Assessment of zeolite 13X and Lewatit® VP OC 1065 for application in a continuous temperature swing adsorption process for biogas upgrading
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Two commercially available CO2-adsorbent materials (i.e., zeolite 13X (13X) and Lewatit® VP OC 1065 (Lewatit)) were evaluated for their applicability in a continuous temperature swing adsorption (TSA) process for biogas upgrading. The equilibrium adsorption characteristics of carbon dioxide and methane were determined by fixed bed and TGA tests. While relatively high CO2 capacities were measured for both materials (3.6 and 2.5 mol kg−1), neither of them was found to adsorb significant amounts of CH4. Lewatit showed to be fully regenerable at 95 °C, whereas for 13X, the regeneration was not complete at this temperature. However, 13X showed no degradation up to 190 °C, whereas Lewatit started to degrade at 110 and 90 °C when exposed to N2 and air, respectively. Fluidization tests showed that Lewatit provides a high mechanical stability, while on the contrary, the tested 13X showed considerable attrition. An equilibrium adsorption model was fitted to the measured CO2 adsorption data. The adsorption model was then integrated into an existing simulation tool for the proposed TSA process to roughly estimate the expectable regeneration energy demand for both materials. It was found that depending on the operating conditions, the regeneration energy demand lies between 0.32–0.54 kWhth/m3prodgas for 13X and 0.71–1.10 kWhth/m3prodgas for Lewatit. Since heat integration measures were not considered in the simulations, it was concluded that the proposed TSA process has a great potential to reduce the overall energy demand for biogas upgrading and that both tested adsorbent materials may be suitable for application in the proposed TSA process.
KeywordsBiogas upgrading CO2 capture TSA Solid sorbents
Biogas is a promising renewable energy source that is considered as carbon neutral since the contained carbon comes from organic matter . It can either be used for power and heat production or preferably sent to an upgrading process to yield biomethane. Biogas pre-dominantly consists of CH4 (40–75%vol) and CO2 (16–60%vol), but depending on the utilized substrate and biological conversion process, traces of different other species can also be present (e.g., H2S, NH3, O2, N2, CO, HC’s or siloxanes) . Thus, the transformation of biogas to biomethane typically requires various gas-cleaning steps for removal of trace components as well as an upgrading step, in which CO2 is removed to yield a biomethane stream with high calorific value. The required biomethane quality (e.g., for injection into the natural gas grid) is defined by national regulations and thus differs between the countries [1, 3]. However, depending on the country and application, biomethane contains 95–97% CH4 and 1–3% CO2 . Due to the significant amount of CO2 that needs to be removed from the raw biogas stream prior to grid injection, the economics of biogas upgrading are typically governed by the CO2 separation step.
Today, there are various CO2 separation processes deployed for biogas upgrading, with the main being amine scrubbing, pressure swing adsorption, physical absorption (water scrubbing, organic solvent scrubbing), and membrane separation [4, 5, 6]. While the individual processes obviously show some differences from a technological point of view, it was pointed out that the corresponding upgrading costs equalize with increasing upgrading capacity [7, 8]. Hence, it can be stated that currently, there is no single optimum biogas upgrading technology available. Instead, the final selection of a specific upgrading technology is mostly governed by local site conditions and by regional quality standards.
