- Email: [email protected]
The role of water on the attenuation of coke deactivation of a SAPO-34 catalyst in the transformation of methanol into olefins
Catalyst Deactivation 1999 B. Delmon and G.F. Froment (Editors) o 1999 Elsevier Science B.V. All rights reserved.
129
The Role o f Water on the A t t e n u a t i o n of Coke D e a c t i v a t i o n of a SAPO-34 Catalyst in the T r a n s f o r m a t i o n of M e t h a n o l into Olefins
A.G. Gayubo, A.T. Aguayo, A.E. Sfinchez del Campo, P.L. Benito and J. Bilbao Departamento de Ingenieria Qulmica, Universidad del Pals Vasco, Apartado 644, 48080 Bilbao, Spain. Telephone: 34-4-6012000, Fax: 34-4-4648500, E-mail: iqp [email protected]
The effect of water concentration on the reaction medium in order to attenuate coke formation on a SAPO-34 has been studied in the transformation of methanol into olefins. From the results obtained in an integral reactor for different values of space time and water concentration in the feed, a kinetic model for deactivation by coking on a SAPO-34 in the 623-748 K range has been proposed. The model takes into account the effect on the deactivation of organic components and water in the reaction medium.
1. INTRODUCTION The silicoaluminophosphate SAPO-34 gives up to 90% selectivity to light olefins in the transformation of methanol (selectivity to ethene and propene is about 60%) in the 623-748 K range. Consequently, this is an encouraging alternative for the use of HZSM-5 zeolites in the MTO process which, nevertheless, has the counterpart of its fast deactivation by coke. Several authors [1-3] have already proven that deactivation is attenuated by feeding water with methanol. The attenuation of coke deactivation by water had already been observed for HZSM-5 zeolite when it was used in the MTG [4] and MTO processes [5]. This effect was also observed in other processes on acidic catalysts, such as in the transformation of pyrolysis products obtained from wood and seed-oil to hydrocarbons on HZSM-5 zeolites [6] and on hybrid catalysts of silica/aluminaHY zeolite and of silica/alumina-HZSM-5 zeolite [7]. Furthermore, the interest in the use of water in the MTO process on SAPO-34 is supported by two circumstances: 1) the fact that water affects slightly to the kinetics of the main reaction [8]; 2) the high hydrothermal stability of the SAPO-34 [9], which is much higher than that of the HZSM-5 zeolite [10,11]. In this paper, the role of water on the deactivation of a SAPO-34 has been studied with the aim of obtaining a kinetic model which accounts for the presence of water. This kinetic model will be required for the optimization of the industrial
130 operation, which is carried out in a fluidized bed reactor with catalyst circulation [12].
2. E X P E R I M E N T A L
The catalyst used in the reactor is prepared by agglomerating the SAPO-34 (of composition H0.o9(Sio.08Alo.51Po.4)O2) (25 wt%) with bentonite (Exaloid) (30 wt%), using fused alumina (Martinswerk) as inert charge (45 wt%). The catalyst is calcined at 848 K for 4 h under N2 stream. Preparation and properties of the catalyst have been detailed in previous papers [8,13]. The kinetic data have been obtained in an isothermal fixed bed reactor, under the following conditions: temperature, 623, 648, 673, 698 and 748 K; space time, between 0.01 and 0.44 (g of catalyst) h (g of methanol) -1 (by changing the catalyst mass); time on stream, 1 h; water/methanol ratio in the feed: 0, 1 and 3, in weight; total flow of liquid feed: 0.40 cm 3 min'l; catalyst particle diameter, between 0.1 and 0.3 mm. The feed-reaction-analysis system (Hewlett-Packard 5890 Series II chromatograph) is controlled by a computer routine. Coke deposited on the catalyst was determined by thermogravimetry (Setaram TAG 24), by combustion with air. Previously, the deactivated bed obtained subsequently to the reaction was subjected to a sweeping treatment with He (100 cm 3 min -1) for 30 min at the reaction temperature.
