ARTICLE pubs.acs.org/IECR
Characterizing Lactic Acid Hydrogenolysis Rates in Laboratory Trickle Bed Reactors Yaoyan Xi,† James E. Jackson,‡ and Dennis J. Miller*,† †
Department of Chemical Engineering and Materials Science and ‡Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, United States ABSTRACT: Representative reaction kinetics are difficult to obtain in multiphase laboratory trickle bed reactors, particularly when the gaseous reactant is rate-limiting, because of mass transport resistances and reactor hydrodynamics in the trickle bed regime. The ruthenium-catalyzed hydrogenolysis of lactic acid to propylene glycol has been examined in trickle bed and batch reactors to better understand the influence of mass transfer and partial wetting and to identify operating conditions where intrinsic kinetic rates can be obtained. At high liquid flow rates and low conversions in the trickle bed reactor, propylene glycol formation rates agree well with intrinsic rates obtained in a stirred batch reactor, with rate independent of feed flow rate or bed configuration in the trickle bed reactor. Application of a mass transport model to the trickle bed reactor at lower flow rates allows rates to be predicted outside the intrinsic kinetic regime. These results provide guidance for proper operation of laboratory trickle bed reactors and make it possible to predict performance in a trickle-bed reactor based on experiments conducted in bench-scale batch reactors.
1. INTRODUCTION The characterization of trickle bed reactors at both the laboratory and the commercial scale has been the subject of numerous investigations.14 Much work has been done to study mass transfer, energy transfer, reaction kinetics, and partial catalyst wetting58 of trickle bed reactors for the purpose of better predicting and characterizing their operation. Even so, trickle bed reactors remain complex unit operations that are particularly subject to the influence of mass transfer and hydrodynamic (e.g., partial wetting) limitations at practical operating conditions. Because of these complications, laboratory and pilotscale trickle bed reaction studies often give results that are difficult to interpret from a fundamental kinetic viewpoint, and scale-up of trickle beds is subject to uncertainty and risk. In contrast to trickle beds, bench-scale batch reactors are easy to operate in an intrinsic kinetic reaction mode and are thus effective tools for evaluating catalysts and conditions for three-phase reactions. With greater control of mass transfer effects via vigorous agitation of fully wetted catalysts and the use of temperature to control reaction rate, batch reactors are efficient and reliable tools. Despite the utility of stirred batch reactors for fundamental reaction characterization, efforts to translate batch reactor results to trickle bed reactor design or analysis have met with limited success.913 Differences in reaction rates of 50% or more are often observed between batch and trickle bed reactors,8 and selectivities can vary widely, unless care is taken to properly define reactor geometry and reactant flow rates in the trickle bed. In this paper, we present an illustration of trickle bed operation where observed reaction rates approach those in a batch reactor operating in the intrinsic kinetic regime. We use the ruthenium-catalyzed hydrogenolysis of lactic acid (2-hydroxypropanoic acid, LA) to propylene glycol (1,2-propanediol, PG), an aqueous-phase reaction system with high selectivity to PG and few byproducts,1417 as a prototype for renewable substrate conversions. We examine a range of tricklebed operating conditions, including some where reaction rate r 2011 American Chemical Society
approaches intrinsic behavior, and then apply a reactor model involving mass transfer and partial wetting to reconcile trickle bed behavior with intrinsic batch data over a wider range of conditions. The goal of this work is to strengthen our ability to predict trickle-bed performance based on batch reaction studies, and thus facilitate more efficient scale-up and design.
