Ind. Eng. Chem. Res. 2010, 49, 11221–11231
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Visbreaking Studies in the Presence of Soaker Internals Rohit P. Kulkarni,† Aniruddha B. Pandit,*,† Jyeshtharaj B. Joshi,† Kamal L. Kataria,† Deepak Tandon,‡ and Man Mohan Kumar‡ Chemical Engineering DiVision, Institute of Chemical Technology, Matunga, Mumbai 400 019, India, and Indian Institute of Petroleum (IIP), Dehradun 248 005, India
Coil-soaker visbreaking is a well-proven technology in terms of lower capital investment, lower operating cost, and low susceptibility to operational upsets. A significant improvement in the performance of this technology can be achieved by incorporating suitable internals in the soaker. In the present work, visbreaking studies using vacuum residues obtained from Indian refineries were performed in a pilot plant equipped with soaker. The soaker was retrofitted with a set of in-house-developed internals. Two different configurations of internals along with a benchmark case of soaker without internals were studied for three vacuum residue feeds. The coil outlet temperature was varied in the range of 410-440 °C, while the soaker outlet pressure was maintained constant at ∼1.2 MPa. The resulting visbroken products, namely, gas (G, C1-C4), gasoline (60-150 °C), light gas oil (LGO, 150-350 °C), vacuum gas oil (VGO, 350-500 °C), and vacuum residue (VR, lumped product consisting of compounds with normal boiling point above 500 °C [500 °C+]) were quantified and characterized in detail. The effect of internals has been studied on the conversion and the selectivity of the visbreaking reactions. The work brings out the advantages of internals in the visbreaking technology. 1. Introduction The quantity of heavy petroleum residues produced during refining is expected to increase in the near future with the progressively increasing heavier nature of the processed crudes.1-3 The residue upgradation technologies available commercially for processing these extra heavy crudes and petroleum residue are solvent deasphalting, visbreaking4,5 delayed coking,6 residue hydroprocessing, and residue fluidized catalytic cracking (RFCC), etc. It is reported that around 33% of the total petroleum residue processing capacity is met through visbreaking.4 In Indian refineries, most of the visbreaking units are operated in coilsoaker mode. The advantages being offered by this configuration over the conventional coil visbreaker are as follows: (i) lower capital investment due to smaller furnace size, (ii) cheaper material of construction due to lower operating temperature, (iii) longer plant run length, (iv) flexibility of handling various types of atmospheric, vacuum residue, and slop oil feeds, and (v) lower susceptibility to operational upsets due to lower operating temperature. However, one of the biggest demerits of this configuration is the presence of intense liquid-phase backmixing prevailing in the soaker drum. Liquid backmixing is caused mainly by buoyancy driven (density difference created by gas hold-up) liquid circulation. The backmixing is detrimental to the overall process since it results in the production of unstable fuel oil due to overcracking of the vacuum residue. Microscopically, vacuum residue is a colloidal mixture of hydrocarbons in which the asphaltenes and resins are peptized in the maltene (oily) phase.7 During visbreaking, four solubility class components are reported to undergo the following reactions: (i) saturates undergo C-C splitting reactions resulting in a reduction in their carbon number, typically from C50 to C30;8 (ii) aromatics and resins undergo dealkylation reaction; (iii) substitutive addition of resins occurs; and (iv) dealkylation and * To whom correspondence should be addressed. Phone: +9122-2414 5616. Fax: +91-22-2414 5614. E-mail: ab.pandit@ ictmumbai.edu.in. † Institute of Chemical Technology. ‡ IIP.
