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)

• General Guideline for determining the number of Pseudo-Components:

Breaking of TBP Curve to Pseudo-components

• Divide the TBP curve into 20 pseudo-components. 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.

Breaking of TBP Curve to Pseudo-components

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= 𝑀𝐴𝐵𝑃

3

𝑆𝐺

• For the pseudo-components; MABP = NBP in Ranking

• Knowing K & NBP, SG can be obtained easily.

• Knowing K = 11.94, calculate the API of the Pseudo-components.

Calculation of Pseuodo-components SG

• 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

Calculation of Pseuodo-components Molecular Weight