Energy & Fuels 1991,5,595-601
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Catalytic Cracking of Canadian Nonconventional Feedstocks. 1. Cracking Characteristics of Gas Oils Derived from Coprocessing Distillate and Shale Oil Siauw H. Ng* and Parviz M. Rahimi Energy Research Laboratories, CANMET, Energy, Mines and Resources Canada, 555 Booth St., Ottawa, Ontario, Canada K I A OGl Received January 16, 1991, Revised Manuscript Received March 27, 1991
Two nonconventional vacuum gas oils (VGOs) featuring high nitrogen concentrations were obtained by vacuum distillation of a coal-oil coprocessing product and a shale oil, respectively. Analyses showed that the coprocessing VGO was more aromatic whereas the shale VGO contained more polar components. The two VGOs were cracked in a microactivity test (MAT) unit a t different temperatures and catalyst/oil ratios over three commercial catalysts containing rare earth-exchanged or ultrastable Y zeolite (REY or USY). Similar to most products from primary upgrading processes, these two VGOs showed comparatively poor cracking results in terms of conversion, gasoline, and coke yields. This was attributed to nitrogen poisoning of catalysts during cracking of either of the two gas oils and also to the relatively high polynuclear aromatic (PNA) content of the coprocessing VGO. Cracking results improved when nitrogen and PNA compounds were partially removed from the VGO's by solvent extraction or clay adsorption. The treated VGOs can be considered as potential fluid catalytic cracking (FCC) feedstocks. Additive coke resulting from the basic nitrogen and the Conradson carbon residue in the gas oils was found to be dependent only on the temperature. The effect of severity on product distribution and the performance of the three catalysts with respect to nonconventional gas oil cracking are demonstrated and discussed. Introduction As conventional Western Canadian oil reserves are declining, Canadian refiners have to depend increasingly on nonconventional feedstocks such as those derived from tar sand bitumens, heavy oils, and resids to produce transportation fuels. Canadian advanced technologies such as the CANMET hydrocracking process1p2have been developed and successfully demonstrated to upgrade these heavy feedstocks effectively and economically. Concurrently, CANMET has shown significant interest in a technology which simultaneously upgrades coal and heavy oils/bitumens for the production of synthetic fuels.*6 This new coprocessing technology is often regarded as a bridge between conventional coal liquefaction and hydrocracking. Switching to coprocessing from coal liquefaction in which the use of recycle solvent is essential does not come as a surprise. Economic indicators show that coprocessing would be economically more favorable than conventional liquefaction at a crude price of approximately $40/bb1.8 One of the major advantages of coprocessing is that the product quality is more compatible with conventional petroleum products compared with coal-derived liquids.' This makes the downstream handling of coprocessing distillates much easier. The upgradability of naphtha which was obtained from coprocessing Cold Lake vacuum bottoms (CLVB) and Forestburg subbituminous coal was studied previously in a bench-scale continuous trickle bed reactor.s Using a commercial Ni-Mo catalyst, heteroatom levels could be lowered to about 1 ppm under mild conditions. The present study was made to further evaluate the crackability of the gas oil fraction (bp 335-525 "C) of the same coprocessing distillate. As expected from most primary upgrading liquid products, the coprocessing gas oil contained also high levels of problematic components. One
* To whom correspondence should be addressed.
Table I. Analyses of Feedstocks coprocessing VGO OAPI 10.7 aromaticity, % 40.0 aniline pt, OC Conradson C, w t % 1.17 total N, ppm 6630 basic N, ppm 2946 total s, wt % 2.79 c,wt % 85.5 9.55 H,wt % 1.20 0,wt % viscosity at 40 O C , cSt 107 UOP K factor 10.8 Ni, ppm 343 "C) with increased conversion. The lines here have a slope close to -1, indicating that the increase in conversion can almost be accounted for by the
__ I 20
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Conversion , w l %
Figure 6. Effect of temperature on HCO selectivity of catalysts
DA-440 and Nova D.
