Kinetic Modeling, Thermodynamic, and Mass-Transfer Studies of

Article pubs.acs.org/IECR

Kinetic Modeling, Thermodynamic, and Mass-Transfer Studies of Gas-Phase Glycerol Dehydration to Acrolein over Supported Silicotungstic Acid Catalyst Amin Talebian-Kiakalaieh and Nor Aishah Saidina Amin* Chemical Reaction Engineering Group (CREG), Faculty of Chemical Engineering, Universiti Teknologi Malaysia (UTM), 81310 Skudai, Johor, Malaysia S Supporting Information *

ABSTRACT: The kinetics of gas-phase glycerol dehydration in a packed-bed reactor over a highly active and stable supported silicotungstic acid catalyst with zirconium oxide and nanosized aluminum oxide (30HZ-20A) was investigated. The kinetic study is based on the optimal reaction conditions determined by response surface methodology. The reaction rate followed first-order kinetics with the activation energy and frequency factor, E = 27.5 kJ/mol and A = 5.35 × 105 s−1, respectively. Based on thermodynamic analysis, the values of ΔH° and ΔS° of the endothermic reaction were 14.70 and 0.09 kJ/(mol K), respectively, and ΔG° = −12.12 kJ/mol. The mass-transfer analysis revealed the pellet sizes of dp < 1 μm proceeded under reaction-limiting conditions. Experimental results confirmed high efficacy of the tested catalyst due to unity effectiveness factor and very low Thiele modulus.

1. INTRODUCTION The demand for fossil fuel applications across various sectors of human life has increased significantly during the last decades. According to recent studies, the number of vehicles worldwide will rise dramatically from 1 to 1.6 billion by 2030.1,2 The huge demand and the environmental concerns caused by the utilization of fossil fuels stimulate the supply of renewable and sustainable fuels such as biodiesel. Consequently, the development and commercial utilization of biodiesel has been significantly enhanced during the past few years.3 For example, biodiesel production underwent rapid annual expansion rates of 28% and 50−80% in Europe and the United States, respectively.4,5 However, the sharp increase in biodiesel production has led to the abundance of low-cost glycerol as the main byproduct from the transesterification process.6,7 Several studies have indicated that glycerol production will exceed actual industrial demand by six times in 2020.8 Thus, studies are being conducted for new applications of glycerol in the chemical industry not only because of its huge availability, but also for its remarkable characteristics such as edibility, nontoxicity, and biodegradablility.9,10 One of the main chemicals that can be produced from glycerol dehydration in the presence of an acid catalyst is acrolein. Conventionally, acrolein is produced from the petroleumbased selective oxidation of propylene. However, petroleum exhaustion is foreseen in the near future. Thus, acrolein synthesis from renewable resources such as glycerol is more prevalent recently. Acrolein is directly used in the production of acrylic acid, superabsorbent polymers, and methionine, which are widely using in the manufacturing of plastics, coatings, adhesives, diapers, and even animal food. Figure S1 reports the number of published research papers on “glycerol dehydration to acrolein” during the past decade. The average number of research papers on glycerol dehydration to acrolein was 26 per year during the last five years, reiterating © XXXX American Chemical Society

the importance of biobased acrolein production process in the chemical industry. However, there is not a single study in the literature on the optimization and kinetic parameter determination of gas-phase glycerol dehydration to acrolein. Both topics are pertinent in the commercialization of the biobased process. The application of modeling tools such as response surface methodology (RSM) can significantly reduce the need for a large number of laboratory tests and associated costs. RSM is a combination of mathematical and statistical techniques and is suitable for evaluating the effects of various reaction variables on the response.11 This method is effective for improving, developing, and optimizing complicated systems. There have been a few reports on the successful optimization of RSM in glycerol conversion processes,12−17 but only one of them is related to the process of dehydration of glycerol to acrolein. Various types of acid catalysts, such as supported zeolites,18−20 heteropoly acids21,22 and metal oxides,23−25 have been reported in the glycerol dehydration to acrolein process. Among these catalysts, supported heteropoly acids (HPAs) displayed remarkable activity due to the strong and easily tunable acidity (Brønsted acid sites) and uniform acidic sites. However, HPAs have low thermal stability and a small surface area. Thus, HPAs are often supported on acidic or natural carriers, such as alumina, zirconia, or silica, to overcome these disadvantages.26−28 Generally, kinetic studies are performed to determine the reaction rates and elucidate the chemical reaction mechanisms on the catalyst surface.29 It appears that Park et al.30 recently published the only kinetic study of gas-phase dehydration of Received: June 16, 2015 Revised: August 5, 2015 Accepted: August 10, 2015

A

DOI: 10.1021/acs.iecr.5b02172 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

0.5 g catalyst (dp = 1−5 μm) sandwiched between plugs of glass wool. Prior to reaction, the catalyst was pretreated at reaction temperature (300 °C) under nitrogen (N2) flow (1200 mL/h) for 1 h. Liquid aqueous glycerol (10 wt %) was fed by a syringe pump with a 2 mL/h flow rate. The liquid was then vaporized in a preheater, mixed with inert carrier gas, and run to the reactor. Gas hourly speed velocity (GHSV) of the inert carrier gas was 1200 h−1. The reaction products and unconverted glycerol were condensed in a water−ice−salt bath (−5 °C) and collected for analysis after 3 h of reaction. N-Butanol was added to the condensed products as the internal standard. The final solution was analyzed by a gas chromatography (GC) instrument equipped with capillary column (DB-Wax: 30 m × 0.53 mm × 0.25 μm) and flame ionization detector (FID). To achieve effective product separation, the column was held at 40 °C for 4 min before the temperature was ramped up to 200 °C at a rate of 12 K/min for 23 min. The glycerol conversion, acrolein selectivity, yield, and carbon balance are defined in eqs 1−4:

