Liquid Residence Time Research Articles

Biofilm mass transfer and thermodynamic constraints shape biofilm in trickle bed reactor syngas biomethanation

Syngas (H2, CO2, CO) produced thermochemically from lignocellulosic biomass is an underexploited source for resource recovery and valorisation through its biological conversion for the production of a wide range of chemicals and fuels. Syngas biomethanation is one such promising bioconversion pathway, displaying interesting features such as high conversion efficiency and product selectivity, as methane is the sole product of the process. The biological conversion of syngas to high purity biomethane is still typically associated with a number of challenges related to the syngas composition, CO toxicity, and gas–liquid mass transfer limitations. In this work, the syngas biomethanation process carried out in trickle bed reactors was investigated at its boundary conditions to explore the limits of the process, focusing on syngas composition and mass-transfer conditions potentially limiting process performance. The process was found to be robust when exposed to CO excess, but highly sensitive to H2 excess, which caused severe inhibition even under small amounts of excess H2. This implies that biomethane purity comparable to natural gas can be achieved by addition of renewable H2, but this requires precise control to avoid process failure. Modulating the liquid recirculation rate and gas residence time allowed for a maximum methane productivity of 9.8 ± 0.5 mmol CH4 h−1 Lreactor−1 with full conversion of H2 and CO at a gas residence time of 1 h. Nevertheless, increasing the gas–liquid mass transfer with increasing liquid recirculation rate did not lead to increased methane productivity, which suggested additional rate-limiting bottlenecks in the process. Careful investigation of other factors potentially limiting the process led to the conclusion that diffusive transport of syngas components in the biofilm was the main bottleneck of the process. This diffusive limitation leads to a scenario of severe substrate scarcity in the biofilm phase that conditions the thermodynamic feasibility of the different biochemical reactions involved in syngas biomethanation. In turn, these thermodynamic constraints were found to drive the stratification of microbial groups and sequential consumption of syngas components along the height of the reactor

CO2 absorption with polyethyleneimine solution intensified by a rotating packed bed with liquid detention

This study explored the absorption of CO2 by the polyethyleneimine (PEI) solution in a rotating packed bed (RPB). In order to boost the CO2 absorption effect, the liquid detention phenomenon in the RPB was utilized to overcome the short residence time of liquid. The effects of different operating parameters on CO2 absorption efficiency (η) and gas-phase volumetric mass transfer coefficient (KGav) in the RPB were investigated. The experimental results indicated that the RPB can greatly intensify the absorption of CO2. η and KGav increased from 3.3 % and 0.041 kmol·m−3·h−1·kPa−1 to 76.0 % and 1.634 kmol·m−3·h−1·kPa−1, respectively, as the rotational speed of the RPB rose from zero to 700 rpm with 250 mL detained liquid. It was also found that the liquid detention phenomenon can significantly elevate η and KGav as a result of extended liquid residence time and increased liquid holdup in the RPB. When the detained liquid volume increased from zero to 250 mL, η and KGav increased by 55.6 % and 113.1 %, respetively. In addition, it was observed that CO2 load in the rich PEI solution increased from 9.650 to 11.189 molCO2/kg PEI while η remained around 85 % after five cycles of absorption–desorption, suggesting an excellent cyclic stability of PEI. This work contributes to the development of viable CO2 capture technologies by intensifying the CO2 absorption process in the RPB with an efficient absorbent.

CFD assessment of internal Vapor/Liquid separator in an ebullated bed reactor (EBR) for heavy oil hydroprocessing

The ebullated-bed reactor (EBR) is an important system used in the upgrading of heavy oil residue into valuable light products in petroleum refinery operations. The assessment of the gas separation region hydrodynamics is very significant to investigate the performance of EBR. An advanced understanding of EBR fluid dynamics will help in improving the design of existing and future commercial units. For this reason, the development of a computational fluid dynamics (CFD) model is very challenging, where understanding the fundamentals of fluid dynamics in EBR is carried out using a three dimensional model and the turbulence model with a standard wall function in cup geometric design. The two models were used to analyze the influence of operational parameters such as recycle ratio, and inlet velocity on gas fraction recycled and separation efficiency. Moreover, the prediction of the regime transitions occurring in the freeboard region of the EBRs was achieved using the two aforementioned models. The results of the fluid dynamics behavior revealed that the inlet velocity has a substantial effect on gas fraction recycled and separation efficiency. The liquid mean residence time scale provided a valuable efficiency result for the separation within the range of expected mean residence times. This investigation, therefore, provided a good framework for the evaluation of future design and gave a better understanding of gas and liquid flow patterns in EBR.

