Experimental Study On Separation Of Liquid And Solid Particles In A Fischer-Tropsch Slurry Bed Reactor

The slurry bed reactor,favored for its simple structure and excellent heat and mass transfer capabilities,is extensively utilized in various domestic low-temperature FischerTropsch(F-T)synthesis plants.Within the slurry bed,the F-T syngas ascends as bubbles,diffuses through the liquid phase,and engages with suspended catalysts to continuously yield liquid hydrocarbons.The efficiency and stability of production in these plants are critically contingent upon the effective and consistent separation of the liquid hydrocarbons and the retention of catalysts post-separation.Systematic experimental research has been conducted to address issues including low separation capacity of liquid products and catalysts in an industrial F-T synthesis slurry bed reactor,as well as the high content of catalyst fines in the slurry.This research aimed to find effective solutions to the aforementioned industrial challenges by thoroughly understanding the theories and operational characteristics of slurry bed filtration separation,and particle classification.The main research contents and results are as follows:1.Experimental study on in-vessel filtration and back-flushing characteristics in slurry bed F-T reactors.A cold model experimental setup,based on the in-vessel filtration system of an industrial F-T synthesis slurry bed reactor,is established to simulate the in-vessel system and back-flushing process.This was done to identify key factors limiting the system’s filtration performance and to explore innovative methods for enhancing separation efficiency by thoroughly investigating filtration and back-flushing characteristics,the impact of reactor flow fields on the filtration process,and the influence of various factors on filtration characteristics.A comparison of the average gas holdup between the filtration process and nonfiltration process revealed that the gas holdup significantly decreased during the filtration process due to gas leakage from the filter.To quantify the amount of gas leakage,the gas leakage flux was defined and a quantitative estimation method developed.The analysis under different operating conditions revealed that increasing the superficial gas velocity increased the gas leakage flux,leading to a decrease in liquid filtration flux.Increasing the filtration differential pressure and reducing particle concentration resulted in an increase in gas leakage flux and liquid filtration flux,but the impact of gas leakage limited the increase in liquid filtration flux.The comparison of liquid filtration flux after gas and liquid back-flushing revealed that liquid back-flushing is superior to gas back-flushing.There is threshold value for both gas and liquid back-flushing pressures under different particle concentration,beyond which the recovery of filtration performance becomes insignificant.Due to continuous contact with the slurry and prolonged waiting time,leading to an early decline in the backflushed filter filtration performance.Gas back-flushing leads to the formation of large air bubble clusters that release from the filter tube pores,causing severe and sustained pressure fluctuations in the bed layer,seriously affecting the operational stability of the slurry bed reactor.2.Experimental study on external cross-flow filtration and back-flushing characteristics of slurry bed F-T reactors.To address the issues of low filtration capacity,gas leakage,ineffective backflushing,and significant bed pressure fluctuations in the in-vessel filtration and backflushing process,an external filtration scheme was proposed.Experimental studies were systematically conducted to investigate the characteristics of external cross-flow filtration and back-flushing,which were then comprehensively compared with in-vessel filtration processes.Analysis revealed that the slurry entering the filter from the top of the slurry bed had a uniform particle concentration and size distribution,creating favorable conditions for external filtration.A comparison of liquid filtration flux and filter cake parameters showed that changes in superficial gas velocity,particle concentration,and filter tube pore size not only affected the slurry circulation velocity but also altered the particle size deposited within the filter cakes.This,in conjunction with the thickness of the filter cake,jointly influenced the performance of the external filtration.Upon increasing the pore size of the filter tubes,a significant increase in liquid filtration flux was observed,and due to the effective interception of the rapidly formed cake layer,there is no substantial or prolonged loss of particles.The back-flushing process was completed almost instantaneously,with extended back-flushing time showing no significant effect on the filtration performance recovery.There was also a threshold value for back-flushing pressure,beyond which no notable improvement in filtration performance was observed.Comparisons with in-vessel filtration revealed that external cross-flow filtration significantly enhanced the liquid filtration flux under various conditions due to the avoidance of gas leakage and maintained a better filtration state for a longer duration.The study concluded with a specific implementation plan for external cross-flow filtration.Compared to internal filtration,the reduction in the number of filtration groups,shortened waiting times,and avoidance of contact with the slurry prevented premature declines in filtration performance.Additionally,cutting out the entire filter and draining the retained liquid before back-flushing eliminated disturbances to the slurry bed layer.3.New method for continuous classification of fine catalyst particles from a slurry bed F-T reactor.During the operation of an industrial F-T synthesis slurry bed reactor,the continuous generation of fine catalyst particles was observed.The presence and increase of these fines had a detrimental impact on the performance of the filter,requiring frequent replacement and disposal of the catalyst.This resulted in significant catalyst waste.To address this issue,a novel approach utilizing a hydrocyclone for continuous classification of catalyst fines was proposed.To verify the feasibility of the proposed technology,a cold model experiment was conducted.Experimental findings indicated that,under high particle concentration,the normal classification of hydrocyclone failed to simultaneously achieve a high fine particle removal ratio and a low coarse particle loss ratio.To address this issue,a "dialysis" continuous classification technology for fine particles,suitable for slurry bed reactor conditions,was proposed.This technology was developed after thoroughly analyzing the particle distribution characteristics and variations in classification performance with separation efficiency in the overflow and underflow.By optimizing the structure and operating conditions of the hydrocyclone,the feasibility of the "dialysis" classification technology was confirmed.Industrial hot side stream tests demonstrated that maintaining a high-temperature slurry could achieve a higher fine particle removal ratio while keeping the coarse particle loss ratio relatively low.A prediction model for "dialysis" technology was established.Although the time for fine particles in the "dialysis" continuous classification process was relatively long,multiple classification cycles significantly reduced the fines content in the slurry without excessive cumulative loss of coarse particles.Given the results of the initial cold model experiments and industrial hot sidestream tests,the new technology received approbation from the commissioned enterprise.Currently,it is undergoing industrial design and specific implementation.The aforementioned research findings not only address the issue of liquid and solid particle separation in the industrial F-T plants but also have broader applications in other similar slurry bed systems.