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Radial behavior in riser and downer during the FCC process
Chemical Engineering and Processing 41 (2002) 259– 266 www.elsevier.com/locate/cep
Radial behavior in riser and downer during the FCC process Rensheng Deng, Fei Wei *, Tengfei Liu, Yong Jin Department of Chemical Engineering, Tsinghua Uni6ersity, Beijing 100084, People’s Republic of China Accepted 20 April 2001
Abstract In this paper, the radial behavior of riser and downer during the FCC process is studied on the basis of simulation results from a two-dimensional dispersion model. Although radial uniformity is observed in both reactors, the radial distribution of products is more uniform in the downer than in the riser. At the same feed conversion, about 5.5 wt.% higher gasoline yield is predicted in the downer due to the less axial backmixing and flatter radial distribution of gas– solids velocity and solids concentration. The simulation results also show that the radial gas dispersion plays an important role in determining the radial distribution of products. The profiles of gasoline in both riser and downer are calculated, and the simulation results of different models compared, showing that the consideration of radial non-uniformity is necessary. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Simulation; Non-uniformity; Radial behavior; FCC; Riser; Downer
1. Introduction The invention and application of novel reactors is an important step in the development of the FCC industry. For example, the riser, which was firstly adopted in the 1960s, exhibits a lot of advantages such as higher gasoline yield and easier process control over conventional fluidized reactors. However, the severe axial backmixing of gas and solids, as well as the significant radial non-uniformity in gas velocity, particle velocity and solids concentration, is realized by people more and more [1]. From 1970 to 1980 some well known companies including Stone & Webster, Mobil and Texaco, proposed a novel reactor-downer, which attracted many research interests quickly and was considered to overcome the drawbacks of the upflow riser in the FCC process [2,3]. There are two aspects usually thought to contribute to the higher gasoline in downer than in riser: one is the lower backmixing of gas and solids; the other is the more uniform radial flow structure. The influences of axial gas backmixing on the gasoline yield are examined by Wei [4] with a one-dimensional model. According to * Corresponding author. Tel.: + 86-10-62785464x8214; fax: +8610-62772051. E-mail address: [email protected] (F. Wei).
their research, there is a gap of 5.l wt.% in gasoline yield between riser and downer at the conversion of 62.0 wt.%. However, there are few reports about the different effects of radial hydrodynamics in riser and downer, which may be of great importance to the product profiles. Furthermore, the radial dispersion behavior in the reactors, concerning with the elimination of the non-uniformity at different local positions, should also be taken into consideration. In this paper, a two-dimensional dispersion model is proposed to simulate the FCC process operated in a riser and a downer both of the industrial scale, and the different radial performances in the two types of reactors are discussed on the simulation results. The emphasis is placed on the contributing factors, performances and results of the radial non-uniformity. 2. The contributing factors of radial non-uniformity
2.1. Radial hydrodynamics of gas and solids in riser and downer The radial hydrodynamics is mainly due to the lateral distribution of gas velocity, particle velocity and solids concentration, which is different in riser and downer. The local gas velocity in the riser can be represented by an Ostwald-de Waele type equation:
0255-2701/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 5 - 2 7 0 1 ( 0 1 ) 0 0 1 4 0 - 4
260
Ug(r)= Ug,av
R. Deng et al. / Chemical Engineering and Processing 41 (2002) 259–266
3n+ 1 n+1
1−
r R
n + 1/n
(1)
Deroum [5] suggested that the index n could be extrapolated to 2.0 for commercial risers. The radial profile of gas velocity in the downer is rather flat compared with that in the empty tube or the riser, as shown in Fig. 1 [6,7]. By correlating the data measured by Cao [7], the ratio of local gas velocity to the cross-section averaged value can be expressed as a function of radial position:
n
Ug 1− r =2 Ug,av R
1/7
exp −0.8
1−r R
0.4
(2)
Yang studied the particle velocity profiles in the dilute phase of the riser by using LDV [6]. More exciting results were obtained by Wei in a high-density riser (solids fraction up to 0.25), which is close to the operating conditions in commercial plants [8]. The radial distribution of particle velocity in the downer is found to be more uniform as shown in Fig. 2 [6,7]. Bai obtained the following equation for the ratio of local velocity to the cross-section averaged value [9]: Vp =6.309F 0.97 m f() r Ug,av
(3)
where f() = − 2.246− 5.086 + 18.262 2 −13.199 3
(4)
By extending the work of Tung [10], Zhang improved the method for calibrating the optical-fiber probe and proposed the following empirical equation to predict the solids concentration in the riser on the basis of their experimental data [11]:
Fig. 2. The radial distribution of particle velocity in downer and riser [6,7].
probe [12], and the results were compared with those from the riser in Fig. 3 [3]. Zhang proved the results in a wider range of axial locations and operating conditions [13], and Lehner found a similar distribution for spherical glass beads with an X-ray computed tomography system [14,15]. An empirical equation for the radial distribution of voidage was provided by Bai [16]: m= m¯ F()
(6)
where F()= 30.62(1− )exp[− 127.6(1− 2)] +
22.8 36.7+(1−)
(7)
The radial solids concentration of FCC particles in the downer was measured by Yang with a fiber optic
In summary, the radial flow structure in risers is typically described by a core-annulus model. Across the cross-section of the reactor, there exist two zones: a central core in which gas velocity and particle velocity are high while the particle concentration and flux are low, and a peripheral annulus, where concentrated
Fig. 1. The radial distribution of gas velocity in downer, riser and empty tube [6,7].
