Low Liquid Velocities Research Articles

Partial Wetting in Trickle Bed Reactors: Measurement Techniques and Global Wetting Efficiency

Three very different techniques for measuring the catalyst wetted fractionor wetting efficiencyin trickle bed reactors have been carried out and compared. The first one based on pressure drop data performed successively in single gas then liquid phases and, then, in a gas−liquid phase is experimentally very simple but did not result in meaningful wetting efficiency estimation. The other ones, based on RTD analysis and on dye adsorption on wetted surfaces, gave similar results and were used to investigate different parameters: liquid velocity and gas molecular weight and pressure by RTD, liquid−solid interaction or affinity (contact angle) and particle shape and diameter by dye fixation. The main operating parameter is the liquid velocity while the effect of gas flow is very weak. Higher solid−liquid affinity (heptane versus water) improves wetting efficiency only at very low liquid velocity (<2 × 10-3 m/s).

Wetting topology in trickle bed reactors

AbstractLocal and average partial wetting efficiencies in trickle‐beds have been directly measured by image processing and PIV analysis. Two different setups have been implemented: a vertical monolayer of pellets where the wetting structure and stability and also local velocity gradient are characterized by PIV, and a fixed bed to analyze (by photography) the axial evolution of wetting over bed cross section after a transient injection of dye. Porous alumina spheres and different liquids have been used to examine the effect of liquid–solid affinity; two liquid distributors, and bed prewetting were investigated at low liquid superficial velocities (0.5 × 10−3–10−2 m/s). Contrary to usual techniques (excepting MRI), not only average wetting efficiencies are derived from the colorimetric tracing, but also local features: axial evolution of wetting, size, and locations of dry zones, distributions of wetting efficiency at a particle scale. All those local data are important to improve reactor models assuming uniform pellet wetting. The effect of liquid–solid affinity is predominant in the case of the monolayer of beads, and, contrary to usual assumption, is still significant for the real trickle bed in the low range of wetting efficiency, corresponding to liquid superficial velocity lower than 2 × 10−3 m/s. © 2007 American Institute of Chemical Engineers AIChE J, 2007

Vertical plates with inclined capillary grooves for redistribution of the liquid phase over the cross-section of packed columns: I. Equations for calculating the liquid flow-rates in the capillary grooves

The uniform distribution of the liquid phase over the cross-section of a packed column is a major prerequisite for its effective operation. Regarding various distributor designs, the best uniformity has been obtained with devices which distribute the liquid into jets with equal flow-rates, distributed uniformly across the column cross-section. The final liquid redistribution, to obtain uniformity over cross-sections of the range of a packing element, takes place in the packing itself or in a specially designed redistribution layer. For this purpose, a packing with inclined capillary grooves, especially suitable for low liquid superficial velocity, is developed. It consists of parallel vertical plates with inclined crossing small-size grooves stamped in them. On the bases of the data obtained from the experimental study of the liquid flow in these grooves, dimensionless equations are obtained which are necessary for calculating the height of the new redistribution layer.

Solid Phase Hydrodynamics of Three-Phase Fluidized Bed Reactors -- A Convective/Dispersive Phenomena

The objective of this work was to investigate the relative contribution of the convective and dispersive mixing mechanisms to the overall solid phase mixing mechanism for three-phase fluidized bed reactors. Noninvasive Radioactive Particle Tracking (RPT) data were obtained at various operating conditions, reactor diameters and particle systems. The structural wake model was updated and consists of three sub-phases: the particle wake and downflow-emulsion phase following the convective mixing mechanism and the vortex-emulsion phase following the dispersive mixing mechanism.The particle velocity mean and STD increased with the superficial liquid and gas velocity as well as the reactor diameter for each particle phase. Therefore, the extent of mixing increased under these operating parameters. The particle phase's holdup and a new Mixing Mechanism Indicator (MMI), however, exhibited a much more complex trend. For the larger reactor (Dc = 0.292 m), the convective mixing mechanism dominated and at low superficial liquid velocity the contribution of the convective mixing mechanism increased with superficial gas velocity. This trend, however, was reversed when the superficial liquid velocity increased. For the smaller reactor (Dc = 0.10 m), the random movement of the solid dominated. Therefore, the extent of mixing and the mixing mechanism did not follow the same trend. The CFD and the mixing model have to follow both those hydrodynamic properties.Relations to estimate mean and STD particle velocity distribution as well as particle phase holdup were developed. Solid in the established region mainly followed the convective mixing mechanism. Random movement of the solid was mostly observed at the top of the bed, but it was also present in the established region and at the bed bottom. The volume of each region depended on the operating conditions, reactor diameter and particle system. Furthermore, the particle wake expulsion frequency range was similar to the wake shedding frequency found in literature. It is, thus, possible to assume that wake shedding was mainly responsible for the solid exchange between particle phases.

