Chapter 7: Petroleum Processes - Detailed Notes

Objectives

  • By the end of these lectures, students should be able to:
    • Describe the origin, composition, and types of crude oil and understand how these properties influence refining strategies.
    • Understand the upstream petroleum industry, including exploration, drilling, production, and crude oil transport, and explain its impact on downstream refining operations.
    • Identify and describe major refining processes, including separation (distillation), conversion (cracking, reforming), and treatment (hydrotreating, desulfurization).
    • List and characterize major refinery products, including fuels, petrochemical feedstocks, and specialty products, along with their specifications and uses.
    • Discuss environmental and sustainability issues related to refining operations, including emission control, waste treatment, and trends toward greener technologies.

Introduction to Petroleum

  • What is Petroleum?
    • Petroleum is a complex mixture of naturally occurring hydrocarbons.
    • It also contains low levels of other compounds such as sulfur, nitrogen, oxygen, and metal-bearing compounds.
  • Why Petroleum?
    • Petroleum, along with other fossil fuels, has been the major source of energy globally since the early 20th century.
  • Theories of Hydrocarbon Formation:
    • Biogenic Theory (Most Widely Accepted):
      • Hydrocarbons are formed from the decomposition of ancient organic matter (animals, plants, algae, microorganisms).
      • This matter is buried under sediments and transformed over millions of years by heat and pressure.
    • Abiogenic Theory (Alternative Theory):
      • Hydrocarbons originate from deep Earth processes involving inorganic carbon sources.
      • Formed under high temperature and pressure in the mantle and migrate upward into the crust.

Oil and Gas Sectors

  • Upstream:
    • Involves exploration, drilling, and production of crude oil and natural gas from underground or underwater reservoirs.
  • Midstream:
    • Focuses on the transportation, storage, and initial processing of oil and gas.
    • Connects production sites to refineries and markets.
  • Downstream:
    • Covers refining crude oil, manufacturing petrochemicals, and distributing finished products like fuels and lubricants to consumers.

Petroleum Upstream Stages

  • Exploration:
    • Geological and geophysical surveys are used to locate potential underground oil or gas reservoirs, followed by drilling an initial test well.
  • Appraisal:
    • Additional wells and data are gathered to evaluate the size, quality, and commercial viability of the discovered reservoir.
  • Development:
    • Infrastructure is designed and installed including drilling of production wells and surface facilities to prepare the field for long-term extraction.
  • Production:
    • Oil and gas are extracted from the reservoir, processed, and sent to market using natural flow or assisted recovery methods.
  • Abandonment:
    • Once a well is no longer economically viable, it is safely sealed and decommissioned, and the site is restored to environmental standards.

Oil Production Stages

  • Primary Recovery:
    • Oil is produced using the natural pressure of the reservoir, which pushes the hydrocarbons to the surface through the well.
  • Secondary Recovery:
    • Water or gas is injected into the reservoir to maintain pressure and displace oil toward the production wells, increasing recovery.
  • Tertiary Recovery (Enhanced Oil Recovery - EOR):
    • Advanced methods such as steam injection, CO_2 flooding, or chemical injection are used to loosen and extract remaining oil from the reservoir.

Types of Crude Oil

  • Crude oil can be classified by:
    • By Density (API Gravity):
      • Type, Description, API Gravity, Characteristics
      • Light Crude Oil, Flows easily; yields more valuable products like gasoline and diesel, > 31.1°, High-value, easy to refine
      • Medium Crude Oil, Moderately dense, 22.3° – 31.1°, Common globally
      • Heavy Crude Oil, Thick and viscous; harder to refine, < 22.3°, Requires more processing.
    • By Sulfur Content:
      • Type, Description, Sulfur Content, Characteristics
      • Sweet Crude Oil, Low sulfur content, < 0.5%, Less corrosive, easier to refine, more environmentally friendly
      • Sour Crude Oil, High sulfur content, > 0.5%, Needs more treatment (desulfurization), common in Middle East
  • API Gravity Note:
    • The higher the API, the lighter the oil is (inversely proportional to density)

