### Browsing by Author "Swartz, Chris"

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- ItemOpen AccessCalculation of chemical and phase equilibria(1996) Meyer, Clifford Alexander; Swartz, ChrisThe computation of chemical and phase equilibria is an essential aspect of chemical engineering design and development. Important applications range from flash calculations to distillation and pyrometallurgy. Despite the firm theoretical foundations on which the theory of chemical equilibrium is based there are two major difficulties that prevent the equilibrium state from being accurately determined. The first of these hindrances is the inaccuracy or total absence of pertinent thermodynamic data. The second is the complexity of the required calculation. It is the latter consideration which is the sole concern of this dissertation.
- ItemOpen AccessControl and optimization of a multiple-effect evaporator(2000) Smith, Patrick D; Swartz, Chris; Harrison, STLFalling commodity prices have reduced the profit margins of Southern African sugar producers. Although these price falls have been severe, they reflect a long-term trend of reducing margins for basic commodity producers during the 20th Century. This trend has forced. producers to closely examine their processes and to look for areas in which improvements in productivity, yield and efficiency can be achieved. Evaporation is the most energy intensive unit operation in the sugar factory, and it is responsible for the removal of most of the water from sugar solution, or juice, which is extracted from the sugar cane. There is also a large potential to lose sucrose at the evaporators due to the high temperatures and long residence times employed there. The smooth control of the evaporators is thus vital to consistent factory operation, and the evaporators are commonly a sugar factory bottleneck. This study developed a control strategy for the particular evaporator configuration found at Triangle Sugar Mill in south eastern Zimbabwe. There are currently several evaporator control strategies being used in the sugar industry. Most of these are an assembly of single loop Proportional Integral Derivative (PID) controllers, which cannot optimally account for the interactions encountered in most evaporator stations. Ideally, any evaporator control system should be able to handle the multiple input multiple output problem while anticipating and handling constraints on inputs and outputs. Several multivariable approaches have been tried, but these usually require a great deal of expensive instrumentation.After a review of the multivariable control literature and testing of several alternative control systems, Dynamic Matrix Control (DMC) was chosen as the bestwsuited control algorithm for the Triangle control problem. A dynamic model of the Triangle evaporator station was, developed to formulate and test the DMC and other controllers. The model was based on a set of differential equations involving mass and energy balances through the evaporators. Real plant data were collected from the SCADA system and the model was tested against this data. After validation the model was. used to record step responses of the process to key input variables. The control system had nine (9) measurable inputs, and three (3) controlled outputs. The objective of the control system was to deliver the maximum amount of consistently high quality symp, within plant constraints. This was formulated in an objective function which seeks to minimize a weighted sum of the errors of syrup concentration from a setpoint, and the fluctuations in juice flowrates. Two alternative formulations were developed, and tested on the plant model.
- ItemOpen AccessDevelopment of a nonlinear predictive control algorithm and its application to flotation(2001) Knights, Benjamin D H; Swartz, Chris; Langson, WilburThis study consists of four clearly defined and interlinked objectives. The first is the development of a method of solving nonlinear optimal control problems. This method is then used to solve the underlying optimal control problems. This method is then used to solve the underlying optimal control problem in a nonliear model predictive control (NMPC) strategy. By way of a case study, and to further understanding of the mechanisms of flotation, a dynamic model of a flotation circuit is developed. This nonlinear dynamic model is then used in the NMPC strategy to simulate the nonlinear predictive control of a flotation cell. An indirect approach is used in the method for solving optimal control problems, applying the Euler-Lagrange equations to transofrm the optimal control problem into a two-point boundary value problem (BVP). Conventionally these problems have been solved using the multiple shooting differential equation solver, or variants of it. In this work the BVP is solved by orthogonal collocation, using the FORTRAN package COLSYS. Stragegies are developed to solve a number of different classes of optimal control problems, including problems with constraints on the control trajectory, and terminal state constraints. An important aspect of this solution method is that it produces a continuous control trajectory over the entire solution horizon. The use of this optimal control problem in the nonlinear model predictive control strategy of Chen and AlgÃ¶wer (1998) is then shown. This strategy is a quasi-infinite horizon NMPC strategy making use of a stabilising terminal penalty cost and terminal state inequality constraint. Stability is guaranteed if the optimal conrol problem is feasible.