glycerol to acrolein at ambient pressure using HZSM-5 and ASPN-40 catalysts. However, the other previous kinetic studies in this field were performed in supercritical water (SCW) conditions without a catalyst or with some simple catalyst utilization,21−34 which has limited potential for industrialization because of the high production costs and inherent technical challenges (e.g., high pressure and temperature). In addition, other similar studies are related to glycerol hydrogenation,35−38 regeneration of catalyst,39 or selectivity of acrolein to allyl alcohol.40 Therefore, the determination of kinetic parameters (reaction rate constant (k), activation energy (Ea), and frequency factor (A)) is very important for process simulation. The objectives of this work are to determine the kinetic parameters of gas-phase glycerol dehydration in a packed-bed reactor (PBR). Next, a brief thermodynamic and mass-transfer limitation analysis are performed. The thermodynamic analysis provides the values for ΔH°, ΔS°, and ΔG°, while the masstransfer limitation evaluates the catalyst efficacy during the glycerol dehydration reaction. Prior to the kinetics, thermodynamics, and mass-transfer studies, the glycerol dehydration was optimized by RSM to investigate the effect of interactions between reaction parameters on the acrolein selectivity. Reaction temperature, glycerol feed concentration, and catalyst loading have been identified as the main reaction parameters.

Conversion Gl , X (%) =

MGl,in feed − MGl,in outlet MGl,in feed

× 100% (1)

SelectivityAC (%) =

2. EXPERIMENTAL SECTION 2.1. Materials. Glycerol (purity >99%), silicotungstic acid (H4SiW12O40.14H2O (HSiW)), aluminum oxide (Al2O3) nanoparticle, and zirconium oxide (ZrO2) were purchased from Sigma-Aldrich (Malaysia) as were other chemicals including acetic acid, allyl alcohol, hydroxyacetone, acetone, propanal, and ethanal at reagent grade. Acrolein at reagent grade was supplied by Scientific Trends (M) Sdn. Bhd. 2.2. Catalyst Preparation. A catalyst with 30 wt % HSiW loading on ZrO 2 was prepared by incipient wetness impregnation. In this method, an aqueous 30 wt % HSiW solution was added dropwise to the ZrO2 support. The slurry was vigorously stirred for 12 h followed by drying at 110 °C for 18 h. The HSiW-ZrO2 supported catalysts are denoted as 30HZ. The final catalyst was prepared by impregnation of γAl2O3 nanoparticles on the 30HZ sample. Initially, a suspension of 30HZ sample (10 mL of water per gram of HSiW) was prepared. Nano γ-alumina was mixed with water (10 mL of water per gram of γ-Al2O3), and the slurry was continuously stirred before the 30HZ mixture with water was added dropwise to the alumina slurry. The mixture was continuously stirred for at least 20 more hours followed by drying at 120 °C for another 18 h. The final catalyst is referred to as 30HZ-20A. 2.3. Catalyst Characterization. The 30HZ-20A catalyst was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field-emission scanning electron microscopy and energy dispersive X-ray techniques (FESEM-EDX), temperature-programmed desorption (NH3TPD), thermogravimetric analysis (TGA), and elemental analyzer (EA). Nitrogen adsorption−desorption was utilized to determine the pore size and surface area by the Brunauer, Emmett, and Teller (BET) method. The details of different characterization results are discussed in the authors’ previous studies.41 Table S1 summarizes some of the physicochemical properties of 30HZ-20A catalyst. 2.4. Catalytic Reaction. The gas-phase dehydration of glycerol was conducted at atmospheric pressure in a vertical packed-bed quartz reactor (30 cm length, 11 mm i.d.) using a

MAC,in product MGl,in feed − MGl,in outlet

× 100% (2)

YieldAC (%) = Conversion Gl (%) × SelectivityAC (%)

(3)

Carbon balance = Sum of selectivities of identified products (4)

where MGl and MAC are moles of glycerol in feed or outlet streams and moles of acrolein, respectively. The experiments were repeated at least three times, and the average results are reported. 2.5. Experimental Design. A five-level, three-factor central composite design (CCD) consisting of 20 experimental runs was investigated in this study. The ranges and the design levels of each variable are summarized in Table 1. Table 1. Experimental Level Coded and Range of Independent Variables coded levels of parameters independent parameters temperature (°C) catalyst amount (g) glycerol feed concentration (wt %)

A B C

−α

−1

0

+1

+α

270 0.1 0.5

285 0.3 5

300 0.5 10

315 0.7 15

330 0.9 20

The performance of the optimization process was evaluated by analyzing acrolein selectivity as the response. The quadratic equation for optimization is given by eq 5: 3

Y = β0 +

3

3

∑ βi xi + ∑ βiixi2 ∑ ∑ i=1

i=1

i=1 j=i+1

βiixixj + ε (5)

where Y is the response abd β0 is constant coefficient; βi, βii, and βij are the linear, quadratic, and second-order interaction coefficient, respectively. xi and xj are independent variables, and B


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Industrial & Engineering Chemistry Research ε is the error. Design Expert (V.6.0.7, State-Ease, Inc.) was applied for regression (ANOVA) analysis.

effect on the acrolein selectivity and only increases the reaction conversion, but the results of the present study confirmed that a higher catalyst loading had an adverse impact on acrolein selectivity. Figure 2 illustrates the interaction effects of the reaction temperature and glycerol feed concentration on the acrolein

3. RESULTS AND DISCUSSION 3.1. RSM Modeling for Glycerol Dehydration to Acrolein. Fitting the data using different linear, two factorial, quadratic, and cubic models revealed that glycerol dehydration to acrolein was properly explained by the quadratic polynomial model. The best predicted acrolein selectivity response model based on coded factors is shown in eq 6: Acrolein selectivity (%) = +88.17 + 1.12A + 0.57B + 1.62C − 4.77A2 − 4.57B2 − 8.83C 2 − 0.93AB + 1.74AC + 1.99BC

(6)

where A is the reaction temperature, B catalyst loading, and C glycerol feed concentration. The F value of 497.84 and value of “Prob > F” less than 0.05 confirmed that the model and model terms (A, B, C, A2, B2, C2, AB, AC, BC) were significant. The low “lack of fit F-value = 4.19” demonstrated the higher accuracy of the predicted model. The high R2 value (0.998) indicated that the quadratic model could adequately represent the actual relationship between reaction parameters and response (Table S2). Figure S2 depicts a close fitting between the actual and predicted values. The number of experiments, the conditions, and the experimental and RSM predicted results are summarized in Table S3. 3.1.1. Effect of Reaction Parameter Interactions on Acrolein Selectivity. The three-dimensional plot (Figure 1)

