Study On Coking Characteristics Of Heavy Oil Residue

The delayed coking process plays an increasingly important role in heavy oil processing technologies in China. As property of feedstocks becomes worse, it is prone to form shot cokes when heavy crude oils are processed. Shot coke production has a big impact on the long period operation and safe production of delayed coking unit. It is necessary to do research on Coke-forming Characteristics of feedstocks in order to optimize the technological parameters and inhibit shot coke formation when the typical heavy feedstocks are processed.Firstly, the impact of properties of the feedstock, operating parameters, and additives on shot coke production is investigated on small-scale delayed coking pilot plant, using four foreign and domestic heavy oil residues including Venezuela vacuum residue as the feedstock and polarized-light microscopy as the main analysis measure. Secondly, the mechanism of shot coke formation is explained from the view of mesophase. Meanwhile, considering the influence of operating parameters on the product distribution and the quality of petroleum coke, the operating parameters processing the Venezuela vacuum residue are given initially.At last, the operating parameters are proved on medium-sized delayed coking pilot plant. Research discovered that properties of feedstocks are essential factors for the formation of shot coke. The feedstocks ( asphaltenes/carbon residue>0.5, H/C atomic ratio<1.5, the colloidal stability parameter<3.5) are prone to form shot cokes. The feedstocks blending with FCC slurry or THN could inhibit shot coke formation, promote the liquid yield and decrease the coke yield. The rising temperature, decreasing pressure and recycle ratio were all able to induce shot coke formation, promote the liquid yield and decrease the coke yield. There are a few shot coke particles when the temperature is 490℃. Lower temperatures can inhibit shot coke formation. Increasing the reaction pressure can suppress shot coke formation. The shot coke formation is inhibited when this value is 0.2MPa(G). An increase in the recycle ratio can suppress shot coke formation. There is no shot coke formation when the recycle ratio is 0.3 (Heavy coking gas oil). An increase in water injection rate can increase vapor velocity in the coking drum and promote shot coke production. When the value is 10% on the small-sized pilot plant and 8% on the medium-sized pilot plant, shot coke occurs. The mechanism of shot coke formation is explained by continuous thermal convertion which was shown as feedstocks→unstable spheres of mesophase→mosaic of mesophase→shot coke. The optimum conditions processing the Venezuela vacuum residue are as follows: temperature 490℃, reaction pressure 0.2MPa(G), recycle ratio 0.3(Heavy coking gas oil), and water injection rate 2%.