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Experimental observation of temperature gradients occurring in a single zeolite pellet
Cht?mnd
Engmanng
ScMce Vd 35 pp 19so Prmted In
@ PupmOrt Press Ltd
2m-24-B Great Bntam
EXPERIMENTAL OBSERVATION OF TEMPERATURE GRADIENTS OCCURRING IN A SINGLE ZEOLITE PELLET Department
J ILAVSKq* and BRUNOVSG of Orgamc Technology, Slovak Techmcal Umvers%y, WI 37 Brabslava, JBnska Street 1, Czechoslovaloa and V HLAVAeEK*
Department of Chemical
Engmeermg,
Insbtute
of Chenucal
Technology,
Suchtitarova Street 1903, Czechoslovakm
166 28 Prague 6.
(Recerued 25 October 1979, accepted 19 Moy 1980) Abstract-An expenmental study has been made of temperature profiles mcldent wlthm a zeohte adsorbent particle m the course of adsorptron of n-heptan Usmg thermocouple wnvs 0 1 mm m dramcter rt has been posstble to measure simultaneously the temperatures of the surface and m the centre It was observed that temperature differences between center of the pellet and amblent stagnant gas may be more than 20°C The results of this work mdlcate that the overheatmg of an adsorbent partnzle IS a very rapld process wluch IS followed by a sbw coolmg of the hot particle Expenmental data are compared with prechchons calculated fi-om the theorettcal model 1 INTRODUCFION
of gases on sohds has been a sublect of mvestlgatlon over the past several decades Id the ma]onty of papers m the field of adsorption the treatment 1s based on the assumption of an isothermal adsorptlon process Smce adsorptlon process are accompanied by heat effects and the adsorption Isotherm may be sensltlve to temperature the tsothermal assumptlon can be cntlcal for certain systems Recently m a theoretical study we have shown that for a number of comhnatlons of adsorbent and adsorbate the mtraparticle and gas-to-sohd temperature gradlents cannot be neglected [l] The mam purpose of the research reported herein was to measure the temperature profiles ansmg during an adsorptlon process Comparison of these results and the mtrapellet and gas-to-sohd temperature differences were used to test the sultablhty of theoretlcal descrrptlon AdsorptIon considerable
3
2 JZXPERIMENTAL
Temperature and adsorbed amount dependences were measured m a Sartonus mlcrobalance vacuum umt The sample of zeohte IS suspended on a long tiun wue m a tube over wluch a furnace may be arranged For nonlsothermal sorption measurements the temperature of the sample can be contmuously recorded The system used expenmentally IS represented schema&ally m Fig 1 Pnor to the expenmental measurements the zeohte particles were preconditIoned at WC for a penod 24 hr under a vacuum 10d torr Measurements were carned out usmg molecular Sieve (d,, = I 052 and 1 094 cm) obtained from VURUP BraWlava The n-heptan used as a sorbate gas was c p grade and was redlsmed before use to ensure lllgh pmty The punty was checked chromatographcally, by refraction Index and ebuhoscoplcally *Author to whom cm-respondence
should he addressed 2475
Fa 1 Measurements of temperature and adsorbed amount dependences 1, suspended parkle used for adsorbed amount measurements. 2, parkle used for temperature measurements. 3. thermocouples. 4, temperature control. 5. electrical furnace The hangdown tube was mmersed m a thermostatically controlled electnc furnace wfuch allowed to keep the temperature ,withm au error -C0 2°C The measurements were camed out at 2 and 15 torr The total volume of the
J ILAVSKY etal
2476
system was large (10 hters) so that the pressure did not change ugmficantly durmg a particular expenment Temperatures were measured with an yon-constantan thermocouple (0 1 mm dia ) Two thermocouples were used to measure the temperature charactenstlcs of the nonisothermal adsorption process One thermocouple was placed m the center of the particle while the second thermocouple was fixed at the particle surface An adsorptlon experiment was carried out as followson an analytical balance a sample IS weighed, and a counter-weight 1s prepared As a counterwe@ an Identical zeohte particle was used After sample and counterweight have been attached to the balance and the balance tubes are in place, the system IS evacuated and heated by an electrical furnace until the sample 1s degassed After the degassmg of the sample the temperature of the furnace IS lowered On approachtng the steady state temperature heptan IS fed m the apparatus The time necessary to adJust the prescribed pressure m the apparatus 1s approx 30 set 3 INTRApeLLET AND GASTo-PELLET T?MPFBATURE DIFFERENCES VALUES OF THE PARA-
