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The typical distillation column which is fed F moles/hr consists of two major components.
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The material is separated in D moles/hr overhead or distillate product consisting of volatile products, and B moles/hr bottom products consisting of non-volatile products.
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Main job is to adjust split between distillate and bottoms by controlling V/F ratio or D/F ratio.
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Consider an example in which feed contains 50% benzene & 50 % toluene, and desired purity is 98% for both products. Normal value of D & B are moles/hr if feed is 100 moles/hr.
If D is increased to 52, overhead purity decreases to 96%.High purity products can be obtained by decreasing D but bottom would be very impure. -
If feed and composition is constant control can be achieved by regulating D or B.
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Bottom flow is regulated by constant level of reboiler(fig a).
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Distillate flow is regulated by level in reflux drum(fig b).
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