| Procedure | Lines | Licensor |
|---|
| Crude Distillation Unit (CDU) [24] Input: crude oil, Output: combustible gases, naphtha (light/heavy), jet fuel, Diesel fuel, heavy oil, residue fluid Purpose: separating crude oil components by molecule length / boiling temperature | It is the first processing step in nearly all petroleum refineries (see image below). The CDU separates the different components of crude oil by their boiling points. - In the desalter salt is being removed from the crude oil. In a single preheat train the crude oil is warmed up [25] by using heat from different procedures on this list or burning fuel from own making, like LPG. [26] After this the crude oil is heated up and led into the distilling column. - In the column, lighter crude oil components travel up. Their molecules are short and thus their boiling points are low. At the very top of the column compounds like methane, ethane, propane, butane, and isobutane [27] leave. They are gases even at room temperature. All of them can explode, when in contact with air/oxygen.
- A little bit lower naphthas exit (naphtha can be described as crude gasoline; there is light naphtha like pentane (C5), hexane (C6) and heptane (C7) and heavy naphtha like octane (C8) up to dodecane (C12)). At room temperature, they are liquids but evaporate (and ignite) quickly. Light naphtha goes to NHT and then to the Penex unit (see below). Heavy naphtha goes to NHT and then to CCR (see below).
- In the middle section of the column compounds from C13 up to C25 exit. They can be used in jet fuel, Diesel fuel and heating oil. (Jet fuel A: C8...C15; jet fuel A-1: C9...C16; jet fuel B and JP-4: C5...15; Diesel fuel: C9...C25; heating oil: C14...C20.) These are liquids at room temperature. They do not evaporate (or ignite) quickly (which makes them "safe" fuel compounds,e.g. jet fuel in aviation).
- Heavier oils (C25 ... C70) and residue (C70 ... C9000) leave at lower exit points of the column. Heavy oil can be used as paraffin wax (C20...C40), as petroleum jelly / vaseline (C28...C60), as motor oil (C20 ...C50) [28] or for making asphalt / bitumen / tar (C40...C150). The longer the molecules are (many carbon atoms and high C-number), the slower they flow (high viscosity). Heavier oils go to the hydrocracking unit, residue fluid to the RFCC unit.
The output streams are called "cuts" (e.g. "side cut", "top cut") and those ones that are not (yet) processed further are called "straight-run" (e.g. "straight-run naphtha" as opposed to naphtha produced in other refinery processes). The main distilling column in Lekki is the biggest in the world (as of 2022) and with 112.5m height even bigger than the rocket Saturn V or the Eko Tower Black Pearl in Lagos. Generally speaking, a tall distilling column has better efficiency, better precision in separating crude oil components (less components that exit at the wrong point for example by turbulence), more exit points, and a higher throughput rate. | 1 | UOP |
| RFCC (Residue Fluid Catalytic Cracking) [29] [30] [31] Input: residue fluid from CDU (long molecule hydrocarbons), air, catalyst, Output: catalyst (to be re-used), flue gas, olefins, isobutane, other short molecule hydrocarbons Purpose: breaking molecules longer than 70 carbon atoms into shorter ones | This cracking process converts residue fluid coming from the CDU into lighter components (see image below). The residue fluid consists of molecules with many carbon atoms (more than 70) and complex ring and branch structures. It is black, viscous, and can barely be evaporated - making it unusable as a fuel without having been "cracked". In the reactor, the residue fluid is brought into contact with a catalyst at high temperature under near-vacuum conditions (which helps to evaporate the fluid). The catalyst in this procedure is an acidic matrix such as crystalline aluminosilicate zeolite. [32] In very simplified terms, the sharp edges of the zeolite crystals cut up the molecules of the residue fluid without themselves changing shape. However, carbon atoms attach themselves to the zeolite crystals in the RFCC process and have to be removed again in the regenerator. The cracking takes place, turning the hot fluid into gas of short-molecule compounds. After this the catalyst is recovered and regenerated. The cracked compounds are fed to a column that separates them according to their boiling point (and thus molecular length, similar to the CDU). [33] Because of the geometry of their molecules, cracking of alkanes always means - resulting excess of carbon atoms e.g. in cokers (in this procedure hardly the case) or
- necessary addition of hydrogen e.g. in hydrocracking (in this procedure not the case) or
- resulting unwanted double covalent bonds in hydrocarbon compounds ("olefins", mainly the case in this procedure).
