Determination of adsorption capacities of N 2 and CO 2 on commercial activated carbon and adsorption isotherm models

. In today's technologies of gas purification systems, adsorption processes offer more advantages than traditional processes (amine absorption and cryogenic distillation). Thanks to advantages such as high efficiency, low energy consumption and ease of operation, the adsorption process plays an important role in today's natural gas purification and carbon capturing processes. In order to bring natural gas to the usage standards and to ensure carbon capture in the emission sources (coal mines, landfills, agricultural activities, etc.) that emit CO 2 -CH 4 as a result of human activities, it is extremely important to purify impurities such as CO 2 and N 2 , which are highly present in the gas mixture. In the study, the adsorption of N 2 and CO 2 gases on activated carbon and the effect of pressure and temperature on adsorption were examined. The operating conditions pressure range was 1-6 bar and temperatures below room temperature. Experimental studies were carried out in laboratory scale adsorption cell system. As a result of the studies, it was determined that the adsorption capacity of activated carbon N 2 and CO 2 increased with pressure. N 2 adsorption capacities were determined between 0.4-7.8 mmol/g and CO 2 adsorption capacities were determined in the range of 2.7-7.4 mmol/g. In addition, Langmuir and Freundlich isotherm models were created, model parameters were examined and the adsorption behaviour of activated carbon for CO 2 and N 2 gases was obtained.


