Heterogeneous Bunsen Reaction nalysis & Experimental study of Chemical absorption of Sulfur dioxide and dissolution of Iodine into aqueous reacting system C. S. R. Prasad, H. Z. Fani, S. B. Menon Chemical Technology Division, ChTG Bhabha tomic Research Centre, Trombay Mumbai 400 085, India
Thermo Chemical Process for H 2 Production HIGH TEMPERTURE NUCLER RECTOR Heat THERMO CHEMICL PROCESS OXYGEN HYDROGEN WTER
Iodine-Sulfur (I-S) Process Reaction Scheme 3 SECTIONS HET Sulfuric acid Decomposition H 2 SO 4 850 o C H 2 O + SO 2 + ½ O 2 H 2 SO 4 HET H 2 O + SO 2 + ½ O 2 cid Production (Bunsen Reaction) OXYGEN 2HI + H 2 SO 4 120 o C I 2 + SO 2 + 2H 2 O WTER 2HI HET I 2 HI Decomposition 2HI 450 o C H 2 + I 2 HYDROGEN
Heterogeneous Bunsen Reaction is the focus of this study Objective is to derive crucial engineering information. Overall reaction rate B. Reaction regimes C. Controlling kinetic/mass transfer resistances D. Coefficients E. Enhancement factor This information helps in chemical reactor selection, flow/contacting scheme choice and design
Region of experimental study Data points of reference SO 2 cc(273.15 K, 101 kpa) - Region Transparent one phase Product solution Experimental points of this study I 2 conc. g/5g of H 2 O bsorption limit of SO 2 by the Bunsen reaction vs. the initial iodine fraction in the absorbent Ref: KORU ONUKI, et at., IS Process For thermo chemical Hydrogen Production JERI - Review 94 006
Sketch of experimental setup
Bunsen reaction stoichiometric : SO 2 + I 2 + 2H 2 O 2HI + H 2 SO 4 with excess of I 2 & H 2 O : SO 2 + xi 2 + y H 2 O 2HI + H 2 SO 4 + (x-1) I 2 + (y-2) H 2 O is represented as follows for analysis SO 2, + I 2, B + H 2 O,q (H 2 SO 4 +HI),Products (g) (aq) B(s) B(aq) (aq) + zb (aq) Products
This multiphase process involving mass transfer and chemical reaction has following steps 1) Diffusion of SO 2 (species ) through gas film 2) Dissolution of Iodine (species B) 3) Diffusion and simultaneous chemical reaction in the liquid film
Concentration profiles based on film theory for gas (sulphur dioxide) liquid (water) solid (Iodine) system Liquid (H 2 O) C B,l Solid (I 2 ) C B,s Gas (SO 2 ) C B,i p,g p,i C,i Reaction zone Fast (pseudo first order) reaction in film with high c B 0 δ Gas film Liquid film Bulk Liquid Liquid film surrounding solid
Gas phase mass transfer rate is given by, r r = k g a = 1 ( + k a g ( pg pi Rate of iodine dissolution is given by, r B Liquid phase reaction rate is given by, r = k k s = 1 c a = r z p H k ae l B g + ) p ( cbs cbl The overall rate of reaction is given by, ) H k (1 1 ε ) ) Gas film resistance Liquid film resistance kinetic resistance Mass balances for the diffusing gas and dissolving solid species B in the liquid film are as follows: d c dx 2 D = k 2 1 c 2 d c dx B DB = zk 2 1 c
Reactor model schematic for analysis G c p out V r volume of G/L emulsion = l V (Batch of Reacting liquid) ( 1 ε ) G C p in t any instant c B is same everywhere in the tank. However c B decreases with time because of reaction with (Yet much more than stoichiometric requirement during most of the batch time). t the start c B =c Bo t the end c B =c Bf Mixed gas and completely mixed liquid.
E=Liquid film enhancement factor = Rate of uptake of with chemical reaction Rate of uptake of for straight mass transfer Enhancement factor for infinitely fast reaction is, DBcBH E i = 1+ zd p in out G C = ( r ) P pin P pout Vl Where, Vr = and ( 1 ε ) Batch time for the conversion of Iodine is calculated by, t = c c p Bo Bf Hatta no is, H a = k1d k l i ( 1 ε ) dc r z p B V r H = By overall mass balance across the reactor, Henry s Law const is, p c
SO 2 absorption rate vs. SO 2 partial pressure for different Iodine loading bsorption rate mol/m 3.sec SO 2 Inlet partial pressure kpa
Batch time calculated vs. Batch time experimental
Typical parameters and values for Bunsen Reaction analysis Batch time (calculated) (min) Batch time (experimental) (min) Gas film resistance (Pa m 3 s/kmol) Liquid film resistance (Pa m 3 s/kmol) Liquid bulk resistance (Pa m 3 s/kmol) Hatta no Enhancement factor (calculated) Enhancement factor (experimental) Solid dissolution parameter t calc t exp 1/(k g a) H/(k l ae) H/(k 1 (1-ε)) Ha E E (k s a p D 2 )/(4k l D B ) 34 30 1.1x10 6 1.0x10 7 7.5x10 2 3.4 2.1 2.2 1.6x10-6
Conclusions 1) SO 2 absorption rate in chemically reacting system of Bunsen Reaction is experimentally studied and found to be a linear function of partial pressure (0-100 kpa) of SO 2 in inlet gas stream at atmospheric pressure. This functional relation is expected to hold good even under prototypical conditions of Bunsen Reaction 2) Multiphase Bunsen Reaction can be viewed as Fast pseudo first order due to high concentration/rate of dissolution of Iodine and reaction zone is located in Liquid film near Gas-Liquid interface 3) Liquid film resistance constitutes ~90% of overall resistance 4) Experimental and theoretical results of this study indicated that this complex reacting system can be analyzed by invoking judicious simplifying assumptions for deriving practical engineering information 5) Rigorous model requires accurate thermodynamic, transport and physical properties 6) This study helps in selection/design of multiphase chemical reactor under prototypical conditions
cknowledgements uthors are grateful to DE, Government of India, Director BRC, Director ChTG for support and encouragement Help rendered by Head, Chemical Technology Division, Head, Chemistry Division, Head Chemical Engineering Division and colleagues in respective divisions is gratefully acknowledged ll members of ChTG who worked very hard for this R&D are specially thanked uthors also express their thanks to IE and JE for the opportunity of participation in this conference
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