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Equilibrium isotherms studies for light hydrocarbons adsorption on 4A molecular sieve zeolite
Author's Accepted Manuscript
Equilibrium isotherms studies for light hydrocarbons adsorption on 4A molecular sieve zeolite Muthanna J. Ahmed, Samar K. Theydan
www.elsevier.com/locate/petrol
PII: DOI: Reference:
S0920-4105(13)00136-8 http://dx.doi.org/10.1016/j.petrol.2013.05.005 PETROL2427
To appear in:
Journal of Petroleum Science and Engineering
Received date: 18 November 2009 Revised date: 30 April 2013 Accepted date: 3 May 2013 Cite this article as: Muthanna J. Ahmed, Samar K. Theydan, Equilibrium isotherms studies for light hydrocarbons adsorption on 4A molecular sieve zeolite, Journal of Petroleum Science and Engineering, http://dx.doi.org/10.1016/j. petrol.2013.05.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Equilibrium isotherms studies for light hydrocarbons adsorption on 4A molecular sieve zeolite Muthanna J. Ahmeda,b†, Samar K. Theydanb a Department of Chemical and Process Engineering -Faculty of Engineering and Built Environment- Universiti Kebangsaan Malaysia, Bangi 43600, Selangor, Malaysia. Email: [email protected] Tel: +60129177261 b Department of Chemical Engineering -University of Baghdad, Baghdad- Iraq. Email: [email protected]
Abstract Equilibrium adsorption isotherms were studied for pure methane, pure ethane, and methane, ethane, and propane mixture on 4A molecular sieve zeolite at 301K. The constant-volume method was used for measuring the pure and the multi-component experimental equilibria data. Various isotherm models were utilized for correlation of pure and multi-component isotherm data. The pure component experimental data agreed well with the Langmuir and Freundlich equations. Both the extended Freundlich and modified extended Langmuir equations correlated the multi component experimental data fairly well. The results of this study showed that ethane was more selectively adsorbed than methane on 4A molecular sieve zeolite, while propane was slowly adsorbed on this zeolite at the studied temperature.
Keywords: Equilibrium adsorption isotherm, 4A molecular sieve zeolite, ethane, methane, propane
1
1. Introduction The light hydrocarbons system such as methane, ethane, and propane system is of ever growing interest because these components appear together in many petroleum gases such as natural gas, deethanizer or depropanizer overhead mixtures. The adsorption separation processes have been largely employed to recover these components with high purities. In the design and optimization of adsorption processes basic experimental equilibria data usually in the form of an adsorption isotherm are required (Qiao et al., 2000). An adsorption isotherm for a single gaseous adsorptive on a solid is the function which relates at constant temperature the amount of substance adsorbed at equilibrium to the pressure or concentration of the adsorptive in the gas phase. This isotherm is useful for indicating the affinity of an adsorbate for a particular adsorbent and to determine the adsorption capacity which is of paramount importance to the capital cost because it indicates the amount of adsorbent required, which also fixes the volume of the adsorber vessels. There are different types of adsorption isotherms as classified by the international union of pure and applied chemistry IUPAC, these isotherms can have very different shapes depending on the type of adsorbent, the type of adsorbate, and the intermolecular interactions between the gas and the surface (Keller, 2005). There are three experimental methods generally used for measuring equilibrium adsorption isotherms, the volumetric method, the gravimetric method, and chromatographic method. The volumetric method is probably the best in terms of flexibility, decent accuracy, and low cost, as compared with the other methods (Gorbach et al., 2004).
2
Several theoretical equations have been used to correlate experimental equilibrium isotherm data for adsorption of pure gases on solids such as the Langmuir isotherm, the Freundlich isotherm, and the BET isotherm. The Langmuir isotherm has been successfully used to describe adsorption equilibria of light hydrocarbons on molecular sieve zeolites (Song et al., 2007). Direct measurement of multi component adsorption isotherms is complicated and tedious. Therefore, many researchers have developed techniques for their estimating from pure component isotherms. The extended Langmuir isotherm, the modified extended Langmuir isotherm and the extended Freundlich isotherm are based on these techniques (Wurster et al., 2000). The study of equilibrium isotherm for the adsorption of light hydrocarbons on different adsorbents has been an important subject for many investigators. Zhu et al. (2000) studied the equilibrium isotherm of methane, ethane, propane, n-butane, and ibutane on silicalite-1. They used a dual-site Langmuir isotherm to fit the experimental equilibrium data. Newalkar et al. (2003) used the volumetric method to measure the adsorption isotherm of methane, ethane and propane on hexagonal mesoporous silica. They used both the Langmuir and Langmuir-Freundlich isotherms to fit the data. Walton et al. (2005) studied the adsorption isotherm of methane, ethane and n-butane on activated carbon using the gravimetric method. Wender et al. (2007) studied the adsorption isotherm of n-alkanes such as n-butane on NaY molecular sieve zeolite using the gravimetric method. Ahmed et al. (2012) studied the adsorption isotherms of propane, n-butane and i-butane on 5A zeolite. The aim of this investigation is to study the pure and multi component adsorption equilibrium isotherm of methane, ethane, and propane on 4A zeolite with the utilization of different isotherm equations for correlation of experimental data.
