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Unit 12 what do we mean by transport phenomena ?Transport phenomena is the collective name given to the systematic and integrated study of three classical areas of engineering science : (i) energy or heat transport ,(ii) mass transport or diffusion ,and (iii) momentum transport or fluid dynamics . 传递现象是工程科学三个典型领域 系统性和综合性研究的总称： 能量或热量传递， 质量传递或扩散， 以及动量传递或流体力学。 Of course , heat and mass transport occur frequently in fluids , and for this reason some engineering educators prefer to includes these processes in their treatment of fluid mechanics . 当 然， 热量和质量传递在流体中经常发生， 正因如此一些工程教育家喜欢把这些过程包含在流 体力学的范畴内。Since transport phenomena also includes heat conduction and diffusion in solids , however , the subject is actually of wider scope than fluid mechanics. 由于传递现象 也包括固体中的热传导和扩散，因此，传递现象实际上比流体力学的领域更广。It is also distinguished from fluid mechanics in that the study of transport phenomena make use of the similarities between the equations used to describe the processes of heat,mass,and momentum transport. 传递现象的研究充分利用描述传热，传质，动量传递过程的方程间的相似性，这 也区别于流体力学。 These analogies,as they are usually called, can often be related to similarities in the physical mechanisms whereby the transport takes place. 这些类推(通常被这么叫)常常可以与传递现象发生的物理机制间的相似性关联起来。 a consequence,an understanding of one As transport process can readily lead to an understanding of other processes. 因此，一个传递过程的 理解能够容易促使其他过程的理解。Moreover,if the differential equations and boundary conditions are the same,a solution need be obtained for only one of the processes since by changing the nomenclature that solution can be used to obtain the solution for any other transport process. 而且， 如果微分方程和边界条件是一样的， 只需获得一个传递过程的解决方案即可， 因为通过改变名称就可以用来获得其他任何传递过程的解决方案。 It must be emphasized , however, that while there are similarities between the transport processes, there are also important differences , especially between the transport of momentum (a vector ) and that of heat or mass (scalars ). 必须强调,虽然有相似之处,也有传递过程之间的差 异,尤其重要的是运输动量(矢量)和热或质量(标量). Nevertheless , a systematic study of the similarities between the transport processes makes it easier to identify and understand the differences between them. 然而,系统地研究了相似性传递过程之间的相似性,使它更容易识别 和理解它们之间的差别。1How We Approach the Subject 怎么研究传递过程？ In order to demonstrate the analogies between the transport processes , we will study each of the process in parallel-instead of studying momentum transport first , then energy transport , and finally mass transport. 为了找出传递过程间的相似性，我们将同时研究每一种传递过程 取代先研究动量传递， 再传热， 最后传质的方法。 Besides promoting understanding , there is another pedagogical reason for not using the serial approach that is used in other textbooks : of the three processes, the concepts and equations involved in the study of momentum transport are the most difficult for the beginner to understand and to use . 除了促进理解之外，对于不使用在其他教科书里用到的顺序法还有另一个教学的原因： 在三个过程中， 包含在动量传递研究中 的概念和方程对初学者来说是最难以理解并使用。Because it is impossible to cover heat and mass transport thoroughly without prior knowledge of momentum transport ,one is forced under the serial approach to take up the most difficult subject (momentum transport) first . 因为在不具 有有关动量传递的知识前提下一个人不可能完全理解传热和传质， 在顺序法的情况下他就被 迫先研究最难的课程即动量传递。On the other hand ,if the subjects are studied in parallel , momentum transport becomes more understandable by reference to the familiar subject of heat transport. 另一方面，如果课程同时被研究，通过参照有关传热的熟悉课程动量传递就变得 更好理解。 Furthermore ,the parallel treatment makes it possible to study the simpler the physical processes that are occurring rather than the mathematical procedures and representations. 而且， 平行研究法可以先研究较为简单的概念， 再深入到较难和较抽象的概念。 我们可以先强调所 发生的物理过程而不是数学性步骤和描述。 For example ，we will study one-dimensional transport phenomena first because equations instead of partial requiring vector notation and we can often use ordinary differential equations instead of partial differential equations ,which are harder to solve . 例如，我们将先研究一维传递现象，因为它在不要求矢量标注下就可以被解 决，并且我们常常可以使用普通的微分方程代替难以解决的偏微分方程。 