Doctoral Degrees (Chemical Engineering)

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Measurement and thermodynamic interpretation of high pressure vapour-liquid equilibrium data.
(1990) Muhlbauer, Andreas Lorenz.
.. Detailed experimental and thermodynamic studies of. the isothermal phase equilibria for the
A study of the phenomenon of bridging of sugarcane bagasse
(1996) Bernhardt, Herbert Wolfgang
This work reports the results of a systematic study of the factors that affect the bridging behaviour of bagasse. It shows that traditional bulk solids theory is inappropriate for predicting bagasse flow mainly because of the impossibility of obtaining a reliable measure of internal friction. It demonstrates the significant influence of fibre length and moisture content on its handling charateristics. Correlations of pertinent bulk properties such as compactibility, tensile strength, surface friction and translation of vertical into horizontal pressure have been developed. These were derived as a result of measurements in equipment that was designed for the purpose. Finally, an empirical model utilising these correlations is proposed by which the likelihood of bridge formation in any piece of bagasse handling equipment can be determined. The validity of the model is assessed by comparison with bridging tests that were performed under controlled conditions.
Dynamic modelling and optimal control of sugar crystallisation in a multi-compartment continuous vacuum pan.
(2002) Love, David John.; Mulholland, Michael.
The objective of this work was to determine the operating conditions which would maximise the crystallisation performance of continuous vacuum pans used in the sugar industry. The specific application investigated in detail is crystallisation of high grade product sugar (A-sugar) in a South African raw cane sugar factory. The optimisation studies are based on a detailed dynamic mathematical model of a continuous pan. Whilst this model is based on the published work of others, the selection of variables and the formulation of the equations have been structured to produce a modular model of an individual compartment with the minimum number of independent variables. The independent variables have also been selected to meet the requirements of both a state-space control formulation and those necessary for the dynamic programming technique of optimisation. The modular compartment models are linked together to model a multi-compartment pan and the steady state model is derived as a special case of the dynamic model. For the model to simulate the conditions in South African sugar factories adequately requires appropriate descriptions of sucrose solubility and growth kinetics. Given the limited applicability of published data, experiments were undertaken to determine these parameters. Sucrose solubility in impure solutions was determined in laboratory tests designed to approach equilibrium by dissolution at conditions approximating those during pan boiling. The dependence of crystal growth rate on the concentration of impurity present in the mother liquor was investigated in both laboratory scale and pilot scale batch pan boiling experiments. The primary dependence of crystal growth rate on the super-saturation driving force was determined by fitting the steady state model to results of tests on an industrial scale continuous pan. The dynamic programming technique was used in conjunction with the mathematical model to determine the operating conditions which maximise steady state crystallisation performance. Using the crystallisation parameters determined for South African conditions, this approach has shown that the conventional wisdom of running with high crystal contents in all compartments of continuous pans boiling A-massecuite is not optimum. Pans should operate at lower crystal contents in earlier compartments, only increasing to higher crystal contents towards the final compartment. The specific values depend on seed conditions, pan design and the solubility and growth kinetics. To reap the benefits of being able to determine the optimum steady state operating condition for a continuous pan, it is necessary to be able to achieve effective steady state operation under industrial conditions. This requires both a steady loading on the pan and effective control of the crystallisation conditions within the pan. To stabilise loading, a strategy has been developed which uses buffer tanks in an optimal way to damp out flow fluctuations. This strategy accommodates multiple buffer tanks in series without the amplification of disturbances that occurs with some of the simpler published techniques. The dynamic behaviour of absolute pressure control and compartment feed control were investigated in an industrial scale pan. This work has demonstrated the importance of high quality absolute pressure control and developed techniques for effective automatic tuning of pan feed controls. As part of this research, computer control systems were developed as tools to provide the appropriate monitoring and control of the experiments undertaken.
Activity of complex multifunctional organic compounds in common solvents.
(2009) Moller, Bruce.; Ramjugernath, Deresh.; Rarey, Jurgen.
The models used in the prediction of activity coefficients are important tools for designing major unit operations (distillation columns, liquid-liquid extractors etc). In the petrochemical and chemical industry, well established methods such as UNIFAC and ASOG are routinely employed for the prediction of the activity coefficient. These methods are, however, reliant on binary group interaction parameters which need to be fitted to reliable experimental data. It is for this reason that these methods are often not applicable to systems which involve complex molecules. In these systems, typically solid-liquid equilibria are of interest where the solid is some pharmaceutical product or intermediate or a molecule of similar complexity (the term complex here refers to situations where molecules contain several functional groups which are either polar, hydrogen bonding, or lead to mesomeric structures in equilibrium). In many applications, due to economic and environmental considerations, a list of no more than 20 solvents is usually considered. It is for this reason that the objective of this work is to develop a method for predicting the activity coefficient of complex multifunctional compounds in some common solvents. The segment activity coefficient approaches proposed by Hansen, MOSCED and the NRTL-SAC models show that it should be possible to “interpolate” between solvents if suitable reference solvents are available (e.g. non-polar, polar and hydrogen bonding). Therefore it is useful to classify the different solvents into suitable categories inside which analogous behaviour should be observed. To accomplish this, a significant amount of data needs to be collected for the common solvents. Data with water as a solvent was freely available and multiple sources were found with suitable data. Both infinite dilution activity coefficient (y∞) and SLE (Solid-Liquid Equilibrium) data were used for model development. The y∞ data were taken from the DDB (Dortmund Data Bank) and SLE data were taken from Beilstein, Chemspider and DDB. The limiting factor for the usage of SLE data was the availability of fusion data (heat of fusion and melting temperature) for the solute. Since y∞ in water is essentially a pure component property it was modelled as such, using the experience gained previously by this group. The overall RMD percentage (in ln y∞) for the training set was 7.3 % for 630 compounds. For the test set the RMD (in ln y∞) was 9.1 % for 25 fairly complex compounds. Typically the temperature dependence of y∞ data is ignored when considering model development such as this. Nevertheless, the temperature dependence was investigated and it was found that a very simple general correlation showed moderate accuracy when predicting the temperature dependence of compounds with low solubility. Data for solvents other than water were very scarce, with insufficient data to develop a model with reasonable accuracy. A novel method is proposed for the alkane solvents, which allows the values in any alkane solvent to be converted to a value in the solvent hexane. The method relies on a first principles application of the solution of groups concept. Quite unexpectedly throughout the course of developing the method, several shortfalls were uncovered in the combinatorial expressions used by UNIFAC and mod. UNIFAC. These shortfalls were empirically accounted for and a new expression for infinite dilution activity coefficient is proposed. This expression is however not readily applicable to mixtures and therefore requires some further attention. The method allows for the extension of the data available in hexane (chosen since it is a common solvent for complex compounds). In the same way as the y∞ data in water, the y∞ data in hexane were modelled as a pure component property. The overall RMD percentage (in ln y∞) for the training set was 21.4 % for 181 compounds. For the test set the RMD (in ln y∞) was 11.7 % for 14 fairly complex compounds. The great advantage of both these methods is that, since they are treated as pure component properties, the number of model parameters grows linearly with the number of groups, unlike with mixture models (UNIFAC, ASOG, etc.) where it grows quadratically. For both the water and the hexane method the predictions of the method developed in this work were compared to the predictions of UNIFAC, mod. UNIFAC, COSMO-RS(OL) and COSMO-SAC. Since water and hexane are not the only solvents of practical interest, a method was developed to interpolate the alcohol behaviour based on the water and hexane behaviour. The ability to predict the infinite dilution activity coefficient in various solvents allowed for the prediction of various other properties, viz. air-water partition coefficient, octanol-water partition coefficient, and water-alcohol cosolvent mixtures. In most cases the predictions of these properties were good, even for the fairly complex compounds tested.
