5.1.2 Furnace

• • The furnace is an important constituent in the crude distillation unit

• Here, fuel oil and fuel gas (heavier products) obtained from the refining process itself are burnt to increase the crude oil temperature.

• Typically in refineries, the crude oil is heated to a temperature that enables overflash conditions in the main crude distillation column.

• The concept of overflash is that the crude is heated to such a temperature that enables an additional 5 % vaporization with respect to the residue product. In other words, the residue fraction vapors amounting to 5 % of the total volume of the crude oil are desired.

• Depending upon the quality of the crude, the desired temperature for the crude oil is about 600 -700°F.

5.1.3 Pre-flash column

• • The crude oil enters the pre-flash column after leaving the furnace

• The pre-flash tower separates the lighter fractions of the already heated crude oil.

• The heavier fractions of the crude oil leave from the bottom section of the pre-flash tower.

• Both lighter and heavier streams emanating from the pre-flash tower are fed to the main crude distillation column at various sections

• Pre-flash column enables better refluxes in the main column by distributing the streams effectively between various processing zones of the crude oil.

• Pre-flash column may or may not be included i.e., it is optional.  In other words, the pre-flash column can be avoided and the heated crude oil from the furnace can be fed to the main column directly.

5.1.4 Main and Secondary distillation columns

• • The distillation columns consisting of both main and secondary crude distillation columns are one of the most complex circuitries in distillation.

• The complex arrangement of distillation columns is based on research carried out with pilot plants and simulation software.

• The crude distillation columns (both main and primary) are regarded to an indirect sequence of thermally coupled distillation sequences to obtain the desired products.

• Effective distribution of vapor and reflux in the main column is a serious issue.

• The effective distribution of vapor and reflux is aided through pump around heat exchanger units.

• Live steam is also used in the recent designs.  The live steam is usually at about 50 psig.

• The basic principle of using live steam stems out from several facts. Firstly, upon condensation, oil and water are very easy to separate. Secondly, steam can take significant amount of heat in terms of enthalpy. Thirdly, steam enables enhancement in relative volatility, a principle that is used in steam distillation laboratory experimental set ups.  These principles together are anticipated to provide good dividends technically.