Ind. Eng. Chem. Res. 2005, 44, 9347-9354
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Reactor Siphon and Its Control of Particle Flow Rate When Integrated into a Circulating Fluidized Bed Guangwen Xu,* Takahiro Murakami, Toshiyuki Suda, and Yoshiaki Matsuzawa Ishikawajima-Harima Heavy Industries (IHI), Ltd., Sin-Nakahara-Cho 1, Isogo-ku, Yokohama 235-8501, Japan
A newly configured siphon, called the reactor siphon, was proposed to fulfill chemical reactions along with its control of the particle circulation rate in a circulating fluidized bed (CFB) mounted with the siphon. The new siphon has two separated seals to prevent bypassing of gases both from the riser to the downcomer and from the downcomer to the riser. This in turn allowed a reaction space, called the reactor of the siphon, to be created between the two seals independently of the riser and downcomer. In a CFB installed with a reactor siphon, the present article further investigated the ways to control the particle circulation rate of the bed. It was found that the pressure difference between the reactor of the siphon and the riser influenced greatly the particle circulation rate. This rendered the circulation rate little varied with the particle amount loaded into the whole bed when the pressure difference was substantially high. Changing the superficial gas velocity in the riser obviously varied the circulation rate, but this gas velocity effect was related to the quoted pressure difference as well. Consequently, the particle circulation rate in the CFBs mounted with the reactor siphon was thought to be subject to the dominance of both the gas velocity in the riser and the pressure difference between the reactor of the siphon and the riser, revealing essentially a controlling mechanism that is much different from that for the CFBs installed with the conventionally configured siphons. The Design For gas-particle fluidizations the siphon refers to a setup used to control the flow of particles from one vessel to another while it simultaneously prevents the gas from intermixing between the two vessels. A typical application of the siphon is to bridge the downcomer and riser of a circulating fluidized bed (CFB) wherein it not only guides the particles to flow from the downcomer to the riser but also suppresses the gas bypassing from the riser to the downcomer and vice verse. Figure 1 illustrates two common configurations of the existing siphons. The necessary basic parts for a siphon include a “riser seal” and a “downcomer seal” that block the gas bypassing from the riser to the downcomer and from the downcomer to the riser, respectively. Meanwhile, a fluidizing gas has to be applied to the siphon to keep the particles inside the siphon flowing. Thus, there has to be an exhaust for the fluidizing gas, which is mainly via either the downcomer (Figure 1, left) or the riser (Figure 1, right) in the existing siphons. A distinctive feature of the existing siphons, as illustrated in Figure 1, is that both the riser and downcomer seals are side by side so that there is no free room between the two seals. Consequently, the fluidizing gas fed to the siphon has to exhaust via either the riser or the downcomer. This makes the siphon hardly an independent reactor to produce gaseous product. In practice, however, chemical reaction is possibly required to take place inside the siphon or, say, the reaction is asked to occur during the move of its involved granular reactant. A typical example may be the regenerationtype gas-solid reaction systems where a kind of granular material (such as catalyst) is circulated between a * Corresponding author. Tel.: 0081-45-759-2867. Fax: 008145-759-2210. E-mail:
[email protected].
Figure 1. Two typical configurations of the existing siphons employed in CFBs. The boxed part with dotted lines is the siphon, which exhausts through exclusively the downcomer (left) or the riser (right).
“main reactor” and a “regenerator” and the two reactors have their own independent exhausts. To completely isolate the gas flows in the main reactor and regenerator, a siphon was commonly installed between the two reactors, as was actually practiced by Golbig and Werther1 for the process of maleic anhydride synthesis. Recently we had another realistic encounter when we intended to develop a two-vessel gasification process for solid fuels (such as biomass and coal) by adopting a pneumatic riser as the char combustor and a bubbling fluidized bed (BFB) that receives hot particles from the riser as the gasifier. A basic requirement for the process is particle circulation through the BFB gasifier, because the hot particles from the riser are the only heat resource for all involved gasification reactions occurring inside the BFB. Consequently, the BFB gasifier is somehow similar to the catalyst regenerator referred to by Golbig and Werther.1 Certainly, mounting a siphon in addition to the BFB gasifier and riser combustor
10.1021/ie050679l CCC: $30.25 © 2005 American Chemical Society Published on Web 11/01/2005
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Figure 2. Configuration example of the newly designed reactor siphon. While the cross section of the siphon can be any appropriate shape, such as a rectangle or a circle, its downcomer seal may also touch the right-side wall of the siphon.
