Biodiesel by Catalytic Reactive Distillation Powered by Metal Oxides

Dec 6, 2007 - sulfated titania, and sulfated tin oxide. Rigorous process ..... water can be separated directly by decanting in the reflux drum when us...
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Energy & Fuels 2008, 22, 598–604

Biodiesel by Catalytic Reactive Distillation Powered by Metal Oxides Anton A. Kiss,*,†,‡ Alexandre C. Dimian,*,†,§ and Gadi Rothenberg*,†,§ Van ’t Hoff Institute for Molecular Sciences, UniVersity of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands, Akzo Nobel Chemicals B.V., Velperweg 76, Arnhem, The Netherlands, and Yellow Diesel B.V., Roetersstraat 35, Amsterdam, The Netherlands ReceiVed May 24, 2007. ReVised Manuscript ReceiVed September 27, 2007

The properties and use of biodiesel as a renewable fuel as well as the problems associated with its current production processes are outlined. A novel sustainable esterification process based on catalytic reactive distillation is proposed. The pros and cons of manufacturing biodiesel via fatty acid esterification using metal oxide solid acid catalysts are investigated. Finding catalysts that are active, selective, and stable under the process conditions is the main challenge for a successful design. The best candidates are metal oxides such as niobic acid, sulfated zirconia, sulfated titania, and sulfated tin oxide. Rigorous process simulations show that combining metal oxide catalysts with reactive distillation technology is a feasible and advantageous solution for biodiesel production.

Table 1. Petroleum Diesel vs Biodiesel

1. Introduction Sustainable energy management is a major concern of the modern society. The increasing energy demand makes the implementation of sustainable fuels a crucial issue worldwide.1,2 Biodiesel has become increasingly attractive because it is made from renewable sources and combines high performance with environmental benefits.3–6 The idea of using vegetable oil as fuel for diesel engines is over a century old. In fact, Rudolph Diesel himself demonstrated the performances of his engine using peanut oil as fuel. Unlike petroleum diesel that contains hydrocarbons, biodiesel consists of a mixture of monoalkyl esters of long-chain fatty acids. These are typically produced by (trans)esterification.7 Biodiesel has several advantages over petroleum diesel: it is safe, renewable, nontoxic, and biodegradable; it contains no sulfur and is a better lubricant.4,8 Despite the chemical differences, these two fuels have similar properties and performance parameters (Table 1).9–12 Along with its technical advantages over petroleum diesel, biodiesel brings several additional benefits to the society: rural revitalization, creation of new jobs, and less global warming. * E-mail: [email protected] (A.A.K.); alexd@ science.uva.nl (A.C.D.); [email protected] (G.R.). Fax: +31 20 525 5604. † University of Amsterdam. ‡ Akzo Nobel Chemicals B.V. § Yellow Diesel B.V. (1) Graedel, T. E. In Handbook of Green Chemistry & Technology; Clark, J. H., Macquarrie, D. J., Eds.; Blackwell: Oxford, 2002; pp 56– 61. (2) Thuijl, E. v.; Roos, C. J.; Beurskens, L. W. M. Energy Research Centre of the Netherlands: Amsterdam, The Netherlands, 2003; p 64. (3) Sheehan, J.; Camobreco, V.; Duffield, J.; Graboski, M.; Shapouri, H. National Renewable Energy Laboratory: Golden, CO, 1998; p 60. (4) Sheehan, J.; Camobreco, V.; Duffield, J.; Graboski, M.; Shapouri, H. National Renewable Energy Laboratory: Golden, CO, 1998; p 314. (5) Demirbas, A. Energy Explor. Exploit. 2003, 21, 475–487. (6) Buczek, B.; Czepirski, L. Inform 2004, 15, 186–188. (7) Maa, F.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1–15. (8) Blumberg, K. O.; Walsh, M. P.; Pera, C. International Council on Clean Transportation, 2003; p 66. (9) Knothe, G.; Gerpen, J. H. v.; Krahl, J. The biodiesel handbook; AOCS Press: Champaign, IL, 2005. (10) Körbitz, W. Renewable Energy 1999, 16, 1078–1083. (11) Lotero, E.; Liu, Y.; Lopez, D. E.; Suwannakarn, K.; Bruce, D. A.; Goodwin, J. G. Ind. Eng. Chem. Res. 2005, 44, 5353–5363. (12) Barnes, L. L. IL Waste Management and Research Center: IL, 2006; Vol. TN06-085.

fuel property

diesel

biodiesel

fuel standard fuel composition kinetic viscosity, mm2/s (at 40 °C) specific gravity, kg/L boiling point, °C flash point, °C cloud point, °C pour point, °C cetane number (ignition quality) stoichiometric air/fuel ratio (AFR) life-cycle energy balance (energy units produced per unit energy consumed)

ASTM D975 C10-C21 HCa 1.3–4.1

ASTM D6751 C12-C22 FAMEa 1.9–6.0

0.85 188–343 60–80 -15 to 5 -35 to –15 40–55

0.88 182–338 100–170 -3 to 12 -15 to 10 48–65

15

13.8

0.83/1

3.2/1

a

HC, hydrocarbons; FAME, fatty acid methyl esters.

