Flux enhancement in crossflow filtration using an unsteady jet

This paper deals with the influence of a new type of unsteadiness in the flow on the permeate flux in crossflow filtration. A pneumatically controlled valve generates intermittent jets from the main flow leading to the formation of large vortices moving downstream along the tubular membrane. The experimental study was carried out by filtering a bentonite suspension through an ultrafiltration mineral membrane. Flux time measurements were taken under steady and unsteady operating conditions. The unsteadiness leads to a permeate flux more than two times higher than in the usual filtration processes.     

Flux decline in crossflow microfiltration and ultrafiltration: experimental verification of fouling dynamics

The theory of fouling dynamics in crossflow membrane filtration is compared with ultrafiltration experiments with suspensions of 0.12 μm silica colloids. It has been experimentally verified that colloidal fouling in crossflow filtration is a dynamics process from non-equilibrium to equilibrium and that the steady state flux is the limiting flux. With the cake concentration c_g identified from an independent experiment and the specific cake resistance calculated by Carman–Kozeny equation, the time-dependent flux and the time to reach steady state in the experiments of this study are correctly predicted with the theory of fouling dynamics.     

Yeast cell microfiltration: Flux enhancement in baffled and pulsatile flow systems

The filtration performance of yeast cell harvesting was greatly improved by using an oscillatory flow mixing technique in both tubular and flat sheet membrane systems; flux increases of several fold were achieved. The effects of various operating parameters such as frequency, amplitude and ratio of net forward flow rate to oscillatory flow rate were investigated. A simple alternative to the dual head pump, which involved the use of solenoid valves for producing flow reversal, was developed. The flux was significantly increased using this new operating mode. As the only increase of power consumption was from inertia loss due to change of flow direction the additional power consumption is minimal.     

Flux enhancement during Dean vortex microfiltration. 8. Further diagnostics

Controlled centrifugal instabilities (called Dean vortices) resulting from flow around a curved channel have been used to reduce both concentration polarization and membrane fouling during microfiltration. These vortices enhance back-migration through convective flow away from the membrane–solution interface and allow for increased membrane permeation rates.As part of an effort to further understand the opportunities and limitations of using Dean vortex microfiltration, we report here on a series of experiments in which the performance of runs in the presence of vortices are compared to that in the absence of vortices. They include changing the following variables: transmembrane pressure, particle suspension concentration, ionic strength of the solution, and the reversibility of changing flow rate (from high rates in the presence of Dean vortices to low rates in the absence of such vortices and the reverse procedure) within a particular experiment. Unexpected results were observed for the ionic strength and flow reversal cases. The flux decreased with increasing ionic strength, passed through a minimum and then increased to a constant value. In the same experiment, the flux decreased when the flow rate was reversed from a high to low and back to a high value, until the flux was not affected by changing flow rate. We explain these interesting observations by contrasting the behavior of aggregated colloids in solution and within the deposited cake layer on the membrane surface. Pressure effects on the cake layer are also considered.     

Predicting limiting flux of skim milk in crossflow microfiltration

Four models for back-transport mechanisms in crossflow microfiltration have been investigated concerning their ability to predict the limiting permeate flux for skim milk. A tubular, ceramic membrane was used to measure the limiting fluxes for a series of crossflow velocities at two temperatures. One of the models — the shear-induced diffusion model — predicts values of the limiting flux close to our experimental values both at 55 and 15°C. The best prediction of the limiting flux is obtained by the empirical relation: flux=Re · 6.94×10⁻¹⁰ m/s.     

