Adsorption Behavior of a Mixture of C13 Isomers onto a Fixed-Bed of

Publication Date (Web): June 5, 2009 ... whereas it passes through a maximum for the asymmetric ones (2- MC12, 3- MC12, 4-MC12), because the asymmetri...
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Ind. Eng. Chem. Res. 2009, 48, 7301–7307

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Adsorption Behavior of a Mixture of C13 Isomers onto a Fixed-Bed of H-ZSM-5 M. A. Uguina,* J. L. Sotelo, J. A. Delgado, and J. I. Go´mez-Cı´vicos Department of Chemical Engineering, Faculty of Chemistry, Complutense UniVersity of Madrid, 28040, Madrid, Spain

Multicomponent liquid-phase adsorption of C13 paraffins (mainly monomethyldodecanes and tridecane) onto a fixed bed of H-ZSM-5 (Si/Al ) 200) has been studied (428 K and 21 barg), obtaining complete breakthrough curves for all the solutes, in order to ensure that equilibrium between the liquid feed and the adsorbent was achieved. The concentration of adsorbates in the feed mixture has been varied by diluting a concentrated mixture of C13 paraffins with a nonadsorbing solvent (2,2,4-trimethylpentane) in different proportions. The relationship between the branching position of the isomer and its adsorption affinity has been studied. The following affinity order is obtained (estimated as adsorbed concentration/liquid concentration): 2-MC12 ≈ 3-MC12 < 4-MC12 < n-C13 < 5-MC12 < 6-MC12. This order can be attributed to the difference in length of the lateral chains of each isomer, considering that a CH3-CH-CH2 group is placed the pore intersections of H-ZSM-5 for the branched isomers, and a CH2-CH2 group for n-C13. For the more symmetric isomers (n-C13, 5-MC12, 6-MC12), the affinity decreases continuously as the concentration of the feed mixture is increased, whereas it passes through a maximum for the asymmetric ones (2- MC12, 3- MC12, 4-MC12), because the asymmetric isomers adsorb weakly if the concentration of the symmetric ones is not very high, but they are displaced from the adsorbed phase as the concentration of the feed mixture is increased. The practical implication of this result is that, if the recovery of both symmetric and asymmetric C13 isomers from kerosene with H-ZSM-5 is desired for the production of tensoactives, it is convenient to use a feed concentration where the displacement of asymmetric isomers is minimal. 1. Introduction In the petrochemical industry, new profitable possibilities for liquid, long-chain monomethyl-paraffins have recently arisen in the field of surfactants manufacture.1 The recovery of such paraffins from different oil fractions can be achieved with high purity by means of zeolite fixed beds operating in adsorption/ washing/desorption repeated cycles. On the grounds of the deep know-how of zeolite 5A for the separation of n-paraffins from kerosene (Molex process), which are used as raw material for the production of LABS (linear alkyl benzene sulfonate), S1 (a silicalite with MFI structure) has been proposed for the selective adsorption of monomethyl-branched paraffins in a process for MABS (modified alkyl benzene sulfonate) production.1 MABS exhibits similar surfactant properties to LABS maintaining a high biodegradability. As silicalite has a wider pore opening than zeolite 5A (about 1.2-1.5 Å), it can adsorb monomethyl branched-paraffins whereas zeolite 5A cannot, and thus the recovery of paraffins useful for LABS and MABS production is enhanced. However, the employment of silicalite for this application is protected by a patent. This problem could be circumvented by using a ZSM-5 zeolite, which has the same pore network as silicalite, but its chemical composition is different, as ZSM-5 zeolites contain aluminum in their structure. The study of multicomponent adsorption equilibrium of the feed mixture is the starting point for the design, scaling, and performance prediction of industrial units intended for this separation. Knowledge on multicomponent liquid adsorption from a fundamental point of view is still very limited, especially onto molecular sieves. The main reasons for this are, among others, the following: (i) Molecular interactions in the fluid can affect adsorption significantly, especially in liquid phase.2,3 In the case of adsorptives diluted in a nonadsorptive solvent (e.g., one * Corresponding author. Phone: +34 91 394 41 13. Fax: +34 91 394 41 14. E-mail address: [email protected].

