Tubular Module Research Articles

Hydrodynamical behaviour of non Newtonian flows in a cross-flow filtration tubular module

Friction factors of Newtonian and non Newtonian fluid flows in a cross-flow filtration module composed of seven tubular membranes connected in parallel were measured during runs with and without suction and modelled by classical laws. With the aim of understanding mass transfer phenomena during cross-flow filtration of non Newtonian fluids, the “flowing” concentration polarisation layer was characterised in terms of concentration and rheological parameters from pressure drops measured under laminar conditions.

Pervaporative extraction of volatile organic compounds from aqueous systems with use of a tubular transverse flow module.: Part I. Composite membrane study

The effect of a woven fiberglass support layer in a composite silicone rubber membrane is examined. Flux reduction during the pervaporation process is attributed to the additional resistance offered by the support layer in both flat sheet and hollow fiber configurations. A 40% reduction (from that of the bare silicone rubber membrane) in toluene flux from a toluene–water feed solution is estimated to occur in the 0.9 mm outer diameter hollow fibers. These fibers are later used in the construction of a tubular transverse flow module to be discussed in Part II of this series.

Pervaporative extraction of volatile organic compounds from aqueous systems with use of a tubular transverse flow module.: Part II. Experimental results

Experimental flux and selectivity results are shown for the toluene–water–silicone rubber system in a tubular transverse flow module. These results are compared to predictions offered from nonlinear pervaporation models based upon modified Flory–Huggins and Lattice Fluid thermodynamics. A comparison of experimental with model results shows good predictions of solute flux for both thermodynamic theories. However, the model was discovered to be very sensitive to calculations of infinite dilution diffusion coefficients and a priori predictions of experimental membrane selectivities were underestimated.

Resistance Analyses for Ultrafiltration in Tubular Membrane Module

The resistance analyses for ultrafiltration of macromolecular solutions by the resistances-in-series model and the modified gel-polarization model, respectively, have been extended to the turbulent ultrafiltration system in this study. The experiments are carried out by ultrafiltrating dextran T500 solutions in a tubular membrane module with membrane material of ZrO2-TiO2/carbon. It is found that the permeate fluxes are predicted very well by these models for both laminar and turbulent systems, and the resistance caused by the concentration polarization/gel layer (R p) can be reduced by increasing the crossflow velocity on the membrane surface. Analysis by the resistances-in-series model showed that, R p decreases with flow velocity with exponents of 0.49 and 0.99 for the laminar system and the turbulent system, respectively.

Kinetic modelling of cadmium removal from phosphoric acid by non-dispersive solvent extraction

In this work it is reported that the kinetic modelling of the separation of cadmium from phosphoric acid by non-dispersive solvent extraction. Using Cyanex 302 as selective extractant, the extraction step was carried out in a hollow fibre module containing polypropylene fibres, whereas the concentration step required a ceramic module with tubular channels due to the high acidity of the backextraction agent. Application of the methodology previously reported by the authors led to the development of a kinetic model with three design parameters, i.e., equilibrium constant of the extraction reaction ( K' e = 6 × 10 3 mol −2/l −2), membrane mass transport coefficient of the extraction module K me=8.33×10 −8 m/s) and of the backextraction module ( K ms=3.33×10 −8 m/s), that described satisfactorily the behaviour of the separation-concentration system. Thus, in this work a new application of the non-dispersive solvent extraction technology is presented, characterising at the same time the behaviour and parameters of a new type of contactor, i.e., a tubular ceramic module.

Airlift crossflow membrane filtration — a feasibility study with dextran ultrafiltration

In this paper, a novel membrane process — airlift crossflow filtration, is developed. It uses the airlift effect to drive liquid flow tangentially across the membrane surface whilst simultaneously making use of the enhancing effect of gas bubbles on membrane performance. The compressed air also provides the transmembrane pressure for the filtration operation. The feasibility of this new concept was demonstrated by the ultrafiltration of a dextran solution, proving that higher permeate fluxes can be achieved with a very low gas injection rate. Compared to conventional single phase crossflow operation at the same liquid flowrate, about a 30% increase in permeate flux was obtained with the airlift crossflow operation without the need of a recirculation pump. Process scale-up for tubular membrane systems was discussed, and a specific design comprising a tubular membrane module with internal circulation was presented. Evaluation of the process energy requirement indicated that this would be a low energy cost process compared to conventional single phase crossflow operation, whilst also saving process capital cost due to the elimination of a recirculating pump. Hence, this process is particularly attractive to cost-sensitive processes such as waste effluent treatment.

