Browsing by Author "Liu, Y.A."
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- Bed dynamics and heat transfer in shallow vibrated particulate bedsMason, Mark Olin (Virginia Polytechnic Institute and State University, 1990)A vibrated bed is a mobile layer of solid particles contained in a vessel that is vibrated vertically. This study investigates bed dynamics and heat transfer from a vertical surface in shallow vibrated beds in absence of aeration. In general, "shallow" means a depth-to-width ratio less than one. In this study, bed depth is 30 mm, and this ratio is about 0.2. All experiments are at 25 hertz and at vibrational amplitudes affording peak accelerations between 2 and 7 times gravity. The study uses spherical glass beads of two densities and "Master Beads," nearly spherical particles of a crude, dense alumina, in size fractions from 63 to 707 micrometers. A disc embedded in the vessel floor, vibrated at 4.5 kilohertz, gives data on bed-vessel separation, showing it to occur later than predicted by plastic, single-mass models. The delay is attributed to bed expansion, monitored by piezoelectric force gauges mounted on floor and wall of the vessel. In large-particle beds, bed-vessel collision occurs simultaneously everywhere. In small-particle beds, exhibiting an uneven top surface, collision occurs first at the side walls and moves toward the center. In small-particle beds, pressure gradients appearing during the bed's free flight drive a horizontal component of particle circulation from the vessel's side walls toward its center. An apparent viscosity of the bed, estimated crudely by pulling a rod through it, influences this component's velocity. In beds of large particles, circulation is almost entirely vertical, a layer of two or three particles moving downward at a wall, and a slow return flow moving upward elsewhere. The study confirms the downward wall motion to be driven by friction. Heat transfer closely follows trends in rate of circulation. Greater dependence upon vibrational intensity is seen in small-particle beds. Values as high as 578 W/m²-K are measured. Comparison of vertical-surface heater geometry with an earlier horizontal tube shows the former to be generally superior for surface-to-bed heat transfer.
- Development of a microreactor system for unsteady-state Fischer- Tropsch synthesisWhiting, Gary Ken (Virginia Polytechnic Institute and State University, 1985)Vibrofluidized microreactor systems have been developed for studies of unsteady-state Fischer-Tropsch synthesis. This development is aimed at preventing carbon deposition on a fused-iron catalyst in a novel reactor called the “heat-tray.” This reactor involves a supernatant gas flowing over a shallow fluidized bed of catalyst particles. Three systems were built: (1) a vibrofluidized-bed microreactor system for obtaining baseline carbon deposition infonnation under industrially important reaction conditions; (2) a sliding-plug vibrofluidized-bed microreactor system for rapid switching of feed gases in the F-T synthesis; and (3) a cold-flow microreactor model for studying the gas mixing characteristics of the sliding-plug vibrofluidized-bed microreactor. The results show that catalyst defluidization occurred under steady-state synthesis conditions below 395°C using a feed gas of H₂/CO ratio of 2:1 or less. Above 395°C, the probability of hydrocarbon chain growth (α) on the fused-iron catalyst was low enough (α < 0.50) to prevent accumulation of high-molecular-weight species that cause defluidization. Carbon deposition was rapid above 395°C when a feed gas of H₂/CO ratio of 2:1 or less was used. Spent catalyst fractions in the form of free-flowing catalyst and "bugdust" were quantitatively analyzed for carbon and iron. Mössbauer spectroscopic analysis of free-flowing catalyst showed mainly Hägg carbide (x-Fe₅C₂) and magnetite (Fe₃O₄) with a smaller fraction present as α-Fe. Scanning electron microscopic analysis of the bugdust revealed a mass of highly porous, fine particles with a high carbon content (18-30 wt%). Cold-flow microreactor model studies show that rapid (on the order of seconds), quantitative switching of feed gases over a vibrofluidized-bed of catalyst could be achieved. Vibrofluidization of the catalyst bed induced little backmixing of feed gas over the investigated flow-rate range of 417 to 1650 actual mm³/s. Further, cold-flow microreactor model studies showed intense solid mixing when a -150+300 µ bed of fused-iron catalyst was vibrofluidized at 24 cycles per second with a peak-to-peak amplitude of 4 mm. The development of this microreactor system has provided an easy way of accurately determining integral fluid-bed kinetics in a laboratory reactor. Further, the unique ability of the microreactor system to rapidly switch feed gases over an intensely-mixed solid has important applications in chemical kinetics and reaction engineering.
