ARTICLE pubs.acs.org/IECR
Glycerol Chlorination in GasLiquid Semibatch Reactor: An Alternative Route for Chlorohydrins Production R. Tesser,*,† M. Di Serio,† R. Vitiello,† V. Russo,† E. Ranieri,† E. Speranza,† and E. Santacesaria† †
Department of Chemistry of the University of Naples “Federico II”, Complesso Universitario Monte S. Angelo, Via Cintia 4, IT 80126 Naples, Italy ABSTRACT: In the present paper, an alternative synthetic way to obtain chlorohydrins, that are commercial products from glycerol feedstock, is presented. This synthetic route to transform glycerol into these high-value chemicals involves the use of gaseous HCl and glycerol in the presence of a carboxylic acid, as catalyst, in order to obtain, as main product, αγ-dichlorohydrin. Monochlorohydrins are also obtained as intermediates reaction products. As shown in a previous work some catalysts are selective in the production of the desired product, while others give monochlorohydrins as the main products. We attempted to correlate the selectivity shown by different carboxylic acids to their pKa, but this correlation seems not of general validity and it still remains a problem to correlate the catalytic behavior with the molecular structure of the catalyst. At this purpose, in this work we have investigated the behavior of a homologous chlorinated series of catalysts, such as the following: acetic, monochloroacetic, dichloroacetic, and trichloroacetic acid, focusing in particular the attention on both activity and selectivity shown by each catalyst. A kinetic model, based on a reliable mechanism, developed in a previous work but implemented for the HCl gasliquid partition has been used for interpreting all the kinetic runs. Then, the obtained kinetic constants have been elaborated by using the Taft equation in the attempt to correlate chemical structure of the catalyst and the activity.
1. INTRODUCTION The increase in biodiesel production by transesterification of vegetable oils1 leads to the production of large amounts of glycerol as a byproduct, more precisely about 10% by weight is obtained from the mentioned reaction.2 More glycerol is produced with respect to the market demand, and many studies have been developed in the world to find new uses for glycerol as raw material. In order to consume the large amount of glycerol coming from biodiesel production plants only two strategies are valuable, that is, to find a new use of glycerol for producing fuels additives or using it as feedstock for commodities. The production of αγ-dichlorohydrin3 is an example of the second type of strategy. As a matter of fact, dichlorohydrins are important intermediates for synthesizing epichlorohydrin and then epoxy resins.4,5 Epichlorohydrin is nowadays produced in industry starting from a mixture of αβ-dichlorohydrin (70%) and αγdichlorohydrin (30%), that are obtained by propylene hydrochlorination.6,7 Recently, it has been proposed a new synthetic route for the preparation of chlorohydrins, by reacting a polyhydroxy aliphatic hydrocarbon with a chlorination agent.8 Santacesaria et al.9 have developed an alternative route for the glycerol chlorination, by using gaseous hydrochloric acid in order to obtain, as the main product, αγ-dichlorohydrin. In this case a reaction scheme of the following type has been ascertained10
can be obtained only from α-monochlorohydrin because chlorine in the β position inhibits the further chlorination as demonstrated in previous works.10 Santacesaria et al. have performed a comparison of the kinetic behaviors of different catalysts, and a correlation between the pKa of the different carboxylic acids and the observed activities and selectivities has been proposed.911 Z. H. Luo et al. developed a kinetic model for glycerol chlorination,12 in the presence of acetic acid as catalyst, that can be considered the classical catalyst for this reaction.8,1316 The proposed kinetic model is based on a simplified monophasic mechanism, that does not consider the formation of the intermediates of the chlorination reaction. It is clear that a more sophisticated model is necessary to properly describe all the chemical and physical phenomena occurring in the chlorination reaction. Moreover, pKa is not sufficient to justify the kinetic behavior of different carboxylic acid catalysts, and a correlation between molecular structure and catalytic activity could be useful to individuate the best catalyst. As a matter of fact, acetic acid gives certainly good performances, but it is too volatile in the reaction conditions and must be replaced by a less volatile catalyst allowing to work also at higher temperature so shortening the reaction time. In the present work, in order to find more insight in the activity-structure correlation we have investigated the behavior of a homologous chlorinated series of catalysts, such as the following: acetic (AA), monochloroacetic (MCA), dichloroacetic (DCA), and trichloroacetic acid (TCA), Special Issue: CAMURE 8 and ISMR 7
