Contents CHAPTER 1. INTRODUCTION..............................................................................................1 1.1
SIGNIFICANCE.........................................................................................................2
1.2
OBJECTIVE OF THESIS...........................................................................................3
1.3
ORGANISATION OF THESIS...................................................................................4
CHAPTER 2. LITERATURE REVIEW....................................................................................5 2.1 OUTLINE........................................................................................................................5 2.2 KINETIC MODELS........................................................................................................5 2.3 REACTION MECHANISM..........................................................................................10 2.4 REACTION CONDITIONS..........................................................................................12 2.4.1 Pressure...................................................................................................................12 2.4.2 Temperature.............................................................................................................13 2.4.3 Space Velocity.........................................................................................................15 2.5 CATALYST....................................................................................................................16 2.5.1 Catalysts Basics.......................................................................................................16 2.5.2. Catalysts Used........................................................................................................18 CHAPTER 3. FIXED BED REACTOR MODELING............................................................25 3.1 PSEDOHOMODENEOUS MODELS (Basic 1-D Model)............................................26 3.1.1 Model Equations.....................................................................................................26 CHAPTER 4. KINETIC AND REACTOR MODELING.......................................................30 4.1 REACTION KINETICS................................................................................................30 4.2 DEVELOPMENT OF MODEL.....................................................................................31 4.2.1 Model Assumptions.................................................................................................31 4.2.2 Model Equations.....................................................................................................32 4.3 SOLUTION TECHNIQUE............................................................................................33
1
CHAPTER 5. RESULTS & DISCUSSIONS...........................................................................35 5.1 VALIDATION OF MODEL...........................................................................................36 5.2 EFFECT OF PRESSURE..............................................................................................38 5.3 EFFECT OF TEMPERATURE......................................................................................44 5.4 EFFECT OF H2/CO2 MOLE RATIO.............................................................................56 CONCLUSION........................................................................................................................57 REFERENCES.........................................................................................................................58
List of Figures Figure 1. Relation between reaction pressure and CO2 conversion and methanol yield from experimental results and thermodynamic predictions .............................................................13 Figure 2. Relationship between reaction temperature and CO 2 conversion amd yield of methanol from experimental results and thermodynamic predictions.....................................14 Figure 3. Relationship between space velocity and CO2 conversion and methanol yield ......15 Figure 4. Rates of methanol formation as a function of space velocity for methanol synthesis over Cu/ZnO/Al2O3catalyst with synthesis gas containing 10 vol% CO2................15 Figure 5. Aspects to be dealt with in the modelling of fixed bed reactors...............................25 Figure 6. Model Results of various parameters at T=498 K & P=50 bar (a) Production of methanol (b) Yield of methanol w.r.t. C (c) Conversion of carbon monoxide (d) Conversion of hydrogen and (e) Temperature along the length of the reactor.................37 Figure 7. Effect of Pressure on the molar flow rates of methanol...........................................38 Figure 8. Effect of Pressure on the conversion of carbon monoxide.......................................39 Figure 9. Effect of Pressure on the yield of methanol w.r.t.C .................................................40 Figure 10. Effect of Pressure on the yield of methanol w.r.t. H2..............................................41 Figure 11. Effect of Pressure on the conversion of Carbon Dioxide.......................................42 Figure 12. Effect of Pressure on the conversion of Hydrogen.................................................43 2
