Cryogenic air separation units (ASUs) are the most widely used technology for industrialscale production of large amounts of high-purity air components. These are highly energyintensive processes, which have motivated the development of demand response strategies to adapt their operation in response to the increased volatility of the energy market. The startup of ASUs warrants particular consideration within this context. ASUs are tightly integrated, thermally and materially, and have slow dynamics. These result in startup times on the order of hours to a day, during which electricity is consumed with limited revenue generation. In the current environment of electricity price deregulation, it may be economically advantageous for ASUs to shut down during periods of high electricity pricing, increasing the occurrences of startups. This presents a promising research opportunity, especially because ASU startup has received relatively little attention in the literature. This thesis investigates the optimal startup of ASUs using dynamic optimization. First, this thesis focuses on startup model development for the multiproduct ASU. Startup model development requires accounting for discontinuities present at startup. Four main discontinuities are considered: stage liquid flow discontinuity, stage vapor flow discontinuities, flow liquid out of sumps and reboilers, and opening and closing valves. Other types of discontinuities accounted for include the change in the number of phases of streams. These discontinuities are approximated with smoothing formulations, using mostly hyperbolic tangent functions, to allow application of gradient-based optimization. The modeling approach was assessed through three case studies: dynamic simulation of a successful startup, dynamic simulation of a failed startup, and dynamic optimization using a least-squares minimization formulation. Following startup model development, this thesis investigates the development of a framework for optimizing ASU startups using readily interpretable metrics of time and economics. For economics, cumulative profit over the startup horizon is considered. Two events are tracked for the definition of time metrics: time taken to obtain product purities and time to obtain steady-state product flows. Novel approaches are proposed for quantifying these time metrics, which are used as objective functions and in formulating constraints. The / Thesis / Doctor of Philosophy (PhD)

I detta examensarbete presenteras en ny metod för att göra schemaläggningsbeslut inom processindustrin och samtidigt ta hänsyn till det dynamiska beteendet hos processen. En modell av en luftsepareringsanläggning som producerar kvävgas och utnyttjar ett rörligt elpris användes för att exemplifiera denna metod. Modellen omfattade en kryogenisk destillationskolonn med en integrerad återloppskokare /kondensator, en multiströms värmeväxlare, en kompressor, två turbiner och en kondensator. Den innehöll 5079 ekvationer och 437 differentiella variabler. Dynamisk optimering användes för att approximera det dynamiska beteendet hos processen vid skiftningar mellan olika driftpunkter. Den registrerade data utnyttjades sedan för att identifiera en reducerad modell som fångade det transienta beteendet hos relevanta processvariabler. Den reducerade modellen bestod av 525 ekvationer och 67 differentiella variabler. Den identifierade modellen visade på god matchning mellan relevanta processvariabler i de simulerade övergångarna och den reducerade modellen. Den reducerade modellen användes för att optimera schemaläggningen av luftsepareringsprocessen så att elkostnaden över en tredagars period minimerades. De optimala resultaten visade på en minskning av kostnaden på 2.6 % jämfört med en konstant produktionstakt. Schemat implementerades och simulerades i den fullt dynamiska modellen över de första 24 timmarna för att jämföra relevanta processvariabler med den reducerade modellen. Resultaten visade på god matchning mellan de båda modellerna. Detta examensarbete visar att en exakt reducerade modell kan användas för att snabbt hitta ett optimalt schema över ett större processystem. Detta genom att kraftigt minska systemets storlek utan att offra noggrannhet av det dynamiska beteendet. / A novel framework for making plant scheduling decisions while considering the plant process dynamics is presented in this thesis. A model of an air separation unit built to supply nitrogen gas and subject to time-varying electricity prices was used to illustrate this framework. The model includes a cryogenic distillation column with an integrated reboiler/condenser, a multi-stream heat exchanger, a compressor, two turbines, and a liquefier. It consisted of 5079 equations and 437 differential variables. The dynamic behavior of the process during operating point transitions was determined using dynamic optimization. This data were used to establish a reduced order dynamic model of the system which captures the transient behavior of relevant process variables. The reduced order model consisted of 525 equations and 67 differential variables. The identified model showed a good fit between the relevant process variables in the simulated transitions and the reduced order model. The air separation unit process schedule was optimized using the reduced order model to minimize electricity cost over a three day time horizon. The optimal result showed a 2.6 % reduction in electricity cost compared to a flat production rate. The optimal schedule was implemented and simulated in the full dynamic model for the first 24 hours to compare the relevant process variables to the reduced model predictions. The result displayed good match between the reduced model and the full dynamic model. This thesis shows that an accurate reduced order dynamic model can be used for quickly finding the optimal schedule of large process systems. This by greatly reducing the size and complexity of the system without sacrificing accuracy of the dynamic behavior. Furthermore, it also shows the economic benefits of the integrating scheduling and control to count for the dynamic behavior of the system.

