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Adsorption phenomena in porous media in presence of moist air
0735-1933/91 $3.00 + .00 Printed in the United States
INT. COMM. HEAT MASS TRANSFER Vol. 18, pp. 71-81, 1991 ©Pergamon Press plc
A D S O R P T I O N P H E N O M E N A I N P O R O U S M E D I A I N PRESENCE OF M O I S T AIR
A.Carotenuto Dipartimento di Ingegneria Industriale Universit-~t degli Studi di Cassino Via Zamosh n*2, 03043 Cassino Italia F. Fucci and G. La Fianza D.E.T.E.C. Universit~ degli Studi Federico II Napoli P.zzale Tecchio n ° 1, 80123 Napoli, Italia (Communicated by J.
OOS,~)
ABSTRACT In the present study the authors show the experimental results for some building
materials as concrete, gypsum, brick, neapolitan yellow tuff, prestressed pine wood. These results are connected, during isothermal conditions, with the adsorbed water content and with a relative humidity variation of the medium air. The authors, consideringprevious results showed,suggestan empiricalcorrelationdescribing,for some materials, the percentual weight variation of the adsorbed water higher than 1.0 kg/kg, for a 0.40-0.90 relative humidity range. Introduction
The knowledge of heat and mass transfer phenomena in walls is very important to the building degradation prevention. These mechanisms are extremely complicated, due to the nature and geometry of the porous material, to the contemporary presence of different kinds of water adsorption and transfer in the porous medium and to the heat, mass and energy fluxes interactions; all these reasons makes the analytical model definition, for a wall thermoigrometric behaviour, very difficult. In the study, a series of experimental results for some building materials are presented. These results show the adsorbed water under equilibrium conditions with the igrometric degree and air temperature variation. An empirical equation which connects these experimental results with those presented by other researchers, is presented.after showing the basic equation of thermofluidodynamic phenomena in 71
72
A. Carotenuto, F. Fucci and O. La Fianza
Vol. 18, No. 1
porous media. These results allow us to determine the adsorbed water with the variation of hygrometric degree and air temperature, for some building materials. Adsorotion vrocess
Liquid adsorption on a solid surface depends on the solid surface capacity (sorbent) to retain a very thin liquid film (sorbate). There are two superficial adsorption mechanisms: chemical and physical adsorption. Physical adsorption shows an higher heat adsorption and a lower pressure and temperature compared to a chemical kind of adsorption. Generally, the sorbate fills the superficial adsorption zone with a monomolecular layer (monolayer adsorption) or with a several overlaid layer (multilayer adsorption). Physical adsorption can be multilayer or monolayer; chemical adsorption can be only monolayer. Working forces, in the physical adsorption mechanism, are the van der Waals forces. These forces are always present whereas the electrostatic ones are active only in case of unstable controions sorbents, as the zeolites. Macroscopically, physical adsorption can be explained by a simple experiment which doesn't take in account the capillarity phenomenon and the salt occurrence [1]. A container of fixed volume V, filled by n gas molecules, is at T temperature; neglecting the solid wall and the gaseus molecules interaction, the p' pressure is defined as: p' = z R T v
(~)
where z is the compressibility factor. The real value of p pressure, considering the previous interactions is defined as [1]: A6
P' = P - ~
(P2 -- Pl) Z R T
(2)
where 5 is the radius of action of the superficial forces, A is the total porous surface, p~ and P2 are respectively the molecule density influenced or not by the surface tension, V is the volume, T the temperature and R is the gas constant. In most cases the AS/V fraction is a small quantity ( 8 is equal to few angstroms and V / A few centimeters ). Anyway, considering this difference, it's clear that the higher the superficial solid area is, compared to the volume, the lower the reduced pressure value will be. On the other hand, the superficial area increase is a
