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INTRODUCTION TO PETROLEUM REFINERY
OPERATIONS
INTRODUCTION
A petroleum refinery is a manufacturing operation where crude petroleum, the
raw material, is converted into usable finished products. In other words, it is the
manufacturing phase of the oil industry. This chapter presents a general
introduction to overall refinery operations as a forerunner to the detailed
information on specific processes and products which follows, and the
technologies that are applied to pressure relief operations. Other chapters cover
the major operations and processes used in refining, and discuss the critical
properties and end uses of the products. However, it should be emphasized that
a refinery is only one of the major phases of the petroleum industry; others being
exploration, production, transportation, and marketing, and a variety of feedstock
chemicals that supply the raw materials for various product lines. Research and
engineering might also be listed, but they are, in reality, a necessary and integral
part of each of the phases.
REFINERY OPERATIONS
The function of the refinery is to convert crude oil into the finished products
required by the market in the most efficient, and hence most profitable manner.
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Pressure Safety Design Practices
The methods employed necessarily vary widely from one refinery to another,
depending on the crude processed, the nature and location of the market, the type
of equipment available, and many other factors. However, for simplification, it
may be considered that all refining processes fall into one of four basic
categories.
The first category is fractionation or distillation. This method of physically
separating a mixture of compounds was the earliest process used in petroleum
refining, and today is still one of the most important. However, since it is not
generally possible to separate the complex petroleum mixtures into individual
compounds, such mixtures are segregated into fractions or "cuts", each of which
is characterized by a carefully controlled boiling range. These cuts are then
further processed or utilized in the refinery operations.
The second basic type of process, essentially chemical in nature, consists of
converting or chemically transforming certain of these "cuts" into products of
higher commercial value. There are many ways of doing this, but all consist
fundamentally of altering the molecular structure of the components. In the case
of a heavy oil, the molecules may be cracked to form lighter, more valuable
products, as for instance in catalytic cracking and coking. On the other hand,
gaseous products may be polymerized or otherwise combined to form liquid
products which may be blended into gasoline. With certain processes, e.g.
catalytic reforming, both cracking and polymerization take place concurrently
with the more desirable de-hydrogenation, hydrogenation, and isomerization
reactions. The net result of all these transformations is the production of mixtures
containing new arrays of hydrocarbons of higher value than the starting
materials.
Nearly all the fractions produced by the processes mentioned above contain
certain objectionable constituents or impurities. The third basic category is,
therefore, treating. This group of processes includes the removal of the
unwanted components, or their conversion to innocuous or less undesirable
compounds. Removal of the impurities is sometimes accomplished by physical
treating, as exemplified by the process for manufacturing kerosene, wherein
sulfur and certain undesirable hydrocarbons are removed by extraction with
liquid sulfur dioxide. Alternatively, the removal may be carried out by converting
the unwanted compounds to a form more readily removed as is done in the
hydrodesulfurization of diesel fuel. Here the sulfur compounds are cracked and
hydrogenated. The sulfur is converted to hydrogen sulfide which can be readily
separated from the heavier diesel oil by fractionation. An example of the
conversion of undesirable components to innocuous compounds which remain in
the product is found in the gasoline sweetening processes. There the mercaptans
present give the product a foul, objectionable odor. The sweetening process
Introduction to Petroleum Refinery Operations
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merely transforms the mercaptans to organic disulfides which are less
objectionable.
Although sulfur is perhaps the commonest and most troublesome of the
impurities found in petroleum, it is certainly not the only one. Substances such
as nickel, vanadium, and nitrogen may also be present in the crude oil. These
impurities are undesirable because of the difficulty they cause during processing
in the refinery or because of some detrimental effect during consumer use of the
product. Furthermore, presence of certain hydrocarbons or certain types of
hydrocarbons may lower the quality of a specific product. It was mentioned that
aromatics are removed from kerosene by SO, extraction. The aromatics have
undesirable burning characteristics and hence the product quality is improved if
these "impurities" are removed. Lube oil treating process such as dewaxing,
deasphalting, and phenol treating also fall into this category.
The fourth basic category is blending of the finished cuts into commercially
saleable products such as motor gasoline, kerosene, lubricating oils, and bunker
fuel oil, according to their specifications.
These four basic categories encompass the fundamental operation of a
refinery. All other activities are carried out to implement them. The
specifications for a given product are established to insure a satisfactory level of
product performance. Specifications can be altered from time to time, but a
product normally must meet the then existing product specifications. Various
crudes on the other hand yield fractions with significantly different properties.