decrease in HCO yield. This is understandable since the yield of LCO, which is one of the two unconverted products, changes very little with conversion. Again, for the shale VGO, some temperature effect is observed. This suggests that more HCO precursors which contain most of the nitrogen in the feed can be cracked into lighter products at higher temperature. Figure 7 illustrates the linear correlation between coke yield and conversion for the two gas oils. At constant conversion, elevated temperature reduces the coke yield as this yield is very sensitive to the &./oil ratio which can be reduced at higher temperature to achieve the same conversion. A t equivalent conversion and temperature, the coprocessing VGO tends to produce more coke due to its refractory nature than the shale VGO when cracked with catalyst DA-440. Between the two catalysts, Nova D apparently has higher coke selectivity (Figure 7B). Unless it was influenced by the matrix, this is somewhat unexpected since Nova D contains zeolite of smaller unit cell
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Catalytic Cracking of Nonconuentional Feedstocks 11 loth G n l n t
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Table IV. Analyses of Upgraded VGO's coprocessing VGO shale VGO DMSO clay DMSO clay recovery, wt % 77.9 82.2 85.3 86.1 O A P I at 15 O C 15.1 27.1 aromaticity, 5% 33.0 18.0 aniline pt, OC 59.0 67.2 0.50 0.40 Conradson C, w t % total N, ppm 2730 1645 5618 3496 basic N, ppm 1255 224 3007 1745 total s, wt % 3.04 0.46 c,wt % 85.7 85.7 H, wt 9% 10.5 12.3 C0.5 0, wt % 33.9 14.7 viscosity at 40 "C, cSt UOP K factor 11.1 11.9
reaction temperature but not on catalyst type. The lower the temperature, the higher is the coke yield. This coke formed at 0 w t % conversion is the "additive" coke which 4 is different from the "catalytic" coke resulting from acidcatalyzed cracking.21 Additive coke is closely related to basic nitrogen as well as Conradson carbon.21 Satterfield 3 et al. suggested that some nitrogen bases can be irreversibly adsorbed on Lewis sites of catalysts and form coke.22 2 Since the coprocessing VGO has a rather high Conradson 20 30 40 50 50 carbon content, this explains why ita additive coke level Conversion, wt % is higher than that of the shale VGO which has twice as Figure 7. Effect of temperature on coke selectivity of catalysts much basic nitrogen as the coprocessing VGO. Making DA-440 and Nova D. the correction for Conradson carbon, the additive coke yield due to basic nitrogen is estimated at 0.85-0.53 w t 3'% at 490-530 "C for the coprocessing VGO and 1.29, 1.07, 0.91, and 0.79 wt 90at 470,490,510, and 530 "C, respectively, for the shale VGO. It is well recognized that octane enhancement in gasoline by a USY zeolite is attributed to its reduced acid site ; 1 2 3 4 5 I 7 1 9 1 0 density. This favors more cracking than hydrogen transfer, P 1.0 , resulting in higher olefinicity of the cracked products.'* In this study, no octane measurement was made on the liquid product. The olefin/paraffin ratio in the C3 C4 2nd.ordsr Conv. = + Cs fraction of the cracked product was higher when the shale VGO was cracked by Nova D than when cracked by 1 2 3 4 5 6 7 1 9 1 0 Coke , wl % DA-440. At 30 wt % conversion and at 470,490 and 510 "C, this ratio is 0.78,0.97 and 1.06 for Nova D, and 0.75, Figure 8. Coke yield versus second-order conversion of feedstocks cracked with catalysts DA-440 and Nova D at various tempera0.88, and 0.97 for DA-440. tures. Based on the above results, it is evident that the two raw gas oils cannot be accepted as FCC feedstocks. Three have been used to achieve higher conversion. The and should produce less coke a t constant c o n v e r s i ~ n . ~ ~methods ~~ The reduction in acid site density resulting from the fmt is to crack the gas oils with GX-30, a much more active catalyst with a higher zeolite content. The other two dealumination of zeolite tends to reduce the surface concentration of coke precursors and increase the rate ratio methods involve upgrading of gas oils by removing some of the nitrogen compounds using either DMSO/acid exof cracking to coke formation. A different result in coke selectivity was observed when a Canadian conventional gas traction or Attapulgus clay adsorption. One disadvantage oil (rich in paraffins) was cracked using the same two of these rejection methods is the accompanying loss of the hydrocarbon value. In general, the clay method is superior catalysts.12 It is likely that the superiority of USY over REY catalysts with respect to coke selectivity depends also to DMSO extraction as the former gives higher weight on the nature of the feedstock. recovery yet it rejects more nitrogen compounds from the Figure 8 shows a linear correlation between the secfeed. ond-order conversion (kinetic conversion) and the coke Table IV shows a general improvement in feedstock yield. For normal FCC feedstocks, the straight lines in this quality, after treatment, except for sulfur content. The plot would pass through the origin closely, indicating that nitrogen levels have declined substantially. The clay very little or no coke was formed a t 0 w t % conversion. method appears to be more effective in removing nitrogen However, for these two feedstocks, the straight lines incompounds especially for the coprocessing VGO. More tersect the x axis at various coke levels depending on the than 90% of the basic nitrogen can be removed compared with about 60% for the shale VGO. A typical Western 5
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(19) Rajagopalan, K.; Peters, A. W. J. Catal. 1987,106,410-416. (20) Ritter, R. E.; Creighton, J. E.; Chin, D. S.;Roberie, T. G.; Wear, C. C. In Catalytic Octane from the FCC;Catalagram,Daviaon, 1986, No. 74. (Note:'Catal am" is published regularly by 'Davison Chemical Division" of W. R.%ace & Co.).
