Masters Degrees (Chemical Engineering)

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Investigation into the effects of froth height in a flotation cell.
(1971) Barker, Ian James.; King, R. P.
Abstract available in PDF file.
Utilisation of bagasse for the production of C5- and C6- sugars.
(1982) Trickett, Richard Charles.; Neytzell-de Wilde, Frank Gerard.
Surplus sugarcane bagasse, estimated at a maximum of 0,9x106 tons/year, represents an annual renewable resource which is readily available at the mill site and is a suitable potential source of alternative fuels and chemical feedstocks. This work contains an extensive literature survey which covers the production of C5- and C6- sugars from lignocelluloses by chemical hydrolysis and the pretreatment of cellulosic materials for enzymatic hydrolysis of the cellulose fraction. This survey was then used to determine the final direction of this research into the utilisation of bagasse for the production of fermentable sugars. It was decided that research should be directed at the dilute acid hydrolysis of the bagasse hemicellulose fraction to determine whether this fraction could be selectively hydrolysed from the complex lignocellulose structure and to obtain xylose yields under different hydrolysis conditions. Acids, especially acetic acid, are liberated from bagasse by steaming at elevated temperatures. In this acid medium the hemicelluloses are hydrolysed and become soluble. Autohydrolysis tests on whole bagasse indicate that hemicellulose hydrolysis becomes significant at temperatures above 140°C. However, the autohydrolysis liquor would still require dilute mineral acid hydrolysis to convert the pentose oligomers to their monomeric forms. Dilute sulphuric and batch hydrolysis of whole bagasse hemicellulose has thus been investigated at a solid to liquid ratio of 1:15 over the following temperature and acid concentrations ranges : 80° to 150°C and 3 to 40 g/l acid. Xylose, glucose, furfural and acetic acid formation and sulphuric acid consumption were monitored during these hydrolyses. Hemicellulose hydrolysis to produce mainly xylose is readily achieved over the entire range of acid hydrolysis conditions tested with little removal of the other bagasse components (lignin and cellulose). At the upper end of the temperature range acid concentrations below 20 g/l are sufficient for hemicellulose hydrolysis due to the effect of temperature on reaction rate. The bagasse hemicellulose consists of two fractions, an easily hydrolysable portion containing 165 mg of potential xylose/g bagasse and a resistant fraction containing 105 mg of potential xylose/g bagasse. A first order reaction model has been developed using the batch acid hydrolysis results. It is based on two hemicellulose fractions reacting simultaneously to give a common product (xylose) and predicts total xylose yield as a function of hydrolysis time for a given set of hydrolysis conditions. The encouraging xylose yields obtained during the batch hydrolyses led to the design of a continuous hydrolysis reactor to process bagasse at low liquid to solid ratios to determine whether xylose yields similar to the batch hydrolysis yields could be obtained at the same hydrolysis conditions. The continuous hydrolyses showed that for the conditions tested the xylose yields are unaffected by the decrease in liquid to solid ratio (down to 3,6:1) and it would appear that reactor performance is still controlled by reaction kinetics. A number of reactor configurations for the industrial production of pentoses from bagasse hemicelluloses are also proposed.
The development and application of combined water and materials pinch analysis to a chlor-alkali plant.
(1989) Gianadda, Paolo.; Buckley, Christopher Andrew.; Brouckaert, Christopher John.
