The Performance of Modified Pumice by TEPA in Adsorption of CO2 in Process Industries

The Performance of modified pumice by TEPA in Adsorption of CO2 in Process Industries

Introduction
The overuse of fossil fuels to supply the fast-growing population of the earth with their needed energy, as well as advanced technologies and industrial development have led to the emission of great amount of greenhouse gases. From among the greenhouse gases, CO2 is of particular importance and accounts for around 60% of the effects of global warming. The best long-term solution to reduce the amount of released CO2 is through its adsorption and sedimentation. As the adsorption stage in carbon capture and storage (CCS) technology is the most expensive phase (70-90 percent of the total costs), conducting research into solid adsorbents and increasing their CO2 adsorption capacity seems reasonable. As a result, adsorbents made of natural and eco-friendly materials, which are economical and do not necessitate the use of complicated synthesis processes are of considerable importance. In order to fulfill such a goal, this study, for the first time, examined the CO2 adsorption capacity of raw (natural) pumice as a green adsorbent. A considerable body of previous research has focused different applications of pumice since 1995. The majority of the studies were related to the removal of pollutants in water and wastewater treatment. After an exhaustive review of the literature, it seems that the available body of research is void of any findings regarding the use of pumice modified with Tetraethylenepentamine (TEPA) as a CO2 adsorbent. Having large contact surface, high porosity (90% on average), and –OH group, this igneous rock seems a suitable choice for the adsorption process. The performance of the adsorbent could be improved if functional groups with high affinity to adsorb CO2 is added to it. Highly porous solids and amine groups can make a very suitable compound to achieve high adsorption rates. According to the recent studies on the selective adsorption of CO2 by amine compounds, TEPA enjoyed the highest adsorption, and therefore was selected in this study as the added substance to pumice.
Materials and Methods
In this study, a new method was used to modify the pumice taken from Maragheh mine. In this method, 0.01 moles of 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane (2.88 grams) to increase adhesion, and 0.01 moles of tetraethylenepentamine (1.89 grams) were mixed in a 50cc beaker containing 10 milliliters of isopropylamine with oxirene ring. The product was used as the modifying agent and was added to powdered pumice (pumicite) at the mass percentage of 6%.
This involved adding 10 milliliters of the solution of water: ethanol (1:10 volume fraction) to 10 grams of the powder and the modifying agent (6%) was added to the beaker while being stirred. The content of the beaker was mixed with 0.01% ammonia solution for 1 hour at 60 degrees Celsius. The sediment was poured on filter paper, rinsed three times with 60% ethanol, and left in the oven for four hours at 60 degrees Celsius to completely dry.
First, the CO2 adsorption capacity of raw pumice and then that of pumice modified with 6% TEPA were measured using the BELSORP-max instrument. Then, the Ideal Adsorption Solution Theory equations were calculated. The analytical equation of spreading pressure is presented based on Toth isotherm:


The selectivity of CO2 to N2 is calculated using the following formula:

The adsorbent performance indicator (API) is calculated using the material balance equation for the three parameters of adsorption capacity, selectivity, and adsorption enthalpy.

According to the following equations, the shares of physical and chemical adsorption on the total amount of adsorption (of the adsorbate on the selected adsorbent) can be calculated.

Results
The results of the XRF test revealed SiO2 and Al2O3 to be the main constituents of pumice. In the XRD results of pumice (from Maragheh) crystal phase was seen when . According to the FT-IR results, in this sample features of SiO4 group was observed at 1033 cm-1, 1037 cm-1, 1048 cm-1, 461 cm-1, and 780 cm-1 wavelengths. The morphology of the sample pumice examined using scanning electron microscope (SEM) demonstrated that in the sample, the amorphous structure of lamella is split into uneven phases and bonds which shows evenly spread pores are extruded in nature. According to the results, the CO2 adsorption capacity of pumice from Maragheh was around 0.230 mmol/g. This figure for the modified pumice was around 0.510 mmol/g, which is twice as much as that of raw (natural) pumice. Increasing the temperature affected the CO2 adsorption capacity negatively and at 298K, 328K, and 348K, the adsorption capacity was calculated to be around 0.510 mmol/g, 0.402 mmol/g, and 0.357 mmol/g, respectively. The values of reduced spreading pressure were measured as molar fractions of the adsorbed CO2 on 6% TEPA modified pumice at 298K and different CO2 concentrations of 5, 15, 25, and 35 percent by volume, and were 0.2, 0.4, 0.5, and 0.6, respectively. Consequently, the adsorbent’s selectivity of CO2 molecules compared to N2 is possible to estimate. The results reflecting the CO2 working capacity after the alteration of the concentration of CO2 revealed that the higher the concentration of CO2 is, the better the modified pumice adsorbent performs. The selectivity of CO2 on modified pumice showed that if the CO2 concentration (partial pressure) rises, the rate of adsorption decreases .This point is justified because molecules of CO2 have high affinity for the sites with more adsorption energy in comparison with N2 molecules. Moreover, when the pressure increases and high-energy sites get full, CO2 and N2 molecules compete to sit on the sites with lower energy (which are of less value in terms of selectivity).When the volume percentages of CO2 were 35 and 25 (which is the common case in cement industry), the rates of selectivity were 2.79 and 3, respectively. When the concentration of CO2 was 15 percent by volume (the common case at coal power plants), the amount of selectivity was equal to 3.76. This amount with CO2 at 5 percent by volume (common in combined cycle and gas turbine power plants) was 4.75.
Discussion and Conclusion
In this study, the experimental results of CO2 adsorption capacity of raw pumice and amine-modified pumice were compared. The natural (raw) pumice demonstrated the rate of CO2 adsorption of 0.230 mmol/g. There was a considerable increase in the amount of CO2 adsorption capacity when pumice was modified using 6% TEPA content (0.510 mmol/g), which showed the adsorbents better performance next to the amine compound. This point has already been proved in several other studies on adsorbents. Upon alterations of the temperature, the adsorption capacity at 298K, 328K, and 348K was higher than that of raw pumice at 298K. Additionally, the highest rate of CO2 adsorption in the modified sample was observed at 298K, which signals that a lower temperature is more favorable for 6% TEPA-modified pumice. The investigation of the effect of concentration of CO2 on the adsorption capacity and API of modified pumice in process units revealed that the lower the concentration of CO2, the better the performance of the adsorbent. In addition, the thermodynamical parameters proved that the process of CO2 adsorption on modified pumice was of the physical adsorption kind and was both exothermic and spontaneous. Despite the lower capacity of CO2 adsorption for pumice in comparison with other synthesized adsorbents, the low cost of production of pumice when compared to other adsorbents, along with its accessibility due to the large number of mines in the country, makes its commercial use justified.