Figure 13. Effect of Temperature on the molar flow rates of methanol...................................45 Figure 14. Effect of Temperature on the yield of methanol w.r.t. C.........................................46 Figure 15. Effect of Temperature on the yield of methanol w.r.t. H2......................................47 Figure 16. Effect of Temperature on the conversion of carbon dioxide...................................48 Figure 17. Effect of H2/CO2 Mole Ratio on (a) Yield of Methanol (b) Molar flow rates of methanol...................................................................................................................................56
List of Tables Table 1. Various kinetic models along with their experimental reaction conditions..................7 Table 2. Various catalysts used along with the reactions, reactor used and the reaction conditions.................................................................................................................................23 Table 3. Frequency Factors of Kinetic Equation......................................................................31 Table 4. Frequency Factors of Enthalpy Equation...................................................................33 Table 5. Industrial Reactor Specification, Catalyst Properties and Feed Conditions...............34 Table 6. Comparison of results with simulated results given by Panahi et al..........................36 Table 7. Comparison of Temperature Effect in Methanol Synthesis........................................49
3
CHAPTER 1. INTRODUCTION
Methanol is one of the most conventional feedstock that is used in various manufacturing processes and a very potential resource of alternative energy. Though it is a conventional studied process, there exists a void on understanding the underlying chemical reactions occurring in these processes. In recent years, it is more effective to produce methanol containing feed gas which is CO2-rich, instead of the traditional CO-rich feed. It is commercially produced from syngas which is a mixture of carbon dioxide, hydrogen and carbon monoxide under high temperature and pressure. The catalyst used mainly is Copper/Zinc (Cu/ZnO) based oxide catalyst. The oxide additives which are generally used include Al2O3, Cr2O3 and ZrO2 [1, 2]. The formation of methanol takes place by virtue of THREE main reactions [3]: The main reactions involved in the production of methanol are Carbon monoxide Hydrogenation CO +2H2 ↔CH3OH
(ΔG = -25.34 kJ.mol-1; ΔH°298= - 90.55 kJ.mol-1)
(1)
Carbon dioxide Hydrogenation CO2 + 3H2 ↔CH3OH + H2O
(ΔG = 3.30 kJ.mol-1; ΔH°298= - 49.43 kJ.mol-1)
(2)
Reverse Water-gas shift reaction CO + H2O ↔ CO2 + H2
(ΔG = -28.60 kJ.mol-1; ΔH°298= 41.12 kJ.mol-1)
(3)
An efficient catalyst is required for the economic operation of methanol synthesis from CO2 allowing high enough yields of methanol. The kinetics involved in the methanol synthesis has been widely studied. Various types of kinetic expressions have been put forward supported by various assumptions considering the phenomenon occurring there in. Catalysts based on Cu/ZnO have been considered to be extremely beneficial for the methanol synthesis process because of their high stability, activity and selectivity which can be further improved by using the promoters and supporters [3, 4]. Foremost studies on the kinetics for the synthesis of methanol were being done in the early1977, and even now, authors are continuously trying the kinetic modelling for the process [3].Although the subject of reaction mechanisms for the methanol synthesis has been studied for many years, but there exists no
1
unified conclusion on one scheme. The role of CO 2 in the synthesis of methanol and the role of the ZnO catalyst are still some areas where voids in exact knowledge exist [4, 5 and 6]. Several efforts have been made for improving the synthesis of methanol after its foundation by B.A.S.F. (Baden Aniline and SODA Factory). They developed new reactor configurations, new, stable and more efficient catalysts, and optimized the reaction parameters viz temperature, pressure and space velocity. The development of catalysts involves efficacious supports ZnO and ZrO2, enhancers viz zirconia, alumina and various contenders like cobalt, gallium, magnesium and boron for enhancing the catalysts activity at changing heat and heating conditions [2, 4]. Since the manufacture of methanol is heat producing exothermic reaction, increased temperatures can improve the amount of methanol produced but because of thermodynamic limitations it can enhance it up to an optimal temperature. Because of these limitations the equilibrium yield decreases with increasingly high temperatures. Hereby, new and improved methods have been developed which prefer synthesis of methanol at low temperatures [2]. However, there is still debate on the reaction mechanism and is still explored.
1.1 SIGNIFICANCE The manufacture of methanol is of great significance industrially. The worldwide methanol product was around “44 MMT per year” in 2010 which increased to “84 MMT per year” in 2012. The World Methanol Cost Study report of the Chemical Market Associates in 2010 stated that the international methanol industry is half-way of the maximum ability build on its past [7]. Another report, in 2011, stated that international methanol consumption increase had been very strong in 2010 and was anticipated to increase at the same rate [8]. It caters to a very wide variety of applications due to its very high demand. It is used as a raw material in the synthesis of very important chemical viz methyl tert-butyl ether, formaldehyde, chloromethane and acetic acid which can be in succession can be used in many application like plastics, paints and plywood to explosives. [9]. Methanol is also used as a transportation fuel in two ways, as methanol as it is or by blending it in petrol.
