1、化 学 反 应 工 程,Chapter 8 Potpourri of Multiple Reactions,Chapter 7 considered reactions in parallel. These are reactions where the product does not react further. This chapter considers all sorts of reactions where the product formed may react further. Here are some examples:,Series,A,R,S,Reversible an
2、d irreversible,Series parallel, or consecutive-competitive,T U,Denbigh system,化 学 反 应 工 程,A R S,Reversible,A,R,S,Reversible network,We develop or present the performance equations of some of the simpler systems and point out their special features such as maxima of intermediates.,化 学 反 应 工 程,proceed
3、 only in the presence of light, that they stop the instant the light is turned off, and that for a given intensity of radiation, the rate equations are,(2),(3),(4),8.1 IRREVERSIBLE FIRST-ORDER REACTIONS IN SERIES,For easy visualization consider that the reactions,(1),化 学 反 应 工 程,Qualitative Discussi
4、on About Product Distribution. Consider the following two ways of treating a beaker containing A: First, the contents are uniformly irradiated; Second, a small stream is continuously withdrawn from the beaker, irradiated(照射), and returned to the beaker; the rate of absorption of radiant energy is th
5、e same in the two cases. The two schemes(计划,方案) are shown in Figs. 8.1 and 8.2.,化 学 反 应 工 程,Figure 8.1 Concentration-time curves if the contents of the beaker are irradiated uniformly.,化 学 反 应 工 程,Figure 8.2 Concentration-time curves for the contents of the beaker if only a small portion of the flui
6、d is irradiated at any instant.,化 学 反 应 工 程,In the first beaker (shown in Fig. 8.1), when the contents are being irradiated all at the same time, the first bit of light will attack A alone because only A is present at the start. The result is that R is formed. With the next bit of light both A and R
7、 will compete; however, A is in very large excess so it will preferentially(优先地) absorb the radiant energy to decompose and form more R.,During this process A disappears and products are formed. Is the product distribution of R and S different in the two beakers? Let us see whether we can answer thi
8、s question qualitatively(定性地) for all values of the rate constants.,化 学 反 应 工 程,Thus, the concentration of R will rise while the concentration of A will fall. This process will continue until R is present in high enough concentration so that it can compete favorably with A for the radiant energy. Wh
9、en this happens, a maximum R concentration is reached. After this the decomposition of R becomes more rapid than its rate of formation and its concentration drops. A typical concentration time curve is shown in Fig.8.1.,化 学 反 应 工 程,In the alternative (二中择一的) way (shown in Fig. 8.2) of treating A, a
10、small fraction of the beakers contents is continuously removed, irradiated, and returned to the beaker. Although the total absorption rate is the same in the two cases, the intensity of radiation received by the removed fluid is greater, and it could well be, if the flow rate is not too high, that t
11、he fluid being irradiated reacts essentially to completion. In this case, then, A is removed and S is returned to the beaker. So, as time passes the concentration of A slowly decreases in the beaker, S rises, while R is absent (缺席的). This progressive change is shown in Fig.8.2.,化 学 反 应 工 程,These two
12、 methods of reacting the contents of the beaker yield different product distributions and represent the two extremes in possible operations, one with a maximum possible formation of R and the other with a minimum, or no formation, of R. How can we best characterize this behavior? We note in the firs
13、t method that the contents of the beaker remain homogeneous throughout, all changing slowly with time, whereas in the second a stream of highly reacted fluid is continually being mixed with fresh fluid. In other words, we are mixing two streams of different compositions called backmix.,No backmix ！,
14、Has backmix !,化 学 反 应 工 程,For irreversible reactions in series the mixing of fluid of different composition is the key to the formation of intermediate. The maximum possible amount of any and all intermediates is obtained if fluids of different compositions and at different stages of conversion are
15、not allowed to mix.,The above discussion suggests the following rule governing product distribution for reactions in series.,No backmix !,化 学 反 应 工 程,As the intermediate is frequently the desired reaction product, this rule allows us to evaluate the effectiveness of various reactor systems. For exam
16、ple, plug flow and batch operations should both give a maximum R yield because here there is no mixing of fluid streams of different compositions. On the other hand, the mixed reactor should not give as high a yield of R as possible because a fresh stream of pure A is being mixed continually with an
17、 already reacted fluid in the reactor.,化 学 反 应 工 程,The following examples illustrate the point just made. We then give a quantitative(定量的) treatment which will verify these qualitative(定性的) findings.,EXAMPLE 8.1 FAVORABLE CONTACTING PATTERNS FOR ANY SET OF IRREVERSIBLE REACTIONS IN SERIES, NOT JUST,