A study of hydrodynamics and mass transfer in small bore deep shaft reactors.
(2010-09-08) Clark, Nigel Norman.
The phenomenology of vertical two phase flow was examined
The effect of solid micro particles on mass transfer in agitated dispersions.
(2008) Soni, Minal.
The industrial application of gas-liquid contactors has made effective design and optimisation of these processes a very important topic. In order to sustain a competitive advantage, rate limiting steps must be clearly understood. Hydrodynamics, heat transfer and mass transfer are complicated features of gas-liquid contactors and require a fundamental understanding. The mechanism of mass transfer in the presence of a small concentration of solid micro particles has been the subject of debate. The adsorption of gas by solid particles ("shuttle mechanism") is the traditional explanation. Recent experimental evidence suggests that the introduction of micro particles removes trace surface active impurities from the system and allows the true mass transfer coefficient to be measured. The objective of this study was to confirm the surfactant removal theory. Mass transfer is a field characterised by imprecise empirical relationships and difficult to obtain experimental parameters. This puts into context the significant challenge posed in preparing the careful set of measurements and analyses presented in this study to lend support to the surfactant removal mechanism. The study began with a review of mass transfer models. These models are based on concepts such as surface renewal and idealised turbulence. It is, however, difficult to choose between the models as they predict similar values despite being based on different mechanisms. The overall mass transfer coefficient is composed of the gas-phase coefficient (kGa) and liquid-phase coefficient (kLa). As the values of the coefficients are comparable and the solubility of oxygen or hydrogen is very Iow, the overall mass transfer coefficient is approximately equal to the liquid side coefficient. The relationship of kL with the diffusion coefficient (D) is one of the limited ways of choosing between the models. Mass transfer models predict k j • u:. D" . n is predicted to be % for a rigid surface (contaminated interface region) and Y2 for a mobile surface (clean interface region). If the surfactant removal mechanism applies, the introduction of solid particles will be accompanied by a reduction of n from % to 1/2. The effect of particles on n can be calculated from precise measurement of kL of gases with significantly different diffusion coefficients. A review of experimental methods was made to find precise methods to characterise mass transfer in the presence of solid micro particles. The chemical sulphite, gas-interchange and pressure step methods were identified as appropriate methods. These were implemented in a stirred cell (0.5 !) and an agitated tank (6 I). The chemical sulphite measurements were used to confirm that the enhancement of kLa is due to an enhancement of kL and not the specific interfacial area (a). Flat surface experiments were made using water and 0.8 M sodium sulphate batches. The reduction of n from % to Y2 was confirmed in both apparatuses after the addition of solid particles. The data were very well correlated and the dependence of kr on the energy dissipation rate per unit volume (e) is similar to the theoretically predicted value of 114 for the exponent. Observation of the reduction of n from % to Y2 was extended to agitated dispersions. The stirred cell kLa data were measured by the gas interchange method and are of excellent quality. The agitated tank results were measured by pressure step methods. The pressure dependence of the polarographic probes affected the precision of the results and the effect was within the experimental uncertainty. The effect of particles on n could not, therefore, be conclusively confirmed in the agitated tank. By relating precisely measured mass transfer coefficients to the diffusion coefficients; the surfactant removal theory is confirmed. The result is valid for a flat mass transfer area as well as for agitated dispersion where the nature of the interface region changes with time due to the accumulation of surfactants on an initially clean interface.
Membrane distillation of concentrated brines.
(2010-10-29) Mariah, Lynette.
Salinity is one of the most critical environmental problems for water scarce countries,
An environmental life cycle assessment of the provision of recycled water in Durban.
(2006) Pillay, Sarushen Dhanapalan.; Buckley, Christopher Andrew.; Friedrich, Elena.
