Tar in Quench Systems: Causes, Consequences, & Care Solutions

During the thermal cracking of hydrocarbon feedstocks (i.e. naphtha, LPG, ethane, shale gas, etc.) to produce ethylene and the subsequent quenching of the cracked feedstock, several secondary substances may also be generated, including a relatively heavy material. This heavy material may be referred to by various names, such as tar or py-tar, and can manifest itself as solids or viscous liquids. This heavy material is condensable in the quench section of the ethylene plant, and can lead to considerable fouling throughout the quench system and dilution steam system. This heavy material may also migrate to the hydrocarbon-water interface in the quench water separator and cause emulsification issues. Fouled quench system components and phase separation issues in the quench water separator can lead to reduced production throughput, operational challenges, reduced run-lengths, and expensive cleaning operations.

When the cracking feedstock is light, such as shale gas, ethane, or LPG, less of this heavy material is generated than when the cracking feedstock is heavy (i.e naphtha), but somewhat counterintuitively, the problems caused by this heavy material can be worse when cracking light feedstock because there is also a reduction in other condensable hydrocarbons that can help solubilize and naturally disperse the heavy material. Management strategies are needed to help mitigate the consequences resulting from this heavy material.

Recently, a novel multifunctional dispersant chemistry has been identified which can assist with the management of this heavy material. By dispersing the heavy material into either the bulk hydrocarbon phase or the bulk caustic phase, the heavy material is not allowed to deposit or foul the quench system. Also, by keeping the heavy material in the bulk phase, it cannot migrate to the oil-hydrocarbon interface of the quench water separator, reducing the heavy material’s contribution to emulsion issues.

This paper will review the likely formation mechanisms for this heavy material, the potential consequences resulting from this heavy material, and possible management strategies for reducing or alleviating these consequences, including the use of the aforementioned novel dispersant chemistry.