Fluidization with Hot Compressed Water in Micro-Reactors

Chemical Engineering Science 60 (2005) 5982–5990 www.elsevier.com/locate/ces

Fluidization with hot compressed water in micro-reactors

B. Potic, S.R.A. Kersten∗, M. Ye, M.A. van der Hoef, J.A.M. Kuipers, W.P.M. van Swaaij

University of Twente, Faculty of Science and Technology, PO Box 217, 7500 AE Enschede, The Netherlands

Received7 December 2004; receivedin revisedform 18 April 2005; accepted18 April 2005 Available online 9 June 2005

Abstract In this paper the concept of micro-fluidized beds is introduced. A cylindrical quartz reactor with an internal diameter of only 1 mm is used ◦ for process conditions up to 500 C and244 bar. In this way, fast, safe, andinherently cheap experimentation is provided.The process that promptedthe present work on miniaturization is gasification of biomass andwaste streams in hot compressedwater (SCWG). Therefore, water is usedas fluidizingagent. Properties of the micro-fluidbedsuch as the minimum fluidizationvelocity ( Umf ), the minimum bubbling velocity (Umb), bedexpansion, andidentificationof the fluidizationregime are investigatedby visual inspection. It is shown that the micro- fluid bed requires a minimum of twelve particles per reactor diameter in order to mimic homogeneous fluidization at large scale. It is not possible to create bubbling fluidization in the cylindrical micro-fluid beds used. Instead, slugging fluidization is observed for aggregative conditions.Conical shapedmicro-reactors are proposedfor improvedsimulation of the bubbling regime. Measuredvalues of Umf and Umb are compared with predictions of dedicated 2D and 3D discrete particle models (DPM) and (semi)-empirical relations. The agreement between the measurements andthe modelpredictionsis goodandthe modelsupports the concept anddevelopment of micro-fluidbeds. ᭧ 2005 Elsevier Ltd. All rights reserved.

Keywords: Chemical reactors; Fludization; Modeling; Supercritical ﬂuid; Micro-ﬂuid bed; Fluidization regime

1. Introduction especially if it involves high temperature andpressure. The objective of this work is, therefore, to develop a new exper- Miniaturization of chemical reactors is receiving increas- imental methodbasedon micro-fluidbedreactors with an ing interest (Kolb andHessel, 2004; Maharrey andMiller, internal diameter of only a few millimetres. Possible appli- 2001). Micro-reactors are specifically suitable for high- cations of micro-fluidbedsare high-throughput screening of throughput screening, on-demand chemical synthesis, and catalysts, establishing the operating window of a certain pro- carrying out reactions under conditions that would nor- cess, or study of specific hydrodynamic phenomena. Investi- mally leadto unsafe operation. In general, miniaturization gating chemical fluidbedprocesses in micro-fluidbeds,for leads to lower capital and operational costs, less energy instance, is preferable over testing in high-throughput fixed consumption, improvedsafety, andless pollution. In this beds.Translating fixedbeddatato fluidbedoperation is not paper, the miniaturization principle is appliedto fluidbed straightforward, particularly when adsorption/desorption of research. At present, laboratory (10–100 ml reactor volume) reactants and products, decompositions on the catalyst, and andpilot-scale (0.1–1000 l reactor volume) research and fast-aging catalysts are involved. Under these conditions, development concerning fluidized bed systems is expensive fixed beds suffer from the fact that strong spatial gradients andlabour intensive. In fact, it is anticipatedhere that this in the composition of the fluidphase andthe catalyst prop- type of research will become too expensive in the future, erties will make interpretation of the data troublesome. The typical characteristics andconditionsof a large-scale fluid bed, being 5 to 15 s gas residence time, 2–3 mixing ∗ Corresponding author. Tel.: +315 34894430; fax: +315 34894738. units, and1–5 mass transfer units ( De Vries et al., 1972; E-mail address: [email protected] (S.R.A. Kersten). Van Swaaij andZuiderweg, 1973 ), shouldbe mimicked