Vol. 18, No. 1
ADSORPTION PHENOMENA IN POROUS MEDIA
73
solid porosity function and therefore, for a porous m e d i u m , a significant pressure decrease, compared to that obtained by the real gases law, can be found. The most widespread adsorption theory is based on the cinetic gas theory. The Langmuir model [2] is the most diffused among the calculation models for isotherm phenomena. It points out the a m o u n t of retained sorbate for a monolayer case. The model assumption are: - the adsorption areas on the sorbent surface are exactly positioned; - all the areas are energetically equivalent; - there are no influences among the sorbent molecules and neighboring sites. with this hypotesis we have: q
=
bp
(3)
qmax l + b p and 1 --
1 =
q
1 +
-
1
-
(4)
qr~x bqmax P
where q/qmax is the adsorbed a m o u n t and the m a x i m u m adsorbable a m o u n t (under saturation condition) rate; b is the adsorption equilibrium constant with temperature and sorbent-sorbate characteristics. E q u a t i o n (3) allows the calculation of qmax and b constants at different temperatures by the experimental data interpolation on a (1/q),(1/p) diagram. A s t u d y for a multilayer, isotherm adsorption model shows some problems due to the sorbate-sorbate interaction. The Brunauer, Emmett and Teller (B.E.T.) equation is based on the following assumptions [3]: - the solid surface is considered as an homogeneus, bidimensional matrix; - a gas molecule is stably adsorbed w h e n it is in contact with an adsorption area; - on the contrary, if a gas molecule contacts a filled area, it will be adsorbed, but a second layer will be formed; - a limit of n strata of adsorbable molecules will mix to the gaseus phase splitting in one layer at time; - all the layers on the first one have the same physical properties. With these assumptions:
q qm
b(p/ps) ( 1 - p / p s ) [ 1 - ( p / p s ) + (bp/p,)]
(5)
74
A. Carotenuto, F. Fucei and G. La Fianza
Vol. 18, No. 1
and P/Ps
1
b- 1 p
q [ 1 - ( P / P s ) ] = bqm + bqm Ps
(6)
where Ps is the vapour saturation pressure and qm is the adsorbed amount for a single layer. Equation (4) allows the evaluation of qm and b constants at different temperatures by the experimental data extrapolation on a (p/Ps)/[q (1-p/ps)],(p/ps) diagram. B.E.T. equation takes in account the experimental data for a 0.05 < p/Ps < 0.35 range. For a porous matrix we have to consider the porous dimensions. Porous media are classified, considering their dimensions, in the following classes: - micropores with equivalent radius below 20 •; - mesopores with equivalent radius from 20A to 50 ,/~; - macropores with equivalent radius above 500 ./k. For macropores it can be assumed that the physical water adsorption theory is still good, if we assume the porous radius and the adsorbed liquid thickness rate as infinite. For mesopores we are in the presence of capillarity, on the contrary for micropores a different theory for the explanation of adsorption processes must be considered [1]. Adsorvtion t~henomena in o o r o u s media in oresence of moat air
In theory, adsorption mechanisms in porous media in presence of moist air should be considered by adsorption models of pure gaseus mixtures, The nitrogen and oxygen adsorption normally acts at very low temperature and very high partial pressure compared to the thermodynamic conditions of water vapor adsorption [4]. Therefore, the previous mono and multilayer adsorption models, for a single component, can be used. The authors studied, experimentally, the adsorption phenomenon for some building materials (cement, neapolitan yellow tuff, brick and prestressed pine wood) at 0.40 - 0.90 relative humidity range. Specimen conditioning Three millimeter thick cylindrical specimens have been obtained by a series of
different core samples (7 cm of diameter). The core samples have been dryed out by air on H2SO4 for five days in a 50°C oven and then weighed at the end of the test [6].
