Pelagia Research Library. Acetaldehyde combustion using platinum catalyst

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1 Available online at Pelaia Reearch Library Der Chemica Sinica, 11, (1): Acetaldehyde combution uin platinum catalyt Sanchita Chauhan ISSN: CODEN (USA) CSHIA5 Univerity Intitute of Chemical Enineerin and echnoloy, Panjab Univerity, Chandiarh, INDIA ABSRAC Catalytic Converter are extenively ued for reduction of the pollutant releaed from variou ource to the atmophere. hee converter become effective in minimiin the pollutant concentration only after they achieve their operatin temperature. Durin thi cold tart period hydrocarbon are releaed untreated to the atmophere. hi paper deal with the oxidation of hydrocarbon acetaldehyde, a VOC durin the cold tart period. Mathematical modelin i a ood way of analyin the converter behaviour without performin very expenive and time conumin experiment. A one-dimenional computer model that take into account the ma and the heat tranfer between the a phae and the catalyt i conidered. he equation form a et of ordinary and partial differential equation, which are non-linear and coupled. hee equation are olved uin Runa kutta and Backward Implicit cheme. Keyword: Acetaldehyde, modelin, converter, quai teady tate, catalyt loadin. INRODUCION VOC are important air pollutant in urban atmophere. hey are defined a oranic compound with hih vapor preure that are eaily vaporized under an ambient temperature and preure condition [1]. hey are releaed motly from vehicular exhaut and indutrial procee. Ethanol-fueled paener car are found to produce hih emiion of aldehyde in comparion to aoline-fueled car []. Ue of catalytic converter ha proved to be a very ood way of reducin emiion from the vehicular exhaut. Modelin of converter a compared to experimental tetin i not only inexpenive but alo time avin [3]. he monolith behaviour durin warm up period can be adequately predicted by uin a one-dimenional model [4]. Pelaia Reearch Library 164

2 Sanchita Chauhan Der Chemica Sinica, 11, (1): In the preent paper modelin for VOC acetaldehyde i conidered in a ceramic cordierite converter. A one dimenional model i ued to imulate the thermal and converion characteritic of the converter operatin under warm up condition. he chane in acetaldehyde concentration due to catalytic oxidation reaction i analyed with repect to operatin temperature and the effect of catalyt loadin on reducin cold tart period i analyed. MAHEMAICAL MODEL In one-dimenional modelin only axial radient are conidered for a concentration, a temperature and catalyt temperature. Aumption made durin modelin include [5]: An irreverible oxidation reaction of VOC acetaldehyde i conidered to be takin place: CH 3 CHO + O CO + H O (1) Catalytic reaction are aumed to be firt order in fuel concentration and zero order with repect to oxyen concentration under lean condition i repreented by [1]: r C = k C () ( E / R ) ( )( ), exp where pre-exponential factor k and activation enery E are taken a m/ and J/mol. A cylindrical channel i conidered in which the catalytic reaction take place. Wahcoat i aumed to be very thin (1µm in thickne) and diffuion in wah coat i nelected. Accumulation of ma and enery in the a phae i neliible. A compared to convective tranport, the axial diffuion of ma and heat tranfer in a phae i very mall, therefore nelected. Noble metal concentration i kept contant alon the lenth of the converter. Catalyt doe not deactivate. Monolith i aumed to be adiabatic. Heat tranfer by radiation within the channel i nelected. Heat exchane between the ubtrate and the urroundin at both the inlet and the outlet face of the monolith i nelected. For a quai teady tate model the time derivative term in a phae ma and enery balance are nelected. C i.e., = (3) t and = (4) t Pelaia Reearch Library 165

3 Sanchita Chauhan Der Chemica Sinica, 11, (1): Ma and enery balance equation for a phae: C v + ks( C C ) = (5) x vρ Cp hs( ) = (6) x Ma and enery balance equation for olid phae: ( r)( C ) = k S( C C ) a, (7) ( )( )( ) ( ) ρ = + + Cp a H r C, hs λ (8) t x Initial and boundary condition: Enterin acetaldehyde concentration at all time (, t) C C = (9) Enterin a temperature at all time (, t) = (1) Solid catalyt temperature at the tart ( x ) = (11), Lain olid catalyt at entry and exit x =, = (1) x x = L, = (13) x he overnin Eq. (5) to (13) are made dimenionle by uin the followin dimenionle variable: C C =, C =, =, z = x L, t t = (14) t Pelaia Reearch Library 166

4 Sanchita Chauhan Der Chemica Sinica, 11, (1): Dimenionle ma and enery balance equation for a and olid phae: C ( E R = C e / ) 1 z γ (15) = α ( ) (16) z z ( E / R ) ( ) = α + + 3C e α 33 δ 3 (17) t where, γ 1, α, α 3, α 33 and δ 3 are dimenionle number a hown in able 1. Dimenionle initial and boundary condition: (, t ) 1. NUMERICAL SCHEME C = (18) (, t ) = (19) ( z,) = () at z =, = (1) z at z = 1., = () z he dimenionle Eq. (15) and (16) are ODE and Eq. (17) i a PDE. A thee equation are coupled; hence they are olved at the ame time. he ODE do not have a time derivative term but a thee equation are coupled with the PDE, the time derivative preent in the later et introduced in the iterated reult for the ODE. he ODE (15) and (16) are olved uin Rune- Kutta method of fourth order and the PDE (17) by uin backward implicit cheme [6]. Pelaia Reearch Library 167