2
It is an excellent promising alternating energy source since it has several advantages such as low emissions, clean burning properties, high energy density, high octane rating, easy transport, high volatility, and has abilities of incorporating in the existing engines with minor infrastructural modifications [3, 10, and 11]. In the fuel cell research applications, methanol is the fuel for direct methanol fuel cells [11]. The world global economy is strongly affected by the methanol industry. It is generating $12 billion in annual economic activity thereby creating vast openings of employment. Another important horizon is the methanol manufacture by addition of hydrogen to carbon dioxide which helps in utilizing the excess atmospheric carbon dioxide and thereby removing one of the main cause of global warming by the reduction of one of the major greenhouse gases [3, 9]. So enhancing the production of methanol and optimizing it by modelling its reaction kinetics is of significant importance due to its numerous benefits as a source of alternative energy and its use in a variety of applications. The disagreement on the reaction scheme for methanol synthesis leads development of new and efficacious kinetic models which can improve the production of methanol and can result in obtaining high yields of methanol with huge profits.
1.2 OBJECTIVE OF THESIS Following objectives were met during the completion of thesis: Formulating a one dimensional mathematical model for methanol production from syngas in a shell and tube fixed bed adiabatic reactor. Solving the mathematical model using equation solver tool in MATLAB along Runge-Kutta-Verner 4th and 5th order with automated increment size for accuracy. Validation of the model by comparing the predicted results with those available in the literature. Performing the simulation of shell and tube fixed bed adiabatic reactor for synthesis of methanol from syngas and to study the effects of following reaction parameters on molar flow rates of hydrogen, carbon monoxide, carbon dioxide, methanol, steam, yield of methanol, conversion of hydrogen and conversion of carbon dioxide
Temperature
3
Pressure
H2:CO2 molar feed ratio
Finding the optimal parametric conditions for the maximum production of methanol
1.3 ORGANISATION OF THESIS The remaining section of the thesis has been organised as follows: In Chapter TWO, various kinetic models proposed by numerous researchers for methanol production is discussed. It gives an idea about the studies which have been forwarded for the production of methanol in recent years. It also describes the reaction mechanism, reaction condition and the various catalysts used for the production of methanol. In Chapter THREE, the one-dimensional fixed bed reactor psuedohomogeneous model is discussed along with the various model equations involved. In Chapter FOUR, the kinetic and the rector modeling of the underlying work is discussed. It gives an idea about the reaction kinetics which is used in this work along with model assumptions and the model equations which help in the development of model. In Chapter FIVE, the effect of various parameters such as temperature, pressure and H2/CO2 mole ratio is discussed. It also forwards the optimal parametric conditions for the maximum production of methanol.
4
CHAPTER 2. LITERATURE REVIEW
2.1 OUTLINE The manufacture of methanol has been accomplished by BASF in 1920. Because of its wide range of applications to which it caters it holds great industrial significance. The worldwide methanol consumption increased from “40.4 MMT” in 2007 to “58.6 MMT” in subsequent 5 years [12].
Because of its high consumption and vast industrial importance, various
explorations have been carried out for improving the production of methanol. A large number of experimental studies have been carried out since decades but some doubts and questions are still unanswered. The main dispute revolves around the identity of active sites and the reaction mechanism (role of CO and CO 2). The following aspects of methanol synthesis kinetics are analysed in the literature review. Kinetic Models Reaction Mechanism Reaction Conditions Catalyst
2.2 KINETIC MODELS The literature proposes numerous kinetic models on which the evaluation of kinetic parameters is performed; every one based on a many facts assumed related to the reaction. Leonov et al. (1973) developed a kinteic model for production of methanol based on Cu/ZnO/Al2O3 catalyst. However effect of CO2 in the feed was not considered by them [13]. Later Klier et al. (1982) and Villa et al. (1985) put forward model equations in which they incorporated the partial pressure of CO2, though it was not considered as the primary reactant [13, 14]. Villa et al. (1982) propounded the model which was developed over the scheme in which production of methanol was from CO only and since carbon dioxide adsorbs strongly at high concentrations a CO2 adsorption term was also included. The empirical rate expressions for methanol synthesis were derived by Takagawa and Ohsugi (1987) in various experimental conditions [15].
