Downcomer liquid velocity

The other two parameters, small clearance under the downcomer and small downcomer top area, have little effect on entrainment flooding, as they are associated with the downcomer only. Downcomer clearance affects downcomer backup, but not downcomer liquid velocity, while downcomer area affects the velocity, but has little effect on downcomer backup. 

Figure .I2 Downcomer aeration, (a) Structure of two phase mixture in the downcomer (b) depen-denco of downcomer and downcomer underflow gas fraction on downcomer liquid velocity, air-water tests. (Data for part b based on M. J. Lockett and A. A, W, Gharani, /. Chem. E. Symp. Ser, 56, p. 2.3/43, The Institution of Chemical Engineers UK, 1979.)...

Figure 8.4 Circulation regime progression in a draught tube internal-loop airlift bioreactor (van Benthum et al., 1999b), where is the downcomer liquid velocity and is the gas slip velocity.

Clear liquid velocity (ft/sec) through the downcomer is then found by multiplying DL by 0.00223. The correlation is not valid if Pl - pv is less than 301b/ft (very high pressure systems). For foaming systems, DL should be multiplied by 0.7. Frank recommends segmental downcomers of at least 5% of total column cross-sectional area, regardless of the area obtained by this correlation. 

VDdsg = Downcomer design velocity, GPM/fT Vioad = Column vapor load factor WFP = Width of tray flow path, in. pL = Liquid density, Ibs/ft pv = Vapor density, Ibs/ft ... 

Table 8-20 gives suggested downcomer clear liquid velocities based on relative foaming characteristics of the fluid on the tray at tray conditions. 

Let the liquid velocity under the downcomer equal the liquid velocity on the tray to give a smooth entry [237]. 

Fig.2 and Fig.3 show the typical liquid velocity and gas hold up distribution in the ALR. From the figures, one notices that the cyclohexane circulates in the ALR under the density difference between the riser and the downcomer. An apparent large vortex appears near the air sparger when the circulating liquid flows fi om the downcomer to the riser at the bottom. In the riser, liquid velocity near the draft-tube is much larger than that near the reactor wall, the latter moved somewhat downward. The gas holdup is nonuniform in the reactor, most gas exists in the riser while only a little appears in the dowmcomer. 

Fig.6 Effect of downcomer diameter on liquid velocity and flowrate...

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. 

In current design practice for downcomers, three parameters are commonly considered liquid residence time, liquid velocity, and downcomer backup. 

Liquid-velocity correlations are generally based on one of two concepts, and it is not always clear which one is the basis of a given correlation. One concept relies on the principle that vapour should not be entrained to the tray below. The other is grounded on the phenomenon of downcomer-inlet-choking caused by the inability of the low-density froth... 

Downcomer liquid load For downcomer design, the liquid load is usually defined as the liquid velocity at the downcomer entrance (m/s or ft/s) ... 

Liquid mixing time decreases sharply for an initial increase in the gas sparging rate and approach an asymptotic value that is determined by the height and diameter of the downcomer and the liquid properties [5]. A higher liquid velocity shortens the gas residence time and results in a decrease of gas holdup and interfacial area. The radial profile of the liquid is parabolic. These are disadvantageous for mass transfer. The mounting of internals in a fixed bed is often used to improve the radial profile of the liquid velocity. This motivates us to mount internals in an EL-ALRs to improve the radial profile of the gas holdup and the liquid velocity and to intensify turbulence. 

Downcomer vapor underflow, In the past, it was thought that downcomer velocity needed to be kept small to avoid vapor being carried under the downcomer apron to the tray below, Thomas (42) and Lockett and Gharani (43), however, showed that some vapor underflow exists even at velocities considered safe. The latter authors showed that the gas fraction in the downcomer underflow depends on downcomer superficial liquid velocity in a manner similar to downcomer gas fraction (Fig. 6.126). 

Head of liquid equivalent to the frictional flow resistance in the downcomer and in the passage of the fluid from the downcomer onto the plate. This head is usually relatively small, but it can be estimated as three, kinetic heads for the liquid, based on the linear liquid velocity at the minimum cross-sectional area for downooming liquid flow, or... 

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