Energy & Fuels 1989,3,603-612 experienced by the shell, $(rJ + $(rp), to the shrinkage experienced by the particle, ro - rp, where ro is the original particle radius and
which is obeyed by the Cas-CaO particles up to about x = 0.50 for lot 1, as seen in Figure 6. Figure 15, which shows (ne), vs x for concrete and steel, shows that (ne),,, does increase dramatically with x . The material properties of the CaO(s) crystals are not known, but its behavior may be reasonably assumed to be intermediate between that of concrete and steel. Also shown in the figure is the tensile strength of A1203(s)crystals, ( ~ g ) ~ 2 0 8which , illustrates the high values attainable for small crystal dimensions where the effect of flaws is min-
603
imized. The tensile strength of the CaO(s) crystals is possibly less than that of A1203(s),but greater than that of concrete. It is seen that failure at 2 0.50 for CaO(s) can occur at a point intermediate between the curves for concrete and steel and at a tensile stress lower than the tensile strength of A1203(s).Hence, this failure (i.e., crack formation and propagation) is expected on the basis of strength of materials consideration. Figure 15 also shows that if the particle size remains constant after x = 0.50 (dashed curve for concrete), lower stresses do develop in the shell.
-
Acknowledgment. This research was funded by the MIT-Exxon Combustion Research Program. Assistance with the analytical techniques from Prisca Chen and Linda Sheehan are gratefully acknowledged. Registry No. Cas, 20548-54-3; CaO, 1305-78-8.
Molecular Transformations in Hydrotreating and Hydrocracking Richard F. Sullivan,* Mieczyslaw M. Boduszynski, and John C. Fetzer Chevron Research Company, 576 Standard Avenue, Richmond, California 94802 Received December 13, 1988. Revised Manuscript Received June 2, 1989
This study involved three hydroprocessing steps. The first step simulated a commercial residuum desulfurization (RDS) process. The second step, in which the residuum-derived vacuum gas oil (RDS-VGO) was hydrotreated, simulated the first stage in a two-stage hydrocracking process. The third step simulated the second stage of a two-stage hydrocracker, operating in an extinction-recycle mode. The study was made by using pilot plants that simulate state of the art refinery units. The focus of this paper is on compositional changes that occurred during hydrocracking of a RDS-VGO to make jet fuel as the primary product. Molecular transformations leading to the buildup of polycyclic aromatic hydrocarbons (PAHs) in the second-stage hydrocracker recycle stream were of particular interest. A combination of high-performanceliquid chromatography (HPLC) with UV/vis diode-array detection and field-ionization mass spectrometry (FIMS) was used to follow changes in composition of VGO through a series of hydroprocessing steps. Detailed compositional data helped unravel some of the complex chemistry of the reactions involved. The results explain why VGO produced in a high-severity RDS process is much harder to hydrocrack than that produced at lower severity. The difference is due to the presence of high-ring-numberPAHs in the high-severity product. These PAHs involve only a small portion of the total process stream. Specific PAHs produced in hydrocrackers have been identified. The PAHs have a unique distribution pattern involving only the most stable PAH isomers of a given ring number. These findings suggest a reaction pathway referred to as the “naphthalene zigzag” to account for the buildup of PAHs in the hydrocracker recycle streams.