The β-monochlorohydrin is formed initially, in a small amount, and together with α-monochlorohydrin and α,β-dichlorohydrin r 2011 American Chemical Society
Received: July 27, 2011 Accepted: November 8, 2011 Revised: November 3, 2011 Published: November 08, 2011 8768
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Table 2. Experimental Runs: Operative Conditions catalyst run catalyst
content (% mol)
glycerol (g)
stirring (rpm)
temperature (°C)
pressure (bar)
1
AA
8.0
150
1200
100
5.5
2
MCA
8.0
150
1200
100
5.5
3
DCA
8.0
150
1200
100
5.5
4
TCA
8.0
150
1200
100
5.5
5
MCA
8.0
150
1200
100
2.0
6
MCA
8.0
150
1200
100
9.0
Table 3. Experimental Run: Products Distribution after 4 h of Reaction (mol %) α
β
α,γ
α,β
AA
0.70
7.66
89.37
2.85
0
MCA
69.98
6.37
10.09
0.33
13.46
DCA
41.94
4.71
1.99
0
51.98
TCA
26.64
3.08
1.47
0
69.43
catalyst
Figure 1. 1 - Hastelloy jacketed reactor, approximated capacity 300 mL; 2 - cylinder of hydrochloric acid; 3 - valve for samples withdrawal of the reaction mix; 4 - magnetic stirrer; 5 - temperature indicator; 6 - pressure indicator; 7 - system for neutralizing the excess HCl (two Drechsel bottles, arranged in series, which contain a solution of sodium hydroxide).
glycerol
Table 1. Physical Properties of the Used Catalysts molar mass
boiling
acidity
(g mol1)
point (°C)
(pKa)
acetic acid
60.05
118119
4.792
monochloroacetic acid
94.5
189.3
2.86
dichloroacetic acid
128.94
194
1.25
trichloroacetic acid
163.39
197
0.77
catalyst
focusing in particular the attention on both activity and selectivity shown by each catalyst. A gasliquid biphasic kinetic model, based on a reliable mechanism, developed in a previous work but implemented for the HCl gasliquid partition has been used for interpreting all the kinetic runs. Then, the obtained kinetic constants have been elaborated by using the Taft equation1720 in an attempt to correlate chemical structure of the catalyst and the activity.
2. EXPERIMENTAL SECTION
Figure 2. Cumulative consumption of hydrochloric acid for the experimental runs performed with different homogeneous catalysts at the same total pressure of 5.5 bar.
2.1. Reagents. All the reagents used for the experimental runs have been purchased from Sigma Aldrich at the highest level of purity available (glycerol anhydrous and catalyst >99%) and were used as received without further purification. Gaseous hydrochloric acid has been purchased from Air Liquide Italy (99.8%). 2.2. Analytical Method. The samples withdrawn (about 10 cm3) were neutralized to eliminate both the dissolved hydrochloric acid and the acid used as catalyst. For the neutralization, a solution containing 2 g of calcium carbonate was added, drop by drop, under stirring keeping the system at 100 °C for about 30 min. Subsequently, the solid was removed by centrifugation, and the composition of the resulting liquid was determined by GC analysis. The column was a CHROMPACK CP Wax, stationary phase of 100% poly ethylene-glycol, length of 30 m. I.D. of 0.25 mm and film thickness of 0.25 μm. The GC was equipped with FID
detector using helium as carrier gas. The other parameters are the following: injector temperature: 250 °C; detector temperature: 280 °C; temperature ramp: 1 min at 40 °C; heating rate, 20 °C/min to 100 °C, then 40 °C/min up to 200 °C, then hold for 10 min. The sample of the reaction mixture was first diluted with methanol in a volumetric ratio of 1:30 and 1 μL of solution is injected into the GC. 2.3. Reactor Setup and Kinetic Runs. All the experiments have been carried out in a 300 cm3 Hastelloy steel reactor equipped with a magnetically driven stirrer. A picture of the fed-batch reactor is reported in Figure 1. Glycerol and catalyst are initially loaded into the reactor. Then, the reactor is closed and heated to the desired temperature. When the desired temperature was reached, HCl is sent to the reactor keeping constant the overall pressure and reading the volumetric-flow rate that compensate the hydrochloric acid consumption by the reaction. During each experimental run, 8769
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Figure 3. Glycerol conversion as a function of time for runs 2, 5, and 6 of Table 2 for MCA catalyst.