This dissertation investigated the questions of sustainable development, in the context of water and sanitation provision, for the eThekwini Municipality. The Durban Water Recycling (DWR) plant, run by Veolia Water, was initially the focus of this investigation. The use of recycled water in Durban has freed potable water supplies for a potential 200 000 new consumers. Industry also benefits as the recycled water is supplied at a lower cost. In order to create a holistic picture of the effect of water recycling, a network incorporating the abstraction, use, re-use and disposal of water in the South Durban Region was investigated. This water supply network was identified consisting of the following units: Inanda Dam, Wiggins Waterworks, the pumping and reticulation network, Durban Southern Wastewater Treatment Works, Durban Water Recycling and the Durban Southern Deep Sea Outfall. For the environmental analysis the Life Cycle Assessment (LCA) tool was chosen. Life cycle assessment is a systematic way to evaluate the environmental impacts of products or processes by following a scientific methodology in which the impacts are quantified. LCA provides objective answers to environmental questions while suggesting more sustainable forms of production and consumption. It is the only tool which has a cradle-to-grave approach and by this it avoids positive ratings for measurements which only consist in the shifting of burdens. The objective of this LCA was twofold. The first was to quantify and evaluate the environmental performance of relevant processes and so help decision makers choose amongst options. The second objective was to provide a basis for assessing potential improvements in the environmental performance of the system. Once these areas and the contributors to the high burdens were identified, improvement options were investigated. One of the key outcomes of this analysis was the development of an electricity index as an indicator of environmental performance for water and wastewater systems. The GaBi 3 software package, which uses the CML (Centre for Environmental Science, University of Leiden, The Netherlands) LCA methodology, was used to compile environmental impact scores for each impact category. For the non standard systems such as Inanda Dam and the Durban Southern Deep Sea Outfall a new way of assessing the impacts was developed. There is an emerging trend to combine the LCA methodology with social issues so as to improve the decision making capability. The social analysis was carried out using an LCA type methodology. The impact categories selected were; job creation and health and health risks. During the course of the study the issue of land displacement arose when investigating the social issues surrounding the construction of a dam. This was then incorporated into the entire study. The system was broken up into sub-systems which were studied separately and then combined to create a holistic picture. Each sub-system was further divided into three stages for analysis; the construction, operation and decommissioning. This method of analysing the system allows for the detailed description of individual process units with the highest social and environmental burden. For example it was identified that the operation of the activated sludge systems at the wastewater treatment works had an environmental burden due to the electricity consumption during this stage. For the impact category of global warming it was discovered that 40% of the total environmental impact of the system could be attributed to the secondary treatment stage at the wastewater works. The construction of the dam had the largest social burden due to the displacement of the communities living in the dam area. The final part of the study was a scenario analysis. The aim of this analysis was to develop a sustainability framework for municipalities seeking to expand their provision of water and sanitation services. Different scenarios for increasing the water supply of a municipality were considered. The environmental impact of each scenario was also investigated. In this stage various options were considered to see how changes in the system affected the environmental profile. Improvements using new, modified or alternate technologies were suggested and their effects calculated. An operating procedure, for the current system, with the lowest environmental impact was also suggested. The results of this research will prove valuable to designers and planners looking to expand existing water supply networks in a sustainable manner. A sustainability framework was developed to complement the existing DWAF framework for municipalities expanding their provision of water and sanitation services. The key findings of this study were: • The quantification of the environmental burdens for the supply of water and sanitation in the eThekwini Municipality first for the individual units then for the system. • An improvement analysis which suggested ways of reducing the environmental burdens of the existing system. • The development of a sustainability framework for a municipality to increase its water and sanitation service levels. • The incorporation of social indicators into the LCA methodology. • The development of a technique that could be incorporated into the LCA methodology, for assessing the toxicity of complex effluents. • The development of a method of evaluating the environmental performance of a water and sanitation system using an electricity index. The thesis provides a holistic view of the abstraction, use, re-use and disposal of water in the eThekwini Municipality and provides a guideline for decision makers when assessing options for expansion or improvement in water supply networks.
Robust equipment for the measurement of vapour-liquid equilibrium at high temperatures and high pressures.
(2004) Harris, Roger Allen.; Ramjugernath, Deresh.; Raal, Johan David.; Letcher, Trevor M.
In this work VLE data was measured on three different pieces of equipment. Measurements were undertaken in the laboratory of Professor Gmehling in Oldenburg, Germany using two different static cells and in the Thermodynamics Research Unit (TRU), University of Natal, South Africa using a specially designed dynamic still. The three pieces of equipment used are as follows: i.) Static apparatus of Rarey and Gmehling (1993), ii.) Static apparatus of Kolbe and Gmehling (1985) as modified by Fischer and Wilken (2001), and, iii.) Dynamic apparatus ofHarris et al. (2003b). In total 370 data points were measured; fourteen sets of VLE data and eight vapour pressure data sets were measured. The work undertaken in Germany measured the systems hexane (1) + N-methylformarnide (2), benzene (1) + N-methylformamide (2), cWorobenzene (1) + N-methylformarnide (2) and acetonitrile (1) + N-methylformamide (2), at 363.15 K using the equipment of Rarey and Gmehling (1993). The systems CO2 (1) + Napthalene (2) at T = 372.45 K, 403.85 K and 430.65 K and CO2 (1) + Benzoic acid (2) at T= 403.28 K, 432.62 K and 458.37 K were measured on the equipment of Kolbe and GmeWing (1985) (as modified by Fischer and Wilken (2001)). Apart from the CO2 (1) + Napthalene (2) system at T = 372.45 K, all the above-mentioned data are new data. The equipment designed in the TRU was designed to operate between 300 and 700 K and between 1 kPa and 30 MPa. The equipment is of the dynamic recirculating VLE still type (DRVS) and is based on the principles of low-pressure stills. The still is constructed from uniquely machined Stainless-steel components and standard commercial Stainless-steel tubing and valves and is computer controlled to operate either isobarically or isothermally. Vapour pressures were measured on the new equipment for n-heptane, n-decane, n-dodecane, n-hexadecane, l-octadecene, 1-hexadecanol and d,l-menthol at low pressures and for acetone at high pressures. These vapour pressure measurements were used as test systems and ranged from 1.00 kPa to 1 000 kPa and from 308.33 K to 583.90 K. Cyclohexane (1) + ethanol (2) at 40 kPa and n-dodecane (1) + l-octadecene (2) at 26.66 kPa were measured as two isobaric VLE test systems. The VLE data measured for d,l-menthol (1) + l-isomenthol (2) at T= 448.15 K and n-dodecane (1) + l-octadecene (2) at P = 3.0 kPa represent new data measured on the equipment. All the VLE systems were modeled. Two data reduction methods were investigated: i.) the combined (r-rf) method, and, ii.) the direct method (H) method. Several different Gibbs excess models (Wilson, NRTL and UNIQUAC), equations of state (PengRobinson and virial) and mixing rules (Huron-Vidal, Wong-Sandler and Twu-Coon) were used in different combinations to find the best fit for the data. The Maher and Smith (1979) method was used to determine infinite dilution activity coefficients from the very smooth data of the N-methylformamide systems. Excess properties were determined for the CO2 (1) + Napthalene (2) and the CO2 (1) + Benzoic acid (2) systems. Although the equipment of Hams et al. (2003b) was able to measure data at high temperatures and elevated pressures, the precission of the data was not as good as was expected. Measuring the system temperature at elevated temperatures was especially problematic. The problem is attributed to the large mass of Stainless-steel used in the construction of the apparatus. To rectify this problem it is suggested that the equipment be modified to be lighter in weight and only capable of measuring VLE at moderate pressures (less than 3 MPa).
Modelling of a vibrating-plate extraction column.
(2010) Rathilal, Sudesh.; Carsky, Milan.
Liquid extraction, sometimes called solvent extraction, is the separation of the constituents of a liquid solution by contact with another insoluble liquid. It belongs to the class of countercurrent diffusional separation processes, where it ranks second in importance to distillation. There are many different types of columns that are available for liquid-liquid extraction and the reciprocating column (RPC) and vibrating plate column (VPE) are two types of mechanically aided columns. This research aims at developing a mathematical model for the prediction of NTUIHETS and the mass transfer coefficient, k-ox for the VPE based on the agitation level of the plates (af- the product of frequency and amplitude of the plate motion), the plate spacing and the flow rates which will allow for the simplification in the design of this type of column. There is a lot of research that has gone into the development of mechanically aided extraction columns but it is limited when it comes to the RPC and VPE and most of this research is devoted to the RPC. The system chosen is the acetone-toluene-water system with the acetone in toluene forming the feed that is dispersed in the column as it moves upward while the water moves as a continuous phase down the column. Experiments were conducted to evaluate the hydrodynamics of the droplets moving up the column (in terms of drop sizes, size distribution and dispersed phase holdup) and to evaluate the mass transfer that occurs (in order to evaluate NTU, HTU and k-ox) as well as the effect of mass transfer on the hydrodynamics of the system while varying the agitation levels and spacing of the plates in the column. Successful models were developed using some of the experimental data and these correlations were verified with additional data.
