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Preparation of Strong Alumina Supports for Fluidized Bed Catalysts
G.Poncelet,P.A.Jacobs,P.Grange and B. Delmon (Editors),Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
583
PREPARATION OF STRONG ALUMINA SUPPORTS FOR FLUIDIZED BED CATALYSTS M.N. Shepeleva, R.A. Shkrabina, Z.R. Ismagilov and V.B. Fenelonov Institute of Catalysis, Siberian Branch of the USSR Academy of Sciences, Pr. Akademika Lavrentieva, 5, Novosibirsk 630090 (U.S.S.R.)
SUMMARY Physico-chemical processes o c c u r i n g i n alumina g r a n u l e s moulded by t h e Hydrocarbon-Ammonia G r a n u l a t i o n Method have been i n v e s t i g a t e d ; o p t i m a l r e a l i s a t i o n c o n d i t i o n s o f main t e c h n o l o g i c a l s t e p s have been e s t a b l i s h e d . As a r e s u l t , h i g h l y s t r o n g s p h e r i c alumina g r a n u l e s w i t h developed s p e c i f i c a r e a and p o r o s i t y , a p p l i c a b l e as a s u p p o r t f o r f l u i d i z e d bed c a t a l y s t , have been o b t a i n e d . INTRODUCTION Many modern technological processes are supplied with energy by combustion of organic fuels. Economically effective and ecologically clean installations with
fluidized bed of the catalyst for flameless combustion were
being deve-
loped within the last years. Highly effective apparatuses of this type are the Catalytic Heat Generators (CHG) developed by the Institute of Catalysis (Siberian Branch of the USSR Ac. Sci.)
[1,2]
.
Catalysts in the course of work in CHG are subjected to at least three kinds of influence: chemical, thermal and mechanical. These factors are interconnected, they
complete and magnify the action of each other, destroying catalyst
granules. The structural-mechanical properties of the supported catalysts are known to be determined at great extent by the properties of a support. The object of this work is the investigation of production conditions of spheric
J'
-alumina,whichpossesses ahigh mechanical strength
as
well as a
highly developed surface area and porosity. It is known, that the preparation conditions and granulation method influence
the product characteristics. Recently the Hydrocarbon-Ammonia Granula-
tion Method (HAG method)
possessing high productivity and facility of techno-
logical parameters regulation has received wide distribution. The distinctive feature of the method is the alumina chemical treatment at several stages, which allows to alter the texture of the initial substance in the necessary direction. In this respect
the HAG method has advantages in comparison with the
widely used method of mechanical moulding.
584
EXPERIMENTAL METHODS Argon thermal desorption was used for the determination of the specific area
A. The pore volume V and the pore size (r, nm) distribution were determined by P
mercury porometer "Porosizer-2300" from the "Micromeretiks" company (USA). Searching for a test of the granules mechanical properties
shows the following pe-
culiarities. Granules work in the CHG fluidized bed is complicated by chemical and thermal factors; granules are subjected not only to the surface friction, but at great extent to impact loadings, and gradual increase of the internal structural macro- and microtensions. The crushing test comprising granules press between two parallel plates has given satisfactory correlation with real speed of granules destruction in the CHG. Therefore, the sample strength was characterized by the average S minimal S . and maximal S values of the individual av' min max granules crushing pressure S. (MPa) in a series of 30 granules. Smin and Smax were calculated from 5 minimal and maximal values of Si. The operating time of a catalyst support has been established to be more than 0.5 year for samples with S 3 25 MPa and S m i n 3 7 MPa. av
RESULTS AND DISCUSSION Raw Material Preparation Pseudoboehmite aluminium hydroxides are usually used as a raw material in the HAG method. We have previously investigated the aluminium hydroxides obtained
by gibbsite dissolution in the alkaly and deposition at pH 8.5-9.0 by nitric acid [5J It was shown in [5-71 that the conditions of aluminium hydroxide
.
synthesis determine the morphology and the structural type of the particles-", as well as the nature of binding between primary particles. A s it was shown by physico-chemical methods, the size of particles obtained from hydroxides synthesized at T 6 4OoC does not exceed 10 nm and the size of the secondary crumbly enough aggregates can exceed 100 nm. The bonds between the particles in such aggregates are mostly of Van-der-Vaals nature. Therefore, the acid treatment at the initial stage of peptization leads to the formation of a disordered system of fine needles and fibres. The dispersion of such mass in ammonia solution leads to rapid coagulation, fine particles of aluminium hydroxide (-
3-4 nm)
being densely packed. Granules of aluminium hydroxide formed in these conditions have fine porous monodisperse
A Z= 250 m2/g, Vp
*
=
structure. After calcination, the alumina with 3 0.3-0.4 cm / g , Sav 3 25 MPa is obtained.
