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Ind. Eng. Chem. Res. 2005, 44, 9642-9645
Continuous Process of Fine Polyols Production in a Trickle-Bed Reactor Laı´sse C. A. Maranha˜ o* and Ce´ sar A. M. Abreu Departamento de Engenharia Quı´mica, Centro de Tecnologia e Geocieˆ ncias, Universidade Federal de Pernambuco, Recife-PE, 50.740-521, Brazil
To produce polyols at small and medium scales via continuous processes, a three-phase system was applied to operate the catalytic hydrogenation of saccharides. A trickle-bed reactor (with a diameter of dR ) 0.042 m and a height of Z ) 0.800 m) with nickel (14.75%)/granulated activated carbon catalyst (2.38 mm < dp < 6.35 mm, where dp is the grain size) was used, in a range of 373-413 K and a hydrogen pressure of 1.22 MPa. The operational conditions, with a liquidphase flow rate of 5.00-15.00 L/h and a gas-phase flow rate of 100.00-300.00 L/h, guaranteed a steady-state saccharide conversion, which was attained after 1 h of operation. Glucose conversions of 44% at 413 K were attained. In the saccharose processing, consecutive productions of monosaccharides and polyols led to yields of 24% for sorbitol and mannitol at 413 K. To describe the continuous process, a heterogeneous model with axial dispersion and considering the partial wetting of the catalyst was considered. 1. Introduction The production of fine chemicals is traditionaly conducted in discontinuous processes that use an apparatus of great volume in relation to the small quantity of the products obtained. Industrial carbohydrate hydrogenation processes are operated in batch reactors that use nickel catalysts to form polyols. These special products are prepared from biomass resources (sugarcane, starch, ...). Polyols such as sorbitol and mannitol are produced from glucose or saccharose hydrogenations from industrial sources of the food processing industry (mainly corn starch and sugarcane byproducts). In industry, nickel or Raney nickel is mainly applied as a catalyst; however, this creates an additional cost to separate the catalyst, which is usually dragged with the fluid, besides the inconveniences of the batch operations. Turek and co-workers1 studied the use of nickel catalysts for carbohydrate hydrogenation with industrial development scopes, with silica as the metal support. Gallezot and co-workers2 studied saccharide hydrogenation in trickle-bed reactors using a ruthenium catalyst, which, as a noble metal, becomes a very expensive process; this dilemma makes nickel an option, although this process choice is subject to lower yields. In the present work, glucose and saccharose hydrogenations were conducted in a trickle-bed reactor in the presence of a carbon nickel-supported catalyst, following the predicted results provided by the steady-state model based on the formulated kinetics from the batch slurry reactor experiments3 and the estimated hydrodynamics and mass transfer parameters from experimental residence time distribution (RTD) measurements with a nonreactive tracer.4 2. Experimental Section The nickel catalyst was prepared in the research laboratory, using an activated granulated carbon (Sp ) * To whom correspondence should be addressed. Fax: 55 81 21267289. E-mail:
[email protected],
[email protected].
Figure 1. Schematic of the trickle-bed reactor, showing the continuous three-phase system.
623.00 m2/g, Carbomafra Co., Brazil) as a support with a granulometry of 4.56 mm. The humid impregnation method with excess solvent was employed, and after slow drying at 333 K for 72 h and at 373 K for 12 h, the catalyst was calcinated and reduced at 773 K under an argon atmosphere, followed by reduction under an argon and hydrogen atmosphere (1:1). The catalyst, after the reduction, was kept under an inert atmosphere. The three-phase system for the continuous saccharide processing is represented in Figure 1, composed by a fixed-bed stainless steel reactor with 0.042 m of internal diameter and a bed height of 0.800 m. A gas-liquid distributor with 0.002-m orifices for the liquid path and tubes with the same internal diameter and a height of 0.020 m for the gas path is located in the top section of the reactor, above the bed. The fixed bed is composed of nickel (14.75%)/granulated carbon catalyst, with an
10.1021/ie050291q CCC: $30.25 © 2005 American Chemical Society Published on Web 08/12/2005
Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005 9643 Table 1. Kinetic Parameters and Adsorption Equilibrium Constants for Saccharide Hydrogenations at Temperatures of 373, 393, and 413 K under a Pressure of 1.22 MPaa
rG )
Value parameter/constant
at 373 K
at 393 K
glucose hydrogenation kG (mol g-1 h-1) 0.31 ( 0.02 0.66 ( 0.04 KG (L/mol) 2.41 ( 0.12 2.09 ( 0.11 saccharose hydrogenation kSac (h-1) kMo (mol g-1 h-1) KMo (L/mol) a
1.24 ( 0.08 1.88 ( 0.09
Data taken from ref 3.
