Radial Non-uniformity Index Research on High-density, High-flux CFB Riser with Stratified Injection

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1 Simulation and Optimization China Petroleum Processing and Petrochemical Technology 2012,Vol. 14, No. 4, pp December 30, 2012 Radial Non-uniformity Index Research on High-density, High-flux CFB Riser with Stratified Injection Geng Qiang 1 ; Wang Lu 1 ; Li Zhichao 1 ; Li Chunyi 1 ; Liu Yibin 1 ; You Xinghua 1, 2 (1. State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao ; 2. Petrochemical Factory of the Yumen Oil-Field Company, PetroChina, Yumen ) Abstract: A high-density, high-flux circulating fluidized bed (CFB) riser (100 mm in ID and m in height) was applied in a wide range of operating conditions (with solid fluxes up to 400 kg/m 2 s and superficial gas velocities up to 12 m/s) to examine its radial non-uniformity dynamics. The solids holdup was determined through the use of a fiber-optic probe at 11 axial levels. The results indicated that under all operating conditions, the high superficial gas velocity and low solid fluxes maintained a low radial non-uniformity index (RNI). The high-density/flux CFB riser had several unique characteristics, so that the peak of the radial solids holdup profile occurred at a position with r/r=0.8. The RNI and solids holdup at the cross-sectional position had a good logarithmic relationship at the low-density condition (with a mean solids holdup of <0.2), and the RNI decreased when the mean solids holdup exceeded 0.2. Investigation of the dynamics of stratified injection revealed that the feed ratio had an important effect on G s and on solids holdup distribution. A novel < shaped axial solids holdup profile was found. G s decreased sharply when the up-flow feed ratio exceeded 0.5, and RNI was lowest when the up-flow feed ratio was 1. Key words: fluidization; high density/flux CFB riser; radial non-uniformity index; stratified injection; feed ratio 1 Introduction Circulating fluidized beds (CFB) are widely used in industrial processes such as coal combustion and fluid catalytic cracking (FCC) processes, as well as in other areas of the chemical, petrochemical, metallurgical, environmental, and energy industries [1-3]. However, particles form aggregates (clusters and particle strands) in industrial CFB risers as a result of flow instability, energy minimization, and particle-wake interactions [4]. As the solids flux (G s, in kg/m 2 s) increases, particle congregation with significant back-mixing is observed, which is probably caused by the lift force that is reduced by shear near the wall in the lowvelocity boundary layer. This back-mixing occurs due to the core-annulus mode of the particle flow [5-6]. The nonuniform solids distribution is obviously reflected in the gas velocity and in the radial gradient characteristic of the solids holdup [1]. This non-uniform solids distribution can affect the flow structure and further influence the reaction rates, the selectivity to desired intermediate products, the mass and heat transfer, the gas-solid contact efficiency, the coking reaction, and the erosion within the riser [2]. Zhu defined the radial non-uniformity index (RNI) parameter to depict this non-uniform quality and provided a direct comparison between different operating conditions [7]. The current study uses RNI as its main tool for analyzing the experimental data that shows the non-uniform features of solids radial profile. Low gas back-mixing is necessary because the expected product is gas. The catalysts require high gas velocity in the riser to reach the plug flow condition because the contact time between gas and solid must be short [8]. Catalytic gas-phase reactions tend to require higher gas velocity, as well as higher solids flux and concentration, than gassolid reactions such as fluid catalytic cracking [9]. In the former case, the G s ranges commonly from 300 kg/m 2 s to kg/m 2 s, with corresponding solids holdup values in the range of 0.1 to 0.25 [10]. High-density circulating fluidized beds (HDCFB) with high solids flux at high gas velocity have received widespread research attention. Further work on HDCFB Corresponding Author: Dr. Li Chunyi, Telephone: ; chyli@upc.edu.cn. 64