Vol. 18, No. 1
A D S O R P T I OPHENOMENA N IN POROUS MEDIA
75
Experimentalapparatus An experimental system used to test the specimen weight variations at a constant temperature and a constant relative h u m i d i t y conditions to the neighboring, has been set up and now schematically demonstrated. The system consists of two independent circuits; the first one is crossed by a thermostatic liquid at a temperature value variyng from 5 to 80°C and the second one is crossed by moist air with fixed thermohygrometric characteristics. The system consists of a cylindrical container of a fixed dimension ( h = 24,5 cm and 0 = 18 cm ), surrounded by a double suitable insulated wall, which contains the specimen hanged to an electrical precision balance. The air space is filled by thermostatic liquid, circulating by a water pump suction, whose temperature varies from 5 to 80°C. In the thermostatic tank, the fixed liquid temperature is obtained by a refrigerator set and by an electrical heater. In the 10 liters of capacity thermostatic tank, a long copper coil, where air passes before lapping the H2SO4 solution, is plunged. The air temperature is checked by the thermostat and is equal to the container temperature, whereas the air humidity is checked by a titration. The air, conditioned according to the experimental model, comes in the container, where it laps the specimen due to a water pump sunction. In the test chamber, a tank has been set. This tank is filled with liquid water by an intake-valve pipe which passes through a kettle closeover flange and the air lapping the specimen be saturated. In that case, the connection of the test chamber with the air circuit is broken off by the two-valve shutdown and the inner air constantly flows, after a fine fractioning by a pumice, due to a contact with water. The electronic balance has been placed into a suitable glass container where the air and the container temperature are the same and at 0.45 relative humidity value. Inside the glass container a little hole has been made to allow the hanging specimen apparatus to pass through. Data recorder device is made by: - an HMD 20Y probe, placed near the specimen, records and transmits the temperature and the air humidity data; in particular the temperature measure shows an uncertainty of + 0.2% whereas the humidity air measure shows an uncertainty of + 2.2 %; - a electric balance with a 4.3 mV/V output, a weight from to 25 g, measures the specimen weight variations. The sensitivity is 100 V/0,1 g then expanded in a 0-100 endscale final signal; four T-type thermocouples placed in a significant point of the system to check its normal running. Data furnished by the experimental apparatus are recorded by a multichannel microvoltmeter Fluke under fixed time. The experimental apparatus is schematically showed in fig.1.
76
A. Carotenuto, F. Fucei and G. La Fianza
Vol. 18, No. 1
Specimens weight variations have been obtained at 20 °C. Values of the moist air temperature and of the moist air relative humidity in the container have been changed (40 - 90%) after obtaining, for the specimens, a fixed thermodynamic equilibrium condition. The equilibrium condition for a dry specimen at the test initial condition was reached in 100 h after having fixed the moist air thermodynamic conditions. Measures have been carried out at an increasing relative humidity, with fixed air temperature, to speed up the equilibrium condition achievement. Therefore, the specimen weight variation test started using the equilibrium conditions of the previous, lower and relative humidity test and waiting to obtain the test stationary conditions. The specimen water weight variation W has been obtained considering the weight specimen 7 measurements at different thermodynamic conditions. We can assume W, variyng (~ and with a fixed 70 (weight of the dry specimen) as: W = ~(7-7°)-100
(kgg)
(7)
Results
Direct comparison with the previous known results for the natural inert concrete, neapolitan yellow tuff and brick hasn't been possible. In fact for these materials there seems to be no data on their hygroscopic behaviour. On the contrary, results obtained for gypsum agree with the results reported in [7]; likewise for the prestressed pine wood dates. Thermohygrometric behaviour of the material examined in laboratory has been compared with the other building materials studied by Luikov [5]. Building materials had been selected under three different classes to clearly present the final results. These classes show the weight water variation of the building materials as function of the moist air thermohygrometric conditions. In particular: - in the first class are included building materials which, during isothermal process, show a W variation less than 1,0 kg/kg. They are: brick, glass wool, artik tuff, limestone and earthenware block; - in the second class are included building materials which, during isothermal process show W variations in the 1.0 - 8,0 k g / k g range. They are: natural inert cement, tripoli brick, slag concrete, sand and cement mortar, foam concrete; - in the third class are included building materials which, during isothermal conditions, show W variations higher than 8.0 kg/kg. They are: wood, prestressed pine wood, gypsum, cork, wood felt and neapolitan yellow tuff.
