PTQ Q4 2023 Issue

multi-ring aromatics, the principal components of fuel oil, are produced in place of solid carbon. Fuel oil components are so-called coke precursors that will produce solid coke eventually. Thermodynamically, the ultimate products of thermal cracking are carbon and hydrogen. By manipulating reactor operating conditions, we max- imise intermediate products. One of the intermediate prod- ucts is ethylene. Thus, reactor design and operations play a vital role in maximising ethylene for any feed. Hence, for high ethylene production, it is preferable to crack feeds with reasonable or high hydrogen content in feed. In that respect, light crudes with high paraffin content also have a higher ethylene potential than heavy crudes; the selection of crude is important for good economic return. The type of compounds present in a crude can be used to illustrate the salient features of crude cracking technology. Altgelt and Boduszynski⁸ studied the type of compounds present in a typical crude as a function of atmospheric equivalent boiling point. A crude characterisation is shown in Figure 3 . In every crude and condensate feed, all these classes of species are present, although the quantity (or concentration) of each species varies in each crude. Every crude contains normal and isoparaffins, mono and multi-ring naphthenes, mono and multi-ring aromatics, hydrocarbons with sulphur, nitrogen, oxygen, and metals (heteroatoms) containing straight chain and/or ring com- pounds. The species that contain heteroatoms are generally called polar species. For any given carbon number, normal paraffin has the lowest boiling point, and polynuclear polar and non-polar compounds have the highest boiling points. For example, Figure 4 shows different C₂₀ species. As shown in Figure 4, nC₂₀H₄₂ (eicosane) boils at 343ºC while C₂₀H1₂ (perylene) boils at 468ºC. C₂₀H12 is a highly condensed molecule with four rings surrounding a benzene ring and has the lowest hydrogen content (4.8 wt%). A four-ring aromatic compound with some side chains, C₂₀H1₈ (butyl pyrene) boils slightly lower than C₂₀H₁₂ (BP=407ºC). C₂₀H₁₂ is a highly hydrogen-deficient molecule compared to nC₂₀H₄₂. Hence C₂₀H₁₂ and C₂₀H1₈ will produce mainly coke rather than ethylene. It is the aim of this technology to remove these species selectively and keep the hydrogen-rich spe- cies in the cracker feed slate. Separation strategies Figure 4 shows that boiling point is one measure of sep- arating hydrogen-rich species from hydrogen-deficient species. By limiting the boiling point, the concentration of multi-ring compounds can be reduced in the cracker feed. Thermodynamically, they cannot be easily removed, but concentrations can be reduced to acceptable levels. Therefore, cut points are critical parameters. A crude dis- tillation column contains many theoretical stages, and the overlapping between various fractions is small. That prin- ciple is followed in the technology developed by Lummus Technology in HOPS, which has been in operation for con- densate cracking for many years. This is further improved in Lummus’ patented crude cracking technology. ⁹ The concept is explained in Figure

90 80

N. parans i. parans Napthenes Aromatics

10 20

PNA

Carbon number

Figure 2 Ethylene yield potential as a function of carbon number

yields and comparable propylene yields to normal paraffins. Naphthenes produce less ethylene and propylene yields than isoparaffins. Aromatics produce very low ethylene and propylene yields. Multi-ring aromatics produce almost no olefins and high amounts of fuel oil that forms coke. The hydrogen content of olefins (C₂H₄, C₃H₆) is 14.3 wt%. The hydrogen content of aromatics is exceptionally low, and multi-ring aromatics have even lower hydrogen content (often <8 wt% H). Therefore, a feed with higher hydrogen content than mono-olefins (14.3%) can eas- ily dehydrogenate (lose hydrogen) and produce an olefin. A feed with lower hydrogen content than an olefin must reject carbon to produce an olefin for carbon-hydrogen bal- ance. Carbon is a solid and hence hydrogen-deficient feeds will shorten the heater run length. This is a simple rule applicable to all types of pyrolysis technologies. Rarely in practice is elemental carbon found in pyrolysis coils. Hydrogen-deficient products such as

nP Multi-ring naphthenes Naphenoaromatics Porphyrin Polar polyfunctional compounds

Naphtha

AGO

VGO

Residue

200

320

430 540

650

Atmospheric equivalent boiling point, ˚C

Figure 3 Carbon number vs atmospheric equivalent boiling point for different classes of species

PTQ Q4 2023