Page 169 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

several disincentives that should be considered before final selection:
1. The highest electric utility demand and cost (assuming that the compressor is electrically driven).
2. The highest cooling utility demand and cost.
3. A condition of special concern, that is, T > 400°C (see Table 6.1).
Note: Compressors are high-speed rotating devices where the loss of material strength and
thermal expansion is critical. It would be expected that the purchase cost of the compressor
would undergo a quantum jump for high-temperature operations.
4. Exceeds the 3:1 pressure ratio provided as a guideline (see Table 6.4).

Alternative -B- is more complex than Alternative -A- because it requires an additional heat exchanger,
but it avoids the condition of special concern in Item 3. The result of using this extra exchanger is a
significant decrease in utilities over Alternative -A-. As a result, it is likely that Alternative -B- would be
preferred to Alternative -A-.

Alternative -C- requires an extra stage of compression and an additional cooler before the second
compressor, something that is not required by Alternative -B-. However, Alternative -C- results in an
additional savings in utilities over Alternative -B-.

The qualitative analysis given above suggests that both Alternatives -B- and -C- are superior to
Alternative -A-. This conclusion is consistent with the two heuristics for compressors in Table 6.4: It is
better to cool a hot gas prior to compressing it, and it is usually desirable to keep the compression ratio

less than 3:1. Before a final selection is made, an economic analysis, which must include both the capital
investment and the operating costs, should be carried out on each of the competing schemes. The
equivalent annual operating cost (EAOC), described in Chapter 10, would be a suitable criterion to make
such a comparison.

You should review the information given in Table 6.4 and convince yourself that you understand the rules,
along with the penalties, remedies, and justifications, for operating equipment under these conditions. You
may also be able to provide additional reasons why operating the equipment in this way would be
justified. You should add these reasons to the list provided along with additional heuristics that are
uncovered as you work problems and gain experience.

6.4 Analysis of Important Process Conditions

In this section, we begin to analyze and to justify the conditions of special concern found in a process
flow diagram. To help with this analysis, it is beneficial to prepare a process conditions matrix (PCM). In
the PCM, all the equipment is listed vertically and the conditions of special concern are listed
horizontally. Each unit is reviewed for conditions of special concern, and a check mark is used to identify
which pieces of equipment have been identified. The PCM for the toluene hydrodealkylation process is
shown in Table 6.5. The information for this PCM was obtained from Chapter 1, and you should verify
that none of the areas of special concern have been missed.

Table 6.5 Process Conditions Matrix for the PFD of the Toluene Hydrodealklyation Process Shown
in Figure 1.5