[工学]化工英语_马正飞版

[工学]化工英语_马正飞版 CONTENTS Characteristics of Special purpose English:........................................................................................ 1 LESSON 15 Materials Science........................................................................................................... 6 LESSON 16 Chemical Process Safety ................................................................................................ 8 LESSON 17 Plant Design and General Considerations................................................................... 11 LESSON 19 Process Reactor Design................................................................................................ 13 LESSON 21 Bioengineering............................................................................................................. 15 LESSON 22 Genetic Engineering..................................................................................................... 18 LESSON 23 How We Digest Carbohydrates .................................................................................... 20 LESSON 26 Green House Effect...................................................................................................... 23 LESSON 28 Nomenclature of Chemical Compounds...................................................................... 26 LESSON 1 Chemical Engineering.................................................................................................... 29 LESSON 3 Unit Operations.............................................................................................................. 33 LESSON 5 Filtration ......................................................................................................................... 35 LESSON 6 Heat Transfer.................................................................................................................. 38 LESSON 7 Absorption of Gases....................................................................................................... 40 LESSON 8 Distillation Operations................................................................................................... 43 LESSON 9 Solvent Extraction.......................................................................................................... 46 LESSON 10 Drying of Solids ........................................................................................................... 49 LESSON 11 Packed Towers.............................................................................................................. 51 Characteristics of Special purpose English: 1: Language 东南大出版社学1 东东意东比东东一～不具有情感容～少使用比东～排比～东等修手法。常出东东看～东念～东内很夸辞会听 的东。 2: Vocabulary 国很从腊来东性强,多拉丁东或希东中派生出。 很个少口东东东～常用东东东代替东东短东: Absorb—take in; discover---find out; assemble---put together. 利用前东后东派生的东多。东东～科技东东占很。1/4 3: Grammar 大量使用名东和名东东东 be very important--- be of great importance 大量使用被东东东 大量使用非东东东东东东 东句东多构 Hey Jude Beatles 嘿,～不要沮东Hey Jude, don't make it bad. Jude 　　 　　唱首悲东的歌曲 舒东自己的心情来Take a sad song and make it better 　　 　　东存放于心将她Remember to let her into your heart 东南大出版社学2 　　 　　生活才更美好会Then you can start to make it better. 　　 　　 嘿不要害怕Hey Jude, don't be afraid Jude 　　 　 生就是要得到你来她You were made to go out and get her. 　　 　 在深藏于心的那一刻将她The minute you let her under your skin, 　　 　　已东东始东的更好你Then you begin to make it better. 　　 　　无东何东～感到痛苦当你And anytime you feel the pain, 　　 　　 嘿停下 　来hey Jude, refrain, Jude 　 　　不要把全世界都在肩上扛Don't carry the world upon your shoulders. 　　 　东东得 东瓜才假东东你懂会装For well you know that it's a fool who plays it cool 强 　 　 　　才把自己的世界东得冷漠会By making his world a little colder. 　　 　　 嘿东东我失望 　Hey Jude don't let me down Jude 　 　已遇东 东在就去东得芳心你她她You have found her, now go and get her. 　　 　　东深藏于心 　将她Remember to let her into your heart, 　 　　生活才更美好 　会Then you can start to make it better. 　 　遇事要拿得起放得下 , 嘿So let it out and let it in, hey Jude, begin, Jude ～振作起来 　　 　一直期待有人同一起成东你你You're waiting for someone to perform with. 　　 　不明白,只有你你 And don't you know that it's just you, hey Jude, you'll do 嘿你 行的Jude 　　 　　未肩东在身上 　来你The movement you need is on your shoulder 　 　 不要消 嘿沉　　Hey Jude, don't make it bad. Jude 　 　　唱首东东的歌曲 东自己振作些Take a sad song and make it better 　　 东南大出版社学3 　　东得心中常东有 　她Remember to let her under your skin 　 　　生活才东得更美好 　　会Then you'll begin to make it better Oh. 　更美好Better better better better better better, Someone like you 东东海另 Adele阿黛东 I heard that youre settled down. 已东君～东事安康。’ That you found a girl and youre married now.遇佳人～不久婚嫁。’ I heard that your dreams came true. 已东君～得东所想。Guess she gave you things, I didnt give to you. 料得是～卿东君望。’ Old friend, why are you so shy? 日知己～何故东皇,旧 Aint like you to hold back or hide from the lie.遮遮掩掩～欲盖彰。弥’ I hate to turn up out of the blue uninvited.客有不速～东非我所想。But I couldnt stay away, I couldnt fight it. 避之不得～遑东相抗。与’’ Id hoped youd see my face& that youd be reminded, 日偶遇～东得依稀东。异’’’ That for me, it isnt over.再无所求～涕零而下。泪’ Never mind, Ill find someone like you. 毋东东东～东有弱水替东海。’ I wish nothing but the best, for you too. 抛却东东～再把相思寄巫山。Dont forget me, I beg, I remember you said:勿忘昨日～亦存君言于肺腑。’ “Sometimes it lasts in love but sometimes it hurts instead” “情堪东永～也善 心潮狂东。”掀 Sometimes it lasts in love but sometimes it hurts instead, yeah. 情堪东永～也善 心潮狂东～然。掀 东南大出版社学4 You know, how the time flies,光东常无踪～东东不敢道荏苒。 Only yesterday, was the time of our lives. 东笑仍如昨～今却孤影东花繁。We were born and raised in a summery haze. 彼东初东手～夏东郁郁衣衫。湿 Bound by the surprise of our glory days. 自东念中～东旧黯喜东光永不。 I hate to turn up out of the blue uninvited.客有不速～东非我所想。But I couldnt stay away, I couldnt fight it. 避之不得～遑东相抗。与’’ I hoped youd see my face and that youd be reminded, 日偶遇～东得依异 had’’ 稀东。 That for me, it isnt over.再无所求～涕零而下。泪’ Never mind, Ill find someone like you. 毋东东东～东有弱水替东海。’ I wish nothing but the best, for you too. 抛却东东～再把相思寄巫山。Dont forget me, I beg, I remember you said:勿忘昨日～亦存君言于肺腑。’ “Sometimes it lasts in love but sometimes it hurts instead” “情堪东永～也善 心潮狂东。”掀 Sometimes it lasts in love but sometimes it hurts instead, yeah. 情堪东永～也善 心潮狂东～然。掀 Nothing compares, no worries or cares. 无可与挂之相提～切莫东心同念。 Regrets and mistakes theyre memories made. 糊涂恋东恨东免～白璧微瑕方可。’’ Who would have known how bittersweet this would taste? 此中酸甜苦咸～世 上东人堪相言, Never mind, Ill find someone like you. 毋东东东～东有弱水替东海。’ I wish nothing but the best, for you too. 抛却东东～再把相思寄巫山。Dont forget me, I beg, I remember you said:勿忘昨日～亦存君言于肺腑。’ “Sometimes it lasts in love but sometimes it hurts instead” “情堪东永～也善 心潮狂东。”掀 东南大出版社学5 Sometimes it lasts in love but sometimes it hurts instead, yeah. 情堪东永～也善 心潮狂东～然。掀 Never mind, Ill find someone like you. 毋东东东～东有弱水替东海。’ I wish nothing but the best, for you too. 抛却东东～再把相思寄巫山。 Dont forget me, I beg, I remember you said:勿忘昨日～亦存君言于肺腑。’ “Sometimes it lasts in love but sometimes it hurts instead” “情堪东永～也善 心潮狂东。”掀 Sometimes it lasts in love but sometimes it hurts instead, yeah. 情堪东永～也善 心潮狂东～然。掀 LESSON 15 Materials Science Materials science is the study of then properties of solid materials and how those properties are determined by a material’s composition and structure. The study encompasses the entire rang of properties, including mechanic, thermal, chemical, electric, magnetic, and optical 东南大出版社学6 behavior. It grew out of amalgam of solid-state physics, metallurgy, and chemistry, since the rich variety of materials properties cannot be understood within the context of any single classical discipline. The optional use of materials in applications such as packaging, construction, magnets, batteries, engines, automobile bodies, insulation, catalytic cracking, electrics, and computers depends on the intelligent explosion of these properties. With a basic understanding of the originals of properties, materials can be selected or designed for an enormous variety of applications, ranging from structural steels to computer microchips. Materials Science is therefore important to many engineering activities such as electronics, aerospace, telecommunications, information processing, nuclear power, and energy conversion. The properties of materials are determined are by their internal structure—that is, the way in which the fundamental parts of the materials are put together. Thus, the atomic structure is the arrangement of the atoms in space, the electron structure is the distribution of the electrons in space and in energy, the defect structure is the distribution of crystal flaws, (such as impurities, vacant atomic sites, and dislocation), and the microscopic structure is the size and arrangement of microscopic grains and precipitates. These structures, and their interactions, are responsible for the behavior of materials. For example, the combination of atomic and electric structure controls the ease with which election can move in or through a solid and therefore determines whether it will be an insulator, a conductor, or a semiconductor; the atomic and defect structure control the ease with which a mechanical disturbance can move through a solid and therefore determine its degree of ductility or brittleness; and the distribution of spinning electrons gives rise to magnetic properties. After World War , economic progress and national defense needs required the development? 