- 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.
Feed and Product characterization
- 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.
Important characterization properties
- API gravity of petroleum fractions is the measure of the 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.
Watson Characterization Factor
- The Watson characterization factor (K) 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.
- 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.
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.
- 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 cease 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.
- For heavier products, they are specified in a desired range and this is achieved by blending appropriate amounts of lighter intermediate products.
- 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 an 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.
- 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.
- Based on chemical analysis and existence of various functional groups, refinery crude can be broadly categorized into various categories summarized as
- 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.
- Below are the structures of Methane(CH4),Ethane(C2H6),Propene(C3H8),Normal Butane(nC4H10) respectively.
Structure below is of Normal Pentane (C5H12)
- 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.
- Below are the structures of Ethylene(C2H4),Propylene(C3H6),Butylene(C4H8).
- 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.
- Below are the structures of Cyclopropane(C3H6),Cyclobutane(C4H8) ,Cyclopentane(C5H10).
- Below are the structures of Cyclohexane(C6H12), Methyl Cyclohexane(C7H14).
- 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) are given below.
- Ortho-Xylene(C8H10),Meta-X ylene(C8H10) are given below
- 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.
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.
- Resins are polynuclear aromatic structures supported with side chains of paraffin and small ring aromatics.
- Their molecular weights vary between 500 – 1500. These compounds also contain sulphur, nitrogen, oxygen, vanadium and nickel.
- 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.
Units in Refinery
The various units presented in the refinery process diagram are categorized as
- Crude distillation unit (CDU)
- Vacuum distillation unit (VDU)
- Thermal cracker
- Fluidized catalytic cracker
- Naphtha splitter
- Alkylation and isomerization
- Gas treating
- Blending pools
- Stream splitters
Crude Distillation Unit (CDU)
- The first essential task for the crude oil consisting of more than 108 compounds is to separate its major components based on boiling point differences.
- This principle is exploited in the crude distillation unit which involves the energy intensive operation.
- Since crude distillation involves the processing of the entire feed, it remains as the most significant operation in a refinery.
Figure. Process flowsheet-a conceptual diagram of the crude distillation unit (CDU) with HEN(heat exchanger network)
The conceptual process flowsheet for the petroleum refinery is shown in the Figure above. It consists of the following important sub-processes:
- Crude desalte
- Pre-flash column
- Crude distillation column supplemented with side columns.
- These columns produce the desired products
- Pump around heat exchanger units
- Heat exchanger network that facilitates energy recovery from hot product and reflux streams to heat the crude oil.
- A critical observation of the overall refinery process block diagram indicates that the straight run gasoline (this is the gasoline obtained from the CDU) does not have good octane number (40 – 60) and needs to be upgraded to obtain the desired octane number (85 – 95). Typically, cracking, reforming and isomerization are regarded as the three most important of processes that contribute towards upgradation the octane number.
- Typically cracking involves the thermal or catalytic decomposition of petroleum fractions having huge quantities of higher molecular weight compounds. Since heat is required, typically cracking reactions are carried out in furnaces that are supplied with either fuel oil or fuel gas or natural gas or electricity as heat source.
- Cracking involves the decomposition of heavier hydrocarbon feedstocks to lighter hydrocarbon feed stocks.
- Cracking can be carried out to any hydrocarbon feedstock but it is usually applied for vacuum gas oil(VGO)
- Cracking can be with or without a catalyst.
- When cracking is carried out without a catalyst higher operating temperatures and pressures are required. This is called as thermal cracking. This was the principle of the old generation refineries.
- Now a days, cracking is usually carried out using a catalyst. The catalyst enabled the reduction in operating pressure and temperature drastically.
Cracking is an endothermic reaction in which:
- Long chain paraffins converted to olefins and olefins
- Straight chain paraffins converted to branched paraffins
- Alkylated aromatics converted to aromatics and paraffins
- Ring compounds converted to alkylated aromatics
- Dehydrogenation of naphthenes to aromatics and hydrogen
- Coke formation due to excess cracking
- Therefore, in principle cracking generates lighter hydrocarbons constituting paraffins, olefins and aromatics. In other words, high boiling low octane number feed stocks are converted to low boiling high octane number products.
