Heat And Mass Transfer Characteristic Of Compound Adsorbents For Adsorption/desorption Process

As a green refrigeration technology, adsorption refrigeration is a good choice for harmonious development of economic, energy and environment because it is driven by the low grade heat and utilizes green refrigerant that has zero ODP and GWP. However, the poor heat and mass transfer performance of the adsorbent bed is the main limitation to the industrialization of adsorption refrigeration system, and besides that, the accuracy of the numerical simulation research on the adsorption system is also badly influenced since there is little knowledge of thermal conductivity and permeability of compound adsorbents for adsorption or desorption process.So for the above issues, this paper focused on the heat and mass transfer enhancement technology and compared the heat and mass transfer performance of physical compound adsorbents solidified by different matrix, and then explained the superiority of the expanded natural graphite (ENG) as a solidified matrix. Above this foundation the thermal conductivity and permeability of compound adsorbents for adsorption/desorption process are tested using the heat and mass transfer experiment system which are set up based on the principle of steady-state heat source method and the Ergun equation. At last, according to the experimental results of the physical compound adsorbent's properties, the numerical simulation research on the adsorption system filled with different kinds of compound adsorbent is conducted, through which the optimal mixing ratio and solidified density is selected. The main content and conclusions are as follows:(1) Three types of activated carbon (AC) adsorbents, i.e. granular AC, solidified AC with chemical binder, and solidified AC with ENG are developed and their thermal conductivity was investigated with the steady-state heat source method and the permeability was tested with nitrogen as the gas source. Results show that the thermal conductivity of granular AC with different sizes almost maintain a constant at 0.36 W·m-1·K-1, while the value modestly increases to 0.40 W·m-1·K-1for the solidified AC with chemical binder. The solidified AC with ENG compound adsorbent shows the best heat transfer performance and their thermal conductivities vary from 2.08 W·m-1·K-1 to 2.61 W·m-1·K-1 according to the different ratios of AC. However, the granular AC and solidified AC with chemical binder show the better mass transfer performance than solidified AC with ENG binder whose permeability changes from 6.98×10-13 m2 to 5.16×10-11 m2, and the value decreases with the increase of ENG ratio which is opposite to the change law of thermal conductivity.(2) A novel the thermal conductivity and permeability test system is set up in order to investigate the heat and mass transfer performance of solidified compound adsorbent under the different equilibrium state of adsorption or desorption. Then solidified compound adsorbents with various ratios between adsorbent and ENG matrix are developed and tested under different ammoniated state. Result shows that the thermal conductivity and the permeability have strong dependence with the ratios and solidified density of the compound adsorbent. Meanwhile, the thermal conductivity and permeability of the chemical compound adsorbents vary significantly with different adsorption quantity of ammonia, while the values for the physical compound adsorbents almost maintain a constant value with different values of adsorption quantity.(3) The numerical simulation research, which focused on the unit tube, is conducted based on the results of thermal conductivity, permeability, and adsorption equilibrium model. And one-dimensional unsteady state differential equation for heat and mass transfer are analyzed as the governing equations. According to the numerical analysis to the kinetic characteristic of the adsorbent bed, the optimal solidified adsorbent is chosen for air conditioning condition and freezing condition based on the optimal COP and SCP. Results show that the optimal solidified activated carbon could improve the SCP by more than 2.7 times if compared with granular activated carbon mainly because of the higher thermal conductivity and reasonable permeability.?At last, it is also suggested that when developing a new type of compound adsorbents, the ratio between adsorbent and matrix must be considered carefully because the performance of the adsorbent system is not only influenced by the heat transfer performance, but also impacted by the mass transfer performance and density of the pure adsorbent.