Formation and Characterization of Deposits in Cyclone Dipleg of a

Publication Date (Web): October 9, 2012 ... Deposits in the cyclone dipleg of a commercial residue fluid catalytic cracking (RFCC) reactor, which are ...
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Formation and Characterization of Deposits in Cyclone Dipleg of a Commercial Residue Fluid Catalytic Cracking Reactor Sung Won Kim,* Ju Wook Lee, Jae Suk Koh, Gyung Rok Kim, Sun Choi, and Ik Sang Yoo Global Technology, SK innovation, 325 Exporo, Yuseong-gu, Daejeon 305-712, Republic of Korea ABSTRACT: Deposits in the cyclone dipleg of a commercial residue fluid catalytic cracking (RFCC) reactor, which are one of the primary problems causing abnormal shut-down of RFCC, were characterized using an analytical approach in order to understand the formation mechanism of the deposit. The main components of the deposits are hydrocarbons and a high portion of inorganic matter, reflecting dipleg conditions with particle flow. The deposit consists of two parts of massive bulk matter which are mainly pure Sb metal and catalyst particles surrounded by filamentous cokes and lumps of carbonaceous matters. Nickel nanopowders on the catalyst surface and from heavy oil in vapor phase catalyze the filament coke, which accelerate the increase in the amount of deposits by filtering heavy oil droplets. The Sb metals, originated from injected Ni passivator in the riser, contribute largely to deposit formation, and this is validated by a simulation of RFCC conditions and a calculation of cyclone collection efficiency. Possible mechanisms for the deposit formation and methods for the reduction of the deposits are proposed.

1. INTRODUCTION Fluid catalytic cracking (FCC) units are used extensively in modern refineries to split longer-chained hydrocarbons into more valuable shorter-chained molecules. Over the past ten years, the residue fluid catalytic cracking (RFCC) process has rapidly evolved as a major means of upgrading heavier oil, because the quality of crude oil forces refineries to process more and more residue.1 The rise of heavy fractions in the feed leads to increase in asphaltene and contaminant metals like vanadium and nickel. The heavy fraction and contaminated metals produce unwanted build-up of carbonaceous deposits as well as decrease conversion from catalyst deactivation. Minimizing the effects of heavy fraction and metals on FCC unit operation has been a major challenge in achieving the economical benefits of better conversion and longer run length in refineries.2,3 Although the use of metal passivators to reduce the undesired effects of nickel and vanadium has become an established practice,4 unwanted buildup of carbonaceous deposits on equipment is becoming a general problem in RFCC units.5−7 Primary problems that affect the run duration occur in the reactor.3,5 A common cause of primary problems is the formation of carbonaceous deposits in the cyclone dipleg, particularly in the secondary cyclone dipleg which has a smaller diameter8 and low catalyst flux. This occurrs on the inner surface of pipes near end of the dipleg. In one case, this deposit built up and plugged the catalyst flow area, causing significant catalyst carryover into the fractionators. In another instance, a lump of heavy coke deposit broke free from the dipleg, blocking off the flapper valve at the end of the dipleg or keeping the valve open, causing malfunction of the cyclone. This resulted in sudden and extreme loss of catalyst inventory, and the unit had to be shut down.3,8 Although many studies have been carried out by academics and industrial personnel to understand the coking problem, major results have been obtained based on laboratory-scale studies, of which results are far from commercial processes.6,9 Some researchers have studied the phenomena of deposit © 2012 American Chemical Society

formations based on cokes from commercial units including steam crackers for ethylene production9−11 and fluid cokers.7,12 However, studies on commercial FCC reactors are relatively sparse. Many companies with FCC units have made little effort to obtain information, as they had not yet realized the importance of this issue. Instead, they concentrated on maintenance efforts, like deposit removal and time efficiency.5 Recently, a few studies2,5,13 have been reported on the formation of deposits at the reactor wall or gas outlet tube of the cyclone in commercial FCC units, where gas flow governs the formation. Catalysts and additive particles greatly affect the formation of deposits in a cyclone dipleg, since the dipleg is used for transferring collected catalysts from the cyclone into the stripper section. Further studies based on analytical approaches are required for providing a comprehensive understanding of deposit formation in the dipleg and insight into the process and operation improvement. In this study, deposits formed in the dipleg of a commercial RFCC reactor, one of the common causes of primary problems causing abnormal shut-down of RFCCs, were characterized. Extensive deposit characterization was carried out with a range of techniques to provide an understanding of the mechanism of deposit formation and methods to improve unit operation.