(Kersten et al., 2004). For quick, safe andcheap experimentation with such a system the micro-fluidbedtechnique has been developed. Sufficient mass balance closure of experiments with a chemical reaction using such a small system has been establishedalreadywith a comparable batch method( Potic et al. 2004a,b). Available data on fluidization with high-pressure CO2 cover a broadrange of fluiddensitiesandviscosities (Marzocchella andSalatino, 2000; Liu et al., 1996 ) and can be used, as a starting point, for the investigation of fluidization with hot compressed water. In fact, using CO2 Fig. 1. Different micro-reactor configurations. of much less severe conditions can approximate the density andviscosity of water usedfor the gasification process. To study the chemical process it is, however, essential that the in micro-systems. It is, however, not necessary that exact actual reaction medium, viz., hot compressed water, can be mapping is achievedat every level. The similarity should usedin the experimental method. be basedon the overall (chemical) performance of the pro- In the present contribution, cylindrical micro-fluid beds cess under consideration. The micro-fluid bed can have ev- are discussed. It is investigated whether or not the classical ery desired shape, ranging from a uniform cylindrical tube design equations for Umf , Umb, Ut andbedexpansion can be to a conical bedanda stagedreactor configuration (see usedfor the micro-system. Fluidizationof A andB powders Fig. 1). In cylindrical micro-beds of only several millime- with water in the range 1–244 bar and20–500 ◦C is reported. tres, it will be difficult to create stable bubbling fluidization, Also the differences and similarities between gas, liquid, and because slug formation will occur immediately after enter- supercritical fluids in fluidization are shown. The experi- ing the aggregative regime. Conical micro-fluidbedsare mental results are comparedwith both literature correlations consideredtosuppress slugging andto simulate a bubbling andthe predictionsof a state-of-the-art 3D Euler–Lagrange bed. Somewhat larger conical fluid beds (1–4 cm), showing (DPM) model (Ye et al. 2004, 2005). intense back mixing of both the solidandthe fluidphase, have been discussed by Kersten et al. (2003). Conical fluid bedscan also be appliedif more fluidphases are to be pro- 2. Experimental set-up cessedtogether. The process that promptedthe present work on miniatur- The central part of the micro-reactor system presented ization is gasification of biomass andwaste streams in hot here is a fluidbedwith an internal diameterof only a few compressedwater (SCWG). SCWG is foreseen to require millimetres (say one to three millimetres). For testing un- fluidbedtechnology operatedin severe conditions( Kersten der high pressure and temperature, all kinds of special al- et al., 2004). Gasification in hot compressedwater is a novel loys (e.g. Inconel) andquartz can be usedas construction process for the conversion of wet biomass andwaste streams material of the fluidbed.In case of quartz, visual inspec- (> 80 wt% moisture) into hydrogen or methane rich gas. The tion of the fluid bed reactor under process conditions is operating pressure is typically above 200 bar andtempera- possible even under extreme circumstances. Due to their tures in the range 300–650 ◦C are considered (Antal et al., small size, the quartz capillaries usedare strong enough 2000; Hao et al., 2003; Kruse et al., 1999; Yu et al., 1993; to withstandextremely high pressures (300 bar) andhigh Schmieder et al., 2000; Potic et al. 2004a,b; Elliott et al., temperatures (900 ◦C). Other advantages of the small di- 1994). The SCWG process is at an early stage of develop- mensions are intensive heat transfer from the surroundings ment. In the laboratory andpilot plant work carriedout so (oven) to the reactor, the very small amount of catalyst far, next to batch autoclave reactors, tubular flow reactors needed, low costs, and the fact that the system is inherently have been usedin connection with a shell andtube heat ex- save. changer. Not much information has been publishedon the The experimental system usedfor this investigation on flu- practical design of a reactor for a commercial plant. Simple idization consisted basically out of an HPLC piston pump, non-fouling model compounds have been used frequently a quartz capillary usedas fluidbed,an electrical oven, and as feedstock. However, thermal composition of real biomass a pressure vessel. Fig. 2 is a schematic representation of (e.g. verge grass paste) results in carbon formation andash the experimental set-up. The flow rate of water was con- deposition (Yoshida et al., 2004). This implies that catalysis trolledaccurately with an HPLC pump (LabAlliance). This is requiredto achieve complete conversion to gases andto pump coulddeliver flow rates of water in the range of steer the product distribution (Antal et al., 2000; Potic et al., 0.01–24 ml/min at a maximum pressure of 500 bar. A quartz 2005). To cope with such fouling conditions in combination capillary of 1 mm ID, 6 mm OD anda length of 1 m was with catalysis an integratedreactor/heat exchanger concept, usedas fluidbed.The actual settledheight of the bedwas basedon fluidbedsandcirculating solids,has been proposed 20 mm. These dimensions lead to an aspect ratio of the bed 5984 B. Potic et al. / Chemical Engineering Science 60 (2005) 5982–5990

