Study On Coking Behavior Of After-Service And Pre-Oxidation Cracking Tube Surfaces And Coking Inhibition Technology

Coking on the inner surface of the cracking tube is the worldwide problem for the production of ethylene by thermal cracking technique, which cannot be avoided. It is of important realism significance for increasing the level of ethylene production, energy saving and emission reduction to develop the effective and feasible technology of inhibiting coke formation.In this paper, the influence of process parameters simulating cracking process on coking behavior of Cr25Ni35 pre-oxidation cracking tube surface was studied. SiO2/S coating was prepared by atmospheric pressure chemical vapor deposition on the inner surface of Cr25Ni35 cracking tube after service. The anti-coking performance of the coating, self-made lab's unique sulfur/phosphorus-containing inhibitor and their combination, as well as N-1 alkali metal inhibitor was investigated respectively. The main research contents and conclusions are as follows:(1) An oxidation layer with spinel structures was formed on the Cr25Ni35 cracking tube surface pretreated by oxidation method. Influence of process parameters on coking behavior of Cr25Ni35 pre-oxidation cracking tube surface was studied, and the rules about cracking time, cracking temperature and coking dilution ratio on coking behavior were mastered. The results show that the diameter increasing rate of filamentous coke was about 0.027μm/min and 0.0051 p,m/℃with increasing the cracking time and cracking temperature, respectively. Increasing the temperature accelerates the transition from catalytic cracking coking to thermal cracking coking. Partial pressure of hydrocarbon and coking rate of coke formation decreased with increasing the diluted ratio.(2) The surface morphology and organizational structure of the inner surface of Cr25Ni35 cracking tube after service were investigated. SiO2/S coating was prepared by atmospheric pressure chemical vapor deposition on the surface. The microstructures, mechanical properties and anti-thermal shock property of coating were also characterized. The results show that the coating was integrity and compact and was composed of three-member ring Si-O-Si structure and amorphous Si-O-S structure. The thermal shock times in water quenching from 900℃were 16.(3) The anti-coking performance of the coating, sulfur/phosphorus-containing inhibitor and their combination were investigated comparatively. The results show that the coating reduced the coke deposition by 65.07% compared with that of the blank tube, and 69.2% for the inhibitor at close range. The coating and the addition of the inhibitor led to a change of coke layer microstructures. The best coking inhibition effect can be obtained when considering the combination of the coating and the inhibitor, which is 98.5%. And the optimal addition dosage is 50 ppm for the sulfur/phosphorus-containing inhibitor.(4) Experiments about the influence of on coke formation were made, and the inhibiting mechanism about alkali metal element was announced. The results show that adding 200 ppm N-1 additive containing alkali metal element reduced the coke deposition by 51.54%. Alkali metal element increased the degree of graphitization, and catalyzed the gasification reaction of coke formed by non-catalytic coking mechanism, which reduced the diameter of filamentous coke to the nanometer level.