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Thermal conductivity measurements in charcoal beds near room temperature
Powder Technology. 19 (1978) 289 - 291 0 Elsevier Sequoia S-4., Lausanne - Printed in the Netherlands
Short
239
Communication
Thermal Charcoal
Conductivity Measurements in Beds near Room Temperature
P_ N_ J. HENRION C.E.N./S.C_K.. (Received
and W_ CLAES
Boeretang
September
200. B-2400
JIoI (Belgium)
23.1977)
Considering the affinity of krypton for charcoal, it is conceivable to keep fission krypton on a granular bed at only a fraction of the pressure present in the container in the absence of this adsorbent. If the pressure were to drop, then krypton containment would be safer and some simplification in the concept of an engineered storage may eventually follow_ Ultimately, the pressure observed in a storage cylinder depends on the state of thermal equilibrium reached with the surroundings, and this is partly determined by the thermal conductivity of the charcoal bed. Reliable values of this property are essential to any evaluation of the procedure. There are many charcoal varieties, and experimental data could not be found in the literature. Besides, theoretical developments for the thermal conductivity of granular materials are either very inaccurate or difficult to apply [l, 2]_ Therefore the conductivity was experimentally determined under various conditions and, in view of the ever growing interest in charcoal beds, we thought these results might be valuable to some readers_
Xxperimental The apparatus consists of a copper cylinder (95 mm i-d_) water-jacketed at 25 “C unless otherwise stated and provided with a central heating element (216 mm) fitted in a copper tube [14 mm o-d_). The heat source provides a homogeneous temperature profile along the axis at about 100 “C above wall temperature_ Charcoal is homogeneously packed in the annular cavity and is insulated above and below by t&on and cork layers 6 cm thick.
Sis chromel-alumel thermocouples are radially located at various depths and levels in the central third of the bed. Only the heat generated in this region is taken into consideration in the calculation, the lower and upper parts of the bed merely guarding the centre from longitudinal heat losses_ The thermocouples are made of wires 50 pm in diameter sheathed in twin-bored alumina rods (1.5 mm o-d.). The distortion introduced in the medium by such thermocouples was neglected. They are, of course, very fragile and call for great care ivhile the process of compaction is under way. However, multiple temperature readings could be achieved in most runs and the linear dependence of In rO/r on the radial temperature was verified repeatedly (r is the radius at the couple junction, r, = 47.5 mm)_ All charcoal samples (about 1 kg) were oven dried for some days at 150 “C and, except for a few measurements in ambient krypton, all measurements took place under dry nitrogen at 1 atmosphere. A small gas flow was maintained during heating to allow both an adsorption and a thermal equilibrium to be established. The flow was progressi-Jely reduced to vanishing values when readings were taken. Granules of material of the selected size range were fed into the column in successive increments under moderate vibration (supplied by a standard laboratory sieving vibrator)_ &Iany commrrcial products are available in coarse granules, several millimetres thick, with a limited size range. Higher bed densities were obtained by a procedure inspired by the work of McGeary [S] . Once formed, the coarse particles bed is “immobilized” by a weight shaped like a perforated cylinder through which a selected size fraction of milled charcoal is fed under gentle vibration into the voids of the coarse structure_ According to McGeary, the average diameter of this fine fraction should be at least 7 times smaller. However, a lower limit is quickly set by the marked tendency of fine charcoal towards agglomeration and clogging of the bed.
