Download Sorption properties of sodium bicarbonate

Barbara WALAWSKA – Inorganic Chemistry Division „IChN” in Gliwice of Fertilizers Research
Institute, Gliwice; Arkadiusz SZYMANEK, Anna PAJDAK – Institute of Advanced Energy Technologies,
Czestochowa University of Technology, Czestochowa; Marzena NOWAK, Bożena HALA – Inorganic
Chemistry Division „IChN” in Gliwice of Fertilizers Research Institute, Gliwice
Please cite as: CHEMIK 2012, 66, 11, 1169-1176
Introduction
From the point of view of different technological processes, the
knowledge of parameters describing the structure of sorbents, such
as surface area or pore size distribution play an important role for
application them in chemical, cement industries, or modification
of mineral raw materials. The sorption properties are determined
by surface area, what binds with surface energy and reactivity [1].
As it is well known, sorbents usually characterize in surface
heterogeneity. That characteristic is due to the presence of varying
sizes and shapes [2]. In general, the grains of fine-grained materials
are of irregular shape, exhibiting a columnar, table, needle-shaped
or lamellar morphology. Less often they can be found in the form of
spherical bodies [1]. There is a close relationship between the type
of pores prevailing in the sorbent and its surface. In accordance with
the IUPAC (International Union of Pure and Applied Chemistry)
recommendation, the surface area of sorbent can be calculated from the
capacity of monolayer, which covers the pores, assuming, that surface
area effectively occupied by particles of adsorbent in total monolayer is
known [3]. Total surface area relates to the unit mass of the adsorbate.
In accordance with th same reccomendations, pores, from the point
of view of their linear dimensions, are classified into micropores (with
a diameter below 2 nm), mesopores (with a diameter from 2 to 50 nm)
and macropores (with a diameter above 50 nm). Macroporous
materials are characterized by a poorly developed surface area from
a few to several m2/g, mesoporous materials – several hundred m2/g,
and microporous materials – up to several thousand m2/g.
Sodium bicarbonate is a sorbent that is increasingly widely used in
purification processes of gaseous products from combustion of solid
fuels. In the seventies of the 20th century, studies on sodium sorbents
were conducted. They included the use of nahcolite (natural sodium
bicarbonate) in the dry flue gas desulphurization process [4]. Nahcolite
changes its microstructure as a result of decomposition at elevated
temperatures, forming an inhomogeneous structure, that is very
reactive in contact with acid gases. Similar properties are possessed
by synthetic sodium bicarbonate. As a result of its thermal activation,
the decomposition of sodium bicarbonate to sodium carbonate
occurs. It influence on decreasing of molar volume of decomposed
sodium bicarbonate (NaHCO3) during releasing of gasous products of
decomposition: carbon dioxide and water vapor, resulting in breaking
apart of compact structure and forming of pores with high surface
area. In respect to different sources of informations, the temperature
of decomposions varies in the range of 60 to 400˚C [5].
Sodium carbonate produced in that proces is characterized
by a more developed surface area in comparison with crystalline
sodium bicarbonate. This translates into an increase in its reactivity.
The process follows the reaction below [6]:
2NaHCO3→Na2CO3+CO2+H2O (1)
The reactivity of sodium bicarbonate depends chiefly on its grain
size and structure [7, 8, 9]. As fine grains react more efficiently, than
larger grains, the examined material was subjected to grinding and
then thermal activation in order to develop its surface area. In the
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present study, the surface area and pore size distribution in modified
sodium bicarbonate was determined, using modern methods for
obtaining the surface topography and structure. The knowledge
of these parameters and their correct interpretation enable the
proper selection and use of sodium bicarbonate for the dry flue gas
desulphurization process.
The presented study on the determination of the sorption
properties of modified sodium sorbent has been carried out within the
development of project nr NR05000910 “Modified sodium bicarbonate
in the processes of dry purification of flue gases from various types of
industrial installations”.
Experimental
Materials
For the examination of the sorption properties of sodium
bicarbonate, baking soda manufactured by Soda Polska Ciech SA, with
characteristics shown in Table 1, was used.
Table 1
Characteristics of sodium bicarbonate
Parameter
Value
Content. %:
NaHCO3
humidity (50oC)
99.9
0.04
Granulation. µm:
< 50
50-100
100-150
150-200
200-250
250-300
200-350
13.9
24.8
25.5
20.0
11.7
3.8
0.3
Average grain. µm
126.0
Bases on the baking soda, four samples were prepared from the
initial material and its mixtures with magnesium stearate, respectively.
