Hydrodenitrogenation property–reactivity correlation

Applied Catalysis A: General 378 (2010) 52–58

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Hydrodenitrogenation property–reactivity correlation Teh C. Ho * Corporate Strategic Research Labs, ExxonMobil Research and Engineering Co., 1545 Route 22 East, Annandale, NJ 08801, United States

A R T I C L E I N F O

A B S T R A C T

Article history: Received 10 October 2009 Received in revised form 26 January 2010 Accepted 31 January 2010 Available online 6 February 2010

This article is about the relationship between the properties of a refinery middle distillate feedstock and its reactivity toward hydrodenitrogenation (HDN). The HDN experiments were conducted with 12 raw middle distillates over a sulfided CoMo/Al2O3 catalyst and with 13 prehydrotreated middle distillates over a sulfided NiMo/Al2O3 catalyst. Of the multitude of physicochemical properties, the feed nitrogen content, which usually reflects the heaviness of the feedstock, is a rough indicator of the HDN reactivity. This suggests a self-inhibition effect whose implications are discussed. ß 2010 Elsevier B.V. All rights reserved.

Keywords: Hydrodenitrogenation Hydrodesulfurization Hydrocracking Property–reactivity correlation Middle distillate Feedstock heaviness

1. Introduction The global demand for middle distillates (200–370 8C boiling range; diesel, jet fuel, etc.) is expected to escalate in coming decades. Concomitant with this trend is the growing need to process low-quality, heavy oils and the increasingly stringent environmental regulations on transportation fuels. The nitrogen content in heavy crude oils generally increases with boiling point [1]. It is thus hardly surprising that the nitrogen content of refinery hydroprocessing feedstocks has been increasing over the years. Hydrotreating and hydrocracking, both being part of hydroprocessing processes, are the two primary producers of clean, highquality middle distillates. Relative to hydrotreating, hydrocracking operates at higher temperatures/pressures and consumes a much higher amount of hydrogen to attain desired levels of heteroatom removal, hydrogenation, and boiling-point reduction. Efforts are being made to increase the middle distillate yield from fluid catalytic cracking (FCC) through modifications to catalyst formulation, operating strategy, and process hardware. The catalysts commonly used in hydroprocessing comprise molybdenum- or tungsten-based sulfides that are promoted by Co and/or Ni [2]. While the active sites on these catalysts perform such reactions as hydrodesulfurization (HDS) and hydrodearomatization with varying degrees of activity and selectivity, they are susceptible to inhibition by organonitrogen (N) species. This is especially the case with the solid acid component of the hydrocracking catalyst.

Indigenous N-compounds are also poisons to both hydroprocessing and FCC catalysts as the strong chemisorption of N-species on active sites leads to coke formation [3]. The objectives of various hydroprocessing processes cannot be achieved unless the N level is reduced to a sufficiently low level. Developments of new or improved hydrodenitrogenation (HDN) catalysts and processes have been and will continue to be important in the years ahead [4]. One of the key considerations in hydroprocessing catalyst and process developments is the relationship between the physicochemical properties of a feedstock and its reactivity toward hydroprocessing reactions such as HDS and HDN. Knowing the relationship is essential to feedstock selection/blending, economics/planning studies, kinetics data interpretation, process modeling, etc. It also helps design model-compound experiments to gain fundamental insights into process chemistry. To be practically useful, such correlations must be expressed in terms of readily available properties. Several quantitative property–reactivity relationships (QPRRs) have been developed for the HDS of middle distillates [5,6 and references therein]. This does not appear to be the case with HDN, however. In light of the above, this article presents a simple QPRR for the HDN of middle distillates. The layout of the article consists of a description of the feedstocks used in the HDN experiments and a brief review of HDS QPRRs, followed by data analysis, and lastly the development of an HDN QPRR. 2. Experimental

* Tel.: +1 908 730 2797; fax: +1 908 730 3314. E-mail address: [email protected]. 0926-860X/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.01.045

The feedstocks and catalysts used in the present work are the same as those used in previous investigations of HDS QPRRs [5,6].

