Introduction to Crude Oil and Petroleum Processing

Abstract

This introduction to crude oil and petroleum processing provides a working knowledge of crude oil properties and refining to make the large array of petroleum-based products we enjoy today. Topics include the composition of crude oil, the crude assay, product properties, and the basic processes used to convert crude to useful products. This chapter sets the stage for the detailed discussions, descriptions, and calculation methods contained in the balance of this handbook.

David S. J. Jones: deceased.

Appendix 1 Procedure for Manual Calculation of an Equilibrium Flash

The equilibrium flash can be calculated using the composition of the feed mixture and the vapor/liquid equilibrium constant of the components in the mixture. This is expressed by the following equation:

$$ L=xf/\left[1+\left(V/L\right)\;K\right] $$

(7)

where:

• L = Total moles/hr of a component in the liquid phase

• xf = Moles/hr of the component in the feed

• V/L = Ratio of total moles vapor to total moles liquid

• K = Equilibrium constant for each component at the temperature and pressure of the flash

The flash calculations can be done using a process simulation program employing appropriate thermodynamic models or manually using a spreadsheet. There are several publications giving values for K. Among these are the charts in Maxwell’s Data Book on Hydrocarbons which are based on fugacities. Others may be found in engineering data books such as “Gas Processors Suppliers Association” (GPSA) which are based on convergence pressures. A rough and ready substitute for K factors is to use the vapor pressure of the component divided by the system pressure. This, however, should not be used for any definitive design work nor in systems which have azeotropes or are near their critical conditions. A method for calculating equilibrium flash vaporization to produce two, primarily, hydrocarbon, phases is given by the following steps. If significant amounts of water are present, more complex procedures are required. Such a “three-phase” flash with water is not included here.

Note that this procedure can be adapted to provide a simple and useful spreadsheet flash calculation, making use of the iterative calculation option that is available in most spreadsheets.

• Step 1. Prepare a table with the first column giving the components making up the feed. The second column will be the composition of the feed in mols/hr. The third column is a listing of the equilibrium constant K for each component at the temperature and pressure of the flash condition. Allow up to three columns following for assumptions of V/L. Each of these three columns should be subdivided into two, the first giving the product of (V/L) K and the second for listing the “L” for each component. Other columns may be added to calculate mole wt of vapor and SG of the liquid phase.

• Step 2. Assume a value for V/L. This is a judged assumption, but start with 1.0 or 0.1 whichever seems to be more realistic. Calculate (V/L) K for each component.

• Step 3. Calculate “L” for each component from the equation:

  $$ L=xf\ /\left[ 1+\left(V/L\right)\;K\right] $$

  (8)

• Step 4. The calculated V/L is now obtained by adding the “L” column and subtracting this value from the total moles of feed in column 1. This subtraction is the vapor moles as calculated. Then the calculated V/L is arrived at by dividing the total V by the total of the “L” column.

• Step 5. The calculation is complete when V/L calculated is equal to V/L assumed. An answer within 5 % is usually acceptable. If the calculated V/L assumed is not within this limit, make another assumption for V/L and repeat steps 2, 3, and 4. For this second assumption try 5, or 0.5, or 0.05, whichever is most appropriate.

• Step 6. If there is still no agreement between assumed and calculated V/L, plot the two trial points (assumed V s calculated) on log graph paper. Draw a straight line through these two points and note where on this line assumed V/L = calculated V/L. This value is the next assumed V/L. Repeat the calculation steps 2–4 using this value; this usually completes the calculation. If it does not then check that the conditions for the flash are within the boiling point and condensing point for the feed.

In the example below, it is desired to determine the amount of vapor and liquid and their compositions in a feed to a fractionator at 112 psig and 300 °F (Table 9).

Table 15 Table 9

Note the components NP260–NP382 are pseudo components having mid-boiling points of 260 °F, 300 °F, 340 °F, and 382 °F respectively. K for these components are based on their vapor pressure and system pressure relationship.

Predicting the EFV Curve from TBP Data. For crude oils and complex mixtures such as the heavy products, the equilibrium vaporization curve can be calculated from the TBP curve using empirical methods given by Edmister or Maxwell. In this work the EFV is based on the method by Maxwell in his book Hydrocarbon Data. This method has been described in the section “Developing the TBP Curve and the Equilibrium Flash Vaporization (EFV Curve from the ASTM Distillation Curve” of this chapter.