Since in RFCC no hydrogen is added, the outgoing compounds belong mostly to the olefins (ethylene, propylene, butylene, pentylene, hexylene etc.). They can be hydrotreated into alkanes (see "NHT" below), alkylated (see "Alky" below") or polymerized into e.g. polyethylene/polypropylene ("plastic"). The RFCC regenerator in Lekki has been the heaviest item on an African road, before it was installed. It also is the heaviest single piece made of metal in the world. The RFCC is one of the most safety-critical areas of a refinery. The main threat is that there must be absolutely no air in the reactor, whereas the working principle of the regenerator, to which the reactor is doubly connected, is based precisely on the supply of air. The constant challenge is not to let the hydrocarbons enter the regenerator via the two connections and not to let air enter the reactor. - In 2015, the regenerator of an Exxon refinery in Torrance, California, exploded due to lack of maintenance (the expander had worn out, a heat exchanger from a different unit was leaking combustible gases into the system and a catalyst slide valve worked insufficiently after too many years of usage) and incorrect actions after a malfunction occurred (decreasing the steam flow, installing a distance ring and "variance" from standard procedure). [34] In 2018, the Superior refinery in Wisconsin exploded, also due to a worn out RFCC catalyst slide valve. [35] | 1 | UOP |
| "Unicracking" (Hydrocracking) in mild hydrocracking mode [36] Input: heavy oil from CDU, hydrogen Output: C4...C12 alkanes, fractionated into light and heavy naphtha, jet fuel, Diesel Purpose: breaking molecules longer than 25 carbon atoms into shorter ones | Hydrocracking [31] [37] [38] [39] "breaks" heavier components (molecules with more than 25 carbon atoms) in the presence of hydrogen into medium-sized molecules (4 - 12 carbon atoms per molecule). [40] [41] Mild hydrocracking (MHC) is cracking with less pressure but by using a catalyst in the presence of hydrogen (the pressure is 60 to 110 bar in comparison to conventional hydrocracking at 200 bar). As a catalyst Zeolite is being used. Unlike hydrotreating, where hydrogen is used to break bonds between carbon and sulphur or carbon and nitrogen, mild hydrocracking uses hydrogen to break bonds between carbon atoms. [42] [43] Safety rules must be observed here. At the Valero refinery in Delaware, two service technicians died in 2005 from nitrogen asphyxiation while trying to retrieve a roll of duct tape from the reactor, only 5 feet (1.5m) below them. [44] Other workers had previously flooded the reactor with nitrogen to remove oxygen but had not put up a specific warning. | 1 | UOP |
| Alkylation ("Alky") Input: propylene, butylene (from RFCC), isobutane Output: isoheptane, isooctane (for blending into gasoline), butane, propane Purpose: merging short hydrocarbons into medium-sized molecules, removing olefins, improving RON (see next line in table) | While cracking splits molecules that are too long for commercial purposes, alkylation "fuses" molecules together that are too short for commercial purposes. Specifically, we are talking about two undesirable "waste products" from the RFCC process (see above): Isobutane on the one hand and propylene or butylene on the other. They are merged into medium-length molecules with one or two branches, the so-called "alkylate", mainly isoheptane and isooctane. (The prefix "iso-" means that the molecule has "branches". In shorthand this is indicated by an added "i", e. g. iC7 for isoheptane. The suffix "-ylene" marks olefins, which are hydrocarbon molecules with one double bond between two of their atoms. They create soot when they burn.) Alkylate is a high-quality gasoline blend because it has good anti-knock properties and burns clean. The alkylation process takes place in the presence of an acid - in the Dangote refinery, this is sulphuric acid (H2SO4). The plant is called a sulphuric acid alkylation unit (SAAU). [45] The main technology for SAAU is the STRATCO process licensed by DuPont. It is a very established technology for alkylation in refineries. In the last ten years, more than 85% of the SAAU capacity added worldwide has been produced using it. A SAAU can be divided into five major sections: reaction, refrigeration, effluent treating, fractionation, and blowdown. In the reactor, the reacting