Evidence for shape-dependent deposition in crossflow microfiltration of microbial cells

Measurements of the mean cell volume and mean cell aspect ratio are reported for suspensions of the polymorphic yeast Kluyveromyces marxianus var. marxianus NRRLy2415 and for cakes of the same microorganism recovered after crossflow microfiltration with a tubular ceramic membrane at fixed trans-membrane pressure and crossflow velocity. The mean cell volume and the mean cell aspect ratio in the cake were determined using image analysis. The mean cell volume was found to be consistently smaller in the cake than in the suspension while tentative evidence was found that the mean cell aspect ratio was also smaller in the cake. Enhanced deposition of less elongated cells is likely to be a consequence of preferential deposition of small cells because, for the microorganism used in this study, the mean cell aspect ratio in a suspension increases with increasing mean cell volume. Close analysis of the data reveals that the mean cell aspect ratio is greater in the cake than would be expected based on the mean cell volume in the suspension, suggesting that the deposition of more elongated cells is favoured over less elongated cells of the same volume.     

Length dependency of flux and protein permeation in crossflow microfiltration of skimmed milk

Crossflow microfiltration of skimmed milk to fractionate casein micelles and whey protein was investigated regarding length dependency of flux and whey protein permeation using a 1.2 m long, 0.1 μm tubular ceramic membrane. A special module consisting of four sections was constructed allowing to assess the effects of membrane length online by measuring flux and permeation of the whey protein β-lactoglobulin as a function of local processing conditions. It was found that under the applied filtration parameters (mean transmembrane pressure Δp_TM,m = 0.5 bar; temperature ? = 55 °C; wall shear stress τ_w = 115 Pa) main parts of the membrane were controlled by a deposit layer. In consequence, the transmission of the whey protein β-lactoglobulin increases from 38% to 87% from membrane inlet to membrane outlet. Results show that a local optimum for protein fractionation exists regarding membrane resistance and process conditions.     

Flux decline behaviors in dead-end microfiltration of activated sludge and its supernatant

The properties of dead-end microfiltration were explored under constant pressure using two types of activated sludge controlled under the condition of different air flow rates. The activated sludge cultured at the air flow rate of 0.15 L min⁻¹ (the anaerobic condition) exhibited a significant flux decline compared with the case of the air flow rate of 2.33 L min⁻¹ (the aerobic condition). It was found from the results of microfiltration of the supernatant separated by centrifugation that the constituents in the supernatant caused a major cake resistance in microfiltration of the activated sludge. The average specific filtration resistance for filtration of the activated sludge was closely consistent with that for filtration of the supernatant at low pressure (49 kPa). However, the cake resistance of the microbial floc in microfiltration of the activated sludge became substantial with increasing filtration pressure because of high compressibility of the microbial floc. Moreover, the foulant and the fouling mechanism in microfiltration of the supernatant were evaluated from both microfiltration test of the supernatant and microfiltration test of the filtrate collected thereby. As a result, the effects of the pore size and material of the microfiltration membrane on the flux decline behaviors in dead-end microfiltration were reasonably elucidated.     

Experimental evaluation of a uniform transmembrane pressure crossflow microfiltration unit for the concentration of micellar casein from skim milk

A uniform transmembrane pressure (UTMP) crossflow microfiltration (CFMF) system maintains a low but uniform transmembrane pressure (ΔP_TM) with high crossflow velocity (CFV), which reduces fouling and cake build-up, and improves the utilization of available filtration area. A CFMF system, with a 0.2 μm nominal pore size ceramic filter, filtration area 0.184 m², was operated in both UTMP and non-UTMP modes. The two modes were compared for their effectiveness in maintaining a steady flux during the separation of casein micelles from skim milk up to a concentration factor (CF) 10 at 50°C. Experiments were performed at an average CFV of 7.2 m s⁻¹ and ΔP_TM from 89 to 380 kPa. Up to CF 4 the non-UTMP mode maintained a slightly better flux and process time than the UTMP mode, but reached the minimally acceptable flux (below 0.005 kg m⁻² s⁻¹) at CF 6. Depending upon the ΔP_TM maintained, the UTMP mode approached the minimal flux at CF 7 or 10 depending upon the combination of ΔP_TM and CFV used. Cake resistance (R_cm) was modified to include the effect of an increase in retentate viscosity with concentration. R_cm increased for the non-UTMP mode and decreased for the UTMP mode with a decrease in the ratio of permeation flux/wall shear stress (J_p/τ_w) (which occurred as the retentate gets concentrated). This indicated that the cake formed during the non-UTMP mode of operation was more compact and durable (harder to erode) than in the UTMP mode. A central composite rotatable design estimated the optimal operating region at a CFV of 7.1 m s⁻¹ and ΔP_TM of 241±10 kPa to achieve maximum flux and a high concentration.     

