Ind. Eng. Chem. Res. 1997, 36, 4543-4548
4543
Novel Designs and Simulations of FCC Riser Regeneration D. Bai,† J.-X. Zhu,*,‡ Y. Jin,† and Z. Yu† Department of Chemical Engineering, Tsinghua University, Beijing 100084, China, and Department of Chemical and Biochemical Engineering, University of Western Ontario, London, Ontario, Canada N6A 5B9
Two novel designs of FCC riser regeneration are proposed in this work. To improve the regeneration performance of a single riser regenerator, the first design allows the regenerating air to be separately supplied at several different levels in the riser. Simulations based on the riser regeneration model previously developed by us show that this method provides good operation performance with improved flexibility and stability, higher solids inventory, and longer solids residence time. By properly adjusting the air flow rates and the regenerated catalyst recycle ratio, this multiple air supply riser regenerator can meet the specifications of most industrial units: reducing the carbon content on the regenerated catalyst to less than 0.1 wt % and controlling the riser temperature under 730 °C. The second design is a two-stage riser FCC regenerator, which connects two riser regenerators in series. This novel design combines the advantages of both the riser regeneration and the conventional two-stage turbulent bed regeneration. It also provides significant advantages over the single riser regenerator: operating the second riser at high temperature without catalyst hydrothermal deactivation and greatly increased regeneration efficiency and operation flexibility. Both novel regeneration technologies have now been patented in China and are being incorporated in the design of the FCCU of the SINOPEC Changling Refinery. 1. Introduction FCC (fluid catalytic cracking) riser regeneration is an innovative technology for catalyst regeneration. It is becoming a key part of modern FCC technology, especially for heavy oil processing (e.g., Avidan and Shinnar, 1990; Avidan et al., 1990; Chen et al., 1994; Squires et al., 1985). Organized by SINOPEC and in cooperation with Beijing Design Institute of SINOPEC, the Fluidization Laboratory of Tsinghua University (FLOTU) has carried out a series of studies on the simulation of riser regenerators (Bai et al., 1991; Gan et al., 1992). The first objective of these studies was to design a pilot riser regenerator with a capacity of 6000 t/a in the Changling Refinery of Baling Petrochemical Corporation. The design was completed in 1993. Initial operation experience has shown that the riser regenerator offers considerable advantages over the conventional turbulent fluidized bed regenerator with high combustion efficiency and intensity. However, both the simulation results and the initial operation experience also revealed some shortcomings of the single riser regenerator. For example, a long riser (up to 30 m) is required, which leads to notable heat loss and increased capital investment. In addition, temperature control becomes difficult. With the large amount of heat generation, a lower entrance temperature is preferred, but too low an entrance temperature will quench the operation. On the other hand, a relatively high entrance temperature will lead to a high temperature at the top of the riser, which will possibly result in the hydrothermal deactivation of catalyst due to steaming from hydrogen combustion. In addition, a previous simulation has also indicated that a single riser regenerator lacks the necessary flexibility and stability in its operation. For example, the operating air velocity is very restricted since a low air flow rate cannot supply enough oxygen * To whom correspondence should be addressed. Phone: (519) 661-3807. Fax: (519) 661-3498. E-mail: jzhu@ julian.uwo.ca. † Tsinghua University. ‡ University of Western Ontario. S0888-5885(97)00126-7 CCC: $14.00
for the regeneration, but a high air flow rate tends to quench the operation at the riser bottom. To overcome the above shortcomings of the single riser regenerator, two novel design concepts are proposed and examined in this study. The first design concept is to split the necessary air flows and to supply them at different levels along a single riser regenerator. This allows the riser to be operated at relatively high gas velocities with enough oxygen supply for the regeneration reactions in most parts of the riser but not too much to cause quenching at the riser bottom. This can also significantly increase the solids inventory in the riser, benefiting the reactions. The second design concept is a two-stage riser regenerator which allows the second riser regenerator to operate at higher temperatures to achieve the high capacity but maintains the first riser at a moderate temperature to prevent the hydrothermal deactivation of the catalyst. An added benefit of this two-stage riser regeneration is that it facilitates the addition of heat exchanger between the two stages. This design combines the advantages of both the high-capacity riser regeneration and the staged operation of the conventional two-stage regeneration technology and is expected to play an important role in the development of residual FCC technology. Both novel regeneration technologies have now been patented in China and are being incorporated in the design of the FCCU of the SINOPEC Changling Refinery. 2. The Previously Developed Riser Regeneration Model and Its Validation For the design and development of FCC catalyst riser regenerators, a one-dimensional model was previously developed by Bai et al. (1997). This model consists of two main parts: the hydrodynamic model for the fast fluidized bed (with adjustable solids mixing patterns) and the kinetic model for the catalyst regeneration. In the hydrodynamic model, the gas is treated as plug flow and the solids are assumed to be either plug flow or complete mixing flow. According to the catalyst regeneration characteristics, one can reasonably assume that © 1997 American Chemical Society
4544 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
Figure 1. Axial voidage profiles in a typical riser for different gas velocities (Bai et al., 1993).