2. EXPERIMENTAL SECTION 2.1. Preparation of Deposit Samples. The RFCC unit, from which deposit samples in this study were acquired, is located in the Ulsan Complex of SK innovation, Korea. The unit has an AR (atmospheric residue) processing capacity of 58 000 BPSD. The RFCC unit consists of a riser, a reactor with a stripper, and a regenerator. The riser utilizes a Suspended Catalyst Separation System (SCSS) to separate catalyst Received: Revised: Accepted: Published: 14279

July 13, 2012 September 3, 2012 October 9, 2012 October 9, 2012 dx.doi.org/10.1021/ie301864x | Ind. Eng. Chem. Res. 2012, 51, 14279−14288

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Figure 1. Typical images of deposit buildup (a) and sample (b) in the dipleg of the secondary cyclone in the RFCC reactor: 1 riser; 2 first cyclone; 3 secondary cyclone; 4 dipleg; 5 reactor.

electron microscopy (SEM) were used to characterize the composition and the morphology of the deposit samples. The SEM (S-4800, Hitachi) is equipped with an energy dispersive spectrometer (EDS: EMAX, Horiba) to obtain the elemental composition of the surface. X-ray fluorescence spectroscopy (XRF: Axios-Petro, PANalytical) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES: Integra, GBC) were used for quantitative analysis of the metal contents of the deposits. Transmission electron microscopy (TEM: Tecnai, FEI) and high-resolution TEM (ARM, JEOL) with energy dispersive spectroscopy were used to obtain information about the nanostructure and elemental and chemical composition of the deposits with nanometer resolution.

particles from cracked products: a vented riser with six sets of two stage cyclones directly connected to the riser as shown in Figure 1. The SCSS is in the reactor, which has a pressure of 2.48 kgf/cm2 and temperature of 527 °C. These directly connected cyclone systems are widely used because of their yield benefits. The dipleg sealing of the first cyclone minimizes the amount of hydrocarbon that enters the stripper and is therefore exposed to long residence time, resulting in overcracking to coke and light gases. The barrel of the first cyclone has a diameter of 1.42 m ID, and the total length of the cyclone is 6.43 m. The barrel of the second cyclone has a diameter of 1.27 m ID, and the total length of the cyclone is 5.74 m. The diameter and the length of the dipleg in the second cyclone are 0.28 m OD and 6.53 m, respectively. The wall of the dipleg is covered with refractory material from the dipleg entrance to 1.5 m above a flapper valve at the end of the dipleg. The deposits were found in all six diplegs of secondary cyclones in inspection of the cyclones after shutdown of the RFCC. The deposits were formed in the entire dipleg but were severe at about 2 m above the end of the dipleg in the secondary cyclone, which corresponds to the point in the moving bed of catalysts or near the boundary between streaming flow from the inlet and moving bed flow to the flapper valve. The deposit samples used in this paper were carefully collected during routine maintenance. The sampling location was where buildup of deposits was severe as shown in Figure 1a, because plugging at the specific location results in big catalysts carryover and following shutdown of the unit. A sample obtained after operation for 2.57 y is prepared in order to characterize the deposit. Additionally, deposit samples after operation for 0.26 y were obtained at the same location in order to compare aging effects. 2.2. Analyses of Deposit Samples. Elemental analysis by an elemental analyzer (EA1110, CE instruments) and scanning

3. RESULTS 3.1. Elemental Analysis of Deposits from Commercial RFCC. Table 1 shows the elemental compositions and H/C Table 1. Elemental Compositions of Deposits in the Cyclone Dipleg of an RFCC Reactor

deposit I deposit II

C [wt %]

H [wt %]

N [wt %]

S [wt %]

inorganic compound [wt %]

H/C atomic ratio [−]

22.1 25.3

0.52 0.46

0.09 0.12

0.85 0.76

76.4 73.4

0.28 0.22

atomic ratios of deposits with different aging times (deposit I: 0.26 y; deposit II: 2.57 y). The two samples were acquired at the same location of the cyclone dipleg after different shutdowns of the same unit to understand the evolution of the deposit composition and structure over time. The content of each component shows different values in the samples. Carbon and hydrogen are the major components, and the 14280

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Table 2. Inorganic Elemental Composition of Deposits in the Cyclone Dipleg and RFCC Catalysts (Inorganic 100% Basis) deposit I deposit II catalyst I catalyst II

SiO2

Al2O3

Fe

Ni

V

Sb

Ti

La

Na

MgO

23.98 23.23 51.71 41.47

16.39 18.19 40.56 56.65

1.46 1.38 0.61 0.40

0.15 0.13

0.08 0.08

54.21 53.28

0.21 0.24 1.52 1.01

0.46 0.50 2.34 0.30

0.11 0.15 0.14 0.14

2.96 2.81 3.12 0.02

Figure 2. (a) SEM of typical deposit. (b) Magnified SEM of particle with filament cokes. (c) Magnified SEM of particle with lump of carbonaceous matter. (d) Magnified SEM of bulk matter in the deposit.

