The Optlmal Design Research Of Internally And Externally Heat-integarted Distillation Column

The separation of ethanol-water mixture with a self-made pilot-plant concentric structured internally heat-integrated distillation column(HIDIC)is investigated experimentally.In the condition of continuous feed operation,the heat transfer rate and heat transfer coefficient at different compression ratios is studied by the method of building heat transfer model and dividing region.The compression ratio which gives minimum reboiler duty and condenser duty can be easily found to be 2.2.In the compression ratio of 2.2 with the maximum heat transfer rate,the zero external reflux or zero reboiler duty do not be realized.Taking the column physical constraint into consideration,the heat transfer area may be the limiting factor.Then the heat transfer area is studied in order to realize the zero external reflux.The optimal design is carried out by the combination methods of thermodynamic analysis and total annual cost(TAC),and the later one is used as the objective function.The results show that the TAC begins to drop as the result of decreasing the number the of the heat exchanges.The more the heat exchangers,the greater the total heat transfer rate and the total heat transfer area.At the same time there exists across-pinch heat transfer when the external reflux is zero.Eliminating the across-pinch heat transfer can reduce the energy consumption and achieve better economic performance.Besides,the analysis of energy efficiency is carried out when the external reflux is zero.When the heat-integrated position is “1” to“4”,the operating cost is lower,i.e.the energy efficiency is high,which indicates that the heat transfer temperature difference is not as larger as better.The larger the heat transfer difference,the lower the energy efficiency.Owing to the result that the condenser duty is less than the reboiler duty,it can infer the condenser is the dominant role,which means that the operation cost is less when the heat-integrated position is close to the main condenser.The optimally heat-integrated configuration is compared with the VRC and conventional distillation column in order to choose the externally heat-integrated distillation column which is fit for the system of ethanol-water.Based on the separation system of benzene-toluene,the minimum total annual cost is obtained when the compression ratio is 2.3.By using the analysis of thermodynamic analysis,the number of heat exchangers is 6 when the total annual cost is at the minimum.Then the heat-integrated position with 6 heat exchangers is optimal to find that the dominant role of the main condenser and main reboiler should be considered when the heat-integrated position is conducted.Afterwards,the analysis of energy efficiency is carried out to obtain the similar principle,i.e.the larger the heat transfer difference,the lower the energy efficiency,which has been described with the system of ethanol-water.The optimally heat-integrated configuration is compared with the VRC and conventional distillation column in order to choose the externally heat-integrated distillation column which is fit for the system of benzene-toluene.