Model-based approach for the plant-wide economic control of fluid catalytic cracking unit

Fluid catalytic cracking (FCC) is one of the most important processes in the petroleum refining industry for the conversion of heavy gasoil to gasoline and diesel. Furthermore, valuable gases such as ethylene, propylene and isobutylene are produced. The performance of the FCC units plays a major role on the overall economics of refinery plants. Any improvement in operation or control of FCC units will result in dramatic economic benefits. Present studies are concerned with the general behaviour of the industrial FCC plant, and have dealt with the modelling of the FCC units, which are very useful in elucidating the main characteristics of these systems for better design, operation, and control. Traditional control theory is no longer suitable for the increasingly sophisticated operating conditions and product specifications of the FCC unit. Due to the large economic benefits, these trends make the process control more challenging. There is now strong demand for advanced control strategies with higher quality to meet the challenges imposed by the growing technological and market competition. According to these highlights, the thesis objectives were to develop a new mathematical model for the FCC process, which was used to study the dynamic behaviour of the process and to demonstrate the benefits of the advanced control (particularly Model Predictive Control based on the nonlinear process model) for the FCC unit. The model describes the seven main sections of the entire FCC unit: (1) the feed and preheating system, (2) reactor, (3) regenerator, (4) air blower, (5) wet gas compressor, (6) catalyst circulation lines and (7) main fractionators. The novelty of the developed model consists in that besides the complex dynamics of the reactorregenerator system, it includes the dynamic model of the fractionator, as well as a new five lump kinetic model for the riser, which incorporates the temperature effect on the reaction kinetics; hence, it is able to predict the final production rate of the main products (gasoline and diesel), and can be used to analyze the effect of changing process conditions on the product distribution. The FCC unit model has been developed incorporating the temperature effect on reactor kinetics reference construction and operation data from an industrial unit. The resulting global model of the FCC unit is described by a complex system of partial-differential-equations, which was solved by discretising the kinetic models in the riser and regenerator on a fixed grid along the height of the units, using finite differences. The resulting model is a high order DAE, with 942 ODEs (142 from material and energy balances and 800 resulting from the discretisation of the kinetic models). The model offers the possibility of investigating the way that advanced control strategies can be implemented, while also ensuring that the operation of the unit is environmentally safe. All the investigated disturbances showed considerable influence on the products composition. Taking into account the very high volume production of an industrial FCC unit, these disturbances can have a significant economic impact. The fresh feed coke formation factor is one of the most important disturbances analysed. It shows significant effect on the process variables. The objective regarding the control of the unit has to consider not only to improve productivity by increasing the reaction temperature, but also to assure that the operation of the unit is environmentally safe, by keeping the concentration of CO in the stack gas below a certain limit. The model was used to investigate different control input-output pairing using classical controllability analysis based on relative gain array (RGA). Several multi-loop control schemes were first investigated by implementing advanced PID control using anti-windup. A tuning approach for the simultaneous tuning of multiple interacting PID controllers was proposed using a genetic algorithm based nonlinear optimisation approach. Linear model predictive control (LMPC) was investigated as a potential multi-variate control scheme applicable for the FCCU, using classical square as well as novel non-square control structures. The analysis of the LMPC control performance highlighted that although the multivariate nature of the MPC approach using manipulated and controlled outputs which satisfy controllability criteria based on RGA analysis can enhance the control performance, by decreasing the coupling between the individual low level control loops operated by the higher level MPC. However the limitations of using the linear model in the MPC scheme were also highlighted and hence a nonlinear model based predictive control scheme was developed and evaluated.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Implementation and performance assessment of a real-time optimization system on a virtual fluidized-bed catalytic-cracking plant

This thesis develops and evaluates RTO implementation in a FCCU virtual plant, taking into account each RTO stage (noise elimination, steady-state detection, data validation, parameter estimation, and optimization). The dynamic data to carry out this analysis were obtained from an FCCU virtual plant based on a dynamic deterministic model developed in Matlab®. The model output data were contaminated with Gaussian and gross errors to simulate measurements from a real plant. For denoising, steady-state detection, data reconciliation, parameter estimation, and optimization, different strategies and algorithms were studied and assessed, while a decentralized PID was proposed for the control system. Finally, the most appropriate strategies for the case study were implemented and their performance was fully evaluated.Resumen: Esta tesis desarrolla y evalúa la implementación de la RTO en una planta virtual de FCCU, teniendo en cuenta cada etapa de una RTO (eliminación de ruido, detección de estado estable, validación de datos, estimación de parámetros y optimización). Los datos dinámicos para llevar a cabo este análisis se obtuvieron de una planta virtual de FCCU basada en un modelo determinista dinámico desarrollado en Matlab®. Los datos de salida del modelo se contaminaron con error de Gauss y error grueso para simular mediciones de una planta real. Para la eliminación de ruido, la detección de estado estable, la reconciliación de datos, la estimación de parámetros y la optimización, se estudiaron y evaluaron diferentes estrategias y algoritmos, mientras que para el sistema de control se propuso un PID descentralizado. Finalmente, se implementaron las estrategias más apropiadas para el estudio de caso y se evaluó su desempeño en conjunto.Maestrí

