Isaac K. Gamwo

BUBBLE COMPUTATION, GRANULAR TEMPERATURES, AND REYNOLDS STRESSES

Bubbles were simulated in a two-dimensional fluidized bed with a constant inlet velocity using two computer codes, the IIT code and the MFIX code. The computational results were compared to the Jung et al. (2005) experiments in a thin bubbling bed of 530 μm glass beads. The use of higher order numerics produces better bubble resolution due to smaller numerical diffusion. The computed bubble sizes and their distributions agreed with the experiments. The simulations show that there is no bubble formation for sufficiently elastic particles. CFD computations and previous experiments show that in the bubbling fluidized beds there exist two random oscillations. The first kind is due to random oscillations of particles and is measured by the conventional granular temperature. The second one is due to motion of bubbles and gives rise to Reynolds type stresses. It is shown that the particle granular temperature is much smaller than the bubble-like granular temperature computed from the average of the normal Reynolds stresses, measured by Cody using a shot noise technique.

Hydrodynamic study in a slurry-bubble-column reactor

Abstract Local gas holdup, bubble diameter and bubble rise velocity in the nitrogen/Drakeol-10 oil system were measured at both laboratory (ambient temperature and pressure) and industrially relevant (high temperature and pressure) conditions using a dual conductivity probe in a slurry-bubble-column reactor. It was found that a constant superficial velocity, the Sauter mean bubble diameter decreases with increasing pressure and temperature. The bubble rise velocity significantly decreases as the pressure increases. Large bubbles rise faster than smaller bubbles. Akita and Yoshidas correlation [1] was utilized to compute the bubble size. Predicted values agree with the experimental data at high temperature.

CFD models for methanol synthesis three-phase reactors: reactor optimization

Abstract Two Computational Fluid Dynamics (CFD) models have been developed for slurry bubble columns. The first model is based on the kinetic theory of granular flow with a measured restitution coefficient in a slurry bubble column. The model was used to predict Air Products/DOE La Porte reactor’s slurry height, gas hold-up and the rate of methanol production. It showed an unfavorable high solids concentration at the bottom of the reactor. The second model with a catalyst viscosity as an input has computed the measured flow patterns and Reynolds stresses in agreement with measurements in a laboratory slurry bubble column. Here, we have rearranged the heat exchangers in the La Porte unit and constructed a CFD model for a baffled reactor that has a higher concentration of the catalyst in the upper portion of the reactor. In this arrangement, the conversion to products is higher than in the La Porte unit, because there is more catalyst in the region of decreased reactant concentration. The baffled arrangement of the heat exchangers prevents the mixing of the catalyst from the upper stage, allowing continued operation of the reactor with a high concentration in the upper stage. Thus, an optimum catalyst concentration is maintained during the course of the production of the liquid fuels.

Hot-gas flow and particle transport and deposition in a candle filter vessel

Abstract Hot-gas flow and particle transport and deposition in an industrial filtration system are studied. The special example of the Siemens-Westinghouse filter vessel at the Power System Development Facility at Wilsonville, Alabama is treated in detail. This tangential flow filter vessel contains clusters of 91 candle filters, which are arranged in two tiers. The upper tier containing 36 candle filters is modeled by six equivalent filters. Seven equivalent filters are used in the computational model to represent the 55 candle filters in the lower tiers. The Reynolds stress turbulent model of FLUENT™ code is used, and the gas mean velocity and root mean square fluctuation velocities in the filter vessel are evaluated. The particle equation of motion used includes drag and gravitational forces. The mean particle deposition patterns are evaluated and the effect of particle size is studied. The computational results indicatethat large particlesof the order of 10 μm or larger are removed from the gas due to the centrifugal forces exerted by rotating flow between the shroud and the refractory.

