Browsing by Subject "Dynamic modeling"
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Item Development of effective medium models for quantification of elastic properties and modeling of velocity dispersion of saturated rocks(2015-12) Sayar, Paul Mikhaël; Torres-Verdín, Carlos; Daigle, Hugh; Spikes, Kyle T; Olson, Jon; Sepehrnoori, KamyElastic effective medium theory (EMT) relates to quantitative rock physics modeling that calculates macroscopic properties of a mixture by incorporating the individual elastic properties, the volume fractions, and the spatial arrangement of the constituents that make up the rock. Despite the valuable merits of effective medium models, these theories exhibit limitations that require further investigation. Common instances are the non-unique configurations of the rock’s elements that give rise to identical wave velocities and the limiting assumption that rocks are purely elastic materials. Consequently, direct applications of classical EMTs can yield inaccurate and non-unique estimates of rock fabric properties that directly affect the assessment of elastic properties. The primary purpose of this dissertation is to improve the reliability of rock physics models based on the use of effective medium theories. In the first part, a rock physics model is developed for reliable estimation of velocities and elastic properties for sandstone-shale laminated rocks that are assumed to be vertical transverse isotropic (VTI). The new model is concerned with the reproduction of typical geological features and petrophysical properties of such formations that exhibit complex rock fabric. Isotropic and anisotropic versions of the self-consistent approximation and the differential effective medium theory, and Backus average are invoked to compute the effective medium’s stiffness tensor. The rock is separated into volumes of sandstone (regarded as isotropic) and shale (regarded as VTI), which are treated separately to reliably reproduce the spatial arrangement of the individual components included in the rock. Shale volumes enclose penny-shaped cracks and clay platelets aligned in the horizontal direction. Total porosity is divided into percolating porosity, isolated pores, and aligned fractures. The new simulation method is implement in three wells in the Haynesville shale and the Barnett shale. Estimates of elastic properties are verified when calculated velocities and sonic logs are in agreement. All relative differences between simulated and measured velocities are below 5.4%. To reduce non-uniqueness, electrical resistivity is calculated with modified effective medium theories and a procedure to compute Stoneley velocity is combined with the rock physics model. A method is advanced to calculate stress distribution and fracture initiation pressure around potential wellbores drilled horizontally in VTI rocks from the stiffness tensor obtained with the improved rock physics model. Effects of degree of anisotropy and elastic properties on fracture initiation pressure are investigated to determine a criterion to locate optimal depths along a vertical well to place a horizontal well. In the second part of the dissertation, an effective medium model is developed for reproduction of four of the main mechanisms of dispersion and attenuation of acoustic waves in saturated rocks. Simple and practical alternatives are introduced for effective medium modeling that account for dispersion mechanisms due to fluid flow inside the pore space. Biot’s flow and squirt flow effects are simulated by the calculation of frequency-dependent equivalent bulk and shear moduli for the solid background of the rock. When equal to the static moduli of minerals that compose the matrix of a rock at low frequencies, dynamic moduli of the solid background become complex at high frequencies and their absolute value increases. Frequency-dependent solid moduli are used as elastic properties of the matrix material in which fluid-filled porous inclusions are then added with dynamic self-consistent approximations for replication of acoustic scattering phenomena due to stiff pores and cracks. Resulting elastic features of the saturated medium calculated with the frequency-dependent effective medium model display viscoelastic behavior. Velocity predictions are conducted on synthetic examples to investigate conditions where dynamic rock physics modeling is necessary to obtain accurate elastic properties.Item Dynamic modeling and analysis of an automatic reconfigurable shipboard power system(2006-05) Park, Jerad Allen; Longoria, Raul G.In future United States Naval warships, all ship power will be converted to electricity and distributed to a complex system of loads through transmission lines. Because these loads are dynamic by nature, it is necessary to develop a control system that can both respond to these changes and rapidly alter the power system interconnections for protection and efficiency. This thesis describes the development of models and analysis methods for evaluating a centralized method of shipboard reconfiguration control. This control utilizes sub-cycle, time-based measurements used to estimate equivalent impedances that characterize the dynamic power system. An optimization algorithm determines steady state switch configurations and a logic routine checks the validity of intermediate switch states. Several test cases are simulated to demonstrate emergency and non-emergency reconfiguration. The speed and accuracy of reconfiguration are dependent on three conditions: current and voltage noise level, the algorithm used to determine equivalent impedances, and the dynamic nature of the load change. With adequate computing resources, careful selection of filter parameters, i.e., the cessation of a transient, and minimal noise, reconfiguration within one cycle is possible.Item Dynamic modeling and analysis of proton exchange membrane fuel cells for control design(2016-05) Headley, Alexander John; Chen, Dongmei, Ph. D.; Wei, Li; Beaman, Joseph J; Ezekoye, Ofodike A; Mullins, Charles BThis dissertation seeks to address a number of issues facing the advancement of Proton Exchange Membrane (PEM) fuel cell technology by improving control-oriented modeling strategies for these systems. Real-time control is a major ongoing challenge for PEM fuel cell technologies, particularly with regards to water and temperature dynamics. This can lead to a number of operational concerns, such as membrane flooding and dehydration, which can seriously diminish the efficiency, reliability, and long term health of the system. To combat these issues, comprehensive models that are capable of capturing the dynamics of the key operating conditions and can be processed in real time are needed. Also, given the inherently distributed nature of the system, such a model would ideally account for the changes in the conditions from cell-to-cell in the stack, which can be very significant. With this goal in mind, the main focus of this dissertation