Measurement of Monobranched Alkane Mobility in the Silicalite

Mar 4, 2008 - ... molecules in the silicalite network reduces greatly the apparent diffusivity of monobranched isomers, questioning the possibility of...
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Ind. Eng. Chem. Res. 2008, 47, 2386-2390

Measurement of Monobranched Alkane Mobility in the Silicalite Framework in the Presence of Dibranched and Linear Molecules A.-C. Dubreuil, E. Jolimaitre,* M. Tayakout-Fayolle, and A. Me´ thivier Institut Franc¸ ais du Pe´ trole (IFP-Lyon), BP 3, 69390 Vernaison, France

This paper presents an experimental method for evaluating the apparent diffusivity of monobranched alkane isomers in mixtures with different molecules, based on the classical inverse chromatography technique and on moments theory. This method, which does not necessitate any complicated model or parameter estimation procedure, has been applied to the study of C6 and C7 alkanes diffusion in silicalite at 185 °C in the liquid phase. Experimental results show that the presence of dibranched molecules in the silicalite network reduces greatly the apparent diffusivity of monobranched isomers, questioning the possibility of separation. However, further adding a fast-diffusing species such as n-heptane in the adsorbent network can compensate for this effect. Although the new method proposed in this paper only gives access to an apparent diffusion coefficient (and not to the self-diffusion coefficient), it is a useful tool for a rapid evaluation of a given adsorbent diffusion selectivity. 1. Introduction Separation of hexane isomers is of great interest for fuel octane number enhancement, since double-branched alkanes have a much higher octane number than their monobranched and normal isomers. One of the most promising methods for this separation is the use of silicalite as a diffusion-based selective adsorbent. Several studies have shown that, in this type of adsorbent, the dibranched isomer pure-component diffusivities are inferior by at least 1 order of magnitude to those of the other isomers.1-3 However, it has also been shown that the presence of slow-diffusing species could slow the diffusion of faster-diffusing molecules (see, for example, the experimental results of Schuring et al.4 or the simulations of Gupta and Snurr5 and Simon6). This phenomenon is amplified at high adsorption loading, that is, industrial operating conditions, and could yield a nonnegligible loss of selectivity in real separation conditions, compared to what is predicted from pure-component data. A better knowledge of the mixture diffusion behavior of these molecules at high adsorption loading is therefore greatly needed. Direct experimental measurements of diffusion coefficients for mixtures at high loading is very complicated and is scarcely found in the literature.4 Another solution is to perform separation experiments with isomer mixtures (for example, breakthrough experiments7,8 or permeation experiments9,10) and to simulate the experimental results using pure-component diffusion coefficients obtained from pure-component experiments. Unfortunately, this procedure is quite long and complicated. Furthermore, the simulated results always depend on the model’s initial hypothesis (such as the form of the thermodynamic equilibrium and the diffusion driving force assumption). In this article, a new method is proposed that enables estimation of the apparent diffusivities of monobranched isomers in the presence of dibranched and normal isomers for different adsorbed phase concentrations. This method, based on the very well-known “moments theory”, is simple to use and does not require any complicated model. Its main drawback is that it only gives access to a mean apparent diffusivity at a given experimental condition, this diffusivity being the same for the two monobranched isomers. Diffusivities of C6 monobranched * To whom correspondence should be addressed. E-mail: [email protected].

alkanes (2-methylpentane (2MP) and 3-methylpentane (3MP)) in the presence of 2,3-dimethylbutane (23DMB), 2,2-dimethylbutane (22DMB), and n-heptane (nC7) were studied in silicalite at 185 °C and at high concentrations. 2. Methodology The method presented in this paper is based on the assumption that the two C6 monobranched isomers (2-methylpentane and 3-methylpentane) have very similar adsorption behaviors in silicalite: their co-adsorption isotherm is linear, their adsorption enthalpies are similar, and their diffusion coefficients are very close. In that case, (a) the mixture diffusion of the two isomers can be assimilated to pure-component diffusion, as is commonly done with radioactive isotopes, and (b) it is therefore possible to use the classical inverse chromatography method coupled with the moments theory to estimate the apparent diffusivity of this pseudounique species. 2.1. Exchange Breakthrough Curves in Liquid Phase. Suppose a column filled with silicalite pellets. The column is initially at equilibrium with one of the isomers in the liquid phase, at a given temperature and pressure. At time t ) 0, a given liquid flow rate of the other isomer is injected into the column. The concentration of the two isomers versus time is measured at the oulet of the column. The resulting breakthrough curve can be interpreted using the very well-known moments theory:12

