Liquid Recirculation Research Articles

Agitation of yield stress fluids by gas injection

AbstractThis study is in line with two previous studies by the same authors on gas injection in yield stress fluids. Gas is injected toward the bottom wall of a prismatic tank containing a yield stress fluid. When rising toward the free surface, trains of bubbles generate fluid recirculation in the tank. Two experimental colorimetric methods are introduced and validated in order to quantify the recirculation liquid flow rate as well as the time evolution of the extent and shape of the mixed volume. The influences of the injection flow rate, fluid rheology, and reactor size have been quantified. Correlations based on the characteristic nondimensional numbers of the flow have been developed to predict the downward liquid flow rate as well as the mixed volume. A model for estimating the mixing time is also developed and compared to experimental results.

An original Arduino-controlled anaerobic bioreactor packed with biochar as a porous filter media

Bioreactors are commonly used apparatuses generally equipped with several built-in specifications for the investigation of biological treatment studies. Each bioreactor test may require different types of specialty such as heating, agitation, re-circulation and some further technologies like online sensoring. Even thought, there are many ready-to-use fabricated bioreactors available in the market with a cost usually over than 1000 €, it is often not possible to access those advanced (but inflexible) systems for many students, young-researchers or small-scale private R&D companies. In this work, a new low cost (≈100€) packed-bed anaerobic bioreactor was developed, and all methodological details including open-source coding and 3D design files are shared with informative descriptions. Some preliminary tests were conducted to verify the developed bioreactor system's credibility in terms of leak-tightness, accurate gas monitoring, temperature controlling, and mass balance (COD-eq) coverage, which all have shown a very promising performance.•A consistent model bioreactor that will be called as “tetrapod” was developed for anaerobic treatment of challenging substrates such as pyrolytic liquids.•Coarse biochar grains were used as an organic packing material to stimulate the microbial bioconversion by increasing the active surface area for the attached-growth anaerobic mixed microbial culture (MMC).•An open-source Arduino based digital gasometer was developed for online monitoring of biogas change in the lab-scale system. Arduino was also used as a digital controller for maintaining pulse-mode liquid recirculation of the bioreactor.

Evaluation of hydraulic mixing performance in a full-scale anaerobic digester with an external liquid recirculation system using CFD and experimental validation

Anaerobic digestion (AD) has become an essential process for sludge treatment and its optimum performance is related to its mixing degree. In this study, a full-scale Anaerobic Digester (ADer) with an external recirculation mixing system was studied via single-phase 3D-CFD simulations to assess the influence of recirculation flow and a 3-blade propeller. The model was validated with inert tracer tests. The design and mixing parameters were studied to characterise the mixing efficiency in different scenarios. The design parameters were assessed first, but wide deviations from the recommended values were found. Local mixing parameters were found to be useful for defining the degree and type of mixing, and are highly recommended in the CFD studies of ADers. A second-order statistical moment was proposed as a global mixing parameter to describe geometrical and local mixing, and to state a reliable homogenisation time.

Effects of process liquid recirculation on material properties of hydrochar and corresponding adsorption of cationic dye

Hydrochar has been gaining tremendous interest as adsorbent materials, however, change in material properties due to process liquid recirculation needs to be explored, since commercial hydrothermal carbonization (HTC) is likely to recycle HTC process liquid in order to preheat the feedstock as well as minimize the wastewater accumulation. Therefore, this study focused on the process liquid recirculation up to 9 times and evaluated how surface functionalities as well as Rhodamine B adsorption capacities change with the number of recycle. HTC of corn stover was performed at a fixed temperature (230 °C) and a fixed residence time (30 mins) with 50 wt% process liquid recirculation. Results indicate that the mass yield for control was 60.6 ± 0.9 wt% which was increased to 66.9 ± 0.0 wt% with HTC process liquid recirculation. Meanwhile, ash was concentrating with HTC process liquid recirculation (from 10.3 ± 1.1% to 13.9 ± 0.3%). The Brunauer, Emmett, and Teller (BET) surface area and total pore volume for control were 6.7 ± 0.1 m2/g and 9.8 × 10−3 cm3/g, respectively, which were increased to 7.4 ± 0.0 m2/g (recycle-1) - 10.8 ± 0.1 m2/g (recycle-9) and 12.4 × 10−3 cm3/g (recycle-1) − 17.3 × 10−3 cm3/g (recycle-9). However, Boehm titration revealed that the recycle-3 retained higher oxygen-containing surface functional group density (231.2 ± 3.9 µmol/g) than control (161.9 ± 2.5 µmol/g) and recycle-9 (132.2 ± 1.5 µmol/g). These morphological characteristics also reflect on the Rhodamine B adsorption capacities, where maximum equilibrium adsorption capacity was found for recycle-3 (30.7 mg/g) followed by recycle-9 (23.6 mg/g) and control (21.4 mg/g) and the adsorption isotherms were following combined Langmuir-Freundlich isotherm.

