Synthesis Of Functional Spherical Polymer Brushes And Its Application For Removal Of Metals From Crude Oil

In recent years,the content of nickel,vanadium,calcium and other metals in crude oil has been remarkably increased.Nickel is usually present in the resins and asphaltenes of crude oil.Nickel often exists in the form of oil-soluble organic salts represented by metalloporphyrin.With the traditional electrical desalination process,it can hardly be removed,which puzzles the industry and academia-How to remove oil-soluble metals in crude oil more effectively?One of the methods is chelation process,which has attracted plenty of concerns.This processing procedure only needs to adopt an additional demetallizing agent to the electric desalination.The advantages are clear,as there is no need for extra equipment nor drastic changes to the existing process.However,current demetallizing agents are mostly composed of strong acidic and common small molecular ligands.Such agents have relatively low coordination abilities with nickel,making it difficult to remove metals from the resins and asphaltenes.Based on the Donnan effect,polymer brushes with core-shell structure were innovatively designed in the present work.In the brushes,hydroxylated micron silica particles is the core,while its shell layer is polymer chains that have strong coordination with nickel.It can greatly increase the probability of ligand-to-metal contact and spatial coordination efficiency,overcoming the shortages of small-molecule ligands.It is highly efficient,non-acidic and recyclable.The scientific issues,research methods and main conclusions are as follows:(1)According to the coordination theory,14 amino acid and non-amino acid ligands are selected.Based on density functional theory(DFT),the binding energy of the complex formed by selected ligand and nickel was calculated.The nickel removal rates obtained from the experiments were compared.The calculation results show that the absolute value of the binding energy of the complex formed by 4-vinylpyridine and nickel is-198.681 kJ/mol,indicating that this complex is the most stable one with the strongest adsorption capacity for nickel among the 14 ligands.The binding energy of macromolecular protoporphyrin and nickel is-2394.876 kJ/mol,and its absolute value is much larger than that of the complex formed by 4-vinylpyridine and nickel.This indicates that it is difficult to remove nickel from porphyrin compounds with small molecule ligands.The experimental value of the nickel removal rate using 4-vinylpyridine is also the highest(22.1%),which is consistent with the DFT calculation results.Combining the results of DFT calculation,electrical desalting experiments and the requirements of the polymer brushes,vinyl pyridine,1-vinyl imidazole and N-vinyl pyrrolidone are selected as ligand functional groups,and the spherical polymer brushes with core-shell structure as crude oil demetallizing agent were proposed.That is,under the Donnan effect,the closer it gets to the silica core,the higher density of ligand functional groups will be in the shell layer,which improves the space coordination efficiency of small molecule ligands and metals,thereby greatly improving the nickel chelation absorption capacity.(2)The internal relationship between the type of ligand on the polymer brushes,the polymer grating ratio,and the metal removal rate were revealed.Using the method of chemical grafting,polyvinyl pyrrolidone polymer brush(PVP@SiO2),poly 2-vinyl pyridine polymer brush(P2VP@SiO2),poly 4-vinyl pyridine polymer brush(P4VP@SiO2)and polyvinyl imidazole polymer brush(PVIm@SiO2)are constructed.Polyvinyl pyrrolidone polymer brush(PVP@SiO2-C)is also successfully prepared using the physical adsorption method.These polymer brushes are used as crude metal demetallizing agents.The results have shown that the demetallization effect of PVP@SiO2 prepared by chemical grafting is significantly better than PVP@SiO2-C prepared by physical grafting.The steric hindrance effect caused by the different substitution positions of nitrogen atoms in the pyridine ring leads to P4VP@SiO2's better outcome than P2VP@SiO2's.In addition,the higher grafting ratio of the polymer on the polymer brush,the more effective ligand functional groups it contains,which is more conducive to the removal of metals in crude oil-the synthesis process has therefore been optimized.Eventually,it is found that the polymer brushes containing the same ligand functional group have a much higher demetallizing effect on crude oil than polymer chains,which proves that the Donnan effect can significantly enhance the chelating adsorption capacity of ligand functional groups to metals.(3)Appropriate electrical desalination process parameters are also vital to the demetallization efficiency of crude oil.Hence,this paper explores the effect of different electrical desalting conditions on the effect of polymer brush demetallization.The results show that with the gradual increase of the amount of polymer brush and the settling time,the demetallization effect will increase first and then tend to relax;with the gradual increase of electric desalination temperature,water/oil ratio,shear emulsification strength,emulsification time and electric field strength,the demetallization effect will increase first and then decrease.(4)In order to investigate the effect of other metals on the nickel removal effect of polymer brushes and compare the adsorption capacity and selectivity of polymer brushes of different systems to nickel,the most representative calcium naphthenate was selected as the model compound to be added to crude oil.Through electrical desalination experiments,the effects of PVP@SiO2,P4VP@SiO2,and PVIm@SiO2 on the removal of nickel and calcium are investigated.The results have shown that the adsorption order of the three polymer brushes for nickel and calcium follows P4VP@SiO2>PVIm@SiO2>PVP@SiO2(strong to weak),and the order of adsorption selectivity for nickel is PVP@SiO2>PVIm@SiO2>P4VP@SiO2(strong to weak).Combined with the experimental and DFT calculation results,models of ligands 4VP,VP,and VIm forming complexes with nickel and calcium are established.The binding energy data shows that the order of adsorption capacity of the three ligands for nickel and calcium is 4VP>VIm>VP(strong to weak),while the adsorption selectivity for nickel follows VP>VIm>4VP(strong to weak).The DFT calculation results are consistent with the experimental results,further reveals the reasons why the three polymer brushes have different adsorption behaviors for nickel and calcium on the molecular scale.(5)In order to investigate the recyclability of the polymer brushes,0.5mol/L hydrochloric acid solution was used to desorb the nickel and calcium.After five electrical desalting-desorption recycle processes,PVP@SiO2,P2VP@SiO2,and P4VP@SiO2 synthesized by chemical grafting can still maintain good removal performance.However,PVP@SiO2-C prepared by physical adsorption illustrates a clear decrease in performance.This result proves that the polymer brushes prepared by chemical grafting have more stable properties and can be recycled and reused through a simple process.This will not only benefit the subsequent wastewater treatment process after the electric desalination but also enhance the economic benefits the polymer brush could gain as a demetallizing agent.