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020248 Get More - Reactions and Separations Get More Out of...

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48 www.cepmagazine.org February 2002 CEP Reactions and Separations ingle-stage distillation systems are widely used in petrochemical, chemical and phar- maceutical plants. This unit operation goes by several names, but is often referred to as a flash drum or evaporator. We will use “evaporator” here, with the understanding that we are not referring to a heat-transfer device, but, instead, to the entire single-stage distillation system (an adiabat- ic “evaporator” system has no source of heat input). A review of the design and operation of this most basic unit operation is available in various sources (1, 2) and will not be covered here. However, we need to ensure that the terminology of evaporators is under- stood, since there are several design features that are common to most such processes. Basic terminology Evaporators typically employ heat to concentrate solutions or to recover dissolved solids by precipitat- ing them from saturated solutions. The scope of our practical solutions will be primarily evaporators of the former classification. A typical industrial evapo- rator has tubular heating surfaces, a vessel to hold in- ventory and disengage vapor from liquid, and a heat exchanger to condense the lighter overheads product. These units can operate at atmospheric or elevated pressures, but are often run under vacuum to reduce the system temperature. This unit operation can be run continuously, semi-batchwise or fully batchwise. In the pharmaceutical industry, it is our empirical ob- servation that most evaporations are either semi- or fully batch. While the product from the evaporation step can either be the overhead or pot contents, the solution to be separated is usually either wide-boiling or has a non-ideal vapor/liquid equilibrium (such as an azeotrope), which results in a more discreet sepa- ration between mixture components. Figure 1 shows a typical evaporator system. A recent capacity initiative at Eli Lilly uncovered several varied and creative methods to reduce cycle time, increase throughput, and/or increase product quality in our semi-continuous train of evaporators. These were based on observations of a series of vacu- um evaporator units that remove and exchange various solvents from a temperature-sensitive, high-boiling product stream (dissolved solute or slurry). These im- provement methods were simplified and generalized to illustrate select ways to boost capacity or reduce cycle time in similar batch distillation operations. In most cases, the practical solutions are applicable to both vac- uum and atmospheric evaporators, and can sometimes be extended to continuous units, as well. Increasing capacity With a basis established as to what an evaporator train may look like, as well as the terminology to be used, discussion can now move into capacity creation and/or recovery. The question typically posed by man- agement is: “How can we increase our capacity?” To answer this, other questions need to be resolved: 1. By how much do we need/want to increase the
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020248 Get More - Reactions and Separations Get More Out of...

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