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Joint 20th AIRAPT – 43th EHPRG, June 27 – July 1, Karlsruhe/Germany 2005
High selective synthesis of acrolein from glycerol in high pressureand high temperature water
Masaru Watanabe,* Toru Iida, Yuichi Aizawa, Hiroshi Inomata, Research Center ofSupercritical Fluid Technology, Tohoku University, Sendai, Japan,
email: [email protected]
Summary
Glycerol reactions were conducted to synthesize acrolein, which is the raw material of acrylicreisn, in high temperature and high pressure water (573 to 673 K, saturated pressure or 34.5MPa) using a batch and a flow apparatus. The main products were always acrolein andacetaldehyde. The experimental results revealed that ca. 80 % selectivity of acrolein wasobtained at 90 % of glycerol conversion with H2SO4 in supercritical condition (673 K and 34.5MPa). We also developed a simple model of glycerol conversion into acrolein and analyzedthe kinetic parameters. The rate constant of acrolein decomposition is always higher thanthat of acrolein formation in the absence of acid catalyst. On the other hand, it was revealedthat the rate constant of acrolein formation overcame that of acrolein decomposition byadding acid in supercritical condition.
Introduction
Waste vegetable oil has gained much attention because of its capability of beingconverted to bio-diesel as a sustainable energy resource. The bio-diesel is generallysynthesized by transesterification of triglyceride into methylester. The byproduct of thetransesterification is glycerol and its effective utilization will be a key issue to promote thisbio-diesel production system.
Glycerol, a C3 compound as same as propane and propylene, has been pointed outto convert into acrolein (Bühler et al., 2002; Ramayya et al., 1987). Bühler et al. (2002)conducted the experiments of glycerol reactions using a flow apparatus in high-temperatureand high-pressure water (HHW) at 622 to 748 K and 25 to 45 MPa and simulated theexperiments using the detail kinetic model. Through the comparison between theexperimental and the computational studies, they revealed that the glycerol reactions inHHW competitively progressed through both ionic and radical reactions. It was suggestedthat the predominance of the ionic or radical depended on temperature and pressure: theionic reaction preferred below the critical point of water (liquid state) and the radical wasrelatively favoured at supercritical region. The kinetic model also indicated that glyceroldehydration into acrolein mainly occurred through ionic reactions. Ramayya et al. (1987)conducted the experiments of glycerol reactions in HHW at 573 K to 623 K of the reactiontemperature and 34.5 MPa of the reaction pressure. They found that acrolein was obtainedwith high selectivity (84 % selectivity of acrolein at 40 % conversion of glycerol) by adding 5mM H2SO4 at 623 K and 34.5 MPa. The above results by Bühler et al. (2002) and Ramayyaet al. (1987) suggested that the acrolein formation from glycerol was controlled bytemperature, pressure, and proton concentration. The experimental data of glycerolconversion in HHW was not enough, in particular, the investigation of an acid catalyst on thisreaction at wide reaction conditions must be demanded to explore the optimum condition foracrolein synthesis from glycerol. In addition, glycerol is a simple model compound ofcarbohydrate and the data of the glycerol reaction in HHW would contribute toward thedevelopment of the other biomass conversion in HHW such as gasification (Bühler et al.,2002).
In this study, glycerol reactions in HHW were examined by a batch (for the slowreaction at 573 K) and a flow apparatus. We focused on the acrolein formation from glycerol.The effects of temperature and H2SO4 on the reactions were investigated. A simple network
Joint 20th AIRAPT – 43th EHPRG, June 27 – July 1, Karlsruhe/Germany 2005
model, of which rate constants were decided by fitting the experimental results, was used toknow the effect of the parameters.
Experimental
Materials
Glycerol was obtained from Aldrich. Acrolein and H2SO4 (1 mol/dm3 aqueoussolution) were purchased from Wako Pure Chemical Co. These regents were used asreceived. Pure water that was distilled after deionization was obtained by with a waterdistillation apparatus.
