DE10019122A1 - Determining gases or vapors dissolved and/or absorbed in solids or on outer or inner surfaces of solid comprises carrying out calorific and dielectric measurements in an impedance calorimeter - Google Patents

Abstract

Determining gases or vapors dissolved and/or absorbed in solids or on outer or inner surfaces of the solid comprises carrying out calorific and dielectric measurements in an impedance calorimeter. Determining gases or vapors dissolved and/or absorbed in solids or on outer or inner surfaces of the solid comprises carrying out calorific and dielectric measurements in an impedance calorimeter. The calorimeter consists of a thermostat (1) containing a thermostat fluid (13), a storage chamber (2) for the gas or vapor (15), a sorption chamber (3), a pressure line (4) connecting the chambers and containing a regulating valve (5), an electrical capacitor (6) filled with the sorbing liquid (7), pipe lines (10) for filling and emptying the chambers, a system of thermocouples (12) arranged in the walls of the chambers, and a measuring sensor (14) for determining the temperature and pressure in the chambers.

Description

introduction

I offer to all who read this patent specification, Jürgen U. Keller, born in Graz, Austria, since 1984 professor of thermodynamics at Department of Mechanical Engineering at the University of Siegen, Germany, my warm regards! The document contains a proposal for a solution of a gas solids problem that has been open for many years Systems, namely, the amount of one dissolved in a solid, i.e. H. either absorbed or, in the case of porous substances, adsorbed gas directly and without additional hypotheses about the gas molecules to measure accessible or inaccessible volumes. I am positive, that the systematic application of what I proposed here Measurement method new insights into the nature of gas-solid Allow systems, and thus to a better physical and technical understanding of these systems will contribute. Therefore I ask everyone who uses this procedure or has any questions with me to get in touch either in writing or electronically and contact me via to report their experiences confidently and frankly.  

Technical field

The invention relates to a method for measuring gases absorbed in solids or adsorbed on their surface according to the preamble of the claim. The method can be used to determine the amount of a gas which is dissolved in a dense solid or to measure the amount of gas which is adsorbed on the surface of pores in a porous solid. Knowing these amounts of gas is necessary to

• a) characterize solids and their with gas solubility to be able to assess related technical properties and or
• b) Basic data for calculation and technical design adsorbed gas separation processes such. B. adsorptive air Laying processes at ambient temperature, solvent recovery extraction processes or exhaust air purification processes to have.

Problem / state of the art

All solids can absorb gases from their environment. With dense substances one speaks of absorption of gases or of their solubility in the solid. With surface-active substances and especially with porous substances like activated carbons, zeolites, porous polymers, metal foams and other substances, gas molecules can accumulate on the outer and inner surfaces of the substances and one speaks of adsorption [1-3]. Both phenomena, the absorption and adsorption of gases in solids, can be reversible or irreversible. In the first case, the gas molecules are only weakly bound to the solid. The binding energy is typically in the range 10 kJ / mol-100 kJ / mol. This phenomenon is called physisorption. In the second case, there is often a chemical reaction between the molecules of the sorbed gas and the solid and one speaks of chemisorption. Both phenomena are of increasing technical importance [4-11]. Their main areas of application are:

• 1. Storage of gases at reduced pressure, e.g. B. methane in Activated carbon or high-purity hydrogen in metallic Alloys as alternative fuels.
• 2. Characterization of porous solids (pore spectrum) by Adsorption of nitrogen or argon at ambient pressure and Boiling temperatures (77 K or 87 K) of these gases.
• 3. When sorbing gas mixtures, the sorbed phase has because of the different interactions of the Gas components with the solid generally another Composition as the space-filling gas phase. This  Circumstance, and other effects that occur with gas mixtures today both on an industrial scale and on a laboratory scale in numerous methods for separating the components of a Gas mixture or for cleaning technical gases, for Recovery of solvents from exhaust air and much more used.
• 4. The conditioning of non-rigid polymeric plastics with supercritical solvents (eg CO ₂ )
  • a) absorption of dyes or
  • b) inclusion of medicinal substances with subsequent controlled release (controlled drug release) or
  • c) to displace residues of the monomer belonging to the polymer.
  The prior art relating to these fields of application of sorption phenomena is reproduced in the literature cited above and specified in the appendix. They are also expressly referred to with regard to the technical terms used below.