would implement the gas separation and solid circulation control between the two reactors simultaneously. The resulting system, however, appeared complicated due to the presence of an additional siphon. Our demand is to operate the BFB gasifier, which has an independent exhaust, also as a siphon to enable its control of the particle flow rate with the progress of chemical reactions. This led to our design of the so-called “reactor siphon” conceptualized in Figure 2.2 In configuration, the newly designed reactor siphon separates its riser seal and downcomer seal to enable a space available between the two seals. This space in turn can serve as a reactor to allow its independent reactant feed (when required) and reacted-gas exhaust. As a siphon, the particles inside the reactor surely have to be fluidized, usually in the bubbling fluidization regime, to convey the particles from the downcomer to the riser. This makes the reactor of the siphon actually a bubbling fluidized bed reactor. The reactor, however, is different from the other BFB reactors commonly encountered. While the reaction in the present case takes place with the particle (including reactant) movement from the downcomer to the riser so that the reaction progress is closely dependent on the residence time of the reactant inside the reactor, in the usual BFB reactors any reactant fed into the bed has basically an infinitely long residence time to maintain its thorough conversion (except for the elutriated fine particles). Therefore, application of the newly designed reactor siphon to CFBs entails a close control of the particle flow rate through the siphon, i.e., the particle circulation rate in the whole CFB system. As a matter of fact, a too high particle flow rate causes the reactant fed to the reactor to have an extremely short residence time inside the reactor, which surely threatens the complete conversion of the reactant. On the other hand, there are different mechanics controlling the particle flow rate in the CFBs mounted with the newly designed siphon when compared to that in the other commonly encountered CFBs (as will be clarified in the succeeding section). Therefore, to clarify theoretically and experimentally the mechanics and ways of controlling the particle flow rate in the CFBs installed with the reactor siphon constitutes the major purpose of the present
work. This clarification is surely the basis for any actual application of the reactor siphon. Another configuration feature of the reactor siphon is the necessarily required partition of the winbox of the siphon. As exemplified in Figure 2, the riser seal and downcomer seal should have generally their own independent compartments for the fluidizing gas supply. Otherwise, a small portion of the reactant gas for the reactor of the siphon, annotated as reactant gas 2 in Figure 2, leaks into the riser and the downcomer. This results not only in a contamination to the gas flow inside the riser but also in a loss of the reactant gas 2 (the latter should become substantial in a commercial case). When partitioning is adopted, the reactant gas 1 for the riser can be applied also to the compartments for the seals, or when it is necessary and possible, the downcomer seal can also accept a sort of inert fluidizing gas. In the following, we will first present some theoretical concerns on the parametric dependence of the particle flow (circulation) rate in the CFBs mounted with the newly designed reactor siphon, in comparison with that in the other commonly encountered CFBs. Succeeding this, an experimental demonstration of the theoretical analysis will be made in a laboratory CFB retrofitted with the reactor siphon. The experiment test will also address the effectiveness of the riser and downcomer seals for preventing gas intermixing. On all of these bases concluding remarks will be finally made to summarize and highlight the technical means for controlling the particle circulation rate in the investigated CFB. Theoretical Concerns Theoretically, we may simply suppose that the particle flow rate from one vessel to another is subject to the force pushing the particles to flow in the desired way. This, when applying to the existing siphons illustrated in Figure 1, causes the particle flow rate into the riser, i.e., Gs,es, to be fully subject to the force pushing the particles from the siphon to the riser, if we do not first consider the possible influences from the flow inside the riser (regarding the latter, an overview is available in Xu and Gao3). This force, because it is impossible to vary the pressures inside the riser and downcomer independently, should come only from the particle bed height inside the downcomer (denoted as Hd in Figure 3) balanced with the resistance from the riser seal. The riser seal’s resistance is conversely proportional to the seal’s opening, say hds or hrs (but hds ) hrs for the conventional siphon). Thus, there must be
Gs,es ∝ f(Hd, hrs-1, ...) ) f(particle load, hrs-1, ...) (1) where the particle bed height Hd in the downcomer is suggested to be subject to the particle amount loaded into the whole CFB, and the ellipsis “...” means all the other influential factors, if there are any, such as the superficial gas velocity Ug inside the riser. As for the newly proposed reactor siphon exemplified in Figure 2 (as well as in Figure 3), the reactor between the riser and downcomer seals serves as a buffer for the conveyance of particles from the downcomer to the riser. Therefore, the variation in Hd would not directly influence the particle flow rate, say, Gs,rs, into the riser but does affect the particle bed height Hs inside the reactor of the siphon due to the consequently varied amount of particles moving into the reactor. Certainly Hs influences the particle flow rate Gs,rs, but it is not exclusive.
