【正文】 entration and the bed density are introduced into Eq. (): where () 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 directly from Eqs. and . However, some patents and technical reports give the conversion as a function of space velocity and temperature rather than presenting fundamental kiic data. To use such data, the space velocity must be carefully defined and interpreted. 50 Heterogeneous Reactions 39。SVIn Eq. (), 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 VF?VFAlthough is closer to the gas residence time than is , there is noadvantage in using for alculations, and Eq. () incorrectly implies that raising would increase the conversion. () () 51 Heterogeneous Reactions FWFV???Other terms that are used when feeding liquids to a reactor are the weight hourly space velocity (WHSV) and the liquid hourly space velocity (LHSV) [3]. Both have units of but are defined differently: 1hr?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 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 B?0AC0AC0ACEven when the space velocity is clearly defined, there may be problems in scaleup or design. It might be thought that if temperature, pressure, and space velocity are kept constant on scaleup, the conversion will be constant. However, as Eq. () shows, a change in or may affect the conversion. A smalldiameter laboratory reactor may have a lower bed density than a large reactor, in which case the large reactor 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。()AAAAC S T RC S T Rdxr C CdtF C x r V C s lo p e VVxtF s lo p eVxttF x t??????? ? ?39。39。39。39。1 Chapter 03 Ideal Reactors BATCH REACTOR DESIGN 1 CONTINUOUSFLOW REACTORS 2 PLUGFLOW REACTORS 3 2 3 ideal reactors ?Stirred tanks ?turbular or packedbed 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 plugflow reactors,or PFRs,there are axial gradients 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 Firstorder reactions 1 Secondorder reactions 2 Consecutive Reactions 3 Parallel Reactions 4 Semibatch Reactions 5 4 Firstorder reactions ?For an irreversible firstorder reation of the type 0l n( )AAOAACtACAAAOktA AOA B CdCr k CdtdCK dtCCktCC C e???? ? ????????() 5 Firstorder reactions ?The result is often given in terms of the fraction converted: 0 ( 1 )1l n ( 1 ) l n ( )1AAC C xx k tx??? ? ? ??() 1l n 2 .31 0 .91l n 4 .61 0 .9 9????????????????6 Secondorder reactions for a unimolecular secondorder reation, 02200111AAAACACA A AtAdCr k CdtdCktC C CxkCx? ? ?? ? ? ????() () () or 7 Secondorder reactions ,A B C??bimolecular reation 000( 1 )BAAAR C CC C x???Where R 1 0AACC x?? () 000 0 0( 1 ) ( )B B AA B A B AC C C xr k C C k C x C C x??? ? ? ?Since 0 ()BAC C R x??and 0AAdC C dx?with A the limiting reactant. Let 8 Secondorder reactions 0 0 0( 1 ) ( )AA A AdC dxr C k C x C R xd t d t? ? ? ? ? ?() 000( 1 ) ( )xtAdx k C d tx R x