1. General Introduction to Refining

1. Crudes
   
   Crude oil is made up of a vast array of different materials with varying compositions. Crude oils are the result of organic processes spanning millions of years. The composition of crudes reflects this fact.
   
   Crudes may be paraffinic, naphthenic, asphaltic, or mixed base in nature. If high in aromatics, they may be called aromatic base crude oils. Designations of this type refer to the basic chemical composition of the crude.
   
   The terms Light versus Heavy refer to the density of crude oils and the products refined from them. Heavy crudes are more dense and have higher final boiling points than Light crudes which have more asphalt. Products refined from Heavy crudes include asphalt, lubricants, waxes, fuel oils. and light fuels (white products). Light crudes yield a higher proportion of light fuels, waxes, lubes, and small amounts of asphalt.
   
   Within these designations are a wide range of different materials with varying levels of different fractions. For example, Arab Heavy crude has approximately 21 percent gasoline fraction and 27 percent bitumen, and Boscan has 3 percent gasoline and 58 percent bitumen. Arab Light has approximately 12 percent bitumen fraction; Nigerian Light has approximately 1 percent , most Russian crudes have 15-22% but are often waxy. Figure 1.
   
   
   
   
   The science of refining, and the differential costs of crudes in various parts of the world, allows some light crudes to be used for bitumen production and some heavy crudes to be used for fuels.
   
   All crudes consist of complex mixtures of hydrocarbons differing in molecular size, structure, weight, and boiling range. The fractions can be separated on this basis.
   
   Further processing to optimize yields of useful fractions is the subject of the complex science of refining.



2. Refinery Processes
   
   A wide range of processes is carried out in a refinery. The following is a brief, simplified overview that is shown schematically in Figure 2.






3. Crude Treatment
   
   Desalting
   The purpose of desalting is to remove salt and reduce corrosion. (The presence of salt in bitumen also has some effects on emulsification causing the viscosity of the emulsion to be raised, often to an intolerable degree). The desalting process involves mixing the crude with wash water at moderate elevated temperatures to dissolve the salt. Water and oil can then be separated in settling tanks using electrostatic (a high voltage current to break the oil in water emulsion) or chemical means (emulsion breakers). This process has the secondary effect of removing suspended particulate matter.
   
   Dewaxing
   Paraffinic crudes often contain microcrystalline or paraffin waxes. The crude may be treated with a solvent such as MEK (methyl-ethyl-ketone) to remove this wax before it is processed. This is not a common practice, however.



4. Fractional Distillation
   
   The first process in refining makes use of the differing boiling ranges of the components of crude oil to separate different fractions. This is called atmospheric distillation and is carried out in an atmospheric fractionation or in distillation towers.
   
   The crude oil is heated in a furnace to 300°C to 350°C (570°F to 660°F) and injected into the tower at just above atmospheric pressure. This results in the stream that enters the column being a mixture of liquid and vapor. The vapors rise up the column until they reach a temperature at which they will condense. The tower has internal sets of steel condensation plates. The plates have holes that allow vapor to rise. Metal caps over these holes deflect vapor downwards so that it bubbles through liquid. This increases tower efficiency and effectively reduces the total number of plates required to achieve separation. This is illustrated in Figure 3.
   
   
   
   
   The vapors that condense on these plates are continuously extracted from the tower via pipes. The lightest vapors are withdrawn from the top of the tower.
   
   At the top of the tower are fractions such as butane and propane which are gases under standard temperature and pressure. Moving down the column is naphtha (feedstock for gasoline and the petrochemical industry), kerosene (jet fuel), gas oil or diesel. The remainder is a long or atmospheric residue that is used for further processing and that contains the asphalt fraction.



5. Reforming
   
   Reforming is a chemical process that rearranges naphtha fractions to achieve a higher octane rating. A process called isomerization changes the molecular form from simple straight chain molecules such as butane, pentane, or hexane into branched molecules.
   
   Reforming produces gasoline components and LPG.



6. Cracking
   
   Higher molecular weight molecules are broken down in catalytic cracking into smaller ones. A high molecular weight residue can be converted into a mixture of heavy oil and smaller molecules. The value of the product is increased in this way. The resulting mixture may be separated by distillation.
   
   Products of catalytic cracking are gasoline, kerosene, LPG, and heating oil.
   
   In the process of Fluid Catalytic Cracking, gas or air is forced through a bed of finely divided catalyst. This creates a fluidized bed. This means that the catalyst flows like a fluid, recirculating and creating maximum efficiency of contact with the molecules that are to be broken down.
   
