Reduce Operating Costs by
Caustic Treating Jet Fuel Streams

The caustic treatment of jet fuel streams using FIBER-FILM™ Contactor has been proven to be practical, reliable, and economical when compared with conventional hydrotreating systems. Low and constant acid number specifications can easily be obtained in one single stage, using a contactor, when the feed contains acid numbers as high as 0.40.

RG Vazquez, Merichem Company

The caustic treatment of jet fuel streams to reduce acidity has been practiced for years. In the past, unfortunately, the refiners experienced numerous problems with emulsions when conventional mixing and settling caustic treatment was used. For this reason, most refiners resorted to a much more capital and energy intensive process: hydrotreating. With this innovative technology, Merichem has demonstrated in many commercial installations that total acidity can be easily reduced with caustic solutions without emulsions when the film-film contactor is used. The following article describes a recent case history.

Installation

In early 1988, Lagoven SA, a subsidiary of Petroleos de Venezuela, installed and started up a 30 000 BPSD caustic treating system to reduce the acidity number from 0.40 to less than 0.015 in its jet fuel stream. Lagoven has a major refinery in Venezuela.

A caustic treating system using the film-film contactors was designed, fabricated in modular form, and delivered by the Merichem Company to replace its existing hydrotreater. This caustic treating system consists of a NAPFINING unit, an AQUAFINING unit. a salt drier, and clay towers. The caustic treating unit and the water wash unit use the contactors. These contactors; are used because they involve non-dispersive contacting of caustic, water, and hydrocarbon phases, thus minimizing the potential for emulsion formation and resulting in minimum caustic and water carry-over and high utilization of caustic solution.

The NAPFINING unit removes naphthenic acids with caustic and consistently reduces the neutralization number of the jet fuel stream to less than 0.015mg KOH/g. Several runs had been conducted at NN as high as 0.50, obtaining a product stream with less than 0.015mg KOH/m. The reaction mechanism is as follows:

RCOOH + NaOH → RCOONa + H₂O

The caustic treated stream leaves the separator vessel substantially free of acidity or entrained caustic droplets.

The AQUAFINING unit is used to eliminate any traces of sodium naphthenates and NaOH before the jet fuel stream passes to the salt drier and clay towers. The salt drier reduces the total water content in the jet fuel stream to protect the clay tower which ensures water separation (WSIM) specification. The salt drier was recommended to guarantee product haze and to extend life of the downstream clay towers, which would otherwise absorb the free water and get consumed.

Figure 1 depicts Lagoven's caustic treating system. Kerosene feed is pumped by centrifugal pumps P-1 or 2 to the NAPFINING unit. Before the untreated kerosene enters the film-film contactor (FFC-1), the hydrocarbon stream passes through one side of two parallel basket strainers (S-1 or 2) to remove any solid particles. The kerosene then flows to the top of FFC- I where it meets the caustic-wetted fiber material. As the kerosene flows downward through the contactor, the acidic impurities, mainly the naphthenic acid, diffuses into the aqueous phase and reacts with the sodium hydroxide to form sodium naphthenate as shown by reaction (1).

The film-film contactor is a static contacting device which improves the removal of the naphthenic acid from the kerosene stream. The removal of this impurity involves mass transfer, or the movement of the mass of the impurity from the hydrocarbon to the aqueous solution.

The contactor increases the rate in which mass transfer occurs. Further explained, the mass transfer rate (M) is the product of three independent variables: M = KAD C where: K is the mass transfer coefficient for the given hydrocarbon and aqueous system; A is the amount of surface contact available for the impurity to pass from the hydrocarbon to the aqueous phase, and D C is the concentration driving force impelling the impurity to leave the hydrocarbon and enter the aqueous phase.

The contactor, containing a multitude of fibres, provides a large amount of interfacial surface (A) which increases the mass transfer rate. At the same time, the aqueous phase is constrained to the fiber material by surface tension, and the stronger the concentration, the stronger the aqueous phase adheres to the fiber. This property of the contactor helps in producing a clean separation of the two phases and producing a kerosene stream substantially free of entrained caustic carryover.

Separator Vessel

After the treated kerosene exits the contactor, it passes through the separator vessel, V-1, and exits at the end opposite FFC-1. The circulating caustic is drawn down and along the fiber material by the interfacial drag force of the kerosene stream, it follows the fiber material to the aqueous layer, and is recirculated by centrifugal pump, P-3 or 4, to the inlet of FFC-l. An automatic flow controller maintains the caustic flow at about 10 per cent volume of the kerosene rate.