Preparation of these hydroxides is connected,however, with certain difficul-
'According to Rebinder [ 8 ] , structures are divided into two main types: coagulative, in which ion-solvate shell on the particle contact places is preserved and crystallizative with point or phase contact between primary particles.
585
ties, for
example, at the stages of washing off alkaline metal
ions and fil-
tration. That is the reason that monodisperse hydroxides are not widely used. Hydroxides obtained by precipitation at T f 4OoC o r mixtures of precipitates obtained at high and low temperatures are used in many researches
[9,10]
.
These hydroxides have contacts between the primary particles of both types; an extent of aggregates packing changes at the next technological stages is determined by their number ratio. In the systems with phase contacts
between the
particles, the peptization does not lead to the aggregates destruction. Macropores preservation between the remained aggregates leads to the formation of lowstrength alumina granules. Because of this, from each hydroxide obtained as in
[9,lO]
, one can prepare alumina granules with strength not exceeding a certain
limiting value, unless special technological methods (e.g.,
high temperature
A s is shown in [5] , for the usually applied aluminium hydroxides, the values of Sav of the obtained gra-
calcination, additive incorporation, etc.) are used.
nules do not exceed 12 MPa. Application of these alumina granules in CHG is not effective, therefore we tried to change the structural type of hydroxide in order to strengthen the final alumina granules. Mechanical activation is known to be one of the ways to increase the solid reactivity. The object of o u r investigation was aluminium hydroxide containing equal mixture of deposits obtained by interaction
between the sodium aluminate and nit-
ric acid at pH 8.7 and temperatures of 20 and 100°C. The phase composition of this hydroxide corresponds to the pseudoboehmite with the range of coherent dissipation
12 nm. Specific area and total pore volume of the sample dried at llO°C are 230 m2 /g and 0.27 cm3 / g respectively, 12% of total pore volume is the
volume of macropores (r > l o 0 nm). The radius distributions of pore volume of aluminium hydroxide before
and
after the treatment in various mills are shown in Fig. 1. It is seen that the treatment in a disk mill does not allow to destroy the secondary aggregates of hydroxide. Large pores are also preserved in the final alumina. The macroporosity of alumina could be removed by grinding intensification, which also increases Sav and Smin substantially, rises the bulk density
and slightly decreasesthe
surface area A ( s e e Table in Fig. 1 ) . We have given in [11] the results of physico-chemical investigation of aluminium hydroxide grinding products. It was shown that the main result of grinding is connected not only with the destruction of the initial aggregates of aluminium hydroxyde, but also with an exchange of strong phase contacts by weak coagulative contacts. This does not practically change the structure of the primary partjkles. Peptization Stage Liquid mass capable t o flow freely from the moulding device spinnerets, is
586
0.4
~~
No1 2
0.3
3 4
Q,
\
d, 9 mkm -
>I00
25 10
-_s,_arPa--av
min
4
2 2 12 17
5 31 35
A, g/ om”
A,
18 /g
0.69 0.70
250 270 240 220
0.84 0.84
I+)
E0
>”
0.2
0.I
Fig. 1. Pore volume radius distribution for initial (1) and grinded (2-4) aluminium hydroxides. Mills: 2 - disk; 3 - ball; 4 - jet. The most abundant particle size, d mkm: 2 - over 100; 3 - 25; 4 - 10.
P’
obtained at this stage by acid treatment of aluminium hydroxide. It should be noted, that basic aluminium salts show thixotropic properties and, therefore, it is necessary to adjust the mass preparation conditions to establish thixotropic setting time
(s) long enough for a free mass flow along the pipelines.
The mass rheological characteristics and properties of the final alumina granules are strongly dependent on the mass preparation conditions, particularly, on the mass maturation time
(m).
During the mass maturation the he-
terogeneous system obtained as a result of the component mixing, becomes homogeneous due to the precipitate swelling. ‘t‘ (m) is influenced by the temperature in reactor
- plastificator.