3. R E S U L T S 3.1. W a t e r e f f e c t o n c o k e d e a c t i v a t i o n
Under the same reaction conditions (methanol concentration in the feed, temperature and space time), coke deposition is higher on the SAPO-34 than in the HZSM-5 zeolite, which is a consequence of the combination of two characteristics of the crystalline structure of the SAPO-34: small diameter of the internal channels (4.4x3.1 A), which may be easily blocked, and cavities constituted by intersections between channels (10x6.7 A), which allow for the arrangement of higher molecular weight structures. The extraction with acetone subsequent to catalyst destruction with an aqueous solution of HF (50 wt%), by following the methodology proposed by Magnoux et al. [14], only removes 10 wt% of the total coke amount. The extracted fraction, once analysed by GC-MS (Hewlett Packard 5989B), contains phenantrene and pyrene, apart from paraffins and naphthenes. A subsequent extraction with pyridine is of minor efficiency and allows for proving the presence of polyaromatics of up to 4 rings, which are derivatives of pyrene, with a molecular weight of up to 400. These results show the impossibility of analysing the coke retained within the channels and cavities of the SAPO-34 by desorption and that coke is more developed and
131
with a higher molecular weight than that deposited within the HZSM-5 zeolites under similar reaction conditions [15]. Coke deposition originates a rapid deterioration of acidity as is ---,r~ ! observed in Figure 1, which 225 = 1 min corresponds to the following 45 min 200 reaction conditions: water in the 25 min feed, 0; temperature, 673 K; space ~175 time, 0.05 (g of catalyst) h (g of 0 methanol)" 1. As is observed, E 150 -,j strongly acidic sites are mainly affected. In a 25 min reaction, sites 125 with an acidity strength above 175 kJ (mol of NH3) -1 disappear. As in 100 this situation the catalyst is 0 0.03 0.06 0.09 0.12 0.15 mmol NH3/g completely inactive for olefin production, a minimum acidity Figure 1. Evolution of catalyst acidity strength within the range between strength distribution with time on 175 and 200 kJ (mol of NH3) -1 is stream. required for this reaction. As coke deposition is sensitive to water content in the reaction medium, it is attenuated under the reaction conditions in which methanol conversion increases and consequently, water concentration increases. This situation takes place as reaction temperature is increased, Figure 2a, and space time is increased, Figure 2b, for constant remaining operating conditions. 11
8
673 K
0.05 gcatalysthlgMeOH 10
=<
7
d6 o r
8
o (J
5
a
b ,
610
i
630
I
I
650
I
I
670
Temperature, K
I
I
690
,I,
..
710
0
i
i
0.1
,t
i
0.2
,i
,
i
.....
0.3
i,,
0.4
W/FMo, gcatalysth/gMeOH
Figure 2. Effect of reaction temperature (a) and of space time (b) on catalyst coke content. Reaction conditions: water in the feed, 0; time on stream, 1 h.
132 With regard to the effect of temperature, mention should be made of cracking of compounds which are intermediate in coke evolution, presumably oligomers, alkylates and paraffins, and alkylated aromatics.
Figure 3 shows the results of coke content on the catalyst for several water contents in the feed for the following reaction conditions: 673 K; space time, 0.05 (g of catalyst) h (g of methanol)-l; time on stream, 1 h. A very important attenuation of coke content is observed.
6
d4 o
2 .....
0
i .......
