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 evaporators between the steam supply and the condenser is called multiple-effect evaporation.
4. General Types of Evaporators *
Horizontal-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 prevent carryover of liquid droplets, and leaves out the top. This type is relatively cheap and is used for nonviscous liquids having high heat-transfer coefficients and liquids that do not deposit scale. Since liquid circulation is poor, they are unsuitable for viscous liquids. In almost all cases, this evaporator and the types discussed below are operated continuously, where the feed enters at a constant rate and the concentrate leaves at a constant 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 liquids. 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 10m 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 recirculated. Contact times can be quite low in this type. In some cases, as when the ratio of feed to evaporation rate is low. natural recirculation of the product through the evaporator is 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-type 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 materials such as orange juice and 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 The 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 connection with a pump between the outlet concentrate line and the feed ling. 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.
(Selected from: Warren L. McCabe, Unit Operations of Chemical Engineering, 5th Edition, McGraw-Hill Inc. , 1993.
Selected from: Christie J. Geankoplis, Transport Processes and Unit Operations, 2nd Edition, 1983. ) 材料14
蒸发
1 介绍
物质的蒸发是一种难挥发性溶质和挥发性溶剂组成的液体的浓缩。在多数的蒸发溶剂是水。蒸发一部分溶剂以获得高浓度溶液是获取浓酒的一种。蒸发区别于干燥，滤渣是一种液体，有时候是高粘性液体，有的时候甚至是固体；蒸发不同于蒸馏，蒸发的蒸汽通常是单组分，甚至当是混合物时，不用再蒸发去分离成部分；蒸发区别于结晶，它是液体的浓缩不是晶体的形成和构建。在一些特定的情况下，例如蒸发卤水来获取普通的盐，蒸发和结晶就相当的不一样了。蒸发有时候是生产一种饱和溶液的晶体的悬浮物。
通常在蒸发中浓液是最有价值的产品，蒸汽被浓缩和去废。在一种特殊的情况下，却反过来。通常蒸发矿物质水用以锅炉给水或者一些特殊工艺要求或者是一些特殊的消费群体。这种方法常称为蒸馏水法，但技术上它是蒸发。大规模的蒸发一直发展用于从海水中取饮用水。冷凝水就是期望的产物。只有一小部分时可以的，其他的又放回到海里。 2 液体特性
蒸发问题的实际解决方法是受到被浓缩液体的特性影响的。液体的特性从简单的热量传递操作和分离工艺是变化很广泛的。一些蒸发液的重要的性能如下：
浓缩 尽管进入蒸发器的液体充分稀释到具备很多的水的性能，一旦浓度增加就会越来越具有自己的性能。密度，黏性和固体含量增加时，要么就是达到饱和状态，要么就是对于传热来说太黏了。继续加热饱和液体，固体含量还将明显的上升，这样浓缩液的沸腾温度就会远高于同压下水的沸点。
发泡 许多的物质尤其是有机物质，蒸发的时候会有泡沫。一种随蒸汽离开蒸发器的泡沫引发了严重的消耗。在极端的情况下可能造成全部的液态物质蒸发出去丢失。
温度灵敏度系数 许多很好的化工医药产品和食物在加热到适当的温度在相当短的时间内就遭到了破坏。集中这些物质的一些特殊方法就是减少温度的同时还要减少加热的时间。
沉淀物 一些液体里的沉淀物在加热的表面，然后总体的系数不断的减少直到蒸发器关闭和管道清理。当这些沉淀物是非常的坚硬和难以溶解的，清理就会变得非常的困难和昂贵。 结构材料 蒸发器是由钢筋制成的。许多的溶剂是对亚铁有损害的。特殊的材料，例如铜，镍，不锈钢，铝，透性石墨和铅会被使用。因为这些材料是很贵，很高的导热率，所以尽量减少其成为设备材料的第一选择。
设计蒸发器的时候液体的其他特性也要被考虑进来。它们其中的一些如比热，浓度，凝固点，蒸发点，毒性，爆炸危害，辐射性和必要的无菌操作。因为需要考虑液体的性质变化方便设计不同类型的蒸发器。对于应对具体问题的蒸发器的选择就要基于液态的特性。
3 单效和多效操作
大部分的蒸发器在蒸汽冷凝金属管中被加热。几乎物质一直在管道里被蒸发流动。通常蒸汽的压强是很低的，低于3 atm abs，在压力小于0. 05 atm abs沸腾液体通常低于中等真空。减小液体沸腾温度，增加蒸汽和沸腾液体的温度差，就增加了蒸发器的传热率。