An important characteristic of diesel fuels is the ability to autoignite, quantified by the cetane number (cetane index). Biodiesel not only has a higher cetane number than petroleum diesel but also has a higher flash point, meaning better and safer performance. Blends of biodiesel and petroleum diesel are designated by a “B” followed by the vol % of biodiesel. B5 and B20, the most common blends, can be used in unmodified diesel engines.5 Remarkably, biodiesel is the only alternative fuel currently available with an overall positive life-cycle energy balance (Figure 1), producing 3.2 units of fuel product energy per unit of fossil energy consumed, compared to barely 0.83 units for petroleum diesel.3 The presence of oxygen in biodiesel (∼10%) improves combustion and reduces CO, soot, and hydrocarbon emissions while slightly increasing the NOx emissions. Figure 2 shows the biodiesel versus petroleum diesel emissions as well as the amount of CO2 per distance produced by various fuels.4,11 One comprehensive study showed that using B20 in trucks and buses would completely eliminate the black smoke released during acceleration.4 There are five primary ways for making biodiesel: (i) direct use and blending of vegetable oil,7 (ii) use of microemulsions

10.1021/ef700265y CCC: $40.75  2008 American Chemical Society Published on Web 12/06/2007

Biodiesel by Catalytic ReactiVe Distillation

Figure 1. Life cycle of diesel vs biodiesel as an environmentally friendly fuel. The CO2 cycle is closed for biodiesel but not for diesel.

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develop a sustainable biodiesel production process based on catalytic reactive distillation using solid acid catalysts (SACs).19 Reactive distillation (RD) integrates reaction and separation in one unit. This intensifies mass transfer and allows in situ energy integration while simplifying the process flowsheet and operation. However, combining the two operations is possible only if the reactions show reasonable conversion and selectivity data at pressures and temperatures that are compatible with the distillation conditions.20–22 Ion-exchange resins were used successfully in low-temperature esterification processes.23–25 Previously, we examined the applicability of a range of alcohol types in the fatty acid esterification process and screened various materials, such as zeolites, ion-exchange resins, heteropoly acids, sulfated carbons, and sulfated zirconia, with potential SAC applications.17,18 Here, we focus on using metal oxides as catalysts. In addition to sulfated zirconia, we have tested sulfated titania, sulfated tin oxide, and niobic acid. We study the effects of calcination temperature and acid concentration on the catalytic activity and integrate the catalyst development in the process design at an early stage. Our objective is a catalytic reactive distillation process powered by metal oxides as green catalysts, with the potential for making low-cost biodiesel. In this Article, we present the key features of our approach, present experimental results and rigorous process simulations, and discuss the possible applications of this new process. 2. Experimental Section

Figure 2. Biodiesel vs petroleum diesel emissions (top). Comparison of CO2 emissions for most common fuels (bottom).

with short-chain alcohols,7 (iii) thermal cracking (pyrolysis) of vegetable oils,13,14 (iv) transesterification of triglycerides7,14 catalyzed by bases, acids, or enzymes,15,16 and (v) esterification of fatty acids with alcohols, using acid catalysts (H2SO4) or solid acids.17,18 The current manufacturing processes, however, have several disadvantages: shifting the equilibrium to fatty esters by using an excess of alcohol that must be separated and recycled, making use of homogeneous catalysts that require neutralization (causing salt waste streams), expensive separation of products from the reaction mixture, and high costs due to relatively complex processes involving one to two reactors and several separation units. Therefore, to solve these problems, we (13) Charusiri, W.; Vitidsant, T. Energy Fuels 2005, 19, 1783–1789. (14) Zappi, M.; Hernandez, R.; Sparks, D.; Horne, J.; Brough, M.; Swalm, D. C.; Arora, S. M.; Motsenbocker, W. D. MSU E-TECH, 2003. (15) Hills, G. Eur. J. Lipid. Sci. Technol. 2003, 105, 601–607. (16) Thum, O. Tenside, Surfactants, Deterg. 2004, 41, 287–290. (17) Kiss, A. A.; Omota, F.; Dimian, A. C.; Rothenberg, G. Top. Catal. 2006, 40, 141–150. (18) Kiss, A. A.; Dimian, A. C.; Rothenberg, G. AdV. Synth. Catal. 2006, 348, 75–81.

