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Glycerol acetylation with acetic acid over Purolite CT-275. Product yields and process kinetics
Renewable Energy xxx (xxxx) xxx
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Glycerol acetylation with acetic acid over Purolite CT-275. Product yields and process kinetics lan c, Ionut Banu a, Gheorghe Bumbac a, Dorin Bombos b, Sanda Velea c, Ana-Maria Ga Grigore Bozga a, * a b c
University Politehnica, Department of Chemical and Biochemical Engineering, 1-7 Polizu Str., 011061, Bucharest, Romania Oil & Gas University, Department of Chemistry, Bd. Bucures¸ti, 39, 100680, Ploiesti, Romania The National Institute for Research & Development in Chemistry and Petrochemistry, ICECHIM, 202 Spl. Independentei, 060021, Bucharest, Romania
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 May 2019 Received in revised form 23 August 2019 Accepted 10 October 2019 Available online xxx
An important category of glycerol derivatives, having multiple practical applications, are its esters with acetic acid (acetates or acetins). The most common preparation method of glycerol acetates is the direct esterification of glycerol with acetic acid in presence of acid catalysts. In this work is investigated the liquid phase glycerol esterification with acetic acid, catalyzed by the commercial Purolite CT-275 ion exchange resin. In this aim, there were performed batch experiments, in a stirred autoclave reactor, at initial acetic acid to glycerol molar ratios between 4 and 9 and temperatures between 70 and 110 C. The experimental data were used to develop a kinetic model, based on the Langmuir Hinshelwood theory. The components activities, involved in the calculation of the reaction rates, were evaluated by the UNIFAC Dortmund method. The proposed kinetic model predicts with good accuracy the products distribution dependencies on the reactants molar ratio and reaction temperature. The calculated values of the Weisz-Prater criteria evidenced influences of the internal diffusion on the esterification process kinetics, over an initial reaction time interval. The proposed kinetic model is directly utilizable in the commercial process analysis and design calculations. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Glycerol Acetylation Acetic acid Kinetic model Internal diffusion
1. Introduction A commercial process largely practiced in the last period is the transesterification of natural triglycerides with inferior alcohols (usually methanol) in the aim of producing biodiesel, a sustainable substitute of petro-diesel, having good combustion efficiency in internal combustion engines. As known, from this process it is obtained also the glycerol as a byproduct, in significant quantities. The estimated production of biodiesel for 2020 is 42 million cubic meters, which corresponds to approximately 4.2 million cubic meters of glycerol [1]. The rise of glycerol supply on the market triggered the efforts to find new valorization technologies, particularly by its conversion into liquid fuels additives and medium tonnage chemicals, replacing petroleum derivatives. Due to its highly functionalized molecule, the glycerol is used in the synthesis of more than a thousand chemical products [1e4]. In
* Corresponding author. E-mail address: [email protected] (G. Bozga).
the last decades there were proposed an increasing number of technologies for glycerol conversion into commodity chemicals, usually based on the heterogeneous catalysis (hydrogenolysis, dehydration, etherification, esterification, oxidation, reforming etc.). Among these, the acetic esters of glycerol, glycerol monoacetate, glycerol diacetate and glycerol triacetate, also called monoacetin (MA), diacetin (DA), and triacetin (TA) respectively, have important practical utilizations: liquid fuels additive, antimicrobial and emulsifying agent in pharmaceuticals and cigarette filters (TA), plasticizer and softening agent (DA); explosives manufacturing, solvent for dye and treatment of animal skin for leather manufacturing (MA). Among the methods applicable for the synthesis of glycerol acetates, the most common way is the direct esterification of glycerol with acetic acid. The main inconvenience of this method, a relatively low TA selectivity, can be avoided by working at high acetic acid to glycerol molar ratios and removing continuously the water product from the reaction mixture (using an entrainer or by applying the catalytic distillation technology). The glycerol esterification with acetic acid is catalyzed by acid substances. It can be
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performed also in the absence of catalysts, due to a slight catalytic effect of acetic acid. However, appropriate transformation rates are achieved only at relatively high temperature and acid excess, at long reaction times, practically the only product being MA [5,6]. Despite of their good catalytic activity, the mineral acids soluble in the reaction mixture (sulfuric, hydrochloric, nitric, p-toluenesulfonic acids etc.) called also homogeneous catalysts [7,8], present the drawbacks of corrosivity, non-reusability and negative environmental impact. As known, these inconveniences can be eliminated by using solid acidic materials as catalysts. An important number of acidic solid compounds have been investigated in the glycerol esterification: the acidic ion-exchange resins [9e13], tungstophosphoric acid [7,14,15], sulfated zirconia [16], silica functionalized with propio-sulfonic acid [17], active carbon and alumina treated with sulfuric acid [18,19], different zeolites [20e22] and mesoporous materials [16,23,24]. The performances of these catalysts in the glycerol acetylation process were compared by Goncalves et al. [20] and reviewed by Okoye and Hameed [6] and Kim et al. [25] respectively. Among the catalysts tested in the direct esterification of glycerol with acetic acid, good catalytic activity, reusability and selectivity towards DA and TA were reported for the ion-exchange resins [11].
1.1. Process kinetics The glycerol esterification consists of three consecutive-parallel steps, having the stoichiometry described by the equations:
G þ AA⇔MA þ W
(1)
MA þ AA ⇔DA þ W
(2)
DA þ AA ⇔TA þ W
(3)
where G-glycerol; AA-acetic acid; W- water. The kinetics of this process has received minimal research attention thus far, being investigated in a small number of published studies. Khayoon et al. [26] developed a LangmuirHinshelwood kinetic model for glycerol acetylation, but the authors do not publish the values of their model parameters. The only detailed kinetic model for this transformation, over Amberlyst-15 as catalyst, was published by Gelosa et al. [27]. In this study, special experiments performed without catalyst (blank experiments) evidenced that the homogeneous reaction rate is negligible as compared to the reaction rate in presence of the catalyst. The authors adopted the Langmuir-Hinshelwood theory, considering the surface reaction between the adsorbed species as controlling step. The kinetic model was successfully used to simulate a chromatographic reactor for the glycerol acetylation with acetic acid (Gelosa et al. [27]). Other authors proposed also few simpler kinetic models for glycerol acetylation. The main kinetic studies available in the open literature and the particularities of the proposed kinetic models are presented in Table 1. In the aim of increasing the DA and TA selectivity, usually it is working in presence of an important excess of acetic acid. This explains the disregard of variations in acetic acid concentration in some kinetic studies, assuming the first order reaction (Zhou et al. [11], Khayoon et al. [26]). As emphasized in the published studies (Okoye et al. [1], Gelosa et al. [27]) the product distribution of the glycerol acetylation with acetic acid is strongly influenced by the process reversibility. Therefore, the accuracy of the chemical equilibrium constants evaluation is one of the main issues in the esterification process modelling. Generally, the chemical equilibrium constants are
calculable from published thermodynamic data, using the Van’t Hoff equation:
DGR ðTÞ ¼ RT ln Keq ; DG0 ðTÞ ¼
s X
ðnj Gf ;j Þ
(4)
j¼1
Keq-chemical equilibrium constant; DGR-standard Gibbs energy variation in the esterification reaction; Gf,j-standard Gibbs energies of formation for species j; nj-stoichiometric coefficient (positive for products and negative for reactants); R-gas constant; Ttemperature. The liquid phase thermodynamic data for the species involved in the glycerol esterification with acetic acid are only partially available. As most tabulated data are given for gaseous state, the corresponding liquid phase values must be calculated by estimation methods, associated with unavoidable inaccuracies. Regarding the chemical equilibrium constant, as highlighted in the published studies (Hassan and Vinjamur [28]), the values deduced from thermodynamic data, are generally uncertain and nonreliable. This is explained by relatively large standard Gibbs energies of formation (Gf,j) for chemical species and small values of standard Gibbs energy variation in the reaction (DGR), correlated with the exponential dependence on equilibrium constant in respect with DGR. Therefore, small errors in the evaluation of Gf,j are conducting to large deviations in equilibrium constant value. This is the reason why many researchers preferred to determine Keq from experi€pken mental data (Gelosa et al. [27], Hassan and Vinjamur [28], Po et al. [29]). The components involved in the glycerol esterification have polar molecules, so that their behavior is expected to feature significant non-idealities. Therefore, in the thermodynamic and kinetic calculations, it is recommended to take into consideration components activities instead of concentrations. The deactivation of sulfonated resin catalysts in esterification reactions is only scarcely investigated in the published works. Okoye et al. [1] survey the possible mechanisms, among which, the most important appear to be the poisoning by in situ formed water, leaching of sulfonic active functional groups from the catalyst surface, and eventually the deposition of high carbon content secondary products. The detrimental effect of the water on the acid resin activity, in glycerol acetylation reaction, was evidenced by Zhou et al. [11], working with Amberlyst-15 ion exchange resin. Liao et al. [33] demonstrated experimentally that the Amberlyst-35 (A-35) resin keeps the catalytic activity practically unchanged, during five reaction cycles of glycerol acetylation at 105 C. The authors are emphasizing that, the A-35 resin catalyst breaks up when the reaction is performed near its limited temperature (120 C), and that it is more sensitive to the temperature rather than reaction time. Glycerol esterification with acetic acid is a reversible process, occurring in three successive steps. The equilibrium limitation is less important for the first step and very important for the third step of triacetin formation. Therefore, when the triacetin is the desired product, modern methods like catalytic distillation (Mufrodi et al. [34]) or chromatographic reactors (Gelosa et al. [27]) could be applied. The objective of this study was to investigate, experimentally and theoretically, the liquid phase glycerol esterification with acetic acid, catalyzed by the commercial Purolite CT-275 ion exchange resin. There were evidenced experimentally the temperature and reactant initial ratio influences on the product yields. A kinetic model was proposed, following the Langmuir-Hinshelwood theory, and considering the real behavior of components in the mixture. The liquid phase activities of the components were calculated by
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Table 1 Published kinetic models for the glycerol esterification with acetic acid.[30e32]..
