Mixed vapours, condensation

The following features, common to all these situations, must be considered in the developing design methods for mixed vapour condensers ... 

To evaluate the true temperature difference (driving force) in a mixed vapour condenser a condensation curve (temperature vs. enthalpy diagram) must be calculated showing the change in vapour temperature versus heat transferred throughout the condenser, Figure 12.48. The temperature profile will depend on the liquid-flow pattern in the condenser. There are two limiting conditions of condensate-vapour flow ... 

Condensation of mixed vapours is considered further in Volume 6, Chapter 12, where it is suggested that the local heat transfer coefficient may be expressed in terms of the local gas-film and condensate-film coefficients. For partial condensation where ... 

It is normal practice to assume that integral condensation occurs. The conditions for integral condensation will be approached if condensation is carried out in one pass, so that the liquid and vapour follow the same path as in a vertical condenser with condensation inside or outside the tubes. In a horizontal shell-side condenser the condensate will tend to separate from the vapour. The mean temperature difference will be lower for differential condensation, and arrangements where liquid separation is likely to occur should generally be avoided for the condensation of mixed vapours. 

Matsouka, M. in Jancic, S. J. and de Jong, E. J. (eds.) Industrial Crystallization 84 (Elsevier, Amsterdam 1984) 357-360. Rates of nucleation and growth of organic condensates of mixed vapours. 

H. Klinger made the di-iodate by mixing 20 grms. of potassium chlorate, 21 grms. of iodine, and 100 c.c. of water in a half-litre tubulated retort with a thermometer fitted in the tubulure, and the neck directed upwards. The mixture is heated by a small flame. The liquid becomes yellow, and violet vapours condense in the neck of the retort. The materials begin to react at about 85°, and the reaction is complete at about 95°. Only a little chlorine is evolved when the liquid is heated up to its b.p. When the colourless liquid is cooled, crystals of the di-iodate separate, and these can be purified by recrystallization from hot water. The yield is over 70 per cent. Some barium di-iodate can be recovered by adding barium chloride to the mother-liquid. 

The unreacted dimethyldichlorosilane with hydrogen chloride is distilled when the temperature in the higher part of the tower is 70-72 °C. Dimethyldichlorosilane vapours condense in refluxer 8, which is cooled with water. The condensate flows into the separation box, from where the most part is returned to reflux the tower, and the rest is sent through rotameter into the chlorinator, where it is mixed with the 2% initiator solution for repeated chlorination. After refluxer 8, hydrogen chloride enters the backflow igurit condenser (not shown in the diagram), where it is purified from the impurity of dimethyldichlorosilane, and is sent for water absorption. 

Deo, P. V., Condensation of Mixed Vapours, Ph.D. Thesis in Chemical Engineering, University of Manchester, Institute of Science and Technology, Manchester, England, 1979. 

A mixture of steam, CS2 and air with a temperature of approximatly 95 °C is sucked off from the CS2-box. Most of the vapour is condensed in the vapour condenser by mixing with water. The water temperature at the outlet of the condenser is approximately 70 to 75 °C, and the same water flows back into the acid water circulation system, to compensate for the water carried out of the system by the product. The feed temperature of the water to the vapour condenser should not be lower than 50 °C to avoid condensation of CS2 inside the vapour condenser. 

The biggest disadvantage of a mixing condenser is the fact that the vapour condensate is mixed with the coolant. This mixing is only acceptable when the condensate is harmless and is to be discarded after use. For all other applications, surface condensers have to be used. 

As mentioned earlier, direct-contact condensers have some advantages against surface condensers. However, the main disadvantage is the mixing of vapour condensate with the coolant. If pure steam is condensed with water as coolant, this may not be a problem. But if it is not permitted to mix cooling water and condensate and in addition there is a fouhng problem, then it is difficult to find an adequate condensation system. On the one hand, direct-contact condensers... 

With vents open to the atmosphere, a cold dense white fog cloud will form down-wind along the ground or sea for several hundred meters. This visible cloud is actually water vapour condensed from the moist atmosphere by the cold LNG vapour. LNG vapour is essentially invisible but as a source of cold it remains within the confines of this visible cloud. As the cloud continues to move downwind, it mixes and warms up with the air. It will eventually become less dense than atmospheric air and rise rapidly away from ground level, or sea surface, and clear of potential ignition sources. If flie entire fog cloud is treated as potentially flammable, this assumption will be on the safe side. 

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. 

Dehumidification of air can be effected by bringing it into contact with a cold surface, either liquid or solid. If the temperature of the surface is lower than the dew point of the gas, condensation takes place and the temperature of the gas falls. The temperature of the surface tends to rise because of the transfer of latent and sensible heat from the air. It would be expected that the air would cool at constant humidity until the dew point was reached, and that subsequent cooling would be accompanied by condensation. It is found, in practice, that this occurs only when the air is well mixed. Normally the temperature and humidity are reduced simultaneously throughout the whole of the process. The air in contact with the surface is cooled below its dew point, and condensation of vapour therefore occurs before the more distant air has time to cool. Where the gas stream is cooled by cold water, countercurrent flow should be employed because the temperature of the water and air are changing in opposite directions. 

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