µ1 )

(

)

1 - i L 1+ K Vi i

σ2 ) 2µ12 DL Vi i i 1 + 1+ LVi L 1 - i 1 - i K

(

)

{

-2

RP2 + 15pDp

K ) p + (1 - p)Kc

(1)

rc2 15Dc

K2 K - p

}

(2)

(3)

Kc is the slope of the coadsorption isotherm of the two isomers, and Dc is the apparent diffusion coefficient of the

10.1021/ie071539+ CCC: $40.75 © 2008 American Chemical Society Published on Web 03/04/2008

Ind. Eng. Chem. Res., Vol. 47, No. 7, 2008 2387

isomers at the given experimental conditions. The significance of the other variables is the classical one and can be found in the Notation section of this paper. Once more, it is very important to note that these equations are only valid if the following hypotheses are respected: (a) The coadsorption isotherm is linear in the whole concentration range (as will be confirmed by the experimental results presented later). (b) The diffusion coefficient is the same for both isomers in the whole concentration range. (c) No temperature variation occurs in the column (the isomers have the same adsorption enthalpy; see, for example, the results of Denayer et al.11). (d) The interstitial velocity does not vary during the experiment (the isomers have the same liquid density). For each breakthrough curve, Kc can be estimated using the experimental first moment and eqs 1 and 3. Dc can then be calculated from the experimental second moments of the curves (if the other mass transfer resistances have been determined independently, as will be described later). Remark: Another way to express the mass transfer resistance in the column is to use the plate theory of chromatography, with the height equivalent to a theoretical plate defined as12

σ2 L µ12

HETP )

(4)

The theoretical expression of the HETP as a function of mass transfer coefficients can be calculated from eqs 2 and 4:

HETP ) 2Vi

2DL + Vi

(

i 1 i 1+ 1 - i 1 - i K

)

-2

{

RP2 + 15pDp

}

rc2 K2 15Dc K - p

(5)

For a nonadsorbable species, eq 5 becomes

HETP(nonadsorbable) ) i 1 i 2DL + 2Vi 1+ Vi 1 - i 1 - i p

(

)( ) -2

RP2 (6) 15pDp

2.2. Multicomponent Breakthrough Curves. The same kind of experiments can be conducted with mixtures. The column is initially at equilibrium with a liquid charge containing a known composition of monobranched and dibranched isomers. At time t ) 0, a second mixture of exactly the same composition is injected into the column, except that the monobranched isomer has been changed (i.e., 3-methylpentane has been replaced by 2-methylpentane). Since the two monobranched isomers have very close thermodynamic adsorption properties, only the adsorbed phase concentration of the monobranched molecules changes during the experiment. The adsorbed phase concentration of all the other species is kept constant. The monobranched isomer breakthrough curve can then be exploited using the same procedure as for the pure-component breakthrough curves (cf. section 2.1). The resulting parameter Dc will then be the apparent diffusion coefficient of the monobranched isomers for the given experimental conditions (that is, for a given adsorbed phase concentration of all the other species). The breakthrough curve for run 5 (see Table 2 for the corresponding experimental conditions) is depicted in Figure 1, as an illustrative example. It can be seen that the 23DMB

Figure 1. Breakthrough curves for run 5 for 23DMB (9), 3MP (×), and 2MP (4); Vi ) 0.85 cm s-1.