2 Geng Qiang, et al. Radial Non-uniformity Index Research on High-density, High-flux CFB Riser with Stratified Injection systems is needed for better comprehension of their advantages and limitations, which in turn will lead to the development of a more reliable reactor scale-up and more cost-effective units. Bi and Zhu described operations with G s >200 kg/m 2 s and solid holdup>0.10 throughout the entire riser. In a series of studies on HDCFB risers, there was an absence of down-flow stream at the wall, a limited tendency to form clusters, and relatively flat axial density profiles without dilute zones [9,11]. Since the HDCFB concept was brought up, a series of work had been done by many researchers including HDCFB system design to achieve high solid holdup [12], effect of different solids feeder and exit riser configuration on flow behavior [13], lateral solids disperation study [14] and flow dynamics in the downer [15]. The stratified injection is an important and novel ingenious approach designed to achieve different reaction environment for the riser [16]. The Research Institute of Petroleum Processing (RIPP), which has developed the latest gas and diesel manufacturing technology, divides the riser into four reaction sections. The stratified injection (with gasoline entering the bottom section, and fresh feed and heavy oil entering the middle section of the riser) can improve the liquefied petroleum gas (LPG) and diesel yields when cracking gasoline and heavy oil feedstocks, respectively. The stratified injection method is also used in FCC process to maximize the production of propylene, a vital component for the manufacture of major petrochemicals and other substances. Cracking heavy feedstock before LCG [17-18], or cracking naphtha before gas oil injection [19], can also improve propylene yield. Li invented a specific reactor type and method for propylene production that employs a combination of heavy and light olefin feeds [20]. Stratified injection style was also applied to SFI, MAXOFIN, MIP processes [21-23]. The above investigations demonstrate that although previous studies have offered an insight into the application of stratified injection in the FCC process, the gas-solids flow behavior has received limited attention thus far. 2 Experimental 2.1 Apparatus The schematic drawing of the experimental apparatus is shown in Figure 1. Experiments were conducted in the riser made of plexiglass, 0.1 m in diameter and m in height. The solid material used in experiments was spent FCC catalyst particles with a mean diameter of 80 μm and a particle density of kg/m 3. The size distribution of FCC particles is shown in Table 1. Compressed air was injected into the riser through a pressure stabilization valve (for maintaining a pressure of 0.17 MPa) and a rotameter. Four compressed air inlets were connected with the distributor symmetrically. Catalysts were transferred to the riser from the down-comer through a 30 o angled pipe for mixing with the air including the pre-lift gas, the pre-fluidized gas, the lower feed gas and the upper feed gas. Catalysts lifted up to the top of the riser were routed to the cyclones, 500 mm in diameter, in which most of the catalysts were removed from the gas phase and returned to the hopper, 480 mm in diameter, with an inventory of kg of catalysts. In order to simulate gas-solid flow of the stratified injection, the upper and lower feed units were set up at a height of 0.8 m and 3.6 m, respec- Figure 1 Scheme of CFB riser unit Table 1 Size distribution of FCC particles determined by screening Mesh size, μm Mass faction, % > <

3 China Petroleum Processing and Petrochemical Technology 2012,14(4):64-72 tively, above the bottom of the riser. This design could effectively improve the solids circulation flux by providing twice the pressure head to achieve the high density/ flux fluidized regime. The operating conditions of superficial gas velocity and the solids circulation rate ranged from 8 to 10 m/s and 200 to 400 kg/m 2 s, respectively. 2.2 Measurement Solids circulating rate was calculated in the measuring system by measuring the difference of height in the measuring tank for a given time period which could be achieved by stopwatch. The G s value can be expressed by the equation: rm h h b Gs = 2 π ( 2 1) ρ (1) t t 2 1 in which r m is the measuring tank radius, m, and r b is the bulk density in the measuring tank, kg/m 3. The solids concentration was measured at 11 axial levels (at 0.53 m, 1.51 m, 2.19 m, 3.42 m, 4.32 m, 5.32 m, 6.42 m, 7.32 m, 8.16 m, 9.06 m, and m, respectively,) and 11 dimensionless radial positions (with r/r=0.00, 0.16, 0.38, 0.50, 0.59, 0.67, 0.74, 0.81, 0.87, 0.92, and 0.97, respectively). The solids concentration was obtained