Flow regime liquid phase

The second section presents a review of studies concerning counter-currently and co-currently down-flow conditions in fixed bed gas-liquid-solid reactors operating at elevated pressures. The various consequences induced by the presence of elevated pressures are detailed for Trickle Bed Reactors (TBR). Hydrodynamic parameters including flow regimes, two-phase pressure drop and liquid hold-up are examined. The scarce mass transfer data such gas-liquid interfacial area, liquid-side and gas-side mass transfer coefficients are reported. 

Figure 6.1 Flow patterns on trays, (a) Froth regime (liquid phase is continuous) (b) spray regime (gas phase is continuous). (Henry Z. Kister, excerpted by special permission from Chemical Engineering, September 8, 1980 copyright , by McGraw-Hill, Inc., New York, NY 10020.)...

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. 

Once the gas is in the downcomer, the liquid has to flow even faster to cause circulation. Gas bubbles are stiU lighter than the liquid and have a buoyant force, which propels them to rise against the flow. The liquid-phase momentum has to provide the power to overcome the buoyant force and create a net downward force in order to cause forward motion and eventual circulation. In effect, a superficial liquid velocity exists at which gas bubbles cau be suspended or are stagnant in the downcomer (regime 2 in Figure 8.4). Hence, this circulation regime is referred to as the transition regime. 

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,... 

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. 

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 ... 

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. 

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. 

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 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... 

Akbar MK, Plummer DA, Ghiaasiaan SM (2003) On gas-liquid two-phase flow regimes in microchannels. Int J Multiphase Flow 29 855-865... 

The forced fluid flow in heated micro-channels with a distinct evaporation front is considered. The effect of a number of dimensionless parameters such as the Peclet, Jacob numbers, and dimensionless heat flux, on the velocity, temperature and pressure within the liquid and vapor domains has been studied, and the parameters corresponding to the steady flow regime, as well as the domains of flow instability are delineated. An experiment was conducted and demonstrated that the flow in microchannels appear to have to distinct phase domains one for the liquid and the other for the vapor, with a short section of two-phase mixture between them. 

GL 27] [R 3] [P 29] By means of sulfite oxidation, the specific interfacial areas of the fluid system nitrogen/2-propanol were determined for different flow regimes [5]. For two types of micro bubble columns differing in micro-channel diameter, interfaces of 9800 and 14 800 m m , respectively, were determined (gas and liquid flow rates 270 and 22 ml h in both cases). Here, the smaller channels yield the multi-phase system with the largest interface. 

---