《工程科技化反》PPT课件.ppt

1、1,Chapter 03 Ideal Reactors,2,3 ideal reactors,Stirred tanks turbular or packed-bed reactors the criterion for ideality in tank reactors is that the liquid be perfectly mixed,which mans no gradients in temperature or concentration in the vessel. in plug-flow reactors,or PFRs,there are axial gradient

2、s of concentration and perhaps also axial gradients of temperature and pressure,but in the ideal PFR there is no axial diffusion or conduction.,3,Batch Reactor Design,4,First-order reactions,For an irreversible first-order reation of the type,(3.2),5,First-order reactions,The result is often given i

3、n terms of the fraction converted:,(3.3),6,Second-order reactions,for a unimolecular second-order reation,(3.6),(3.5),(3.4),or,7,Second-order reactions,bimolecular reation,Where R 1,(3.7),Since,and,with A the limiting reactant. Let,8,Second-order reactions,(3.8),Integration between limits gives,(3.1

4、0),9,Second-order reactions,R=2.0,10,Consecutive Reactions,The rates of reaction depend on the concentration of B in the liquid phase, which is a function of gas solubility, pressure, and agitation conditions. we are often concerned with the relative reaction rates and the selectivity, which do not

5、depend on the concentration of B , if the reaction orders are the same for both reactions. The reactions are treated as pseudo-first-order, and equations are developed for an ideal batch reactor with irreversible first-order kinetics:,11,Consecutive Reactions,(3.13),(3.12),The concentration of A fal

6、ls exponentially, as was shown earlier in Eq. (3.2):,(3.11),12,Consecutive Reactions,The material balance for product C is,Combining these equations gives,(3.15),(3.14),13,Consecutive Reactions,If no C is present at the start, integration of Eq. (3.15) gives,The concentration of C goes through a max

7、imum with time, can be found by differentiating Eq. (3.16) and setting the derivative to zero:,(3.16),(3.17),14,Consecutive Reactions,The concentration of D is obtained by a material balance:,(3.18),(3.19),(3.20),15,Consecutive Reactions,maximum of 67% conversion of A is 86.5% and the selectivity is

8、 77%.,the conversion is 63% and the yield of C is 56% for a selectivity of 89%.,16,Parallel Reactions,Selectivity effects can also be important with parallel reactions having different reaction orders. Consider the case where the main reaction is first order to both reactants and the byproduct react

9、ion is second order to one of the reactants:,byproduct formation,main reaction,17,Parallel Reactions,The local or instantaneous selectivity is the ratio of to the total rate of consumption of A:,(3.21),(3.22),18,Parallel Reactions,Higher selectivity could be achieved by decreasing the concentration

10、of A in the initial charge, but this would give a lower concentration of C in the final product and increase separation costs.,19,Semibatch Reactions,The reactor might be two-thirds to three-quarters full at the start, and the fluid volume increases as A is added and no product withdrawn. Figure 3.4

11、 shows the calculated concentration curves for the same kinetics and as for Figure 3.3, with the feed of A at a slow, constant rate for 14 hours,20,Semibatch Reactions,the initial selectivity is very high but decreases as decreases and increases slightly. After the feed is stopped, the selectivity a

12、gain increases,21,CONTINUOUS-FLOW REACTORS,22,CONTINUOUS-FLOW REACTORS,Operating a stirred-tank reactor with continuous-flow of reactants and products (a CSTR) has some advantages over batch operation. The reactor can make products 24 hours a day for weeks at a time, whereas for a typical cycle, the

13、 batch reactor is producing only about half the time. In the CSTR, temperature control is easier because the reaction rate is Constant, and the rate of heat release does not change with time, as it does in a batch reactor.,23,CONTINUOUS-FLOW REACTORS,Finally, conversion and selectivity may vary from

14、 day to day with a batch reactor, and they are more likely to be constant with a CSTR and a good control system. The main disadvantage of continuous operation is that the reaction rate is nearly always lower than the average rate for a batch reaction. In most cases, the batch reaction rate decreases

15、 as the conversion increases, and in the CSTR the reaction rate is the same as the final reaction rate in the batch reactor. For high conversions, the final rate may be several-fold lower than the average rate and the average residence time in the CSTR must then be several-fold greater than the reac

