
CHAPTER ONE INTRODUCTION 1.1 THE SCOPE OF THERMODYNAMICS The word thermodynamics means heat power, or power developed from heat, rellecting its origin in the analysis of steam engines. As a fully developed modem science, thermodynamics deals with transformations of energy of all kinds from one form to another. The general restrictions within which all such transformations are observed to occur are known as the first and second laws of thermodynamics. These laws cannot be proved in the mathematical sense. Rather, their validity rests upon experience. Given mathematical expression, these laws lead to a network of equations from which a wide range of practical results and conclusions can be deduced. The universal applicability of this science is shown by the fact that it is employed alike by physicists, chemists, and engineers. The basic principles are always the same, but the applications differ. The chemical engineer must be able to cope with a wide variety of problems. Among the most important are the determination of heat and work requirements for physical and chemical processes, and the determination of equilibrium conditions for chemical reactions and for the transfer of chemical species between phases. TheQl'odynamic considerations by themselves are not sufficient to allow calculation of the rates of chemical or physical processes. Rates depend on both driving force and resistance. Although driving forces are thermodynamic vari­ ables, resistances are not. Neither can thermodynamics, a macroscopic-property formulation, reveal the microscopic (molecular) mechanisms of physical or chemical processes. On the other hand, knowledge of the microscopic behavior 1 ] INTRODUCTION 10 CHEMICAL ENGINEERING THERMODYNAMICS INTRODUCTION ~ of matter can be useful in the calculation of thermodynamic properties. Such as there are atoms in 0.012 kg of carbon-I 2. This is equivalent to the "gram mole" property values are essential to the practical application of thermodynamics; commonly used by chemists. numerical results of thermodynamic analysis are accurate only to the extent that Decimal mUltiples and fractions of SI units are designated by prefixes. Those the required data are accurate. The chemical engineer must deal with many in common use are listed in Table 1.1. Thus we have, for example, that I cm = chemical species and their mixtures, and experimental data are often unavailable. 10-2 m and I kg = 103 g. Thus one must make effective use of correlations developed from a limited data Other systems of units, such as the English engineering system, use units that base, but generalized to provide estimates in the absence of data. are related to SI units by fixed conversion factors. Thus, the foot (ft) is defined The application of thermodynamics to any real problem starts with the as 0.3048 m, the pound mass (Ibm) as 0.45359237 kg, and the pound mole (lb mol) identification of a particular body of matter as the focus of attention. This quantity as 453.59237 mol. of matter is called the system, and its thermodynamic state is defined by a few measurable macrosCopic properties. These depend on the fundamental dimensions of science, of which length, time, mass, temperature, and amount of 1.3 FORCE substance are of interest here. The SI unit of force is the newton, symbol N, derived from Newton's second law, which expresses force F as the product of mass m and acceleration a: 1.2 DIMENSIONS AND UNITS F=ma The fundamental dimensions are primitives, recognized through our sensory The newton is defined as the force which when applied to a mass of I kg produce, perceptions and not definable in terms of anything simpler. Their use, however, an acceleration of I m s -2; thus the newton is a derived unit representin~ 2 requires the definition of arbitrary scales of measure, divided into specific units I kgms- . of size. Primary units have been set by international agreement, and are codified In the English engineering system of units, force is treated as an additional as the International System of Units (abbreviated SI, for Systeme International). independent dimension along with length, time, and mass. The pound force (Ib,: The second, symbol s, is the SI unit of time, defined as the duration of is defined as that force which accelerates I pound mass 32.1740 feet per second 9,192,631,770 cycles of radiation associated with a specified transition of the per second. Newton's law must here include a dimensional proportionalit) cesium atom. The meter, symbol m, is the fundamental unit of length, defined constant if it is to be reconciled with this definition. Thus, we write as the distance light travels in a vacuum during 1/299,792,458 of a second. The I kilogram, symbol kg, is the mass of a platinum/iridium cylinder kept at the F=-ma International Bureau of Weights and Measures at Sevres, France. The unit of go temperature is the kelvin, symbol K, equal to 1/273.16 of the thermodynamic whencet temperature of the triple point of water. A more detailed discussion of tem­ perature, the characteristic dimension of thermodynamics, is given in Sec. 1.4. 