REFINERY PRODUCTSLECTURE 3

CRUDE DISTILLATION CURVES

When a refining company evaluates its own crude oils to determine the most desirable processing sequence to obtain the required products, its own laboratories will provide data concerning the distillation and processing of the oil and its fractions.

In many cases, information not readily available is desired concerning the processing qualities of crude oils.

In such instances, true boiling point (TBP) and gravity-midpercent curves can be developed from U.S. Bureau of Mines crude petroleum analysis data sheets.

U.S. Bureau of Mines crude petroleum

analysis

The U.S. Bureau of Mines has carried out Hempel distillations on thousands of crude oil samples from wells in all major producing fields.

Although the degree of fractionation in a Hempel assay is less than that in a 15:5 distillation, the results are sufficiently similar that they can be used without correction.

If desired, correction factors developed by Nelson can be applied. The major deficiency in a Bureau of Mines assay is the lack of information concerning the low-boiling components.

The materials not condensed by watercooled condensers are reported as ‘‘distillation loss.’’

An estimate of the composition of the butane and lighter components is frequently added to the low-boiling end of the TBP curve to compensate for the loss during distillation.

Boiling point at 760 mmHg versus boiling point at 40 mmHg

Crude distillation curve

The Bureau of Mines analysis is reported in two parts:

The first is the portion of the distillation performed at atmospheric pressure and up to 527°F (275°C) end point.

The second at 40 mmHg total pressure to 572°F (300°C) end point. The portion of the distillation at reduced pressure is necessary to prevent excessive pot temperatures, which cause cracking of the crude oil.

The distillation temperatures reported in the analysis must be corrected to 760 mmHg pressure.

Generally, those reported in the atmospheric distillation section need not be corrected, but if carried out at high elevations it may also be necessary to correct these.

TBP and gravity-midpercent curves. Hastings Field, Texas crude: gravity, 31.7°API; sulfur, 0.15 wt%.

The 572°F (300°C) end point at 40 mmHg pressure corresponds to 790°F(421°C) at 760 mmHg.

Refinery crude oil distillation practices take overhead streams with end points from 950 to 1050°F (510 to 566°C) at 760 mmHg.

Estimates of the shape of the TBP curve above 790°F (421°C) can be obtained by plotting the distillation temperature versus percentage distilled on probability graph paper and extrapolating to 1100°F (593°C) .

The data points above 790°F (421°C) can be transferred to the TBP curve.

The gravity mid-percent curve is plotted on the same chart with the TBP curve.

The gravity should be plotted on the average volume percent of the fraction, as the gravity is the average of the gravities from the first to the last drops in the fraction.

For narrow cuts, a straight-line relationship can be assumed and the gravity used as that of the mid-percent of the fraction.

CRUDE DISTILLATION

The crude stills are the first major processing units in the refinery.

They are used to separate the crude oils by distillation into fractions according to boiling point so that each of the processing units following will have feedstocks that meet their particular specifications.

Higher efficiencies and lower costs are achieved if the crude oil separation is accomplished in two steps:

first by fractionating the total crude oil at essentially atmospheric pressure; then by feeding the high-boiling bottoms fraction (topped or atmospheric reduced crude) from the atmospheric still to a second fractionator operated at a high vacuum.

The vacuum still is employed to separate the heavier portion of the crudeoil into fractions because the high temperatures necessary to vaporize the topped crude at atmospheric pressure cause thermal cracking to occur, with the resulting loss to dry gas, discoloration of the product, and equipment fouling due to coke formation.

Typical fraction cut points and boiling ranges for atmospheric and vacuum still fractions are given in Tables .

Relationships among the volume-average, molal-average, and mean-average boiling points of the crude oil fractions are shown in Figures.

Nitrogen and sulfur contents of petroleum fractions as functions of original crude oil contents are given in Figures.

DESALTING CRUDE OILS

If the salt content of the crude oil is greater than 10 lb/1000 bbl (expressed as NaCl).

The crude requires desalting to minimize fouling and corrosion caused by salt deposition on heat transfer surfaces and acids formed by decomposition of the chloride salts.

In addition, some metals in inorganic compounds dissolved in water emulsified with the crude oil, which can cause catalyst deactivation in catalytic processing units, are partially rejected in the desalting process.

The trend toward running heavier crude oils has increased the importance of efficient desalting of crudes. Until recently, the criterion for desalting crude oils was 10 lb salt/1000 bbl (expressed as NaCl) or more, but now many companies desalt all crude oils.

Boiling Ranges of Typical Crude Oil Fractions

Reduced equipment fouling and corrosion and longer catalyst life provide justification for this additional treatment. Two-stage desalting is used if the crude oil salt content is more than 20 lb/1000 bbl and, in the cases where residua are catalytically processed, there are some crudes for which three-stage desalting is used.

TBP Cut Points for Various Crude Oil Fractions

Mean average boiling point of petroleum fractions

Molal average boiling point of petroleum fractions

Nitrogen distributions in

crude oil fractions

Sulfur content of products from

miscellaneous U.S. crude oils

Sulfur content of products from

West Texas crude oils

Sulfur content of products from

California crude oils

Sulfur content of products from

Middle East crude oils