Vol. 18, No. 1
A D S O R P T I OPHENOMENA N IN POROUS MEDIA
'
-
77
t
FIG. 1 Experimental apparatus: (E. B electric balance, T. termostatic tank, H. HMD probe, W.P. water pump, C H2SO4 tank, S specimen) In fig. 2 the the W variations (for the materials of the first class) under isotherm conditions as (~ function, are presented. These curves are included in the second and third class of physical adsorption mechanisms made by Braunauer et al.[4]. These adsorption curves are generally considered for materials with an high range of porous dimensions. In these materials increasing the hygrometric degree, a change from a monolayer adsorption to a multilayer adsorption till a capillary condensation is observed. By the examined data we have: 1
m=a(~+d W
(8)
which relates, with good approximation, the amount of adsorbed water with the hygrometric degree variation. This is for isothermal processes and for ~ value higher than 0,4 as showed in 3a, 3b and 3c diagram. In tab. I coefficient a and b for different building materials and correlation coefficient of data are showed.
78
A. Carotenuto, F. Fucci and G. La Fianza
1,5
A earthenwareblock • limestone o brick glass wool Q artik t u f f
W
Vol. 18, No. I
/ /
~
1,0
O~
0,0014
0,6
0,8
1,0
FIG. 2 The W variations as ¢# function for building materials ( W < 1.0 k g / k g )
W
• glasswool • limestone a earthenwareblock
T~
1
0, 0,4
"%"-..
° ~
0,6
0,8
1,0
FIG. 3a The 1 / W variations as (~ function for building materials ( W < 1.0 k g / k g )
Vol. 18, No. 1
ADSORPTION PHENOMENA IN POROUS MEDIA
2~
• natural inert cement • cement mortar • foam concrete
1
W 1,6
• tripoli bconcriCrkte
1,2 0~ 0,4'
0,4
~6
~8
¢
1~
FIG. 3b The 1/W variations as ¢ function for building materials ( 1.0 < W < 8.0 kg/kg )
1
] '~
a cork O gypsum
~
• neapolitan yellow tuff
~
W
0,2
• p~r~sed wood
0'100! 0, 4
0;6
0;s
1,0
FIG. 3c The 1/W variations as ~ function for building materials ( W > 8.0 kg/kg)
79
80
A. Carotenuto, F. Fucci and G. La Fianza
v.-4
~.~
. ~ o
~
f¢
,.d
<
0
~
Vol. 18, No.
~.
Vol. 18, No. 1
ADSORPTION PHENOMENA IN POROUS MEDIA
81
In this study the authors showed an experimental apparatus to test the water content, during isothermal conditions, for some building materials considering the air relative humidity variation. The experimental apparatus seems reliable. Some structural expedients were done to widen the experimental range till 0.90 of relative humidity value. The authors, considering the obtained data and some previous results, suggest a building material classification related to their hygroscopic behaviour. The authors show an empirical correlation for these classes of materials. This relation determines, during isothermal conditions, the medium weight variation at a 0.40 - 0.90 relative humidity range. References
[1] [2] [3] [4] [5] [6] [7]
C.H.Hsieh., H.J.Jr.Romey, Vapour pressure lowering in geothermal system, Soc.Pet.Eng.J, pp.156-167, (1983). I.Langmuir , "The constitution and fundamental properties of solid and liquids, part. 1, Solids J. Am. Chem. Soc., 38, 2221-95, (1916). S.Brunauer, P.H.Emmett, E.Teller , Adsorption of gases in multimolecular layers, ]. Am. Chem. Soc., 60, 309, (1938). M.Douglas, Ruthven, Principles of adsorption and adsorption processes, c. 3 - 4, John Wiley & Sons, (1984). A.V.Luikov, Heat and mass transfer in capillary porous bodies, P e r g a m o n Press, (1966). P.J.Sereda, R.F.Feldman, Wetting and drying of porous materials, Canadian Building Digest, NRC DBR, Ottawa, CBD 130, october 1970. P.Bondi, P.Stefanizzi, Rilevamento sperimentale di parametri caratteristici del trasporto di umidit~ in materiali porosi: apparecchiature e primi risultati, La Termotecnica, pp.35-39, giugno (1989).
