A First Course on Kinetics and Reaction Engineering. Class 19 on Unit 18

Transcription

1 A Firs ourse on Kineics and Reacion Engineering lass 19 on Uni 18

2 Par I - hemical Reacions Par II - hemical Reacion Kineics Where We re Going Par III - hemical Reacion Engineering A. Ideal Reacors B. Perfecly Mixed Bach Reacors Reacion Engineering of Bach Reacors Analysis of Bach Reacors Opimizaion of Bach Reacor Processes. oninuous Flow Sirred Tank Reacors D. Plug Flow Reacors E. Maching Reacors o Reacions Par IV - Non-Ideal Reacions and Reacors 2

3 Reacion Engineering wih Bach Reacors Operaion lean Prepare and harge Process according o Proocol Drain Produciviy Turnaround ime Processing (reacion) ime Reacor Design Problems Sizing and Processing Proocol Opimizaion Oher Reacion Engineering Tasks Simulae he enire process (e. g. for auomaing conrols) Evaluae he effec of some change in he proocol Reasons for using Bach Reacors Flexibiliy Small Quaniies of Produc Precise onrol Disadvanages Labor Inensive Bach o Bach onsisency No Suied o Producing Large Quaniies Imporance of Physical Undersanding You will reain your physical undersanding much longer han an equaions-based undersanding A physical undersanding may allow you o eliminae some design alernaives wihou having o solve he design equaions I will be easier o make creaive new (paenable) discoveries if you have a sound physical undersanding 3

4 Major AFoKaRE Problem Types and How o Idenify Them Reacion Mechanism Problems In a reacion mechanism problem one is ypically given a macroscopically observed (also called overall or apparen) reacion along wih a mechanism and asked o generae a rae expression for he macroscopically observed reacion rae. Age Funcion Problems In an age funcion problem one is ypically given daa for he response of a laboraory reacor o eiher a sep change or an impulse simulus and asked o use hose daa o deermine wheher he laboraory reacor obeys he assumpions of one of he ideal flow reacor models (STR or PFR). Kineics Daa Analysis Problems In a kineics daa analysis problem, one is ypically given a se of kineics daa for a given reacion, he ype of ideal reacor used o gaher hose daa and a descripion of he reacor and how i was operaed. One is hen asked eiher o find a rae expression ha describes he daa, or, more commonly o es wheher a given rae expression gives an accurae represenaion of he daa. Qualiaive Reacion Engineering Problems In a qualiaive reacion engineering problem, one is ypically given he reacion(s) ha is(are) aking place and some informaion abou hem along wih he ype of reacor being used and some informaion abou how ha reacor is operaed. One is hen usually asked o qualiaively describe or skech how one (or more) quaniies will vary during he operaion of he reacor. In paricular, one is no asked o calculae quaniies or o plo calculaed quaniies (as opposed o making a qualiaive skech). 4

5 A General Approach o Solving Qualiaive Reacion Engineering Problems Read hrough he problem saemen and idenify he ype(s) of reacor(s) being used he reacor operaing procedure being used (isohermal vs. adiabaic, seady sae vs. ransien, ec.) he ype of reacion(s) aking place (reversible/irreversible, ypical, auo-caalyic, produc inhibied, ec.) he quaniies whose variaion you are asked o describe Skech a plo of reacan concenraion(s), produc concenraions, emperaure, reacion rae and oher quaniies of ineres versus ime (for a bach reacor) or space ime (for a flow reacor) Draw ses of axes for he plos Deermine he iniial values of each of hese quaniies (a he sar of he reacion or inle o he reacor) and add o he corresponding plo 5

6 Deermine he iniial slope of he plos of hese quaniies by considering he firs small incremen in ime (or space ime) and add o he corresponding plo - Do he reacan concenraions, produc concenraions and emperaure increase or decrease during his inerval? - Will hose changes cause he reacion rae o increase or decrease during his inerval? - Do he quaniies of ineres increase or decrease during his inerval? - Will hose changes cause he equilibrium conversion o increase or decrease during his inerval? - if comparing wo or more sysems, for each plo, deermine he which sysem will have he larges slope, he second larges slope, ec. Deermine he curvaure of he plos by considering he nex small incremen in ime (or space ime) and add o he corresponding plo - Do he reacan concenraions, produc concenraions, emperaure and rae change by a greaer or lesser amoun han during he preceding inerval? - Do he quaniies of ineres change by a greaer or lesser amoun han during he preceding inerval? Deermine wheher coninuing he iniial rends will resul in he rae asympoically approaching equilibrium - If no, infer wha mus happen so ha he sysem approaches equilibrium properly (i. e. so he rae progressively decreases o zero) and add o he corresponding plos If comparing wo or more sysems, deermine he relaive magniudes of he equilibrium concenraions and emperaures in order o ascerain wheher or no he curves for he sysems being compared cross each oher Use he plos o answer he quesions posed in he problem 6

7 7 Quesions?