Figure 4. Selectivity to monochlorohydrins as a function of time for runs 2, 5, and 6 of Table 2 for MCA catalyst.
samples are periodically withdrawn, in order to have the evolution of the reaction products during the time. After 4 h of reaction the hydrochloric acid in excess is flushed by N 2 through traps containing NaOH solutions, the stirring is stopped, and the system is brought back to room temperature. All the adopted operating conditions and properties of the used catalysts set for the experimental runs are listed in Table 1 and in Table 2.
3. RESULTS AND DISCUSSION In this work, we have studied in detail the influence of some experimental variables on the activity and selectivity of the glycerol chlorination reaction, such as the type of the used catalysts and the influence of HCl pressure. For what concerns the comparison of the studied catalysts, a summary of the catalytic tests performed is reported in Table 2. The results, expressed in terms of products distribution after 4 h of reaction time, are reported in Table 3 for the runs performed at constant pressure of 5.5 bar (see Table 2). As it is possible to observe, pKa of the catalysts seems to have a strong influence on the reaction rate. We have for the activity that AA > MCA > DCA > TCA,
ARTICLE
Figure 5. Selectivity to dichlorohydrins as a function of time for runs 2, 5, and 6 of Table 2 for MCA catalyst.
while the corresponding pKas are as follows: 4.75 > 2.85 > 1.48 > 0.7. In conclusion glycerol conversion decreases by increasing the chlorine atoms numbers in the catalyst. This fact can be better observed by considering the experimental data trends reported in Figure 2, where the cumulative consumption of HCl monitored during the experimental runs is reported. The experimental data trends confirm that acetic acid is much more active than MCA, DCA, and TCA. For what concerns, the influence of the pressure on activity and the selectivity we have focused our attention on MCA, being that this catalyst is more active than DCA and TCA and having a lower volatility with respect to AA. For this purpose, some runs have been performed, working at different pressures, by using MCA as catalyst and monitoring both the glycerol conversion and the selectivity to mono- and dichlorhydrins. In Figure 3 it can be seen that an increase in the HCl pressure leads to a markedly increase in glycerol conversion. In Figure 4, it can be seen that the selectivity to monochlorohydrins decreases with pressure, while, on the contrary, selectivity to dichlorhydrins increases as it can be seen in Figure 5. Moreover, Figure 2 shows that in the first part of the reaction the hydrochloric acid consumption is higher, because there are present two separated effects that are as follows: (i) the HCl migration from the gaseous to the liquid phase and (ii) the reaction rate effect. Then the consumption slowly declines for MCA, DCA, and TCA with a smaller slope, and a smooth decrease of the HCl consumption proceeds through the rest of the experimental run. The high rate of HCl consumption observed in the initial part of the runs suggests that in some cases gasliquid mass transfer could be operative. For describing mass transfer rates HCl solubility data in the reaction environment are necessary. These data are necessary also because HCl concentration appears in the kinetic laws. Solubility data of this type are not available in the literature, and experimental data have been collected by us. 3.1. Mass Transfer and HCl Solubility. First of all, the solubility tests have been experimentally performed by imposing a HCl pressure on the mixture of the chosen composition recording, from time to time, the HCl flow-rate. The experimental results are reported in Figure 6 in terms of cumulative consumption, while in Table 4 are reported all the experimental conditions for the mentioned runs. 8770
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Figure 6. Solubility tests: dots represent the experimental data, lines the simulations curves.
