Material balances on multiple-unit processes Overall material balance on a simple unit was described and formulated. However, chemical plants are formed often several process units where raw material enters into the process exits as product or by-product. In this section we consider multiple-unit processes where more than one unit is involved processing one or more than one chemical compounds or species. In this situation we need to write down several material balance equations to solve the problem. It is very important to keep track of all species individually and label all the stream and units. When facing multiple-unit processes, remember: • •
When analysing multiple-unit processes, carry out degree-of-freedom analyses on the overall process and on each subsystem. Only consider the streams that intersect the boundary of the system under consideration.
• Do not begin to write and solve equations for a subsystem until you have verified that it has zero degrees of freedom.
Material Balances with Recycle Recycle stream is used to separate reagent from product and recycle back to the process to reduce waste or increase conversion.
• Reactor can have low efficiency, but the overall efficiency of the process can be high due to the recycling of reactants • Recovery of un-reacted material and returned to inlet feed gives greater product quality and lower waste •
Recovery of a catalyst from the reactor
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Controlling feed conditions to a process by recycling some of the product
• Recycle only economically feasible if extra equipment costs are less than the savings made on extra reactant costs