Mathematical modeling of flow and kinetics in a reactor for dilute-acid hydrolysis of cellulose particles

Amathematical model to simulate the dilute-acid hydrolysis process of cellulose particles is presented. In this model, the mass is treated as a mixture of different components. A test case is considered for which transport equations for components are developed and solved together with the momentum equation for the fluid flow. To solve the model equations, a commercially available flow solver was used. All input data were taken from previously published works. For the small static mixer considered as test geometry, the result, in terms of the conversion of the cellulose particles, was reasonable. With input parameters that are relevant to a plant-size reactor, the model can be used to predict the conversion of both cellulose and hemicellulose particles.     

A comprehensive kinetic model for dilute-acid hydrolysis of cellulose

Acid-catalyzed hydrolysis is controlled not only by temperature and acid concentration but also by the physical state of the cellulose. Under low temperature and acid condition the cellulose structure stays in stable crystalline form. Therefore, the prevailing reaction mode is endwise hydrolysis. Glucose then becomes the main sugar product. However, when temperature and/or acid concentration is raised to a certain level, the cellulose structure becomes unstable by breakage of hydrogen bonding, the primary force that holds the cellulose chains. Once the crystalline structure of the cellulose is disrupted, acid molecules can penetrate into the inner layers of the cellulose chains. In support of this hypothesis, we have experimentally verified that a substantial amount of oligomers is formed as reaction intermediates under extremely low-acid and high-temperature conditions. We also found that the breakage of hydrogen bonds occurs rather abruptly in response to temperature. One such condition is 210°C, 0.07% H₂SO₄. Glucose, once it is formed in the hydrolysate, interacts with acid-soluble lignin, forming a lignin-carbohydrate complex. This occurs concurrently with other reactions involving glucose such as decomposition and reversion. On the basis of these findings, a comprehensive kinetic model is proposed. This model is in full compliance with our recent experimental data obtained under a broad range of reaction conditions.     

Kinetics of glucose decomposition during dilute-acid hydrolysis of lignocellulosic biomass

Recent research work in-house both at Auburn University and National Renewable Energy Laboratory has demonstrated that extremely low concentrations of acid (e.g., 0.05–0.2 wt% sulfuric acid) and high temperatures (e.g., 200–230°C) are reaction conditions that can be effectively applied for hydrolysis of the cellulosic component of biomass. These conditions are far from those of the conventional dilute-acid hydrolysis processes, and the kinetic data for glucose decomposition are not currently available. We investigated the kinetics of glucose decomposition covering pH values of 1.5–2.2 and temperatures of 180–230°C using glass ampoule reactors. The primary factors controlling glucose decomposition are the reaction medium, acid concentration, and temperature. Based on the experimental data, a kinetic model was developed and the best-fit kinetic parameters were determined. However, a consistent discrepancy in the rate of glucose disappearance was found between that of the model based on pure glucose data and that observed during the actual process of lignocellulosic biomass hydrolysis. This was taken as an indication that glucose recombines with acid-soluble lignin during the hydrolysis process, and this conclusion was incorporated accordingly into the overall model of glucose decomposition.     

A process economic approach to develop a dilute-acid cellulose hydrolysis process to produce ethanol from biomass

Successful deployment of a bioethanol process depends on the integration of technologies that can be economically commercialized. Pretreatment and fermentation operations of the traditional enzymatic bioethanol-production process constitute the largest portion of the capital and operating costs. Cost reduction in these areas, through improved reactions and reduced capital, will improve the economic feasibility of a large-scale plant. A technoeconomic model was developed using the ASPEN Plus^TN modeling software package. This model in cluded a two-stage pretreatment operation with a co-current first stage and countercurrent second stage, a lignin adsorption unit, and a cofermentation unit. Data from kinetic modeling of the pretreatment reactions, verified by bench-scale experiments, were used to create the ASPEN Plus base model. Results from the initial pretreatment and fermentation yields of the two-stage system correlated well to the performance targets established by the model. The ASPEN Plus model determined mass and energy-balance information, which was supplied, to an economic module to determine the required selling price of the ethanol. Several pretreatment process variables such as glucose yield, liquid: solid ratio, additional pretreatment stages, and lignin adsorption were varied to determine which parameters had the greatest effect on the process economics. Optimized values for these key variables became target values for the bench-scale research, either to achieve oridentify as potential obstacles in the future commercialization process. Results from this modeling and experimentation sequence have led to the design of an advanced two-stage engineering-scale reactor for a dilute-acid hydrolysis process.     

