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 reduced crude 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.
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