Three-phase upward flow in a vertical pipe

In this paper we develop an approach to design a three-phase, gas–solid–liquid flow system that transports pneumatically scarified solid particles, including sticky ones, through a vertical pipe. The proposed system permits the introduction and maintenance of a liquid film that coats the pipe’s inner wall and acts as a lubricant that ensures sticky particles continue to move upward without permanently adhering to the pipe wall. The system’s operating conditions fall within the boundaries of the annular dispersed region on a typical flow pattern map of vertical flow of a gas–liquid mixture. High gas superficial velocities combined with low liquid superficial velocities characterize such a region. A combination of a modified one-dimensional, two-fluid annular dispersed flow model and a one-dimensional pneumatic conveying model is shown to describe this transport process satisfactorily. Solution of the combined models produces all the necessary design parameters including power requirements and superficial velocities of the two-fluid media needed to transport a given amount of solid particles. Results of model calculations are compared with rare three-phase flow data obtained prior to the development of the present model, by an independent experimental team that used the physical conditions of the present approach. Reasonable agreement justifies the use of the combined model for engineering design purposes.

Laser-induced fluorescence (LIF) studies of liquid–liquid flows. Part II: Flow pattern transitions at low liquid velocities in downwards flow

This paper describes the use of Laser-induced fluorescence to investigate liquid–liquid vertical pipe flow in a closed experimental circuit at very low mixture velocities. The dynamic evolution of annular, mixed and dispersed flows were observed, as well as the associated physical processes such as wave coalescence and the formation of secondary dispersions. The experimental results presented in this paper suggest that as the flow velocity was increased, the flow undergoes a transition from annular to wavy-annular to mixed (or transition) flow, and finally dispersed flow. The results also indicated that secondary dispersions, such as w/o/w and o/w/o, occur in the dispersed core of a mixed flow. In transient flow (in which the flow velocity is decreased), the structure of the flow was very complex and the formation of ‘slugs’ could occur, which resemble the situation in gas–liquid flows.

Separation of Organic−Water Mixtures by Co-current Vapor−Liquid Pervaporation with Transverse Hollow-Fiber Membranes

The advantages of a liquid−vapor feed in a pervaporation unit are presented in comparison to a single-phase feed pervaporation unit that uses interstage heating. The comparison is based on calculations for the dehydration of isopropyl alcohol (IPA) from 13 to 1 wt % in a transverse system with hollow-fiber membranes. Because of the presence of the vapor phase, high mass- and heat-transfer coefficients in combination with low pressure drops are achieved. Vapor also supplies the heat for the evaporation and expansion of the permeating components through the membrane. Mass-transfer coefficients from the liquid to the membrane surface were measured using an electrochemical method. At low liquid superficial velocities, mass transfer increases up to 4-fold as compared to single-phase flow. Both the enhancement in mass and heat transfer to the membrane and the energy supply to the liquid result in a reduction of the required membrane area of 45%. This reduction in membrane area combined with the avoidance of interstage heat exchangers results in a marked reduction of capital costs, thus significantly expanding the application window of pervaporation.

Flow pattern, pressure drop and void fraction of two-phase gas–liquid flow in an inclined narrow annular channel

