Condensed film

While L2 films ultimately extrapolate to some limiting area at high pressures, that area is usually found to be some 20% larger than the cross-sectional area of a hydrocarbon chain taken from X-ray data, or 10% greater than the respective condensed film (0.22 versus 0.205 nm ). 

Condensed films are composed of densely packed, highly oriented molecules with little mobility and low compressibility. Unlike the gaseous films, the ti-A 

FIGURE 8.15. As pressure is applied to a cxjndensed flhn, the adsorbed molecules can rearrange to a small extent by a change in packing structure (e.g., cubic to hexagonal). Beyond that point added pressure will result in flhn buckling.  

FIGURE 8.16. A monomolecular film of a straight chain carboxylic acid such as myristic acid on distilled water will show a sharp transition in the n-A curve as the head groups become more closely packed. That kind of transition may be viewed as something like a reversed sublimation in which the film passes from gaseous to solid condensed without passing through the liquid expanded state. With very careful experimental work, it is sometimes possible to identify an intermediate liquid expanded phase as illustrated for myristic acid on 0.1 N HCl. 

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A plot of G x versus composition is shown in Fig. IV-22 for condensed films of octadecanol with docosyl sulfate. Gaines [241] and Cadenhead and Demchak [242] have extended the above approach, and the subject has been extended and reviewed by Barnes and co-workers (see Ref. 243). 

Fig. IV-22. Excess free energy of mixing of condensed films of octadecanol-docosyl sulfate at 25°C, at various film pressures. Top curve t = 5 dyn/cm bottom curve ir = 50 dyn/cm intermediate curves at 5-dyn/cm intervals. The curves are uncorrected for the mixing term at low film pressure. (From Ref. 246.)...

It is evident that boundary lubrication is considerably dependent on the state of the monolayer. Frewing [48] found that, on heating, the value of fi rose sharply near the melting point sometimes accompanied by a change from smooth to stick-slip sliding. Very likely these points of change correspond to the transition between an expanded film and a condensed film in analogy with... 

It is known that even condensed films must have surface diffusional mobility Rideal and Tadayon [64] found that stearic acid films transferred from one surface to another by a process that seemed to involve surface diffusion to the occasional points of contact between the solids. Such transfer, of course, is observed in actual friction experiments in that an uncoated rider quickly acquires a layer of boundary lubricant from the surface over which it is passed [46]. However, there is little quantitative information available about actual surface diffusion coefficients. One value that may be relevant is that of Ross and Good [65] for butane on Spheron 6, which, for a monolayer, was about 5 x 10 cm /sec. If the average junction is about 10 cm in size, this would also be about the average distance that a film molecule would have to migrate, and the time required would be about 10 sec. This rate of Junctions passing each other corresponds to a sliding speed of 100 cm/sec so that the usual speeds of 0.01 cm/sec should not be too fast for pressurized film formation. See Ref. 62 for a study of another mechanism for surface mobility, that of evaporative hopping. 

The adsorbed layer at G—L or S—L surfaces ia practical surfactant systems may have a complex composition. The adsorbed molecules or ions may be close-packed forming almost a condensed film with solvent molecules virtually excluded from the surface, or widely spaced and behave somewhat like a two-dimensional gas. The adsorbed film may be multilayer rather than monolayer. Counterions are sometimes present with the surfactant ia the adsorbed layer. Mixed moaolayers are known that iavolve molecular complexes, eg, oae-to-oae complexes of fatty alcohol sulfates with fatty alcohols (10), as well as complexes betweea fatty acids and fatty acid soaps (11). Competitive or preferential adsorption between multiple solutes at G—L and L—L iaterfaces is an important effect ia foaming, foam stabiLizatioa, and defoaming (see Defoamers). 

The Reynolds number of the condensate film (falling film) is 4r/ I, where F is the weight rate of flow (loading rate) of condensate per unit perimeter kg/(s m) [lb/(h ft)]. The thickness of the condensate film for Reynolds number less than 2100 is (SflF/p g). ... 

Vertical Tubes For the following cases Reynolds number < 2100 and is calculated by using F = Wp/ KD. The Nusselt equation for the heat-transfer coefficient for condensate films may be written in the following ways (using liquid physical properties and where L is the cooled lengm and At is — t,) ... 

The Diikler theory is applicable for condensate films on horizontal tubes and also for falling films, in general, i.e., those not associated with condensation or vaporization processes. 

Vertical In-Shell Condensers Condensers are often designed so that condensation occurs on the outside of vertical tubes. Equation (5-88) is valid as long as the condensate film is laminar. When it becomes turbulent. Fig. 5-10 or Colburns equation [Tran.s. Am. Jn.st. Chem. Ertg., 30, 187 (1933-1934) maybe used. 