Development and critical evaluation of group contribution methods for the estimation of critical properties, liquid vapour pressure and liquid viscosity of organic compounds.
(2006) Nannoolal, Yash.; Ramjugernath, Deresh.; Rarey, Jurgen.
Critical properties, liquid vapour pressures and liquid viscosities are important thermophysical properties required for the design, simulation and optimisation of chemical plants. Unfortunately, experimental data for these properties are in most cases not available. Synthesis of sufficiently pure material and measurements of these data are expensive and time consuming. In many cases, the chemicals degrade or are hazardous to handle which makes experimental measurements difficult or impossible. Consequently, estimation methods are of great value to engineers. In this work, new group contribution methods have been developed for the estimation of critical properties, liquid vapour pressures and liquid viscosities of non-electrolyte organic compounds. The methods are based on the previous work of Nannoolal (2004) & Nannoolal et al. (2004) with minor modifications of structural group definitions. Critical properties, viz. critical temperature, critical pressure and critical volume, are of great practical importance as they must be known in order to use correlations based on the law of corresponding states. However, there is a lack of critical property data in literature as these data are difficult or in many cases impossible to measure. Critical property data are usually only available for smaller molecules of sufficient thermal stability. The proposed group contribution method for the estimation of critical properties reported an average absolute deviation of 4.3 K (0.74%), 100 kPa (2.96%) and 6.4 cm3.mol1 (1.79%) for a set of 588 critical temperatures, 486 critical pressures and 348 critical volumes stored in the Dortmund Data Bank (DDB (2006)), respectively. These results were the lowest deviations obtained when compared to ten well known estimation methods from literature. In addition, the method showed a wider range of applicability and the lowest probability of prediction failure and leads to physically realistic extrapolation when applied to a test set of components not included in the training set. For the estimation of the critical temperature using the new method, knowledge about the normal boiling point is required. If there is no information on the latter property, then the previous group contribution estimation method can be employed for estimation. Because of their great importance in chemical engineering, liquid vapour pressures have received much attention in literature. There is currently an abundance of experimental data for vapour pressures, especially for smaller molecules, but data are scarce or of low quality for larger and more complex molecules of low volatility. The estimation of liquid vapour pressures from molecular structure has met with very limited success. This is partly due to the high quality predictions required for vapour pressures for use in the design of for example distillation columns. This work presents a new technique for the estimation of liquid vapour pressures by developing a two-parameter equation where separate parameters model the absolute value and slope while at the same time the equation is able to approximate the nonlinearity of the curve. The fixed point or absolute value chosen was the normal boiling point for which a large amount of experimental data is available. A group contribution estimation of the slope was then developed which showed nearly no probability of prediction failure (high deviation). Employing experimental normal boiling points in the method, an absolute relative deviation of 6.2% in pressure for 1663 components or 68835 (68670 from DDB and 165 from Beilstein) data points was obtained. This result is in comparable accuracy or slightly higher in deviation than correlative models such as the Antoine and DIPPR equations (direct correlations). A test of the predictive capability by employing data that were not used in the training set also showed similar results. Estimations are possible up to the inflection point or a reduced normal boiling temperature of ±1.2. If there is no information about the experimental normal boiling point, two options are recommended to obtain this value. The first and more reliable is back-calculation using the known boiling point at other pressures and the estimated slope of the vapour pressure equation. Results in this case are similar to cases where experimental normal boiling points were used. The second possibility is to estimate the normal boiling point using the method developed previously. In this case, an absolute relative deviation of 27.0% in pressure is obtained. The saturated liquid viscosity is an important transport property that is required for many engineering applications. For this property, experimental data are limited to mostly simple and more common components and, even for these components the data often cover only a small temperature range. There have been many different approaches to estimate liquid viscosities of organic compounds. However, correlative and empirical methods are often the only or preferred means to obtain liquid viscosities. The technique used for the estimation of the liquid viscosity is similar to that in case of liquid vapour pressures, i.e. a two-parameter equation models the absolute value, slope and the non-linearity of the curve. As there was no convenient reference point at a standard viscosity available to model the absolute value (viscosity reference temperature), an algorithm was developed to calculate this temperature which was chosen at a viscosity of 1.3 cP. This work then presents a group contribution estimation of the slope and using calculated or adjusted reference temperatures, an absolute relative deviation of 3.4% in viscosity for 829 components or 12861 data points stored in the DDB was obtained. This result is in comparable accuracy or slightly higher in deviation than correlative models such as the Andrade and Vogel equations (direct correlations). The estimation method has an upper temperature limit which is similar to the limit in case of liquid vapour pressures. If no data are available for a viscosity close to 1.3 cP then, as in case of the vapour pressure estimation method, the temperature can be back calculated from data at other viscosity values. Alternately, the viscosity reference temperature can be estimated by a group contribution method developed in this work. This method reported an average absolute deviation of 7.1 K (2.5%) for 813 components. In case both the slope and absolute value were estimated for the liquid viscosity curve, an average absolute deviation of 15.3 % in viscosity for 813 components or 12139 data points stored in the DDB was obtained. The new method was shown to be far more accurate than other group contribution methods and at the same time has a wider range of applicability and lower probability of prediction failure. For the group contribution predictions, only the molecular structure of the compound is used. Structural groups were defined in a standardized form and fragmentation of the molecular structures was performed by an automatic procedure to eliminate any arbitrary assumptions. To enable comparison, chemical family definitions have been developed that allow one to automatically classify new components and thus inform the user about the expected reliability of the different methods for a component of interest. Chemical family definitions are based on the kind and frequency of the different structural groups in the molecule.
Development of novel apparatus for vapour-liquid equilibrium measurements at moderate pressures.
(2006) Reddy, Prashant.; Ramjugernath, Deresh.; Raal, Johan David.