Two-phase flow pattern, pressure drop and void fraction in horizontal and inclined upward air–water two-phase flow in a mini-gap annular channel are experimentally studied. A concentric annular test section at the length of 880 mm with an outer diameter of 12.5 mm and inner diameter of 8 mm is used in the experiments. The flow phenomena, which are plug flow, slug flow, annular flow, annular/slug flow, bubbly/plug flow, bubbly/slug–plug flow, churn flow, dispersed bubbly flow and slug/bubbly flow, are observed and recorded by high-speed camera. A slug flow pattern is found only in the horizontal channel while slug/bubbly flow patterns are observed only in inclined channels. When the inclination angle is increased, the onset of transition from the plug flow region to the slug flow region (for the horizontal channel) and from the plug flow region to slug/bubbly flow region (for inclined channels) shift to a lower value of superficial air velocity. Small shifts are found for the transition line between the dispersed bubbly flow and the bubbly/plug flow, the bubbly/plug flow and the bubbly/slug–plug flow, and the bubbly/plug flow and the plug flow. The rest of the transition lines shift to a higher value of superficial air velocity. Considering the effect of flow pattern on the pressure drop in the horizontal tube at low liquid velocity, the occurrence of slug flow stops the rise of pressure drop for a short while, before rising again after the air velocity has increased. However, the pressure does not rise abruptly in the tubes with θ = 30° and 60° when the slug/bubbly flow occurs. At low gas and liquid velocity, the pressure drop increases, when the inclination angles changes from horizontal to 30° and 60°. Void fraction increases with increasing gas velocity and decreases with increasing liquid velocity. After increasing the inclination angle from horizontal to θ = 30° and 60°, the void fraction appears to be similar, with a decreasing trend when the inclination angle increases.

Relationship between enamel erosion and liquid flow rate

The association of severe erosion with certain habits of consuming soft drinks prompted a study of erosion in relation to fluid flow. In the main experiments, citric acid solution (pH 3.2) was directed at polished enamel surfaces at flow rates of 26, 67 or 126 ml min(-1) through outlets of diameters 1.0, 2.6 or 5.0 mm for up to 20 min. Erosion depth increased with time of exposure and total volume of solution, with increasing flow rate and with decreasing outlet diameter. Scanning electron microscopy of acid-treated surfaces and of the subsurface enamel indicated that the clearance of dissolution products, rather than the mechanical dislodgement of partly dissolved crystal bundles, was the main process involved. For an exposure time of 20 min, erosion increased with (liquid velocity)(1/4). In a secondary experiment, it was found that the erosive potential of a low-erosive drink did not increase with increasing liquid velocity. The rapid increase in the rate of erosion caused by citric acid at low liquid velocities shows that reproducible stirring is essential for precision and intercomparison of in vitro studies. It also accounts for the rapid erosion seen when erosive drinks are consumed from a straw placed labial to the anterior teeth, or are "swished" between the teeth.

Hydrodynamics of Taylor Flow in Vertical Capillaries: Flow Regimes, Bubble Rise Velocity, Liquid Slug Length, and Pressure Drop

Two-phase flow hydrodynamics in vertical capillaries of circular and square cross sections were experimentally studied, using air as the gas phase and water, ethanol, or an oil mixture as the liquid phase. The capillary hydraulic diameters ranged from 0.9 mm to 3 mm, with the superficial gas and liquid velocities covering a span of 0.008−1 m/s, which is typical of that obtained in monolith reactors. Using a high-speed video camera, four distinct flow regimes were observed within the range at which experiments were conducted: bubbly, slug-bubbly, Taylor, and churn flows. Annular flow was observed at excessively high gas and low liquid flow rates, well beyond those of interest to this study. Based on the definition of a two-class flow regime, the combination of two parametersthe slip ratio (S) and the ratio of the superficial gas velocity to two-phase superficial velocity (UG/UTP)was observed to be suitable for determining the transition from homogeneous flow to nonhomogeneous flow. The influence of capillary geometry, capillary hydraulic diameter, and fluid properties on bubble rise velocity (Vb) were investigated and determined to be of little significance. Furthermore, a new and simplified correlation for predicting Vb and, by implication, the gas holdup (εG) was proposed. Liquid slug lengths were also experimentally studied, using a correlation that was developed to estimate them. Pressure drop experiments were also performed, and the peculiar phenomenon of negative frictional pressure drops was observed at very low liquid velocities. By defining a new dimensionless quantitythe pressure factor, FEa flow-regime-dependent method for estimating the total pressure drop in two-phase vertical capillary flows was developed.