(21) Cimbalo, R. N.; Foster, R. L.; Wachtel, S. J. Oil Cas J. 1972, 70(20), 112. (22) Satterfield, C. N.; Modell, M.; Hines, R. A.; Dederck, C. I. Ind. Eng. Chem. R o d . Res. Deo. 1978,17, 141.
Ng and Rahimi
600 Energy & Fuels, Vol. 5, No. 4, 1991 50 60
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Canadian conventional gas oil (IPPL) has approximately the following analysis:12 25 "API;17% aromaticity; 90 "C aniline point; 0.5 wt % Conradson C; lo00 ppm total N; 300 ppm basic N 0.5 wt % S; 12.0 UOP K factor. It is thus apparent that the clay-treated coprocessing VGO is still comparatively heavy and refractory, although its basic nitrogen level is acceptable whereas the clay-treated shale VGO still has a high basic nitrogen concentration which is about 6 times higher than that of the IPPL gas oil. Figure 9 shows a significant improvement in conversion for the two treated VGO's cracked over DA-440 and for the untreated feeds cracked over GX-30. It can be seen that, at an equivalent cat./oil ratio, the conversion is influenced by factors in the following order: feedstock quality, catalyst activity, and reaction temperature. Figure 9B shows that, for clay-treated shale VGO, more than 70 wt % conversion can be achieved at high severity even when the less active catalyst DA-440 is used. The major advantage of using a high-activity nitrogenresistant FCC catalyst is that the cost of a pretreatment process is eliminated or greatly reduced. Comparing the performances of GX-30 and DA-440 for the two untreated feedstocks, one may conclude that at equivalent severity GX-30 gives about 10 wt % more conversion than DA-440 (Figure 9). This is indeed a significant increase. Figures 10 and 11depict the product distributions of the raw and treated VGO's cracked a t different conditions at 60 wt % conversion. Note that, for the raw VGOs cracked by DA-440, extrapolated values were used with good confidence as their selectivity curves coincide with or are parallel to others obtained at different conditions. For the raw coprocessing VGO, Figure 10 shows that irrespective of the temperature change both DA-440 and GX-30 give almost identical product distributions, although the latter
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Figure 11. Product distribution at 60 w t % conversion of raw and upgraded shale VGO's cracked with catalysts DA-440and GX-30 at 510 and 530 O C . is much more active. Their cracking characteristics are signified by the low gasoline yield and the high HCO and coke yields. Upgrading with DMSO extraction significantly increases gasoline yield and cuts coke formation by one half. Further improvement in yields can be achieved with clay treatment. In this case, temperature has only a minor effect on product selectivity as the governing factor is not the basic nitrogen but the refractoriness of the oil. In contrast, a high basic nitrogen content is the key problem with the raw and DMSO-treated shale VGO's. Raising the temperature from 510 to 530 "C tends to crack more HCO into LCO and dry gas with slightly lower gasoline production (Figure 11). Only when the basic nitrogen level is further decreased with clay treatment (from 4570 to 1745 ppm) does it start to substantially increase gasoline and LCO yields at the expense of HCO, dry gas, and coke yields. The experiments show that, for clay-treated shale oil, about 45 wt % gasoline can be obtained at 73 wt % conversion. This is a significant improvement in product yield over the raw shale oil. Again, it is noticed that at the same temperature (530 "C)both GX-30 and DA-440 show the same product selectivity for the raw shale oil. This is not surprising since the zeolites involved, which are the most active ingredients in these FCC catalysts, have been hydrothermally deactivated to
Catalytic Cracking of Nonconventional Feedstocks VGO coprocessing raw clay-treated overall shale raw clay-treated overall
case 1 2 1
2
Energy & Fuels, Vol. 5, No. 4, 1991 601
Table V. Effect of Catalyst Deactivation on Product Yields' recoverv, wt % basic N. DDm conversion! wt % gasoline.' w t %
coke.' w t %
100.0 82.2 100.0
2946 224
47.0 62.0 51.0
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'Catalyst DA-440,cat./oil = 6,temp 530 "C. *Primarily from Figure 9. cFrom selectivity curves, e.g., Figures 4 and 7 for raw gas oils.