outlet concentration is stable during the whole run, except for a slight perturbation corresponding to the breakthrough time of the monobranched isomers. This perturbation indicates that a very small amount of 23DMB is desorbed, due to a slight difference between the 2MP/23DMB and 3MP/22DMB coadsorption isotherms. Integrating this perturbation versus time shows that this amount represents less than 0.01 molecule/unit cell, and is therefore negligible. Such perturbations can be seen on several breakthrough curves, but the quantities adsorbed (or desorbed) are never significant, meaning the two isomers have very close coadsorption isotherms regarding 23DMB. Furthermore, the breakthrough curves of the two monobranched isomers are symmetric, showing that the two molecules have perfectly similar behaviors. 2.3. External Mass Transfer Estimation. As shown by eq 2, the second moment of the breakthrough curves depends on all the mass transfer resistances potentially present in the column (axial dispersion DL, macropore resistance Dp, and micropore resistance Dc). To evaluate the micropore diffusion coefficient Dc, all the other mass transfer resistances have to be estimated beforehand. To do so, we have used a nonadsorbable species, triisopropylbenzene (TiPB), whose molecular diameter (8.6 × 10-10 m) is too large to enter the silicalite porosity. DL was estimated from the variation of the second moment of the TiPB breakthrough curve versus fluid velocity. Dp was estimated from TiPB’s second moment. It is assumed that these coefficients are the same for all species. 2.4. Adsorbed Phase Concentration Estimation. To estimate the apparent diffusion coefficients of the monobranched isomers, 2MP/3MP exchange breakthrough curves are carried out. As explained before, the adsorbed phase concentrations of the other species do not varysand therefore cannot be estimateds during these experiments. Consequently, the adsorbed phase concentrations of each species have to be evaluated independently. To do so, breakthrough curves of the mixtures to be studied were measured prior to each experiment using pure 2-methylpentane as solvent. The adsorbed quantities of the different isomers were then calculated from the first moments of the different species. 3. Experimental Section 3.1. Adsorbent Characterization. The silicalite crystals used in this study were supplied by Zeolyst International. The Si/Al ratio, measured by X-ray fluorescence, is 500 ( 50%. The few H+ cations initially present in the zeolite framework were replaced by Na+ cations using the conventional ion exchange technique and subsequent washing. Scanning electron micros-

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Ind. Eng. Chem. Res., Vol. 47, No. 7, 2008 Table 1. Physical and Geometric Characteristics of the Adsorbent and the Column Silicalite Crystals

Figure 2. Schematic view of the testing device.

mean radius, rc Si/Al ratio Dubinin volume crystallinity rate structural density

0.75 × 10-6 m 500 ( 50% 0.141 mL/g 96% 1.79 g/mL

Silicalite Pellets morphology extrudate diameter extrudate length mean equivalent diameter, RP binder ratio pellet density

7 × 10-4 m (7-8) × 10-4 m 5 × 10-4 m 20% 1.29 g/mL

Adsorbent Column length diameter section mass of adsorbent (after activation) interstitial porosity, i macroporous porosity, p

1m 7.5 × 10-3 m 0.4415 × 10-4 m2 37.13 g 0.355 0.28

Table 2. Experimental Conditions for the Different Runs feed composition (wt %)

Figure 3. HETP versus velocity for run 1 (9, monobranched isomers; 4, TiPB).

copy showed that the crystals are spherical with a mean crystal radius of rc ) 0.75 × 10-6 m. This zeolite was pelletized with a silica binder at the Institut Franc¸ ais du Pe´trole, and then cut and sieved. Resulting pellets were small cylinders with a diameter of 7 × 10-4 m and a mean length of 8 × 10-4 m. For model simplification, the particles were supposed to be spherical with a mean radius of RP ) 0.5 × 10-3 m. The binder ratio, determined from n-hexane adsorption gravimetric uptake experiments performed with both crystals and pellets, is 20%. The pellet porosity was determined by mercury porosimetry. Prior to experiments, the sample was activated in a nitrogen stream for 3 h at 300 °C. 3.2. Experimental Setup and Procedure. The dynamic adsorption unit is described in Figure 2. A stainless steel column (L ) 1 m and S ) 0.4415 × 10-4 m2) is packed with a known mass of pellets and placed into the oven. First the column is filled with the so-called solvent, containing the first monobranched isomer. Then the feed, containing the second monobranched isomer, is injected into the column, until complete breakthrough (outlet concentration ) inlet concentration). For a given feed composition, different feed flow rates were tested (from 3 to 15 mL/min) depending on each feed composition. All the experiments were performed at 185 °C and 35 bar (to maintain the fluid in the liquid phase). Liquid fractions of the effluent were collected and analyzed by a gas chromatograph with an FID detector and a PONA analytical column. 2-Methylpentane (2MP) and 3-methylpentane (3MP) were purchased from Fluka Chemika, 2,3-dimethylbutane (23DMB) and 2,2-dimethylbutane (22DMB) were purchased from SAFC, and n-heptane (nC7) was purchased from SDS. The specified purities were over 99%. Triisopropylbenzene (TiPB) was purchased from Acros Organics with a specified purity over 97%. All these adsorbates were used without further purification.

