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Distributed addition of gaseous reactants in fluidized beds Al-Sherehy, Fahad A.

Abstract

The development of new chemical technologies is moving toward unconventional reactor design to improve old technologies and to solve new challenges. One configuration that requires attention involves the distributed addition of reactants along the reactor height. For some reactions, e.g. partial oxidation, this has potential in overcoming safety and thermodynamics constraints, while improving selectivity. One of the most important technologies is partial oxidation of paraffins to olefins. Ethane oxidative dehydrogenation to ethylene is a good example. However, this technology is constrained by a safety concern, since the reactor feed consists of ethane and oxygen such that composition might fall inside the explosion limits. In this case, the reactor feed composition should be changed to a composition far outside explosion limits. As a result, the reactor performance may be well below optimal, rendering the process uneconomic. To avoid such constraints one reactant may be fed in a distributed manner along the bed height. In the current study, the effect of feed distribution on the reactor performance and hydrodynamics is investigated by studying: horizontal jet penetration, pressure fluctuations, gas mixing and reaction. Most of the experiments were carried out in a column of 152 mm diameter at atmospheric temperature and pressure with fluid catalytic cracking catalyst particles, while reactor experiments were conducted in an industrial pilot plant of diameter 97 mm operating at a temperature of 265 °C and a pressure of 202.6 kPa absolute with oxidative dehydrogenation catalyst particles. The results provide a better understanding of the effect of injecting secondary gas through one or several injection points along the bed height. Significant changes in a fluidized bed hydrodynamics were encountered when part of the total gas flow was distributed along the fluidized bed height compared to the case when all the gas is fed through the main distributor. For constant total gas flow, introducing secondary gas decreases the primary gas velocity, leading to lower bed expansion and smaller bubbles. Based on the pressure fluctuations measurements, fluidization quality generally improved when secondary gas was distributed over several levels along the bed. Such a configuration helps to control the bubble size inside the fluidized bed. The secondary gas concentration profile at the injection level was almost uniform for higher secondary gas injection velocity and lower primary gas velocity. However, the concentration profile was not uniform for lower injection velocities or when the primary feed superficial gas velocity was high. In addition, secondary gas becomes readily mixed with bed material after a short distance above the injection level. Back mixing is primarily influenced by the primary feed superficial gas velocity rather than by the secondary gas injection velocity. Experiments show that regardless of the secondary gas injection velocity, the secondary gas was barely detected below the injection level at low UP. However, at higher Up it was detectable. Comparing axial dispersion coefficients and mass transfer coefficients for a bed with all the gases fed through the windbox with a bed where part of the total gas was distributed at one or more higher levels, axial dispersion and mass transfer decreased significantly. The effect of distributing oxygen along a fluidized bed reactor for ethane oxidative dehydrogenation reaction was studied experimentally in a pilot-scale unit. The results confirmed that oxygen distribution along the reactor height allows the reactor to be operated safely with promising performance. Two models are proposed to simulate the process of gaseous feed distribution along a fluidized bed reactor height. Both models provide good fits to the experimental gas mixing data. In addition, one of the models was validated by the reaction experiments data and provided a good fit to the measured reactor performance. These models provide a good starting point for future application of distributed feed to fluidized bed reactors.

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