3 Study of Mixture Equilibria of Methane and Krypton on 5A Zeolite
Adsorption and Ion Exchange with Synthetic Zeolites Downloaded from pubs.acs.org by UNIV OF MISSOURI COLUMBIA on 08/05/18. For personal use only.
K. F. LOUGHLIN University of Petroleum and Minerals, P.O. Box 144, UPM Box 25, Dhahran, Saudi Arabia G. D. ROBERTS Department of Chemical Engineering, The University of New Brunswick, P.O. Box 4400, Fredricton, N.B., Canada, E3B 5L1
Mixture equilibria models, derived from pure component models, inherit the primary assumptions on which the original isotherm is based, augmented by further additions. Limitations applicable to the original pure component models also generally apply to the multicomponent models. For instance isotherms derived based on localized behaviour in the pure state have this same premise in the multicomponent state. Among the simple adsorption theories for both pure and multicomponent systems, the Langmuir models envisage localized behaviour, the B.E.T. isotherms multilayered behaviour, the Polyani theory the potential theory, and among isotherms specifically applicable to molecular sieves, the Ruthven isotherm models (1, 2) assume non-localized sorption within the cavities. A mixture equilibria model which is not based on any specific isotherm model does not have the limitations expressed in the derivation of such a model. The Ideal Adsorbed Solution Theory of Myers et a l . (3) is based on the equivalence of the spreading pressures and does not presuppose any isotherm model. In fact, in the original papers, Glessner and Myers (4) stipulated that the model should only be applied using raw equilibrium data to calculate the spreading pressures. This is a very restrictive covenant and does not permit the use of the model for predictive purposes other than that for which data are available. However various authors have applied the Ideal Adsorbed Solution Theory (IAST) model using isotherm models (5, 6) quite satisfactorily. The isotherm model of Schirmer et a l . (7.) for sorption i n molecular sieves i s based on s t a t i s t i c a l thermodynamics i n which the configuration integrals describing the sorption behaviour are extracted from the available data. The model does not presuppose any specific kind of sorption mechanism. The multicomponent form of this isotherm derived by Loughlin and Roberts (8) i s also not limited to any particular sorption mechanism, 0-8412-0582-5/80/47-135-055$05.00/0 © 1980 American Chemical Society
56
SYNTHETIC ZEOLITES
and may be used, analogously to IAST, to calculate the multicomponent equilibria for different sorption mechanisms. In this paper we report experimental and theoretical results on the sorption of methane and krypton on 5A zeolite. The sorption of methane i n the 5A cavity i s reported to be nonlocalized (9), whereas that of krypton i s localized at a cavity site and window site (10). The multicomponent form of the isotherm of Schirmer et a l . i s used to interpret the experimental data and to predict mixture equilibria at other concentrations. Theory In the model of Schirmer et a l . (T.) for sorption i n zeolites, the canonical partition function q(i) i s expressed as 4
g(i)X = (P/ffl)
1
^
(1)
L,conf
assuming the residual contributions of moments, rotations and internal motions are unchanged in moving from the gas to the sorbed phase. They replace the integral expressions for q^ f by f i n i t e sums as ' con
P/P qUU
1
.k
= (τ75^
)1
^
S. . Τ - Ε.. E
X
P
U
C
-
J
S T
^
]
}
(
2
)
where the S^* are constants representing the (T,V ) standard difference of entropy, and the E.. are energy constants for corresponding levels. The resulting isotherm i s 0
J
m
P/P
. k
S..T - E..
(3)
Cm 1 + Σ 1=1
P/P . k S.,T - E. ί ^ ) Σ expiiC ^ ^]} / o j=l 4
1
T
T
R T
For a cavity which i s energetically homogeneous the isotherm reduces to m ±
P/P i(
Ι
λ
T7T^
m
P/P
. }1
S.T - E. exp{i[
1
X R T
]> (U)
1 +
i=i
^
S.T - E. ^ " c - h s H ^
3.
