The Oil Drum

63 comments on Refining 101: Summer Gasoline

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Robert Rapier on March 17, 2007 - 2:46am

Naphtha does not necessarily have a low RVP. I've had light naphthas pass through my hand that were being sold because they could not blended into even 11 PSI gas. (pentane hexane rich streams)

I know that I shouldn't generalize, but that makes it easier to understand without getting bogged down into details. When we start getting into fine details, the eyes of the audience start to glaze over. I have personally never seen high RVP naphthas, but this is of course based on numerous factors. Where are your cut points defined? Are you calling something naptha that someone else would call light straight run? What I have seen is that the RVP of straight run naphtha is very low - down in the 2-3 psi range.

The key to all of this isn't just butane. In summer you have to cut the front end off of all the components to keep light ends out of the blend by adjusting cutpoints in all the fractionators. The entire distillation specification shifts heavier.

It isn't all about butane, but it is about shifting lighter components out in the summer and back in during the winter, and butane tends to be the most important component. But if you were of a mind to, and the economics supported it, you could run your units exactly the same year round and just add butane in the fall and take it back out in the spring.

Glad to see you point out though just how much butane can be fit into winter gas. The double whammy of extra production capabitily on top of lower demand in winter is the main reason prices fall hard in fall (despite election year conspiracy theories).

Exactly. You can always have a fall hurricane screw up this trend, but more often than not it is very cyclical: In the fall you have lower demand and higher supplies because of the butane addition, and in the spring it is the reverse.

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OilcoEx on March 17, 2007 - 3:45am

the word that usually comes after light straight run is ....
naphtha (sometimes gasoline). At least in my oil boiling education. Top cut is LSR, next HSR, then jet, diesel and so on. Whoever wrote the wikipedia page must have been to the same jargon school.

http://en.wikipedia.org/wiki/Image:RefineryFlow.png

I tossed most of my spec file when it molded after I retired so I can't drag out typical naphtha actuals. But I did find a sample crude unit distillation for AL. Unrectified gas was shown as 22 PSI RVP(eyeball 85->230 cut). Rectified 10.
(eyeball 100-->230 F). For a Texas crude another assay shows RVP of 7 on the LSR "gasoline". The world wide Dow Open Spec Naphtha calls for 13 RVP max though this is a notoriously loose contract. For trading purposes, we generally assumed naphtha had about the same RVP as gasoline to estimate the loses from vaporization en route.

I kinda doubt anyone would just take summer gas and jam 8% more butane in to make winter though my blending experience is limited to tank farm stocks for a trading house. I'm too lazy to start WAGing VLIs or Driveability indexes but I have to believe those might go a little wonky. Not to mention the octane #'s for nC4 are 89.6/93.8 so you'd be giving away octane something rotten. I always felt sorry for the real refinery mogas blenders. Lots of components, crappy price forecasts, pressure to hit octane with nil giveaway etc, etc. And by the way, never run out or have tanks too full.

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Robert Rapier on March 17, 2007 - 4:24am

I always felt sorry for the real refinery mogas blenders. Lots of components, crappy price forecasts, pressure to hit octane with nil giveaway etc, etc. And by the way, never run out or have tanks too full.

Prior to my current job (Process Engineering Team Leader over North Sea projects), I was doing blending and scheduling in a refinery in Montana. I have done a few hundred gasoline blends over the past few years.

You are correct that octane giveaway is a constant concern, and units are tweaked to balance everything. But the vast majority of the difference in summer and winter blends boils down to butane.

You should post here more often. We need more people who understand the energy industry and are willing to share that knowledge. Of course you may get the occasional "Big Oils sucks, and so do you", but most people here will engage and are seriously interested in learning new things.

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OilcoEx on March 17, 2007 - 5:40pm

thanks

I worked in a Process Design shop for 6-8 years on FCCs, visbreakers, H2 plants, hydrocrackers/treaters, some upstream processing (nat gas treating) etc. Had lunch every day with the Shale oil folks, the reformer experts, the crude unit designers and had a little rub off as can be expected. When the fun stopped in the mid 80's I found a way to get into the S&D end of things and ended up trading for a large bank's oil group. That didn't take long to get old.

I wander here from time to time. Comment when I think I have something useful to add. I don't worry much about negative feedback of the "big oil sucks" variety. What I do sometimes find offputting is the "we'e all going to die" pessimism. The problem is large but I can't accept the Kunstler collapse model.