On the comprehension of the gas split in loop seal devices

Powder Technology 408 (2022) 117777 Available online 29 July 2022 0032-5910/© 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/). On the comprehension of the gas split in loop seal devices M. Su´ arez-Almeida * , A. G´ omez-Barea Chemical and Environmental Engineering Department, Escuela T´ ecnica Superior de Ingeniería, University of Seville, Camino de los Descubrimientos s/n, 41092 Seville, Spain HIGHLIGHTS GRAPHICAL ABSTRACT •Gas splitting into the chambers of a loop seal is a key issue difficult to predict. •The gas distribution was experimentally assessed in an isolated loop seal. •Loop seal operation as non-mechanical valves leads to non-ideal gas-solids flows. •Non-ideal gas-solids flow pattern establishes the gas splitting into the chambers. •The study gives insights on how to control the solids circulation with a loop seal. ARTICLE INFO Keywords: Loop seal Solids circulation Supply chamber Recycle chamber Standpipe Fluid dynamics ABSTRACT Gas distribution through the chambers of a loop seal in circulating fluidized bed (CFB) units is still difficult to predict and often misunderstood. Dedicated experiments were carried out in an isolated loop seal to assess how the gas fed into a loop seal is distributed under different aeration modes (through the recycle chamber, the supply chamber and both chambers), aeration flowrates and solids fluxes. Experimental results shed light on the nonideality of the gas-solids flows occurring in these units and, in combination with theoretical considerations, it is demonstrated why semi-empirical models cannot effectively describe the performance of loop seals operating as non-mechanical valves. Recommendations are given for the optimization of the operation of a loop seal as a non-mechanical valve in a solids circulating loop. A method is proposed to characterize the performance of a loop seal in a CFB unit from measurements of that loop seal in isolation mode. 1. Introduction Non-mechanical valves constitute a superior choice for solids circulating and gas sealing in CFB units operated with Geldart B particles, due to the absence of moving parts, low cost and operational simplicity [1,2]. Compared to L-valves and J-valves, loop seals present several advantages: safer operation due to the isolation of the standpipe from the pressure fluctuations in the riser, operational capability both as nonmechanical valve and as a solids circulation device, and more flexible design to allocate devices like heat exchangers in CFB units such as boilers. Although loop seals have been successfully used in industrial processes (mainly in CFB boilers and catalytic reactors), there is still a lack of knowledge to enable the design and operation on a fundamental basis. One controversial point is whether a loop-seal performs merely as * Corresponding author. E-mail address: [email protected] (M. Su´ arez-Almeida). Contents lists available at ScienceDirect Powder Technology journal homepage: www.journals.elsevier.com/powder-technology https://doi.org/10.1016/j.powtec.2022.117777 Received 3 May 2022; Received in revised form 11 July 2022; Accepted 22 July 2022 Powder Technology 408 (2022) 117777 2 an automatic solids flow device or as a non-mechanical valve with the capability of regulating the solids flux in a CFB loop. While most authors recognize the capacity of the loop seal for controlling solids circulation under some conditions [3–7], some argue that the solids flux is merely governed by the riser aeration [8]. The design and operating conditions of the loop seal determine which of these statements apply for a given CFB unit. In large CFB boilers, where the loop seal is rather small compared to the combustor chamber (riser), the solids flux is usually controlled by the fluid dynamics of the riser. However, the operation of the loop seal as a non-mechanical valve could be crucial in emerging fluidized beds like those dealing with technologies such as chemicallooping combustion and reforming, thermochemical energy storage based on gas-solids reversible reactions, or the production of renewable energy in hybrid processes like steam gasification of biomass in dual fluidized bed gasifiers using solar energy [9–11]. These technologies require solids circulating and gas sealing but also high flexibility of solids circulation to adapt the operation to different scenarios. Designing and optimizing loop seals for fluidized bed applications in the near future requires a robust and comprehensive knowledge of the fluid dynamics of these units. Some attempts have been made, both under modeling and experimental work, to assess the performance of the loop seal, but clear and general conclusions are still lacking. As a result, loop seal systems are still empirically designed, often ignoring fundamental considerations, thereby hindering optimization of these devices. This paper intends to clarify existing knowledge of the fluid dynamics of loop seals on the basis of conclusions obtained from experimental work conducted in an isolated loop seal cold flow model. Dedicated experiments were carried out to assess, through direct measurements, the gas split through the chambers of the loop seal as a function of the gas flowrate, the location of the aeration points and the solids flux. The experimental results, together with their implications for the design and operation of loop seals within solids circulating systems, are discussed and compared with the existing literature. In addition, the way in which results from an isolated loop seal should be used to predict the performance of and to design a loop seal coupled to a CFB unit is presented. 