Batch experiments
The experimentswithout H2SO4 at 573 Kwere carried out bymeans of a SS 316stainless steel tubebomb reactor with aninner volume of 6 cm3.Figure 1 shows theschematic diagram ofthe reactor and theprocedure. The batchtype reactor was constructed of an SS316 tube (105 mm length, 8.5 mm i.d., 12.7 mm o.d.)and Swagelok connectors (a reducing union and a cap) that are made of. SS316. Thereducing union was sealed by 1/16-inch plug. The loaded amount of glycerol was 0.1 g at allthe batch experiments and it was corresponded to 0.18 mol/dm3 of the concentration ofglycerol in the reactor. The loaded amount of water was 4.3 g (ca. 720 kg/m3). The loadedamount of water correspond to the density of the saturated liquid phase at 573 K. Thereactor containing the sample solution was purged air in it by Ar gas and sealed. The reactorthen was submerged into the heating bath and the reaction started. Immersing the reactor inwater bath stopped the reaction.
Flow experiments
Figure 2 shows theflow apparatus. Two HPLCpumps were employed tofeed pure water and asample solution. All theparts of the flow apparatus(the lines and theconnectors) were made ofSS316. The reaction cellwas constructed of the1/16-inch line (0.59 mm i.d.,1.59 mm o.d.) made ofSS316 and its lengthranged from 10 to 50 m asto the reaction times. Thepre-heater and the reactor were heated to keep a desired temperature by electrical air ovens.In order to achieve a rapid heating (milli-second order) at the mixing point, the pure waterwas heated up to more than 50 K higher than the reaction temperature in the pre-heater. Adouble piped heat exchanger was employed to cool down the reaction effluent rapidly.Pressure was maintained with a backpressure regulator. Temperatures of the several pointsof the apparatus (the outlet of the pre-heater, the inlet of the reactor, the mixing point, the
Figure 2 Flow apparatus
Figure 1 Batch reactor and procedure
Joint 20th AIRAPT – 43th EHPRG, June 27 – July 1, Karlsruhe/Germany 2005
outlet of the reactor, and the outlet of the heat exchanger) were measured by K-typethermocouples. The system pressure was measured by electrical pressure gauge at thepoints both before and after the reactor. Temperature and pressure were recorded by a datalogger. The concentration of glycerol in the reaction cell varied from 0.05 or 0.25 mol/dm3.When the effect of H2SO4 on the reaction was examined, glycerol was dissolved in H2SO4
solution and the acidic glycerol solution was fed to the apparatus. The glycerol concentrationin the acidic solution was adjusted to be the same as that in the acid-free solution. TheH2SO4 concentration in the reaction cell was set 1 or 5 mmol/dm3. The reaction time wascontrolled by changing the reactor length and the flow rate of the total influent.
The reaction temperatures were 573, 623, and 673 K. Pressure was set at 34.5 MPaas same pressure as the experiments by Ramayya et al (1987). The influent of the reactorwas considered to be pure water when the reaction time was calculated because the densityof the mixture at the reaction conditions could not be obtained. However, the assumption ofinfluent as pure water would be reasonable because of the low-concentration of sample. Thereaction time in the reaction cell of the flow apparatus determined by the following equation:
(1)
where F is volume flow rate, m3/s; V is the volume of the reactor, m3; ρ0 is density of water atthe ambient temperature and pressure, kg/m3; and ρTP is density of water at the experimentaltemperature and pressure, kg/m3. The reaction time ranged from 5 sec to 80 sec. Theexperimental conditions of the flow experiments also listed in Tables 1 and 2 with thecondition of the batch experiments and the experimental results.
Analyses and definitions
The liquid products in the recovered solution were analyzed by HPLC (JASCO), whichhad RI and UV as detectors, with KS-802 column (Shodex) and GC-FID (Hewlett-Packard,HP6890) with HP-5 column (Hewlett-Packard). The total amount of carbon in the watersolution was evaluated by a total organic carbon detector (Shimadzu, model TOC-5000A).