3
2. Experimental Work 2.1 Materials Different gases such as methane, ethane, and propane were used as adsorbates. Methane and ethane (supplied by State Enterprise for Petrochemical Industries) were of purity higher than 99.5%. The composition of propane gas (supplied by Al-Durra Refinery) was: 3.81% methane, 7.92% ethane, and 88.27% propane. 4A molecular sieve zeolite (supplied by Laporte Industries Ltd) of particle density 1352 g/l and an average particle size 0.09 cm3 was used as adsorbent. Nitrogen (locally supplied) of purity greater than 99% was used as a carrier gas of gas-solid chromatography analyzer. 2.2 Apparatus A schematic sketch of the apparatus is outlined in Fig. 1. It is comprised of two iron pressure vessels connected together by (0.24 cm I.D. and 0.32 cm O.D.) stainless steel tubing. Both the reservoir and adsorption chambers have 0.02 l volume. The gas pressure was measured by a Heise bourdon gauge with a diameter of 43 cm and a pressure range from 0-6891 kPa. A water bath was employed to provide a constant temperature environment for both the reservoir and the adsorber. The bath was thermo-stated and vigorously mixed by using a magnetic stirrer thermo-stat hot plate (Gallenkamp). The temperature of the bath was continuously measured and recorded by a digital recorder (Dacq TR-2721) and a thermocouple wire, calibrated with mercury thermometer. Both chambers were evacuated by (Leybold-heraeus D8A) vacuum pump and (Acco Helicoids) gauge pressure prior to each experiment. The feed supplied from cylinder was regulated by (Victory) pressure regulator. The gas composition, in the case of the multi component mixture equilibrium runs, were measured by a gas-solid chromatography (GC) unit consist of: thermal
4
conductivity detector (TCD) (Varian vista 6000), integrator (Packard 602), and recorder (Packard 621). The GC carrier gas flow rate was measured by soap film wet flow-meter. 2.3 Experimental procedures The activated adsorbent (10g) was packed into the adsorption vessel and the apparatus, as shown in Fig.1, was evacuated to less than 3pa for three hours. This pretreatment, prior to each equilibrium measurements, was enabled us to make adsorption measurements without changing the adsorbents. After the adsorbent had been regenerated, the vacuum pump was switched off. The water bath with electrical stirrer thermo-stated hot plate was set at the desired temperature of adsorption 301K. Pure gas was introduced into the reservoir vessel through valve V1 until the desired concentration was reached. Valve V1 was then closed and the initial concentration Cri recorded when steady. Then valve V2 was opened and the system allowed reaching equilibrium to record final concentration Crf. For the multi component gas the reservoir vessel was isolated from the system, after reaching the equilibrium, by closing Valve V2. Then a sample of the gas was analyzed through Valve V4 by GC section to determine the concentration of each component. For both cases, the amount of gas adsorbed at equilibrium q was determined from a mass balance. The amount of a component in the reservoir vessel before adsorption is equal to the amount remaining in this vessel after adsorption plus the amount in the gas phase in the adsorber vessel plus the amount adsorbed in the adsorber vessel, then:
q=
( C ri − C rf ) Vr − (C rf ε o V ) a
W 5
(1)
The over all bed void fraction εo was determined by measuring the methanol volume Vl required to fill the cylinder volume Vc packed with the zeolite weight W, then: εo =
Vl Vc
(2)
3. Mathematical model Langmuir, Freundlich, and BET equations were used to correlate experimental equilibrium isotherms data for pure components. To correlate experimental equilibrium isotherms data for multi components, extended Langmuir, modified extended Langmuir, and extended Freundlich equations were used. 3.1. Langmuir isotherm The simplest and still the most useful isotherm for both physical and chemical adsorption is the Langmuir isotherm (Langmuir, 1915) which can be described as follows: q =
q m BC 1 + BC
(3)
This isotherm is derived by assuming a homogeneous surface with no interaction between adsorbate molecules, dynamic equilibrium between the adsorption and desorption, and the maximum adsorption corresponding to a complete monolayer. 3.2. Freundlich isotherm The Freundlich isotherm reflects the variation in heat of adsorption with coverage where it can be expressed as: q = KC
n
(4)
This isotherm is derived by assuming a heterogeneous surface with a non-uniform distribution of the heat of adsorption over the surface (Thomas and Crittenden, 1989).
6
3.3. BET isotherm The most widely used isotherm dealing with multilayer adsorption was derived by Brunauer, Emmet, and Teller (1938), and is called the BET isotherm. This isotherm can be written as follows:
q =
C C* C ⎞⎛ C ⎞⎤ − 1) * ⎟ ⎜ 1 − * ⎟ ⎥ C ⎠⎝ C ⎠⎦
qmK B ⎡⎛ ⎢⎜ 1 + ( K B ⎣⎝
(5)
The classical BET equation, although used for many years to evaluate surface areas and heats of adsorption on many adsorbent-adsorbate systems, is not applicable to zeolites. Basically, this equation was developed for multilayer adsorption.
3.4. Extended Langmuir isotherm The extension of the Langmuir isotherm for pure component to the description of multi component adsorption phenomena was first proposed by Markham and Benton (1931). This isotherm is based on the assumption that for each gaseous component an equilibrium exists between the amount adsorbed at the surface and the concentration of that component in the gas phase. The general form of the extended Langmuir equation for each component in the mixture is: qi =
q mi B i C i
(6)
n
1 + ∑ B jC j=1
j
Broughton (1948) observed that the extension of the Langmuir isotherm to adsorption from binary adsorbate system is thermodynamically consistent only for the special case where there is no competition between the two components on the same binding sites.
3.5. Modified extended Langmuir The original extended Langmuir isotherm was improved by Jain and Snoeyink whose modification was based on the hypothesis that adsorption without competition
7
occurs on some sites and with competition on the other sites. On this basis, the following equation was described: qi =
q mi B ii C i
(7)
n
1 + ∑ B ij C j=1
j
Further, it was assumed that the number of sites for non competition adsorption would be proportional to the difference between the maximum loadings of the species (Wurster et al., 2000).
3.6. Extended Freundlich isotherm Extended Freundlich isotherm can be deduced as special case of the relationship proposed by Fritz and Schluender (1981) for multi component adsorption equilibria. This isotherm is in the following form:
qi =
K iC i
n i + n ii
n
∑ K ij C j=1
j
(8)
n ij
This equation has been used to fit the experimental data for equilibrium adsorption of two and three adsorbates with satisfactory results. A nonlinear lest-squares regression program based on Gauss-Newton method was used to fit Eqs (3)-(8) to experimental equilibrium isotherms data. This program gave the parameters of each equation and the agreement between experimental and calculated adsorption isotherms in terms of correlation coefficient.
4. Results and discussion 4.1 Pure component adsorption isotherms The experimental equilibrium data for pure methane and ethane adsorption on 4A molecular sieve zeolite, calculated from Eqs (1) and (2), and for pure propane, taken from Grande et al. (2006), are fitted with Langmuir, Freundlich, and BET equations.