Materials and Instrumentation. Characterization of mixed metal oxides was performed by atomic emission spectroscopy with inductively coupled plasma atomization (ICP-AES) on a CE Instruments Sorptomatic 1990. NH3-TPD was used for the characterization of acid site distribution. Sulfated zirconia (0.3 g) was heated up to 600 °C using He (30 mL min-1) to remove adsorbed components. Then, the sample was cooled at room temperature and saturated for 2 h with 100 mL min-1 of 8200 ppm NH3 in He as carrier gas. Subsequently, the system was flushed with He at a flow rate of 30 mL min-1 for 2 h. The temperature was ramped up to 600 °C at a rate of 10 °C min-1. A thermal conductivity detector (TCD) was used to measure the ammonia desorption profile of NH3. Textural properties were established from the nitrogen adsorption isotherm determined after degassing at 200 °C under a vacuum at 5–10 mbar. The catalyst surface area was calculated using the BET equation, and the pore volume was determined at a relative pressure of 0.98. The pore size was calculated using the Barrett–Joyner– Halenda (BJH) method. All process simulations were performed using the AspenTech AspenOne 2004 engineering suite.26 Procedure for Fatty Acid Esterification. The experimental results are presented on the use of solid catalysts in esterification of dodecanoic acid (C12H24O2) with methanol (CH4O), propanol (C3H8O), or 2-ethylhexanol (C8H18O). Reactions were performed using a system of six parallel reactors (100 mL)sSTEM OmniReacto Station 6100, with modular design and interchangeable heating blocks, glassware, and reflux heads. Reaction progress was monitored by gas chromatography (GC). GC analysis was performed using an InterScience GC-8000 gas chromatograph with a (19) Clark, J. H. Acc. Chem. Res. 2002, 35, 791–797. (20) Schoenmakers, H. G.; Bessling, B. Chem. Eng. Prog. 2003, 42, 145–155. (21) Subawalla, H.; Fair, J. R. Ind. Eng. Chem. Res. 1999, 38, 3696– 3709. (22) Taylor, R.; Krishna, R. Chem. Eng. Sci. 2000, 55, 5183–5229. (23) Steinigeweg, S.; Gmehling, J. Ind. Eng. Chem. Res. 2003, 42, 3612– 3619. (24) Scala, C. v.; Fässler, P.; Maus, E.; Bailer, O.; Gerla, J.; Meszaros, I. CHIMIA Int. J. Chem. 2003, 57, 799–801. (25) Schmitt, M.; Scala, C. v.; Moritz, P.; Hasse, H. Chem. Eng. Process. 2005, 44, 677–685. (26) AspenTech AspenOne 2004: User Guide & Reference Manual; Aspen Technology Inc.: Cambridge, U.K., 2004.

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Table 2. Matrix of Experimental Conditions catalyst

catalyst amount (wt %)

temperature (°C)

reactant ratioa

H2SO4 Amberlyst-15 Cs2.5H0.5PW12O40 Nb2O5 · 5H2O (HY-340) ZrO2/SO42- (SZ) TiO2/SO42- (STi) SnO2/SO42- (SSn)

0 (noncatalyzed) 0.5b 1 2 3 5b 10b

120 130 140 150 160b 180b

1:1 2:1 3:1 5:1

a The reactant ratio is expressed as the initial molar ratio of alcohol to acid. b These experiments were performed only for sulfated zirconia (SZ) or noncatalyzed reaction.

