100 。 Third order(n=3) Second order (n=2) =2 EA =1 0 1 223 10 First order and() (n=1) 1 0 2 n 0.5 n=0.25 1 0.01 0.1 1.0 6 1-XA
6
For n=2 (1-X4” m XA --x广- 、n-1 XA X 1-X4)-1-X1-X1-1-X》1-x 7
7 ( ) ( ) ( ) ( ) ( ) (( ) ) ( ) ( ) ( )( ( )) A A A A A A A A A A p n A m n A A p n A m n A X X X X X X X X X X n X X X C C n − = − − − = − − − = − − − = − − − − = = − − − − 1 1 1 1 1 1 1 1 1 1 1 1 1 1 For 2 2 1 1 2 1 0 1 0
Above figure shows 1.For any particular duty and for all positive reaction orders the mixed reactor is always larger than the plug flow reactor.The ratio of volumes increases with reaction order. 2.When conversion is small,the reactor performance is only slightly affected by flow type.The performance ratio increases very rapidly at high conver- sion;consequently,a proper representation of the flow becomes very impor- tant in this range of conversion. 3.Density variation during reaction affects design;however,it is normally of secondary importance compared to the difference in flow type. Remember that the essential factor for reactor size is reaction rate, which is controlled only by concentration of reactant A,besides temperature. The order of reaction,conversion,expansion factor are all related to concentration. 8
8 Above figure shows Remember that the essential factor for reactor size is reaction rate, which is controlled only by concentration of reactant A, besides temperature. The order of reaction, conversion, expansion factor are all related to concentration
Variation of reactant ratio for second-order reaction A+B->products,M=CBo/CAo (3.13) 一rA=-rB=KCACB behave as second-order reactions of one component when the reactant ratio is unity.Thus -rA=kCACB=kC when M=1 (3) On the other hand,when a large excess of reactant B is used then its concentration does not change appreciably(Ca=CBo)and the reaction approaches first-order behavior with respect to the limiting component A,or -rA kCACB =(kCBO)CA =k'CA when M>1 (4) Thus in Fig.6.1,and in terms of the limiting component A,the size ratio of mixed to plug flow reactors is represented by the region between the first-order and the second-order curves
9 • Variation of reactant ratio for second-order reaction
·General graphical comparison ·For a nth-order Any rate curve reaction(n>0),it can be seen that Area Tm/CAO mixed flow Area Tp/CAO always needs a larger volume than does plug XA flow for any Figure 6.2 Comparison of performance of mixed flow and plug flow reactors for any reac- given duty. tion kinetics. 10
10 • General graphical comparison • For a nth -order reaction(n>0), it can be seen that mixed flow always needs a larger volume than does plug flow for any given duty