the same degree as reflected by their unit cell sizes (Table 111). The higher zeolite content in GX-30 enhances this catalyst's activity but not selectivity compared with DA440. To further evaluate the catalyst poisoning effect, let us consider two cases. Case 1is simply a direct cracking of the raw VGO whereas case 2 involves separation of the gas oil into two fractions prior to their individual cracking under the same conditions as case 1. In the latter case, assume one portion contains less nitrogen and is of better quality whereas the other portion has, for simplicity, negligible conversion because of either its high nitrogen level or its refractory nature. Overall cracking yield is thus calculated based on the yield of the good quality oil and its weight recovery. Table V shows the product yields at 530 "C and cat./oil ratio of 6, using catalyst DA-440. It can be seen that after rejecting about 18 w t % of the raw material, the clay-treated coprocessing VGO has an insignificant nitrogen content and shows improved cracking result. Overall, the conversion and gasoline yields are higher by about 4 units than the corresponding yields of the untreated oil. These increases reflect clearly the poisoning effect of basic nitrogen and coke on the catalyst. For the shale VGO, the differences in yield between the upgraded and the untreated oils are even higher (10-12 units). These differences demonstrate the net nitrogen poisoning effect on the catalyst as both the raw and treated oils gave the same coke yield of 6.6 wt %.
Conclusions Although the coprocessing VGO and shale VGO were quite different compositionally, they had one common feature, i.e., poor crackability with somewhat similar cracking patterns in MAT tests. This was attributed to their high nitrogen contents and the refractory nature of the coprocessing gas oil. The stronger temperature dependence observed on most of the yield curves of the shale VGO seemed to support this view. The additive coke of
the shale gas oil was produced predominantly from basic nitrogen whereas that of the coprocessing VGO was from both Conradson carbon and basic nitrogen. Compared with DA-440, Nova D was less active in cracking nonconventional feedstocks, perhaps due to the small pores of the matrix. At equivalent conversion and temperature, Nova D produced more gases and coke. As well, it enhanced the olefin content in the liquid and gaseous products as a result of dealumination of the zeolite. The most active catalyst GX-30 gave higher conversion and better yields of the converted products at equivalent severity. However, at constant conversion, GX-30's behavior was identical with that of DA-440 at the same temperature. The two gas oils could be upgraded either by DMSO extraction or Attapulgus clay adsorption. The latter is more effective in removing basic nitrogen compounds particularly from the coprocessing VGO. Partial removal of the problematic components greatly enhanced the processability of the two gas oils and made them potential FCC feedstocks. It should be pointed out that the cracking yields obtained from the MAT reactor should be treated cautiously as MAT data show only directional changes. MAT testing cannot simulate the dynamic operation of a commercial rise reactma In short, the number of active catalytic sites in a given MAT reactor is fixed whereas that in a riser reactor is constantly changing as the catalyst/oil ratio changes in response to the varying environment. Thus, to support the findings in this study, more reliable data should be obtained from the circulating riser pilot unit.
Acknowledgment. We thank our colleagues Dr. J. Kriz and Dr. T. de Bruijn for useful discussions. We also thank Dr. A. Johnson of Sheridan Technical Associates for directing some of the experiments. The provision of equilibrium catalysts for this study by W. R. Grace & Co., Davison Chemical Division, is gratefully acknowledged. (23)Mott, R.W.Oil Cas J. 1987,Jan 26,73.
