GASOLINE MEROX UNIT

PROCESS PRINCIPLESGENERAL

For the Minalk unit (and for all sweetening type units), the mercaptans are converted directly to disulfides which remain in the product; there is no reduction in total sulfur content. Because the vapor pressures of disulfide are so low relative to those of mercaptans, their presence is much less objectionable.

The Merox Minalk process relies on a small, continuous injection of an appropriate strength alkali solution rather than gross intermittent alkali saturation of catalyst bed as used in conventional Merox fixed bed sweetening. This continuous, small injection of alkali, such as dilute sodium hydroxide solution (2-5 wppm), is held to a minimum but enough to provide the needed alkalinity. In this way mercaptans are oxidized to disulfides and do not enter into a peroxidtion reaction in case of insufficient alkalinity.

For Minalk Merox systems, pretreatment is rarely required. The need for pretreatment depends on feed characteristics, treated product specifications, and end-use. For charge stocks such as light gasolines, where more than low ppm levels of hydrogen sulfide are presenet, the charge stock passed through a guard caustic scrubber to ensure essentially complete removal of H2S. Hydrogen sulfide does not poison the catalyst but it competes with mercaptans for the available catalytic sites and available oxygen. Post treatment is not required on Minalk sweetened streams.

MEROX PROCESS OVERVIEW

The UOP Merox Process is an efficient and economical catalytic process developed for the chemical treatment of petroleum distillates for removal of sulfur present as mercaptan (Merox Extraction) or conversion of mercaptan sulfur to another much less objectionable form (Merox Sweetening). The name “Merox” is an acronym derived from MERcaptan OXidation. The process relies on a special catalyst to accelerate the oxidation of mercaptans to disulfides at or near economical product rundown temperature. The MINALK unit is a sweetening process.

Depending on the application, extraction and sweetening are used either separately or in combination. The Merox process is based on the ability of a catalyst or catalysts composed of iron group metal chelates in an alkaline environment to promote the oxidation of mercaptans to disulfides using air as the source of oxygen. The reaction proceeds at an economically practical rate at normal temperatures for refinery rundown streams. For light stocks, operating pressure is controlled slightly above the bubble point to assure liquid phase operation; for heavier stocks, operating pressure is normally set to keep air dissolved in the reaction section.

Applications of the Merox Process

Typical Hydrocarbon Streams Applicable Merox TreatmentGas, Natural Gas, Fuel Gas ALPG (C2, C3, C4) APentanes A, B, C, ELight, Straight-Run Naphtha A, B, C, ELight, Thermally Cracked Naphtha A, DFBR, Straight-Run Naphtha C, D, EFBR, FCC Cracked Naphtha B, C, EHeavy, FCC Cracked Naphtha C, EHeavy, Straight Run Naphtha DAviation Turbine Fuel D, FGasoline D, FDistillate Fuel (to 350ºC EP) D

Key to Treatment MethodsA Mercaptan Removal (Extraction)B Liquid-Liquid Mercaptan Conversion (Sweetening)C Minimum Alkali (Minalk) Fixed-Bed Mercaptan Conversion (Sweetening)D Fixed-Bed Mercaptan Conversion (Sweetening)E Caustic-Free Fixed Bed Mercaptan Conversion for Gasoline (Sweetening)F Caustic-Free Fixed Bed Mercaptan Conversion for Kerosine (Sweetening)

NOTE: FBR = Full Boiling Range

2.3 REACTION CHEMISTRY

To fully understand the Merox Process and how it can be applied to such a broad range of treating problems, it is necessary to review the basic chemical reaction of the process and how it is applied in a variety of way. UOP may have been remiss in using the mane “Merox” to designate a variety of applications of single catalyed chemical reaction. The use of a single name to designate what may appear to be multiplicity of processes has unfortunately been the cause of some confusion in understanding the various applications of the procss.

Merox Reactions

The name “Merox” originates from the function of the process itself ; namely the conversion of mercaptans by oxidation

MERcaptan OXidation

The word “mercaptan” is descriptive name applied over 140 years ago to organic compounds with a sulfhydryl function (-SH) and derived from the Latin mercurium captans, due to their mercury seizing properties. Today’s literature, however, assigns the

word “thiol” for this functional group. In the common system used by the petroleum industry, mercaptans are named for after their analogous alcohol counterparts. Thus, CH3SH, by the common system, is methyl mercaptan just as one would name an alcohol having formula CH3OH methyl alcohol ; the strict formal name is “methanethiol”. Likewise, n-butyl mercaptan becomes I-butanethiol, t-butyl mercaptan becomes 2-methyl-2-propanethiol. The aryl mercaptans are commonly called thiothenols, while in the formal system as used by Chemical Abstracts, these compounds are benzenethiol, toluenethiol, etc. This discussion uses the common mercaptan terminology as practiced in the petroleum industry.

