CDU

A crude distillation unit is the first major processing vessel at any oil refinery, designed to separate crude oil into useful fractions by boiling point. It is a tall, cylindrical furnace-heated column, typically 30 to 60 meters high and 3 to 4 meters in diameter, operating continuously at atmospheric or slightly elevated pressure. The crude enters a fired heater where it is heated to around 350 to 400 degrees Celsius, then fed into the distillation column where temperature decreases from bottom to top. Lighter hydrocarbon vapors rise and condense at progressively cooler trays, while heavier fractions remain at the base, creating distinct product streams without chemical change.

The outputs from a CDU are determined by boiling range. Gases escape from the overhead and are sent for further processing; naphtha, kerosene, and gas oil are drawn from different tray levels; and residual fuel oil or vacuum gasoil feedstock settles at the bottom. Some installations feed the atmospheric residue directly to a vacuum distillation unit to recover additional medium fractions that would otherwise be lost. The arrangement and number of product drawoff points varies with refinery design and crude type, but all CDUs operate on the same separation principle.

Operational challenges are substantial. Crude composition changes with source, affecting product yields and requiring operators to adjust feed rate, heater outlet temperature, and reflux ratios. Fouling and coking occur when heavier molecules deposit on hot internal surfaces, reducing efficiency and requiring periodic shutdown for cleaning. Vapor recovery is critical because overhead vapors contain valuable gasoline and fuel gas; uncontrolled release represents both economic loss and environmental violation. Furnace tube corrosion from sulfur compounds in crude is managed through material selection and sulfidation rates monitoring.

The CDU is the refinery's workhorse and primary profit lever. Its capacity and product slate determine the entire refinery's throughput and product mix. A modern CDU processes 50,000 to 300,000 barrels per day depending on refinery size. Capital costs are substantial, efficiency is measured in fractions of a percent, and reliability directly affects margins. Engineers speak of CDU throughput, crude assay, product recovery, and heater duty as routine operational metrics.

The term crude distillation unit became standard once petroleum refining shifted from batch to continuous distillation in the early twentieth century. It distinguishes the process from vacuum distillation (which handles the bottoms product) and from secondary units like catalytic crackers and hydrotreaters that upgrade specific fractions. In commercial language, a refinery's size is often quoted as its CDU capacity in barrels per day.