Flow field in a shrinking-bed reactor for pretreatment of cellulosic biomass

A shrinking-bed reactor was designed by the National Renewable Energy Laboratory to maintain a constant bulk packing density of cellulosic biomass. The high solid-to-liquid ratio in the pretreatment process allows a high sugar yield and avoids the need to flush large volumes of solution through the reactor. The shrinking-bed reactor is a promising pretreatment reactor with the potential for scale-up for commercial applications. To scale up the shrinking-bed reactor, it is necessary to understand the flow pattern in the reactor. In this study, flow field is simulated with computational fluid dynamics using a porous medium model. Different discrete “snapshots” and multiple steady states are utilized. The bulk flow pattern, velocity distribution, and pressure drop are determined from the simulation and can be used to guide reactor design and scale-up.     

Use of computational fluid dynamics simulations for design of a pretreatment screw conveyor reactor

Computational fluid dynamics simulations were employed to compare performance of various designs of a pretreatment screw conveyor reactor. The reactor consisted of a vertical screw used to create cross flow between the upward conveying solids and the downward flow of acid. Simulations were performed with the original screw design and a modified design in which the upper flights of the screw were removed. Results of the simulations show visually that the modified design provided favorable plug flow behavior within the reactor. Pressure drop across the length of the reactor without the upper screws in place was predicted by the simulations to be 5 vs 40 kPa for the original design.     

Simulation of Polymer Reactors Using the Compartment Modeling Approach

A compartment modeling approach based on computational fluid dynamics (CFD) simulations is applied to a simplified static mixer geometry. Compartments are derived from velocity fields obtained from cold CFD simulations. This methodology is based on the definition of periodic flow zones (PFZ) derived from the recurrent flow profile within the static mixer. In general, PFZ can be characterized by two different compartments: flow zones with hydrodynamic behavior of a tubular reactor and dead zones exhibiting a more continuous stirred tank reactor‐like characteristic. In CFD studies the influence of changing fluid properties, for example viscosity, on flow profile due to polymerization progress is considered. In the deterministic compartment model, the continuous flow profile within the static mixer is transformed to basic reactor models interconnected via an exchange stream. To reduce model complexity and the number of model parameters, constant volumes of compartments are assumed. Changes in hydrodynamics are considered by a variable exchange flow rate as a function of Re manipulating residence time in compartments. Simulation studies show the influence of decreasing exchange flow rates with polymerization progress, as Re decreases, resulting in a greater increase of viscosity in dead zones. The reactor performance is qualitatively represented by the simulation results.     

Shrinking-bed model for percolation process applied to dilute-acid pretreatment/hydrolysis of cellulosic biomass

For many lignocellulosic substrates, hemicellulose is biphasic upon dilute-acid hydrolysis, which led to a modified percolation process employing simulated two-stage reverse-flow. This process has been proven to attain substantially higher sugar yields and concentrations over the conventional single-stage percolation process. The dilute-acid pretreatment of biomass solubilizes the hemicellulose fraction in the solid biomass, leaving less solid biomass in the reactor and reducing the bed. Therefore, a bed-shrinking mathematic kinetic model was developed to describe the two-stage reverse-flow reactor operated for hydrolyzing biphasic substrates, including hemicellulose, in corn cob/stover mixture (CCSM). The simulation indicates that the shrinking-bed operation increases the sugar yield by about 5%, compared to the nonshrinking bed operation in which 1 reactor volume of liquid passes through the reactor (i.e.,t = 1.0). A simulated optimal run further reveals that the fast portion of hemicellulose is almost completely hydrolyzed in the first stage, and the slow portion of hemicellulose is hydrolyzed in the second stage. Under optimal conditions, the bed shrank 27% (a near-maximum value), and a sugar yield over 95% was attained.     

