In an LC-FiningTM hydroprocessor, the orientation of the free surface in the plenum chamber and the efficiency of the internal gas-liquid separator impact the amounts of the gas and the liquid passing through individual bubble cap risers in the grid. Various gas and liquid flowrates were thus passed through a single bubble cap riser to determine resulting bubble properties. Bubble sizes correlated well with energy dissipation rate and gas-to-liquid velocity ratio to the power of −0.4 and 0.6, respectively, in agreement with theory on the turbulent breakup of dense dispersions. Bubbles were also influenced by the gas-liquid distributor geometry, where the bubble cap produced larger bubbles than a perforated plate. Finally, the size of bubbles sparged in the plenum marginally affected overall gas holdup, suggesting that a bubble cap distributor redesign should be prioritized over upstream gas introduction equipment to optimize the performance of the hydroprocessor.

A new multiphase fluid dynamics model for a commercial ebullated bed hydroprocessor was developed. The impact of the gas-liquid distribution system is now explicitly included through new submodels for bubble size distribution and drag coefficients. The size distribution submodel is coupled with the existing gas-liquid separation submodel to better predict recycled gas and liquid flow rates. Either the mass of the catalyst inventory or recycle pump curve can be specified as inputs to converge the model; the former is not always well known during operation in which case the latter can be used after making a few assumptions. A sensitivity analysis was performed to study the impact of fresh treat gas velocity, catalyst mass, phase properties, and reactor internals on recycled gas and liquid flow rates, bubble size distribution, and bed liquid holdup. A 0.2 mm shift in bubble size distribution toward larger sizes was found to significantly increase bed liquid holdup, suggesting that distributor modification/redesign could help improve the capacity of the hydroprocessor.

This study investigated the effect of pressure on individual bubble drag for a contaminated polydisperse swarm. Bubble size distributions and individual bubble drag coefficients were experimentally determined at four different pressures (0.14, 0.46, 2.00, 4.00 MPa) to establish if there is a relationship between bubble size distribution and individual drag coefficient and quantify the effect of pressure on bubble drag. The analysis is limited to bubbles with diameters between 0.4 and 6 mm. Individual bubble drag coefficients were found to decrease with pressure at constant gas hold-up. This effect is attributed to changes in bubble size distribution driven by pressure. As pressure increased, the bubble size distribution shifted towards smaller sizes. Relatively speaking, these smaller bubbles induce marginal liquid perturbations and could potentially dampen turbulence from other sources, resulting in reduced drag. A new drag model has been proposed to improve CFD simulation of bubble columns at elevated pressures. (c) 2020 Elsevier Ltd. All rights reserved.

The Vietnam War, widely considered the worst foreign policy debacle in American history, remains the most controversial event of the twentieth century. Much criticism for Vietnam involvement stems from two sources: 1) disapproval with how American leadership conducted the war, and 2) disagreement over the reason for the conflict in the first place. Few historians, if any, dispute the first criticism. The historical community remains divided, however, in terms of a definitive position on the basis or origin for the conflict. For a holistic approach to the origin of the Vietnam War, one must first elucidate the conception of American intervention in the region, including “why” and “how” it arose. Any analysis of American involvement in Vietnam must begin with President Truman and his administration following the conclusion of the Second World War. This can only be accurately accomplished viewed in the context of US foreign policy during the Cold War. The initial American involvement in Southeast Asia in the context of the developing Cold War must be thoroughly examined to more fully understand the origins of the Vietnam War. Considering the increasingly complex situation in Southeast Asia following the Second World War, Truman and his administration acted consistently, bearing in mind the vested interests of the United States, their Allies, and the people of Southeast Asia, in light of the threat of Communist expansion in the region and across the world.