Geoffrey W. Barton

A quantitative comparison between chemical dosing and electrocoagulation

Abstract A renewed interest in electrocoagulation has been spurred by the search for reliable, cost-effective water treatment processes. This technology delivers the coagulant in situ as the sacrifcial anode corrodes, due to an applied potential, while the simultaneous evolution of hydrogen at the cathode allows for pollutant removal by flotation. By comparison, conventional chemical dosing typically adds a salt of the coagulant, with settling providing the primary pollutant removal path. This paper provides a quantitative comparison of these two approaches based on turbidity removal associated with a clay pollutant. Chemical coagulation was evaluated via jar tests using aluminium sulphate (alum). This proved more effective than electrocoagulation under acidic conditions (pH ∼4) and low coagulant levels (4 mg-Al l −1 being the minimum able to effectively destabilise the colloidal clay particles). Highly effective coagulation was observed at intermediate alum dosage levels (4–20 mg-Al l −1 ), where the isoelectric point occurred at pH ∼7.8. Three operating stages (lag, reactive and stable) were identified in a batch electrocoagulation reactor with the operating current determining the pollutant removal rate. At the isoelectric point, which occurs during the reactive stage, the greatest turbidity reduction occurs, indicating aggregation by a sorption mechanism (compared to the charge neutralisation as in the case of chemical coagulation). During the stable stage, continued precipitation of aluminium hydroxide and a decrease in turbidity indicated a sweep coagulation mechanism. The highest current (2 A) reduced the pollutant level in the shortest time, 1% residual turbidity after 30 min, though the highest efficiency (in terms of pollutant removed per unit of aluminium added) was achieved at the lowest current (0.25 A).

Steady-state modelling of chemical process systems using genetic programming

Complex processes are often modelled using input-output data from experimental tests. Regression and neural network modelling techniques are commonly used for this purpose. Unfortunately, these methods provide minimal information about the model structure required to accurately represent process characteristics. In this contribution, we propose the use of Genetic Programming (GP) as a method for developing input-output process models from experimental data. GP performs symbolic regression, determining both the structure and the complexity of the model during its evolution. This has the advantage that no a priori modelling assumptions have to be made. Moreover, the technique can discriminate between relevant and irrelevant process inputs, yielding parsimonious model structures that accurately represent process characteristics. Following a tutorial example, the usefulness of the technique is demonstrated by the development of steady-state models for two typical processes, a vacuum distillation column and a chemical reactor system. A statistical analysis procedure is used to aid in the assessment of GP algorithm settings and to guide in the selection of the final model structure.

Operation of semi-batch emulsion polymerisation reactors: Modelling, validation and effect of operating conditions

A detailed dynamic model was developed for a styrene emulsion polymerisation semi-batch reactor to predict the evolution of the product particle size distribution (PSD) and molecular weight distribution (MWD) over the entire range of monomer conversion. A system exhibiting zero-one kinetics was employed, with the model comprising a set of rigorously developed population balance equations to predict monomer conversion, PSD and MWD. The modelling equations included diffusion-controlled kinetics at high monomer conversion where the transition from the zero-one regime to a pseudo-bulk regime occurs. The model predictions were found to be in good agreement with experimental results. Both particle growth and the PSD were found to be strongly affected by the monomer feedrate. Reactor temperature had a major influence on the MWD which was, however, insensitive to changes in the monomer feedrate. These findings were confirmed experimentally. As a result, it seems reasonable to propose that the use of the monomer feedrate to control the PSD and the reactor temperature to control the MWD are appropriate in practical situations. Consequently, an optimal monomer feed trajectory was developed off-line (using the validated reactor simulation) and verified experimentally by producing a polymer with specific PSD characteristics.

Influence of field evaporation on Radial Distribution Functions in Atom Probe Tomography

This paper examines the extraction of structural information in the form of Radial Distribution Functions (RDFs) using Atom Probe Tomography (APT) data. These functions are generated in a highly efficient manner, thus allowing for the analysis of large data sets typical of APT. Experimental RDF calculations were performed for crystalline aluminium and a Mg65Cu25Y10 bulk metallic glass. For the pure aluminium sample, significant pair distance information was extracted, the quality of which was found to vary throughout the data set. Through a novel analysis procedure, the measured total RDF was used to map the local pair distance quality about each reconstructed atom. Surprisingly, the RDF quality maps indicated improved pair distance quality around poles and zone lines. In the case of the metallic glass, however, significant pair correlations were not discernible within the data set, despite short-range ordering being observed using TEM diffraction. The lack of correlations is thought to be associated with a non-uniform ion desorption sequence, as observed in this study. This affects the uniform evaporation assumption that is implicit in current 3D APT reconstruction procedures.

