Hydrodynamic Operating Conditions Research Articles

Scale formation in NF/RO: mechanism and control

Scale formation of soluble salts is one of the major factors limiting the application of nanofiltration (NF) and reverse osmosis (RO) membranes. This article reviews the scale formation mechanisms in membrane systems, methods to retard scale formation, and a new hybrid system consisting of MF-NF/RO. Two distinct mechanisms in NF/RO fouling by scale formation including surface and bulk crystallization have been identified and investigated. The hydrodynamic operating conditions as well as module geometry determines which fouling mechanism is dominant. An increase in solute concentration at the membrane surface by concentration polarization is responsible for surface crystallization. Conventional methods for scale control only retard the rate of scale formation and their performances are unpredictable. On the other hand, using a MF-NF/RO hybrid system for continuous removal of crystal particles from the retentate stream appears to be effective at high recovery of permeate. When applying the MF-NF/RO hybrid system, substantial improvement in flux is observed in spiral wound module, whereas it is negligible in case of the tubular module. This is because the microfilter could only removes crystals formed in the retentate through the bulk crystallization that is the dominant fouling mechanism in the spiral wound module.

Hollow fiber ultrafiltration membranes with enhanced flux for humic acid removal

In this study, polyethersulfone (PES) hollow fiber ultrafiltration membranes with enhanced flux have been developed for removal of humic acid. The membranes were spun from dope solutions containing PES/poly(vinyl pyrrolidone) (PVP)/ N-methyl-2-pyrrolidone (NMP)/additive(s) by using the dry-jet wet-spinning process. Effects of additive such as water, LiNO 3 and 1,2-propanediol (1,2-PD) in dope and air gap length were investigated. Characterization of the membranes in terms of pure water flux and apparent retention for humic acid were conducted at a steady state under almost identical hydrodynamic operating conditions. The experimental results showed that all additives in the dope dramatically enhanced the flux of resultant membranes but reduced the retention depending on the additive. For 1,2-PD as an additive, the fiber with an inner skin spun under an air gap of 600 mm had higher both flux and retention than the wet-spun fiber with an outer skin. However, for LiNO 3 as an additive, 2 mm increase in air gap significantly enhanced both flux and retention. 1,2-PD enhanced the membrane flux but reduced the retention much significantly than LiNO 3. The best membrane had a pure water flux of 110 × 10 −5 L m −2 h −1 Pa −1 and the nominal MWCO 6000 with a retention rate of over 97% for humic acid. The SEM images revealed that the designed morphology of hollow fiber membranes could be obtained by adjusting the relative coagulation rate at each surface of the nascent fiber (adjusting composition of the bore fluid and air gap length in this study), resulting in a single skin at either outside or inside and a porous surface on the other side.

Removal of 17β Estradiol and Fluoranthene by Nanofiltration and Ultrafiltration

With the recent emergence of endocrine disrupting compounds as an important potable drinking water and reclaimed wastewater quality issue, the removal of two estrogenic compounds (17β-estradiol and fluoranthene) by nanofiltration and ultrafiltration membranes was investigated. A less hydrophobic organic compound model species [parachlorobenzoic acid (PCBA)] was tested. 17β-estradiol (E2), fluoranthene, and PCBA were applied to the membrane in the presence and absence of natural organic matter (NOM). Both batch adsorption and dead-end stirred-cell filtration experiments indicated that adsorption is an important mechanism for transport/removal of relatively hydrophobic compounds, and is related to the octanol-water partition coefficient (\iK\dO\dW) values. All filtration measurements were performed approximately the same permeate flow rate in order to minimize artifacts from concentration polarization varied with different hydrodynamic operating conditions at the membrane interface. The percent removal by dead-end stirred-cell filtration ranged from 10 to >95% depending upon membrane pore size/hydrophobicity and presence/absence of NOM at an initial concentration ranging from 0.1 to 0.5 μM. Additional batch adsorption experiments with radio-label (³H) E2 at lower concentrations ranging 0.025 to 5 nM showed that E2 removal due to adsorption was independent of its initial concentration. Adsorption occurs both on the membrane surface and interior membrane pore surfaces. However, adsorption was insignificant for PCBA (log \iK\dO\dW=2.7), but removal presumably occurred due to electrostatic exclusion. Partition coefficients (log \iK) of 0.44 to 4.86 measured in this study increased with log \iK\dO\dW and membrane pore size.

