Study On The Effect Of Distillation Technology On Removal Of Polycyclic Aromatic Hydrocarbons In Coal Tar Pitch

Road construction is closely related to the growth of the national economy and the improvement of people's living standards.Coal tar pitch(CTP)used as road material,has good wetting and adhesion properties to various types of broken stones,and the road paved with coal tar mixture has the advantages of oil resistance,corrosion resistance and large friction coefficient,etc.In addition,CTP is rich in resource and inexpensive in price in China,and its economic benefit is better than petroleum asphalt.But due to the fact that the content of toxic polycyclic aromatic hydrocarbons(PAHs)in CTP exceeds the relevant environmental standards,which can bring human health problems and environmental pollution,it's necessary to remove toxic PAHs in CTP.In order to achieve the aim removing toxic PAHs in CTP,CTP was distilled in atmospheric and vacuum pressure in the paper.First of all,Aspen Plus software was used to simulate the distillation process,which shows the feasibility and the possible effect of the experiments.After that,the removal efficiency of distillation technology was obtained by atmospheric and vacuum distillation test,and the conditions ofdistillation process were optimized at the same time.Next,the basic properties,water soluble matter content,equivalent toxicity of CTP and residues were measured,analyzed and compared.Finally,by means of elemental analysis,GC-MS,FT-IR,XRD and TG/DTG,the composition and structure of the distillation residues were characterized.16 priority PAHs by EPA is regarded as the toxic PAHs in the paper.The main conclusions are as follows:(1)PAHs in JYCTP were enriched by solvent extraction,and the solvent was selected according to the principle of "like dissolves like".In this paper,toluene was used to extract the PAHs,of which average extraction rate was 76.13%.By GC analysis,14 EPA priority PAHs were detected in toluene extracts,which don't contain acenaphthylene,and treat benzo[b]fluoranthene and benzo[k]fluoranthene as one.(2)In distillation simulation completed by Aspen Plus software,the samples were characterized by means of the combination of virtual and real components;the FLASH2 module in Separators was selected as unit operation module of distillation;the NRTL-RK model was the most suitable physical property method.Due to the loss of fractions and chemical reaction,the experimental results are slightly lower than the simulation results(3)The results of atmospheric and vacuum distillation experiments are as follows: With the increase of distillation temperature,the yield of the fractions increased,in which the Species and contents of PAHs increased.The PAHs content of the 2~3 rings in fractions remained unchanged at about 435 DEG C,and the PAHs of the 4~5 rings began to evaporate at about 405 DEG C.The yield of residues decreased with the increase of temperature,in which the content of PAHs decreased and the PAHs of the 2~3 rings were not detected after 435 DEG C.The effect of vacuum distillation was better than that of atmospheric distillation.When the distillation temperature was 425 DEG C,the total removal rate of PAHs in vacuum distillation residue was 49.82%.(4)With the increase of distillation temperature,the toluene insoluble matter and coking value of distillation residues increased.The softening point of the residues rose,and the softening point of the vacuum residues was greater than that of the atmospheric residues before 405 DEG C,and less after 405 DEG C.The extraction rate of residues decreased.At the same temperature,the extraction rate of vacuum residues was higher than that of atmospheric residues,and the difference increased with the increase of temperature.The equivalent toxicity of the residues was also reduced,and the reduction rate was about 40% at 425 DEG C.The content of PAHs in the residual water soluble matter was decreased obviously,and the content of PAHs in the water soluble fraction of vacuum residues was lower than that of atmospheric residues,which was reduced to zero at 425 DEG C.(5)The components and structures of distillation residues were characterized by elemental analysis,GC-MS,FT-IR,XRD and TG/DTG.Experimental results show: In terms of component variation,the number of aromatic hydrocarbons in solvent extract of distillation residues at 425 DEG C was more than that of JYCTP,and the relative content of aromatic hydrocarbons with 6 rings also increased.The difference of the species and the content was mainly due to the occurrence of low cyclic condensation and methyl substitution reaction.In terms of structural change,the C/H ratio and aromaticity()of distillation residues increased.The strength of characteristic peak of aromatic skeleton structure and aromatic ring substituted in distillation residues increased.The aromatic ring spacing(d002)decreased,the number of layers of aromatic ring(Me)and the stack height(Lc)increased,and the diameter of the aromatic lamellae decreases first and then increases.All the above changes from vacuum distillation residues were lower than those of atmospheric residue.