No description available.

Deposition of amorphous silica (SiO2) and calcium oxalate (CaOx) on the calandria tubes of juice evaporators cause serious processing problems in Australian cane sugar mills. The removal of these deposits by mechanical and chemical means is a timeconsuming and costly experience. The cost of downtime and chemical cleaning can be several million dollars per year for the Australian sugar industry. The interactions between CaOx and SiO2 have not been investigated previously because conventional studies only address fouling by individual components. The present work evaluates their interactions using two experimental approaches: batch tests for assessing kinetic and thermodynamic behaviour, and fouling-loop experiments for examining composite fouling behaviour under different operating conditions. The above two approaches were employed both in the absence and in the presence of sugar to elucidate the effect of sugar on composite fouling mechanisms and to determine the controlling species responsible for composite fouling. The composite fouling experiments were performed in a novel closed-loop circulation system simulating the effect of feed composition of successive stages of evaporation cycle in a single run. In addition, the fouling-loop system was operated in a constant composition mode to study the effects of thermal hydraulic conditions on composite fouling. The combined information obtained from both the batch and fouling-loop tests in this study offer a unique insight into the mechanisms of composite fouling of CaOx and SiO2. Some of the highlights of the obtained results are as follows: ??? Identification of a complex interactive process in calcium oxalate monohydrate ??? silica (COM-SiO2) systems by investigation of the kinetics and thermodynamics of COM-SiO2 coprecipitation in water and sugar solutions, and an understanding of the mechanisms of these interactions; ??? Development of a novel fouling-loop system, which is simple, efficient and cost effective for the study of the effect of juice composition on scale formation in various stages of juice evaporation; ??? Elucidation of composite fouling mechanisms, e.g., a feed composition dependent fouling mechanism is proposed; ??? Isolation and verification of the existence of certain species in composite deposits, which is known to be thermodynamically unstable. In other words, it is established that calcium oxalate trihydrate is stable under certain conditions; ??? Evaluation of the role of thermal hydraulic operating parameters in determining the characteristics of subcooled flow boiling heat transfer and in determining the strength of the composite deposit; ??? Development and validation of an empirical model to predict the subcooled flow boiling heat transfer coefficients in water and sugar solutions; ??? Development of an analytical model incorporating the effects of operating parameters for COM and SiO2 composite fouling in sugar solutions. This model predicted the experimental data better than available models. Results of this work are significant, not only because they have made a valuable contribution to advance the fundamental understanding of heat exchanger fouling, but also because they may play a key role in the development of scale control and removal strategies to minimize the composite fouling in Australian sugar mill evaporators. For example it was found that, in order to effectively minimize the rate of composite fouling and reduce the scale tenacity, it would be necessary to control thermal hydraulic operating conditions, especially the fluid velocity, and to adjust the initial CaOx/SiO2 supersaturation ratio to the optimum value. To achieve the optimal CaOx/SiO2 ratio, certain device can be developed to sequentially measure oxalic acid and SiO2 concentrations in juice so that the correct proportions of chemicals can be added. Model simulations of the composite fouling rate may also effectively and economically provide comparative and relevant information essential for process optimisation and evaporator design