by the PV-56 D optical fiber probes (manufactured by the Institute of Process Engineering, Chinese Academy of Sciences). The measurement mechanism of the optical fiber probe is shown in Figure 2. The probe receives the light signal reflected from the particles that is simultaneously emitted by the light source in the probe. The optical signals will then be converted into voltage signals by a photo-multiplier and acquired by the PCbased data sampling system. It is of great importance that the fiber probe should be calibrated to exactly transform voltage signals to the solids concentrations before being used. The conversation from voltage signal to solids concentration signal is shown in Figure 3. More details can be referenced in Li s paper [24-25]. Although the standard deviation could reflect the fluctuations of solids radial profile to some extent, it failed to depict the contrast between different sections in the total riser. As a result a novel parameter RNI was defined by an equation hatched out by Zhu and Manyele [7] to achieve the contrasting function. Standard deviation ratio between the real radial distribution and the most nonuniform radial distribution of solids concentration under the same cross-sectional average solids concentration was calculated by the following equation: σ ( ε s ) RNI( ε s ) = = σ ( ε ) max s σ ( ε ) ε ( ε ε ) s s smf s where ε smf means solids concentration or solids holdup at minimum fluidization conditions. In this circulation fluidized bed, we obtained its value to be equal to 0.57 through experiments. (2) Figure 2 Principle for measurements by the optical fiber probe Fibre for light emitting; fibre for light receiving 66

4 Geng Qiang, et al. Radial Non-uniformity Index Research on High-density, High-flux CFB Riser with Stratified Injection clusters near the wall [1, 7, 26]. Figure 3 Relationship between solids concentration signal and voltage signal 8 m/s; 10 m/s; 12 m/s 3 Results and Discussion 3.1 Effect of operating conditions on RNI Superficial gas velocity and solids flux are the primary factors affecting the radial profile of solids. Our study applied the RNI parameter to reveal the effect of operating conditions on the uniformity of the solids radial profile. RNI changed from 0 to 1 as the concentration of the radial distribution of solids became increasingly non-uniform. The effect of superficial gas velocity and solids holdup on RNI is illustrated in Figure 4. In general, the solids radial concentration profile is more or less non-uniform under all operating conditions. At a constant G s (300 kg/m 2 s), an increasing U g can result in a decreasing RNI as a major trend, showing that higher U g can benefit the uniformity because it reduces the average solid holdup and weakens the core-annulus structure. With respect to the energy balance, the concentration in the core region was small and the gas-solid flow, which was basically in a dispersed state of particles, was more fully developed, so the influence of operating conditions was relatively small. In the wall region, solids holdup was high, and in combination with the wall effect the particles tended to gather more firmly. A greater energy is required to maintain the particle motion in the wall area [26]. Thus, changes in operating conditions affected the particle motion at the wall more directly, and higher gas velocity decreased the wall effect and lowered the concentration of solids, making the radial profiles more uniform. At a constant U g (10 m/s), the value of RNI increased with a higher G s, due to the formation of significant amount of 200 kg/m 2 s; 300 kg/m 2 s; 400 kg/m 2 s Figure 4 Influence of operating conditions on RNI At the axial position, the riser was divided into three parts, namely: the dense suspension upflow region, the flow regime transition region, and the fully developed region (as seen in Figure 5). Under a high solids flux condition, the dense suspension upflow (which was not a net downflow at the wall, with e s >0.2) was obtained in the bottom region [12], and the RNI was relatively small compared with the transition region, which had higher back-mixing that resulted in non-uniformity situations. The RNI decreased as the height of the transition region increased because the solids concentration remained constant in the core region and increased in the wall region. Figure 7 illustrates this regulation when the height is 4.32 m. Increasing the superficial gas velocity lengthens the fully developed region [27], so the RNI remains at a low level within a large range with higher U g. However, near the exit of the riser, RNI suddenly increases mostly because of exit restraints, resulting in a certain degree of back-mixing. 67

