The effectiveness of the reverse dead-end MBR versus radial flow MBR designs was assessed using real, complex lignocellulosic biomass, namely date seeds (DSs). A detailed kinetic model was developed to predict the dynamic behavior of the radial flow tubular MBR, and the kinetic parameters were estimated from the experimental data.
Introduction
Statement of the Problem
Research Objectives
This is supposed to improve the kinetics of the reaction and lower the cost of the process. Experimental analysis is used to prove this concept and determine the dynamic model of the system in the integrated membrane MBR system.
Hypothesis and Novelty Statement
A model will be developed that combines reaction kinetics with product separation, helping to better understand the system. Using the waste biomass hydrolysis system, which is abundantly available in the UAE, will be another new aspect of this work that will present real results.
Lignocellulosic Bioethanol Production
- Ethanol Feedstock
- Lignocellulose
- Date Seeds
- Lignocellulose to Bioethanol Process
- Pretreatment
- Hydrolysis
- Enzyme Kinetics and Modeling
To account for the processive action of the enzyme, Equation (1) was modified to include multiple cycles of the catalytic action of cellobiohydrolase, as shown in Equation (2). The inhibition of the enzyme during the processive hydrolysis, Equation (3), was proposed in the assessment of the hydrolysis of the microcrystalline cellulose, which is composed of amorphous and totally hydrolyzable cellulose, which reported that the decrease in the kinetics was due to the enzyme-substrate irreversible binding [87].
![Figure 1: Schematic diagram of the lignocellulosic structure composed of cellulose, hemicellulose, and lignin [21]](https://thumb-ap.123doks.com/thumbv2/azpdfco/10576161.0/32.893.150.738.736.941/figure-schematic-diagram-lignocellulosic-structure-composed-cellulose-hemicellulose.webp)
Challenges of lignocellulose Enzymatic Hydrolysis and Potential Solutions
- Heterogeneous Mixture
- Enzyme Inhibition
- Immobilization: Solution for Heterogeneous Mixture Challenge
- Membrane Technology: Solution for Product Inhibition Challenge
- MBRs Configurations
- Membrane Selection
- Key Factors Affecting the Performance of MBRs
Nevertheless, at a higher solids loading, the increase in the initial phase of the reaction was greater. In addition, because the enzyme must be used in a soluble form, due to the heterogeneous nature of the reactant, it is only used for a single pass in the STRs and is then lost with the wastewater. The shear stress exerted on the enzymes could also be reduced in MBRs, further preserving the activity of the enzymes.
To overcome the caking of the substrate on the ultrafiltration surface, a modified configuration with multiple membranes system was proposed. The success of an MBR in effectively retaining the enzymes while easily permeating the product depends on the type and properties of the membrane used in the reactor [130]. The pumping of the reaction suspension has also been proposed to slow down substrate accumulation, by redistributing the molecules on the membrane surface, and thus controlling the rate of accumulation.
Materials and Methods
Chemical and Enzymes
MBR with Inverted Dead-end Filtration Concept
- The MBR Design
- Standard Cellulose Hydrolysis with Product Separation
- Kinetic Model
To minimize this effect, in this study the membrane was placed on top of the reaction cell, which contrasts with the dead-end filtration concept. The weight of the water in the upper chamber provided an additional support to the membrane against the flow of water entering from the bottom chamber. The reaction temperature was maintained by covering the bottom zone of the reaction cell with insulated heating tape (Thermolyne, Sigma) equipped with a thermocouple connected to a temperature controller (TC4S-14R).
The ranges and levels of the input variables for the enzymatic cellulose hydrolysis used in the Minitab software are presented in Table 4. The sum of the three calculated amounts was then divided by the amount of substrate initially added to the lower chamber. The dynamic model of the system included the following seven steps: (1) enzyme diffusion from the bulk of the lower zone to the substrate surface, (2) enzyme binding to the substrate, both productive and nonproductive, (3) the reaction from productive binding produces the product, (4) desorption of the product from the enzyme surface to the bulk, which represents the product inhibition effect, (5) diffusion of the product from the bulk to the membrane surface, (6) diffusion of the product through the membrane , and (7) diffusion of the product from the other side of the membrane to the bulk in the upper zone.
Radial-flow Tubular MBR
- Radial-flow MBR Design
- Enzymatic Hydrolysis with Product Separation
- Kinetic Modeling
Flow diagrams of the inverted dead end and radial flow MBRs are shown in Figure 9. In the heterogeneous system of lignocellulosic hydrolysis, the reaction proceeds through several steps: (1) enzymes diffuse from the bulk to the substrate surface, (2) enzyme - substrate complexes are formed under which the enzyme molecules can either bind to hydrolyzable or non-hydrolyzable parts of the cellulose, resulting in productive or non-productive bonds, respectively, (3) productive bonds result in product formation, whereas the non-productive bonds result in loss of activity of the attached enzyme molecules until they detaches, representing a substrate-inhibiting effect, (4) products are produced from the productive enzyme-substrate complex, where delay in desorption of the products from the enzyme-active sites represents product-inhibiting effect, (5) diffusion of the desorbed products from the bulk to the membrane surface, (6) diffusion of the products through the membrane and (8) diffusion of the product from the other side of the membrane to the outer cylinder. Because of the mixing in the inner cylinder, convection from the bulk to a surface, being of the substrate or membrane, was assumed to be instantaneous.