东南大出版社学7 of sophisticated materials, and it was soon apparent that an integration of the knowledge and methods of metallurgy, chemistry, and physics was essential for their development. The field of semiconductor electronics was a prime example of this. The basic work was done by physicists, who were oriented toward the analysis of electronic properties of pure, sample solids. But the successful production, of good semiconductor devices required a knowledge of defect structure, traditionally the province of the metallurgist, and the importance of impurity control was in many respects a problem of chemistry. By 1960 the integration of the three fields into a new activity was well under way. In the late 1950s the Advanced Research Projects Agency of the U.S. Department of Defense, in cooperation with research universities, sponsored an open competition to establish government-supported research laboratories at a limited number of university to pursue the integrated study of materials and to educate graduate students in the new field. A dozen such facilities were set up in the United Sates. The methods of materials science have been extended to the study of polymers, glasses, ceramics, amorphous metals, and even biological materials such as bone. The simple concept of relating properties to structure has resulted in an astonishing variety of advanced materials of great utility. LESSON 16 Chemical Process Safety In 1978, Robert M. Solow, an economist at the Massachusetts Institute of Technology, received the Nobel Prize in economics for his work in determining the source of economic growth. Professor Solow concluded that the bulk of an economy’s growth is the result of technological advances. It is reasonable to conclude that the growth of an industry is also dependent on technological 东南大出版社学8 advances. This is especially true in the chemical industry, which is entering an era of more complex processes: higher pressure, more reactive chemicals, and exotic chemistry.More complex processes require more complex safety technology. Many industrialists even believe that the development and the application of safety technology is actually a constraint on the growth of the chemical industry. As chemical process technology becomes more complex, chemical engineers will need a more detailed and fundamental understanding of safety. H.H. Fawcett has said that to know is to survive and to ignore fundamentals id to court disaster.Since 1950, significant technological advances have been made in chemical process safety. Today, safety is equal in importance to production and has developed into a scientific discipline which includes many highly technical and complex theories and practices. Examples of the technology safety include: ;,aHydrodynamic models representing two-phase flow through a vessel relief. ;,bDispersion models representing the spread of toxic vapor through a plant after a release. ;,cMathematical techniques to determine the various ways that processes can fail, and the probability of failure. Recent advances in chemical plant safety emphasize the use of appropriate technological tools to provide information for making safety to decision with respect to plant design and operation. The word safety used to means the older strategy of accident prevention through the used of hats, safety shoes, and a variety of rules and regulations. The main emphasis was on worker safety. Much more recently, safety has been replaced by loss prevention. This term includes hazard identification, technical evaluation, and the design of new engineering features to prevent loss. The words safety and loss prevention will be used synonymously throughout for 东南大出版社学9 convenience. Safety, hazard, and risk are frequently-used terms in chemical process safety. Their definitions are: ;,aSafety or loss prevention is the prevention of accidents by the use of appropriate technologies to identifiers the hazard of a chemical plant and to eliminate them before an accident occurs. ;,bA hazard is anything with potential for producing an accident. ;,cRisk is the probability of a hazard resulting in an accident. Chemical plants contain a large variety of hazards. Fires, there are the usual mechanical hazards that cause worker injuries from tripping failing, or moving equipment. Second, there are chemical hazards. These include fire and explosion hazards, reactivity hazards, and toxic hazards. As will be shown later, chemical plants are the safest of all manufacturing facilities. However, the potential always exists for an accident of catastrophe proportions. Despite substantial safety programs by the chemical industry, headlines of the type shown in Figure continue to appear in newspapers. A successful safety program requires several ingredients. These ingredients are a)Safety knowledge b)Safety experience c)Technical competence d)Safety management support e)Commitment 东南大出版社学10 LESSON 17 Plant Design and General Considerations The general term plant design includes all engineering aspects involved in the development of either a new, modified, or expanded industrial plant. In this development, the chemical engineer will be making economic evaluations of new processes, designing individual pieces of equipment for the proposed new venture, or developing a plant layout for coordination of the overall operation. Because of these many design duties, the chemical engineer is many times referred to here as a design engineer. On the other hand, a chemical engineer specializing in the economic aspects of the design is often referred to as a cost engineer. In many instances, the process engineering is used in connection with economic evaluation and general economic analyses of 东南大出版社学11 industrial processes, while process design refers to the actual design of the equipment and facilities necessary for carrying out the process. Similarly, the meaning of plant design is limited by some engineers to items related directly to the complete plant, such as plant layout, general service facilities, and plant location. The purpose of the text is to present the major aspect of plant design as related to the overall design project. Although one person cannot be an expert in all the phases involved in plant design, it is necessary to be acquainted with the general problems and approach in each of the phases. The process engineer may not be connected directly with the final detailed design of the equipment and the designer of the equipment may have little influence on a decision by management as to whether or not a given return on an investment is adequate to justify construction of a complete plant. Nevertheless, if the overall design project is to be successful, close teamwork is necessary among the various groups of engineers working on the different phases of the project. The most effective teamwork and coordination of efforts are obtained when each of the engineers in the specialized groups is aware of the many functions in the overall design project. The development of the overall design project involves many different design considerations. Failure to include these considerations in the overall design project may, in many instances, alter the entire economic situation so drastically as to make the venture unprofitable. Some of the factors involved in the development of a complete plant design include plant location, plant layout, materials of construction, structural design, utilities, buildings, storage, materials handling, safety, waste disposal, federal, state, and locate laws or codes, and patents. Record keeping and accounting procedures are also important factors in general design considerations, 东南大出版社学12 and it is necessary that the design engineer be familiar with the general terminology and approach used by accountants for cost and asset accounting. The development of a design project proceeds in a logical, organized sequence requiring more and more time, effort, and expenditure as one phase leads into the next. It is extremely important, therefore, to stop and analyze the situation carefully before proceeding with each subsequent phase. Many projects are discarded as soon as the preliminary investigation or research on the original idea is complete. The engineer working on the project must maintain a realistic and practical attitude in advancing through the various stages of a design project and not be swayed by personal interests and desires when deciding if further work on a particular project is justifiable. Remember, if the engineer’s work is continued on through the various phases of a design project, it will eventually end up in a proposal that money be invested in the process. If no tangible return can be realized from the investment, the proposal will be turned down. Therefore, the engineer should have the ability to eliminate unprofitable ventures before the design project approaches at a final-proposal stage. LESSON 19 Process Reactor Design Every chemical engineer should know not only the processes of manufacturing chemicals but also the design and operation of the equipment needed to carry out the processes. This equipment can be divided into two main groups. The first group consists of the equipment used to purify or separate raw material to the extent necessary to obtain optimum yields. In as much as the properties of the substances after processing remain the same as those of the raw 东南大出版社学13 materials, these separations are essentially physical treatment steps; the design and operation of the physical separation equipment are studied in unit operations. The second group consists of chemical reactors in which the processed raw materials react to create products with entirely new physical and chemical properties. This is a chemical treatment step. When we look at chemical processes, we see that every process behaves as shown in Fig.1. That is to say, the raw materials go through a series of physical separation devices and then enter the chemical reactors in which the transformation is carried out. From the reactors, the reaction mixtures which contain the new desired products, side undesired products, and unreacted reactants will be once more purified or separated to obtain products of high quality, to recover the potentially valuable unreacted raw materials, and to destroy or separate the side products. In most cases, recycling the reaction mixtures from the last groups to the first group of the physical separation devices increased the yield of the reaction. Here, we are primarily concerned with the chemical treatment step. We will discuss the type of the reactor needed, provisions for exchange of energy with surroundings, and operation conditions such as temperature, pressure, flow rates, and compositions. Some economic considerations of important in process reactor systems are the optimum operating conditions, cost analysis and profitability, the stability of the reaction, the control of the reaction, the material construction, and scale-up problems. Whenever necessary, some of these factors will be briefly described. In the design of a process reactor, a chemical engineer must consider the following: 1.What reactor will occur in the reactor? 2.How fast could the reaction go? 东南大出版社学14 3.What type and size should the reactor be? What operating temperature, pressure, compositions, and flow rates should be selected? 