Simulation And Optimization Of Variable Load Operation In Air Separation Unit

When the air separation unit is operated under the normal gas condition with variable load(load reduction),the oxygen output is generally required to be reduced to the minimum under the condition that the nitrogen output remains unchanged and the purity of each product meets the standard.On the one hand,the oxygen emission caused by abnormal production of the main line(iron making and steel-making)can be reduced,and the cost of the air separation unit can be reduced on the other hand.In recent years,due to the sluggish iron and steel products market,the slow production rhythm of the main line and the periodic imbalance of oxygen consumption in production,the oxygen emission rate remained high,but the nitrogen consumption did not decrease.In the previous variable load(load reduction)operation of the 40000 Nm~3/h air separation unit,when the nitrogen output is satisfied and the purity of each medium product is within the standard range,the minimum oxygen output can be reduced to 38000 m~3/h,and no load reduction operation with oxygen output less than 38000 m~3/h has been carried out.First of all,the passage combed the process of air separation unit,combined with the previous experience in variable load(load reduction)operation and the operation record data at variable load,summed up that the influence of air compressor back pressure and expansion on variable load operation is more prominent.Secondly,the convergence conditions of air compressor system and expander system are selected.According to the design data and actual data,Aspen Plus simulation software is used to model and simulate the whole process of large-scale air separation unit.Firstly,under the condition that other conditions remain unchanged,the change of product output and purity when the pressure of air compressor system changes in the air separation unit is simulated and calculated;secondly,the change of product medium and purity when the expansion volume changes in the expander system is simulated and calculated.Thus,the change trend of air compressor system and expander system with each medium output is obtained when the large-scale air separation unit is under variable load(load reduction),and its principle is analyzed to lay a theoretical foundation for the actual operation scheme.In the actual operation process of 2019,due to the adjustment of air separation unit operation mode and slow production rhythm of main line,it is conducive to the simulation practice.At the same time,according to the change of data in the simulation and based on the safety range of each parameter adjustment,the operation scheme is prepared and issued to the post personnel for implementation.The simulation results are applied to No.9 air separation unit in practice.After that,each process of the air separation unit runs smoothly,and the oxygen output is reduced from 38000 m~3/h to 37000 m~3/h.If the influence of nitrogen production is not considered,the minimum can reach 36000 m~3/h.Finally,the operation method of each part of the parameters in practice is compiled into standardized operation instructions for the staff to learn and reference for operation.