Efficiency Flow Regimes

Generally, the minimum wetting rate is at 0-5 to 2 gpm/ft2 for random packings, and 0.1 to 0.2 gpm/ft2 for structured packings (Sec. 8.2.15). It follows that point A is usually a distributor turndown limit. Regardless of which limit point A represents, it is extremely sensitive to maldistribution (Fig. 8.16b). When liquid distribution is poor, it will take more liquid to wet the entire bed, and point A will shift to the right. If distribution is very poor, point A may never be observed, and the curve will have no flat region at all. A V-shaped curve is not uncommon, and is indicative of poor distribution. 

Region A-B (Fig. 8.16a) has turbulent liquid film, good wetting of the packing, good mass transfer, and essentially constant efficiency. This region is ideal for packed column design and operation. 

Raising gas velocity past point B moves column operation into the loading region. Initially, efficiency improves because of the greater liquid holdup (region B-E), but this improvement is short-lived. As the flood point is approached, the efficiency passes through a maximum (point E), and then drops (region E-C) because of excessive entrainment. 

Packed towers are usually designed for region A-B. Although region 

B-F gives the highest efficiency, it is usually avoided in design because of Hie proximity of the flood point. In practice, operation is normally stable and design efficiency or better is achieved throughout region A-F. 

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The propulsor, in a steady situation, has to provide the thrust equivalent to the resistance of the ship. The prediction of this balance between ship resistance and propeller thrust is a complex process because the water flow in the stern region of the ship interacts with the flow through the propeller. The ship resistance may be modified by the action of the propeller, while the propeller efficiency usually is influenced by the disturbed flow regime near the stern of the ship. 

Stirred tanks are modeled assuming that both phases are well mixed. Tray columns are usually modeled as well mixed on each tray so that the overall column is modeled as a series of two-phase, stirred tanks. (Distillation trays with tray efficiencies greater than 100% have some progressive flow within a tray.) When reaction is confined to a single, well-mixed phase, the flow regime for the other phase makes little difference but when the reacting phase approximates piston flow, the flow regime in the other phase must be considered. The important cases are where both phases approximate piston flow, either countercurrent or cocurrent. 

This chapter will only deal with the possible gas transport mechanisms and their relevance for separation of gas mixtures. Beside the transport mechanisms, process parameters also have a marked influence on the separation efficiency. Effects like backdiffusion and concentration polarization are determined by the operating downstream and upstream pressure, the flow regime, etc. This can decrease the separation efficiency considerably. Since these effects are to some extent treated in literature (Hsieh, Bhave and Fleming 1988, Keizer et al. 1988), they will not be considered here, save for one example at the end of Section 6.2.1. It seemed more important to describe the possibilities of inorganic membranes for gas separation than to deal with optimization of the process. Therefore, this chapter will only describe the possibilities of the several transport mechanisms in inorganic membranes for selective gas separation with high permeability at variable temperature and pressure. 

The conductance of any system depends on the nature of the gas flow at higher pressures a viscous flow regime prevails and at lower pressures (p < 10 Torr) a molecular flow regime. Turbulent flow, seldom considered when assessing the efficiency of a vacuum system, is encountered only when the pressure in the system is close to atmospheric pressure. As a general rule, if the efficiency of the system is adequate for the viscous flow regime then it will also be suitable for turbulent flow. 

The Reynolds number is based on superficial velocity. This equation is proposed for applications with organic liquids such as n-hcxane, light petroleum fractions, and similar species. In the trickle flow regime, the increase in the gas flow rate leads to a decrease in the wetting efficiency (Burghardt et al., 1995). 

In areas where sand or sediment production is anticipated, it should be removed pnor to flowing through a standard CPI Because of the required laminar flow regime, plate coalescers arc efficient sand-settling devices... 

In the first slit, the liquid wets the wall with a film of uniform thickness the gas being in the central core (wet slit). The second slit is visited exclusively by the gas (dry slit). The high-pressure-and high-temperature-wetting efficiency, liquid hold-up and pressure-drop data reported in the literature for TBR in the trickle-flow regime were successfully forecasted by the model. 

The average error for the 105 data points is -7.1% with a standard deviation of the error of 26.1%. The data (65 points ) for external contacting efficiency in the trickle-flow regime (11. 18, 20, 21) can be correlated by ... 

Prior to discussing scaleup, the factors that affect tray efficiency need to be addressed. These factors are addressed in Secs. 7.3.1 to 7.3.3. Considerations relevant to the effect of flow regime were previously discussed in Secs. 6.4.4 and 6.4.5. 

Some limitations to this procedure are mentioned above and in Ref. 208. In addition, it has been reported that reflux ratio may have a marked effect on Oldershaw column efficiency (213) and this variation must be studied carefully, especially if measurements are not conducted at total reflux. Also note that the formation of the cellular flow regime is composition-dependent (116,206), and it may occur under some conditions, but not under others. 

Conceptually similar results were demonstrated by Krutzer et al. [14], who measured the orthokinetic coagulation rate under laminar Couette flow and isotropic turbulent flow (as well as other flow conditions). Despite equal particle collision rates, significance differences were observed in the overall rates indicating different collision efficiencies (higher collision efficiencies were found under a turbulent flow regime). Thus, identical chemical properties of a dispersion do not determine a single collision efficiency the collision efficiency is indeed dependent upon the physical transport occurring in the system. 

For highly exothermic reactions, it is necessary to maintain the temperature at a desired level. In general, the heat-transfer coefficient is proportional to (Pg/F)0 25-033, and the actual power cost varies linearly with (Pt/V) these dependencies lead to an optimum value of the reactor volume. For these reactions, when the demand for mass transfer is not important, the radial-flow turbine (curved blades) can be used for an efficient heat transfer. Optimum stirring conditions in the laminar flow regime have been evaluated by Zlokarnik and Judat (1988) and Pawlowski and Zlokarnik (1972). 

A more detailed explanation about the use of tracer methods to evaluate contacting efficiency, the relationships needed to interpret tracer response data, the experimental methodology, and various results are given by Mills and Dudukovic (41). It suffices to say here that the following correlations based upon Reynolds and Galileo numbers were determined by El-Hisnawi to represent the available data on small porous packings in the trickle-flow regime ... 

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