- These very much depend upon the feed stock and type of cracking (thermal /catalytic ) used.
- Cracking is a gas phase reaction. Therefore, entire feedstock needs to be vaporized.
- It was observed that short reaction times (to the order of 1 – 3 seconds only) provide good quality product and less coke formation.
- For vacuum gas oil, thermal cracking requires operations at 600°C and 20 atms gauge pressure.
- For vacuum gas oil, catalytic cracking is usually carried out at 480°C and 0.7 – 1 atms gauge pressure.
- Acid treated silica-alumina was used as catalyst.
- 20 – 80 mesh size catalysts used for FCCR and 3 – 4 mm pellets used for MBRs
- During operation, poisoning occurs with Fe, Ni, Vd and Cu
- The process technology consists of two flowsheets namely the cracking coupled with main distillation column and stabilization of naphtha.
Figure above shows Flow sheet of Catalytic Cracking process
- Feed enters the cracking reactor.
- Old generation refineries used moving bed reactors
- Now a days, fluidized catalytic cracking (FCC) reactors are used.
- The cracked product from the reactor enters a main distillation column that produces unstabilized naphtha, light gas oil, heavy gas oil, slurry and gas.
Fluidized catalytic cracking reactor (FCCR)
The basic principle of the FCCR is to enable the fluidization of catalyst particles in the feed stream at desired pressure and temperature.
- Another issue for the FCCR is also to regenerate the catalyst by burning off the coke in air.
- Therefore, the reactor unit should have basically two units namely a reactor (FCCR) and a catalyst regenerator (CR).
- The FCCR consists essentially of two important components in a sophisticated arrangement. These are the riser and the cyclone unit assembled in a reactor vessel.
Figure above shows Fluidized Catalytic Cracking Reactors
- In the riser (a long tube), the feed is allowed to get in contact with the hot catalyst.
- The hot catalyst is enabled to rise through lift media in the riser. The lift media is usually steam or light hydrocarbon gas.
- The riser contact time is about 250 milliseconds.
- The riser is eventually connected to cyclone units.
- The cyclone units receive the catalyst and finished product. The catalyst that enters the cyclone unit is fully coked and needs to be sent to a regenerator to regain its lost activity.
- After cyclone operation (which separates the hydrocarbon vapors and catalyst as a solid fluid operation), the catalyst falls down to the vessel that houses the riser and cyclone units.
- The catalyst in the vessel is subjected to stream stripping in which direct contact with steam is allowed to remove hydrocarbons from the catalyst surface.
Catalyst regenerator (CR)
- The spent catalyst which is relatively cold enters the regenerator unit.
- Here air enters the vessel through a sparger set up.
- The catalyst is subsequently burnt in the air. This enables both heating the catalyst (which is required to carry out the endothermic reaction) and removing the coke so as to regain the activity of the coke.
- The catalyst + air after this operation will enter the cyclone separator unit. Unlike the FCCR, the CR does not have a riser. Therefore, air enters a dense phase of catalyst and also enables the movement of the catalyst to a dilute phase of catalyst + air
- The cyclone separators separate the flue gas and catalyst as a solid fluid operation.
- The activity regained catalyst is sent to the riser through a pipe.
- During this entire operation, the catalyst temperature is increased to 620 – 750°C
- The flue gas is obtained at 600 - 760°C and is sent for heat recovery unit to generate steam.
The Reforming involves enhancing the octane number of the product
- Heavy naphthas are used are typical feed stocks
- The reaction is carried out on a catalyst
- Reforming reaction produces hydrogen as a by product which is used elsewhere in the refinery
- Usually Platinum supported on porous alumina is used as a catalyst
- Catalyst activity enhanced using chloride
Paraffin isomerisation takes place
- Naphthene isomerisation also takes place to produce cycloalkanes
- Cyclo alkanes undergo dehydrogenation to generate aromatics
- Dehydrocyclization takes places to convert side chained alkanes to cyclo alkanes and hydrogen
- In summary lower octane number feeds are converted to high octane products
- The reformate thus produced has high octane and aromatics (benzene, toluene and xylene) content.
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