A new equation-oriented process model for multistream heat exchangers (MHEX) is presented with a special emphasis on handling phase changes. The model internally uses the pinch concept to ensure the minimum driving force criteria. Streams capable of phase change are split into substreams corresponding to each of the phases. A novel disjunctive representation is proposed that identifies the phases traversed by a stream during heat exchange and assigns appropriate heat loads and temperatures for heat integration. The disjunctive model can be reformulated to avoid Boolean (or integer) variables using inner minimization and complementarity constraints. The model is suitable for optimization studies, particularly when the phases of the streams at the entry and exit of the MHEX are not known a priori. The capability of the model is illustrated using two case studies based on cryogenic applications. V V C 2011 American Institute of Chemical Engineers AIChE J, 58: 190-204, 2012 Keywords: heat transfer, mathematical modeling, optimization IntroductionHeat integration in the chemical process industry is usually performed by a sequential strategy. The first step in this strategy is to design and optimize the process while assuming that all the heating and cooling loads will be supplied by the utilities. Once the process conditions (pressure, temperature, and flowrates of streams) are known, heat integration can be performed in the subsequent step, using techniques such as the problem table 1 or LP/MILP transshipment model. 2 The literature also suggests an alternate simultaneous strategy that performs the heat integration while optimizing the process. 3,4 Although the simultaneous strategy is much more difficult to implement and solve, it can lead to larger economic benefits. 5A multistream heat exchanger (MHEX) is a single process unit in which multiple hot and cold streams exchange heat simultaneously. MHEXs are very common in cryogenic applications where heat transfer equipment need to be kept compact and well-insulated while recovering heat from streams at very small temperature driving forces. 6 Use of a MHEX to perform such heat transfer tasks often leads to substantial savings in both energy and capital cost. MHEXs are traditionally analyzed using composite curves, a thermodynamic concept used in heat integration called pinch analysis. The streams in an MHEX are multicomponent and typically undergo phase changes. An important issue concerning the use of the pinch concept (or heat integration) for design or optimization of MHEXs is how to handle the nonlinear variation in heat capacity-flowrates when a stream changes phase while exchanging heat, particularly when the phases are not known a priori.There are a few noteworthy contributions on handling streams with phase changes. Ponce-Ortega et al. 7 proposed a new approximation to the logarithmic mean temperature difference, which handles matches involving phase changes in the heat exchanger network. Their approach assumes Correspondence concerning this article shoul...
Self-condensation of cyclohexanone yields an isomeric mixture of compounds, which has important industrial applications. In this work, the intrinsic kinetics for this reaction was generated in a stirred batch reactor at atmospheric pressure over the temperature range of 353−373 K. The effect of various parameters such as temperature and catalyst loading was investigated, and also the reusability of the catalyst was studied. Further, the performance of various catalysts was evaluated. The presence of water as a reaction product inhibits the reaction rate, leading to lower conversion levels. The reaction was studied with special emphasis toward variation of selectivity with conversion. An activity-based kinetic model was developed for the reacting system, which represents the data quite well. Reactive distillation can be advantageously used to enhance the selectivity toward the desired product.
Alkylates are a class of probable replacements for MTBE as gasoline additives that can be produced by dimerization of isobutene (to isooctene) with subsequent hydrogenation. The characteristics of the dimerization reaction make it a potential candidate for reactive distillation. The dimer, being heavier than C 4 , can be maintained at a low concentration level in the reactive zone by simultaneous distillation, thereby suppressing the subsequent oligomer-producing reactions. In this work, the influence of important design and operating parameters on the performance of the reaction in a hybrid reactive distillation column is studied through process simulations. the results show that a high selectivity toward diisobutene can be achieved along with adequate temperature control in the presence as well as absence of polar components. Multiple steady states are observed in some cases that introduce additional complexities in the determination of the optimal windows for certain parameters. The process seems economically attractive, as it is capable of utilizing the existing reactive distillation assets and the feedstock for MTBE production by suitable revamping.
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