In order to design technical sorption processes, ie adsorption or absorption processes with solids according to the above examples (1-4), the so-called sorption equilibrium, ie the gas mass dissolved in a given amount of solids depending on the pressure, temperature and type of gas, must be known as precisely as possible his. The traditional methods of determining this size, namely
Volumetry / Manometry [1, 2, 12]
Gravimetry [1, 2, 12]
Oscillometry [12, 14]
do not allow to determine the absolute mass of the gas in the solid (m ^a ). Rather, only that can
so-called reduced mass

Ω = m ^a - ρ ^f V ^v (1)

be measured. Here Ω means a measured variable or a number to be calculated from measured values, m ^a the sorbate mass sought, ρ ^f the mass density of the gas surrounding the solid and V ^v the volume of the solid that is not accessible to the gas molecules. This volume is generally not known, so that the relationship (1) contains two unknown quantities that cannot be determined from (1) at the same time! The second term on the right side of equation (1) in volumetry and oscillometry is physically determined by the extensivity of matter, that is, its property of fulfilling space. In gravimetry, this term is caused by the buoyancy of the solid in the surrounding gas atmosphere.

With homogeneously absorbing solids, such as. B. polymers, V ^{v means} the volume of the whole solid. This is also generally not known, even in the case of simple geometric shapes of the sorbing polymer sample, since on the one hand it is compressed by the external pressure of the gas, and on the other hand it can be swollen from the inside by the sorption of gas molecules. This swelling effect can occur with individual substances such. B. Makrolon 2400 when gassed with CO ₂ at 298 K even at moderate pressures of approx. 50 bar up to 30% of the original volume of the sample in air at atmospheric pressure [14, 15].

In the case of porous substances such as activated carbons and zeolites, the inaccessible volume cannot be measured volumetrically or gravimetrically, since part of the gas molecules is basically adsorbed when penetrating into the pore system of the solid and the interface between the gas phase and the adsorbate / solid system can no longer be clearly defined. The "Gibbs dividing surface" [1, 2] introduced by JW Gibbs in this connection at the time does not solve this problem, since it can only be defined for flat adsorbing surfaces, the inner surface of practically all, especially technical ones Adsorbents, but usually have a complex geometric structure, which can only be approximated by areas of fractal dimension (1 ≦ D ≦ 3) [16], or a suitable pore model with a statistical distribution function [3, 9]. In addition, porous substances can also be compressed by the external gas pressure, but can be expanded by the adsorption of gas molecules. It should also be noted that large gas molecules cannot penetrate parts of the pore system, especially in micro- and submicropores [1-3, 17], Fig. 1. The term inaccessible volume therefore also depends on the type of gas used and can therefore not be defined independently of the substance, especially for gas mixtures!

In technical practice, the so-called helium volume hypothesis is used in this situation: The solid sample to be investigated is exposed to a helium atmosphere, it is assumed that helium is not or negligibly adsorbed, and the inaccessible volume V ^v is approximated by the z . B. Helium volume V ^He measured by a helium pycnometer:

V ^V ≅ V ^He (2)

This hypothesis has been found for low gas pressures (p ≦ 0.1 MPa) and Temperatures around or above ambient temperature (T ≧ 293 K) in about proven. For higher pressures and also in principle however, it leads to difficulties and is therefore problematic [18-20]. For one thing, experiments have shown that helium is also effective in Room temperature can be adsorbed by solids. To the others have also been observed to undergo this sorption process can span hours, days, and weeks, so rapid mes solutions with commercial helium pycnometers usually none Equilibrium, but only one (incompletely characterized) Result in non-equilibrium state!