Downstream Industry

  • Upstream industry produces crude oil and natural gas, which goes to the downstream industry for further refining and processing.
  • What is a Refinery?
    • It is an industrial complex to convert crude oil into usable products.
    • Involves separation, conversion, treatment, and blending.
  • Why Do We Need Refining?
    • Crude oil is not directly useful.
    • Refineries break it down and upgrade components.
    • There's a mismatch between natural yield and market demand (e.g., too much heavy oil, not enough gasoline).
  • The focus in this chapter is on crude oil

Oil Refining

  • Input: Crude Oil
  • Processes:
    • Separation (distillation)
    • Conversion (cracking, reforming)
    • Treatment (HDS, sweetening)
    • Blending and storage
  • Output: Marketable Products

Key Unit Operations in Oil Refining

  • Atmospheric Distillation
  • Vacuum Distillation
  • FCC (Fluid Catalytic Cracking)
  • Hydrocracking
  • Reforming
  • Hydrotreating

Oil Refining Products

  • Liquefied Petroleum Gas (LPG):
    • A mix of propane and butane used primarily as fuel for heating and cooking. C3 - C4
  • Naphtha:
    • A volatile, flammable liquid used as a feedstock in petrochemical plants or blended into gasoline. C5 - C9
  • Gasoline (Petrol):
    • A high-energy fuel used mainly in internal combustion engines for cars and light vehicles. C5 - C{12}
  • Kerosene:
    • A middle distillate used for lighting, heating, and as a feedstock for jet fuel. C9 - C{16}
  • Diesel Fuel:
    • A dense fuel used in diesel engines for heavy-duty vehicles, trucks, buses, and some cars. C{10} - C{20}
  • Fuel Oil (Heavy Fuel Oil):
    • A residual product used to power ships, industrial furnaces, and electricity generation units. C{20} - C{70+}
  • Lubricating Oils:
    • Oils used to reduce friction and wear in engines, machines, and industrial equipment. C{20} - C{50}
  • Asphalt (Bitumen):
    • The heaviest product from refining, used for paving roads and waterproofing roofs. C_{70+}

Atmospheric Distillation

  • Also known as “Crude Distillation Unit (CDU)”.
  • It is at the front-end of the refinery.
  • Receives high flow rates, hence its size and operating cost are the largest in the refinery.
  • Many crude distillation units are designed to handle a variety of crude oil types.
    • The process starts by desalting and removing the impurities from the crude oil.
    • This process aims to provide the primary fractionation of crude oil based on boiling point.

Atmospheric Distillation - Process Description

  • Pre-treatment:
    • Crude is pumped from storage tanks and freed from sediments and water by gravity.
    • Salts present in crude oils are removed by electrostatic separation (desalting).
  • Pre-heating and furnace heating:
    • Crude is preheated using hot product streams to 120 – 150°C, improving energy efficiency.
    • Crude is heated further to 330 – 385°C in a furnace to achieve partial vaporization before entering the column.
  • Distillation Column Operation:
    • In the distillation column, crude oil components are separated by their boiling points
    • Heavier components condense and are collected lower in the column, while lighter components travel upward and are collected at higher points.

Vacuum Distillation

  • Vacuum distillation processes atmospheric residue to produce light, medium, and heavy vacuum gas oils.
  • By reducing the operating pressure, further distillates are recovered at lower temperatures.
  • Vacuum units generally prepare feedstock for FCC and hydrocracking units.
  • The main distillated from vacuum units are called vacuum gas oil (VGO).
  • Vacuum column bottom is known as vacuum residue or short residue.
  • Why Vacuum?
    • Reduces boiling points by lowering pressure.
    • Prevents thermal decomposition of heavy fractions.

