Low temperature drying of pomace in spout and spout-fluid beds

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Journal of Food Engineering 79 (2007) 1179–1190

Low temperature drying of pomace in spout and spout-fluid beds

L. Marmo *

Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, C.so Duca Degli Abruzzi, 24 10129 Torino, Italy

Received 18 October 2005; accepted 10 April 2006Available online 6 May 2006

Abstract

The solid residue that originates from virgin olive oil production has a consistency that falls between a paste and a particulate solid,consists of two phases: the crushed olive kernel and the fruit pulp. The kernel particles can be used as high quality solid fuel, while oil canbe extracted from the pulp residue. Two technological problems should be tackled to optimize the extraction process: drying the solidresidue at low temperature and separating the two phases.

The pomace was dried batchwise in a 0.17 m diameter spouted bed and continuously in a 0.29 m diameter draft tube spout-fluid bedwith independent control of the air flow rate to the spout and to the annulus. Experimental drying tests were carried out. The dryingkinetics were measured during each test in the 0.17 m diameter bed. The solids circulation rate was measured in the larger unit, undera wide range of air flow rates to both the spout and the annulus. An operation regime map has been obtained from a characterization ofthe quality of the fluidization under a wide range of hydrodynamical conditions. The drying efficiency of both apparatuses was studied.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Low temperature drying; Olive pomace flow regimes; Spout-fluid bed; Draft tube; Solid circulation; Biomasses

1. Introduction

The solid residue of olive oil processing (pomace) stillcontains a certain amount of low grade oil. As noted byGog}us and Maskan (2006), the industrial drying processescommonly adopted to reduce the water content of the pom-ace to a level useful for the oil extraction consists in coun-tercurrent rotary drying facilities fed with air at hightemperature (250 �C). The high temperature adoptedcauses pomace hydrolysis and reduce the oil quality (Fre-ire, Figueiredo, & Ferrao, 1999). As a consequence (Gog}us& Maskan, 2006) highlighted the need for low temperaturedrying processes and proposed a try dryer.

In this work the spouted bed (SB) and the draft-tubespout-fluid bed (DTSFB) are proposed to dry the olivepomace at low temperature.

0260-8774/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jfoodeng.2006.04.034

* Tel.: +39 11 564 4697; fax: +39 11 564 4699.E-mail address: [email protected]

Spouted beds (SB) are well known for handling coarseparticulates or even sticky materials, and for being able toobtain a regular solids circulation (Mathur & Epstein,1974), while offering some advantages over conventional flu-idized beds (Bridgwater, 1985). Spouted beds are thereforesuitable for different physical and chemical operations, rang-ing from coating, which has extensively been studied fornuclear and pharmaceutical applications (Bridgwater,1985; De Oliveira, Freire, & Coury, 1997; Jono, Ichikawa,Miyamoto, & Fukumori, 2000; Mathur & Epstein, 1974;Olazar, Jose, & Bilbao, 1996; Pissinati & Pereira Oliveira,2003), for catalyzed reactions, coal gasification and combus-tion (Konduri, Altwicker, & Morgan, 1995, 1999; Lim et al.,1988; Olazar, Arandes, Zabala, Aguayo, & Bilbao, 1997;Piccinini, Grace, & Mathur, 1979; Rovero, Piccinini, Grace,Epstein, & Brereton, 1983; Stocker, Eng, Svrcek, & Behie,1989) and for solids to fluid mass transfer operations, suchas drying (Arsenijevic, Grbavcic, & Garic-Grulovic, 2004;Di Bartolo, Marmo, Rovero, & Mazzarino, 1996; Hattori,Morimoto, Yamaguchi, Onezawa, & Arai, 2001; Izadifar& Mowla, 2003; Khoe & Van Brakel, 1983; Nagaraju,

mailto:[email protected]

Notation

cpaspecific heat of air [J/kg/K]

d diameter [m]E energy [J]c gas compression factor [–]Hsd disengagement height [m]m mass [kg]_m mass flow rate [kg/s]_mw specific water evaporation rate

[kgwater/m3 reactor]

mw,ds effective evaporation [kgwater/kgdry pomace]mw,ds effective evaporation [kgwater/kgdry pomace/s]mw,sp specific evaporation [kgwater/kgdry air]mw,spa specific annular evaporation

[kgwater/kgannulus dry air]mw,sp,sat specific evaporation at saturation

[kgwater/kgdry air]P pressure [Pa]Q gas flow rate [m3/s]bQ heat per unit mass [J/kg]R gas constant [J/mol/K]

T temperature [K]t time [s]U gas velocity [m/s]X solid moisture content [–]W compression energy [J/kg]Ws solids circulation rate [kg/s]

Subscripts

0 initiala ambientair airf fluidizationfin finalfines finesI internalin inletms minimum spoutingout outlets spouting