- ItemOpen AccessDynamic simulation of the leaching and adsorption sections of a gold plant(1992) Schubert, Joachim Hans; Swartz, ChrisThis dissertation describes the development of a dynamic simulator for the leaching and adsorption sections of a gold plant. In contrast to the milling stage which precedes the leaching and which is a purely mechanical process, leaching and adsorption are hydrometallurgical processes which are of particular interest to chemical engineers. Leaching is a well-defined chemical processes in which gold is dissolved out of the rock by reaction with cyanide ions. The leaching process occurs in a series of stirred tank reactors and is easily modelled. The adsorption process is far more challenging to model. The adsorption occurs on carbon particles which are mixed into the pulp and this gives rise to the name carbon-in-pulp (CIP). The actual adsorption of the gold cyanide complex on the lattice structure of the carbon particles is a surface phenomenon which, while it has not been totally defined, can yet be described by conventional rate processes. The adsorption also takes place in a cascade of stirred tank reactors, but the occasional pumping of carbon up the cascade and the resulting counter-current movement of carbon and pulp present modelling challenges. A dynamic simulator was regarded necessary for this process to determine what the short and long term effects of process disturbances are. While steady state models have been developed before, they are not able to describe the transient responses to such changes. Disturbances are all too common on an operating plant and as a result the plant never truly reaches a steady-state. Any control strategy for the plant must necessarily be developed by taking the transient responses into consideration. Another requirement was for the simulator to be flexible enough to be adapted quickly to various plants. It was also to be able to read in any applicable and easily available information from plant data files and to use the data to recreate reasonably accurate outputs. The simulator is written as a collection of ordinary differential equations each of which is a mole balance of one of the components (or state variables) in the system. The mole balances include the effect of chemical reactions between the various reactants describing the production and depletion of these components. The hydrodynamics of the bulk pulp phase are also accounted for by considering the amount of all components within process units and the movement of components between the units. Various factors affecting the two sections of the plant have been investigated, most of which have been considered in theory or were included in simulators by earlier investigators. Some aspects, such as attrition and screen overflows, have been included in a dynamic simulator for the first time. Attrition was found to have a major effect on the efficiency of the adsorption process by levelling out the gold solution profile and thereby reducing the rate of loading on coarse carbon. Other inefficiencies are the result of unsteady operation, especially of wildly fluctuating feed flowrates which make the addition of reagents difficult to control, and various process upsets in the CIP such as screen breakages and overflows, which allow loaded carbon to move downstream with the pulp.
- ItemOpen AccessH to infinity optimal control of a counter-current process(1991) Olivier, Brendan; Braae, Martin; Swartz, ChrisThe extraction of gold from ore that has been mined is the most important part of the process which eventually produces gold bullion. The process most commonly used today is that of carbon-in-pulp gold extraction (CIP). One of the main reasons for this is that it is the most economically efficient method of extracting gold from ore. The process uses activated carbon to absorb gold from a cyanide leach solution. Slurry containing the gold bearing ore and the activated carbon flow in a counter-current fashion. This counter-current flow enables a high percentage of the gold to be recovered. Gold can then be recovered through an elution process. Large amounts of activated carbon are used in the process and a formal multi variable control study of the adsorption section of the CIP process could provide further economic savings by extracting more gold with controlled amounts of carbon. A study was performed to identify the chemical mechanisms involved in the adsorption section of a CIP plant. It was felt that the workings of the process could best be established by designing a simple simulator of the process. The simulator was designed with four reactor tanks, in which the carbon absorbs gold from the leached slurry. The simulator uses a continuous transfer of carbon. In order to fully understand the operation of the HERIG, a simulation study was performed. This simulated model was a simplified version of the actual rig. The level changes of COLD water in the tanks were assumed to be instantaneous and the heat transfer coefficients were assumed to be the same for all four of the tanks. The calculation of the heat transfer coefficients was investigated thoroughly and care was taken to obtain accurate values. The simulator designed was a lumped parameter model. The pipes containing the HOT stream were divided into many small section, in each of which a constant temperature was assumed. A sum of the contributions of all the sections submerged under the COLD water was used to calculate the heat transferred into the COLD water. The COLD water in the tank is stirred continuously and is assumed to be at a constant temperature. The level of COLD water in each tank on the HERIG represents the mass of carbon in each of the tanks on a CIP plant. A change in the HOT water pipe temperature (concentration of Au in slurry) was examined as a function of a change in the level of COLD water in a tank (mass of carbon in a reactor). A steady state and dynamic analysis verified that trends observed from the CIP model were in fact mimicked by the trends observed on the HERIG. It was then decided to perform a formal control study of the HERIG, since the numerous similarities found between the CIP and HERIG enabled relevant conclusions to be drawn about the control of CIP from the control of the HERIG.