Pinch Analysis, in the broadest sense, is concerned with the optimal use of resources (material or energy) in a multi-process system. Pinch Analysis based techniques have emerged for water systems over the past decade. A major assumption that has been made in applying these techniques is that a process system can be segregated into a set of process streams and a set of water streams. With this distinction in place, only the water streams are considered in the Pinch Analysis with the process streams represented implicitly. This approach has obvious limitations in situations where a clear distinction between process streams and water streams cannot be made. The chlor-alkali process is an example of a system in which the clear distinction between process streams and water streams cannot be made. Water is intrinsically involved in the process, serving as a carrier medium for raw materials and eventually becoming part of the products produced by the complex. Hydrochloric acid and caustic soda are reagents which are both used within and produced by the complex. These reagents are required by the process at a range of concentrations and the concentrated reagent is diluted to the required concentrations using demineralised water. Within the chlor-alkali complex, a number of effluents containing the reagent species are available and are typically sent to drain. It is conceivable that these effluents might be recovered and used for dilution purposes instead of demineralised water. This would bring about a reduction in the amount of water and concentrated reagent used and the amount of effluent produced by the complex. Given the economic value of these reagents relative to water, their recovery, if feasible, is likely to dominate the optimal water-use and effluent generation strategy. Current Water Pinch Analysis theory relies on the distinction being made between process streams and water streams and does not consider the recovery of reagents or the presence of desirable species within the system. In addition, the assumption is made that species are non-reactive; reactive species such as hydrogen chloride and sodium hydroxide, fall outside the scope of the current theory. The objectives of this study have included the development of an approach which is able to address these limitations of the existing theory. This approach, termed Combined Water and Materials Pinch Analysis seeks to identify optimal use strategies for raw materials and reagents, in addition to water-use and effluent generation. The approach combines mathematical programming with conceptual insights from Water Pinch Analysis. The approach is based on the optimisation of a superstructure which represents the set of all possible flow configurations for water, reagents and raw materials between the various operations within the process system; this problem is solved as a nonlinear programming (NLP) problem using standard optimisation tools. The application of the developed approach to the Sasol Polymers chlor-alkali complex at Umbogintwini, south of Durban, has been a further objective of this study. Given the variety of process operations present within the complex, which differ both in terms of their physical structure and function, individual process models for these operations were required. These models were described in terms of four basic functional elements, namely, mixing, flow separation, component separation and reaction, and incorporated into the superstructure. Given the complexity of the problem, the process system was divided into three subsystems which were optimised in isolation from each other. These results were subsequently integrated to reflect the performance of the subsystems in combination with each other. The results showed a potential reduction of 14% in water-use and 42% in effluent production by the complex, relative to the existing operating configuration. Amongst other savings in material use, the results indicated a 0.2% reduction in the use of salt, a 1.6% reduction caustic soda use and an 8.3% reduction in the use of hydrochloric acid. Economically, the potential saving identified was R 945 727 per annum, based on operating costs in the year 2000. The final objective of this study was the interpretation of the pinch as it relates to the Combined Water and Materials Pinch Analysis problem. A general definition of the pinch was proposed; according to this definition, the pinch corresponds to that constraint or set of constraints which limits the performance of the system, that is, prevents it from further improvement. For the Combined Water and Materials Pinch Analysis problem, this performance is measured in terms of the operating cost. This definition is thus a departure from its usual thermodynamic interpretation of the pinch; in addition, the pinch is defined in terms of a constraint or a set of constraints instead of a point. These constraints are identified by an analysis of the marginal values provided by the optimisation algorithm. Marginal values are also used as a means of identifying process interventions which may be effected such that the performance of the system may be improved further.
Mathematical modelling and experimental study of the kinetics of the acid sulphite pulping of eucalyptus wood.
(1992) Watson, Edward.; Wright, Dave.; Mulholland, Michael.
The chemistry of the batch cooking process at Sappi Saiccor, relating to both the pulp and liquor, was investigated with the aim of using kinetic expressions to develop an improved process control model. The mill produces dissolving pulps using the acid sulphite method. Three process reactions were identified as important: cellulose hydrolysis, delignification and hemicellulose dissolution. Of these, cellulose hydrolysis is the most important since the primary aim is to achieve a targeted cellulose degree of polymerisation (DP) or viscosity (DP is commonly expressed in terms of this measurement). This is directly determined by the rate of this reaction during the cook, and the acidity of the cooking liquor was found to be the key factor. As existing equipment was not suitable for obtaining the data required to perform a kinetic analysis, a pilot plant was constructed. A commercially available probe was used for the first time to measure pH directly. The measured acidity is not directly equivalent to hydrogen ion activity at these temperatures and pressures; however, since the conditions of each cook are similar the errors incurred were found to be constant from cook to cook. The probe was found to be prone to drift due to ageing and this was accounted for by using an 'on line' calibration based on a liquor analysis. The kinetics of the cellulose hydrolysis reaction were determined using the on-line measurement of acidity and the concept of degradation increase (DI) which relates the reduction in DP value to the rate at which the polymeric chains are split. Delignification and hemicellulose dissolution were examined, since it is beneficial to maximise these reactions to reduce the quantities of chemicals consumed during the bleaching process. A model for controlling cooks to a set target cellulose DP value within a set time was developed based on the reaction kinetics. This was capable of predicting cooking conditions required with sufficient accuracy to control the cellulose DP value to within ±6 cp SNIA on the viscosity scale.
Residence time methods for modelling and assessing the performance of water treatment processes.
(1995) Barnett, Jacqueline Lisa.; Buckley, Christopher Andrew.