4.Is the production economical? The first question deals with the thermodynamics from which the equilibrium composition of the reaction mixture can be estimated. The second concerns the process kinetics from which the rate constant of a reaction can be predicted. The third accounts for mass and energy balances in the reaction system. The incorporation of the second and third determines the type and size of the reactor required for certain reactions. The fourth question considers the economics of the process from which the optimum operating conditions can be obtained. In order to fulfill these requirements, we nee information, knowledge, and experience from a variety of areas: thermodynamics, chemical kinetics, fluid mechanics, heat transfer, mass transfer, and economics. LESSON 21 Bioengineering Bioengineering is the application of engineering knowledge to the fields of medicine and biology. The bioengineer must be well grounded in biology and have engineering knowledge that is broad, drawing upon electrical, chemical, mechanical, and other engineering disciplines. The bioengineer may work in any of a large range of areas. One of these is the provision of artificial limbs, and supportive or substitute organs. In another direction, the bioengineer may use engineer methods to achieve biosynthesis of animals or plant products—such as for fermentation processes. Before World War the field of bioengineering was essentially unknown, and little? communication or interaction existed between the engineer and the life scientist. A few exceptions, however, should be noted. The agricultural engineer and the chemical engineer, 东南大出版社学15 involved in fermentation processes, have always been bioengineers in the broadest sense of the definition since they deal with biological systems and work with biologists. The civil engineer, specializing in sanitation, has applied biological principles in the work. Mechanical engineers have worked with the medical profession for many years in the development of artificial limbs. Another area of mechanical engineering that falls in the fields of bioengineering is the air-conditioning field. In the early 1920s engineers and physiologists were employed by the American Society of Heating and Ventilating Engineers to study the effects of temperature and humidity on humans and to provide design criteria for heating and air-conditioning systems. Today there are many more examples of interaction between biology and engineering, particularly in the medical and life-support fields. In addition to an increased awareness of the need for communication between the engineer and the associate in the life sciences, there is an increasing recognition of the role the engineer can play in several of the biological fields, including human medicine, and, likewise, an awareness of the contribution biological science can make toward the solution of engineering problems. Much of the increase in bioengineering activity can be credited to electrical engineers. In the 1950s bioengineering meetings were dominated by sessions devoted to medical electronics. Medical instrumentation and medical electronics continue to be major areas of interest, but biological modeling, blood-flow dynamics, prosthetics, biomechanics (dynamics of body motion and strength of materials), biological heat transfer, biomaterials, and other areas are now included in conference programs. Bioengineering developed out of specific desires or needs: the desire of surgeons to bypass the heart, the need for replacement organs, the requirements for life support in space, and many 东南大出版社学16 more. In most cases the early interaction and education were a result of personal contacts between physician, or physiologist, and engineer. Communication between the engineer and the life scientist was immediately recognized as a problem. Most engineers who wandered into the field in its early days probably had an exposure to biology through a high-school course and no further work. To overcome this problem, engineers began to study not only the subject matter but also the methods and techniques of their counterparts in medicine, physiology, psychology, and biology. Much of the information was self-taught or obtained through personal association and discussions. Finally, recognizing a need to assist in overcoming the communication barriers as well as to prepare engineers for the future, engineering schools developed course and curricula in bioengineering. Some important branches of bioengineering as follows: Medical engineering concerns the application of engineering Medical engineering principles to medical problems, including the replacement of damaged organs, instrumentation, and the systems of health care, including diagnostic applications of computers. This includes the application of engineering principles to Agricultural engineering the problems of biological production and to the external operations and environment that influence this production. Bionics is the study of living systems so that the knowledge gained can be applied Bionics to the design of physical systems. Biochemical engineering includes fermentation engineering, Biochemical engineering application of engineering principles to microscopic biological systems that are used to create 东南大出版社学17 new products by synthesis, including the production of protein from suitable raw materials. This concerns the application of engineering, physiology, Human-factors engineering and psychology to the optimization of the human-machine relationship. Also called bioenvironmental engineering, this field Environment health engineering concerns the application of engineering principles to the control of the environment for the health, comfort, and safety of human beings. It includes the field of life-support systems for the exploration of outer space and the ocean. LESSON 22 Genetic Engineering This slightly misleading term covers the recent development of techniques concerned with producing what are effectively controlled mutations—deliberate evolution. The techniques, often colorfully called “gene splicing”, are methods of constructively rearranging the genetic code to produce an organism with new, desirable characteristics. In the case of a simple microbe, this might involve introducing the ability to produce a certain chemical. Somewhere on the DNA molecule is the information (gene) concerned with this desired quality or product. The “engineering” is concerned with “cutting out” that part of the string, and joining or grafting this into another organism. This is not, of course, done with a knife, but chemically by using special enzymes. The discovery of this way to “restriction enzymes”, by Stanley Cohen and Herbert Boyer in the early 1970s, supplied a tool that would cut the long DNA molecule into a fixed number of defined fragments. The enzyme does this by recognizing and attacking certain specific nucleotide 东南大出版社学18 sequences in the DNA chain. There are now well over 300 of these enzymes in general use, each “tuned” to a different sequence. “Splicing” is done with another set of enzymes called “litigation” enzymes, which can stick the fragments back together. All this talk of “cutting” and “splicing” perhaps suggests delicate surgery, but it must be remember that all these functions are in practice carried out chemically in solution. The molecular biologist/genetic engineer often ends up with a “soup” composed of fragments of the genetic code. Somewhere in there is the coding for the desired characteristic, but unneeded sequences will also be present. There are a number of techniques that have been developed to help in the selection of those fragments he wishes to process. If a DNA sample is placed on a plate of gel under appropriate chemical conditions and a weak electric current is passed across the plate, the fragments will to “migrate” in the direction of the positive pole. The smaller ones move faster, and after several hours they are all arranged in a neat line from the shortest to the longest. Electrophoretic separation can be used to determine the sequence of “bases” in a DNA fragment or to separate specific molecules from one another. The DNA synthesizer (or “gene machine”) is a fairly recent development that promises greatly to expand the scope of the gene-splicer. Since there are only four components (however complicated their permutations and combinations) to the DNA string, it has been possible to construct an apparatus that can create a specific sequence of DNA, a synthetic gene. When a desired fragment is identified or synthesized it can be re-inserted into a “host” organism. Genetic engineers tend to chose micro-organisms with thoroughly known characteristics. The resulting “engineered” organism is then “cloned” from a single cell grown 东南大出版社学19 and multiplied in the usual way and if the process is successful it will breed true-a new organism with an added ability engineered in. To get DNA into a “host” a “vector” must be used. Usually this is a small piece of DNA which is attached to the new DNA and when inserted “recombines” again with the chromosomal DNA using natural processes usually involved in the cell’s reproduction. The process has two distinct aims. It can simply be used to increase the production capability of the microbe concerned. This, known as “amplification”, consists of increasing the number of gene sequences devoted to the production of the desired substance. Alternatively, an entirely new production ability can be grafted in. Genetic material from literally any source, plant or animal, can be inserted into a microbe whose growth and fermentation characteristics are thoroughly well known. LESSON 23 How We Digest Carbohydrates While any of the monosaccharides, glucose or fructose for example, easily penetrates the intestinal wall to enter the bloodstream, neither disaccharides nor the larger carbohydrates normally get through the intestinal barrier. They are too large. To assimilate these larger carbohydrates, from maltose and sucrose to starch, we must first clip them down to their component monosaccharides, which are able to pass through the intestinal wall and into the bloodstream. (4) As we’ve seen, in forming the larger carbohydrate molecules the individual 东南大出版社学20 monosaccharide rings connect to each other with loss of a molecule of water. (4) To reverse this process, to cleave the disaccharides and the polysaccharides to their component monosaccharides, we must return the water to the molecules through hydrolysis. Our bodies carry out this hydrolysis through the catalytic action of our digestive enzymes. Enzymes, you’ll recall, act as biological catalysts that allow chemical reactions to take place more rapidly or under milder conditions than they might otherwise, but are not consumed themselves. In cleaving the nutrient polysaccharides quickly and efficiently in the relatively mild chemical environment of our digestive