run

3MP/2MP

TiPB

1 2 3 4 5 6 7 8 9

91.9 84.9 70 50 35.8 20.6 50.1 20.1 29.7

8.1

22DMB

23DMB

nC7

Vi (m/s)

Dc (m2 s-1)

10.7

0.7-1.4 0.4-1.4 0.7-2.2 0.5-1.8 0.7-1.1 0.4-0.8 1.4 1.4 0.7-1.4

3.2 × 10-15 2.6 × 10-15 2.2 × 10-15 1.2 × 10-15 6.7 × 10-16 2.7 × 10-16 1.2 × 10-15 4.0 × 10-16 4.1 × 10-15

15.1 30 50 64.2 79.4 49.9 79.9 59.6

All the physical and geometric characteristics of the adsorbent and the column are summarized in Table 1. 4. Results and Discussion Several mixtures of monobranched and dibranched C6 isomers have been studied. For each mixture, breakthrough experiments have been run at different flow rates in the range from 3 to 15 mL/min. All these runs are summarized in Table 2, along with the values of the apparent diffusion coefficients of the monobranched isomers estimated for each run. A typical breakthrough curve is given in Figure 1. 4.1. Macropore Resistance and Axial Dispersion Estimation. HETP values as a function of interstitial velocity for TiPB and for pure monobranched isomers (run 1) are depicted in Figure 3. DL and Dp were estimated by comparing this curve with eq 6 for the nonadsorbable TiPB (DL was estimated using the curve ordinate at V ) 0 and Dp using the slope of the curve). Different conclusions can be drawn from these curves: (a) The axial dispersion contribution to the total HETP is very small: the curves are linear in our velocity range and tend toward an ordinate of around 3 × 10-3 m for small velocities. For liquid flowing through a packed bed, axial dispersion is mainly due to turbulent mixing. In that case, the axial dispersion coefficient is12 DL ≈ RPVi, and the axial dispersion contribution to the HETP can be calculated using eq 5, giving HETP ) 2DL/ Vi ≈ 2RP. For our pellets, this formula gives a value of 8 × 10-4 m. The axial dispersion contribution is therefore greater than the theoretical value. This discrepancy might be due to turbulent mixing in the experimental setup downstream or upstream the column. It is difficult to conclude because the axial dispersion contribution is so small that the HETP values are within the experimental incertitude (as can be seen in Figure 3).

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Figure 4. Influence of dibranched molecule concentration on monobranched isomer apparent diffusivity for 23DMB (9), 22DMB (0), and 23DMB + nC7 (4).

(b) Both macropore and micropore diffusion contribute to the total mass transfer resistance: the HETP for TiPB is nonnegligible but is still smaller than the one of the monobranched isomers. Dp was estimated from the slope of the TiPB curve using eq 6: Dp ) (3 ( 1) × 10-9 m2‚s-1. Assuming that diffusion in the macropores takes place by molecular diffusion (Dp ) Dm/τ), and using the Wilke-Chang correlation13 to calculate the molecular diffusion coefficient (Dm ) 1.5 × 10-8m2‚s-1), the macroporous tortuosity can be estimated: τ ) 4.5 ( 1.5. This tortuosity factor is quite consistent with the values that can be found in the literature.14-16 The estimated value of Dp is therefore realistic and was kept for the whole study. 4.2. Monobranched Isomer Diffusivity versus Dibranched Adsorbed Phase Concentration. The apparent diffusion coefficients of the monobranched isomers were estimated from multicomponent breakthrough curves (runs 1-9) using eq 5, as explained in the Methodology section. Axial dispersion was neglected and the macropore diffusion coefficient was fixed as specified above. The results can be seen in Figure 4. The monobranched isomer apparent diffusivity decreases as the dibranched isomer concentration in the zeolite network increases. 2MP normalized breakthrough curves for different runs (but for the same interstitial velocity) are shown in Figure 5. It appears clearly that, the higher the 23DMB concentration in the feed, the more dispersed are the curves. This “tailing” of the breakthrough curves is characteristic of a growing difficulty for 2MP to diffuse in the zeolite framework. Also visible in Figure 5 is the difference between the two dibranched isomers: for the same concentration in the feed, 22DMB induces a greater dispersion of the 2MP breakthrough curves than 23DMB. The HETP values corresponding to run 8 (79.9% of 22DMB) are on the order of 10-1 m. Hence, for these experiments, the column represents only 10 theoretical plates (total column length is 1 m). This is usually defined as the minimal number of theoretical plates to be able to accurately evaluate the mass transfer resistance. Moreover, the experimental points are much more scattered than for the other curves. For a concentration of 1 molecule/unit cell, the monobranched isomer apparent diffusivity is divided by 4 with 23DMB and by 10 with 22DMB. This is in agreement with literature purecomponent diffusion data, which show that in silicalite the 22DMB diffusivity is much smaller than the one of 23DMB.1 For 23DMB adsorbed phase concentrations above 1.2 molecules/

