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Ind. Eng. Chem. Res. 2005, 44, 318-324
Conversion, Selectivity, and Kinetics of the Dehydration of 1-Pentanol to Di-n-Pentyl Ether Catalyzed by a Microporous Ion-Exchange Resin F. Cunill,* J. Tejero, C. Fite´ , M. Iborra, and J. F. Izquierdo Departament d’Enginyeria Quı´mica i Metal‚lu´ rgia, Universitat de Barcelona, c/n Martı´ i Franque` s 1, E-08028 Barcelona, Spain
Selectivity, yield, and kinetics of the liquid-phase dehydration of 1-pentanol to di-n-pentyl ether were studied experimentally over a commercial gel-type ion-exchange resin with 4% of crosslinking in the temperature range 120-150 °C at 1 MPa in a continuous stirred tank reactor. Selectivities to ether higher than 96% were obtained at all temperatures for space times greater than 5 g h mol-1. However, the obtained maximum yield is only 0.22 of 1. The best kinetic model stems from a mechanism in which 1-pentanol, adsorbed on one center, reacts with 1-pentanol from the liquid phase to give the ether adsorbed. The surface reaction is assumed to be the rate-limiting step, in which only one center takes part. Introduction Changes in diesel fuel properties can significantly affect car emissions and, therefore, provide benefits in meeting increasingly stringent diesel exhaust regulations. These regulations establish limits for unburned hydrocarbons, carbon monoxide, nitrogen oxides, particulates, and smoke. To reduce such emissions, reformulated fuels are expected to be characterized by higher cetane number, lower density, and lower aromatics, polyaromatics, and sulfur contents with respect to the present ones. Several options, such as selective blending, upgrading refining processes, and use of cetane improver additives, are possible from technical and economic standpoints.1,2 Using reformulated diesel fuels containing appropriate high-quality components may be a good chance to meet such goals. In comparison with short and branched ethers used in gasoline, which have a good octane number but a poor cetane number, ethers for diesel have to be linear with a relatively long chain.1-3 The use of oxygenates to produce cleaner burning diesel fuels is not new. Alcohols, ethers, acetals, esters, etc., are frequently presented in the patent literature as fuel extenders, cetane boosters, or low-temperature behavior improvers and smoke suppressants. However, comprehensive studies were performed some years ago.1-3 These studies observed that linear ethers with a chain of nine or more carbon atoms showed the best compromise between blending cetane number and blending cold flow properties, which are the most important parameters to assess the quality of a diesel fuel. Among the tested ethers, di-n-pentyl ether (DNPE) turned out to be one of the most promising because it is also very effective in reducing emissions4 and could be produced industrially from C4 feedstocks via n-pentyl alcohol, obtained in its turn by selective hydroformylation of linear butenes. Diesel specifications in the European countries are the most stringent in the world. They mandated that
until 2005 the allowed minimum cetane number was 51.5 Although the blending cetane number of DNPE2,3,4 is about 109, a preliminary estimation of the costs showed that the use of such ether is not competitive. At this moment, however, it cannot be forecasted what the minimum cetane number will be in the next decade. It is understood that an increase will take place for the next 5-year period, and in cases where the increase was important, products such as DNPE could have an important role for refiners. Furthermore, it is necessary to bear in mind the environmental benefits obtained by using ethers as a result of both emission reductions and aromatics dilution. As mentioned above, DNPE can be produced by bimolecular dehydration of 1-pentanol on acid catalysts. The main secondary reaction is the intramolecular dehydration to produce the respective alkene. In addition, the alcohol can react with the formed alkene to give branched ethers. A scheme of the reacting system is showed in Figure 1. Note the large number of possible byproducts formed according to the presence of alkenes. Moreover, in the reaction scheme the addition reactions of alcohols to alkenes present in the system to produce other ethers, as showed in Figure 1, has to be considered. Thus, if a high selectivity to DNPE is desired, it will be necessary to use a selective catalyst to minimize the production of alkenes. In a previous work6 performed in a batch reactor, we found that, on the basis of 1-pentanol conversion, selectivity to DNPE, and initial reaction rates at 150 °C, gel-type acidic resins, which swell moderately in the reaction medium, were the more suitable catalyst for the main reaction. The aim of this paper is to study the dehydration of 1-pentanol to DNPE in a continuous tank reactor using a gel-type resin at different temperatures and alcohol flow rates to determine selectivities and reaction rates. A kinetic model in terms of component activities for the commercial catalyst used is proposed. Experimental Section
* To whom correspendence should be addressed. Tel.: +34 93 402 1304. Fax: +34 93 402 1291. E-mail: cunill@ angel.qui.ub.es.
