Crude oil characterisation and atmospheric and vacuum distillation

Why it matters

Every refinery starts with the crude distillation unit (CDU) and vacuum distillation unit (VDU), and every downstream unit is sized from the crude assay. Indian refiners process a basket of imported crudes, so knowing how API gravity, sulphur, the TBP curve and characterisation factors predict yields, corrosion and processing needs is basic to refinery work and a steady source of GATE questions.

Key ideas

Crude oil is a mixture of paraffins, naphthenes (cycloparaffins) and aromatics, with sulphur, nitrogen, oxygen compounds, metals (Ni, V), salts and water.

Characterisation properties.

• API gravity: higher API = lighter crude. Light crudes are above about 31 °API, heavy crudes below about 22 °API.
• Sulphur: "sweet" crude has under about 0.5 wt % S, "sour" crude more. Sulphur sets hydrotreating load and corrosion.
• TBP (true boiling point) distillation: a batch distillation with many stages and high reflux; it plots temperature against volume % distilled and is the basis for predicting product cut yields. The ASTM D86 test is a quick, low-separation distillation used for product specs; its curve is flatter than the TBP curve.
• Watson (UOP) characterisation factor K: indicates the chemical nature. K around 12.5–13 is paraffinic, about 11.5–12 mixed/intermediate, about 10.5–11.5 naphthenic, and about 10 strongly aromatic.
• Others: pour point (wax), viscosity, Conradson carbon residue (coke-forming tendency), salt content, total acid number (naphthenic-acid corrosion), metals (catalyst poisons), flash point.

Desalting. Crude is mixed with 3–8 % wash water at about 120–150 °C and the emulsion is broken in an electrostatic desalter. Salts (mainly NaCl, MgCl₂, CaCl₂) would hydrolyse to HCl in the furnace and corrode the column overhead, and foul exchangers.

Atmospheric distillation (CDU).

1. Crude is preheated against hot products in a preheat train, then in a fired furnace to about 350–370 °C. Higher temperature would crack the crude.
2. It flashes into the column flash zone at slightly above atmospheric pressure (about 1–2 bar). Vapour rises through 30–50 trays.
3. Products are drawn as side streams and sent to side strippers, where steam strips light ends to meet flash point. Pumparound circuits remove heat at intermediate heights, which reduces overhead condenser load and lets heat be recovered at useful temperature.
4. Steam injected at the bottom lowers the hydrocarbon partial pressure and strips light material from the residue.

Typical TBP cut ranges (vary by refinery): gas and LPG (C1–C4); light naphtha (C5 to about 90 °C); heavy naphtha (about 90–180 °C, reformer feed); kerosene/ATF (about 160–250 °C); diesel or gas oil (about 250–370 °C); atmospheric residue (above about 370 °C).

Vacuum distillation (VDU). Atmospheric residue cannot be heated much beyond about 400 °C without thermal cracking (coke and gas). The VDU runs at an absolute pressure of roughly 25–100 mmHg in the flash zone (lower at the top), produced by steam ejectors or vacuum pumps, so heavy material vaporises at lower temperature. Columns are wide (low vapour density) and use packing or low-pressure-drop trays. Products: light and heavy vacuum gas oils (feeds to FCC or hydrocracker), lube oil distillates in lube refineries, and vacuum residue (bitumen, coker, visbreaker feed). Loss of vacuum raises boiling points, so the furnace must run hotter, risking cracking and poor lift.

Formulas

°API = 141.5 / SG − 131.5, SG = 141.5 / (°API + 131.5)

• SG: specific gravity at 60 °F / 60 °F (dimensionless). Water is 10 °API.

K = (T_B)^(1/3) / SG

• T_B: mean average boiling point in degrees Rankine (°R = 1.8 × °C + 491.67). Used to judge paraffinic or aromatic character and in property correlations.

Mass flow = volume flow × SG × ρ_water

• ρ_water = 999.0 kg/m³ at 60 °F; 1 barrel (bbl) = 0.158 987 m³.

Cut yield (vol %) = TBP vol % at upper cut point − TBP vol % at lower cut point

Worked examples

Example 1 (standard): API and throughput. A crude has SG 0.85. Find its API gravity and the mass throughput of a 100 000 bbl/day CDU.

1. °API = 141.5 / 0.85 − 131.5 = 166.47 − 131.5 = 34.97 ≈ 35.0.
2. Volume = 100 000 × 0.158 987 = 15 898.7 m³/day.
3. Mass = 15 898.7 × 0.85 × 999.0 = 1.350 × 10⁷ kg/day = 13 500 t/day.

Answer: 35.0 °API; about 13 500 t/day (≈ 562 t/h).

Example 2 (GATE level): characterisation factor and cut yield. A fraction has a mean average boiling point of 450 °C and SG 0.876. The crude's TBP curve gives 50 vol % distilled at 270 °C and 70 vol % at 370 °C. Find K, classify the fraction, and find the diesel (270–370 °C) yield from 100 000 bbl/day.

1. T_B = 1.8 × 450 + 491.67 = 1301.67 °R.
2. (1301.67)^(1/3) = 10.919.
3. K = 10.919 / 0.876 = 12.46, so the fraction is paraffinic.
4. Diesel cut = 70 − 50 = 20 vol %, so 0.20 × 100 000 = 20 000 bbl/day.

Answer: K ≈ 12.5 (paraffinic); about 20 000 bbl/day of diesel cut.

Common mistakes

• Thinking a higher API means a heavier crude; it is the reverse.
• Using °C or K instead of °R in the Watson K formula.
• Confusing TBP and ASTM D86 curves; only TBP gives sharp cut yields.
• Saying vacuum distillation is used because heavy fractions "will not boil"; they would crack before boiling at atmospheric pressure.
• Forgetting the role of stripping steam: it lowers partial pressure, not total pressure.
• Skipping desalting in the process sequence; chlorides cause overhead corrosion.

For GATE CH

Expect API–SG conversions, Watson K and classification, the sequence desalter → furnace → CDU → VDU, the function of side strippers, pumparounds and steam, the reason for vacuum operation, and which fraction goes where (naphtha to reformer, VGO to FCC/hydrocracker, residue to coker or bitumen). Numericals are usually short: API, K, cut volumes from a TBP curve and mass–volume conversions.

Quick check

1. What is the API gravity of water?
2. Why is crude desalted before distillation?
3. What does a Watson K of about 10 indicate?
4. Why are vacuum columns much wider than atmospheric columns of the same capacity?

Answers: 1. 10 °API; 2. to prevent HCl formation and overhead corrosion and to reduce fouling; 3. a highly aromatic material; 4. vapour density is very low at vacuum, so the vapour volume per unit mass is large.