In this research, the adsorbed natural gas storage system is investigated both experimentally and theoretically for its enhanced storage capacity and effective thermal management of the adsorbent bed. The major findings of this study are summarized as follows:
1. The adsorption isotherms of methane/activated carbon systems were measured for temperatures ranging from (5 to 75) °C and pressures up to 2.5 MPa. Three different types of activated carbon samples were used in the measurement of adsorption uptake data which are useful in designing the ANG storage system. The following findings are obtained from the experimental uptake results.
a) An improved accuracy of about (10 to 15) % has been observed in the present uptake measurement than the earlier studies (Lozano-Castelló et al., 2002c;
Himeno et al., 2005; Saha et al., 2007) for similar adsorbate-adsorbent pairs.
b) Among the carbon samples used for isotherm experiment, the Maxsorb III sample presented the highest uptake capacity because of its high specific surface area and pore volume.
c) The Dubinin-Astakhov isotherm model was observed to be more appropriate to fit the methane uptake data on the carbon samples due to (i) the accountability of heterogeneity parameter and (ii) the consideration of adsorbed phase volume.
Therefore, this model is selected for the thermodynamic analysis of the adsorbed phase of methane as well as for the parametric study of the ANG storage system.
2. The adsorption kinetics of the methane/Maxsorb III pair was determined using the volumetric apparatus. The transient uptake values were found to be well-described by the modified Linear Driving Force (LDF) model (Loh et al., 2010c) that accounts for the non-isothermal phenomena during uptake measurement. This modified kinetics model provides a convenient basis for the analysis of non- isothermal charge and discharge processes of the ANG storage system.
3. The adsorption isotherms of the methane/Maxsorb III pair were measured at temperatures for the cryogenic ranges using a purpose-built cryostat. The uptake values were regressed with the Dubinin-Astakhov isotherm model (with a regression error of 6 %) considering the adsorbed phase specific volume. These isotherm results are useful to estimate the gas charging pressure for the ANG storage vessel, especially when the natural gas is considered to charge from the LNG terminal at near cryogenic temperatures.
4. The theoretical frameworks for the thermodynamic properties are described in Chapter 3 where the thermodynamic relations for the adsorbed phase are developed from the rigor of adsorption thermodynamics by incorporating the micropore filling theory for carbonaceous materials. The proposed expressions provide better analysis than the recent study by Chakraborty et al. (2009) as the effect of adsorbed phase volume is taken into consideration.
5. The adsorbed phase thermodynamic quantities, such as specific volume, heat of adsorption, specific heat capacity, enthalpy, and entropy, were evaluated from the measured isotherm data of methane/Maxsorb III pair along with the other activated carbon samples cited from the literature. The evaluated values exhibit strong dependence of the adsorption parameters on the adsorbed phase thermodynamic
quantities along with the pressure and temperature. These values are also useful in the analysis of charge and discharge processes of the ANG storage system.
6. The ANG storage system with internal thermal control based on finned type heat exchanger in the adsorbent bed has been theoretically modeled for the cyclic processes. The transient pressure and temperature profiles were simulated which demonstrate that both the charge and discharge processes are notably enhanced due to the insertion of fins and tubes in the adsorbent bed. It can be seen that the system performance is improved by shortening the charge period due to the quick removal of adsorption heat and by maximizing the delivered amount of methane due to the heat supply to the adsorbent bed during the discharge cycle. The volumetric capacity of more than 130 V/V can be reached and it is approximately five times of storage capacity by the CNG method for similar operating conditions.
7. A prototype of the ANG storage system with fin and tube arrangement in the activated carbon bed was fabricated and experiments were performed for different operating conditions. Water was circulated through the heat exchanger tubes during the cyclic processes at different flow rates and inlet temperatures. The experimental results assess that the storage capacity was increased by 17 % with charge period of less than 10 minutes when water was circulated to remove the adsorption heat.
Similarly, the gas delivery was improved by 7 % during the discharge process due to the supply of heat from external source. Thus, the overall capacity was enhanced by 24 % in storing and delivering gas, although the fins and tubes occupied some volumes (about 13 %) of the activated carbon bed that necessitates the optimization of the heat exchanger design for effective thermal management. The experimental results agreed well with the simulation for both the charge and discharge processes.
From the above findings, it can be said that microporous activated carbons with larger surface area and micropore volume are required for higher storage capacity of the ANG vessel and the heat exchanger arrangement in the packed activated carbon bed can be an effective means for enhanced temperature management during both the charge and discharge processes.