T.C. Ho / Applied Catalysis A: General 378 (2010) 52–58 Table 1 Raw middle distillates in Set A1 and their nitrogen contents and final boiling point (95%). Feed description 1. 2. 3. 4. 5. 6. 7. 8.

620+ Arab medium virgin Gas oil Virgin 620 Arab medium virgin Full Arab medium Heavy gas oil 170 Arab medium virgin 570+ Arab medium virgin

Nf, ppm

95% FBP, 8F

134 61 67 30 61 306 12 69

696 671 656 641 679 738 598 682

Table 2 Raw middle distillates in Set A2 and their nitrogen contents and final boiling point (95%). Feed description

Nf, ppm

95% FBP, 8F

9. 450–580 8F fraction 10. 580–700 8F fraction 11. Light cycle oil B 12. Light coker gas oil

591 2061 1051 1674

600 720 656 732

Table 3 Descriptors definition and designation, Set A. Definition 1 2 3 4 5 6 7 8 9 10 11 12

Definition

Total sulfur, wt.% (Sf) % nonthiophenic S in Sf (S0) Nonthiophenes, ppm (S1) Thiophenes, wppm (S2) Benzothiophenes (S3) Dibenzothiophenes (S4) b-Dibenzothiophenes I (S5) b-Dibenzothiophenes II (S6) 3- & 4-ring S (S7) Unassigned 1- & 2-ring S (S8) Total nitrogen, ppm (Nf) Basic nitrogen, ppm (BN)

13 14 15 16 17 18 19 20 21 22 23 24

Hydrogen, wt.% (H) 2- & 3-ring S in aromatics (SA) 1-Ring aromatics (1R) 2-Ring aromatics (2R) 3-Ring aromatics (3R) Naphthenes (NP) Total aromatics (TA) Total paraffins (TP) API gravity, 60 8F (8API) Bromine number, cg/g (BM) I/5 GCD, avg. temperature 8F (I/5) 99.5/FBP GCD, avg. temp. 8F (FBP)

53

The HDN data analyzed in this work were obtained with two very different sets of feedstocks, denoted as Sets A and B. Set A contains 12 widely different raw middle distillates including both virgin and cracked feedstocks [5]. This set can be subdivided into two distinct groups, designated as A1 and A2. The feed N contents in the A1 group are relatively low. Table 1 lists the nature of the feeds in the A1 group. Also listed in Table 1 are the feed nitrogen contents (Nf) and the final (95% volume) boiling point (FBP) measured from GC distillation (GCD). The FBP is taken as a measure of the heaviness of a feedstock. The A2 group has four high-N feeds as shown in Table 2. Two of the feeds (numbers 9 and 10 in Table 2) were ‘‘artificially’’ prepared by fractionating a virgin middle distillate into 580–700 and 450–580 8F boiling cuts. The former has an exceptionally high Nf of 2061 ppm, while the latter has 591 ppm. Also included in this group are a light catalytic cycle oil (LCO) and a coker gas oil containing 1051 and 1674 ppm N, respectively. Each feedstock in Set A was characterized by 24 property descriptors that are listed in Table 3. The relative HDN reactivity of a given feed, denoted by kHDN, is defined as the apparent first-order volumetric rate constant measured from tests with a commercial CoMo/Al2O3 catalyst in a fixed-bed reactor. The reaction conditions used were 250 psig H2, 0.74 LHSV (liquid hourly space velocity), 650 8F, and 600–1000 standard cubic feet H2/barrel oil [5]. Tables 4 and 5 show the HDN reactivities and the corresponding descriptor data for the A1 and A2 feedstocks, respectively. The wide diversity of the feedstocks in Set A can be exemplified by the ranges of the following five descriptors: sulfur content of the feed (Sf), 0.3–2.7 wt.%; nitrogen content, 12–2061 wppm; API gravity, 16.5–38.68; aromatic content, 18.2–78.6 wt.%; and FBP, 641–738 8F. Set B contains 13 prehydrotreated middle distillates. As such, their properties do not vary as widely as those in Set A and their sulfur contents are comparatively low. Table 6 shows that each feedstock was characterized by 10 property descriptors [6]. For perspective, the ranges of the aforementioned five descriptors are as follows: sulfur content, 250–1567 ppm; nitrogen content, 27– 174 ppm; API gravity, 31.6–39.88; aromatic content, 20.6–35.1 wt.%; and FBP, 656–783 8F. As expected, on average the nitrogen and

