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
•
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
•
Note the difference between fresh feed (F) to the process plant and feed to the reactor (A) A (feed to the reactor) = F (fresh feed to the process) + R (Recycle)
• Separation processes are rarely perfect. Product stream will contain un-reacted material, recycle stream will contain some product
Material Balances with Bypass A procedure that has several features in common with recycle is bypass in which a fraction of the feed to a process unit is diverted around the unit and combined with the output stream from the unit. By varying the fraction of the feed that is bypass, we can vary the composition and properties of the product.
• • •
Part of the stream skips one or more stages. Used to control the final composition of a product Solution procedures similar to that for recycle stream
Bypass and recycle calculations are approached exactly the same manner: the flowchart is drawn, and label and overall balances around the process unit or the stream mixing point following the process unit are used to determine unknown parameters.
Material Balance with Purge
Need to remove inerts at the same rate as they are entering Recycle and purge have the same composition Inerts are TIE components which are present in only ONE inlet and ONE outlet stream at the same flowrate Flow of inert material in feed (F) = flow of inert material in purge stream (P) at steady state