Design Optimization And Experimental Research Of Self-enhancement Multi-stage Air Compression Process Driven By Heat Recovery In Air Separation Units

Cryogenic air separation units(ASUs)are characterized by high energy consumption and complex technology,and the investment and energy costs account for a relatively high proportion of the total cost in petrochemical,metallurgical and other industries.Taking the steel industry as an example,the energy consumption costs of ASUs account for 15-20% of the total cost of iron and steel enterprises.Focusing on Chinese “carbon peak” and “carbon neutral" goal,further reducing the energy consumption of ASUs is crucial to achieving energy conservation and emission reduction.From the perspective of the development history of cryogenic ASUs,for the current sixth-generation ASUs,it is very difficult to further reduce the compressor discharge pressure.And other core machine technology like molecular sieves,booster expanders,and structured packing used to reduce system pressure loss is basically mature.As the main energy-consuming unit related to the two energy-saving methods,the air compression process has a huge amount of residual heat,but the air separation system lacks a matching demand side,and there is a significant "unbalanced supply and demand" contradiction of residual heat.It is urgent to explore a new compression heat utilization method for ASUs.Based on this,this paper proposes an energy-saving air compression process based on process waste heat adapted to local conditions and onsite utilization to realize the application of air compression process energy-saving technology.The system principle,economic and technical feasibility are verified separately from the aspects of system design,matching optimization,and experimental verification.theoretical design and experimental verification basis for realizing the industrial application of energy-saving technology using waste heat in the air compressor process.A theoretical design and experimental verification basis are provided for realizing the industrial application of energy-saving technology using compression heat.The main contents of this paper are as follows:1)A self-enhanced multi-stage air compression process was proposed and designed.A targeted comprehensive performance evaluation method was established,the feasibility of the system principle was verified.A self-enhanced multi-stage air compression process based on the organic Rankine vapor compression system(ORC-VCR)is proposed and established,without changing the basic layout of the air compression process,based on the characteristics of large and separated heat at each stage of compression outlet.By recovering the heat from the multi-stage compression outlet to drive the refrigeration system,and cool the compressed inlet air at all stages to achieve the purpose of reducing compression power consumption.In order to achieve the comprehensive performance evaluation of the process,this paper provides various evaluation indicators of the system based on the first law,the second law of thermodynamics and the system economic evaluation.The effects of energy distribution,changes in operating conditions and parameter settings on the thermodynamics and economic performance of the system are discussed separately.The result shows that the energy saving rate of the 60,000-scale ASU can reach 3.6%,and the annual cost saving is 2.73 million yuan,which verifies the principle and feasibility of the system.2)A performance optimization scheme based on the firefly algorithm was established,which revealed the matching optimization mechanism of internal system factors and the performance influence mechanism of external environmental factors,and verified the economic feasibility of the system.The internal factors that mainly affect the performance of the self-enhanced air compression process due to the waste heat recovery and cooling capacity distribution at all stages is analyzed.The main factors that affect the system performance are the quantity of waste heat and the distribution of refrigeration capacity at the outlet and inlet of the secondary compression.On this basis,a firefly optimization algorithm with high and low temperature evaporators as the energy distribution carrier was established.The heat exchange size of each evaporator was used as the optimization parameter,the life cycle cost saving LCS and the energy saving average cost LCOE were the optimization goals.For the self-enhanced multi-stage air compression process designed for typical areas Hangzhou and Yinchuan,the energy saving rate ESR after optimized can reach 4.3% and 3.7%,respectively,the life cycle cost saving LCS can reach 66.95 and 55.9 million yuan,respectively,and the payback period are 4.1 and 4.3 years respectively,which verifies the economic feasibility of the process system.3)The first ORC-VCR experiment platform in China was designed and built,the experiment was carried out to study the system's stable working condition design method and system performance influence law,which verified the technical feasibility of the system.The ORC-VCR experimental system was built,and the stability test of air floatation liquid supply,balance of axial force,and motor cooling performance were carried out according to the stable operating conditions of the coaxial expansion compressor.For bearing fluid supply,the bearing pressure difference in the expansion compressor can be controlled between 0.53-0.63 MPa,and the air-floating fluid supply is sufficient.For motor cooling performance,the temperature of the expansion compressor chamber can be controlled around 40 ? in 30 minutes through PID control.For axial force balance,the motor assist method is adopted to ensure that the axial force in the expansion compressor is within the safety range,and the vibration value of the machine cavity does not exceed 0.2 mm/s.In terms of refrigeration performance research,the COP of the system basically shows a linear decline with the increase of the evaporating pressure ratio between high and low temperature PH/PL.The working load of the expansion compressor can be adjusted between 40% and 100% of the normal working condition through the throttle valve.In the basic working condition of 26,000 rpm and 0% bypass valve opening,the cooling capacity of the system is 14.2 k W,the cooling temperature is 14.6 ?,and the maximum system COP is 0.63,which basically meets the requirements of the theoretically optimized system for cooling capacity.