The compositions of the deposits are compared with two FCC catalysts with different Al/Si ratios used during the unit run to understand where each component comes from. A distinctive difference between deposits and FCC catalysts is that the deposits contain considerable amounts of different metals like Ni, V, Fe, and, particularly, Sb. Ni, V, and some of the Fe come from the oil droplets in the vapor phase, which were carried over from the feedstock.14 Antimony seems to come from the nickel passivator which is used to reduce the dehydrogenation activity of nickel on the catalyst. Other metals come from the catalysts, of which Al2O3 and SiO2 are the main components. Nickel and vanadium are present in the atmospheric residue (AR) feed at a ratio of about 1:1.5, which is compared with 1.6−1.9:1 in Table 2. Iron content in the deposits is high, possibly due to the feedstock and catalyst component. In comparison of different aged deposits, deposits I and II show similar inorganic elemental compositions, which indicates that evolution of inorganic matter in the deposit is not shown over time. 3.2. Macro- and Micromorphology of Deposits. The macrograph of the deposit sample on the inner wall of the cyclone dipleg is shown in Figure 1b. The deposit appears mostly dark and locally light gray. Layers of carbonaceous matter are observed in the direction of catalyst flow in the dipleg, and the outer layers are brittle. It seems that viscous oil and a particle mixture are laminated and solidified on the dipleg wall. The SEM micrographs at the surface of the aged deposit sample (deposit II) in the cyclone dipleg are shown in Figure 2. Figure 2a shows a typical micrograph of the deposit. The

carbon content of pure coke reaches more than 93 wt % on an ash-free basis. This indicates that the coke is severely carbonized and its condensation degree is very high.5 The contents of an inorganic compound are much higher than about the 30 wt.% value found for carbonaceous deposits from cyclone gas ducts reported in previous studies,5,6,12 because the deposit components are related to the exposure state of catalyst flow per space volume. The dipleg of the secondary cyclone, where captured catalysts from the cyclone are transferred to the stripper in moving or fluidized bed flow, is exposed to high catalyst fraction. The H/C atomic ratios of the samples are more than 0.22. The FCC catalyst generally produces about 7 wt % hydrogen in the coke after cracking reaction of AR feedstock, which corresponds to a H/C atomic ratio of 0.90.14 Compared to the H/C ratio of coke on the FCC catalyst, the lower H/C of the deposits also indicates that the coke fraction experienced a severe carbonization condition.2 In comparison of the differently aged deposits, deposit II, with extended aging time, features lower H/C atomic ratio than deposit I. Aging leads to severe dehydrogenation over time at high temperature. Deposit II has experienced a long dehydrogenation process in the dipleg. However, the degree of decrease in H/C ratio between deposits I and II is small compared to the H/C of coke on the catalyst after reaction. This indicates that the dehydrogenation process evolved relatively quickly in the beginning, then slowed, as reported by previous results of lab-scale tests6 and analyses of commercial deposits.5 Table 2 shows the inorganic elemental composition of deposit samples with different aging time in the cyclone dipleg. 14281

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Figure 3. Energy dispersive spectrometer analyses of particle rich region (a) and bulk matter part (b) in a deposit.

Two-dimensional elemental mapping is useful in helping to obtain the distribution of a constituent in a structure and to identify the constituent on a microscale.5 EDS was used for elemental mapping of the deposit cross section. The elemental maps of the surface of the deposit are shown in Figure 3. In the magnified image of an area of spherical particles in Figure 3a, carbon (white spots) is evenly spread over the surfaces of all the particles. Al, Si, and oxygen, the main components of the catalyst, are also distributed across the image. The distribution areas of Al, Si, and oxygen are well-matched with the shape of inorganic particles, and this indicates that the inorganic particles originated from the FCC catalyst and that the catalysts are covered with coke. However, Mg, another main component of catalyst I, is concentrated on some particles, suggesting that the particles are mixtures of catalysts I and II, and that all catalysts contribute to the formation of the deposits irrespective of catalyst type. Another feature is that Sb (antimony) regions are observed between particles as fragments and are distinctively separated from catalyst regions with Al and Si. This indicates that the Sb, as components in deposits, is not related to the FCC catalysts. In the image of the bulk matter in Figure 3b, the bulk matter region is clearly distinct from the catalyst region. The region of inorganic matter consists mainly of Sb, where a small amount of carbon is evenly distributed across the Sb layer. Interestingly, components of the catalyst are rarely observed in the layer, and Sb shows nonuniformity in distribution. This indicates again that the Sb contributes to the formation of deposits independently, and the stacking of Sb on the deposit is not continuous. 3.3. Identification of Filament Coke in Deposits. The formation of filament coke is a key feature in the organization of carbon structure in the deposit in the cyclone dipleg. The identification of filament coke is important to understand the

