Page 160 - Plant design and economics for chemical engineers

1 3 4 PLANT DESIGN AND ECONOMICS FOR CHEMICAL ENGINEERS

calculations are repeated in the same sequence:
SO2 and SO3 = 169.2
SO4 = 402.2
SOS = so6 = 571.4
S14 = 169.2*0.5 = 84.6
Again, SO5 and S14 have changed. These new values are now used as the
calculations are again repeated:
so2 = SO3 = 197.4
SO4 = 469.3
SO5 = SO6 = 666.7
s14 = 98.7
This sequence of calculations is repeated until SO.5 and S14 do not change
significantly. The final results are, as obtained in Example 3:
SO2 = SO3 = 225.6
SO4 = 536
SO5 = 761.6
S14 = 112.8
The styrene feed was used as the starting point in this example; the stream
feed could have been chosen just as well. If it is, the same recycle loops will be
identified, but a somewhat different calculation sequence will be identified. This is
recommended as an exercise for the reader.

Many flow-sheeting programs perform the partitioning, solution ordering,
and tearing functions discussed above and present the user with one or more
choices of solution sequence and tear variables. FLOWTRAN, however, does
not do this. The user must identify the recycle loops, the calculation sequence,
and the tear streams. The preceding example illustrated their identification and
selection.

PROBLEMS
1. A Hock diagram for a gas processing plant is shown below. The purpose of this
process is to recover ethane and propane (+ butane) from a natural-gas stream
because these components have more value as chemicals than as fuel. This problem is
based upon the 1987 AIChE Student Contest Problem.
Natural gas is to be fed to the process at a rate of 2.5 X lo6 g-mol/h. The
feed-gas composition and product-gas specifications are given on page 135.