5 Sanchita Chauhan Der Chemica Sinica, 11, (1): able 1: Mathematical expreion for the dimenionle number Dimenionle number γ 1 α α 3 ( ) α 33 δ 3 Mathematical expreion Lak v SLh v( ρcp ) al ( H ) k C λ ShL λ ( ρcp ) L λ t RESULS AND DISCUSSION At the tart of the operation the converter i at 5 C and exhaut a enterin the converter at 4 C heat the converter. he catalytic reaction tart once the converter ha attained the operatin temperature. Due to the reaction a decreae in the concentration of acetaldehyde i oberved. he inlet concentration of acetaldehyde (1ppm) ha a dimenionle value of 1. Reult a hown in Fiure1 repreent the onet of catalytic reaction for acetaldehyde and Fiure how the variation in exit concentration of acetaldehyde with time baed on converter catalyt loadin. Fiure 1. Effect of olid catalyt temperature on converion of acetaldehyde Fiure 1 how the effect of olid temperature on the converion of acetaldehyde. Initially when the converter i at 5 C, there i neliible converion of acetaldehyde. On heatin the converter by the incomin hot a at 4 C the converter finally reache it operatin temperature. he reult how that for the olid temperature of about C converion of acetaldehyde i only Pelaia Reearch Library 168

6 Sanchita Chauhan Der Chemica Sinica, 11, (1): However beyond thi temperature, a noticeable converion of acetaldehyde take place. About 5% and 8% converion are brouht about at catalyt temperature of 7 C and 36 C repectively. he reult indicate that after the operatin temperature i achieved the catalytic reaction bein and the heat releaed by the reaction further increae the olid catalyt temperature. hi brin about a fater converion of acetaldehyde. Fiure. Variation of exit concentration of acetaldehyde with time for different catalyt loadin Fiure. how the effect of catalyt loadin on the converion of acetaldehyde. hree different cae are analyed with catalyt loadin 68 cm Pt/cm 3, 536 cm Pt/cm 3 and 17 cm Pt/cm 3. At dimenionle time 3.4 the exit concentration of acetaldehyde are.96,.81 and.6744 for catalyt loadin 68 cm Pt/cm 3, 536 cm Pt/cm 3 and 17 cm Pt/cm 3. More deviation i oberved in the exit concentration a time increae. At dimenionle time 7.5 the exit concentration of acetaldehyde are.515,.515 and.633 for catalyt loadin 68 cm Pt/cm 3, 536 cm Pt/cm 3 and 17 cm Pt/cm 3. he reult how that hiher the catalyt loadin fater i the converion of acetaldehyde. CONCLUSION At ambient condition the oxidation of acetaldehyde i not appreciable durin the cold tart period of the converter, due to low initial temperature of the converter. So ome time i required to preheat the converter to it operatin temperature after which the catalytic reaction et initiated and brin about converion of acetaldehyde. Freh catalyt with relatively hiher catalyt loadin brin about a fater converion of acetaldehyde a compared to in ue catalyt with lower value of catalyt loadin. NOMENCLAURE a C C p E catalytic urface area per unit reactor volume (cm /cm 3 ) concentration of the pecie (mole/cm 3 ) pecific heat (J/ K) activation enery for catalytic reaction (J/mole) Pelaia Reearch Library 169

7 Sanchita Chauhan Der Chemica Sinica, 11, (1): h - H k k L R S t v x z λ ρ heat tranfer coefficient (J/cm K) heat of reaction (J/mole) ma tranfer coefficient (cm/) rate contant for catalytic reaction (cm/) lenth of monolith (cm) a contant (J/mole K) eometric urface area per unit reactor volume (cm /cm 3 ) temperature (K) time () a velocity (cm/) axial coordinate (cm) dimenionle axial coordinate thermal conductivity (J/cm K) denity (/cm 3 ) Subcript and Supercript olid a dimenionle value initial condition Acknowledement he upport of Prof. V. K. Srivatava (Academy of Buine & En. Science, Vijay Naar, Ghaziabad) for hi invaluable input and advice i ratefully acknowleded. REFERENCES [1] Wan P, ran DV, Sakurai VM, Kameyama H, Applied Catalyi A: General, 8, 35, 15. [] R.W. McCabe, P.J. Mitchell, Ind En Chem Prod Re Dev, 1983,, 1. [3] P.M. Lian, M.D. Shane, S. Son, A.A. Adamczyk, P. Li, SAE Paper , 1999, 136. [4] R.H. Heck, J. Wei, J.R. Katzer, AIChE J, 1976,, 477. [5] S. Chauhan, V.K. Srivatava, Computer and Mathematic with Application, 8, 55, 319. [6] S. Chauhan, V.K. Srivatava, Chemical Product and Proce Modelin, 8, 3, Article 1, 1. Pelaia Reearch Library 17