4.7 Balances on Reactive Processes
4.7 a. Balances on Molecular and Atomic Species
General balance equation for a steady-state process: input + generation = output + consumption.
Molecular Balance: Products: generation = output
Reactants: input = output + consumption
Atomic Balance: input = output
In this section a number of different balances can be written, but the formula that we are used to:
Input = Output can no longer always be used. The balance equation for each specie included in the
reaction must include a generation term (products) or a consumption term (reactants). Balances on
atomic species can be written input = output, because atoms can’t be created (generation = 0) nor
destroyed (consumption = 0).
4.7 b. Independent Equations, Independent Species, and Independent Reactions
Degree-of-freedom analysis:
number of material balances for a nonreactive process = number of independent species involved in
the process
4.7 c. Molecular Species Balances
If it is a molecular specie balance, the balance on the reactive specie must contain generation and/or
consumption terms.
The degree-of-freedom analysis is as follows:
No. unknown labeled variables + No. independent chemical reactions - No. independent molecular
species balances - No. other equations relating unknown variables = No. degrees of freedom
One consumption/ generation term calculated for each specie. Consumption/ generation terms for
all species can be determined from stoichiometric equation.
Video explaining Molecular Specie Balances:
http://www.youtube.com/watch?v=ddtW9G1oUxI
4.7 d. Atomic Species Balances
All balances on atomic species: Input = Output
No additional degrees of freedom needs to be contributed.
The degree-of-freedom analysis is as follows:
No. unknown labeled variables - No. independent atomic specie balances - No. molecular balances
on independent nonreactive species - No. other equations relating unknown variables = No. degrees
of freedom
Video explaining Atomic Specie Balances:
http://www.youtube.com/watch?v=NDm4FRt2fyM
4.7 e. Extent of Reaction
Third way to determine unknown molar flow rates for a reactive process: Expressions for each
product specie flow rate in terms of extents of reaction. Substitute known feed and product flow
rates, and solve for the extents of reaction and the remaining reactive specie flow rates.
Degree-of-freedom analysis:
No. unknown labeled variables + No. independent reactions (one extent of reaction for each) - No.
independent reactive species (one equation for each species in terms of extends of reaction) - No.
independent nonreactive species (one balance equation for each) - No. other equations relating
unknown variables = No. degrees of freedom
Video explaining Extent of Reaction:
http://www.youtube.com/watch?v=YusSU0jlOUk
4.7 f. Product Separation and Recycle
Overall Conversion =
–
Single- Pass Conversion =
4.7 g. Purging
Some processes involve recycling. When a material remains in a recycle stream the substance would
continuously enter the process and will therefore steadily accumulate. A purge stream must
withdraw a portion of the recycle stream to prevent a buildup.
Videos explaining Purge streams:
http://www.youtube.com/watch?v=47tPvx3lOKc (Part 1)
http://www.youtube.com/watch?v=gOuOxozb4pE (Part 2)
Summary:
http://www.youtube.com/watch?v=jyso8NSytWw
http://www.youtube.com/watch?v=MSzTIRAv5io
http://www.youtube.com/watch?v=tQyrSvll_nc