HVGO Cutpoint/Yield improvement

[lgmarti]

I was wondering what other refineries do to optimize their HVGO cutpoint/yield. In my refinery the strategy is to always maximize the heater outlet temperature. I have noticed, however, that the actual yield does not necesarily increase with the higher duty/temp and that sometimes maximizing the heater increases the tower vacuum (3-5mmHG). Can someone tell me how they maximize their HVGO yield? I think that I may be hurting myself by always maximizing the heater outlet Temp/duty.

 

[25362]

You say the vacuum increases on heater increased firing. Do you mean the column absolute pressure increases, and the vacuum drops by 3-5 mm Hg ?

 

[sayedm]

Maximizing the heater outlet temp may increase the cracking in the heater, as a result more noncondensable gases will be produced, these gases will lower your vacuum and as well your HVGO cut, also reduce your unit run time.

In my refinery we try to improve the vacuum in the column, you will get more evaporatoin and deeper cut, improving the vaccum however is a design issue, in your case reducing the noncondensable gases and some operational adjustments may improve the cut.

You did not mension what temp you are using,current cut point and column vacuum, one of them may cause the problem.

 

[JPR02]

Operational-wise...lowering the vacuum in the vacuum tower at constant COT can help to increase the cut-point.
Is the overhead condensers performing? Are there any signs of fouling?

From the design point of view, packed tower generally gives a lower delta P drop. Is it a packed tower you are using?

 

[lgmarti]

The maximum heater outlet temperature is 775F. Yesterday, we were step-testing the heater and we increased its duty 14% to a 775F heater outlet. The HVGO yield did not improved and we lost 3-4 mmHg of vacuum (we started at 14.5mmHG and ended up at 18.3mmHg). However, one of my operators (bless his soul!) increased the bottom liquid quench last night thus decreasing the tower btm temperature from 690F to 676 F. The tower vacuum then improved by 2 mmHg (current P = 15.98 mmHg). I may add that this is a dry unit and that the ejector system is working as design. The tower has packing. We can't adjust the ejectors to improve vacuum. Everyone here is convinced that increasing heater outlet temperature will always increase yield. I just wish I could prove it in paper! Any ideas of things we should be doing to proof that our yield is optimized?

 

[sayedm]

The way I see it is you have cracking in the bottom of your column due to the high temp. The cracking increase the noncondensable gasses and the load on the vacuum system.

Our unit is not a dry unit ( we use steam ),but the vacuum we have is 25-27 inHg60F ( if my memory serves me right; with the units that is ). We use a three stage steam ejector system.

IF you can increase the quinch temp or circulation rate there is a chance to improve the vacuum by lowering the cracking rate.

On the vacuum system check if you are in optimal conditions, Ejector systems are constant mass flow not vol. so if you have more overhead products or underperforming washing sections you may overload the system.Do a mass balance on the top section and check the ejector curves.

 

[kaypeegee]

In an upgrading refinerey "optimizing" HVGO cut-point does not necessarily mean "maximizing" it.

Cutting deeper into the resid mean the marginal bbl of HVGO recovered is much poorer quality than the average quality of the HVGO stream.

The marginal value of this stream needs to be compared with alternative modes of operation of down stream processing units e.g. is it better to increase FCC conversion rather than increasing feed rate using a poor quality feed.

This is a fairly complicated optimization best done by a refinery-wide modelling tool. As well as changing coil outlet temp and quench rate, pumparound and wash oil rates also affect HVGO yield and quality.

Note that cutting too deep into the resid in a unit that is not designed for it can lead to excessive coking in the column and furnace.

If it "optimum" to maximize HVGO yield then performing a few test runs on the unit a various conditions is usually he best way to confirm the best process conditions.

 

[25362]

As repeatedly said above, there is a limit to vacuum "cutting", and sometimes solvent deasphalting combined with vacuum distillation becomes a better economical option to remove HVGO fractions from vacuum residue.

 

[25362]

Having said that, one mustn't forget that companies such as Buss-SMS GmbH, Butzbach, Germany, offer their three-stage vacuum residue short path distillation (VRSD) to markedly improve yields of VGOs, changing the overall refinery economy. See Hydrocarbon Processing July 1993 on Process Technology.