Modelling and simulation of batch pressure filter cycles

Modelling and simulation of batch pressure filter cycle

Computer based selection of solid/liquid separation equipment

Filtration and separation technology contains numerous heuristics, evidenced by consulting industrial reference books such as Solid/Liquid Separation Equipment Scale-Up. A majority of industrial process engineers need to possess wide ranging knowledge covering many unit operations and rarely have the opportunity to gain in-depth specialist knowledge of filtration and separation technology. Consequently the large number of heuristics that have evolved in the technology are confusing. Solid/liquid separation technology, whether it be in the areas of selection or design, is best dealt with by software designed to run interactively, so that the engineer can input data and receive a result rapidly. An expert system such as pC-SELECT can be used to ensure the correctness of input data as far as this is possible, and it can utilise interactive graphics facilities to show effects of changes in variables or to allow the engineer access to calculations to make value judgements. To be most effective the software must be a well-chosen mix of algorithms, expert system, and input information by the engineer. There exist a number of charts which serve as a guide to the approach to equipment selection, the better ones of which consider a variety of possible eventualities and indicate where decisions must be made. These charts generally have been devised by experts to be fairly comprehensive and are of value to the solid/liquid separation expert. They also illustrate the near impossibility of combining comprehensiveness with useability when so much interacting information is presented in written form. Purchas introduced a general guide for the non-specialist, which is a valuable aid to one confronted with this confusing and complex area. This guide is adopted, suitably extended and adapted, for use in the software pC-SELECT which incorporates features of the type discussed in this paper

Crossflow electroacoustic separations

Experimental data are presented to show how imposed force fields can reduce flux decline during the crossflow microfiltration of aqueous, mineral based suspensions. Both electric and ultrasonic fields, employed individually or in combination, help prevent particle accumulation at the separating surface. This allows fluid removal rates an order of magnitude higher than those obtained in comparable tests without imposed force fields to be achieved. Such process intensification is demonstrated to have the added benefits of lower overall power requirements, reduced pumping requirements and smaller filtration areas

Computer software for the simulation of solid/liquid separation equipment

This paper details aspects of Filter Design Software® (FDS), Windows® software for the selection and simulation of solid/liquid separation equipment as well as the analysis of test data. FDS has been developed in collaboration with multi-national companies spanning a wide range of industrial sectors to provide a comprehensive calculation, education and training tool that maintains a balance between ease of use, level of knowledge conveyed and comprehensibility. The selection module of the FDS compares up to 7 user-defined selection criteria with information contained in databases to produce a numerically ranked list of potentially suitable equipment. The FDS allows access to text and pictorial descriptions of more than 70 equipment types and hyperlinks provide more specific equipment manufacturer details via the internet. The data analysis module facilitates interactive analysis of leaf filtration, jar sedimentation and piston press test data. Calculations are performed in a hierarchical manner using the available information, if some data are not measured then the FDS performs the best possible analysis using approximations. The results of an analysis can be used to refine (shorten) a list of selected equipment or provide scaleup information for equipment simulation. Results in the paper concentrate on the equipment simulation capabilities of FDS. More than twenty types of vacuum and pressure filters can be simulated which potentially involve combinations of cake formation, compression and gas deliquoring, and washing. By way of example, the operation of a pressure Nutsche filter that is required to process a pharmaceutical product is simulated and the predicted influence of crystal formation and other operating parameters on the filter cycle are shown. Simulations quantify how crystal form can detrimentally influence all phases of a cycle and lead to, for instance, slower filtration and wetter cakes

Understanding flux decline in crossflow microfiltration. Part 2 - Effects of process parameters

Further results from an experimental study of membrane fouling and permeate flux decline during crossflow microfiltration are presented. A computer controlled microfilter and a variety of well characterised particulate solids and polymeric membranes were used to acquire a range of data over typical operating conditions. Example data highlight influences of the process parameters filtration pressure, crossflow velocity, suspension concentration, and particle surface charge, and demonstrate the interdependence of the process operating conditions with particle size, size distribution and shape. Many of the results obtained are discussed with respect to existing literature data which are apparently contradictory, but the current data provides explanations for these contradictions and enable conclusions to be drawn

Understanding flux decline in crossflow microfiltration. Part 3 - Effects of membrane morphology

The influences of membrane type and composition on fouling in crossflow microfiltration are discussed with relation to data obtained from sequences of computer controlled experiments. A number of commercially available polymeric membranes were identified, characterised and challenged with particulate streams of known size, shape and surface charge at a range of well defined, constant process conditions. The flux declines observed during microfiltration are related to the known characteristics of the particle stream and the filtering membrane septum. The fine particles in the feed suspension are shown to control the rate of filtration, and render the rate insensitive to membrane pore size or size distribution. The greatest rate of filtration is obtained with membranes whose pore sizes are smaller than the finest particles in the feed stream. Effects of membrane hydrophilicity/phobicity are short lived, and surface charge effects are secondary

Microfiltration enhancement by electrical and ultrasonic force fields

‘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

Membrane characterisation: the need for a standard

Biologists have been aware for at least 200 years that membranes in living creatures set up and maintain concentration differences between different regions of an organism, without which life could not occur. The search for synthetic membranes able to bring about useful separations in industry was a conscious attempt to mimic nature; early successes were limited, and industrial membrane technology has been a major development over the last 10 to 20 years. Now there are at least a dozen widely differing processes that use membranes to bring about separations. When the liquid to be treated contains colloidal particles or aggregated molecular structures larger than about 0.01 μm in size, it is appropriate to think of it as a suspension rather than a solution. Such large particles undergo relatively minor Brownian motion and sediment appreciably, if rather slowly, under gravity. In order to separate them from the suspending liquid, which is usually a true solution and may contain molecular solutes, it is appropriate to think of filtration as an alternative to, for example, centrifugation. Conventional filters made from compacted fibres or powders are available for removing particulates smaller taken about 10 μm from liquid feeds. For this purpose, polymer technologists have developed techniques to cast films using a combination of solvent and precipitants that contain controllably small pores of fairly narrow size range. These ‘membrane microfilters' are used extensively for the filtration of bacteria and colloidal particles and are coming up against increasing competition from a rapidly growing range of ceramic microfilters. Methods of characterising the membranes are diverse and no standard exists between different manufacturers for the measurement of properties such as pore size and size distribution, pore shape, asymmetry, permeability and wettability. These properties, together with measured permeate flux decline and rejection data, are the ones that most affect the potential suitability of a membrane to a particular industrial application

Crossflow micro- and ultrafiltration augmented by electric and ultrasonic force fields

An experimental study of field assisted crossflow filtration has shown that electric and ultrasonic fields, either in isolation or in combination, reduce membrane fouling. Particle liquid interfacial phenomena are used to advantage with the imposed fields to remove fouling layers and enhance flux rates. Synergistic effects were observed when the fields were applied simultaneously. Lower crossflow velocities can be utilised which implies that pumping costs, heat transfer in recirculation loops, and the degradation of shear sensitive streams can be reduced

Utilising particle-liquid interfacial phenomena to augment crossflow microfiltration

Results from an experimental study of field assisted crossflow microfiltration are presented. Both electric and ultrasonic fields, either in isolation or in combination, can reduce membrane fouling by utilising particle-liquid interfacial phenomena. Synergistic effects were observed when the fields were applied simultaneously. Lower crossflow velocities can be utilised when force fields are employed, implying that pumping costs, heat transfer in recirculation loops, and the degradation of shear sensitive streams can be substantially reduced