流体力学与传热件evaporation.ppt

华南理工大学 化工学院 伍钦 ceqwu@ 020Chapter 5evaporation The objective of evaporation is to concentrate a consisting of a nonvolatile solute and a volatile solvent. Evaporation is conducted by vaporizing a portion of the solvent to produce a concentrated solution or thick liquor. Evaporation 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 . Evaporation differs from crystallization in that emphasis is placed on concentrating a solution rather than forming and building crystals. Liquid characteristics The practical solution of an evaporation problem is profoundly affected by the character of the liquor to be concentrated. Concentration As the concentration increases, the solution becomes more and more individualistic. The density and viscosity increase with solid content. The boiling point of the solution may also rise 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. Foaming 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. Temperature sensitivity Many fine chemicals , pharmaceutical products, and foods are damaged when heated to moderate temperatures for relatively short time . scale Some 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. Single- and multiple-effect operation Most evaporators are heated by steam condensing on metal tubes. Nearly always the material to be evaporated flows inside the tubes. When a single evaporator is used, the vapor from the boiling liquid is condensed and discarded, the method is called single-effect evaporation. Add