Table 4. Solubility Tests: Experimental Conditionsa
a
XGlymol
XH2Omol
Xαmol
run
pressure (bar)
S-1
2.0
1
0
0
S-2
5.5
1
0
0
S-3
5.5
0.33
0.33
0.33
Table 6. Calculated and Experimental Solubilitiesa of Some Known Substances temperature (°C)
experimental21
calculated with PSRK
ethanol
0 20
838 756
1165 926
methanol
0
1092
2647
20
877
1578
0
829
1554
20
724
1259
substances
Xi is the molar fraction referred to the component i.
H2O
Table 5. HCl Solubility Concentration in the Different Compounds Present in the Reaction Media
a
value [mol/cm3] KHClW
0.01127
KHClGly
0.01291
KHClαβ KHClα
0.00671 0.01008
KHClαγ
0.00676
Then HCl solubility at 100 °C was estimated by calculation adopting the PSRK state equation and using CHEMCAD v.6.3. From these calculations it is possible to obtain estimated values for the HCl equilibria concentrations in the liquid phase. These values are reported in Table 5, for the different pure compounds present in the reaction media. The solubility in a multicomponent reaction mixture can be calculated as the sum between the product of the molar fraction of the i component for the related solubility contributions solubility ¼
i ∑ xi 3 KHCl
ð2Þ
In this way, the Henry’s constant can be calculated as follows HHCl ¼
solubility P
each solubility run. This factor corrects the discrepancy between experimental solubility and PSRK prediction. The simulations of the experimental runs after the introduction of the corrective γ factor are reported in Figure 6, while the values of γ obtained by regression analysis are respectively 0.266, 0.201, and 0.308 for runs 1, 2, and 3. A further confirmation of the validity of this approach has been made on different systems available in the literature21 considering the solubilities of HCl in respectively methanol, ethanol, and water. In Table 6 are compared the experimental solubility data and the values estimated with the PSRK equation. As it can be seen, also in those cases solubilities from the PSRK equation are always overestimated, and this justifies the introduction of the corrective factor γ. For what concerns the mass-transfer rate expression, we have adopted the double-film theory developed by Whitman assuming, as a first approximation, that no resistance to mass transfer is given by the gas-side film. In this way, it is possible to write a mass-transfer rate expression for HCl in the liquid film as follows JHCl ¼ kl 3 a 3 ð½HCl ½HClÞ ¼ β 3 ð½HCl ½HClÞ½¼ mol=ðcm3 3 minÞ
ð3Þ
with P being the reactor’s pressure. The values obtained with this predictive approach resulted always higher than the experimental ones. Therefore, the solubility function reported in (2) has been multiplied by a factor γ, that has been regressed on the experimental data for
Solubilities expressed in g of HCl/1000 g of solvent.
ð4Þ
where [HCl]* represents the equilibrium concentration at the gasliquid interphase evaluated as ½HCl ¼ HHCl 3 P 8771
ð5Þ
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Figure 7. Glycerol chlorination performed with acetic acid at 5.5 bar. Lines represent simulations, while points represent the experimental data.
For this reaction set it is possible to write the following kinetic rate laws, all expressed in mol/(cm3•min) r 1 ¼ k1 3 ½Cat 3 ½Gly 3 ½HCl k1 3 ½Cat 3 ½H2 O 3 ½α
ð6Þ
r2 ¼ k2 3 ½Cat 3 ½Gly 3 ½HCl
ð7Þ
r3 ¼ k3 3 ½Cat 3 ½Gly 3 ½α k3 3 ½Cat 3 ½H2 O 3 ½α, γ
ð8Þ
r4 ¼ k4 3 ½Cat 3 ½Gly 3 ½α
ð9Þ
In order to describe the evolution with the time of each chemical compound, it is then possible to write the mass balance as follows moles accumulated ¼ molesðreacted þ transferred sampledÞ½ ¼ mol 3 min1 ð10Þ
Figure 8. Glycerol chlorination performed with monochloroacetic acid at 5.5 bar. Lines represent simulations, while points represent the experimental data.