deposit consist of two parts of massive bulk matter and spherical particles about 20−50 μm in diameter. A clear boundary between the two parts is observed. This size distribution of the spherical particles is well-matched with that of catalyst inflow into the second cyclone from the first cyclone, with 99% capture efficiency of the catalyst with range of 10−120 μm (average size 60 μm).15 All particles are involved in the formation of deposits irrespective of size. When the particle-rich part is magnified, two types of carbonaceous particles are observed as shown in Figures 2b and c. One is particles with filamentous coke which are mainly found at the inner region of the deposit. The filaments are grown on the surface of the particles and are coiled or helical. They are almost uniform in diameter, which is about 380 nm. They are very similar to cokes formed in a steam cracker for ethylene production. The formation of filament cokes on catalysts and metal surfaces in the FCC unit have been reported in previous studies2,13 without giving full discussion. A deep analysis is required to provide an understanding of the mechanism of deposit formation. The other part is particles with lumps of carbonaceous matter (Figure 2c). The lumps are stuck to filaments or particle surfaces. The size of the lumps is less than 10 μm, which is much less than the general oil droplet size of about 60 μm from the feed nozzle in the riser.14 This indicates that the lump is not directly from the oil droplets from the feed nozzle but from heavy oil droplets with higher boiling temperature than reactor temperature after cracking reaction of the feedstock in the riser. When the bulk matter part is magnified, aggregates with smooth surface are observed, as shown in Figure 2d. The shape is irregular, and the size cannot be defined. Each aggregate seems to stick together weakly. From the morphology, the aggregates seem not to be catalysts found in FCC units. 14282

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Figure 4. Effect of aging on filament coke formation on the catalyst surface in the deposit: (a) spent FCC catalyst, (b) sample of deposit I, (c) sample of deposit II.

Figure 5. High-resolution TEM images of representative filament cokes from deposit II (a and b) and EDS of particle (c) and SAED (d) analyses of filament coke from deposit II.

formation mechanism of the deposit. The effects of aging on the filament coke formation on the catalyst surfaces are shown in representative SEM micrographs of the deposits I and II as in Figure 4. The aged morphology is compared with spent FCC catalyst as a reference condition (Figure 4a), which is catalyst immediately coked after cracking reaction and sampled in the stripper. The spent catalyst is simply covered with a coke layer, and the coked surface is relatively smooth. However, thin filament cokes are observed in the particle surface of deposit I (Figure 4b). The surface is dimpled and seems to be covered with lumps of coke, suggesting the possible contribution of physical condensation of oil drops and subsequent polymerization in aging.6 In deposit II with extended aging time, the number of the filament cokes increases, and the thickness and length increase compared to deposit I (Figure 4c). This

indicates that gaseous hydrocarbons are catalyzed by metal components in catalyst particles to form the carbon filament, which grow in radius and length by a complex catalytic and physical process like the coke formed in the ethylene cracker.2 Another feature is that the filament coke is sometimes seen to peel from the particle as in Figure 4c, since adhesion of the coke to particles is rather poor. The filament coke in steam crackers has been reported in many studies, but analysis of the inner structure has been sparse. To identify the organization of carbon structures and to verify the chemical and physical process in the mechanism of the filament coke, TEM and EDS analyses were conducted. A representative high-resolution TEM image of filament coke from deposit II is shown in Figure 5. A thin line and small particles are observed in the filament. Many small particles with 14283

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Figure 6. TEM images of carbonaceous matter on catalyst (a and b) and EDS analysis of lump in the matter (c) from deposit II.

steel surface in the FCC reactor affect the formation of catalytic coke. However, iron was rarely detected in the filament cokes in the present work, and the wall of the cyclone dipleg is covered with refractory material (alumina), indicating the formation of catalytic coke is related not to the wall material, but to the metal on the catalyst. An analysis of the TEM images with EDS is shown in Figure 6 to give direct visualization of nanomorphology and information on the local components of the catalyst surface for tracing the cause of the catalytic coke formation. Some lumps (bright parts) with size larger than 10 nm are observed in the carbon region on the surface as in Figure 6a and b. Figure 6c presenting EDS analysis of a lump (dotted circle in Figure 6a) shows that the lump is nickel combined with a small amount of Sb because other components are originated from coke and catalyst as shown in Table 2. This finding is well matched with Figure 5c, indicating the catalytic coke in the filament is directly related to the nickel powder. As described in Table 2, Ni comes from the droplets in the vapor phase, which were carried over from the feedstock. The Ni accumulates quantitatively on the catalyst particles, and it is concentrated near the particle edge.16 Under FCC conditions, nickel can exist in both an oxized (+2 valence state) and reduced (0 valence state) form. The oxidized form is easily reduced in the FCC reactor and dipleg conditions. The reduced form is more active in the promotion of dehydrogenation reaction leading to coke formation and is more mobile within a catalyst particle.17