I
2
i
I
4
t
I
|
6
8
Xwo
Figure 3. Effect of water concentration in the feed on the catalyst coke content. It has been proven (GC-MS analysis after supercritical extraction of the coke with CO2) that when water is fed the coke is less evolved. In the same way, deterioration of the physical properties and of catalyst acidity is attenuated. The more consistent explanations of the effect of water when the reaction conditions here studied and the properties of the SAPO-34 are taken into account are: a) the transformation of Lewis sites into Bronsted sites [16]; b) the inhibition of oligomerization of the olefins adsorbed on the acidic sites, due to the competition of steam in the adsorption [2]. 3.2. T h e role of w a t e r in t h e k i n e t i c m o d e l The kinetic scheme proposed (with elemental steps) is shown in Figure 4. The validity of this scheme, which is a simplification of that proposed by Bos et al. [3], has been proven in a previous paper [8], in which the following kinetic constants have been obtained for the single steps:
kl= 0.709 108 exp(-21800/RT)
(1)
k2= 0.637 107 exp(-18100/RT)
(2)
methanol + DME 1
k3= 0.228 106 exp(-14600/RT)
(3)
k4= 0.231 105 exp(-14200/RT)
(4)
9~
ethene
v~
propene
,4 3
rest of products
Figure 4. Kinetic scheme.
133 Among the kinetic models studied for the deactivation, the following is proposed, which takes into account that deactivation is dependent on the total concentration of the organic compounds referred to the total mass, x w. da kdXT ad = kd ad - d--t-= (1 + kwx w) [1 +(1 + kw)Xw]
where: a=
[dXi / d(W / FMo)]t
(6)
[dX i / d(W / FMo)]o
In eq. (5) kw is the constant for the attenuation of deactivation due to water, which competes with the evolving coke precursors in the adsorption on acidic sites. The method for calculation of the kinetic parameters has consisted of solving the mass conservation equations for each component in the reactor under the plug flow assumption: dX i d~
(5)
W ri,oa FMo
= ~
~
0.4
Xi 0.3
'
l
\ ~" \ /= \V ~I~
0.2
.... "ll
l
. ethene o propene x restof HC = MeOH+DME
kd = 1.76 107 exp(-15600/RT)
(8)
kw = 3.35 10 s exp(-14000/RT) (9) d= 1.5
(10)
0.6 0.4
0.1
0.2 , ~ _ m . . . m
,
o 0
0.2
0.4
0.6
.m
_ ,,,,
0.8
timeon stream,h Xi
1
t b
~
XA
0.8
0.3
For solving the set of (n-l) differential equations corresponding to the mass conservation equations of the (n1) components of the kinetic scheme, Figure 4, a calculation program in FORTRAN, which uses the subroutine DGEAR of IMSL library, was developed. The values calculated for the kinetic parameters are:
XA
0.8
xwo0
0.4
(7)
1
~ ~Y"
-
propene _ x rest of HC = MeOH+DME
0.6
Xwo= 3
0.4
o
O.2
0.1
0.2
0
0.2
0.4
0.6
timeon stream,h
0.8
Figure 5. Evolution with time on stream of the mass fraction of the reaction components, for different values of water concentration in the feed. Temperature, 698 K. Space time, 0.1 (g of catalyst) h (g of methanol) "1.
134 This value of deactivation order is greater than unity, which implies that the chemisorbed coke partially blocks the porous structure and restricts diffusion of reaction medium components. The validity of the kinetic model is shown in Figures 5 and 6, where the evolution with time on stream of the mass fraction of the components, on a water-free basis, has been plotted for given operating conditions. Points are experimental results and lines have been calculated with the proposed kinetic model.
0.4
a .....
~,
.
.
.
ethene o propene x rest of HC
0.3
1
.
I
0.8 | ! 0.6
t
0.2 48 K
0.1
o
0.4 0.2
.
0
0.2
.
.
0.4
.
.
.
'" -
0.6
~
0.8
0
1
time on stream, h 1
0.4
xi
XA 0.8
0.3
\ / , kX~ L=~
0.2
4. CONCLUSIONS SAPO-34 gives yields of light olefins over 90%, which are maintained with time, in spite of the fact that deactivation by coke deposition is important in the first minutes and affects production. The coke blocks the internal channels of the SAPO-34 crystals and subsequently blocks the mesopores of the binder (bentonite). The coke nature is mainly paraffinic, although polycyclic aromatics have been found. Coke deposition mainly blocks sites of acidic strength over 175 kJ mo1-1.