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Meantime, the particle bed height Hs in the reactor can be formulated with
Hs ∝ f[(P3 - P4), Hd] ) f[(P3 - P4), particle load] (3) so that eq 2 becomes
Gs,rs ∝ f[(P3 - P4), particle load, hrs-1, ...]
Figure 3. Schematic diagram of the experimental CFB installed with a rectangular reactor siphon between the riser and downcomer.
An important characteristic of the reactor siphon is that both the reactor of the siphon and the riser have their own independent exhausts, which allows independent control of the pressures in two such vessels. For example, in Figure 3 (the employed experimental rig) the adjustment of the valve mounted in the reactor’s exhaust line allows P3 inside the reactor to be controlled without respect to P4 inside the riser (riser seal, in fact). This in turn develops a pressure difference (P3 - P4) between the reactor and the riser (or the riser seal) to vary the force pushing the particles to move from the reactor to the riser. Thus, the quoted pressure difference would certainly influence the particle flow rate Gs,rs as well as the particle bed height Hs. Furthermore, the separation of the riser seal from the downcomer seal makes Gs,rs subject to the opening hrs of the riser seal, whereas the opening hds of the downcomer seal does not directly work on Gs,rs. Changing hds (in reasonable values) varies Hd and in turn Hs, but the variation in Hs should be generally slight, if there is a fixed total particle amount in the siphon, because the cross section of the reactor (for Hs) is much bigger than that of the downcomer (for Hd). Overall, we then have
Gs,rs ∝ f[(P3 - P4), Hs, hrs-1, ...]
(2)
(4)
Compared to eq 1 for Gs,es, eq 4 reveals a distinctively different mechanics for controlling Gs,rs. That is, Gs,rs is subject not only to the particle load and the opening hrs of the riser seal but also to the pressure difference (P3 - P4) between the reactor and the riser seal of the reactor siphon. In particular, the pressure difference (P3 - P4) is likely an overwhelmingly important factor influencing Gs,rs, because any effect of the particle load on Hs and Gs,rs is possibly compensated for with a correspondingly adjusted (P3 - P4). Therefore, the knowledge that we now have, such as reported in refs 3 and 6, about the particle flow rate control (i.e., Gs,es) for the CFBs mounted with one of the existing siphons illustrated in Figure 1, cannot be directly applied to the CFBs installed with the newly designed reactor siphon conceptualized in Figure 2. Nonetheless, the particle flow rate Gs,rs in the investigated case of this work should also vary with the superficial gas velocity Ug in the riser, being similar to the observation on Gs,es. As for Gs,es, it is commonly suggested that the solid circulation rate be an exclusive function of Ug. This relationship can be valid to Gs,rs only when the pressure difference (P3 - P4) is given. In this sense, we may believe that the factors dominating Gs,rs have to be both Ug and (P3 - P4). In addition, the split of the gas flow fed to the riser (i.e., Ug) into two streams at different bed positions to form a primary flow and a secondary flow, and the superficial gas velocity U0 in the siphon itself, may also influence the particle circulation rate, irrespective of what kind of siphon is utilized. Consequently, in the experiments detailed herein the parameters, including those correlated in eq 4, Ug, U0 (shown with U0/Umf), and the split ratio R for Ug (secondary flow over total flow), are all tested to demonstrate their influences on Gs,rs. Via these tests it was expected to grasp the general technical means for manipulating the investigated particle circulation rate. Although the circulation rate data themselves may be closely relative to the experimental apparatus adopted (including size, geometry, etc.), the clarified parametric dependences and thus the resulting technical means for solid flow rate control would be general and universal to the CFBs mounted with the proposed reactor siphon. Experimental Section The experimental setup used, which is schematically shown in Figure 3, was a CFB mounted with the newly designed reactor siphon between the riser and the downcomer. The riser was rectangularly shaped and had a height of 2700 mm and a cross section of 280 × 270 mm2 (see A-A view in Figure 3). As an additional means to adjust the particle circulation rate (i.e., Gs,rs), there was a secondary airflow supply on the riser at 550 mm high (air being used as the fluidizing gas). The downcomer was a cylindrical pipe of 120 mm in diameter. The reactor siphon had a rectangular section of 380 × 610 mm2 (see A-A view) and a height of 560 mm
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Figure 4. Parameters varying with pressure difference (P3 - P4) between the reactor and riser seal of the reactor siphon under different particle loads: (a) Gs,rs, (b) Hs, (c) Hd, and (d) ∆Pr.
