Replacement of MP steam with LP steam in HCN side stripper in FCC

[PetroDDP]

Heavy cracked naphtha which is produced in Fludiesed cataltic cracker plant having IBP is 150 deg C and FBP is 220 degc.Is there any technical issue replacement of MP steam with LP steam for energy saving.

Thanks and Regards,

 

[georgeverghese]

FBP control is done in the FCC main frac column, while IBP is controlled in the HCN draw side stripper. Stripping steam should be some dT above 150degC.

dT1 in the side stripper - say 10 to 15degC
dT2 for letdown through steam supply valve - say 5degC

Total supply temp = 150+15+5 = 170degC, which is sat steam at approx 7barg = 100psig ( at least)

 

[25362]

"Stripping steam should be some dT above 150 degC"? Why?

 

[25362]

PetroDDP.
Here are my thoughts:

The switch from MPS to superheated LPS (whatever pressure levels those acronyms mean) has been frequently done for economic reasons.

The superheat of the LPS is needed for two main reasons:

A cooler steam reduces the stripping efficiency by slightly cooling the naphtha, which, to compensate, would require an increase in flow rate, more friction drop and more load to the top condensers. Therefore superheating the LPS would enable to keep the same steam mass flow rate.
Another reason is that although throttling saturated steam through control valves produces some superheat, do not use saturated steam for stripping, because it may contain (and usually does) water that could evaporate violently damaging the stripper internals.

 

[PetroDDP]

Thanks to all.
Regards,

 

[georgeverghese]

Agreed superheated steam at the same temp as the current "MP steam" being used would help to maintain the IBP spec at no loss in tray stripping efficiency.

On the other hand, superheated low pressure steam at some lower temp than the current MP steam may also be possible if there are enough trays to make up for the coincident loss in tray efficiency ( due to lower mass transfer rate on the top trays operating at a lower temp) on an existing stripper.