ethyl acetate product stream are all kept within specifications despite the load or set point changes.