Introduction
usually petroleum oils that are very complex mixtures of hydrocarbons and heteroatom-containingcompocds. The The reaction chemistry of pure hydrocarbons over hyreaction chemistry can be quite different from that which drotreating and hydrocracking catalysts has been studied occurs with individual pure compounds. Hydrocracking extensively as discussed in a number of review arti~les.l-~ (HCR) catalysts are usually dual functional-that is, they However, feeds to commercial hydroprocessing units are contain both an acidic cracking component and a hydrogenation component. The relationship between these catalytic components can be altered by the preferential (1) Bolton, A. P. Hydrocracking,Isomerization, and Other Industrial Processes. In Zeolite Chemistry and Catalysis; &bo, J. A., Ed.; ACS adsorption of reactant hydrocarbons and heteroatomMonograph 171; American Chemical Society: Washington, DC, 1976; pp containing compounds on catalytic sites. Furthermore, 714-779. bimolecular reactions such as disproportionation and al(2) Choudhary, N.; Saraf,D. N. Hydrocracking: A Review. Ind. Eng. kylation make the reaction chemistry exceedingly comChem. Prod. Res. Deu. 1976,14(2), pp 74-83. (3) Langlois, G.E.;Sullivan, R. F. Chemistry of Hydrocracking. In plicated. Refining Petroleum for Chemicals; L. J. Spillane, L. J., Leftin, H. P., Even with modern analytical techniques, it is impossible Eds.; Advances in Chemistry 97; American Chemical Society: Washingto identify all of the individual compounds present in ton, DC, 1970, pp 38-67. 0887-062418912503-0603$01.50/0 0 1989 American Chemical Society
604 Energy & Fuels, Vol. 3, No. 5, 1989
high-boiling gas oils fed to hydroprocessing units.4 Therefore, feedstocks are usually characterized by gross properties such as gravity, aniline point, boiling range, hydrogen content, and heteroatom contents (nitrogen, sulfur, oxygen). Hydrocarbon types are usually grouped into classes such as paraffins, naphthenes, and aromatics, as determined by a variety of ugroup-typen methods. These analyses are all useful in characterizing feedstocks for comparison purposes. However, they may be inadequate for predicting the ease or difficulty of hydroprocessing one feedstock compared to another. In this paper, we will show examples of how a combination of modern analytical techniques can be applied to explain the differences in the behavior of two residuumderived vacuum gas oil (VGO) feedstocks in a series of catalytic hydroprocessing steps. Detailed compositional data will be presented that help to unravel some of the complex chemistry of the reactions involved. VGOs produced in residuum-upgrading processes are becoming increasingly important hydrocracker feedstocks because of the large number of residuum-conversionunits in operation or under construction throughout the world. Residuum hydrotreating, residuum hydrocracking, and coking processes all produce VGOs that require futher upgrading. In general, VGOs from cracked residua are more difficult to further crack than straight-run (SR) gas oils of the same boiling range.5 This is probably because easy-to-crack bonds are broken in the primary conversion processes. Residuum-derived VGOs typically contain high concentrations of condensed polycyclic aromatic hydrocarbons (PAHs) and hydroaromatics, which are known to be difficult feeds to hydroprocess. Therefore, the reaction chemistry of residuum-derived VGOs is currently of particular importahce to the petroleum industry. Residuum desulfurization (RDS) runs are usually controlled to maintain a target sulfur concentration in the product. As a hydrodesulfurization catalyst deactivates, it is necessary to increase the catalyst temperature to keep the amount of desulfurization constant. As the temperature increases, the amount of cracking of the higher boiling portion of the residuum increases. Therefore, as an RDS run progresses, the amount of cracked residuum in the VGO fraction of the product increases. Typical average catalyst temperatures in commercial RDS units are usually between about 350 and 425 "C. The maximum amount of cracking occurs at the highest temperature reached, that is, at the end of the RDS run. In order to compare the effect of the residuum-cracking severity on the downstream HCR steps, we selected RDS-VGOs from the two extremes as feeds for HCR tests-the first was from the start of the RDS run (low severity); the second was from the end of the run (high severity). These two samples will be referred to as SOR RDS-VGO and EOR RDS-VGO, respectively. Experimental Section Pilot Plant Tests. An experimental study was made to investigate compositional changes that occurred during the hydrocracking of a residuum-derived VGO to make jet fuel as the primary product. This study was made by using pilot plants simulating state of the art refining units. The processing sequence is shown in Figure 1. (4) Boduszynski, M. M. Composition of Heavy Petroleums. 2. Molecular Characterization. Energy and Fuek 1988,2,597-613 and references therein. ( 5 ) Bridge, A. G.; Jaffe, J.; Powell, B. E.; Sullivan, R. F. Isocracking Heavy Feeds for Maximum Middle Distillate Production. In 1983 Proceedngs-Refining Department; American Petroleum Institute: Washington, DC, 1983;Vol. 62, pp 7W36.