Introduction
The increase in energy demand and the depletion of fossil fuel reserves faster than predicted are the biggest problem of today's energy economy.In addition to this economic-oriented problem, the biggest environmental problem of conventional power generation systems is greenhouse gas emissions.Flue gas emissions of conventional systems are the main component of global CO2 emissions, as of February 2023, according to data from the Mauna Loa observatory, its concentration in the atmosphere is 420.57ppm [1].However, CH4 gas which ranks second among antropogenic greenhouse gas emissions, is 28 times more effective than CO2, and its concentration in the atmosphere exceeded 1893 ppb in January 2020 [2,3].The sources of CH4 emissions as a result of human activities are coal mines, landfills, livestock fattening activities and agriculture.However, CH4 gas is an important source of fossil fuels.The fact that it emits less CO2 per unit of energy when burned compared to coal and petroleum products has led to its being considered a cleaner energy source and sources containing CH4 for its production to be at the forefront of national energy strategies [4].For this reason, the purification of the streams in coal mines, which have CH4 emission, and the production of natural gas suitable for use is an important subject of today's researches.
The methane in the gas stream in coal mines is called coal bed methane.It is a type of unconventional natural gas, and the content of the gas stream is highly dependent on the geological characteristics and location of the coal bed.Coal bed methane, while being complex and heterogeneous, basically contains CH4 and largely CO2 and N2.Depending on the location of the coal mine, H2O and various hydrocarbon compounds may also present [5,6].Due to the high demand for natural gas, coal bed methane containing CH4 must be retained and purified by appropriate processes.In order to CH4 to be supplied to pipelines, the gases CO2 and N2 in its content must be removed.If the sales gas limit values are exceeded, the natural gas stream is purified by a process called sweetening [7].The content of CO2 and N2 reduces the calorific value of the gas, and CO2 is an acidic gas and also causes corrosion in pipelines.Therefore, impurities in the gas stream are undesirable.
Natural gas purification technologies include amine absorption, cryogenic distillation, membrane systems and adsorption processes.In today's technologies, CO2 removal with amine absorption is carried out in large-scale facilities with high energy demand in amine/water and regeneration.Likewise, under cryogenic operating conditions (80bar, -170℃) can be separated from natural gas by N2 with a very high energy requirement.In membrane systems and natural gas purification, the fact that membrane production is very costly as a result of system-specific material needs and gas molecule-specific optimization conditions is a big problem of these systems.In the adsorption process, pressure swing adsorption (PSApressure swing adsorption) systems are the preferred in laboratory and pilot scale systems for CO2 capture in the flue gas outlets of steam reformer output gas, natural gas and traditional fossil fuel power generation systems.It has advantages over conventional systems (absorption and cryogenic distillation) such as low energy consumption, high efficiency, being environmentally friendly and simplicity [7,8].
The functioning of PSA systems consists of two basic stages.The first stage is the adsorption of the adsorbate gas on the solid adsorbent until it becomes saturated.The second stage is the desorption/release of the adsorbted component and its preparation for the next cycle.The heart of the PSA system is the selection of an adsorbent with high selectivity to the adsorbate gas, as it is the most important determinant of PSA efficiency.This is because the achievement of a high separation/purification efficiency depends on the affinity of the adsorbate gas depending on its molecular properties and the electrical properties between gas and the adsorbent [9].There are lots of adsorbents that are manufactured with some commercial names in zeolite groups such as 13X zeolite, 4A, 5A, Silicalite, Hβ so on.Also, carbon molecular sieves group has CMS-3K as a commercial adsorbent.In the literature, metal organic frameworks (MOFs) are presented Cu-MOF, MIL-101, DD3R, ETS-4 MOF-508b as an adsorbent for purification studies.Especially activated carbons are the most used adsorbents in industry and literature, and also this group has many commercial varieties like Xtrusorb A754, Norit-RB2, Norit-RB3, BPL, and F-400.Although many natural and synthetic adsorbent classes are included in the literature in the purification of natural gas, activated carbons, carbon molecular sieves, zeolites and metal organic frameworks (MOF) are the most preferred material groups due to their low energy requirements in regeneration.The fact that activated carbons have a high surface area, well-developed pore structure, high adsorption capacity at high pressure values, strong-durable structure (skeleton) and also that they can be easily obtained from cheap agricultural wastes allow them to be used effectively as adsorbents.In addition, the fact that the functional groups or organic bonds they have can be changed by pretreatment for further expands their use, contributing to the improvement of surface/electrical properties [10,11,12].
The mechanisms of adsorption are basically a process by which the adsorbate gas attaches physically to a solid-porous adsorbent with weak Van der Waals forces, and chemically thanks to chemical bonds.For adsorption applications, adsorption isotherm is an important thermodynamic equilibrium relationship that indicates the amount of gas adsorbed on an adsorbent as a function of pressure at a constant temperature.Determining how adsorption mechanisms occur is of great importance for designing adsorption systems.Many mathematical adsorption models are used to determine adsorption mechanisms and equilibrium capacities.Modeling of adsorption data with isoterm models is the most appropriate and widely used method.Furthermore, adsorption isotherm models are also helpful in acquiring knowledge about adsorption capacity, which is important in evaluating the performance of adsorbents.The most commonly used isotherm models are Langmuir, Freundlich, Sips, Toth, Temkin, and BET (Brunauer, Emmett, Teller) [13].
This work presents experimental results for adsorption capacities of N2 and CO2 on a commercial activated carbon by laboratory scale adsorption cell system.The obtained results are compared with literature.In the parametric study, the effect of pressure and temperature on capacity was examined experimentally in the adsorption cell system.Using the data obtained after experimental studies, Langmuir and Freundlich isotherm models and adsorption behavior were investigated and model parameters were determined and their physical meanings were explained.

Material and Method
In the experimental study, the effect of pressure and temperature on the equilibrium adsorption capacity of pure CO2 and N2 gases on commercial activated carbon was examined.Granular activated carbon in the particle size range of 0.6 mm-2.38 mm taken from DamlaNem Kimya Ltd. (Sincan, Ankara) was used as adsorbent.High purity N2 (99.99%) and CO2 (99.99%) gases taken from Samtaş A.Ş. were used as adsorbate gases.Pore size distribution and surface area of activated carbon were determined by characterization studies.In the experiments, equilibrium adsorption capacities were calculated by volumetric method.The experiments were carried out in the adsorption cell system.The pore properties of commercial activated carbon are presented in detail in Section 2.1, and the volumetric method and the adsorption cell system are presented in detail in Section 2.2.