Modeling of a continuous pretreatment reactor using computational fluid dynamics

Computational fluid dynamic simulations are employed to predict flow characteristics in a continuous auger driven reactor designed for the dilure acid pretreatment of biomass. Slurry containing a high concentration of biomass solids exhibits a high viscosity, which poses unique mixing issues within the reactor. The viscosity increases significantly with a small increase in solids concentration and also varies with temperature. A well-mixed slurry is desirable to evenly distribute acid on biomass, prevent buildup on the walls of the reactor, and provides an uniform final product. Simulations provide flow patterns obtained over a wide range of viscosities and pressure distributions, which may affect reaction rates. Results provide a tool for analyzing sources of inconsistencies in product quality and insight into future design and operating parameters.     

Design of a fluidized bed reactor for microencapsulated urease

A fluidized bed reactor was designed, constructed, and tested for handling microencapsulated urease. The working volume of the reactor was 10 mL, with a minimum fluidization velocity of 7.7×10⁻⁵ m/s. An even suspension of the microcapsules was obtained at fluid velocities between 1.5×10⁻⁴ and 6.0×10⁻⁴ m/s without breakage of the shear-sensitive microcapsules. The mixing behavior in the reactor was evaluated using pulse input tracer experiments and the hydrolysis rates of urea in continuous flow experiments were evaluated under various operating conditions.     

Comparison of yellow poplar pretreatment between NREL digester and sunds hydrolyzer

Single-stage cocurrent dilute acid pretreatments were carried out on yellow poplar (Liriodendron tulipifera) sawdust using an as-installed and short residence time modified pilot-scale Sunds hydrolyzer and a 4-L bench-scale NREL digester (steam explosion reactor). Pretreatment conditions for the Sunds hydrolyzer, installed in the NREL process development unit (PDU), which operates at 1 t/d (bone-dry t) feed rate, spanned the temperature range of 160 – 210°C, 0.1 – 1.0% (w/w) sulfuric acid, and 4-10-min residence times. The batch pretreatments of yellow poplar sawdust in the bench-scale digester were carried out at 210 and 230°C, 0.26% (w/w) sulfuric acid, and 1-, 3-, and 4-min residence times. The dilute acid prehydrolysis solubilized more than 90% of the hemicellulose, and increased the enzymatic digestibility of the cellulose that remained in the solids. Compositional analysis of the pretreated solids and liquors and mass balance data show that the two pretreatment devices had similar pretreatment performance.     

Enhanced simulation of an RF ion funnel including gas turbulence

Electrodynamic ion funnels are used to enhance the transmission of ions in electrospray‐based ion injection systems in 0.1 to 30 Torr pressure range. Jet disrupters are commonly employed to prevent droplets and high pressure jets from entering subsequent vacuum regions. This study presents the simulation and testing of an ion funnel containing a jet disrupter using computational fluid dynamics (CFD) and SIMION ion trajectory simulations. Traditional modeling approaches have utilized approximations for the bulk fluid flow fields without including the time‐varying nature of the turbulent flow present in the system, thus yielding idealized results. In this study, the fluid flow fields are calculated using CFD. In an effort to include time dependence, a random velocity vector, whose magnitude is proportional to the square root of the turbulence kinetic energy, was calculated at each time step and added to the velocity of the background gas. These simulations predicted that the transmitted ion current is effectively modulated by the variation of the jet disrupter voltage. The addition of the random velocity vector produced results that closely matched the experiments. The simulations yielded the dependence of the transmission on the jet disrupter voltage, and the voltage necessary for maximum ion throughput was accurately predicted. In addition, the magnitude of the predicted transmission closely matched that of the experimental results. This modeling approach could be extended to similar ion transport and filtering systems in which the effects of turbulent fluid flow cannot be ignored. Copyright © 2015 John Wiley & Sons, Ltd.     

