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Kinetics of Aqueous Extraction of Hemicelluloses from Spruce in an Intensified Reactor System Henrik Grenman,† Kari Er€anen,† Jens Krogell,‡ Stefan Willf€or,‡ Tapio Salmi,† and Dmitry Yu. Murzin*,† †
Laboratory of Industrial Chemistry and Reaction Engineering, Process Chemistry Centre, Department of Chemical Engineering, Åbo Akademi University, Biskopsgatan 8, FI-20500 Åbo/Turku, Finland ‡ Laboratory of Wood and Paper Chemistry, Åbo Akademi Process Chemistry Centre, Porthansgatan 3, FI-20500 Åbo/Turku, Finland ABSTRACT: The aqueous extraction of hemicelluloses has gained increasing interest with new emerging applications for hemicelluloses in the modern forest-based biorefinery concept. The extraction kinetics play a key role in their industrial utilization. The traditional kinetic studies and models for the selective dissolution of softwoods, however, always incorporate high concentrations of sodium hydroxide and sodium sulfide relevant to pulping, and the kinetics in pure water is left outside the scope of these investigations. Aqueous extraction of hemicelluloses from spruce sapwood was investigated with a new cascade reactor setup, which was developed for intensified investigation of solid-liquid dissolution kinetics. The experiments were performed at 150-170 °C with a particle size of 1.25-2 mm and solid loads of about 6.25 g of dry wood/L in the kinetic regime of intrinsic kinetics. The pH of the liquid phase was measured during the reaction. The selectivity of the dissolution and degradation of hemicelluloses was examined qualitatively. The total concentrations of arabinose (Ara), xylose (Xyl), galactose (Gal), glucose (Glc), mannose (Man), rhamnose (Rha), glucuronic acid (GlcA), 4-O-methylglucuronic acid (4-O-MeGlcA), and galacturonic acid (GalA) were analyzed from the liquid and solid phases during the reaction. The dissolution was observed to be highly temperature dependent, and degradation of the compounds was observed. A kinetic model was developed for the overall extraction of the hemicelluloses. The activation energy was determined to be 135 kJ/mol. No significant influence of the pH on the actual extraction kinetics was observed, even though it influences strongly the degradation of the compounds through hydrolysis.
1. INTRODUCTION Solid-liquid reactions are involved to a large extent in the chemical industry of today. Many fields of industry, e.g., hydrometallurgy, fuel production, agrochemicals, and pharmaceuticals, employ solid-liquid reactions at some point of production, which explains the vast research in the field. The research of the reaction kinetics has long been a subject of extensive investigations due to its importance in the design and optimization of the production processes. A large sector of industry, which is dependent almost solely on a solid-liquid process in production, is the chemical pulping industry, where the ultimate aim is to remove lignin, but unfortunately for the production of pulp, a large part of the hemicelluloses are dissolved simultaneously, too. The basic production methods used commonly today, i.e., the kraft process and soda pulping, date back to the 19th century, to the days long before chemical engineering made its breakthrough in the modeling and optimization of industrial production. With the development of chemical engineering, these processes have been optimized and enhanced tremendously. However, some aspects still rely partly on empirically developed experimental optimization without a comprehensive understanding of the underlying chemical and physical phenomena. One of these aspects is the kinetics of the selective dissolution of wood chips in the cooking liquor (often called delignification), which is the core process in pulp production. There are several reasons for this: the kinetics of solid-liquid reactions is more difficult to investigate than the kinetics of homogeneous systems due to such factors as the determination of surface areas, combined control of diffusion and intrinsic kinetics, and the heterogeneity of the materials. Moreover, r 2011 American Chemical Society
