Back Mixing Research Articles

Computational Fluid Dynamics Simulation of Hydrodynamics in a Two-Stage Internal Loop Airlift Reactor with Contraction-Expansion Guide Vane.

Global circulation and liquid back mixing adversely affect the continuous production of a multistage internal airlift loop reactor. A contraction-expansion guide vane (CEGV) is proposed and combined with a two-stage internal loop airlift reactor (TSILALR) to suppress the liquid back mixing between stages. A computational fluid dynamics (CFD) simulation is conducted to evaluate the performance of the CEGV in the TSILALR. The bubble size distribution and turbulent flow properties in the TSILALR are considered in the CFD simulation by using the population balance model and RNG k-ε turbulence model. The CFD model is validated against the experimental results. The deviations in the gas holdup and mean bubble diameter between the simulation and experimental results are less than 8% and 6%, respectively. The streamlines, flow pattern, bubble size distribution, and axial liquid velocity in the TSILALRs with and without the CEGV at superficial velocities of 0.04 and 0.08 m/s are obtained by CFD simulation. It has been shown that the CEGV generated local circulation flows at each stage instead of a global circulation flow in the TSILALR. The average global gas holdup in the TSILALR with a CEGV increased up to 1.98 times. The global gas holdup increased from 0.045 to 0.101 and the average axial velocity in the riser decreased from 0.314 to 0.241 m/s when the width of the CEGV increased from 50 to 75 mm at the superficial gas velocity of 0.08 m/s.

Hydrodynamics and mass transfer in Kühni extraction columns

This work provides a review of hydrodynamic characteristics and mass transfer in the K?hni extraction columns. The experiments, as reported in the literature, were performed in the presence and absence of mass transfer. The results showed that the Sauter mean drop diameter was strongly affected by the rotor speed and interfacial tension, whereas the effects of the dispersed and continuous velocities were negligible. Empirical correlations for the Sauter mean drop diameter, taken from the literature, were discussed. It was experimentally determined that the dispersed-phase holdup depended to a great extent on the rotor speed, mass transfer direction between the phases, physical characteristics of fluids in the liquid-liquid system, and the dispersed-phase flowrate whereas it increased with the increase in mixing in the two-phase system and the ratio of phase flowrates. On the other hand, it has been shown that the mass transfer rate increases with increasing the level of back mixing. It was found that the mass transfer coefficient depends on the rotor speed and the direction of mass transfer between the phases. At the same time, it has been shown that the mass transfer coefficient depends relatively little on the phase flowrates. An empirical correlation was proposed for prediction of the overall mass transfer coefficient based on dimensionless numbers. Also, novel empirical correlations for prediction of the Sherwood number in the continuous phase were presented based on the dispersed-phase holdup, Reynolds number, and mass transfer direction between the phases. Empirical correlations based on dimensionless numbers can be considered as a useful tool for the design of the K?hni columns.

Discussion on product oil on-line back mixing technology based on a product oil pipeline

The mixed oil treatment of Product oil is an important task of pipeline enterprises. The paperconcluded that the on-line back mixing technology was feasible, compared treatment methods from the perspective of safety and economic benefits. TheProduct oil pipeline is taken as the research object in this paper, studied the concentration distribution of mixed Product oil and amount of mixed Product oilunder operation and shutdown condition, and brief studied Product oil On-line back mixing technology. Combined with the demand and pipeline length, the mixing test is determined at CD station and CQ station. The operation process as follow: First, Product oilsamples areextracted from the pipeline. Second, the index parameters are detected after the oil samples are mixed with different proportions, and obtaining the blending potential under the theoretical situation, the result show that the mixing potential of gasoline is greater than that of diesel. Third, the field test do with arbitrarily mixed oil download scheme, which study two mixed oil surfaces. The required fund of dealing with the mixed oil is obtained combined with the download amount of mixed oil.Finally, comparing with the funds of the previous contaminated treatment methods, it is concluded that the On-line Back Mixing technology has great advantages in both safety and economic benefits.