Section Thermo “A” Window couple Reactor TI Distributor Pressure Section PI sensor Thermocouple “A”

Glass window Pressurized Rea- vessel ctor

Electrical oven

HPLC pump Water tank

Fig. 2. Scheme of the experimental set-up. of 20, which is high comparedto industrialpractice. Twenty system was pressurizedby pumping water, with the valve millimetres fixedbedheight was selectedin orderto keep closed, into the quartz capillary and tubing until the pressure the error on the measurement of the fluidbedlevel below prevailing inside the pressure vessel was reached. Then the 10%. This was necessary to obtain accurate estimates of valve was openedandthe nitrogen in the vessel maintained Umf and Umb from the expansion curve. In case of homo- the pressure in the system. Due to the much higher volume geneous fluidization, it is known that the aspect ratio does of the vessel comparedto the capillary (500:1), the pressure not influence the fluidization behaviour to a large extent. For remainedalmost constant duringoperation. By employing bubbling fluidization, the aspect ratio is an important pa- this semi-batch methodof operation, the needfor a com- rameter. However, the overall heat andmass transfer rates plex two-phase pressure-release valve was omittedandlarge of beds with high and low aspect ratio may approach each samples couldbe collectedat nearly constant conditions. other by tuning of the operating conditions. Monitoring of the pressure was done with the built-in sensor Some 30 cm of length before andafter the bedwere re- of the HPLC pump anda pressure sensor placedat the en- quiredto account for heating the incoming water andto pre- trance of the pressure vessel. During operation, the tempera- vent back-mixing effects of coldwater from the downstream ture was measuredwith three thermocouples (tip = 0.5mm) side. placedon the outer wall of the capillary andone thermo- Application of a conventional gas distributor turned out couple (tip = 0.5 mm) inside the reactor. The latter was the not to be possible in such a small system. To solve this one usedas support for the quartz ball serving as a distrib- problem, a quartz ball (∼ 0.9 mm) that just fittedinsidethe utor. One of the three thermocouples on the outer wall was 1 mm ID capillary placedon a thermocouple was usedas an placedat the location of the fluidbed.The temperature dif- alternative to a regular distributor plate. Raining of bed par- ference between this thermocouple andthe one insidethe ticles through the gap between the inner wall andthe ball bednever exceededmorethan 3 ◦C during fluidization and distributor was never observed. The quartz capillary set-up their average value was definedasthe bedtemperature. Ad- was heatedin an electrical oven. A glass window was placed ditionally, the axial temperature profile inside the quartz tube in this oven at the location where the fluidbedwas posi- was measured, in the absence of the fluid bed, by a movable tioned.This allowedfor visual inspection of the fluidbed thermocouple insertedvia the top. By these measurements andphotography. A ruler was placedbehindthe fluidbedfor it was shown that up to 20 cm downstream the ball distribu- measurement of the bedheight. Connections of the quartz tor the temperature inside the capillary was constant for all capillary to the steal tubing, connecting it to the pump and investigatedflow rates. the pressure vessel, were placedoutsidethe oven. The cap- Sandwith a densityof 2450 kg /m3 was as bedmate- illary was connectedto the pressure vessel via a one-way rial. Four different particle sizes were used: 60–70 m, valve in order to prevent that the content of the pressure 80–90 m, 100–150 m, and150–250 m. vessel couldflow back into the reactor. Nitrogen was used To investigate the effect of the reactor diameter the 1 mm to pressurize this vessel. Upstream the pressure vessel, the diameterquartz tube was replacedby tubes of 12 and26 mm. B. Potic et al. / Chemical Engineering Science 60 (2005) 5982–5990 5985