systems, these molecular catalysts act very much like the platinum and palladium catalysts that help remove unburned hydrocarbons from automobile exhausts. An enzyme that enables us to digest both maltose and starch is maltase, which our bodies produce in sufficient quantities to allow us to digest the starch we eat. As you might infer from this single example, an enzyme’s name usually resembles the name of the substance it acts on. For the simple disaccharides, just replace the –ose ending of the sugar with –ase to get the name of the enzyme that hydrolyses the disaccharide. Maltase helps us hydrolyze the link of maltose and starch. We’ve seen that the combination of two glucose units through a β link produces cellobiose, which resembles two links of the cellulose chain. Since we don’t produce any of the digestive enzyme that hydrolyses the β linkage, cellobiase, we don’t digest cellobiose or cellulose. For humans, starch constitutes a digestible carbohydrate, while cellulose is one of the indigestible carbohydrates that from a large part of the fiber, bulk or roughage of our diets. Grass, leaves, and other plant material, all of which are indigestible in our own intestines, 东南大出版社学21 provide metabolic energy to crows, goats, sheep, and other ruminants, and to termites and similar insects, simply because the digestive systems of these animals harbor microorganisms that produce the needed cellobiase. We human don’t have the required enzyme, and so we can’t live on grass and wood. Foods rich in fiber include fruits, vegetables, bran, and nuts. For good health it’s recommended that these be part of our daily diets. Dietary fiber seems to reduce the risk of cancer, especially cancer of the colon. How and why dietary fiber might have this beneficial effect are uncertain. One bit of speculation holds that the secret of fiber may lie in its mechanical effect on our large intestine. Fiber absorbs much water as it passes through us and it assumes considerable bulk. This bulk may stimulate the intestine and promote a rapid transit of the fibrous bulk through and out of our bodies. If this rapid passage of fiber also speed the elimination of other cancer-causing substances, then their contact time with intestinal tissue is shortened as well, and their opportunity for acting on intestinal tissue and generating cancer is diminished. On the other hand, another possibility may simply be that as we increase the proportion of fiber-containing foods in our diets we necessarily decrease the proportion of meats and other fatty foods. Since diets high in fats seem to be associated with cancer, this hypothesis could easily account for the protective effect of fiber. 东南大出版社学22 LESSON 26 Green House Effect A large amount of carbon dioxide gets introduced into the Atmosphere from fossil fuel 13burning, furnaces and breathing of animals. From fossil fuel alone, more than 2.5*10 tons of CO is emitted into the atmosphere each year. Not all of the CO injected into the atmosphere 22 remains there; about half of it gets utilized by plant life or absorbed by water of the oceans. Part of the CO, dissolved in the ocean may get precipitated or incorporated in marine organisms. In 2 this respect aquatic plants in the ocean are playing an important role in maintaining CO2 equilibrium between the atmosphere and the surface layers of the ocean (up to 100 meters deep). Part of the CO, taken up by terrestrial plants gets deposited in dead vegetation and humus on 2 the forest floor. Some of it, in the form of organic plant parts, has been eaten by herbivorous animals and gets deposited on or in the soil. However, much of CO is still left in the atmosphere. 2 An increase in atmospheric carbon dioxide will influence the photosynthesis, and consequently on plant growth by its direct fertilizing effect, especially in hot tropical environments and a longer growing season in temperate regions. This potential fertilizing effect should be exploitable by using modified crop varieties and agriculture practices to compensate for the disadvantageous effects of temperature increase. Carbon dioxide emitted by volcanoes during several billion years was at least 4000 times that still present in the air. On a global time scale, the known amounts of CO, in limestone and fossil sediments suggest that normal residence time 2 of CO in the atmosphere has been probably around 100000 years.2 Carbon dioxide acts confined exclusively to troposphere. In dense concentration it can act a serious pollutant. The temperature at the surface of the earth has been maintained by the energy balance of the sun’s rays that strike the planet and the heat that gets radiated back into the 东南大出版社学23 space. Some of the sun’s rays that penetrate the thick layer of CO, are able to strike the earth 2 and get converted into heat. The heated earth is able to reradiate this absorbed energy as radiations of longer wavelengths. Much of this does not pass through CO layer to outer space 2 but gets absorbed by the CO and water in the atmosphere and adds to the heat that has been 2 already present. Thus the earth’s atmosphere heats up. This phenomenon is termed as the greenhouse effect. Carbon dioxide thus acts like the glass of a green house and on a global scale, tends to warm the air in the lower levels of the atmosphere. In the “greenhouse effect”, increase in the CO concentration means more infrared 2 radiation is trapped and re-emitted back to the earth producing a build up of infrared radiation in the atmosphere and a mean global temperature rise. Estimates of the rise in temperature with increasing CO concentration range from 0.1?to 4.9?, with a mean around 2? for doubling 2 3the CO concentration to 600ppm. As there is a logarithmic relationship between surface 2 temperature and CO concentration it is likely that a temperature will be reached, beyond which 2 further increases of CO levels have little effect. A maximum increase of 2.5K has been 2 suggested. Since 1945 the mean global temperature has dropped suggesting that either the greenhouse effect is not a satisfactory explanation, or some other effect is beginning to dominate. Aerosols in the size range 0.1-5μm, could reduce incoming radiation by 10% through back scattering and any increase could lead to cooling of the atmosphere. The rate of increase of atmosphere turbidity has been greater than the increase in CO levels, and it could be that this is 2 the more important factor today. It is not yet possible to adequately explain the change in the temperature over the last century, but at least an increase in the CO and aerosol content of the 2 atmosphere has the potential to alter the world’s climate. Pollutants such as NO, CH and 24 东南大出版社学24 CClF, which have strong infrared absorptions, could also influence the mean global 3 temperature. An increased heating of earth would cause recede of glaciers, disappearance of ice caps such as those found over Antarctic and Greenland, and rise in ocean level. In fact, it has been estimated that if all the ice on the earth should melt, 200 feet of water would be added to the surface of all oceans, and low-lying coastal cities, such as Bangkok and Venice would get inundated. Only a rise in sea level of 50-100 cm caused by ocean warming would be able to flood low-lying lands, in Bangladesh and west Bengal. Further, due to the much warmer tropical oceans because of increased carbon dioxide, there have been likely to be more hurricanes and cyclones. According to Mr. Stephan Keckes, at Yugoslavian marine biologist, heat of the United-Nations Environment Program’s Center on ocean and coastal areas, within 30 years, rising seas will be able to wash away entire countries and flood cities from Boston to Bombay. He calculated that seas will rise by 1.5 to 3.5 meters within three decades. In Bangladesh alone 15million people will have to move or drown. He says that little can be done about rising, oceans beyond studying the phenomenon and preparing for the worst. According to G. N. Plass, if the carbon dioxide content to the atmosphere gets doubled, the average surface temperature of the earth would rise 6.5?. However, this would not happen because of reflection of some of the sun’s heat back into the space by densely accumulating particulate contaminants such as smoke and dust from industrial and automobile exhaust. If carbon dioxide continues to accumulate, it may disallow the cooling effect of particulate contaminants and as a consequence the earth’s temperature may rise again. 东南大出版社学25 LESSON 28 Nomenclature of Chemical Compounds The necessity of giving each compound a unique name requires a richer variety of terms that are available with descriptive prefixes such as n- and iso-. The naming of organic compounds is facilitated through the use of formal systems of nomenclature. Nomenclature in organic chemistry is of two types: common and systematic. Common names for organic compounds originate in many different ways but share the feature that there is no necessary connection between name and structure. The name that corresponds to a specific structure must simply be memorized, much like learning the name of a person. Systematic names, on the other hand, are keyed directly to molecular structure according to a generally agreed upon set of rules. The most widely used standards for organic nomenclature have evolved from suggestions made by a group of chemists assembled for that purpose in Geneva in 1892 and have been revised on a regular basis by the International Union of Pure and Applied Chemistry (IUPAC). The IUPAC rules govern all classes of organic compounds but are ultimately based on alkane names. Compounds in other families are viewed as derived from alkanes by appending functional groups to, or otherwise modifying, the carbon skeleton. Table 1 IUPAC Names of Unbranched Alkanes 东南大出版社学26 Alkane formula name alkane formula name CHmethane CH(CH)CHoctane4 363 CHCHethane CH(CH)CHnonane33 373 CHCHCHpropane CH(CH)CHdecane323 383 CH(CH)CHbutane CH(CH)CHpentadecane3223 3133 CH(CH)CH pentane CH(CH)CH icosane3233 3183 CH(CH)CHhexane CH(CH)CH tricontane3243 3283 CH(CH)CHheptane CH(CH)CH hectane3253 3983 The IUPAC rules assign names to unbranched alkanes according to the number of their carbon atoms. Methane, ethane, and propane are used for CH, CHCH, and CHCHCH, 433323 respectively. The unbranched alkane CHCHCHCH is defined as butane, not n-butane as 3223 given above. (the n- prefix is not in systematic IUPAC nomenclature.) Beginning with five-carbon chains, the names of unbranched alkanes consist of a Latin or Greek stem corresponding to the number of carbons in the chain followed by the suffix –ane. Some examples are given in Table. A group of compounds, such as the unbranched alkanes, that differ from one another by successive introduction of CH groups constitute what is called a homologous series.2 Alkanes with branched chains are named on the basis of the name of the longest continuous chain of canbon atoms in the molecule, called the parent. The alkane shoun has seven carbons in its longest continuous chain and is therefore named as a derivative of heptane, the unbranched alkane that contains seven carbon atoms. The position of the CH (methyl) substituent on the 3 seven-carbon chain is specified by a number (3-), called a locant, obtained by successively numbering the carbons in the parent chain starting at the end of nearer the branch. The compound is therefore is called 3-methylheptane. 