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10/27/99

ChE 182 Major 2 Styrene Production
Thank you for your analysis of the intermittent problems in our styrene production facility, Unit 500. The ratio controller on H-501 has been fixed and the process is once again running at design conditions. The process flow diagram is shown in Figure 1. Tables 1 and 2 show the design conditions for Unit 500. In order to decrease the cost ofmanufacture for the styrene process, it is desirable to lower the steam input to the reactor, possibly lower the steam temperature, and optimize the economics for selectivity for styrene. It has also been suggested to purchase a new heat exchanger to use Stream 25 to preheat and vaporize the ethylbenzene in Stream 2, Stream 3, or Stream 23. Your assignment is to determine optimal placement and operatingconditions for this improvement. As always, you may include additional process modifications that are profitable. The existing production rate and purity of styrene must be maintained. Limitations on the reactors are a maximum temperature of 1000 K, a maximum pressure of 2.5 bar, and a minimum pressure of 0.75 bar. It is required that the temperature and pressure entering the three-phase separatorbe maintained at existing values. The pressure can be maintained by adding either a valve or a small pump before the flash. It is possible that the temperature can be maintained by adjusting the utility flows in E-503 through E-505. If this is not possible, there is a spare, unused heat exchanger that can be put into service. Since we already have this heat exchanger, there is no capital cost;however, you should include the required utility cost in the incremental EAOC. The objective function is the incremental EAOC, as defined in Chapter 5 of your textbook1. The only incremental capital cost is that for the new heat exchanger. You decide exactly where to place it. Assume that the minimum approach temperature allowable in this new heat exchanger is 50°C. The incremental savings in costof manufacture will be from reduced lowpressure steam use (use cost per kg in Table 3.4), reduced ethylbenzene feed ($0.56/kg), reduced natural gas use in H-501 (Table 3.4), and possibly high-pressure steam in E-501 (use cost per GJ in Table 3.4). Assume the cost of manufacture changes by the sum of the incremental changes in the costs of low-pressure steam, ethylbenzene, natural gas, andhigh-pressure steam. Use an internal hurdle rate of 15% before taxes, with an equipment life of 10 years. Since optimization of the feed section will change the flowrates, temperatures, and concentrations of each stream, you also must analyze all units from the feed through E-505 to determine the conditions (T and P) of all streams through Stream 15. This analysis must be quantitative, detailed, and notdone on Chemcad except for the reactor. You may assume that the all stream physical properties are unchanged from the design case. Clearly, the separation section will also be affected. For this preliminary evaluation, you should analyze qualitatively the effect the changes in the feeds to the separation section will have on the units in the separation section.

H-501 steam heater E-501 feedheater

E-502 interheater R-501 styrene reactor

R-502 styrene reactor

E-503 product cooler

E-504 E-505 V-501 C-501 product product three- comprescooler cooler phase sor separator

P-501A/B waste water pump

T-501 E-506 benzene reboiler toluene column

E-507 P-502A/B V-502 condenser reflux reflux drum pump

T-502 E-508 E-509 P-503 A/B styrene reboiler condenser reflux column pumpP-504 A/B benzene toluene pump P-505 A/B P-506 A/B styrene recycle pump pump

29
hps

23 P-506 A/B 19

1 ethylbenzene

2 E-501

3 8

9 C-501 26 P-504 A/B

hydrogen

16 R-501 T-501
FIC

E-507
cw

21

benzene toluene

H-501 7 5 4 lps 6

ratio

10

E-502 25 to steam plant

V-501
17

FIC

V-502

LIC

20

11

P-502 A/B T-502
LIC FIC

E-509
cw

ratio...
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