Table 4 HDN reactivities and feed properties, Set A1. Feed

kHDN 1, Sf 2, S0 3, S1 4, S2 5, S3 6, S4 7, S5 8, S6 9, S7 10, S8 11, Nf 12, BN 13, H 14, SA 15, 1R 16, 2R 17, 3R 18, NP 19, TA 20, TP 21, API 22, BM 23, I/5 24, FBP

1

2

3

4

5

6

7

8

0.21 1.919 25 4710 0 1075 1382 2939 1109 779 7196 134 9 12.97 7.73 8.54 5.14 1.81 37.78 23.2 39.01 31.4 10 600 727

0.68 1.414 32 4660 0 3894 682 873 313 286 3732 61 33 13.21 8.27 8.65 5.86 1.0 35.46 23.78 40.77 34.8 12 407 710

0.53 0.938 28 2670 0 3069 480 549 84 136 2452 67 37 13.42 5.27 6.69 5.54 0.74 36.99 18.23 44.76 36 10 423 703

0.87 1.284 34 4400 0 3839 921 630 69 77 2904 30 19 13.38 6.46 7.75 6.58 0.86 35.4 21.67 42.94 35.4 10 434 658

0.5 1.294 1 129 0 3499 870 1098 465 132 6747 61 27 13.35 8.44 8.02 7.57 0.99 34.16 25.01 40.83 35.5 7 422 712

0.07 0.297 19 554 9 143 254 525 248 89 1148 306 101 13.01 2 8.79 11.42 2.24 47.59 24.46 27.97 30.4 3 455 795

0.82 0.953 25 3610 0 3997 135 38 0 39 1711 12 0 13.56 5.49 8.52 7.07 0.38 31.74 21.49 46.80 38.6 8 364 733

0.79 1.55 33 5040 0 1677 1072 1512 586 96 5527 69 35 13.14 8.63 4.15 8.31 1.07 35.56 22.14 42.29 34.3 9 435 714

T.C. Ho / Applied Catalysis A: General 378 (2010) 52–58

54 Table 5 HDN reactivities and feed properties, Set A2.

also defined as the apparent first-order volumetric rate constant. Table 7 lists the data on the 10 property descriptors and the corresponding kHDN. To help set the stage for developing a QPRR for HDN, the following section is a recapitulation of HDS QPRRs.

Feed

kHDN 1, Sf 2, S0 3, S1 4, S2 5, S3 6, S4 7, S5 8, S6 9, S7 10, S8 11, Nf 12, BN 13, H 14, SA 15, 1R 16, 2R 17, 3R 18, NP 19, TA 20, TP 21, API 22, BM 23, I/5 24, FBP

9

10

11

12

1.10 2.715 53 14,400 228 10,259 133 19 0 0 3761 591 462 12.85 7.79 11.78 8.34 0.45 37.78 28.36 33.86 33.4 22 432 620

0.26 3.171 55 17,300 18 3271 1036 1488 527 419 7651 2061 906 12.48 9.91 10.61 7.38 2.08 39.27 30.00 30.73 27.8 29 395 781

0.48 1.496 27 4000 0 2654 1214 1645 705 197 4545 1051 971 13.19 9.52 7.27 6.85 1.16 35.05 24.80 40.15 33.5 8 493 717

0.10 2.735 16 4460 385 10,663 572 793 355 438 9684 1674 764 11.38 13.48 22.27 7.12 1.66 43.56 44.53 11.91 25 68 374 839