above its distributor, with a nonpartitioned winbox. Surely, the partition itself of the winbox would not vary the preceding eqs 2-4 and in turn the tests intended, given that the fluidization statuses in the riser and downcomer seals are equivalently maintained for the case with a partition (i.e., by keeping similar gas velocities for all compartments). Nonetheless, to the nonpartition configuration a siphon-to-riser gas leakage surely occurs. In our apparatus this leakage was found to be about 10% of the total gas fed to the siphon, which was thus treated as a part of the reported Ug. Both the riser and the downcomer were made of Plexiglas, while the reactor siphon was of iron with Plexiglas windows on its top and two sides neighboring the riser and downcomer seals. The riser and the reactor of the siphon shared a common bagfilter and a common induction fan. Thus, in the exhaust line of the reactor a valve was installed to adjust the pressure, P3, inside the reactor. On the downcomer there was a butterfly valve to measure the particle circulation rate Gs,rs by detecting the height of the accumulated particles on the valve in a specified interval of time. The reactor siphon had a fixed opening hds of 150 mm for its downcomer seal, whereas the hrs for its riser seal was optionally 150 or 300 mm. This thus allowed the influence of hrs to be examined. Another important configuration parameter for the reactor siphon was the height Hos of the particle outlet of the siphon opened on the sidewall of the riser seal. Generally, a higher Hos may lead to a lower Gs,rs under a specified particle load, but this height would not vary much with the facilities once an suitable value was approved. Thus, in Figure 3 the outlet was fixed at 390 mm above the siphon’s distributor, with a Plexiglas pipe of 120 mm in diameter to extend it to the riser. The particles tested were silica sand, whose Sauter mean diameter was 104 µm and density was 2500 kg/ m3. The voidage at the minimum fluidization was suggested to be 0.4, resulting in a terminal velocity ut of 0.61 m/s and a minimum fluidization velocity Umf of 0.023 m/s calculated from the force balance equations
reported in Xu and Li4 for a single particle and the particle bulk, respectively. Consequently, the examined superficial gas velocity Ug in the riser was varied from 1.40 to 2.00 m/s (all over ut). No Ug above 2.0 m/s was tested because we had a relatively large bed area of the riser (280 × 270 mm2) but conversely a limited capacity of the air compressor. This, however, would not hinder the intended exploration. At all the gas velocities examined the only cyclone of the experimental setup enabled an efficient particle capture, which was verified by measuring the particle amounts holding in the facility before and after the experiment. Airflow rates into the riser and siphon, including the secondary airflow to the riser, were measured using digitalized orifice meters. U-typed water barometers measured the pressures along the riser and the reactor of the siphon, but the reported pressures in the article were confined to P1 to P4 annotated in Figure 3. The particle bed heights Hd and Hs were both manually measured with rulers and were treated as dependent parameters in the coming result section. The fact is also that both such bed heights varied with the pressure difference (P3 P4), as will be shown below in our results. The sealing functions of the riser and downcomer seals were confirmed with a CO2 tracer. When a CO2tracer stream (from a CO2 cylinder) was input into the space above the riser seal from the position annotated as P4 in Figure 3, no obvious CO2 presence was identified in the gas flow sampled from the location indicated as P3 (CO2 was monitored in a continuous infrared CO/CO2 analyzer). Conversely, there was also no CO2 release detected from P4 when the CO2 tracer stream was applied to P3. Therefore, we believed that the two seals performed well to make the devised configuration be just the expected integration of a siphon and a reactor. Results and Discussion Dominance of (P3 - P4). Figure 4 shows a few parameters, say, Gs,rs (Figure 4a), Hs (Figure 4b), Hd
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(Figure 4c), and ∆Pr (Figure 4d) measured under three different particle loads (85, 100, and 115 kg) and with given gas velocities for the riser (Ug ) 1.76 m/s) and siphon (U0/Umf ) 4.0). Under each individual load the valve in the exhaust line of the reactor of the siphon was adjusted to vary the pressure P3 inside the reactor (see Figure 3) and in turn the pressure difference (P3 P4). Theoretically, a higher P3 leads to a higher (P3 P4), which causes not only a larger force to push the particles from the reactor into the riser seal but also a higher resistance toward the granular flow from the downcomer to the reactor. A straightforward result is that the particle bed height inside the reactor (Hs) must decrease, while that in the