Research On Process And Mechanism For Deep Removal And Resource Utilization Of Thiophene From Coking Benzene By Alkylation And Adsorption Methods

Benzene is an important chemical feedstock. As one of its main sources, the coking benzene is rich in China, but its utilization range is greatly limited by the relatively bacfiward refining process. Nowadays, its industrialized refining processes mainly include the sulfuric acid washing, the catalytic hydrogenation and the extractive distillation. Unfortunately, those processes have some disadvantages, for example, the low desulfurization efficiency and environmental pollution caused by the waste acid, the high operation costs and consumption of H2, the big consumption of extracting agent and poor cyclic utilization. Based on the above background, a novel refining process of coking benzene with the advantages of high desulfurization precision, easy operation, low environmental pollution and thiophene recyclability was suggested. First, the olefins and thiophene in coking benzene, which is previously washed by sodium hydroxide aqueous solution to remove the phenols, were simultaneously removed by catalytic alkylation reaction. And then, the residual thiophene in benzene was deeply removed to below1ppm by adsorption method using the sorbent. The sorbent adsorbed thiophene was used to prepare the polythiophene-zeolite composite to recycle thiophene. Based on the experimental researches, including the preparation of catalyst for thiophene alkylation, the preparation and regeneration of sorbents for the deep removal of thiophene, adsorptive mechanism for thiophene and the preparation of polythiophene composite, the main results are shown as follows.(1) AlCl3/silica gel catalysts were prepared by airtightly grafted method in the autoclave and their catalytic activities for the alkylation of thiophene (700mg/L) with1-hexane (4690mg/L) in benzene were studied. The effects of preparation conditions, such as grafted temperature, mass ratio of AICl3to silica gel support, grafted time and thermal treatment temperature of the silica gel, on the activity of AlCl3/silica gel catalyst were inverstigated. XPS, SEM, EDX and BET techniques were used to characterize the physical and chemical properties of prepared catalyst samples, and the gas chromatographs coupled with flame photometric detector and mass spectrometer were used to quantitatively determine the thiophene in benzene and qualitatively measure the reaction products, respectively.The results show that AICl3can be effectively anchored by reacting with the silanol groups on the silica gel surface to form the Six(OH)y_aOaAlCl(3-a) species. The results show that the catalytic effect of AICl3/silica gel catalyst on the alkylation reaction of thiophene with1-hexane is obvious and the main reaction product is alkylthiophene. The activity of catalyst can be impacted by the preparation conditions by adjusting the content and distribution of the active component over the AlCl3/silica gel catalyst. The optimum preparation conditions of AlCl3/silica gel catalyst are that AICl3is grafted on silica gel support, which has been thermally treated at400℃for3h, at180℃for4h with the mass ratio of0.20for AlC13to silica gel support. The loading ratio of AICl3on silica gel can reach to about64wt%and the optimal efficiency removing thiophene from benzene is94.23%when the ratio of liquid to catalyst is20mL/g.(2) S10Cu, S10Zn, S10Ni, S10Ce and S10Ag sorbents, with10wt%Cu, Zn, Ni, Ce and Ag metal components respectively supported on γ-Al2O3carrier, were prepared by incipient-wetness impregnation method. CuYE1, ZnYE1, NiYE1, CeYE1and AgYE1sorbents, with the above metal ions respectively modified NaY zeolite, were also prepared. And the static adsorption experiments for removing thiophene from benzene were carried out in the thiophene-benzene solution with thiophene concentration of500mg/L to choose the suitable sorbent.The results show that the desulfurization activity of all γ-Al2O3sorbents modified by different metal components obviously increases and follows the order of S10Ag> S10Ni> S10Ce> S10Cu> S10Zn. The active components of sorbents from Cu, Zn, Ni, Ce nitrates loaded on γ-Al2O3carrier are their oxides. Besides Ag2O, the products of silver nitrate thermal decomposition in sorbent prepared still have Ag0and Ag-O-Al species. The assistant ultrasound in the process of sorbent preparation can not only shorten the impregnation time, but also enrich the pore structure of sorbent and improve the size and distribution of the Ag species, which is favorable to the removal of thiophene from benzene. The desulfurization capacity of sorbent changes with the Ag content loaded. The S15AgU sorbent with15% quality content of Ag prepared by ultrasound-assisted impregnation method has the highest desulfurization efficiency. It could reduce the thiophene concentration to1.7mg/L from500mg/L at room temperature and ambient