Neste trabalho foram estudadas 10 corridas em duas usinas siderúrgicas semi-integradas, de aço baixa liga para uso em construção mecânica. Em cada uma das corridas foram retiradas 9 amostras, cada uma ao final de uma etapa do processo de produção: após a remoção de escória na panela, antes da desgaseificação a vácuo, após a desgaseificação a vácuo, após a adição de arame de Al, após a adição de arame de CaSi, após a adição de arame de S, após o fim da turbulência no distribuidor no lingotamento contínuo, 30 minutos após o fim da turbulência no distribuidor no lingotamento contínuo, e 60 minutos após o fim da turbulência no distribuidor no lingotamento contínuo. As amostras foram preparadas metalograficamente e analisadas ao microscópio eletrônico de varredura (MEV) com espectrometria de dispersão de energia (EDS), a fim de se identificar as inclusões presentes no aço em cada etapa do processo. Com isto pode-se fazer a caracterização das inclusões encontradas em cada etapa do processo e a sua classificação segundo a composição química e morfologia. Através da comparação da composição química das inclusões encontradas ao final do refino e no lingotamento contínuo foi possível verificar uma tendência de formação de inclusões de espinélio, e através da composição química das inclusões encontradas no lingotamento contínuo foi possível identificar em quais das corridas estudadas houve a presença de inclusões de aluminatos de cálcio formados no estado líquido. / In the present work it was studied 10 heats in two steelworks, of low alloyed steel for use in mechanical construction. From each heat were taken 9 samples, each one of them at end of one production stage: after deslagging in the ladle; before vacuum degassing; after vacuum degassing; after Al wire addiction; after CaSi wire addiction; after S wire addiction; after the end of tundish turbulence at continuously casting; 30 minutes after the end of tundish turbulence at continuously casting; and 60 minutes after the end of tundish turbulence at continuously casting. Samples were metallographic prepared and analyzed by scanning electronic microscopic (SEM) with energy dispersive X-ray spectroscopy (EDX), in order to identify the inclusions present in steel in each process stage. This allowed the founded inclusions in each process stage to be characterized and classified according to chemical composition and morphology. By comparing founded inclusions chemical composition at end of refining and continuous casting was possible to observe a tendency of formation of spinel inclusions, and by founded inclusions chemical composition in continuous casting was possible to identify in which studied heats there were presence calcium aluminates inclusions formed in the liquid state.

The advancement in computational thermodynamics can help researchers to test their hypotheses regarding complex steelmaking operations in a more quantified manner. The main aim of the current work was to use develop a kinetic model that can predict changes in steel, slag and inclusions during ladle refining and use this model as a tool to develop better understanding of the steelmaking process itself. The important reactions during ladle refining are: steel-refractory reaction, slag-refractory reaction, flotation of inclusions to slag, steel-inclusion reaction, steel-slag reaction and inclusions originating from slag. The chemical reactions between two phases were considered to be mass transfer controlled. The macro-processing feature in FactSage was used to do multiple equilibrium calculations and calculate the change in steel, slag and inclusion composition. Targeted experiments and industrial trials were conducted to find model parameters. For laboratory experiments, the rate of magnesium-transfer to oxide inclusions in steel due to steel-crucible and steel-slag reaction was studied. It was concluded that the presence of spinel layer on MgO crucible at the steel-crucible reaction can help in significantly reduce the rate of Mg pick-up due to steel-crucible reaction. For industrial trials, a comparison between the rate of steel-slag reaction and inclusion flotation rate showed that the steel-slag reaction could be significantly slowed due to slag inhomogeneity. The kinetic model was also used to identify artifacts in steel and slag sampling during ladle refining. One of the main limitations of the kinetic model was the over-prediction of calcium pick-up in steel due to steel-slag reaction. Induction furnace experiments were conducted using MgO, ZrO2 and CaO crucible with different slag composition and silicon concentration to study the extent of calcium pick-up due to steel-slag and steel-crucible reactions. The steel-CaO crucible equilibrium experiment was used to estimate Ca-O interaction parameter. The equilibrated steel was reoxidized with known amount of oxygen to allow all the dissolved calcium to precipitate as oxide inclusions. Inclusion analysis of sample taken after reoxidation was used to estimate dissolved calcium in steel. The measured dissolved calcium was used to estimate Ca-O interaction parameter. A private database for liquid steel was created in FactSage and used for kinetic modeling of laboratory scale steel-slag-crucible experiments. The use of private database for kinetic model helped in avoiding excess calcium pick-up in steel due to steel-slag reaction. However, the model and database should be tested for conditions where significant calcium pick-up is experimentally observed. In the present work, the inclusion removal was assumed to be first order reaction with fixed rate constant. In practice, the inclusion removal is expected to be a more complicated process of agglomeration and flotation. Similarly, the steel-inclusion reactions were considered in equilibrium for each time step of calculation. Sometimes, the composition difference inside single inclusions was found. Some characterization tools were used that could be useful in future to study the agglomeration of inclusions and composition differences inside single inclusion. The agglomeration behavior of inclusions at the steel-argon interface inside confocal laser scanning microscope was compared to the agglomeration in bulk samples from laboratory and industrial steel samples. The size and morphology of inclusion clusters were studied using X-ray micro CT. The composition and morphology of single inclusion was studied using focused ion beam methods: Ga-FIB instrument and plasma-FIB instrument.