hydrocarbons are brought into contact with the catalyst sulphuric acid at a temperature of 15.6 °C (60 °F). Then the feedstocks are treated to remove impurities, especially water. The feedstock is cooled in the refrigeration section and the light hydrocarbons are discharged from the plant. Then acid, alkyl sulfates, and di-alkyl sulfates are removed from the effluent stream to prevent corrosion and fouling in the downstream process. To maintain the desired strength of spent acid, a small amount of fresh acid is continuously added to the reactor. In the fractionation section, the superfluous isobutane is recovered and the remaining hydrocarbons are separated into the desired products. The spent acid is degassed in an acid blowdown drum and the acid effluent is neutralized with caustic in a scrubber before being flared. The spent acid is stored and discharged at regular intervals. Here, too, safety rules must be observed. On 22 November 2016, a fire occurred in a refinery in Baton Rouge due to maintenance work on a 30-year-old valve that had to be operated manually. 900kg of isobutane escaped and formed an explosive gas cloud that ignited at a welding machine that was left switched on (20m away). 4 workers sustained serious burns. [46] In Philadelphia, Pennsylvania, on 21 June 2019, propane leaked from a manifold at an alkylation plant and ignited, resulting in three extremely violent explosions. This launched several pieces of equipment, including a 19-tonne container that crashed 700 m from its original location. [47] | 1 | DuPont |
| Continuous Catalyst Regeneration (CCR) Platforming [48] [49] [50] Input: heavy naphtha from NHT, hydrogen Output: Toluene, Xylene (high octane gasoline components), hydrogen surplus Purpose: improving the RON of gasoline | In high-compression engines, some fuel compounds tend to ignite before the spark plug delivers the spark. This is called "engine knocking" and is undesirable. For each component there is a measure that indicates how "knock-proof" it is: the RON or octane rating. Since naphtha has an octane rating of around 90, but modern engines require RON 95 or 98, some of the naphtha must be converted into components that have a RON of over 98 and blended into petrol. These components are the aromatic compounds ("BTX"): molecules with ring structures and double covalent bonds. They have a RON of 120 (Xylene) and 120...146 (Toluene). [51] The conversion of heavy naphtha into these aromatic compounds takes place in the CCR (see images in the gallery below). This process converts linear molecules with at least 7 carbon atoms into ring-shaped Toluene and Xylene by withdrawing hydrogen from them. Hydrogen is an important by-product of CCR and is used in other procedures on this list or can be sold for hydrogen technology. Before entering the CCR unit, the feed of heavy naphtha has passed through the NHT, and C5 and C6 have been removed. (If C6 is not removed, the CCR process produces benzene, which is both toxic and carcinogenic.) In the CCR itself, hydrogen is added at 4 to 45 atm and 495°C to 525°C. The process is highly endothermic, which requires constant re-heating between several reactor chambers. The incoming naphtha must also be free of sulphur since it damages the catalyst (this is called "nickel catalyst poisoning"). Water also damages the catalyzer as it washes out chlorides. After the chemical reaction, the stream goes into a separator which extracts excess hydrogen. After this, a stabilizing column divides the stream into lighter molecule components (methane, ethane, propane) and the high octane "reformate", similar to a distilling column. [52] Besides Dangote's CCR, the alternative, SR platforming ("semi-regenerative", "fixed-bed"), should be mentioned. However, CCR is considered to be more technically advanced (e.g. the catalyst does not wear out as quickly). [49] | 1 | UOP |