INT. COMM. HEAT MASS TRANSFER Vol. 18, pp. 71-81, 1991 ©Pergamon Press plc
A D S O R P T I O N P H E N O M E N A I N P O R O U S M E D I A I N PRESENCE OF M O I S T AIR
A.Carotenuto Dipartimento di Ingegneria Industriale Universit-~t degli Studi di Cassino Via Zamosh n*2, 03043 Cassino Italia F. Fucci and G. La Fianza D.E.T.E.C. Universit~ degli Studi Federico II Napoli P.zzale Tecchio n ° 1, 80123 Napoli, Italia (Communicated by J.
OOS,~)
ABSTRACT In the present study the authors show the experimental results for some building
materials as concrete, gypsum, brick, neapolitan yellow tuff, prestressed pine wood. These results are connected, during isothermal conditions, with the adsorbed water content and with a relative humidity variation of the medium air. The authors, consideringprevious results showed,suggestan empiricalcorrelationdescribing,for some materials, the percentual weight variation of the adsorbed water higher than 1.0 kg/kg, for a 0.40-0.90 relative humidity range. Introduction
The knowledge of heat and mass transfer phenomena in walls is very important to the building degradation prevention. These mechanisms are extremely complicated, due to the nature and geometry of the porous material, to the contemporary presence of different kinds of water adsorption and transfer in the porous medium and to the heat, mass and energy fluxes interactions; all these reasons makes the analytical model definition, for a wall thermoigrometric behaviour, very difficult. In the study, a series of experimental results for some building materials are presented. These results show the adsorbed water under equilibrium conditions with the igrometric degree and air temperature variation. An empirical equation which connects these experimental results with those presented by other researchers, is presented.after showing the basic equation of thermofluidodynamic phenomena in 71
72
A. Carotenuto, F. Fucci and O. La Fianza
Vol. 18, No. 1
porous media. These results allow us to determine the adsorbed water with the variation of hygrometric degree and air temperature, for some building materials. Adsorotion vrocess
Liquid adsorption on a solid surface depends on the solid surface capacity (sorbent) to retain a very thin liquid film (sorbate). There are two superficial adsorption mechanisms: chemical and physical adsorption. Physical adsorption shows an higher heat adsorption and a lower pressure and temperature compared to a chemical kind of adsorption. Generally, the sorbate fills the superficial adsorption zone with a monomolecular layer (monolayer adsorption) or with a several overlaid layer (multilayer adsorption). Physical adsorption can be multilayer or monolayer; chemical adsorption can be only monolayer. Working forces, in the physical adsorption mechanism, are the van der Waals forces. These forces are always present whereas the electrostatic ones are active only in case of unstable controions sorbents, as the zeolites. Macroscopically, physical adsorption can be explained by a simple experiment which doesn't take in account the capillarity phenomenon and the salt occurrence [1]. A container of fixed volume V, filled by n gas molecules, is at T temperature; neglecting the solid wall and the gaseus molecules interaction, the p' pressure is defined as: p' = z R T v
(~)
where z is the compressibility factor. The real value of p pressure, considering the previous interactions is defined as [1]: A6
P' = P - ~
(P2 -- Pl) Z R T
(2)
where 5 is the radius of action of the superficial forces, A is the total porous surface, p~ and P2 are respectively the molecule density influenced or not by the surface tension, V is the volume, T the temperature and R is the gas constant. In most cases the AS/V fraction is a small quantity ( 8 is equal to few angstroms and V / A few centimeters ). Anyway, considering this difference, it's clear that the higher the superficial solid area is, compared to the volume, the lower the reduced pressure value will be. On the other hand, the superficial area increase is a
