M.Sc. (50/50) Research project submitted to School of Chemical and Metallurgical Engineering, Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, South Africa July 2017 / This study and the work done involves investigating the effects of different parameters on the decarburization process of high carbon ferrochromium melts to produce medium carbon ferrochrome, and takes into account the manipulation of the different parameters and thermodynamic models based on actual plant data. Process plant data was collected from a typical plant producing medium carbon ferrochrome alloys using AODs. The molten alloy was tapped from the EAF and charged into the AOD for decarburization using oxygen and nitrogen gas mixtures. The gases were blown into the converter through the bottom tuyeres. Metal and slag samples and temperature measurements were taken throughout the duration of each heat. The decarburization process was split into two main intervals namely first stage blow (where carbon content in the metal bath is between 2-8 wt. % C) and second stage blow (carbon mass% below 2 wt. %). The first and second blow stages were differentiated by the gas flow rates whereby the first stage was signified by gas flow ratio of 2:1 (O2:N2), whilst the stage blow had 1:1 ratio of oxygen and nitrogen respectively. The effect of Cr mass% on carbon activity and how it relates to rate of decarburization was investigated, and the results indicated that an increase in Cr 66.54 – 70.5 wt. % reduced carbon activity in the metal bath from 0.336 – 0.511 for the first blowing stage. For the second blowing stage, the increase in Cr mass % of 67.22 – 71.65 wt. % resulted in an increase in C activity from 0.336 – 0.57. The trend showed that an increase in chromium composition resulted in a decrease in carbon activity and the same increase in Cr mass% resulted in reduced carbon solubility. Based on the plant data, it was observed that the rate of decarburization was time dependent, that is, the longer the decarburization time interval, the better the carbon removal from the metal bath. An interesting observation was that the change in carbon mass percent from the initial composition to the final (Δ%C) decreased from 10.18 – 8.37 wt. % with the increase in Cr/C ratio from 8.37 – 10.18. This effect was attributed to the chromium affinity for carbon and the fact that an increase in chromium content in the bath was seen to reduce activity of carbon. It was also observed that the effect of the Cr/C ratio was more significant in the first stage of the blowing process compared to the second blowing stage. A mass and energy balance model was constructed for the process under study to predict composition of the metal bath at any time interval under specified plant conditions and parameters. The model was used to predict the outcome of the process by manipulating certain parameters to achieve a set target. By keeping the gas flow rates, blowing times, gas ratios and initial metal bath temperature unchanged, the effect of initial temperature on decarburization in the converter was investigated. The results showed that the carbon end point with these parameters fixed decreased with increasing initial temperature, and this was supported by literature. The partial pressure of oxygen was observed to increase with decrease in C mass % between the first and second blow stages. For the second stage blow the partial pressure changed from 5.52*10-12 – 2.1*10-10 and carbon mass % increased from 0.754 – 2.99 wt. %. A carbon mass % of 7.87 had an oxygen partial pressure of 4.51*10-13 whilst a lower carbon content of 1.53 wt. % had an oxygen partial pressure of 8.06*10-11. The CO partial pressure however increased with increase in carbon composition in the metal bath. When the oxygen flow rate increased, a corresponding increase in the carbon removed (Δ%C) was observed. For the first stage of the blowing process, an increase in oxygen flow rate from 388.67 – 666.5Nm3 resulted in an increase in carbon removed from 5.06 – 7.28 wt. %. The second blowing stage had lower oxygen flow rates because of the carbon levels remaining in the metal bath were around +/- 2 wt. %. In this stage oxygen flow rates increased from 125 – 286.67 Nm3 and carbon removed (Δ%C) from 0.16 – 2.093 wt. %. The slag showed that an increase in basicity resulted in an increase in Cr2O3 in the slag. As the basicity increased from 0.478 – 1.281, this resulted in an increase in Cr2O3 increase from 0.26 – 0.68. Nitrogen solubility in the metal bath was investigated and it was observed that it increased with increasing Cr mass %. The increase in nitrogen solubility with increasing Cr mass % was independent of the nitrogen partial pressures. / MT2018