Figure 2. Reaction schemes for the hydrogenation of carbohydrates.
average diameter of 0.0046 m , bed density of 1105.30 kg/m3, and porosity of 0.47. During the reaction, the saccharide solution with a concentration of 100.00 g/L was fed into the reactor by a piston pump under a pressure of 1.22 MPa at the top of the reactor with a flow rate of 5.00 L/h from a 50-L tank, ensuring a continuous operation time of 7 h, while the hydrogen was provided under pressure at the top of the reactor with a flow rate of 300.00 L/h at 413 K. The liquid accumulates on the distributor, maintaining a constant level while it trickles over the bed. After reaching the steady state indicated by the reproducibility of the results, new conditions are imposed and the procedures are repeated. The heating of the reactor was external through electrical resistances maintained actionated by a proportional integral differentiation (PID) controller. A condenser provided sample collection at room temperature along the reactor and at the outlet of the bed. 3. Results and Discussion For the purpose of seeking cost reduction and higher productivity for this process, which is traditionally batch-operated, saccharide processing in three-phase systems in the trickle-bed reactor provided a scaleup of the discontinuous three-phase process of the saccharide hydrogenation to a continuous process. The kinetic and adsorption parameters of the saccharide hydrogenation processes in the presence of Ni(14.75 wt %)/activated carbon catalyst were established, resulting from the batch experiments with glucose and saccharose performed in a slurry reactor (Table 1).3 On the basis of the experimental evidence, two reaction schemes (Figure 2) were proposed in which the reaction steps were identified. Based on the batch experiments with glucose and saccharose,3 kinetic models were proposed in which the reaction rates were expressed by
rMo )
(1)
(1 + KH2CH2)(1 + KGCG)
rSac )
at 413 K
1.56 ( 0.13 1.11 ( 0.16 2.00 ( 0.09
kGKH2KGCH2CG kSacKH2KSacCH2CSac
(2)
(1 + KH2CH2) kMoKH2KMoCH2CMo
(3)
(1 + KH2CH2)(1 + KMoCMo)
The mass-balance equations of the batch operations that contain the reaction rates were fit to experimental kinetic data, and the values of the kinetic parameters and adsorption equilibrium constants were estimated (see Table 1). The reaction schemes describe the reaction processes that were applied to the operation conditions in the continuous trickle-bed reactor, which establish predictions according to a continuous model. The continuous process of saccharide hydrogenation was operated with a constant liquid and gas flow rate. After 1 h, when the process reached a steady state (i.e., when the analysis of the samples at the bed points and at the bed outlet reproduced the results continuously), the conditions were modified and the process was repeated until the new steady state was reached. At temperatures of 373, 393, and 413 K, a liquid flow rate of 5.00 L/h, and gas flow rates of 100.00 and 300.00 L/h, a conversion of up to 44% at the bed outlet for the glucose hydrogenation was obtained. The hydrodynamic parameters and mass-transfer quantification were obtained from the nonreactive experimental dynamic analysis (RTD experiments) with the trickle-bed reactor. A dynamic heterogeneous model was formulated to describe the nonreactive experiments. The solution of the model equations was expressed as a transfer function that contains the physical parameters (kL, Dax, hL, fe; see Nomenclature section for definitions). A numerical optimization procedure that associates the transfer function solution and the moment method provided optimized values of the parameters. Under the conditions studied in the reactive process in the continuous reactor, values for the Sherwood and Peclet numbers (Sh and Pe, respectively) and for the wetting efficiency (fe) were estimated.4 The results are presented in Table 2 for the two gas flow rates. A heterogeneous steady-state model that included axial dispersion was adopted to describe the isothermal reactive process associated with the hydrodynamic and mass-transfer phenomena, considering partial wetting of the catalytic fixed bed. Balance equations for the saccharides and products (polyols, monosaccharides) were formulated, where Ci (i ) glucose, saccharose, monosaccharides, polyols) are the concentrations of the liquid-phase flowing down through the bed with a porosity in the z-direction. Table 2. Dimensionless Parameters Related to the Mass Transfer, the Axial Dispersion, and the Partial Wetting Adopted for the Trickle-Bed Reactora QG (L/h)
Sh
Pe
fe
100.00 300.00
12.04 12.40
3.63 3.39
0.38 0.34
a Additional parameter values for the calculation were as follows: liquid flow rate, QL ) 5.00 L/h; temperature, T ) 298 K; and pressure, P ) 10.13 × 10-2 MPa.
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Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005
For the glucose hydrogenation:
Dax
d2CG 2
dz
- uL
ηGk′GCG dCG )0 dz 1 + KGCG
(4)
for boundaries
z ) 0; CG0- ) CG0, CG0- ) CG0+ z ) Z;
Dax dCG uL dz
dCG )0 dz
where Dax and uL are the axial dispersion and the superficial velocity in the liquid phase, respectively. The last term represents the reaction rate considering a Langmuir-Hinshelwood (L-H) kinetic model that leads to a nonlinear equation where k′G is the apparent kinetic constant of glucose (k′G ) kGKH2KGPH2HH2-1Fcat) and KG is the equilibrium adsorption constant of glucose. ηG is the effectiveness factor for the trickle-bed reactor, which considers the partial wetting of the bed;5 this factor is expressed as
ηG )
fe/φG[coth(3φG/fe) - (fe/3φG)]
Figure 3. Hydrogenation of glucose at 1.22 MPa and 413 K in a trickle-bed reactor.