3. HDS property–reactivity relationship The approach for developing a QPRR has been detailed elsewhere [5–7]. Briefly, it consists of three steps. A chemometric analysis was performed to sift through the large number of property descriptors and their linear combinations. This was followed by identifying the most influential descriptors. The thusidentified dominant descriptors were then used to construct a compact correlation collapsing the data. For HDS, this approach identified two competitive adsorption regimes, as discussed below. 3.1. High-sulfur regime To reduce sulfur from a feedstock in Set A to a product sulfur level of, say, 400–600 ppm, the feedstock’s API gravity is the primary indicator of its HDS reactivity. Specifically, in this not-sodeep HDS regime, the HDS reactivity, measured as the 1.5th order volumetric rate constant, scales as HDS reactivity / A2:18 S0:31 N0:2

Here the three normalized (dimensionless) feed descriptors are defined as: A = API/38.6, S = sulfur in DBTs/5746, and N = Nf/2061, with DBTs = DBT + b-DBT + bb-DBT [5]. Eq. (1) implies that the inhibiting effect of heavy aromatics is far more important than that of N-species. Also of note is that while steric hindrance around the sulfur heteroatom is not the dominant factor, it does play a role. In fact, it is more influential than the feed nitrogen content. As a relevant aside, for both HDS and HDN of heavy gas oil (FBP  1000– 1150 8F), the dominant feed property is the concentration of threeplus-ring aromatics [8,9].

Table 6 Descriptors definition and designation, Set B. No.

Definition

1 2 3 4 5 6 7 8 9 10

Total sulfur in feed (Sf), ppm Total nitrogen in feed (Nf), ppm Total polynuclear aromatics (PNAs), wt.% Total aromatics, wt.% Final (95%) average boiling point, 8F API gravity (60 8F), 8API Sulfur in non-b-DBTs, ppm Sulfur in b-DBTs, ppm Sulfur in bb-DBTs, ppm Total DBT, ppm

(1)

3.2. Low-sulfur regime Now turn to the HDS of 13 prehydrotreated feedstocks in Set B, which signifies a deep HDS regime in which the targeted product sulfur level is less than 10 ppm. In this deep HDS regime, the most influential property turns to be the feed nitrogen content [6]. Moreover, the HDS reactivity, determined as the 1.2-order rate constant, decreases linearly with increasing Nf, namely

aromatic contents in Set B are much lower than those in Set A. Also, Set B’s feedstocks are more saturated as can be seen from the API gravity. Moreover, this set of feeds has a disproportionately high level of alkyldibenzothiophenes (alkyl-DBT) with at least one substituent located at the position b to the sulfur heteroatom (the 4 and 6 positions). For convenience, let 4-alky-DBT and 4,6dialkyl-DBT be represented by b-DBT and bb-DBT, respectively. To treat Set B feeds, a commercial NiMo/Al2O3 catalyst was run at 650 psig H2 pressure, 0.65–1.2 LHSV, and a hydrogen treat gas rate of 450 standard ft3/barrel oil [6]. The relative HDN reactivity kHDN was

HDS reactivity ¼ k  aNf

(2)

where k and a are constants for a given family of feeds over a given catalyst. This behavior can be derived from a simple competitive

Table 7 HDN reactivities and properties of prehydrotreated distillates, Set B. Feed

Rel. kHDN

Sf, ppm

Nf, ppm

PNAs, wt.%

Total aroms

FBP 95%, 8F

API gravity

Non-b-DBT

b-DBT

bb-DBT

Total DBT

1 2 3 4 5 6 7 8 9 10 11 12 13

1 0.8 1.37 0.5 0.98 1.4 1.67 1.13 0.9 0.73 0.65 0.63 0.45

410 634 775 747 250 266 390 462 325 272 293 1567 522

97 27 53 155 61 46 44 66 37 72 123 164 174

11.1 12.7 10.5 17.4 16.1 9.2 11.9 9.0 9.9 8.8 16.6 12.2 12.3

28.8 30.5 27.0 28.2 35.1 22.6 24.1 28.8 26.1 27.3 27.8 20.6 23.9

691 687 703 735 701 695 727 670 656 722 751 740 783

35.7 39.8 36.2 31.6 37.0 37.6 35.4 35.7 36.8 33.4 32.8 35.6 34.1

40 100 46 52 25 14 42 35 31 18 15 130 26

92 98 78 122 48 35 94 145 104 39 26 294 97

224 195 62 226 123 88 88 190 64 62 89 238 106

356 393 186 400 196 137 224 370 199 119 130 662 229

T.C. Ho / Applied Catalysis A: General 378 (2010) 52–58

Fig. 1. Sulfur concentration (wt.%) in the liquid product (Sp) vs. 1/LHSV. HDS proceeds from an aromatics-inhibited regime (I) to an organonitrogen-inhibited regime (II). The lines of demarcation are arbitrarily drawn for illustration; they depend on catalyst, feedstock, and reaction conditions.

