Cracked naphtha reactivity and its influence on acid catalyst deactivation
Creators: Uzcategui Barrios, Giselle
Publisher: name: University of Alberta Library
Year: 2022

<p>Acid catalyzed olefin-aromatic alkylation is an alternative process to hydrotreating to reduce the olefin content thermally cracked naphtha produced during the partial upgrading of bitumen. Acid catalysis is not commonly employed for conversion of untreated cracked naphtha. This motivated the study of the reactivity of cracked naphtha and its role on the deactivation of the acid catalyst employed in the olefin-alkylation process. <br>
The instances in which cracked naphtha has led to complications during its processing were reviewed. It was found that oxidative and thermal conditions could trigger the initiation of free radical reactions that promoted the formation of heavy compounds or deposits. It was possible to distinguish between the products from oxidative instability and thermal instability. Compounds with a carbon that is simultaneously in the allylic and benzylic position were found to be particularly reactive and could initiate free radical reactions at 250–350 °C in the absence of oxygen.<br>
The deactivation of the amorphous silica-alumina (ASA) acid catalyst used in the olefin-aromatic alkylation process were first studied by analyzing the deposits of the spent catalysts after reaction with thermally cracked naphtha at different conditions in the range 250–350 °C, WHSV of 0.5–2 h-1, in a pilot scale unit. The main cause of deactivation was deposit formation caused by a combination of catalytic and thermal reactions. The location of most of the deposits depended on the operating conditions, including at the reactor outlet. Another outcome was the understanding that nitrogen bases were not the main cause of catalyst deactivation. The nitrogen was only found present in deposits in the inlet of the reactor and at very low concentrations. <br>
To study if free radical chemistry was occurring in cracked naphtha at 200–300 °C, we used a probe molecule, α-methylstyrene (AMS).  AMS facilitated detection of free radical reactions products. Heating of cracked naphtha and AMS mixtures resulted in the formation of compounds linked to free radical termination, like cumene and AMS dimers. Control reactions of AMS ruled out AMS self-initiation as explanation for the observations. The results suggested that compounds in the cracked naphtha initiated the reaction network leading to the observed products, partly because AMS act as a hydrogen acceptor and participate in molecule induced radical formation.<br>
There have been indications that cyclopentenes can undergo reactions that accelerate the formation of carbonaceous deposits. After verifying the existence of cyclopentenes in the cracked naphtha, model compounds were used to investigate how cyclopentene could lead to formation of deposits on an ASA catalyst, and how it compared to the effect of linear olefins. It was found that cyclopentene had a more significant effect on the formation of deposits than 1-hexene. Reactions of cyclopentene on an ASA catalyst at 300 °C yielded bicyclic compounds like decalin and octalin, which indicated that hydrogen transfer was part of the reaction network, since decalin had to be the result of these reactions. It also indicated that cyclopentenes and dienes may share a pathway to bicyclic compounds leading to the formation of deposits on the catalyst surface.<br>
Following the insights gained during the analysis of the spent catalyst samples, it was hypothesized that the adsorption of nitrogen bases was a dynamic process. A series of experiments was done to assess if compounds in the cracked naphtha were competing with nitrogen bases for the adsorption on the acid sites. For this, a pyridine-saturated catalyst was subjected to different temperatures in the presence of cracked naphtha and heptane, and the amount of pyridine desorbed was quantified in the liquid product. It was found that, at all temperatures studied, more pyridine was desorbed towards the cracked naphtha compared to heptane. There were contributions of partitioning of the previously adsorbed pyridine between the solid and liquid, as well as competitive adsorption in the case of the cracked naphtha. The adsorption of compounds from cracked naphtha was confirmed with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), where it was determined that alkyl aromatics were present on the catalyst surface.</p>