THERMAL CONVERSION PROCESSES

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1 5 THERMAL CONVERSION PROCESSES

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3 5.1 Coking Introduction Coking is a thermal cracking process in which a low value residual oil, such as an atmospheric or vacuum residue, is converted into valuable distillate products, off-gas and petroleum coke. Coking allows the refiner to significantly reduce the production of low value fuel oil. Most modern units today are designed and operated to maximize the yield of distillate products and produce fuel grade coke as a by-product. This type of coker represents the majority of coker installations. Some specialized plants, on the other hand, are designed to process special feeds and to produce high value anode grade coke or needle coke. These units are normally of small capacity, particularly the needle cokers. Two different classes of coking processes are implemented commercially: delayed coking and fluid coking. Delayed coking represents the largest combined capacity and is the most widely encountered process. Fluid Coking and Flexicoking, offered by ExxonMobil Research and Engineering, consist of a class of coking processes that are less widely practiced compared to delayed coking. A discussion of Fluid Coking and Flexicoking is provided in section Delayed coking is a semi-continuous process. Though the coking process is continuous, the coke removal, handling and disposal are carried out in a batch manner. The feed is heated to the reaction temperature in a direct fired heater, and subsequently transferred to the coke drums. The coking reaction is delayed until the heated feed is transferred into the coke drums where the residence time is long enough for the coking reactions to go to completion. Coke is deposited in the drum and the cracked vapour product exits the drum from the top, and enters the downstream fractionator. Coke is removed from the drum by taking the drum off-line. In order to achieve near steady state unit operation, the coke drums operate in pairs such that one drum is in filling mode, while the other is off-line for decoking Evolution of the coking process and its role in the refinery The delayed coking process represents a natural evolution from earlier thermal cracking processes. During the late Nineteenth century, refineries employed batch distillation techniques. Since the temperature of the batch still and the residence time were not well controlled, the oil often underwent thermal decomposition. Coke accumulated in the vessel and was removed manually. Later developments included use of multiple stills in series to produce different boiling range products. In this arrangement, the first still was operated at the highest temperature to flash the majority of the crude oil. Coke accumulated in the first still and was removed using manual techniques. During the 1920s, development of continuous distillation processes and improved thermal cracking processes, such as the Burton process, paved the way for the basic delayed coking process. The Burton process, developed by Standard Oil of Indiana, was used to produce gasoline from gas oil. A by-product of this process was petroleum coke. As demand for gasoline in the United States increased and demand for heavy fuel oil (essentially atmospheric reduced crude) decreased, refiners began to utilize the thermal conversion of VOLUME II / REFINING AND PETROCHEMICALS 213

4 THERMAL CONVERSION PROCESSES these residue fractions. Coke drums downstream of the reaction furnace were employed to collect the increased yield of petroleum coke. The delayed coking process was commercially demonstrated by Standard Oil of Indiana at the Whiting refinery in The term delayed was attributed to the fact that the coking reaction is delayed until after the heated feed is transferred into the coke drums where adequate residence time is provided for the coking reactions to reach completion. In the early stages, manual decoking methods were employed. Development of hydraulic decoking methods began in the 1930s and continues to the present day. Early developments included the use of drilling bits and high pressure hydraulic cutting nozzles to remove coke. Using a two-drum system, in which one was filled and the other emptied at the same time, it was possible to operate in a semi-continuous fashion. The growth in demand for motor gasoline from the 1950s through the 1970s saw an increase in the number of delayed coking units constructed that allowed the refiner to convert residual fuel oil stocks into gasoline and gas oil. The gas oil provided an additional feed to the fluid catalytic crackers which had become the predominant gasoline production units in the refinery. Improvements to the delayed coking process are still being made. These improvements are offered by various process licensors and specialized equipment suppliers. Improvements of the mechanical type generally address increased furnace run length, decreased decoking cycle time, improved operator safety and advances to allow for larger diameter coke drums and increased capacity. Process improvements are offered to allow the processing of very heavy residues and to increase liquid yields. Residual conversion refinery The economics of a refinery can be considerably improved by the addition of a residual light naphtha isomerate C 3 -C 4 LPG crude oil heavy naphtha reformate kerosene kerosene/jet fuel diesel gasoline diesel LVGO naphtha diesel jet fuel HVGO gas oil C 3 -C 4 olefins alkylate coker naphtha light gasoline diesel LCGO heavy gasoline HCGO coke light cycle oil slurry oil Fig. 1. Residual conversion refinery based on delayed coking. LVGO, Light Vacuum Gas Oil; HVGO, Heavy Vacuum Gas Oil. 214 ENCYCLOPAEDIA OF HYDROCARBONS