The objective of this study was to provide a technique, based on the residence time distribution of a process, for modelling, assessing and improving flow in the processes of water and waste water treatment works. The technique should be accessible to the staff managing and operating the works. From a review of the literature, a preference was given for the experimental method used for determination of the tracer response, including choice of tracer and tracer addition and monitoring. Data analysis techniques were reviewed, and the method of time domain fitting was developed into a computer program, IMPULSE. IMPULSE provided a tool for analysis of residence time data, and removed the constraint of numerical complexity. Using the building blocks of IMPULSE, a realistic flow model can be constructed from tracer data and evaluated. IMPULSE allows a quantitative comparison of models proposed for a process, and provides the parameters of the models. These parameters quantify the non-idealities in a process. A knowledge of the non-idealities provides a basis for decision-making when modifying a process. The results of tracer experiments performed on some water and waste water treatment processes were analysed using IMPULSE. The results showed that collection of good experimental data was critical to the success of the analysis. It is proposed that a guide be produced which draws out the main points raised in the study, including collection of tracer data and use of IMPULSE. The guide should be accessible and easily understandable to the staff managing and operating water and waste water treatment works.
Modelling of the tubular filter press process.
(2000) Mullan, David James.; Buckley, Christopher Andrew.
The objective of this project was to develop a suitable procedure for the design, control and optimisation of the Tubular Filter Press. To this end, the following objectives were defined for this study: • To extend or improve upon the constant pressure compressible cake filtration model, predictive solution procedure, and standard laboratory characterisation techniques requlred to obtain the empirical model parameters, presented in Rencken (1992). A new generalised area contact constant pressure compressible cake filtration model was developed for both the internal cylindrical and planar filtration geometries. The model utilises a heuristically developed area contact function which relates the interparticle contact area to the solids compressive pressure within the cake. If the area contact is zero, the model reduces to the conventional point contact model as presented in Rencken ( 1992). The sludge used in this investigation was found to exhibit a negligible degree of area contact. A new pseudo variable pressure solution procedure was developed, that is an extension of the constant pressure solution procedure, to account for the initial variable pressure stage of the Tubular Filter Press operation. The pseudo variable pressure solution procedure was found to account accurately for the initial filtration behaviour observed during the pressurisation period of the Tubular Filter Press. However for the normal operation of the Tubular Filter Press. the difference between the output of the pseudo variable pressure and constant pressure solution procedures, was found to be insignificant. Wall friction in compression-permeability (C-P) cell tests was identified as a main source of error. The significance of wall friction was investigated using a specially constructed C-P cell. that enabled the transmitted pressure through the cake sample to be measured. The accuracy of the characterisation which had been corrected for the effects of wall friction, was found to improve the prediction of the filtration behaviour of the sludge significantly. The direct shear test was identified and documented as a feasible experimental procedure to determine the coefficient of earth pressure at rest. The coefficient of earth pressure is unique to the non-planar filtration geometries. The coefficient of earth pressure at rest was determined for the sludge used in this investigation. • To incorporate the constant pressure compressible cake filtration model and the associated predictive solution procedures into a user-friendly computer programme that will facilitate the design and optimisation of full-scale plants. The predictive solution procedures were incorporated into the Windows 95 computer programme, COMPRESS, that can be used for any constant pressure compressible cake dead-end filtration application where the filtration geometry is planar or internal cylindrical. A control and optimisation strategy for the continuous operation of the Tubular Filter Press has been proposed. To develop a regressive solution procedure, and incorporate this procedure into a user-friendly computer progranune, that will enable the empirical model parameters. normally obtained from standard laboratory-scale tests, to be obtained from actual filtration data. A regressive solution procedure was developed that utilises a direct search optimisation technique that is an extension of the COMPLEX method. The regressive solution procedure was incorporated into the Windows 95 program, REGRESS. The program utilises filtration data from any dead-end constant pressure filtration application of either planar or internal cylindrical geometry. REGRESS provides an effective means for determining the true physical or plant specific filtration characteristics of the sludge. The regressive solution procedure also enables the parameters specific to the new area contact model to be determined. The sludge characterisation obtained from regressing on filtration data was found to be a significant improvement in predicting the filtration behaviour, than the characterisation obtained from the standard non-filtration laboratory-scale methods, even after the C-P cell data had been corrected for the effects of wall friction. The programs COMPRESS and REGRESS should greatly assist in the design. control and optimisation of the Tubular Filter Press process.
Monoethanolamine : suitability as an extractive solvent.
(2000) Harris, Roger Allen.; Letcher, Trevor M.; Ramjugernath, Deresh.; Raal, Johan David.
