Investigation Into The Optimal Design Of Refining Vacuum Furnace In Atmospheric-vacuum Distillation Unit

Due to the high operating temperature and heavy oil processing, many refining vacuum furnaces improperly designed have problems of coke formation in atmospheric-vacuum distillation unit. Especially in deep-cut operation, refining vacuum furnaces usually face short operating life cycles because of coking problems. In the present study, we aimed to provide theoretical basis for the optimal design of industrial furnaces by optimizing the process side condition and numerically simulating both the process side and flue gas side field.A new steam injection technology was proposed from the perspective of steam injection condition optimization. Theoretical calculations were performed by the use of commercial software Petro-SIM and the results were validated against onsite data. Comparison was made between the new technology and existing process. The best two-phase flow pattern and control of the optimal oil film temperature were obtained. The results showed that the new technology was effectively in improving the two-phase flow pattern and was adjustable when oil type and operating condition changed, at the same time decrease the tendency of coking problem in the furnace. The principle of the optimal steam injection was proposed as: the best steam injecting point was at the oil bubble point area, and the optimal steam injection rate was the smallest rate that guarantees correct two-phase flow pattern in all the tubes.A full-scale geometry model of the radiant section and the governing equations for the steady state and compressible flue gas system were developed for predicting the flow field, temperature field, combustion and radiative heat transfer. A fixed tube skin temperature profile taken from the calculation by Petro-SIM based on onsite data was applied and considered to be part of the boundary conditions of the simulations. The Flamelet model was employed for combustion modeling due to its high precision in predicting the NO emission, since it calculated O radical concentration from the flamelet library. The CFD calculation showed good agreement with onsite data, and analysis was made for the flue gas flow field, temperature field and inner relationship between the two as well as the NO emission in the radiant section.A horizontal cylindrical geometry was constructed to represent a segment of fired furnace tube, and a three-dimensional CFD model was developed for predicting the two-phase flow, heat transfer, phase change and thermal reactions inside the tubes of fired furnace. The oil system was represented by the SARA pseudo-components. A thermal reaction kinetic net from the reference was modified and the calculation was compared with experimental data, the results showed that the modified kinetic net sufficed in simulating the coking process inside the tubes. Detailed insights into the flow field, temperature field, phase separation and coke formation rate inside the tube were obtained at different severities of petroleum type and tube wall temperature.The new steam injection technology proposed in the present study was of great significance for coke prevention in refining vacuum furnace, and the analyzing approach provided theoretical guide for the process side optimization of the furnace. The CFD model frameworks developed for the simulation of both the process side and flue gas side could be employed to assess the designing scheme of the furnace, and guide the optimal design of the tubing shape and arrangement, the chamber structure, operating condition et al.