Simulation of anaerobic biodegradation process in a tubular bioreactor with a biofilm layer: Steady-state and unsteady-state conditions

In this paper, anaerobic biodegradation process in a tubular bioreactor with an inner biofilm layer for steady-state and unsteady-state conditions are simulated. The effects of various parameters including bioreactor diameter, fraction of active biomass transferred to liquid phase, and residence time of the liquid on bioreactor performance are examined. Simulations indicate that decreasing diameter of bioreactor leads to increasing degree of conversion of the substrate in liquid phase and decreasing dimensionless concentration of the substrate in biofilm. With an increase in the fraction of active biomass transferred to liquid, substrate concentrations in liquid and biofilm slightly vary. Increased residence time of the liquid phase results in the degree of conversion of substrate goes up, but substrate concentration in biofilm lowers a little. In addition, it is found that biomass concentration of liquid phase is boosted with decreased bioreactor diameter and increased residence time of liquid. A proportional-integral controller is designed and the tuned parameters of KP=−0.131 and KI=0.02 are obtained using genetic algorithm. It is observed that controller regulate well the degree of conversion of the substrate within 120 s for both servo and regulatory modes.

Hydrophobic C60-modified PVDF membrane with micro-nano structures for mitigating CaSO4 scaling in direct contact membrane distillation (DCMD)

Membrane scaling is a non-negligible problem for membrane distillation (MD) in the treatment of high-salinity wastewater (e.g., calcium sulfate and calcium carbonate). In this study, fullerene (C60) was adhered to a polyvinylidene fluoride (PVDF) membrane using a polydimethylsiloxane (PDMS) solution to create a micro-nano structure, fabricating the C60/PDMS-PVDF membrane. This membrane was then re-immersed in a PDMS solution to further reinforce the C60 nanoparticles on the surface, forming the PDMS/C60/PDMS-PVDF membrane. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the membrane surface morphology. Fourier transform infrared (FT-IR) was used to explore the membrane chemical composition. Liquid entry pressure (LEP) and water contact angle (WCA) measurements were employed to assess the membrane wettability. The results showed that C60-modification created the micro-nano structure on the membrane surface. The WCA of the PVDF membrane was 112.7 ± 2.4°, with a sliding angle far greater than 90°, whereas the PDMS/C60/PDMS-PVDF membrane exhibited a WCA of 147.9 ± 3.2° and a sliding angle of 6.3°. Compared to the virgin PVDF membrane, the LEP value of the PDMS/C60/PDMS-PVDF membrane increased from 68.4 ± 2.7 kPa to 87.2 ± 3.3 kPa. In direct contact membrane distillation (DCMD), a CaSO4 solution (2 g/L) was used to test the membrane performance. After 48 hours of operation, the flux of the PDMS/C60/PDMS-PVDF membrane remained at approximately 3.57 kg/(m2·h), and the salt rejection rate was over 99.96 %. This could be attributed to the C60 modification forming a micro-nano structure on the membrane surface, which trapped an air layer. This air layer could reduce the contact area between the crystal and the membrane, decrease nucleation probability, and shorten the residence time of liquid on the membrane surface. Additionally, the PDMS/C60/PDMS-PVDF membrane exhibited superior stability during the ultrasonic destruction experiment due to the encapsulation of PDMS. In summary, a novel hydrophobic membrane with a micro-nano structure was fabricated through C60 modification, offering superior anti-scaling properties and stability in treating high-salinity wastewater.

