Sub-Product

Petroleum pitch (or asphalt) is a residue from heat treatment and distillation of petroleum fractions. More specifically it is the residue from extractive refining of vacuum resid (also named tar, or straight-run bitumen) to extract deasphalted oil leaving asphalt as a residual product^[1].

It is solid at room temperature, consists of a complex mixture of numerous predominantly aromatic and alkyl-substituted aromatic Hydrocarbons, and exhibits a broad softening range instead of a defined melting temperature. The hydrogen aromaticity (ratio of aromatic to total hydrogen atoms) varies between 0.3 and 0.6. The aliphatic hydrogen atoms are typically present in alkyl groups substituted on aromatic rings or as naphthenic (cycloaliphatic) hydrogen^[1].

The molecules within petroleum pitch typically contain 17-65 carbon atoms with an average of 29 carbon atoms, and 12-40 hydrogen atoms with an average of 20^[2].

The overall molecular weight ranges from approximately 200 to 2,000 Da^[3]. A typical isotropic petroleum pitch has a number-average molecular weight (Mn) of 516^[4].

References

1. 'petroleum pitch' in IUPAC Compendium of Chemical Terminology, 3rd ed. International Union of Pure and Applied Chemistry; 2006. Online version 3.0.1, 2019.
2. Pengcheng Chen et al., Petroleum pitch: Exploring a 50-year structure puzzle with real-space molecular imaging, Carbon, Volume 161, 2020, Pages 456-465, ISSN 0008-6223. (for properties of petroleum pitch)
3. Barbara Apicella et al., Analysis of Petroleum and Coal Tar Pitches as Large PAH, CHEMICAL ENGINEERING TRANSACTIONS, VOL. 57, 2017.
4. E. G. Cervo, M. C. Thies, 30^th May 2017, Control of the Molecular Weight Distribution of Petroleum Pitches via Dense-Gas Extraction, Chemical Engineering & Technology, Volume 30, Issue 6, Special Issue:Hight Pressure Technology, June, 2007, Pages 742-748.