Flow regime liquid

The areas concerned with hydrodynamics in trickle beds include flow regimes, liquid distribution on the solid (catalyst) packing, pressure drop, liquid holdup, and, more generally, the effect of the physical properties of the liquid and gas phases on all hydrodynamic properties. 

General aspects Flow regimes, liquid holdup, two-phase pressure drop, and wetting efficiency... 

A description of the flow phenomena in this type of process is very complicated and is outside the scope of this book (see reviews by Shah 1979, and Shah and Sharma, 1987). Phenomena that have been studied include the various flow regimes, liquid and gas hold-ups, pressure drop, the distribution of liquid and gas flows, effective solids wetting, axial mixing, etc.. However, it is important to remenber that for chemical reactor development one can measure, separately, the mass transfer and the integral performance of the three-phase system on a small scale, using the same solid particles that are going to be used on the larger scale. 

Charpentier, J.C, M, Bakos and P, Le Goff, "Hydrodynamics of Two-Phase Concurrent Down Flow Regimes, Liquid Axial Dispersion and Dead Zones", Proc, 2nd Conference on Appl. Physical Chem., Vol, 2, (1971) 31-47, Veszprem, Hungary,... 

Consider a separation device, tubular or otherwise, filled with a liquid assume also that the inlet pipe filled with the liquid is connected to a pump (Figure 6.1.1) at the inlet of the separation device, which is assumed to be horizontal. As the pump runs, it introduces mechanical energy into the liquid, which is driven into the device against whatever flow resistance is offered. As the liquid moves through the device, its hydrostatic pressure is reduced due to frictional losses. The relation between the hydrostatic pressure drop AP encountered by the liquid and its volumetric flow rate Q depends on the flow regime, liquid viscosity /i, the flow channel geometry and... 

The effect of chemical reaction in reducing the effect of variation of the liquid rate on the rate of absorption in the laminar-flow regime was illustrated by the evaluation of the rate of absorption of chlorine in ferrous chloride solutions in a wetted-waU column by Gilliland, Baddoiir, and White [Am. In.st. Chem. Eng. J., 4, 323 (1958)]. 

Discharge Flow Regimes Upon developing a puncture in either the vessel or a line attached to the vessel, as in Fig. 26-62, the subsequent depressurization can cause a volatile liqmd to flash and develop bubbles in the liquid. These bubbles cause an expansion, or. 9well, which raises the two-phase, or frothy, level. If the puncture is in the vapor space of a vessel or on a line from the vapor space, the discharge will be at least initially all vapor. This is the simplest discharge case and is treated here as a special case. 

Figure 26. Flow regime map for horizontal gas and liquid flows (D - droplet X - stratification M - mixed B - bubble).

In addition to flow regime, hold-up and pressure drop are two other important parameters in two-phase gas-liquid flows. Hold-up is defined as the relative portion of space occupied by a phase in the pipe. It can be expressed on a time or space average basis, with the actual method chosen depending on the intended use of the hold-up value, and the measurement method employed. There are numerous correlations in the literature for hold-up, but most are based upon a pressure drop-hold-up correlation. The following expression is a widely recognized empirical relationship between hold-up and pressure drop ... 

An extensive treaunent of gas-liquid flows encountered in industry applications, along with numerous design correlations can be found in Volume 3 of the Encyclopedia of Fluid Mechanics - Gas-Liquid Flows (N. P. Cheremisinoff, editor. Gulf Publishing Co, Houston, TX, 1986). Further discussions in this volume can be found in Chapter 4 with regard to flow regimes typically encountered in bubble columns and similar devices. 

The first two eases represent the smallest and largest vent sizes required for a given rate at inereased pressure. Between these eases, there is a two-phase mixture of vapor and liquid. It is assumed that the mixture is homogeneous, that is, that no slip oeeurs between the vapor and liquid. Furthermore, the ratio of vapor to liquid determines whether the venting is eloser to the all vapor or all liquid ease. As most relief situations involve a liquid fraetion of over 80%, the idea of homogeneous venting is eloser to all liquid than all vapor. Table 12-3 shows the vent area for different flow regimes. 

