Refinery Alkylation Basics | RefinerLink
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Refinery Alkylation Basics

By Process Pro Eric

Jan 08, 2020
 

Breaking down some reaction fundamentals in Refinery Alkylation.

 
 

No one can deny the importance of an Alky unit within the integration of a U.S. oil refinery.  Whether benefits come from upgrading LPG to gasoline, or offsetting toxics within your gasoline blend pool, the Alkylation process produces liquid gold that all refiners love.  This is true whether you operate an HF Alky or a Sulfuric Alky plant. 

 

 

Despite the high value a refinery Alky unit brings, many people still treat the process like a big black box.  Everyone can say that high I/O and high acid strength are good, but not many refiners really understand why. 

 

I’m not going to explain every single variable in operating an Alky unit in this article, but I’ll cover some basics to move the competence needle a notch.   

 

 

 

The Alkylation process is organic chemistry at its finest.  In the geekiest form, it is a bimolecular nucleophilic substitution reaction where an olefin molecule reacts with an isobutane molecule to form a branched chain paraffin in the presence of an acid catalyst. 

 

Ok, blah…I did you no favors in demystifying the black box with that last paragraph.  However, I did this to prove a point.  We can indeed simplify all of this chemistry jargon into something that makes sense.  Let’s start from the beginning and break things down into steps.

 

The alky unit receives olefin feed from upstream cracking units such as the FCC and Coker. Olefins, by definition, are unstable molecules because of the double bond created by an extra set of electrons.  This reactivity sets the stage for the alkylation reaction.

 

Step 1: Cation Formation

To start, let’s take a butene olefin in an HF Alky unit.  The olefin molecule first reacts with the HF acid catalyst.

  • Fluorine is a very electronegative atom and disassociates from hydrogen easily
  • The olefin double bond attracts the hydrogen atom from the HF acid molecule
  • The olefin molecule becomes a cation (positively charged molecule) as it bonds to hydrogen  
  • A fluoride ion is formed when the hydrogen is released from the HF molecule

 

    

 

 

Step 2: Cation Re-arrangement

Before combining with an olefin molecule, the butane cation re-arranges itself to form an isobutane cation.  This occurs through a hydride and a methyl shift.

      

 

 

Step 3: Pairing

The Isobutane cation then reacts with another olefin molecule to create a larger cation

  • The Isobutane cation is electron deficient so seeks out an olefin molecule
  • The olefin molecule has an extra electron pair that it can donate to the isobutane cation

 

    

 

 

Step 4: Chain Termination

To successfully complete the refinery alkylation process, the Alkyl Cation created in Step 3 must now absorb a hydrogen atom from an isobutane molecule. 

  • Isobutane is very important in the alkylation reaction as it transfers hydrogen very readily.  This occurs because isobutane has a tertiary hydrogen, thus driving the importance of monitoring Alky I/O. 
  • An 8 carbon alkylate molecule (2,2,4-trimethylpentane) is created as a final product
  • An Isobutane cation is also created, thus propagating the next alkylation reaction again

 

   

 

 

Step 5: Catalyst recovery

Since the role of HF is to act as a catalyst and not as a reactant, the fluoride ions eventually recombine with hydrogen ions to reform a HF molecule again. 

 

 

 

Now with the basics of the refinery alky process understood, we can now explain the importance of alky process variables.  I’ll address the two most important variables that an alky process engineer monitors religiously. 

 

I/O Ratio:  The alky reaction depends on isobutane quite heavily as seen above.  Isobutane forms a cation that reacts with an olefin molecule to create a larger chain.  Isobutane also provides the hydrogen atom to terminate the alkylation reaction. 

 

Without sufficient Isobutane available, the alkylation reaction can continue to form undesirable longer chain polymers.  These polymers not only degrade the value of alkylate in gasoline blending, but can also form acid soluble oils (ASOs) that dilute acid strength.  

                                                                                                               

Acid Strength:  Acid acts as a catalyst in a refinery alky reaction, and requires a minimal amount to enable the reaction to occur.  As acid strength declines, undesirable side reactions occur, and can cascade on itself in a “runaway” manner that consumes all of the acid in the unit. 

 

An example of an undesirable side reaction is the creation of organic fluorides in an 

 

HF Alkylation unit.  With insufficient acid strength, the fluoride ion may not disassociate from the butane cation formed in Step 1. 

 

This creates a Flurobutane molecule that consumes acid and leaves the unit with product.                     

                                                       

Under typical circumstances it is normal to see some level of organic fluorides in LPG product (i.e. propane and butane), but a lesser extent in alkylate product.  In an acid runaway situation, the amount of organic fluorides in the alkylate product elevates as acid is consumed and exits with the alkylate. 

 

By reviewing the fundamentals of the refinery alkylation reaction, you can now apply this knowledge in understanding how the different refinery alky products are formed.  Use of the 5 Steps above can explain what happens when you alkylate different types of olefin feedstocks.

 

At the basic level you can see how different products are obtained when alkylating Propylenes, Butenes, and Amylenes.  It’s intuitive to derive the production of C7, C8, and C9 alkylates.  

 

It’s even more fascinating to understand how different isomers of alkylate are produced from the different isomers of olefin feed.  That’s right…1-butene, cis 2-butene, trans 2-butene, and isobutene all produce different types of alkylates.  From 2,2,3-trimethylpentane to 2,4 dimethylhexane, there’s another sub-level of chemistry geekiness that I’ll save for a later discussion. 

 

For now, I trust that the basic chemistry lesson above is enough to demystify the black box a bit.

 
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  • :   You show a butene molecule becoming an isobutane cation and then combining with another butene. So, you're showing the reaction to be butene molecule + butene molecule = alkylate molecule. You show isobutane's only contribution is to donate a hydrogen ato

    Aug 08, 2013

  • Process Pro Eric :   Josh, I started with a C4= turning into a cation to demonstrate how the acid catalyst initiates the reaction mechanism. At steady state, iC4 supports the alkylation reaction as a reagent by becoming the cation.

    Aug 10, 2013

  • Tony Evans :   Another plus point of the Alkylation process, is that the Alkylate product has low Octane sensitivity, thus has the capability to minimize Octane giveaway in finished product.

    Feb 08, 2016

  • José Luis Rodríguez Miller :   HF detectors are a very important part for this Alkylation Unit. They shall be failure proof.

    Feb 14, 2016

  • Chaz :   Can anyone help me with step 3? The arrow from the isobutane cation is drawn to the center carbon of the isobutene, however this would form 2,2,3,3, tetramethylbutane and not 2,2,4 TMP as shown. I was curious why the negative charge on the olefin would not be more stable at the center of the olefin and thus prefer the tetramethylbutane reaction.

    Feb 08, 2018

  • Steve :   When was the last HF unit built in the US?

    Jul 15, 2019

  • Tony Z :   I would guess last HF alky grassroots construction around the mid 70's in the US

    Aug 25, 2019

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