condensation reactions of asphaltenes occur. The relative rates of these reactions are found to be the function of visbreaking severity and feed characteristics. The first two reactions reduce the solubility (solubilization capability) parameter of the maltene phase, while the other two reactions increase the aromaticity of asphaltenes. Above a certain threshold conversion, asphaltenes start flocculating and result in unstable fuel oil (visbroken vacuum residue). In a qualitative approach to these changes, Fainberg et al.8 have correlated feed stability as a function of AS/(AR + Rs) and have shown a good correspondence. Moreover, in extreme situations, liquid-phase backmixing may significantly reduce the selectivity toward mid-distillates due to underutilized reactor volume. To minimize the liquid backmixing in the soaker drum, the following different approaches have been reported in the literature:9-11 (1) compartmentalization of the soaker with sieve plates or radial baffles; (2) incorporation of steam/inert gas spargers along the periphery of the soaker; (3) operation of the soaker in the homogeneous regime, i.e., at higher pressure, or use of antifoaming chemicals; (4) design modification of distributor and vessel heads to eliminate dead zones. The above aspects and hydrodynamics of soaker in the presence of internals are reviewed extensively by Joshi et al.5 Similarly, the studies reported by Kumar et al.12 using residence time distribution (RTD) technique for a visbreaker pilot plant concluded there are some advantages of a sectionalized soaker, such as the following: (i) reduction in backmixing (by 3-5 times); (ii) increased LPG + gasoline and gas oil yield by 10-16 and 11-19%, respectively, under an identical temperature gradient; and (iii) better fuel oil stability. However, their studies are primarily focused on confirming the suitability of radiotracer for high-temperature application, estimating the mean residence time in soaker during actual operation and the quantification of backmixing in a sectionalized soaker. The present work is a forward step and is intended to examine the crackability of different vacuum residues in the presence of in-house-developed internals. To achieve this, visbreaking experiments are carried out on a pilot plant using three vacuum
10.1021/ie901954z 2010 American Chemical Society Published on Web 09/30/2010
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Table 1. Properties of Feedstock property
BHSR
VB feed
AMSR
source density, d154 pour point,°C viscosity, cSt at 100 °C at 135 °C av molecular wt C/H ratio nC5 insolubles, wt % nC7 insolubles, wt % toluene soluble asphaltenes, wt % CCR, wt % sulfur, wt % stability metals, ppm V Ni Fe H-C analysis, wt % saturates naphthenic aromatics polar aromatics nC7 asphaltenes
BPCL, Kochi 1.0199 +72
IOCL, Mathura 1.0176 +39
HPCL, Mumbai 1.0232 +42
1034.55 141.76 1267.0 7.17 16.85 6.67 6.64
526.5 102.3 871.5 7.50 13.19 8.90 7.72
1631.45 218.97 800.2 6.59 17.91 9.59 6.50
19.81 0.84 P value 1.5
19.8 4.29 merit No. 2
21.84 4.43 merit No. 2
29.30 36.55 22.90
9.00 21.65 82.00
14.76 67.85 6.40 10.47
15 66.60 6.74 11.00
27.09 39.05 29.37 4.49
residues, namely, Bombay high short residue (BHSR), Arab mix short residue (AMSR), and visbreaker feed (VB feed) obtained from operational units of Indian refineries. The visbroken residue is fractioned into industrially important boiling cuts, identified as gas (C1-C4), gasoline (60-150 °C), light gas oil (LGO, 150-350 °C), vacuum gas oil (VGO, 350-500 °C), residue consisting of compounds with normal boiling point above 150 °C (150 °C+ residue), residue consisting of compounds with normal boiling point above 350 °C (350 °C+ residue), and residue consisting of compounds with normal boiling point above 500 °C (500 °C+ residue). These fractions are quantified and characterized in detail. The variation in the product yields as a function of the geometry of the internals, operating variables, and product properties is discussed in detail. 2. Feedstock Characterization The vacuum residues obtained from the operational units of refineries were first homogenized and dehydrated. The residue dehydration (water removal) was performed by heating the barrel (top lid open) containing residue using “barrel bottom electrical heating plate” and “electrical heating jacket”. The vacuum residue was stirred continuously post melt condition. The temperature was monitored using a temporary thermocouple, and heat input to the barrel was controlled using an automatic on-off controller. The heating was continued until the temperature ∼ 110 °C is attained. It ensured complete removal of water from the vacuum residue. The entire operation required 4-6 h, depending on the type