2r-solvent-switch

2r-solvent-switch - Reactions and Separations Reduce...

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56 www.cepmagazine.org January 2002 CEP Reactions and Separations atch distillation is commonly applied to switch solvents in a production process. Sol- vents might be exchanged to crystallize the final product or to conduct a subsequent reac- tion. The most common procedure usually imitates what was easiest for the chemist who developed the process in the lab. First, the batch is boiled down to remove much of the original solvent. Next, the replacement solvent is charged to the batch. Then, the batch is boiled down again to remove the remainder of the original solvent. The last two steps may be repeated to minimize the amount of original solvent left in the batch. With this method, the batch ends up mainly in the replacement solvent, with some trace left of the original solvent. Some of the replacement solvent also co-distills and is lost to the distillate. The loss of replacement solvent is a raw material cost, and often a waste-disposal cost. This article describes a simple modification to mini- mize the loss of replacement solvent. Rather than charging the replacement solvent prior to the start of the final distillation, the replacement solvent should be added continuously to maintain a constant level during the batch distillation. Batch distillation basics Mathematically, batch distillation is modeled by a differential equation (Table 1). The equations in the last row of Table 1 become: y dL = L dx + x dL (1) which is solved to give the familiar Rayleigh equation: (2) For a simple one-pot distillation with no reflux, y is the equilibrium vapor concentration corresponding to the liquid concentration, x . This derivation is found in standard references (1, p. 13-96; 2, pp. 1–2; 3; 4, pp. 305–307 ) . Constant-level batch distillation A constant-level batch distillation is also modeled by a differential equation (Table 2). The derivation is simplified by doing the component balance on the orig- ln L f L i = dx y x x i x f In simple takeover distillations to switch process solvents, operating at a constant level can reduce the loss of the replacement solvent to the distillate by 50–70%. Reduce Solvent Usage in Batch Distillation B Michael J. Gentilcore, Tyco Healthcare — Mallinckrodt Table 1. Simple batch distillation differential-material balance. Total Material Component Moles In 0 Moles Out dV y dV Moles Accumulated dL d ( Lx ) = L dx + x dL In – Out = Accumulation 0 – dV = 0 – y dV = + Total Material Component Moles In dF x dF = 0 × dF = Moles Out Moles Accumulated dL = d(Lx) = + x dL = L dx In – Out = Accumulation dF – dV = 0 = Table 2. Constant-level batch distillation differential-material balance.
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CEP January 2002 www.cepmagazine.org 57 inal solvent, and assuming that the replacement solvent is pure. The equations in the last row of Table 2 become: y dF = L dx (3) which is solved to give the equation: (4) This equation is not found in standard references, al- though it should be. Its economic benefits will be shown
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This note was uploaded on 12/29/2011 for the course CHE 128 taught by Professor Scott,s during the Fall '08 term at UCSB.

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2r-solvent-switch - Reactions and Separations Reduce...

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