It was shown, that at T
=
+
20-25OC L (m) makes
up 1.0-1.5 days. The temperature rise leads to a substantial speeding up of the mass maturation ( such masses
(m)
=
4-10 hrs). However, the alumina granules obtained from
possess not only the increased strength (Sav = 30-40 MPa) but also
fine porosity, which complicates the drying and calcination stages and decrease s the granules water stability. r-
L
If M(a)
(m) also depends on the amount of acid added, M(a),
g-m/g-m of alumina. /
equals to 0.06-0.08and the solid phase concentration is 25-30%, L (m)
makes up 1.0-1.5 days as necessary. The decrease of M(a) leads to the increase of
(m) and vice versa. the time of mass thixotropic setting. A s it
Besides, M(a) influences was shown in our experiments,
5
(s) should be within the range of 15-60 min.
587
We have established the dependence of the optimal value of M(a)
<
z
at which opt (m) are within the required limits on the aluminium hydroxide pro-
(s) and perties and mass preparation conditions.
The formation of the stable disperse
system from the structurated precipita-
te or gel is known to be caused by the formation of the double electric layer on particles surface.
Let us consider now the peptizator distribution in
the bulk of aluminium hydroxide. The concentration of peptizator added after mixing with aluminium hydroxide is:
3 V are mass (kg) and volume (m ) of the peptizator, respectively; pep' Pep Wo is moisture content evaluated by drying at l l O ° C (kg H20 / kg A1203); m is mass concentration of A1 0 in hydroxide (kg); is liquid phase density (kg/ 2 3 3 m ). The mass balance equation calculated on the basis of A1203 is the following:
where m
2 is specific sorption of substance - peptizator (kg/m ) ; A is speci2 fic area of A 1 0 (m / g ) ; C is equilibrium peptizator concentration in the in2 3 P 3 termicellar liquid of hydroxide (kg/m ). In the left part of eq. (2) the first component expresses the liquid consumption for sorption and chemical interaction with aluminium hydroxide particles, the second component - for creation of equilibrium concentration of pepti-
where
d
zator in intermicellar liquid. The value of M(a ) can be expressed as follows opt
where M and MA1203 Pep vely
.
are molecular masses of peptizator and A1203, respecti-
The formula ( 3 ) can be simplified by taking into consideration the following: (1) while using the nitric acid, the ratio MAl / Mpep is equal to 1.619; (2) A values of oxides and hydroxides of pseudoboeh&& structure (A ) are close h enough; (3) the value of V /m is small. The formula (3) after simplification Pep looks like:
588
A
1;
”
E
4
M (a,opt)
0.9
L)
1
7 0.84
24
- 250 I
I
M (ai
0.08
0.06
A
Fig. 2. Dependence of the average strength Sav, the bulk density and the specific area A of alumina granules on the M(a) value for the plastificated mass with alumina concentration 28%
According to the experimental data, 3 kg/m
.
= (4
‘I 1)
kg/m2; C P
=
(14 + - 2)
S o , if the aluminium hydroxide is treated to M(a) = M(a ) , the time needed opt for mass maturation and the time of mass thixotropic setting are achieved,
which establishes high mechanical properties of alumina
granules. As it can be
seen from Fig. 2, the acid treatment of hydroxide up to M(a the alumina
granules characteristics.
opt
) d o e s not change
Sphere Formation and Coagulation The preparation
liquid
of spheric alumina
granules occurs in the column with two
layers: the upper layer is hydrocarbon, the lower one is a coagulant
SO-
lution. In the upper layer of hydrophobic liquid the mass drop is subjected to the surface tension forces which tighten it into the sphere. The granules hardening proceeds in the course of a coagulant diffusion into the volume of sphere and structure formation. The ammonia solution is usually used as a coagulant. For high strength alumina granules preparation it is necessary to establish the complete interaction between mass and coagulant. It was shown that while mould0.1 ing of masses with alumina concentration being less than 25% and M(a)&
589
granules hardening is completed in 30-40 s in 16-19% ammonia solution. If M(a)
is increased, it is necessary to increase the contact time up to 60-80 s.
and to rise the concentration of the ammonia solution. Granules Thermal Treatment The granules drying was carried out by several ways: in air, in a drying box, in aggregates with moving ribbon and heaters over it. The experimental results are presented in Table 1 .