Continuous kinetics of 2-hydroxyisobutryic acid esterification using solid acid granular and monolithic activated carbon catalysts

Kinetics of 2-hydroxyisobutyric acid (2-HIBA) esterification with ethanol was studied using sulfonated carbon catalysts in granular (sGAC) and monolith (sACM) forms in a continuous flow packed bed reactor. 2-HIBA is a potential biobased carboxylic acid and its ester a sustainably produced industrial solvent. The catalyst monolith form, made from renewable carbon, offers significantly higher space time yields and conversions, yet little kinetic data or models are available for its use, especially under non-ideal conditions. The effect of ethanol/2-HIBA molar ratio (44, 10, 3.6 and 1.5) at different liquid residence times (3–9 min) was investigated using sGAC. Kinetics of sACM esterifications were investigated at a molar ratio of 3.6 by varying the residence time (2–7 min) and compared with the sGAC. The sACM displayed higher ester yields, acid conversions, and significantly higher reaction rates and space–time-yields (2.1–2.8 ×) compared to the sGAC due to higher surface area to volume ratio. The sACM reaction rates and turnover frequency were comparable to solid acid resins and sulfated silica and zirconia. Pseudo-homogeneous, Eley-Rideal and Langmuir-Hinshelwood models were applied to fit the kinetic data, and the reaction system parameters were obtained through regression analysis. The LH dual active site model resulted in the lowest mean relative error for both 2-HIBA and ester concentration residuals. Adsorption/desorption equilibrium constants for 2-HIBA, ethanol, and ester, were higher compared to water suggesting hydrophobic regions in the sGAC/sACM. Estimation of the external and internal mass transfer criteria indicated negligible effects of mass transfer. With further development sACM and the kinetic model can be used for reactor design of continuous esterification processes using solid acid monolith catalysts.

Liquid flow field and residence time distribution in a baffleless oscillatory flow coil reactor

Baffleless coils can operate as continuous oscillatory flow reactors to produce pharmaceutical products where process intensification is intended. A method to evaluate mixing performance and its impact on reaction conversion is residence time distribution (RTD) analysis. Liquid macromixing in a long baffleless coil reactor (L = 24 m, dt = 4.57 mm) at ranges of oscillation amplitude (11.75–58.75 mm), frequency (0–3.68 Hz), and net flow rate (30–120 g⋅min−1) was investigated based on measured RTDs fitted to the skewed normal distribution model. The impact of operating conditions on the RTD was supported by analyzing the liquid flow field derived from CFD simulations. The RTD variance under different oscillation conditions suggests a near plug flow pattern, although deviations from ideality such as skewness and long tailing were always observed. The model parameters, along with the RTD variance, were correlated to the operating conditions lumped in an oscillatory Dean number that used the flow amplitude as characteristic length (Deox). Values of Deox varied between 150 and 11000, and the RTD variance at the different net flow rates was minimized for 300 < Deox < 600, as also evidenced from the minimum cross-sectional variance of the simulated time-averaged axial velocity profile. Normalizing the data allowed comparison to similar coil studies with different geometrical dimensions and operating conditions, leading to generalized correlations for the RTD model parameters. Finally, chemical conversion was assessed for first- and second-order single phase reactions at different Damköhler numbers using the segregation and maximum mixedness micromixing models combined with the fitted RTDs. The performance of the coil remained consistent with a plug flow reactor despite its departures from ideal flow.

Flow effects on sonochemical oxidation activity in a 20 kHz ultrasonic horn system

