Applied Petrochemical Research (2018) 8:131–139 https://doi.org/10.1007/s13203-018-0204-y REVIEW New trends in improving gasoline quality and octane through naphtha isomerization: a short review Salman Raza Naqvi1 · Ayesha Bibi1 · Muhammad Naqvi2 · Tayyaba Noor1 · Abdul-Sattar Nizami3 · Mohammad Rehan3 · Muhammad Ayoub4 Received: 8 October 2017 / Accepted: 18 June 2018 / Published online: 27 June 2018 © The Author(s) 2018 Abstract The octane enhancement of light straight run naphtha is one of the signifcant solid acid catalyzed processes in the modern oil refneries due to limitations of benzene, aromatics, and olefn 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 efect of various contaminants on catalyst activity. In addition, dif- ferent technologies used for isomerization process are evaluated and compared by diferent process parameters. Keywords Catalyst · Isomerization · Light naphtha · Octane number · Oil refneries Introduction The heterogeneous catalysts are used more than homo- geneous catalysts due to their high reactivity during the Today, there is a consensus to enhance fuels quality to process and ease of catalyst separation [5]. The reusabil- reduce their detrimental impacts on the environment and ity of catalysts is critical from fnancial and environmental human health [1]. As a result, restrictions are imposed on prospects [6]. Moreover, the diferent types of reactions and gasoline to reduce its benzene, cyclic compounds, heavy their mechanisms taking place at the active reaction sites aromatics, and olefn concentrations along with the removal of the catalysts are the key factors in the isomerization pro- of tetramethyl lead [2]. However, the octane number of aro- cess. Therefore, to achieve the required yield and conversion matics and olefns is relatively high, so reducing their con- of n-parafn, the catalyst should have a suitable shape and centration in gasoline causes a decrease in octane number compositional characteristics, especially correct ratios of and, consequently, fuel quality [3, 4]. Therefore, the pro- metal and acid [7]. cess of isomerization is gaining signifcant attention in the The process of light naphtha isomerization mainly bases petroleum refneries to increase the fuel octane number [5]. on the chlorinated platinum promoted alumina catalysts [3]. However, the process mainly depends on the catalysts and These active catalysts function even at a low temperature, how efcient they are in their activity and selectivity [2]. which is a favorable condition for the process of isomeriza- tion. In earlier times, aluminum chloride was used as a cata- lyst in isomerization to produce iso-butane [4]. Afterward, many other catalysts have been developed but operated at a * Salman Raza Naqvi [email protected] high temperature with reduced conversion of naphtha due to their lower activity. Therefore, highly active dual function 1 School of Chemical and Materials Engineering, National catalysts are developed in recent years that can be operated University of Sciences and Technology, H‑12, Islamabad, at a lower temperature with high conversion of naphtha [6]. Pakistan The types of catalysts used for the isomerization process 2 Department of Energy, Building and Environment, Future of light naphtha are mixed metal oxide- and zeolite-based Energy Center, Mälardalen University, Västerås, Sweden catalysts [2]. 3 Centre of Excellence in Environmental Studies (CEES), King The suitability of catalyst type depends on the isomeri- Abdulaziz University, Jidda, Saudi Arabia zation process temperature. For example, the Pt-chlorin- 4 Department of Chemical Engineering, Universiti Teknologi ated alumina is the most active catalyst for isomerization, PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia Vol.:(0123456789)1 3 132 Applied Petrochemical Research (2018) 8:131–139 and they are operated at lower temperatures (20–130 °C). Key factors infuencing the isomerization The mixed metal oxide catalysts are less reactive than Pt- process chlorinated alumina base catalyst, so they are operated at a relatively higher temperature (around 150 °C) [6–8]. Feed to isomerization unit and its characteristics The chlorinated alumina catalysts produce compounds with higher octane number. However, the chlorinated Pt/ Light straight run naphtha (LSRN) is rich in n-pentane and Al2O3 catalyst requires a promoter to enhance its activity n-hexane. Other feedstock to the isomerization process may [10]. These catalysts are highly susceptible to contami- come from thermal processes like visbreaking, delayed nation during their use and can lead to catalyst poison- coking, fuid catalytic cracking (FFC) of naphtha, light