In the maleic anhydride process shown in Figure B.5.1, the temperature of Stream 6 at the outlet of the feed heater, H- 601, needs to be maintained. For this system, do the following:

1. Devise a control scheme for maintaining the temperature of Stream 6 by manipulating the flowrate of the natural gas to the furnace (i.e., firing rate of H-601). The flowrate of combustion air must be maintained at a desired ratio to the flowrate of the natural gas.

2. In an alternative scenario, it is decided that, for maintaining the furnace temperature, fuel oil will be used to minimize the consumption of natural gas. When the fuel oil alone is not sufficient to maintain the temperature, natural gas will be fired along with the fuel oil. The flowrate of combustion air should be regulated such that the desired ratios with respect to the flowrate of the natural gas and the fuel oil are both maintained. Devise a control scheme to achieve this control objective. What type of control system(s) did you design? As the fuel oil and the natural gas have very different heating values, what challenge would you expect in achieving a satisfactory performance with this control scheme?

Figure B.5.1

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V-601 C-601 Inlet Air Compressor Benzene Feed Drum Dibutyl Phthalate Make-up Air Benzene C-601 P-601A/B E-601 Benzene Benzene Feed Feed Pumps Vaporizer V-601 E-601 P-601 A/B Temperature, °C 116 H-601 P-602 A/B Feed Molten Heater Salt Circ. Pumps E-602 hps bfw P-603 A/B E-602 R-601 P-603A/B E-603 T-601 Molten Reactor Dibutyl Reactor MA Salt Cooler Products of Combustion H-601 air ng P-602 A/B hps E-604 V-602 P-604 A/B E-605 MA Tower ΜΑ MA MA Makeup Effluent Scrubber Condenser Reflux Reflux Reboiler Pumps Cooler Drum Pumps R-601 E-605 bfw T-601 hps E-603 T-602 E-606 Dibutyl Dibutyl Scrubber Tower CW/ E-604 (LIC Condenser Drum Pumps X-DLICV-602 P-604 A/B T-602 V-603 P-605 A/B Dibutyl Dibutyl Reflux Reflux E-607 E-606 CW/ FICV-603 P-605A/B P-606 A/B E-607 P-606 A/B Dibutyl Dibutyl Reboiler Recycle Pumps Off-Gas to Incinerator To Maleic Anhydride Purification