the UNIFAC Dortmund method. Based on the experimental data, there was analyzed the influence of the internal diffusion on the esterification process kinetics, using the Weisz-Prater approach. The values of the Weisz-Prater criteria evidenced a limitation effect of internal diffusion, over an incipient time interval. At the best of our knowledge, this is the first kinetic study of glycerol acetylation over Purolite CT-275, developing a kinetic model which considers the non-idealities of the reaction mixture, accompanied by the analysis of internal mass transport step limitation over the entire reaction time interval. 2. Experimental In all experiments there were used glycerol and acetic acid of analytical purity. As catalyst, we used commercial Purolite CT-275, a macroporous strong acid resin catalyst (macroporous polystyrene crosslinked with divinylbenzene), kindly supplied by Purolite
Romania, having the characteristics given in Table 2 (available on the manufacturer website, at the address https://www.purolite. com/product/ct275). Before use, the resin was washed several times with distilled water and with ethanol, finally dried in an oven under vacuum at 95 C, for 12 h. The acidity of the catalyst was
Table 2 Characteristics of the catalyst (Purolite CT-275). Property
Value
Exchange capacity (dry) Bulk density Particle size range Surface area Average pore diameter Particle porosity Maximum temperature limit
5.2 Eq Hþ/kg 760 kg/m3 0.425 ÷ 1.20 mm 24 m2/g 652 Å 0.38 130 C
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checked by the procedure described by Dijs et al. [35], confirming the producer’s specifications. A sieving analysis (in dry state) using meshes of different openings led to the particles size distribution given in Table 3. This corresponds to an average value of catalyst particle diameter of 0.6 mm. SEM images of catalyst particle shape and structure are provided in Fig. 1 A and B. From separate experiments, conducted using glycerol-acetic acid mixtures, having compositions typical for esterification experiments, we estimated an average swelling ratio (volumes ratio of swelled to dry particles) of 1.47. The esterification experiments were carried out in liquid phase, batch-wisely in a stainless-steel autoclave (Berghof) of 300 mL, provided with a heat transfer jacket, a magnetically driven mixing impeller and standard transmitters for internal pressure, temperature and impeller rotation frequency. The working temperature was controlled by circulating, through the reactor jacket, of low volatility thermal oil, fed from a thermo-regulated bath. The glycerol esterification experiments were performed under a nitrogen atmosphere at 10 bar, temperatures between 70 and 110 C, initial acetic acid to glycerol molar ratios ranging from 4 to 9, stirrer rotation speed 1000 rpm and a catalyst loading of 0.065 g/g glycerol. The composition analyses were performed by gas chromatography (Varian CP-3800, VF-5ms capillary column, 30 m 0.25 mm x 0.25 mm). FID and injector temperatures were fixed at 280 C. GC analyses were carried out according to the following temperature program: initially, the oven temperature was set at 125 C, then the temperature increased from 125 C to 250 C with a rate of 10 C/min, and held at 250 C for 5 min. The concentrations of glycerol, diacetin and triacetin were determined by using suitable calibration curves, obtained by the external standard method, using glycerol, diacetin and triacetin of known concentrations. As pure monoacetin was not available, the concentration of MA was calculated by molar balancing between the reacted glycerol and DA and TA products respectively. In each experiment, there were charged into autoclave the appropriate amounts of acetic acid and ion exchange resin, which were heated to approximately 70 C. At this temperature, it was fed the glycerol in the desired molar ratio and the autoclave was sealed and pressurized with nitrogen at 10 bar, starting the timing of the experimental run. Further, the autoclave heating was continued until a pre-specified temperature level was reached, which was kept constant till the final reaction. The temperature evolutions were measured and registered rigorously in each experiment. In the discussion below, the temperature regime of each experiment will be referred by the final temperature. The time evolution of the reaction mixture composition was determined by periodical sampling. The composition analyses were performed at least twice for each sample, to minimize the measurement errors. The concentration measurements, performed at different reaction times, were used to calculate the glycerol conversion (fraction of consumed glycerol) and the yields of the three esters, using the relation:
yield of J ¼ 100
moles J in the mixture ; J ¼ MA; DA; TA initial moles of glycerol (5)
Table 3 Catalyst particle size distribution. Size range, mm
0e0.425
0.425e0.63
0.63e0.8
0.8e1.0
1.0e1.2
wt %
0.9
62.3
12.4
24
0.4
3. Results and discussion The acetylation experiments are characterized by a fairly good reproducibility, as resulted from the replicate test presented in Fig. 2 (two experiments). The experimental results are presented graphically, as time dependencies of glycerol conversion and diacetin and triacetin yields, in Figs. 3e6 (the points in these figures represent experimental data). Our experimental study showed that the Purolite CT275 resin is an active catalyst for glycerol acetylation. Among the three reactions, the first one (glycerol esterification) is fastest and the last reaction, of DA acetylation, occurs most slowly. Equilibrium conversion of glycerol (almost total at initial molar ratios AA to G higher than 6) is practically achieved, in our working conditions, after approximately 2 h. Nevertheless, the overall stabilization of products concentrations (approaching the equilibrium composition) was achieved in much longer reaction times. The effect of initial AA/G molar ratio can be observed from Figs. 3e5. By increasing the initial molar ratio from 4 to 9, at a temperature of 100 C and a reaction time of 350 min, the DA yield increases from 0.46 to 0.58 and the TA yield increases from 0.08 to 0.24. The reaction temperature effect on the observed acetylation process kinetics is more complex, cumulating the influences of the temperature on the rate constant and chemical equilibrium composition respectively. As observed from Figs. 4 and 6, by increasing the temperature from 100 C to 110 C, at an initial molar ratio of 6:1 and a reaction time of 350 min, the reaction rates accelerate on the first time interval (the slopes of the curves are increased), but the final yields do not change significantly (TA yield increases only slightly, from 0.15 to 0.18, whereas the DA yield practically does not change). The maximum TA yield obtained in our experiments was 0.3, for the AA/G ¼ 9 and temperature 110 C (the corresponding DA yield being 0.56). 3.1. Evaluation of the influence of physical steps on the overall kinetics In the solid catalyzed esterification processes, the chemical reaction occurring on the internal catalyst surface is preceded by the mass transfer of reagents from the bulk liquid to the external surface of catalyst pellet (external diffusion) and the reactants transport inside the pores network of the pellet (internal diffusion). As the experimental results evidenced that the glycerol consumption in the reaction (1) has the most rapid kinetics, we investigated the diffusional limitations only for this reaction. 3.1.1. External diffusion The influence of the external diffusion step on the process kinetics is mainly dependent on the rotation frequency of the stirrer. Zhou et al. [11] found that, for liquid phase glycerol esterification with acetic acid over Amberlyst-15, (AA/G molar ratios between 3 and 9 and temperatures up to 100 C), the limitation induced by the external diffusion step is negligible for rotation speeds over 800 rpm. Similarly, Gelosa et al. [27] reported a negligible influence of the external diffusion step, for the same process investigated at temperatures up to 100 C, at stirrer speeds over 600 rpm. Consequently, we assumed that, for a rotation speed of 1000 rpm in our experiments, the influence of this step is also negligible. 3.1.2. Internal diffusion The Purolite CT-275 ion exchange resin is a macroporous, strongly acidic resin, consisting in a matrix of polystyrene highly crosslinked with divinylbenzene. The acidity is provided by the sulfonic acid groups (-SO3H) linked essentially to the aromatic carbons. Physically, the ion-exchange beads (having the diameters
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Fig. 1. SEM images of Purolite CT 275 resin particles (A e commercial particles shapes; B e microscopic structure).
Fig. 2. Reproducibility test for glycerol acetylation (AA/G ¼ 6; T ¼ 110 C).
Fig. 3. Time variation of glycerol conversion and product DA and TA yields. The points represent measured values and the solid lines the calculated ones (initial molar ratio AA:G ¼ 4; Temperature 94 C).
Fig. 4. Time variation of glycerol conversion and product DA and TA yields. The points represent measured values and the solid lines the calculated ones (initial molar ratio AA:G ¼ 6; Temperature 100 C).
Fig. 5. Time variation of glycerol conversion and product DA and TA yields. The points represent measured values and the solid lines the calculated ones (initial molar ratio AA:G ¼ 9; Temperature 100 C).
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mixture, DG, by the glycerol binary diffusion coefficient in acetic acid (as suggested by Gelosa et al. [27]). This was calculated as an average of the values predicted by the Wilke-Chang, Tyn- Calus and Hayduk-Minhas methods (Reid et al. [40]). The viscosity of the mixture, involved in these calculations, was evaluated also as an average using correlations and data published by Reid et al. [40] and Viswanath [41]. Based on the data published by Soto et al. [42] for Purolite CT275 resin and the swelling ratio evaluated in our experiments, we estimated a porosity of the swelled bead, ε ¼ 0.55. The tortuosity of the swelled pellet pores network was selected from literature data, t ¼ 3. The internal diffusion has a negligible limitation effect on the overall kinetics, if WPc < 1 (Bischoff [43]).