Separation processes are fundamental to all chemical engineering industries. Solvent separation, either liquid-liquid extraction or extractive distillation, is a specialised segment of separation processes. Solvents can be used either to optimise conventional distillation processes or for azeotropic systems, which can not be separated by conventional means. This work focuses on the performance of monoethanolamine (MEA) as a solvent in extractive distillation. Furthermore, the methodology of solvent evaluation is also studied. The preliminary assessment of solvent selection requires the determination of selectivity factors. The selectivity factor is defined as follows: P• = y,." . y, where y" is the activity coefficient at infinite dilution of the solute in the solvent. Subscript 1 and 2 refer to solute 1 and 2. A large selectivity factor implies enhanced separation of component 1 from 2 due to the solvent. Activity coefficients at infinite dilution were determined experimentally (gas-liquid chromatography) and predicted theoretically (UNIFAC group contribution method) for twenty-four solutes at three temperatures. Solutes used were alkanes, alkenes, alkynes, cyclo-alkanes, aromatics, ketones and alcohols. Most of this experimental work comprises data for systems which have not been measured before. Predicted and experimental values for y' were compared. For systems such as these (with polar solvents and non-polar solutes), UNIFAC results are not accurate and experimentation is vital. The experimental selectivity factors indicated tihat MEA could be an excellent solvent for hydrocarbon separation. Three binary azeotropic systems were chosen for further experimentation with MEA n-hexane (1) - benzene (2): fJ,~ = 31. Compared to other industrial solvents this is one of the largest values and MEA could serve as an excellent solvent. cyclohexane (1) - ethanol (2): fJ,~ = 148. This high value indicates an excellent solvent for this system. Acetone (1) - methanol (2): fJ,~ = 7.7. Further work involved vapour-liquid equilibrium experimentation at sub-atmospheric pressures in a dynamic recirculating stil l. The binary components with a certain amount of MEA were added to the still. The vapour and liquid mole fractions for the binary azeotropic components were measured and plotted on a solvent-free basis. The results are summarised below: n-hexane - benzene: Amount MEA added to still feed: 2%. MEA improved separability slightly. Further addition of MEA resulted in two liquid phases forming. cyclohexane - ethanol: Amount MEA added to still feed: 5% and 10%. Two liquid phases were formed for cyclohexane rich mixtures. Addition of MEA improved separability but did not remove the azeotrope. acetone - methanol: Amount MEA added to still feed : 5%, 10% and 20%. The ternary mixture remained homogenous and separability improved with addition of MEA. The binary azeotrope was eliminated. Due to the hetrogenous nature of the cyclohexane - ethanol system liquid-liquid equilibrium experimentation was performed to complete the analysis. Viable separation processes are possible for (a) cyclohexane - ethanol mixtures and for (b) acetone - methanol mixtures using MEA as the solvent. Comparison of various solvents used for the separation of acetone from methanol was possible by constructing equivolatility curves for the ternary systems. Results showed that MEA may possibly be the best solvent for this extractive distillation process. This study provides the following results and conclusions: • New thermodynamic data, important for the understanding of MEA in the field of solvent separations, was obtained. • Results show that the UNIFAC contribution method cannot be used to accurately predict polar solvent - non-polar solute y«> values. Experimentation is essential. • Selectivity factors indicate that MEA could be an excellent solvent for hydrocarbon separation. • The separation of the azeotropic cyclohexane - ethanol mixture is possible with a combination of extractive distillation and liquid-liquid extraction or simply liquid-liquid extraction using MEA as the solvent. • The separation of the azeotropic acetone methanol mixture is possible with extractive distillation using MEA as the solvent. The solvent MEA is possibly the best solvent for this separation.
Microbial oxidation of arsenite in gold mine effluent.
(2000) Ramdial, Anusha.; Kuwornoo, D. K.
No abstract available.
Comparison of an anaerobic baffled reactor and a completely mixed reactor : start-up and organic loading tests.
(2000) Mudunge, Reginald.; Buckley, Christopher Andrew.