8
The calculated constants for the three isotherms equations along with the correlations coefficients values are presented in Table 1. This table shows that the Langmuir equation correlates with a correlation coefficient of 0.998, 0.997, and 0.992 for methane, ethane, and propane, respectively. While the Freundlich equation correlates with correlation coefficient of 0.987, 0.994, and 0.981 for methane, ethane, and propane, respectively. The BET equation fails to correlate experimental equilibrium data for pure components. Although, the Langmuir and Freundlich equation give good fitting, the best fit is achieved with the Langmuir equation. Thus, the experimental equilibrium data is correlated by the Langmuir equation and presented in Figs 2-4. . These figures show that at a given gas phase concentration the adsorbed phase concentration of ethane is greater than methane, which means that ethane is more selectively adsorbed than methane on 4A molecular sieve zeolite. This is in agreement with Song et al. (2007) who showed that at a given concentration and temperature, the amount of the adsorbed hydrocarbon component increased with molecular weight. The study of the equilibrium isotherm indicates that the adsorption capacity increases with increasing the inlet concentration of the adsorbate, but it reaches nearly a steady value at high concentration. This means that the equilibrium is of the favorable type. The above results are also observed by Pakseresht et al. (2002) who showed that the equilibrium isotherm data for adsorption of pure methane on 5A molecular sieve zeolite could be fitted by the Langmuir isotherm with good correlation coefficient. Different researchers showed that both the Langmuir and Freundlich equations were succeeded to simulate pure component adsorption isotherms in different molecular sieve zeolites (Tezel and Apolonatos, 1993). These results are not surprising because
9
the BET isotherm deals with multilayer adsorption, while the Langmuir and Freundlich isotherm deal with monolayer adsorption, such as in zeolites.
4.2 Multi component adsorption isotherms The experimental equilibrium data for multi-component adsorption, calculated from Eqs (1) and (2), are fitted by extended Langmuir, modified extended Langmuir, and extended Freundlich equations. The calculated constants for the three isotherm equations along with the correlation coefficient values are presented in Table 2. This table shows that the extended Freundlich equation correlates with correlation coefficients of 0.999, 0.999, and 0.994 for methane, ethane, and propane, respectively. While the modified extended Langmuir equation correlates with a correlation coefficient of 0.998, 0.994, and 0.911 for methane, ethane, and propane, respectively. The extended Langmuir equation fails to correlate experimental equilibrium data for multi components mixture. Therefore, both the modified extended Langmuir and the extended Freundlich equations succeed to predict the adsorption equilibrium isotherm for the multi component mixture. The extended Freundlich equation, however, give a best correlation. Thus, the experimental equilibrium data is correlated by the extended Freundlich equation and presented in Fig. 4. This figure shows that ethane is more selectively adsorbed than methane. Propane is less selectively adsorbed on 4A molecular sieve zeolite (Grande and Rodrigues, 2004). Different researchers showed that the extended Freundlich equation, although empirical, had greater flexibility in characterizing adsorbent heterogeneity and resulted in better multi component adsorption isotherm predictions. These observations are not surprising because the extended Langmuir assumes that there is
10
no competition of adsorbates on the same site, while the other isotherms take into account this competition.
5. Conclusions Experimental equilibrium isotherm data for pure and multi component adsorption showed that 4A molecular sieve zeolite had more affinity with ethane than methane, while it had low affinity with propane at the studied temperature. Equilibrium isotherms for the pure component adsorption of methane, ethane, and propane on 4A molecular sieve zeolite were fitted successfully by both Langmuir and Freundlich equations. Extended Freundlich and modified extended Langmuir equations provided good predictions for adsorption of methane, ethane, and propane mixture on 4A molecular sieve zeolite.
Acknowledgement We gratefully acknowledge Baghdad University and chemical engineering department for assist and support of this work.
References Ahmed M.J., Mohammed A.H. A.K., Kadhum A.H., 2012. Prediction of multi component equilibrium isotherms for light hydrocarbons adsorption on 5A zeolite, Fluid Phase Equilibria 313, 165– 170. Broughton D.B., 1948. Adsorption isotherms for binary gas mixtures. Ind Eng Chem., 40(8), 1506–1508. Brunauer S., Emmett P.H., Teller E., 1938. Adsorption of gasses in multimolecular layers, J. Am. Chem. Soc., 60, 309-319. Fritz, W. and Schluender, E.U., 1981. Competitive adsorption of two dissolved organics onto activated carbon - I, Chem. Eng. Sci., 36, 721-730.
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Gorbach A., Stegmaier M., Eigenberger G., 2004. Measurement and modeling of water vapor adsorption on zeolite 4A: equilibria and kinetics, Adsorption, 10(1), 2946. Grande C.A. and Rodrigues A.E., 2004. Adsorption kinetics of propane and propylene in zeolite 4A, Chem. Eng. Res. & Des., 82(A12), 1604-1612. Grande C.A., Cavenati S., Barcia P., Rodrigues A.E., 2006. Adsorption of propane and propylene in zeolite 4A honeycomb monolith, Chem. Eng. Sci., 61, 3053-3067 Keller J.U., 2005. Gas Adsorption Equilibria: Experimental methods and adsorptive isotherms, Springer Science, Boston. Langmuir I., 1915. Chemical reactions at low pressures, J. Am. Chem. Soc., 37, 11391167. Markham E.C. and Benton A.F., 1931. The adsorption of gas mixtures by silica. J Am Chem Soc., 53, 497–506. Newalkar B.L., Choudary N.V., Turaga U.T., Vijayalakshmi R.P., 2003. Adsorption of light hydrocarbons on HMS type mesoporous silica, Microporous & Mesoporous Materials, 65, 267-276. Pakseresht S., Kazemeni M., Akbarnejad M.M., 2002. Equilibrium isotherms of CO, CO2, CH4 and C2H4 on the 5A molecular sieve by a simple volumetric apparatus, Sep. & Purif. Tech., 28, 53-60. Qiao S., Wang K., Hu X., 2000. Study of Binary adsorption equilibrium of hydrocarbons in activated carbon using micro pore size distribution, Langmuir, 16(11), 5130-5136. Song L., Sun Z., Duan L., Gui J., 2007. Adsorption and diffusion properties of hydrocarbons in zeolites, Microporous & Mesoporous Materials, 104, 115-128.