In this work, a novel experimental apparatus has been designed, constructed and commissioned for the measurement of VLE at pressures up to 750 kPa and temperatures up to 600 K. The project undertaken represents a complete re-working of the design of Harris (2004), which was plagued by irregularities in the equipment operation and in the acquisition of experimental data. As in the work of Harris (2004), the design of the apparatus presented here is based upon the highly successful glass VLE still design of Raal (Raal and Muhlbauer, 1998). The novel apparatus is principally constructed from machined 316 stainless steel and features sight glasses in strategic positions to allow for an observation of the fluid flow characteristics in specific sections of the apparatus. The key criteria that encompassed the design of the equipment were expediency, operational efficiency and versatility in the acquisition of reliable VLE data. An initial test of the performance of the equipment was achieved through the measurement of pure-component vapour pressures of selected hydrocarbons (n-alkanes and a cycloalkane) and alkanols. The test system for vapour-liquid equilibrium (P-T-x-y) measurements with the novel apparatus was that of cyclohexane + ethanol at a pressure of 40 kPa. Good agreement between the literature and the experimental data was observed. Isobars for the cyclohexane + ethanol system at 69.8 kPa, 97.7 kPa and ISO kPa were also measured. The latter constitutes new data that have been measured for this system. Novel vapour-liquid equilibrium data were also obtained for the systems of I-propanol + 2-butanol, I-propanol + n-dodecane and 2-butanol + ndodecane at temperatures of 373.15 K, 393.15 K and 423.15 K. For the very high relative volatility alkanol + n-dodecane systems, uncertainties in the measurement of the vapour phase (y) resulted in only P-T-x experimental data being presented here, where the vapour phase composition was computed with the Wilson equation. The theoretical treatment of the experimental VLE data was achieved through a combination of the gamma-phi and the phi-phi approaches in the fitting of the VLE data to various thermodynamic models. In the gamma-phi method, a variety of activity coefficient models (Wilson, T-K Wilson, NRTL, UNIQUAC and modified UNIQUAC) together with the truncated virial equation of state were employed to find the best fit for the data. In the phi-phi method, the isothermal data sets were treated with the Peng-Robinson-Stryjek-Vera equation of state with the original Huron-Vidal (HV) and the modified Huron-Vidal mixing rules (MHVI and MHV2) in the correlative procedure. Thermodynamic consistency testing was also performed with the Direct Test of Van Ness (1995) to assess the quality of the experimental P-T-x-y VLE data sets measured in this study.
Treatment of dye wastewaters in the anaerobic baffled reactor and characterisation of the associated microbial populations.
(2002) Bell, Joanne.; Buckley, Christopher Andrew.
There is potential for the anaerobic baffled reactor (ABR) to be implemented on-site for pre-treatment of coloured wastewaters. The implementation of waste minimisation and cleaner production strategies in industry will result in the production of smaller volumes of concentrated wastewaters. With implementation of the ABR, the concentrated waste stream could be pre-treated, with an acclimated biomass, which should facilitate sufficient degradation such that the effluent could be discharged to sewer for further treatment. The ABR is a high-rate compartmentalised anaerobic bioreactor, the design of which promotes the spatial separation of microorganisms. The use of molecular teclmiques to characterise the microbial populations and the dynamics of these populations with time and/or changing operating conditions will add to the current understanding of the process, which is based on the biochemical pathways and chemical analyses. This knowledge will allow for optimisation of the design of the ABR. The hypothesis of the horizontal separation of acidogenesis and methanogenesis through the ABR was proven. Changes in the HRT affected the operation of the reactor, however, recovery from these upsets was almost immediate and operation of the reactor was stable. Two synthetic dye waste streams, one food dye (tartrazine) and one textile dye (Cl Reactive Red 141), and a real industrial dye wastewater, were treated in separate laboratory-scale ABRs. These investigations showed that successful treatment of a highly coloured wastewater is possible in the ABR. The design of the ABR facilitates efficient treatment of concentrated dye wastewaters by protecting the sensitive methanogens from the inhibitory dye molecules and promoting efficient colour and COD reduction. The molecular-based method, fluorescent in situ hybridisation, allowed the direct identification and enumeration of microbial populations active in the ABR. In all of the reported investigations, there was a definite shift in the microbial populations through the ABR, with a predominance of eubacteria in the first compartments (acidogenesis) and archaea (methanogenesis) in the later compartments. The number of compartments involved in each depended on the strength of the substrate (organic loading rate - OLR). A combination of FISH probing, and the analysis of 98 archaeal l6S rDNA clone inserts provided useful descriptions of the methanogens actively involved within each compartment. These showed a predominance of the Methanosaeta spp., particularly in the last compartments of the reactor. Methanogens present in the first four compartments consisted of species of Methanobacterium and Methanospirillum, a relatively unstudied methanogen Methanomethylovorans hollandica, and an unidentified short filamentous species.
The development of a method for the inclusion of salinity effects into environmental life cycle assessments.
(2003) Leske, Anthony.; Buckley, Christopher Andrew.