1.5 3.0

2.5 1.0 [-] 0 )/H

0 2.0 ln (U) 0.5 (H-H 1.5

0.0 1.0 0 5 10 15 20 25 -0.6 -0.5 -0.4 -0.3 -0.2 (a)U [mm/s] (b) ln (ε)

◦ Fig. 3. Measuredexpansion curve of homogeneous ﬂuidizationin a 1 mm ID ﬂuidbed.(a) (H − H0)/H0 versus U, (b) ln(U) versus ln(ε). T = 320 C, 3 P = 160 bar, f = 682 kg/m , dp = 80–90 m.

2.1. Measurements Due to the limitation of the pump with respect to minimum achievable flow rate (0.01 ml/min), determination of Umf A digital camera was used to take snapshots of the flu- was principally not possible for densities below 100 kg/m3. idization behaviour. All derived numbers in this work were The minimal flow rate of 0.01 ml/min corresponded to a basedon visual observations. linear velocity in the range 2–14 mm/s for densities below In case of homogeneous fluidization (investigated range 100 kg/m3, whereas according to the Abrahamsen and Gel- 3 f = 500–1200 kg/m ), Umf , Ut , and n were determined dart relation, Umf shouldrange between 2 and3 mm/s for from the bedexpansion curve. The superficial velocity was these conditions. For the density range 100–230 kg/m3 it fixedby the flow rate of the HPLC pump andthe prevailing turnedout that Umf could also not be determined. The rea- temperature andpressure in the fluidbed.The minimum ve- son for this was that the observedbedexpansion between the locity increment corresponded to an increase of the flow rate fixedbedstate andthe slugging velocity was only 2–3 mm, of liquidwater by 0.01 ml/min. The bedexpansion curve which was too little to serve as basis for an accurate esti- was constructedby readingthe height of the bedat different mate of Umf . The minimum bubbling velocity was defined velocities. The error on reading the bed level was 1–2 mm. as the average value of the velocity at which the first slugs By measurement of the amount of bedmaterial andthe fixed were observedandthe velocity before that one. bed height, the initial (fixed bed) porosity was determined. The voidageandthe superficial velocity were correlatedac- cording to the Richardson and Zaki equation: 3. Discrete particle model U n = ε (1) Ut The micro-fluidbedwas modeledbythe volume-averaged Navier–Stokes equations (Kuipers et al., 1992) combined Fig. 3 shows a typical measuredexpansion graph andthe with the Newtonian equations of motion for a single particle derived Richardson and Zaki curve. Umf was determined by taking into account possible collisions with other particles extrapolating the expansion curve to the fixedbedheight. andthe confining walls. In this study, the contact force be- Extrapolation was necessary, because in all cases, Umf cor- tween two particles was calculatedby use of the soft-sphere responded to flow rates below the lower limit of the HPLC model (Cundall and Strack, 1979). The details can be found pump. Due to the needfor extrapolation andthe error on elsewhere (Ye et al. 2004, 2005). Comparedto the codere- reading the bed height, the statistical error on Umf was cently described (Ye et al. 2004, 2005), one additional force, around50%. Extrapolation was not requiredfor determin- namely the added mass force, was added to allow simu- ing Ut and n, andconsequently the relatederror was much lation of systems involving high-density fluidizing agents. lower, namely ca. 10%. The added mass force is essential if the fluid is a dense gas Bedexpansion was measuredvisually also in case or a liquid. There are different expressions for the added of fluidization with water vapour (investigated range mass forces in the literature (Auton et al., 1988; Chang and 3 Maxey, 1995; Maxey andRiley, 1983; Clift et al., 1978 ). In f = 16–230 kg/m ). For superficial velocities higher than 12 mm/s aggregative fluidization, in the form of slugs, was this study, we followed the derivation of Maxey and Riley, observedin this regime. In case of slugging, the height of where the added mass force was given by the expanded bed was more difficult to determine. A time- ( − ) averagedbedheight taken over a time periodof 1 min was D v u Fm =−Cmf Vp (2) then used. Dt 5986 B. Potic et al. / Chemical Engineering Science 60 (2005) 5982–5990