O presente trabalho teve como objetivo avaliar os efeitos de celulases comerciais na superfície de fibras celulósicas branqueadas de eucalipto, bem como o impacto destes efeitos durante o refino. Após tratamento enzimático realizado em laboratório sob condições controladas, as polpas de fibra seca foram submetidas a refino em moinho PFI em vários níveis de energia e os resultados foram avaliados por meio dos ensaios físico-mecânicos realizados em folhas manuais formadas em laboratório e ensaios de morfologia de fibra, que tinham como objetivo evidenciar alguns efeitos provocados pelas enzimas utilizadas como auxiliar de refino. As análises dos dados obtidos mostraram que para a fibra seca tratada com produto denominado Maxymize 2530, foi possível obter valores 31% maiores de resistência à tração e 21% maiores de resistência interna (Scott Bond), valores estes sem refino mecânico. Estes valores foram similares aos encontrados para a fibra nunca seca, o que mostrou que a enzima pôde compensar o efeito \"Hornification\" causado pelo processo de secagem da fibra. Para a fibra seca pré-tratada com celulase e refinada em moinho PFI, foi possível obter um valor de Scott Bond de 100 lb.ft/in² com um consumo específico de energia 36% menor quando comparado com a fibra seca refinada em moinho PFI sem pré-tratamento enzimático. O mesmo aconteceu para os valores de resistência a tração, onde se obteve um valor de 70 kNm/kg com consumo específico de energia 57% menor. O planejamento experimental realizado contribuiu para a otimização de dosagem e variáveis do pré-tratamento enzimático, e com isso tornou possível a viabilização financeira da aplicação de celulases como auxiliar de refino. Todos os resultados foram ilustrados por imagens de Microscopia Eletrônica de Varredura (MEV), que evidenciou o aumento do nível de fibrilação externa das fibras tratadas com enzimas. / This study aimed to evaluate the effects of commercial cellulases on the surface of bleached eucalyptus fibers, as well as the impact of these effects during refining. After enzymatic treatment performed in the laboratory under controlled conditions, the dried pulps were subjected to refining in PFI mill at various energy levels and the results were evaluated by physical-mechanical tests performed on handsheets formed in laboratory and morphology fiber tests, which were intended to show some effects caused by the enzymes used as an aid to refining. The data analysis showed that for the dry fiber treated with compound Maxymize 2530, it was possible to obtain values 31% higher tensile strength and 21% higher Scott Bond test, these values without mechanical refining. These values are similar to those found for the never dried fiber, which showed that the enzyme could compensate the \"Hornification\" caused by the drying process of the fiber. To the dried fiber pre-treated with cellulase and refined in a PFI mill, it was possible to obtain a value of Scott Bond 100 lb.ft /in² with a specific energy consumption was 36% lower when compared to the dried fiber refined in a PFI mill without pre -enzymatic treatment. The same happened to the values of tensile strength, which presented a value of 70 kNm/kg with a specific energy consumption with 57% lower. The experimental design carried out contributed to the optimization of dosage and variables of enzymatic pre-treatment, and thereby made possible the financial feasibility of the application of cellulases as an aid to refining. All results are illustrated by images of scanning electron microscopy (SEM), which showed an increase in the level of external fibrillation of fibers treated with enzymes.