2. Theory Gas distribution along the loop seal (Fig. 1) is the key parameter determining the performance of the system, i.e. the flow state of the supply chamber (SC)/standpipe/downcomer (usually regarded as a dense bed in minimum fluidization or a moving bed) and the regulation capability of the solids circulation. This issue has been a matter of concern over the years when trying to model and predict the performance of loop seals. A summary is given below to bring to light the interpretations and assumptions made by different authors that often contradict each other. 2.1. Gas distribution and aeration mode Gas distribution along the loop seal has traditionally been assessed based on estimations from standpipe pressure and height measurements (assumed to be in a defluidized state) [12–15] and through the use of gas tracers [7,16–20], but the gas flowrate has never been directly measured until now. Most authors agree that the majority of the air fed to the loop seal leaves the unit through the RC (provided the SC is not fluidized) [3,7,14,21]. It has been experimentally demonstrated that increasing the solids circulation leads to a higher resistance for gas to flow up through the standpipe. For a given loop seal aeration, there is a threshold in the solids velocity, at which gas is dragged by solids in the standpipe [13,14,16], resulting in a higher flowrate of gas through the RC than that fed through the loop seal. Moreover, when the solids circulation increases as a result of an increase in the loop seal aeration, there is a critical aeration flowrate at which the downward gas flow through the standpipe is maximum (or the upward gas is minimum if the loop seal is operated under no dragging). Beyond this critical aeration, the downward gas flowrate rapidly decreases, the flow direction reverses upward and the standpipe becomes fluidized [15,17]. The aeration mode has a significant effect on gas distribution along the loop seal (i.e., on its performance) and has been the subject of many studies. Although many combinations have been analyzed, the performance of the loop seal when aerated under the simplest mono-chamber modes, i.e., separately through the SC and through the RC, has received less attention. The effect of mono-chamber aeration on the solids circulation was firstly assessed in [22] leading to unclear differences between the aeration modes and, later, in [23], where it was concluded that aerating through the SC is more effective for controlling the solids circulation, since it results in a higher solids flux for a given aeration flowrate. A more dedicated study was carried out in [24] where the SC aeration resulted in the highest solids circulation, while the RC resulted in a wider range of control of the solids circulation. Although these works demonstrate that a loop seal can operate under any monochamber aeration, conclusions on the operation of the loop seal under its simplest aeration modes are still unclear. The experiments carried out in this work (detailed in Section 3.2) intend to fill this gap. The reason behind the higher efficiency of the aeration through the SC in the control of solids circulation rests on the gas distribution and its effect on the loop seal performance as a non-mechanical valve. The operation of the loop seal as a non-mechanical valve takes place when the standpipe is under moving bed regime (also referred to as a transitional packed bed) and the pressure drop per unit length is controlled by drag forces between gas and solid particles. This pressure drop is regulated by the gas-solids relative velocity (estimated, for instance, through an expression such as that proposed by Ergun, Eq. (5)). Under this operation mode, a change in the gas-solids relative velocity results in a change in the pressure gradient along the standpipe, which needs to reallocate the solids to keep the pressure balance around the CFB loop, leading to a change in the solids circulation. Therefore, the range of solids circulation control of a non-mechanical valve is determined by the ability to change the gas-solids relative velocity before it reaches the minimum fluidization velocity (related to the flow of solids). Feeding the air through the SC leads to an increased amount of gas flowing through the standpipe (compared to the same air fed through the RC) and therefore, to a higher pressure drop in the standpipe and solids circulation. More details on the operation of the loop seal in a CFB loop and Fig. 1. Sketch of the main parts of a loop seal together with the distribution of the total fed gas (Q LS ) between the two chambers. M. Su´ arez-Almeida and A. G´ omez-Barea Powder Technology 408 (2022) 117777 3 on the gas split under different aeration modes are given in Section 4.2.1 and Section 4.1.3, respectively. The principle of the operation of loop seals has been a matter of discussion over the years, resulting in a wide variety of studies with different approaches. Traditionally, aeration through the RC was considered essential, while that of the SC simply constituted an optional way of aiding the solids flow down the standpipe and ensure smooth operation [21,25,26]. Moreover, some authors still wrongly state that to operate the standpipe in a non-fluidized mode, the aeration through the SC should not exceed that of minimum fluidization [27,28]. In addition to the previously mentioned works, the superior performance of the SC aeration for controlling the solids circulation is also recognized by [3,5,19], but only in [19] is the possibility of operating under SC monochamber aeration recognized. Although operational limitations, such as the appearance of slugging reported in [25] for a DFB gasifier, tend to tarnish the advantages of aerating through the SC, new designs have already been proposed to stabilize the operation [29]. Vertical aeration through the SC/standpipe has been extensively studied as a way to optimize the performance of the loop seal as a non-mechanical valve [29–31], promoting the gas flowing through the standpipe and avoiding the gas bypassing through the opening. Other works have assessed different aerations combining both bottom and side aerations [32,33], although the results are not comprehensive enough for generalization. An extensive analysis of the operation of the loop seal in a DFB unit is carried out in [7], where different aeration modes are analyzed and the superior performance of aerating through the SC is also demonstrated for these units (in which the gas distribution along the loop seal plays an important role since the leakage between reactors needs to be avoided [7,20]). 