Modelling of mass transfer in separation of hydrocarbon mixtures and modernization of industrial columns

A system of differential equations of mass transfer with an interfacial mass source for multicomponent distillation in a column with random and structured packings in the film mode is presented. The interfacial source is associated with the matrix of volumetric mass transfer coefficients as well as a driving force for mass transfer. The diffusion fluxes of mass of components are written in the form of Fick’s law with elements of the matrix of coefficients of molecular diffusion in the gas (vapor) phase. To simplify the mathematical model, one dimensional equations for the diffusion model of the structure of flows in the gas and liquid phases with the interfacial sources and coefficients of back (longitudinal) mixing of gas and liquid flows are used. The Dankwerts boundary conditions are applied to the system of equations. To calculate the back mixing coefficients, experimental data obtained for the selected types of packings are used. When modelling mass transfer in hydrocarbon mixtures consisting of several dozen components, the system of equations for the diffusion models is solved for the key components and fractions of the oil and gas condensate (OGC) mixture stabilization column at an industrial enterprise. A brief description of the technological scheme is given. Scientific and technical solutions are developed to replace obsolete sieve trays in the stabilizer column with a new structured corrugated roll packing having a rough surface. It is found that a packing height of eight meters is sufficient for separating OGC in a given composition range. As a result of implementation of the new packing, the content of isopentane in the stable condensate decreases and its concentration in the broad fraction of light hydrocarbons increases from 9 wt.% to 11.7 wt.%. The implementation of packing in the stabilizer column meets the requirements of the technical specifications.

Experimental and Numerical Study of Gas-Liquid Flow in a Sectionalized External-Loop Airlift Reactor

The external loop airlift reactor (EL-ALR) is widely used for gas-liquid reactions. It’s advantage of good heat and mass transfer rates compared to conventional bubble column reactors. In the case of fermentation application where a medium is highly viscous and coalescing in nature, internal in riser helps in the improvement of the interfacial area as well as in the reduction of liquid- phase back mixing. The computational fluid dynamic (CFD) as a tool is used to design and scale-up of sectionalized external loop airlift reactor. The present work deals with computational fluid dynamics (CFD) techniques and experimental measurement of a gas hold-up, liquid circulation velocity, liquid axial velocity, Sauter mean bubble diameter over a broad range of superficial gas velocity 0.0024 ≤ UG ≤ 0.0168 m·s−1. The correlation has been made for bubble size distribution with specific power consumption for different plate configurations. The effects of an internal on different mass transfer models have been completed to assess their suitability. The predicted local mass transfer coefficient has been found higher in the sectionalized external loop airlift reactor than the conventional EL-ALR.

Solid Circulating Velocity Measurement in a Liquid–Solid Micro-Circulating Fluidised Bed

Liquid–solid circulating fluidised beds (CFB) possess many qualities which makes them useful for industrial operations where particle–liquid contact is vital, e.g., improved heat transfer performance, and consequent uniform temperature, limited back mixing, exceptional solid–liquid contact. Despite this, circulating fluidised beds have seen no application in the micro-technology context. Liquid–solid micro circulating fluidised bed (µCFBs), which basically involves micro-particles fluidisation in fluidised beds within the bed of cross-section or inner diameter at the millimetre scale, could find potential applications in the area of micro-process and microfluidics technology. From an engineering standpoint, it is vital to know the solid circulating velocity, since that dictates the bed capability and operability as processing equipment. Albeit there are several studies on solid circulating velocity measurement in CFBs, this article is introducing the first experimental study on solid circulating velocity measurement in a CFB at micro-scale. The experimental studies were done in a novel micro-CFB which was fabricated by micro milling machining 1 mm2 cross-section channels in Perspex and in a 4 mm2 cross-section micro-CFB made by additive manufacturing technology. Soda-lime glass and polymethyl methacrylate (PMMA) micro-particles were employed as solid materials and tap water as the liquid medium. The digital particle image velocimetry (PIV) method was used as a measurement technique to determine the particle velocity in the micro-CFB system and validated by the valve accumulation technique using a novel magnetic micro-valve. The measured critical transition velocity, Ucr, is comparable to the particle terminal velocity, i.e., the normalised transition velocity is approximately 1 in line with macroscopic systems results and our previous study using simple visual observation. As in macroscopic CFB systems, Ucr decreased with solid inventory (1–9%) and finally becomes stable when the solid inventory is high enough (10–25%) and it increases with a reduction in particle size and density.

