- Email: [email protected]
Gas and particle circulation in an internally circulating fluidized bed membrane reactor cold model
Chemical Engineering Science 64 (2009) 2599 -- 2606
Contents lists available at ScienceDirect
Chemical Engineering Science journal homepage: w w w . e l s e v i e r . c o m / l o c a t e / c e s
Gas and particle circulation in an internally circulating fluidized bed membrane reactor cold model Donglai Xie a,c,∗ , C. Jim Lim b,c , John R. Grace b,c , Alaa-Eldin M. Adris c a
MOE Key Laboratory of Enhanced Heat Transfer and Energy Conservation, South China University of Technology, Guangzhou 510640, PR China Department of Chemical and Biological Engineering, The University of British Columbia, 2360 East Mall, Vancouver, Canada V6T 1Z3 c Membrane Reactor Technologies Ltd., Suite 1800-200 Granville Street, Vancouver, Canada V6C 1S4 b
A R T I C L E
I N F O
Article history: Received 28 July 2008 Received in revised form 12 February 2009 Accepted 19 February 2009 Available online 9 March 2009 Keywords: Hydrogen Fluidization Membrane Gas circulation Particle circulation Cold model
A B S T R A C T
A cold model study was carried out of an internally circulating fluidized bed membrane reactor for hydrogen production by steam reforming. This configuration facilitates a novel process in which air is introduced for autothermal operation, while avoiding diluting the hydrogen in the gas stream by nitrogen. A column of diameter 230 mm and height 2.4 m contained a membrane box of square cross-section. Gas circulation was investigated by injecting helium tracer into the top of the bed above the membrane box and detecting its concentration in the annulus and lower membrane box by a thermal conductivity detector. Solids circulation was determined from the downward velocity of particles in the annulus. The major resistance to solids circulation was found to be at the passage connecting the outer downflow to the inner upflow compartment. A ring distributor helped overcome this resistance. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Widespread usage of hydrogen, if generated in an advantageous manner, could alleviate growing concerns about the world's energy supply, security, air pollution, and greenhouse gas emissions. Hydrogen is also a major industrial commodity, used as an intermediate in a number of chemical and metallurgical processes, for example in the production of ammonia and methanol, upgrading of heavy hydrocarbons, iron ore reduction and food processing. Although it is the most abundant element in the universe, hydrogen does not exist naturally in large quantities or high concentrations. Most of the world's hydrogen is generated at large petroleum and chemical plants by steam reforming or partial oxidation of natural gas in parallel fixed bed reactors within huge top-fired or side-fired furnaces, coupled with pressure swing absorption (PSA) for hydrogen purification. Other methods of producing hydrogen such as gasification of biomass, biological production and electrolysis of water are not currently economic at large scales. In conventional steam reforming, high temperatures (typically > 850 ◦ C) are achieved by hanging fixed bed reactor tubes inside a gas-burning furnace. Heat must be transferred inwards through
∗ Corresponding author at: MOE Key Laboratory of Enhanced Heat Transfer and Energy Conservation, South China University of Technology, Guangzhou, 510640, PR China. Tel./fax: +86 20 22236985. E-mail address: [email protected] (D. Xie). 0009-2509/$ - see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2009.02.032
the walls of the tubes. Expensive metal alloys are required for the tubes to withstand the resulting high temperature gradients. Catalyst particles are several millimeters at least in diameter to minimize the pressure drops through the beds, causing strong internal diffusional resistances (Grace et al., 2005). To overcome these disadvantages, fluidized bed and membrane separation technologies have been combined in fluidized bed membrane reactors (FBMR) (Adris et al., 1991, 1994) for hydrogen production from methane in a single-step process. Many aspects of FBMR have been investigated. Adris and Grace (1997) studied the scale-up and practical issues; Roy et al. (1999) explored autothermal operation of FBMR; Chen et al. (2003, 2004) studied steam reforming of heptane and higher hydrocarbons. Several reactor models (e.g. Abba et al., 2003; Adris et al,. 1997; Chen et al., 2003; Dogan et al., 2003; Grace et al., 2001; Patil et al., 2007) have also been developed to simulate FBMR processes. Experimental studies in a commercial FBMR unit were reported by Chen et al. (2007). A review of FBMR developments was provided by Deshmukh et al. (2007). Since the net steam methane reforming (SMR) reactions are endothermic, heat is required to sustain the reactions by one or both of (1) External heat exchange. (2) Air, enriched air or oxygen addition to generate heat by exothermic oxidation of species like CH4 , CO, and H2 . When the latter method is employed, it is desirable to use air, rather than enriched air or pure oxygen, as the oxidant source to reduce the
2600
D. Xie et al. / Chemical Engineering Science 64 (2009) 2599 -- 2606
operating cost and energy penalties. However, when air is selected, nitrogen is also introduced into the reactor, decreasing the hydrogen partial pressure in the gas stream, thereby reducing the driving force for diffusion of hydrogen through the membrane. To compensate for this shortcoming and to maintain uniform reactor temperature, an internally circulating fluidized bed membrane reactor (ICFBMR) system is investigated in this paper. 2. Conceptual design of the ICFBMR and challenges The conceptual design of the ICFBMR is illustrated in Fig. 1. Catalyst particles are transported upwards by gas in the central membrane baffle zone where SMR reactions take place to produce hydrogen, which is then withdrawn after permeation through the membrane panels. Air is injected into an oxidation zone at the top of the bed above the membrane baffles or into the outer annulus of the reactor. Much of the heat generated by the oxidation reactions is absorbed by the catalyst particles, which circulate downwards through the outer annulus area. At the bottom, the heated solids are re-entrained into the central reforming (upflow) zone by the reactant gas of steam and methane entering the bottom of the reactor. The excess sensible heat of the circulating catalyst reaching the bottom then provides the heat needed for the endothermic steam reforming. Similar reactor configurations can be used for processes that are reversible and thermodynamically limited, such as propane dehydrogenation to propylene, cyclohexane to benzene, and ethyl benzene to styrene. To design such reactors, the hydrodynamic features of the bed need to be understood, especially the solids and gas circulation
patterns. Important questions needing clarification addressed in this paper are: 1. Quantity of nitrogen reaching the reforming zone. The nitrogen concentration in the SMR zone must be as low as possible so as not to appreciably reduce the permeation of hydrogen through the membrane surfaces. 2. Circulation rate of catalyst particles recycle between the oxidation and SMR zones. The solids circulation rate must be sufficient to ensure that the temperature difference between the oxidation and SMR zones is within tolerable limits. For the hydrodynamic study of the ICFBMR, a pilot scale cold model was constructed at the University of British Columbia. Extensive experiments were carried out to study the gas and solids circulation patterns and obtain quantitative data for the design of such reactors. 3. Experimental set-up and methodology 3.1. Basic experimental set-up As shown in Fig. 1, the cold model reactor was equipped with an air distributor, a cyclone (later replaced by a sintered metal filter) and a 2.4 m tall column with “dummy” membrane box of square cross-section inside. Air was supplied to the cold model from an air compressor. An acrylic pipe of 230 mm inner diameter served as the column. The dummy membrane box was made of Plexiglas, with four identical membrane sections or modules, each 330 mm high with a cross-section of 142 mm2 . Six 9.5 mm thick Plexiglas equally spaced plates were inserted into each section form the overall membrane box. The four sections were joined end-to-end in an aligned (i.e. parallel) manner. The cross-section of the membrane box is shown in Fig. 2. Two panels were removed in some tests to study the influence of the core cross-sectional area on solids circulation. Air was introduced by four separately controlled distributors: (a) a main distributor in the center of the bottom cap, directing the gas to the bottom of the membrane box; (b) a side distributor directing gas to the outer annulus region; (c) a top ring distributor; (d) a bottom ring distributor. The main distributor contained 16 nozzles made of 6 mm O.D. plastic tubes, in four rows with four in each row. Each nozzle had four 1.6 mm diameter holes, pointing at 45◦ downward to the horizontal. The side distributor consisted of eight plastic 6 mm O.D. tubes, two in each quadrant. Each side distributor tube was drilled with two 1.6 mm diameter holes, again pointing at 45◦ downward to the horizontal. The two ring distributors were fabricated from 3.2 mm copper tubes and installed around the outside air distributor nozzles, as shown in Fig. 1. The top ring contained sixteen uniformly distributed 0.4 mm diameter holes, whereas sixteen 0.8 mm diameter holes were drilled in the bottom ring, with their centrelines all pointing inwards toward the axis of the column. When installed, the bottom of the membrane box was immediately above the side distributor nozzle openings. Fig. 3 is a photo of the bottom cap illustrating the relative positions of these distributors. Fluidized catalytic cracking (FCC) particles of Sauter mean diameter 65 m and density 1600 kg/m3 were selected for the experiments based on dimensionless scaling parameter studies on the operation conditions between the high temperature reactor and the cold model (Boyd et al., 2007). These particles had a minimum fluidization velocity of 0.0032 m/s and a loose bed voidage of 0.45 in air at atmospheric temperature and pressure. 3.2. Gas circulation measurements
Fig. 1. Schematic (side view) of internally circulating fluidized bed membrane reactor and the cold model. All dimensions are in mm.