Sullivan et al.
7 345'C-
285%-
U 4 54O'ci
285'Ct
I
v 285'Ci
Figure 1. Diagram of RDS-VGO hydrocracking. The first hydroprocessing step simulated a commercial RDS proceas. The catalyst was a conventional commercial RDS catalyst consisting of groups VI and VI11 metals supported on alumina. The feedstock to this unit was a Middle Eastern atmospheric residuum (AR),boiling above about 370 "C (700 OF). Product from this RDS step was then distilled to obtain a VGO, referred to as RDS-VGO, with a nominal boiling range of 345-540 "C (650-1000 O F ) . The sulfur content of the RDS-VGO was about 0.13%, compared to 2.7% for that of the SR-VGO fraction of the same boiling range. The nitrogen content of the RDS-VGO was 625 ppm, about the same as that of SR-VGO. The next hydroprocessing step simulated the first stage in a two-stage HCR process. In this step, the RDS-VGO was hydrotreated to remove nitrogen and sulfur to about 0.3 and 1ppm, respectively. An appreciable amount of hydrocracking occurred in this step, and the product was distilled to separate the 285 "C(550OF-) fraction from the 285 "C+ bottoms. The third and final hydroprocessing step simulated the second stage of a two-stage hydrocracker, operating in an extinctionrecycle mode. In this operating mode, the hydrotreated 285 OC+ RDS-VGO from the previous step was hydrocracked at conditions selected to maximize the production of jet fuel with a 285 "C end point. The product was distilled, and the remaining 285 "C+ bottoms was combined with fresh feed and recycled to extinction. Both stages of the hydrocracker employed commercial catalysts consisting of groups VI and VI11 metals on amorphous supports containing silica and alumina. In general, amorphous HCR catalysts with a high ratio of hydrogenation activity to acidity are selective for cracking VGOs to middle distillates (jet and diesel fuels). Those with more acidity relative to their hydrogenation activities are selective for production of naphthas (boiling below about 200 OC)! Because the goal in these studies was to make jet fuel (150-285 "C), the catalysts used in both HCR stages had relatively high ratios of hydrogenation activity to acidity. Analytical Methods. The analytical approach used in this study involved an extensive use of high-performance liquid chromatography (HPLC) with W/visphotodiodemay detection, followed by detailed characterization of the HPLC fractions in terms of homologous series of compounds and their carbon number distributions by using field ionization mass spectrometry (FIMS). A more detailed discussion of this analytical approach can be found el~ewhere.~ In the past, the characterization of complex hydrocarbon mixtures in terms of homologous series ("2series") using mass spectrometry (MS) has been limited due to the fact that the Z value in the general formula CnHZn+Z is affected by both the number of rings and the number of double bonds. The 2 value can be calculated as follows: 2 = 2[1- ( R DB)]
+
where R = the number of rings and DB = the number of double bonds. Thus,compounds having the same s u m of rings and double bonds could not be distinguished when routine Ms methods were used. This is illustrated by the examples in Chart I. The examples in Chart I show that a physical separation of compounds having the same formula, but different structures, is necessary prior to MS analysis. This problem is particularly pronounced when one is dealing with hydroprocessed streams, (6)Sullivan, R.F.; Meyer, J. A. Catalyst Effecta on Yields and Product Properties in Hydrociacking. In Hydrocracking and Hydrotreating; Ward, J. W., Qadar, S. A., Eds.; ACS Symposium Series 20 American Chemical Society: Washington, DC, 1975;pp 28-51,
Molecular Transformations i n Hydroprocessing Chart I' CH,,,,
W
alkylnaphthalenes
akyltetracycldxnzenes CnHzfez
00
H2)x
akylpyrenes O x
@CH~1.
