# Application of Nanofluids in CO\({}_{2}\) Absorption: A Review
## 1 Introduction
Despite chemical absorption, there is no need for heat in physical absorption, and the solvent is recovered by pressure reduction in these processes [14; 15; 16]. However, physical solvents have some drawbacks; they are sensitive to the partial pressure of acid gas (acid gas partial pressure should be high), they hardly meet H\({}_{2}\)S specifications, the concentration of inert gases must be low [14]. Some of the commonly used physical solvents are NMP (N-Methyl-2-Pyrrolidone), DEPG (Dimethyl Ether of Polyethylene Glycol), and PC (Propylene Carbonate) [17]. The cryogenic processes is another CO\({}_{2}\) absorption process that takes advantage of the intrinsic properties of components of a gas mixture; CO\({}_{2}\) capture is done using the distinct desublimation and condensation properties of CO\({}_{2}\) and its carrier gas. The high purity of CO\({}_{2}\) (99.99%) is achieved in a highly efficient (99.99%) process, compared to other CO\({}_{2}\) capture techniques [18]. In this process, a CO\({}_{2}\) phase change occurs in blue gases, after several stages of compression and cooling. Other constituents of the mixture will invariably experience phase change, too. CO\({}_{2}\) might be separated as a liquid or a solid along with other components and can be further purified by distillation. However, the absorbent pores are at risk of blockage by the water from other components, and in this case, the cost of CO\({}_{2}\) capture will increase [19]. One of the commercial gas separation and purification processes is membrane separation. This highly efficient, environmentally friendly technique is utilized in some gas purifying systems like natural gas sweetening and air separation and can compete with the other absorption systems for CO\({}_{2}\) removal from blue gas [20]. The concept of membrane separation is very simple. Nevertheless, in the case of post-combustion capture, some membrane properties like permeability and selectivity, limit its utilization. As an example, 95% purification of CO\({}_{2}\) with a 90% CO\({}_{2}\) capture rate is not feasible with a single-stage membrane. When talking about using membrane separation in gas-transport pipeline systems for CO\({}_{2}\) capture, we face two conflicting effects, high energy consumption in low membrane areas and high investment cost for large membrane areas [21]. Adsorption is a promising alternative to chemical adsorption processes; the process in which one or more components are removed by attachment to a solid surface is called adsorption. The basis of adsorption is the intermolecular interactions between solid materials surface (like activated carbon or molecular sieve) and the adsorbed gas (like CO\({}_{2}\)). Single or multilayer adsorption depends on the variables of the partial pressure of the absorbed gas, temperature, adsorbent pore sizes, and surface forces [22]. Activated carbon, metal-organic frameworks, zeolites, and microporous organic polymers are solid absorbents that have some superiorities in CO\({}_{2}\) adsorption capture. The lower energy consumption of CO\({}_{2}\) capture by adsorption is attracting increasing attention. Gas streamswith different CO\({}_{2}\) content can be treated with temperature vacuum swing (TVS), vacuum swing adsorption (VSA), temperature swing adsorption (TSA), electrical swing adsorption (ESA), and pressure swing adsorption (PSA) [23]. In addition, there are other processes for CO\({}_{2}\) absorption, such as Ionic Liquids (ILs), electrochemical conversion, photochemical, thermochemical, and biochemical, using organic/metallic (organometallic) catalysts on chemical and hybrid rings. Even though all of these methods are promising in CO\({}_{2}\) absorption, they are still being evaluated with respect to the obstacles pertinent to cost, energy, and also their application at large scales [24; 25]. Over the recent years, CO\({}_{2}\) absorption through nanofluid has captured interest due to its unique advantages in adjusting physical and chemical properties and its special applications and high specific area.