Cryogenic Adsorption of Methane and Carbon Dioxide on Zeolites 4A

Sep 25, 2014 - Adsorption equilibrium of CH4 and CO2 on commercial zeolite 4A and ... Experiments were performed at different total pressures (1 and 1...
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Cryogenic Adsorption of Methane and Carbon Dioxide on Zeolites 4A and 13X Carlos A. Grande* and Richard Blom SINTEF Materials and Chemistry, Post Office Box 124 Blindern, N0314 Oslo, Norway S Supporting Information *

ABSTRACT: The utilization of adsorption processes operating at low temperatures can be interesting in the context of production of liquefied natural gas (LNG), where they can constitute a lower energy alternative as hybrid technologies with cryogenic distillation. This paper provides the necessary parameters to design an adsorption process for selective removal of CO2 from methane at low temperatures to satisfy LNG specifications, with particular emphasis on a temperature swing adsorption (TSA) process. Adsorption equilibrium of CH4 and CO2 on commercial zeolite 4A and zeolite 13X is reported at cryogenic temperatures: 198, 208, 223, 248, and 279 K. Carbon dioxide is much more adsorbed than methane, and CO2 isotherms are extremely steep at low temperatures. In the studied low-temperature range, it was observed that zeolite 4A has a very different behavior toward CH4 and CO2; adsorption of methane is entirely controlled by diffusion (kinetic control), while adsorption of CO2 is mostly controlled by the shape of the isotherm (equilibrium control). Adsorption breakthrough curves of a mixture of 1.5% CO2 and 98.5% CH4 were measured in the zeolite 4A adsorbent at 204 K to identify transport phenomena at such low temperatures and verify if adsorption equilibrium can be described on the basis of pure component data. Experiments were performed at different total pressures (1 and 10 bar) and different flow rates.

1. INTRODUCTION It is widely known that the anthropogenic emissions of greenhouse gases are causing the so-called “climate change” effects. The utilization of natural gas for production of energy and as fuel is associated with reduced CO2 emissions while being able to provide a secure energy supply. For this reason, natural gas consumption has been increased over the years, now being responsible for almost 24% of the global energy demand and with sustained growth.1 Natural gas is composed mostly by methane and other hydrocarbons but can also contain important amounts of other “undesired” components, such as H2S, CO2, and N2. When natural gas has an important amount of CO2, an alternative that was considered to reduce the environmental footprint of exploration is to remove CO2 and reinject it, with the additional possible benefit of enhanced gas recovery.2 One of the possibilities to remove contaminants from natural gas is by cryogenic distillation.3,4 This is of particular interest when the final product is liquefied natural gas (LNG). In the case of having a natural gas with a high content of CO2, the separation should be carried out using multiple distillation columns, where the content of CO2 is sequentially decreased. In such a case, the bulk removal of CO2 will happen in one column, while one or more other columns will be used to polish the gas to LNG specifications (>50 ppm of CO2). As demonstrated,4 if the initial content of CO2 is 50%, the cryogenic distillation can be composed by three columns operating in series, with the inlet composition of the last column having 0.9% CO2. One possible alternative would be to use an adsorption process to polish this stream to 50 ppm, replacing the last distillation column. However, to design an adsorption process, fundamental adsorption equilibrium and kinetic data are required because © 2014 American Chemical Society

