Packed Bed flooding.pdf - Youngstown State University's Personal ...

More documents

Recommendations

Info

(f) (a) This is remarkably similar to the empirical two-fluid atomizer relationships of El-Shanawany and Lefebvre [J. Energy, 4, 184 (1980)] and Jasuja [Trans. Am. Soc. Mech. Engr., 103, 514 (1981)]. For example, El-Shanawany and Lefebvre give a relationship for a prefilming atomizer: D32 = 0.0711(σ/ρG) 0.6 (1/velocity) 1.2 (1 + L/G)(Dnozzle) 0.4 (ρL/ρG) 0.1 + 0.015[(µL) 2 /(σ×ρL)] 0.5 (Dnozzle) 0.5 (1 + L/G) (14-199) where µL is liquid viscosity. According to Jasuja, D32 = 0.17(σ/ρG) 0.45 (1/velocity) 0.9 (1 + L/G) 0.5 (Dnozzle) 0.55 + viscosity term (14-200) [Eqs. (14-198), (14-199), and (14-200) are dimensionally consistent; any set of consistent units on the right-hand side yields the droplet size in units of length on the left-hand side.] The second, additive term carrying the viscosity impact in Eq. (14- 199) is small at viscosities around 1 cP but can become controlling as viscosity increases. For example, for air at atmospheric pressure atomizing water, with nozzle conditions Dnozzle = 0.076 m (3 in) velocity = 100 m/s (328 ft/s) L/G = 1 El-Shanaway measured 70 µm and his Eq. (14-199) predicted 76 µm. The power/mass correlation [Eq. (14-198)] predicts 102 µm. The agreement between both correlations and the measurement is much better than normally achieved. (b) (h) (c) (d) (e) (g) (i) PHASE DISPERSION 14-95 FIG. 14-87 Charactersitic spray nozzles. (a) Whirl-chamber hollow cone. (b) Solid cone. (c) Oval-orifice fan. (d) Deflector jet. (e) Impinging jet. ( f) Bypass. (g) Poppet. (h) Two-fluid. (i) Vaned rotating wheel. Rotary Atomizers For rotating wheels, vaneless disks, and cups, there are three regimes of operation. At low rates, the liquid is shed directly as drops from the rim. At intermediate rates, the liquid leaves the rim as threads; and at the highest rate, the liquid extends from the edge as a thin sheet that breaks down similarly to a fan or hollow-cone spray nozzle. As noted in Table 14-19, rotary devices have many unique advantages such as the ability to handle high viscosity and slurries and produce small droplets without high pressures. The prime applications are in spray drying. See Masters [Spray Drying Handbook, Wiley, New York (1991)] for more details. Pipeline Contactors The correlation for droplet diameter based on power/mass is similar to that for two-fluid nozzles. The dimensionless correlation is D32 = 0.8(σ/ρG) 0.6 (1/velocity) 1.2 (Dpipe) 0.4 (14-201) (The relation is dimensionally consistent; any set of consistent units on the right-hand side yields the droplet size in units of length on the lefthand side.) The relationship is similar to the empirical correlation of Tatterson, Dallman, and Hanratty [Am. Inst. Chem. Eng. J., 23(1), 68 (1977)] σ � ρG D 32 ∼ � � 0.5 (1/velocity) 1 (Dpipe) 0.5 Predictions from Eq. (14-201) align well with the Tatterson data. For example, for a velocity of 43 m/s (140 ft/s) in a 0.05-m (1.8-inch) equivalent diameter channel, Eq. (14-201) predicts D32 of 490 microns, compared to the measured 460 to 480 microns.
14-96 EQUIPMENT FOR DISTILLATION, GAS ABSORPTION, PHASE DISPERSION, AND PHASE SEPARATION FIG. 14-88 Droplet-size distribution for three different types of nozzles. To convert pounds per square inch gauge to kilopascals, multiply by 6.89; to convert gallons per minute to cubic meters per hour, multiply by 0.227. (Spraying Systems Inc.) Entrainment due to Gas Bubbling/Jetting through a Liquid Entrainment generally limits the capacity of distillation trays and is commonly a concern in vaporizers and evaporators. Fortunately, it is readily controllable by simple inertial entrainment capture devices such as wire mesh pads in gravity separators. In distillation towers, entrainment lowers the