6.1.2.2.2 Chemical Properties and Composition
Under normal conditions petroleum coke has a low reactivity. At higher temperature (up to 1400 °C under calcination conditions), an expansion of the crystallite size up to 2.5–3.5 nm takes place because of the cracking of side chains. At the same time the original hydrogen content of approximately 3.5 wt% is reduced to below 0.1 wt%. Under graphitization conditions (approximately 2900 °C), good crystallized forms of petroleum coke (needle coke character) are transformed into graphites. These graded changes in quality of petroleum coke with temperature demonstrate the structure, character, and classification of the product very likely.
The dominant element in petroleum coke is carbon. The hydrogen content is around 3.5 wt%. Sulfur and nitrogen from the feed components concentrate in the petroleum coke (S content of petroleum coke 0.1–7.5 wt%; N content 0.2–1.5 wt%). The V and Ni contents (0.001–0.1 wt%) of the feed streams remain almost entirely in the petroleum coke. The Na, Ca, and Mg content in the petroleum coke is the sum of the content of these cations in the crude oil and of their concentration in the cutting water (Section 6.1.1).
The level of polycyclic aromatics in petroleum coke is about 250 mg/kg. Benzo (a) pyrene makes up about 25 mg/kg. The amount of heavy metals, apart from V and Ni, is insignificant (<1 mg/kg). In general the quality of the feed components and the production conditions determine the quality of the product.
Petroleum coke is generally considered nontoxic and nonhazardous. According to the 21st ATP (Adaptation of Technical Progress of the European Substance Directive) and CONCAWE product dossier No. 93/105 and 95/95 [3], petroleum coke is considered a nondangerous product. In the Second ATP (Adaption to Technical Process to the CLP Regulation) Commission Regulation EU No. 286/2011, Brussels, Petroleum Coke. In the guidance “Classification and Labelling of Petroleum Substances (CLP) according to the EU Dangerous Substances Directive (CONCAWE recommendations August 2001),” the summary of classification and labeling recommendations for petroleum coke is that no classification and labeling required. The American Petroleum Institute (API) in the report “Petroleum Coke Category Analysis and Hazard Characterization (December 2007)” gives the conclusions: Both green coke and calcined coke have a low potential to cause adverse effects to the environment and human health, with the exception of noncompound‐specific, insoluble particle‐portal‐of‐effects. The level of polycyclic aromatics in petroleum coke is about 250 mg/kg. Benzopyrene makes up about 25 mg/kg. The amount of heavy metals, apart from V and Ni, is insignificant (<1 mg/kg). In general the quality of the feed components and the production conditions determine the quality of the product.
6.1.2.3. Production
In 2004, about 60 × 106 t/a of petroleum coke were produced worldwide. The production rate is increasing from 110 × 106 t/a in 2013 and is estimated to reach 150 × 106 t/a in 2017. The main reason for the increase in petroleum coke production are the projects that started in 2005, to refine Orinoco crudes [4] (largest petroleum reserve in the world) and Athabasca oil sands [5]. Both are heavy crudes with a high coke yield. About three quarters of the worldwide petroleum coke production is available on the market. One quarter will be reused by the producer [6, 7]. The available petroleum coke in market is about 10% of the quantity of blast furnace coke (hard coke), produced from hard coal [8]. More than 92% of the petroleum coke is produced in delayed cokers; about one third of the feed streams is incurred as petroleum coke. Residues and crack components (see Section 6.1.2.3.1.1) are used in delayed cokers in more than one hundred refineries. Due to the reaction conditions, net coke production from fluid cokers and flexicokers is only about 5–10 wt% of the feed material. The number of these production units is much lower. As of 2013, about 15 fluid cokers and 7 flexicokers were in production worldwide. About 20–25% of 700 refineries worldwide are equipped with delayed cokers. The number of delayed coker units is more than 150 worldwide. From 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, Middle East, and Western Europe.
Anode‐grade calcined petroleum coke is the more important product with regard to margin and value. In 2014 this production rate was 29 × 10 t/a.
Table 6.1.2.1 and Figure 6.1.2.1 list the worldwide locations for petroleum coke production, the area production rates for petroleum coke, and the progress of delayed coker projects in different areas [6, 7, 9, 10].
6.1.2.3.1 Production Processes
6.1.2.3.1.1 Delayed Coking
Coking is an effective way for an integrated refinery to process the residues of crude oil. Coking processes are normally carried out to minimize coke and maximize liquid products. Production of liquid products drives economics except for specialty cokers (like needle coke). The first atmospheric coking processes were started in 1928. Coking, in general, has become more important over the years because traditional heavy fuel oil demand has declined and the demand for lighter products has increased. This has made it more attractive to convert the heavy fuel oil components into better marketable light, clean components. The present major source of production of marketable coke is the delayed coking process developed by Standard Oil. Conventional delayed coking is shown in the flow diagram in Figure 6.1.2.2. A delayed coke complex is depicted in Figure 6.1.2.3.
The refinery residues are heated in a furnace (b) up to about 485–500 °C. After passing a fractionator tower where middle distillates are topped off, they are fed into one of the coking chambers. The energy gained in the furnace passages is sufficient to release the cracking reaction in the filling time of the coking chamber (delayed coking). The reaction zone is also called mesophase, because the transition from liquid residue molecules to solid crystalline coke structure occurs here. The pressure in the coking chamber can be varied between 2 bar and 6 bar. In the furnace, steam is dosed to the residue to inhibit the formation of coke. The vapors in the coking chamber – gases, naphtha, middle distillates, and heavy coker gas oil – are routed to the fractionator for separation, then fractionated, and forwarded for downstream units for posttreatment. The heavy coker gas oil is recycled as coker feed or is used in other refinery processes such as hydrocracker or gas oil hydrotreater or as feed to fluid catalytic cracking unit [11].
Table 6.1.2.1 Refineries with petroleum coke production 2005.
| Refinery | Estimated production Mta 2005 | Typical analysis % S | Current FOB price | US$ |
|---|---|---|---|---|
| North America | ||||
| East Coast | ||||
| Motiva, Delaware City | 0.70 | 5.6 | 34 | 15–18 |
| Valero Energy, Paulsboro | 0.50 | 5.0 | 100 | No export |