Jonathan J. Stickel

Particle concentration and yield stress of biomass slurries during enzymatic hydrolysis at high-solids loadings.

Effective and efficient breakdown of lignocellulosic biomass remains a primary barrier for its use as a feedstock for renewable transportation fuels. A more detailed understanding of the material properties of biomass slurries during conversion is needed to design cost‐effective conversion processes. A series of enzymatic saccharification experiments were performed with dilute acid pretreated corn stover at initial insoluble solids loadings of 20% by mass, during which the concentration of particulate solids and the rheological property yield stress (τy) of the slurries were measured. The saccharified stover liquefies to the point of being pourable (τy ≤ 10 Pa) at a total biomass conversion of about 40%, after roughly 2 days of saccharification for a moderate loading of enzyme. Mass balance and semi‐empirical relationships are developed to connect the progress of enzymatic hydrolysis with particle concentration and yield stress. The experimental data show good agreement with the proposed relationships. The predictive models developed here are based on established physical principles and should be applicable to the saccharification of other biomass systems. The concepts presented, especially the ability to predict yield stress from extent of conversion, will be helpful in the design and optimization of enzymatic hydrolysis processes that operate at high‐solids loadings. Biotechnol. Bioeng. 2009; 104: 290–300

Laboratory-scale method for enzymatic saccharification of lignocellulosic biomass at high-solids loadings.

BackgroundScreening new lignocellulosic biomass pretreatments and advanced enzyme systems at process relevant conditions is a key factor in the development of economically viable lignocellulosic ethanol. Shake flasks, the reaction vessel commonly used for screening enzymatic saccharifications of cellulosic biomass, do not provide adequate mixing at high-solids concentrations when shaking is not supplemented with hand mixing.ResultsWe identified roller bottle reactors (RBRs) as laboratory-scale reaction vessels that can provide adequate mixing for enzymatic saccharifications at high-solids biomass loadings without any additional hand mixing. Using the RBRs, we developed a method for screening both pretreated biomass and enzyme systems at process-relevant conditions. RBRs were shown to be scalable between 125 mL and 2 L. Results from enzymatic saccharifications of five biomass pretreatments of different severities and two enzyme preparations suggest that this system will work well for a variety of biomass substrates and enzyme systems. A study of intermittent mixing regimes suggests that mass transfer limitations of enzymatic saccharifications at high-solids loadings are significant but can be mitigated with a relatively low amount of mixing input.ConclusionEffective initial mixing to promote good enzyme distribution and continued, but not necessarily continuous, mixing is necessary in order to facilitate high biomass conversion rates. The simplicity and robustness of the bench-scale RBR system, combined with its ability to accommodate numerous reaction vessels, will be useful in screening new biomass pretreatments and advanced enzyme systems at high-solids loadings.

Calculating sugar yields in high solids hydrolysis of biomass.

Calculation of true sugar yields in high solids enzymatic hydrolysis of biomass is challenging due to the varying liquid density and liquid volume resulting from solid solubilization. Ignoring these changes in yield calculations can lead to significant errors. In this paper, a mathematical method was developed for the estimation of liquid volume change and thereafter the sugar yield. The information needed in the calculations include the compositions of the substrate, initial solids loading, initial liquid density, and sugar concentrations before and after hydrolysis. All of these variables are measurable with conventional laboratory procedures. This method was validated experimentally for enzymatic hydrolysis of dilute sulfuric acid pretreated corn stover at solid loadings up to 23% (w/w). The maximum relative error of predicted glucose yield from the true value was less than 4%. Compared to other methods reported in the literature, this method is relatively easy to use and provides good accuracy.

Data smoothing and numerical differentiation by a regularization method

While data smoothing by regularization is not new, the method has been little used by scientists and engineers to analyze noisy data. In this tutorial survey, the general concepts of the method and mathematical development necessary for implementation for a variety of data types are presented. The method can easily accommodate unequally spaced and even non-monotonic scattered data. Methods for scaling the regularization parameter and determining its optimal value are also presented. The method is shown to be especially useful for determining numerical derivatives of the data trend, where the usual finite-difference approach amplifies the noise. Additionally, the method is shown to be helpful for interpolation and extrapolation. Two examples data sets were used to demonstrate the use of smoothing by regularization: a model data set constructed by adding random errors to a sine curve and global mean temperature data from the NASA Goddard Institute for Space Studies.

A constitutive model for microstructure and total stress in particulate suspensions

Constitutive equations for concentrated suspensions that explicitly account for the development of anisotropy in the microstructure are not generally available, even for relatively simple systems of hard spheres suspended in a Newtonian medium. Here, we use a directionally dependent mean-free path length and a truncated Cartesian tensor expansion to define a second-order structure tensor for systems of suspended particles. This tensor captures the principal nature of the microstructure. A semiempirical differential equation is developed for the structure tensor, with representation theorems being used to insure frame indifference. A separate equation is proposed to relate the stress tensor to the structure and rate of strain tensors. These coupled equations model structure and stress in both steady and time-dependent viscometric flows. Results from Stokesian dynamics simulations are used to demonstrate the utility of this modeling approach. The simulations were for monodisperse suspensions in an infinite ...

A mechanistic model for enzymatic saccharification of cellulose using continuous distribution kinetics. II: Cooperative enzyme action, solution kinetics, and product inhibition.