If one accepts the helium volume hypothesis (2), it gives an expression for the mass sorbed in the solid from equation (1)

m ^a = Ω + ρ ^f V _He . (3)

Strictly speaking, this is not the absolute sorbed gas mass, but only the so-called Gibbs excess mass [1, 2, 3, 12]. To completely sorbed gas mass to get approximately when m ^a bands according to (3) even with a correction factor:

This factor takes into account the intrinsic volume of the sorbed gas molecules in the solid. The size ρ ^L ₀ is the liquid density of the gas in a suitable standard state, e.g. B. the liquid state at the triple point of the sorptive gas [12]. The choice of this size also contains a certain arbitrariness! This of course makes it difficult, for example, to compare measured values according to (3) or (4) with analytical values for the sorbed mass derived from molecular models of the solid and the sorption process of the gas!

In summary, it can be said that it is with conventional Methods not possible today, sorbed gas masses without hypotheses to measure and that it would therefore be very desirable to have a method develop who can do this.  

Presentation of the invention

The method according to the invention consists in the simultaneous measurement of the heat (Ω _U ) and the dielectric polarization (Ω _DE ) which occur in a solid of mass (m ^S ) when gassed with a sorptive gas of the total mass (m *) with the aid of an impedance calorimeter , Fig. 1, [1, 21]. From the two measured variables (Ω _U , Ω _DE ), other system-specific auxiliary variables such as pressure (p), temperature (T *) and the two masses (m *, m ^S ), the dielectric sorbed state equation can be used to determine or absorb the solid sorbed mass (m ^a ) adsorbed on its inner surface can be calculated:

m ^a = M ^a (Ω _U , Ω _DE , m *, m ^S , T *). (5)

Here M ^{a is} a function of the variables given in (5), which approximately has the following form:

In this respect, the quantities (u ^f , α ^f ) mean the mass-related, ie specific internal energy and dielectric polarizability of the sorptive gas [21, 22], (u ^a ₀ , α ^a ₀ ) the specific internal energy and dielectric polarizability of the sorbed Phase in a freely selectable reference state (index "0"). In principle, a liquid or a gas state of the sorptive gas can also be selected as such. The size K means only the temperature (T *) of the overall system, cf. Fig. 1, and the physico-chemical nature of the sorptive gas and the sorbent solid (index "S") dependent constant. Their value must be determined from the totality of all measurements available via a system using a Gaussian error minimization method.

The impedance calorimeter (IK) is shown in Fig. 1. It consists of a thermostat ( 1 ) which contains two pressure chambers, a gas storage chamber ( 2 ) and a sorption chamber ( 3 ). Both chambers are connected to each other by a pressure line ( 4 ) which contains a regulating valve ( 5 ). The sorption chamber ( 3 ) contains a plate condenser ( 6 ), [21], between the plates of which the pellet or powder-like sorbent solid ( 7 ) can be filled. Instead of a plate capacitor, a cylindrical capacitor can also advantageously be used. The capacitor ( 6 ) is connected to electrical lines ( 8 ), which are led through the walls of the sorption chamber ( 3 ), to an electrical impedance analyzer, which contains a voltage source ( 9 ). These can be used to apply a direct voltage or an alternating voltage of preceding frequency (ω) to the capacitor.

The storage and sorption chambers are provided with pressure lines ( 10 ) through which sorptive gas can be led into the chambers or removed from them again with the aid of a vacuum pump ( 11 ). The walls of the chambers ( 2 ) and ( 3 ) contain numerous thermocouples ( 12 ), with the help of which the inflow or outflow of heat from the chambers into the surrounding thermal bath fluid ( 13 ), e.g. B. nitrogen, water, methanol, oil can be measured. The chambers ( 2 ), ( 3 ) also contain sensors ( 14 ) for measuring pressure and temperature in the chambers. The lines ( 8 ), the thermocouples ( 12 ) and the measuring sensors ( 14 ) are connected to corresponding electronic evaluation devices ( 18 ) in which the measuring signals are converted into the physical measured variables Ω _u , Ω _DE .

