The crucial role of viscosity in petroleum residues
Better understanding petroleum residues
In petroleum refining, residues appear as a by-product of crude oil distillation, where the heaviest fractions condense at high temperatures. Petroleum residues are the most viscous fractions resulting from this refining process. These dense, high molecular weight hydrocarbons are utilized in bitumen, coke, and lighter fractions production through cracking and viscoreduction steps.
Due to their high energy content, energy production and maritime transport are also outlets for these residues. Like many products processed in this industry, viscosity is an essential characteristic. For heavy fractions, viscosity influences the proper execution and efficiency of unit operations.
Thus, precise viscosity measurement is essential for efficient treatment and use.
Sources and production process of heavy residues
Residues are primarily produced in three stages:
• Through atmospheric distillation of heavy and extra-heavy crude oils,
• Then by vacuum distillation,
• And finally, by catalytic cracking and visbreaking processes.
Similar residues also exist in other industries, resulting from the destructive distillation of coal or organic materials such as wood. The term “heavy residues” also includes products referred to as “tar” or “pitch.”
The nature and quantity of residues depend on the quality of the crude oil being processed during its refining. Crude oil is classified by its API gravity, a fundamental property that defines its density and viscosity—two essential characteristics for the petroleum industry.
This classification distinguishes four types of crude oil:
• Light crude oil, with a density less than 31.1° API, which flows easily and requires minimal treatment.
• Medium crude oil, with a density between 31.1° and 22.3° API, with moderate viscosity.
• Heavy crude oil, with a density between 22.3° and 10° API, which is much more viscous.
• Extra-heavy crude oil, with a density lower than 10° API, which is almost solid at room temperature and requires intensive processing.
Depending on the API gravity of the distilled oil, the residue extraction rate will vary; extra-heavy crude oil produces more residues than light crude oil.
Stages of residues production
The main stages producing residues during crude oil refining are:
Fractional distillation
Crude oil is first heated, and its components are separated according to their boiling points. The volatile fractions are collected at the top of the distillation column, while the heavier fractions are collected at the bottom.
Fractional distillation occurs in two steps. The first distillation takes place at atmospheric pressure and allows for initial separation. The column bottom product of the atmospheric distillation is sent to the second vacuum distillation column.
Vacuum distillation reduces boiling temperatures, allowing further separation without decomposing components. The bottom products of both distillations, particularly the vacuum distillation bottoms, are sources of residues.
Catalytic cracking
This step breaks down high molecular weight hydrocarbons produced by distillation into smaller molecules. Cracking reduces viscosity and increases the added value of the products.
Visbreaking
Similar to catalytic cracking, it is a thermal cracking process with similar goals and outcomes.
Bitumen production
Bitumen, also called asphalt in some countries, consists of fractions with high boiling points, around 500°C. The first step involves treating the distillation bottom to remove compounds with lower boiling points.
Coking
Coking is another method of transforming heavy residues, allowing for the extraction of lighter hydrocarbons and the production of coke, which is primarily composed of carbon and exists in solid form.
Blending
Blending involves mixing heavy residues with a much lighter, low-viscosity fraction, typically from a cracking stage.
The goal of this step is to mix in appropriate proportions to produce residual fuel oil or heavy fuel oil. These standardized viscosity products are used as fuel by industries and thermal power plants or as fuel for ship engines.
Impact of viscosity on industrial transformations and the use of heavy residues
Viscosity influences several aspects of the transformation, handling, and use of residues and products derived from them:
Pumping efficiency
Viscosity varies depending on the temperature, quality of crude oil, and upstream transformation stages.
Residue viscosity is particularly high. To ensure flow and transfer from one point to another in the industrial setup, viscosity must remain within ranges defined during equipment sizing for efficient pumping. The best solution for controlled viscosity is to measure it directly online using a process viscometer.
Monitoring and controlling transformation stages of residues
This is particularly relevant for cracking stages, including those in the visbreaker, deasphalting, fluidization (“cutback bitumen”), fluxing (“fluxed bitumen”), and modification (polymer-modified bitumen (PMB) and crumb rubber modified bitumen (CRMB)), which result in bitumen.
Online viscosity measurement allows for monitoring the quality of the feedstock entering the manufacturing unit and characterizing the quality of the finished product.
Blending control
When residues have no other use, they are transformed into residual fuel oil by diluting them with light fractions to obtain heavy fuel. Measuring viscosity, particularly at the reference temperature, is essential to control the solvent or light fraction addition and ensure the heavy fuel has the correct viscosity value that characterizes the final product.
Atomization and combustion
When used as fuel or combustion fuel, petroleum residues and their derived products are usually sprayed either in the combustion chamber of a boiler or in that of an internal combustion engine.
For each injection and atomization system, droplet size depends on the fluid’s viscosity. Excessively high viscosity can hinder atomization, leading to incomplete combustion, overconsumption, increased harmful gas emissions, and frequent maintenance. Efficient spraying! is crucial for optimal combustion in power plants and industrial boilers. Inline viscometers that measure viscosity at process temperature are widely used to maintain viscosity at the desired level.
How Sofraser viscometers optimize petroleum residue Management
Sofraser’s vibrating viscometers ensure real-time monitoring of the viscosity of petroleum residues and their derivatives, guaranteeing optimal treatment and operational efficiency. They offer several advantages:
• High-precision measurement even at extreme temperatures.
• Immediate responsiveness to process variations, reducing waste and energy consumption.
• Robust design suited to harsh industrial environments.
• Improved blending and refining control by maintaining optimal viscosity levels.
• Continuous online monitoring to avoid inefficiencies during transport and combustion.
Sofraser product range
Sofraser’s viscometer range is perfect for processing and using residues and allows for the selection of a solution that best fits the installation. Two different solutions can be considered:
The MIVI viscometers and THERMOSET analyzers are adaptable to all fluids and process conditions, offering a wide range of options and configurations. They provide users with reliable, long-lasting, and safe measurements.
The MIVI Viscometer
With ATEX, IECEx, FM, JIS, and KGS certifications, using explosion-proof and intrinsic safety protection modes, this viscometer is the solution for implementation in any hazardous areas found in refineries, as well as in petroleum storage, processing, and usage facilities.
With its high-temperature option, the MIVI sensor easily adapts to the hottest streams, providing maximum sensitivity regardless of the viscosity level measured, from low viscosities to high-viscosity applications.
Benefits for heavy residue applications:
• No wear parts, operating for years or even decades without maintenance.
• Sensor vibration helps delay fouling and facilitates online cleaning.
• No measurement drift.
• No disassembly required between two periodic maintenance shutdowns and even during.
Thermoset analyzers
The THERMOSETS, viscosity at reference temperature analyzers , benefits from all the advantages of the MIVI sensor and does not need the hot oil bathor furnaces required by other measuring principles.
• The THERMOSET LT is the most economical solution for dynamic viscosity measurement on the market and operates on a temperature cycle principle, recording viscosity when it reaches the reference value.
• The Thermoset CF, which also operates on this principle, is the all-in-one solution simplifying design, even potentially eliminating the sample conditioning loop.
• The THERMOSET KV is designed to continuously measure, directly and simultaneously, kinematic viscosity, dynamic viscosity, and density. This allows for direct correlation with ASTM D445 without the need for an additional densitymeter.
Products similar to and derived from residues:
• Rust inhibitors and lubricants
• Bitumen
• Road bitumen
• Residual fuel
• Heavy fuels
• Tar
• Boiler oil and process fuel
• Foundation insulation and protection
• Asphalt lubricants
• Roofing materials
• Petrolatum
• Pitch
• Cable coating products
• Waterproofing products
