Unification

The lightest molecular weight products of other units in the refinery are further processed through unification. Both alkylation and polymerization use a combination of heat, pressure, and catalysts to react and combine hydrocarbon molecules that contain double bonds. In an alkylation unit, light petroleum gases (LPGs) containing double bonds are reacted with isobutane in the presence of a catalyst, producing alkylate, a high performance fuel containing larger iso- hydrocarbons such as iso-octane or iso-nonane.

Polymerization units operate in a similar manner, except polymerization reactions do not have an isobutane component. Inside the reactor, LPGs are reacted in the presence of a catalyst to form larger hydrocarbon molecules. These heavier molecules are collectively known as polymer gasoline. Polymer gasoline has physical properties similar to other gasoline components, meaning it can be blended with gasoline produced from units throughout the refinery.

After both alkylation and polymerization reactions are carried out, the products are passed through various separators to isolate the unreacted LPGs, waste materials, and the desired products.

Production

Catalysts are used in alkylation units to help the reactions proceed at a lower temperature, decreasing the possibility of polymerization of the LPG reactants and favoring the alkylation reaction between LPGs and isobutane.

Hydrofluoric acid and sulfuric acid are two catalysts that are commonly employed in alkylation units. Hydrofluoric acid can be used for higher temperature reactions and all feed compositions. Sulfuric acid is used for lower temperature reactions but due to an unfavorable side reaction, it cannot be used with LPG feeds that contain ethylene gas.

The hydrocarbon feed to alkylation units consists of two components: Isobutane and LPGs that contain double bonds. The isobutane feed often comes from the isomerization reaction unit, while the LPG feed originates from the either the initial distillation column or the cracking units. Conditions in the reactor will vary based on which catalyst is being used and the composition of the LPGs. A hydrofluoric acid catalyzed reaction can be run at a higher temperature because the hydrocarbons are less likely to form polymers, which is an undesirable side reaction for the alkylation process.

Polymerization units also employ a catalytic acid. Rather than using sulfuric or hydrofluoric acid, polymerization units use phosphoric acid. Combinations of phosphoric acid and solid pellets can be used to carry out specific reactions, but as a general rule, most catalytic polymerization units use liquid phosphoric acid.

Unlike the alkylation unit, the polymerization unit's hydrocarbon feed consists solely of LPGs that are obtained from either the initial distillation column or the cracking units. Hydrocarbon feed is combined with the acid catalyst inside the reactor. The temperature inside the reactor needs to be closely monitored in a polymerization unit for two reasons. First, proper reaction conditions need to be maintained to ensure that the product being formed is limited to dimers, trimers, or tetramers (reacting two, three, or four individual molecules of LPG). Second, the polymerization reaction is highly exothermic, meaning the reaction generates a large amount of heat. The reaction could quickly result in a major safety issue if the heat being released isn't properly managed.

Purification

After the hydrocarbon-catalytic acid mixture has passed through the reactor, it enters a liquid settler , where it will gradually separate out into a hydrocarbon-rich phase on top and an acid-rich phase on bottom. The acid-rich phase is recycled back into the alkylation unit feed to be reused as a catalyst. The hydrocarbon-rich phase is sent to a distillation column .

Alkylate, the desired product, is removed through the bottom of the distillation column, while various by-products and unreacted material are removed through the top section of the column. After distillation, alkylate is sent to another unit in the refinery where it is blended with fuel products from other units to create the final gasoline mixture, which is sent to storage and eventually sold. Unreacted material and by-products are first sent to a stripper to ensure that there is little to no remaining acid catalyst present in the gas. After stripping, the unreacted materials and by-products are passed through a second fractionator. Unreacted materials such as isobutane are recycled back to the feed entering the alkylation unit. By-products such as propane and butane are sent to other units in the refinery to be treated and stored.

In a polymerization unit, the hydrocarbon-catalytic acid mixture undergoes a purification process similar to that of the alkylation unit. Taking advantage of liquid phase equilibrium properties, the hydrocarbons are separated from the acid catalyst using a liquid settler. The acid is recycled and sent back to the entrance of the reactor to be recombined with the reaction feed. Flash drums or distillation columns are used to further separate the hydrocarbon mixture into polymer gasoline, unreacted LPGs, and by-products such as propane and butane. The light petroleum gases are recycled and sent back to the entrance of the reactor, while by-products and polymer gasoline are sent to storage.

Acknowledgements

Sierra Monitor Corporation , Milpitas, CA

Monroe Environmental Corporation , Monroe, MI

ROBATEL, Inc. , Pittsfield, MA

References

"Petroleum Refining Process." OSHA Technical Manual. Section IV: CHapter 2. United States       Department of Labor, Occupational Safety & Health Administration, n.d. Web. 13 Mar.       2016. https://www.osha.gov/dts/osta/otm/otm_iv/otm_iv_2.html

Speight, James G. The Chemistry and Technology of Petroleum. 4th ed. Boca Raton: CRC       Press, 2007. Print.

Developers

Jackson Irwin