Petroleum Products and Their Uses
Basic refinery process
Each refinery is uniquely designed to process specific crude oils into selected products . In order to meet the business objectives of the refinery , the process designers selects from an array of basic processing units . In general , these units one of the three functions :
1) separating the many types of hydrocarbon present in crude oils into fractions of more closely related properties
2) chemically converting the separated hydrocarbons into more desirable reaction products , and
3) purifying the products of unwanted elements and compounds .
Process involving the conversion of a liquid into vapour that is subsequently condensed back to liquid form . It is exemplified at its simplest when from a kettle becomes deposited as drops of distilled water on a cold surface . Distillation is used to separate liquids from non-volatile solids , as in the separation of alcoholic liquors from fermented materials , or in the separation of two or more liquids having different boiling points , as in the separation of gasoline , kerosene , and lubricating oil from crude oil . Other industrial applications include the processing of such chemical products as formaldehyde and phenol and the desalination of seawater . The distillation process appears to have been utilized by the earliest experimentalists . Aristotle mentioned that pure water is made by the evaporation of seawater. Pliny the Elder described a primitive method of condensation in which the oil obtained by heating rosin is collected on wool placed in the upper part of an apparatus known as still.
Most methods of distillation used by industry and in laboratory research are variations of simple distillation . This basic operation requires the use of a still or retort in which a liquid is heated , a condenser to cool the vapour . and a receiver to collect the distillate . In heating a mixture of substances , the most volatile or the lowest boiling distils first , and the others substequently or not at all . This simple apparatus is entirely satisfactory for the purification of a liquid contaning non-volatile material and is reasonable adequate for separating liquids of widely divergent boiling points . For laboratory use , the apparatus is commonly made of glass and connected with corks , rubber bungs , or ground-glass joints . For industrial applications , larger equipment of metal or ceramic is employed.
The primary process for separating the hydrocarbon components of crude oil is fractional distillation . Crude oil distillers separate crude oil into fractions for subsequent processing in such units as catalytic reformers , cracking units , alkylation units , or cokers . In turn , each of these more complex processing units also incorporates a fractional distillation tower to separate its own reaction products.
Modern crude oil distillation units operate continiously over long periods of time and are much larger than the fractional distillation units employed in chemical or other industries . Process rates are commonly delineated in American barrels ; units capable of processing 100,000 barrels per day are commonplace , and the largest units are capable of charging more than 200,000 barrels per day
The principles of a modern crude oil distillation unit are shown in the figure . Crude oil is withdrawn from storage tanks at ambient temperatureand pumped at a constant rate though a series of heat exchangers in order to reach a temperature of about 120C . A controlled mount of fresh water is introduced , and the mixture is pumped into a desalting drum , where it passes through an electrical field and a saltwater phase is separated . The desalted crude oil passes through additional heat exchangers and then through steel alloy tubes in a furnace . There it is heated to a temperature between 315C and 400C , depending on the type of crude oil and cracking unit . At the end products desired . A mixture of vapour and unvaporized crude oil passes from the furnace into the fractionating column , a vertical cylindrical tower as much as 45 meters high containing 20to 40 trays are of the sieve or valve type . Sieve trays are simple perforated plates with small holes about 5 to 6 millimetres in diameter . Valve trays are similar , except the perforations are covered by small metal disks that restricted the flow through the perforations under certain process conditions .
The oil vapors rise up through the column and are condensed to a liquid in a water or air cooled condenser at the top of the tower . A small amount of gas remains uncondensed and is piped into refinery fuel-gas system . A pressure control value on the fuel gas line maintains fractionating column pressure at the desired figure , usually near atmospheric pressure . Part of the condensed liquid , called reflux , is pumped back into the top of the column and descends from tray to tray , contaning rising vapours as they pass through the slots in the trays . The liquid progressively absorbs heavier constituents from the vapour and ,in turn , gives up lighter constituents to the vapour phase . Condensation andrevaportion takes place on each tray . eventually an equilibrium is reached in which there is a continual gradation of temperature and oil properties throughout the column , with the lightest constituents on the top tray and heaviest on the bottom . The use of reflux and vapour –liquid contacting trays distinguishes fractional distillation from simple distillation columns.
As shown in the figure , intermediate products , or sidestreams , are withdrawn at several points from the column . In addition , modern crude distillation units employ intermediate reflux streams . Sidestreams are known as intermediate products because they have properties between those of the top or overhead product and those of products issuing from the base of the column . Typical boiling ranges for various streams are as follows : light straight-run naphtha ; heavy naphtha , 90-165C; crude kerosene .150-245C; light gas oil , 215-315C.
Unvaporized oil entering the column flows downward over a similar set of trays in the lower part of the column ,called stripping trays ,which act to remove any light constituents remaining in the liquid . Steam is injected into the bottom of the column in order to reduce the partial pressure of the hydrocarbons and assist in the separation . Typically a single sidestream is withdrawn from the stripping section : heavy gas oil , with a boiling range of 285-370C . The residue that passes from the bottom of the column is suitable for blending into industrial fuels . alternately , it may be further distilled under vacuum conditions to yield quantities of distilled oils for manufacture into lubricating oils or for use as feedstock in a gas oil cracking process.