of vacuum residue feedstock. The residues were then characterized for their different physicochemical properties according to ASTM and IP standards (Table 1). Among these vacuum residues, BHSR showed an exceptionally high pour point (+72 °C) possibly due to the presence of a higher percentage of saturates. Further, the characterization factor (KUOP) estimated for these residues indicated BHSR as paraffinic type (KUOP ) 12.39) while AMSR and VB feed as naphthenic type (KUOP ) 11.68 and 11.86, respectively). For feed and visbroken residue stability determination, a merit number test (IFP 3024-82) was found suitable for AMSR and VB feed. IFP merit number test (IFP-3024-82, severity test for thermal reactions by spot method and residue stability determination) was performed by dissolving the residue under test in a mixture of o-xylene + 2,2,4-trimethylpentane in
different proportions, and the resulting solution is spotted for ring appearance (asphaltene separation from bulk phase) on Whatman filter paper No. 2. It works well for residues with a low wax content. In the case of BHSR (a high wax content residue) the appearance of a ring at all proportions of o-xylene + 2,2,4-trimethylpentane mixture indicated the BHSR feed to be unstable. This appearance of a ring may be attributed to isolation of higher wax content instead of the expected asphaltene flocculation from the feed. When the stability of BHSR was verified using Shell P-value method (1400-2), no appearance of flocculated asphaltenes was observed until the P-value was equal to 1.5. As a result, the Shell P-value method was explicitly used for determining the stability of the BHSR feedstock and its visbroken residue. However, the method requires an expertise for investigating the onset of instability. For merit test stability, in the present work, samples with merit numbers up to 7 (o-xylene:2,2,4-trimethylpentane ) 7:3) were considered as stable samples while those with merit numbers in the range of 7-8 were considered as borderline, and above 8 the samples were said to be unstable.13 It is to be noted that a lower merit number equal to 2 for VB feed and AMSR indicates their greater stability; i.e., these feedstocks require a greater quantity of 2,2,4-trimethylpentane (paraffinic solvent) for asphaltene flocculation. The studies on the effects of crude types on visbreaker conversion reported by Kuo14 indicated that the stability factor (SF) is proportional to the feed aromatics (AR) and the resin content (Rs) and inversely proportional to the saturate (Sa) and CCR content. The same is denoted by the following relationship, SF )
AR + Rs Sa + CCR
(1)
In the present work, the lowest SF is observed in the case of BHSR (SF ) 1.46), whereas VB feed (SF ) 2.15) and AMSR (SF ) 2) showed higher SF. Thus, the higher stability of these feeds can be attributed mainly to the proportions of aromatics to saturates. The same is also evident from the higher crackability (i.e., 3.8 and 4.0 wt %) exhibited by VB feed and AMSR. The variation visbroken residue stability is discussed in a later section. Similarly, the lower stability observed in the case of BHSR feed was primarily due to a high content of saturates, i.e., 27.09 wt % (refer to Table 1) and lower aromatic to saturate ratio. Thus, during P-value determination, the surplus quantity of n-cetane that BHSR can tolerate was lower due to preexisting saturates. The above measured stability and compositional hypothesis was in congruence to the extent of permissible conversion (i.e., 2.5 wt %) exhibited by the BHSR feed. H-C analysis (SARA) of the feedstock was performed by column elution chromatography. In this method approximately 10 g of vacuum residue sample was dissolved in 300 mL of n-heptane solvent (dilution ratio, 1:30). The n-heptane insoluble asphaltenes were removed over filter paper (Whatman No. 42), and the n-heptane soluble portion was subjected to elution chromatography. The variation observed in the nC7 asphaltenes under H-C type analysis and nC7 insolubles reported in Table 1 was mainly attributed to the residue sample quantity. The sample taken for H-C type analysis was 10 g while 1 g for nC7 insoluble determination only using Soxhlet extraction method (IP 143). The other factors likely to have affected the variation in the asphaltene content were contact time and nonstandardized procedure for washing the asphaltene filter cake using solvent.15,16 However, the discussion pertaining to as-
Ind. Eng. Chem. Res., Vol. 49, No. 22, 2010
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Figure 1. Schematic representation of visbreaking pilot plant. Table 2. Geometrical Details of Coil and Soaker equipment type furnace coils
soaker
soaker internals
