Study On Particle Deposition On The Outer Wall Of Vortex Finder And Separation Performance In A Cyclone Separator

Residua Fluidized Catalytic Cracking(RFCC)is one of the major processes in refineries for the production of fuel oils.However,with the rise of the resid blending percentage,coke deposition within RFCC units is getting more and more severe,which has been one of the main challenges the refineries facing.Coke formation process in the RFCC disengagers relates to a complicated sequence of chemical and physical changes,and involves in the following two aspects,one is the chemical reaction mechanism of coking,and the other is the movement to the wall,the adhesion,the deposition and the solidification on the wall of oil droplets and/or catalyst particles.However,the present studies were primarily concerned with the former,and less attention had been paid to the latter.Aimed at the latter,by methods of experimental study,theoretical analysis and numerical simulation,the influences of wall roughness on separation performance and particles deposition on the outer wall of the vortex finder of cyclone separator were investigated.Especially the advantages of separation and mitigation of coke deposition on the vortex finder outerwall of the two designed cyclone separators were discussed.The main contents and results of this article are as follows:(1)For the first time,through the cold model experiment and simulation method,the law of wall roughness on the particle deposition on the outer wall of vortex finder of cyclone was studied in a strong swirl field.Cold-model particle deposition experiments showed that the larger the roughness was,the easier the particles with a diameter of10?m or less deposited,and the median particle diameter increased slightly.However,the wall roughness had little effect on circumferential distribution of the deposited particles on the outerwall.Furthermore,the particles deposition layer showed an asymmetric distribution characteristics.In the favorable pressure gradient zone,the deposited particles kept tight,and their diameters were very small with almost less than10?m.In the adverse pressure gradient zone,the deposited particles were loose and their diameters were relative bigger.Within a certain distance from the top plate of the separator,there existed a section of non-grain deposition area.Increasing roughness or decreasing inlet gas velocity would reduce the non-grain deposition area.Roughness greatly reduced tangential velocity near the outer wall of vortex finder(range from 0?15mm)and increased the space of the adverse pressure gradient zone.Which would lead to the increase of the thickness of the boundary layer on the outer wall both in the favorable pressure gradient zone and adverse pressure gradient zone.As a result,the capacity of insufficient flow scouring decreased and particles deposited easily.Due to the increase of the adverse pressure gradient zone,the backflow increased,more particles would enter the boundary layer to increase the chance of deposition.The roughness had little effect on flow field in separation space,but it would increase the short-circuit flow to a certain extent.Fine particles easily escaped from the vortex finder through the short-circuit flow,resulting in the decrease of separation efficiency.The critical St_cr of the deposited particles was 0.126.When St<St_cr of the particles,the probability of particles deposited was higher.Generally,most of the particles deposited on the outer wall of the vortex finder had a diameter of less than 10?m.The results of the cold model experiment of particle deposition were consistent with the actual production.(2)Secondly,in this article,separation performance and flow field of cyclone separators were investigated at a wide range of roughness by the means of experimental measurement and simulation.The results showed increasing the roughness would lead to the decease of tangential velocity and pressure drop due to the friction between fluid and wall.The maximum pressure drop could reach 38%in the range of roughness(0.01mm?2 mm)in our experiment.Separation efficiency had a critical value with the increase of roughness,and increasing roughness would reduce the maximum efficiency inlet velocity.The simulated results indicated that the axial velocity near wall in separation space was almost not affected by roughness,however,the upward axial velocity in the center region increased.The particles trajectory was significantly influenced by roughness,especially for particles with the diameter of 5?m or less.The influence of roughness on the internal flow field of different size separators was consistent.But the flow field of large size separators was much more affected by the same relative roughness.Based on our research,the Muschelknautz pressure drop model was modified for wall friction coefficient under pure gas flow.Compared with the published literature and experimental data,it had good applicability and accuracy with the maximum relative error less than 9%.(3)Then,from the point of view of uniforming flow field in annular space and eliminating the pressure gradient zone and the pressure gradient zone along the outer wall of vortex finder,the swirl vanes cyclone separator was designed to suppress the particle deposition.Numerical simulation and cold model experiment were carried out to study the aspects of restraining particle deposition and guaranteeing the separation performance.The results showed that,due to the multi-inlet structure the favorable pressure gradient zone and adverse pressure gradient zone near the wall of the vortex finder were eliminated.And the deposition of particles on the outer wall of vortex finder was greatly reduced.Compared with the conventional PV separator,it had a uniform particle size distribution in the circumferential deposition.Increasing the inlet velocity or reducing the particle concentration would reduce the median diameter of deposited particles of the two separators.As well,the distribution of particles in the annular space of the two separators was extremely uneven,especially the larger particle,particularly in swirl vanes separator.The radial pressure gradient was smaller than that of the PV separator,which could suppress the compaction of deposited particles effectively.The simulation showed that although the pressure drop of dual-passage swirl vane cyclone separator was higher,it had more advantages than that of the one-passage swirl vane cyclone separator in restraining particle deposition and improving separation efficiency.(4)Finally,for the purpose of destroying boundary layer near the outer wall of the vortex finder,a slotted vortex finder was designed.The results revealled that particles deposition was significantly reduced with conical slotted vortex finder.Due to the flow escaping through the slots,the low-velocity boundary layer was destroyed and tangential velocity near the outerwall increased.In addition,the slots not only reduced the flow resistance loss,but also controlled particles escapement due to inertial separation.The upper flow shielding formed on the outer wall of vortex finder,which eliminated the short-circuit flow at the end of the vortex finder,restricted the escape of fine particles through the short-circuit flow and improved the separation efficiency.Conical slotted vortex finder with convex platform structure could reduce the top dust ring.It could reduce the probability of fine particles escaping through slot and conveying particles to the outer wall of vortex finder,improve the separation efficiency and reduce the source of deposited particles.Simulation results indicated that,compared with the straight-tube vortex finder,tangential velocity of the flow was higher near the outer wall of the vortex finder due to the slot.However,the tangential velocity in the separation space of the slot vortex finder was lower than that of the straight-tube vortex finder due to the decrease of the flow entering the separation space.On the side of the outer wall near the roof,the"secondary vortex"rotates around the vortex finder and the vortex center at the same time moved forward as"twisting flow".It had been confirmed by the cold-model particle deposition experiments and industrial practices that the particles were not easily deposited here.Based on the pressure drop distribution theory proposed by Chen Jianyi,the pressure drop composition of the cyclone separator with conical slotted vortex finder was analyzed,and the pressure drop model under pure gas flow condition was established by modeling the loss of each part.Compared with the experimental data,the established pressure drop model had a good accuracy.Finally,the cold model test and numerical simulation of straight cylinder slotted vortex finder were carried out in D900 mm cyclone separator.The results showed that the straight cylinder slotted vortex finder could not only effectively inhibit the particle deposition on the outerwall of the vortex finder,but also improve efficiency and reduce pressure drop.The results provide a basis for further industrial development.