However, among the currently deployed technologies, amine scrubbing shows some outstanding advantages. In amine-scrubbing plants, the pre-treated biogas is passed through an absorber column where it is contacted with an aqueous amine solution (typically monoethanolamine (MEA), diethanolamine (DEA), or methyldiethanolamine (DMEA)). In the absorber, the CO2 present in the biogas stream is absorbed by the solvent and reacts chemically with the dissolved amine components. The CO2-rich solvent is continuously sent to the stripper where it is heated up (> 120–150 °C) to desorb the CO2 again and to obtain a regenerated, CO2-lean solvent that can be sent back to the absorber. Since the utilized solvents show a high affinity and selectivity towards CO2, the biogas stream can be treated at low operating pressure. Thus, further compression of the biogas upstream to the absorber is typically not needed which gives a significant benefit, especially if the required pressure level of the application that utilizes the biomethane is low. Furthermore, in amine-scrubbing units, the CO2 separation efficiencies and consequently the methane contents in the obtained biomethane are high while at the same time the methane slip to the stripper off-gas can be kept at a very low level [1, 2]. Consequently, the achievable methane recovery rates are high and the methane emissions of the process are low enough to avoid the need for further downstream gas treatment measures. The major disadvantage of this technology, however, results from the high heat input at relatively high temperature that is required for solvent regeneration in the stripper. Furthermore, due to the contact with the aqueous amine solution, the obtained biomethane stream is saturated with water and needs to be dried before it can be sent to compression. Another typical problem associated with this process is that the degradation products of the utilized solvents could lead to equipment corrosion and to harmful emissions of aerosols and nitrosamines. To improve the amine scrubbing process, current research is focusing on reducing the energy demand in the stripper through the development of new amine solvents and improved heat integration measures in the process. Research is also carried out on ways to utilize excess heat, for the regeneration of the solvent, with new systems proposed for biogas upgrading that can be regenerated at lower temperature compared to the conventionally used amine solvents [9, 10].
More recently, it has been proposed to utilize solid adsorbent materials instead of aqueous amine solutions. Similar to the amine-scrubbing process, the adsorbent materials can be utilized in a continuous temperature swing process for selective CO2 separation. The major difference, however, is that the expected separation energy demand of the adsorption process could be significantly smaller. This is mainly due to the facts that no water evaporation occurs in the stripper column and that, compared to the conventionally used aqueous amine solutions, the adsorbent material also has a lower heat capacity. It has been further noted that in an adsorption process, the corrosion and emission issues are less critical and could be even almost eliminated if the right adsorbent materials are used . Another benefit of the adsorption process is that the utilized adsorbents provide large specific surface areas that can facilitate prompt reactions between the active adsorption sites and the gas-phase CO2. Consequently, the process equipment could be built smaller which could entail further reductions of the CO2 separation costs by reducing the associated investment costs.
The basic set-up of a continuous temperature swing adsorption (TSA) process for biogas upgrading consists of an adsorber, operating at low temperatures, and a desorber, operating at higher temperatures. In the adsorber, the solid sorbent material selectively adsorbs the CO2 present in the pre-treated biogas stream. Since adsorption is an exothermic process, the adsorber needs to be cooled in order to maintain the desired operating temperature of typically 40–70 °C. The CO2-loaded adsorbent is continuously extracted from the adsorber and sent into the desorber. The desorber needs to be heated, in order to provide the sensible heat for the colder adsorbent stream, coming from the adsorber and to drive the endothermic desorption of CO2. A stripping gas may be utilized in the desorber to further promote CO2 desorption by keeping the CO2 partial pressure low. This allows for deep regeneration of the adsorbent material, which is essential to achieve high CO2 separation efficiencies. In post-combustion CO2 capture applications, steam has been proposed as suitable stripping agent, since it can easily be separated from the desired CO2 product by downstream condensation of the desorber off-gas. In biogas upgrading applications, air may be used as stripping agent instead, to save on operational costs for steam production. However, for some adsorbent materials, the presence of oxygen in combination with higher operating temperatures might lead to increased adsorbent degradation [12, 13]. Hence, depending on the utilized adsorbent material, the desorber inventory and operating temperature could be limited in case air is used as stripping agent.
