Reactor design for minimizing product inhibition during enzymatic lignocellulose hydrolysis: II. Quantification of inhibition and suitability of membrane reactors

Product inhibition of cellulolytic enzymes affects the efficiency of the biocatalytic conversion of lignocellulosic biomass to ethanol and other valuable products. New strategies that focus on reactor designs encompassing product removal, notably glucose removal, during enzymatic cellulose conversion are required for alleviation of glucose product inhibition. Supported by numerous calculations this review assesses the quantitative aspects of glucose product inhibition on enzyme-catalyzed cellulose degradation rates. The significance of glucose product inhibition on dimensioning of different ideal reactor types, i.e. batch, continuous stirred, and plug-flow, is illustrated quantitatively by modeling different extents of cellulose conversion at different reaction conditions. The main operational challenges of membrane reactors for lignocellulose conversion are highlighted. Key membrane reactor features, including system set-up, dilution rate, glucose output profile, and the problem of cellobiose are examined to illustrate the quantitative significance of the glucose product inhibition and the total glucose concentration on the cellulolytic conversion rate. Comprehensive overviews of the available literature data for glucose removal by membranes and for cellulose enzyme stability in membrane reactors are given. The treatise clearly shows that membrane reactors allowing continuous, complete, glucose removal during enzymatic cellulose hydrolysis, can provide for both higher cellulose hydrolysis rates and higher enzyme usage efficiency (kg_product/kg_enzyme). Current membrane reactor designs are however not feasible for large scale operations. The report emphasizes that the industrial realization of cellulosic ethanol requires more focus on the operational feasibility within the different hydrolysis reactor designs, notably for membrane reactors, to achieve efficient enzyme-catalyzed cellulose degradation.     

Design of a reactor bioleach process for refractory gold treatment

Abstract: CRA has been developing bioleaching for the treatment of low-grade refractory gold resources. In lhe course of dcveloping a biolcach process for a pyrite concentrate at Bougainville Copper Limited (BCL), CRA has confronted the myriad of problems associated with proving a concept at a small scale, to the design of a conceptual flowsheet. A phased programme was initiated to develop the project. Laboratory scale batch studies indicated that the pyrite concentrate was amenable to bacterial leaching and subsequent cyanidatkm gold recovery. Large scale continuous leaching was then performed to delincate the major operating wtriables. In conjunction with this programme, CRA has also been addressing the problem of reactor scale-up. The success of the bioleach process is dependent on the design of large, energy-efficient reactors, with reactor sizes of the order of 1000 m³ contemplated. Results from these scale-up studies are presented in this paper.     

Scale-down studies on the hydrodynamics of two-liquid phase biocatalytic reactors

The maintenance of constant interfacial area per unit volume is a key parameter for the successful scale-up of two-liquid phase bioconversion processes. To date, however, there is little published information on the hydrodynamics of such systems and a suitable basis for scale-up has yet to be defined and verified. Here we report power input and hydrodynamic data for a whole-cell bioconversion process using resting cells of Rhodococcus R312 to catalyse the hydration of a poorly water-soluble substrate 1,3-dicyanobenzene (1,3-DCB). Experiments were performed in geometrically similar 3-L and 75-L reactors, each fitted with a three-stage Rushton turbine impeller system. The two-phase system used comprised of 20% v/v toluene dispersed in 0.1 M aqueous phosphate buffer containing up to 10 g(ww) x L(-1) of resuspended biocatalyst and 20 g x L(-1) 1,3-DCB. The power input to the 3-L reactor was first determined using an air-bearing technique for both single-phase and two-phase mixing. In both cases, the power number attained a constant value of 11 at Re>10,000, while the measured power inputs were in the range 0.15-3.25 kW x m(-3). Drop size distributions and Sauter mean drop diameters (d(32)) were subsequently measured on-line in both reactors, using an in-situ light-backscattering technique, for scale-up on the basis of either constant power input per unit volume or constant tip speed. At both scales d(32) decreased with increasing agitation rate, while the drop size distributions obtained were log-normal. All the measured d(32) values were in the range of 30-50 microm, with the lowest values being obtained in systems with biocatalyst present. In all cases, constant power input per unit volume was found to be the most suitable basis for scale-up. This gave virtually identical d(32) values and drop size distributions at both scales. A number of correlations were also identified that would allow reasonable prediction of d(32) values for various agitation rates at each scale. While the results obtained are for a particular phase system, the scale-down methodology presented here would allow the rapid evaluation of other bioconversion processes in the 3-L reactor with a 25-fold reduction in scale. In this way, potential problems that might be encountered at the larger scale, such as the carryover of antifoam from the fermentation stage, could be quickly and efficiently identified.     

