Petroleum Refining Overview
In this lecture, we present a brief overview of the petroleum refining, a prominent process technology in process engineering.
3.1 Crude oil
Crude oil is a multicomponent mixture consisting of more than 108 compounds. Petroleum refining refers to the separation as well as reactive processes to yield various valuable products. Therefore, a key issue in the petroleum refining is to deal with multicomponent feed streams and multicomponent product streams. Usually, in chemical plants, we encounter streams not possessing more than 10 components, which is not the case in petroleum refining. Therefore, characterization of both crude, intermediate product and final product streams is very important to understand the processing operations effectively.
3.2 Overview of Refinery processes
Primary crude oil cuts in a typical refinery include gases, light/heavy naphtha, kerosene, light gas oil, heavy gas oil and residue. From these intermediate refinery product streams several final product streams such as fuel gas, liquefied petroleum gas (LPG), gasoline, jet fuel, kerosene, auto diesel, lubricants, bunker oil, asphalt and coke are obtained. The entire refinery technology involves careful manipulation of various feed properties using both chemical and physical changes.
Conceptually, a process refinery can be viewed upon as a combination of both physical and chemical processes or unit operations and unit processes respectively. Typically, the dominant physical process in a refinery is the distillation process that enables the removal of lighter components from the heavier components. Other chemical processes such as alkylation and isomerisation are equally important in the refinery engineering as these processes enable the reactive transformation of various functional groups to desired functional groups in the product streams.
3.3 Feed and Product characterization
The characterization of petroleum process streams is approached from both chemistry and physical properties perspective. The chemistry perspective indicates to characterize the crude oil in terms of the functional groups such as olefins, paraffins, naphthenes, aromatics and resins. The dominance of one or more of the functional groups in various petroleum processing streams is indicative of the desired product quality and characterization. For instance, the lighter fractions of the refinery consist of only olefins and paraffins. On the other hand, products such as petrol should have high octane number which is a characteristic feature of olefinic and aromatic functional groups present in the product stream.
The physical characterization of the crude oil in terms of viscosity, density, boiling point curves is equally important. These properties are also indicative of the quality of the product as well as the feed. Therefore, in petroleum processing, obtaining any intermediate or a product stream with a defined characterization of several properties indicates whether it is diesel or petrol or any other product. This is the most important characteristic feature of petroleum processing sector in contrary to the chemical process sector.
The product characterization is illustrated now with an example.Aviation gasoline is characterized using ASTM distillation.The specified temperatures for vol% distilled at 1 atm.Are 158oF maximum for 10 % volume, 221oF maximum for 50 % volume and 275oF maximum for 90% volume. This is indicative of the fact that any product obtained in the refinery process and meets these ASTM distillation characteristics is anticipated to represent Aviation gasoline product.However, other important properties such as viscosity, density, aniline product, sulphur density are as well measured to fit within a specified range and to conclude that the produced stream is indeed aviation gasoline.
3.4 Important characterization properties
Numerous important feed and product characterization properties in refinery engineering include
3.4.1 API gravity
API gravity of petroleum fractions is a measure of density of the stream. Usually measured at 60oF, the API gravity is expressed as
o API = [ 141.5/specific gravity ] – 131.5
where specific gravity is measured at 60o F.
According to the above expression, 10oAPI gravity indicates a specific gravity of 1 (equivalent to water specific gravity). In other words, higher values of API gravity indicate lower specific gravity and therefore lighter crude oils or refinery products and vice-versa. As far as crude oil is concerned, lighter API gravity value is desired as more amount of gas fraction, naphtha and gas oils can be produced from the lighter crude oil than with the heavier crude oil. Therefore, crude oil with high values of API gravity are expensive to procure due to their quality.
3.4.2 Watson characterization factor
The Watson characterization factor is usually expressed as
Where TB is the average boiling point in degrees R taken from five temperatures corresponding to 10, 30, 50,70 and 90 volume % vaporized.
Typically Watson characterization factor varies between 10.5 and 13 for various crude streams. A highly paraffinic crude typically possesses a K factor of 13. On the other hand, a highly naphthenic crude possesses a K factor of 10.5. Therefore, Watson characterization factor can be used to judge upon the quality of the crude oil in terms of the dominance of the paraffinic or naphthenic components.
3.4.3 Sulfur content
Since crude oil is obtained from petroleum reservoirs, sulphur is present in the crude oil. Usually, crude oil has both organic and inorganic sulphur in which the inorganic sulphur dominates the composition. Typically, crude oils with high sulphur content are termed as sour crude. On the other hand, crude oils with low sulphur content are termed as sweet crude. Typically, crude oil sulphur content consists of 0.5 – 5 wt % of sulphur. Crudes with sulphur content lower than 0.5 wt % are termed as sweet crudes. It is estimated that about 80 % of world crude oil reserves are sour.
