Kinetic Study of Glycerol Etherification with Isobutene - Industrial

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Kinetic Study of Glycerol Etherification with Isobutene Jingjun Liu, Bolun Yang, and Chunhai Yi* Department of Chemical Engineering, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an Shaanxi 710049, People’s Republic of China ABSTRACT: The etherification of biodiesel based glycerol with isobutene catalyzed by an ion-exchange resin (NKC-9) was studied in a stirred batch reactor. The effects of temperature, catalyst loading, and feed composition on the product distribution were investigated. A kinetic model based on Eley−Rideal mechanism was developed according to the experimental results. In this model, three consecutive etherifications and a dimerization of isobutene were taken into account; isomers in the product were lumped as monoethers (ME), diethers (DE), and dimers of isobutene (DIB); glycerol and isobutene were thought to be adsorbed on the active sites of catalyst and the surface reactions were treated as the rate determine step; the acceleration function of ME to the reactions was also considered. The activities of components were employed in rate equations with the activity coefficients calculated by UNIFAC. Model parameters including reaction and adsorption equilibrium constants as well as rate constants were estimated. The model reliability was validated through the prediction of glycerol conversion and ether yields under different conditions.

1. INTRODUCTION The severe energy crisis and environmental problems boost the production and use of biodiesel. This renewable and ecofriendly alternative fuel, usually referred to as fatty acid esters, is mainly produced by transesterification of vegetable oil or animal fat with low carbon alcohol. In the process, glycerol is also generated as an inevitable byproduct, in amount of approximately 10 wt % of the feedstock. Glycerol cannot be directly added to biodiesel as fuel because of its low solubility and poor thermal stability. The traditional glycerol market is very limited. Therefore, it is emergent to develop new technology to convert the redundant glycerol to high value-added products. As reviewed by Zhou et al.,1 Zheng et al.,2 and Behr et al.,3 a series of alternative routes to transform glycerol to valuable products had been developed, such as selective oxidation to produce C1−C3 products, selective hydrogenolysis to gain propanediol or ethylene glycol, catalytic dehydration to acrolein, pyrolysis and gasification to syngas or hydrogen, transesterification or esterification to monoglycerides, etherification to oxygenates, and so on. Among them, energy transformation, which can convert large amounts of bioglycerol to fuel, presents the main direction and is expected to grow. Etherification of glycerol to produce fuel additives drew considerable attention in recent years. Isobutene, tert-butanol,4−6 or ethanol7,8 was employed as another feedstock in the process. tert-Butanol was proved to be not suitable for etherification with glycerol because the generated water deactivated catalyst4 and the glycerol conversion was limited by equilibrium.6 However, isobutene was most favored for the process. Etherification of glycerol (G) with isobutene (IB) forms mono-tert-butyl ethers of glycerol (ME), di-tert-butyl ethers of glycerol (DE) and tri-tert-butyl ether of glycerol (TE). ME cannot be mixed with fuel due to its high polarity, which leads to phase separation. DE and TE, the objective products, were reported to have very high octane numbers and were considered as alternatives to gasoline additives such as MTBE and ETBE.9,10 Studies also indicate that these glycerol derivate © 2013 American Chemical Society

ethers have good burning properties with reduced pollutant and particulate matter emissions when blended to biodiesel or diesel fuel.11,12 Besides, the addition of DE and TE can improve low temperature performance and viscosity of biodiesel.13 Through etherification with isobutene, the surplus glycerol is transformed to excellent compound for the formulation of gasoline, diesel, and biodiesel fuels. Predecessors concentrated mainly on active and selective catalyst screening for the etherification process. A wide variety of catalysts were prepared and evaluated, including p-toluenesulfonic acid,14 silicotungstic acid,15 acid ion-exchange resins (Amberlyst type),4,9,14 zeolites (HY, Hβ),4,16 sulfonated peanut shell,17 several supported acid catalysts,18,19 and so on. The main objects of these works are to ensure good selectivity of the desired ethers at high glycerol conversion and to find the corresponding operation conditions for the process. Up to now, very few works are about kinetics, to the best of our knowledge. Melero et al.20 correlated the glycerol conversion and selectivity of different ethers with the reaction temperature and initial isobutene/glycerol molar ratio using second-order polynomial equations. Di Serio et al.21 measured the equilibrium constants for the glycerol etherification and the rate constants for isobutene dimerization at 363 K with an isobutylene/glycerol ratio of 2:1 and a pressure of 15 bar. Behr et al.22,23 suggested a simplified kinetic model using few experimental data under two reaction temperatures with homogeneous catalyst. Klepácǒ vá et al.24 studied the kinetics of this system using dioxane as solvent and only some of the model parameters were solved. Both Behr and Klepácǒ vá employed pseudohomogeneous model with power exponential rate equations based on concentration to fit the observed rates. However, heterogeneous catalysts are preferred, and solvent is not necessary in the Received: Revised: Accepted: Published: 3742