16、tion time in a batch reactor. The average residence time in the CSTR is t =V/F. The ratio of CSTR residence time to batch residence time is readily derived for simple kinetic models. For a first order reaction in a CSTR, the steady-state material balance is,24,CONTINUOUS-FLOW REACTORS,In terms of fr

17、action converted,in - out,(3.23),(3.24),(3.25),(3.26),(3.27),In-out = amount reacting,25,CONTINUOUS-FLOW REACTORS,or,Equations (3.26), (3.27), and (3.28) are equivalent, and they are used when solving for , t, or x. To compare the batch and CSTR times, the equation for a batch first-order reaction,

18、Eq. (3.3) or Eq. (3.29), is used with Eq. (3.27):,(3.28),(3.29),(3.30),26,CONTINUOUS-FLOW REACTORS,27,CONTINUOUS-FLOW REACTORS,28,CONTINUOUS-FLOW REACTORS,For the CSTR,From Figure 3.5,or,(3.31),(3.32),(3.33),(3.34),29,CONTINUOUS-FLOW REACTORS,30,Reactors in Series,31,Reactors in Series,CSTR-1,CSTR-2

19、,CSTR,32,Reactors in Series,When the two reactors are used in series, the total volume is proportional to the sum of the rectangular area ebgf and cdhg in Figure 3.8. With several reactors in series, the total volume would approach that for a plugflow reactor.,(3.36),(3.35),(3.37),(3.38),33,Reactors

20、 in Series,When the reactors are equal in size and operate at the same temperature, theequation is,(3.39),(3.40),34,Reactors in Series,For a very large number of tanks, the conversion approaches that for a plug-flow reactor or a batch reactor. With three tanks in series, the total time is 50% more t

21、han for plug flow if the desired conversion is 90% and for five tanks the time is only 25% greater.,35,Reactors in Series,It is easy to show that for a first-order reaction and two tanks, the volume should be equal:,Taking, for example,:,Any other combination with the same total time gives higher .

22、Forexample, if and ;,36,Temperature Optimization,When a sequence of reactions produces a mixture of products, the selectivity for the main product is a major factor in choosing reaction conditions. We have shown that the ratio of reactant concentrations and the conversion can affect the selectivity,

23、 particularly when the main and byproduct reactions have different reaction orders. When the reactions have different activation energies, the selectivity will also depend on the temperature. If the main reaction has the higher activation energy, raising the temperature will increase the selectivity

24、 and also decrease the time needed to reach the desired conversion. The best operating temperature cannot be chosen from just the kinetics but depends on other factors, such as the cost of supplying or removing heat, vaporization losses, corrosion rate, and safety considerations.,37,Temperature Opti

25、mization,When the byproduct reaction has a higher activation energy than the main reaction, the selectivity is improved by reducing the temperature. However, this means a greater reaction time for a batch reactor or a larger reactor for a flow system. The temperature chosen is again a compromise bas

26、ed on the reactor size, raw material costs, and the cost of product separation. However, for an existing CSTR and a fixed feed rate, an optimum temperature can be defined as the temperature that gives the greatest yield of the main product. Increasing the temperature increases the conversion but dec

27、reases the selectivity, so the yield goes through a maximum, as shown in the following example.,38,Temperature Optimization,39,Temperature Optimization,40,Temperature Optimization,41,Temperature Optimization,42,PLUG-FLOW REACTORS,43,PLUG-FLOW REACTORS,elements of the fluid are assumed to pass throug

28、h the reactor with no mixing all elements spend the same time in the reactor. With a packed-bed reactor, the velocity profile is complex and changing with distance, as the fluid flows around and between the particles. However, when the bed depth is many times the particle diameter (L/dp 40), the res

29、idence time distribution of the fluid is quite narrow, and plug flow can be assumed.,44,Homogeneous Reactions,(3.41),To compare the equations for the PFR with a batch reactor, the molar feed rate is expressed as the volume feed rate F times the concentration of A:,(3.42),45,Homogeneous Reactions,(3.