the chemistry of wood is very complex, comprising interlinked macromolecules which are cleaved into numerous smaller fragments during the process. The shear number of different compounds makes it impossible to obtain reaction models consisting of elementary steps only, but still, improvement can be made in the understanding of the overall process. However, some works have been published in which models have been proposed for the influence of a certain parameter on the extraction rate or with the aim of modeling the overall reactions.1-11,14-28 Probably the best known one is the H-factor model, which describes the effect of temperature on the extent of delignification.1 More complex models were developed later on to incorporate the influence of the concentrations of different compounds. A common methodology has been to divide the dissolution process or the dissolved components at different stages into categories, e.g., initial, bulk, and residual.2-7 Different reaction orders have been determined for the different phases in the different regimes of the extraction. Some of the models assume that the different phases (of lignin and carbohydrates) are present in the wood from the beginning of the reaction and that they react simultaneously, resulting in parallel reactions, while others assume that the phases are formed during the extraction, resulting in consecutive reaction kinetics. Another approach used to describe the extraction process is to use time- or conversiondependent rate constants, which basically leads to the Received: September 21, 2010 Accepted: February 8, 2011 Revised: February 1, 2011 Published: March 03, 2011 3818
dx.doi.org/10.1021/ie101946c | Ind. Eng. Chem. Res. 2011, 50, 3818–3828
Industrial & Engineering Chemistry Research assumption that the dissolving compounds consist of an infinite number of components, reacting with a continuous distribution of rate constants and activation energies depending on the stage of the reaction.8-10 Moreover, Yang et al. have published a model which treats the compounds in wood as lumped chemical species, e.g., lignin and carbohydrates, and which incorporates surface adsorption and reaction from a chemical engineering point of view.11 They obtained satisfactory fits to the data of Santos et al., as well as the data Wilder and Dalenski, by assuming that the mass transfer was negligible when thin wood chips were used.12,13 Dang et al. applied an Avrami type model to the dissolution process and found the kinetics of extraction to be inversely proportional to the chip thickness to the power m depending on the type of wood species.14,15 Some of the models presented above aim at characterizing the influence of only one or a few parameters on the extraction kinetics of lignin and hemicelluloses; others aim at developing an overall model. The traditional kinetic models for pulping, however, always incorporate high concentrations of sodium hydroxide and sodium sulfide relevant to pulping, and the kinetics in pure water or low concentrations of these traditional reagents is left outside the scope of these studies and models. Aqueous extraction of hemicelluloses from various raw materials has also been investigated in studies concerned with utilizing the hemicelluloses instead of the cellulose. The hydrolysis rates of hardwoods such as eucalyptus, sugar maple, red oak, Douglas fir, hard maple, and aspen16-23 as well as other sources of hemicelluloses such as mixed agricultural residue, almond shells, sugar cane bagasse, and corn cob16,23-28 have been studied. Kinetic models have also been developed for these extraction processes based on the severity approach, where a number of factors are lumped into a single parameter, partly comparable with the time- or conversion-dependent rate constants used in the models for pulping. Pseudohomogeneous kinetic models have been also employed, where an Arrhenius type temperature dependence of the rate can be incorporated. Sch€adel et al.29 and Carrasco et al.23 demonstrated that the structures, compositions, and hydrolysis rates of hemicelluloses from different sources vary significantly, which makes it very challenging to find a general model for hemicellulose extraction independent of the raw material. Aqueous extraction from softwoods has not gained as much attention as that from hardwoods and agricultural raw materials, in contrast to pulping studies where softwoods are at the core of the research. The main hemicellulose present in softwood is O-acetylgalactoglucomannan (AcGGM for short); the amount is usually 10-25% (w/w) in stem wood. The degree of polymerization (DP) has been shown to be approximately 100-150.30,31 This water-soluble hemicellulose has a promising biological activity and physicochemical properties suitable for various applications in, for example, food, animal feed, health, papermaking, textile, and cosmetics industries.31,32 The utilization of these heteropolysaccharides has gained an increasing interest during the past years with the strive for developing modern forest-based biorefineries. In current operations in kraft pulping, this raw material is combusted with the black liquor for energy production. Aqueous or mild solvent extraction of the hemicelluloses could, in practice be performed with existing pulping equipment, prior to delignification. Several different solvents, such as chlorous acid, bisulfate, dimethyl sulfoxide, sodium hydroxide, potassium hydroxide, and boric acid, have been studied, but according to current knowledge, the best solvent for extracting unmodified
