摘要
随着人们的环境保护意识不断增强以及越来越严格的环境法规的颁布,世界各国对石油及其产品中含硫量的控制也越来越严格。目前常规的石油脱硫工艺普遍存在工艺复杂、设备要求苛刻、投资运行费用高等问题,特别是对油品中的低浓度噻吩型硫化物的深度去除还比较困难。因此,探索新的石油脱硫方法显得非常必要。
针对石油中所存在的噻吩型硫化物不易去除的问题,本研究首次提出并探索了以γ射线辐照技术为基础的辐射转化方法,通过其作用将石油中的噻吩硫转化为相对容易去除的硫化物,以便于其进一步去除。本文以十二烷为模拟油品,研究了其中的噻吩型硫化物在γ射线辐照下的转化反应规律及作用机制等问题,并取得了如下重要结论:
单独以γ射线对油品进行辐照处理时,辐射对噻吩型硫化物有明显的转化作用。当辐射剂量率为3.8kGy/h、总辐射剂量为160kGy时,浓度为500mg/L的苯并噻吩(BT)及二苯并噻吩(DBT)转化率分别约为74%和33%。同等辐射剂量条件下,初始浓度越低时,DBT的转化率越高;剂量率越高,DBT的转化率也越高;氧气对DBT的辐射转化有一定抑制作用。噻吩型硫化物中的硫在单独辐射作用下的主要转化产物为二硫化物。
考察了一些化学添加物对噻吩型硫化物的辐射转化作用的影响。当以四氯化碳作为辐射敏化剂,其只对二甲基噻吩和苯并噻吩的转化有比较明显的促进作用,而对DBT的辐射转化作用不明显。而在反应体系中添加双氧水和乙酸以后,DBT的转化率得到了显著地提升。当油相、双氧水、乙酸的比例为3:1:1,辐射剂量为200kGy左右时,二苯并噻吩的转化率在90%以上,而且几乎全部转化为二苯并砜。
制备并筛选了部分催化剂,考察了其在辐射共同作用下对DBT的转化作用。结果发现,负载在γ-氧化铝上的氧化钴或氧化锆均具有明显的催化活性。在剂量率为3.9kGy/h,辐射剂量为179.2kGy时,500mg/L的DBT在钴催化剂或锆催化剂的作用下,转化率分别达到了80%和95%以上。不同的制备条件对DBT的辐射转化率有直接的影响。有催化剂存在时,辐射剂量率越低,DBT的转化率反而越高;通气条件可显著促进DBT的转化。DBT在辐射与催化剂共同作用下转化的主要产物是二苯并砜,且该产物通过在催化剂上的吸附而从油相中分离,达到脱硫效果。另外,对催化剂的寿命及再生方法进行了初步考察。通过X-射线衍射、X-射线电子能谱、扫描电子显微镜等技术对催化剂进行表征,结果表明射线对催化剂的晶体结构、元素价态以及表面形态等都没有影响。一级反应动力学模型可以较好地描述DBT的辐射转化规律,催化剂可使反应速率明显加快、辐射能量利用率显著提升。
本文研究初步探明了噻吩型硫化物在不同辐射条件下的转化规律。所探索出辐射结合催化的方法具有较好应用潜力,可望为石油的深度脱硫提供一条新途径。
As the environmental standard is getting stricter, the limitation values of sulfur content in oil and oil products have kept decreasing worldwide. However, there exist many problems with the conventional petroleum desulfurizaiton technologies, such as complicated techniques, rigorous operation requirements and high investment. Especially, it is very difficult to remove low concentration thiophenes compounds in the oil. Therefore, it is necessary to develop a novel desulfurization technology.
In the present study,γ-rays radiation method was put forward and investigated to convert thiophenes to any other compounds that were easily removed from the oil. Dodecane was used to simulate oil to study the radiated conversion of thiophene type compounds, and the kinetic model was studied.
The results indicated that the gamma-rays radiation was effective to convert thiophenes compounds, and the conversion efficiency went up with the increase of the radiation dose. When the radiation dose rate was 3.8 kGy/h and the radiation dose was about 160 kGy, the conversion efficiency of benzothiophene (BT) and dibenzothiophene (DBT) was about 74% and 33%, respectively. At the same radiation dose, the conversion of DBT was higher at the lower initial DBT concentration or the higher radiation dose rate. The main conversion product of DBT was bisulfide, and oxygen in air showed slight inhibition to the conversion of DBT in the merely radiation system (without additives or catalysts).
Tetrachloromethane displayed an improvement on the conversion efficiency of dimethyl thiophene and BT as a sensibilizer in the presence of gamma radiation at room temperature. The results also showed that the mixture of hydrogen preoxide and acetic acid appeared to be synergic with the radiation to convert DBT. When the ratio of oil: hydrogen preoxide: acetic acid was 3:1:1 and the radiation dose was about 200 kGy, the conversion efficiency of DBT was over 90%.
The catalysts that could enhance the conversion of DBT in the simulated petroleum were selected. Among the tested catalysts, cobalt oxide and zirconium oxide impregnated onγ-alumina showed high catalytic activities under gamma-rays irradiation. The preparation conditions of the catalysts were also optimized experimentally. The main factors that affected the conversion of dibenzothiophene were studied. The results showed when the applied radiation dose rate was 3.9kGy and the radiation dose was about 179.2 kGy, the conversion efficiency of DBT was over 80% and 95% in the presence of cobalt oxide and zirconium oxide, respectively. The main conversion product of DBT was Dibenzo-sulfone.
The effect of gamma-rays irradiation upon the structure of the catalyst was investigated by XRD, XPS and SEM techniques. The catalyst appeared to be stable under the gamma-rays irradiation except for the surface coverage by the oxidized organic compound. In addition, the possible mechanism for the synergistic effect of gamma-rays irradiation and catalyst was proposed.
The first-order rate model could be used to well describe the conversion reaction kinetics of DBT. The reaction constant and G factor have been compared in different experiment condition. It is obvious that the reaction constant and G factor was higher when the catalysts were employed.
According to the present study, the method by means of gamma-rays radiation to convert thiophene type compounds has been understood. The combination of the radiation with the catalyst has been proved to be an effective way to convert DBT to Dibenzo-sulfone, which could be easily removed from the oil. This study has provided a potential alternative to the deep petroleum desulfurization in the future.
引文
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