Measuring process

To prepare for a measurement, the sorbing solid ( 7 ) is filled into the plate condenser in the activated state or with fixed presorption, the sorption chamber ( 3 ) is closed and both pressure chambers ( 2 ), ( 3 ) with the valve ( 5 ) open via the vacuum pump ( 11 ) evacuated. The valve ( 5 ) is then closed and the sorptive gas is filled into the gas storage chamber ( 2 ) via one of the pressure lines ( 10 ). After reaching the bath temperature (T = T *), the mass (m *) in the storage container ( 2 ) can be calculated by measuring the pressure and using the thermal state equation of the sorptive gas. If you now open the valve ( 5 ), the sorptive gas ( 15 ) flows partially from the storage chamber ( 2 ) into the sorption chamber ( 3 ), where it is usually cooled by the expansion (exception: hydrogen, example!) And partially sorbed in the solid ( 7 ). This releases heat of sorption, which changes both the temperature of the solid now loaded with sorptive gas and that of the surrounding gas. Due to the temperature differences that occur in both the storage chamber ( 2 ) and in the sorption chamber ( 3 ) compared to the thermostatic fluid ( 13 ), heat flows ( _SV , _AV ) now occur via the thermocouples ( 12 ) and with the help of the measurement data acquisition device ( 18 ) can be measured and deliver the thermal signal (Ω _U ) of the sorptive process after time integration. After reaching the thermal equilibrium, ie constant and everywhere the same temperature in the whole system, an electrical alternating voltage is applied to the plates of the capacitor ( 6 ) and with the help of the impedance analyzer ( 9 ) the (generally complex) impedance of the gas loaded solid measured. This gives the dielectric measurement signal (Ω _DE ). From both measured variables (Ω _U , Ω _DE ), as explained in the previous section, cf. Equations (5), (6) that calculate the mass sorbed in the solid.

To carry out further measurements at other pressures, the supply of sorptive gas in the storage container ( 2 ) must be adjusted accordingly. In this process, both connection attempts by increasing the pressure and expansion attempts by partially blowing off the sorptive gas can be carried out.

Reference list

Fig.

1

1

thermostat

2

Gas storage room

3rd

Sorption chamber, contains the sorbing solid, mass (m ^S

)

4

Pressure line

5

Regulating valve

6

capacitor

7

Solid, sorbent

8th

Electric lines

9

Impedance analyzer and voltage source

10th

Pressure lines

11

Vacuum pump

12th

Thermocouples

13

Thermostatic fluid

14

Sensor

15

Sorptive gas,
Mass in the pantry (

2

) is filled: m *
Mass in equilibrium in both chambers (

2

) and (

3rd

) together there is: m ^f

,

16

Sorbed gas mass (m ^a

)

17th

Electrical cables for thermocouples (

12th

)

18th

Measurement data acquisition device
T * thermostat temperature

literature

[1] SING; KSW, GREGG, SJ Adsorption, Surface Area and Porosity Academic Press, London etc., 1982
[2] ROUQUÉROL, F., ROUQUÉROL, J., SING, KSW Adsorption by Powders & Porous Solids Academic Press, San Diego etc., 1999 ISBN 0-12-598920-2
[3] DO, DD Adsorption Analysis: Equilibria and Kinetics, Imperial College Press, World Scientific Publishing Co., Singapore, 1998.
[4] DABROWSKI, A., Ed. Adsorption and its Applications in Industry and Environmental Protection, Vols. 1 and 2, Elsevier, The Netherlands, 1998.
[5] A Bibliographical Update (1997), Adsorption Science & Technology, 16 (1998), 331-357.
[6] YANG, RT Gas Separation by Adsorption Processes, Imperial College Press, Singapore, 1998.
[7] SUZUKI, M. Adsorption Engineering, Elsevier, Amsterdam etc., 1990.
[8] STAUDT, R. (ed.) Technical Sorption Processes Keller-Festschrift (1), Progress Reports VDI, Series 3, Process Engineering, Vol. 554, VDI-Verlag, Düsseldorf, 1998.
[9] STAUDT, R. (Ed.) Adsorption by Porous Solids Keller-Festschrift (2), Progress Reports VDI, Series 3, Process Engineering, Vol. 555 VDI-Verlag, Düsseldorf, 1998.
[10] MEUNIER, F., Editor Fundamentals of Adsorption, Proceedings of the 6 ^th