Product yield (mol%) and selectivity (%) of carbon compound were evaluated fromcarbon base as shown below:
(2)
(3)
Results
Without H2SO4
Table 1 Experimental conditions and results (without H2SO4)
€
τ =V ⋅ ρTPF ⋅ ρ0
€
Product yield [mol%] = An amount of carbon atom in a productThe amount of carbon atom in the loaded glycerol
×100
T, KPressure,
MPaConcentration,
mol/dm3 Time, sTOC,mol%
Glycerol,mol%
Acrolein,mol%
Acetaldehyde,mol%
Formaldehyde,mol%
Hydroxyacetone,mol%
Allylalcohol,mol%
Selectivity ofacrolein, %
573 Saturated 0.18 600 105.99 98.19 3.04 0.02 > 100573 Saturated 0.18 1800 103.57 96.52 4.22 0.28 0.05 0.03 > 100573 Saturated 0.18 3600 103.70 95.06 5.57 1.00 0.16 0.19 0.10 > 100623 34.5 0.05 9.68 99.28 98.60 2.35 0.05 > 100623 34.5 0.05 14.35 98.82 97.09 2.80 0.07 96623 34.5 0.05 28.69 96.51 94.95 3.25 0.04 64673 34.5 0.05 10.47 102.00 89.46 15.72 0.78 > 100673 34.5 0.05 14.06 98.91 89.12 15.41 0.94 > 100673 34.5 0.05 21.33 99.48 85.14 13.38 1.50 90
TOC, Glycerol, and Products YieldConditions
€
Selectivity [%] = Amount of carbon atom in the productAmount of carbon atom in the converted glycerol
×100
Joint 20th AIRAPT – 43th EHPRG, June 27 – July 1, Karlsruhe/Germany 2005
Table 1 shows the experimental conditions and the results in the absence of H2SO4.The products that were identified and quantified in this study are acrolein, acetaldehyde,formaldehyde, hydroxyacetone, and allylalcohol. Bühler et al. (2002) reported thatethanol, methanol, and propionaldehyde were also main products. In the present study,these compounds could not been detected at all the conditions. The acrolein selectivity wassometimes over 100 % at lower conversion of glycerol, as shown in Table 1. This was dueto the accuracy of quantification of high amount glycerol was possibly ± 1 %.
Glycerol conversion at 573 K was quit slow reaction such that 5 mol% of glyceroldisappeared at 60 min (3600 s). The main product was acrolein, however, 1 mol% ofacetaldehyde was obtained at 60 min. The acrolein yield was 3 mol% and the glycerolconversion was 5 mol% at 29 s and 623 K. The selectivity of acrolein was 64 %. Insupercritical region (673 K), the quantitative acrolein formation was observed, that is, 14mol% of acrolein was gained at 15 mol% of glycerol conversion (21 s). The selectivity ofacrolein at 673 K was kept high (90 %) even at the longer reaction time (21 s), however, theyield of acrolein was low, only 13 mol%. Furthermore, the yield of acrolein decreased withincreasing the conversion of glycerol. This means that acrolein converted into something.
With H2SO4
Table 2 shows the reaction conditions and the results in the presence of H2SO4. Inthis table, the selectivity of acrolein was sometimes more than 100 % by the same reason inthe acid-free condition. The identified and quantified products were acrolein, acetaldehyde,formaldehyde, hydroxyacetone, and allylalcohol, as well as the results without H2SO4.
Table 2 Experimental conditions and results (with H2SO4)
Addition of H2SO4 accelerated glycerol reaction at all the reaction time. The yield ofacrolein was always much higher than those of the other products and the primary productwas always acrolein also in the presence of H2SO4.
The selectivity of acrolein formation was always around 60-70 % at 573 K. Theconversion of glycerol at 573 K was 10 mol% at 83 s and then the yield of acrolein was 5mol%.
