Research On Operating Characteristics And Control Strategy Of Compressed Air Drying System Using Liquid Desiccant

Combining the liquid desiccant dehumidificaton technology with the needs of compressed air drying,the waste heat produced by compressor is used to solution regeneration,which saves the energy consumption required by traditional freeze drying,thereby improving the energy efficiency of the compressor unit greatly.Analytical studies have shown that the dew point of compressed air can reach-20℃ when using liquid desiccant,thus replacing the traditional freeze drying method and cover the technology blank area with dew point of-20℃～ 0℃.A prototype of a compressed air drying system using liquid desiccant is built in previous paper,which including dehumidifier,regenerator and waste heat recovery device,and the drying performance of the system has been initially explored.This paper will integrate and improve the research content on the basis of existing results.The main research content and results are as follows:Firstly,compare the causticity of KCOOH,a new type of liquid desiccant,with the halogen inorganic salt solutions used in liquid desiccant dehumidifier.Then,based on the effect of the solution’s physical and chemical properties on the drying performance of the system,comparing the difference between KCOOH and inorganic salt solutions from the vapor pressure,density/viscosity,and evaluate the performance of the liquid desiccants with indicators such as drying/regeneration performance and solution cost.The results show that the drying performance of KCOOH solution is worse than Li Cl slightly,and humidity of compressed air outlet is 0.2g/kg higher than Li Cl solution on average.However,combined with corrosion,regeneration performance and solution cost,the comprehensive performance of KCOOH solution is far superior to Li Cl.In summary,the application of KCOOH has great advantages.Secondly,establish a dynamic mathematical model of the system,and compared with experimental data: the change trend of the compressed air outlet humidity/temperature and solution temperature during the startup phase is consistent with the experimental data.The relative error between the simulated and the experimental value of the humidity is within ±5%,and the relative error between the air temperature and the solution temperature is less than ±3%;and under the parameter adjustment working conditions where the solution flow step increases and the solution temperature decreases,The relative error between the simulated and the experimental data is within ±7%.This shows that the dynamic mathematical model established in this paper has high prediction accuracy and reliability.Subsequently,the operating characteristics were analyzed based on the prototype of the compressed air drying system using liquid desiccant.In terms of startup time,the operating parameters have a greater impact on it.The startup time increases with the increase of compressed air pressure and solution concentration,but decrease with the the solution flow rate and the quality of the solution in the storage tank;In terms of the impact of operating characteristics due to the start and stop of the air compressor,the air compressor is affected by the pressure protection module,and its pressure changes periodically.When the system is in stable state,the change period of humidity is the same as the pressure change,and the overall fluctuation range is within 0.1g/kg;the heat production of the air compressor is affected by its intermittent operation,which leads to a periodic change in the temperature of the solution after heat absorption.During a start-stop cycle,the temperature of the solution regeneration in summer is between 42°C and 67°C,which basically meets the needs of regeneration.In system’s regulation,the humidity of compressed air can be regulated by adjusting the parameters on the air and solution side.The analysis results show that the compressed air flow rate is usually affected by the air use terminal and it needs to be kept as stable as possible,so it cannot be used as an active adjustment method;while the solution flow rate can play a certain effect only in a small flow range;Solution concentration and temperature are key parameters that directly affect the properties of the solution,they can be controlled by setting the solution buffer tank and adjusting the cooling performance of the solution cooler.When the humidity is greater than the set upper limit,increase the cooling water flow to reduce the temperature of the solution or increase the concentration of the solution by adding a concentrated solution to decrease the humidity;conversely,when the humidity is monitored below the lower limit,the cooling water flow can be reduced to increase the temperature of the solution to increase the humidity.Finally,the power consumption of the air compressor and the proportion of solution regeneration heat are analyzed.Taking the Nanjing area as an example,the power consumption of the air compressor is the largest in July,and the recoverable residual heat of the air compressor is also the largest.According to the established heat exchanger efficiency model,the temperature of the KCOOH solution after the absorption of the residual heat can reach 68℃～73℃ and 46℃～51℃,and the proportion of solution regeneration heat can reach 60%～80% and 34%～55% in summer and winter respectively.Whether in summer or winter,the temperature of the solution heated by the air compressor can meet the requirements of solution regeneration.