Industrial Process Design

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Industrial Process Design

Process design and optimization are fundamental steps to ensure the economic sustainability of chemical processes. Suitable description of the performance of the various unit operations of the plant should be made and integrated, to understand the mutual interferences both under steady state conditions and under unsteady operation. Process simulation tools are helpful to compute both situations. The stationary case can be simulated with packages dealing with material and energy balances applied to each unit and extended to the whole plant. More complex is the unsteady state case, which needs dynamic modeling to describe the time-dependent evolution of the system. Separation equipment are simulated using algorithms embedded in process simulators, which may be more or less complex and adequate to represent the system with the desired approximation. A key issue in this case is the correct choice of a thermodynamic package able to cope with the system complexity in sufficient detail, in order to make reliable previsions on phase equilibria and components partition among them. Reactors are more critical to describe, since they represent an unicum for the particular case under investigation. When introducing a reactor in a flow sheet, different options become available, as for instance the following examples from Aspen Plus. The Stoichiometric and Yield reactors require some definition of the reactions taking place and their extent (e.g., in form of yield of different products). This is easily applied to available experimental result, but it does not allow tuning freely the process variables, since their influence on reactor performance is actually missing. Equilibrium or Gibbs reactors are useful to define the dependence on process variables, but they consider the reactor at equilibrium, so reactor sizing is not allowed. Furthermore, kinetically limited situations cannot be correctly described. Finally, the Batch, Plug-Flow (PFR, in case filled with catalyst as packed bed reactor) and Continuously Stirred Tank (CSTR) reactors are the most flexible options, which allow a full description of the process under variable conditions and proper sizing of the reactor. However, to perform these calculations, a suitable reaction set and the relative kinetic model are needed.

 

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Industrial Chemistry