NRB: Do the calculations suggested in the degree-of-freedom analysis

Material Balance Calculations

As suggested in the previous lesson, once the degrees of freedom analysis yields a value of zero, you have a roadmap for solving the material balance equation.


The equations suggested by the Degrees of Freedom analysis (# of species = # of balance equations; # of process specifications; # of physical constraints) MUST be the equations used to solve the material balance!

General Procedure for (single unit) Material Balances


Choose a basis of calculation from among the given data. That  is, choose one of the given stream amounts (batch) or flowrates (continuous or semi-batch) OR (if no stream amounts or flowrates are given) choose something convenient (like 100 mol(/s) or 100 g(/s)).


Its called conservation of MASS not VOLUME, do NOT choose a volume as your basis!


Draw the flowchart. (Follow the procedure previously outlined.)


Remember, to fully specify a stream, you must know BOTH the total amount of the flow within the stream and the composition of the stream (label what you DON'T know with variables). (Alternatively, you can know the flow of each component separately.)


Do the degrees of freedom analysis. (Follow the procedure previously outlined.) Can you match the number of unknowns with the number of equations? (i.e., is the DofF = 0?)


Remember, if they DO NOT match, you may have a problem! If there are too few equations, you CAN'T solve it. If there are too many, you MAY solve the problem, but then you must CHECK THE ANSWER against your additional equations (the problem may be overspecified).


If DofF is zero then simply do the algebra for each of the equations suggested by the DofF!


Do the calculations suggested in the degree-of-freedom analysis