东南大出版社学27 1623457 CHCHCHCHCHCHCH 3-methylheptane322223 CH3 When there are two or more identical substituents, replicating prefixes (di-, tri-, tetra-, etc.) are used, along with a separate locant for each substituent. Different substituents, such as an ethyl (-CHCH) and a methyl (-CH) group, are cited in alphabetical order. Replicating prefixes 233 are ignored when alphabetizing. In alkanes numbering begins at the end nearest the substituent that appears first on the chain so that the prefix numbers are as low as possible. Methyl and ethyl are examples of alkyl groups. IUPAC nomenclature rules for naming alkyl groups beyond these two extend to cover even very complex structures. The IUPAC rules are unambiguous in the sense that there is no possibility that two different compounds will have the same name. CH3 CHCHCHCHCCHCHCH34-ethyl-2,4-dimethyloctane32222 CHCHCH323 东南大出版社学28 LESSON 1 Chemical Engineering Chemical engineering is the development of processes and the design and operation of plants in which materials undergo changes in physical or chemical state on a technical scale. Applied throughout the process industries, it is founded on the principles of chemistry, physics, and mathematics. The laws of physical chemistry and physics govern the practicability and efficiency of chemical engineering operations. Energy changes, deriving from thermodynamic considerations, are particularly important. Mathematics is a basic tool in optimization and modeling. Optimization means arranging materials, facilities, and energy to yield as productive and economical an operation as possible. Chemical engineering is as old as the process industries. Its heritage dates from the fermentation and evaporation processes operated by early civilizations. Modern chemical engineering emerged with the development of large-scale, chemical-manufacturing operations thin the second half of the 19 century. Throughout its development as an independent discipline, chemical engineering has been directed toward solving problems of designing and operating large plants for continuous production. thManufacture of chemicals in the mid-19 century consisted of modest craft operations. Increase in demand, public concern at the emission of noxious effluents, and competition 东南大出版社学29 between rival processes provided the incentives for greater efficiency. This led to the emergence of combines with resources for larger operations and caused the transition from a craft to a science-based industry. The result was a demand for chemist with knowledge of manufacturing processes, known as industrial chemist or chemical technologists. The term chemical engineer was in general use by about 1900. Despite its emergence in traditional chemicals manufacturing, it was through its role in the development of the petroleum industry that chemical engineering became firmly established as a unique discipline. The demand for plants capable of operating physical separation processes continuously at high levels of efficiency was a challenge that could not be met by the traditional chemist or mechanical engineer. A landmark in the development of chemical engineering was the publication in 1901 of the first textbook on the subject, by George E. Davis, a British chemical consultant. This concentrated on the design of plant items for specific operations. The notion of a processing plant encompassing a number of operations, such as mixing, evaporation, and filtration, and of these operations being essentially similar, whatever the product, led to the concept of unit operations. This was first enunciated by the American chemical engineer Arthur D. Little in 1915 and formed the basis for a classification of chemical engineering that dominated the subject for the next 40 years. The number of unit operations—the building blocks of a chemical plant—is not large. The complexity arises from the variety of conditions under which the unit operations are conducted. The unit approach suffered from the disadvantage inherent in such classifications: a ?restricted outlook based on existing practice. Since World War , closer examination of the 东南大出版社学30 fundamental phenomena involved in the various unit operations has shown these to depend on the basic laws of mass transfer, heat transfer, and fluid flow. This has given unity to the diverse unit operations and has led to the development of chemical engineering science in its own right; as a result, many applications have been found in fields outside the traditional chemical industry. Study of the fundamental phenomena upon which chemical engineering is based has necessitated their description in mathematical form and has led to more sophisticated mathematical techniques. The advent of digital computers has allowed laborious design calculations to be performed rapidly, opening the way to accurate optimization of industrial processes. Variations due to different parameters, such as energy source used, plant layout, and environmental factors, can be predicted accurately and quickly so that the best combination can be chosen. Chemical Engineering Functions. Chemical engineers are employed in the design and development of both processes and plant items. In each case, data and predictions often have to be obtained or confirmed with pilot experiments. Plant operation and control is increasingly the sphere of the chemical engineer rather than the chemist. Chemical engineering provides an ideal background for the economic evaluation of new projects and, in the plant construction sector, for marketing. Branches of Chemical Engineering. The fundamental principles of chemical engineering underlie the operation of processes extending well beyond the boundaries of the chemical industry, and chemical engineers are employed in a range of operations outside traditional areas. Plastics, polymers, and synthetic fibers involve chemical reaction engineering problems 东南大出版社学31 in their manufacture, with fluid flow and heat transfer considerations dominating their fabrication. The dyeing of a fiber is a mass-transfer problem. Pulp and paper manufactures involve considerations of fluid flow and heat transfer. While the scale and materials are different, these again are found in modern continuous production of foodstuffs. The pharmaceuticals industry presents chemical engineering problems, the solutions of which have been essential to the availability of modern drugs. The nuclear industry makes similar demands on the chemical engineer, particularly for fuel manufacture and reprocessing. Chemical engineers are involved in many sectors of the metals processing industry, which extends from steel manufacture to separation of rare metals. Further applications of chemical engineering are found in the fuel industries. In the second thhalf of the 20 century, considerable numbers of chemical engineers have been involved in space exploration, from the design of fuel cells to the manufacture of propellants. Looking to the future, it is probable that chemical engineering will provide the solution to at least two of the world’s major problems: supply of adequate fresh water in all regions through desalination of seawater and environmental control through prevention of pollution. 东南大出版社学32 LESSON 3 Unit Operations Chemical engineering has to do with industrial processes in which raw materials are changed or separated into useful products. The chemical engineer must develop, design, and engineer both the complete process and the equipment used; choose the proper raw materials; operate the plants efficiently, safely, and economically; and see to it that products meet the requirements set by the customers. Chemical engineering is both an art and a science. Whenever science helps the engineers to solve a problem, science should be used. When, as is usually the case, science does not give a complete answer, it is necessary to use experience and judgment. The professional stature of an engineer depends on skill in utilizing all sources of information to reach practical solutions to processing problems. The range and variety of processes and industries that call for the services of chemical engineers are both great. The field is not one that is easy to define. The processes described in standard treatises on chemical technology and the process industries give the best idea of the field of chemical engineering. An economical method of organizing much of the subject matter of chemical engineering is based on two facts: (1) although the number of individual processes is great, each one can be broken down into a series of steps, called operations, each of which in turn appears in process after process; (2) the individual operations have common techniques and are based on the same principles. For example, in most processes solids and fluids must be moved, heat or other 东南大出版社学33 forms of energy must be transferred from one substance to another, and tasks like drying, size reduction, distillation, and evaporation must be performed. The unit operation concept is this: by studying systematically these operations themselves—operations which clearly cross industry and process lines—the treatment of all processes is unified and simplified.The strictly chemical aspects of processing are studied in a companion area of chemical engineering called reaction kinetics. The unit operations are largely used to conduct the primarily physical steps of preparing the reactants, separating and purifying the products, recycling unconverted reactants, and