Properties of activated carbon as an adsorbent
Granular commercial activated carbon with a particle size range of 0.6-2.38 mm was used in the experiments.BET analysis was performed on Quantochrome brand Autosorb-1 model N2 adsorptiondesorption device in order to determine particle size distribution, pore properties and surface area.N2 adsorption/desorption isotherm is presented in Figure 1.The N2 adsorption/desorption behavior of commercial activated carbon was found to be consistent with type IV adsorption isotherm.The hysterisis behavior seen at a relative pressure of about 0.4 is type H3.H3 type isotherms are generally observed in materials with narrow space and irregular pore 2 E3S Web of Conferences 433, 01004 (2023) https://doi.org/10.1051/e3sconf/202343301004REEE 2023 structure [14].The Type IV behavior conforms to the adsorption isotherms, where the adsorption heat of the first layer is higher than the condensation temperature and where the capillary greatest.The occurrence of hysterisis is an indication that the isotherms of adsorption and desorption follow different pathways.This is explained by the emptying of narrow pores from wide pore openings at the surface.Generally, adsorption isotherms in micro-and mesoporouscontaining solids conform to type IV [15].
The average pore diameter of BJH (Barret-Joiner-Halenda) and activated carbon is 10.1 nm and 3.7 nm for mesopores, while it was also determined that there are micropores of 1.3 nm size in the structure.The average pore size distribution results are given in Figure 2. The BET-specific surface area value is 868.8 m 2 /g.The pore diameter was 1.194 nm and the pore volume was 0.4658 cm 3 /g.

Volumetric method and adsorption cell system
In this study, the adsorption capacities of N2 and CO2 on a commercial activated carbon and the effect of pressure on the capacities were investigated by laboratory scale adsorption cell system.Adsorption tests were performed at pressures ranging from 1 bar to 6 bar.In addition, operating temperatures were kept lower than room temperature with a range of 12°C-18°C.
Figure 3 presents a schematic representation of the adsorption cell system.The system has an adsorbent cell made of stainless steel.At the top of the cell, there is a gas inlet-outlet line and two valves for the opening and closing process.At the top of the gas outlet line, a Keller model Leo 2 model digital manometer (0.1 precision) is placed to read the internal pressure of the cell.On the left side of the adsorption cell there are cylinders for adsorbate gases.One GAST DOA-P504-BN model vacuum pump is connected to the output current line of the cell.
Volumetric method was used to determine adsorption capacities.In the volumetric method, the amount of gas remaining in the system at the end of the experiment is calculated by filling the adsorbate gas into the system and observing the pressure drop and subtracting the amount of gas remaining in the system at the end of the experiment.For this, it is necessary to first calculate how much volume the adsorbent will occupy inside the cell and the volume in which the gas can be found.All equations used in the volumetric method are from Eq.1 to Eq. 8.
The adsorption cell length is 14 cm and the diameter is 6.53 cm.The total volume considering the inlet and outlet pipes of the system (Eq. 1) was calculated as 549.3 cm 3 when the volume of cells and pipes was considered.Considering that the required amount of adsorbents filled into the cell, the volume of adsorbents in the cell was calculated with Eq. 2 using particle properties.When the cell is filled with an adsorbent, the volume in which the adsorbate gas can be found is called Vgas.Accordingly, the volume of gas was Since experiments were carried out at values higher than atmospheric pressure in determining adsorption capacities, the behavior of gases must be taken into account as the actual gas behavior.For this, the compressibility factor of the gases (Eq.4) was determined according to the operating temperature and pressure.At the beginning of the experiments the moles of the gas in the cell was calculated using Eq. 5.After the system comes to equilibrium the mole of the gas in the cell was calculated with Eq. 6.The difference between the total amount of adsorbed gas hence was calculated with Eq. 7. Equilibrium adsorption value was defined in Eq. 8.
If the gas pressure in the first state fed to the cell was P1 and , the moles of the gas in the 1 st state was calculated with Eq. 5.The pressure value after the system comes to equilibrium is called P2 and the mole of the gas in the 2 nd state was calculated with Eq. 6.It is the difference between the total amount of adsorbed gas and the mole amounts in the system in the 1 st and 2 nd states, it was calculated with Eq.7.Equilibrium adsorption value is defined as the amount of gas that 1gram adsorbent substance can adsorb.It was calculated with Eq. 8.
Z= Z (Pr, Tr) = (PV)/(nRT) (4) nadsorbed = n1-n2 (7) nadsorbed,eq = nadsorbed gas/ gram adsorbent (8) The adsorbent (activated carbon) whose capacity was going to be determined was dried in an oven at 95 °C for 2.5 hours beforehand.It was kept in a desiccator for testing.In the beginning of the experiment, the adsorbent was quickly filled into the cell so that its contact with air was kept minimal.After making sure that the adsorbent bed was equal everywhere in the cell, the cover of the cell was closed.Then, it was vacuumed with the help of the vacuum pump connected to the output stream and the pressure in the system was provided to be -0.7 bar.After the vacuum, the gas was filled to the cell and pressure drop in the system was observed with time.When the system pressure had reached equilibrium, the experiment was ended.