2.2. Modeling barriers Loop seals may be considered the heart of a CFB plant. As a result, models appear to be excellent tools, not only to understand the performance of the loop seal, but also to optimize its operation within the solids circulating loop. Many modeling attempts have been made over the years, but certain features of these units have prevented them from achieving success. This section presents the barriers found when modeling the fluid dynamics of a loop seal and reviews different works dealing with loop seals modeling. The fluid dynamics of an isolated loop seal, such as that presented in Fig. 2, can be studied by a simple model based on the pressure balance along the unit. The unit in Fig. 2 is fed by a constant flow of solids (F s ) and a gas flowrate (Q LS ), while the solids inventory is adapted by the height of solids in the SC (h SC ) and, to a lesser extent, by the bed expansion in the RC, to balance the pressure along the loop seal. Assuming that the pressure drop along the opening is small compared to that of the chambers, it follows, ΔPSC =ΔPRC +ΔPyx (1) The RC always needs to be fluidized to allow the solids circulation thus, the pressure drop is given by the hydrostatic pressure of the head of solids along the RC up to the weir (h RC ), ΔPRC = ρ p(1− ε RC)hRC g(2) The bed voidage ( ε RC ) can be estimated considering the bubble fraction (δ) as, ε RC =δ+ (1−δ) ε mf (3) A bubble fraction equal to zero represents the operation of the RC under incipient fluidization. The expansion of the bed can be calculated with a model estimating the expansion behavior in bubbling beds such as that in [34], δ=1 1+1.3(0.15+(ug,RC −us,RC ) ε RC −umf )1/3 0.26+0.7e−3300 dp((ug,RC −us,RC) ε RC −umf )−0.8 (4) Assuming that the gas-solids relative velocity in the SC is that allowing the operation under moving bed regime, the pressure drop is given by the modified Ergun equation [35], where the bed voidage can be regarded, as a first approach, constant and equal to that at minimum fluidization. ε SC = ε mf (6) When the superficial gas-solids relative velocity in the SC, (u g,SC +u s, SC ) ε SC , is higher than the minimum fluidization velocity, the SC operates under fluidized flow. In this case, the modeling approach followed for the RC Eqs. (2)–(4) can be applied to the SC, but taking into consideration the gas-solids relative velocity in the SC and the height of solids in the SC. However, this operation is rarely applied to loop seals operating in actual circulating loops since, apart from entailing higher costs regarding the increased need for fluidizing agent, operating the SC under vigorous bubbling can lead to instabilities in the operation, affecting the efficiency of the cyclone in CFB units and leading to contamination/dilution of the reacting system in DFB units. The actual solids and gas velocities in both chambers are defined as, us,RC =Fs ρ p(1− ε RC)ARC (7) us,SC =Fs ρ p(1− ε SC)ASC (8) Fig. 2. Solids and gas velocities and fed gas distribution in the two chambers of a loop seal. The downcomer is considered an extension of the SC in this sketch; F s represents the flowrate of solids and P x and P y the pressure at the inlet and outlet of the SC and RC, respectively, being P y >P x . ΔPSC =(150 (1− ε SC)2 ε SC2 μ g ϕdp2(ug,SC +us,SC)+1.75 (1− ε SC) ε SC ρ g ϕdp(ug,SC +us,SC)2)hSC (5) M. Su´ arez-Almeida and A. G´ omez-Barea Powder Technology 408 (2022) 117777 4 ug,RC =u0,RC ε RC (9) ug,SC =u0,SC ε SC (10) where the superficial gas velocities are given by considering the gas flowing through each chamber, the sum of them being equal to the introduced gas through the loop seal, Q LS , u0,RC =QRC ARC (11) u0,SC =QSC ASC (12) QLS =QSC +QRC (13) To sum up, this simple model accounts for 13 equations and 14 unknowns (ΔP RC , ΔP SC , ε RC , ε SC , δ, u g,RC , u g,SC , u s,RC , u s,SC , u 0,RC , u 0,SC , h SC , Q RC and Q SC ), the rest of the parameters constituting known data. Obviously, the same balance results from considering the SC under fluidized flow. Therefore, it can be concluded that an additional relation is necessary to solve the model, predicting the gas split through the chambers of a loop seal. Gas distribution through the loop seal is determined by the resistance to gas-solids flow offered by the SC, the horizontal passage (opening) and the RC. A proper model of these resistances by a momentum balance, considering both gas and solid phases, provides the required closure relation. Under certain operating conditions, there is a limiting resistance, enabling the simplified treatment of the loop seal model. However, the operating conditions and geometry of loop seals found in the literature greatly vary from each other, making a comprehensive generalization of loop seal performance difficult. For these reasons, none of the models developed to date are general enough to predict the performance of loop seals. There are only two published models that, although unsuccessful, theoretically