Human capital is the only resource within an organisation that can learn. Developing high levels of competence in employees is one of the most challenging issues in organisations. Off–the–Job training programs either miss the mark or are too far away from the performance setting to have the desired impact on employee competence. Studies have shown that unstructured On–the–Job Training (OJT) leads to increased error rate, lower productivity and decreased training efficiency, compared to structured On–the–Job Training(S–OJT). The proven efficiency and effectiveness of S–OJT make it especially suitable to meet this challenge. Though S–OJT has been around for a while there has not been a proper integration of technology into the process. Every training approach, including S–OJT, is merely a means to an end, not an end in itself. The use of S–OJT helps to develop consistent appropriate levels of employee competence. When employees have these competencies e.g. better knowledge of the production processes, they can increase productivity, complete projects on time, lower defect rates, or achieve other outcomes of importance. These are the outcomes that matter to the organisation and the effectiveness of S–OJT should be judged from this perspective. Researchers have consistently found that one way to improve learners' success is to increase the frequency of exams. Classes meet for a set number of times. An instructor's decision to give more exams typically means that students have less time for learning activities during class meetings. How then can one have the best of both worlds, increasing the number of assessments and at the same time having enough time for learning activities? This can only be accomplished by integrating computer–based assessment into S–OJT programs. Computer–based testing and training can provide flexibility, instant feedback, an individualised assessment and eventually lower costs than traditional written examinations. Computerised results create opportunities for teaching and assessment to be integrated more than ever before and allow for retesting students, measuring growth and linking assessment to instruction. This research aims to evaluate the effectiveness of integrating computer–based testing and training into S–OJT programs using the Air Separation unit of Sasol Synfuels as a case study. The null hypothesis is used to investigate the draw backs of OJT and S–OJT programs. A framework is also developed for the effective integration of CBT into S–OJT programs. / Thesis (M.Ing. (Development and Management))--North-West University, Potchefstroom Campus, 2011.

Human capital is the only resource within an organisation that can learn. Developing high levels of competence in employees is one of the most challenging issues in organisations. Off–the–Job training programs either miss the mark or are too far away from the performance setting to have the desired impact on employee competence. Studies have shown that unstructured On–the–Job Training (OJT) leads to increased error rate, lower productivity and decreased training efficiency, compared to structured On–the–Job Training(S–OJT). The proven efficiency and effectiveness of S–OJT make it especially suitable to meet this challenge. Though S–OJT has been around for a while there has not been a proper integration of technology into the process. Every training approach, including S–OJT, is merely a means to an end, not an end in itself. The use of S–OJT helps to develop consistent appropriate levels of employee competence. When employees have these competencies e.g. better knowledge of the production processes, they can increase productivity, complete projects on time, lower defect rates, or achieve other outcomes of importance. These are the outcomes that matter to the organisation and the effectiveness of S–OJT should be judged from this perspective. Researchers have consistently found that one way to improve learners' success is to increase the frequency of exams. Classes meet for a set number of times. An instructor's decision to give more exams typically means that students have less time for learning activities during class meetings. How then can one have the best of both worlds, increasing the number of assessments and at the same time having enough time for learning activities? This can only be accomplished by integrating computer–based assessment into S–OJT programs. Computer–based testing and training can provide flexibility, instant feedback, an individualised assessment and eventually lower costs than traditional written examinations. Computerised results create opportunities for teaching and assessment to be integrated more than ever before and allow for retesting students, measuring growth and linking assessment to instruction. This research aims to evaluate the effectiveness of integrating computer–based testing and training into S–OJT programs using the Air Separation unit of Sasol Synfuels as a case study. The null hypothesis is used to investigate the draw backs of OJT and S–OJT programs. A framework is also developed for the effective integration of CBT into S–OJT programs. / Thesis (M.Ing. (Development and Management))--North-West University, Potchefstroom Campus, 2011.