5 COKING conversion unit, such as a delayed coker. The delayed coker converts the vacuum residue into more valuable lighter products and petroleum coke. A schematic diagram of a residual conversion refinery is shown in Fig. 1 (see also Chapter 1.1). The vacuum residue is processed in a delayed coker rather than blending into fuel oil. The coker produces a wide range of products, which must be processed further in the refinery along with the other intermediate streams. The Heavy Coker Gas Oil (HCGO) is hydrotreated as feedstock to the FCC (Fluid Catalytic Cracking) unit while the Light Coker Gas Oil (LCGO) is hydrotreated and blended into the diesel pool. The off-gas and the un-stabilized naphtha are further processed in the vapour recovery unit to produce fuel gas, C 3 -C 4 LPG, and C 5 naphtha products. Petroleum coke is a by-product from the coker unit. The overall yields from a residual conversion refinery that processes a blend of 50:50 Arabian Light and Arabian Heavy compared to the yields from a refinery without a coker are presented in Table 1. Coker design The design features of the coker vary depending on the type of coke to be produced. Cokers producing anode coke are usually subjected to more severe temperature and pressure conditions. They typically include smaller diameter coke drums, high-pressure jet pumps, etc. Cokers producing needle coke operate at even higher pressures and temperatures. In addition, the recycle amount is also generally high (typically greater than 50%). Current designs are mostly fuel grade cokers, designed to operate at low pressures and low recycle ratios in order to maximize liquid product yields Process chemistry Coking reactions Delayed coking is a thermal cracking process, with complete conversion of the vacuum residue to solid petroleum coke and hydrocarbon products that are lighter than the feed. Petroleum coke does not consist of a single, simple chemical compound, nor is it a form of pure elemental carbon, although it approaches the latter (SRI Consulting, 1971; Ballard et al., 1981). It can be described as an impure mixture of elemental carbon and hydrogen compounds, in which the carbon to hydrogen ratio is very high, often in excess of 20 by weight. The ratio increases to well over 1,000 when the coke is calcined. Table 1. Overall refinery yields based on 50:50 Light Arabian and Heavy Arabian crude mix (100,000 bbl/d crude rate) Overall refinery yields (without coker) Overall refinery yields (with coker) Product (liquid volume %) (liquid volume %) C 3 components C 4 components Gasoline Jet fuel Diesel Fuel oil Coke (t/d) 1,075 Sulphur (t/d) During the coking process, many different chemical reactions occur simultaneously. Thus, a precise explanation of the reaction mechanism is difficult. The principal reactions can be summarized as follows: Decomposition of large molecules into smaller molecules, including free radicals. Free radicals, which are highly reactive and short-lived species, react with other hydrocarbons, combine with other free radicals resulting in termination, or decompose further to olefins and smaller radicals, and so on. Thermal cracking of heavy stocks proceeds stepwise through a series of progressively lower molecular-weight products, for example, heavy gas oil to light gas oil to gasoline to gas with reactions occurring simultaneously. The other secondary reactions occurring in coking are polymerization and condensation. The decomposition and polymerization reactions result in the formation of polycondensed aromatic compounds. When these planar compounds rearrange and become stacked in a fixed direction, the state is called the mesophase (or liquid crystal state). With further heating and increased interfacial forces, mesophase spheres form and grow into droplets dispersed in the oil. The spheres continue to grow and coalesce into bulk mesophase. Further heating results either in mosaic or fibrous coke formation. The coke structure can be related to the chemical and molecular VOLUME II / REFINING AND PETROCHEMICALS 215

6 THERMAL CONVERSION PROCESSES composition of the feedstocks. The most critical operational factors are the temperature, residence time and gas-flow rate. In summary, the reaction mechanism of delayed coking is complex, but three distinct steps occur in the entire coking process: in the furnace, the feed is partially vaporized and mildly cracked; in the coke drum, the vapours crack as they pass through the drum; and the heavy liquid feed, given the temperature and residence time in the coke drum, is simultaneously cracked to vapour, polymerized and condensed to coke. Structure of petroleum coke Numerous studies have been performed to explain the formation and the structure of a coke. Petroleum cokes have an unordered crystalline structure. In contrast to resin and asphaltene molecules, the structural model of the carbenes and carboids that constitute most of petroleum coke probably consist, not of individual members capable of being broken up on heating, but of condensed polycyclic nuclei having short methyl and other side chains. Heterocompounds (with O, S, N, etc.) may be present in both the side chains and the ring structures. The dimensions and ordering of the coke crystals are the principal factors in determining its physical properties (thermal conductivity, electrical conductivity, density, etc.), while the type of side chains ( CH 3, SH, H) influences its chemical reactivity. In addition, coke contains 2-10% of adsorbed intermediate decomposition products, which have an effect on coke calcining technology. Needle coke formation mainly involves the polymerization and condensation of the aromatics to such a degree that coke is eventually formed. The coke produced is more crystalline in appearance than asphaltic coke obtained from conventional vacuum residues. Heterocompounds are present in very small amounts and play only a small role in the coking mechanism Coking processes Delayed coking Delayed coking represents the largest number of commercial coking units. A simplified flow diagram of the delayed coking process is shown in Fig. 2. Main process sections The delayed coking flow scheme can be divided into the following sections: a) coking; b) fractionation; c) vapour recovery; d) closed blowdown; and e) coke removal. Coking Fresh feed (residue) is delivered into the bottom of the fractionator where it is mixed with the Fig. 2. Simplified delayed coking flow scheme. BFW, Boiler Feed Water. vapours to recovery slop oil make-up water water water coke drums fractionation tower steam sour water off-gas LPG naphtha LCGO HCGO steam BFW steam pumparound coke residue feed coker heating BFW 216 ENCYCLOPAEDIA OF HYDROCARBONS

