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1 Moleules 7, 1, 9-9 Full Pape moleules ISSN Modeling of Aetylene Pyolysis unde Steel Vauum Cabuizing Conditions in a Tubula Flow Reato Rafi Ullah Khan *, Siegfied Bajoh, Fank Gaf and Raine Reimet Engle-Bunte-Institut, Beeih Gas, Edöl und Kohle, Engle Bunte Ring 1, Univesität Kalsuhe, D-711 Kalsuhe, Gemany; s: siegfied.bajoh@iw.uni-kalsuhe.de, fank.gaf@iw.uni-kalsuhe.de, aine.eimet@iw.uni-kalsuhe.de * Autho to whom oespondene should be addessed. afi_ullah.khan@iw.uni-kalsuhe.de Pape pesented at ECSOC-1, 1- Novembe Reeived: Febuay 7; in evised fom: 1 Mah 7 / Aepted: 1 Mah 7 / Published: Mah 7 Abstat: In the pesent wok, the pyolysis of aetylene was studied unde steel vauum abuizing onditions in a tubula flow eato. The pyolysis tempeatue anged fom C to 1 C. The patial pessue of aetylene in the feed mixtue was 1 and mba, espetively, while the est of the mixtue onsisted of nitogen. The total pessue of the mixtue was 1. ba. A kineti mehanism whih onsists of seven speies and nine eations has been used in the ommeial omputational fluid dynamis (CFD) softwae Fluent. The speies tanspot and eation model of Fluent was used in the simulations. A ompaison of simulated and expeimental esults is pesented in this pape Keywods: Aetylene; Pyolysis; Modeling; Simulation; Cabuizing Intodution Amongst the many appliations of aetylene, low pessue abuizing of steel is an up and oming one. The pyolysis of aetylene has been studied by many eseahes unde vaious onditions, depending upon the appliation being onsideed. Detailed kineti mehanisms poposed in liteatue

2 Moleules 7, 1 91 may onsist of hundeds of speies and eations. Typially, these investigations inlude shok tube pyolysis of aetylene [1 1] and pyolysis in flow systems [1 1]. The deposition of pyolyti abon fom vaious hydoabons has also been desibed by some eseahes [1 ]. Although the detailed kineti mehanisms ae onsideed to be moe auate and eliable, thei use is mostly limited to ideal flow models. Use of suh detailed kineti mehanisms is not easy with tanspot models in CFD softwaes with uent omputational hadwae and algoithms. Nevetheless, CFD softwaes nomally implement Navie-Stokes equations and ae used fo modeling the flow field, even when the geomety is omplex. Opeational kineti mehanisms o edued vesions of detailed mehanisms an be used with CFD softwaes fo modeling the eating flow. Vauum abuizing with aetylene involves the pyolysis of aetylene unde low pessue, whih podues solid pyolyti abon and soot along with othe hydoabon gases, making the flow field moe omplex. Diffusion of abon into the sufae of steel takes plae, whih is ontinued until the desied abon ontent and ase depth ae ahieved. In the pesent wok, the pyolysis of aetylene was studied unde suh onditions suitable fo vauum abuizing of steel. Results and Disussion The measued pyolysis poduts inlude CH, C H, C H, C H, C H.. The amount of abon C (s) in the fom of soot was alulated fom a abon mass balane of the measued gas phase poduts. The podut yields have been plotted as a funtion of tempeatue. The expeimental esults ae ompaed with the simulation esults of Fluent vesion.. The amount of hydogen was also alulated by a mateial balane and is also ompaed with the simulation esults. The tempeatue vaies along the eato length. Figue 1 shows the tempeatue pofiles in the eato at thee diffeent ontolle tempeatues values. Figue 1. Tempeatue pofiles at seleted ontolle tempeatues T.

