Chapter 5 Absorption and Stripping

Transcription

1 Chaper 5 sorpion and Sripping 5.1 Inroducion In asorpion (also called gas asorpion, gas scruing, or gas washing), here is a ransfer of one or more species from he gas phase o a liquid solven. The species ransferred o he liquid phase are referred o as solues or asorae. sorpion involves no change in he chemical species presen in he sysem. sorpion is used o separae gas mixures, remove impuriies, or recover valuale chemicals. The operaion of removing he asored solue from he solven is called sripping. sorers are normally used wih srippers o permi regeneraion (or recovery) and recycling of he asoren. Since sripping is no perfec, asoren recycled o he asorer conains species presen in he vapor enering he asorer. When waer is used as he asoren, i is normally separaed from he solue y disillaion raher han sripping. Exi gas o 25 C, 90 kpa Liquid asoren o 25 C, kpa kmol/h Waer kmol/h rgon 6.9 O N Waer 22.0 ceone 0.05 Feed gas o 25 C, kpa kmol/h rgon 6.9 O N Waer 5.0 ceone Exi liquid o 22 C, kpa kmol/h O N Waer 1,926.0 ceone Figure Typical asorpion process. ypical indusrial operaion for an asorpion process is shown in Figure The feed, which conains air (21% O 2, 78% N 2, and 1% r), waer vapor, and aceone vapor, is he gas 1 J. D. Seader and E. J. Henley, Separaion Process Principles,, Wiley, 2006, pg

2 leaving a dryer where solid cellulose aceae fiers, we wih waer and aceone, are dried. ceone is removed y a 30-ray asorer using waer as he asoren. The percenage of aceone removed from he air sream is % lhough he major componen asored y waer is aceone, here are also small amouns of oxygen and nirogen asored y he waer. Waer is also sripped since more waer appears in he exi gas han in he feed gas. The emperaure of he exi liquid decreases y 3 o C o supply he energy of vaporizaion needed o srip he waer. This energy is greaer han he energy of condensaion lieraed from he asorpion of aceone. Three approaches have generally een employed o develop equaions used o predic he performance of asorers and asorpion equipmen: mass ransfer coefficiens, graphical soluion, and asorpion facor. The use of mass ransfer coefficien is covered in Chaper 2.2. The graphical soluion is simple o use for one or wo componens and provides explici graphical presenaion of he inerrelaionships of he variales and parameers in an asorpion process. However he graphical echnique ecomes very edious when several solues are presen and mus e considered. The asorpion facor approach can e uilized eiher for hand or compuer calculaion. sorpion and sripping are conduced mainly in packed columns and plae columns (rayed ower) as shown in Figure Packed column 2 Plae column 3 Figure Equipmen for asorpion and sripping. 2 (ug ) 3 hp:// (ug ) 5-2

3 5.2 Single-Componen sorpion Mos asorpion or sripping operaions are carried ou in couner curren flow processes, in which he gas flow is inroduced in he oom of he column and he liquid solven is inroduced in he op of he column. The mahemaical analysis for oh he packed and plaed columns is very similar. L x V y L V L V x y Figure Counercurren asorpion process. The overall maerial alance for a counercurren asorpion process is L + V L + V (5.2-1) where V vapor flow rae L liquid flow rae, op and oom of ower, respecively The componen maerial alance for species is L x + V y L x + V y (5.2-2) where y mole fracion of in he vapor phase x mole fracion of in he liquid phase For some prolems, he use of solue-free asis can simplify he expressions. The solue-free concenraions are defined as: x 1 x mole fracion of in he liquid mole fracion of non- componens in he liquid (5.2-3a) 1 y y mole fracion of in he vapor mole fracion of non- componens in he vapor (5.2-3) If he carrier gas is compleely insolule in he solven and he solven is compleely nonvolaile, he carrier gas and solven raes remain consan hroughou he asorer. Using 5-3

