Why Control Tower Pressure

Why are distillation towers designed with controls that fix the tower pressure  

Naturally, we do not want to overpressure the tower and pop open the safety relief valve. Alternatively, if the tower pressure gets too low, we could not condense the reflux. Then the liquid level in the reflux drum would fall and the reflux pump would lose suction and cavitate. But assuming that we have plenty of condensing capacity and are operating well below the relief valve set pressure, why do we attempt to fix the tower pressure Further, how do we know what pressure target to select  

I well remember one pentane-hexane splitter in Toronto. The tower simply could not make a decent split, regardless of the feed or reflux rate selected. The tower-top pressure was swinging between 12 and 20 psig. The flooded condenser pressure control valve, shown in Fig. 6.1, was operating between 5 and 15 percent open, and hence it was responding in a non-linear fashion (most control valves work properly only at 20 to 75 percent open). The problem may be explained as follows. 

The liquid on the tray deck was at its bubble, or boiling, point. A sudden decrease in the tower pressure caused the liquid to boil violently. The resulting surge in vapor flow promoted jet entrainment, or flooding. 

Alternately, the vapor flowing between trays was at its dew point. A sudden increase in tower pressure caused a rapid condensation of this vapor and a loss in vapor velocity through the tray deck holes. The resulting loss in vapor flow caused the tray decks to dump. 

Either way, erratic tower pressure results in alternating flooding and dumping, and therefore reduced tray efficiency. While gradual 

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While we realize that distillation towers are designed with a control scheme to fix the tower pressure, why is this necessary ... 

Another reason to raise tower pressure is to permit higher reflux rates. If the pressure controller in Fig. 3.1 is set too low, then during hot weather, when condenser capacity becomes marginal, the level in the reflux drum will be lost. If we then raise the pressure set point, the drum will refill—but why ... 

The most intriguing question is why the plant s safety equipment and controls, which should liave been designed to cope with a chemical tliat is known to be subject to violent reactions, did not work. Tlie MIC tank liad tliree safety devices. A pipe leading from tlie tank contained a valve tliat was set to rupture if the pressure in the tank e.xceeded 40 psi. Beyond the valve there were two other safety devices. One was a flare (tower) to bum escaping gas. The other was a vent gas scrubber, a tower packed with loose niaterial tlirough wliich a solution of caustic soda could be poured onto rising MIC to decompose it. On December 3, 1984, tlie operator of the factory s control room noticed tliat the pressure in tlie MIC tank (see Figure 1.3.2) luid risen from 2 to 30 psi, and in a... 

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