Investigation On Gas-Liquid Mixing And Dispersion Facilities Of A Hydroprocessing Reactor

Catalytic hydroprocessing is a mature technology practiced in the petroleum refining industry for upgrading a variety of hydrocarbon streams ranging from straight-run naphtha to vacuum residua or even heavy crude oils. Nowadays, the hydroprocessing technology is of vital importance to the refining industry, as a result of the growing market of high value petroleum products and decreasing availability of light oils. Since the introduction of hydroprocessing technology, catalyst suppliers have made significant advances in improving the relative activities of hydroprocessing catalysts. However, the design of hydroprocessing reactors has not advanced at the same pace as the development of hydroprocessing catalysts, and the existing reactors are not capable of utilizing the benefits of the high activity catalysts. To develop novel types of internals with excellent performance, mixing and dispersion of gas and liquid for internals (gas-liquid distributor and quench box) of the fixed bed hydroprocessing reactor was investigated in this paper.To overcome shortages of the traditional gas-liquid distributor, a novel gas-liquid separated flow distributor was designed in this paper, which was supposed to show low sensitivity to the tray levelness and distribute liquid uniformly. The relationship between the breakup of liquid and the liquid distribution uniformity, and effects of operating conditions on the pressure drop of the novel distributor were studied through cold model experiments. Furthermore, the liquid distribution uniformity of a distribution tray, which was composed of37gas-liquid separated flow distributors, was investigated. Results show that, there exists a critical gas volume flow rate for the atomization of liquid. Liquid can be distributed uniformly only when the gas volume flow rate is higher than the critical value. The relative liquid distribution nonuniformity of the distribution tray is lower than5%when the gas volume flow rate is higher than the overall critical value, which reveals the uniform distribution of liquid. Besides, a brief correlation between the pressure drop of the novel distributor and Reynolds number of two phases was proposed. After that, experiments were conducted to investigate effects of geometry structures on the hydraulic performance of the gas-liquid separated flow distributor, such as sensitivity to the tray levelness, liquid breakup performance and pressure drop. Results show that, the optimal comprehensive performance of the gas-liquid separated flow distributor can be obtained, when two gas-inlet holes are drilled as well as the center distance between gas-inlet and liquid-inlet is spaced at approximately55millimeters.To enhance the gas-liquid and liquid-liquid mixing efficiency of the quench box, a novel structure based on supergravitational swirling flow was proposed, which owns multiple swirling flow structures. To reach supergravitational swirling flow, the Froude number of liquid in the horizontal swirling tube should be greater than unity. Through computational fluid dynamics (CFD) simulation and using a high-speed camera, the supergravitational swirling flow was visualized. Based on the Higbie’s penetration theory, CFD simulation was conducted to estimate the gas-liquid mass-transfer coefficient, and was compared with the oxygen absorption experiment. It shows the gas flow rate plays a key role in the gas-liquid mixing performance in the quench box. Experiments were also carried out to investigate effects of operating conditions on the pressure drop of the two-phase flow in quench box, and a correlation was proposed accordingly. Furthermore, to study effects of operating conditions on the micromixing process, cold model experiments were carried out based on the competitive and consecutive reaction between1-naphthol and diazotized sulphanilic acid, and the segregation index was used to characterize the micromixing performance. Results show that, the segregation index decreases sharply with the increase of the gas flow rate, while increases with increasing liquid flow rate. This indicates that intimate contact between gas and liquid can be realized in the novel designed quench box, while the turbulence intensity and micromixing efficiency of the liquid can be notablely improved by taking full advantage of the gas phase’s driving effect. The relationship between segregation index and micromixing time was obtained according to the incorporation model. Under the operating conditions of this work, the micromixing time was calculated to be40～100ms, which is far less than the average residence time and is of the same order of magnitude as the mechanically stirred tank. Hence, it illustrates that the composition and temperature of liquid should be perfectly homogenized at outlet of the supergravitational swirling-type quench box, and the novel quench box owns excellent liquid-liquid mixing performance.In addition, to investigate mixing and heat transfer capability of the supergravitational swirling-type quench box, gas-liquid and liquid-liquid heat transfer processes at industrial conditions were investigated through CFD simulation. Results show that, gas-liquid heat exchange rate will be increased significantly with the increase of gas flow rate, which shows the same tendency as the conclusions regarding studies of the gas-liquid mass transfer. Meanwhile, the simulation results of liquid-liquid heat transfer process show that, the maximum temperature difference of liquid at outlet of the supergravitational swirling-type quench box is lower than2K and is reduced by97%compared with that of the inlet.