Shrinking-bed model for percolation process applied to dilute-acid pretreatment/hydrolysis of cellulosic biomass

For many lignocellulosic substrates, hemicellulose is biphasic upon dilute-acid hydrolysis, which led to a modified percolation process employing simulated two-stage reverse-flow. This process has been proven to attain substantially higher sugar yields and concentrations over the conventional single-stage percolation process. The dilute-acid pretreatment of biomass solubilizes the hemicellulose fraction in the solid biomass, leaving less solid biomass in the reactor and reducing the bed. Therefore, a bed-shrinking mathematic kinetic model was developed to describe the two-stage reverse-flow reactor operated for hydrolyzing biphasic substrates, including hemicellulose, in corn cob/stover mixture (CCSM). The simulation indicates that the shrinking-bed operation increases the sugar yield by about 5%, compared to the nonshrinking bed operation in which 1 reactor volume of liquid passes through the reactor (i.e.,t = 1.0). A simulated optimal run further reveals that the fast portion of hemicellulose is almost completely hydrolyzed in the first stage, and the slow portion of hemicellulose is hydrolyzed in the second stage. Under optimal conditions, the bed shrank 27% (a near-maximum value), and a sugar yield over 95% was attained.     

Modeling the enzymatic hydrolysis of dilute-acid pretreated Douglas Fir

Glucose yield from the enzymatic hydrolysis of cellulose was investigated as a function of cellulase enzyme loading (7–36 filter paper units [FPU]/g cellulose) and solids concentration (7–18% total solids) for up to 72 h on dilute sulfuric-acid pretreated Douglas Fir. The saccharification was performed on whole hydrolysate with no separation or washing of the solids. Enzyme loading had a significant effect on glucose yield; solids concentration had a much smaller effect even at higher glucose concentrations. The data were used to generate an empirical model for glucose yield, and to fit parameters of a cellulose hydrolysis kinetic model. Both models could be used for economic evaluation of a separate hydrolysis and fermentation process.     

COS hydrolysis in the presence of oxygen： Experiment and modeling

A mathematical model of COS hydrolysis on Al2O3, with fouling of catalyst, has been developed. Kinetic studies were carried out in a fixed bed reactor under atmospheric pressure and low temperature （40-70℃）. The effects of the COS inlet concentration, temperature, and relative humidity were analyzed. Experimental results of breakthrough curves were used to obtain kinetic parameters, which accounted for effects of S deposition on the inner-face of the catalyst. The model described the experimental breakthrough curves satisfactorily and well explained the performance of COS hydrolysis in the presence of oxygen. The exothermic heat of adsorption and activation energy, assuming Arrhenius type of temperature dependence of the equilibrium constant, were determined. Activation energy of COS hydrolysis and H2S oxidation were 35.9 kJ/mol, 19.6 kJ/mol; adsorption heat of H2O and H2S on Al2O3 were 45.1 and 60.1 kJ/mol respectively. Deactivation coefficient （α） was used to quantify the behavior of COS hydrolysis at different operating conditions. The effect of relative humidity on α is significant in the relative humidity range under study. Experimental data accorded well with model data in the studied range.     

Two-stage dilute-acid pretreatment of softwoods

Whole treechips obtained from softwood forest thinnings were pretreated via single-and two-stage dilute-sulfuric acid pretreatment. Whole-tree chips were impregnated with dilute sulfuric acid and steam treated in a 4-L steam explosion reactor. In single-stage pretreatment, wood chips were treated using a wide range of severity. In two-stage pretreatment, the first stage was carried out at low severity tomaximize hemicellulose recovery. Solubilized sugars were recovered from the first-stage prehydrolysate by washing with water. In the second stage, water-insoluble solids from first-stage prehydrolysate were impregnated with dilute sulfuric acid, then steam treated at more severe conditions to hydrolyze a portion of the remaining cellulose to glucose and to improve the enzyme digestibility. The total sugar yields obtained after enzymatic hydrolysis of two-stage dilute acid-pretreated samples were compared with sugar yields from single-stage pretreatment. The overall sugar yield from two-stage dilute-acid pretreatment was approx 10% higher, and the net enzyme requirement was reduced by about 50%. Simultaneous saccharification and fermentation using an adapted Saccharomyces cerevisiae yeast strain further improved cellulose conversion yield and lowered the enzyme requirement.     