In the past, there have been many solid sorbent materials tested and proposed for CO2 capture, in either post- or pre-combustion capture applications . So far, several solid adsorbent materials have been in use already for pressure swing adsorption processes, where high adsorption capacities under elevated pressure are required [18, 19]. Among the most commonly applied adsorbent types are activated carbons and zeolites [17, 18, 20, 21, 22, 23, 24, 25]. Activated carbons are well-studied adsorbent materials for CO2 capture and are commonly applied because of their low price and easy availability. However, due to their relatively low CO2 capacity at ambient pressure and at temperatures above 40 °C, they may not be suitable for application in the considered TSA process [17, 24, 25, 26]. On the contrary, zeolite-type adsorbents, also known as molecular sieves, typically show a very high CO2 capacity and a high selectivity at the mentioned operating conditions. They are porous crystalline aluminosilicates, where the aluminum atoms introduce negative framework charges, which are compensated with exchangeable cations in the pore space . As the type of the cation influences the strength of the electric field inside the pores, as well as the available pore volume, the cation is the essential factor that determines whether a zeolite is suitable for adsorption of a certain molecule or not . In total, there are more than 230 different types of zeolites known by now, which differ in their SiO2/Al2O3 ratio and the type and amount of applied cations . Li et al.  tested and evaluated six different adsorbents for the separation of CO2 and CH4. Out of these six different zeolites, NaX and CaA were identified as the most promising adsorbents due to their high capacity for CO2 uptake and their good CO2/CH4 selectivity of 76 (NaX) and 74(CaA). In another study conducted by Montanari et al. , the adsorption capacity of CO2 and CH4 on NaX was also tested and they rated the CH4 adsorption as negligible.
More recently, solid amine sorbents have been proposed and developed as new adsorbent class for post-combustion CO2 capture applications [13, 29, 30, 31]. In general, these materials consist of a porous polymeric, organic, or inorganic support, on which functional amine groups are immobilized. The amine can be either physically adsorbed (e.g., by wet impregnation), or covalently bound to the support’s surface . However, for the application in a TSA process, the thermal stability of the material is of special importance. Therefore, the latter adsorbent type with amine groups covalently bond to the support may be preferred, as they showed to be more stable [17, 32]. Similar as within liquid amine systems, bulk gas CO2 can be selectively separated through chemical reaction with the active amine sites. Thus, it can be expected that the achievable methane purities and recovery rates in a TSA biogas upgrading process that utilizes amine-functionalized adsorbent materials are similarly high as for amine-scrubbing processes. Nevertheless, as most of the solid amine sorbents are proposed for post-combustion capture, there is hardly any rating in the literature regarding CH4 adsorption on these materials.
In this work, a zeolite and a solid amine-type sorbent are evaluated for application in the proposed TSA system. The first part of the material evaluation included measurements of CO2 and CH4 adsorption isotherms, for assessment of the adsorption selectivity as well as an evaluation of the mechanical and thermal stability of both adsorbent materials. In the second part of the material evaluation, the experimentally derived adsorption data was used to establish mathematical adsorption models for both materials. The adsorption models were then introduced into an existing process simulation tool that has previously been used for a thermodynamic evaluation of the TSA process for CO2 capture from flue gas streams . Finally, process simulations were performed to assess the expectable regeneration energy demand of the TSA biogas upgrading process for both the tested materials and the different operating conditions.
Material properties of the tested adsorbents
Cross-linked polystyrene functionalized with primary amines
Clay bound NaX
Average particle diameter
Average pore diameter
630–710 kg m−3
641 kg m−3
From the group of solid amine sorbents, the ion-exchange material Lewatit® VP OC 1065 (hereafter called “Lewatit”), from the manufacturer Lanxess, was chosen as second potential adsorbent for the proposed TSA process. The material consists of a polystyrene polymer, which is cross-linked with divinylbenzene and functionalized with primary amine groups, through a phthalimide process . This adsorbent was proposed for example by Veneman et al.  for application in continuous TSA CO2 capture processes. In several studies, this material was considered as thermally and mechanically stable and showed excellent CO2 capture properties [29, 34, 35]. These previous studies assessed the adsorption behavior of CO2 and H2O on Lewatit for the purpose of carbon capture from flue gas. However, so far, Lewatit has not been considered for utilization in biogas upgrading applications, except by Sutanto et al. in their recently published study . Hence, only little information on CO2/CH4 separation properties of Lewatit are available. This fact triggered the adsorption tests performed with Lewatit in this work.