Evaluating the potential of adjusting axial back mixing in continuous manufacturing of solid oral dosage forms

Evaluating the potential of adjusting axial back mixing in continuous manufacturing of solid oral dosage forms

Upgrading Siberian (Russia) crude oil by hydrodesulfurization in a slurry reactor: A kinetic study

Hydrodesulfurization (HDS) of sour crude oil is an effective way to address the corrosion problems in refineries, and is an economic way to process sour crude oil in an existing refinery built for sweet oil. In the current study, the HDS of Siberian crude oil was carried out in a slurry reactor. The Co–Mo, Ni–Mo, and Ni–W catalysts supported on γ-Al 2 O 3 were compared at the temperature of 340 °C and the pressure of 4.5 MPa. The HDS activity follows the order of Co–Mo > Ni–Mo > Ni–W at a high concentration of H 2 S, and the difference between Co–Mo and Ni–Mo becomes insignificant at a low concentration of H 2 S. The influence of reaction temperature 320–360 °C and reaction pressure 3–5.5 MPa was investigated, and both play a positive role in the HDS reaction. A kinetic model over Ni–Mo/Al 2 O 3 in the slurry reactor was established. The activation energy is estimated as 60.34 kJ·mol −1 ; the orders of sulfur components and hydrogen partial pressure are 1.43 and 1.30, respectively. The kinetic parameters are compared with those in a trickle-bed reactor, implying that the mass transfer is greatly enhanced in the slurry reactor. The back mixing effect is present in the slurry reactor and can be reduced by a multi-stage design, which would lead to higher reactor efficiency in industrial application. • Sweetening Siberian crude oil (HDS) meets the requirement of local refinery in Daqing. • A kinetic model over Ni–Mo/Al 2 O 3 in the slurry reactor is established. • The orders of sulfur component and hydrogen are 1.43 and 1.30, respectively. • The activation energy is 60.34 kJ ∙ mol −1 . • Mass and heat transfers are enhanced in the slurry reactor.

Optimisation of interaction parameters for CFD modelling of multiphase flow using NR method

Accurate modelling of fluid and particle flow is essential for reliable predictions of gas–solid reactions in the multiphase reactor. Due to the promising features such as short contact time, less back mixing of solids and low residence time, downer column has been used in industries than conventional riser configuration. However, the effective simulation and validation of the hydrodynamic characteristics have been scarce. A RANS model with an appropriate Eulerian-Eulerian approach poses a challenging task of choosing the hydrodynamic simulation parameters, such as i) particle-particle restitution coefficient, ii) particle-wall restitution coefficient and iii) specularity coefficient. The objective of the present study was to optimise these model parameters to obtain close validation with experimental data so that accurate downer hydrodynamics can be predicted. The error obtained from validating the experimental data with the CFD simulation output is our function which needs to be minimised. Newton Raphson multi-parameter root finding method was used for this purpose. The Jacobian evaluation required multiple CFD simulations at each iteration. A 61% reduction in error was achieved after 3 iterations. Thus, the optimised model parameters of hydrodynamics used for further modelling study in predicting hydrodynamic characteristics of a 200 mm diameter 8 m column. Highlights Validation of hydrodynamics of multiphase flow in downer employing Eulerian-Eulerian Approach Optimisation of hydrodynamic interactive model parameters viz., Restitution and Specularity coefficient coupled with Newton Raphson multi-parameter root finding and Jacobian Method. Agreement between experimental and simulation with optimised modelling parameters

Non-ideal flow model for a tubular reactor and a stirred tank reactor in-series, using stimulus-response techniques

