Distillation pressure control uses either mass or energy balances around the unit. Mass methods control pressure by regulating the amount of flow into or out of the tower. Energy methods regulate the heat flow into and out of the tower. In general, mass flow methods control the tower vapor inventory. This may be directly by throttling the vapor rate out of the system or indirectly by manipulating downstream equipment that evacuates gas from the system. Energy methods control the heat flux in the overhead condenser. Temperature (on either side of the condenser) and effective surface area are used to vary heat flux.
In general, the best control method uses a control function physically as close as possible to the controlled variable. Since the control objective is usually tower overhead pressure or condensate drum pressure, our control configurations will look at different ways to control pressure by varying condenser duty and overhead product rates. The literature includes systems that control overhead pressure by reboiler duty, however, these are unusual and will not be covered here.
Most of the control systems shown are in current use in the
hydrocarbon processing industries. Other industries may have specific
control options available that are not shown here. Both vacuum
systems and pressure systems are shown.
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