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Petroleum refining industry

 

Petroleum refining industry extensively involves heterogeneous solid catalytic processes. The major catalytic processes are shown in Fig 1.

 

Petroleum Refining (Part - 1) - Chemical Engineering

Fig. 1. Various solid catalytic processes in petroleum refining industries

 

Catalytic cracking

 

Catalytic cracking of gas oil is among the most important catalytic processes and major source of fuels. This process involves simultaneous occurrence of various reactions. Some typical reactions in catalytic cracking process are shown in Fig. 2.

 

Petroleum Refining (Part - 1) - Chemical Engineering

Petroleum Refining (Part - 1) - Chemical Engineering
Fig. 2. Some typical reactions in catalytic cracking process

 

Process

Usually catalytic cracking of gas oil is carried out at 500-5500C at 2 -3 atm pressure, in a fluidized bed reactor . In the fluidized bed reactor, most of the cracking occurs in the transport-line (riser). Catalyst to oil weight ratio is usually maintained at 4-6 with residence time of 2-6 seconds in the riser reactor. The gasoil feed stock is usually consist of C20-C40 in the boiling range of 350 -5500C. Catalyst circulation rate of about 6 ton/m3 of total feed is maintained for smooth operation. The schematic diagram of a typical fluidized catalytic cracking reactor is shown in Fig. 3.

 

Catalyst

Common cracking catalysts are amorphous SiO2-Al2O3 and zeolites. Zeolites are alumino-silicates with well- defined crystalline structures having molecular size pores that give rise to its shape selective properties. Zeolites are discussed in detail in later sections. The product distribution in catalytic cracking process also depends on the type of catalyst. The SiO2-Al2O3 catalyst gives higher alkene yield while zeolite catalyst result in higher aromatic yield.

 

The typical commercial cracking catalyst is a mixture of zeolite and SiO2–Al2O3. The catalyst can contain 3-25 wt.% zeoliteY. The zeolite is usually ion exchanged with rare earth ions such as La+3or Ce+3 to provide additional thermal stability. The balance of mixture is SiO2 -Al2O3, similar to the original amorphous cracking catalyst. The function of acidic SiO2 -Al2O3 is to crack the large feed molecules to a size which can diffuse into the zeolite channels for further reactions. The alumino-silicate matrix also protects the zeolite from poisons and attrition. The catalyst particles are of 40-100 µm in diameter with pore sizes in the range of 8-10 nm. Surface area of the zeolite ranges from 600-800 m2/g, whereas, the surface area of the alumino-silicate matrix is in the range of 100-300 m2/g.

 

Additives are added to cracking catalyst and constitute about 5 wt. % of catalyst. Commonly used additives include octane boosting additives such as ZSM-5, metal passivators, SOx reducing agents, and CO oxidation catalyst. Addition of 1-3 wt. % of ZSM-5 increases octane number while decreases gasoline yield. Nickel increases gas and coke selectivities while vanadium destroys the zeolite lowering the activity. These undesirable effects of Ni can be minimized by adding a passivator such as a compound of antimony and bismuth to the process stream to react selectively with the Ni to form a catalytically inactive Ni-Sb or Ni-Bi species. These are added commonly as organometallic solutions to the process stream.

 

Petroleum Refining (Part - 1) - Chemical Engineering
Fig. 3. Schematic diagram for catalytic cracking in Riser reactor.

 

Mechanism and kinetics

Cracking reactions involve C-C bond rupture via formation of carbocations. Reaction is catalyzed by acid sites on SiO2–Al2O3 and zeolites catalysts. Carbocations are formed on Bronsted and Lewis acid sites of the catalysts. A typical mechanism for catalytic cracking of alkane initiated by protonation is given below:

 

Petroleum Refining (Part - 1) - Chemical Engineering

A detailed micro kinetic model for gas oil cracking involve hundreds of elementary reaction steps for which kinetic parameters are to be determined. Due to this complexity of cracking process, the kinetic study of these reactions is extremely difficult. For simplification, the reaction steps and products are combined into groups of species. These groups are called lumps and each lump is considered as an independent entity. This is known as lumped kinetic technique.

 

A three lumped model proposed by Wojciechowski and Corma [3] for gas oil cracking is shown in Fig. 4. Simple parallel- series kinetic network involves reactions of gas oil to gasoline /diesel (path 1) or to gases/coke (path 2). The gasoline and diesel can further react to give gas and coke.

