Ind. Eng. Chem. Res. 2010, 49, 6323–6331
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Partial Hydrogenation of Soybean Oil in a Piston Oscillating Monolith Reactor Yogesh G. Waghmare, Alan G. Bussard, Robert V. Forest, F. Carl Knopf, and Kerry M. Dooley* Department of Chemical Engineering, Louisiana State UniVersity, Baton Rouge, Louisiana 70803
The partial hydrogenation of soybean oil was carried out in a novel piston oscillating monolith reactor (POMR). POMR performance was studied under the application of low frequency (0-17.5 Hz) and amplitude (2.5 mm) vibrations at 110 °C and 0.41 MPa H2 using a Pd/Al2O3 monolith catalyst. Results show observed rate improvements of up to 220% for 17.5 Hz, 2.5 mm piston oscillations over low frequency pulsing conditions. For comparison purposes, the reaction was also carried out in a stirred tank reactor using the monolith catalyst. The POMR showed better activity at an equivalent power per unit volume when compared to a stirred tank. With the monolith catalyst, both external and internal mass transfer limitations exist. Using standard diffusionreaction calculations and measurements over a range of particle sizes it was shown that the vibrations improve external mass transfer rates as well as internal transport within the washcoat. The POMR showed equal or better serial pathway selectivity than a stirred tank, except at the highest frequency, but gave higher trans fatty acid formation. Introduction The heterogeneous catalyzed hydrogenation of edible oils has long been an important process for the food industry, because it provides improved resistance to oxidation and better textural properties (e.g., a higher melting point). This reaction is traditionally carried out in a three-phase agitated tank at 0.1-0.7 MPa and 150-200 °C using a Ni-based catalyst present as a slurry.1 While Ni-based catalysts are prevalent in industrial applications, supported Pd catalysts have also been investigated because of their higher activity, allowing for lower catalyst loadings and temperatures.2,3 Several continuous flow laboratory reactors have also been studied for this reaction, such as trickle beds,4,5 tubular reactors,6 and bubble columns.7-9 Winterbottom et al.9 showed a packed bubble column exhibited less trans product formation than a slurry bubble column, possibly due to a more plug-like flow pattern. Boger et al.10 noted a similar selectivity effect for soybean oil hydrogenation with a monolithic catalyst, compared to a slurry stirred tank, but they attributed this effect to differences in mass transfer. While a high catalyst activity is desired, the selectivity to the intermediate mono- and diunsaturated triglycerides in the serial hydrogenation pathway is also important. The reaction has historically been operated in the external gas mass transferlimited regime to avoid excessive hydrogenation.1 Furthermore, recent health concerns regarding the adverse effects of trans fatty acids (TFA) on LDL/HDL cholesterol ratios has spurred research into minimizing the formation of TFAs. Raw soybean oil has no trans-fat content and so the formation of TFAs is the result of stereoisomerization. Previous work has shown that a higher hydrogen concentration on the catalyst surface lowers the rate of TFA formation.11 The easiest way to increase the surface H2 concentration is by increasing the H2 pressure and the rate of agitation. Unfortunately, this will in turn promote the formation of saturates by serial reactions.11 The effects of large intraparticle concentration gradients have also been investigated. It has been shown that both the serial pathway selectivity and the stereoselectivity decrease when the reaction is pore diffusion-limited with respect to the triglycerides;12,13 under these conditions the partially hydrogenated triglycerides * To whom correspondence should be addressed. E-mail: dooley@ lsu.edu.
