Coking of HP tubes in ethylene steam cracking plant and its mitigation

Coke deposited on the inner wall of industrial HP cracker tubing during the cracking of ethane has been investigated in detail to elucidate the mechanism of the coking process. The outer wall of the deposit, adjacent to the metal tube, consists of filamentary coke. Lamellar coke occupies the central zone of the deposit, while the inner zone, adjacent to the reaction gases, is comprised of coke particles. The formation of the filamentary coke at the tube wall is closely related to composition of the tube material, but both the lamellar coke and the coke particles are derived from the gas phase reactions so bear no relation to the tube material. The difference in morphology between the lamellar and particulate types of coke is attributed to changes in the coking process. Possible ways to alleviate or prevent coking of the tube include reduction of the catalytic activity of the material at the inner wall of the tube and improvement of the ethylene steam cracking process.