Permeate flux enhancement by pressure and flow pulsations in microfiltration with mineral membranes

We have investigated the possibility of permeate flux enhancement with mineral membranes using pressure and flow pulsations superimposed on the inlet flow of the filtration module. These pulsations are generated by a piston in a cylinder; various pressure wave shapes, generated by controlling the piston motions, have been tried. One wave form (fast piston return followed by a fast forward stroke) was found to yield the largest permeate flux increase, up to 45%, at 1 Hz frequency and with stroke volumes smaller than the internal volume of the membrane. Carefully chosen pulsation decrease the hydraulic power dissipated in the retentate per unit volume of permeate by up to 30%.     

Concentration gradient generator using a convective-diffusive balance

Since biomolecular concentration gradients are an important factor in various biological phenomena, many concentration gradient generators utilizing microfluidic techniques have been devised in order to establish stable in vitro biomolecular concentration gradients, each with their own strengths. However, all the fundamental issues, simplicity in apparatus, dynamic controllability, and convection-free conditions, have not been addressed simultaneously. In this paper, we describe a novel way of establishing concentration gradients using a convective-diffusive balance in a counterflow configuration. With the help of this method, stable, reproducible concentration gradient profiles can be formed in under a minute and the shape of the profiles are easily predicted with a simple mathematical formula.     

Investigation of heterogeneous electrochemical processes using multi-stream laminar flow in a microchannel

A multi-component microfluidic electrochemical cell is shown to be a useful analytical tool for probing complex coupled processes in electrolytic systems. We recently reported an enzymatic signal amplification phenomenon that may provide increased sensitivity when detecting bio-analytes (M. S. Hasenbank, E. Fu and P. Yager, Langmuir, 2006, 22, 7451-7453), but to fully harness this method requires an improved understanding of the underlying electrochemical and chemical processes. We use spatial control of electrolyte streams on patterned conductive substrates in a microfluidic platform to elucidate the coupling of homogeneous chemical steps to heterogeneous electrochemical charge transfer processes. Because the gold surface was observable using SPR imaging, electrochemical phenomena could be monitored optically in real time. Based on these and additional results, we propose a mechanism for the novel amplification phenomenon that involves direct electron transfer between surface-immobilized enzyme molecules and the gold surface. This improved understanding of the underlying mechanism should enable the future implementation of this phenomenon in signal amplification schemes for highly sensitive lab-on-a-chip biosensors.     

Electrically enhanced crossflow membrane filtration of oily waste water using the membrane as a cathode

The effect of an external electric field on the flux in crossflow membrane filtration of a model oily waste water was studied using a carbon fibre – carbon composite membrane as a cathode. Limiting fluxes for low flow rate increased significantly under the conditions studied, from 75 l/m² h without an electric field to more than 350 l/m² h using an electric field. The experimentally determined increase in the limiting flux showed good agreement with the theoretical value of 430 l/m² h calculated using a simple model. The limiting flux increase was affected by the electrophoretic mobility of the oil droplets and the applied electric field strength. When there were no cakes without an electric field due to the high flow rate, the flux increase when using an electric field under at the same conditions was minor. The critical electric field strength was determined, and experimentally obtained values were corresponded with calculated values. Decreasing the crossflow velocity above the critical electric field strength increased the flux, or had no effect, depending on the size of the particles. Permeate quality was also improved to some extent when using the electric field, and a membrane with a large pore size could be used when an electric field was applied. The main disadvantage in using the membrane as a cathode was foaming at the membrane surface causing decrease in the flux enhancement as the conductivity of the feed increased. It was not possible to restore the flux to the original value by applying an electric field after filtration of the oil emulsion without an electric field. An intermittent electric field was thus not efficient enough for keeping the flux at high level.     