To determine the gas circulation and mixing pattern, helium tracer was injected into the top oxidation zone above the membrane
D. Xie et al. / Chemical Engineering Science 64 (2009) 2599 -- 2606
2601
Table 1 Position of helium sampling probes. Probe #
Location and horizontal position
Height above air distributor (m)
1 2 3 4
Annulus (10 mm from column Annulus (10 mm from column SMR zone (50 mm from baffle SMR zone (50 mm from baffle
0.83 0.42 0.95 0.25
wall) wall) edge) edge)
Table 2 Comparison of visually measured particle velocity with velocity measured by tethered glass bead falling in the annulus (mm/s).
Fig. 2. Cross-section (top view) of membrane module with two panels removed.
Velocity by glass bead (average ± stand deviation)
Velocity by visual measurement (average ± stand deviation)
7.9 ± 0.7 9.0 ± 0.6 16.4 ± 0.8
7.6 ± 0.6 8.5 ± 0.7 14.3 ± 0.8
lus and the SMR zones. The positions of these probes are listed in Table 1. The gas sent to the TCD system was maintained at 6×10−6 Nm3 /h, while the gas flow rate in each quadrant of the annulus was 0.5 Nm3 /h and that in the core at least 5 Nm3 /h. Hence the effect of sampling gas extraction on the gas flow field was negligible. When helium was injected into the bed, the TCD system monitored the helium concentration at the sampling position. A change of voltage signals from the TCD system could be observed a certain time after the helium injection. Voltages were averaged over several minutes after the signals reached a statistically steady value. 3.3. Solids circulation measurements
Fig. 3. Photograph of the cold model cap with distributors. The black line shows the position of the top ring distributor.
baffle. Its concentration was then detected in both the annulus and SMR zones by a thermal conductivity detector (TCD) system consisting of sampling probes, a TCD cell, two rotameters, a mixing tank and a vacuum pump. The system was calibrated to give the volumetric percentage of helium from the measured voltage signals. The cold model column was equipped with several ports where sampling probes could be inserted into both the annu-
The solids circulation rate was estimated by following the descent of identifiable particles in the annulus through the transparent wall to determine their velocity, and then multiplying by the crosssectional area of the annulus, assuming plug flow of particles in the annulus area. This assumption works reasonably well for a circulating fluidized bed so long as the wall is smooth and the particles are descending in moving packed bed flow base on the study of Burkell et al. (1988). To reduce the error, the velocity was measured multiple times for each quadrant and an average value was taken for that quadrant. The overall particle velocity was then averaged over all four quadrants. Quadrant-to-quadrant velocity differences were within 15% of the overall particle velocity. To verify the measurements, a tethered wooden cylinder (13.2 mm diameter and 2500 kg/m3 density) and a tethered glass bead (diameter: 10 mm, length: 18 mm, density: 655 kg/m3 ) were also deployed to measure the bulk downward velocity in the annulus. For these measurements, the bead or cylinder was lowered onto the surface of the bed above the center of the annulus from the top of the column by a fine fishing line, which was then made slack. The object then descended with the particles in one quadrant of the annulus. The time for the bead or cylinder to travel a known distance was then recorded by a stopwatch. Particle velocities measured by different methods for the same operating conditions are compared in Tables 2 and 3. The velocities measured by the wood cylinder and glass bead are seen to be similar to those determined visually at the wall. Given this validation of the measurement technique, the solids velocities below are based on the visual method.
2602
D. Xie et al. / Chemical Engineering Science 64 (2009) 2599 -- 2606
Table 3 Comparison of the visually measured particle velocity with velocity measured by wooden cylinder in the annulus (mm/s). Velocity by wood cylinder (average ± stand deviation)
Velocity by visual measurement (average ± stand deviation)
5.2 ± 0.5 6.6 ± 0.6 10.8 ± 0.8
4.5 ± 0.5 7.4 ± 0.7 10.6 ± 0.9
4. Experimental results and discussion 4.1. Solids circulation The major driving force for solids circulation is the entrainment of particles by rapidly rising dilute flow through the central core, caused by the gas entering through the main distributor. It is restrained by flow resistance along the solids flow pathways—the core membrane baffle channels, top transition from core to annulus, the annulus quadrants, and bottom passage from annulus to the core. To achieve a high solids circulation rate, a high driving force and low flow resistances are required. Many factors are expected to affect the solids circulation rate, including operating factors like the outside air flowrate, main air flowrate, and geometric factors, such as the core cross-sectional area, annulus cross-sectional area, membrane baffle module arrangement, and particle physical properties. To maintain the temperature difference between the oxidation and SMR zones within tolerable limits, the solids circulation rate should be high. The relationship between the downward particle velocity and temperature difference can be predicted given the reactor geometry, solids density, solids heat capacity and the heat required by the hydrogen production reactions. A typical case was presented by Boyd et al. (2007). 4.1.1. Influence of electrostatic charge on solids circulation In the experiments, air was supplied from a compressor located nearby, with its humidity controlled by passing air through a silica gel drier. It was observed during operation that considerable electrostatic charges were generated due to particle–particle and particle–wall interactions. This caused particles to agglomerate, reducing the solids circulation rate. Electrostatic charging was more serious at higher superficial gas velocities. It can be seen in Figs. 4 and 5 that the solids circulation significantly improved after adding 0.5% weight of Larostat 511 particles, an anti-static agent. The error bars in the figures show 90% confidence intervals. In the high temperature high pressure hydrogen production reactors, electrostatic charges are unlikely to be a problem. Hence in the subsequent cold model tests, Larostat particles were always added to ensure maximum solids circulation unaffected by static charges. 4.1.2. Influence of main air flowrate on solids circulation The influence of the main air flowrate on the downward particle velocity is portrayed in Fig. 4. The tested main airflow range was 6.9–20.6 Nm3 /h, corresponding to superficial gas velocities of 0.23–0.69 m/s in the core if all of the main air enters the core. In this range, solids circulation increases with increasing main air flowrate. A higher airflow in the main stream causes more particles to be carried upwards in the reforming zone per unit time, leading to a higher downward particle velocity in the annulus. 4.1.3. Influence of outside air flowrate on solids circulation The influence of the outside flowrate on the downward particle velocity is shown in Fig. 5. In these tests, the outside air varied from 0 to 3.0 Nm3 /h, corresponding to superficial gas velocities of 0–39 mm/s in the annulus, assuming that all outside air entered the
Fig. 4. Influence of main air flowrate and anti-static agent on solids velocity of descent at outer wall. (Qs = 1.2 Nm3 /h, Qrb = 0, Qrt = 0.)
Fig. 5. Influence of outside air flowrate and anti-static agent on solids velocity of descent at outer wall. (Qm = 12 Nm3 /h, Qrb = 0, Qrt = 0.)
annulus area. When there was no outside gas flow, the particles in the annulus stopped flowing, and the core area functioned as a regular bubbling bed. This indicates that the outside air facilitated solids descent in the annulus and that it was essential to establish the solids circulation. In the range tested, solids circulation increased with increasing outer air flowrate. 4.1.4. Influence of annulus cross-sectional area on solids circulation To study the influence of annulus cross-sectional area on solids circulation, a 203 mm I.D. Plexiglas column was constructed to replace the 230 mm I.D. acrylic pipe surrounding the same draft box as before. The new column provided an annulus cross-sectional area of 0.012 m2 , while that of the 230 mm I.D. was 0.021 m2 . Since the annulus area changed significantly, the solids circulation rate, rather than the downward particle velocity, is the parameter of interest
D. Xie et al. / Chemical Engineering Science 64 (2009) 2599 -- 2606
Fig. 6. Influence of annulus cross-sectional area on solids velocity of descent at outer wall. (Qs = 1.2 Nm3 /h, Qrb = 0, Qrt = 0.)