The aim of the investigation was to compare the performance of an anaerobic baffled reactor (ABR) with a completely mixed anaerobic reactor (CMAR). The ABR was operated with a hydraulic retention time (HRT) of 20 h while the CMAR was operated at 20 d. A control experiment was conducted with a CMAR operated at a constant hydraulic retention time and substrate feed concentration. During the first phase, the start-up performance of the ABR and CMAR were compared. In the second phase of the study the steady state COD removals were compared. The laboratory completely mixed anaerobic reactor was a 20L glass vessel with a stirrer coming in through the neck. A second type of reactor, anaerobic baffled reactor (ABR) was also operated. The ABR was a rectangular perspex box with internal vertical baffles alternately hanging and standing. The baffles divide the reactor into eight compartments with a total working volume of 7.5 L. Each baffle is angled at about 45Q to distribute the flow towards the centre of the upcomer. The reactors were seeded with raw sewage and allowed to stand for 3 days after which a continous feed of sucrose and basal salts was commenced. The initial HRT for the ABR and the CMAR were 60 h and 30 days respectively. When the reactors reached steady state (pH, gas production, gas composition and alkalinity), the HRT was reduced in a stepwise fashion (ABR 60 h to 35 h to 20 h and CMAR 35 d to 30 d to 20 d). At the final HRT the COD removals were similar (67 %). The ABR took 120 d to attain final steady state while the CMAR took 200 d. The organic loading tests were undertaken with a stepwise increase (doubling) in the influent substrate concentration. The feeding commenced at an organic loading rate (OLR) of 4.8 kg/m(3).d for the ABR. The flow rate (HRT) into both reactors and other parameters were kept constant (HRT of 20 h and 20 d for ABR and CMAR respectively). The substrate concentration was increased from 4 gCOD/L (4.8 kg/m(3).d) to 64 gCOD/L (76.8 kg/m(3).d) for the ABR. For the CMAR it was increased from 4 gCOD/L (0.25 kg/m(3).d) to 32 gCOD/L (2 kg/m(3).d). The method used was to increase the organic loading rate until the reactors failed. Since the two reactors had different operating HRTs, the tests began when both had the same COD removal rate of about 60 % COD reduction. The same parameters as in the start-up period were monitored for both reactors. The CMAR had a COD removal efficiency ca. 70 %, which did not fluctuate when OLR was increased. The ABR reached a maximum COD removal of 80 %. An increase in the OLR led to an initial decrease in the COD removal until the biomass recovered and the high COD (80 %) removal rates resumed. The ABR reached a maximum OLR of 76.8 kg/m(3).d whilst the CMAR reached a maximum OLR of 2.0 kg/m(3).d. The investigations showed that the ABR could be operated at higher organic loads than the CMAR and give the same organic removal rate. This verified the importance of increasing the SRT/HRT ratio in anaerobic reactors. The CMAR, however, proved to be stable to changes in the influent feed strength, as there was no immediate noticeable changes in the gas production.
Mathematical modelling of the dynamical interactions between killer and sensitive wine yeast subjected to nutritional stress.
(2000) Vadasz, Alisa S.; Abashar, Mohamed Elbashir Elamin.; Gupthar, Abindra Supersad.
No abstract available.
Simultaneous neutral sulphite semichemical pulping of hardwood and softwood.
(2000) Antonides, Floris.; Hunt, John.
The work described in this thesis was aimed at obtaining a better understanding of the neutral sulphite semichemical pulping process employed by Mondi Kraft's Piet Retief mill, and to investigate ways to improve this process. The unique feature of the process in this mill is that hardwood and softwood species are pulped simultaneously in a continuous digester. The pulping trials described were carried out in a laboratory batch digester which was build as a part of this project. Pulps were evaluated for yield, Hypo number as an indication of the residual lignin content and strength properties. The first part of the experimental work focused on the effect that different pulping variables have on the process and the resulting pulp. Variables investigated were the chemical charge, pulping temperature, chip composition and anthraquinone dosage. The second part of the work was to investigate ways in which the process can be improved. In particular it was investigated whether it would be advantageous to pulp hardwood and softwood separately and mix the two pulps together after pulping. The effect of changing to an alkaline sulphite process was also briefly investigated. It was found that the current process is optimized as far as the chemical charge and pulping temperature is concerned. Increasing the softwood percentage used to 50 % (from current value of 41 %) increases the tear strength, whilst decreasing it to 30 % increases the tensile strength of the resulting pulp. It was also determined that increasing the AQ dosage from 0.1 % to 0.5 % might bring savings in chemical costs. It is suggested that this is investigated in a mill trial. It was further found that pulping the two species separately improves the tear strength of the pulp by about 20 % compared with that which was pulped simultaneously. The results indicate that no benefits concerning the chemical costs, pulping temperature, pulp yield, burst strength or tensile strength are to be gained from separate pulping. Preliminary results indicated that significant strength increases and possible chemical cost savings are to be gained by changing from a neutral sulphite to an alkaline sulphite process. Further work to determine the reproducibility of these results, as well as the effect of different chemical charge and ratios is suggested.