TABLE 1 The influence of drying conditions on alumina Drving of Granules <
-
T (OC)
Method ~~
z (h)
Alumina A(mL/g)
granules characteristics Characteristics
Vp(cm3/g)
Sav(MPa) Smin(MPa)
Smax(MPa)
~~~
in air 20 in a drying 110 box under heat200 ers the same 40-200
48
240
0.31
41.8
20.7
59.2
4
240
0.27
23.5
6.8
38.9
0.5 0.5
2 40 240
0.25 0.28
22.8 31.3
6.0 12.8
39.3 45.3
It is seen that the drying mode does not practically affect the specific area A and pore volume V
but it influences strongly the granule strength. This signiP' ficant strength change at the unchanged porosity could be probably explained
with an increase of residual microtensions with the rise of drying speed. So rapid drying under heaters at T >lOO°C
.
leads to the decrease of S The strongav est granules are obtained while drying in air. This widely recommended method
cannot be applied to continuous technology. It was found that lowering of the initial temperature and its gradual increase lead
to the increase of S and av
'mine strengthens the granules due to the The thermal treatment at T >200°C transfer of coagulative structure to the crystallization one. The calcination of granules with bidisperse
or wide porous structure does not present
great
difficulties. Calcination of fine porous samples is more complicated. It was shown that the reduction of temperature rise and granule bed height
as well as
the increase of calcination time and temperature (up to 75OoC) facilitate the slow moisture removal, granules shrinking, fine pore sintering. As a result, simultaneous rise of V
P
and Sav is observed, the specific area slightly drops.
CONCLUSIONS The investigation of the physico-chemical processes taking place at the main
590
stages of alumina moulding by hydrocarbon-ammonia method enables us to develop the scientific background of preparation of granules with different characteristics, to establish the optimal conditions for each stage, to improve the process apparatus. It was shown that the preparation of strong alumina
gra-
nules requires the directed conducting of all the technological steps. Direct interrelation between the properties of the initial aluminium hydroxides and structural-mechanical properties of the final alumina granules was established. Preparation of fibrous pseudoboehmite in mild precipitation conditions allows to prepare from it the fine porous strong alumina granules. Formation of pseudoboehmite in the form of well crystallized needles and plates unable to react with acid-peptizator requires the introduction of the intensive grinding into the technological process. The investigation of peptization process shows that this stage breaks ground granules. So-
for textural and mechanical characteristics of the final alumina
lid phase concentration, nature and amount of acid-peptizator determine the rheological properties of mass and extent of dispersion of secondary aggregates of aluminium hydroxide. The properties of substance-coagulant and the residential time of granules in ammonia solution influence the completion of coagulative hardening. The conditions of granule thermal treatment allow to increase the amount of contacts between particles and aggregates in granule, strengthen these contacts due to transformation of coagulative type into the phase one. Alumina
granules prepared by the developed method posess the specific area
and porosity necessary for the incorporation of the required amount of the active component and can be applied as a support for fluidized bed catalysts. REFERENCES 1
G.K. Boreskov, E.A. Levitskii and Z.R. Ismagilov. Zh. Vsesouznogo Khim. Obshchestva, 29 (1984) 379-385. 2 Z.R. Ismagilov, in: D.N. Saraf and D. Kunzry (eds.) Proc. Intern. Conf. on Advances in Chem. Eng., Kanpur, January 4-6, 1989, Tata McGrow Hill Publ. Co. Ltd, New Delhi, 1989, pp. 310-315. 3 USA Patent 2805206 (1953). 4 Ya.R. Katsobashvili and N.S. Kurkova, Zh. Priklad. Khim. 39 (1966) 24242429. 5 M.N. Shepeleva, V.B. Fenelonov, R.A. Shkrabina and E.M. Moroz, Kinet. Katal. 27 (1986) 1202-1207. 6 M.N. Shepeleva, R.A. Shkrabina, L.G. Okkel, V.I. Zaikovskii, V.B. Fenelonov and Z.R. Ismagilov, Kinet. Katal. 29 (1988) 195-200. 7 Z.R. Ismagilov, M.N. Shepeleva, R.A. Shkrabina and V.B. Fenelonov, Appl. Catal (in press). 8 P.A. Rebinder, Physical and Chemical Mechanisms of Dispersed Structures, Nauka, Moskva, 1966, p . 3 . 9 M.D. Efros, A.V. Tabulina and N.V. Ermolenko, Izv. Akad. Nauk BSSR, 1 (1971) 9-13. 10 E.A. Vlasov, I.A. Rizak and E.A. Levitskii, Kinet. Katal., 5 (1972) 13111314. 11 M.N. Shepeleva, Z.R. Ismagilov, R.A. Shkrabina, E.M. Moroz, V.B. Fenelonov and V.I. Zaikovskii, Kinet. Katal.(in press).