geometrical details total number of coils internal diameter, mm number of preheater coils total length of reaction coil, mm internal diameter, mm length TL/TL, mm soaker volume, × 10-3 m3 cold oil liquid residence time, min type 12 (0.5) average free area, % HC/DC ratio type 16 (0.5) average free area, % HC/DC ratio
6 6 3 11000 100 400 4 20 12 0.5 16 0.5
phaltene variation in visbroken residue is based on nC7 insolubles determined by IP 143 method (1 g sample). The other physicochemical characteristics were found comparable for AMSR and VB feed, except the kinematic viscosity and percentage of nC5 insolubles. All these characteristics have been summarized in Table 1. 3. Experimental Procedure The schematic representation of visbreaking pilot plant is shown in Figure 1. While the geometrical details of coils and soaker are given in Table 2. The pilot plant essentially consisted of three sections, namely, (i) feed section, (ii) reaction section, and (iii) separation section. In the feed section, the vacuum residue and gas oil charge were preheated and homogenized in two separate feed tanks. Visbreaking experiments were conducted with two different sets of internals, with axially varying free area and also for the base case of soaker without any internals. The coil outlet temperature (COT) which was the same as the soaker inlet temperature was varied in the range of
410-440 °C, while the other operational variables such as vacuum residue flow rate (3.33 × 10-6 m3 s-1), soaker outlet pressure (1.2 MPa), and water injection rate (∼1 wt % feed) were kept constant throughout the study. The gas oil was used for plant start-up and flushing purpose. Initially, the hot gas oil was charged to the first coil (C1) of the furnace section at the required flow rate using a reciprocating type metering pump. The pump head was adjusted to maintain a desired pressure in the cracking coils while the soaker outlet pressure was controlled with the help of a high-precision pneumatic valve. Approximately 1 wt % water was injected continuously in the first coil (C1) using a dosing pump. The purpose of water injection was to increase the volumetric flow rate and induce turbulence in the reaction coils to prevent coking inside the coils. The feed gas oil was then passed through the furnace section consisting of six stainless steel helical coils connected in series. The first three coils (C1, C2, and C3) were preheating coil, while the remaining coils (C4, C5, and C6) were reaction coils. Each coil was heated separately by immersing it in molten salt bath placed on the hydraulic platform. The electrical power input to each salt bath was regulated using a PID controller with coil outlet temperature as a set point. The outlet of the last reaction coil was connected to the (upflow) soaker drum. Electrical heat tracing was provided around the soaker to compensate for the heat loss to the surrounding. The gas oil leaving the soaker was then fed to the flash vessel located in product recovery section. The flash vessel facilitated the separation of lighter hydrocarbons that were further condensed in a lighters receiver. During the experimental run, the coil, soaker and salt bath temperatures were continuously monitored, and after achieving the steady state (no change in temperature profile), the gas oil feed was stopped and the vacuum residue was charged to the coil. Any variations in the operating variables due to feed switching were constantly monitored, and the plant was allowed to attain a new
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Figure 2. Typical steady-state axial temperature profile for coil and soaker.
steady state. A typical steady-state temperature profile recorded across coil and soaker are shown in Figure 2. The visbroken residue from the flash vessel bottom was collected in a tar accumulator. The tar and distillates from lighters receiver were cooled and weighed. Similarly, the cumulative volume of gases leaving the lighters receiver was measured using a gas meter. The gas samples were collected and analyzed. In each experimental run, about 99 wt % material was achieved. The routine calibration of thermocouples and pressure transducers and the standardization of the sampling procedure were ensured for reproducible results. The experimental procedure was repeated for each vacuum residue feed and a set of internals. 4. Reaction Product Analysis 4.1. Gas Analysis. The analysis of gaseous product was performed using a gas chromatograph (GC), equipped with a packed column (30% squalein, 5 m long) and flame ionization detector (FID). The analysis was done only for the hydrocarbon type constituents. However, the gases such as H2S, CO, and CO2 that are also found to be present in the visbreaker gas in trace quantities were not analyzed, and thus, the results shown should be considered as semiquantitative and indicative of trends. The molar composition for a few gas samples collected during the pilot plant runs for BHSR, AMSR, and VB feed is shown in Table 3. It is seen that methane is a major constituent present in the gas. Similar observations were reported for visbreaking of Maya, Hondo, Arabian light asphalt, and vacuum residue.17,18 Further, in the case of BHSR the mole percent of C3 components is found to be higher than C2 components, whereas a reverse trend is observed for AMSR and VB feed. This fairly agrees
