Effect of mixing on enzymatic hydrolysis of cardboard waste: saccharification yield and subsequent separation of the solid residue using a pressure filter

Development of a simple and versatile process for commercial and municipal lignocellulosic waste conversion into fermentable sugars

Lignocellulosic wastes are valuable feedstock in biorefinery thanks to their high sugars content and low level of fibers intricacy. However, their massification is often a limiting parameter in the development of industrial projects. Hence, this study aims to develop an efficient process enabling the conversion of several waste streams within the same process line. Several pretreatment and enzymatic hydrolysis parameters were firstly evaluated with Old Corrugated Cardboards (OCC) as a model substrate. A chemical free pretreatment followed by an enzymatic hydrolysis (Cellic Ctec 3 enzymatic cocktail at 0.06 g of cocktail per g of Total Sugars (TS)) efficiently depolymerized OCC into monomeric sugars (0.50 g/gTS) consequently fermented into ethanol (0.24 g/gTS). Then, the suitability of this process was validated for sugars production from Pulp and Paper (P&P) sludge (0.48 g/gTS), sieved toilets papers (0.40 g/gTS), the Organic fraction of municipal solid waste (0.37 g/gTS) and Waste Wood B (0.08 g/gTS).

Hydrolysis and fermentation steps of a pretreated sawmill mixed feedstock for bioethanol production in a wood biorefinery

The aim of this work was to demonstrate the feasibility of second-generation bioethanol production using for the first time a sawmill mixed feedstock comprising four softwood species, representative of biomass resource in Auvergne-Rhône-Alpes region (France). The feedstock was subjected to a microwave-assisted water/ethanol Organosolv pretreatment. The investigation focused on enzymatic hydrolysis of this pretreated sawmill feedstock (PSF) using Cellic® Ctec2 as the enzyme, followed by fermentation of the resulting sugar solution using Saccharomyces cerevisiae strain. The cellulose-rich PSF with 71% w/w cellulose content presented high saccharification yields (up to 80%), which made it perfect for subsequent fermentation; this yield was predicted vs. time up to 5.2% w/v PSF loading using a mathematical model fitted only on data at 1.5%. Finally, high PSF loading (7.5%) and scaleup were shown to impair the saccharification yield, but alcoholic fermentation could still be carried out up to 80% of the theoretical glucose-to-ethanol conversion yield.

Mixing effects on the kinetics and the dynamics of two-phase enzymatic hydrolysis of hemicellulose for biofuel production

This work uses a coupled experimental and modeling approach to explore the effects of macro- and micro-mixing on the kinetics and the dynamics of two-phase enzymatic hydrolysis of hemicellulose. Reactor mixing does not alter the non-competitive nature of product inhibition in hemicellulose hydrolysis by endoxylanase, but produces stronger inhibition that reduces the soluble sugar yield by 8-14.5%, as the mixing speed increases from 0 to 200 rpm. The kinetic constants (K_m, V_max, K_x) assume mass-transfer disguised values at 0-200 rpm. An optimal mixing strategy, comprising of 55-70 min of initial rapid convective macromixing followed by diffusive micromixing (without any macromixing) for the rest of the hydrolysis, increases xylose and reducing sugar yields by 6.3-8% and 13-20%, respectively, over continuous mixing at 200 rpm, for 1-5 mg/ml substrate loading at an optimum enzyme to substrate ratio of 1:20, with an energy saving of 94-96% over 24 h.

Effects of dilute-acid pretreatment conditions on filtration performance of corn stover hydrolyzate

The reaction conditions used during dilute-acid pretreatment of lignocellulosic biomass control the carbohydrate digestion yield and also hydrolyzate properties. Depending on the conversion route of interest, solid-liquid separation (SLS) may be required to split the hemicellulose-rich liquor from the cellulose-rich insoluble solids, and slurry properties are important for SLS. Corn stover was pretreated at different reaction conditions and the slurries were assessed for conversion yield and filtration performance. Increasing pretreatment temperature reduced the solids mean particle size and resulted in slower slurry filtration rates when vacuum filtered or pressure filtered. Corn stover pretreated at 165°C for 10min and with 1% H₂SO₄ exhibited the highest xylose yield and best filtration performance with a no-wash filtration rate of 80kg/hm² and cake permeability of 15x10^-15.

