Study of the Adsorption of Nitrogen Compounds for Diesel Fuel Production Using Three Commercial Materials

The aim of this work was to study the removal of nitrogen (N-) compounds for ultra-low sulfur diesel (ULSD) production purposes by using three commercial materials: selexsorb® CDX (CDX), silica gel (SG), and activated carbon VG-077 (VG-077). The experiments were conducted in three steps: (1) Adsorption of quinoline, indole, or carbazole from model fuels (300wppm in a 1:1g/g n-hexadecane:toluene mixture) in a batch setup; adsorption of N- compounds from straight run gas oil (SRGO): (2) in a batch setup, and (3) in a fixed-bed column (FBC). Langmuir and Freundlich models were considered for modeling the adsorption isotherms of quinoline, indole, carbazole from model fuels and N-compounds from SRGO. Clark and Thomas models were employed for fitting the FBC experimental data. In the batch experiments, the adsorption capacities (qm) for neutral N-compounds were 0.795mmol/g (VG-077), 0.287mmol/g (CDX), and for basic N-compounds were 0.708mmol/g (SG), 0.385mmol/g (CDX), and 0.242mmol/g (VG-077). The goodness of the fit for the Langmuir and Freundlich models strongly depended on the N-compound and the adsorbent when treating model fuels. Treating SRGO, VG-077 presented a higher qm (0.855mmol/g) than the other two materials (0.687mmol/g (SG) and 0.372mmol/g (CDX)), which is understandable because neutral N-compounds represent 75% of nitrogen in this fuel. The Langmuir model was better than the Freundlich model to reproduce the adsorption isotherms properly with SRGO (R2>0.9966). Besides, the pseudo second-order model performed better than the pseudo first-order model to simulate the adsorption rates in almost all cases, although the comparison was not straightforward, because the values depended highly on the N-compound and the material studied. In the FBC experiments, the Clark model agreed better with the experimental data than the Thomas model did. VG-077 achieved the highest N-adsorption (0.38mmol/g vs. 0.18mmol/g (SG) and 0.15mmol/g (CDX)). However, according to the preliminary estimation carried out, a higher adsorption capacity is still required for commercial application of this process.