(5)
1 + (φG/ShLG)[coth(3φG/fe) - (fe/3φG)]
where fe is the wetting efficiency and φG is the Thiele modulus (φG ) [(rGdp2)/(6DeffG)CG]1/2). For the saccharose hydrogenation:
Dax Dax
d2CMo dz
2
d2CSac
- uL
Figure 4. Hydrogenation of saccharose at 1.22 MPa and 413 K in a trickle-bed reactor.
dCMo k′MoCMo + ηMo k′SacCSac )0 dz 1 + KMoCMo (7)
ride, and polyol productions, at 1.22 MPa and 413 K. The applied heterogeneous nonlinear model predicts that the experimental results have an average precision on the order of 7.40%.
(
-u
Dax
dCSac - ηSack′SacCSac ) 0 dz
(6)
dz
2
d2CPo dz2
)
dCPo k′MoCMo )0 + ηPo dz 1 + KMoCMo
-u
(8)
for boundaries
z ) 0; CSac0- ) CSac0+ z ) Z;
Dax dCSac uL dz
dCSac dCMo dCPo ) ) )0 dz dz dz
CSac0- ) CSac0; Cj+ )
Dax dCj ; j ) Mo, Po uL dz
where k′Sac (k′Sac ) kSacKH2PH2HH2-1Fcat) and k′Mo (k′Mo ) kMoKMoKH2PH2HH2-1Fcat) are the apparent kinetic constants, and ηSac, ηMo, and ηPo are the effectiveness factors (η ) ηi(φi,ShLi)),5 based on the corresponding components (i ) Sac, Mo, Po, respectively). To solve the nonlinear equation system, the finite difference implicit method was used. Figures 3 and 4 present the comparison between the calculated (line) and experimental (scatter) concentration axial profiles in the continuous trickle-bed reactor for the glucose and saccharose hydrogenations with sorbitol, monosaccha-
4. Conclusions By predicting the scaleup of the catalytic processes from discontinuous to continuous operations, keeping in mind the polyol production, the development of the saccharide hydrogenation process into a continuous three-phase reactor was achieved. To achieve this accomplishment, the employment of the trickle-bed reactor that was operating a catalytic process under a continuous flow regime was analyzed. Under moderate operation conditions (1.22 MPa, 413 K), a conversion of 44% was obtained for the glucose continuous hydrogenation, with a polyol selectivity of 99.31%. Yields of 24% in sorbitol and mannitol for the saccharose hydrogenation were observed under the same conditions, indicating a possibility to develop a system to process saccharide continuously using higher pressures (up to 2.54 MPa) and lower liquid flow rates to obtain higher conversions. It was observed that, in the batch process, 0.49 mol of sorbitol from a 0.50-L glucose solution of 0.56 mol/L was obtained after 3 h of reaction at 2.44 MPa and 413 K, whereas 3.55 mol of sorbitol were obtained in the continuous operation at the same temperature, after the same time and under lower pressure (1.22 MPa).
Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005 9645
Nomenclature
Literature Cited
C ) concentration dp ) particle diameter (m) Dax ) axial dispersion coefficient (m2/s) DL ) molecular diffusion coefficient (m/s) fe ) wetting efficiency HH2 ) Henry constant for hydrogen KG ) glucose adsorption equilibrium constant (L/mol) KH2 ) hydrogen adsorption equilibrium constant (L/mol) KSo ) sorbitol adsorption equilibrium constant (L/mol) k ) kinetic constant (s-1) kt,LS ) liquid-solid mass transfer coefficient (m/s) Pe ) Peclet number; Pe ) ZuL/(Dax) RTD ) residence time distribution Sh ) Sherwood number; Sh ) kt,LSdp/DL u ) superficial velocity (m/s) V ) volume (m3) z ) reactor length (m) (mol/m3)
Greek Letters ) bed porosity Fcat ) catalyst specific mass
(1) Turek, F.; Chakrabarti, R. K.; Lange, R.; Geike, R.; Flock, W. On the experimental study and scale-up of three-phase catalytic reactors. Hydrogenation of glucose on nickel catalyst. Chem. Eng. Sci. 1983, 38, 275-281. (2) Gallezot, P.; Nicolaus, N.; Fle`che, G.; Fuertes, P.; Perrard, A. Glucose hydrogenation on ruthenium catalysts in a trickle-bed reactor. J. Catal. 1998, 180, 1. (3) Maranha˜o, L. C. A.; Sales, F. G.; Pereira, J. A. F. R.; Abreu, C. A. M. Kinetic evaluation of polyol production by three-phase catalytic hydrogenation of saccharides. React. Kinet. Catal. Lett. 2004, 81, 169. (4) Maranha˜o, L. C. A. Processo contı´nuo de hidrogenac¸ a˜o catalı´tica de sacarı´deos em reator de leito gotejante. Ph.D. Thesis, UNICAMP, Brazil, 2001. (5) Sakornwimon, W.; Sylvester, N. D. Effectiveness factors for partially wetted catalysts in trickle-bed reactors. Ind. Eng. Chem. Proc. Res. Dev. 1980, 19, 103-108.
Received for review March 1, 2005 Revised manuscript received July 12, 2005 Accepted July 13, 2005 IE050291Q
