Flow of Gas and Liquid

Practically all the available data concern conditions suitable for catalysis, with small packing particles (small spheres, cylinders, and the like) which are the catalyst supports. Very complete reviews of pressure drop and mass-transfer for such situations are available [100, 103]. 

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The flows of gas and liquid need not be concentric for aerosol formation and, indeed, the two flows could meet at any angle. In the cross-flow nebulizers, the flows of gas and sample solution are approximately at right angles to each other. In the simplest arrangement (Figure 19.11), a vertical capillary tube carries the sample solution. A stream of gas from a second capillary is blown across this vertical tube and creates a partial vacuum, so some sample solution lifts above the top of the capillary. There, the fast-flowing gas stream breaks down the thin film of sample... 

Minimum allowable capacity of a column is determined by the need for effective dispersion and contacting of the phases. The types of plates differ in their ability to permit Tow flows of gas and liquid. A cross-flow sieve plate can operate at reduced gas flow down to a point where liquid drains through the perforations and gas dispersion is inadequate for good efficiency. Valve plates can be operated at veiy... 

Packed-bed scrubbers m be construc ted for either vertical or horizontal gas flow. Vertical-flow units (packed towers) commonly use countercurrent flow of gas and liquid, although cocurrent flow is sometimes used. Packed scrubbers using horizontal gas flow usually employ cross-flow of liquid. 

Three basic fluid contacting patterns describe the majority of gas-liquid mixing operations. These are (1) mixed gas/mixed liquid - a stirred tank with continuous in and out gas and liquid flow (2) mixed gas/batch mixed liquid - a stirred tank with continuous in and out gas flow only (3) concurrent plug flow of gas and liquid - an inline mixer with continuous in and out flow. For these cases the material balance/rate expressions and resulting performance equations can be formalized as ... 

The most important safety devices in a production facility are the pressure relief valves, which ensure that pipes, valves, fittings, and pressure vessels can never be subjected to pressures higher than their design pressures. Relief valves must be designed to open rapidly and fully, and be adequately sized to handle the total flow of gas and liquids that could potentially cause an overpressure situation. They relieve the pressure by routing this stream to a safe location where it can be vented to atmosphere or burned. 

Glaser and Lichtenstein (G3) measured the liquid residence-time distribution for cocurrent downward flow of gas and liquid in columns of -in., 2-in., and 1-ft diameter packed with porous or nonporous -pg-in. or -in. cylindrical packings. The fluid media were an aqueous calcium chloride solution and air in one series of experiments and kerosene and hydrogen in another. Pulses of radioactive tracer (carbon-12, phosphorous-32, or rubi-dium-86) were injected outside the column, and the effluent concentration measured by Geiger counter. Axial dispersion was characterized by variability (defined as the standard deviation of residence time divided by the average residence time), and corrections for end effects were included in the analysis. The experiments indicate no effect of bed diameter upon variability. For a packed bed of porous particles, variability was found to consist of three components (1) Variability due to bulk flow through the bed... 

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. 

Where the concentration of the solute is small, say less than 10 per cent, the flow of gas and liquid will be essentially constant throughout the column, and the height of packing required, Z, is given by ... 

Another aspect of the process design is the relative rates of flow of gas and liquid, G and L, respectively. This is considered in the following example. 

The catalyst is a fixed bed. Flows of gas and liquid are cocurrent downwards. Liquid feed is at a such a low rate that it is distributed over the packing as a thin film and flows by gravity, helped along by the drag of the gas. This mode is suited to reactions that need only short reaction times, measured in seconds, short enough to forestall undesirable side reactions such as carbon formation. In the simplest arrangement the liquid distributor is a... 

Schematic representation of a bubble column with cocurrent flow of gas and liquid...

The drop in pressure for the flow of gas and liquid over packings is discussed in Chapter 4. It is important to note that, during operation, the tower does not reach flooding conditions. In addition, every effort should be made to have as high a liquid rate as possible, in order to attain satisfactory wetting of the packing. 

The independent variables involved in the simultaneous flow of gas and liquid phases are the viscosities, densities, and mass flows of the individual phases and the interfacial tension between the two phases. To these variables the channel diameter and shape must be added, and the inclination for horizontal flow the acceleration of gravity must be considered, and in vertical flow the elevation will be a factor. Finally, tube roughness may be an important variable. 

Fixed-bed reactors Trickle-flow reactor (TFR) This is a tubular flow reactor with a concurrent down-flow of gas and liquid over a fixed-bed of catalyst (Figure 3.10). Liquid trickles down whereas the gas phase is continuous. This reactor is mainly used in catalytic applications. Typical application examples of this reactor type are the following HDS of heavy oil fractions and catalytic hydrogenation of aqueous nitrate solutions. 

Packed Bubble Bed Reactor (BBR) This is a tubular flow reactor with concurrent up-flow of gas and liquid (Figure 3.11). The catalyst bed is completely immersed in a continuous liquid flow while gas rises as bubbles. Some applications of BBR are the catalytic denitrification of aqueous nitrate solutions and the hydrogenation processes. 

Hanratty and Hershman (H2) 1961 Jeffreys theory (J4) for roll-wave transition on a liquid surface is applied to the cocurrent flow of gas and liquid at small slopes. Experimental data on the initiation and growth of waves on various liquids with and without surfactant additives. 

Hewitt (H7), 1961 Extension of Dukler treatment (D12, D13) to case of upward cocurrent flow of gas and liquid film, with heat transfer. Computer solutions presented. 

Fig. 4.18. Flow regimes for co-current downwards flow of gas and liquid through a fixed bed of particles, (a) Non-foaming liquids. (b) Foaming liquids. L, G are liquid, gas flowrates per unit area (kg/mJs) X = Kpc/p.itH /pwater) ...

A variant on the froth contact is the reverse jet contactor (Example 22), which can be considered as an upside-down distillation tray operated above the flooding velocity in cocurrent flow of gas and liquid. It is limited to one stage. 

With a gas superficial velocity of 1.5 m/s, for equal mass flow of gas and liquid, with gas density 0.001 of liquid density, and with 500- im-diameter droplets falling at a terminal settling of 2.5 m/s, Eq. (14-179) gives a fractional holdup of liquid of... 

The bench-scale hydrogenation unit was designed for operation at up to 3,000 psig and 450° C, with once-through flow of hydrogen, down-flow of gas and liquid over a fixed-bed catalyst. Reactor temperature, pressure, and hydrogen flow and liquid level in the high-pressure product separator were controlled automatically. Liquid product was withdrawn periodically from a low-pressure... 

The three-stage rotating disk reactor is illustrated in Fig. 30. Each stage consists of one cylindrical and two conical elements and is connected to the next stage by another cylindrical element with a relatively small diameter. Fluid motion and gas dispersion are achieved by a rotating flat plate that contains holes at its outer edge to generate gas bubbles. The reactor can be used for cocurrent and countercurrent flow of gas and liquid or slurry. 

For a cocurrent flow of gas and liquid at steady state, the governing liquid-phase differential equations for species A and C in dimensionless form are... 

Many empirical equations for predicting pressure gradients in countercurrent flow of gas and liquid are available in the literature.17,31,36 The pressure drop in countercurrent flow can be represented by an equation of the Carman-Kozeny type for flow through packed beds, Below the flooding point, the following equation is suggested36 and has been shown to agree well with experimental data ... 

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