2.2 Experimental set-ups
2.2.1 Fixed bed reactor tests
2.2.2 TGA measurements
For the TGA measurements, a NETZSCH STA 409 PC Luxx® was used. The TGA has two different gas inlets, one for the protective gas stream over the balance and a second one for the reactive gas stream, which both are mixed at the entry to the sample chamber. For the protective gas stream, only inert gas, such as N2, may be used and the volume stream must be greater than the reactive gas stream. The reactive gas stream used in the experiments contained either CO2, CH4, or any mixture thereof, as well as a N2/O2 mixture for the regeneration and thermal stability tests. In the present measurements, a total gas stream of 100 ml min−1 was chosen, whereof a constant protective stream of 55 ml min−1 N2 was used. The flows of the individual gas species were controlled via five MFCs. Prior to each adsorption test, a conditioning step was performed to desorb any pre-adsorbed CO2 and moisture. Therefore, the sample was heated to 95 °C under a N2 stream and kept at these conditions for about 1.5 h. This time span was enough to reach a constant mass signal at the end of the conditioning step. Afterwards, the sample was cooled to the desired adsorption temperature. As soon as the signal was stable, the composition of the reactive gas stream was adjusted and the adsorption test started. For all tests performed with CO2, a mass increase of the adsorbent sample was detectable immediately after CO2 was introduced into the sample chamber. It was assumed that the adsorbent sample reached its equilibrium adsorption capacity for the prevailing test conditions when there was no further mass change detectable. After adsorption equilibrium was reached, either another adsorption step at a higher concentration of the adsorptive or a desorption step for complete regeneration of the adsorbent sample followed. For the CO2 and CH4 isotherms, the reactive gas concentration was increased stepwise, from 2 to 45%. The reactive gas stream consisted of the desired CO2 or CH4 concentration, which was balanced with N2 to a total stream of 45 ml min−1. The measurements with mixed gases of CO2 and CH4 were performed with a CO2 gas stream of 4 ml min−1 and a CH4 stream of 41 ml min−1. To simulate air, an O2 stream of 20 ml min−1 was mixed with a total of 80 ml min−1 N2.
Each measurement was performed twice: once with the sample and a second time without the sample. Afterwards, the gravimetric signal of the empty measurement was subtracted from the measurement with sample, to correct for influences of changing operating temperatures and gas compositions on the buoyancy of the sample probe. The corrected mass change per adsorption step was then related to the dry sample mass (i.e., the sample mass after the conditioning step) to obtain the specific equilibrium loading of the adsorbent material. The used gases, within the TGA measurements, were CO2 grade 4.8, CH4 grade 4.5, N2 grade 5.0, and O2 grade 5.0.
2.2.3 Fluidization tests
The minimum fluidization velocity Umf was determined experimentally in the same test rig, by a stepwise decrease of the superficial gas velocity from above to below fluidization conditions (i.e., from a fluidized bed to a fixed bed) while measuring the resulting pressure drop over the test rig. First, this procedure was conducted without material to determine the pressure drop of the gas distributor that needs to be subtracted from the measured pressure drops obtained with a bed material. The minimum fluidization velocity was then determined by intersecting the obtained linear correlation, between the superficial gas velocity and the bed pressure drop over the fixed bed, with the constant pressure drop obtained for the fluidized bed of adsorbent material.
2.3.1 Adsorption equilibrium modeling
The adsorption models by Langmuir and Toth were used in this work to derive suitable mathematical descriptions of the experimentally determined CO2 adsorption equilibria data of both adsorbent materials . The Langmuir model assumes a monolayer adsorption (Eqs. (1 and 2)) on the adsorbent material. The temperature-dependent Toth model is a semi-empirical model, which also accounts for sub-monolayer coverage (Eqs. (3–7)). In the Langmuir model, the maximum adsorption capacity qmax corresponds to a complete mono-layer coverage of the adsorbent and is thus temperature independent. The parameter ΔHADS reflects the heat of adsorption, which is considered to be equal for each adsorption site and the parameter b, the so-called Langmuir constant, reflects the affinity of the attraction of an adsorbate to the surface.
