Petroleum Refinery

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Petroleum Refining

Petroleum Refining is the process of taking crude oil and reprocessing it into a number of different products

Crude Oil - a yellow/black liquid found underneath the surface of the earth which contains various hydrocarbons and other liquid organics

Typical refineries are very large industrial complexes made up of many primary and auxiliary units

No two refineries are identical in that each handles varying amounts of crude oil and produces vary amounts of products depending on location and demand

What is it?

ProductsLight Distillates

Liquid Petroleum Gas (LPG)

Gasoline (Petrol)

Light and Heavy Naphtha

Middle Distillates

Kerosine

Diesel Fuels

Heating Fuels

Other Light Fuels

Heavy Distillates

Heavy Fuel Oils

Bunker Fuel Oil

Others (not all listed)

Petroleum Naphthas

Solvents

Elemental Sulfur

Petrochemicals

Asphalt and Tar

General Flow DiagramSince gasoline is the most desired product, that is what a majority of the crude oil is used to produce

Aside from air and water, hydrogen gas and H2S are frequently used and are thus produced on site

Crude Oil Distillation UnitThis is the first step for all refineries

The crude oil is heated up until it boils, and then, by using fractional distillation, the heavier hydrocarbons (higher BP) can be separated from the light ones (lower BP)

As the gases travel up the column, they will condense and can be pumped out

No reactions occur within the distillation column

Crude Oil Distillation UnitThe products by temperature range and chain length are:

< 40 oC - Petroleum Gas (C1 - C3)

40 - 200 oC - Gasoline (C4 - C12)

200 - 250 oC - Kerosene, Jet Fuel (C12 - C16)

250 - 300 oC - Heating Oils (C15 - C18)

300-370 oC - Lubricating Oils (C19 +)

370+ oC - Residue, Asphalt (C25 +)

Vacuum DistillationVacuum distillation units are found in 80+% of refineries

At very low pressure, the leftovers from the initial distillation tower can be vaporized without cracking

These oils can later be cracked with the aid of catalysts to control the product formation

They are primarily used to produce addition gasoline

No reaction occur during vacuum distillation

Naphtha HydrotreaterNaphtha - a flammable oil made up of various hydrocarbons with sulfur atoms

By treating the naphtha fraction with hydrogen, refineries are able to desulfurize the hydrocarbons

The purpose of this is to reduce the SOx emissions generated from using the fuels produced

Nitrogen and Olefins will be removed as well

Primary Reactions

C-S + 3/2 H2 > R-C-H + H2S

C=C + H2 > H-C-C-H

C-N + 2 H2 > C-H + NH3

Catalytic ReformingCatalysts are used to convert the desulfurized naphtha into higher-octane molecules know as reformates

N.B. - Octane rating measure the compression which can be withstood before ignition

The reformates are rearranged and cracked versions of the hydrocarbons

These are a component of the final gasoline product

Primary Reactions:

R-R + H2 > R-H + R-H

R-C-R > C-R-R (and similar rearrangements)

AlkylationConversion of isobutane and butylene into alkylate, another high-octane component of the gasoline product

The process is typically done in sulfuric acid of hydrofluoric

The product alkylate can either be used directly or farther alkylated in order to produce a higher premium product for aviation fuels

Primary Reaction: butene + isobutane > 2,2,4 - trimethylpentane

IsomerizationThe two distinct isomerization process are butane isomerization and pentane/hexane isomerization

In this process, the straight chains are catalyticlly rearranged into their branched isoparaffins of a higher octane number

For low temperature butane isomerization, aluminium chloride and HCl are typically used, whereas with high temperature isomerization, platinum or other metal catalysts are used

The reaction is simply a catalytic rearrangement similar to the following: R-C-R > C-R-R

The above is a generic sense of the rearrangement. The catalyst will determine the exact isomerization product

Distillate HydrotreaterSimilar to the Naphtha hydrotreater, it uses hydrogen gas to desulfurize other fractions of the crude oil distillate in order to reduce the production of Sox gases from use

It will also remove nitrogen and aromatics; exactly what depends on the feed

Primary Reactions:

C-S + 3/2 H2 > R-C-H + H2S

C-N + 2 H2 > C-H + NH3

Merox (Mercaptan Oxidizer)

These units also desulfurize fraction of the crude oil distillation, specifically liquid petroleum gas, kerosene, and jet fuels, but by oxidizing undesired mercaptans (also known as thiols, the R-S-H group)

The overall reaction can be written:

Amine Gas Treater & Claus UnitThese two units are used in conjunction to create elemental sulfur from the H2S byproduct of the hydrotraters

This process makes up the majority of all sulfur production worldwide

The Amine Gas treater also reduces the CO2 levels, which when present in gas, sharply reduces its markability

Primary Reactions:

RNH2 + H2S > RNH3+ + SH- (Amine Gas Treater)

2 H2S + O2 S2 + 2 H2O (Claus Unit)

Fluid Catalytic CrackingConverts heavy, high-boiling fractions into lighter products with a higher octane rating to be used in gasoline

This used to be done by thermal cracking, but the catalytic cracking allows the refineries to control the product they produce, and as such is now the common method

Carbon is deposited on the catalyst in this process and reduces the performance of the catalyst

This catalyst coke is burnt off in a regenerator

HydrocrackerUses hydrogen gas to break large carbon chains into smaller ones of a higher octane rating

The process works by pumping the hydrogen gas and crude oil distillate at high pressures over towers of catalyst

The main products of this are jet fuel and diesel

Primary Reaction:

R-R + H2 > R-H + RH

VisbreakerThis unit is responsible for thermally cracking the heavy residual oils of the vacuum distillation into more valuable products with a reduced viscosity

While they are old, straight forward units, they will continue to be present in refineries for many years to come with more modernized designs that help decrease energy losses and increase conversion efficiencies

Thermal Cracking Reaction:

2C-C > [ 2C+

+ C-C ] > 2C + C=C

The initial and final molecule will be longer than ethyls and methyls, but it is the same idea

Delayed CokingConverts heavy residual oils into petroleum coke, a carbonaceous solid, in addition to naphtha and other gas oils

The coke is referred to as fuel grade, meaning it is high in sulfur and metals) and anode grade (low in sulfur and metals)

Coke that is primarily carbon can be sold for use in fuel, whereas anode grade coke sees use in the aluminium and steel industries

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