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.  An acid number specification can be obtained in a single stage using this technology

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

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 neutralization 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 ionize and produce a mildly acidic environment but to a lesser extent than naphthenic acids. The most noticeable characteristic of mercaptans is their strong, unpleasant odor 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, color, 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.

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:

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
• D 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.

In the case of caustic treating systems, small droplets of caustic solution in the hydrocarbon increase the rate of mass transfer. Small droplets, however, have the disadvantage of taking longer to separate or settle out of the hydrocarbon, increasing the difficulty of the next essential operation in any treating job, which is to separate the aqueous phase from the treated hydrocarbon.

The most frequently encountered problem with treating systems in the oil industry is caustic carryover with the treated hydrocarbon. Settlers associated with dispersive mixing must be quite large to avoid aqueous phase carryover. Stokes Law can be used to size the settler once the caustic droplet size is known, Quite often, settlers are undersized for economic reasons.

As hydrocarbon market demand grows, and throughputs must be increased through the settler, the settling time becomes even more inadequate and many more unsettled caustic droplets remain in the treated hydrocarbon. The presence of caustic in the treated hydrocarbon can cause multiple problems and, if allowed to go unchecked, can result in a loss of product acceptability in the market. The first step usually taken when caustic carryover becomes unacceptable is to input less mixing energy, thus creating larger droplets which settle more rapidly. This diminishes mass transfer surface area (A) which reduces the mass transfer rate and treating efficiency. Quite often, decreasing the caustic concentration of the treating solution will reduce caustic carryover. This diminishes the equilibrium constant (K) which, in turn, reduces the mass transfer rate and treating efficiency.

At some point, as treating efficiency diminishes, the treating operation cannot afford further reductions in mixing energy or caustic concentration.

It was the dilemma of caustic carryover faced by the oil industry that Merichem addressed in its research programs some 20 years ago. The answer that evolved was FIBER-FILM™ Contactor technology - a new and more efficient method of creating interfacial surface area that would avoid dispersion while at the same time allowing equilibrium constants and concentration driving force (D C) to be increased.

The FIBER-FILM™ Contactor is a static contacting device that produces nondispersive contacting of the caustic and hydrocarbon phases and improves the removal of acidic impurities from the hydrocarbon stream. This prevents emulsion formation and results in minimum caustic carryover and high utilization of the caustic solution.

The contactor, containing a multitude of fibers, provides a large amount of interfacial surface area (A) which increases the mass transfer rate. At the same time, the aqueous phase is constrained to the fiber material by surface tension forming a film on each fiber that contacts, but never mixes with, the hydrocarbon phase. Consequently, separation of phases becomes a simple and efficient step in the process.

In addition, the stronger the concentration, the greater the tendency for the aqueous phase to adhere to the fiber. This property of the contactor helps in producing a clean separation of the two phases yielding a hydrocarbon stream substantially free of entrained caustic.

NAPFINING Technology

There are two important steps in wet treating of jet fuel: total acidity reduction and mercaptan oxidation. The processing step required when total acidity must be reduced is a weak caustic prewash which is designed specifically to extract strongly acidic compounds such as H₂S but in particular naphthenic acids from the jet fuel.

The typical neutralization number specifications vary from 0.005mg KOH/g of hydrocarbon to as high as 0.10, depending on the product market or downstream process requirements. In addition, the removal of H₂S and light mercaptans (if any) ensure that the product jet fuel will meet the copper and silver strip corrosion specifications.

The purpose of Merichem's NAPFINING technology is to extract naphthenic acids from distillate fractions, such as kerosene/jet fuel, to ensure that the final product acid number specification will be met. This process not only reduces product corrosivity but also protects the downstream sweetening system.

One of the major problems in operating conventional naphthenic acid extraction systems is the formation of stable emulsions of sodium naphthenate in the treated hydrocarbon. Merichem has demonstrated in many commercial installations that the total acidity can be easily reduced with caustic solutions without creating emulsions when the FIBER-FILM™ Contactor is used.

The conventional mixing/settling mechanism with electrostatic precipitation, historically used for naphthenic acid extraction with caustic, has been one of the major contributors to problematic jet fuel production for many refiners. In fact, it has led some to abandon caustic treating in lieu of the more expensive hydrotreating.

Sodium naphthenate has a great tendency to emulsify with the jet fuel, producing a stable emulsion, sometimes called a rag or soap, which is very difficult to break and certainly not in the time provided in most conventional caustic treating systems. Therefore, entrainment of the caustic phase can be excessive when dispersive mixing devices are used.

If these soaps get into downstream mercaptan sweeteners, they adversely affect their performance. These soaps also will cause the jet fuel to fail other specifications, such as water separation index, if allowed to remain in the finished jet fuel product. Quite often the system can only be made to work by adjusting operating variables, such as caustic strength, and spending to inefficient set points that substantially increase operating costs. Therefore, the NAPFINING step is critical to successful jet fuel production.

The following example of a recent commercial NAPFINING installation illustrates this point.