the change in the gas mole flow rate during the regeneration reaction is insignificant and is neglected. The specific heat capacity of the catalyst is taken as the same as that of the coke, and the pressure in the regenerator is taken as constant given the low pressure drop across the riser. Simulations using this model show excellent agreement with the operation of a 5 × 104 t/a, 8-m riser regenerator in the Dagang Refinery on the east coast of China. 3. Riser Regenerator with Multiple Air Supplies
Figure 2. Illustration of the design concepts of the multiple air supply riser regenerator.
3.1. Design Concept and Considerations. As discussed above, the main factors affecting the catalyst regeneration in the riser regenerator are the gas velocity and temperature. For the benefit of high-efficiency coke combustion, high catalyst inventory and high temperatures are desirable. The appropriate gas velocity should ensure the riser to operate at a stable fast fluidization regime, supply enough oxygen to the reactions, and maintain a reasonably high solids inventory. However, the solids inventory is inversely proportional to the air supply rate, so it is difficult to maintain a proper balance. As conceptually shown in Figure 1, hydrodynamic studies for risers have indicated that the solids inventory in the bed, which is proportional to the solids holdup (1 - ), commonly increases with decreasing gas velocity for a given solids circulation rate. A low gas velocity (curve OA) would lead to high solids inventory but may not be able to provide enough oxygen necessary for the reactions. This will limit the reaction efficiency and lower the reaction temperature due to the limited amount of heat generation. The lower temperature, in turn, will further slow down the reaction and lead to a higher coke content on the regenerated catalyst. On the other hand, a high gas velocity will significantly reduce the solids inventory (curves BC and DE), which will not only decrease the contact time between gas and solids but also lower the mixture temperature at the riser entrance, which can possibly lead to the quenching of the reaction at the riser bottom. In order to solve the problem, a new design is proposed based on our understanding of the hydrodynamics of fluidized bed risers. In this design, the gas supplied to a single riser regenerator is divided into several streams, as shown in Figure 2 (with three air supplies). A portion of the gas with a velocity Ug1 is introduced into the riser from the bottom. The value of Ug1 is selected such that the bed is operated just above the transition point from turbulent to fast fluidization to obtain the maximum solids concentration in the lower part of the riser (curve OA in Figure 1). When the gas meets with the spent catalyst from the FCC reactor and
the recycled regenerated catalyst from the riser top, the low gas flow rate in the first section ensures a reasonably high mixture temperature at the entrance. Combined with the high solids concentration in this section, the regeneration reaction is greatly enhanced. Above the first section, a secondary air is supplied to the riser at a level where the oxygen supplied by the primary air is nearly consumed (e.g., less than 2-4 mol %) in order to continue the regeneration. At this point, the gas velocity increases to Ug2 and the solids holdup decreases (curve BC in Figure 1). Because of the exothermic reaction, the catalyst temperature in the first section has risen up to such a level that mixing with a small amount of cold gas at the secondary air injection point will not significantly change the bed temperature. In the second section, the relatively high temperature and the newly added fresh oxygen further stimulate the reactions to a higher degree of conversion. In the third section (if it is still necessary), a tertiary air supply is added at a higher axial position where the oxygen is about to be completely consumed. This brings the reactions to an oxygen-rich atmosphere again and is especially beneficial for the combustion of the hardburning coke contained within the catalyst pores. This new design with multiple air supply offers many significant advantages over the single riser regenerator: (1) higher solids inventory and longer solids residence time (which leads to a shorter riser); (2) higher temperatures at the entrance; (3) improved gas-solids contacting; and (4) efficient distribution of air (oxygen). As a result, the performance of the riser regenerator is expected to be significantly improved by this type of multiple air supply arrangement. Simulation is carried out for this multiple air supply operation using the model developed by Bai et al. (1997). The simulation conditions are the following: riser height, 30 m; riser diameter, 0.3 m; catalyst, CRC-1; spent catalyst feed rate, 30 kg/(m2‚s); H/C ratio on the spent catalyst, 0.089; air temperature, 40 °C; spent catalyst temperature, 490 °C; reaction pressure, 253.25 kPa.