Figure 2. Effect of glycerol concentration and reaction temperature interactions on acrolein selectivity.

selectivity while the catalyst loading (0.5 g) was constant. The perfect bowl shape confirmed the significant effect of the reaction temperature and glycerol concentration on the selectivity. It can be seen clearly that increasing the reaction temperature and glycerol concentration up to the optimal values of 300 °C and 10.25 wt % dramatically increased the acrolein selectivity from about 25.4% to 88.3%. However, the acrolein selectivity significantly reduced beyond these optimal values. High glycerol concentration was reported as one of the main reasons for reduction of catalyst activity and stability due to condensation of glycerol on the catalyst surface particularly at lower temperatures (glycerol boiling point is 290 °C).44 In addition, Paul et al.17 reported that some part of the feed (glycerol) was not catalytically converted in dehydration reaction because of the high concentration of glycerol. Consequently, some amount of produced acrolein was consumed to form acetal because of the presence of noncoverted glycerol. Thus, the acrolein selectivity decreased. The effect of reaction parameter interactions on the glycerol conversion is discussed in Section S1 and Figure S3. 3.1.2. Optimization of Process Parameters. The dehydration of glycerol was optimized by RSM to obtain the highest acrolein selectivity. The predicted selectivity was 88.3% at optimum conditions, which are 10 wt % glycerol feed concentration, 300 °C reaction temperature, and 0.5 wt % catalyst loading. RSM could accurately predict and model the glycerol dehydration to acrolein process with only 0.57% error as the experimental acrolein selectivity was 88.8%. 3.2. Thermodynamic Analysis. Acrolein is produced during gas-phase dehydration of glycerol over a supported silicotungstic acid (30HZ-20A) catalyst in a packed-bed reactor. The stoichiometric reaction is shown in eq 7:

Figure 1. Effect of reaction temperature and catalyst amount interactions on acrolein selectivity.

represents the acrolein selectivity as a function of the catalyst loading and reaction temperature while the glycerol feed concentration remained constant at 10 wt %. Increasing the temperature from 270 to 300 °C and also the catalyst loading from 0.1 to 0.5 wt % significantly enhanced the acrolein selectivity up to 88.3%. However, a catalyst loading above 0.5 wt % and reaction temperature above 300 °C imposed a negative effect on the acrolein selectivity. A high reaction temperature increases the coke deposition on the catalyst surface and significantly decreases the catalyst activity.42 Yadav et al.43 report that increasing the catalyst amount has no serious

C3H8O3(A) ⇔ C3H4O(B) + 2H 2O(C) C

(7)


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Industrial & Engineering Chemistry Research Table 2. Calculated Equilibrium Constants (Kc*) at Different Temperaturesa T (°C) 280 300 320 340

Xeq (%)b 80.0 82.6 83.2 85.0

Sel (%)c 70.2 76.0 68.3 59.5

yield (%)d

MW*f

MACe

56.2 62.3 56.7 50.6

0.518 0.532 0.538 0.549

21.60 21.62 21.64 21.66

MGlg

KC*h

0.122 0.108 0.102 0.091

× × × ×

1.98 2.30 2.46 2.82

CBi 3

10 103 103 103

95.2 96.8 96.0 97.3

Reaction conditions: 10 wt % glycerol concentration, 0.5 g catalyst loading weight, 2 mL/h glycerol feed flow rate, and 20 mL/min carrier gas (N2) flow rate at various reaction temperatures. bEquilibrium conversion. cAcrolein selectivity at equilibrium. dAcrolein yield at equilibrium. eFinal moles of acrolein. fMW* = MW + Msolvent is final moles of water plus a constant mole of water (solvent) in the product side. gFinal moles of glycerol. h Equilibrium constant with taking into account the constant water (solvent) concentration in the product side. iCarbon Balance: sum of the selectivities of identified products. a

The thermodynamic analysis of the glycerol dehydration was carried out at the optimum reaction conditions reported with additional data at three other temperatures: 280, 320, and 340 °C. Figure S4 depicts acrolein selectivity and glycerol conversion versus time-on-stream indicating catalyst deactivation caused by sintering. Beyond 12 h of reaction time, the catalyst and the system reached stability and steady-state, respectively. For a continuous system, the equilibrium state is achieved at a long residence time. In this study, the residence time was calculated to be 90 s, long enough to consider that equilibrium has been reached.45,46 The equilibrium molar concentrations of glycerol, acrolein, and water were determined experimentally to calculate the equilibrium constant (Kc*) according to eq 8: [B][C*]2 K C* = [A]

Figure 3. Plot of equilibrium constant (KC) versus temperature (T).

As a result, ΔG° = −12.12 kJ/mol, indicating that the dehydration reaction is a spontaneous reaction. To the best of the authors’ knowledge, there are no reports on the experimental calculation of enthalpy (ΔH°), entropy (ΔS°), and Gibbs free energy change (ΔG°) for gas-phase dehydration of glycerol to acrolein in the literature. 3.3. Kinetic Study. 3.3.1. Theoretical Background. The PBR assumes an idealized view of the motion of a fluid, whereby all the fluid elements move with a constant velocity through the parallel stream lines. The steady-state continuity equation is a simple ordinary differential equation due to the uniformity of conditions in a cross-section. In fact, the molar balance over a differential volume element for reactant A involved in a single reaction can be reported by eq 11:50,51

(8)

where [A], [B], and [C*] are the molar concentrations of glycerol, acrolein, and water, respectively. However, we have taken into account the excess water in the feed (solvent) and that the KC* calculated is modified KC by considering excess water in the feed. Table 2 summarizes all the details for KC*calculation at different reaction temperatures. The results indicate that by increasing the reaction temperature from 280 to 340 °C, KC* increased steadily from 1.98 × 103 to 2.82 × 103 as the glycerol dehydration to acrolein is an endothermic reaction. A similar trend is reported elsewhere.47 The KC* values also indicate that the glycerol dehydration reaction is irreversible within the temperature range investigated. The temperature dependency of Kc* is given by eq 9: ΔS° ΔH ° 1 ln K C* = − R R T

