PTQ Q4 2022 Issue

Fuel gas amine treatment LPG amine treatment

Treated ue gas

Treated LPG

Stabiliser

Stabilised n aphtha Treatment / renery pool Vacuum slops Diesel Treatment / refinery pool

Vacuum Unit

Thermal cracker

Delayed coker unit

VGO

Hydrotreater

FCC DCO

Calcined needle coke

Calciner

Green coke Vacuum residue / To fuel

Figure 2 Schematic for needle coke production

the feedstock after hydrotreating may have to undergo a thermal pretreatment step to recover the aromaticity lost by hydrotreating. A preliminary schematic for needle coke production is shown in Figure 2 . There are four distinct steps in the transition of isotropic liquid feedstock to anisotropic graphitisable carbon: Nucleation → Growth → Coalescence → Bulk mesophase  Nucleation Isotropic, aromatic feedstock is heated and undergoes thermal cracking followed by polymerisation, condensation, and de-alkylation to form larger, aromatic molecule spheres  Growth These spheres enlarge and continue to grow, producing sheets of large polynuclear aromatic molecules referred to as beta resins or mesophase  Coalescence Under the right conditions, the spheres coalesce and eventually line up parallel to each other to form a second liquid layer – the bulk mesophase  Bulk mesophase As the bulk mesophase condenses to solid coke, it is sheared by the evolving gas and forms a highly anisotropic coke product that, when crushed, appears as needles. The conversion to mesophase is a func- tion of the molecular weight of the mesophase material and the composition of the liquid from which it is formed. More aromatic mother liquids will hold the aromatic mesophase molecules in solution longer, thereby achieving higher molecular weight molecules. At the end of the coking cycle, the coke drum typically contains material ranging from premium needle coke (at the bottom of the drum) to unreacted feedstock (at the top of the drum). Post-treatment ‘drying’ techniques are typically applied to allow this mass to continue to react at some predetermined temperature. Gas evolution post- treatment will help deform the uncoked material into the needle coke structure. Additionally, the post-treatment step produces higher strength coke, improves friability by reducing fines content, and reduces the coke volatile com - bustible matter (VCM). The post-treatment step also lowers the coke CTE which is desirable and an important specification for electrodes

used in electric arc furnaces. A long coking cycle time (i.e., more than 32 hours) with an extended post-treatment step (6+ hours) is typically required. The post-treatment step includes the medium, temperature, and time required and can be fine-tuned by pilot testing. Considerations for repurposing the coker Several considerations must be taken when repurposing a fuel-grade or anode-grade coker into a needle-grade coker. First, the refiner must decide if needle coke will be produced in a campaign mode (a few months per year alternating with fuel-grade mode) or if it will be a long- term operation. This decision will impact feedstock prepa- ration and coker design in terms of variability in operating conditions, design pressures and temperatures, capacity turndown capabilities, equipment/hydraulics limitations, etc., as the feedstock rates and operating conditions to produce needle coke are very different compared to fuel- grade coke operation. Product impact on downstream units (i.e., minimum-to-zero HCGO product make) must also be considered. Since the typical feedstock to the coking unit (i.e., vacuum residue) is not suitable for making needle coke, alternate uses or dispositions for the current feedstock must be found. Simultaneously, potential feedstock availability to make needle coke must be confirmed. Screening studies, including feedstock texture analysis, are recommended to confirm the feedstock’s propensity to make the anisotropic structure of the coke. These studies give the refiner greater confidence in meeting needle coke specifications, especially when trying to optimise a distillate cut and/or blending dif- ferent feedstocks. Once a feedstock or feedstocks are selected, a feasibil- ity study, including pilot plant testing, is recommended to identify the best feedstock to optimise the needle coke quality and quantity. Pilot plant testing would also be used to confirm the unit configuration and if feed preparation/ pre-treatment is required, determine target operating con- ditions, and develop the theoretical product yield and qual- ity estimates. Needle coke quality could also be confirmed through coke production, calcination, and CTE testing.

PTQ Q4 2022