:
_
xi
=
o propene_ x rest of HC "MeOH+DME
~
0.1
0
" " 0.6 i 0.4
698 K
0.2
0.2
0.4
0.6
0.8
1
time on stream, h
1
0.4
xi
XA 0.8
0.3 / 0.2
/ 0.1
\\
l
0
" " i
T48 K
' 0.4
~
,
0
propene x rest of HC -MeOH+DME
o
~'k~ //x~,
'
0.2
,
'
0.4
,
"
'
.
.
0.6
.
.
0.8
--
0.6
0.2 0
1
time on stream, h
Figure 6. Evolution with time on stream of the mass fraction of the reaction components, for different values of temperature. Space time, 0.1 (g of catalyst) h (g of methanol) -1. Water concentration in the feed, Xwo= 1.
135 Coke formation is attenuated by the presence of water in the reaction medium and, consequently, by dilution of methanol with water in the feed, or by operating conditions that favour the formation of water as a product, as are: increase in temperature and space time. The role of water in the reaction medium is not that of an inert diluting the feed (decreasing the concentration of coke precursors). Water inhibits the evolution of coke precursors as it competes with them in the adsorption on acidic sites. Irreversible deactivation is not appreciated under successive reactionregeneration cycles. The kinetic model for deactivation, eq. (5) is valid in the wide range of operating conditions used in this work and, by combining it with the kinetic model of the main reaction, it is useful for the design of a fluidized bed reactor with catalyst circulation. The design methodology is similar to that proposed in a previous paper for the MTG process [17].
Acknowledgement This work was carried out with the financial support of the Department of Education, University and Research of the Basque Country (Project No. PI96/10) and of the Ministry of Education and Culture of the Spanish Government (Project DGICYT PB96-1478).
5. NOTATION a FMo kd, kw
Catalyst activity, eq. (6) " Mass flow of methanol in the feed, g h -1. Kinetic constant for coke deactivation and constant for attenuation of deactivation due to water. rio reaction rate for formation of component i at zero time on stream, g h 1 (g of catalyst) 1 T Temperature, K. t Time on stream, h. W Catalyst weight, g. XA Composition of oxygenates (methanol and dimethyl ether) in the reaction medium, on a water-free basis. Xi,xi Composition of i component in the reaction medium, on a water-free basis and referred to the total mass. Xw, Xwo Mass fraction of water, on a water-free basis, in the reaction medium and in the feed. dimensionless longitudinal coordinate of the reactor.
136 REFERENCES
J. Liang, H. Li, S. Zhao, W. Guo, R. Wang and M. Ying, Appl. Catal., 64 (1990) 31. A.J. Marchi and G.F. Froment, Appl. Catal., 71 (1991) 139. 3. A.N.R. Bos and P.J.J. Tromp, Ind. Eng. Chem. Res., 34 (1995) 3808. 4. C.D. Chang, Hydrocarbons from Methanol, H. Heinemann (ed.), Marcel Dekker, Inc., New York, 1983. S.W. Kaiser, U.S. Patent No. 4 499 327 (1985). 6. J.D. Adjaye, R.K. Sharma and N.N. Bakhshi, in Advances in Thermochemical Biomass Conversion, A.V. Bridgwater (ed.), Blackie A&P, London, 1992, p. 1032.. S.P.R. Katikaneni, J.D. Adjaye and N.N. Bakhshi, Energy & Fuels, 9 (1995) 599. A.G. Gayubo, A.T. Aguayo, A.E. S~nchez del Campo, A.M. Tarrio and J. Bilbao, Ind. Eng. Chem. Res., in press. Y. Watanabe, A. Koiwai, H. Takeuchi, S. Hyodo and S. Noda, J. Catal., 143 (1993) 430. 10. V. S. Nayak and V. R. Choudhary, Appl. Catal., 10 (1984) 137. 