Methodology considering surface roughness in UV water disinfection reactors

Water disinfection making use of an ultraviolet (UV) reactor is an attractive procedure because it does not produce any by-products. In this work, the effects of pipe roughness on the performance of a closed-conduit water disinfection UV reactor were investigated. In order to incorporate the surface roughness effects, a simple, stable, highly accurate model, better than any iterative approximation, was adopted in the numerical simulations. The analysis was carried out on the basis of two performance indicators: reduction equivalent dose (RED) and system dose distribution. The analysis was performed using a commercial computational fluid dynamics (CFD) tool (ANSYS Fluent). The fluence rate within the UV reactor was calculated using UVCalc3D. The pipe surface roughness resulted in longer pathogen residence times and higher dose distribution among the pathogens. The effect of pipe surface roughness on RED depends on the Reynolds number and relative roughness. Pipe surface roughness plays an important role because UV reactors for water disinfection operate at moderate Reynolds numbers. In addition, the positioning of the UV lamp in the reactor plays an important role in determining the RED of the reactor. Search criteria for lamp-positioning are also proposed in the current work. The proposed CFD methodology can be used to analyse the performance of closed-conduit reactors for water disinfection by UV.     

Effects of slight variations in nutrient loadings on pore plugging in soil columns

The high nutrient concentrations that would exist near the nutrient injection well during the application of cometabolicin situ bioremediation may lead to the development of significant quantities of biomass at this point in the subsurface. This biomass can decrease the porosity of the soil to such an extent that nutrient injection is no longer possible. In this work, experiments were conducted using a porous media biofilm reactor, operated under constant substrate loading conditions, such that the pressure drop across the reactor was allowed to increase to maintain a constant volumetric flow rate through the reactor. Results suggest that biomass production, and hence biofilm thickness, near the injection feed port is highly sensitive to substrate loading. In addition, these variations in biofilm thickness produce dramatic differences in the pressure drop that is attained across the reactor. Use of the Kozeny-Carman equation can be used to predict that once a critical depth has been exceeded, the pressure drop across the bed will increase exponentially within biofilm depth. This result means that pressure is not a reliable indicator of the onset of pore plugging.     

Dilute-acid pretreatment of corn stover using a high-solids percolation reactor

Pretreatment of corn stover by dilute sulfuric acid was investigated using a laboratory percolation (flowthrough) reactor operated under high-solids conditions. The effects of reaction conditions and operating parameters on the performance of the percolation reactor were investigated seeking the optimal range in which acceptable levels of yield and sugar concentration could be attained. It was demonstrated that 70–75% recovery of xylose and 6 to 7% (w/w) xylose concentration were attainable. The high sugar concentration was obtained as a result of dense packing of dry corn stover and the low liquid throughput. Xylose was mostly unreacted, rather than decomposed. The cellulose and the unreacted xylan of treated corn stover were both effectively hydrolyzed by a “cellulase” enzyme preparation that also exhibits some activity on xylan. The xylose yield was affected significantly by the flow rate under the same reaction time and conditions. This behavior appears to be related to sugar decomposition, mass transfer resistance, and the fact that acid is neutralized by the buffering components of the biomass.     

Performance of a magnetically stabilized bed reactor with immobilized yeast cells

This paper is focused on the possibility to apply the magnetic stabilization technique in bioprocessing. The feasibility of a continuous ethanol fermentation process with immobilizedSaccharomyces cerevisiae cells in a magnetically stabilized bed (MSB) was demonstrated. The fermentation processes were carried out in an external magnetic field, transverse to the fluid flow. The flexibility to change the bed expansion owing to the independent change of the fluid flow and the field intensity (the “magnetization FIRST” mode) permitted the creation of fixed beds with different particle arrangements, which affected the bed porosity, the effective fluid-particle contact area, and the mass transfer processes on the particle-fluid interface. As a result, higher ethanol concentration, ethanol production, and glucose uptake rates than in conventional packed bed reactor were reached.     