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4545 Table 2. Comparison of Regenerator Performance between Single (Ug ) 3.0 m/s), Double (Equivalent Gas Flow Rates of Ug ) 2.0 and 1.0 m/s Introduced at x ) 0 and 15 m), and Triple (Equivalent Gas Flow Rates of Ug ) 2.0, 0.5, and 0.5 m/s Introduced at x ) 0, 10, and 20 m) Air Supply Systems (a ) 2, Cs ) 1.00 wt %) air supply catalyst inventory, kg mean catalyst residence time, s temp at riser entrance, °C temp at riser exit, °C C content on regenerated catalyst, wt % O2 concn in flue gas, mol %
single double
triple
112.1 17.6 595.4 687.9 0.2280 2.03
164.1 25.8 610.8 699.3 0.1902 1.23
154.0 24.2 609.2 697.0 0.1978 1.39
Table 3. Comparison of Regenerator Performance between Single (Ug ) 4.2 m/s), Double (Equivalent Gas Flow Rates of Ug ) 3.0 and 1.4 m/s Introduced at x ) 0 and 15 m), and Triple (Equivalent Gas Flow Rates of Ug ) 3.0, 1.0, and 0.5 m/s Introduced at x ) 0, 10, and 20 m) Air Supply Systems (a ) 2, Cs ) 1.20 wt % and with Constant Exit Oxygen Concentration at Approximately 3 mol %) air supply catalyst inventory, kg mean catalyst residence time, s temp at riser entrance, °C temp at riser exit, °C total gas flow rate, m/s C content on regenerated catalyst, wt % O2 concn in flue gas, mol %
Figure 3. Regeneration performance comparison between single (cruve A), double (curve B), and triple (curve C) air supply systems (see Table 2 for detailed conditions). Table 1. Comparison of Regenerator Performance between Single (Ug ) 4.5 m/s), Double (Equivalent Gas Flow Rates of Ug ) 3.0 and 1.5 m/s Introduced at x ) 0 and 15 m), and Triple (Equivalent Gas Flow Rates of Ug ) 3.0, 1.0, and 0.5 m/s Introduced at x ) 0, 10, and 20 m) Air Supply Systems (a ) 2, Cs ) 1.25 wt %) air supply catalyst inventory, kg mean catalyst residence time, s temp at riser entrance, °C temp at riser exit, °C C content on regenerated catalyst, wt % O2 concn in flue gas, mol %
single double
triple
57.5 9.0 633.2 763.3 0.1018 2.7
85.5 13.4 655.1 777.1 0.0509 2.1
80.2 12.6 651.6 772.2 0.0695 2.2
3.2. Simulation Results and Discussion. The typical simulation results for single (at x ) 0 m), double (at x ) 0 and 15 m), and triple (at x ) 0, 10, and 30 m) air supply riser regenerator systems are compared in Figure 3 for a total equivalent gas velocity of 3.0 m/s. The recycle ratio (regenerated/spent catalyst rates, Gsg/ Gss) is fixed at 2. The carbon contents on the spent catalyst are 1.0 wt %. The key simulation results are summarized in Tables 1 and 2. From Figure 3, which corresponds to the results in Table 2, we can see that for systems with double or triple air supplies, the oxygen concentration experiences a sudden “jump” and the temperature suffers a slight drop but quickly regains its level at the secondary and tertiary air supply points. Examining the curves of the oxygen concentration and the carbon content, whose slopes correspond to the reaction rates, one notices that the reaction rates become faster when changing from single to double and eventually to triple air supply operation. This can be
single double
triple
63.4 10.0 625.6 748.1 4.2 0.1302 3.0
87.0 12.7 645.5 762.5 4.5 0.0512 3.0
82.7 13.0 642.8 758.9 4.4 0.0735 3.0
attributed to the increased solids inventory, residence time, and temperature (see Tables 1 and 2). Consequently, the triple air supply system can produce a carbon content as low as 0.05 wt % for the regenerated catalyst, a value desirable for an industrial FCC unit. The single air supplying system is, however, unable to obtain such a low carbon content on the regenerated catalyst under these simulation conditions. In practical operation, it is usually preferable to keep the oxygen concentration constant at the riser exit. In this case, it is necessary to increase the total gas flow rate somewhat with the increased number of air supply points. The computed results for a fixed oxygen concentration at the riser exit (3 mol %) are tabulated in Table 3. It is seen that the solids inventory, solids residence time, and temperature for the multiple air supply systems are still higher than for the single air supply system, even with a slightly increased process capacity. The increased oxygen concentration speeds up the reactions, resulting in good regeneration performance in comparison with a single air supply system. It can also be noted from