11. T. Sano, N. Yamashita, Y. Iwami, K. Takeda and Y. Kawakami, Zeolites, 16 (1996) 258. 12. B.V. Vora, T.L. Marker, P.T. Barger, H.R. Nilsen, S. Kvisle and T. Fuglerud, Stud. Surf. Sci. Catal., 107 (1997) 87. 13. A.E. Shnchez del Campo, A.T. Aguayo, A.G. Gayubo, A.M. Tarrio and J. Bilbao, Ind. Eng. Chem. Res., 37 (1998) 2336. 14. P. Magnoux, P. Cartraud, S. Mignard and M. Guisnet, J. Catal., 106 (1987) 242. 15. P.L. Benito, A.G. Gayubo, A.T. Aguayo, M. Olazar and J. Bilbao, Ind. Eng. Chem. Res., 35 (1996) 3991. 16. J.P. van den Berg, J.P. Wolthuizen and J.H.C. Hooff, in Proc. 5th Conf. on Zeolites, L.V.C. Rees (ed.), Heyden, London, 1980, p. 649. 17. J.M. Ortega, A.G. Gayubo, A.T. Aguayo, M. Olazar and J. Bilbao, Ind. Eng. Chem. Res., 37 (1998) 4222. o
~
.
~
0
129
The Role o f Water on the A t t e n u a t i o n of Coke D e a c t i v a t i o n of a SAPO-34 Catalyst in the T r a n s f o r m a t i o n of M e t h a n o l into Olefins
A.G. Gayubo, A.T. Aguayo, A.E. Sfinchez del Campo, P.L. Benito and J. Bilbao Departamento de Ingenieria Qulmica, Universidad del Pals Vasco, Apartado 644, 48080 Bilbao, Spain. Telephone: 34-4-6012000, Fax: 34-4-4648500, E-mail: iqp [email protected]
The effect of water concentration on the reaction medium in order to attenuate coke formation on a SAPO-34 has been studied in the transformation of methanol into olefins. From the results obtained in an integral reactor for different values of space time and water concentration in the feed, a kinetic model for deactivation by coking on a SAPO-34 in the 623-748 K range has been proposed. The model takes into account the effect on the deactivation of organic components and water in the reaction medium.
1. INTRODUCTION The silicoaluminophosphate SAPO-34 gives up to 90% selectivity to light olefins in the transformation of methanol (selectivity to ethene and propene is about 60%) in the 623-748 K range. Consequently, this is an encouraging alternative for the use of HZSM-5 zeolites in the MTO process which, nevertheless, has the counterpart of its fast deactivation by coke. Several authors [1-3] have already proven that deactivation is attenuated by feeding water with methanol. The attenuation of coke deactivation by water had already been observed for HZSM-5 zeolite when it was used in the MTG [4] and MTO processes [5]. This effect was also observed in other processes on acidic catalysts, such as in the transformation of pyrolysis products obtained from wood and seed-oil to hydrocarbons on HZSM-5 zeolites [6] and on hybrid catalysts of silica/aluminaHY zeolite and of silica/alumina-HZSM-5 zeolite [7]. Furthermore, the interest in the use of water in the MTO process on SAPO-34 is supported by two circumstances: 1) the fact that water affects slightly to the kinetics of the main reaction [8]; 2) the high hydrothermal stability of the SAPO-34 [9], which is much higher than that of the HZSM-5 zeolite [10,11]. In this paper, the role of water on the deactivation of a SAPO-34 has been studied with the aim of obtaining a kinetic model which accounts for the presence of water. This kinetic model will be required for the optimization of the industrial
130 operation, which is carried out in a fluidized bed reactor with catalyst circulation [12].