Performance Comparison of Some Classes of Flexible Flow Shops and Job Shops

Flexible manufacturing systems (FMSs) for two-stage production may possess a variety of operating flexibilities in the form of tooling capabilities for the machines and alternative routings for each operation. In this paper, we compare the throughput performance of several flexible flow shop and job shop designs. We consider two-stage assembly flow shops with m parallel machines in stage 1 and a single assembly facility in stage 2. Every upstream operation can be processed by any one of the machines in stage 1 prior to the assembly stage. We also study a similar design where every stage 1 operation is processed by a predetermined machine. For both designs, we present heuristic algorithms with good worst-case error bounds and show that the average performance of these algorithms is near optimal. The algorithms presented are used to compare the performance of the two designs with each other and other related flexible flow shop designs. It is shown, both analytically and experimentally, that the mode of flexibility possessed by a design has implications on the throughput performance of the production system.     

Anaerobic dechlorination of pentachlorophenol in fixed-film and upflow anaerobic sludge blanket reactors using different inocula

Longterm performance and stability of two upflow anaerobic sludge blanket (UASB) reactors inoculated with granular sludge and treating a synthetic waste water containing pentachlorophenol (PCP) and phenol were studied. A similar system consisting of two fixed-film reactors inoculated with anaerobic digested sewage sludge were further studied. One reactor in each series received glucose in addition to the phenols. Dechlorination of PCP proceeded via two different dominating pathways in the respective reactor systems, suggesting that two distinct microbial populations were present, probably originating from the different inocula. Dechlorinating activity was maintained for more than 18 months in the UASB reactors and was generally higher than in the fixed-film reactors. In the fixed-film reactors, dechlorination of PCP suddenly decreased after 15.5 months of operation compared to earlier performance. Since no operational parameters had been changed, this indicated that the enriched culture was unstable on a longterm basis. Addition of yeast extract to the medium restored activity. General process stability in both reactor systems was clearly enhanced by the addition of glucose and was superior in the UASB/granular sludge system. The better performance and the higher stability in the UASB/granular sludge reactor highlights the importance of thorough screening of inocular prior to start-up of processes treating waste waters containing xenobiotic compounds.Abbreviations PCP pentachlorophenol - TeCP tetrachlorophenol - TCP trichlorophenol - DCP dichlorophenol - UASB upflow anaerobic sludge blanket - HRT hydraulic retention time     

Efficacy of a hot washing process for pretreated yellow poplar to enhance bioethanol production

Cost reductions for pretreatment and bioconversion processes are key objectives necessary to the successful deployment of a bioethanol industry. These unit operations have long been recognized for their impact on the production cost of ethanol. One strategy to achieve this objective is to improve the pretreatment process to produce a pretreated substrate resulting in reduced bioconversion time, lower cellulase enzyme usage, and/or higher ethanol yields. Previous research produced a highly digestible pretreated yellow poplar substrate using a multistage, continuously flowing, very dilute sulfuric acid (0.07% (w/v)) pretreatment. This process reduced the time required for the bioconversion of pretreated yellow poplar sawdust to ethanol. This resulted in a substantially improved yield of ethanol from cellulose. However, the liquid volume requirements, steam demand, and complexity of the flow-through reactor configuration were determined to be serious barriers to commercialization of that process. A reconfigured process to achieve similar performance has been developed using a single-stage batch pretreatment followed by a separation of solids and liquids and washing of the solids at a temperatures between 130 and 150 degrees C. Separation and washing at the elevated temperature is believed to prevent a large fraction of the solubilized lignin and xylan from reprecipitating and/or reassociating with the pretreated solids. This washing of the solids at elevated temperature resulted in both higher recovered yields of soluble xylose sugars and a more digestible pretreated substrate for enzymatic hydrolysis. Key operating variables and process performance indicators included acid concentration, temperature, wash volume, wash temperature, soluble xylose recovery, and performance of the washed, pretreated solids in bioconversion via simultaneous saccharification and fermentation (SSF). Initial results indicated over a 50% increase in ethanol yield at 72 h for the hot washed material as compared to the control (no washing, no separation) and a 43% reduction of in the bioconversion time required for a high ethanol yield from cellulose     