rejected by molecular sieve effect), solvent could even inhibit adsorption when adsorptive-solvent interactions are stronger than adsorptive-adsorbent ones.4 (ii) The frequently found assumption that zeolites are perfectly rigid adsorbents should be taken with caution for adsorptives with a channel diameter to molecular size ratio near one.5 In some cases, bulky adsorbed molecules could deform the surrounding channel structure in some degree.6-9 Pore deformation induced by temperature should also be taken into account.10,11 (iii) The difficulty in determining experimentally and/or modeling multicomponent liquid adsorption on a fundamental basis.12-17 The study of the multicomponent adsorption in the liquid phase is usually tedious (comprising high equilibration contact times for batch techniques and/or complexity of operation and control of continuous-flow techniques), and correspondingly, experimental data for these systems are scarce, especially for mixtures including more than two adsorptives.18 The extensively studied thermodynamics of vapor/liquid systems do not apply to solid/(adsorbed) liquid equilibrium.15 Moreover, in the case of zeolites, the adsorbed phase may deviate significantly from ideality even for low adsorbate occupancies, because entropic effects and/or lateral interactions are far from negligible.16,17 The Ideal Adsorbed Solution Theory (IAST) approach was successfully applied to multicomponent adsorption onto several zeolitic structures.12,14,19 However, this approach should be still taken with caution when it is applied to adsorption of paraffins on zeolites and zeotypes. Configurational Bias Monte Carlo (CBMC) simulation has recently been demonstrated to be a powerful tool for predicting multicomponent paraffin adsorption equilibrium onto MFI structure. Nevertheless, this model is well developed only for silicalite-1. Significant difficulties also arise when considering the effect of extraframework cations, such as the uncertainty about the

10.1021/ie900419k CCC: $40.75  2009 American Chemical Society Published on Web 06/05/2009

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Figure 1. Pore matrix of ZSM-5 zeolites. The internal pore surface (only partially shown) has white color. (a) Frontal view. (b) Upper view. (c) Left view. The images were generated with the software available at the website of the International Zeolite Association (IZA-SC).

most probable location of Al atoms and the corresponding mobile cations20 which depends on the synthesis procedure as well as the Si/Al molar ratio.21,22 In industrial practice, it is common to use MFI zeolites with Al in its structure.23 Adsorption isotherms of several pure paraffins onto zeolitic adsorbents have been obtained in the literature.24-26 However, experimental data on the competitive adsorption of long paraffins (above C10) on these adsorbents are very scarce in the literature, especially in the liquid phase. Therefore, it is clear that the obtention of experimental data for these systems is an interesting subject from a fundamental point of view, because they are required for validating the predictions of the theoretical models proposed to explain the observed results. In a previous work,23 the adsorption of a mixture of C13 paraffins (where the main components are monomethyldodecanes and tridecane) onto several zeolitic adsorbents with different structures (BEA, MFI, AEL) and Si/Al ratios was studied. The objective of the previous work was to see the effect of pore size and Si/Al ratio on the adsorption of monomethyldodecanes and tridecane, when the pore size is wider than the one of zeolite 5A. It was observed that the MFI structure is the best one to adsorb monomethyldodecanes and tridecane, rejecting other paraffins with higher degree of branching, and that the Si/Al ratio in H-ZSM-5 zeolites has little effect on the adsorption of these components for Si/Al ratios above 40, so that these zeolites have the same performance as silicalite. The C13 mixture (obtained by catalytic isomerization of tridecane) was used as a simplified model mixture of kerosene, because the vast number of components in kerosene makes it difficult to analyze the results if real mixtures are employed. The previous study was carried out in a batch contactor at 298 K to reduce the experimentation time, as several batch experiments could be carried out simultaneously. However, the Molex process is carried out in fixed-bed equipment at higher temperatures (175