The projected cost for the enzymatic hydrolysis of cellulosic biomass continues to be a barrier for the commercial production of liquid transportation fuels from renewable feedstocks. Predictive models for the kinetics of the enzymatic reactions will enable an improved understanding of current limitations, such as the slow‐down of the overall conversion rate, and may point the way for more efficient utilization of the enzymes in order to achieve higher conversion yields. A mechanistically based kinetic model for the enzymatic hydrolysis of cellulose was recently reported in Griggs et al. ( 2011 ) (Part I). In this article (Part II), the enzyme system is expanded to include solution‐phase kinetics, particularly cellobiose‐to‐glucose conversion by β‐glucosidase (βG), and novel adsorption and product inhibition schemes have been incorporated, based on current structural knowledge of the component enzymes. Model results show cases of cooperative and non‐cooperative hydrolysis for an enzyme system consisting of EGI and CBHI. The model is used to explore various potential rate‐limiting phenomena, such as substrate accessibility, product inhibition, sterically hindered enzyme adsorption, and the molecular weight of the cellulose substrate. Biotechnol. Bioeng. 2012; 109:676–685.

A mechanistic model for enzymatic saccharification of cellulose using continuous distribution kinetics I: Depolymerization by EGI and CBHI

A mechanistically based kinetic model for the enzymatic hydrolysis of cellulosic biomass has been developed that incorporates the distinct modes of action of cellulases on insoluble cellulose polymer chains. Cellulose depolymerization by an endoglucanase (endoglucanase I, EGI) and an exoglucanase (cellobiohydrolase I, CBHI) is modeled using population‐balance equations, which provide a kinetic description of the evolution of a polydisperse distribution of chain lengths. The cellulose substrate is assumed to have enzyme‐accessible chains and inaccessible interior chains. EGI is assumed to randomly cleave insoluble cellulose chains. For CBHI, distinct steps for adsorption, complexation, processive hydrolysis, and desorption are included in the mechanistic description. Population‐balance models that employ continuous distributions track the evolution of the spectrum of chain lengths, and do not require solving equations for all chemical species present in the reacting mixture, resulting in computationally efficient simulations. The theoretical and mathematical development needed to describe the hydrolysis of insoluble cellulose chains embedded in a solid particle by EGI and CBHI is given in this article (Part I). Results for the time evolution of the distribution of chain sizes are provided for independent and combined enzyme hydrolysis. A companion article (Part II) incorporates this modeling framework to study cellulose conversion processes, specifically, solution kinetics, enzyme inhibition, and cooperative enzymatic action. Biotechnol. Bioeng. 2012; 109:665–675.

A Simplified Method for the Measurement of Insoluble Solids in Pretreated Biomass Slurries

The biochemical conversion of cellulosic biomass to liquid transportation fuels includes the breakdown of biomass into its soluble, fermentable components. Pretreatment, the initial step in the conversion process, results in heterogeneous slurry comprised of both soluble and insoluble biomass components. For the purpose of tracking the progress of the conversion process, it is important to be able to accurately measure the fraction of insoluble biomass solids in the slurry. The current standard method involves separating the solids from the free liquor and then repeatedly washing the solids to remove the soluble fraction, a laborious and tedious process susceptible to operator variations. In this paper, we propose an alternative method for calculating the fraction of insoluble solids which does not require a washing step. The proposed method involves measuring the dry matter content of the whole slurry as well as the dry matter content in the isolated liquor fraction. We compared the two methods using three different pretreated biomass slurry samples and two oven-drying techniques for determining dry matter content, an important measurement for both methods. We also evaluated a large set of fraction insoluble solids data collected from previously analyzed pretreated samples. The proposed new method provided statistically equivalent results to the standard washing method when an infrared balance was used for determining dry matter content in the controlled measurement experiment. Similarly, in the large historical data set, there was no statistical difference shown between the wash and no-wash methods. The new method is offered as an alternative method for determining the fraction of insoluble solids.

Particle morphology characterization and manipulation in biomass slurries and the effect on rheological properties and enzymatic conversion.

An improved understanding of how particle size distribution relates to enzymatic hydrolysis performance and rheological properties could enable enhanced biochemical conversion of lignocellulosic feedstocks. Particle size distribution can change as a result of either physical or chemical manipulation of a biomass sample. In this study, we employed image processing techniques to measure slurry particle size distribution and validated the results by showing that they are comparable to those from laser diffraction and sieving. Particle size and chemical changes of biomass slurries were manipulated independently and the resulting yield stress and enzymatic digestibility of slurries with different size distributions were measured. Interestingly, reducing particle size by mechanical means from about 1 mm to 100 μm did not reduce the yield stress of the slurries over a broad range of concentrations or increase the digestibility of the biomass over the range of size reduction studied here. This is in stark contrast to the increase in digestibility and decrease in yield stress when particle size is reduced by dilute‐acid pretreatment over similar size ranges.

Response of elastoviscoplastic materials to large amplitude oscillatory shear flow in the parallel-plate and cylindrical-Couette geometries

Most investigations of large amplitude oscillatory shear (LAOS) rheometry to date have presumed uniform shear. The study of structured materials would especially benefit from LAOS rheometry but require the use of the larger gaps and roughened surfaces in parallel-plate and cylindrical-Couette geometries. However, in these geometries, the shear profiles are not homogeneous throughout the deformation field. For elastoviscoplastic materials undergoing LAOS in these geometries, both elastic and viscoplastic deformations may occur simultaneously, complicating the data analysis. By means of model simulations, we provide a comprehensive picture of a model elastoviscoplastic material undergoing oscillatory shear deformation in the parallel-plate and cylindrical-Couette geometries, and we compare the oscillatory signals to those obtained from a uniform-shear field. Both displacement-controlled and torque-controlled oscillatory flows were simulated. We show that using popular linear formulas for mapping displacemen...