The following variables are important in the design of crude columns:
1. The nature of the crude—water content, metal content, and heat
stability. The heat stability of the crude limits the temperature to
which crude can be heated in the furnace without incipient cracking.
2. Flash zone operating conditions—flash zone temperature is limited
by advent of cracking; flash zone pressure is set by fixing the reflux
drum pressure and adding to it to the line and tower pressure drop.
3. Overflash is the vaporization of crude over and above the crude
overhead and sidestream products. Overflash is generally kept in
the range of 3-6 LV% (LV = Liquid Volume). Overflash prevents
coking of wash section plates and carryover of coke to the bottom
sidestream and ensures a better fractionation between the bottom
sidestream and the tower bottom by providing reflux to plates
between the lowest sidestream and the flash zone. A larger overflash
also consumes larger utilities; therefore, overflash is kept to
a minimum value consistent with the quality requirement of the
bottom sidestream.
4. In steam stripping, the bottom stripping steam is used to recover
the light components from the bottom liquid. In the flash zone of
an atmospheric distillation column, approximately 50-60% of crude
is vaporized. The unvaporized crude travels down the stripping
section of the column containing four to six plates and is stripped
of any low boiling-point distillates still contained in the reducedcrude by superheated steam. The steam rate used is approximately
5-101bs/bbl of stripped product.1 The flash point of the stripped
stream can be adjusted by varying the stripping steam rate.
5. Fractionation is the difference between the 5% ASTM curve of
a heavy cut and the 95% point on the ASTM curve of a lighter
cut of two adjacent side products. A positive difference is called
a gap,2 and a negative difference is called an overlap.
The design procedures used for atmospheric and vacuum distillation
are mostly empirical, as crude oil is made of a very large number of
hydrocarbons, from methane to asphaltic pitch. The basic data required,
refinery crude distillation column, and a brief overview of the design
procedures follow.
1. The nature of the crude—water content, metal content, and heat
stability. The heat stability of the crude limits the temperature to
which crude can be heated in the furnace without incipient cracking.
2. Flash zone operating conditions—flash zone temperature is limited
by advent of cracking; flash zone pressure is set by fixing the reflux
drum pressure and adding to it to the line and tower pressure drop.
3. Overflash is the vaporization of crude over and above the crude
overhead and sidestream products. Overflash is generally kept in
the range of 3-6 LV% (LV = Liquid Volume). Overflash prevents
coking of wash section plates and carryover of coke to the bottom
sidestream and ensures a better fractionation between the bottom
sidestream and the tower bottom by providing reflux to plates
between the lowest sidestream and the flash zone. A larger overflash
also consumes larger utilities; therefore, overflash is kept to
a minimum value consistent with the quality requirement of the
bottom sidestream.
4. In steam stripping, the bottom stripping steam is used to recover
the light components from the bottom liquid. In the flash zone of
an atmospheric distillation column, approximately 50-60% of crude
is vaporized. The unvaporized crude travels down the stripping
section of the column containing four to six plates and is stripped
of any low boiling-point distillates still contained in the reducedcrude by superheated steam. The steam rate used is approximately
5-101bs/bbl of stripped product.1 The flash point of the stripped
stream can be adjusted by varying the stripping steam rate.
5. Fractionation is the difference between the 5% ASTM curve of
a heavy cut and the 95% point on the ASTM curve of a lighter
cut of two adjacent side products. A positive difference is called
a gap,2 and a negative difference is called an overlap.
The design procedures used for atmospheric and vacuum distillation
are mostly empirical, as crude oil is made of a very large number of
hydrocarbons, from methane to asphaltic pitch. The basic data required,
refinery crude distillation column, and a brief overview of the design
procedures follow.