 

Petroleum Refining (Part - 1) - Chemical Engineering

Fig. 4. Three Lumped kinetic model for catalytic cracking of gas oil

 

The corresponding reaction kinetics was proposed to be first order with respect to each hydrocarbon component of the feed reacting by path 1 or 2.

 

Petroleum Refining (Part - 1) - Chemical Engineering

To account for rapid catalyst decay, each of the rate constants was modified using a time–on-stream functions:

 

Petroleum Refining (Part - 1) - Chemical Engineering

 

Deactivation

The major sources of catalyst deactivation are

► Coke formation
► Sintering
► Deactivation by metals such as Ni, 

Coke formation on the catalyst surface is the major deactivation source for catalytic cracking catalysts. Very high rate of coke formation results in very low residence time of catalysts in the reactor and high regeneration frequency. The metal residue in the feed can also affect the performance and activity of the catalysts. Nickel increases gas and coke selectivities while vanadium is reported to destroy zeolites and lowers the activity. The effects of metals can be prevented by adding metal passivators. Antimony or bismuth compound can be added which form Ni-Sb or Ni- Bi alloy. Magnesium orthosilicate is also added to form MgO-V2O5-SiO2. At severe conditions of high temperature and steam, zeolite structure gradually deteriorates.

Petroleum Refining (Part - 1) - Chemical Engineering

Petroleum Refining (Part - 1) - Chemical Engineering

   Petroleum Refining (Part - 1) - Chemical Engineering  
Fig. 5. Lump kinetic models for catalytic cracking (a) Five lump kinetic models [4] (b) seven lump kinetic models [5]

 

Hydrocracking

This process has wide range of applications including upgradation of petrochemical feedstock, improvement of gasoline octane number, production of high quality lubricants etc. The process upgrades the original feedstock by increasing its overall hydrogen-to-carbon ratio and decreasing the average molecular weight.

 

Hydrocracking is extensively used for simultaneously cracking and hydrogenating low value gas oil, containing high amount of cyclic polyaromatic and naphthenic compounds, to produce high value products such as gasoline, diesel or jet fuel. Hydrocracking involves multiple reactions such as ring opening, cracking, dealkylation and isomerization with simultaneous saturation due to presence of hydrogen. The major advantage of this process is higher selectivity towards cracking of polyaromatics to desired fuels such as gasoline, diesel or jet fuel and less production of lower hydrocarbons. This is in contrast with catalytic cracking process which gives rise to considerable amount of lower alkene products. Some of the typical reactions are shown in Fig. 6.

Petroleum Refining (Part - 1) - Chemical Engineering

Petroleum Refining (Part - 1) - Chemical Engineering
Fig. 6. Typical hydrocracking reactions

 

Upon substituting the modified rate constants for k1 and k2 into equations (1) and inserting this expression in equation for plug flow and integrating, an expression was obtained by Wojciechowski and Corma [3] which relates instantaneous conversion Xto the time of reaction twhile accounting for volume expansion εA.

Petroleum Refining (Part - 1) - Chemical Engineering

b = constant and P= catalyst to oil ratio

This above model although useful for modeling the kinetics of gas oil cracking, was highly empirical and could not be generalized for kinetics of catalytic cracking.

A five lump kinetic model (Fig 5a) was proposed to describe the gas oil catalytic cracking (FCC) process by Ancheyta-Juárez et al.[4]. The model contained eight kinetic constants, including one for catalyst deactivation. The model included unconverted gas oil, gasoline, LPG, dry gas and coke as lumps. The activation energies of the various steps were in the range of 9-13 kcal/mol. Even seven lump kinetic models for fluid catalytic cracking, as shown in Fig. 5b, are reported in literature. The shown seven-lump model involved residual oil, heavy lump, light lump, gasoline, liquefied petroleum gas, dry gas and coke [5]. The VGO, HFO and LFO in Fig 5b are vacuum gas oil, heavy fuel oil and light fuel oil respectively.