Hydroconversion

  • Hydroconversion is a term used to describe all different processes in which hydrocarbon reacts with hydrogen.
  • It includes hydrotreating and hydrocracking.
    1. Hydrotreating is used to describe the process of the removal of sulphur, nitrogen and metal impurities in the feedstock by hydrogen in the presence of a catalyst.
    2. Hydrocracking is the process of catalytic cracking of feedstock to products with lower boiling points by reacting them with hydrogen.

Hydrotreating

  • Hydrotreating is one of the major processes for the cleaning of petroleum fractions from impurities such as sulphur, nitrogen, oxy-compounds, chlorocompounds, aromatics, waxes and metals using hydrogen.
  • Hydrotreating achieves the following objectives:
    1. Removing impurities, such as Sulphur, Nitrogen and Oxygen for the control of a final product specification or for the preparation of feed for further processing (naphtha reformer feed and FCC feed);
    2. Removal of metals, usually in a separate guard catalytic reactor when the organo-metallic compounds are hydrogenated and decomposed, resulting in metal deposition on the catalyst pores (e.g. atmospheric residue desulphurization (ARDS) guard reactor); and,
    3. Saturation of olefins and their unstable compounds.

Hydrotreating - Main Role

  • The main role of hydrotreating can be summarized as follows:
    1. Meeting finished product specification.
      • Kerosene, gas oil and lube oil desulphurization.
      • Olefin saturation for stability improvement.
      • Nitrogen removal.
      • De-aromatization to improve the quality of the fuel.
    2. Feed preparation for downstream units:
      • Naphtha is hydrotreated for removal of metal and sulphur.
      • Sulphur, metal, polyaromatics and Conradson carbon removal from vacuum gas oil (VGO) to be used as FCC feed.
      • Pretreatment of hydrocracking feed to reduce sulphur, nitrogen and aromatics.

Hydrotreating Reactions

  • Hydrotreating reactions can be classified as follows:
    1. Desulphurization
    2. Denitrogenation
    3. Deoxidation
    4. Hydrogenation of chlorides
    5. Hydrogenation of olefins
    6. Hydrogenation of aromatics

Hydrocracking

  • Hydrocracking is a catalytic hydrogenation process in which high molecular weight feedstocks are converted and hydrogenated to lower molecular weight products.
  • The catalyst used in hydrocracking is a bifunctional one:
    • It is composed of a metallic part, which promotes hydrogenation; Hydrogenation removes impurities in the feed such as sulphur, nitrogen and metals.
    • and an acid part, which promotes cracking; Cracking will break bonds, and the resulting unsaturated products are consequently hydrogenated into stable compounds.

Hydrocracking Reactions

  • Hydrocracking reactions can be classified as follows:
    • Heavier compounds → lighter compounds

Catalytic Reforming

  • Catalytic reforming is the process of transforming C7 – C{10} hydrocarbons with low octane numbers to aromatics and iso-paraffins which have high octane numbers.
  • The process can be operated in two modes:
    • a high severity mode to produce mainly aromatics (80–90 vol%)
    • a middle severity mode to produce high octane gasoline (70% aromatics content).
  • Catalytic reforming process: Low RON → High RON

Catalytic Reforming - Role of the Reformer

  • The catalytic reformer is one of the major units for gasoline production in refineries. It can produce 37wt% of the total gasoline pool.
  • The straight run naphtha from the crude distillation unit is hydrotreated to remove sulphur, nitrogen and oxygen which can all deactivate the reforming catalyst.
  • The hydrotreated naphtha (HTN) is fractionated into light naphtha (LN), which is mainly C5 – C6, and heavy naphtha (HN) which is mainly C7 – C9 hydrocarbons.
  • Hydrogen, produced in the reformer can be recycled to the naphtha hydrotreater, and the rest is sent to other units demanding hydrogen.

