Condensing vapor dropwise condensation

Film-type condensation is more common and more dependable. Dropwise condensation normally needs to be promoted by introducing an impurity into the vapor stream. Substantially higher (6 to 18 times) coefficients are obtained for dropwise condensation of steam, but design methods are not available. Therefore, the development of equations for condensation will be for the film type only. 

In a i-l. three-necked flask are mixed 150 g. (r.63 moles) of /3-hydroxyethyl methyl sulfide (p. 54) (Note i) and 200 g. of dry chloroform (Note 2). The flask is placed on a steam bath and is fitted with a dropping funnel, a mechanical stirrer, and a condenser. The condenser is fitted with a trap to remove the vapors of hydrogen chloride and sulfur dioxide (page 2). A solution of 204 g. (1.7 moles) (Note 3) of thionyl chloride in 200 g. (135 cc.) of dry chloroform is added dropwise to the /3-hydroxyethyl methyl sulfide over a period of about two hours (Note 4). The reaction mixture is stirred vigorously during this addition and for about four hours after the addition is complete. The chloroform is distilled on the steam bath and the residue is distilled under reduced pressure. The yield is 135-153 g- (75 5 per cent of the theoretical amount) of a product boiling at 55-s6°/3o mm- (Note 5). 

In a i-l. round-bottomed flask are placed 12 g. (0.5 gram atom) of magnesium powder, 37 g. (0.5 mole) of tert.-bvlyl alcohol, and 100 g. of anhydrous ether (Note 1). The flask is fitted with an addition tube, one arm of which bears a reflux condenser, and the other arm a dropping funnel. While the mixture is being shaken by hand, a solution of 55 g. (0.7 mole) of acetyl chloride (Note 2) in 50 g. of anhydrous ether is added dropwise (Note 3). A lively reaction gradually ensues with evolution of hydrogen, mixed with ether vapor and a little hydrogen chloride (Note 4). After all the acetyl chloride has been added, the reaction mixture is allowed to stand in a pan of cold water for one hour (Note 5). After another hour at room temperature the mixture is warmed in a water bath at 40-450 for one-half hour in order to complete the reaction. 

To a flask equipped with stirrer, condenser, and dropping funnel is added 40.0 gm (0.61 gm-atom) of zinc dust in 40 ml of absolute ethanol. After the condenser outlet is connected to a series of Dry Ice-acetone traps cooled to —80° to —70°C, the mixture is heated to gentle reflux while 20.3 gm (0.0852 mole) of l,2-dibromo-l,l-difluoropropene in 30 ml of 95% ethanol is added dropwise over a 2 hr period. The reaction mixture is heated for an additional hour, and then the gaseous products which are condensed in the Dry Ice traps are purified by several vaporization distillations to afford 3.6 gm (56 %) b.p. 

Dimethyl Tellurium1 (Potassium Hydroxide-Tin(II) Chloride Method A mixture of 100 g (1.78 mol) potassium hydroxide and 180 m/ dimethyl sulfoxide is heated on a water bath. After addition of 10 g (53 mmol) tin(Il) chloride and 60 g (0.47 mol) tellurium powder, the mixture is heated at 120" for 20 h. The mixture is cooled to 40° and 157 g (1.1 mol) methyl iodide are added dropwise over a period of 2 h. The mixture is then heated at 70° for 2 h and then distilled until the condensing vapor reaches a temperature of 100°. The organic layer of the residue is separated from the aqueous layer. The organic layer is dried and distilled at atmospheric pressure yield 97% b.p. 93-94". 

Consider a vertical flat plate exposed to a condensable vapor. If the temperature of the plate is below the saturation temperature of the vapor, condensate will form on the surface and under the action of gravity will flow down the plate. If the liquid wets the surface, a smooth film is formed, and the process is called film condensation. If the liquid does not wet the surface, droplets are formed which fall down the surface in some random fashion. This process is called dropwise condensation. In the film-condensation process the surface is blanketed by the film, which grows in thickness as it moves down the plate. A temperature gradient exists in the film, and the film represents a thermal resistance to heat transfer. In dropwise condensation a large portion of the area... 

Because of the higher heat-transfer rates, dropwise condensation would be preferred to Him condensation, but it is extremely difficult to maintain since most surfaces become wetted after exposure to a condensing vapor over an extended period of time. Various surface coatings and vapor additives have been used in attempts to maintain dropwise condensation, but these methods have not met with general success to date. Some of the pioneer work on drop condensation was conducted by Schmidt [26] and a good summary of the overall problem is presented in Ref. 27. Measurements of Ref. 35 indicate that the drop conduction is the main resistance to heat flow for atmospheric pressure and above. Nucleation site density on smooth surfaces can be of the order of 10 sites per square centimeter, and heat-transfer coefficients in the range of 170 to 290 kW/m2 °C [30,000 to 50,000 Btu/h ft2 °F] have been reported by a number of investigators. 

Two distinct forms of condensation are observed Jilni condensation and dropwise condensation. In film condensation, the condensate wets the surface and forms a liquid film on the surface that slides down under the influence of gravity. The tliickne.ss of the liquid film increases in the flow direction as more vapor condenses on the film. This is how condensation normally occurs in practice. In dropwise condensation, the condensed vapor forms droplets on the surface instead of a continuous film, and the surface is covered by countless droplets of varying diameters (Fig. 10-20). 

Under some surface conditions, the condensate does not form a continuous film. Droplets are formed which grow, coalesce, and then run from the surface. As a fraction of the surface is always directly exposed to the vapor, film resistance is absent, and heat-transfer coefficients, which may be ten times those of film condensation, are obtained. This process is known as dropwise condensation. Although highly desirable, its occurrence, which depends upon the wettability of the surface, is not predictable and cannot be used as a basis for design. 

Condensation occurs whenever a vapor, a vapor mixture, or a vapor containing a noncondensable gas is brought into contact with a surface below the dew point or saturation temperature of the vapor. The condensed liquid is most likely to form a continuous film covering the cooled surface. In some cases, however, dropwise condensation is possible if the fluid does not wet the surface. Filmwise condensation is encountered in most industrial applications and is the only mode of condensation we shall consider further. 

An intimate mixture of 150 g. (0.64 mole) of dry e-benzoyl-aminocaproic add (p. 20) and 26.4 g. (0.85 gram atom) of dry red phosphorus is placed in a i-l. three-necked flask provided with a separatory funnel, an air-cooled condenser connected through a calcium chloride tube to a water trap, and a mechanical stirrer (Note i). The reaction flask is surrounded by an ice-salt mixture, the stirrer started, and 408 g. (131 cc. 2.55 moles) of dry bromine added dropwise from the separatory funnel. When all the bromine has been added, the cooling bath is removed. The mixture is warmed slowly at first and finally heated on a steam bath, with stirring, until the bromine vapors have practically disappeared. The hot mixture is poured slowly into 400 cc. of water in a i-l. beaker with hand stirring. The viscous acid bromide reacts with the water with the evolution of heat, and the solid acid is formed. The lumps are pulverized, the mixture is replaced in the original reaction flask, and the whole treated with a slow stream of sulfur dioxide to remove excess bromine. The solid product is filtered on a Buchner funnel, washed with... 

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