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Characterization and Identification of the most Refractory Nitrogen Compounds in Hydroprocessed Vacuum Gas Oil Peter Wiwel,† Berit Hinnemann,*,† Angelica Hidalgo-Vivas,† Per Zeuthen,† Bent O. Petersen,‡ and Jens Ø. Duus‡ Haldor Topsøe A/S, NymølleVej 55, DK-2800 Kgs. Lyngby, Denmark, and Carlsberg Laboratories, Gamle Carlsberg Vej 10, DK-2500 Valby, Denmark
There is currently a growing need to hydroprocess heavier and tougher crude oils with increased nitrogen content. Therefore, hydrodenitrogenation (HDN) has become a critical hydroprocessing reaction, making it essential to gain insight into which nitrogen-containing compounds are the most difficult to treat. In the present article, we describe the identification of nitrogen compounds in severely pretreated feed for hydrocracking (HC). The nitrogen compounds in the N-slip to the hydrocracker are isolated and concentrated on solid-phase extraction (SPE) columns and identified by gas chromatography mass spectrometry (GC-MS), gas chromatography with atomic emission detection (GC-AED), and nuclear magnetic resonance (NMR) spectroscopy. Density functional theory (DFT) calculations support the structural identification and are further used to investigate the reactivity. We find that the most refractory organic nitrogen compounds in the N-slip belong to the family of 4,8,9,10-tetrahydrocyclohepta[def]carbazoles. These molecules are slightly more basic than other carbazoles and thus are likely to have an impact on the performance of the downstream catalysts; however, their very low reactivities make them extremely difficult to remove under normal hydrotreating conditions. 1. Introduction The current economic conditions and oil reserve situation worldwide have resulted in a growing interest in processing heavy oils and even extra-heavy oils with a much higher nitrogen content. It is our experience that there has been a 20% increase in the nitrogen content of feeds to hydrocrackers over the past years.1,2 Removal of nitrogen is essential to prevent catalyst poisoning in downstream refinery processes, such as hydrocracking (HC), catalytic cracking, and reforming. Organic nitrogen is removed catalytically by hydrodenitrogenation (HDN), which is generally the most difficult hydrotreatment reaction.3 Most of the nitrogen is present as heterocycles with multiple aromatic rings. Basic compounds are mainly 6-membered-ring nitrogen compounds, such as quinolines and benzoquinolines. Nonbasic compounds are mainly 5-membered-ring compounds, such as indoles and carbazoles. Some chemical structures are shown in Table 1 for reference. Half of the total nitrogen is typically concentrated in the heaviest 30% of heavy feeds,2 with carbazole compounds substituted at position 1 being the most abundant.4 The high relative concentrations of 1-substituted carbazoles in crudes are attributed to preferential migration of nitrogen compounds with sterically shielded N centers through layers of sediments. Diand trimethylcarbazoles with substitution at position 1 have been observed to be the most predominant. The problem of nitrogen compound inhibition has received considerable attention because the effects influence both process and catalyst development. Organic nitrogen compounds have a significantly negative kinetic effect on hydrotreating reactions such as hydrodesulfurization (HDS), on other hydrogenolysis reactions, and on hydrogenation reactions. Many general studies
related to nitrogen compound inhibition have been performed with the primary objective of improving the knowledge of HDS and HDN.5-9 In the area of hydrotreated stream compositions, prior work identified alkylcarbazoles as low-reactivity N compounds,10,11 but little information on the relative rates of individual compound conversions was gained because of the difficulties in quantifying the results. More recent studies have focused on identifying the most refractive and inhibiting compounds in gas oil hydrotreatment1,4 and assessing their negative effect on hydroprocessing catalysts, particularly on the hydrotreating catalyst HDS activity. The poisoning of the more acidic catalysts employed in hydrocracking caused by nitrogen compounds is even more severe, and the detrimental effect is reflected in the performance of the hydrocrackers. This study focuses on the characterization of the most refractory nitrogen compounds in vacuum gas oil (VGO) hydrotreatment and on understanding which of the compounds present in the N-slip are the worst poisons to the HC catalysts. The consequence of introducing organic nitrogen onto the HC catalyst is a reduction in conversion and liquid volume yields. The increase in operating temperature required to maintain conversion reduces the cycle length and can affect the Table 1. Structures of Selected Organic Nitrogen Compounds
* To whom correspondence should be addressed. Tel.: +45 4527 2130. Fax: +45 4527 2999. E-mail:
[email protected]. † Haldor Topsøe A/S. ‡ Carlsberg Laboratories. 10.1021/ie901473x 2010 American Chemical Society Published on Web 02/25/2010
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Table 2. VGO Feed Properties property
method
