Page 82 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

toluene and benzene fluctuate in phase with each other, but the toluene price ($0.648/kg) is currently
elevated relative to that of benzene ($0.657/kg). In general, toluene disproportionation has been the
preferred process for benzene production over the last two decades.

Examples 2.1 and 2.2 make it apparent that a better approach to evaluating the margin for a process would
be to find cost data for the feed and product chemicals over a period of several years to get average
values and then use these to evaluate the margin. Another important point to note is that there are often two
or more different chemical paths to produce a given product. These paths may all be technically feasible;
that is, catalysts for the reactions and separation processes to isolate and purify the products probably
exist. However, it is the costs of the raw materials that usually play the major role when deciding which
process to choose.

2.4 Step 3—The Recycle Structure of the Process

The remaining three steps in building the process flow diagram basically involve the recovery of
materials and energy from the process. It may be instructive to break down the operating costs for a
typical chemical process. This analysis for the toluene process is given in Chapter 8, Example 8.10. From

the results of Example 8.10, we can see that raw material costs (toluene and hydrogen) account for
(60.549)/(87.3) × 100 = 69% of the total manufacturing costs. This value is typical for chemical
processes. Peters and Timmerhaus [9] suggest that raw materials make up between 10% and 50% of the
total operating costs for processing plants; however, due to increasing conservation and waste
minimization techniques this estimate may be low, and an upper limit of 75% is more realistic. Because
these raw materials are so valuable, it is imperative that we be able to separate and recycle unused
reactants. Indeed, high efficiency for raw material usage is a requirement of the vast majority of chemical
processes. This is why the generic block flow process diagram (Figure 2.4) has a recycle stream shown.
However, the extent of recycling of unused reactants depends largely on the ease with which these
unreacted raw materials can be separated (and purified) from the products that are formed within the
reactor.

2.4.1 Efficiency of Raw Material Usage

It is important to understand the difference between single-pass conversion in the reactor, the overall
conversion in the process, and the yield.

(2.1)

(2.2)