Systematic Approach to Chemical Process Design: Application to Ethanol Production From Ethylene

Wednesday, October 19, 2011
Exhibit Hall B (Minneapolis Convention Center)
Kate Harboe, Yu Jiang and Alessandra Pennati, Chemical and Biochemical Engineering, Technical University of Denmark, Lyngby, Denmark

Ethanol is a very important chemical as it is a very common solvent used in industrial processes and widely used as an alternative fuel. It is expected that the importance of ethanol will increase and that ethanol consumptions will grow significantly in the coming years because of its energy and environmental advantages compared to gasoline.

A plant for producing 190 proof ethanol from ethylene is designed by applying a systematic method. The systematic method divides the process design work into 12 sequential tasks that include selection of process alternatives, development of a base case followed by economic and environmental evaluations and improvements of the base case through heat/mass integration and optimization. This procedure can be applied to design or analysis of any other chemical process. The design of the ethanol production process was developed in the MSc course on Process Design at DTU.

The base case selected is direct catalytic hydration of ethylene at high temperature and pressure, which is currently the most widely used process in industry. Generation of diethyl ether from ethanol also occurs as a side reaction. The ethylene feed contains impurities of methane and propylene. Both the byproduct and the impurities make the downstream separation of the product quite complex. Separation is done by a sequence of distillation units and ethanol at azeotropic composition is obtained as the product. According to the 12-tasks design procedure, first information about the product and process was obtained in tasks 1-2. A preliminary process flowhseet was obtained in task 3 using a modified Douglas-approach for flowsheet synthesis. Next, design decisions related to separation factors, reactor operating conditions, product purity, etc., were made and further refined in moving from tasks 4-7. In tasks 4 mass balance is performed. After operating conditions are set in task 5, energy balances are performed in tasks 6-7. All simulations are made with PRO/II. Tasks 8-9 make the sizing and economic evaluation calculations. At this point, the base case design is obtained, which is then further refined and improved with respect to heat integration and process optimization (tasks 10-11). In the final task 12, the environmental impact of the process design is evaluated together with some of the key sustainability measures. In addition to PRO/II, the following software are used: ICAS (for property prediction, analysis) and ECON (cost and economic analysis).

It is estimated that the annual profit of base case is 37 million US$ with a payback time of 3 years for a production rate of 72,287 metric tons/yr. The capital and operating costs are divided into the individual cost items. It is found that compressors are the most expensive equipment (59.5% of the total purchased equipment cost) and that raw material accounts for 65.8 % of the total operating cost, while steam represents the 39.6% of utility cost. This information is then used as target for process improvement by heat integration and process optimization, which then further increases the annual profit and reduces the payback time. The environmental impact analysis identifies impact due to the release of chemicals and points to the need of better control mechanisms. The 12-tasks design method helped to perform the process design related work systematically and efficiently, where first design decisions were made based on collected information and then verified through simulations.

This design project work was done under the supervision of Professor Rafiqul Gani.

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