provide a relation that could be used to close the model above [36,37]. Both approaches set in the RC the closure relation, considering the RC as that which is limiting the flow of solids and therefore, setting the gas distribution. However, the RC is always under fluidized flow, so it never limits the flow of solids and both approaches fail on a fundamental basis. In [36] the sharp-crested weir theory was applied to the solids overflow from a loop seal leading to an empirical relation. However, it is not clear from publications [1,36] if the solids flow rate is related to the height of the expanded bed above the weir or to the height of the weir. Only the latter option would enable to close the model since the expanded bed above the weir is unknown. For a given solids flux, the relation allows for calculation of the voidage in the RC (since the voidage in the bed above the weir is assumed to be equal to that in the RC), thereby establishing the gas distribution. This theory has received neither more attention nor experimental validation over the years. In the model developed in [37] the required closure relation is provided by a global force balance in the recycle chamber. However, it can be verified by the equations of the model that the gas-solids relative velocity in the RC, the voidage at the RC and, therefore, the pressure drop along the RC, are independent of the operating conditions, so the model is not able to capture the measure trends. This model intended to describe the operation of a loop seal aerated only through the RC with the standpipe operating under moving bed regime. Later, a similar approach was extended to assess the operation of a loop seal aerated by both the RC and the SC in which both chambers were assumed to be fluidized [38]. Although both models are contestable, they represent the only attempts at comprehensively describing the fluid dynamics of a loop seal considering differences in the operation due to the aeration mode. In [39] a momentum balance in the SC is used as the closure relation to determine the gas division through a standpipe joined to a fluidized bed. This approach is applicable to narrow standpipes (where the solidswall friction may be significant) connected in series to a fluidized bed, but it is rarely valid in loop seal non-mechanical valves in circulating devices (with wider standpipes connected to the RC by a horizontal passage which offers significant resistance to gas-solids flow). In most models the resistance through the horizontal passage (opening) is either neglected [10,37,38,40] or considered as that between two joined fluidized beds [41]. No published work has considered the resistance through the opening as that limiting the flow for setting the gas split in loop seal units. Other modeling studies have applied assumptions for the gas distribution or semi-empirical correlations. The first attempts at modeling dealt with validating experimental measurements from an isolated loop seal [21] and analyzing the performance of a loop seal in a CFB loop [42]. Both assumed a constant fraction of gas flow through the RC equal to 9.5% as correlation of experimental measurements in [21]. In [43] the performance of the loop seal was assessed (to describe the fluid dynamics of a CBF unit) using an empirical correlation of the pressure drop with the solids flux, particle properties and loop seal diameter. This approach may fit measurements of a particular unit, but it does not provide a comprehensive understanding of the performance of a loop seal on a fundamental basis. A model based on the pressure drop of the defluidized supply chamber and the horizontal channel was developed in [44] to estimate the pressure at the bottom of the recycle chamber of an isolated loop seal. Based on the same unit, a model was developed in [40] to estimate the concentration of solids in the RC and the height of solids in the SC based on a force balance in the RC. In both models the gas distribution was arbitrarily set, not being subjected to any fundamental or empirical consideration. The model proposed in [45] is based on a function tending to a constant value as the aeration is increased, whose parameters are obtained through correlation of experimental data. Once the parameters are found, the model is able to predict the solids circulation in a loop seal as a function of the fluidization number (ratio between the actual and minimum fluidization velocity). Although the authors attempted to provide the model a fundamental background, it is far from describing the performance of a loop seal from a fundamental point of view. A general law to model loop seals has yet to be proposed. None of the models developed to date are general enough to predict the performance of loop seals. Those based on correlations are only applicable to the units from which the correlated measurements were obtained, while those based on theoretical approaches have failed in their attempts since they have not been able to ascertain the limiting factors leading to the gas split between the chambers (key aspects, such as the aeration mode, have been overlooked). The experimental work presented here clearly demonstrates that theoretical prediction of the operation of a loop seal requires a more careful modeling effort than those offered by the models developed to date. 3. Experimental 3.1. Experimental setup An experimental cold flow model was used to study the fluid dynamics of a loop-seal. The system was isolated from an existing CFB to dedicatedly study the gas and solid motions in the different parts of the loop seal. The rig, presented in Fig. 3, consists of a hopper joined to a solids discharge system (pipe and valve for controlling the solids flux) and a loop seal joined