The sulphur content in the crude oil is responsible for numerous hydrotreating operations in the refinery process. Strict and tighter legislations enforce the production of various consumer petroleum products with low quantities of sulphur (in the range of ppm). Presently, India is heading towards the generation of diesel with Euro III standards that indicates that the maximum sulphur content is about 500 ppm in the product. This indicates that large quantities of inorganic sulphur needs to be removed from the fuel. Typically, inorganic sulphur from various intermediate product streams is removed using hydrogen as hydrogen sulphide.
A typical refinery consists of good number of hydrotreaters to achieve the desired separation. The hydrotreaters in good number are required due to the fact that the processing conditions for various refinery intermediate process streams are significantly different and these streams cannot be blended together as well due to their diverse properties which were achieved using the crude distillation unit. More details with respect to the hydrotreating units will be presented in the future lectures.
3.4.4 TBP/ASTM distillation curves
The most important characterization properties of the crude/intermediate/product streams are the TBP/ASTM distillation curves. Both these distillation curves are measured at 1 atm pressure. In both these cases, the boiling points of various volume fractions are being measured. However, the basic difference between TBP curve and ASTM distillation curve is that while TBP curve is measured using batch distillation apparatus consisting of no less than 100 trays and very high reflux ratio, the ASTM distillation is measured in a single stage apparatus without any reflux. Therefore, the ASTM does not indicate a good separation of various components and indicates the operation of the laboratory setup far away from the equilibrium.
Viscosity is a measure of the flow properties of the refinery stream. Typically in the refining industry, viscosity is measured in terms of centistokes (termed as cst) or saybolt seconds or redwood seconds. Usually, the viscosity measurements are carried out at 100oF and 210oF. Viscosity is a very important property for the heavy products obtained from the crude oil. The viscosity acts as an important characterization property in the blending units associated to heavy products such as bunker fuel. Typically, viscosity of these products is specified to be within a specified range and this is achieved by adjusting the viscosities of the streams entering the blending unit.
3.4.6 Flash and fire point
Flash and fire point are important properties that are relevant to the safety and transmission of refinery products. Flash point is the temperature above which the product flashes forming a mixture capable of inducing ignition with air. Fire point is the temperature well above the flash point where the product could catch fire. These two important properties are always taken care in the day to day operation of a refinery.
3.4.7 Pour point
When a petroleum product is cooled, first a cloudy appearance of the product occurs at a certain temperature. This temperature is termed as the cloud point. Upon further cooling, the product will ceases to flow at a temperature. This temperature is termed as the pour point. Both pour and cloud points are important properties of the product streams as far as heavier products are concerned. For heavier products, they are specified in a desired range and this is achieved by blending appropriate amounts of lighter intermediate products.
3.4.8 Octane number
Though irrelevant to the crude oil stream, the octane number is an important property for many intermediate streams that undergo blending later on to produce automotive gasoline, diesel etc.Typically gasoline tends to knock the engines. The knocking tendency of the gasoline is defined in terms of the maximum compression ratio of the engine at which the knock occurs. Therefore, high quality gasoline will tend to knock at higher compression ratios and vice versa. However, for comparative purpose, still one needs to have a pure component whose compression ratio is known for knocking. Iso-octane is eventually considered as the barometer for octane number comparison. While iso-octane was given an octane number of 100, n-heptane is given a scale of 0. Therefore, the octane number of a fuel is equivalent to a mixture of a iso-octane and n-heptane that provides the same compression ratio in a fuel engine. Thus an octane number of 80 indicates that the fuel is equivalent to the performance characteristics in a fuel engine fed with 80 vol % of isooctane and 20 % of n-heptane.
Octane numbers are very relevant in the reforming, isomerisation and alkylation processes of the refining industry. These processes enable the successful reactive transformations to yield long side chain paraffins and aromatics that possess higher octane numbers than the feed constituents which do not consist of higher quantities of constituents possessing straight chain paraffins and non-aromatics (naphthenes).
3.5 Crude chemistry
Fundamentally, crude oil consists of 84 – 87 wt % carbon, 11 – 14 % hydrogen, 0 – 3 wt % sulphur, 0 – 2 wt % oxygen, 0 – 0.6 wt % nitrogen and metals ranging from 0 – 100 ppm. Understanding thoroughly the fundamentals of crude chemistry is very important in various refining processes. The existence of compounds with various functional groups and their dominance or reduction in various refinery products is what is essentially targeted in various chemical and physical processes in the refinery.
