1、化 学 反 应 工 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 ra
2、pidly component i forms or disappears in a given environment as a function of the conditions there, or,This is a differential expression.,化 学 反 应 工 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 the co
3、nditions 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, dep
4、ending 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 i
5、s the main problem of reactor design.,化 学 反 应 工 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
6、 two categories. These types are shown in Fig. 4.1.,化 学 反 应 工 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 学 反 应 工 程,The steady-state flow reactor is ideal for industrial purp
7、oses when 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 ind
8、ustry.,The batch 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 学 反 应 工 程,The semibatch reactor is a flexible system but is
9、more difficult 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 stee
10、l production.,The 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 学 反 应 工 程,Figure 4.2 Material balance for an element of volume of the reactor.,化 学 反 应 工 程,(1),Where the composition within the reactor is unif
11、orm (independent of position), the accounting may be made over the whole reactor.,化 学 反 应 工 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 variou
12、s reactor types 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 semibatc
13、h reactor all four terms may have to be considered.,化 学 反 应 工 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 学 反 应 工 程,Figure 4.3 Energy balance for an element of volume of the reactor.,化 学 反 应 工 程,T
14、he material 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
15、 temperature and 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
16、that we can arrive at the design well fitted for the purpose at hand.,化 学 反 应 工 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
17、 there are two related 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 学 反 应 工 程,Figure 4.4 Symbol used for batch reac
18、tors.,化 学 反 应 工 程,(3),Figure 4.5 Symbols used for flow reactors. To relate the changes in B and R to A we have,化 学 反 应 工 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 reactio
19、n. In addition, we require 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 relate
20、d as follows:,化 学 反 应 工 程,To follow changes in the other components we have,(6),化 学 反 应 工 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
21、 key. Then for ideal gas behavior,化 学 反 应 工 程,化 学 反 应 工 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,
22、 say B, note that,化 学 反 应 工 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 学 反 应 工 程,SOLUTION,First sketch what is known (see Fig. E4.1).,Figure E4.1,化 学 反 应 工 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 学 反 应 工 程,PROBLEMS p?, ?,