An Othmer recirculating still and a standard packed column fractionating unit were used in determining the equilibrium flash vaporization and true boiling point curves respectively, for eight samples of crude oils from producing oil-fields in Alberta. The EFV curves, at 10 mm. absolute pressure were also obtained for these samples. Determinations at atmospheric pressure were used for modifying Okamoto and Van Winkle's correlation, which allows the prediction of the EFV curves from the TBP curves, for application to crude oils. The correlation relates the 50 percent points and the slopes of the two curves. In spite of its simplicity, it results in predicted curves of good accuracy. It is believed that this method is suitable for a large number of crudes and can give dependable phase equilibrium data. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

In the Betts process for lead electrorefining the noble impurities originally present in the bullion form a strong and adherent layer of slimes. Within this layer the established ionic concentration gradients can lead to secondary reactions. The following processes were analyzed from a thermodynamic perspective: (A) hydrolysis of the acid (B) precipitation of secondary products (C) reaction of noble compounds. The nature of the concentration gradients within the slimes layer and related secondary processes was studied by using transient electrochemical techniques which include: (A) current interruption, (B) AC impedance, and (C) a variation of SACV (Small Amplitude Cyclic Voltammetry). These studies were complemented by: (A) physico-chemical data on electrolyte properties, (B) "insitu" and "industrially recovered" slimes electrolyte compositions, (C) SEM and X-ray diffraction analysis of the slimes layer. For comparison purposes the electrochemical behaviour of "pure" Pb electrodes was also studied. Upon current interruption the anodic overpotential decays, first abruptly, (as the uncompensated ohmic drop disappears) and then slowly (due to the presence of a back E.M.F. created by ionic concentration gradients that decay slowly). Current interruption measurements showed that: (A) concentration gradients exist across the slimes layer, (B) inner solution potentials within the slimes layer can be larger than those measured from reference electrodes located in the bulk electrolyte, (C) secondary products can shift the inner solution potential to negative values which reverse upon re-dissolution and (D) ionic diffusion is seen upon current interruption but it is complex and difficult to model due to the presence of processes that can support the passage of internal currents. The anodic polarization components were obtained by analyzing the potential and current dependance upon application of a small amplitude sinusoidal waveform. This dependance was found to be linear in the low overpotential region (< 250mV). Thus, upon subtraction of the uncompensated ohmic drop, the remaining polarization is due to the "apparent" ohmic drop of the slimes electrolyte and to liquid junction and concentration overpotentials. These components are directly linked to the electrolysis conditions and to the slimes layer structure. Furthermore, the ratio of these components can be used to obtain the point at which the precipitation of secondary products starts. Changes in this ratio can also be related to the anodic effects caused by the presence of addition agents. AC impedance measurements performed in the presence of a net Faradaic current showed that the impedance increases uniformly as the slimes layer thickens up to the point at which noble impurities start to react. Three electrical analogue models were used to describe the impedance spectra. A steady-state mathematical model that predicts concentration and potential gradients across the slimes layer was developed. Only when a position dependent eddy diffusion term was incorporated in the numerical solution, were reasonable local ionic concentrations and overpotentials obtained. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

No description available.