18、Which contacting pattern of Figs. E8.1, when properly operated, can give a higher concentration of any intermediate, the contacting pattern on the left or the one on the right?,化 学 反 应 工 程,Figure E8.1 a,b,c,d,化 学 反 应 工 程,SOLUION,Focusing on the mixing rule for reactions in series, that the extent of
19、 mixing of streams of different composition should be minimized, we reason (v.推论)for part (a): The left pattern is better; in fact it is the best possible flow scheme.for part (b): Looking at Figs.6.5, 6.6, and 6.16, 6.17 of Chapter 6 we see that the left is closer to plug flow for both first- and s
20、econd-order reactions. So we generalize this to any positive order reaction.,化 学 反 应 工 程,for part (c): The right pattern is better because it is closer to plug flow. for part (d): Turbulent flow has less intermixing of fluids of different ages, less bypassing; hence, the right scheme is better.,Note
21、. In the quantitative(定量的) analysis that follows we verify this general and important rule.,化 学 反 应 工 程,In Chapter 3 we developed the equations relating concentration with time for all components of the unimolecular-type reactions,in batch reactors. The derivations(推导) assumed that the feed containe
22、d no reaction products R or S. If we replace reaction time by the space time, these equations apply equally well for plug flow reactors,Quantitative Treatment, Plug Flow or Batch Reactor.,化 学 反 应 工 程,(3.47) or (6),(3.49) or (7),thus,化 学 反 应 工 程,The maximum concentration of intermediate and the time
23、at which it occurs is given by,(3.52) or (8),where k1 k2,(3.51) or (9),This is also the point at which the rate of formation of S is most rapid.,化 学 反 应 工 程,Figure 8.3a, prepared for various k2 / k1 values, illustrates how this ratio governs the concentration-time curves of the intermediate R. Figur
24、e 8.3b, a time-independent plot, relates the concentration of all reaction components; also see Eq.37.,化 学 反 应 工 程,Figure 8.3 a,b Behavior of unimolecular-type reactions(a) concentration-time curves, and (b) relative concentration of the reaction components.,in a plug flow reactor:,化 学 反 应 工 程,Quant
25、itative Treatment, Mixed Flow Reactor. Let us develop the concentration-time curves for this reaction when it takes place in a mixed flew reactor. This may be done by referring to Fig.8.4. Again, the derivation will be limited to a feed which contains no reaction product R or S. By the steady-state
26、material balance we obtain for any component,Input = output + disappearance by reaction,(4.1) or (10),化 学 反 应 工 程,Figure 8.4 Variables for reactions in series ( no R or S in the feed ) occurring in a mixed flow reactor.,化 学 反 应 工 程,Which for reactant A becomes,or,Noting that,(11),we obtain for A, on
27、 rearranging (for rA= k1CA),(12),For component R the material balance, Eq.10, becomes,or,化 学 反 应 工 程,With Eqs.11 and 12 we obtain, on rearranging,(13),CS is found by simply noting that at any time,hence,(14),The location and maximum concentration of R are found by determining . Thus,化 学 反 应 工 程,Whic
28、h simplifies neatly to give,(15),The corresponding concentration of R is given by replacing Eq. 15 in Eq.13. On rearranging, this becomes,(16),化 学 反 应 工 程,Typical concentration-time curves for various k2 / k1 values are shown in Fig.8.5a. A time-independent plot, Fig.8.5b, relates the concentrations
29、 of reactant and products.Remarks on Performance Characteristics, Kinetic Studies, and Design. Figures 8.3a and 8.5a show the general time-concentration behavior for plug and mixed flow reactors and are an aid in visualizing(显现) the actual progress of the reaction.,化 学 反 应 工 程,Figure 8.5 a,b Behavio
30、r of unimolecular-type reactionsin a mixed flow reactor: (a) concentration-time curves, and (b) relative concentration of the reaction components.,化 学 反 应 工 程,Comparison of these figures shows that except when the plug flow reactor always requires a smaller space time than does the mixed reactor to
31、achieve the maximum concentration of R. In addition, for any reaction the maximum obtainable concentration of R in a plug flow reactor is always higher than the maximum obtainable in a mixed reactor (see Eqs.16 and 8). This verifies (验证) the conclusions arrived at by qualitative reasoning (推理).,化 学
32、反 应 工 程,Figures 8.3b and 8.5b, time-independent plots, show the distribution of materials during reaction. Such plots find most use in kinetic studies because they allow the determination of k2/k1 by matching the experimental points with one of the family of curves on the appropriate graph. Figures
33、8.13 and 8.14 (P190 and 191) are more detailed representations of these two figures. Though not shown in the figures, CS can be found by difference between CA0 and CA+ CR .,化 学 反 应 工 程,Figure 8.6 presents the fractional yield curves for intermediate R as a function of the conversion level and the ra