Purity monitoring for ProtoDUNE-SP

The Deep Underground Neutrino Experiment is a next-generation long-baseline neutrino oscillation experiment based on liquid argon time projection chamber technology. DUNE-s single-phase prototype ProtoDUNE-SP at CERN finished its two-year Phase-1 running in July 2021, successfully collected test-beam and cosmic ray data. A key aspect of LArTPC calibration is the lifetime of drift electrons. A purity monitor is a miniature TPC measuring the lifetime of electrons generated from the photocathode via the photoelectric effect. It enables continuous monitoring of the detector status, especially when filling the cryostat and when liquid argon recirculation systems operate. The purity monitoring system in ProtoDUNE-SP Phase-1 monitored liquid argon purity throughout its entire lifetime. It is essential to the experiment’s successful commissioning, operation, and data taking. This poster discusses the design, implementation, and results of purity monitors and plans.

Direct visualisation of bubble-particle interactions in presence of cavitation bubbles in an ultrasonic flotation cell

In this study, the role of cavitation bubbles in the bubble-particle interaction dynamics was investigated using high-speed imaging. A lab-scale conceptual flotation cell was designed involving an ultrasonic field with adjustable power output and a continuous liquid recirculation stream. Coarse size range hydrophobic glass Ballotini particles were suspended in the cell by the combined action of a confined vortex and turbulence created by periodic ultrasonic pulses. Ultrasonic pulses in the liquid medium led to the generation of numerous fine size range cavitation bubbles which were observed to form particle clusters. These particle clusters were observed attaching to relatively larger size carrier bubbles forming stable bubble-particles aggregates, which subsequently floated. It was noted that a favourable state of bubble-particle interactions was achievable to produce such stable bubble-particles aggregates by suitably controlling the input power and period of the applied ultrasonic pulse.

Investigation on the elimination of geyser in a cryogenic pipe by a recirculation method

• A recirculation structure is designed to successfully eliminate the geyser in a cryogenic pipe. • The geyser-elimination physics and the mechanism of recirculation formation are revealed. • A sine-wave-like oscillation process is found to exist in the initial process of the recirculation. • An improved approach is proposed to enhance the recirculation ability and reduce the oscillation amplitude by as much as 55%. A geyser elimination approach using recirculation methods is investigated in the present work, which is essential to guarantee the integrity of facilities. Both experimental and numerical methods are adopted in the investigation using LN 2 as working fluids. A recirculation pipe is designed to eliminate geysers in a cryogenic vertical pipe. The performance of the recirculation method is comprehensively explored involving its formation mechanism and oscillation physics. A deeper understanding of the liquid recirculation is achieved. The physics of the ambiguous oscillation phenomenon is revealed. It is concluded that by breaking down the energy storage pattern inside the cryogenic pipe, geysers can be perfectly eliminated by the recirculation method. Liquid recirculation in the pipeline is found to be initiated firstly by the liquid density difference between the vertical pipe and recirculation pipe and dominated by liquid flashing in the recirculation pipe. A negative feedback connection between void fraction and mass flow rate is the reason for the oscillation in its initial process. Besides, an effective method is proposed to enhance the recirculation ability as well as to attenuate the unstable oscillation by imposing a gradually increased heat flux. It is found that the oscillation amplitude could be reduced by as much as 55%.

