10 replies to this topic

[mrobles1942]

Hi

I need to develop a  project consisting to upgrade one  existing VDU in order to maximize distillates recovery while maintaining throughput as defined in the design case.It`s necessary to achieve the following  two objectives:

 Yield a stream of LLVGO (very light vacuum gas oil) suitable to be sent to the diesel pool

 Yield extra HVGO (heavy vacuum gas oil) against bottom product (Deep cut operation)
 

The Heavy Vacuum Gasoil (HVGO) stream will be sent to the catalytic cracking unit (FCC), complying with the low metal content and C Conradson specification of this unit.

I have no doubt how to get the first of the two objective.However I´m concerned about  the second one. I think this depend of the right design of the section of the column between the flash zone and the extraction plate  of HVGO.
Taking in account the ascending gases from the flash zone have VR in vapor phase, due to vaporization in a single equilibrium stage , and in liquid phase, due to mechanical liquid carry over (“entrainment”), I have  the following  idea how to make the design:
1) To remove the VR in liquid phase ,by coalescence, in the wash zone, keeping  one slop wax recirculation through the wash oil bed, with the idea to have sufficient flow rate to achieve a good wetting of  the bed
2) To remove the VR in vapor phase in a new independent  bed with sufficient fractionation capacity ( F- Factor) located immediately above of the wash zone.

This new bed would be feeding  by  HVGO reflux for condensing and fractionating the slop wax, that it would fall to the washing zone where il should be purged.
I think the idea to use only one bed( the wash zone bed) with HVGO  reflux  is insufficient to get the two objectives of eliminating the VR in gas and liquid phase, due to the usual tendency  to minimize the overflash or HVGO reflux
Would welcome comments about  this approach of design

Best regard

Roure

[PingPong]

For outsiders to understand the situation correctly, two sketches are required:

one sketch showing the present column layout with beds, pumparounds, et cetera,

and another sketch showing the future column layout including the modifications you have in mind.

[mrobles1942]

Attached you can find the sketches

Attached Files

•  VDU 001.jpg   126.78KB   11 downloads

[mrobles1942]

Current situation

Attached Files

•  VDU 002.jpg   126.52KB   3 downloads

[mrobles1942]

Sorry.Now is attached current situation  VDU Current Situation 001.jpg   145.18KB   10 downloads

[PingPong]

Just some first impressions:

 

I don't see the new LLVGO product (diesel quality) in the Future Situation. Is it going to be drawn from a new column? Or what?

 

This new wash bed with liquid circulation: is its height taken from the existing wash bed, or do you feel confident that you can take it from the flash zone?

Although I am sure this has been done before, there are probably better ways to handle the de-entraiment issue. Note also that such liquid circulation bed hardly contribututes to the fractionation between HVGO and VR but does cost precious bed height. Consider instead a state-of-the-art feed vane type inlet device (design depends on whether the feed nozzle is straight or tangental) to reduce entrainment at the source, and a modified wash bed with new structured packing in the top and modern grid type in the bottom, and recycle the slop wax from the bottom of the bed to the heater inlet. By using a reduced internal cross section for that wash bed (possible thanks to the higher hydraulic capacity of the modern packings) the liquid load (GPM/ft² or m³/h/m²), and consequently de-entraiment, at the bottom of the wash bed can be maximised.

 

 

To increase the HVGO yield you need to increase the vaporization at the column inlet, and consequently at the heater outlet. Moreover the pressure drop over the transfer line will increase due to more vapor in the future (assuming same unit capacity and feedstock). You indicate a COT=425 ^oC both for Current and Future Operation, implying that it can in future be done with the same COT as present?

A COT of 425 ^oC is already high, so to increase HVGO yield you may need to decrease heater outlet pressure by decreasing flash zone pressure. To achieve that you may need to replace all packing in all column beds by structured packing with lower pressure drop. Vacuum system may also need modification if column top pressure is to be decreased in other to decrease heater and flash zone pressures.

To help achieve more HVGO yield you may also need to consider replacing the stripping trays by more modern designs and/or increase stripping steam to increase stripout.

You can recycle the dirty slop wax (drawn from chimney tray above the flash zone) over the heater as a means to increase HVGO yield, but if it turns out that that dirty slop wax is mainly VR then it probably makes little sense.

Question is whether the heater can handle the additional duty required for higher HVGO yield, and whether the transfer line can handle the increased vapor volumetric flowrate (stay below sonic velocity).

Edited by PingPong, 16 February 2014 - 09:21 AM.

[mrobles1942]

I dont need a new column to recover the light front end of the LVGO. (LLVGO to the Diesel Pool).
I have in mind how to do it modifying the existing VDU.
There are two options .You can see them on the sketches attached
But my main problem is to maximize HVGO, with a deep cut, meeting the specification of the conversion units (FCC and Hydrocracker): Metals and C.C.
My ideas are to remove with the Wash oil bed the VR entrainment in liquid drops (like fog) and to install a new fractionating bed, above the wash oil bed, to remove the VR in vapor phase with HVGO reflux
At present there is only the Wash oil bed, without slop wax recirculation , with HVGO reflux
Thanks
My best regard

Attached Files

•  VDU 003.jpg   135.07KB   4 downloads

[PingPong]

There are two options .You can see them on the sketches attached

Aha, so you are going to put a new piece of column shell on top of the existing to provide additional height for LLVGO fractionation. Okay.

 

 

My ideas are to remove with the Wash oil bed the VR entrainment in liquid drops (like fog) and to install a new fractionating bed, above the wash oil bed, to remove the VR in vapor phase with HVGO reflux

Do you mean that you are going to insert a new piece of column shell between the existing wash oil bed and HVGO PA section to accomodate a new fractionation bed?