FA − (FA + dFA ) + rA dV = 0

For a continuous system, the molar flow rate is defined by eq 12: FA = FA0(1 − XA )

(9)

(12)

Therefore, for the reactant (glycerol), the reaction rate is shown in eqs 13 and 14:

where ΔS°, ΔH°, R, and T are standard entropy at 298 K (kJ/ (mol K)), standard enthalpy at 298 K (kJ/mol), universal gas constant (J/(mol K)) and reaction temperature (K), respectively. The standard entropy (ΔS°) and enthalpy (ΔH°) of the reaction can be determined from the equilibrium constant (Kc) at different temperatures by plotting ln Kc versus 1/T (K−1). According to eq 9, ΔH° can be determined from the slope and ΔS° from the intercept as illustrated in Figure 3, where the values of ΔS° and ΔH° are 0.09 kJ/(mol K) and 14.70 kJ/mol, respectively. The same procedure for finding the standard entropy and enthalpy is reported in previous studies.48,49 The standard Gibbs free energy change (ΔG°) can be found from the standard values of enthalpy and entropy according to eq 10. ΔG° = ΔH ° − T ΔS°

(11)

rA =

moles of reacted glycerol dF = A (unit catalyst bed volume)(unit time) dV

⇒ − rA =

dX d(V /FA0)

(13)

(14)

where −rA is glycerol reaction rate (mol/(m s)), XA the glycerol conversion, V the catalyst bed volume (m3), and FA0 the glycerol molar feed flow rate. The irreversible reaction of glycerol dehydration to acrolein is shown in eq 15: 3

Glycerol (A) → Acrolein (B) + 2Water (C)

(15)

Glycerol dehydration to acrolein was performed in the presence of a solvent (water). Therefore, the solvent starting

(10) D


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Industrial & Engineering Chemistry Research material ratio (κ) should be taken into account. Furthermore, for every mole of glycerol in the feed, three moles of products (1 mol acrolein and 2 mol of water) are produced. Therefore, the expansion factor (δA) is another constant which should be used in the calculations. Thus, the total molar flow rate (FT) can be reported as eqs 16−18:

⇒ C BS =

(16)

⇒FT = FA0[1 − XA + (1 + δA )XA + κ ]

(17)

⇒FT = FA0(1 + δAXA + κ )

(18)

⇒ CCS =

⇒CS =

The Langmuir−Hinshelwood−Hougen−Watson (LHHW) model is applied to find the reaction rate equation. The LHHW mechanism includes adsorption of the glycerol (A), the reaction between adsorbed reactants (glycerol) on the catalyst active site (S), and finally the desorption of acrolein (B) and water (C) from the catalyst surface. Hence, the following steps were taken to delineate the rate equations:

−rA =

(22)

(23)

k3

BS ⇔ B + S

−rA =

(24)

k −3

(25)

k −4

⇒ CAS = K1CACS

K1CA +

CB K3

+

CC K4

+1

(34)

k 2K1CAC t,s 3 ⎡ ⎢⎣K1CA +

CB K3

+

CC K4

⎤3 + 1⎥ ⎦

(35)

k 2K1CAC t,s 3 [K1CA + 1]3

(36)

(37)

where kSR is apparent reaction rate constant equal to k2K1Ct,s3. The LHHW model confirmed that the glycerol dehydration reaction rate is a function of glycerol (A) concentration only. A similar procedure for calculation of reaction rate is also reported elsewhere.35 Therefore, the reaction rate based on the overall dehydration reaction (eq 15) was calculated by the differential method. Data were collected at various reaction temperatures and feed flow rates to determine the reaction rate constants and confirm that the reaction is first-order. 3.3.2. Reaction Rate Equation (rA). The rate expression in eq 37 can be expressed as the power-law form as in eq 38:

(26)

If the adsorption and desorption steps are very fast during the above reaction mechanisms, then concentration of the adsorbed species can be determined by assuming the adsorption and desorption steps are at equilibrium. Therefore, the concentrations of all adsorbed species are determined by eqs 27−29: −rAds = k1CACS − k −1CAS = 0 ⇒ CAS =

C t,s

−rA = k SR CA

If the surface reaction is the rate-controlling step during dehydration reaction, the rate of reaction is given by eq 26: −rA = k 2CASCS2

(33)

The reaction rate can be expressed as eq 37 by assuming K1CA ≪ 1 in gas-phase reactions:

k4

2CS ⇔ 2C + 2S

(32)

However, at initial conditions (t = 0, −rA = −rA0; therefore CB = CC = 0). Thus, the reaction rate is obtained as eq 36:

k2

k −2

C BCS C C + C S + CS K3 K4

where Ct,s is the total available active sites. Therefore, the rate of reaction (eq 35) is determined by substituting Cs from eq 34 into eq 30:

k1

AS + 2S ⇔ BS + 2CS (Rate‐determining step)

(31)

⎞ ⎛ C C ⇒C t,s = CS⎜⎜K1CA + B + C + 1⎟⎟ K3 K4 ⎠ ⎝

(21)

k −1

(30)

⇒C t,s = K1CACS +

Thus, the relationship between glycerol concentration (CA, mol/m3), total molar concentration (CT, mol/m3), and conversion (XA) is shown in eq 21:

A + S ⇔ AS

(29)

C t,s = CAS + C BS + CCS + CS (20)

1 − XA CT (1 + δAXA + κ )

1 CCCS K4

k −4 CCCS k4

The value of Cs can be determined from the site balance according to eqs 31−34:

The mole fraction of glycerol is described in eq 20:

CA =

(28)

−rA = k 2K1CACS3

(19)

(1 − XA ) yA = (1 + δAXA + κ )