7 COKING recycle material from the wash section. The total feed is then pumped from the fractionator bottom to the coker heater. In the heater, the feed is rapidly heated to the coking temperature and sent to the coke drums. Coking reactions occur in the coke drum producing coke and light hydrocarbon vapours. Coke accumulates in the coke drums as it forms over the period of the coking cycle, while the hydrocarbon vapours exit the drum at the top and are sent to the bottom of the fractionator. Gas oil is injected into the coke drum overhead line to quench the product vapours and minimize coking in the line. High pressure steam or condensate is injected into the heater tubes in order to maintain a minimum fluid velocity, reduce residence time and thus minimize coking in the heater tubes. Fractionation The most conventional fractionator design has shed decks above the feed zone with a trayed wash section immediately above the decks. The coke drum vapours pass through the shed decks, wash section and enter the gas oil fractionation section where a circulating gas oil pumparound is used to remove heat and to condense the gas oil vapours. With low recycle designs, the internals near the bottom of the tower are minimized due to the potential for coking deriving from the low wash oil rates. An open spray chamber design is commonly employed in such designs. Heavy Coker Gas Oil (HCGO) is withdrawn as a total draw-off. A portion of the coker gas oil is pumped back to the wash section below as wash oil. The heavier portion of the coke-drum vapours condenses in the wash section to form the recycle, which is mixed with the fresh feed and returned to the heater. The coker gas oil pumparound heat is typically used to preheat fresh feed, provide reboil heat in the vapour recovery towers and to generate steam. The next side-draw product, Light Coker Gas Oil (LCGO), is steam stripped in a side stripper to remove the light ends, cooled and sent to storage. A portion of the unstripped LCGO is used as lean sponge oil in the secondary absorber of the vapour recovery section. Rich sponge oil is returned to the fractionator for recovery of the absorbed hydrocarbons. The fractionator overhead vapours are partially condensed in an overhead condenser. The uncondensed vapour is separated in the overhead drum and sent to the vapour recovery section for LPG (Liquefied Petroleum Gas) recovery. A part of the condensed liquid is returned as reflux to the top of the fractionator and the remaining overhead liquid is sent to the vapour recovery section for stabilization. Sour water collected in the overhead drum is sent off-site for treating. Vapour recovery A simplified flow scheme of the vapour recovery is shown in Fig. 3. The fractionator wash water fractionator overhead liquid sponge absorber fuel gas to treating light gas oil rich light gas oil fuel gas fractionator vapour wet gas compressor sour water primary absorber LPG to treating wash water stripper debutanizer sour water naphtha to hydrotreating Fig. 3. Vapour recovery flow scheme. VOLUME II / REFINING AND PETROCHEMICALS 217

8 THERMAL CONVERSION PROCESSES overhead vapour is compressed and cooled, and the resulting vapour and liquid streams are fed to an absorber-stripper. The vapour is fed to the bottom of the absorber while the liquid is fed to the top of the stripper. The fractionator overhead liquid stream is introduced into the top of the absorber as lean oil. Normally, this lean oil is insufficient to achieve the desired LPG recovery, and therefore a portion of the stabilized naphtha from the downstream debutanizer is cooled and recycled to the top of the absorber as supplemental lean oil. The bottoms from the stripper, containing C 3 s and heavier, flows to the debutanizer where LPG is recovered as an overhead liquid product, and C 5 as bottom product. The C 5 product is cooled and sent to storage. The overhead C 3 -C 4 LPG is further treated to remove sulphur compounds, including hydrogen sulphide, mercaptans, etc., and sent for further processing. The overhead gas from the absorber containing mostly C 2 and lighter and some unrecovered C 3 s is fed to the bottom of the sponge absorber where it comes into contact with lean sponge oil. Any C 5 and heavier hydrocarbons present in the absorber off-gas are recovered in the sponge absorber and returned to the fractionator as rich sponge oil. The sponge absorber overhead off-gas is finally treated with an amine solution to remove hydrogen sulphide before discharging into the refinery fuel gas system. Closed blowdown The closed blowdown system, shown in Fig. 4, is used to separate and recover hydrocarbon and steam vapours generated during the coke drum steaming and cooling operations. The coke drum blowdown vapours are condensed in the blowdown scrubber by contact with circulating oil drawn from the bottom of the blowdown scrubber. The uncondensed vapour, mostly steam and light hydrocarbon vapours, is condensed in the overhead condenser before entering the blowdown drum. In the blowdown drum, light oil is separated from the steam condensate and pumped to the refinery slops system, while the recovered water is pumped off-site, initially for further treating in a sour water stripper and later sent to the clear water tank for reuse in coke cutting. The vent gas from the blowdown water separator is returned to the wet gas compressor or other suitable refinery hydrocarbon recovery systems. The net bottoms from the blowdown scrubber, containing wax tailings, are removed and returned either to the fractionator or sent to the refinery slops system. Coke removal The coke drum filled with coke is taken offline, steam stripped, and quenched by water. The vapours generated during steaming and quenching are routed to the blowdown scrubber for Fig. 4. Closed blowdown system. condenser blowdown drum to vapour recovery coke drum blowdown LCGO make-up blowdown scrubber slop oil tank water make-up to sour water flash drum coke handling return water slop oil to reprocessing heavy oil to fractionator jet water to coke drums quench water to coke drums quench water tank 218 ENCYCLOPAEDIA OF HYDROCARBONS