290
Packing (g 1-l)
Thermal cond(watt cm -’ deg-’
424
30 32-6
(b) type I + type I milted, 75< <180 m
520 548
41-5 43
(c) II as received
549
26.5 26.6
(d) II <1_4 mm fraction
535
26-4
(e) II >1.7
545
30 28.3 29.1 28.6
Bed description
(a) Charcoal
(f)
type I
mm fraction
same as (e); outer wall at 40 “C
545
29-5
621
35.8
(h) ii > l-7 mm; under krypton
545
19.2 20.8
(i)same as (h); outer wall at 0 “c
545
20-9
(j)
545
20s
267
16.3
(g) II >1_7 nun f type I75<
Cl60
JI.m
same as (h); o-w_ at 40 “C
(k) type III
Results
x 104)
and discussion
Theresults hzvebeencolkctedinTablel_ Three commercial types of charcoal were examin~,andtheirspecificationsareas follows:typeI,RBL4granules,NoritN.V., Amersfoort, The Netherlands; type II, PCNSX16,Chemviron,Pittsburgh,US-A~ typeIlI,Elorit,coarsepowder,NoritN-V., Amersfoort, These materials have been cha?z&erkedinthislaboratory,and data suchasnitrogenadsorptionisothermsand mercury penetration curves are available_ The three types of charcoals examined vary considerab~yintheirporosi~distribution.Accordingly,apparentparticledensities (determined by mercury densimetry) are O-67,0.97andO_58g/cm3 fortypeI,IIand IIIrespectiveIy_Ithaslongbeenrecoguized thatthethermal conductivityoffiagmented non-poroussolidsdependsprimarilyonthe volume fraction occupied by thesolids Cl]_ Particlesizewasshowntobeofksser importance 123,Thepoorconductivity observed infragmentedgoodconductorsiscon~~~ bytheheattransferintheinterkialareas betweens~lidpartZcks_Wemaytherefore .supposethattherelatZv~ysmaIlvariaGonsin tEe(large)conductivityofindividu~particles
Fig_ l_ Thermal conductivity of charcoal behasa function of the volume fxaction of the particlea (regarded as non-porous). Letters near experimental pointsrefertodabzinTable1. linkedwithorigin
andpreparationwouldbe of lesser imporkmce for the thermal conductivity ofabed, Accordingly, calculation ofcoalvolumefractionsinthevariousbeds wasbasedontheapparent~articledemStiesr.notherwords.inthiscaIculationpartick5 were~~asnon-porous.~viewIsput totbetesbinF'ig_l.Afirdapproximatio~for
291 TABLE
2
ambient
Red desaiption
Thermal _“d_ (watt cm deg.-’ x 104)
Cbamoaltype1+intecstitialAI powder, ESaco cl50 m TypeI+gmphite lOO< <180 m. unknown
47
Origill
Type I + graphite EG 3851 lf2<
50.7
gas [2],and the reduction in conductivity observed with krypton makes it ckuthatgaseousambience isessentialtothe definitionofconductivity in granularbeds_ Table2reportsonthefailuretoimprove thecharcoalbed conductivitysubstanWy byintergranularfillingwithverygoodconductorsinafragmentedstate.
<5OOpm
47 REFERENCES
the conductivity ofany charcoal bed may probablybeobtained~omthiscurveunder ambient nitrogen or air if the apparentpartitledensity has already been determined. Thetransferofheatatsolidinterfacelevel isgreatly influenced bythenatureofthe
1 T. E. W. Schumann and V_ Voss, Fuel, 13 (8) (1934) 249. 2 A. V. Luikov, A. G. Shashkov and L. L. Vasiliev, tit_ J_ Heat Mass Transfer. 11(1968) 117_ I Am_ Ce-_ &xc_. 44 (10) 3 R. K_ McGc.ay. (1961) 513_
Short
239
Communication
Thermal Charcoal
Conductivity Measurements in Beds near Room Temperature
P_ N_ J. HENRION C.E.N./S.C_K.. (Received
and W_ CLAES
Boeretang
September
200. B-2400
JIoI (Belgium)
23.1977)
Considering the affinity of krypton for charcoal, it is conceivable to keep fission krypton on a granular bed at only a fraction of the pressure present in the container in the absence of this adsorbent. If the pressure were to drop, then krypton containment would be safer and some simplification in the concept of an engineered storage may eventually follow_ Ultimately, the pressure observed in a storage cylinder depends on the state of thermal equilibrium reached with the surroundings, and this is partly determined by the thermal conductivity of the charcoal bed. Reliable values of this property are essential to any evaluation of the procedure. There are many charcoal varieties, and experimental data could not be found in the literature. Besides, theoretical developments for the thermal conductivity of granular materials are either very inaccurate or difficult to apply [l, 2]_ Therefore the conductivity was experimentally determined under various conditions and, in view of the ever growing interest in charcoal beds, we thought these results might be valuable to some readers_
Xxperimental The apparatus consists of a copper cylinder (95 mm i-d_) water-jacketed at 25 “C unless otherwise stated and provided with a central heating element (216 mm) fitted in a copper tube [14 mm o-d_). The heat source provides a homogeneous temperature profile along the axis at about 100 “C above wall temperature_ Charcoal is homogeneously packed in the annular cavity and is insulated above and below by t&on and cork layers 6 cm thick.
