(120b) Ethyl Lactate Production Using Semicontinuous Distillation with Reaction in an Auxiliary Vessel and Pervaporation

At the 2005 Spring AIChE Distillation Symposium, we introduced a novel concept that combined semicontinuous distillation with reaction in a middle vessel (SDRMV) to produce 2,4-dimethyl-1,3-dioxolane in a forced cyclic process [1]. At the Cincinnati Meeting of AIChE (November 2005), we showed that this system was also economically preferred to traditional batch and continuous alternatives for a wide range of production rates [2]. Now, we present a new application of integrated semicontinuous distillation, reaction, and pervaporation for the production of ethyl lactate from ethanol and lactic acid.

Semicontinuous distillation uses one or more middle vessels integrated with a distillation column in a forced cycle consisting of a sequence of operating modes. With this method, three or more chemical species can be separated with just one column at purities higher than a sidedraw would permit. Unlike divided wall columns, only a single shell is needed, and so the column is simple and control is easier. Reaction can take place in the middle vessel or in an auxiliary vessel, which when integrated with the distillation column, allows for either the simultaneous removal of products or the recycling of unreacted reagents to help push the reaction forward. This method is sometimes preferable to reactive distillation (reaction on the trays of the column) for exothermic processes or where the bubble-point temperatures inside the column do not favor fast reactions or high yields.

In this work, equimolar amounts of ethanol and lactic acid are reacted to chemical equilibrium in a CSTR to form equimolar amounts of water and ethyl lactate, which is the desired product. The reaction is exothermic and reversible with a well-known azeotrope existing between ethanol and water. Once the reaction approaches chemical equilibrium, the contents of the reactor (containing all four species) are moved to a middle vessel that is integrated with a distillation column, and the CSTR is recharged. The middle vessel feeds to and receives a sidedraw from the column simultaneously.

Throughout the cycle, ethanol and water are collected in the distillate and are separated with a pervaporation unit utilizing a hydrophilic membrane. This permits a low-energy separation without complications due to the azeotrope. The water-rich permeate is collected and removed from the system, while the ethanol-rich retentate is recycled to the CSTR, pushing the reaction forward. Lactic acid is collected in the bottoms product of the column and recycled to the CSTR at decreasing flow rates, eventually approaching zero flow. At this point, the process resembles a fed-batch distillation with a sidedraw. Any ethyl lactate product collected in either stream is kept within the process since the streams are recycled to the CSTR, thus achieving zero product loss. Since water, ethanol, and lactic acid are removed from the middle vessel during this process, the middle vessel becomes highly concentrated with ethyl lactate approaching the end of the cycle, when the contents are collected as product.

The flow rates and compositions of the feed, distillate, and bottoms vary throughout the process and require a special model-based control system to maintain purity constraints and prevent the column from failing due to flooding or weeping. A sequence of operating modes are needed to ensure seamless operation from mode-to-mode within the cycles, so that unlike batch distillation, there are no startup or cooldown phases. Thus, the process is semicontinuous in nature.

The process is simulated rigorously through integration of the dynamic MESH equations for the column, mass and energy balance equations for the tanks, and empirical reaction equations for the CSTR. The pervaporation model is derived from a mass-transfer-based bulk flow model for continuous systems and is adapted for use in a dynamic environment. Physical properties are modeled rigorously through calls to Aspen Properties 2004.1.

The results demonstrate the feasibility of using semicontinuous distillation to separate a four species mixture into three mixtures of controllable purity using a single distillation column. This requires less capital expenses than using alternative batch or continuous processes. Additionally, when integrated with a reaction unit and pervaporation unit, the system can help increase reaction yield and circumvent simple distillation boundaries associated with the azeotropes. Furthermore, the operation of the column without startup or cooldown phases between the batches permits a higher rate of production to be achieved for processes of comparable size. Thus, the process is a good candidate for use at intermediate production rates between those typically associated with batch and continuous processes.

1. Adams TA, Seider WD. A novel concept: Semicontinuous reactive distillation. 2005 AIChE Spring Nat'l. Meet., 2005. Atlanta, GA, United States: American Institute of Chemical Engineers, New York, NY 10016-5991, United States.

2. Adams TA, Seider WD. Semicontinuous Reactive Distillation for Specialty Chemicals Production: Economic Comparison with Batch and Continuous Processing. 2005 AIChE Annual Meet., 2005. Cincinnati, OH, United States: American Institute of Chemical Engineers, New York, NY 10016-5991, United States.