Coking

Abstract

Currently, conventional oil is used as the main source for the petrochemical industry. However, conventional oil’s capacity is declining, and that source will probably be exhausted in the near future. Heavy oil and petroleum residues have become a suitable alternative source to meet global energy demand. However, heavy oil and oil residues require many upgrading processes before turning to be valuable products. Among the various upgrading processes, delayed coking, which is capable of processing any residue at a low investment cost, garnered tremendous importance. Petroleum coke is one of the coking products that is divided into three types: shot coke, sponge coke and needle coke, depending on the feed properties and operating conditions of the process. Needle coke is used as a valuable product in the production of graphite electrodes used in electric arc furnace (EAF) for melting scrap metal and producing steel.

1. Introduction

The petroleum industry provides most of the world’s energy needs and has been the world’s most important energy source since the mid-1950s because of its high energy density, easy transportability and relative abundance [1]. Due to rapid population growth, the consumption of fuels, energy, and petrochemical products has increased sharply [2]. At present, light crude oil reserves are the main source of energy that meets global energy demand due to high quality and low production costs. Nevertheless, light crude oil reserves are declining. Such a rapid decline in light crude oil reserves poses great challenges to meeting the world’s energy needs. Over the past few decades, renewable, nuclear and bioenergy have been developing rapidly; however, these resources are costly and insufficient in meeting energy demands, especially for transportation [3]. Therefore, refineries have to depend increasingly on unconventional feedstocks such as heavy oils, oil residues, and bitumen to supply the increasing demand for fuels [1]. The fundamental characteristics of heavy crude oil are low American Petroleum Institute gravity (API), low economic value, high viscosity, and high asphaltenes content which makes it more difficult to transport and process than conventional crude oil [4]. This fact leads to an emphasis on the upgrading of heavy and residual oil. The purpose of upgrading heavy oil and residues is to convert feedstock with high boiling point and low H/C ratio to low boiling point distillate fractions and higher H/C ratio and to eliminate hetero atoms such as sulphur, nitrogen, and metals to Environmentally acceptable levels. To achieve this goal, hydrocarbon molecules are exposed to thermal and catalytic cracking reactions during the upgrading processes [5]. According to the approaches to achieve higher H/C ratios, upgrading technologies can be divided into carbon rejection and hydrogen addition processes. Carbon rejection rejects the carbon into carbonaceous product (coke) to obtain lighter products (with a high H/C ratio) in these processes. On the other hand, hydrogen addition processes such as hydrocracking involve the reaction of raw materials with an external source of hydrogen in the presence of a catalyst, which leads to an overall increase in the H/C ratio [6]. Hydrogen addition processes have higher quality and yield of desired products. However, these processes require the participation of hydrogen and catalysts, which leads to higher investment and operating costs compared to carbon rejection processes. In contrast, carbon rejection processes are superior to hydrogen addition processes in terms of simplicity and operating costs, and therefore have many units in the world [7, 8]. Petroleum residues processing capacity indicates that the major portion (approximately 63 wt.%) of petroleum residues are upgraded by thermal processes such as visbreaking and delayed coking [2].

2. Carbon rejection processes

Carbon rejection technologies have been used by refineries since 1913 to upgrade various hydrocarbon feeds. These technologies include visbreaking, gasification, and coking processes. visbreaking and coking technologies can be generally applied to all residual feeds because they are not limited to constraints such as metal content and coke-forming tendencies as in the case of catalytic processes for upgrading. In a carbon rejection process, the feeds (larger molecules) are heated under inert atmospheric pressure to fracture them into smaller molecules [2, 9, 10].

2.1 Visbreaking

Visbreaking remains the oldest and least costly of the upgrading option and is only used in areas where heavy fuel oil is used to generate electricity and fuel ships. Visbreaking is a process in which residues are slowly cracked to reduce viscosity, and its main product is fuel oil, which has a dwindling market and provides low margins. This is a very low conversion process, and 15–20 wt. % residues are converted into lighter fractions. The yield of gas and gasoline together is generally limited to a maximum of about 7 wt. % as the cracking reactions are arrested to prevent asphaltene flocculation. Current interest in visbreaking is in those areas where motor fuel demand is relatively low. Vacuum residue and atmospheric residue can be used as feedstock for the visbreaking process [2, 6, 8, 10].

2.2 Gasification

The Texaco Gasification Process (TGP) was developed in the late 1940s. This process involves the complete cracking of residues into gaseous products, which has received less attention than other processes. Residual gasification is done at high temperatures (>1000°C) and synthetic gas (hydrogen and carbon monoxide), carbon black, and ash are the major products. It was modified in the 1950s for heavy oil feeds, in the 1970s for solid feeds such as coal and in the 1980s for petroleum coke. Almost from the beginning, this process has been attractive for hydrogen production. gasification can be used by refineries to produce hydrogen, increase the yield of high-value products, eliminate the production of high sulphur fuel oil, minimize the environmental effects of refineries (reduce the emission of NO_x and SO_x pollutants) and process a wider range of crude oil [2, 9, 11].