1 C BCS K3

The new equation for the reaction rate is generated in eq 30 by substituting the adsorbed species of A, CAS in eq 26:

Similarly, the total molar concentration can be obtained from eq 19: C T = CA0(1 + δAXA + κ )

k −3 C BCS k3

−rdes = k4CCS2 − k −4CC 2CS 2 = 0 ⇒ CCS =

FT = FA0(1 − XA ) + FA0(1 + δA )XA + FA0κ   Solvent Glycerol Products

⏟

−rdes = k 3C BS − k −3C BCS = 0 ⇒ C BS =

k1 CACS k −1

−rA = kACA n

(27) E

(38) DOI: 10.1021/acs.iecr.5b02172 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 4. Linear relationship between reaction rate (rA) and log((1 − XA)/(1 + δAXA + κ)) at different reaction temperatures: (a) 280 °C, (b) 300 °C, (c) 320 °C, and (d) 340 °C.

where n is the reaction order and ksr = kA. The logarithm form of eq 38, shown in eq 39, is used to determine the reaction order (n) and reaction rate constant (kA) ⇒log( −rA ) = log kA + n log CA

be rounded up to 1. The reaction orders (n), intercepts, and reaction rate constants (k) at different temperatures are listed in Table 3.

(39)

Table 3. Reaction order (n) and rate constant (k) at different temperatures

By substituting CA from eq 20 into eq 38, the final logarithm form is illustrated by eq 40: ⎞ ⎛ 1 − XA ⇒log rA = log kA + n log C T + n log⎜ ⎟ ⎝ 1 + δAXA + κ ⎠ (40)

The slope of the straight line in the log(rA) versus log((1 − XA)/(1 + δAXA + κ)) plot provides the reaction order (n), whereas the reaction rate constant can be calculated from the intercept. The glycerol dehydration to acrolein was carried out at four different temperatures (280, 300, 320, and 340 °C), and for each reaction temperature, a series of experiments were performed with different feed flow rates (2, 5, and 10 mL/h). Figure S5a−d depicts the dependency of glycerol conversion (XA) versus Vcat/FA at the different reaction temperatures. Quadratic functions were fit to the data with R2 = 1. The equation related to the glycerol consumption rate (rA) at each flow rate and temperature can be calculated by determining the first derivative of the corresponding quadratic equation at a given Vcat/FA. The quadratic functions and the (rA) equations are listed in Table S4. 3.3.3. Reaction Order (n) and Reaction Rate Constant (k). With the availability of δ, κ, CT,and rA, log(−rA) could be plotted versus log((1 − XA)/(1 + δAXA + κ)). The equations and procedures for finding δ, κ, and CT are reported in sections S2−S4 of Supporting Information. The reaction order (n) and reaction rate constant (k) were calculated from the slope of the straight line and the intercept according to eq 40. Table S4 summarizes the required information for determining the reaction rate constant (k) and reaction order (n). Figure 4a−d confirms that the glycerol dehydration to acrolein followed first-order kinetic because all the slopes can

T (°C)

280

300

320

340

slope (n) intercept CT = P/RT (mol/m3) k (× 103 s−1)

1.05 4.52 19.82 1.30

1.00 4.60 21.20 1.86

1.10 4.72 21.97 1.95

1.03 4.72 20.49 2.40

3.3.4. Activation Energy (Ea) and Frequency Factor (A). The last step in the kinetic study is the determination of the activation energy (Ea) and frequency factor (A). Based on the Arrhenius equation (eq 41), ln(k) versus inverse of temperature (1/T) is plotted in Figure 5. As a result, the Ea (calculated from the slope) and the frequency factor (calculated from the intercept) were 27.5 kJ/mol and 5.35 × 105 s−1, respectively.

ln(k) = −

Ea + ln A RT

(41)

Figure 5. Arrhenius plot for overall glycerol dehydration reaction to acrolein. F


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Figure 6. (a) Relationship between Thiele modulus (ϕ1) and effectiveness factor (η) and (b) relationship between effectiveness factor (η) and pellet size (dp) for glycerol dehydration to acrolein at 300 °C over 30HZ-20A catalyst with small (dp = 1 μm) and large (dp= 100 μm) sizes.

As mentioned earlier, there is no report in the literature on the activation energy (Ea) and frequency factor (A) for gasphase glycerol dehydration to acrolein in PBR. However, the results of this study are compared with the only three available studies that report the Ea for glycerol dehydration to acrolein in SCW conditions. The 27.5 kJ/mol activation energy obtained in this study is lower than the results reported in the literature. The values of the activation energy were reported as Ea = 146 kJ/mol at 300−400 °C temperature and 34.5 MPa pressure by Watanabe et al.,31 Ea = 140 kJ/mol at 300−360 °C and 25 MPa pressure by Ott et al.,32 and Ea = 39.6 kJ/mol at 200−400 °C reaction temperature range and 30 MPa pressure by Qadariyah et al.33 3.4. Effectiveness Factor for 30HZ-20A Catalyst with Two Different Pellet Sizes. In this section, the evaluation of the effect of the catalyst size (30HZ-20A) on the glycerol dehydration to acrolein reaction is presented. The effectiveness factors (η) for a small and large pellet size of the 30HZ-20A catalyst were determined in the current study. To evaluate the effect of the pellet size on the catalyst activity in detail, the effectiveness factor (η) and Thiele modulus (ϕ1) for glycerol dehydration to acrolein were calculated using the effective diffusivity (Deff) and reaction rate constant (k1 = 1.86 × 103 s−1) at 300 °C. The gas diffusivity (DAB) is calculated by eq 42:52 DAB = 1.173 × 10−16(φMB)

T μB VA 0.6

Deff =

ε DAB τ

(43)

The Thiele modulus (ϕ1) and effectiveness factor (η) are calculated based on eqs 44 and 45:55,61 φ1 = R

η=

k1 Deff

3 (ϕ coth φ1 − 1) φ12 1

(44)

(45)

The relationships between the Thiele modulus (ϕ1) and effectiveness factor (η) for small (dp= 1 μm) and large (dp = 100 μm) pellet sizes are shown in Figure 6a,b. Indeed, the effectiveness factor, η = 1, indicates that the glycerol dehydration to acrolein over a small catalyst (dp = 1 μm) proceeds under reaction-controlling conditions. In addition, for the Thiele modulus, ϕ < 0.1, the reaction performs under reaction-limiting conditions. However, for dp = 100 μm, the low effectiveness factor (η = 0.3) confirms the presence of masstransfer limitation (diffusional condition).