The work presented in this thesis stemmed out of the apparent lack of a method for incorporating salinity effects into environmental life cycle assessments. Salination of the water resources is a well-known problem in South Africa, and is of strategic concern. Any environmental decision support. tool that does not allow the evaluation of salinity effects therefore has limited applicability in the South African context. The starting-point for the work presented in this thesis was to evaluate existing impact categories, and the characterisation models used to calculate equivalency factors for these impact categories, in an attempt to incorporate salinity effects into existing categories and/or characterisation models. The types of effects that elevated (above normal background levels) dissolved salt concentrations have on the natural and man-made environment were evaluated, and it was concluded that, although there was some overlap with existing impact categories, some of the salinity effects could not be described by existing impact categories. It was also concluded that there are clear and quantifiable causal relationships between releases to the environment and salinity effects. A separate salinity impact category was therefore recommended that includes all salinity effects, including; aquatic ecotoxicity effects, damage to man-made environment, loss of agricultural production (livestock and crops), aesthetic effects and effects to terrestrial fauna and flora. Damage to the man-made environment is evaluated in terms of effects on equipment and structures, interference with processes, product quality and complexity of waste treatment, and is used as an indicator for the environmental consequences derived from the caused additional activity in the man-made environment. Once a conceptual model for a separate salinity impact category had been formulated, existing characterisation models were evaluated to determine their applicability for modelling salinity effects. Salination is a global problem, but generally restricted to local or regional areas, and in order to characterise salinity effects, an environmental fate model would be required in order to estimate salt concentrations in the various compartments, particularly surface and subsurface water. A well-known environmental fate and effect model was evaluated to determine if it could be used either as is, or in modified form to calculate salinity potentiaIs for LCA. It was however concluded that the model is not suitable for the calculation of salinity potentials, and it was therefore decided to develop an environmental fate model that would overcome the limitations of existing model, in terms of modelling the movement of salts in the environment. In terms of spatial differentiation, the same approach that was adopted in the existing model was adopted in developing an environmental fate model for South African conditions. This was done by defining a aunit South African catchmenta (including the air volume above the catchment), which consists of an urban surface; rural agricultural soil (and associated soil moisture); rural natural soil (and associated moisture), groundwater (natural and agricultural) and one river with a flow equal to the sum of the flows of all rivers in South Africa, and a concentration equal to the average concentration of each river in the country. A non steady-state environmental fate model (or, hydrosalinity model) was developed that can predict environmental concentrations at a daily time-step in all the compartments relevant to the calculation of salinity potentials. The environmental fate model includes all the major processes governing the distribution of common ions (sodium, calcium, magnesium, sulphate, chloride and bicarbonate) in the various compartments, and described as total dissolved salts. The effect factors used in the characterisation model were based on the target water quality ranges given by the South African Water Quality Guidelines in order to calculate salinity potentials. The total salinity potential is made up of a number of salinity effects potentials, including; damage to man-made environment, aquatic ecotoxicity effects, damage to man-made environment, loss of agricultural production (livestock and crops), aesthetic effects and effects to terrestrial fauna and flora. The total salinity potentials for emissions into the various initial release compartments are shown in the table below. Initial release compartment Atmosphere River Rural natural surface Rural agricultural surface Total salinity potential (kg TDS equJkg) 0.013 0.16 0.03 1.00 The salinity potentiaIs are only relevant to South African conditions, and their use in LeA in other countries may not be applicable. This, in effect, means that the life cycle activities that generate salts should be within the borders of South Africa. It has been recognised that the LCA methodology requires greater spatial differentiation. Salination is a global problem, but generally restricted to local or regional areas on the globe, and it is foreseen that local or regional salinity potentials would need to be calculated for different areas of the earth where salinity is a problem. The LCA practitioner would then need to know something about the spatial distribution of LCA activities in order to apply the relevant salinity potentials. The LCA practitioner should also take care when applying the salinity potentials to prevent double accounting for certain impacts. Currently, this is simple because no equivalency factors exist for common ions, or for total dissolved salts as a lumped parameter. The distribution of salinity potentials, which make up the total salinity potential, appears to be supported by the environmental policies and legislation of South Africa, in which irrigation using saline water is listed as a controlled activity, and subject to certain conditions. The major recommendations regarding further work are focussed on the collection of data that will allow further refinement of the model, and to decrease the uncertainty and variability associated with the results. The values of the published equivalency factors are dependent on the mathematical definition of the local or regional environment, and these values have been calculated for Westem European conditions. Equivalency factors may vary by several orders of magnitude, depending on how the local or regional conditions have been defined. It is therefore recommended that the model developed in this work ultimately be included into a global nested model that can be used to calculate equivalency factors for other compounds, including heavy metals and organic compounds. This would result in equivalency factors for all compounds that are relevant to South Africa.
Optimal design of a secondary milling circuit for treating chromite-rich UG-2 platinum ores.
(2011) Maharaj, Lakesh.; Pocock, Jonathan.; Loveday, Brian Kelsey.
Extraction of platinum group elements (PGE) is a major source of revenue in South Africa and the reserves represent about 75 per cent of world reserves. Most of the remaining Platinum Group Mineral (PGM) reserves are located in the UG-2 chromitite layer of the Bushveld Igneous Complex. Platinum concentrators experience significant losses of PGE in their secondary milling circuits due to insufficient liberation of platinum-bearing particles. The chromium oxide (Cr2O3) content in UG-2 concentrates is typically 3%, which results in operational problems in the downstream smelting process. Ways of improving the design of the secondary milling circuit were investigated, with the purpose of improving PGE recovery and reducing Cr2O3 entrainment in the subsequent flotation stage. Batch-scale laboratory and pilot plant tests were carried out to investigate the optimal design of a secondary milling circuit configuration. The optimal design consisted of a conventional hydrocyclone to de-slime the feed, followed by gravity separation with a spiral concentrator circuit to separate the ore into lights (silicates-rich) and heavies (chromite-rich) fractions. Separate milling of the light and heavy fractions made it possible to grind the silicate-rich fraction finer and to avoid over-grinding of the chromite. The total milling energy was redistributed between the silicates and chromite ball mills with 88% of the energy input to the silicates mill and 12% to the chromite mill thus reducing chromite over-grinding. The effects on the recovery of PGE, and the entrainment of Cr2O3 were measured in combined batch rougher flotation tests. The results indicated a 2% improvement in the secondary rougher flotation PGE recovery for the densifier underflow sample as compared to the standard MF-2 circuit, and most significantly the Cr2O3 entrainment was reduced by over 30% overall. Attritioning of the chromite-rich heavies fraction and ball milling of the silicates-rich lights fraction resulted in a 52% reduction of Cr2O3 in the rougher flotation concentrate and a 0.4% increase in PGE recovery (0.4%) as compared to the standard circuit. The improved reduction in chromite entrainment may be attributed to the lower fines generation with attritioning (52.8%- 106μm) as compared to ball milling with a 12% energy input (83.6% -106μm). Over 50% of the chromite minerals remained in the +106μm of the attritioned heavies product as compared with 21% for the ball milled spiral heavies stream. This accounted for a significant proportion of the overall chromite reduction in the flotation concentrate and supported the motivation for the inclusion of a separate grinding circuit for the chromite and silicate particles. Pilot plant testwork on a VHG (very high grade) spiral concentrator circuit followed by laboratory milling and rougher flotation tests confirmed the above conclusions. A 3.7% improvement in PGE recovery was noted with a 32% Cr2O3 reduction in the secondary rougher flotation concentrate as compared to the standard circuit. The statistical reliability of the laboratory and pilot plant data were quantified at various stages of the testwork due to the heterogeneous nature of the feed material and representative sampling. The repeat analyses on selected flotation tests for the high grade ore revealed that the variances were below 0.5%, 4%, and 7% for the head grades, PGE and Cr2O3 recoveries respectively. The flotation results for the standard and significantly improved milling circuits had variances in the 4E recoveries for the low grade ore and pilot plant ore of below 5.5% and 1% respectively. Low variances (<1%) in the Cr2O3 recoveries were noted for the low grade and pilot plant ores. A preliminary cost estimate was undertaken based on the pilot plant data to determine what value the proposed circuit could add for an additional 3.7% PGE recovery. An additional revenue of approximately R50 000 per day could result based upon the platinum mineral recovery only. The other precious metals, i.e. palladium and rhodium were neglected and would further increase the overall revenue. The minimum payback period for the estimated capital investment would be approximately 4 years. This confirmed the benefit of this improved secondary milling circuit design as a viable option. A closed-circuit operation of the silicates mill should offer more significant benefits compared to the open circuit option; however, this was not considered in the current testwork. This project has confirmed the benefit of separate ball milling and the use of a spiral concentrator as an effective gravity separation device in the secondary milling circuit for a chromite-rich (>50%) UG-2 platinum ore.
