过程装备与控制工程专业英语翻译14

Reading material 14Evaporation1． IntroductionThe objective of evaporation is to concentrate a solution consisting of a nonvolatile 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. Evaporation differs from drying in that the residue is a liquid-sometimes is highly viscous one-rather than a solid; it 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; it differs from crystallization in that emphasis is placed on concentrating a solution rather than forming and building crystals. In certain situations, e.g., in the evaporation of brine to produce common salt, the line between evaporation and crystallization is far from sharp. Evaporation sometimes produces a slurry of crystal in a saturated mother liquor.Normally, in evaporation the thick liquor is the valuable product and the vapor is condensed and discarded. In one specific situation, however, the reverse is true. Mineral-bearing water often is evaporated to give a solid-free product for boiler feed, for special process requirements, or for human consumption. This technique is often called water distillation, but technically it is evaporation. Large-scale evaporation processes have been developed and used for recovering potable water from seawater. Here the condensed water is the desired product. Only a fraction of the total water in the feed is recovered, and the remainder is returned to the sea.2. Liquid CharacteristicsThe 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 judgment 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 as follows. Concentration Although the thin liquor fed to an evaporator may be sufficiently dilute to have many of the physical 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 much higher than that of water at the same pressure.FoamingSome materials, especially organic substances, foam during vaporization. A stable foam accompanies the vapor out of the evaporator, causing heavy entertainment. In the extreme cases, the entire mass of liquid may boil over into the vapor outletand be lost.Temperature sensitivity Many fine chemicals pharmaceutical products, and foods 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.Scale Some solutions deposit scale on the heating surface. The overall coefficient then steadily diminished, until the evaporator must be shut down and the tubes cleaned. When the scale is hard and insoluble, the cleaning is difficult and expensive.Materials of construction Whenever possible, evaporator are made of some kind of steel. Many solutions, however, attack ferrous metals or are contaminated by them. Special materials such as copper, nickel, stainless steel, aluminum, imperious graphite, and lead are then used. Since these materials are expensive, high heat transfer rates become especially desired to minimize the first cost of the equipment. Many other liquid characteristics must be considered by the designer of an evaporator. Some of these are specific heat, heat of concentration, freezing point, gas liberation on boiling, toxicity, explosion hazards, radioactivity, and necessity for sterile operation. Because of the variation in liquor properties, many different evaporator designers have been developed. The choice for any specific problem depends primarily on the characteristics of the liquid.3. Single and multiple-effect operationMost evaporators are heated by steam condensing on the metal tubes. Nearly always the material to be evaporated flows inside the tubes. Usually the steam is at low pressure, below 3 atm abs; often the boiling liquid is under moderate vacuum, at pressure down to about 0.05 atm abs. Reducing the boiling temperature of the liquid increase the temperature difference between the steam and the boiling liquid and thus increase the heat transfer rate in the evaporator.When a single evaporator is used, the vapor from the boiling liquid is condensed and discarded. This method is called single-effect evaporation, and although it is simple, it utilizes steam ineffectively. To evaporate 1 kg water from a solution call for from 1 to 1.3 kg of steam. If the vapor from one evaporator is fed into steam chest of a second evaporator and the vapor from second is then sent to a condenser, the operation becomes double-effect. The heat in the original steam is reused in the second effect, and the evaporation achieved by a unit mass of steam fed to the first effect is approximately doubled. Additional effects can be added in the same manner. The general method of increasing the evaporation per kilogram of steam by using a series of evaporator between the steam supply and the condenser is called multiple-effect evaporation.4. General types of evaporatorHorizontal-tube natural circulation evaporator the horizontal bundle of heating tubes is similar to the bundle of tubes in a heat exchanger. The steam enters into the tubes, where it condenses. The steam condensate leaves at the other end of the tubes. The boiling liquid solution covers the tubes. The vapor leaves the liquid surface, often goes through some deentraining device such as a baffle to preventcarryover of liquid droplets, and leaves out the top. This type is relatively cheap and is used for no viscous liquid having high heat transfer coefficients and liquids that do not deposit scale. Since liquid circulation is poor, they are unsuitable for viscous liquid. In almost all cases, this evaporator and the types discussed below are operating continuously, where the feed enters at a constant rate and the concentrate leaves at a concentrate rate.Vertical-type natural circulation evaporator in this type of evaporator, vertical rather than horizontal tubes are used, and the liquid is inside the tubes and the steam condenses outside the tubes. Because of boiling and decreases in density, the liquid rises in the tubes by natural circulation and flows downward through a large central open space or downcomer. This natural circulation increases the heat transfer coefficient. It is not used with viscous liquid. This type is often called the short-tube evaporator. A variation of this is the basket type. Where vertical tubes are used, but the heating element is held suspended in the body so there is an annular open space as the downcomer. The basket type differs from the vertical natural circulation evaporator, which has a central instead of annular open space as the downcomer, this type is widely used in the sugar, salt, and caustic soda industries.Long-tube vertical-type evaporator since the heat transfer coefficient on the steam side is very high compared to that on the evaporating liquid side, high liquid velocities are desirable. In a long-tube vertical-type evaporator the liquid is inside the tubes. The tubes are 3 to 10 m long and the formation of vapor bubbles inside the tubes causes a pumping action giving quite high liquid velocities. Generally, the liquid passes through the tubes only once and is not reticulated. Contact times can be quite low in this type. In some case,as when the ratio of the feed to evaporation rate is low.Natural recirculation of the product through the evaporators done by adding a large pipe connection between the outlet concentrate line and the feed line. This is widely used for producing condensed milk. Falling-film evaporator a variation of the long tube type is the falling-film evaporator, wherein the liquid is fed to the top of the tubes and flows down the walls as a thin film. Vapor-liquid separation usually takes place at the bottom. This type is widely used for concentrating heat-sensitive material such as orange juice and the other fruit juices, because the holdup time is very small (5 to 10 s or more).and the heat-transfer coefficients are high.Forced-circulation type evaporator this liquid film heat transfer coefficient can be increased by pumping to cause forced circulation of the liquid inside the tubes. This could be done in the long tube vertical type by adding a pipe concentrate with a pump between the outlet concentrate line and the feed line. However, usually in a forced-circulation type, the vertical tubes are shorter than in the long-tube type. Also, in other cases a separate and external horizontal heat exchanger is used. This type is very useful for viscous liquids.阅读材料14蒸发1、介绍蒸发的目的是浓缩不易挥发的溶质和易挥发的溶剂组成的溶液。