downcomer (Hd) has to increase on increasing (P3 - P4). The data shown in Figure 4b,c corroborate these. The increase in (P3 - P4) also elevated the particle circulation rate Gs,rs in Figure 4a, although the corresponding Hs plotted in Figure 4b gradually decreased. This reveals essentially that as for the reactor siphon the particle bed height in its reactor (i.e., Hs) is not dominant to the particle flow rate from the siphon to the riser. Under a given particle load, a lower Hs means a reduced particle amount kept in the reactor of the siphon. Mass balance then must increase the particles inside the riser, as is verified in Figure 4d, where the pressure drop ∆Pr across the whole riser ()P1 - P2), which is indicative of the particle amount inside the column, gradually increases with raising (P3 - P4). The particle load refers to the available particle stuff for forming the particle beds inside the reactor of the siphon and inside the downcomer. Thus, under a given pressure difference (P3 - P4), a higher particle load should lead to a higher particle bed height Hs as well as a higher Hd. Figure 4b,c basically corroborates this anticipation, but we see also from Figure 4b that the Hs heights tended to overlap each other when (P3 - P4) was high, for example, over 90 mmAq for the particle loads of 100 and 115 kg (b, 0) and over 130 mmAq for 85 and 115 kg (], 0). The result indicates that, at sufficiently high (P3 - P4) values, the increased amount of particles with the higher particle load is mostly reserved into the riser and downcomer, as is actually shown in the figure with the evidently increased ∆Pr (Figure 4d) and Hd (Figure 4c) on increasing (P3 - P4) and particle load. A similar variation tendency is exhibited also in Figure 4a for the particle circulation rate Gs,rs. There, Gs,rs is obviously higher for a higher particle load at relatively low (P3 - P4), such as up to 120 mmAq, but comes closer with increasing pressure difference. This demonstrates further that under a given Ug the pressure difference (P3 - P4) likely dominates Gs,rs so that the particle load manifested a detectable influence on Gs,rs only when (P3 - P4) was relatively low. Nonetheless, the value of (P3 - P4) has to be in an appropriate range. In Figure 4 its value varies from -100 to 250 mmAq. However, we can see (through matching Figure 4a with Figure 4b) that no Gs,rs was plotted in Figure 4a for negative (P3 - P4) at the particle load of 100 kg and for (P3 - P4) of about 40 mmAq at 85 kg (i.e., the points highlighted with a dashed-line box in Figure 4b). The fact is that the values of pressure P4 corresponding to these (P3 - P4) values in the reactor of the siphon were too low (or P4 , P3), which caused the particle bed inside the riser seal to be unable to reach the seal’s outlet at the height Hos, say, 390 mm in Figure 3 for the tested apparatus. In turn, there
should be no particle circulation inside the system. However, a steady particle circulation was observed at the particle load of 115 kg even when (P3 - P4) was as low as -80 mmAq. The higher Hs in this case, as shown in Figure 4b (0 vs b), is responsible for the result. That is, even if (P3 - P4) was the same or lower, the higher Hs had to lead to a larger total pushing force to maintain a relatively higher particle bed height over Hos inside the riser seal (since the pushing force is basically the sum of (P3 - P4) and the pressure owing to the weight of particles bed inside the reactor). With the same cause, Figure 4 clarifies that no Gs,rs was available until (P3 P4) ) 100 mmAq at the particle load of 85 kg (]), whereas the Gs,rs was steadily measurable from (P3 P4) of about zero under the higher particle load of 100 kg (b). On the other hand, the failure in measuring the Gs,rs comes also from a (P3 - P4) that is too big. A too big (P3 - P4) possibly leads the bed height Hs in the reactor to be lower than the opening of the riser seal (hrs) or that of the downcomer seal (hds). Then, the siphon loses its function of sealing up the gas intermixing between the riser and reactor, when Hs < hrs, or between the reactor and downcomer once Hs < hds. Therefore, in Figure 4 the lowest Hs tested was 200 mm, which was above, although very close to, the opening hrs ()150 mm) of the riser seal (hds was 150 mm for the employed experimental setup). In summary, one can see that for the reactor siphon the pressure difference (P3 - P4) between its reactor and riser seal dominates the particle circulation rate in the CFB mounted with the siphon, if the gas velocity for the riser is specified. Nonetheless, the values of (P3 P4) must be in a valid range for maintaining a particle flow through the siphon and simultaneously for its seal functions of preventing the interflow of gas among the riser, siphon, and downcomer. Broadening the valid range of (P3 - P4) can be achieved by increasing the particle load since it allows the CFB to be operated at lower, even minus (P3 - P4) (see