A Comprehensive Guide to Petroleum Refining Processes and Properties
How to Measure the Severity of the Visbreaking Process
1. Viscosity Measurements: Compare the viscosity of the oil before and after visbreaking; a greater reduction indicates higher severity.
2. Temperature: Higher temperatures during the process usually result in more significant viscosity reduction.
3. Yield and Composition of By-Products: A higher yield of lighter fractions suggests a more severe visbreaking process.
Why Multi-Grade Oils are Preferred Lubricants for Modern Motors
Each oil has two SAE numbers indicating its viscosity at different temperatures: the lower number represents the oil’s viscosity at 0°F, and the higher number represents its viscosity at 210°F. This dual-number system allows the oil to perform well across a broad range of operating temperatures. For example, 20W-40, 10W-30, and 20W-50 are types of multi-viscosity oils.
Advantages of Reformed Gasoline Over Thermally Cracked Gasoline
Higher Octane Rating: Reformed gasoline has a higher octane rating compared to thermally cracked gasoline. This means it can withstand higher compression in the engine without knocking, leading to better engine performance and efficiency.
Reduced Sulfur Content: Reformed gasoline generally has lower sulfur content, which helps in reducing harmful emissions and improving air quality.
The Importance of Space Velocity in Catalytic Cracking
Space velocity is the ratio of the volumetric flow rate of the liquid stock to the catalyst volume in the reaction zone. Increasing space velocity reduces the conversion rate, but this can be offset by using a more active catalyst or raising the temperature. Conversely, lower space velocity improves conversion rates, but this effect is also influenced by the catalyst’s activity and the reaction temperature. The contact time between the stock and the catalyst is directly related to space velocity.
Why Vacuum Towers are Wider and Shorter
Using vacuum improves separation efficiency by increasing relative volatility, which reduces the required number of plates. However, vacuum also raises vapor load, necessitating more plates to compensate for shorter contact time. Despite this, the increased efficiency generally outweighs the need for more plates, leading to a shorter column. To maintain constant viscosity with a higher flow rate under vacuum, the column diameter should be increased.
Benefits of Zeolite as a Catalyst in Catalytic Cracking
- High activity
- Good fluidizing properties
- High gasoline yield
- Low coke yield
Catalytic Cracking Process Variables
- Temperature
- Heat effect
- Pressure
- Space velocity
- Catalyst to charge ratio
How Viscosity Affects Diesel Fuel Performance
Very low viscosity in diesel fuel leads to excessive atomization and poor penetration, and it doesn’t provide adequate lubrication for high-pressure fuel pump parts. Conversely, high viscosity causes difficulties in atomization and requires longer injection times.
The Effect of Pressure on the Thermal Cracking Process
Increasing pressure in the cracking process does not impact the rate of decomposition but accelerates polymerization due to reduced volume during the reaction. Consequently, higher pressure results in cracking products with fewer unsaturated hydrocarbons.
The Effect of Temperature on the Thermal Cracking Process
As the temperature increases, the reaction rate increases (kinetics), meaning the maximum yield is reached in a shorter time. However, the rate of secondary reactions also increases. As the temperature increases, the maximum gasoline yield decreases (but with a higher octane number), and gas yield increases.
What are Viscosity Index Improvers?
Viscosity Index Improvers are substances added to low viscosity oils to increase their viscosity at temperatures above zero while minimally affecting it below zero. These additives, such as polyisobutylene and methacrylate polymers, help transform low-viscosity oils into high-viscosity oils with a smooth viscosity-temperature curve. Polyisobutylene, with a molecular weight of 15,000 to 25,000, offers good solubility and thermal stability. Methacrylate polymers have molecular weights ranging from 2,700 to 17,000. However, these polymers often have limited thermal and mechanical stability.
Why Lubricating Oil Needs Regular Replacement
Lubricating oils deteriorate over time due to two main causes: oxidation by atmospheric oxygen, particularly when the oil is hot, which follows a free radical chain mechanism, and contamination from suspended impurities such as metallic particles from engine surfaces or dust from the environment.
The Importance of Oxygenates in Gasoline
Oxygenates are added to gasoline to improve its anti-knock properties, which allows for higher compression ratios and improved engine efficiency. They also help to reduce harmful emissions, such as carbon monoxide.
Understanding Potential Gum in Gasoline
Potential gum refers to the gum that might form in gasoline during storage. It indicates the stability of gasoline and its resistance to gum formation. High potential gum can lead to deposits in the engine and fuel system, potentially causing performance issues.
The Role of Pressure Distillation in Refineries
Pressure distillation is used in refineries to separate gases or volatile materials. Examples include stabilization and fractionation of light hydrocarbons.
The Importance of Steam in Catalytic Cracking
In catalytic cracking, steam serves several important functions:
- Preventing coke buildup on the catalyst
- Regenerating the catalyst
- Facilitating hydrogen transfer reactions
- Optimizing product yields
This maintains catalyst efficiency and improves the quality of the cracked products.
Important Requirements for Jet Engine Fuel
- High Calorific Value: It should have a high calorific value per unit mass and per unit volume.