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AcGGM is water at neutral or slightly acidic pH.31-39 However, in order to make it a viable option, the extraction kinetics should be determined quantitatively. Moreover, the influence of the extraction conditions on both the liquid-phase and solid-phase products should be closely examined under precisely controlled conditions. The extraction process should not deteriorate the pulp quality, and the kinetics of hemicellulose degradation should be well understood. The basis for developing kinetic models for any reaction is to have reliable data on the studied phenomena. This can rarely be obtained from industrial data, since the reaction rate is usually heavily influenced by the mass and heat transfer process. This is also the case in chemical pulping, in which both external mass transfer and internal mass transfer influence the process strongly. It was demonstrated by Chiang et al.6 that, even with wood meal, the experimental conditions should be very turbulent in order to overcome the external mass transfer effects. This might be one of the reasons for somewhat contradictory results reported in the literature for the dissolution kinetics in the kraft process. Several models are based on data obtained from the literature, where experiments have been performed with various setups and methods, but the means and sufficiency of the agitation is not always investigated or reported. For example, Kleinert2 reports that the reactor tubes were subjected to gentle shaking in a glycol bath. Moreover, vigorous mechanical mixing performed with an impeller might influence mechanically the reaction rate by causing mechanical removal of compounds from the surface of the chips. Precise temperature control and measurement is another key issue when investigating the kinetics. Typical digesters used in the extraction studies, even at laboratory scale, operate with rather long heating and cooling times, making the extraction of reliable kinetic data very challenging. This is partly due to the fact that these reactors have initially not been designed for kinetic studies, but rather have been designed for evaluating the influence of the initial reaction conditions and some process parameters on the properties of the final product, pulp. The current research is devoted to obtaining systematically reliable kinetic data on the extraction of spruce chips (Picea abies), starting with aqueous extraction and continuing with increasing concentrations of selected solvents. The work is focused on evaluating the overall kinetics in the absence of external mass and heat transfer restrictions and providing a possibility to quantify and distinguish between the intrinsic kinetics and internal mass transfer effects in a systematic manner. The novel reactor setup constructed for detailed dissolution studies is described here in detail. Experimental results and modeling of aqueous extraction of hemicelluloses from spruce sapwood are reported in the current work. These results are interesting due to several aspects. The topic of extracting and utilizing hemicelluloses is a very interesting and very relevant topic in the biorefinery research. Even though the aqueous extraction of hemicelluloses has been investigated previously, the kinetics of hemicellulose dissolution from spruce is not a thoroughly studied subject, having still much room for improvement in experimental data and modeling. It also lays the foundation for an overall model for the selective extraction from spruce, including the influence of traditionally used and novel solvents. These studies will be made with the same equipment and procedure. Moreover, the experiments demonstrate the practical applicability of the intensified reactor system.
2. REACTOR SETUP The entire reactor system is constructed of stainless steel. All the components are connected to each other with pipes with an 3819
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Industrial & Engineering Chemistry Research
Figure 1. Simplified scheme of the reactor setup. The reactors are numbered from 1 to 5.
