Analysis of composite adsorber with graphite nanofiber based nanofluid as coolant

Abstract

The use of composite adsorbent with nano particle additives having high surface area can lead to enhanced heat transfer, higher adsorption capacity and faster rate leading to better adsorber performance. The objectives of the present study were to investigate the effectiveness of nanofluid as a cooling fluid of a composite adsorber by carrying out nanofluid characterization and to investigate adsorption-desorption characteristics of a composite adsorbent for ammonia as an adsorbate.An experimental set up with tube-in-tube type configuration was constructed to monitor the thermal characteristics (heat transfer from and to the adsorber) and ammonia adsorption/desorption rates. The composite of calcium chloride with activated carbon and expanded graphite nanoparticles was made having different concentrations of CaCl2 ranging from 40 % to 80 % in the composite and expanded natural graphite (ENG) content ranging from 0-3 % by weight. The desorption characteristics were studied at different temperatures from 70° C to 110° C. The cooling fluid with and without graphite nanofibers (GNF) was circulated in the inner tube at different flow rates from 5 LPH to 25 LPH.From the experimentation on adsorption-desorption characteristics, it was found that the total amount of ammonia adsorbed increased from 133 g for 50:50 composition of CaCl2:AC to 156 g for CaCl2:AC composition of 50:50 with the addition of 3 % ENG nanoparticles. Also the adsorption process time reduced from 22 minutes to 18 minutes. For 50 % CaCl2 in the composite the temperature at which the maximum desorption occurred was 70°C.So, desorption could occur at lower temperature, which is advantageous as it increases the number of options for heat sources to be used. When nanofluid was used as a cooling fluid of the composite adsorber, the total amount of ammonia adsorbed increased from 79.411 g for basefluid to 90.206 g for 0.4 % graphite nanofiber nanofluid for flow rate of 25 LPH. With the use of composite adsorbent of CaCl2:AC of 50:50 composition and addition of expanded natural graphite (ENG) by 3% and when 0.4 % graphite nanofiber nanofluid was used as a cooling fluid, the maximum ammonia was adsorbed up to the bed length of 600 mm only out of the total bed length of 1 m indicating the possibility of reduction in the size of an adsorber.