Fig. 3. The radial distribution of solids concentration in downer and riser [3].
2.5 + 311)
m= m¯ (0.191 +
(5)
R. Deng et al. / Chemical Engineering and Processing 41 (2002) 259–266
solids flow down with a gas velocity close to zero. For the downer, the radial flow structure can be characterized by three regions: a dilute core region (05 r/RB 0.85), where the local gas velocity, particle velocity and solids concentration are rather uniform; a relative dense annular region (0.855r/R 50.96), where all three variables have their maxima; a wall region (r/R ] 0.96), where all these variables decrease towards the wall [1,17]. From the discussion above, it can be concluded that the radial flow structure in the downer is much more uniform than that in the riser. The reduction of nonuniformity in the downer reduces the backmixing in the reactor and, therefore, benefits the gasoline yield in the FCC process greatly.
2.2. Radial mixing beha6ior in riser and downer Mixing behavior is quite important for heat transfer and mass transfer. The axial dispersion will lead to a broader residence time distribution (RTD) and consequently reduce the gasoline yield, while the radial dispersion can smoothen the non-uniformity at different radial positions and consequently benefit the gasoline yield. According to Wei [4], the axial gas Peclet number in the riser is typically about 4, while in the downer it can reach 100. Wei [18] also measured the axial solids dispersion with the phosphorescent particle tracer technique in both riser and downer and pointed out that the axial solids Peclet number is around 100 in the downer while about 3–4 in the riser. Koenigsdorff [19] stated that the radial Peclet number of solids in the riser ranges from 100 to 300. The radial solids dispersion in the downer was studied by Wei [20] and the dispersion coefficient Dr,p ranging from 0.0026 to 0.0113 m2/s was reported, with the corresponding Pelcet number from 70 to 300. However, according to the work of Deroum [5] and Landeghem [21], the radial catalyst mixing can be neglected for kinetic reasons, only the catalyst hold-up distribution is important for the modeling in this situation. Wei [22] studied the radial gas mixing with continuous tracer point injection. The radial gas dispersion coefficients they measured in the downer (0.0015–0.003 m2/s) are comparable with the one in the riser measured by Adams [23] (0.0005– 0.0030 m2/s), and changes little with the operating conditions. They also found that the Per ranges from 100 to 300, of the same order of magnitude as in the riser. It can be concluded that the greatly reduced axial gas and solids backmixing in the downer will benefit the conversion and selectivity of such rapid reactions like FCC, while the excellent radial gas and solids dispersion can provide the downer with excellent heat and mass transfer ability across the section. As a result, the
261
gas–solids flow in the downer will be much closer to plug flow than in the riser, and so higher gasoline yield can be expected at the same operating conditions. In this paper, the radial Peclet number is typically set at 200 both for downer and riser, while the effects of different Peclet number are also examined.
3. The effect of the radial non-uniformity
3.1. Model building Besides the hydrodynamics and mixing behavior, the reaction kinetics are also important for the development of the reactor model. Great progress has been made in the reaction kinetic models for FCC in recent years, among which the lump kinetics models firstly developed by Weekman [24] are the most successful and attractive. As a preliminary attempt, the four-lump reaction kinetic model proposed by Gianetto [25] is adopted here, with reaction network shown in Fig. 4. The gas oil will be cracked into gasoline, gas and coke, then the gasoline can be further converted into gas and coke, but gas and coke will never turn into other products. By applying the mass balance to an infinitesimal unit in the reactor, the following equation is obtained:
R 1 ( (yi H ( 2yi (yi 1 r + − + (− ri )= 0 2 Per r (r (r Pea (L (L Ug The boundary conditions are: inlet (L= 0):
yi(0−) = yi(0+) −
outlet (L= H): center (r=0): wall (r=1):
1 (yi Pea (L
(yi =0 (L (yi =0 (r
(yi =0 (r
(8)
(9)
0+
(10) (11) (12)
It should be mentioned that the equations above are valid for both riser and downer, while the ordinates in
Fig. 4. Kinetic model of FCC.
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R. Deng et al. / Chemical Engineering and Processing 41 (2002) 259–266
It can be seen that there is significant non-uniformity in both riser and downer. The products gasoline, gas and coke are higher near the wall than in the center, while the distribution of the feed gas oil has the opposite trend. Comparing with the downer, the non-uniformity in the riser is far more significant. The gap of gasoline concentration between the wall and the center is about 15 wt.% in the riser but only 5 wt.% in the downer. To evaluate the non-uniformity more precisely, a non-uniformity index similar to the indices adopted by Bai [16] for radial solids concentration profile and radial particle velocity profile is defined for products S 2(gasoline)= 2
&
1
(ygasoline − y¯ gasoline)2d
(13)
0
Fig. 5. Radial distribution of products at X= 4.5 m in the riser.
the vertical dimension are in the opposite direction. The equations are numerically calculated with an orthogonal collocation method, which can provide faster convergence speed and higher accuracy than finite difference methods. The reactor simulated is a commercial plant with a height of 45 m and a diameter of 1 m. The reaction is supposed to occur at 550°C. The average solids concentration is set at 47.8 kg/m3, and the inlet gas velocity is 6.0 m/s.
