Alteration

Alteration processes are used to upgrade the octane number of products obtained throughout the refining process. A higher octane number means that the fuel can withstand a greater pressure before detonation, increasing the performance of the fuel in high-pressure environments such as that in jet engines or internal combustion engines.

Reforming and isomerization are two commonly used alteration processes. Both processes use catalysts to carry out the reaction. The difference between the two processes is that catalytic reforming can be used with a wide variety of feeds, whereas isomerization is usually limited to butane, pentane, and hexane feeds. Reforming and isomerization follow similar paths through their reaction units. First, the feed is pre-heated with a furnace. Next, the pre-heated feed is passed through a catalytic reactor. Finally, the reactor products are passed through separators and distillation columns, separating the desired hydrocarbon products from the waste.

Pre-Heating The Feed

are used to pre-heat a mixture of hydrocarbon feed and hydrogen up to high temperatures, ranging from 450°C to 520°C. The hydrocarbon feed usually consists of what is known as "straight-run gasoline", meaning that it comes straight from the refinery's distillation column. The hydrocarbon feed is sometimes treated to remove components such as sulfur or nitrogen that could poison the reaction catalyst. Direct-fired furnaces can also be inserted in between individual reactors to reheat the reactants and make up for the heat lost due to the highly endothermic nature of the reforming reaction.

(Copyright Nutec Bickley, Santa Catarina, Mexico)

Production

There are three types of reactors used in catalytic reforming: Fixed-bed, fluidized-bed and moving-bed reactors. Temperatures range from 400°C to 900°C, and pressures can range from 200 to 1000 psi. Despite the use of different catalysts and reactor conditions, all three reactors can effectively carry out the reformation process. Based on the composition of the feed, four separate reactions will occur throughout the process. Dehydrogenation results in the formation of aromatic structures where there were previously hydrocarbon rings, producing hydrogen gas as a by-product. Dehydrocyclization converts straight chain hydrocarbon molecules into ring-shaped aromatic hydrocarbons, also producing hydrogen gas a byproduct. Isomerization converts straight chain hydrocarbon molecules into branched or iso- hydrocarbon molecules such as iso-octane. Hydrocracking breaks apart the largest hydrocarbon molecules into smaller molecules, which is comparable to the reaction that takes place during thermal and catalytic cracking.

The catalyst used in reforming reactions will vary based on the composition of the feed, pre-treatment of the feed, and the reactor type. For example, platinum catalysts cannot be used with feeds that have not been pre-treated to remove sulfur, as the sulfur will poison the catalyst, leading to highly reduced yields and increased costs to replace the poisoned catalyst. Due to the high reaction temperatures, layers of coke (pure carbon) will build up on the catalyst, gradually leading to deactivation because of reduced surface area. To regenerate the catalyst, it is reacted in the presence of oxygen to remove the coke layer.

Isomerization reactions also use a catalyst. Commonly used catalysts include platinum or aluminum chloride with anhydrous hydrochloric acid. The catalytic reactions that take place in isomerization are more selective than reforming reactions. Molecules are only being rearranged; they are not being cracked or dehydrogenated.

(Copyright Falmouth Products Inc., Falmouth, MA)

Fixed-bed reactors are used for a reforming process known as continuous catalytic regeneration, or CCR. Utilizing a reactor known as a swing reactor, CCR allows refineries to keep multiple reactors on-stream while other reactors are swung off-stream for catalyst regeneration. Reactor conditions are at high temperature (450°C to 550°C) and pressure (200 to 1000 psi). The conditions can vary based on the catalyst and feed composition. Platinum is the most common catalyst utilized in CCR units. In addition to a high yield of reformate, the CCR process also provides a constant supply of hydrogen, due its continuous operation.

In a fluidized-bed reactor configuration, a separate reactor is integrated with the main reforming reactor, allowing for continuous regeneration of the catalyst. Molybdena (Mo2O3) catalyst is used for this reaction. Reactor conditions can range from 480°C to 950°C and 200 to 300 psi.

In moving-bed reactors, a catalyst consisting of Cobalt-Molybdate pellets flows down the reactor under the influence of gravity. The feed, which consists of hydrocarbon vapor and hydrogen gas, moves upward against the flow of catalyst. Reactor conditions can range from 425°C to 480°C and around 400 psi.

Isomerization reactions are carried out in fixed bed reactors, with temperatures ranging from 40°C to 480°C and pressures ranging from 150 to 1000 psi. Butane isomerization often takes place in the lower end of the temperature range while pentane and hexane isomerization takes place in the higher end.

Purification

Hydrogen is separated from the hydrocarbon products after the mixture has passed through all of the reactors. After separation, the hydrogen gas is split into two streams - one hydrogen stream is recycled back into the reforming unit feed, and the second hydrogen stream is piped to other units in the refinery where it can be utilized for processes such as chemical manufacturing or energy production.

The hydrocarbon products are separated in a placed at the end of the reforming unit. The two major products are reformate and light petroleum gases (LPGs). Reformate is removed through the bottom of the distillation column and sent to storage, where it can be blended with other gasoline components to produce a valuable high-octane product. LPGs, such as butane and propane, are removed from the top of the distillation column and sent to other processing units in the refinery.

After passing through the isomerization reaction unit, the products go through a purification process similar to a reforming unit's purification process. First, hydrogen is separated from the hydrocarbons. Any unreacted hydrogen is recycled back into the isomerization unit feed. After the hydrogen separation, the hydrocarbons are passed through a distillation column. Iso- hydrocarbons such as isobutane are removed from the bottom of the column and sent to storage. Unreacted hydrocarbon feed is removed from the top of the column and recycled back to the start of the isomerization unit to be combined with the original feed.

(Copyright Sulzer Chemtech Ltd., Switzerland)

Acknowledgements

Sierra Monitor Corporation , Milpitas, CA

Falmouth Products Inc. , Falmouth, MA

Nutec Bickley , Santa Catarina, Mexico

Sulzer Chemtech Ltd. , Switzerland

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

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