CFD Assistant Design Of The Struvite Fluidized Bed Reactor And Experimental Research

Phosphorus is an essential element of life.As the global demand continues to increase,the existing phosphate rock can be only used for about 100 years,furthermore,due to the one-way mobility of phosphorus and uneven distribution of phosphate rock,the problem of phosphorus shortage is becoming more and more prominent.At the same time,phosphorus is the main controlling factor of eutrophication in water,and eutrophication caused by excessive phosphorus not only seriously endangers the healthy development of agriculture,fishery,tourism and other industries,but also poses a great threat to the safety of food and drinking water.Struvite crystallization(MgNH4PO4·6H2O)method can simultaneously remove high concentration of nitrogen and phosphorus efficiently in water,meanwhile,the struvite products were only slightly soluble in water and soil humidity environment and the release rate of its nutrient is slower than most chemical fertilizer,which makes it a high grade phosphorus slow-release fertilizer.The struvite phosphorus recovery attracted widespread attention in recent years and is considered to be a most promising commercial phosphorus resource recovery technology.At present,classical multistage and newly developed awl tube fluidized bed reactors are two most common styles of fluidized bed reactor.Although they both have good prospects for application,on the one hand,the difficulty and cost of machining of the equipments are increased because of the complicated fluidization segment and restrictions of material selection;On the other hand,the local supersaturation at the feed port is one of the factors leading to the explosive nucleation of the crystal in the device,which makes the growth of the individual crystallite decreased,thus inhibiting the formation of large size of struvite pellets;in addition,the existing of massive crystallites causes a poor settlement of the effluent simultaneously,and a large volume of sedimentation tank is needed to settle the microcrystalline,but this will significantly increase the cost of the construction and operation of the device.To solve the problems above,through the combination of computational fluid dynamics and experimental research,our research successfully developed a compact fluidized bed reactor.In the new device,the fluidization segment is only made up of a vertical tube with the same diameter and can promote the formation of large size pellets through the optimization of inlet discreteness by controlling the local supersaturation in it,and the controlling of the excessive concentration by new inlet discreteness makes it possible for the reactor to operate without an additional sedimentation tank at both low and high supersaturation.The main conclusions are as follows:(1)Based on the principle of CFD,the cold model for the optimization of fluidization segment of struvite fluidized bed reactor was established using Fluent:First of all,the drag model Huilin-gidaspow and turbulence model Dispread were selected as the most suitable models in the simulation system;Then the applicability of the optimized cold model to the system were verified by the comparing of simulation and experimental results(height of struvite pellets bed),according to the results:the difference between heights of struvite pellets bed given by simulation and experiment were all less than 20%for different sizes struvite pellets and rising velocities,which indicates that the cold model is suitable to the simulation of the research system;(2)The established cold model was used to simulate turbulence intensity and pellets stratification in three fluidized segments,and the results shows:the turbulence intensity is uniform in the new developed one-stage reactor and the pellets stratification is the best of all,which proves that it has potential granulation ability.Then the simulative wastewater experiments were conducted with the new processing device to further verify its granulation ability,and the experimental results showed that the new device has a good soluble phosphate and total phosphorus removal rate with an additional settling tank,and can produce much struvite products with uniform pellet size;(3)To reduce the inhibition of formation of large struvite pellets caused by excessive concentration of materials at the feed port,hot simulation and experimental research were combined to further optimize the structure of the one-stage struvite fluidized bed reactor.First,a thermal model is established by coupling the population balance model to simulate the supersaturation in the inlet assembly.According to the results,split inlet assembly has a good effect on the controlling of local supersaturation:compared with opposed inlet assembly,the distribution of supersaturation is more uniform and the maximum supersaturation is lower in split inlet assembly.At the same time,the experimental results also proves that the split inlet assembly can effectively control the local supersaturation,thus increasing the amount of large struvite pellets.The subsequent experiments aiming at eliminating sedimentation tank showed:the combination of split inlet assembly and return port at the side of top storage tank can produce much large struvite pellets in the absence of sedimentation tank,and can withstand the impact of high supersaturation.Meanwhile,the combination of opposed inlet assembly and return port at the side of top storage tank can also produce much large struvite pellets in the absence of sedimentation tank at low supersaturation,but the granulation capacity was inhibited by high supersaturation;(4)Product characteristics and security are two important aspects that must be taken into consideration in the process of resource recovery,this study investigated the transfer behavior of heavy metals during the struvite recovery from swine wastewater,and the feasibility of the new design of one-stage fluidized bed reactor for real wastewater treatment was further verified.First,the concentration and distribution of heavy metals in swine wastewater were investigated and the results showed:the concentration of Zn,Mn and Cu was the highest in swine wastewater,and the value are all above 200 ?g/L,the high concentration of Zn,Cu was mainly introduced by the heavy metal additives and Mn was mainly came from soil.Meanwhile,except for Mn,HMs in SW were mainly distributed in suspended phase,and for much of the HMs-ss were rushed out by the ascending streams in FBR,the HMs contents in Struvite-FBR were much lower than those of Struvite-BsTR.For Struvite-BSTR only,its HMs contents were mainly controlled by hydrodynamics rather than thermodynamics:most HMs would be further decreased as the increase of upflow rate,while the recycled ratio almost took no effect.For StrruviteBSTR,the reduction can be achieved by the pretreatment process of centrifugation.For both,the main ways of HMs accumulated in Struvite-FBR and Struvite-BSTR are both related to organic matters and acid radicals in SW,and the transfer of HMs during struvite crystallization can be divided into two steps:1)HMs was first concentrated in SS(before pH adjustment)or precipitates(after pH adjustment)in the form of metal precipitates or organically bounded compounds and 2)co-precipitated with struvite.