This dissertation includes two accounts of rigorous modeling of petroleum refinery modeling using rigorous reaction and fractionation units. The models consider various process phenomena and have been extensively used during a course of a six-month study to understand and predict behavior. This work also includes extensive guides to allow users to develop similar models using commercial software tools. (1) Predictive Modeling of Large-Scale Integrated Refinery Reaction and Fractionation Systems from Plant Data: Fluid Catalytic Cracking (FCC) Process with Planning Applications: This work presents the methodology to develop, validate and apply a predictive model for an integrated fluid catalytic cracking (FCC) process. We demonstrate the methodology by using data from a commercial FCC plant in the Asia Pacific with a feed capacity of 800,000 tons per year. Our model accounts for the complex cracking kinetics in the riser-regenerator and associated gas plant phenomena. We implement the methodology with Microsoft Excel spreadsheets and a commercial software tool, Aspen HYSYS/Petroleum Refining from Aspen Technology, Inc. The methodology is equally applicable to other commercial software tools. This model gives accurate predictions of key product yields and properties given feed qualities and operating conditions. This work differentiates itself from previous work in this area through the following contributions: (1) detailed models of the entire FCC plant, including the overhead gas compressor, main fractionator, primary and sponge oil absorber, primary stripper and debutanizer columns; (2) process to infer molecular composition required for the kinetic model using routinely collected bulk properties of feedstock; (3) predictions of key liquid product properties not published alongside previous related work (density, D-86 distillation curve and flash point); (4) case studies showing industrially useful applications of the model; and (5) application of the model with an existing LP-based planning tool. (2) Predictive Modeling of Large-Scale Integrated Refinery Reaction and Fractionation Systems from Plant Data: Continuous Catalyst Regeneration (CCR) Reforming Process: This work presents a model for the rating and optimization of an integrated catalytic reforming process with UOP-style continuous catalyst regeneration (CCR). We validate this model using plant data from a commercial CCR reforming process handling a feed capacity of 1.4 million tons per year in the Asia Pacific. The model relies on routinely monitored data such ASTM distillation curves, paraffin-napthene- aromatic (PNA) analysis and operating conditions. We account for dehydrogenation, dehydrocyclization, isomerization and hydrocracking reactions that typically occur with petroleum feedstock. In addition, this work accounts for the coke deposited on the catalyst and product recontacting sections. This work differentiates itself from the reported studies in the literature through the following contributions: (1) detailed kinetic model that accounts for coke generation and catalyst deactivation; (2) complete implementation of a recontactor and primary product fractionation; (3) feed lumping from limited feed information; (4) detailed procedure for kinetic model calibration; (5) industrially relevant case studies that highlight the effects of changes in key process variables; and (6) application of the model to refinery-wide production planning. / Ph. D.

Submitted in fulfilment of the academic requirements for M-Tech in Chemical Engineering, ML Sultan Technikon, 2001. / The purpose of this investigation was to identify and then optimise the factors that affect the decolourisation during the carbonatation process. Batch carbonatation tests, carried out in a pilot plant in the laboratory, were performed to establish the effect of selected factors on the decolourisation of melt. / M

Energy carriers, such as coal, natural gas and biomass, can be converted by Fischer-Tropsch technology into synthetic crude (syncrude). Fischer-Tropsch derived syncrude can then be refined to transportation fuels, such as motor-gasoline, jet fuel and diesel fuel. These fuels meet the same specifications as crude oil derived transportation fuels. Conventional refining technologies have to be adapted to deal with Fischer-Tropsch syncrudes, because they differ significantly from crude oil with respect to composition. Some of the key differences are the high concentration of oxygenates and olefins and absence of sulphur in Fischer-Tropsch syncrude. Imposing a crude oil refining methodology on syncrude can lead to unwieldy and expensive refineries. Yet, despite an abundance of literature of Fischer-Tropsch synthesis, there is little literature that deals with the refining of Fischer-Tropsch syncrude. The present study investigated current refining practice for both crude oil and Fischer-Tropsch syncrude in order to identify fundamental differences in their refining focus and conversion behaviour. This was followed by a critical evaluation of the compatibility of syncrudes from high temperature Fischer-Tropsch (HTFT) and low temperature Fischer-Tropsch (LTFT) synthesis with the chemistry and catalysis of various conversion processes. The conversion processes that were evaluated include isomerisation, oligomerisation, etherification, alkylation, metathesis, hydrogenation, hydroisomerisation, hydrocracking, catalytic cracking, coking, thermal cracking, catalytic reforming and dehydration. The recommendations from the technology evaluation provided the foundation for the development of Fischer-Tropsch syncrude based refinery designs. Refinery designs were developed to determine configurations that would maximise the production of on-specification motor-gasoline, jet fuel and diesel fuel respectively. It could be shown that less complex refinery designs were required to refine Fischer-Tropsch syncrude to motor-gasoline and jet fuel, than were required for crude oil refining. It was also shown that on a molecular level Fischer-Tropsch syncrude is unsuited for maximising the production of Euro-4 type diesel fuel. The present study illustrates the advantage of considering fundamentals in developing refineries specifically for Fischer-Tropsch syncrude, rather than imposing crude oil design practises on Fischer-Tropsch syncrude refinery designs. / Thesis (PhD (Chemical Engineering))--University of Pretoria, 2008. / Chemical Engineering / unrestricted