Superficial gas flow rate

Pressure-drop data obtained in these tests were correlated empirically by defining a friction-factor based on gas-phase properties and superficial gas-flow rates. This friction-factor was plotted against the group Glijll/Ggi G with the water-oil feed mass-ratio as a parameter. 

For systematic study of several gas-liquid chemical reactions using a laboratory model bubble-cap column, Sharmaet al. (S23) have shown that the presence of electrolytes, size of caps, type of slots, ionic strength, liquid viscosity, and presence of solids do not affect the mass-transfer rates. These rates chiefly depend on the gas and liquid flow rates (S23, M2). The influence of the superficial gas flow rate on ki, and Icq is indicated in Fig. 20. Interfacial area a" per unit area of plate (or per unit floor area) for plate diameters varying between 0.15 and 1.20 m have been grouped in Fig. 21, which with the following correlations (S23) can be used to scale up bubble-cap plates up to 2 or 3 m in diameter ... 

For packed beds Sh and Re are related to the average particle diameter and to the superficial gas flow rate u (volume flow divided by cross sectional area) ... 

The superficial gas flow rate (volumetric flow rate/cross sectional area). 

In the older correlations, the effects of dissolved electrolytes and gas pressure were neglected. It appeared that for larger vessels, with diameters > 0.15 m, the most important independent variables are the superficial gas flow rate u, the acceleration of gravity g, and the density p, the viscosity i and the surface tension a of the liquid (with the gas). It appears that these can be grouped in two... 

Van Dierendonck (1971) found that the gas-bubble fraction in a bubble column (for columns with a diameter > 0.15 m) is a function of one process parameter, the superficial gas flow rate w, and physical parameters only ... 

Figure 4.11. The bubble holdup versus the superficial gas flow rate, for three values of the transition gas flow rate u = 0.015, 0.05 and 0,10 m/s,...

Considering the nature of the bubble behaviour, one would expect that the specific surface area a could be related to the superficial gas flow rate very much as the bubble holdup, as expressed by eqs. (4.56a) and (4.56b) ... 

In the practice of chemical reactor development, one is usually interested in the effects of process variables and scale, for a given gas/liquid system. For given geometrical ratios of a stirred vessel, the specific surface area should a function of the impeller speed and the net superficial gas flow rate. Empirical relations are often expressed in terms of the specific energy dissipation of the impeller e and the gas load. Correlations for electrolyte solutions, taken from the literature cited above, can be summarized as follows ... 

For reactor scale-up for a given system, one may proceed as follows. First, a stirred bench scale reactor should be chosen with standardized dimensions. The net superficial gas flow rate should be kept within the indicated limits. Mass transfer measurements (with or without chemical reaction) should be carried out under realistic conditions. The influence of the stirrer speed should be measured accurately over a wide range. The superficial gas flow rate may also be varied, e.g., between 1 and 4 cm/s. With these experiments the constants in eq. (4.62) can be determined. The resulting equation may be used with confidence for the larger scale. The variables e and w, on the larger scale, should be within the limits of the bench scale tests ... 

A CSTR of 3 1 is used for a gas/liquid reaction. The gas is dispersed by a turbine impeller. The reaction rate appears to be proportional to the interfacial area. The mean residence time is 1000 s, the stirrer speed is 10 s and the superficial gas flow rate is 0.005 m/s. ( e wishes to carry out the process in a geometrically similar reactor of 1.5 m, under the same conditions. What stirrer speed is required ... 

Ghirardini et al. (1991) measured the specific surface area (a) with the help of a rapid chemical reaction (see section 5A.2J)y and found the following empirical correlation with the superficial gas flow rate in the leg and its diameter, for aqeous solutions... 

Om larger scales, stirred gas/liquid reactors can often be replaced by bubble columns. The reason is obvious the superficial gas flow rate increases proportionally to the linear dimensions of the reactor vessel. When it becomes more than about 0.1 m/s, the use of a stirrer is no longer practical. By adjusting the height/diameter ratio the superficial gas flow rate can be adjusted. When it is in the order of 0.2 m/s (for gases with densities comparable to air), a good dispersion is usually obtained. One should be prepared for the risk of foam formation, however. 

This simply means that the gas should have the same mean residence time. When we choose the dimensions of die bed as follows diameter 2.1 m, height 3 m, the superficial gas flow rate has to be about 0.4 m/s, which seems reasonable. 

Figure 4A.1. Mass transfer capacity, expressed as number of transfer units, as a function of bed height, for three values of the superficial gas flow rate, (0.05 - 0.2 mis), see eq. (4A.4).

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