9 COKING Fig. 5. Single-fired vs. double-fired coker heater. single-fired type peak flux average 1.8 double-fired type peak flux average 1.2 outlet outlet old burners bushy flames outlet outlet modern low NO x burners long, thin flames hydrocarbon and steam recovery. Coke is removed from the drum by hydraulic decoking. Various coke handling methods are in use, including coke pit loading, coke pad loading, direct rail car and hydraulic coke handling. The most commonly used methods are coke pit and coke pad handling. Major design considerations Coker heater The coker heater provides the necessary heat to the feed in order to reach the coking reaction temperature. There are two principal types of heater design in use today: single-fired heaters or a double-fired heaters (Fig. 5). In modern cokers the double-fired heater designs are mainly used, in which the heat input is from both sides of the tube. This arrangement allows higher average heat flux, resulting in lower peak temperatures and shorter residence time. Heater tube metallurgy is also being enhanced. New designs employ steel alloys containing 9% Cr and 347 SS tubes, which permit higher skin temperatures and allow longer run lengths to be achieved. The cold-oil velocities vary from 1.8 to 2.4 m/s, and the average radiant heat flux is approximately 43,000 W/m 2. Steam injection in the radiant section, particularly when processing heavy feeds, is common in modern cokers. Steam is injected in order to increase the fluid velocity, thereby greatly reducing the residence time and the potential for coking in the heater tubes. Coke drum Coke drum sizing is governed by the superficial vapour velocity, cycle time and the allowable outage. The vapour velocity typically determines the drum diameter, while the cycle time sets the drum volume. The allowable drum vapour velocity is a function of the vapour density and the foaming tendency of the feedstock. Typical drum vapour velocities are in the range of 0.1 to 0.2 m/s, although some units run at velocities higher than 0.2 m/s. The drum outage, which is the disengaging height between the top tangent line and the maximum coke level in the drum, is typically in the range of 4 to 6 m. The actual outage is determined based on the type and origin of feedstock, its foaming tendency and the operating conditions. The foaming in the drum is controlled by the addition of anti-foam chemicals (generally as a mixture with a distillate fluid) during the last few hours of the fill cycle. The coke drum level, which is indicative of the progress of coking in the drum and preparation for the drum switching, is monitored by a nuclear backscatter level instrument mounted on the outside of the coke drums. These are also used to detect the foam levels as the drum fills up. In the past, cokers were designed with coking cycle times of 20 to 24 hours (overall drum cycle of hours). Modern designs and retrofits use shorter cycle time of the order of hours. The cycle time schedule sets the total volume required for the coke drum, which, for a given drum diameter, essentially sets the overall dimensions for the coke drum. Coke drums of about 9 m diameter are currently in commercial use. The coke drum shell is fabricated from alloy steel (typically 1-Cr and 0.5- Mo) with a stainless steel (410S, Cr) cladding. A switch valve, usually a four-way ball valve, located at the drum inlet is used to switch the feed between the drums. The switch valve also allows the drum to be bypassed during unit start-up and shutdown. VOLUME II / REFINING AND PETROCHEMICALS 219