A demanda do silício de grau solar (SiGS) utilizado na indústria fotovoltaica cresce atualmente em uma média de 35% ao ano e a projeção para a próxima década é de constante crescimento. Entretanto, a disponibilidade de matéria-prima para a produção do SiGS não tem crescido na mesma taxa, resultando em uma elevação de seu custo. Logo, a obtenção de rotas alternativas menos onerosas para a produção do SiGS torna-se crucial, principalmente sendo o Brasil o 3o maior produtor mundial de silício de grau metalúrgico. Tendo como objetivo a purificação do silício, o presente trabalho de pesquisa apresenta um estudo detalhado da solidificação direcional do silício, que é uma das etapas utilizadas em todas as rotas alternativas atualmente pesquisadas com o objetivo de purificação. Este estudo foi realizado em um forno do tipo Bridgman vertical, onde o silício líquido contido em um molde foi extraído da região quente do forno, resultando em uma solidificação direcional controlada e ascendente. Dois tipos de silício foram escolhidos como material de entrada para o processo de refino: o silício de grau metalúrgico e o silício de grau metalúrgico pré-refinado por lixiviação. Quatro velocidades de extração do molde entre 5 a 110 mm/s foram empregadas nos ensaios. A macrossegregação de impurezas nos lingotes obtidos foi medida através de análises químicas e a estrutura metalúrgica observada através de macro e micrografias. Os lingotes solidificados com menor velocidade de extração, onde se observa evidências de uma interface sólido-líquido plana, apresentam um grau de refino superior aos de maior velocidade, que mostram evidências de uma interface celular ou dendrítica. As análises químicas mostram que, após a solidificação direcional, os teores de Fe, Al, Cr e Ti medidos nas regiões da base e central dos lingotes atingem níveis especificados para o SiGS por alguns autores. / The demand for solar grade silicon (SiGS) in the photovoltaic industry has grown 35% per year and a constant growth rate is predicted for the next decade. The availability of feedstock to produce SiGS, however, has not grown at the same rate, resulting in increasing SiGS cost. Consequently, the development of less expensive alternative processing routes is of paramount importance to the photovoltaic industry in general, and particularly to Brazil, which is the 3rd largest producer of metallurgical grade silicon. The objective of the present work is to study the directional solidification of silicon, which is a step adopted in virtually all alternative processing routes under development to refine silicon. The directional solidification of silicon was carried out in a vertical Bridgman furnace, where liquid silicon was poured into a quartz mold, which was extracted at constant velocity from the hot region of the furnace, resulting in ascending directional solidification. Two types of feedstock silicon were used, namely, a metallurgical grade silicon that was refined by leaching prior to directional solidification and metallurgical grade silicon as-received from the manufacturer. The quartz molds with liquid silicon were extracted at four different velocities in the range from 5 to 110 mm/s. In the solidified silicon ingots, the macrosegregation of elements were measured using accurate analytical techniques, whereas the metallurgical structures were observed in macro and micrographs. The ingots solidified at lower extraction velocities, at which evidences of a planar solidliquid interface were observed, showed greater refining effect than those solidified at larger velocities, which showed evidences of a cellular or dendritic solid-liquid interface. The contents of Fe, Al, Cr, and Ti measured at the base and center portions of the solidified ingots comply with the specifications of SiGS proposed by some authors.

O preenchimento dos atuais requisitos industriais para as propriedades das ligas Al-Si exige o controle dos processos de fundição e o conhecimento das relações entre as variáveis de processo e as propriedades finais desejadas. Sabe-se que as propriedades finais dependem fortemente da macroestrutura, especificamente da forma e tamanho dos grãos e da distribuição de porosidade. Embora se conheça a importância do estudo da formação da macroestrutura bruta de solidificação, existem diversos aspectos que foram pouco estudados. O presente trabalho tem como objetivo principal estudar o efeito da adição de inoculante na macroestrutura de grãos, particularmente na transição colunar-equiaxial, formada durante a solidificação unidirecional das ligas Al-3%Si e Al-7%Si. Foram realizados diversos ensaios de solidificação unidirecional ascendente em um forno de aquecimento resistivo adicionando-se diferentes quantidades de inoculante na forma de uma liga-mãe comercial de composição nominal Al-3%Ti-1%B. A solidificação unidirecional foi realizada num cadinho de aço inoxidável com base refrigerada a água e com topo e paredes laterais isolados termicamente. A macroestrutura da seção longitudinal dos lingotes obtidos foi revelada e parâmetros térmicos foram calculados a partir de curvas de resfriamento coletadas por termopares posicionados no interior do cadinho durante a solidificação. O aumento da quantidade de inoculante adicionada resultou em um conjunto de lingotes que apresentaram macroestruturas compostas quase completamente de grãos colunares até estruturas completamente equiaxiais. Sem a adição de inoculante, as macroestruturas mostraram uma transição colunar-equiaxial que não ocorre abruptamente, ao passo que a adição de inoculante resultou no aparecimento de uma região mista contendo simultaneamente grãos colunares e equiaxiais. O aparecimento desta região mista e a diminuição do tamanho dos grãos equiaxiais ocorreram na liga Al-7%Si para maiores quantidades de inoculante do que na liga Al-3%Si. / The development of various industrial sectors, such as the automotive and the aerospace industry, demand premium aluminum castings with strict control of mechanical properties. To fulfill these requirements, the control of solidification processes and the knowledge of relations between the processing variables and the casting properties are necessary. The mechanical properties of a cast part depend strongly on its macrostructure, which are