Effect of bubble size and frequency on the permeate flux of gas sparged ultrafiltration with tubular membranes

Gas sparged ultrafiltration experiments are performed using a tubular membrane module with solutions of dextran and human serum albumin (HSA) as the test media. Air is injected, in a controlled manner with the ability to adjust bubble size and frequency independently, into the membrane module to create a gas–liquid two-phase crossflow operation. The effects of bubble size and frequency on the permeate flux of the sparged ultrafiltration are studied experimentally. It is found that the permeate flux increases with the bubbling frequency in the examined range. The effect of bubble size on flux can be divided into two regions, an increasing region for smaller bubbles and a plateau region for larger slugs. The results are discussed on the basis of bubble wake hydrodynamics.

Hybrid membrane filtration process for leachate treatment

A hybrid technology known as the ultrafiltration-biologically active carbon (UF-BAC) process that amalgamates adsorption, biodegradation and membrane filtration is found to be highly efficient for treatment of landfill leachates. The process employs bioactive powdered activated carbon (PAC) with a leachate-acclimated microbial culture for the simultaneous sorption and biodegradation of organic constituents. Tubular cross-flow ultrafiltration membrane modules separate out colloids and microorganisms, and a high quality permeate is obtained. Batch biokinetic studies were performed for the two leachates to evaluate their extent of biodegradability and biodegradation kinetics. The process efficiencies for both leachates were in the range of 95–98% in terms of TOC removal, and exceeded 97% for specific organic pollutants. The UF-BAC process compared well with the PACT process in terms of organic removal, and produced higher quality effluent in terms of suspended solids (100% removal). The study demonstrated that addition of 1% PAC mitigated permeate flux deterioration attributed to membrane fouling and concentration polarization, and enhanced permeate transport. Possible mechanisms for flux amelioration are discussed.

Flux enhancements with gas sparging in downwards crossflow ultrafiltration: performance and mechanism

In this study gas-liquid two-phase crossflow ultrafiltration was studied in downwards flow condition. A commercially available tubular membrane module was installed vertically and the feed solution and the injected gas bubbles flow downwards inside the membrane tubes. The permeate flux achieved in such an operation was compared to conventional single phase ultrafiltration and also to gas sparged upwards crossflow operation. The effect of operating parameters, including liquid and gas flow rates, transmembrane pressure (TMP) and feed concentrations, was examined with dextran solutions. Flux increases of up to 320% were achieved with gas sparging in the experimental study compared to single liquid phase crossflow ultrafiltration. This enhancement is much more profound when the liquid phase is in laminar flow, particularly when the concentration polarisation is more severe, for example, at a high TMP, a higher feed concentration, and a low liquid crossflow velocity. The enhancing effect is not very significant when liquid flow itself is turbulent. The flow pattern proved to be an important parameter and the operation could be optimised to achieve maximum enhancement with minimum gas injection.

Process factors during removal of oil-in-water emulsions with cross-flow microfiltration

The results from an experimental study on the separation of oil from oily waters are presented. The investigations were focused on the problem of the process factors that affect the permeate flux. The following parameters were taken into account: membrane type, module type, operating conditions such as transmembrane pressure, recirculation velocity, feed concentration and temperature. The continuous monitoring of the permeate flux enabled the flux decline to be observed in each case. The experimental method compared the cross flow with dead-end runs under various operating conditions. The performance of the polymeric membranes Millipore 0.45 μm and Gelman 0.1 μm were compared with the ceramic membranes Ceramesh 0.1 μm and Membralox 0.1 μm. The flat plate module Pleiade and tubular module membralox were used. The flux-time curves were analyzed using a modified form of Hermia's method and the surface renewal concept to investigate the reason for flux decline. The conditions under which pore-blocking is important were identified using a combination of pore blocking and cake filtration formulae. The results of the experiments show that the ceramic membranes demonstrated better performance. The permeate flux was higher and more stable. Ceramic membranes were less susceptible to fouling and much more sensitive to control by means of operating conditions.

