Kinetics and reaction steps of autothermal methanol steam reforming over CuO-ZnO-Al2O3 catalyst
Hyun Chan Lee, Jitae Lee, Dong Hyun Kim*
Department of Chemical Engineering, Kyungpook National University
Daegu, 702-701, Korea
Methanol can be regarded as a hydrogen carrier as it can be easily reformed with steam (SRM) to produce hydrogen by
The reaction is endothermic and the reaction heat can be supplied by external heating or by oxidizing part of the methanol in the catalyst bed. The possible heat generation reactions are
The reaction (2) is partial oxidation (POX) and (3) is total oxidation (TOX) of methanol. Combination of either of the reactions, POX or TOX, with SRM can make an autothermal reaction (ATR) system.
As the heat is generated and consumed in the same catalyst bed, the ATR system can have a smaller, simpler reactor design, suitable for mobile hydrogen generation. Majority of studies on ATR assumed POX as the main oxidation reaction [1-5]. But a few studies have indicated TOX as the main oxidation reaction [6,7]. The objective of this study is to experimentally find the dominant oxidation reaction and its kinetics, as this information is critical in proper design of an ATR reactor.
The reactor was made of 10 cm copper tube (3.175 mm OD, 1.55 mm ID). The catalyst in the reactor was a commercial CuO-ZnO-Al2O3 (Synetics 33-5) of a size of 0.3 – 0.43 mm (0.13 g). The feed rate was 150 ml/min (CH3OH/H2O= 25/37.5 ml/min, O2 = 0 (SRM), 1.5 – 5.5 ml/min (ATR)), balance He). The measured temperature profile along the length of the reactor was flat due to the short length and the high thermal conductivity of the 1/8 inch Cu tube. The reaction products were mainly H2 and CO2 with traces of CO.
3. Results and discussions
Fig. 1. shows the methanol conversion for various feed conditions. At temperatures below 240 C, SRM rate is seen to be much faster than ATR rate. This temperature region corresponded to incomplete O2 conversion in the reactor as seen in Fig. 2. At above 240 C, the ATR rate became appreciable and comparable to the SRM rate. A power-law rate expression for the oxygen reaction rate was obtained by least square fitting of the experimental data of Fig. 2.
The oxygen rate is well described as a half-order reaction with an activation energy of 128 kJ/mol.
If oxygen is consumed by POX (Eq. (2)), 1 mole of oxygen consumes 2 moles of methanol. Or if oxygen is consumed by TOX (Eq. (3)), 1 mole of oxygen consumes 0.67 moles of methanol. We plotted the amount of oxygen and methanol consumption in Fig. 3. For each oxygen flow rate, the ratio of methanol/oxygen consumption followed the line of TOX to around 50 % oxygen conversion. At higher oxygen conversions, the ratio deviated from TOX and increased rapidly with increasing oxygen conversion. This can be explained by SRM occurring in parallel with TOX in the reactor. Under any conditions, the ratio of methanol/oxygen reaction ratio never followed the POX ratio, showing that POX as described by Eq. (2) does not occur directly over the catalyst.
In autothermal reforming (ATR) of methanol, the reaction between oxygen and methanol is experimentally found to be the total oxidation (TOX), Eq. (3), and hydrogen is formed only by the steam reforming of methanol (SRM), Eq. (1). No experimental evidence for the occurrence of the partial oxidation reaction (POX), Eq. (2), has been observed. It is thus concluded that ATR consists of TOX and SRM. The kinetics of TOX over the CuO-ZnO-Al2O3 is obtained as a half-order reaction with respect to the oxygen partial pressure with an activation energy of 128 kJ/mol.
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