The concentration of H2SO4 and glycerol was changed to explore the effect of thesefactors on the reaction at 623 K. Addition of 1 mmol/dm3 of H2SO4 to the reaction at 0.05mol/dm3 of glycerol concentration slightly enhanced the reaction (see also Table 1). Theyield of acrolein was 10 mol% was achieved at 20 % of glycerol conversion (20 s) and theselectivity of acrolein was 46 %. Increase of H2SO4 concentration from 1 mmol/dm3 to 5mmol/dm3 brought the enhancement of glycerol conversion (43 mol% of glycerol conversionat 30 s). However, the increase of H2SO4 concentration was not effective for the selectivity ofacrolein: 23 mol% of acrolein was obtained at 45 mol% of glycerol conversion (30 s at 0.05
T, KPressure,
MPaConcentration,
mol/dm3H2SO4,
mmol/dm3 Time, sTOC,mol%
Glycerol,mol%
Acrolein,mol%
Acetaldehyde,mol%
Formaldehyde,mol%
Hydroxyacetone,mol%
Allylalcohol,mol%
Selectivity ofacrolein, %
573 34.5 0.25 5 16.60 103.27 95.61 2.80 0.17 0.03 64573 34.5 0.25 5 22.41 103.26 95.05 3.45 0.19 0.04 70573 34.5 0.25 5 33.26 103.11 94.41 3.90 0.19 0.06 70573 34.5 0.25 5 41.35 102.52 93.87 4.44 0.14 0.05 72573 34.5 0.25 5 55.54 102.06 92.39 4.82 0.17 0.06 63573 34.5 0.25 5 82.62 105.01 90.95 5.13 0.07 0.24 0.06 57623 34.5 0.05 1 5.86 105.59 97.82 6.87 0.14 0.25 > 100623 34.5 0.05 1 7.29 104.62 96.50 7.97 0.14 0.36 > 100623 34.5 0.05 1 9.73 103.38 95.49 8.67 0.28 0.46 0.03 > 100623 34.5 0.05 1 14.51 104.95 86.67 9.34 0.54 0.07 0.88 70623 34.5 0.05 1 19.48 96.59 78.03 10.00 0.74 0.11 1.08 46623 34.5 0.05 5 7.38 105.03 84.97 12.92 0.53 0.05 1.29 0.03 86623 34.5 0.05 5 9.86 104.25 83.48 14.52 0.88 0.05 1.48 0.04 88623 34.5 0.05 5 14.67 104.38 77.70 17.00 1.61 0.10 2.08 0.03 76623 34.5 0.05 5 19.71 101.11 73.25 19.32 2.31 0.28 1.93 0.03 72623 34.5 0.05 5 29.09 95.74 56.62 23.04 4.60 0.40 3.08 0.04 53623 34.5 0.25 5 14.58 106.87 73.43 26.97 1.98 0.67 1.98 0.04 > 100623 34.5 0.25 5 19.46 106.08 65.02 32.10 2.92 1.08 2.66 0.06 92623 34.5 0.25 5 29.59 104.15 54.65 39.09 4.79 1.88 3.04 0.08 86673 34.5 0.05 5 10.32 88.42 7.85 74.29 14.30 5.59 0.04 4.46 81673 34.5 0.05 5 13.70 85.63 6.02 74.11 18.22 6.59 0.05 3.18 79673 34.5 0.05 5 20.20 79.77 4.12 66.18 22.86 7.78 0.06 1.94 69
TOC, Glycerol, and Products YieldConditions
Joint 20th AIRAPT – 43th EHPRG, June 27 – July 1, Karlsruhe/Germany 2005
mol/dm3 of glycerol concentration and 5 mmol/dm3 of H2SO4) and thus the selectivity ofacrolein at this condition was 53 %. The conversion of glycerol at 0.25 mol/dm3 of glycerolconcentration at 30 s (45 mol%) was almost the same as that at 0.05 mol/dm3 at 30 s (43mol%). On the other hand, the yield of acrolein was increased with increasing glycerolconcentration, that is, 23 mol% of acrolein at 0.05 mol/dm3 of glycerol (at 30 s) was obtainedand 40 mol% of acrolein at 0.25 mol/dm3 of glycerol. Although 86 % of selectivity wasobtained at 30 s (at 0.25 mol/dm3 of glycerol concentration and 5 mmol/dm3 of H2SO4), theselectivity of acrolein decreased with increasing reaction time, namely glycerol conversion (>100 % selectivity of acrolein at 15 s, 92 % at 20 s, and 86 % at 30s, respectively).
The high yield (74 mol%) and high selectivity (81 %) of acrolein were obtained at theexperiments at 673 K, 10 s, 0.05 mol/dm3 of glycerol concentration, and 5 mmol/dm3 ofH2SO4 concentration. Increase of reaction time lead to the decrease of acrolein selectivity.Higher temperature would be suitable for the high-selective acrolein synthesis. Glycerolconcentration must bring the positive factor to promote the acrolein synthesis. Furthermore,pressure would affect the acrolein formation in supercritical region. To make the mechanismclearer, some studies on these effects are now undergoing in more detail.