3.2. Kinetic Interpretation of the Experimental Runs. The collected experimental data have been interpreted with a gasliquid kinetic model that is able to describe all the physical and chemical phenomena that occurs during the reaction. In particular, a gasliquid fed-batch reactor scheme has been considered, by taking into account both the HCl mass transfer from the gas to the liquid phase and the reaction rates of the reaction network. We have considered the following reaction scheme
Gly þ HCl T α
ð1aÞ
Gly þ HCl f β
ð2aÞ
α þ HCl T α, γ
ð3aÞ
α þ HCl f α, β
ð4aÞ
Starting from this balance equation, it is possible to write the following differential equations for respectively the liquid and the gas phase. Liquid phase dnGly ¼ r1 3 VR FGly ð11Þ dt
Reactions 2a and 4a have been considered irreversible, because they are far from equilibrium, according to our previous studies.9,10 8772
dnα ¼ ð þ r1 þ r3 þ r4 Þ 3 V R Fα dt
ð12Þ
dnα, γ ¼ þ r3 3 VR Fα, γ dt
ð13Þ
dnβ ¼ þ r2 3 VR Fβ dt
ð14Þ
dnα, β ¼ þ r4 3 VR Fα, β dt
ð15Þ
dnH2 O ¼ ð þ r1 þ r2 þ r3 þ r4 Þ 3 VR FH2 O dt
ð16Þ
dnHCl ¼ ð r1 r2 r3 r4 Þ 3 VR þ JHCl 3 VR FHCL dt
ð17Þ
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Figure 9. Glycerol chlorination performed with dichloroacetic acid at 5.5 bar. Lines represent simulations, while points represent the experimental data.
Figure 11. Glycerol chlorination performed with monochloroacetic acid at 2.0 bar. Lines represent simulations, while points represent the experimental data.
system calculated considering the atmosphere in the reactor as an ideal gas constituted by pure HCl, whose value can be calculated as follows PTOT ¼
Figure 10. Glycerol chlorination performed with trichloroacetic acid at 5.5 bar. Lines represent simulations, while points represent the experimental data.
Gas phase dnHClgas ¼ FHCl IN JHCl 3 VR dt
ð18Þ
where Fi represents the molar flow-rate related to the component i as a rectangular pulse-function, necessary to consider the sampling amount (each run is characterized by 2 samples of respectively 20 cm3). While FHClIN represents the inlet gaseous HCl molar flow-rate calculated as follows FHCl IN ¼ KP 3 ðPSET PTOT Þ
ð19Þ
Equation 19 represents the inlet HCl molar flow rate evaluated as proportional to the pressure gradient between the actual and the desired pressure. Here PSET represents the set point pressure experimentally imposed, while PTOT is total pressure of the
nHClgas 3 R 3 T V GAS
ð20Þ
Here VGAS is the volume of the gas head space expressed in cm3. The experimental runs have been submitted to mathematical regression analysis, determining, on the experimental data, both the kinetic and mass transfer constants, for all the involved reactions. All the obtained agreements, in terms of products distributions in molar percentages, are reported in Figures 711, while in Table 7 all the obtained parameters are listed. As it can be seen, the model is able to describe properly all the collected experimental data. The first four runs are characterized by a relatively high β value; this fact represents an indication that the gasliquid mass transfer has not a limiting effect for these runs. Moreover, run 5 differs from the others for the β value, because very probably this is the only experimental run performed at 2.0 bar. Furthermore, as it is possible to observe in Table 7, the reverse kinetic constants of reactions 1a and 3a can be considered negligible. For this reason, all the reactions occurring in the reaction network can be considered irreversible. A further interesting effect that can be pointed out by observing the kinetic constants is related to the selectivity of each catalyst to give mono- and dichlorohydrin. In fact, in Table 8, by comparing the k1/k2 ratios for all the catalytic systems MCA resulted much more selective to α-monochlorohydrin with respect to the other catalysts. Moreover, k3/k4 ratios show that all catalysts are selective to give αγ-dichlorohydrin instead of αβdichlorohydrin. 3.3. Property-Structure Study. In order to correlate the activity of the used catalysts with the related chemical structure, a first attempt has been made by considering the dependence of the kinetic constants, obtained by mathematical regression analysis on the fed-batch experimental runs, reported in Table 7, with the pKa values.9 In this way, according to some authors that studied similar problems,11 a rough linear dependence has been obtained as it can be seen in Figure 12. The correlation between 8773
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Table 7. List of the Parameters Obtained by Mathematical Regression Analysis on the Experimental Data k1a
run 1
a
17022
k2a
k3a
k4a
1793.67
20332.70
645.14
k‑3a
βb
1.76 106
5.74 107
3.00
7
k‑1a
2
2896.72
223.40
422.87
12.99
1.82 10
5.55 107
2.99
3
986.59
102.79
143.08
1.93 105
1.40 106
5.75 107
3.25
4
509.43
57.95
129.61
1.13 104
1.14 107
5.72 107
2.38
5
2896.72
223.40
422.87
12.99
1.82 107
5.55 107
0.025
Expressed in [(cm3/mol)2 min1]. b Expressed in [min1].