Microreactor-based mixing strategy suppresses product inhibition to enhance sugar yields in enzymatic hydrolysis for cellulosic biofuel production

A novel microreactor-based energy-efficient process of using complete convective mixing in a macroreactor till an optimal mixing time followed by no mixing in 200-400μl microreactors enhances glucose and reducing sugar yields by upto 35% and 29%, respectively, while saving 72-90% of the energy incurred on reactor mixing in the enzymatic hydrolysis of cellulose. Empirical exponential relations are provided for determining the optimal mixing time, during which convective mixing in the macroreactor promotes mass transport of the cellulase enzyme to the solid Avicel substrate, while the latter phase of no mixing in the microreactor suppresses product inhibition by preventing the inhibitors (glucose and cellobiose) from homogenizing across the reactor. Sugar yield increases linearly with liquid to solid height ratio (r_h), irrespective of substrate loading and microreactor size, since large r_h allows the inhibitors to diffuse in the liquid away from the solids, thus reducing product inhibition.

A discretized model for enzymatic hydrolysis of cellulose in a fed-batch process

In the enzymatic hydrolysis of cellulose, several phenomena have been proposed to cause a decrease in the reaction rate with increasing conversion. The importance of each phenomenon is difficult to distinguish from batch hydrolysis data. Thus, kinetic models for the enzymatic hydrolysis of cellulose often suffer from poor parameter identifiability. This work presents a model that is applicable to fed-batch hydrolysis by discretizing the substrate based on the feeding time. Different scenarios are tested to explain the observed decrease in reaction rate with increasing conversion, and comprehensive assessment of the parameter sensitivities is carried out. The proposed model performed well in the broad range of experimental conditions used in this study and when compared to literature data. Furthermore, the use of data from fed-batch experiments and discretization of the model substrate to populations was found to be very informative when assessing the importance of the rate-decreasing phenomena in the model.

Rate-constraining changes in surface properties, porosity and hydrolysis kinetics of lignocellulose in the course of enzymatic saccharification

BACKGROUND

Explaining the reduction of hydrolysis rate during lignocellulose hydrolysis is a challenge for the understanding and modelling of the process. This article reports the changes of cellulose and lignin surface areas, porosity and the residual cellulase activity during the hydrolysis of autohydrolysed wheat straw and delignified wheat straw. The potential rate-constraining mechanisms are assessed with a simplified kinetic model and compared to the observed effects, residual cellulase activity and product inhibition.

RESULTS

The reaction rate depended exclusively on the degree of hydrolysis, while enzyme denaturation or time-dependent changes in substrate hydrolysability were absent. Cellulose surface area decreased linearly with hydrolysis, in correlation with total cellulose content. Lignin surface area was initially decreased by the dissolution of phenolics and then remained unchanged. The dissolved phenolics did not contribute to product inhibition. The porosity of delignified straw was decreased during hydrolysis, but no difference in porosity was detected during the hydrolysis of autohydrolysed straw.

CONCLUSIONS

Although a hydrolysis-dependent increase of non-productive binding capacity of lignin was not apparent, the dependence of hydrolysis maxima on the enzyme dosage was best explained by partial irreversible product inhibition. Cellulose surface area correlated with the total cellulose content, which is thus an appropriate approximation of the substrate concentration for kinetic modelling. Kinetic models of cellulose hydrolysis should be simplified enough to include reversible and irreversible product inhibition and reduction of hydrolysability, as well as their possible non-linear relations to hydrolysis degree, without overparameterization of particular factors.

A multiscale three-zone reactive mixing model for engineering a scale separation in enzymatic hydrolysis of cellulose

This multiscale three-zone reactive mixing model provides a theoretical framework for engineering a scale separation in batch enzymatic hydrolysis of cellulose to strategize significant leaps in glucose yields. Formulated using the Liapunov-Schmidt method of the classical bifurcation theory, our model explores the multiscale spatiotemporal dynamics between the fundamental processes of macromixing (convection) and micromixing (diffusion) of the enzymes (Endoglucanase, Exoglucanase, β-glucasidase) and reducing sugars, adsorption and desorption of enzymes on the solid cellulosic substrates, and the product-inhibited liquid and solid phase enzymatic reactions that depolymerize microcrystalline cellulose (Avicel). The model is validated for a range of substrate loadings (2-5%) using our experimental results for the two asymptotic cases of no mixing and continuous mixing, as well as for the macro/micro scale-separated optimal mixing strategy that increases the glucose yield by up to 26% by macromixing completely for an initial period followed by micromixing for the remaining duration of the hydrolysis.