Figure 4a). In addition, decreasing the height Hos of the siphon’s particle outlet, which is opened on the sidewall of the riser seal, also enables (P3 - P4) to be at rather lower values (referring to the last paragraph), but in practice the Hos must be sufficiently high in order to seal the gas inside the riser and to avoid the possible bypassing of the gas into the reactor. Similarly, smaller hrs and hds values can accept rather lower Hs values to allow the operation to adapt to rather higher (P3 - P4) (see Figure 4b), but a too small hrs or hds should block the particle move from the siphon to the riser or from the downcomer to the siphon, respectively. Influences of Other Parameters. Figure 5 compares the measurements under two different openings hrs of the riser seal, 150 and 300 mm. The measurements were carried out by varying the pressure difference (P3 - P4), noting its dominance over Gs,rs as well as the other parameters clarified in the preceding section. The figure shows that between the two tested hrs openings there was no obvious difference in Gs,rs (Figure 5a, left Y). The consistency for all the other measured parameters, i.e., Hs (Figure 5a, right Y), Hd (Figure 5b, left Y), and ∆Pr (Figure 5b, right Y), further verify the little influence of hrs varying from 150 to 300 mm. Nonetheless, the result is conditional. Rather small hrs might reduce the particle circulation rate Gs,rs. An extreme case is hrs ) 0, which surely stops the particle circulation completely. Practically, however, one may
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Figure 5. Influence of riser seal opening of reactor siphon on parameters plotted in Figure 4 (but here the examined openings are over 150 mm).
hardly adopt such a hrs smaller than 100 mm. Hence, Figure 5 actually clarifies that, as long as hrs remains above 150 mm, its variation should not significantly affect the particle circulation in the whole system. This result would be similarly valid for hds. The implication is that in this case the seal’s resistance toward the particle flow through the siphon is much lower than the pushing force stemming from (P3 - P4) and the particle bed inside the reactor of the siphon (the latter is proportional to Hs). The opening of a seal determines the passage or gate area for its controlled particle flow. Thus, when considering the preceding result in a more general sense, we can say that maintaining the ratio of the seal’s gate area to the cross-sectional area of the siphon (including the areas occupied by two seals) is essential. As for our employed bed (in Figure 3), this ratio corresponding to the examined smaller seal opening of 150 mm was 0.18 and 0.24 for the riser and downcomer seals, respectively. Thus, keeping the quoted area ratio above 0.18 may be a general criterion for designing the seals of other differently sized reactor siphons. Meanwhile, it is noteworthy that the cross-sectional area ratios of the riser and downcomer seals to the siphon is 0.08 and 0.05, respectively, for our facility. The fluidization velocity U0 in the siphon may also influence the particle circulation rate. Figure 6 clarifies this by comparing the results measured under three different U0/Umf values, where Umf refers to the minimum fluidization velocity estimated from the force balance equation of particle bulk reported in Xu and Li.4 When U0 was close to Umf, say, U0/Umf ) 2.0 in Figure 6, there was a lower particle circulation rate Gs,rs (Figure 6a, left Y) under each specified (P3 - P4), whereas the rate varied little once U0/Umf was increased from 4.0 to 8.0 (O vs ], 0). The result indicates that a too-low U0 somehow leads to low fluidity of particles inside the siphon, which in turn reduces the particle flow rate through the setup. However, the particles’ fluidity inside a fluidized bed likely becomes the same as U0 increases to make the fluidized bed sufficiently dynamic and
Figure 6. Influence of gas velocity for reactor siphon all the parameters plotted in Figure 4. The Umf used was estimated using the force balance equation for particle bulk reported in Xu and Li.4
turbulent.5 This consequently causes Gs,rs to vary little with U0, given that the velocity is high enough, such as at U0/Umf greater than 4.0 according to Figure 6a. Meanwhile, Figure 6 clarifies that, corresponding to the lower Gs,rs under U0/Umf ) 2.0, there was a lower particle bed height Hd (Figure 6b, left Y), whereas both Hs (Figure 6a, right Y) and ∆Pr (Figure 6b, right Y) did not exhibit much difference among all the tested U0/ Umf values. Under specified (P3 - P4) and gas velocity Ug both Hs and ∆Pr should be subject basically to the particle load, thus making them vary little with U0/Umf. The lower Hd under U0/Umf ) 2.0 is again relative to the lower fluidity of particles inside the reactor of the siphon at this low U0, which causes the downcomer seal a to have higher resistance toward the particle flow (herein the “seal resistance”). On the other hand
