Ni/Zr/ kieselguhrkatalizörü ile karbon dioksit metanasyonu

Abstract

Sunulan çalışmada Ni/Zr/Kieselguhr katalizörü ile C02 metanasyon kinetiği 368-508 K aralığında ve atmosfer basıncında incelendi. Tepkime hız denklemi üstel yasaya göre yazıldı ve tepkime mertebesi, C02 için n=0.6 olarak bulundu. rCÖ2=1.9xlö7£XP(-92094/l?21) C°(7o2(0-6) (l-JT) (0-^mol/kg,s Tepkime hız denklemi Langmuir-Hinshelwood mekanizmasına göre aşağıdaki şekilde oluşturuldu. 5784 SXP(-85704/J?D Pm rca = - --^ (mol/kg,s) C0z l+5.23x10"10 EXP(63443/J?r) Pc<% Tepkime ürünü olan CH4'ın yanında, daha yüksek parafinler (C2H6) ile, olefinlerin (C3H6) tepkime hızına etkisi olmadığı; su buharının ise geri dönüşümlü olarak CH4 oluşumunu %5 oranına yaklaşan miktarlarda düşürdüğü belirlendi. Bu çalışmanın sonuçlarına göre, C02 hidrojenasyonuyla CH4 oluşumu yanında düşük hızlarda yürüyen homojen su gazı tepkimesiyle CO oluşumu ihmal edilebilir. Yüksek hidrokarbonların oluşumu da gerçekleşmemektedir. Kullanılan katalizörde Ni etkin olarak rol oynamakta, buna karşın Zr hidrojenasyon etkisi göstermemektedir. C02 ve CO metanasyonu birlikte gerçekleştirildiğinde CO metanasyonu öncelikle tamamlanmaktadır

Methanation reactions are the final step of SNG (Substitute Natural Gas) production f rom coal. Coal is gasified by partial combustion, usually in the presence of steam. The primary reactions are: C + 1/2 02 > CO (1) C + H20 * CO + H2 (2) When the hydrogen to carbon monoxide ratio in synthesis gas is equal to ör greater than 3, the conversion of carbon monoxide and hydrogen to methane can be described by the reaction: 3 H2 + CO f:-^---^ CH4 + H2O (3) Methane is also formed by hydrogenation of carbon oxides in two other reactions: 2 H2 + 2CO f=====^ CH4 + C02 (4) 4 H2 + CO2 r=====^ CH4 + 2H20 (5) However, hydrogenation of carbon dioxide, Reaction (5), does not occur in the presence of carbon monoxide. Also, Reaction (4) can be considered to be a combination of Reaction (3) and the water gas shift Reaction (6): CO + H20 ^-----^ C02 + H2 (6) IX Although the water gas shift reaction does not produce methane, it is an important reaction in methanation chemistry, altering the H2/CO ratio with far-reaching effects on reaction products. Formation of elemental carbon by heterogeneous decomposition of carbon monoxide during methane synthesis (Boudard reaction) is an undesired side reaction: 2 CO «======? C02 + C (7) it leads to carbon deposits and reduced the catalyst activity. Also heterogeneous decomposition of methane as the reverse reaction of methane formation from the elements can cause carbon formation: CH4 «ı======» C + 2H2 (8) From thermodynamic considerations, carbon formation by the Boudard reaction (7) can be avoided only if the CO content in the reactor feed gas does not exceed 0.1 %vol. This thermodynamic limitation is, however, unrealistic for practical systems as a great number of tests have shown. As reaction rate of CO methanation is much higher than that of the Boudard reaction, the CO hydrogenation is predominate. Residual CO concentration is so low that there is no more driving force for the Boudard reaction thus making carbon formation impossible. The heterogeneous decompositionof methane into carbon and hydrogen (8) can be neglected. First, hydrogen counteracts methane decomposition and, second, reaction velocity (8) is negligibly low. X The idea tîıat C02 reacts by a mechanism not involving intermediate CO formation was proposed by Medsford [2]. + 2H2 OH -H20 +H2 -H20 C0~ >. H2 - C^ - .> CH20 > CH3OH ». OH +H2 CH2 * CH4 This mechanism was further developed by Pichler [2] who proposed the following sequence:

