Application Study On Solution Dehumidification Technology In Closed-loop Drying System

Closed-loop drying is a drying method in which the drying medium circulating in a closed loop repeatedly undergoes the process of carrying moisture and dehumidifying.The closed-loop drying process has almost no exhaust gas emissions,which can achieve safe and environmentally friendly production.It is a green drying technology with great potential.Dehumidification is an indispensable and important part for the dryer exhaust(drying medium after carrying moisture)to regain its moisture carrying capacity,and it is also a part with large energy consumption.Reducing the energy consumption of the dehumidification process has become the key to the large-scale industrial application of closed-loop drying technology.Solution dehumidification is a dehumidification method in which the partial pressure difference of the moisture between the gas to be dehumidified and the dehumidification solution(hereinafter referred to as the solution)pushes the moisture into the solution to reduce the humidity of the gas.Due to the low partial pressure of moisture on the surface of the solution,the drying medium can obtain lower humidity and higher temperature than conventional condensing dehumidification,thereby obtaining greater moisture carrying capacity and drying driving force,and reducing energy consumption.In addition,direct contact between the dryer exhaust and the solution can also reduce the dust content.The thesis proposes to apply the solution dehumidification technology to the closed-loop drying process,and designs a solution dehumidification system suitable for drying the exhaust.The system is mainly composed of solution dehumidification subsystem,solution regeneration subsystem and low temperature waste heat recovery subsystem.The main equipment of the solution dehumidification subsystem is the solution dehumidification tower.The dryer exhaust contacts the solution countercurrent in the dehumidification tower,and then returns to the drying system after dehumidification.The solution regeneration subsystem is composed of a solution regeneration tower and a compression heat pump.The heat required for regeneration(evaporative dehumidification)of the solution after absorption of moisture comes from the latent heat of the moisture vapor recovered by the heat pump system.The low-temperature waste heat recovery subsystem uses an absorption heat pump to recover part of the sensible heat of the regenerated solution to generate hot water or heat and dehumidify the drying medium.The solution dehumidification system can greatly reduce dehumidification energy consumption.A mathematical model of the solution dehumidification system is established,and potassium formate(HCOOK)solution is used as the dehumidification solution.The influence of the humidity and temperature of the dryer exhaust entering the dehumidification tower,the temperature,the mass concentration of the solution entering the dehumidification tower,and the liquid-gas ratio of the dehumidification tower on the COP(the ratio of the system's output energy to the input energy)and SMER(the ratio of the water removed by the system to the energy consumed)are simulated and analyzed.The results show that: the temperature of the solution entering the dehumidification tower has a greater impact on the system's COP and SMER.Although the increase in the humidity of the tower's dryer exhaust greatly increases the heat output of the system,the COP of the system shows a slight downward trend.An adiabatic packed tower solution dehumidification experiment platform is designed and established.Using potassium formate solution as the dehumidification solution,the flow rate and humidity of the exhaust entering the dehumidification tower,the flow rate,temperature and mass concentration of the solution entering the dehumidification tower are experimentally studied for the dehumidification capacity,dehumidification efficiency,gas output humidity of the dehumidification tower,the output solution temperature of the dehumidification tower,and the mass transfer and heat transfer coefficients.The results show that: the dryer exhaust flow rate and the solution flow rate entering the dehumidification tower have a great influence on the dehumidification efficiency;under the experimental conditions,the minimum dehumidification efficiency is 50%,and the highest reaches 85.1%;the minimum temperature of the dehumidification tower solution is 45.0 ?,the highest reaches 66.2 ?.According to the experimental data fitting,the correlations of the coupled heat and mass transfer coefficients under the experimental conditions are obtained.The relative error between the calculated values and the experimental values of mass and heat transfer coefficients is within 15%.According to the research results,the preliminary process design of the ammonium phosphate closed-loop spray drying system using potassium formate solution for dehumidification is carried out and compared with the closed-loop spray drying system using condensing dehumidification.The results show that under the design conditions,the initial investment of equipment for the closed-loop spray drying system using potassium formate solution dehumidification is large,but the annual operating cost can be saved by about 25.2%,and the system can obtain hot water which is 1.85 times heat by consuming a small amount of high-temperature driving heat source.The results have reference value for the industrial application of closed-loop drying system using solution dehumidification,and provide a useful way to further reduce the energy consumption of the closed-loop drying process.