10 THERMAL CONVERSION PROCESSES Coker fractionator The coker fractionator separates the coke drum vapours into various products, including wet gas, gasoline, LCGO, and HCGO. The bottom section of the fractionator, up to the HCGO draw-off pan, is highly prone to coking due to the entrained coke particles as well as the high-temperature vapourphase coking. Modern designs therefore minimize tower internals at the bottom of the fractionating tower, with some designs using an open spray chamber below the HCGO pan. A slotted standpipe in the bottom of the fractionator is used for collecting the coke particles and to provide passage to the heater charge pump. Also, a separate coke removal system, consisting of a circulating pump and coke filters, is employed in order to remove the accumulated coke from the bottom of the fractionator to minimize unit downtime. Coker recycle Coker recycle is one of the key operating variables in a coker to control the HCGO end point as well as to reduce the coking propensity of the heavy feed in the heater tubes. Recycle is produced at the fractionator bottom by condensing the heavier portion of the coker gas oil, which is then mixed with the fresh feed and sent to the heater. Higher recycle produces more coke at the expense of gas oil yields, however the HCGO end point decreases, and other impurities like Conradson Carbon Residue (CCR) and metals are also reduced. In fuel grade cokers, where maximizing liquid yields is the primary objective, low to ultra-low recycles are used. Cokers with recycle less than 5% are considered ultra-low recycle operations. Other coking processes Delayed coking represents the largest number of commercially practised coking units. Fluidized coking processes are a specialized class of coking processes that consume part of the coke produced to supply the necessary endothermic heat of reaction for thermal cracking. The Fluid Coking and Flexicoking processes, licensed by ExxonMobil Research and Engineering (EMRE), are in commercial use. Detailed information on these technologies and their applications can be found in an article by EMRE (Hammond et al., 2003). Fluid Coking A generic flow diagram of the Fluid Coking process is shown in Fig. 6. This figure and the description that follows apply to the Fluid Coking reactor system only. The fractionation, vapour recovery and coke-handling systems are similar to those used in the delayed coking process. The reaction section entails two primary vessels: a coking reactor and a heater. A scrubber, located on top of the reactor, preheats the fresh feed, cools the reactor effluent vapours, removes coke particles entrained by the vapours, and condenses the heavy recycle stream. The hydrocarbon conversion reactions occur in the reactor. Feed enters the reactor into the fluidized bed of coke. Stripping steam is injected at the bottom of the reactor, and reaction product vapours fluidize the bed as they rise toward the reactor cyclone and scrubber. New coke produced from the cracking reactions is deposited on the coke particles in the reactor bed. In product to fractionator flue gas to CO boiler Blowdown scrubber The blowdown scrubber recovers and provides primary separation of the hydrocarbons and the steam that are generated during the coke drum steaming and cooling operations. The blowdown system includes a blowdown scrubber, overhead condenser, water separator, circulating oil cooler, bottom heater and the associated pumps. The drum blowdown temperature varies from a maximum of 450 C at the start of the cooling cycle to about 150 C near the end of the cycle. Below 150 C, the drum effluent bypasses the scrubber and is sent directly to the blowdown overhead condenser. A demulsifying agent is added to the blowdown overhead water separator to aid the oil/water separation. Another significant function of the blowdown system is to handle the coke drum emergency relief discharge during any drum overpressure event. 220 feed net coke hot coke air steam cold coke Fig. 6. Simplified Fluid Coking scheme. ENCYCLOPAEDIA OF HYDROCARBONS

11 COKING order to supply heat and maintain reactor temperature, hot fluidized coke is circulated from the heater to the top of the reactor through the hot coke line. To provide bed level control in the reactor, cold coke from the bottom of the reactor stripper section is circulated back to the heater through the cold coke line. Each of the transfer lines consists of a standpipe, a sharp angle bend, an angle riser, and a vertical riser. The heater normally operates at about ºC and slightly above atmospheric pressure. The cold coke from the reactor is heated by direct contact with hot gas. Heat required to support the coking reaction is obtained by partially combusting a portion of the gross coke produced in the reactor. The flue gas, which contains mostly carbon monoxide (CO), sulphur oxides (SOx) and inerts, can be utilized in a CO-boiler. The net coke produced by the process exits the heater. Flexicoking The objective of the Flexicoking process is to further reduce the amount of net coke produced in the reactor by utilizing a gasifier to convert the net coke to a synthetic gas. The gasifier is also used to heat the circulating coke and supply the heat required for the coking reaction. A flow diagram showing the Flexicoking process is provided in Fig. 7. The Flexicoking process produces a large quantity of low heating-value gas. This gas is normally cooled, treated to remove coke fines and processed to remove H2S. The gas can be utilized in burners designed to handle low heating-value gas (approx MJ/m3). Table 2. Typical yields from Fluid Coking and Flexicoking (4.4 API and 24.4 wt% carbon residue feedstock) Product yields Fluid Coking Flexicoking Gross coke (wt%) Net coke (wt%) Butanes and lighter (wt%) C5-510 C (vol%) Low BTU gas (FOE-vol%) FOE, Fuel Oil Equivalent All coking processes are severe thermal conversion processes and accomplish that conversion using much the same reaction mechanisms. The gross coke quantities produced from delayed coking, Fluid Coking and Flexicoking processes are similar. The heat input required to achieve thermal conversion is also similar among the processes. Fluid Coking consumes about 20% of the gross coke produced to supply heat required for the coking reaction. The Flexicoking process consumes additional coke to produce a synthetic gas product. About 90-97% of the gross coke produced is consumed by the Flexicoking process. Typical yields for processing an Arabian vacuum residue provided by the process licensor (ExxonMobil Research and Engineering) are shown in Table 2. The yield of liquid product and gross coke production are the same. The net coke Fig. 7. Simplified low BTU gas Flexicoking scheme. product to fractionator steam Venturi scrubber tertiary cyclones feed dry coke fines ejector wet coke fines purge coke steam air VOLUME II / REFINING AND PETROCHEMICALS steam 221