Cavitation erosion tests on a 2D hydrofoil using surface-mounted obstacles

A new cavitation erosion testing procedure in high-speed cavitation tunnel is presented. The method is based on surface-mounted obstacles on a plane-convex hydrofoil leading edge. This experimental set-up is able to accelerate the accumulation of damage on specimens located behind the obstacles and presents several advantages. Measurable cavitation erosion damage has been obtained for a thermal spray coating and a stainless steel 13.4 after 75 h of operation. The hydrodynamic operating conditions have been an inlet flow velocity of 35 m/s, a sigma value of 1 and an incidence angle of 3°. Final erosion results have been compared with similar data obtained with the vortex cavitation generator (VCG) and the vibratory apparatus as a continuation of a previous study, see [Wear 233–235 (1999) 65]. The cavitation induced vibration monitoring provides a way to explain the apparent discrepancy of the erosion data between the tests with the high-speed cavitation tunnel and the vibratory apparatus.

Transport characteristics of wastewater effluent organic matter in nanofiltration and ultrafiltration membranes

This study demonstrates the transport characteristics of wastewater effluent organic matter (EfOM) through membrane pores using a four-parameter (intrinsic mass transfer coefficient ( k i ), solute concentration near the membrane surface ( C m ), solute permeability ( P m ), and reflection coefficient (σ) model based on thermodynamics, concentration polarization (CP) and hydrodynamic operating conditions represented by a J 0 / k ratio (the ratio of initial permeate flux ( J 0 ) to a back diffusional mass transfer coefficient ( k )). EfOM transport characteristics through the pores of four different membranes (a nanofiltration (NF)/ultrafiltration (UF) polymeric pair and two ceramic UF membranes with different molecular weight cutoffs (MWCOs)) were different; the NF polymeric membrane exhibited either convection- or diffusion-dominant conditions, while the UF membranes exhibited convection-dominant conditions in terms of EfOM transport through membrane pores. A critical J 0 / k ratio (representing a transitional condition between diffusion- and convection-dominant transport of solute) was found for the examined NF membrane with a MWCO of 250 Daltons. Four different parameters ( k i , C m , P m , and σ) were determined by the model to be informative to elucidate the various interactions between EfOM and the tested membranes. EfOM characteristics (size, structure, and functionality) and membrane properties (MWCO, surface/pore charge in terms of zeta potential, and module configurations) were revealed to play a major role in EfOM rejection and flux decline under convection-dominant conditions, as compared to diffusion-dominant conditions.

Impact of coagulation conditions on the in-line coagulation/UF process for drinking water production

An in-line coagulation (without settling)/UF process has been studied to improve membrane performance and water quality for surface water treatment. Using coagulation before UF increases permeate quality; the extent of dissolved organic matter removal is controlled by the coagulation step. Efficient coagulation conditions for a coagulation/settling process can be applied for the in-line coagulation/UF process and membrane fouling is reduced. Floc cake resistance is lower than resistance due to the unsettled floc and the uncoagulated organics. For an inside-out hollow-fiber system, the impact of coagulation dose depends on filtration mode: for instance, in cross-flow with a feed-and-bleed configuration, a reduction of coagulant dose induces an increase of the mass transfer resistance even in quasi-stable hydrodynamic operating conditions.

The relationship between flux decline of NF membranes with NOM transport characteristics: convection vs. diffusion

Recent studies have shown that NF membranes can effectively remove natural organic matter (NOM). However, NOM also serves as an organic foulant to membranes, resulting in flux decline. In this study, the flux decline of NF membranes was analyzed through the transport characterization of NOM in NF membrane pores. The transport of NOM through NF membrane pores was influenced by either convection or diffusion, depending on the applied hydrodynamic operating conditions, as represented by the ratio J 0/ k ( J 0 represents the initial pure water flux, and k is the mass transfer coefficient) [1]. Different behaviors of flux decline, and flux recovery of tested membranes could be obtained: depending on which transport mechanism (either convection or diffusion) dominates during NOM filtrations. The removal (or rejection) behavior of NOMs, by NF membranes, was evaluated with respect to the ratio J 0/ k under a dominant transport condition, along with the performance of the flux decline effects.