   Thermophore catalytic cracking uses high temperature and a static bed. The catalyst pore size is the length of chain required.
   
   Hydrocracking uses high pressure in the presence of hydrogen.



7. De waxing
   
   In the process of dewaxing, waxy molecules are removed from heavy distillate fuel cuts or lube distillates. The Mobil Distillate Dewaxing process (MDDW) uses a zeolite catalyst to convert low quality gas oil into high quality diesel.
   
   The Mobil Lube Dewaxing process (MLDW) is used instead of the MEK process in some refineries for dewaxing raffinates to produce lube base stocks.



8. Vacuum Distillation
   
   In vacuum distillation, the atmospheric residue from the atmospheric tower is fractionated into different boiling (viscosity) levels. This produces lubricant distillates that are processed further through furfural (to extract aromatics) and MEK units (to remove wax) to produce lubricant base stocks. A residue, known as vacuum tower bottoms or vacuum residue, remains after the lighter fractions are boiled off. This vacuum residue may be further processed into brightstock lube oil, asphalt, and aromatic extracts.



9. Visbreaking
   
   Vacuum residue may also be broken down. by thermal cracking to distillate and fuel oil by the Visbreaking process.



10. Coking
    
    Application of high heat to vacuum residue will produce coke.



11. Blowing
    
    Vacuum residue may be blown with air (with or without a catalyst) to produce industrial bitumens. This will be considered in detail later.

Bitumen Manufacturing Methods

The manufacturing methods that are used to produce bitumen reflect one of two things: the quality of the crude as a bitumen feed or the other products in the refinery.

Four primary methods are used to manufacture bitumen:

• Atmospheric Distillation
• Vacuum Distillation
• Solvent Refining
• Blowing or Air Rectification

a) Atmospheric Distillation

The crude is preheated via a heat exchanger after desalting and dewaxing. It is then heated to about 400°C (750°F) by pumping it rapidly through a coiled tube exposed to direct heat in a pipe still or furnace.

It is then continuously delivered to the flash zone of the atmospheric distillation tower. The most volatile components are drawn from the top of the tower and the less volatile from the sides. Steam is often used to assist this process. Figure 3 shows the overall schematic.

This process is only capable of producing finished bitumens from very heavy crudes such as Boscan (58 percent asphalt residue). The atmospheric residue is used directly as bitumen. A "topped crude" will be produced for subsequent vacuum distillation in most cases.

b) Vacuum Distillation

Figure 4 shows a schematic layout for a vacuum tower. The vacuum tower may be used for two purposes: to distil! an atmospheric residue from any other source or, to top or extract kerosene from a cutback bitumen.

Lube Oil Operation
Atmospheric residue must be processed under vacuum to avoid thermal cracking. The vacuum allows lower temperatures to be used. The normal overhead pressure is approximately 21 mm of Hg and approximately 48mm of Hg in the flash zone.

In this process, the residue is introduced into the tower after heating. The — residue enters a flash zone at approximately 400°C after heating in a furnace. The temperature is set according to the volatility of the feedstock. Superheated steam is injected to maintain temperature and increase the feed rate. The partial steam pressure also increases volatility, aids separation, and minimizes cracking.

In the flash zone, light and heavy gas oils are flashed off and the bitumen, the highest boiling component, is the residue. This is often called Vacuum Residue or Vacuum Tower Bottoms (VTB).

The VTB may be a finished bitumen. That is. it may meet certain paving or industrial specifications without further processing. This will depend on the crude and the processing parameters used. Rarely is a refinery run for bitumen production. In lube oil refineries, the requirements of lube production must be balanced against the requirements for bitumens. A lighter feed is required for lube production. A heavier feed is preferable for bitumen. The processing parameters must be set in operation of the vacuum tower to take these different needs into account.

As a result, the VTB are used either for further processing or as blend stock.

Cut Back Processing
In this process, a finished bitumen that has had up to 30 percent kerosene added at another source is refractionated to remove the kerosene. The process is shown schematically in Figure 5.

The advantage of such a system is that it allows product from one refinery to supply and manufacturing flexibility. In this case, cutback is made from certified bitumen at and moved toanother4 location by road, pipeline or by ship where it is stored in a tank and allowed to dewater.

The cutback is heated by heat exchangers and injected into the flash zone of the vacuum tower. Liquid flows through the stripping section below the flash zone against a flow of stripping steam. Vapor flows up the tower and is refined and condensed in three packed sections. Two kerosene streams, dirty and clean, are withdrawn from the tower. The clean stream is stored and the dirty stream is reprocessed elsewhere.