This procedure is also justified by the fact that many of the practical problems of transport phenomena can be solved by one-dimensional models. 加上传递现象的许多实际问题可以通过一维模型解决的这样一 个事实，这种处理做法也是合理的。 2.Why Should Engineers Study Transport Phenomena? 为什么工程师要研究传递现象？ Since the discipline of transport phenomena deals with certain laws of nature , some people classify it as a branch of engineering . 因为传递现象这个学科牵扯到自然界定则，一些人就把 它划分为工程的一个分支。 For thisreason the engineer , who is concerned with the economical design and operation of plants and equipment , quite properly should ask how transport phenomena will be of value in practice . 正因如此，对于那些关心工厂和设备设计和操作经济 性的工程师而言，十分应该探知在实际中传递现象如何起到价值作用。There are two general types of answers to those questions . 对于那些问题有两种通用型答案。The first requires one to recognize that heat ,mass ,and momentum transport occur in many kindsof engineering , e.g., heat exchangers ,compressors ,nuclear and chemical reactors, humidifiers, air coolers ,driers , fractionaters , and absorbers. 第一种要求大家认识到传热， 传质和动量传递发生在许多工程设 备中，如热交换器，压缩机，核化反应器，增湿器，空气冷却器，干燥器，分离器和吸收器。 These transport processes are also involved in the human body as well as in the complex processes whereby pollutants react and diffuse in the atmosphere. 这些传递过程也发生在人体 内以及大气中污染物反应和扩散的一些复杂过程中。It is important that engineers have an understanding of the physical laws governing these transport processes if they are to understand what is taking place in engineering equipment and to make wise decisions with regard to its economical operation . 如果工程师要知道工程设备中正在发生什么并要做出能达到经济性 操作的决策，对主导这些传递过程的物理定律有一个认识很重要。 The second answer is that engineers need to be able to use their understanding of natural laws to design process equipment in which these processes are occurring . 第二种答案是工程师 需要能够运用自然定律的知识设计包含这些过程的工艺设备。To do so they must be able to predict rates of heat ,mass , or momentum transport . 要做到这点，他们必须能够预测传热，传质，或动量传递速率。For example, consider a simple heat exchanger , i.e., a pipe used to heat a fluid by maintaining its wall at a higher temperature than that of the fluid flowing through it . 例 如， 考虑一个简单的热交换器， 也就是一根管道通过维持壁温高于流经管道的流体温度 来加热流体。The rate at which heat passes from the wall of the pipe to the fluid depends upon a parameter , etc. 热量从管壁传递到流体的速率取决于传热系数，传热系数反过来取决于管的 大小，流体流速，流体性质等。Traditionally heat-transfer coefficients are obtained after expensive and time-consuming laboratory or pilot-plant measurements and are correlated through the use of dimensionless empirical equations. 传统上传热系数是在耗费和耗时的实验室或模 范工厂的测量之后获得并且通过使用一维经验方程关联起来。Empirical equations are equations that fit the data over a certain range; they are not based upon theory and cannot be used accurately outside the range for which the data have heen taken . 经验方程是适合一定数据范围 的方程，它们不是建立在理论基础上而且在应用数据的范围外不能被精确使用。 The less expensive and usually more reliable approach used in transport phenomena is to predict the heat-transfer coefficient from equations based on the laws of nature . 使用在传递现象 中比较不耗费和通常较为可靠的方法是从以自然定律为基础的方程中预测传热系数。The predicted result would be obtained by a research engineer by solving some equations (often on a computer ). 预测的结果将由一个研究工程师通过解一些方程获得（常常在电脑上）A design engineer would then use the equation for the heat-transfer coefficient obtained by the research engineer . 设计工程师再使用由研究工程师获得的关于传热系数的方程。 Keep in mind that the job of designing the heat exchanger would be essentially the same no matter how the heat-transfer coefficients were originally obtained. 要记住无论传热系数是怎么 得来的设计热交换器的工作将基本上是一样的。For this reason ,some courses in transport phenomena emphasize only the determination of the heat-transfer coefficient and leave the actual designprocedure to a course in unit operations . 正因如此，传递现象的一些课程只强调传热系 数的决定而把真正的设计步骤留给单元操作中的一个课程。 is of cource a “practical “ matter It to be able to obtain the parameters , i.e., the heat-transfer coefficients that are used in design , and for that reason a transport phenomena course can be considered an engineering course as well as one in science . 当然，能获得参数也就是设计中使用的传热系数是事实，并正因此，一个传 递现象课程可被视为一个工程课程或一个科学课程。 