Measurment of CO2 adsorption isotherms and kinetics of activated carbons suitable for the development of CO2 based adsorption cooling systems

Abstract

Adsorption of CO2 on activated carbons is widely utilized in gas storage systems and adsorption-based cooling systems. Selection of CO2-activated carbon pairs for gas storage and cooling systems is mainly influenced by adsorption isotherms, adsorption kinetics, thermodynamic properties and operating conditions. The elaborated research work aims at experimental and theoretical studies on CO2 adsorption of various activated carbons namely Norit RB3 (steam activated rod) (AC1), Norit Darco (100 mesh size) (AC2) and Norit Darco (12×20 US mesh size) (AC3) and coconut shell based activated carbon (CSAC) for the deveolpment of CO2 based adsorption cooling systems. In the present study, the CO2 adsorption isotherms of activated carbons are measured using a Sievert’s type experimental setup at wide range of temperatures from 273 to 358 K. Experimental data of CO2 adsorption isotherms is modelled using Langmuir, Tóth, Dubinin-Astakhov (D-A) and modified Dubinin-Astakhov model. Pseudo-saturation pressure of CO2 plays a significant role in the estimation of thermodynamic properties of supercritical gas adsorption, and it is evaluated using measured adsorption isotherms data. The thermodynamic properties namely isosteric heat of adsorption at surface loading, Gibbs free energy, and entropy are evaluated using classical Clausius-Clapeyron equation and Dubinin-Astakhov model for both subcritical and supercritical phases of CO2 respectively. Adsorbed phase specific heat capacity, entropy, and enthalpy are also estimated. The adsorption kinetics of CO2 on activated carbons (AC1, AC2, AC3 and CSAC) are measured at different supply pressures (1, 5, 10 and 20 bar) and temperatures (298, 308, 318 and 338 K) using volumetric method. Activation energies at different pressures are calculated by fitting the Arrhenius equation to the adsorption kinetics data. Analysis of mechanism of interaction between CO2 molecules and adsorbents during the adsorption process is attempted using interparticle and Boyd’s diffusion models. It has been found that the film-diffusion is an important rate controlling step in CO2 adsorption process.The adsorption based cooling/heating systems have been paid more attention owing to free of moving parts, simpler control, lesser operational cost and utilization of waste heat. They are non-toxic and eco-friendly as compared to the mechanical vapor compression cooling systems. CO2 has emerged as a next generation promising refrigerant for an adsorption-based cooling system for the possible applications in commercial/residential cooling, vehicle air-conditioning, and large refrigeration systems. The measured adsorption isotherms data and estimated thermodynamic properties are used for the thermodynamic analysis of single-stage and single-effect two-stage adsorption cooling system. The single-stage CO2 based cooling system performance parameters in terms of maximum theoretical specific cooling effect (SCE) and coefficient of performance (COP) are estimated for the driving heat source temperature of 80°C at four cooling temperatures of 0, 5, 10 and 15°C with a fixed adsorption/condenser temperature of 25°C. All the four activated carbons are found suitable for developing CO2-activated carbon pairs based cooling system. The maximum theoretical value of SCE and COP are obtained as 25.87 kJ/kg and 0.10 respectively. In addition, the performance of the adsorption cooling cycle in terms of the maximum theoretical SCE and COP of single-effect two-stage adsorption cooling system using CSAC at different driving heat source temperatures (80°C for single-stage and 65°C for single-effect two-stage) and evaporator temperatures (0, 5, 10 and 15°C) is estimated. The maximum theoretical values of SCE and COP are obtained as 9.26 kJ/kg and 0.06 for single-stage and 8.85 kJ/kg and 0.04 for single-effect two-stage cooling system respectively. This thesis presents important data related to experimental measurement and theoretical modelling of CO2 adsorption isotherms and kinetics of activated carbons. Important thermodynamic properties are estimated from adsorption isotherms and kinetics data. Single-stage and single-effect two-stage adsorption refrigeration cycles are analyzed thermodynamically. The results obtained will add value to the knowledge base of CO2 adsorption isotherms and kinetics, thermodynamic properties of activated carbons and CO2 based adsorption cooling systems.