°C), as the viscosity of the feed mixture must be low enough to reduce the pressure drop along the beds. The objective of this work is to study the adsorption behavior of C13 paraffins (a similar mixture to the one studied in the previous work) onto H-ZSM-5 (Si/Al ) 200) in conditions nearer to those of the Molex process. In this case, a fixed-bed contactor has been employed, obtaining complete breakthrough curves for all the solutes, in order to ensure that equilibrium between the liquid feed and the adsorbent was achieved. The temperature of the experiments has been set to 155 °C, because the viscosity of the mixture is quite similar to that at 175 °C (according to simulated results), and the possibility of cracking reactions with aluminum sites (which was observed experimentally at 175 °C) is eliminated. It is important to note that the adsorption of monomethyldodecanes cannot be studied using pure compounds. No equilibrium data have been reported up to date for those components onto H-ZSM-5 (except for the ones presented in our previous work), and pure monomethyldodecanes are not commercially available at present. 2. Experimental Section Agglomerated H-ZSM-5 (Si/Al molar ratio ) 200, binder (pure Al2O3)/zeolite ratio ) 0.26 g/g) was supplied by Su¨dChemie as cylindrical pellets. After calcination for 6 h at 550 °C, they were crushed and screened. The pore matrix of this zeolite (Figure 1) is composed of straight channels that are intersecting with sinusoidal channels with a free pore diameter of 0.53 nm × 0.56 nm and 0.51 nm × 0.55 nm, respectively. The length of the sinusoidal channels and straight channels (between intersections) is 0.67 and 0.5 nm,27 respectively, and the diameter of the intersections (assuming an spherical shape) is about 0.9 nm.28 The pore size distribution of the meso- and macropore volume of the zeolite pellets is given in Figure 2, estimated by Hg porosimetry. The fraction with size between

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Table 1. Experimental Conditions for the Adsorption Runs C13 mixture/2,2,4 volumetric flow zeolite bed run TM-C5 proportion (g/g) rate (cm3/min) mass (g) EBCT (s) 1 2a 2b 2c 2d 2e 2f 2g 3 4 5

0.003

0.012

0.041 0.070 0.110

9.6 2.4 6.0 6.0 6.0 6.0 9.6 9.6 6.0 2.4 2.4

2.162 2.162 3.540 7.053 7.053 5.405 8.648 8.648 11.70 11.70 11.70

10 40 26 52 52 40 40 40 87 217 217

Table 2. Composition of the Studied C13 Mixture compound

Figure 2. Pore size distribution of the H-ZSM-5 zeolite pellets studied in this work estimated by Hg porosimetry.

0.294 and 1.19 mm was chosen for the fixed bed experiments in the setup shown in Figure 3. This setup consists of a fixed bed adsorption column (1) 1.6 m long, with an inner diameter of 4.9 mm, spirally arranged with an outer diameter of 35 mm. The flow rate from storage tanks (6A, 6B, or 6C) was controlled by a positive displacement pump (5). Temperatures at the bed inlet (7) and exit (8) were also measured. Pressure at the bed exit was measured (14) and set to 21-22 barg with a backpressure regulator (BPR; 15), to keep the liquid phase inside the bed at the specified bed temperature (155 °C). All the tubing, vessels, valves, and fittings were made from AISI 316 steel. Temperature was measured with K-type thermocouples. Several experiments were performed, changing the proportion of 2,2,4-trimethylpentane, flow rate and empty bed contact time (EBCT), as shown in Table 1. In a typical experiment, the bed was first regenerated by flowing helium for 16 h at 300 °C and afterward it was cooled to 80 °C. The desired steady flow rate, pressure, and temperature were achieved by circulating pure solvent (2,2,4-trimethylpentane), which does not adsorb onto MFI zeolites. After that, a

2-methyldodecane 3-methyldodecane 4-methyldodecane 5-methyldodecane 6-methyldodecane total monomethyldodecanes total dimethylundecanes total ethyldodecanes total trimethyldecanes total polymethyl tridecane total identified