Advantages of counter-current operation for hydrodesulfurization in trickle bed reactors

The deep desulfurization of oil fraction is a central matter of concern to every refinery. Hydrogen sulfide is the product of hydrodesulfurization reaction and it is the inhibiter of the reaction. When products inhibit the reaction, the counter-current operation is expected to have an advantage over the co-current operation. Hydrodesulfurization of vacuum gas oil in a trickle bed reactor was simulated for both models of co-current operation and counter-current operation. The models were simulated on high and low gas and liquid velocities. Hydrogen sulfide was affected by mass transfer resistance in both gas-liquid and liquid-catalyst interface. The other component mass transfer resistances were negligible. When the deep desulfurization was required, simulation results showed that the counter-current operation was superior to the co-current operation in organic sulfur conversion

An Investigation of Two-Phase Slug Flow in Inclined Pipelines

Two-phase slug flow in inclined pipelines can cause significant practical operating problems. When slugs flow in an inclined pipeline that contains sections of different inclinations, they undergo a change of length and slug flow characteristics as the slug moves from section to section. In addition, slugs can be generated at low elbows, dissipate at top elbows and shrink or grow in length as they travel along the pipe. A mathematical model and a computer program were developed to simulate these phenomena. The model was based on the sink/source concept at the pipeline connections. A connection between two pipeline sections of different slopes was conveniently called elbow. An elbow accumulates liquid as a sink, and releases liquid as a source. The sink/source has a characteristic capacity of its own. This capacity is positive if the liquid can indeed be accumulated at the elbow or negative if the liquid is actually drained away from the elbow. The hydrodynamic flow model was also used to calculate the film liquid holdup in horizontal and inclined pipelines. The model can successfully predict the liquid film holdup if the liquid film height is assumed to be uniform through the gas pocket. The overall effect of a hill or terrain on slug flow depends on the operating flow rates and pipeline configurations. For special cases of near constant slug frequency corresponding to moderately high superficial liquid and gas velocities, this effect was found to be small. The film liquid holdup decreased with increasing pipe diameter. The unit slug length increased at the upstream inclined pipes and decreased at the downstream inclined pipes with increasing pipe diameter. The possibility of pseudo-slug generation was increased at large pipe diameters even at high sink capacities. At low sink capacities, no pseudo-slugs were generated at high superficial velocities. The slug flow characteristics were more affected by low superficial gas and liquid velocities, large pipe diameters and shallow pipeline inclinations.

Estimation of particle concentration profiles in a three-phase fluidized bed from experimental data and using the wake model

Particles with a size distribution in the range of 34 to 468 µm were fluidized in a three-phase bed using low liquid and gas velocities. Particle size distribution and pressure profile measurements were carried out at different locations in the bed in order to study the influence of fluid velocities on segregation and dispersion of particles in different size classes. The influence of gas velocity on particle mixing was analyzed in terms of internal solid fluxes, calculated by means of the wake model. Based on the experimental results, different particle distribution patterns were identified. Although no significant tendencies were observed for radial profiles, particles of different sizes have significantly different axial profiles, which are mainly affected by the velocity of the liquid phase. Thus, depending on the liquid velocity, smaller particles reach a maximum concentration at different bed heights.

細管における垂直上昇気液二相流の伝熱・流動特性(沸騰,熱工学部門一般講演)

Void fraction, frictional pressure loss and heat transfer coefficient were measured for vertical upward heated air-water two-phase flow in a 1.03mm I.D. stainless steel tube to investigate heat transfer and flow characteristics of two-phase flow in minichannel. Flow regimes were observed using high speed video camera. Void fraction was correlated with drift flux model better than Smith's correlation when α>0.7, but did not agree with both correlations when α<0.7. Frictional pressure losses agreed with Chisolm-Laird's equation at relatively large liquid Reynolds number. At low liquid Reynolds number, experimental results were close to Mishima-Hibiki's modified Chisolm-Laird's equation. Pressure loss was affected by flow regime at low liquid Reynolds number. Heat transfer coefficient fairly agreed with the results of ordinary inner diameter tube except for low liquid velocities.

Dynamic characteristics of bed collapse in threephase fluidized beds

) were fluidized by cocurrent flow of deionized water and air. The transient behavior of the bed collapse after cut-off the gas supply to the bed was monitored by a video camera (30 frames/s). The dense bed surface height was measured from the image of videotape. At lower liquid velocity, the dense bed surface increases with the elapsed time and then reaches a bed height, whereas at higher liquid velocity the dense bed surface increases sharply with the elapsed time, then decreases and reaches the bed height corresponding to the liquid-solid fluidized beds (water-polymer beads).