diffuse more slowly from the pores, giving them opportunity to react further.1 Pore diffusion limitations with respect to hydrogen have the opposite effect, improving selectivities.14 While agitated tank slurry reactors are now common for this process, it would be beneficial if a structured catalyst could be used instead, obviating an additional separation step. The presence of catalyst particles or dissolved transition metal is particularly troublesome for a food product. Previous work on three-phase structured reactors has shown they are a viable alternative for gas mass transfer-limited reactions such as hydrogenations.15,16 The two most common such systems are monoliths operated in the slug (Taylor) flow regime and trickle beds. Boger et al.10 did an economic evaluation of a process where a monolith reactor is used for the hydrogenation of edible oil and showed that cost reductions up to 40% can be achieved when compared to the conventional slurry reactor process. Monolith reactors in slug flow show improved surface wetting compared to trickle beds, which suffer from rivulet formation and radial gradients in liquid concentrations at the catalyst external surface. Improvements in trickle bed performance are possible when inducing pulsed flows, through the periodic modulation of the liquid feed flow.17-23 By alternating between gas- and liquidrich conditions over the surface of the catalyst, the gradients in the gaseous reactant’s concentration can be reduced and the rate of mass transfer increased. Under these conditions, catalyst monolith reactors exhibit alternating gas and liquid slugs passing over the catalyst surface, with the hydrodynamics approaching plug flow. However the liquid phase reactants used in these systems have typically been characterized by low molecular weights and viscosities. Therefore, the logical extension of soyoil hydrogenation to a structured catalytic system is complicated by the higher viscosity and its effect on the mass transfer. One magnetic resonance imaging study has shown that a sucrose solution with twice the viscosity of water increases the film thickness surrounding gas slugs in monoliths,24 presumably reducing mass transfer. A piston oscillating monolith reactor (POMR) has previously been used to show activity enhancements of up to 84% and equal or better selectivity for the hydrogenation of alpha-methyl styrene (AMS) to cumene, compared to a stirred tank reactor at the same conditions.25 These improvements result from low
10.1021/ie902000e 2010 American Chemical Society Published on Web 06/25/2010
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Figure 1. Schematic of the piston oscillating monolith reactor (POMR) reactor body and flowsheet of system. H.E. ) heat exchanger.
frequency/amplitude oscillations that enhance external gas mass transfer to the surface, while also altering the surface wetting behavior. The liquid films are apparently reduced in thickness for at least part of the cycle. This work is an extension of the oscillating reactor system, exploring a more complex reacting system of higher viscosity and with a more complex reaction network than in AMS hydrogenation. Experimental Section Catalyst Preparation. The Pd/γ-Al2O3 catalyst was prepared by an ion-exchange technique. Pseudoboehmite (UOP V-250) was first calcined at 500 °C in flowing air to convert it to the gamma phase. The Pd precursor, PdCl2, was converted to Pd(NH3)4(NO3)2 by dissolving in excess NH4OH and NH4NO3 at a pH of 11. Excess Cl- was removed from this solution by contacting with an ion-exchange resin (Amberlite IRA-400, Rohm and Haas). The solution was then contacted with γ-Al2O3 at a temperature of 60 °C overnight. The sample was filtered, dried at 80 °C, and then calcined at 500 °C in flowing air. The ion exchange step was repeated two more times with intermediate drying and calcination steps. Finally, the catalyst was reduced at 130 °C in 10% H2/N2. This gave a final Pd loading of 0.5 wt % by ICP-AES. The catalyst had a BET surface area of 290 m2/g, a pore volume of 0.46 cm3/g, and an average pore diameter of 10 nm. Dispersion as measured by H2 chemisorption was 74%. The catalyst was washcoated on 200 cpsi cordierite monolith supports (5 × 5 × 1.2 cm3, 0.13 cm channel diameter) from an aqueous slurry of 25 wt % solids, maintained at pH 3.5-4 by the addition of 0.1 M HNO3. The slurry was ball milled for 90 min before the washcoating step in order to lower the average alumina particle size to 0.5 Hz, was to diminish the internal resistance. Similarly, it is possible to compute the ratio CHs 2/C*H2 for the stirred autoclave using eq 23. The overall volumetric mass transfer coefficient for the autoclave reactor was calculated using the correlation proposed by Albal et al.44 Sh ) 1.41 × 10-3Sc0.5ReI0.67We1.29