alkylpentacyclcphenanthrenes
= 0, 1, 2, 3, etc.
which, as it will be shown later, contain a variety of polycyclic aliphatic, hydroaromatic, and aromatic hydrocarbons. The analytical approach used in this study overcomes this problem by use of HPLC methods capable of producing welldefined fractions that then can be analyzed by FIMS in terms of both "2series" (2in CnHzn+2)and their carbon number distributions. The detailed description of the HPLC methods used in this work is beyond the scope of this paper. Instead, a brief outline of the separation and characterization steps is given below. The first step involved a quantitative HPLC-UV survey using an analytical ZORBAX-NH2 column (4.6 mm i.d. X 25 cm; 5-pm particle size) with heptane (isocratic) followed by heptanemethylene chloride (gradient) as a mobile phase a t a flow rate of 2.5 mL/min. The quantitation of results (acquired by a Hewlett-Packard Model 1040M photodiode-array detector, a Nelson Analytical 760 Series interface connected to a HewlettPackard 9920 computer system, and a Hewlett-Packard 7550A plotter) was performed by using calibration data and computer programs developed at Chevron Research. This step, HPLC-UV survey, was used to identify aromatic ring types and to determine concentrations of various aromatic ring-number fractions in hydroprocessed streams. The second step involved preparative HPLC separation on a dual ZORBAX-NH2/ZORBAX-SIL column (each 21.2 mm i.d. X 25 cm; 8-pm particle size) with heptane (isocratic) followed by heptane-methylene chloride (gradient) as a mobile phase a t 20 mL/min flow rate, using a switching column technique. This separation step produced the following fractions: (1)saturates, (2) monoaromatics, (3) diaromatics, (4) triaromatics, (5) tetraaromatics, and (6) pentaaromatics and greater aromatics. The cut points between the aromatic ring-number fractions were determined by using a photodiode-array detector output. Those fractions were collected, solvents were removed, and the recovered fractions were analyzed by FIMS. The FIMS analyses were performed a t SRI International using previously described procedures.' The computer processing and interpretation of the FIMS data were done a t Chevron Research. The HPLC-UV method, involving the high efficiency ZORBAX-NH2 column and UV/vis photodiode-array detector, is of particular importance as it provides a separation according to the number of double bonds in the aromatic ring system with relatively little effect on the separation due to alkyl substituents (alkyl chains and/or naphthenic rings). Furthermore, the photodiode-array detector allows for identification of aromatic "ring cores" by collecting full wavelength spectra. This greatly facilitates interpretation of the FIMS spectra of the various aromatic ringnumber fractions. In related studies, deposits found in commercial hydrocrackers were extracted with methylene chloride in a Soxhlet apparatus, and the extracts were analyzed for the large PAHs (seven rings and larger) by using nonaqueous reversed-phase HPLC with a photodiode-array detector. The column packing was a polymeric octadecyl bonded phase (Vydac 201TP5),which has been shown (7) Buttrill, S. E., Jr. Final Technical Report, SRI Project PYU 8903, 1981;SRI International, Menlo Park, CA.
Energy & Fuels, Vol. 3, No. 5, 1989 605 Table I. Comparison of RDS-VGO Feed Properties first stage second stage inspections SOR EOR SOR EOR 25.7 26.5 34.5 33.2 gravity, OAPI 0.859 0.900 0.896 specific gravity 0.852 (16 O C f 16 "C) 102 87 89 aniline point, O C 104 1.2 1310 1240 1.2 sulfur, ppm 600 0.46 750 0.25 nitrogen, ppm hydrogen, w t % 12.83 12.77 13.97 13.83 63 78