2. E X P E R I M E N T A L
The catalyst used in the reactor is prepared by agglomerating the SAPO-34 (of composition H0.o9(Sio.08Alo.51Po.4)O2) (25 wt%) with bentonite (Exaloid) (30 wt%), using fused alumina (Martinswerk) as inert charge (45 wt%). The catalyst is calcined at 848 K for 4 h under N2 stream. Preparation and properties of the catalyst have been detailed in previous papers [8,13]. The kinetic data have been obtained in an isothermal fixed bed reactor, under the following conditions: temperature, 623, 648, 673, 698 and 748 K; space time, between 0.01 and 0.44 (g of catalyst) h (g of methanol) -1 (by changing the catalyst mass); time on stream, 1 h; water/methanol ratio in the feed: 0, 1 and 3, in weight; total flow of liquid feed: 0.40 cm 3 min'l; catalyst particle diameter, between 0.1 and 0.3 mm. The feed-reaction-analysis system (Hewlett-Packard 5890 Series II chromatograph) is controlled by a computer routine. Coke deposited on the catalyst was determined by thermogravimetry (Setaram TAG 24), by combustion with air. Previously, the deactivated bed obtained subsequently to the reaction was subjected to a sweeping treatment with He (100 cm 3 min -1) for 30 min at the reaction temperature.
3. R E S U L T S 3.1. W a t e r e f f e c t o n c o k e d e a c t i v a t i o n
Under the same reaction conditions (methanol concentration in the feed, temperature and space time), coke deposition is higher on the SAPO-34 than in the HZSM-5 zeolite, which is a consequence of the combination of two characteristics of the crystalline structure of the SAPO-34: small diameter of the internal channels (4.4x3.1 A), which may be easily blocked, and cavities constituted by intersections between channels (10x6.7 A), which allow for the arrangement of higher molecular weight structures. The extraction with acetone subsequent to catalyst destruction with an aqueous solution of HF (50 wt%), by following the methodology proposed by Magnoux et al. [14], only removes 10 wt% of the total coke amount. The extracted fraction, once analysed by GC-MS (Hewlett Packard 5989B), contains phenantrene and pyrene, apart from paraffins and naphthenes. A subsequent extraction with pyridine is of minor efficiency and allows for proving the presence of polyaromatics of up to 4 rings, which are derivatives of pyrene, with a molecular weight of up to 400. These results show the impossibility of analysing the coke retained within the channels and cavities of the SAPO-34 by desorption and that coke is more developed and
131
with a higher molecular weight than that deposited within the HZSM-5 zeolites under similar reaction conditions [15]. Coke deposition originates a rapid deterioration of acidity as is ---,r~ ! observed in Figure 1, which 225 = 1 min corresponds to the following 45 min 200 reaction conditions: water in the 25 min feed, 0; temperature, 673 K; space ~175 time, 0.05 (g of catalyst) h (g of 0 methanol)" 1. As is observed, E 150 -,j strongly acidic sites are mainly affected. In a 25 min reaction, sites 125 with an acidity strength above 175 kJ (mol of NH3) -1 disappear. As in 100 this situation the catalyst is 0 0.03 0.06 0.09 0.12 0.15 mmol NH3/g completely inactive for olefin production, a minimum acidity Figure 1. Evolution of catalyst acidity strength within the range between strength distribution with time on 175 and 200 kJ (mol of NH3) -1 is stream. required for this reaction. As coke deposition is sensitive to water content in the reaction medium, it is attenuated under the reaction conditions in which methanol conversion increases and consequently, water concentration increases. This situation takes place as reaction temperature is increased, Figure 2a, and space time is increased, Figure 2b, for constant remaining operating conditions. 11
8
673 K
0.05 gcatalysthlgMeOH 10
=<
7
d6 o r
8
o (J
5
a
b ,
610
i
630
I
I
650
I
I
670
Temperature, K
I
I
690
,I,
..