to a cyclone which collects the circulating solids leaving the system into a bin. Two rotameters are used to control the gas flowrate separately, one to feed the gas to the supply chamber (SC) and another one to the recycle chamber (RC) (the wind boxes of the SC and RC are completely separated from each other). The gas leaving the M. Su´ arez-Almeida and A. G´ omez-Barea Powder Technology 408 (2022) 117777 5 cyclone, i.e., that passing through the RC, is measured using another rotameter, which requires a valve at the inlet to stabilize the float. A measuring tape is set along the downcomer to measure the height of solids during the tests. Fifteen pressure taps are allocated along the loop seal. A data logger and a computer are used to continuously (every second) register the pressure measurements. The downcomer has a 0.093 m internal diameter, SC and RC have a cross section of 0.119 ×0.130 m 2 while the section of the opening is 0.130 ×0.055 m 2 . The height of the SC above the distributor is 0.270 m and that of the RC until the discharging pipe is 0.155 m. The measuring tape is located just above the SC but the downcomer is 0.060 m below this point (0.02 m along the junctions and 0.04 m inside the SC). The gas distributor is a 4 mm thick perforated plate with 45 holes of 2 mm diameter in each chamber and a fine metallic grid to avoid the falling of solids back to the plenum. The pressure drop through the distributor was below 15 mbar for all the tests. The bed material was glass beads with a particle size of 150–250 μ m, a density of 2503 kg/m 3 (Geldart B) and an experimental minimum fluidization velocity at room temperature of 0.04 m s −1 . Air at room temperature was used as fluidizing agent. 3.2. Experiments During the tests the solids flux was kept constant while varying the gas flowrate fed to the system, as well as the aeration mode. Solids fluxes of 5, 10 and 15 kg m −2 s −1 (referring to the SC cross section) were tested at three different aeration modes (only RC, only SC and both chambers). The same gas flowrate was introduced through each chamber in experiments aerated under both chambers. For each test the pressure at taps presented in Fig. 3 was continuously measured and registered for two minutes of operation after reaching the steady state. The latter was considered to be attained when the height of solids in the downcomer reached a constant value on the measuring tape. The gas distribution along SC and RC was calculated by the difference between the introduced air and that leaving the system through the RC. Two sets of tests were carried out: one with the outlet rotameter (and the outlet valve) to measure the gas leaving the RC and, another, removing both the outlet rotameter and valve, to assess the effect of this outlet constriction on the gas and solids distribution along the loop seal. In those tests carried out with the outlet rotameter, the outlet valve was always kept in the same position. A minimum of 2 tests were conducted for each of the operating conditions to guarantee repeatability. The tests carried out (145 in total, 108 with the outlet valve and 37 without outlet valve) are summarized in Table 1. 4. Results and discussion Experiments were carried out for constant solids fluxes varying the aeration flowrate. For a given solids flux, after a change in the air flowrate fed, the solids inventory in the system varied, adapting the pressure throughout the unit to allow the operation under the new conditions. This operation provides insights into the behavior of an isolated loop seal, but the knowledge gained can be used to describe the performance of the loop seal connected to a CFB unit where (most of the time) the solids inventory is kept constant while the solids flux is the variable to be controlled. Experimental results obtained in the isolated loop seal under constant solids fluxes are presented in Section 4.1, while discussion on how this information is used in a loop seal coupled to a CFB unit is discussed in Section 4.2. Fig. 3. Experimental rig (numbered dots represent the pressure taps; dot number 1 is never covered by solids so it measures the atmospheric pressure). Table 1 Summary of tests (fed gas velocity referred to the cross section of a single chamber; G s referred to the cross section of the SC). Aeration mode Air flowrate (m 3 h −1 ) Fed gas velocity (m s −1 ) Number of tests G s =5 kg m −2 s −1 G s =10 kg m −2 s −1 G s =15 kg m −2 s −1 Tests with outlet rotameter SC 3.3 0.059 2 – – 3.5 0.063 2 2 2 3.7 0.066 3 – – 4.0 0.072 2 2 2 4.3 0.077 2 – – 4.6 0.083 2 2 2 4.9 0.088 2 – – 5.2 0.093 2 2 2 6.8 0.122 3 2 2 SC +RC 3.9 0.070 3 2 2 4.5 0.081 2 2 2 5.0 0.090 2 2 2 5.5 0.099 2 2 2 7.2 0.129 2 2 2 RC 3.8 0.068 2 2 3 4.3 0.077 2 2 2 4.8 0.086 3 2 4 5.3 0.095 3 2 4 6.8 0.122 2 2 2 Tests without outlet rotameter SC 3.3 0.059 3 – 4 3.8 0.068 2 – 2 5.0 0.090 2 – 4 6.5 0.117 2 – 2 RC 3.6 0.065 2 – 2 4.2 0.075 2 – 2 5.0 0.090 2 – 2 6.5 0.117 2 – 2 M. Su´ arez-Almeida and A. G´ omez-Barea Powder Technology 408 (2022) 117777 6 4.1. Experimental results 4.1.1. Tests with the outlet rotameter The gas split between the chambers of the loop seal can be understood by examination of Fig. 4. The gas flowrate through the RC (Fig. 4 (a)) and SC (Fig. 4(b)) is plotted as a function of the total flowrate fed into the loop seal for the three solids fluxes and three aeration modes investigated. In agreement with previous studies [16], the solids flux has a significant effect on the gas distribution. For a given aeration flowrate, the air flowing through the RC increases as the solids flux is increased, due to the higher amount of air dragged in the SC by the downwards flow of solids. Moreover, it can be observed that the increase in gas dragging is not