2.3.2. Adsorption

This is the process of removing a component from a mixture using a solid surface. Unlike absorption processes, the formation of physical or chemical bonds takes place between the solid phase adsorbent surface and CO2. The intermolecular forces between the solid surfaces and gas are the driving force for adsorption [31]. Single or multiple layers of the gas can be absorbed based on adsorbent pore size, temperature, pressure and surface force [54].

At first, a column is filled up with the adsorbent. Then, the gas stream bearing CO2 is passed through this column. The CO2 adheres to the solid surface of the absorbent during the flow to the saturation of the adsorbent. When the surface becomes saturated with CO2, it is removed and desorbed through different cycles for CO2 adsorption [31].

Generally, four different regeneration cycles are used for single bed CO2 adsorption. They are pressure swing adsorption (PSA), temperature swing adsorption (TSA), electrical swing adsorption (ESA), and vacuum swing adsorption (VSA). In temperature swinging adsorption, the temperature of the adsorbent is raised up to the point at which the chemical bonds are broken. CO2 gets released at that point. An additional requirement of energy in this process makes this method costlier [54]. Also, this process is time consuming due to heating the adsorbent bed for desorption and cooling it again to make it ready for adsorption [48]. This can be done quickly using electrical swing adsorption. Here, low voltage electric current is passed through the adsorbent to heat up the adsorbent using joule effect. ESA makes it possible to regenerate the adsorbent fast, but it requires high-grade electrical energy instead of low-grade heat energy used in TSA [55].

In the case of pressure swinging adsorption, the pressure of the adsorbent is reduced to accomplish this consequence. Vacuum swing adsorption is a specialized PSA cycle which is used if the feed gas pressure is close to the ambient pressure. The extra energy required for achieving high pressure in PSA can be minimized using VSA [48]. Here, a partial vacuum is used at the downstream of the feed stage to draw the low-pressure feed gas. These cycles can be used in combination with one another. Plaza et al. [56] provided a model of VSA process using aspen plus for post combustion carbon capture. A schematic of the regeneration processes is shown in Figure 8.

**Figure 8.** Schematic diagram of different adsorption regeneration cycles: (**a**) TSA (**b**) PSA (**c**) VSA (**d**) ESA [57].

Pressure swinging operation is favorable when the partial pressure of the CO2 is high whereas temperature swinging adsorption is favorable if the concentration of CO2 is low in the gas stream. PSA will take a much longer time if the concentration of CO2 is low [58]. The adsorption process is more preferable because of its high adsorption capacity at normal pressure and temperature, long-term stability, low regeneration cost, high rate of adsorption, and lower energy requirement [59].

The focus of research on this process is to find a suitable sorbent to separate CO2 from the gas stream. Various substances like zeolites, activated carbons, molecular sieves, hydrotalcites, and metal-organic framework materials have been investigated [60]. Garcia et al. showed [18] that the partial pressure of CO2 is the most influential variable when activated carbon is used as absorbent. Sorption-enhanced water gas shift combines adsorption of CO2 with the water gas reaction. This method is more economical and more energy efficient than amine scrubbing in absorption [61]. Hydrotalcite-based materials are more suitable for adsorption at high temperatures. These materials exhibit improved result when used in a sorption-enhanced WGS reactor for better carbon capture [62].