they are not available in the literature. In fact, despite the large volume of work existing for adsorption of methane and carbon dioxide on almost all kinds of porous solids, most of the work was focused on the removal of CO2 for natural gas and biogas upgrading at temperatures higher than 273 K. The group by May in Australia has been focusing on adsorption of a natural gas compound at low temperatures.5−7 Because the content of CO2 to be removed is quite low (in percentage), a temperature swing adsorption (TSA) process can be idealized.8 This process might have an inherent advantage in the cost of regeneration, that is, the utilization of a “hot fluid” that can be at ambient temperature. Because of this technological choice (TSA), our measurements were focused on zeolites. At such a low temperature, a very strong adsorption of CO2 was expected, and utilization of other technologies, such as pressure swing adsorption (PSA), will require extreme vacuum to desorb CO2 and proceed to regeneration. In this paper, we report adsorption equilibrium of pure carbon dioxide and methane in two different commercial zeolites: zeolite 4A (Fluka) and zeolite 13X (Grace). Isotherms were measured at five different temperatures between 198 and 279 K at pressures ranging from 0 to 1 bar. The initial point of adsorption isotherms of zeolite 4A (in the very low pressure range and, thus, Henry zone) was used to estimate the diffusion parameters. Breakthrough curves of a mixture of 1.5% CO2 and 98.5% CH4 were measured in zeolite 4A at a temperature of 204 K at different total pressures (1 and 10 bar). Received: August 13, 2014 Revised: September 23, 2014 Published: September 25, 2014 6688

dx.doi.org/10.1021/ef501814x | Energy Fuels 2014, 28, 6688−6693

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The Virial isotherm is given by8 qeq exp(Aq + Bq2 + Cq3) P= Ki

2. EXPERIMENTAL SECTION Adsorption equilibrium of pure CH4 and CO2 was measured in a Belsorp Max instrument (Japan) on zeolite 4A (Fluka) and zeolite 13X (Grace) extrudates. The measurements were carried out at 198, 208, 223, 248, and 279 K for all adsorbents. CH4 with purity of >99.9995% and CO2 with purity of >99.9992% were employed without further treatment. Degassing of the samples was performed at 593 K under vacuum for 12 h. The transient variation of pressure of the first adsorption equilibrium point was recorded and used to estimate the diffusion parameters. Adsorption equilibrium criteria were that the pressure should have variations lower than 0.3% for a period longer than 9999 s. Breakthrough curves were measured in the breakthrough curve setup shown in Figure 1. Helium (purity higher than 99.999%) was

(1)

where Ki is the adsorption equilibrium constant and A, B, and C are the Virial coefficients. The adsorption constant is calculated by K i = K i∞ exp( −ΔHi /RT )

(2)

The temperature dependency of the Virial coefficients was calculated according to

A = A 0 + A1/T

(3)

B = B0 + B1/T

(4)

C = C0 + C1/T

(5)

The Virial isotherm has an extension to predict multicomponent adsorption equilibrium.9 Ki N

Figure 1. Simplified scheme of the experimental setup used for breakthrough curve measurement. Abbreviations are as follows: MFC, mass flow controller; V1, four-way valve; V2, on−off valve; T, temperature sensor; and BPR, back-pressure regulator.

temperature (K) pressure (bar) flow rate (cm3/min)

run 1 204 1 67

j=1 N

j=1 k=1

N

(6)

The mixed Virial coefficients can be calculated by different mixing rules. In this work, we have used the following: (A i + A j )

Aij =

(7)

2

Bijk = Cijkl =

run 3 204 10 67

N

j=1 k=1 l=1

Table 1. Column Dimensions and Experimental Conditions Used for Breakthrough Experiments 0.0975 0.0091 3.7255 0.015 0.985 run 2 204 10 260

N

exp[∑ Aij qj+ ∑ ∑ Bijk qjqk +

∑ ∑ ∑ Cijklqjqkql]

used as inert gas. Helium and the premixed mixture (1.5% CO2 and 98.5% CH4) were introduced to the system by two different mass flow controllers (Bronkhörst, Netherlands). The temperature control was performed using a cooler filled with ethanol (Julabo, Germany). The adsorption column dimensions are listed in Table 1, together with the

column length (m) column diameter (m) adsorbent weight (g) CO2 molar fraction CH4 molar fraction

N

qi

Pi =

(Bi + Bj + Bk ) (8)

3 (Ci + Cj + Ck + Cl) 4

(9)