3.2. The non-uniformity in radial products distribution The distribution of the radial products in riser and downer are shown in Figs. 5 and 6, respectively. The axial location is at 4.5 m, one-tenth of the whole reactor height.
From the data in Figs. 5 and 6, the indices for riser and downer are 0.00223 and 0.000256, respectively. While considering that the average gasoline concentration is different in riser and downer, the dimensionless nonuniformity index is more important S 2(gasoline) | 2(gasoline)= (14) (y¯ gasoline)2 The corresponding values for riser and downer are 0.03146 and 0.00218, respectively. The former is about 15 times of the latter, which shows that the non-uniformity is more severe in riser than in downer. The analysis of indices for other products follows the same rules. The difference of radial product distribution between the riser and the downer is due to the unlike radial hydrodynamics in these two reactors. In the core section of the riser where solids are dilute and gas velocity is high, the reaction rate is predicted to be much lower than in the annulus section, with high solids concentration and low gas velocity, and hence significant nonuniformity of products profiles. The more uniform radial flow structure of both gas and particle phases in the downer leads to a more uniform radial distribution of products.
3.3. The effects of radial dispersion
Fig. 6. Radial distribution of products at X= 4.5 m in the downer.
Due to the radial flow structure and the reaction kinetics, the radial non-uniformity of products stated above is inevitable. However, it is the existence of radial dispersion that prevents such non-uniformity in the FCC reactors from accumulating and resulting in broader residence time distribution and more severe backmixing. Figs. 7 and 8 show the radial gasoline profiles at different radial gas Peclet number in the riser and the downer, respectively. The curves in the figures correspond to axial positions at 1/10, 1/4, 1/2 of the whole reactor length and at the exit. It can be seen that the non-uniformity diminishes along the reactor direction, obviously due to radial gas dispersion.
R. Deng et al. / Chemical Engineering and Processing 41 (2002) 259–266
263
riser and downer is more remarkable, the gap of gasoline weight fraction between the wall and the center is about 12% in the riser at the exit, while it is less than 1% in the downer. According to Arena [26], the lateral mixing in CFB reactors of industrial dimensions is relatively poor (Per is about 200), which means that the product profiles will be mainly determined by the reaction rates related
Fig. 7. Radial distribution of gasoline yield along the reactor length under different Per in the riser.
In the case of low radial Peclet number (such as Per =10), the radial non-uniformity of gasoline concentration is quite small in both reactors. When Per reaches 200, the gasoline is significantly uneven in the radial dimension at the inlet region of both reactors, but such unevenness tends to diminish in the rear section in the downer while it remains in the riser. If Per is up to 1000, the difference of radial non-uniformity between
Fig. 8. Radial distribution of gasoline yield along the reactor length under different Per in the downer.
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R. Deng et al. / Chemical Engineering and Processing 41 (2002) 259–266
Fig. 9. Gasoline yield at different Per in the riser and downer.
directly to the radial hydrodynamics. Although the radial mixing is comparable, the uniform radial flow structure in downer ensures uniform radial products profiles and the radial dispersion reduces the minor unevenness to a great extent, while the limited radial dispersion cannot eliminate the great non-uniformity caused by the core-annulus flow structure in the riser. If the radial dispersion were excellent (for example, Per = 10), the radial distribution of products in the riser could also be improved greatly.
4. The results of the non-uniformity
4.1. Feed con6ersion and gasoline yield at different Per The gas radial Peclet number varies with the operation conditions over a limited range (Per 100 – 400). The variation in radial dispersion behavior will affect the gasoline yield of the FCC process, as shown in Fig. 9. It can be seen that Per affects the riser and downer to a different extent. In the downer, the gasoline yield changes little with the radial Peclet number, while it reduces from 36.2 to 34.2 wt.% in the riser when Per increases from 100 to 400. For the downer reactor, it means that the gasoline yield will not reduce apparently, even for a severe radial dispersion resistance under some operating conditions, which is sure to be an advantage for the application of downer. However, maintaining excellent radial dispersion is very important for risers if high gasoline yield is desired, and it will be effective to adopt some techniques to improve the radial dispersion, such as the introduction of internals in the risers.
4.2. The product concentration profiles in riser and downer The gasoline profiles in both riser and downer are shown in Figs. 10 and 11, respectively. It can be seen that there are great differences between the distributions of gasoline concentration in the two reactors. In the axial dimension, the gasoline rises sharply at the inlet section and then slows down in the riser, yet in the downer it goes up less steeply but reaches a higher value than that in the riser at the exit. The most significant differences lie in the radial dimension, where the radial non-uniformity in the riser is much higher than in the downer. Even at the outlet, the unevenness is obvious in the riser, but there is almost a flat radial gasoline distribution after about 15 m from the inlet to the downer.
Fig. 10. Gasoline profiles in the riser reactor.
R. Deng et al. / Chemical Engineering and Processing 41 (2002) 259–266
Fig. 11. Gasoline profiles in the downer reactor.