size in the range of 3−6 nm follow the thin line. Some particles are on the line, but most are near the line. The line is broken at some region and appears again (Figure 5b). Interestingly, the broken lines head for one particle as in Figures 5a and c, suggesting that the thin line is related to the particles. It can be clearly observed that the nanostructure of particle region is well ordered with crystallinity. From the analysis of the chemical composition of particle in the filament by TEM-EDS (circle in Figure 5c), the particle was determined to be nickel combined with small amount of Sb. The nanoscale features of the thin line are clearly seen in the dark image (Figure 5d), and lattice spacing is evident, which suggests that the line is crystalline and catalytic coke. A thick amorphous coke layer surrounds the catalytic coke, indicating that the formation mechanisms of the two coke regions are different. These results from visual observation of the two coke regions become much clearer in Figure 5d, where selected area electron diffraction (SAED) analyses are presented. The SAED pattern of the catalytic coke region (square A) shows complete crystalline structure, whereas the surrounding region (square B) shows only a broad and diffuse 002 ring in the SAED pattern indicating amorphous structure. It is concluded that the catalytic coke in the filament is possibly related to the Ni particles. Most studies9−11 on cokes from steam crackers have reported that material components of the tube wall like Ni and Fe affect the formation of filament cokes. Some studies2,13 on coke from FCC reactors have speculated that iron and nickel on catalyst particles or on the 14284

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The mobility leads to agglomeration of the metal. The nickel agglomerates on catalysts, which are tied up in the dipleg, cause the formation of catalytic cokes over long periods. However, all Ni agglomerates in the filament seem not to be from the catalysts surfaces, when observed in the TEM images of filament coke from deposit I in Figure 7. In the initial filament,

Figure 7. High-resolution TEM image of filament coke from deposit I.

some Ni agglomerates are observed around the lateral surface of the filament, indicating they come directly from the condensation on the filament of droplets in the vapor phase. On the basis of the findings from Figures 5−7, the formation process of filament coke in FCC cyclone dipleg can be deduced. Coke precursors like olefins and aromatics are adsorbed at Ni agglomerates on the surfaces of catalysts and converted into coke by a surface reaction like dehydrogenation under the reactor conditions. Carbon atoms dissolve in and diffuse through the Ni powder. The catalytic surface of Ni powder is lifted at the tip of the growing column of carbon with steady state deposition of carbon over a long period. The filament column grows in the longitudinal direction (linear growth) by catalytically assisted decomposition, and in thickness by condensation of heavy oil droplets or adsorption of free radicals on the coke surface.18 However, the column of catalytic carbon stops growing when the Ni surface becomes covered and encapsulated by carbon,19 indicating blocking of the catalytic active metal cluster. However, some carbon filaments with Ni powders, which are incompletely covered by carbon, slowly grow and capture oil droplets with Ni powder over a long period. The additional Ni powder continuously grows the coke filament catalytically. The growth is accelerated over time because of the enhancement of filtering action by filament coke with increased surface.20 3.4. Identification of Piles of Antimony in Deposits. Another key feature in the organization of the deposits in the cyclone dipleg is the existence of massive bulk antimony. Until now, the identification of antimony aggregates in the cyclone dipleg of FCC reactors has not been reported to our knowledge, because it has been shown that the unconverted Ni passivator goes to the regenerator within catalyst fines or to the main column bottom.21 Thus, it is important to understand the formation mechanism of the Sb aggregates in the deposit. A typical SEM micrograph of the antimony and catalyst mixture in a deposit sample (deposit II) is shown in Figure 2a. The antimony-rich region, which consists of small aggregates, is distinguished from the catalysts-rich region, where parts of the antimony region exist in places as shown in Figure 3a. When magnifying after the mounting and ion milling of the sample (Figure 8a), small antimony grains with size of ca. 10 μm exist separately and seem not to be combined with catalyst particles.

Figure 8. Typical SEM (a), TEM (b), and SAED (c) images of antimony and catalysts mixture of deposit II.