The residue is a finished bitumen and may be used as is or adjusted to harder grades by blowing (see Air Rectification).

c) Solvent Refining

This takes advantage of the complex internal solubility parameters of bitumen. An alkane injected into the bitumen disrupts the dispersion of components and causes the polar constituents to precipitate. Two similar techniques are employed:

• Propane Deasphalting
• ROSE Supercritical Process

Propane Deasphalting
Propane deasphalting is extensively used. In this process (see Figure 6), a mixture of overflash and VTB from the vacuum distillation tower is introduced in the upper section of the PDA unit. The ratio of these components is variable and dependent on other refinery considerations but can be from (VTB:OF) 1:0 to 2:1. Overflash contains little, if any, asphaltenes and as the proportion of overflash increases, the level of asphaltenes decreases. However, this is not significant for crudes of approximately 8 percent to 9 percent asphaltenes.

The propane (or sometimes propane/butane mixtures) is introduced from the bottom. The process runs under pressure to maintain the propane in a liquid form (approximately 30 bar). The treat rate is approximately 500 percent (five times as much propane as feed). Temperature is approximately 60°C (top approximately 75°C and bottom approximately 47°C) and is largely controlled by the propane coming in. Steam heating is provided at the top. The paraffinic oils dissolve with some of the naphthenic and aromatic oils at these temperatures. The temperature at the top of the tower is close to the critical temperature for propane and the aromatic and naphthenic oils separate. The flow down the tower creates reflux and aids the separation of the resins and asphaltenes into a sharp fraction.

The asphaltene and resin mixture is called Propane Deasphalted Asphalt (PDA) or PD Tar. This is taken off as a suspension and the propane is recovered. Steam stripping may be used. Oils taken off near the top are known as Deasphalted Oil or DAO. The DAO is furfural extracted to produce Bright Stock oil. Aromatic extract is recovered from the furfural and can be used as a blending material with PDA and VTB for finished bitumens (see Figure 7).

It is very important that bitumens blended in this way conform to the controlled compositions required for field performance. This will be considered later.

ROSE Supercritical Process
ROSE supercritical process is a natural progression from propane deasphalting and allows the separation of bitumens into their base components (resins, asphaltenes, and oils) for recombination to optimum properties (see Figure 8). Only three refineries in the United States currently use this process.

Propane, butane, and pentane may be used as the solvent depending on the feedstock and the desired compositions.

A mixer is used to blend residue with liquefied solvent at elevated temperature and pressure. The blend is pumped into the first stage separator where, through counter current flow of solvent, the asphaltenes are precipitated, separated, and stripped of solvent by steam. The overhead solution from the first tower is taken to a second stage where it is heated to a higher temperature. This causes the resins to separate. The final material is taken to a third stage and heated to a supercritical temperature. This makes the oils insoluble and separation occurs.

This process is very flexible and allows precise blending to required compositions.

d) Air Blowing/Air Rectification

For light crudes, or where industrial bitumens are required, blowing a bitumen with air, or bringing it into contact with air, at elevated temperatures is used to increase the viscosity to meet specification. This process is often termed oxidation. However, only a small amount of oxygen is actually incorporated in the bitumen during the process.

The process may be controlled by controlling temperature, air rate, and the use of the catalyst. The properties of the blown asphalt are highly dependent on the composition of the feedstock and the degree of blowing.

Although air blowing has been used extensively for paving asphalts in the past, it is now out of favor due to its tendency to create binders that are brittle, particularly at low temperatures. Air rectification (a more controlled, light blowing process) is useful for paving grade bitumens but requires careful attention to the feedstocks used and the degree of blowing.

Air blowing remains the method of manufacture of industrial asphalts.

Chemistry of Blowing

Non-Catalytic Blowing
The reactions that occur in air blowing are as follows:

• Reactions that increase molecular size and degree of association of polar compounds. These include formation of carbonyl compounds in the form of carboxyl groups, esters, anhydrides, and ketones. In aging, in field or oven situations less than 150°C, the ketones, anhydrides, and sulphoxides are the main products. In air blowing, the combination of air and temperature (200°C to 275°C) results in most of the oxygen being incorporated as esters. This fact allows condensation reactions that lead to linking of polar molecules such as asphaltenes. It also increases polarity resulting in increased molecular associations and structure.
  
  The result is an increase in molecular weight and concentration of asphaltenes and a significant increase in viscosity.
  
• Volatilization may occur as light ends are driven off.
  
• Reactions that do not increase molecular size. These include dehydrogenation and formation of cyclic compounds with water as a by-product.
  