January 6, 2013 January 31, 2013 February 6, 2013 February 6, 2013 dx.doi.org/10.1021/ie400050v | Ind. Eng. Chem. Res. 2013, 52, 3742−3751

Industrial & Engineering Chemistry Research

Article

of the etherification product, which was separated to gain pure monoethers, diethers, triether, and isobutene dimers by water extraction and vacuum distillation.25 The purities of the gained ME, DE, TE, and DIB were all greater than 99% on gas chromatography mass spectroscopy (GCMS). The resin was sieved to four sizes including 0.2−0.4 mm, 0.4−0.6 mm, 0.6−0.8 mm, and 0.8−0.9 mm to study the effect of internal diffusion on the etherification. Different stirring speeds, varying from 800 to 1400 rpm, were employed to investigate the effect of external diffusion. Kinetic experiments were carried out at four temperatures of 343 K, 353 K, 363 K, and 373 K with an initial isobutene/glycerol mole ratio of 4 and a catalyst loading of 5.7 wt % to the mass of glycerol. Effects of feed composition and catalyst loading were also explored experimentally. 2.4. Analytical Methods. Quantity analysis was done on a GCMS instrument (GCMS-Plus 2010, Shimadzu), equipped with an Rxi-1 ms capillary column of length 30 m, film thickness 0.25 μm, and diameter 0.25 mm. The MS working in the SIM model was used as a detector. All products and glycerol in the liquid mixture were detected with GCMS and quantified using external standard method. The amount of isobutene was calculated through material balance. 2.5. Experimental Data Treatment. (1) The complex reaction system consists of nine reactants. Since these isomers show similar properties, they were lumped as monoethers (ME), diethers (DE), and isobutene dimers (DIB). This supposition significantly decreased the number of reactants and reactions reasonably. (2) The conversion and yield used in the subsequent discussion are calculated by ni ,0 − ni ,t Xi = × 100% (i = G, IB) ni ,0 (1)

process. A more detailed and precise kinetic model is needed for subsequent reactor and process design. This paper is devoted to provide a comprehensive kinetic analysis to the etherification of glycerol with isobutene. Experiments were performed in a batch reactor at 343−373 K catalyzed by a commercial resin in liquid phase. Three etherification reactions and one dimerization of isobutene were detected and taken into consideration. An Eley−Rideal (ER) based model was proposed to describe the kinetic performance. Besides, effects of catalyst loading and isobutene/glycerol molar ratio were investigated experimentally.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Catalyst. Liquid isobutene (>99.9%) was obtained from China National Petroleum Corporation, Lanzhou, China. Glycerol (>99.0%) and ethanol (>99.7%) were obtained from Tianjin Fuyu Fine Chemical Co., Ltd., China. All chemicals were used without further purification. A commercial macroporous strong acidic ion-exchange resin, NKC-9, purchased from the Chemical Plant of Nankai University was used in kinetic experiments. The resin consists of styrene-divinylbenzene base and sulfonic acid active sites. The ion-exchange capacity, specific surface area, and pore volume are 4.7 mol H+/kg, 77 m2/g, and 0.27 mL/g, respectively, according to the manufacturer. NKC-9 was washed with ethanol and then dried at 383 K under vacuum overnight before use. 2.2. Equipment. The reaction was carried out in a 1-L stainless batch reactor. A temperature controller along with an outside jacket and a built-in condensing coiler guaranteed the reaction temperature within a deviation of ±0.3 K from the set point. Reactants were stirred by a mechanical stirrer driven by a motor, the speed of which could be changed continuously. Several inlets for feed and outlets for sampling and exhaust were equipped to the reactor. Other equipment included a liquid isobutene cylinder, a dosing metering pump, and a high-pressure nitrogen gas cylinder, and so on. 2.3. Experimental Procedure and Conditions. Unless otherwise stated, all experiments were carried out using the following procedures. A measured amount of catalyst and glycerol were added to the reactor at room temperature. The reactor was sealed, and the air in it was swept twice with nitrogen. The reactor was pressurized to 2 MPa with nitrogen to ensure a liquid feeding of isobutene and liquid phase reaction under all reaction temperatures. Stirring was started, and the reactant was heated to the reaction temperature; liquid isobutene was pumped in, and then the reaction was timed. The pressure was 3.2−4 MPa, varying with the volume and composition of the reactant during the reaction. Liquid samples about 2 mL were withdrawn at certain times without stopping stirring to ensure the compositions were the same with those in the reactor, while catalyst particles were prevented from being brought out with liquid sample by a gauze element packed on the outlet. The sampling port was washed with ethanol repeatedly and dried after sampling. Most of the isobutene in the samples volatilized, and only components that were liquid under atmosphere at room temperature were collected and analyzed. Samples from the reactor were diluted about 1000 times using ethanol for quantity analysis. Before kinetic experiments, some synthesis and separation experiments were made to get pure or relatively pure reaction products to be used in quantity analysis. In synthesis experiments, factors such as reaction temperature, reaction time, and feed composition were controlled to get a favorable composition