Real-time observer modelling of a gas-phase ethylene polymerisation reactor.
(2000) Thomason, Richard.; Mulholland, Michael.
The desire for precise polymer property control, minimum wastage through grade transitions, and early instrument fault detection, has led to a significant effort in the modelling and control of ethylene polymerisation world-wide. Control is difficult due to complex inter-relationships between variables and long response times from gas to solid phase. The approach in this study involves modelling using the kinetic equations. This forms the basis of a scheme for real-time kinetic parameter identification and Kalman filtering of the reactor gas composition. The scheme was constructed off-line and tested on several industrial polymer grades using historical plant data. The scheme was also converted into a form for use on the linear low-density polyethylene plant, Poly 2, at POLlFIN Limited. There proved to be no difficulty in the identification step, but the Kalman filter requires more tuning for reliable fault detection. The software has been commissioned on-line and results from the POLlFIN plant match the off-line model exactly.
Inverse internal model control of an ethylene polymerisation reactor using artificial neural networks.
(2001) Dunwoodie, Ryan.; Mulholland, Michael.
An artificial neural network is a mathematical black-box modelling tool. This tool can be used to model complex non-linear multivariable processes. In attempting to create an inverse process model of an industrial linear low density polyethylene reactor, several interesting results were encountered. Both time-invariant algebraic and time-invariant dynamic models could adequately represent the process, provided an identified 50-minute time lag was taken into account. A novel variation of the traditional IMC controller was implemented which used two inverse neural network process models. This was named Inverse Internal Model Control (IIMC). This controller was initially tested on a real multivariable pump-tank system and showed promising results. The IIMC controller was adapted to an on-line version for the polymer plant control system. The controller was run in open loop mode to compare the predictions of the controller with the actual PID ratio controllers. It was hoped that by incorporating neural network models into the controller, they would take the non-linearity and coupling of the variables into account, which the present PID controllers are unable to do. The existing PID controllers operate on separate loops involving the two main feeds (co-monomer and hydrogen) to the reactor, which constitute aspects of the control system in which the scope for advanced control exists. Although the control loop was not closed, the groundwork has been laid to implement a novel controller that could the operation of the plant.
Environnmental life cycle assessment of potable water production.
(2001) Friedrich, Elena.; Buckley, Christopher Andrew.
This study investigates and compares the environmental burdens of two different methods for producing potable water by using the environmental life cycle assessment (LCA). The first method, for the production of potable water, is used by Umgeni Water at their Wiggins Waterworks and it involves conventional processes. The second method is based on a South African membrane technology and currently it is used in three pilot plants around the country. The life cycle concept gives the means understand the environmental impacts associated with a product. process or activity by considering all life-cycle stages, from cradle- to-grave. Formal methodologies for conducting such studies have been developed and in this project the methodological framework endorsed by the International Organisation for Standardisation (ISO) 14040 series of standards has been used. By using this methodology and by tracing all processes involved in the production of potable water, it was found that the main contribution towards the environmental burdens of potable water is due to electricity generation. This conclusion is valid for both methods in vestigated. and as a result the recommendations focus on increasing the energy efficiency of waterworks in order to increase their overall environmental performance.
Pyrolysis of chlorinated organic chemicals.
(2001) Pillay, Kleantha.; Arnold, David R.; Ramjugernath, Deresh.