Kinetic Intersections as a Source of Information on Rate Coefficients

C

Framework for operability assessment of production facilities: an application to a primary unit of a crude oil refinery

This work focuses on the development of a methodology for the optimization, control and operability of both existing and new production facilities through an integrated environment of different technologies like process simulation, optimization and control systems. Such an integrated environment not only creates opportunities for op¬erational decision making but also serves as training tool for the novice engineers. It enables them to apply engineering expertise to solve challenges unique to the process industries in a safe and virtual environment and also assist them to get familiarize with the existing control systems and to understand the fundamentals of the plant operation. The model-based methodology proposed in this work, starts with the implementation of first principle models for the process units on consideration. The process model is the core of the methodology. The state of art simulation technologies have been used to model the plant for both steady state and dynamic state conditions. The models are validated against the plant operating data to evaluate the reliability of the models. Then it is followed by rigorously posing a multi-optimization problem. In addition to the basic economic variables such as raw materials and operating costs, the so-called “triple-bottom-line” variables related with sustainable and environmental costs are incorporated into the objective function. The methodologies of Life Cycle Assessment (LCA) and Environmental Damage Assessment (EDA) are applied within the optimization problem. Subsequently the controllability of the plant for the optimum state of conditions is evaluated using the dynamic state simulations. Advanced supervisory control strategies like the Model Predictive Control (MPC) are also implemented above the basic regulatory control. Finally, the methodology is extended further to develop training simulator by integrating the simulation case study to the existing Distributed Control System (DCS). To demonstrate the effectiveness of the proposed methodology, an industrial case study of the primary unit of the crude oil refinery and a laboratory scale packed distillation unit is thoroughly investigated. The presented methodology is a promising approach for the operability study and optimization of production facilities and can be extended further for an intelligent and fully-supportable decision making

