DIESEL HYDRO TREATING UNIT The purpose of removing the sulphur is to reduce the sulphur dioxide (SO2) emissions that result from using those fuels in automotive vehicles, aircraft, railroad locomotives, ships, gas or oil burning power plants, residential and industrial furnaces, and other forms of fuel combustion.Another important reason for removing sulphur from the naphtha streams within a petroleum refinery is that sulphur, even in extremely low concentrations, poisons the noble metal catalysts (platinum and rhenium) in the catalytic reforming units.

In petroleum refineries, the hydrogen sulphide gas is then subsequently converted into by-product elemental sulphur or sulphuric acid. In fact, the vast majority of the 64,000,000 metric tons of sulphur produced worldwide in 2005 was by-product sulphur from refineries and other hydrocarbon processing plants.

A diesel hydro treating unit unit in the petroleum refining industry is also often referred to as a hydrotreater.

The industrial hydrodesulfurization processes include facilities for the capture and removal of the resulting hydrogen sulfide (H2S) gas. In petroleum refineries, the hydrogen sulfide gas is then subsequently converted into byproduct elemental sulfur or sulfuric acid (H2SO4). In fact, the vast majority of the 64,000,000 metric tons of sulfur produced worldwide in 2005 was byproduct sulfur from refineries and other hydrocarbon processing plants.

Hydrogenation is a class of chemical reactions in which the net result is the addition of hydrogen (H). Hydrogenolysis is a type of hydrogenation and results in the cleavage of the C-X chemical bond, where C is a carbon atom and X is a sulfur (S), nitrogen (N) or oxygen (O) atom. The net result of a hydrogenolysis reaction is the formation of C-H and H-X chemical bonds. Thus, hydrodesulfurization is a hydrogenolysis reaction. Using ethanethiol (C2H5SH), a sulfur compound present in some petroleum products, as an example, the hydrodesulfurization reaction can be simply expressed as

Ethanethiol + Hydrogen

→ Ethane + Hydrogen sulfide

C2H5SH + H2 → C2H6 + H2S

In an industrial hydrodesulfurization unit, such as in a refinery, the hydrodesulfurization reaction takes place in a fixed-bed reactor at elevated temperatures ranging from 300 to 400 °C and elevatedpressures ranging from 30 to 130 atmospheres of absolute pressure, typically in the presence of a catalyst consisting of an alumina base impregnated with cobalt and molybdenum (usually called a CoMo catalyst). Occasionally, a combination of nickel and molybdenum (called NiMo) is used, in addition to the CoMo catalyst, for specific difficult-to-treat feed stocks, such as those containing a high level of chemically bound nitrogen.

AMINE GAS TREATIG UNIT :

Amine gas treating, also known as gas sweetening and acid gas removal, refers to a group of processes that use aqueous solutions of various alkyl-amines (commonly referred to simply as amines) to remove hydrogen sulphide (H2S) and carbon dioxide (CO2) from gases.[1][2] It is a common unit process used in refineries, and is also used in petrochemical plants, natural gas processing plants and other industries.Processes within oil refineries or chemical processing plants that remove hydrogen sulphide and/or mercaptans are commonly referred to as sweetening processes because they result in products which no longer have the sour, foul odours of mercaptans and hydrogen sulphide. There are many different amines used in gas treating: Monoethanolamine (MEA)

Diethanolamine (DEA)

Methyldiethanolamine (MDEA)

Diisopropylamine (DIPA)