Fig. 6. Time variation of glycerol conversion and product DA and TA yields. The points represent measured values and the solid lines the calculated ones (initial molar ratio AA:G ¼ 6; Temperature 110 C).
practically between 0.4 mm and 1.2 mm), are agglomerates of geltype microparticles (microspheres) having the diameter below 0.1 mm, interspersed with macropores, as resulting from the image presented in Fig. 1B. So, in a macroreticular resin bead are present two types of pores: micropores inside the microparticles and macropores in the spaces between microparticles. It is appreciated that about 5% of the acidic (-SO3H) groups are placed on the outer surface of microparticles, the remainder being located inside the microparticles. Published studies are hypothesizing that, due to their small dimensions, the process kinetics inside the microparticles should not be limited significantly by the internal diffusion (Chakrabarti and Sharma [36], Xu and Chuang [37], Ramírez et al. [38]). 3.1.2.1. Classical Weisz-Prater criterion. The limitation influence of the internal diffusion on the process kinetics at the level of a catalyst pellet is usually evaluated using the classical Weisz-Prater (WP) criterion [39]: ðvÞ
WPc ¼
rrG;obs L2 Deff CG
; L ¼ Rp =3
(6)
Rp-radius of the catalyst pellet; CG e bulk liquid glycerol concentration (kmol/m3); Deff - the effective diffusion coefficient of glycerol inside the catalyst pore network. ðvÞ The observed transformation rate of glycerol (rrG;obs ) was calculated from the measured time dependence of glycerol conversion (XG) using the relation: ðvÞ
rrG;obs ¼ rp
nG;0 dXG mcat dt
(7)
mcat-amount of dry catalyst in the reaction mixture (kg); rp-density of the dry pellet (kg/m3); nG,0 einitial quantity of glycerol (kmol); The effective diffusion coefficient of glycerol, inside the catalyst pore network, was calculated by the relation:
ε Deff ¼ DG ;
t
(8)
Considering the limited accuracy of existing correlations for the prediction of diffusion coefficient in multicomponent liquid mixtures and the existence of an acetic acid excess in the liquid during the period of interest for our study (that of glycerol consumption), we approximated the molecular diffusion coefficient in the reaction
3.1.2.2. The Bischoff Weisz-Prater criterion. The expression of the Weisz-Prater criterion (6) is based on the generalization of the results particular for the first order reaction kinetics. Bischoff [43] extended the WP criterion for a more general reaction kinetics, based on the idea of a generalized Thiele modulus which makes the internal effectiveness factor to follow essentially the same curve for all reaction rate forms. For reaction rate expressions having the mathematical form:
rrðvÞ ¼ kv gðCG Þ
(9)
Bischoff developed the WP type criterion: ðvÞ
WPg ¼
rrG;obs L2 gðCG Þ ð CG 2 Deff gðCÞ dC
(10)
CG;eq
Similarly, the overall catalytic process kinetics is considered free of internal diffusion limitation, if WPg is smaller than unity (WPg < 1). More conservative limits were proposed to be 0.3 for second order reactions and 0.6 for first order ones (Mears [44]). As will be shown in the next section of this work (paragraph 4), we have developed a Langmuir-Hinshelwood type reaction rate expression for glycerol consumption (r1), having the form given in the relations (19). In this expression, the reaction rate dependence on the glycerol concentration is an implicit one, included by the intermediate of species activities (aj):
gðCG Þ ¼
aW aG aAA aMA Ka;1 !2 ; aj ¼ Fj ðCG Þ; kv ¼ k1 rcat P 1 þ KJ aJ
(11)
J
Z CG The integral, I ¼ gðCÞ dC, appearing in the relation (10) was CG;eq calculated by integrating, on the interval [CG,eq, CG], the system of differential equation (12). The first of these equations, describing the dependencies of species concentrations in respect with glycerol concentration, are deduced from the mass balance equation (23). The last one is a transform of the integral to be calculated.
dCj rrj ¼ ; C ¼ CG;eq ; dC rrG dI ¼ gðCÞ; C ¼ CG;eq ; dC
Cj ¼ Cj;eq ; j ¼ AA; MA; DA; TA; W I¼0 (12)
The equilibrium concentration (CG,eq) was calculated based on the chemical equilibrium constants, determined as described below. The values of the two WP criteria WPc and WPg, for three of our experiments are presented in Fig. 7. As observed, the values of
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values as high as 4.5 for glycerol and 1.7 for mono-acetate. Therefore, it is more properly to define rate expressions in activities, instead of concentrations of the species. Considering this issue, we developed a kinetic model using components activities, following €pken et al. [29] and the general approaches used in the studies of Po Gelosa et al. [27] respectively. The chemical mechanism of the glycerol acetylation over ion exchange resin catalysts is explained by Okoye et al. [1] and Zhou et al. [11] respectively. In the development of the kinetic model we considered that the bulk liquid phase reactions (homogeneous kinetics) have a negligible contribution to the overall kinetics. This hypothesis is supported by the observations of Zhou et al. [11] and Gelosa et al. [27] respectively, in glycerol acetylation studies over Amberlyst-15 resin (slightly less acid than Purolite CT-275). We developed activity-based rate expressions for the reactions (1) to (3) using the Langmuir-Hinshelwood (LH) theory. Published kinetic studies evidenced that the controlling steps of the surface esterification reactions over acidic resin catalysts are the chemical reactions occurring between adsorbed molecules of reactants (dual site mechanism) (Okoye and Hameed [6], Gelosa et al. [27]). In this hypothesis, assuming that the reactants adsorption and products desorption steps occur close to equilibrium, one obtains the approximate relations:
Fig. 7. Weisz Prater criteria values in time, for three experiments. In the legend are given the working values of reaction temperature and initial AA/G molar ratio).
WPg criterion are significantly different of the WPc ones, except for highest AA/G molar ratio (presumably due to a kinetic behavior better approaching the first order one, at the highest acetic acid excess). The values of WPg criterion are higher over the entire investigated interval, this suggesting that the classical expression (6) is underestimating, in this case, the limitative effect of internal diffusion. The values of WP criteria are evidencing limitation effects of internal diffusion during an initial time interval, corresponding to the glycerol depletion time. For the highest AA/G ratio and working temperature, due to the highest glycerol consumption rate, the duration of the diffusion control interval is the shortest. On the other hand, for lower molar ratio and temperature, the WPg values indicate a smaller influence of internal diffusion, extending over a larger time, due to a relatively slower consumption rate of glycerol. As already underlined, in this analysis it was considered only the glycerol esterification to MA, which occurs faster than the MA and DA esterification reactions. Consequently, the latter reactions are expected to be less influenced by internal and external diffusion steps, as compared with the glycerol esterification. Our results, regarding the influence of internal diffusion, are in accord with the findings of Gelosa et al. [27], for the glycerol esterification with acetic acid, using Amberlyst-15 resin as catalyst. The authors evidenced possible internal diffusion effects on glycerol esterification kinetics, even for milder conditions (60 C).
QJ
KJ ¼
aJ QS
;
J ¼ G; AA; MA; DA; TA; W
KJ-adsorption equilibrium constant; QJ-fraction of active sites on the catalyst surface occupied with component J; QS- fraction of the free active sites on the catalyst surface; aJ e activity of the component J in the bulk liquid phase. The overall balance of active sites is expressed by the equation:
QS þ QG þ QMA þ QDA þ QTA þ QW ¼ 1
QJ ¼
KJ aJ P ; J ¼ G; AA; MA; DA; TA; W 1 þ KJ aJ
The rate expressions of the three acetylation reactions, occurring on the catalyst surface, have the expressions:
r1 ¼ k1;S QG QAA
As already mentioned, due to the molecule polarity, the behavior of the chemical species in liquid mixture presents significant non-idealities that should be accounted for in the kinetic study. At typical compositions for this transformation, the activity coefficients, calculated by UNIFAC Dortmund method (Constantinescu and Gmehling [45] and the references therein), attain
r1 ¼
P 1 þ KJ aJ J
!2 ; r2 ¼
P 1 þ KJ aJ J
!2
; r3 ¼
(15)
J
QMA QW
r2 ¼ k2;S QMA QAA
aW k2 aMA aAA aDA Ka;2
(14)
From the previous two equations are obtained the fractions of adsorbed species:
4. Development of the kinetic model
aW k1 aG aAA aMA Ka;1
(13)
r3 ¼ k3;S QDA QAA
! (16)
Keq;s;1
QDA QW
!
