OVERVIEW AND RECENT DEVELOPMENTS IN EFFECTIVE PARTICLE DECONTAMINATION BY WASHING PROCESSES
H. Anlauf (pages 20-25)
Washing of disperse solid systems is a widely used and complex operation that is embedded in the process chain. The target of the washing operation can be either the purification of the solid particles or to recover the substances dissolved in the liquid as completely as possible. In many cases hazardous substances have to be removed from solid waste materials before depositing them. Washing is often not only a question of environmental protection but also a possibility for the recovery of valuable substances to recycle them in the process and decrease the demand of raw materials. Since the washing step is mostly integrated into other process operations, it is subject to numerous demands exceeding by far the primary demands, like good separation results and a low specific wash liquid demand. Because of the phenomenology and limits of different washing processes it is difficult to choose the appropriate washing process. Additionally, the transport mechanisms of some washing processes are not yet sufficiently understood.
A broad overview of the existing washing processes, the numerous parameters, and the demands they have to meet is given. Included here are well known examples from different separation technologies for permeation and dilution washing in co-current or counter-current mode as well as new highly efficient and recently developed washing processes. Based on a categorization of the washing processes and the classification of these processes according to their macroscopic transport phenomena, one can compare the existing washing processes and show their individual limits. This can be used as a rough guideline for the choice of an appropriate washing process within the complete process chain of solid/liquid separation.
POLYMERIC AND CERAMIC NANOFIBRES
J. Steffens (pages 26-28)
Nanofibre is a broad phrase generally referring to a fibre with a diameter less than 1 μm. While glass fibres have existed in the sub-micron range for some time and polymeric meltblown fibres are just beginning to break the micron barrier, 0.25 μm diameter electrospun nanofibres have been manufactured and used commercially for air filtration applications for more than twenty years. Several value-added nonwoven applications, including filtration, barrier fabrics, wipes, personal care, medical and pharmaceutical applications may benefit from the interesting technical properties of commercially available nanofibres and nanofibre webs.
This paper is a review of the electrospinning process for making nanofibres and nonwoven nanofibre webs from synthetic fibre-forming polymers. The resulting physical characteristics of nanofibre webs will be discussed. In order to provide a useful context for the nonwovens industry, nanofibre webs will be compared to both meltblown and spunbond nonwovens. The description and comparison of the properties should provide product designers in the nonwovens industry with the tools to generate product and applications ideas about new uses for nanofibres.
THREE STEPS TO A NEW FILTRATION QUALITY: MODERNISATION OF FILTER PLANTS BY REVAMPING
T. Langeloh and R. Bott (pages 29-32)
The standard job of a filter is to separate solids from a liquid and to provide: (a) well dewatered dry filter cakes; (b) clear, particle-free filtrate; (c) washed, clean filter cakes; (d) high and constant liquid and solids flow rates; (e) easy to handle bulks; in a reliable operation without failures e.g. by a clogged or damaged filter cloth. However, filter plants are often not operated under optimum conditions and the results often do not meet the requirements. The discrepancy becomes critical if a filter is used up to its capacity limit or beyond its capability, because of increased production rates, higher quality requirements, the introduction of modern process control or a change of product characteristics, for example, the particle size distribution.
In this situation the decision has to be made whether the production target shall be reached with a new filter or even with retrofitting of the existing filter plant. To answer the question it is necessary to assess the potential of improvement and to have concrete ideas as to which specific measures have to be taken for an effective filter upgrading. For this reason BOKELA developed a 3-step optimisation programme for drum, disc, belt, pan, cartridge, leaf filters or filter presses which is clearly structured in 3 phases with exactly defined decision points. According to the programme numerous filter plants have been already optimised in different industries leading to improved product qualities and operational reliability and capacity increases up to 100%.