2603
Fig. 8. Influence of core cross-sectional area and outside flowrate on solids velocity of descent in the annulus. (Qm = 12 Nm3 /h, Qrb = 0, Qrt = 0.)
Table 4 Comparison of 4-panel and 6-panel membrane modules.
Box material Box cross-section Box corner Panel material Gap width between neighbouring panels Total box height
Fig. 7. Influence of core configuration and main flowrate on solids velocity of descent at outer wall. (Qs = 1.2 Nm3 /h, Qrb = 0, Qrt = 0.)
with respect to solids circulation. It can be established from Ws = p up Aa (1 − a )
(1)
The measured solids circulation rates from the 203 mm I.D. column are compared with those from the 230 mm I.D. column in Fig. 6. It can be concluded that the influence of annulus crosssectional area was negligible for the conditions covered. 4.1.5. Influence of core cross-sectional area on solids circulation To study the influence of core cross-sectional area on solids circulation, two of the six vertical plates were removed, as shown in Fig. 2, providing ∼30% more open area in the core. The measured downward particle velocity with the 4-panel membrane box is compared in Figs. 7 and 8 with that for the original 6-panel box. The experimental data indicate that the core cross-sectional area had negligible influence on the solids circulation. When the main air flowrate
4-panel plywood box
6-panel Plexiglas box
6.4 mm thick plywood 142 mm×142 mm Chamfered 9.5 mm thick plywood 19 mm
9.5 mm thick Plexiglas 142 mm×142 mm Sharp 9.5 mm thick Plexiglas 9.5 mm
1.43 m
1.74 m
remained unchanged, the superficial gas velocity decreased as the cross-sectional area of the core increased, in turn reducing particle entrainment. On the other hand, the flow resistance to solids circulation decreased. These two counteracting effects on solids circulation likely approximately balanced each other, with the result that there was little or no net influence on solids circulation for the conditions studied. To further verify the influence of core cross-sectional area on solids circulation, a 142 mm×142 mm plywood box with chamfered corners was built and installed into the cold model. Unlike the 6panel Plexiglas box, this one had only 4 plates. Information on the geometry of these two boxes is provided in Table 4. Downward particle velocities for both boxes are compared in Figs. 9 and 10. Again, there is no evidence that the core crosssectional area had a significant influence on the solids circulation. 4.1.6. Influence of ring distributors on solids circulation The experimental data indicated that neither the core nor the annulus cross-sectional area significantly influenced the solids circulation. It was therefore suspected that the major flow resistance for solids circulation resides at the transition from outer downflow to inner upflow. Two ring distributors were therefore installed around the outside air distributor nozzles, as shown in Figs. 1 and 3. The centerlines of all holes of the ring distributors pointed inwards toward the axis of the column. It was anticipated that the flow from these ring distributors would help the solids move around the bottom of the baffle assembly. Fig. 9 shows the influence of top ring air flowrate on solids circulation for Qm = 5.1 Nm3 /h, Qs = 0, Qrb = 1.5 Nm3 /h. In the air
2604
D. Xie et al. / Chemical Engineering Science 64 (2009) 2599 -- 2606
Fig. 9. Influence of top ring air flowrate and core configuration on solids velocity of descent at the outer wall. (Qm = 5.1 Nm3 /h, Qs = 0 Nm3 /h, Qrb = 1.5 Nm3 /h.)
Fig. 10. Influence of bottom ring flowrate and core configuration on solids velocity of descent in the annulus. (Qm = 5.1 Nm3 /h, Qs = 0 Nm3 /h, Qrt = 1.4 Nm3 /h.)
flowrate range tested, solids circulation increased significantly with increasing airflow through the top ring distributor. This figure also shows that with airflow through the bottom ring of 1.5 Nm3 /h, solids circulated continuously, even when there was no airflow through the outside distributor and top ring. This indicates that the bottom ring distributor could also replace the side air distributor. Fig. 10 shows the influence of the bottom ring air flowrate on solids circulation for Qm = 5.1 Nm3 /h, Qs = 0, Qrt = 1.4 Nm3 /h. At the given main air flowrate and top ring flowrate, when the bottom ring flowrate varied from 0 to 0.3 Nm3 /h, the downward particle velocity increased by approximately 5 mm/s. Beyond 0.3 Nm3 /h, the bottom ring flowrate had negligible further influence on solids circulation. For an air flow through the top ring of 1.4 Nm3 /h, even with no flow of air through the side distributor and bottom ring, solids continued to circulate in the cold model, indicating that the top ring distributor can again serve as a side air distributor.
Fig. 11. Influence of main and top ring flows on downward particle velocity of descent in the annulus. (Qs = 1.2 Nm3 /h, Qrb = 0 Nm3 /h.)
Fig. 12. Influence of outside and top ring flows on downward particle velocity of descent in the annulus. (Qm = 12 Nm3 /h, Qrb = 0 Nm3 /h.)
Figs. 11 and 12 show the influence of the top ring flows on the downward particle velocity for the 6-panel Plexiglas membrane box, with no flow through the bottom ring. It can be seen that introduction of air via the top ring significantly improved the solids circulation. 4.1.7. Influence of membrane baffle arrangement on solids circulation All previous experiments were carried out with all of the membrane panels aligned. There was concern that gas might bypass through one or more compartments with this aligned configuration. If this were the case, it might be possible to obtain a more uniform gas distribution among the compartments by rotating successive modules through 90◦ (rotation around a vertical axis). The 6-panel Plexiglas membrane box was employed with the bottom section at right angles to the three upper modules. With this configuration, a significant amount of gas was observed passing
D. Xie et al. / Chemical Engineering Science 64 (2009) 2599 -- 2606
Fig. 13. Influence of main air flow on tracer gas concentration at probes whose positions are given in Table 1. (Qs = 1.2 Nm3 /h, Qrb = 0, Qrt = 0.)
through the outer annulus area, making visual measurement of downward particle velocities difficult. This suggested that the staggered configuration hindered the main upward airflow in the central SMR zone, and some of the gas from main air distributor bypassed the core through annulus. Given also that the staggered configuration would likely promote catalyst attrition, no further experiments were carried out on this configuration. 4.2. Gas circulation In the gas circulation experiments, steady state helium concentration was measured in the core region with helium injected continuously into the top region of the bed (just above the top box) to determine the circulation of the injected gas induced by solids circulation. 4.2.1. Influence of main air flowrate on gas circulation The influence of the main air flowrate on the tracer gas concentration in both the annulus and central SMR zones with helium injected above the baffle assembly is shown in Fig. 13. C0 is the helium volumetric concentration in the reactor if the tracer were perfectly mixed with all of the air introduced at the bottom, i.e. C0 =
Qhe × 100 Qm + Qs + Qhe
(2)
The helium concentration in the top region of the annulus (probe 1) matches C0 , indicating that the top oxidation zone is well mixed. However, as desired, the helium concentration at the bottom of annulus (probe 2) was less than that at the top of the annulus. The concentration differences between the top and bottom of the annulus decreased with increasing main air flowrate. When the main air flowrate exceeded 12 Nm3 /h, the difference was negligible. The helium concentrations in the central SMR zone (probes 3 and 4) were 5–12 times lower than the theoretical well-mixed helium concentration C0 . This indicates that the ICFBMR design concept is successful in limiting re-circulation of nitrogen introduced with the air into the reforming central core.
2605
Fig. 14. Influence of outside air flow on tracer gas concentration at probes whose positions are given in Table 1. (Qm = 10.2 Nm3 /h, Qrb = 0, Qrt = 0.)