% w/w 9.12 15.66 17.75 14.72 13.74 71.09 12.00 8.40 1.56 0.72 4.37 98.1

valve (12) was turned so that the adsorption feed (6A) was pumped into the bed, taking this moment as the initial time. Complete breakthrough curves were determined by taking liquid samples at the exit of the installation. Samples were analyzed by a Perkin-Elmer A/S gas chromatograph equipped with a flame ionization detector (FID; 17). Experiments were carried out with a mixture of C13 isomers diluted with 2,2,4-trimethylpentane in different proportions. The C13 mixture was supplied by PETRESA (Spain), and it was obtained by catalytic isomerization of tridecane. Its composition is shown in Table 2. A low proportion of 1,3,5-trimethylbenzene was also added as a nonadsorbible tracer (0.5% w/w), whose breakthrough curve served to calculate the average residence time (tRES) using the following equation:

Figure 3. Experimental setup: (1) adsorption column; (2, 3, 4) temperature controlled electrical heaters; (5) pump; (6A, 6B, 6C) storage tanks; (7) inlet temperature sensor; (8) bed outlet temperature sensor; (9, 10, 11, 12, 13) ball valves; (14) outlet pressure indicator; (15) B.P.R.; (16) rotameter; (17) gas chromatograph (autosampler).

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tRES ) tF -

∫

tF

0

[c/c0]tracer dt

(1)

where tF is the time at which the bed inlet and exit concentrations are considered equal. The adsorbed amount of each component (mads, g) was calculated from the mass balance expressed by mads ) FfQv

c0 [[t - tRES] 100 F

∫

tF

0

[c/c0] dt]

(2)

It is also important to note that equilibrium was achieved in all cases because the sampling period was long enough to reach the inlet concentration at the bed outlet for all the solutes. The total concentration of adsorbible components was kept below 12% to ensure that the assumption that the volumetric flow rate is constant in eq 2 is reasonably valid for all the experiments. Experimental adsorption capacity (q) was expressed as number of adsorbed molecules per zeolite unit cell (molec/u.c.). In a reproducibility test (runs 2a-g, where the amount of adsorbent in the bed was varied, fixing the composition of the feed mixture), a maximum deviation of (0.058 molec/u.c was obtained. These experiments were performed to check that the adsorption capacity at equilibrium does not depend on the contact time between the liquid and the adsorbent, which is confirmed by these results. The average relative errors in the estimation of the adsorbed concentrations of n-C13, 2-MC12, 3-MC12, 4-MC12, 5-MC12, and 6-MC12 were (4.1, 18, 26, 12, 4.7, and 3.9%, respectively.

Figure 4. Experimental breakthrough curves, run 2e. (a) Paraffin lumps. (b) Monomethyldodecanes.

3. Results and Discussion Figure 4a shows the breakthrough curves obtained for the trimethyldecanes, dimethylundecanes, ethylundecanes, and monomethyldodecanes lumps, and Figure 4b, the breakthrough curves corresponding to the five monomethyldodecanes and tridecane for run 2e. The rest of the runs exhibited similar behavior (Figures 5-7 show the breakthrough curves for runs 3, 4, and 5). As it can be seen in these figures, only monomethyldodecanes and tridecane were significantly adsorbed showing breakthrough times clearly larger than the one of the nonadsorbing tracer. To confirm the rejection of the trimethyldecanes, dimethylundecanes and ethylundecanes, a separate adsorption experiment with the conditions of run 2c was performed, which included adsorption followed by washing with 2,2,4-trimethylpentane for a period long enough to remove the adsorption feed retained in bed voids (≈1.5tRES). After that, a mixture of pentane/2,2,4-trimethylpentane 40/60% w/w was fed to the bed in order to desorb the paraffins adsorbed in the first step. Ethylundecanes and trimethyldecanes were completely purged during the washing step, indicating that these paraffins were excluded from the zeolite pores by molecular sieve effect. All the monomethyldodecanes presented breakthrough times higher than the one of the tracer, which means that they were significantly adsorbed. However, a clear displacement of 2-, 3-, and 4-methyldodecanes by the rest of the isomers resulted in lower adsorption capacities at equilibrium of the monomethylparaffins with the branch position near the extreme of the main C-chain. In run 2d (Figure 8), even 5-methyldodecane was displaced slightly by 6-methyldodecane, revealing a strong preference for this monomethylparaffin with the methyl branch nearest to the center of the main C-chain. The adsorbed concentration of each isomer obtained in the breakthrough experiments with different feed concentrations is given in Table 3.

