《专业英语Lesson .docx》由会员分享,可在线阅读,更多相关《专业英语Lesson .docx(15页珍藏版)》请在三一办公上搜索。
1、专业英语 Lesson Lesson 10 Thermodynamics Thermodynamics is the physics of energy, heat, work, entropy and the spontaneity of processes. Thermodynamics is closely related to statistical mechanics from which many thermodynamic relationships can be derived. While dealing with processes in which systems exc
2、hange matter or energy, classical thermodynamics is not concerned with the rate at which such processes take place, termed kinetics. For this reason, the use of the term “thermodynamics” usually refers to equilibrium thermodynamics. In this connection, a central concept in thermodynamics is that of
3、quasistatic processes, which are idealized, “infinitely slow” processes. Time-dependent thermodynamic processes are studied by non-equilibrium thermodynamics. 1 Applications of Thermodynamics There are two major applications of thermodynamics, both of which are important to chemical engineers: (1) T
4、he calculation of heat and work effects associated with process as well as the calculation of the maximum work obtainable from a process or the minimum work required to drive a process. (2) The establishment of relationships among the variables describing systems at equilibrum. The first application
5、 is suggested by the name thermodynamics, which implies heat in motion. Most of these calculations can be made by the direct implementation of the first and second laws.Examples are calculating the work of compressing a gas, performing an energy balance on an entire process or a process unit, determ
6、ining the minimum work of separating a mixture of ethanol and water, or evaluating the efficiency of an ammonia synthesis plant. The application of thermodynamics to a particular system results in the definition of useful properties and the establishment of a network of relationships among the prope
7、rties and other variables such as pressure, temperature, volume, and mol fraction. Actually, application 1 would not be possible unless a means existed for evaluating the necessary thermodynamics property changes required in implementing the first and second laws. The property changes are calculated
8、 from experimentally determined data via the established network of relationships. Additionally, the network of relationships among the variables of a system allows the calculation of values of variables which are either unknown or difficult to determine experimentally from variables which are eithe
9、r available or easier to measure. For example, the heat of vaporizing a liquid can be calculated from measurements of the vapor pressure at several temperatures and the densities of the liquid and vapor phase at several temperatures, and the maximum conversion obtainable in a chemical reaction at an
10、y temperature can be calculated from calorimetric measurements performed on the individual substances participating in the reaction. 2 The nature of Thermodynamics A. Thermodynamics is a science that includes the study of energy transformations and of the relationships among the physical properties
11、of substances that are affected by these transformations. 1. Definition is broad and vague. 2. Mechanical engineers typically focus on power and refrigeration devices such as steam power plants, fuel cells, nuclear reactors, etc. 3. Chemical engineers typically focus on phase equilibria and chemical
12、 reactions and the associated properties. 4. Element which really sets thermodynamics apart from other sciences is the study of energy transformations through heat and work. B. Thermodynamic properties can be studied either by studying macroscopic or microscopic behavior of matter. 1. Classical ther