1180 L. Marmo / Journal of Food Engineering 79 (2007) 1179–1190

Murthy, Ramalaksmi, & Srinivasa Rao, 1997; Olazar et al.,1996; Osorio Revilla, Lopez-Suarez, & Gallardo Velazquez,2004a, Osorio Revilla, Elias-Serrano, & Gallardo Velaz-quez, 2004b; Shuhama, Aguiar, Oliveira, & Freitas, 2003;Tulasidas, Kudra, Raghavan, & Mujumdar, 1993; Viswana-than, Lyall, & Raychaudhuri, 1986). Some applicationsconcerning desulfurization in combustion processes havealso been presented (Xu, Guo, Kaneko, & Kato, 2000).The momentum transferred from the gas phase to the solidsis probably the main feature of spouting: this permits a goodand regular contact between the two phases. A limitationthat has persisted over the years is encountered whenscale-up (Le, Lim, & Grace, 1997) is needed and when a welldefined gas distribution through the mass of particulate sol-ids is required to promote mass transfer from the solids tothe fluid. This limitation is even more obvious when wetand sticky materials are treated in the fluidization units: inthis situation, a separate control of gas flows to the spout(mainly acting as a mixing means, besides promoting effec-tive mass transfer, though for a very short time) and to theannulus (where equilibrium between the moving phasescan be easily reached, thus stopping the phenomenon) isdesirable (Hattori et al., 2001). The addition of a draft tube(Arsenijevic et al., 2004; Hattori et al., 2001; Ijichi, Miyau-chi, Uemura, & Hatate, 1998; Ishikura, Nagashima, & Ide,2003; Ji, Tsutsumi, & Yoshida, 1997, 1998; Khoe & VanBrakel, 1983; Muir, Berruti, & Behie, 1990; Osorio Revillaet al., 2004a; Osorio Revilla et al., 2004b; Stocker et al.,1989) has proven to act as a beneficial constraint and helpsin terms of a better definition of both the gas distributionand the solids motion pattern during various processes suchas drying (Khoe & Van Brakel, 1983; Tulasidas et al., 1993),

coal gasification, combustion (Konduri et al., 1995,Konduri, Altwicker, & Morgan, 1999) pyrolysis of hydro-carbons (Stocker et al., 1989) and production of pharma-ceuticals (Fukumori & Ichikawa, 1997; Littman, Morgan,& Morgan, 1997). A paste-like agglomerate surely benefitsfrom such a geometrical configuration which assures theintermixing of the solids and close contact with the dryingfluid medium. The flow characteristics of spouted bedsequipped with a non-porous draft tube (SBDT) was revisedby Ijichi et al. (1998) and recently by Ishikura et al. (2003),while some papers have studied solid circulation in thebed, and the hydrodynamics and the flow regimes that occurinside SB and SBDT (Dogan, Freitas, Lim, Grace, & Luo,2000; Freitas, Dogan, Lim, Grace, & Luo, 2000, 2004; Ishik-ura et al., 2003; Ji et al., 1997, Ji, Tsutsumi, & Yoshida, 1998;Larachi, Grandjean, & Chaouki, 2003; Muir et al., 1990;Olazar et al., 1996; Yurong, Guangbo, Bouillard, & Huilin,2004). Modifications to the classic scheme were adopted byLim et al. (1988), and by Hattori et al. (2001) who adopted asecond air feed in the annulus of their spout-fluid bed (SFB)which was sometimes equipped with draft tubes (SFBDT).The effect of the entrainment length, the spout and fluidizingair flow rate on the solid circulation rate and on the amountof gas bypass were studied by Muir et al. (1990), Olazar, SanJose, Izquierdo, deSalazar, and Bilbao (2001), and by Ijichiet al. (1998). These authors showed that the fluidizing air canoffer some benefits by increasing the solid circulation rateand reducing the gas bypass from the spout versus the annu-lus and vice-versa. The spout-fluid bed, which has also beenadopted in this work, gives a better gas–solid contact sincethe gas that percolates in the annulus results in a higher flowthan in the classic configuration.

L. Marmo / Journal of Food Engineering 79 (2007) 1179–1190 1181

2. Experimental apparatus

Two drying rigs were used in the experimentation: a 0.17ID spouted bed with a 60� inclined angle cone was oper-ated with 4.5 kg of pomace with a bulk density of570 kg m�3, 40% moisture on a wet basis (kg water/kgwet pomace) and a coarse particle size of between 4 and8 mm. The SB was operated batchwise at a superficialvelocity of U = 1.15Ums and with an inlet diameterDi = 18 mm. The preliminary results can be found in DiBartolo et al. (1996).

