Study On Dynamic Accumulation Operation Mode Of Continuous Distillation

Distillation is the most mature technology in industry and has the largestthroughput. But when deal with the mixture with low concentration of impuritiesusing continuous distillation, if the product purity was assigned, the recovery of mainproduct will increase with an increase number of stages while the investment of theequipment will increase. The recovery of main component decrease with a decreasenumber of stages, because the impurity flowrate will carry much main component.The small flowrate and the loss of main component don’t exist in batch distillation,but the handling capacity of batch distillation is far lower than continuous distillation.In order to solve the two shortcomings, dynamic accumulation operation mode ofcontinuous distillation is proposed and studied in this paper.Firstly, the operation scheme of dynamic accumulation continuous distillation isproposed. This operation scheme consist three steps.1) The distillate flow rate isequal to the feed flow rate, the bottom flow rate is0. In this step, heavy component isenriched in the bottom. Stop this step when the purity of light component in theaccumulator decreases to0.999.2) Keep all the conditions still and draw the bottomholdup out quickly. Stop this step when the holdup is not much enough support theboil-up steam.3) Increase the reflux ratio to make the hold up back to the initial level.The mathematical model of dynamic accumulation continuous distillation was built,and MATLAB was used to simulate the process and investigate the influence ofdifferent parameters.Secondly, the first step of dynamic accumulation operation mode of continuousdistillation was simulated with n-hexane-cyclohexane system using ChemCAD. Thesimulation results show that dynamic accumulation operation mode of continuousdistillation has higher light component recovery (99.9999%) than continuousdistillation (99.9%) with45stages and5.8reflux ratio, and the disparity becomesmore obviously when the number of stages decreased to25, the former becomes99.16%and the later becomes96.47%. When using dynamic accumulation operation mode ofcontinuous distillation, the system is easier to operate, the control is more convenient.At the same time, a higher purity of cyclohexane could be withdrawn from the bottom, which indicates this operation mode is suitable for the mixture with a lowconcentration of main component.At last, the dynamic accumulation operation mode of continuous distillationexperiment was conducted using ethanol-n-propanol and methanol-water system. Inthe experiment using ethanol-n-propanol system, the feed has a99%volume fractionof ethanol, and the temperature of bottom increased from78.4℃to96.4℃, the massfraction of n-propanol in bottom increased from1.02%to92.94%. The temperature ofthe top maintained about78.4℃and the product purity maintained above99.9%. Theresult of experiment agreed well with the simulation result. The experiment confirmedthat dynamic accumulation operation mode of continuous distillation has advantage indealing with mixtures with low impurities concentration.