At present, South Africa has inadequate technology to destroy its hazardous waste, with approximately 18000 litres of chlorinated hazardous waste stored in this country. Approximately 800 tons of banned or obsolete chemicals are to be sent to Pontypool. Wales, for incineration, at a considerable cost. Because of the toxic nature of chlorinated waste and their long-term effects on the environment , a sustainable method of dealing with this type of waste is essential. Gas phase destruction of methylene chloride, trichlorobenzene and lindane by pyrolysis (i.e. heating in the absence of oxygen) was attempted. Destruction was effected by high temperature thermal degradation of molecules into free radicals. These radicals then combine to form hydrogen chloride and carbon as major products. This method was chosen so as to eliminate the possible formation of highly toxic oxygenated derivatives such as polychlorinated dibenzofurans and dibenzodioxins that can be formed during incineration if strict control is not excercised. The reactor assembly was built in the Department of Chemical Engineering at the University of Natal. 11 incorporates aspects of many different previously designed reactors, as discussed in the text. Heat for the reactions was supplied by induction. A high frequency induction unit supplied current to a copper coil. The resulting magnetic field induced current to flow in a susceptor housed within the copper coil. The susceptor in this case was a graphite tube, which served as both the heating element and the thermal radiation source, in addition to forming the walls of the reaction zone. Up and down stream processes were designed and experiments were carried out in which reaction temperatures (348-1400°C) and residence times (1.3-5.6 seconds) were varied. Destruction efficiencies of 100% and 99.99% were obtained for methylene chloride and trichlorobenzene respectively, with inert argon used as the carrier gas. These destruction efficiencies comply with the 99.99% stipulated by the United States Conservation and Recovery Act. A cause for concern was the formation of chlorinated benzenes and naphthalenes. Destruction of lindane proved unsuccessful due to limitations in the vapourisation and feed system and will have to be investigated further. The method of induction heating was evaluated to be 98.9% thermally efficient. Raw material and utility consumption per ton of waste destroyed by the pyrolysis process was compared to values for incineration as well as the plasma arc and catalytic extraction processes. Consumption for pyrolysis compares favourably with all three processes and suggests that the process could be competitive. Claims to the success of the technology on a wide scale are limited by the small number of compounds that were successfully pyrolysed. Results do however indicate much promise for this technology to be used as a fi nal chlorinated waste destruction unit on an existing process. Modifications to the existing reactor to improve product recovery and analys is will allow for temperature and residence time optimisation for a variety of wastes. Additional in strumentation and process control will allow for kinetic studies to be undertaken in future. This project should be considered as the first step in an ongoing series of research and subsequent improvements to the technology presented here.
Mathematical modelling of climbing film evaporators.
(2001) Peacock, Stephen David.; Starzak, Maciej.
Climbing film evaporators are in widespread use in the South African sugar industry, with the vast majority of the local sugar mills currently utilising these evaporators as first effect vessels in multiple effect evaporator sets. However, it is generally considered that the performance of these evaporators has not been maximised, and that improvements could be achieved by proper optimisation of the operating parameters. Unfortunately, very little comprehensive design information has been published in the literature. owing to the complexity of the heat transfer and hydrodynamic interactions in the evaporator tube. Attempts at performance improvement have been hampered by the lack of any theory to explain fully the effects of the operating parameters and physical properties of the feed liquor on the performance of the evaporator. In this study. a mathematical model of the climbing film evaporator system was developed in order to assess the effects of changing operating conditions on evaporator performance, based on as solid a theoretical foundation as cunendy possible. The model was tested against experimental data from a pilot plant climbing film evaporator and this experimental data was used to enhance the accuracy of the model by means of process identification. Because of the complexity of the model and the extensive computational time required for its solution, a simplified evaporator model was also developed, based on Iinearisation of the system of ordinary differential equations describing the climbing film evaporator system. This simplified model was used to predict trends in evaporator behaviour under various operating conditions.
Multipurpose separation and purification facility.
(2001) Sewnarain, Reshan.; Ramjugernath, Deresh.
A waste acid stream is being produced by a local petrochemical company (SASOL) at a rate of 10 000 -12 000 tons per annum and contains approximately 44-mole % butyric acid, 20 % isobutyric acid and 10 % valeric acid. Whilst this stream is currently being incinerated, SASOL has requested an investigation into the possibility of separating and purifying butyric acid and isobutyric acid from this waste acid stream. The goal of this project was to determine a separation and purification route for butyric acid and isobutyric acid from SASOL'S waste acid stream. In order to achieve this, vacuum distillation and freeze crystallization were chosen for the recovery and purification of the acids respectively. Vapour-liquid equilibrium data for key component pairs present in the waste acid stream (propionic acid + butyric acid, isobutyric acid + butyric acid, butyric acid + isovaleric acid and butyric acid + hexanoic acid) were experimentally determined in a dynamic VLE still. The measured VLE data was successfully correlated us ing the gamma-phi approach. with the NRTL activity coefficient model representing the liquid phase and the virial equation of state describing the vapour phase. Using these equations. the VLE data obtained from the experimental work was then regressed to provide interaction coefficients for the NRTL model. which were then used in the Hysys process simulator to explore a range of design alternatives for distillation. Hysys simulations showed that greater than 80 % butyric acid and isobutyric acid can be recovered from the waste acid stream in a single distillation column containing 18 theoretical stages and an optimum reflux ratio of 3.8. The simulation was performed at a pressure of 58kPa and a maximum operating tempe rature of 150°C. Batch distillation experiments performed in a batch rectification column at 250kPa recovered more than 90% of both the butyric acid and isobutyric acid from a 450ml sample of the waste acid stream. A subsequent batch experiment concentrated the recovered acids into a distillate containing more than 95 % butyric acid and isobutyric acid combined. To investigate freeze crystallization as a suitable operation for purifying butyric acid and isobutyric acid a solid-liquid phase equilibrium curve for the system was generated us ing the Van Hoft equation. The generated curve showed that butyric acid and isobutyric acid could be theoretically purified (>98%) by operating two crystallizers at -20°C and -55°C respectively. A simple freeze crystallization experiment produced butyric acid with greater than 94% purity. An economic feasibility study conducted on the process showed that separation and purification of the acids by this process (distillation and crystallization) could create a business opportunity with revenue of approximately R47 million per annum. Preliminary estimates for capital investment amounted to approximately R5.4 million. for which the payback period was estimated at less than one year.
