Experimental investigation of porous carbon for cooling and desalination applications

The study addresses the growing need for sustainable energy-efficient cooling and water desalination. Conventional technologies are energy-intensive and rely on fossil fuels. Adsorption cooling and desalination (ACD) systems can be powered by low-temperature heat sources (solar, geothermal, waste heat), have no moving parts, and can deliver high-quality desalinated water. Typical systems comprise two or more adsorbent beds, a condenser, and an evaporator, and can operate in single- or two-stage configurations. In ACD cycles, heat of adsorption and condensation are removed via cooling water, while evaporation requires an external cooling load; in adsorption desalination with heat recovery (AD-HR), condenser heat is recovered to drive evaporation. Research focuses on enhancing coefficient of performance (COP), specific cooling power (SCP), and specific daily water production (SDWP) through system design and, critically, advanced adsorbent materials with higher water adsorption capacity. This work aims to enhance ACD performance by chemically treating and activating Maxsorb III (Max) activated carbon—first acid-treated with HCl and then impregnated with ammonium carbonate ((NH₄)₂CO₃)—and evaluating its equilibrium and kinetic water adsorption behavior and modeled system performance (with and without heat recovery).

Prior studies show that ACD/AD-HR performance can be improved by system configurations (e.g., multi-bed, heat recovery) and by optimizing operating temperatures. Two-stage air-cooled ACD can operate in hot climates but may underperform compared to single-stage water-cooled systems. Four-bed AD-HR cycles can operate at 50 °C and produce significantly more water than conventional two-bed cycles. Chemical activation (acid/alkali) increases surface area and pore volume of adsorbents such as clays, zeolites, vermiculite, silica gel, and MOFs, often improving water uptake and system COP/SCP/SDWP. Composite adsorbents combining carbons with salt hydrates (e.g., LiBr, CaCl₂, MgCl₂, Na₂SiO₃) substantially enhance water adsorption capacity (up to ~1.1 kg/kg in some carbon–salt composites) and associated performance metrics. These findings motivate exploring chemical activation and salt impregnation of high–surface-area carbons like Maxsorb III for dual cooling–desalination applications.

Materials and preparation: Maxsorb III (AX21-type, Kansai Coke & Chemicals Co. Ltd., Japan) was selected for its high porosity and low density. Pretreatment involved mixing 10 g of Max with 2 M HCl for 12 h at room temperature with stirring, leaching, washing, and drying at 150 °C for 12 h. Activated Max was prepared by impregnating 2.1 g of dried Max with 30 wt% (NH₄)₂CO₃ aqueous solution at room temperature for 24 h, filtering, and drying at 150 °C for 12 h (ammonium carbonate evaporates around 58 °C), to a constant weight. Characterization: SEM was used to examine microstructure. XRD (Bruker Axs-D8 Advance, Cu Kα, λ=1.5406 Å, 2θ=10–80°) characterized structure. N₂ adsorption–desorption at −196 °C (Quantachrome TouchWin) provided BET surface area (SBET at P/Po≈0.3) and total pore volume (V0.99 at P/Po≈0.99); samples were degassed at 150 °C for 10 h. Water vapor adsorption was measured via a dynamic vapor sorption apparatus (Tabbin Institute for Metallurgical Studies, Egypt). For adsorption isotherms (Ad-Iso), relative humidity (RH) was stepped 5–90% in 18 steps at controlled adsorption temperatures; each step reached stability when mass change ≤0.002 g for 15 min. For adsorption kinetics (Ad-Kin), RH was ramped to 90% and held, recording mass vs. time from 0–850 s, repeated for Ad-T = 25–55 °C. Samples were pre-degassed at 100 °C under N₂ (99% purity) for 10 h. Modeling: Experimental equilibrium data were fitted with the Dubinin–Astakhov (D–A) model; kinetics were fitted with a linear driving force (LDF) model. Fitted parameters were used in a previously validated lumped-parameter model of two-bed ACD and AD-HR cycles (including mass/energy balances for beds, evaporator, condenser; heat exchanger outlet temperature relation; and performance integrals). Performance indicators computed: SCP, COP, and SDWP for ACD; SDWP and gained output ratio (GOR) for AD-HR. Typical operating conditions examined included adsorption temperature ~25 °C, desorption temperature 75–85 °C, condenser pressure ~3.2 kPa, evaporator pressures 1.0 kPa (dual cooling+desalination) and 2.25 kPa (desalination), and half cycle time around 350 s.