| Penex isomerization processes [53] Input: light, "normal" naphtha Output: gasoline blends with a RON of 92 (isomerized naphtha) Purpose: improving the RON of gasoline | Similar to the CCR, the Penex isomerization improves the octane rating of fuel, in this case of light naphtha. In a refinery, light naphtha can come from the distillation column ("straight-run naphtha") or from other processes, for example, the cracking units. Since Bonny crude oil is a light oil, it has a high percentage of light naphtha (C5, C6, and C7). Light naphtha with 5 or 6 carbon atoms per molecule (pentane/C5, hexane/C6) has an octane rating (RON) of 50 ... 60, while gasoline at the pump station has a RON of 95 or 98. For this reason, refinery operators convert C5 and C6 into compounds with a higher RON, taking advantage of the fact that branched hydrocarbons (which begin with "Iso-" in the name) have a higher RON value than linear (so-called "normal") ones. If a molecule arranged in a row is "rearranged" so that it branches at least once, its RON value increases. This process is called "isomerization". This process isomerizes light naphtha (pentane (nC5), hexane (nC6)) into higher-octane, branched molecules (iC5, iC6). The Penex process uses fixed-bed catalysts with chlorides. In a UOP Penex unit the feedstock passes a deisopentanizer (which extracts isopentane), then a reaction chamber with a fixed-bed catalyst, then a deisohexanizer (which extracts isohexane) and finally a Molex technology column. The end product has a RON of ca. 92. An isomerization unit is also safety critical. 2005, in Texas City, the distilling column of a isom unit erupted and then ignited, killing 15 employees and injuring 180 people. [54] This happened due to a faulty measuring instrument as well as overworked and underqualified personnel. It is the worst accident the US Chemical Safety Boar d has investigated. | 1 | UOP |
| Merox treatment ("Sweetening") Input: sulphides (corrosive, foul smell) in jet fuel or combustible gas Output: di-sulphides (harmless, smell of garlic, onions or not at all) to be fractionated out, hydrogen Purpose: removing corrosive sulphide compounds | Sulphur compounds in crude oil are a big problem for operators of refineries, even if the crude oil is low in sulphur, like Bonny oil. These sulphuric compounds are called "mercaptans" and cause so-called "fouling": in continuous operation, the sulphur decomposes all metallic objects with which it comes into contact and deposits elsewhere together with soot and rust as plaque on the inside of pipes, e.g. heat exchangers - leading to blockages and leaks. (In 2012, a pipe of the CDU in the Chevron refinery in Richmond, California, burst and caused a serious fire. The carbon steel of the lining had eroded due to sulphur in the sidecut product from the CDU. [55] ) Operators of refineries, therefore, try to eliminate sulphur from their products, even if legal regulations do not stipulate this in this form - for example for kerosene or LPG gas. (1997, during the era of dictator Abacha, "foul fuel" was sold at Mobil and National Oil gas stations in Nigeria. [56] The bad fuel was poisonous for humans and animals and damaged the engines of the cars. Concluding from the description, this was most certainly caused by the mercaptans in the fuel, which had not been removed.) - In the Merox process, the highly corrosive sulphides are converted into the harmless di-sulphides - a process also known as "sweetening" because (at least the larger-molecule) di-sulphides do not have the foul ("rotten eggs"/"rotten vegetables") odour of sulphides (the shorter molecule di-sulphides smell like garlic or onions). Each sulphur atom is thereby bonded to another sulphur atom and thus largely ineffective against metals. Disulphides resemble, in a very simplified way, two snakes (sulphides) that bite into each other, subsequently cannot let go and are safely eliminated together. - This is achieved by adding caustic soda or ammonia. The lye removes a hydrogen atom from the sulphur atom in the sulphide, after which it bonds to a hydrogen-free sulphur atom of another sulphide molecule. Di-sulphide molecules are therefore almost twice as long as sulphide molecules (see image below), have a higher boiling temperature and can thus be easily fractionated out of the product. [57] [58] | | UOP |