Determination of mass transport characteristics for natural organic matter (NOM) in ultrafiltration (UF) and nanofiltration (NF) membranes

This study is mainly concerned with establishing a reliable method of the quantitative analysis of natural organic matter (NOM) transport characteristics through ultrafiltration (UF) and nanofiltration (NF) membranes with molecular weight cutoffs of 8000 (GM) and 250 (ESNA), respectively. Filtrations were conducted with a cross-flow filtration unit and hydrodynamic operating conditions were controlled by a J0/k ratio (the ratio of initial permeate flux [J0] to a back diffusional mass transfer coefficient [k]). A four-parameter (the apparent mass transfer coefficient [ka], the solute concentration near the membrane surface [Cm], the solute permeability [Pm], and the reflection coefficient (σ) model based on concentration polarization and irreversible thermodynamics was used to manipulate experimental results quantitatively. With the values of the determined parameters, the transport characteristics of NOM due to different solution chemistries such as pH and ionic strength through UF/NF membrane pores were investigated. This model was also used to demonstrate the effects of NOM structure (hydrophobic/transphilic/hydrophilic) on transport through the membranes, with XAD-8/4 resins fractionation and isolation procedures. Four parameters estimated through the model were revealed to be relevant to elucidate the behaviors of NOM in membranes and corresponding transport-related results were in good agreement with the theoretical descriptions related to the interactions between NOM molecules and membrane surface/pores.

Optimization of hydrodynamic operating conditions of alkaline adsorbers of carbon dioxide in fine purification of coke oven gas

Optimization of hydrodynamic operating conditions of alkaline adsorbers of carbon dioxide in fine purification of coke oven gas

Effect of operating conditions on CaSO 4 scale formation mechanism in nanofiltration for water softening

The effect of the operating condition, such as transmembrane pressure and crossflow velocity on the scale formation pathways, was investigated in nanofiltration (NF) of a CaSO 4 solution. The examination of scanning electron microscopy of a fouled membrane surface provided strong evidence that there are two mechanisms in NF fouling (e.g., scale formation on the membrane surface) in nanofiltration; surface and bulk crystallization. The hydrodynamic operating conditions determined which fouling mechanism was dominant. The fouling due to surface crystallization augmented with an increase in operating pressure, but reduced with an increase in flow velocity. Concentration polarization was found to be responsible for this type of fouling. On the other hand, the extent of fouling by bulk crystallization appeared to be the most important at intermediate crossflow velocity and higher operating pressure (e.g., higher flux) because both secondary nucleation effect and deposition probability depends on crossflow velocity and permeation rate. An experimental and theoretical method for differentiating one mechanism from another was explored in order to elucidate the dominant fouling mechanism according to the hydrodynamic conditions.

Membrane filtration of natural organic matter: comparison of flux decline, NOM rejection, and foulants during filtration with three UF membranes

Three ultrafiltration membranes [thin-film composite, polyethersulfone (PES), and sulfonated PES] which have different chemistries and nominal relative molecular mass cut-offs (MWCO), were compared in terms of their characteristics of flux decline, rejection of natural organic matter (NOM), and the adsorbed foulants, with two very different (relatively hydrophilic and hydrophobic) NOM-containing source waters. To facilitate comparison, the ratio ( J o k ) of the initial pure water flux J o to the estimated boundary layer mass transfer coefficient k was used to define similar initial hydrodynamic operating conditions for the three different membranes. The membranes showed differences in NOM rejection when filtering hydrophilic NOM-source water, and also exhibited differences in flux decline with the hydrophobic NOM-source water. Flux decline and NOM rejection were quantified using a resistances-in-series model and effective molecular mass cut-off, respectively. Non-charged NOM fractions (hydrophilic and hydrophobic neutrals/bases) were found to be significant foulants for these negatively charged membranes.

Interactions Between Natural Organic Matter (NOM) and Membranes: Rejection and Fouling

This work demonstrates that interactions between membranes and natural organic matter (NOM) include rejection by both steric and electrostatic exclusion, and fouling by adsorption. NOM rejection and fouling are influenced by both NOM characteristics (molecular weight, aromaticity/humic content, and charge) and membrane properties (molecular weight cutoff (MWCO), hydrophobicity, and charge) as well as hydrodynamic operating conditions represented by a f/k ratio (the ratio of permeate flux (f) to a diffusional back-transport mass transfer coefficient (k)).