4. CONCLUSIONS Optimization and kinetics of gas-phase glycerol dehydration to acrolein in a PBR were studied over supported silicotungstic acid catalyst by zirconium dioxide and nanosized alumiun oxide (30HZ-20A) at ambient pressure. Response surface methodology was applied to determine the optimal reaction parameters (reaction temperature, glycerol feed concentration, and catalyst loading). RSM could accurately (R2 = 0.998) predict the maximum selectivity (88.3%) to acrolein at optimal conditions of 10 wt % glycerol feed concentration, 0.5 wt % catalyst loading, and 300 °C. Thermodynamic studies revealed the glycerol dehydration reaction was highly endothermic. Kinetic study delineated the reaction followed first-order kinetics with respect to glycerol. The results of the mass-transfer studies revealed that reaction over small-sized catalysts (dp < 1 μm) proceeded under reaction-limiting conditions because of effectiveness factor (η) and Thiele modulus (ϕ1) equal to η = 1 and ϕ1= 0.1, respectively. The findings of the current research provides useful data for future simulation and commercialization purposes.

(42)

where MB and μB are the molecular weight and viscosity of solvent B, which is water. The feed was aqueous glycerol in water (10 wt %). VA is the solute (glycerol) molar volume at its boiling temperature.52,53 Finally, the association parameter, ϕ, is taken to be 2.6 for water.54,55 According to the nitrogen adsorption−desorption characterization results (Table S1), the pellet porosity (εp), which is equal to εp = (pore volume)/(total volume), can be calculated.56 Therefore, after calculation of porosity (ε = 0.1), we can simply use the equation τ = 1 − 0.5 ln(ε) for calculation of tortuosity (τ).56−58 The calculated amount of tortuosity (τ = 2) is in good agreement with previous reports.52−55,59,60 Tortuosity is usually in the range of 2−4 for catalysts with conventional morphology. The effective diffusivity, Deff, is then calculated with eq 43 as 4.12 × 10−8 m2/ s:52 G


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ammoxidation over Sb-(Fe,V)-O. Appl. Catal., B 2013, 132−133, 170−182. (15) Zhou, L.; Al-Zaini, E.; Adesina, A. A. Catalytic characteristics and parameters optimization of the glycerol acetylation over solid acid catalysts. Fuel 2013, 103, 617−625. (16) Guillon, C.; Liebig, C.; Paul, S.; Mamede, A.; Holderich, W.; Dumeignil, F.; Katryniok, B. Ammoxidation of allyl alcohol − a sustainable route to acrylonitrile. Green Chem. 2013, 15, 3015. (17) Paul, S.; Heyte, S.; Katryniok, B.; Garcia-Sancho, C.; MairelesTorres, P.; Dumeignil, F. REALCAT: A new platform to bring catalysis to the lightspeed. Oil Gas Sci. Technol. 2015, 70, 455−462. (18) de Oliveira, A. S.; Vasconcelos, S. J. S.; de Sousa, J. R.; de Sousa, F. F.; Filho, J. M.; Oliveira, A. C. Catalytic conversion of glycerol to acrolein over modified molecular sieves: Activity and deactivation studies. Chem. Eng. J. 2011, 168, 765−774. (19) Possato, L. G.; Diniz, R. N.; Garetto, T.; Pulcinelli, S. H.; Santilli, C. V.; Martins, L. A comparative study of glycerol dehydration catalyzed by micro/mesoporous MFI zeolites. J. Catal. 2013, 300, 102−112. (20) Costa, B. O. D.; Peralta, M. A.; Querini, C. A.Gas phase dehydration of glycerol over, lanthanum-modified beta-zeolite. Appl. Catal., A 2014, 472, 53−63. (21) Stosic, D.; Bennici, S.; Sirotin, S.; Stelmachowski, P.; Couturier, J.; Dubois, J.; Travert, A.; Auroux, A. Examination of acid−base properties of solid catalysts for gas phase dehydration of glycerol: FTIR and adsorption microcalorimetry studies. Catal. Today 2014, 226, 167−175. (22) Shen, L.; Yin, H.; Wang, A.; Feng, Y.; Shen, Y.; Wu, Z.; Jiang, T. Liquid phase dehydration of glycerol to acrolein catalyzed by silicotungstic, phosphotungstic, and phosphomolybdic acids. Chem. Eng. J. 2012, 180, 277−283. (23) Yan, W.; Suppes, G. J. Pressure Packed-Bed Gas-Phase Dehydration of Glycerol to Acrolein. Ind. Eng. Chem. Res. 2009, 48, 3279−3283. (24) Shen, L.; Yin, H.; Wang, A.; Lu, X.; Zhang, C. Gas phase oxidehydration of glycerol to acrylic acid over Mo/V and W/Voxide catalysts. Chem. Eng. J. 2014, 244, 168−177. (25) Massa, M.; Andersson, A.; Finocchio, E.; Busca, G.; Lenrick, F.; Wallenberg, L. R. Performance of ZrO2-supported Nb- and W-oxide in the gas-phase dehydration of glycerol to acrolein. J. Catal. 2013, 297, 93−109. (26) Atia, H.; Armbruster, U.; Martin, A. Influence of alkaline metal on performance of supported silicotungstic acid catalysts in glycerol dehydration towards acrolein. Appl. Catal., A 2011, 393, 331−339. (27) Zhu, S.; Qiu, Y.; Zhu, Y.; Hao, S.; Zheng, H.; Li, Y. Hydrogenolysis of glycerol to 1,3-propanediol over bifunctional catalysts containing Pt and heteropolyacids. Catal. Today 2013, 212, 120−126. (28) Katryniok, B.; Paul, S.; Capron, M.; Lancelot, C.; Bellere-Beca, V.; Rey, P.; Dumeignil, F. A long-life catalyst for glycerol dehydration to acrolein. Green Chem. 2010, 12, 1922−1925. (29) Einaga, H.; Tokura, J.; Teraoka, Y.; Ito, K. Kinetic analysis of TiO2-catalyzed heterogeneous photocatalytic oxidation of ethylene using computational fluid dynamics. Chem. Eng. J. 2015, 263, 325− 335. (30) Park, H.; Yun, Y. S.; Kim, T. Y.; Lee, K. R.; Baek, J.; Yi, J. Kinetic of the dehydration of glycerol over acid catalysts with an investigation of deactivation mechanism by coke. Appl. Catal., B 2015, 176, 1−10. (31) Watanabe, M.; Iida, T.; Aizawa, Y.; Aida, T. M.; Inomata, H. Acrolein synthesis from glycerol in hot-compressed water. Bioresour. Technol. 2007, 98, 1285−1290. (32) Ott, L.; Bicker, M.; Vogel, H. Catalytic dehydration of glycerol in sub- and supercritical water: a new chemical process for acrolein production. Green Chem. 2006, 8, 214−220. (33) Qadariyah, L.; Mahfud; Sumarno; Machudah, S.; Wahyudiono, S.; Sasaki, M.; Goto, M. Degradation of glycerol using hydrothermal process. Bioresour. Technol. 2011, 102, 9267−9271.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b02172. Additional experimental and computational details and results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +607-553-5579. Fax: +607-558-8166. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express their sincere gratitude to the Ministry of Science, Technology and Innovation (MOSTI), Malaysia for supporting the project under Project 03-01-06-SF0963.

