THE SEPARATION OF METHANE-ETHANE MIXTURES ON ACTIVATED CARBON
A.Tafazolia, Sh. Fatemia* , F. Jadidia a
Department of Chemical Engineering, University of Tehran,
[email protected] , *
[email protected],
[email protected]
Abstract
The single component isotherm adsorption experiments of methane and ethane was carried out on a kind of activated carbon at room temperature. The isotherms were well fitted on Langmuir equation. The ideal selectivity of C2H6/CH4 on activated carbon showed possibility of studied adsorbent for separation ethane from methane. The dynamic adsorption experiment of mixed gases was performed at room temperature and pressure in a fixed bed of activated carbon and breakthrough curve of each component was obtained for a feed containing gases 90% CH4 and 10% C2H6. The significant difference between the breakthrough times of each component confirmed the capacity of this kind activated carbon for effective separation of ethane from natural gas.
Key words: adsorption; activated carbon; natural gas; ethane
1-Introduction The purification of natural gas to the best use of methane as energy source is a relevant subject. Natural gas is approximately 80-99% methane, with the remainder being mostly other hydrocarbons (ethane, propane, butane, etc.) as well as some nitrogen, oxygen, water, CO2, sulfur and various contaminants. After purification of natral gas from CO2 and other contaminated, product of this process
is piped into homes for domestic heating and cooking purposes called domestic gas containing methane and even some ethane which caused to bad Impacts on economical issues. In this study we are going to remove ethane from natural gas after purification, with activated carbon in a small isothermal fixed bed. The ability of activated carbons to separate gases is demonstrated by the work of Jiintgen[1] who examined the various applications of carbonaceous adsorbents and concluded that their
microporous structure influenced the rates at which components are adsorbed and desorbed[2].This paper examines whether an activated carbon offers the properties required for separation of ethane-methane. 2-experimental The experiments consist of two stages; the first part of the experiments is focused to the equilibrium adsorption study of the pure gases; methane and ethane to derive the capacity, selectivity and isotherm equations on used activated carbon utilized in this study which carried out at ambient tempreture and various pressures (3, 5, 7, 10, 15, 20 bar). The adsorbed amount was measured by volumetric method at equilibrium condition. At the second stage, dynamic adsorption is considered in a packed bed of mentioned adsorbent at both adsorption and desorption modes for a mixture of the gases to obtain the breakthrough curves and breakthrough times of methane and ethane. 2-1 material The gases; methane (99.995% pure) and ethane (purity>99.5%), were used as the adsorbate, Helium (99.999% pure) as an inert gas for the dead volume corrections and as the carrier gas . The properties of activated carbon that used in this research illustrated in table 1. Table 1. properties of activated carbon adsorbent[]
Sp. Gr. Ave. pellet length Ave. pellet diameter
Bulk density Surface area
1.8-2.1 5.6 mm 2.1 mm 551 kg/m3 670 m2/gr
2.2. Apparatus and Procedure A general arrangement of the adsorption apparatus is shown in Fig.1 which was employed for adsorption and gas analysis In order to determine the adsorption capacity and selectivity of C2H6/CH4, the adsorbents were placed in the adsorption cell. After pretreatment of the adsorbent, the adsorption tests were carried out at different pressures, to obtain the equilibrium state and the isotherm curve.
Fig. 1- Schematic diagram of experimental apparatus: A: Sample gases (C2H6 or CH4); B: Carrier gas (Helium); C: Mixing vessel D: Pressure sensor; E: Vent; F: Pressure sensor; G: Reference gas vessel; H: Adsorption sell; I: Vacuum pump
3-result and discussion 3-1 Isotherms The single adsorption equilibrium isotherms of methane and ethane at ambient temperature(298K) are obtained from experimental results and presented in Fig. 2. Solid lines correspond to the Langmuir model while markers are due to the experiments. It is observed that this model can fits the experimental data sufficiently. The values of Langmuir parameters obtained by regression are listed in Table 1.
1.4
1.2
4.5 4
1
3.5 0.8 y/y0
q(mmol/gr)
3 2.5
0.6
2 1.5
0.4
CH4
CH4
1
C2H6
0.5
0.2
0
0 0
5
10
15
20
25
Fig 2. Adsorption isotherms of methane; ¡ and ethane; the curves are the L models plotted by their parameters
Table 2. Langmuir Adsorption Isotherm parameters for CH4 and C2H6 onto activated carbon Components
qm (mmol/gr)
k(1/bar)
R2
CH4
3.97
0.18
0.9967
C2H6
4.18
0.78
0.9983
298
Where qm and k are the constants of Langmuir equation[3]
q=
Kq m p 1 + Kp
0
20
40
60
80
100
120
140
160
180
tim e (m in)
P(bar)
T(k)
C2H6
L-model
(1)
3-2 breakthrough curves The breakthrough curves of methane and ethane are obtained and presented in Fig.3 as the ratio of outlet molar fraction over inlet composition.
Fig 3. Breakthrough curves of methane and ethane, on activated carbon at adsorption stage
The results show that in adsorption stage, C2H6 is more strongly adsorbed whereas methane is passed through the bulk phase at early times. Significant difference between the breakthrough times of methane and ethane shows that methane can completely exit until breakthrough time of ethane is achieved. 4-conclusion A kind of activated carbon was examined for separation of CH4 and C2H6 in a fixed bed column at static and dynamic conditions. The static experiments were used for equilibrium adsorption and calculating the isotherm models and its parameters. Dynamic adsorption was carried out for the mixed gases of 90% CH4 and 10% C2H6 (by volume) with a feed flow rate of 20 Nml/min over 5 g of activated carbon at atmospheric pressure and 298K and Beakthrough curves obtained. The results showed effluent of methane from the first time of beginning the experiment although ethane was appeared after 30 min in the outlet stream. The dynamic experiments showed good capability of ethane separation from natural gas.
Reference 1- H. Jiingten, Carbon 15, 273 (1977). 2- H. Jiingten, K. Knoblausch, II. Mauzner, H. J. Schroger andD. Zundorf. Proc. 4th (1974) Carbon and Graphite Conf. p.441. London (1976). 3- D. D. Do, "Adsorption Analysis: Equilibria and Kinetics", Imperial College Press, London, (1998).