In the Toth model, the maximum loading n s is temperature dependent and compared to the Langmuir model, an additional parameter t is added. The parameter t is a characterization for the systems heterogeneity. The suffix 0 for the parameters t0 and b0 indicates that they have to be found at a reference temperature T0, whereby in this work, T0 was selected with 343 K. ΔH again represents the heat of adsorption, whereas ΔH0 reflects the heat of adsorption at zero coverage. The parameters α and χ are constant parameters to describe the temperature dependency of the corresponding model parameters t and ns.
2.3.2 TSA process modeling
The gas feeding rates chosen for the simulations were 12 Nm3 h−1 and 7 Nm3 h−1 for the adsorber and desorber, respectively, based on typical operation conditions in a bench-scale unit . The CO2 content of the raw biogas was assumed to be 40% in accordance with the average composition of biogas derived from anaerobic digestion of agricultural waste [1, 2, 5]. Ambient air at 20 °C is used as stripping agent in the desorber column, whereas a blower was incorporated in the model to provide the required pressure elevation of that stream. The blower power is, however, not added to the process regeneration energy demand. The specific thermal regeneration energy requirements of the TSA process that are reported in this work were determined by summation of the heating demands in the individual desorber stages and by relating this heating demand to the product gas stream leaving the adsorber column. As pointed out above, the heating demand in the individual desorber stages arise from the latent heat demand for desorption of CO2 and the sensible heating demand for lifting the temperature of the incoming sorbent and stripping gas streams to the desorber operating temperature.
In the process simulations performed in this work, the adsorber stages were considered to be operating at a constant temperature of 50 °C. The desorber stages were also considered to be operated at the same temperature, but in contrast to the adsorber, the desorber temperature was varied in a temperature range of 80–120 °C to assess the impact of varying temperature swings on the derived regeneration energy demand. Simultaneously, to the variation of the desorber temperature, a variation of the residual CO2 content in the adsorber off-gas (i.e., the biomethane quality) has been performed. Due to different legislations that are in place among European countries, the biomethane standards can differ significantly with respect to the residual CO2 content in the upgraded biomethane stream. For instance, the lowest limit of the residual CO2 content for gas grid injection varies from ≤ 2%vol for gas grids in Austria  to up to 10.3%vol in the Netherlands . In accordance to these values, the residual CO2 content in the off-gas stream of the adsorber column has been varied from 2 to 10%vol.
3 Results and discussion
3.1 Experimental results
3.1.1 CO2 capacity
For the measurements in the TGA, the pre-dried Lewatit, which was prepared for the fixed bed measurements, was used. During the first conditioning step, an additional weight loss of 16.8% was measured, which led to the conclusion that the drying was not complete. Therefore, the pure CO2 adsorption isotherms were repeated in TGA. It is expected that the material dried in the pre-conditioning step in the FB is the same as that detected in the TGA. As the loading is referred to the dry sample mass, which was weighted after the 4 h of drying in the FB, it was decided to correct the data points obtained from the FB measurements. This correction was performed by reducing the dry sample weight, on which the adsorbed CO2 mass was balanced, by 16.8%. In Fig. 7, the corrected FB values are displayed. For future measurements, FB is considered as a reliable source for adsorption measurements, as long as the drying step is extended according to the moisture content of the sample. For 13X, no additional mass loss was measured in the TGA, which matches the indication in the material delivery datasheet, provided by the supplier, that 13X is dry. As the isotherms for 13X fit very well to literature references [20, 38] and no additional drying occurred, it was not repeated in TGA but taken from the FB results.