5 China Petroleum Processing and Petrochemical Technology 2012,14(4): m/s; 10 m/s; 12 m/s 200 kg/m 2 s; 300 kg/m 2 s; 400 kg/m 2 s Figure 5 Axial profiles of solids holdup under different operating conditions Figure 6 The radial profiles of solids concentration at different U g 8 m/s; 10 m/s; 12 m/s Figure 6 shows the effect of superficial gas velocity on radial profiles at different heights (i. e.: near the bottom of the riser, near the lower feeder, and near the upper feeder). We divided the radial section into two parts. The core region (at r/r=0 0.5), where solids holdup was less, and the wall region (at r/r= ), where solids holdup was more, constituted the typical core annulus structure, which was significant with the height. The solids holdup in the wall region was more sensitive to changes in operating conditions than that in the core region. An increasing superficial velocity lowered the solids holdup, but it was nearly constant in the wall region. This can also be elucidated in terms of the energy balance, as it has been previously mentioned. The summit of the radial profiles curve is located near the position at r/r=0.8, but not close to the position at r/r=1 as reported by other studies on low-density or low-flux conditions. In this study, we found out that when the operating conditions achieved high density or high flux, some gas flow occurred along the wall. The traditional explanation of the core-annulus structure is attributed to the wall effect, such as the interaction between particles and the wall, and the particle velocity decreases as the resistance of particle motion increases. The high density or flux of the CFB reactor may break this wall effect, shifting the resistance region from the wall to the position at r/r=0.8. However, this potential explanation requires further investigation. 3.2 Effect of feed ratio on G s and RNI Figure 7 plots the changes in solids flux for different upflow feed rates. In general, the solids flux is constant under certain operating conditions for up-flow feed ratio. However, when the up-flow ratio increased to 0.45, the solids flux decreased rapidly from 470 kg/m 2 s to 400 kg/m 2 s. To find out the reason, we designed three different feed patterns, i. e.: a single up-flow feed, a single down-flow feed, and a feed with 0.43 up-flow ratio under the same operating conditions (with U g =10 m/s, and G s =300 kg/m 2 s). The axial profiles of the solids holdup at the bottom of riser depicted in Figure 8 show the values of the solids holdup in a decreasing order: single up-flow feed >stratified feed>single down-flow feed. In the case of the single up-flow ratio, the pre-lift and pre-fluidized gas has little ability to carry solids at the bottom of the riser due to decreasing velocity and dynamic head, resulting in a lower drag force, back-mixing of particles, and higher solids holdup. Thus, the regenerated catalysts flowing into the riser are faced with a greatest resistance. Furthermore, the axial profile of the solids holdup in the 68

6 Geng Qiang, et al. Radial Non-uniformity Index Research on High-density, High-flux CFB Riser with Stratified Injection stratified feed style was different from that of the traditional riser (single down-flow feed style), which has a typical L-shaped axial solids distribution profile. The stratified feed style had a unique < shape of axial solids distribution profile mainly because it was introduced to two inlets at different positions, and the carryover of particles in gas could be completed in relay, which differed from the traditional one inlet configuration of the riser. The axial profiles of solids holdup of traditional risers have the following characteristics as reported in the literature. When presenting a typical S shape, it has a significant transition section, and the scope of the transition section increases with the extension of G s. When presenting a typical L shape, it has a dense phase at the bottom and a dilute region at the top. At the same time, the solids holdup of both axial profiles of the traditional riser stays at 0.05 in the fully developed region [28]. Although the stratified-feed-style riser had no evident transition section and axial solids holdup, it was affected Figure 7 The influence of up-flow feed ratio on the