Vol. 18, No. 1
ADSORPTION PHENOMENA IN POROUS MEDIA
73
solid porosity function and therefore, for a porous m e d i u m , a significant pressure decrease, compared to that obtained by the real gases law, can be found. The most widespread adsorption theory is based on the cinetic gas theory. The Langmuir model [2] is the most diffused among the calculation models for isotherm phenomena. It points out the a m o u n t of retained sorbate for a monolayer case. The model assumption are: - the adsorption areas on the sorbent surface are exactly positioned; - all the areas are energetically equivalent; - there are no influences among the sorbent molecules and neighboring sites. with this hypotesis we have: q
=
bp
(3)
qmax l + b p and 1 --
1 =
q
1 +
-
1
-
(4)
qr~x bqmax P
where q/qmax is the adsorbed a m o u n t and the m a x i m u m adsorbable a m o u n t (under saturation condition) rate; b is the adsorption equilibrium constant with temperature and sorbent-sorbate characteristics. E q u a t i o n (3) allows the calculation of qmax and b constants at different temperatures by the experimental data interpolation on a (1/q),(1/p) diagram. A s t u d y for a multilayer, isotherm adsorption model shows some problems due to the sorbate-sorbate interaction. The Brunauer, Emmett and Teller (B.E.T.) equation is based on the following assumptions [3]: - the solid surface is considered as an homogeneus, bidimensional matrix; - a gas molecule is stably adsorbed w h e n it is in contact with an adsorption area; - on the contrary, if a gas molecule contacts a filled area, it will be adsorbed, but a second layer will be formed; - a limit of n strata of adsorbable molecules will mix to the gaseus phase splitting in one layer at time; - all the layers on the first one have the same physical properties. With these assumptions:
q qm
b(p/ps) ( 1 - p / p s ) [ 1 - ( p / p s ) + (bp/p,)]
(5)
74
A. Carotenuto, F. Fucei and G. La Fianza
Vol. 18, No. 1
and P/Ps
1
b- 1 p
q [ 1 - ( P / P s ) ] = bqm + bqm Ps
(6)
where Ps is the vapour saturation pressure and qm is the adsorbed amount for a single layer. Equation (4) allows the evaluation of qm and b constants at different temperatures by the experimental data extrapolation on a (p/Ps)/[q (1-p/ps)],(p/ps) diagram. B.E.T. equation takes in account the experimental data for a 0.05 < p/Ps < 0.35 range. For a porous matrix we have to consider the porous dimensions. Porous media are classified, considering their dimensions, in the following classes: - micropores with equivalent radius below 20 •; - mesopores with equivalent radius from 20A to 50 ,/~; - macropores with equivalent radius above 500 ./k. For macropores it can be assumed that the physical water adsorption theory is still good, if we assume the porous radius and the adsorbed liquid thickness rate as infinite. For mesopores we are in the presence of capillarity, on the contrary for micropores a different theory for the explanation of adsorption processes must be considered [1]. Adsorvtion t~henomena in o o r o u s media in oresence of moat air
In theory, adsorption mechanisms in porous media in presence of moist air should be considered by adsorption models of pure gaseus mixtures, The nitrogen and oxygen adsorption normally acts at very low temperature and very high partial pressure compared to the thermodynamic conditions of water vapor adsorption [4]. Therefore, the previous mono and multilayer adsorption models, for a single component, can be used. The authors studied, experimentally, the adsorption phenomenon for some building materials (cement, neapolitan yellow tuff, brick and prestressed pine wood) at 0.40 - 0.90 relative humidity range. Specimen conditioning Three millimeter thick cylindrical specimens have been obtained by a series of
different core samples (7 cm of diameter). The core samples have been dryed out by air on H2SO4 for five days in a 50°C oven and then weighed at the end of the test [6].