tray efficiency, and 1 pound of entrainment per 10 pounds of liquid is sometimes taken as the limit for acceptable performance. However, the impact of entrainment on distillation efficiency depends on the relative volatility of the component being considered. Entrainment has a minor impact on close separations when the difference between vapor and liquid concentration is small, but this factor can be dominant for systems where the liquid concentration is much higher than the vapor in equilibrium with it (i.e., when a component of the liquid has a very low volatility, as in an absorber). As shown by Fig. 14-90, entrainment droplet sizes span a broad range. The reason for the much larger drop sizes of the upper curve is the short disengaging space. For this curve, over 99 percent of the entrainment has a terminal velocity greater than the vapor velocity. For contrast, in the lower curve the terminal velocity of the largest particle reported is the same as the vapor velocity. For the TABLE 14-20 Exponential Dependence of Drop Size on Different Parameters in Two-Fluid Atomization Relative Surface Atomizer velocity tension Gas density 1 + L/G dimension Jasuja (empirical for small nozzle) −0.9 0.45 −0.45 0.5 0.55 El-Shanawany and Lefebvre (empirical for small nozzle) −1.2 0.6 −0.7 1 0.40 Tatterson, Dallman, and Hanratty (pipe flow) −1 0.5 −0.5 0.5 Power/mass −1.2 0.6 −0.6 0.4 0.4 settling velocity to limit the maximum drop size entrained, at least 0.8 m (30 in) disengaging space is usually required. Note that even for the lower curve, less than 10 percent of the entrainment is in drops of less than 50 µm. The coarseness results from the relatively low power dissipation per mass on distillation trays. This means that it is relatively easy to remove by a device such as a wire mesh pad. Over 50 percent is typically captured by the underside of the next higher tray or by a turn in the piping leaving an evaporator. Conversely, though small on a mass basis, the smaller drops are extremely numerous. On a number basis, more than one-half of the drops in the lower curve are under 5 µm. These can serve as nuclei for fog condensation in downstream equipment. Entrainment E is inherent in the bubbling process and can stem from a variety of sources, as shown by Fig. 14-89. However, the biggest practical problem is entrainment generated by the kinetic energy of the flowing vapor rather than the bubbling process. As vapor velocity approaches the flooding limit [Eq. (14-168)], the entrainment rises approximately with (velocity) 8 . Pinczewski and Fell [Trans. Inst. Chem Eng., 55, 46 (1977)] show that the velocity at which vapor jets onto the tray sets the droplet size, rather than the superficial tray velocity. The power/mass correlation predicts an average drop size close to that measured by Pinczewski and Fell. Combination of this prediction with the estimated fraction of the droplets entrained gave a relationship for entrainment, Eq. (14- 202). The dependence of entrainment with the eighth power of velocity even approximates the observed velocity dependence, as flooding is approached. constant(velocity) E = (14-202) 8 (ρG) 4 �� (Af) 3 (ρL −ρG) 2.5 σ 1.5 (Here E is the mass of entrainment per mass of vapor and A f is the fractional open area on the tray.) When flooding is defined as the condition that gives E of 1, the flood velocity is estimated by Eq. (14-203).
Page 1 and 2: Previous Page 14-58 EQUIPMENT FOR D
Page 3 and 4: 14-60 TABLE 14-13 Characteristics o
Page 5 and 6: 14-62 EQUIPMENT FOR DISTILLATION, G
Page 21 and 22: 14-78 (a) (b) (c) FIG. 14-71 High-v
Page 37: 14-94 EQUIPMENT FOR DISTILLATION, G
Page 43 and 44: 14-100 EQUIPMENT FOR DISTILLATION,

Packed Bed flooding.pdf - Youngstown State University's Personal ...

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?