The measured
temperature
profiles at the surface
temperature difference was approx 2&25”C and the mtrapellet difference was =S”C A comparison of mtrapellet temperature profiles for two different diameters of the particle are shown m Rg 3 It 1s evident that for a particle wrth higher diameter the center temperature IS higher A slmdar non-isothermal behavior of a zedite particle was also observed for a lower pressure p = torr One of the goals of this paper was to show whether it 1s possible to get an agreement between measured and calculated temperature and adsorbed amount profiles supposing that an a priori model IS used The magmtudes of the physical parameters are presented m Table I The values of the dimensIontess parameters are reported in Table 2 In order to evaluate the parameter o It IS necessary to determme the values of the parameters Le and B1 Since it IS difficult to estimate a pnon the values of the diffusion coefficient and the gas-to-sohd coefficient as well we have estimated the value of w u postenorr based on temperature and adsorbed amount measurements 4 CALCULATION OF PROFILES OF TEMPERATURE AND ADSORBED AMOUNT
and
in the center of a zeoltte partrcle are drawn m Figs 2(a) and 2(b), respectively From these figures may be mfer-
red that the temperature increases suddenly and a temporal hot spot occurs The maximum temperature was already reached after l-2 mm This observation 1s m accordance with the hterature[2] The hot spot temperature IS followed by a slow cooling process In a certain pellet the measurement of temperature profiles was repeated five times The measured temperature profiles m each run are also drawn m Figs 2(a) and 2(b) In this particular experiment the maximum gas-to-pellet
A comparison of the temperature profiles expenmentally evaluated and theoretIcally calculated for parameters from Table 2 IS presented m Fig 4(a) It IS obvious from this figure that the posItion of the hot spot and a part of the curve representmg the coolmg process may be descnbed, however, the calculated hot spot temperature 1s higher Thrs calculation 1s based on the simplified model [3] From the simplified model may be inferredI that two parameters exhibit a substantial effect on the behavior of temperature profiles-namely the product w = 3 Le BE/S and K, The effect of the former parameter IS shown m
95.
90 -
POI 0
Fii
2(a)
10
20
Repeated
measurements 1052 cm) 1-M
30
40
50
60
t [mln]
of temperature tn the center of the partde (Tn = 75”C, p = 15 torr, d,, = measurements, 24th meawrements, 3-5th meawrements
L
70
Expertmental
90
0
Fw
90
L
0 Fg
10
2(b)
Repeated
20
observatton
of temperature
30
measurements of temperature at the particle l-3rd measurements, 2-4th measurements,
40
2477
gradlcnts
50
60
t
[InIll]
90
surface (T0=75“C, p = IS torr, dP = 1,052cm) 3-5th measurements
I
10 3 Comparison
20 of the temperature
L
30
40
50
m the center of the partlcle for two different partlcles (i, =0985cm
60
1.
t [rnlll]
052 cm, 2.
90
2478
J
kAVSKY
ef ai
0
40
t
[mln]
60
Fig 4(b) Comparison of theoretlcal and measured profiles of dlmenstonless adsorbed amount 1, measured adsorbed amount, 2, calculated adsorbed amount for parameters from Table 2
I
0
20
40
t [rnlnl
60
Fig 4(a) Compartson of theoretrcal and measured temperature profiles 1, measured temperature m the center of the partrcle, 2, measured temperature at the particle surface, 3, calculated temperature for parameters from Table 2
Table
20
I Values of physrcal parameters m the system n-heptan-
Fig 5 For a higher value of this product the theofitlcal value of the hot spot temperature approaches the experlmental value The effect of the parameter K~ IS shown m Ftg 6 Here the calculations were performed for Y, = 0 85, 0 88 and 0 91 Evidently the problem ts very sensltlve to the value of the parameter K, For the value K~ =0 85 the agreement between measurements and calculation IS satisfactory
molecular sieve Calsit SA 5 CONCLUSION
Adsorbent partrcle thermal conducttvuy density specific heat
porosity diameter
Adsorpbon Isotherm
In thts paper an attempt has been made to compare the expertmental measurements with theoretical predIctIons Since the experimental observations Indicated that the mtrapellet temperature gradients are not very Important a slmphfied model was selected for theoretlcal cal-
A, = 0 36 W/m’K. p = I@40 kg/m’, C, = %3 J/kg”K,
r=O22, Cr,= 1052mm.
