Page 146 - Plant design and economics for chemical engineers

COMPUTER-AIDED DESIGN 121

components as liquids and leave the first three in the gaseous state. Gaseous
mixtures are very difficult to separate; when they are combustible, as here,
experience indicates that it is usually best to use them as a fuel. Property data also
show that water and the liquid hydrocarbons are highly insoluble in each other and
thus will split into two liquid phases that can be separated easily. The reactor
effluent should be condensed and then separated into the following streams: water
(reuse if possible, or dispose of if not), benzene and toluene (these two do not
need to be separated unless they are to be used separately), ethylbenzene (to be
recycled to the reactor), styrene (the desired product), and gaseous fuel. It is
assumed at this stage that the separations between components are complete.
Mass balances for the process are recalculated using the extent-of-reaction
values from above and assuming that all unreacted ethylbenzene (EB) is recycled
and converted to products. On the basis of 1 lb-mol(lO4 lb) of styrene product, the
calculations are:
Feed EB flowrate = (1 lb-mol styrene/h)
* (0.5 mol EB/0.47 mol styrene)
* (106 lb EB/lb mol)
= 112.8 lb/h
The EB feed rate to the reactor must be twice this, because only one-half of the
feed reacts per pass and the EB recycle rate equals the feed rate; Thus,
Reactor EB feed rate = 225.6 lb/h
The steam feed rate is then = (225.6/106 mol EB/h)
* (14 mol steam/m01 EB)
* (18 lb steam/m01 steam)
= 536 lb/h
The by-product benzene plus toluene and the fuel gas rates are calculated, from
Eqs. (2) and (3), to be 5.2 and 3.6 lb/h, respectively. The raw materials and
product values can be compared again. The benzene-toluene mixture is valued at
$O.lO/lb and the fuel gas at $O.l8/lb (both based on heating value); condensed
water has no value. The value of the feeds, ethylbenzene and steam, is $33.54/104
lb of styrene. The value of the product plus the by-products and fuel gas is
$44.84/104 lb of styrene. The value of the outputs still exceeds that of the inputs,
but the margin has narrowed.
The process, as identified so far, is shown in Fig. 4-3.
Step 3. Separation processes. The separation after condensation into a
water phase, a hydrocarbon phase, and a gas phase is accomplished by gravity in
one or two holding tanks (other phase-separation methods are available and could
be considered in a more detailed design). The hydrocarbon phase needs to be
separated into three fractions: (1) benzene plus toluene, (2) ethylbenzene, and (3)
styrene. Experience shows that components with boiling points near ambient
temperature (between, say, 0 and 200°C) and with adequate differences between
their boiling points (at least 5°C) are usually most economically separated by
distillation. An ordinary fractional distillation column yields two products; two
distillations are required to produce the three hydrocarbon products in this
process. Experience again provides guidance in planning this separation: First,