Effect of liquid recirculation flow on sonochemical oxidation activity in a 28 kHz sonoreactor

Sonochemical oxidation activity may be significantly enhanced by optimizing the geometric factors of a sonoreactor and implementing additional physical actions, such as mechanical mixing and gas sparging. This study investigates the effects of liquid recirculation flow on sonochemical oxidation reactions. This was carried out through experimental testing with a 28 kHz bath-type sonoreactor under various liquid heights and flow rates, ranging from 1λ to 4.0λ and 1.5–6.0 L/min, respectively. The potassium iodide (KI) dosimetry and sonochemiluminescence methods were used in the experiment. With an increase in the liquid height/volume, the pseudo zero-order kinetic constant and the mass of triiodide (I3−) ions fluctuated. The optimal liquid height was 2.0λ, 2.5λ, and 3.0λ, based on the appropriate formation of a cavitation active zone in the reactor. The introduction of a liquid recirculation flow led to a large reduction in sonochemical activity due to the shrinkage of the cavitation active zone. However, the sonochemical activity increased at higher flow rates through the capture of ultrasonic energy at the bottom zone. This increase was attributed to the formation of a strong and expanded active zone limited to the reactor bottom to the height of the recirculation flow. The results demonstrate that applying a high rate liquid flow adjacent to the transducer module may be beneficial for enhanced sonochemical activity.

A pressurized hydrogenotrophic denitrification reactor system for removal of nitrates at high concentrations

A pressurized hydrogenotrophic denitrification reactor to remove nitrate from groundwater was recently presented. To enable treatment of nitrate-concentrated brines, we demonstrate here the removal of high nitrate concentrations without nitrogen gas accumulation in the reactor’s headspace over time. In order to eliminate nitrogen gas build up in the pressurized reactor’s headspace, the main unsaturated flow pressurized reactor was connected to an external degassing unit via liquid recirculation. Pressurized liquid from the main reactor was intermittently discharged to the degassing unit where dissolved N2 gas effervesced and escaped to the atmosphere. The degassed effluent water was continuously recirculated back to the main reactor to maintain steady-state nitrogen and hydrogen gas partial pressures. Denitrifying rates of up to 6 g N/L-reactor/d were achieved in the reactor system under freshwater and brine conditions. Nitrate removal was over 97 %, with nitrate effluent concentrations lower than 10 mg N/L for influent concentrations of about 400 mg N/L. The reactor system maintained steady-state nitrogen and hydrogen gas partial pressures using low external recirculation rates at low reactor total pressures (< 5 atm). At high denitrification rates, dissolved nitrogen concentrations in the reactor were super saturated (125–150%) while on the other hand, dissolved hydrogen was undersaturated (30–60%) due to intense bacteria uptake from the water. These dynamic conditions reduced the external recirculation flow requirement for degassing and resulted in much better hydrogen utilization than expected (greater than 93 %).

Immersion Effect on the Anaerobic Degradation and the Rheological Properties of Straw-Cattle Manure (SCM) at 440 L Batch Pilot Scale Reactor

The effects of the solid bed immersion in the anaerobic digestion of straw-cattle manure (SCM) were studied using two batch reactors of 2 m of solid height and 440 L of total volume. The reactors were operated in parallel with total (R1_100%) and partial solid height immersion (R2_74%) and no liquid recirculation. Recovered methane yield was 31.1% higher in R1 than in R2, 33.7 and 23 Nm3 CH4 t−1SCM respectively. The volatile solids (VS) and fiber degradation was studied in layers distributed each 0.5 m of the initial solid bed height profile; VS removal was measured at 16% at 2 m height in contrast to 39.9% at the reactor bottom. VS removal was related with hemicellulose and cellulose biodegradation, maximal hemicellulose and cellulose degradation in the studied layers were 68.2 and 49.5% respectively. Physical and rheological changes of the solid phase were measured between the SCM and the digestate. Macropores volume was reduced from 30.4% to values between 0.82 and 5.57%, this decrease was related to the water content and the fiber degradation state in each layer. Similarly, yield stress values obtained with the slump test depends on total solids content and fiber degradation state, yield stress values ranged from 1.41 and 2.23 kPa for raw and digested SCM. Moreover, values of cohesion and the friction angle were between 1.5 and 2.8 kPa and 15.6 and 47.7° respectively. Physical and rheological properties of digested SCM depends on the solid degradation state and the water availability through the material’s height profile.