A spout-fluid bed with a 0.29 m internal diameter,equipped with a draft tube (SFBDT) permitted the dryingtests to be run continuously. This apparatus is depicted in

Fig. 1. The process flow-diagram

Figs. 1 and 2. Both units were operated by heated air fromthe mains. Two rotating compressors (C1 and C2) fed atank (T) at a pressure of 8 bar; the compressed air was thendried at a dew point of �4 �C (D) and the pressure wassubsequently regulated via a pressure control valve (PCV)to about 1.5 bar. The total gas flow was metered using arotameter (FE1) via a gate regulation valve (HCV) andheated to the desired temperature by an electrical heater(H). Two streams were then derived to feed the spout(Qs, spouting air flowrate) and the annulus (Qf, fluidizationair flowrate). The flow rates were controlled by individualvalves (HCV2 and HCV3) and measured by calibrated ori-fices (FE2 and FE3). A noticeable improvement, as far asthe gas flow measurements are concerned, was achieved

of the spout fluid bed drier.

Fig. 2. Details of the 0.29 I.D. unit: mechanical sketch, fluidization gas.


by positioning a honeycomb upstream to each disk. Thestainless steel piping was thermally insulated. A relevantnumber of components were ground-connected to dis-charge any static electricity.

Fig. 2(a)–(c) provide construction details of the SFBDT:the first drawing is a mechanical overview of the equip-ment; Fig. 2(b) highlights the gas repartition devices as wellas the rising spouting nozzle; the last sketch depicts theinternal parts (draft tube, thermocouple positioning, coni-cal deflector). The 0.29 m I.D. vessel was made of a Perspex1.65 m high pipe with a 62� included angle cone. The coneitself acted as a gas distributor, thanks to a regular pitch ofabout 100 holes with an internal diameter of 1 mm, drilledperpendicularly to the cone surface; the surrounding coni-cal wind-box was equipped with a pressure safety valve(PSV, Fig. 1). The spouting air entered the unit via a32 mm nozzle, whose elevation was suitably set to meetthe hydrodynamic requirements. A non-porous draft tube(40 mm I.D., 50 mm E.D., 430 mm high) was held in placeby two rows of radial spikes placed 120� apart connected tothe cylindrical vessel. The vertical positioning of the drafttube was chosen according to the pomace agglomerationconditions. The best compromise was found setting the dis-engagement height at 0.06 m. A conical impinger, placed500 mm above the draft tube exit, acted as a target to con-fine the spout vertically, deflect the coarse particles andreduce the carryover of the fines. A bag filter (F) acted asa particle collector downstream to the fluidization unit.

The apparatus was equipped with six J-type thermocou-ples: downstream to the heater, at the gas inlet, at three dif-ferent depths in the bed and in the freeboard above theconical deflector to measure the mean value of the temper-ature at the gas exit (Fig. 2(c)). The thermocouples placedin the annulus could be radially moved along the annulusradius in order to monitor the temperature in the wholemoving bed. An on-purpose feeding device was constructed

to feed the solids at a steady rate, the pomace having quitea variable moisture content (in the 35–50% range in weight)and consequently undergoing inconstant severe agglomera-tion. A 50 mm feeding tube discharged the material some600 mm above the free surface of the bed at the mid radiusof the annulus. The discharge opening was positioned onthe opposite side to the feeding port to minimize the pom-ace bypass. The pomace over-flew a gate at the wall andwas collected in a side bin.

3. Experimental procedure

3.1. Batch drying

The 0.17 m SB was operated with a batch of 4.5 kg pom-ace inventory, which initially corresponds to a 0.43 m beddepth. The pomace has a bulk mean density of 570 kg/m3

at a moisture content of 40% (wet basis). The coarse parti-cle size ranged from 2 to 4 mm. It should be specified that,while the kernel can be dimensionally classified, the pulp,which consists above all of fibrous material and containsmost of the water in the wet pomace, tends to agglomeratewith a variable consistency, therefore it is quite hard todefine a proper particle size for it. Moreover, due to thecrushing process for oil extraction, the size distributionand the sphericity of the material depend on the mechani-cal action it has undergone.

The composition of the pomace, taken from Martin-enghi (1948), is shown in Table 1. The diameter distribu-tion depicted in Fig. 3 shows a strong bimodal shape dueto the presence of the kernel and to the fact that the pulphas very different sizes. The pulp roughly represents 40%by weight of the wet material (40% moisture content).

The spouted unit was operated at a superficial velocityU = 1.15Ums. Although it has been shown by Konduriet al. (1999) that stable spouting can be achieved by

Fig. 4. Pomace moisture content versus time at different inlet airtemperatures. Dc = 0.17 m, H = 0.49 m, U ffi 1.15Ums, pomace inven-tory = 4.5 kg.

Fig. 3. Granulometric distribution of the pomace: solid line = pulp,dotted line = kernel.

Table 1Main components in pomace according to Martinenghi (1948)

Moisture 30%Oil 5%Pulp 30%Kernel 35%

Water 30%Nitrogen substances 4%Fatty substances 5%Non-nitrogen substances 18%Cellulose 37%Ash 6%


operating the bed at a spouting velocity above 1.5–2.5 Ums,in our case, stable spouting was attained at lower gas veloc-ities. It is our opinion that this is due to the presence of thepulp, which, being made up of fines, leads to a diminishingof the minimum spouting velocity.