30、43),Equation (3.42) can then be presented using the space velocity SV or the space time , which is the reciprocal of the space velocity:,where,volumetric feed rate,reactor volume,(3.44),46,Heterogeneous Reactions,For a heterogeneous catalytic reaction in an ideal packed-bed reactor, the material bal

31、ance is written for a differential mass of catalyst, dW. The basic equation for the conversion of the key reactant A is the same as for any type of reaction, or combination of reactions, including reversible reactions.,For,or,A,B+C,Or,47,Heterogeneous Reactions,where,Moles A fed/hr,r=total moles A c

32、onsumed/hr,kg W=mass of catalyst,Integration of the rate equation gives the mass of catalyst needed per unit feed rate of A for a specified conversion.,(3.46),48,Heterogeneous Reactions,The reactor volume is determined from the mass of catalyst and the bed density :,(3.47),The dimensions of the reac

33、tor are not fixed by these equations, The reactor dimensions are selected to give reasonable proportions and a tolerable pressure drop. Often the mass velocity is chosen first, which gives the cross-sectional area, and the bed length is determined from the required volume.,49,Heterogeneous Reactions

34、,To relate the conversion to the space velocity, the feed concentration and the bed density are introduced into Eq. (3.46):,where,(3.48),It is not really necessary to use the concept of space velocity in designing a reactor, since the mass of catalyst needed and the bed volume are determined directl

35、y from Eqs. 3.46 and 3.47. However, some patents and technical reports give the conversion as a function of space velocity and temperature rather than presenting fundamental kinetic data. To use such data, the space velocity must be carefully defined and interpreted.,50,Heterogeneous Reactions,In Eq

36、. (3.48), the space velocity is defined using the volumetric flow rate at the entrance to the reactor, but it could be based on the volume of gas at standard conditions:,Another definition is based on the void volume of the reactor 2, which corresponds to,Although is closer to the gas residence time

37、 than is , there is noadvantage in using for alculations, and Eq. (3.50) incorrectly implies that raising would increase the conversion.,(3.49),(3.50),51,Heterogeneous Reactions,Other terms that are used when feeding liquids to a reactor are the weight hourly space velocity (WHSV) and the liquid hou

38、rly space velocity (LHSV) 3. Both have units of but are defined differently:,pounds of feed/hr,pounds of catalyst,=,WHSV,volume of liquid/hr,volume of reactor,=,LHSV,The LHSV is sometimes used when feeding liquids that are vaporized in a preheater before entering the reactor, and of course the LHSV

39、is much lower than the SV based on the actual vapor flow to the reactor. Sometimes WHSV is based on the feed rate of one reactant rather than the total feed rate,52,Heterogeneous Reactions,Even when the space velocity is clearly defined, there may be problems in scaleup or design. It might be though

40、t that if temperature, pressure, and space velocity are kept constant on scaleup, the conversion will be constant. However, as Eq. (3.48) shows, a change in or may affect the conversion. A small-diameter laboratory reactor may have a lower bed density than a large reactor, in which case the large re

41、actor might have a higher conversion for the same SV. Doubling would double r if the reaction is first order, and the conversion would not change; but for other orders the effects of would not cancel, and the conversion could change.,53,Adiabatic Reactors,Reactions on solid catalysts are often carri

42、ed out in adiabatic reactors if there is little change in selectivity or the rate of catalyst aging with temperature. The reactor is generally a large-diameter cylindrical vessel containing one or more beds of catalytic particles supported on grids or heavy screens, as shown in Figure 3.12a. Another

43、 type of reactor has one or more annular beds of catalyst with radial flow of gas either inward or outward , as shown in Figure 3.12b.,54,Adiabatic Reactors,55,Adiabatic Reactors,The first step in reactor design is to calculate the equilibrium conversion as a function of temperature for a given pres

44、sure and feed ratio. For a bimolecular reversible reaction such as,(3.54),56,Adiabatic Reactors,At steady state, the energy released is equal to the increase in sensible heat of the feed stream, since there is no heat loss to the surroundings in an adiabatic reactor.,The heat capacity of the catalys

45、t and the reactor wall are not included in the heat balance, since once the steady-state temperature profile is established, the solids cannot store any more energy, and all the heat released must be absorbed by the flowing gas. From Eqs. (3.55) and (3.36),(3.56),(3.55),57,Adiabatic Reactors,since, another form of Eq. (3.57) is,58,Adiabatic Reactors,59,Adiabatic Reactors,If a higher conversion is needed, the gases ar