Ind. Eng. Chem. Res. 2008, 47, 8471–8477
8471
Reactions of Organic Sulfur and Nitrogen Compounds in the FCC Pretreater and the FCC Unit Peter Ø. Vistisen and Per Zeuthen* Haldor Topsøe A/S, Lyngby, Denmark
The effect of hydrotreating VGO (FCC pretreatment) at different severities was examined with respect to FCC performance. Furthermore, the reaction mechanisms for conversion of the organic sulfur and nitrogen compounds found in the pretreated FCC feedstocks during FCC processing were studied. The studies were carried out under realistic conditions in a large pilot plant hydrotreating unit and in an FCC pilot plant operating with a circulating catalyst. It was found that the sulfur compounds in the FCC gasoline (i.e., thiophenes and benzothiophenes) were formed from recombination reactions between H2S and olefins generated in the FCC unit. Similarly, the nitrogen compounds found in the FCC gasoline (i.e., anilines and indoles) were formed from recombination reactions between ammonia and olefins. These results show that the sulfur and nitrogen compounds in the FCC gasoline are mostly the same with only minor differences in the relative concentration of the individual compound. This phenomenon is not dependent on the feed charged to the FCC unit but is a result of equilibrium reactions between olefins, H2S and ammonia, respectively, and the temperature in the FCC reactor. 1. Introduction The more stringent specifications introduced in many countries make the removal of nitrogen and sulfur in particular from diesel and gasoline more and more imperative. The quality of the products from the FCC unit is thus becoming increasingly important, since FCC gasoline is a major compound in the overall gasoline pool, and since the mid-distillate product LCO is often blended into the overall diesel pool. It is very important to have as much information as possible about the origin of the sulfur compounds found in FCC gasoline and LCO, since the sulfur content is either reduced by upstream desulphurization or by post-treatment processes. To improve our understanding of the sulfur and nitrogen reaction network and the origin of the FCC gasoline sulfur impurities, a comprehensive study was recently completed at Haldor Topsøe A/S, where different FCC feeds and products were compared and characterized in detail. The objective was to study the effect of feed pretreatment on the quality of FCC products particularly with respect to sulfur and nitrogen composition. For many years, it was believed that the organic sulfur compounds found in cracked gasoline were intermediate products from the breakdown of larger heterogenic species found in the VGO feed to the FCC units. Very few studies focused on the formation of organic nitrogen compounds in the FCC gasoline. However, it was believed that the mechanism was the same as that seen for the formation of sulfur compounds. Post-treatment processes were developed to desulphurize the FCC gasoline with a minimum loss of valuable octane number or hydrogenation of the lighter gasoline range olefins. The results of the studies described in this paper clearly showed the benefit of removing the organic sulfur compounds as well as the organic nitrogen compounds upstream of the FCC unit (e.g., by FCC pretreatment processes). 2. Impact of Nitrogen on the FCC Catalyst FCC feed pretreatment offers the refiner a unique opportunity to comply with increasingly stringent environmental regulations * To whom correspondence should be addressed. Tel.: +45 45278554. Fax: +45 45272999. E-mail:
[email protected].
and to improve FCC operations. Reduction of FCCU feed nitrogen will lower the product nitrogen. Additionally, aromatic saturation of the feed will improve FCCU yields as polynuclear aromatics are saturated toward monoaromatic rings, depending on the conditions in the FCC pretreater. Nitrogen removal from the feed will help increase the activity of the FCC catalyst circulating in the unit. It is well-known that nitrogen, particularly basic nitrogen, will act to neutralize the acid sites of the zeolitic catalyst. When basic nitrogen compounds neutralize an acid site, this site will lose its ability to promote cracking. 3. Control of Gasoline Sulfur The sulfur content of FCC gasoline may considerably affect a refinery’s gasoline pool sulfur level. Traditional ways of reducing sulfur content include feed hydrotreating, naphtha hydrofinishing, and lowering of the end point of FCC naphtha. Hydrotreating and hydrofinishing are capital intensive. Hydrofinishing also lowers the octane number of FCC gasoline.1 Lowering the gasoline cut point may reduce the sulfur content but may also significantly lower the gasoline volume, and it may be difficult to find room for the high-sulfur heavy gasoline without further processing. Thus, there is a strong incentive for refiners and catalyst manufacturers to find ways to reduce gasoline sulfur within the FCC process itself.2-6 The objective of this paper is to discuss the sulfur and nitrogen chemistry, study the mechanism of formation of the gasoline sulfur and nitrogen compounds and to propose possible ways of lowering the FCC gasoline sulfur content. It is the general belief that sulfur in gasoline is present as stable fragments formed by dealkylation partial cracking of higher molecular weight sulfur compounds.1,7-11 In a study,12 it was shown that the concentration of sulfur in gasoline only to a very low degree depends on conversion but that the dependence increases with increasing temperature in the FCCU. FCC catalysts with higher hydrogen transfer activity tend to produce less sulfur in gasoline.13,14 Catalysts containing REY, REUSY, and Lewis acid materials were developed as gasoline sulfur reduction catalysts or additives.15-17 The mechanism of the sulfur reduction additives was explained by these materials
10.1021/ie8006616 CCC: $40.75 2008 American Chemical Society Published on Web 10/08/2008
8472 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 Table 1. Operating Conditions as well as Analysis of the Hydrotreater Feed and Product Properties pilot plant hydrotreater condition #2 (low N and S) L-FEED 358 85 1.1 500 0.8796 260/395/532
n-FEED 396 85 0.2 500 TOSCA
BN-FEED
0.9088 284/411/545
H-FEED 358 85 2.2 500 0.8819 259/396/533
0.8502 192/341/497
0.8504 192/341/496
2.26
0.1947
0.0835