Table 1 Parameters usedin the DPM simulations

Parameters Value

Particle number 36000 (3D) and3440 (2D) Particle diameter 85 m (3D) and80 m (2D) Particle density 2495 kg/m3 (3D) and2600 kg /m3 (2D) Initial bedheight 2.00 cm (3D) and1.36 cm (2D) − CFD time step 2.0 × 10 6 s Number of CFD cells 4 × 4 × 100 (3D) and4 × 100 (2D) Coefﬁcient of restitution 0.9 (3D) and0.95 (2D) Coefﬁcient of friction 0.2 (3D) and0.1 (2D) Fluidtemperature 873.16 K; 773.16 K; and673.16 K Fluidpressure Varying from 0.6 to 28.4 MPa

Fig. 4. Photographs of (a) homogeneous fluidization for different flow with ◦ rates at 300 C and 160 bar and, (b) slugging fluidization at the same ◦ D j velocity 370 C and160 bar. = 2450 kg/m3, dp = 80–90 m. = + u ·∇. (3) p Dt jt

In most previous studies, the added mass coefficient Cm was simply assumedto equal 0.5, being the theoretical value of fluidization with water vapour slugging is observed. The for a sphere moving in an unbounded fluidum. This value, slugs can be clearly seen in the capillaries (see Fig. 4b). however, is in addition, only valid for very dilute systems The bed level is fluctuating under slugging conditions where the influences of neighbouring are not important. In (see Fig. 4b). this research, the added mass coefficient was taken from the (2) Minimum number of particles per reactor diameter: Zuber equation for the added mass force of a particle in a ho- It is expectedthat there shouldbe minimum number of par- mogeneous dispersion in an non-rotational flow (Batchelor, ticles present per reactor diameterfor a micro-fluidbedto 1988), given by mimic a large-scale bed under otherwise identical conditions. To investigate this, glass balls of 60–200 m diameter 3 − 2ε Cm = . (4) have been fluidized with water at ambient conditions in three 2ε glass tubes with different internal diameters, viz. 26 mm, Together with the added mass force, there also exists a his- 12 mm and, 1 mm. The homogeneous expansion curves of tory force, which is also known as memory force or Bas- these fluidbedshave been recordedwhileusing sandparti- set force. It takes into account the vorticity diffusion in the cles of different size. Unfortunately, the data cannot be com- surrounding fluid, and the disturbance effect caused by the paredover the full velocity range. Due to limitations of the acceleration of the sphere. The calculation of this force is pump with respect to its minimum achievable flow rate, the complicatedsince it involves the integral of the derivative lowest velocities in the 12 mm and26 mm bedcouldnot be of fluidvelocity andparticle velocity. In all simulations re- reachedin the 1mm capillary. portedin this paper the Basset history force was neglected. Fig. 5a shows the bedexpansion andthe voidageversus In the presentedsimulations, the geometries of the system the velocity for the 1 mm andthe 12 mm ID bedwhile us- were taken as either a box with a size of 1 × 1 × 40 mm3 in ing particles of 60–70 m diameter. It can be seen that the 3D or a rectangle with a size of 1.5 × 50 mm2 in 2D. The curves obtainedin the 1 mm andthe 12 mm bedare nearly choice of such simplifiedgeometries was madeto enable the identical. Also for the particles of 80–90 m, there is a good DPM simulations. The parameters usedin the simulations agreement between the expansion curves obtainedin the are summarizedin Table 1. The water properties were taken three beds of different sizes, although the agreement seems from the database of the National Institute of Standards and less satisfactory than for the 60–70 m particles. In the case Technology (NIST) in the form of polynomials. of particles of 100–150 m, expansion in the 1 mm beddif- fers significantly from expansion in the 12 mm bed. This deviation is ascribed to wall effects. Finally, when using par- 4. Results ticles larger than 150 m, it has been observedthat the particles appear to move irregularly in clusters in stick-slip flow (1) Examples of visual observations:InFig. 4 snapshots anda fluidizedstatecannot be clearly recognizedanymore. of the micro-fluidbedin operation are presented. Fig. 4a These results leadus to the conclusion that micro-fluidbeds shows a typical series of homogeneous fluidization experi- need 12 or more particles per diameter in order to resemble ments with liquidwater in which the velocity is increased. a large-scale fluidbedin case of homogeneous fluidization. 3 The length of the expandedbedfordifferent velocities is (3) Fluidization with water: f = 500–1020 kg/m : usedfor the derivation of the expansion curve. In the case Measurements have been done in the range 20–360 ◦C and B. Potic et al. / Chemical Engineering Science 60 (2005) 5982–5990 5987

