Page 144 - Plant design and economics for chemical engineers

COMPUTER-AIDED

(2 lb) requires 1 lb-mol of ethylbenzene (106 lb); Thus, on a basis
styrene, the value of the products is
104*0.42 + 2*0.30 = $44.28
while the value of the ethylbenzene is
106*0.25 = $26.50
The product value exceeds that of the raw materials, so R&Q@
process to be profitable and the process synthesis may continue. %&c@#.I
carrying out the process-the equipment, utilities, labor, overhead, etc.-has not’
been accounted for, nor has the fact that the conversion of reactant to product
necessarily will be less than 100 percent.
The chemistry of the process is considered more thoroughly. Reaction (1) is
an equilibrium reaction; it does not go to completion. The reaction is conducted.in
the gas phase over a solid, ferric-oxide based, catalyst. It is carried out at about
1-atm total pressure (low pressure favors the products of this equilibrium) and a
temperature around 600°C (1112°F). Steam is added to lower the partial pressure
of the products, thereby further favoring their formation. The reaction is
endothermic, with the heat required being supplied by the steam. A steam-to-
ethylbenzene molar ratio of 14 to 1 is representative. At typical process
temperatures other reactions occur, including
C,H,-C,H, + C,H, + C2H, (2)
1 benzene ethylene
and
C,H,-C,H, + H, = C,H,-CH, + CH, . (3)
to1uene methane
Reactions (2) and (3) consume ethylbenzene without producing the desired styrene;
they also produce by-products that must be separated from the product and
disposed of or utilized.
The process feeds are ethylbenzene and steam and the products are
condensed steam, styrene, benzene, toluene, hydrogen, methane, and ethylene.
Including the steam, valued at $O.Ol/lb, adds $2.52 to the total feed cost, raising it
to $29.02 per 104 lb of styrene product. The value of the products still exceeds that
of the inputs.
Our knowledge of the process at this point is shown schematically in Fig. 4-2.
Step 2. Recycle structure. It is necessary to have information on the extent
of each of the three reactions at the reactor conditions. These extents could be
calculated by using complete reaction-kinetic information, and this probably would
be done in a more complete design of the process. Fogler presents examples of
such models.? Process-invention programs, however, avoid lengthy calculations by
simplifying the models used. A chemical reactor might be modeled by expressing
the extent of each reaction as a function of the conversion-per-pass of a key
reactant or, even more simply, as a constant. Simplified models are derived from
detailed data or models of process operations and reasonable assumptions

tH. S. Fogler, “Elements of Chemical Reaction Engineering,” Chapter 9, Prentice-Hall, Englewood
Cliffs, NJ, 1986.