# 05 MULTIPLE UNITS.pdf - Balances on Multiple-Unit Processes...

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Balances on Multiple-Unit Processe s

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Draw a flowchart, and fill in all given variables values. Choose a basis of calculation an amount or flow rate one of the process streams Label unknown stream variables on the chart Do the problem book keeping => # unknown variables - # independent equations? Convert known stream volumes or volumetric flow rates to mass or molar quantities, using tabulated densities or gas law. If you are given a mixed mass and mole units for a stream, convert all quantities to one basis. If any information is given in the problem statement that has not been used in labeling the flowchart, translate it into equations in the variables defined. Write material balance equations and solve them. If a stream quantity or flow rate Q g was given in the problem statement and another value Q c , was either taken as a basis or calculated from this stream, scale the balanced process by the ratio Q g /Q c to obtain the final result. Procedure for Material Balance Calculations
Introduction to Multiple-Unit Processes Industrial processes rarely involve just one unit Redefining system portion of a process that can be enclosed within a hypothetical box (boundary) The inputs and outputs are the process stream that intersect the system boundary Keeping track of material flows for overall processes Keeping track of material flow of all individual units Definition of system : arbitrary choice Recommended solving method Overall Balances Balances on Subsystems

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Example of Typical PFD
Boundary system – System (overall balance) ……. Boundary A (3 inputs, 3 outputs) Sub-system (balance on each process unit) Stream mixer …… Boundary B (2 inputs, 1 output) Unit 1 (e.g. reactor) …… Boundary C (1 input, 2 outputs) Splitter …… Boundary D (1 input, 2 outputs) Unit 2 (e.g. mixer) …… Boundary E (2 inputs, 1 output) Unit 1 Unit 2 Product 1 Product 2 Product 3 Feed 3 Feed 2 Feed 1 B A E D C Balances on Multiple Unit - Boundary

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Number of independent equations = number of unit processes For one component system For every unit, 1 independent equation can be made. Mixer/Splitter is considered as one unit process For the above system we can only have 4 independent equations Choose (4 from 5, i.e., A, B, C, D, E). The remaining can be used as checking. Unit 1 Unit 2 Product 1 Product 2 Product 3 Feed 3 Feed 2 Feed 1 B A E D C Balances on Multiple Unit One Component
Useful Information: Do the overall material balance first for the whole system. to determine unknown process feed and product stream variables balances on various process subsystem may be used to determine interior stream variables In choosing subsystem, select the boundaries having intersect streams containing few unknown variables.

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