A membrane reactor for methanol synthesis

Methanol synthesis from CO 2 and H 2 is a promising chemical energy storage reaction for hydrogen. The methanol yield of the synthesis is limited by the thermodynamic equilibrium at the temperatures required by state-of-the-art catalysts. Substantial conversion improvements would be achieved by selective product separation from the catalyst bed of a synthesis reactor. A perfluorinated cation exchange material (Nafion ®, DuPont) is evaluated for use as a vapour permeation membrane at temperatures up to 200°C. The permselectivities for methanol and water with respect to hydrogen depend on the counter ion in the polymer, and drop as a function of temperature. With respect to long term stability and performance, lithium is best suited as a counter ion, with permselectivities of 32 for water, and 5.6 for methanol, respectively. Permeability and permselectivity are found to increase with increasing partial pressures of both methanol and water vapours. The capability of the Li-Nafion membrane to separate products from a commercially available catalyst bed operating on CO 2 and H 2 at 200°C is demonstrated using a simple tubular membrane reactor module.

Removal of benzene traces from contaminated water by vacuum membrane distillation

Vacuum membrane distillation was investigated as a possible alternative technique for the removal of volatile organic compounds from contaminated water. A mechanism for the trans-membrane removal of benzene was postulated and corroborating experimental evidence is presented. Batch experiments were conducted on dilute benzene-water mixtures using a tubular polypropylene membrane module. The concentration within the feed reservoir was monitored over time and the results analysis was conducted from an overall mass transfer coefficient perspective. The impact of operating variables such as feed concentration, flow rate, temperature and downstream pressure was investigated. A previously published case study was used as a basis for comparing volatile organic compound removal by vacuum membrane distillation to air stripping in a packed tower.

Purification of oily wastewater by ultrafiltration

Investigations have been carried out for purification of oily wastewater by a combination of ultrafiltration/reverse osmosis processes. A tubular ultrafiltration system using cellulosic and non-cellulosic membranes was tested with typical oily wastewater collected from harbour and simulated emulsion without any pretreatment. Both membrane types produced a permeate with an oil content generally less than 10 mg/dm 3. Rejection of chemical oxygen demand (COD) is 80% for ultrafiltration treatment. The permeate is generally of acceptable quality for direct sewer discharge. For further treatment of UF permeate, reverse osmosis was applied with tubular B1 PCI modules using cellulose acetate membranes. The rejection of COD after ultrafiltration/reverse osmosis treatment increased up to 98.5% and total dissolved solids (TDS) to 95.7%. Reverse osmosis treatment will permit reuse of the treated water as process water.

Flowsheet simulation of ultrafiltration and reverse osmosis processes

A membrane separation model for tubular module reverse osmosis and ultrafiltration processes was developed in this work. The membrane area of a process can be calculated by this model and the stream matrix of a process can be determined by a process simulation program. In this work the unicorn simulation program was used. The membrane separation model calculates permeate flux and rejection of the solute in small increments of the membrane tube over the entire range of the tube and the process. Calculation of the permeate flux and rejection can be performed by polynomial equations fitted to the experimental data, or by equations based on mass transfer models. The finely porous model and the statistical mechanical model were used. Some experimental data are also needed for determining the parameters of the mass transfer model equations by parameter fitting. The membrane separation model was tested by the simulation of reverse osmosis of aqueous ethanol and acetic acid solutions, and ultrafiltration of aqueous sodium carboxymethylcellulose (CMC) and poly(vinylpyrrolidone) (PVP) solutions. Multistage recycle separation processes with four or five recycle stages were used as test processes. The values of the calculation parameters (i.e., iteration accuracy of the recycle stream, iteration acceleration factor and the number of increments of the membrane tube) giving reliable results with minimum computing time were determined.

Ultrafiltration preparation of pectinolytic enzymes from citric acid fermentation broth

The introduction of ultrafiltration to the classical scheme of citric acid production allows the combination of two technological cycles: citric acid production and the recovery of pectinolytic enzymes from the fermentation broth. Both products are partially purified and concentrated. The investigation was carried out using a tubular ultrafiltration module with polyacrylonitrile membranes with adequate transport-separation properties. The ultrafiltration process was optimized and the concentration of fermentation broth was carried out after primary diafiltration. The retentate obtained was also concentrated by means of a thermal method and permeate containing citric acid was then concentrated using a reverse osmosis DDS plate-frame module (De Danske Sukkerfabrikker, Denmark).