当单效操作使用时，蒸发气体冷凝丢弃。这种虽然简单采用无效蒸汽的称作单效操作。蒸发1kg水需要1到1.3kg的蒸汽。如果蒸汽从一个蒸发器里出来进入到第二个蒸发器的蒸汽室然后从第二个蒸发器输送到冷凝管，这个操作就叫做双效操作。原来蒸汽的热量将产生第二次效果，这样获得的单位质量的蒸汽将是第一次的两倍。同样的方法可以获得更好的效果。通常增加每千克蒸汽的含量的方法是在蒸汽供应和冷凝之间使用一系列的蒸发器，这种方法称为多效蒸发。 4 一般类型的蒸发器
水平管道自然循环蒸发器 水平束加热管类似于换热器管道。蒸汽进入浓缩管道。蒸汽冷凝留在管道的一端。沸腾溶液充满管道。蒸汽留在液体表面，通过一些例如挡板的装置防治携带液滴出去。这种类型相对便宜，用于高传热系数无黏性无沉淀液体。因为液体循环是很恶劣的，所以不适合黏性液体。几乎在所有的事例中，这种蒸发器和以下讨论的类型都是连续操作的，物料以一定的速率进去，冷凝液以一定的速率出去。
垂直类型自然循环蒸发器 这种类型的蒸发器，液体是在管道里面，浓缩蒸汽是在管道的外面。因为沸腾，密度增加。循环管道液体上升通过下水管流下。这种自然循环增加了热转换系数。它不用于黏性溶液。这种类型的蒸发器称为短管蒸发器。这是一个变化的类型，但是悬浮在加热元件是举行阀体内,有一个环形开放空间的下水道。篮子类型区别于垂直自然循环管道，有一个中央代替环形开放空间的下水道。这种类型广泛用于于糖，盐和氢氧化钠工业。
长管垂直类型蒸发器 因为相比较蒸发液体方面蒸汽方面的热转换系数是非常的高的，高液体流速是非常令人满意的。在长管垂直类型蒸发器中液体是在管道里面的。管道可达3到10米泡沫的形成原因内蒸汽管抽行动给予相当高的流速。通常，液体流经管道只有一次不会再循环。这种类型的接触时间将会非常的短。在一些例子中，当物料的蒸发率非常的低，自然循环中的产物就会就会在出口和进口之间增加大量的管道连接。这在生产浓缩牛奶中经常用。
降膜型蒸发器 长管型变化就是降膜型蒸发器，液体由管得顶端流入沿墙壁像薄膜一样流下。蒸汽液的分离在底部发生。这种类型广泛使用于集中热敏性材料，例如橙汁和一些水果饮料，因为堵塞时间非常的小（5到10秒）热转换系数非常的高。
力循环型蒸发器 液体膜的热转化系数能够通过泵引起液体在管道内部的循环而增加。通过在长垂直管道的出口集中线和物料进口之间增加管道连接也可以达到。通常，在力循环类型中垂直管道比长管道类型的要短。在其它的一些分离和外部卧式换热器中也被用到。这种类型对于黏性液体是非常有用的。
（选自：Warren L. McCabe, 化学工程单元操作第5版，麦格劳希尔集团, 1993.
选自： Christie J. Geankoplis,传递过程和单元操作第2版, 1983. )
Reading Material 15
Chemical Industry and Environmental Protection
How can we reduce the amount of waste that is produced? And how can we close the loop by redirecting spent materials and products into programs of recycling? All of these questions must be answered through careful research in the coming years as we strive to keep civilization in balance with nature. 1. Life Cycle Analysis
Every stage of a product's life cycle has an environmental impact, starting with extraction of raw materials, continuing through processing, manufacturing, and transportation, and concluding with consumption and disposal or recovery. Technology and chemical science are challenged at every stage. Redesigning products and processes to minimize environmental impact requires a new philosophy of production and a different level of understanding of chemical transformations. Environmentally friendly products require novel materials that are reusable, recyclable, or biodegradable; properties of the materials are determined by the chemical composition and structure. To minimize waste and polluting by-products, new kinds of chemical process schemes will have to be developed. Improved chemical separation techniques are needed to enhance efficiency and to remove residual pollutants, which in turn will require new chemical treatment methods in order to render them harmless. Pollutants such as radioactive elements and toxic heavy metals that cannot be readily converted into harmless materials will need to be immobilized in inert materials so that they can be safely stored. Finally, the leftover pollution of an earlier, less environmentally aware era demands improved chemical and biological remediation techniques.
