Cryogenic Solid Solubility Measurements for HFC-32 + CO2 Binary Mixtures at Temperatures Between (132 and 217) K

Accurate phase equilibrium data for mixtures of eco-friendly but mildly-flammable refrigerants with inert components like CO2 will help the refrigeration industry safely employ working fluids with 80 % less global warming potential than those of many widely-used refrigerants. In this work, a visual high-pressure measurement setup was used to measure solid–fluid equilibrium (SFE) of HFC-32 + CO2 binary systems at temperatures between (132 and 217) K. The experimental data show a eutectic composition of around 11 mol % CO2 with a eutectic temperature of 131.9 K at solid–liquid–vapour (SLVE) condition. Measured SLVE and solid–liquid equilibrium data were used to tune a thermodynamic model implemented in the ThermoFAST software package by adjusting the binary interaction parameter (BIP) in the Peng–Robinson equation of state. The tuned model represents the measured melting points for binary mixtures with a root mean square deviation (RMSD) of 3.2 K, which is 60 % less than achieved with the default BIP. An RMSD of 0.5 K was obtained using the tuned model for the mixtures with CO2 fractions over 28 mol % relative to an RMSD of 3.4 K obtained with the default model. The new property data and improved model presented in this work will help avoid solid deposition risk in cryogenic applications of the HFC-32 + CO2 binary system and promote wider applications of more environmentally-friendly refrigerant mixtures

Enhanced Photocatalytic Degradation of Tetracycline-Class Pollutants in Water Using a Dendritic Mesoporous Silica Nanocomposite Modified with UiO-66

Tetracyclines (TTCs), a widely used group of antibiotics in agriculture and animal husbandry, cause water pollution and the emergence of antibiotic-resistance genes. This study reports the synthesis of a metal-organic framework nanocomposite of UiO-66 based on modified dendritic fibrous nanosilica to act as a photocatalyst for the degradation of doxycycline (DOX) and TTC as drug model pollutants in water. This nanocomposite demonstrated about three times better photodegradation performance than UiO-66 due to a decreased electron-hole recombination rate, increased conductivity, and decreased band gap, leading to the higher pollutant reduction efficiency. Structural and morphological analyses were performed on the nanocomposite, and various influencing parameters, including sample pH, catalyst dose, and irradiation time, were studied on the photocatalytic degradation of DOX and TTC to optimize the photodegradation process. At the optimum condition, the maximum photodegradation of 97.2 ± 3.1% was achieved for solutions containing 200 mg·L-1 each drug. The results showed that the proposed photocatalyst is stable and effective in eliminating TTC-class pollutants from water and wastewater with high efficiency and fast kinetics. The reusability of the catalyst was examined, and no significant decrease in the efficiency of the catalyst was observed after five times

Comparative Study between Regression and Soft Computing Models to Maximize the Methane Storage Capacity of Anthracite-Based Adsorbents

Adsorbed natural gas (ANG) technology is a safe and low-cost approach for natural gas storage. Improving the volumetric adsorption capacity of adsorbents in the ANG tank can enhance the fuel density and make this technology cost-effective compared to other available CH4 storage approaches. For this purpose, the present research focuses on maximizing CH4 uptake on low-cost and available anthracite-based carbon materials via experimental and analytical investigations. The effect of preparation variables of the chemical agent (KOH) impregnation ratio to the anthracite (2.6-4.3 g/g), activation temperature (666-834 °C), and retention time (39-140 min) on the specifications of the coal-based activated carbons (ACs) and their CH4 adsorption capacity were examined experimentally. The results were analyzed through empirical models, including response surface methodology (RSM), our in-house developed models, namely, regularization networks (RN) and adaptive neuro-fuzzy interface systems. The statistical assessment revealed that all three established models could effectively predict the methane adsorption capacity of the carbon samples based on their preparation conditions. The superior performance of our in-house RN is dedicated to its robust theoretical backbone in the regularization theory. Finally, the carbon sample prepared under the optimized preparation conditions, based on the RSM and genetic algorithm, showed the highest CH4 uptake of 175 cm3(STP)/cm3. Based on the authors' knowledge, the volumetric CH4 capacity of the optimized AC is one of the highest values reported in the literature among different classes of the adsorbent

Dynamic simulation and experimental performance of an adsorbed natural gas system under variable charging conditions