Epoxidation of trans-stilbene in a microfluidic plasma reactor

Novel organic synthesis routes that circumvent the need for a catalyst and reduce unwanted by-products are highly sought by industry. A novel microfluidic plasma reactor that generates a dielectric barrier discharge (DBD) plasma in the vicinity of the gas-liquid interface and facilitate a chemical reaction at the interface of microbubbles has been used for trans-stilbene epoxidation. Three different operating strategies were implemented to optimise the transfer of species selectivity: single pass, multi-pass and continuous recirculation. The effect of initial trans-stilbene concentration, oxygen content in the feed gas mixture and reaction time on the epoxide formation was studied to optimise the chemical reaction. The optimum operating conditions were found to be short bubble-liquid contact times (~2 s) with frequent exposure to freshly generated microbubbles containing reactive species by continuous liquid recirculation, and under these conditions the overall epoxide yield was ~94% with an overall epoxide selectivity of 10:1.

CFD analysis of blade coating from a reservoir onto a horizontal substrate using a homogeneous two‐phase model

AbstractA two‐phase numerical analysis is performed of two‐dimensional, laminar, liquid flow from a reservoir onto a horizontal, rigid, moving substrate. The homogeneous two‐phase model in commercial CFD code CFX is used to model the liquid plus a region containing both liquid and air near the phase interface and downstream of the blade region. Slow convergence due to surface tension modelling required initialization from intermediate results omitting surface tension. Comparisons made with two previous numerical analyses demonstrate the performance of this approach. For a particular upstream reservoir geometry, the effects of changing the substrate speed and the liquid properties from a Newtonian fluid to a Carreau‐Yasuda non‐Newtonian fluid on the pressure field and the downstream film height are studied. The details of the liquid recirculation in the reservoir and the overall pressure distribution are discussed. New results are presented for the meniscus position and contact angle, which are fully predicted by the two‐phase approach. The meniscus position was strongly influenced by substrate speed and liquid properties, whereas the contact angle did not change significantly with changes in substrate speed and dynamic viscosity for the Newtonian fluid nor with changes in the functional parameters for the non‐Newtonian fluid.

Development of ejector performance map for predicting fixed-geometry two-phase ejector performance for wide range of operating conditions

Recent studies have expanded the operational envelope of ejector by utilizing both pressure recovery and liquid recirculation mechanisms. This necessitates finding a consistent representation of fixed-geometry ejector performance data using only operational variables for predicting ejector performance. Taking inspiration from the compressor performance maps, the study explores the appropriate motive, performance, and characterization variable for representing ejector performance while conducting experiments on a transcritical CO2 ejector system. The existing ejector efficiency is revisited by conducting ejector power analysis. Two possible motive variables, along with a new ejector efficiency is introduced. The trends of existing and new ejector efficiencies are studied in controlled motive variable experiments while changing suction flow conditions. The ejector performance can be represented with the help of a single curve, termed as ejector performance map, using the new ejector efficiency as performance variable and the volumetric entrainment ratio as characterization variable. The prediction accuracy for both the single-phase and the two-phase suction inlet conditions are evaluated while developing performance map using only the single-phase suction inlet data points. The methodology can predict 89.3% of the data within 20% accuracy. The applicability of the methodology is assessed for other datasets involving different refrigerants, cycle architectures, and applications. The proposed ejector performance map can be utilized in numerical ejector system analysis for investigating new cycle architectures based on ejector experimental data, thus, improving system model fidelity. It can also help system design engineers in making ejector system selection decisions after thorough system performance analysis.

Downflow bubble column electrochemical reactor (DBCER): In-situ production of H2O2 and O3 to conduct electroperoxone process