When measuring the adsorption equilibrium data, the approach to equilibrium of the first point was used to estimate the diffusion kinetics of pure gases. In such a case, the system behaves as a batch system with a finite volume. We are assuming that the process is isothermal (which cannot be verified because of the lack of temperature measurements) and also that the isotherm is quite linear in the very low pressure range. Under those conditions, it is possible to derive an analytical expression to describe the kinetics of adsorption for long times8,10

run 4 204 1 195

experimental conditions used to measure the breakthrough curves. Within the adsorption column, there is a K thermocouple located in the center of the column at 0.05 m (50 mm) from the feed inlet. The total pressure is controlled by a manual back-pressure regulator (Omega, U.K.), and the exit gases are measured by mass spectroscopy (MS, Thermo, Waltham, MA). Degassing of the adsorbent was carried out by maintaining the sample at 593 K for a period of at least 12 h under a flow of helium at atmospheric pressure.

∞ exp( −pn 2 Kμ , it ) q = 1− 6∑ Λ 2 qeq n = 1 9 1 − Λ + (1 − Λ)pn

(10)

where q and qeq are the amounts adsorbed changing with time and after reaching equilibrium, respectively, and Kμ,i is the diffusion constant, Kμ,i = Dμ/rμ2. The factor describing the variation of the concentration with time (Λ) is given by

3. THEORETICAL SECTION The Virial isotherm was used to fit the adsorption equilibrium data. The main reason for this selection is that the adsorption process used for this application might cover a wide range of temperatures. If other models with variable maximum loading have been used, the theoretical extension to multicomponent systems is not straightforward. In that case, binary adsorption equilibrium might be required to validate the model, making the initial design more complex.

Λ=

C ini − Ceq C ini

(11)

To solve eq 10, the roots of the following equation are required:11 tan(pn ) = 6689

3pn 3+

( Λ1 − 1)pn2

(12)

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4. RESULTS AND DISCUSSION 4.1. Adsorption Equilibrium of Pure Gases. Adsorption equilibrium of pure CH4 and CO2 on zeolite 13X is shown in Figure 2, and adsorption on zeolite 4A is reported in Figure 3.

Figure 2. Adsorption equilibrium isotherms of (a) CH4 and (b) CO2 on zeolite 13X at different temperatures. Solid lines are the fitting of the Virial model.

It has been discussed in the literature that, to obtain an acceptable description of the diffusion in short times, at least 40 roots of eq 12 are required.12 The data regression of adsorption equilibrium and uptake kinetics were carried out in Scilab 5.4 (www.scilab.org) using the Nelder Mead method for minimization of the error function. The sum of square deviations was used as an error function for adsorption equilibrium and the average of residuals for uptake kinetics. Fixed-bed breakthrough curve experiments were simulated by a well-known mathematical model used to describe binary adsorption based on pure component data. The mathematical model assumes that the flow in the fixed bed is plug flow, includes two linear driving force equations for diffusion within the macro- and micropores of the zeolite extrudates, and uses three different energy balances (gas phase, solid phase, and column wall). Furthermore, pressure drop is described by the Ergun equation. For this initial study, the gas phase was assumed to behave as ideal. The mathematical model is provided as Supporting Information of this publication. Details of the mathematical model can be found elsewhere.13−16 The mathematical model was solved using gPROMS 3.7 (PSE Enterprise, U.K.). The centered finite difference method (CFDM) of second order over a uniform grid of 400 intervals was the numerical method used. Such a large amount of intervals was required because of the strong steepness of the CO2 isotherms.

Figure 3. Adsorption equilibrium isotherms of (a) CH4 and (b) CO2 on zeolite 4A at different temperatures. Solid lines are the fitting of the Virial model.