8 Analysis of a Reacan-Inhibied Reacion Suppose he caalyic reacion (1) below is reacan inhibied wih a rae expression of he form shown in equaion (2). The reacion is irreversible, and K is very, very small in magniude. Predic, qualiaively, how he rae and he conversion will vary as a funcion of isohermal bach reacion ime (a) if PA 0 = PB 0 and (b) if PA 0 > PB 0. A + B Y + Z! (1) r! = kp B K + P A In his problem we are given informaion abou a reacion and he reacor in which ha reacion akes place. We are asked o make qualiaive predicions abou he reacor s performance This is a qualiaive reacion engineering problem Read hrough he problem saemen and idenify he ype(s) of reacor(s) being used he reacor operaing procedure being used (isohermal vs. adiabaic, seady sae vs. ransien, ec.) he ype of reacion(s) aking place (reversible/irreversible, ypical, auo-caalyic, produc inhibied, ec.) he quaniies whose variaion you are asked o describe (2) 8

9 Read hrough he problem saemen and idenify he ype(s) of reacor(s) being used: a bach reacor he reacor operaing procedure being used (isohermal vs. adiabaic, seady sae vs. ransien, ec.): isohermal, bach reacors are always ransien he ype of reacion(s) aking place (reversible/irreversible, ypical, auo-caalyic, produc inhibied, ec.): irreversible reacion he quaniies whose variaion you are asked o describe: wan variaions of r and fa Skech a plo of reacan concenraion(s), produc concenraions, emperaure, reacion rae and oher quaniies of ineres versus ime (for a bach reacor) or space ime (for a flow reacor) Draw ses of axes for he plos 9

10 Reacans Producs T Temperaure r Rae fb 1 onversion 0 10

11 11 Deermine he iniial values of each of hese quaniies (a he sar of he reacion or inle o he reacor) and add o he corresponding plo

12 12 Deermine he iniial values of each of hese quaniies (a he sar of he reacion or inle o he reacor) and add o he corresponding plo - since he reacor is isohermal, we don need o consider emperaure - in case (a), boh reacan concenraions are posiive and equal, boh reacan concenraions are zero, he rae is posiive and he conversion is zero. - in case (b), boh reacan concenraions are posiive wih A greaer han B, boh reacan concenraions are zero, he rae is posiive and he conversion is zero.

13 ase (a) Reacans Producs T Temperaure A or B Y or Z r Rae fb 1 onversion 0 13

14 ase (b) Reacans Producs T Temperaure A Y or Z B r Rae fb 1 onversion 0 14

15 15 Deermine he iniial slope of he plos of hese quaniies by considering he firs small incremen in ime (or space ime) and add o he corresponding plo - Do he reacan concenraions, produc concenraions and emperaure increase or decrease during his inerval? - Will hose changes cause he reacion rae o increase or decrease during his inerval? - Do he quaniies of ineres increase or decrease during his inerval? - Will hose changes cause he equilibrium conversion o increase or decrease during his inerval? - if comparing wo or more sysems, for each plo, deermine he which sysem will have he larges slope, he second larges slope, ec.

16 Deermine he iniial slope of he plos of hese quaniies by considering he firs small incremen in ime (or space ime) and add o he corresponding plo - Do he reacan concenraions, produc concenraions and emperaure increase or decrease during his inerval? in boh cases he reacan concenraions decrease and he produc concenraions increase - Will hose changes cause he reacion rae o increase or decrease during his inerval? only he reacan concenraion affec he rae In case (a) he rae will say he same because K is negligibly small and P A and PB will cancel In case (b) he rae will decrease because he denominaor will be ~ consan while he numeraor decreases - Do he quaniies of ineres increase or decrease during his inerval? in boh cases conversion will increase (iniial slope posiive) - Will hose changes cause he equilibrium conversion o increase or decrease during his inerval? since he emperaure is consan, he equilibrium conversion will no change during he reacion - if comparing wo or more sysems, for each plo, deermine he which sysem will have he larges slope, he second larges slope, ec. in case (a) he rae will approximaely equal k, in case (b) i will approximaely equal kpb/k we can ell which rae is larger, so we can deermine he relaive slopes for he 2 cases 16

17 ase (a) Reacans Producs T Temperaure A or B Y or Z r Rae fb 1 onversion 0 17

18 ase (b) Reacans Producs T Temperaure A Y or Z B r Rae fb 1 onversion 0 18

19 19 Deermine he curvaure of he plos by considering he nex small incremen in ime (or space ime) and add o he corresponding plo - Do he reacan concenraions, produc concenraions, emperaure and rae change by a greaer or lesser amoun han during he preceding inerval? - Do he quaniies of ineres change by a greaer or lesser amoun han during he preceding inerval?