710
0
i
i
0.1
,t
i
0.2
,i
,
i
.....
0.3
i,,
0.4
W/FMo, gcatalysth/gMeOH
Figure 2. Effect of reaction temperature (a) and of space time (b) on catalyst coke content. Reaction conditions: water in the feed, 0; time on stream, 1 h.
132 With regard to the effect of temperature, mention should be made of cracking of compounds which are intermediate in coke evolution, presumably oligomers, alkylates and paraffins, and alkylated aromatics.
Figure 3 shows the results of coke content on the catalyst for several water contents in the feed for the following reaction conditions: 673 K; space time, 0.05 (g of catalyst) h (g of methanol)-l; time on stream, 1 h. A very important attenuation of coke content is observed.
6
d4 o
2 .....
0
i .......
I
2
i
I
4
t
I
|
6
8
Xwo
Figure 3. Effect of water concentration in the feed on the catalyst coke content. It has been proven (GC-MS analysis after supercritical extraction of the coke with CO2) that when water is fed the coke is less evolved. In the same way, deterioration of the physical properties and of catalyst acidity is attenuated. The more consistent explanations of the effect of water when the reaction conditions here studied and the properties of the SAPO-34 are taken into account are: a) the transformation of Lewis sites into Bronsted sites [16]; b) the inhibition of oligomerization of the olefins adsorbed on the acidic sites, due to the competition of steam in the adsorption [2]. 3.2. T h e role of w a t e r in t h e k i n e t i c m o d e l The kinetic scheme proposed (with elemental steps) is shown in Figure 4. The validity of this scheme, which is a simplification of that proposed by Bos et al. [3], has been proven in a previous paper [8], in which the following kinetic constants have been obtained for the single steps:
kl= 0.709 108 exp(-21800/RT)
(1)
k2= 0.637 107 exp(-18100/RT)
(2)
methanol + DME 1
k3= 0.228 106 exp(-14600/RT)
(3)
k4= 0.231 105 exp(-14200/RT)
(4)
9~
ethene
v~
propene
,4 3
rest of products
Figure 4. Kinetic scheme.
133 Among the kinetic models studied for the deactivation, the following is proposed, which takes into account that deactivation is dependent on the total concentration of the organic compounds referred to the total mass, x w. da kdXT ad = kd ad - d--t-= (1 + kwx w) [1 +(1 + kw)Xw]
where: a=
[dXi / d(W / FMo)]t
(6)
[dX i / d(W / FMo)]o
In eq. (5) kw is the constant for the attenuation of deactivation due to water, which competes with the evolving coke precursors in the adsorption on acidic sites. The method for calculation of the kinetic parameters has consisted of solving the mass conservation equations for each component in the reactor under the plug flow assumption: dX i d~
(5)
W ri,oa FMo
= ~
~
0.4
Xi 0.3
'
l
\ ~" \ /= \V ~I~
0.2
.... "ll
l
. ethene o propene x restof HC = MeOH+DME
kd = 1.76 107 exp(-15600/RT)
(8)
kw = 3.35 10 s exp(-14000/RT) (9) d= 1.5
(10)
0.6 0.4
0.1
0.2 , ~ _ m . . . m
,
o 0
0.2
0.4
0.6
.m
_ ,,,,
0.8
timeon stream,h Xi
1
t b
~
XA
0.8
0.3
For solving the set of (n-l) differential equations corresponding to the mass conservation equations of the (n1) components of the kinetic scheme, Figure 4, a calculation program in FORTRAN, which uses the subroutine DGEAR of IMSL library, was developed. The values calculated for the kinetic parameters are:
XA
0.8
xwo0
0.4
(7)
1
~ ~Y"
-
propene _ x rest of HC = MeOH+DME
0.6
Xwo= 3
0.4
o
O.2
0.1
0.2
0
0.2
0.4
0.6
timeon stream,h
0.8
Figure 5. Evolution with time on stream of the mass fraction of the reaction components, for different values of water concentration in the feed. Temperature, 698 K. Space time, 0.1 (g of catalyst) h (g of methanol) "1.