2.3 Coking

Coking is a process in which raw materials are thermally decomposed into products with lower boiling points. Different types of coking processes include delayed coking, fluid coking, and flexicoking. Delayed coking is the most common technology used in petroleum refineries to produce petroleum coke. More than 90% of petroleum coke is produced by this process. The main reasons are the relatively low investment cost and the claims of a better quality of liquid products compared with the fluid or flexicoking process [12].

2.3.1 Delayed coking

In the delayed coking process, the general goal of such a technology is to maximise liquid product yield while minimising coke production. The inherent flexibility of the delayed coking process for handling various feedstocks gives the refinery a promising solution to the problem of decreasing residual fuel demand and takes advantage of the attractive economics of upgrading it to more valuable lighter products. A refinery with a delayed coker is called a ‘zero resid refinery’ that can convert various feedstocks to valuable engine fuels while eliminating unsold refinery flows that are environmentally unfriendly. Disadvantages of this technology can be the abundant production of coke, low yield of liquid products, and highly aromatic products which require post-treatment. Another disadvantage of delayed coking is that it is a more expensive process than solvent deasphalting. Environmental pollution from coke particles is also a concern. In this process, 20–30 wt.% coke is also produced as a by-product. Although coke is accepted as a by-product of coking processes, excessive coke formation is economically disadvantageous because the value of coke is much lower than that of distillates. Even considering these disadvantages, delayed coking is the most frequently preferred process for refiners to residue processing because of the low investment cost [2, 6, 8, 9, 10].

Delayed coking is a severe form of thermal cracking process that operates at low pressures, without the use of hydrogen and catalysts, and falls in the temperature range of 450–500°C. Delayed coking is highly efficient in rejecting mineral solids and metals as well as some organic nitrogen and sulphur in the coke. The name ‘delayed’ derives from the fact that cracking reactions are given enough time (long) to form coke in coke drums. The first commercial delayed coker was started in 1930 at Standard Oil’s Whiting refinery [12, 13].

The global trend of processing heavy raw materials in delayed cokers, in order to obtain maximum yield of liquid products, has led to the production of coke with fuel grade that contains large amounts of sulphur and metals. Fuel grade coke, once considered a by-product of waste, is now an important fuel for the cement industry and electricity generation [6].

2.3.1.1 Process description of delayed coking

A schematic flow diagram of the delayed coking is shown in Figure 1. The process includes a fractionator, furnace, two coke drums, and stripper. the feedstock is charged directly to the fractionator, where it is heated, and the lighter fractions are removed as middle distillates. The bottom of the fractionator is pumped to the coking furnace and then heated to the temperature range of 485–500°C. The heated feedstock (liquid−vapour mixture) enters one of the pairs of coking drums, where the cracking reactions continue. The energy obtained in the furnace passages is sufficient to perform the cracking reaction when the coking drum is filled. In the furnace, steam is injected to prevent the formation of premature coking. In addition, to prevent the formation of coke in the furnace, short residence time and high mass velocity in the furnace are required. Overhead stream in the coking drum; gases, naphtha, middle distillates and coker heavy gas oil are sent to the fractionator for separation, then separated and sent to downstream units for post-treatment and coke deposits on the inner surface. For continuous operation, two coke drums are used; while one is onstream, the other is decoking. The typical volume of a modern coke drum is about 1000 m³, with a size range of 5–9 meters in diameter and a height range of 20–45 meters. The temperature in the coke drum ranges from 415 to 465°C and the pressure varies between 2 and 6 bar. Coker heavy gas oil is recycled as a coker feed and combined with fresh preheated feed and fed to the furnace, or used in other refining processes such as hydrocracker or gas oil hydrotreater or as a catalytic fluid cracking feed. The Coke drum is usually onstream about 24 hours before filling with porous coke. Figure 2 shows a section of a coke drum and shows how coke forms during a delayed coking operation. The material at the bottom of the coke drum is fully carbonised, creating a porous structure through which gases and liquids can pass. The top layer is not fully carbonised until it is exposed to heat for a long time. Some foam forms on the top of the drum, so foam forming can be prevented by injecting anti-foam materials (silicone oil) into the coke drums during the last 5 or 6 hours of the coking cycle. It is important to prevent the carryover of foam into vapour lines. Level indicators are useful for detecting the position of liquid or foam in the drum. After steaming and cooling the coke drum, the coke is removed by drilling and cutting with high-pressure (up to 340 bar) water jets [6, 12, 13, 14, 15, 16, 17].

Figure 1.

Flow sheet of delayed coking [modified from 12].

Figure 2.

Coke formation in coke drum of a delayed coking unit [6].

Decoking operation of the drum (Figure 3) involves the following steps:

1. The coke deposit is cooled with water.

2. One of the heads of the coking drum is removed to make it possible to drilling of a hole through the deposit centre.

3. A hydraulic cutting machine, which uses multiple high-pressure water jets, is inserted into the hole and wet coke is removed from the drum [6].