Figure 5. Experimental breakthrough curves, run 3. (a) Paraffin lumps. (b) Monomethyldodecanes.

In order to evaluate the adsorbent affinity to each isomer, since their fluid concentrations are different, it is better to consider the adsorbed concentration/fluid concentration ratio instead of the adsorbed concentration. Results for the individual affinities as a function of fluid concentration are shown in Figure 9. From these results, the following affinity order is obtained for the different isomers: 2-MC12 ≈ 3-MC12 < 4-MC12 < n-C13

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Figure 8. Experimental breakthrough curves for monomethyldodecanes, run 2d. Table 3. Experimental Adsorption Capacities at Equilibrium liquid concentration

adsorbed concentrationa

c component

n-C13 Figure 6. Experimental breakthrough curves, run 4. (a) Paraffin lumps. (b) Monomethyldodecanes. 2-MC12

3-MC12

4-MC12

5-MC12

6-MC12

q

run

g/dm3

g/100 g

molecules/u.c.b

1 2a-ga 3 4 5 1 2a-g 3 4 5 1 2a-g 3 4 5 1 2a-g 3 4 5 1 2a-g 3 4 5 1 2a-g 3 4 5

0.091 0.362 1.20 1.96 2.99 0.189 0.754 2.50 4.09 6.23 0.324 1.29 4.28 7.00 10.7 0.369 1.47 4.87 7.97 12.2 0.304 1.22 4.04 6.61 10.1 0.284 1.14 3.77 6.16 9.38

0.763 0.877 0.832 1.01 0.893 0.023 0.311 0.063 0.054 0.121 0.001 0.354 0.058 0.001 0.159 0.129 0.684 0.483 0.601 0.331 1.64 2.01 2.08 1.91 1.90 3.28 3.96 4.42 5.25 5.60

0.239 0.275 0.261 0.318 0.280 0.007 0.097 0.020 0.017 0.038 0.0004 0.111 0.018 0.0004 0.050 0.040 0.214 0.151 0.188 0.104 0.515 0.630 0.651 0.600 0.596 1.03 1.24 1.39 1.65 1.76

a Runs 2a-g: averaged values. b Assuming a silicalite-like unit cell comprising 96 Si atoms and 192 O atoms.

Figure 7. Experimental breakthrough curves, run 5. (a) Paraffin lumps. (b) Monomethyldodecanes.

< 5-MC12 < 6-MC12. It is clear that the adsorption affinity increases with the degree of symmetry of the isomer. This result agrees with our previous results obtained in a batch contactor at lower temperature (298 K),23 which can be attributed to the fact that the most symmetric isomers fit better in the MFI structure, leaving a CH3-CH-CH2 group in the intersection between channels,14 whereas the remaining linear chains (two C5 chains for 6-MC12, C4 and C6 chains for 5-MC12) occupy pore lengths from to 0.38 to 0.63 nm, (considering that each

C-C bond of a linear paraffin occupies a pore length of 0.1268 nm17) smaller than the distance between intersections (Figure 1), which favors the attractive interaction with the channel pore walls because an adequate distance between the electronic cloud of the molecule and that of pore walls is allowed. According to the Lennard-Jones theory, if this distance is too short, the repulsion energy increases, and if it is too long, the attraction energy decreases. The lower adsorption affinity of n-C13 with respect to 5- and 6-MC12 can be attributed to the absence of methyl group in this molecule. Thus, although n-C13 can adsorb placing a CH2-CH2 group in an intersection, leaving C5 and C6 chains which fit into channels quite well, the lack of methyl branch group in the intersection reduces the interaction energy. The low affinities for the less symmetric isomers (2-MC12,

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concentration). This result suggests that the asymmetric isomers are compelled to adopt a high-energy conformation in the presence of the symmetric ones which leads to a relatively weak interaction with the internal zeolite surface, so that they are adsorbed only if the external driving force (liquid concentration) is high enough. As the concentrations of all of the isomers is increased, the symmetric isomers displace the asymmetric ones in all the adsorption sites, which results in the maximum capacities observed in Figure 10 for the asymmetric isomers. From these results, it is reasonable to predict that at high liquid concentrations of the feed mixture the asymmetric isomers are completely excluded from the zeolite matrix. 4. Conclusions

Figure 9. Experimental affinity at equilibrium calculated as q/c for each isomer (tridecane and monomethyldodecanes): T ) 428 K; P ) 21 barg.

