I.W. Cumming

Prediction of steady state crossflow filtration using a force balance model

Abstract A model to predict steady state flux of a crossflow filter is presented. A frictional force balance is applied to a particle at the cake surface — it is assumed that if the ratio of net axial force to net normal force exceeds the coefficient of friction, deposition does not occur. Unlike previous frictional models, both hydrodynamic and interparticle forces are considered. Experiments filtering spherical monodisperse latex particles with sizes of 0.55 and 1.8 micrometers were used to test the theory. The variables examined were membrane type, crossflow, pressure, particle diameter and system pH. The model successfully predicts both the linear nature of permeate velocity dependence on shear and the associated gradient as being a function of particle diameter. The system can be characterized by a single empirical parameter, the form of which is dictated by the force balance.

Pore design and engineering for filters and membranes

In filtration, the concept of pore size is not easy to define. In microfiltration, there are numerous advantages in employing a surface filtering membrane, rather than one relying on depth filtration mechanisms from a tortuous pore flow channel. Modern manufacturing techniques provide means to produce surface filtering membranes. For filtration, it is shown that a suitable pore design is an array of long thin slots. An analysis of fluid flow through the slots suggests that a short slot is adequate, but experimental data with suspended material indicates that slot length is important. Using long slots and careful control of the flow through the membrane it is possible to filter deforming particles such as oil drops from water.

Particulate fouling of surface microfilters with slotted and circular pore geometry

During microfiltration it is possible to obtain a permeate rate equal to that of the clean liquid permeation rate under certain critical conditions. This occurs when a deposit does not foul the surface of the membrane and internal deposition of material within the filter does not occur. Surface filters do not possess an internal structure, therefore, the only particulate fouling possible is that on the surface. Experiments in a stirred cell compared the surface fouling of two types of true surface filters: a commercially available track-etched filter with circular pores 10 μm in diameter and a filter with slotted pores 10μm×420 μm in size. When using a challenge suspension containing dilute latex particles of diameters 15 μm, and less, the circular pore membrane exhibited a critical flux at 260 l m−2 h−1. Under the same conditions, the critical flux for the slotted membrane was in excess of 1500 l m−2 h−1. Surface fouling was removed by back-flushing for both filters. These results indicate that slotted pores are less likely to suffer from particles bridging the pores leading to cake deposition, or secondary membrane formation, and that further development of filters with slotted pores of smaller slot widths would be worthwhile.

Prediction of deposit depth and transmembrane pressure during crossflow microfiltration

Most crossflow microfiltrations of suspensions of significant solids volume concentration exhibit non-Newtonian flow behaviour, and may lead to the deposition of appreciable cake depths. The cake depth may even extend to the full filter tube diameter, thus blocking any further filtration. Crossflow filtration modelling also requires knowledge of the cake depth, and the transmembrane pressure. Experiments have been performed for both the crossflow microfiltration of non-Newtonian talc suspensions, and an investigation of the pressures and flow rates in impermeable tubes of diameters similar to the crossflow filters. The investigation also illustrates how the transmembrane pressure drop can be corrected to take account of differences in cross-sectional area between where the pressure measurements are recorded and the filter flow channel. A method is demonstrated for the estimation of the depth of deposit on a fouled filter tube from pressure and flow measurements, when combined with a knowledge of the suspension rheology. The pseudo-equilibrium steady-state flux has been shown to correlate with shear stress at the filter or deposit surface, after the above corrections have been performed.

Crossflow microfiltration for mineral suspension thickening and washing

Abstract Crossflow microfiltration is not normally applied to mineral suspension thickening and washing. However, its use is feasible and relatively high filtrate flux rates can be achieved. Thus it may find application for suspensions that are otherwise difficult to process such as clays and slimes. A semi-empirical model of flux rate with the shear stress on the deposit on the filter is shown to be applicable for the three finely divided minerals investigated here. The model has been extended to provide a relation for deposit depth with operating pressure. This agreed with experimental data when filtering at pressures over 1 bar, but needs to be developed further for operation at lower pressure. There are several operating modes that may be used for process operation, the selection of the most appropriate one can be based on some simple testwork to obtain the necessary empirical constants coupled with the design and operating equations derived from overall mass balances. The mass balances are based on the assumption that the crossflow system can be treated as a well mixed tank. The experimental evidence suggests that this is a reasonable assumption for both the thickening and diafiltration studies reported here.

A Model to Predict the Performance of an Electrochemical Ion Exchange Cell

Electrochemical ion exchange (EIX) is a recently developed electrical process for removing ions from solution by adsorbing them on to an ion exchange membrane. These ions can be subsequently eluted from the ion exchange material by polarity reversal. A model of the process has been developed and tested against experimental results. The model gives predictions close to the measured data over a range of operating conditions for different values of pH, ion concentration, flow and current density.

Integrated knowledge based system for process synthesis

Abstract The combined use of heuristics such as expert systems, databases, mathematical process simulators, equipment sizing and cost estimation is a potential way of exploring improved chemical process synthesis. We report on the development of a software that integrates knowledge based system with HYSYS process simulator and Icarus economic evaluator utilising knowledge from existing processes to obtain heuristic rules. The structure and the systematic procedure of the proposed Integrated Knowledge Based System (IKBS) have been discussed. The prototype IKBS has been applied for the selection of reactor systems for the ethylene oxide and ethylene glycol manufacturing processes. Analysis by the software suggests the use of two reactor systems and a list of suitable reactors. The list contained new and currently used reactors in addition to the recommended reactors by industrial research.

Slotted pore microfilters for oil/water filtration, fractionation and aggressive filtration environments

Abstract In the UK, researchers have been investigating ways of separating oil and other particles from water. They have developed and designed a new filter that uses microfiltration media that are similar to very fine sieves. Furthermore, there is no internal deposition of solids, and the pressure that is required to pass liquids through the media is low. This article provides details of this technology, which could be used on offshore oil rigs to vastly reduce the amount of oil discharged into the worlds oceans.

Hydraulic performance of an annular plunging jet reactor

Agas-liquid reactor is described, in which a plunging jet issues into a confined downcomer column, entraining headspace gas as it impinges onto the surface of a receiving pool of the same liquid. The two-phase mixture flows down the confining column and then rises through a concentric annulus, before disengaging in a separation vessel. Such a design gives increased gas-liquid contact times compared to jets plunging into open pools. Another feature of the current design is that it is straightforward to recycle both the gas and liquid phases from the separation, using only a single pump, thus ensuring almost complete utilization of the gaseous feed. A hydraulic model of the reactor is presented and tested against experimental data over a range of liquid flow rates and nozzle sizes. The agreement between the predictions and the experimental data indicates that the model provides a good basis for the design of this type of gas-liquid reactor. It is also shown that downcomers with diameter greater than 24 mm appeared to have little effect on the gas entrainment rate, when nozzles of 5 to 12 mm diameter were used. Smaller downcomers significantly reduced the gas volumetric entrainment flow rate.