Several experimental runs were carried out at various airtemperatures ranging from 60 to 100 �C. The minimumspouting velocity was varied throughout each run, in orderto accomplish a decrease in Ums due to the change in thematerial properties provoked by the decrease in the mois-ture content. The Ums was very high at the beginning ofthe run, when the pomace had very sticky characteristicsdue to the moisture content. During the drying process,the moisture content decreased remarkably and the mate-rial therefore also increased in mobility to a great extent.This resulted in a remarkable decrease in the minimumspouting velocity and hence the air flow rate was continu-ously adjusted to maintain a value of 1.15 * Ums. At thesame time, the power supply to the air heater was dimin-ished to keep the temperature at the bed inlet constant.The temperature of the material inside the bed was mea-sured continuously by means of a thermocouple. The mea-surements were made in the annulus.

In order to monitor the drying process, some small pom-ace samples (with a weight of about 1 g) were withdrawnfrom the annulus at different times. Since segregation phe-nomena can involve the solid phase in a spouted bed

(McNab & Bridgwater, 1979; Rovero, 1983), the samplingpoint was accurately chosen in order to pick samples thatwere representative of the whole inventory.

The residual moisture in the samples was measured bydrying them at 105 �C for 12 h in a oven. This procedurepermitted samples with a stable weight to be obtained.

The process was extended until the humidity reached avalue close to equilibrium at room temperature. This mois-ture content prevented any fermentation process. Themoisture content versus time profiles are given in Fig. 4with the air temperature as an operative variable. All theexperiments were performed in the falling period of thedrying rate. It appears that the higher the air temperature,the faster the drying rate (moisture decline versus time) ofthe material. This means that, as expected, the drying ratewas faster as the temperature of the drying mediumincreased, hence the productivity of the dryer rose withan increase in the air temperature or, which is the sameresult, less time is needed to lower the moisture contentto a given value. Since increasing the air temperatureresults in higher energy consumption to heat the unit vol-ume of air, but at the same time the time needed and hencethe volume of air are lowered, the energy consumption forpumping decreases. An optimization problem thereforearises when the energy consumption is considered to bethe most important variable.

The energy consumption was evaluated to determine theoptimal temperature. The consumption was divided intotwo contributions, the energy required to heat the air andthe energy required for blowing purposes.

The heating term was calculated as the heat necessary toraise the air temperature from Ta to T (Eq. (1)).

Ta was assumed to be 15 �C.bQ ¼ cpaðT � T aÞ ð1Þ

The mechanical energy requirement was calculated asthe energy necessary for the adiabatic compression fromthe ambient condition to the inlet pressure Pin (Eq. (2)):

Fig. 5. Overall energy requirement, U = 1.15Ums, wet pomace inventory4.5 kg.

Fig. 6. Regime map in the 0.29 m I.D. spout-fluid bed unit: seven zonescan be distinguished according to the definitions given in Table 2, pomaceInventory: 8 kg, Hsd = 0.06 m.


W ðtÞ ¼ RT a

cc� 1

� �P inðtÞ

P a

� � c�1cð Þ� 1

!ð2Þ

where Pin is a function of time since the air flow decreaseswhile the pomace dries and this results in a decrease in theinlet pressure.

The total energy consumption is therefore calculated asfollows:

EðtÞ ¼Z t

0

ðbQ þ W ðtÞÞ _mairðtÞdt ð3Þ

Fig. 5 provides a quantification of the overall energyrequirement versus the moisture content of the batch ofpomace. The lowest energy consumption was obtainedfor an air temperature close to 80 �C. The energy evaluatedfor a fixed bed unit is given as a reference, (this resulted tobe about 26% lower than the optimal SB condition): thelower requirement, however, does not necessarily corre-spond to an easier process operation, since a fluidizedbed permits a continuous, smooth and well mixedfunctioning.

4. Continuous drying

4.1. Spout fluid bed with draft tube (SFBDT)

The effect of the addition of a draft tube on hydrody-namics has extensively been discussed (Ijichi et al., 1998;Khoe & Van Brakel, 1983; Muir et al., 1990). The drafttube is a tube that is aligned to the spout direction, so thatthe spout can enter the draft. The main function of thedraft is to support the solids in the annulus, so that thebed height can be increased in comparison to conventionalspouted beds and the pressure drop can decrease. In thepresence of a draft tube, the spout gas can no longer perco-late from the spout to the annulus, thus the gas–solid con-tact decreases (Khoe & Van Brakel, 1983). The draftbottom is located at a certain distance from the spout inlet.This distance is called the disengagement height. As the gascan no longer percolate to the annulus, the solid cannotenter the spout region unless through the disengagement

height. As a consequence, the disengagement height hasan important influence on the solids circulation rate.

In SBDT, a large percentage of the spout gas, as high as70% (Muir et al., 1990), can percolate through the disen-gagement height towards the annulus. In the SFBDT, theaddition of an auxiliary gas injection in the annulus leadsto beneficial effects because it can reduce the gas bypassfrom the spout to the annulus, and at the same time itfavours the entrainment of the solids towards the drafttube, and hence increases the solids circulation rate andthe mass transfer that occurs in the draft.

