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1 May 2002

In-line RI applications enrich refinery ops

Lube oil processing: The refractive index of lube oils relates to the viscosity index (VI), which determines the quality of lube oil. The crude oil type also influences the final VI of lube oil.

The challenge to the refiner is the selection and blending of the crude type to optimize the production of the desired lube oil. An in-line refractometer can help control the final quality or final VI of the lube oil.

Several types of process changes can affect VI, including the blend stock rates and extraction tower temperature. There is a finite residence time for the effect of a given process change to pass through the process unit.

However, lab samples often cannot take place fast enough to avoid a deleterious effect to the process. Only an in-line sensor can establish the correlation between the rapid process changes of VI that can lead to an adjustment to the efficiency of the process.

Sulfuric acid alkylation: The alkylation process used in refining is the reaction of isobutane with an olefin to form an isoparaffin compound with superior stability and antiknock characteristics.

The compounds improve the octane rating of aviation gasoline and motor fuel. In the sulfuric acid alkylation unit, sulfuric acid helps convert isobutane into isooctane.

The process depletes the acid’s strength. It is important to measure the acid strength to prevent a runaway reaction that occurs at low acid levels. In-line monitoring provides earlier warning than titrating samples in the lab or using a hydrometer and allows dramatic reduction of the actual runaway set point value.

Amine/Acid gas sweetening: Refineries that process high sulfur or sour crude oil generate hydrogen sulfide (H₂S), a pollutant that the refinery must capture. An acid gas unit uses a water-based amine solution to remove the H₂S.

There are three types of amines: monoethanolamine (MEA), diethanolamine (DEA), and methyldiethanolamine (MDEA), with each having a unique relationship between RI and concentration.

MDEA absorbs the greatest amount of H₂S, while MEA absorbs the least. Due to the increasing amount of sour crude oil processing, most amine units use either DEA ($0.50/pound) or MDEA ($1 to $1.50/pound) because of their higher absorption.

RI sensors can install in three locations—rich amine, lean amine, and the reflux accumulator—to reduce amine consumption and improve the process.

Scrubber applications: Scrubbers remove unwanted components from process streams. The removal may be necessary because the product is corrosive or its discharge is limited due to environmental regulation.

Acids are a commonly removed component. Removing the acid prevents system corrosion, meets environmental regulation, and improves production. Sodium hydroxide (caustic) neutralizes the sulfuric acid.

Fresh or makeup caustic adds as it depletes. An in-line monitor system ensures that the sudden drop in pH that can occur when the caustic concentration falls in these kinds of reaction doesn’t.