Petroleum Refining Process

Overview of petroleum refining process Process name Action Method Purpose Feedstock(s) Product(s) FRACTIONATION PROCESSES Atmospheric distillation Separation Thermal Separate fractions Desalted crude oil Gas, gas oil, distillate, residual CONVERSION PROCESSED--DECOMPOSITION Catalytic cracking Alteration Catalytic Upgrade gasoline Gas oil, coke distillate Gasoline, petrochemical feedstock Coking Polymerize Convert vacuum residuals Hydro-cracking Hydrogenate Convert to lighter HC's Gas oil, cracked oil, residual Lighter, higher-quality products Visbreaking Decompose reduce viscosity Atmospheric tower residual Distillate, tar CONVERSION PROCESSES--UNIFICATION Alkylation Combining Unite olefins & isoparaffins Tower isobutane/ cracker olefin Iso-octane (alkylate) Polymerizing Unite 2 or more olefins Cracker olefins High-octane naphtha, petrochemical stocks

Overview of petroleum refining process Process name Action Method Purpose Feedstock(s) Product(s) FRACTIONATION PROCESSES CONVERSION PROCESSES--ALTERATION OR REARRANGEMENT Catalytic reforming Alteration/ dehydration Catalytic Upgrade low-octane naphtha Coker/ hydro-cracker naphtha High oct. Reformate/ aromatic Isomerization Rearrange Convert straight chain to branch Butane, pentane, hexane Isobutane/ pentane/ hexane TREATMENT PROCESSES Desalting Dehydration Absorption Remove contaminants Crude oil Desalted crude oil Hydrotreating Hydrogenation Remove impurities, saturate HC's Residuals, cracked HC's Cracker feed, distillate, lube

1. UNIFICATION (Alkylation) Alkylation is basically the INTRODUCTION of one hydrocarbon group into another hydrocarbon molecule. It is inverse of cracking process, because it increases the chain length In oil refining, ISOBUTANE is alkylated with the low molecular weight olefins (propene/butene,..), (means isobutane is combined with olefin) in the presence of strong catalyst, low temperature and high pressure.

Alkylate is mixture of HIGH octane number, branched chain paraffin hydrocarbons. It is premium blending stock because it has exceptionally good antiknock properties. Since crude oil contains only upto 40% of the hydrocarbon constituents in the gasoline range, refiners use FCC process to convert high molecular weight olefins into smaller and more volatile compounds. While Alkylation process transforms low molecular-weight alkenes and iso-paraffins molecules into large iso-paraffins with high octane number.

Reaction Mechanism of Acid catalyzed alkylation Step I:  Reaction of olefin (here we use example of Propene) with an acid to form carbo-cation named here propyl cation. In this reaction addition of H+ ion to the double bond of alkene takes place. Means the double bond breaks up and one of the carbon atoms picks up the H+ ion; The cation of carbon atom is named as “carbo-cation”.

Reaction of carbo-cation with isobutene to form Step II:  It takes place in two further steps: Reaction of carbo-cation with isobutene to form higher carbon atom carbo-cation: This Carbo-cation now is more reactive and has 3carbon atoms directly attached to it and known as tertiary carbo-cation.

There are 3 types of carbocations depending upon the number of carbon atoms directly attached to the carbon atom having positive charge. (i) Primary carbocation (p-carbocation): having single carbon atom attached (ii) secondary carbocation (sec-carbocation): having two carbon atoms attached, e.g.: propyl cation (iii) tertiay carbocation (ter-carbocation): having three carbon atoms attached The order of their reactivity is: Ter-carbocation > sec-carbocation > p-carbocation

Step III:  It takes place in two further steps: (a) Picks up H+ ion from Acid to form ALKYLATE or Rearranges to form a new carbocation for further chain reaction.

(b) Reaction of butyl cation with propene to form long chain cation (b) Reaction of butyl cation with propene to form long chain cation. Means double bond break up and cation adds to one of the carbon atom to form a long chain cation.

Reaction Mechanism of Acid catalyzed alkylation Overall reaction:    Acid sulfuric acid (H2SO4) or hydrofluoric acid (HF) is represented as symbol “H+X-” where H+ = H+ X- = F-/ HSO4-

Refinery Process for Alkylation Alkylation plant uses catalyst hydrofluoric acid HF, known as HF-plant, or sulfuric acid H2SO4: known as sulfuric plant. The HF is very volatile, and difficult to use, so this plant is not commonly used while mostly common is sulfuric plant. The sulfuric plant consist of 7 main parts: the chillers, the reactor, the acid separator, the caustic wash and three distilling plants as shown in figure 2.