Forced fluid flow, when properly aligned with the geometry of a 20 kHz horn-type sonoreactor, can increase its sonochemical efficiency (SE). We designed a flow-through reactor configuration to evaluate the effect of fluid dynamics on sonochemical activity. Using particle image velocimetry (PIV), we mapped the velocity profiles within the sonoreactor under flow rates up to 350 ml/min, and ultrasound power density from 142 to 498 W/L. KI dosimetry and sonochemiluminescence (SCL) are conducted to correlate fluid motion with chemical activity.At an ultrasound power density below 241 W/L, placing the probe near the water surface with a 288 ml/min flow rate maximized radical generation. Conversely, above 402 W/L, maximum SE is achieved with the probe near the vessel’s bottom at the same flow rate. At power densities below 241 W/L, external flow increases the liquid residence time in acoustically active zones from 0.16 to an average of 6.3 s, yielding enhanced chemical activity. Above 402 W/L, the introduction of forced flow broadens the acoustic jet, relocating bubbles from the active zone below the probe’s tip to the area around its neck. The expansion of this active zone leads to increased SE.A significant discrepancy is noted between Electron Paramagnetic Resonance (EPR) and KI dosimetry in measuring hydroxyl (OH.) radical concentrations. EPR suggests that most of these radicals are lost to recombination, highlighting the rapid dynamics of radical generation and recombination into hydrogen peroxide (H2O2) at interfacial regions.

Microreactor designed for efficient plasma-liquid segmented flows.

Microreactors were designed for gas-liquid plasma chemical processes and operated under segmented flows in a high aspect ratio (8.76) rectangular microchannel. First, the hydrodynamics of the gas-liquid flows generated at a T-junction was investigated for fifteen solvents commonly used in organic synthesis. The classical literature scaling laws were revised to describe the dependence of bubble and slug lengths, and bubble residence time on the liquid nature by introducing their liquid vapour pressure. Liquid film thickness and liquid residence time were estimated from residence time distribution experiments. Secondly, plasma could be successfully generated in these segmented flows for all the liquids. Due to the plasma dissipation of thermal energy, gas phase temperature increased and induced the lengthening of bubbles and the decrease in bubble residence time. The flow pattern was also impacted by the gas temperature increase. A flow map describing the evolution of the flow pattern under plasma conditions was built, enabling prediction of the flow pattern based on the liquid boiling point and dielectric constant. These microreactors have demonstrated great potential, and by adapting the synthesis solvent or the operating plasma conditions, they could find promising applications in gas-liquid plasma chemical processes.

Determination of Liquid Residence Time Characteristics in Spinning Disk Reactors via a Computational Approach

Determination of Liquid Residence Time Characteristics in Spinning Disk Reactors via a Computational Approach

Hydrothermal pretreatment of Platanus orientalis Linn fallen leaves: effects of process conditions on anaerobic digestion

ABSTRACT Platanus orientalis Linn is widely distributed as a famous street tree, and its fallen leaves are a rich biomass resource. The fallen leaves of Platanus orientalis Linn (FLPOL) can be resourcefully treated by generating biogas through anaerobic digestion. This study aimed to assess the anaerobic digestion (AD) performance of FLPOL under different hydrothermal pretreatment (HTP) conditions (solid–liquid ratio 2–10%, residence times 0.5–2.5 h, and temperatures 120–200°C). The solid–liquid ratio, residence time, and temperature determined by energy balance calculation were 6%, 2.0 h, and 180°C, respectively, and the energy balance was 1.823 kJ/g biomass. Under these conditions, the methane yield of the process water was 56.85 mL/g·VS. The methanogenic potential, hydrolysis rate, maximum methanogenic rate, and the lag period obtained by the Fitzhugh first-order kinetic model and the modified Gompertz model were 101.803 mL/g·VS, 0.130 d−1, 14.207 mL/(g·VS·d) and 1.332 d, respectively. The research results suggest that HTP has an important role in promoting the large-scale AD application of FLPOL.

Design and evaluation of co-currently illuminated two-phase bubbly flow photochemical reactors