13、modynamics treats matter as a continuum and studies the macroscopic behavior of matter 2. Statistical thermodynamics studies the statistical behavior of large groups of individual particles. It postulates that observed physical property behavior (e.g., T, p, H, ) is equal to the appropriate statisti
14、cal average of a large number of particles. C. Thermodynamics is based upon experimental observation. 1. Conclusions of observations have been cast as postulates or laws. 2. Our study of thermodynamics will consider five laws or postulates; two dealing with energy transformation and three dealing wi
15、th properties. Energy Transformation Laws: a. First Law of Thermodynamics-Energy is conserved. (You cant win!) b. Second Law of Thermodynamics-Takes many forms. In essence it says that energy has different “quality” and processes only spontaneously proceed in one direction. It isnt possible to conve
16、rt all of the energy of a system into work. Property Relationship Laws: c. Zeroth Law of Thermodynamics-When each of two systems is in thermal equilibrium with a third system, they are also in thermal equilibrium with each other. d. Third Law of Thermodynamics-The “entropy” of a perfect crystal is z
17、ero at absolute zero temperature. e. State Postulate-The state of a simple, single phase thermodynamic system is completely specified by two independently variable, intensive properties. D. Energy Conversion and Efficiency 1. A primary concern in thermodynamics is energy conversion and a measure of
18、energy conversion success is called the efficiency. For energy consuming or producing devices it is called the thermal efficiency: 2. Example efficiencies: a. Automobile b. Gas Turbine d. Solar Cell e. Fuel Cells f. Electric Motor 12-25% 12-16% 38-41% 12% 40-60% 90% c. Steam power plant 3 Definition
19、s and Thermodynamic Vocabulary A. Thermodynamic System 1. Definition-A three dimensional region of space bounded by arbitrary surfaces (which may be real or imaginary and may change size or shape) which delineate the portion of the universe we are interested in. a. Closed System is a system that is
20、closed with respect to the flow of matter, e.g., fixed, closed volume. A closed system is defined by a fixed quantity of mass. b. Open System is a system that is open with respect to the flow of matter such as a compressor. The system is defined by an imaginary volume surrounding the region of inter
21、est. The surface of this volume is called the control or sigma (s ) surface. Mass, heat, work and momentum can flow across the control surface. c. Isolated System is a system that is not influenced in any way by the part of space which is external to the system boundaries. No heat, work, mass or mom
22、entum can cross the boundary of an isolated system. (N, V, U) are fixed and constant in a closed system. d. Simple System is a system that does not contain any internal adiabatic, rigid and impermeable boundaries and is not acted upon by external forces. e. Composite System is a system that is compo
23、sed of two or more simple systems. B. Property 1. Definition-A characteristic of a system. a. Primitive Property is a property that can in principle be specified by describing an operation or test to which the system is subjected. Examples include mechanical measurements (e.g., pressure, volume, and
24、 thermometric temperature T) and heat capacity. b. Derived Property is a property that is mathematically defined in terms of primitive properties. c. Intensive Property is a property that is independent of the extent of or mass of the system. Examples are T, P, density, (x), etc. d. Extensive Proper