4.2.2. Influence of outside air flowrate on gas circulation The influence of outside air flowrate on helium concentration in both the annulus and inner SMR zones is shown in Fig. 14. With the outside gas flow rate less than 1.2 Nm3 /h, the helium concentration in the annulus was as low as in the SMR zone. Visual observation showed that under these conditions, the particles in the annulus almost stopped. Hence there were very few particles circulating to carry the tracer gas downwards. When the outside flowrate exceeded 1.2 Nm3 /h, the annulus tracer gas concentration at the top (probe 1) was close to C0 , whereas at the bottom (probe 2) it was less than C0 . This implies that the outside airflow hindered the tracer gas from traveling downwards. From the above experimental observations, a likely picture describing the mechanism of gas and solids circulation in the cold model unit may be constructed. In the central membrane baffle zone, particles are transported upwards by gas from the main (central) distributor. The driving force causing the ascent of these particles is the drag imposed by the gas, which readily overcomes the gravity of particles. Greater airflow in the main stream increases the solids circulation rate. In the top oxidation zone, the particles are carried out of the top of the central baffle zone and fall down on the annulus due to the reduced gas velocity there. Helium is introduced and mixed with particles and air near the top of the annulus. In the annulus zone, particles, mixed with air and helium, descend due to gravity from the top of the annulus zone. At the same time, air from the side distributor, likely supplemented by some of the air from the ring distributors, travels upwards counter to the particles from the bottom of the annulus zone. Gas is exchanged between these two streams, leading to a higher helium concentration in the upper part of the annulus (near probe 3) and less helium in the lower part (near probe 4). The side air from the side distributor or ring distributors keeps the gas velocity relative to the particles above that required for minimum fluidization, facilitating solids movement in the annulus. Since the suspension density in the annulus exceeds that in the core, the pressure at the bottom of the annulus is higher than that at the same height in the core, driving particles from the outer downflow region to the inner upflow region. When the ring distributors are employed, the horizontal air jets from the ring distributors push the solids inwards, significantly assisting the solids circulation. The particles descending in the annulus carry some helium with them,
2606
D. Xie et al. / Chemical Engineering Science 64 (2009) 2599 -- 2606
which is further diluted by air from the main distributor when it reaches the inner upflow region. 5. Conclusions and recommendations 1. For the range of conditions investigated, the major resistance to solids circulation resides neither inside the membrane assembly, nor in the annulus, but at the bottom of the baffle assembly, i.e. in the transition from outer downward flow to inner upward flow. 2. For the conditions studied, solids circulation increased with increasing main and outside airflows. The top and bottom ring distributors can provide gas to the outer quadrants. Introduction of gas from the top ring distributor reduced the major flow resistance in the transition from outer downflow to inner upflow, thereby improving the solids circulation significantly. 3. After being injected into the bed above the baffle assembly, most of the injected gas left the bed without re-circulating back to the bottom with the circulating solids. For the internally circulating bed configuration, the tracer gas concentration in the core membrane baffle area was 5–12 times less than the well-mixed concentration in the upper (oxidation) zone. The cold model experimental results indicate that the novel ICFBMR concept provides a promising configuration for circulating heat to the bottom of a fluidized bed membrane reformer with only minor reduction of the driving force for hydrogen permeation by recirculation of nitrogen introduced with the combustion air. This concept has been patented (Grace et al., 2006) and tested at high temperatures and pressures (Grace et al., 2005; Boyd et al., 2005; Xie et al., 2006). Notation Aa C0 Qhe Qm Qrb Qrt Qs up Ws
annulus cross-sectional area, m2 theoretical helium concentration, % flowrate of helium injection, Nm3 /h air flowrate through main distributor, Nm3 /h air flowrate through bottom ring distributor, Nm3 /h air flowrate through top ring distributor, Nm3 /h air flowrate through outside distributor, Nm3 /h particle downward velocity in annulus, m/s solids circulation rate, kg/s
Greek letters
a p
annulus bed voidage, dimensionless particle density, kg/m3
Acronyms FBMR FCC ICFBMR PSA TCD
fluidized bed membrane reactors fluidized catalytic cracking internally circulating fluidized bed membrane reactor pressure swing absorption thermal conductivity detector
Acknowledgments We are grateful to employees of Membrane Reactor Technologies and Noram Engineering, who contributed to the success of the ICFBMR cold model tests. References Abba, I.A., Grace, J.R., Bi, H.T., 2003. Application of the generic fluid bed reactor model to the FBMR process for steam methane reforming with oxygen input. Industrial and Engineering Chemistry Research 42, 2736–2745. Adris, A.M., Grace, J.R., 1997. Characteristics of fluidized bed membrane reactors (FBMR)-Scale-up and practical issues. Industrial and Engineering Chemistry Research 36, 4549–4556. Adris, A.M., Elnashaie, S.S., Hughes, R., 1991. Fluidized bed steam reforming of methane. Canadian Journal of Chemical Engineering 69, 1061–1070. Adris, A.M., Lim, C.J., Grace, J.R., 1994. The fluidized bed membrane reactor FBMR system: a pilot scale experimental study. Chemical Engineering Science 49, 5833–5843. Adris, A.M., Lim, C.J., Grace, J.R., 1997. The fluidized bed membrane reactor for steam methane reforming: model verification and parametric study. Chemical Engineering Science 52, 1609–1622. Boyd, D.T., Grace, J.R., Lim, C.J., 2005. Hydrogen from an internally circulating fluidized bed membrane reactor. International Journal of Chemical Reactor Engineering 3, A58, Available at: http://www.bepress.com/ijcre/ vol3/A58. Boyd, T., Lim, C.J., Grace, J.R., Adris, A.M., 2007. Cold modelling of an internally circulating fluidized bed membrane reactor. International Journal of Chemical Reactor Engineering 5, A26, Available at: http://www.bepress.com/ijcre/ vol5/A26. Burkell, J.J., Grace, J.R., Zhao, J., Lim, C.J., 1988. Measurement of solids circulation rate in circulating fluidized beds. In: Basu, P., Large, J.F. (Eds.), Circulating Fluidized Bed Technology II. Pergamon, Oxford, pp. 501–509. Chen, Z., Yan, Y., Elnashaie, S.S.E.H., 2003. Modeling and optimization of a novel membrane reformer for higher hydrocarbons. A.I.Ch.E. Journal 49, 1250–1265. Chen, Z., Yan, Y., Elnashaie, S.S.E.H., 2004. Hydrogen production and carbon formation during the steam reforming of heptane in a novel circulating fluidized bed membrane reformer. Industrial and Engineering Chemistry Research 43, 1323–1333. Chen, Z., Grace, J.R., Lim, C.J., Li, A., 2007. Experimental studies of pure hydrogen production in a commercialized fluidized-bed membrane reactor with SMR and ATR catalysts. International Journal of Hydrogen Energy 32, 2359–2366. ¨ L., et al., 2007. Membrane assisted fluidized Deshmukh, S.A.R.K., Heinrich, S., Morl, bed reactors: potential and hurdles. Chemical Engineering Science 62, 416–436. Dogan, M., Posarac, D., Grace, J.R., Adris, A.M., Lim, C.J., 2003. Modeling of autothermal steam methane reforming in a fluidized bed membrane reactor. International Journal of Chemical Reactor Engineering 1, A2, Available at: http://www.bepress.com/ijcre/vol1/A2. Grace, J.R., Li, X., Lim, C.J., 2001. Equilibrium modeling of catalytic steam reforming of methane in membrane reactors with oxygen addition. Catalysis Today 64, 141–149. Grace, J.R., Elnashaie, S.S.E.H., Lim, C.J., 2005. Hydrogen production in fluidized beds with in-situ membranes. International Journal of Chemical Reactor Engineering 3, A41, Available at: http://www.bepress.com/ijcre/vol3/A41. Grace, J.R., Lim, C.J., Adris, A., Xie, D., Boyd, D., Brereton, C., Wolfs, W., 2006. Internally circulating fluidized bed membrane reactor system. US Patent 7,141,231. Patil, C.S., Annaland, M.S., Kuipers, J.A.M., 2007. Fluidised bed membrane reactor for ultrapure hydrogen production via methane steam reforming: experimental demonstration and model validation. Chemical Engineering Science 62, 2989–3007. Roy, S., Pruden, B.B., Adris, A.M., Grace, J.R., Lim, C.J., 1999. Fluidized bed steam methane reforming with oxygen input. Chemical Engineering Science 54, 2095–2102. Xie, D.L., Grace, J.R., Lim, C.J., 2006. Development of internally circulating fluidized bed membrane reactor for hydrogen production from natural gas. Journal of Wuhan University of Technology 11, 252–257.