with the results of batch reactor studies reported by Kataria et al.13 However, no dependency was observed between soaker internals and the gas composition over the temperature range covered in this work. 4.2. Liquid Product Separation and Analysis. The visbroken tar collected during the experiment was transferred to a distillation flask. A mild heating of tar accumulator was required for complete transfer of visbroken tar to the distillation flask. The water entrained in the liquid distillates was removed in a decanter. The liquid distillates were then added to the visbroken tar, and the resulting mixture was subjected to TBP distillation (ASTM D2892) using an Oldershaw column (15 theoretical stages) to separate 60-150 °C fraction and 150 °C+ residue. This residue was further subjected to atmospheric and (partial) vacuum distillation to separate 150-350 °C fraction and produce 350 °C+ residue. This residue was further distilled under full vacuum to separate 350-500 °C fraction and 500 °C+ residue. The separated boiling fractions, i.e., 60-150 and 150-350 °C, were also analyzed using ASTM D86 distillation. Similarly, the vacuum gas oil cut (350-500 °C) was analyzed using ASTM D1160 distillation. This step facilitated in quantifying the boiling curve overlap and the necessary redistribution in the respective fraction for final material balance. The liquid distillate products were also analyzed for specific gravity (ASTM D4052), sulfur content (ASTM D4294), and micro-carbon residue (MCR) content (ASTM D4530). The other important properties such as pour point, fluorescence indicative absorption analysis (ASTM D1319) for gasoline, bromine number (ASTM D1159), and aniline point (ASTM D611) were also determined for a few samples. However, more emphasis was given on studying the variation in the sulfur content with the extent of feed conversion. The collected residue samples, i.e., 150 °C+, 350 °C+, and 500 °C+, were checked for their stability using the methods as per stated in section 2. In addition, the important properties such as density (ASTM D4052), viscosity (ASTM D445), pour point (ASTM D97), sulfur (ASTM D4294), MCR (ASTM D4530), nC5 and nC7 insoluble asphaltenes (IP 143), and flash point (ASTM D93) were also analyzed. 5. Conversion and Yields In industrial units, cracking severity is often expressed in terms of the conversion of vacuum residue to gas and gasoline. Gas and gasoline are predominantly present in the vapor phase at visbreaker operating conditions and hence require very high activation energy for further cracking. Because visbreaking is a mild liquid-phase cracking phenomena, these lower carbon molecules appear to be more difficult to crack under the visbreaking severity range and thus they behave as a stable product.13 Hence, in the present work, the conversion to gas +
Table 3. Gas Analysis for given feed and run number BHSR 19 % FA HC/DC ratio soaker inlet temp, °C composition, mol % C1 C2′ + C2 C3′ + C3 C4′ + iC4 nC4 C5 and above total
20
23
24
400
21 12 0.5 410 420
430
50.93 15.77 20.44 1.99 6.78 4.08 100
48.56 15.35 20.14 2.04 7.55 6.36 100
47.35 16.53 19.39 2.12 7.63 6.99 100
50.17 16.53 19.75 2.07 7.00 4.25 100
AMSR 26
410
25 16 0.5 420
51.56 15.69 20.42 1.85 6.46 4.16 100
48.55 18.12 20.62 3.40 7.01 2.30 100
14
40
41 12 0.5 410 420
56
430
15 100 410 420
48.67 16.30 19.54 2.20 7.40 5.78 100
53.45 15.91 20.58 3.16 4.91 1.99 100
37.98 21.68 20.63 1.45 10.91 7.35 100
50.58 15.62 20.05 3.26 5.94 4.54 100
40.94 23.10 20.47 1.44 8.92 5.12 100
VB Feed 48
410
57 58 100 420 430
50.87 22.98 19.06 1.42 5.66 0.00 100
47.84 24.88 22.12 1.68 3.49 0.00 100
51.79 26.02 19.48 0.62 2.22 0.00 100
43
410
49 12 0.5 420
44.48 23.14 21.70 3.96 6.59 0.00 100
46.35 24.33 22.75 3.28 3.28 0.00 100
44
45
410
52 16 0.5 420 430
44.68 22.76 21.71 3.76 7.10 0.00 100
47.35 24.72 22.63 1.82 3.49 0.00 100
48.23 24.24 21.59 3.28 3.03 0.00 100
45.57 24.63 21.63 4.72 3.22 0.00 100
46 100 410 420 51.93 23.79 22.10 0.84 1.33 0.00 100
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Table 4. Effect of Soaker Inlet Temperature on Product Yields run no.