4.3. The comparison between the results of different models The two-dimensional dispersion model can be changed into a one-dimensional model if the radial non-uniformity in flow structure is ignored, or a plug flow model if the axial dispersion is also neglected. Comparison between the simulation results calculated from different models and the data obtained from a commercial riser in Shengli Petrochemical Company can give us a more comprehensive understanding of the reactors and the FCC process, as shown in Table 1. At the same average solids concentration, the feed conversion and products yields are almost the same in the two reactors when the plug flow model is adopted. By taking the axial backmixing into consideration in the one-dimensional model, the conversion and gasoline yield drop about 1.0 and 0.6 wt.%, respectively, in the downer, while they drop about 7.9 and 4.2 wt.%, respectively, in the riser. Furthermore, if the radial non-uniformity is considered, the conversion in the downer drops 0.1 wt.% and the gasoline drops about 0.25 wt.%, while the corresponding reduction in the riser is about 1.9 and 2.1 wt.%, respectively. It can be seen that the calculated data from the two-dimensional model is close to that from the commercial unit.
265
From the data above, it can be concluded that both aspects, low axial backmixing and uniform radial flow structure, contribute to the higher gasoline yield in the downer, although the former seems to play a more important role. The data also show that the existence of axial backmixing and radial non-uniformity also damage the selectivity of gasoline. The simulation results for the riser and downer in the FCC process with a two-dimensional dispersion model show that there is a gap of 5.5 wt.% in the gasoline yield between the two reactors, consistent with the experimental results of Kauff [27] that the gasoline yield in the downer is 6.6% (vol.%) higher than in the riser. Considering the enormous scale of the FCC industry, the benefit of the downer is impressive. However, it should also be mentioned that there are still obstacles in the commercial application of downer reactors. One of the main shortcomings is that the solids concentration in the downer is lower than in the riser. To reach the same conversion of feed, a higher solids reflux may be necessary. At the same time, a higher temperature and a novel catalyst with higher activity will also be helpful to solve the problem. 5. Conclusions The radial behavior of riser and downer in the FCC process are compared in this paper. The radial profiles of products in the downer are more uniform than those in the riser, which are mainly determined by the radial hydrodynamics and radial mixing behavior. The flatter radial flow structure, together with the less axial gas backmixing, leads to about 5.5 wt.% higher gasoline yield in the downer than that in the riser. It shows that the downer is an ideal reactor to replace the riser currently used in the FCC process. Appendix A. Nomenclature Fr H L n Pea
Froude number, dimensionless reaction height (m) axial position (m) index (dimensionless) gas axial Peclet number (dimensionless)
Table 1 Comparison of the results from different models (unit: wt. fraction) Reactor
Downer
Riser
Commercial riser
Model
Plug flow
1-dimension
2-dimension
Plug flow
1-dimension
2-dimension
From Shengli Company
Conversion Gasoline Gas Coke
0.8012 0.4168 0.3457 0.0387
0.7912 0.4106 0.3424 0.0382
0.7901 0.4081 0.3439 0.0382
0.8014 0.4166 0.3461 0.0387
0.7239 0.3741 0.3148 0.0350
0.7052 0.3532 0.3188 0.0341
0.7123 0.3578 0.3133 0.0412
R. Deng et al. / Chemical Engineering and Processing 41 (2002) 259–266
266
Per Pers r (−ri ) R Re S2 Ug Ug, av Vp yi y¯ i |2
gas radial Peclet number (dimensionless) particle radial Peclet number (dimensionless) radial position (m) formation rate of i (dyi /dt) reactor radius (m) Reynold number (dimensionless) non-uniformity index (wt. fraction) local gas velocity (m/s) average gas velocity across section (m/s) particle velocity (m) yield of product i (wt. fraction) average yield of i across section (wt. fraction) non-uniformity index (dimensionless)
Greek letters m¯ average voidage across section, dimensionless voidage (dimensionless) m voidage (dimensionless) radial ratio (r/R)
References [1] J. Zhu, Z. Yu, Y. Jin, J.R. Grace, A. Issangya, Can. J. Chem. Eng. 73 (1995) 662 –677. [2] J. Zhu, F. Wei, in: Preprints of Fluidization VIII Tours, France, 1995, pp. 907 –915. [3] Z. Wang, D. Bai, Y. Jin, Powder Technol. 70 (1992) 271 – 275. [4] F. Wei, X. Ran, R. Zhou, G. Luo, Y. Jin, Ind. Eng. Chem. Res. 33 (1997) 5049 –5053. [5] C. Deroum, D. Nevicato, M. Forissier, G. Wild, J.R. Bernard, Ind. Eng. Chem. Res. 36 (1997) 4504 –4515. [6] Y. Yang, Y. Jin, Z. Yu, H. Bi, AIChE Symp. Ser. 89 (296) (1993) 81 – 90.
