Simulation And Analysis Of Operation Flexibility Of Divided Wall Column For Ternary Distillation

Distillation is a basic unit and the most reliable separate operation in chemical process industries.However,distillation is a high energy consuming and low thermodynamic efficiency process.Therefore,energy saving of distillation is widely concerned.Thermal coupling is a common energy-saving method for distillation operation.The most common thermal coupling distillation columns used in industries are the side-rectifier and side stripper configurations.While divided-wall column(DWC)or fully thermally coupled distillation column,or Petlyuk configuration has drawn much attention since the last decade.Divided-wall column(DWC)as a fully thermally coupled distillation column,can achieve the thermal coupling within a single column shell for separating multi-components mixtures.As a consequence,DWC can decrease drastically the investment and operation costs.Studies have shown that DWC can save more than 30% energy compared to traditional column system for separating ternary mixtures.DWC is of coupling of multi-separation tasks in nature,and can obviously increase control difficulties in operation.Thus,it is the main reason why industrial application of DWC has been limited.The operation and control of a DWC is more complex than those for traditional columns because of its coupling feature.However it exhibits more flexibility due to the extra degree of freedoms than the traditional columns.In this dissertation,seven DWCs are designed for separating different feed compositions of ideal mixtures of n-pentane,n-hexane and n-heptane.And their operational flexibilities are investigated by rigorous simulation using Aspen Plus software.The flexibilities of the columns in terms of the change of feed compositions re described in triangle composition diagram.In the investigation,the feed composition is changed until the products specification cannot be satisfied by changing the operating conditions.This dissertation compares the operation flexibility of divided-wall column with traditional column.The results show that the operation flexibilities of the DWCs designed for different feed composition are different.The operation flexibility of a DWC is higher than that of traditional column.The effect hydraulic pressure drop on the flexibility of a DWC is also investigated in the dissertation.The operation flexibilities of partial thermally coupled distillation columns,including side rectification and side stripping are also compared.The operation flexibilities of partial thermally coupled distillation columns and DWCs are analyzed using minimum vapor flow rate principle,and the results are consistent with those by rigorous simulation..The results of the present study can be helpful in better understanding of the operability characters of the DWC,and can also provide reference for design and operation practice for DWCs.