Current two-phase flow photochemical reactors are lacking energy efficiency required for industrial applicability. As an alternative, co-currently illuminated bubbly flow photochemical reactors are proposed. These have the advantage of running at higher absorbance values, thereby wasting less photons, improving cost efficiency, and decreasing gas holdup by separating gas and liquid residence time in the reactor. A mathematical model was used in the study in combination with photosensitized reactions, which yielded three coupled differential equations and four non-dimensional groups governing the reactor performance. A parameter study yielded an optimal operating point at a quantum photon balance (ϕtotρ) of 1, an absorbance (AS) of at least 1, a chemical quenching parameter (κA) of less than 0.1, and a sensitizer stability ratio (ϕS/fS) of less than 1. Optimal absorbance was determined via the use of a ray tracing study, yielding a theoretical optimal absorbance of 1.75 using 7 µM of rose bengal. The modelling results are experimentally validated, and the model was shown to be suitable across a wide range of operating conditions. Optimal absorbance was experimentally evaluated to match the modelling results, higher absorbance values lead to more local absorbance, throttling reactor performance. With high reagent concentrations, the reaction was found to be liquid side mass transfer limited. Guidelines are presented to design this novel type of reactor and ways to improve the reactor performance in scaled up versions. The reactor was compared against the current state-of-the-art reactors, yielding an improvement PSTY by 4.5 times and an increased reagent/sensitizer ratio by 100 times, both drastically decreasing operational cost of the reactor.

Enhanced Treatment of Basic Red 46 by Ozonation in a Rotating Packed Bed with Liquid Detention

This study investigated the use of ozone in a rotating packed bed (RPB) with liquid detention for the treatment of Basic Red 46 (BR-46). Liquid detention means that liquid accumulates at the lower section to a certain level in the RPB, which leads to longer liquid residence time and greater liquid holdup in the packing and cavity in the RPB. The experimental results showed that the presence of liquid detention in the RPB significantly enhanced the BR-46 treatment effect and ozone absorption rate. With 200 mL of liquid detention in the RPB, the decolorization rate, COD degradation rate, and ozone absorption rate were 34.7%, 62.8%, and 80.0% higher than those without liquid detention. The effects of the rotational speed of the RPB, ozone concentration, initial BR-46 concentration, liquid and gas flow rates on BR-46 degradation were also investigated, and it was found that the high-gravity environment is beneficial to the degradation of BR-46. These results suggest that with the utilization of the liquid detention phenomena in the high-gravity devices, the applications of the high-gravity technology can be extended to the processes where a long liquid residence time is required.

Epoxidation of light olefin mixtures with hydrogen peroxide on TS-1 in a laboratory-scale trickle bed reactor: Transient experimental study and mathematical modelling

Direct epoxidation of light olefin mixtures with hydrogen peroxide and titanium silicalite (TS-1) catalyst is interesting from a pure scientific viewpoint and it might have a considerable economic and environmental advantages. Direct epoxidation of olefin mixtures implies a significant process intensification as the separation steps of alkenes prior to the epoxidation process is avoided. Binary and ternary mixtures of ethene, propene and 1-butene were selectively epoxidized with hydrogen peroxide in the presence of TS-1 as the heterogeneous catalyst. Methanol was used as the solvent. Transient and stationary kinetic experiments were conducted in a laboratory-scale trickle bed reactor, which was operated under atmospheric pressure and isothermal conditions at 15–55 °C. The experiments were designed to investigate the kinetics of the epoxidation of light olefins at different temperatures, alkene-to-hydrogen peroxide molar ratios as well as gas and liquid residence times in the trickle bed reactor. The recorded transient responses of the reaction products confirmed that the components in the alkene mixtures are epoxidized simultaneously. The epoxide selectivity was 90 % or higher in most experiments; ring-opening products formed from the epoxides and methanol were observed as by-products. The concentration maxima of epoxides during the transient experiments were explained by accumulation of the epoxide products inside the catalyst, which resulted in partial blocking of active sites. After-treatment of the catalyst with methanol and nitrogen completely restored the initial activity. Kinetic modeling was applied, based on presumed elementary steps on the TS-1 surface to describe the behavior of the system at steady state conditions. The trickle bed reactor was described with a dynamic multiphase model, taking into account the kinetic and mass transfer effects. The numerical values of the activation energies, kinetic constants and mass transfer parameters were estimated by nonlinear regression analysis. The model gave a satisfactory description of the experimental data.