Tables 1-3 that when the carbon content on the spent catalyst is higher (i.e., g1.2 wt %), the temperature at the riser exit often exceeds 750 °C. Since most FCC catalysts face the danger of hydrothermal deactivation under such a high temperature, this would be a serious problem for RFCC (residual FCC) processes where the coke contents are always high. With the multiple air supply design, the present work shows that the temperature at the riser exit can be lowered to below 730 °C by adjusting the ratios of the gas flow rates at the different levels, while a low carbon content of below 0.1 wt % on the regenerated catalyst can still be obtained. Though an increase in the total gas flow rate is sometimes necessary, a relatively low gas velocity at the riser bottom can still be maintained by adjusting the air flow rate to ensure a higher mixture temperature, in order to reduce the risk of riser quenching due to a low temperature, a benefit resulting from the flexibility of the multiple air supply riser regenerator design.
4546 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 Table 4. Simulation Results with an External Heat Exchanger under Fixed Exit Temperature of 730 °C (Cs ) 1.2 wt %) air supply regenerated catalyst recycle ratio gas velocities, m/s x ) 0-5 m x ) 5-10 m x ) 10-15 m temp at riser exit, °C temp at riser entrance, °C C content on regenerated catalyst, wt % O2 concn in flue gas, mol % heat removed,a °C
single triple triple triple 2
2
3
4
4.0 4.0 4.0 730 613 0.19
3.0 4.0 4.5 730 618 0.11
3.0 4.0 4.5 730 642 0.084
3.0 4.0 4.5 730 660 0.07
2.65 7.9
3.60 11.6
3.24 10.5
3.08 8.8
a Heat removed is expressed in terms of the temperature drop across the external heat exchanger.
Most industrial FCC regenerators are equipped with external heat exchangers for maintaining heat balance. For heavy oil FCC processes, it is usually necessary to control the temperature at the regenerator exit to below 730 °C. This requirement is generally met when the carbon content on the spent catalyst is relatively low (i.e., 1.2 wt %). This can be done by removing heat from the recycled regenerated catalyst. With the temperature at the riser exit fixed at 730 °C, a series of calculations were carried out to simulate this situation. As shown in Table 4, a good regeneration performance with low carbon content (0.1 wt %) on the regenerated catalyst can be obtained by the multiple air supply operation. 4. Two-Stage Riser Regenerator 4.1. Design Concept and Consideration. Another novel design concept is to employ a two-stage riser regenerator to overcome the shortcomings associated with the single riser regenerator such as the long riser and the difficulty in temperature control. This design connects two riser regenerators in series and therefore has the advantages of the riser regeneration and conventional two-stage regeneration. A new patented design of this technology is illustrated in Figure 4 (Jin et al., 1993). The idea is to combust almost all the hydrogen and a portion of the coke under a relatively low temperature (e700 °C) in the first riser regenerator. The low combustion temperature in the first riser can effectively prevent the catalyst hydrothermal deactivation from steaming generated from hydrogen combustion. At the top of the first stage regenerator, solids are separated and then introduced into the bottom of the second riser (a portion of the solids need to be returned to the bottom of the first riser to maintain the inlet temperature for the first riser). For high carbon content spent catalyst, an external heat exchanger may be installed between the two risers to remove the excess heat. In the second riser, the slow-burning residual coke is combusted more effectively under a high temperature (commonly above 800 °C) in a dry environment with excess air. This allows the coke to be burned very thoroughly with a minimum amount of catalyst hydrothermal deactivation. At the top of the second riser, the regenerated catalyst with the required carbon content is separated and then introduced to the FCC reactor (a short contact downer reactor as shown in this design package). Since the temperature at the bottom of the second riser is already high enough after the first-
Figure 4. Newly patented quick-contacting reaction (downer), two-stage riser regeneration system for FCC process (Jin et al., 1993).