Granulation of activated sludge in a laboratory upflow sludge blanket reactor

The creation of anoxic granulated biomass has been monitored in a laboratory USB (Upflow Sludge Blanket) reactor with the volume of 3.6 L. The objective of this research was to verify the possibilities of post-denitrification of residual NO₃-N concentrations in treated wastewater (denitrification of 10-20 mg L⁻¹ NO₃-N) and to determine the maximum hydraulic and mass loading of the granulated biomass reactor. G-phase from biodiesel production and methanol were both tested as external organic denitrification substrates. The ratio of the organic substrate COD to NO₃-N was 6. Only methanol was proven as a suitable organic substrate for this kind of reactor. However, the biomass adaptation to the substrate took over a week. The cultivation of anoxic granulated biomass was reached at hydraulic loading of over 0.35 m h⁻¹. The size of granules was smaller when compared with results found and described in literary reports (granules up to 1 mm); however, settling properties were excellent and denitrification was deemed suitable for the USB reactor. Sludge volume indexes of granules ranged from 35-50 mL g⁻¹ and settling rates reached 11 m h⁻¹. Maximum hydraulic and mass loadings in the USB reactor were 0.95 m3 m⁻² h⁻¹ and 6.6 kg m⁻³ d⁻¹. At higher loading levels, a wash-out of the biomass occurred. Presented at the 35th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 26–30 May 2008.     

Modeling of acetylene pyrolysis under steel vacuum carburizing conditions in a tubular flow reactor

In the present work, the pyrolysis of acetylene was studied under steel vacuum carburizing conditions in a tubular flow reactor. The pyrolysis temperature ranged from 650 degrees C to 1050 degrees C. The partial pressure of acetylene in the feed mixture was 10 and 20 mbar, respectively, while the rest of the mixture consisted of nitrogen. The total pressure of the mixture was 1.6 bar. A kinetic mechanism which consists of seven species and nine reactions has been used in the commercial computational fluid dynamics (CFD) software Fluent. The species transport and reaction model of Fluent was used in the simulations. A comparison of simulated and experimental results is presented in this paper.     

Influence of the Electromagnetic Forces on Momentum and Heat Transfer in a 3-Phase ac Plasma Reactor

A new 3-phase ac plasma reactor has been developed within the framework of research on hydrocarbon cracking for the production of carbon black and hydrogen. ^(1,2) One of the main characteristics of the system is related to the 3-phase, 50 Hz ac current plasma generator which induces a very particular arc motion affecting the heat and mass transfer inside the reactor. In a first step, the general flow inside the reactor in the absence of hydrocarbon injection has been studied. A simplified approach to characterize the heat and mass transfer inside the reactor is presented in this paper. The arc zone analysis is carried out simultaneously by a theoretical analysis of the electromagnetic forces and by an ultrahigh-speed cine-camera analysis. The flow in the reactor is modeled with a CFD commercial code. Results are compared with experimental temperature measurements.     

Biochemical properties of genetic recombinant xylanase II

An ultraviolet (UV) coil reactor was designed and used for the online sterilization of cheese whey. Its microbial destruction efficiency was compared to that of the conventionally used UV reactor. Both reactors have the same geometry (840 ml volume and 17 mm gap size) and were tested at 11 flow rates of 5, 10, 15, 20, 25, 30, 35, 40, 50, 60, and 70 ml/min. The results obtained from this study showed that despite of its high turbidity, cheese whey could be sterilized using UV radiation if the proper reactor design and flow rate are used. The performances of the UV reactors were governed by the flow rate and the hydraulics of flow inside the reactor. The flow was laminar in both the reactors, as the Reynolds number was in the range of 1.39–20.10. The phenomenon of Dean Flow was observed in the coil reactor and the Dean number was in the range of 1.09–15.41. Dean vortices resulted in higher microbial destruction efficiency in the coil reactor in a shorter retention time. The rate of microbial destruction was found to be exponential in the conventional reactor and polynomial in the coil reactor. Increasing the flow rate from 5 ml/min to 70 ml/min decreased the microbial destruction efficiency of the conventional reactor from 99.40 to 31.58%, while the microbial destruction efficiency in the coil reactor increased from 60.77% at the flow rate of 5 ml/min to 99.98% at the flow rate of 30 ml/min and then decreased with further increases in flow rate reaching 46.2% at the flow rate of 70 ml/min. The maximum effluent temperatures in the conventional and coil reactors were 45.8 and 46.1°C, respectively. Fouling in the coil reactor was significantly less compared to the conventional reactor. The extent of fouling was influenced by flow rate and reactor’s hydraulics.     