Results and Discussion
As a result of the experimental study, the effect of pressure and temperature on the adsorption of CO2 and N2 on commercial activated carbon was compared with the literature studies and presented in Sections 3.1 and 3.2.With the experimental data obtained, Langmuir and Freundlich isotherm models were obtained and the model parameters were determined.The theoretical assumptions of the models and the physical outputs of the model parameters for the adsorption process are described in Section 3.3.

N2 adsorption results
In N2 adsorption experiments, experiments were carried out at 1 bar working pressure and at different temperatures.The N2 adsorption capacity of activated carbon at 18°C was determined as 0.31 mmol N2/g AC.In adsorption studies carried out under room temperature conditions, the N2 adsorption capacity of various activated carbons was reported as approximately 0.3 mmol N2/g AC [2,[16][17][18].The result obtained at 18 ℃ was found to be in good agreement with the literature.At a constant adsorption pressure of 1 bar, the N2 adsorption capacity increased as the temperature was decreased.The N2 capacity at 15°C was calculated as 0.31 mmol N2/g AC, at 14°C the N2 capacity was 0.32 mmol N2/g AC, and at 13°C the N2 capacity was calculated as 0.42 mmol N2/g AC.In the study of Qu et al., the adsorption capacity for 1 bar-10 ℃ is 0.65 mmol N2/g AC [19].Considering the low temperature effect in the study, it is expected that the capacity is larger than the capacity at 13℃, 14℃ and 15℃, but it also supports that the adsorption takes place physically [20][21][22][23][24].In Figure 4, the relationship between the N2 adsorption capacities of activated carbon and temperature is given.In the adsorption experiments carried out at 2 bar and 14 ℃, the N2 adsorption capacity of activated carbon was determined as 0.7 mmol N2/g AC.In the study of Qian et al., the N2 adsorption capacity at 2 bar pressure and 25°C is 0.6 mmol N2/g AC, and at 5°C the adsorption capacity is 0.81 mmol N2/g AC [17].Likewise, in the study of Wu et al., the N2 adsorption capacity on activated carbon was stated as 0.6 mmol N2 /g AC for 2 bar-20°C [21].Accordingly, the adsorption capacity at 14°C was higher (>0.6 mmol N2/g AC) than the adsorption capacity at 25°C, but also lower as expected at a low temperature of 5°C.Low temperature values affect the capacity positively because the physical adsorption is an exothermic process [20,21].
In the adsorption experiments carried out at 3 bar and 14 ℃, the N2 adsorption capacity of activated carbon was determined as 1.1 mmol N2/g AC.In the adsorption experiments carried out at 4 bar and 14 ℃, the N2 adsorption capacity of activated carbon was determined as 3.4 mmol N2/g AC.In the studies of Wu et al. ( 2015) and Qian et al. ( 2021), the N2 adsorption capacity was stated as 0.7 and 0.8 mmol N2/g AC, respectively, in experiments at 3 bar-20°C [21].With the effect of low temperature, its capacity at 14℃ is higher than 0.8 mmol N2/g AC.In the study of Qu et al. (2020), the N2 capacity is 1.1 mmol N2/g AC at 3 bar-10°C and 0.85 mmol N2/g AC at 25°C [19].Accordingly, the capacity of 1.1 mmol N2/g AC and 3.4 mmol N2/g AC was found to be compatible with the literature, considering that for 14℃ it is close to 10℃ and yet the pressure is higher.
In the adsorption experiments carried out at 4 bar and 5 bar (14 ℃), the N2 adsorption capacities of activated carbon were determined as 3.4 mmol N2/g AC and 4.9 mmol N2/g AC, respectively.In the adsorption experiments carried out at 6 bar and 14 ℃, the N2 adsorption capacity of activated carbon was determined as 7.8 mmol N2/g AC.In the study of Park et al., the adsorption capacity was specified as 1.3 and 1.5 bar, respectively, at 20℃ -5 and 6 bar.Considering that the temperature is quite low in the experimental study, it is expected that the capacity value will be higher [25].As the working pressure increases, the adsorption capacity of N2 on activated carbon increases (Figure 5).The relationship of adsorption capacity with pressure is in good agreement with the literature [2,16,[26][27][28].The results of the N2 adsorption capacities on activated carbon are presented in Table 1.