This work presents an experimental study on a laboratory scale about the hydrodynamic behavior of fluid flow in a tubular reactor and a stirred tank reactor in-series and varying their arrangement. The experiments were carried out using stimulus-response techniques with a tracer solution of sodium chloride in unit pulse form. The experimental results allowed obtaining the Residence Time Distribution curves Rθ vs θ for different reactor arrangements. The results obtained are similar to those reported by Levenspiel for a battery in-series with a plug flow reactor and a perfectly stirred reactor. The difference found with the studied system is that these show dead water in both equipment. The arrangement of the reactors does not modify the graph of Residence Times Distribution obtained, similar to the ideal situation presented in the literature consulted. As a result of the experimental study and the adjustment of the data, a non-ideal flow model was obtained for different kinds of reactors with dead water, connected in-series and with different arrangements. The simulation of the system for different degrees of back mixing in the system reflects a behavior similar to that of series of reactors of this type, but with ideal behavior.

A Mathematical Model and Design Calculation of a Thermal Deaerator with a Bubbling Storage Tank

A mathematical model of mass transfer in an atmospheric bubbling deaerator storage tank at thermal power stations (TPS) to calculate the removal efficiency of dissolved gases from water is considered. For mathematical simulation and calculation of the deaerator efficiency, a system of differential equations of mass and heat transfer is written in a local form for the liquid phase of the layer with volume interphase sources of mass and heat. Mass transfer of the dissolved gases from water to the vapor phase is taken into account using volume interphase sources and the balance equations. Parameters of the interphase sources and correlations for calculating the eddy viscosity of the liquid phase depending on bubbling conditions are given. Results from the system of equations' numerical solution are presented and compared with the available experimental data on the final concentration of dissolved oxygen at the deaerator outlet. Curves of the efficiency of dissolved oxygen removal depending on the operating conditions and design characteristics of the bubbling process are presented, and scientific-and-engineering solutions have been developed to upgrade the bubbling storage tank. It has been found that an increase in gas velocity sharply raises the deaeration efficiency to a certain value (depending on the specified initial parameters), and the efficiency growth is then retarded. Similarly, increasing the length of a bubbling device improves the efficiency in removal of dissolved oxygen from water. One of the causes of a decrease in the water deaeration efficiency is likely to be back mixing of the flow. Upgrading of the deaerator tank is in the installation of perforated baffles across the flow to considerably reduce back mixing of the liquid phase. This increases the driving force of mass transfer, and its efficiency improves by five to 20% depending on the steam velocity. The presented mathematical model can be generalized to a wide range of bubbling apparatuses with a high two-phase layer.

Axial dispersion and CFD models for the extraction of levulinic acid from dilute aqueous solution in a Kühni column with 2-methyltetrahydrofuran solvent

1D axial dispersion and 3D CFD models for the extraction of levulinic acid from dilute aqueous solution by applying 2-methyltetrahydrofuran as a solvent are presented. The models are validated by comparison with the measured levulinic acid concentration profile data obtained in a bench-scale Kühni column. The 1D model contains NRTL parameters for the system levulinic acid-water-2MTHF. Correlations for drop size and hold-up for Kühni columns were taken from literature. The values for overall mass transfer coefficient ranged from 1.4E-5 to 2.2E-5 ms−1, and increased as a function of the rotor speed. The fitting of the column performance resulted in a very good prediction of the solute concentration profiles in the extraction column, and the average absolute value of relative error for the 1D model was 23%. CFD model visualized the column performance at the column height of 150.5–160 cm giving valuable information on back mixing, phase velocities, dispersed phase volume fraction, and mass transfer. Dispersed phase volume fraction and mass transfer contours revealed, that the mass transfer rate (app. 0.25 g L−1s−1) is at its highest just below the rotor, and that there are blind spots in the compartments close to the extractor and just above each down comer. Values for the dispersed phase volume fraction are highest in the same area where the mass transfer reaches the highest values. The highest slip velocity values (app. 0.03 m−1) are located in the tip of each compartment partition plates.General correlations, such as hold-up and drop size correlations, can successfully be applied in levulinic acid-water-2MTHF system reported in this work. The 1D axial dispersion model proved to be valuable tool for scale-up purposes, and CFD model, despite the long time needed for each simulation, gave useful information for the design purposes.

