(55f) Coke and Microreactors: a Contradiction? - Catalytic Cracking of Propane in a Microreactor Heated by Catalytic Combustion

Coke and Microreactors: A Contradiction? - Catalytic Cracking of Propane in a Microreactor Heated by Catalytic Combustion

The cracking of propane to generate hydrogen for fuel cell systems is promising due to the availability of excess C3/C4 fractions from oil refinery and a resulting high hydrogen purity which is a demanded for applications like auxiliary power units, battery chargers or mobile generators. Microchannel reactors offer the potential to cope with sudden hydrogen demand and load changes.

Although it is commonly agreed that coke or soot formation has to be avoided in microreactors, we can demonstrate in this study the successful application of a microchannel reactor to produce hydrogen by propane cracking. The used reactor contains four passages. The reactor operates in transient mode, i.e. two intervals. In passage 1, filled with stainless steel mesh, propane is cracked in interval 1 and hydrogen is formed, but also soot is formed. In passage 2, also filled with a stainless steel mesh, soot is simultaneously oxidized in interval 1. In interval 2, the formed soot is oxidized in passage 1, and propane is cracked in passage 2. Between the two intervals, the former respective cracking passage has been flushed in the experiments by nitrogen for about half a minute for safety reasons. Passage 3 serves as supply of simulated anode-off-gas to passage 4 but it could be also used to supply CO from coke burning. Passage 3 is connected to passage 4 by micro holes to prevent homogeneous reaction. Air is added in passage 4 to perform the catalytic combustion.

By mass spectrometry, the product gas stream in the two intervals was analyzed. It can be shown that propane can successfully be cracked reaching an hydrogen yield of more than 70 % for about 8 minutes. In the second interval the generated soot can be oxidized within 20 minutes in air. The balance shows that the heat of oxidizing soot and of oxidizing anode-off-gas is sufficient to drive the endothermal cracking process. The oxidation of soot can be accelerated by impregnation of the stainless steel mesh with V₂O₅ without decreasing cracking activity.

Fig. 1: Propane crack reactor                                    Fig. 2: product gas of passage 1 of one complete cycle