Thin layer molecularly imprinted microfiltration membranes by photofunctionalization using a coated alpha-cleavage photoinitiator.

A novel approach towards thin-layer molecularly imprinted polymer (MIP) composite membranes was developed based on using benzoin ethyl ether (BEE), a very efficient alpha-scission photoinitiator. The triazine herbicide desmetryn was used as the template, and a mixture of the functional monomer 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS) and the cross-linker N,N'-methylene-bis-acrylamide (MBAA) in methanol was copolymerised via photoinitiation followed by deposition on the surface of either hydrophobic or hydrophilically precoated polyvinylidene fluoride (PVDF) microfiltration membranes. Blanks were prepared under identical conditions, but without the template. Especially, the degree of functionalization (DF) of the PVDF membranes with poly(AMPS-co-MBAA), the membrane permeabilities and non-specific vs. MIP-specific template binding from aqueous solutions during fast filtration were studied in detail to evaluate the effects of the preparation conditions, in particular the coating of the membrane surface with the photoinitiator prior to UV irradiation and the influence of the precoated hydrophilic layer on PVDF. Significant template specificities of the MIP membranes compared with the blanks were only achieved for the preparations including coating the two types of PVDF membranes with BEE. In contrast, a homogeneous photoinitiation of the copolymerisation in the membrane pore volume yielded functional layers with similar DF but with only non-specific desmetryn binding. All data clearly indicate the significant contribution of MIP stabilization by the support material in layers of optimum thickness to the MIP specificity. Main advantages of the novel approach are the potential to synthesize MIP composite membranes by controlled deposition onto any kind of polymer support, and the very fast MIP preparations due to a very efficient photoinitiator and small MIP layer thickness. Due to the mechanical and chemical stability in combination with high permeabilities, thin-layer MIP composite membranes have a large application potential, e.g., in solid phase extraction.     

Product selectivity control induced by using liquid-liquid parallel laminar flow in a microreactor

Product selectivity control based on a liquid-liquid parallel laminar flow has been successfully demonstrated by using a microreactor. Our electrochemical microreactor system enables regioselective cross-coupling reaction of aldehyde with allylic chloride via chemoselective cathodic reduction of substrate by the combined use of suitable flow mode and corresponding cathode material. The formation of liquid-liquid parallel laminar flow in the microreactor was supported by the estimation of benzaldehyde diffusion coefficient and computational fluid dynamics simulation. The diffusion coefficient for benzaldehyde in Bu(4)NClO(4)-HMPA medium was determined to be 1.32 × 10(-7) cm(2) s(-1) by electrochemical measurements, and the flow simulation using this value revealed the formation of clear concentration gradient of benzaldehyde in the microreactor channel over a specific channel length. In addition, the necessity of the liquid-liquid parallel laminar flow was confirmed by flow mode experiments.     

Thermal performance enhancement in a miniature channel using different passive methods

Journal of Thermal Analysis and Calorimetry - In the present work, different passive methods have been numerically investigated to improve the thermal performance in a miniature channel. The heat...     

Multielement analysis of minor components using a 14 MeV neutron generator

14-MeV neutron activation has been used for the determination of F, Al, Si, P, K, Mn, Mo and W in minerals and steel. Most of these elements have been estimated simultaneously through gamma-ray spectrometry. The evaluation of the effects of interferences is discussed and the sensitivities are given. The main features of the neutron generator fabricated in this Centre and used in this work are briefly described.