visbreaking

Visbreaking, short for viscosity breaking, is a noncatalytic thermal cracking process used in petroleum refining to reduce the viscosity of heavy residuum and other high-viscosity feeds. The goal is to convert a portion of the heavy molecules into lighter, more handleable products, improving flow properties and enabling further processing or easier storage and transport.

In a visbreaker unit, the feed is heated in a furnace or heated coil to high temperatures, typically in the range of about 450 to 550°C, for a short residence time ranging from seconds to a few minutes. Because the process is noncatalytic and relatively mild compared with other thermal or catalytic cracking options, the cracking is predominantly random and produces a mix of lighter hydrocarbons (gases, naphtha, and gas oil) along with a viscous residuum. The product slate is then cooled and separated into gas, light and middle distillates, cycle oils, and a residual asphalt-like stream.

Common feedstocks include atmospheric and vacuum residues, vacuum tower bottoms, and heavy vacuum gas oils. The primary products are lower-viscosity liquids suitable for downstream upgrading or blending, with a portion of the feed remaining as a heavier resid that can be used as asphalt or directed to further processing.

Advantages of visbreaking include relatively low capital cost and simplicity, with the ability to upgrade heavy feeds and integrate with existing refinery infrastructure. Disadvantages include nonselective cracking, potential increases in sulfur and metals in the lighter products, coke formation risks at higher severities, and generally lower conversion compared with catalytic or more severe thermal processes. Visbreaking is often used to prepare feeds for catalytic cracking, hydrocracking, or cokers.