Condensation of vapour mixtures

In the process industry, vapour mixtures are frequently liquified with all the components being present in the liquid phase. Alternatively, the vapour mixture may contain inert gases that do not condense. The influence of this type of mixture on the heat transfer in condensation has already been dealt with in section 4.1.4. Therefore, the variation in the heat transfer for a condensate that contains all the components, in larger or smaller amounts, still has to be discussed. 

As the components with the higher boiling points normally condense first out of a vapour mixture, and the vapour left is lacking in these components, a concentration profile which varies along the flow direction forms. This decisively affects the heat transfer. 

This suggests, that for the calculation of the heat and mass transfer, it would be sensible to subdivide the flow path into individual sections, and then solve the mass and energy balances for each section, taking the laws of heat and mass transfer into consideration. Calculations of this type for binary mixtures are only possible with the assistance of a computer. In the discussions presented here we want to limit ourselves to an explanation of the fundamental physical processes along with an illustration of the decisive balance equations. As the process of condensation in multicomponent mixtures with more than two components is similar to that in binary mixtures, we will limit ourselves to the consideration of binary mixtures. 

When a binary mixture, whose boiling and dew point lines are shown in Fig. 4.19, condenses on a cooled wall of temperature -j90, a condensate forms, Fig. 4.19b, which is bounded by the vapour. At the phase interface, a temperature i i develops, which lies between the temperature dc of the vapour far away from the wall and the wall temperature d0. If the vapour is saturated, its temperature is i9a = 0S corresponding to Fig. 4.19a. The temperature profile in the vapour and condensate are illustrated in Fig. 4.19c. 

In general, at the phase interface the components with the higher boiling points preferentially change from vapour to condensate. As a result of this the vapour mixture at the phase interface contains more of the more volatile component than at a large distance away from it. A concentration profile like that presented in Fig. 4.19d develops. The concentration of the more volatile component increases towards the phase interface. At steady-state, the molecules of the more volatile 

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Similarly to Nusselt s film condensation theory, in the condensation of vapour mixtures, the heat flux transferred increases with the driving temperature difference — 1 0. According to Nusselt s film condensation theory, the heat transfer coefficient decreases with the driving temperature difference according to a ( oo- o) 1/4 (4-12). The heat flux increases in accordance with q co — q)3/4. Fig. 4.21a shows clearly that a minimum for the transferred heat flux exists at a certain temperature oa. This is because the temperature difference dj — r)(j between the condensate surface and the wall, which is decisive for heat transfer, also assumes a minimum this can be explained by the boiling diagram, Fig. 4.21. 

Hulden, B., Condensation of Vapours from Gas-Vapour Mixtures. An Approximate Method of Design, Chem. Eng. Science, V. 7, p. 60 (1957). 

Cot. BURN, A.P. Proceedings of the General Discussion on Heat Transfer, September, 1951. p. [.Problems in design und research on condensers of vapours and vapour mixtures. (Inst, of Mech. Eng. and Am. Soc. Mech. Eng.). 

Table 16.7 shows typical values for partial heat transfer coefficients that can be used in preliminary design. The assumed values should be checked in the final design. Particularly attention should be given to the situations involving two-phase mixtures, hydrogen-rich gases and condensation of vapours with non-condensable gases. 

The component parts include processes such as the separation of liquid mixtures by distillation or liquid-liquid extraction, and of solids from liquids by filtering, sedimentation or centrifuging the heating, cooling and evaporation of liquids and the condensation of vapours the drying of solids and the chemical conversion of a continuous stream of solid or fluid material. 

CH2 CH CH0. a colourless, volatile liquid, with characteristic odour. The vapour is poisonous, and intensely irritating to eyes and nose b.p. 53"C. It is prepared by the distillation of a mixture of glycerin, potassium sulphate and potassium hydrogen sulphate. It is manufactured by direct oxidation of propene or cross-condensation of ethanal with meth-anal. 

The material to be steam-distilled (mixed with some water if a solid compound, but not otherwise) is placed in C, and a vigorous current of steam blown in from D. The mixture in C is thus rapidly heated, and the vapour of the organic compound mixed with steam passes over and is condensed in E. For distillations on a small scale it is not necessary to heat C if, however, the flask C contains a large volume of material or material which requires prolonged distillation, it should be heated by a Bunsen burner, otherwise the steady condensation of steam in C will produce too great a volume of liquid. 

The set-up of Fig. 11, 41, 3 ensures the complete condensation of the steam when a rapid flow of steam is necessary for satisfactory results, and is useful in the distillation of large volumes of liquids of low vapour pressure, such as nitrobenzene. Thus the flask A containing the mixture may be of 3-litre capacity and B may be a 1-litre flask the latter is cooled by a stream of water, which is collected in a funnel and conducted to the sink. The receiver C must be of proportionate size all stoppers... 

