| Fischer-Tropsch synthesis (FTS) has been powerfully supported by government for its narrowing the increasing gap between supply and demand of crude oil. Improving the gasoline quantity and quality is significant for maximizing the FTS economic benefit, because gasoline is the main product of FTS. Olefins accounts for 70 w% in the condensation product which is obtained from the HTFT synthesis tail gas through the initial separating of ethene and propene. Oligomerization is an efficient path to expand gasoline output and improve gasoline quality. In the oligomerization process of Sasol company in South Africa, kieselguhr solid phosphoric acid catalyst (KSPAC) is used as industrial catalyst for its excellent performance,90% conversion of alkene and 80% gasoline selectivity. However, the key technology of preparing KSPAC is monopolized by UOP company. The existing problem of KSPAC prepared at home is support pulverizing that leads to the deactivation of it. Independently producing KSPAC of super performance is a very urgent problem to solve.In this article, optimized kieselguhr and phosphoric acid were selected as support and active phase, respectively. Catalysts were synthesized by blending methods through impregnation, drying, calcination and activation process. Free phosphoric acid amount was determined by chemical titration, and phase composition was detected by XRD characterization. Preparation parameters, such as phosphoric acid concentration and content, additive content, impregnation temperature, calcination temperature and so on, were adjusted by single factor experiments, through which Si/P and temperature were modified leading to silicon phosphates composition changes. The relation between catalytic performance and phase composition was investigated, and the optimized catalysts preparation conditions were obtained as well.In order to control the silicon phosphates composition, the effects of preparation conditions on it were investigated. The modulation of phosphoric acid concentration and content, additive content resulted in the Si/P changes, the decreasing of which was benefit for the formation of SiP2O7 in the range from 0.31 to 1. The main product of silica impregnated in phosphoric acid at temperature from 170℃ to 180℃ was SiH2(PO4)2. There were two phase transformation points 310℃ and 420℃ during calcination process. SiH2(PO4)2 transformed to SiP2O7 at 310℃ and Si5O(PO4)6 transformed to SiP2O7 at 420℃. The silicon phosphates composition was less affected by phosphoric acid concentration. With the increasing of phosphoric acid content, additive content, impregnation temperature and calcination temperature, Si/P declined and temperature raised. Correspondingly, SiP2O7 content ascended and Si5O(PO4)6 content dropped. The catalyst with 11% Si5O(PO4)6 in it performed well, whose propene conversion was larger than 75%. When Si5O(PO4)6 content was in the range from 11% to 15%, and relevant free phosphoric acid amount was in the range from 15% P2O5 to 33% P2O5, catalyst conversion was larger than 72%.The optimum preparation conditions were obtained as following, proper phosphoric acid concentration from 104% H3PO4 to 115% H3PO4, phosphoric acid content from 80% to 80.5%, appropriate additive content less than 1%, impregnation temperature from 170 to 180℃, calcination temperature from 370 to 420℃. Catalysts prepared at above conditions having propene conversion larger than 72%, and C9,C12 selectivity was larger than 80%.The conversion of optimum catalyst was larger than 81%, which could be improved by 20% compared with chemical equilibrium conversion 100%. However, the specific activity of optimum catalyst was five times of the industrial catalyst. Gasoline selectivity was about 80%. The lifetime evaluation showed that catalyst activity was stable during 60 hours reaction, while this of industrial catalyst was 49 days. |
PDF Full Text Request