Hydrocracking

What is Hydrocracking? Hydrocracking is the conversion process of higher boiling point petroleum fractions to light hydrocarbons (mainly gasoline and jet fuels) in the presence of a catalyst

Hydrocracking Hydrocracking is catalytic cracking in the presence of hydrogen. The extra hydrogen saturates, or hydrogenates, the chemical bonds of the cracked hydrocarbons and creates isomers with the desired characteristics. Hydrocracking Hydrocracking is also a treating process, because the hydrogen combines with contaminants such as sulphur and nitrogen, allowing them to be removed

Feed The straight run Coker gas oil FCC cycle oil Hydrocracking Uses high pressure and large amount of hydrogen Hydrogenation reaction suppress coke formation Products high in liquid yield and low in sulfur and olefins

Process: Hydrocracking The two reactions that occur are Cracking (endothermic) and Hydrogenation (exothermic). Heat of cracking is supplied by the hydrogenation reaction  autothermal reaction.

Why Hydrocracking? The increasing demand for gasoline and jet fuel compared to diesel fuel and home heating oils was a dominant factor in the development of hydrocracking process Hydrogen as a byproduct of catalytic reforming process was available in large amounts and relatively cheap

Benefits of Hydrocracking Hydrocracking is one of the most versatile process, which facilitate product balance with the market demand Other advantages are: Very high gasoline yield High octane numbers Production of large amount of isobutane Supplementing FCC (Fluid Catalytic Cracking) to upgrade heavy stocks, aromatics and coker oils

Hydrocracking vs FCC Fluid Catalytic Cracking (FCC) takes more easily cracked paraffinic atmospheric and vacuum gas oil Hydrocracking is capable of using aromatics and cycle oils and coker distillates as feed (these compounds resist FCC) Cycle oils and aromatics formed in catalytic cracking (FCC) are satisfactory feedstock for hydrocracking Middle distillate and even light crude oil can also be used as feedstock for hydrocracking

Hydrocracking processes The fresh feed is mixed with hydrogen gas and recycle gas (high in hydrogen content) and passed through a heater to the first reactor If the feed is high in sulfur and nitrogen a guard reactor is employed to convert sulfur to hydrogen sulfide and nitrogen to ammonia to protect precious catalyst in the following reactor. Jet fuel Two-Stage Hydrocracking gasoline

Hydrocracking processes Hydrocracking reactors are operated at high temperatures to produce materials with boiling point below 400 F The reactor gaseous effluent goes through heat exchangers and a high pressure separator where the hydrogen rich gases are separated and recycled to the first stage. Jet fuel Two-Stage Hydrocracking gasoline

Hydrocracking processes The liquid product from the reactor is sent to a distillation column where C1-C4 and lighter gases are taken off and the gasoline, jet fuel, naphta and/or diesel fuel streams are removed as liquid side streams. The distillation bottom product is sent to the second hydrocracker Jet fuel Two-Stage Hydrocracking gasoline

Hydrocracking Reactions There are hundreds of simultaneous chemical reactions occurring in hydrocracking It is assumed that the mechanism of hydrocracking is that of catalytic cracking with hydrogen superimposed In catalytic cracking the C – C bond is broken, while in hydrogenation, H2 is added to a carbon- carbon double bond Cracking is an endothermic reaction Hydrogenation is an exothermic reaction

Hydrocracking Reactions Cracking and hydrogentation are complementary as shown below

Hydrocracking reactions Aromatics which are difficult to process in FCC are converted to useful products in Hydrocrackers.

Hydrocracking Reactions Cracking provides olefins for hydrogenation and Hydrogenation provides heat for cracking The overall reaction provides an excess of heat as hydrogenation produces much larger heat than the heat required for cracking operation Therefore the overall process is exothermic and quenching is achieved by injecting cold hydrogen into the reactor and apply other means of heat transfer, e.g. intermediate heat exchanger Isomerization is another type of reaction, which occurs in hydrocracking

Hydrocracking Reactions As a result of isomerization, the olefinic products formed are rapidly hydrogenated which provides high octane isoparaffins The volumetric yield can be as high as 125% as the hydrogenated products have a higher API gravity Hydrocracking reactions are normally carried out at an average catalyst temperature between 550 and 750 F

Hydrocracking Reactions The reactor pressure ranges from 8275 – 13800 kPa (1200 to 2000 psig) Large quantity of hydrogen is circulated in order to prevent excessive catalyst fouling Catalyst poisons are removed from the feedstock to enhance catalyst life The feedstock may be hydrotreated to reduce the sulfur and nitrogen levels as well as metals In recent designs, the first reactor in the reactor train may be used for sulfur and nitrogen removal

Reaction Kinetics Kinetic modeling of reactions in hydrocracking is difficult due to the complexity of feedstock and products In general, adsorption of hydrogen to the active sites on the catalyst surface can be shown as H2 + S H2.S kA k-A S = vacant adsorption site on the catalyst surface kA = adsorption rate constant k-A = desorption rate constant

Reaction Kinetics The rate of adsorption ra is proportional to the concentration of active sites Ca and the partial pressure of hydrogen ra = kA pH2 Ca The desorption rate would be rd = k-A CH2S The net rate of adsorption is ra - rd = kA pH2 Ca - k-A CH2S The total concentration of active sites is constant CT = Ca + CH2S

Reaction Kinetics At equilibrium, the net rate will be zero CH2S = (KA PH2 CT )/(1 + KA PH2) Where KA = kA/k-A Once the molecules have been adsorbed, they undergo several possible types of surface reactions The rate constant is also related to activation energy of the reaction In general, the rate of hydrocracking follows a first order kinetics

Catalysts Characteristics of good catalyst: - ability to produce desirable product and not coke - selective to valuable products (e.g. high octane gasoline). - stable, so it does not deactivate at the high temperature levels in regenerators. - resistant to contamination

Catalysts Hydrocracking catalysts are dual functional, (having metallic and acidic sites) promoting cracking and hydrogenation reactions. The main reactions are: Cracking Hydrogenation of unsaturated hydrocarbons obtained from cracking Hydrogenation of aromatic compounds Hydrogenolysis (breaking C-C bond by the addition of hydrogen) of naphthenic structure

Catalysts Cracking is promoted by metallic sites of the catalyst. Acid sites transform the alkenes formed into ions Hydrogenation reactions are also occur on metallic sites Both metallic and acidic sites take part in the hydrogenolysis reactions To minimize coke formation a proper balance must be achieved with the two sites on the catalyst (depending on the conditions of the operation)

Catalysts High temperatures lead to more reactions on acidic sites while increase in hydrogen partial pressure enhances hydrogenation on metallic sites Conventional catalysts are composed of transition metals deposited on acidic sites. The metals are those from group VIII (e.g. molybdenum, cobalt, nickel,…)

Catalysts Three classes: acid-treated natural aluminosilicates amorphous synthetic silica-alumina combinations crystalline synthetic silica-alumina catalyst called zeolite or molecular sieves

Catalysts Zeolite-based catalyst is one of the most commonly used catalysts in hydrocracking The use of zeolite catalyst minimizes coke formation and improves catalyst stability Zeolites have large concentration of Brunsted acid sites which enhances their hydrocracking activity Zeolites also need lower temperatures to achieve a specified conversion Amorphous -alumina is also widely applied as a catalyst support due to its mechanical and thermal stability and porous structure

Advantages of using Zeolite as Catalyst Higher activity Higher gasoline yield Higher octane number Lower coke yield Increased isobutane production Higher conversion without overcracking

End of Chapter Six