(i) Materials. 1-Pentanol (98.5% pure; water 97%), supplied by Fluka, and water were used for analysis. The catalyst was the commercial microporous ionexchange resin CT-224 from Purolite Co. (Bala Cynwyd, PA). It is an oversulfonated resin with an ion-exchange capacity determined by titration of 5.34 equiv of H+ (kg of dry resin)-1 and a cross-linking degree of about 4% divinylbenzene. The average bead size is 0.75 mm, and the surface area is 0.92 m2 g-1. Before use, the catalyst was dried at 378 K and under vacuum (0.1 mmHg) for 3 h and the residual content of water was less than 0.5 wt %. The recommended maximum operating temperature is 150 °C. (ii) Apparatus. The experiments were carried out in a stainless steel continuous 100 mL autoclave equipped with a magnetic stirrer and mixing baffles. The temperature was controlled to within (0.1 K by an electric furnace. An electronic back-pressure regulator (Bronkhorst Hi-Tec P-702C, Veenendaal, The Netherlands) kept the whole reactor system at 1 MPa with nitrogen, resulting in a completely liquefied reaction mixture. Nitrogen was used to impel the liquid feed into the reactor from a reservoir equipped with a suitable membrane, which separated the liquid from the gas. The flow rate of alcohol was controlled by an electronic mass flow device (Bronkhorst L2C2-FA-22-P). One of the outlets of the reactor was connected directly to a liquid sampling valve commanded by software. When activated, 0.2 µL of pressurized liquid was injected into a gas-liquid chromatograph (GLC). LabVIEW 5.0 software controlled and registered experimental flow rates, pressures, and temperatures. The experiments were finished automatically if the system performance deviated from programmed set points.
(iii) Analysis. The composition of the reacting mixture was analyzed by a HP6890A GLC apparatus equipped with a thermal conductivity detector, allowing measurement of the water content, and commanded by the ChemStation HP system controlled by the LabVIEW software. A 50 m × 0.2 mm × 0.5 µm methylsilicone capillary column was used to separate and determine 1-pentanol, DNPE, water, and byproducts: C5 alkenes (1-pentene, 2-pentene, and methylbutenes), branched ethers [di-2-pentyl ether, 1-(1-methylbutoxy)pentane, 1-(2-methylbutoxy)pentane, and 2-(2-methylbutoxy)pentane], 2-methyl-1-butanol, and 2-pentanol. The column was temperature-programmed with a 6-min initial hold at 318 K followed by a 30 K min-1 ramp up to 453 K and held for 5 min. Helium was used as the carrier gas at a total flow rate of 30 mL min-1. (iv) Procedure and Calculations. The necessary parameters for an experimental session were introduced to the computer program that controlled the reactor system. The temperature ranged from 120 to 150 °C, the 1-pentanol flow rate from 15 to 40 g h-1, and the amount of dried catalyst from 0.5 to 4 g. After the input procedure, the equipment was started to perform the experiments in a sequential way. Online analysis of the product effluent started after a period equal to 3 times the residence time of the reactor. After three repetitive chromatographic results, it was considered that the system reached the stationary state. If so, the next programmed experiment started. All experimental data were stored for subsequent calculations. For each experiment, the 1-pentanol conversion, selectivity to DNPE, DNPE yield, and reaction rate were computed. The overall fractional yield of DNPE with respect to 1-pentanol was selected as a measure of the selectivity to DNPE. It is defined as SDNPE ) molar flow rate of reacted 1-pentanol to form DNPE molar flow rate of reacted 1-pentanol
(1) Likewise, the selectivity to alkenes and branched ethers different from DNPE was defined in a similar way. The yield of DNPE with respect to 1-pentanol is defined as YDNPE ) molar flow rate of 1-pentanol reacted to form DNPE molar flow rate of fed 1-pentanol
(2) Taking into account that the agitation speed is high enough, we assume that the 1-pentanol reaction rate can be computed from the performance equation for an ideal continuous stirred tank reactor:
XP W ) FP -rP
(3)
To estimate the experimental error, an experiment was doubled at 150 °C. Water, DNPE, and 1-pentanol molar fractions, similar to the conversion of 1-pentanol and reaction rates, were accurate to within (4%. Fulfillment of material balances was also checked, and some runs that do not fulfill it were discarded. Results and Discussion (i) Selectivities and Yield. First of all, note that byproduct production was lower than that obtained in
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Figure 2. Selectivity to DNPE at assayed temperatures.