- Low Volatility: It should not vaporize at the low pressure of high altitudes.
- Low Solidification Temperature: It should not solidify at extremely low temperatures, such as -50°C at 30,000 ft.
- Low Viscosity: It should have a low viscosity that remains reasonably low at these low temperatures.
The Significance of Pour Point in Lubricating Oil
The pour point is the lowest temperature at which a liquid oil (L.O.) remains fluid. A low pour point is desirable and is largely determined by the oil’s paraffin wax content. To achieve this, L.O. should undergo dewaxing treatment in the refinery. The pour point is determined by lowering the temperature until the oil no longer flows from a horizontal tube. The L.O. should be used at a temperature higher than its pour point to ensure it remains fluid.
Effects of Asphaltenes, Resin, and Oil on Asphalt Properties
- Asphaltenes: An increase in asphaltenes makes asphalt harder.
- Resins: Resins improve the elasticity of asphalt and its cementing properties.
- Oils: Oils make asphalt frost-resistant.
The Importance of Ignition Lags in Diesel Engine Performance
The time between the beginning of fuel injection and ignition is called the delay period or ignition lag. A good diesel fuel should self-ignite easily. A shorter time lag indicates a better fuel with a higher cetane number and vice versa.
Delayed Coking in Petroleum Refining
The delayed coking process is a refining process developed to minimize refinery yields of residual fuel oil by severe thermal cracking of stocks such as vacuum residual and thermal tar.
Selective Thermal Cracking
In selective thermal cracking, the refractory stock is cracked for a longer time or at a higher temperature, while less stable stocks are cracked at a lower temperature. Selective cracking is used for feedstocks with a wide boiling point range.
The Effect of Aniline Point on Diesel Oil Ignition Quality
The aniline point is the lowest temperature at which aniline and a diesel fuel sample mix completely. A low aniline point suggests the presence of many aromatic compounds, while a high aniline point indicates a higher amount of paraffins. For optimal diesel fuel, a high aniline point is preferred.
How to Remove Coke Deposits from a Surge Drum
- Cooling by water
- Drill a hole through the center of the deposits
- Insert multiple jet water into the hole
Reasons for Catalyst Deactivation
- Impurities in fresh catalyst
- Impurities backed up by feed
- High temperature
- Coke deposits
Points to Consider When Selecting a Catalyst for Catalytic Cracking
- Porosity
- High absorbency
- Stability under physical impact of loading and thermal shocks
- Resistance to air, steam, and sulfur compounds
Difference Between Gasoline and Diesel Engines
| Feature | Gasoline Engine | Diesel Engine |
|---|---|---|
| Fuel Type | Gasoline (volatile and lighter) | Diesel (heavier and less volatile) |
| Compression Stroke | Lower compression ratio; air-fuel mixture compressed before ignition | Higher compression ratio (up to 21:1); air compressed first, then fuel injected |
| Ignition | Spark plug | Compression ignition (fuel self-ignites) |
| Power Stroke | Combustion pressure increases smoothly | Higher pressure (6-10 MPa), potential for knocking |
| Key Properties | Lower viscosity fuel, suitable for high speeds | Higher viscosity fuel, high torque applications, requires good lubrication |
How Viscosity Affects Diesel Fuel Performance
Very Low Viscosity:
- Excessive atomization and poor fuel penetration
- Poor lubrication for moving parts in the high-pressure fuel pump
High Viscosity:
- Difficult atomization
- Longer injection times
The Effect of Pressure on the Thermal Cracking Process
- Primary Reaction (Decomposition): Pressure does not affect the rate.
- Secondary Reaction (Polymerization): Increasing pressure accelerates polymerization due to decreased volume, resulting in products with fewer unsaturated hydrocarbons.
- Feedstock Use: Higher pressure allows using higher boiling point feedstocks and facilitates homogenization.
The Role of Steam in Petroleum Towers
In Topping Towers:
- Steam Stripping: Steam strips light components from side stream products, increasing their flash point and stabilizing them during storage.
In Vacuum Distillation:
- Boiling Point Reduction: Steam lowers the effective partial pressure of oil vapors, reducing the boiling point.
The Importance of Pour Point in Lubricating Oil
- Pour Point: The lowest temperature at which the oil remains fluid.
- Paraffin Wax: Increases pour point, making the oil less fluid at low temperatures. Can decompose at high temperatures, diluting the oil and increasing friction.
- Testing and Usage: Determined by cooling the oil until it no longer flows. Use at temperatures higher than its pour point.
Why Vacuum Towers are Wider and Shorter
- Efficiency of Separation: Vacuum increases relative volatility, improving separation efficiency and requiring fewer plates (shorter column).
- Higher Vapor Load: Vacuum increases vapor load and velocity. To maintain allowable vapor velocity, the diameter must be increased.
The Importance of Space Velocity in Catalytic Cracking
- Definition: Ratio of liquid stock volumetric flow rate to catalyst volume in the reaction zone.
- Effect on Conversion: Increasing space velocity decreases conversion (can be offset by higher catalyst activity or temperature). Decreasing space velocity increases conversion at lower temperatures and catalyst activities.