Figure 3.

Steps of decoking operation [14].

Most cokers were originally designed for a 20- to 24-hour coking cycle. In the late 1980s and early 1990s, the coking cycle time was reduced to 16–20 hours. In the late 1990s, it dropped to 14 hours. A typical time cycle in delayed coking is shown in Table 1.

Operation	Time, h
Coking	24
Decoking	24
Switching drums and steaming out	3
Cooling with water	3
Draining water	2
Hydraulic decoking	5
Reheading and testing	2
Warming up	7
Spare time	2
Total cycle time	48

Table 1.

Time cycle for delayed coking [6, 14].

2.3.1.2 Delayed coking process variables

Delayed coking process variables include process operating variables, feedstock properties and engineering variables. Furnace outlet temperature, coke drum pressure and recycle ratios are the main operating variables that affect not only the coke yield but also its properties. Increasing the drum pressure leads to a higher coke yield and a slight increase in gas yield, because more molecules, even in the gas oil range, contribute to coke formation by remaining in the liquid phase. It also reduces the sulphur content of coke. However, refinery economics requires operating at minimal coke formation. As the temperature of the furnace and drum increases, due to the removal of more volatile matter, the yield of coke reduces and the higher quality and harder coke is produced. However, it can cause cutting problems during decoking. Lower temperatures produce more coke, but lower quality. Therefore, the temperature at the furnace outlet must be optimized to form a minimum amount of coke in the furnace coils. To reduce the formation of coke in furnace coils, steam is injected into the furnace before the critical decomposition zone. However, the coke produced by steam injection in this process is more isotropic, that is, of lower quality. The recycle ratio has the same pressure effect as in delayed coking units, which varies from 1.03 to 1.30. The highest values are used in commercial units that produce premium coke, while the lowest values are used in delayed coking units where the goal is to maximise distillate yields. In addition, reducing the recycle ratio causes low-quality coke because the concentration of asphaltenes in the reaction mixture is higher [6, 12, 13, 14, 15, 16, 17].

Delayed coking units for processing vacuum residues are designed to operate under operating conditions that maximise liquid distillates yield and minimise coke production. These operating conditions include lower pressures, higher temperatures, and a lower recycle ratio. Feedstock variables are characterization factors and conradson carbon that affect product yields. Engineering variables also affect process performance, including mode of operation, capacity, and equipment used in coking and handling equipment. Operating variables have practical constraints that prevent further changes. Also, the constraints for each will be different with the type of feed consumed [14]. The effect of operating variables on coke yield and quality is shown in Table 2.

Variable	Effect on
	Coke yield	Coke quality
Increase drum pressure	Increase	Variable
Increase drum temperature	Decrease	Improve
Increase coker recycle ratio	Increase	Improve to maximum
Thermal crack recycle	Increase	Improve

Table 2.

Effect of operating variables on the yield and quality of coke [6].

2.3.1.3 Delayed coking feedstock

The delayed coking process can be applied to all residues in general, as they are not limited to constraints such as metal, sulphur, and asphaltene content. Heavy residues such as atmospheric and vacuum residue usually enter the delayed cokers, however, there are many raw materials that have been used as delayed coker feedstock for years. These feedstocks include:

1 - Gilsonite.

2 - Lignite pitch.

3 - Crack components (visbroken tar, cycle oil, decant oil or thermal crack tar).

4 - Refinery hazardous wastes.

5 - Deasphalted residues (pitch).

6 - Coal oils.

7 - Used plastic materials (recycling).

8 - Topped bitumen.

9 - C₃ to C₆ asphalt or lube oil extracts [6, 12, 15, 18]

2.3.1.4 Delayed coker yield prediction

In general, the products of the delayed coking process (based on vacuum residue feed) include gas (approximately 13 wt. %), naphtha (approximately 11 wt. %), middle distillate (approximately 45 wt. %), and green petroleum coke (approximately 31 wt. %).

The yield of products from delayed coking depends on the feed composition, in particular the amount of micro carbon residue (MCR) or Conradson carbon residue (CCR) content. Product yields can be estimated using the correlation based on the weight percentage of Conredson carbon residue (wt. % CCR) in the vacuum residue [14].