Contents lists available at ScienceDirect
Chemical Engineering Science journal homepage: w w w . e l s e v i e r . c o m / l o c a t e / c e s
Gas and particle circulation in an internally circulating fluidized bed membrane reactor cold model Donglai Xie a,c,∗ , C. Jim Lim b,c , John R. Grace b,c , Alaa-Eldin M. Adris c a
MOE Key Laboratory of Enhanced Heat Transfer and Energy Conservation, South China University of Technology, Guangzhou 510640, PR China Department of Chemical and Biological Engineering, The University of British Columbia, 2360 East Mall, Vancouver, Canada V6T 1Z3 c Membrane Reactor Technologies Ltd., Suite 1800-200 Granville Street, Vancouver, Canada V6C 1S4 b
A R T I C L E
I N F O
Article history: Received 28 July 2008 Received in revised form 12 February 2009 Accepted 19 February 2009 Available online 9 March 2009 Keywords: Hydrogen Fluidization Membrane Gas circulation Particle circulation Cold model
A B S T R A C T
A cold model study was carried out of an internally circulating fluidized bed membrane reactor for hydrogen production by steam reforming. This configuration facilitates a novel process in which air is introduced for autothermal operation, while avoiding diluting the hydrogen in the gas stream by nitrogen. A column of diameter 230 mm and height 2.4 m contained a membrane box of square cross-section. Gas circulation was investigated by injecting helium tracer into the top of the bed above the membrane box and detecting its concentration in the annulus and lower membrane box by a thermal conductivity detector. Solids circulation was determined from the downward velocity of particles in the annulus. The major resistance to solids circulation was found to be at the passage connecting the outer downflow to the inner upflow compartment. A ring distributor helped overcome this resistance. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Widespread usage of hydrogen, if generated in an advantageous manner, could alleviate growing concerns about the world's energy supply, security, air pollution, and greenhouse gas emissions. Hydrogen is also a major industrial commodity, used as an intermediate in a number of chemical and metallurgical processes, for example in the production of ammonia and methanol, upgrading of heavy hydrocarbons, iron ore reduction and food processing. Although it is the most abundant element in the universe, hydrogen does not exist naturally in large quantities or high concentrations. Most of the world's hydrogen is generated at large petroleum and chemical plants by steam reforming or partial oxidation of natural gas in parallel fixed bed reactors within huge top-fired or side-fired furnaces, coupled with pressure swing absorption (PSA) for hydrogen purification. Other methods of producing hydrogen such as gasification of biomass, biological production and electrolysis of water are not currently economic at large scales. In conventional steam reforming, high temperatures (typically > 850 ◦ C) are achieved by hanging fixed bed reactor tubes inside a gas-burning furnace. Heat must be transferred inwards through
∗ Corresponding author at: MOE Key Laboratory of Enhanced Heat Transfer and Energy Conservation, South China University of Technology, Guangzhou, 510640, PR China. Tel./fax: +86 20 22236985. E-mail address: [email protected] (D. Xie). 0009-2509/$ - see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2009.02.032
the walls of the tubes. Expensive metal alloys are required for the tubes to withstand the resulting high temperature gradients. Catalyst particles are several millimeters at least in diameter to minimize the pressure drops through the beds, causing strong internal diffusional resistances (Grace et al., 2005). To overcome these disadvantages, fluidized bed and membrane separation technologies have been combined in fluidized bed membrane reactors (FBMR) (Adris et al., 1991, 1994) for hydrogen production from methane in a single-step process. Many aspects of FBMR have been investigated. Adris and Grace (1997) studied the scale-up and practical issues; Roy et al. (1999) explored autothermal operation of FBMR; Chen et al. (2003, 2004) studied steam reforming of heptane and higher hydrocarbons. Several reactor models (e.g. Abba et al., 2003; Adris et al,. 1997; Chen et al., 2003; Dogan et al., 2003; Grace et al., 2001; Patil et al., 2007) have also been developed to simulate FBMR processes. Experimental studies in a commercial FBMR unit were reported by Chen et al. (2007). A review of FBMR developments was provided by Deshmukh et al. (2007). Since the net steam methane reforming (SMR) reactions are endothermic, heat is required to sustain the reactions by one or both of (1) External heat exchange. (2) Air, enriched air or oxygen addition to generate heat by exothermic oxidation of species like CH4 , CO, and H2 . When the latter method is employed, it is desirable to use air, rather than enriched air or pure oxygen, as the oxidant source to reduce the
2600
D. Xie et al. / Chemical Engineering Science 64 (2009) 2599 -- 2606
operating cost and energy penalties. However, when air is selected, nitrogen is also introduced into the reactor, decreasing the hydrogen partial pressure in the gas stream, thereby reducing the driving force for diffusion of hydrogen through the membrane. To compensate for this shortcoming and to maintain uniform reactor temperature, an internally circulating fluidized bed membrane reactor (ICFBMR) system is investigated in this paper. 2. Conceptual design of the ICFBMR and challenges The conceptual design of the ICFBMR is illustrated in Fig. 1. Catalyst particles are transported upwards by gas in the central membrane baffle zone where SMR reactions take place to produce hydrogen, which is then withdrawn after permeation through the membrane panels. Air is injected into an oxidation zone at the top of the bed above the membrane baffles or into the outer annulus of the reactor. Much of the heat generated by the oxidation reactions is absorbed by the catalyst particles, which circulate downwards through the outer annulus area. At the bottom, the heated solids are re-entrained into the central reforming (upflow) zone by the reactant gas of steam and methane entering the bottom of the reactor. The excess sensible heat of the circulating catalyst reaching the bottom then provides the heat needed for the endothermic steam reforming. Similar reactor configurations can be used for processes that are reversible and thermodynamically limited, such as propane dehydrogenation to propylene, cyclohexane to benzene, and ethyl benzene to styrene. To design such reactors, the hydrodynamic features of the bed need to be understood, especially the solids and gas circulation
patterns. Important questions needing clarification addressed in this paper are: 1. Quantity of nitrogen reaching the reforming zone. The nitrogen concentration in the SMR zone must be as low as possible so as not to appreciably reduce the permeation of hydrogen through the membrane surfaces. 2. Circulation rate of catalyst particles recycle between the oxidation and SMR zones. The solids circulation rate must be sufficient to ensure that the temperature difference between the oxidation and SMR zones is within tolerable limits. For the hydrodynamic study of the ICFBMR, a pilot scale cold model was constructed at the University of British Columbia. Extensive experiments were carried out to study the gas and solids circulation patterns and obtain quantitative data for the design of such reactors. 3. Experimental set-up and methodology 3.1. Basic experimental set-up As shown in Fig. 1, the cold model reactor was equipped with an air distributor, a cyclone (later replaced by a sintered metal filter) and a 2.4 m tall column with “dummy” membrane box of square cross-section inside. Air was supplied to the cold model from an air compressor. An acrylic pipe of 230 mm inner diameter served as the column. The dummy membrane box was made of Plexiglas, with four identical membrane sections or modules, each 330 mm high with a cross-section of 142 mm2 . Six 9.5 mm thick Plexiglas equally spaced plates were inserted into each section form the overall membrane box. The four sections were joined end-to-end in an aligned (i.e. parallel) manner. The cross-section of the membrane box is shown in Fig. 2. Two panels were removed in some tests to study the influence of the core cross-sectional area on solids circulation. Air was introduced by four separately controlled distributors: (a) a main distributor in the center of the bottom cap, directing the gas to the bottom of the membrane box; (b) a side distributor directing gas to the outer annulus region; (c) a top ring distributor; (d) a bottom ring distributor. The main distributor contained 16 nozzles made of 6 mm O.D. plastic tubes, in four rows with four in each row. Each nozzle had four 1.6 mm diameter holes, pointing at 45◦ downward to the horizontal. The side distributor consisted of eight plastic 6 mm O.D. tubes, two in each quadrant. Each side distributor tube was drilled with two 1.6 mm diameter holes, again pointing at 45◦ downward to the horizontal. The two ring distributors were fabricated from 3.2 mm copper tubes and installed around the outside air distributor nozzles, as shown in Fig. 1. The top ring contained sixteen uniformly distributed 0.4 mm diameter holes, whereas sixteen 0.8 mm diameter holes were drilled in the bottom ring, with their centrelines all pointing inwards toward the axis of the column. When installed, the bottom of the membrane box was immediately above the side distributor nozzle openings. Fig. 3 is a photo of the bottom cap illustrating the relative positions of these distributors. Fluidized catalytic cracking (FCC) particles of Sauter mean diameter 65 m and density 1600 kg/m3 were selected for the experiments based on dimensionless scaling parameter studies on the operation conditions between the high temperature reactor and the cold model (Boyd et al., 2007). These particles had a minimum fluidization velocity of 0.0032 m/s and a loose bed voidage of 0.45 in air at atmospheric temperature and pressure. 3.2. Gas circulation measurements
Fig. 1. Schematic (side view) of internally circulating fluidized bed membrane reactor and the cold model. All dimensions are in mm.