n = c,
1094mm,
boexdbl/Tlp l+boexpf-b,/T)p
[K, Pal
u, = 1450 mollm’, b, = 5000°K. t-AH~)=4157OJ/mol, o$ = 1314 5 mollm’, co = 0 6912 mol/m3
*In thus type of absorptron Isotherm the coefficrent u,, which represents the hmtt value of adsorbed amount, does not depend on the temperature This fact IS not in agreement with expertmentaf observattons, however, for the sake of stmphctty the temperature dependence of a, was not considered The values of parameters were calculated by nonlinear least square technique
Table 2 Estimated values of the dunensronless parameters m=25. 1-000012. o= 1437. P=OI57, u,=09Ottand us=0
1lC
21 1oc
9c
8a
20
40
Rg 5 Effect of the parameter W(K, = 0 91)
t
[ml n]
Go
I, o = 25, 2, o = 35
ExperImental observatron of temperature grahents
2479
value of the parameter o must be estimated a postenon based on expenmentally measured temperature and adsorbed amount dependences It IS apparent that problems associated wtth modelmg of a nonisothermal sorption process are slmllar as those from heterogeneous catalysis Certain parameters can be readily estimated a pnon, while other must be evaluated (I postenon Nevertheless, the model suggested seems to be capable of predlctmg the quahtatlve as well as quantitative behavlour of a sorption process occurmg m a single sorbent particle and based on the u postenon corrected parameters the mode1 may be used for mterpolabon of experimental data NOTATION a
80 -
a8 a&&t
I
0
I
I
20
40
t
[mtn]
60
FX 6 Effect of the parameter K,(O = 25) I, K, = o 85, 2, K, = 088.3, K, =091 culations Based on our former results this slmphficatlon seems to be fully JustIfied[3] A number of governmg parameters m the mode1 considered may be calculated a pnon, e g the parameters a, /? and 6 For slmphclty. we have chosen the LangmuIr Isotherm, however, the nonhnear least square analysis revealed that the constant a, depends on the temperature In our calculations we have used an average value m a gven temperature regon Of course, It IS possible to estimate the temperature dependence of a, from nonlinear least square analysis This modified Langmulr isotherm may be used for descnptlon of heat and mass transfer in a sorbent particle, however, formidable equations would result We have preferred the former alternative It 1s very d&cult to esttmate a pnon a rehable value of the dlffuslon coefficient We feel that a correctlon of the estimated value of the dtffuslon coefficient from the measured profiles of adsorbed amount IS the most relrable way so far The value of the gas-to-sohd heat transfer coefficient can be estimated as a value for a stagnant gas, however, thts value must be multlphed by a certam factor (m our case = 4) because of free convection and radiation effects As a result the
BI co C,
dP ( - A H,,) LX
p T
To
adsorbed amount equlhbnum adsorbed amount parameters m the Langmurr equation Blot number gas phase concentration m the bulk flow specific heat of sorbent diameter of particle heat of adsorption Lewis number partial pressure of adsorbate temperature temperature m the bulk flow
Greek symbols a dlmenslonless /3 8 E K,,Q p AS w
adsorption energy dlmensionless adiabatic temperature nse dlmenstonless parameter (see [l]) porosity of particle dlmenstonless parameters m adsorption NOtherm density of sorbent parttcle thermal conductlvlty of sorbent particle dlmenslonless parameter (see [3])
REFERENCES
[II Brunovska A, HlavhEek V , Ilavsky J and Valtqm J , Chem Engng SCI 1978 33 1385 [21 RudobaSta C P and Planovsklj A N , I Appf Chem 1976 49 1249 (m RussIan) [33 Brunovskfi J and Hlava&k V , Ciwm Engng Scr . . A,. llavsk$ . . . w De pulnlsne4l
Engmanng
ScMce Vd 35 pp 19so Prmted In
@ PupmOrt Press Ltd
2m-24-B Great Bntam
EXPERIMENTAL OBSERVATION OF TEMPERATURE GRADIENTS OCCURRING IN A SINGLE ZEOLITE PELLET Department
J ILAVSKq* and BRUNOVSG of Orgamc Technology, Slovak Techmcal Umvers%y, WI 37 Brabslava, JBnska Street 1, Czechoslovaloa and V HLAVAeEK*
Department of Chemical
Engmeermg,
Insbtute
of Chenucal
Technology,
Suchtitarova Street 1903, Czechoslovakm
166 28 Prague 6.