4.2. Regime mapping

Detailed knowledge of the solids circulation rate isimportant to spout binary mixtures of particles with anon-porous draft tube, in order to ascertain both the effec-tiveness of the gas–solid contact and the heat transfer char-acteristics. Some studies of the regimes that occur in SBhave been proposed in literature by Dogan et al. (2000),Freitas et al. (2000, 2004). In these works pressure fluctua-tions were used to study the flow regimes. Eight differentregimes were discovered. The hydrodynamical behaviourof the SFBDT was also extensively studied in the presentwork to provide a basic description of the regimes thatoccurred when the bed was operated with pomace. In thelight of these aims, a visual observation of the flow behav-ior was used to identify the various regimes. As expected,two variables, the fluidization flow rate from the conicalgas distributor (Qf) and the spouting flow rate from thenozzle (Qs), affected the flow regimes to a great extent.When these two variables are varied, many different fluid-ization conditions take place in the bed, from the fixedbed regime, in which no movement of the particles canbe observed, and the air flows through the bed material,to bubbling fluidization with large bubbles rising in theannulus and strong mixing of the solid phase. In order todetermine the most appropriate operative condition, twomaps of the regimes (Figs. 6 and 7) were drawn by playingwith the two gas flow rates and visually monitoring the

Fig. 7. Regime map in the 0.29 m I.D. spout-fluid bed unit: seven zonescan be distinguished according to the definitions given in Table 2, pomaceinventory: 12 kg, Hsd = 0.06 m.


onset of the solids motion, bubble formation, spout heightand the overall stability. The two maps correspond to dif-ferent solid inventories, equal to 8 and 12 kg, respectively.

A very high number of batch runs, characterized by thetwo pomace inventories, which resulted in bed depths of0.40 and 0.49 m, respectively, were performed. During theruns, the solids mass was regularly checked to replace thefines conveyed outside together with the outlet gas. Themapping was performed with dry pomace in order to workunder reproducible conditions. Since wet pomace is muchstickier than a dry material, the operation of the dryer withmoist material is much more difficult and the regimeschange according to the moisture content. A mapping withdry material, however, allows a good comprehension of thebehaviour of the apparatuses to be attained. We howeveroperated with a wet material, since the same regimes canbe found under both dry and wet conditions.

Table 2 defines the different zones that make up the mapand the corresponding behaviour of the spout fluid bed.The optimal hydrodynamical condition was devised at asmooth solid circulation, moderate spouting and minimumfluidization flow rate, thus saving both in terms of mechan-ical blowing energy and drying medium utilization.

Beside the optimal conditions, seven zones with nonoptimal conditions were also identified, as can be seen inTable 2, that was built operating with the 8 kg inventory.

Table 2Working regimes in the spout-fluid bed induced by varying Qs and Qf

Regime Condition Description

{1} Fixed bed No solids movement is detected{2} Internal spout A small amount of solids moves

close to the nozzle{3} Unstable spout Irregular spouting with a spout

height <0.2 m{4} Excessive spouting Spout height �0.5 m{5} Optimal fluidization 0.2 m < spout height <0.5 m{6} Bubbling bed Bubbles rise along the annulus{7} Unstable spout/bubbling

bedBubbling in the annulus causesinstability in the spout

The regime map for the 8 kg inventory, which gives theextension of the zones related to the various regimes on theQf versus Qs plane, according to the definition given inTable 2, is depicted in Fig. 6. The tests were carried outboth increasing and decreasing the gas rates in order toavoid hysteresis and the observations were prolonged longenough to assess the steady state.

Different transitions between the regimes, which dependon the path the system moves on, were identified. It can beseen in Fig. 6 that, if the Qs is increased at constant Qf, threepaths can be followed: low (1), medium (2) and high Qf (3).

With reference to path 1, the unit operates as a fixed bed{regime1} at low Qs. A first transition occurs when aninternal spout forms with an increase in Qs. The systementers the internal spout regime {2}, where only the inter-nal part of solids circulate, while a layer of particulatematerial close to the vessel wall is still steady. A furtherincrease in the spouting air brings the system to a secondtransition, leading to the excessive spouting regime {4}with a very high spout that results in the solid particlesbeing deflected by the conical target. This transition is, ofcourse, not clear cut. The spout height instead increaseswhen the Qs is increased in the internal spout regime.

