Solid Phosphoric Acid (SPA) is commercially used for the oligomerisation of C2-C5 olefins to produce motor-gasoline. It is also used for the alkylation of benzene and toluene with propene to produce cumene and cymene. It is a critical unit in a Fischer-Tropsch (FT) fuels refinery for the production of motor gasoline [1]. Despite SPA oligomerisation being a mature technology, it's application in FT refining has brought new insights with regard to the products formed from the highly olefinic FT feed material. The olefinic motor gasoline produced must also be hydrogenated to meet the olefin specifications of the final gasoline product, which results in a large drop in RON value from the olefin rich gasoline (96 RON) to the hydrogenated gasoline (40-90 RON) [2]. It is therefore critical to monitor fuel properties during laboratory test work and understand how molecules that have a high RON value even after hydrogenation can be produced and retained.

For economic reasons laboratory test work is always done at the smallest scale possible. This sometimes introduces experimental difficulties unrelated to commercial scale operation. When dealing with a gaseous feed that can be liquefied during the process, pressure control becomes difficult. This can be overcome by co-feeding a non-condensible inert (like nitrogen) or by changing the hardware. Again for reasons of cost, the former is preferred. The commercial plant, however, does not make use of nitrogen and the addition of nitrogen further complicates matters in that it contributes to shifting the reaction to the gas phase through the entire test reactor. In commercial operation the feed material is in the gas phase at inlet reaction conditions but as more condensed products is formed in the reactor the phase composition changes, eventually becoming liquid phase only. This complicates the hydrodynamic description of the reaction system [3].

This difference in reaction phase composition was investigated to determine if the reaction system was sensitive to the hydrodynamics, resulting in changes with respect to product fuel properties, using the nitrogen as co-feed in laboratory reactors. The liquid phase testing consisted of pumping the liquefied gas (excluding the nitrogen co-feed) to the reactor under reaction pressure and modifying the product work-up section by removing the high pressure knockout and level control section. The product stream from the reactor then passes through a single control valve for pressure control. In both cases liquid and gaseous products were collected at atmospheric conditions and the liquid product was hydrogenated to determine final fuel properties.

Conversions obtained showed some differences for the two modus operandi and small differences were observed in the final hydrogenated fuel properties. The reaction phases were investigated showing clear differences in the hydrodynamic behaviour of the two feed cases and the molecular product distribution was also investigated, to determine and explain changes in the product composition.

[1] Steynberg, A.P.; Dry, M.E. Fischer-Tropsch Technology; Elsevier: Amsterdam, 2004. [2] De Klerk, A.; Engelbrecht, D.J.; Boikanyo, H. Ind. Eng. Chem. Res. 2004, 43, 7449. [3] Golombok, M.; De Bruijn, J. Trans. IChemE, 2000, 78A, 145.