5
One of the kinetic models was given by Graaf et al. (1988) in which carbon monoxide and carbon dioxide hydrogenation was taken into account. They got around 48 reaction pathways by appropriate assumptions of various basic reactions to be rate controlling and next, by using statistical discrimination they selected the best possible kinetic model. Also the water gas shift reaction was considered along with above motioned reactions. [16]. Mcneil et al. (1989) derived a CO2 expression rate which was rooted on more mechanist data and conditions written in literature. [17]. Skyzypek et al. (1991) showed through experiments that the production of methanol chooses CO 2 upon carbon monoxide as the carbon source and hence developed their kinetic models based on the reactions (2) & (3) [18]. Askgaard et al. (1995) gave a kinetic model for synthesis of methanol and evaluated the kinetic parameters using the surface science studies and gas phase thermodynamics. They discovered out that the calculated rates compared well with the measured rates when they were extrapolated to actual working conditions [19].Froment and Buschhe (1996) developed a steady state kinetic model by conducting experiments which were supported by a comprehensive reaction path and network considering carbon dioxide to be the primary feed of carbon in the process of methanol synthesis. The models took into accounted the effect of various parameters such as pressure, gas phase composition and temperature and on the production rates of methanol even beyond their own experimental conditions [13]. Kutoba et. al (2001) also developed the kinetic equations for the synthesis of methanol assuming the hydrogenation of carbon dioxide to be the principal reaction. The authors propounded that that yield values obtained from their equations compared well with the experiments conducted in a test plant so their equations were reasonably accurate [20] Setnic and Levec (2001) put forward model for kinetics of liquidized phase of methanol manufacture. They demonstrated that the yield of methanol is directly related to the CO2 amount only [21]. Rozovskii and Lin (2003) propounded a couple of reaction steps for building hypothetical model for kinetics that satisfy the data from experimentation as well. Two separate compositions of gas were used, one rich carbon monoxide and second with CO 2 for testing their models applicability of their. They propounded that both the schemes were efficacious with a carbon dioxide enriched mixture but the scheme 1 kinetic model failed in
6
matching the experimental data when a carbon monoxide enriched mixture was used. [22]. Lim et al. (2009) derived an inclusive kinetic model which consisted of forty-eight reaction rates which were based on the various possible rate controlling reactions. By estimating the parameters they demonstrated, from the forty-eight rate of reactions, methoxy species surface reaction step was controlling for addition of hydrogen to CO, formate intermediates hydrogenation was controlling for the hydrogenation of CO 2 and formate intermediate formation
was
controlling
step
for
reverse
water
gas
shift
reaction
using
Cu/ZnO/Al2O3/Zr2O3catalyst [3]. Mavrikakis and Grabow (2011) developed an inclusive micro kinetic theoretical setup utilizing density functional theory calculations for dealing with the uncertain quantities concerning active sites nature and the reaction mechanism [23]. The following table puts together the numerous models dealing with kinetics forwarded in literature along with the conditions of the reactions carried out experimentally.