EVALUATION OF ELECTROSTATIC FILTER MEDIA FOR AEROSOL SAMPLING AND ANALYSIS
M.M. Abdel-Salam (pages 33-40)
Aerosol particles are released into the atmosphere by different processes with significant effects on human health. During the last few decades there has been increasing interest in the measurement of aerosol particles using a wide range of different aerosol samplers and aerosol filtration is a widely used method for measuring airborne particles. Electrostatic filters have the advantage of high aerosol collection efficiency accompanied by low resistance to airflow when compared to the traditional high resistance fibrous and membrane filters commonly used in aerosol sampling.
This paper investigates the suitability of 3M electrostatic filter media for aerosol sampling and analysis in comparison to some conventional aerosol sampling filters, such as glass fibre and membrane filters. The areas investigated include resistance to airflow, aerosol collection efficiency, loading capacity, weight stability and chemical properties. The 3M electrostatic filter media was found to be suitable for aerosol sampling and analysis (gravimetric and elemental chemical analyses) and was as good as the traditional filters widely used in aerosol sampling.
MEASUREMENT OF BIOAEROSOL FILTRATION EFFICIENCY IN AN ASHRAE 52.2 FILTER TEST FACILITY
W. Tang, M.A. Ramakrishnan, S. Goyal and T. Kuehn (pages 40-44)
A real-time monitor of bioaerosols, Fluorescence Laser Aerodynamic Particle Sizer (FLAPS), was evaluated for its ability to detect bioaerosols and measure bioaerosol filtration efficiency. Biological spores (Bacillus subtilis and Cladosporium sphaerospermum) were aerosolized and injected into a wind tunnel based on ASHRAE Standard 52.2. The sensitivity of the FLAPS was adjusted by the photomultiplier tube (PMT) gain to distinguish between bioaerosols and non-biological aerosols such as polystyrene latex and potassium chloride particles. Suspensions of biological spores were carefully prepared to remove interference with culture media. Aerosol concentrations upstream and downstream of a clean synthetic media filter (MERV-14) with electrostatic charge were monitored by FLAPS and by an Andersen impactor. The calculated filtration efficiency from the measurement of FLAPS was 96% for B. subtilis and 100% for C. sphaerospermum. These results agree with those obtained with the Andersen impactor (91% for B. subtilis and 99.5% for C. sphaerospermum).
PULSED ELECTRIC FIELD ASSISTED SOLID/LIQUID EXPRESSION OF AGRO-FOOD MATERIALS: TOWARDS A NOVEL ENVIRONMENTALLY FRIENDLY TECHNOLOGY E. Vorobiev, L. Praporscic and N. Lebovka (pages 45-49)
In this paper we analyse the kinetics and mechanisms of solid/liquid expression combined with a pulsed electric field (PEF). Experiments were performed with raw agro-food materials (slices of fresh carrots and apples, whole grapes), and with agro-food wastes. The layer of particles was formed in the laboratory filter press cell, connected to a PEF generator. The PEF treatment of different intensity (<1000 V/cm) and duration (10-300 ms) was applied before solid/liquid expression (as a pretreatment) or during expression (intermediate treatment).
The PEF application caused electropermeabilisation of cell membranes and enhanced considerably the solid/liquid expression from studied agro-food materials (from 30-50% before treatment to 70-85% after the treatment). The simplified empirical model of solid/liquid expression fitted the experimental data well for both untreated and electrically treated materials. The PEF treatment also influenced the quality of juices expressed from raw materials. The PEF treated juices were more limpid and pure compared to the untreated juices, probably due to selectivity of extraction from electrically permeated cells. Such juices can be purified and clarified more easily than untreated juices. Therefore, it can be speculated that the novel technology comprising the combination of PEF and solid/liquid expression will generate less quantity of industrial wastes compared to existing transformation technologies.