4 L o denoe he flow rae of he nonvolaile and V o denoe he carrier gas flow rae, he maerial alance for solue ecomes L + V L + V (5.2-4) or L V + L V (5.2-5) The maerial alance for solue can e applied o any par of he column. For example, he maerial alance for he op par of he column is L V L + V (5.2-6) In his equaion, and are he mole raios of in he liquid and vapor phase, respecively, a any locaion in he column including a he wo erminals. Equaion (5.2-6) is L called he operaion line and is a sraigh line wih slope V when ploed on - coordinaes. The equilirium relaion is frequenly given in erms of he Henry s law consan which can e expressed in many differen ways: P HC mx Kx (5.2-7) In his equaion, P is he parial pressure of species over he soluion and C is he molar concenraion wih unis of mole/volume. The Henry s law consan H and m have unis of pressure/molar concenraion and pressure/mole fracion, respecively. K is he equilirium consan or vapor-liquid equilirium raio. Tale lis Henry s law consan m for various gases in waer. Tale Henry s Law consan for Gases in waer 4 (m 10-4 am/mole fracion) T( o C) CO 2 CO C 2 H 6 C 2 H 4 He H 2 H 2 S CH 4 N 2 O Example solue is o e recovered from an iner carrier gas B y asorpion ino a solven. The gas enering ino he asorer flows a a rae of 500 kmol/h wih y 0.3 and leaving he asorer wih y Solven eners he asorer a he rae of of 1500 kmol/h wih x 4 Geankoplis, C.J., Transpor Processes and Separaion Process Principles, 4 h ediion, Prenice Hall, 2003, pg Hines,. L. and Maddox R. N., Mass Transfer: Fundamenals and pplicaions, Prenice Hall, 1985, pg

5 The equilirium relaionship is y 2.8 x. The carrier gas may e considered insolule in he solven and he solven may e considered nonvolaile. Consruc he x-y plos for he equilirium and operaing lines using oh mole fracion and solue-free coordinaes. Soluion The flow raes of he solven and carrier gas are given y L L (1 x ) 1500( ) kmol/h V V (1 y ) 500(1 0.3) 350 kmol/h The concenraion of in he solven sream leaving he asorer can e deermined from he following expressions: x Moles in L Moles in L + L Moles of in L Moles of in L + Moles of in V Moles of in V Moles of in L Moles of in V y Moles in V Moles in + Moles in V Moles in V V 0.01 V Moles of in V /(1 0.01) kmol/h Moles of in L kmol/h x Moles in L Moles in + L L For he solue free asis: x 1 x y 1 y, x x x x

6 y y y y The equilirium curves in oh mole fracion and solue-free coordinaes are calculaed from he following procedures: 1) Choose a value of x eween and 0.10 x 2) Evaluae he corresponding 1 x 3) Evaluae y 2.8 x y 4) Evaluae he corresponding 1 y The operaing lines in oh mole fracion and solue-free coordinaes are calculaed from he following procedures: 1) Choose a value of x eween and x 2) Evaluae he corresponding 1 x 3) Evaluae L L + V V 4) Evaluae he corresponding y 1 + The following Mala codes plo he equilirium and operaing lines in oh mole fracion and solue-free coordinaes % Example xelinspace(0.001,0.1); ye2.8*xe; exe./(1-xe);eye./(1-ye); xlinspace(0.001,0.0898); Lar1498.5;Var350; x./(1-x);.0898;0.4286; LoVLar/Var; LoV*+-LoV*; y./(1+); plo(xe,ye,x,y,'--') legend('equilirium curve','operaing line',2) xlael('x');ylael('y') Tile('Equilirium and Operaing lines on mole fracion coordinaes') figure(2) plo(e,e,,,'--') legend('equilirium curve','operaing line',2) 5-6

7 xlael('');ylael('') Tile('Equilirium and Operaing lines on solue free coordinaes') The equilirium relaion in he mole fracion coordinaes is a sraigh line while he operaing line in he solue-free coordinaes is a sraigh line. Normally he equilirium relaion is no a sraigh line in he mole fracion coordinaes. Therefore i is advanage o use solue-free coordinaes ecause he operaing line will always e sraigh. 5-7

8 () (B) (C) c Figure Limiing condiions for asorpion process. c The driving force for mass ransfer ecomes zero whenever he operaing line inersecs or ouches he equilirium curve. This limiing condiion represens he minimum solven rae o recover a specified quaniy of solue or he solven rae required o remove he maximum amoun of solue. In Figure he inersecion of he equilirium and operaing lines occurs a he oom of he asorer. This condiion defines he minimum solven rae o recover a specified quaniy of solue. This minimum solven rae can e calculaed from he following expression: L V min (5.2-8a) In Figure 5.2-2B, he inersecion of he equilirium and operaing lines occurs a he op of he asorer. This condiion represens he solven rae required o remove he maximum amoun of solue. This solven rae can e calculaed from he following expression: L V max (5.2-8) Equaion (5.2-8) is exacly he same as Eq. (5.2-8a) excep in his case he oom composiions are fixed so ha he maximum slope of he operaing line occurs when he operaing line inersecs he equilirium curve a he op of he column. Figure 5.2-2C shows he case when he operaing line ecomes angen o he equilirium curve. The minimum liquid-o-vapor raio for his case can e deermined from L V min c c (5.2-9) In his equaion, c and c are he coordinaes of he angen poin. 5-8