- Hydrodynamics and heat transfer in shallow fluidized bedsYang, Jyh-Shing (Virginia Polytechnic Institute and State University, 1986)The use of shallow fluidized beds for heat exchange has been suggested because they give high bed-to-surface heat transfer rate and require very low bed pressure. However, in comparison with research on deep fluidized beds, only relatively few studies have been devoted to heat transfer in shallow beds, and results from the available literature are often inconsistent. This study represents an integrated research on the hydrodynamics and bed-to-surface heat transfer in shallow beds. The results from this study provide the quantitative basis for the design and efficient operation of shallow fluidized-bed heat-recovery systems. Based upon their physical appearance, shallow fluidized beds have been categorized into nine different types. A "phase diagram" (plot of superficial gas velocity versus static bed height) can be used to delineate the ranges of fluidization variables within which each type of shallow beds will be seen. Pressure-drop data in gas flowing upward through a shallow bed reflect pressure recovery in jets formed immediately above a gas distributor at the bottom of the bed. Pressure-recovery data provide an effective means of distinguishing a shallow bed from a deep one, and suggest that the power consumption across a fluidized bed can be reduced dramatically by dividing a single deep bed into many multi-staged shallow beds. A computerized light probe has been developed for measurements of particle volume-fraction distribution and its statical fluctuation (standard deviation). These data have been shown to quantitatively define: (1) different types of shallow beds; (2) relative magnitude of solid mixing; (3) bed surface and bed height; and (4) jet penetration depth. Based upon observations of the hydrodynamic behavior of shallow fluidized beds, three regions can be identified for heat-transfer applications: a jet-affected region at the bottom, a free-board region at the top, and, sandwiched between theses, a homogeneous region. Only heat-transfer data in the homogeneous region are sufficiently well-behaved to be subjected to quantitative correlation in terms of fluidization variables. For relatively coarse particles (Geldart's Group B particles) the vigor of solid mixing can be the most important factor in affecting the heat-transfer performance. Bed voidage and static electricity effects are found to be important for smaller and/or lighter particles (i.e., Geldart's Group A particles).
- Prolog and artificial intelligence in chemical engineeringQuantrille, Thomas E. (Virginia Tech, 1991)This dissertation deals with applications of Prolog and Artificial Intelligence (AI) to chemical engineering, and in particular, to the area of chemical process synthesis. We introduce the language Prolog (chapters 1-9), discuss AI techniques (chapters 10-11), discuss EXSEP, the EXpert System for SEParation Synthesis (chapters 12-15), and summarize applications of both AI and Artificial Neural Networks (ANNs) to chemical engineering (chapters 16-17). We have developed EXSEP, a knowledge-based system that performs separation process synthesis. EXSEP is a computer-aided design tool that can generate flowsheets using any combination of high-recovery (sharp) and low-recovery (nonsharp) separations, using a variety of separation methods with energy and mass separating agents. EXSEP generates separation process flowsheets using a unique plan-generate-test approach that incorporates computer-aided tools and techniques for problem representation and simplification, feasibility analysis of separation tasks, and heuristic synthesis and evolutionary improvement. A difficult problem in knowledge-based approaches to chemical engineering is the "quantitative or deep knowledge dilemma." Experience has shown that a strictly qualitative knowledge approach to chemical process synthesis is insufficient. However, including rigorous quantitative analysis into an expert system is cumbersome and impractical. EXSEP overcomes this deep-knowledge dilemma through a unique knowledge representation and problem-solving strategy that includes shortcut design calculations. These calculations are used as a feasibility test for all separations; no separation is chosen by EXSEP unless it is deemed as thermodynamically feasible through this quantitative, deep-knowledge, engineering analysis. We apply EXSEP for the flowsheet synthesis of several industrial separations problems. The results show that EXSEP successfully generates technically feasible and economically attractive process flowsheets accurately and efficiently. EXSEP is also user-friendly, and can be readily applied by practicing engineers using a personal computer. In addition, EXSEP is developed modularly, and can be easily expanded in the future to include additional separation methods.
