**6. Factors that affect the storage capacity of natural gas in CNT**

In general, carbon nanotubes (SWCNT with a diameter of 2040 nm and MWCNT with a diameter of 3060 nm) are stable in the temperature range of 25– 600°C. Also, an increase in the thermal capacity of carbon nanotubes has been reported by shortening the length of CNT diameter nanotubes in the range of 60– 100 nm. While thermal stability increases with increasing nanotube length [6]. In a conducted simulation work to study the storage capacity of SWCNT at different temperatures and pressures, it was observed that methane is weakly adsorbed in SWCNT. Results showed that as pressure increases, the amount of methane adsorbed on SWCNT increased but as temperature increased, the amount of methane adsorbed on SWCNT decreased. It was reported that the binding energies for methane on the defected SWCNT increased by about 56% over the defect-free SWCNT showing that the presence of defects on the structure of nanotubes increases its methane adsorption capacity. Furthermore, for the encapsulated methane molecules inside the defected nanotubes, results showed about 68% increase in binding energy compared with the confined molecules in the defect-free nanotubes. It was pointed out that introducing surface curvatures in the nanotubes could reduce the binding energy between the methane molecules and the substrate. Thus, some factors that affect methane adsorption in SWCNT are pressure, temperature, structural defects, and curvatures. The methane storage capacity of MWCNT has been studied by several researchers. One set of results showed that a type of MWCNT strongly adsorbed methane at a maximum value of 5.44 mmol/g at a temperature of 283.15 K and a pressure of 40 bars. It was also reported that increasing pressure increased the amount of methane adsorbed, while increasing temperature decreased the amount of adsorbed methane. Another report has it that treating MWCNT with acids such as HCl and HNO3 improves its methane adsorption capacity. The results of experiments conducted using acid-treated MWCNT and untreated MWCNT at the same pressures revealed that acid treatment of nanotubes enhances methane adsorption capacity especially at low pressures. In another work, the methane adsorptions on MWCNT treated with sulfuric and nitric acids, nitric acid, and alkaline were compared with the methane adsorption capacity of untreated MWCNT. Reported results showed that sulfuric and nitric acid–treated MWCNT adsorbed more methane than all other treated and untreated nanotubes. This was followed by nitric acid–treated case before the alkaline-treated nanotubes. The methane adsorption capacity on all the treated nanotubes was higher than the untreated cases showing that treating MWCNT with acids enhances its methane adsorption capacity. This work also showed that increase in pressure increases methane adsorption while increase in temperature decreases methane adsorption on MWCNT [2]. A group of scientists found in their research that the initial slopes of isotherms increased sharply, which indicates that active sites are created on the adsorbent during the functionalization step. Existence of functional groups on MWCNTs causes the adsorption capacity to increase gas at low pressures. At low pressures, adsorption on MWCNTs is affected by the fluid adsorbent interaction; therefore, functional groups led to increased fluid adsorbent interactions. But at higher pressures, fluid interactions become more important than fluid adsorbent interactions, and the role of functional groups is reduced [9].
