time required to heat a catalytic bed

[rgrokkam]

Dear All,
I've noticed catalytic fixed bed reactors being heated to specified temperature(start up) by either hydrogen or nitrogen. How can I estimate the time and quantity of hydrogen/nitrogen required to heat the bed to a specified temperature? Also ,what factors go into choosing between hydrogen/nitrogen?

Thanks

 

[dcasto]

You need the heat capcity of the catalyst (zeolite based are about .22BTU/lb-f). mass of cataylst times heat capcity time temperature change is BTU required.

Then do the same for all the steel in the vessels and lines.

Add these two and add about 10% for heat loss to get the BTU's required. Next you need to know either time you you want to do the heating in to get the heat input rate OR the heat rate you have available to calculate time to heat.

As for H2 or N2, its a cost thing. Since N2 is far cheaper I'd use it. N2 has a higher heat capacity on a molar density than H2. OK, put it this way. H2 has a 13.6 times higher heat capacity per pound, but it takes 14 times as many moles or cubic feet to get the same mass.