Adaptive dynamic matrix control for a multivariable training plant.
(2001) Guiamba, Isabel Remigio Ferrao.; Mulholland, Michael.
Dynamic Matrix Control (DMC) has proven to be a powerful tool for optimal regulation of chemical processes under constrained conditions. The internal model of this predictive controller is based on step response measurements at an average operating point. As the process moves away from this point, however, control becomes sub-optimal due to process non-linearity. If DMC is made adaptive, it can be expected to perform well even in the presence of uncertainties, non-linearities and time-vary ing process parameters. This project examines modelling and control issues for a complex multivariable industrial operator training plant, and develops and applies a method for adapting the controller on-line to account for non-linearity. A two-input/two-output sub-system of the Training Plant was considered. A special technique had to be developed to deal with the integrating nature of this system - that is, its production of ramp outputs for step inputs. The project included the commissioning of the process equipment and the addition of instrumentation and interfacing to a SCADA system which has been developed in the School of Chemical Engineering.
Optimization of a multi-level steam distribution system by mixed integer non-linear programming.
(2001) Saunion, Roland.; Mulholland, Michael.
The objective of this project is to optimize the SAPREF oil refinery steam distribution in which imbalances between the various levels presently require the venting of steam from the lowest level. The overall steam balance shows that the problem originates from an excess of high·pressure (HP) steam production for too few medium pressure steam users and turbines. We proposed to solve this problem by considering the replacement of selected steam turbines with electrical drives. Given a set of demands of electricity, mechanical power and steam at various pressure levels, the objective is to recommend configuration changes to minimize overall cost. This is not a trivial problem, as steam not passed down through turbines to lower levels can create a shortage there, so a combination of replacements is required. The variables of the problem are both decision variables on every steam turbine and continuous variables, such as flows and enthalpies. These decision variables are integer variables, 0 or 1 for every steam turbine. Depending on whether it is kept on steam use or replaced with an electrical drive, these variables are as follows: E = 0: keep the existing steam turbine E - 1: switch it to an electrical drive. A complete and realistic model of this utility section must be constructed in order to represent the actual distribution accurately. This model will include an objective function to minimize, some equality and inequality constraints, and some cost functions. If we want this model to be accurate, we shall have to deal with nonlinearities to avoid simplifications, and these non-linearities could lead to infeasabilities or sub-optimal solutions. So we are facing a typical MTNLP (Mixed Integer Non-Linear Programming) problem to find optimal configuration changes which will maximize the return on investment, meeting the electrical, mechanical and steam demands of the refinery. In order to solve this difficult optimization problem we shall use the user-friendly package GAMS (General Algebraic Modeling System).
Modelling and control of a co-current sugar dryer.
(2001) Lacave, Benoit.; Mulholland, Michael.
The drying of sugar is the last step in the recovery of solid sugar from sugar-cane. To ensure that the sugar can be transported and stored, the final moisture content leaving the sugar mill must be carefully controlled. Data spanning periods of normal plant operation were collected at the Tongaat-Hulett Ltd Darnall sugar mill. These measurements were reconciled to achieve instantaneous mass and energy balances across the sugar dryer. Using these measurements, a general model has been developed to simulate the sugar drying. It includes ten compartments through which the sugar and drying air flow, with a mass and energy balance in each compartment. It was assumed that a "film" around the sugar crystal is supersaturated, and that crystallisation is still occurring. A sorption isotherm determining the equilibrium moisture content of the sugar, at which point mass transfer ceases, was included. The model has been matched to process measurements by adjusting the heat and mass transfer coefficients. A Dynamic Matrix Controller was developed and tested off-line on the model, using the reconciled measurement sequences. The controller manipulated the inlet air temperature in order to control the exit sugar moisture content. The model predictive control format successfully dealt with the large process dead-time (5 minutes).

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