At first glance, it might appear reasonable to select crudes to best match the
product needs of each refinery. Many times, however, this is not economical as
the money saved in eliminating various conversion and treating processes is offset
by other factors. These might include crude availability, price, and transportation
or specialty product requirements. A refinery is a sophisticated multi-component
process operated in overall balance. The balance is set by economic
considerations with the major variables being crude oil, process costs, and final
products. It is thus easier to see why (1) no two refineries are exactly alike, (2)
various conversion and purification processes are required, and (3) crude
selection is important.
TYPES OF REFINERIES
Each refinery is designed to manufacture products as economically as possible
based on the best knowledge available with regard to end product needs, future
expansion plans, crude availability and other pertinent factors.
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Pressure Safety Design Practices
A basic modern refinery which does not produce lubricating oils or
chemicals is commonly referred to as a fuel products refinery. It is designed to
produce primarily motor gasoline, distillate fuels (diesel oil, jet fuel, and heating
oil), and bunker (residual) fuel oil. The fuel products refineries can be considered
basic and minimum as regards refinery product and processing requirements.
Hydroskimming and conversion are the two major variations of this type
refinery. There is a wide range of conversion levels. The term maximum
conversion type has no precise definition but is often used to describe a level of
conversion , where there is no net fuel oil manufacture. A fuel products refinery
with specialities may manufacture lubricating oils, asphalts, greases, solvents,
waxes and chemical feed stocks in addition to the primary fuel products. The
number and diversity of products will naturally vary from one refinery to
another.
Refineries produce chemical feed stocks for sale to the chemical affiliates
and do not have responsibility for the manufacture of chemical products directly.
Both operations may be carried out at the same physical location but the
corporate product responsibilities are usually separate.
FUEL PRODUCTS REFINERY
Hydroskimmer
A hydroskimmingrefinery lends itself to locations where the market demands for
the major fuel products (gasoline, gas oil, and residual fuel oil) approximate the
quantities of these products obtainable by distillation from the available crudes.
A typical hydroskimming refinery would include the following:
1. Atmospheric Pipestill
2. Powerforming (Catalytic Naphtha Reforming)
3. Light Ends Recovery - Fractionation
4.Treating and Blending
Figure 1 shows a simplified flow plan for a typical hydroskimming refinery.
The atmospheric pipestill performs the initial distillation of crude oil into gas,
naphtha, distillates, and residuum. The naphtha may be separated into gasoline
blending stock, solvents, and Powerformer feed. The distillates include kerosene,
jet fuel, heating oil and diesel oil. The residuum is blended for use as bunker fuel
oil.
The Powerforming unit is required to upgrade virgin naphtha to produce
high octane gasoline. Powerforming is a fixed bed catalytic reforming process
employing a regenerable platinum catalyst. In the process, a series of reactions
Introduction to Petroleum Refmery Operations
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takes place. The most important of these is aromatization; other reactions include
isomerization, cracking, hydrogenation, and polymerization. The desired product
is of approximately the same boiling range as the feed, but the molecules have
been rearranged or reformed into higher octane compounds.
Light ends recovery and fractionating equipment is necessary after the
Powerformer and on the pipestill overhead stream to separate the effluent
mixtures into the desired boiling range cuts.
Hydrofining is used to reduce sulfur and/or other impurities and to improve
odor, color, and stability of the pipestill fractions. Hydrofining is a fixed-bed
catalytic process using a regenerable cobalt molybdate catalyst in a hydrogen
atmosphere. The hydrogen is produced by the Powerformer with supplemental
hydrogen manufactured if necessary. The difficulty of hydrofining
(desulfurization) increases with increase in the hydrocarbon boiling point.
Naphthas are generally desulfurized up to 99+ % by hydrofining while the
maximum desulfurization of distillates is usually 90 % .
The components produced by the process sequence outlined above are
blended as required to meet final product rates and qualities.
Conversion
The hydroskimming type refinery is used where the gasoline demand is
substantially lower and hence the final product demand is close to that yielded by
single stage distillation. In areas where the demand for gasoline is relatively high,
conversion processing is required. The minimum processes for a fuel products
refinery designed would typically include:
1. Atmospheric and Vacuum Crude Distillation
2. Catalytic Gas Oil Cracking
3. Powerforming
4. Light Ends Recovery - Fractionation
5. Treating and Blending
Figure 2 shows a simplified flow plan for a typical conversion type refinery.
