5 replies to this topic

[gegio1960]

Dear Experts,

I'm trying to look for specific design/field experiences relevant to Vapour Recovery Units (VRU) connected to loading of Gasolines.

The selected technology includes Adsorption coupled to Absorption.

The suggested washing liquid is Heavy Gasoline (eg Reformate, Heavy Cracked Naphtha) but, on the base of experience in similar plants in other Refineries, someone would like to investigate if Kerosene or Gasoil could have better (i.e. more economic) performances.

Without any specific experience, I've tried to simulate the washing column in the various cases. The main results of this preliminary investigation are as follows:

- Reformate appears to be too light: the column works as a Stripper, the opposite of an Absorber.

- At equal volumetric flowrates, Kerosene seems to be better than Gasoil (i.e. Kero absorbs more vapours than Gasoil).

Since there are thousand of installations of this kind in the world....

Before going ahead with the analysis I'd like to compare these results with real experiences.

Also references to specific papers, articles, books are welcome.

Thank you in advance,

gegio

[PingPong]

I don't have any experience with this kind of systems, but let me give my opinion anyway:

 

- Reformate appears to be too light: the column works as a Stripper, the opposite of an Absorber.

That seems logical, as reformate is not a heavy gasoline, but a full range. It contains C5 and C6 and even a little C4, while heavy usually refers to C7 and heavier. If the refinery has a reformate splitter, the heavy reformate might be suitable. However if it were my refinery I would not allow any reformate to be used in such system as it contains a very high amount of aromatics which are very unhealthy, as we all know. I assume the offgas is basically air that is finally sent to atmosphere?

- At equal volumetric flowrates, Kerosene seems to be better than Gasoil (i.e. Kero absorbs more vapours than Gasoil).

That is to be expected, because what matters in absorption is the molar flowrate of the liquid.

 

When selecting the absorber liquid, you should also consider what to do with the liquid from the absorber bottoms.

 

Like in an FCCU gasplant it is also possible to have two absorber columns in series: the first one using heavy naphtha, the second one using a gasoil (LCO in case of an FCCU) as sponge oil.

[gegio1960]

Thank you, Ping Pong.

I agree about avoiding circulation of Reformate but it seems to be the most common choice, according to the "licensor".

Under HSE aspect I'd prefer to use Gasoil instead of Kerosene.

The absorber bottom will be returned to the storage tank. The content of the tank will be reprocessed when the vapor pressure of the liquid has increased too much (how much?).

I really would like to see answers from people with specific experience :-)

[PingPong]

In the absence of expert opinions, I will throw in a few more remarks:

 

If the VRU is existing and designed for use of naphtha/reformate, then the use of a higher viscosity gasoil will result in less theoretical stages in the absorber column.

Liquid viscosity has big impact on mass transfer between gas and liquid, both in trays and packing.

 

If you keep circulating the liquid over the same tank, then the performance of the absorber will gradually decrease as the concentration of lights in the lean oil (liquid from that tank) increases in time. Better would be to use two tanks, one for the lean and one for the rich oil (absorber bottoms liquid). But if the liquid is to be reprocessed anyway, it seems better to do that continuously by sending the rich oil not to the tank but continuously to the reprocessing unit. Note however that the liquid will be saturated with air, no matter how you do it.

Edited by PingPong, 30 July 2015 - 09:31 AM.

[gegio1960]

Dear Ping Pong, I want to thank you for your brainstorming.

You're right. In fact the foreseen tanks are two.

The rich washing fluid will be reprocessed, since the O2 content is negligible and it will be further diluted during the reprocessing.

There are more concerns about possible MTBE recirculation. At the end, it will go to the Naphtha.

The unit is new and, based on simulations, the increase of equilibrium stages doesn't improve significantly the purification.

 

Once more, I'd like to see contributions from specific experience....

[Napo]

Dear Gegio 1960,

You can search in google: gasoline vapor recuperation unit, there are some patents about your question.

In fact, the selection of absorption fluid is function of the GVRU type: membrane or activated charcoal.

Napo.