CO2 adsorption results
In the adsorption experiments carried out at 1 bar and 14 ℃, the CO2 adsorption capacity of activated carbon was determined as 2.7 mmol CO2/g.In Fan (2016) and Wu (2015) studies, the CO2 capacity of Norit is 2.27 mmol CO2/g Norit (1 bar-@25℃) and the CO2 capacity of activated carbon is 2.5 mmol CO2/g Norit (1 bar-@20℃), respectively.[21,29].Likewise, when the literature studies and 1 bar test results are compared, the CO2 adsorption capacity of activated carbon is above 2 mmol CO2/g AC as expected.
In Figure 6, the relationship between the CO2 adsorption capacities of activated carbon and temperature is given.When the capacity value of the adsorption carried out at 14℃ is compared with the findings in the literature, it is seen that the capacity is higher at temperatures lower than room temperature compared to the values reported at higher temperatures.In studies on activated carbons in the literature, it has been observed that the CO2 capacity decreases at high temperatures.This is due to the exothermic nature of the adsorption process.In the exothermic process, the molecular diffusion rate and surface adsorption energy increase with increasing temperature, leading to the possibility of chemical adsorption rather than physical adsorption [30][31][32].In the adsorption experiments carried out at 2 bar and 14 ℃, the CO2 adsorption capacity of activated carbon was determined as 3.1 mmol CO2/g AC.In the experiments carried out at 3 bar in Fan (2016) and Wu (2015) studies, the CO2 adsorption capacity was stated as 3 mmol CO2/g AC [21,29].In the adsorption experiments carried out at 3 bar and 14 ℃, the CO2 adsorption capacity of activated carbon was determined as 3.6 mmol CO2/g AC.In the literature, the CO2 adsorption capacity at room temperature and 3 bar conditions is reported to be about 4 mmol CO2/g AC [21,29].Experiment results were expected to be close to 4 mmol CO2/g AC, yet a maximum of 3.6 mmol CO2/g AC CO2 was adsorbed.The slight difference between the experimental results and the literature may be due to the differences in the organic bonds and functional groups that the activated carbon samples naturally contain.
In the adsorption experiments carried out at 4 bar and 14 ℃, the CO2 adsorption capacity of activated carbon was determined as 4.6 mmol CO2/g AC.In the literature, the CO2 adsorption capacity at room temperature and 4 bar conditions is approximately 4-5 mmol CO2/g AC [21,29].In the adsorption experiments carried out at 5 bar and 14 ℃, the CO2 adsorption capacity of activated carbon was determined as 6.2 mmol CO2/g AC.In the literature, the CO2 capacity of activated carbon at room temperature and 5 bar conditions has been reported as 5.2 mmol CO2/g AC [25].In the adsorption experiments carried out at 6 bar and 14 ℃, the CO2 adsorption capacity of activated carbon was determined as 7.4 mmol CO2/g AC.The results obtained at 5 bar and 6 bar are higher than the reported values in literature due to temperature difference (this study:14 • C, literature value: room temperature).As the working pressure increases, the CO2 adsorption capacity on the activated carbon increases (Figure 7).The results of CO2 adsorption capacities on activated carbon are presented in Table 2. Basically, the important point in the separation of N2-CO2 containing streams by the adsorption process is the surface properties of the selected adsorbent.Different dipole-quadrupole forces and molecular polarization energies of gas molecules affect the adsorption process.The fact that the adsorbent surface is more heterogeneous for CO2 adsorption and the presence of various ions and functional groups in the adsorbent structure increases the interaction of the molecule with the surface [33,34].Besides, the polarizability of the CO2 molecule is higher than that of the N2 gas (Table 3).This is one of the important features that increase the interaction of the CO2 molecule with the surface.Therefore, increased surface interaction due to surface heterogeneity and high polarizability allows the CO2 adsorbent capacity to increase [35][36][37].The molecular properties of CO2 and N2 are given in Table 3.The kinetic diameter of gas molecules is also important for separation with adsorption.Kinetic diameters of the molecules are CO2:0.33nm; N2:0.364nm;CH4:0.38 nm; H2O:0.265nm;O2: 0.346 nm; in the presence of SO2 (0.360 nm) in the gas stream, the adsorption precedence was observed as H2O< CO2<O2<SO2<N2<CH4 [38][39][40].The molecular diameter of CO2 is 0.33 nm, which is considerably smaller than the molecular diameter of N2.Considering the very high surface area and pore volume of activated carbons, the probability of small-scale CO2 molecules penetrating into the pores is also high [41].The relatively small molecular diameter of CO2 leads to its high adsorption capacity.