The principles of vacuum distillation resemble those of fractional distillation ,except that larger diameter columns are used to maintain comparable vapour velocities at reduced operating pressures . A vacuum of 50 to 100 millimetres of mercury absolute is produced by a vacuum pump or stream ejector.
The primary advantage of vacuum distillation is that it allows for distilling heavier materials at lower temperatures than those that would be required at atmospheric pressure , thus avoiding thermal cracking of the components . Firing conditions in the furnace are adjusted so that oil temperatures usually do not exceed 425C . The residue remaining after vacuum distillation , called bitumen , may be further blended to produce road asphalt or residual fuel oil , or it may be used as a feedstock for thermal cracking or coking units . Vacuum distillation units are essential parts of the many processing schemes designed to produced lubricants .
An extension of the distillation process , superfraction employs smaller-diameter columns with a much larger number of trays and reflux ratios exceeding 5: 1 With such equipment it is possible to isolate a very narrow range of components or even pure compounds . Common applications involve the separation of highpuritysolvents such as isoparaffins or of individual aromatic compounds for use as petrochemicals.
Gaseous refinery products include hydrogen , fuel gas , ethane , and propane or lpg . Most of the hydrogen is consumed in refinery desulfurization facilities; small quantities may be delivered to the refinery fuel system . Refinery fuel gas usually has a heating value similar to natural gas and is consumed in plant operations . Periodic variability in heating value makes it unsuitable for delivery to consumer gas systems . Ethane may be recovered from the refinery fuel systems use as a petrochemical feedstock . Liquefied petroleum gas , or lpg is a convenient , portable fuel or domestic heating and cooking or light industrial use .
Motor gasoline , or petrol , must meet three primary requirements . It is provide an even combustion pattern , start easily in cold weather , andmeet prevailing environmental requirements .
In order to meet the first requirements ,gasoline must burn smoothly in the engine without premature detonation , or knocking . Severe knocking can dissipate power output and even cause damage to engine . When gasoline engines become powerful in 1920’s , it was discovered that some fuels knocked more readily than others . Experimental studies led to the determination that , of the standard fuels available at that time , the most extreme knock was produced by a fuel composed of pure normal heptane , while the least knock was produced by pure isooctane . This discovery led the development of the octane scale for defining gasoline quality . Thus , when a motor gasoline gives the same performance in a standard knock engine as a mixture of 90 percent isooctane and 10 percent normal heptane , it is given an octane rating of 90 .
There are two methods for carrying out the knock engine test . Research octane is measured under mild conditions of temperature and engine speed (49 C and 600 revolutions per minute) , while motor octane is measured under more several conditions . For many years the research octane number was found to be more accurate measure of engine performance and was usually quoted alone . Since the advent of unleaded fuels in the mid-1970’s ,however , motor octane measurements have frequently been found to limit actual engine performance . As a result a new measurement ,road octane number which is a simple average of the research and motor values , is most frequently used to define fuel quality . Automotive gasolines generally range from research octane number 87 to 100 , while gasoline for piston-engine aircraft ranges fromresearch octane number 115 to 130.
Each naptha component that is blended into gasoline is tested separately for its octane rating . Reformate , alkylate , polymer , and cracked naphtha , as well as butane , all rank high on this scale , while straight-run naphtha may rank at 70 or less . In the 1920s it was discovered that the addition of tetraethyl lead would substantially enhance the octane rating of various naphthas . Each naphtha component was found to have a unique response to lead addivities , some combinations being found to be synergistic and others antagonistics . This gave rise to very sophisticated techniques for designing the optimal blends of available components into desired grades of gasoline
The advent of leaded , or ethyl , gasoline led to the manufacture of high-octane fuels and became universally employed throughout the world after World War 2 . Lead is still an essential component of high-octane aviation gasoline , but , beginning in 1975 , environmental legislation in the United States restricted the use of lead additivies in automotive gasoline . Similar restrictions have since been adopted in most developed countries . The required use of lead-free gasoline has placed a premium on the construction of new catalytic reformers and alkylation units for increasing yields of high-octane gasoline ingredients and on the exclusion of low-octane naphthas from the gasoline blend .
High-volatile and Low-volatile components
The second major criterion for gasoline –that the fuel be sufficiently volatile to enable the car engine to start quickly in cold weather – is accomplished by the addition of butane , a very low-boiling paraffin , to the gasoline blend . Fortunately , butane is also a high –octane component with a little alternate economic use , so its application has historically been maximized in gasoline . Another requirement , that a quality gasoline have a high energy content , has traditionally been satisfied by including higher-boiling components in blend . However , both of these practices are now called into question on environmental grounds . The same high volatility that provides good starting characteristics in cold weather can lead to high evaporative losses of gasoline during refuelling operations , and the inclusion of high-boiling components to increase the energy content of the gasoline can also increase the emission of unburned hydrocarbons from engines on start-up . As a result , since 1990 gasoline consumed in USA has been reformulated to meet stringent new environmental standards . Among these changes are the inclusion of some oxygenated compounds in order to reduce the emission of carbon monoxide and nitrogen oxides .