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REFERENCES

(1) Russbueldt, B. M. E.; Hoelderich, W. F. New rare earth oxide catalysts for the transesterification of triglycerides with methanol resulting in biodiesel and pure glycerol. J. Catal. 2010, 271, 290−304. (2) Venugopal, A.; Sarkari, R.; Anjaneyulu, C.; Krishna, V.; Kumar, M. K.; Narender, N.; Padmasri, A. H. Influence of acid-base sites on ZnO−ZnCr2O4 catalyst during dehydrocyclization of aqueous glycerol and ethylenediamine for the synthesis of 2-methylpyrazine: Kinetic and mechanism studies. Appl. Catal., A 2014, 469, 398−409. (3) Wang, C.; Jiang, H.; Chen, C.; Chen, R.; Xing, W. Solvent effect on hydrogenolysis of glycerol to 1,2-propanediol over Cu−ZnO catalyst. Chem. Eng. J. 2015, 264, 344−350. (4) Liao, X.; Zhu, Y.; Wang, S.; Li, Y. Producing triacetylglycerol with glycerol by twosteps: esterification and acetylation. Fuel Process. Technol. 2009, 90, 988−93. (5) Dunn, R. O.; Bagby, M. O. Low-temperature properties of triglyceride-based dieselfuels: transesterified methyl esters and petroleum middle distillate/esterblends. J. Am. Oil Chem. Soc. 1995, 72, 895−903. (6) Jang, H.; Bae, K.; Shin, M.; Kim, S. M.; Kim, C.; Suh, Y. Aromatization of glycerol/alcohol mixtures over zeolite H-ZSM-5. Fuel 2014, 134, 439−447. (7) Tymchyshyn, M.; Yuan, Z.; Xu, C. Direct conversion of glycerol into bio-oil via hydrotreatment using supported metal catalysts. Fuel 2013, 112, 193−202. (8) Christoph, R.; Schmidt, B.; Steinberner, U.; Dilla, W.; Karinen, R. Glycerol, Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH: Weinheim, 2006. DOI: 10.1002/14356007.a12_477.pub2. (9) Talebian-Kiakalaieh, A.; Amin, N. A. S.; Hezaveh, H. Glycerol for renewable acrolein production by catalytic dehydration. Renewable Sustainable Energy Rev. 2014, 40, 28−59. (10) Esteban, J.; Fuente, E.; Blanco, A.; Ladero, M.; Garcia-Ochoa, F. Phenomenological kinetic model of the synthesis of glycerol carbonate assisted by focused beam reflectance measurements. Chem. Eng. J. 2015, 260, 434−443. (11) Draper, N. R.; John, J. A. Response-surface design for quantitative and qualitative variables. Technometrics 1988, 30, 423−8. (12) Zakaria, Z. Y.; Amin, N. A. S.; Linnekoski, J. Optimization of catalytic glycerol steam reforming to light olefins using Cu/ZSM-5 catalyst. Energy Convers. Manage. 2014, 86, 735−744. (13) Pathak, K.; Reddy, K. M.; Bakhshi, N. N.; Dalai, A. K. Catalytic conversion of glycerol to value added liquid products. Appl. Catal., A 2010, 372, 224−238. (14) Liebig, C.; Paul, S.; Katryniok, B.; Guillon, C.; Couturier, J.; Dubois, J.; Dumeignil, F.; Hoelderich, W. Glycerol conversion to acrylonitrile by consecutive dehydration over WO3/TiO2 and H


Article


(53) Bas, L. E. The molecular volumes of liquid chemical compounds; David McKay Co., Inc.: New York, 1915. (54) Gemo, N.; Biasi, P.; Canu, P.; Salmi, T. O. Mass transfer and kinetics of H2O2 direct synthesis in a batch slurry reactor. Chem. Eng. J. 2012, 207−208, 539−551. (55) Geankoplis, C. J. Transport Processes and Separation Process Principles, 4th ed.; Pearson Education International: Upper Saddle River, NJ, 2009. (56) Malekshahian, M.; De Visscher, A.; Hill, J. M. A non-equimolar mass transfer model for carbon dioxide gasification studies by thermogravimetric analysis. Fuel Process. Technol. 2014, 124, 1−10. (57) Delgado, J. A simple experimental technique to measure tortuosity in packed beds. Can. J. Chem. Eng. 2006, 84, 651−655. (58) Tien, R. H.; Turkdogan, E. T. Incomplete pore diffusion effect on internal burning of carbon. Carbon 1970, 8, 607−621. (59) Potemkin, D. I.; Snytnikov, P. V.; Belyaev, V. D.; Sobyanin, V. A. Preferential CO oxidation over Cu/CeO2−x catalyst: Internal mass transport limitation. Chem. Eng. J. 2011, 176−177, 165−171. (60) Armatas, G. S. Determination of the effects of the pore size distribution and pore connectivity distribution on the pore tortuosity and diffusive transport in model porous networks. Chem. Eng. Sci. 2006, 61, 4662−4675. (61) Fogler, H. S. Elements of Chemical Reaction Engineering, 4th ed.; Pearson International: Upper Saddle River, NJ, 2009.

