Research On Synthesis Of Multicomponent Thermally Coupled Distillation Based Separation System

The synthesis of distillation separation system with the utilization of process system engineering methodology is one of the most important energy saving and efficiency enhancement technologies in refining and chemical industry.However,the traditional distillation system synthesis methods are faced with numerous challenges because of the new demands by industrial upgrading and research progress in new technologies.On the one hand,large-scale clustering layout and the extension of high-value-added industrial chains have led to the diversified sources and requirements of separation.On the other hand,the rapid development of energy-saving and other separation technologies provides more combinations possibilities for separation flowsheet.Therefore,this paper is focused on the synthesis framework for structure-constrained thermally coupled distillation sequences,multiple feed sources and multicomponents products complex distillation systems and hybrid separation systems,as well as the dimension reduction methods for large scale industrial problems,which could provide methodologies for the development of energy-efficient distillation-based complex separation flowsheets.The main research and results are as follows:(1)The industrial application bottleneck of the multicomponent thermally coupled flowsheet given by the existing methods is its high complexity and difficulty in implementation.To solve this problem,the flowsheet complexity regulation methodology with coupled degree as measurement index is proposed.The research reveals mechanisms for valid coupled degree generation,load transfer and energy integration,as well as their energy saving principles.Considering the existing thermally coupled distillation configuration applied in the real projects,seven constraint rules for process structure regulation are developed through the proposed coupled degree analysis.Furthermore,a mathematic programming model for structure-constrained thermally coupled distillation sequences synthesis based on state-task network superstructure is formulated.The results of examples show that the unnecessary complexity is substantially decreased by the tradeoff between energy-saving effect,investment cost and system coupled degree,meanwhile up to 30%in energy saving can be achieved through optimization of thermally coupled mechanisms and position.Besides,feed characteristic and investment cost proportion have critical impact on the optimal position and number of thermally coupled structure.Furthermore,a distillation sequence synthesis case study of six component zeotropic mixture is carried out and the result shows that not only the practicability of flowsheet is guaranteed,but also the solution space and solving time are sharply decreased through eliminating the infeasible solutions by coupled degree regulation rules.(2)The existing algorithm synthesis methods of complex distillation systems could not handle multiple feed sources and multi-specification products separation problems.Thus,the new concepts of the super column and the mass-energy stage is proposed to explore more structural possibilities of the distillation system.The proposed mass-energy stage state-space network superstructure has the advantages of complete solution space,strong versatility,and extendability,which overcomes the essential defects of the column section based state-task framework.The distribution network and stages-cascade process operator embedded in the framework has the function of both blending and separation,which could handle multiple feed sources and mixed products easily.Based on the rigorous MESH equations,a MINLP model is established with the total annualized cost as the objective function,where a series of solution strategies are proposed including binary variables continuty,nonlinear function reformulation,and random perturbation of initial value guesses in specific boundary.The results of examples show that the proposed rigorous model is accurate and reliable.The total annualized cost could be reduced by 27.7%by introducing new structures such as multiple feeds/draws,by-pass blending and thermally links.For the mixed products problem,fully thermally coupled distillation is not an inevitable optimal configuration so it is necessary to adopt a more complete superstructure.The results of the reported double-feeds example show that the integrated design can be reduced by 14.4%over the total annualized cost,demonstrating the ability and industrial application value of the proposed method.Furthermore,the aforementioned methodology is extended into the batch distillation process,the results show that the proposed state-time-space superstructure and general disjunctive dynamic optimization model can simultaneously trade off operation period,column configuration,design parameters and operation strategies,providing a new optimization method for pollution reduction and energy saving of batch distillation units.(3)The synthesis problem of distillation based separation system with multiple technologies has been divided into binary and multiple scenarios sub-problems.By introducing the new concept of improvement factor,the synthesis of multi-component thermally coupled distillation sequences which considering additional binary technologies is realized through the minor modification of the model in the second chapter.The proposed method overcome the shortcomings(high workload and the loss of the best solution)associated with the step-by-step screening method.The results show that there is a threshold for the application of the binary improved technology meanwhile the threshold in thermally coupled sequences is higher than that in conventional sequences,which provides theoretical direction for binary technologies selection.Taking the distillation-extraction hybrid system as a demo,a new idea of extracting agent-component incidence matrix is proposed for generating a reduced state-task tree network and further developed into a general methodology for the synthesis of hybrid technology separation sequences.The result of example shows that multi-component additional separation technology have the reconstruction effect on the distillation sequence and the significant cost advantages for complex and difficult to separate mixture.(4)Aiming at combination explosion problems caused by large-scale components and products in industrial projects,a methodology consists of multicomponent products lumping rules and stable central point dimension reduction is proposed to convert the industrial scale problems into solvable small-scale sub-problems.An real separation flowsheet development industrial project for 800,000 t/yr continuous catalytic reforming unit(31components and 14 products)is illustrated and which could be converted into three subsystems by the proposed dimension reduction method.The obtained optimal structure constrained thermally coupled and simple column sequences increase the annualized sales by 359 million yuan compared with that of the conventional sequence.Besides,the profit of the thermally coupled and simple column sequences are increased by 22.35%and 18.07%respectively compared with that of the conventional sequence,while the energy consumption of thermally coupled sequence is 19.57%lower than that of the simple column sequence.The results demonstrates the effectiveness of the proposed methodology in solving the large-scale separation system synthesis problems and its theoretical and practical value in development of new efficient energy-saving and value-added separation flowsheet.