Petroleum Refinery Alkylation Process

ACID ALKYLATION IMPROVES GASOLINE PROPERTIES

Motor fuel alkylation using liquid hydrofluoric (HF) or sulfuric acids (H2SO4) is one of the oldest catalytic processes used in the petroleum refinery operations. The purpose of the alkylation is to improve motor and aviation gasoline properties (higher octane) with up to 90% lower emissions compared to conventional fuel.

The problem with HF is that the catalyst forms a hazardous air pollutant when released as a superheated liquid, while H2SO4 does not. Therefore nearly 90% of all alky units built since 1990 have adopted the H2SO4 technology.

WORLDWIDE BOOM OF ALKYLATION PROJECTS

Owing to high gasoline prices and reformulated gasoline regulations, the refineries invest in additional alkylation capacity. There is a construction boom in new alkylation units and in smaller retrofit projects, such as in replacing radioactive nuclear and coriolis meters with K-Patents Process Refractometers.

The leading H2SO4 alky units licensor is DuPont (Stratco) with it's 90% market share. Another licencor in the market is EMRE (Exxon Mobile Research Engineering, formely K.W. Kellogg).

BASICS OF ALKYLATION PROCESS

In the process, isobutylene is alkylated with low molecular weight olefins (propylene, butylene and pentylene) in the presence of a strong acid catalyst to form alkylate (the premium higher octane gasoline blending stock). The reaction is carried out at moderate temperatures in a two phase reaction. The phases separate spontaneously, so the acid phase is vigoriously mixed with the hydrocarbon phase to form higher molecular weight isoparafinic compounds.

After the reactor, the mixture is moved to an acid separation vessel where the liquid phases are separated into the acid and the hydrocarbon portion. The acid is then recycled back to the reactor.

MONITORING OF ACID STRENGTH CONTINUOUSLY

The problem involved in the sulphuric acid iso-butylene alkylation process is that sulphuric acid concentration is critical to complete consumption of the iso-butylene. A highly variable concentration of iso-butylene in the feed stock upsets the sulfuric acid content in the process.

It is important to determine the proper amount of acid to feed to the process. This is done by combining the routine sample titration analysis with a continuous acid monitoring using K-Patents Process Refractometer. Without the continuous measurement, the board operator would typically wait for a confirming titration result before making any changes to acid flow.

The continuous monitoring can prevent panic between the titration measurements. K-Patents refractometer better indicates any gradual changes in the acid flow control, which generally reduces unit acid consumption. It also helps prevent the scenario that's most unwanted by operating personnel, the acid runaway, a situation simply referred to as "when acid becomes wild".

The acid runaway may happen when the acid strength goes below 85-87%. Then the reactions between olefins and iso-butene turn into reactions of olefins only, producing polymers known as "acid sludge, ASO or red oil".

The initial acid concentration is typically 85-100% and the temperature 15°C (59°F). The benefits of K-Patents continuous monitoring include substantial cost savings associated with reduced acid consumption, and smooth alkylate production without acid runaways.

Alloy 20 can be used as sensor wetted material at high acid concentration (over 90%). Also Hastelloy C is available.