The objective of this thesis is to provide an extensive description of the correlations in some of the most crucial sub-processes for hard coal fired IGCC with carbon capture (CC-IGCC). For this purpose, process simulation models are developed for four industrial gasification processes, the CO-shift cycle, the acid gas removal unit, the sulfur recovery process, the gas turbine, the water-/steam cycle and the air separation unit (ASU). Process simulations clarify the influence of certain boundary conditions on plant operation, performance and economics. Based on that, a comparative benchmark of CC-IGCC concepts is conducted. Furthermore, the influence of integration between the gas turbine and the ASU is analyzed in detail. The generated findings are used to develop an advanced plant configuration with improved economics. Nevertheless, IGCC power plants with carbon capture are not found to be an economically efficient power generation technology at present day boundary conditions.

Cette thèse porte sur la modélisation, pour le contrôle, des profils de compositions dans les colonnes de distillation cryogénique. Nous obtenons un modèle non-linéaire de convection-diffusion par réduction d'un modèle d'équations-bilans singulièrement perturbé. Du point de vue de l'automatique, nous nous intéressons à la stabilité des profils de compositions résultants, ainsi qu'à leur observabilité. Du point de vue du procédé, la nouvauté de notre modèle réside dans la prise en compte d'une efficacité de garnissage dépendant des conditions d'opération de la colonne. Le modèle est validé par des comparaisons avec des données de fonctionnement dynamique issues d'une unité de séparation réelle, pour la séparation d'un mélange binaire. Sur le cas plus complexe d'une cascade de colonnes séparant un mélange ternaire, le modèle montre une grande sensibilité aux erreurs d'estimation des taux de reflux. Des résultats adaptés du champ de la chromatographie nous permettent de relier cette sensibilité à des erreurs d'estimation des vitesses d'ondes de compositions cohérentes. En parallèle, nous proposons et testons également un modèle de fonctions de transfert simple (fondé sur des gains statiques et des retards purs uniquement) pour les petites dynamiques de compositions, qui dépend explicitement de valeurs mesurables ou observables sur le procédé / This thesis addresses the problem of modeling the composition profiles dynamics inside cryogenic distillation columns, for control applications. We obtain a non-linear convection-diffusion model from the reduction of a singularly perturbed mass-balances model. In the control theory framework, we consider the stability of the resulting composition profiles and their observability. From the process viewpoint, we express the novelty of our model in terms of operating-conditions dependent packing efficiency. The model is validated against real dynamic plant data for a binary separation case. On a more complex, ternary separation columns cascade, the model shows highly sensitive to reflux rate estimation errors. Result adapted from the field of chromatography allows us to interpret this sensitivity in terms of erroneous coherent composition waves speeds. In parallel, we also propose and test a simple transfer functions model (based on static gains and pure delays only) for small composition dynamics, which explicitly depends on measurable or observable process data.

The objective of this thesis is to provide an extensive description of the correlations in some of the most crucial sub-processes for hard coal fired IGCC with carbon capture (CC-IGCC). For this purpose, process simulation models are developed for four industrial gasification processes, the CO-shift cycle, the acid gas removal unit, the sulfur recovery process, the gas turbine, the water-/steam cycle and the air separation unit (ASU). Process simulations clarify the influence of certain boundary conditions on plant operation, performance and economics. Based on that, a comparative benchmark of CC-IGCC concepts is conducted. Furthermore, the influence of integration between the gas turbine and the ASU is analyzed in detail. The generated findings are used to develop an advanced plant configuration with improved economics. Nevertheless, IGCC power plants with carbon capture are not found to be an economically efficient power generation technology at present day boundary conditions.