[7] C. Cao, Y. Jin, Z. Yu, Z. Wang, in: A.A. Avidan (Ed.), Circulating Fluidized Bed Technology IV, AIChE, New York, 1994, pp. 406 – 413. [8] F. Wei, H. Lin, G. Yang, Z. Wang, Y. Jin, in: Y. Jin, et al. (Eds.), Sixth China-Japan Symposium on Fluidization, Science Press, Beijing, China, 1997, pp. 121 – 126. [9] D. Bai, Y. Jin, Z. Yu, N. Gan, J. Chem. Ind. Eng. China (English Edition) 6 (2) (1991) 171 – 181. [10] Y. Tung, J. Li, M. Kwauk, in: M. Kwuak, D. Kunii (Eds.), Fluidization, Science Press, Beijing, 1988, pp. 139 – 145. [11] W. Zhang, Y. Tung, F. Johnsson, Chem. Eng. Sci. 46 (12) (1991) 3045 – 3052. [12] Y. Yang, Y. Jin, Z. Yu, Z. Wang, in: M. Kwauk, M. Hasatani (Eds.), Fluidization’91: Science and Technology, Science Press, Beijing, 1991, pp. 66 – 75. [13] H. Zhang, J.-X. Zhu, M.A. Bergougnou, Chem. Eng. Sci. 54 (1999) 5461 – 5470. [14] P. Lehner, K.-E. Wirth, Chem. Eng. Sci. 54 (1999) 5471 –5483. [15] P. Lehner, K.-E. Wirth, Can. J. Chem. Eng. 77 (1999) 199 –206. [16] D. Bai, Y. Jin, Z. Yu, N. Gan, in: P. Basu, M. Horio, M. Hasatani (Eds.), Circulating Fluidized Bed Technology III, Pergamon Press, Toronto, 1991, pp. 157 – 162. [17] H. Zhang, J.-X. Zhu, M.A. Bergougnou, Can. J. Chem. Eng. 77 (1999) 194 – 198. [18] F. Wei, J. Zhu, Chem. Eng. J. 64 (1996) 345 – 352. [19] R. Koenigsdorff, J. Werther, Powder Technol. 82 (1995) 317 – 329. [20] F. Wei, Z. Wang, Y. Jin, Z. Yu, W. Chen, Powder Technol. 81 (1994) 25 – 31. [21] F.V. Landeghem, D. Nevicato, I. Pitaullt, M. Forissier, P. Turlier, C. Deroum, J.R. Bernard, Applied Catalysis A: General 138 (1996) 381 – 405. [22] F. Wei, Y. Jin, Z. Yu, J. Liu, Chem. Eng. Technol. 18 (1995) 59 – 62. [23] V.W. Weekman, Ind. Eng. Chem. Proccess Design Dev. 8 (3) (1969) 385 – 391. [24] V.W. Weekman Jr, Ind. Eng. Chem. Process Design Dev. 8 (3) (1969) 385 – 391. [25] A. Gianetto, H. Faraq, A. Blasetti, H.I. de Lasa, Ind. Eng. Chem. Res. 33 (1994) 3053. [26] U. Arena, in: J.R. Grace, A.A. Avidan, T.M. Knowlton (Eds.), Circulating Fluidized Beds, Blackie Academic & Professional, London, UK, 1997, pp. 101 – 103. [27] D. Kauff, D. Bartholic, C. Steves, M. Keim, NPRA Annual Meeting, 1996
Radial behavior in riser and downer during the FCC process Rensheng Deng, Fei Wei *, Tengfei Liu, Yong Jin Department of Chemical Engineering, Tsinghua Uni6ersity, Beijing 100084, People’s Republic of China Accepted 20 April 2001
Abstract In this paper, the radial behavior of riser and downer during the FCC process is studied on the basis of simulation results from a two-dimensional dispersion model. Although radial uniformity is observed in both reactors, the radial distribution of products is more uniform in the downer than in the riser. At the same feed conversion, about 5.5 wt.% higher gasoline yield is predicted in the downer due to the less axial backmixing and flatter radial distribution of gas– solids velocity and solids concentration. The simulation results also show that the radial gas dispersion plays an important role in determining the radial distribution of products. The profiles of gasoline in both riser and downer are calculated, and the simulation results of different models compared, showing that the consideration of radial non-uniformity is necessary. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Simulation; Non-uniformity; Radial behavior; FCC; Riser; Downer
1. Introduction The invention and application of novel reactors is an important step in the development of the FCC industry. For example, the riser, which was firstly adopted in the 1960s, exhibits a lot of advantages such as higher gasoline yield and easier process control over conventional fluidized reactors. However, the severe axial backmixing of gas and solids, as well as the significant radial non-uniformity in gas velocity, particle velocity and solids concentration, is realized by people more and more [1]. From 1970 to 1980 some well known companies including Stone & Webster, Mobil and Texaco, proposed a novel reactor-downer, which attracted many research interests quickly and was considered to overcome the drawbacks of the upflow riser in the FCC process [2,3]. There are two aspects usually thought to contribute to the higher gasoline in downer than in riser: one is the lower backmixing of gas and solids; the other is the more uniform radial flow structure. The influences of axial gas backmixing on the gasoline yield are examined by Wei [4] with a one-dimensional model. According to * Corresponding author. Tel.: + 86-10-62785464x8214; fax: +8610-62772051. E-mail address: [email protected] (F. Wei).