stage regeneration, no external solids circulation is necessary for the second riser. Clearly, the common problems associated with single riser FCC catalyst regeneration such as catalyst hydrothermal deactivation and the difficulty in excess heat removal and temperature control can be satisfactorily solved in this twostage riser regeneration system. The regeneration performance of this two-stage riser is simulated using the riser regeneration model previously developed by Bai et al. (1997). In this case, the parameters at the first riser exit are taken as the initial conditions of the second riser. (There would be a temperature change if an external heat exchanger is present.) Simulation conditions are the same as in section 3.1 for multiple air supplies except there are two risers, each 15 m high. 4.2. Simulation Results and Discussion. Figure 5 shows the simulation result when 56 wt % of the coke is combusted in the first-stage regenerator. In this case, the first-stage regenerator combusts approximately 80 wt % of the hydrogen contained in the catalyst. The gas velocity is adjusted so that the oxygen concentration at the exit of the first riser is approximately 2% (Figure 5c). It is seen that a temperature of 665 °C (Figure 5b) and a carbon content of 0.53 wt % (Figure 5a) are achieved at the exit of the first riser. For the second riser, the reactions take place under oxygen-rich atmosphere and higher temperatures. At the riser height of 6 m, the temperature and the carbon content have achieved 750 °C (Figure 5e) and 0.05 wt % (Figure 5d), respectively, and the hydrogen has been completely combusted. This indicates that the second riser can be significantly shorter than 15 m. Table 5 summarizes the simulation results for the three different levels of coke combustion in the first riser. It is seen that with the increase in the amount of coke combustion in the first riser, the temperature increases at the first riser exit but decreases at the second riser exit. The predicted carbon content on the regenerated catalyst can reach as low as 0.0001 wt %, providing very clean catalyst. Apparently, excellent regeneration performance can be achieved by adjusting the amounts of coke combustion between the two risers.
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4547
Figure 5. Axial distributions of carbon content, temperature, and oxygen concentration in the first (a, b, and c; a ) 2.0, Ug ) 2.7 m/s) and second (d, e and f, a ) 0.0, Ug ) 2.7 m/s) risers with 56 wt % coke combustion in the first riser (Cs ) 1.2 wt %). Table 5. Simulation Results of Two-Stage Riser Regenerator (Cs ) 1.2 wt %; First Riser, a ) 2.5, Ug ) 3.2 m/s; Second Riser, a ) 0, Ug ) 2.0 m/s) coke combustion in the first riser, wt % hydrogen combustion in the first riser, wt % at the first riser exit C content, wt % hydrogen concn, wt % O2 concn, wt % temp, °C at the second riser exit C content, wt % hydrogen concn, wt % O2 concn, wt % temp, °C
56 80
70 89
81 94
0.53 0.02 2.0 665
0.36 0.012 1.5 704
0.23 0.006 0.4 728
0.0046 0.0 4.4 767
0.0002 0.0 9.8 753
0.0001 0.0 14.0 737
Note that the exit oxygen concentration given in Table 5 becomes higher with the increasing amount of coke combustion in the first riser, because an identical gas velocity is imposed for all these conditions for easy comparison. A more desirable value of the exit oxygen concentration can be obtained by adjusting the gas velocities between the two riser. It is worthwhile to point out that although 20 wt % of the hydrogen combustion in the second riser seems to be not very significant, it is still possible for the hydrothermal catalyst deactivation to occur. An alternative to overcome this problem is to increase the extent of coke combustion (and hydrogen combustion at the same time) in the first riser. As shown in Table 5, the hydrogen combustion in the first riser is approximately 90 wt % when the amount of coke combustion in the first riser is increased to 70 wt %. Then, the small