The atmospheric P/S residuum can be fed to a vacuum pipestill. The vacuum
tower enables the refiner to cut deeper into the crude, at the same time avoiding
high temperatures (above about 750 O F ) which cause thermal cracking with
resultant deposition of coke and tarry residues in the equipment.
The vacuum gas oil produced by vacuum distillation is fed to a catalytic
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crackmg unit for conversion into high octane gasoline blending stock. Byproducts
are gas, distillate, cycle gas oil, and fractionator bottoms. The process uses a
fluidized catalyst system. The catalyst is circulated continuously between the
reactor where cracking takes place and the regenerator where the coke deposited
on the catalyst is burned off. The major competing process is hydrocracking
which offers greater conversion and flexibility but usually requires a higher
investment.
Hydrocracking is a fixed bed catalytic process which cracks and
hydrogenates hydrocarbon feeds. The process consumes large quantities of
hydrogen and a hydrogen plant is usually necessary to support the operation.
Practically any stock can be hydrocracked, including refractory feeds which
resist conversion by other processes. In general, the very heavy residuum from
the vacuum pipestill does not make good quality feed for catalytic cracking. In
the refinery shown it is blended into residual fuel oil. Many times, however, the
market for large volumes of residual fuel oil does not exist. When this is the
case, additional conversion units are added to further process the vacuum pipestill
bottoms. In other words, the higher the conversion of the refinery the more
lighter fractions are produced. The relative levels of conversion vary from
refinery to refinery.
A typical maximum conversion type refinery is shown in Figure 3. The
higher conversion levels are obtained by ad&tional processing of the bottoms
and/or light ends. To increase conversion of the bottoms the amount and/or
severity of processing is increased. The resulting fuel oil levels may decrease to
zero. Included here in addition to the basic components of a conversion refinery
may be fluid coking, delayed coking, and/or visbreaking. These processes are
basically thermal cracking processes for reducing the volume and viscosity of the
vacuum residuum while producing appreciable quantities of lighter products.
Each of the three processes is commercially used with selection based on
particular needs at a given refinery. Some of the various characteristics include:
1. Coking-Delayed Coking and Fluid Coking are the two major variations
of this process. Fluid coking produces less coke as compared with delayed coking
and hence yields a better product distribution. That is, for a given product slate
less crude is converted into coke. The coke produced by fluid coking, however,
is of little value as it consists of fine hard particles in contrast to large pieces for
delayed coke. This difference in size and texture is important to electrode
manufacturers who historically have used delayed coke.
2. Visbreaking is the least expensive of the cracking processes but is limited
to the lowest conversion of perhaps 20 to 25% of the feed to 680 "F material.
Introduction to Petroleum Refmsry Operations
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To obtain light ends conversion, alkylation and polymerization are used to
increase the relative amounts of liquid fuel products manufactured. Alkylation
converts olefins, (propylene, butylenes, amylenes, etc.), into high octane gasoline
by reacting them with isobutane. Polymerization involves reaction of propylene
and/or butylenes to produce an unsaturated hydrocarbon mixture in the motor
gasoline boiling range.
An old variation of the conversion type is a catalytic combination unit.
Development of this scheme was necessitated by the rising cost of refinery
construction after World War I1 and by the great demand for capital for postwar
expansion. The scheme reduced the investment and operating costs for refining
equipment. The basic feature of the combination unit lies in the integration of the
fractionation facilities of the reduced crude distillation and catalytic cracking
sections.
A FUEL PRODUCTS REFINJ3RY WITH SPECIALTIES
A fuel products refinery with specialties may manufacture products such as
lubricating oils, asphalts, greases, solvents, waxes and chemical feed stocks in
addition to the primary fuel products. The number and diversity of products will
naturally vary from one refinery to another, but for purposes of discussion a fuel
products refinery with specialties may include many of the following processes.
1. Two-Stage Crude Distillation (Atmospheric and Vacuum) - The vacuum
stage can be used alternately to produce heavy gas oil for catalytic cracking feed
or raw lube distillate cuts for lubricating oil manufacture.
2. Virgin Naphtha Catalytic Reforming (Powerforming) - This technique is
used for the production of high octane motor gasoline, or as a source of aromatic
compounds.