internals, % FA (HC/D)
av soaker inlet temp, °C
14 15 16 24 25 26 19 20 21 23
no internals
410 420 430 410 421 432 400 410 420 430
18 20 58 37 38 39 40 41 42
no internals
500 °C+, wt %
VGO, wt %
LGO, wt %
gasoline, wt %
gas, wt %
3.44 5.27 8.94 4.74 8.27 11.13 4.67 6.01 8.19 10.12
2.63 3.99 6.69 2.59 4.80 7.33 2.88 4.18 6.77 10.90
0.60 1.13 1.75 0.52 1.05 1.91 0.69 0.86 1.76 2.53
0.43 0.79 1.43 0.68 1.16 2.17 0.62 1.16 1.85 3.23
5.03 7.90 10.26 7.05 9.63 12.06 6.85 9.55 14.10
3.99 6.96 9.02 3.86 6.09 10.89 4.10 6.94 11.67
0.82 1.65 2.57 0.86 1.67 2.52 0.91 1.78 3.11
0.64 1.25 2.45 0.97 1.78 2.96 1.10 1.89 3.38
8.50 10.08 13.19 10.58 13.12 15.78 8.25 11.18 15.41
3.58 5.43 9.12 4.08 7.50 8.70 4.67 6.97 10.75
0.98 1.72 2.95 0.93 1.68 3.00 1.04 2.02 3.11
0.68 1.14 1.86 1.03 1.70 2.56 0.86 1.63 2.86
Feed: BHSR
16 (0.5) 12 (0.5)
92.90 88.83 81.19 91.46 84.71 77.46 91.14 87.79 81.43 73.22 Feed: AMSR
409 419 429 411 421 430 410 420 429
16 (0.5) 12 (0.5)
89.53 82.24 75.70 87.26 80.83 71.57 87.04 79.83 67.74 Feed: VB Feed
45 46 17 43 44 52 48 49 55
no internals
408 419 430 409 419 430 411 420 430
16 (0.5) 12 (0.5)
86.26 81.63 72.88 83.38 76.00 69.96 85.18 78.19 67.87
gasoline (compounds with normal boiling point less than or equal to 150 °C [150 °C-]) is considered as a basis for studying the property variation (extremity) and is defined as x150°C- )
(
)
Wgas + Wgasoline × 100 Wfeed
(2)
The other conversions are defined as x350°C- )
x500°C- )
(
(
)
Wgas + Wgasoline + WLGO × 100 Wfeed
)
(3)
Wgas + Wgasoline + WLGO + WVGO × 100 Wfeed
(4)
The weight percent yield of the ith species is defined as yi )
( )
Wi × 100 Wfeed
(5)
where i ) gas, gasoline, LGO, or VGO, etc. 6. Results and Discussion 6.1. Product Yields. The effect of soaker inlet temperature on product yields of visbroken BHSR, AMSR, and VB feed for identical sets of internals is shown in Table 4. For all types of vacuum residues, the product yields were found to increase with soaker inlet temperature. Further, for identifying the secondary cracking of visbreakets, the individual product yields were plotted against 150 °C- conversion as shown in Figure 3. A scatter observed in the VGO yields indicates a nonlinear dependency for 150 °C conversion products, whereas gas,
gasoline, and LGO yields showed a linear trend with respect to conversion. The nonlinear trend observed for VGO could be due to the secondary cracking of larger molecules present in this boiling range which require lower activation energy, thus increasing their cracking with the process severity.19,20 6.2. Effect of Internals on Conversion. The effect of percent free area of internals on conversion to 150 °C- and 350 °Cproduct for different soaker inlet temperatures is shown in Figure 4 and Figure 5, respectively. In all of the experiments, the axial temperature difference across the soaker was maintained at ∼10 °C. It was observed that, under an identical temperature gradient, the conversion to 150 °C- and 350 °C- fraction increased with reduction in the percent free area of internals. The following explanation can be given for this enhancement in the conversion. The soaker can be viewed as a gas-liquid bubble column reactor consisting of multiple circulation cells with net liquid exchange between the adjacent cells. The liquid recirculation velocity is found proportional to the superficial gas velocity (VG), which in turn depends on the extent of vacuum residue conversion to lighter hydrocarbons and gases. RTD studies12 conducted on visbreaker pilot plant using radiotracer technique showed the lowest transportation lag of tracer for the case of soaker without internals. Further, to determine the extent of backmixing (weeping) in the soaker, the time varying tracer concentration at the soaker outlet was fitted using an axial dispersion model (ADM) with dimensionless Peclet number (Pe) as an adjustable variable. The backmixing coefficient (f) is inversely proportional to Pe. For three different conversions studied Pe varied in the range of 0.98-1.31. This lower range of Pe confirmed a strong recirculatory pattern, i.e., a high degree of backmixing in the soaker as a result of vapor (gas + steam + low boiling hydrocarbon) formation. Thus, in the absence of internals the soaker behavior was close to a single continuous stirred tank reactor (CSTR), i.e., flat axial concentration and temperature
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Figure 3. Variation in product yields with 150 °C- conversion: O, BHSR; +, AMSR; ×, VB feed.