ing. Therefore, the feedstock is required to be preheated reformate, light hydrocracked naphtha and from conden- to obtain the required products [1, 8, 9]. Several studies sate naphtha. The feedstock also contains some fractions of have been carried out with varying amount of different benzene, olefns, and heavy hydrocarbon compounds (C7+). metal content in the catalyst and the experimental results Therefore, the feedstock is characterized by X-factor that showed that Ni–Pt mixed metal oxide (0.2% Pt and 0.4% determines the number of heavy hydrocarbons, mainly C 6 n + Ni) showed lesser activity and yield of -paraffin than cyclic and C 7 compounds, present in the feedstock. Feed- Pd–Pt (0.2% Pt and 0.4% Pd) mixed metal oxide catalyst stock with a high value of X-factor will have a lower value [8–10]. of octane number of the product. X-factor is estimated using The isomerization process has low capital cost and the following equation: produces a more valuable product with optimum specifi- cations such as research octane number (RON) and aro- X-factor = (wt.% Methylcyclopentane) + (wt.% Benzene) matics concentrations than other methods [2]. Whereas + (wt.% cyclohexane) + wt.%C+ . 7 (1) reforming produces more aromatic compounds and less isoparaffin than isomerization [5]. The product produced Key isomerization reactions and their conditions by isomerization has a small difference between RON and motor octane number (MON) [8]. The increasing The key reactions taking place in the isomerization unit is worldwide significance of isomerization in oil refineries listed in Table 1. Isomerization is a limited equilibrium and encourages the researchers to conduct detailed reviews on exothermic reaction [9]. According to Le Chatelier’s prin- this topic. This study, therefore, aims to examine the role ciple, such reactions are favorable at low temperatures. The of various catalysts in the isomerization of light naphtha. complete conversion of n-parafn to iso-parafn is not pos- A special focus is given to the reaction mechanism at the sible due to equilibrium limitations [11]. The heat of reac- + active catalyst sites, and the effect of different contami- tion for isomerization, C 7 hydrocracking, naphthene ring nants on catalyst activity. Furthermore, the current pro- opening, and benzene saturation are 2200, 11,000, 11,000 cess challenges and possibilities of process optimization and 50,000 kcal/kg-mol, respectively. These values show are also discussed. that benzene saturation is a highly exothermic reaction and reduces the yield of the isomerates. Therefore, there should be a limited fraction of benzene in the feedstock [11–13]. Table 1 Important reactions take place in isomerization unit and their features Major reactions Reactions Features Isomerization n-C5H12 → i-C5H12 It is a desirable reaction in the isomerization process It is favorable at low temperature and high pressure [35] n-C6H14 → 2-methylpentane It enhances the octane number of the product n-C H → 3-methylpentane 6 14 It is exothermic in nature 2-methylpentane → 2.3 dimethyl butane It takes place at both acidic and metallic site of the catalyst Benzene saturation C6H6 + 3H2 → CH It is a desirable reaction It is highly exothermic in nature and produce a lot of heat It takes place at the metallic site of the catalyst Hydrocracking n-C7 + H2 → Gas It takes place at higher temperature [36] It decreases the product yield It takes place at acidic site of catalyst Naphthene ring opening MCP + H2 → 2-MP It is favored at high temperature It takes place at the metallic site of the catalyst 1 3 Applied Petrochemical Research (2018) 8:131–139 133 Table 1 shows the various reactions taking place in the For LSRN, the iso ration is defned as isomerization reactors. Benzene saturation is the fastest reac- n iC5∕C5P = iC5∕iC5 + C5. tion that takes place at the active metal site of the catalyst and (3) top of the reactor. The heat energy released by the reaction is Process variables for isomerization approximately 50 times greater than the heat energy released by the isomerization. The isomerization reaction takes place The critical process parameters for the isomerization of the at both metallic and acidic sites. The reaction starts at the LSRN are temperature, pressure, liquid hourly space veloc- metallic site and then continues at the acidic site and follows ity (LHSV), feedstock composition, H 2/hydrocarbon ratio the mechanism in which frstly there is dehydrogenation of and catalyst promoter [1]. The temperature of the process alkane at the metallic site, and alkene will form. Afterward, should be kept low as isomerization is an exothermic reac- there will be a rearrangement of