Interactions between natural organic matter (nom) and membranes: Rejection and fouling

This work demonstrates that interactions between membranes and natural organic matter (NOM) include rejection by both steric and electrostatic exclusion, and fouling by adsorption. NOM rejection and fouling are influenced by both NOM characteristics (molecular weight, aromaticity/humic content, and charge) and membrane properties (molecular weight cutoff (MWCO), hydrophobicity, and charge) as well as hydrodynamic operating conditions represented by a f/k ratio (the ratio of permeate flux (f) to a diffusional back-transport mass transfer coefficient (k)).

Droplet stabilization in high holdup fraction suspension polymerization reactors

This paper presents a study on the evolution of the drop size of a methyl methacrylate suspension in an aqueous agitated medium in the absence and in the presence of polymerization reaction. The influence of the impeller speed, of the fraction of monomer and of the surfactant concentration on the Sauter diameter has been determined for the non-reactive system and a correlation between the diameter and these parameters has been established. The evolution of the Sauter mean diameter has been explained by the mean of the variation of the interfacial tension of the system. In the case where a reaction occurs, the properties of the organic phase evolve, that may modify the equilibrium between coalescence and breakage. Consequently, the surfactant concentration has to be higher than without reaction. If this concentration is sufficient, the bead size remains nearly constant during all the reaction. It is defined by the initial properties of the binary system and by the hydrodynamic operating conditions. If the concentration is too low, the bead diameter increases during the reaction especially in the gel effect zone. The final size depends not only on the hydrodynamic factors, but also on the kinetic parameters. Moreover, the hydrolysis ratio of the surfactant has an effect on the bead size due to the quantity adsorbed on the organic surface.

Functional geometry of ciliated tentacular arrays in active suspension feeders

Parallel tentacular structures with lateral cilia that produce suspension-feeding and respiratory flows occur repeatedly in many diverse taxonomic groups. We use a computational hydrodynamic model of flow through ciliated tentacles to simulate flow rates through ciliated tentacle arrays. We examine the functional relationship of one performance measure, flow rate per unit length of array, to geometrical variables, such as cilia length, cilia tip speed and the gap between adjacent tentacles, and to hydrodynamic operating conditions, such as adverse pressure drops across the array. We present a scaling and interpolation scheme to estimate flow rates for a wide range of geometries that span many taxa. Our estimates of flow rate can be coupled with the hydrodynamic characteristics of biological piping systems to understand design trade-offs between components of these systems. As a case study, we apply the model to the blue mussel Mytilus edulis by investigating the effect on performance of changes in the gap between neighboring tentacles. Our model suggests that the observed gaps between tentacles in M. edulis reflect flow-maximizing geometries. Even relatively weak adverse pressure drops have strong effects on flow-maximizing geometries and flow rates. One consequence is that an intermediate range of pressure drops may be unfavorable, suggesting that animals may specialize into high-pressure and low-pressure piping systems associated with differences in organism size and with their strategy for eliminating depleted water.

Cavitation Nuclei and Cavitation Inception of Marine Propellers: State of the Art at the Dawn of the 21st Century.

The important role of nuclei on cavitation inception and cavitation development is now well known. Several recent studies have allowed a precise correlation between cavitation inception for different types of cavitation on marine propellers and nuclei distributions(20th ITTC report). In a standard cavitation facility the nuclei distribution will depend on the hydrodynamic operating conditions, and can change during whole cavitation tests. Then, to be able to transpose the model results to full-scale, it is necessary to determine precisely the nuclei distribution corresponding to each cavitation operating point. An illustration is given by analyzing the influence of the operating conditions on a complete cavitation inception <<bucket>>. Moreover, as nuclei govern the whole cavitation process, nuclei measurements are also conducted at sea. The main purpose of all these studies is the accurate prediction of full-scale cavitation behavior from precise transposition laws. This is possible only when nuclei distributions are determined.