G s value by the operating conditions in the fully developed region. As the superficial velocity increased from 8 m/s to 10 m/s, and to 12 m/s, the solids holdup in the fully developed region decreased from 0.15 to 0.10, and to 0.05, respectively. Similar to changes in G s, the feed ratio also has a great influence on the RNI, as shown in Figure 9. Generally, an increase in the up-flow feed ratio decreases the RNI, indicating that the radial distribution of solids becomes more uniform. Fan [29] divided the nozzle jet into two parts: the mainstream and the secondary flow. The synergy of both parts resulted in a much sharper solids holdup distribution gradient due to a steeper velocity gradient and intense shearing. This effect exacerbated the annual-core structure, giving rise to an increase in RNI. We divided the riser into two parts: (1) the down-flow feed coverage area (0.8 m to 3.6 m), and (2) the up-flow feed coverage area (>3.6 m), based on the position of the nozzle. Therefore, higher solids holdup (0.24 to 0.29) and the lack of nozzle jet effect are the main factors for determining the least value of RNI in the down-flow feed coverage area applicable to the single up-flow feed style. In our study, a significant drop of RNI occurred at the position that was away from the down-flow feed nozzle in the other two feed styles. When the height exceeded that of the upflow feed nozzle, the value of RNI in the single downflow feed style changed little and that in the other two styles began to climb back from the trough showing some growing trend. Furthermore, the axial position of each inflection point came closer to the up-flow feed nozzle with Figure 8 Axial profiles of solids holdup with three different feed patterns Up-flow feed ratio=0; Up-flow feed ratio=0.4; Up-flow feed ratio=1 Figure 9 Influence of feed ratio on RNI Up-flow feed ratio=0; Up-flow feed ratio=0.4; Up-flow feed ratio=1 69

7 China Petroleum Processing and Petrochemical Technology 2012,14(4):64-72 an increasing up-flow feed ratio, since the effect of the nozzle on the gas-solid flow became more obvious as the up-flow feed ratio increased. 3.3 Relationship between RNI and mean solids holdup The relationship between RNI and mean solids holdup under the low-density condition has been investigated by other researchers [7, 26]. The experimental results showed that RNI and solids holdup at the cross-sectional position had a good logarithmic relationship under the low-density condition (with mean solids holdup being below 0.2). As illustrated in Figure 10 (a), this regulation was not suitable for all operating conditions, since the value of RNI showed a downward trend when the solids holdup was >0.2. This indicates that at high suspension densities, the high density is conducive to the uniform radial profiles when there is no downward flux at the wall and the coreannulus structure is absent or not obvious [22]. In the case of low-density conditions where solids holdup was below 0.2, the correlation between RNI and mean solids holdup was obtained within the experimental range: RNI= ln(e s ) (3) To further analyze the radial profile of solids, the regulations in three different feed styles are presented in Figure 10 (b), where they experienced the same operating conditions (at U g =10 m/s, and G s =300 kg/m 2 s). In general, similar trends between RNI and solids holdup indicated that solids holdup was a good response to uniformity in spite of different feed styles. However, the radial profiles of solids holdup in the traditional riser (with up-flow feed ratio=0) were relatively less uniform as compared to other feed styles. The stratified feed style had good uniformity, particularly at high solids holdup values (0.2 to 0.3), which provided the experimental foundation for industrial applications such as maximum propylene production process via stratified feed. RNI is not significantly sensitive to operating conditions, as illustrated in Figure 10 (c). RNI remains constant when the solids holdup is typically less than RNI increases with gas velocity and lower solids fluxes at solids holdup values being up to The peak of the curve is typically at a solids holdup value of 0.2, which is the dividing line between low-density and high-density CFB. These results also show that the high-density CFB is conducive to uniformity. Up-flow feed ratio=0; Up-flow feed ratio=0.4; Up-flow feed ratio=1 U g =10 m/s, G s =300 kg/m 2 s; U g =12 m/s, G s =300 kg/m 2 s; U g =10 m/s, G s =200 kg/m 2 s Figure 10 Curves of RNI changing with solids concentration 4 Conclusions (1) Based on a series of contrasting STD data from various operating conditions, RNI was obtained for analyzing the radial non-uniformity behaviors of solids in the CFB riser. 70