583
PREPARATION OF STRONG ALUMINA SUPPORTS FOR FLUIDIZED BED CATALYSTS M.N. Shepeleva, R.A. Shkrabina, Z.R. Ismagilov and V.B. Fenelonov Institute of Catalysis, Siberian Branch of the USSR Academy of Sciences, Pr. Akademika Lavrentieva, 5, Novosibirsk 630090 (U.S.S.R.)
SUMMARY Physico-chemical processes o c c u r i n g i n alumina g r a n u l e s moulded by t h e Hydrocarbon-Ammonia G r a n u l a t i o n Method have been i n v e s t i g a t e d ; o p t i m a l r e a l i s a t i o n c o n d i t i o n s o f main t e c h n o l o g i c a l s t e p s have been e s t a b l i s h e d . As a r e s u l t , h i g h l y s t r o n g s p h e r i c alumina g r a n u l e s w i t h developed s p e c i f i c a r e a and p o r o s i t y , a p p l i c a b l e as a s u p p o r t f o r f l u i d i z e d bed c a t a l y s t , have been o b t a i n e d . INTRODUCTION Many modern technological processes are supplied with energy by combustion of organic fuels. Economically effective and ecologically clean installations with
fluidized bed of the catalyst for flameless combustion were
being deve-
loped within the last years. Highly effective apparatuses of this type are the Catalytic Heat Generators (CHG) developed by the Institute of Catalysis (Siberian Branch of the USSR Ac. Sci.)
[1,2]
.
Catalysts in the course of work in CHG are subjected to at least three kinds of influence: chemical, thermal and mechanical. These factors are interconnected, they
complete and magnify the action of each other, destroying catalyst
granules. The structural-mechanical properties of the supported catalysts are known to be determined at great extent by the properties of a support. The object of this work is the investigation of production conditions of spheric
J'
-alumina,whichpossesses ahigh mechanical strength
as
well as a
highly developed surface area and porosity. It is known, that the preparation conditions and granulation method influence
the product characteristics. Recently the Hydrocarbon-Ammonia Granula-
tion Method (HAG method)
possessing high productivity and facility of techno-
logical parameters regulation has received wide distribution. The distinctive feature of the method is the alumina chemical treatment at several stages, which allows to alter the texture of the initial substance in the necessary direction. In this respect
the HAG method has advantages in comparison with the
widely used method of mechanical moulding.
584
EXPERIMENTAL METHODS Argon thermal desorption was used for the determination of the specific area
A. The pore volume V and the pore size (r, nm) distribution were determined by P
mercury porometer "Porosizer-2300" from the "Micromeretiks" company (USA). Searching for a test of the granules mechanical properties
shows the following pe-
culiarities. Granules work in the CHG fluidized bed is complicated by chemical and thermal factors; granules are subjected not only to the surface friction, but at great extent to impact loadings, and gradual increase of the internal structural macro- and microtensions. The crushing test comprising granules press between two parallel plates has given satisfactory correlation with real speed of granules destruction in the CHG. Therefore, the sample strength was characterized by the average S minimal S . and maximal S values of the individual av' min max granules crushing pressure S. (MPa) in a series of 30 granules. Smin and Smax were calculated from 5 minimal and maximal values of Si. The operating time of a catalyst support has been established to be more than 0.5 year for samples with S 3 25 MPa and S m i n 3 7 MPa. av
RESULTS AND DISCUSSION Raw Material Preparation Pseudoboehmite aluminium hydroxides are usually used as a raw material in the HAG method. We have previously investigated the aluminium hydroxides obtained
by gibbsite dissolution in the alkaly and deposition at pH 8.5-9.0 by nitric acid [5J It was shown in [5-71 that the conditions of aluminium hydroxide
.
synthesis determine the morphology and the structural type of the particles-", as well as the nature of binding between primary particles. A s it was shown by physico-chemical methods, the size of particles obtained from hydroxides synthesized at T 6 4OoC does not exceed 10 nm and the size of the secondary crumbly enough aggregates can exceed 100 nm. The bonds between the particles in such aggregates are mostly of Van-der-Vaals nature. Therefore, the acid treatment at the initial stage of peptization leads to the formation of a disordered system of fine needles and fibres. The dispersion of such mass in ammonia solution leads to rapid coagulation, fine particles of aluminium hydroxide (-
3-4 nm)
being densely packed. Granules of aluminium hydroxide formed in these conditions have fine porous monodisperse
A Z= 250 m2/g, Vp
*
=
structure. After calcination, the alumina with 3 0.3-0.4 cm / g , Sav 3 25 MPa is obtained.