20 Deermine he curvaure of he plos by considering he nex small incremen in ime (or space ime) and add o he corresponding plo - Do he reacan concenraions, produc concenraions, emperaure and rae change by a greaer or lesser amoun han during he preceding inerval? in case (a) he rae says he same (no curvaure) he reacan concenraions decrease by he same amoun (no curvaure) he produc concenraions increase by he same amoun (no curvaure) in case (b) he rae is smaller han in he previous inerval he reacan concenraions decrease by less (concave up) he reacan concenraions increase by less (concave down) since he reacan concenraions decrease by less, he rae will decrease by less (concave up) - Do he quaniies of ineres change by a greaer or lesser amoun han during he preceding inerval? in case (a) he conversion increases by he same amoun (no curvaure) in case (b) he conversion will increase by less (concave down) 20

21 ase (a) Reacans Producs T Temperaure A or B Y or Z r Rae fb 1 onversion 0 21

22 ase (b) Reacans Producs T Temperaure A Y or Z B r Rae fb 1 onversion 0 22

23 23 Deermine wheher coninuing he iniial rends will resul in he rae asympoically approaching equilibrium

24 Deermine wheher coninuing he iniial rends will resul in he rae asympoically approaching equilibrium - in case (a) he rend canno coninue, leading o equilibrium reacan concenraions will become zero conversion will become greaer han 1 rae will never equal zero - in case (b) he rend can coninue concenraion of B will go o zero concenraion of A will become consan conversion will become 1 rae will equal zero 24

25 ase (a) Reacans Producs T Temperaure A or B Y or Z r Rae fb 1 onversion 0 25

26 ase (b) Reacans Producs T Temperaure A Y or Z B r Rae fb onversion

27 Deermine wheher coninuing he iniial rends will resul in he rae asympoically approaching equilibrium - in case (a) he rend canno coninue, leading o equilibrium reacan concenraions will become zero conversion will become greaer han 1 rae will never equal zero - If no, infer wha mus happen so ha he sysem approaches equilibrium properly (i. e. so he rae progressively decreases o zero) and add o he corresponding plos 27

28 Deermine wheher coninuing he iniial rends will resul in he rae asympoically approaching equilibrium - in case (a) he rend canno coninue, leading o equilibrium reacan concenraions will become zero conversion will become greaer han 1 rae will never equal zero - If no, infer wha mus happen so ha he sysem approaches equilibrium properly (i. e. so he rae progressively decreases o zero) and add o he corresponding plos A some poin, he K in he denominaor will become significan so ha he numeraor decreases more han he denominaor and he rae will sar o decrease more and more (concave up) Evenually he P A in he denominaor will become negligible afer which he rae will decrease less and less (inflecion poin hen concave down) During his whole ime he rae is decreasing in each successive ime inerval; as a consequence he reacan concenraions will decrease by less and less (concave up) he produc concenraions will increase by less and less (concave down) he conversion will increase by less and less (concave down) These rends can coninue unil equilibrium is reached he rae will equal zero, he concenraions and conversion equal 1 28

29 ase (a) Reacans Producs T Temperaure A or B Y or Z r Rae fb onversion

30 If comparing wo or more sysems, deermine he relaive magniudes of he equilibrium concenraions and emperaures in order o ascerain wheher or no he curves for he sysems being compared cross each oher Use he plos o answer he quesions posed in he problem 30

31 Qualiaive Analysis wih Parallel Reacions Suppose irreversible, parallel reacions (1) and (2) ake place isohermally in a bach reacor wih kineics as indicaed. Prepare a hree slide presenaion ha describes and qualiaively jusifies approaches you migh employ in order o improve he seleciviy for D. A + S D! r = 10 e -10/T A S! (1) A + S U! r = 10 e -15/T A (S) 2! (2) 31

32 Qualiaive Analysis wih Parallel Reacions Suppose irreversible, parallel reacions (1) and (2) ake place isohermally in a bach reacor wih kineics as indicaed. Prepare a hree slide presenaion ha describes and qualiaively jusifies approaches you migh employ in order o improve he seleciviy for D. A + S D! r = 10 e -10/T A S! (1) A + S U! r = 10 e -15/T A (S) 2! (2) Insananeous seleciviy parameer: D is favored by - low emperaure - low S Low reacion emperaure will rade off rae of conversion versus seleciviy o D; use he lowes accepable emperaure Low iniial concenraion of S will rade off conversion and ime per bach versus seleciviy o D Oher approaches Find a caalys ha favors producion of D S D/U = r D r U If reacion is soluion phase, find a solven ha favors producion of D = 10e 10 T A S 10e 15 T A = e 2 S 5 T S 32

33 Par I - hemical Reacions Par II - hemical Reacion Kineics Where We re Going Par III - hemical Reacion Engineering A. Ideal Reacors B. Perfecly Mixed Bach Reacors Reacion Engineering of Bach Reacors Analysis of Bach Reacors Opimizaion of Bach Reacor Processes. oninuous Flow Sirred Tank Reacors D. Plug Flow Reacors E. Maching Reacors o Reacions Par IV - Non-Ideal Reacions and Reacors 33