Experimental investigation of porous carbon for cooling and desalination applications

The global demand for sustainable solutions to energy and water scarcity is driving research into efficient desalination and cooling technologies. Conventional methods are energy-intensive and rely heavily on fossil fuels. Adsorption cooling and desalination (ACD) systems offer a promising alternative, utilizing low-temperature heat sources or waste energy to produce both cooling and desalinated water. These systems typically consist of adsorption beds, a condenser, and an evaporator, sometimes incorporating multiple evaporators and operating in single or two-stage configurations. Performance is enhanced by improving the coefficient of performance (COP), increasing cooling effect, and maximizing specific daily water production per ton of adsorbent (SDWP). Research efforts focus on improving system design, heat transfer, and developing novel adsorbent materials with high adsorption capacity (Ad-C). Previous studies have explored various adsorbents, including silica gel (SG), and investigated the impact of chemical modifications such as acid/alkaline activation to enhance their properties. This research builds upon these prior studies by investigating the potential of chemically modified Maxsorb III activated carbon as a superior adsorbent for ACD systems.

Numerous studies have explored the optimization of ACD systems. Researchers have investigated the hybridization of AD systems with thermal desalination plants, and significant advancements have been made in developing new adsorbents and improving existing ones. Ng et al. demonstrated the linear relationship between SDWP and evaporator/heat source temperatures. Chakraborty et al. optimized ACD system performance by modifying silica gel's surface thermodynamic properties. Mitra et al. showed that a two-stage ACD system with air cooling has lower performance than single-stage water-cooled systems but can operate under hot climate conditions. Thu et al. showed a four-bed system with heat recovery produces significantly more freshwater. Extensive research has also been conducted on the effects of acid/alkaline treatment on various adsorbents like clay, zeolite, vermiculite, SG, and metal-organic frameworks (MOFs) to increase their surface area and water adsorption capacity. Studies have shown significant improvements in Ad-C following acid activation with HCl, leading to enhanced COP and SCP values. The incorporation of salt hydrates into carbon composites has also been explored to further boost Ad-C. This work reviews and builds on the significant body of existing knowledge by exploring the potential of improved activated carbon.

The study employed Maxsorb III (Max), a commercially available activated carbon, as the base adsorbent. The Max was first subjected to acid treatment using 2 M HCl for 12 hours at room temperature to remove impurities and increase surface area. Following washing and drying, the acid-treated Max was then impregnated with 30 wt.% (NH₄)₂CO₃ solution for 24 hours at room temperature to further enhance its adsorption capacity. The resulting materials (raw Max, Max/HCl, and Max/(NH₄)₂CO₃) were characterized using several techniques. Scanning electron microscopy (SEM) was used to examine the materials' microstructure and morphology. X-ray diffraction (XRD) was used to study the crystalline structure of the materials. Nitrogen (N₂) adsorption-desorption isotherms were employed to determine the surface area (SBET) and total pore volume (V0.99) using a Quantachrome TouchWin™ instrument. A custom-built water vapor adsorption analyzer (details described in Ref. 59) was used to measure the water vapor adsorption isotherms and kinetics of the modified and unmodified Max samples at various temperatures. The experimental data obtained were fitted to established models. The Dubinin-Astakhov (D-A) model was used to describe the water adsorption equilibrium, while the linear driving force (LDF) model was used to represent the adsorption kinetics. The fitted parameters from these models were then employed in a previously validated lumped parameter model of a two-bed ACD system (with and without heat recovery) to estimate the system's performance parameters, including specific cooling power (SCP), coefficient of performance (COP), and specific daily water production (SDWP). The performance of the ACD system was analyzed with varying cycle times and desorption temperatures. For the AD-HR system (adsorption desalination with heat recovery), the gained output ratio (GOR) was also evaluated.

SEM analysis revealed that the (NH₄)₂CO₃ treatment significantly altered the surface microstructure compared to the HCl treatment. XRD analysis showed no change in the crystalline structure of Max after the treatments, indicating that the modifications primarily affected the surface properties. The N₂ adsorption-desorption isotherms revealed that Max/(NH₄)₂CO₃ exhibited the highest N₂ uptake, indicating increased porosity. Water adsorption isotherms revealed that Max/(NH₄)₂CO₃ exhibited the highest water adsorption capacity (0.52 kg.kg⁻¹ at 25 °C and 0.9 P/Ps), exceeding both Max/HCl and raw Max. The adsorption kinetics showed a similar trend, with Max/(NH₄)₂CO₃ exhibiting the fastest uptake. The D-A model fitted the experimental isotherm data well, and the LDF model accurately represented the adsorption kinetics. Using these fitted parameters in the validated ACD model, the Max/(NH₄)₂CO₃-based ACD cycle demonstrated superior performance, achieving a SCP of 373 W.kg⁻¹, a COP of 0.63, and an SDWP of 13.2 m³.ton⁻¹ per day at an optimal cycle time. The Max/HCl-based system showed lower performance. The performance of the ACD system improved with increasing desorption temperature. The Max/(NH₄)₂CO₃-based AD-HR cycle exhibited even higher performance, reaching an SDWP of 22.5 m³.ton⁻¹ per day and a GOR of 0.7 at the optimal cycle time. The SDWP increased with desorption temperature. A direct comparison with previous studies using silica gel as the adsorbent revealed that the Max/(NH₄)₂CO₃ outperforms silica gel, achieving approximately double the SDWP, SCP, and COP in dual cooling and desalination applications.

The significant improvement in the performance of the ACD and AD-HR systems using the modified Maxsorb III adsorbent demonstrates the effectiveness of the HCl pretreatment and (NH₄)₂CO₃ activation. The increased surface area and pore volume of Max/(NH₄)₂CO₃ resulting from the treatment provided more adsorption sites for water molecules, leading to enhanced adsorption capacity. This translated directly into improved cooling and desalination performance as evidenced by the higher SCP, COP, and SDWP values. The heat recovery in the AD-HR system significantly amplified the water production, highlighting the potential of integrating energy-efficient design strategies. The superior performance of the Max-based system compared to silica gel-based systems suggests a promising alternative for ACD applications. The findings provide valuable insights for optimizing ACD system design and material selection for enhanced energy efficiency and water production.

This study successfully demonstrated the potential of chemically modified Maxsorb III activated carbon for improved ACD and AD-HR system performance. The Max/(NH₄)₂CO₃ composite showed significantly enhanced adsorption capacity and resulted in substantially higher SCP, COP, and SDWP compared to both untreated and HCl-treated Max, as well as silica gel-based systems. The results highlight the importance of adsorbent material optimization for efficient ACD applications. Future research should explore further modifications of Maxsorb III and other porous carbon materials using different activation methods and composite formulations to achieve even higher adsorption capacities and system performance.

The study focused on a specific type of activated carbon and a particular activation method. The generalizability of the findings to other types of adsorbents or different activation techniques remains to be explored. The experimental setup and model used in the study may have introduced some uncertainties or limitations that could affect the accuracy of the results. Future studies could benefit from more rigorous validation of the mathematical model used in this study. The study primarily focused on laboratory-scale experiments, and scaling up the process to an industrial scale could present additional challenges that need to be addressed.