539
20
Solvent Processes
20.1 INTRODUCTION
Solvent deasphalting (SDA) processes are a major part of refinery operations and are not often
appreciated for the tasks for which they are used (Bland and Davidson, 1967; Hobson and Pohl,
1973; Speight and Ozum, 2002; Hsu and Robinson, 2006; Gary et al., 2007; Speight, 2011). In the
solvent deasphalting processes, an alkane is injected into the feedstock to disrupt the dispersion of
components and causes the polar constituents to precipitate. Propane (or sometimes propane/butane
mixtures) is extensively used for deasphalting and produces a deasphalted oil (DAO) and propane
deasphalter asphalt (PDA or PD tar) (Dunning and Moore, 1957). Propane has unique solvent properties; at lower temperatures (38°C–60°C; 100°F–140°F), paraffins are very soluble in propane, and
at higher temperatures (about 93°C; 200°F), all hydrocarbons are almost insoluble in propane.
A solvent deasphalting unit processes the residuum from the vacuum distillation unit and produces deasphalted oil (DAO), used as feedstock for a fluid catalytic cracking unit, and the asphaltic
residue (deasphalter tar and deasphalter bottoms) which, as a residual fraction, can only be used to
produce asphalt or as a blend stock or visbreaker feedstock for low-grade fuel oil. SDA processes
have not realized their maximum potential. With on-going improvements in energy efficiency, such
processes would display its effects in combination with other processes. SDA allows removal of
sulfur and nitrogen compounds as well as metallic constituents by balancing yield with the desired
feedstock properties (Dunning and Moore, 1957; Ditman, 1973).
Most of the metals present in refinery feedstocks are concentrated in the asphaltene constituent, and, by using SDA, it is possible to recover significant amounts of partially demetallized and
deasphalted oil (DAO) from the residues that can be processed in fluid catalytic cracking units
(Chapter 19) or in hydrocracking units (Chapter 22).
The main advantages of SDA are relatively low investment and operational costs, while its principal limitation is that to obtain DAO with a low content of pollutants (in particular, sulfur, nitrogen, metals, and coke-forming constituents), the yield must be limited (Ditman, 1973). As a result,
significant quantities of by-products are generated (asphaltene constituents plus resin constituents)
that can be used as components of low-quality fuels or as sources of carbon for the production of
synthesis gas and, therefore, hydrogen in suitable gasification units (Chapter 23).
The application of the SDA process to heavy crude oil and tar sand bitumen upgrading has led to
the development of a variety of technologies in which the typical SDA unit is combined with other
processes (Chapter 15).
20.2 COMMERCIAL PROCESSES
Petroleum processing normally involves separation into various fractions that require further
processing in order to produce marketable products. The initial separation process is distillation
(Chapter 17) in which crude oil is separated into fractions of increasingly higher boiling range
fractions. Since petroleum fractions are subject to thermal degradation, there is a limit to the
temperatures that can be used in simple separation processes. The crude cannot be subjected to temperatures much above 395°C (740°F), irrespective of the residence time, without encountering some
thermal cracking. Therefore, to separate the higher molecular weight and higher boiling fractions
from crude, special processing steps must be used.
20
Solvent Processes
20.1 INTRODUCTION
Solvent deasphalting (SDA) processes are a major part of refinery operations and are not often
appreciated for the tasks for which they are used (Bland and Davidson, 1967; Hobson and Pohl,
1973; Speight and Ozum, 2002; Hsu and Robinson, 2006; Gary et al., 2007; Speight, 2011). In the
solvent deasphalting processes, an alkane is injected into the feedstock to disrupt the dispersion of
components and causes the polar constituents to precipitate. Propane (or sometimes propane/butane
mixtures) is extensively used for deasphalting and produces a deasphalted oil (DAO) and propane
deasphalter asphalt (PDA or PD tar) (Dunning and Moore, 1957). Propane has unique solvent properties; at lower temperatures (38°C–60°C; 100°F–140°F), paraffins are very soluble in propane, and
at higher temperatures (about 93°C; 200°F), all hydrocarbons are almost insoluble in propane.
A solvent deasphalting unit processes the residuum from the vacuum distillation unit and produces deasphalted oil (DAO), used as feedstock for a fluid catalytic cracking unit, and the asphaltic
residue (deasphalter tar and deasphalter bottoms) which, as a residual fraction, can only be used to
produce asphalt or as a blend stock or visbreaker feedstock for low-grade fuel oil. SDA processes
have not realized their maximum potential. With on-going improvements in energy efficiency, such
processes would display its effects in combination with other processes. SDA allows removal of
sulfur and nitrogen compounds as well as metallic constituents by balancing yield with the desired
feedstock properties (Dunning and Moore, 1957; Ditman, 1973).
Most of the metals present in refinery feedstocks are concentrated in the asphaltene constituent, and, by using SDA, it is possible to recover significant amounts of partially demetallized and
deasphalted oil (DAO) from the residues that can be processed in fluid catalytic cracking units
(Chapter 19) or in hydrocracking units (Chapter 22).
The main advantages of SDA are relatively low investment and operational costs, while its principal limitation is that to obtain DAO with a low content of pollutants (in particular, sulfur, nitrogen, metals, and coke-forming constituents), the yield must be limited (Ditman, 1973). As a result,
significant quantities of by-products are generated (asphaltene constituents plus resin constituents)
that can be used as components of low-quality fuels or as sources of carbon for the production of
synthesis gas and, therefore, hydrogen in suitable gasification units (Chapter 23).
The application of the SDA process to heavy crude oil and tar sand bitumen upgrading has led to
the development of a variety of technologies in which the typical SDA unit is combined with other
processes (Chapter 15).
20.2 COMMERCIAL PROCESSES
Petroleum processing normally involves separation into various fractions that require further
processing in order to produce marketable products. The initial separation process is distillation
(Chapter 17) in which crude oil is separated into fractions of increasingly higher boiling range
fractions. Since petroleum fractions are subject to thermal degradation, there is a limit to the
temperatures that can be used in simple separation processes. The crude cannot be subjected to temperatures much above 395°C (740°F), irrespective of the residence time, without encountering some
thermal cracking. Therefore, to separate the higher molecular weight and higher boiling fractions
from crude, special processing steps must be used.