In chemical reactor, reactions takes place under controlled conditions, the engineer selects reactor design variables to optimize the performance of the process and quality of product. Some variables include:
Temperature and pressure: these variables maximize the conversion of raw materials to desired product, using the relation proposes by Charles, the relation between temperature and pressure can be programmed to optimize the design. Some system can operate at a constant pressure while some vary in operation. Whichever case, the main constrain is the temperature of reaction. Example is the reactor processing hydrocarbon gas may operate at 500 -600˚c and 400bar. At higher temperature, special materials may be placed under consideration in reactor design
Reactor volume: reactors differ by size and volume.reactor size is chosen on the basis of the volume of material to be processed as well as the time required for the reaction. These relationships can play a vital role:
Residence time (T): this refers to the time required for a material to stay in the reactor. For continuous reactor,
A fast reaction requires low reactor volume.
Mode of operation: this deals on the type of process flow a reaction may take; such as batch, semi batch and continuous process. Bate reactor have low capital investment and flexibility in operation. Continuous flow is cheaper to operate on a long run while semi batch is sometimes used in polymer synthesis fermentation reaction or.
Mixing pattern: the extent of mixing is essential as regards to the mode of operation in which the process is operated on. The mixing can be carried out in a continuous stirred tank reactor Continuous flow is sometimes designed as a plug flow reactor which the concentration changes with distance.
SELECTION OF DESIGN VARIABLES
The choice of your design determines the type of variables to choose, such as feed flow rates, stream compositions and many more. The selection requires the application of phase rule principles in order to determine the degree of freedom.
Nd = Nv – Nr
Nd is the degree of freedom, Nv is number of variables and Nr is number of design relationships (independent equations).
Example is seen in a binary flash distillation, having Nd relationship as (C+4) where C is the number of components, if C=6 with 4 as fixed variables, it means that the designer has to optimize the system with the two remaining variables. With the degree of freedom available, the out-stream compositions can be calculated using material or mass balance relations.
Selecting a particular design variable can be straightforward and easy while some selections can still be complex that may require the use of simultaneous equations, linear programming, dynamic programming and software to carry out some iterative calculations of formulated objective functions of some sub systems in the design.
Introduction to chemical process, Regena M. Murphy ( principles, analysis and synthesis). MC-Graw Hill international editions 2007.
Optimization of chemical process, Edgar, Hummeiblua and lasdon, second edition. MC-Graw Hill international editions 2001