profile. Moreover, with visbreaking reactions being first order with respect to feedstock concentration,13 this flat concentration profile resulted in lower product yields for the case of soaker without internals. After incorporating 16% free area internals, Pe was increased to 3.06-3.07, i.e., flow approaching plug type. Further, 12% free area internals raised Pe to a new range of 3.28-3.80. Thus, reducing the percent free of internals improved the axial concentration and temperature gradient profile and consequently increased the theoretical number of CSTR to 4. The overall result is radial uniformity in temperature, reduction in weeping, and narrow residence time distribution and, consequently, enhancement of product yields. The effect of internals was more pronounced in the case of BHSR followed by AMSR and VB feed. The possible reason for this can be derived from kinetic studies of low-severity visbreaking reported by Kataria et al.13 These authors have proposed a five lump seven parameter parallel-series reaction model for capturing the essential mechanism of thermal cracking reactions occurring during the visbreaking. The prevalence of secondary cracking of formed VGO to lower boiling fractions probably would have shifted the apparent reaction order above unity. Second, over the studied temperature range, BHSR indicated the lowest 350 °C- conversion. This means lower gas holdup in the case of BHSR and consequently higher effective reactor volume available for liquid-phase cracking reactions. Hence, the effect of backmixing was prominent in the case of BHSR. Further, ASPEN Plus software was used for determining the volumetric flow rate of vapor in equilibrium with visbroken liquid for each conversion level over the range of temperature and pressure
covered in this work. The assay data, i.e., boiling curve, specific gravity, and viscosity, at standard condition for each collected fraction were provided as an input to the software. The RK-Soave property package was selected for determining the thermodynamic equilibrium. The superficial gas and liquid velocities estimated from the volumetric flow rate were found to be in the range of 1-2 and 150 °C+ residue. In the case of BHSR, the viscosity ratio for 150 °C+ residue decreased initially and approached a local minimum of 0.28 at 2.3 wt % conversion. The viscosity reduction in this region was primarily due to the C-C splitting reactions. Further, the viscosity ratio increased to 0.39 for corresponding conversion of 3.62 wt %, which indicates an increase in the proportion of
Figure 7. Viscosity ratio as a function of 150 °C- conversion: ], 150 °C+ residue; 0, 350 °C+ residue; 4, 500 °C+ residue.
hard to crack material in the residue and the onset of vacuum oil instability. A further decrease in the viscosity ratio above this conversion may be attributed to the increased population of low-boiling distillates (bp LGO > gasoline) was probably due the Cali-S bond cleavage, which tends to occur even at low process severity. Further conversion of vacuum residue results in prevalence of Cali-Cali splitting and dealkylation reactions over Cali-S splitting reactions, thereby reducing the proportion of sulfur in the distillates. For aromatic feeds, viz., AMSR and VB feed, the total sulfur also followed an identical trend; i.e., VGO > LGO > gasoline. However, for AMSR, no significant variation in the sulfur content of gasoline and LGO was observed with an increase in the conversion. However, a marginal increase in the sulfur content of VGO was likely, due to the cracking of sulfur moieties present in the residue as peripheral and pericondensed rings. Nevertheless, for VB feed a slight decrease in sulfur content of Ga and LGO fractions can be attributed to the prevalence of Cali-Cali splitting reactions over Cali-S bond cleavage.21 Table 5 reveals the variation in the sulfur content of visbroken residue over the studied severity range. The sulfur content of visbroken residue was found to be higher than the feed, except for the 150 °C+ residue of BHSR. The total sulfur content followed the order 500 °C+ > 350 °C+ > 150 °C+ residue. Though, the distillates have revealed the presence of sulfur compounds due to thermal desulphurization, this effect was masked for residue due to the concentration of sulfur bearing compounds in the heavier (residual) portion. 6.5.2. Asphaltenes and MCR Content. The combustion products of fuel oil are a significant source of air pollution. One of the main reasons for incomplete combustion of fuel oil is asphaltenes formed during thermal cracking of vacuum residue. The vacuum residues employed in this study can be categorized as low sulfur (S < 1 wt %) and high sulfur (S > 2 wt %) vacuum residues.8 For example, BHSR is a low-sulfur vacuum residue,
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Figure 8. Variation in the total sulfur content of distillates: ], gasoline; 0, LGO; 4, VGO. Table 5. Visbroken Residue Total Sulfur Content total sulfur, wt % +
feed used
feed
500 °C residue
350 °C+ residue
150 °C+ residue
BHSR AMSR VB feed
0.84 4.43 4.29
0.87-0.94 5.10-5.83 4.90-5.42
0.83-0.89 5.00-5.43 4.72-5.10
0.79-0.86 4.95-5.26 4.60-4.87
whereas AMSR and VB feed are high-sulfur vacuum residues. The variation in asphaltene content of 150 °C+ residue as a function of 150 °C- conversion is shown in Figure 9. The asphaltene content was found to increase with an increase in 150 °C- conversion. This can be attributed to (i) dealkylation of aromatic rings and dehydrogenation of hydroaromatics through hydrogen-transfer reactions permitting the aggregation of these molecules into bigger molecules and (ii) cracking of
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Figure 10. Variation in MCR content of 150 °C+ residue with conversion: O, BHSR; +, AMSR; ×, VB feed.