$$\mathit{Gas}\left(C_4^{-}\right)\mathit{wt}.\%=7.8+{0.144}^{\ast }\left(\mathit{wt}.\%\mathit{CCR}\right)$$E1

$$\mathit{\text{Naphtha}}\mathit{wt}.\%=11.29+{0.343}^{\ast }\left(\mathit{wt}.\%\mathit{CCR}\right)$$E2

$$\mathit{\text{Light Naphtha}}\mathit{wt}.\%={0.3322}^{\ast }\left(\mathit{\text{Naphtha}}\mathit{wt}\%\right)$$E3

$$\mathit{\text{Heavy Naphtha}}\mathit{wt}.\%={0.6678}^{\ast }\left(\mathit{\text{Naphtha}}\mathit{wt}\%\right)$$E4

$$\mathit{\text{Coke}}\mathit{wt}.\%={1.6}^{\ast }\left(\mathit{wt}.\%\mathit{CCR}\right)$$E5

$$\mathit{Gas}\mathit{\text{Oil}}\mathit{wt}.\%=100-\left(\mathit{Gas}\mathit{wt}.\%+\mathit{\text{Naphtha}}\mathit{wt}.\%+\mathit{\text{Coke}}\mathit{wt}.\%\right)$$E6

$$\mathit{\text{Light Cycle}}\mathit{Gas}\mathit{\text{Oil}}\mathit{wt}.\%={0.645}^{\ast }\left(\mathit{Gas}\mathit{\text{Oil}}\mathit{wt}.\%\right)$$E7

$$\mathit{\text{Heavy Cycle}}\mathit{Gas}\mathit{\text{Oil}}\mathit{wt}.\%={0.355}^{\ast }\left(\mathit{Gas}\mathit{\text{Oil}}\mathit{wt}.\%\right)$$E8

The gaseous compounds from the delayed coking process typically include methane, ethane, propane, butane, carbon monoxide, carbon dioxide, hydrogen, nitrogen, hydrogen sulphide and ammonia, the composition of which depends on the type of feed and the operating conditions.

2.3.1.5 Types of coke and their properties

Depending on the properties of feedstock and the operating conditions of the delayed coking process, different types of the coke can be produced. Coke can be distinguished by its morphology. Typically, coke can be divided into spherical shot coke (isotropic, amorphous, with almost no pores), sponge coke (semi-isotropic), and needle coke (anisotropic, regular crystalline structure, containing numerous fine pores and crystal sizes in the order of 4–7 nm). Either, according to its use, can be divided to fuel grade coke (cement industry and power generation), anode grade coke (aluminium production) or electrode grade coke (steel production). The differences between these types of coke are not always very clear. Due to the heterogeneity within the coke drum, one coke type may contain certain values of another coke type. Therefore, sponge coke may contain some shot coke and needle coke may contain some sponge coke [6, 15, 19]. Types of coke resulting from the delayed coking process with their optical structure are shown in Figure 4.

Figure 4.

Delayed coke types and optical textures. a: Needle coke, b: sponge coke, c: shot coke [20].

Petroleum coke can be in two forms, green petroleum coke and calcined petroleum coke. Petroleum coke obtained without calcination is called green coke. Coke calcination is done in a furnace to remove remaining hydrocarbons by heating green coke to about 1300–1500°C. During calcination, the coke decomposes further, and the carbon to hydrogen ratio increases from about 20 in green coke to 1000 for calcined coke [18].

Typical properties for different types of coke are shown in Table 3:

Properties	Fuel-grade green coke	Anode-grade calcined coke	Calcined needle coke
Sulphur (wt. %)	3–7.5	1.7–3.5	<0.5
Ash (wt. %)	0.1–0.3	0.1–0.4	<0.5
Nickel (ppm)	—	165–200	7
Vanadium (ppm)	200–400	120–350	—
Volatile matter (wt. %)	14 maximum	0.5	0.5
Bulk density (g/cm^3)	—	0.87	—
Real density (g/cm^3)	—	2.05	2.1–2.14

Table 3.

Typical properties for different types of coke [12].

2.3.1.5.1 Shot coke

Shot coke comprises dense low porosity spherical clusters with 2–10 mm diameters, frequently present as agglomerates up to the size of basketballs. These large agglomerates are fragile and can be broken easily; however, the small spheres are very hard. Shot coke is obtained from petroleum precursors with high resin and asphaltene and low API gravity, and it is less valuable than sponge coke. High velocities in the reactor are required to produce shot coke with spherical particles. Given that a very turbulent condition is required for the formation of shot coke, shot-coke production in the laboratory is difficult, because surface velocities are very low [14, 19].

2.3.1.5.1.1 Variables affecting shot-coke formation

The variables which impact coke structure are the quality of the feedstock and the operating variables including pressure, temperature, vapor velocity, and recycle ratio.

• Feedstock quality:

Different authors agree that the feedstock properties associated with the production of shot coke are asphaltene content and Conradson carbon residue content. Researchers claim that the tendency to produce shot coke increases when the ratio between the asphaltene content and the Conradson carbon residue content approaches 0.5. Moreover, the characterisation of vacuum residues from different heavy oil sources shows that this ratio (asphaltene content/Conradson carbon content) is equal to or higher than 0.5; therefore, if the operating conditions are favourable, the formation of shot coke is likely when these feedstocks are processed.

Another fact that shows that the feedstock quality has an important impact on the coke structure is the use of decanted oil mixed with vacuum residue. Decanted oil is the residual product from the fluid catalytic cracking (FCC) process. This hydrocarbon stream is highly aromatic (more than 70% aromatics) and its incorporation into the coker with the feedstock (between 15% and 20% of the total feedstock) suppresses shot-coke formation. This suppressing action can be related to the solubility effect of the aromatics on the asphaltenes, although, this has not been shown experimentally [6, 14, 19].

