6104
Ind. Eng. Chem. Res. 2006, 45, 6104-6110
Aging of Carbonaceous Deposits from Heavy Hydrocarbon Vapors Zhiming Fan and A. Paul Watkinson* Department of Chemical & Biological Engineering, The UniVersity of British Columbia, 2360 East Mall, VancouVer, Canada V6T 1Z3
An aging study of fresh deposits from a laboratory-scale bitumen coker was conducted at the temperature of 550 °C to examine the evolution of deposit composition and structure over time and to rationalize the differences between the laboratory deposits and industrial samples. Although the fresh laboratory deposits are much different from the industrial deposits, after days to weeks of aging at elevated temperature, the H/C ratio, thermal behavior, 13C solid-state nuclear magnetic resonance (13C NMR), and diffuse-reflectance infrared Fourier (DRIFT) spectra of the laboratory deposits become very similar to those of the graphitic industrial deposits. The differences in morphology which remain after aging are attributed to the difference in hydrodynamic conditions during the deposit laydown. The various techniques studied yielded a consistent picture of the evolution from the heavy fluid phase components which initially deposit from the vapor to the massive graphitic deposit found in the industrial coker cyclone exit tube. 1. Introduction An investigation undertaken to examine the causes of fluid coker cyclone exit line fouling included studies of the effects of process variables on deposition in test sections downstream of a bench scale coking unit.1,2 For these laboratory experiments, the main cause of the fouling was reported to be physical condensation of heavy hydrocarbon vapors, which has also been implicated in transfer line fouling in naphtha crackers for olefins production. Characterization of the laboratory deposits has shown major differences from those of an industrial coker cyclone exit line.3 The laboratory deposits were collected after 6 h running in the bench-scale fluid coker unit, whereas the industrial samples had been aging in place in the presence of hydrocarbons at temperatures around 500 °C for an undetermined time period of many months. To interpret behavior in industrial units, and to relate short-term laboratory results to long-term industrial operation, it is important to understand the aging process as fresh deposits undergo reaction over long periods at elevated temperature. The overall aims of this work are to examine the evolution of deposit composition and structure over time and to rationalize the differences between the laboratory deposits and industrial samples. 2. Experimental Methods 2.1. Materials. Deposit samples were collected from the long test section of the bench-scale fluid coker cyclone fouling unit after running 6 h.2,3 The four samples (B19, B20, B30, and B40) were collected under cooling operating conditions.3 Insufficient quantities of deposits for further aging experiments could be collected under heating conditions. Samples B19 and B20 are deposits from the same bitumen feed ATB-A, and samples B30 and B40 are from another bitumen feed ATB-B,2 in which the ATB-A has much more heavy fractions, more carbon residue, and lower H/C atomic ratio than that of feed ATB-B. 2.2. Aging of Fresh Deposits. Weighed samples are placed in a glass tube liner (10 mm o.d.) and then inserted into a stainless steel tube (7.8 cm long, 12.7 mm o.d.) with Swagelok caps at the two ends. The tube was purged with nitrogen under * To whom correspondence should be addressed. Tel: 1-6048222741. Fax: 1-604-822-6003. E-mail:
[email protected].
