Dry point problem 1

[essenwood]

Hi guys,
I was reading one of the old report in the refinery; which indicated that coker naphtha feed had contributed to the fouling of the feed/effluent Heat Exchanger of the Naphtha Hydrotrating unit. The report mentioned that the poor heat transfer has increased the fuel gas consumption at the heater; due to "dry point" problem faced by the reactor heater.
What is exactly a "dry point problem"; in this context?
I am familiar with the term distillation drypoint for solvents; but am unsure the of the usage of this term for this particular HEX fouling problem.

Thanks guys.

 

[25362]

Taken from Chapter 21, "Fired Heaters: Process side" in A Working Guide To Process Equipment by Lieberman and Lieberman, McGraw-Hill.

Dry-point deposits: Depending on the service, the liquid fed into heater tubes is likely to contain small amounts of dissolved or suspended solids, coke, salts, and corrosion products that may precipitate out of the flowing fluid due to low tube-side velocities.

The concentration of these solids increases as the liquid vaporizes; when the liquid reaches 100% vaporization, i.e., its dry point, the solids may stick to the wall of the tubes.

As guidelines, the minimum recommended tube-side velocity to retard this sticking effect are

[→] For two-phase vapor/liquid flow, 20 fps
[→] For liquid-only flow, 8 fps.

 

[essenwood]

25362:
Your explanation was much appreciated.
Just to confirm some more details:
Can I control the problem by shifting as much possible the dry point outside of the heater tubes? i.e. clearing up the fouling of the feed/effluent HEX?; such that the vaporization happens there. The tube side velocity would be then be another step to retard the sticking effect (if feed/effluent HEX is good: that would make the task easier)?

Thanks a lot.

 

[25362]

A thought: by totally vaporizing the feed to the heater while in the exchanger may help the heater since there wouldn't be any dry point in its tubes. Then, as you say, the right velocity might keep it clean.

However, if there remains, say, 10% liquid in the heater feed, one may encounter:

a. a dry point as above;
b. flow r[é]gimes that may induce phase segregation,
as in the annular r[é]gime in which the liquid
creeps along the periphery of the tubes while the vapor
flows at the center carrying some liquid as a mist.
A slow-moving liquid might induce its own thermal
degradation with a harmful buildup of deposits.

As I said, just a thought.