Hd ∝ Hs + height equivalent to P3 height equivalent to seal resistance (5) Thus, when Hs and P3 remain unchanged, a higher resistance must cause a lower Hd. For the other tested higher U0 values (0, ]), the nearly identical fluidity of the particles inside the reactor implies similar seal resistance, having in turn little differentiated Hd values in Figure 6b (left Y). Nonetheless, the preceding results about the influences of U0/Umf are probably valid only in the tested range of U0/Umf (i.e., not over 8.0). For example, further increasing U0 may cause the fluidized bed inside the reactor to have high voidages and to increase consequently Hs with raising U0/Umf. Notwithstanding, the U0/Umf in practical reactor siphons should be as low as possible to avoid substantial particle elutriation from the reactor. In light of this, we thus believe that the tested range of U0/Umf and its corresponding results are practically representative and typical. Considering the entire CFB, its particle circulation is surely subject to the gas velocity Ug in the riser. For our case, however, the action of Ug is as well relative to
Ind. Eng. Chem. Res., Vol. 44, No. 24, 2005 9353 Table 1. Influence of Superficial Gas Velocity in the Riser on the Parameters Shown in Figure 4a P3 - P 4 [mmAq]
Ug [m/s]
Gs,rs [kg/(m2‚s)]
Hs [mm]
Hd [mm]
∆Pr [mmAq]
190 191 135 135
1.40 1.76 1.76 2.00
18.6 21.0 16.8 18.7
185 190 240 240
430 450 440 450
195 173 73 65
a Other conditions for the data: particle load ) 90 kg; h rs ) 150 mm; U0/Umf ) 4.0; R ) 0.0.
Figure 7. Influence of secondary airflow to riser on all the parameters plotted in Figure 4.
the pressure difference (P3 - P4), as shown in Table 1 by the measured Gs,rs. In Table 1, Gs,rs is higher at higher Ug when (P3 - P4) is specified, but Ug of 1.40 m/s at (P3 - P4) ) 190 mmAq (first row) and that of 2.00 m/s under (P3 - P4) ) 135 mmAq (fourth row) resulted in the nearly same Gs,rs. The higher Ug of 2.00 m/s did not lead to a higher Gs,rs, which is surely due to the lower (P3 - P4) for this test. Meanwhile, Table 1 demonstrates that under the same (P3 - P4) the higher the Ug and the lower the pressure drop ∆Pr across the riser. This obviously refers to a plausible result. Besides, we can see from Table 1 that increasing Ug likely slightly elevates Hd and Hs when (P3 - P4) is given. It should be mainly relative to the lower ∆Pr at the higher Ug, which reduces the particle amount inside the riser and in turn causes the potential increases in Hs and Hd. In CFB combustors the staged air is usually adopted to reduce NOx emissions from combustion. This split of fluidizing gas would also vary the particle circulation rate, as is experimentally shown in Figure 7. There, the measurement was conducted by varying the ratio R of the secondary airflow (fed at 500 mm high; see Figure 3) to the total airflow under a given velocity Ug of 1.76 m/s (Ug is the total airflow rate). During the test, the valve in the exhaust line of the reactor siphon was fixed so that (P3 - P4) would remain basically constant (this was verified in our experiment). As anticipated, the particle circulation rate Gs,rs gradually decreased with increasing the ratio R (left Y). Meanwhile, the particle bed heights Hs (]) and Hd (0) also slightly decreased, whereas the pressure drop ∆Pr (O) across the riser increased. Under a given Ug, increasing the secondary airflow means to decrease the primary airflow. Thus, more particles should be retained to the bed section below the inlet of the secondary airflow. Its consequence is in turn a denser bed there and thus the increase of ∆Pr with rising R. Figure 7 especially reveals that ∆Pr had a quick increase when R was elevated from 0.2 to 0.5. This may be indicative of the rapid transition of the concurrent gas-solid flow in the riser bottom from
the dilute transport (suspension) flow to a dense fluidization, such as the turbulent or bubbling fluidization. This transition was widely recognized to be accompanied by a sudden increase in the bed density, which was termed the phenomenon of “choking”.7-9 With more particles (higher ∆Pr) inside the riser, reduced amounts of particles should be available for the particle beds inside the siphon and downcomer, which has to lower Hd and Hs when increasing R. A higher R leads to a lower primary airflow, which results not only in a denser bed in the riser bottom but also in fewer particles carried into the bed section above the inlet of the secondary airflow. This consequently decreases the particle circulation rate Gs,rs, even if the total velocity Ug above the secondary airflow is identical. In addition, the lowered Hs with higher R would also contribute, although slightly, to the observed lower Gs,rs. Concluding Remarks A newly configured siphon, called the reactor siphon, for a circulating fluidized bed (CFB) was designed to implement chemical reaction (or reactions) along with its control of the particle