1(lb,} =.!. x 1(lbm} x 32.1740(ft)(s}-2 The measure of the amount of substance is the mole, symbol mol, defined as the go amount of substance represented by as many elementary entities (e.g., molecules) and Table 1.1 Prefixes for SI units go = 32.1740(lbm)(ft)(lb,}-'(s}-2 The pound force is equivalent to 4.4482216 N. Fraction or Since force and mass are different concepts, a pound force and a pound ma" multiple Prelix Symbol are different quantities, and their units cannot be cancelled against one another 10-9 nano n When an equation contains both units, (Ib,) and (Ibm), the dimensional constanl 6 10- micro P. go must-also appear in the equation to make it dimensionally correct. 3 10- milli m Weight properly refers to the force of gravity on a body, and is therefon 10-2 centi c correctly expressed in newtons or in pounds force. Unfortunately, standards 01 10' kilo k 10' mega M 10' giga G t Where English units are employed, parentheses enclose the abbreviations of all units. 4 INTRODUCTION TO CHEMICAL ENGINEERING THERMODYNAMICS INTRODUCTION 5 mass are often called "weights," and the use of a balance to compare masses is Thus a uniform tube, partially filled with mercury, alcohol, or some other called "weighing." Thus, one must discern from the context whether force or can indicate degree of "hotness" simply by the length of the fluid column. mass is meant when the word "weight" is used in a casual or informal way. Howev,er, numerical values are assigned to the various degrees of hotness by art,itr,ary definition. Example 1.1 An astrQnaut weighs 730 N in Houston, Texas, where the local acceler­ For the Celsius scale, the ice point (freezing point of water saturated with ation of gravity is 9 = 9.792 m S-2. What is the mass of the astronaut, and what does 'r at standard atmospheric pressure) is zero, and the steam point (boiling point he weigh on the moon, where 9 = 1.67 m s-21 .:~pure water at standard atmospheric pressure) is 100. We may give a thermometer SOLUTION Letting a = g, we write Newton's law as a numerical scale by immersing it in an ice bath and making a mark for zero at the fluid level, and then immersing it in boiling water and making a mark for F=mg 100 at this greater fluid level. The distance between the two marks is divided into whence 100 equal spaces called degrees. Other spaces of equal size may be marked off F 730N I below zero and above 100 to extend the range of the thermometer. m = - = = 74.55 N m- S' 9 9.792ms' All thermometers, regardless of fluid, read the same at zero and 100 if they 2 Since the newton N has the units kgms- , this result simplifies to are calibrated by the method described, but at other points the readings do not usually correspond, because fluids vary in their expansion characteristics. An m = 74.55 kg arbitrary choice could be made, and for many purposes this would be entirely This mass of the astronaut is independent of loc~tion, but his weight depends on the satisfactory. However, as will be shown, the temperature scale of the SI system, local acceleration of gravity. Thus on the moon his weight is with its kelvin unit, symbol K, is based on the ideal gas as thermometric fluid. Since the definition of this scale depends on the properties of gases, detailed Fmoon = mOmoon = 74.55 kg x 1.67 m S-2 discussion of it is delayed until Chap. 3. We note, however, that this is an absolute or scale, and depends on the concept of a lower limit of temperature. Fmoon = 124.5 kg m s-' = 124.5 N Kelvin temperatures are given the symbol T; Celsius temperatures, given the symbol t, are defined in relation to Kelvin temperatures by To work this problem in the English epgineering system of units, we convert the astronaut's weight to (lb,) and the values of 9 to (ft)(s)-'. Since 1 N is equivalent to tOC = T K - 273.15 O.2248090b,) and 1 m to 3.28084(ft), we have: The unit of Celsius temperature is the degree Celsius, nc, equal to the kelvin. Weight of astronaut in Houston = 164.1 (lb,) However, temperatures on the Celsius scale are 273.15 degrees lower than on the gHn~'nn = 32.13 and gmoon = 5.48(ft)(s)-' Kelvin scale.
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