Vol. 18, No. 1
A D S O R P T I OPHENOMENA N IN POROUS MEDIA
75
Experimentalapparatus An experimental system used to test the specimen weight variations at a constant temperature and a constant relative h u m i d i t y conditions to the neighboring, has been set up and now schematically demonstrated. The system consists of two independent circuits; the first one is crossed by a thermostatic liquid at a temperature value variyng from 5 to 80°C and the second one is crossed by moist air with fixed thermohygrometric characteristics. The system consists of a cylindrical container of a fixed dimension ( h = 24,5 cm and 0 = 18 cm ), surrounded by a double suitable insulated wall, which contains the specimen hanged to an electrical precision balance. The air space is filled by thermostatic liquid, circulating by a water pump suction, whose temperature varies from 5 to 80°C. In the thermostatic tank, the fixed liquid temperature is obtained by a refrigerator set and by an electrical heater. In the 10 liters of capacity thermostatic tank, a long copper coil, where air passes before lapping the H2SO4 solution, is plunged. The air temperature is checked by the thermostat and is equal to the container temperature, whereas the air humidity is checked by a titration. The air, conditioned according to the experimental model, comes in the container, where it laps the specimen due to a water pump sunction. In the test chamber, a tank has been set. This tank is filled with liquid water by an intake-valve pipe which passes through a kettle closeover flange and the air lapping the specimen be saturated. In that case, the connection of the test chamber with the air circuit is broken off by the two-valve shutdown and the inner air constantly flows, after a fine fractioning by a pumice, due to a contact with water. The electronic balance has been placed into a suitable glass container where the air and the container temperature are the same and at 0.45 relative humidity value. Inside the glass container a little hole has been made to allow the hanging specimen apparatus to pass through. Data recorder device is made by: - an HMD 20Y probe, placed near the specimen, records and transmits the temperature and the air humidity data; in particular the temperature measure shows an uncertainty of + 0.2% whereas the humidity air measure shows an uncertainty of + 2.2 %; - a electric balance with a 4.3 mV/V output, a weight from to 25 g, measures the specimen weight variations. The sensitivity is 100 V/0,1 g then expanded in a 0-100 endscale final signal; four T-type thermocouples placed in a significant point of the system to check its normal running. Data furnished by the experimental apparatus are recorded by a multichannel microvoltmeter Fluke under fixed time. The experimental apparatus is schematically showed in fig.1.
76
A. Carotenuto, F. Fucei and G. La Fianza
Vol. 18, No. 1
Specimens weight variations have been obtained at 20 °C. Values of the moist air temperature and of the moist air relative humidity in the container have been changed (40 - 90%) after obtaining, for the specimens, a fixed thermodynamic equilibrium condition. The equilibrium condition for a dry specimen at the test initial condition was reached in 100 h after having fixed the moist air thermodynamic conditions. Measures have been carried out at an increasing relative humidity, with fixed air temperature, to speed up the equilibrium condition achievement. Therefore, the specimen weight variation test started using the equilibrium conditions of the previous, lower and relative humidity test and waiting to obtain the test stationary conditions. The specimen water weight variation W has been obtained considering the weight specimen 7 measurements at different thermodynamic conditions. We can assume W, variyng (~ and with a fixed 70 (weight of the dry specimen) as: W = ~(7-7°)-100
(kgg)
(7)
Results
Direct comparison with the previous known results for the natural inert concrete, neapolitan yellow tuff and brick hasn't been possible. In fact for these materials there seems to be no data on their hygroscopic behaviour. On the contrary, results obtained for gypsum agree with the results reported in [7]; likewise for the prestressed pine wood dates. Thermohygrometric behaviour of the material examined in laboratory has been compared with the other building materials studied by Luikov [5]. Building materials had been selected under three different classes to clearly present the final results. These classes show the weight water variation of the building materials as function of the moist air thermohygrometric conditions. In particular: - in the first class are included building materials which, during isothermal process, show a W variation less than 1,0 kg/kg. They are: brick, glass wool, artik tuff, limestone and earthenware block; - in the second class are included building materials which, during isothermal process show W variations in the 1.0 - 8,0 k g / k g range. They are: natural inert cement, tripoli brick, slag concrete, sand and cement mortar, foam concrete; - in the third class are included building materials which, during isothermal conditions, show W variations higher than 8.0 kg/kg. They are: wood, prestressed pine wood, gypsum, cork, wood felt and neapolitan yellow tuff.
