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Synthesis, Design And Operating Strategies For Batch Reactive Distillation

Abstract
Batch reactive distillation (BRD), combines the flexibility of batch processing with the advantages of reactive distillation and can offer advantages over conventional batch processing for small to medium volume production. BRD is also important in process development for reactive distillation. The essence of reactive distillation is the integration between reaction and distillation. There are two limiting cases for integration between reaction and distillation, (1) no integration and (2) full integration. In this thesis, we have shown that by analyzing these two limiting cases, and combining the information obtained from two limiting cases in a novel way, we can develop a partial integrated design with an appropriate amount of integration and an appropriate operation mode, which can provide advantages over both conventional design without integration and a fully integrated design. We apply this approach in two realistic examples, i.e. (1) isopropyl acetate synthesis and (2) 1, 1-dimethoxyethane production. For the isopropyl acetate synthesis, we compare the vapor-liquid and liquid-liquid features of the phase equilibrium to nbutyl acetate and amyl acetate systems, and show the similarities and differences among them. In particular, we show why the isopropyl system is more difficult than the others and how to use the known information in two limiting cases to develop a semi-batch reactive distillation (SBRD) with partial integration, which overcomes the reactive azeotrope brought by full integration and results in a more efficient process for the production of isopropyl acetate than has been previously known. In SBRD, the loss of isopropanol can be substantially decreased, the purity of water can be improved, and the total refluxing or recycling of organic distillate can be avoided. The resulting SBRD can provide 20% higher production efficiency than BRD. For the example of 1, 1-dimethoxyethane (DMA) synthesis, following the approach, we develop a partially integrated BRD, which can take advantage of integration to overcome the reaction equilibrium limitation, meanwhile avoid a distillation boundary brought by over-integration. This leads to a high-purity product, which is unattainable in a fully integrated reactive distillation process without special and expensive treatment of the methanol/DMA azeotrope. In this thesis, we also address selectivity issues in BRD, and provide new results describing the impact of key operating parameters: the reflux or reboil ratio and the Damköhler number (Da ). We show that selectivity improvements in BRD are limited for high values of Da or for high values of the reflux or reboil ratio and that selectivity is enhanced as Da or reflux or reboil ratio is decreased. However, decreasing Da will cause conversion loss, which can be mitigated by increasing reflux ratio (or reboil ratio) at expense of selectivity. Consequently, there is an optimum value of reflux or reboil ratio that gives a maximum yield for systems operated at low or moderate Da . We show sample results for a system of serial isomerization reactions and for the synthesis of ethylene glycol. For the isomerization, we show that for BRD with a constant heating rate, the additional heat released by reaction can improve selectivity at expense of conversion. For ethylene glycol synthesis, we show that at a low reboil ratio, integration between reaction and distillation has a negative impact on both conversion and selectivity by causing separation of the reactants. We also show that decreasing the reboil ratio near the end of the BRD can increase the removal rate of ethylene glycol and thereby improve selectivity. This operating strategy is different from the common practice in distillation of increasing the reboil ratio near the end of a batch or cut.
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campus
article
dissertation
Date
2010-09-01
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