Catalytic Reforming - Octane Number

  • The research octane number (RON) is defined as the percentage by volume of iso-octane in a mixture of iso-octane and n-heptane that knocks with some intensity as the fuel is being tested
  • A list of the RON of pure hydrocarbon is given in the table aside.
  • It is seen from this appendix that the RON of paraffins, iso-paraffins and naphthenes decrease as the carbon number of the molecule increases. Aromatics have the opposite trend.

Catalytic Reforming Reactions

  • Catalytic Reforming reactions can be classified as follows:
    1. Naphthene Dehydrogenation of Cyclohexanes
    2. Paraffin Dehydrogenation
    3. Dehydrocyclization
    4. Isomerization

Fluidized Catalytic Cracking (FCC)

  • The fluidized catalytic cracking (FCC) unit is the heart of the refinery and is where heavy low-value petroleum stream such as vacuum gas oil (VGO) is upgraded into higher value products, mainly gasoline and C3/C4 olefins.

Fluidized Catalytic Cracking - Fluidization

  • When a fluid flows upward through a packed bed of catalyst particles at low velocity, the particles remain stationary
  • As the fluid velocity increases, the pressure drop increases. Upon further increases in velocity, a balance of pressure drop times the cross-sectional area equals the gravitational forces on the particles’ mass. Then the particles begin to move. This is the minimum fluidization velocity as shown in the figure.
  • As the velocity increases, the bed expands and bed porosity increases while the pressure drop remains practically unchanged.

Fluidized Catalytic Cracking - Reactions

  • The main reaction in the FCC is the catalytic cracking of paraffin, olefins, naphthenes and side chains in aromatics. The VGO undergoes the desired ‘primary cracking’ into gasoline and LCO
  • A secondary reaction also occurs, which must be limited, such as a hydrogen transfer reaction which lowers the gasoline yield and causes the cycloaddition reaction. The latter could lead to coke formation.

Fluidized Catalytic Cracking - Configuration

  • The basic configuration of the FCC unit is a reactor (riser) and a regenerator.
  • The catalyst is circulated between them where it is deactivated in the riser and regenerated in the regenerator.
  • There are two basic types of FCC units in use today:
    • The ‘Side-by Side’ type is one in which the reactor and regenerator are separate vessels adjacent to each other.
    • The Stacked or Orthoflow type reactor is mounted on the top of the regenerator.

Hydrocracking vs FCC - Differences

  • Differences Between Catalytic Cracking and Hydrocracking

Product Blending

  • Refining processes do not generally produce commercially usable products directly, but rather semi-finished products which must be blended in order to meet the specifications of the demanded products.
  • The main purpose of product blending is to find the best way of mixing different intermediate products available from the refinery and some additives in order to adjust the product specifications.
  • For example, gasoline is produced by blending a number of components that include alkylate, reformate, FCC gasoline and an oxygenated additive such as methyl tertiary butyl ether (MTBE) to increase the octane number.

Residue Upgrading

  • There are plenty of other processes that may take place in refineries, depending on the type of crude oil feed and the targeted product.
  • The table below shows some other processes used to upgrade residues to more valuable products.

Oil and Gas Sector in Bahrain

  • Bapco Energies (previously Nogaholding) is leading the oil and gas sector in Bahrain with several operating companies including:
    • Bapco Upstream
    • Bapco Refining
    • Bapco Gas / Bapco Gas Expansion
    • GPIC (partially owned by Bapco Energies)
  • The total daily crude oil production in Bahrain is around 190,000 barrels per day from two oil fields:
    • Awali (Bahrain) Field: Approximately 40,000 barrels per day .
    • Abu Safah Field: Approximately 150,000 barrels per day.
  • Bapco Refining recently commissioned a new expansion (Bapco Modernization Program) which raised the total refining capacity from 267,000 to 380,000 barrels per day.

Environmental Aspects in Refining

  • The refinery industry has demanding environmental management challenges to protect water, soil and the atmosphere from refinery pollution.
  • For example, petroleum refineries use relatively large volumes of water, especially for cooling systems.
  • The table below lists typical wastes from a refinery.