Acid Regeneration - Alkylation Spent Acid
February 25, 2002

Introduction

Alkylation of isobutane with C₃-C₅ olefins is generally considered to be the best method of producing high octane number gasolines for motor vehicles.  The classic reaction involves isobutylene and isobutane to produce trimethyl pentane (isooctane), which has a defined octane number of 100.

      CH₃                 CH₃                      CH₃     CH₃
       |                      |                           |          |
CH₃-C=CH₂ + CH₃-CH-CH₃    -->  CH₃-C-CH₂-CH-CH₃
                                                       |
                                                       CH₃

The above reaction is a catalyst based reaction that has been used throughout the world since the phaseout of lead based octane gasoline additives.  Concentrated sulphuric acid in the range of 88% to 92.5% H₂SO₄ is used as a catalyst for the reaction with approximately an equal amount of water and organics making up the remaining portion.

In theory, a catalyst promotes a chemical reaction and is not consumed or modified by the reaction.  In the alkylation reaction, the acid is diluted as a result of side reactions and feed contaminants.   The alkylation process ‘consumes’ approximately 0.18 to 0.27 kgs (0.4 to 0.6 lbs) per gallon of alkylate.  The loss of sulphuric acid is made up by fresh concentrated (98.5 wt%) sulphuric acid and a bleed stream of spent acid is removed from the process.  The spent acid stream undergoes a series of process steps to remove entrained hydrocarbons which are returned to the process with the spent acid being sent to storage.

The acid concentration in the alkylation process must be strictly controlled to prevent runaway reactions from occurring.  A runaway reaction will occur when the acid strength is between 85 to 87% H₂SO₄.   At the acid concentration, the reaction conditions do not favour the alkylation reaction between olefins and isobutane.  Instead, olefins will react with each other to form conjunct polymers which are also known as acid sludge, acid soluble oil and read oil.

The conjunct polymers are soluble in sulphuric acid which has the effect of decreasing the concentration of the sulphuric acid.   As the acid concentration decreases, the reaction to form additional conjunct polymers is favoured and the reaction accelerates.

Another reaction that occurs is the oxidation of polymers by sulphuric acid.  The polymer is oxidized to a tar like substance and sulphuric acid is reduced to water and SO₂.  

These reactions can occur within the alkylation process unit or in the spent acid storage tanks.