PTQ Q4 2022 Issue

Green

Calcined

Graphite Artifact

Sulphur, wt% Nitrogen, wt% Nickel, ppmw

0.5 0.7 5–7

0.5 0.5

- - - - - -

-

Ash, wt%

0.1 max

0.1 max

Volatile combustible matter (VCM), wt% (dry basis)

6 max

-

Real density, g/cc Flexural strength, psi

- - - - -

2.10–2.14

- - - -

2500

Coefficient of thermal expansion (CTE), x 10 -7 / °C

2.5 (30–125°C)

Electrical resistivity, Ohm, x 10 -6

320

Granulometry, wt%

+6 Tyler max

Table 1 Typical needle coke properties

yield (minimise coke product yield), high-quality needle coke and maximum coke yield are the primary goals during needle coke production. The processing capacity for needle coking units is typically much less than fuel-grade coking units due to feedstock availability and operating conditions. Needle coke quality is achieved through mesophase management controlled by feedstock quality and operating conditions. Feedstocks promoting the production of needle coke are aromatic tars with low sulphur and metals content such as decant oils, thermal tars, or other multi-ring aro- matics with short aliphatic side chains. Feedstock characteristics In general, a highly aromatic feedstock must be used to pro- duce premium-quality, readily graphitisable needle coke. The feedstock must also be low in sulphur and nitrogen to avoid ‘puffing’ during the electrode manufacturing process. Puffing causes internal cracks in the electrode and signifi - cantly reduces its strength. The feedstock must also be low in ash to avoid interfer- ence with the growth and coalescence of the mesophase required to achieve the anisotropic structure of needle coke. In some cases, pretreatment processing may be necessary to achieve the ultimate feedstock required. However, having the right feedstock does not guarantee a high-quality nee- dle coke since the quality is also a function of the processing path and proper selection of operating conditions. Typically, pilot plant tests are conducted to confirm the feedstock and operating conditions required for needle coke production. Table 2 shows typical needle coke feedstock properties. Operating conditions Temperature, pressure, recycle rate, coking time, and post-treatment affect the quality of needle coke. The most

important of these conditions is temperature since this ultimately determines the reaction rate and viscosity of the reacting medium. The optimum temperature for needle coke formation depends on the feedstock used and is usu- ally determined by pilot testing. Too high or too low tem- perature would result in an isotropic coke. Needle cokers are normally operated at higher coke drum pressures, higher coil outlet temperatures, higher recycle rates, longer coking times, and lower superficial velocities than fuel or sponge/anode cokers. The operating param- eters are set to increase the fluidity and hence reaction time of the mesophase while increasing the yield of needle coke. Higher recycle also leads to higher coke density and improved coke quality (i.e., lower contaminants due to dilu- tion by increased coke make and lower CTE). However, excessively high pressures and recycle rates are not recom- mended because of diminishing returns, and the additional quantity of light material retained in the drum at these con- ditions can detrimentally affect the final coke properties. Table 3 compares typical DCU operating conditions for the various coke types. General processing steps for quality needle coke production For some feedstocks, especially ones derived from coal tar pitch, pretreatment of the feedstock may be necessary for needle coke production. For example, solids or quino- line insoluble (QI) must be removed by filtration or solvent extraction as these components hinder mesophase coalesc- ing. Distillation of the feedstock may also be required to remove the light-end components and optimise the distil- late cut/quality to be used as feedstock. For feedstocks high in sulphur and nitrogen, hydrotreating will also be required to reduce sulphur and nitrogen content. In some cases,

Parameter

Fuel coke

Anode coke

Needle coke

Parameter

Value

Aromatic carbon, wt% of total carbon 75 min. Sulphur content, wt% 0.5 max. Nitrogen content, wppm 700-1000 max. Quinoline Insolubles (QI), wt% 0.1 max. Table 2 Typical needle coke feedstock properties

Temperature, °F (°C) Pressure, psig (kg/cm²g)

910-930 (488-498)

925-950 (496-510)

940-950 (504-510) 60-120 (4.2-8.4)

15 (1.05)

18-60 (1.3-4.2)

Recycle, LV%

0-5

0-50

60-150

Coking time, hours

9-18

24

32-36

Table 3 Comparison of typical DCU operating conditions for various coke types

78

PTQ Q4 2022

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