 

Process

The process is typically carried out in a series of fixed bed reactors at 300-4500C and 100-200 atm. The process is associated with large heat release due to exothermic hydrogenation reactions which dominate the endothermic cracking reactions. The major disadvantage of the process is the requirement of very high pressure of hydrogen with large energy consumption making the process rather expensive. A typical two-stage hydrocracker is shown in Fig. 7. In first stage 40-50 vol% of the feed is hydrocracked. The first stage also acts as a hydrotreater, where poisonous nitrogen and sulfur compounds are partially hydrogenated. The effluent from the first stage reactor passes through heat exchangers to a high pressure separator where hydrogen-rich gases are separated and recycled. The liquid from the separator is fed to a fractionating tower and the tower bottoms form the feed to the second stage. Usually fixed-bed reactors with liquid down flow are used.

 

Petroleum Refining (Part - 1) - Chemical Engineering

Fig. 7. Schematic diagram of two stage hydrocracking process

 

Catalysts

The hydrocracking catalyst has to be a bi-functional catalyst:

– Acid sites to catalyze cracking reactions

– Metal sites catalyzing hydrogenation

 

Catalyst choice depends on the nature of feed and desired product distribution. CoO-MoO3-Al2O3has been widely used for hydrocracking of heavy feed stocks such as residual raffinate, solvent deasphalted residual oil and vacuum residue. Base metals (Co, Mo, Ni, W) supported on Al2O3-SiO22 and zeolite are used for producing lubricating oils and middle or heavy distillate. Typical catalysts consist of 2% Co 7 % Mo, 6% Ni, 20% W on Al2O3-SiO2. The Ni/SiO2-Al2O3 increased conversion of heavy polynuclear compounds in the feed. Pt or Pd supported on zeolites are used for clean and pretreated feeds and are highly selective towards gasoline, diesel or jet fuel. Typical catalyst is 0.5 wt % Pt or Pd on zeolites prepared by ion exchanges. Ni-Mo-zeolite and Ni-W-zeolite catalysts are used for maximizing gasoline and gas oil production respectively. The Ni-W-impregnated rare earth exchanged X-type zeolite was found to be more resistant to nitrogen and structurally more stable. Catalyst poisoned by deposition of coke and other materials is usually regenerated by burning off the deposits.

 

Mechanism and kinetics

As discussed earlier hydrocracking of petroleum feedstock also involves a complex network of reactions of large number of components. Reviews [6, 7] on kinetics of hydrocracking of heavy oil fractions include various lumped kinetic models. Callejas and Martinez [8] for hydrocracking of residue over Ni-Mo/γ-Al2O3 catalyst in CSTR at 12.5 MPa hydrogen pressure and 375-415°C temperature, used three lumps model (Fig 8a). The three lumps were atmospheric residue (AR), light oils (LO) and gases. The kinetics of hydrocracking of vacuum distillates was studied by Orochko et al. [9] over an alumina–cobalt molybdenum catalyst using a first-order kinetic scheme as shown in Figure 8b. The rate of a first-order heterogeneous catalytic reaction was expressed by the following equation:

 

Petroleum Refining (Part - 1) - Chemical Engineering

where α = rate constant, t = reaction time, y = total conversion and b = inhibition factor due to adsorption reaction products on catalysts.

 

Sanchez et al. [10] proposed a five lump kinetic model (Fig 8c) for moderate hydrocracking of heavy oil in a fixed bed downflow reactor over Ni-Mo/Al2O3 catalyst. The lumps were unconverted residue, vacuum gas oil, distillates, naphtha, and gasses. The model included 10 kinetic parameters. Krishna and Saxena [11] reported a detailed kinetic model with seven lumps (Fig 8d) considering cuts of different temperatures. The lumps were sulfur compounds, heavy and light aromatics, heavy and light naphthenes, heavy and light paraffins. The pseudo components were considered light if they were formed from fractions with boiling points lower than the cut temperature. Sulfur compounds were considered to be a heavy lump.

 

Deactivation

Catalyst deactivation mainly occurs by deposition of coke on catalysts surface that can be removed by periodic burn-off. The other source of deactivation is presence of sulfur and nitrogen metals in feed. The S and N metals can be removed by feed pretreatment.

Petroleum Refining (Part - 1) - Chemical Engineering

Petroleum Refining (Part - 1) - Chemical Engineering
Fig. 8. Various lumped kinetic models for hydrocracking reactions (a) Three lump model [8] (b) Four lump model [9] (c) Five lump model [10] (d) Seven lump model [11]

 

 

 

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