Laurent Sehabiague

Prediction of the Gas Holdup in Industrial-Scale Bubble Columns and Slurry Bubble Column Reactors Using Back-Propagation Neural Networks

The total gas holdup and the holdup of large gas bubbles were predicted in bubble column reactors (BCRs) and slurry bubble column rectors (SBCRs) using two Back-Propagation Neural Networks (BPNNs). Over 3880 and 1425 data points for gas holdup and Large gas bubble holdup respectively, covering wide ranges of gas-liquid-solid physical properties, operating variables, reactor geometry, and gas sparger type/size, were employed to develop, train and validate the two neural networks. The developed BPNN for gas holdup has a topology of [14,9-7,1] and was able to predict the trained and untrained data with an average absolute relative error (AARE), standard deviation, and regression coefficient (R2) of 16, 19 and 90%, respectively. The developed BPNN for large gas bubble holdup has a topology of [14,8,1] and was capable of predicting the trained and untrained data with AARE, standard deviation, and R2 of 10, 14 and 93%, respectively. The BPNNs were then used to predict the effects of pressure, superficial gas velocity, temperature and catalyst loading on the total syngas holdup for Low-Temperature Fischer-Tropsch (LTFT) synthesis carried out in a 5 m ID SBCR. The predicted total syngas holdup appeared to increase with increasing reactor pressure, superficial gas velocity and the number of orifices in the gas sparger. The predicted syngas holdup, however, was found to decrease with increasing catalyst loading and reactor temperature. Also, under similar LTFT operating conditions (P = 3 MPa, T = 513 K, CW = 30 and 50 wt%), the total syngas holdup values predicted for H2/CO ratio of 2:1 and cobalt-based catalyst are consistently lower than those obtained for H2/CO ratio of 1:1 and iron oxide catalyst in the superficial gas velocity range from 0.005 to 0.4 m/s. These predictions are in perfect agreement with reported literature trends, which underscore the reliability and validity of the developed BPNNs in predicting the total syngas holdup and the holdup of large gas bubbles in large-scale bubble columns and SBCRs operating under industrial conditions.

Hydrodynamic and Mass Transfer Characteristics in a Large-Scale Slurry Bubble Column Reactor for Gas Mixtures in Actual Fischer–Tropsch Cuts

Abstract The hydrodynamics (gas holdup, Sauter mean bubble diameter, d 32) and the overall volumetric liquid-side mass transfer coefficients (kLa) were measured in a large-scale (0.29 m ID, 3 m high) slurry bubble column reactor (SBCR) for He/N2 gaseous mixtures, as surrogates for syngas, in three different Fisher–Tropsch (F-T) products (liquid paraffins mixture, light F-T cut and heavy F-T cut) in the presence and absence of three different solids (spent iron oxides catalyst, alumina powder and Puralox alumina). The effects of pressure (10–30 bar), temperature (up to 500 K), superficial gas velocity (0.14–0.26 m/s), solid concentration (0–20 vol.%) and gas density on these design parameters were investigated. The experimental data revealed that increasing the reactor pressure or gas density increased the gas holdup and decreased d 32, by increasing the population of the small gas bubbles, which increased the overall kLa values for all the gas mixtures used in the three F-T cuts under most of the operating conditions employed. Increasing temperature increased the gas holdup in the three F-T cuts, except for N2-light F-T cut, where the gas holdup values remained almost constant from 400 to 500 K. Increasing the slurry concentration decreased the gas holdup and increased d 32, mainly for gaseous mixtures with high He mole fractions, which decreased the overall kLa under all conditions used. Increasing the gas superficial velocities (UG ) increased the gas holdup and kLa values, even though d 32 was found to increase or decrease with increasing UG . Increasing the He mole fraction in the He/N2 gaseous mixture at constant pressure led to low gas holdup and high d 32 which decreased kLa values, and under similar operating conditions, kLa values of He as a single gas were always lower than those of N2 as a single gas. Increasing the He mole fraction in the He/N2 gaseous mixture at constant density, however, was found to have negligible effect on the gas holdup, d 32 and subsequently on the overall kLa. The gas holdup, the overall kLa and the population of the small gas bubbles for N2 in the liquid paraffins mixture were greater than those in the light F-T cut. Operating the SBCR with the heavy F-T cut resulted in the lowest gas holdup and the largest gas bubbles size which led to the lowest gas–liquid interfacial area and consequently, the lowest kLa values. Also, under the operating conditions investigated, the behavior of overall kLa for the gases used in the three F-T cuts in the presence and absence of the three solids employed was controlled by that of the gas–liquid interfacial area (a). Using the data obtained, two novel empirical correlations for predicting the gas holdup and the overall kLa for gases specifically in F-T cuts are proposed.

Modeling and Simulation of a Fischer–Tropsch Slurry Bubble Column Reactor Using Different Kinetic Rate Expressions for Iron and Cobalt Catalysts

Abstract A user-friendly simulator based on a comprehensive computer model for slurry bubble column reactor (SBCR) was used to predict the performance of a conceptual commercial-scale (9-m ID and 50-m height) SBCR for Fischer–Tropsch (F–T) synthesis. Novel correlations for predicting the hydrodynamic and mass transfer parameters; a new relationship between the axial dispersion of large gas bubbles and their average diameter; and up to ten different kinetic rate expressions available in the literature for iron and cobalt-based catalysts were included in the simulator. The effects of operating conditions, including catalyst concentration, pressure, temperature, H2/CO ratio, and superficial gas velocity on the SBCR performance were predicted using the simulator for both types of catalysts. The predictions showed that the performance of the reactor was strongly dependent on the type of catalyst and the kinetic rate expression used.