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How can I calculate the optimal velocity in furnace tubing? At our gasoil/kero hydrotreater we operate usually at low throughput, but we keep the recycle gas at a higher value than needed for the reaction, to prevent the coking of furnace tubes. I guess that the optimal recycle gas amount could be calculated, but I don't know how to do it.

Some additional info: It's the unit manager's explanation that he doesn't want to decrease recycle gas to prevent heater coking. We are usually running on low throughput with 4-500 Nm3/m3 H2/CH ratio. In the last cycle we had pressure drop problems on our reactor, we found solid deposit on top of the bed. We performed a furnace coke burning process during the last turnaround, and found that there was some significant coking in the furnace. Our licensors suggestion is, that H2/CH ratio should be approx. 5 times the H2 consumption. Based on this, 100-200 Nm3/m3 would be enough, but we are running often at 400-500 ratio, which is way higher than suggested.
19/12/2015 A: RUPAM MUKHERJEE, ENGINEERS INDIA LIMITED, rupammukherjee86@gmail.com
It is true that the reactor charge heaters in units like these often require a minimum mass velocity below which the chances of coking increases. The exact value depends on specific heater designer and the heater coil size and turndown criteria.
12/11/2013 A: Satish Angadi, Haldor topsoe, satish.angadi@gmail.com
Theoretically, yes, recycle gas rate can be decreased. for doing so, you need other factors, like mass-flux in reactor beds etc., need to investigate.
Some catalyst licensor do not recommend reducing RG-flow, when you decrease the liquid flow rate, to avoid mal-distribution etc.
When you have more H2/CH in heater coils, chances for coke formation will reduce.
18/09/2013 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Hydrogen circulation requirements are normally dictated by the reactor rather than the furnace. You need plenty of excess hydrogen to help get good distribution and to minimize coking in the catalyst bed. I would set my minimum recycle gas flow based on licensor recommendations for the reactor rather than any furnace considerations.