yi =

ni,t nG,0

yDIB =

× 100%

(i = ME, DE, TE) (2)

nDIB, t nIB,0

× 100% (3)

where Xi is the conversion of component i, yi is the yield of component i, ni,t is the number of moles of component i at t, and ni,0 is the initial number of moles of component i.

Figure 1. Effect of catalyst particle size on the glycerol conversion. Stirring speed, 1400 rpm; temperature, 373 K; catalyst loading, 5.7 wt %; MR, 4. 3743

dx.doi.org/10.1021/ie400050v | Ind. Eng. Chem. Res. 2013, 52, 3742−3751


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3. RESULT AND DISCUSSION

are shown in Figures 1 and 2, respectively. It can be known from these figures that the diffusion emerges the same effect to the conversion of glycerol when the stirring speed is above 1200 rpm and the catalyst particle size is under 0.6 mm. Both increasing the catalyst particle size and decreasing the stirring speed would decrease the reaction rates. Hence, all following experiments were conducted with the agitation speed of 1400 rpm and catalyst particle size of 0.4−0.6 mm. 3.2. Effect of Reaction Temperature. Figure 3 shows the changes in product composition with time in different temperatures. The yield of mono-tert-butyl ethers of glycerol increases almost linearly with time at first and then decreases after reaching a maximum value. The reduction of ME yield is attributed to the formation of DE and TE. The yield of DE increases slowly at initial stage of the reaction and then increases dramatically at the time when the maximum ME yield is achieved. The TE yield maintains a very low level in the beginning and then increases quickly with the increase of DE. The final yields of ME, DE, and TE become stable, as a character of equilibrium reaction. The variation progress of product yields verifies the consecutive transformation from glycerol to ME, to DE and then to TE. The dimerization of isobutene proceeds all the time with an obvious acceleration appearing after the highest ME yield was achieved. The variation curve of IB conversion with time is made up of three different stages. Isobutene is mainly conversed by etherification at the first stage and dimerization at the third stage. Yet both etherification and dimerization consume isobutene at the second stage. So, the conversion of IB increases more rapidly in the middle of the etherification than in the ends.

3.1. Effect of Diffusion. To evaluate the influence of mass transfer resistance, reactions were performed using catalysts of different sizes under different stirring speeds with an isobutene/ glycerol feeding molar ratio (MR) of 4 and a catalyst loading of 5.7 wt % to the mass of glycerol. The reaction temperature is the highest (373 K) in our investigating range (343−373 K) to make sure that the mass transfer rate is not a limit to the overall rate of the reaction under all temperatures. The effects of catalyst particle size and stirring speed on glycerol conversion

Figure 2. Effect of stirring speed on the glycerol conversion. Catalyst particle size, 0.4−0.6 mm; temperature, 373 K; catalyst loading, 5.7 wt %; MR, 4.