DB-1 capillary column (30 m × 0.21 mm). GC conditions: isotherm at 40 °C (2 min), ramp at 20 °C min-1 to 240 °C, isotherm at 240 °C (4 min). The injector and detector temperatures were set at 240 °C. Reaction profiles were measured for both noncatalyzed and catalyzed reactions, at several temperatures exceeding 100 °C (below 100 °C and in the absence of mixing, the liquids separate before equilibrium is reached). The catalyst concentration in the reaction mixture was varied from 0 to 5 wt %. The initial alcohol/ acid molar ratio used was varied from 1:1 up to 5:1. Table 2 shows the matrix of experimental conditions. Double-distilled water was used in all experiments. Unless otherwise noted, chemicals were purchased from commercial companies and were used as received. Dodecanoic acid 98 wt % (GC), methanol, propanol, and 2-ethylhexanol 99+ wt % were supplied by Aldrich, zirconil chloride octahydrate 98+ wt % by Acros Organics, 25 wt % NH3 solution and H2SO4 97% from Merck. Niobic acid (HY-340, Nb2O5 · nH2O) was provided by Companhia Brasileira de Metalurgia e Mineração-CBMM. The sulfated metal oxides were prepared according to literature recipes.27–32 Procedure for Preparation of Metal Oxides. The sulfated metal oxides (zirconia, titania, and tin oxide) were synthesized using a two-step method. The first step consists of hydroxylation of zirconium, titanium, and tin complexes. The second step is the sulfonation with H2SO4 followed by calcination in air at various temperatures. Example: Sulfated tin oxide. Sn(OH)4 was prepared by adding a 25% aqueous NH3 solution to an aqueous solution of SnCl4 (Aldrich, >99%, 50 g in 500 mL) until the pH reached 9–10. The precipitate was filtered, washed, and then suspended in a 100 mL aqueous solution of 4% CH3COONH4. The precipitate was filtered, washed, and dried for 16 h at 140 °C. Then, 1 N H2SO4 (15 mL of H2SO4 per 1 g of Sn(OH)4) was added to prepare SO42-/ SnO2 and the precipitate was filtered, washed, and dried for 16 h at 140 °C, and calcined in air for 4 h. Testing for Catalyst Leaching. The leaching of catalyst was studied in an organic and an aqueous phase. First, a sample of fresh sulfated zirconia catalyst (0.33 g) was stirred with water (50 mL) while measuring the pH development in time. After 24 h, the acidity was measured by titration with KOH. The suspension was then filtered and treated with a BaCl2 solution to test for SO42- ions. In a second experiment, the catalyst was added to an equimolar mixture of reactants. After 3 h at 140 °C, the catalyst was recovered from the reaction mixture, dried at 120 °C, and finally stirred in 50 mL of water. The pH was measured and the suspension titrated with a diluted solution of KOH after 24 h. Sulfate ions in the suspension were determined qualitatively with BaCl2 (at 140 °C, the reaction mixture does not split into two liquid phases because the water (27) Jiang, Y. X.; Chen, X. M.; Mo, Y. F.; Tong, Z. F. J. Mol. Catal. A: Chem. 2004, 213, 231–234. (28) Patel, A.; Coudurier, G.; Essayem, N.; Vedrine, J. C. J. Chem. Soc., Faraday Trans. 1997, 93, 347–353. (29) Furuta, S.; Matsuhashi, H.; Arata, K. Appl. Catal., A 2004, 269, 187–191. (30) Kamiya, Y.; Sakata, S.; Yoshinaga, Y.; Ohnishi, R.; Okuhara, T. Catal. Lett. 2004, 94, 45–47. (31) Sun, Y.; Ma, S.; Du, Y.; Yuan, L.; Wang, S.; Yang, J.; Deng, F.; Xiao, F.-S. J. Phys. Chem. B 2005, 109, 2567–2572. (32) Yadav, G. D.; Murkute, A. D. J. Catal. 2004, 224, 218–223.

evaporates). In a third experiment, the same procedure was repeated at 100 °C when the reaction mixture segregates and a separate aqueous phase is formed.

