Preparation Of Biomass-based Solid Adsorbent And Its Dehumidification Performance In Fixed Bed

Under the dual pressures of energy crisis and environmental pollution,energy consumption and environmental protection are getting more and more attention.Solar energy,as the main source of different renewable energy,is inexhaustible,clean,and does not bring about environmental pollution.Solar drying system that uses solar energy as a heat source is favorable for the drying of agricultural and sideline products.In order to improve the thermal efficiency of the solar drying system,the high temperature and high humidity air flow after the solar drying system must be dealt with solid adsorption dehumidification technology and then returned to the solar air collector.Therefore,the development of solid desiccant adsorbent and the performance of fixed-bed adsorption dehumidification system are very important for solar drying system.In this paper,walnut shells,one kinds of agricultural wastes,as a biomass,were used as raw materials to prepare activated carbon by a two-step physical activation method including the pyrolysis carbonization of the starting materials and the CO₂physical activation process of the resulting chars.The effects of preparation conditions on pore structure and water vapor adsorption performance of walnut shell activated carbon were investigated by single factor method.The physical and chemical properties of walnut shell activated carbon were characterized by thermogravimetric analysis,automated gas sorption analyzer and scanning electron microscope.The relationship between the pore structure and water vapor adsorption performance was analyzed and the optimal preparation conditions were determined.Based on these studies,various activation methods were used to compare the pore structure distribution and water vapor adsorption performance of walnut shell activated carbons.The programmable constant temperature and humidity chamber was used to test the water vapor non-equilibrium adsorption and regeneration process of activated carbons.A miniaturized fixed-bed dynamic adsorption system was built and employed to investigate the water vapor adsorption isotherms of walnut shell activated carbons at different temperatures.Biomass-based solid dehumidification adsorbent was prepared by granular walnut shell activated carbon and corn starch as a binder.The effect of binder mass ratio on thermal conductivity,water vapor adsorption and pore structure parameters of the adsorbent was identified.A fixed-bed adsorption dehumidification system was built.The influences of inlet air moisture content,flow rate and temperature on the dehumidification performance of fixed-bed adsorption dehumidification system was analyzed.Furthermore,the moving speed of mass transfer zone,gas diffusion coefficient,gas phase mass transfer coefficient and total mass transfer coefficient of this system were further calculated and analyzed,which ensured the fixed-bed adsorption dehumidification system had high dehumidification efficiency and high stability.The main conclusions are as follows:（1）According to the pyrolysis kinetic analysis results of walnut shell,the optimal carbonization temperature and heating rate during the carbonization process was700°C and 10°C/min,respectively.The pyrolysis activation energies of walnut shell at different heating rates were 30.16–64.86 kJ/mol.Based on non-equilibrium adsorption performance of water vapor,the optimal activation temperature and time of walnut shell activated carbon were 900℃and 3 h,respectively.Under this preparation condition,the adsorption capacity and specific surface area of activated carbon were0.3824 g/g and 1228 m²/g,respectively.The pore size distribution was dominated by ultra-micropores less than 0.7 nm.And water vapor adsorption performance and pore structure development of walnut shell activated carbon had good consistency.On the basis of this conclusion,the water vapor adsorption performance of activated carbon was optimized by different activation methods.The effects of CO₂ physical activation method,KOH chemical activation method,and CO₂+KOH combined physical and chemical activation method on the pore structure and water vapor adsorption performance of activated carbon were compared.The result showed that the CO₂+KOH combined physical and chemical activation method was considered to be the best method.The specific surface area,total pore volume,and water vapor adsorption capacity of the optimal activated carbon were 2326 m²/g,1.075 cm³/g,and 0.6754 g/g,respectively.Compared with the activated carbon by CO₂ physical activation method,these three parameters of the optimal activated carbon had increased by about 89%,86%,and 77%,respectively.（2）The results of curing experiments showed that the optimal mass binder ratio was 20%.The thermal conductivity,water vapor non-equilibrium adsorption capacity and specific surface area of the biomass-based solid dehumidification adsorbent were0.2263 W/（m·K）,0.5562 g/g and 2314 m²/g,respectively.Compared with the granular activated carbon,the non-equilibrium adsorption capacity of water vapor decreased by about 18%and its thermal conductivity increased by about 71%.The dispersion and agglomeration of powdered walnut shell activated carbon was solved by solidification experiments.It naturally followed that the problems of floating and agglomeration of granular walnut shell activated carbon could be solved and a biomass-based solid dehumidification adsorbent would be the potential candidates for dehumidification and heat recovery in solar drying system.（3）The dehumidification capacity of this adsorbent in fixed-bed adsorption dehumidification system increased significantly with the increase of inlet air moisture content.When the inlet air moisture content increased from 19.827 g/kg to 29.740 g/kg,the dehumidification capacity increased by about 53%.Secondly,the inlet air flow had a greater influence on the dehumidification time.When the inlet air flow rate decreased from 50 m³/h to 150 m³/h,the effective dehumidification time was reduced by about75%.And the mass transfer coefficient of the mobile phase was doubled.Thirdly,the dehumidification capacity of this system decreased significantly with the increase of the inlet air temperature.When the inlet air temperature of the fixed bed adsorption dehumidification system decreased from 40℃to 60℃,the dehumidification capacity was reduced by about 54%.The total mass transfer coefficient was increased by 6.5times also at the same time.At last when the regeneration temperature was 70℃,the exhausted adsorbent could be completely regenerated in 34 min.