Experts in filtration and separation

We advance and disseminate knowledge in the design and use of filtration and separation techniques in industry, commerce and other walks of life.

Volume 14, Issue 2 Abstracts from the FILTRATION journal

Shireesh Pankaj, Dominic Baron, Trevor Sparks and Richard Lydon (pages 84-86)

AzurtexTM and PrimaporTM is a family of filter media products from Clear Edge filtration that incorporate a microporous coating onto, and into, the surface. Over the last number of years, Clear Edge has developed this product for a variety of fine and ultrafine filtration applications, including titanium dioxide, kaolin, gold tailings etc. Clear Edge has developed a body of knowledge in fine filtration applications and built-up a strong scientific understanding using its state-of-the-art laboratories to understand the chemistry and structural properties required for solid/liquid filtration.

It is shown that microporous coatings improve blinding resistance, enhance throughput and improve filtrate clarity; these properties are mainly attributed to the surface filtration effects. The gain in productivity is also reflected by increased retention efficiencies and abrasion resistance of the coated media. The findings from a number of case studies show that the most economically viable solution for fine industrial process filtration is microporous coated media.

Riina Salmimies (pages 86-91)

In this paper it is shown how the ceramic filter media used for the dewatering of iron ore, both hematite and magnetite concentrates, can be regenerated to full initial permeability using oxalic acid washing. However, regeneration of the filter medium might not always be consistent with particle dissolution as this process could have stopped but changes in performance of the filter medium can still be seen. Furthermore, the dissolution of particles could still be ongoing although most of the initial permeability was already restored. Results indicate that changes in pore size distribution explain the increase in permeability. As more pore volume is liberated, so permeability increases, although, such data cannot differentiate between complete and partial blocking of the medium pores.

Jose Angel Sorrentino (pages 92-100)

Capillary pressure curves have been determined for a long time in both filtration technology and oil reservoir evaluation with different aims, but with quite similar modelling tools. It has been always accepted that the existence of a capillary pressure curve is the result of the pore size distribution (PSD) prevailing in the porous medium, otherwise, all pores in the porous bulk (cake or reservoir) would drain or fill to the same pressure. However, modelling of the capillary pressure curve has been mainly done in accordance with the work of Corey and Brooks1. This approach is based on the demonstrated fact that a double logarithmic plot of capillary pressure vs. pore saturation leads to a straight line, except in the neighbourhood of saturation (S) equal to one, where a concavity is present. Although that form has not been detected due to insufficient resolution of the measurement, it has led to the use of a very simple model based on the power function which has been extensively used by oil-related researchers to calculate relative permeabilities.

This paper incorporates the quite simple concept that if the capillary pressure curve represents a PSD then a distribution function should be used to fit it. Therefore, different distribution functions were applied. Normal and log-normal functions fail to fit the well known, straight line behaviour of the double log plot. Power functions satisfy this criterion, but only the Rosin-Rammler-Sperling-Bennett (RRSB) function was able to produce linear behaviour as well as a concavity near S = 1. Although mathematical handling of the RRSB function is more difficult, presently available computer programs provide the required capability. This approach was used to fit capillary pressure data obtained from filter cakes constructed from custom mixtures of limestone particles that were prepared by varying geometrical standard deviations between 1 and 3 to ensure a broad data spectrum.

The use of the proposed model allows a general plot for reduced saturation that yields reasonably consistent capillary pressure curves for all the tested mixtures. The parameters of the RRSB distribution were correlated with PSD data: Sr and pk’ as a function of the Sauter mean diameter (xSV) and the PSD index, ξ, with the PSD geometrical standard deviation, σg,3.

Audrey Forthomme, Aurélie Joubert, Yves Andrès, Xavier Simon, Philippe Duquenne, Denis Bemer and Laurence Le Coq (pages 100-108)

Microbial aerosols are captured by HVAC filters and can colonise those filters in appropriate conditions. The aim of the study is firstly to examine the influence of humidity at fixed temperature on microbial growth for two HVAC filtering media (synthetic and glass fibre), and secondly to estimate the microbial release downstream of these filters after microbial growth. An experimental setup has been developed to homogeneously contaminate several filters simultaneously by nebulising a microbial consortium composed of bacteria cells and fungal spores.

After filter contamination, the growth of microorganisms onto the filters and measurements of microbial release from the filters were studied after latency in specific conditions of humidity and temperature during 48 h and 168 h (which simulate stops of HVAC systems during a weekend and a holiday week). Microbial growth onto the filters was evaluated after extraction by CFU counting. In order to study release the tested filters were placed in a column where airflow, representing a restart of the HVAC ventilation, was imposed. The microorganisms released downstream of the tested filters were recovered by a high efficiency collection filter and evaluated by CFU counting.

Maksym Loginov, Nikolai Lebovka and Eugene Vorobiev (pages 109-120)

This article describes an experimental procedure and data analysis for characterising the filterability of mineral suspensions with the help of a new method of multistage centrifugation (MC). MC is based on the combination of two experiments: centrifugal sedimentation-consolidation in a centrifuge tube with an impermeable bottom, and centrifugal compression-permeability in a centrifuge tube with a permeable bottom (filtration cell). The experiments were performed at different centrifugal accelerations with the help of the analytical photocentrifuge. The application of MC requires a relatively small quantity of suspension (a few millilitres).

The analysis of experimental data yields the constitutive equations, namely the dependence of local permeability on particle volume fraction, and the pressure dependencies of local filter cake dryness and local specific cake resistance. MC was used for characterisation of the model suspension of calcium carbonate. Application of MC allowed the estimation of specific resistance in the pressure range 104 to 105 Pa. The values of specific cake resistance, obtained with the help of MC, were in good correspondence with those determined from conventional constant pressure filtration experiments. The pressure dependence of specific cake resistance obtained by MC was also comparable with that estimated for a lower pressure region (103 to 104 Pa) by the method of centrifugal consolidation as proposed by Loginov et al.1

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