Control of Gas Temperature by Bypassing

All of these systems have some common control loops. The system pressure is controlled by manipulating the fresh feed of A (F0A). The concentration controller with ratio control is used to control reactor inlet gas composition by manipulating the fresh feed of B (F0B). Bypassing (Fhy) around the FEHE is used to control gas mixture temperature Tmix. Reactor inlet temperature (Tin or T ) is controlled by manipulating the furnace heat input QF. The setpoints of these two temperature controllers are the same, and the controller output signals are split-ranged so that bypassing and furnace heat input cannot occur simultaneously. 

The temperature 7 mix of the blended stream of gas from the heat exchanger and gas bypassing the heat exchanger is controlled by manipulating bypass flow-rate Fh y. 

Catalyst bed input gas temperatures are measured with three thermocouples in the top of each bed. They are controlled by adjusting the amount of gas being bypassed around the catalytic converter s gas cooling devices, Fig. 22.2. 

The temperature inlet the third catalyst bed is controlled by means of the bypass around the loop boiler feed water preheater, adjusting the gas temperature at converter inlet. 

A conventional control structure for this process, which works for low to moderate activation energies, is shown in Figure 9. The flowrate Fob of gaseous fresh feed of B is manipulated to control system pressure. The flowrate Fqa of gaseous fi esh feed of A is ratioed to Fob and the ratio is reset by the composition controller, which maintains the composition of A in the circulating gas stream at yuA = 0.5. A bypass stream around the FEHE controls the furnace inlet temperature. Furnace firing controls reactor inlet temperature. Separator drum temperature is controlled by heat removal in the condenser (typically the cooling water valve is wide open to minimize drum temperature). Liquid product comes off on drum level control. 

Ammonia feed is shut off by a low temperature switch when the operating flue gas temperature drops below the minimum recommended value. This prevents deactivation of the catalyst from ammonium bisulfate deposition. This control feature is also applied during system startup and shutdown. An economizer bypass is used to maintain the flue gas temperature above the minimum recommended SCR operating temperature and an SCR bypass is shown (though not often provided). The SCR bypass is used to protect the SCR catalyst during startup and shutdown when the flue gas temperature can be below its dew point. Economizer bypasses are used at the Chambers, Indiantown, and Keystone power plants (Franklin, 1993). 

The unit was built in a loop because the needed 85 standard m /hour gas exceeded the laboratory capabilities. In addition, by controlling the recycle loop-to-makeup ratio, various quantities of product could be fed for the experiments. The adiabatic reactor was a 1.8 m long, 7.5 cm diameter stainless steel pipe (3 sch. 40 pipe) with thermocouples at every 5 centimeter distance. After a SS was reached at the desired condition, the bypass valve around the preheater was suddenly closed, forcing all the gas through the preheater. This generated a step change increase in the feed temperature that started the runaway. The 20 thermocouples were displayed on an oscilloscope to see the transient changes. This was also recorded on a videotape to play back later for detailed observation. 

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