(Recerued 25 October 1979, accepted 19 Moy 1980) Abstract-An expenmental study has been made of temperature profiles mcldent wlthm a zeohte adsorbent particle m the course of adsorptron of n-heptan Usmg thermocouple wnvs 0 1 mm m dramcter rt has been posstble to measure simultaneously the temperatures of the surface and m the centre It was observed that temperature differences between center of the pellet and amblent stagnant gas may be more than 20°C The results of this work mdlcate that the overheatmg of an adsorbent partnzle IS a very rapld process wluch IS followed by a sbw coolmg of the hot particle Expenmental data are compared with prechchons calculated fi-om the theorettcal model 1 INTRODUCFION
of gases on sohds has been a sublect of mvestlgatlon over the past several decades Id the ma]onty of papers m the field of adsorption the treatment 1s based on the assumption of an isothermal adsorptlon process Smce adsorptlon process are accompanied by heat effects and the adsorption Isotherm may be sensltlve to temperature the tsothermal assumptlon can be cntlcal for certain systems Recently m a theoretical study we have shown that for a number of comhnatlons of adsorbent and adsorbate the mtraparticle and gas-to-sohd temperature gradlents cannot be neglected [l] The mam purpose of the research reported herein was to measure the temperature profiles ansmg during an adsorptlon process Comparison of these results and the mtrapellet and gas-to-sohd temperature differences were used to test the sultablhty of theoretlcal descrrptlon AdsorptIon considerable
3
2 JZXPERIMENTAL
Temperature and adsorbed amount dependences were measured m a Sartonus mlcrobalance vacuum umt The sample of zeohte IS suspended on a long tiun wue m a tube over wluch a furnace may be arranged For nonlsothermal sorption measurements the temperature of the sample can be contmuously recorded The system used expenmentally IS represented schema&ally m Fig 1 Pnor to the expenmental measurements the zeohte particles were preconditIoned at WC for a penod 24 hr under a vacuum 10d torr Measurements were carned out usmg molecular Sieve (d,, = I 052 and 1 094 cm) obtained from VURUP BraWlava The n-heptan used as a sorbate gas was c p grade and was redlsmed before use to ensure lllgh pmty The punty was checked chromatographcally, by refraction Index and ebuhoscoplcally *Author to whom cm-respondence
should he addressed 2475
Fa 1 Measurements of temperature and adsorbed amount dependences 1, suspended parkle used for adsorbed amount measurements. 2, parkle used for temperature measurements. 3. thermocouples. 4, temperature control. 5. electrical furnace The hangdown tube was mmersed m a thermostatically controlled electnc furnace wfuch allowed to keep the temperature ,withm au error -C0 2°C The measurements were camed out at 2 and 15 torr The total volume of the
J ILAVSKY etal
2476
system was large (10 hters) so that the pressure did not change ugmficantly durmg a particular expenment Temperatures were measured with an yon-constantan thermocouple (0 1 mm dia ) Two thermocouples were used to measure the temperature charactenstlcs of the nonisothermal adsorption process One thermocouple was placed m the center of the particle while the second thermocouple was fixed at the particle surface An adsorptlon experiment was carried out as followson an analytical balance a sample IS weighed, and a counter-weight 1s prepared As a counterwe@ an Identical zeohte particle was used After sample and counterweight have been attached to the balance and the balance tubes are in place, the system IS evacuated and heated by an electrical furnace until the sample 1s degassed After the degassmg of the sample the temperature of the furnace IS lowered On approachtng the steady state temperature heptan IS fed m the apparatus The time necessary to adJust the prescribed pressure m the apparatus 1s approx 30 set 3 INTRApeLLET AND GASTo-PELLET T?MPFBATURE DIFFERENCES VALUES OF THE PARA-
The measured
temperature
profiles at the surface
