Hemicellulose hydrolysis model

This model was applied to the same data for batch and flowthrough systems with and without acid addition as for the previous two models, and some of the xylan conversion predictions calculated from the data and concentration predictions via Eq. 8 are summarized in Figs. 5 and 6 for batch and flowthrough systems, respectively. Tables 4 and 2 present the parameters and the SSE values for the branched pore model, respectively. Overall, although some data are better matched than others, hemicellulose hydrolysis models based on mass transfer alone can predict performance in batch and flow systems as well as, if not better than, reaction-only models. In addition, the changes in mass transfer coefficient with flow are consistent with expectations for a mass transfer model but not for strictly a chemical reaction. 

Data and kinetic models of dilute acid pretreatment ate vital to provide a foundation for undostanding hemicellulose hydrolysis and the cause of enhanced performance by flow system systems. Initial hemicellulose hydrolysis models were adapted from Saeman s first-order homogeneous kinetic model of cellulose hydrolysis in a dilute acid batch system (30) and later modified to include two different fractions of hemicellulose, one of which is more easily... 

Initial Evaluation of Simple Mass Transfer Models to Describe Hemicellulose Hydrolysis in Corn Stover... 

In the present study, two mass transfer models were adapted from other applications, and preliminary comparisons were made to conventional reaction-only models to assess their abilities to describe hemicellulose hydrolysis inbatch and flowthrough reactors. Particular attention was paid to including production and diffusion of oligomers in these models with the intent of exploring whether this approach holds promise for explaining the performance of batch and flowthrough systems in a more consistent manner. 

A branched pore leaching model as applied to release of water-soluble carbon from soil incorporates reaction to soluble compounds coupled with pore diffusion within the solids and leaching into the bulk solution. Application of such a model appears to describe hemicellulose hydrolysis reasonably well but not significantly better than chemical reaction only or simple leaching models. 

These results could suggest that what has been traditionally been described as "biphasic" behavior may reflect a combination of chemical reaction and mass transfer effects, with each limiting xylan reaction and removal at different stages or modes of operation. This effect might be better described by a model that incorporates reaction of solids to form soluble species as a function of temperature and acid concentration coupled with a second mass transfer step that is affected by flow. On this basis, we plan to investigate whether the pore leaching model could be simplified and adapted in this way to better describe hemicellulose hydrolysis. 

Both Eqs. 3 and 4 show that hydrogen ion concentration should affect the rate of hemicellulose hydrolysis, but the neutralization capacity of biomass is not always taken into consideration in models reported in the literature. Neutralization is caused by basic minerals containing potassium, sodium, calcium, iron, and other cations present in biomass reacting with sulfuric acid and reducing available hydrogen ions stoichiometrically (17) ... 

Traditional hemicellulose hydrolysis kinetic models cannot account for a change in hemicellulose sugar yields with solids concentration and suffer from inconsistencies that bring into question their mechanistic accuracy. Thus, although current models can be usefiil for a given flow regime, their ability to describe different systems such as flowthrough reactors on a consistent basis is unproven (29). 

A model which includes the effects of mass transfer on hemicellulose hydrolysis has been developed by Brennan and Wyman with the following equations applied to describe a batch system (62). 

Jacobsen, S Ji., Wyman, C.E., 1999. Hemicellulose and cellulose hydrolysis models for application to current and novel pretreatinent processes. Applied Biochemistry and Biotechnology 84—86, 81—96. 

Kim and Lee [14] have reviewed the kinetics of acid-catalyzed hydrolysis of hemicellulose and provide a useful tabulation of previously published kinetic rate constants. There is wide variation in reported rate constant values, reflecting again the differences in the substrate and pretreatment conditions, the model employed and the method by which the effective acid concentration (or weight percent) was determined [16]. Unfortunately, only a few researchers have performed detailed kinetic studies in which both hydrolysis and decomposition rates are determined. Kim and Lee [14] have carried out the only study including both the fast and slow xylan fractions and xylose decomposition. 

Although percolation reactors have been in use extensively over several decades, it was not until 1983 that the first theoretical model of this type of reactor was introduced [5]. The model was developed for sequential first-order reactions in order to assess the performance in hydrolysis of hemicellulose. As an unsteady reactor, the model involves a partial differential equation with the following parameters kinetic parameter a = k2/kj operational parameter (3 = kiL/u, T = ut/L, where L is the bed length and u is the liquid flow velocity. 

Figure 5a reveals that this depolymerization model shows trends consistent with data but is not particularly accurate. However, a modified model was developed in which die right-hand side of Equation [5] was multiplied by a first-order reactivity term, a = with a being the reactivity, ka a proportionality constant, and t the time, and application of this model did a much better job of describing the time course of xylan hydrolysis, as shown in Figure 5b. The fact that declining bond reactivity describes the data well suggests that the original assumption that all xylan bonds are broken equally at random is not valid. An obvious next step is the determination of individual bond energies within the hemicellulose molecule and incorporation of this information into a modified model.

We will discuss here an example which concerns kinetic modeling of hemicellulose O-acetyl-galactoglucomannan (Fig. 11.25) hydrolysis [4]. 

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