Effect of modulation of enzyme inactivation on temperature optimization for reactor operation with chitin-immobilized lactase

Temperature is a critical variable to be optimized in any enzymatic process, producing opposite effects on enzyme activity and inactivation rate. Temperature functions for all kinetic and inactivation parameters were validated for chitin-immobilized yeast lactase (CIL). Enzyme inactivation was described by a two-stage series mechanism. The effect of galactose and lactose on inactivation was determined in terms of modulation factors that were positive for galactose and negative for lactose. Modulation factors were mild functions of temperature in the first stage and strong functions in the second stage of enzyme inactivation, where galactose positive modulation factors increase while lactose negative modulation factors decrease with temperature. Temperature-explicit functions for kinetic and inactivation parameters were incorporated into a scheme to optimize temperature in the simulation of a continuous packed-bed reactor operation with chitin-immobilized lactase, based on an annual cost objective function. Optimum temperature was 20°C at enzyme replacement of 25% residual activity, and increased only slightly at higher replacement frequencies. The effect of modulation factors on reactor design and temperature optimization is presented and discussed. Software for temperature optimization that allows the introduction of variations in all parameters and operational criteria to perform sensitivity analysis was developed.     

The significance of the initiation process parameters and reactor design for maximizing the efficiency of microbial fuel cells

Microbial fuel cells (MFCs) can be used for electricity generation via bioconversion of wastewater and organic waste substrates. MFCs also hold potential for production of certain chemicals, such as H₂ and H₂O₂. The studies of electricity generation in MFCs have mainly focused on the microbial community formation, substrate effect on the anode reaction, and the cathode’s catalytic properties. To improve the performance of MFCs, the initiation process requires more investigation because of its significant effect on the anodic biofilm formation. This review explores the factors which affect the initiation process, including inoculum, substrate, and reactor configuration. The key messages are that optimal performance of MFCs for electricity production requires (1) understanding of the electrogenic bacterial biofilm formation, (2) proper substrates at the initiation stage, (3) focus on operational conditions affecting initial biofilm formation, and (4) attention to the reactor configuration.     

Continuous-flow biodiesel production using slit-channel reactors

Slit-channel reactors are reactors whose active surface areas are orders of magnitude higher than those of micro-reactors but have low fabrication costs relative to micro-reactors. We successfully produced biodiesel with different degrees of conversion using homogeneous catalyst in the slit-channel reactor. The reactor performance shows that percent conversion of soybean oil to biodiesel increases with channel depth, as expected, due to more efficient mixing. Shallow slit-channels require short average residence times for complete product conversion. Present results show that the slit-channel reactor provides an improved performance over traditional batch reactors using homogeneous sodium alkoxide catalyst. It is aimed to couple the reactors with solid catalysts in converting soybean oil to biodiesel and implementation method is suggested. The cost advantages resulting from the ease of fabrication of slit-channel reactors over micro-reactors and how these factors relate to the oil conversion efficiency to biodiesel are briefly noted and discussed.     

Operating conditions of a 200l staged vertical reactor for bioconversion of wheat straw by Phanerochaete chrysosporium

Bioconversion of wheat straw using Phanerochaete chrysosporium was carried out in a 200l staged vertical reactor. The bioconversion process was characterized by measuring the percentage degradation of lignin and cellulose, and increment in crude protein content. The effect of airflow rate, inoculum amount and wheat straw loading on bioconversion was investigated using a statistical experimental design. An analysis of variance was performed to determine response surfaces. The quality of bioconversion indicated by an optimization index called the desirability coefficient had the highest value of 0.75 for the fifth day of cultivation. This corresponded to an operating condition of 1.5kg wheat straw per stage using an inoculum amount of 0.38g (100g dry wheat straw)(-1) and an airflow rate of 15lmin(-1). The lignin and cellulose degradation achieved at this operating condition was 27% and 29%, respectively. A ratio of 3 for the weight of wheat straw to inoculum amount gave the highest crude protein of 5.9% on dry weight basis. Among the variables investigated, the airflow rate exhibited a significant effect on the quality of bioconversion. Our results indicate that the quality of bioconversion may be controlled by implementing a predetermined airflow rate schedule.     