PARTICULATE EMISSIONS FROM FILTER-BASED GAS CLEANING DEVICES AND THEIR CHARACTERIZATION ACCORDING TO PM2.5 CRITERIA
G. Kasper, J. Binnig and J. Meyer (pages 49-54)
We report on a study of particle emissions from pulse-jet cleaned filter media and filter ageing effects of up to 10,000 cycles, which were obtained in laboratory tests on a VDI test rig, using two measurement systems adapted specifically to determine size selective data according to PM2.5. Time resolved measurements were made with an optical particle counter recalibrated (for the test dusts Pural SB and NF) to directly read dust mass vs. aerodynamic diameter according to the PM2.5 transmission curve. In parallel, a cyclone was used as pre-separator in combination with an analytical filter to obtained emitted dust mass averaged over a number of cycles.
Regarding PM2.5 emission levels, one can draw different conclusions from the same data, depending on whether total emissions per cycle or average concentrations per cycle are reported. The results also illustrate how ageing depends on the type of dust and the duration of ageing cycles. Ageing progressed more rapidly with the coarse, free flowing dust. The 5 s ageing cycles prescribed by VDI and ASTM protocols are too short.
THE PRESSURE-DRIVEN MEMBRANE BIOREACTOR vs. THE ION EXCHANGE MEMBRANE BIOREACTOR FOR THE REMOVAL OF TOXIC ANIONS FROM DRINKING WATER SUPPLIES: ADVANTAGES AND LIMITATIONS
S. Velizarov, A. Barreiros, C.T. Matos, M.A. Reis and J.G. Crespo (pages 54-58)
Two membrane bioreactor configurations, the pressure-driven membrane bioreactor and the ion exchange membrane bioreactor (IEMB), were tested for the removal of toxic oxyanions from drinking water. The results based on a case study – nitrate removal at a treated water production rate of 30 L m-2 h-1 – showed that in both bioreactors the desired water quality in terms of nitrate and nitrite can be achieved. The pressure-driven membrane bioreactor allows for possibly higher water production rates and uses relatively cheaper membranes, however, the water quality control in terms of TOC requires on-line monitoring and a strict regulation of the carbon source addition. This configuration also requires an initial start-up period before achieving a steady state operation because the water and microbial culture are in direct contact.
The ion exchange membrane bioreactor offers the advantage of selectively removing the target anion and producing TOC-free water with practically no initial start-up period. Due to the physical separation of the microbial culture from the water stream, the very low diffusion coefficient of ethanol through the Neosepta ACS membrane used, and the membrane-attached biofilm acting as an additional reactive barrier to ethanol penetration, secondary pollution of the treated water was avoided. The development of cheaper anion exchange membrane (possibly in a hollow fibre form) would make the IEMB process economically competitive.
LINKING DEWATERING PARAMETERS FROM TRADITIONAL, FLUID MECHANICAL AND GEOTECHNICAL THEORIES
R. de Kretser and P. Scales (pages 60-66)
Due to the wide range of research fields requiring an understanding of solid/liquid separation, a range of theories and parameter sets have evolved in parallel. Discussion of the approach from one field of research is seldom discussed in the context of the other. This paper attempts a reconciliation of the key approaches in terms of their respective similarities, advantages and inter-conversion of the key parameters on which they are based. The advantages of such a reconciliation both enables a greater understanding of dewatering in general and provides the ability to readily convert data or utilise test methods from one field of research within another.
Keywords: Specific cake resistance; hindered settling factor; flux density; compressive yield stress; solids stress; consolidation coefficient; permeability; compressibility.
NANOFLUIDICS IN FILTRATION AND PARTICLE PROCESSING
H. Nirschl, R. Wengeler and B. Schäfer (pages 67-74)
The subject of this paper is to give an insight into the fluid dynamics when nanoscale or submicron particles are handled in filtration or separation processes. The differences of the forces on particles in macroscale and nanoscale are presented and discussed in detail. The differences can be separated according to particle-particle interaction, the interaction with the surrounding fluid and the influences of external fields. It is evident that the behaviour of nanoparticles suspended in a fluid depends strongly on the surface properties of the suspended material.
As examples, the flow through a nanoscale packed bed and the dispersion process of nanoscale particles is discussed. It is shown that the permeation through a nanoscale structure is influenced by electrostatic and electroviscous effects. Electrostatic effects have an influence on the aggregate structure while the electroviscous effects directly influence the permeation.