S, wt % N, ppm (w/w) H, wt % SG 60/60 °F flash point, °C pour point, °C viscosity at 50 °C, cSt viscosity at 100 °C, cSt aromatics 1 ring, wt % 2 ring, wt % 3+ ring, wt % N basic, ppm (w/w) Fe/Ni/V
D 4294 D 4629 D 4808 D 4052 D 93 D 5949 D 445/446 D 445/446 IP 391
VGO feed 2.2960 1394 12.16 0.9267 211 40 44.1 8.1 16.2 8.74 14.89 493 nonbasic N compounds > olefins > thiophenic sulfur > NH3 > aromatics > H2S . paraffins. Furthermore, a detrimental effect to the hydrogenation step in the hydrodesulfurization, hydrocracking, and saturation of other molecules is also expected, as concluded from real feed experiments on VGO that show that heavy organic nitrogencontaining compounds are the major inhibitors to the hydrogenation function during HDS. This was evaluated in another set of experiments by preparing several different feedstocks, where total and basic nitrogen were removed from a hydrotreated VGO feed with properties similar to those of the VGO used for the identification experiments. The method used to remove the nitrogen compounds is reported elsewhere.4 The sulfur concentration of the feeds was not modified. The sulfur and nitrogen concentrations of the feeds are shown in Table 9. The VGOs were further hydrotreated under the same conditions, and the results are presented in Figure 10. With these experiments, we demonstrated that the HDS rate constant for hydrotreating of the more refractory S compounds in VGO increased by a factor of 2-5, depending on the degree of nitrogen removal.26 Although we did not determine the distribution of the nitrogen compounds and therefore cannot ascribe the observed effect solely to the identified 4,8,9,10-tetrahydrocyclohepta[def]carbazole, these experiments clearly show the detrimental effect of nitrogen compounds. The example shown in Table 10 illustrates the consequences of increasing the N-slip in a hydrocracker from about 20 to 80
Figure 10. Hydrotreatment of low-nitrogen VGOs (conditions: T ) 360 °C, H2/oil ) 590 Nm3/m3, P ) 80 bar, and LHSV ) 0.8 h-1).
Ind. Eng. Chem. Res., Vol. 49, No. 7, 2010 Table 10. Example of Low and High N-Slip to the Hydrocracking Catalyst
N-slip, ppm (w/w) pressure, barg HC temperature, °C H2 consumption, wt % FF conversion, wt % C3 + C4 yields, wt % C5 yields, wt % jet yields, wt % diesel yields, wt %
low N-slip
high N-slip
20 150 395 2.9 base base base base base
80 150 395 2.5 -17% -2% -9% -7.6 +0.5%
ppm to the conversion and product yields. At constant HC reaction temperature, an increase of only 60 ppm reduces the conversion by approximately 17%. It would be necessary to increase the HC temperature by about 8 °C to keep a constant conversion. The increase in reaction temperature would reduce the cycle length of this hydrocracker by 35%. 4. Conclusions The detection and identification of a group of very refractory nitrogen-containing organic compounds present in hydrotreated products that persists even after high-severity HDN were achieved by concentrating and purifying the N species in the 340-370 °C distillate fraction through the use of GC-MS, GCAED, and NMR spectroscopy. DFT studies confirmed the structural assignment of the C15H13N molecule by NMR spectroscopy by pointing at the 4,8,9,10-tetrahydrocyclohepta[def]carbazole molecule as the most stable isomer. The small structural changes upon addition of the cyclohepta ring predicted by DFT seemed to be sufficient to render the 4,8,9,10tetrahydrocyclohepta[def]carbazole molecule more difficult to hydrogenate at the C4 position. This difference in hydrogenation might be one of the factors responsible for the low reactivity. The impact of these compounds on the HC catalyst has been discussed, although a more precise quantification of the effect remains to be presented. The extent of the negative effect is, of course, a function of the resistance of a catalyst to nitrogen poisoning; however, the implication is that an optimum N content exists, as it was established in the present study that hydrotreating to a nitrogen content below 10 ppm (w/w) is very costly. Acknowledgment The authors thank Dr. Michael Brorson for giving the 7-ring carbazole isomer a systematic name, Dr. Simon I. Andersen for his help during the SPE work, Dr. D.Whitehurst for suggestions for the molecular structure, Dr. Kim G. Knudsen for ongoing discussions throughout the identification and characterization process, and Vivian Grindsted for doing most of the separation work. Literature Cited (1) Zeuthen, P.; Knudsen, K. G.; Whitehurst, D. D. Organic nitrogen compounds in gas oil blends, their hydrotreated products and the importance to hydrotreatment. Catal. Today 2001, 65, 307–314. (2) Bettati, A.; Zeuthen, P.; Hidalgo-Vivas, A. Nitrogen Tolerance of Hydrocracking Catalysts. Presented at the Topsoe Hydrocracking Catalyst and Technology Seminar, Copenhagen, Denmark, September 28-29, 2005. (3) Topsøe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating Catalysts: Science and Technology; Springer Verlag: New york, 1996. (4) Wiwel, P.; Knudsen, K. G.; Zeuthen, P.; Whitehurst, D D. Assessing Compositional Changes of Nitrogen Compounds during Hydrotreating of Typical Diesel Range Gas Oils. Ind. Eng. Chem. Res. 2000, 39, 533–540.
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ReceiVed for reView September 18, 2009 ReVised manuscript receiVed January 27, 2010 Accepted February 3, 2010 IE901473X