Fresh 3-7°Be caustic solution is charged continuously to the NAPFINING unit. The fresh caustic is always passed through a basket strainer to remove particles larger than 150 micron. The fresh caustic makeup concentration is maintained by means of a caustic conductivity analyzer controlling the water makeup rate to the system. The spent caustic is removed on level control to the spent caustic storage tanks. The fresh caustic makeup rate is set to maintain a 50 to 60 per cent spending level of the caustic to sodium naphthenate. Because of the high spending, and thus the quality of this stream, it can be easily marketable for naphthenic acid recovery.

A variable sampler is included in the NAPFINING vessel to locate and control the emulsion layer should it occur. This layer can be drawn off to either the spent caustic tank or sewer through a draw-off line located at the caustic and kerosene interface.

The treated kerosene stream is then introduced to the top of the second film-film contactor (FFC-2) where it contacts metallic fibres which are wetted with a recycled water solution circulated by means of pumps, P-5 and P-6, on flow control. The kerosene and the water solution flow concurrently downward through the contactor shroud wherein the slight amount of entrained sodium naphthenate and NaOH are removed. The washed kerosene leaves the contactor, passes through and exits the separator vessel (V-2) at the end opposite from FFC-2, and flows to the salt drier.

The water solution containing traces of naphthenates adheres to and follows the fiber material downward until it reaches the water layer in the bottom of the separator vessel. The water solution is recycled by means of centrifugal pumps on automatic flow control back to the top of FFC-2 at a rate approximately equal to 20 per cent volume of the kerosene stream.

Fresh water addition is continuously pumped on flow control from the fresh water storage tank, TK-1, by means of pumps P-7 and 8. Before the fresh water enters the AQUAFINING unit, it passes through one of two basket strainers (S-3 and 4) to remove solids. The fresh water addition rate is controlled to maintain less than 300ppm wt of Na+ in the spent water stream.

A cooler (E-1) ahead of the AQUAFINING unit was included in this design to lower the temperature of the kerosene stream near the temperature in the storage tanks. Due to the cooling of the kerosene stream, additional water dropout in the AQUAFINING unit occurs thus preventing this from occurring in the product tanks.

Before the kerosene enters the clay towers, it passes through a salt drier (Figure 2) to remove free and some soluble water and further clean the product. The kerosene flows up through the salt bed while the water is removed from the bottom of the drier as a brine solution. The salt bed is approximately l7ft deep.

 

The salt bed will be topped off with fresh salt approximately every three to four months. No more than one day will be required to refill the drier with fresh salt. Pressure drop through the salt drier is typically less than 5psi.

After being dried in the salt drier, the kerosene flows down through two parallel beds of Attapulgus clay. This final processing step removes any solids, moisture, soaps, emulsions, and surfactants. This clay treating step will ensure that the treated kerosene product stream will meet the haze, color stability, and water separation specifications. The clay towers are 14ft by 24ft T/T length with a clay bed of approximately 21 ft deep.

Clay filtering is considered a necessity by refineries in order to produce military jet fuels such as JP-4 and Nato F-35. The WSIM, color, and stability specifications of these fuels are quite stringent and very much scrutinized by the users of these fuels.

The use of film-film technology plays a major role in ensuring that the clay towers will function properly without excessive pressure drops and for long periods. The clay towers for Lagoven were designed for approximately a 12-month life based on WSIM improvement. However, Lagoven has found that the clay needs to be replaced approximately every five months due to its saturation with water.

The pressure drop across the clay bed usually determines the life of the treater before replenishment is required. Pressure drop will be caused by the quantity of particles in the kerosene and the quantity of water which causes caking of the bed.

Large quantities of free or dissolved water should be avoided since it shortens the life of the clay. The maximum recommended delta pressure is 25psi. This high delta pressure has not been reached at Lagoven. Clay has been replaced due to a product stream being out of specification on appearance.

The estimated annual requirements for utilities, chemicals, and manpower for a caustic treating system, bases on 30 000 BPSD, feed NN = 0.40, and 350-day operating year, are presented in Table 2.

The US Gulf Coast sum total of all operating costs for utilities, chemicals, and manpower excluding chemical additives required by Nato-35 specifications amount to less than $0.04/BBL of jet fuel produced. This compares with a much higher US Gulf Coast cost for hydrotreating of $0.50 to $0.75/BBL.

Lagoven refinery has had this caustic treating system in operation for over a year. Table 1 shows some typical feed and product characteristics at design flow rates. As clearly shown, the refinery that produces a good quality jet fuel at the lowest cost will be in the best position to supply the growing market.

This article is being published with the approval of the Lagoven Amuay Refinery the Merichem Company appreciates the assistance provided by Lagoven personnel, in particular Mr. Cirilo Gonzalez who was the engineer in charge of the design and initial operations of the system.

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Ramiro G Vazquez is the Development Manager for the Proprietary Technology Group at the Merichern Company. He has worked for the company for eight years in several engineering and managerial capacities. He previously worked for Exxon at its Baytown refinery. He has an MSc in chemical engineering from the University of Houston.