In fact , there are some cases in which the design engineer might use the methods and equations of transport phenomena directly in the design of equipment . 实际上， 在设备设计中有 一些情况下设计工程师可能直接使用传递现象的方法和方程。An example would be a tubular reactor ,which might be illustrated as a pipe ,e.g., the heat exchanger described earlier, with a homogeneous chemical reaction occurring in the fluid within . 一种情况就是设计可以被称为管 道的管式反应器， 前面所提过的热交换器， 如， 在它里面的液相中发生着一个均相化学反应。 The fluid enters with a certain concentration of reactant and leaves the tube with a decreased concentration of reactant and an increased concentration of product . 流体以一定浓度的反应物 流进并以浓度降低的反应物和浓度增加的产物流出反应管。 If the reaction is exothermic , the reactor wall will usually be maintained at a low temperature in order to remove the heat generated by the chemical reaction . 如果反应是放热的， 为了移除化学反应生成的热量反应器壁通常维持在一个低的温度。Therefore the temperature will decrease with radial position , i.e.,with the distance from the centerline of the pipe . 因此沿径向 方向也就是说随离管道中心线距离的增大， 温度降低。 Then , since the reaction rate increases with temperature , it will be higher at the center ,where the temperature is high , than at the wall , where the temperature is low . 再者，因为反应速率随温度升高而增大，在温度高的中心处的 反应速率高于温度低的管壁处的反应速率。Accordingly ,the products of the reaction will tend to accumulate at the centerline while the reactants accumulate near the wall of the reactor . 结果， 反应产物将倾向于在中心线处积累而反应物在靠近管壁处积累。 Hence , concentration as well as temperature will vary both with radial position and with length . 因此，沿径向和横向浓 度和温度都将改变。To design the reactor we would need to know ,at any given length , the mean concentration of product . 为了设计反应器我们需要知道在任意给定的管长下产物的平均浓 度。Since this mean concentration is obtained from the point values averaged over the cross section , we actually need to obtain the concentration at every point in the reactor , i.e., at every radial position and at every length . 由于这个平均浓度是将整个反应器内每个点的浓度平均 起来得到的，实际上我们需要得到反应器内每个点的浓度，也就是说，在每个径向和横向位 置。But to calculate the concentration at every point we need to know the reaction rate at every point , and to calculate the rate at every point we need to know both the temperature and the concentration at every point ! 但是为了计算每个点的浓度我们需要知道每个点处的反应速 率，而为了计算每个点处的速率我们需要知道温度和浓度！ Furthermore, to calculate the temperature we also need to know the rate and the velocity of the fluid at every point . 而且， 为了 计算温度我们也要知道每个点处的反应速率和速度。We will not go into the equations involved ,but obviously we have a complicated set of partial differential equations that must be solved by sophisticated procedures, usually on a computer. 我们将不得到所包含的方程， 但显然 有一组必须由精细繁琐的步骤解决的复杂偏微分方程 （通常在电脑上） It should be apparent 。 that we could not handle such a problem by the empirical design procedures used in unit operations courses for a heat exchanger . 我们不能通过用于单元操作课程中关于热交换器的 经验设计步骤来解决这样一个问题，应该是明显的。Instead the theory and mathematical procedures of transport phenomena are essential ,unless one wishes to go go the expense and take the time to build pilot plants of increasing size and measure the conersion in each . 然而传递现象 的理论和数学步骤是必不可少的， 除非一个人愿意花金钱和时间去建立规模不断扩大的模范 工厂并测出每一个工厂的产率。 Even then the final scale-up is precarious and uncertain.即便 最后的扩大规模是靠不住和不确定的。 Of course ,not all problems today can be solved by the methods of transport phenomena. 当 然， 并非今天所有的问题都能通过传递现象的方法解决。 However, with the development of the computer ,more and more problems are being solved by these methods . 然而，随着电脑科技的 发展， 越来越多的问题通过这些方法正被解决。 engineering students are to have an education If that is not become obsolete , they must be prepared, through an understanding of the methods of transport phenomena , to make use of the computations that will be made in the future . 如果工程 学学生要得到一个不过时的教育， 他们必须通过理解传递现象的方法准备好去充分利用将在 未来形成的计算机计算。Because of its great potential as well as its current usefulness , a course in transport phenomena may ultimately prove to be the most practical and useful course on a students undergraduate career. 由于其极大的潜能及当前的实用性， 在一个大学生的在校学习 生涯中，传递现象这门课程或许最终证明是最实用和有用的课程。