Page 54 - Chemical engineering design

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FUNDAMENTALS OF MATERIAL BALANCES
Solution
Method: write out the equation using letters for the unknown number of molecules of
each reactant and product. Make a balance on each element. Solve the resulting set of
equations.
A C 2 H 4 C B Cl 2 C C O 2 D D C 2 H 3 Cl C E H 2 O
Balance on carbon
2A D 2D, A D D
on hydrogen
4A D 3D C 2E
A
substituting D D Agives E D
2
on chlorine
A
2B D D, hence B D
2
on oxygen
E A
2C D E, C D D
2 4
putting A D 1, the equation becomes

1
1
1
C 2 H 4 C Cl 2 C O 2 D C 2 H 3 Cl C H 2 O
2 4 2
multiplying through by the largest denominator to remove the fractions
4C 2 H 4 C 2Cl 2 C O 2 D 4C 2 H 3 Cl C 2H 2 O

2.6. CHOICE OF SYSTEM BOUNDARY
The conservation law holds for the complete process and any sub-division of the process.
The system boundary defines the part of the process being considered. The flows into
and out of the system are those crossing the boundary and must balance with material
generated or consumed within the boundary.
Any process can be divided up in an arbitrary way to facilitate the material balance
calculations. The judicious choice of the system boundaries can often greatly simplify
what would otherwise be difficult and tortuous calculations.
No hard and fast rules can be given on the selection of suitable boundaries for all types
of material balance problems. Selection of the best sub-division for any particular process
is a matter of judgement, and depends on insight into the structure of the problem, which
can only be gained by practice. The following general rules will serve as a guide:
1. With complex processes, first take the boundary round the complete process and if
possible calculate the flows in and out. Raw materials in, products and by-products
out.
2. Select the boundaries to sub-divide the process into simple stages and make a balance
over each stage separately.
3. Select the boundary round any stage so as to reduce the number of unknown streams
to as few as possible.