PILOT-SCALE ULTRAFILTRATION OF CONCENTRATED PROTEIN PRECIPITATE SUSPENSIONS: THE EFFECT OF CONCENTRATION AND FLUID DYNAMICS

Abstract The effects of transmembrane pressure (TMP) and trans-cartridge pressure drops on the concentration of protein precipitate suspensions are evaluated in a hollow fibre device. A sharp decline in the filtrate flux occurs above a critical concentration, the value of which depends on the hydrodynamic operating conditions, and this is shown to coincide with the system entering a state of pressure-independent flux, commonly known as “gel polarization”. The effect of TMP, cross-flow velocity and concentration on the onset of pressure independence are examined in more detail in a series of nitrations carried out at constant concentration. Increasing cross-flow velocity enhanced fluxes for a given TMP in accordance with the standard film model, but increasing concentration was found unexpectedly to enhance fluxes at relatively low concentrations. Two possible explanations for this are discussed; one depends on the interaction of the large protein precipitate aggregates with the soluble protein and one on ...

Lipase production by immobilized Candida rugosa cells

Immobilization of Candida rugosa cells on a solid support for extracellular lipase production has been explored. The use of Ca-alginate beads and of mixed matrix of polyurethane foam/Ca-alginate beads enabled us to operate a batch and a continuous four-phase fluidized bed bioreactor. Cells co-entrapped together with polyurethane into Ca-alginate did not show higher lipase production levels than the cells entrapped in Ca-alginate gels. The addition of gum arabic to the medium greatly enhanced lipase production without affecting the hydrodynamic operating conditions significantly. This fact demonstrates that the reactor system is limited in terms of organic substrate dispersion and direct contact with cells.

Étude du comportement mécanique d'une pale de turbine kaplan en régime transitoire

A very comprehensive series of tests has been run on a big Kaplan turbine at the Beauchastel power plant on the Rhone with the following main objects in view : __ (i) To try out special techniques, for example sticking strain gauges in high velocity flows, fitting multi-conductor wires inside the turbine hub, among the blade control links and through the governor tube, designing a slip ring for low background noise or a radio transmission system between the turbine runner and power house, etc.; (ii) Direct turbine runner blade load and stress measurement ; (iii) Comparison of stresses measured under various normal and abnormal operating conditions. Normal (i.e. everyday) operation is considered to include no-load running, stopping, emergency shut-down, and routine running, whilst cases of abnormal operation either seldom occur (runaway) or are of a new kind (relief or sluice operation). With the results of these tests, blade behaviour can be investigated as follows : __ a) By measurement of local stresses due to hydrodynamic forces acting on the blade; b) By calculation of the total hydrodynamic load components from measured local dynamic data and static calibration results. This also enables a link to be established between these results and the model data, and the model tests to be used for types of operation liable to be dangerous on the prototype. Special instrumentation used comprised thirteen strain gauge bridges connected to five-channel frequency modulation transmitting equipment and an eight-channel slip ring providingdata straight from the blade. AlI this information was recorded for both steady and transient conditions. As a general rule, the measured hydrodynamic stresses were low. At partial runaway condition with twice the normal running speed and half the rated discharge, the highest bending stress was 8.5 kg/sq.mm at a point on the blade centre-line a quarter of a runner radius in from the blade tip. These values would certainly be very much higher at the absolute maximum runaway condition. All the extreme values recorded were associated with major variations in the hydrodynamic operating conditions, e.g. with the guide vanes near the end of their closing travel, the flow cut off by the gate, maximum speed, etc. Compression and tensile stresses recorded under transient conditions at the blade leading edge near the root of the blade were low and invariably less than those observed for steady operation. Normal stress fluctuations were up to ± 25 % at low frequency (1 to 2 cycles/second). Alternating stresses were apt to occur with comparatively high frequencies (30 to 60 cycles/second), being of a low level (± 0.3 kg/sq.mm). They were recorded by all the gauges simultaneously when the turbine cut out at low speed (0.75 n0) and acted as a pump in the draught tube with the downstream in the process of closing. The components of the total hydrodynamic load acting on the blade and the coordinates of its P.O.A. were calculated from these results for both steady and transient conditions. A check on the values found in this way showed them to be representative to within 10 % of the actual blade stress values. The inference from these calculations is that the total hydrodynamic load component normal to the blade depends on the same relationships as bending stress, confirming that the latter play a decisive part compared to torsional stresses.