a previous study using a bath reactor.6 Figure 2 shows the selectivities as a function of the space time at different temperatures. No alcohols other than 1-pentanol and its impurity 2-methyl-1-butanol were detected for the range of investigated temperatures. Besides, 2-methyl-1-butanol hardly reacts at lower temperatures, and its concentration remains constant throughout the experiments. At 120 °C, neither alkenes nor branched ethers were detected, and as a result, selectivity toward DNPE was 1. Analyses at 130 °C showed no alkenes, but very small amounts of 1-(2-methylbutoxy)pentane, obtained by intermolecular dehydration between 1-pentanol and its impurity 2-methyl-1-butanol, were detected at the highest residence time. In these cases, selectivity for DNPE decreased about 0.01. At 140 °C, there are no alkenes present and the only detected byproduct was also 1-(2methylbutoxy)pentane, but now it was present for any space time. DNPE selectivity at low space time is about 0.96, and it increased quickly with space time. Finally, at 150 °C, besides 1-(2-methylbutoxy)pentane, other branched ethers such as 1-(1-methylbutoxy)pentane, 2-(1-methylbutoxy)pentane, and 2-(2-methylbutoxy)pentane appeared. The presence of these ethers could be explained by taking into account the complex network of reactions shown in Figure 1, in which 1-pentanol impurities, 2-methyl-1-butanol and water, are also involved. When intramolecular dehydration of 1-pentanol takes place, 1-pentene is produced; this can give 2-pentanol upon rehydration, which at its turn reacts quickly with itself or with 1-pentanol, forming the ethers previously mentioned. As the data suggest, selectivity to DNPE is quite high, with the majority of values between 0.98 and 1. Thus, we can conclude by considering also the values of selectivity obtained for high conversions in batch experiments,6 higher than 0.955, that the catalyst used is a good catalyst to prepare DNPE from 1-pentanol dehydration. The swelling of the used resin in a polar medium like in dehydrations could explain the higher selectivity to DNPE. The gel phase of the resins swells because of the interaction of the alcohol and water with the resin. In this way, in a flexible swollen state, the resin can accommodate the reaction intermediate without steric hindrance and ether formation is favored. The ether is formed by the nucleophilic attack of alcohol on the oxonium ion in a bimolecular reaction9 (SN2 type). Polymer chains are probably located in such a way that the precise concentration and orientation of active sites are appropriate to form DNPE reaction intermediates but not 1-pentene. The dehydration to pentenes occurs through a monomolecular reaction of elimination (E1 type). If the resin is not swollen, polymer chains are close, the SN2 reaction is limited to a great extent by
Figure 3. Experimental DNPE yield at different temperatures.
