Jump to content



Featured Articles

Check out the latest featured articles.

File Library

Check out the latest downloads available in the File Library.

New Article

Product Viscosity vs. Shear

Featured File

3-Stage Propane Ref Performance

1

Water Carryover From Water Boot To Downstream Tower


9 replies to this topic
Share this topic:
| More

#1 colt16

colt16

    Gold Member

  • Members
  • 114 posts

Posted 11 August 2017 - 12:54 AM

Dear all,

 

We recently had an issue where a drum which is feeding hydrocarbon to a tower had its water from the water boot carried over to the hydrocarbon side. 

 

The small amount of water overloaded the overhead of the downstream tower. Even though the amount of water small, our study shows that the duty demand in the overhead increased by 48% which obviously cannot be provided by the existing condenser. 

 

I'd like to know if any of you have faced this situation before and what did you do to prevent such incidents? We only have one LT at the water boot of the drum. This seems to have failed. 



#2 Bobby Strain

Bobby Strain

    Gold Member

  • Members
  • 2,144 posts

Posted 11 August 2017 - 06:31 PM

If it truly costs money in interruption, then you should consider installing instrumentation to alert of a high water level in the boot. And the instrument should be separate from the level controller. Until such measure can be implemented, you should make sure the level control components are adequately serviced. And, of course, we don't know what "small" is.

 

Bobby



#3 colt16

colt16

    Gold Member

  • Members
  • 114 posts

Posted 13 August 2017 - 08:21 PM

We don't have a flowmeter of the water boot flow. But let's say if the water boot flow control valve suddenly closes we expect carry over of 27 m3/h to a hydrocarbon feed of around 500 m3/h. So that is about 5% water content. 



#4 Bobby Strain

Bobby Strain

    Gold Member

  • Members
  • 2,144 posts

Posted 13 August 2017 - 11:07 PM

That is not a small amount of water! This should result in lighter material in the bottoms stream. The condenser won't necessarily be overloaded, but the reboiler will likely not provide the heat to properly fractionate. You have to be careful when simulating this system when you suddenly have a water phase present. In any event, you should correct the deficiency in you instrumentation.

 

Bobby



#5 colt16

colt16

    Gold Member

  • Members
  • 114 posts

Posted 14 August 2017 - 01:20 AM

In the column simulation we have used the SURE algorithm that is good for this sort of water phase present on trays. 

 

You are right we should not allow this. My question is besides providing an independent LT, what are some other ways we can prevent this sort of water carryover. 



#6 Bobby Strain

Bobby Strain

    Gold Member

  • Members
  • 2,144 posts

Posted 14 August 2017 - 09:12 AM

There is no reason to do more. There seems to be no hazard from such occurrence to warrant a redundant separator. And your steady state simulation doesn't reflect the transient behavior when conditions suddenly change. Continuous water input gives a much different operation.

 

Bobby



#7 vjmromero

vjmromero

    Brand New Member

  • Members
  • 7 posts

Posted 14 August 2017 - 09:22 PM

Try to rate existing accumulator / condenser due to increased amount of reflux draw. Note that at increased overhead duty, residence time of hydrocarbon in the drum will be less.

 

If you find out that the drum is indeed undersized, then you can check where in the normal operating condition deviate. For example,

 

1. What causes the increased in overhead duty?

a. increase in heat input to tower?

b. decrease in internal reflux or pumparound duty (if there is)?

c. decrease in performance of overhead exchangers? possibly due to fouling?

d. decrease in overhead cut point? etc.

e. increase in feed input vs. max design?

check also design basis

 

From there, you can know what will be the solution.



#8 colt16

colt16

    Gold Member

  • Members
  • 114 posts

Posted 14 August 2017 - 10:51 PM

For the record I am just saying there is nothing wrong with the existing equipment. The water carryover is a misoperation. 

 

Hence, the discussion been sought here is how to prevent rather than re-rating equipment for such a scenario. 



#9 vjmromero

vjmromero

    Brand New Member

  • Members
  • 7 posts

Posted Yesterday, 02:48 AM

Hi Colt16,

 

I thought the drum you're referring to is an overhead drum. From what i have read, the service is a " feed surge drum".

 

I assume that you are using pneumatic instrument for water level control of your boot water. Even the level instrument is obsolete, as long as the controller is "controlling" and the boot level is controlled within acceptable level ranges, then there shouldn't have a water carryover problem.

 

For surge drums, it is normal not to put flow meters, and even use pneumatic controllers. For me to share some information, kindly answer the ff:

 

1. What is the upstream unit? or service?

2. Is it typical to have water carryover from feed? Maybe the issue here is the upstream unit

 

Normally, level alarms are used to warn the operator if there is a possible water carryover.



#10 colt16

colt16

    Gold Member

  • Members
  • 114 posts

Posted Yesterday, 11:23 AM

Hi Colt16,

I thought the drum you're referring to is an overhead drum. From what i have read, the service is a " feed surge drum".

I assume that you are using pneumatic instrument for water level control of your boot water. Even the level instrument is obsolete, as long as the controller is "controlling" and the boot level is controlled within acceptable level ranges, then there shouldn't have a water carryover problem.

For surge drums, it is normal not to put flow meters, and even use pneumatic controllers. For me to share some information, kindly answer the ff:

1. What is the upstream unit? or service?
2. Is it typical to have water carryover from feed? Maybe the issue here is the upstream unit

Normally, level alarms are used to warn the operator if there is a possible water carryover.

It is not a surge drum. It is the overhead drum of the upstream tower which has a continuous wash water injection for the fin fans condenser. The distillate of this tower is sent to another tower. During this LT failure it closed the water boot valve which resulted in water carryover.

Level alarm is there but once water enters the downstream tower a lot of issues creep up with the tower. The temperature in the tower drops and reboiler and condenser are unable to provide enough duties. We tested in simulation with SURE algorithm which considers free water in trays and even 1.5 m3/h carryover will require condenser cooling increase by 48%. Water will also be present on all trays which floats hydrocarbon on top of trays and prevent proper mass transfer.

I'd like to know if anyone else has such an experience and what was done to prevent this scenario.

Edited by colt16, Yesterday, 11:24 AM.





Similar Topics