studied, AMSR and VB feed showed linear variation in MCR content with conversion. However, in the case of BHSR a scatter in MCR content was observed. The possible reason cannot be given as of now. 6.5.3. Pour Point. Visbreaking has an additional advantage of lowering the pour point of the cracked residue, thereby reducing the quantities of cutterstock required to meet the necessary fuel oil specifications. For stable 150 °C+ visbroken residue the reduction in pour point by 24, 21, and 27 °C (from +72 to +48 °C, from +42 to +21 °C, and from +39 to +12 °C) can be achieved for BHSR, AMSR, and VB feed, respectively. The significant pour point reduction (by 24 °C) observed in the case of BHSR was possibly due to the higher initial percentage of saturates which undergo C-C splitting reactions. For 350 °C+ residue no appreciable variation in the pour point was observed. The pour point of 500 °C+ residue was found to be higher than the feed (BHSR, from +69 to +87 °C; AMSR, from +48 to +78 °C; VB feed, from +42 to +72 °C), possibly due to the removal of distillates resulting in the concentration of refractory and heaver molecules. 7. Conclusions
Figure 9. Variation in asphaltene content of 150 °C+ residue with conversion: 0, n-heptane insoluble asphaltenes; 4, n-pentane insoluble asphaltenes.
resins forming polynuclear aromatics which condense to form asphaltenic compounds and/or dealkylation of resins, decreasing their solubility in pentane and heptane.15 For BHSR, nC5 and nC7 insoluble asphaltenes increased steeply with slopes (indicative/rate of increase) equal to 1.95 and 1.94, respectively. For AMSR the corresponding slopes were 0.9 and 0.82, respectively. Consequently, for VB feed the slopes were 0.98 and 0.69. The observed variation for these vacuum residues was in close agreement with those reported by Fainberg et al.;8 i.e., feedstock with low sulfur content showed a sharp increase in the asphaltenes while high sulfur feeds showed relatively lower rise in the asphaltene content. Additionally, concentration of polynuclear aromatics in the visbroken residues (as a result of distillation) could have contributed to the percentage increase in asphaltenes for these feeds. Further, the variation in MCR content of 150 °C+ residue with conversion is shown in Figure 10. For the range of severity
(1) Gas, gasoline, and LGO product yields were found to increase linearly with the cracking severity, whereas VGO product was found susceptible to secondary cracking reactions at higher severity levels. (2) Under identical temperature conditions, reducing the percent free area of the internals increased the product yields. The effect of internals was more pronounced in the case of BHSR followed by AMSR and VB feed. (3) Both the internal sets, i.e., 12 (0.5) and 16 (0.5), were found suitable for sectionalization of the existing or newly designed soaker. (4) Among the studied feeds BHSR showed the least crackability. The limiting conversion to obtain stable visbroken residue for BHSR, AMSR, and VB feed were found to be 2.5, 4, and 3.8 wt %, respectively. (5) The viscosity reduction was found to be higher in the case of AMSR, while BHSR and VB feed showed comparable viscosity ratios. Acknowledgment The authors are thankful to the Centre for High Technology (CHT), New Delhi, India for providing the financial help and also to Indian Institute of Petroleum, Dehradun, India, for
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providing the experimental and analytical facilities. The authors R.P.K. and K.L.K. are grateful to the CHT for providing the research fellowship. Nomenclature % FA ) percent free area AR ) aromatics (wt %) AS ) asphaltenes (wt %) CCR ) conradson carbon residue (wt %) DC ) soaker diameter (m) f ) backmixing coefficient HC ) length of section (m) LGO ) light gas oil (wt %) nC5 ) n-pentane insolubles (wt %) nC7) n-heptane insolubles (wt %) Pe ) peclet number Rs ) resins (wt%) Sa ) saturates (wt%) SF ) stability factor defined in eq 1 VG ) superficial gas velocity (m s-1) VGO ) vacuum gas oil (wt %) Wi ) weight of ith species (kg) x150°C- ) gas + gasoline conversion (wt %) x350°C- ) gas + gasoline + LGO conversion (wt %) x500°C- ) gas + gasoline + LGO + VGO conversion (wt %) yi ) yield of ith species (wt %) Subscript i ) species gas, gasoline, LGO, VGO
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ReceiVed for reView December 8, 2009 ReVised manuscript receiVed April 15, 2010 Accepted August 25, 2010 IE901954Z
