In fluid coking, the heavy crude oil residue is sprayed onto a bed of hot coke. The feed is thermally cracked and new coke layers are formed on the coke particles. The fluid coke particles are small (100–150 μm), round, and of onion‐like structure. This coke is partly consumed during heating up in the cycling phase of the process. The surplus coke is removed from the process and burned in power plants or cement kilns. This coke is highly isotropic, rich in ash and sulfur, and not used in the carbon and graphite industry for these reasons. Flexicoking (see petroleum coke 6.1.2) is a combination of fluid coking with coke gasification [5].
Delayed coking (→petroleum coke, Section 6.1.2) is the by far most frequently applied technology in oil refineries to produce solid coke. More than 90% of petroleum coke is produced by this method [6]. The main reasons are the comparatively low investment capital cost and claims of better quality of liquid products compared with the fluid or flexicoking process.
The coke can be distinguished by its morphology. Typically, the coke is classified into spherical shot coke (isotropic), sponge coke (semi‐isotropic), and needle coke (anisotropic) or according to its usage as fuel coke grade, anode coke grade for aluminum production, or needle coke grade for steel production. The limits between these coke grades are not always very sharp. Due to inhomogeneities inside the coke drum, one coke grade can contain certain amounts of another coke grade. Thus, sponge coke can contain some shot coke and needle coke can contain some sponge coke.
The quality of the coke for carbon manufacturing depends on the quality of the feed and the process parameters during coking. Feedstocks like vacuum residues from crude oils produce less anisotropic coke than more aromatic feedstock. In addition, if these residues contain significant levels of sulfur and metals, the coke is not satisfactory for specialty carbon applications and is generally used as fuel only. Lower sulfur and metal content residues can satisfy the needs of coke requirements for carbon anode production in the aluminum industry and thus are used in large quantities for this application. This feedstock is seldom modified prior to its use in the coker.
Needle coke production requires special emphasis on the coker feed quality. Suited feeds are aromatic refinery streams, very low in asphaltene content, such as decant oils derived from FCC [7–9], thermal cracking, or pyrolysis tars from steam crackers. Besides petroleum residues, also CTP can be used to produce needle coke. Important for the production of needle coke is the low ash content of the coker feed. Today these feeds are deashed in most cases.
The product from the delayed coker is so‐called green petroleum coke. This green coke has to be calcined prior to its usage for the production of industrial carbons (see petroleum coke 6.1.2). Calcination is carried out at about 1500–1800 K, generally in a rotary kiln, sometimes also in a rotary hearth calciner [10]. Rotary kiln calciners range in capacities from 60 000 to more than 200 000 t of calcined coke per year. The heat treatment of the green coke during calcination is necessary to reduce the amount of unconverted volatile matter in the green coke. Uncalcined coke would undergo strong shrinkage during carbon artifact production, resulting in severe damage of the carbon product during its baking.
In addition to the quality of the liquid coker feed and the coking conditions, the coke quality is determined by the calcination parameters such as heat‐up rate, final calcination temperature, time at the maximum temperature, and the atmosphere in the kiln [11].
The calcination condition like the heating gradient above 800 K has a high impact on the porosity of the coke and the coke sizing. Both parameters are important for its later use during the manufacturing of graphite electrodes, carbon anodes, or other products.
Figure 6.1.1.3 Optical micrographs of petroleum needle coke (a), an anode‐grade petroleum coke (b), and isotropic CTP coke (c).
Green petroleum coke with high sulfur levels (>3 wt%) and metallic impurities is used as green fuel grade in power plants or is burned in calciners for cement production. Due to the shortage of good anode grades, the aluminum smelters had to widen their coke specifications in regard to metal and sulfur content in the last years.
Figure 6.1.1.3 shows an optical micrograph of an anode‐grade coke compared with a needle coke grade. The differences manifest themselves in the orientation and extension of the flow texture domains. The needlelike appearances of parallel flow domains explain the name needle coke. Essential for this morphology are the formation and coalescence of mesophase spherules and their orientation by upstreaming liquids and gaseous cracking products in the delayed coker drum.
Some typical physical properties of calcined petroleum cokes are summarized in Table 6.1.1.1.
Historically the coefficient of thermal expansion (CTE) of a laboratory test artifact has been the predominant quality parameter for needle cokes, which are used to produce graphite electrodes for steel production in the electric arc steel furnace. Significant improvements in this parameter have been obtained since the 1970s, as indicated in Figure 6.1.1.4.
Table 6.1.1.1 Typical coke data for various grades.
| Property | Petroleum needle coke | Coal‐tar pitch needle coke | Petroleum anode coke | Semi‐isotropic pitch coke | Isotropic pitch coke |
|---|---|---|---|---|---|
| CTE (m/μm K) | 0.31 | 0.30 | 1.3 | 2.3 | 4.0 |
| Real density (g/cm3) | 2.153 | 2.146 | 2.088 | 2.106 | 2.016 |
| Vibrated bulk density (g/cm3) | 0.78 | 0.75 | 0.77 | 0.89 | 0.96 |
| Sulfur (%) | 0.42 | 0.26 | 1.00 | 0.28 | 0.32 |
| Nitrogen (%) | 0.2 | 0.53 | 0.84 | 0.77 | 0.88 |
| Hydrogen (%) | 0.031 | 0.041 | 0.051 | 0.096 | 0.123 |
| Ash (%) |
0.03
|