The solid lines in the figures correspond to the fitting of the Virial model. We did attempt to fit the data with the multi-site Langmuir model17 but found that regression reported an unrealistically large number of sites per CO2 (>20). Therefore, we have not included that modeling approach here. The Virial model can fit the data well. The heat of adsorption obtained with the fitting is quite lower when compared to other zeolites, such as 13X and 5A, which is higher than 40 kJ/mol.14,18,19 However, it has been previously reported that the heat of adsorption of CO2 on zeolite 4A is in the same order as the heat of adsorption obtained in this work.19 The heat of adsorption obtained by the Virial model might also be different from other values reported particularly in the conditions used for the measurements reported in this work.20,21 The parameters of the fitting of the Virial model to the experimental data are shown in Table 2. The amount adsorbed of both gases on zeolite 13X is considerably higher than on zeolite 4A. It is also noticeable that the initial slope of the CO2 isotherms is extremely steep, and this would result in a very hard regeneration of the material. In fact, using any of these materials for a PSA application will not be viable because of the enormous amount of power consumed by a vacuum pump that reaches a very low vacuum required for regeneration. 6690

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that the response of the system is not influenced by the temperature, which does not mean that the diffusion coefficient does not change with the temperature. The system used for measurements behaves as a “finite-volume” batch reactor, where the concentration changes with time. In the particular case of adsorption of CO2 at low temperatures, where the amount adsorbed is considerable, the variation of the concentration is very important, making parameter Λ (defined by eq 11) deviate strongly from zero, where infinite volume batch reactors can be assumed. Under such cases, the diffusion coefficient is also related to the degree of concentration variation of the system.

The extreme difficulty of measuring these data has to be mentioned. To obtain full regeneration of the zeolite between measurements, an intermediate heating to around 373 K was required to regenerate the sample (with the consequent problems of removing ice and drying ethanol to avoid safety problems). Moreover, the measurements of adsorption equilibrium of methane were very time-consuming, requiring over 20 000 s to reach one adsorption equilibrium point. 4.2. Uptake Kinetics of CO2 and CH4 in Zeolite 4A. The approach to adsorption equilibrium of the first adsorption equilibrium of CO2 in zeolite 4A at 198 and 248 K is reported in Figure 4. The overlapping of both curves is evident, showing

Figure 5. Breakthrough curve of a mixture of CO2/CH4 in a fixed bed filled with zeolite 4A measured at 204 K and 1 bar. Solid lines correspond to the prediction of the mathematical model. Conditions are reported in Table 1 (a, run 1; b, run 4).

Figure 4. Uptake kinetic experiments of (a) CO2 and (b) CH4 in zeolite 4A at low temperatures measured in a finite-volume batch equipment. Solid lines represent the fitting of the mathematical model.

Table 2. Parameters of the Virial Isotherm Model for the Adsorption of Pure CO2 and CH4 on Zeolites 13X and 4A Adsorption Equilibrium Parameters zeolite 13X parameter

zeolite 4A

CH4

Ao (kg/mol) A1 (kg K mol−1) Bo [(kg/mol)2] B1 [(kg/mol)2 K] Co [(kg/mol)3] C1 [(kg/mol)3 K] −1 K∞ kPa−1) i (mol kg ΔHi (kJ/mol)

CO2

0.164 7.518 20.488 −989.949 0.154 −2.001 −56.025 290.705 −0.008 0.191 10.010 −25.650 4.44 × 10−6 4.66 × 10−3 18.176 26.804 Diffusion Parameter (k, s−1) in Zeolite 4A

CH4

CO2

0.438 −47.076 −0.118 7.839 0.034 16.751 2.97 × 10−6 18.759

5.809 368.478 −0.226 −260.705 −0.032 35.679 4.70 × 10−1 28.000

1.0 × 10−3 1.7 × 10−5

CO2, 198 K CH4, 198 K 6691

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Figure 6. Breakthrough curve of a mixture of CO2/CH4 in a fixed bed filled with zeolite 4A measured at 204 K and 10 bar. Solid lines correspond to the prediction of the mathematical model. Conditions are reported in Table 1 (a, run 2; b, run 3).