Vinylacetic acid. Place 134 g. (161 ml.) of allyl cyanide (3) and 200 ml. of concentrated hydrochloric acid in a 1-htre round-bottomed flask attached to a reflux condenser. Warm the mixture cautiously with a small flame and shake from time to time. After 7-10 minutes, a vigorous reaction sets in and the mixture refluxes remove the flame and cool the flask, if necessary, in cold water. Ammonium chloride crystallises out. When the reaction subsides, reflux the mixture for 15 minutes. Then add 200 ml. of water, cool and separate the upper layer of acid. Extract the aqueous layer with three 100 ml. portions of ether. Combine the acid and the ether extracts, and remove the ether under atmospheric pressure in a 250 ml. Claisen flask with fractionating side arm (compare Fig. II, 13, 4) continue the heating on a water bath until the temperature of the vapour reaches 70°. Allow the apparatus to cool and distil under diminished pressure (compare Fig. II, 20, 1) , collect the fraction (a) distilling up to 71°/14 mm. and (6) at 72-74°/14 mm. (chiefly at 72 5°/ 14 mm.). A dark residue (about 10 ml.) and some white sohd ( crotonio acid) remains in the flask. Fraction (6) weighs 100 g. and is analytically pure vinylacetic acid. Fraction (a) weighs about 50 g. and separates into two layers remove the water layer, dry with anhydrous sodium sulphate and distil from a 50 ml. Claisen flask with fractionating side arm a further 15 g. of reasonably pure acid, b.p. 69-70°/12 mm., is obtained. 

The composition of the vapour in equilibrium with a miscible liquid mixture at any temperature, e.g. on heating during distillation, will be enriched by the more volatile components. The composition of the liquid phase produced on partial condensation will be enriched by the less volatile components. Such fractionation can have implications for safety in tliat tlie flammability and relative toxicity of the mixtures can change significantly. 

Mass transfer may take place from a mixture of gases, such as the condensation of water from moist air. In this instance, the water vapour has to diffuse through the air, and the rate of mass transfer will depend also on the concentration of vapour in the air. In the air-water vapour mixture, the rate of mass transfer is roughly proportional to the rate of heat transfer at the interface and this simplifies predictions of the performance of air-conditioning coils [1,5, 9]. 

In the previous discussion it has been assumed that the vapour is a pure material, such as steam or organic vapour. If it contains a proportion of non-condensable gas and is cooled below its dew point, a layer of condensate is formed on the surface with a mixture of non-condensable gas and vapour above it. The heat flow from the vapour to the surface then takes place in two ways. Firstly, sensible heat is passed to the surface because of the temperature difference. Secondly, since the concentration of vapour in the main stream is greater than that in the gas film at the condensate surface, vapour molecules diffuse to the surface and condense there, giving up their latent heat. The actual rate of condensation is then determined by the combination of these two effects, and its calculation requires a knowledge of mass transfer by diffusion, as discussed in Chapter 10. 

In the design of a cooler-condenser for a mixture of vapour and a permanent gas, the method of Colburn and Hougen(66) is considered. This requires a point-to-point calculation of the condensate-vapour interface conditions T( and P . A trial and error solution is required of the equation ... 

The separation of liquid mixtures by distillation depends on differences in volatility between the components. The greater the relative volatilities, the easier the separation. The basic equipment required for continuous distillation is shown in Figure 11.1. Vapour flows up the column and liquid counter-currently down the column. The vapour and liquid are brought into contact on plates, or packing. Part of the condensate from the condenser is returned to the top of the column to provide liquid flow above the feed point (reflux), and part of the liquid from the base of the column is vaporised in the reboiler and returned to provide the vapour flow. 

The correlations given in the previous sections apply to the condensation of a single component such as an essentially pure overhead product from a distillation column. The design of a condenser for a mixture of vapours is a more difficult (ask. 

Condensation of only part of a multicomponent vapour mixture, all components of which are theoretically condensable. This situation will occur where the dew point of some of the lighter components is above the coolant temperature. The uncondensed component may be soluble in the condensed liquid such as in the condensation of some hydrocarbons mixtures containing light gaseous components. 

The cooling process of the mixture can be represented on the enthalpy diagram of Fig. 6.13. Point A represents the starting condition after thermalization at 4.2K. The A-B line corresponds to the decrease of enthalpy in a J-T heat exchanger. In point B, we have a coexistence of liquid and vapour. The percentage of liquid decreases in the isoenthalpic expansion B-C which produces a decrease of p and T. The remaining vapour is condensed in the still exchanger (line C-D). 

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