• Operating variables

Operating variables refer to the pressure, temperature, vapour velocity, and recycle ratio within the coker.

Pressure: Reduction of the coker pressure favours the formation of shot coke.

Temperature: Higher temperatures favour shot-coke formation, and temperature change of 5°C or less can either suppress or promote shot-coke formation. In a commercial delayed coking unit, the heater outlet temperature varies between 490 and 500°C. However, scaling down of these units is reached by operating the small-scale units at lower temperatures, which may vary between 417 and 450°C.

Vapour Velocity: The feedstock flow is not an important variable that affects product yields in delayed coking technology, but this variable is an important parameter for shot-coke formation because it impacts the vapour superficial velocity, which is thought to give a spherical shape to shot-coke particles. The vapour superficial velocities in commercial delayed coking units are between 0.12 and 0.21 m/s. These vapour velocities are so high that they are not achieved in laboratory-scale units.

Recycle Ratio: It is calculated with the following expression:

$$\mathit{RR}=\mathrm{HF}/\mathrm{FF}$$E9

HF is the flow of the heater. After mixing the recycling flow with fresh feed at the bottom of the main fractionator, it is measured at the heater inlet. FF is the fresh feed stream that is measured before pumping the processed feedstock into the main fractionator. Both flows are measured in barrels per day.

The recycle ratio in delayed coking units varies from 1.03 to 1.30. The highest values are used in commercial units that produce needle coke, while the lowest values are used in delayed coke units where coke yields should be minimised [6, 14, 15, 18, 19].

2.3.1.5.2 Sponge coke

Sponge coke is the most common form of green coke. Sponge coke is a friable solid material with pores on the surface and internal cavities connecting the pores, which is due to the evolution of gas from the liquid in the coke drum. The structure of this coke causes good drainage of water from the coke drums and easy cutting of the coke bed with water jets. This coke is typically derived from crude oil, which contains numerous cross-linkages. The diffusion of gas bubbles into the coke drum may also cause some spongy coke. In fact, sponge coke is a combination of sponge and shot structures. Most sponge coke is used to fuel boilers. Some low-sulphur, low-metal sponge coke can be used to make anodes used in aluminium production [6, 14].

2.3.1.5.3 Needle coke

Using the proper feedstocks, optimal design techniques, and operating parameters, delayed coking can be used to produce needle coke, a specialized and rare product in the refining and coke production industry.

Producing good quality needle coke is not easy, because the control of several parameters is necessary to control the production process. In other words, it is a control process of several parameters. Needle coke is a premium coke made from special petroleum feedstocks. The needle coke has a silvery-grey appearance that has a broken crystalline needle-like structure, highly ordered, microcrystalline, under a light microscope. The observed optical texture is called flow domain. Needle coke has anisotropic components such as fine fibrous and leaflet structure. This coke has long, thin cavities that result from the gas bubbles released by the solid coke itself. This high-quality coke can only be produced from feedstocks of high purity (low metals and sulphur) and with high aromatic compounds, such as cycle oil from the fluid catalytic cracking unit. In addition, a long filling time is required for the solid coke in the coke drum to react and release the gases. This type of coke cannot be produced from vacuum residue [6, 14, 15, 21].

2.3.1.5.3.1 Needle coke applications

Natural graphite is a limited source. It is estimated that 800 million tons can be mined worldwide. Only 10 to 15% of natural graphite is actually graphite carbon. Most of it is amorphous and contains minerals or silicate metals. In contrast, needle coke is continuously produced with high graphitizable content and low impurity concentration [12].

It was generally accepted that needle coke can be divided into two types according to the different feedstocks and named coal-based needle coke and petroleum-based needle coke. Excellent physical and chemical properties of needle coke such as high mechanical strength, high electrical conductivity (strong oxidation resistance), high thermal conductivity, high density as well as low thermal expansion coefficient (good abrasion resistance/heat shock resistance), low ash and sulphur content, low volatility, low energy consumption and easy graphitizable make needle coke an excellent raw material to obtain high-quality artificial graphite [12, 22].

There are two methods, basic oxygen furnace (BOF) and electric arc furnace (EAF), for steel production. Coal, iron, and limestone are used to produce steel in the BOF method. However, in the EAF method, an electric current passes through the graphite electrodes to convert the steel scrap into molten steel. Approximately 70% of world steel is produced by the BOF method and 30% by the EAF method. EAF has historically been the fastest growing sector of the global steel industry, with EAF steel production amounting to about 20 million tonnes per year in 1950, and EAF steel production expanded rapidly after 1950, and it exceeded 100 million tons in the 1970s. Needle coke, produced in the delayed coking process of petroleum oil refineries, was later developed in 1960 and commercialised in 1970. Finally, EAF steel production in 2020 reached about 550 million tons [12].

