Adsorbed Natural Gas
Group 12-20
Why Adsorbed Natural Gas?
The best choice for methane storage.
Safe. Efficient. Cost saving.
Metal Organic Framework (MOF)
Best material as adsorbent?
At present, there are three ways for methane storage, Compressed Natural Gas (CNG) and Liquefied Natural Gas (LNG) are two of them.
However, a pressure of 20 MPa is needed for the compression and liquefaction process. Hence safty issues exist, and great amounts of energy are lost during the process. In comparision, the third way, Adsorbed Natural Gas (ANG) is relatively safer, more efficient and cost saving.
The third choice for the adsorbent is Metal Organic Framework (MOF). From the diagram we can see the molecular structure of MOF, which consists of inorganic metal ions of clusters, and organic ligands that connect the metal clusters together. Gas molecules will be adsorbed by the porous structure.
Metal Organic Framework (MOF) has many advantages, such as high plasticity, high specific surface area (SSA) and gas molecules can be charged and discharged easily. It can also act as a catalyst as well.
Molecular sieve has low specific surface area (SSA) and low hydrophilicity. Hence it is highly inefficient for methane storage.
Activated carbon, as an alternative option, has high specific surface area (SSA), and it can convert natural gas into methane clathrates for convenient storage.
Literature review — Adsorption
By definition, adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface. It can be categorised as physical and chemical adsorption. Adsorption can be used in many areas and can function for both industrial and domestic uses. For example, adsorbents can be used to adsorb and store gases transportation or used as the stationary phase for chromatography. Adsorption can also be applied for purification of air in houses after painting by adsorbing formaldehyde and other hazardous gases. In this project, adsorption is used for the storage of natural gas and experiments to calculate the efficiency of adsorbents.
Adsorption studies are carried for adsorbates at liquid or gas states. By studying the research paper, Adsorption Studies of Methylene Blue Dye from Aqueous Solution onto Phaseolusaureus Biomaterials (Jerikar D. B., A. A. Pathan, M. Farooqui, 2014), it was learnt that for adsorption of liquid adsorbates, there are three major factors that affect the eventual effect and efficiency of adsorption which are the contact time of adsorption, the dose of adsorbent and the initial concentration of the adsorbate. Controlling variables was the methodology taken to conduct the experiment. The temperature also had to be controlled at 298±3 K.
The amount of dye removed per unit weight of adsorbent (qt) and the dye-removal efficiency (R) can be calculated using the formulae below.
For adsorption of gases, the methodology used for carrying out the studies was similar and the formulae for calculation of the amount and efficiency of adsorption are still applicable. However, the experiment data in Progress In Adsorbed Natural Gas Technology In China (Chen J., Qu M., Xu W., 2002) showed that the variables in the experiment were different from the adsorption of liquid adsorbates. The specific surface area, bulk density of adsorbent and the diameter of nano pores of the adsorbent (activated carbon in most cases) were taken into consideration instead of dose and initial concentration. The temperature and pressure of the experiments were still controlled at 298K and 3.5 MPa respectively.
Adsorption has quite many differences from absorption. Adsorption can be affected by temperature and especially favoured by low temperature while temperature does not affect the result of absorption. Adsorption is a exothermic process whereas absorption is endothermic. Most importantly, adsorption is a phenomenon at the surface of adsorbent only but absorption happens at both the surface and the inside structure of absorbent, thus a bulk phenomenon. The reaction rates are different between adsorption and absorption as well.
In this project regarding storing natural gas, the experiment set-up and variable-controlling methodology can be learnt from the previous research done by Chen Jinfu, Qu Mei and Xu Wendong and improved by increasing the variety of adsorbents to collect more data with less inaccuracy. Due to the irreversibility of most chemical adsorption cases, this project will be researching on physical adsorption of natural gas using porous materials including activated carbon and zeolite.
Literature review — Adsorbents and methane storage
According to past research (Xie H., Liu Y., Feng B., Chu W., 2014) studied, two methods were recommended to store methane, the major component of natural gas. The first way is to liquefy (LNG) or to compress natural gas (CNG) to store it in a smaller container and at a higher density. At the same time, LNG or CNG possesses a higher energy density. The energy density CNG/LNG has is the highest at present, however, due to its high-level of energy loss during compression and the great safety risk present ruing transportation, this method is disadvantaged. Thus, this group will focus on the succeeding method, which is ANG (Adsorbed Natural Gas).
The research done by Xie suggested molecular sieve as the first material for adsorption. However, research have showcased that the low specific surface (<1000 m2/g) area and the hydrophilicity of its surface highly limited the efficiency of the adsorption.
The second kind of material is dry activated carbon (AC), which has a higher specific surface area (3000 m2/g), and the third one is wet activated carbon. Compared with dry AC, wet AC converts the natural gas into natural gas hydrates (NGH) or clathrates. In this way, it largely improved the ability of adsorption by 63%. Besides, by adding water, its storage capacity will also be improved up to 200 V(STP)/V and will occupy only a small volume in the container of ANG. Moreover, using wet AC simplifies the process of desorption by just reducing the pressure. Thus wet AC has a great potential for future application.
The last sort of material used is metal-organic framework (MOF). It is the best material present to adsorb methane. The storage capacity can be up to 265 V(STP)/V. This method is using complexation between organic ligand and transitional metal ions to create a stronger attracting force such that more natural gas can be adsorbed.
The current research areas are mainly regarding two crucial challenges. The first one is lowering the cost and complexity of production process. As MOF and wet activated carbon are still at the experimental level, the cost and producing process will limit the application of these two kinds of materials. The second challenge is on how to maximise the specific surface area, so that more natural gas can be carried within a smaller amount of adsorbents. To find a simpler way and use cheaper materials for methane adsorption are the main objectives of this project.