Distillation is one of the most important separation processes in the chemical industry. Retrofit design of distillation processes receives much attention from chemical engineers. Retrofit design for debottlenecking distillation processes is one of the most important objectives in the retrofit of distillation processes. The existing approaches for retrofit design for debottlenecking distillation processes can be classified into: (a) replacing the existing internals with high capacity and/or high efficiency internals; (b) improving the performance of the existing equipment by making some necessary modifications, such as adjusting the operating conditions, adding some extra equipment, etc. The retrofit cost of class (b) is often lower than class (a). However, there is little work that provides both qualitative and quantitative tools for the methods of this second approach to retrofit design. In this thesis, new methods are proposed to solve the problems of debottlenecking distillation columns, by carrying out minor modifications to the columns. Both qualitative and quantitative guidelines and methods are developed. To analyse the hydraulic performance of distillation columns, we propose a new graphical tool to identify the bottlenecks of a distillation column when the throughput of the column is to be increased. This tool can also be used to evaluate, in both qualitative and quantitative ways, the potential modifications that can be applied to increase the throughput of the column. Several guidelines, based on the performance of a distillation column and the characteristics of the mixture being separated, are proposed to evaluate potential modifications. The potential modifications discussed in this thesis include: adjusting the operating conditions, such as, the thermal condition of the feed, operating pressure, etc.; adding extra equipment, such as side heat exchangers, heat pumps, etc.; increasing the number of theoretical stages, using high efficiency and/or high performance internals, for example. The influence of the operating pressure on the throughput of distillation columns-is investigated. This work reveals that at low pressures, increasing the operating pressure can increase the throughput of a distillation column. At high pressures, decreasing the operating pressure can increase the throughput of a distillation column; and at moderate pressures, pressure has little influence on the throughput of a distillation column. This thesis provides a clear and complete picture of the influence of the operating pressure on the throughput of distillation columns. Downcomer flooding, which is another limitation of the capacity of a frayed distillation column at high pressure or high liquid flow rate, is also addressed. Some minor modifications are suggested, which can be used to increase the capacity of the downcomer. The selection of suitable column internals is also addressed in this thesis. A two-stage procedure is proposed to select new internals. In retrofit design, the existing internals should be re-used if possible to reduce costs. The operating conditions can be adjusted to increase the capacity of the internals. Several examples are studied to illustrate the proposed methods. The results show that the proposed methods can solve the problem of debottlenecking distillation processes successfully, effectively and relatively easily.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:506285 |
| Date | January 2000 |
| Creators | Liu, Zhi-Yong |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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