Supporting the catalyst
The active catalyst is usually supported on a ceramic material. The three types of support commercially available are:
- aluminium oxide
- calcium aluminate
- magnesium aluminate
The catalyst support helps produce a material that has a :
- high surface area
- high catalytic activity
high resistance to breaking up in hostile operating conditions
The reforming reaction happens on the surface of the nickel sites on the ceramic support. This reaction is so fast that the overall rate of reaction is actually controlled by how quickly the reacting gases can get to the catalyst surface rather than the reaction at the surface. This is decided by the rate at which reactant gases can diffuse through the film of gases on the surface of the catalyst.
It is important for efficient working of the reforming process that there should be effective transfer of heat from outside the tube holding the catalyst through to the gases passing inside it.
One of the problems that can arise is the formation of a stationary gas film inside the tube wall that reduces the rate of heat transfer. The thickness of the stationary gas film can be minimised by using smaller catalyst particles. The problem with this is that smaller catalyst particles make gas flow more difficult and result in a gas pressure drop from one end of the tube to the other. The trick is to design a catalyst shape that allows high heat transfer with a low pressure drop.
The reaction pipes run through the furnace. Flames hat the furnace. It is important that the heat transfer through to the steam/methane mixture is effiecient
The traditional reforming catalyst shape is a ring. The smaller the ring the higher the catalytic activity and the better the heat transfer properties but the higher the pressure drop as the gases pass through the reformer tube. Catalyst shapes have been developed with four or more holes in them to increase the geometrical surface area and hence the catalytic activity. The geometric surface area of a catalyst may be between 250 and 400 square metres per cubic metre.
The normal four hole catalyst has become the standard with a mini four hole shape being used for severe duties and rings used for lighter duties. Sometimes more than one type of catalyst shape is used in the same tube.
Catalyst pellets used in industrial processes