directly related to the grain size and distribution of porosity. Although the study of the macrostructure formation is important to establish the relationship between processing variables and properties, some important effects were not studied thoroughly. The main objective of the present work is to study the effect of inoculant addition on the grain macrostructure, particularly the columnar-to-equiaxial transition, of Al-3%Si and Al-7%Si alloys solidified directionally. Several experiments of directional upwards solidification have been carried out in a electric resistance furnace. Different contents of a master-alloy (Al-3%Ti-1%Ti) traditionally used for grain refining in the aluminum industry were added to the melt before solidification. The directional solidification occurred in a stainless steel crucible with a water-cooled base and with lateral walls and top with thermal insulation. The macrostructures of the longitudinal section of ingots were revealed and thermal parameters, such as isotherm velocity and temperature gradients were calculated from the cooling curves measured during solidification by thermocouples inserted within the crucible. An increase in the inoculant addition resulted in a series of different ingot macrostructures that ranged from completely columnar to completely equiaxed grain structures. Without inoculant addition, the columnar-to-equiaxed transition region was abrupt, whereas the addition of inoculant gave rise to a mixed structure of columnar and equiaxed grains. The amount of inoculant that had to be added to the Al-7%Si to cause this mixed structure and decrease the equiaxed grain size was nearly one order o magnitude larger than the amount of inoculant added to the Al-3%Si alloy.

O trabalho disserta sobre a investigação dos mecanismos de degradação da placa refratária utilizada no sistema de válvula gaveta, o qual controla o fluxo de aço líquido da panela da aciaria para o distribuidor. As placas estudadas eram do tipo Al2O3-ZrO2-C. Na investigação foram caracterizadas e analisadas placas novas e usadas. Também foi feito teste de corrosão estático para avaliação da interação química das placas com a escória proveniente do processo de refino secundário. Além disso, foi feito experimento em laboratório para avaliar o desgaste das placas devido a presença de resquícios de escória entre as placas no processo de abertura da válvula. Tal experimento foi realizado nos moldes do teste de resistência de argamassa usado por Leroy (EQUIPE DE FURNAS, 1997). Para tanto foram usadas técnicas de caracterização como Microscopia Eletrônica de Varredura (MEV), Espectroscopia de Dispersão em Energia (EDS), Fluorescência de raios x (FRX) e Difração de raios x (DRX). Por meio de análises foi possível propor explicações para os mecanismos de degradação, como a corrosão devido ao contato com a escória e a formação de desgaste em forma de lingueta devido ao uso de válvula semiaberta Principalmente, a partir dos experimentos realizados foi possível identificar um mecanismo de degradação ainda não abordado na literatura: o desgaste do material das placas ocasionado pela abertura da válvula gaveta quando as placas estão ligadas por escória solidificada. Devido à falta de metodologia específica para abordar esse tema, adotou-se um método usado para avaliar a resistência de cimetos e argamassas na investigação sobre a influência da adesão da escória na degradação das placas. Tal método, chamado de tração direta (L.A.Farias, 2016), mostrou uma degradação intensa das placas. A reação química entre a placa aluminosa e a escória é a responsável pela adesão entre as duas placas e a escória. A adesão por escória mostrou que pode ser tão resistente quanto a ligação entre as partículas do refratário, que teve partículas retiradas no teste de tração direta. Assim, conclui-se que a interação química entre a placa e a escória, aliada a movimentação de abre e fecha do conjunto de placas quando a escória já solidificou, são mecanismos importantes na degradação das placas Al2O3-ZrO2-C de válvula gaveta. / This master’s dissertation investigates the deterioration of one of the parts that make up the sliding gate system of a steelmaking ladle. This part controls the liquid steel flow from the ladle to the tundish. These parts are called plates and may be made of different refractory ceramic materials. However, the plates used in this study are made of Al2O3-ZrO2-C. New parts and after service parts were analyzed. A static corrosion test was performed to analyze the chemical interaction between the refractory plates and the slag derived from steelmaking refining process. A laboratory experiment was carried out to evaluate wear due to remnants of slag between the sliding gate plates during the opening and closing of the gate. Such experiment was performed based on Mortar Resistance test used by Leroy. To analyze the materials, characterization techniques such as Scan Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), X-ray fluorescence (XRF), and X-ray diffraction were used. Based on the results of these analyses, a series of deterioration mechanisms were presented, such as the corrosion due to slag contact, and intensification of wear due to the use of semi-open valve. Mainly, from the performed experiments it was possible to identify a deterioration mechanism, that has not been discussed in literature yet This wear mechanism happens when the sliding gate plates are bonded by solidified slag, which is caused by opened valves, and leads to the extraction of particles from the surface of the plates. Due the lack of methodology to evaluate this kind of deterioration mechanism, a methodology based on mortar testing was developed. This method, called direct tensile test (L.A.Farias, 2016), showed intense deterioration. The chemical reaction between aluminum oxide refractory plate and the slag is responsible for the adhesion between plates and slag. This adhesion may be as strong as adhesion between the particles of the refractory, since refactory particles were removed in the test. Thus, it was concluded that the chemical interaction between refractory plate and slag, alied to the opening and closing mechanism are important in the deterioration of the Al2O3-ZrO2-C plates of the slide gate of the ladle.

