Visbreaker

Visbreaking is a thermal cracking process used in oil refineries to reduce the viscosity of heavy residual oils, making them more suitable for further processing or sale as fuel oils. The process involves the mild cracking of these heavy oils at temperatures between 450°C and 500°C in the presence of a short residence time to produce a mixture of lighter oils, gas, and coke. The term "visbreaking" is derived from "viscosity breaking," reflecting the process's primary goal of reducing the oil's viscosity.

Process Overview

Visbreaking is conducted in a so-called visbreaker unit, which typically operates in one of two modes: the soaker mode or the furnace mode. In the soaker mode, the feedstock is held at the cracking temperature in a soaker drum for a period to allow the cracking reactions to proceed. In the furnace mode, the feedstock is rapidly heated in a furnace then immediately quenched to stop the cracking reactions. The choice between these modes depends on the desired product quality and refinery configuration.

The main reactions occurring during visbreaking are thermal cracking reactions, where large hydrocarbon molecules are broken down into smaller ones. This process reduces the molecular weight of the residue, thereby decreasing its viscosity and making it easier to handle and process. However, visbreaking is a relatively mild form of thermal cracking, designed to minimize the production of undesirable by-products like coke and dry gas.

Products

The primary products of visbreaking are:

• Lighter oils, which can be blended into fuel oils or further processed in other refinery units.
• Tar, which has reduced viscosity and can be used as a feedstock for other processes or sold as a fuel oil component.
• Gases, including methane, ethane, propane, and butenes, which can be used as refinery fuel or feedstock for petrochemical processes.
• A small amount of coke, which deposits on the reactor surfaces and must be periodically removed.

Applications

Visbreaking is primarily used to reduce the viscosity of heavy fuel oils, making them easier to transport and use. It also increases the yield of more valuable light and middle distillates from heavy crude oil fractions, improving overall refinery economics. Additionally, visbreaking can serve as a feed pretreatment step for more severe cracking processes, such as fluid catalytic cracking (FCC) or hydrocracking, by reducing feedstock viscosity and coking tendencies.

Environmental and Safety Considerations

The visbreaking process involves high temperatures and potentially hazardous materials. It requires careful control and monitoring to prevent equipment fouling and to ensure the safety of operations. Additionally, the gases produced can contain sulfur and nitrogen compounds, which must be treated to meet environmental regulations regarding emissions.

See Also

• Oil refinery
• Thermal cracking
• Fluid catalytic cracking
• Hydrocracking
• Petroleum product

References

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