Modeling of glycerol production by fermentation in different reactor states

A kinetic model of glycerol production by fermentation with the osmophilic yeast Candida krusei was studied firstly by analogies to published works. Considering that the glycerol produced competes with glucose, as a second carbon source for energy maintenance, mathematical models of glucose utilization and glycerol accumulation were modified further. By adjusting only two variable macrokinetic parameters, K_S and β, the model simulations could fit experimental data well when the reactor was changed from Airlift Loop Reactor in different scale or airlift mode to Stirred Vessel. To avoid a significant reduction in glycerol production in the latter fermentation stage, the final condition of the fermentation, determined by the concentration ratio of glycerol to glucose, was also investigated in four different Reactor States. The kinetic models and simulation results can provide certain reference for scale up of glycerol production by fermentation.     

A modeling study of anaerobic biofilm systems: II. Reactor modeling

A rigorous steady-state model of anaerobic biofilm reactors taking into account acid-base and gas-phase equilibria in the reactor in conjunction with detailed chemical equilibria and mass transfer in acetate-utilizing methanogenic biofilms is presented. The performances of ideal completely stirred tank reactors (CSTRs) and plug-flow reactors, as well as reactors with nonideal hydraulic conditions, are simulated. Decreasing the surface loading rate increases the acetate removal efficiency, while decreasing the influent pH and increasing the buffering capacity improves the removal efficiency only if the bulk pH of the reactor shifts toward more optimal values between 6.8 to 7.0. The reactor can have negative or positive removal efficiencies depending on the start-up conditions. The respiration coefficient plays a critical role in determining the minimum influent pH required for reactor recovery after failure. Having multiple CSTRs-in-series generally increases the overall removal efficiency for the influent conditions investigated. Monitoring of the influent feed quality is critical for plug-flow reactors, becasue failure of the initial sections of the reactor may cause a cascading effect that may lead to a rapid reactor failure. (c) 1995 John Wiley & Sons, Inc.     

Modeling of antibiotics production in magneto three-phase airlift fermenter

A mathematical model is developed to describe the performance of a three-phase airlift reactor utilizing a transverse magnetic field. The model is based on the complete mixing model for the bulk of liquid phase and on the Michaelis-Menten kinetics. The model equations are solved by the explicit finite difference method from transient to steady state conditions. The results of the numerical simulation indicate that the magnetic field increases the degree of bioconversion. The mathematical model is experimentally verified in a three-phase airlift reactor with P. chrysogenum immobilized on magnetic beads. The experimental results are well described by the developed model when the reactor operates in the stabilized regime. At relatively high magnetic field intensities a certain discrepancy in the model solution was observed when the model over estimates the product concentration.     

Anaerobic Reactor Design Concepts for the Treatment of Domestic Wastewater

Since the earlier anaerobic treatment systems, the design concepts were improved from classic reactors like septic tanks and anaerobic ponds, to modern high rate reactor configurations like anaerobic filters, UASB, EGSB, fixed film fluidized bed and expanded bed reactors, and others. In this paper, anaerobic reactors are evaluated considering the historical evolution and types of wastewaters. The emphasis is on the potential for application in domestic sewage treatment, particularly in regions with a hot climate. Proper design and operation can result in a high capacity and efficiency of organic matter removal using single anaerobic reactors. Performance comparison of anaerobic treatment systems is presented based mostly on a single but practical parameter, the hydraulic retention time. Combined anaerobic reactor systems as well as combined anaerobic and non-anaerobic systems are also presented.     

Bio-oil from photosynthetic microalgae: Case study

Microalgae possess the potential to produce bio-oils, carbohydrates, protein, amino acids and other value added products, each of which increase its value as a crop. Unfortunately, proven systems do not yet exist for commercial scale production. System designs have generally not adequately accounted for water and energy use at scale, as well as byproduct markets, and thus yielded systems that are both unaffordable and unsustainable. We address energy and water use by presenting a straightforward microalga-to-bio-oil production process and then characterize system performance using steady-state water and energy balances. Practical limitations to commercial production of bio-oils from photosynthetic microalgae are proposed and conclusions drawn regarding system potential for assumed biomass productivities. As this is a theoretical analysis of a generic process and in practice many of the bottlenecks presented remain to be solved, it is our intent that the analysis framework presented herein can be applied to future systems that propose such solutions.     