Novas rotas para obtenção de silício grau solar a partir de silício grau metalúrgico estão em desenvolvimento e a solidificação direcional é uma etapa presente em todos os processos propostos. O refino de silício por solidificação direcional baseia-se no fenômeno de macrossegregação das impurezas. Experimentos de solidificação direcional transiente foram realizados em condições estáticas utilizando um equipamento projetado no âmbito desse trabalho. A partir de um experimento de referência, foram avaliadas as influências da alteração do material da base do cadinho, altura do lingote e condição de resfriamento do forno. Para estudo das condições de solidificação e dos mecanismos envolvidos no fenômeno de macrossegregação de solutos foi proposto e implementado um modelo matemático. Esse modelo considera as equações gerais de transporte no caso unidirecional e o transporte de espécies químicas por difusão macroscópica e convecção. A convecção foi tratada a luz da teoria da camada estagnada a frente da interface sólido-líquido. Variáveis como velocidade da interface sólido-líquido, gradiente de temperatura, perfis de concentração de soluto e de fração de sólido foram obtidos com o modelo matemático utilizando as temperaturas medidas no silício durante os experimentos de solidificação como condições de contorno do modelo. Os resultados experimentais mostraram que sob algumas condições foram obtidos lingotes com macroestrutura típica de solidificação unidirecional e, além disso, as microestruturas mostraram evidências de macrossegregação de solutos. Os resultados do modelo matemático mostraram que a solidificação ocorreu em diferentes condições de velocidade da interface sólido-líquido e gradiente de temperatura nos experimentos. Os resultados obtidos com a utilização do modelo matemático mostraram que a convecção teve papel fundamental no fenômeno de macrossegregação de solutos. / New process routes are under development to obtain solar grade silicon from metallurgical grade one, and the directional solidification is an essential step in all proposed process routes. The silicon refining by directional solidification is based on the impurities macrosegregation phenomena. Transient solidification experiments were conducted under a static condition in a furnace projected for this work. From a reference experiment it was analyzed the effects of the changing the material of the crucible base, the ingot height and the cooling condition of the furnace. A mathematical model was proposed and implemented in order to study the solidification conditions and the main mechanisms regarding the macrosegregation phenomena. The mathematical model considers the conservation equations in one direction and the transport of chemical species occurs by diffusion and convection. The convection was treated using the diffusion layer theory. The velocity of solid-liquid interface, temperature gradient and profiles of solute concentration and solid fraction were obtained using the temperatures on silicon during the solidification experiments as boundaries conditions of the model. The experimental results showed that under some conditions it was obtained ingots with typical unidirectional macrostructure and, besides, showed in the microstructure evidences of macrosegregation. The mathematical model results showed that the solidification took place under different conditions of solid-liquid interface velocities and temperature gradient in the experiments. The results from the mathematical model showed that convection plays an essential role in the macrosegregation phenomena.