The received CO2 isotherms for Lewatit match the capacities reported by Yu et al. , who reported 2.15 mol kg−1 (40 °C, pCO2 0,15 bar) and Sutanto et al.  (Toth model by Sutanto et al. displayed in Fig. 7) but are below the capacities reported by Veneman et al. . At 60 °C, the model presented by Sutanto et al. shows an exact overlap with the measured equilibrium loadings. However, the model predicts slightly larger equilibrium loadings at higher and lower equilibrium loadings at lower operating temperatures.
CO2/CH4 mixed gas adsorption compared to pure CO2 adsorption capacity at 40 °C
CO2 loading (wt%)
CO2 loading (wt%)
3.1.3 Thermal degradation
In contrast to the findings for Lewatit, 13X did not show a significant degradation (i.e., mass loss) up to 190 °C, regardless of the gas atmosphere (i.e., N2 or air). This is also in line with the information provided in the material datasheet of 13X. Hence, it was concluded that the desorber operating temperature in the TSA process would not be limited by the thermal stability of 13X.
Next to adsorbent degradation, the desorber operating temperature may also influence the CO2 desorption rates and consequently adsorbent regeneration in general. Preferably, the adsorbent material shows a sufficient regeneration rate at moderate desorber operating temperatures. Therefore, measurements with two adsorption cycles (at 40 °C with a CO2 partial pressure of 0.12 bar), each followed by a desorption step with air, were performed. Based on the results obtained from the thermal degradation tests, the desorption step was performed at 95 °C for 1 h.
3.1.5 Fluidization tests and mechanical stability
3.2 Modeling results
3.2.1 Adsorption equilibrium model
For further evaluation of the proposed TSA process, it is necessary to find a mathematical model that describes the adsorption properties of both materials at all relevant CO2 partial pressures and operating temperatures. Therefore, an adsorption model was selected to mathematically describe the CO2 adsorption behavior of both adsorbent materials. At first, the commonly known adsorption model of Langmuir (see Eqs. (1 and 2) in Sect. 2.3.1) was applied.
Langmuir isotherm parameters
qmax (mol kg−1)
1.17 × 10−9
3.23 × 10−6
ΔHADS (kJ mol−1)
For 13X, the Langmuir model already represented a good fit to the measured isotherms. For description of the CO2 equilibrium adsorption behavior (isotherms) on Lewatit, the Langmuir model was, however, not sufficient to consistently describe the obtained adsorption data (see Fig. 7). Therefore, the adsorption model proposed by Toth (see Eqs. (3–6) in Sect. 2.3.1) was applied instead. This model was already proposed by Veneman et al.  for mathematical description of the CO2 adsorption isotherms for Lewatit.
Toth isotherm parameters for own model fitting, compared to other model fittings for Lewatit by Sutanto et al. and Veneman et al.
3.2.2 TSA process simulations
As mentioned before, air was used as stripping gas for the biogas upgrading simulation, instead of steam, as proposed by Pröll et al.  for post-combustion CO2 capture applications. This helps to reduce the energy demand of the TSA process significantly, in case the separated CO2 is not required at concentrated form for further utilization. Besides that point, it is known from literature that the capacity of 13X is reduced dramatically when water is present in the adsorption process . It is thus questionable if the TSA process could be operated without pre-drying step upstream to the adsorber in case 13X is used as adsorbent material. On the contrary, the CO2 capture performance of Lewatit may even improve if water is present in the system . Hence, the application of an amine functionalized adsorbent material may even allow for a simultaneous removal of various components (i.e., CO2, H2O, H2S ) in a single apparatus. Also not considered in the simplified model used in this work are the methane recovery rates that can be expected from the process. However, from the methane adsorption properties obtained in this work, it can be expected that methane slip should be smaller or even eliminated completely in case Lewatit is used as adsorbent material.
While the intention of this work was to get a rough idea of the regeneration energy demand for further comparison of two different adsorbent materials, all of the abovementioned aspects need to be considered to get a clearer view on the techno-economic figures of the proposed TSA biogas upgrading process.