• Reactions that decrease molecular size. These include separation of side branches and formation of blown distillates, water, and carbon dioxide.

In blowing, the chemical reaction process is stepwise. Peroxides and hydroperoxides are produced initially and progress to ester and carbonyl group formation.

The formation of carbonyl and ester products is favored at the lower temperatures; molecular size increase is optimized at approximately 250°C. The effect is increased as the aromaticity of the feed is increased.

When fractionated after blowing, asphalt will usually exhibit a significant increase in asphaltenes, a slight increase in resins, and a reduction in aromatic oils.

Catalytic Blowing
"Catalytic" blowing is not strictly catalytic as the additive is used up in the reaction. The blowing time is reduced, however, and the softening point/penetration relationship is altered higher penetration for a given softening point (see Figure 9) by. Common catalyst types are ferric chloride and phosphorus pentoxide. Acids such as sulphuric and hydrochloric have also been used.

The catalyst is added via the blowing air or into the charge at about 0.1 percent to 0.5 percent. The catalyst is maintained in suspension by the air blower.

The catalysts appear to work by promoting reactions that build molecular weight. That is, they favor increases in polarity and in asphaltene level.

Processes

Blowing may be batchwise or continuous.

Batchwise
In a batchwise process, preheating takes place in a tube furnace or heater. Air supply is via a blower or compressor. A vertical tower is preferred. Fumes must be disposed of. This is best done by incineration, but water scrubbers and knockout drums have been used (see Figure 10). Temperature is the most important variable (see Figure 11). Preheating takes place at temperatures from 200°C to 230°C. After the air has been injected, the exothermic reaction increases temperature until some means is introduced to control it. Water or steam sprays on the asphalt surface are used. The end point may be monitored by viscosity, penetration, or softening point.

Continuous
There are two related processes that may be used for blowing or air rectification: the conventional and the turbo oxidizer process. The first is well suited to industrial bitumens but requires careful handling for air rectification. The second has superior control and performs both functions well.

In the conventional system (see Figure 12) bitumen is heated by a furnace to approximately 250°C and injected into the tower. Air is introduced via an air sparger. This gives bubbles of air through the bitumen. A stirrer may be fitted. The air sparger system is not ideal because air bubbles coalesce and the oxidizing effect is uneven. This causes some areas to be more hardened than others. Air rates vary with the product and are largely determined by experience or laboratory trials.

The reaction is exothermic and can go out of control at temperatures of approximately 275°C. To avoid this. a water curtain is often sprayed on the bitumen with some steam to control foaming. Product quality is controlled by regular sampling. Gases produced in the process are incinerated. Soot may build up and is a fire hazard unless removed periodically.

The product is drawn from the bottom of the drum into a surge drum to control product level. The product is cooled using heat exchangers.

In the turbo oxidizer process, similar conditions are used but the hardware allows greater control (see Figure 13).

In the Biturox system (a turbo oxidizer), the air is blown into the bottom of from the top and run down to the base. The process is kept under temperature control by water injection into these pipes. The air is injected through horizontal pipes running from the vertical pipes and distributed by a multistage turbine. This ensures that the bubbles are fine and maximizes reaction rate.

As the bubbles rise they are redispersed by additional turbines. This means that air injection is optimized with little excess; even mixing occurs. This results in little or no "cooking" because there is little oxygen in the waste gases. As the reaction rate is optimized, the temperature of operation can be minimized and the blowing is even as a result. That is, the total bitumen is evenly oxidized rather than some parts being heavily oxidized and others remaining relatively untouched.

As discussed above, the polycondensation reactions and carbonyl forming reactions are favored at lower temperatures; hence, the viscosity increase is also optimized.

It is reported that Fraas points of industrial grade bitumens are significantly lower (up to 20°C) when blown on such equipment in comparison to traditional towers. This may be attributed to a more dispersed polar phase of lower, but more even molecular weight. This effect is also observed for roadpave grades

Feedstocks For Blowing/Air Rectification

The physical properties of the final blown or air rectified bitumen depend on the feedstock and the degree and method of blowing. This may be related to compositional changes during blowing. See Bitumen Chemistry and Composition section.