adsorption model based on the Langmuir–Hinshelwood formulism [6]. Physically, k represents an intrinsic HDS reactivity in that it is measured in the absence of N inhibition. The constant a is an inhibition parameter reflecting the combined effect of the adsorption–desorption and the denitrogenation reactivity of adsorbed N-species [6]. 3.3. Change of dominant property Patching the two regimes together indicates that the HDS property–reactivity correlation undergoes a transition in going from the high-sulfur regime to the low-sulfur regime. Schematically, this is illustrated in Fig. 1, which plots ln Sp vs. 1/LHSV where Sp is the sulfur content in the effluent liquid. The desulfurization trajectory is concave upward because as 1/LHSV increases, reactive sulfur species get desulfurized first and the feedstock becomes progressively more difficult to desulfurize. The result is that the overall HDS kinetics can be described as nth-order with n > 1. Qualitatively, Fig. 1 shows that the two regimes, I and II, are separated by a transition zone, with the lines of demarcation being arbitrarily drawn. In regime I, polynuclear aromatics are the dominant inhibitor because of their high concentrations. After passing through the transition zone, most of polynuclear aromatics are severely hydrogenated. The residual nitrogen heterocycles emerge as the dominant inhibitor for the HDS of b-DBT and bbDBT, which is very sensitive to organonitrogen inhibition [10–13]. In the HDS of two severely prehydrotreated oils, van Looij et al. observed a profound N inhibition effect with Nf as low as 30 ppm [14]. Model-compound experiments have shown that the HDS rate of 4,6-diethyl-DBT decreases precipitously upon the addition of a trace amount of 3-ethylcarbazole (5 wppm as N atom) to the feed. By contrast, the HDS of DBT is only moderately inhibited by 80 wppm feed N [15]. In addition to blocking the active sites for HDS, the adsorbed N-compounds slow down the hydrogen activation process, thus limiting the supply of surface hydrogen for hydroprocessing reactions [16]. Note that the HDS of b-DBT and bb-DBT is a hydrogen-intensive process.

55

Fig. 2. HDN reactivity kHDN vs. 95% final boiling point (95% FBP) for raw middle distillates (Set A). Solid circles denote low-nitrogen feeds in A1. Open circles denote high-nitrogen feeds in A2.

singled out as the most influential property governing the HDN reactivity. Fig. 2 shows a fairly good linear correlation, considering the diversity of the feedstocks. The open circles in Fig. 2 are the data for the four feeds in the A2 group. An examination of the relation between Nf and FBP reveals that within each group, Nf correlates with FBP, as can be seen from Fig. 3. This suggests that for a set of similar raw middle distillates, Nf can be used as an approximate indicator of HDN reactivity. This is indeed the case for both A1 and A2 groups, as displayed in Fig. 4. Here the semi-logarithmic scale is used in order to include both groups in Fig. 4. The kHDN vs. Nf relation is also approximately linear on the linear scale for each group. The above observations, when contrasted with those on HDS, indicate two distinguishing characteristics of the HDN process. One is that the adsorption of nitrogen species is so fast and strong that the presence of aromatic and sulfur species has little, if any, influence on HDN. The other is that the effect of steric hindrance around the heteroatom is not as pronounced in HDN as in HDS [17– 19]. It is hardly surprising to see that Fig. 5 shows little correlation between the 12 pairs of HDN and HDS reactivities of the raw middle distillates.

