Browsing by Author "Thomas, Benku"

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    Cold model of a vibrated-bed microreactor capable of varying Peclet number at fixed weight hourly space velocity providing a tool for simulating an important feature of the reaction kinetic scene in large catalytic fluid beds
    Benge, G. Gregory (Virginia Tech, 1992-03-04)
    A cold-flow model of a vibrated-bed microreactor has been designed and tested with capability for varying the level of gas dispersion (characterized by axial Peclet number) at a fixed weight hourly space velocity (WHSV). A tool has thus been provided whereby an important feature (viz., gas dispersion) of the reaction kinetic scene in large catalytic fluid beds can be simulated on a microscale, using approximately 5 grams of catalyst. Realization of a hot design of the microreactor (a task for another student) should permit the industrial chemist or chemical engineer, working at laboratory bench scale, quickly and inexpensively, to determine the sensitivity of a cataly1ic reaction to fluid-bed gas dispersion. The new microreactor exploits the coherent-expanded (C-E) vibrated-bed state, and is perhaps the first technical use of this state. The C-E state is achieved by subjecting a shallow layer of a fine powder to vertical sinusoidal vibration. The microreactor comprises a rectangular horizontal duct, 12.7 mm in height, 25.4 mm in width, variable in length, and with nonporous floor and walls. The microreactor is charged with a powder, such as fluid cataly1ic cracking (FCC) catalyst, at a compacted depth of I mm, and is vibrated at ~15 Hertz and amplitude of ~3 mm. Under influence of this vibration, the powder expands, displaying the C-E state. Between a phase angle of ~50° and an angle of ~150°, the powder assumes a depth of ~4 mm (i.e., expanded 4-fold from its compacted depth). Later, in each vibration cycle, the powder expands further. At ~300° phase angle, the powder reaches ~12.7 mm (i.e., collides with the roof of the microreactor duct).
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    Sensitivity of methanol-to-olefin reaction to axial gas dispersion: determination in a vibrated-bed microreactor
    Tshabalala, Samuel Nhlanganiso (Virginia Tech, 1995)
    A microreactor has been developed to study the sensitivity of Methanol-To-Olefin (MTO) reaction to axial gas dispersion. It comprises a rectangular horizontal duct, 12.7 mm in height, 25.4 mm in width, variable in length, charged with a 1-mm-deep layer of HZSM-5 catalyst. The microreactor is constructed of aluminum alloy and heated with cartridge heaters. A glass reference duct with the same catalyst loading is mounted directly above the microreactor to provide visual check of the coherent-expanded vibration state achieved by vibrating the setup at 24 hertz and 4.3-mm vertical displacement. In this. state, catalyst powder expands to full duct height during a portion of each vibration cycle, with intense vertical mixing of powder and little horizontal. Axial gas dispersion coefficient varies linearly with superficial gas velocity, and axial Peclet Number (Peax) can be studied over a wide range of values simply by varying duct length while holding weight hourly space velocity constant. Conducting the MTO reaction in microreactors of 7.62-, 15.24-, and 22.86-cm length (Peax = ca. 2, 9, and 19 respectively) revealed the reaction to be sensitive to axial gas mixing. Trend in light olefin yield versus Peax agrees with earlier turbulent fluid-bed data. Loss in olefins with increase in axial gas dispersion (decrease in Peax) suggests that a circulating fluid bed may be the preferred reactor for this reaction. Researchers can use the microreactor to determine, quickly and inexpensively, how reaction outcomes vary with axial gas dispersion. The microreactor could help R&D managers to avoid expense of a fluid bed R&D effort where an economically significant outcome of a reaction is acutely sensitive to axial gas dispersion, and where a fixed bed is an acceptable alternative.
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    Shallow vibrated particulate beds - bed dynamics and heat transfer
    Thomas, 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.