2. Manufacturing with Minimal Environmental Impact
Discharge of waste chemicals to the air, water, or ground not only has a direct environmental impact, but also constitutes a potential waste of natural resources. Early efforts to lessen the environmental impact of chemical processes tended to focus on the removal of harmful materials from a plant's waste stream before it was discharged into the environment. But this approach addresses only half of the problem; for an ideal chemical process, no harmful by-products would be formed in the first place. Any discharges would be at least as clean as the air and water that were originally taken into the plant, and such a process would be \
Increasing concern over adverse health effects has put a high priority on eliminating or reducing the amounts of potentially hazardous chemicals used in industrial processes. The best course of action is to find replacement chemicals that work as well but are less hazardous. If a substitute cannot be found for a hazardous chemical, then a promising alternative strategy is to develop a process for generating it on-site and only in the amount needed at the time.
Innovative new chemistry has begun delivering environmentally sound processes that use energy and raw materials more efficiently. Recent advances in catalysis, for example, permit chemical reactions to be run at lower temperatures and pressures. This change, in turn, reduces the energy demands of the processes and simplifies the selection of construction materials for the processing facility. Novel catalysts are also being used to avoid the production of unwanted by-products.
3. Control of Power Plant Emissions
Coal-, oil-, and natural-gas-fired power generation facilities contribute to the emissions of carbon
monoxide, hydrocarbons, nitrogen oxides, and a variety of other undesired by?products such as dust and traces of mercury. A rapidly increasing array of technologies are now available to reduce the emissions of unwanted species to meet national or local standards. Chemists and chemical engineers have made major contributions to the state of the art, and catalytic science is playing a critical role in defining the leading edge.
The simultaneous control of more than one pollutant is the aim of some recently developed catalyst or sorbent technologies. For example, catalytic methods allow carbon monoxide to be oxidized at the same time that nitrogen oxides are being chemically reduced in gas turbine exhaust. Other research efforts are aimed at pilot-plant evaluation of the simultaneous removal of sulfur and nitrogen oxides from flue gas by the action of a single sorbent and without the generation of massive volumes of waste products. 4. Environmentally Friendly Products
Increased understanding of the fate of products in the environment has led scientists to design \and 1950s, new products were introduced that were based on synthetic surfactants called branched alkylbenzene sulfonates. These detergents had higher cleaning efficiency, but it was subsequently discovered that their presence in waste water caused foaming in streams and rivers. The problem was traced to the branched alkylbenzene sulfonates; unlike the soaps used previously, these were not sufficiently biodegraded by the microbes in conventional sewage treatment plants. An extensive research effort to understand the appropriate biochemical processes permitted chemists to design and synthesize another new class of surfactants, linear alkylbenzene sulfonates. The similarity in molecular structure between these new compounds and the natural fatty acids of traditional soaps allowed the j1 microorganisms to degrade the new formulations, and the similarity to the branched alkylbenzene sulfonates afforded outstanding detergent performance.
Novel biochemistry is also helping farmers reduce the use of insecticides. Cotton plants, for example, are being genetically modified to make them resistant to the cotton bollworm. A single gene from a naturally occurring bacterium, when transferred into cotton plants, prompts the plant to produce a protein that is ordinarily produced by the bacterium. When the bollworm begins to eat the plant, the protein kills the insect by interrupting its digestive processes. 5. Recycling
Increasing problems associated with waste disposal have combined with the recognition that some raw materials exist in limited supply to dramatically increase interest in recycling. Recycling of metals and most paper is technically straightforward, and these materials are now commonly recycled in many areas around the world. Recycling of plastics presents greater technical challenges. Even after they are separated from other types of waste, different plastic materials must be separated from each other. Even then, the different chemical properties of the various types of plastic will require the development of a variety of recycling processes.
Some plastics can be recycled by simply melting and molding them or by dissolving them in an appropriate solvent and then reformulating them into a new plastic material. Other materials require more complex treatment, such as breaking down large polymer molecules into smaller subunits that can subsequently be used as building blocks for new polymers. Indeed, a major program to recycle plastic soft drink bottles by this route is now in use.
A great deal of research by chemists and chemical engineers will be needed to successfully