Energy Efficient Hybrid Separation Processes

Distillation accounts for a large percentage of the energy used in the manufacturing industry. As energy costs rise, hybrid separation strategies sstrategies that combine one or more separation techniques with distillation s are attracting attention as a means of saving energy. Examples of hybrid separation schemes include extraction followed by distillation, reactive distillation, adsorption/distillation, and others. In this work, the energy efficiency of hybrid separation schemes is studied using the novel concept of shortest separation lines. Hybrid separation of acetic acid and water using extraction/distillation is used to show that the shortest separation lines correctly define the target extract compositions for the extractor and lead to the most energy efficient hybrid separations. A global optimization strategy, which uses a mixture of feasible and infeasible subsets of constraints to avoid the discrete nature of the feasible region, is presented for directly computing the most energy efficient hybrid separation schemes. Batch and continuous distillation and crystallization have been the workhorses for separations in the petroleum, chemical, pharmaceutical, and other industries for many years, and this is unlikely to change. These unit operations, as well as others, will remain the primary means of separation in many industries for the foreseeable future. Other separation techniques like chromatography and membrane separation simply cannot provide the purity and volume to be competitive. However, distillation consumes significant amounts of energy. While some believe that these unit operations are mature technologies and that there is little to be gained from research in separations such as distillation and crystallization, we disagree with this viewpoint for two reasons. First, with the recent significant increase in global energy demands and every indication that demand will remain high, it is important to consider ways of designing new separation processes and retrofitting existing ones so they are energy efficient. Hybrid separations such as extraction followed by distillation and reactiVe distillation can often be used to reduce the energy costs of conVentional distillation alone. Second, the approach taken in this work is a direct outgrowth of recent results that shed new light on residue curves and distillation lines and it is unlikely that we would have uncovered the proposed characterization of energy efficient separations without our initial results. Lucia and Taylor 1 have recently presented a geometric methodology for finding exact boundaries in separation processes and shown that for ternary mixtures all separation boundaries are given by the locally longest residue curves that run from a given unstable node to all reachable stable nodes. See Figure 1sin which the numbers associated with each residue curve represent the distance from any unstable node to all reachable stable nodes. For this illustration of ethanol/ethyl acetate/water at 1 atm, the liquid phase was modeled by the UNIQUAC equation and the vapor phase was assumed to be ideal. The associated binary interaction parameters for the UNIQUAC model can be found in the Appendix. For four-component mixtures, boundaries are local maxima in surface areas, while for five or more components boundaries correspond to local maxima in volumes. This geometric theory has led to an efficient feasible path optimization algorithm for computing exact separation boundaries for a wide variety of batch or continuous separations. Moreover, rigorous proof and a number of challenging numerical illustrations have been used to validate the theory. Motivation and Overview of this Work. Recent increases in the demand for energy on the world market have resulted in serious concern over the high energy costs associated with distillation. Hybrid separation schemes (i.e., extraction/distillation, reactive distillation, adsorption/distillation, and so on) represent one way of reducing the energy costs of distillation alone. The motivation for this work comes from our fundamental belief that there is a connection between the length of residue curves (or distillation lines) and the energy needed to perform a given separation. In particular, we began with the intuitive