Gas superficial velocities

The superficial gas velocity Uq is a description of the amount of gas present in the reactor volume and is defined as the volumetric gas flow rate per unit cross-sectional area of the reactor. This definition is easily quantifiable and has been used by many researchers as a correlation parameter for gas holdup and gas-liquid mass transfer. Most researchers cited in this chapter, with the exception of Linek et al. (2005a), have proposed a positive correlation of superficial gas velocity with gas-liquid mass transfer however, its specific influence on mass transfer is often confusing. 

If the impeller is operated below a minimum tip speed (2.25 m/s for RT), the reactor hydrodynamics are dominated by the gas flow and the reactor acts as a bubble column. At this point, gas-liquid mass transfer has an exclusive dependence on the superficial gas velocity (Charpentier, 1981). Since the intent of the STR is to provide agitation that would be superior to gas sparging alone, STRs are operated such that impeller agitation dominates the hydrodynamics (Nishikawa et al., 1981). 

The superficial gas velocity is often recognized to influence gas-liquid mass transfer through gas holdup (Nocentini et al., 1993) and its influence on the interfacial surface area (Garcia-Ochoa and Gomez, 2004). It is generally assumed that the interfacial surface area can be increased by entraining more gas in the reactor, and this results in increased gas dispersion and gas-Uquid mass transfer. 

Superficial gas velocity has an influence over the collision frequency. If more gas is present, there is a higher probability of colhsion (Martin et al., 2008b). Coalescence efficiency and drainage rate depend on the film properties which are a function of the liquid properties. The collision force, however, is the controlhng factor because the bubble diameter is a function of the power input (Bouaifi et al., 2001 Nocentini et al., 1993). 

increasing the superficial gas velocity may initially increase because there will be more gas bubbles and a larger gas-liquid interfacial area. However, further increasing superficial gas velocity could lead to bubble coalescence, which would increase the average bubble diameter and bubble rise velocity and lower the gas residence time (Moilanen et al., 2008). All of these factors would lower 

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Pig. 22. Schematic representation of typical pressure drop as a function of superficial gas velocity, expressed in terms of G = /9q tiQ, in packed columns. O, Dry packing , low Hquid flow rate I, higher Hquid flow rate. The points do not correspond to actual experimental data, but represent examples. 

Fig. 1. Fluidized-bed behavior where U is the superficial gas velocity and is the minimum fluidization velocity (a) packed bed, no flow (b) fluid bed,...

Terminal Velocity. The single-particle terminal velocity, U, is the gas velocity required to maintain a single particle suspended in an upwardly flowing gas stream. A knowledge of terminal velocity is important in fluidized beds because it relates to how long particles are retained in the system. If the operating superficial gas velocity in the fluidized bed far exceeds the terminal velocity of the bed particles, the particles are quickly removed. 

Flue particles ia a fluidized bed are analogous to volatile molecules ia a Foiling solution. Therefore, the concentration of particles ia the gas above a fluidized bed is a function of the saturation capacity of the gas. To calculate the entrainment rate, it is first necessary to determine what particle sizes ia the bed can be entrained. These particles are the ones which have a terminal velocity less than the superficial gas velocity, assuming that iaterparticle forces ia a dilute zone of the freeboard are negligible. An average particle size of the entrainable particles is then calculated. If all particles ia the bed are entrainable, the entrained material has the same size distribution as the bed material. 

Normally vessels are designed with the gas outlet location well above TDH. If circumstances force operation with a bed height so that the outlet is below TDH, an equivalent velocity, an effective velocity higher than the actual superficial gas velocity, is used ia the above calculation. The effective gas velocity can be determined from Figure 19 (27). 

Fig. 11. Flow regimes for air—water in a 2.5-cm horizontal pipe where is superficial Hquid velocity and is superficial gas velocity.