Hydrolysis and composition of recovered fibres fractionated from solid recovered fuel

Fibres fractionated from solid recovered fuel (SRF), a standardised market combustion fuel produced from sorted waste, were considered as a source of lignocellulosic fermentable sugars. The fibre yield from four samples of SRF was 25-45%, and the separated material consisted of 52-54% carbohydrates, mainly glucan, with a high content of ash (12-17%). The enzymatic digestibility of recovered fibres was studied at low and high solids loading and compared with model substrates containing only chemical and mechanical pulps. Above 80% hydrolysis yield was reached at 20% solids loading in 48 h, but variation was observed between different samples of recovered fibres. Surfactants were found to improve the hydrolysis yield of recovered fibres especially in tumbling-type of mixing at low solids loading, where hydrolysis was found to stagnate without surfactants. The results suggest that SRF is a potential source of easily digestible lignocellulosic carbohydrates for use in biorefineries.

Performance and techno-economic assessment of several solid-liquid separation technologies for processing dilute-acid pretreated corn stover

Solid-liquid separation of pretreated lignocellulosic biomass slurries is a critical unit operation employed in several different processes for production of fuels and chemicals. An effective separation process achieves good recovery of solute (sugars) and efficient dewatering of the biomass slurry. Dilute acid pretreated corn stover slurries were subjected to pressure and vacuum filtration and basket centrifugation to evaluate the technical and economic merits of these technologies. Experimental performance results were used to perform detailed process simulations and economic analysis using a 2000 tonne/day biorefinery model to determine differences between the various filtration methods and their process settings. The filtration processes were able to successfully separate pretreated slurries into liquor and solid fractions with estimated sugar recoveries of at least 95% using a cake washing process. A continuous vacuum belt filter produced the most favorable process economics.

Enzymatic saccharification of pretreated wheat straw: comparison of solids-recycling, sequential hydrolysis and batch hydrolysis

In the enzymatic hydrolysis of lignocellulose materials, the recycling of the solid residue has previously been considered within the context of enzyme recycling. In this study, a steady state investigation of a solids-recycling process was made with pretreated wheat straw and compared to sequential and batch hydrolysis at constant reaction times, substrate feed and liquid and enzyme consumption. Compared to batch hydrolysis, the recycling and sequential processes showed roughly equal hydrolysis yields, while the volumetric productivity was significantly increased. In the 72h process the improvement was 90% due to an increased reaction consistency, while the solids feed was 16% of the total process constituents. The improvement resulted primarily from product removal, which was equally efficient in solids-recycling and sequential hydrolysis processes. No evidence of accumulation of enzymes beyond the accumulation of the substrate was found in recycling. A mathematical model of solids-recycling was constructed, based on a geometrical series.

A novel mixing strategy for maximizing yields of glucose and reducing sugar in enzymatic hydrolysis of cellulose

This work explores the effects of mixing on enzymatic hydrolysis of cellulose to innovate a novel mixing strategy that maximizes glucose and reducing sugar yields for production of cellulosic ethanol while reducing the power required for reactor mixing. Batch experiments of cellulose hydrolysis are performed under aseptic conditions for 72 h at various substrate loading (2-6% wt./vol.), where the reactor mixing is terminated after different intervals of time ranging from 0 to 72 h. We find that initial mixing for a certain 'optimal mixing time' followed by no mixing for the rest of the reaction time maximizes glucose and reducing sugar yields. We report a maximum of 26% and 31% increase in glucose and reducing yields, respectively, in case of optimal mixing over continuous mixing for 2% substrate loading. We obtain an algebraic expression that predicts that the optimal mixing time increases exponentially with substrate loading.

Characteristics of enzyme hydrolysis of cellulose under static condition

The effect of enzyme loading under static and agitated conditions was investigated. Enzymatic hydrolysis of 10 w/v% de-lignified cellulose slurry such as filter paper, avicel and pulp was conducted under agitated (120 rpm) and static condition, and the enzyme loading ranging from 1.2 to 120 mg-protein/g-dry substrate. Under the agitated condition, the final sugar concentration decreased with the decreasing enzyme loading. Under the static condition, the final sugar concentration was maintained even if the enzyme loading was decreased. The above phenomenon was caused by a rapid precipitation of cellobiohydrolase 2 (CBH2) under the agitated condition, which was not observed under the static condition. The hydrolysis experiments using enzymes containing different ratios of cellobiohydrolase 1 (CBH1) and CBH2 under the static condition suggested that preservation of CBH2 and its synergism with CBH1 is essential for static condition's characteristics, and for efficient hydrolysis of cellulose.