Application of a depolymerization model for predicting thermochemical hydrolysis of hemicellulose

Literature data were collected and analyzed to guide selection of conditions for pretreatment by dilute acid and water-only hemicellulose hydrolysis, and the severity parameter was used to relate performance of different studies on a consistent basis and define attractive operating conditions. Experiments were then run to confirm performance with corn stover. Although substantially better hemicellulose sugar yields are observed when acid is added, costs would be reduced and processing operations simplified if less acid could be used while maintaining good yields, and understanding the relationship between operating conditions and yields would be invaluable to realizing this goal. However, existing models seldom include the oligomeric intermediates prevalent at lower acid levels, and the few studies that include such species do not account for the distribution of chain lengths during reaction. Therefore, the polymeric nature of hemicellulose was integrated into a kinetic model often used to describe the decomposition of synthetic polymers with the assumption that hemicellulose linkages are randomly broken during hydrolysis. Predictions of monomer yields were generally consistent with our pretreatment data, data reported in the literature, and predictions of other models, but the model tended to overpredict oligomer yields. These differences need to be resolved by gathering additional data and improving the model.     

Enzymatic kinetic of cellulose hydrolysis

The ethanol effect on the Trichoderma reesei cellulases was studied to quantify and clarify this inhibition type. To determine inhibition parameters of crude cellulase and purified exoglucanase Cel7A, integrated Michaelis-Menten equations were used assuming the presence of two inhibitors: cellobiose as the reaction product and ethanol as a possible bioproduct of cellulose fermentation. It was found that hydrolysis of cellulose by crude enzyme follows a model that considers noncompetitive inhibition by ethanol, whereas Cel7A is very slightly competitively inhibited. Crude cellulase is much more inhibited (K _iul=K _icl=151.9 mM) than exoglucanase Cel7A (K _icl=1.6 × 10¹⁵ mM). Also, calculated inhibition constants showed that cellobiose inhibition is more potent than ethanol inhibition both for the crude enzyme as well as exoglucanase Cel7A.     

Optimization of Brewery's spent grain dilute-acid hydrolysis for the production of pentose-rich culture media

Dilute-acid hydrolysis of brewery's spent grain to obtain a pentose-rich fermentable hydrolysate was investigated. The influence of operational conditions on polysaccharide hydrolysis was assessed by the combined severity parameter (CS) in the range of 1.39–3.06. When the CS increased, the pentose sugars concentration increased to a maximum at a CS of 1.94, whereas the maximum glucose concentration was obtained for a CS of 2.65. The concentrations of furfural, hydroxymethylfurfural (HMF), as well as formic and levulinic acids and total phenolic compounds increased with severity. Optimum hydrolysis conditions were found at a CS of 1.94 with >95% of feedstock pentose sugars recovered in the monomeric form, together with a low content of furfural, HMF, acetic and formic acids, and total phenolic compounds. This hydrolysate containing glucose, xylose, and arabinose (ratio 10∶67∶32) was further supplemented with inorganic salts and vitamins and readily fermented by the yeast Debaryomyces hansenii CCMI 941 without any previous detoxification stage. The yeast was able to consume all sugars furfural, HMF, and acetic acid with high biomass yield, 0.68C-mol/C-mol, and productivity, 0.92 g/(L·h). Detoxification with activated charcoal resulted in a similar biomass yield and a slight increase in the volumetric productivity (11%).     

Enzymatic hydrolysis of protein: Mechanism and kinetic model

The bioreaction mechanism and kinetic behavior of protein enzymatic hydrolysis for preparing active peptides were investigated to model and characterize the enzymatic hydrolysis curves. Taking into account single-substrate hydrolysis, enzyme inactivation and substrate or product inhibition, the reaction mechanism could be deduced from a series of experimental results carried out in a stirred tank reactor at different substrate concentrations, enzyme concentrations and temperatures based on M-M equation. An exponential equation dh/dt = aexp(-bh) was also established, where parameters a and b have different expressions according to different reaction mechanisms, and different values for different reaction systems. For BSA-trypsin model system, the regressive results agree with the experimental data, i.e. the average relative error was only 4.73%, and the reaction constants were determined as K _m = 0.0748 g/L, K _s = 7.961 g/L, k _d = 9.358/min, k ₂ = 38.439/min, E _a = 64.826 kJ/mol, E _d = 80.031 kJ/mol in accordance with the proposed kinetic mode. The whole set of exponential kinetic equations can be used to model the bioreaction process of protein enzymatic hydrolysis, to calculate the thermodynamic and kinetic constants, and to optimize the operating parameters for bioreactor design. __________ Translated from Journal of Tianjin University, 2005, 38(9) (in Chinese)     