Figure 2 Alkylation unit flow diagram

Refinery Process for Alkylation The chillers: are used to reduce the working temperature and to maintain high pressure. Since acid is very strong, highly reactive, it releases heat, which may cause explosion. So temperature must be controlled to be low 10-30oC. High pressure is maintained in order to keep the reaction mixture in liquid form.

The reactors: mixture is then pumped into the reactor The reactors: mixture is then pumped into the reactor. The reaction time for the alkylation process is relatively long, so the reactors used here are very large. The residence time for the reaction is 25-30 minutes to assure that olefins are in good contact with isobutane and acid, to promote the reaction. The acid separators: also called acid Settler. The mixture then moves to this chamber, where hydrocarbon mixture gets completely separated from the acid like water and oil. Hydrocarbons are drawn off from the top, the acid is drawn off the bottom and then recycled back to the feed stock (raw material Or crude oil).

The caustic wash: the Hydrocarbons (HC) from the acid separators have some traces of acid in it, which must be removed. This mixture is then washed with caustic soda (NaOH), which neutralize the acid and removes from the mixture. Fractionators: 3 fractionators separate the alkylate and saturated gases. Any unreacted isobutene is recycled back to the feed stock. During the alkylation some reactions also takesplace. Because there are lot of molecules forming and reacting, so there are small amount of propane, butane also forms.

2. Polymerization- UNIFICATION It is the process in which light olefins, e.g., ethene, propene, butene are induced to combine/polymerize with itself to produce a single branched molecule of two/three times their original molecular weight having same elements in the same proportion as the original molecule. The reaction takes place in the presence of catalyst: phosphoric acid, high pressure and temperature in the range of 300° and 450° F. 17

The product of polymerization is known as DIMATE. For example: Propene polymerizes to form isohexene This process is similar to Alkylation. Here the reaction plant is named as POLY PLANT or DIMER plant.

Description Polymerization in the petroleum industry is the process of converting light olefin gases including ethylene, propylene, and butylene into hydrocarbons of higher molecular weight and higher octane number that can be used as gasoline blending stocks. It is similar to the Alkylation process. Polymerization combines two or more identical olefin molecules to form a single molecule with the same elements in the same proportions as the original molecules. Polymerization may be accomplished thermally or in the presence of a catalyst at lower temperatures.

Polymerization The reaction can takes place through various types of mechanisms, such as free-radical type (involve free-radical formation), cationic mechanism (involve carbo-cation formation), Anionic mechanism (involve carbo-anion formation). And the final product in Termination step can either have double bond in it (by rearrangement of carbon chain) or with the single bond (like in alkylation process).

Mechanism of Reaction Step I(Initiation)  Reaction starts with the addition of acid (H+) to the butene. The double bond breaks up and picks up the H+ ion to form butyl cation.  

Mechanism of Reaction Step II Addition of propyl cation to the propene: double bond breaks up and adds to the carbon centre with positive charge and forms long chain carbo-cation

Mechanism of Reaction Step III (termination) The unstable iso-hexyl cation picks up H+ ion to terminate the reaction and forms stable iso-hexane molecule.

Details for the Refinery process of polymerization The olefin feedstock is pretreated to remove sulfur and other undesirable compounds. In the catalytic process the feedstock is either passed over a solid phosphoric acid catalyst or comes in contact with liquid phosphoric acid, where an exothermic polymeric reaction occurs. This reaction requires cooling water and the injection of cold feedstock into the reactor to control temperatures between 300° and 450° F at pressures from 200 psi to 1,200 psi. The reaction products leaving the reactor are sent to stabilization and/or fractionator systems to separate saturated and unreacted gases from the polymer gasoline product.

Details for the Refinery process of polymerization In the petroleum industry, polymerization is used to indicate the production of gasoline components, hence the term "polymer" gasoline. Furthermore, it is not essential that only one type of monomer be involved. If unlike olefin molecules are combined, the process is referred to as "copolymerization." Polymerization in the true sense of the word is normally prevented, and all attempts are made to terminate the reaction at the dimer or trimer (three monomers joined together) stage. However, in the petrochemical section of a refinery, polymerization, which results in the production of, for instance, polyethylene, is allowed to proceed until materials of the required high molecular weight have been produced.

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