The organization of antimony grains becomes clear in visual observation of the grain by the TEM image (Figure 8b) and structure analysis (Figure 8c) with SAED of the square part of the antimony region in Figure 8b. Antimony grains (bright image) are connected by a carbonaceous layer (dark image) without any contacts between grains, indicating the antimony grains are entrapped in the coke. The composition of the antimony grain is pure Sb. These results of Figure 8 are very interesting considering that the antimony compound injected in AR feeds as Ni passivator in FCC risers. The Ni passivator blended with AR is a colloidal dispersion of antimony pentoxide (Sb2O5) in water. The Sb2O5 has size in the range of 20−50 nm (average size: 32 nm), which is measured by a particle size analyzer (90Plus, Brookhaven Instruments). The injected Sb2O5 combines with NiO on catalysts in the riser and is converted to Ni−Sb alloy like NiSb2O6.22 However uncombined Sb2O5 is reduced into Sb2O3 and metal Sb under reactor conditions with hydrogen above 460 °C.23,24 Sb2O5 has lower melting point (380 °C) than reactor 14285

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Figure 9. (a) SEM with EDS analysis from simulated test about Sb grain formation. (b) Effect of particle size on cyclone collection efficiency of single particle.

Figure 10. Schematic diagram about mechanism of deposit formation.

dispersive spectrometer) analysis. The Sb has the shape of grains, similar to Figures 8b and c, with size larger than a few micrometers, suggesting uncombined Sb2O5 with Ni is possibly agglomerated into grains and converted to pure Sb metal under FCC conditions. Even though Sb grains are formed, the size is too small to be captured in the cyclone because the second cyclone is designed to capture more than 90% of catalysts in the size range of 20−50 μm. To understand the pile formation of Sb grains, collection efficiencies (η) of single particle for FCC catalyst and Sb grain are calculated by eq 1.25

temperature and easily agglomerates, but reduced Sb has higher melting point (630 °C). This indicates that enlarged agglomerates become solid grains which possibly settle down in stagnant zones. To prove the formation of piles of Sb grains in the deposit, a simulated experiment with Ni passivator and a calculation of collection in cyclone of Sb grains were carried out. A colloidal liquid of Ni passivator (Baker Industrial Chemicals), which is used in FCC reactors and consists of Sb2O5, was heated at 520 °C in nitrogen flow to prevent oxidation to simulate FCC reactor conditions. The heat-treated Ni passivator is converted into pure Sb metal as shown in Figure 9a with EDS (energy 14286

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covered with heavy oil have been easily stuck on the film due to slow catalyst movement and high viscosity of the oil.2 The liquid film is gradually carbonized into coke by condensing and polymerization with dehydrogenation, which easily converts multiaromatics into poly nuclear aromatics as a coke precursor.11 The coke precursor seems to have a role of bridging between the wall and particles, and between particles.27 The series of reactions seems to slowly proceed after the initial rapid devolatilization and formation of a polyaromatic nucleus in the long aging history.5 The deposits with the mechanism have two features. A feature with the differences from the deposits at the gas outlet tube of the cyclone in the commercial FCC unit is that the Ni powders on the catalyst surface and from heavy oil in vapor phase were involved in the formation of deposits. The Ni powders directly catalyze light hydrocarbon product and form the filament coke. The formation of filament cokes on catalyst surfaces contributed to the acceleration of increase in the amount of deposits with increase of time because of the enhancement of oil droplet filtering action with increased surface.20 The bushy filament cokes seem to be cross-linked and produce a network of catalysts, in addition to mechanical bridges by the asphaltene fraction of heavy oil.5,27 The other feature of the deposit is that pure Sb grains contribute to the formation of deposits. The Ni passivator or antimony colloidal solution is added in proportion to the amount of nickel present in the feed. The solution’s laydown efficiency on the catalyst is in the range of 75−85%,21 indicating about 20% would not meet with catalysts. While the injection rate of the antimony solution is constant on a daily scale, the nickel content in the feed and operating condition including catalyst flow in the riser change every moment in commercial units. Also, mixing of aqueous antimony solution with AR feed is incomplete, indicating the solution can be injected into the riser in droplet form. The above reasons lead to a momentary increase of uncombined Sb2O5, and local stacking of Sb on the deposit in the dipleg. The contribution of Sb metal to the deposit formation was as high as about 11% in volume considering density from Table 2. Recently, Kim et al.5 proposed methods to minimize the deposit formation on the gas outlet tube of reactor cyclones in RFCC. The methods include the decrease of heavy fraction in the feed stock, keeping the API (American Petroleum Institute) gravity of the feedstock high, improvement of the physical properties of catalysts, and design changes of the feed atomizer and the cyclone. The proposed methods are believed to alleviate the deposit formation in the dipleg as well as the gas outlet tube of the cyclone. On the basis of the proposed mechanism and available findings considering dipleg conditions, the following further modifications would likely minimize the deposit formation in dipleg formation of reactor cyclones. First, decreasing Ni metal content in the feedstock can lead to reduced coke formation in the deposit. Decrease of Ni content indicates alleviating the formation of filament coke in the initial coking step and mitigating the increasing of liquid viscosity by dehydrogenation.9 Also, the dosage of Ni passivator can be reduced by decreasing Ni content in the feedstock. Second, the management of Ni passivator can reduce coking problems. Control of the injection rate considering Ni content, choice of passivator with smaller antimony powder considering lay-down efficiency, and proper mixing of aqueous antimony solution with the AR feed are believed to relieve the rapid growth of deposits. Finally, design and operation improvements of the dipleg are helpful to overcome coking in the deposit.

