MEROX PROCESS DESCRIPTION

 MEROX process equipment  Pretreatment

 Extraction section  Sweetening  Post treatment  MEROX catalysts

The MEROX process licensed by M/S Universal Oil Products Co., (UOP), USA, is for the chemical treatment of LPG, gasoline and distillates to remove mercaptans into disulfides. The removal of mercaptans may be either partial or full. The chemical treatment is based on the ability of MEROX catalysts to promote the oxidation of mercaptan to disulfide using air as the source of oxygen. The overall reaction is as follows:

2RSH + 1/2O2 RSSR + H2O

The oxidation is carried out in the presence of an aqueous alkaline solution such as sodium hydroxide or potassium hydroxide. The reaction proceeds at an economical rate at normal rundown temperature of refinery streams.

Low molecular weight mercaptans are soluble in caustic solution and therefore when treating LPG and light gasoline fractions, the process can be used to extract mercaptan to the extent, they are soluble in caustic. Extraction of mercaptan reduces the sulphur content of the treated product. Alternatively mercaptans can be converted to disulfides without removing any sulphur from the treated stock in which case the operation is referred to as sweetening. In the treatment of heavier boiling fractions such as heavy naphtha and kerosene only sweetening is possible.

4.2.1.

PRETREATMENT

Petroleum fractions may contain hydrogen sulfide and stocks boiling higher than 180°C may also contain naphthenic acids. Hydrogen sulfide is not a catalyst poison as such, but will dilute the caustic containing MEROX catalyst by reacting with caustic. Further it blocks some of the catalyst activity sites slowing down the normal reaction and also consumes part of the oxygen available. Hence, it is recommended that hydrogen sulfide is removed by washing with dilute alkali solution before the distillate is sent to reactor for treatment.

Naphthenic acids also interfere with treating operations and must be removed prior to treatment. The reactor contains caustic and if naphthenic acids are not removed, they form sodium naphthenates, which coat the catalyst and block the pores. For removal of naphthenic acids, the procedure used is to wash with dilute caustic. Dilute caustic is used so as to avoid formation of emulsions. There could, however, be some carry-over of haze depending on the acidity of stock treated. The haze can easily be removed by coalescing through a sand filter.

Feedstock, where carry-over of water from distillation units can be expected must be passed through a coalescer for removal of suspended water prior to caustic wash, which would otherwise dilute the caustic used for pretreatment.

4.2.2.

EXTRACTION SECTION

As previously stated, low molecular weight mercaptans are caustic soluble and can easily be removed by washing with caustic in a counter current tower. Improved extraction is favored by:

(i) Low temperature

(iii)Lower molecular wt. of mercaptans

Type of mercaptans, viz. normal mercaptans are easily extractable, tertiary mercaptans least extractable and secondary being in between.

The mercaptan enters the caustic solution and reacts as follows:

RSH + NaOH  NaSR + H2O

This being a reversible reaction the degree of completion of reaction is governed by normal equilibrium laws.

The sodium mercaptide is readily oxidized to disulfide in the presence of MEROX catalyst as shown:

2NaSR + l/2O2 + H2O  2NaOH + RSSR

This is not a reversible reaction and the reaction rate is sped up by: (i) Raising the temperature

(ii) Use of excess air

(iii)Increasing the intimacy of contact (iv) Increasing the catalyst concentration

The oxidation of mercaptides is carried out in oxidizer in the presence of MEROX catalyst. The disulfides oil, which is formed, separates out from caustic, as it is insoluble in caustic. Caustic can be reused for extraction. The presence of MEROX catalyst in extraction caustic does not however, affect the amount of mercaptans extracted and extraction is dependent only on parameters explained earlier.

4.2.3.

SWEETENING

Sweetening can be defined as conversion of mercaptan sulphur present in a hydrocarbon stream to disulfide sulphur without actually reducing sulphur content of treated stock. The sweetening process is based on the ability of MEROX catalyst to promote the oxidation of mercaptans to disulfide using air as the source of oxygen. The reaction is as follows:

RSH + NaOH  NaSR + H2O

2NaSR + l/2O2 + H2O  2NaOH + RSSR

As can be seen from reactions, the oxidation is carried out only in the presence of alkali solution.

The Sweetening can be accomplished either as solid bed sweetening, where the hydrocarbons and caustic are simultaneously controlled over a solid support impregnated with MEROX catalyst or as liquid-liquid sweetening, where hydrocarbon, air and caustic containing MEROX catalyst are simultaneously controlled in a mixer.

Solid bed sweetening consists of a reactor, which contains a bed of activated charcoal impregnated with MEROX catalyst and kept wet with caustic solution. Impregnation of catalyst on bed is achieved by dissolving the catalyst with ammonia solution and pumping ammonia solution over charcoal. Air is injected ahead of reactor and in the presence of MEROX catalyst the mercaptans are oxidized to disulfide. The reactor is followed by a settler, which serves as reservoir of caustic. Caustic is intermittently circulated from the settler over the catalyst bed to wet the charcoal.

For liquid-liquid sweetening, the most common type of mixer used is the orifice plate mixer, which is a vessel, fitted with a series of plates with orifices. The vessel provides adequate residence time and the orifice plates create enough turbulence to bring

about the intimate contact between hydrocarbons, caustic, catalyst and air. The higher the molecular weight or the more highly branched the mercaptan is, the more difficult it is to accomplish necessary mixing. Hence heavy gasoline and Kerosene may have to be treated using fixed bed reactor.

4.2.4.

POST TREATMENT

The product from the MEROX reactor will at times contain caustic haze. Post treatment is required if the product is to go to storage, clear and bright. In most cases provision of caustic settler and sand filter is adequate to remove caustic haze. However, for treatment of ATF, which has to meet stringent specifications caustic must be removed by water wash after caustic settling. Water wash removes entrained caustic as well as water soluble surfactants. Water wash is followed by a salt filter to remove entrained water and part of the dissolved water. This may be followed by clay filter to remove copper and water insoluble surfactants, if present in feed.

4.2.5.

MEROX CATALYSTS

There are two types of MEROX catalyst, each one being used for specific service. Catalyst FB is to be used on units equipped with solid bed sweetening reactors. Catalyst WS is used for liquid-liquid sweetening in mixers. This is a caustic dispersible catalyst. This is also used for oxidation of extraction caustic in oxidizers.