25、ty is a property whose value for the system is dependent upon the mass or extent of the system. Examples are the enthalpy, internal energy, volume, etc. e. Specific Property is an extensive property per unit mass. Specific properties are intensive. f. State Property is a property that only depends o
26、n the thermodynamic state of the system, not the path taken to get to that state. C. State of a System 1. Thermodynamic State-The condition of the system as characterized by the values of its properties. 2. Stable Equilibrium State is a state in which the system is not capable of finite spontaneous
27、change to another state without a finite change in the state of the surroundings. a. Many types of equilibrium must be fulfilled - thermal, mechanical, phase (material) and chemical. 3. State Postulate: The equilibrium state of a simple closed system can be completely characterized by two independen
28、tly variable properties and the masses of the species contained within the system. D. Thermodynamic Process 1. Definition-A transformation from one equilibrium state to another. a. Quasi-static Process is a process where every intermediate state is a stable equilibrium state. b. Reversible Process i
29、s one in which a second process could be performed so that the system and surroundings can be restored to their initial states with no change in the system or surroundings. 1) Reversible processes are quasi-static but quasi-static processes are not necessarily reversible. 2) A quasi-static process i
30、n a simple system is also reversible. 3) Some factors which render processes irreversible are friction, unrestrained expansion of gasses, heat transfer through a finite temperature difference, mixing, chemical reaction, etc. E. Thermodynamic Path 1. The specification of a series of states through wh
31、ich a system passes in a process. a. Isothermal - ? T = 0 b. Isobaric - ? P = 0 c. Adiabatic - Q = 0 d. Isochoric - ? V = 0 e. Isentropic - ? S = 0 (Adiabatic and Reversible) f. Isenthalpic - ? H = 0 g. Cyclic - same initial and final states. h. Polytropic - pVk = constant 4 The Laws of Thermodynami
32、cs First law of Thermodynamics The first law of thermodynamics is simply a statement of the conservation of energy. The sum of all the energy leaving a process must equal the sum of all the energy entering, in the steady state. The laws of conservation of mass and energy are followed implicitly by e
33、ngineers designing and operating processes of all kinds. Unfortunately, taken by itself, the first law has led to much confusion when attempting to evaluate process efficiency. People talk of energy conservation being an important effort, but in fact, no effort is required to conserve energyit is na
34、turally conserved. The conclusions which can be drawn from the first law are limited because it does not distinguish among the various energy forms. Shaft work introduced by a reflux pump will leave a column as heat to the condenser just as readily as will heat introduced at the reboiler. Some engin
35、eers have fallen into the trap of lumping all forms of energy together in attempting to determine process efficiency. This is obvious not justifiedthe various energy forms have different costs. Second law of Thermodynamics / Entropy plays a critical role in thermodynamic analysis, because it is the
36、missing factor that we were seeking to allow us to predict the direction of change in atomic or molecular systems. The essential result constitutes the second law of thermodynamics, which can be stated in several ways, not all of them obviously equivalent, but in fact all of them providing the same
