Vaporization in transfer line

[Anadrago]

Hi everyone,
We have a naphtha/diesel/Gasoil distillation unit, and distillation expert give us the entire simulation in ASPEN Plus. The distillation tower seams to work; I know that we need from 3 to 10% of overflash that according our heavy naphtha 91% is just fine.
Nevertheless, the transfer line, from heater to column have also 91% of vaporization, and our company design manual says that we must to take care of vaporization (not up to 60%) to avoid heater coking. Now, some other process engineers are proposing to put a control valve in the transfer line to help the expansion and vaporization in that point, which I haven’t seen before in other facilities.
Besides that I just have the VDU transfer line criteria to design with critical velocity, even though I haven design one. The actual path of this line has horizontal and vertical stretches instead the shortest ride with different diameters.
That’s why I ask you:
1. It is possible to have more than 60% of vaporization outside the heater, taking care the film temperature of the heavy naphtha?
2. To put a control valve in the transfer line is correct?
3. What design criteria I should follow to design transfer line in this kind of facilities?
The plant is new, the heavy naphtha comes from gas that goes to a Cryogenic Plant.
Thanks in advance for your help.

 

[georgeverghese]

One option would be to recycle some portion of the tower bottoms ( or some intermediate tower product with low vaporisation fraction at the heater exit temp ) product with back to heater feed so that 60% vaporisation is maintained ?

Depending on the feedstream composition, installing a backpressure control valve on the heater exit may or may not help to maintain the 60% limit prescribed to avoid coking on the heater tubes.

 

[EmmanuelTop]

I'm sorry but your post is all over the place. You first say it is a "naphtha/diesel/Gasoil distillation unit", then you speak about VDU, and at the end in your first question you are asking about film temperatures of heavy naphtha? I have no idea how your distillation unit looks like, what is the feed and what are the products.

Also, talking about overflash relative to naphtha 91% (distillation D86?) makes no sense. Overflash is related to quality of the first product above the flash zone, which cannot be Naphtha, based on your description.

You really need to post a sketch/drawing of this unit.

I don't know how you have calculated 91% of feed vaporization, but again we don't know the feed, the heater temperature, and operating pressure.

Dejan IVANOVIC
Process Engineer, MSChE