A demanda por energia fotovoltaica vem aumentando a razão de mais de 20% ao ano no mercado internacional nos últimos dez anos. O silício com pureza entre 99,999% e 99,99999% é utilizado na fabricação de células fotovoltaicas. O silício metalúrgico tem pureza entre 98,5% e 99%. Este estudo visa investigar o refino a vácuo como um processo alternativo de menor custo para se obter o silício para células fotovoltaicas. Este processo pode remover o fósforo do silício, que é um dos elementos prejudiciais à célula fotovoltaica. Isso permitiria agregar valor à produção brasileira de silício metalúrgico, que alcança um preço de aproximadamente US$2,5 por quilo, enquanto o silício para células fotovoltaicas varia entre US$20 e 60 por quilo. Foram realizados experimentos de fusão em forno de indução a vácuo, variando parâmetros como temperatura, tempo e pressão. O teor de fósforo caiu de 33 ppm para cerca de 0,1 ppm e os resultados foram comparados com um modelo matemático da literatura. Conclui-se que o refino por este processo é tecnicamente viável. / The demand for photovoltaics is increasing at a ratio over 20 % per year in the international market in the last ten years. Silicon with purity of 99.999 % and 99.99999 % is used in the manufacture of photovoltaic cells. The purity of metallurgical silicon is between 98.5% and 99%. This study aims to investigate the vacuum refining process as a lower cost alternative to obtain silicon for photovoltaic cells. This process can remove phosphorus from silicon, which is a harmful element to the photovoltaic cell. This would add value to Brazilian production of metallurgical silicon, that reaches a price of approximately U.S.$ 2.5 per kilogram, while the silicon for photovoltaic cells varies between U.S.$ 20 and 60 per kilo . Melting experiments were performed in a vacuum induction furnace by varying such parameters as temperature, time and pressure. The phosphorus content dropped from 33 ppm to about 0.1 ppm and the results were compared with a mathematical model from literature. It is concluded that refining of this process is technically feasible.

<p>Den svenska regeringen satsar på småföretagande på landsbygden som omfattar bland annat gårdsbutiker och livsmedelsförädlande företag. Uppsatsen studerar denna bransch i Stockholms län och för att hitta de underliggande faktorerna till framgång används en kvalitativ studie. En serie intervjuer med Herrvik, Järna Mejeri, Nilssons Köttaffär och Saltå Kvarn bygger upp det empiriska innehållet och från dessa intervjuer studeras vilka strategival som gjorts inom respektive fokusområde. Med hjälp av produktklassificeringarna bekvämlighetsprodukter, preferensprodukter, shoppingprodukter och specialitetsprodukter dras slutsatser om företagens förmåga att hålla sig till sin målklassficering.</p><p>Analysen visar att Saltå Kvarn, det företag som har vuxit sig störst, hör till klassificeringen preferensprodukter och har i princip optimal strategi över alla fokusområden. Enda avvikelsen är i marknadsföring. För alla företag utom Saltå Kvarn är målklassificeringen specialitetsprodukter och de fokusområden som konsekvent inte uppnår detta för företagen är produktstrategi och prisstrategi. Tolkningen är att företagens största problem är hur man skapar ett kundinflytande som kunderna är beredda att betala ett högre pris för. Detta är också nyckeln till framgång för dessa småföretag. Social inbäddning är enligt teorin en stark plattform för att lösa detta problem.</p> / <p>The Swedish government is investing on small entrepreneurship on the countryside involving farm shops and food refining businesses. The essay studies this line of business in the Stockholm region and a qualitative study is used to find the underlying variables of success. A series of interviews with Herrvik, Järna Mejeri, Nilssons Köttaffär and Saltå Kvarn build up the empirical content and studied from these interviews are what strategic choices have been made within respective focus area. By support from the product classifications comfort-, preference-, shopping- and specialty products conclusions are made about the companies' ability to stick to their goal classification.</p><p>The analysis shows that Saltå Kvarn, the company that has grown largest, belongs in the classification preference products and holds practically optimal strategy over all focus areas. The only exception is in marketing. For all companies besides Saltå Kvarn the goal classification is specialty products and the focus areas that consistently do not achieve this for the companies are product- and pricing strategy. The interpretation of this is that the companies' greatest problem is how to create customer influence that the customers are prepared to pay a higher price for. This is also the key for success for these small entrepreneurs. Social inbeddedment is according to theory a strong platform for solving this problem.</p>

