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New trends in improving gasoline quality and octane through naphtha isomerization: a short review
Applied Petrochemical Research (2018). https://doi.org/10.1007/s13203-018-0204-y
pp 1–9
Ayesha Bibi 1, Muhammad Naqvi 2, Tayyaba Noor 1, Abdul-Sattar Nizami 3, Mohammad Rehan 3, Muhammad Ayoub 4
1. School of Chemical and Materials Engineering, National University of Sciences and Technology, Islamabad, Pakistan
2. Department of Energy, Building and Environment, Future Energy Center, Mälardalen University, Västerås, Sweden
3. Centre of Excellence in Environmental Studies (CEES), King Abdulaziz University, Jidda, Saudi Arabia
4. Department of Chemical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, Malaysia
Abstract
The octane enhancement of light straight run naphtha is one of the significant solid acid catalyzed processes in the modern oil refineries due to limitations of benzene, aromatics, and olefin content in gasoline. This paper aims to examine the role of various catalysts that are being utilized for the isomerization of light naphtha with an ambition to give an insight into the reaction mechanism at the active catalyst sites, and the effect of various contaminants on catalyst activity. In addition, different technologies used for isomerization process are evaluated and compared by different process parameters.
Introduction  
Today, there is a consensus to enhance fuels quality to reduce their detrimental impacts on the environment and human health [1]. As a result, restrictions are imposed on gasoline to reduce its benzene, cyclic compounds, heavy aromatics, and olefin concentrations along with the removal of tetramethyl lead [2]. However, the octane number of aromatics and olefins is relatively high, so reducing their concentration in gasoline causes a decrease in octane number and, consequently, fuel quality [3, 4]. Therefore, the process of isomerization is gaining significant attention in the petroleum refineries to increase the fuel octane number [5]. However, the process mainly depends on the catalysts and how efficient they are in their activity and selectivity [2]. 
The heterogeneous catalysts are used more than homogeneous catalysts due to their high reactivity during the process and ease of catalyst separation [5]. The reusability of catalysts is critical from financial and environmental prospects [6]. Moreover, the different types of reactions and their mechanisms taking place at the active reaction sites of the catalysts are the key factors in the isomerization process. Therefore, to achieve the required yield and conversion of n-paraffin, the catalyst should have a suitable shape and compositional characteristics, especially correct ratios of metal and acid [7].  
The process of light naphtha isomerization mainly bases on the chlorinated platinum promoted alumina catalysts [3]. These active catalysts function even at a low temperature, which is a favorable condition for the process of isomerization. In earlier times, aluminum chloride was used as a catalyst in isomerization to produce iso-butane [4]. Afterward, many other catalysts have been developed but operated at a high temperature with reduced conversion of naphtha due to their lower activity. Therefore, highly active dual function catalysts are developed in recent years that can be operated at a lower temperature with high conversion of naphtha [6]. The types of catalysts used for the isomerization process of light naphtha are mixed metal oxide- and zeolite-based catalysts [2]. 
The suitability of catalyst type depends on the isomerization process temperature. For example, the Pt-chlorinated alumina is the most active catalyst for isomerization, and they are operated at lower temperatures (20–130 °C). The mixed metal oxide catalysts are less reactive than Pt-chlorinated alumina base catalyst, so they are operated at a relatively higher temperature (around 150 °C) [6, 7, 8]. The chlorinated alumina catalysts produce compounds with higher octane number. However, the chlorinated Pt/Al2O3 catalyst requires a promoter to enhance its activity [10]. These catalysts are highly susceptible to contamination during their use and can lead to catalyst poisoning. Therefore, the feedstock is required to be preheated to obtain the required products [1, 8, 9]. Several studies have been carried out with varying amount of different metal content in the catalyst and the experimental results showed that Ni–Pt mixed metal oxide (0.2% Pt and 0.4% Ni) showed lesser activity and yield of n-paraffin than Pd–Pt (0.2% Pt and 0.4% Pd) mixed metal oxide catalyst [8, 9, 10]. 
The isomerization process has low capital cost and produces a more valuable product with optimum specifications such as research octane number (RON) and aromatics concentrations than other methods [2]. Whereas reforming produces more aromatic compounds and less isoparaffin than isomerization [5]. The product produced by isomerization has a small difference between RON and motor octane number (MON) [8]. The increasing worldwide significance of isomerization in oil refineries encourages the researchers to conduct detailed reviews on this topic. This study, therefore, aims to examine the role of various catalysts in the isomerization of light naphtha. A special focus is given to the reaction mechanism at the active catalyst sites, and the effect of different contaminants on catalyst activity. Furthermore, the current process challenges and possibilities of process optimization are also discussed. 
Free full text: https://link.springer.com/article/10.1007/s13203-018-0204-y
source: https://link.springer.com/article/10.1007/s13203-018-0204-y
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