An integrated computational fluid dynamics an kinetics study of ozonation in water treatment.
(2005) Huang, Tzu Hua.; Buckley, Christopher Andrew.; Brouckaert, Christopher John.
Computational fluid dynamic (CFO) modelling has been applied to examine the operation of the prc-ozonation system at Wiggins Waterworks, operated by Umgeni Water in Durban, South Africa. Ozonation is employed in water treatment process primarily to achieve the oxidation of iron and manganese, the destruction of micro-organisms and the removal of taste and odour causing compounds. It also aids in the reduction of the colour of the final water, enhancement of algae removal and possible reduction of coagulant demand. A hydrodynamic model has been satisfactorily verified by experimental tracer tests. The effect of the gas injection was modelled by increasing the level of turbulence intensity at the ozone contactor inlet. The model prediction of the overall tracer response corresponded closely to the experimental results. The framework of ozone reaction modelling was subsequently established using values of rate constants from the literature. An accurate prediction of the ozone concentration profile requires the application of the correct ozone kinetics involved. In raw waters, the depletion of ozone is influenced by the presence of natural organic matters (NOM). The observed ozone decay was found in good agreement using the pseudo first-order rate law. By measuring the total organic carbon (TOC) as a surrogate for NOM, the experimentally determined rate constants can be calculated to account for the effects of the ozone doses and the water quality. The characterisation study also aimed to provide sufficient information on ozone depletion and to be operated easily, without the lengthy and costly analyses ofa detailed kinetics study. The predicted profile of residual ozone concentration suggests the current operating strategy can be improved to optimise the ozone utilisation. The proposed monitoring point was suggested to be at the end of second companment where most ozone reactions have been completed. By coupling the transport equations of the target compounds with their chemical reaction rates, the concentration profile of these compounds such as ozone can be predicted in order to assist the understanding of an operation and to attain better interpretation of experimental results.
An investigation of advanced oxidation processes in water treatment.
(2001) Schwikkard, Gavin Wyatt.; Buckley, Christopher Andrew.
The deteriorating water quality in South Africa and changing legislation requiring the industrial implementation of waste minimisation and pollution prevention technologies has highlighted the need for the investigation of new effluent treatment technologies such as advanced oxidation processes. This investigation details the evaluation of ultrasound, an emerging advanced oxidation process, to degrade organic compounds during water treatment. The objectives of the investigation included the design of a suitable ultrasonic laboratory reactor to investigate ultrasound chemistry and the sub-processes occurring during sonication. Atrazine was used as a model compound to compare the performance of ultrasound with that of ozone and hydrogen peroxide, already established advanced oxidation processes. Recommendations have also been made for the scale-up of ultrasonic processes. A 500 mL ultrasonic cell containing an ultrasonic horn as an energy source was designed and constructed. The measurement of hydrogen peroxide concentration was used as a tool to indicate the process conditions under which the formation of free radical reactions during sonication are enhanced. These include the application of oxygen and air sparging or the addition of a commercial source of hydrogen peroxide. It was found that oxygen sparging and a high acoustic power input should be used in ultrasonic processes with a short retention time, and conversely, that air sparging and a lower acoustic energy source should be used in processes with a long retention time. A flow loop system should be considered to maximise oxidation both within and beyond the sonicated zone, gas sparging should only occur within the sonication zone else the degradation of hydrogen peroxide is encouraged. Ultrasound is most effectively applied in water treatment as a pretreatment stage in combination with other technologies and not as a stand-alone process. Atrazine was used. as a model compound to compare the performance of ultrasound with ozone because of its persistence in the environment and resistance to degradation. Atrazine was degraded during sonication and ozonation. degradation increased wim the addition of hydrogen peroxide. Ozone decomposition (and hence free radical reactions) was enhanced when ozone was combined with ultrasound or hydrogen peroxide. Enhanced ozone decomposition during ozonation combined with sonication is due to the conditions (high temperatures and pressures) as well as the free radical reactions occurring within the collapsing cavitation bubbles and at the gas-liquid interface. The enhancing effect of combining ultrasound with ozone was greatest at the low ozone concentrations typically applied during water treatment. Atrazine degradation during sonication and ozonation is predominantly due to the reaction with hydroxyl radicals. Atrazine degradation products identified using gas chromatography and mass spectrometry were deethylatrazine. hydroxyatrazine and deethyldeisopropylatrazine (tentatively identified).
Design of a static micro-cell for phase equilibrium measurements : measurements and modelling = Conception d'une micro-cellule pour mesures d'é́́́quilibres de phases : mesures et mod́élisation.
(2011) Narasigadu, Caleb.; Ramjugernath, Deresh.; Naidoo, P.; Coquelet, Christophe.; Richon, Dominique.
