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1 Floating pressure strategy or control is a technique pioneered by I Shir.skey (4,5) in which the column is trimmed or controlled to operate at the minimum possible pressure. This technique achieves significant energy savings during periods of favorable ambient i conditions.

| For most systems, relative volatility increases as pressure is i reduced, making separations easier; the ensuing reduction in reflux | ratio accounts for a lower reboiler duty as pressure reduces. This lower duty benefit is usually counteracted by an increase in latent heat, and a small reduction in tray efficiency as the pressure is lowered. In general, the effect of pressure on relative volatility dominates, and reboiler duty can be minimized by operating at the ! lowest practicable pressure.

Reduction of distillation pressure is usually restricted by condenser capacity. However, condensers are normally designed to operate at j ambient conditions prevailing on a hot summer day, and during winter | periods, when air and cooling water temperatures are lower, spare. | capacity becomes available. This spare capacity can be utilized for I pressure reduction and consequent energy savings.-I

! The extent to which pressure can be lowered is determined by the spare capacity available in the condenser, but is often limited by j other restrictions, such as availability of sufficient pressure to | transfer products to the next unit, or by the onset of flooding. In ; most cases, these restrictions would not eliminate energy savings from floating pressure operation, but rather restrict the extent to | which column pressure can be reduced.

; APPLICATION Floating pressure does not always lead to significant | energy savings. Highest energy savings occur when the ratio of | operating reflux to minimum reflux is high, relative volatility is j low, the rate of change of volatility with temperature is high, and j product purity is high. Negligible energy savings occur when : relative volatility is high and its rate of change with temperature I is low. If the ratio of operating to minimum reflux ratio is low, significant energy savings can only be achieved when relative volatility is very low and its rate of change with temperature is unusually high. A more detailed set of application guidelines has been presented elsewhere (6).

When the column includes feed preheaters or interreboilers, floating pressure can lead to even larger energy savings than those achievable from enhancing relative volatility alone. Reduction of column pressure lowers the equilibrium temperature at the point of interreboiling or preheating. If the preheat or interreboil is restricted by exchanger AT or area, lowering this temperature will increase the preheater or interreboiler capacity, which would enable additional cuts in reboil du-ty. Note that the above would not apply I if the exchanger employs one of the column products as the heating medium (eg feed-bottom interchanged , because reducing column pressure would also reduce the heating medium temperature.

CONTROL SYSTEM Floating pressure car. be implemented as an operating strategy or a? a control system. Shinskey (5/ developed the floating pressure control method, in which a valve-position controller (Figure 4.14) is used to adjust the set point of the pressure controller. This valve position controller keeps the condenser control valve as fully open (or fully shut, depending on the control system) as practicable, without going "off control".

The valve position controller acts very slowly, so that the pressure controller can achieve its normal pressure control function. Figure 4.14 demonstrates the response to sudden rain (assuming an uninsulated column) . The pressure controller takes corrective action immediately, and maintains column pressure at the initial upset. The valve position controller slowly takes over an reduces the pressure set point over the next 30 minutes.

Floating pressure control is a strategy very commonly implemented in advanced (computer) control systems.

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' S.net ST— 3^ '5"* Fig. 4-14bThi column pressure ui II mo it exponentially to a neu steady state as trie bypass lalve returns to a nearly closed poi trion.

Fig. 4-l*tThe vahe-positton controller slouly adjusts the pressure set point to keep the condenter fully loaded in the lung term.

(F.G. Shinskey, "Distillation by courtesy, McGraw-Hill Book

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' S.net ST— 3^ '5"* Fig. 4-14bThi column pressure ui II mo it exponentially to a neu steady state as trie bypass lalve returns to a nearly closed poi trion.

Control",1984, Company.)

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