Vertical in-tube condensers are often designed for reflux or knock-back application in reactors or distillation columns. In this case, vapor flow is upward, countercurrent to the hquid flow on the tube wall the vapor shear ac4s to tliicken and retard the drainage of the condensate film, reducing the coefficient. Neither the fluid dynamics nor the heat transfer is well understood in this case, but Sohman, Schuster, and Berenson [J. Heat Transfer, 90, 267-276... 

Figure 1 Condensing film coefficients. (By permission, D. Q. Kern, Process Heat Transfer, first edition, McGraw-Hill Book Co., 1950.)...

Figure 10-67A. Condensing film coefficients outside horizontal or vertical tubes. (Used by permission Kern, D.Q. Process Heat Transfer, Ed., 1950. McGraw-Hill, Inc. All rights reserved.)...

Figure 10-67B. Correlation of McAdams representing the condensing film coefficient on the outside of vertical tubes, integrated for the entire tube length. This represents the streamline transition and turbulent flow conditions for Prandtl numbers 1 and 5. Do not extrapolate Prandtl numbers, Pr beyond 5. (Used by permission Engineering Data Book II 1984, Wolverine Tube, Inc.)...

Nofe These Condensate Film Coefficients for Verticol Surfaces ore Restricted to GV/i f =1090. To Obtain Theoreticol hem for Horizontoi Tubes,use the above Nomogram and Multiply Result by 0.8. ... 

Figure 10-70. Condensate film coefficients—vertical or horizontal. (Used by permission Devore, A. Petroleum Refiner, V. 38, No. 6, 1959. Gulf Publishing Company, Houston, Texas. All rights reserved.)...

Estimate film temperature of fluid on the outside of tubes and determine At across condensing film. 

Check the assumed temperature drop across the condensate film. At. 

The mean temperature of condensate film before subcooling ... 

H = heat transfer coefficient ratio, h /hN h i = effective heat transfer film coefficient, Btu/hr-ff-°F hNu condensing film coefficient by Nusselt equation Btu/hr-ff-°F... 

Calculate the tube-side film coefficient, hj, and reference it to the outside of the tube, hj. Calculate the condensing film coefficient ... 

M, = average molecular weight of vapor, dimensionless Po = partial pressure of vapor at the condensate film, °F Po = partial pressure of vapor in gas body, atm L = temperature of condensate film, °F tg = temperature of dry gas (inerts), °F L = temperature of water, °F = latent heat of vaporization, Btu/lb... 

L = temperature of condensate film, °F p = vapor pressure of condensate at L, psia or atm... 

Because no condensation (theoretically) occurs in this section, no heat is diffused through condensate film. 

Medwell, J. O. and A. A. Nicol, Surface Roughness Effects on Condensate Films, ASME-AlChE Heat Trans. Conference and Exhibit, Los Angeles, California, Aug. (1965), Paper No. 65-HT-43. 

Rodriguez, E, andj. C. Smith, When Non-Condensables Are Present Make a Nomograph to Find the Condensate Film Temperature, Chem. Eng, Mar. 10, (1958) p. 150. 

Whereas a film formed in dry air consists essentially of an anhydrous oxide and may reach a thickness of 3 nm, in the presence of water (ranging from condensed films deposited from humid atmospheres to bulk aqueous phases) further thickening occurs as partial hydration increases the electron tunnelling conductivity. Other components in contaminated atmospheres may become incorporated (e.g. HjS, SO2, CO2, Cl ), as described in Sections 2.2 and3.1. 

Aqueous environments will range from very thin condensed films of moisture to bulk solutions, and will include natural environments such as the atmosphere, natural waters, soils, body fluids, etc. as well as chemicals and food products. However, since environments are dealt with fully in Chapter 2, this discussion will be confined to simple chemical solutions, whose behaviour can be more readily interpreted in terms of fundamental physicochemical principles, and additional factors will have to be considered in interpreting the behaviour of metals in more complex environments. For example, iron will corrode rapidly in oxygenated water, but only very slowly when oxygen is absent however, in an anaerobic water containing sulphate-reducing bacteria, rapid corrosion occurs, and the mechanism of the process clearly involves the specific action of the bacteria see Section 2.6). 

There are many special factors controlling atmospheric bimetallic corrosion that entitle it to separate treatment. The electrolyte in atmospheric corrosion consists of a thin condensed film of moisture containing any soluble contaminants in the atmosphere such as acid fumes from industrial atmospheres and chlorides from marine atmospheres. This type of electrolyte has two characteristics which are summarised in a paper by Rosenfel d . 

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