is the development and experimental validation of control-oriented modeling techniques for PEM fuel cell stacks. The first major work in this study was the verification of a relative humidity model in response to varying loads. Through this work, a multiple control volume (CV) approach was developed and experimentally validated to model the distribution of operating conditions more accurately while keeping the computational expense sufficiently low. To optimize the modeling efforts, further analysis of the temperature and vapor distribution was performed starting from first principles. This led to the creation of various techniques to optimally size CVs based on the parameters and operating conditions of the system in question. Finally, it was noted throughout the testing that the performance of the membrane electrolyte assemblies in the test stack declined significantly from their initial state. To compensate for this, a Kalman filter was implemented to quantify the membrane degradation. SEM analysis of membranes from the test stack confirmed the validity of this technique. This work can be used to significantly improve real-time models for PEM fuel cells for model-based control applications.Item Dynamic modeling of post-combustion amine scrubbing for process control strategy development(2016-05) Walters, Matthew Scott; Rochelle, Gary T.; Edgar, Thomas F.; Baldea, Michael; Akella, Maruthi R; Chen, EricIntensified process designs with advanced solvents have been proposed to decrease both capital and operating costs of post-combustion carbon capture with amine scrubbing. These advanced flowsheets create process control challenges because process variables are designed to operate near constraints and the degrees of freedom are increased due to heat recovery. Additionally, amine scrubbing is tightly integrated with the upstream power plant and downstream enhanced oil recovery (EOR) facility. This work simulated an amine scrubbing plant that uses an intercooled absorber and advanced flash stripper configuration with aqueous piperazine to capture CO2 from a 550 MWe coal-fired power plant. The objective of this research was to develop a process control strategy that resulted in favorable closed-loop dynamics and near-optimal conditions in response to disturbances and off-design operation. Two models were created for dynamic simulation of the amine scrubbing system: a medium-order model of an intercooled absorber column and a low-order model of the entire plant. The purpose of the medium-order model was to accurately predict the absorber temperature profile in order to identify a column temperature that can be controlled by manipulating the solvent circulation rate to maintain a constant liquid to gas ratio. The low-order model, which was shown to sufficiently represent dynamic process behavior through validation with pilot plant data, was used to develop a plantwide control strategy. A regulatory control layer was implemented and tested with bounding cases that represent either electricity generation requirements, CO2 emission regulations, or EOR constraints dominating the control strategy. Satisfying the operational and economic objectives of one system component was found to result in unfavorable dynamic performance for the remainder of the system. Self-optimizing control variables were identified for the energy recovery flowrates of the advanced flash stripper that maintained good energy performance in off-design conditions. Regulatory control alone could not satisfactorily achieve the set point for CO2 removal rate from the flue gas. A supervisory model predictive controller was developed that manipulates the set point for the stripper pressure controller in order to control removal. The straightforward single-input, single-output constrained linear model predictive controller exhibited a significant improvement compared to PI control alone.Item Dynamic modeling, optimization, and control of monoethanolamine scrubbing for CO2 capture(2012-08) Ziaii Fashami, Sepideh; Rochelle, Gary T.; Edgar, Thomas F.; Seibert, A F.; Masada, Glenn Y.; Freeman, Benny D.This work seeks to develop optimal dynamic and control strategies to operate post combustion CO2 capture in response to various dynamic operational scenarios. For this purpose, a rigorous dynamic model of absorption/stripping process using monothanolamine was created and then combined with a simplified steady state model of power cycle steam turbines and a multi-stage variable speed compressor in Aspen Custom Modeler. The dynamic characteristics and interactions were investigated for the plant using 30% wt monoethanolamine (MEA) to remove 90% of CO2 in the flue gas coming from a 100 MW coal-fired power plant. Two load reduction scenarios were simulated: power plant load reduction and reboiler load reduction. An ACM® optimization tool was implemented to minimize total lost work at the final steady state condition by adjusting compressor speed and solvent circulation rate. Stripper pressure was allowed to vary. Compressor surge limit, run off condition in rich and lean pumps, and maximum allowable compressor speed were found as constraints influencing the operation at reduced loads. A variable speed compressor is advantageous during partial load operations because of its flexibility for handling compressor surge and allowing the stripper and reboiler to run at optimal conditions. Optimization at low load levels demonstrated that the most energy efficient strategy to control compressor surge is gas recycling which is commonly applied by an anti-surge control system installed on compressors. Trade offs were found between initial capital cost and optimal operation with minimal energy use for large load reduction. The examples are, designing the stripper in a way that can tolerate the pressure two times larger than normal operating pressure, over sizing the pumps and over designing the compressor speed. A plant-wide control procedure was used to design an effective multi-loop control system. Five control configurations were simulated and compared in response to large load variations and foaming in the stripper and the absorber. The most successful control structure was controlling solvent rate, reboiler temperature, and stripper pressure by liquid valve, steam valve, and compressor speed respectively. With the investigated disturbances and employing this control scheme, development of an advanced multivariable control system is not required. This scheme is able to bring the plant to the targeted set points in about 6 minutes for such a system designed initially with 11 min total liquid holdup time.Frequency analysis used for evaluation of lean and rich tanks on the dynamic performances has shown that increasing the holdup time is not always helpful to damp the oscillations and rejecting the disturbances. It means there exists an optimum initial residence time in the tanks. Based on the results, a 5-minute holdup can be a reasonable number to fulfill the targets.