Figure 2. Picture of the reactor setup.
inner diameter of 9.7 mm. The pieces are connected and the flow paths are controlled with Swagelok valves. A simplified scheme of the reactor system is shown in Figure 1, and a picture of the reactor system is displayed in Figure 2. Five 200 mL Parr reactors are connected in series to form a cascade reactor. The effective volume of each reactor is 178 mL due to the built-in liquid injection system. The liquid flow is led through a pipe with an inner diameter of 10.2 mm from the top to the bottom of the reactor and released into the reactor at the bottom through eight circular openings of 3 mm diameter, which spread the liquid flow uniformly (Figure 4). The liquid flows through the reactor from the bottom up in about 2-3 s depending on the flow rate, causing a turbulent flow. A net, with a hole size of 0.6 mm (optionally smaller), is placed in the top of the reactor to prevent any solids from moving further into the reactor system. The liquid flow is circulated through a 2000 mL Parr vessel, which is placed before the pump. Each of the 200 mL Parr reactors is equipped with a heating jacket with a heating capacity of 500 W, while the 2000 mL Parr vessel is equipped with a 2000 W heating jacket. Moreover, heating cables (Horst) with a total length of 10 m and a heating capacity of 1300 W are wrapped around the pipelines connecting the different components, in order to ensure proper and evenly distributed heating in the system. The temperature is measured continuously inside each of the 200 mL reactors and recorded on a PC. The temperature of each Parr reactor, including the 2000 mL vessel, as well as the temperature between the last reactor and the 2000 mL vessel, is measured for controlling purposes. The
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temperature control is performed with PID controllers (Winkler WRT2000X) attached to the thermocouples and the heating devices. The liquid is circulated with a Bran et Luebbe Novados H2 diaphragm pump, which can tolerate 180 °C and sodium hydroxide concentrations up to 20%. Moreover, the pump ensures a constant flow rate independent of the pressure drop and total volume of the system. The maximum pumping capacity is 221 L/h. A flow meter, produced by ABB (Model FAM 541EIY0FI, maximum 300 L/h), is placed after the pump in order to monitor the flow through the system. The pressure of the system is measured and recorded online before the first reactor and after the last reactor in order to determine the pressure drop over the system. The reactor system can be equipped with a custom-made glass reactor of equal size to the Parr reactors. This is utilized to visually observe how the solid particles behave at different flow rates, which makes it possible to optimize the flow through the reactors depending on the solids used in the experiment, i.e., particle size, density, etc. 2.1. Experimental Operation. In the initial stages of the experiment, the reactors are filled with the solid material and optionally with a predetermined amount of liquid, e.g., distilled water, to wet the particles before the reaction, usually overnight to ensure complete wetting. The system is filled with a predetermined quantity of the solvent, which is circulated in bypass mode; i.e., the solvent does not flow through the reactors at this stage. The liquid phase is then heated to the desired temperature in the bypass mode before any reaction takes place. Once the solvent temperature has stabilized, the reactors can be rapidly heated to the desired temperature, after which the solvent flow is directed through the reactors and the reactions are started. The large liquid flow through the reactors ensures turbulent conditions as well as evenly distributed liquid-phase concentrations throughout the reactors. The conditions are thus as if the particles were in a large batch reactor under very turbulent conditions. At a desired point in time, one of the reactors is again bypassed, stopping the flow through the reactor, after which it is rapidly quenched to around 50-80 °C to stop the reaction and to lower the pressure and enable the opening of the reactor. Both the liquid- and solid-phase samples can thus be rapidly collected for analysis. The amount of the samples is quite large (∼50 mL of liquid and ∼4 g of dry solid), enabling several different analyses of both phases with various techniques. The remaining four reactors can be stopped and sampled in a similar manner to obtain a total of five liquid- and solid-phase samples in one experiment. Several liquid-phase samples can also be collected into a cooled container during the reaction if needed. 2.2. Operational Characteristics. 2.2.1. Temperature Profile. The temperature is continuously measured inside each reactor. Once the temperature inside the reactors reaches 80 °C, the preheated liquid flow is led through the reactors and the reaction is thus started. The temperature is stabilized within a few minutes at the desired level with the aid of the effective heating and six PID regulators. After a predetermined time, the last reactor in series is bypassed and rapidly quenched, stopping the reaction in about 1 min (