To determine the gas circulation and mixing pattern, helium tracer was injected into the top oxidation zone above the membrane
D. Xie et al. / Chemical Engineering Science 64 (2009) 2599 -- 2606
2601
Table 1 Position of helium sampling probes. Probe #
Location and horizontal position
Height above air distributor (m)
1 2 3 4
Annulus (10 mm from column Annulus (10 mm from column SMR zone (50 mm from baffle SMR zone (50 mm from baffle
0.83 0.42 0.95 0.25
wall) wall) edge) edge)
Table 2 Comparison of visually measured particle velocity with velocity measured by tethered glass bead falling in the annulus (mm/s).
Fig. 2. Cross-section (top view) of membrane module with two panels removed.
Velocity by glass bead (average ± stand deviation)
Velocity by visual measurement (average ± stand deviation)
7.9 ± 0.7 9.0 ± 0.6 16.4 ± 0.8
7.6 ± 0.6 8.5 ± 0.7 14.3 ± 0.8
lus and the SMR zones. The positions of these probes are listed in Table 1. The gas sent to the TCD system was maintained at 6×10−6 Nm3 /h, while the gas flow rate in each quadrant of the annulus was 0.5 Nm3 /h and that in the core at least 5 Nm3 /h. Hence the effect of sampling gas extraction on the gas flow field was negligible. When helium was injected into the bed, the TCD system monitored the helium concentration at the sampling position. A change of voltage signals from the TCD system could be observed a certain time after the helium injection. Voltages were averaged over several minutes after the signals reached a statistically steady value. 3.3. Solids circulation measurements
Fig. 3. Photograph of the cold model cap with distributors. The black line shows the position of the top ring distributor.
baffle. Its concentration was then detected in both the annulus and SMR zones by a thermal conductivity detector (TCD) system consisting of sampling probes, a TCD cell, two rotameters, a mixing tank and a vacuum pump. The system was calibrated to give the volumetric percentage of helium from the measured voltage signals. The cold model column was equipped with several ports where sampling probes could be inserted into both the annu-
The solids circulation rate was estimated by following the descent of identifiable particles in the annulus through the transparent wall to determine their velocity, and then multiplying by the crosssectional area of the annulus, assuming plug flow of particles in the annulus area. This assumption works reasonably well for a circulating fluidized bed so long as the wall is smooth and the particles are descending in moving packed bed flow base on the study of Burkell et al. (1988). To reduce the error, the velocity was measured multiple times for each quadrant and an average value was taken for that quadrant. The overall particle velocity was then averaged over all four quadrants. Quadrant-to-quadrant velocity differences were within 15% of the overall particle velocity. To verify the measurements, a tethered wooden cylinder (13.2 mm diameter and 2500 kg/m3 density) and a tethered glass bead (diameter: 10 mm, length: 18 mm, density: 655 kg/m3 ) were also deployed to measure the bulk downward velocity in the annulus. For these measurements, the bead or cylinder was lowered onto the surface of the bed above the center of the annulus from the top of the column by a fine fishing line, which was then made slack. The object then descended with the particles in one quadrant of the annulus. The time for the bead or cylinder to travel a known distance was then recorded by a stopwatch. Particle velocities measured by different methods for the same operating conditions are compared in Tables 2 and 3. The velocities measured by the wood cylinder and glass bead are seen to be similar to those determined visually at the wall. Given this validation of the measurement technique, the solids velocities below are based on the visual method.
2602
D. Xie et al. / Chemical Engineering Science 64 (2009) 2599 -- 2606
Table 3 Comparison of the visually measured particle velocity with velocity measured by wooden cylinder in the annulus (mm/s). Velocity by wood cylinder (average ± stand deviation)
Velocity by visual measurement (average ± stand deviation)
5.2 ± 0.5 6.6 ± 0.6 10.8 ± 0.8
4.5 ± 0.5 7.4 ± 0.7 10.6 ± 0.9
4. Experimental results and discussion 4.1. Solids circulation The major driving force for solids circulation is the entrainment of particles by rapidly rising dilute flow through the central core, caused by the gas entering through the main distributor. It is restrained by flow resistance along the solids flow pathways—the core membrane baffle channels, top transition from core to annulus, the annulus quadrants, and bottom passage from annulus to the core. To achieve a high solids circulation rate, a high driving force and low flow resistances are required. Many factors are expected to affect the solids circulation rate, including operating factors like the outside air flowrate, main air flowrate, and geometric factors, such as the core cross-sectional area, annulus cross-sectional area, membrane baffle module arrangement, and particle physical properties. To maintain the temperature difference between the oxidation and SMR zones within tolerable limits, the solids circulation rate should be high. The relationship between the downward particle velocity and temperature difference can be predicted given the reactor geometry, solids density, solids heat capacity and the heat required by the hydrogen production reactions. A typical case was presented by Boyd et al. (2007). 4.1.1. Influence of electrostatic charge on solids circulation In the experiments, air was supplied from a compressor located nearby, with its humidity controlled by passing air through a silica gel drier. It was observed during operation that considerable electrostatic charges were generated due to particle–particle and particle–wall interactions. This caused particles to agglomerate, reducing the solids circulation rate. Electrostatic charging was more serious at higher superficial gas velocities. It can be seen in Figs. 4 and 5 that the solids circulation significantly improved after adding 0.5% weight of Larostat 511 particles, an anti-static agent. The error bars in the figures show 90% confidence intervals. In the high temperature high pressure hydrogen production reactors, electrostatic charges are unlikely to be a problem. Hence in the subsequent cold model tests, Larostat particles were always added to ensure maximum solids circulation unaffected by static charges. 4.1.2. Influence of main air flowrate on solids circulation The influence of the main air flowrate on the downward particle velocity is portrayed in Fig. 4. The tested main airflow range was 6.9–20.6 Nm3 /h, corresponding to superficial gas velocities of 0.23–0.69 m/s in the core if all of the main air enters the core. In this range, solids circulation increases with increasing main air flowrate. A higher airflow in the main stream causes more particles to be carried upwards in the reforming zone per unit time, leading to a higher downward particle velocity in the annulus. 4.1.3. Influence of outside air flowrate on solids circulation The influence of the outside flowrate on the downward particle velocity is shown in Fig. 5. In these tests, the outside air varied from 0 to 3.0 Nm3 /h, corresponding to superficial gas velocities of 0–39 mm/s in the annulus, assuming that all outside air entered the
Fig. 4. Influence of main air flowrate and anti-static agent on solids velocity of descent at outer wall. (Qs = 1.2 Nm3 /h, Qrb = 0, Qrt = 0.)
Fig. 5. Influence of outside air flowrate and anti-static agent on solids velocity of descent at outer wall. (Qm = 12 Nm3 /h, Qrb = 0, Qrt = 0.)
annulus area. When there was no outside gas flow, the particles in the annulus stopped flowing, and the core area functioned as a regular bubbling bed. This indicates that the outside air facilitated solids descent in the annulus and that it was essential to establish the solids circulation. In the range tested, solids circulation increased with increasing outer air flowrate. 4.1.4. Influence of annulus cross-sectional area on solids circulation To study the influence of annulus cross-sectional area on solids circulation, a 203 mm I.D. Plexiglas column was constructed to replace the 230 mm I.D. acrylic pipe surrounding the same draft box as before. The new column provided an annulus cross-sectional area of 0.012 m2 , while that of the 230 mm I.D. was 0.021 m2 . Since the annulus area changed significantly, the solids circulation rate, rather than the downward particle velocity, is the parameter of interest
D. Xie et al. / Chemical Engineering Science 64 (2009) 2599 -- 2606
Fig. 6. Influence of annulus cross-sectional area on solids velocity of descent at outer wall. (Qs = 1.2 Nm3 /h, Qrb = 0, Qrt = 0.)