a low flow rate for about 2 min. Ceramic wool was used to plug the open end of the glass tube, to reduce nitrogen escape while the top-end-cap was tightened to seal the samples in the nitrogen atmosphere. A number of sealed tube reactors were placed together in a muffle oven at 550 °C, individual reactors were removed and cooled, and the samples were weighed after different aging times of the order of days. 2.3. Characterization of Deposit Samples. Elemental analysis, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy, 13C solid-state nuclear magnetic resonance (13C NMR), and solvent extraction were used to characterize the composition and structure of the fresh and aged deposits. The different instrumental conditions are the same with those in ref 3. 3. Results and Discussion 3.1. H/C Ratio Changes and Weight Loss during Aging. The elemental composition and H/C atomic ratio of fresh and aged deposits for different aging times at 550 °C are given in Table 1. The fresh deposits which were formed under cooling operation conditions (vapor temperature, 448-520 °C; tube wall, 425-500 °C) have similar elemental compositions. All four fresh deposits are similar in composition in terms of the contents of carbon, hydrogen, sulfur, and nitrogen. The H/C atomic ratio is about 0.54-0.59. Industrial deposits have much lower H/C atomic ratio (0.16-0.21) compared with fresh laboratory deposits. The H/C atomic ratio of the fresh samples decreases dramatically from 0.54-0.59 during the first 2 days, especially in the first day. After samples were aging 2 days, the H/C ratios drop to 0.2-0.3 and then do not change much. Scatter is expected at low H/C ratios because of analytical difficulties at low hydrogen values. The drop of H/C ratio with the aging time can be approximately expressed by a first-order exponential decay equation, where t is the aging time (days):
(
H/C ) 0.26 + 0.31 exp -
t 0.18
)
(1)
For deposits aged over 2 days, the H/C atomic ratios approach those of the industrial samples. Figure 1 gives the total weight loss versus aging time for deposits B30 and B40. After aging over 5 h, the weight loss is
10.1021/ie060526d CCC: $33.50 © 2006 American Chemical Society Published on Web 08/05/2006
Ind. Eng. Chem. Res., Vol. 45, No. 18, 2006 6105 Table 1. Elemental Composition and H/C Ratio of Fresh, Aged, and Industrial Depositsa deposits
C (wt %)
H (wt %)
fresh aged 12 hs aged 1 day aged 2 days aged 8 days fresh aged 1 day aged 10 days aged 18 days aged 23 days
85.42 82.85 86.44 82.92 86.33 85.54 87.82 84.83 83.68 81.35
4.24 2.41 2.35 1.78 1.84 4.05 2.48 1.96 1.38 1.73
S (wt %)
N (wt %)
H/C (atomic)
Sample B19 nd nd nd nd nd 6.44 4.62 3.66 4.20 4.21
1.69 1.76 1.73 1.81 1.61 nd nd nd nd
0.59 0.35 0.33 0.26 0.26 0.57 0.34 0.28 0.20 0.26
6.27 nd nd nd 1.95 nd nd
nd 1.9 1.83 1.70 nd 1.74 1.80
0.58 0.33 0.25 0.27 0.25 0.21 0.30
nd nd nd nd nd nd
nd nd nd nd nd nd
0.54 0.26 0.28 0.17 0.21 0.16
Sample B30 fresh aged 5 hs aged 12 hs aged 1 day aged 3 days aged 13 days aged 20 days
84.92 79.24 87.53 81.24 86.92 85.22 75.50
4.10 2.21 1.80 1.84 1.84 1.74 1.80
fresh aged 2 days aged 11 days aged 24 days sample B sample D
84.89 86.70 89.28 85.51 88.75 87.85
3.80 1.86 2.08 1.18 1.57 1.18
Sample B40
a
nd means that the item was not determined.
Figure 2. Weight loss of fresh deposits with aging time in the TGA. Figure 1. Deposit weight loss with the aging time in tube reactor.
about 10 wt % and does not change over about 24 days. The weight loss occurs because of the release of volatile components either initially present in the deposits or which crack off from the side chains in the deposits. The volatile release was evident from the black color of the ceramic wool after aging. The presence of this material in the wool may also have contributed to the scatter of the weight loss data. Thermogravimetry was employed to monitor the weight loss on line during the first 12 h aging time to investigate how the release of volatiles proceeds in the initial aging periods. Samples of 5-15 mg were heated under a nitrogen flow rate of 50 mL/ min to 550 °C from ambient temperature at a heating rate of 10 °C/min and then kept at 550 °C for 12 h. The weight loss results are given in Figure 2. All the fresh deposits have similar thermal properties: the weight loss curves show a sharp drop within the first 2 h and then subsequently decrease gradually. The total weight loss percentages (12-14 wt %) are consistent with the muffle oven aging study. There is very little weight loss below 300 °C (