Figure 3. Effect of temperature on product distribution. Catalyst loading, 5.7 wt %; MR, 4. 3744



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Figure 4. Effect of catalyst loading. Temperature, 363 K; MR, 4.

The variation curve of glycerol conversion is sunken rather than raised at the beginning of the reaction (refer to Figure 3), which means the reaction is accelerated for some reason before the highest conversion is achieved. As mentioned above, the formation of DE, TE, and DIB all speed up when the maximum ME yield is obtained. Thus, the formation of ME was thought to accelerate the etherifications and dimerization. Influence of temperature on the reaction was investigated over the range of 343−373 K with an isobutene/glycerol feeding molar ratio of 4 and a catalyst loading of 5.7 wt % to the mass of glycerol. As expected, increasing temperature accelerates all reactions. Conversions of reactants and yields of products increase rapidly with temperature before equilibrium approaching. The maximum yield of ME, and knee points of DE, TE, and DIB yield curve appear earlier at higher temperature. The highest final glycerol conversion, DE yield, and TE yield are achieved at the lowest reaction temperature, indicating the exothermic property of the three etherification reactions. Actually, the conversions of glycerol at different temperatures are tightly close after a certain reaction time. The decrease of ME yield with the increasing temperature is ascribed to the large consumption of ME to form DE at low temperature. As can be seen in Figure 3, a slight reduction of TE yield and addition of ME and DE arise at the end of the reaction. The changes are more obvious at higher temperature. They are attributed to equilibrium shifting caused by the decrease of isobutene concentration due to the dimerization. 3.3. Effect of Catalyst Loading. To study the effect of catalyst loading on the reaction, experiments was carried out under a serial of catalyst loadings (3.7 wt %, 4.8 wt %, 5.7 wt %,

and 7.3 wt % based on the mass of glycerol) at 363 K with an isobutene/glycerol feeding molar ratio of 4. Figure 4 shows the composition change with time under different catalyst loadings. It can be known from the figure that both dimerization and etherification rates increase with the increase of catalyst loading. Higher conversions of reactants and yields of glycerol ethers are gained with more catalyst loading before the approach of equilibrium. The conversions of glycerol got under different catalyst loadings trend to the same equilibrium value in the end. The dimerization of isobutene is far away from equilibrium in our study. Therefore, the more catalyst is loaded, the more DIB is obtained, and the concentration of isobutene decreases with the increase of catalyst loading. Consequently, the final yields of glycerol derivates, depending on chemical equilibrium and the concentration of isobutene, are irregular with the change of catalyst loading as a result of equilibrium shifting. 3.4. Effect of Initial Isobutene/Glycerol Feeding Molar Ratio. Influence of another important factor, feed composition, was also studied experimentally. Product distributions under different feeding molar ratios of isobutene to glycerol are revealed by Figure 5. According to the stoichiometry, high isobutene/ glycerol feeding molar ratio enhances the glycerol conversion. However, a large excess of isobutene would lower the concentration of other reactants, leading to a decrease of reaction rates and then the glycerol conversion. These deductions are confirmed by experimental results, as demonstrated in Figure 5. The highest glycerol conversion is achieved under an isobutene/ glycerol feeding molar ratio of 4. However, the differences of glycerol conversions under different feed compositions are inconspicuous. 3745



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Figure 5. Effect of glycerol/isobutene feeding molar ratio (MR). Temperature, 363 K; catalyst loading, 5.7 wt %.