Vol. 18, No. 1
A D S O R P T I OPHENOMENA N IN POROUS MEDIA
'
-
77
t
FIG. 1 Experimental apparatus: (E. B electric balance, T. termostatic tank, H. HMD probe, W.P. water pump, C H2SO4 tank, S specimen) In fig. 2 the the W variations (for the materials of the first class) under isotherm conditions as (~ function, are presented. These curves are included in the second and third class of physical adsorption mechanisms made by Braunauer et al.[4]. These adsorption curves are generally considered for materials with an high range of porous dimensions. In these materials increasing the hygrometric degree, a change from a monolayer adsorption to a multilayer adsorption till a capillary condensation is observed. By the examined data we have: 1
m=a(~+d W
(8)
which relates, with good approximation, the amount of adsorbed water with the hygrometric degree variation. This is for isothermal processes and for ~ value higher than 0,4 as showed in 3a, 3b and 3c diagram. In tab. I coefficient a and b for different building materials and correlation coefficient of data are showed.
78
A. Carotenuto, F. Fucci and G. La Fianza
1,5
A earthenwareblock • limestone o brick glass wool Q artik t u f f
W
Vol. 18, No. I
/ /
~
1,0
O~
0,0014
0,6
0,8
1,0
FIG. 2 The W variations as ¢# function for building materials ( W < 1.0 k g / k g )
W
• glasswool • limestone a earthenwareblock
T~
1
0, 0,4
"%"-..
° ~
0,6
0,8
1,0
FIG. 3a The 1 / W variations as (~ function for building materials ( W < 1.0 k g / k g )
Vol. 18, No. 1
ADSORPTION PHENOMENA IN POROUS MEDIA
2~
• natural inert cement • cement mortar • foam concrete
1
W 1,6
• tripoli bconcriCrkte
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FIG. 3c The 1/W variations as ~ function for building materials ( W > 8.0 kg/kg)
79
80
A. Carotenuto, F. Fucci and G. La Fianza
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Vol. 18, No.
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Vol. 18, No. 1
ADSORPTION PHENOMENA IN POROUS MEDIA
81
In this study the authors showed an experimental apparatus to test the water content, during isothermal conditions, for some building materials considering the air relative humidity variation. The experimental apparatus seems reliable. Some structural expedients were done to widen the experimental range till 0.90 of relative humidity value. The authors, considering the obtained data and some previous results, suggest a building material classification related to their hygroscopic behaviour. The authors show an empirical correlation for these classes of materials. This relation determines, during isothermal conditions, the medium weight variation at a 0.40 - 0.90 relative humidity range. References
[1] [2] [3] [4] [5] [6] [7]
C.H.Hsieh., H.J.Jr.Romey, Vapour pressure lowering in geothermal system, Soc.Pet.Eng.J, pp.156-167, (1983). I.Langmuir , "The constitution and fundamental properties of solid and liquids, part. 1, Solids J. Am. Chem. Soc., 38, 2221-95, (1916). S.Brunauer, P.H.Emmett, E.Teller , Adsorption of gases in multimolecular layers, ]. Am. Chem. Soc., 60, 309, (1938). M.Douglas, Ruthven, Principles of adsorption and adsorption processes, c. 3 - 4, John Wiley & Sons, (1984). A.V.Luikov, Heat and mass transfer in capillary porous bodies, P e r g a m o n Press, (1966). P.J.Sereda, R.F.Feldman, Wetting and drying of porous materials, Canadian Building Digest, NRC DBR, Ottawa, CBD 130, october 1970. P.Bondi, P.Stefanizzi, Rilevamento sperimentale di parametri caratteristici del trasporto di umidit~ in materiali porosi: apparecchiature e primi risultati, La Termotecnica, pp.35-39, giugno (1989).