- ItemOpen AccessInclusion of input saturation in the design of dynamically operable plants(2000) Baker, Rhoda; Swartz, ChrisDynamic operability reflects the quality with which a plant can be controlled using feedback, and is a function of both the design of the plant and its associated control system. A plant designed on the basis of steady-state considerations alone could exhibit poor dynamic characteristics, leading to a loss of economic performance and a reduced capacity to effectively handle safety and environmental constraints. This motivates the need for the development of quantitative techniques for dynamic operability assessment, as well as its incorporation into procedures for process plant design. Optimization-based approaches to dynamic operability assessment permit simultaneous consideration of performance-limiting factors of nonminimum phase characteristics, input constraints and model uncertainty, and also provide considerable flexibility in the choice of performance criteria, decision variables and constraints. Recent work has incorporated operability requirements as constraints within a single optimal plant design problem formulation (Mohideen et at., 1997; Bahri et at.,1996). Young and Swartz (1997) considered the rigorous inclusion of input saturation effects in optimizing control. Actuator saturation introduces discontinuities in the system model and, to avoid potential problems using a sequential optimization approach, two alternative formulations were proposed for solving the problem within a simultaneous solution framework. Input saturation discontinuities were handled by the introduction of slack variables and their inclusion in either bilinear or mixed-integer constraints resulting in a nonlinear or mixed-integer linear programming problem respectively. The formulations were applied to a linear system with dead time to find the economically optimal operating point for a controller with fixed structure and tunings when disturbance deviations are taken into account. It was shown that using a strictly linear controller in this case would lead to an overly conservative estimate of the feasible operating range and consequently, a suboptimal operating point.
- ItemOpen AccessOn the operability of heat exchanger networks(2000) Hattingh, Caleb; Swartz, ChrisThe dynamic operability of processes refers to the degree to which plants may be satisfactorily controlled. This report presents a study of the operability of heat exchanger networks (HENs).The integration of heat exchange systems such as HENs typically results in significant steady-state cost savings which is the motivation for their implementation. However, such integration may lead to problems in the dynamic operation of the system if the operability of HENs is not considered. Operability analysis techniques are presented that provide a quantitative measure of the operability of HENs that is related to the minimum integral setpoint error of a closed-loop HEN under a step disturbance. The different operability analysis techniques are specified by ,u sing different controller types which are optimally tuned in an optimization framework. The different controllers include PI (proportionalintegral) control, MPC (model predictive control), optimal linear control (via Qparametrization) and an optimal open-loop control strategy that represents the best possible closed-loop performance.
- ItemOpen AccessOperability analysis of an industrial communication circuit(2000) Seaman, David Richard; Swartz, ChrisBibliography: leaves 117-120.