34、te constant ratio. These curves clearly show that the fractional yield of R is always higher for plug flow than for mixed flow for any conversion level.,化 学 反 应 工 程,Figure 8.6 Comparison of the fractional yields of R in mixed flow and plug flow reactors for the unimolecular-type reactions,化 学 反 应 工
35、程,A second important observation in this figure concerns the extent of conversion of A we should plan for. If for the reaction considered k2/k1 is much smaller than unity, we should design for a high conversion of A and probably dispense with (不用) recycle of unused reactant. However, if k2/k1 is gre
36、ater than unity, the fractional yield drops very sharply even at low conversion. Hence, to avoid obtaining unwanted S instead of R we must design for a very small conversion of A per pass, separation of R, and recycle of unused reactant.,化 学 反 应 工 程,8.2 FIRST-ORDER FOLLOWED BY ZERO-ORDER REACTION,Le
37、t the reactions be,(17),For batch or plug flow with integration gives,(18),化 学 反 应 工 程,The maximum concentration of intermediate, , and the time when this occurs is found to be,(20),and,(21),and,(19),化 学 反 应 工 程,Figure 8.7 Product distribution for the reactions,化 学 反 应 工 程,8.3 ZERO-ORDER FOLLOWED BY
38、 FIRST-ORDER REACTION,Let the reactions be,(22),for batch or plug flow with integration gives,(23),化 学 反 应 工 程,and,(24),(25),The maximum concentration of intermediate, , and the time when this occurs is found to be,(26),(27),and,化 学 反 应 工 程,Figure 8.8 Product distribution for the reactions,化 学 反 应 工
39、 程,8.5 REVERSIBLE REACTIONS,Solution of the equations for successive reversible reactions is quite formidable even for the first-order case; thus, we illustrate only the general characteristics for a few typical cases. Consider the reversible first-order reactions,A,R,S,and,B,U,T,(28),(29),化 学 反 应 工
40、 程,Figures 8.9 and 8.10 display the concentration time curves for the components in batch or plug flow for different values of the rate constants.,Figure 8.9 shows that the concentration of intermediate in reversible series reactions need not pass through a maximum, while Fig.8.10 shows that a produ
41、ct may pass through a maximum concentration typical of an intermediate in the irreversible parallel reaction; however, the reactions may be of a different kind.,化 学 反 应 工 程,Figure 8.9 Concentration-time curves for the elementary reversible series reactions,From Jungers et al.(1958), p. 207.,A,R,k3,S
42、,k1,k2,k4,化 学 反 应 工 程,Figure 8.10 Concentration-time curves for the elementary reversible parallel reactions,A,R,S,k3,k1,k2,k4,From Jungers et al. (1958), p.207.,化 学 反 应 工 程,A comparison of these figures shows that many of the curves are similar in shape, making it difficult to select a mechanism of
43、 reaction by experiment, especially if the kinetic data are somewhat scattered. Probably the best clue to distinguishing between parallel and series reactions is to examine initial rate data data obtained for very small conversion of reactant. For series reactions the time-concentration curve for S
44、has a zero initial slope, whereas for parallel reactions this is not so.,化 学 反 应 工 程,8.6 IRREVERSIBLE SERIES-PARALLEL REACTIONS,Multiple reactions that consist of steps in series and steps in parallel are called series-parallel reactions. From the point of view of proper contacting, these reactions
45、are more interesting than the simpler types already considered because a larger choice of contacting is usually possible, leading to much wider differences in product distribution.,化 学 反 应 工 程,Thus, design engineers are dealing with a more flexible system and this affords them the opportunity to dis
46、play their talents(才干) in devising(设计,发明) the best of the wide variety of possible contacting patterns. Let us develop our ideas with a reaction type that represents a broad class of industrially important reactions. We will then generalize our findings to other series-parallel reactions.,化 学 反 应 工
47、程,For the reaction set consider the successive(连续的) attack of a compound by a reactive material. The general representation of these reactions is,(30),or,化 学 反 应 工 程,Where A is the compound to be attacked, B is the reactive material, and R, S, T, etc., are the polysubstituted materials formed during
48、 reaction. Examples of such reactions may be found in the successive substitutive(取代的) halogenation (卤化) (or nitration硝化) of hydrocarbons, say benzene or methane, to form monohalo, dihalo, trihalo, etc. , derivatives(衍生物) as shown below:,C6H6,+HNO3,C6H5Cl,+Cl2,+Cl2,C6Cl6,C6H6,+Cl2,+HNO3,+HNO3,C6H5NO
49、2,C6H3(NO2) 3,CH4,+Cl2,CH3Cl,+Cl2,+Cl2,CCl4,化 学 反 应 工 程,Another important example is the addition of alkene oxides, say ethylene oxide(环氧乙烷), to compounds of the proton donor class such as amines(胺), alcohols, water, and hydrazine(肼,联氨) to form monoalkoxy(单烷氧基的), dialkoxy, trialkoxy, etc., derivatives, some examples of which are shown in P184.,Such processes are frequently bimolecular, irreversible, hence second-order kinetically. When occurring in the liquid phase they are also essentially constant-density reactions.,