(24)
where the Sherwood number is Sh ) kLa dI2/D, and the other dimensionless quantities are defined as usual. Using eq 24, kLa was found to be ∼11 s-1 at 2000 rpm. For the smallest catalyst size, this gives CHs 2/C*H2 ) 0.99. This shows that the reaction is not external transport-limited for the powder catalyst in the stirred tank. Since significant external and internal mass transfer resistances exist in the POMR, improvements in POMR performance with increasing frequency suggest that either or both resistances are decreased. Improvements in the rate of reaction due to enhancements in the external mass transfer have already been shown in our previous work,25 and can also be discerned from the increasing kova values (Table 6). The possibility of improving reaction rates by altering the internal transport also exists. Recently Bakker et al.45 showed that the alternating flow of gas bubbles and liquid slugs in a monolith can enhance internal convection and diffusion in a macroporous structured catalyst. They used a honeycomb monolith structure made up of interlocked elongated mullite grains with interstitial voids of ∼45 µm size. The silica used for the coating had an average pore size of 4.2 µm. They did not investigate the effect of frequency or of alternating gas-liquid slug flow. In a previous study, Chandhok et al.46 achieved up to 2 orders of magnitude enhancement in kLa with pulsating flow through a liquid membrane, at frequencies in the 1 Hz range. Rice47 and Leighton and McCready48 explained that the enhancement of diffusive transport in pores due to oscillating flows can result from enhanced Taylor dispersion within the pores. Leighton and McCready48 showed that such dispersion can explain quantitatively the enhancements observed in liquid membrane transport. They proposed that the enhancement is a function of Womersley number R ) r(ω/υ)0.5, with r as pore radius, and the Schmidt number Sc ) υ/De:
( ) (1 - β4 T (β) )
K ∆x )1+ De r
2
T1(β)
(25)
2
where β ) R · Sc0.5 T1(β) ) ber β ber′ β + bei β bei′ β T2(β) ) ber β bei′ β + ber′ β bei β K is the apparent diffusion coefficient due to pulsations, De the normal effective diffusivity, ∆x is the amplitude of fluid pulsation penetrating inside the pores, and the functions ber β and bei β are Kelvin functions, related to the Bessel function I0 by I0(β i-1/2) ) ber β + i bei β . This equation is only valid for a limiting case of R f 0 and Sc f ∞, but β of order one. For this study, R ≈ 10-5, Sc ≈ 104, and β ≈ 0.1.
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Table 7. Transport Enhancements in the POMR f (Hz)
r (µm)
∆x (µm)
K/De
(K/De)fHz/(K/De)0.5Hz
0.5 0.5 0.5 0.5 8 8 8 8 17.5 17.5 17.5 17.5
10 10 1 1 10 10 1 1 10 10 1 1
300 30 300 30 300 30 300 30 300 30 300 30
360 5 2700 28 720 8 3600 37 790 9 9900 100
1.0 1.0 1.0 1.0 2.0 1.8 1.3 1.3 2.2 1.9 3.7 3.6
Using the above equation, predicted enhancements in pore diffusion transport were calculated for our runs (Table 7). Values of macropore radius (r) were obtained from SEM images of washcoated monoliths, which showed both larger (∼10 µm average radius) and smaller (∼1 µm average radius) pores. To obtain the upper and lower bounds, two extremes were considered for ∆x. The largest possible value of ∆x was taken as the product of washcoat thickness and tortuosity which gives a value of 300 µm. The conservative estimate was obtained by assuming that the pulsing fluid is only able to penetrate to a distance of one tenth of the washcoat thickness (∆x ) 30 µm). Calculated values of the transport enhancement, K/De, for the combination of two different pore radii and two different ∆x values are reported in Table 7. The calculated values of K/De for r ) 10 µm and ∆x ) 30 µm are 5-9. Using our lowest practical frequency of 0.5 Hz, the observed improvement in the performance relative to this frequency is the ratio (K/De)f Hz/ (K/De)0.5Hz (keeping r and ∆x constant). For r ) 10 µm and ∆x ) 30 µm, this ratio is 1.8 and 1.9 for 8 and 17.5 Hz, respectively. These values can be compared to the actual ratios of observed rate constants obtained in the POMR, namely 1.6 and 2.2 for 8 and 17.5 Hz. Therefore the enhancement theory results can scale reasonably with the experimental observations for some choice of ∆x. With better estimates of r and ∆x, it might be possible to predict