12 THERMAL CONVERSION PROCESSES produced using the Flexicoking process for the same feedstock is substantially reduced compared to the Fluid Coking process. Capital costs for a fluid coker are approximately the same as those for a delayed coker. On the other hand, the capital costs for a flexicoker are greater (30-40%) than for a fluid coker because of the need for an additional gasifier vessel, gas clean-up and the requirement for a larger air blower. The major utility cost for a fluid coker or flexicoker is associated with the air blower Process variables This section provides a description of coker feedstocks, product yields and quality of the various coker products and the variables that affect the yields and product qualities. Feedstocks Delayed cokers can process practically any heavy oil material in the refinery. While the typical feedstock to a coker is a straight-run vacuum residue, a variety of other refinery residual feedstocks and intermediate products can also be processed in the delayed coker. The ability of a coker to handle a variety of feedstocks is demonstrated by the range of the gravity ( 5 to 15 API) and carbon residue (4 to 40 wt%) of the materials it can process. The feedstocks to a coker can be classified into the following main categories: Straight-run residual material, such as the atmospheric and vacuum tower bottoms, from the distillation processes and asphaltenes produced from deasphalting. Heavy aromatic stocks such as the decant or slurry oil produced from FCC units, thermal tars from thermal cracking units, aromatic extracts from lube operations, and pyrolysis tars from ethylene plants. Other materials such as visbroken tars, slop oils, tank sludge bottoms, and coal tar pitches, etc. The above feedstock classification leads to different qualities of the by-product coke produced. In addition to the origin and upstream treatment, the feedstock properties that affect the yields and product quality are: the specific gravity, CCR, and the content of sulphur, metals and asphaltenes. These properties determine the quality of coke produced as well as the entire slate of the coker products. The properties of some typical feedstocks are summarized in Table 3. Operating variables The three primary operating variables that affect product yield and quality are: coke drum pressure, recycle ratio, and coke drum temperature. The operating conditions are selected depending on the feedstock quality and the process objectives. The conditions vary significantly between the three types of coking operations, depending on the overall economic objectives. Coke drum pressure The reference pressure at which coking reactions take place is generally considered to be the operating pressure at the top of the coke drum. The pressure is actually controlled at the reflux drum near the top of the coker fractionator. Increasing coke drum (coking) pressure increases coke yields, reduces liquid yields and reduces the gas oil end point. Increasing coke drum pressure also increases gas and gasoline yields. Table 3. Typical coker feedstock characteristics Anode coke Fuel grade coke Needle coke Feedstock Vacuum residue Vacuum residue Slurry oil Thermal tar Feedstock source African crude 50:50 Light/Heavy Arabian Mix FCC Thermal cracker Specific gravity at 15 C API gravity Conradson carbon (wt%) Sulphur (wt%) Vanadium (ppm) Nickel (ppm) ENCYCLOPAEDIA OF HYDROCARBONS

13 COKING Table 4. Effect of low pressure and low recycle on coking yields (20.5% carbon residue feedstock) Past designs Current trend Drum pressure (bar) Recycle ratio (vol%) 10 5 Coke yield (wt%) C 5 plus liquid yield (vol%) In old designs, drum operating pressures of 2 bar were common for sponge coke production, while today, cokers are being designed and revamped to operate at pressures as low as 1 bar. Table 4 shows the effects of drum operating pressure and recycle on delayed coker yields at a given drum temperature (Sloan et al., 1992; Bansal et al., 1993). With heavier feeds containing high CCR and asphaltenes, reducing the drum pressure and recycle can potentially contribute to shot coke formation. To produce anode grade or needle coke, higher operating pressures are employed and generally justified because of the higher value coke produced from those units. While anode cokers are typically limited to 2 to 3 bar pressures, it is not uncommon to operate needle cokers at much higher pressures, 4 to 6 bar. Recycle ratio The recycle ratio represents the amount of recycle material (typically 540 C plus) produced at the bottom of the coker fractionator and recycled back (along with the fresh feed) to the heater and coke drum for additional cracking. As the recycle ratio is increased, the resulting effects are an increased coke yield, reduced liquid yield, and a lower gas oil end point. The higher coke yield leads to more gas and gasoline yields also. Higher recycle rates produce cleaner HCGO with lower end point, carbon residue and metals. In the past, conventional cokers were designed with recycle ratios of 10 to 15% in order to produce cleaner gas oils, as limited by the capability of the downstream units to handle contaminants. Today, with advances in hydrotreating, hydrocracking and fluidized catalytic cracking technologies, greater amounts of HCGO contaminants can be tolerated and cokers are designed with recycle ratios of 5% or lower, with many operated at recycle rates of 2 to 3%. When producing anode or needle grade coke, higher recycle ratios are commonly used and justified because of the higher value coke produced. While anode cokers are typically limited to recycle ratios of 25 to 30%, needle cokers often run with recycle ratios of 50 to 80%. Coke drum temperature Coke drum temperature is a key operating variable in a delayed coker. Although the drum temperature is not directly controlled, the narrow range of furnace outlet temperatures at which a given feed must be run is critical for the smooth operation of the unit and the maintenance of reasonable furnace run lengths. Too low a furnace outlet temperature leads to incomplete coking reactions in the drum, which results in the production of soft coke. A very high temperature, on the other hand, produces a hard coke that would be difficult to remove from the coke drum. A higher drum temperature can only be achieved through a higher furnace outlet temperature, which could result in excessive furnace tube coking and the need for frequent tube cleaning. Also, the drum overhead system (overhead piping, valves up to the quench point) may be subject to excessive coking which could lead to increased unit downtime. The optimum temperature at which the drum must be operated for a given feed is a compromise between the yield benefits, plant operability and hardware limitations. With anode or needle grade coke production, higher drum temperatures are required to produce a coke with the required properties. While drum temperatures for anode cokers are only slightly higher (5 C), needle cokers typically run at much higher drum temperatures, in the 450 to 460 C range. It should be recalled that fuel grade cokers typically run at a drum temperature of about C. With these key operating variables in mind, the range of coker operating conditions can vary significantly, as shown in Table 5. Table 5. Range of coker operating conditions Fuel grade Anode coke Needle coke Drum pressure (bar) Recycle ratio (vol%) Drum temperature ( C) VOLUME II / REFINING AND PETROCHEMICALS 223