flow from the downcomer to riser of the bed. By isolating its riser seal and downcomer seal, the new siphon has a reactor between the two seals, which is independent of the gas flows inside the riser and downcomer, to enable an independent exhaust of reacted gas (i.e., gaseous product) from the reactor. This in turn rendered the pressure inside the reactor of the siphon independently controllable so that a pressure difference between the reactor and the riser (the former minus the latter) can be developed to control the particle flow rate through the siphon. Because of the existence of the pressure difference, the mechanics controlling the particle circulation rate in the CFB mounted with the newly designed reactor siphon is distinctively different from that in the CFBs installed with the other conventionally configured siphons. Theoretical analysis and experimental demonstration both showed that the quoted pressure difference likely dominates the particle circulation rate when the gas velocity in the riser is specified. This thus mitigated the effect of the particle load in the whole CFB on the circulation rate such that the effect appeared pronounced only in the case that the pressure difference was relatively low, such as not much over 0 mmAq. The experimental study demonstrated further that the particle circulation rate in the CFB mounted with the reactor siphon considerably increased with elevating the superficial gas velocity in the riser but obviously decreased with raising the split ratio of the velocity into a secondary gas flow. The superficial gas velocity in the siphon hindered the particle flow through the siphon to lead to somehow lower particle circulation rates when the velocity was too low, for example, below 4 times of the minimum fluidization velocity of the tested particles. (Note: The minimum fluidization velocity referred to was calculated with the force balance equation for particle bulk proposed in ref 4.) Varying the opening of the riser seal of the siphon from 150 to 300 mm little changed the particle circulation rate, indicating essentially that an area ratio above 0.18 of the seal gate to the siphon cross section is enough to keep the particles passing through the siphon without much blockage at the seal. The result is surely applicable also to the downcomer seal of the reactor siphon. Consequently, the particle circulation rate in the CFB mounted with the newly proposed reactor siphon can
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be first manipulated through adjusting the superficial gas velocity to the riser under well-defined pressure differences, for example, not far from 0 mmAq between the reactor of the siphon and the riser. At a given gas velocity, raising the pressure difference can also considerably elevate the particle circulation rate, but it lowers the particle bed height inside the reactor. Thus, the pressure difference has to be in an appropriate range so that its highest value does not cause the particle bed inside the reactor to be lower than the openings of the riser and downcomer seals, and its lowest value is able to maintain the particles inside the riser seal to catch the siphon’s particle outlet leading to the riser (opened on the sidewall of the riser seal). Varying the particle load in the entire CFB would enable an adjustment of the particle circulation rate, but the means can be effective only when the above-mentioned pressure difference is relatively low (such as not much over 0 mmAq). Under specified pressure difference and gas velocity for the riser, the particle circulation rate is subject as well to the split ratio of the gas to the riser for forming the secondary flow. In this case the control is similar to that adopted in the CFBs mounted with other conventionally configured siphons. Acknowledgment The authors are grateful to Mr. Yukio Oda and Mr. Minoru Asai of IHI for their help in the experiment. Nomenclature Gs,es ) particle circulation rate in the CFB with the existing siphon, kg/(m2‚s) Gs,rs ) particle circulation rate in the CFB with the reactor siphon, kg/(m2‚s) hds ) opening of the downcomer seal, m hrs ) opening of the riser seal, m Hd ) particle bed height in the downcomer, m Hos ) height of particle outlet of the siphon, m Hs ) particle bed height in the reactor of the siphon, m
Pi ) pressure for a local position i ()1, 2, 3, 4) corresponding to riser bottom, riser top, inside the reactor, and inside the riser seal, kg/(m‚s2) ∆Pr ) pressure drop across the riser ) P1 - P2, kg/(m‚s2) ut ) particle terminal velocity, m/s U0 ) superficial gas velocity in the siphon, m/s Ug ) superficial gas velocity in the riser, m/s Umf ) minimum fluidization velocity, m/s Greek Letters R ) ratio of the secondary gas flow to total gas flow into the riser
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Received for review June 10, 2005 Revised manuscript received September 11, 2005 Accepted September 29, 2005 IE050679L