12
Tezel F.H. and Apolonatos G., 1993. Chromatographic study of adsorption for N2, CO, and CH4 in Molecular Sieve Zeolites, Gas Sep. & Purif., 7(1), 11-17. Thomas W.J. and Crittenden D.B., 1998. Adsorption technology and design, Butterworth-Heinemann, London. Walton K.S., Cavalcante C.L., Levan M.D., 2005. Adsorption equilibrium of alkanes on a high surface area activated carbon prepared from brazilian coconut shells, Adsorption, 11, 107-111. Wender A., Barreau A., Lefebvre C., DiLella A., 2007. Adsorption of N-alkanes in faujasite zeolites: molecular simulation study and experimental measurements, Adsorption, 13, 439-451. Wurster D.E., Alkhamis A.K., Matheson L.E., 2000. Prediction of adsorption from multicomponent solutions by activated carbon using single-solute parameters, AAPS Pharm. Sci. Tech., 1(3), 79-93. Zhu W., Kapteijn F., Moulijn J.A., 2000. Equilibrium adsorption of light alkanes in silicalite-1 by the inertial microbalance technique, Adsorption, 6, 159-167.
Nomenclature Notation B
: Langmuir equation coefficient (l/mmole)
BET
: Brunauer, Emmet, and Teller
Bii, Bij
: Modified extended Langmuir equation coefficients (l/mmole)
C
: Gas phase concentration (mmole/l)
IUPAC : International union of pure and applied chemistry K
: Freundlich equation coefficient (ln/mmolen-1.g)
KB
: BET equation coefficient
Kij
: Extended Freundlich and Sheindorf equations coefficient
13
MSZ
: Molecular sieve zeolite
n
: Freundlich equation parameter
nii, nij
: Fritz and Extended Freundlich equations parameters
q
: Adsorbed phase concentration (mmole/g)
V
: Volume (l)
W
: Mass of adsorbent (g)
εo
: Over all bed void fraction
Subscripts a
: Adsorber vessel
c
: Cylinder
i
: Component i
j
: Component j
l
: Methanol
m
: Mono-layer
n
: Component n
r
: Reservoir vessel
rf
: Final condition of reservoir vessel
ri
: Initial condition of reservoir vessel
*
: Saturation condition
14
Highlights
Adsorption of pure and multi component for methane, ethane and propane on 4A zeolite were studied.
constant-volume method was adopted for measuring of data.
Langmuir and Freundlich equations correlate the pure component data fairly well.
Extended Freundlich and modified extended Langmuir provide good predictions for mixture data.
4A zeolite had more affinity with ethane than methane, while it had low affinity with propane.
15
Fig. 1, Schematic diagram of adsorption equilibrium measurement apparatus
Adsorbed phase concentration q (mmole/g)
1
0.8
0.6
0.4
0.2
Exp. The. 0 0
20
40
60
80
Gas phase concentration C (mmole/l)
Fig.2, Adsorption equilibrium isotherm of pure methane on 4A MSZ at 301K correlated with Langmuir equation
16
100
Adsorbed phase concentration q (mmole/g)
2.5
2
1.5
1
0.5 Exp. The. 0 0
20
40
60
80
100
Gas phase concentration C (mmole/l)
Fig.3, Adsorption equilibrium isotherm of pure ethane on 4A MSZ at 301K correlated with Langmuir equation
Adsorbed phase concentration q (mmole/g)
0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 Exp.
0.02
The. 0 0
10
20
30
40
50
60
70
Gas phase concentration C (mmole/l)
17
80
90
Fig.4, Adsorption equilibrium isotherm of pure propane on 4A MSZ at 301K correlated with Langmuir equation (Exp. data from Grande et al (2006))
Fig.5, Adsorption equilibrium isotherm of multi component mixture (3.81%CH4, 7.92%C2H6, 88.27%C3H8) on 4A MSZ at 301K correlated with extended Freundlich equation Graphical abstract
18
Table 1, Pure component adsorption isotherm results Pure component isotherm results correlated with Langmuir equation adsorbate
qm
B
C. Coef.
methane
1.297
0.025
0.998
ethane
2.549
0.091
0.997
propane
0.343
0.011
0.992
Pure component isotherms results correlated with Freundlich equation adsorbate
K
n
C. Coef.
methane
0.098
0.499
0.987
ethane
0.650
0.292
0.994
propane
0.007
0.711
0.981
Pure component isotherms results correlated with BET equation adsorbate
P* (kPa)
qm
KB
C.Coef.
methane
215
2.582
8836
-1.81
ethane
208
4.564
8710
-1.07
propane
205
0.106
15994E4
-0.20
Table 2, Multi component adsorption isotherm results for (3.81%CH4, 7.92%C2H6, 88.27%C3H8) mixture
19
Multi component isotherm results correlated with extended Freundlich equation adsorbate
Ki
ni
Ki1
Ki2
Ki3
ni1
ni2
ni3
C.Coef
methane
0.098
0.499
1
0.002
5.16
0.738
4.52
0.044
0.999
ethane
0.650
0.291
14.08
1
0.05
0.209
0.69
3.187
0.999
propane
0.007
0.710
-0.122
-0.224
1
1.700
-1.57
0.720
0.994
Multi component isotherm results correlated with modified extended Langmuir equation adsorbate
qmi
Bi1
Bi2
Bi3
C.Coef.
methane
1.296
0.013
-0.037
0.086
0.998
ethane
2.548
0.315
0.022
-0.089
0.994
propane
0.342
0.916
-0.503
0.060
0.911
Multi component isotherm results correlated with extended Langmuir equation adsorbate
qmi
B1
B2
B3
C.Coef.