4.3. Breakthrough Curves of the CH4/CO2 Mixture in Zeolite 4A. After adsorption equilibrium and kinetics of pure gases were measured, it is necessary to test the validity of the mathematical model that will be used for prediction of the behavior of the binary mixture in a TSA process. For this reason, four breakthrough curves were measured at two different pressures: 1 and 10 bar. The measurement at 1 bar intended to keep the system under isothermal conditions and with the total velocity relatively constant because it was observed that methane adsorption proceeds very slowly. Breakthrough curves measured using different flow rates of a mixture with 1.5% CO2 balanced by CH4 (98.5%) are shown in Figure 5. The recorded temperature showed no variation during the experiment, and thus, the temperature data are not presented here. The solid lines in the figure correspond to the prediction of the mathematical model using only pure component adsorption data reported in Table 2. Breakthrough curves of the same mixture were also performed at 10 bar with two different flow rates. Results are presented in Figure 6. Note that, in this case, the temperature variation of the system was around 1.5 K mostly because of adsorption of carbon dioxide. Similar good-quality prediction of the mathematical model (using parameters measured in this work reported in Table 2) is observed. It can be concluded that the adsorption equilibrium and kinetic data reported here can be used in the mathematical model proposed to describe the behavior of a binary mixture of CO2 and CH4 in zeolite 4A under cryogenic conditions. These data can be used to design an adsorption process under such conditions.

The variation of the concentration at a low temperature (198 K) is much higher than at a higher temperature (248 K). When a high gas concentration is injected to the volume containing the adsorbent, it is difficult to confidently assume that the temperature within the adsorbent particle is constant (isothermal behavior). Unfortunately, in the equipment used, it is not possible to measure locally the temperature of the zeolite extrudate.22 Because the first point of adsorption equilibrium is at a low pressure (less than 1.5 Pa for all temperatures), we have assumed that the temperature variation is not considerable, and thus, the system behaves isothermally. A similar problem of the finite system (to a much lower extent) was observed with methane. Diffusion data of methane at 198 and 223 K are reported in Figure 4. Methane adsorption was also very slow; the reason why it is simpler to believe that the extrudate may be under isothermal conditions during the measurement. For these reasons, the diffusion constant estimated with this method is not very accurate and can be considered as approximate with correctness in the order of magnitude. The estimated diffusion parameters for CO2 and CH4 in zeolite 4A are reported in Table 2. The obtained values are higher than in previously reported work,19 but that might be a consequence of a higher activation temperature used in this study. It should be noticed that these measurements indicate that time to reach equilibrium is very fast for CO2 (5 h) is required to achieve CH4 adsorption equilibrium. For this reason, the high selectivity of the adsorbent toward CO2 is boosted by kinetic effects.23 6692

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(4) Berstad, D.; Nekså, P.; Anantharaman, R. Energy Procedia 2012, 26, 41−48. (5) Watson, G.; May, E. F.; Graham, B. F.; Trebble, M. A.; Trengove, R. D.; Ida Chan, K. J. Chem. Eng. Data 2009, 54, 2701−2707. (6) Jensen, N. K.; Rufford, T. E.; Watson, G.; Zhang, D. K.; Ida Chan, K.; May, E. F. J. Chem. Eng. Data 2012, 57, 106−113. (7) Saleman, T. L. H.; Watson, G. C. Y.; Rufford, T. E.; Hofman, P. S.; Ida Chan, K.; May, E. F. Adsorption 2013, 19, 1165−1180. (8) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; Wiley: New York, 1984. (9) Taqvi, S. M.; LeVan, M. D. Ind. Eng. Chem. Res. 1997, 36, 2197. (10) Kärger, J.; Ruthven, D. M. Diffusion in Zeolites and Other Microporous Solids; Wiley: London, U.K., 1992. (11) Crank, J. The Mathematics of Diffusion; Clarendon Press: Oxford, U.K., 1956. (12) Haynes, P. D.; Lucas, S. K. ANZIAM J. 2007, 48, 503−521. (13) Da Silva, F. A.; Silva, J. A.; Rodrigues, A. E. Adsorption 1999, 5, 229−244. (14) Cavenati, S.; Grande, C. A.; Rodrigues, A. E. Energy Fuels 2006, 20, 2648−2659. (15) Grande, C. A.; Lopes, F. V. S.; Ribeiro, A. M.; Loureiro, J. M.; Rodrigues, A. E. Sep. Sci. Technol. 2008, 43, 1338−1364. (16) Ribeiro, R. P.; Grande, C. A.; Rodrigues, A. E. Ind. Eng. Chem. Res. 2011, 50, 2144−2156. (17) Nitta, T.; Shigetomi, T.; Kuro-Oka, M.; Katayama, T. J. Chem. Eng. Jpn. 1984, 17, 39−45. (18) Mulloth, L. M.; Finn, J. E. Carbon Dioxide Adsorption on a Zeolite 5A Designed for CO2 in Spacecraft Cabins; National Aeronautics and Space Administration (NASA): Washington, D.C., 1998; Final Report NASA/TM-1998-208752. (19) Ahn, H.; Moon, J.-H.; Hyun, S.-H.; Lee, C.-H. Adsorption 2004, 10, 111−128. (20) Al-Muhtaseb, S. A.; Ritter, J. A. J. Phys. Chem. B 1999, 103, 8104−8115. (21) Al-Muhtaseb, S. A.; Ritter, J. A. Ind. Eng. Chem. Res. 1998, 37, 684−696. (22) Grande, C. A.; Rodrigues, A. E. Chem. Eng. Res. Des. 2004, 82, 1604−1612. (23) Habgood, H. W. Can. J. Chem. 1958, 36, 1384−1397.