their research, there is a gap of 5.l wt.% in gasoline yield between riser and downer at the conversion of 62.0 wt.%. However, there are few reports about the different effects of radial hydrodynamics in riser and downer, which may be of great importance to the product profiles. Furthermore, the radial dispersion behavior in the reactors, concerning with the elimination of the non-uniformity at different local positions, should also be taken into consideration. In this paper, a two-dimensional dispersion model is proposed to simulate the FCC process operated in a riser and a downer both of the industrial scale, and the different radial performances in the two types of reactors are discussed on the simulation results. The emphasis is placed on the contributing factors, performances and results of the radial non-uniformity. 2. The contributing factors of radial non-uniformity
2.1. Radial hydrodynamics of gas and solids in riser and downer The radial hydrodynamics is mainly due to the lateral distribution of gas velocity, particle velocity and solids concentration, which is different in riser and downer. The local gas velocity in the riser can be represented by an Ostwald-de Waele type equation:
0255-2701/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 5 - 2 7 0 1 ( 0 1 ) 0 0 1 4 0 - 4
260
Ug(r)= Ug,av
R. Deng et al. / Chemical Engineering and Processing 41 (2002) 259–266
3n+ 1 n+1
1−
r R
n + 1/n
(1)
Deroum [5] suggested that the index n could be extrapolated to 2.0 for commercial risers. The radial profile of gas velocity in the downer is rather flat compared with that in the empty tube or the riser, as shown in Fig. 1 [6,7]. By correlating the data measured by Cao [7], the ratio of local gas velocity to the cross-section averaged value can be expressed as a function of radial position:
n
Ug 1− r =2 Ug,av R
1/7
exp −0.8
1−r R
0.4
(2)
Yang studied the particle velocity profiles in the dilute phase of the riser by using LDV [6]. More exciting results were obtained by Wei in a high-density riser (solids fraction up to 0.25), which is close to the operating conditions in commercial plants [8]. The radial distribution of particle velocity in the downer is found to be more uniform as shown in Fig. 2 [6,7]. Bai obtained the following equation for the ratio of local velocity to the cross-section averaged value [9]: Vp =6.309F 0.97 m f() r Ug,av
(3)
where f() = − 2.246− 5.086 + 18.262 2 −13.199 3
(4)
By extending the work of Tung [10], Zhang improved the method for calibrating the optical-fiber probe and proposed the following empirical equation to predict the solids concentration in the riser on the basis of their experimental data [11]:
Fig. 2. The radial distribution of particle velocity in downer and riser [6,7].
probe [12], and the results were compared with those from the riser in Fig. 3 [3]. Zhang proved the results in a wider range of axial locations and operating conditions [13], and Lehner found a similar distribution for spherical glass beads with an X-ray computed tomography system [14,15]. An empirical equation for the radial distribution of voidage was provided by Bai [16]: m= m¯ F()
(6)
where F()= 30.62(1− )exp[− 127.6(1− 2)] +
22.8 36.7+(1−)
(7)
The radial solids concentration of FCC particles in the downer was measured by Yang with a fiber optic
In summary, the radial flow structure in risers is typically described by a core-annulus model. Across the cross-section of the reactor, there exist two zones: a central core in which gas velocity and particle velocity are high while the particle concentration and flux are low, and a peripheral annulus, where concentrated
Fig. 1. The radial distribution of gas velocity in downer, riser and empty tube [6,7].
Fig. 3. The radial distribution of solids concentration in downer and riser [3].
2.5 + 311)
m= m¯ (0.191 +
(5)
R. Deng et al. / Chemical Engineering and Processing 41 (2002) 259–266
solids flow down with a gas velocity close to zero. For the downer, the radial flow structure can be characterized by three regions: a dilute core region (05 r/RB 0.85), where the local gas velocity, particle velocity and solids concentration are rather uniform; a relative dense annular region (0.855r/R 50.96), where all three variables have their maxima; a wall region (r/R ] 0.96), where all these variables decrease towards the wall [1,17]. From the discussion above, it can be concluded that the radial flow structure in the downer is much more uniform than that in the riser. The reduction of nonuniformity in the downer reduces the backmixing in the reactor and, therefore, benefits the gasoline yield in the FCC process greatly.
2.2. Radial mixing beha6ior in riser and downer Mixing behavior is quite important for heat transfer and mass transfer. The axial dispersion will lead to a broader residence time distribution (RTD) and consequently reduce the gasoline yield, while the radial dispersion can smoothen the non-uniformity at different radial positions and consequently benefit the gasoline yield. According to Wei [4], the axial gas Peclet number in the riser is typically about 4, while in the downer it can reach 100. Wei [18] also measured the axial solids dispersion with the phosphorescent particle tracer technique in both riser and downer and pointed out that the axial solids Peclet number is around 100 in the downer while about 3–4 in the riser. Koenigsdorff [19] stated that the radial Peclet number of solids in the riser ranges from 100 to 300. The radial solids dispersion in the downer was studied by Wei [20] and the dispersion coefficient Dr,p ranging from 0.0026 to 0.0113 m2/s was reported, with the corresponding Pelcet number from 70 to 300. However, according to the work of Deroum [5] and Landeghem [21], the radial catalyst mixing can be neglected for kinetic reasons, only the catalyst hold-up distribution is important for the modeling in this situation. Wei [22] studied the radial gas mixing with continuous tracer point injection. The radial gas dispersion coefficients they measured in the downer (0.0015–0.003 m2/s) are comparable with the one in the riser measured by Adams [23] (0.0005– 0.0030 m2/s), and changes little with the operating conditions. They also found that the Per ranges from 100 to 300, of the same order of magnitude as in the riser. It can be concluded that the greatly reduced axial gas and solids backmixing in the downer will benefit the conversion and selectivity of such rapid reactions like FCC, while the excellent radial gas and solids dispersion can provide the downer with excellent heat and mass transfer ability across the section. As a result, the
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gas–solids flow in the downer will be much closer to plug flow than in the riser, and so higher gasoline yield can be expected at the same operating conditions. In this paper, the radial Peclet number is typically set at 200 both for downer and riser, while the effects of different Peclet number are also examined.