methane
1.297
0.025
0.091
0.011
-0.675
ethane
2.549
0.025
0.091
0.011
-578.6
propane
0.343
0.025
0.091
0.011
0.231
20
Equilibrium isotherms studies for light hydrocarbons adsorption on 4A molecular sieve zeolite Muthanna J. Ahmed, Samar K. Theydan
www.elsevier.com/locate/petrol
PII: DOI: Reference:
S0920-4105(13)00136-8 http://dx.doi.org/10.1016/j.petrol.2013.05.005 PETROL2427
To appear in:
Journal of Petroleum Science and Engineering
Received date: 18 November 2009 Revised date: 30 April 2013 Accepted date: 3 May 2013 Cite this article as: Muthanna J. Ahmed, Samar K. Theydan, Equilibrium isotherms studies for light hydrocarbons adsorption on 4A molecular sieve zeolite, Journal of Petroleum Science and Engineering, http://dx.doi.org/10.1016/j. petrol.2013.05.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Equilibrium isotherms studies for light hydrocarbons adsorption on 4A molecular sieve zeolite Muthanna J. Ahmeda,b†, Samar K. Theydanb a Department of Chemical and Process Engineering -Faculty of Engineering and Built Environment- Universiti Kebangsaan Malaysia, Bangi 43600, Selangor, Malaysia. Email: [email protected] Tel: +60129177261 b Department of Chemical Engineering -University of Baghdad, Baghdad- Iraq. Email: [email protected]
Abstract Equilibrium adsorption isotherms were studied for pure methane, pure ethane, and methane, ethane, and propane mixture on 4A molecular sieve zeolite at 301K. The constant-volume method was used for measuring the pure and the multi-component experimental equilibria data. Various isotherm models were utilized for correlation of pure and multi-component isotherm data. The pure component experimental data agreed well with the Langmuir and Freundlich equations. Both the extended Freundlich and modified extended Langmuir equations correlated the multi component experimental data fairly well. The results of this study showed that ethane was more selectively adsorbed than methane on 4A molecular sieve zeolite, while propane was slowly adsorbed on this zeolite at the studied temperature.
Keywords: Equilibrium adsorption isotherm, 4A molecular sieve zeolite, ethane, methane, propane
1
1. Introduction The light hydrocarbons system such as methane, ethane, and propane system is of ever growing interest because these components appear together in many petroleum gases such as natural gas, deethanizer or depropanizer overhead mixtures. The adsorption separation processes have been largely employed to recover these components with high purities. In the design and optimization of adsorption processes basic experimental equilibria data usually in the form of an adsorption isotherm are required (Qiao et al., 2000). An adsorption isotherm for a single gaseous adsorptive on a solid is the function which relates at constant temperature the amount of substance adsorbed at equilibrium to the pressure or concentration of the adsorptive in the gas phase. This isotherm is useful for indicating the affinity of an adsorbate for a particular adsorbent and to determine the adsorption capacity which is of paramount importance to the capital cost because it indicates the amount of adsorbent required, which also fixes the volume of the adsorber vessels. There are different types of adsorption isotherms as classified by the international union of pure and applied chemistry IUPAC, these isotherms can have very different shapes depending on the type of adsorbent, the type of adsorbate, and the intermolecular interactions between the gas and the surface (Keller, 2005). There are three experimental methods generally used for measuring equilibrium adsorption isotherms, the volumetric method, the gravimetric method, and chromatographic method. The volumetric method is probably the best in terms of flexibility, decent accuracy, and low cost, as compared with the other methods (Gorbach et al., 2004).
2
Several theoretical equations have been used to correlate experimental equilibrium isotherm data for adsorption of pure gases on solids such as the Langmuir isotherm, the Freundlich isotherm, and the BET isotherm. The Langmuir isotherm has been successfully used to describe adsorption equilibria of light hydrocarbons on molecular sieve zeolites (Song et al., 2007). Direct measurement of multi component adsorption isotherms is complicated and tedious. Therefore, many researchers have developed techniques for their estimating from pure component isotherms. The extended Langmuir isotherm, the modified extended Langmuir isotherm and the extended Freundlich isotherm are based on these techniques (Wurster et al., 2000). The study of equilibrium isotherm for the adsorption of light hydrocarbons on different adsorbents has been an important subject for many investigators. Zhu et al. (2000) studied the equilibrium isotherm of methane, ethane, propane, n-butane, and ibutane on silicalite-1. They used a dual-site Langmuir isotherm to fit the experimental equilibrium data. Newalkar et al. (2003) used the volumetric method to measure the adsorption isotherm of methane, ethane and propane on hexagonal mesoporous silica. They used both the Langmuir and Langmuir-Freundlich isotherms to fit the data. Walton et al. (2005) studied the adsorption isotherm of methane, ethane and n-butane on activated carbon using the gravimetric method. Wender et al. (2007) studied the adsorption isotherm of n-alkanes such as n-butane on NaY molecular sieve zeolite using the gravimetric method. Ahmed et al. (2012) studied the adsorption isotherms of propane, n-butane and i-butane on 5A zeolite. The aim of this investigation is to study the pure and multi component adsorption equilibrium isotherm of methane, ethane, and propane on 4A zeolite with the utilization of different isotherm equations for correlation of experimental data.