Figure 10. Multicomponent adsorption equilibrium data for monomethyldodecanes and tridecane: T ) 428 K; P ) 21 barg. Lines are a guide to the eye.

3-MC12, 4-MC12) may be attributed to the fact that their lateral chains are too long to accommodate well in the channels. These molecules can adsorb placing a CH3-CH-CH2 group in the intersections, but the long lateral chains are compelled to protrude in an adjacent intersection. For the 4-MC12 isomer, the pore length occupied by the C7 lateral chain is 0.76 nm, which is longer that the length of channels (Figure 1). For 2and 3-MC12, it is clear that the lack of contact with channel pore walls is more pronounced. Hence, the attractive interaction is weaker, which results in an affinity lower than the one of n-C13. Another important difference between the symmetric and asymmetric isomers (from this point on, n-C13, 6-MC12, and 5-MC12 will be considered as symmetric isomers, and 2-MC12, 3-MC12, and 4-MC12, as asymmetric ones to simplify the discussion) is the shape of the experimental affinity-concentration curves shown in Figure 9. For the symmetric ones, the affinity decreases continuously as liquid concentration increases, whereas for the asymmetric ones it passes through a maximum value. This difference is also reflected on the shape of the adsorption isotherms depicted in Figure 10. For the asymmetric isomers, the initial part of the isotherm is unfavorable (the slope of the isotherm increases with the liquid

Multicomponent liquid-phase adsorption equilibrium of several C13 isomers (tridecane and monomethyldodecanes) onto a fixed bed of H-ZSM-5 has been analyzed. After evaluating the affinity of the adsorbent to each isomer as the adsorbed concentration/liquid concentration ratio, the following affinity order is obtained: 2-MC12 ≈ 3-MC12 < 4-MC12 < n-C13 < 5-MC12 < 6-MC12. The higher affinity toward 5-MC12 and 6-MC12 can be attributed to fact that they fit better in MFI structure than the others, leaving a CH3-CH-CH2 group in the intersection between channels, whereas the remaining linear chains are well accommodated inside the channels. For the less symmetric isomers (2-MC12, 3-MC12, 4-MC12), the lateral chains are not commensurate with the channels, which results in an affinity lower than that of n-C13. The dependence of affinity on liquid concentration is quite different for the symmetric isomers and the asymmetric ones. For the symmetric ones, the affinity decreases continuously as the liquid concentration increases, and for the asymmetric ones, it passes through a maximum value. This difference is also reflected on the shape of the adsorption isotherms, showing a maximum for the asymmetric isomers. This behavior can be attributed to the fact that the asymmetric isomers are adsorbed weakly if the adsorbed concentration of the symmetric isomers is not very high, but they are compelled to adopt high-energy conformations which reduce the interaction force with the zeolite internal surface. The displacement of the asymmetric isomers by the symmetric ones in the adsorbed phase becomes more important as the concentration of all the isomers in the feed mixture increases. The practical implication of this result is that, if the recovery of both symmetric and asymmetric C13 isomers from kerosene with H-ZSM-5 is desired for MABS production, it is convenient to use a feed concentration where the displacement of asymmetric isomers is minimal. Acknowledgment The financial support of Petroquı´mica Espan˜ola S.A. (PETRESA) is gratefully acknowledged. Nomenclature c ) solute concentration, g/dm3 c0 ) feed concentration, g/dm3 mads ) adsorbent mass, g q ) adsorbed concentration, molecules/u.c. Qv ) volumetric flow, cm3/min t ) time, s tF ) equilibration time in the fixed-bed installation, s tRES ) average residence time in the installation, s Ff ) liquid density, g/dm3

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ReceiVed for reView March 13, 2009 ReVised manuscript receiVed May 13, 2009 Accepted May 20, 2009 IE900419K