1 1 particle size 60-70micron 0 0 0 0 )/H )/H 0 0

-1 -1 1mm voidage 1mm expansion 1mm voidage 12mm voidage ln(e), ln(H-H 1mm expansion ln(e), ln(H-H 12mm expansion -2 12mm voidage -2 26mm voidage 12mm expansion o 26mm expansion

-10 -9-8 -7 -6 -5 -10 -9 -8 -7 -6 -5 (a) ln(U) (b) ln(U)

1 particle size 100-150micron

0 0 )/H 0

-1 1mm voidage 1mm expansion 12mm voidage ln(e), ln(H-H 12mm expansion -2 26mm voidage o 26mm expansion

-10 -9-8 -7 -6 -5 (c) ln(U)

Fig. 5. Homogeneous expansion and voidage curves recorded at ambient conditions using water as ﬂuidizing agent. Results obtained in reactors with an 3 internal diameterof 1, 12, and26 mm are shown. Sandparticles of: (a) 60–70 m, (b) 80–90 m, and(c) 100–150 m are used. p = 2450 kg/m , U in mm/s.

1–221 bar. In this density range, the observed fluidization is 100 always particulate. With increasing velocity, homogeneous expansion of the bedis observedandbubbles (slugs) are Ut never present. These observations are in agreement with the discrimination number (Dn) criterion of Liu et al. (1996) Haider & Levenspiel which states that for Dn < 104, fluidization is homogeneous. 10 For the conditions presented here, Dn is always smaller than 3500. Umf In the considered range, the viscosity of water at a certain U [mm/s] density hardly depends on the prevailing temperature and ε 1 Ergun ( 0=0.50) pressure. This allows constructing a characteristic velocity map with the fluiddensityon the x-axis. Such a plot for 3 particles with dp =85 m and p =2450 kg/m is presented in Fig. 6. As mentionedbefore, the measurement error for Umf 0.1 is rather large. Nevertheless, it can be concluded that the 500 600 700 800 900 1000 3 measured Umf values are in reasonable goodagreement fluid density [kg/m ] with predictions obtained on basis of the Ergun equation Fig. 6. Umf and Ut measuredfor water in the 1 mm ID FB versus f . (Ergun, 1952), providedthatthe measuredfixedbedporos- 3 3 f =500–1050 kg/m , p =2450 kg/m , dp =80–90 m. Measurements ity of 0.5 is usedin the calculations. The sharp increase of as well as correlations are plotted. one decade in Umf when going down in density from 1020 to 900 kg/m3 as predictedbytheory is also observedexper- imentally. Values of Ut derived from the expansion graphs Correlations for Umf and Ut are basedon measurements are close, although systematically higher, to theoretical val- in larger fluidbedsranging from several centimetres to me- ues calculatedaccordingto ( Haider and Levenspiel, 1989). tres. Because the measurements obtainedin the 1 mm fluid 5988 B. Potic et al. / Chemical Engineering Science 60 (2005) 5982–5990

experimental data for Umb DPM model Umf 400°C DPM model Umb 400°C DPM model Umf 500°C DPM model Umb 500°C DPM model Umf 600°C DPM model Umb 600°C 25