the enhancements in intraparticle transport possible in a POMR or other pulsed reactor. Conclusions It was shown that by using low frequency and low amplitude mechanical oscillations, up to a 220% enhancement in the hydrogenation rate of soybean oil can be achieved in a POMR. On a power per unit volume basis, the POMR performs better than a stirred tank. Experiments with powdered catalysts having much shorter diffusion lengths did show higher reaction rates in the stirred tank. Experimental observations confirmed by standard mass transfer calculations showed that the reaction is intraparticle diffusion-limited for larger catalyst sizes including the monolith in the stirred tank. For the POMR, calculations showed that both external and internal mass transfer limitations exist. As compared to the stirred tank, the POMR generally showed better serial pathway selectivities in soybean oil hydrogenation. However, the formation of trans fatty acids was higher with the monolith catalyst. A theory proposed by Leighton and McCready47 for pulsed diffusion in liquid membranes was used to calculate the transport enhancement in the monolith catalyst washcoat. The theoretical predictions were in order of magnitude and qualitative agreement with experimental observations. So we conclude that it is possible to achieve significant internal transport enhancements in the washcoat of a monolith using low amplitude and low
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frequency pulsation in the fluid. The pulsations are propagated into the pore space, to some extent. Improved internal transport combined with the enhanced external mass transfer rates combine to give higher activity for the POMR. According to the theory of Leighton and McCready,47 the smaller the pore diffusivity and the larger the ratio of amplitude to pore size are, the greater the enhancement is. This result suggests that for higher amplitudes than used here, and for reactions of even heavier molecules, the POMR would be an ideal reactor operating with no transport limitations, but further investigation is required to systematically discern the effects of pulse frequency and amplitude on internal diffusion in the POMR. Acknowledgment This work was partially supported by NSF GOALI-0754397. A.B. acknowledges NSF IGERT-0436759 for a fellowship. We acknowledge Cassidy Sillars and Joe Bell for experimental assistance. Nomenclature a ) interfacial area/volume, m2/m3 A ) piston amplitude, m C ) concentration, mol/m3 C* ) saturation concentration, mol/m3 d ) diameter, m D ) molecular diffusivity, m2/s f ) pulsing frequency, 1/s g ) gravitational constant, m/s2 IV ) iodine value, dimensionless k ) rate constant, 1/min K ) apparent diffusivity, m2/s L ) characteristic length, m m ) mass of sample, g N ) normality, mol/L NI ) impeller speed, 1/s No ) impeller constant (0.8 for marine propeller), dimensionless NH2 ) moles of H2 consumed per unit volume, mol/m3 P ) pressure, MPa Pv ) Power per unit volume, kg/(m · s3) rH2 ) rate of hydrogen consumption, mol/(gPd · min) RH2 ) rate of hydrogen consumption, mol/(gcat · s) r ) pore radius, m R ) gas constant, J/(K · mol) S ) selectivity, dimensionless t ) time, s T ) temperature, K T1,T2 ) functions in eq 25, dimensionless u ) superficial velocity, m/s V ) reaction mixture volume, m3 V0 ) titration volume for blank test, mL Vs ) titration volume for sample, mL W ) catalyst weight, g ∆x ) amplitude of fluid pulsation, m Greek Letters R ) Womersley number, dimensionless β ) R · Sc0.5Z, dimensionless δ ) liquid film thickness, m ε ) hold-up, dimensionless εc ) catalyst porosity, dimensionless η ) effectiveness factor, dimensionless Φ ) Weisz-Prater modulus, dimensionless µ ) viscosity, Pa · s υ ) kinematic viscosity, m2/s
F ) density, kg/m3 Fcc ) catalyst loading, kg/m3 σ ) surface tension, N/m τ ) tortuosity, dimensionless ω ) pulsing frequency (2πf), rad/s Dimensionless Numbers Ca ) Capillary number ()µuTP/σ) Re ) Reynolds number ()FuLdch/µ) ReI ) impeller Reynolds number ()FNIdI2/µ) Sc ) Schmidt number ()µ/FD) Sh ) Sherwood number ()kL/D) We ) Weber number ()FNI2dI3/σ) Subscripts b ) bubble c ) catalyst ch ) channel DB ) double bonds G ) gas GL ) gas-liquid GS ) gas-solid H2 ) hydrogen I ) impeller L ) Liquid LS ) liquid-solid obs ) observed ov ) overall p ) particle Pd ) Palladium S ) solid TG ) triglycerides TP ) two phase surf ) surface UC ) unit cell
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ReceiVed for reView December 17, 2009 ReVised manuscript receiVed May 21, 2010 Accepted June 15, 2010 IE902000E