2603
Fig. 8. Influence of core cross-sectional area and outside flowrate on solids velocity of descent in the annulus. (Qm = 12 Nm3 /h, Qrb = 0, Qrt = 0.)
Table 4 Comparison of 4-panel and 6-panel membrane modules.
Box material Box cross-section Box corner Panel material Gap width between neighbouring panels Total box height
Fig. 7. Influence of core configuration and main flowrate on solids velocity of descent at outer wall. (Qs = 1.2 Nm3 /h, Qrb = 0, Qrt = 0.)
with respect to solids circulation. It can be established from Ws = p up Aa (1 − a )
(1)
The measured solids circulation rates from the 203 mm I.D. column are compared with those from the 230 mm I.D. column in Fig. 6. It can be concluded that the influence of annulus crosssectional area was negligible for the conditions covered. 4.1.5. Influence of core cross-sectional area on solids circulation To study the influence of core cross-sectional area on solids circulation, two of the six vertical plates were removed, as shown in Fig. 2, providing ∼30% more open area in the core. The measured downward particle velocity with the 4-panel membrane box is compared in Figs. 7 and 8 with that for the original 6-panel box. The experimental data indicate that the core cross-sectional area had negligible influence on the solids circulation. When the main air flowrate
4-panel plywood box
6-panel Plexiglas box
6.4 mm thick plywood 142 mm×142 mm Chamfered 9.5 mm thick plywood 19 mm
9.5 mm thick Plexiglas 142 mm×142 mm Sharp 9.5 mm thick Plexiglas 9.5 mm
1.43 m
1.74 m
remained unchanged, the superficial gas velocity decreased as the cross-sectional area of the core increased, in turn reducing particle entrainment. On the other hand, the flow resistance to solids circulation decreased. These two counteracting effects on solids circulation likely approximately balanced each other, with the result that there was little or no net influence on solids circulation for the conditions studied. To further verify the influence of core cross-sectional area on solids circulation, a 142 mm×142 mm plywood box with chamfered corners was built and installed into the cold model. Unlike the 6panel Plexiglas box, this one had only 4 plates. Information on the geometry of these two boxes is provided in Table 4. Downward particle velocities for both boxes are compared in Figs. 9 and 10. Again, there is no evidence that the core crosssectional area had a significant influence on the solids circulation. 4.1.6. Influence of ring distributors on solids circulation The experimental data indicated that neither the core nor the annulus cross-sectional area significantly influenced the solids circulation. It was therefore suspected that the major flow resistance for solids circulation resides at the transition from outer downflow to inner upflow. Two ring distributors were therefore installed around the outside air distributor nozzles, as shown in Figs. 1 and 3. The centerlines of all holes of the ring distributors pointed inwards toward the axis of the column. It was anticipated that the flow from these ring distributors would help the solids move around the bottom of the baffle assembly. Fig. 9 shows the influence of top ring air flowrate on solids circulation for Qm = 5.1 Nm3 /h, Qs = 0, Qrb = 1.5 Nm3 /h. In the air
2604
D. Xie et al. / Chemical Engineering Science 64 (2009) 2599 -- 2606
Fig. 9. Influence of top ring air flowrate and core configuration on solids velocity of descent at the outer wall. (Qm = 5.1 Nm3 /h, Qs = 0 Nm3 /h, Qrb = 1.5 Nm3 /h.)
Fig. 10. Influence of bottom ring flowrate and core configuration on solids velocity of descent in the annulus. (Qm = 5.1 Nm3 /h, Qs = 0 Nm3 /h, Qrt = 1.4 Nm3 /h.)
flowrate range tested, solids circulation increased significantly with increasing airflow through the top ring distributor. This figure also shows that with airflow through the bottom ring of 1.5 Nm3 /h, solids circulated continuously, even when there was no airflow through the outside distributor and top ring. This indicates that the bottom ring distributor could also replace the side air distributor. Fig. 10 shows the influence of the bottom ring air flowrate on solids circulation for Qm = 5.1 Nm3 /h, Qs = 0, Qrt = 1.4 Nm3 /h. At the given main air flowrate and top ring flowrate, when the bottom ring flowrate varied from 0 to 0.3 Nm3 /h, the downward particle velocity increased by approximately 5 mm/s. Beyond 0.3 Nm3 /h, the bottom ring flowrate had negligible further influence on solids circulation. For an air flow through the top ring of 1.4 Nm3 /h, even with no flow of air through the side distributor and bottom ring, solids continued to circulate in the cold model, indicating that the top ring distributor can again serve as a side air distributor.
Fig. 11. Influence of main and top ring flows on downward particle velocity of descent in the annulus. (Qs = 1.2 Nm3 /h, Qrb = 0 Nm3 /h.)
Fig. 12. Influence of outside and top ring flows on downward particle velocity of descent in the annulus. (Qm = 12 Nm3 /h, Qrb = 0 Nm3 /h.)
Figs. 11 and 12 show the influence of the top ring flows on the downward particle velocity for the 6-panel Plexiglas membrane box, with no flow through the bottom ring. It can be seen that introduction of air via the top ring significantly improved the solids circulation. 4.1.7. Influence of membrane baffle arrangement on solids circulation All previous experiments were carried out with all of the membrane panels aligned. There was concern that gas might bypass through one or more compartments with this aligned configuration. If this were the case, it might be possible to obtain a more uniform gas distribution among the compartments by rotating successive modules through 90◦ (rotation around a vertical axis). The 6-panel Plexiglas membrane box was employed with the bottom section at right angles to the three upper modules. With this configuration, a significant amount of gas was observed passing
D. Xie et al. / Chemical Engineering Science 64 (2009) 2599 -- 2606
Fig. 13. Influence of main air flow on tracer gas concentration at probes whose positions are given in Table 1. (Qs = 1.2 Nm3 /h, Qrb = 0, Qrt = 0.)
through the outer annulus area, making visual measurement of downward particle velocities difficult. This suggested that the staggered configuration hindered the main upward airflow in the central SMR zone, and some of the gas from main air distributor bypassed the core through annulus. Given also that the staggered configuration would likely promote catalyst attrition, no further experiments were carried out on this configuration. 4.2. Gas circulation In the gas circulation experiments, steady state helium concentration was measured in the core region with helium injected continuously into the top region of the bed (just above the top box) to determine the circulation of the injected gas induced by solids circulation. 4.2.1. Influence of main air flowrate on gas circulation The influence of the main air flowrate on the tracer gas concentration in both the annulus and central SMR zones with helium injected above the baffle assembly is shown in Fig. 13. C0 is the helium volumetric concentration in the reactor if the tracer were perfectly mixed with all of the air introduced at the bottom, i.e. C0 =
Qhe × 100 Qm + Qs + Qhe
(2)
The helium concentration in the top region of the annulus (probe 1) matches C0 , indicating that the top oxidation zone is well mixed. However, as desired, the helium concentration at the bottom of annulus (probe 2) was less than that at the top of the annulus. The concentration differences between the top and bottom of the annulus decreased with increasing main air flowrate. When the main air flowrate exceeded 12 Nm3 /h, the difference was negligible. The helium concentrations in the central SMR zone (probes 3 and 4) were 5–12 times lower than the theoretical well-mixed helium concentration C0 . This indicates that the ICFBMR design concept is successful in limiting re-circulation of nitrogen introduced with the air into the reforming central core.
2605
Fig. 14. Influence of outside air flow on tracer gas concentration at probes whose positions are given in Table 1. (Qm = 10.2 Nm3 /h, Qrb = 0, Qrt = 0.)
