HUA GONG YUAN LIChapter 5 evaporation The objective of evaporation is to concentratea consisting of a nonvolatile solute and avolatile solvent. Evaporation is conducted by vaporizing a portion of the solvent to produce a concentrated solution or thickliquor.华南理工大学 化工学院 伍钦 ceqwutom.com 020-87114377HUA GONG YUAN LIlEvaporation differs from distillation in that the vapor usually is a single component, and even when the vapor is a mixture ,no attempt is made in the evaporation step to separate the vapor into fractions .华南理工大学 化工学院 伍钦 ceqwutom.com 020-87114377HUA GONG YUAN LIlEvaporation differs from crystallization in that emphasis is placed on concentrating a solution rather than forming and building crystals.Normally ,in evaporation the thick liquor is the valuable product and the vapor is condensed and discarded.华南理工大学 化工学院 伍钦 ceqwutom.com 020-87114377HUA GONG YUAN LILiquid characteristicsThe practical solution of an evaporation problem is profoundly affected by the character of the liquor to be concentrated. Some of the most important properties of evaporating liquids are as follows.华南理工大学 化工学院 伍钦 ceqwutom.com 020-87114377HUA GONG YUAN LIConcentration As the concentration increases, the solution becomes more and more individualistic. The density and viscosity increase with solid content.lContinued boiling of a saturated solution causes crystals to form; these must be removed or the tube clog.华南理工大学 化工学院 伍钦 ceqwutom.com 020-87114377HUA GONG YUAN LIlThe boiling point of the solution may alsorise considerably as the solid content increases , so that the boiling temperature of a concentrated solution may be much higher than that of water at the same pressure.华南理工大学 化工学院 伍钦 ceqwutom.com 020-87114377HUA GONG YUAN LIFoaming A stable foam accompanies the vapor out of the evaporator, causing heavy entrainment. In extreme cases the entire mass of liquid may boil over into the vapor outlet and be lost.华南理工大学 化工学院 伍钦 ceqwutom.com 020-87114377HUA GONG YUAN LITemperature sensitivityMany fine chemicals , pharmaceuticalproducts, and foods are damaged whenheated to moderate temperatures for relatively short time .华南理工大学 化工学院 伍钦 ceqwutom.com 020-87114377HUA GONG YUAN LIscaleSome solutions deposit scale on the heating surfaces. The over-all coefficient then steadily diminishes, until the evaporator must be shut down and the tubes cleaned.华南理工大学 化工学院 伍钦 ceqwutom.com 020-87114377HUA GONG YUAN LISingle- and multiple-effect operationMost evaporators are heated by steam condensing on metal tubes. Nearly always the material to be evaporated flows inside the tubes.Reducing the boiling temperature of the liquid increasing the temperature difference between the stream and the boiling liquid and thereby increases the heat-transfer rate in the evaporator华南理工大学 化工学院 伍钦 ceqwutom.com 020-87114377HUA GONG YUAN LIWhen a single evaporator is used, the vaporfrom the boiling liquid is condensed and discarded, the method is called single-effectevaporation.If the vapor from one evaporator is fed into the stream chest of a second evaporator and vapor from the second is then sent to a condenser, the operation become double- effect.华南理工大学 化工学院 伍钦 ceqwutom.com 020-87114377HUA GONG YUAN LIAdditional effects can be added in the same manner. A series of evaporators between the stream supply and the condenser is called multiple- effect evaporator.华南理工大学 化工学院 伍钦 ceqwutom.com 020-87114377HUA GONG YUAN LI Types of evaporatorsThe chief types of stream-heated tubular evaporator in uses today are:l Short-tube evaporatorsl Long-tube vertical evaporators forced-circulationUpward-flow (climbing-film)down-ward-flow (falling-film)华南理工大学 化工学院 伍钦 ceqwutom.com 020-87114377HUA GONG YUAN LIOnce-through and circulation evaporatorsEvaporators may be operated either as once- through or circulation units.In once-through operation the feed liquor passes through the tubes only once, releases the vapor, and leaves the unit as thicker liquor. All the evaporation is accomplished in a single pass.华南理工大学 化工学院 伍钦 ceqwutom.com 020-87114377HUA GONG YUAN LIIn circulation evaporators a pool of liquid is held within the equipment. Incoming feed mixes with the liquid from the pool, and mixture passes through the tubes.Unevaporated liquid discharged from thetubes returns to the poolAll short-tube and forced-circulation evaporators are operated in this way.华南理工大学 化工学院 伍钦 ceqwutom.com 020-87114377HUA GONG YUAN LIShort-tube evaporatorsIn the older types of evaporators the tubes are “short”.In the short-tube vertical evaporator shown in Figvaporfeed Stream interconcentratecondensateCentral downcomer华南理工大学 化工学院 伍钦 ceqwutom.com 020-87114377HUA GONG YUAN LIThe stream condenses outside the tubes. The tube bundle contains a large central downcomer, the cross-sectional area of which is 25 to 40 per cent of the total cross-sectional area of the tubesMost of the boiling takes plac