Table 8. Ratios between the Kinetic Constants Obtained by Mathematical Regression on the Experimental Runs Performed by Using Different Catalysts run
catalyst
k1/k2
k3/k4
k3/k1
k4/k1
1
AA
9.49
31.52
1.19
0.0379
2
MCA
12.97
32.55
0.15
0.0045
3
DCA
9.60
7.40 106
0.15
1.96 108
8.79
1.10 10
0.25
2.22 107
4
TCA
6
Table 10. Taft Values δ and G* δ
F*
k1
0.21786
0.76789
k2 k3
0.44228 1.08825
0.93646 1.78066
Figure 13. Taft’s equation steric contribution diagram for the different used catalysts. Figure 12. Plot of the kinetic constant against the pKa of each catalyst.
Table 9. Taft’s Parameters catalyst
σ*
Es
AA
0.00
0.00
MCA DCA
1.05 1.94
0.19 1.54
TCA
2.65
2.06
ln(k3) and pKa is not so good, because, at low pKa, characteristic for the less active catalysts, only a negligible amount of α,γdichlorohydrin can be obtained. This gives place to a great uncertainty in ln(k3) data correlation. Following on this analysis, we have tried to deepen the calculations by using the generalized Taft equation1720 provided by the following relation logðki =k0 Þ ¼ F 3 σ þ δ 3 Es
ð21Þ
The original approach of Taft equation considers a propertystructure study on the reagents of a chemical reaction. In our work, this equation has been applied to the investigated catalysts, considering that the catalyst is very probably involved in the reactions forming esters with glycerol and monochlorohydrin. For more details about the reaction mechanism see our previous works on the topic.9,10 According to those mechanisms, as wellknown from organic chemistry textbooks carboxylic group is a better leaving group than hydroxyl and this is probably the intimate mechanism of this reaction. The Taft equation allows to calculate two contributes relative to the reactivity of the adopted catalyst, that are respectively F* and δ. These two contributes represent the sensitivity to both the electronic nature of the substituents (F*) and the steric effect (δ). To apply this approach, it is necessary to know both the σ* and Es values of the various substituents introduced in the acetic acid, this last assumed as a reference molecule. These values can be taken from the literature and are reported in Table 9.20 These two contributions represent respectively the polar and the steric substituent constants. It is now possible to apply the Taft eq 21, by determining through a simultaneous fitting on the kinetic constants 8774
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predictive mixed approach. It has been verified that PSRK equation gives always overestimated values of HCl solubility and a corrective factor has been determined. At last, a previously developed kinetic model9,10 has been improved by introducing also the hydrogen chloride gasliquid partition. This model has successfully been applied to the performed fed-batch runs, describing with a good approximation all the experimental data. Thanks to this model, it was possible to evaluate the kinetic parameters, usefully employed to perform the property-structure study based on the use of the Taft equation.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
Figure 14. Taft’s equation polar contribution diagram for the different used catalysts.
reported in Table 7, the values of δ and F*. The obtained values are reported in Table 10. In Figures 13 and 14 it is possible to appreciate the linear trends obtained for both the steric and polar contribution of the Taft equation. These plots clearly show that polar and steric effects are both important in all the hydrochlorination occurring reactions. By observing the parameters reported in Table 10 it results that the polar effect has a greater influence on the kinetic behavior with respect to the steric one. The steric effects are different for reactions 1a and 2a of the reaction scheme, because the β position is more hindered, while we have 2 equivalent OH in the α position. This represents an improvement with respect to consider pKa as a unique factor influencing activity and selectivity in the mentioned reactions as we have made in our previous works9,10 and in Figure 12.