8 Geng Qiang, et al. Radial Non-uniformity Index Research on High-density, High-flux CFB Riser with Stratified Injection (2) The effect of operating conditions (U g and G s ) on RNI in high density or in fluxes was investigated. Increasing U g and decreasing G s resulted in a lower RNI value, indicating that those conditions were conducive to radial uniformity. (3) The summit of the solids radial profile curves was located near the position at r/r=0.8, when the CFB was operated under high-density conditions. (4) RNI and solids holdup had different regulations compared with previous studies. In contrast to the monotonically increasing relationship between RNI and solids holdup under low-density conditions (at solids holdup < 0.2), a decreasing trend was formed when the solids holdup was > 0.2. (5) Stratified injection nozzles and the feed ratio had an influence on the values of G s and RNI. When the up-flow feed ratio was over 0.47, the value of G s decreased noticeably. Increasing the up-flow feed ratio decreased the RNI, and a unique < shaped axial solids distribution profile was found. (6) RNI was not significantly sensitive to operating conditions. The high-density CFB was conducive to the uniformity of the solids radial profile. Nomenclature r radial distance from the riser axis, m G s solids circulation rate/solids fluxes, kg/ (m 2 s) U g superficial gas velocity, m/s r m measuring tank diameter, m h 2 -h 1 difference of measuring tank height, m t 2 -t 1 difference in measuring time, s r b bulk density in the measuring tank, kg/m 3 ε smf solids holdup at minimum fluidization condition. σ(ε s ) standard deviation of solids holdup σ max (ε s ) maximum standard deviation of solids holdup Acknowledgements: The authors gratefully appreciate the financial support of the National Program on Key Basic Research Project (973 Program) of China (no. 2012CB215000). References [1] Manyele S V, Zhu J, Khayat R E, et al. Analysis of the chaotic dynamics of a high-flux CFB riser using solids concentration measurements [J]. China Particuology, 2006, 4(3/4): (in Chinese) [2] Zhu H, Zhu J. Characterization of fluidization behavior in the bottom region of CFB risers [J]. Chemical Engineering Journal, 2008, 141(1/2/3): [3] Guan G, Fushimi C, Ishizuka M, et al. Flow behaviors in the downer of a large-scale triple-bed combined circulating fluidized bed system with high solids mass fluxes [J]. Chemical Engineering Science, 2011, 66(18): [4] Manyele S V, Pärssinen J H, Zhu J. Characterizing particle aggregates in a high-density and high-flux CFB riser [J]. Chemical Engineering Journal, 2002, 88(1/2/3): [5] Yan Chaoyu, Fan Yiping, Lu Chunxi, et al. Solids mixing in a fluidized bed riser [J]. Powder Technology, 2009, 193(1): [6] Mahmoudi S, Baeyens J, Seville J. The solids flow in the CFB-riser quantified by single radioactive particle tracking [J]. Powder Technology, 2011, 211(1): [7] Zhu J, Manyele S V. Radial Nonuniformity Index (RNI) in fluidized beds and other multiphase flow systems [J]. The Canadian Journal of Chemical Engineering, 2001, 79(2): [8] Zhu J, Bi H T. Distinctions between low density and high density circulating fluidized beds [J]. The Canadian Journal of Chemical Engineering, 1995, 73(5): [9] Grace J R, Issangya A S, Bai D, et al. Situating the highdensity circulating fluidized bed [J]. AIChE Journal, 1999, 45(10): [10] Issangya A S, Grace J R, Bai D, et al. Further measurements of flow dynamics in a high-density circulating fluidized bed riser [J]. Powder Technology, 2000, 111(1/2): [11] Malcus S, Cruz E, Rowe C, et al. Radial solid mass flux profiles in a high-suspension density circulating fluidized bed [J]. Powder Technology, 2002, 125(1): 5-9 [12] Kim S W, Kirbas G, Bi H, Lim C J, et al. Flow behavior and regime transition in a high-density circulating fluidized bed riser [J]. Chem Eng Sci, 2004, 59(18): [13] Kim J, Tachino R, Tsutsumi A. Effects of solids feeder and riser exit configuration on establishing high density circulating fluidized beds [J]. Powder Technology, 2008, 187(1): [14] Ran X, Wei F, Wang Z, et al. Lateral solids dispersion in a high-density riser with swirling air flow [J]. Powder Technology, 2001, 121(2/3):

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