Based on chemical analysis and existence of various functional groups, refinery crude can be broadly categorized into about 9 categories summarized as
3.5.1 Paraffins: Paraffins refer to alkanes such as methane, ethane, propane, n and iso butane, n and iso pentane. These compounds are primarily obtained as a gas fraction from the crude distillation unit.
3.5.2 Olefins: Alkenes such as ethylene, propylene and butylenes are highly chemically reactive. They are not found in mentionable quantities in crude oil but are encountered in some refinery processes such as alkylation.
3.5.3 Naphthenes: Naphthenes or cycloalkanes such as cyclopropane, methyl cyclohexane are also present in the crude oil. These compounds are not aromatic and hence do not contribute much to the octane number. Therefore, in the reforming reaction, these compounds are targeted to generate aromatics which have higher octane numbers than the naphthenes.
Cyclopropane(C3H6) Cyclobutane(C4H8) Cyclopentane(C5H10)
Cyclohexane(C6H12) Methyl Cyclohexane(C7H14)
3.5.4 Aromatics: Aromatics such as benzene, toluene o/m/p-xylene are also available in the crude oil. These contribute towards higher octane number products and the target is to maximize their quantity in a refinery process.
Benzene(C6H6) Tolune(C7H8) Para-X ylene(C8H10)
Ortho-Xylene(C8H10) Meta-X ylene(C8H10)
3.5.5 Napthalenes: Polynuclear aromatics such as naphthalenes consist of two or three or more aromatic rings. Their molecular weight is usually between 150 – 500.
Organic sulphur compounds: Not all compounds in the crude are hydrocarbons consisting of hydrogen and carbon only. Organic sulphur compounds such as thiophene, pyridine also exist in the crude oil. The basic difficulty of these organic sulphur compounds is the additional hydrogen requirements in the hydrotreaters to meet the euro III standards. Therefore, the operating conditions of the hydrotreaters is significantly intense when compared to those that do not target the reduction in the concentration of these organic sulphur compounds. Therefore, ever growing environmental legislations indicate technology and process development/improvement on the processing of organic sulphur compounds.
3.5.6 Oxygen containing compounds: These compounds do not exist 2 % by weight in the crude oil. Typical examples are acetic and benzoic acids. These compounds cause corrosion and therefore needs to be effectively handled.
3.5.7 Resins: Resins are polynuclear aromatic structures supported with side chains of paraffins and small ring aromatics. Their molecular weights vary between 500 – 1500. These compounds also contain sulphur, nitrogen, oxygen, vanadium and nickel.
3.5.8 Asphaltenes: Asphaltenes are polynuclear aromatic structures consisting of 20 or more aromatic rings along with paraffinic and naphthenic chains. A crude with high quantities of resins and asphaltenes (heavy crude) is usually targeted for coke production.
3.6 Technical Questions:
1. Explain how crude quality affects the topology of refinery configuration?
Ans: This is a very important question. Usually, refinery crudes are characterized as light, moderate and heavy crudes. Light and moderate crudes are typically targeted for gas, naphtha, diesel, light and heavy gas oil fractions. Heavy crudes are targeted for coke and residue product streams. Therefore, the crude quality does affect the topology of the refinery configuration. According to the choice of the crude available, refineries are classified into four types namely
Those that target fuels. This is very prominent in a country like India
Those that target coke. This is very much targeted for refineries that supply coke as an important raw materials to other industries such as steel, catalysts etc.
According to the desired product pallete, the refinery configuration and hence topology is affected with the crude quality.
2. What is the basic difference between a chemical and a refinery process?
Ans: A chemical process essentially involves streams whose composition is fairly known. As far as refinery processes are concerned, their chemical constituents are not exactly known but are estimated as functions of various measurable properties such as viscosity, cetane number, octane number, flash point, TBP/ASTM distillation etc.
Therefore, refinery process technology should accommodate the details pertaining to these issues in addition to the technology issues.
3. Why are refinery process flow sheets very complex?
Ans: Well, large refineries involve the production of about 30 to 40 refinery products with diverse specifications and needs of the consumers. And necessarily all these products are derived from the crude oil in a complex way. Due to complicated physical and chemical processes that are sequentially applied for various refinery process streams, refinery process flow sheets are very complex.
4. How to analyze refinery process flow sheets in a simple way?
1. Functional role of each process/operation
2. Plausible changes in property characteristics such as octane number or viscosity etc.
This way refinery technology will be easy to understand with maturity.
5. Relate the important crude oil cuts and associated products?
i. Fuel gas
ii. Jet fuel
i. Jet fuel
(d) Light gas oil
i. Auto Diesel
ii. Tractor Diesel
iii. Home heating oil
(e) Heavy gas oil
i. Commercial heating oil
ii. Industrial heating oil
i. Bunker oil