3
2. Experimental Work 2.1 Materials Different gases such as methane, ethane, and propane were used as adsorbates. Methane and ethane (supplied by State Enterprise for Petrochemical Industries) were of purity higher than 99.5%. The composition of propane gas (supplied by Al-Durra Refinery) was: 3.81% methane, 7.92% ethane, and 88.27% propane. 4A molecular sieve zeolite (supplied by Laporte Industries Ltd) of particle density 1352 g/l and an average particle size 0.09 cm3 was used as adsorbent. Nitrogen (locally supplied) of purity greater than 99% was used as a carrier gas of gas-solid chromatography analyzer. 2.2 Apparatus A schematic sketch of the apparatus is outlined in Fig. 1. It is comprised of two iron pressure vessels connected together by (0.24 cm I.D. and 0.32 cm O.D.) stainless steel tubing. Both the reservoir and adsorption chambers have 0.02 l volume. The gas pressure was measured by a Heise bourdon gauge with a diameter of 43 cm and a pressure range from 0-6891 kPa. A water bath was employed to provide a constant temperature environment for both the reservoir and the adsorber. The bath was thermo-stated and vigorously mixed by using a magnetic stirrer thermo-stat hot plate (Gallenkamp). The temperature of the bath was continuously measured and recorded by a digital recorder (Dacq TR-2721) and a thermocouple wire, calibrated with mercury thermometer. Both chambers were evacuated by (Leybold-heraeus D8A) vacuum pump and (Acco Helicoids) gauge pressure prior to each experiment. The feed supplied from cylinder was regulated by (Victory) pressure regulator. The gas composition, in the case of the multi component mixture equilibrium runs, were measured by a gas-solid chromatography (GC) unit consist of: thermal
4
conductivity detector (TCD) (Varian vista 6000), integrator (Packard 602), and recorder (Packard 621). The GC carrier gas flow rate was measured by soap film wet flow-meter. 2.3 Experimental procedures The activated adsorbent (10g) was packed into the adsorption vessel and the apparatus, as shown in Fig.1, was evacuated to less than 3pa for three hours. This pretreatment, prior to each equilibrium measurements, was enabled us to make adsorption measurements without changing the adsorbents. After the adsorbent had been regenerated, the vacuum pump was switched off. The water bath with electrical stirrer thermo-stated hot plate was set at the desired temperature of adsorption 301K. Pure gas was introduced into the reservoir vessel through valve V1 until the desired concentration was reached. Valve V1 was then closed and the initial concentration Cri recorded when steady. Then valve V2 was opened and the system allowed reaching equilibrium to record final concentration Crf. For the multi component gas the reservoir vessel was isolated from the system, after reaching the equilibrium, by closing Valve V2. Then a sample of the gas was analyzed through Valve V4 by GC section to determine the concentration of each component. For both cases, the amount of gas adsorbed at equilibrium q was determined from a mass balance. The amount of a component in the reservoir vessel before adsorption is equal to the amount remaining in this vessel after adsorption plus the amount in the gas phase in the adsorber vessel plus the amount adsorbed in the adsorber vessel, then:
q=
( C ri − C rf ) Vr − (C rf ε o V ) a
W 5
(1)
The over all bed void fraction εo was determined by measuring the methanol volume Vl required to fill the cylinder volume Vc packed with the zeolite weight W, then: εo =
Vl Vc
(2)
3. Mathematical model Langmuir, Freundlich, and BET equations were used to correlate experimental equilibrium isotherms data for pure components. To correlate experimental equilibrium isotherms data for multi components, extended Langmuir, modified extended Langmuir, and extended Freundlich equations were used. 3.1. Langmuir isotherm The simplest and still the most useful isotherm for both physical and chemical adsorption is the Langmuir isotherm (Langmuir, 1915) which can be described as follows: q =
q m BC 1 + BC
(3)
This isotherm is derived by assuming a homogeneous surface with no interaction between adsorbate molecules, dynamic equilibrium between the adsorption and desorption, and the maximum adsorption corresponding to a complete monolayer. 3.2. Freundlich isotherm The Freundlich isotherm reflects the variation in heat of adsorption with coverage where it can be expressed as: q = KC
n
(4)
This isotherm is derived by assuming a heterogeneous surface with a non-uniform distribution of the heat of adsorption over the surface (Thomas and Crittenden, 1989).
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3.3. BET isotherm The most widely used isotherm dealing with multilayer adsorption was derived by Brunauer, Emmet, and Teller (1938), and is called the BET isotherm. This isotherm can be written as follows:
q =
C C* C ⎞⎛ C ⎞⎤ − 1) * ⎟ ⎜ 1 − * ⎟ ⎥ C ⎠⎝ C ⎠⎦
qmK B ⎡⎛ ⎢⎜ 1 + ( K B ⎣⎝
(5)
The classical BET equation, although used for many years to evaluate surface areas and heats of adsorption on many adsorbent-adsorbate systems, is not applicable to zeolites. Basically, this equation was developed for multilayer adsorption.
3.4. Extended Langmuir isotherm The extension of the Langmuir isotherm for pure component to the description of multi component adsorption phenomena was first proposed by Markham and Benton (1931). This isotherm is based on the assumption that for each gaseous component an equilibrium exists between the amount adsorbed at the surface and the concentration of that component in the gas phase. The general form of the extended Langmuir equation for each component in the mixture is: qi =
q mi B i C i
(6)
n
1 + ∑ B jC j=1
j
Broughton (1948) observed that the extension of the Langmuir isotherm to adsorption from binary adsorbate system is thermodynamically consistent only for the special case where there is no competition between the two components on the same binding sites.
3.5. Modified extended Langmuir The original extended Langmuir isotherm was improved by Jain and Snoeyink whose modification was based on the hypothesis that adsorption without competition
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occurs on some sites and with competition on the other sites. On this basis, the following equation was described: qi =
q mi B ii C i
(7)
n
1 + ∑ B ij C j=1
j
Further, it was assumed that the number of sites for non competition adsorption would be proportional to the difference between the maximum loadings of the species (Wurster et al., 2000).
3.6. Extended Freundlich isotherm Extended Freundlich isotherm can be deduced as special case of the relationship proposed by Fritz and Schluender (1981) for multi component adsorption equilibria. This isotherm is in the following form:
qi =
K iC i
n i + n ii
n
∑ K ij C j=1
j
(8)
n ij
This equation has been used to fit the experimental data for equilibrium adsorption of two and three adsorbates with satisfactory results. A nonlinear lest-squares regression program based on Gauss-Newton method was used to fit Eqs (3)-(8) to experimental equilibrium isotherms data. This program gave the parameters of each equation and the agreement between experimental and calculated adsorption isotherms in terms of correlation coefficient.
4. Results and discussion 4.1 Pure component adsorption isotherms The experimental equilibrium data for pure methane and ethane adsorption on 4A molecular sieve zeolite, calculated from Eqs (1) and (2), and for pure propane, taken from Grande et al. (2006), are fitted with Langmuir, Freundlich, and BET equations.