Int. Conf. on Fundamentals of Adsorption, Giens, 1998, Elsevier, Paris, New York etc., p. 1266, 1998.
[11] VDI guidelines (3674 E), 1996 Exhaust gas purification by adsorption Process gas and exhaust gas purification GG Börger, Bayer AG (coordinator).
[12] KELLER, JU, F. DREISBACH, H. RAVE, R. STAUDT, M. TOMALLA Measurement of Gas Mixture Adsorption Equilibria of Natural Gas Compounds on Microporous Sorbens, Adsorption, 5 (1999), 199-214.
[13] KELLER, JU Theory of Measurement of Gas Adsorption Equilibria by Rotational Oscillations, Adsorption, 1 (1995), 283-290.
[13a] RAVE, H., R. STAUDT, JU KELLER Measurement of Gas-Adsorption Equilibria Via Rotational Oscillations, J. of Thermal Analysis and Calorimetry, 55 (1999), 601-608.
[14] RAVE, H. Measurement of sorption equilibria of gases on solids with the help of slow vibrations of a rotating pendulum Progress reports VDI, series 3, process engineering, vol. 000, VDI-Verlag, Düsseldorf, in print, 2000.
[15] RAVE, HO KOELSCH, R. SCHWARZER, R. STAUDT, JU KELLER Oscillometric-Volumetric Measurements of Gas Solubilities in Swelling Glassy Polymers, AIChE Journal, paper No 6304, submitted, March 2000.
[16] MANDELBROT, BB The Fractal Geometry of Nature, Freeman, New York, 1983.
[17] IUPAC Recommendations 1994: J. ROUQUÉROL, D. AVNIR; CW FAIRBRIDGE; DH EVERETT, JH HAYNES, N. PERNICONE, JDF RAMSAY, KSW SING, KK UNGER: Recommendations for the Characterization of Porous Solids, Pure & Appl. Chem. 66 (1994) 8, 1739-1785.
[18] STAUDT, R., SALLER, G., TOMALLA, M., KELLER JU A Note on Gravimetric Measurements of Gas-Adsorption Equilibria, Ber. Bunsenges. Phys. Chem., 97: 98-105 (1993).
[19] STAUDT, R., BOHN, S., DREISBACH, F., KELLER, JU Gravimetric and Volumetric Measurements of Helium Adsorption Equilibria on Different Porous Solids, Proceedings of IV Int. Conference on Porous Solids (COPS IV) p. 261-266, Bath 1996, B. Mc Enany et al., Eds., The Royal Society of Chemistry, Special Publication No. 213, 1997.
[20] ROBENS, E., JU KELLER, CH MASSEN, R. STAUDT Sources of Error in Sorption and Density Measurements, J. of Thermal Analysis and Calorimetry, 55 (1999), 383-387.
[21] STAUDT, R., RAVE, H., KELLER, JU Impedance Spectroscopic Measurements of Pure Gas Adsorption Equilibria on Zeolites, Adsorption, 5 (1999), 159-167.
[22] FROHLICH, H. Theory of Dielectrics, Oxford University Press 2 ^nd

. Edition, New York etc., 1986.