By contrast, high isobutene/glycerol feeding molar ratio remarkably promote the formation of ‘high ethers’ (DE and TE). The yields of DE and TE increase by about 0.2 and 0.1, respectively, when the MR is increased from 3 to 6.7. The remarkable reduction of ME yield with the increase of MR is attributed to its transformation to DE under large MR. Although the dimerization of isobutene is accelerated with the increase of MR in the later period, a heavily aggravation of dimerization is avoided during the observed reaction time. 3.5. Kinetic Modeling. 3.5.1. Reaction Analysis. In the etherification of glycerol with isobutene, monoethers including 3-tert-butoxyl-1,2-propanediol (ME1) and 2-tert-butoxyl-1,3propanediol (ME2), diethers including 1,3-di-(tert-butoxyl)-2propanol (DE1) and 1,2-di-(tert-butoxyl)-3-propanol (DE2), and triether of 1,2,3-tri-(tert-butoxyl)-propane (TE) were formed. Only dimerization of isobutene was detected as a side reaction, which produced 2,4,4−3-methyl-1-pentene (DIB1) and 2,4,4− 3-methyl-2-pentene (DIB2). The dimerization was considered as an irreversible reaction as no equilibrium restriction was observed in experiments. Figure 6 illustrates the reaction scheme of the system, including etherification and oligomerization. 3.5.2. Model Development. To model the reaction system, the typical Eley−Rideal (ER) mechanism was used at first, but it did not give satisfactory simulation results. Therefore, a modified ER model (the extending model) considering the acceleration function of ME to the reactions demonstrated in section 3.2 was proposed. The extending model assumes that only glycerol and isobutene are adsorbed on the active sites while the adsorption ME, DE, TE, and DIB are negligible, the rate determining steps are the surface reactions.

Adsorption:

G + S ↔ G·S

(4)

IB + S ↔ IB·S

(5)

Surface reaction: IB·S + G·S ↔ ME + 2S

(6)

IB·S + ME ↔ DE + S

(7)

IB·S + DE ↔ TE + S

(8)

2IB·S → DIB + 2S

(9)

The rate equations are based on activities instead of concentrations to revise the nonideality of the liquid phase. The activity coefficients are calculated by UNIFAC. It is prudent to point out that, in the activity coefficient calculation, the major component (of large amounts) is chosen to stand for the lumped species, of which the isomers generally have some different groups. Considering the discussion above, rate equations of the model are obtained. Rate equations are as follows:

3746

rME = k1θIBθG − k 2aMEθ 2 − k 3θIBaME + k4aDEθ

(10)

rDE = k 3θIBaME − k4aDEθ − k5θIBaDE + k6a TEθ

(11)

rTE = k5θIBaDE − k6a TEθ

(12)

rDIB = k 7θIB2

(13)



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described by the Langmuir isotherm (eq 16), where Ki is the adsorption equilibrium constant of component i. θi =

K iai 1 + ∑ K iai

(16)

Equations 10−15 are rate equations of the basic ER model. A term, (1 + kcaME), is multiplied to these rates to depict the acceleration of ME to the reactions in our extending model. Substituted in the adsorption isotherm and rewritten the equations, the rates for the formation of each component are obtained. ⎡ k ′(a a − a /K ) k 3′(aIBaME − aDE /Ke2) ⎤ ME e1 ⎥ rME = ⎢ 1 IB G − 1 + K GaG + KIBaIB ⎦ ⎣ (1 + K GaG + KIBaIB)2 × (1 + kcaME) ⎡ k ′(a a − aDE /Ke2) − k5′(aIBaDE − a TE /Ke3) ⎤ rDE = ⎢ 3 IB ME ⎥ 1 + K GaG + KIBaIB ⎦ ⎣

Figure 6. Reaction pathway of the etherification process.

rIB = −k1θIBθG + k 2aMEθ 2 − k 3θIBaME + k4aDEθ − k5θIBaDE + k6a TEθ − 2k 7θIB2

(14) rTE =

rG = −k1θIBθG + k 2aMEθ 2

(17)

(15)

in which ki is the reaction rate constant, ai is the activity of component i, and θ is the concentration of unoccupied active sites. The adsorbed concentration of component i, θi, is

× (1 + kcaME)

(18)

k5′(aIBaDE − a TE /Ke3)(1 + kcaME) 1 + K GaG + KIBaIB

(19)

rDIB =

2 k 7′aIB (1 + kcaME)

(1 + K GaG + KIBaIB)2

(20)

Figure 7. Modeled values as a function of measured values in terms of conversions or yields. Catalyst loading, 5.7 wt %; MR, 4. 3747



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⎛ −Δa Hi ⎞ ⎟ K i = kai exp⎜ ⎝ RT ⎠

⎡ − k ′(a a − a /K ) − 2k ′a 2 ME e1 7 IB rIB = ⎢ 1 IB G (1 + K GaG + KIBaIB)2 ⎣ − k 3′(aIBaME − aDE /Ke2) − k5′(aIBaDE − a TE /Ke3) ⎤ ⎥ + 1 + K GaG + KIBaIB ⎦ × (1 + kcaME)

rG =

(21)