LOUGHLIN AND ROBERTS
CH
k
57
& Kr Mixture Equilibria
The index m, representing the maximum sorption i n the cavity, may be derived from considerations of the z e o l i t i c cavity volume and molar volume of the sorbate, as ι < ν/β. However Schirmer et a l . (7.) state that a maximum of 6 terms has been used i n their work. The extension of this model to a multicomponent sorbate is straightforward, and presented elsewhere by the authors (8K The binary isotherm equation i s η
m
Ρ /Ρ Λ
,
Λ
S/Γ - Ε
4
Α ΟχΙ / Β o j _ _£ £-]} Σ Σ ΐ ί - ^ Γ (-fjf-y expUD RT j=o i=o ο ο N
A /
C
/ Ρ Γ
1
.
( ) 5
η m Ρ /Ρ . Ρ /Ρ . S,T - E« A o x i / Β' o j rprJ: £ Σ Σ (-^Γ expU[ RT -]} j=o i=o ο ο Λ
Ώ
Ί
1 7
E
£
=
7
[
i
E
A /
+
A£
+
^
(
V
7
)
and the summations are now carried out over a l l values satisfying the restriction i $ + j g &2 ^ 3 * ^ ^ on the basis of thermodynamic considerations. In our optimization —
i d
>
, ,
m
58
SYNTHETIC ZEOLITES
we i n i t i a l l y specified the energy vector components equal (E;L = Eg = E3 = EJJJ) and equivalent to the negative of the heat of sorption where known. Apparatus, Materials and Procedure The apparatus, which i s shown diagrammatically i n Figure 1, i s similar to that discussed i n the paper by Loughlin et a l . ( l l ) with two modifications. Chambers Ε and C are connected by a second path containing an impeller pump, which i s magnetically s t i r r e d , and the Barocel pressure transducer has been removed. A 100 cm Eberbach cathetometer, subdivided to 1 mm, and having a vernier capable of being read to 0.1 mm, i s used to read a manometer for pressure measurement. The remaining equipment i s the same as reported previously ( l l ) . The sorbent was Linde 5A powder and the sorbates were Matheson research grade methane (purity 99% + ) and Matheson research grade krypton (purity 99-9955 +)· The procedure adopted was similar to that reported earlier ( l l ) except that the mixture measurements were a l l performed at 97.36 kPa and the gas phase was continuously circulated between chambers Ε and C during runs to ensure uniformity of composition. The pressure was recorded using the manometer i n chamber A, opened to chamber C; to ensure uniformity of gas phase composition, the mercury reservoir R was period i c a l l y raised and lowered, exchanging the gas i n the top of chamber A with that i n chamber C. Results and Discussion Mixture equilibria isotherms were obtained for the methane-krypton Linde 5A system at 238,255 and 271K at a t o t a l pressure of 97.36 kPa. At each experimental point, the gas phase composition (Y^, Yg) and the number of moles of each component adsorbed were recorded. By reversing the order i n which the gases are introduced a very sensitive test of reproducibility i s provided. The experimental data are shown i n Figures 2a to 2f. The order of contacting, whether starting with krypton or methane, i s indicated by a 0, t or Χ, X· symbol. The experimental data shown are independent of the order of contacting, demonstrating the consistency of the data. The range of sorbate coverage i s from 15$ at the highest temperature (271K) to k0% at the lowest temperature (238K). For this system saturation coverage i s 12 molecules/cavity for either component. At 271 and 255K the t o t a l amount adsorbed increases continuously with increasing mole fraction of methane, whereas at 238K the t o t a l amount adsorbed passes through a maximum at 60% methane i n the adsorbed phase. This difference i s also reflected i n the X, Y diagram which i s symmetric at 271
LOUGHLIN AND ROBERTS
CHj & Kr Mixture Equilibria
Figure 1. Schematic of apparatus: A, calibrated variable-volume mercury burette; B reference volume; C, main chamber; D, mixing pump; E, adsorption chamber; F, reference chamber; G, constant temperature baths; H, mercury manometer; J, cold-cathode gauge; P, Pirani vacuum gauge; R, mercury reservoir t