3. Results An industrial solid acid esterification catalyst must fulfill several conditions that seem trivial on the laboratory scale. It should have high activity and selectivity, as byproducts are likely to render the process uneconomical, and it should be watertolerant and stable at relatively high temperatures, which are required for increased reaction rates. Additionally, it must be a readily available and inexpensive material. Considering these conditions and previous literature reports, we investigated metal oxides with strong Brønsted acid sites and high thermal stability (up to 200–250 °C). The literature study shows a large range of SACs available: zeolites,33,34 ion-exchange resins,35–37 heteropoly acids,38,39 carbon-based catalysts,18,38 sulfated metal oxides,27,40–46 and niobium-based catalysts.47–49 On the basis of the literature reviews and our previous experimental screening,17,18 we focused on metal oxide catalysts based on Zr, Ti, Sn, and Nb. Fatty acid esterification using solid acids is not yet well established in industry, as it is much more difficult to find a suitable SAC for long-chain acid esterification compared to shorter acids such as acetic acid. In contrast to liquid acids that possess well-defined acid properties, SACs may contain a variety of acid sites.19,50,51 Usually, SACs are categorized by their Brønsted or Lewis acidity, the strength and number of sites, and the textural properties of the support.52 A water-tolerant catalyst is required, since the nonideal mixture may segregate into an organic and an aqueous phase that may easily deactivate the catalyst.33 In a typical reaction, equivalent amounts of dodecanoic acid and 2-ethylhexanol were reacted at 160 °C in the presence of 1 wt % solid acid catalyst (eq 1). We define the percentage (33) Okuhara, T. Chem. ReV. 2002, 102, 3641–3665. (34) Kirumakki, S. R.; Nagaraju, N.; Narayanan, S. Appl. Catal., A 2004, 273, 1–9. (35) Harmer, M. A.; Farneth, W. E.; Sun, Q. J. Am. Chem. Soc. 1996, 118, 7708–7715. (36) Harmer, M. A.; Sun, Q.; Vega, A. J.; Farneth, W. E.; Heidekum, A.; Hoelderich, W. F. Green Chem. 2000, 2, 7–14. (37) Harmer, M. A.; Sun, V. Appl. Catal., A 2001, 221, 45–62. (38) Okuhara, T. Catal. Today 2002, 73, 167–176. (39) Matsuda, H.; Okuhara, T. Catal. Lett. 1998, 56, 241–243. (40) Ardizzone, S.; Bianchi, C. L.; Ragaini, V.; Vercelli, B. Catal. Lett. 1999, 62, 59–65. (41) Corma, A.; Martinez, A.; Martinez, C. Appl. Catal., A 1996, 144, 249–268. (42) Omota, F.; Dimian, A. C.; Bliek, A. Chem. Eng. Sci. 2003, 58, 3159–3174. (43) Omota, F.; Dimian, A. C.; Bliek, A. Chem. Eng. Sci. 2003, 58, 3175–3185. (44) Sohn, J. R.; Seo, D. H.; Lee, S. H. J. Ind. Eng. Chem. 2004, 10, 309–315. (45) Sohn, J. R.; Lee, S. H. Appl. Catal., A 2004, 266, 89–97. (46) Sohn, J. R. J. Ind. Eng. Chem. 2004, 10, 1–15. (47) Carniti, P.; Gervasini, A.; Biella, S.; Auroux, A. Chem. Mater. 2005, 17, 6128–6136. (48) Reguera, F. M.; Araujo, L. R. R. D.; Picardo, M. C.; Bello, F. O.; Scofield, C. F.; Pastura, N. M. R.; Gonzalez, W. D. A. Mater. Res. 2004, 7, 343–348. (49) Tanabe, K. Catal. Today 2003, 78, 65–77. (50) Harmer, M. A.; Farneth, W. E.; Sun, Q. AdV. Mater. 1998, 10, 1255. (51) Harmer, M. A. In Handbook of Green Chemistry & Technology; Clark, J. H., Macquarrie, D. J., Eds.; Blackwell: Oxford, 2002; pp 86– 119. (52) Waal van de, J. C.; Bekkum van, H. In Supported catalysts and their applications; Sherrington, D. C., Kybett, A. P., Eds.; Royal Society of Chemistry: Cambridge, U.K., 2001; pp 27–37.

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Figure 3. Esterification of dodecanoic acid with 2-ethylhexanol (left) and methanol (right), noncatalyzed reaction at different temperatures.

Figure 5. Reaction profiles for the esterification of dodecanoic acid with 2-ethylhexanol: (left) using sulfated zirconia catalyst calcined at 400–800 °C; (right) using sulfated zirconia, titania, and tin oxide catalysts. Table 3. Catalyst Characterization

catalyst sample Cs2.5H0.5PW12O40 Nb2O5 (HY-340) ZrO2/SO42-/650 °C TiO2/SO42-/550 °C SnO2/SO42-/650 °C

Figure 4. Esterification of dodecanoic acid with 2-ethylhexanol (left) at 130 °C using liquid and solid acid catalysts (2 wt %); (right) noncatalyzed and catalyzed (0.5–10 wt % SZ catalyst) reaction profiles.

fractional conversion as X % ) 100{1 - ([acid]final/[acid]initial)} and normalize the amount of catalyst to the total amount of reactants, that is, Wcat % ) Mcat/(Macid + Malcohol). Using metal oxides as catalysts, no byproducts were observed by GC analysis. For all of the catalysts described here, the selectivity was assessed by testing the formation of side products in a suspension of catalyst in alcohol (e.g., sulfated zirconia in pure 2-ethylhexanol) under reflux for 24 h. No ethers or dehydration products were detected by GC analysis.

(1) Figure 3 shows the reaction profiles for the noncatalyzed esterification of dodecanoic acid with 2-ethylhexanol and methanol, respectively. This background reaction should be kept in mind when comparing the efficiency of the various solid catalysts. Zirconia compounds are well-known for their industrial applications in a variety of processes.30,53–55 Zirconia can be modified with sulfate ions to form a superacidic catalyst, depending on the treatment conditions.32 In our experiments, sulfated zirconia (SZ) outperformed other solid acid catalysts (Figure 4, left). SZ showed good thermal stability as well as high activity and selectivity for the esterification of fatty acids with a variety of alcohols ranging from 2-ethylhexanol to methanol. By increasing the amount of catalyst, the reaction rate, hence conversion after a certain time, can be further (53) Chu, W.; Hu, J.; Xie, Z.; Chen, Q. Catal. Today 2004, 90, 349– 353. (54) Clark, J. H.; Monks, G. L.; Nightingale, D. J.; Price, P. M.; White, J. F. J. Catal. 2000, 193, 348–350. (55) Ecormier, M. A.; Wilson, K.; Lee, A. F. J. Catal. 2003, 215, 57– 65.

