Gasoline alkylation desulfurization over Amberlyst 35 resin: Influence of methanol and apparent reaction kinetics

Abstract

Gasoline desulfurization is receiving attention worldwide due to the increasing stringent regulations on sulfur content for environmental protection purpose. As conventional hydrotreating technology leads to significant octane number loss and processing costs, the gasoline alkylation desulfurization process, which consists of weighing down the sulfuric compounds by catalytic alkylation with olefins present in the feed and distillation followed by, is a rather attractive way. In this paper, firstly alkylation of thiophenic compounds was researched over macroporous sulfonic resin Amberlyst 35 in methanol presence to increase the selectivity of catalyst, then kinetics of thiophenic sulfurs alkylation in FCC gasoline was researched without and with methanol. Results found that appropriate methanol (⩽2 wt.% methanol in model gasoline and ⩽1 wt.% methanol in FCC gasoline) could inhibit olefins oligomerization significantly without influence on the conversions of thiophenic compounds. The alkylation of thiophenic sulfurs could be described as pseudo first order reaction regardless of the existence of methanol. The introduction of methanol decreases the reaction rate constant and increases the activation energy of alkylation reactions.

Introduction

Sulfur present in the fuels leads to SO_x air pollution generated by vehicle engineers. Moreover, sulfur is a well-known poison for catalytic converters [1]. In order to minimize the negative health and environmental effects of automotive exhaust emissions, the sulfur level in motor fuels is minimized [2]. Sulfur limits of <150 ppm by weight (ppmw) for gasoline have been introduced into China from January 1, 2010, and the European Union has issued regulations that required refineries to reduce the sulfur content of gasoline to <10 ppmw in 2009 [3]. However, the issues of gasoline deep desulfurization are becoming more serious because the crude oils refined are getting higher in sulfur contents and heavier in density [4].

At present, hydrodesulfurization (HDS) is the most commonly used method of sulfur reduction of fossil fuels in refineries, which can remove most sulfur compounds by converting them to hydrogen sulfide [5]. The major problem for deep desulfurization of gasoline is that the conventional hydrotreating technology results in a significant reduction of octane number due to saturation of olefins in naphtha from fluid catalytic cracking (FCC), which also causes higher hydrogen consumption [4]. Therefore, the most recent gasoline desulfurization studies have considered desulfurization processes circumventing the need of the use of hydrogen, which includes alkylation, extraction, precipitation, oxidation, adsorption, etc. [2], [4], [6], [7].

Among the most promising non-hydrodesulfurization technologies, the alkylation of thiophenic compounds with olefins contained in the same feed, which was first patented by Mobil Oil Corporation [8], is an interesting way to eliminate the heavier alkylated sulfur molecules by further distillation [9]. As is well known that the alkylation of thiophenic compounds occur through the formation of carbocation [3], acid catalysts are needed to achieve the reactions. Previous researches have proposed different catalysts either with Brönsted or Lewis acidity, including various zeolites (HY, USY, HMCM-22, Hβ) [3], [10], [11], [12], [13], [14], silica-supported phosphoric acid (SPA-11) [15], supported heteropolyacids (HPW/SiO₂) [9], [16], etc.

However, even though the nature of the acid catalyst and the alkylation conditions have been chosen very carefully, the secondary reactions such as olefins oligomerization are almost unavoidable, since they take place via the same mechanism as thiophene alkylation. The oligomerization byproduct such as C₁₅ + hydrocarbons with boiling point exceeding the boiling range of gasoline would decrease the product yield of gasoline, therefore, the formation of oligomers with high boiling point should be inhibited while decreasing olefins conversions.

Oxygenated compounds such as ethers which are produced by the reaction of methanol and tertiary olefins have good potential for use as environmental enhancing additives for transportation fuels [17]. As both methanol etherification and thiophene alkylation reactions could be catalyzed by macroporous sulfonic resins, the combination of the two reactions could obtain gasoline with high octane number and low sulfur content in theory. Previous researchers [18], [19], [20] have investigated the influence of etherification on olefins dimerization, and found that alcohol could improve the selectivity of dimerization significantly. However, no papers about the influence of etherification on thiophene alkylation have been published. Therefore, in this paper, alkylation of thiophene with isoamylene using Amberlyst 35 resin as catalysts was carried out in a batch stirred tank reactor in methanol presence, and then the influence of methanol on alkylation reactions of thiophenic compounds in FCC gasoline was also evaluated. Furthermore, kinetics of thiophenic sulfurs alkylation with olefins present in FCC gasoline was researched without and with methanol, including the determination of reaction rate constant and activation energy, and a comparison of reaction kinetics parameters under the two kinds of conditions was also made.