DOF process flowsheet

1. QUESTION ONE
A simplified flow sheet for the manufacture of sugar is shown in Figure 2.7.
Sugar cane (S1) is fed to a mill where a syrup is squeezed out, and the resulting gases (S2)
contain 80% pulp. The syrup (S3) containing finely divided pieces of pulp is fed to a screen,
which removes all of the pulp in a waste stream (S5) and produces a clear syrup (S4)
containing 15% sugar and 85% water. The evaporator produces water (S6) and a heavy syrup
(S7) containing 40% sugar.
The crystallizer produces water (S8) and 1000 lb/h of pure sugar crystals (S9).
The objective is to determine all unknown flow rates and compositions and to calculate a
process efficiency as lb of sugar obtained per lb of sugar cane. Can you think of another
meaningful definition of efficiency for the process?
Define each unit as a processing system in order to expose all streams for analysis. Following
is summary for all of the systems using mass fractions and total mass flow rates as the
variables:

2. QUESTION TWO
A condensed version of the process for the manufacture of formaldehyde is shown in Figure
2.9. Methanol is oxidized to formaldehyde using air as the source of oxygen.
CH3OH +1/2O2  HCHO + H2O
The reactor is a fixed bed containing an iron–molybdenum oxide catalyst. The reactor feed,
stream 4, contains 8 mol% methanol and 10 mol% oxygen. The methanol is completely
converted in the reactor. A purge stream for nitrogen is taken off the overhead product from
the separator. The purge contains no water. The bottom product from the separator is a
37 wt% solution of formaldehyde in water. The additional water required for this product is
supplied to the separator in stream 5.
Calculate the flow rates of the streams that have labels. Since we are not concerned with the
unlabeled streams, we may group together the two splitters and the reactor together with the
separator. Therefore, we can make mass balances on three equipment groups. Note that
stream 3 and stream 7 have the same composition, but possibly different flows and that
stream 7 is the only exit for nitrogen. Stream 4 therefore contains no water or formaldehyde.