Inputs/initial costs of steel production through the EAF method include scrap steel, electricity, and graphite electrodes. There is no known alternative to graphite electrodes used in the EAF method of steel production. Needle coke is a major component in the production of graphite electrodes. The main application of needle coke is in the graphite electrode industry, and it can be purchased for 1500–3000 $/ton. In addition, needle coke is also used in the production of graphite cathodes in the aluminium industry. Electrodes made of needle coke need to withstand temperatures above 3000°C. Global steel production on the EAF is expected to grow. This has led to a similar increase in consumption of graphite electrodes. It is expected to eventually increase the consumption of needle coke [6, 12].

Needle coke is now widely used as a carbon filler for the production of graphite electrodes in the steel industry for smelting scrap metal for recycling in an electric arc furnace (EAF), cathodes required for smelting aluminium, anodes for commercial lithium-ion batteries, electric machines and some inherent parts of mobile phones, electrode materials for high energy density supercapacitors, anode materials for high-performance sodium-ion batteries, adsorbents, isotropic graphite, nuclear graphite, perovskite solar cell, carbon substitute super-activated carbon, graphene precursors, aerospace and other functional materials are used. Graphite electrodes have a low coefficient of thermal expansion (CTE), which is defined as an increase in length per unit temperature increase. Low CTE values indicate anisotropic needle coke, while high values indicate an isotropic shot coke [6, 12, 22, 23].

In terms of grade, needle coke is divided into an intermediate, premium, and super premium needle coke. As shown in Table 4, their difference is in the amount of thermal expansion coefficient and sulphur content.

Property	Quality grade
	Super premium	Premium	Intermediate
Coefficient of thermal expansion (CTE), *10^−7/°C	<2.0	2.0–3.0	3.1–4.0
Sulphur content, wt.%	<0.5	<0.6	<0.8
Real density, gm/cc	>2.12	>2.12	>2.12
Ash content, wt.%	<0.1	<0.2	<0.2
H content, wt.%	<0.03	0.03–0.05	0.03–0.05

Table 4.

Typical calcined needle coke specification [6, 12].

2.3.1.5.3.2 Feedstocks quality for needle coke production

Precursors for needle coke production have historically been limited to available residues whose aromatic molecular composition naturally predisposes them to form highly anisotropic carbon during carbonisation. However, further requirements of the feedstock include:

• Low ash content

• Low quinoline insoluble (QI) content

• Low asphaltene content

• Reduced content of stable nitrogen or sulphur heterocyclics

• Low oxygenate content

• Low air and carboxy reactivity of the coke during calcination

Coal-based needle coke is made from Coal Tar Pitch, refined coal tar pitch, refined coal liquefied pitch, and coal extraction. Petroleum-based needle coke is usually obtained by delayed coking of residual oil, petroleum bitumen, oxidized petroleum bitumen, and Fluidised Catalytic Cracker Decant Oil [18].

The chemical and physical properties considered in choosing a proper feedstock for the production of needle coke are summarised as follows:

1. Feedstock should have high aromaticity with 60–85% aromatic carbon aromaticity;

2. Feedstock should be of high initial boiling point, over 250°C with not more than 25–30% of material boiling below 360°C;

3. Feedstock should have low API gravity;

4. Feedstock should have low sulphur content preferably below 1 wt. % due to the concern for product quality;

5. Feedstock should have low metal, asphaltenes, and CCR content [6].

2.3.2 Fluid coking

Although the delayed coking process has been selected for large-scale operations, they are more attractive for processing the small volumes of residues due to the safety issues involved in decoking the drums at the end of each cycle. In addition, by reducing the retention time of cracked vapours, the yields of coking distillation products can be improved. To simplify the handling of the coke and to enhance product yields, Exxon developed a continuous process in the mid-1950s called fluidized bed coking (or fluid coking), in which the residence time was shorter, with more liquid and less coke. However, in this process, the products have lower quality. Fluid coking is a fluidized bed process developed by fluid catalytic cracking (FCC) technology, except that no catalysts are used and heavy feedstocks such as atmospheric and vacuum residues, residues of catalytic cracking units and oil sand bitumen turn into light products. In fluid coking, about 6% of the coke is burned to provide heat to the process, while the net coke yield is 70 to 75% of delayed coking. The yields of products resulting from fluid coking are determined by feed properties, fluidized bed temperature, and residence time in the bed [12, 14, 15, 16, 17].

An example of the material balance for fluid coking of Arab light vacuum residue is given in Table 5.

Feed	Products	Yield wt.%
Arab light Vaccum residue (22 wt.% CCR)	Reactor Gas, $C_4^{-}$	11 wt.%
	Coker Naphtha (C5–221°C)	15–20 wt.%
	LCGO1 (221–343°C)	12–14 wt.%
	HCGO2 (343–524°C)	35–36 wt.%
	Fuel gas	0.02 FOEB3 bbl Feed
	Net coke	21 wt.%

Table 5.