- Structure/porosity: XRD showed a persistent graphite hump (~42.5°) with no structural change after HCl treatment or (NH₄)₂CO₃ activation, indicating no crystallinity change and no aging effects. N₂ isotherms indicated microporosity (IUPAC I-type, pore size <2 nm). BET and pore volume (Table 1): Raw Max SBET 2873.61 m²/g, V0.99 1.55 cm³/g; Max/HCl 2612.37 m²/g, 1.41 cm³/g; Max/(NH₄)₂CO₃ 3065.18 m²/g, 1.65 cm³/g. - Water adsorption equilibrium: At 25 °C and P/Ps=0.9, Max/(NH₄)₂CO₃ achieved the highest uptake, ~0.52 kg/kg (reported up to 0.53 kgH₂O/kg), followed by Max/HCl (~0.42 kg/kg). D–A model fitted parameters were obtained and used to estimate isosteric heat of adsorption. - Kinetics: Initial rates were comparable among samples; Max/(NH₄)₂CO₃ exhibited the fastest kinetics, followed by Max/HCl, with raw Max the slowest. LDF fits showed excellent agreement across adsorption temperatures. - Adsorption capacity swing (AC) at operating pressures: Dual-purpose (Pevap=1 kPa) AC values: Max/(NH₄)₂CO₃ 0.27 kg/kg; Max/HCl 0.17 kg/kg; Raw Max 0.07 kg/kg. Desalination-only (Pevap=2.25 kPa) AC values: Max/(NH₄)₂CO₃ 0.47 kg/kg; Max/HCl 0.30 kg/kg; Raw Max 0.15 kg/kg. AD-HR cycles exhibited roughly double the water vapor capacity compared to ACD cycles. - Modeled ACD performance (Ad-T 25 °C, De-T 85 °C): At half cycle time 350 s, Max/(NH₄)₂CO₃ achieved SCP ≈ 373 W/kg, COP ≈ 0.63, SDWP ≈ 13.2 m³·ton⁻¹·day⁻¹. Max/HCl delivered ~200 W/kg SCP, COP ~0.53, SDWP ~7 m³·ton⁻¹·day⁻¹; raw Max delivered ~147 W/kg SCP, COP ~0.50, SDWP ~5.2 m³·ton⁻¹·day⁻¹. SCP and SDWP increased with higher desorption temperature; COP peaked around 75–85 °C. - Modeled AD-HR performance (Ad-T 25 °C, De-T 85 °C): Max/(NH₄)₂CO₃ achieved SDWP ≈ 22.5 m³·ton⁻¹·day⁻¹ and GOR ≈ 0.7 at half cycle time 350 s. SDWP increased with desorption temperature, while GOR slightly decreased. - Benchmarking: Max/(NH₄)₂CO₃ produced approximately double the cooling and desalination performance of silica gel (SG)-based systems under comparable conditions (De-T 85 °C).

The research question was whether chemical pretreatment and salt impregnation of high–surface-area activated carbon (Maxsorb III) could enhance adsorption capacity and kinetics sufficiently to improve ACD/AD-HR system performance at low-grade heat inputs. The results show that (NH₄)₂CO₃ activation increased porosity and, more importantly, water uptake and kinetics, yielding substantially greater capacity swing at relevant operating pressures. When these equilibrium and kinetic improvements are propagated through a validated cycle model, the activated Max delivers higher SCP, COP, and SDWP than both raw and acid-treated Max, and outperforms typical silica gel adsorbents by roughly a factor of two for dual-effect cooling and desalination. The AD-HR configuration further doubles effective adsorption capacity relative to ACD by exploiting condenser–evaporator heat recovery, translating into higher SDWP at similar regeneration temperatures. Collectively, the findings demonstrate that tailored carbon–salt activation enables efficient ACD/AD-HR operation using sub-100 °C heat sources, advancing sustainable water–energy solutions.

This work demonstrates that Maxsorb III activated carbon, when acid pretreated and impregnated with ammonium carbonate, attains high water adsorption capacity (up to ~0.52–0.53 kg/kg at 25 °C, 0.9 P/Ps) and fast kinetics. Using D–A and LDF fits in a validated two-bed cycle model, the activated Max achieves superior dual cooling–desalination performance (SCP ~373 W/kg, COP ~0.63, SDWP ~13.2 m³·ton⁻¹·day⁻¹) and high desalination output with heat recovery (SDWP ~22.5 m³·ton⁻¹·day⁻¹, GOR ~0.7) at low regeneration temperatures (≤85 °C). Compared with silica gel, the proposed material approximately doubles combined cooling and water production under similar conditions. Future research should further improve adsorption capacity via alternative activation strategies and composite formulations with different salt hydrates, and explore broader operating conditions and system integrations to maximize low-grade heat utilization.

System-level performance (SCP, COP, SDWP, GOR) was estimated using a previously validated mathematical model based on experimentally fitted isotherm and kinetic parameters; full-scale experimental ACD/AD-HR cycle testing with the modified Max was not reported here. The study focused on a single salt loading (30 wt% (NH₄)₂CO₃) and specific preparation/operating conditions; broader optimization (e.g., different salt hydrates, loadings, long-term cycling stability) was not detailed in this work.