The main influences on the penetration/softening point relationship are:

• Composition of the feedstock
• Temperature of column
• Residence time
• Air-to-feed ratio
• Reaction efficiency

Achieving low penetration with high softening point for industrial bitumens requires a different feedstock from that for manufacturing of road pave grades (see Figure 14). The road pave grade feed is different in that it is:

• of lower viscosity
• blown through an interval of 100 open units maximum
• rich in aromatics so that the final product, though higher
in asphaltenes, is well dispersed
• blown with shorter residence times (continuous versus batch)
• blown with a lower air rate
• blown at lower temperatures (250°C to 270°C)

This combination effectively controls the final composition of the bitumen. The penetration/softening point (Ring and Ball) relationships are unique to the compositions achieved. It is possible that the final composition of a given initial mixture may never achieve the required range. For example, the feeds for some road pave grades cannot be blown to industrial grades.

Blending

Blending of bitumens of different grades made from the same feedstock can allow intermediate grades to be produced. This is not necessarily a simple proportioning process and laboratory work should always be carried out to determine correct blend ratios.

If the feedstocks and the manufacturing route are similar, the penetration at 25°C of a blend of bitumens A and B can be approximated by:

Log pen (mix) = fraction A x log pen (A) + fraction B x log pen (B)

The softening point (SP) may be approximated by:

SP (mix) = fraction A x SP A + fraction B x SP B

The viscosity may be approximated by using:

Volume (mix)X log (log viscosity (mix))
= Volume AX log (log viscosity A) + Volume BX log (log viscosity B)

Feedstocks from different processes may be blended. The proportions that may be blended are strictly determined by the composition of each material. Usually, for example, PDA is thermally susceptible and must be used sparingly.

Blending of components gives an opportunity to optimize bitumen properties. The Strategic Highways Research Program (SHRP) has carried out work that shows that blending can alter properties significantly. This is, of course, another way of saying that bitumen components must be balanced. The work was carried out on materials from the ROSE process using butane extracted pitch and resins. DAO was the final blend material.

Effect of Manufacture on Composition

This discussion is based on the considerations covered above and in detail in the module on Bitumen Chemistry and Composition.

a) Triangular Diagram

The composition of bitumen, as shown by the different fractionation techniques and conceptualized in the microstructural model, is good evidence that the basic approach of measurement of polarity differences by asphaltene/resin and oil measurement is sound. These components may be easily and directly measured using either clay-gel analysis or the latroscan instrumented chromatography method can be used directly to plot a triangular diagram of composition.

The effect of the components has been discussed in detail earlier. The as- phaltenes are the highest polarity material and usually have the highest molecular weight. They form the structural part of the asphalt. The resins are lower polarity and associate less, but they are partially responsible for structure. The oils are a mixture of aromatic and saturated species. The aromatic oils are the main dispersive material. The asphaltenes and the saturates are very diverse materials and can only coexist in a thermodynamically stable system because of the balance of the rest of the components.

The triangular diagram is a valuable technique for checking that this balance exists.

The diagram in Figure 15 may be divided into three main areas for bitumen:

• Sol bitumens are low in asphaltenes content (less than 5 percent by heptane) and are generally high in aromatic oils. They behave in a Newtonian manner; that is, they are viscous liquids. Their major use is as saturants.
• Sol/gel bitumens are intermediate types. They have moderate asphaltene levels (5 percent to 25 percent by heptane) and a balance of components that yield a viscoelastic material. That is, a material that flows at high temperatures and is an elastic solid at low temperatures, with intermediate behavior in between. These are used mainly as road pave asphalts.
• Gel bitumens are high in asphaltenes (more than 25 percent by heptane) and are elastic solids with high degrees of structure. These are industrial grades.

b) Effect of Crude

Heavy crudes of high asphaltene content, with a good balance of properties, may be fractionated for direct use as road pave bitumens. Boscan, Arab Heavy, and Arab Light can produce usable bitumens directly from the Vacuum Tower.

Light crudes of low asphaltene content can produce sol bitumens directly from the vacuum tower.

Very heavy, naturally occurring, asphalts may qualify as gel bitumens. For example, Gilsonite is a rock asphalt that is almost pure asphaltene.

Most crudes will need some processing to make a range of grades.

c) Effect of Processing on Composition

Fractionation techniques do not affect the general composition of the asphalt fraction. If the asphalt is of a usable composition it will remain so. If the asphalt does not have a usable composition or, if more than one grade is required, then further processing will be necessary.

The effect of air rectification and blowing, and of propane deasphalting, is shown in Figure 16. It is clear from this diagram that air rectification gives the best opportunity to tailor bitumen composition. The ROSE process, which breaks the asphalt up into all the component streams, is also suitable.

The industrial grade bitumens at the gel end must be blown, but at the sol end may be blended from lighter oils with PDA or VTB.

In all cases, the regions on the triangular diagram must be established from field correlation.