Source: HANDBOOK OF PETROLEUM REFINING PROCESSES

CHAPTER 11.3

UOP MEROX PROCESSG. A. DziabisUOP LLC Des Plaines, Illinois

INTRODUCTIONThe UOP* Merox* process is an efficient and economical catalytic process developed for the chemical treatment of petroleum fractions to remove sulfur present as mercaptans (Merox extraction) or to directly convert mercaptan sulfur to less-objectionable disulfides (Merox sweetening). This process is used for liquid-phase treating of liquefied petroleum gases (LPG), natural-gas liquids (NGL), naphthas, gasolines, kerosenes, jet fuels, and heating oils. It also can be used to sweeten natural gas, refinery gas, and synthetic gas in conjunction with conventional pretreatment and posttreatment processes. Merox treatment can, in general, be used in the following ways:

To improve lead susceptibility of light gasolines (extraction) To improve the response of gasoline stocks to oxidation inhibitors added to prevent gum formation during storage (extraction and sweetening) To improve odor on all stocks (extraction or sweetening or both) To reduce the mercaptan content to meet product specifications requiring a negative doctor test or low mercaptan content (sweetening) To reduce the sulfur content of LPG and light naphtha products to meet specifications (extraction) To reduce the sulfur content of coker or fluid catalytic cracking (FCC) C -C olefins to save on acid consumption in alkylation operations using these materials as feedstocks or to meet the low-sulfur requirements of sensitive catalysts used in various chemical synthesis processes (extraction)3 4

PROCESS DESCRIPTIONThe UOP Merox process accomplishes mercaptan extraction and mercaptan conversion at normal refinery rundown temperatures and pressures. Depending on the application,*Trademark and/or service mark of UOP.

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UOP MEROX PROCESS 11.32SULFUR COMPOUND EXTRACTION AND SWEETENING

extraction and sweetening can be used either singly or in combination. The process is based on the ability of an organometallic catalyst to promote the oxidation of mercaptans to disulfides in an alkaline environment by using air as the source of oxygen. For light hydrocarbons, operating pressure is controlled slightly above the bubble point to ensure liquid-phase operation; for heavier stocks, operating pressure is normally set to keep air dissolved in the reaction section. Gases are usually treated at their prevailing system pressures.

Merox Extraction Low-molecular-weight mercaptans are soluble in caustic soda solution. Therefore, when treating gases, LPG, or light-gasoline fractions, the Merox process can be used to extract mercaptans, thus reducing the sulfur content of the treated product. In the extraction unit (Fig. 11.3.1), the sulfur reduction attainable is directly related to the extractable-mercaptan content of the fresh feed. In mercaptan-extraction units, fresh feed is charged to an extraction column, where mercaptans are extracted by a countercurrent caustic stream. The treated product passes overhead to storage or downstream processing. The mercaptan-rich caustic solution containing Merox catalyst flows from the bottom of the extraction column to the regeneration section through a steam heater, which is used to maintain a suitable temperature in the oxidizer. Air is injected into this stream, and the mixture flows upward through the oxidizer, where the caustic is regenerated by converting mercaptans to disulfides. The oxidizer effluent flows into the disulfide separator, where spent air, disulfide oil, and the regenerated caustic solution are separated. Spent air is vented to a safe place, and disulfide oil is decanted and sent to appropriate disposal. For example, the disulfide oil can be injected into the charge to a hydrotreating unit or sold as a specialty product. The regenerated-caustic stream is returned to the extraction column. A small amount of Merox catalyst is added periodically to maintain the required activity.

FIGURE 11.3.1

Merox mercaptan-extraction unit.

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Merox Sweetening In sweetening units, the mercaptans are converted directly to disulfides, which remain in the product; the total sulfur content of the treated stock is not reduced. Merox sweetening can be accomplished in four ways:

Fixed-bed processing with intermittent circulation of caustic solution (Fig. 11.3.2) Minimum-alkali fixed-bed (Minalk*) processing, which uses small amounts of caustic solution injected continuously (Fig. 11.3.3) Caustic-Free Merox* treatment for gasoline (Fig. 11.3.4) and kerosene (Fig. 11.3.5) Liquid-liquid sweetening (Fig. 11.3.6)

