The use of electrode probes in determinations of filter cake formation and batch filter scale-up

A well controlled apparatus was used to obtain new data for constant pressure cake formation in a batch leaf filter which could be fitted with electrode type sensing probes. For aqueous suspensions of calcite and talc, the influence of the intrusive electrodes on cake formation was quantified with respect to filtration parameters. As the electrode length and diameter were altered such that the projected cross-sectional area occupied up to 2.6% of the filter area, the average specific cake resistance (as determined by classical filtration theory) was observed to increase by up to 49% and the average cake porosity decreased by up to 21%. When the electrodes were small relative to the dimensions of the filter cell, their influence on macroscopic cake properties was low. The additional information which electrical resistance measurements provide is discussed in relation to the results and observations by previous researchers and the benefits to filter test analysis and batch filter calculations are highlighted. For moderately compressible cake filtrations it is postulated that the incorrect assessment of cake porosity can typically account for an ~12% error in scale-up calculations for batch filters

The modelling and simulation of solid-liquid filtration processes

The modelling and simulation of solid-liquid filtration processe

The role of field-assisted techniques in solid/liquid separation

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 development of the more pertinent assisted separation techniques and illustrates their performance 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

Selection, scale-up and process simulation of filters in the new millenium

This article details aspects of a new, generic approach to the selection, scale-up and process simulation of filtration and separation equipment. An interactive computer software package capable of identifying a likely separation device is outlined. Choosing the example of cake filters, the beneficial use of computer software and automated data acquisition in laboratory apparatus is illustrated. It is shown how the typical experimental data generated can be analysed in a consistent manner by computer software to generate scale-up data. The article also indicates how this information can subsequently be used within a family of dedicated process modelling software packages to simulate detailed filter cycle operations on batch and continuous cake filters. The approach has general application within process and related industries and offers integration with manufacturers data via the internet and the world wide web

Pressure leaf filter control and the prediction of performance

The widespread reliance on heuristics for the design and specification of particle/fluid separation devices has prompted a new approach to pressure filtration which incorporates the principles of mechatronics. By combining accurate experimentation with classical filtration models it is shown how the performance of a deadend pressure leaf filter can be predicted over a range of process conditions. An experimental apparatus is described along with data which illustrate its versatility and accuracy. Experimental data obtained from the apparatus with aqueous mineral suspensions are shown to compare favourably with theoretical predictions of important design parameters such as cake height and cumulative volume of filtrate. Preliminary results from unique constant flow filtration experiments are also presented where the air pressure within the filter was controlled through a combination of flow & pressure transducers and an electronically adjusted pressure regulator. Their combination allowed filtrations performed under different pressure/flow regimes to be compared as identical suspension characteristics could be maintained

Using mechatronics technology to assess pressure filtration

This paper describes some recent (initial) developments in laboratory scale cake filtration technology which may ultimately lead to the production of standard equipment for assessing filtration performance. The principles of mechatronics, which integrates electronics, computers, process control and mechanical systems, have been used to provide a state-of-the-art pressure filtration apparatus capable of performing repeatable experiments over a range of pressure/flow regimes commonly encountered in industrial filtrations. Data obtained at constant pressure and proportionally controlled variable pressure are presented to illustrate the scope of the apparatus and the benefits of removing operator interference and damaging pumping operations from filtration experiments. It is shown how scale-up parameters (obtained for aqueous mineral suspensions) can be used to successfully predict constant pressure filtration performance and the problems which can arise when predictions of more compressible variable pressure filtrations are made using data obtained at constant pressure

The influence of sedimentation during downward cake filtration

Data for near incompressible cake formations with simultaneous settling are presented. Aqueous calcite suspensions exhibiting similar median particle size, but different size distributions, were filtered over a range of constant pressures. For each experiment the time dependent history of filtrate removal and the particle size distributions of cake samples at different spatial positions were measured. These data were compared with predictions from a new mathematical model that divides cake formation into a range of discrete time steps. Cake growth due to filtration and sedimentation were considered to proceed simultaneously, but separately, with the additive results predicting the change in cake thickness during a time step. Account was taken of the changing effects of suspension concentration on settling rate and the transient influence of size distribution on specific cake resistance. The model is shown to quantitatively predict the influence of feed particle size distribution on cake formation and filtrate removal rates and favourable comparisons are made with values recorded in experiments. For the experimental conditions investigated, sedimentation is shown to contribute up to one third of the cake resistance in a filtration test. At lower pressures and with wider size distributions, larger particles from the feed tended to accumulate near the filter medium and in some cases a minimum cake resistance was observed toward a mean cake height. For higher pressures, however, the effect of particl

The control of pressure in constant rate cake filtration

The widespread reliance on heuristics for the design and specification of particle/fluid separation devices has prompted a new approach to pressure filtration incorporating the principles of mechatronics. A unique experimental apparatus is described and used to obtained filtration data for aqueous mineral suspensions forming either incompressible or moderately compressible filter cakes. Data for constant flow filtrations are presented where the air pressure within the filter is controlled through flow & pressure transducers and an electronically adjusted pressure regulator. Their combination allowed filtrations performed under different pressure/flow regimes to be compared. It is shown how scale-up data obtained from constant pressure filtrations can produce erroneous predictions of constant rate filtration behaviour, particularly as cake compressibility increases

Cake filter scale-up, simulation and data acquisition - a new approach

This paper details the capability of a unique, automated filtration apparatus and the newly developed Filter Design Software (FDS) which facilitate equipment selection, scale-up and simulation through an integrated experimental and theoretical approach. By way of example, experimental data were obtained with the apparatus over constant, variable and stepped pressure regimes. Inherent suspension properties were maintained throughout by utilising a computer controlled pressure controller and cake formation was monitored by micropressure transducers capable of providing up to seven independent measures of liquid pressure within 3.3 mm of the filter medium surface. For constant pressure and moderately compressible talc cakes the liquid pressure increased with cake height in a non-linear manner and generally exhibited a concave profile. When a pressure step was applied following a period of constant pressure filtration, the cake structure typically required up to 30 s to reach a new pseudoequilibrium state. During this time the reciprocal filtrate flow rate vs. filtrate volume plot was nonlinear and the liquid pressures in the cake increased rapidly before remaining nearly constant. When the cake was thicker or the pressure step larger, the liquid pressure measured closer to the filter medium remained either constant following the increase in pressure or increased slowly over the 360 s duration of the pressure step which indicates potential difficulties with the stepped pressure test. The filtration data were analysed using FDS to obtain scale-up coefficients and the impact of using incorrect scale-up coefficients on likely filter performance at the process scale is shown. The simulation capabilities of FDS are also highlighted through a case study in which, by way of example, the influence of crystal formation and other operating parameters on the filter cycle for a pharmaceutical product are shown. Simulations quantify how crystal form can detrimentally influence all phases of a cycle and lead to, for instance, slower filtration and wetter filter cakes

Predicting the performance of pressure filters

As a step towards removing heuristics from the design and specification of filters, it is shown how accurate experimentation and modelling can be utilised to predict the performance of a pressure leaf filter over a range of experimental conditions. An experimental apparatus combining the principles of mechatronics with a dead-end pressure leaf filter is outlined, along with sample data which outline its versatility. Experimental data obtained from the apparatus are compared with predictions given by classical filtration theories incorporated within flexible computer simulations. Batch experimental results obtained at constant pressure with aqueous suspensions of calcite, talc and zinc sulphide are shown to match well with theoretical predictions of the important design parameters cake height and cumulative volume of filtrate, particularly when cake compressibility is lower. Constant pressure data are further used to predict constant flow filtrations where the applied pressure is manipulated via computer software