Yield of fluid coker process [14].

¹

Light Coker Gas Oil.

²

Heavy Coker Gas Oil.

³

Fuel Oil Equivalent Barrels.

2.3.2.1 Process description of fluid coking

Fluid coking is a thermal cracking process consisting of a fluidized bed reactor and a fluidized bed burner. A flow diagram is shown in Figure 5. Vacuum residue is preheated and fed to a scrubber that operates at 370°C above the reactor for coke fine particle recovery. The heavy hydrocarbons in the feed are recycled with the fine particles to the reactor as slurry recycle. The heavy vacuum residue feed is injected through nozzles to a fluidized bed of coke particles. Cracking reactions take place in the reactor at a temperature of 500–550°C, and the feed is converted to vapour and lighter gases, which enter the scrubber after passing through the cyclones at the top of the reactor and go to the fractionator column. Steam enters from the bottom of the reactor to remove heavy hydrocarbons from the coke surface. The evolution of vapour from the cracking of the feed, and the addition of steam, gives intense mixing of the coke particles within the reactor. The coke formed in the reactor flows continuously to the burner, where it is heated to 593–677°C and burns with partial combustion of 15–30% of the coke by injecting air into the burner. Coke combustion produces flue gases with low heating value (20 BTU/SCF), which are rich in CO and H₂. Parts of the heated coke particles are returned to the reactor to provide energy for the endothermic cracking reactions and to maintain the reactor temperature. After cooling, the remaining coke is removed from the process as a stream of fine particles of ‘petroleum coke’ and is burned in power plants or cement industries. This coke is very isotropic, rich in ash and sulphur and therefore not used in the carbon and graphite industry [12, 16, 17].

Figure 5.

Flow sheet of fluid coking [modified from 12].

The lower limit on operating temperature for fluid coking is set by the behaviour of the fluidized coke particles. If the conversion to coke and light ends is too slow, then the coke particles become sticky and agglomerate within the reactor. This phenomenon occurs in localised zones of the reactor, likely near the nozzles that inject the (colder) liquid bitumen feed, giving rise to chunks of coke that fall to the bottom of the bed. For this reason, optimising the method for introducing feed into the reactor is crucial. In addition, excellent heat transfer in the fluidized bed helps to reduce hotspots, which allows the reactor to operate at a higher temperature to cause more cracking of volatile matters. These factors generally reduce coke yields and increase the yields of gas oil and olefins compared to the delayed coking process. One disadvantage of the fluid coking process is the high rate of coke accumulation inside the unit. The reactor operates in a fouling mode, so coke deposits continuously on the interior surfaces during operation. The reactor must be shut down for a month or more every 2 or 3 years to remove the accumulated coke, which can grow to be as thick as 1 meter on the interior walls of the coker. The second disadvantage is the emission of significant amounts of hydrogen sulphide and sulphur dioxide from the reactor burner [16, 17].

At first, it was thought that the fluid coking process would replace the delayed coking process in the market, but so far this has not happened.

2.3.3 Flexicoking

The decline in coke markets derived from delayed coking and fluid coking due to constraints in sulphur emissions encouraged the development of flexicoking. Burning coke to generate process heat (Figure 6) liberates the sulphur in the coke as hydrogen sulphide and sulphur dioxide gases. The off-gas stream from the coke burner also contains CO, CO₂ and N₂. An alternate approach is to use a coke gasifier which can convert the carbonaceous solids to a mixture of CO, CO₂ and H₂ without producing SO₂. Flexicoking was designed by ExxonMobil as a fluid coking modifier that was introduced in 1976 in Japan. This process combines fluid coking with coke gasification, which, similar to fluid coking, is a fluidized bed process developed from catalytic fluid cracking technology. A fluidized bed is added to the process, which acts as a gasifier in which coke from the heater is reacted with steam and air in a fluid-bed gasifier to produce a gas of low heating value (20–40 BTU/sCF) and significantly reduces coke production. Yields of liquid products are the same for flexicoking and fluid coking because the coking reactor is unaltered, but up to 97% of the coke can be converted to gas by steam and air in a gasifier. Air is injected into the gasifier to maintain temperatures of 830–1000°C, but injected air is not enough to burn the entire coke. Under these conditions, the sulphur in the coke is converted to hydrogen sulphide, which can be scrubbed from the gas prior to combustion elsewhere. After removal of the hydrogen sulfide, a typical gas product contains 18% CO, 10% CO₂, 15% H₂, 51% N₂, 5% H₂O and 1% CH₄. Petroleum coke is removed, and economical fuel gas is available for use at the refinery. Due to the high initial investment and mechanical cost, only seven units were built worldwide. The main drawback of gasification is the requirement for a large additional reactor, especially if the high conversion of the coke is required [12, 14, 15, 16, 17].

Figure 6.

Flow sheet of flexicoking [modified from 12].

