(446z) Development of Activated Carbons for Adsorption Heat Pumps

Adsorption heat pumps (AHPs) system have been receiving a lot of attentions due to the ability of heating and cooling using relatively low-level heat sources. Activated carbon (AC)-ethanol pair is considered to be promising for AHP because of high ethanol adsorption capacity of ACs and a relatively high vapor pressure of ethanol even at low temperature levels. To further improve performances of ACs as an adsorbent for AHP (e.g. effective adsorption amount of ethanol, adsorption/desorption kinetics, and cycle-ability during the AHP operations), there are various modification factors such as surface area, pore size and the distribution, surface functional groups, pore depth and packing density, to be considered and optimized. Among them, the surface functionalities and pore structures are considered to strongly influence on adsorption behavior of polar ethanol molecules. To clarify the influence of these factors, model ACs having controlled pore structures and surface functionalities have to be developed.

In this study, ACs having different content of surface functionalities but similar pore structure or similar content of surface functionalities but different pore structure were prepared by heat treatments in H₂ and/or careful adjustments of carbonization and KOH activation conditions. Then, the ethanol adsorption performances of the prepared ACs were investigated by using a gravimetric adsorption apparatus.

It was found that abundant surface functionalities decreased the adsorption amount of ethanol and shortened the apparent adsorption equilibrium time due to the diffusional hindrance of ethanol molecules by strong interaction between the surface functional groups and ethanol molecules. On the other hand, as was predicted by our GCMC simulation, the AC having micropores of 1.6 nm in pore width was found to show the highest effective adsorption amount in the AHP operation range (P/P₀ = 0.1-0.3), which reached to 1.7 times higher than ultrahigh surface area commercial AC, Maxsorb III. Furthermore, the superior ethanol adsorbability of the prepared AC was maintained upon adsorption/desorption cycling without remarkable pore structural and morphological degradations at least up to 10 cycles.

We believe that the findings confirmed a potential of the AC-ethanol pair for AHPs.