Effect of Liquid and Solid Nature

The effect of liquid and solid nature on the hydrodynamic and mass transfer parameters of N2 is shown in Figures 54 through 60. Under similar operating conditions, the nature of the liquid and solid phases appeared to have a significant impact on the gas holdup, the gas bubbles size and the overall volumetric mass transfer coefficient.

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Figure 55: Effect of Gas Composition on εG (a), d32 (b) and kLa (c) under Constant Pressure (triangles) or Gas Density (circles)

(a)

(c)

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Figure 56: Effect of Gas Nature and Composition on the Gas Bubbles Size Distribution in Molten Reactor Wax (a) CS = 0 vol.%, T = 450 K ; (b) CS = 3 vol.%, T = 400 K

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The gas holdup and the Sauter mean bubble diameter of N2 obtained in the paraffins mixture and in the reactor wax were similar. Figure 54 shows that the least fraction of large gas bubbles was obtained when using the Puralox particles in the paraffins liquid mixture. In the molten reactor wax, however, the population of both large and small gas bubbles increased, which led to similar Sauter mean bubble diameters as shown in Figure 60. Also, the population of small bubbles with diameters db < 3 mm is slightly greater in the paraffins mixture than in the molten reactor wax

and as a result the gas holdup in the paraffins mixture is greater than in the reactor wax. The kLa

values of N2 in the paraffins liquid mixture are found to be greater than in the reactor wax. When increasing the solid concentration, however, kLa values in both liquids became similar. This is

expected as the diffusivity of N2 and therefore its mass-transfer coefficient kL is greater in the

paraffins mixture than in the reactor wax under the same temperature according to the correlation by Erkey et al. [259]. Moreover, at low solid concentrations the gas-liquid interfacial area is similar in both liquids due to similar gas holdup and gas bubbles size. At high solid concentrations, on the other hand, the gas bubbles size in the paraffins liquid mixture increases with increasing the solid concentration at a greater rate than in the reactor wax, and therefore the gas-liquid interfacial area becomes smaller in the paraffins liquid mixture than in the reactor wax and as a result kLa values in both liquids become closer as can be observed in Figure 60.

Operating the SBCR with the molten Sasol wax led to lower gas holdup values and larger gas bubbles which result in low interfacial area and consequently kLa values. Figure 54 shows that

using molten Sasol wax with alumina particles increased the population of large gas bubbles (db

> 3 mm) and greatly reduced the population of the small gas bubbles (db < 3 mm). Even though

when using the iron oxides particles more gas bubbles coalescence is expected than with alumina, in this case, only a small fraction of gas bubbles (db < 1 mm) does not coalesce. This

led to high values of the Sauter mean bubble diameter and low gas holdup in the Sasol wax as shown in Figure 60. These findings confirm that operating the SBCR with a heavy F-T liquid, composed of long hydrocarbon chains having high density and viscosity; will lead to larger gas bubbles and lower values of gas holdup and kLa than those to be expected in a lighter F-T Liquid

with shorter hydrocarbon chains.

In the case of the Sasol wax, the use of alumina particles led to greater εG (see Figure 57) and kLa (see Figure 58) values, and lower d32 values (see Figure 59) than when using Iron oxides (FeOx) particles under similar operating conditions. The Iron oxides particles were smaller and

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denser than the alumina ones (see sections 4.2.4.1 and 4.2.4.2). The decrease of gas holdup with the heavier particles (FeOx) is in agreement with some literature finding [151]_{. The smaller gas} holdup and larger gas bubbles obtained in the case of FeOx can be due to the higher viscosity of the slurry phase observed when using FeOx as compared with that of Al2O3, which is probably due to the smaller size of the FeOx particles. As explained previously, higher viscosities lead to large gas bubbles [146] _{and small gas holdup} [124, 128, 131, 137-139, 144, 145, 147-149] _{and k}

La values [121, 140-

143]_{. Under similar conditions the decrease of gas holdup and the increase of the gas bubble size} resulted in the decrease of the gas-liquid interfacial area and consequently kLa when using FeOx.

Figure 57: Effect of Solid Nature on εG of N2-Sasol wax-FeOx/Al2O3

The liquid-solid systems studied showed that increasing the solid particles concentration decreased the gas holdup and increased the Sauter mean bubble diameter and consequently led to low kLa values as presented in Figures 44 and 60. These figures also show that the impact of

solid concentration on the hydrodynamics and mass transfer of the SBCR is dependent on the nature of the liquid and solid phases used. The Alumina particles in the molten Sasol wax appeared to have less impact on the SBCR behavior than that of FeOx particles. Also, the

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alumina particles in the molten Sasol wax have less effect on the SBCR behavior than that of the alumina Puralox particles in the molten reactor wax and in the paraffins mixture.

Figure 58: Effect of Solid Nature on d32 of N2-Sasol wax-FeOx/Al2O3

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Figure 60: Effect of Liquid and Solid Nature on εG (a), d32 (b) and kLa (c)

(a)

(c)

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