Retrofit Design For Debottlenecking Complex Distillation Columns

Distillation is one of the most important separation processes in the chemical industry. It is widely used in chemical industry, petrochemical, fine chemical industry, etc. The purpose of retrofit distillation system includes: increasing throughtput of distillation columns, reducing consumption of energy, improving product quality or using new technology. 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 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. However, there is little work that provides both qualitative and quantitative tools for the methods of the second group problems. Recently, the concept of the hydraulic indicator—Fraction of Utilised Area (FUA) was proposed by Liu and Jobson(Liu & Jobson, Trans IChemE, 2004, 82(A1): 3–9). The authors used the method to investigate the retrofit design of simple distillation columns. The results showed that the method can provide both qualitative and quantitative design guidance for the second group problems mentioned above.As the development of technology, various forms complex distillation columns have been widely used. In this paper, the FUA curve is used to analyse the increasing throughtput of the complex distillation columns. The main contents are listed as follows:Firstly, effect of adjusting feed thermal conditions on the capacity of the complex distillation columns is investigated. The FUA curve of the base case can be used to identify the bottleneck of an existing complex distillation column when its throughput is to be increased. If the FUA curve is unbalanced, there may exist some opportunity to increase throughput by changing the feeds thermal condition. If the FUA curve is balanced, there is no opportunity to increase throughput of the complex distillation column by changing the feeds thermal condition.Secondly, effect of adding side heat exchanger and heat pump on the capacity of complex distillation columns is investigated. According to the shape of the FUA curve, the suitable location and duty of side heat exchanger and/or heat pump of a complex distillation column can be identified. The effects of using the modifications on the capacity of the complex column is also investigated.Thirdly, effect of changing the composition of feeds on the capacity of the complex distillation columns is investigated. According to the shape of the FUA curve, the effect of the composition of feeds on the throughput of the complex distillation column is investigated. Several literature examples are studied to illustrate the methods. The results show that the FUA curve can solve the problem of debottlenecking distillation processes successfully, effectively and relatively easily.