Fixed-Bed Sweetening (Conventional). Fixed-bed sweetening (Fig. 11.3.2) is normally employed for virgin or thermally cracked chargestocks having endpoints above about 120C (248F). The higher-molecular-weight and more branched mercaptan types associated with these higher-endpoint feedstocks are only slightly soluble in caustic solution and are more difficult to sweeten. The use of a fixed-bed reactor facilitates the conversion of these types of mercaptans to disulfides. Fixed-bed sweetening uses a reactor that contains a bed of specially selected activated charcoal impregnated with nondispersible Merox catalyst and wetted with caustic solution. Air is injected into the feed hydrocarbon steam ahead of the reactor, and in passing through the catalyst bed, the mercaptans in the feed are oxidized to disulfides. The reactor is followed by a settler for separation of caustic and treated hydrocarbon. The settler also serves as a caustic reservoir. Separated caustic is circulated intermittently to keep the catalyst bed wet. The frequency of caustic circulation over the bed depends on the difficulty of the feedstock being treated and the activity of the catalyst. An important application of this fixed-bed Merox sweetening is the production of jet fuels and kerosenes. As a result of the development of the Merox fixed-bed system, jet fuels and kerosenes (also diesel and heating oils) can be sweetened at costs that are incomparably lower than those of the simplest hydrotreater. The same basic process flow just described is used. However, because of other particular jet-fuel quality requirements, some pretreatment and posttreatment are needed whenever any chemical sweetening process is used. Fixed-Bed Sweetening (Minalk). This Merox sweetening version is applied to feedstocks that are relatively easy to sweeten, such as catalytically cracked naphthas and light virgin naphthas. This sweetening design achieves the same high efficiency as*Trademark and/or service mark of UOP.

FIGURE 11.3.2

Fixed-bed Merox sweetening unit.

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UOP MEROX PROCESS 11.34SULFUR COMPOUND EXTRACTION AND SWEETENING

conventional fixed-bed sweetening but with less equipment and lower capital and operating costs. The UOP Merox Minalk process (Fig. 11.3.3) relies on a small, controlled, continuous injection of an appropriately weak alkali solution rather than the gross, intermittent alkali saturation of the catalyst bed as in conventional fixed-bed Merox sweetening. This small injection of alkali provides the needed alkalinity so that mercaptans are oxidized to disulfides and do not enter into peroxidation reaction, which would result if the alkalinity were insufficient. Caustic-Free Merox. A different version of the Merox family is the Caustic-Free Merox process for sweetening gasoline and kerosene (Figs. 11.3.4 and 11.3.5). This technology development uses the same basic principles of sweetening in which the mercaptans are catalytically converted to disulfides, which remain in the treated hydrocarbon product. The Caustic-Free Merox catalyst system consists of preimpregnated fixed-bed catalysts, Merox No. 21* catalyst for gasoline and Merox No. 31* catalyst for kerosene, and a liquid activator, Merox CF.* This system provides an active, selective, and stable sweetening environment in the reactor. The high activity allows the use of a weak base, ammonia, to provide the needed reaction alkalinity. No caustic (NaOH) is required, and fresh-caustic costs and the costs for handling and disposing of spent caustic are thus eliminated. The actual design of the Caustic-Free Merox unit depends on whether it is used on gasoline or kerosene. The reactor section is similar to the previously mentioned fixed-bed systems, conventional and Minalk, except for the substitution of a different catalyst, the addition of facilities for continuous injection of the Merox CF activator, and replacement of the caustic injection facilities with ammonia injection facilities, anhydrous or aqueous. For kerosene or jet fuel production, the downstream water-wash system is modified to improve efficiency and to ensure that no ammonia remains in the finished product. Other posttreatment facilities for jet fuel production remain unchanged. Liquid-Liquid Sweetening. The liquid-liquid sweetening version (Fig. 11.3.6) of the Merox process is not generally used today for new units as refiners switch to the more

*Trademark and/or service mark of UOP.

FIGURE 11.3.3 sweetening unit.

Fixed-bed minimum-alkali Merox

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FIGURE 11.3.4 Caustic-Free Merox sweetening for gasoline.

FIGURE 11.3.5

Caustic-Free Merox sweetening for kerosene jet fuel.

active fixed-bed systems. Hydrocarbon feed, air, and aqueous caustic soda containing dispersed Merox catalyst are simultaneously contacted in a mixing device, where mercaptans are converted to disulfides. Mixer effluent is directed to a settler, from which the treated hydrocarbon stream is sent to storage or further processing. Separated caustic solution from the settler is recirculated to the mixer. A small amount of Merox catalyst is added periodically to maintain the catalytic activity. In general, liquid-liquid sweetening is applicable to virgin light, thermally cracked gasolines and to components having endpoints up to about 120C (248F). The mercaptan types associated with catalytically cracked naphthas are easier to oxidize than those contained in light virgin or thermal naphthas, and therefore liquid-liquid sweetening has been successfully applied to catalytically cracked gasolines having endpoints as high as 230C (446F). The various applications of the Merox process on different hydrocarbon streams are summarized in Table 11.3.1.