煤化程度对煤基固体酸结构及其水解纤维素性能的影响

利用煤具有缩合芳环、脂肪侧链及含氧官能团的结构特点,采用不同煤化程度的煤为碳源,在不同炭化温度下制备了煤基固体酸催化剂（CSA）。通过XRD、FT-IR、¹³C NMR对催化剂结构进行了表征。以还原糖和葡萄糖的产率为考察指标,探讨了煤化程度和炭化温度对煤基固体酸非均相催化水解纤维素的影响。结果表明,煤作为碳源具有传统碳源所不具备的结构优势,煤基固体酸碳层片上除含有磺酸基、酚羟基和羧基外,还含有传统碳基固体酸不具备的桥键（-O-、-CH₂-）和侧链（-CH₃、-OCH₃、-CH₂CH₃）。除磺酸基外,其余均由煤的结构演化而来。随着炭化温度的升高,催化剂的芳香度增大、活性基团的种类和数量减少、磺酸基密度逐渐下降,且随着煤化程度增加,煤基固体酸结构可调性降低,所需要的最佳炭化温度也逐渐降低。不同种类的煤基固体酸在水解纤维素过程中表现出了较高的活性,其中,霍林河煤基固体酸的活性最高。水解活性受催化剂芳香片层大小及堆叠高度、片层之间桥键和磺酸基密度等因素的影响,是众多活性基团协同作用的结果。     

反相悬浮与非均相水解法合成阴离子聚丙烯酰胺的研究

采用反相悬浮与非均相水解相结合的方法合成了分子量大于 １０７ 的阴离子聚丙烯酰胺（ＡＰＡＡ）。研究了水解度（ＨＤ）与水解时间及体系 叫的关系、不同水醇比（Ｖ＿水／Ｗ＿醇）条件下 ＨＤ与时间的关系，ＨＤ与温 度的关系，同时研究了ＡＰＡＡ在溶液中的粘性行为，讨论了分子量的 测定方法。     

Enzymatic hydrolysis of cellulose I is greatly accelerated via its conversion to the cellulose II hydrate form

Cellulose II hydrate was prepared from microcrystalline cellulose (cellulose I) via its mercerization with 5 N NaOH solution over 1 h at room temperature followed by washing with water. The structure of cellulose II hydrate changed to that of cellulose II after drying. Compared with cellulose II, cellulose II hydrate exhibited a slightly (8.5%) expanded structure only along the direction. The hydrophobic stacking sheets of the cellulose II were conserved in the cellulose II hydrate, and water molecules could be incorporated in the inflated two-chain unit cell of cellulose II hydrate. Enzymatic hydrolysis of cellulose I, cellulose II hydrate, and cellulose II was carried out at 37 °C using solutions comprising a mixture of cellulase and β-glucosidase. The hydrolysis of cellulose II hydrate proceeded much faster than the hydrolysis of the other two substrates, while the saccharification ratio of cellulose II was only slightly higher than that of cellulose I. The alkaline mercerization treatment was also applied to sugarcane bagasse. After its direct mercerization, the cellulose in bagasse was converted from cellulose I to cellulose II hydrate, and then to cellulose II after drying. Similar to in the case of microcrystalline cellulose, the rate of the enzymatic hydrolysis of the mercerized bagasse without drying (cellulose II hydrate) was much faster than the enzymatic hydrolysis of the other two substrates. Thus, the wet forms of cellulose and cellulosic biomass after mercerization, and after hydrolysis with cellulolytic enzymes, afforded superior products with extremely high degradability.     

