Superficial velocity riser

In the Fig.4, it can be seen that the gas hold-up in both riser and downcomer decreases with increasing the draft-tube horn-mouth diameter and approaches the maximum when the draft-tube hom-mouth diameter is 1.05m. However, due to the gas hold-up decreases more in the downcomer, the gas hold-up difference between the downcomer and the riser increases. Therefore, the apparent density difference between the riser and the downcomer enhances, causing higher liquid superficial velocity in the downcomer and in the riser With increasing the hom-mouth diameter. Fig.5 also shows that the existence of hom-mouth promotes the ability to separate gas from liquid and decreases the amount of gas entrained into the downcomer. 

Fig.6 and Fig.7 illustrate the effect of draft-tube diameter on liquid superficial velocity, liquid circulating flowrate and gas hold-up. Results show that the liquid superficial velocity in the riser increases with increasing the draft-tube diameter while the liquid velocity in the... 

Eulerian two-fluid model coupled with dispersed itequations was applied to predict gas-liquid two-phase flow in cyclohexane oxidation airlift loop reactor. Simulation results have presented typical hydrodynamic characteristics, distribution of liquid velocity and gas hold-up in the riser and downcomer were presented. The draft-tube geometry not only affects the magnitude of liquid superficial velocity and gas hold-up, but also the detailed liquid velocity and gas hold-up distribution in the reactor, the final construction of the reactor lies on the industrial technical requirement. The investigation indicates that CFD of airlift reactors can be used to model, design and scale up airlift loop reactors efficiently. 

The first oil-catalyst contact is essential. The mixing temperature (about 530-600 °C) is very difficult to measure and is about 20-80 °C higher than the riser exit temperature (4, J). The superficial gas velocity at the inlet is several meters per second, much higher than the terminal velocity of the catalyst, which is about 0.20 m/s. The catalyst and the gas are transported upwards together but at a different superficial velocity, u (6). These two velocities are related by the slip velocity (sv), defined as follows ... 

Expansion Factor Flow Increase along Riser Height Because of cracking, the flow rate, the volume, and the superficial velocity of the gas increase. For mean molecular sizes, the following values can be used A, 40 O, 20 E, 9 and G, 3 (27). Then, for a given weight of feed (A), the volumetric flow rate (Q) will be inversely proportional to these mean molecular sizes. Thus ... 

For a given flow (in tons per day) of feedstock to be processed, if MW increases, the number of moles decreases, the superficial velocity of the gas decreases, and the residence time of the gas in the riser increases. These two effects are then opposed the first decreases XA and the second increases it. In any case, the MW of the feedstock must be known or estimated. 

Temperature along the Riser Height Temperature (T) affects the cracking (k.) and deactivation (VO constants (or functions). It also influences the gas density and, as a consequence, the flow rates, the superficial velocities, and the residence time of the gas in the riser. Its effect on ki makes the product distribution (gas, coke, etc.) dependent on T. 

Figure 2 Solid mass flux G, as a function of the superficial velocity uo in the riser...

Conversely, a wider range of superficial gas velocities than that of the pilot plant was chosen to increase the flexibility of the riser-downcomer hold-up ratio. Notably, the industrial-scale plant operates with superficial velocities that are generally higher than those used in the pilot plants, but within the original design range. Further room for maneuver was added by increasing the head of the recirculation compressor relative to that in use at the pilot plants. 

The experiments reported by Horio et al (1988) were used to validate the EMMS/DP drag model for Eulerian—Lagrangian approaches. The riser was simplified with 2D, rectangular domain. In the simulations, air enters the riser from the bottom with a uniform superficial velocity, the top of the domain is set to be a pressure oudet for the gas phase, and the entrained particles are directly returned to the riser from the bottom to maintain the... 

For the sake of developing commercial reactors with high performance for direct synthesis of DME process, a novel circulating slurry bed reactor was developed. The reactor consists of a riser, down-comer, gas-liquid separator, gas distributor and specially designed internals for mass transfer and heat removal intensification [3], Due to density difference between the riser and down-comer, the slurry phase is eirculated in the reactor. A fairly good flow structure can be obtained and the heat and mass transfer can be intensified even at a relatively low superficial gas velocity. 

Figure 3 shows the radial profile of the gas holdup in the riser with increasing superficial gas velocity under different solid holdups. The gas holdup increases with increasing superficial gas velocity at the different solid holdups. At a low superficial gas velocity, the liquid velocity... 

Air was used as the gas phase and was introduced into the system through a distributor with holes of diameter 1 mm and a perforation of 0.25%. The superficial gas velocity, based on the riser cross-section area, varied from 0.0067 and 0.0535 m/s. 

Figure 7 Cross-sectional view of the riser with left downwards moving particles, and right upwards moving particles at G = 260 kg/m2s and indicated superficial gas velocity (U). The plots show all the particle locations over the height of the viewed section, integrated over the time of the run (Van de Velden et al, 2008).

Vc = linear velocity of gas in riser, reversal area, or annulus of bubble cap (maximum value) or in sieve hole, ft/s = maximum allowable superficial linear velocity of gas (based on net cross-sectional area of tower for vapor flow), ft/s, see Eq. (3)... 

Fig. 21. Computed and measured radial profiles of (a) solids concentration and (b) axial solids velocity in a CFB riser for three superficial gas velocities (U = 7.5 m/s, U = 10.0 m/s, and U = 15.0 m/s) at a constant mass flux of 300 kg/(m s). Riser diameter D = 0.0536 m, physical properties of the particles diameter, 126 pm density, 2540 kg/m. ...

FIGURE 12.17 Some simulation results for gas-solids flows in a riser, (a) Axial particle diameter profiles for different superficial gas velocities (Vsijp) (from Mathiesen et al., 2000). (b) Radial solid volume fraction profiles at 0.7 m from the bottom, Vsup = I.Oms (from Mathiesen et al., 2000). (c) Radial solids volume fraction profiles at different heights solid line and circles 1.86 m short-dashed line and squares 4.18 m long-dashed line and diamonds 5.52 m (from Neri and Gidaspow, 2000). 

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