- ItemOpen AccessOptimal scheduling of an integrated batch and continuous process system(2000) MacRosty, R D M; Swartz, ChrisAn industrial problem has been posed involving the determination of the optimal scheduling and sequencing of a batch process system. The plant incorporates both batch and continuous units and is further complicated by the existence of a nonlinear relationship between the processing rate and the lot size in the continuous plant. The primary goal of this project is to investigate the application of a rigorous mathematical approach to determine the optimal operating policy of a complex processing system. In broad terms, scheduling involves the allocation of a limited number of resources to ensure the completion of a set of tasks, in an optimal way. The problem is formulated as a mathematical model through the use of mixed-integer programming. The difficulty associated with this type of work is in describing the plant using constraints to develop a mathematical model which both accurately reflects the plant and renders the problem solvable. The model must incorporate the connectivity of the plant and prevent resource-task allocation conflicts. A survey of the pertinent literature has been conducted and is reviewed in Chapter 2. Here the principles of batch scheduling are addressed. The focus of the literature review is on the formulation of such problems into a mathematical representation. A description of two fundamentally different approaches for the formulation of short-term scheduling problems is presented. Other aspects of the review include the commercially available software used in the solution of these problems, campaign planning and on-line scheduling. The work in this thesis initially focuses on the scheduling of a simplified version of the industrial plant. The simplified version consists exclusively of batch processing units. The motivation for this was to develop a formulation which incorporated the unique characteristics of the full problem but was computationally easier to solve. A number of scenarios were conducted which show both the flexibility of the formulation and the ability of the formulation to reschedule tasks when faced with different operating conditions. In the discussion of these scenarios, issues such as the computational efficiency and the implications of the results are addressed. The next step was to add a continuous plant upstream to the simplified batch process. The continuous plant exhibits a nonlinear relationship between the processing rate and the efficiency of operation. An approximation of the non-linearity is proposed and further scenario studies are carried out. Finally, the full problem is tackled but due to the great computational expense required to solve the problem an alternative method is proposed. This method is based on a series of random-type scenarios, which serve as an alternative as well as benchmark to the solutions obtained via rigorous optimization.
- ItemOpen AccessAn optimizing control strategy for milling circuit operation(1998) Hart, Jarrod Rex; Swartz, ChrisIt has been recognized that significantly more benefit can be derived by operating processes at the economically optimal process conditions than by solely improving the regulatory control performance. This has been the motivation for an increased focus on optimizing control strategies where the set points are adjusted on-line in accordance with an economic criterion. Since plant conditions change, this adjustment needs to be made on a continual basis, but at a frequency lower than that at which inputs are manipulated at the regulatory control level. In the operation of tumbling mills, the fractional filling of the mill with slurry has a marked effect on the efficiency of the breakage process within the mill. The selection of the set point for this variable is critical in the optimization of throughput and energy efficiency in a grinding circuit. Unfortunately, the optimal value for this set point is a function of both circuit and ore characteristics, which means that it varies as the feed and plant characteristics change. This means that periodic re-evaluation of this optimal value needs to be performed in order to fully maximize circuit throughput. The mechanism for seeking this optimum on-line forms the basis for this thesis. Continuous evaluation of the function relating throughput to mill loading (if that were possible) would reveal the optimal set point for the load which could be regularly updated. A recent strategy tackled this issue by assuming power draw from the mill motors is related to circuit, throughput. With the aim to optimize throughput, power consumption can be used as a yardstick by which the load set point (set point for fractional filling in mill) can be selected. The load set point is simply adjusted periodically to optimize the power consumption and correspondingly, the circuit through put. The key proposal in this thesis is a strategy that employs a model of the plant to assist and improve the selection of the load set point.
- ItemOpen AccessParticle size estimation of hydrocyclone overflow(2000) Smith, Vincent Combemere; Swartz, ChrisThis dissertation describes the development of a robust hydrocyclone particle size estimation model that will form the basis of an industrial soft-sensor. Aside from increased throughput, efficient product regulation constitutes the primary function of a milling circuit control system. Before the milling circuit product size can be regulated, it should be measured or estimated. The particle size estimation algorithm developed provides a reliable estimate of the product size and will compliment or replace conventional size measurement devices.