amount of hydrogen left for the second stage can no longer cause significant hydrothermal catalyst deactivation. In this case, the carbon content is already below 0.02 wt % and the temperature approaches about 750 °C at 5 m of the second riser, suggesting a 5-m second riser is sufficient. 5. Conclusion Based on a previously developed mathematical model for FCC riser regeneration, two novel riser regenerator schemes to improve regeneration performance are proposed and simulated. It is predicted that without a significant change to the equipment configuration, the performance of a single riser regenerator can be greatly improved by splitting and feeding the air flow at several different axial locations along the riser. By properly adjusting the air flow rates and the regenerated catalyst to spent catalyst recycle ratio (a ) Gsg/Gss), this multiple air feed riser regenerator can meet the industrial specifications, i.e., carbon content on the regenerated catalyst be less than 0.1 wt % and the riser temperature be lower than 730 °C. In order to regenerate catalyst with a high coke content (e.g., residual FCC process), a new two-stage riser FCC regenerator consisting of two risers connected in series is proposed. The main advantages of this design are the separately controlled air flow rates and temperatures for the two risers and the much higher temperature operation without hydrothermal catalyst deactivation in the second riser, resulting in high regeneration efficiency and operation flexibility.
4548 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
Acknowledgment This work is partially funded by SINOPEC and the Chinese National Nature Science Foundation. We are also grateful to Professor G.-H. Yang, President of University of Petroleum (China), for providing the detailed regeneration kinetics. Nomenclature a ) ratio of regenerated to spent catalyst flow rates ()Gsg/ Gss) Cc ) carbon content on regenerated catalyst, wt % CO2 ) oxygen mole fraction, mol % Cs ) carbon content on spent catalyst, wt % Gsg ) regenerated catalyst flow rate (based on the crosssectional area of the riser), kg/(m2‚s) Gss ) spent catalyst flow rate (based on the cross-sectional area of the riser), kg/(m2‚s) T ) temperature, °C Ug ) superficial gas velocity (at standard condition of 273.15 K and 101.3 kPa), m/s x ) axial coordinate above the distributor, m
Literature Cited Avidan, A. A.; Shinnar, R. Development of Catalyst Cracking Technology: A Lesson in Chemical Reactor Design. Ind. Eng. Chem. Res. 1990, 29, 931-942.
Avidan, A. A.; Edwards, M.; Owen, H. Innovative Improvements Highlight FCC’s Past and Future. Oil Gas J. 1990, 88 (2), 3358. Bai, D.; Gan, J.; Jin, Y.; Yu, Z.-Q. Simulation of Coke Combustion in an FCC Riser Regenerator. Petrol. Refin. 1991, 21 (10), 2027. Bai, D.; Jin, Y.; Yu, Z.-Q. Flow Regimes in Circulating Fluidized Beds. Chem. Eng. Technol. 1993, 16, 307-313. Bai, D.; Zhu, J.-X.; Jin, Y.; Yu, Z.-Q. Simulation of FCC Catalyst Regeneration in a Riser Regenerator. Chem. Eng. J. 1997, submitted. Chen, J.; Cao, H.; Liu, T. In Catalyst Regeneration in Fluid Catalytic Cracking. In Fast Fluidization; Kwauk, M., Ed.; Academic: New York, 1994; pp 389-420. Gan, J.; Bai, D.; Jin, Y.; Yu, Z.-Q. Analyses on the Main Factors Affecting the Coke Combustion Process in an FCC Riser Regenerator. Petrol. Refin. 1992, 22 (4), 26-33. Jin, Y.; Yu, Z.-Q.; Bai, D.; Wei, F. Chinese Patent No. 93219972 0, 1993. Squires, A. M.; Kwauk, M.; Avidan, A. A. Science 1985, 230, 13291337.
Received for review February 7, 1997 Revised manuscript received July 31, 1997 Accepted August 4, 1997X IE970126D
X Abstract published in Advance ACS Abstracts, October 1, 1997.
