boiling point (i.e., mid boiling point) and its gravity in
API. Care must be made to
establish these properties as accurately as possible. It will form the basis of any
design process or definitive study work that may be carried out on this unit.
Developing the Side Stream Product TBPs
The first step is to divide the crude feed TBP curve into the product yield (in percent
volume) and in terms of that product temperature cut range. This starts with
establishing the cut point of the residue, that is the temperature on the TBP curve
at which it is intended to separate the total distillate from the residue. Then the
distillate portion of the TBP is divided into its product cuts such as gas oils,
kerosene, and the total overhead naphtha or some other combination of products.
The volumes of these cuts and temperature ranges represent the yield of each
product that will be produced as a percentage of the whole crude.
From the cut points of each of the products, their ASTM curves are developed
using the method described in the “Introduction” chapter to this handbook. The
initial boiling point (IBP) of each cut (except the overhead naphtha) is fixed by
the fractionation capability of the distillation unit. This term “fractionation
capability” for a crude oil distillation unit is measured as the difference in
temperature between the 95 % volume point of the lighter cut’s ASTM distillation
and the 5 % volume point of the adjacent heavier cut. This difference may be
positive (gap) or negative (overlap). A gap indicates good separation while the
overlap indicates poor separation. The ability to separate the fractions efficiently
decreases as the products become heavier. Thus, one can expect an ASTM gap
between the overhead product and the first side stream to be around 25
F, while
that between the third and fourth side stream may have an ASTM gap around – 10
F
(an overlap of 10
F).
The side stream TBP curves are now developed using these concepts and
following these stepwise procedures.
Step 1. Establish the cuts and cut ranges on the crude TBP that represent the
products that will be produced in this unit. For example, the overhead product
will be a full-range naphtha and will contain all the gas in the crude and the
distillate to a cut point of 400
F. The first side stream will be a kerosene cut
beginning at 400 and ending at 500
F on the crude TBP curve. The next side
stream will be a gas oil boiling between 500
F and 650
F on the TBP curve.
These will have yields as % volume on crude from the TBP curve. As an
example, typical cuts are given as Fig. 4.
From this figure:
Naphtha (gas to 400
F) = 30 %vol on crude.
Kerosene (400–500
F) = 10.5 %vol on crude.
Gas oil (500–650
F) = 14.5 %vol on crude.
Residue (+650) = 45.0 %vol on crude.
Step 2. Predict the ASTM end point and the ASTM 90 % point using the figures
given in the chapter “▶ Introduction to Crude Oil and Petroleum Processing” of
this handbook for the full-range naphtha (i.e., the overhead product).
Atmospheric and Vacuum Crude Distillation Units in Petroleum Refineries
131
API. Care must be made to
establish these properties as accurately as possible. It will form the basis of any
design process or definitive study work that may be carried out on this unit.
Developing the Side Stream Product TBPs
The first step is to divide the crude feed TBP curve into the product yield (in percent
volume) and in terms of that product temperature cut range. This starts with
establishing the cut point of the residue, that is the temperature on the TBP curve
at which it is intended to separate the total distillate from the residue. Then the
distillate portion of the TBP is divided into its product cuts such as gas oils,
kerosene, and the total overhead naphtha or some other combination of products.
The volumes of these cuts and temperature ranges represent the yield of each
product that will be produced as a percentage of the whole crude.
From the cut points of each of the products, their ASTM curves are developed
using the method described in the “Introduction” chapter to this handbook. The
initial boiling point (IBP) of each cut (except the overhead naphtha) is fixed by
the fractionation capability of the distillation unit. This term “fractionation
capability” for a crude oil distillation unit is measured as the difference in
temperature between the 95 % volume point of the lighter cut’s ASTM distillation
and the 5 % volume point of the adjacent heavier cut. This difference may be
positive (gap) or negative (overlap). A gap indicates good separation while the
overlap indicates poor separation. The ability to separate the fractions efficiently
decreases as the products become heavier. Thus, one can expect an ASTM gap
between the overhead product and the first side stream to be around 25
F, while
that between the third and fourth side stream may have an ASTM gap around – 10
F
(an overlap of 10
F).
The side stream TBP curves are now developed using these concepts and
following these stepwise procedures.
Step 1. Establish the cuts and cut ranges on the crude TBP that represent the
products that will be produced in this unit. For example, the overhead product
will be a full-range naphtha and will contain all the gas in the crude and the
distillate to a cut point of 400
F. The first side stream will be a kerosene cut
beginning at 400 and ending at 500
F on the crude TBP curve. The next side
stream will be a gas oil boiling between 500
F and 650
F on the TBP curve.
These will have yields as % volume on crude from the TBP curve. As an
example, typical cuts are given as Fig. 4.
From this figure:
Naphtha (gas to 400
F) = 30 %vol on crude.
Kerosene (400–500
F) = 10.5 %vol on crude.
Gas oil (500–650
F) = 14.5 %vol on crude.
Residue (+650) = 45.0 %vol on crude.
Step 2. Predict the ASTM end point and the ASTM 90 % point using the figures
given in the chapter “▶ Introduction to Crude Oil and Petroleum Processing” of
this handbook for the full-range naphtha (i.e., the overhead product).
Atmospheric and Vacuum Crude Distillation Units in Petroleum Refineries
131