Preparation of these hydroxides is connected,however, with certain difficul-
'According to Rebinder [ 8 ] , structures are divided into two main types: coagulative, in which ion-solvate shell on the particle contact places is preserved and crystallizative with point or phase contact between primary particles.
585
ties, for
example, at the stages of washing off alkaline metal
ions and fil-
tration. That is the reason that monodisperse hydroxides are not widely used. Hydroxides obtained by precipitation at T f 4OoC o r mixtures of precipitates obtained at high and low temperatures are used in many researches
[9,10]
.
These hydroxides have contacts between the primary particles of both types; an extent of aggregates packing changes at the next technological stages is determined by their number ratio. In the systems with phase contacts
between the
particles, the peptization does not lead to the aggregates destruction. Macropores preservation between the remained aggregates leads to the formation of lowstrength alumina granules. Because of this, from each hydroxide obtained as in
[9,lO]
, one can prepare alumina granules with strength not exceeding a certain
limiting value, unless special technological methods (e.g.,
high temperature
A s is shown in [5] , for the usually applied aluminium hydroxides, the values of Sav of the obtained gra-
calcination, additive incorporation, etc.) are used.
nules do not exceed 12 MPa. Application of these alumina granules in CHG is not effective, therefore we tried to change the structural type of hydroxide in order to strengthen the final alumina granules. Mechanical activation is known to be one of the ways to increase the solid reactivity. The object of o u r investigation was aluminium hydroxide containing equal mixture of deposits obtained by interaction
between the sodium aluminate and nit-
ric acid at pH 8.7 and temperatures of 20 and 100°C. The phase composition of this hydroxide corresponds to the pseudoboehmite with the range of coherent dissipation
12 nm. Specific area and total pore volume of the sample dried at llO°C are 230 m2 /g and 0.27 cm3 / g respectively, 12% of total pore volume is the
volume of macropores (r > l o 0 nm). The radius distributions of pore volume of aluminium hydroxide before
and
after the treatment in various mills are shown in Fig. 1. It is seen that the treatment in a disk mill does not allow to destroy the secondary aggregates of hydroxide. Large pores are also preserved in the final alumina. The macroporosity of alumina could be removed by grinding intensification, which also increases Sav and Smin substantially, rises the bulk density
and slightly decreasesthe
surface area A ( s e e Table in Fig. 1 ) . We have given in [11] the results of physico-chemical investigation of aluminium hydroxide grinding products. It was shown that the main result of grinding is connected not only with the destruction of the initial aggregates of aluminium hydroxyde, but also with an exchange of strong phase contacts by weak coagulative contacts. This does not practically change the structure of the primary partjkles. Peptization Stage Liquid mass capable t o flow freely from the moulding device spinnerets, is
586
0.4
~~
No1 2
0.3
3 4
Q,
\
d, 9 mkm -
>I00
25 10
-_s,_arPa--av
min
4
2 2 12 17
5 31 35
A, g/ om”
A,
18 /g
0.69 0.70
250 270 240 220
0.84 0.84
I+)
E0
>”
0.2
0.I
Fig. 1. Pore volume radius distribution for initial (1) and grinded (2-4) aluminium hydroxides. Mills: 2 - disk; 3 - ball; 4 - jet. The most abundant particle size, d mkm: 2 - over 100; 3 - 25; 4 - 10.
P’
obtained at this stage by acid treatment of aluminium hydroxide. It should be noted, that basic aluminium salts show thixotropic properties and, therefore, it is necessary to adjust the mass preparation conditions to establish thixotropic setting time
(s) long enough for a free mass flow along the pipelines.
The mass rheological characteristics and properties of the final alumina granules are strongly dependent on the mass preparation conditions, particularly, on the mass maturation time
(m).
During the mass maturation the he-
terogeneous system obtained as a result of the component mixing, becomes homogeneous due to the precipitate swelling. ‘t‘ (m) is influenced by the temperature in reactor
- plastificator.