 

I don't know how much % more HVGO you intend to produce in future, or what future TBP cutpoint you mean with Deep-Cut. And it is not clear to me why you expect that in future entrainment will be a much bigger problem than today.

 

It is not standard pratice to have a circulating wash oil bed as you propose and I fear you overestimate its benefit. It will take up column height which in my opinion is better used for improved HVGO/VR fractionation and moreover extra bed height will also help de-entrainment. It will not reduce the wash oil (HVGO reflux) requirement, what you seem to expect. Yes, it will catch most entrainment, but so will extra bed height in the existing wash oil bed, for the rest it is just a wetted section with the same dirty liquid on top as at its bottom. Entrainment from that section into the new washoil fractionation bed is therefor the same dirty stuff as if the wetted section was not there. Therefor, if you decide to go ahead with this, I advise you to consider a trough type distributor instead of spray nozzles above the wetted section.

 

In the current operation (and many other VDU's all over the world) most of the washoil to the top of the bed is vaporized by the hot vapor from the flash zone. The HVGO reflux shall therefor be high enough to have sufficient liquid load at any point in the bed to flush down entrainment and avoid dry spots where coke would deposit. And that is where the real problem is: operators don't always know or understand this and think they can maximise HVGO product by reducing HVGO reflux. They look at the slop wax (dirty washoil) flow meter and think it's enough, but don't realise that that flow contains entrained residue and in the middle of the bed there is hardly any reflux due to vaporization. Result is lousy fractionation and coke formation inside the bed.

 

In your proposed design it will not be different. If you think that all you need to do to produce more HVGO is reduce the HVGO reflux and add a dirty washoil circulation, that does not really do anything to improve fractionation or reduce washoil vaporization, you will be in for a bad surprise.

Edited by PingPong, 16 February 2014 - 04:49 PM.

[mrobles1942]

 VDU from technical Bibliography.pdf   122.14KB   44 downloadsI appreciate very much your comments

Yes I´m going to provide additional height for LLVGO production.On the narrowest part of the column it is necessary to install two new beds: one for controlling the fractionation LLVGO / LVGO and the other to condense LLVGO (about 15% of LVGO). This is the easiest part to solve.

About the second part :” To maximize the HVGO yield” ,  the target  is produce  HVGO with  95% TBP cut point  = 560ºC ( 1040ºF) “on spec”.

The idea is to reduce the overflash improving the fractionation between HVGO and VR. I don’t like to spend to much HVGO reflux because that mean more  overflash  amount which penalizes the HVGO yield.

Two things must be improved:       

 1) the VR liquid entraînement and

 2) the VR that vaporizes in  flash zone

I think it is necessary to install two bed to do that. One could be one demister wetted with small amount of HVGO in order to remove the fog (“ small amount of  VR liquid”) and the other to control the HVGO/VR fractionation. For now I've forgotten the idea of ​​using recirculating of slop wash.

 I understand to wash is to clean, without fractionation,  for that reason I didn’t want to use HVGO for cleaning, knowing that it is part of the overflash

Best regard

MRobles

 

[PingPong]

the target  is produce  HVGO with  95% TBP cut point  = 560ºC ( 1040ºF) “on spec”.

I think you mean that the 95% TBP point of the HVGO is to be 560 oC max. That is not the same as the TBP cutpoint. And why is that 560 oC a specification for feed to an FCCU? Or is it a feed spec for a VGO HDS or MHC upstream the FCCU?
But 560 oC is rather low, so I would not call that Deep Cut.

One could be one demister wetted with small amount of HVGO in order to remove the fog (“ small amount of  VR liquid”) and the other to control the HVGO/VR fractionation.

So now you want to install a wetted demister above the wash section, just under the HVGO chimney drawoff tray? Is there no demister yet? Normally there would be a (dry) demister just under the chimney tray, flush with the bottom of the HVGO drawoff pan so it does not take up column height. Problem with your idea is that it does take up column height, which you don't have in an existing column. Moreover there will not be a VR mist above the wash section, provided it is properly designed, and operated with sufficient HVGO washoil reflux. The dry demister that others use is really meant to knock down the HVGO mist from the spray nozzles (not to knock down VR) so that all HVGO washoil reflux that is measured to go to the bed does indeed enter the bed.

Moreover I don't see how that wetted demister will reduce the overflash, which seems to be your (only?) way to increase HVGO product yield. The washoil flow at any point in the wash bed shall be at least 0.5 m³/h/m² or 0.2 GPM/ft² to assure that VR entrainment from the FZ is flushed down and no coke is formed inside the bed. As most of the washoil is vaporized in the bed the required HVGO reflux to the top of the bed has to be much bigger than the overflash. People who do not understand that think that they are wasting a lot of HVGO but that is not really so: the HVGO vaporised in the bed is again condensed in the PA bed above, and the remaining overflash (slop wax) can be recycled over the heater (if you put in the pump, lines and valves for it).

[Zubair Exclaim]

Let me get it right...... you want to completelty remove your wash oil section

 

getting rid of the wash oil bed is not a goood idea it suppreses the entrained asphaltenic and other problem creating cracked products from travelling up the column, also safegaurding your trays above and product contamination

 

you might get a  lower tray efficiency with such operaton over a period of time

 

 

However it also is higly relient on what type of Crude you have i.e, PONA analysis ...

 

 

do you guys have a higher wax content ?

 

if so then dont remove your wash section at all