2 replies to this topic

[AlanC079]

I have a handful of questions around distillation reboiler.  <1 year refinery experience, no prior distillation experience.  

 

Debutanizer column.  The reboiler on this system fouls regularly (I don't know exact frequency, but less than once a year I believe).  

1.  The reboiler is designed with 3 pumps in parallel for the circulation.  The reboiler is designed for 529,000 lb/hr liquid inlet, 298 psig.  Density is 29 lb/ft3.  Converting this to barrels per day I get 77,969 BPD.  We only run one pump with a recirc rate around 25,000 BPD.

• What are the consequences for running lower than designed recirculation rates?  
• I feel like this could be a significant factor in our fouling issue?
• I am working to try to get recirculation up, by turning on additional pump but we need to fix our flow meters first to help ensure we are balancing the rate between the pumps. 
• Anything I need to be concerned about as increasing the recirculation rate?  I'd expect to need to watch temperature and ensure the steam supply is adequate.  

 

2.  This reboiler also has one 10" inlet and two 10" outlets. Outlet design basis is 182,000 lb/hr vapor & 347,000 lb/hr liquid.  I'm being directed to look at replacing the reboiler so it would have one inlet and one outlet or two inlet and two outlet - basically the same inlet and outlet sizing.  The feeling is this is a bad design and part of our fouling issue. 

• Is this really a bad design?
• I've never seen a heat exchanger with different inlet/outlet designs, but from some googling, it seems like this is used to help with the liquid/vapor volume returning to column.  I feel like the more likely issue is the low recirculation rate? 

 

[deft clay]

To be clear, we have steam on the tubeside and petroleum bottoms on the shell side? And is the issue fouling on the outside of the tubes? Or water fouling on the inside of the tubes? Both?

 

For S&T exchangers, keeping speed high enough generally matters and is a known way to decrease fouling. For one, the film temperature can get high if there is not enough flow to mix, in this case causing coking. Also, fouling that might normally go out of the exchanger with the stream might instead drop out and pile up on the bottom of the shell.

 

Have you run this in HTRI or a similar equipment sizing program?

I don't totally understand the question about the reboiler design. These outlets are generally sized for very low pressure drop, which can be the bottleneck in getting vapor back to the column. I've seen a few that do have 2 outlets which join together before entering the column.

[AlanC079]

Sorry, I should have included, you are correct - Petroleum on shell side, steam on tube side.  And the two 10" outlets do recombine to a 10" line before returning to the column. 

 

I have not run through any sizing program - I am not familiar with HTRI.  Would HYSYS be an option for sizing (although I'd need to figure out how to use this too)?

 

 

In regards to question #2:   I feel like my supervisor is just randomly picking on a cause of fouling (one inlet, two outlet) because it looks different than other reboilers, so he thinks the design is bad.  So, I'm trying to get some feedback on the design and if it might actually be okay and why (I don't expect anyone to know absolutely, just conceptually - it could make sense to use in this scenario).  I like the suggestion to run through a sizing calculator, if I can find one and figure out how to use.  

 

It seems to me the low recirculation rate, from question 1, is likely a more significant issue than the inlet/outlet design from question 2.