It was shown, that at T
=
+
20-25OC L (m) makes
up 1.0-1.5 days. The temperature rise leads to a substantial speeding up of the mass maturation ( such masses
(m)
=
4-10 hrs). However, the alumina granules obtained from
possess not only the increased strength (Sav = 30-40 MPa) but also
fine porosity, which complicates the drying and calcination stages and decrease s the granules water stability. r-
L
If M(a)
(m) also depends on the amount of acid added, M(a),
g-m/g-m of alumina. /
equals to 0.06-0.08and the solid phase concentration is 25-30%, L (m)
makes up 1.0-1.5 days as necessary. The decrease of M(a) leads to the increase of
(m) and vice versa. the time of mass thixotropic setting. A s it
Besides, M(a) influences was shown in our experiments,
5
(s) should be within the range of 15-60 min.
587
We have established the dependence of the optimal value of M(a)
<
z
at which opt (m) are within the required limits on the aluminium hydroxide pro-
(s) and perties and mass preparation conditions.
The formation of the stable disperse
system from the structurated precipita-
te or gel is known to be caused by the formation of the double electric layer on particles surface.
Let us consider now the peptizator distribution in
the bulk of aluminium hydroxide. The concentration of peptizator added after mixing with aluminium hydroxide is:
3 V are mass (kg) and volume (m ) of the peptizator, respectively; pep' Pep Wo is moisture content evaluated by drying at l l O ° C (kg H20 / kg A1203); m is mass concentration of A1 0 in hydroxide (kg); is liquid phase density (kg/ 2 3 3 m ). The mass balance equation calculated on the basis of A1203 is the following:
where m
2 is specific sorption of substance - peptizator (kg/m ) ; A is speci2 fic area of A 1 0 (m / g ) ; C is equilibrium peptizator concentration in the in2 3 P 3 termicellar liquid of hydroxide (kg/m ). In the left part of eq. (2) the first component expresses the liquid consumption for sorption and chemical interaction with aluminium hydroxide particles, the second component - for creation of equilibrium concentration of pepti-
where
d
zator in intermicellar liquid. The value of M(a ) can be expressed as follows opt
where M and MA1203 Pep vely
.
are molecular masses of peptizator and A1203, respecti-
The formula ( 3 ) can be simplified by taking into consideration the following: (1) while using the nitric acid, the ratio MAl / Mpep is equal to 1.619; (2) A values of oxides and hydroxides of pseudoboeh&& structure (A ) are close h enough; (3) the value of V /m is small. The formula (3) after simplification Pep looks like:
588
A
1;
”
E
4
M (a,opt)
0.9
L)
1
7 0.84
24
- 250 I
I
M (ai
0.08
0.06
A
Fig. 2. Dependence of the average strength Sav, the bulk density and the specific area A of alumina granules on the M(a) value for the plastificated mass with alumina concentration 28%
According to the experimental data, 3 kg/m
.
= (4
‘I 1)
kg/m2; C P
=
(14 + - 2)
S o , if the aluminium hydroxide is treated to M(a) = M(a ) , the time needed opt for mass maturation and the time of mass thixotropic setting are achieved,
which establishes high mechanical properties of alumina
granules. As it can be
seen from Fig. 2, the acid treatment of hydroxide up to M(a the alumina
granules characteristics.
opt
) d o e s not change
Sphere Formation and Coagulation The preparation
liquid
of spheric alumina
granules occurs in the column with two
layers: the upper layer is hydrocarbon, the lower one is a coagulant
SO-
lution. In the upper layer of hydrophobic liquid the mass drop is subjected to the surface tension forces which tighten it into the sphere. The granules hardening proceeds in the course of a coagulant diffusion into the volume of sphere and structure formation. The ammonia solution is usually used as a coagulant. For high strength alumina granules preparation it is necessary to establish the complete interaction between mass and coagulant. It was shown that while mould0.1 ing of masses with alumina concentration being less than 25% and M(a)&
589
granules hardening is completed in 30-40 s in 16-19% ammonia solution. If M(a)
is increased, it is necessary to increase the contact time up to 60-80 s.
and to rise the concentration of the ammonia solution. Granules Thermal Treatment The granules drying was carried out by several ways: in air, in a drying box, in aggregates with moving ribbon and heaters over it. The experimental results are presented in Table 1 .