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The calculated constants for the three isotherms equations along with the correlations coefficients values are presented in Table 1. This table shows that the Langmuir equation correlates with a correlation coefficient of 0.998, 0.997, and 0.992 for methane, ethane, and propane, respectively. While the Freundlich equation correlates with correlation coefficient of 0.987, 0.994, and 0.981 for methane, ethane, and propane, respectively. The BET equation fails to correlate experimental equilibrium data for pure components. Although, the Langmuir and Freundlich equation give good fitting, the best fit is achieved with the Langmuir equation. Thus, the experimental equilibrium data is correlated by the Langmuir equation and presented in Figs 2-4. . These figures show that at a given gas phase concentration the adsorbed phase concentration of ethane is greater than methane, which means that ethane is more selectively adsorbed than methane on 4A molecular sieve zeolite. This is in agreement with Song et al. (2007) who showed that at a given concentration and temperature, the amount of the adsorbed hydrocarbon component increased with molecular weight. The study of the equilibrium isotherm indicates that the adsorption capacity increases with increasing the inlet concentration of the adsorbate, but it reaches nearly a steady value at high concentration. This means that the equilibrium is of the favorable type. The above results are also observed by Pakseresht et al. (2002) who showed that the equilibrium isotherm data for adsorption of pure methane on 5A molecular sieve zeolite could be fitted by the Langmuir isotherm with good correlation coefficient. Different researchers showed that both the Langmuir and Freundlich equations were succeeded to simulate pure component adsorption isotherms in different molecular sieve zeolites (Tezel and Apolonatos, 1993). These results are not surprising because
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the BET isotherm deals with multilayer adsorption, while the Langmuir and Freundlich isotherm deal with monolayer adsorption, such as in zeolites.
4.2 Multi component adsorption isotherms The experimental equilibrium data for multi-component adsorption, calculated from Eqs (1) and (2), are fitted by extended Langmuir, modified extended Langmuir, and extended Freundlich equations. The calculated constants for the three isotherm equations along with the correlation coefficient values are presented in Table 2. This table shows that the extended Freundlich equation correlates with correlation coefficients of 0.999, 0.999, and 0.994 for methane, ethane, and propane, respectively. While the modified extended Langmuir equation correlates with a correlation coefficient of 0.998, 0.994, and 0.911 for methane, ethane, and propane, respectively. The extended Langmuir equation fails to correlate experimental equilibrium data for multi components mixture. Therefore, both the modified extended Langmuir and the extended Freundlich equations succeed to predict the adsorption equilibrium isotherm for the multi component mixture. The extended Freundlich equation, however, give a best correlation. Thus, the experimental equilibrium data is correlated by the extended Freundlich equation and presented in Fig. 4. This figure shows that ethane is more selectively adsorbed than methane. Propane is less selectively adsorbed on 4A molecular sieve zeolite (Grande and Rodrigues, 2004). Different researchers showed that the extended Freundlich equation, although empirical, had greater flexibility in characterizing adsorbent heterogeneity and resulted in better multi component adsorption isotherm predictions. These observations are not surprising because the extended Langmuir assumes that there is
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no competition of adsorbates on the same site, while the other isotherms take into account this competition.
5. Conclusions Experimental equilibrium isotherm data for pure and multi component adsorption showed that 4A molecular sieve zeolite had more affinity with ethane than methane, while it had low affinity with propane at the studied temperature. Equilibrium isotherms for the pure component adsorption of methane, ethane, and propane on 4A molecular sieve zeolite were fitted successfully by both Langmuir and Freundlich equations. Extended Freundlich and modified extended Langmuir equations provided good predictions for adsorption of methane, ethane, and propane mixture on 4A molecular sieve zeolite.
Acknowledgement We gratefully acknowledge Baghdad University and chemical engineering department for assist and support of this work.
References Ahmed M.J., Mohammed A.H. A.K., Kadhum A.H., 2012. Prediction of multi component equilibrium isotherms for light hydrocarbons adsorption on 5A zeolite, Fluid Phase Equilibria 313, 165– 170. Broughton D.B., 1948. Adsorption isotherms for binary gas mixtures. Ind Eng Chem., 40(8), 1506–1508. Brunauer S., Emmett P.H., Teller E., 1938. Adsorption of gasses in multimolecular layers, J. Am. Chem. Soc., 60, 309-319. Fritz, W. and Schluender, E.U., 1981. Competitive adsorption of two dissolved organics onto activated carbon - I, Chem. Eng. Sci., 36, 721-730.
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Gorbach A., Stegmaier M., Eigenberger G., 2004. Measurement and modeling of water vapor adsorption on zeolite 4A: equilibria and kinetics, Adsorption, 10(1), 2946. Grande C.A. and Rodrigues A.E., 2004. Adsorption kinetics of propane and propylene in zeolite 4A, Chem. Eng. Res. & Des., 82(A12), 1604-1612. Grande C.A., Cavenati S., Barcia P., Rodrigues A.E., 2006. Adsorption of propane and propylene in zeolite 4A honeycomb monolith, Chem. Eng. Sci., 61, 3053-3067 Keller J.U., 2005. Gas Adsorption Equilibria: Experimental methods and adsorptive isotherms, Springer Science, Boston. Langmuir I., 1915. Chemical reactions at low pressures, J. Am. Chem. Soc., 37, 11391167. Markham E.C. and Benton A.F., 1931. The adsorption of gas mixtures by silica. J Am Chem Soc., 53, 497–506. Newalkar B.L., Choudary N.V., Turaga U.T., Vijayalakshmi R.P., 2003. Adsorption of light hydrocarbons on HMS type mesoporous silica, Microporous & Mesoporous Materials, 65, 267-276. Pakseresht S., Kazemeni M., Akbarnejad M.M., 2002. Equilibrium isotherms of CO, CO2, CH4 and C2H4 on the 5A molecular sieve by a simple volumetric apparatus, Sep. & Purif. Tech., 28, 53-60. Qiao S., Wang K., Hu X., 2000. Study of Binary adsorption equilibrium of hydrocarbons in activated carbon using micro pore size distribution, Langmuir, 16(11), 5130-5136. Song L., Sun Z., Duan L., Gui J., 2007. Adsorption and diffusion properties of hydrocarbons in zeolites, Microporous & Mesoporous Materials, 104, 115-128.