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UOP MEROX PROCESS 11.36SULFUR COMPOUND EXTRACTION AND SWEETENING

FIGURE 11.3.6

Liquid-liquid Merox sweetening unit.

TABLE 11.3.1

Merox Process Applications Merox type Extraction Extraction Extraction, extraction plus sweetening Extraction, liquid-liquid sweetening, Minalk sweetening, caustic-free sweetening Liquid-liquid sweetening Caustic-free sweetening Extraction plus sweetening, Minalk sweetening, fixed-bed sweetening, caustic-free sweetening Fixed-bed sweetening Caustic-free sweetening Fixed-bed sweetening

Hydrocarbon stream Gas LPG Natural gas liquids Light naphtha Medium or heavy naphtha Full-boiling-range naphtha Kerosene or jet fuel Diesel

Merox Process Features Relative to other treating processes, the Merox process has the following advantages. Low Operating Cost and Investment Requirement. The noncorrosive environment in which the process operates requires no alloys or other special materials, thus minimizing investment. In many applications, investment is essentially nil because of the ease of converting existing equipment to Merox treating. Ease of Operation. Merox process units are extremely easy to operate; usually, the air-injection rate is the only adjustment necessary to accommodate wide variations in feed rate or mercaptan content. Labor requirements for operation are minimal. Proven Reliability. The Merox process has been widely accepted by the petroleum industry; many units of all kinds (extraction, liquid-liquid, and fixed-bed sweetening) have been placed in operation. By early 2002, more than 1700 of these UOP Merox units had been licensed.

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Minimal Chemical-Disposal Requirements. Caustic consumption by atmospheric CO2 , excessive acid in the feedstock, and accumulation of contaminants are the only reasons for the occasional replenishment of the caustic inventory. Proven Ability to Produce Specification Products. Product deterioration as a result of side reactions does not occur nor does any addition of undesirable materials to the treated product. This fact is especially important for jet-fuel treating. In the Merox process, sweetening is carried out in the presence of only air, caustic soda solution, and a catalyst that is insoluble in both hydrocarbon and caustic solutions and cannot therefore have a detrimental effect on other properties that are important to fuel specifications. High-Efficiency Design. The Merox process ensures high catalyst activity by using a high-surface-area fixed catalyst bed to provide intimate contact of feed, reactants, and catalyst for complete mercaptan conversion. The technology does not rely on mechanical mixing devices for the critical contact. State-of-the-art Merox technology has no requirement for continuous, high-volume caustic circulation that increases chemical consumption, utility costs, and entrainment concerns. High-Activity Catalyst and Activators. Active and selective catalysts are important in promoting the proper mercaptan reactions even when the most difficult feedstocks are processed. For the extraction version of the process, UOP offers a high-activity, watersoluble catalyst, Merox WS,* which accomplishes efficient caustic regeneration. As a result, chemical and utility consumption is minimized, and mercaptans are completely converted. For the sweetening version of the Merox process, UOP offers a series of catalysts and promoters that provide the maximum flexibility for treating varying feedstocks and allow refiners to select which catalyst system is best for their situation.

PROCESS CHEMISTRYThe Merox process in all its applications is based on the ability of an organometallic catalyst to accelerate the oxidation of mercaptans to disulfides at or near ambient temperature and pressure. Oxygen is supplied from the atmosphere. The reaction proceeds only in an alkaline environment. The basic overall reaction can be written:Merox catalyst

4RSH

O2 2RSSRAlkalinity

2H2O

(11.3.1)

where R is a hydrocarbon chain that may be straight, branched, or cyclic and saturated or unsaturated. Mercaptan oxidation, even though slow, reportedly occurs whenever petroleum fractions containing mercaptans are exposed to atmospheric oxygen. In effect, the Merox catalyst speeds up this reaction, directs the products to disulfides, and minimizes undesirable side reactions. In Merox extraction, in which mercaptans in the liquid or gaseous feedstocks are highly soluble in the caustic soda solution as solvent, the mercaptan oxidation is done outside*Trademark and/or service mark of UOP.