Keq;s;2
QTA QW Keq;s;3
(17)
! (18)
From the relations (15) to (18), there were derived the rate expressions:
aW k3 aDA aAA aTA Ka;3 P 1 þ KJ aJ
!2
(19)
J
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k1 ¼ k1s KG KAA ;
k2 ¼ k2s KMA KAA ;
k3 ¼ k3s KDA KAA :
In the expressions presented above, Keq,s,i and Ka,i are chemical equilibrium constants for reaction i, defined in respect with concentrations of adsorbed species and activities in the bulk liquid respectively. To limit the complexity of reaction rate expressions, we assumed that, among the components of the mixture, the most strongly adsorbed on the catalyst surface are the water, glycerol and acetic acid. This hypothesis is in accord with the observations of Gelosa et al. [27], acquired from adsorption experiments on the Amberlyst-15 resin catalyst. Consequently, the terms corresponding to MA, DA and TA in the summation appearing in the denominator (adsorption term) of the rate expressions (19) were neglected:
X KJ aJ yKW aW þ KG aG þ KAA aAA
(20)
J
The apparent rate constants k1, k2 and k3, involved in the expressions (19), are temperature dependent, following Arrhenius type functions:
ki ¼ km;i
E z expð i Þ ; R
1 1 z¼ ; 363 T
i ¼ 1; 2; 3 ;
dnJ ¼ mcat rrJ ; dt
t ¼ 0; nJ ¼ nJ0 ;
J¼
the reaction i. The kinetic model so developed is characterized by the parameters km,1, km,2, km,3, E1,E2, E3, KG, KAA, KW, DGR,1, DGR,2 and DGR,3. € pken et al. [29] suggested a method to determine the Po adsorption equilibrium constants, KJ, from separate experiments, determining the swelling ratio of catalyst particles in the pure components of reaction mixture. However, due to difficulties to find all the glycerol esters in pure state, this method is hardly applicable for this process. Therefore, we estimated the three adsorption equilibrium constants, KG, KAA, and KW (as average values on the working interval), from experimental data, simultaneously with the other parameters of the model. As already stated, the standard free energy variations (DGR,i) appearing in the calculation of the chemical equilibrium constants (Ka,i) could be calculated using thermodynamic relations and published data. However, as already emphasized, this approach is leading to important errors for liquid phase reactions, due to lack of reliable liquid phase thermodynamic data. Usually, the standard free energy variations DGR,i are estimated from the experimentally measured compositions for long reaction times in isothermal €pken et al. [29]). This was conditions (Hassan and Vinjamur [28], Po the approach followed also in this work. We measured reaction mixture compositions at reaction times of minimum 8 h, at rigorously controlled temperature and used them to calculate species activities and further the equilibrium constants, Ka,i, of the three reactions. The values of equilibrium constants (Ka,i), obtained in different working conditions, were used to estimate DGR,1, DGR,2 and DGR,3 using the relation (22), by the least squares method (Table 4). The
(23)
nTA ¼ nG0 ðnG þ nMA þ nDA ; nAA ¼ nAA;0 ðnMA þ 2nDA þ 3nTA
(24)
(25)
nW ¼ nAA;0 nAA
(26)
The value of the formation rate for a species J was calculated from the relation:
(22)
DGR,i - standard free energy (Gibbs energy) variation associated to
MAG; DAG
nJ-moles number of species J in the bulk liquid; rrJ-formation rate of species J (kmole/kg/s); mcat-mass of the dry catalyst in the mixture. The triacetin (TA), acetic acid (AA) and water (W) number of moles in the mixture were calculated from the stoichiometric relations:
(21)
Ei - apparent activation energies; km,i - rate constants values at 363 K. The chemical equilibrium constants Ka,1, Ka,2 and Ka,3 are also temperature dependent:
DGR;i ; i ¼ 1; 2; 3 Ka;i ¼ exp RT
calculated and measured values of the equilibrium constants are graphically compared in Fig. 8. Hassan and Vinjamur [28] found that the chemical equilibrium constants for esterification of organic acids with mono-hydroxylic alcohols, at 298 K, vary between 1.91 and 4.58. As order of magnitude, the values of the chemical equilibrium constants we determined for the glycerol esterification with acetic acid (including also the activity coefficients) are in an acceptable agreement with this general observation. The kinetic parameters km,1, km,2, km,3, E1, E2, E3, KG, KAA and KW were estimated from experimental data by using a non-linear least squares method (function “lsqcurvefit” of Matlab software). In this aim we used the mass balance equations specific for the perfectly stirred batch reactor:
rrJ ¼
3 X
ðniJ ri Þ
(27)
i¼1
niJ e stoichiometric coefficient of species J in the reaction i (negative for reactants). To estimate the kinetic parameters, we used nine sets of measured values for glycerol conversion and ester products yields in time, for a duration of approximately 6 h, at different temperatures and AA/G ratios, few of which being already discussed above. The estimated values of the kinetic model parameters and the corresponding 95% confidence intervals are presented in Table 5. These estimates correspond to a correlation coefficient, R2 ¼ 0.98. In the same table are given, for comparison, the activation energies found in other studies using Amberlyst-15 as catalyst (a similar ion exchange resin). As observed, the activation energies found in our study are slightly higher but not very different of the published ones. The predictions of the glycerol conversion and glycerol esters yields are compared with measured data in Figs. 3e6 (and Figs. S1eS5 in the section Supplementary Information), for different AA/G ratios and reaction temperatures. As can be observed, the calculated values are in good agreement with the experimental data, this demonstrating, along with the small size of statistical intervals and a relatively high correlation coefficient, the Table 4 Estimated values of the standard free energy variations in the three reactions. Reaction, i
(1)
(2)
(3)
DGR,i (J/mol) Ka,i at 90 C
5117.9 5.45
960.9 1.37
3371.4 0.33
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I. Banu et al. / Renewable Energy xxx (xxxx) xxx
Fig. 8. Estimated (lines) and experimental (points) values of the chemical equilibrium constants.
Table 5 Estimated values of kinetic parameters (R2 ¼ 0.98). Parameter
km,1 km,2 km,3 E1/R E2/R E3/R KG KAA KW
Value
0.0162 (1 ± 0.003) 0.0118 (1 ± 0.008) 0.003 (1 ± 0.015) 7650 (1 ± 0.005) 3198 (1 ± 0.027) 3030 (1 ± 0.064) 5.4 (1 ± 0.004) 2.5 (1 ± 0.005) 10.0 (1 ± 0.008) 6.82,(1 ± 0.0111)
Units
kmol/(kg$s) kmol/(kg$s) kmol/(kg$s) K K K e e e
Published values Zhou et al. [11]
Gelosa et al. [27]
6886 3883 1672
7500 2100 2200
good adequacy of the proposed kinetic model. As already underlined, the parameters of the developed kinetic model include the internal mass transport limitations, occurring during a temporary incipient reaction time interval. The proposed model is directly useable for process calculations in industrial operating conditions (using commercial catalyst Purolite CT-275), not being necessary to calculate the temporary limitation effects of the mass transport step. This is avoiding the necessity to evaluate theoretically the internal diffusion limitation, involving uncertainties in the prediction of transport properties, by published correlations.
5. Conclusions The commercial Purolite CT-275 ion exchange resin proved a good catalytic activity in the glycerol esterification with acetic acid. Working batch-wisely at a relatively low resin load (0.065 g/g glycerol), initial reactants ratios of minimum 4 and temperatures over 95 C, equilibrium glycerol conversions were approached in 100 min and triacetin equilibrium yields in 260 min reaction times. It was observed that, on the working interval, the initial AA/G ratio has a stronger impact on the DA and TA yields, as compared with the temperature. Increasing the initial molar ratio from 4 to 9 (at 100 C and a reaction time of 350 min), the DA yield increased from 0.46 to 0.58 and the TA yield from 0.08 to 0.24. A temperature increment of 10 C (from 100 to 110 C) led to a TA yield increase
9
from 0.15 to 0.18 and an insignificant increase of DA yield. The experimental data are also evidencing a faster glycerol esterification to MA, as compared with the following steps of MA and DA transformations. The theoretical evaluation of the internal diffusion effects on glycerol transformation kinetics indicated the existence of an initial reaction time interval, which is superposing on the glycerol transformation time, where the internal diffusion has a significant limitation effect on the glycerol consumption kinetics. An important step in the study of the intrinsic kinetics of glycerol acetylation is the prediction of the chemical equilibrium constants. Preliminary trials for estimation of the three chemical equilibrium constants, based on thermodynamic relations and published data, failed to obtain reliable results, evidencing the necessity of experimental equilibrium data. A kinetic model of the glycerol acetylation over Purolite CT-275 has been developed, based on independent equilibrium and kinetic experiments performed in a batch laboratory reactor. The developed LH reaction rate expressions, including species activities and considering as controlling steps the chemical reactions of adsorbed species on the catalyst surface, describe adequately the time evolutions of the glycerol conversion and of the product yields, measured in batch experiments. The proposed kinetic model is useful for the prediction of glycerol acetylation process performance, conducted in different operation regimes. At our knowledge, this work represents the first kinetic study of glycerol acetylation over Purolite CT-275 and the first kinetic modelling of this process, considering the non-idealities of the reaction mixture, accompanied by a thorough analysis of internal mass transport limitation, during the entire reaction time. Acknowledgments This work has been funded by Romanian Government, Executive Unit for Financing Higher Education, Research, Development and Innovation (UEFISCDI), Romania Bridge Grant Research Project PNeIIIeP2-2.1-BG-2016-0324 through the Financial Agreement 80BG/2016. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.renene.2019.10.060. References [1] P.U. Okoye, A.Z. Abdullah, B.H. Hameed, Renew. Sustain. Energy Rev. 74 (2017) 387e401. [2] E.R. Bozga, V. Plesu, G. Bozga, C.S. Bildea, E. Zaharia, Rev. Chim. 62 (6) (2011) 646e654. [3] I. Banu, G. Guta, C.S. Bildea, G. Bozga, Environ. Eng. Manag. J. 14 (3) (2015) 509e517. [4] A.R. Trifoi, P.S¸. Agachi, T. Pap, Renew. Sustain. Energy Rev. 62 (2016) 804e814. [5] J. Bonet, J. Costa, R. Sire, J.-M. Reneaume, A.E. Ples¸u, V. Ples¸u, G. Bozga, Food Bioprod. Process. 87 (3) (2009) 171e178. [6] P.U. Okoye, B.H. Hameed, Renew. Sustain. Energy Rev. 53 (2016) 558e574. [7] M.A. Betiha, H.M.A. Hassan, E.A. El-Sharkawy, A.M. Al-Sabagh, M.F. Menoufy, H.E.M. Abdelmoniem, Appl. Catal. B Environ. 182 (2016) 15e25. [8] K.B. Ghoreishi, N. Asim, M.A. Yarmo, M.W. Samsudin, Chem. Pap. 68 (9) (2014) 1194e1204. [9] I. Dosuna-Rodriguez, C. Adriany, E.M. Gaigneaux, Catal. Today 167 (1) (2011) 56e63. [10] I. Dosuna-Rodriguez, E.M. Gaigneaux, Catal. Today 195 (1) (2012) 14e21. [11] L.M. Zhou, T.H. Nguyen, A.A. Adesina, Fuel Process. Technol. 104 (2012) 310e318. [12] S. Kale, S.B. Umbarkar, M.K. Dongare, R. Eckelt, U. Armbruster, A. Martin, Appl. Catal. Gen. 490 (2015) 10e16. [13] I. Banu, G. Bozga, G. Bumbac, A. Vintila, S. Velea, A.-M. Galan, M. Bombos, O. Blajan, A.C. Crucean, Rev. Chim. 70 (7) (2019) 2325e2329. [14] P. Ferreira, I.M. Fonseca, A.M. Ramos, J. Vital, J.E. Castanheiro, Appl. Catal. B Environ. 91 (1e2) (2009) 416e422. [15] M. Balaraju, P. Nikhitha, K. Jagadeeswaraiah, K. Srilatha, P.S.S. Prasad,
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Glycerol acetylation with acetic acid over Purolite CT-275. Product yields and process kinetics lan c, Ionut Banu a, Gheorghe Bumbac a, Dorin Bombos b, Sanda Velea c, Ana-Maria Ga Grigore Bozga a, * a b c
University Politehnica, Department of Chemical and Biochemical Engineering, 1-7 Polizu Str., 011061, Bucharest, Romania Oil & Gas University, Department of Chemistry, Bd. Bucures¸ti, 39, 100680, Ploiesti, Romania The National Institute for Research & Development in Chemistry and Petrochemistry, ICECHIM, 202 Spl. Independentei, 060021, Bucharest, Romania
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 May 2019 Received in revised form 23 August 2019 Accepted 10 October 2019 Available online xxx
An important category of glycerol derivatives, having multiple practical applications, are its esters with acetic acid (acetates or acetins). The most common preparation method of glycerol acetates is the direct esterification of glycerol with acetic acid in presence of acid catalysts. In this work is investigated the liquid phase glycerol esterification with acetic acid, catalyzed by the commercial Purolite CT-275 ion exchange resin. In this aim, there were performed batch experiments, in a stirred autoclave reactor, at initial acetic acid to glycerol molar ratios between 4 and 9 and temperatures between 70 and 110 C. The experimental data were used to develop a kinetic model, based on the Langmuir Hinshelwood theory. The components activities, involved in the calculation of the reaction rates, were evaluated by the UNIFAC Dortmund method. The proposed kinetic model predicts with good accuracy the products distribution dependencies on the reactants molar ratio and reaction temperature. The calculated values of the Weisz-Prater criteria evidenced influences of the internal diffusion on the esterification process kinetics, over an initial reaction time interval. The proposed kinetic model is directly utilizable in the commercial process analysis and design calculations. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Glycerol Acetylation Acetic acid Kinetic model Internal diffusion
1. Introduction A commercial process largely practiced in the last period is the transesterification of natural triglycerides with inferior alcohols (usually methanol) in the aim of producing biodiesel, a sustainable substitute of petro-diesel, having good combustion efficiency in internal combustion engines. As known, from this process it is obtained also the glycerol as a byproduct, in significant quantities. The estimated production of biodiesel for 2020 is 42 million cubic meters, which corresponds to approximately 4.2 million cubic meters of glycerol [1]. The rise of glycerol supply on the market triggered the efforts to find new valorization technologies, particularly by its conversion into liquid fuels additives and medium tonnage chemicals, replacing petroleum derivatives. Due to its highly functionalized molecule, the glycerol is used in the synthesis of more than a thousand chemical products [1e4]. In
* Corresponding author. E-mail address: [email protected] (G. Bozga).
