050264 Reduce Costs

050264 Reduce Costs - Reactions and Separations Reduce...

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64 www.cepmagazine.org May 2002 CEP Reactions and Separations istillation is the primary separation pro- cess used in the chemical processing in- dustries (CPI). While this unit operation has many advantages, one drawback is its significant energy requirement. The dividing-wall distillation column (DWC) offers an alternative to conventional distillation towers, with the possibility of savings in both energy and capital costs. For example, two applications of DWC technolo- gy designed by UOP are now part of a new UOP linear alkyl benzene (LAB) complex that saved 9% of the total fractionation energy used in this com- plex. Based on a present world LAB production of 2.6 million m.t./yr, and a fuel value of $10.8/million kcal, a $12.8 million annual energy savings would be possible if these DWCs were used in place of conventional multicolumn trains in every complex. In addition, the uninstalled equipment cost for the total complex is reduced by about 10%. The capital cost savings result from the reduction in the quantity of equipment ( i.e. , one column, reboiler, condenser, etc., instead of two of each). There are also indirect benefits: a DWC requires less plot area and, therefore, shorter piping and electrical runs, a smaller storm runoff system and other associated benefits. The flare loads are reduced because of the smaller heat input and less fire-case surface, leading to a smaller flare system. While a DWC is not suitable for every situation, clearly it can be an attractive alternative to conven- tional distillation. This article will review recent de- velopments in DWC technology and provide guide- lines for the design of these columns. Finally, two applications will be discussed to illustrate the de- sign process. DWC background A DWC is not a new concept, having been intro- duced in 1949 (1) . However, lack of reliable design methods, and concerns about the operation and con- trol of these columns have prevented their widespread application. Work being done in both academia and industry is helping to address these concerns. Several authors provide some good back- ground for understanding the theory behind Petlyuk columns and/or DWCs (2–11) . A recap is provided here to show how a single DWC can replace an ex- isting two-column sequence. ABC split and Petlyuk evolution Consider a mixture consisting of three compo- nents, A , B and C , where A is the lightest and C the heaviest. Figure 1a shows how this separation would be accomplished in a direct sequence of two distillation columns. For some mixtures, for in- stance when B is the major component and the split between A and B is roughly as easy as the split be- tween B and C , this configuration has an inherent thermal inefficiency (Figure 2). In the first column, the concentration of B builds to a maximum at a These distillation columns can significantly reduce capital and energy expenses vs. conventional multicolumn arrangements.
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050264 Reduce Costs - Reactions and Separations Reduce...

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