化學反應工程英文課件

1、Chapter 1 Overview of Chemical Reaction Engineering,1.1 Chemical process illustration Every industrial chemical process is designed to produce economically a desired product from a variety of starting materials through a succession of treatment steps. Figure 1.1 shows a typical situation,化 學 反 應 工 程

2、,化 學 反 應 工 程,Fig.1.1 Typical chemical process,化 學 反 應 工 程,Physical treatment of feed,The raw materials undergo a number of physical treatment steps to put them in the form in which they can be reacted chemically. Then they pass through the reactor. The products of the reaction must then undergo furt

3、her physical treatmentseparations, purification, etc.for the final desired product to be obtained,Design of equipment for the physical treatment steps is studied in the unit operations,化 學 反 應 工 程,In this book we are concerned with the chemical treatment step of a process. Economically this may be a

4、n inconsequential(不合理的) unit, perhaps a simple mixing tank. Frequently(常常), however, the chemical treatment step is the heart of the process, the thing that makes or breaks the process economically,化 學 反 應 工 程,Design of the reactor is no routine(常規(guī))matter, and many alternatives(二中擇一, 可供選擇的辦法) can be

5、 proposed for a process. In searching for the optimum it is not just the cost or the reactor that must be minimized. One design may have low reactor cost, but the materials leaving the unit may be such that their treatment requires a much higher cost than alternative designs. Hence, the economics of

6、 the overall process must be considered,化 學 反 應 工 程,Reactor design uses information, knowledge, and experience from a variety of areas: Thermodynamics（熱力學） Chemical kinetics（化學動力學） Fluid mechanics（流體力學） Heat transfer, mass transfer（傳熱,傳質(zhì)） Economics（經(jīng)濟學,化 學 反 應 工 程,1.2 Performance equation,Figure1.2

7、Information needed to predict what a reactor can do,化 學 反 應 工 程,Contacting Pattern or how materials flow through and contact each other in the reactor, how early or late they mix, their clumpiness or state of aggregation. By their very nature some materials are very clumpyfor instance, solids and no

8、ncoalescing liquid droplets,Kinetics or how fast things happen. If very fast, then equilibrium tells what will leave the reactor. If not so fast, then the rate of chemical reaction, and maybe heat and mass transfer too, will determine what will happen,化 學 反 應 工 程,Output = input, kinetics, contacting

9、 (1,This is called the performance equation (or design equation). Why is this important? Because with this expression we can compare different designs and conditions, find which is best, and then scale up to larger units,化 學 反 應 工 程,1.3 Classification of Reactions,There are many ways of classifying

10、chemical reactionsIn the view of chemical reaction engineering probably the most useful scheme is the breakdown (= classify ) according to the number and types of phases involved,1.3.1 Homogeneous and Heterogeneous reaction systems,Homogeneous reaction, it takes place in one phase alone. (homo- :相同的

11、, hetero- :不同的, 相異的) Heterogeneous reaction, it requires the presence of at least two phases to proceed at the rate that it does,化 學 反 應 工 程,1.3.2 Catalytic and Non-catalytic reaction systems,Catalytic reaction, whose rate is altered by materials that are neither reactants nor products. These foreig

12、n materials, called catalysts, need not be present in large amounts,Catalysts act somehow as go-betweens(媒介), either hindering(阻礙) or accelerating (加快) the reaction process. But it does not change the equilibrium of chemical reaction,化 學 反 應 工 程,Non-catalytic reaction: absence of catalyst,Tablel.1 s

13、hows the classification of chemical reactions according to our scheme with a few examples of typical reactions for each type,化 學 反 應 工 程,1.3.3 Other ways for reaction classification,reactor type (BR, PFR, MFR, fixed-bed, moving-bed, fluidized-bed ) heat effects (endothermic, exothermic) reaction mec

14、hanism (molecule reaction, free-radical reaction, ion reaction) molecule number of reacting together (single-, two-, or more-molecule reaction) order of reaction (first, second, ) reaction expression (simple or single reaction, complex or multiple reaction) method of heat transfer (adiabatic operati

15、on, nonadiabatic operation,化 學 反 應 工 程,1.4 Variables Affecting the Rate of Reaction,Homogeneous systems temperature, pressure, and composition Heterogeneous systems T, P, C, and heat and mass transfer,The rate of mass transfer can become important,The rate of heat transfer may also become a factor,T

16、hese heat and mass transfer effects become increasingly important the faster the rate of reaction，and in very fast reactions，such as burning flames, they become rate controlling. Thus, heat and mass transfer may play important roles in determining the rates of heterogeneous reactions,化 學 反 應 工 程,1.5

17、 Definition of Reaction Rate,How to define the rate of a reaction? First we must select one reaction component for consideration and define the rate in terms of this component i. If the rate of change in number of moles of this component due to reaction is dNi/dt, then the rate of reaction in its va

18、rious forms is defined as follows,Based on unit volume of reacting fluid,2,化 學 反 應 工 程,Based on unit mass of solid in fluid-solid systems,Based on unit interfacial surface in two-fluid systems or based on unit surface of solid in gas-solid systems,3,4,化 學 反 應 工 程,Based on unit volume of solid in gas

19、-solid systems,Based on unit volume of reactor, if different from the rate based on unit volume of fluid,5,6,化 學 反 應 工 程,In homogeneous systems the volume of fluid in the reactor is often identical to the volume of reactor. In such a case V and Vr are identical and Eqs.2 and 6 are used interchangeab

20、ly,In heterogeneous systems all the above definitions of reaction rate are encountered, the definition used in any particular situation often being a matter of convenience,化 學 反 應 工 程,Relations between different rate definitions,It is very important to express a reaction with the proper form of rate

21、. For example, the reaction of C7 olefin catalytic cracking can be completed in 2030 milliseconds，it maybe need several seconds in fluidized-bed reactor,or,化 學 反 應 工 程,Speed of Chemical Reactions,Figure1.3 indicates the relative rates at which reactions occur. To give you an appreciation(感謝, 感激, 正確評

22、價) of the relative rates or relative values between what goes on in sewage(污水) treatment plants and in rocket engines, this is equivalent to 1 sec to 3 years. With such a large ratio, of course the design of reactors will be quite different in these cases,化 學 反 應 工 程,化 學 反 應 工 程,EXAMPLE 1.1 THE ROCK

23、ET ENGINE,A rocket engine, Fig. E1.1, burns a stoichiometric mixture of fuel (liquid hydrogen) in oxidant (liquid oxygen). The combustion chamber is cylindrical, 75 cm long and 60 cm in diameter, and the combustion process produces 108 kg/s of exhaust gases (H2O). If combustion is complete, find the

24、 rate of reaction of hydrogen and of oxygen,化 學 反 應 工 程,SOLUTION,Let us evaluate terms. The reactor volume and the volume in which reaction takes place are identical. Thus,We want to evaluate,化 學 反 應 工 程,Next, let us look at the reaction occurring,So from Eq. (i,molar stoichiometry 1 0.5 1 molecular weight: 2gm 16gm 18gm,Therefore,H2 used = 6 kmol/s; O2 used = 3 kmol/s,and the rate of reaction is,化