4.2.2. Influence of outside air flowrate on gas circulation The influence of outside air flowrate on helium concentration in both the annulus and inner SMR zones is shown in Fig. 14. With the outside gas flow rate less than 1.2 Nm3 /h, the helium concentration in the annulus was as low as in the SMR zone. Visual observation showed that under these conditions, the particles in the annulus almost stopped. Hence there were very few particles circulating to carry the tracer gas downwards. When the outside flowrate exceeded 1.2 Nm3 /h, the annulus tracer gas concentration at the top (probe 1) was close to C0 , whereas at the bottom (probe 2) it was less than C0 . This implies that the outside airflow hindered the tracer gas from traveling downwards. From the above experimental observations, a likely picture describing the mechanism of gas and solids circulation in the cold model unit may be constructed. In the central membrane baffle zone, particles are transported upwards by gas from the main (central) distributor. The driving force causing the ascent of these particles is the drag imposed by the gas, which readily overcomes the gravity of particles. Greater airflow in the main stream increases the solids circulation rate. In the top oxidation zone, the particles are carried out of the top of the central baffle zone and fall down on the annulus due to the reduced gas velocity there. Helium is introduced and mixed with particles and air near the top of the annulus. In the annulus zone, particles, mixed with air and helium, descend due to gravity from the top of the annulus zone. At the same time, air from the side distributor, likely supplemented by some of the air from the ring distributors, travels upwards counter to the particles from the bottom of the annulus zone. Gas is exchanged between these two streams, leading to a higher helium concentration in the upper part of the annulus (near probe 3) and less helium in the lower part (near probe 4). The side air from the side distributor or ring distributors keeps the gas velocity relative to the particles above that required for minimum fluidization, facilitating solids movement in the annulus. Since the suspension density in the annulus exceeds that in the core, the pressure at the bottom of the annulus is higher than that at the same height in the core, driving particles from the outer downflow region to the inner upflow region. When the ring distributors are employed, the horizontal air jets from the ring distributors push the solids inwards, significantly assisting the solids circulation. The particles descending in the annulus carry some helium with them,
2606
D. Xie et al. / Chemical Engineering Science 64 (2009) 2599 -- 2606
which is further diluted by air from the main distributor when it reaches the inner upflow region. 5. Conclusions and recommendations 1. For the range of conditions investigated, the major resistance to solids circulation resides neither inside the membrane assembly, nor in the annulus, but at the bottom of the baffle assembly, i.e. in the transition from outer downward flow to inner upward flow. 2. For the conditions studied, solids circulation increased with increasing main and outside airflows. The top and bottom ring distributors can provide gas to the outer quadrants. Introduction of gas from the top ring distributor reduced the major flow resistance in the transition from outer downflow to inner upflow, thereby improving the solids circulation significantly. 3. After being injected into the bed above the baffle assembly, most of the injected gas left the bed without re-circulating back to the bottom with the circulating solids. For the internally circulating bed configuration, the tracer gas concentration in the core membrane baffle area was 5–12 times less than the well-mixed concentration in the upper (oxidation) zone. The cold model experimental results indicate that the novel ICFBMR concept provides a promising configuration for circulating heat to the bottom of a fluidized bed membrane reformer with only minor reduction of the driving force for hydrogen permeation by recirculation of nitrogen introduced with the combustion air. This concept has been patented (Grace et al., 2006) and tested at high temperatures and pressures (Grace et al., 2005; Boyd et al., 2005; Xie et al., 2006). Notation Aa C0 Qhe Qm Qrb Qrt Qs up Ws
annulus cross-sectional area, m2 theoretical helium concentration, % flowrate of helium injection, Nm3 /h air flowrate through main distributor, Nm3 /h air flowrate through bottom ring distributor, Nm3 /h air flowrate through top ring distributor, Nm3 /h air flowrate through outside distributor, Nm3 /h particle downward velocity in annulus, m/s solids circulation rate, kg/s
Greek letters
a p
annulus bed voidage, dimensionless particle density, kg/m3
Acronyms FBMR FCC ICFBMR PSA TCD
fluidized bed membrane reactors fluidized catalytic cracking internally circulating fluidized bed membrane reactor pressure swing absorption thermal conductivity detector
Acknowledgments We are grateful to employees of Membrane Reactor Technologies and Noram Engineering, who contributed to the success of the ICFBMR cold model tests. References Abba, I.A., Grace, J.R., Bi, H.T., 2003. Application of the generic fluid bed reactor model to the FBMR process for steam methane reforming with oxygen input. Industrial and Engineering Chemistry Research 42, 2736–2745. Adris, A.M., Grace, J.R., 1997. Characteristics of fluidized bed membrane reactors (FBMR)-Scale-up and practical issues. Industrial and Engineering Chemistry Research 36, 4549–4556. Adris, A.M., Elnashaie, S.S., Hughes, R., 1991. Fluidized bed steam reforming of methane. Canadian Journal of Chemical Engineering 69, 1061–1070. Adris, A.M., Lim, C.J., Grace, J.R., 1994. The fluidized bed membrane reactor FBMR system: a pilot scale experimental study. Chemical Engineering Science 49, 5833–5843. Adris, A.M., Lim, C.J., Grace, J.R., 1997. The fluidized bed membrane reactor for steam methane reforming: model verification and parametric study. Chemical Engineering Science 52, 1609–1622. Boyd, D.T., Grace, J.R., Lim, C.J., 2005. Hydrogen from an internally circulating fluidized bed membrane reactor. International Journal of Chemical Reactor Engineering 3, A58, Available at: http://www.bepress.com/ijcre/ vol3/A58. Boyd, T., Lim, C.J., Grace, J.R., Adris, A.M., 2007. Cold modelling of an internally circulating fluidized bed membrane reactor. International Journal of Chemical Reactor Engineering 5, A26, Available at: http://www.bepress.com/ijcre/ vol5/A26. Burkell, J.J., Grace, J.R., Zhao, J., Lim, C.J., 1988. Measurement of solids circulation rate in circulating fluidized beds. In: Basu, P., Large, J.F. (Eds.), Circulating Fluidized Bed Technology II. Pergamon, Oxford, pp. 501–509. Chen, Z., Yan, Y., Elnashaie, S.S.E.H., 2003. Modeling and optimization of a novel membrane reformer for higher hydrocarbons. A.I.Ch.E. Journal 49, 1250–1265. Chen, Z., Yan, Y., Elnashaie, S.S.E.H., 2004. Hydrogen production and carbon formation during the steam reforming of heptane in a novel circulating fluidized bed membrane reformer. Industrial and Engineering Chemistry Research 43, 1323–1333. Chen, Z., Grace, J.R., Lim, C.J., Li, A., 2007. Experimental studies of pure hydrogen production in a commercialized fluidized-bed membrane reactor with SMR and ATR catalysts. International Journal of Hydrogen Energy 32, 2359–2366. ¨ L., et al., 2007. Membrane assisted fluidized Deshmukh, S.A.R.K., Heinrich, S., Morl, bed reactors: potential and hurdles. Chemical Engineering Science 62, 416–436. Dogan, M., Posarac, D., Grace, J.R., Adris, A.M., Lim, C.J., 2003. Modeling of autothermal steam methane reforming in a fluidized bed membrane reactor. International Journal of Chemical Reactor Engineering 1, A2, Available at: http://www.bepress.com/ijcre/vol1/A2. Grace, J.R., Li, X., Lim, C.J., 2001. Equilibrium modeling of catalytic steam reforming of methane in membrane reactors with oxygen addition. Catalysis Today 64, 141–149. Grace, J.R., Elnashaie, S.S.E.H., Lim, C.J., 2005. Hydrogen production in fluidized beds with in-situ membranes. International Journal of Chemical Reactor Engineering 3, A41, Available at: http://www.bepress.com/ijcre/vol3/A41. Grace, J.R., Lim, C.J., Adris, A., Xie, D., Boyd, D., Brereton, C., Wolfs, W., 2006. Internally circulating fluidized bed membrane reactor system. US Patent 7,141,231. Patil, C.S., Annaland, M.S., Kuipers, J.A.M., 2007. Fluidised bed membrane reactor for ultrapure hydrogen production via methane steam reforming: experimental demonstration and model validation. Chemical Engineering Science 62, 2989–3007. Roy, S., Pruden, B.B., Adris, A.M., Grace, J.R., Lim, C.J., 1999. Fluidized bed steam methane reforming with oxygen input. Chemical Engineering Science 54, 2095–2102. Xie, D.L., Grace, J.R., Lim, C.J., 2006. Development of internally circulating fluidized bed membrane reactor for hydrogen production from natural gas. Journal of Wuhan University of Technology 11, 252–257.