3. The effect of the radial non-uniformity
3.1. Model building Besides the hydrodynamics and mixing behavior, the reaction kinetics are also important for the development of the reactor model. Great progress has been made in the reaction kinetic models for FCC in recent years, among which the lump kinetics models firstly developed by Weekman [24] are the most successful and attractive. As a preliminary attempt, the four-lump reaction kinetic model proposed by Gianetto [25] is adopted here, with reaction network shown in Fig. 4. The gas oil will be cracked into gasoline, gas and coke, then the gasoline can be further converted into gas and coke, but gas and coke will never turn into other products. By applying the mass balance to an infinitesimal unit in the reactor, the following equation is obtained:
R 1 ( (yi H ( 2yi (yi 1 r + − + (− ri )= 0 2 Per r (r (r Pea (L (L Ug The boundary conditions are: inlet (L= 0):
yi(0−) = yi(0+) −
outlet (L= H): center (r=0): wall (r=1):
1 (yi Pea (L
(yi =0 (L (yi =0 (r
(yi =0 (r
(8)
(9)
0+
(10) (11) (12)
It should be mentioned that the equations above are valid for both riser and downer, while the ordinates in
Fig. 4. Kinetic model of FCC.
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It can be seen that there is significant non-uniformity in both riser and downer. The products gasoline, gas and coke are higher near the wall than in the center, while the distribution of the feed gas oil has the opposite trend. Comparing with the downer, the non-uniformity in the riser is far more significant. The gap of gasoline concentration between the wall and the center is about 15 wt.% in the riser but only 5 wt.% in the downer. To evaluate the non-uniformity more precisely, a non-uniformity index similar to the indices adopted by Bai [16] for radial solids concentration profile and radial particle velocity profile is defined for products S 2(gasoline)= 2
&
1
(ygasoline − y¯ gasoline)2d
(13)
0
Fig. 5. Radial distribution of products at X= 4.5 m in the riser.
the vertical dimension are in the opposite direction. The equations are numerically calculated with an orthogonal collocation method, which can provide faster convergence speed and higher accuracy than finite difference methods. The reactor simulated is a commercial plant with a height of 45 m and a diameter of 1 m. The reaction is supposed to occur at 550°C. The average solids concentration is set at 47.8 kg/m3, and the inlet gas velocity is 6.0 m/s.
3.2. The non-uniformity in radial products distribution The distribution of the radial products in riser and downer are shown in Figs. 5 and 6, respectively. The axial location is at 4.5 m, one-tenth of the whole reactor height.
From the data in Figs. 5 and 6, the indices for riser and downer are 0.00223 and 0.000256, respectively. While considering that the average gasoline concentration is different in riser and downer, the dimensionless nonuniformity index is more important S 2(gasoline) | 2(gasoline)= (14) (y¯ gasoline)2 The corresponding values for riser and downer are 0.03146 and 0.00218, respectively. The former is about 15 times of the latter, which shows that the non-uniformity is more severe in riser than in downer. The analysis of indices for other products follows the same rules. The difference of radial product distribution between the riser and the downer is due to the unlike radial hydrodynamics in these two reactors. In the core section of the riser where solids are dilute and gas velocity is high, the reaction rate is predicted to be much lower than in the annulus section, with high solids concentration and low gas velocity, and hence significant nonuniformity of products profiles. The more uniform radial flow structure of both gas and particle phases in the downer leads to a more uniform radial distribution of products.
3.3. The effects of radial dispersion
Fig. 6. Radial distribution of products at X= 4.5 m in the downer.
Due to the radial flow structure and the reaction kinetics, the radial non-uniformity of products stated above is inevitable. However, it is the existence of radial dispersion that prevents such non-uniformity in the FCC reactors from accumulating and resulting in broader residence time distribution and more severe backmixing. Figs. 7 and 8 show the radial gasoline profiles at different radial gas Peclet number in the riser and the downer, respectively. The curves in the figures correspond to axial positions at 1/10, 1/4, 1/2 of the whole reactor length and at the exit. It can be seen that the non-uniformity diminishes along the reactor direction, obviously due to radial gas dispersion.
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riser and downer is more remarkable, the gap of gasoline weight fraction between the wall and the center is about 12% in the riser at the exit, while it is less than 1% in the downer. According to Arena [26], the lateral mixing in CFB reactors of industrial dimensions is relatively poor (Per is about 200), which means that the product profiles will be mainly determined by the reaction rates related
Fig. 7. Radial distribution of gasoline yield along the reactor length under different Per in the riser.
In the case of low radial Peclet number (such as Per =10), the radial non-uniformity of gasoline concentration is quite small in both reactors. When Per reaches 200, the gasoline is significantly uneven in the radial dimension at the inlet region of both reactors, but such unevenness tends to diminish in the rear section in the downer while it remains in the riser. If Per is up to 1000, the difference of radial non-uniformity between
Fig. 8. Radial distribution of gasoline yield along the reactor length under different Per in the downer.
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Fig. 9. Gasoline yield at different Per in the riser and downer.
directly to the radial hydrodynamics. Although the radial mixing is comparable, the uniform radial flow structure in downer ensures uniform radial products profiles and the radial dispersion reduces the minor unevenness to a great extent, while the limited radial dispersion cannot eliminate the great non-uniformity caused by the core-annulus flow structure in the riser. If the radial dispersion were excellent (for example, Per = 10), the radial distribution of products in the riser could also be improved greatly.