Possible effects of air sparging for nanofiltration of salted solutions

Among the different ways to improve the flux in ultrafiltration or micro filtration, air sparging recently appeared as an interesting process for different applications. Air sparging consists in injecting air in the concentrate compartment during filtration time. A gas/liquid two-phase flow is then obtained at the membrane surface. This study focused on the use of air sparging in nanofiltration membranes. Experiments were performed with a lab-scale flat sheet module. Previous results showed that air sparging allows flux enhancement in the case of liquid/liquid suspensions and liquid/solid suspensions, for which the main limiting phenomenon is particle deposit. The objective here was to characterise the effects of gas sparging for a salt (CaCl 2)/water solution when mass transfer is limited by concentration polarisation or osmotic effects. At high concentrations, air sparging has no effect on the permeate flux not on CaCl 2 retention factor. This was explained by the fact that, for CaCl 2 solutions until 10 g.L −1, and even for a low liquid velocity, the transfer limitation is due to osmotic effects and no polarisation concentration occurs. In that case, no benefice of turbulence increase can be expected.

A new principle for creation of packings operating at extremely low liquid superficial velocity

The lack of packings able to operate at extremely low liquid superficial velocities does not allow to carry out mass transfer processes at full countercurrent flow at too low concentrations of the component to be absorbed. For this reason, the full countercurrent flow in many important processes is replaced by division of the packing into layers and re-circulation of the absorbent in each of them. The latter increases the expenses for the construction of the apparatus with parallel decreasing the process driving force. A method for creation of packings with effective surface equal to their total surface even at liquid superficial velocities that tend towards zero is described. The construction of such effective packing is given. The volumetric mass transfer coefficient when the mass transfer is controlled by the gas-side boundary layer has been received from the results of the absorption of sulfur dioxide into a solution of sodium hydroxide. Data for the pressure drop and liquid hold-up of the packing are also presented. The comparison of the new packing with other types of highly efficient packings shows its great advantages.

Quantitative Flux Analysis of Gas–Liquid Two-Phase Ultrafiltration

The gas–liquid two-phase upward-flow ultrafiltration in a hollow-fiber membrane module with dextran aqueous solution as tested solution is studied in this work. The permeate fluxes are measured under various feed concentrations, superficial liquid velocities, superficial gas velocities, and transmembrane pressures. The flux increases with the increase in liquid velocity, gas velocity, and the transmembrane pressure, and decreases with the increase in the feed concentration. The flux enhancement by gas slugs is particularly significant when the ultrafiltration system is operated at a lower liquid velocity. The resistance-in-series model combined with the modified gel polarization model is applied to obtain the mass transfer coefficient of the gas–liquid two-phase ultrafiltration, and the dimensional analysis technique is used to find the correlation equation of the mass transfer coefficient. Furthermore, the flux equation for gas-sparging ultrafiltration is proposed. The predicted fluxes agree well with the experimental data.

New Packings for Operation at Extremely Low Liquid Superficial Velocity

It is well known that the column apparatuses operating at full countercurrent flow ensure maximum driving force for mass transfer when equilibrium processes are used. It is also known, from the mass balance, that the ratio between gas and liquid superficial velocity, in case of countercurrent flow, is determined from the initial and end concentrations of the absorbed component in both phases. In many cases, especially in purification of waste gases, when the initial gas concentration is very low and its solubility is high, the necessary calculated liquid superficial velocity is extremely low. The lack of packings able to operate effectively at these conditions requires a division of the packing bed into layers with recirculation of the liquid phase in each of them, i.e., a refusal of the principle of complete countercurrent flow takes place. The paper shows the possibility to use this principle even at extremely low liquid superficial velocity.

Biological treatment of mixed industrial wastewaters in a fluidised bed reactor

A study has been carried out on the operating parameters that influence the biodegradation of petroleum and brewery wastewaters, with a low-density biomass support. The biodegradation rate of a mixture of two wastes was compared with that of the separate wastes. A low aspect ratio reactor was employed, and this made it possible to operate at low superficial gas and liquid velocities. The gas distributor used created a fluid flow pattern similar to that of a draft tube, which enhanced axial mixing. At a particles loading of 12% (v/v), the optimum superficial gas velocity was 2.5 cm/s for the mixture. The interstice structure of the biomass-support particles, improved microbial attachment due to the resulting large surface area. There was a low biomass concentration when petroleum wastewater was treated alone, however, for a mixture of petroleum and brewery wastewaters, an increase in the concentration was observed. There was a higher gas hold up in the mixture than in the petroleum wastewater, but lower than in the brewery wastewater. An improved biodegradation was achieved when a mixture of brewery and petroleum wastewaters was treated, and this gave an indication that nutrient deficient wastes can be treated together with phosphate and nitrate rich food industry wastewaters.