Microstructured Polymer Optical Fibres: New Opportunities and Challenges

ABSTRACT Microstructured polymer optical fibres [mPOF] were first developed in 2001, and have attracted attention in part because the range of fabrication techniques possible with polymers has allowed novel structures to be made that cannot be made simply in other materials. Their material properties also offer attractive possibilities as polymers can contain a much larger variety of dopants than glass. In this article, we review progress on some of the major challenges of this technology: particularly the need to reduce fibre losses, and report on some recent developments including the fabrication of the first hollow core mPOF. Some initial investigations into changing the material properties are reviewed.

Fabrication of microstructured optical fibers-part II: numerical modeling of steady-state draw process

By combining theoretical, numerical, and experimental analyses, this paper examines the continuous draw process that underpins the fabrication of microstructured optical fibers (MOFs) with the aim of quantifying the impact of material properties and drawing conditions on the hole structure in the finished fiber. First, by treating the continuous draw process as a steady-state isothermal extensional flow of a Newtonian material, three-dimensional (3-D) modeling clearly demonstrates how a combination of force effects can lead to dramatic hole deformation in the neck-down region-i) surface tension contributing to hole size collapse (particularly if the fiber contains small holes and is drawn slowly over a long distance), while ii) viscous effects are the major contributor to hole shape changes (particularly in cases where different size holes are in close proximity within the overall structure). Then the central role of the neck-down region in hole deformation is examined via nonisothermal numerical analysis. Results indicate that the shape of the neck-down region is highly sensitive to the viscosity profile and thus to temperature gradients. Finally, it is shown that predicted hole deformations agree well with experimental measurements made in drawing polymethylmethacrylate MOFs.

Theoretical, Numerical, and Experimental Analysis of Optical Fiber Tapering

An optical fiber taper is fabricated by heating and stretching a fiber. The resulting taper shape is important as it strongly affects optical performance. In this paper, the tapering process of solid optical fiber is modeled and analyzed under several heating and stretching conditions. The fiber material is assumed to be of non-Newtonian inelastic type. The results show that for a given heating profile, the shape of a tapered fiber is independent of the material properties and the stretching conditions applied at the fiber ends, and a section of uniform waist can be formed as long as the extensional deformation rate in a section of the heating zone is position-independent. Different shapes of fiber tapers can only be achieved by using different heating profiles. Therefore, spatially uniform heating of the fiber within the heating zone is of critical importance for producing a taper with a uniform waist. This is particularly true if the fiber material has a low deformation temperature

Inferential conversion monitoring and control in emulsion polymerisation through calorimetric measurements

Abstract A dynamic model has been developed describing the emulsion polymerisation of styrene within a batch/semi-batch stirred tank reactor (BSTR). This model includes the initiation, propagation and termination steps for styrene polymerisation, along with the relevant mass balance equations (including those for polymeric radicals) and energy balance equations—the latter covering heat of reaction, internal and external heat transfer effects, as well as external heat losses. The resulting set of (differential/algebraic) equations was solved for both species concentrations and temperature profiles as functions of time. Experiments were conducted in a laboratory BSTR instrumented with platinum resistance thermal transducers and gravimetric conversion measurement devices. The model predictions compared well with inferential calorimetric measurements which were validated using experimental gravimetric data. Subsequent implementation of a model-based optimal control strategy resulted in a 13% relative increase in monomer conversion, together with a 28% reduction in batch time.

Modeling and control of a food extrusion process

This contribution reports on the process systems engineering analysis of a food extrusion process leading towards the development and implementation of an on-line process control scheme. A versatile dynamic model of the process predicting the quality variables product gelatinisation, specific mechanical energy, specific volume and residence time has been developed and fitted to experimental data. The model is used to investigate the practicality of an adaptive inferential estimator (Guilandoust et al., 1987) for predicting infrequently measured quality variables on-line. The simulated performance of simple PI and model predictive control of these quality variables is compared.

Determination of the generic rank of structural matrices

Abstract A new algorithm for determining the generic rank of structural matrices is presented. The use of the algorithm in determining whether a system is structurally controllable is discussed and it is found to be adaptable to the much wider problem of control system synthesis such as the elimination of excess manipulated variables or control objectives.