4. HDN property–reactivity relationship With the preceding section as a prelude, we now look at the corresponding situation in HDN. For the 12 high-sulfur raw distillates (Set A), it turns out that the heaviness of the feed can be

Fig. 3. Feed nitrogen concentration (ppm) vs. feed heaviness (95% FBP) for raw middle distillates (Set A). Solid circles denote low-nitrogen feeds in A1. Open circles denote high-nitrogen feeds in A2.

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T.C. Ho / Applied Catalysis A: General 378 (2010) 52–58

Fig. 4. HDN reactivity kHDN vs. feed nitrogen concentration (ppm) for raw middle distillates (Set A). Solid circles denote low-nitrogen feeds in A1. Open circles denote high-nitrogen feeds in A2.

Fig. 5. Normalized HDN reactivity vs. HDS reactivity for raw middle distillates (Set A). Solid circles denote low-nitrogen feeds in A1. Open circles denote high-nitrogen feeds in A2.

Fig. 7. Feed heaviness (95% FBP) vs. feed nitrogen concentration (ppm) for prehydrotreated middle distillates (Set B).

Fig. 8. Normalized kHDN vs. feed nitrogen concentration (ppm) for A1 feeds (*), A2 feeds (~), and Set B feeds (*). The two lines are not obtained from regression.

As for the 13 prehydrotreated, low-sulfur distillates in Set B, Nf is identified as the most influential correlating property for HDN reactivity, as shown in Fig. 6. Fig. 7 indicates that there also exists a certain degree of correlation between Nf and FBP. It transpires from the forgoing development that when it comes to HDN, Nf is the most important property in both regimes I and II. To pursue this point further, we merge Fig. 6 with Fig. 4 to obtain Fig. 8 in which kHDN is normalized within each of the A1, A2, and B groups. Note that the pseudo-first-order rate constant kHDN is independent of Nf. As Fig. 8 shows, the behaviors of the two sets of low-N feeds, A1 (raw distillates) and B (prehydrotreated feeds), are similar. This is satisfying considering that the nature of the feeds, catalysts (CoMo/Al2O3 vs. NiMo/Al2O3), and reaction conditions are very different. This perhaps should not come as a surprise given the strong adsorption affinity of the catalyst for N-species. 5. Discussion Fig. 6. HDN reactivity kHDN vs. feed nitrogen concentration (ppm) for prehydrotreated middle distillates (Set B).

The above results, taken together, lead to the following suppositions. First, for a given set of similar feedstocks, feed