steric hindrance, and the occurrence of the E1 type increases. More details about the effect of the structure of the resin on selectivity to DNPE can be found elsewhere.6 Figure 3 shows the DNPE yield versus space time at different temperatures. The effects of these two variables can be seen. The DNPE yield increases upon an increase of either the residence time or temperature. However, it is worth noticing that the obtained maximum yield is only 0.228, which means that only 0.228/2 ) 0.114 mol of DNPE would be obtained from 1 mol of 1-pentanol fed to the reactor. For an industrial application, it would be necessary to increase both the space time and temperature without exceeding the maximum temperature for catalyst thermal stability. (ii) Kinetics Aspects. As mentioned earlier, runs were performed at 120-150 °C. Because at working temperatures thermal stability is a key point for long operation periods, the chance that experimental results were influenced by thermal deactivation was first assessed. It is known that, in aqueous liquid media, thermal deactivation can take place by means of two mechanisms: hydrolysis of sulfonic groups with H2SO4 release; and formation of sulfone bridges between adjacent polymer chains.8,9 For gel-type resins, such as CT-224 used in this work, it was found9 that their halflives were longer than 800 h at 120-150 °C, which indicates that thermal deactivation is a rather slow process in the aqueous liquid phase. Moreover, kinetic runs on CT-224 heated in a vacuum show that it is necessary to keep the reaction system at high temperature for a long period for the resins to become deactivated. To test the thermal deactivation of the resin, an experiment lasting 77 h at 170 °C was performed, and the loss of sulfonic groups was about 2%. All of these facts ensure that the thermal deactivation of CT-224 resin for short experiments lasting less than 10 h at 150 °C was negligible. Preliminary experiments were carried out to ensure that experimental data were free of external masstransfer effects.10 Because commercial bead sizes were used, the internal mass-transfer influence cannot be excluded. By considering that resins swell sizably in polar liquid media, accessibility of 1-pentanol to sulfonic groups would not be seriously hindered at the reaction conditions.11,12 Indeed, a previous work6 showed that for this reaction using different resins, including CT-224, all active sites are equally accessible. Besides, a rough assessment and discussion of the effect of the internal mass transfer was checked further when the apparent activation energy was estimated. So, henceforward, unless otherwise stated, we will assume that the measured reaction rates are the intrinsic ones.
Ind. Eng. Chem. Res., Vol. 44, No. 2, 2005 321
Prior to kinetic modeling, the deviation from the ideal behavior for the reacting system was assessed. Activity coefficients were estimated through the UNIFAC predictive method for all compounds at the explored temperature and concentration ranges. Because the activity coefficient of DNPE varies from 2 to 2.7 and that of water from 3.6 to 4.4, whereas the activity coefficient of 1-pentanol is very close to unity, the system can be considered to behave as clearly nonideal. In addition, a previous kinetic study on the dehydration of linear alcohols13 used activities instead of concentrations. Consequently, we decided to express the kinetic model in terms of activities. The kinetic model considered in this work is based on that most commonly proposed in the literature for dehydration reactions of alcohols catalyzed by ionexchange resins: the Langmuir-Hinshelwood-Hougen-Watson (LHHW) model and its derived form, the Eley-Rideal (ER) model. In these models, one of the parameters to be determined is the exponent of the adsorption term related to the number of active centers participating in the ratelimiting step. In kinetic studies, different values for this exponent can be found, ranging most commonly from 1 to 3, which means that one, two, or three active centers take part in the rate-limiting step according to the LHHW or ER formalism. What is not clear in the literature is how many sulfonic groups form an active center. In this context, Wesley and Gates14 reported that up to seven sulfonic groups could take part in benzene propylation. Ancilotti et al.15 pointed out a third-order dependence of the rate of the methyl tert-butyl ether synthesis reaction on the