Figure 2. Mixture equilibria diagrams for sorption of CH and Kr on linde 5A sieve at 730 torr; (Ο,Φ,χ,*) data of Roberts, ( ) theoretical curves calculated using Equation 5 k
60
SYNTHETIC ZEOLITES
and 255K but d i s t i n c t l y asymmetric at 238K (see Figures 2d, 2e and 2 f ) . It appears that at the lowest temperature no separation i s obtained above 60% sorbate concentration of methane i n the adsorbed phase. Roberts (l£0 indicates that each experimental point took 3 or k days to attain equilibrium i n this portion of Figure 2f, whereas for the other experimental data equilibrium was achieved i n under six hours. He postulates that during this time there may have been a zero d r i f t i n the thermistor for measuring composition. The system methane-krypton 5A was selected for study because previous pure component studies for each of these sorbates on Linde 5A zeolite indicate that the sorption mechanisms are significantly different. According to the experimental and theoretical studies of Ruthven and Loughlin (£), the sorption of methane i s nonlocalized within the 5A cavity, whereas the studies of Ruthven and Derrah (12) indicate that the sorption of krypton appears to be localized at either of two sites, the window sites capable of sustaining 3 molecules per cavity or the cavity site capable of sustaining 9 molecules per cavity. As both molecules have similar molecular volumes, and as the Henry constants are not too dissimilar, i t was anticipated that the effect of the localized non-localized behaviour would reveal some interesting facets of the sorption mechanisms. Unfortunately, most of the experimental data i s at a low concentration, due to apparatus limitations, being lower than 3 molecules/cavity at 271K, approximately 3 molecules/cavity at 255K, and only being greater than 3 molecules/cavity at 238K. The f i r s t two systems (271 and 255K) apparently show no effect of the interaction, whereas the latter system at 238K appears to show significant interaction, and this i s the only system where krypton w i l l apparently occupy both sites significantly. More data at a higher concentration are desirable to elucidate the effects of localized non-localized interaction. Pure component experimental data for sorption of methane and krypton on 5A zeolite at 238, 255, and 271K, and i n the pressure range of 0 to 97.36 kPa were also obtained during this work (shown i n Figures 3 and k). Further sorption data for methane on 5A zeolite (10_ 13, ), and for krypton on 5A zeolite (10, 15) are also plotted for other temperatures, a l l of which appear to be consistent. These experimental data were used to derive the energy and entropy parameters i n equation h for the isotherm model of Schirmer et a l . by a minimization of a sum of squares optimization procedure. The resulting optimized parameters and for sorption of methane and krypton on 5A zeolite are shown i n Figure 5 and are presented i n Table 1. The calculated energy parameters 22000 joules/mole for methane and - l6,725.0 joules/mole for krypton were independent of the amount adsorbed and agree with 9
CH
LOUGHLIN AND ROBERTS
k
& Kr Mixture Equilibria
PRESSURE (KPdJ
Figure 3. Equilibrium sorption data for CH on pure 5A zeolite; parameter is temperature; data of (x) Roberts, (O) Derrah, (O) Loughlin, (A) Lederman; ( ) theoretical curves calculated using Equation 4 and data in Table I k
PRESSURE (KPd)
5000 10000
Figure 4. Equilibrium sorption data for Kr on pure 5A zeolite; parameter is temperature; data of (x) Roberts, (O) Derrah, ( Q ot 271 K) Barrer et al.; ( ) theoretical curves calculated using Equation 4 and data in Table I
Figure 5. Variation of integral molar energy of sorption with coverage for (x) methane and (Θ) Kr on 5A zeolite? (ED data of Rolniak for CH ; ( ) theo retical curve for CH calculated using Equation 10 for Ruthven model k
0.2