linear but increases at higher solids fluxes. Both behaviors are observed for all the aeration modes. Furthermore, it is seen that the gas split strongly depends on the aeration mode. For a given air flowrate fed to the loop seal, the highest gas flow through the RC is obtained when aerating through the RC, and the lowest by aerating through the SC. For a given solids flux, increasing the air flowrate through the SC leads to an almost constant gas flowrate through the RC. A considerable decrease is only observed when the SC gets fluidized, suggesting that once bubbles are formed in the SC, the gas finds lower resistance flowing through the bubbles than through the opening and the RC, resulting in a reduction of the gas circulation through the RC, even after increasing the total aeration. As expected, the flowrate of air at which the SC gets fluidized is higher as the solids flux is increased. When the loop seal is aerated through both chambers or through the RC, the air flowing through the RC always increases with the increase in the aeration flowrate. Fig. 4(c) presents the flow of air circulating through the opening, which is defined as positive when circulating from the SC to the RC. It is observed that, for a given solids flux, when the loop seal is aerated through the SC and through both chambers, the gas flowrate through the opening is almost constant and positive for all the aeration flowrates tested. On the contrary, when the loop seal is aerated through the RC, the gas circulates from the RC to the SC (except for the lowest aeration flowrate at a solids flux of 15 kg m −2 s −1 , at which the gas is dragged from the SC to the RC) and it is higher as the air flowrate fed to the loop seal is increased. During the tests it was observed that when aerating only through the SC, the RC was always a dense bed with no bubbles, while vigorous bubbling appeared as the gas flowrate was increased for the other aeration modes. This observation is evident from Fig. 4(d), where the fluidization number in the RC (ratio of actual to minimum fluidization velocity) is shown to be well below 1.5 and practically constant as the aeration increases through the SC, whereas it greatly increases when aerating through the RC. However, bubbles are expected to appear in the RC when aerating only through the SC at high enough solids fluxes as a result of the gas dragging (this was not observed in our experiments due to the limited range of solids fluxes tested). A better understanding of the influence of aeration modes is obtained by analyzing Fig. 5 where the height of solids in the downcomer, the pressure at the bottom of the RC (tap 12) and that at the bottom of the SC (tap 10) are presented for the three aeration modes tested. The constriction introduced by the pressure drop of the valve prior to the Fig. 4. Air flowing (a) through the RC, (b) SC and (c) opening, and (d) fluidization number in the RC (superficial gas velocity in the RC to minimum fluidization velocity considering the solids flux) against the total flowrate of air fed to the loop seal for tests with the outlet rotameter, under different solids fluxes (G s : kg m −2 s −1 , referring to the cross section of the SC) and aeration modes (RC: only RC, RC +SC: both chambers, SC: only SC). M. Su´ arez-Almeida and A. G´ omez-Barea Powder Technology 408 (2022) 117777 7 outlet rotameter needs to be considered when interpreting the experimental results from Fig. 5. Logically, the pressure drop through the valve is higher as the gas flowrate through the RC increases. Fig. 5(a-c) shows that, for a given aeration, the increase in the solids flux increases the height of the downcomer, while, for a given solids flux, the height of the downcomer as the aeration increases, follows different patterns depending on the aeration mode. The height continuously decreases when the gas is fed through the SC and through both chambers (although not significantly), while it presents a minimum when aerating through the RC. These findings are discussed below. When the loop seal is aerated through the SC, for a given solids flux, the amount of air flowing through the RC is almost constant (as shown in Fig. 4) leading to a fairly constant pressure drop in the outlet valve. As a result, the pressure is constant at the bottom of the RC (tap 12, Fig. 5(d)). The gas circulating through the SC increases consistently with the air flowrate fed to the SC, but the maximum pressure of the system does not change (tap 10, Fig. 5(g)). Therefore, the height of solids in the downcomer decreases, keeping the pressure constant while increasing the pressure gradient due to the higher relative velocity. It is also observed that the height of solids in the downcomer tends to a constant value at high gas flowrates since, once the SC gets fluidized, the pressure drop remains roughly constant with the air flowrate. The aeration of the loop seal through the RC leads to the highest height of solids in the downcomer for a given flowrate of fed gas, as seen by comparison of Fig. 5(a) and Fig. 5(b). This is due to the lower gas flowing through the SC when aerating through the RC (Fig. 4(b)) (compared to the same aeration flowrate fed through the SC), which results in a higher height of solids to compensate for the pressure drop of the system. The height decreases with the increase in aeration flowrate through the RC, while the increase in the gas flow through the SC results in a higher pressure drop than that required to compensate for the pressure drop introduced by the outlet valve. For higher aerations, the pressure drop given by the outlet valve becomes so high that the increase in the gas flow through the SC is not enough to compensate for it and the column of solids starts building