14 THERMAL CONVERSION PROCESSES Table 6. Typical delayed coker yields Anode coke Fuel grade coke Needle coke Feedstock Vacuum Residue Vacuum Residue Slurry Oil Thermal Tar Feedstock source African crude Arabian mix FCC Thermal Cracker Yields (wt%) Dry gas C 3 -C 4 components Gasoline (C C) LCGO HCGO Coke Total Table 7. Delayed coker product properties based on 50:50 light Arabian and heavy Arabian crude mix Coker naphtha LCGO HCGO Specific gravity at 15 C API gravity Sulphur (wt%) Nitrogen (wt%) Bromine number Cetane index 40 RON 80 Carbon residue (wt%) 0.3 PONAs (vol%) Paraffins 45.0 Olefins 30.0 Naphthenes 10.0 Aromatics 8.0 RON, Research Octane Number; PONAs, Paraffin Olefin Naphtenes Aromatics Product yields Typical coker yields for conventional residual oils as well as those for needle coker feedstocks are summarized in Table 6. Included in the table are the following yield cases: low sulphur and low CCR residue feedstocks suitable for anode grade production; high sulphur and high CCR residue feed with high metals that produce a fuel grade coke; and highly aromatic feedstocks that produce needle coke. These yield estimates are developed using the KBR (Kellogg Brown and Root) yield models. Coker product properties Coker products are set primarily by the refinery product slate, product specifications, and the ability of the refinery process units to handle their further processing. The estimated properties for various coker products are summarized in Table 7 for the Arabian crude feed blend. The product treatment steps and the end usage are summarized in Table 8. In general, all coker products are highly olefinic. The bromine number, which is indicative of the degree of olefinicity, ranges between 10 and 70. The sulphur and nitrogen are distributed among the various products with coke retaining a major portion of the feed sulphur and nitrogen. Essentially all feed metals are retained by the coke. 224 ENCYCLOPAEDIA OF HYDROCARBONS

15 COKING Table 8. Coker product treating steps and end use Product Treating step End use C 3 -C 4 olefins LPG Mercaptan extraction Alkylation feed Light naphtha Mercaptan extraction Gasoline blending Isomerization feed Heavy naphtha Hydrodesulphurization Reforming feed Gasoline blending LCGO Hydrodesulphurization Diesel blending HCGO Hydrodesulphurization Hydrocracking FCC feed Coker gas includes hydrocarbons such as methane, ethane, and ethylene. Also, a small amount of hydrogen produced is present in the dry gas. This gas is usually produced as wet gas from the gas separator, containing most of the coker LPG and some heavier hydrocarbons and must be processed in a Vapour Recovery Unit (VRU) to recover LPG and a gasoline product. Coker gasoline is recovered and stabilized in the VRU and sent for hydrotreating and, ultimately, for catalytic reforming for octane improvement before blending with the finished gasoline product. Light coker gas oil is typically blended with other diesel range materials from other refinery units and sent for further hydrotreating for low sulphur ( 500 ppm) or ultra-low sulphur ( 10 ppm) diesel production. Heavy coker gas oil is also sent for hydrotreating or hydrocracking along with the virgin vacuum gas oil and is ultimately sent as feed to the FCC unit. Coke quality The coke quality not only determines the unit economics but also influences its operability, operations, reliability, maintenance and safety. Typical coke properties are summarized in Table 9. There are essentially three grades of coke currently being produced in the petroleum industry: regular grade sponge coke, widely used in the aluminium industry for the manufacture of electrodes (also called anode coke); high grade needle coke, a premium coke used to manufacture electrodes for the steel industry; and fuel grade coke, used primarily in power and cement plants as fuel. Regular (anode) coke Many delayed cokers produce a regular grade coke also known as anode coke. This coke has a sponge-like structure, is porous and exhibits structural consistency. Typically, anode grade coke has less than 3 wt% sulphur and no more than 350 ppm by weight total metals. The anode coke is generally produced from paraffinic or asphaltic materials. The quality of coke produced can vary significantly depending on the residue feed being processed. Generally, the feed sulphur and metals content need to be sufficiently low so that the produced coke can meet the desired specifications. Typical specifications for the anode coke are shown in Table 10. Table 9. Typical coke properties Property Value Sulphur (wt%) 7.0 Nitrogen (ppm) 6,000 Volatile material (wt%) Vanadium (ppm) 141 Nickel (ppm) 489 Bulk density (kg/m 3 ) 880 Table 10. Typical anode coke specifications Specification (wt%) Green coke Calcined coke Moisture Volatile Combustible Material (VCM) Sulphur Silicon Iron Nickel Ash Vanadium Bulk density (kg/m 3 ) Real density (g/cm 3 ) 2.06 VOLUME II / REFINING AND PETROCHEMICALS 225

