DETERMINATIONS OF MEMBRANE SWELLING AND THE EFFECT ON FILTRATION PERFORMANCE
Steve Tarleton (pages 152-160)
This paper describes an apparatus for in-situ determinations of swelling where a linear inductive probe and electronic column gauge with an overall resolution of 0.1 µm was used for measurements of seven variants of polyacrylonitrile (PAN)/polydimethylsiloxane (PDMS) composite nanofiltration membranes in a range of alkane, aromatic and alcohol solvents. The unswollen membranes incorporated PDMS layers between 1 and 10 μm nominal thickness and were manufactured with a radiation and/or thermal crosslinking step.
The tested membranes exhibited a range of swelling dependent on the degree of crosslinking, the initial PDMS layer thickness and the type of solvent. With no applied pressure the PDMS layer on some radiation crosslinked membranes swelled as much as ~170% of the initial thickness whilst other membranes were restricted to a maximum swelling of ~80%. When a pressure up to 2000 kPa was applied to a membrane then swelling could be reduced to ~20% of the value obtained at zero applied pressure. By vertically stacking up to 3 membrane samples it was possible to determine the swelling of PDMS layers as thin as 1 μm, although higher imposed pressures rendered some results unreliable as the measurement resolution of the apparatus was approached. The typical influences of membrane swelling in crossflow membrane filtration are also shown.
AN INVESTIGATION INTO THE ROLE OF FACTORS INFLUENCING PRESSURE DROP IN PULSE-JET FABRIC FILTRATION
A.K. Choudhary and A. Mukhopadhyay (pages 160-169)
The present investigation elucidates the role of pulse cleaning parameters and filter media design on various time progression differential pressure parameters (peak pressure, residual pressure and cake pressure). The experimental study shows that at steady state, pressure parameters are more susceptible to change with a change in pulse cleaning operating parameters. The effect of all pulse parameters on the pressure values are stabilized with time as the needlefelt media acquires a stabilized state. In a stable state, the role of pulse pressure and pulse cycle, and their interaction, are prominently increased which influences all the pressure parameters investigated. However, the roles of pulse duration diminish substantially. With progression of the filtration process, the impact of bag height and bag seam line spacing on the pressure parameters becomes very small/insignificant.
COUPLING CENTRIFUGE DRAINAGE WITH MICROWAVE DRYING: DESCRIPTION OF CAKE DESATURATION BEFORE DRYING
Xabier Apaolaza, Marc Valat, Pascal Ginisty, Wahbi Jomaa and Alain Sommier (pages 169-176)
Centrifuge drainage is an operation which finds applications in the pharmaceutical, chemical and textile industries. It is an efficient and economic process, and is frequently used before thermal drying to eliminate most of the liquid retained in the cake at low energy cost. Moreover, the combination of mechanical and thermal operations in the same machine enables, in some cases, the optimisation of productivity, energy efficiency and safety aspects and to reduced cleaning fluid volumes and handling operations requirements. In this study, the interest of coupling centrifugal drainage and microwave drying is brought into focus. Some of the technical aspects are also addressed.
QUANTIFYING CHANGES IN FILTER CAKE STRUCTURE DURING AN ABRUPT TRANSITION
Kuhan Chellappah (pages 177-185)
The constant pressure filtration of aqueous cellulose fibre and binary fibre/titania (rutile) suspensions was studied. With some filtrations, abrupt transitions in cake structure induced disturbances in the expected filtrate flow. These transitions were only observed at pressures above ~400 kPa and when the cake consisted of loosely networked solids. Changes in solids composition altered the nature of cake restructuring. A theoretical framework was developed to perform quantitative assessment during an abrupt transition. In this framework it was assumed that cake restructuring was due to changes in pore radius and/or cake porosity. Estimates of the magnitudes involved during these changes were calculated. Changes of larger magnitude are required to account for the observed filtration behaviour if the restructuring is more localised within the cake. The primary mechanism causing the transitions was suggested to be migration of fines which led to formation of preferential flow channels in some cases and blinding of channels in others.
CROSSFLOW MICROFILTRATION OF CHLORELLA SP. SUSPENSIONS
Kuo-Jen Hwang and Lu-Lin Chou (pages 185-192)
A kind of algae, Chlorella sp., is concentrated using plane-sheet crossflow microfiltration in this study. The Chlorella sp. cells with sizes ranging from 3 to 12 µm were filtered using a 0.1 µm membrane made of mixed cellulose ester. The effects of operating conditions, such as crossflow velocity and trans-membrane pressure, on the filtration flux and cake properties were measured and are discussed. The filtration flux increased with increasing crossflow velocity and trans-membrane pressure. The overall filtration resistance is mainly contributed by the cake formed from the algae cells. The order of magnitude of the average specific filtration resistance of cake formed by Chlorella sp. is as high as 1013 m-1, and the cake compressibility is ca. 0.74. An increase in crossflow velocity led to higher wall shear stress which resulted in reduced cake mass.
A force balance model for cell deposition is applied to obtain the relationship between cake mass and operating conditions. The cake mass decreases linearly with the ratio of wall shear stress to filtration flux. The filtration flux can be estimated using the basic filtration equation once the cake mass and filtration resistance are analysed. The calculated results of filtration flux agree fairly well with experimental data.