the last decades there were proposed an increasing number of technologies for glycerol conversion into commodity chemicals, usually based on the heterogeneous catalysis (hydrogenolysis, dehydration, etherification, esterification, oxidation, reforming etc.). Among these, the acetic esters of glycerol, glycerol monoacetate, glycerol diacetate and glycerol triacetate, also called monoacetin (MA), diacetin (DA), and triacetin (TA) respectively, have important practical utilizations: liquid fuels additive, antimicrobial and emulsifying agent in pharmaceuticals and cigarette filters (TA), plasticizer and softening agent (DA); explosives manufacturing, solvent for dye and treatment of animal skin for leather manufacturing (MA). Among the methods applicable for the synthesis of glycerol acetates, the most common way is the direct esterification of glycerol with acetic acid. The main inconvenience of this method, a relatively low TA selectivity, can be avoided by working at high acetic acid to glycerol molar ratios and removing continuously the water product from the reaction mixture (using an entrainer or by applying the catalytic distillation technology). The glycerol esterification with acetic acid is catalyzed by acid substances. It can be
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performed also in the absence of catalysts, due to a slight catalytic effect of acetic acid. However, appropriate transformation rates are achieved only at relatively high temperature and acid excess, at long reaction times, practically the only product being MA [5,6]. Despite of their good catalytic activity, the mineral acids soluble in the reaction mixture (sulfuric, hydrochloric, nitric, p-toluenesulfonic acids etc.) called also homogeneous catalysts [7,8], present the drawbacks of corrosivity, non-reusability and negative environmental impact. As known, these inconveniences can be eliminated by using solid acidic materials as catalysts. An important number of acidic solid compounds have been investigated in the glycerol esterification: the acidic ion-exchange resins [9e13], tungstophosphoric acid [7,14,15], sulfated zirconia [16], silica functionalized with propio-sulfonic acid [17], active carbon and alumina treated with sulfuric acid [18,19], different zeolites [20e22] and mesoporous materials [16,23,24]. The performances of these catalysts in the glycerol acetylation process were compared by Goncalves et al. [20] and reviewed by Okoye and Hameed [6] and Kim et al. [25] respectively. Among the catalysts tested in the direct esterification of glycerol with acetic acid, good catalytic activity, reusability and selectivity towards DA and TA were reported for the ion-exchange resins [11].
1.1. Process kinetics The glycerol esterification consists of three consecutive-parallel steps, having the stoichiometry described by the equations:
G þ AA⇔MA þ W
(1)
MA þ AA ⇔DA þ W
(2)
DA þ AA ⇔TA þ W
(3)
where G-glycerol; AA-acetic acid; W- water. The kinetics of this process has received minimal research attention thus far, being investigated in a small number of published studies. Khayoon et al. [26] developed a LangmuirHinshelwood kinetic model for glycerol acetylation, but the authors do not publish the values of their model parameters. The only detailed kinetic model for this transformation, over Amberlyst-15 as catalyst, was published by Gelosa et al. [27]. In this study, special experiments performed without catalyst (blank experiments) evidenced that the homogeneous reaction rate is negligible as compared to the reaction rate in presence of the catalyst. The authors adopted the Langmuir-Hinshelwood theory, considering the surface reaction between the adsorbed species as controlling step. The kinetic model was successfully used to simulate a chromatographic reactor for the glycerol acetylation with acetic acid (Gelosa et al. [27]). Other authors proposed also few simpler kinetic models for glycerol acetylation. The main kinetic studies available in the open literature and the particularities of the proposed kinetic models are presented in Table 1. In the aim of increasing the DA and TA selectivity, usually it is working in presence of an important excess of acetic acid. This explains the disregard of variations in acetic acid concentration in some kinetic studies, assuming the first order reaction (Zhou et al. [11], Khayoon et al. [26]). As emphasized in the published studies (Okoye et al. [1], Gelosa et al. [27]) the product distribution of the glycerol acetylation with acetic acid is strongly influenced by the process reversibility. Therefore, the accuracy of the chemical equilibrium constants evaluation is one of the main issues in the esterification process modelling. Generally, the chemical equilibrium constants are
calculable from published thermodynamic data, using the Van’t Hoff equation:
DGR ðTÞ ¼ RT ln Keq ; DG0 ðTÞ ¼
s X
ðnj Gf ;j Þ
(4)
j¼1
Keq-chemical equilibrium constant; DGR-standard Gibbs energy variation in the esterification reaction; Gf,j-standard Gibbs energies of formation for species j; nj-stoichiometric coefficient (positive for products and negative for reactants); R-gas constant; Ttemperature. The liquid phase thermodynamic data for the species involved in the glycerol esterification with acetic acid are only partially available. As most tabulated data are given for gaseous state, the corresponding liquid phase values must be calculated by estimation methods, associated with unavoidable inaccuracies. Regarding the chemical equilibrium constant, as highlighted in the published studies (Hassan and Vinjamur [28]), the values deduced from thermodynamic data, are generally uncertain and nonreliable. This is explained by relatively large standard Gibbs energies of formation (Gf,j) for chemical species and small values of standard Gibbs energy variation in the reaction (DGR), correlated with the exponential dependence on equilibrium constant in respect with DGR. Therefore, small errors in the evaluation of Gf,j are conducting to large deviations in equilibrium constant value. This is the reason why many researchers preferred to determine Keq from experi€pken mental data (Gelosa et al. [27], Hassan and Vinjamur [28], Po et al. [29]). The components involved in the glycerol esterification have polar molecules, so that their behavior is expected to feature significant non-idealities. Therefore, in the thermodynamic and kinetic calculations, it is recommended to take into consideration components activities instead of concentrations. The deactivation of sulfonated resin catalysts in esterification reactions is only scarcely investigated in the published works. Okoye et al. [1] survey the possible mechanisms, among which, the most important appear to be the poisoning by in situ formed water, leaching of sulfonic active functional groups from the catalyst surface, and eventually the deposition of high carbon content secondary products. The detrimental effect of the water on the acid resin activity, in glycerol acetylation reaction, was evidenced by Zhou et al. [11], working with Amberlyst-15 ion exchange resin. Liao et al. [33] demonstrated experimentally that the Amberlyst-35 (A-35) resin keeps the catalytic activity practically unchanged, during five reaction cycles of glycerol acetylation at 105 C. The authors are emphasizing that, the A-35 resin catalyst breaks up when the reaction is performed near its limited temperature (120 C), and that it is more sensitive to the temperature rather than reaction time. Glycerol esterification with acetic acid is a reversible process, occurring in three successive steps. The equilibrium limitation is less important for the first step and very important for the third step of triacetin formation. Therefore, when the triacetin is the desired product, modern methods like catalytic distillation (Mufrodi et al. [34]) or chromatographic reactors (Gelosa et al. [27]) could be applied. The objective of this study was to investigate, experimentally and theoretically, the liquid phase glycerol esterification with acetic acid, catalyzed by the commercial Purolite CT-275 ion exchange resin. There were evidenced experimentally the temperature and reactant initial ratio influences on the product yields. A kinetic model was proposed, following the Langmuir-Hinshelwood theory, and considering the real behavior of components in the mixture. The liquid phase activities of the components were calculated by
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Table 1 Published kinetic models for the glycerol esterification with acetic acid.[30e32]..