pore pore diameter volume max/mean/ sulfur surface area (m2/g) (cm3/g) calcd (nm) content (%) 163 176 118 129 100

0.135 0.162 0.098 0.134 0.102

2/5.5/3 –/–/3.7 4.8/7.8/7.5 4.1/4.3/4.2 3.8/4.1/4.1

N/A N/A 2.3 2.1 2.6

increased (Figure 4, right). Thus, sulfated zirconia is suitable for RD applications where high activity is required in a short time. The concentration of the H2SO4 used for sulfated zirconia preparation did not affect the catalytic activity. Conversely, the calcination temperature has a tremendous effect: the optimal calcination temperature was 600–700 °C (Figure 5, left). In a separate set of experiments, we tested the catalyst reusability and robustness. After five consecutive runs, the activity dropped to ∼90% of the original value and (importantly) remained constant thereafter. Recalcination of the used catalyst restored its original activity. Considering the promising results with sulfated zirconia, we tested also the applicability of sulfated titania and tin oxide. These catalysts performed slightly better than SZ, showing a several percent increase in acid conversion (Figure 5, right). However, SZ is cheaper and readily available on the industrial scale. The catalytic activity of SZ can be enhanced by preparing it from using a chlorosulfonic acid precursor dissolved in an organic solvent, instead of the conventional H2SO4 impregnation.32 Other metals, such as iron, can also be added to enhance the activity.53 The catalyst characterization results are given in Table 3. The values are in very good agreement with literature data.40,56 As expected, higher sulfur content corresponds to higher acidity of the catalyst and consequently higher catalytic activity. In addition, the pore size plays an important role as the reactants and the products must be able to fit inside the catalyst to take full advantage of the total surface area available. The pore sizes of metal oxides are sufficiently large (>2 nm) to facilitate the mass transfer into and from the catalyst pores. This compensates for their lower acidity compared to other solid acids.17,18 The leaching of catalyst was studied in an organic and an aqueous phase. From the leaching tests, it can be concluded that sulfated zirconia is not deactivated by leaching of sulfate groups when water is present in the organic phase, but it is easily deactivated in pure water or aqueous phase. Other solid acid catalysts are also deactivated by the presence of water or (56) Yadav, G. D.; Nair, J. J. Microporous Mesoporous Mater. 1999, 33, 1–48.

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Figure 6. Reaction pathways and possible products (left). FAME production by esterification with methanol in a reactive distillation column (right).

in an aqueous phase. There are several methods to prevent aqueous phase formation and leaching of acid sites: (1) using an excess of one reactant, (2) working at low conversions, and (3) increasing the temperature to a value exceeding the boiling point of water, preserving the catalyst activity and driving the reaction to completion. Applying Reactive Distillation to Biodiesel Synthesis. Reactive distillation is a valuable process intensification technique that can be applied successfully to the manufacture of biodiesel, since the reactions leading to the end-product are controlled by the chemical equilibrium. This is particularly advantageous in esterification-type processes, when the feed has a high content in fatty acids. This is the case with waste fats (cooking oil, yellow grease, tallow, etc.) or with fatty acids obtained by a hydrolysis prestep, as in a supercritical process. By combining reaction and separation into a single unit, one can shift the reaction equilibrium toward the key product (ester) by continuous removal of byproduct (water), instead of using an excess of reactant. The secondary reactions (etherification or dehydration) can be avoided by using a selective solid catalyst such as sulfated metal oxides. The process based on RD does not produce waste salt streams. Figure 6 shows a RD process for FAME manufacturing based on reactive distillation. The reactive distillation column consists of a core reactive zone completed by rectifying and stripping separation sections, whose extent depends on the separation behavior of the reaction mixture. Since methanol and water are much more volatile than the ester and acid, these will separate easily in the top such that no supplementary rectifying zone is needed. Similarly, no stripping zone is necessary. In this way, the reactive zone can cover practically the whole column. The operating pressure deserves particular attention. This determines the reaction temperature, which should be compatible with the catalytic activity. For mixed oxides, the convenient temperature is in the range 130-150 °C when methanol is used as the alcohol. This corresponds to a pressure of 6-10 bar. In addition, the reboiler temperature should not exceed the limit imposed by the thermostability of the product, usually 200 °C. This constraint can be fulfilled by letting a certain amount of methanol in the bottoms, which is further recovered and recycled by a simple flash. The top stream from the RDC contains water and alcohol in a proportion dictated by the reflux policy. When methanol is used, Omota et al.42,43 demonstrated that a large excess is not necessary, the column being operated optimally at a low alcohol reflux, or even at reflux with fatty acid when water can be separated directly by decanting. In this case, the column operates