Keywords: Nanofluidics; nanoparticles; electrostatic and electroviscous effects; packed bed; suspension.
INVESTIGATION OF FILTRATION CONDITIONS FOR NANOFILTRATION OF REACTIVE DYE PRINTING WASTEWATER
N.P.R. Andersen, M.L. Christensen, K. Keiding and I. Petrinić (pages 75-81)
One of the major problems concerning textile wastewaters is the coloured effluent from rinsing and washing processes. Although the dye concentration is low, compared to many other chemicals found in this type of wastewater, the water quality is insufficient for reuse or discharge. Application of nanofiltration and reverse osmosis to reduce or remove dyestuff from reactive dyeing operations has been widely explored in the literature and is now applied in many plants worldwide. Less attention, however, has been paid to the membrane filtration of reactive dye printing wastewater containing high amounts of thickeners (alginate).
In this study synthetic wastewater, with a composition similar to reactive dye printing wastewater produced during the washing of printed fabric, has been subjected to nanofiltration. The effects of dye, thickener and salt concentration on the separation efficiency and permeate flux has been investigated. All experiments were carried out with an NFT-50 membrane in a plate and frame module applying constant crossflow and filtration pressures ranging from 200 to 1500 kPa.
The results showed that the presence of alginate in the wastewater markedly decreased the permeate flux compared to wastewater without alginate but with the same amount of dye and salts. Calculations indicated that the reduced permeate flux originated exclusively from concentration polarization (CP) and not membrane fouling. The retention of dye was high (>99%) at all pressures, while the retention of salt, expressed as retained conductivity, increased with pressure from 65% at 200 kPa to 85% at 700 kPa (crossflow 240 L/h). However, at higher pressure and crossflow the retention of salt decreased due to more pronounced CP. Thus, for the filtration of reactive dye printing wastewater, a high retention of dye and salt could be obtained at filtration pressures that were significantly lower than those normally used when filtering textile wastewaters.
Keywords: Nanofiltration; alginate; retention efficiencies; process optimisation; water reuse.
IMPROVING CFD SIMULATIONS OF LUBRICANT OIL FIBROUS FILTER MEDIA
M.J. Lehmann and P.K. Hermar (pages 82-86)
Modelling lubricant oil filter media and its particle collection efficiency is still a demanding but elusive goal. Two key areas of difficulty are the availability of appropriate models for particle collections in simulation programs and the characterisation of the fibrous media. The widely used CFD program FLUENT calculates particle tracks based on the motion of the mass point of a particle. Consequently a particle is only counted as deposited when its centre hits a fibre surface. This model is not appropriate for liquid filtration modelling, as the dominant collection mechanism is interception. Therefore, we enhanced FLUENT by our own subroutines to account for the perimeter of a particle and its distance to a fibre surface which resulted in improved prediction of efficiency for particle removal from liquids.
The challenge to model the fibrous structure of the media remains. Many recent studies attempt to model the 3D fibre structure in 2D by random placement of cylinder cross sections vs. the prior over-simplified approach using perfectly spaced fibres. However, this approach still does not generate a realistic structure and still over predicts pressure drop. So the current work investigates the influence of semi-randomly arranged fibre structures. The resulting fibre structures are more realistic than those created by fully-random placement. Simulations comparing the pressure drop and collection efficiency of these types of structures are shown.
The results demonstrate the effect of the different kinds of simulated 2D fibre structures on pressure drop and collection efficiency. A shift from fully random to semi-random structures results in changes to flow patterns and generally leads to a significant decrease of pressure drop. Furthermore, changing from a staggered fibre grid to more locally aggregated fibre structures increases the media usage through its depth. However, moving from fully random to semi-random structures causes decreases in collection efficiency. The results and discussion briefly illustrate these effects and the challenges for future media modelling.
Keywords: CFD-simulation; fibrous filter; filter media; filter design; modelling.