Strategies for the design and operation of enzymatic reactors for the degradation of highly and poorly soluble recalcitrant compounds

The presence of recalcitrant compounds in both wastewaters and soils is an important environmental problem. Oxidative enzymes from white-rot fungi have been successfully utilised for the in vitro degradation of xenobiotics, such as the azo dye Orange II and the polycyclic aromatic hydrocarbon anthracene (compounds with high and low solubilities, respectively). Two different reactor configurations are proposed: (i) an enzymatic membrane reactor for the treatment of soluble compounds, consisting of a continuous stirred tank reactor coupled to an ultrafiltration membrane to facilitate the retention and recycling of enzyme; and (ii) a two-phase enzymatic reactor for the degradation of poorly soluble compounds, consisting of an immiscible solvent, which contains the contaminant at high concentrations, and the aqueous phase containing the enzyme and cofactors involved in the catalytic cycle. In this paper, factors affecting the design and operation of both systems are discussed, and experimental results concerning the efficiency and stability of the processes are presented.     

Improving the production yield and productivity of 1,3-dihydroxyacetone from glycerol fermentation using <Emphasis Type="Italic">Gluconobacter oxydans</Emphasis> NL71 in a compressed oxygen supply-sealed and stirred tank reactor (COS-SSTR)

In this study, a compressed oxygen gas supply was connected to a sealed aerated stirred tank reactor (COS-SSTR) bio-system, leading to a high-oxygen pressure bioreactor used to improve the bio-transformative performance in the production of 1,3-dihydroxyacetone (DHA) from glycerol using Gluconobacter oxydans NL71. A concentration of 301.2 ± 8.2 g L^?1 DHA was obtained from glycerol after 32 h of fed-batch fermentation in the COS-SSTR system. The volumetric productivity for this process was 9.41 ± 0.23 g L^?1 h^?1, which is presently the highest obtained level of glycerol bioconversion into DHA. These results show that the application of this bioreactor would enable microbial production of DHA from glycerol at the industrial scale.     

Antibiotics production in a fluidized bed reactor utilizing a transverse magnetic field

A mathematical model is developed to describe the performance of a three-phase fluidized bed reactor utilizing a transverse magnetic field. The model is based on the axially dispersed plug flow model for the bulk of liquid phase and on the Michaelis-Menten kinetics. The model equations are solved by the explicit finite difference method from transient to steady state conditions. The results of the numerical simulation indicate that the magnetic field increases the degree of bioconversion. The mathematical model is experimentally verified in a three-phase fluidized bed reactor with Penicillium chrysogenum immobilized on magnetic beads. The experimental results are well described by the developed model when the reactor operates in the stabilized regime. At low and relatively high magnetic field intensities certain discrepancy in the model solution is observed when the model over estimates the product concentration.     

Strategies for the design and operation of enzymatic reactors for the degradation of highly and poorly soluble recalcitrant compounds

The presence of recalcitrant compounds in both wastewaters and soils is an important environmental problem. Oxidative enzymes from white-rot fungi have been successfully utilised for the in vitro degradation of xenobiotics, such as the azo dye Orange II and the polycyclic aromatic hydrocarbon anthracene (compounds with high and low solubilities, respectively). Two different reactor configurations are proposed: (i) an enzymatic membrane reactor for the treatment of soluble compounds, consisting of a continuous stirred tank reactor coupled to an ultrafiltration membrane to facilitate the retention and recycling of enzyme; and (ii) a two-phase enzymatic reactor for the degradation of poorly soluble compounds, consisting of an immiscible solvent, which contains the contaminant at high concentrations, and the aqueous phase containing the enzyme and cofactors involved in the catalytic cycle. In this paper, factors affecting the design and operation of both systems are discussed, and experimental results concerning the efficiency and stability of the processes are presented.     

Mathematical modelling of immobilized living yeast cell reactor for sugar bioconversion to ethanol

In the present work, a mathematical model was developed regarding the immobilized living yeast cell reactor for sugar bioconversion to ethanol. The model, composed of a system of ordinary differential equations (ODEs) enables the computation of the paramters involved in the steady state reactor behaviour. Comparing the values computed through the integration of this mathematical model with the experimental data, it has been shown its capacity to describe sufficiently accurate the steady state behaviour of the continuous fixed film bioconversion reactor.