the UNIFAC Dortmund method. Based on the experimental data, there was analyzed the influence of the internal diffusion on the esterification process kinetics, using the Weisz-Prater approach. The values of the Weisz-Prater criteria evidenced a limitation effect of internal diffusion, over an incipient time interval. At the best of our knowledge, this is the first kinetic study of glycerol acetylation over Purolite CT-275, developing a kinetic model which considers the non-idealities of the reaction mixture, accompanied by the analysis of internal mass transport step limitation over the entire reaction time interval. 2. Experimental In all experiments there were used glycerol and acetic acid of analytical purity. As catalyst, we used commercial Purolite CT-275, a macroporous strong acid resin catalyst (macroporous polystyrene crosslinked with divinylbenzene), kindly supplied by Purolite
Romania, having the characteristics given in Table 2 (available on the manufacturer website, at the address https://www.purolite. com/product/ct275). Before use, the resin was washed several times with distilled water and with ethanol, finally dried in an oven under vacuum at 95 C, for 12 h. The acidity of the catalyst was
Table 2 Characteristics of the catalyst (Purolite CT-275). Property
Value
Exchange capacity (dry) Bulk density Particle size range Surface area Average pore diameter Particle porosity Maximum temperature limit
5.2 Eq Hþ/kg 760 kg/m3 0.425 ÷ 1.20 mm 24 m2/g 652 Å 0.38 130 C
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checked by the procedure described by Dijs et al. [35], confirming the producer’s specifications. A sieving analysis (in dry state) using meshes of different openings led to the particles size distribution given in Table 3. This corresponds to an average value of catalyst particle diameter of 0.6 mm. SEM images of catalyst particle shape and structure are provided in Fig. 1 A and B. From separate experiments, conducted using glycerol-acetic acid mixtures, having compositions typical for esterification experiments, we estimated an average swelling ratio (volumes ratio of swelled to dry particles) of 1.47. The esterification experiments were carried out in liquid phase, batch-wisely in a stainless-steel autoclave (Berghof) of 300 mL, provided with a heat transfer jacket, a magnetically driven mixing impeller and standard transmitters for internal pressure, temperature and impeller rotation frequency. The working temperature was controlled by circulating, through the reactor jacket, of low volatility thermal oil, fed from a thermo-regulated bath. The glycerol esterification experiments were performed under a nitrogen atmosphere at 10 bar, temperatures between 70 and 110 C, initial acetic acid to glycerol molar ratios ranging from 4 to 9, stirrer rotation speed 1000 rpm and a catalyst loading of 0.065 g/g glycerol. The composition analyses were performed by gas chromatography (Varian CP-3800, VF-5ms capillary column, 30 m 0.25 mm x 0.25 mm). FID and injector temperatures were fixed at 280 C. GC analyses were carried out according to the following temperature program: initially, the oven temperature was set at 125 C, then the temperature increased from 125 C to 250 C with a rate of 10 C/min, and held at 250 C for 5 min. The concentrations of glycerol, diacetin and triacetin were determined by using suitable calibration curves, obtained by the external standard method, using glycerol, diacetin and triacetin of known concentrations. As pure monoacetin was not available, the concentration of MA was calculated by molar balancing between the reacted glycerol and DA and TA products respectively. In each experiment, there were charged into autoclave the appropriate amounts of acetic acid and ion exchange resin, which were heated to approximately 70 C. At this temperature, it was fed the glycerol in the desired molar ratio and the autoclave was sealed and pressurized with nitrogen at 10 bar, starting the timing of the experimental run. Further, the autoclave heating was continued until a pre-specified temperature level was reached, which was kept constant till the final reaction. The temperature evolutions were measured and registered rigorously in each experiment. In the discussion below, the temperature regime of each experiment will be referred by the final temperature. The time evolution of the reaction mixture composition was determined by periodical sampling. The composition analyses were performed at least twice for each sample, to minimize the measurement errors. The concentration measurements, performed at different reaction times, were used to calculate the glycerol conversion (fraction of consumed glycerol) and the yields of the three esters, using the relation:
yield of J ¼ 100
moles J in the mixture ; J ¼ MA; DA; TA initial moles of glycerol (5)
Table 3 Catalyst particle size distribution. Size range, mm
0e0.425
0.425e0.63
0.63e0.8
0.8e1.0
1.0e1.2
wt %
0.9
62.3
12.4
24
0.4
3. Results and discussion The acetylation experiments are characterized by a fairly good reproducibility, as resulted from the replicate test presented in Fig. 2 (two experiments). The experimental results are presented graphically, as time dependencies of glycerol conversion and diacetin and triacetin yields, in Figs. 3e6 (the points in these figures represent experimental data). Our experimental study showed that the Purolite CT275 resin is an active catalyst for glycerol acetylation. Among the three reactions, the first one (glycerol esterification) is fastest and the last reaction, of DA acetylation, occurs most slowly. Equilibrium conversion of glycerol (almost total at initial molar ratios AA to G higher than 6) is practically achieved, in our working conditions, after approximately 2 h. Nevertheless, the overall stabilization of products concentrations (approaching the equilibrium composition) was achieved in much longer reaction times. The effect of initial AA/G molar ratio can be observed from Figs. 3e5. By increasing the initial molar ratio from 4 to 9, at a temperature of 100 C and a reaction time of 350 min, the DA yield increases from 0.46 to 0.58 and the TA yield increases from 0.08 to 0.24. The reaction temperature effect on the observed acetylation process kinetics is more complex, cumulating the influences of the temperature on the rate constant and chemical equilibrium composition respectively. As observed from Figs. 4 and 6, by increasing the temperature from 100 C to 110 C, at an initial molar ratio of 6:1 and a reaction time of 350 min, the reaction rates accelerate on the first time interval (the slopes of the curves are increased), but the final yields do not change significantly (TA yield increases only slightly, from 0.15 to 0.18, whereas the DA yield practically does not change). The maximum TA yield obtained in our experiments was 0.3, for the AA/G ¼ 9 and temperature 110 C (the corresponding DA yield being 0.56). 3.1. Evaluation of the influence of physical steps on the overall kinetics In the solid catalyzed esterification processes, the chemical reaction occurring on the internal catalyst surface is preceded by the mass transfer of reagents from the bulk liquid to the external surface of catalyst pellet (external diffusion) and the reactants transport inside the pores network of the pellet (internal diffusion). As the experimental results evidenced that the glycerol consumption in the reaction (1) has the most rapid kinetics, we investigated the diffusional limitations only for this reaction. 3.1.1. External diffusion The influence of the external diffusion step on the process kinetics is mainly dependent on the rotation frequency of the stirrer. Zhou et al. [11] found that, for liquid phase glycerol esterification with acetic acid over Amberlyst-15, (AA/G molar ratios between 3 and 9 and temperatures up to 100 C), the limitation induced by the external diffusion step is negligible for rotation speeds over 800 rpm. Similarly, Gelosa et al. [27] reported a negligible influence of the external diffusion step, for the same process investigated at temperatures up to 100 C, at stirrer speeds over 600 rpm. Consequently, we assumed that, for a rotation speed of 1000 rpm in our experiments, the influence of this step is also negligible. 3.1.2. Internal diffusion The Purolite CT-275 ion exchange resin is a macroporous, strongly acidic resin, consisting in a matrix of polystyrene highly crosslinked with divinylbenzene. The acidity is provided by the sulfonic acid groups (-SO3H) linked essentially to the aromatic carbons. Physically, the ion-exchange beads (having the diameters
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Fig. 1. SEM images of Purolite CT 275 resin particles (A e commercial particles shapes; B e microscopic structure).
Fig. 2. Reproducibility test for glycerol acetylation (AA/G ¼ 6; T ¼ 110 C).
Fig. 3. Time variation of glycerol conversion and product DA and TA yields. The points represent measured values and the solid lines the calculated ones (initial molar ratio AA:G ¼ 4; Temperature 94 C).
Fig. 4. Time variation of glycerol conversion and product DA and TA yields. The points represent measured values and the solid lines the calculated ones (initial molar ratio AA:G ¼ 6; Temperature 100 C).
Fig. 5. Time variation of glycerol conversion and product DA and TA yields. The points represent measured values and the solid lines the calculated ones (initial molar ratio AA:G ¼ 9; Temperature 100 C).
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mixture, DG, by the glycerol binary diffusion coefficient in acetic acid (as suggested by Gelosa et al. [27]). This was calculated as an average of the values predicted by the Wilke-Chang, Tyn- Calus and Hayduk-Minhas methods (Reid et al. [40]). The viscosity of the mixture, involved in these calculations, was evaluated also as an average using correlations and data published by Reid et al. [40] and Viswanath [41]. Based on the data published by Soto et al. [42] for Purolite CT275 resin and the swelling ratio evaluated in our experiments, we estimated a porosity of the swelled bead, ε ¼ 0.55. The tortuosity of the swelled pellet pores network was selected from literature data, t ¼ 3. The internal diffusion has a negligible limitation effect on the overall kinetics, if WPc < 1 (Bischoff [43]).