Vapour-Liquid Equilibrium (VLE), Liquid-Liquid Equilibrium (LLE) and Vapour-Liquid-Liquid Equilibrium (VLLE) are of special interest in chemical engineering as these types of data form the basis for the design and optimization of separation processes such as distillation and extraction, which involve phase contacting. Of recent, chemical companies/industries have required thermodynamic data (especially phase equilibrium data) for chemicals that are expensive or costly to synthesize. Phase equilibrium data for such chemicals are scarce in the open literature since most apparatus used for phase equilibrium measurements require large volumes (on average 120 cm3) of chemicals. Therefore, new techniques and equipment have to be developed to measure phase equilibrium for small volumes across reasonable temperature and pressure ranges. This study covers the design of a new apparatus that enables reliable vapour pressure and equilibria measurements for multiple liquid and vapour phases of small volumes (a maximum of 18 cm3). These phase equilibria measurements include: VLE, LLE and VLLE. The operating temperature of the apparatus ranges from 253 to 473 K and the operating pressure ranges from absolute vacuum to 1600 kPa. The sampling of the phases are accomplished using a single Rapid-OnLine-Sampler- Injector (ROLSITM) that is capable of withdrawing as little as 1μl of sample from each phase. This ensures that the equilibrium condition is not disturbed during the sampling and analysis process. As an added advantage, a short equilibrium time is generally associated with a small volume apparatus. This enables rapid measurement of multiple phase equilibria. A novel technique is used to achieve sampling for each phase. The technique made use of a metallic rod (similar in dimension to the capillary of the ROLSITM) in an arrangement to compensate for volume changes during sampling. As part of this study, vapour pressure and phase equilibrium data were measured to test the operation of the newly developed apparatus that include the following systems: • VLE for 2-methoxy-2-methylpropane + ethyl acetate at 373.17 K • LLE for methanol + heptane at 350 kPa • LLE for hexane + acetonitrile at 350 kPa • VLLE for hexane + acetonitrile at 348.20 K New experimental vapour pressure and VLE data were also measured for systems of interest to petrochemical companies. These measurements include: • VLE for methanol + butan-2-one at 383.25, 398.14 and 413.20 K ABSTRACT • VLE for ethanol + butan-2-one at 383.26, 398.23 and 413.21 K • VLE for ethanol + 2-methoxy-2-methylbutane at 398.25 and 413.19 K • VLE for ethanol + 2-methylpent-2-ene at 383.20 K These measurements were undertaken to understand the thermodynamic interactions of light alcohols and carbonyls as part of a number of distillation systems in synthetic fuel refining processes which are currently not well described. Two of these above mentioned systems include expensive chemicals: 2-methoxy-2-methylbutane and 2-methylpent-2-ene. The experimental vapour pressure data obtained were regressed using the extended Antoine and Wagner equations. The experimental VLE data measured were regressed with thermodynamic models using the direct and combined methods. For the direct method the Soave-Redlich-Kwong and Peng-Robinson equations of state were used with the temperature dependent function (α) of Mathias and Copeman (1983). For the combined method, the virial equation of state with the second virial coefficient correlation of Tsonopoulos (1974) was used together with one of the following liquid-phase activity coefficient model: TK-Wilson, NRTL and modified UNIQUAC. Thermodynamic consistency testing was also performed for all the VLE experimental data measured where almost all the systems measured showed good thermodynamic consistency for the point test of Van Ness et al. (1973) and direct test of Van Ness (1995).
A study of the cane sugar diffusion process.
(1972) Rein, Peter William.; Woodburn, Edward T.
A mathematical model has been developed to represent the extraction of sugar from sugar cane in the diffusion process. As a consequence of the structure of prepared cane, extraction is postulated to occur via two first order relations in parallel. The model was found to represent accurately the extraction behaviour observed in two different experimental configurations. Experiments on a laboratory scale were undertaken to elucidate the mechanism and basic rates of extraction. The results are consistent with the postulate that extraction takes place by a combination of washing and molecular diffusion. The diffusion process was simulated experimentally in a fixed-bed pilot plant diffuser. Model parameter values estimated from this data provide evidence of the effect of liquid hydrodynamics on the extraction process. These results have been interpreted in terms of liquid holdup, liquid velocities and the efficiency of liquid- solid contacting. It is shown how the model can be applied to full scale diffusers of the moving-bed type, which will enable prediction of performance and the choice of optimum operating conditions.
A study of comminution in a vertical stirred ball mill.
(1993) Tuzun, Mustafa Asim.; Dawson, M. F.; Edwards, R. I.
A 20 litre experimental batch and continuous test rig and 5 litre batch and 50 litre continuous test rigs for stirred ball milling were built at the University of Natal and Mintek respectively. All the mills featured a grinding vessel with a central shaft equipped with pins and a torque measurement system. A washed chrome sand from the Bushveld Igneous Complex was used for the grinding experiments. Particle size analysis of products was performed using standard sieves and a Malvern Particle Sizer. Batch tests were run in the 20 litre stirred ball mill to achieve efficient grinding conditions. The effects of grinding conditions such as pulp density, media size, media density and shaft rotation speed and mill design parameters such as ball load, pin spacing and pin diameter on product size, power consumption and media wear were studied. It has been shown that the median size of the product can be calculated by the Charles' Energy-Size Equation. The stirred ball mill has been found to be more energy efficient than the tumbling ball mill. An energy reduction of 50% was possible for a product size of 6 microns when the stirred ball mill was employed instead of the tumbling ball mill. The energy input per ton of grinding media in the stirred ball mill could be 10 times higher than for the tumbling ball mill. Although during coarse grinds the media wear was higher in the stirred ball mill than in the tumbling mill, it became less so as grinding proceeded and for a product median size of 4.8 microns it was the same. Using a 5 litre batch mill, an experimental programme was designed to study the comminution characteristics of the stirred mill. A factorial design was prepared with the following parameters, which influence grinding in the stirred ball mill: pulp density, pin tip velocity and ball density and size. The energy required for grinding the chromite sand in the stirred ball mill was determined by the use of Charles' Equation. The findings were in agreement with the results predicted by this equation. It was shown that the Rosin-Rammler size distribution equation was a suitable procedure for presenting and comparing grinding data obtained from the stirred ball mill. The factors that had the greatest effect on grindability were, in order of importance: ball size, pin tip velocity and ball density. Interactions between grinding parameters were negligible. results implied that accurate predictions can be made to determine the grinding conditions required to achieve a desired product specification. An attempt was made to study the grinding kinetics the chromite are using the mass population·balance model. Grinding tests were performed with two mono size fractions ·53+38 and -38+25 microns and natural feed ·100 microns using various pin tip velocities, ball densities and within the normal stirred ball milling operating range. relationship between the ball diameter and the particle was explained by the "angle of nip" theory which applied for roller crushers. It was shown that the particle giving the maximum breakage rate was directly proportional to the ball diameter. Estimated grinding kinetic parameters from monosize provided a good basis for predictions of natural feed. However, the breakage rate obtained from monosize tests appeared to be lower than those from the natural feed It was found that if the selection and breakage functions were determined by monosize tests, it was possible to modify selection function parameters by back-calculation which gave the best fit to the natural feed size. A good correlation was obtained between the experimental and product distributions using a population-balance model. The links between the empirical model combining Charles' and Rosin-Rammler equations and the first-order batch grinding equation were also shown. The stirred ball mills were operated in batch and continuous mode. The median size of the products from the batch stirred ball mill experiments closely matched those of the continuous grinding experiments under similar grinding conditions. Using a salt solution as a tracer material, an attempt was made to estimate the residence time distribution based on a simplified analysis of the motion of the water in tile mill. The current scale-up methods for the stirred ball mill are discussed. A torque model was developed for given shaft geometry and ball relating the power rements of the stirred ball mill to the following prime design and operating parameters : mill diameter, mill height, pin tip velocity and effective density of the mill load. The basic assumptions underlying the model were that the mill content behaved as a fluidised bed, consequently a P effg h type model for the pressure was applied throughout the grinding media bed the effective charge velocity was proportional to the pin tip velocity. It was found that pin spacing, pin diameter and ball diameter significantly affected the mill torque. A semi-empirical torque model was derived to include these parameters. The relationships formulated from these models were shown to be in excellent agreement with experimental results.

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