Modeling And Simulation Of Air Separation Unit Operational Flexibility Analysis

Air separation has the characteristics of periodicity,intermittentness or imbalance of downstream users’ oxygen demand.Frequent load changes require air separation researchers to have a more complete understanding of the operational feasibility of air separation equipment,especially the internal relationship between operating variables and product quality.This thesis constructs a flexible analysis proposition for the operation of an air separation unit taking the oxygen and nitrogen liquefaction process as an example,establishes a full air separation process model including a multi-stream heat exchanger,and analyzes the feasible domains and operations of the main operational variables of the air separation process flexible,with a view to providing a reasonable basis for theoretical research and industrial applications.The work of this article mainly includes the following aspects:(1)The design and operation of the flexible analysis proposition and the calculation method of the flexible index are comprehensively reviewed,and the flexible index and the number of feasible points of the industrial device are determined as the two flexible analysis indexes.Take the heat exchange network as an example to compare the design and operation flexibility analysis,and compare the difference between the two in the number of variables,the number of equat ions and the degrees of freedom.The effects of adjusting the cooling capacity of the public works,outlet temperature constraints and heat exchanger area on the feasible region and the flexibility index are discussed.Through comparison,it is found that due to the limitation of equipment parameters,the operation feasible range is much smaller than the design feasible range.Among them,the outlet temperature constraint and the heat exchange area have a greater influence on the adjustment operation feasible range.It is determined that when the appropriate outlet temperature is controlled at about ± 3 K,the ratio of the flexibility index and the feasible area to the temperature fluctuation is the largest.When the heat exchanger area directly related to the product stream is 1.2 times the regular value,the flexibility index and the feasible area are largest.(2)The Stage-Wise(SW)superstructure modeling method is used to establish an equivalent heat exchange network model for multi-stream heat exchangers.13 double-flow heat exchangers are used to adjust the split ratio and obtain the heat exchanger area through parameter estimation.The operation analysis of the stream heat exchanger shows that the average error between the simulation result and the original process is about 0.3%.(3)Taking the oxygen and nitrogen liquefaction process in the air separation plant as the research object,replacing the multi-stream heat exchanger with an equivalent heat exchange network to construct an operation model for process simulation.The single-variable and multiple-variable operational feasible domains are analyzed.The single-variable feasible domains except for the outlet pressure of the expander are all within the regular value of about ± 4%.The flexibility index of the feasible region on the plane of two feed flows are all 0.1,and the main limiting factor is the power of the turbocharged turboexpansion unit.The operation model effectively completes the operation process simulation and flexibility analysis.