concentration of sulfonic groups for isobutene-methanol equimolar mixtures and a fourth-order one when isobutene is in excess. However, Rehfinger and Hoffmann16 reported a second-order dependence. These discrepancies could be interpreted if we consider a cluster of sulfonic groups as an active center in the LHHW kinetic model. The number of sulfonic groups that form a cluster would depend on the resin and on the reactants.18 Notwithstanding, in a previous work,6 we found out that even for low conversions all of the sulfonic groups are accessible and take part in the reaction, which means that the resin is swollen and probably the hydrogen bridges between sulfonic groups are broken, and in this case we can assume that only one sulfonic group forms a center. Under the consideration of the adsorption-reactiondesorption process, we can assume that the best kinetic model for the reaction could stem from the LHHW mechanism
P + σ T Pσ 2Pσ + (n - 2)σ T Dσ + Wσ + (n - 2)σ Dσ T D + σ Wσ T W + σ or from the analogue ER one, which differs only in the surface reaction step, in which one 1-pentanol molecule would react from the liquid phase with one adsorbed 1-Pentanol molecule:
Pσ + P + (n - 1)σ T Dσ + Wσ + (n - 2)σ
For both mechanisms, in the surface reaction step one or more additional active sites could take part. Assuming that the surface reaction is the rate-limiting step, which is well accepted for this type of catalysis, the LHHW formalism leads to the following basic kinetic model:
(
)
aDaW K r) (1 + KPaP + KDaD + KWaW)n kˆ KP2 aP2 -
(4)
In the case of the ER model, a very similar equation is obtained:
(
)
aDaW K r) (1 + KPaP + KDaD + KWaW)n kˆ KP aP2 -
(5)
The n exponent of the denominator of eqs 4 and 5 is the number of active sites that take part in the surface reaction step. The more plausible values 2 and 3 have been considered. However, models with n equal to unity have also been considered for ER models, and for LHHW models only from an empirical point of view. On the other hand, the experimental conversion level for 1-pentanol was typically kept lower than 15%, except for a few experiments at 150 °C and for high space time, in which a conversion of 20% was reached. Now then, because equilibrium preliminary experiments have shown that equilibrium conversions are greater than 80%, it can be assumed that the extent of reverse reaction is negligible and only some discrepancies at 150 °C could be expected in the kinetic model-fitting procedure. Consequently, eq 4 reduces to
rLHHW )
kˆ KP2aP2 (1 + KPaP + KDaD + KWaW)n
(6)
and eq 5 becomes
rER )
kˆ KPaP2 (1 + KPaP + KDaD + KWaW)n
(7)
To endorse the suitability of these models and to outline the form of the kinetic equation, a qualitative analysis of the effect of the compound activities aP and aD on the reaction rate was performed. It has to be kept in mind that in the present case activity values are interdependent because no diluents were added. A plot of that effect is presented in Figures 4 and 5, from which the following conclusions can be drawn: 1. aP enhances the reaction rate in the entire range explored and for all temperatures. This fact indicates that 1-pentanol would enlarge the driving force term, placed in the numerator of the kinetic equation, because it would promote the forward reaction. 2. aD appears to inhibit the reaction rate in the entire range explored. As for the lowest temperature, the conversion of the reaction is less than 3.6%, the effect of the reverse reaction is negligible, and the inhibitor effect of the ether could only be explained through their contribution to the denominator of the kinetic model.
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Ind. Eng. Chem. Res., Vol. 44, No. 2, 2005 Table 1. Kinetic Models Tested for Liquid-Phase Synthesis of DNPE, with n Values Ranging from 1 to 3 type
Figure 4. Effect of 1-pentanol activity on the reaction rate at experimental temperatures.
model class I
model class II
1
r ) AaP2-n
r)
2
aP2 r)A n aD
r)
3
aP2 r)A n aW
r)
4
r)
5
r)
6
r)
7
r)
AaP2
r)
(aP + BaD)n AaP2
r)
(aP + BaW)n AaP2
r)
n
(aD + BaW)
AaP2 (aP + BaD + CaW)
n
r)
k1aP2 (1 + KPaP)n k1aP2 (1 + KDaD)n k1aP2 (1 + KWaW)n k1aP2 (1 + KPaP + KDaD)n k1aP2 (1 + KPaP + KWaW)n k1aP2 (1 + KDaD + KWaW)n k1aP2 (1 + KPaP + KDaD + KWaW)n
ing that kinetic constants and adsorption equilibrium constants fulfill the following temperature dependence: Figure 5. Effect of DNPE activity on the reaction rate at experimental temperatures.
