(532d) Kinetics and Reaction Steps of Autothermal Methanol Steam Reforming over CuO-ZnO-Al2O3 Catalyst

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

1.      Introduction

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

     kJ/mol               
(1)

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

     kJ/mol            
(2)

     kJ/mol           
(3)

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. 

     kJ/mol   (4)

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.  

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 0.3
– 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.

  mol/kg
s         (5)

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.

4. Conclusions

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 isobtained as a
half-order reaction with respect to the oxygen partial pressure with an
activation energy of 128 kJ/mol.

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