3 Moleules 7, 1 9 Figue. Mole fation ontous of C H at 9 C and mba patial pessue of aetylene. Figue. Mole fations ontous of C (s) at 9 C and mba patial pessue of aetylene. Figue. Compaison of simulated vs expeimental esults at 1 mba aetylene patial pessue. CH sim soot sim 1 CH exp 1 soot exp 9 1 % Yield based on abon % Yield based on abon % Yield based on abon CH sim CH exp. CH sim CH exp. CH sim CH-exp CH sim. CH exp volume % 1 1 H sim H exp

4 Moleules 7, 1 9 Figues and show two of the typial mole fation ontous obtained fom simulations fo the two impotant speies C H and C(s) at 9 C and mba patial pessue of aetylene in the eato. Figue shows the ompaison of expeimental and simulation esults fo aetylene at 1 mba patial pessue fo a ontolle tempeatue vaiation of C to 1 C. The abon ontent aied by unonveted aetylene in the mixtue deeases fom 99 % at C to 7 % at 1 C fo mba, epesenting a % onvesion of aetylene to othe poduts at the outlet, as shown in Figue. The seond majo and impotant omponent aying abon among the pyolysis poduts is the soot, fo whih esults ae shown fo 1 mba as well as fo mba patial pessue of aetylene. The peentage of abon C (s) in the fom of soot ineases with an inease in tempeatue. The fomation of C H and C H ineases up to a tempeatue of 9 C and then gadually deeases at highe tempeatues. CH and C H ae also fomed, but the abon ontent in these ompounds is less than 1 % unde these expeimental onditions. Figue. Compaison of simulated vs expeimental esults at mba aetylene patial pessue. % Yield based on abon CH sim CH exp % Yield based on abon soot sim soot exp CH sim CH exp CH sim CH exp 1 sim H exp H % Yield based on abon CH sim CH exp CH sim CH-exp volume % Reation Mehanism and CFD Model The eation mehanism shown in Table 1 onsists of seven speies whih ae the majo poduts of aetylene pyolysis unde vauum steel abuizing onditions. These inlude C (s), whih epesents the solid abon in the fom of soot, and hydoabons, onsisting of CH, C H, C H, C H, and C H, along with H. The oveall mehanism onsists of nine eations. The estimated Ahenius

5 Moleules 7, 1 9 paametes, ativation enegies and poposed eation ates ae also shown in Table 1. A -D gid onsisting of x ells ( ells in total) has been used to epesent a eato length of mm with a diamete of mm. The speies tanspot and eation model in Fluent was used fo modeling the hemisty. The mehanism disussed above was implemented though a use defined funtion (UDF) in Fluent. The opeating pessue was set equal to 1. ba while inlet tempeatue and veloity bounday onditions wee used aoding to the expeimental measuements. Table 1. Poposed eation mehanism and kineti paametes of aetylene pyolysis. Rate onstant k = A i i e EA,i RT No. Reation Rate Expession A i (mol, m, se) E A, i (kj/mol). C H + H C = k C H C H CH + H CH + H CH CH CH + H CH C s + H ( ) CH + CH CH 7 C H CH + CH CH + CH CH 9 CH C( s ) + H 1 1 H H. = k C H = k C H. 1 H.1 = k CH. 1 k CH = H 1. = k C H. 1.7 = k7 C H = k C H C. 1 H 9 k = 9 C H H As the eato is not opeated unde isothemal onditions, a tempeatue pofile was neessay to model the tempeatue field. A mathematial fit in the fom of a polynomial shown in equation (1) below was used to desibe the tempeatue pofile in the simulations: T ( T, x) = ( a x + b x + ) T + d x + e x + f (1) whee T(T,x) epesents the tempeatue as a funtion of the ontolle tempeatue T and the position x along the eato length, wheeas a, b,, d, e and f ae the polynomial oeffiients obtained by least squaes fit to the expeimental esults. This tempeatue pofile was also implemented though a use defined funtion (UDF) and ompiled befoe loading into Fluent using the default poedues in the softwae. A typial tempeatue pofile implemented in Fluent is shown in Figue 1. The solution was onveged to speies esiduals of 1 - o less so that thee was no futhe vaiation of these esiduals. The onvegene was fast and ahieved in less than iteations. Conlusions Aetylene pyolysis has been modeled by using simplified hemisty with a tanspot model unde the patiula onditions of the industial poess of vauum abuizing of steel. Compaison of simulated and expeimental esults shows satisfatoy ageement unde tehnial opeating onditions.