Impact of Dense Internals on Fluid Dynamic Parameters in Bubble Column

AbstractIndustrial gas-liquid processes such as oxidation, hydrogenation, Fischer-Trospch synthesis, liquid-phase methanol synthesis, and nuclear fission are exothermic in nature; the reactor of choice for such processes is, therefore, a bubble column equipped with heat exchanging internals. In addition to maintaining the desired process temperature, the heat exchanging vertical tube internals are used to control flow structures and liquid back mixing. The present work reports the experimentally measured gas hold-up, mean liquid velocity and liquid phase turbulent kinetic energy, using the Radioactive Particle Tracking (RPT) technique, in a 120 mm diameter bubble column equipped with dense vertical tube internals covering 23 % of the total cross-sectional area of the column. The effect of superficial gas velocity (44–265 mm/s) on gas hold-up, mean liquid velocity and turbulent kinetic energy is presented and discussed. It has been inferred from the experimental results that the vertical tube internal located at the center of the column plays a vital role in affecting the hydrodynamics when compared to the conventional internal configurations reported in the literature. For the chosen dense internal configuration, the cross-sectional distribution of the gas holdup, mean liquid velocity and turbulent kinetic energy show asymmetry for all the superficial gas velocities investigated. The overall gas holdup and the liquid turbulence increases with an increase in the superficial gas velocity. The strong liquid circulation velocities have been seen upon the insertion of the dense internals.

Modeling the Separation of Microorganisms in Bioprocesses by Flotation

Bioprocesses for the production of renewable energies and materials lack efficient separation processes for the utilized microorganisms such as algae and yeasts. Dissolved air flotation (DAF) and microflotation are promising approaches to overcome this problem. The efficiency of these processes depends on the ability of microorganisms to aggregate with microbubbles in the flotation tank. In this study, different new or adapted aggregation models for microbubbles and microorganisms are compared and investigated for their range of suitability to predict the separation efficiency of microorganisms from fermentation broths. The complexity of the heteroaggregation models range from an algebraic model to a 2D population balance model (PBM) including the formation of clusters containing several bubbles and microorganisms. The effect of bubble and cell size distributions on the flotation efficiency is considered by applying PBMs, as well. To determine the sensitivity of the results on the model assumptions, the modeling approaches are compared, and suggestions for their range of applicability are given. Evaluating the computational fluid dynamics (CFD) of a dissolved air flotation (DAF) system shows the heterogeneity of the fluid dynamics in the flotation tank. Since analysis of the streamlines of the tank show negligible back mixing, the proposed aggregation models are coupled to the CFD data by applying a Lagrangian approach.

Pressure equalization approach for flow uniformity in microreactor with parallel channels

Numbering-up using parallel channels helps to achieve higher processing capacity while retaining the advantages of microreactors. However it may also lead to non-uniformity in flow across all the channels. Here we report the CFD simulations and experimental observations on the extent of uniformity in the flow from an assembly of several parallel channels. In the conventional parallel channel geometry, smaller channel size (<300 m) as well as higher fluid viscosity is seen to give better flow uniformity. Inequality in pressure distribution at the inlet of a large number of parallel channels is overcome by having two ‘pressure equalization slots’ at an equal distance from inlet and outlets that open in the respective manifolds. Having such an arrangement helped to reduce the standard deviation in the flow by almost 90% when compared to the conventional geometry. However the modification was seen to increase the extent of back mixing to some extent.

Separation and energy efficiency of packed apparatuses for purifying gases from aerosols

A mathematical model of the turbulent transport of liquid aerosol particles has been considered using an approach in which the transport of a finely dispersed phase is conditionally described as a diffusion process with mass-transfer theory equations. The main parameter in this formulation is the coefficient of the velocity of particle transport (turbulent migration), which is an analogue of the mass transfer coefficient. The use of flow pattern models with a bulk source of the mass of sedimenting particles in a random packed bed and wire-mesh demisters is demonstrated. The equations for calculating the separation zone length and the process efficiency have been derived with allowance for the inlet section and the back mixing of the gas flow. Some calculation results are given for the aerosol separation efficiency and the energy coefficient compared with experimental data.