5. CONCLUSION Adsorption equilibrium of CO2 and CH4 was measured at cryogenic temperatures (198−279 K). The data can be welldescribed by the Virial equation. Adsorption of methane on zeolite 13X is very fast and important; thus, this material is not recommended to be used in a TSA process for CO2 removal at low temperatures. On the other side, methane diffusion in zeolite 4A is extremely slow, boosting the selectivity of zeolite 4A toward carbon dioxide. Breakthrough curves of a binary CO2 and CH4 mixture were measured at different pressures, confirming that the behavior of the binary mixture can be predicted using pure component parameters determined by standard techniques. This paper provides all necessary parameters to evaluate the feasibility of the TSA process at low temperatures to remove low amounts of CO2 from methane.

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ASSOCIATED CONTENT

S Supporting Information *

Mathematical model used for simulation of breakthrough curves. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +47-93207532. Fax: +47-22067350. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This paper is based on the results from the research project “A Green Sea”, performed under the Petromaks program. The authors acknowledge the partners: Statoil, Gassco, Petrobras, and the Research Council of Norway (200455/S60) for their support.

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NOMENCLATURE A = first Virial coefficient (kg/mol) Aij = mixture of the first Virial coefficient (kg/mol) B = second Virial coefficient [(kg/mol)2] Bijk = mixture of the second Virial coefficient [(kg/mol)2] C = third Virial coefficient [(kg/mol)3] Ceq = equilibrium concentration in uptake experiments (mol/m3) Cijkl = mixture of the third Virial coefficient [(kg/mol)3] Cini = initial concentration in uptake experiments (mol/m3) Ki = adsorption constant (mol kg−1 kPa−1) P = pressure (kPa) pn = roots of the diffusion equation q = amount adsorbed (mol/kg) qeq = equilibrium amount adsorbed (mol/kg) Kμ,i = diffusion time constant, Dμ/rμ2 (s−1) Λ = variation of the concentration with time

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REFERENCES

(1) BP. BP Statistical Review of World Energy; BP: London, U.K., June 2013; http://www.bp.com/content/dam/bp/pdf/statistical-review/ statistical_review_of_world_energy_2013.pdf. (2) Neeraas, B. O.; Maråk, K. A. Energy efficiency and CO2 emissions in LNG value chain. Proceedings of the 2nd Trondheim Gas Technology Conference; Trondheim, Norway, Nov 2−3, 2011. (3) Hart, A.; Gnanendran, N. Energy Procedia 2009, 1, 697−706. 6693

dx.doi.org/10.1021/ef501814x | Energy Fuels 2014, 28, 6688−6693