134 This value of deactivation order is greater than unity, which implies that the chemisorbed coke partially blocks the porous structure and restricts diffusion of reaction medium components. The validity of the kinetic model is shown in Figures 5 and 6, where the evolution with time on stream of the mass fraction of the components, on a water-free basis, has been plotted for given operating conditions. Points are experimental results and lines have been calculated with the proposed kinetic model.
0.4
a .....
~,
.
.
.
ethene o propene x rest of HC
0.3
1
.
I
0.8 | ! 0.6
t
0.2 48 K
0.1
o
0.4 0.2
.
0
0.2
.
.
0.4
.
.
.
'" -
0.6
~
0.8
0
1
time on stream, h 1
0.4
xi
XA 0.8
0.3
\ / , kX~ L=~
0.2
4. CONCLUSIONS SAPO-34 gives yields of light olefins over 90%, which are maintained with time, in spite of the fact that deactivation by coke deposition is important in the first minutes and affects production. The coke blocks the internal channels of the SAPO-34 crystals and subsequently blocks the mesopores of the binder (bentonite). The coke nature is mainly paraffinic, although polycyclic aromatics have been found. Coke deposition mainly blocks sites of acidic strength over 175 kJ mo1-1.
:
_
xi
=
o propene_ x rest of HC "MeOH+DME
~
0.1
0
" " 0.6 i 0.4
698 K
0.2
0.2
0.4
0.6
0.8
1
time on stream, h
1
0.4
xi
XA 0.8
0.3 / 0.2
/ 0.1
\\
l
0
" " i
T48 K
' 0.4
~
,
0
propene x rest of HC -MeOH+DME
o
~'k~ //x~,
'
0.2
,
'
0.4
,
"
'
.
.
0.6
.
.
0.8
--
0.6
0.2 0
1
time on stream, h
Figure 6. Evolution with time on stream of the mass fraction of the reaction components, for different values of temperature. Space time, 0.1 (g of catalyst) h (g of methanol) -1. Water concentration in the feed, Xwo= 1.
135 Coke formation is attenuated by the presence of water in the reaction medium and, consequently, by dilution of methanol with water in the feed, or by operating conditions that favour the formation of water as a product, as are: increase in temperature and space time. The role of water in the reaction medium is not that of an inert diluting the feed (decreasing the concentration of coke precursors). Water inhibits the evolution of coke precursors as it competes with them in the adsorption on acidic sites. Irreversible deactivation is not appreciated under successive reactionregeneration cycles. The kinetic model for deactivation, eq. (5) is valid in the wide range of operating conditions used in this work and, by combining it with the kinetic model of the main reaction, it is useful for the design of a fluidized bed reactor with catalyst circulation. The design methodology is similar to that proposed in a previous paper for the MTG process [17].
Acknowledgement This work was carried out with the financial support of the Department of Education, University and Research of the Basque Country (Project No. PI96/10) and of the Ministry of Education and Culture of the Spanish Government (Project DGICYT PB96-1478).
5. NOTATION a FMo kd, kw
Catalyst activity, eq. (6) " Mass flow of methanol in the feed, g h -1. Kinetic constant for coke deactivation and constant for attenuation of deactivation due to water. rio reaction rate for formation of component i at zero time on stream, g h 1 (g of catalyst) 1 T Temperature, K. t Time on stream, h. W Catalyst weight, g. XA Composition of oxygenates (methanol and dimethyl ether) in the reaction medium, on a water-free basis. Xi,xi Composition of i component in the reaction medium, on a water-free basis and referred to the total mass. Xw, Xwo Mass fraction of water, on a water-free basis, in the reaction medium and in the feed. dimensionless longitudinal coordinate of the reactor.
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