Fig. 6. Time variation of glycerol conversion and product DA and TA yields. The points represent measured values and the solid lines the calculated ones (initial molar ratio AA:G ¼ 6; Temperature 110 C).
practically between 0.4 mm and 1.2 mm), are agglomerates of geltype microparticles (microspheres) having the diameter below 0.1 mm, interspersed with macropores, as resulting from the image presented in Fig. 1B. So, in a macroreticular resin bead are present two types of pores: micropores inside the microparticles and macropores in the spaces between microparticles. It is appreciated that about 5% of the acidic (-SO3H) groups are placed on the outer surface of microparticles, the remainder being located inside the microparticles. Published studies are hypothesizing that, due to their small dimensions, the process kinetics inside the microparticles should not be limited significantly by the internal diffusion (Chakrabarti and Sharma [36], Xu and Chuang [37], Ramírez et al. [38]). 3.1.2.1. Classical Weisz-Prater criterion. The limitation influence of the internal diffusion on the process kinetics at the level of a catalyst pellet is usually evaluated using the classical Weisz-Prater (WP) criterion [39]: ðvÞ
WPc ¼
rrG;obs L2 Deff CG
; L ¼ Rp =3
(6)
Rp-radius of the catalyst pellet; CG e bulk liquid glycerol concentration (kmol/m3); Deff - the effective diffusion coefficient of glycerol inside the catalyst pore network. ðvÞ The observed transformation rate of glycerol (rrG;obs ) was calculated from the measured time dependence of glycerol conversion (XG) using the relation: ðvÞ
rrG;obs ¼ rp
nG;0 dXG mcat dt
(7)
mcat-amount of dry catalyst in the reaction mixture (kg); rp-density of the dry pellet (kg/m3); nG,0 einitial quantity of glycerol (kmol); The effective diffusion coefficient of glycerol, inside the catalyst pore network, was calculated by the relation:
ε Deff ¼ DG ;
t
(8)
Considering the limited accuracy of existing correlations for the prediction of diffusion coefficient in multicomponent liquid mixtures and the existence of an acetic acid excess in the liquid during the period of interest for our study (that of glycerol consumption), we approximated the molecular diffusion coefficient in the reaction
3.1.2.2. The Bischoff Weisz-Prater criterion. The expression of the Weisz-Prater criterion (6) is based on the generalization of the results particular for the first order reaction kinetics. Bischoff [43] extended the WP criterion for a more general reaction kinetics, based on the idea of a generalized Thiele modulus which makes the internal effectiveness factor to follow essentially the same curve for all reaction rate forms. For reaction rate expressions having the mathematical form:
rrðvÞ ¼ kv gðCG Þ
(9)
Bischoff developed the WP type criterion: ðvÞ
WPg ¼
rrG;obs L2 gðCG Þ ð CG 2 Deff gðCÞ dC
(10)
CG;eq
Similarly, the overall catalytic process kinetics is considered free of internal diffusion limitation, if WPg is smaller than unity (WPg < 1). More conservative limits were proposed to be 0.3 for second order reactions and 0.6 for first order ones (Mears [44]). As will be shown in the next section of this work (paragraph 4), we have developed a Langmuir-Hinshelwood type reaction rate expression for glycerol consumption (r1), having the form given in the relations (19). In this expression, the reaction rate dependence on the glycerol concentration is an implicit one, included by the intermediate of species activities (aj):
gðCG Þ ¼
aW aG aAA aMA Ka;1 !2 ; aj ¼ Fj ðCG Þ; kv ¼ k1 rcat P 1 þ KJ aJ
(11)
J
Z CG The integral, I ¼ gðCÞ dC, appearing in the relation (10) was CG;eq calculated by integrating, on the interval [CG,eq, CG], the system of differential equation (12). The first of these equations, describing the dependencies of species concentrations in respect with glycerol concentration, are deduced from the mass balance equation (23). The last one is a transform of the integral to be calculated.
dCj rrj ¼ ; C ¼ CG;eq ; dC rrG dI ¼ gðCÞ; C ¼ CG;eq ; dC
Cj ¼ Cj;eq ; j ¼ AA; MA; DA; TA; W I¼0 (12)
The equilibrium concentration (CG,eq) was calculated based on the chemical equilibrium constants, determined as described below. The values of the two WP criteria WPc and WPg, for three of our experiments are presented in Fig. 7. As observed, the values of
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values as high as 4.5 for glycerol and 1.7 for mono-acetate. Therefore, it is more properly to define rate expressions in activities, instead of concentrations of the species. Considering this issue, we developed a kinetic model using components activities, following €pken et al. [29] and the general approaches used in the studies of Po Gelosa et al. [27] respectively. The chemical mechanism of the glycerol acetylation over ion exchange resin catalysts is explained by Okoye et al. [1] and Zhou et al. [11] respectively. In the development of the kinetic model we considered that the bulk liquid phase reactions (homogeneous kinetics) have a negligible contribution to the overall kinetics. This hypothesis is supported by the observations of Zhou et al. [11] and Gelosa et al. [27] respectively, in glycerol acetylation studies over Amberlyst-15 resin (slightly less acid than Purolite CT-275). We developed activity-based rate expressions for the reactions (1) to (3) using the Langmuir-Hinshelwood (LH) theory. Published kinetic studies evidenced that the controlling steps of the surface esterification reactions over acidic resin catalysts are the chemical reactions occurring between adsorbed molecules of reactants (dual site mechanism) (Okoye and Hameed [6], Gelosa et al. [27]). In this hypothesis, assuming that the reactants adsorption and products desorption steps occur close to equilibrium, one obtains the approximate relations:
Fig. 7. Weisz Prater criteria values in time, for three experiments. In the legend are given the working values of reaction temperature and initial AA/G molar ratio).
WPg criterion are significantly different of the WPc ones, except for highest AA/G molar ratio (presumably due to a kinetic behavior better approaching the first order one, at the highest acetic acid excess). The values of WPg criterion are higher over the entire investigated interval, this suggesting that the classical expression (6) is underestimating, in this case, the limitative effect of internal diffusion. The values of WP criteria are evidencing limitation effects of internal diffusion during an initial time interval, corresponding to the glycerol depletion time. For the highest AA/G ratio and working temperature, due to the highest glycerol consumption rate, the duration of the diffusion control interval is the shortest. On the other hand, for lower molar ratio and temperature, the WPg values indicate a smaller influence of internal diffusion, extending over a larger time, due to a relatively slower consumption rate of glycerol. As already underlined, in this analysis it was considered only the glycerol esterification to MA, which occurs faster than the MA and DA esterification reactions. Consequently, the latter reactions are expected to be less influenced by internal and external diffusion steps, as compared with the glycerol esterification. Our results, regarding the influence of internal diffusion, are in accord with the findings of Gelosa et al. [27], for the glycerol esterification with acetic acid, using Amberlyst-15 resin as catalyst. The authors evidenced possible internal diffusion effects on glycerol esterification kinetics, even for milder conditions (60 C).
QJ
KJ ¼
aJ QS
;
J ¼ G; AA; MA; DA; TA; W
KJ-adsorption equilibrium constant; QJ-fraction of active sites on the catalyst surface occupied with component J; QS- fraction of the free active sites on the catalyst surface; aJ e activity of the component J in the bulk liquid phase. The overall balance of active sites is expressed by the equation:
QS þ QG þ QMA þ QDA þ QTA þ QW ¼ 1
QJ ¼
KJ aJ P ; J ¼ G; AA; MA; DA; TA; W 1 þ KJ aJ
The rate expressions of the three acetylation reactions, occurring on the catalyst surface, have the expressions:
r1 ¼ k1;S QG QAA
As already mentioned, due to the molecule polarity, the behavior of the chemical species in liquid mixture presents significant non-idealities that should be accounted for in the kinetic study. At typical compositions for this transformation, the activity coefficients, calculated by UNIFAC Dortmund method (Constantinescu and Gmehling [45] and the references therein), attain
r1 ¼
P 1 þ KJ aJ J
!2 ; r2 ¼
P 1 þ KJ aJ J
!2
; r3 ¼
(15)
J
QMA QW
r2 ¼ k2;S QMA QAA
aW k2 aMA aAA aDA Ka;2
(14)
From the previous two equations are obtained the fractions of adsorbed species:
4. Development of the kinetic model
aW k1 aG aAA aMA Ka;1
(13)
r3 ¼ k3;S QDA QAA
! (16)
Keq;s;1
QDA QW
!