One of the most critical economic issues for a petroleum refiner is selecting the optimal combination of components to produce final gasoline products . Gasoline blending is much more complicated than a simple mixing of components . First , a typical refinery may have as many as 8 to 15 different hydrocarbon steams to consider as blend stocks . These may range from butane , the most volatile component , to a heavy naphthaand include several gasoline naphthas from crude distillation , catalytic cracking , and thermal processing units in addition to alkylate , polymer , and reformate . Modern gasoline may be blended to meet simultaneously 10 to 15 different quality specifications , such as vapour pressure ; initial , intermediate , and final boiling points ; sulfur content ; colour ; stability; aromatics content ; olefin content; octane measurements for several different portions of the blend ; and other local governmental or market restrictions . Since each of the individual components contributes uniquely in each of these quality areas and each bears a different cost of manufacture , the proper allocation of each component into its optimal disposition is of major economic importance . In order to address this problem , most refiners employ linera programming , a mathematical technique that permits the rapid selection of an optimal solution from a multiplicity of feasible alternative solutions . Each component is characterized by its specific properties and cost of manufacture , and each gasoline grade requirement is similary defined by quality requirements and relative market value . The linear programming solution specifies the unique disposition of each component to achieve maximum operating profit . The next step is to measure carefully the rate of each component to the blend and collect it in storage tanks for final inspection before deliver it for sale . Still , the problem is not fully resorved until the product is actually delivered into customer’ tanks . Frequently , last-minute changes in shipping schedules or production qualities require the reblending of finished gasoline or the substitution of a high-quality grade for one or more immediate demand even though it may generate less income for the refinery .
Though its use as an illuminant has greatly diminished , kerosene is still used extensively throughout the world in cooking and space heatingand is the primary fuel for modern jet engines . When burned as a domestic fuel , kerosene must produce a flame free of smoke and odour . Standard laboratory procedures test these properties by burning the oil in special lamps . All kerosene fuels must satisfy minimum flash-point specifications to limit fire hazards in storage and handling .
Jet fuels must burn cleanly and remain fluid and free from wax particles at the low temperatures experienced in high-altitude flight . The conventional freeze-point specification for commercial jet fuel is –50C . The fuel must also be free of any suspended water particles that might cause blockage of the fuel system with ice particles . Special-purpose military jet fuels have even more stringent specifications.
The principle end use of gas oil is as diesel fuel for powering automobile , truck , bus, and railway engines . In a diesel engine , combustion is induced by the heat of compression of the air in the cylinder under compression . Detonation , which leads to harmful engine knocking in a gasoline engine ,is a necessity for the diesel engine . A good diesel fuel starts to burn at several locations within the cylinder after the fuel is injected . Once the flame has initiated , any more fuel entering the cylinder ignites at once .
Straight-chain hydrocarbons make the best diesel fuels . In order to have a standard reference scale . the oil is matched against blends of cetane and alpha methylnaphthalene , the latter of which gives very poor engine performance . High-quality diesel fuels have cetane rating of about 50 , giving the same combustion characteristics sa a 50-50 mixture of the standard fuels . The large , slower engines in ships andstationary power plants can tolerate even heavier diesel oils . The more viscous marine diesel oils are heated to permit easy pumping and to give correct viscosity at the fuel injectors for good combustion.
Until the early 1903s , standars for diesel fuel quality were not particulary stringent . A minumum cetane number was critical fro transportation uses , but sulfur levels of 0,3 to 0,5 weight percent by weight were common in most markets . With the advent of more stringent exhaust emission controls , however , diesel fuel qualities came under increased scrutiny . In the USA , diesel fuel is generally restricted to a maximum sulfur level of 0,054 weight percent , and regulations have restricted aromatic content as well . The limitation of aromatic compounds requires a much more demanding scheme of processing individual gas oil components than was necessary for earlier highway diesel fuels.
Furnace oil consists largely of residues from crude oil refining . These are blended with other suitable gas oil fractions in order to achieve the viscosity required for convenient handling . As a resiude product of significant quantity that commands a market price lower than the cost of crude oil .
Because the sulfur contained in the crude oil is concentrated in the residue material , fuel oil sulfur levels naturally vary from less than 1 to as much as 6 percent . The sulfur level is not critical to the combustion process as long as the fuel gases do not impinge on cool surfaces . However , residual fuels may contain large quantities of heavy metal such as nickel and vanadium ; these produce ash upon burning and can foul burner systems. Such contaminants are not easily removed and usually lead to lower market prices for fuel oils with high metal contents.
In order to reduce air pollution , most industrialized countries now restrict the sulfur content of fuel oils . Such regulation has led to the construction of residual desulfurization units or cokers in refineries that produce these fuels .