Umb(A&G correlation)

5 superficial fluid velocity [mm/s]

Umf(A&G correlation) 0 0 50 100 150 200 250 density [kg/m3]

3 Fig. 7. Measured Umb versus the density. p = 2450 kg/m , dp = 80–90 m. Umf and Umb versus the density for 400, 500, and ◦ 600 C according to the A&G correlation and the 3D DPM model. For the appliedconditions,see Table 1. Fig. 8. Snapshots of the simulation results in a 2D supercritical water fluidized bed. Particle diameter d = 80 m, particle density=2600 kg/m3, bedare reasonably well describedbythese correlations, it fluidtemperature T =773 K, fluidpressure p=153.0 bar, andfluiddensity can be concludedthatlarge-scale fluidizedoperatedinthe 50 kg/m3. Superficial fluidvelocities U are 0.0, 12, 24, 36, and48 mm/s homogeneous regime can be mimickedin micro-fluidbeds. respectively. However, for this to holdthe micro-fluidbedshouldsatisfy the condition of D/dp > 12 (see above). 3 (4) Fluidization with water vapour: f =16 to 230 kg/m : As mentionedbefore, Umf cannot be determined for these Measurements have been done as well in the range conditions and Umb is estimatedfrom the velocity at which 345–485 ◦C and50–245 bar. In the densityregion below the first slugs are observed(see Section 2.1). Measuredval- 90 kg/m3, only slugging is observed. This region cor- ues of Umb are comparedwith the Abrahamsen andGeldart responds to discrimination numbers exceeding 4 × 104, relation and predictions of a 3D DPM model (see Fig. 7). which is indicative for the aggregative fluidization regime. It shouldbe noticedthat the A& G correlation must be ex- For higher densities, two regimes are observed. From zero trapolated outside the validated regime for the conditions velocity up to approximately 2 × Umb, fast slugging is investigatedhere. In the DPM model,the fluidvelocity is observed. Upon increasing the velocity, the slugs become increasedlinearly with time. At the point where the overall smaller andtheir frequency increases. Eventually, this leads pressure drop equals the bed weight per unit area, the min- to fluidization in which the slugs cannot be observed any- imum fluidization velocity is reached. Via visualization of more andthe bedlevel becomes constant. For instance, for the computedresults using animation techniques, the min- water vapour of 113 kg/m3 such a regime is observedfor imum bubbling velocity can be established. At Umb larger velocities higher than 26 mm/s. This phenomenon has been voidsappearedin the generatedpictures of the fluidbed. described also by Li on the basis of simulations with a The details of the DPM model are presented elsewhere (Ye Euler–Lagrange CFD model (Li, 2003). The corresponding et al. 2004, 2005). Both the A&G relation andthe DPM Dn values for 100 kg/m3 and U 2 × Umf range from model predict that, in the investigated density regime, Umf 0.5 × 104 up to 1.8 × 104 describing homogeneous and and Umb hardly (10%) depend on the temperature (viscos- transitional regimes. ity) for a certain water vapour density (pressure). Within the It is not possible to create bubbling fluidization in the experimental error, the experimental estimates of Umb agree cylindrical micro-fluid beds used. Conical shaped micro- with the DPM results, which are also somewhat higher than reactors, geometrically similar to those of Kersten et al. predictions from the empirical relations of A& G (see Fig. 7). (2003), will be developedfor improvedsimulation of bub- (5) 2D simulation results: In addition, 2D simulations bling andthree-phase fluidizationas may prevail in catalyt- have been carriedout to visualize the flow patterns andthe ical gasification in hot compressedwater. transition of the flow regimes. Snapshots of the simulated B. Potic et al. / Chemical Engineering Science 60 (2005) 5982–5990 5989

3. Despite the small diameter of the fluid bed, the expansion of the homogeneous fluidbedis similar to those of larger beds provided Dt /dparticles > 12. 4. 2D and 3D soft-sphere simulations, including added mass effects for high density fluids, were able to simulate the micro-fluid beds well. In the future, this model will be used to assist the further development of micro-fluidbedreactors.

