Jan-1997

Caustic treatment of jet fuel streams

Caustic treatment of jet fuel streams using Fiber-Film Contactors has proven to be practical and reliable, compared with conventional systems

Patricia Forero and Felipe J Suarez, Merichem Company
Abe J duPont, National Petroleum Refiners of South Africa

Article Summary

There are many important aspects of chemically treating kerosene fractions with sodium hydroxide solutions (caustic) for the removal of naturally occurring contaminants in the production of jet fuels. A treating process consisting of several steps is often necessary to meet acidity, mercaptan, and other specifications required for the upgrading of these fractions to jet fuel products.

The acidity specification of jet fuel is measured by its neutralisation number, or total acid value, which can be related to its corrosion potential on equipment and engines.

Naphthenic acids are the main contributors to acidity of jet fuels. Their name is generic for a family of compounds that belong to the broader category of carboxylic acids where one, or a combination, of saturated ring hydrocarbons, have the organic acid (COOH) radical attached to one of the carbon atoms.

Although naphthenic acids are naturally found in most crude oils, fortunately for refiners they create little processing difficulties because their concentration is typically quite low. However, there are several important crude oil sources in the world where this is not the case, such as in Peru and Venezuela in South America; Trinidad in the Caribbean; California and Louisiana in the USA; mainland China’s Sheng Li and Xing Xiang crudes; and in some European crudes such as those produced in Romania, as well as new finds in the North Sea.

Mercaptan is the generic name for a family of organic compounds where a sulphur and a hydrogen atom (SH) are bonded to one of the carbon atoms in the molecule. The hydrogen atom in the SH radical can ionise and produce a mildly acidic environment but to a lesser extent than naphthenic acids. The most noticeable characteristic of mercaptans is their strong, unpleasant odour even when their concentration is only a few parts per million.

It is interesting to note that jet fuel fractions that are derived from crude oils containing large amounts of naphthenic acids seldom contain significant quantities of mercaptans. Likewise, the opposite is also true. Furthermore, there are a few crude oils that contain neither in significant quantities. In general, Middle Eastern, Mexican and US West Texas crudes are high in mercaptan content.

South American crudes, as expected because of their high acid content, do not contain significant amounts of mercaptans. However, most refiners process a variety of mixed crude oils that will require them to deal with both types of impurities in their jet fuel treating operations.

One of the fastest growing refinery product market demands is jet fuel, often called turbine fuel. Air travel is projected to continue growing in popularity in the years to come and the refinery that produces jet fuel at the lowest cost will be in the best position to compete in this market. A refiner that produces high quality jet fuels can find attractive markets for his product throughout the world.

Jet fuels must meet very stringent international specifications because they are used by airlines all over the world who, regardless of where they land and refuel, must purchase quality and safe fuels. Among the numerous specifications are acidity, aromatics, olefins, naphthalene, smoke point, sulphur, mercaptan, freeze point, colour, and water separation index.

As is readily apparent to those familiar with caustic treating, some of these specifications are not affected in any way, since the compounds affecting the specifications do not react with caustic. Aromatics, olefins, smoke point, sulphur content, and freeze point are such specifications.

The refinery production of jet fuel varies from simply withdrawing a side stream product from the crude oil fractionator that requires no additional treating or cleanup, to caustic treating followed by water washing, salt drying, and clay filtration; and, finally, to hydrotreating the product so that it can meet all jet fuel specifications.

Hydrotreating requires a much greater capital investment (10 to 20 times) and involves much higher operating costs (20 to 50 times) than “wet treating”, which is the phrase often used to denote caustic treating, with the attendant clean-up processes. For these reasons, refineries avoid hydrotreating whenever possible.

However, hydrotreating can produce jet fuel from most crude oils, whereas, wet treating is limited to jet fuels that already meet the specifications not affected by caustic treating. Table 1 provides a cost comparison of caustic treating and hydrotreating.

Principles of caustic treating
The removal of any impurity involves mass transfer or, in the case of caustic treatment, the movement of the impurity from the hydrocarbon to the aqueous solution. The rate at which this mass transfer occurs is the product of three independent variables:
M = KA ∆C
where
K is the mass transfer coefficient for the given hydrocarbon and aqueous system
A is the amount of surface area available for the impurity to pass from the hydrocarbon to the aqueous phase
∆C is the concentration driving force impelling the impurity to leave the hydrocarbon and enter the aqueous phase.

In the conventional treating mechanism, devices such as mix valves and static mixers create interfacial surface by dispersive mixing to generate droplets of one phase in the second phase. The outside surface of each droplet provides the mass transfer surface. However, the sphere is the shape with the least surface area per unit volume of any other shape – the very opposite condition demanded for high mass transfer rates.

To create the most interfacial surface area possible from a given volume, considerable sheer energy must be imparted to form as many small droplets as possible.