Liquid mean residence time (MRT) in rotating packed bed (RPB) by empirical correlation and residence time distribution (RTD) method using computational fluid dynamics (CFD) simulation

Rotating packed bed (RPB) belongs to a HIGee technology, a process intensification device that can provide better mass transfer rate due to the generation of hyper-gravity under the influence of centrifugal force. While determining the efficiency of the RPB, the MRT of the liquid plays a vital role. The MRT of the RPB is very small and can be tuned in accordance with the mass transfer rate of the solvent to achieve the required outlet concentration of the absorbed gas. There exist two methods, i.e., empirical correlation and the residence time distribution (RTD) method. The applicability of both methods still needs to be investigated for better prediction of MRT in RPB. The current study compares the MRT of the two of the most widely employed techniques, i.e., MRT by empirical correlation and the RTD approach using the Computational Fluid Dynamics (CFD). The difference between the MRT by both methods lies between 30-38%. The results show that the RTD better predicts the MRT in the RPB as compared to the Burns empirical correlation.

Interplay of Particle Suspension and Residence Time Distribution in a Taylor–Couette Crystallizer

In small-scale continuous crystallization, particle suspension and residence time distribution are critical factors determining operability and product quality. Here, the Taylor–Couette crystallizer stands out for its high flexibility. Its characteristic vortex structure intensifies local mixing, thus improving the suspension and simultaneously narrowing the residence time distribution, whereby these effects can be adjusted by operating and design parameters. However, the operating window is limited by the prerequisite of sufficient particle suspension. In this study, we investigated the suspension behavior and its impact on the attainable liquid phase residence time distribution and the flow regimes observed. For this purpose, the just-suspended rotation rate was visually determined for different design and operating parameters. A correlation was regressed from experimental data, showing that this rotation rate was mainly affected by the radius ratio of the rotor and stator. In addition, the liquid phase residence time distribution was measured by tracer experiments in regions of sufficient suspension, validating a correlation from the literature. With a combination of both correlations, the design parameters of the apparatus can thus be optimized according to the goal of, for example, a narrow residence time distribution in the suspended state.

Crystallization in Fluidized Bed Reactors: From Fundamental Knowledge to Full-Scale Applications

A review is presented on fifty years of research on crystallization in fluidized bed reactors (FBRs). FBRs are suitable for recovery of slightly soluble compounds from aqueous solutions, as it yields large, millimeter sized particles, which are suitable for reuse and permits low liquid residence times in the timescale of minutes. Full-scale applications for water softening have been applied since the 1980s, and since then, new applications have been developed or are in development for recovery of phosphorus, magnesium, fluoride, metals, sulfate, and boron. Process integration with membrane, adsorption, and biological processes have led to improved processes and environmental indicators. Recently, novel FBR concepts have been proposed, such as the aerated FBR for chemical-free precipitation of calcium carbonate, the seedless FBR to yield pure particulate products, a circulating FBR for economic recovery and extended use of seeds, as well as coupled FBRs for separation of chiral compounds and FBRs in precipitation with supercritical fluids. Advances are reported in the understanding of elementary phenomena in FBRs and on mathematical models for fluid dynamics, precipitation kinetics, and FBR systems. Their role is highlighted for process understanding, optimization and control at bench to full-scale. Future challenges are discussed.

Transient cooling performance of R134a flash-evaporation spray enhanced by cold air jet: A novel heat transfer enhancement method in laser dermatology