Control of ethanol production and monitoring of membrane performance by mass-spectrometric gas analysis in the coupled fermentation-pervaporation of whey permeate

A coupled fermentation-pervaporation process was operated continuously with on-line mass spectrometric gas analysis monitoring of product accumulation on both the upstream and the downstream sides of the membrane. Efficient coupling of the fermentation with pervaporation was attained when a steady state of ethanol production and removal was achieved with whey permeate containing high concentrations of lactose (>8%) or by controlled lactose additions that also compensated for loss of liquid due to pervaporation. The combined system consists of a tubular membrane pervaporation module, directly connected to a stirred fermentor to form one circulation loop, kept at 38°C, with both units operating under computer control. Mass spectrometric gas analysis of the CO2 gas evolved in the fermentor and the ethanol and water in the pervaporate on the downstream side of the membrane enabled us to follow the production of ethanol and its simultaneous removal. Membrane selectivity was calculated on-line and served to monitor the functioning of the membrane. Batch-wise-operated fermentation-pervaporation with Candida pseudotropicalis IP-513 yielded over 120 gl−1 of concentrated ethanol solution using supplemented whey permeate containing 16% lactose. A steady state lasting for about 20 h was achieved with ethanol productivity of 20 g h−1 (approx. 4 g l−1 h−1). Membrane selectivity was over 8. Controlled feeding of concentrated lactose suspension in the whey permeate (350 g l−1) resulted in the continuous collection of 120–140 g l−1 of ethanol pervaporate for 5 days, by which time salt accumulation hampered the fermentation. Medium refreshment restored the fermentative activity of the yeast cells and further extended the coupled process to over 9 days (200 h), when reversible membrane fouling occurred. The membrane module was exchanged and the combined process restarted.

An investigation on the configuration of inserts in tubular ultrafiltration module to control concentration polarization

The flux through a tubular ultrafiltration membrane has been improved using inserts with different configurations in the module. It is established that the introduction of inserts, irrespective of their configuration, leads to better flux in comparison with that in the empty tube. Inserts with variable cross-sectional area show better performances than do smooth cylindrical ones. The interruption of the formation of a gel layer by the inserts with variable cross-sectional area is attributed to the periodical change in the feed flow velocity and the tangential stress on the membrane surface.

An Investigation into UF Systems in the Pretreatment of Seawater for RO Desalination

Low-cost tubular and capillary polyethersulphone ultrafiltration modules have been developed and their use in the pretreatment of seawater for reverse osmosis desalination investigated. Mechanical and chemical cleaning regimes were evaluated for the restoration of the productivity of fouled membranes. The cost-effectiveness of using these UF membranes for pretreatment to RO, was found to depend on a combination of membrane configuration, type and frequency of cleaning regime and average productivity.

Japan's Aqua Renaissance '90 Project

Aqua Renaissance' 90 is a six year R & D project for water reuse and energy recovery supported by the Japanese Ministry of International Trade and Industry. The objective is to develop low cost treatment processes utilizing bioreactors coupled with membrane separation units to produce reusable water from industrial waste water and sewage. Polymeric and ceramic membranes in capillary, hollow fiber, tubular and plate and frame modules have been tested in conjunction with bioreactors on a number of actual waste water and sewage streams. This paper will focus on membrane performance and power consumption. Recent results from test plant operation indicate that treatment of concentrated waste water by membrane separation units uses less than a third of the conventional power requirement.

Potential Applications of Pulsed Flow for Minimizing Concentration Polarization in Ultrafiltration

Abstract Various methods have been proposed to control or minimize concentration polarization and membrane fouling. A brief review of these alternatives is presented in this paper. Flow pulsation as a means of improving transmembrane flux has been studied experimentally by a few investigators. A mathematical model is developed to evaluate the performance of a tubular membrane module under oscillatory flow conditions. Besides the effect of osmotic pressure and axial pressure variation, the model considers the convective-diffusive mass transport without decoupling the momentum equation from the solute continuity equation. Model equations are solved by a finite difference method as part of an iterative solution. Model predictions of transmembrane flux with experimental data are found to be in good agreement. By flow pulsing, it is possible to improve the transmembrane flux by more than 68% at pulsing frequency of 60 cycles · min−1. An analysis of extra power requirement for flow oscillation shows that the ga...