Claims (10)

1. A method for determining gases or vapors sorbed in solids, ie either dissolved or absorbed, or deposited or adsorbed on outer or inner surfaces of the solid by a combined caloric and dielectric measurement in a patented instrument called an impedance calorimeter, FIG. 1 consisting of a thermostat ( 1 )

• - a thermostatic fluid ( 13 ),
• - a storage chamber ( 2 ) for the sorbing gas or steam ( 15 ),
• - a sorption chamber ( 3 ),
• a pressure line ( 4 ) which connects the storage chamber ( 2 ) and the sorption chamber ( 3 ) and contains a regulating valve ( 5 ),
• an electrical capacitor ( 6 ) which is filled with the sorbing solid ( 7 ) and is connected via electrical lines ( 8 ) to a voltage source combined with an impedance analyzer ( 9 ),
• - Pipes ( 10 ) for filling and emptying the chambers ( 2 ) and ( 3 ) via a gas supply system (not shown) or a vacuum pump ( 11 ),
• - A system of thermocouples ( 12 ), which are arranged evenly distributed in the walls of the chambers ( 2 ) and ( 3 ) and allow heat flows into the chambers through the electrical voltages ( 17 ) to a measurement data acquisition device ( 18 ), or out of them as a result of temperature differences between the sorptive gases ( 15 ) located in the chambers and the thermostatic fluid ( 13 ) contained in the thermostat ( 1 ) outside the chambers and
• - Sensor ( 14 ) for determining the pressures and temperatures in the chambers ( 2 ) and ( 3 );
• - And in that for measuring the amount of gas sorbed (m ^a ) ( 16 ) after filling the sorbent solid ( 7 ) in the condenser ( 6 )
• - The two chambers ( 2 ) and ( 3 ) are evacuated with the valve ( 5 ) open via the vacuum pump ( 11 ), then
• - The valve ( 5 ) is closed and
• - The storage chamber ( 2 ) is filled with sorptive gas ( 15 ), then
• - The valve ( 5 ) is opened and
• - The occurring heat flows _SV and _AV measured with the help of thermocouples ( 12 ) and the measurement data acquisition device ( 18 ) and combined to form a calorimetric measurement signal Ω _U and further
• - After setting the pressure and temperature equilibrium between the two chambers ( 2 ) and ( 3 ), a dielectric signal Ω _{DE is} measured by means of the impedance analyzer ( 9 ), from which then together with the signal Ω _U and the total mass (m *) of Sorptive gas ( 15 ) originally filled into the storage chamber ( 2 ), the mass (m ^a ) of the gas sorbed in the solid ( 7 ) is calculated.

2. Measuring method according to claim 1, characterized, that the sorbent solid is either a powder, a pellet bulk or a sample. 3. Measuring method according to claim 1, characterized, that the sorbent solid to be examined is either rigid or polymeric material swelling in the sorption process can be chemical in nature. 4. Measuring method according to claim 1, characterized, that the measurement in the pressure range 0 <p <500 bar and in the temperature range 77 K <T <773 K takes place. 5. Measuring method according to claim 1, characterized in that the electrical impedance of the solid ( 7 ) filled capacitor ( 6 ) at a frequency in the range
0 <w <100 MHz
is measured. 6. Measuring method according to claim 1, characterized in that the number of thermocouples used in the pressure chambers ( 2 ) and ( 3 ) is between 10 ^-3 pieces / cm ² and 100 pieces / cm ² outer container surface. 7. Measuring method according to claim 1, characterized in that when measuring in the thermostat ( 1 ) as thermostatic fluid ( 13 ) either air or water without or with freezing-preventing additives such as methanol, or thermostatic oils with high, ie above 200 ° C, boiling points or liquid nitrogen or another cold brine is used to achieve low temperatures below 0 ° C. 8. Measuring method according to claim 1, characterized in that after calculating the sorbed gas mass (m ^a ), ( 16 ) from a mass balance for the sorptive gas, the volume of the solid in the sorbate state, ie including the amount of gas absorbed, can be calculated. 9. Measuring method according to claim 1, characterized in that after calculation of the sorbed gas mass (m ^a ), ( 16 ) from an energy balance and the caloric measurement signal (Ω _U ), the specific energy of the gas adsorbed in the solid ( 7 ) can be calculated. 10. Measuring method according to claim 1, characterized in that after calculation of the sorbed gas mass (m ^a ), ( 16 ) from the dielectric signal (Ω _DE ), the dielectric polarizability of the gas molecules adsorbed in the solid ( 7 ) can be calculated.