The Merox process in all of its applications is based on the ability of an organmetallic catalyst to accelerate the oxidation of mercaptans to disulfides at or near ambient temperatures and pressures. Oxygen is supplied from the atmosphere. The reaction will proceed only in an alkaline environment. The overall reaction can be written :

RSH + 1/4 O2 1/2 RSSR + 1/2 H2O

R is a hydrocarbon chain which may be straight, branched, or cyclic. These chains may be saturated or unsaturated. In most petroleum fractions, there will be a mixture of mercaptans to the extent that the R chain might have 1, 2, 3, ……10 or more carbon atoms in the chain. When this reaction occurs, two different mercaptans might enter the reaction. This is indicated by showing R and R’ in the reaction. The reaction is then written:

2 R’SH + 2 RSH + O2 2 R’SSR + 2 H2O

This reaction occurs spontaneously, but at a very slow rate, whenever any sour mercaptan bearing distillate is exposed to atmospherix oxygen.

Detailed Sweetening Chemistry

It is believed on the basis of Licensor (UOP) studies, that the mercaptan, or at least the thiol (-SH) functional group, first transfers to the aqueous alkaline phase and then combines with the catalyst. The simultaneous presence of oxygen causes this mercaptan – catalyst complex to oxidize, yielding a disulfide molecule and water.

Figure 2-1

Mercaptide at Interface

The purpose of the caustic is in two folds :

1) To transfer the mercaptan, or at least the thiol portion of the mercaptan, to the aqueous phase, and

2) To supply the alkaline environment needed for the reaction to proceed in the direction desired.

This system works efficiently for all the lower molecular weight mercatpans which

partially soluble in caustic soda solution.

It is the function of the catalyst not only to accelerate the oxidation without allowing side reactions to occur.

The most important point to remember, and the point that makes the Merox process unique, is that the Merox reaction occurs in the aqueous phase. Most of the other mercaptan oxidation processes utilize oxidation reactions which occur in the oil phase. An oil phase reaction is much less desirable because there are many competing oxidation reactions which can occur in the oil phase to leave contaminants in the product.

2.4 MEROX CATALYSTS

The fixed bed Merox process has the Merox catalyst deposited upon the extended hydrophilic surface of specially selected activated charcoal. The function of the activated charcoal is to provide a vast surface area where mercaptans, catalyst, caustic, and oxygen may come into contact for completion of the sweetening reaction. To provide the alkaline medium, the charcoal is kept saturated with caustic solution. The fixed bed process utilizes a type of Merox catalyst that is not dispersible in caustic so as to prevent casutic from removing the catalyst from the charcoal surface.

The Minalk version of the fixed bed Merox process utilizes the same catalyst/charcoal reactor bed as other fixed bed Merox processes. A brief description of Merox catalysts and reagents for the Minalk process is given below. Additional information can be found in Section 14 (Safety) of this manual.

MEROX FB is the catalyst reagent used to impregnate the specially selected activated carbons to form fixed bed Merox catalysts. Merox FB is supplied as a dark blue water slurry which is packaged in 9.5 liter(2.5 gallon) sealed polyethylene bottle. Each bottle contains 2.5 kg(5.5 lb) of active ingredient. MEROX FB requires no additional handling or mixing prior to use. It is recommended that the bottle of reagent be well shaken before dispensing to avoid settling and ensure proper addition to the process. The specific steps involved in actually adding the reagents to the unit described in Section 13 (Special Procedure) of this manual.

It is recommended that personnel handling Merox FB wear protective clothing such as that worn when handling caustic soda solutions, i.e., rubber gloves, apron, and eye protection.

MEROX No. 8 is finished catalyst for use in fixed bed sweetening. It is an activated carbon which has been impregnated with Merox FB reagent. The catalyst is supplied in polyethylene bags packaged 200 liter (55 gallon) steal drums or in supersacks containing 1,133 liters (40 cubic feet). Merox No.8 is loaded directly into the fixed bed reactor and requires no additional impregnation or activation prior to use.