H OH +H2 +H2 C +. » C^ - >. CH20 + H20 ». |i "X. ^ ^

H ^OH O +H2 CH3OH ». CH4 + H20 Vlasenko and Yuzefovich [2] conclude that the most probable scheme for the formation of CH4 from C02 and H2 appears to be öne in which the formation of complexes of a type corresponding to the enol form of formaldehyde takes place initially. 21 ] + 2e + H2 - > 2[H]~ OH 2 [H]" + C02 ». [HCOOH] * 21 ] + C^ + 2e (slow) OH OH C( + H2 - * HCOH + H20 OH H-C-OH + H2 > CH2 + H20 CH2 4- H2 » CH4 XI In this study, the kinetics of carbon dioxide methanation over Ni/Zr/Kieselguhr catalyst at 368-508 K and atmospheric pressure was studied. Gas mixtures of 0.23, 0.46 and 1.39 carbon dioxide %vol in hydrogen were used. Carbon dioxide was diluted with nitrogen. The kinetic studies were conducted in a integral ractor system. Hydrogen and carbon dioxide were regulated independently before mixing prior to entering the reactor. The dilution of carbon dioxide with nitrogen helped maintain isothermal conditions. The reactor was constructed 2.9 mm I.D. glass tubing, and normally about 0.2 g of catalyst was contained in the reactor between glass wool. The nickel catalyst was GIRDLER G-69 (Süd- Chemie) obtained from Perkin Elmer. The catalyst was activated in a hydrogen stream (30 cmVmin) at 623 K for 2 h before cooling to the reaction temperature. The data were recorded after minumum waiting period of 10 min. The activation of the catalyst was checked with a special run during the collection of data. Helyum was used for keeping the catalyst when it was not in use. At steady state conditions the effluent gas was analyzed with Gas Chromatograph (On line and Routine). Metan, carbon dioxide and carbon monoxide were calculated seperately in the routine analysis. Conditions of Chromatographs : 1) On line: Perkin Elmer 990, Porapak Q column, 2m, 1/8", 60°C, F.I.D detector. 2) Routine: a- CH4 analysis: Packard 419, Activated Alumina column, 2m, 1/8", 80°C, F.I.D detector. b- C02 analysis: Perkin Elmer F-ll, Porapak Q column, 4m, 1/8",60°C, T.C.D. detector. c- CO analysis: Perkin Elmer 8300, Porapak Q column, 2m, 1/8", 35°C, F.I.D. detector, Ni/Zr Catalyst. XII The reaction rate equation was derivated accrding to power law. The reaction order with respect to carbon dioxide was 0.6. i co^l. 9 xlOn 'EXP '{-9209 4 /RT) C°co2i0-6) (1-X) {Q-e)mol/kg. s Also, the experimental results were described as a kinetic equation known as Langmuir-Hinshelwood kinetics, as Eley-Rideal mechanisms and also as Hougen-Watson models. 57 84 £XP{-85704/i?T) PT r_n = is {mol/kg. s) c°2 1+5. 23xl0-x0 EXP {634:43 /RT) Pco It was determined that metan, higher paraffins (C2H6) and olefins (C3H6) had not effect on the reaction rate, at the other side water vapour decreased the reaction rate in an insignificant amount. According to the results of this study; during hydrogenation of carbon dioxide to methane, the formation of carbon monoxide by reverse shift reaction is negligible. There is no other hydrocarbon products except methane. In the catalyst used, in this study, Zr has no hydrogenation effect in spite of this Ni is essentially active element. In the methanation of carbon dioxide and carbon monoxide, carbon monoxide methanation is completed first.