upwards, explaining the minimum found in Fig. 5(b). It is notable that, in spite of the higher pressure at the bottom of the RC (at tap 12) when aerating through the RC, Fig. 5(e), (compared with that when aerating by the SC, Fig. 5(d)) caused by the higher pressure drop introduced by the outlet valve, it is observed that for low gas flowrates the pressure is lower at the bottom of the SC (Fig. 5(h)). The reason is that the pressure drop across the opening (P 10 -P 12 ) is much lower when aerating through the RC (Fig. 6), compensating for the higher pressure drop given by the valve at low aerations. As expected, an intermediate behavior is observed as the aeration is applied through both chambers. The height of solids in the downcomer (Fig. 5(c)) is in between that obtained for the SC and the RC aerations. The trend of the solids height profile is closer to that obtained when aerating through the SC since, for all the air flowrates tested, the circulation of gas through the SC led to a higher pressure drop than that required to compensate for the increase in pressure drop given by the outlet valve (leading to a decrease in the height of solids). According to the measurements from the gas circulation through the opening (Fig. 4(c)) the behavior of pressure drop through it (P 10 -P 12 ) could be anticipated a priori: (i) a rather constant pressure drop should be measured when aerating through the SC/both chambers (since the gas circulation is roughly independent of aeration flowrate for fixed solids flux), and (ii) a negative and decreasing pressure drop should be Fig. 5. Height of the downcomer (a, b, c) and pressure at taps 15 (d, e, f) and 10 (g, h, i) against the air fed through the different aeration modes for tests carried out with the outlet rotameter under the different solids fluxes (G s : kg m −2 s −1 , referring to the cross section of the SC). M. Su´ arez-Almeida and A. G´ omez-Barea Powder Technology 408 (2022) 117777 8 measured while increasing the aeration through the RC (since more gas circulates from 12 to 10). However, as shown in Fig. 6, where the pressure drop measured across the opening is plotted as a function of the aeration flowrate (Fig. 6(a)) and of the actual gas flowing through the opening (Fig. 6(b)), the actual behavior apparently differs from that expected. Despite the scattering of measurements, it is clearly seen that the pressure drop is higher when aerating through the SC and it decreases, for the three aeration modes, with the increase in the aeration flowrate to the loop seal. The latter contradicts statement (i). However, it can be explained by considering the irregular gas-solid flow through the opening. At low aerations the solids at the bottom of the SC are very compacted, favoring gas shortcuts instead of a homogenous distribution. As the aeration is increased, the solids are softened, allowing for a better distribution of the gas through the cross section of the horizontal passage and the opening. When taking a shortcut, a given gas flowrate through the opening leads to higher pressure drop due to the higher gassolids relative velocity (narrower effective cross section). The same gas flowrate under a homogeneous gas-solid flow distribution leads to lower gas-solids relative velocity resulting in lower pressure drop. Finally, it is observed that the trend of the pressure drop with the aeration through the RC is consistent with that expected in statement (ii), although the expected negative pressure drops were only registered for the highest air flowrate tested. The ability of the different aeration modes to change the solids flux in a CFB unit is assessed in this work through the increase in the standpipe pressure gradient (see Section 4.2.1 for details on the operation of loop seals in CFB systems). This increase in the pressure gradient is evaluated by the change in the pressure drop measured between taps 10–8, which are always covered by solids. Measurements reported in Fig. 7(a) show that the pressure gradient through the SC increases for all the aeration modes but the highest slope is observed when aerating only through the SC. This implies that, in a CFB unit, the aeration of the loop seal through the SC allows for controlling of the solids circulation with the lowest aeration and solids inventory. The maximum solids flux given by the Fig. 6. Pressure drop through the opening against (a) the total flowrate of air fed to the loop seal and (b) the flowrate of air flowing through the opening for tests with the outlet rotameter under different solids fluxes (G s : kg m −2 s −1 , referring to the cross section of the SC) and aeration modes (RC: only RC, RC +SC: both chambers, SC: only SC). Fig. 7. Pressure drop through the SC between taps 10–8 against the flowrate of air fed to the loop seal (a) and measured (Exp.) and estimated (Est.) pressure drop through the SC between taps 10–8 against the superficial gas-solids relative velocity in the supply chamber (b) for tests with the outlet rotameter under different solids fluxes (G s : kg m −2 s −1 , referring to the cross section of the SC) and aeration modes (RC: only RC, RC +SC: both chambers, SC: only SC), only tests with the SC under moving bed regime are considered in (b). M. Su´ arez-Almeida and A. G´ omez-Barea Powder Technology 408 (2022) 117777 9 loop seal operated as a non-mechanical valve is that of the SC under fluidized regime (from this point forward there is no change in the pressure drop of the SC/downcomer), which is reached at approximately 5 m 3 /s when aerating through the SC and 7 m 3 /s when aerating through both chambers while, for the flowrates of air tested, it is never reached when feeding the air through the RC. As a way to assess the quality of the gas distribution across the SC, the pressure drop between taps 10–8 have been estimated using the Ergun equation (Eq. (5)), assuming a constant bed voidage equal to that at incipient fluidization, for tests with a gas-solids relative velocity below that of minimum fluidization. Results are presented in Fig. 7(b) where high discrepancies are observed for the lowest pressures, corresponding to those tests in which the gas is not well-distributed across the section of the SC. This is consistent with the explanations provided above: the gas short-cuts the bed at low gas flows, passing through a narrower section than that of the SC, which results in a higher pressure drop compared to that estimated from the Ergun equation. A comprehensive qualitative analysis of the misdistribution of gas and solids in the loop seal operation is given in Section 4.1.3. 