Nevus of Ota is a dermal hyperplastic pigmented disease with a morbidity rate of 0.1–0.6% in the Asian population. Laser surgery with the assistance of cryogen spray cooling (CSC) is a potential clinical treatment for nevus of Ota. However, for Asians with high melanin content, it is necessary to enhance cooling capability without the increment of refrigerant amount. In this study, a R134a flash-evaporation cryogen spray cooling coupled with cold air jet (CSC-CAJ) was developed to improve the cooling capacity. Epoxy resin was employed as a skin phantom to investigate the heat transfer characteristics of CSC-CAJ with different air jet parameters. Air jet parameters of CSC-CAJ are recommended to be Ta = -7 ℃, Va = 3 m3/h Za = 10 mm and Φa = 30°. Compared to CSC, CSC-CAJ could reduce the minimum surface temperature from -39 to -42.1 ℃, and the average value of convective heat transfer coefficient (h) of CSC-CAJ could increase from 8 to 12.9 W/(m2·k) or 61.3% for the recommended parameters. The liquid film residence time (τ) decreases from 301 to 234 ms or 23.3%. In addition, based on the visualization observation and surface heat transfer measurement, the frost formation phenomenon on the surface and the heat transfer enhancement mechanisms are well understood. Four modes, including Leidonfrost mode, no runoff mode, runoff mode and evaporation mode, are proposed to describe the four stages of CSC-CAJ. A non-dimensional correlation of normalized Nusselt number (Nu*) and time (t*) is summarized. The primary heat transfer enhancement mechanism of CSC-CAJ is found to be the increase in cryogen evaporation rate and the ratio of heat removal from the phantom surface to phase change latent heat.

The Impact of Preservation Techniques on Methane-Arrested Anaerobic Digestion of Nutrient-Rich Feedstocks.

The carboxylate platform is a promising biomass-to-energy pathway that uses methane-arrested anaerobic digestion (MAAD) to convert biomass to carboxylic acids, which can be chemically converted to industrial chemicals and liquid fuels. Lignocellulose is an energy-rich carbon source, but lacks nutrients necessary for microbial growth. Chicken manure (rural waste) and sewage sludge (urban waste) are rich in nitrogen and useful macronutrients; therefore, co-digesting these wastes with lignocellulose improves MAAD performance. However, waste nutrients must be digested immediately, or preserved. This study investigated the effects of various preservation techniques - frozen (fresh), air-dried, and baked - on chicken manure and sewage sludge. Batch experiments were performed with office paper (carbon source) and chicken manure or sewage sludge (nutrient source) with different methods of preservation. Fresh substrates produced higher acid yields and biomass conversion (the amount of biomass consumed during digestion) than dried substrates. Baked chicken manure showed reduced conversion and total acid production, which suggests that oven-drying reduces digestibility. From the batch data, the Continuum Particle Distribution Model (CPDM) predicted results of a four-stage countercurrent digestion. The data are displayed on maps showing the impact of liquid residence time (LRT) and volatile solids loading rate (VSLR) on conversion and product concentration. Co-digesting office paper and wet chicken manure at a non-acid volatile solid (NAVS) concentration of 300g/Lliq, the model predicted a high total acid concentration of 52.8g/L and conversion of 0.89g NAVSdigested/NAVSfed at a volatile solid loading rate of 4g/(Lliq·day) and liquid retention time of 35days.

Numerical analysis of the influence of packing corrugation angle on the flow and mass transfer characteristics of cryogenic distillation

The corrugation angle of structured packing is an important structural parameter that affects the mass transfer efficiency and pressure drop of distillation process. However, at present, the detailed effects of corrugation angle on the distillation performance of the cryogenic fluids are seldom reported due to the difficulties in conducting cryogenic experiment, which needs to be further revealed for the optimizing of cryogenic structured packing. In this work, a three-dimensional CFD model combing VOF method was established to investigate the flow characteristics of LOX in Mellapak 250 packing under different packing corrugation angles. The effects of corrugation angle on the component mass transfer rate of cryogenic fluids of oxygen and nitrogen were analyzed in detail. Results show that under the combined action of interfacial area, liquid film thickness and liquid residence time, the mass transfer rate decreases by about 30% with the increase of angle from 45° to 60°, while the decrease of wet pressure drop can reach 81.1%, much higher than the reduction ratio of mass transfer rate. It shows 60° packing has application potential and can be used in the middle part of the cryogenic distillation column where mass transfer force is very large, reducing pressure drop loss without substantially sacrificing mass transfer efficiency. 60° packing with higher specific area can also be applied. In addition, it is found that 50° packing can greatly reduce the loss of pressure drop (41.1%) with only a small amount of mass transfer rate loss (8.8%), which shows a good application prospect in cryogenic distillation.