Analysis of a method of maximizing the usefiilness of smaH pilot units in achieving similitude is described in Reference 67. The pilot unit should be designed to produce fully developed large bubbles or slugs as rapidly as possible above the inlet. UsuaHy, the basic reaction conditions of feed composition, temperature, pressure, and catalyst activity are kept constant. Constant catalyst activity usuaHy requires use of the same particle size distribution and therefore constant minimum fluidization velocity which is usuaHy much less than the superficial gas velocity. Mass transport from the bubble by diffusion may be less than by convective exchange between the bubble and the surrounding emulsion phase. 

Circulating fluidized-beds do not contain any in-bed tube bundle heating surface. The furnace enclosure and internal division wall-type surfaces provide the required heat removal. This is possible because of the large quantity of soflds that are recycled internally and externally around the furnace. The bed temperature remains uniform, because the mass flow rate of the recycled soflds is many times the mass flow rate of the combustion gas. Operating temperatures for circulating beds are in the range of 816 to 871°C. Superficial gas velocities in some commercially available beds are about 6 m/s at full loads. The size of the soflds in the bed is usually smaller than 590 p.m, with the mean particle size in the 150—200 p.m range (81). 

Operational Considerations. The performance of catalytic incinerators (28) is affected by catalyst inlet temperature, space velocity, superficial gas velocity (at the catalyst inlet), bed geometry, species present and concentration, mixture composition, and waste contaminants. Catalyst inlet temperatures strongly affect destmction efficiency. Mixture compositions, air-to-gas (fuel) ratio, space velocity, and inlet concentration all show marginal or statistically insignificant effects (30). 

Fluidized This is an expanded condition in which the sohds particles are supported by drag forces caused by the gas phase passing through the interstices among the particles at some critical velocity. It is an unstable condition in that the superficial gas velocity upward is less than the terminal setting velocity of the solids particles the gas... 

Ue = liquid velocity relative to the gas, often approximately the terminal velocity of droplets (see Sec. 6 lor estimation) L/g = superficial gas velocity = droplet diameter... 

It is often helpful to use the relationship between and superficial gas velocity (t/sg) and the rise velocity of a gas bubble relative to the liquid velocity (U + Ui, with L/l defined as positive upward) ... 

AP is the pressure drop, cm of water Pg is the gas density, g/cm Ap is the total projected area of an entire row of baffles in the direction of inlet gas flow, cm" and At is the duct cross-sectional area, cm". The value jd is a drag coefficient for gas flow past inclined flat plates taken from Fig. 14-113, while L/ is the actual gas velocity, cm/s, which is related to the superficial gas velocity by U = L/g/cos 0. It must be noted that the angle of incidence 0 for the second and successive rows of baffles is twice the angle of incidence for the first row. Most of Calverts work was with 30° baffles, but the method correlates well with other data on 45° bafiles. 

FIG. 14-115 Experimental collection efficiencies of rectangular impactors. C is the Stokes-Ciinningbam correction factor Pp, particle density, g/ond U, superficial gas velocity, approaching the impactor openings, cm/s and ig, gas viscosity, P. Calveri, Yung, and Leung, NTIS Puhl. PB-24S050 based on Mercer and Chow, J. Coll. Interface Sci., 27, 75 (1.96S).]... 

Otner Collectors Tarry particulates and other difficult-to-handle hquids have been collected on a dry, expendable phenol formaldehyde-bonded glass-fiber mat (Goldfield, J. Air Pollut. Control A.SSOC., 20, 466 (1970)] in roll form which is advanced intermittently into a filter frame. Superficial gas velocities are 2.5 to 3.5 m/s (8.2 to 11.5 ft/s), and pressure drop is typically 41 to 46 cm (16 to 18 in) of water. CoUection efficiencies of 99 percent have been obtained on submicrometer particles. Brady [Chem. Eng. Prog., 73(8), 45 (1977)] has discussed a cleanable modification of this approach in which the gas is passed through a reticulated foam filter that is slowly rotated and solvent-cleaned. 

V/ Filtration velocity (superficial gas velocity tbrougb filter) m/min ft/min... 

FIG. 18-26 IXpk al curve for ma.s.s tran.sfer coefficient K.,a as i mixer power and superficial gas velocity. 

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