Introduction
The emission of gaseous products of combustion into the atmosphere, mainly Carbon dioxide (CO
2) is regarded as a major cause of global warming and climate change, through the so-called greenhouse effect [
1]. Currently, 85 % of total world demanded energy is supplied by thermal power plants fed by fossil fuels, including coal, oil and gas. They account for about 40 % of total CO
2 emissions [
2]; Yang et al. [
3]; [
4]. Among the ways to control, reduce or mitigate this effect, the capture of CO
2 from flue gasses of industrial combustion processes and its storage in deep geological formations is now being considered as a serious option [
5‐
7].
A number of adsorption processes are used commercially for adsorbent process, including pressure swing adsorption (PSA), vacuum pressure swing adsorption (VPSA), and thermal or temperature swing adsorption (TSA). A number of research works have been done using the processes mentioned above on different types of adsorbent materials. Recent developments have demonstrated that PSA is a promising option for separating CO
2 due to its ease of applicability over a relatively wide range of temperature and pressure conditions, its low energy requirements, and its low capital investment cost (Agarwal et al. [
8]). Many studies concerning CO
2 removal from various flue gas mixtures by means of PSA processes have been addressed in the literature. Prior to the design of an adsorption process, selecting an appropriate adsorbent with high selectivity and working capacity, as well as a strong desorption capability, is key to separating CO
2. As a result, a wide variety of adsorbents like activated carbon, zeolites, silica gel, activated alumina, urea–formaldehyde and melamine–formaldehyde resins, poly-ethyleneimine and hollow fiber carbon membranes based adsorbents, etc. have been investigated for this purpose [
9]; Sircar et al. [
10]; [
11,
12,
17]. Recent development shows an improvement in adsorbent materials with higher adsorption capacity and selectivity like Activated carbon honeycomb monolith—Zeolite 13X hybrid system, zeolites NaKA and nano-NaKA, FAU zeolites and zeolite 13X prepared from bentonite [
13‐
16].
The PSA process is based on preferential adsorption of the desired gas on a porous adsorbent at high pressure, and recovery of the gas at low pressure. Thus, the porous sorbent can be reused for subsequent adsorption. PSA technology has gained interest because of the low energy requirements and low capital investment costs. The low recovery rate of CO
2 is one of the problems reported with the PSA process [
18]. Development of regenerable sorbents that have high selectivity, adsorption capacity, and adsorption/desorption rates for CO
2 capture is critical for the success of the PSA process. Cost of the sorbent is also a major factor that needs to be considered for the process to be economical [
19,
20].
The adsorption method of choice for many zeolite molecular sieves is PSA, although some experiments have employed a combined pressure and temperature swing adsorption (PTSA) process
(Ruthven et al. [
21]; [
22,
23]. It has been reported that a particular TSA and PSA cycle conditions would result in higher expected working capacity with an increase in feed temperature. Zeolites have shown promising results for the separation of CO
2 from gas mixtures and can potentially be used for the PSA process. Natural zeolites are inexpensive and can be viable sorbents if they work for the process application [
24]. It has also been reported that using AC as an adsorbent material, the adsorption capacity can increase till 30 Bar and become steady after 30–35 bars [
25].
Based on the literatures available, PSA seems to be the best option for separating CO2 from flue gas due to its ease of applicability over a relatively wide range of temperature and pressure conditions. A number of sorbents like zeolite, activated alumina, activated carbons, etc. have been utilized and cost of the sorbents play a vital role for the process to be economical. In this paper, low cost and abundantly available locally, coconut fiber based AC was employed as the sorbent materials and compared with commercial zeolites. Work had been done to develop a process in which CO2 was adsorbed from a gas stream containing ~13.8 vol. % of CO2 onto zeolite 13X, zeolite 4A and AC by means of PSA process. The system was tested for five different adsorption and desorption cycles in order to determine the adsorbent bed’s regeneration efficiencies. Kinetics and adsorption thermodynamics parameters have also been calculated.
Conclusions
The CO2 adsorption experiments were conducted using gravimetric method at different temperatures and pressures. From this study, it was concluded that the CO2 adsorption isotherm obtained in this study followed general gas adsorption behavior, demonstrating that the CO2 adsorption capacity increases with increasing pressure and decreases with increasing temperature. The adsorption isotherm follows a type-I isotherm classification according to IUPAC, representing a monolayer adsorption mechanism. Among the three adsorbents tested, zeolite 13X offers the highest adsorption capacity, and AC provides the lowest capacity at temperatures ranging from 25 to 60 °C and pressures up to 1 bar. The experimental data of CO2 adsorption were fitted with Langmuir and Freunlich isotherm models. It was found that Langmuir model showed the best fit with the zeolite 13X and zeolite 4A while Freunlich model provided excellent fit with AC. The thermodynamics parameter were calculated from Van’t Hoff`s equation and concluded that the adsorption experiment were exothermic in nature for the three adsorbents. There is no full regeneration for zoelites while a full regeneration can be achieved with AC.