Adsorbed natural gas (ANG) technology is considered a cost-effective and sustainable energy storage system that can offer a leading clean and environmentally friendly combustion fuel. Despite the benefits of ANG systems, still, there are some challenges in simulation of these systems accurately under actual conditions. The actual charging condition of ANG vessel with variable gas flow rate was simulated and experimentally validated for the first time. For this purpose, we proposed a new time-dependent equation to monitor methane's variable injection flow rate into the vessel. Dynamic methane storage was experimentally tested to validate the simulation results using a custom-built pressurised ANG vessel (∼300 cm3) filled with various in-house prepared adsorbents (i.e. AC1 and AC2) under the loading condition of 40 bar and 298 K. Also, the thermal behaviour of the ANG vessel was studied via experimental observations. A 2D distributed dynamic model, solved by COMSOL Multiphysics software, was developed to assist the simulation in predicting pressure and temperature variations inside the ANG bed. Analysis of the ANG vessel's performance exhibited higher thermal fluctuations attributed to the adsorbent with superior isothermal methane storage capacity. Due to the low thermal conductivity of both adsorbents, a significant temperature rise was observed in the central region of the bed. Sensitivity analysis shows that increasing the length and diameter of the ANG tank leads to a longer required time for charging the tank up to the desired pressure and relative decreases in the temperature profile. Moreover, increasing heat capacity of adsorbent from 800 to 1350 J/kg.K caused 37% reduction in the temperature variations and 7.7% enhancement in gravimetric methane storage efficiency

Modification of activated carbon using nitration followed by reduction for carbon dioxide capture

Activated carbon (AC) samples were modified using nitration followed by reduction to enhance their CO2adsorption capacities. Besides characterization of the samples, investigation of CO2 capture performance was conducted by CO2 isothermal adsorption, temperature-programmed (TP) CO2 adsorption, cyclic CO2adsorption-desorption, and dynamic CO2 adsorption tests. Almost all modified samples showed a rise in the amount of CO2 adsorbed when the comparison is made in unit surface area. On the other hand, some of the samples displayed a capacity superior to that of the parent material when compared in mass unit, especially at elevated temperatures. Despite ∼65% decrease in the surface area, TP-CO2 adsorption of the best samples exhibited increases of ∼10 and 70% in CO2 capture capacity at 30 and 100°C, respectively

Temperature dependence of adsorption hysteresis in flexible metal organic frameworks

"Breathing" and "gating" are striking phenomena exhibited by flexible metal-organic frameworks (MOFs) in which their pore structures transform upon external stimuli. These effects are often associated with eminent steps and hysteresis in sorption isotherms. Despite significant mechanistic studies, the accurate description of stepped isotherms and hysteresis remains a barrier to the promised applications of flexible MOFs in molecular sieving, storage and sensing. Here, we investigate the temperature dependence of structural transformations in three flexible MOFs and present a new isotherm model to consistently analyse the transition pressures and step widths. The transition pressure reduces exponentially with decreasing temperature as does the degree of hysteresis (c.f. capillary condensation). The MOF structural transition enthalpies range from +6 to +31 kJ·mol-1 revealing that the adsorption-triggered transition is entropically driven. Pressure swing adsorption process simulations based on flexible MOFs that utilise the model reveal how isotherm hysteresis can affect separation performance

Prediction of solid formation conditions in mixed refrigerants with iso-pentane and methane at high pressures and cryogenic temperatures

High boiling-point components in mixed refrigerants can improve the performance of natural gas and hydrogen liquefaction facilities. However, such heavy compounds can freeze out from the refrigerant mixture, posing blockage and plant shutdown risks for cryogenic heat exchangers. To improve the predictions of these conditions, freezing and melting temperatures of pure iso-pentane and (methane + iso-pentane) binary systems were measured at temperatures down to 87.5 K and pressures up to 13 MPa. The iso-pentane melting data are compared with predictions of a thermodynamic model embedded in the ThermoFAST software package. Adjusting the model's fusion molar volume change parameter to force agreement with the measurements reduced the deviations of the experimental data from the model by over 90% relative to the default parameter value. The measured melting data for binary mixtures were used to confirm solubility predictions for iso-pentane in mixed refrigerants. Adding 20 mol% iso-pentane to a methane-rich refrigerant increases the available duty for cooling natural gas or hydrogen from (313–123) K by a factor of three. This improvement outweighs the risk of freeze-out in this refrigerant with the melting temperature being 98 K, which is 15 K lower than the minimum temperature needed for LNG production