Modeling of Langmuir and Freundlich Isotherms
The Langmuir isotherm model is used to describe equilibrium adsorption conditions, especially gas-solid interaction of adsorbent materials.The Langmuir model is based on some assumptions about the adsorption mechanism.These assumptions are that the entire surface of the adsorbent is homogeneous, all active sites of the adsorbent have equal energy and their distribution on the material is homogeneous, the adsorption energy is 6 E3S Web of Conferences 433, 01004 (2023) https://doi.org/10.1051/e3sconf/202343301004REEE 2023 constant, monolayer adsorption is explained, and there is no interaction between the adsorbate molecules [13,36,42].Langmuir model is expressed in Eq.9.Where Θ is fractional coverage; q is the amount adsorbed; qm is the maximum adsorption capacity; B is called as affinity constant and it describes the ratio of the adsorption rate and desorption rate; P is the pressure [13,43].Langmuir model can be linearized like in Eq. 10.
Θ =q/qm = (BP)/(1+BP) (9) Pe/q = Pe/qm + 1/Bqm (10) The Freundlich isotherm model is widely used in adsorption, presenting the physical and multilayer adsorption process (Eq.11).Also, it is an empirical equation.k and n are Freundlich constant parameters.q is the amount adsorbed and P is the pressure [13].The Freundlich isotherm model can also be linearized.Linear model is presented in Eq. 12. Langmuir and Freundlich model parameters of CO2 and N2 adsorption in experiments with activated carbon are presented in Table 4.By using the experimental data on the adsorption of both gases, their compatibility with the linear model equations was investigated.q = kP 1/n ( 11) Langmuir model affinity constants (B) were calculated as 5.48 bar -1 for CO2 gas and 0.27 bar -1 for N2 gas.The affinity constant is an indicator of how strongly the adsorbate molecules will adhere to the surface.When B>1, it is stated that the adsorption rate is higher than the desorption rate [44].It has been observed that the affinity constant for the CO2 gas to be adsorbed on the activated carbon is higher than the N2 gas.This supports the fact that the CO2 capacity of activated carbon is higher than the capacity even at low pressures.
With the Langmuir isotherm model, the experimental data were compatible with the capacities at 1, 2 and 3 bar pressure values.Non-ideal conditions in isotherms are usually due to the heterogeneity of the adsorbent surface.Such heterogeneities are generally observed when using activated carbon [45].Langmuir model graphs created for CO2 and N2 adsorption are presented in Figure 8 and Figure 9, respectively.At low pressures, the experimental data showed a better fit with the Langmuir model, and deviations in the isotherm occurred at values higher than 3 bar pressure.At low adsorption pressures, the filling in the micropores takes place in a single layer.In high pressure adsorption, the probability of multi-layer filling in the pores instead of a single layer is very high.Another experimental result supporting the possibility of multilayer adsorption on activated carbon was observed for CO2 adsorption in the Freundlich model.In the Freundlich isotherm model, the parameter k is an indicator of whether the adsorption in the process is more physical or chemical.Larger k values indicate greater physisorption compared to chemisorption [45][46][47].In the adsorption of CO2 and N2 gases, the k values were determined as 1.63 and 1.01, respectively.Although it is known that the polarizability and quadripole moment of CO2 molecules are higher compared to N2 (Table 3), the k value of CO2 adsorption was also found to be higher than the k value of N2 adsorption.CO2 molecules are better adsorbed on the activated carbon surface due to the higher electrical field.High k value and high electrical field properties are in good agreement with each other in CO2 adsorption.
The Freundlich isotherm has no Henry's law behavior at low pressures and a finite limit when pressure is high.It can be applied in gas phase systems with heterogeneous surfaces, provided that the pressure range is not too wide.It is generally valid in the narrow range of adsorption data.It has been stated that since it cannot be applied in the entire pressure range, it is 7 E3S Web of Conferences 433, 01004 (2023) https://doi.org/10.1051/e3sconf/202343301004REEE 2023 possible to choose and limit the data [42].Since the behavior at low pressures is not compatible, data at 1 bar pressure were not used in the creation of the Freundlich model.The data on CO2 and N2 adsorption and Freundlich isotherm model compatibility are presented in Figure 10  The Freundlich model showed better agreement with the experimental data at high pressures.Since this model can define multilayer adsorption, the model created for CO2 (Figure 10) has been studied with 5 experimental data.Thus, it is stated that multilayer adsorption can take place on activated carbon compared to monolayer adsorption.This also helps in explaining the way and the order in which the pores are filled in the pore size distribution and adsorption.The filling of pores in activated carbons or activated carbon fibers begins with the filling of micropores.Then, a multilayered film is formed on the mesopore walls, and finally, the empty spaces inside the mesopores are filled by capillary condensation.There are two stages in filling the micropores.In the first stage, the sizes are similar to the diameters of adsorbent molecules and are the dominant mechanism for smaller micropores.This process is due to the overlap of the plane surface energy corresponding to the gas-solid interaction energy and the potential fields originating from the neighbour walls of the pore.In the second step, the adsorbed gas molecules prefer to combine with other previously adsorbed molecules rather than completing a monolayer through interaction with the pore walls [48].