Modelling centrifugal partition chromatography separation behavior to characterize influencing hydrodynamic effects on separation efficiency

In addition to the selection or adjustment of phase systems to gain a suitable partition coefficient for the target molecule, the separation efficiency in centrifugal partition chromatography (CPC) is strongly influenced by the hydrodynamic interactions of the mobile and the stationary phase in the chambers. Thus, the hydrodynamic interactions must be investigated and understood in order to enhance a CPC separation run. Different hydrodynamic effects like mass transfer, back mixing and the non-ideal behavior of stationary phase, which cannot be determined directly, are known, but quantifying these effects and their influence on separation performance is barely achieved. In order to understand their influence, a physically detailed mathematical model of a CPC chamber was developed. The model includes a parameter representing the hydrodynamic effects mentioned above and is able to determine the parameters significance by fitting them to separation experiment data. The acquired knowledge is used to correlate the effects of the hydrodynamic influences on the separation performance and can be used to forecast hydrodynamic and separation behavior in a CPC device.

Influence of particle size on the exit effect of a full-scale rolling circulating fluidized bed

ABSTRACTThis paper investigated the influence of particle size on the exit effect of a full-scale rolling circulating fluidized bed (CFB) by using the Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) method. The gas–solid two-phase flow of the full-scale rolling CFB was compared with that of a simplified rolling CFB. Thus, the exit effect of the full-scale rolling CFB was clarified. In the air phase, a peak of air axial velocity vya was observed when the full-scale rolling CFB reached the maximum angular displacement. The particle phase possessed back mixing and radial exchange phenomena at the top and bottom of the full-scale rolling CFB, respectively. However, those phenomena were not obvious at the top and bottom of the simplified rolling CFB. The mechanism of the above-mentioned exit effect was then clarified by analyzing the forces acting on the particles under different particle sizes. Finally, the increases in particle size lead to the intensification of the peak of vya, particle back mixing, and radial exchange phenomena. Therefore, the intensity of the exit effect of the gas–solid two-phase flow increased as the particle size increased. The results suggested that the small particles had the potential to promote the purification rate of the full-scale rolling CFB on account of its small exit effect.

Modeling of the maleic anhydride circulating fluidized bed reactor

Abstract Maleic Anhydride (MA) production by selective oxidation of n-butane in a Multi-Tubular Fixed Bed reactor is constrained by the flammability limits. The use of Fluidized Bed circumvents this constraint but suffers from high back mixing. The Circulating Fluidized Bed (CFB) reduces the back mixing of the catalyst and enhances the catalyst activity by reactivating the catalyst. A simple reactor model with suitable hydrodynamic and kinetic models at the operating conditions of both the pilot and commercial MA CFB reactors is proposed. A dispersion model with variable gas density is used for modeling the hydrodynamics of these reactors. The reactors' configurational complexities are also accounted for in the model. The data fitting exercise of the proposed reactor model is performed. The proposed method simultaneously minimizes the total bias based on the individual biases for each component of the multi component reactor system and the total error in a multi-objective framework.

A study on continuous and batch electrocoagulation process for fluoride removal

AbstractContinuous and batch electrocoagulation processes (CEC and BEC) for treating fluoride-containing solution were investigated and compared. The poor mixture and uneven distribution of flocs during BEC lead to a lower removal efficiency, when compared with CEC. Under whole range of current densities, the superiority of CEC is obvious. However, at high residence time, CEC process, which has intense back mixing has a lower removal efficiency when compared with BEC. Also it was found that even at same charge loading, the CEC differ a lot with BEC. The influences of charge loading and initial pH on CEC process were investigated with the consideration of utilization rate of electro-generated Al element. The molar ratio of F/Al in sludge presents a trend of gradual decline with the increment of charge loading. The molar ratio of (OH + F) to Al element in sludge conforms the fact that the F− is removed not only by Al(OH)3-xFx(s) precipitation but also by the absorption of aluminum flocs. The influent at the...