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rather as a reactive absorber. This result was confirmed later experimentally.23 However, the operation should be safer with a small excess of alcohol, 10–20%, such to cope with feed variability. The excess of methanol can be recovered by simple stripping with vapor distillate, which is recycled to the bottom of the RDC column. When using ethanol and propanols, the formation of water azeotrope is inevitable. This aspect can be handled conveniently by using a suitable entrainer, such as cyclohexane. Note that water can be separated directly by decanting in the reflux drum when using a high-boiler alcohol, such as 2-ethylhexanol. The solubility of alcohol in water is so small that quantitative separation can be achieved.24 This property can be advantageously exploited by so-called dual esterification, with light and heavy alcohol. Employing methanol and 2-ethylhexanol allows the quantitative removal of water on top. The column can be operated near the atmospheric pressure while keeping the temperature profile at 130 °C. Since 2-ethylhexanol is typically a waste alcohol from the manufacturing of butanols, this process makes sense from an economic viewpoint. Reactive distillation can also be used in the esterification stage. However, there is a fundamental difference with the esterification reaction, namely, the fact that both products, fatty ester and glycerol, are heavies and cannot be separated by distillation. In this case, shifting equilibrium can be ensured only by using a large alcohol excess by internal reflux, 2 times or more the necessary stoichiometric amount. In addition, the use of a reactive distillation offers a good environment for heat integration: the reboiler can be driven with energy from process waste, while the condenser can recover it as hot stream or by raising steam, to be used for driving other operations. Note that using a tubular reactor with external methanol recycle is a good alternative too. Actually, two reactors with intermediate glycerol separation are needed to shift the equilibrium to the desired product distribution, as a low content in monoglycerides. As result, when the feed consists of triglyceride oil containing a high proportion of free fatty acids, a good reaction setup is a reactive distillation column for esterification followed by two tubular reactors for transesterification. Conceptual Process Design and Process Simulations. To complement the experimental studies, we carried out a detailed rigorous simulation study of the biodiesel production process under reactive distillation conditions. By combining reaction and separation into a RD unit, one can shift the reaction equilibrium toward products by continuous removal of reaction products, instead of using an excess of reactant.23,42,43 The secondary reactions (etherification or dehydration) can be avoided by using a selective solid catalyst such as sulfated metal oxides. The process based on RD has no additional separation steps and produces no waste salt streams, as water is the only byproduct (Figure 6). The process simulations were performed using the AspenTech AspenOne 2004 engineering suite.26 Analysis of physicochemical properties shows high boiling points for acid and esters which both will go to the bottom of the RD column (Table 4). The conversion of fatty acid to ester should be complete, since they cannot be separated conveniently by distillation. The UNIQUAC property model was used (UNIFAC (Dortmund modified) can also be successfully applied23). Fresh reactants were fed to the reactive distillation column in a stoichiometric ratio. The RD column has 14 stages and is operated at a very low reflux ratio (0.01–0.1 kg/kg). A higher reflux ratio is detrimental, as it brings back water byproduct into the column, hence decreasing the fatty acid

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Energy & Fuels, Vol. 22, No. 1, 2008 603

Table 4. Normal Boiling Points of Chemical Species Involved in the Process chemical name

chemical formula

Mw (g/mol)

Tb (K)

Tb (°C)

dodecanoic (lauric) acid methanol 2-ethylhexanol methyl dodecanoate 2-ethylhexyl dodecanoate water

