Steady-state And Dynamic Control Of Divided-Wall Column Prefractionation And Extraction Of Aromatics For Naphtha Separation

Naphtha is one of the main raw materials for solvent oil production.In the conventional naphtha extraction process,naphtha feedstocks contain C5 light components and C9 heavy components.These components not only increased the device load and energy consumption of the extraction system,reduced the aromatics content of aromatic products,but also caused excessive consumption of extraction solvent.The pretreatment of naphtha feedstocks schemes include two types.One is only removing the light component,the other is simultaneously remove the light and heavy components.In this paper,a lumped model for aromatic extraction feedstock was proposed at first.The whole process of aromatics extraction of the two pretreatment schemes was simulated,and the results showed that the later scheme had more industrial advantages,but it has a higher energy consumption.Then,rigrious simulation model of DWC and the conventional separation sequence were built for the separation of the naphtha.Results showed that DWC can reduce energy consumtion about 13.7%lower than that of the 2-column separation sequence.On this basis,this paper continues to propose an optimization naphtha aromatics extraction process,that is,DWC is applied to pretreat the naphtha raw materials,and a striping column is added in the process to improve the mass fraction of aromatics in the mixed aromatics.The aromatics mass fraction can be increased from 68.36%to 91.07%,and the total energy consumption could be decreased by 9.61%compared with the two column separation process.The dynamic control of DWC was studied in order to further demonstrate the feasibility of the application of DWC in naphtha aromatics extraction process.Based on the results of steady-state simulation,three control structures,including composition control(CC),temperature control(TC)and differential temperature control(DTC),were built in Aspen Dynamic.Control results of the three controlled structures showed that all of them can provide effective control in face of ±10%disturbances of naphtha feed flow rate and naphtha feed compositions.The CC structure employed three control loops.One loop used reflux to control the concentration of isomerized C6 in the distillate.Another control loop used the reboiler duty to control the concentration of normal C9 in the bottom product.The sidestream control loop emplored the sidetream flowrate to control the concentraction of aromatic C8.Results showed that the CC structure could handle all disturbances of feed flow rate and feed compositions.The difference of the key component between the final value and the steady value is small.Therefore,the aromatic C6 in the distillate could be controlled well.The yield of the intermediate components,C6 to C8 cuts,in the sidestream could reach 99.0%.After that,the TC structure was built and tested.This strucuture could also handle the disturbances.However,the bias of product purities was not satisfied.At last,the DTC structure was built.Results showed that the DTC structure owns advantages over the other two control methods.The bias of product purities of DTC was close to that of the composition control method,while its settle-out time is similar to the time of the TC structure.