Conclusion
The aim of the study was to find a high capacity and cheaper adsorbent alternative to PSA systems, which are suitable technologies for the capture and purification of coal bed methane stream.In the study, the equilibrium adsorption capacities of CO2 and N2 gases of a commercial activated carbon were investigated in a laboratory scale cell adsorption system.It was seen that as the adsorption pressure was increased, the equilibrium adsorption capacity increased.In experiments carried out at different temperatures, it was observed that low adsorption temperatures positively affected the adsorption capacity.This showed that CO2 and N2 gases were adsorbed on activated carbon by physical adsorption.Besides, it was stated that single-layer adsorption in the low pressure region could be defined with the Langmuir model and multi-layer adsorption could be defined with the help of the Freundlich model as the pressure increases.The capture and purification of coal bed methane, which is an important source of CO2 and CH4 emissions, offers an alternative approach to today's energy and greenhouse gas problems.The design of pressure swing adsorption systems (PSA) for the capture and purification of coal bed methane and the development of suitable adsorbents for these systems are extremely important.This concept candidates to ensure national energy security.

Fig 2 .
Fig 2. Average pore size distribution of commercial activated carbon

3 E3S
Web of Conferences 433, 01004 (2023) https://doi.org/10.1051/e3sconf/202343301004REEE 2023found by subtracting from the total volume the volume occupied by the adsorbent.If the amount of the adsorbent to be used, particle density and void fraction are known, the volume in the cell can be calculated for each adsorbent.

Fig 4 .
Fig 4. Change of the adsorption capacity of activated carbon N2 with temperature (@ 1bar)

Fig 8 .Fig 9 .
Fig 8. Fit of the Langmuir model and experimental data for CO2 adsorption

and Figure 11 .Fig 10 .Fig 11 .
Fig 10.Fit of Freundlich model and experimental data for CO2 adsorption