In the context of water remediation, advanced oxidation processes have been proven to be an effective solution. In most of the cases, however, the reaction systems are usually highly expensive, because of the addition of chemical substances or energy consumption. This usually constrains their application at an industrial scale. This has motivated several researchers to develop technologies able not only to intensify the processes but able also to increase the sustainability of the whole process. In this context, this work aimed to assess a relatively novel technology, a Downflow Bubble Column Electrochemical Reactor (DBCER), in the mineralization of a rather typical organic pollutant, phenol.The studied variables were: current density (20–60 mA/cm2), electrolyte concentration (0.025–0.1 M), liquid recirculation rate (4.7 and 6 L/min) and pH (3 and 7). The response variables were total organic carbon (TOC), phenol and by-products concentration, oxidant species concentration (O2, H2O2 and O3).It was demonstrated that the DBCER with BDD electrodes allows not only the production of •OH, but also the in situ production of O2, H2O2 and O3 (without the addition of any gas) and more importantly their utilization to conduct an electro-peroxone process.The highest mineralization degree was around 75% under pH 3, 60 mA/cm2, 4.7 L/min and an electrolyte concentration of 0.05 M. Under these conditions, it was figured out that the phenol oxidation occurs mainly by ozone attack and the main remaining compound was oxalic acid. Although at pH 7 the mineralization degree was lower than at pH 3, it was demonstrated by a biotoxicity study on Cyprinus carpio that the original toxicity was significantly decreased.

Food waste treatment through anaerobic co-digestion: Effects of mixing intensity on the thermohydraulic performance and methane production of a liquid recirculation digester

This paper investigates the thermohydraulic performance and methane production of a Liquid recirculation Mesophilic Digester with Submerged solid waste (LMDS) under different mixing strategies. A combination of food waste and cow dung was used as a substrate in the co-digestion experiments. After 24 days of digestion, we reported a higher specific methane yield (301.84 Nm3 CH4 tvs−1) when gentle mixing was adopted, compared to intense mixing (221.32 Nm3 CH4 tvs−1). As for the unmixed co-digestion, it produced the lowest methane yield (112.88 Nm3 CH4 tvs−1). Furthermore, the thermal study showed that the gentle mixing caused less heat loss from the LMDS (20.90 kWh) compared to the intense mixing (40.21 kWh). Moreover, CFD results showed that intense mixing caused a higher pumping energy consumption (through increasing pressure drop) without inducing a significant improvement in the mixing inside the LMDS. Additionally, the liquid flow fields, force distribution, and percolation of the submerged waste were qualitatively evaluated for the considered mixing strategies. Based on these findings, we presented several recommendations aimed at reducing thermal and hydraulic energy losses while guaranteeing optimal methane production in LMDS digesters.

Effect of superficial gas velocity and ratio of bed volume to reactor volume of inverse fluidized bed biofilm reactor on the removal of ammonia-nitrogen from wastewater

Hydrodynamic parameters of an inverse fluidized bed biofilm reactor (IFBBR) have been studied using spherical polypropylene (PP) particles having average diameter and density of 5.63 mm and 920 kg/m3 respectively. Gas-phase holdup (g) was analyzed for various ratios of settled bed volume to reactor volume (Vb/Vr) and superficial gas velocities (Ug) with a liquid recirculation velocity (Ul) of 0.0021 m/s. The g values were found to increase with Vb/Vr ratios up to a certain limit and then decrease with further increase of Vb/Vr ratios. The effect of Ug, Vb/Vr ratios, and initial concentration of ammonia-nitrogen (NH4 +-N) on the removal of NH4 +-N from synthetic wastewater were studied. The optimal values of Vb/Vr ratio and Ug were found to be 0.380 and 0.0085 m/s respectively for all initial NH4 +-N concentrations. Complete removal of NH4 +-N was achieved in 8 to 44 hours for different initial NH4 +-N concentrations. It was also observed that with the increase in initial NH4 +-N concentrations, the nitrification decreases.

Intensification of methane production from waste frying oil in a biogas-lift bioreactor

Biogas upgrading from anaerobic digestion of waste frying oils (WFO) was accessed in this study. For that, two bioreactors (Rb-biogas-lift bioreactor with gas and liquid recirculation, Rc-control reactor with liquid recirculation) were fed three times per week, with a mixture of WFO, glycerol and volatile fatty acids (VFA). Rb produced 1.4 times more biogas with higher methane content (79%) than Rc (67%). Higher relative abundance of hydrogenotrophic methanogens (34%–39%) was observed in Rb, when compared to Rc (16%–21%). The relative abundance of Sprochaetia class, which includes some homoacetogens/syntrophic acetate oxidizing genera, was also higher in Rb. This work shows that biogas recirculation applied in the biogas-lift bioreactor facilitated WFO degradation, most probably due to the selective enrichment of hydrogenotrophic methanogens. Recirculation of CO2 present in the biogas and reverse homoacetogenesis (i.e. syntrophic acetate oxidation) seem to be the main factors involved in the stimulation of hydrogenotrophic methanogenesis.

