Page 101 - Synthetic Fuels Handbook

88 CHAPTER THREE

3.4 HEAVY OIL REFINING

The demand for petroleum and petroleum products has shown a sharp growth in recent
years (Speight, 2007) this could well be the last century for petroleum refining, as we
know it. The demand for transportation fuels and fuel oil is forecast to continue to show a
steady growth in the future. The simplest means to cover the demand growth in low-boiling
products is to increase the imports of light crude oils and low-boiling petroleum products,
but these steps may be limited in the future.
Over the past three decades, crude oils available to refineries have generally decreased in
API gravity. There is, nevertheless, a major focus in refineries on the ways in which heavy
feedstocks might be converted into low-boiling high-value products (Khan and Patmore,
1997). Simultaneously, the changing crude oil properties are reflected in changes such as
an increase in asphaltene constituents, an increase in sulfur, metal, and nitrogen contents.
Pretreatment processes for removing such constituents or at least negating their effect in
thermal process they would also play an important role.
Difficult-to-refine feedstocks, such as heavy oil and residua, are characterized by low
API gravity (high density) and high viscosity, high initial boiling point, high carbon resi-
due, high nitrogen content, high sulfur content, and high metals content. In addition, to
these properties, the heavy feedstocks also have an increased molecular weight and reduced
hydrogen content (Speight, 2007).
The limitations of processing these heavy feedstocks depend to a large extent on the ten-
dency for coke formation and the deposition of metals and coke on the catalyst. However,
the essential step required of refineries is the upgrading of heavy feedstocks, particularly
residua (McKetta, 1992; Dickenson et al., 1997). In fact, the increasing supply of heavy
crude oils is a matter of serious concern for the petroleum industry. In order to satisfy
the changing pattern of product demand, significant investments in refining conversion
processes will be necessary to profitably utilize these heavy crude oils. The most efficient
and economic solution to this problem will depend to a large extent on individual country
and company situations. However, the most promising technologies will likely involve the
conversion of vacuum bottom residual oils, asphalt from deasphalting processes, and super-
heavy crude oils into useful low-boiling and middle distillate products.
Upgrading heavy oil upgrading and residua began with the introduction of desulfuriza-
tion processes (Speight, 1984, 2000). In the early days, the goal was desulfurization but, in
later years, the processes were adapted to a 10 to 30 percent partial conversion operation,
as intended to achieve desulfurization and obtain low-boiling fractions simultaneously, by
increasing severity in operating conditions. Refinery evolution has seen the introduction of
a variety of residuum cracking processes based on thermal cracking, catalytic cracking, and
hydroconversion. Those processes are different from one another in cracking method, cracked
product patterns, and product properties, and will be employed in refineries according to their
respective features. Thus, refining heavy feedstocks has become a major issue in modern
refinery practice and several process configurations have evolved to accommodate the heavy
feedstocks (RAROP, 1991; Shih and Oballa, 1991; Khan and Patmore, 1997).
Technologies for upgrading heavy feedstocks can be broadly divided into carbon rejection
and hydrogen addition processes. Carbon rejection redistributes hydrogen among the various
components, resulting in fractions with increased H/C atomic ratios and fractions with lower
H/C atomic ratios. On the other hand, hydrogen addition processes involve reaction of heavy crude
oils with an external source of hydrogen and result in an overall increase in H/C ratio. Within
these broad ranges, all more common upgrading technologies can be subdivided as follows:
1. Carbon rejection: Delayed coking, fluid coking, flexicoking, residuum fluid catalytic
cracking, and heavy oil cracking.
2. Hydrogen addition: LC-Fining, H-Oil.