Study On Propene Oligomerization With Solid Phosphoric Acid Catalyst

Fischer-Tropsch synthesis，as one of the most significant techniques inefficient and clean coal conversion, has got great support from the government.The main product of high temperature Fischer-Tropsch synthesis is gasoline. Thebyproducts are rich in light olefins. The way of light olefins treatment plays acrucial role in boosting the output of gasoline and improving the gasoline qualityand increasing the economic benefit of Fischer-Tropsch synthesis process. A lightolefin mixture is produced from the tail gas which is obtained from the hightemperature Fischer-Tropsch synthesis. Olefin oligomerization process can yieldhigh value-added byproducts and increase the overall income. The key factor ofolefin oligomerization process is the development of catalyst. At present, catalystused for olefin oligomerization process maturely is solid phosphoric acid catalyst.However, its core technology has only been mastered by America UOP company.The independent production of solid phosphoric acid catalyst with highconversion, good selectivity and long service life is the main problem that isneeded to study. In this paper, phosphoric acid and optimized kieselguhr were selected asactive component and carrier respectively. Solid phosphoric acid catalysts wereobtained by mixing kieselguhr with concentrated phosphoric acid followed bycalcination at high temperature. Among the light olefin mixtures, propene is lessactive. So propene is selected as the feed material. The laboratory equipment forsolid phosphoric acid catalyst tests was a fixed-bed reactor with high pressure.Solid phosphoric acid catalysts with different compositions were obtained byaltering preparation parameters. The crystal structure of the silicon phosphate wasstudied by X-ray diffraction (XRD). The amount of acid and acid strength weredetected by ammonia temperature-programmed desorption (NH3-TPD). Thecrushing strength (force required to break a tablet by compression) was measuredin weights of force using a Tablet Hardness Tester. The activity of solidphosphoric acid catalyst with different compositions was evaluated and the effectof silicon phosphates on catalytic performance was investigated. The life time ofsolid phosphoric acid catalyst we prepared was tested in the fixed bed reactor.Propene conversion and product selectivity were analysed when the test wasfinished. The reason of solid phosphoric acid catalyst deactivation was studied bythe change of acidity and silicon phosphate compositions during the propeneoligomerization process. The activation conditions such as temperature, time andthe amount of water were adjusted by single factor experiments to change theacidity of free phosphoric acid spreading over the solid phosphoric acid catalyst.The relationship between acidity and the selectivity of propene oligomerization product was investigated.The effect of weight hourly space velocity (WHSV) on the propeneconversion was studied under210oC and4MPa. The results showed that with theWHSV of4.5h-1, the propene conversion of solid phosphoric acid catalyst is thehighest. When the activation conditions were constant, the activity evaluationresults of solid phosphoric acid catalysts with different compositions indicatedthat solid phosphoric acid catalyst with relative content of51%Si5O(PO4)6and49%SiP2O7showed the best activity, which is higher than the solid phosphoricacid catalyst T-99used in the domestic industry. The propene conversion is ashigh as93.9%.Water existing in the feedstock, the life time of solid phosphoric acid catalystwas evaluated and the results showed the propene conversion was99%andmaintained for110h. When stopping feeding water, the activity of solidphosphoric acid catalyst decreased gradually. During the life time experiment ofsolid phosphoric acid catalyst, the activity of propene oligomerization was instable state as well as the selectivity of liquid products. Acidity of solidphosphoric acid catalyst at different stages during the life time experiment showedthat an appropriate acid amount of the solid phosphoric acid catalyst is needed tocatalyze propene oligomerization and maintain the activity of the oligomerizationreaction efficiently. Propene conversion increased with acid strength. The life timeof solid phosphoric acid catalyst could be forecast according to the generation rateof silica. In the case of water in the feedstock and accurate control, the silicon phosphates were hydrolyzed to produce certain amount of acid gradually and thestationary state of solid phosphoric acid catalyst with high propene conversionmay be prolonged. The crushing strength experiments showed that the crushingstrength of solid phosphoric acid catalyst remained relatively stable in the life timeexperiment, which is the basic guarantee for the stability of the catalyst structure.According to the experiment results, there is no definite relationship between theoligomers selectivity and acid amount and strength.