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

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

OPTIMIZATION OF SYNTHESIS AND CHARACTERIZATION OF PALM SHELL-BASED BIO-CHAR AS A BY-PRODUCT OF BIO-OIL PRODUCTION PROCESS

In this study the optimum preparation conditions of bio-char were achieved as a by-product of the bio-oil production process from oil palm shell as an agricultural waste material. To investigate the possibility of utilizing bio-char as an adsorbent for wastewater treatment and other applications, a central composite design was applied to investigate the influence of carbonization temperatures, nitrogen flow rates, particle sizes of precursor, and duration on the bio-char yield and methylene blue adsorption capacity as the responses. Methylene blue was chosen in this study due to its wide application and known strong adsorption onto solids. Two quadratic models were developed for the responses and to calculate the optimum operating variables providing a compromise between yield and adsorption. From the analysis of variance, temperature was identified as the most influential factor on each experimental design response. The predicted yield and adsorption capacity was found to agree satisfactorily with the experimental values. A temperature of 400°C, nitrogen flow of 2.6 L/min, particle size of 1.7 mm and time of 61.42 min were found as the optimum preparation conditions and near to the optimal bio-oil production variables

Natural gas density measurements and the impact of accuracy on process design

The liquefaction of natural gas is an energy intensive process, requiring at least 5% of the energy associated with methane's lower heating value. Key to estimating and optimizing these energy requirements are process simulations which rely upon calculated thermophysical properties of the natural gas. In particular, the prediction of thermophysical properties of natural gas mixtures at pressure-temperature conditions close to the mixture's critical point or cricondenbar is challenging but important as often natural gas processes operate close to these conditions. In this work, we present a comprehensive study of two natural gas related systems: (CH4 + C3H8 + CO2) and (CH4 + C3H8 + C7H16) with n-heptane fractions up to 15 mol%. High accuracy measurements of densities, at temperatures from 200 K to 423 K and pressures up to 35 MPa are presented. The extensive experimental data collected for these mixtures were compared with the GERG-2008 equation of state, as implemented in the NIST software REFPROP. The relative deviations of the measured densities from those calculated using the GERG-2008 model range between (−2 to 4)% for all mixtures, presenting a systematic dependent on mixture density and n-heptane content. Finally, a case study is presented that probes the impact of the accuracy of density on the pinch point in a simulated LNG heat exchanger. An uncertainty in the density of 1% is shown to cause significant 30% reduction in the minimum approach temperature difference, suggesting that accurate thermophysical property calculations are key to reducing over-design of processing plant

Phase equilibrium studies of high-pressure natural gas mixtures with toluene for LNG applications

© 2020 Elsevier B.V. To prevent possible freeze out in the main cryogenic heat exchanger (MCHE) used in liquefied natural gas (LNG) plants, new and accurate phase equilibrium data are required to improve the predictive reliability of existing models, in particular cubic equations of state (EOS). In this work, the vapor-liquid equilibrium (VLE) of a ternary methane + propane + toluene (methylbenzene) mixture was studied over a wide range of conditions with toluene as the minor component in both the liquid and vapor phases. Measurements were conducted along different isochoric paths at temperatures between (213 and 298 K) and pressures up to 8.3 MPa, to obtain data at conditions relevant to the operation of LNG scrub columns. The measured VLE data were compared to results calculated with the HYSYS Peng Robinson (PR) equation of state (EOS) that is used widely in LNG industry. The amount of toluene in the vapor phase was found to be under-predicted by the HYSYS PR EOS by an average of around 77% at lower temperatures, with the error increasing as temperature and toluene concentration decreased. The current work demonstrates that the HYSYS PR EOS as well as other cubic EOS substantially under-predict the possible toluene content of saturated vapours that could be present in the overhead of the LNG scrub column. Using the ThermoFAST model recently developed and optimised for the calculation of solid-liquid equilibrium conditions in LNG production, this work further demonstrates that the 77% increase in the toluene content of a saturated vapor entering the MCHE, corresponds to a 7 K increase in the solid formation temperature, which could significantly increase the likelihood of a blockage in the MCHE and thus possible shutdown of the LNG plant. The experimental and modelling work presented here underscores the importance of improving predictions of the allowable threshold concentration of heavy components in fluids entering cryogenic heat exchangers in LNG plants