Figure 5. 2MP normalized breakthrough curve for run 1 (0% 23DMB) (4), run 2 (15.1% 23DMB) (9), run 6 (79.4% 23DMB) (0), and run 8 (79.9% 22DMB) ([); Vi ) 1.7 cm s-1.

unit cell, the monobranched isomer apparent diffusivity seems to drop even faster, but it is difficult to conclude with only one experimental point. This result questions the feasibility of the separation processs at least under our experimental conditions. To reach a final product with a high purity in dibranched isomers, the separation has to take place even as these molecules are highly concentrated in the liquid phase. Our results show that the separation selectivity, that is, the diffusivity ratio between dibranched and monobranched molecules, drops as the feed is concentrated in dibranched isomers. The monobranched isomers have more and more difficulties entering the zeolite network, and can therefore no longer be separated from the dibranched isomers. It is important to note that since monobranched molecules are more favorably adsorbed in silicalite, dibranched isomer adsorbed phase concentrations never reach very high values in our experiments. As an example, the liquid phase corresponding to 1.5 molecules/unit cell of 23DMB contains over 80 wt % 23DMB (run 6). 4.3. Influence of nC7 Adsorption. In order to test the influence of a third fast-diffusing species, n-heptane was added in the feed (it is well-known that the diffusivity of normal alkanes in silicalite is higher than the one of monobranched alkanes1,17). Run 9 was performed with 10 wt % nC7 and 60 wt % 23DMB in the feed, corresponding to respectively 1.5 and 0.9 molecules/unit cell in the adsorbed phase (nC7 is much more favorably adsorbed in silicalite than 23DMB). As in the previous experiments, only the monobranched isomers are exchanged during run 9. As can be seen in Figure 4, the resulting monobranched isomer apparent diffusion coefficient is equivalent to the pure monobranched isomer diffusivity. The influence of the slow-diffusing species on the behavior of the monobranched molecules that was pointed out previously is no longer visible, even though the dibranched concentration in the adsorbed phase is equivalent. It seems that the effect of the slowdiffusing dibranched molecules is compensated by the presence of the fast-diffusing nC7. The monobranched isomer apparent diffusion can be slowed down by a dibranched isomer or accelerated by a normal alkane. As a consequence, a separation process is conceivable if an additional fast-diffusing species is present in the adsorption bedsconsidering of course that the dibranched isomer diffusivity is unchanged. If a SMB (simulated moving bed) process is chosen, this species may be the desorbent.

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5. Conclusion Apparent diffusion coefficients of monobranched C6 alkanes have been measured in silicalite at 185 °C, in the presence of dibranched isomers and n-heptane. The measurements were made according to a new, very simple to implement procedure based on classical inverse chromatography and on the moments theory. It has been shown that the mobility of the monobranched isomers is greatly reduced as slow-diffusing dibranched molecules are present in the solid network. The effect of 22DMB is even greater than the one of 23DMB, in agreement with previous literature results. Further adding fast-diffusing nC7 enables regaining the pure-component mobility of the original monobranched isomers. Notation Dc ) apparent diffusion coefficient in zeolite micropores (m2 s-1) DL ) axial dispersion coefficient (m2 s-1) Dm ) molecular diffusion coefficient in the liquid phase (m2 s-1) Dp ) macroporous diffusion coefficient in the binder (m2 s-1) HETP ) height equivalent to a theoretical plate (m) L ) column length (m) Kc ) Henry’s thermodynamic adsorption constant Vi ) intersitial velocity (m s-1) RP ) pellet mean radius (m) rc ) crystal mean radius (m) S ) column section (m2) Greek Synbols µ1 ) first moment of the breakthrough curves (s) σ2 ) variance of the breakthrough curves (s2) i ) interstitial porosity of the bed p ) pellet macroporous porosity τ ) macropore tortuosity Literature Cited (1) Cavalcante, C. L., Jr; Ruthven, D. M. Adsorption of branched and cyclic paraffins in silicalite. 2. Kinetics. Ind. Eng. Chem. Res. 1995, 34, 185.