MEROX PLUS activator, a ready to use liquid additive, is a commercially proven high activity promoter. It can be used for improving product quality, extending catalyst life, and reducing air and caustic consumption.

PROCESS VARIABLES

The purpose of this section is to discuss the major Merox process variables and their effect on performance. Specific emphasis will be placed on identifying the optimum operating conditions. Although feedstock quality is obviously variable, it will not be discussed here. The Merox operator will generally have little, if any, control over the feedstock.

The following represent the five primary process variable. Each will be discussed as an independent with all other held constant.

C atalystO xygenA akalinityC ontactH eat

3.1 CATALYST CONCENTRATION

For the Minalk Merox unit, the catalyst concentration is not a variable. The charcoal should be impregnated with 1.4 kg of UOP Merox FB Reagent per cubic foot of charcoal. Over the course or a run, catalyst activity will gradually decline. When deactivation reaches an intolerable level (when product quality is not meeting product specification), a reactivation with additional catalyst should be conducted. This is described in detail Section 13, Special Procedures.

3.2 OXYGEN INJECTION

Oxygen is supplied to all versions of the Merox process in the form of compressed atmospheric air. The stoichiometric, or theoretical, amount of oxygen necessary is 0.25 pound per pound of mercaptan sulfur. One cubic foot of atmospheric air at standard conditions of temperature and pressure contains about 0.018 pounds of oxygen, therefore, about 14.2 SCF (standard cubic feet) of air is needed for each pound of mercaptan sulfur to be oxidized (0.84 Nm3 per kilogram). It is of course, necessary to have at least a slight excess of oxygen present but it is always recommended to keep the excess to a minimum. The amount of excess air injection necessary is highly dependent upon the individual feedstock, but normally ranges between 1.2 to 2.0 times the theoretical requirement. Thus, a good initial injection rate at 1.5 times theoretical may be calculated from the formula below:

(Air) = 1.5 (K) (Feed Rate) (S.G.) (RSH) / (10,000)

Where : English Metric

Air = Air Injection Rate SCFH Nm3/hK = Constant 2.07 8.35Feed Rate = Feed Flow Rate BSD m3/hS.G. = Specific Gravity @ 60 ℉ 15.5 ℃RSH = Mercaptan Sulfur wt-ppm wt-ppm

For Minalk and other fixed bed units the air injected must be completely dissolved in the hydrocarbon; otherwise a separate air phase will exist within the fixed bed and eventually lead to channeling. The most important factor for solubility is pressure on the system. Thus sufficient pressure to dissolve the required air must be maintained at all times. Pressure is in fact secondary variable, since pressure is directly proportional to the solubility of air in hydrocarbon.

ALKALINITY

3.3 Alkalinity is needed to selectively drive the mercaptan oxidation reaction. Alkalinity is provided by aqueous sodium hydroxide, or caustic, solution. The mercaptans encountered in the feed to Minalk units are typically so readily oxidized that very little alkalinity is required. Thus, only sufficient caustic need be injected to maintain the drain effluent from 9-12 pH. About 2-10 wt-ppm of NaOH at caustic concentrations of 3-5 degrees Be’ (2-3 wt %) is normally sufficient.However, one other factor must be considered. Thiophenol, when present with olefins, as in catalytically cracked gasolines, can be easily oxidized to a thioetherhydrope oxide gum precursor. Caustic, by transferring the thiophenols to the aqueous phase will prevent this peroxidation. Thus, sufficient quantity of caustic must be injected to “tie up” all of the thiophenols present in the feed until it can be converted to disulfide. Experience has shown that this quantity of caustic injection need not necessarily be a stoichiometric amount. A single molecule of caustic may well arrest hydroperoxidation for many molecules of thiophenol. The tendency to form gum via this mechanism may be monitored with the peroxide number test. A sufficient quantity of caustic injection should be maintained to yield a peroxide number of 0.3 or less.Of course, any acids present in the feed lead to caustic neutralization. As caustic is neutralized, it must be replaced in order to maintain acceptable operation.

3.4 CONTACT

Contact may be defined as the intimacy of mixing of the reactants. Contact is accom-plished by on of several means. Contact can be characterized by duration of mixing time, by volumetric ratio of reactants, or by differential pressure, which implies degree of dis-persion. With a fixed bed flow restriction, the differential pressure is directly related to the linear velocity so that velocity is often taken to be the process variable.