When path (2) is considered, where Qf is moderatelyincreased compared to path (1), the system again operatesas a fixed bed at low Qs. An increase in Qs results in a firsttransition, with the system entering the unstable spoutingregime {3}, but this increase is not quite sufficient to ensurea steady solids circulation. A similar regime was observedby Muir et al. (1990) in their half-cylindrical spout-fluidbed. In that case, this happened when the spout enteredthe draft tube ‘‘as a stream of large bubbles passing through

the bed of solids’’ and separated due to a ‘‘collapse of annu-

lar particles into the spout’’. Under these conditions theyobserved a slug flow of solids inside the draft tube, whichresulted in unstable spouting. A second transition bringsthe system to the optimal fluidization conditions {5}. Thearea is located in the middle of the map at Qf in the 24–58 m3/h range, corresponding to a superficial velocity of0.1–0.244 m/s and Qs between 24 and 32 m3/h, which thencorresponds to a superficial velocity of 0.1–0.135 m/s.When Qs is further increased, the spout height increasesand the excessive spout regime is reached.

When path (3) is considered, which means high Qf, thesystems acts as a fixed bed at low Qs, the first transitionresults in the incipient bubbling regime, which is in a notwell defined area in the Qf Qs plane, followed by the bub-bling bed regime where the air rises in the annulus as largebubbles.

The experiments conducted with an inventory equal to12 kg gave a similar map, which is reported in Fig. 7. Ifthe two figures are compared, two differences can be noted.With the 12 kg inventory, the incipient bubbling regimewas not observed clearly, moreover the optimal fluidizationzone extended toward high values of Qf, and probably‘‘absorbed’’ the incipient bubbling zone. The optimal fluid-ization regime extended to very low values of Qs, much

Fig. 9. Effect of the fluidization air on the solids circulation rate, pomaceinventory 8 kg, Qs acts as a parameter, Hsd = 0.06 m.


lower than the case of the 8 kg inventory, suggesting that,in this case, Ums was much lower. This result agrees withthe findings of Ijichi et al. (1998) who showed that anincrease of the mass of solids in the bed produces anincrease of the solid circulation rate. It should also notedthat this second map was built with pomace from a differ-ent crushing campaign, therefore some difference in thefines content could exist. It was experimentally demon-strated by Ishikura, Shinohara, and Funatsu (1982) andIshikura et al. (2003), that in the case of a DTSB, anamount of fines as small as 1% by weight can lower Ums

to 1/4 of the original value.

5. Solids circulation measurements

The solids circulation rate WS can affect the perfor-mance of an SBDT to a great extent. The effect of the diam-eter of the draft tube, of the length of the entrainment zoneand of Qs and Qf on the solid circulation rate was studiedby Muir et al. (1990) in a 0.2 m diameter half unit.

In the present work, the solids circulation rate was mea-sured at each point in the map regime by deflecting the par-ticles transported in the draft tube into a dogleg shapedpipe ending with a small bag where the solids were col-lected to weigh the mass after a given sampling time. Thesolids inventory and the moisture content in the pomacewere the same as in the experiments that provided themap regime. Figs. 8 and 9 give the effect on the solids cir-culation rate as a function of the spouting air flow rate Qs

and the fluidization flow rate Qf, respectively; in the sameplots, the other variable acts as a parameter. It can benoticed that the increase in Ws versus Qs is monotonic,hence an increase in the spouting air flux results, in therange explored in this work, in an increase in the solids cir-culation rate. This result confirms that as Us increases, theentrainment of solid particles in the entrainment zoneincreases, and therefore Ws increases monotonously withUs, as previously found in SB (Mathur & Epstein, 1974)and in DTSB (IIjichi et al., 1998; Muir et al., 1990).

On the contrary, the solids circulation rate versus Qf isnot so well defined. The trend slightly increases at low

Fig. 8. Effect of the spouting air flow rate on the solids circulation rate,pomace inventory 8 kg, Qf acts as parameter, Hsd = 0.06 m.

Qf, and a sort of plateau or even a maximum can be iden-tified when Qf exceeds 58 Nm3/h. If this value is exceeded, aslight decrease in the Ws is observed at the higher Qs. Thesevalues of Qs and Qg roughly correspond to the boundary ofthe optimal fluidization area with the excessive spout andthe bubbling bed areas in the regime map (Figs. 6 and 7).The increase in Ws, as observed at low Qf, has an analo-gous trend in the case of porous (Ishikura et al., 2003)and non porous draft tubes (Muir et al., 1990), and canbe attributed to the aeration of the solids in the annulus,which promotes higher flowability of the solid particlesfrom the annulus to the entrainment zone. On the contrary,when a excessive high air flow rate is supplied to the annu-lus, the bubbles that form at the bottom of the bed slow thepassage of the particles in the entrainment zone.

These results are an indirect confirmation of the needfor, and the correctness of, a regime map and show thatboth the bubbling onset in the annulus and the excessivespout have a negative effect on the regular pattern of solidscirculation.

The solids circulation rate measurements were extendedto the whole map of the regimes and an attempt to drawisocirculation curves was then made. The map, of course,only has the main purpose of providing a rough idea ofthe behaviour of the hydrodynamics of the DTSFB. Morein depth knowledge can be obtained when measurementsare made with solids with more well defined characteristics.The results for the 8 kg inventory case are reported inFig. 10: these curve are indices of the mixing promoted inthe bed for the various regimes.

