A basic question realated to catalytic conversion.
We run a catalytic conversion at say 500C, the feed is gas that is converted to a product over a catalyst bed (fixed bed reactor). Must the gas be heated to reaction temperature (500) or is it enough if the catalyst is heated to temperature. Energy is then used to keep the bed at temperature because of the cooling effect of the gas.
So is the hot catalyst surface enough to make reaction happen independent of the gas temperature?
Heating the catalyst bed to the temperature of e.g. 500oC does not guarantee the same temperature of the gaseous substrates flowing in contact with it. Thereafter, the bed will be cooled. I am also wondering how you are going to heat the catalyst bed cheaply and without losses. Additional question: is the process exothermic? If so, autothermal reactors of various structures have been known for many years, where the heat accumulated in the reactants is used to heat the substrates. As a result, apart from the start-up stage, heating of the substrates can be partially or even completely eliminated. However, it is important how great the heat of reaction is.
The reactor is a steel reactor (SS316) inserted in full length in a tubular oven with the catalyst bet in the middle of the reactor. It is a lighly endothermic reaction so yes some cooling will happen.
But in the energy calulation is expected that the flowing gas is heated to reaction temperature (500). Up to me the heterogenous catalyst (metals on metaloxides) is easier to heat up and keep at temperature than the the flowing gas. The gas will be heated too, but is it necessary to heat it to reaction temperature?
THe contact time gas-solid catalyst is rather short so up to me the cooling effect from gas is low compared to energy needed to heat a flowing gas.
Henrik Romar, After further explanation, the problem becomes clearer. If the reaction is slightly endothermic, this changes the situation because the heat of reaction cannot be used. The structure of the reactor - a tube with a catalyst in the middle placed in a tubular oven - indicates that the gas is heated along the entire tube (I don't know why after the bed). The catalyst is also heated. As I guess, you want to redesign the system and heat only the middle part of the tube with the catalyst. It is possible and will save energy consumption, but may slow down the reaction, especially in the initial part of the catalyst bed. So why not preheat the gas using the heat accumulated in the products. Additionally, it will reduce the cost of energy.
The reason for my question is that it was calculated to heat the gas to the reaction temperature. This will need a lot of energy and problems with heat transfer into the flowing gas.
Is it wrong to assume that catalyst bed will have the proper temperature. Gas will be heated but not up to reaction temp. Reactor temperature must be set over reaction temp to compensate for gas temperatur and endothermic effect.
Henrik Romar, The most expensive stage is the start-up, where both the bed is cold and the raw material is cold. However, I do not see any significant difference between continuously heating the gas and keeping the temperature of the catalyst bed constant. In both cases, direct contact with the heating medium is impossible and energy is transported through the wall of the exchanger or reactor. In the case of an endothermic process, the exchanger can be placed in front of the bed (gas reheating) and behind the bed (preheating). This will increase one-time investment costs, but will lower operating costs and reduce heat loss during the flow, due to the shortening of the hot gas flow path. Furthermore, the catalyst particles will be heated to a greater extent by the flowing gas and to a lesser extent by direct contact with the reactor wall. If the particles are spherical, the contact area will be very small and may not provide sufficient energy transfer. How high the gas/bed temperature will be compared to the minimum one, will depend on how much energy is absorbed during the reaction, whether the catalyst is resistant to high temperatures and whether the reaction is reversible.
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