UNIT 14 Separation operations achieve their objective by the creation of two or more coexisting zones which differ in temperaturepressure, compositionandor phase stateEach molecular species in the mixture to be separated reacts in a unique way to differing environments offered by these zonesConsequentlyas the system moves toward equilibriumeach establishes a different concentration in each zoneand this results in a separation between the species 分离过程通过形成在温度、压力、组成和相态上有区别的两个或多个共存区来达到它的目的。每一种待分离的在混合物中的分子种类以一种独特的17方式与这些区域提供的不同的环境相互作用。结果是随着系统向平衡移动每个物质都在每个区域形成不同的浓度这样就可以使这些物质的到17分离。 The separation operation called distillation utilizes vapor and liquid phases at essentially the same temperature and pressure for the coexisting zonesVarious kinds of device such as dumped or ordered packings and plates or trays are used to bring the two phases into intimate contactTrays are stacked one above the other and enclosed in a cylindrical shell to form a columnPackings are also generally contained in a cylindrical shell between hold-down and support plates 利用在共有区中拥有相同温度和压力的汽油和液相的分离操作叫做分离。不同种类的装置例如规整填料和乱堆整料和塔板或塔盘被用来使两相充分接触。塔盘是一个堆着一个排列的并且被装入到一个圆柱形的外壳中形成一个塔。填料通常也被装入一个在压具板和支撑板之间的圆柱形外壳中。 1 Continuous Distillation The feed materialwhich is to be separated into fractionsis introduced at one or more points along me column shellBecause of the difference in gravity between vapor and liquid phases, liquid runs down me columncascading from tray to traywhile vapor floats up the columncontacting liquid at each tray. 1连续蒸馏 将要被分离成馏分的进料通过一个或多个沿着管壳的点被引入到塔中。由于气相和液相之间重力的不同液体在塔中从一个塔盘到另一个塔盘呈阶梯形向下流动。同时气体在塔中向上流动并与液相在每个塔盘接触。 Liquid reaching the bottom of the column is partially vaporized in a heatedreboiler to provide boil-upwhich is sent back up the columnThe remainder of the bottom liquid is withdrawn as bottomsor bottom productVapor reaching the top of the column is cooled and condensed to liquid in the overhead condensePart of this liquid is returned to the column as reflux to provide liquid overflow. The remainder of the overhead stream is withdrawn as distillate, or overhead product. 液体到达塔底被一个加热的再沸器部分汽化来提供蒸汽。蒸汽重新返回塔中。塔底的残余物作为釜液或塔底产品被取出。达到塔底的气相在塔底冷凝器中被冷却并冷凝成液体。部分被冷凝的液体回流到塔中从而提供液体溢流。塔顶馏分的剩余物作为馏分或塔顶产品被取出。 This overall flow pattern in a distillation column provides countercurrent contacting of vapor and liquid stream on all the trays through the columnVapor and liquid phases on a given tray approach thermalpressureand composition equilibriums to an extent dependent www.docin.comupon the efficiency of the contacting tray 在蒸馏塔中的整个流动模式为气相和液相提供了一个在每一个塔盘逆流接触的机会。气相和液相在特定的塔盘上达到热量压力以及组成的平衡这在一定程度上取决于接触的塔板的效率。 The lighter(lower-boiling)components tend to concentrate in the vapor phase, while the heavier(higher-boiling)components tend toward the liquid phaseThe result is vapor phase that becomesricher in light components as it passes up the column and a liquid phase that becomes richer in heavy components as it cascades downward. The overall separation achieved between the distillate and the bottoms depends primarily on therelative volatilities of the components, the number of contacting trays, and the ratio of the liquidphase flow rate to the vapor-phase flow rate. 低沸点的轻组分倾向于气相中富集而同时高沸点的重组分倾向于在液相中富集。这个结果是当它沿着塔向上移动的时候气相变得富含轻组分而当液体沿着塔积极向下移动的时候液相变得富含重组分。要在塔顶产品和釜液之间达到整个分离主要依赖于组分的相对挥发度接触塔盘的数目和液相和气相流动的比例。 If the feed is introduced at one point along the column shell the column is divided into an upper sectionwhich is often called the rectifying section, and a lower sectionwhich is often referred to as the stripping section. These terms become rather indefinite in multiple-feed columns and columns from which a product sidestream is withdrawn somewhere along the column length in addition to the two end-product streams 如果进料从塔壳的某一个点引入这个塔被分成一个向上部分这个部分通常叫做精馏段还有一个向下的部分这个部分叫做提馏段。这些术语在多口进料塔中变得相当不确定。侧线产品沿着塔的某个地方被取中的塔会有两个最终产品馏分。 