temperature difference was approx 2&25”C and the mtrapellet difference was =S”C A comparison of mtrapellet temperature profiles for two different diameters of the particle are shown m Rg 3 It 1s evident that for a particle wrth higher diameter the center temperature IS higher A slmdar non-isothermal behavior of a zedite particle was also observed for a lower pressure p = torr One of the goals of this paper was to show whether it 1s possible to get an agreement between measured and calculated temperature and adsorbed amount profiles supposing that an a priori model IS used The magmtudes of the physical parameters are presented m Table I The values of the dimensIontess parameters are reported in Table 2 In order to evaluate the parameter o It IS necessary to determme the values of the parameters Le and B1 Since it IS difficult to estimate a pnon the values of the diffusion coefficient and the gas-to-sohd coefficient as well we have estimated the value of w u postenorr based on temperature and adsorbed amount measurements 4 CALCULATION OF PROFILES OF TEMPERATURE AND ADSORBED AMOUNT
and
in the center of a zeoltte partrcle are drawn m Figs 2(a) and 2(b), respectively From these figures may be mfer-
red that the temperature increases suddenly and a temporal hot spot occurs The maximum temperature was already reached after l-2 mm This observation 1s m accordance with the hterature[2] The hot spot temperature IS followed by a slow cooling process In a certain pellet the measurement of temperature profiles was repeated five times The measured temperature profiles m each run are also drawn m Figs 2(a) and 2(b) In this particular experiment the maximum gas-to-pellet
A comparison of the temperature profiles expenmentally evaluated and theoretIcally calculated for parameters from Table 2 IS presented m Fig 4(a) It IS obvious from this figure that the posItion of the hot spot and a part of the curve representmg the coolmg process may be descnbed, however, the calculated hot spot temperature 1s higher Thrs calculation 1s based on the simplified model [3] From the simplified model may be inferredI that two parameters exhibit a substantial effect on the behavior of temperature profiles-namely the product w = 3 Le BE/S and K, The effect of the former parameter IS shown m
95.
90 -
POI 0
Fii
2(a)
10
20
Repeated
measurements 1052 cm) 1-M
30
40
50
60
t [mln]
of temperature tn the center of the partde (Tn = 75”C, p = 15 torr, d,, = measurements, 24th meawrements, 3-5th meawrements
L
70
Expertmental
90
0
Fw
90
L
0 Fg
10
2(b)
Repeated
20
observatton
of temperature
30
measurements of temperature at the particle l-3rd measurements, 2-4th measurements,
40
2477
gradlcnts
50
60
t
[InIll]
90
surface (T0=75“C, p = IS torr, dP = 1,052cm) 3-5th measurements
I
10 3 Comparison
20 of the temperature
L
30
40
50
m the center of the partlcle for two different partlcles (i, =0985cm
60
1.
t [rnlll]
052 cm, 2.
90
2478
J
kAVSKY
ef ai
0
40
t
[mln]
60
Fig 4(b) Comparison of theoretlcal and measured profiles of dlmenstonless adsorbed amount 1, measured adsorbed amount, 2, calculated adsorbed amount for parameters from Table 2
I
0
20
40
t [rnlnl
60
Fig 4(a) Compartson of theoretrcal and measured temperature profiles 1, measured temperature m the center of the partrcle, 2, measured temperature at the particle surface, 3, calculated temperature for parameters from Table 2
Table
20
I Values of physrcal parameters m the system n-heptan-
Fig 5 For a higher value of this product the theofitlcal value of the hot spot temperature approaches the experlmental value The effect of the parameter K~ IS shown m Ftg 6 Here the calculations were performed for Y, = 0 85, 0 88 and 0 91 Evidently the problem ts very sensltlve to the value of the parameter K, For the value K~ =0 85 the agreement between measurements and calculation IS satisfactory
molecular sieve Calsit SA 5 CONCLUSION
Adsorbent partrcle thermal conducttvuy density specific heat
porosity diameter
Adsorpbon Isotherm
In thts paper an attempt has been made to compare the expertmental measurements with theoretical predIctIons Since the experimental observations Indicated that the mtrapellet temperature gradients are not very Important a slmphfied model was selected for theoretlcal cal-
A, = 0 36 W/m’K. p = I@40 kg/m’, C, = %3 J/kg”K,
r=O22, Cr,= 1052mm.
