Transition Between Flow Regimes

Froth-Spray Froth-spray transition has been investigated for sieve trays using a variety of techniques. The gradual nature of this transition bred a multitude of criteria for defining it, and made its correlation difficult. Excellent overviews were given by Lockett (Distillation Tray Fundamentals, Cambridge University Press, Cambridge, England, 1986) and Prado, Johnson, and Fair [Chem. Eng. Progr. 83(3), p. 32 (1987)]. Porter and Jenkins [I. Chem. E. Symp. Ser. 56, Summary Paper (1979)] presented a simple correlation for the froth-to-spray transition.

The terms of this equation are in English units and are explained in the Nomenclature. This correlation is based on the premise that froth-to-spray transition occurs when the entrainment vs. liquid load relationship passes through a minimum (see "Entrainment"). Alternatively, it was argued that the minimum represents a transition from the froth regime to a partially developed spray region (Kister, Pinczewski, and Fell, Paper presented in the 90th National AIChE Meeting, Houston, April 1981). If this alternative argument is valid, then when the correlation predicts froth, it is highly unlikely that the column operates in the spray regime; but when it predicts spray, the column may still be operating in the froth regime. Recent entrainment studies by Ohe [Distillation 2005: Topical Conference Proceedings, AIChE Spring National Meeting, p. 283, Atlanta (April 10-13, 2005)] argue that the entrainment minima represent minimum liquid residence times on the tray, and are unrelated to the froth-spray transition.

A second correlation is by Pinczeweski and Fell [Ind. Eng. Chem. Proc. Des. Dev. 21, p. 774 (1982)]

The terms of Eq. (14-128) are in English units and are explained in the Nomenclature. The exponent n is calculated from Eq. (14-84). Equation (14-128) is based on transition data obtained from orifice jetting measurements for the air-water system and on entrainment minimum data for some hydrocarbon systems.

A third recent correlation by Johnson and Fair (loc. cit.) is

U* = C1 Pg0'50 pL'692 c006Af°-25 ^ N— j0 05dh01 (14-129)

where U* = gas velocity through active area at inversion, m/s pG = gas density, kg/m3 PL = liquid density, kg/m3 C = surface tension, mN/m Af = hole/active area ratio

FIG. 14-39 Vapor crossflow channeling. Note entrainment near the tray middle and outlet, and weep near the tray inlet. (Kister, H. Z., K. F. Larson, and P. Madsen, Chem. Eng. Prog., Nov. 1992, p. 86; reproduced with permission.)

q/Lw = liquid flow, m3/(s-m) weir dh = hole diameter, mm C\ = 0.0583 for 25.4-mm overflow weirs = 0.0568 for 50.4-mm overflow weirs = 0.0635 for 101.6-mm overflow weirs

Froth-Emulsion Froth-emulsion transition occurs [Hofhuis and Zuiderweg, I. Chem. E. Symp. Ser. 56, p. 2, 2/1 (1979)] when the aerated mass begins to obey the Francis weir formula. Using this criterion, the latest version of this transition correlation is

Lwhc

The terms of this equation are in English units and are explained in the Nomenclature; hc is calculated from the Hofhuis and Zuiderweg (loc. cit.) equation.

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