(132b) Multiple Steady States in Oxidative Steam Reforming of Methanol (OSRM) over CuO-ZnO-Al2O3 Catalyst

Young Shin Jang, Jung Hyun Kim, Jitae Lee, Dong Hyun Kim*

Department of Chemical Engineering, Kyungpook National University

Daegu, 702-701, Korea

1. Introduction

Fuel cell cars are operated on compressed hydrogen. The pressure of the stored hydrogen in the cars is extremely high, around 700 bar, to extend the range between refueling. Nevertheless the range is still relatively short (~ 400 km, Hyundai fuel cell car) compared to ordinary cars using gasoline or diesel. Moreover the fuel cell cars need a new social infrastructure of hydrogen stations. These problems as well as safety issues of handling highly compressed flammable gas act as barriers for widespread use of fuel cell cars. An alternative to storing hydrogen onboard is to produce it onboard from readily available liquid fuel.

Methanol can be regarded as a hydrogen carrier as it can be easily reformed with steam (SRM) to produce hydrogen by

CH₃OH + H₂O →CO₂ + 3H₂, ΔH= 49.5 kJ/mol (1)

The reaction is endothermic and the SRM reactor has a heat exchange type structure to supply the reaction heat efficiently. On the other hand the reaction heat can be supplied in situ by oxidizing a part of methanol by adding O₂ into the SRM feed. This is called the oxidative steam reforming of methanol (OSRM). As the amount of the heat generation depends on the O₂ content in the feed, an autothermal system (ATR), in which the net enthalpy change is zero, can be made

CH₃OH + 0.8 H₂O + 0.10 O₂ →CO₂ + 2.8 H₂, ΔH= 0 kJ/mol (2)

In practice, the O₂ content relative to CH₃OH is higher than 0.1 to account for the heat loss of the OSRM reactor. There is still disagreement in the literature on how the O₂ reacts with CH₃OH. Two reactions have been suggested.

CH₃OH + 0.5 O₂ →CO₂ + 2 H₂, ΔH= - 192.5 kJ/mol (3)

CH₃OH + 1.5 O₂ →CO₂ + 2 H₂O, ΔH= - 676 kJ/mol (4)

Majority of studies on OSRM assumed Eq. (3), partial oxidation of methanol (POM), as the heat generating reaction [1-5], while a few studies have shown that Eq. (4), combustion of methanol (COM), as the oxidation reaction [6,7]. It is one of the objective of this study to experimentally determine which of the two reactions takes place during OSRM.

The exothermic reaction, Eq. (3) or Eq. (4), can give rise to a multiplicity in steady states of OSRM. In the present study, we first report such multiple steady states of OSRM observed experimentally and analyze the states to unravel the heat generation and removal in the OSRM reactor.

2. Experimental

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 0.2 - 0.3 mm (50 mg) diluted with the same volume of glass beads of the same size. The feed rate was 100 ml/min (CH₃OH:H₂O: O₂:He= 15:20:3:62). The reaction temperature was measured at the outer surface of the reactor. The temperature variation along the reactor was negligible because of the high thermal conductivity of the reactor and the short catalyst bed (~ 4 cm). The reaction product was analyzed online with a gas chromatography. The reaction products were mainly H₂ and CO₂ with less than 0.5 % CO.

3. Results and discussions

Fig. 1. shows the methanol conversion with respect to the reaction temperature. With increasing temperature from 160 ^oC, the conversion followed branch I until 250 ^oC where the conversion (X) was 0.13 and the products were H₂O and CO_2, showing that COM took place in the branch I. Above 250 ^oC, the conversion jumped to B at 275 ^oC where the conversion was 0.77 and with further increase in the temperature, the conversion followed the branch II from B to C. On decreasing the reaction temperature from C, the conversion followed first branch II and then to branch III until D where the reaction temperature was 220 ^oC and the conversion was 0.19. A further decrease in the temperature from D quenched the reaction to E where T=209 ^oC and X=0.02.

The O₂ conversion (not shown) was almost 1.0 at A. If O₂ were consumed by POM, Eq. (3), the methanol conversion would have been at least 0.4. The observed X=0.13 at A only corresponds to the combustion of methanol, disproving that POM is an independent single reaction. On branches II and III, the O₂ conversion was always complete and H₂ was produced in proportional to the methanol conversion, showing that SRM followed COM in the catalyst bed.

The multiple steady states were analyzed in terms of heat generation and removal of the reactor [8]. For this purpose, we obtained experimentally the kinetics of COM and the overall heat transfer coefficient between the catalyst bed and its surrounding. The details of the analysis and comparison between the simulation and experiments will be presented at the conference.

4. Conclusions

In oxidative steam reforming of methanol (OSRM), the reaction between oxygen and methanol is experimentally found to be combustion of methanol (COM), Eq. (4), 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 (POM), Eq. (3), has been observed. Multiple steady states of OSRM were experimentally observed and analyzed to be the results of balancing the heat generated by combustion and the heat consumed by steam reforming as well as the heat loss to the reactor surrounding.

References

1. S.Velu , K. Suzuki, M.P. Kapoor, F. Ohashi, T. Osaki, Applied Catal. A: General 213 (2001) 47.

2. J. B. Wang, Chia-Hao Li, Ta-Jen Huang, Catalysis Letters 103(2005).

3. M. Turco G. Bagnasco, U. Costantino, F. Marmottini, T. Montanari, G. Ramis, G. Busca, J. of Catal. 228 (2004) 56.

4. M. Turco, G. Bagnasco, C. Cammarano, P. Senese, U. Costantino, M. Sisani, Applied Catal. B: Env. 77 (2007) 46.

5. S. Patel, K.K. Pant, Applied Catal. A: General 356 (2009) 189.

6. J. Agrell, M. Boutonnet, J. L.G. Fierro, Appl. Catal. A: General, 253 (2003) 213.

7. M. Lyubovsky, S. Roychoudhury, Appl. Catal. B: Environmental, 54 (2004) 203.

8. H. Fogler, Elements of Chemcal Reaction Engineering, 4^th ed. Prentice Hall (2006) 533.