Discussion
In order to knowthe reaction mechanism,the simple networkmodel was developed.Figure 3 shows themodel. As proposed byBühler et al. (2002), theprimary reactions ofglycerol in HHW weredehydration into acroleinand C-C bond splittinginto acetaldehyde. In addition, the other reaction such the formation of allylalcohol was alsoconsidered to be a primary reaction (Bühler et al., 2002). In the present study, acrolein andacetaldehyde were always primary and main products, and the small amount of allylalcoholwas also obtained. Thus we accept the concept proposed by Bühler et al (2002), as shownin Fig. 3. The selectivity and yield of acrolein reduced at longer reaction as shown in Tables1 and 2. This means that the formed acrolein was converted into the other products.According to this model, the formation rate of acrolein was represented by the followingequation.
(4)
The formation of acrolein was sensitive to the concentrations of glycerol and H2SO4 asdescribed in the above Results section. Thus, the dependency of the formation rate on theseconcentrations must be taken into consideration as shown in Eq. 4. However, there were notenough experimental data at this moment. Therefore, the concentration dependency of bothorganics was set to first-order and the proton concentration term was included in each rateconstant (Eq. 5). We determined the kinetic parameters (k’1 and k’4) by fitting theexperimental data.
(5)
Figures 4 and 5 display the fitting results without H2SO4 and that with H2SO4, respectively.In order to know the kinetic parameters for the rate constants, we plotted the logarithm value
Figure 3 Simple reaction network model of glycerol reaction
€
d[Acrolein]dt
= k1[Glycerol]a[H +]b − k4[Acrolein]
c[H +]d
€
d[Acrolein]dt
= k1'[Glycerol] − k4
' [Acrolein]
Joint 20th AIRAPT – 43th EHPRG, June 27 – July 1, Karlsruhe/Germany 2005
of the rate constant against the reverse value of temperature (Arrhenius plot). Figures 6 and7 are the Arrhenius plot of the rate constants without H2SO4 and those with H2SO4.
Table 3 lists the kinetic parameters that were decided from the slops of the Arrhenius plots(Figs. 6 and 7).
The rate constants of acroleinformation from glycerol (k’1) without H2SO4
were always lower than those of acroleindecomposition (k’4) over all theexperimental temperature range, as shownin Fig. 6. This means that acroleinproduced from glycerol rapidly disappearedat the experimental conditions studied.The activation energy (Ea) of k’1 withoutH2SO4 was slightly higher than that of k’4(see in Table 3). Thus, the formation rateof acrolein from glycerol would becomehigher than that of acrolein decomposition at higher temperature, however, the temperaturewhere k’1 exceeds k’4 without H2SO4 is 1900 K, too high temperature.
In contrast, the temperature where the rate constant of acrolein formation (k’1) withH2SO4 exceeded that of acrolein decomposition (k’4) was 623 K as shown in Fig. 7. Theformed acrolein was relatively stable because of the slower decomposition rate and thus thehigh selectivity and high yield of acrolein was achieved in supercritical water.
The model in the present study is too rough to understand the reaction mechanismand the optimum condition. The effect of temperature, pressure, glycerol concentration, and
Table 3 Kinetic parameters
k A, 1/s Ea, kJ/molwithout H2SO4k'1 1.0E+19 247.5k'4 1.0E+18.6 231.6with H2SO4k'1 1.0E+11.7 146.4k'4 1.0E+2.1 1
Figure 5 Fitting results with H2SO4
Figure 6 Arrhenius plot without H2SO4 Figure 7 Arrhenius plot with H2SO4
Figure 4 Fitting results without H2SO4
Joint 20th AIRAPT – 43th EHPRG, June 27 – July 1, Karlsruhe/Germany 2005
H2SO4 concentration is experimentally examined now. A detailed reaction kinetic model willbe developed on the basis of the further experimental results at the various conditions.
Conclusion
In this study, it was revealed that 74 mol% of acrolein yield and 81 % of its selectivity wasobtained with acid catalyst in supercritical condition (673 K). We developed a simple modelof acrolein formation. The rate constant of acrolein formation overcame that of acroleindecomposition by adding acid in supercritical condition.
References
Bühler, W., Dinjus, E., Ederer, H. J., Kruse, A., Mas C. 2002. Ionic reactions and pyrolysis ofglycerol as competing reaction pathways in near- and supercritical water. J. Supercrit. Fluids22, 37-53.
Ramayya, S., Brittain, A., DeAlmeida, C., Mok, W., Antal, Jr., M. J. 1987. Acid-catalyseddehydration of alcohols in supercritical water. Fuel 66, 1364-1371.