(34) Akizuki, M.; Oshima, Y. Kinetics of Glycerol Dehydration with WO3/TiO2 in Supercritical Water. Ind. Eng. Chem. Res. 2012, 51, 12253−12257. (35) Sharma, R. V.; Kumar, P.; Dalai, A. K. Selective hydrogenolysis of glycerol to propylene glycol by using Cu:Zn:Cr:Zr mixed metal oxides catalyst. Appl. Catal., A 2014, 477, 147−156. (36) Vasiliadou, E. S.; Lemonidou, A. A. Kinetic study of liquid-phase glycerol hydrogenolysis over Cu/SiO2 catalyst. Chem. Eng. J. 2013, 231, 103−112. (37) Xia, S.; Nie, R.; Lu, X.; Wang, L.; Chen, P.; Hou, Z. Hydrogenolysis of glycerol over Cu0.4/Zn5.6xMgxAl2O8.6 catalysts: The role of basicity and hydrogen spillover. J. Catal. 2012, 296, 1−11. (38) Zhou, Z.; Li, X.; Zeng, T.; Hong, W.; Cheng, Z.; Yuan, W. Kinetics of Hydrogenolysis of Glycerol to Propylene Glycol over CuZnO-Al2O3 Catalysts. Chin. J. Chem. Eng. 2010, 18, 384−390. (39) Patience, G. S.; Farrie, Y.; Devaux, J.; Dubois, J. Oxidation Kinetics of Carbon Deposited on Cerium-Doped FePO4 during Dehydration of Glycerol to Acrolein. Chem. Eng. Technol. 2012, 35, 1699−1706. (40) Wei, H.; Gomez, C.; Liu, J.; Guo, N.; Wu, T.; Lobo-Lapidus, R.; Marshall, C. L.; Miller, J. T.; Meyer, R. J. Selective hydrogenation of acrolein on supported silver catalysts: A kinetics study of particle size effects. J. Catal. 2013, 298, 18−26. (41) Talebian-Kiakalaieh, A.; Amin, N. A. S. Supported silicotungstic acid on zirconia catalyst for gas phase dehydration of glycerol to acrolein. Catal. Today 2015. DOI: 10.1016/j.cattod.2015.01.045. (42) Atia, H.; Armbruster, U.; Martin, A. Dehydration of glycerol in gas phase using heteropolyacid catalysts as active compounds. J. Catal. 2008, 258, 71−82. (43) Yadav, G. D.; Sharma, R. V.; Katole, S. O. Selective Dehydration of Glycerol to Acrolein: Development of Efficient and Robust Solid Acid Catalyst MUICaT-5. Ind. Eng. Chem. Res. 2013, 52, 10133− 10144. (44) Haider, M. H.; Dummer, N. F.; Zhang, D.; Miedziak, P.; Davies, T. E.; Taylor, S. H.; Willock, D. J.; Knight, D. W.; Chadwick, D.; Hutchings, G. J. Rubidium- and caesium-doped silicotungstic acid catalysts supported on alumina for the catalytic dehydration of glycerol to acrolein. J. Catal. 2012, 286, 206−213. (45) Andersson, M. E.; Gardfeldt, K.; Wangberg, I. A description of an automatic continuous equilibrium system for the measurement of dissolved gaseous mercury. Anal. Bioanal. Chem. 2008, 391, 2277− 2282. (46) Stum, W.; Morgan, J. J. Aquatic chemistry: Chemical Equilibria and Rates in Natural Waters, 3rd ed.; Wiley-Interscience Publication: New York, 1996. (47) Liu, L.; Ye, X. P.; Bozell, J. J. A comparative review of petroleum-based and bio-based acrolein production. ChemSusChem 2012, 5, 1162−1180. (48) Nanda, M. R.; Yuan, Z.; Qin, W.; Ghaziaskar, H. S.; Poirier, M.; Xu, C. C. Thermodynamic and kinetic studies of a catalytic process to convert glycerol into solketal as an oxygenated fuel additive. Fuel 2014, 117, 470−477. (49) Rahaman, M.; Graca, N. S.; Pereira, C. S. M.; Rodrigues, A. E. Thermodynamic and kinetic studies for synthesis of the acetal (1,1diethoxybutane) catalyzed by Amberlyst 47 ion-exchange resin. Chem. Eng. J. 2015, 264, 258−267. (50) Liu, L. Roles of Non-thermal Plasma in Gas-phase Glycerol Dehydration Catalyzed by Supported Silicotungstic Acid. Ph.D. Dissertation, University of Tennessee, Knoxville, TN, 2011. http:// trace.tennessee.edu/utk_graddiss/992. (51) Vorgelegt, V. Optimization of the dehydration of glycerol to acrolein and a scale up in a pilot plant. Ph.D. Disseration, University of Technology, RWTH Aachen, 2007. (52) Delidovich, I. V.; Moroz, B. L.; Taran, O. P.; Gromov, N. V.; Pyrjaev, P. A.; Prosvirin, I. P.; Bukhtiyarov, V. I.; Parmon, V. N. Aerobic selective oxidation of glucose to gluconate catalyzed by Au/ Al2O3and Au/C: Impact of the mass-transfer processes on the overall kinetics. Chem. Eng. J. 2013, 223, 921−931. I


Kinetic Modeling, Thermodynamic, and Mass-Transfer Studies of

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