| Naphtha Hydrogen Treatment (NHT, "Hydrotreatment" or "Hydrodesulfurization") [59] [60] Input: Naphtha from CDU with sulphur and nitrogen contaminations, hydrogen Output: light naphtha for Penex, heavy naphtha for CCR, ammonia, hydrogen sulphide Purpose: removing sulphur or nitrogen components and olefins from gasoline blends (mainly) | Naphtha is a mixture of short but fluid hydrocarbon molecules (5 to 15 carbon atoms per molecule), some of which have inclusions of sulphur or nitrogen or have undesirable double bonds between two atoms (olefins). Some of it comes from the CDU ("straight-run naphtha") or from other processes in the refinery. In this procedure, naphtha reacts with hydrogen in the presence of a catalyst like cobalt-molybdenum (or nickel-molybdenum for low-sulphur crude oil) at relatively high temperatures and moderate pressures. (Reactor conditions for a naphtha hydrotreater unit are around 205-260˚C with a 25-45 bar pressure. [59] ) NHT converts olefins (like hexylene), nitrogen, oxygen, metals and sulphur compounds into products, which can be utilized in other processes. [61] The main purpose of the NHT is to remove sulphur and nitrogen (see images in the gallery below). In a combustion engine, these fuel contaminations turn into NOx and SOx and are harmful to human health and the environment. Sulphur and nitrogen compounds in naphtha can also deactivate the catalyst in reforming procedures like CCR and therefore must be removed prior to catalytic reforming. NHT consists mainly of a heater, a hydrogen injector, a fixed-bed reactor, and a separator column, in which the desulfurized naphtha and gases like hydrogen and hydrogen sulfide are divided (see image in the gallery below). - The last section of NHT can be a "slide stripper". The slide stripper separates ("strips") light naphtha (pentanes, hexanes) from heavy naphtha. [60] The light naphtha goes to the Penex unit to be isomerized. The slide stripper consists mainly of a stripping column and a reflux drum. [61] Like the RFCC, the NHT is a safety-critical area of a refinery. 2010 the NHT heat exchanger of the Tesoro refinery in Anacortes ruptured, causing an explosion and killing 7 workers. The heat exchanger had suffered long-term effects of "high-temperature hydrogen attack". [62] The maintenance staff had ignored the leaking of the heat exchanger before the explosion, considering it to be "normal". [62] | 1 | UOP |
| Naphtha splitter column | It splits hydrotreated naphtha into light and heavy naphtha, very similar to a distilling column or CDU (see above). [63] While the Dangote refinery lists this column as a separate unit, it probably is identical to the "slide stripper" mentioned in section "Naphtha hydrogen treatment" further up in this table. | | |
| Diesel Hydrotreatment (DHDT) Input: C9...C24, hydrogen Output: C9...C24, hydrogen sulphide Purpose: removing sulphur and nitrogen components from Diesel | Diesel Hydrotreatment (DHDT) removes impurities like sulphur and nitrogen from diesel oil in the presence of a catalyst and hydrogen converting it into diesel fuel. [64] Diesel fuel has 9 to 24 carbon atoms per molecule (C9 ... C24), while petrol has 4 to 12 carbon atoms per molecule (C4...C12). [61] Diesel oil also has a higher sulphur content than naphtha. The process of hydrotreating diesel oils is similar as but much more complex than NHT, primarily due to the addition of the regenerative amine system, which recovers excess hydrogen gas and removes hydrogen sulfide via diethanolamine (DEA). [60] | | |
| SCANfining Input: Sulphur-rich naphtha from the RFCC or MHC Output: naphtha Purpose: removing sulphur components | Naphtha is not only produced during distillation in the CDU ("straight-run naphtha") but also during cracking (RFCC and MHC). This naphtha from cracking is particularly rich in sulphur. To protect the refinery from harmful sulphur compounds and to comply with international quality regulations, they must be removed. The above-mentioned naphtha hydrotreatment is cost-intensive and not very efficient with high sulphur content like in FCC-naptha. This is why the SCANfining process was developed in 1994. It is a low-pressure process and therefore, depending on the existing economic factors, not as capital-intensive as hydrotreatment (NHT). [65] In addition, it does not affect other properties of the naphtha (such as olefin content, nitrogen content, etc.) as much as NHT. It is therefore a more "selective" process for sulphur removal than NHT. [66] SCANfining (Selective Catalytic Naphtha hydrofining) uses an Al2O3 catalyst (RT-225) with a low metals content and high dispersion. | | EMRE (Exxon) |