R = 8.314 J·mol−1·K−1

−k1′(aIBaG − aME /Ke1)(1 + kcaME) (1 + K GaG + KIBaIB)2

dni mcat Q E dt

(23)

min

ni is the molar number of component i, mcat is the mass of catalyst, and QE is the ion exchange capacity of the catalyst. Since reliable equilibrium constants are unavailable in literatures, they are introduced into model parameters as well as reaction rate constants. Effects of temperature on rate constants and equilibrium constants are described by the following equations: ⎛ E ⎞ ki = k 0i exp⎜ − i ⎟ ⎝ RT ⎠

(26) (27)

where R is the gas constant, k0i is the pre-exponential factor, Ei is the activation energy, ΔaHi is the adsorption enthalpy of component i, ΔrH0i is the standard enthalpy change for reaction and ΔrS0i is the standard entropy change for reaction. Model parameters were solved by minimizing the sum of absolute relative error between the measured and calculated compositions of the reaction mixture, shown as eq 28. The accuracy of the model was expressed by the standard deviation (σ) of calculated values in terms of conversion or yield (vcal,i) with respect to experimental data (vexp,i).

(22)

In the equations, kc is the parameter describing the acceleration function of ME; the apparent rate constants are determined as k1′ = k1KGKIB, k3′ = k3KIB, k5′ = k5KIB and k7′ = k7K2IB; the reaction equilibrium constants are Ke1 = k1KGKIB/k2, Ke2 = k3KIB/k4 and Ke3 = k5KIB/k6. The reaction rates (mol·h−1·(mmol H+)−1) are defined by eq 23. ri =

−Δr Hi0 Δ S0 + r i RT R

ln K ei =

(25)

⎛ |ncal, i − ni| ⎞ ER = sum⎜ ⎟ ni ⎝ ⎠ N

σ=

∑ i

(28)

(vcal, i − vexp, i)2 N−1

(29)

Experimental data gained in different reaction temperatures shown in Figure 3, were employed to solve the model parameters. Comparison of modeled values with measured values in different terms is presented in Figure 7. As an example, Figure 8 shows the modeled composition change with time

(24)

Figure 8. Comparison between modeled and measured conversion or yield changes with time at 363 K. Catalyst loading, 5.7 wt %; MR, 4. 3748



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deviation. The standard deviation of conversions and yields calculated by the extending model is σ = 0.0197; however, the standard deviation is σ = 0.0455 for the basic ER model results at 363. The correlation coefficients (r) between experimental and calculated values are labeled on the figures. It turns out that the proposed model gives an accurate prediction to the measured composition. The estimated model parameters are reported in Table 1, Table 2 and Table 3, with their 95% confidence intervals. Table 1 lists the pre-exponential factors and activation energies for rate constants. The activation energies in the formation of ME, DE, and TE are in the general range of etherification reaction. Equilibrium constants at different temperatures were also solved as model parameters and were used to fit eq 26 to obtain standard enthalpy changes and standard entropy changes of etherification reactions. The results are presented in Table 2. The standard enthalpy changes of the three etherification reactions are all less than zero, which confirms their exothermic property. The adsorption equilibrium constants of glycerol and isobutene were obtained in the model solving. Correlating the adsorption equilibrium constant to temperature using eq 25 gained the adsorption enthalpy. The kc was estimated and related to temperature (eq 30).

Table 1. Estimated Pre-exponential Factors and Activation Energies for Etherification and Dimerization reaction formation of ME formation of DE formation of TE dimerization

k 0i (mol ·h−1·(mmolH+)−1) (1.7 (3.9 (4.1 (1.5

± ± ± ±

0.3) 1.3) 1.2) 0.8)

× × × ×

109 1010 106 1013

Ei (kJ·mol−1) 46 55 28 76

± ± ± ±

1 3 3 4

Table 2. Estimated Standard Enthalpy Changes and Standard Entropy Changes for Etherification reaction

Δr Hi0 (kJ·mol−1)