- Shallow vibrated particulate beds - bed dynamics and heat transferThomas, Benku (Virginia Polytechnic Institute and State University, 1988)Particulate beds which are mobilized and expanded by the application of mechanical vibrations are called vibrated beds. These beds are generally defined as shallow, if the depth-to-width ratio is less than unity. The dynamics of shallow vibrated beds and the heat transfer from immersed tubes to such beds are investigated using a vibrational frequency of 25 Hz. The vibration equipment is designed to minimize distortions in the applied displacement waveform. Transducers used are of a sufficiently high frequency response to accurately follow the variation of bed properties over a vibrational cycle. An electronic circuit is designed to exactly phase-match data collected by a transducer with the vibrational displacement. The circuit may also be used to trigger a strobe lamp at any phase angle, thus permitting an accurate examination of the evolution of bed characteristics over a cycle. Measurements of floor pressures beneath the bed, indicate cyclic characteristics, caused by the bed motion. Horizontal floor-pressure gradients cause the bed to pile up or bunker within the vessel. In bunkered beds, particle motion is determined by horizontal gas flows, and a compaction wave which propagates diagonally through the bed during the bed-vessel collision. In non-bunkered beds, particle motion is driven largely by wall friction. The observed instant of bed-vessel separation lags the theoretical prediction by several degrees, most likely because of bed expansion associated with the bed lift-off. Different "states" of shallow vibrated beds are identified, each with a unique set of characteristics. One state which exists in ultra-shallow beds of depths between 6 and 15 particle diameters is characterized by a high porosity and good gas-solid interaction, making it potentially useful for studies of reaction kinetics. Surface-to-bed heat-transfer coefficients are measured for Master Beads and glass beads, and found to vary with particle size and vibrational intensity. Heat-transfer coefficients as high as 484 W/m²-K are obtained. Heat transfer depends on particle circulation and the formation of air gaps which periodically surround the heater surface. A simplified theoretical formulation for the heat-transfer coefficient appears to qualitatively predict observed trends in heat transfer.
- Systematic synthesis of sloppy multicomponent separation sequencesCheng, Shueh-Hen (Virginia Polytechnic Institute and State University, 1987)An important process-design problem in multicomponent separations is separation sequencing, which is concerned with the selection of the best method and sequence for a separation system. Essentially all of the published work on this subject has been limited to high-recovery or sharp separations, in which each component to be separated appears in one and only one product stream. In industrial practice, however, it is often useful to permit components that are being separated to appear in two or more product streams. This type of separation results in products that have overlapping components and is called nonsharp or sloppy separations. The present work proposes and demonstrates a simple and practical approach to the systematic synthesis of sloppy multicomponent separation sequences. The task of synthesizing sloppy multicomponent separation sequences is inherently more complicated than that of synthesizing sharp separation sequences as identification of infeasible splits and stream splitting, and transformation of infeasible product sets into equivalent feasible product sets are examples of some difficult tasks involved. A successful synthesis strategy calls for the development of an effective and flexible framework for representing the synthesis problem and for analyzing the feasibility of component splits. In this thesis, we propose a "component assignment diagram (CAD)" for problem representation. It is shown that the use of a CAD allows the design engineer to consider many alternative solutions (or sequences) and eliminate all infeasible component splits. Further, a "separation specification table (SST)" is proposed for feasibility analysis. In particular, the use of an SST provides a means to : (i) properly define and specify key and nonkey components; (ii) quickly identify feasible and infeasible splits; (iii) effectively deal with fuel products with unmatched compo- nent specifications; and (iv) systematically consider sloppy separations with multiple split points. One difficult problem arising from the design of multicomponent distillation columns for sloppy separations is to appropriately specify the distributions of non-key components in both overhead and bottoms products. Despite the importance of these specifications, there is very little information available on this subject in the literature. This thesis reports the results from a comparative study of rigorous simulation and shortcut modeling of multicomponent distillation columns for sloppy separations. One objective was to obtain improved quantitative understanding and practical design insights into the characteristics of nonkey distributions through a shortcut modeling based upon the Fenske equation. One method proposed in this work for synthesizing sloppy multicomponent products is a heuristic method that involves a two-phase approach. The first phase is concerned with the feasibility analysis of splits pertinent to a CAD with the aid of an SST. The second phase is to specify systematically a subsequent split by applying heuristics, an activity that involves the sequential application of several "rank-ordered" heuristics. A unifying approach is proposed and demonstrated for the synthesis of sloppy multicomponent product sets. Its objective is to generate equally good initial separation schemes, featuring as many as three characteristically different sequences, including all-sharp, all-sloppy, and both sharp and sloppy (i.e., mixed separation). The proposed methods have been applied to a number of industrial separation problems. The results show that the new methods offer an extremely useful means for design engineers to generate a number of good initial sequences for obtaining sloppy multicomponent product sets prior to the ultimate separator optimization and heat integration.