As liquid flowrate inereases beyond the minimum required to suspend the partieles, the bed expands. Unfortunately, however, the expansion index n is not eonstant over all flow regimes but is a funetion of the flow. Thus... 

Weekman and Myers (W2) examined the fluid-flow characteristics of cocurrent downward flow of gas and liquid. The pulsing effect first noted by Larkins et al. was also observed in this work. Pressure-drop data could be correlated satisfactorily by a relation similar to those used for two-phase flow in pipes. Surface-active agents were observed to have a pronounced influence upon flow regime transition and pressure drop. 

The above experimenters have used the technique described to obtain flow rate measurements of the liquid wall-film at various mass velocities, tube dimensions, etc., and some typical results from Staniforth and Stevens (S7) are shown in Fig. 7. Also shown are the values of burn-out heat flux obtained at the four different mass velocities indicated. It can be seen that the liquid-film flow rate decreases steadily with increasing heat flux until at burn-out the flow rate becomes zero or very close to zero. We thus have confirmation of a burn-out mechanism in the annular flow regime which postulates a liquid film on the heated wall diminishing under the combined effects of evaporation, entrainment, and deposition until at burn-out, the film has become so thin that it breaks up into rivulets which cause dry spots and consequent overheating. 

Consideration will now be given to the various flow regimes which may exist and how they may be represented on a Flow Pattern Map to the calculation and prediction of hold-up of the two phases during flow and to the calculation of pressure gradients for gas-liquid flow in pipes. In addition, when gas-liquid mixtures flow at high velocities serious erosion problems can arise and it is necessary for the designer to restrict flow velocities to avoid serious damage to equipment. 

The regions over which the different types of flow can occur are conveniently shown on a Flow Pattern Map in which a function of the gas flowrate is plotted against a function of the liquid flowrate and boundary lines are drawn to delineate the various regions. It should be home in mind that the distinction between any two flow patterns is not clear-cut and that these divisions are only approximate as each flow regime tends to merge in with its neighbours in any case, the whole classification is based on highly subjective observations. Several workers have produced their own maps 4 8. ... 

The relation between c and / and X (defined by equation 5.1) is shown in Figure 5.4, where it is seen that separate curves are given according to the nature of the flow of the two phases. This relation was developed from studies on the flow in small tubes of up to 25 mm diameter with water, oils, and hydrocarbons using air at a pressure of up to 400 kN/m . For mass flowrates per unit area of U and G for the liquid and gas, respectively, Reynolds numbers Rei L d/fii ) and Rec(G d/fia) may be used as criteria for defining the flow regime values less than 1000 to 2000, however, do not necessarily imply that the fluid is in truly laminar flow. Later experimental work showed that the total pressure has an influence and data presented by Gr1H ITH(i9) may be consulted where... 

Chapter 9 is devoted to regimes of capillary flow with a distinct interface. The effect of certain dimensionless parameters on the velocity, temperature and pressure within the liquid and vapor domains are considered. The parameters corresponding to the steady flow regimes, as well as the domains of flow instability are defined. 

In Chap. 5 the available data related to flow and heat transfer of a gas-liquid mixture in single and parallel channels of different size and shape are presented. These data concern flow regimes, void fraction, pressure drop and heat transfer. The effects of different parameters on flow patterns and hydrodynamic and thermal characteristics of gas-liquid flow are discussed. 

Figure 5.3e shows the situation when the air velocity was increased to Ugs = 20 m/s. It is seen from this figure that the liquid bridges in churn flow disappeared and a liquid film formed at the side walls of the channel with a continuous gas core, in which a certain amount of liquid droplets existed. The pressure flucmations in this case became relatively weaker in comparison with the case of the churn flow. The flow pattern displayed in Fig. 5.3f indicates that as the air velocity became high enough, such as Ugs = 85 m/s, the liquid droplets entrained in the gas core disappeared and the flow became a pure annular flow. It is also observed from Fig. 5.3f that the flow fluctuation in this flow regime became weaker than that for the case shown in Fig. 5.3e, where Ugs = 20 m/s. 

From the visual studies on the flow patterns for circular, trapezoidal and rectangular channels it may be concluded that as the tube diameter decreases, transitions between flow regimes occur at different combinations of superficial gas and liquid velocities. 

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