6 Moleules 7, 1 9 Futhe investigations fo omplex geometies ae neessay to pove the validity of the model fo the appliation as a pedition tool to ontol the abuizing poess in ommeial plants. Expeimental A eami eato with an inne diamete of mm, oute diamete of mm and a length of mm was used in the pesent investigation. Thee is a eami filte at the outlet of the eato whih an sepaate entained solid abon fom the gas steam. The pyolysis poduts in the gas phase wee measued by gas homatogaphy. The tempeatue pofile was measued at the ente of the eato in an inteio eami pipe with an oute diamete of mm. Aknowledgements The finanial assistane of the Highe Eduation Commission of Pakistan (HEC) and the Geman Aademi Exhange Sevie (DAAD) ae gatefullly aknowledged. Refeenes 1. Wu, C. H.; Singh, H. J.; Ken, R. D. Pyolysis of aetylene behind efleted shok waves. Intenational. Int. J. Chem. Kin. 197, 19, Fenklah, M.; Dugapasad, M. B.; Taki, S.; Matula. R. Soot fomation in shok-tube pyolysis of aetylene, allene, and 1, -butadiene. Combust. Flame 19,, Fenklah, M.; Clay, D. W.; Gadine, W. C.; Stein. S. E. Detailed kineti modeling of soot fomation in shok-tube pyolysis of aetylene. Po. Combust. Inst. 19,, Fenklah, M. Reation mehanism of soot fomation in flames. Phys. Chem. Chem. Phys.,, -7.. Bak, M. H. Mehanism of the Pyolysis of Aetylene. Can. J. Chem. 1971, 9, Kiefe J. H.; Von Dasek, W. A. The mehanism of the homogeneous pyolysis of aetylene. Int. J. Chem. Kinet. 199,, Gay, I. D.; Kistiakowsky, G. B.; Mihael, J. V.; Niki, H. Themal deomposition of aetylene in shok waves. J. Chem. Phys. 19,, Kuse, T.; Roth, P. Kinetis of C Reations duing High-Tempeatue Pyolysis of Aetylene. J. Phys. Chem. A. 1997, 11, Tanzawa, T.; Gadine, W. C., J. Reation mehanism of the homogeneous themal deomposition of aetylene. J. Phys. Chem.. 19,, Laskin, A.; Wang, H. On initiation eations of aetylene oxidation in shok tubes- A quantum mehanial and kineti modeling study. J. Chem. Phys. Lett. 1999,, Kiefe, J. H.; Sidhu, S. S.; Ken, R. D.; Xie, K.; Chen, H.; Hading, L. B. The homogeneous pyolysis of aetylene II: the high tempeatue adial hain mehanism. J. Combust. Si. Teh.. 199,, Böhm H.; Jande, H. PAH fomation in aetylene-benzene pyolysis. Phys. Chem. Chem. Phys. 1999, 1,

7 Moleules 7, Noinaga, K.; Deutshmann, O.; Hüttinge, K. J. Analysis of gas phase ompounds in hemial vapo deposition of abon fom light hydoabons. Cabon,, Dimitijevi, S. T.; Pateson, S.; Paey. P. D. Pyolysis of aetylene duing visous flow at low onvesions; influene of aetone. J. Anal. App. Pyol.,, Cullis, C. F.; Fanklin, N. H. The pyolysis of mixtues of aetylene with othe hydoabons. J. Combust. Flame 19,, Kestinin, A. V. On the Kinetis of Heteogeneous Aetylene Pyolysis. Kinet. Catal., 1, Hu, Z. J.; Hüttinge, K. J. Mehanisms of abon deposition-a kineti appoah. Cabon,, Hu, Z. J.; Hüttinge, K. J. Influene of the sufae aea/volume atio on the hemisty of abon deposition fom methane. Cabon, 1, Noinaga, K.; Hüttinge, K. J. Kinetis of sufae eations in abon deposition fom light hydoabons. Cabon, 1, Zhang, W. G.; Hu, Z. J.; Hüttinge, K. J. Chemial vapo infiltation of abon fibe felt: optimization of densifiation and abon miostutue. Cabon,, by MDPI ( Repodution is pemitted fo nonommeial puposes.