Gas-liquid operation

Figure 9-54. Typical pressure drop—capacity curves for countercurrent gas-liquid operation, for Koch Flexipac . Used by permission of Koch Engineering Co., Inc., Bull. KFP-4. Parameter lines on charts are gpm/ft. ...

Sie ST, Lebens PJM. Monolithic reactors for countercurrent gas-liquid operation. In Cybulski A, Moulijn JA, eds. Structured Catalysts and Reactors. New York Marcel Dekker, 1998 305-321. 

Const, (power) dissipation energy per unit vessel volume Const, impeller discharge flow energy Turbulent dispersion Gas-liquid operation Reaction requiring microscale mixing... 

Monolithic catalyst carriers are state-of-the-art in exhaust gas cleaning, for example in automobiles, DeNOx or removal of VOCs. To minimize diffusion length and to increase the geometric surface area, monoliths with small-diameter channels have been developed which can be produced easily by extrusion, followed by calcination. In the past few years the application of monoliths in gas-liquid operation has been investigated intensively [10-13]. 

In monolithic catalyst carriers with wider channels, the hquid forms a film on the channel walls, whereas in the core of the channel a continuous gas phase exists. As shown by Lebens [10], countercurrent gas-liquid operation is now possible, and shows certain advantages over the countercurrent trickle bed operation. Typical channel diameters are 3-5 mm, and the geometric surface areas are between 550 and 1000 m2 m 3. Below the flooding point, almost no hydrodynamic interaction between the gas and hquid can be observed for example, the RTD is the same for both co-current and countercurrent operation. Apart from some surface waves, the film flow is completely laminar. 

Due to an always high water contents at the reactor inlet, the eventual internal water loop (as described in Fig. 8.3) should not be a problem. However, if co-current and countercurrent operation is compared (Fig. 8.28), a clear difference between these two operation modes is found only for the water concentrations, whereas the gain in space-time yield is limited to around 10 %. This can be attributed to the high recirculation rate of the liquid, whereas in both experiments the gas phase was vented after one pass. Therefore, the gas-liquid operation was of the cross-flow-type rather than pure co-current or countercurrent To identify the effect of pure co-current or countercurrent operation, the gas phase should also be recycled. 

Mesh Microcontactor A mesh microcontactor contains a microstructured plate with regular circular openings through which separate gas and liquid streams come into contact [270,271]. Stability of the interface and prevention of breakthrough are achieved by adjusting the pressure. Gas-liquid operation requires a low gas flow... 

These ideas of impeller flow, head and power input as related to operating variables have some merit for a qualitative description of the effects of the operating variables on the process. However, it requires extensive experience, and usually actual experiments, to decide whether a system performance is favored by a particular combination of flow and head. (Rushton and Oldshue (R12) note that high values of Q/3Care preferred for blending and solid suspension, low ratios for liquid-liquid and gas-liquid operations.) This approach still requires the systematic study of impeller speed and diameter as process variables. 

An analysis is made of the factors which pose a limit to representative downscaling of catalyst testing in continuous fixed-bed reactors operated with either gas or gas-liquid flow. Main limiting factors are the axial dispersion and, in the case of gas-liquid operation, also the contacting of the catalyst. The effects of catalyst and reactor geometries are quantified, and boundaries for safe operation are indicated. 

Many equipment possibilities exist for gas-liquid operations. They are outlined in Table 1 with their main operational characteristics (at least in air-water systems) presented in Table 2. However, the remainder of this section will deal exclusively with agitated vessels containing low-viscosity liquids in which turbulent flow is achieved Re = > --lO )-... 

The value of g/u in Eq. (6) can easily fall as low as 0.4 for 6-blade flat-blade disk turbines (Fig. 6) and downflow pitched-blade turbines and hydrofoils (Table 3). However, for modern impellers with parabolic concave blades (e.g., Scaba SRGT, Chemineer BT6, Lightnin R-130), g/u typically falls to only about 0.9 (and then only at high FIq values). The ability to deliver high power makes these impellers highly suitable for gas-liquid operations. 

Multiple impellers are often used in gas-liquid operations. Assuming that the lowest impeller is used for the primary gas dispersion, the upper ones are not loaded by all the gas entering through the spar-ger F lo] Pqj. purpose of power demand estimation, it can be assumed that upper impellers experience about half the total gas rate. Correlations to estimate... 

For gas-liquid operations there is another relationship called the K factor which relates the effect of gas rate on power level. Figure 26 illustrates a typical K factor plot which can be used for estimation. Actual calculation of K factor in a particular case involves very specific combinations of mixer variables, tank variables, and fluid properties, as well as the gas rate being used. 

Static mixers are also used for continuous gas-liquid operations (see Section 9.9). The orientation of the mixer is important. A vertical orientation with both gas and liquids passing cocurrently downward is desirable. Considerable vendor information is available on gas-liquid dispersion in static mixers. 

The operations which include humidification and dehumidification, gas absorption and desorption, and distillation all have in common the requirement that a gas and a liquid phase be brought into contact for the purpose of diffusional interchange between them. The equipment for gas-liquid contact can be broadly classified according to whether its principal action is to disperse the gas or the liquid, although in many devices both phases become dispersed. In principle, at least, any type of equipment satisfactory for one of these operations is suitable for the others, and the major types are indeed used for all. For this reason, the main emphasis of this chapter is on equipment for gas-liquid operations. 

Design sieve-tray towers for gas-liquid operations. 

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