Δr Si0 (J·mol−1·K−1)

formation of ME formation of DE formation of TE

−32 ± 4 −17 ± 2 −19 ± 3

−82 ± 12 −51 ± 6 −67 ± 8

Table 3. Estimated Adsorption Enthalpies of Glycerol and Isobutene adsorbed molecule

kai

Δa Hi (kJ·mol−1)

glycerol isobutene

0.0040 ± 0.0001 0.0010 ± 0.0006

−19 ± 1 −15 ± 2

⎛ 33827 ⎞ ⎟ kc = 3.66 × 10−5 exp⎜ ⎝ RT ⎠

(solid line) at 363 K. The dash line is a typical fitting of the basic ER model to the experimental data, which emerges obvious

(30)

Figure 9. Comparison of experimental values with the model estimated values in terms of conversion or yield. Temperature: 363 K. 3749



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ΔrS0i standard entropy change for reaction, J·mol−1·K−1 vexp,i experimental value (in terms of conversion and yield) vcal,i model calculated value (in terms of conversion and yield) Xi conversion of component i yi yield of component i θ concentration of unoccupied active sites θi surface concentration of component i σ standard deviation

3.6. Model Validation. Experimental data obtained under different catalyst loadings and different isobutene/glycerol molar ratios, demonstrated in Figures 4 and 5, were used to examine the kinetic model. Figure 9 compares the modeled values with the measured values in experiments. The standard deviation of values calculated by the proposed model with these experimental data is σ = 0.0312 and the correlation coefficients are close to 1 (r ≥ 0.9782). The good consistency of modeled values with experimental data validates the effectiveness of the model. Some other models were also tested in kinetic study. Multicomponent adsorptions, such as adsorption of ME and DE were considered. However, the obtained adsorption equilibrium constants were very small for components except glycerol and isobutene.

Abbreviations

4. CONCLUSION The reaction kinetics of glycerol etherification with isobutene to produce fuel additives was studied experimentally. Only the dimerization of isobutene was found as a side reaction. An extending model based on Eley−Rideal mechanism predicted adequately the experimental data in terms of conversions and yields. Reaction and adsorption equilibrium constants were also estimated as model parameters using experimental data. Activation energies for all reactions, and standard enthalpy changes and standard entropy changes for etherification reactions were determined by data fitting and calculation.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-29-82663189. Fax: +86-29-82668789. E-mail: chyi@ mail.xjtu.edu.cn.

DE di-tert-butyl ethers of glycerol DE1 1,3-di-(tert-butoxyl)-2-propanol DE2 1,2-di-(tert-butoxyl)-3-propanol DIB dimer of isobutene DIB1 2,4,4−3-methyl-1-pentene DIB2 2,4,4−3-methyl-2-pentene ETBE ethyl tert-butyl ether ER Eley−Rideal G glycerol IB isobutene ME mono-tert-butyl ethers of glycerol ME1 3-tert-butoxyl-1,2-propanediol ME2 2-tert-butoxyl-1,3-propanediol MTBE methyl tert-butyl ether MR isobutene/glycerol feeding molar ratio S active site of catalyst TE tri-tert-butyl ether of glycerol

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this work from the National Basic Research Program of China (973 Program, No. 2009CB219906), National Natural Science Foundation of China (No. 21276203), and Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20110201130002) is gratefully acknowledged.

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NOMENCLATURE ai activity of component i Ei activation energy, kJ·mol−1 ΔaHi adsorption enthalpy of component i, kJ·mol−1 ΔrH0i standard enthalpy change for reaction, kJ·mol−1 kc the parameter describing acceleration function of ME ki reaction rate constant, mol·h−1·(mmolH+)−1 ki′ apparent rate constant, mol·h−1·(mmolH+)−1 k0i pre-exponential factor, mol·h−1·(mmolH+)−1 Ki adsorption equilibrium constant of component i Kei reaction equilibrium constant mcat mass of catalyst, g ni molar number of component iin experiment, mol ncal,i calculated molar number of component i, mol N number of experimental points (conversion and yield measurements) QE ion exchange capacity of the catalyst, mol H+/kg r correlation coefficient ri reaction rate of component i, mol·h−1·(mmolH+)−1 R gas constant, J·mol−1·K−1 3750



Article

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Kinetic Study of Glycerol Etherification with Isobutene - Industrial

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