AnSBBR Applied to a Personal Care Industry Wastewater Treatment: Effects of Fill Time,Volume Treated Per Cycle,and Organic Load

A study was performed regarding the effect of the relation between fill time, volume treated per cycle, and influent concentration at different applied organic loadings on the stability and efficiency of an anaerobic sequencing batch reactor containing immobilized biomass on polyurethane foam with recirculation of the liquid phase (AnSBBR) applied to the treatment of wastewater from a personal care industry. Total cycle length of the reactor was 8 h (480 min). Fill times were 10 min in the batch operation, 4 h in the fed-batch operation, and a 10-min batch followed by a 4-h fed batch in the mixed operation. Settling time was not necessary since the biomass was immobilized and decant time was 10 min. Volume of liquid medium in the reactor was 2.5 L, whereas volume treated per cycle ranged from 0.88 to 2.5 L in accordance with fill time. Influent concentration varied from 300 to 1,425 mg COD/L, resulting in an applied volumetric organic load of 0.9 and 1.5 g COD/L.d. Recirculation flow rate was 20 L/h, and the reactor was maintained at 30 °C. Values of organic matter removal efficiency of filtered effluent samples were below 71% in the batch operations and above 74% in the operations of fed batch followed by batch. Feeding wastewater during part of the operational cycle was beneficial to the system, as it resulted in indirect control over the conversion of substrate into intermediates that would negatively interfere with the biochemical reactions regarding the degradation of organic matter. As a result, the average substrate consumption increased, leading to higher organic removal efficiencies in the fed-batch operations.     

CFD Simulation of a Hydrogen／Argon Plasma Jet Reactor for Coal Pyrolysis

A Computational Fluid Dynamics (CFD) model was formulated for DC arc hydrogen/argon plasma jet re-actors used in the process of the thermal H2/Ar plasma pyrolysis of coal to acetylene. In this model, fluid flow, convective heat transfer and conjugate heat conductivity are considered simultaneously. The error caused by estimating the inner-wall temperature of a reactor is avoided. The thermodynamic and transport properties of the hydrogen/argon mixture plasma system, which are usually expressed by a set of discrete da-ta, are fitted into expressions that can be easily implemented in the program. The effects of the turbulence are modeled by two standard k-ε equations. The temperature field and velocity field in the plasma jet reactor were calculated by employing SIMPLEST algorithm. The knowledge and insight obtained are useful for the design improvement and scale-up of plasma reactors.     

Modeling on the Momentum and Heat/Mass Transfer Characteristics of an Argon Plasma Jet Issuing into Air Surroundings and Interacting with a Counter-Injected Argon Jet

Modeling study is performed to reveal the momentum and heat/mass transfer characteristics of a turbulent or laminar plasma reactor consisting of an argon plasma jet issuing into ambient air and interacting with a co-axially counter-injected argon jet. The combined-diffusion-coefficient method and the turbulence-enhanced combined-diffusion-coefficient method are employed to treat the diffusion of argon in the argon–air mixture for the laminar and the turbulent regimes, respectively. Modeling results presented include the streamline, isotherm and argon mass fraction distributions for the cases with different jet-inlet parameters and different distances between the counter-injected jet exit and the plasma torch exit. It is shown that there exists a quench layer with steep temperature gradients inside the reactor; a great amount of ambient air is always entrained into the plasma reactor; and the flow direction of the entrained air, the location and shape of the quench layer are dependent on the momentum flux ratio of the plasma jet to the counter-injected cold gas. Two quite different flow patterns are obtained at higher and lower momentum flux ratios, and thus there exists a critical momentum flux ratio to separate the different flow patterns and to obtain the widest quench layer. There exists a high argon concentration or even ‘air-free’ channel along the reactor axis. No appreciable difference is found between the turbulent and laminar plasma reactors in their overall plasma parameter distributions and the quench layer locations, but the values of the critical momentum flux ratio are somewhat different.