The primary source of contact is a fixed bed unit is the charcoal bed itself. The porous structure of the charcoal provides vast interfacial surface area necessary for contact be-tween Gasoline, caustic, and catalyst. The variable used to characterize contact is resi-dence time in the charcoal bed.

Residence time is often expressed in terms of volume of feed per hour, per volume of cat-alyst bed. The common term for this is liquid hourly velocity, LSHV. As residence time increases (LHSV decreases), mercaptan increases (Figure 3-1)

Figure 3-1Sweetening

As the catalyst bed accumulates running time, the charcoal pores tend to plug and be-come blinded by extremely heavy hydrocarbon molecules. This reduces contact and is largely the cause of fixed bed deactivation. Fixed bed deactivation results in higher mer-captan content in the product which will not respond to corrective action. As a result, re-duced feed rates for lower LHSV or catalyst bed reactivation will be required.

3.5 HEAT (TEMPERATURE)Minalk fixed bed sweetening is favored by higher temperature. Increasing the Gasoline feed temperature increases the rate of mercaptan conversion. However, increasing the temperature will also enhance other oxidation reactions, which may be undesirable be-cause of color deterioration, gum precursor formation, and premature catalyst deactiva-tion. Therefore, the feed temperature should be kept as low as possible while still main-taining the desired degree of mercaptan conversion; normally this is in the range of 38 ℃ to 50 ℃. Temperature is also limited by storage considerations such as flash point or va-por losses. Feed temperatures in excess of 60℃ for long periods should be avoided be-cause of metallurgical considerations.

3.6 FEED QUALITYSuccessful MINALK unit operations requires the removal or neutralization of any com-pounds or impurities that could cause catalyst deactivation, interference, or degradation of the treating chemicals and solvents.

3.6.1 Elemental Sulfur

Elemental sulfur, once formed, is very difficult to remove. None of the equipment in a Merox unit is capable of removing elemental sulfur. Every precaution should be taken to prevent its formation in the first place. If it is found to be present in a fee-stock, its origin should be quickly identified and corrective action taken.

3.6.2 Hydrogen Sulfide

Low concentrations of H2S may be removed by caustic prewashing, that is, by washing the hydrocarbon stream with dilute solution of caustic soda. Dilute caustic solutions are necessary to prevent the precipitation of sodium sulfide(Na2S) crystals. Economic considerations usually limit the practical range of H2S removal by this method to hydrocarbon streams containing not more than 100 wt. Ppm H2S. As dis-cussed in Section 2, H2S cannot normally be tolerated in Merox feedstocks in con-centrations exceeding 5 wt-ppm.

3.6.3 Carbon Dioxide

Carbon dioxide (CO2) is commonly present in light hydrocarbon fractions, mainly as a result of contamination, which occurs in catalytic cracking. However, CO2 is an acidic contaminant and thus may be simultaneously removed with H2S caustic wash-ing or ethanolamine extractive absorption. Since it is an acid, it must not be over-looked when setting pretreatment equipment operating parameters.

A small amount of CO2 is also naturally present in the atmospheric air (~350 ppmv of CO2) used in the Merox process.

PROCESS FLOW AND CONTROL

The flow scheme for Minalk unit is very simple. Air and caustic are injected into the hydrocarbon feed stream, which then enters the Minalk reactor downflow. Sweetened product, essentially free of entrained caustic, is withdrawn directly from the side of the fixed bed near the bottom of the Reactor(15-R-501)

The coalesced aqueous phase collects at the bottom of the vessel and is discharged via a small drain pot to sewer or to the spent caustic disposal system. The charcoal bed is an excellent coalescer. A flow diagram of the Minalk unit is shown in Figure 4-1.

Figure 4-1Fixed Bed Minimum Alkali Merox Sweetening Unit

The hydrocarbon feed and process air to the Minalk unit enter on flow control and sweetened product leaves on pressure control. This spent caustic leaves the drain pot on level control.Air is injected into the hydrocarbon stream via an Air Mixer(15-MX-501). This ensures a unifirm dissolution of air in the hydrocarbon just before entering the Reactor(15-R-501). The air injection line is provided with a cutoff valve which stops the air flow on low hydrocarbon flowrate. An excessive amount of air injection for a given feed rate can cause the formation of separate air phase at the top of Reactor(15-R-501) which will promote channeling in the catalyst bed.

A continuous injection of dilute caustic is required to maintain a minimum level of alkalinity in the Reactor(15-R-501). Caustic is injected through an atomizing nozzle to ensure uniform distribution in the feed.