3. Light Ends Recovery, Fractionation, and Conversion - Propylenes and
butylenes may be recovered for feed to a polymerization plant for production of
high octane gasoline; or chemicals. Butylenes and isobutane may be desired for
use in an alkylation plant where they are combined to make aviation gasoline and
motor gasoline blendstocks. Propanes and butanes may be recovered in
essentially pure form for sale as liquefied petroleum gases. It may be profitable
to recover ethylene for chemical production. Certain of the light ends
components, particularly ethylene, propylene, and butadiene are so in demand
that processes such as steam cracking are employed specifically for their
Introduction to Petroleum Refinery Operations
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production.
4. Fuel Products Treating- a. Sweetening - This is a process for improving
odor of gasolines, kerosenes, and heating oils. The foul smelling mercaptans are
converted into bisulfides whose odor is much less objectionable. Among the types
in use are copper chloride, hypochlorite, Merox, Mercapfining, and air inhibitor
sweetening.
b. Hydroprocessing - The nomenclature system with regard to hydrogen
processing is quite confusing with an array of labels involving trade names, terms
such as mild, medium, and severe, high and low pressure. Choice of terminology
varies widely from company to company.
A wide variety of petroleum fractions may be treated at elevated temperature
and pressure with hydrogen in the presence of a catalyst to reduce sulfur,
improve stability, odor, combustion characteristics, appearance, and to convert
heavy fractions to lighter more valuable products. The most severe form of
hydroprocessing as discussed previously is hydrocracking. For fuel products
treating, however, two less severe hydroprocessing operations are used,
hydrofining and hydrotreating.
Hydrofining usually involves only minor molecular changes of the feed with
hydrogen consumption in the range of about 100 to 1,0oO cu.ft./bbl. Typical
applications include desulfurization of a wide range of feeds (naphtha, light and
heavy distillates, and certain residua) and occasional pretreatment of cat cracker
feeds.
Hydrotreating essentially involves no reduction in molecular size with
hydrogen consumption less than about 100 cu. ft./bbl. Primary application is to
remove small amounts of impurities with typical uses including naphtha and
kerosene hydrosweetening.
c. SO, Extraction - This is a method of solvent extraction with liquid SO, to
remove aromatic hydrocarbons and cyclic sulfur compounds. It is used to
improve the burning qualities of kerosene and diesel fuels, and to reduce sulfur.
This process has practically been supplanted by other solvent extraction or by
hydrotreating.
5. Fluid catalytic Cracking.
6. Hydrocracking.
7. Residuum Conversion - Included here may be fluid coking, delayed
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Pressure Safety Design Practices
coking, visbreaking, and residuum hydroprocessing.
8 . Solvent Deasphalting - This is the solvent extraction of virgin residuum
to remove asphaltenes or other tarry constituents. The deasphalted oil may be
further processed into lubricating oils and greases, or used as cat cracking feed.
9. Lubricating Oil Manufacture - This will usually consist of the following
processes:
&. Solvent Deasphalting.
b. Phenol Treating - An extraction process for removal of aromatic asphaltic
and sulfur compounds from the lube cut.
c. Solvent Dewaxing - Waxy lube is diluted with a solvent such as propane
or methyl ethyl ketone (MEK), and cooled to crystallize the wax which is then
removed by filtration.
10. Grease Manufacture - Selected lube oil fractions 2::: blended with
various metallic soaps to produce high viscosity lubricating greases.
11. Wax Manufacture - A waxy distillate cut frox i(ude or the wax
byproduct from lube oil dewaxing is first deoiled. Resulting low oil content wax
is hydrofined for color improvement and fractionated into appropriate melting
point grades.
12. Asphalt Manufacture - Saleable asphalts are produced from the residua
of selected crudes. The residuum itself may be sold as straight reduced cuts to
make it easier to handle, producing the so called cut-back asphalts. Another
variation is air blown or oxidized asphalts for improved tenacity, greater
resistance to weathering, and decreased brittleness. Emulsified asphalts are made
for application at relatively low temperatures.
13. Chemical and Other Specialty Manufacture - A wide variety of products
may be derived from petroleum feed stocks, including such diverse materials as
alcohols, butyl rubber, sulfur, additives, and resins. Other specialties such as
solvent naphthas, white oils, Isopars, Varsol, may also be produced. As indicated
previously the respective chemical affiliate usually has responsibility for products
broadly classified as petrochemicals.
There are many other processes used in refineries not mentioned here. The
list above is intended only to emphasize the wide diversity of processing which
is common to petroleum refining and to introduce in a very general way some of
the more important of these processes. Also it must be emphasized that only
fundamental principles of refinery operations have been discussed and modem
manufacturing techniques vary widely from company to company.