Doctor of Philosophy

dissertationThree phase ebullieted bed reactors are appropriate for processing petroleum residues, bitumen and bitumen derived liquids. The length of a three phase ebullieted bed reactor, which is long at the commercial scale, can be reduced, for process research and development studies in the laboratory, by decreasing the superficial liquid velocity and solid particle size. It is necessary to maintain same phase holdups of the commercial reactor, to simulate the process kinetics in a laboratory reactor. Similarity criteria that ensured identical phase holdups in commercial and laboratory units were identified, through extensive similitude studies. These criteria required the equality o f six dimensionless numbers. It was impractical to establish all the parameters in the set of dimensionless numbers at the desired values for reacting systems. Therefore, a procedure was developed to achieve similarity by varying a minimum number of parameters, such as liquid and gas superficial velocities and particle size. This resulted in two conditions, which when satisfied yielded essentially equal holdups in the two reactors. These criteria and procedure were validated using the generalized wake model and experimental data for three phase systems. The similitude studies identified the importance of the bubble rise velocity for scale down. Two different approaches were developed to predict the bubble rise velocity in three phase ebullieted beds. In the first approach, a mathematical model was developed to predict the volume of a single bubble generated at an orifice in a gas-liquid system at a constant gas flow rate. The model was based on a rigorous bubble closure mechanism and incorporated the interaction between the primary bubble and subsequent bubbles formed at the orifice at high gas flow rates. The model also calculated the distance traveled by the bubble from the orifice before it detached. The model is applicable for both viscous and nonviscous liquids and for systems over wide ranges of hydrodynamic properties. The model was validated by comparison with the available experimental data and it was found that this model represented an improvement over previous models. This model was used to approximate the value of the bubble size in a high pressure three phase ebullieted bed with small solid particles. In the second approach the concept of effective bubble rise velocity was introduced. The generalized wake model equations were manipulated to give correlations for the effective bubble rise velocity at atmospheric pressure. The parameters for the correlations were liquid and gas superficial velocities, liquid viscosity, surface tension and solid particle size. These correlations were categorized as per the type of three phase system, solid particle size and liquid and gas superficial velocities. Flow transition liquid velocities for various three phase systems were identified. Forms of the correlations were explained by addressing various hydrodynamic phenomena for three phase ebullieted beds such as flow regimes and their transitions, flow transition liquid velocity, solid wettability, bubble behavior, apparent bed viscosity and the effect of solid particles. The performance of the correlations was tested with experimental phase holdup data. The influence of pressure on bubble behavior and bubble rise velocity in a three phase ebullieted bed was considered. This led to the introduction of a pressure factor in the bubble rise velocity correlations. The modified correlations were used to predict the bubble rise velocity in three phase ebullieted bed operating at high temperature and high pressure. The predictions of the modified bubble rise velocity correlations were evaluated, using the concept of drift flux, against experimental plots available from the literature. The trends of drift flux vs. gas holdup in the plots were found satisfactory. Values of the gas and liquid densities, liquid viscosities and surface tensions at high temperature and high pressure were required for reactor scale down. A plot for temperature versus weight fraction distilled up to 813 K was obtained by simulated distillation for the native bitumen. A method was then developed to extrapolate the low temperature (813 K -) SIMDIS curve to high temperature (813 K + ) region by matching the measured value of specific gravity of the native bitumen with the specific gravity calculated from the extrapolated curve. The extrapolated SIMDIS curve was used to develop a predictive correlative procedure for estimating the viscosity and surface tension of bitumen fractions and bitumen at high temperature and high pressure. The predictive method identified a new mixing rule for fractions of heavy feeds, where the viscosities of the individual fractions vary over a few orders of magnitude. An overall procedure for scaling down a commercial three phase ebullieted bed reactor to a laboratory scale was then developed. The procedure ensured reduction in reactor length and maintained identical phase holdups and bubble rise velocity in both the reactors. The space velocity in the laboratory reactor was adjusted to achieve similar intraparticle mass transfer as the commercial reactor. Using the methods mentioned above for calculating the bubble rise velocity and physical properties of the feed and the overall scale down procedure, a detailed design of a laboratory scale three phase ebullieted bed reactor was carried out. This reactor can be used to carry out process development studies for hydrotreating/hydrocracking of bitumens and bitumen derived liquids in the laboratory, under conditions similar to the commercial reactor

Modeling and Simulation of Cluster Formation Effects in Riser Section of Fluid Catalytic Cracking Unit

The riser reactor is a highly effective reactor for fast gas-solid reaction systems. In spite of extensive research in this area, the degree of understanding of these types of reactors is different. In this work, mathematical model for riser reactor is developed based on the conservation equations for non-isothermal riser reactor linked with hydrodynamics. The cracking reaction is described based on four lump kinetic models and the hydrodynamic is based on cluster based approaches. The advantage of this work is the model developed based on the concept of cluster formation. Resulting riser FCC models calculate flow and reaction parameters including conversion rates and product yields to determine performance. The resulting riser model is simulated using numerical method of Dormand-Prince, a member of Runge-Kutta family of ordinary differential equation (ODE) solvers, via MATLAB Environment. Simulation results of the base case riser model agree with plant data sufficiently well with majority of the data deviation lies between 1 and 5%. Simulation studies were also performed using the model to encompass the effect of inlet catalyst temperature, and catalyst-to-oil (CTO) ratio on reactor performance. The gasoline yield did not show direct relation with inlet catalyst temperature due to secondary reaction. Increasing CTO ratio increases conversion and other products. Further increase of CTO ratio beyond 10 did not increase the conversion and yield of gasoline due to increase in coking. These findings are useful to determine coking limit for CTO ratio and its cost. Finally, the effect of cluster formation on riser performance was investigated. Conversion was increased by 9% with the formation of cluster and an additional densification by 25% due to residence time of cluster increased. The reason for higher conversion may be explained by the formation of cluster which increases the residence time of catalyst inside the reactor. However, the formations of cluster had inverse effect on the production of gasoline, which drops by 5%, due to high temperature drop attained and higher residence time of catalyst. In summary, the objectives of this study, which are to develop mathematical model and build understanding on the parameters that influence the performance of riser reactor, have been achieved

Analysis and structural design of on-line process optimization systems.

Imperial Users onl