Notation

3 2 Ar Archimedes number, Ar = gdp(p − f )f / , dimensionless Cm added mass coefficient, dimensionless D reactor inside diameter, m Dn discrimination number, Dn = Ar/Remf (p − f )/ f , dimensionless dp particle diameter, m Fig. 9. Snapshots of the simulation results in a 2D supercritical water F fluidized bed. Particle diameter d =80 m, particle density=2600 kg/m3, m added mass force, N 2 fluidtemperature T = 773 K, fluidpressures are, from left to right, 34.5, g acceleration of gravity = 9.81 m/s 67.1, 97.5, 126.3, 153.0, 178.1, 201.4, 223.2, and262.4 bar, respectively, k slope, m/s2 which corresponds with fluid densities of, from left to right, 10, 20, 30, mf mass, kg 40, 50, 60, 70, 80, and100 kg /m3. Superficial fluidvelocity U is 36 mm/s. n numerical exponent in Richardson–Zaki equation, dimensionless Remf Reynolds number at minimum fluidization fluidization behaviour at a set pressure of 153.0 bar are velocity, Remf = Umf dpf /, dimensionless shown in Fig. 8. At first, homogeneous bedexpansion is ob- t time, s served.Then voidstructures in the top andbottom of the u local gas velocity, m/s bedcan be clearly observed,but no obvious bubbles appear. U fluidvelocity, mm/s At a velocity of 36 mm/s, more or less clear bubbles can be Umb minimum bubbling velocity, mm/s found. For a still higher fluid velocity, the bubbles develop Umf minimum fluidization velocity, mm/s quickly andform clear slugs. The snapshots of the fluidiza- Ut terminal velocity, mm/s tion at the same fluidvelocity but at different fluidpressures v particle velocity, m/s are shown in Fig. 9. It can be seen that with an increasing pressure, the fluidization behaviour becomes more “smooth” andlarge bubbles or slugs are transformedinto micro-voids. Greek letters Visually this may be interpretedas homogeneous fluidization. ε voidfraction, dimensionless 3 f fluiddensity, kg /m 3 p particle density, kg/m 5. Conclusions fluidviscosity, Pa s The conclusions of this work can be summarizedas fol- lows: References 1. A quartz micro-fluidbedset-up has been developed ◦ andoperatedup to 500 C and244 bar. With these reactors, Antal, M.J., Allen, S.G., Schulman, D., Xu, X., Divilio, R.J., 2000. biomass gasification in hot compressed water can be studied. Biomass gasification in supercritical water. Industrial and Engineering The methodis safe andcheap. Visual observation of the bed Chemistry Research 39, 4040. Auton, T.R., Hunt, J., Prud’Homme, M., 1988. The force excerted on a position andfluidizationregime is possible as well as the body in inviscid unsteady non-uniform rotating flow. Journal of Fluid occurrence of different phases, carbon deposition in case of Mechanics 197, 241. gasification, etc. Full control of pressure, temperature, and Batchelor, G.K., 1988. A new theory of the instability of a uniform flow rates is possible andrelatively easy. fluidized bed. Journal of Fluid Mechanics 193, 75. 2. Homogeneous fluidization, slug flow, and more or less Chang, E.J., Maxey, M.R., 1995. Unsteady flow about a sphere at low to moderate Reynolds number. Part 2: accelerated motion. Journal of homogeneous turbulent beds have been observed roughly FluidMechanics 303, 133. in line with the Dn criterion of a Liu et al. andempirical Clift, R., Grace, J.R., Weber, M.E., 1978. Bubbles, Drops andParticles. correlations. Academic Press, New York. 5990 B. Potic et al. / Chemical Engineering Science 60 (2005) 5982–5990

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