2. MATERIALS Lactic acid (88.9%, J.T. Baker), HPLC-grade water (99.99%, J. T. Baker), propylene glycol (99.5%, Jade Scientific, Inc.), ethylene glycol (99.0%, Spectrum, Inc.), and sulfuric acid (98%, Columbus Chemical Industries) were all used as-received. Ultrahigh purity gases used in experiments, hydrogen (99.999%), helium (99.999%), and oxygen (99.99%), were produced by Linde Gas, LLC. Inert glass beads (150220 μm) were purchased from Sigma-Aldrich Co. The catalyst used for LA hydrogenolysis is a 5 wt % ruthenium on coal-based activated carbon (15 þ 30 mesh, average particle diameter dp = 1.4 103 m; Calgon Carbon Corp). The catalyst was prepared by dropwise addition to the dry activated carbon support of a predetermined quantity of solution just sufficient to fill the internal porosity of the activated carbon support and containing an amount of RuCl3 equivalent to 5 wt % Ru metal on the carbon support. The incipiently wetted support was then subjected to slow drying at 90 °C, heating in He, and finally reduction at 300 °C overnight in 0.7 MPa H2 in the trickle bed reactor.17 The catalyst was then passivated in 2% O2 in Ar for 30 min prior to removal for use either in the batch or in the trickle bed reactor. The prepared catalyst has a dry bulk density of about 400 kg/m3 and a particle density of 800 kg/m3. Received: November 16, 2010 Accepted: March 29, 2011 Revised: March 24, 2011 Published: March 29, 2011 5440
dx.doi.org/10.1021/ie1023194 | Ind. Eng. Chem. Res. 2011, 50, 5440–5447
Industrial & Engineering Chemistry Research
Figure 1. Trickle bed reactor configuration.
3. APPARATUS AND PROCEDURES 3.1. Trickle Bed Reactor. The trickle bed reactor consists of a 316 Stainless Steel tube of 0.025 m outside diameter (OD) and an inside diameter dR = 0.013 m with a length Z = 0.45 m (Figure 1). The reactor features an oil jacket to control temperature and an internal, centered thermowell to facilitate bed temperature measurements during reaction. A Chromel-Alumel thermocouple (Omega), 1.5 mm 0.6 m in length, can be moved within the thermowell to measure the axial temperature profile in the bed. Reactor temperature is maintained by a Julabo SE-6 recirculating oil bath that circulates silicon oil through the reactor jacket. In the reactor, pressure is maintained constant (8.3 MPa in most runs) via a back pressure regulator, and hydrogen flow rate is monitored by a Porter mass flow controller. Liquid feed solution is fed to the reactor through a Bio-Rad HPLC pump. The catalyst was packed into the trickle bed reactor between a 24-cm layer of glass beads (below) to support the catalyst particles and an 8-cm layer of stainless steel beads (above) to help preheat and distribute the liquid feed prior to contacting the catalyst. In this study, three catalyst beds were used: one containing 19.0 g (45 cm3) of catalyst, another 9.1 g (22.5 cm3) of catalyst, and a third containing 9.1 g (22.5 cm3) of catalyst diluted with 22.5 cm3 inert glass beads as fines. The method of packing and the inert glass bead diameter (dfines ∼ 0.2 mm) for the diluted bed were chosen in accordance with those suggested by Tsamatsoulis et al.18 to enhance wetting of the catalyst. Following the packing of a catalyst and beads into the trickle bed, the catalyst was re-reduced by heating the trickle bed from room temperature at 1.5 °C/min to 563 K and kept for 3 h in 0.7 MPa hydrogen flowing at 100 cm3(STP)/min. After reduction, the trickle bed was cooled to reaction temperature under hydrogen flow. Once the system pressure and hydrogen flow rate were stable, the liquid feed pump was started, and downward, co-current flow of hydrogen and LA solution were established in the trickle bed reactor.
ARTICLE
Initial LA concentration for all experiments in the trickle bed reactor was 1.0 M, and hydrogen pressure was maintained constant at 8.3 MPa. Hydrogen flow rate was adjusted to maintain a 2.5:1 molar feed ratio of hydrogen to LA in all experiments. It generally took 23 h to reach steady state in the trickle bed reactor. 3.2. Batch Reactor. Batch reactions were carried out in a Parr 5000 Multireactor System containing six 75 mL reactors controlled with a Parr 4871 process controller. For batch reactions, catalyst was ground to a fine powder (