16 THERMAL CONVERSION PROCESSES Needle coke This form of coke is the most valuable of all the various petroleum cokes produced. It is used primarily for the production of electrodes for the steel industry (electric arc furnaces). The coke is uniquely characterized by properties such as its low sulphur and metals content, low Coefficient of Thermal Expansion (CTE), its needle like crystalline structure and high electrical conductivity. All premium needle cokes have a low CTE. Typical needle coke specifications are shown in Table 11. Needle coke production requires a special feedstock, which is typically high in aromatics, and low in asphaltenes, sulphur and metals. In addition, the coker unit must be operated at conditions that will provide the best premium needle coke quality. Typical feedstocks include slurry or decant oil from FCC units or thermal tars from gas oil thermal cracking units. In addition, aromatic extracts from lube operations, pyrolysis tars from ethylene plants and some coal tar pitches are considered potential coker feeds for needle coke production. Fuel grade coke With the current trend in refineries to process heavy crude oils, the industry continues to see a major shift in terms of coke quality. Many new and existing cokers have switched to processing heavy crude oils in order to achieve improved refining economic margins. This change results in the production of poorer quality coke that is not suitable for anode production. Due to very high sulphur, metals and other impurities present in the heavy feedstocks, the coke produced is only suitable for fuel purposes, either in power plants or the cement industry. This coke is thus referred to as fuel grade Support process operations The support (auxiliary) operations include some of the key mechanical operations associated with the delayed cokers and the coke calcining processes: Decoking operation (removal of coke from coke drums). Automated drum unheading (removal of the top and bottom heads). Hydraulic decoking by high pressure water jets. Coke receiving and water drainage. Quench water management. Coke calcining. In the following section we will briefly describe the coke calcining process. The other operations are essentially mechanical and are too specific for the purposes of this work. Coke calcining Petroleum coke (green coke), either anode or needle coke, is calcined in rotary kilns; such processes are frequently performed outside the petroleum refinery. The characteristics of the calcined coke depend primarily on the properties of the green coke fed to the calciner as well as the major operating variables, such as rate of heating, hot zone (calcining) temperature, residence time and rate of cooling. Calcined anode coke is used mostly in the manufacture of anodes for the aluminium industry. Consumption of calcined coke in specific user industries varies considerably as shown in Table 12. Typical specifications of green coke vs. calcined coke are presented in Table 10 and Table 11. In a typical coke calcining plant, the green coke is calcined in a rotary kiln. Process heat is supplied through a fuel burner. Another source of the process heat is the volatiles which are released in the kiln. Cokes with varying amounts of the volatile matter can be burned in the kiln. From the kiln, the Table 11. Typical needle coke specifications Specification (wt%) Green coke Calcined coke Moisture Volatile Combustible Material (VCM) Sulphur Ash Bulk density (kg/m 3 ) CTE ( C) Real density (g/cm 3 ) 2.11 Table 12. Usage of calcined coke User industry Calcined coke usage (kg/kg) Aluminium 0.5 Silicone carbide 1.4 Phosphorous 1.8 Calcium carbide 0.69 Graphite ENCYCLOPAEDIA OF HYDROCARBONS

17 COKING calcined coke is discharged into a rotary cooler where it is quenched with direct water sprays at the cooler inlet. Additional cooling is accomplished by pulling a stream of ambient air through the cooler. From the discharge of the cooler, the calcined coke is conveyed to storage silos. References Ballard W.P. et al. (1981) Thermal cracking, in: McKetta J.J. (editor in chief) Encyclopaedia of chemical processing and design, New York, Marcel Dekker, ; v. XIII. Bansal B.B. et al. (1993) Design and economics for low pressure delayed coking, in: Proceedings of the National Petroleum Refiners Association annual meeting, San Antonio (TX), March. Hammond D.G. et al. (2003) Review of fluid coking and flexicoking technologies, in: Proceedings of the American Institute of Chemical Engineers Spring national meeting, New Orleans (LA), 30 March-3 April. Sloan H.D. et al. (1992) Delayed coking has a role in clean fuels environment, «Fuels Reformulation», July-August. SRI Consulting (1971) Petroleum coke, Process Economics Program Report 72. Bharat B. Bansal Joseph A. Fruchtbaum Aldrich H. Northup Rao Uppala Kellogg, Brown & Root Houston, Texas, USA VOLUME II / REFINING AND PETROCHEMICALS 227

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