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1、Chapter 1 Overview of Chemical Reaction Engineering 1.1 Chemical process illustrationEvery 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.化化 学学 反反 应应 工
2、工 程程化化 学学 反反 应应 工工 程程Fig.1.1 Typical chemical process化化 学学 反反 应应 工工 程程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 un
3、dergo further physical treatmentseparations,purification,etc.for the final desired product to be obtainedDesign of equipment for the physical treatment steps is studied in the unit operations.化化 学学 反反 应应 工工 程程In this book we are concerned with the chemical treatment step of a process.Economically th
4、is may be an 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.化化 学学 反反 应应 工工 程程Design of the reactor is no routine(常规)matter,and many alternatives(二中择一,可供选择的办法
5、)can be 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
6、of the overall process must be considered.化化 学学 反反 应应 工工 程程 Reactor design uses information,knowledge,and experience from a variety of areas:Thermodynamics（热力学）（热力学）Chemical kinetics（化学动力学）（化学动力学）Fluid mechanics（流体力学）（流体力学）Heat transfer,mass transfer（传热（传热,传质）传质）Economics（经济学）（经济学）化化 学学 反反 应应 工工 程程1
7、.2 Performance equation:Figure1.2 Information needed to predict what a reactor can do.化化 学学 反反 应应 工工 程程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 ve
8、ry clumpyfor instance,solids and noncoalescing 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.化化 学学 反反 应应 工工 程程Ou
9、tput=input,kinetics,contacting (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.化化 学学 反反 应应 工工 程程1.3 Classification of Reactions There
10、 are many ways of classifying 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 systemsHomogeneous reaction,it takes place in one
11、phase alone.(homo-:相同的相同的,hetero-:不同的不同的,相异的相异的)Heterogeneous reaction,it requires the presence of at least two phases to proceed at the rate that it does.化化 学学 反反 应应 工工 程程1.3.2 Catalytic and Non-catalytic reaction systemsCatalytic reaction,whose rate is altered by materials that are neither reactan
12、ts nor products.These foreign materials,called catalysts,need not be present in large amounts.Catalysts act somehow as go-betweens(媒介),either hindering(阻碍)or accelerating(加快)the reaction process.But it does not change the equilibrium of chemical reaction.化化 学学 反反 应应 工工 程程Non-catalytic reaction:absen
13、ce of catalystTablel.1 shows the classification of chemical reactions according to our scheme with a few examples of typical reactions for each type.化化 学学 反反 应应 工工 程程1.3.3 Other ways for reaction classification reactor type(BR,PFR,MFR,fixed-bed,moving-bed,fluidized-bed)heat effects(endothermic,exoth
14、ermic)reaction mechanism(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 opera
15、tion,nonadiabatic operation)化化 学学 反反 应应 工工 程程1.4 Variables Affecting the Rate of ReactionHomogeneous systems temperature,pressure,and compositionHeterogeneous 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.Th
16、ese 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 化化 学学 反反 应应 工工 程程1.
17、5 Definition of Reaction RateHow 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 vario
18、us forms is defined as follows.Based on unit volume of reacting fluid,(2)化化 学学 反反 应应 工工 程程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)化化 学学 反反 应应 工工 程程Based on unit volume of so
19、lid in gas-solid systems,Based on unit volume of reactor,if different from the rate based on unit volume of fluid,(5)(6)化化 学学 反反 应应 工工 程程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 use
20、d interchangeably.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.化化 学学 反反 应应 工工 程程Relations between different rate definitions:It is very important to express a reaction with the
21、proper form of rate.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化化 学学 反反 应应 工工 程程Speed of Chemical Reactions Figure1.3 indicates the relative rates at which reactions occur.To give you an appr
22、eciation(感谢,感激,正确评价)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.化化 学学 反反 应应 工工 程程化化 学学 反反 应应 工工
23、 程程EXAMPLE 1.1 THE ROCKET ENGINEA 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 complet
24、e,find the rate of reaction of hydrogen and of oxygen.化化 学学 反反 应应 工工 程程SOLUTIONLet us evaluate terms.The reactor volume and the volume in which reaction takes place are identical.Thus,We want to evaluate化化 学学 反反 应应 工工 程程Next,let us look at the reaction occurring,So from Eq.(i)molar stoichiometry 1 0
25、.5 1molecular weight:2gm 16gm 18gm Therefore,H2 used=6 kmol/s;O2 used=3 kmol/sand the rate of reaction is化化 学学 反反 应应 工工 程程EXAMPLE 1.2 THE LIVING PERSONA human being(75kg)consumes about 6000 kJ of food per day.Assume that the food is all glucose and that the overall reaction isO2 come from air and CO
26、2 come out from the process of human breathing.Find mans metabolic rate(the rate of living,loving,and laughing)in terms of moles of oxygen used per m3 of person per second.化化 学学 反反 应应 工工 程程Let us evaluate the two terms in this equation.First of all,from our life experience we estimate the density of
27、 man to be SOLUTIONWe want to find(i)Therefore,for the person in question化化 学学 反反 应应 工工 程程Next,noting that each mole of glucose consumed uses 6 moles of oxygen and releases 2816 kJ of energy.We see that we needInserting into Eg.(i)Note:compare this value with those listed in Figure 1.3.化化 学学 反反 应应 工工 程程PROBLEMSP9 1.3