(1)

where ρp is particle density, Q is the volumetric gas rate (4.36 m3/s) at the cyclone inlet, d is particle diameter, n is the number of turns, μ is the gas viscosity (0.019 cP), W is the width of the entrance (0.37 m), r2 is the radius of the cyclone barrel (0.64 m), and r1 is the radius of the gas duct (0.28 m), respectively. The collection efficiency of FCC catalyst decreases with decrease of particle size as in Figure 9b. The collection efficiency is less than 50% under 20 μm, indicating most catalysts with size less than 20 μm are carried over to the main column out of the cyclone exit. This result is well matched with that of spherical particles in the deposit with diameters larger than 20 μm. However, Sb particles have 4 times higher efficiency than FCC catalyst and 50% of collection efficiency at 12 μm, because the density of metal Sb (ρp = 6697 kg/m3) is higher than that of the FCC catalyst (ρp = 1969 kg/m3). The high density of Sb makes the efficiency high, and the smaller Sb grains are captured in the cyclone. The captured Sb grains travel into dipleg, and part of them pile up on the deposit. From the result, it is conjectured that transition from Sb2O5 to Sb occurs in the FCC riser because Sb2O5 has low melting point (380 °C) and low density (3780 kg/m3), indicating Sb2O5 should be liquid in the riser and possibly goes out to main column.

4. DISCUSSION There is a large body of literature on the mechanisms of coke formation and deposition in steam crackers.2,10,11 These mechanisms are based on gas flow conditions of light gaseous hydrocarbons. Recently, a few studies2,5 have proposed the formation mechanism of deposits at the reactor wall or gas outlet tube of cyclones in commercial FCC units, where heavy hydrocarbon gas flow is closely related to deposit formation. However, the conditions in the cyclone dipleg with massive catalyst flow are totally different from the conditions with high volume fraction of gas. The overall picture becomes clear based on available results, literature, and findings from various analyses. A schematic diagram about the deposit formation mechanism including accumulation manner in Figure 10. Unconverted heavy oil droplets of feedstock and cracked heavy product are formed after reactions on FCC catalysts in the riser reactor. The heavy oil with higher boiling points than reactor temperature covers coke on the surface of catalysts or travels independently as droplets. The catalysts, which are not captured in the first cyclone because of small size, are captured in the second cyclone and go down to the flapper valve at the end of the dipleg. The counter-weighted flapper valve repeats open and closing operations in respond to the injected weight of catalysts from the cyclone.14 The catalysts in the dipleg of the second cyclone easily lose fluidity or defluidized because the catalysts have intergranular cohesive characteristics of Geldart C particles26 from removal of large particles in the first cyclone, and their flow becomes jerky and moving bed from defluidization in long residence time due to lower catalyst flux than in the first cyclone dipleg. Meanwhile, heavy nonvolatile oil droplets are exposed to the dipleg inner surface. The oil droplets easily collide with the surface, and they are physically condensed on the surface. The condensation of oil droplets could form a liquid film on the surface, and catalysts 14287