Strategies for decreasing the O2 content in the upgraded biogas purified via microalgae-based technology

Microalgae-bacteria consortium based technology using a High Rate Algal Pond (HRAP) interconnected to an Absorption Bubble Column (ABC) has emerged as an environmentally friendly promising option to upgrade biogas. However, the oxygenic photosynthesis of microalgae induces oxygen contamination in upgraded biogas, which could limit its further applications. Several strategies were proposed to favor the oxygen desorption and oxygen uptake in parts and accessories of the upgrading system. The effect of the volumetric ratio liquid recirculation rate/biogas rate (L/G = 5.0, 1.0 y 0.5) was evaluated in conjunction with the application of a novel accessory called Open Trickling Column (OTC). The O2 content in upgraded biogas was around 2.1%v, attaining CO2 removal efficiencies around 90%, at L/G ratio of 1.0 during diurnal and nocturnal periods. The inclusion of an OTC at the previous L/G, enhanced 54% the removal of O2 by stripping and uptake compared with the basal condition. Mass balances of H2S and methane showed that L/G > 1.0 favored the complete oxidation of H2S but promoted the loss of methane in dissolved form. Additionally the effect of increasing linear velocity of liquid broth in the lab-scale HRAP (from 15 cm s−1 to 20 cm s−1) showed to improve the O2 stripping with a consequential increase of biomass concentration under steady-state (from 0.7 to 1.4 g L−1) besides achieving O2 content in the upgraded biogas around 1.5%v.

General correlations for gas-liquid mass transfer in laminar slug flow

Mass transfer is an important phenomenon that can be highly leveraged through hydrodynamics. Slug flow is frequently associated with the enhancement in transport phenomena, and so, mass transfer from a single Taylor bubble to the flowing liquid is numerically studied. This CFD work validates a recently published study using pure oxygen bubbles and expands it to pure bubbles of other compounds (different Schmidt numbers) for different Reynolds numbers. The simulations were performed with VOF + PLIC to track the gas-liquid interface. Flow and mass fields were solved simultaneously based on the “species transport” model and imposing a constant concentration at the interface.The results confirmed that the solute around the bubble remains in a thin layer along the nose and film regions. At the bubble rear, it accumulates in the wake due to the closed liquid recirculation characteristic of laminar flow. Local and global mass transfer coefficients were also determined for these characteristic hydrodynamic regions. The data showed a higher impact of the nose and film on the overall mass transfer, so a general correlation between Sherwood, Reynolds and Schmidt numbers was derived for each of these regions. The developed correlations offer a good prediction of the mass transfer (deviations around 10–20%).

A systematic assessment of aeration rate effect on aerobic degradation of municipal solid waste based on leachate chemical oxygen demand removal

Aeration is one mainstream technique to accelerate municipal solid waste (MSW) degradation in landfills. The determination of an appropriate aeration rate is critical to the design and operation of a landfill aeration system. In this study, we analyze 132 waste degradation tests reported in forty one studies in the literature. We use L min−1 kg−1 dry organic matter (L min−1 kg−1 DOM) as the uniform unit to quantify the aeration rates in all tests. The first order rate coefficient for chemical oxygen demand (COD) removal in leachate (kCOD) is selected as the parameter to characterize MSW degradation process. We further divide aerobic tests into five aerobic groups base on the respective aeration rates, i.e., <0.02, 0.02–0.1, 0.1–0.3, 0.3–1, and >1 L min−1 kg−1 DOM. With an increase in the aeration rate, the kCOD increases first and then decreases. The aeration rate between 0.1 and 0.3 L min−1 kg−1 DOM has the best enhancement on the kCOD. The kCOD values are not much higher than the anaerobic and semi-aerobic tests when the aeration rates are <0.1 L min−1 kg−1 DOM, because such aeration rates may be lower than the actual oxygen consumption rates. An aeration rate >0.3 L min−1 kg−1 DOM reduces the kCOD likely due to excess water evaporation and ventilation cooling. Among the analyzed results, the aeration rate is the most related to the kCOD in principal component analysis than the other factors, including liquid recirculation and addition, waste total density, waste degradation level, and waste initial temperature.