Thermophysical Properties of
Petroleum Fractions and Crude Oils
Introduction
• Design of Petroleum Refinery requires the
knowledge of certain Thermophysical properties
Introduction
• Crude oil has complex composition of
petroleum fractions
• Calculation methods for pure hydrocarbons
are not applicable
• Correlations are developed to estimate the
properties of petroleum fractions
Basic Input Data
Such correlations require basic input data from
LAB experiments:
Specific Gravity (SG)
Boiling Point Curve (ASTM or True Boiling
Point Distillation)
Refractive Index (n)
Molecular Weight (M)
True Boiling Point Distillation
• More detailed characterization of petroleum
fractions
• Performed in column with 15 plates and reflux
ratio of 5
• Higher degree of separation than ASTM
distillation
• IBP is Lower than that of ASTM distillation
• FBP is higher than that of ASTM distillation
Conversion between ASTM & TBP
Distillation
Both Temperatures are in
Rankin
T(R)=(T(C)+273)*1.8
Or
T(R) = (1.8T(C)+32)+460
Excel Example
Pseudo-Components
• Calculations involving crude oil and petroleum
fractions require composition of each process
stream
• Since the actual components are not known,
petroleum fractions are characterized as
mixture of pseudo components
Pseudo-Components
• They are defined over boiling point range over
the TBP distillation curve.
• Each pseudo-component corresponds to
several unknown actual compounds.
Pseudo Components
• Each pseudo-component is characterized by
– Normal Boiling Point (NBP, treated as MABP)
– Specific Gravity
– Molecular Weight
• Once this parameters are characterized,
Pseudo-component can be treated as any
defined component for the Thermophysical
parameters Calculations
Breaking of TBP Curve to Pseudo-components
• The following cut-points are reasonable for
most refining calculations
TBP range is defined by (IBP-FBP)
Breaking of TBP Curve to Pseudo-components
• General Guideline for determining the number
of Pseudo-Components:
Breaking of TBP Curve to Pseudo-components
• Divide the TBP curve into 20 pseudocomponents. Calculate the liquid volume
percentage of each pseudo-component.
Breaking of TBP Curve to Pseudo-components
Note that: TBP curve extends to 95 volume
percent distilled only. To Obtain the NBP of the
final cuts, it has to be extrapolated to 100 %
distilled; by fitting polynomial curve using Excel
and extrapolating it.
Excel Continued
Calculation of Pseuodo-components SG
• The K factor is assumed constant for all
fractions, and equal to its value for the whole
Crude Oil.
• K=
3
𝑀𝐴𝐵𝑃
𝑆𝐺
• For the pseudo-components; MABP = NBP in
Ranking
• Knowing K & NBP, SG can be obtained easily.
Calculation of Pseuodo-components SG
• Knowing K = 11.94, calculate the API of the
Pseudo-components.
Calculation of Pseuodo-components
Molecular Weight
• The molecular weight is obtained from the
following correlation
• Where M is the Mwt
• Tb is the NBP in K
• SG is the specific gravity