The principal objective of this study was to investigate process simulation and optimization of an existing offshore natural gas process which needed profitability improvements. Optimization was done using two alternative approaches: a global approach (response surface methodology) or a local approach (successive quadratic programming). The global approach was characterized by process performance at selected case study points throughout the feasible operating region and made use of global information, while the local approach was characterized by numerical iterative computation driven by local information in the neighborhood of a single point in the design space. A Box-Behnken design was used as a second-order response surface design for the estimation of correlation between process simulator design variables and an economic objective function, and the estimation model was then optimized. In the local approach, a process simulator (ASPEN PLUS) with optimization capabilities was used. From the investigation, three major design variables were identified that had significant effects on the objective function of maximum product sales value. The three variables were: the compression ratio of the Production Compressor, the heat duty of the Gas/Gas Exchanger, and the compression ratio of the Expander. The results indicated that profitability of the offshore natural gas process could be improved by greatly (about 60%) increasing the production of raw condensate, even though this meant slightly lower (about 4%) sales of natural gas as a main product. The improvement found, however, was only 7% when the design variables were limited by currently installed equipment. The global approach was found to converge more consistently because once the global quadratic model was calculated, convergence to its unique optimum was simple. On the other hand the local approach had non-unique termination points due to the small effects of some design variables. The global approach provided better engineering insight since the effects of each design variable could be easily calculated from the quadratic model. However, the global approach was less efficient in terms of engineering manpower because of the time required to identify the quadratic model. / Graduation date: 1995

The thesis is focussed on low consistency (LC) refining of mechanical pulp. Theresearch included evaluations of energy efficiency, development of pulpproperties, the influence of fibre concentration on LC refining and effects of rotorposition in a two-zoned LC refiner. Trials were made in mill scale in a modern TMP line equipped with an MSDImpressafiner for chip pre-treatment, double disc (DD) first stage refining and aprototype 72-inch TwinFlo LC refiner in the second stage. Tensile index increasedby 8 Nm/g and fibre length was reduced by 10 % in LC refining at 140 kWh/adtgross specific refining energy and specific edge load 1.0 J/m. Specific lightscattering coefficient did not develop significantly over the LC refiner. The above mentioned TMP line was compared with a two stage single disc highconsistency Twin 60 refiner line. The purpose was to evaluate specific energyconsumption and pulp properties. The two different process solutions were testedin mill scale, running similar Norway spruce wood supply. At the same tensileindex and freeness, the specific energy consumption was 400 kWh/adt lower in theDD-LC concept compared with the SD-SD system. Pulp characteristics of the tworefining concepts were compared at tensile index 47 Nm/g. Fibre length was lowerafter DD-LC refining than after SD-SD refining. Specific light scattering coefficientwas higher and shive content much lower for DD-LC pulp. The effects of sulphite chip pre-treatment on second stage LC refining were alsoevaluated. No apparent differences in fibre properties after LC refining werenoticed between treated and untreated pulps. Sulphite chip pre-treatment iniiicombination with LC refining in second stage, yielded a pulp without screeningand reject refining with tensile index and shives content that were similar to nonpre-treated final pulp after screening and reject refining. A pilot scale study was performed to investigate the influence of fibreconcentration on pulp properties in LC refining of mechanical pulps. MarketCTMP was utilised in all trials and fibre concentrations were controlled by meansof adjustments of the pulp consistency and by screen fractionation of the pulp. Inaddition, various refiner parameters were studied, such as no-load, gap and baredge length. Pulp with the highest fibre concentration supported a larger refinergap than pulp with low fibre concentration at a given gross power input. Fibreshortening was lower and tensile index increase was higher for long fibre enrichedpulp. The results from this study support the interesting concept of combiningmain line LC refining and screening, where screen reject is recycled to the LCrefiner inlet. It has been observed that the rotor in two-zoned refiners is not always centred,even though pulp flow rate is equal in both refining zones. This leads to unequalplate gaps, which renders unevenly refined pulp. Trials were performed in millscale, using the 72-inch TwinFlo, to investigate differences in pulp properties androtor positions by means of altering the pressure difference between the refiningzones. In order to produce homogenous pulp, it was found that uneven plate gapscan be compensated for in LC refiners with dual refining zones. Results from thedifferent flow rate adjustments indicated that the control setting with similar plategap gave the most homogenous pulp.