Optimization of steam explosion to enhance hemicellulose recovery and enzymatic hydrolysis of cellulose in softwoods

A combination of Douglas fir heartwood and sapwood chips were steam pretreated under three conditions as measured by the Severity Factor (log R_o), which incorporated the time, temperature/pressure of pretreatment. By adjusting the steam pretreatment conditions, it was hoped to recover the majority of the hemicellulose component as monomers in the water-soluble stream, while providing a cellulosic-rich, water-insoluble fraction that could be readily hydrolyzed by cellulases. These three conditions were chosen to represent either high hemicellulose sugar recovery (low severity [L], log R_o=3.08), high-enzyme hydrolyzability of the cellulosic component (high severity [H], log R_o=4.21), and a compromise between the two conditions (medium severity [M], log R_o=3.45). The medium-severity pretreatment conditions (195°C, 4.5 min, 4.5% SO₂ logR_o=3.45) gave the best compromise in terms of relatively high hemicellulose recovery after stream pretreatment and the subsequent efficiency of enzymatic hydrolysis of the water-insoluble cellulosic fraction. The percent recovery of the original hemicellulose in the water-soluble fraction dropped significantly when the severity was increased (L-76.8%, M-64.7%, and H-37.5%). However, the ease of enzymatic hydrolysis of the cellulose-rich, water-insoluble fraction increased with increasing severity (L-24%, M-86.6%, and H-97.9%). Although more severe pretreatment conditions provided optimum hydrolysis of the cellulosic component, less severe conditions resulted in better recovery of the combined hemicellulose and cellulosic components.     

复合分子筛的合成及其在纤维素水解反应中的应用

采用水热晶化法制备了HUSY@MFI核壳结构复合分子筛。通过XRD、SEM、N_2吸附-脱附、NH_3-TPD及吡啶吸附红外等手段表征催化剂的结构和性质。结果表明,HUSY@MFI晶粒在形貌上呈椭球状,表面是鳞片状结构的MFI型分子筛,里面是光滑的HUSY型分子筛,焙烧模板剂前几乎没有或只有很少量的N_2能进入其孔道结构,致密的壳层MFI覆盖在HUSY型分子筛表面,形成了新的弱酸位,而中强酸强度和酸类型并没有受到影响,复合分子筛的表面酸量及总酸量减少。将所制备的HUSY@MFI复合分子筛催化剂应用于以离子液体1-乙基-3-甲基咪唑氯盐([Emim]Cl)为溶剂的纤维素水解反应中,与HUSY催化的纤维素水解相比,HUSY@M FI复合分子筛催化纤维素水解反应的速率较慢,葡萄糖收率由30.9%提高到41.3%。     

Heterogeneous soft acid catalysis of the sucrose hydrolysis

Using a micro-calorimetrical DSC we have compared the acid-catalyzed inversion of sucrose in homogeneous and heterogeneous systems. Acetic acid was chosen as catalyst for homogeneous system, and several carboxylic cationites were used as heterogeneous catalysts. The kinetic apparent parameters (A, E, k _ap) for all the systems were calculated from DSC data with Friedmann’s method and catalytic constant, k₃₂₃^cat, was further inferred. We found that the specific catalyst efficiency, q _cat, in heterogeneous system is over 5000 times higher than in case of homogeneous ones. The activity of heterogeneous carboxylic systems is still about 30 times larger than those of a strong mineral acid in homogeneous catalysis. The results indicate the high efficiency of heterogeneous systems for soft acid catalysis of the sucrose hydrolysis.     

Kinetic model for micellar catalysed hydrolysis of esters-biomolecular reactions

The cationic micelles of cetyltrimethylammonium bromide, cetyldibenzylammonium chloride and cetylpyridinium chloride stabilize the tetrahedral intermediate formed in the hydrolysis of carboxylic esters (e.g.p-nitrophenylbenzoate) to a greater extent, preferring a mechnism, than the anionic intermediate formed in the hydrolysis ofm-nitrophenyl-N-N-diphenylphosphorodiamidate, which prefers aE _1cB mechanism. The co-operative index,n ₁, calculated for these reactions is greater than 1 indicating that the substrate induced micellation is responsible for the observed catalysis. Based on the present kinetic model for a bimolecular reaction the fraction of substrate and nucleophile bound to the micelle have been calculated. The above results suggest that reaction occurs between the substrate solubilised into the micelle and the nucleophile residing at the Stern layer rather than at the micelle-water interface. The equilibrium constant, and critical micelle concentration evaluated using the present model are in agreement with the values obtained by using earlier models, suggesting a method of evaluating these parameters from kinetic data only.