Table 1. Numerous kinetic models along with their experimental reaction conditions. Operati
Autho
ng
Kinetic Model
Conditio
r,
Refere nce
Year
ns 493-533 K;
r CH OH =k 3
40-55
(
0.5
p CO p H
0.34
2
0.66
pCH
3
−
OH
pCH OH 3
0.4
¿
p CO pH K 2 2
Leonov
)
et al.,
13
1973
atm K
498-523
r CH OH =const 3
K; 75 atm
(
[
3 redox
pCO pCO
( )( ( )] (
1+ K redox
2
pCO p H −
2
pCO
2
3
¿
K2
) n
F + K CO pCO ) 2
pCH OH p H 3
p3H
7
p CH OH
3
p CO
( )(
1 + K ' pCO − ¿ K1 2
3
2
2
2O
))
2
Klier et al., 1982
14
f CO f 2H −
f CH OH 3
K ¿2
2
r CH OH = 3
3
( A+ B f CO+C f H +G f CO ) 2
2
Villa
N/A
et al., r RWGS =
f CO f H −f CO f H 2
1985
O K ¿3
2
2
M
Operati
Autho
ng
Kinetic Model
Conditio
r, Year
ns
(
k 1 K CO c CO c3H/2 − r CH OH =
2
c CH OH c
1 /2
( 1+k CO c CO + k CO c CO ) c H 2
K;
2
k 2 k CO cCO c H − 2
2
2
2
( ) O
1 /2 H2
K
cH
2
2
1/ 2 2
+
bar
KH
2
r ¿CH OH =
(
2
( ) 2
O
K 1/H 2
c CH OH c H 3
c
2
et al.,
cH O
1988
2
3
1 /2
( 1+k CO c CO + k CO c CO ) c H 2
2
8
2
) ( )
3 /2 H2
+
K
2
O
eq 3
KH K
nce
Graaf
2
k 1 K CO c CO c3H/2 −
Refere
O
K eq 2
( 1+ k CO cCO +k CO cCO ) c H 2
KH
+
2
2
)
c H O c CO
2
r H O=
15-50
K eq 1
3
1 /2 H2
3
483-518
15
2
O
1 /2 H2
cH
2
O
16
(
k '1 K CH K 2H K 2H K CO pCO p 2H − 2
r= 483-513
3 /2
2
p CH OH K eq 3
3/ 2
) '
K CH K H K H K CO p CO p CO + K CO pCO + K H p H 2
2
2
2
2
2
K; 2.89-
McNeil
} {p} rsub {{H} rsub {2}} rsup {2} right )} right ]} over {{{K} et al.,rsub {{H} 17rsub {2}} rsup {1
4.38
1989
MPa
+¿=k−11 K
3 /2 5
K
−1 8
K 9 K 10 K 11
483-563 K; 1-4 bar
pH po
3/ 2
p CO 2 θ po ¿
( )( ) 2
2
Askgaar
r¿
d
1 pCH OH pH O 2 −¿=k −11 K 35 / 2 K −1 θ¿ 8 K 9 K 10 K 11 K G p 3/H 2 p1o /2 r¿ 3
et al.
2
1995
2
[
1 pCH OH p H ¿ K p3H pCO
( )
k '5 a k '2 K 3 K 4 K H pCO p H 1− 2
r MeOH = 453-553
((
KH O 1+ K8 K9 K H 2
2
2
3
2
2
2
pH O +√ K H p H K H O pH O pH
)( )
2
2
]
O
2
2
2
2
2
3
)
K;
[ (
k '1 pCO 1−K ¿3
bar
2
r RWGS =
((
KH O 1+ K8 K 9 K H 2
p H O p CO p H pCO 2
2
Conditio
2
)]
pH O +√ K H pH K H pH
and
2
)( ) 2
2
2
1996 3
2
O
pH
2
O
)
Autho Kinetic Model
r, Year
ns
9
13
,
2
Operati ng
Froment Bussche
15-51
19
Refere nce
{
k M P ( CO 2 ) P ( H 2 ) − R M=
P ( CH 3 OH ) P ( H 2 O )
[K
P ( H 2)] 2
M
[ 1+ K CO P ( CO 2 ) + K H O P ( H 2 O ) ]
473-548
2
}
2
Kubota
2
et al.,
K; 4.9MPa
{
k R P ( CO 2) − RR =
P ( CO ) P ( H 2 O )
[K
P2 ( H 2 ) ]
R
2001
}
[ 1+ K CO P ( CO2 ) + K H O P ( H 2 O ) ] 2
2
(e −ERT )
c CO ( c H −c H
Me
r Me= A Me
473-513
2
(1+ A
2
2
,eq
) 2
( )c e −EW RT
W
H2 O
)
Setinc
K;
and
34-41
Levec,
bar
20
r H O =A H O 2
2
( e
−EH O RT 2
)
cCO −cCO ,eq 2
(1+ A
W
2001
2
( )c −EW RT
e
H2O
21
)
Rozovsk r=
513 K;
K 3 p H ( 1− pm p H O ) 2
1+ K−2 p H O +
5.2 MPa
i
2
2
K −2 p H O
And
( K 1 pCO )
Lin,
2
2
526-533
(
k A K CO K H 2 K CH , CO p CO p 2H −
K;
2
r A=
5 MPa
2
3
)
et al.,
0.5 ( 1+k CO p CO ) (1+ K 0.5 H pH + K H O pH O) 2
k B K CO rB=
pCH OH K PA
2003 Lim
(
2
K 0.5 H p CO p H − 2
2
2
pH
2
2
2009
2
pCO p H O K PB 2
)
0.5 2
0.5 0.5 ( 1+ k CO pCO ) (1+ K H p H + K H O p H O ) ( 1+k CO p CO ) 2
2
2
10
2
2
2
22
3
k C K CO K H K CH ,CO 2
rC=
(1+ K
2
0.5 H2
0.5 H2
(
pCO p2H − 2
p CH OH p H O K PC 3
pH
2
p + K H O pH 2
2
O
2
)
2
) ( 1+ k CO p CO ) 2
2
2.3 REACTION MECHANISM Synthesis of methanol occurs via three reaction namely: CO hydrogenation CO2 hydrogenation Reverse water-gas shift reaction A number of studies have been on the kinetics involved in the synthesis of methanol using Copper-supported catalysts for many years now even then there are controversies with regards mechanisms involved in the reactions. The role of carbon dioxide was one of the major concerns in the synthesis of methanol. Inceptive kinetic studies on the production of methanol by Leonov et al. & Natta et al. neglected any contribution from CO2. CO