E.S. Tarleton

Membrane fouling prevention in crossflow microfiltration by the use of electric fields

Abstract From an analysis of fluid velocity and electric field profiles the trajectory of particles through crossflow microfiltration units has been calculated. A tubular geometry filter leads to the most effective use of electrical power, when it is used as an aid to prevent membrane fouling. Results for plate, tubular and mutitube filters are given. Some experimental data from a tubular filter are presented, which demonstrate typical effects of the important parameters.

Factors affecting the formation and properties of wet compacts

Abstract New data are reported on the formation of wet compacts from slurries. The compaction data are analysed through two consecutive mechanisms, filtration and consolidation. Process design parameters for each mechanism are obtained from the models which might allow calculations to be made from small-scale experiments. The magnitude and dependence of the constitutive parameters on pressure, pH and surface charge, particle size and shape, and the nature of the particle—particle interactions are shown through the experimental results.

Colloidal fouling of microfiltration membranes during the treatment of aqueous feed streams

Abstract Experimental data has been obtained from a computer controlled crossflow microfiltration apparatus. Polymer membranes of differing properties have been used to filter suspensions carrying colloidal and fine particles of known shape, size, surface charge and chemical composition. The process parameters (feed concentration, crossflow velocity and pressure difference) have been changed systematically, and flux decline curves measured for the matrix of particle, suspension, and process properties. This paper presents results from the experimental programme, and in the light of these results explains some of the apparent anomalies which have been presented in previous publications.

THE ROLE OF FIELD-ASSISTED TECHNIQUES IN SOLID/LIQUID SEPARATION

Abstract The separation of finer particle suspensions into constituent solid and liquid components is difficult to achieve. Field-assisted separations which utilise the forces generated by applied electric, acoustic or magnetic fields are becoming an increasingly viable alternative to the more conventional, frequently ill-suited, techniques used presently. An overview is given of the more pertinent assisted separation techniques and their performance is illustrated with experimental data obtained by the author. The data show how improved separation rates can be achieved with imposed force fields, often at lower overall energy inputs per unit of product. Current and possible future applications of assisted separations are discussed both in terms of equipment performance and economic considerations.

Microfiltration enhancement by electrical and ultrasonic force fields

Abstract ‘Assisted’ filtration techniques are emerging as technical alternatives to conventional separations. Experimental data presented in this paper show how electric and ultrasonic fields can assist microfiltration by reducing the flux decline caused by membrane fouling. Effects of the fields acting individually and in combination are illustrated, together with the influence of other filtration parameters.

The effects of scale and process parameters in cake filtration

A well controlled and instrumented apparatus has been used for a systematic investigation of cake growth in pressure leaf filtration. Both incompressible and compressible systems have been filtered over a range of constant pressures to evaluate the effects of applied pressure, initial suspension concentration, time of filtration, particle surface charge (interpreted through suspension pH) and scale of filtration. Sample data show how aqueous calcite and zinc sulphide systems behave in different manners dependent on both the relative magnitudes of the process parameters and apparently on the scale at which a filtration is performed. The data, analysed by consistent procedures through the general filtration equation, indicate that the scale-up ‘constants’ frequently used in filter design may vary with scale and how variations in measured cake properties can be seen as scale is altered. The results are discussed in the context of filter design.

Electro-acoustic crossflow microfiltration

Abstract Results from an experimental study of field-assisted crossflow microfiltration are presented. It was found that both electric and ultrasonic fields, either in isolation or in combination, can reduce membrane fouling by an amount dependent on the applied field strengths, acoustic frequency, suspension concentration, liquid viscosity, particle size and particle surface charge. Synergistic effects were observed when the fields were applied simultaneously. When force fields are used in microfiltration lower crossflow velocities can be employed. This implies that pumping costs, heat transfer in recirculation loops and the degradation of shear sensitive streams can be substantially reduced.

Modelling, simulation and process design of the filter cycle

Abstract Taking a vacuum belt filter as a basis, the authors show how different stages of solid-liquid separation interact and affect one another in an attempt to point a way towards more rational design of a separation system. The models used are based on fundamental concepts of varying complexity, and sufficiently developed to facilitate design calculations. They can also be readily incorporated into process integration and optimisation packages, and form part of a suite of computer software packages being developed at Exeter University.

Using mechatronics for the interpretation and modelling of the pressure filter cycle

An experimental apparatus combining the principles of mechatronics with a dead-end pressure leaf filter is described. The computer controlled and sequenced apparatus was used to monitor performance during the cake formation, gas dewatering and displacement washing phases of filter cycles. Data relating to cake properties and liquor transport rates were acquired through electrical resistance measurements and suitable transducers. The data illustrate the versatility and accuracy of the apparatus and they are used to compare with predictions given by existing theories, particularly for cake formation. Batch experimental results obtained with aqueous suspensions of calcite are shown to compare favourably with theoretical predictions for important design parameters such as cake height and cumulative volume of filtrate whereas data for more compressible talc systems tended to show more deviation. The work presented highlights the benefit of a mechatronics approach to experimentation and emphasizes the importance of data acquisition and system control to future developments in filter design.

A framework methodology for the simulation and sizing of diaphragm filter presses

Abstract A framework for simulating filter cycles is presented, based on a blend of tested theoretical models and accepted design procedures. Output from the simulator predicts correctly the general effects of process variables. Some sample results are used to illustrate these when it is applied in a general sense to a diaphragm (variable volume) filter press.