dx.doi.org/10.1021/ie301864x | Ind. Eng. Chem. Res. 2012, 51, 14279−14288

Industrial & Engineering Chemistry Research

Article

(9) Cai, H.; Krzywicki, A.; Oballa, M. C. Coke formation in steam crackers for ethylene production. Chem. Eng. Process. 2002, 41, 199− 214. (10) Albright, L. F.; Marek, J. C. Coke formation during pyrolysis: Roles of residence time, reactor geometry, and time of operation. Ind. Eng. Chem. Res. 1988, 27, 743−751. (11) Albright, L. F.; Marek, J. C. Mechanistic model for formation of coke in pyrolysis units producing ethylene. Ind. Eng. Chem. Res. 1988, 27, 755−759. (12) Fan, Z.; Watkinson, A. P. Formation and characteristics of carbonaceous deposits from heavy hydrocarbon coking vapors. Ind. Eng. Chem. Res. 2006, 45, 6428−6435. (13) Gao, J.; Xu, C.; Gao, D.; Wang, H. Coking mechanisms within RFCC disengagers. Petroleum Sci. Technol. 2004, 22, 601−615. (14) Wilson, J. W. Fluid Catalytic Cracking Technology and Operation; Penn Well Publishing Co.: Tulsa, OK, 1997. (15) Kim, S. W.; Kim, G. R.; Shin, J. W.; Yoo, I. S.; Kang, H. S.; Park, S. H. Fluidization technology for stable startup of commercial FCC unit. Fluidization XIII, Gyungju, Korea, May 16−21, 2010; pp 773− 780. (16) Kugler, E. L.; Leta, D. P. Nickel and vanadium on equilibrium cracking catalysts by imaging secondary ion mas spectroscopy. J. Catal. 1988, 109, 387−395. (17) Petti, T. F.; Tomczak, D.; Pereira, C. J.; Cheng, W. Investigation of nickel species on commercial FCC equilibrium catalystsimplications on catalyst performance and laboratory evaluation. Appl. Catal., A 1998, 169, 95−109. (18) Kopinke, F. D.; Zimmermann, G.; Nowak, S. On the mechanism of coke formation in steam cracking. Carbon 1988, 26, 117−124. (19) Purwono, S.; Silveston, P. L. Time influence on reactivity and morphology of coke deposits formed during thermal cracking of hydrocarbons. Carbon 1993, 31, 651−660. (20) Graff, M. J.; Albright, L. F. Coke deposition from acetylene, butadiene and benzene decompositions at 500−900 °C on solid surfaces. Carbon 1982, 20, 319−330. (21) Sadeghbeigi, R. Fluid Catalytic Cracking Handbook; Gulf Publishing Co.: Houston, TX, 1995. (22) Bohmer, R. W.; McKay, D. L.; Knopp, K. P. Fluid cracking catalyst metals passivation, Development and application. Fluid Catal. Cracking II, ACS Symp. Ser. 1991, 452, 183−197. (23) Klestchov, D.; Burmistrov, V.; Sheinkman, A.; Pletnev, R. Composition and structure of phases formed in the process of hydrated antimony pentoxide thermolysis. J. Solid State Chem. 1991, 94, 220−226. (24) Chernogorenko, V. B.; Lynchak, K. A.; Klimak, Z. A.; Morozova, R. A. Kinetics of antimony trioxide reduction by hydrogen. Effect of atomic hydrogen. Powder Metall. Met. C+ 1978, 17, 419−421. (25) Danielson, J. A. Air Pollution Engineering Manual; Public Health Service Publishing, No.999-AP-40: U.S. Government Printing Office, U.S.A., 1967. (26) Geldart, D. Types of gas fluidization. Powder Technol. 1973, 7, 285−292. (27) Wilbrink, M. W. L.; Michels, M. A. J.; Vellinga, W. P.; Meijer, H. E. H. Rigidity percolation in dispersions with a structured viscoelastic matrix. Phys. Rev. E 2005, 71, 013402.

Short residence time of catalysts in the dipleg by increasing the catalyst flow from modifications of the dipleg diameter and weights of the flapper valve, a decrement of wall roughness by change of the refractory material and fluidity increase by modification of catalysts physical properties are believed to have great benefit in alleviating the deposit formation.

5. CONCLUSIONS Characterization of deposits in the cyclone dipleg of the commercial RFCC reactor was carried out by analytical approaches. The main components of the deposits are hydrocarbons and a high portion of inorganic matter, reflecting dipleg conditions with particles flow. The deposit consists of two parts of massive bulk matter, which are mainly pure Sb metal and catalyst particles surrounded by filamentous cokes and lumps of carbonaceous matter. We found that Ni nanopowders on catalyst surfaces and from heavy oil in vapor phase catalyze the filament coke, which accelerate the increase in the amount of deposits by the filtering of heavy oil droplets. The Sb metals that originate from injected Ni passivator in the riser contribute largely to deposit formation, and this was validated by simulation test of RFCC conditions and calculation of cyclone collection efficiency. Possible mechanisms of the deposit formation are related to the condensation and polymerization of heavy oil droplets, fine catalyst properties and their flow, sticking of catalyst, formation of filament coke by Ni on catalyst surfaces, and captured Sb metal grains in the cyclone. Management of Ni contents in the feedstock and Ni passivator and design and operation improvements of the cyclone dipleg are suggested as methods to minimize the formation of deposits in addition to previously proposed methods.

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*Tel.:+82-42-609-8314. Fax:+82-42-609-8804. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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dx.doi.org/10.1021/ie301864x | Ind. Eng. Chem. Res. 2012, 51, 14279−14288