Nonisothermal Simulation of Flows in the Hot-Gas Filter Vessel at Wilsonville

A numerical simulation of nonisothermal gas flows in the hot-gas filter vessel at the Power Systems Development Facility in Wilsonville, Alabama is presented. The gas velocity and thermal simulations are based on the Reynolds stress transport turbulence model of the FLUENT TM commercial CFD computer code. While earlier modeling studies were limited to isothermal conditions, in this study, the energy transport equation was solved in addition to the mass and momentum equations. The gas flow and temperature field inside the filter vessel were also studied. Results reveal that the gas flow shows strong rotating flow regions outside the shroud and in the upper and lower parts of the body of the vessel. It is also shown that the temperature distribution is nonuniform with somewhat higher temperatures in the upper part of the filter. The simulated results qualitatively agree with the experimental field observations of the filter vessel. filter vessel numerical simulation nonisothermal FLUENT

ULTRASONIC CHARACTERIZATIONS OF GAS HOLDUP IN A BUBBLE COLUMN REACTOR

An indirect method of measuring gas holdup in gas-liquid bubble column reactors has been developed. This technique is based on the analysis of the ultrasonic wave transmitted through the two-phase flow. Gas holdup measurements have been made on water-nitrogen bubble system at ambient conditions. The data clearly show that the attenuation of the sound is a well-defined function of the gas holdup in the bubble column for homogeneous flow regime only (i.e., the superficial gas velocity is 4 cm/sec or less).

Measurements of solids concentration in a three-phase reactor by an ultrasonic technique

An ultrasonic technique was developed to measure the concentration of solids in a three-phase slurry reactor. Preliminary measurements were taken on slurries consisting of water, glass beads, and nitrogen bubbles. The data show that the speed and attenuation of the sound are well defined functions of the solid and gas concentrations in the slurries. A simple model is proposed to correlate the concentration of solids with the measured characteristics of the ultrasonic signals.

Ultrasonic measurement of solids concentration in an autoclave reactor at high temperature

Abstract An ultrasonic technique was developed to measure the slurry concentration in an autoclave reactor. Preliminary measurements were conducted on slurries consisting of molten FT-200 wax, glass beads, and nitrogen bubbles at a typical Fischer–Tropsch (FT) synthesis temperature of 265 °C. The data show that the velocity and attenuation of the sound are well-defined functions of the solid and gas concentrations in the molten FT-200 wax. The results suggest possibilities for directly measuring solids concentration during operation of a three-phase slurry reactor under the reaction temperature and with molten FT-200 wax.

Experimental Investigation and Molecular-Based Modeling of Crude Oil Density at Pressures to 270 MPa and Temperatures to 524 K

A predictive crude oil density model reliable over a wide range of temperature and pressure conditions is increasingly important for the safe production of oil and accurate estimation of oil reserves. While hydrocarbon density data at low-to-moderate temperatures and pressures are plentiful, data and validated models that have reasonable predictive capability for crude oil at extreme temperatures and pressures are limited. In this investigation, we present new experimental density data for crude oil sample obtained from the Gulf of Mexico region. Density data are measured at pressures to 270 MPa and temperatures to 524 K. These conditions simulate those encountered from ultra-deep formations to platforms. These density data points are then used to validate both empirical-based and molecular-based equations of state models. Results show that the molecular-based perturbed-chain statistical associating fluid theory (PC-SAFT) models, without the use of any fitting parameters, predict the crude oil density within 1% of the experimental data. These results are superior to the density predictions obtained with the high-temperature, high-pressure, volume-translated cubic equations of state.

Temperature Distribution in a Demonstration-Scale Filter Vessel With and Without Ash Bridging

The influence of ash bridging on the temperature distribution of the ceramic filters of a demonstration-scale filter vessel is analyzed. The Reynolds stress turbulence model of FLUENT™ code is used to study the gas flow behavior inside the filter vessel. Particle equations of motions are employed, and transport and deposition of the micron-size aerosols are studied. Computational results predict that ash bridging leads to a non-uniform temperature distribution along the ceramic candle filters in the bridging region. The analyses of ash bridging deposition on the internal surfaces of the filter show that the absence of ash bridging tends to promote the deposition of particles of 10 μm on the surfaces.