2.3.3.1 Process description of flexicoking

In the process, the viscous feedstock enters the scrubber for direct-contact heat exchange with the overhead product vapours from the reactor. Lower-boiling overhead constituents in the scrubber go to a conventional fractionator and also to light ends recovery. The feedstock is thermally cracked in the reactor fluidized bed to a range of gas and liquid products and coke. The typical bed temperature is 510–540°C. Vapour products resulting from the conversion reactions in the bed pass through the cyclone separators, which remove most of the entrained coke and return it to the reactor bed. The cyclone outlets discharge the vapor product directly into a scrubber, where the heavy liquid is used to scrub out the remaining coke dust and condense unconverted high-boiling fractions. The dust-laden liquid is recycled as ‘a slurry cycle’ to the reactor with the feed. The scrubbed vapour is sent to the coker fractionator, where the stream is split into gas, naphtha, distillate and heavy gas oil streams. The heater is located between the reactor and the gasifier, and it serves to transfer heat between the two vessels. The heater temperature is controlled by the rate of coke circulation between the heater and the gasifier. Adjusting the air rate to the gasifier controls the unit inventory of coke, and the gasifier temperature is controlled by steam injection into the gasifier. Excess coke is converted to a low-heating value gas in a fluid-bed gasifier with steam and air. The air is supplied to the gasifier to maintain temperatures of 830–1000°C, but is insufficient to burn all the coke. The heater transfers heat from the gasifier overhead gas to coke, which in turn supplies the heat of reaction in the reactor. The heater bed temperature is approximately 610°C. Coke is continuously circulated between the three vessels to transfer heat and maintain vessel inventories. A typical gas product, after the removal of hydrogen sulfide, contains carbon monoxide (CO, 18%), carbon dioxide (CO₂, 10%), hydrogen (H₂, 15%), nitrogen (N₂, 51%), water (H₂O, 5%) and methane (CH₄, 1%) [12, 14, 15, 16, 17].

In the oxidation zone of the gasifier, the following reactions take place very rapidly [14]:

$$C+0.5O_2\to \mathit{CO}$$E10

$$\mathit{CO}+0.5O_2\to {\mathit{CO}}_2$$E11

In the reduction zone, the following reactions take place slowly:

$$C+H_{2}O\to \mathit{CO}+H_2$$E12

$$H_{2}O+\mathit{CO}\to {\mathit{CO}}_2+H_2$$E13

Delayed coking is the most commonly used process among all commercial coking processes. More than 92% of petroleum coke is produced in the delayed coking process; About one-third of feed streams are produced in the form of petroleum coke. Due to the reaction conditions, net coke production from fluid cokers and flexicokers is only about 5–10 wt.% of the feed material. About 20–25% of 700 refineries worldwide are equipped with delayed cokers. Of the 140 US refineries in operation, 55 have delayed coker units. Most of the petroleum coke is produced in the United States, followed by China, South America, Canada, India, the Middle East and Western [6, 12].

Coke produced by delayed coker is a marketable product, while coke produced by fluid coker and flexicoker is burned to meet the reactor heat needs and feed preheat.

3. Conclusion

At present, light crude oil reserves are the main source of energy that meets global energy demand due to high quality and low production costs. Decline in light crude oil reserves poses great challenges to meeting the world’s energy needs. Heavy oil and oil residues have become a suitable alternative source to meet global energy demand. According to the approaches to achieving higher H/C ratios, upgrading technologies can be divided into carbon rejection and hydrogen addition processes. However, the cost of hydrogen addition processes is much higher than carbon rejection processes, because the production of hydrogen and the catalysts used in hydrogen addition processes are very expensive. Carbon rejection technologies have been used by refineries since 1913 to upgrade various hydrocarbon feeds. In a carbon rejection process, raw materials are heated to high temperatures to crack large hydrocarbons into smaller ones. Coking (delayed, fluid and flexi) is one of the types of carbon rejection processes. Delayed coking has been chosen by many refineries as an upgrading process due to its low investment cost and the inherent flexibility of the process to process any residuals. In this process, 20–30 wt.% coke is produced as a by-product. Depending on the properties of the raw materials and the operating conditions of the delayed coking process, different types of the coke can be produced. Typically, coke can be divided into spherical shot coke, sponge coke, and needle coke. Using the proper feedstocks, optimal design techniques, and operating parameters, delayed coking can be used to produce needle coke, a specialized and rare product in the refining and coke production industry. Needle coke is a premium coke made from special petroleum feedstocks. There are two methods, BOF and EAF, for steel production. Coal, iron, and limestone are used to produce steel in the BOF method. However, in the EAF method, an electric current passes through the graphite electrodes to convert the steel scrap into molten steel. There is no known alternative to graphite electrodes used in the EAF method of steel production. Needle coke is a major component in the production of graphite electrodes. The main application of needle coke is in the graphite electrode industry. Global steel production on the EAF is expected to grow. This has led to a similar increase in consumption of graphite electrodes. It is expected to eventually increase the consumption of needle coke.