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Tezel F.H. and Apolonatos G., 1993. Chromatographic study of adsorption for N2, CO, and CH4 in Molecular Sieve Zeolites, Gas Sep. & Purif., 7(1), 11-17. Thomas W.J. and Crittenden D.B., 1998. Adsorption technology and design, Butterworth-Heinemann, London. Walton K.S., Cavalcante C.L., Levan M.D., 2005. Adsorption equilibrium of alkanes on a high surface area activated carbon prepared from brazilian coconut shells, Adsorption, 11, 107-111. Wender A., Barreau A., Lefebvre C., DiLella A., 2007. Adsorption of N-alkanes in faujasite zeolites: molecular simulation study and experimental measurements, Adsorption, 13, 439-451. Wurster D.E., Alkhamis A.K., Matheson L.E., 2000. Prediction of adsorption from multicomponent solutions by activated carbon using single-solute parameters, AAPS Pharm. Sci. Tech., 1(3), 79-93. Zhu W., Kapteijn F., Moulijn J.A., 2000. Equilibrium adsorption of light alkanes in silicalite-1 by the inertial microbalance technique, Adsorption, 6, 159-167.
Nomenclature Notation B
: Langmuir equation coefficient (l/mmole)
BET
: Brunauer, Emmet, and Teller
Bii, Bij
: Modified extended Langmuir equation coefficients (l/mmole)
C
: Gas phase concentration (mmole/l)
IUPAC : International union of pure and applied chemistry K
: Freundlich equation coefficient (ln/mmolen-1.g)
KB
: BET equation coefficient
Kij
: Extended Freundlich and Sheindorf equations coefficient
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MSZ
: Molecular sieve zeolite
n
: Freundlich equation parameter
nii, nij
: Fritz and Extended Freundlich equations parameters
q
: Adsorbed phase concentration (mmole/g)
V
: Volume (l)
W
: Mass of adsorbent (g)
εo
: Over all bed void fraction
Subscripts a
: Adsorber vessel
c
: Cylinder
i
: Component i
j
: Component j
l
: Methanol
m
: Mono-layer
n
: Component n
r
: Reservoir vessel
rf
: Final condition of reservoir vessel
ri
: Initial condition of reservoir vessel
*
: Saturation condition
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Highlights
Adsorption of pure and multi component for methane, ethane and propane on 4A zeolite were studied.
constant-volume method was adopted for measuring of data.
Langmuir and Freundlich equations correlate the pure component data fairly well.
Extended Freundlich and modified extended Langmuir provide good predictions for mixture data.
4A zeolite had more affinity with ethane than methane, while it had low affinity with propane.
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Fig. 1, Schematic diagram of adsorption equilibrium measurement apparatus
Adsorbed phase concentration q (mmole/g)
1
0.8
0.6
0.4
0.2
Exp. The. 0 0
20
40
60
80
Gas phase concentration C (mmole/l)
Fig.2, Adsorption equilibrium isotherm of pure methane on 4A MSZ at 301K correlated with Langmuir equation
16
100
Adsorbed phase concentration q (mmole/g)
2.5
2
1.5
1
0.5 Exp. The. 0 0
20
40
60
80
100
Gas phase concentration C (mmole/l)
Fig.3, Adsorption equilibrium isotherm of pure ethane on 4A MSZ at 301K correlated with Langmuir equation
Adsorbed phase concentration q (mmole/g)
0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 Exp.
0.02
The. 0 0
10
20
30
40
50
60
70
Gas phase concentration C (mmole/l)
17
80
90
Fig.4, Adsorption equilibrium isotherm of pure propane on 4A MSZ at 301K correlated with Langmuir equation (Exp. data from Grande et al (2006))
Fig.5, Adsorption equilibrium isotherm of multi component mixture (3.81%CH4, 7.92%C2H6, 88.27%C3H8) on 4A MSZ at 301K correlated with extended Freundlich equation Graphical abstract
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Table 1, Pure component adsorption isotherm results Pure component isotherm results correlated with Langmuir equation adsorbate
qm
B
C. Coef.
methane
1.297
0.025
0.998
ethane
2.549
0.091
0.997
propane
0.343
0.011
0.992
Pure component isotherms results correlated with Freundlich equation adsorbate
K
n
C. Coef.
methane
0.098
0.499
0.987
ethane
0.650
0.292
0.994
propane
0.007
0.711
0.981
Pure component isotherms results correlated with BET equation adsorbate
P* (kPa)
qm
KB
C.Coef.
methane
215
2.582
8836
-1.81
ethane
208
4.564
8710
-1.07
propane
205
0.106
15994E4
-0.20
Table 2, Multi component adsorption isotherm results for (3.81%CH4, 7.92%C2H6, 88.27%C3H8) mixture
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Multi component isotherm results correlated with extended Freundlich equation adsorbate
Ki
ni
Ki1
Ki2
Ki3
ni1
ni2
ni3
C.Coef
methane
0.098
0.499
1
0.002
5.16
0.738
4.52
0.044
0.999
ethane
0.650
0.291
14.08
1
0.05
0.209
0.69
3.187
0.999
propane
0.007
0.710
-0.122
-0.224
1
1.700
-1.57
0.720
0.994
Multi component isotherm results correlated with modified extended Langmuir equation adsorbate
qmi
Bi1
Bi2
Bi3
C.Coef.
methane
1.296
0.013
-0.037
0.086
0.998
ethane
2.548
0.315
0.022
-0.089
0.994
propane
0.342
0.916
-0.503
0.060
0.911
Multi component isotherm results correlated with extended Langmuir equation adsorbate
qmi
B1
B2
B3
C.Coef.
methane
1.297
0.025
0.091
0.011
-0.675
ethane
2.549
0.025
0.091
0.011
-578.6
propane
0.343
0.025
0.091
0.011
0.231
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