Systematic Design and Optimization of Acrylonitrile Production

Systematic Design and Optimization of Acrylonitrile Production

Steen M. Christensen, Anna Leth-Espensen, Marina Fedorova*

Computer Aided Process Engineering Center, Department of Chemical and Biochemical Engineering, Technical University of Denmark, DK-2800 Lyngby, Denmark

*mfad@kt.dtu.dk

Acrylonitrile is an important starting material for a wide range of polymers and copolymers and is found in numerous everyday products like conveyer belts, Lego blocks, and fabric material. In this project, which is part of a MSc Process Design course at the Technical University of Denmark, a production facility of acrylonitrile is designed. The objective is to design a process for the production of 100 000 tons of acrylonitrile per year.  The designed process must be environmentally acceptable, must apply heat integration and be optimal with respect to cost.

The production of acrylonitrile is nowadays performed through the Sohio process. This process is a reaction between ammonia, air, and propene that react through the catalytic mechanism of ammoxidation.  Other production methods for acrylonitrile including the ethylene cyanohydrine process and the ammoxidation of propane are also possible. A new process that could be interesting in the future is the ammoxidation of propane, because of the lower cost of propane compared to propylene. However, the selectivity for product in this process is too low. Therefore this project focuses mainly on designing and optimizing the known Sohio process.

 The design is done systematically from generating a simple flow sheet to a rigorous model of the plant. The simple flow sheet is created by the Hierarchical Decomposition Douglas method. The design has then been optimized to find a more sustainable solution, both with respect to economics and the environment. Also, heat integration has been performed to reduce the energy consumption. 

With respect to the economical optimization especially the by-products have been taken into account. In the production of acrylonitrile two other saleable by-products are produced though in considerably smaller amounts than acrylonitrile. One is hydrogen cyanide which is converted into acetone cyanohydrin for easy shipping and another is acetonitrile. The purification of these two by-products and the acrylonitrile is a difficult process, which has been considered in detail in this project. The separation methods include neutralization, absorption by water, stripping, decanting, distillation, reactive distillation and a pressure swing distillation column of which especially the last two are interesting and crucial in order to obtain the required purity of the acetonitrile.

Several alternatives to the base case design were obtained after performing optimization and sustainability and LCA analyses. With respect to the environmental optimization especially the recycle of process water and definition of the optimum pressure and temperature settings for the distillation columns save energy and resources.