In this study, Lewatit® VP OC 1065 and Zeolite 13X have been evaluated for an application in a continuous TSA process for biogas upgrading. For this, the CO2 adsorption isotherms, the adsorbent selectivities towards CO2 adsorption, and the thermal and mechanical stability were measured for both materials. In a further step, the measured CO2 adsorption isotherms of both materials were used to derive model parameters of a suitable mathematical adsorption model. The adsorption models were then incorporated into an existing simulation tool for the proposed multistage fluidized bed TSA process for continuous biogas upgrading. Based on the performed material assessment and the performed process simulations, the following conclusions can be drawn.
Both materials show a sufficiently high CO2 adsorption capacity. 13X has a higher capacity for CO2 partial pressures above 0.12 bar, whereas Lewatit shows higher capacities at pressures below 0.12 bar. On the other hand, at low partial pressures, Lewatit shows a high CO2 capacity at practical adsorption temperatures as well as a low CO2 capacity at the desired operating temperatures of the desorber.
Neither Lewatit nor 13X showed a significant CH4 adsorption capacity, which indicates that a highly selective CO2 removal from biogas should be possible with both adsorbent materials. This in turn means that it is possible to achieve high purities and methane recovery rates in a TSA process that is operated with either one of the two adsorbent materials.
13X shows no degradation at higher temperatures and is stable to temperatures of at least 190 °C. Lewatit started to degrade at 110 °C in N2 atmosphere and even at 90 °C in air atmosphere. Consequently, it is concluded that the operating temperature and the solid residence time in the desorber of the TSA reactor need to be limited in order to avoid high make-up streams of Lewatit.
The mechanical stability of Lewatit is given, and no weight loss during fluidization was encountered. The tested 13X is not suitable for an application in a fluidized bed, as it showed high abrasion and breakage when it was fluidized.
In general, both materials showed to be promising adsorbents for utilization in a TSA biogas upgrading process. Nevertheless, the 13X-type adsorbent tested in this work is not recommended for the proposed process, operating in fluidized beds, due to its low mechanical stability, unless this property is enhanced.
The data obtained in the experimental part of this work was used to establish a mathematical adsorption model with the respective fitting parameters. The obtained Toth adsorption models showed excellent alignment with the measured data and were further implemented to an existing process simulation model.
First, results from the process simulation with both materials revealed that 13X requires almost half of the energy for regeneration than Lewatit mainly because of the difference in their heat of adsorption. Nevertheless, for both adsorbent materials, the derived regeneration energy demands were well in the range of those reported for existing technologies if though the simplified process simulation tool used in this work did not account for any heat integration measures.
In a next step, further research will be dedicated to evaluate and quantify the benefits of different heat-integration measures to the process. In addition, co-adsorption of water and other species will be assessed for both materials and the corresponding impact on the process energy demand will be quantified. Furthermore, at least one of the adsorbent materials will be tested in continuous operation using an existing TSA bench-scale unit at TU Wien and raw biogas from a biogas facility in Austria.
Open access funding provided by TU Wien (TUW).
α Toth parameter
b Temperature dependent Langmuir / Toth parameter (bar−1)
b∞ Langmuir parameter at infinite temperature (bar−1)
b 0 Toth parameter at reference temperature (bar−1)
χ Toth parameter
ΔH ADS Heat of adsorption Langmuir (J mol−1)
ΔH 0 Toth heat of adsorption at zero surface coverage (J mol−1)
n s Temperature dependent maximum Loading Toth (mol kg−1)
n s0 Maximum loading Toth at reference temperature (mol kg−1)
p CO2 Partial pressure of CO2 (bar)
q Adsorption uptake (mol kg−1)
R Universal gas constant (J molK−1)
ϴ Fractional loading (−)
T 0 Reference temperature (K)
T Temperature (K)
t Temperature dependent Toth parameter
t 0 Toth parameter at reference temperature T0
U mf Minimum fluidization velocity
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