As the reaction proceeds, hydrolysis proceeds from the outer layers of the substrate to the inner layers. Assuming that the permeability of the product from the reaction cell (inner tube) is due to pure convection, the total amount of reducing sugars produced can be represented by equation (36), which is the sum of the mass of total reducing sugars accumulated in the reaction cell and diffused from it to the outer cylinder . The amount of total reducing sugars produced was recalculated using Equation (36), and the influence of each parameter was evaluated using Equation (37) [173].
Glucose Permeation Analysis
The significance of the estimated kinetic parameters on the total reducing sugars produced was examined by sensitivity analysis. where P1 represents the total amount of total reducing sugars produced with the originally estimated parameter (change 0%) and P2 is the newly estimated amount with the changed parameter.
Analytical Methods
- Total Reducing Sugars Analysis
- Protein Analysis
- Enzyme Assay
- Biomass Characterization
- Membrane Characterization
Whereas the acid-soluble lignin was determined by measuring the absorbance of the liquid filtrate at 205 nm. Fine Coater (JEOL) to increase the conductivity of the non-conductive catalyst and to prevent the build-up of electrostatic charge on the surface of the sample. To detect internal fouling or changes in the internal structure of the membrane, cross-sectional images of PES-30 were observed using SEM (JCM-5000; NeoScope).
Therefore, internal fouling is expected to be more significant as the MWCO increases and becomes closer to the size of the cellulase. Samples of PES membranes before and after use were coated with gold using a JFC-1600Auto Fine Coater (JEOL) to increase the conductivity of the non-conductive catalyst and to prevent the build-up of electrostatic charge at the sample surface. XRD analysis was also used to measure changes in the crystalline structure of the membranes.
Results and Discussion
Inverted Dead-end MBR
- Selective Glucose Permeation
- Effect of the Process on the Membranes
- Enzymatic Hydrolysis with Product Separation
- Effects of the Substrate Concentration and Water Flowrate on Standard
- Evaluation of the Kinetic Model Parameters
- Statistical Analysis
To eliminate the possibility of cellulase being pushed into the pores of the membrane, resulting in internal fouling and a decrease in the bulk concentration of the enzyme, the cellulase concentration in the reaction cell was measured while using the PES-10 membrane. The absence of internal contamination was further confirmed by measuring the enzyme concentration in the bottom cell at the beginning and at the end of the reaction. The above findings were further confirmed using XRD analysis, which was performed to examine changes in the crystalline structure of the membrane.
The slight decrease in the curve observed towards the end of the observation period for the substrate concentration of 2.7 g/l was due to the dilution effect. The Polymath software was used to determine the numerical values of the parameters in equation (15). These results were consistent with the 3D surface plot (Figure 20), which also showed the positive effect of water flow and the negative effect of substrate concentration on yield.
Radial-flow Tubular MBR Analysis
- Biomass Characterization and the Effect of Pretreatment on Substrate
- Enzymatic Hydrolysis with Product Separation
- Effects of the Substrate Concentration and Water Flowrate on DSs
- Kinetic Model
- Sensitivity Analysis
- Statistical Analysis
However, as the water flow rate increased to 1.2 mL/min (τ = 10.4 h), the effect becomes more obvious and the amount of total reducing sugars produced increased significantly. For example, hydrolysis of NaOH-pretreated DS with product separation in dead-end inverted MBR resulted in 10.8% production yield after 8 h compared to only 3.5% under the same conditions but without product separation, as is shown in this paper. The effects of water flow rate and substrate concentration on the total yield of reducing sugars after 8 hours in the radial flow MBR are shown in Figure 27B.
Substrate inhibition can be seen in the yield at 39.6 g/L, reduced at a flow rate of 1.2 mL/min. This leads to faster unproductive binding and a decrease in the total production of reducing sugars [171]. A more significant effect of substrate concentration on the effect of water flow, as indicated by the p-values, can also be observed in the 3D surface plot.
Glucose Permeation
- Parametric Study
- Statistical Analysis
The effects of an increasing initial glucose concentration at a constant water flow rate of 0.4 mL/min (τ = 31.3 h) and increasing water flow rate at a constant initial concentration of 40 g/L, on glucose permeation rate through PES-30 membrane are shown. The effect of increasing the MWCO of the membrane on the glucose permeation rate at a constant glucose concentration and a water flow rate of 40 g/L and 0.4 mL/min (τ = 31.3 h), respectively, is shown in Figure 32C. It can be seen that increasing the membrane MWCO resulted in a slight increase in the permeation rate.
However, both the quadratic term of water flux (𝑋𝑋22) and the interaction between MWCO and water flux showed a negative contribution to the permeability rate. As shown in Table 14, the glucose permeability rate predicted by the statistical model at 3 hours resulted in an R2 value of 0.99. Under these optimal conditions, a maximum permeation rate of 4.5 g/h could be achieved, which was very close to that determined experimentally under the conditions of glucose concentration of 53.3 g/L and water.
Conclusion and Future Perspectives
Conclusion
Future Perspectives
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Sen: "Enumeration of Monosugar Inhibition Characteristics in the Kinetics of Enzymatic Cellulose Hydrolysis." Biochemistry of the process. Koukios: "Relation of pretreatment effect to enzymatic hydrolysis of straw." Biotechnology and bioengineering. Saddler: "Effect of Shaking Regime on the Rate and Extent of Enzymatic Hydrolysis of Cellulose." Journal of Biotechnology.
![Figure 3: A schematic diagram of the bioconversion of lignocellulosic biomass to fermentable glucose [80]](https://thumb-ap.123doks.com/thumbv2/azpdfco/10576161.0/41.893.233.704.555.1077/figure-schematic-diagram-bioconversion-lignocellulosic-biomass-fermentable-glucose.webp)
