- The Filtration Society - http://www.filtsoc.org -

Volume 11, Issue 3

A REVIEW OF EQUIPMENT SELECTION AND PROCESS DESIGN IN SOLID/LIQUID SEPARATION
Steve Tarleton (pages 151-161)

This paper presents a survey of the methods of equipment selection and process design for solid/liquid separation. It is shown how a combination of automated analysis, laboratory scale test work and computer aided calculations provide the most reliable results. Current best practice is highlighted, both in terms of the computer software available (e.g. Filter Design Software) and the advantages offered by automated filter apparati for data generation. Worked examples and example data are presented to show the capability of the overall process.

CLOGGING OF INDUSTRIAL PLEATED HIGH EFFICIENCY PARTICULATE AIR (HEPA) FILTERS IN THE EVENT OF FIRE IN A CONFINED AND VENTILATED FACILITY
Victor Mocho and François-Xavier Ouf (pages 162-167)

The IRSN and AREVA NC are currently conducting a fire research programme to improve knowledge of the clogging of industrial pleated high efficiency particulate air (HEPA) filters in order to develop an empirical model for clogging of such filters by combustion aerosols. The model must insofar as possible be independent of the nature of the fuel. This paper discusses the influence of various ‘direct’ factors such as the filtration velocity, mass of deposited aerosols per filter area, diameter and morphology of the combustion particles, condensate content of the aerosols, and ‘indirect’ factors such as the air flow feeding the fire and its oxygen content, which influence evolution in the aeraulics of a clogged filter.

OUTGASSING CHARACTERISTICS OF HEATED PTFE MEMBRANE FILTERS WITH A THERMOPLASTIC SUPPORT
Jacob Swanson and David Pui (pages 168-172)

Airborne molecular and particle contamination in clean rooms reduces product quality and yield. In this study we investigated the outgassing characteristics of heated, expanded polytetrafluoroethylene (ePTFE) membrane filters that were supported by a thermoplastic layer because these filters represent a potential source of contamination. Two filter types were evaluated: ePTFE membrane filters supported by PTFE and ePTFE membrane filters supported by a polyester laminate. Filters were heated in a cyclic fashion by varying the temperature of the particle free nitrogen challenge gas. During heating, the concentration and size distributions of particles downstream of the filter were measured.

The results show that at a temperature of 200°C, high concentrations (>107 particles/cm3) of ~20 nm particles formed downstream of the polyester filter due to the nucleation of organic vapours that outgassed during heating. Additional experiments showed that after the outgassing ceased and the filter was cooled, subsequent heating would generate more particles. Filters with PTFE support that were cyclically heated in a similar fashion generated no particles at 200°C and very few particles at 260°C suggesting they are more suitable for use in a manufacturing clean room when elevated temperatures are present.

CHALLENGE TESTING FILTERS USING CERTIFIED MICROSPHERES
Graham Rideal and Jamie Storey (pages 172-177)

Challenge testing is a long established method of testing the performance of filters by measuring their ability to trap particles of known size. Traditionally, sands and test dusts have been used but more recently a range of narrow particle size distribution spherical particles have been developed, which can give higher precision and more repeatable results. The pore sizes that can be tested are only limited by the sizes of the challenge particles available so filters from millimetres down to nanometres can be measured. Being spherical, there is not an orientation element to the filtration as in the case of irregular dust particles but the biggest problem, particularly in the sub-micron range, is finding a sizing technique that gives sufficient resolution to detect small changes in size as a result of passing through the filter. Furthermore, spherical particles should give results independent of the method of analysis.

This brief illustrated review summarises the challenge test methods using the new Whitehouse Scientific spherical particles. In addition to the traditional single parameter result, the cut point of the filter, the latest evolution of the method is also shown to provide information on the pore size distribution in a filter.

TRANSITION BEHAVIOUR OF FILTER LOADING CHARACTERISTICS FOR SUPERMICROMETRE OIL-COATED ARTICLES
Ta-Chih Hsiao and Da-Ren Chen (pages 177-187)

Liquid-coated particles are often encountered in ambient and industrial working places. The loading behaviour (i.e. the characteristics of filter pressure drop vs. loaded particle mass per unit area) of filters for liquid-coated particles is expected to be different from those for the cases loading solid only or liquid only particles. In this study, a system capable of generating oil coated particles in stable concentrations was first developed for filter loading testing. A series of filter loading experiments were then performed to investigate the transitional behaviour of filter loading for oil-coated particles. It is observed that the general oil-coated particle loading behaviour of filters transits from that of solid only particles to that of oil only ones as the liquid volume percentage in test particles increases.

Our study shows the transitional loading behaviour highly depends on the overall particle size, and property of filter media and coating liquids. We concluded that the loading behaviour is dominated by the effect of liquid surface tension when the liquid volume percentage in coated particles is less than 50%. Under the above oil-coated particle conditions, the filter loading curves for particles with various liquid percentages can be merged into one via the solid core particle surface area. When the oil volume percentage in coated particles is more than 50%, the effect of liquid viscosity becomes dominant and the loading curves of various liquids follows the order of the oil viscosity.

‘BUBBLE BOBBLE’ IN THE LAUTER TUN – A NEW WAY FOR MASH SEPARATION IN THE BREWHOUSE
Johannes Tippmann, Hans Scheuren, Jens Voigt and Karl Sommer (pages 188-192)

Beer production starts in the brewhouse where milled malt is mixed with water. Relevant ingredients of the malt are converted and dissolved into the wort. The solids have to be separated from the remaining liquid which is a step done in the lauter tun and still a very time consuming process. The developed system is the result of two principles, evaporation of unwanted flavouring substances and the prevention of the blocking of the filter cake. The approach described in this paper was the integration of a bubble generator in the lauter tun which blows nitrogen bubbles (inert gas, not oxidising) through the mash and the filter cake.