Below the curve at 55 kg/s, the solids flow rate dropssharply because the system enters the unstable spout area(see the map in Fig. 6) where is very difficult to operatethe apparatus at constant conditions. This demonstratesthat this apparatus cannot be properly operated at a solidflow rate lower than a given minimum.

5.1. Drying

The drying process in the SFBDT was carried out con-tinuously since this way of running the process allows the

Fig. 11. Temperature of the inlet air and of the material during acontinuous drying run, pomace inventory 12 kg, bulk moisture 10%.

Fig. 10. Solids flow rate with respects to Qs and Qf, inventory: 8 kg,Hsd = 0.06 m.


reactor to be operated with a low moisture content pomaceinventory with which is therefore easily fluidized because itis not as sticky as the wet material. Since the reactorroughly behaves like a perfectly mixed reactor, the mois-ture content of the material inside the DTSFB is equal tothe moisture of the discharged material. Hence 10% on adry basis was assumed as the moisture of the pomace insidethe reactor. It should be noted that, according to the resultsof the batch runs (Fig. 4), the lower the moisture content,the lower the drying rate. Hence the lower the moisturecontent, the less efficient the process is expected to be.For this reason continuous runs with 10% moisture contentwere assumed as a target instead of the 4% adopted in thebatch runs. The pomace feeding rate was adjusted, accord-ing to the total air flow rate (Qs + Qf) and to the inlet airtemperature in order to achieve stationary conditions.The total solid inventory was constant, since a continuousdischarge of dry material was provided by the overflowlocated at the freeboard of the bed. The drying processwas controlled through a continuous measurement of thetemperature inside the bed, in the position shown inFig. 2(c).

The temperatures measured in a typical run at the inletand inside the bed are depicted in Fig. 11. It can beobserved that, except for a short transitory at the beginningof the run, the temperature measured inside the bed (T3–T5) is equal to the air temperature at the bed exit, down-stream to the shield (T6). This is assumed to indicate thatno significant bypass occurs through the draft tube at thegiven conditions. The air that flows through the draft alsoundergoes mass and heat exchange to the same extent asthe air flowing through the annulus. Since the system canbe considered adiabatic, the temperature decrease onlydepends on the amount of water that is evaporated. Inthe case of a lower mass transfer in the draft tube com-pared to the annulus, a higher temperature of the airemerging from the draft than of the air emerging fromthe annulus should result. In this case, the air temperatureat the bed exit would result to be higher than the tempera-ture inside the bed.

Several experimental runs were performed at an air inlettemperature of 55 and 68 �C. The bed was operated with asolid inventory of 12 kg of dry pomace.

The pomace inventory and the moisture content at thebeginning and at the end of the run, the amount of pomacefed and discharged and the respective moisture contentwere measured at each experimental run.

The material balances at the dryer were written for thedry material and for the water:

m0ð1� X 0Þ þZ

t_minðtÞð1� X inðtÞÞdt

¼ mfð1� X fÞ þZ

t_moutðtÞð1� X outðtÞÞdt

þZ

t_mfinesðtÞð1� X finesðtÞÞdt ð4Þ

m0X 0 þZ

t_minðtÞX inðtÞ ¼ mf X f þ

Zt

_moutðtÞX outðtÞdt

þZ

t_mfinesðtÞX finesðtÞdt ð5Þ

The amount of evaporated water was calculated for eachexperimental run by solving Eqs. (4) and (5). The balanceswere solved both for the whole run, in order to validate it,and for the steady state period of the process; hence, in thiscase, the transient at the beginning was disregarded. Fromthis ‘‘steady state balance’’, which accounts for the amountof evaporated water and for the air released into the dryer,the following parameters were calculated:

(a) the specific evaporation mw,sp, which represents theratio between the amount of evaporated water andthe volume of air fed to the reactor,

(b) the specific annular evaporation mw,spa, which repre-sents the ratio between the amount of water evapo-rated and the volume of air fed to the annulus,

(c) the effective evaporation mw,ds, which represents theamount of water evaporated per unit mass of drypomace, per unit time (this is analogous to whatwas defined by Viswanathan et al. (1986).

Table 3Results of the continuous drying runs, pomace inventory 12 kg, bulkmoisture 10%

Operatingconditions

Run N� Units

1 2 3 4 5

Qs 14.7 14.1 14.2 14.9 15 Nm3/hQf 65.4 69.7 71 66.1 99 Nm3/hT 68 68 55 55 55 �C_min 1.3 1.3 1.3 1.1 1.3 g/smw,sp 18.2 17.1 15.52 15.03 11.47 gw/Nm3

air

mw,ds 111.7 109.8 104.79 96.57 108.42 gw/kgdry pomace/hmw,spa 22.3 20.5 18.53 17.14 13.21 gw=Nm3

fluidization air

_mw 58.70 57.5 53.14 48.93 52.59 kgw/m3h


(d) the specific water evaporation rate _mw, which repre-sents the rate of water evaporation per unit volumeof the reactor.