Sis chromel-alumel thermocouples are radially located at various depths and levels in the central third of the bed. Only the heat generated in this region is taken into consideration in the calculation, the lower and upper parts of the bed merely guarding the centre from longitudinal heat losses_ The thermocouples are made of wires 50 pm in diameter sheathed in twin-bored alumina rods (1.5 mm o-d.). The distortion introduced in the medium by such thermocouples was neglected. They are, of course, very fragile and call for great care ivhile the process of compaction is under way. However, multiple temperature readings could be achieved in most runs and the linear dependence of In rO/r on the radial temperature was verified repeatedly (r is the radius at the couple junction, r, = 47.5 mm)_ All charcoal samples (about 1 kg) were oven dried for some days at 150 “C and, except for a few measurements in ambient krypton, all measurements took place under dry nitrogen at 1 atmosphere. A small gas flow was maintained during heating to allow both an adsorption and a thermal equilibrium to be established. The flow was progressi-Jely reduced to vanishing values when readings were taken. Granules of material of the selected size range were fed into the column in successive increments under moderate vibration (supplied by a standard laboratory sieving vibrator)_ &Iany commrrcial products are available in coarse granules, several millimetres thick, with a limited size range. Higher bed densities were obtained by a procedure inspired by the work of McGeary [S] . Once formed, the coarse particles bed is “immobilized” by a weight shaped like a perforated cylinder through which a selected size fraction of milled charcoal is fed under gentle vibration into the voids of the coarse structure_ According to McGeary, the average diameter of this fine fraction should be at least 7 times smaller. However, a lower limit is quickly set by the marked tendency of fine charcoal towards agglomeration and clogging of the bed.
290
Packing (g 1-l)
Thermal cond(watt cm -’ deg-’
424
30 32-6
(b) type I + type I milted, 75< <180 m
520 548
41-5 43
(c) II as received
549
26.5 26.6
(d) II <1_4 mm fraction
535
26-4
(e) II >1.7
545
30 28.3 29.1 28.6
Bed description
(a) Charcoal
(f)
type I
mm fraction
same as (e); outer wall at 40 “C
545
29-5
621
35.8
(h) ii > l-7 mm; under krypton
545
19.2 20.8
(i)same as (h); outer wall at 0 “c
545
20-9
(j)
545
20s
267
16.3
(g) II >1_7 nun f type I75<
Cl60
JI.m
same as (h); o-w_ at 40 “C
(k) type III
Results
x 104)
and discussion
Theresults hzvebeencolkctedinTablel_ Three commercial types of charcoal were examin~,andtheirspecificationsareas follows:typeI,RBL4granules,NoritN.V., Amersfoort, The Netherlands; type II, PCNSX16,Chemviron,Pittsburgh,US-A~ typeIlI,Elorit,coarsepowder,NoritN-V., Amersfoort, These materials have been cha?z&erkedinthislaboratory,and data suchasnitrogenadsorptionisothermsand mercury penetration curves are available_ The three types of charcoals examined vary considerab~yintheirporosi~distribution.Accordingly,apparentparticledensities (determined by mercury densimetry) are O-67,0.97andO_58g/cm3 fortypeI,IIand IIIrespectiveIy_Ithaslongbeenrecoguized thatthethermal conductivityoffiagmented non-poroussolidsdependsprimarilyonthe volume fraction occupied by thesolids Cl]_ Particlesizewasshowntobeofksser importance 123,Thepoorconductivity observed infragmentedgoodconductorsiscon~~~ bytheheattransferintheinterkialareas betweens~lidpartZcks_Wemaytherefore .supposethattherelatZv~ysmaIlvariaGonsin tEe(large)conductivityofindividu~particles
Fig_ l_ Thermal conductivity of charcoal behasa function of the volume fxaction of the particlea (regarded as non-porous). Letters near experimental pointsrefertodabzinTable1. linkedwithorigin
andpreparationwouldbe of lesser imporkmce for the thermal conductivity ofabed, Accordingly, calculation ofcoalvolumefractionsinthevariousbeds wasbasedontheapparent~articledemStiesr.notherwords.inthiscaIculationpartick5 were~~asnon-porous.~viewIsput totbetesbinF'ig_l.Afirdapproximatio~for
291 TABLE
2
ambient
Red desaiption
Thermal _“d_ (watt cm deg.-’ x 104)
Cbamoaltype1+intecstitialAI powder, ESaco cl50 m TypeI+gmphite lOO< <180 m. unknown
47
Origill
Type I + graphite EG 3851 lf2<
50.7
gas [2],and the reduction in conductivity observed with krypton makes it ckuthatgaseousambience isessentialtothe definitionofconductivity in granularbeds_ Table2reportsonthefailuretoimprove thecharcoalbed conductivitysubstanWy byintergranularfillingwithverygoodconductorsinafragmentedstate.
<5OOpm
47 REFERENCES
the conductivity ofany charcoal bed may probablybeobtained~omthiscurveunder ambient nitrogen or air if the apparentpartitledensity has already been determined. Thetransferofheatatsolidinterfacelevel isgreatly influenced bythenatureofthe
1 T. E. W. Schumann and V_ Voss, Fuel, 13 (8) (1934) 249. 2 A. V. Luikov, A. G. Shashkov and L. L. Vasiliev, tit_ J_ Heat Mass Transfer. 11(1968) 117_ I Am_ Ce-_ &xc_. 44 (10) 3 R. K_ McGc.ay. (1961) 513_