and H2 were only taken into consideration as the main reactant [13]. Klier et al. in 1982 propounded the formation of methanol primarily takes place from carbon monoxide and hydrogen which gets adsorbed on the catalyst and carbon dioxide acts only as a promoter. It was recommended that the synthesis of methanol was highest at a CO 2/CO proportion of 2:28 administered by balancing the retarding effect due to very strong adsorption of CO 2 and the promoting effect of carbon dioxide [14]. In the other study by Liu et al. they carried out inceptive rate experiments which were performed in a batch reactor for determining the feed composition effect on the synthesis rate of methanol and inappropriate results were
11
obtained. They demonstrated that on increasing the CO2 pressure the formation rate of methanol also increased. Later on in the next year, they demonstrated a fine study and propounded that the hydrogenation of carbon dioxide as main step in the methanol production at low conditions of temperature, conversion and if the water is absent but at increased conditions of conversion, temperature, and presence of water, methanol was mainly a product of hydrogenation of carbon monoxide [25]. Chinchen et al. accounted that carbon dioxide is the main reactant in the synthesis of methanol by utilizing
14
C-labelled reactants [26]. The empirical rate expressions for the
methanol synthesis was derived by Takagawa and Ohsugi in 1987 under various experimental conditions. They demonstrated that in the beginning of the reaction the synthesis rate with the increase in CO2/CO proportion but later it decreased with the increasing ratio and started forming water. They asserted that their results were very similar with both Liu et al. and Klier et al. [15]. The experimental study by McNeil et al. demonstrated that the feed containing 2 mole % carbon dioxide in the feed yields an optimum production rate of methanol. The also propounded that at lower temperatures there is more contribution of CO 2 to formation of methanol. Unlike earlier studies, they derived a mechanistic information based rate expression. It included the study of CO2 affecting the rate as a producer as well as an inhibitor [17]. A class of researchers headed by Rozovskii et al. demonstrated that carbon monoxide doesn’t hydrogenate directly to methanol. In earlier work, by use of 14C labelling and in a recently conducted work utilizing the technique of Temperature Programmed Desorption they reported that the synthesis of methanol takes place through CO 2 hydrogenation [22]. Fujita et al. conducted a methanol synthesis study in a flow reactor at atmospheric pressure. They found that methanol produced from CO 2 via formate species hydrogenation appeared on Co and Cu formed methanol via formate species hydrogenation formed on ZnO. The rates of carbon dioxide hydrogenation came out much intense as compared to the hydrogenation of CO. The study accounted that existence of H2O and former reactivity differences and the later discussed formate species primarily were behind the change in the synthesis rates of methanol from CO2 & CO. Sun and co-workers in 1988 analysed synthesis of methanol & water gas shift reaction utilizing IR technique, subsequently concluded that the major pathway in the synthesis of methanol was the hydrogenation of carbon dioxide and CO 2/CO hydrogenation reactions. 12