| LPG Splitter | Propane-Propylene (PP) Splitters transform Liquefied Petroleum Gas (LPG) to the heavier Polymer Grade Propylene (PGP) and the lighter propane by using thermocompression. [67] At a pressure of 11 bar and 34oC, propylene is gaseous and propane is liquid. The mixture is brought to this state in a distillation column, with propylene collecting at the top and propane at the bottom. The derived propylene has a purity of 99.5%, which corresponds to the highest quality level "Polymer Grade Propylene" (PGP). [68] The propane has a purity of over 90%. | | |
| FCCG desulfurization treatment [69] | Flue gas treatment reduces the number of pollutants produced from the combustion of fossil fuels. It can contain pollutants such as particulates, mercury, sulfur dioxide and carbon dioxide. [70] | 1 | EMRE (Exxon) |
| Butamer process [71] [72] | There is only a limited market for butane as it comes from the CDU. Isobutane, on the other hand, is used in the alkylation process to produce alkylate and is therefore a blending component of petrol used at the pump. Isobutane is therefore commercially more interesting than butane. In the Butamer process (Butane isomerization), butane is therefore converted into isobutane. In principle, it is similar to the Penex process, but it involves gases rather than liquids. The Butamer process is a high-efficiency, cost-effective means of isomerizing normal butane (nC4) to isobutane (iC4). [73] | 1 | UOP |
| Sulphur recovery [74] (SRU) | SRU recovers sulfur-containing, poisonous acidic gas. The most commonly used process is the Claus Process, in which the incoming gas burns with oxygen and is refrigerated. This recovers sulfur from the burnt gas. [75] | 2 | Jacobs |
| Hydrogen plant | It usually consists of a Steam Methane Reformer (SMR) and a hydrogen purification system. Preheated natural gas (or Refinery Off Gas) is introduced across a catalyst to produce a 75–80% hydrogen stream, then is puried (by a MEA scrubber or a PSA unit) to produce 99% pure hydrogen. [76] | 2 | Air Liquide |
| Polymerization | Polypropylene and polyethylene are the two most well known commodity plastics produced through the polymerization of propylene/ethylene with catalysts. [77] | 2 | INEOS |
| Gas plant [78] Input: combustible gas from the CDU or RFCC, Output: LPG gas, propane, butane | The gas plant extracts the heavier and more valuable gases out of gas coming from the distillation units and other process units. There are two types of gas plant. A saturated gas plant treats gas streams that contain only saturated hydrocarbons (mainly methane, ethane, propane and butane). An unsaturated gas plant treats gas from cracking units (RFCC, hydrocracking, coker) that contains unsaturated hydrocarbons (olefins / alkenes like propene / propylene and butene / butylene). [79] The gas plant leads the more valuable heavy components back into product blending or to conversion and sends the lighter and less valuable gases to the refinery fuel system where they are burned for heating processes like in the endothermic CCR platforming / reforming. The gas plant consists of a number of process units like: [79] - De-ethanizer - In an absorption tower methane and ethane are separated in a pressurized environment from heavier components, which are absorbed into a naphtha stream. Naphtha before having absorbed the heavier gases is called lean oil, after the absorption it is called fat oil [80] The light gases go to the sponge absorption, the fat oil to the de-butanizer.
- Sponge absorption - The stream of the light components (methane and ethane) coming from the de-ethanizer is fed to another absorption tower where a kerosene stream (sponge oil) extracts lean oil that might have been carried over.
- De-butanizer - Fat oil coming from the de-ethanizer is heated and sent to a fractionation tower which separates the propane and butane from the naphtha.
- De-propanizer - The propane / butane stream from the de-butanizer is sent to a second fractionation tower to separate propane from butane.
- De-isobutanizer - The butane from the de-propanizer is divided in a very tall column into normal butane and isobutane [79]
- Amine treatment - Fuel gas contains acidic gases like hydrogen sulfide and carbon dioxide, hence its name "sour gas". The fuel gases for the refinery are compressed and treated with amine (like DEA / "diethanolamine") which removes hydrogen sulfide and carbon dioxide from the fuel gas. (After the amine treatment the gas is called "sweet gas".) This protects the health of people living around and working in the refinery. [81]
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