Keq;s;2
QTA QW Keq;s;3
(17)
! (18)
From the relations (15) to (18), there were derived the rate expressions:
aW k3 aDA aAA aTA Ka;3 P 1 þ KJ aJ
!2
(19)
J
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k1 ¼ k1s KG KAA ;
k2 ¼ k2s KMA KAA ;
k3 ¼ k3s KDA KAA :
In the expressions presented above, Keq,s,i and Ka,i are chemical equilibrium constants for reaction i, defined in respect with concentrations of adsorbed species and activities in the bulk liquid respectively. To limit the complexity of reaction rate expressions, we assumed that, among the components of the mixture, the most strongly adsorbed on the catalyst surface are the water, glycerol and acetic acid. This hypothesis is in accord with the observations of Gelosa et al. [27], acquired from adsorption experiments on the Amberlyst-15 resin catalyst. Consequently, the terms corresponding to MA, DA and TA in the summation appearing in the denominator (adsorption term) of the rate expressions (19) were neglected:
X KJ aJ yKW aW þ KG aG þ KAA aAA
(20)
J
The apparent rate constants k1, k2 and k3, involved in the expressions (19), are temperature dependent, following Arrhenius type functions:
ki ¼ km;i
E z expð i Þ ; R
1 1 z¼ ; 363 T
i ¼ 1; 2; 3 ;
dnJ ¼ mcat rrJ ; dt
t ¼ 0; nJ ¼ nJ0 ;
J¼
the reaction i. The kinetic model so developed is characterized by the parameters km,1, km,2, km,3, E1,E2, E3, KG, KAA, KW, DGR,1, DGR,2 and DGR,3. € pken et al. [29] suggested a method to determine the Po adsorption equilibrium constants, KJ, from separate experiments, determining the swelling ratio of catalyst particles in the pure components of reaction mixture. However, due to difficulties to find all the glycerol esters in pure state, this method is hardly applicable for this process. Therefore, we estimated the three adsorption equilibrium constants, KG, KAA, and KW (as average values on the working interval), from experimental data, simultaneously with the other parameters of the model. As already stated, the standard free energy variations (DGR,i) appearing in the calculation of the chemical equilibrium constants (Ka,i) could be calculated using thermodynamic relations and published data. However, as already emphasized, this approach is leading to important errors for liquid phase reactions, due to lack of reliable liquid phase thermodynamic data. Usually, the standard free energy variations DGR,i are estimated from the experimentally measured compositions for long reaction times in isothermal €pken et al. [29]). This was conditions (Hassan and Vinjamur [28], Po the approach followed also in this work. We measured reaction mixture compositions at reaction times of minimum 8 h, at rigorously controlled temperature and used them to calculate species activities and further the equilibrium constants, Ka,i, of the three reactions. The values of equilibrium constants (Ka,i), obtained in different working conditions, were used to estimate DGR,1, DGR,2 and DGR,3 using the relation (22), by the least squares method (Table 4). The
(23)
nTA ¼ nG0 ðnG þ nMA þ nDA ; nAA ¼ nAA;0 ðnMA þ 2nDA þ 3nTA
(24)
(25)
nW ¼ nAA;0 nAA
(26)
The value of the formation rate for a species J was calculated from the relation:
(22)
DGR,i - standard free energy (Gibbs energy) variation associated to
MAG; DAG
nJ-moles number of species J in the bulk liquid; rrJ-formation rate of species J (kmole/kg/s); mcat-mass of the dry catalyst in the mixture. The triacetin (TA), acetic acid (AA) and water (W) number of moles in the mixture were calculated from the stoichiometric relations:
(21)
Ei - apparent activation energies; km,i - rate constants values at 363 K. The chemical equilibrium constants Ka,1, Ka,2 and Ka,3 are also temperature dependent:
DGR;i ; i ¼ 1; 2; 3 Ka;i ¼ exp RT
calculated and measured values of the equilibrium constants are graphically compared in Fig. 8. Hassan and Vinjamur [28] found that the chemical equilibrium constants for esterification of organic acids with mono-hydroxylic alcohols, at 298 K, vary between 1.91 and 4.58. As order of magnitude, the values of the chemical equilibrium constants we determined for the glycerol esterification with acetic acid (including also the activity coefficients) are in an acceptable agreement with this general observation. The kinetic parameters km,1, km,2, km,3, E1, E2, E3, KG, KAA and KW were estimated from experimental data by using a non-linear least squares method (function “lsqcurvefit” of Matlab software). In this aim we used the mass balance equations specific for the perfectly stirred batch reactor:
rrJ ¼
3 X
ðniJ ri Þ
(27)
i¼1
niJ e stoichiometric coefficient of species J in the reaction i (negative for reactants). To estimate the kinetic parameters, we used nine sets of measured values for glycerol conversion and ester products yields in time, for a duration of approximately 6 h, at different temperatures and AA/G ratios, few of which being already discussed above. The estimated values of the kinetic model parameters and the corresponding 95% confidence intervals are presented in Table 5. These estimates correspond to a correlation coefficient, R2 ¼ 0.98. In the same table are given, for comparison, the activation energies found in other studies using Amberlyst-15 as catalyst (a similar ion exchange resin). As observed, the activation energies found in our study are slightly higher but not very different of the published ones. The predictions of the glycerol conversion and glycerol esters yields are compared with measured data in Figs. 3e6 (and Figs. S1eS5 in the section Supplementary Information), for different AA/G ratios and reaction temperatures. As can be observed, the calculated values are in good agreement with the experimental data, this demonstrating, along with the small size of statistical intervals and a relatively high correlation coefficient, the Table 4 Estimated values of the standard free energy variations in the three reactions. Reaction, i
(1)
(2)
(3)
DGR,i (J/mol) Ka,i at 90 C
5117.9 5.45
960.9 1.37
3371.4 0.33
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Fig. 8. Estimated (lines) and experimental (points) values of the chemical equilibrium constants.
Table 5 Estimated values of kinetic parameters (R2 ¼ 0.98). Parameter
km,1 km,2 km,3 E1/R E2/R E3/R KG KAA KW
Value
0.0162 (1 ± 0.003) 0.0118 (1 ± 0.008) 0.003 (1 ± 0.015) 7650 (1 ± 0.005) 3198 (1 ± 0.027) 3030 (1 ± 0.064) 5.4 (1 ± 0.004) 2.5 (1 ± 0.005) 10.0 (1 ± 0.008) 6.82,(1 ± 0.0111)
Units
kmol/(kg$s) kmol/(kg$s) kmol/(kg$s) K K K e e e
Published values Zhou et al. [11]
Gelosa et al. [27]
6886 3883 1672
7500 2100 2200
good adequacy of the proposed kinetic model. As already underlined, the parameters of the developed kinetic model include the internal mass transport limitations, occurring during a temporary incipient reaction time interval. The proposed model is directly useable for process calculations in industrial operating conditions (using commercial catalyst Purolite CT-275), not being necessary to calculate the temporary limitation effects of the mass transport step. This is avoiding the necessity to evaluate theoretically the internal diffusion limitation, involving uncertainties in the prediction of transport properties, by published correlations.
5. Conclusions The commercial Purolite CT-275 ion exchange resin proved a good catalytic activity in the glycerol esterification with acetic acid. Working batch-wisely at a relatively low resin load (0.065 g/g glycerol), initial reactants ratios of minimum 4 and temperatures over 95 C, equilibrium glycerol conversions were approached in 100 min and triacetin equilibrium yields in 260 min reaction times. It was observed that, on the working interval, the initial AA/G ratio has a stronger impact on the DA and TA yields, as compared with the temperature. Increasing the initial molar ratio from 4 to 9 (at 100 C and a reaction time of 350 min), the DA yield increased from 0.46 to 0.58 and the TA yield from 0.08 to 0.24. A temperature increment of 10 C (from 100 to 110 C) led to a TA yield increase
9
from 0.15 to 0.18 and an insignificant increase of DA yield. The experimental data are also evidencing a faster glycerol esterification to MA, as compared with the following steps of MA and DA transformations. The theoretical evaluation of the internal diffusion effects on glycerol transformation kinetics indicated the existence of an initial reaction time interval, which is superposing on the glycerol transformation time, where the internal diffusion has a significant limitation effect on the glycerol consumption kinetics. An important step in the study of the intrinsic kinetics of glycerol acetylation is the prediction of the chemical equilibrium constants. Preliminary trials for estimation of the three chemical equilibrium constants, based on thermodynamic relations and published data, failed to obtain reliable results, evidencing the necessity of experimental equilibrium data. A kinetic model of the glycerol acetylation over Purolite CT-275 has been developed, based on independent equilibrium and kinetic experiments performed in a batch laboratory reactor. The developed LH reaction rate expressions, including species activities and considering as controlling steps the chemical reactions of adsorbed species on the catalyst surface, describe adequately the time evolutions of the glycerol conversion and of the product yields, measured in batch experiments. The proposed kinetic model is useful for the prediction of glycerol acetylation process performance, conducted in different operation regimes. At our knowledge, this work represents the first kinetic study of glycerol acetylation over Purolite CT-275 and the first kinetic modelling of this process, considering the non-idealities of the reaction mixture, accompanied by a thorough analysis of internal mass transport limitation, during the entire reaction time. Acknowledgments This work has been funded by Romanian Government, Executive Unit for Financing Higher Education, Research, Development and Innovation (UEFISCDI), Romania Bridge Grant Research Project PNeIIIeP2-2.1-BG-2016-0324 through the Financial Agreement 80BG/2016. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.renene.2019.10.060. References [1] P.U. Okoye, A.Z. Abdullah, B.H. Hameed, Renew. Sustain. Energy Rev. 74 (2017) 387e401. [2] E.R. Bozga, V. Plesu, G. Bozga, C.S. Bildea, E. Zaharia, Rev. Chim. 62 (6) (2011) 646e654. [3] I. Banu, G. Guta, C.S. Bildea, G. Bozga, Environ. Eng. Manag. J. 14 (3) (2015) 509e517. [4] A.R. Trifoi, P.S¸. Agachi, T. Pap, Renew. Sustain. Energy Rev. 62 (2016) 804e814. [5] J. Bonet, J. Costa, R. Sire, J.-M. Reneaume, A.E. Ples¸u, V. Ples¸u, G. Bozga, Food Bioprod. Process. 87 (3) (2009) 171e178. [6] P.U. Okoye, B.H. Hameed, Renew. Sustain. Energy Rev. 53 (2016) 558e574. [7] M.A. Betiha, H.M.A. Hassan, E.A. El-Sharkawy, A.M. Al-Sabagh, M.F. Menoufy, H.E.M. Abdelmoniem, Appl. Catal. B Environ. 182 (2016) 15e25. [8] K.B. Ghoreishi, N. Asim, M.A. Yarmo, M.W. Samsudin, Chem. Pap. 68 (9) (2014) 1194e1204. [9] I. Dosuna-Rodriguez, C. Adriany, E.M. Gaigneaux, Catal. Today 167 (1) (2011) 56e63. [10] I. Dosuna-Rodriguez, E.M. Gaigneaux, Catal. Today 195 (1) (2012) 14e21. [11] L.M. Zhou, T.H. Nguyen, A.A. Adesina, Fuel Process. Technol. 104 (2012) 310e318. [12] S. Kale, S.B. Umbarkar, M.K. Dongare, R. Eckelt, U. Armbruster, A. Martin, Appl. Catal. Gen. 490 (2015) 10e16. [13] I. Banu, G. Bozga, G. Bumbac, A. Vintila, S. Velea, A.-M. Galan, M. Bombos, O. Blajan, A.C. Crucean, Rev. Chim. 70 (7) (2019) 2325e2329. [14] P. Ferreira, I.M. Fonseca, A.M. Ramos, J. Vital, J.E. Castanheiro, Appl. Catal. B Environ. 91 (1e2) (2009) 416e422. [15] M. Balaraju, P. Nikhitha, K. Jagadeeswaraiah, K. Srilatha, P.S.S. Prasad,
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Please cite this article as: I. Banu et al., Glycerol acetylation with acetic acid over Purolite CT-275. Product yields and process kinetics, Renewable Energy, https://doi.org/10.1016/j.renene.2019.10.060