Equilibrium-Stage Concept Energy and masstransfer processes in an actual distillation column are much too complicated to be readily modeled in any direct wayThis difficulty is circumvented by the equilibriumstage modelin which vapor and liquid streams leaving an equilibrium stage are in complete equilibrium with each other and thermodynamic relations can be used to determine the temperature of and relate the concentrations in the equilibrium streams at a given pressure. A hypothetical column composed of equilibrium stages (instead of actual contact trays)is designed to accomplish the separation specified for the actual columnThe number of hypothetical equilibrium stages required IS then converted to a number of actual trays by means of tray efficiencieswhich describe the extent to which the performance of an actual contact tray duplicates the performance of an equilibrium stage 平衡级的概念 实际的蒸馏塔中的能量以及质量传递过程太复杂以至于不能从一种方式直接做成模型。这个难题可以通过平衡级模型避免。在平衡级模式中气相和液相达到一个完全的平衡并且热力学关系可以被用于测量温度以及相关的浓度在某个特定压力下的平衡流中。由平衡级组成的一个特定的塔。代替了实际过程中的接触塔盘被设计用来完成实际塔的特定的分离。需要的假想得平衡级的数目通过塔盘效率的方法转化成一个实际塔盘的数目。塔盘效率描述了实际的接触塔盘的行为复制平衡级行为的程度。 Use of the equilibrium-stage concept separates the design of a distillation column into three major steps(i)Thermodynamic data and methods needed to predict equilibriumphase compositions are assembled(ii)The number of equilibrium stages required to accomplish a specified separationor the separation that will be accomplished in a given number of equilibrium stagesis calculated(iii)The number of equilibrium stages is converted to an equivalent number of actual contact trays or height of packingand the column diameter is determined www.docin.com. 平衡级概念的使用 将蒸馏塔的设计分为三个主要的步骤热力学数据和方法被收集用来预测相平衡组分的。需要完成一个指定的分离或者在一个给定平衡级数目的情况下平衡级的数目需要被计算。平衡级的数被转化为等价的实际接触塔盘的数目或填料的高度。而且塔径被决定。 All separation operations require energy input in the form of heat or work. In the conventional distillation operationenergy required to separate the species is added in the form of hear to the reboiler at the bottom of the columnwhere the temperature is highestAlso, heat is removed from a condenser at the top of the column, where the temperature is lowest. This frequently results in a large energy-input requirement and low overall thermodynamic efficiency. With recent dramatic increases in energy costs, complex distillation operations that offer higher thermodynamic efficiency and lower energy-input requirements are being explored. 所有的分离操作需要以热量或者做功的形式的能量的输入。在传统的蒸馏操作中用来分离物质的能量被以热量与形式注入到塔底的再沸器中塔底的温度用最高的。热量从塔顶的冷凝器中被移走了在那里温度是最低的这就导致了大量的能量的输入以及低的热力学效率。随着最高近能耗上的显著增加能够有更高的热力学效率以及较低能量输入的复杂的蒸馏技术正在被开发。 Related Separation Operations The simple and complex distillation operations just described all have two things in common :(i) both rectifying and stripping sections are provided so that a separation can be achieved between two components that are adjacent in volatility; and (ii) the separating agent (MSA) such as in liquid-liquid extraction. Sometimes, alternative single- or multiple-stage vapor-liquid separation operations, of the types shown in Fig. 3-2, may be more suitable than distillation for the specified task. 相关的分离操作工作 我们刚刚描述的简单的和复杂的蒸馏操作都有两个共同点。被提供的精馏段和提馏段都可以使挥发度相近的两个组分被分离。分离仅仅被能量的输入输出影响不被加入任何的质量分离剂所影响例如液液萃取。一些时候可选择的单级或者多级气液分离操作像在图3-2展示的类型也许比指定任务的蒸馏更适合。 2.Batch Distillation Batch distillation, which is the process of separating a specific quantity (the charge) of a liquid mixture into products, is used extensively in small production units that may have to serve for many mixtures. When there are N components in the feed, one batch column will suffice where N-1 simple continuous-distillation columns would be required间歇蒸馏.就是将一个特定量的液油混和物分离成产品,把它们广泛的用于实验室,还用于有许多类型混和物的产量少的生产单元.当进料有N种组分时,一个间歇蒸馏塔就可以满足,而却有N-1个连续蒸馏塔被需要. Many larger installations also feature a batch stillMaterial to be separated may be high in solids contentor it might contain tars or resins that would plug or foul a continuous unitUse of a batch unit can keep solids separated and permit convenient removal at the termination of the process许多大型设备也以间歇蒸馏塔为特点.待分离的物质可能固体含量高,或者有可能含有焦油或树脂,这些东西会阻塞或弄脏连续蒸馏.使用间歇蒸馏可以使固体.
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