A small amount of magnesium stearate (below 0.5 %) facilitated
the process of grinding, without the influence on further thermal
modification.
Analytical methods and apparatus
Sodium bicarbonate, as a crystalline product, is characterized
by a poorly developed porous structure. The purpose of the
mechanical activation was to mill it and increase its active surface
area. A mixture of baking soda with magnesium stearate prepared
in an Eirich mixer was ground in a 160 UPZ impact mill at a rotor
rotational speed of 11400 rpm and a material batching rate of 60 kg/h.
Samples with the mean size – 13.17 μm were obtained.
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Sorption properties of sodium bicarbonate
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The grain size measurement of the test samples, before and
after milling, was made on a Beckman analyzer confirming to the
ISO requirements for grain size determination by the laser diffraction
method [10]. The examination of grain size composition was made,
using a wet measuring module.
Sodium bicarbonate, without and after mechanical activation,
was subjected to thermal activation successively in the range of
temperatures from 100°C to 400°C in a Nabetherm laboratory
furnace for a duration of 30 minutes. The surface areas of the
samples after thermal activation were obtained by experimental
determinations of the low-temperature nitrogen adsorption
isotherms. The adsorption data were obtained by the Brunauer,
Emmet and Teller methods using a Gemini VII 2390 analyzer of
surface area. The analyzer measures the volume of gas adsorbed
at the temperature of -196°C in the relative pressure range
of 0.03 – 0.5. The BET surface analysis enabled the determination
of the micro and mesopores of analysed sorbents.
The surface area of chosen samples of sorbents activated thermally
at temperatures: 100 and 250˚C (Tab.2.) was measured. A method
used for determination the parameters of the porous structure for
macropores was the porosimetric analysis. An Auto Pore IV 9500
(Automated Mercury Porosimeters) was employed for this analysis.
The device serves also for determination, among others, the total
pore area, pore size distribution, percentage porosity, density, and
the transfer properties of pores.
Based on the BET and porosimetric analyses, the surface areas of
the samples, i.e. the magnitude of the external surface (considering the
surface area of the solid body and open pores, not taking into account
closed pores) per unit material mass, m2/g, were determined. Also the
distribution of pore volume as a function of pore size was established.
It allowed the classification of the material to be made in accordance
with the IUPAC recommendation.
The structure of the sorbent was also analized, using a Scanning
Microscope Philips XI 30 ESEM with analizer EDS Edax (SEM). A device
was designed for examining the surface morphology of solid bodies
on a micro- and nano-scale. It makes it possible to represent the
surface of the examined samples and determine the mean length and
width of pores in the samples. From each sample, a test preparation
was made and then sputter coated with gold. Using an SE (Secondary
Electron) detector, images were recorded at a zoom from 1000
to 20000x. EDS X-ray microanalyses were made at selected points
on the samples. The examinations were carried out in a high-vacuum
mode at a voltage of 15 kV.
To assess the sorption properties of sorbents, the test of
reactivity was performed. The testing method involved a cycle of
tests used for determination the reactivity indexes (Ri) and absolute
sorption (Ci). The indices were detected from the Ahlstrom test.
The reactivity test was carried out by desulphuring the flue gas in the
model conditions. It contained sulfur dioxide (SO2 – 5091 mg/m3),
oxygen (O2 – 3 %), carbon dioxide (CO2 – 16 %) and nitrogen,
keeping constant temperature in the reaction chamber. At the
outlet of the reaction chamber the concentractions of individual
gases were measured, using exhaust gases analyzer – Mahiak.
After completion of the reactivity tests, the chemical composition
of the flue gas desulfurization products were determined. Using
the analyzer LECO SC-144, the amount of sulfur and carbon was
obtained. Using standard chemical methods, the percentage of
active compounds was measured.
Investigation results
The sorptive properties of sorbents in the processes of dry
purification of flue gases are determined mainly by their surface area.
Both, the mechanical (grinding) and thermal (temperature above
150 ˚C) activations are significant in increasing the surface area of
1174 •
sodium bicarbonate. To determine the influence of these activations,
surface areas of sorbents after mechanical and thermal activation in
the range of temperatures: 100 – 400 ˚C were compared (Fig. 1).
Fig. 1. Effect of mechanical and thermal activation of the surface area
(BET) of sodium sorbent.
