Strategies For Controlling The Externally Heat-Integrated Double Distillation Columns (EHIDDIC): Synthesizing And Analyzing

In the separation process of distillation, the upper section ofdistillation column known as rectifying section releases heat, while thelower stripping section requires heat supply. If the heat from strippingsection can be removed to the rectifying section, energy saving andthermodynamic efficiency can be improved. Based on the above target, theexternally heat-integrated distillation columns (EHIDDiC) was developed,which consisted of two individual distillation columns. The heat istransfered using heat exchangers located between the rectifying section ofone column (high pressure column) and the stripping section of the other(low pressure column).In recent years, as a rising star in chemical engineering, the EHIDDiCdraws lots of attention. It has been sufficiently proved that the EHIDDiChas a potential of energy saving and high thermodynamic efficiency.Numerous papers can be found about EHIDDiC, but few of them areconcerning the operating and control of EHIDDiC.The objective of thiswork is to synthesize and analyze novel control strategies for EHIDDiC. Two essential characters of EHIDDiC are mainly considered. First,EHIDDiC is a parallel unit. The high pressure column and the low pressurecolumn have same products in both overhead and bottom, while the feed issplitted into two streams and are fed to them separately. Hence, mixing thetop or the bottom same products are two strategies in the design of controlsystem. Second, taking the simplified EHIDDiC (S-EHIDDiC) into account,the duty of heat exchangers can be used as manipulated variables in thenovel control shceme. The inputs and outputs variables are matched upthrough RGA analysis and control structures are fixed. In separating thebenzene/toluene mixtures, the comparative closed-loop evaluation isconducted between the proposed three control schemes and theconventional one separetely.The results indicated that the top blended scheme and bottom blendedscheme with three control loops are better than the conventional four loopscontrol. What is more, the top blended configuration appears to be superiorto the bottom one. For the S-EHIDDiC, the disturbances can be handledand somewhat improvement in dynamic performance is achieved. However,there may be some restriction in adjusting the heat duty. Since the stageliquid level is closely connected with the heat exchanger, over adjustmentof heat exchanger duty may result in flooding or weeping in the stage.Inaddition, it could be a better choice to mix the top or bottom products ofS-EHIDDiC based on the results of the mixed configuration. The dynamic results also proved that S-EHDDiC is a feasible way to realize theEHIDDiC.