That is to say, its adsorption could not be negligible, and its value would be greater than that of 1-pentanol. As a consequence, it has been assumed that the best kinetic equation is derived from eq 6 or eq 7. On the basis of these equations, all possible derived kinetic equations have been considered and, for systematic purposes, grouped into two classes, depending on the number of unoccupied active centers: (i) Class I, for which the number of unoccupied active centers is considered to be negligible. This fact implies that the unity present in the denominator (the absorption term) can be removed. Now, the only possible values for n are 2 for eq 6 and 1 for eq 7. The last case represents an ER mechanism in which the ether or the water is adsorbed. (ii) Class II, where that hypothesis is rejected. In a second step, the adsorption terms are considered to be alternately negligible. The models obtained are shown in Table 1. For models of class I, the surface reaction kinetic constant, kˆ , and adsorption equilibrium constants, KP, KD, and KW, have been grouped into factors, called A, B, and C, for mathematical fitting purposes. The particular form in which constants are grouped depends on the mechanism (LHHW or ER) and the neglected adsorption term, if any. Concerning the models of class II, k1 is equal to kˆ KP2 for LHHW models and to kˆ KP for ER models. Kinetic models of Table 1 were fitted to the experimental data. The purpose was to find out the set of parameter values that minimizes the sum of squares of residuals between experimental and predicted data. The temperature dependence was considered by assum-
[ (T1 - T1h )] 1 1 K ) exp(d ) exp[-d ( - )] T T h kˆ ) exp(b1) exp -b2 i
1
2
(8) (9)
Fitted parameters were b’s and d’s. The exponential form of the first factor and the subtraction of the mean experimental temperature have been included in eqs 8 and 9 in order to obtain a lower correlation between fitted parameters. It is obvious that any combinations multiplication or divisionsof kinetic and adsorption equilibrium constants yields the same form of temperature dependence but other parameter values. From a mathematical point of view, the most suitable model is the one in which the minimum sum of squares, random residuals, and low parameter correlation is obtained with the minimum number of fitted parameters. On the other hand, these parameters should have a physicochemical meaning. For instance, at increasing temperature, the reaction rate constant should increase, and the adsorption equilibrium constant should decrease. In agreement with that, the activation energy should be positive and the adsorption enthalpies and entropies negative. Figure 6 shows the goodness of fit values for the different kinetic models, in terms of the sum of squares of residuals (SQ). In that plot, a value of unity in the y axis corresponds to the minimum of squares, i.e., the best fit. As can be seen, four models yield a minimum SQ value: class I, types 4 and 7, and class II, types 4 and 7, in all cases with n equal to unity. A closer look at those kinetic models leads to the discovery of some common features: (i) both 1-pentanol and DNPE contribute to the adsorption term and (ii) exponent n equals 1 in all cases. It stands out that any other kinetic model with exponent n > 1 shows a remarkable lack of fit with respect to the previous models. Therefore, in the fol-
Ind. Eng. Chem. Res., Vol. 44, No. 2, 2005 323 Table 2. Correlation Matrix of Estimated Parameters b1 b2 b3 b4
b1
b2
b3
b4
1 -0.134 0.919 -0.482
-0.134 1 0.019 0.814
0.919 0.0199 1 -0.474
-0.482 0.814 -0.474 1
0.95 (absolute value). The most important correlation corresponds to the pair b1 and b3. As a consequence, the proposed kinetic equation is
Figure 6. Comparison of the goodness of fit of the assayed kinetic models in terms of SQmin/SQ. The best fit corresponds to the maximum SQmin/SQ value, equal to unity.
AaP2 r) aP + BaD
(10)
with
[ (T1 - T1h )]
A ) exp(b1) exp -b2 and
[ (T1 - T1h )]
B ) exp(b3) exp -b4
Optimal values for the fitted parameters, with their respective standard error, are
A ) exp(-4.468 ( 0.023) ×
[
(
)]
(
)]
exp -(17597 ( 520) Figure 7. Correspondence between experimental and calculated reaction rates at all temperatures, according to the selected model (I-4, with n ) 1).