The thermal activation has an significant influence on the surface
area of analyzed sorbents, much less influence has mechanical activation.
At the temperature range: 20 – 100 ˚C, a small surface area of sorbents
is the result of the lack or not complete decomposition of the sodium
bicarbonate. The significant increment of surface area occurs at the
temperature range: 100 – 150˚C, touching the maximum values at the
temperature range: 200 – 25˚C. Noted differences in surface area are
related with occuring of pores created during thermal activation. At
the temperature range:
250 – 300˚C decreasing in surface area is noticed, the results
achieved at 400˚C are similar to the values obtained at the beginning
range of the activation temperatures. It can be probably related with
softening process and closing of opened pores after treating the sorbent
with high temperatures. The obtained surface area values of analyzed
sorbents after thermal activation suggests, that most wide pores occur
in their structure. The surface area of sorbents, reaching a few m2/g
proves their macroporous character. To confirm that statement, the
structural analysis, using a scanning microscope, was analized. Four
samples of sorbents, differing in surface area, obtained in the result of
their activation (according to Tab. 2.), were estimated.
Table 2
Samples NaHCO3 used for the test
Mechanical
Thermal activation
activation
Sample
Type of sample
P1
NaHCO3
none
none
P2
mixture of
NaHCO3 with magnesium stearate
grinding
none
P3
mixture of
NaHCO3 with magnesium stearate
grinding
100°C by time 0.5 h
P4
mixture of
NaHCO3 with magnesium stearate
grinding
250°C by time 0.5 h
In order to establish the surface structure of sodium bicarbonate,
the porous structure parameters, such as the surface area and the
volume pore distribution function, were determined. The investigations
were carried out experimentally by the BET nitrogen adsorption
method (that determines micro- and mesopores) and by the mercury
porosimetry method (that determines macropores). The obtained
surface area values are summarized in Figure 1. Sodium bicarbonate
not activated thermally (P1, P2) has a very poorly developed surface
nr 11/2012 • tom 66
at 100°C did not cause well developed pores to form on all the
grain surfaces. It is most likely that the transformation of NaHCO3
to Na2CO3 occurred here partially and only on some grain surfaces.
Very numerous lamellar and columnar crystals of a size of several
micrometers formed on the sample grain surface.
The volume pore size distribution function determined
by the mercury porosimetry method, confirmed these observation
(Fig.3.). The area under the curves in the diagrams yields here
the volume of open pores. The impact and thermal modification
at 100°C (P3) resulted in the opening of pores with diameters of
70-160 nm, derived from the activation of shallow pore spaces
with undifferentiated shapes. The pores formed were distinguished
by a narrow range of sizes and quite a small volume. After heating
up to 250°C (P4), crystals occuring on the surface of sorbent,
disappeared, whereas the pores developed slightly better and
almost completely covered nearly all surfaces of grains. Additionally
pores with diameters of 160-200 nm occured. The pores were
characterized by a narrow pore size range and a large volume,
which indicates the opening of deep porous spaces with a well
developed structure.
Fig.2. Surface area of sodium bicarbonate determined by the BET
method and mercury porosimetry method
Fig. 3. The pore volume distribution of sodium bicarbonate
determined by the mercury porosimetry method
The EDS spectrum recorded at the point situated on a smooth
grain’s surface of the sample (P3) not covered with pores is the
spectrum of NaHCO3 (Fig.4). Also the composition of the lamellar
and columnar crystals corresponds to NaHCO3. In contrast, the EDS
spectrum from the grain’s surface of sample (P4) with well developed
pores corresponds to the spectrum of Na2CO3 (Fig.5).
Photo 1. Comparison of surface area of unmodified and modified
sodium bicarbonate – magnification 10 000x
The structural analysis using a scanning microscope (Photo 1)
showed that the grains of the samples not subjected to thermal
treatment had a relatively smooth surface and do not exhibit
porosity, characterizing in compact crystalline surface, free from
pores and cracks. It can be proved by obtained values of surface
area. In the sample subjected to grinding, thermal activation
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Fig.4. EDS spectrum of NaHCO3 from the point on a smooth
surface of the grain (100°C)
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area, which is indicative of poor sorption properties of the material
in that form. Activation at the temperature of 100°C (P3) resulted in
developing the surface area values by 10 times (up to approx. 2-3 m2/g).