controlling the energy transfer into or out of the chemical reactor. The unit operations are as applicable to many physical processes as to chemical ones. For example, the process used to manufacture common salt consists of the following sequence of the unit operations: transportation of solids and liquids, transfer of heat, evaporation, crystallization, drying, and screening. No chemical reaction appears in these steps. On the other hand, the cracking of petroleum, with or without the aid of a catalyst, is a typical chemical reaction conducted on an enormous scale. Here the unit operations—transportation of fluids and solids, distillation, and various mechanical separations—are vital, and the cracking reaction could not be utilized without them. The chemical steps themselves are conducted by controlling the flow of material and energy to and from the reaction zone.Because the unit operations are a branch of engineering, they are based on both science and experience. Theory and practice must combine to yield designs for equipment that can be fabricated, assembled, operated, and maintained. A balanced discussion of each operation requires that theory and equipment be considered together. A number of scientific principles 东南大出版社学34 and techniques are basic to the treatment of the unit operations. Some are elementary physical and chemical laws such as the conservation of mass and energy, physical equilibria, kinetics, and certain properties of matter. Their general use is described in the remainder of this text. Other special techniques important in chemical engineering are considered at the proper places in the text. LESSON 5 Filtration The separation of solids from a suspension in a liquid by means of a porous medium or screen which retains the solids and allows the liquid to pass is termed filtration.In general, the process of the medium will be larger than the particles which are to be removed, and the filter will work efficiently only after an initial deposit has been trapped in the medium. In the chemical laboratory, filtration is often carried out in a form of Buchner funnel, and the liquid is sucked through the thin layer of particles using a source of vacuum: in even simple cases the suspension is poured into a conical funnel with a filter paper. In the industrial equivalent of such a operation, difficulties are involved in the mechanical handling of much larger quantities of suspension and solids. A thicker layer of solids has to form and, in order to achieve a high rate of passage of liquid through the solids higher pressures will be needed, and it will be necessary to provide a far greater area. A typical filtration operation shows the filter medium, its production and the layer of solids, or filter cake which has already 东南大出版社学35 formed. The volumes of the suspensions to be handled will vary from the extremely large quantities involved in water purification and ore handling in the mining industry to relatively small quantities in the fine chemical industry where the variety of solids will be considerable. In most instances in the chemical industry it is the solids that are required and their physical size and properties are of paramount importance. Thus the main factors to be considered when selecting equipment and operating conditions are: a) The properties of the liquid, particularly its viscosity, density and corrosive properties. ,b) The nature of the solidits particle size and shape, size distribution, and packing characteristics. c) The concentration of solids in suspension. d) The quantity of material to be handled, and its value. e) Whether the valuable product is the solid, the fluid, or both. f) Whether it is necessary to wash the filtered solids. g) Whether very slight contamination caused by contact of the suspension or filtrate with the various components of the equipments is detrimental to the product. h) Whether the feed liquor may be heated. i) Whether any form of pretreatment would be helpful. Filtration is essentially a mechanical operation and is less demanding in energy than evaporation or drying where the high latent heat of the liquid, which is usually water, has to be provided. In the typical operation the cake gradually builds up on the medium and the resistance to flow progressively increases. During the initial period of flow, particles are deposited in the surface layers the cloth to form the true filtering medium. This initial deposit 东南大出版社学36 may be formed from a special initial flow of precoat material. The most important factors on which the rate of filtration then depends will be: a)The drop in pressure from the feed to the far side of the filter medium. b)The area of the filtering surface. c)The viscosity of the filter. d)The resistance of the filter cake. e)The resistance of the filter medium and initial layers of cake. The type of filtration described above is usually referred to as cake filtration the proportion of solids in the suspension is large and most of the particles are collected in the filter cake which can subsequently detached from the medium. Where the proportion of solids is very small, as for example in air or water filtration, the particles will often be considerably smaller than the pores of the filter medium and will penetrate a considerable depth before being captured; such a process is called deep bed filtration. In the next lesson the types of filtration equipment will be considered. 东南大出版社学37 LESSON 6 Heat Transfer Practically all the operations that are carried out by the chemical engineer involve the production or absorption of energy in the form of heat. The laws governing the transfer of heat and the types of apparatus that for their main object the control of heat flow are therefore of great importance. When two objects at different temperatures are brought into thermal contact, heat flows from the object at the higher temperature to that at the lower temperature. The net flow is always in the direction of the temperature decrease. The mechanisms by which the heat may flow are three: conduction, convection, and radiation. Conduction If a temperature gradient exists in a continuous substance, heat can flow unaccompanied by any observable motion of matter. Heat flow of this kind is called conduction. In metallic solids, thermal conduction results from the motion of unbound electrons, and there is close correspondence between thermal conductivity and electrical conductivity. In solids, which are poor conductors of electricity, and in most liquids, thermal conduction results from the transport of momentum of individual molecules along the temperature gradient. In gases conduction occurs by the random motion of molecules, so that heat is “diffused” from hotter regions to colder ones. The most common example of conduction is heat flow in opaque solids, as in the brick wall of a furnace or the metal wall of a tube. Convection When a current or macroscopic particle of fluid crosses a specific surface, such as the boundary of a control volume, it carries with it a definite quantity of enthalpy. Such a flow of enthalpy is called a convection flow of heat or simply convection. Since convection is a macroscopic phenomenon, it can occur only when forces act on the particle or stream of fluid 东南大出版社学38 and maintain its motion against the forces of friction. Convection is closely associated with fluid mechanics. In fact, thermodynamically, convection is no considered as heat flow but as flux of enthalpy. The identification of convection with heat flow is a matter of convenience, because in practice it is difficult to separate convection from true conduction when both are lumped together under the name convection. Examples of convection are the transfer of enthalpy by the eddies of turbulent flow and by the current of warm air flowing across and away from an ordinary radiator. Natural and forced convection The forces used to create convection currents in fluids are of two types. If the currents are the result of buoyancy force generated by differences in density and the differences in density are in turn caused by temperature gradients in the fluid mass, the action is called natural convection. The flow of air across a heated radiator is an example of natural convection. If the currents are set in motion by the action of a mechanical device such as a pump or agitator, the flow is independent of density gradients and is called forced convection. The two kinds of force may be active simultaneously in the same fluid, and natural and forced convection then occur together. Radiation Radiation is a term given to the transfer of energy through space by electromagnetic waves. If radiation is passing through empty space, it is not transformed into heat or any other form of energy nor is it diverted from its path. If, however, matter appears in its path, the radiation will be transmitted, reflected, or absorbed. It is only the absorbed energy that appears as heat, and this transformation is quantitative. For example, fused quartz transmits practically all the radiation that strike it; a polished opaque surface or mirror will reflect most of the radiation impinging on it; a black or matte surface will absorb 东南大出版社学39 most of the radiation received by it and will transform such absorbed energy quantitatively into heat. Monatomic and diatomic gases are transparent to thermal radiation, and it is quite common to find that heat is flowing through masses of such gases both by radiation and by conduction-convection. The two mechanisms are mutually independent and occur in parallel, so that one type of heat flow can be controlled or varied independently of the other. Conduction-convection and radiation can be studied separately and their separate effects added together in cases where both are important. In very general terms, radiation becomes important at high temperatures and is independent of the circumstances of the flow of the fluid. Conduction-convection is sensitive to flow conditions and is relatively unaffected by temperature level. LESSON 7 Absorption of Gases The removal of one or more selected components from a mixture of gases by absorption into a suitable liquid is the second major operation of chemical engineering that is based on 东南大出版社学40 interphase mass transfer controlled largely by rates of diffusion. Thus, acetone can be recovered from an acetone-air mixture by passing the gas stream into water in which the acetone dissolves while the air passes out. Similarly, ammonia