- Height equivalent to a theoretical plate (HETP) -Operating line based on material balance -Two-film theory - Determination of column packed height

Transcription

1 Lecture 10. Absorption and Stripping (2) [Ch. 6] Rate-Based BasedMethodforPackedCoumns - Height equivaent to a theoretica pate (HETP) -Operating ine based on materia baance -Two-fim theor - Determination of coumn packed height - Reation between TU, HTU & t, HETP

2 Packed Coumns Packed coumn : a vesse containing one or more sections of packing - Liquid fows downward as a fim or as dropets between packing eements - Vapor fows upward through the wetted packing, contacting the iquid Packed coumns are continuous, differentia-contacting t devices : do not have phsica distinguishabe, discrete stages better anazed b mass-transfer modes than b equiibrium-stage concepts

3 Theoretica Pate n practice, packed-tower performance is often presented on the basis of equivaent equiibrium stages Height equivaent to a theoretica pate (HETP) packed height T HETP number of equivaent equiibrium stages HETP concept has no theoretica basis : HETP is difficut to generaize and is a function of the tpe of packing, the sstem being separated and the hdrauics of the coumn Eperimenta or vendor-suppied vaues are used [Eampe] Bioethano recover from a CO 2 -rich vapor umber of equiibrium pate : 6.1 (from equiibrium-stage method) 1.5-inch meta Pa rings are used for random packings : HETP = 2.25 ft T = HETP ft t t

4 Operating Line Absorber Stripper L in, in V out, out L in, in V out, out L V L V V in, in L out, out V in, in L out, out Materia baance L V L V in in out out L V L V in in out out Assuming diute soutions, V V in V out V and L L in L out L Operating ine L L out L L in in out V V V V

5 Two-Fim Theor (1) A concentration gradient eists in each fim Phsica equiibrium at the interface Voumetric mass transfer coefficient, i ka is common used kg mo ka 3 s m packing mo fraction a : mass transfer area per unit voume of packed bed Buk gas phase composition or p c Gas nterface or c or p Liquid Buk iquid phase composition or c p

6 Two-Fim Theor (2) At stead state absorption, without chemica reactions, (rate of soute mass transfer across gas fim) = (rate of soute mass transfer across iquid fim) r k a( ) k a( ) ( ) ka ( ) ka Mass transfer resistance in gas phase is ver ow : Liquid-fim controing process Mass transfer resistance in iquid id phase is ver ow : Gas-fim controing process Gas-phase driving force Liquid-phase driving force (, ) A F C (, ) E D (, ) B (, ) Rate of mass transfer can be increased b promoting turbuence and/or increasing dispersion

7 Two-Fim Theor (3) Mass transfer in terms of overa driving force r K a( ) K a( ) : fictitious vapor moe fraction in equiibrium with in buk iquid : fictitious iquid moe fraction in equiibrium with in buk vapor Ka ka ka Ka ka ka 1 K a 1 K a 1 k a 1 k a K k a 1 Kk a ED K BE CF 1 FB K For diute soutions when the equiibrium curve is a near straight ine

8 Rate-Based Method (1) Determination of coumn packed height : common invoves overa gas-phase coefficient, i K a Liquid has a strong affinit for the soute (mass transfer resistance is most in the gas phase) Differentia materia baance for a soute Vd K a( ) Sd V out out L in in Cross section, S K as T K ast in d d 0 V V out V L d T Packed height T V in d KaS out V in in +d V +d L L out out

9 Rate-Based Method (2) B Chiton and Coburn, T HOGOG V HOG Overa height ht of a (gas) transfer unit, HTU KaS OG in out d ntegration of TU in d out OG in out Overa number of (gas) transfer units, TU d (1 KV / L) ( KV / L) K out n{[( A 1) / A][( in Kin ) /( out Kin )] (1/ A)} ( A1)/ A in K L L out in V V L /( KV ) A

10 Reation between TU, HTU & t, HETP When the operating and equiibrium ines are straight and parae HETP OG HOG t When the operating and equiibrium i ines are straight but not parae TU t HETP OG t H OG n(1/ A) (1 A) / A n(1/ A) (1 A) / A TU t