n = c,
1094mm,
boexdbl/Tlp l+boexpf-b,/T)p
[K, Pal
u, = 1450 mollm’, b, = 5000°K. t-AH~)=4157OJ/mol, o$ = 1314 5 mollm’, co = 0 6912 mol/m3
*In thus type of absorptron Isotherm the coefficrent u,, which represents the hmtt value of adsorbed amount, does not depend on the temperature This fact IS not in agreement with expertmentaf observattons, however, for the sake of stmphctty the temperature dependence of a, was not considered The values of parameters were calculated by nonlinear least square technique
Table 2 Estimated values of the dunensronless parameters m=25. 1-000012. o= 1437. P=OI57, u,=09Ottand us=0
1lC
21 1oc
9c
8a
20
40
Rg 5 Effect of the parameter W(K, = 0 91)
t
[ml n]
Go
I, o = 25, 2, o = 35
ExperImental observatron of temperature grahents
2479
value of the parameter o must be estimated a postenon based on expenmentally measured temperature and adsorbed amount dependences It IS apparent that problems associated wtth modelmg of a nonisothermal sorption process are slmllar as those from heterogeneous catalysis Certain parameters can be readily estimated a pnon, while other must be evaluated (I postenon Nevertheless, the model suggested seems to be capable of predlctmg the quahtatlve as well as quantitative behavlour of a sorption process occurmg m a single sorbent particle and based on the u postenon corrected parameters the mode1 may be used for mterpolabon of experimental data NOTATION a
80 -
a8 a&&t
I
0
I
I
20
40
t
[mtn]
60
FX 6 Effect of the parameter K,(O = 25) I, K, = o 85, 2, K, = 088.3, K, =091 culations Based on our former results this slmphficatlon seems to be fully JustIfied[3] A number of governmg parameters m the mode1 considered may be calculated a pnon, e g the parameters a, /? and 6 For slmphclty. we have chosen the LangmuIr Isotherm, however, the nonhnear least square analysis revealed that the constant a, depends on the temperature In our calculations we have used an average value m a gven temperature regon Of course, It IS possible to estimate the temperature dependence of a, from nonlinear least square analysis This modified Langmulr isotherm may be used for descnptlon of heat and mass transfer in a sorbent particle, however, formidable equations would result We have preferred the former alternative It 1s very d&cult to esttmate a pnon a rehable value of the dlffuslon coefficient We feel that a correctlon of the estimated value of the dtffuslon coefficient from the measured profiles of adsorbed amount IS the most relrable way so far The value of the gas-to-sohd heat transfer coefficient can be estimated as a value for a stagnant gas, however, thts value must be multlphed by a certam factor (m our case = 4) because of free convection and radiation effects As a result the
BI co C,
dP ( - A H,,) LX
p T
To
adsorbed amount equlhbnum adsorbed amount parameters m the Langmurr equation Blot number gas phase concentration m the bulk flow specific heat of sorbent diameter of particle heat of adsorption Lewis number partial pressure of adsorbate temperature temperature m the bulk flow
Greek symbols a dlmenslonless /3 8 E K,,Q p AS w
adsorption energy dlmensionless adiabatic temperature nse dlmenstonless parameter (see [l]) porosity of particle dlmenstonless parameters m adsorption NOtherm density of sorbent parttcle thermal conductlvlty of sorbent particle dlmenslonless parameter (see [3])
REFERENCES
[II Brunovska A, HlavhEek V , Ilavsky J and Valtqm J , Chem Engng SCI 1978 33 1385 [21 RudobaSta C P and Planovsklj A N , I Appf Chem 1976 49 1249 (m RussIan) [33 Brunovskfi J and Hlava&k V , Ciwm Engng Scr . . A,. llavsk$ . . . w De pulnlsne4l