4. The results of the non-uniformity
4.1. Feed con6ersion and gasoline yield at different Per The gas radial Peclet number varies with the operation conditions over a limited range (Per 100 – 400). The variation in radial dispersion behavior will affect the gasoline yield of the FCC process, as shown in Fig. 9. It can be seen that Per affects the riser and downer to a different extent. In the downer, the gasoline yield changes little with the radial Peclet number, while it reduces from 36.2 to 34.2 wt.% in the riser when Per increases from 100 to 400. For the downer reactor, it means that the gasoline yield will not reduce apparently, even for a severe radial dispersion resistance under some operating conditions, which is sure to be an advantage for the application of downer. However, maintaining excellent radial dispersion is very important for risers if high gasoline yield is desired, and it will be effective to adopt some techniques to improve the radial dispersion, such as the introduction of internals in the risers.
4.2. The product concentration profiles in riser and downer The gasoline profiles in both riser and downer are shown in Figs. 10 and 11, respectively. It can be seen that there are great differences between the distributions of gasoline concentration in the two reactors. In the axial dimension, the gasoline rises sharply at the inlet section and then slows down in the riser, yet in the downer it goes up less steeply but reaches a higher value than that in the riser at the exit. The most significant differences lie in the radial dimension, where the radial non-uniformity in the riser is much higher than in the downer. Even at the outlet, the unevenness is obvious in the riser, but there is almost a flat radial gasoline distribution after about 15 m from the inlet to the downer.
Fig. 10. Gasoline profiles in the riser reactor.
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Fig. 11. Gasoline profiles in the downer reactor.
4.3. The comparison between the results of different models The two-dimensional dispersion model can be changed into a one-dimensional model if the radial non-uniformity in flow structure is ignored, or a plug flow model if the axial dispersion is also neglected. Comparison between the simulation results calculated from different models and the data obtained from a commercial riser in Shengli Petrochemical Company can give us a more comprehensive understanding of the reactors and the FCC process, as shown in Table 1. At the same average solids concentration, the feed conversion and products yields are almost the same in the two reactors when the plug flow model is adopted. By taking the axial backmixing into consideration in the one-dimensional model, the conversion and gasoline yield drop about 1.0 and 0.6 wt.%, respectively, in the downer, while they drop about 7.9 and 4.2 wt.%, respectively, in the riser. Furthermore, if the radial non-uniformity is considered, the conversion in the downer drops 0.1 wt.% and the gasoline drops about 0.25 wt.%, while the corresponding reduction in the riser is about 1.9 and 2.1 wt.%, respectively. It can be seen that the calculated data from the two-dimensional model is close to that from the commercial unit.
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From the data above, it can be concluded that both aspects, low axial backmixing and uniform radial flow structure, contribute to the higher gasoline yield in the downer, although the former seems to play a more important role. The data also show that the existence of axial backmixing and radial non-uniformity also damage the selectivity of gasoline. The simulation results for the riser and downer in the FCC process with a two-dimensional dispersion model show that there is a gap of 5.5 wt.% in the gasoline yield between the two reactors, consistent with the experimental results of Kauff [27] that the gasoline yield in the downer is 6.6% (vol.%) higher than in the riser. Considering the enormous scale of the FCC industry, the benefit of the downer is impressive. However, it should also be mentioned that there are still obstacles in the commercial application of downer reactors. One of the main shortcomings is that the solids concentration in the downer is lower than in the riser. To reach the same conversion of feed, a higher solids reflux may be necessary. At the same time, a higher temperature and a novel catalyst with higher activity will also be helpful to solve the problem. 5. Conclusions The radial behavior of riser and downer in the FCC process are compared in this paper. The radial profiles of products in the downer are more uniform than those in the riser, which are mainly determined by the radial hydrodynamics and radial mixing behavior. The flatter radial flow structure, together with the less axial gas backmixing, leads to about 5.5 wt.% higher gasoline yield in the downer than that in the riser. It shows that the downer is an ideal reactor to replace the riser currently used in the FCC process. Appendix A. Nomenclature Fr H L n Pea
Froude number, dimensionless reaction height (m) axial position (m) index (dimensionless) gas axial Peclet number (dimensionless)
Table 1 Comparison of the results from different models (unit: wt. fraction) Reactor
Downer
Riser
Commercial riser
Model
Plug flow
1-dimension
2-dimension
Plug flow
1-dimension
2-dimension
From Shengli Company
Conversion Gasoline Gas Coke
0.8012 0.4168 0.3457 0.0387
0.7912 0.4106 0.3424 0.0382
0.7901 0.4081 0.3439 0.0382
0.8014 0.4166 0.3461 0.0387
0.7239 0.3741 0.3148 0.0350
0.7052 0.3532 0.3188 0.0341
0.7123 0.3578 0.3133 0.0412
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Per Pers r (−ri ) R Re S2 Ug Ug, av Vp yi y¯ i |2
gas radial Peclet number (dimensionless) particle radial Peclet number (dimensionless) radial position (m) formation rate of i (dyi /dt) reactor radius (m) Reynold number (dimensionless) non-uniformity index (wt. fraction) local gas velocity (m/s) average gas velocity across section (m/s) particle velocity (m) yield of product i (wt. fraction) average yield of i across section (wt. fraction) non-uniformity index (dimensionless)
Greek letters m¯ average voidage across section, dimensionless voidage (dimensionless) m voidage (dimensionless) radial ratio (r/R)
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