Aminoethoxyethanol (diglycolamine) (DGA) BASIC REACTION CHEMISTRY OF HYDROTREATINGBasics of HydrotreatingHydrotreating of diesel stream from various units is mainly done to meet thestringent diesel fuel specification in force from time to time.The hydrotreating of any hydrocarbon stream is done to achieve followingobjectives* Sulfur removal* Nitrogen removal* Aromatic saturation* Olefin saturation* Metals removal* CCR/RCR removal* CrackingThe diesel boiling range hydrotreatment is mainly done to achieve the sulfur andaromatic specification. For VGO and heavier fraction hydrotreatment in additionto S, N and aromatic saturation, metals removal , CCR reduction becomesimportant. Some cracking is also achieved (though not desirable some timesdue to product yield loss) due to operating severity and feed quality.We will briefly describe some of the basic reactions for some of the abovereactions.2.2 Sulfur RemovalSulfur is found in various molecular forms in hydrocarbon stream. H2S is themain by product due to desulfurization of the hydrocarbon stream. HDS reactionproduces moderate heat release and consumes approximately (20 Nm3/m3offeed ) of hydrogen for 1 % wt sulfur removal from feed. Some of the basic sulfurcompound reactions are described below Mercaptan : RSH + H2 --------> H + H2S Sulfides: RSR' + H2 --------> RSH + R'H ORRSR' + 2H2 --------> RH + R'H + H2S Thiophene:

4C4H4S + 12 H2 ------> C4H9SH + C4H6 + C4H8 + C4H10 + 3H2SC4H9SH + C4H6 + C4H8 + C4H10 + 3H2S + 4H2 ------> 4C4H10 + 4H2SC4H4S + 2H2------> C4H8S + H2 -----> C4H9SH + H2 ------> C4H10 + H2SNitrogen RemovalNitrogen is found in many forms in hydrocarbon molecules. Nitrogen isgenerally present in unsaturated molecules. Cracked streams contain moreunsaturated molecules and hence contain higher nitrogen compounds. Cokerand FCC diesel fractions have higher level of nitrogen than SR diesel fraction.HDN reaction is irreversible reaction and by product NH3. HDN producesmoderate heat release like the HDS reactions. Typical hydrogen consumptionfor HDN reaction is approximately 0.7 Nm3/m3 of feed per 100 PPM nitrogenremoved. Some of the HDN reactions areCyanides HCN + 3H2 CH4 + NH3Amines RNH2 + H2 RH + NH3Pyrrole C4HSN + 2 H2 C4H9NC4H9N + H2 C4H9NH2C4H9NH2 + H2 C4H10 + NH32.4 Olefin saturationOlefins saturate rapidly and completely. Only traces of olefins are usuallypresent in straight run fractions. Cracked streams from coker and FCC unitscontains higher levels of olefins.Olefin saturation is highly exothermic reaction. Biggest contributor to heatrelease. Higher hydrogen partial pressure and lower temperature favourformation of saturate compounds. No by-product generation due to olefinsaturation. Typical hydrogen consumption for olefin saturation reaction is1.5 Nm3/m3 of feed per 1 wt % olefins. Some of the olefin saturation reactionis given belowRCH=CH2 + H2 RCH2CH3RCH=CHCH=CH2 + H2 R(CH2)3CH3Cycloolefins + H2 Cycloparaffins2.5 Aromatic SaturationAromatic saturation contributes significantly to hydrogen consumption andheat release. Aromatic saturation depends on many factors like the type ofaromatic in feed, operating pressure, LHSV and temperature.Aromatic saturation is limited by equilibrium. Cracked streams from FCC andCoker units has more potential for aromatic saturation than straight run dieselfraction. LCO and other cracked streams contain more of the poly aromaticwhich is easier to saturate under normal hydrotreating conditions. It will bedifficult to saturate the mono aromatics in the feed.2.2Aromatic saturation is favored by high hydrogen partial pressure and loweroperating temperature. Aromatic saturation helps in lowering the product densityand improving the Cetane number of the product.Typical hydrogen consumption for aromatic saturation is 27 Nm3/m3 of feed pereach one percent reduction in aromatic content. No by-products generated due tothese reaction.

The CCR removal and cracking are not major reactions associated with dieselfraction hydrotreatment at moderate pressure. These reactions will be critical forhigh severity operation while treating VGO and heavier fraction.