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Need For Butterfly Valve At Thermosyphon Reboiler Inlet

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#26 Zubair Exclaim

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Posted 02 April 2014 - 05:58 AM

Principaly i would agree with Paul ...but i too have seen similar dsign by co worker where he had no details for the actual piping plans and no substantiated inlet line losses... not a best practice but helps some time.



#27 PingPong

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Posted 02 April 2014 - 12:49 PM

In very low pressure applications (vacuum), a small static head might be enough to cause a deep subcooling, which, in turn, results in very long heating zone (50% or more) and consumes large portion of heat duty (30% or more).
Better not use a thermosiphon in vacuum service, but use kettle or falling film evaporator reboiler.

 

Given that the reboiler is always oversurfaced when clean, and more so if the client also specifies a capacity margin of say 10%, I considered that unstable surging could be a real problem and the valves were a simple remedy. B
Clean or overdesign do not have any impact on the amouint of vapor produced. It is the column control system that adjusts the heating medium so that the reboiler produces exactly the vapor that the column wants.

 

However care should be taken when sizing the vertical return line from reboiler to column (horizontal reboiler) and the number and diameter of tubes in a vertical reboiler, so as to assure stable two-phase flow over the range of design flow to turndown. Oversizing the cross sectional area of the vertical line(s) can cause unstable two-phase flow at turndown.



#28 xavio

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Posted 02 April 2014 - 07:06 PM

 

In very low pressure applications (vacuum), a small static head might be enough to cause a deep subcooling, which, in turn, results in very long heating zone (50% or more) and consumes large portion of heat duty (30% or more).
Better not use a thermosiphon in vacuum service, but use kettle or falling film evaporator reboiler.

 

It is exactly what is written in numerous design guidelines. In reality, however, thermosyphons are used for the whole range of pressure. I have seen thermosyphons operating at  2-10 torr without any problems.

It is common in petrochemical industry to choose thermosyphon whenever possible until the calculation (or owner) mandates otherwise.

I would choose falling film film evap reboiler for viscous and heat-sensitive liquids, not merely because of vacuum service.

 

Good discussion so far!



#29 breizh

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Posted 03 April 2014 - 12:51 AM

Interesting discussion indeed !

Consider reading this paper about Design and Operation of Thermosyphon  reboiler .

 

Breizh



#30 narendrasony

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Posted 16 April 2014 - 06:49 AM

This discussion has turned out to be most interesting.

 

I have put in butterfly valves on the inlets to thermosyphon reboilers in several cases, with the intention of giving the operator the option of dealing with unstable circulation. In particular this can arise on turndown, as explained by Sachin. In a number of projects the client has specified a turndown to 25% of capacity as part of the basis of design. Given that the reboiler is always oversurfaced when clean, and more so if the client also specifies a capacity margin of say 10%, I considered that unstable surging could be a real problem and the valves were a simple remedy. But I accept the reasoning of Ping Pong and others that in practice the unit can work over a wide range of vaporisation.

 

Paul

 

Thanks for very interesting thread and for different perspectives.

 

I've heard of these valves, but never had a chance to come across them practically and I'm skeptical about their use for turn down purpose. It will be interesting to know if turn down to ~25% could be really achieved with a valve on reboiler inlet line. It would probably require to almost fully close the valve. Why not use Kettle reboiler for such low turndowns.

 

Art Montemayor has mentioned about "U-tube" oscillations inherent to vertical thermosyphons in the following thread:

http://www.cheresources.com/invision/topic/1480-reboilers-kettle-vs-vertical-thermosiphon/

 

A restriction in the liquid line can correct this problem by providing damping. These oscillations may be accentuated by disturbance in the process parameters such as liquid level, column pressure, composition, temperature driving force etc.

 

As per the attachment provided by Breizh in Post-29, this instability can be corrected by decreasing exit flow resistance or temperature driving force also.

 

 

Regards

Narendra



#31 Neelakantan

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Posted 19 April 2014 - 02:53 AM

As  designer many young engineers may feel a need for "safe" killing of excess flow/load in the reboiler when the overall column inlet is running at a turn-down condition.  this comes probably as a regular engineering practice in some companies;  but as mentioned, the column hydraulics and heat load, especially around a thermosiphon reboiler is not a steady state; in a particular LPG column (where the bottom product is stabilized condensate (c5+) and the reflux product liquid is LPG, I have seen the steam flow around the vertical thermosiphon reboiler hunting between two limits when the column runs at 30% load;  the column temperature and product quality is NOT controlled by the reboiler vapour temperature but rather a control tray temperature (3 trays above the bottom); 

 

this variation occurred because the reboiler tends to collect heaviers and evaporation load would go up and the steam load would go up; as the heavier vaporise the temperature would increase in the control tray pulling down the steam flow; this will go over a two-hour cycle;  the product (condensate) would however vary in the RVP very little and we continued to get good product.

 

One suggestion by our engineering company (at that time I was in production and not in design!) was to introduce low pressure-drop butter fly valve and watch the column pressure-drop; we did not do the change and so per-se I have no operational experience on the valve.

 

Dynamically a good design of a column with a matching reboiler (not too much oversized, intentionally or by mistake) will tend to auto-adjust (self-regulate?) the reboiler duty within a range; however a HCV is allowable; read paulhorth's explanation:

 

http://www.cheresour...yphon-reboiler/

 

regards

neelakantan






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