may be removed from an ammonia-air mixture by absorption in water. In each of these examples the process of absorption of the gas in the liquid can be treated as a physical process, the chemical reaction having no appreciable effect. However, when oxides of nitrogen are absorbed in water to give nitric acid, or when carbon dioxide is absorbed in a solution of sodium hydroxide, a chemical reaction occurs, the nature of which influences the actual rate of absorption. Absorption processes are therefore conveniently divided into two groups, those in which the process is solely physical and those where a chemical reaction is occurring. In considering the design of equipment to achieve gas absorption, the main requirement is that the gas be brought into intimate contact with the liquid, and the effectiveness of the equipment will largely be determined by the success with which it promotes contact between the two phases.The most useful concept of the process of absorption is given by two-film theory. According to the theory, material is transferred in the bulk of the phases by convection currents, and concentration differences are regarded as negligible except in the vicinity of the interface between the phases. On either side of this interface it is supposed that the currents die out and that there exist a thin film of fluid through which the transfer is effected solely by molecular diffusion. This film will be slightly thicker than the laminar sub-layer, because it offers a resistance equivalent to that of the whole boundary layer. According to Fick’s Law the rate of transfer by diffusion is proportional to the concentration gradient and to the area of interface over which the diffusion is occurring. 东南大出版社学41 The general form of equipment is similar to that described for distillation, and packed and plate tower are generally used for large installations. The method of operation is not the same. In absorption, the feed is a gas and is introduced at the bottom of the column, and the solvent is fed to the top, as a liquid; the absorbed gas and solvent leave at the bottom, and the unabsorbed components leave as gas from the top. The essential difference between distillation and absorption is that in the former the vapor has to be produced in each stage by partial vaporization of the liquid which is therefore at its boiling point, whereas in absorption the liquid is well below its boiling point. In distillation there is a diffusion of molecules in both directions, so that for an ideal system equimolecular counter-diffusion exists, but in absorption gas molecules are diffusing into the liquid, and the movement in the reverse direction is negligible. In general, the ratio of the liquid to the gas flowrate is considerably greater in absorption than in distillation with the result that layout of the trays is different in the two cases. Furthermore, with the higher liquid rates in absorption, packed columns are much more commonly used. As a result of the wide variety of purpose and specification of absorbers, a large number of quite different types have been built and used. The various types of gas-absorption equipment may be divided roughly into three groups: a) packed towers, b) bubble-cap towers, and c) miscellaneous types, including spray chambers and agitated vessels. The objective of each design, however, is the provision for intimate contact of gas and liquid over a large interphase surface; with low first cost and low operating and maintenance expense. The first cost includes foundations; tower shell; packing materials; solvent charge, 东南大出版社学42 pumps, blowers, piping ducts, accessory heaters, coolers, and heat exchangers; and solvent-recovery system, if needed. The operating costs include power for circulating gas and solvent, maintenance, labor, steam for regenerating the solvent, cooling water, solvent make-up, and the value of the material which remains unabsorbed and is lost. Abbreviated methods for choosing the economically optimum operating conditions and equipment dimensions are needed. LESSON 8 Distillation Operations Distillation is a process in which a liquid or vapor mixture of two or more substances is separated into its component fractions of desired purity by the application and removal of heat. It is well known that pure liquids exhibit different volatilities (i.e. vapor pressure) at a given temperature, and thus if heat applied to a liquid mixture of these substances, the vapor 东南大出版社学43 ,so generated will be richer in the more volatile substancesthose having higher vapor pressures. If this vapor is condensed, it should be clear that a certain amount of purification will be achieved. This is the basic principle underlying a distillation operation.A distillation process may be classified in one of two ways: Binary distillation refers to the separation of two substances and Multicomponent distillation involves more than two substances. A typical stage-type distillation column consists of a vertical shell with a number of equally spaced trays mounted inside of it. Each tray contains two conduits, one on each side, called downcomers. Liquid flows through these downcomers by gravity from each tray to the one below. A weir on one side of the tray maintains the liquid level at a suitable height on that tray. A variety of tray types are available commercially. The simplest is a sieve tray, a sheet of metal containing a number of perforations that are provided for vapor flow. The flow of vapor must be sufficiently high to prevent weeping of liquid through the holes. For years bubble-cap trays were the best known vapor-liquid contacting devices in the chemical and petroleum industries. In recent years valve trays have gained much popularity in distillation operation.The vertical shell is connected by suitable piping to a heating device called a reboiler, which provides the necessary vaporization for the distillation operation, and to a condenser, which condenses the overhead vapors. There are a number of different types of reboilers and condensers in use. The vertical shell together with the condenser and reboiler constitute a distillation column. Let us consider how the column operates. Assume, for simplicity, that the feed is a binary liquid mixture that is to be separated into two relatively pure products. Feed enters (more or 东南大出版社学44 less) the central portion of the column on a tray called the feed tray. It flows down by gravity from the tray to tray below. The liquid from tray 1 flows into the base of the column and then into a reboiler, where it is partially vaporized. The unvaporized liquid is one of the products of the distillation operation. It is called the bottoms product, and is removed from the reboiler. The bottoms product has the highest concentration of the least volatile substance and thus its temperature, which is also the temperature of the vapor generated in the reboiler, is the highest of any location in the column. Since the volatilities of the two substances involved in the distillation process are different, the vapor generated in the reboiler is richer in the more volatile component. This vapor rises and comes into contact with the descending stream of liquid on each tray, beginning with that on tray 1. The mixing of warmer vapor with the liquid results in the transfer of heat and mass, and the net result is some vapor vaporization of the more volatile component and condensation of a thermally equivalent amount of the less volatile component. Thus the feed is “stripped” of its more volatile component as it flows downward and it becomes more and more concentrated in the less volatile substance. The feed tray and the trays below it constitute what is called the stripping section. The feed tray and the trays above it constitute what is called the distillate section. The vapor rising from the feed tray comes into contact with a liquid that is more concentrated in the more volatile substance on each tray where some of the more volatile substance vaporizes at the expense of some of the less volatile substance, which condenses. Thus the vapor becomes “enriched” in the more volatile substances as it flows up the column. The vapor from the top tray contains a higher concentration of the more volatile substance than anywhere else in the column. It is condensed in a total condenser, a part of it is removed as the distillate product, and the remainder is 东南大出版社学45 returned to the column as reflux. This liquid, which is the purest in terms of the more volatile component, contacts the ascending vapor rising from the feed tray and the heat and mass transfer processes occur as described earlier. The reflux stream combines with the feed and serves as the liquid phase in the stripping section. It should be clear from the description of the distillation operation that the basic steps consist of repeated contact of vapor and liquid phases on the trays. In the ideal case, the liquid and vapor phases leaving a given tray would be in thermodynamic equilibrium. LESSON 9 Solvent Extraction Separation of two or more component of a liquid solution is one of the commonest of chemical-engineering problems. The most usual procedures are evaporation, fractional crystallization, and distillation, in which separation is accomplished by taking advantage of the differing solubilities or volatilities of the components. Alternatively, it is often possible to accomplish the desired separation by bringing the liquid in contact with a second liquid which selectively removes one or more of the components of the solution so treated. Separation is 东南大出版社学46 accomplished because certain of the components are more readily soluble than others in the solvent employed. The solution and the solvent must not be completely miscible, since the purpose is to effect the desired separation by mechanical separation of the two liquid phases. The separation is accomplished without vaporization, though evaporation or distillation is usually required to recover the separated components from the two liquid product streams. Solvent extraction also refers to the treatment of a solid with a solvent, as in the extraction of oil from cottonseed. The present text, however, will be confined to solvent extraction processes in which a liquid solution is treated with a liquid solvent phase (called liquid-liquid extraction). One of the common applications of extraction is in the separation of compounds differing as to chemical type but difficult to separate