TABLE 1 The influence of drying conditions on alumina Drving of Granules <
-
T (OC)
Method ~~
z (h)
Alumina A(mL/g)
granules characteristics Characteristics
Vp(cm3/g)
Sav(MPa) Smin(MPa)
Smax(MPa)
~~~
in air 20 in a drying 110 box under heat200 ers the same 40-200
48
240
0.31
41.8
20.7
59.2
4
240
0.27
23.5
6.8
38.9
0.5 0.5
2 40 240
0.25 0.28
22.8 31.3
6.0 12.8
39.3 45.3
It is seen that the drying mode does not practically affect the specific area A and pore volume V
but it influences strongly the granule strength. This signiP' ficant strength change at the unchanged porosity could be probably explained
with an increase of residual microtensions with the rise of drying speed. So rapid drying under heaters at T >lOO°C
.
leads to the decrease of S The strongav est granules are obtained while drying in air. This widely recommended method
cannot be applied to continuous technology. It was found that lowering of the initial temperature and its gradual increase lead
to the increase of S and av
'mine strengthens the granules due to the The thermal treatment at T >200°C transfer of coagulative structure to the crystallization one. The calcination of granules with bidisperse
or wide porous structure does not present
great
difficulties. Calcination of fine porous samples is more complicated. It was shown that the reduction of temperature rise and granule bed height
as well as
the increase of calcination time and temperature (up to 75OoC) facilitate the slow moisture removal, granules shrinking, fine pore sintering. As a result, simultaneous rise of V
P
and Sav is observed, the specific area slightly drops.
CONCLUSIONS The investigation of the physico-chemical processes taking place at the main
590
stages of alumina moulding by hydrocarbon-ammonia method enables us to develop the scientific background of preparation of granules with different characteristics, to establish the optimal conditions for each stage, to improve the process apparatus. It was shown that the preparation of strong alumina
gra-
nules requires the directed conducting of all the technological steps. Direct interrelation between the properties of the initial aluminium hydroxides and structural-mechanical properties of the final alumina granules was established. Preparation of fibrous pseudoboehmite in mild precipitation conditions allows to prepare from it the fine porous strong alumina granules. Formation of pseudoboehmite in the form of well crystallized needles and plates unable to react with acid-peptizator requires the introduction of the intensive grinding into the technological process. The investigation of peptization process shows that this stage breaks ground granules. So-
for textural and mechanical characteristics of the final alumina
lid phase concentration, nature and amount of acid-peptizator determine the rheological properties of mass and extent of dispersion of secondary aggregates of aluminium hydroxide. The properties of substance-coagulant and the residential time of granules in ammonia solution influence the completion of coagulative hardening. The conditions of granule thermal treatment allow to increase the amount of contacts between particles and aggregates in granule, strengthen these contacts due to transformation of coagulative type into the phase one. Alumina
granules prepared by the developed method posess the specific area
and porosity necessary for the incorporation of the required amount of the active component and can be applied as a support for fluidized bed catalysts. REFERENCES 1
G.K. Boreskov, E.A. Levitskii and Z.R. Ismagilov. Zh. Vsesouznogo Khim. Obshchestva, 29 (1984) 379-385. 2 Z.R. Ismagilov, in: D.N. Saraf and D. Kunzry (eds.) Proc. Intern. Conf. on Advances in Chem. Eng., Kanpur, January 4-6, 1989, Tata McGrow Hill Publ. Co. Ltd, New Delhi, 1989, pp. 310-315. 3 USA Patent 2805206 (1953). 4 Ya.R. Katsobashvili and N.S. Kurkova, Zh. Priklad. Khim. 39 (1966) 24242429. 5 M.N. Shepeleva, V.B. Fenelonov, R.A. Shkrabina and E.M. Moroz, Kinet. Katal. 27 (1986) 1202-1207. 6 M.N. Shepeleva, R.A. Shkrabina, L.G. Okkel, V.I. Zaikovskii, V.B. Fenelonov and Z.R. Ismagilov, Kinet. Katal. 29 (1988) 195-200. 7 Z.R. Ismagilov, M.N. Shepeleva, R.A. Shkrabina and V.B. Fenelonov, Appl. Catal (in press). 8 P.A. Rebinder, Physical and Chemical Mechanisms of Dispersed Structures, Nauka, Moskva, 1966, p . 3 . 9 M.D. Efros, A.V. Tabulina and N.V. Ermolenko, Izv. Akad. Nauk BSSR, 1 (1971) 9-13. 10 E.A. Vlasov, I.A. Rizak and E.A. Levitskii, Kinet. Katal., 5 (1972) 13111314. 11 M.N. Shepeleva, Z.R. Ismagilov, R.A. Shkrabina, E.M. Moroz, V.B. Fenelonov and V.I. Zaikovskii, Kinet. Katal.(in press).