(2) Jolimaitre, E.; Tayakout-Fayolle, M.; Jallut, C.; Ragil, K. Determination of mass transfer and thermodynamic properties of branched paraffins in silicalite by inverse chromatography technique. Ind. Eng. Chem. Res. 2001, 40, 914. (3) Zhu, W.; Kapteijn, F.; Moulijn, J. A. Diffusion of linear and branched C6 alkanes in silicalite-1 studied by the tapered element oscillating microbalance. Microporous Mesoporous Mater. 2001, 47, 157. (4) Schuring, D.; Koriabkina, A. O.; De Jong, A. M.; Smit, B.; Van Santen, R. A. Adsorption and diffusion of n-hexane/2-methylpentane mixtures in zeolite silicalite: Experiments and modeling. J. Phys. Chem. B 2001, 105, 7690. (5) Gupta, A.; Snurr, R. Q. A study of pore blockage in silicalite zeolite using free energy perturbation calculations, J. Phys. Chem. B 2005, 109, 1822. (6) Simon, J. M. Kinetics of Adsorption of Pure and Mixtures of Linear and Branched C6 Alkanes Onto Silicalite by Non-Equilibrium Molecular Dynamics. Proceedings of the AIChE Annual Meeting, San Francisco, CA, NoV 12-17, 2006. (7) Jolimaitre, E.; Ragil, K.; Tayakout-Fayolle, M.; Jallut, C. Separation of mono- and dibranched hydrocarbons on silicalite. AIChE J. 2002, 48, 1927. (8) Tayakout, M.; Jolimaitre, E.; Dubreuil, A. C.; Methivier, A. Characterization of Multicomponent Counter-Diffusion in Silicalite: Application to C6 Isomers in Liquid Phase. Proceedings of the AIChE Annual Meeting, San Francisco, CA, NoV 12-17, 2006. (9) Sommera, S.; Melin, T.; Falconer, J. L.; Noble, R. D. Transport of C6 isomers through ZSM-5 zeolite membranes. J. Membr. Sci. 2003, 224, 51. (10) Krishna, R.; Paschek, D. Permeation of hexane isomers across ZSM-5 zeolite membranes. Ind. Eng. Chem. Res. 2000, 39, 2618. (11) Denayer, J. F. M.; Baron, G. V.; Souverijns, W.; Martens, J. A.; Jacobs, P. A. High-temperature low-pressure adsorption of branched C5C8 alkanes on zeolites Beta, ZSM-5, ZSM-22, Zeolite Y, and Mordenite. J. Phys. Chem. B 1998, 102, 4588. (12) Ruthven, D. M. Principles of adsorption and adsorption processes; Wiley: New York, 1984. (13) Poling, B. E.; Prausnitz, J. M.; O’Connell, J. P. The properties of gases and liquids, 5th ed.; McGraw-Hill: New York, 2000. (14) Garcia-Ochoa, F.; Santos, A. Effective diffusivity under inert and reaction conditions. Chem. Eng. Sci. 1994, 49, 3091. (15) Hashimoto, K.; Smith, J. M. Macropore diffusion in molecular sieve pellets by chromatography. Ind. Eng. Chem. Fundam. 1973, 12, 353. (16) Wang, C. T.; Smith, J. M. Tortuosity factors for diffusion in catalyst pellets. AIChE J. 1983, 29, 132. (17) Xiao, J.; Wei, J. Diffusion mechanism of hydrocarbons in zeolitess II. Analysis of experimental observations. Chem. Eng. Sci. 1992, 47, 1143.

ReceiVed for reView November 13, 2007 ReVised manuscript receiVed January 23, 2008 Accepted January 24, 2008 IE071539+