Evaporation and destillation
Back to EFFICIENCY FINDER
The objective of evaporation is to concentrate a solution consisting of a non-volatile solute and a volatile solvent. In the overwhelming majority of evaporations the solvent is water. Evaporation is conducted by vaporizing a portion of the solvent to produce a concentrated solution of thick liquor. Normally, in evaporation the thick liquor is the valuable product and the vapour is condensed and discarded (Unit Operations of Chemical Engineering, McCabe, Smith, Harriot, 5th Edition, McGrawHill). The objective of distillation is to separate a liquid mixture into fractions of different boiling points by application of heat. Compounds in the liquid mixture are separated according to their volatility. Evaporation differs from drying on that the residue is a liquid- sometimes a highly viscous one- rather than a solid; it differs from distillation in that the vapour usually is a single component and even when the vapour is a mixture, no attempt is made in the evaporation step to separate the vapour into fractions (Unit Operations of Chemical Engineering, McCabe, Smith, Harriot, 5th Edition, McGrawHill).
2. FIELD OF APPLICATION
Industrial evaporation and distillation are energy intensive processes which are carried out in a wide range of industries, including chemicals, foodstuffs, minerals, paper, textiles and many others.
3. DESCRIPTION OF TECHNIQUES, METHODS AND EQUIPMENT
Characteristics of Evaporating Liquids (Unit Operations of Chemical Engineering, McCabe, Smith, Harriot, 5th Edition, McGrawHill) The practical solution of an evaporation problem is profoundly affected by the character of the liquor to be concentrated. It is the wide variation in liquor characteristics (which demands judgement and experience in designing and operating evaporators) that broadens this operation from simple heat transfer to a separate art. Some of the most important properties of evaporating liquids are the following:
1. Concentration Although the thin liquor fed to an evaporator may be sufficiently dilute to have many of the physical properties of water, as the concentration increases the solution becomes more and more individualistic. The density and viscosity increase with solid content until either the solution becomes saturated or the liquor becomes too viscous for adequate heat transfer. Continued boiling of a saturated solution causes crystals to form; these must be removed or the tubes clog. 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 higher than that of water at the same pressure. 2. Foaming Some materials, especially organic substances, foam during vaporization. A stable foam accompanies the vapour out of the evaporator, causing heavy entrainment. In extreme cases the entire mass of liquid may boil over into the vapour outlet and be lost. 3. Temperature sensitivity Many fine chemicals, pharmaceutical products and food are damaged when heated to moderate temperatures for relatively short times. In concentrating such materials special techniques are needed to reduce both the temperature of the liquid and the time of heating. 4. Scale Some solutions deposit scale on the heating surface. The overall coefficient then steadily diminishes, until the evaporator must be shut down and the tubes cleaned. When the scale is hard and insoluble the cleaning is difficult and expensive. 5. Construction materials Whenever possible, evaporators are made of some kind of steel. Many solutions, however, attack the ferrous metals or are contaminated by them. Special materials such as copper, nickel, stainless steel, aluminium, impervious graphite and lead are then used. Since these materials are expensive, high heat-transfer rates become especially desirable to minimize the first cost of the equipment.
Evaporation equipment (Unit Operations of Chemical Engineering, McCabe, Smith, Harriot, 5th Edition, McGrawHill) Chief-types of steam-heated tubular evaporators in use today are: 1. Long-tube vertical evaporators: a) Upward flow (climbing-film) b) Downward flow (falling-film) c) Forced circulation 2. Agitated-film evaporators.
Principal methods of distillation (Unit Operations of Chemical Engineering, McCabe, Smith, Harriot, 5th Edition, McGrawHill) In practice, distillation may be carried out by either two principal methods: 1. The first method is based on the production of a vapour by boiling the liquid mixture to be separated and condensing the vapours without allowing any liquid to return to the still .There is then no reflux: a) Flash distillation: Flash distillation consists of vaporizing a definite fraction of the liquid in such a way that the evolved vapour is in equilibrium with the residual liquid, separating the vapour from the liquid and condensing the vapour. b) Batch distillation: In some plants, volatile products are recovered from liquid solution by batch distillation. The mixture is charged to a still or reboiler and heat is supplied through a coil or through the wall of the vessel to bring the liquid to the boiling point and then vaporize part of the batch. In the simplest method of operation, the vapours are taken directly from the still to a condenser. 2. The second method is based on the return of part of the condensate to the still under such conditions that this returning liquid is brought into intimate contact with the vapours on their way to the condenser. Either of these methods may be conducted as a continuous process or as a batch process: a) Continuous distillation with reflux (rectification): Flash distillation is used most for separating components that boil at widely different temperatures. It is not effective in separating components of comparable volatility, since then both the condensed vapour and residual liquid are far from pure. By many successive redistillations, small amounts of some nearly pure components may finally be obtained, but this method is too inefficient for industrial distillations when nearly pure components are wanted. Methods now used in both laboratory and plant apply the principle of rectification. In a distillation column, the vapour produced is condensed in a condenser and the liquid is vaporized in a reboiler. Part of these streams is recycled in the distillation column. The recycling liquid stream form the condenser is called reflux. By using plates inside the distillation column, the gas and liquid mixtures are brought into intimate contact and their concentrations tend to move toward an equilibrium state. Some of the more volatile component is vaporized from the liquid, decreasing its liquid concentration, while some of the less volatile component is condensed from the vapour, increasing its vapour concentration. If no azeotropes are encountered, both overhead and bottom products may be obtained in any desired purity if enough plates and adequate reflux are provided.
4. RELEVANT PARAMETERS TO ENERGY
Temperature Pressure Medium Evaporating liquid characteristics (e.g. concentration, temperature sensitivity)
Back to EFFICIENCY FINDER