37、message. Here are some of them: 1. Heat does not spontaneously flow from a cold body to a hot body. 2. Spontaneous processes are not thermodynamically reversible. 3. The complete conversion of heat into work is impossible without leaving some effect elsewhere. 4. It is impossible to convert heat int
38、o work by means of a constant temperature cycle. 5. All natural processes are accompanied by a net gain in entropy of the system and its surroundings. This last statement is most useful to us. Let us write dSnet = dSsystem + dSsurroundings Then the second law says dSnet 0 (spontaneous processes) dSn
39、et = 0 (reversible processes) 第十课 热力学 热力学是能源,热,工作,熵和物理过程的自发性。热力学是密切相关的许多热力学关系,从可以得到的统计力学。 虽然在这系统交换物质或能量,古典热力学不与速率等过程发生,有关程序处理称为动力学。由于这个原因,对“热力学“的使用通常是指平衡态热力学。在这方面,在热力学的核心概念是,准静态过程,这是理想化的,“无限慢“的进程。随时间变化的热力学过程,研究了非平衡热力学。 1热力学的应用 有两个主要应用热力学,这两者都是重要的化学工程师: 热量和工作的影响,计算与过程,以及相关的最大工作从一个进程或最低工作需要驾驶过程索取计算。 中
40、描述的平衡系统变量关系的建立。 第一个应用是热力学所建议的名称,这意味着在运动中的热量。这些计算大部分可以由第一和第二定律。例如直接执行制成的计算压缩气体的工作,对整个表演过程或过程的单位能量平衡,确定分离混合物的最低工作乙醇和水,或评价氨合成工厂的效率。 在热力学中的应用有用的属性定义特定系统的结果,以及一间的性质,如压力,温度,体积,摩尔分数和其他变量的关系网络的建立。其实,应用一不可能,除非必要手段热力学性质评估在执行第一和第二定律需要修改的存在。该属性的变化是通过计算的关系建立的网络实验确定的数据。此外,在一个系统中的变量关系网络允许的变量的都是未知的或者难以确定的变量是从可用或易于衡
41、量实验值的计算。例如,液体的蒸发热可以计算出从蒸气压力测量在不同温度下和在不同温度下的液体和气相的密度,最大转换在任何温度下发生化学反应,可以计算出索取热测量反应表现在个人参与物质。 2 热力学性质 热力学是一门科学,其中包括能源之间的转换和由这些变革影响物质的物理性质的关系研究. 1定义是广泛和模糊。 2机械工程师通常集中在电力和制冷设备,如蒸汽发电厂,燃料电池,核反应堆等 3. 化学工程师通常侧重于相平衡和化学反应和相关的属性。 4. 元素确实不同于其他科学的热力学除了是能量的转换,通过热和工作研究 (2) 热力学性质或者可以研究通过研究宏观或微观物质的行为. 1. 经典热力学视为一个连续
42、和研究物质的宏观行为问题 2. 统计热力学研究了单颗粒大集团的统计行为。它假设,观察到的物理性质的行为等于适当的大量粒子的统计平均值 (3) 根据热力学实验观察。 1. 观察结论已经转换为假设或法律。 2. 我们的热力学研究将考虑五个法律或假设,两个处理能源转化和处理三个属性。 能量变化规律: A.热力学第一定律-能量是守恒的。 B.第二热力学定律-有多种形式。它在本质上说,能源有不同的“质量”和自发过程只在一个方向进行。这是不可能转化为工作的一个系统的所有能量 性质关系的定律: C.热力学第零规律-当两个系统的每一个与第三个系统热平衡,他们还与对方热平衡。 D.热力学第三定律- 在“熵“是一
43、个完美的晶体在绝对零度的温度为零。 E 一个简单的单相的热力学系统的状态完全是两个独立的变量所指定的,密集的特性。 (4)能源转换与效率 1热力学主要关心的是能量转换和能量转换的成功措施是所谓的效率。对于能源消耗或生产设备,它被称为热效率 效率=耗能量/供能量 2。例如效率: A.汽车12-25 B.燃气轮机12-16 C.火力发电厂38-41 D.太阳能电池12 E.燃料电池的40-60 F.电机90 3定义和热力学词汇 A.热力系统 1.界定 - 以三个区域的三维空间任意曲面界的界定是对我们感兴趣的宇宙的一部分 a.封闭的系统是一个是相对于物质,例如,固定,封闭的体积流量封闭系统。一个封闭
44、的系统是指由一个固定的数量质量。 b.开放系统是一个开放的系统,相对于物质,如压缩机流量。该系统被定义为一个假想的周边利益区域的体积。本卷的表面称为控制或六西格玛表面。质量,热量,工作和势头能够控制表面流过。 c.隔离系统是一个不以任何部分的空间是外部的系统边界的方式影响系统。无热,工作质量或势头能够跨越一个孤立的系统边界。 是固定的,在一个封闭的系统常数。 d.简单的系统是一个不包含任何内部绝热,刚性和不渗透的边界,而不是由外部势力采取行动时。 e.复合系统是一种由两个或多个简单的系统组成的系统。 B.性质 定义 -一个系统的特点。 a.原始属性是一个可以在原则上通过描述一个操作或测试,该系
45、统是受指定的属性。例子包括机械测量和热容量。 b.所得财产是一个数学上的原始属性来定义的属性。 c.集约物业是一个属性,它是对系统范围或质量无关。例子有温度,压力,密度 d.大量财产属性,其值是该系统是建立在大规模或系统的依赖程度。例子是焓,内能,体积等 e.具体属性是每单位质量的广泛的财产。具体属性密集。 f.国有资产是一个属性,只有在该系统,不采取该国得到的热力学状态的路径依赖。 简单的系统。 C.系统的状态 1.热力学状态-为按照其物业价值为特征的制度条件。 2.稳定的平衡态,是一个国家在该系统也不是没有在有限的环境状态改变有限自发变化到另一个状态的能力。许多类型的平衡,必须满足- 热,
46、机械,相和化学。 3.状态假设:一个简单的封闭系统的平衡状态可以完全特点是两个独立的变量属性和系统内包含的物种的种类。 D热力过程 定义 -从一个平衡状态转变为另一种。 a.准静态过程是一个过程,每一个中间状态,是一种稳定的平衡状态。 b.可逆过程是在其中一个可以执行第二个进程,使系统及周边地区可以恢复由于没有系统或环境的变化对它们的初始状态。 1)可逆过程是准静态的,而是准静态过程并不一定是可逆的。 2)在一个简单的系统准静态过程也是可逆的。 3)有些因素是摩擦呈现不可逆转的进程,无节制膨胀气体,通过一个有限的温差传热,混合,化学反应等 E热力学路径一个国家的一系列规范,通过这些系统通过在一
47、个进程。 a.等温 -? T = 0时 b.恒压- ? p = 0型 c.绝热- Q=0 d.定容- ? =0 e.熵- ? S =0 f.焓- ?H =0 g.循环-相同的初始和最终状态。 h. -PVT=常数 4.热力学规律 热力学第一定律 在热力学第一定律是一个简单的节约能源的声明。所有的离开过程中的能量之和必须等于所有的能量的总和,在进入稳定状态。对质量和能量守恒定律隐含遵循设计和运行各种程序工程师。不幸的是,本身采取的第一部法律,导致很多混乱时,试图评估过程的效率。人们都说,作为一个重要的能源节约工作,但事实上,没有努力是必需的,以节省能源,它是自然保护。 哪些可以从第一定律得出的结
48、论是有限的,因为它没有区分各种能源形式。轴泵的工作介绍会回流热刚刚离开那样容易将热量在冷凝器再沸器推出了一列。一些工程师已经陷入了各种能源结块陷阱一起试图确定过程的效率。这是显而易见的不合理,各种能源形式有不同的成本。 热力学第二定律 热力学熵中起着关键作用的分析,因为它是缺少的因素,我们正在寻求让我们来预测在原子或分子系统改变方向。结果构成的基本热力学,可从几个方面说明,不是所有的人都明显相当于第二定律,但实际上他们都提供相同的消息。 下面是其中一些: 1.热量不会自发地从寒冷的身体流动到热的身体。 2.热力学自发过程不可逆的。 3.热能转化为工作的转换是不可能完全不留一些影响其他地方。 4.这是不可能转化为工作的恒温循环热。 5.所有的自然过程都伴随着一个在系统和它的周围熵净收益。 这最后的陈述是对我们最有用的。让我们写dSnet= dSsystem+ dSsurroundings 然后,第二定律说 dSnet0 dSnet=0