C12H24O2 CH4O C8H18O C13H26O2 C20H40O2 H2O

200 32 130 214 312 18

571 338 459 540 607 373

298 65 186 267 334 100

conversion by shifting the equilibrium back to reactants. To reduce the amount of dodecanoic acid in the final product, the fatty acid is fed above and methanol below the reactive zone (mid 8 stages), respectively. By allowing about 10% alcohol in the bottom stream, the reboiler temperature in the RD column can be maintained around 200 °C. When an excess of alcohol is used, the maximum reaction rate is located at the top of the column, with total acid conversion in the bottom but partial conversion of alcohol in the top. For the optimal reflux rate and stoichiometric reactant ratio, the maximum reaction rate is located in the center of the column, providing complete conversion of both reactants at the ends of the column. The composition and temperature profiles in the reactive distillation column are shown in Figure 7. Note that the reactants are fed at the same temperature as on the feed reactive stage. The residence time in the column is ∼10 min, much less than the retention times of 60–90 min in the conventional processes reported.57–59 Depending on the reflux ratio and total production rate of FAME, the energy requirement for such a process is ∼150 kcal/kg of fatty ester. At a catalyst loading of 15%, the production of fatty ester can exceed 21.4 kg of ester · kg of catalyst-1 · h-1, or 3250 kg of ester/h · m3 reactor. The column diameter is only 0.1 m for a pilot plant producing 1880 tons/ year of fatty ester. 4. Discussion The process we propose has several advantages over conventional biodiesel production processes: (1) short reaction time and high unit productivity, (2) no excess alcohol requirements, (3) lower capital costs due to the small size of RDC and no need for additional separation units, and (4) no neutralization and separation of the catalyst, as solid acids are used instead of homogeneous catalysts. Further investigations on the process efficiency and in-depth economic analysis of the RD system are required in order to fully establish its impact on the biodiesel production. In contrast to conventional reactors, the RD column has some hydraulic constrains that limit the maximum residence time (∼20–60 min) of the liquid phase. That means not only increased production rates, but also that a highly active catalyst is required.42,43 Adding an entrainer to the system leads to reduced catalyst loading.60 The reaction mechanism for the heterogeneous acid-catalyzed esterification was reported to be similar to the homogeneously catalyzed one.61 However, there is a major difference concerning (57) Darnoko, D.; Cheryan, M. J. Am. Oil Chem. Soc. 2000, 77, 1269– 1272. (58) Noureddini, H.; Harkey, D.; Medikonduru, V. J. Am. Oil Chem. Soc. 1998, 75, 1775–1783. (59) Peterson, C.; Cook, J.; Thompson, J.; Taberski, J. Appl. Eng. Agric. 1999, 18, 5–11. (60) Dimian, A. C.; Omota, F.; Bliek, A. Chem. Eng. Process. 2004, 43, 411–420.

Figure 7. Simulated profiles in the reactive distillation column (RDC): liquid composition (left) and temperature and ester generation (right). Table 5. Oil-Producing Crops plant

latin name

kg of oil/hectare

cashew nut soybean coffee sunflower peanut rapeseed jojoba brazil nut coconut palm oil

Anacardium occidentale Glycine max Coffea arabica Helianthus annuus Arachis hypogaea Brassica napus Simmondsia chinensis Bertholletia excelsa Cocos nucifera Elaeis guineensis

148 375 386 800 890 1000 1528 2010 2260 5000

the relationship between the surface hydrophobicity and the catalyst’s activity.18 “Reaction pockets” are created inside a hydrophobic environment, where the fatty acid molecules can be absorbed and react further. Hydrophobic surfaces are preferred to avoid the covering with water of the solid acid surface which prevents the adsorption of organic materials. Metal oxide catalysts have large pores (Table 3) and therefore do not limit the diffusion of the fatty acid molecules. They do not leach under the reaction conditions and suppress secondary reactions such as etherification or dehydration. The sulfated metal oxides are good catalysts, with strong acid sites and high thermal stability. Typical raw materials of choice are palm oil in Brazil and Malaysia, sunflower oil in France and Italy, and soybean oil in the U.S. Table 5 lists the most common oil-producing crops.62 Considering the land required for producing the current energy demands, biodiesel is not able to completely replace petroleum fuels. However, it is an excellent complementary fuel, contributing to a combined strategic approach of using alternative energy sources. 5. Conclusions Biodiesel can be produced by esterification of fatty acids using solid acid catalysts. The activity and selectivity of these catalysts is determined by the pore size, as well as by the density of acid sides and the surface hydrophobicity. Catalysts based on metal oxides such as niobia, zirconia, titania, and tin oxide proved to be the best candidates. They are active and selective. The calcination temperature is the key parameter in the preparation of these catalysts, as it heavily affects the density of acid sites, and surface hydrophobicity. Our simulations show that biodiesel can be produced by a sustainable continuous process based on catalytic reactive distillation. The integrated design shifts the chemical equi(61) Koster, R.; van der Linden, B.; Poels, E.; Bliek, A. J. Catal. 2001, 204, 333–338. (62) Tickell, J. From the fryer to the fuel tank: the complete guide to using Vegetable oil as an alternatiVe fuel, 3rd ed.; Tickell Energy Consulting: Tallahassee, FL, 2002.

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librium to completion and preserves the catalyst activity, by continuously removing the water byproduct. Manufacturing of biodiesel by reactive distillation as a multifunctional reactor can be applied to a variety of fatty acids and alcohols, the actual applications depending on the feedstock at hand. The process proposed in this work can dramatically improve

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the biodiesel synthesis and reduce the number of downstream steps. Acknowledgment. We thank Dr. M. C. Mittelmeijer-Hazeleger and Ing. J. Beckers for technical support. EF700265Y