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1、化化 学学 反反 应应 工工 程程Chapter 4 Introduction to Reactor Design 4.1 GENERAL DISCUSSION So far we have considered the mathematical expression called the rate equation which describes the progress of a homogeneous reaction.The rate equation for a reacting component i is an intensive measure,and it tells how
2、 rapidly component i forms or disappears in a given environment as a function of the conditions there,orThis is a differential expression.化化 学学 反反 应应 工工 程程 In reactor design we want to know what size and type of reactor and method of operation are best for a given job.Because this may require that t
3、he conditions in the reactor vary with position as well as time,this question can only be answered by a proper integration of the rate equation for the operation.This may pose difficulties because the temperature and composition of the reacting fluid may vary from point to point within the reactor,d
4、epending on the endothermic or exothermic character of the reaction,the rate of heat addition or removal from the system,and the flow pattern of fluid through the vessel.In effect,then,many factors must be accounted for in predicting the performance of a reactor.How best to treat these factors is th
5、e main problem of reactor design.化化 学学 反反 应应 工工 程程Equipment in which homogeneous reactions are effected can be one of three general types:the batch,the steady-state flow,and the unsteady-state flow or semibatch reactor.The last classification includes all reactors that do not fall into the first two
6、 categories.These types are shown in Fig.4.1.化化 学学 反反 应应 工工 程程Figure 4.1 Broad classification of reactor types.(a)The batch reactor.(b)The steady-state flow reactor.(c),(d),and(e)Various forms of the semibatch reactor.化化 学学 反反 应应 工工 程程The steady-state flow reactor is ideal for industrial purposes wh
7、en large quantities of material are to be processed and when the rate of reaction is fairly high to extremely high.Supporting equipment needs are great;however,extremely good product quality control can be obtained.As may be expected,this is the reactor that is widely used in the oil industry.The ba
8、tch reactor is simple,needs little supporting equipment,and is therefore ideal for small-scale experimental studies on reaction kinetics.Industrially it is used when relatively small amounts of material are to be treated.化化 学学 反反 应应 工工 程程The semibatch reactor is a flexible system but is more difficu
9、lt to analyze than the other reactor types.It offers good control of reaction speed because the reaction proceeds as reactants are added.Such reactors are used in a variety of applications from the calorimetric titrations in the laboratory to the large open hearth(炉膛)furnaces for steel production.Th
10、e starting point for all design is the material balance expressed for any reactant(or product).Thus,as illustrated in Fig.4.2,we have 化化 学学 反反 应应 工工 程程Figure 4.2 Material balance for an element of volume of the reactor.化化 学学 反反 应应 工工 程程(1)Where the composition within the reactor is uniform(independe
11、nt of position),the accounting may be made over the whole reactor.化化 学学 反反 应应 工工 程程Where the composition is not uniform,it must be made over a differential element of volume and then integrated across the whole reactor for the appropriate flow and concentration conditions.For the various reactor typ
12、es this equation simplifies one way or another,and the resultant expression when integrated gives the basic performance equation for that type of unit.Thus,in the batch reactor the first two terms are zero;in the steady-state flow reactor the fourth term disappears;for the semibatch reactor all four
13、 terms may have to be considered.化化 学学 反反 应应 工工 程程In nonisothermal operations energy balances must be used in conjunction with material balances.Thus,as illustrated in Figure 4.3,we have(2)化化 学学 反反 应应 工工 程程Figure 4.3 Energy balance for an element of volume of the reactor.化化 学学 反反 应应 工工 程程The materia
14、l balance of Eq.1 and the energy balance of Eq.2 are tied together by their third terms because the heat effect is produced by the reaction itself.When we can predict the response of the reacting system to changes in operating conditions(how rates and equilibrium conversion change with temperature a
15、nd pressure),when we are able to compare yields for alternative designs(adiabatic versus isothermal operations,single versus multiple reactor units,flow versus batch system),and when we can estimate the economics of these various alternatives,then and only then will we feel sure that we can arrive a
16、t the design well fitted for the purpose at hand.化化 学学 反反 应应 工工 程程Symbols and Relationship between CA and XA For the reaction aA+bB rR,with inerts iI,Figs.4.4 and 4.5 shows the symbols commonly used to tell what is happening in the batch and flow reactors.These figures show that there are two relate
17、d measures of the extent of reaction,the concentration CA and the conversion XA.However,the relationship between CA and XA is often not obvious but depends on a number of factors.This leads to three special cases,as follows.化化 学学 反反 应应 工工 程程Figure 4.4 Symbol used for batch reactors.化化 学学 反反 应应 工工 程程
18、(3)Figure 4.5 Symbols used for flow reactors.To relate the changes in B and R to A we have 化化 学学 反反 应应 工工 程程(4)Special Case 2.Batch and Flow Systems of Gases of Changing Density but with T and Constant.Here the density changes because of the change in number of moles during reaction.In addition,we r
19、equire that the volume of a fluid element changes linearly with conversion,or V=V0(1+AXA).Special Case 1.Constant Density Batch and Flow Systems.This includes most liquid reactions and also those gas reactions run at constant temperature and density.Here CA and XA are related as follows:化化 学学 反反 应应
20、工工 程程To follow changes in the other components we have(6(6)化化 学学 反反 应应 工工 程程Special Case 3.Batch and Flow Systems for Gases in General(varying,T,)which react according to Pick one reactant as the basis for determining the conversion.We call this the key reactant.Let A be the key.Then for ideal gas b
21、ehavior,化化 学学 反反 应应 工工 程程化化 学学 反反 应应 工工 程程For liquids or isothermal gases with no change pressure and density and the preceding expressions simplify greatly.For high-pressure nonideal gas behavior replace by ,where z is the compressibility factor.To change to another key reactant,say B,note that 化化
22、学学 反反 应应 工工 程程EXAMPLE 4.1 A BALANCE FROM STOICHIOMETRY Consider a feed CA0=100,CB0=200,Ci0=100 to a steady-flow reactor.The isothermal gas-phase reaction is If CA=40 at the reactor exit,what is CB,XA and XB there?化化 学学 反反 应应 工工 程程SOLUTION First sketch what is known(see Fig.E4.1).Figure E4.1 化化 学学 反反 应应 工工 程程Next recognize that this problem concerns Special Case 2.So evaluate and .For this take 400 volumes of gas Then from the equation in the text 化化 学学 反反 应应 工工 程程PROBLEMSp?,?