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UOP MEROX PROCESS 11.38SULFUR COMPOUND EXTRACTION AND SWEETENING

the extraction environment. Therefore, a mercaptan-extraction step is followed by oxidation of the extracted mercaptan. These steps are: RSHOil phase

NaOH NaSRAqueous phase Aqueous phase Merox catalyst

H2O

(11.3.2)

4NaSRAqueous phase

O2

2H2O 4NaOH

2RSSR

(11.3.3)

Aqueous phase Oil phase (insoluble)

According to these treating steps, the treated product has reduced sulfur content corresponding to the amount of mercaptan extracted. In the case of Merox sweetening, in which the types of mercaptans in the feedstocks are difficult to extract, the sweetening process is performed in situ in the presence of Merox catalyst and oxygen from the air in an alkaline environment. UOP studies have shown that the mercaptan, or at least the thiol ( SH) functional group, first transfers to the aqueous alkaline phase (Fig. 11.3.7) and there combines with the catalyst. The simultaneous presence of oxygen causes this mercaptan-catalyst complex to oxidize, yielding a disulfide molecule and water. This reaction at the oil-aqueous interface is the basis for both liquid-liquid and fixed-bed sweetening by the Merox process and can be written:Merox catalyst

4RSHOil phase

O2 2RSSRAlkalinity Oil phase Merox catalyst

2H2O

(11.3.4)

2RSR 2RSHOil phase

O2 2RSSRAlkalinity Oil phase

2H2O

(11.3.5)

Equation (11.3.5) represents the case in which two different mercaptans may enter into this reaction. Petroleum fractions have a mixture of mercaptans so that the R chain may have any number of carbon atoms consistent with the boiling range of the hydrocarbon feed.

FIGURE 11.3.7 face.

Mercaptide at inter-

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UOP MEROX PROCESSUOP MEROX PROCESS

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Because the process is catalytic, essentially catalyst and caustic soda are not consumed. This fact is borne out by commercial experience, in which actual catalyst consumptions are low. Consumption is due mainly to fouling by certain substances and loss through an occasional purge of dirty or diluted caustic solution and a corresponding makeup of fresh caustic to maintain effective caustic concentration.

PRODUCT SPECIFICATIONSThe only product specification applicable to Merox treating is the mercaptan sulfur content of the product because the Merox process per se has no effect on the other properties of the feedstock being treated. Generally, therefore, the Merox process is used to reduce the mercaptan sulfur content, and thereby the total sulfur content, when the process is applied to gases and light stocks in the extraction mode of operation. In the case of heavier chargestocks that require the sweetening mode of operation, the only product specification applied is the mercaptan sulfur content (or sometimes also the doctor test); the total sulfur contents of the untreated feed and the treated product are the same. Merox-treated products may be finished products sent directly to storage without any further processing or intermediate products that may require either blending into finished stocks or additional processing for making other products. Table 11.3.2 lists typical quality specifications for treating applications of the Merox process.

PROCESS ECONOMICSSample economics of the UOP Merox process in 2002 dollars on the basis of 10,000 barrels per stream day (BPSD) capacity for various applications are given in Table 11.3.3. The capital costs are for modular design, fabrication, and erection of Merox plants. The estimated modular cost is inside battery limits, U.S. Gulf coast, FOB point of manufacturer. The estimated operating costs include catalysts, chemicals, utilities, and labor.

PROCESS STATUS AND OUTLOOKThe first Merox process unit was put on-stream October 20, 1958. In October 1993, the 1500th Merox process unit was commissioned. Design capacities of these Merox units range from as small as 40 BPSD for special application to as large as 140,000 BPSD and total more than 12 million BPSD. The application of the operating Merox units is distributed approximately as follows:

25 30 30 15

percent percent percent percent

LPG and gases straight-run naphthas FCC, thermal, and polymerization gasolines kerosene, jet fuel, diesel, and heating oils

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TABLE 11.4.2

TABLE 11.3.2 Feed Type Gases, LPG, NGL NGL, LN 502,000 10 .... 510 50natural gas liquid; gas

Quality Specifications for the Merox Process

Characteristics MN, HN 505,000 10 0.01 510 510 10 505,000 10 0.01 301,000 1 0.01

FBR gasoline

Jet fuels

Kerosene 301,000 1 0.01 10

Diesels 50800