At the temperature of 250°C (P4), a further increase in surface area
was observed up to approx. 7-9 m2/g. Figure 2 illustrates the dynamics
of development of the sodium bicarbonate sample surface area values,
as determined by the BET method and the mercury porosimetry
method, respectively.
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Literature
Fig.5. EDS spectrum of Na2CO3 from the surface
of the well-developed pore (250°C)
The last stage of assessment of the sorption properties of sodium
products provided for carrying the reactivity tests, which were
performed on the laboratory scale. The reactivity index (Ri) and the
absolute sorption index (Ci) were determined. For the assessment
of reactivity, the Alhstrom scale was used (Tab.3). The test results
are given in Table 4. Taking into account the obtained results, both
– the modified and unmodified sodium bicarbonate were classified as
exquisite sorbents.
Table 3
Alhstrom’s reactivity scale
Rating sorbent
Ri, mol/mol
Ci, g/kg
excellent
< 2.5
> 120
very good
2.5 – 3.0
100 -120
good
3.0 – 4.0
80 - 100
sufficient
4.0 – 5.0
60 - 80
low quality
> 5.0
< 60
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(74), 12-17.
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Translation into English by the Author
Barbara WALAWSKA – Ph.D., graduated from the Faculty of Technology
and Chemical Engineering, Silesian University of Technology (1974). She is an
adjunct in Inorganic Chemistry Division „IChN” in Gliwice of Fertilizers Research Institute. She is interested in inorganic technology and environmental
protection.
e-mail: [email protected], tel.: (32) 231-30-51÷54
Table 4
Sorption properties of sodium bicarbonate
Indicator
Ri , mol/mol
Ci, g/kg
Score Alhstrom’s
scale
P1
1.7
150
Excellent
P2
1.6
157
Excellent
Sample
Conclusions
For the assesment of the reactive properties of sodium bicabonate
used in the processes of dry purification of flue gases, the analysis of
surface structure, including surface area and morfology of pores were
done. The investigations of the simultaneous effect of both: mechnical
(grinding) and thermal activation on the development of surface area
have shown a large increement in surface area and porosity because
of the opening of pores on the grains surface. The structural analyses
of samples, using a scanning microscope confirmed the results of
the surface area examinations conducted by the BET and mercury
porosimetry methods. The surface area measured by these methods
can have different values. The surface area, measured by the mercury
porosimetry method (determining the macropores) obtained after the
complete decomposition of the ground sorbent at the temperature
of 250°C, is larger, approx. 9 m2/g, than the surface area determined
by the BET method (micro- and mesopores), approx. 7 m2/g.
The increase of surface area values was from 0.07 m2/g for non
– modified sorbent to 7-9 m2/g indicated the high reactivity of sodium
bicarbonate. It was confirmed by Alhstrom tests. The reactivity indices
were determined the high sorption potential of both unmodified and
mechanically modified sodium bicarbonate.
1176 •
Arkadiusz SZYMANEK – Sc.D., Professor of Czestochowa University of
Technology, graduated from the Faculty of Civil and Environmental Engineering of Czestochowa University of Technology (1995). He obtained Ph.D.
degree (2000) from the Institute of Thermal technique and Fluid Mechanics
of Wroclaw University of Technology and habilitated in 2009 on Energeticko
Stratelna Fakulta, Zilinska Universita. In 2010 he was an associated professor
on Czestochowa University of Technology. Nowadays He is employed on the
position of professor in the Institute of Advanced Power Technology from
the Faculty of Engineering and Environmental Protection of Czestochowa
University of Technology. Manyfold He has received the number of accolades
from rector of Czestochowa University of Technology. Research topics: power engineering. He is the author and co-author of 2 monographs, 2 didactic
scripts, over 90 original reviews, publications presented during national and
foreign conferences.
e-mail: [email protected], tel.: (34) 325‒09‒33.
Anna PAJDAK – M.Sc., graduated from the Faculty of Engineering and
the Environment Czestochowa University of Technology (2004). She is PhD
student at the Institute of Advanced Energy Technologies of the university. He
is co-author of the publication in the scientific – technical press and poster
presented at the national conference. Interests: chemical engineering.
e-mail: [email protected]
Marzena NOWAK – M.Sc., graduated from Faculty Chemistry, Silesian
University of Technology (2009). She is an technologist in Inorganic Chemistry
Division „IChN” in Gliwice of Fertilizers Research Institute. She is interested
in inorganic technology.
e-mail: [email protected]
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