The results are summarised in Table 3. It is interesting tocompare mw,sp to the theoretical amount of water needed toadiabatically saturate the hot air fed to the dryer. Thisvalue, defined as mw,sp sat, was devised from the Mollierchart and resulted to be 18.72 g H2O=m3

air and 15.43 gH2O=m3

air, respectively, at 68 and 55 �C. All the tests, withthe exception of number 5, basically brought the air tocomplete saturation, while test 5 did not bring the dryingmedium to saturation. This is probably due to the highvalue of Qf which results in a certain bubbling of the annu-lus with a reduced contact between the phases.

When mw,spa is considered, which is of course higher invalue than mw,sp, it appears that the computed value ishigher than the saturation capacity of the air. This effectcould account both for a certain mass transfer betweenthe gas and the pomace in the draft tube and for somecross-flow of the air introduced through the nozzle (henceaccounted for as Qs) that flows through the annulus ofthe bed.

It is also interesting to note that mw,ds is greatly influ-enced by the operative conditions. At the temperature of68 �C, values around 110 gw/kgdry pomace/h were obtained,while tests 3 and 4 run at 55 �C returned a lower value. Test5, on the contrary, demonstrated a value close to those cal-culated at 68 �C, in spite of the lower saturation of the airthat was obtained. These results demonstrate that, at ahigh gas flow rate, the air does not saturate and hencethe thermal efficiency of the operation is lower, but theeffectiveness of the drying of the solids could be higher.However, if _mw, the total evaporative capacity of the appa-ratus is observed, and if the results from tests 1 and 2 arecompared with tests 3, 4 and 5 it can be seen that the over-all evaporative capacity of the apparatus increases with anincrease in the air temperature. On the contrary, theincrease in the gas flow rate adopted in test 5 does notimprove the overall evaporation to any great extent, atleast with respects to test 3, and this effect depends onthe lower contact efficiency which results in an incompleteair saturation.

These results agree with those reported by Khoe & VanBrakel (1983) who found that, in a DTSB, the gas–solidheat transfer occurs mainly in the draft tube and in thegas recirculation zone and hence the drying efficiency isstrongly influenced by the particle holdup (whichdetermines the heat exchange area) in the tube. In our case,the increase in Qs seems to decrease the solid holdup in thedraft tube and hence decreases the heat exchange and thedrying efficiency. Since this happens when Qs is increased,this result also indicates a certain bypass of the gas fromthe annulus to the draft tube. Analogous considerationswere drawn by Arsenijevic et al. (2004).

Hattori et al. (2001) instead found that the drying effi-ciency in the DTSFB was unaffected by the gas flow rate.Their experimental apparatus, which was equipped with aflat air distributor at the bottom, without any nozzle, washowever probably able to achieve a distribution of the airflow that was independent of the total flux.

6. Conclusions

Olive pomace was dried under various operative condi-tions in a conventional spouted bed and in a spout fluidbed equipped with a draft tube. The spouted bed was oper-ated batchwise, the spout fluid bed was operatedcontinuously.

Due to the high moisture content of the material, thebatch procedure is quite difficult to run, at least at thebeginning, since wet material is sticky and tends to agglom-erate. It is instead much easier to operate continuously,when the bed is made up of almost dry pomace.

In the case of the batch operation, two contributions tothe energy consumption were identified: the first is relatedto the heating of the air and the second is related to theblowing of the air. When the first contribution increases,due to an increase in the air temperature, the seconddecreases due to the lower quantity of air needed.

It was demonstrated that an optimal temperature of theinlet air can be found to optimise the overall energyconsumption.

The spout fluid bed flow regimes were studied in aspouted bed equipped with a non porous draft tube. Anoptimal fluidisation regime and seven non-optimal regimeswere identified. The solids circulation rate was measured inall the regimes and a circulation map was built. This workconfirmed the finding by Ijichi et al. (1998) that an increaseof the bed weight produces an increase of the solid circula-tion rate.

Several continuous drying tests in the spout fluid beddemonstrated that, in the optimal fluidization regime, theair is almost completely saturated by the moisture. Underthese conditions the contact between the phases is highenough to allow the saturation of the air that rises intothe draft tube and of the air that rises in the annulus.

At high fluidization air rates, the dryer operates in thebubbling bed regime. Under this condition, bubblingoccurs in the annulus with incomplete saturation of the


drying medium occurs. When the bed is operated in thebubbling bed regime, some improvement in the mw,ds canbe obtained, but the overall evaporation rate of the beddoes not increase.

Acknowledgements

The author would like to recall Alberto Mattia, a livelyyoung chemical engineer, who made a sound contributionto this experimental work and who sadly passed awayprematurely.

Professor N. Piccinini and G. Rovero are also acknowl-edged for the useful discussions and suggestions.

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Low temperature drying of pomace in spout and spout-fluid beds