Visbreaker

Visbreaker

Diagram of a visbreaker unit

A visbreaker is a type of chemical engineering process unit used in petroleum refineries to reduce the viscosity of heavy residual oils. The primary purpose of a visbreaker is to break down large hydrocarbon molecules into smaller, more valuable products such as fuel oil and gasoline. This process is a form of mild thermal cracking.

Process Description

The visbreaking process involves heating the heavy residual oil feedstock to a high temperature, typically between 450°C and 500°C, in a furnace. The heated oil is then passed through a soaking drum where the thermal cracking reactions occur. The residence time in the soaking drum is carefully controlled to achieve the desired level of cracking without producing excessive amounts of coke.

The cracked products are then rapidly quenched to stop the cracking reactions. The resulting mixture is separated into different fractions using a fractionating column. The main products of the visbreaking process include:

• Gasoline
• Diesel fuel
• Fuel oil
• Gas

Types of Visbreaking

There are two main types of visbreaking processes:

• Furnace Visbreaking: In this type, the cracking reactions occur primarily in the furnace tubes. The feedstock is heated to a high temperature and then immediately quenched.

• Soaker Visbreaking: In this type, the feedstock is heated to a lower temperature and then held in a soaking drum for a longer period to allow the cracking reactions to occur.

Advantages and Disadvantages

The visbreaking process offers several advantages:

• Increased Yield: It increases the yield of valuable lighter products from heavy residual oils.
• Reduced Viscosity: It reduces the viscosity of the residual oil, making it easier to handle and transport.
• Lower Operating Costs: Compared to other cracking processes, visbreaking is relatively simple and has lower operating costs.

However, there are also some disadvantages:

• Limited Conversion: The conversion of heavy molecules to lighter products is limited compared to more severe cracking processes like fluid catalytic cracking.
• Coke Formation: There is a risk of coke formation, which can foul equipment and reduce efficiency.

Applications

Visbreakers are commonly used in refineries to process heavy residual oils that are not suitable for further processing in other units. The products from visbreaking can be used as feedstock for other refining processes or blended into finished products.

Related Pages

• Petroleum refining processes
• Cracking (chemistry)
• Fluid catalytic cracking
• Coking (chemistry)