pressure, with the desulfurization efficiency of more than99%, when the ratio of solution to sorbent was4mL/g.Among the zeolite sorbents, which is the modified NaY by different metal ions, the CeYE2sorbent has the best desulfurization proformce. It can remove almost all the thiophene from benzene at room temperature and ambient pressure with the ratio of solution to sorbent was4mL/g, which is better than that of S15AgU sorbent. The results of XRD and BET characterization indicate that the preparation conditions mainly influence on the crystallinity, specific surface area of CeYE2sorbent. And the optimal CeYE2sorbent can be obtained when NaY zeolite is ion-exchanged in0.1mol/L Ce(NO3)3solution at100℃for4h, calcined at700℃for2h, and then repeated the above steps.(3) The effect of operation parameters, such as the particle size of CeYE2sorbent, the flow rate and concentration of solution and the temperature of fixed bed, on the adsorption of thiophene was investigated. The results show that those operation parameters affect the capacity of CeYE2removing thiophene from benzene by changing the mass transfer resistance in the inner and external diffusion process and the adsorption type. The suitable operation parameters are the particle size of0.2～0.3mm, the solution flow rate of0.15mL/min and the room temperature. CeYE2sorbent can completely remove the thiophene from benzene under the optimum conditions and the maximum adsorption capacity" of thiophene is2.88mg/g with the thiophene concentration of500mg/L.(4) Thiophene, tetrahydrothiophene, cyclohexane and benzene were selected as the model compounds and their adsorption capacity over CeYE2sorbent was measure to investigate the adsorption mechanism of thiophene and the competitive effect caused by benzene through XRD, FT-IR and XPS characterizations. The results show that the thiophene can be adsorbed on CeYE2sorbent via three manners, i.e., the S-Ce connection, π-complexation between thiophene molecular and cerium ions, and the connection between molecular and acid sites on CeYE2sorbent, while benzene acts through the later two manners. The competitive effect on thiophene adsorption brought out by benzene can be attributed to the competition of π-complexation adsorption sites, the acid sites and the steric hindrance. The adsorption capacity to thiophene of CeY300and CeY550sorbents, which prepared by reducing the valence state of cerium on CeYE2sorbent in hydrogen atmosphere at300℃and550℃respectively, was determined to study the effect of valence state of cerium on thiophene adsorption. The results show that Ce(III) is more favorable for the thiophene adsorption, compared with Ce(IV).(5) The used CeYE2after adsorbing thiophene (marked with TY) was chosen to study the effects of regeneration atmosphere and temperature on its regeneration proformance. And mass spectrometer (MS) was used to detect the products during its regeneration process. The results show that the regeneration atmosphere and temperature paly an important role on the regeneration property through the effluences of the carbon and sulfur content of TY sample and its structure. Moreover, during the regeneration process, the sulfur in TY sample can easily releases in air, oxygen and hydrogen atmosphere, while is hardly in argon atmosphere; For the carbon, air and oxygen atmosphere are very favorable for its release but the argon atmosphere is not useful. During the heating process, the catalystic cracking reactions of thiophen in TY sample firstly occur, and the products can be transformed into different compounds by followed reaction in different atmosphere, when the temperature is higher than400℃. And the temperature would bring out the effect of collapse on the structure of sorbent, which goes against its regeneration proformance. The most suitable regeneration condition is under air atmosphere at500℃for5h. And under this condition, the desulfurizaiton precision of regenerated sorbent remains the same as that of fresh CeYE2sorbent, and its breakthrough capacity for thiophene after three times adsrorption-regeneration circles still holds the69.44%of fresh sorbent.(6) CeYE2sorbent adsorbed thiophene from benzene in fixed bed adsorption experiment (TBY) was used as the raw material and series of polythiophene-CeY composite materials with different content of polythiophene were prepared. The results of FT-IR and SEM show that used CeYE2sorbent after removing thiophene from benzene can be used to prepare the polythiophene-CeY composite material, in which the polythiophene has α-α connection mode between thiophene molecules. The results of XRD, TG and conductivity characterization show that the polythiophene-CeY composite material remains the crystal structure of Y zeolite and its thermal stability and specific conductance are all better than those of polythiophene. The sample prepared by thiphene and TBY with the ratio of2.5:1g/mL has the highest specific conductance of1.001μS/m.