lowing discussion, only models with exponent n equal to unity will be considered. Figure 7 shows the plot of calculated vs experimental reaction rates for model I-4 with n ) 1, for all experiments. Similar plots are obtained for models I-7, II-4, and II-7, with n ) 1. Therefore, it can be stated that the goodness of fit is satisfactory. Further, discrimination proceeds with the examination of fitted parameters. Model I-4 has the lower number of parameters, namely, four, and as a consequence, it is beginning to look like the best one. Model I-7 has two additional parameters: the first four are practically identical with those of model I-4, and the additional two confer to the water adsorption summand a nonsignificant contribution to the adsorption term (about a 10-10% of the contribution). Concerning models II-4 and II-7, the convergence of the iterative process in the fit to the experimental data is unsatisfactory because of the very large cross correlation, and consequently large error, between the fitted parameters. Thus, with respect to the mathematical criteria, model I-4 with n ) 1 has been selected as the best kinetic model. One of the most important aspects in the fitting procedures, and frequently underestimated, is the determination of the error of parameters. A very large error could point toward an inappropriate model or a nonsignificant parameter that would have to be excluded from the kinetic equation. In this way, the error of parameters of eq 10 will be given below. When more than two parameters are involved, it is worth discussing the correlation for them. Table 2 shows the correlation matrix for the four estimated parameters. It can be deduced that correlation is not a serious problem in our case because all of the values of the matrix are less than
1 1 T (K) 408.15
and
B ) exp(1.718 ( 0.075) ×
[
exp -(6235 ( 975)
1 1 T (K) 408.15
where also the standard error of the parameters has been included. It is noticeable that, although water is very polar and tends to be adsorbed on the sulfonic groups, it does not appear in the adsorption term. The most plausible explanation is that water actually occupies a large amount of active centers, but practically constant, without a significant variation of its extent, in such a manner that different water activities do not affect significantly the number of active centers on which water is adsorbed. Hence, aW does not affect the adsorption term. From the parameter values, some physicochemical properties can be calculated. The temperature dependence of factor A in eq 10 gives the following true activation energy: 146 ( 4 kJ mol-1. This value is higher than that obtained in bath experiments (100 ( 8 kJ mol-1) using different resins,6 but it allows us to assume a negligible effect of the internal mass-transfer processes. Otherwise, the apparent activation energy would be much lower than 100 kJ mol-1, which is not the case, even if the large error of the activation energy is taken into account. Another factor that can back up the negligible effect of the internal mass transfer is the high porosity obtained in the swollen state for the gel resin6 CT-224 in the presence of water, 57%, compared with a practical null porosity in the dry state, which could lead to easy accessibility to the sulfonic groups of the gel phase of the resin without resistance. The parameter values of factor B in eq 10 are related to the adsorption enthalpies and entropies of 1-pentanol
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and DNPE as follows: ∆HD - ∆HP ) 51 ( 8 kJ mol-1 and ∆SD - ∆SP ) 14.3 ( 0.6 J K-1 mol-1. Conclusions DNPE can be obtained by dehydration of 1-pentanol using a gel-type acidic resin with an excellent selectivity, higher than 96%, in the temperature range 120-150 °C. Alkenes only appeared in small amounts at the highest temperature. On the other hand, at high space time, selectivity to DNPE remains practically constant and close to 98%. The yield to DNPE can be raised by increasing the space time and operating at the maximum available temperature. A kinetic model in terms of compound activities, based on a mechanism in which the surface reaction between one adsorbed 1-pentanol molecule and one 1-pentanol molecule from the liquid phase is the rate-determining step, describes satisfactorily the kinetic data. The apparent activation energy obtained for the etherification reaction is about 146 kJ mol-1, a value slightly higher than that in the literature data. Acknowledgment The authors are thankful for financial support from the State Scientific & Technological Research Office of Spain, DGICYT (PB96-0394). Nomenclature aj ) activity of compound j FP ) input flow rate, mol h-1 kˆ ) intrinsic rate coefficient, mol h-1 g-1 k1 ) apparent rate coefficient, mol h-1 g-1 K ) thermodynamic equilibrium constant Kj ) adsorption equilibrium constant of compound j ∆H ) adsorption enthalpy variation, kJ mol-1 n ) number of active centers that take part in the surface reaction step r ) -rP ) consumption reaction rate of 1-pentanol, mol h-1 g-1 SDNPE ) selectivity to DNPE ∆S ) adsorption entropy variation, J mol-1 K-1 SQ ) sum of squares of residuals W ) dry load of resin, g XP ) conversion of 1-pentanol YDNPE ) DNPE yield Subscripts D ) DNPE, di-n-pentyl ether min ) minimum P ) 1-pentanol W ) water
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Resubmitted for review June 1, 2004 Revised manuscript received October 13, 2004 Accepted October 18, 2004 IE030755P