4.1.2. Comparison of tests with and without the outlet rotameter Tests without the outlet rotameter were conducted to compare the performance of the loop seal with and without restriction to flow at the exit of the RC (i.e., the difference of pressure drop (P 15 -P 1 )). As the gas distribution cannot be directly measured in tests without the outlet rotameter, the comparison is made qualitatively through the height of solids in the downcomer and the pressure drops throughout the system, for the same operating conditions (aeration mode and flowrate). In tests without the outlet valve, the maximum pressure of the isolated loop seal rig (that of tap 10 or 12, depending on the operating conditions) either remains constant or slightly decreases (due to the RC bed expansion) as the gas flowrate fed to the loop seal is increased, but it never increases as it does when operating with the outlet valve. As a result, the height of solids in the downcomer always decreases with the air flowrate, even for the aeration through the RC (Fig. 8) (in contrast to the increase observed in the operation with the outlet rotameter to compensate for the high pressure drop given by the outlet valve). Moreover, as expected, a lower level is reached by solids in the downcomer in tests carried out without the outlet valve. The highest difference in heights is observed for tests aerated through the RC as are those, in tests with the outlet rotameter, with the highest circulation of gas through the RC, leading to a higher pressure drop at the outlet valve (represented by pressure at tap 15, Fig. 9(a)). Fig. 9(a-c) compares the pressure drop through the RC, the SC and the opening for tests with and without the outlet valve and a solids flux of 5 kg m −2 s −1 . The pressure drop through the RC (represented by pressure drop 12–13 in Fig. 9(b)) tends to be lower in tests without the outlet valve due to bed expansion, consistently with a higher flowrate of gas through the RC. This behavior is more prevalent in tests carried out for a solids flux of 15 kg m −2 s −1 (not reported here) where more gas flows through the RC due to the solids dragging. The pressure drop in the SC cannot be assessed so easily since it also depends on the pressure drop across the opening, which is difficult to evaluate. Although accurate estimations of the gas distribution along the loop seal are not possible in tests without the outlet rotameter, it is clear from Fig. 9 that the performance of the loop seal follows the same trends with and without the outlet valve. The conclusion is that, although the actual gas-solids distribution is affected by the pressure drop along the other components of the CFB loop, the pattern of the resistance of the loop seal unit to gassolids flow (resistance curve) is characterized, to a large extent, by its geometry. 4.1.3. Implications of gas-solids flow patterns in the operation and modeling of loop seals The above experimental results showed that the aeration mode, together with the geometry of the unit, establishes the distribution of the gas fed through the loop seal and, therefore, the performance of the loop seal under different aeration and solids flowrates. It was also demonstrated that the non-homogeneous gas distribution and the presence of stagnant zones of solids are essential to understand (and to predict) the gas split between the chambers. Fig. 10 and Fig. 11 show a representation of the non-ideal gas and solids flow patterns observed when operating the loop seal under monochamber aerations, SC and RC, respectively. The stagnant zone formed at the bottom of the SC is reduced with the increase in aeration through the SC until disappearing once it becomes fluidized Fig. 10(a-c). At low solids flows the RC is a dense bed close to incipient fluidization, while bubbles start flowing close to the central baffle as the solids flow is increased, due to the higher contribution of the gas dragged by solids (Fig. 10(d)). Although not represented in figures, it is clear from Fig. 10 (d) that when aerating through the SC the gas is spread into the RC through the upper part of the opening, leading to a lower stagnant zone in the RC. Fig. 11 shows that the operation of the loop seal when aerated through the RC is characterized by a huge stagnant zone in the SC, but good gas distribution across the RC, leading to vigorous bubbling as the aeration is increased. Although the death zone in the SC gets smaller both with increasing the aeration and the solids circulation, it always remains sizable (Fig. 11(c)). An intermediate behavior is obtained when Fig. 8. Downcomer height against the flowrate of air fed to the loop seal for tests carried out with and without the outlet rotameter (nv), under different aeration modes (RC: only RC, SC: only SC) and solids flux (G s : kg m −2 s −1 , referring to the cross section of the SC) of 5 (a) and 15 (b). M. Su´ arez-Almeida and A. G´ omez-Barea