4. CONCLUSIONS In the present paper, starting from acetic acid catalyst as the reference molecule, the effect of introducing one, two, or three chlorine atoms in the molecule on the activity and selectivity in glycerol hydrochlorination reaction has been investigated. In particular, we observed that pKa of the catalyst has an important role in defining activity and selectivity. For a good activity and selectivity to 1,3-dichlorohydrin a pKa in the range between 4 and 5 seems to be the optimal choice. Acetic acid has this characteristic and is surely a good catalyst but is not the ideal catalyst for industrial purposes for its relatively low boiling point (117 °C) near to the reaction temperature. This renders the catalyst recycle more difficult. It is important, therefore, to find less volatile catalysts maintaining high both activity and selectivity. According to our experience on many different catalysts, the criterium of the choice based only on the pKa value is not sufficient for predicting the catalyst behavior. An improvement is represented by the Taft equation approach. It has been shown by applying the Taft equation to the homologous series, acetic, mono chloroacetic, dichloro acetic, and trichloro acetic acids, that both the polar and steric effect are important in determining the catalyst behavior in terms of activity and selectivity. In this work acetic acid has been used as a useful reference molecule for the reactivity-structure analysis. In this work has also been studied the HCl solubilities in the reaction environment by adopting an experimental plus
’ ACKNOWLEDGMENT Thanks are due to MIPAF (Italian Ministry of Agricultural, Food and Forest Policies) “Project AGROPROM New technologies for the production of biodiesel from waste oil and fats sources” D.M. 246/2007 (23/10/2007) and 16912/7303/10 (23/7/2010) for the financial support. ’ LIST OF SYMBOLS AA acetic acid MCA monochloroacetic acid DCA dichloroacetic acid TCA trichloroacetic acid HCl solubility in the j compound [mol/cm3] KHClj γ solubility correction factor mass transfer rate for HCl in the aquous film [mol/ JHCl (cm3 3 min)] β mass transfer rate constant [min1] [J] concentration of the J component [mol/cm3] [HCl]* HCl film concentration [mol/cm3] kinetic rate [mol/(cm3 3 min)] ri moles of the j component [mol] nj molar fraction of the j component [-] xj rectangular-pulse function molar flow-rate of the j Fj component [mol/min] FHClIN molar HCl flow-rate [mol/min] P pressure [bar] V volume [cm3] T temperature [°C] ’ REFERENCES (1) http://www.assocostieribiodiesel.com/ (2) Park, Y. M.; Lee, D. W.; Kim, D. K.; Lee, J. S.; Lee, K. Y. The heterogeneous catalyst system for the continuous conversion of free fatty acids in used vegetable oils for the production of biodiesel. Catal. Today 2008, 131, 238. (3) Atia, H.; Armbruster, U.; Martin, A. Dehydration of glycerol in gas phase using heteropolyacid catalysts as active compounds. J. Catal. 2008, 258, 71–82. (4) Gilbeau, P. Manufacturing epichlohydrin-based resins. PCT Int. Appl, WO2011054769 A2 20110512, 2011. (5) Gilbeau, P. Processbfor manufacturing an epoxy resin. PCT Int. Appl, WO2011054770 A1 20110512, 2011. (6) Carra, S.; Santacesaria, E.; Morbidelli, M. Synthesis of epichlorohydrin by elimination of hydrogen chloride from chlorohydrins. 1. Kinetic aspects of the process. Ind. Eng. Chem. Process Des. Dev. 1979, 18, 424–427. 8775
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dx.doi.org/10.1021/ie201629z |Ind. Eng. Chem. Res. 2012, 51, 8768–8776
