The crude flow rate to the CDU determines the capacity of the whole refinery.
A crude column is typically designed for 80% loading, which means that the unit can be operated at 20% throughput more than the design value.
The capacity of the column is limited by the vapor flow rate with a velocity between 2.5 and 3.5 ft/s (0.76 and 1.07 m/s). The vapor flow rate increases as the vapors rise from the flash zone to the overhead.
To keep the vapor velocity within the limits mentioned above, the wraparounds, which are installed at several points along the column, extract heat from the column.
This results in condensing the rising vapours and reducing the vapour velocity.
To expand crude capacity, the most used technique is to introduce a pre-flash column before the crude heater.
The crude oil after preheating in the hot products and pumparound heat exchangers is flashed into a column where the lightest products are removed.
The bottoms from the pre-flash column are introduced into the crude heater and then to the crude column.
The amounts of the light ends in the crude are now less, and this reduces the vapor loading up the column. Although the unit throughput is increased, the furnace duty is not increased, since the crude rate going to the furnace is not affected due to the removal of the light ends.
Pre-flash columns are also introduced in the original design of the CDU when the crude oil is light, and when it contains a lot of light ends in the naphtha range.