Redefining Gasoline RVP Giveaway | RefinerLink
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Redefining Gasoline RVP Giveaway

By Optimization Specialist Robert

Feb 13, 2017
 

You can still have a suboptimal gasoline blend even when RVP giveaway is zero. Are you measuring the right factors for blend optimization?

 
 

Every refiner should know that minimizing RVP giveaway is a fundamental aspect of gasoline blending.  Regardless of season, the uplift of butane into the gasoline pool has large benefits.  Most refiners; however, do not understand that zero spec giveaway can still mean big bucks left on the table. 

 

                What is this madness that I’m talking about? 

 

            How can no giveaway equate to suboptimal blending?  

 

All engineers and gasoline blenders can agree that all molecules blend differently.  At the most basic level the vapor pressure of two isomers can drastically differ.  Normal 

butane has a vapor pressure of 52 psig while isotbane has a vapor pressure of 72 psig.  Since RVP does not blend linearly, every 1 barrel of isobutane blended into a gasoline pool displaces 1.5 barrels normal butane.  


That can be worth $5 Million per year in today’s economic environment!

 


Since isobutane generally has a higher value than normal butane, blending isobutane into gasoline has a double whammy effect of gasoline volume reduction and margin reduction. 

 

 


I started with an obvious example and no refiner intentionally blends isotubane in place of normal butane unless normal butane is unavailable. 


That being said, many refiners unintentionally engage in this behavior without knowing about it.   To understand if you fall into this category, you must first

evaluate how well you monitor your blend components, specifically the refinery butane pool. Testing the butane pool RVP on a weekly basis can be a good starting point, but it’s best to perform a GC analysis. 


While a stream RVP will indicate directionally if there are impurities in the refinery butane pool, a GC will indicate what type of impurity and how much.  A high amount of isobutane can be masked by C5+ material, so using RVP alone can be misleading. 

 

Another benefit of GC analysis is differentiating between saturate and unsaturate molecules. 


Recall that isobutane has a RVP of 72 psig.  1-butene in comparison has a RVP of 62 psig. 


Both molecules have a RVP higher than 



that of normal butane, but the economics of blending isobutane vs 1-butene can differ considerably.  This can depend on a wide variety of factors that include:

 

  • time of year  (summer vs winter)
  • value of octane
  • product specification (RBOB vs Conventional)
  • and many more…

 

Having established the incentive to measure RVP blend performance differently, I’ll now address what can be done differently.  It is a common fallacy to place the blame of blend sub-optimization on shoulder of the gasoline blender.  Blend sub-optimization results from the combination of flawed blend planning and imperfect blend execution.  Often times refiners should look at upstream unit performance to truly address the shortcomings in blend optimization.

 

 

“Blend sub-optimization results from the combination of

 

flawed blend planning and imperfect blend execution”

 

Since blenders cannot monitor everything upstream of the component tanks, the responsibility really lies with unit process engineers.  Yes, process engineers truly have a large influence in how well blenders can perform their jobs.  Based on the concepts shared above, here are the following areas that process engineers can focus on:

 

Depropanizers: 

There’s nothing worse than getting propane into the refinery butane pool.  When this happens, it’s highly unlikely that the gasoline blends are even on spec, let alone worrying about being sub-optimized.  It’s still possible, particularly during winter blending, to have a dabble of propane and remain on spec, so just be sure that this is not happening to you.

 

Debutanizers: 

Having a sloppy cut between the debut overhead (C4-) and bottoms (C5+) may not necessarily be a bad thing.  An engineer should just understand the importance of each cut to understand how to optimize it. 

 

On a Sats Debut, the overhead stream produces a heartcut butane stream.  In the winter time it’s likely fine to let C5+ slip overhead since the butanes will all get blended into the gasoline pool.  During the summer sending C5s overhead will result in selling gasoline molecules at a highly discounted butane stream.  Conversely, dropping C4s into the Debut bottoms is not often times a question of blend optimization, but rather blend feasibility. 

 

On an Unsats Debut, managing this column operation has a larger range of economic impacts.  The split between overhead and bottoms affects variables such as alkylation economics, refinery octane loss, and RVP blend optimization.  One should not only focus on the molecule distribution for each product stream, but also pay attention the end disposition of the stream.  If you happen to have an unsats LPG stream routed to gasoline blending, explore if this is the best alternative.  While olefins have a favorable blending octane, the RVP penalty may be significant. 

 

DeIsobutanizers & Isostrippers:

As illustrated above, the split between normal butane and isobutane can have a significant impact on blend economics.  Excessing isobutane out the butane draw not only wastes valuable isobutane, but also has a significant impact on gasoline RVP optimization.

 

 

In conclusion, I hope that you all have a better appreciation for gasoline RVP blending.  For refiners who have good performance in minimizing RVP giveaway, opportunity may still exist to better optimize the pool. 

 

Instead of focusing purely on finished blend RVP, refiners should monitor molecule composition.  Furthermore, all refiners should understand that blend optimization does not only concern the refinery gasoline blender – it should involve everyone, from process engineers to planning analysts.

 
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