T.C. Ho / Applied Catalysis A: General 378 (2010) 52–58

nitrogen content can be used as an approximate indicator of the relative HDN reactivities of the feedstocks. Since we are concerned with the removal of total nitrogen, this supposition implies a selfinhibiting effect that can manifest itself in different ways. This can be illustrated from the results obtained from model-compound experiments. The first example is that the HDN reaction orders for quinoline, acridine, carbazoles, and tetrahydroquinoline are less than unity [4 and references therein], suggesting self-inhibition. The second example is that self-inhibition can also arise from competition among indigenous N-species for active sites. A case in point: acridine inhibits the HDN of 1,4-dimethylcarbazole [20]. This is so because acridine is a six-membered N-heterocycle and hence has a higher basicity. The third example is that some partially hydrogenated N-heterocycles, which are generated as HDN intermediates, are more inhibiting than their parent Nheterocycles. Some known instances are 1,2,3,4,5,6,7,8-octahydroacridine vs. acridine [20], dimethyltetrahydrocarbazoles vs. 1,4-dimethylcarbazole [20], tetrahydroquinoline vs. quinoline [21,22], and indoline vs. indole [22]. Moreover, it was found in the HDS of DBT over a sulfided CoMo/Al2O3 catalyst that the inhibition is more severe with a mixture of quinoline, indole, and carbazole than with any of the individual constituent N-species at the same nitrogen concentration [23]. Given the above, if one uses the Langmuir–Hinshelwood formulism to model HDN kinetics, the denominator of the rate expression should also account for the stronger inhibition by Nintermediates. In so doing, one may find that the inhibition severity increases as the reactions proceed. Indeed, this was observed in real-feed hydrotreating on the hydrogenation sites of a commercial hydrotreating catalyst [24]. The second supposition is that as a first approximation, the relationship between kHDN and Nf is a linear one, analogous to Eq. (1). There thus exists an apparent parallel between HDN and deep HDS (regime II in Fig. 1). A subtle distinction here is that while nitrogen inhibition is a ‘‘curse’’ for deep HDS, self-inhibition in HDN is part of nitrogen removal process. Borrowing the physical meaning of a in Eq. (1) that was developed for deep HDS, one expects that the N self-inhibition effect also reflects the interplay of Nf, adsorption/desorption, and the reactivity of adsorbed N. The relevance of N reactivity can be seen from the observation that light, highly basic N-species, such as alkylpyridines, are rapidly adsorbed. Yet they do not have much inhibiting power because the subsequent C–N cleavage is commensurately facile [25]. The third proposition is that N self-inhibition is associated with the fact that Nf usually goes hand in hand with feed heaviness. Since adsorption on metal sulfide surface in a high-pressure trickle-bed reactor involves surface ‘‘condensation’’ and ‘‘wetting’’, the inhibition power of an adsorbate should be closely tied to its heaviness. In the hydrogenation of polynuclear aromatics, the adsorption equilibrium constant is a linear increasing function of aromatic ring number and alkyl side chain length [26]. Even on solid acid catalysts, the inhibiting power of N-compounds is more influenced by adsorbate’s heaviness than by basicity [7]. Heavy six-membered N-species, which are highly basic, are readily adsorbed and do not come off the catalyst surface easily [11,27,28]. The same is true for heavy, non-basic, five-membered N-heterocycles such as alkylcarbazoles [13]. Once they become adsorbed, they linger on the catalyst surface because of the slow denitrogenation rate [29]. Finally, it is important to distinguish and quantify six- and fivemembered nitrogen heterocycles in hydroprocessing catalyst/ process developments [30]. This can be achieved by using the electrospray ionization mass spectrometry (ESI-MS) capable of detecting trace levels (low ppm) of polar species [31]. When the ESI is operated in the positive ion mode, it selectively ionizes species

57

Fig. 9. Nitrogen concentration (ppm) vs. average boiling point for a feedstock and its hydrotreated products at different hydrogen pressures and over KF-702 and KF-840 catalysts [25].

that can be easily protonated, such as six-membered nitrogen heterocycles (e.g., acridine). In the negative ion mode, it can selectively ionize species that can be easily deprotonated, such as five-membered N-heterocycles. In this sense, five-membered Nheterocycles are weakly acidic. ESI-MS experiments have indicated that five-membered N-compounds are more difficult to denitrogenate than six-membered N-compounds. Carbazoles are the most-difficult-to-denitrogenate compounds among the five-membered N-compounds. The HDN reactivity of alkylcarbazoles decreases with the number of methyl substituent [13,32,33]. More model-compound kinetic experiments are needed to gain a better understanding of the catalytic chemistry of the HDN of alkylcarbazoles. To illustrate some of the above points, we show a relatively inaccessible figure of Krause and Joutsimo [25] as Fig. 9 here. The figure plots the nitrogen content vs. boiling-point distributions for an LCO before and after hydrotreating at 370 8C over supported CoMo (KF-702) and NiMo (KF-840) sulfide catalysts. LCO is a mostdifficult-to-treat middle distillate. Referring to Fig. 9, the lowboiling nitrogen species can be easily denitrogenated with both catalysts at 30 bar hydrogen pressure. In contrast, the highboiling portion of the feed has a far higher concentration of nitrogen that is difficult to remove even at 50 bar over KF-840. Of particular note is that the concentrations of some heavy nitrogen species actually increase after hydrotreating. These species should be partially hydrogenated high-boiling N-heterocycles. They may be more inhibiting than their parent molecules. Fig. 10 indicates that the residual nitrogen compounds are mostly alkylcarbazoles [25]; a similar result was also reported by Hudson [34]. The low-boiling quinolines are absent because they react away on the catalyst surface [32] in spite of their high adsorption affinity.

Fig. 10. GC chromatograms of nitrogen compounds in hydrotreated liquid product [25].

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