Dynamic And Control Of Internally Heat Integrated Middle Dividing Wall Column

Dividing wall column(DWC)is an effective process strengthening design,which are capable of separating ternary mixtures in one unit.Although DWC has obvious energy saving potential compared with conventional distillation sequence(CDS)composed of multiple distillation columns,irreversibility in the separation process is still strong.It can still be further intensified through the introduction of internally heat integrated technology.As the product of the process strengthening and integration of internally heat integrated technology and DWC,internally heat integrated middle dividing wall column(IHIMDWC)uses two compressors and throttle valves to ensure the temperature difference and pressure difference between the upper and lower parts(upper part includes common rectifying section and two section beside the partition above the feed location,and lower part contains two section beside the partition under the feed location and common stripping section)of DWC so as to realize the internal heat transfer of DWC and reduce the irreversibility of the separation operation.Depending on the arrangement of internal heat integration at one side or both sides of the partition above/under the feed location,four topological configurations can be derived,which are IHIMDWC-S,IHIMDWC-L,IHIMDWC-U and IHIMDWC-A.Although IHIMDWC has obvious advantages in energy saving and cost reduction compared with conventional distillation sequence(CDS)and DWC,the influence of complex mass and heat transfer processes on the dynamic performance of IHIMDWC in the separation process is still unknown.It is necessary to explore the dynamic characteristics of IHIMDWC and design appropriate control schemes to provide theoretical bases for its transformation into the actual chemical production process.Taking the separation of two groups of ternary mixture systems(methanol,ethanol,1-propanol and n-pentane,cyclopentane,2-methylpentane)as illustrative examples,the dynamic and control of IHIMDWC are studied.The main difference between the two separation systems lies in the difference of freedom caused by the different number of reboiler and condenser in the steady-state optimal process.On this basis,the corresponding IHIMDWC decentralized temperature control systems are presented respectively.The interaction between the control loops were quantitatively analyzed by the relative gain array(RGA),and the open loop and closed loop dynamic responses of the four topologies were compared under different disturbances,so as to explore the influence of different heat integrated modes on the operability of the systems.The open loop dynamic response results show that IHIMDWC has serious nonlinearity and poor process controllability due to the strong coupling of matter and energy inside the system.Among the four different topologies,IHIMDWC-A has the worst open loop response performances.It is assumed that the excessive deepening of heat coupling worsens the dynamic characteristics.The RGA results show that the interaction between the control loops in the decentralized temperature control schemes for IHIMDWC are small,which preliminarily verifies the rationality of the scheme.The closed loop response results show that the interference of feed flow and feed components can be effectively suppressed,and the operability of IHIMDWC is clearly defined from the dynamic point of view.The optimal topological structure of IHIMDWC should be selected according to the thermodynamic characteristics of specific separation system and the main disturbance source.For the separation of ternary mixture of methanol,ethanol and n-propanol,the optimal topology is IHIMDWC-S if the main disturbance comes from feed flow,and IHIMDWC-U if the main disturbance comes from feed compositions.For the separation of ternary mixture of n-pentane,cyclopentane and 2-methylpentane,IHIMDWC-L is the optimal topology for both sources of feed disturbance.