by distillation because their volatilities do not differ greatly. An example is the separation of aromatic and paraffin hydrocarbons in the solvent refining lubricating oils. In other cases solvent extraction is employed because the components are heat-sensitive and tend to decompose at the ordinary temperatures of distillation or evaporation. Penicillin, streptomycin, and other biologicals produced in dilute solutions may be concentrated and purified by solvent extraction in order that the pure products may be recovered by fractional crystallization or precipitation. In some cases extraction may prove economical in instances where distillation is also entirely practical. Thus Othmer and Trueger claim heat savings for the solvent extraction of acetone and ethyl alcohol from dilute aqueous solution, as compared with standard rectification practice for these separations. This claim is based on the fact that moderately concentrated extracts may be obtained by the use of proper solvents and that the heat 东南大出版社学47 requirement for distillation from these extracts may be less than for distillation of the original dilute aqueous solutions. The solvent extraction process involves the four operations of: a)bringing solvent and solution into intimate contact; b)separation of the two phases; c)removal and recovery of solute from the extract phase; d)removal and recovery of solvent from each phase, usually by distillation.Contacting may be accomplished in any of the several types of equipment, such as baffle-plate mixers which employ impinging jets of the two liquid streams, agitated vessels containing the liquids, plate columns, packed, tower, or centrifugal contactors. Separation may be accomplished by simple settling tanks or by means of centrifuges. The difficulty encountered in separating the phases is usually greatest when the phases are dispersed to a high degree in the contacting equipment. A large difference between the densities of the two phases tends to make separation easy, but the presence of emulsifying agents may cause more trouble in the separation process than a small density difference. After separation of the phases, the solvent is usually recovered by ordinary distillation of the solvent layer, termed the “extract”, and of the treated solution, termed the “raffinate”. 东南大出版社学48 LESSON 10 Drying of Solids The discussions of drying in this text are concerned with the removal of water from process materials and other substances. The term drying is also used to refer to removal of other organic liquids, such as benzene or organic solvents, from solids. Many of the types of equipment and calculation methods discussed for removal of water can also be used for removal of organic liquids. The wet solid is dried by passing a stream of heated gas across or through it. The hot gas serves to transfer heat to the solid by convection and to remove the evaporated vapor. If the hot gas is supplied to the system at a constant temperature and humidity it is observed that the drying process occurs in two distinct stages. Initially the rate of drying is constant and then at some moisture content it begins to diminish and continues to do so progressively until it is zero when the material is completely dry. The moisture content at which the drying rate beings to diminish is known as the critical moisture content but the change generally tends to occur gradually over a range of moisture content. In some cases the initial moisture content may below the critical and the drying will be then be entirely falling-rate with no constant rate. The falling-rate curves themselves may be concave or convex or may approximate to a straight line; they are inflected when a change of physical form occurs-for example when shrinking and cracking occur or when a skin forms on the surface of partly dried material. The constant-rate drying period corresponds to the situation when the surface of the solid is wet with the liquid and in a given system the rate of drying is controlled entirely by the drying conditions; which in the case of pure convection drying are simply the velocity, temperature, and humidity of the drying gas. Thus if these are constant the rate of drying is constant; the 东南大出版社学49 rate of migration of liquid from inside the solid to the surface at which evaporation occurs is such that it does not in any way limit the process. In the falling-rate period the rate of migration of liquid to the surface has decreased so that it controls the rate of drying. The surface is now no longer fully wetted and as the rate of migration decrease the effect of the external conditions progressively diminishes and the rate is merely a reflection of lack of uniformity in the rate of migration of liquid to the surface. The critical moisture content represent the range of moisture content over which the rate of migration to the surface equals the rate of evaporation from the surface and a mean value can be taken for the calculation purposes. The critical moisture content will be dependent upon the external drying conditions.Drying methods and processes can be classified in several different ways. Drying processes can be classified as batch, where the material is inserted into the drying equipment and drying proceeds for a given period of time, or as continuous, where the material is continuously added to the dryer and dried material continuously removed. Drying processes can also be categorized according to the physical conditions used to add heat and remove water vapor: (1) in the first category, heat is added by direct contact with heated air at atmospheric pressure and the water vapor formed is removed by the air; (2) in vacuum drying, the evaporation of water proceeds more rapidly at low pressures, and the heat is added indirectly by contact with a metal wall or by radiation (low temperatures can also be used under vacuum for certain materials that may discolor or decompose at higher temperatures); and (3) in freeze drying, water is sublimed from the frozen material. 东南大出版社学50 LESSON 11 Packed Towers The most common type of absorption equipment is the packed tower. It consists of a vertical shell set on an adequate foundation and filled with one of numerous types of inert packing materials. The operation is usually countercurrent, the solvent being distributed over the packing at the top of the tower and passing down over the packing in thin liquid films, while the gas passes up through the free space between the wetted particles of packing. In place of one very tall tower it is customary to employ several shorter towers in series, the gas passing from the top of the first tower to the bottom of the second, etc., while the liquor is pumped in the opposite direction, as from the bottom of the second to the top of the first, etc. This type of operation is common practice in the low-pressure absorption of nitric oxide in the 东南大出版社学51 manufacture of nitric acid. In some cases the liquor rate is necessarily small compared to the gas flow, and if the tower has a cross section adequate to handle the gas, there may not be enough liquid to wet the packing thoroughly. One method of overcoming this difficulty is to recirculate the liquor through the tower so that the quantity of liquor circulated hourly is several times the actual net hourly throughput. When this is done in a single tower, the advantage of countercurrent action is largely lost, and the use of several shorter towers is resorted to in an effort to simulate true countercurrent action. With this design the liquor moves from tower to tower, countercurrent to the flow of gas. If the liquor throughput is small compared to the liquor recirculated, the change in liquor concentration from top to bottom of any one tower will be small, and it is not important that the gas pass up through each tower. In order to save on gas mains in cases the gas may be allowed to pass alternately up and then down through successive towers. It is advisable to have the diameter small compared to the packed height, and where the ratio of diameter to height is greater than one-fifth, special care must be taken to ensure proper initial distribution of gas and liquor flow through the packing. Even after being distributed uniformly over the top of the packing the liquid has a tendency to flow toward the tower wall. In the tall tower it may be necessary to divide the packing into several sections with a device above each which will collect the liquid from the wall and redistribute it to the packing. The packing rests on a brick checkerwork or metal grid, below which the gas is introduced. A part of the weight of the packing may be supported by the shell in a tall tower, but the 东南大出版社学52 packing support should be strong enough to bear the entire weight. A crude packing support may be made by stacking several large pieces of packing on each other. The packing may be crushed by alternate contraction and expansions of the shell if cyclic temperature changes occur. Above the packing is the liquor-feed distributor, which may be any one of several types. If the liquid enters through the same openings by which the gas leaves, the cross-sectional area provided for gas flow must be sufficient to prevent flooding at the distributor. A better plan is to provide separate openings for the liquid and gas. For adequate initial distribution, the liquid should be subdivided into a number of separate streams. This can be done by installing a system of troughs and weirs of by introducing the main liquid stream onto a horizontal tray having several liquid-overflow tubes extending through the tray. The tops of these tubes must be leveled carefully during fabrication and installation, but the adjustment is not so critical if each tube is notched at the top, where the liquid overflows. Since absorption towers find wide application in chemical plants, particularly for acid manufacture, a common construction material is acidproof ceramic ware. The acidproof brick for the shells of the larger towers are bonded with an acidproof mortar, usually having a sodium silicate base. In general the porous materials are more resistant to temperature changes, and denser materials more resistant to acid. In addition to packing material and shells for the smaller towers it is possible to obtain acidproof chemical-stoneware pipes, liquor-distributing plates, valves, pumps, and blowers. Pipes, valves, and other small pieces may also be had in hard rubber. 东南大出版社学53 东南大出版社学54