Hydrogenation of formate species was found to be the rate determining step. They recommended that the addition of carbon monoxide brings down the production process activation energy, in addition to influencing the path of reaction [5]. In the other study by Sahibzada et al. they demonstrated that the basic rate for the hydrogenation of carbon dioxide was 20 times than the hydrogenation of CO and at CO 2> 1%, was the primary source for methanol synthesis. They accounted for production of methanol increased linearly with increase in amount of CO2 in the products absence [26]. Further, Ostrovskii established the part played by carbon dioxide in the production of methanol analysed the mechanism of formation of methanol on Cu/Zn comprising promoter in various experimental setups and demonstrated that main feed for production of methanol was CO2 [27]. Thereafter, Lim et al. performed an inclusive study in which they assumed CO 2 and CO to get absorbed on various Cu spaces and adsorbing H2O on a ZnO site. It was propounded that the rate of hydrogenation of CO 2 was slower than the rate of hydrogenation of CO which brought down the rate of methanol formation but since carbon dioxide decreases the rate of water-gas sift reaction, therefore the production of DME decreases, which is a byproduct of methanol. It was henceforth summarized that the synthesis rate of methanol can be secondarily improved by finding an optimal concentration of CO 2. Among the various studies, they affirm to be the first one reporting the carbon dioxide role in the production of methanol, recommending a kinetic mechanism relating the hydrogenation reactions of CO and CO2 [3]. In a neoteric study by the same authors, they evaluated the effect of CO 2 fraction on the methanol yield by using the kinetic model developed by them and they even formulated a way for optimizing and maximizing the formation rate of methanol including CO2 fraction and temperature profile into the account [28].
2.4 REACTION CONDITIONS The primary reactions conditions which are taken into account in the formation of methanol are pressure, space velocity and temperature.
2.4.1 Pressure The formation of methanol was carried in beginning at extremely increased pressures when BASF incepted it in 1920’s. Lately, ICI reduced the pressures from 50-100 atm utilizing 13
a catalyst based on Cu/ZnO/Al2O3 [9]. Graaf et al.in 1988 analysed the kinetic model for methanol formation from carbon dioxide, carbon monoxide and hydrogen over the similar catalyst and thereby formulated a kinetic model which was carried out at pressures of 15-50 atm. They affirmed that the low pressure kinetic model for the synthesis of methanol to be more accurate in representing and comparing the experimental values to the models proposed previously [29]. Deng et al. reported that the synthesis of methanol could also be operated at 20 atm using the catalyst based on Cu/ZnO/Al2O3 [9]. Xin et al. propounded that it is beneficial to use high pressure for the hydrogenation of carbon dioxide as represented by Figure 1 [31].
Figure 1. Relation between reaction pressure and CO2 conversion and methanol yield from experimental results and thermodynamic predictions [30].
However, the production cost tends to increase with very high pressures and are also unsafe. Therefore, current efforts are being going on to decrease pressure by developing novel catalysts without affecting the methanol yield.
14
2.4.2 Temperature Synthesis of methanol is generally at 493-573 K [17, 19]. Since, CO 2 and CO hydrogenation steps are exothermic; with increase in temperature their rate increase but only up to a limited temperature. The rates start to decline at higher temperatures due to reduction in the equilibrium constant with the reduction in temperature. Hereby extremely high temperatures are unsuitable. Bill et al. showed that the yield of methanol increases with the temperature but only up to 493 K [9]. Likewise, Xin et al. founded that highest yield and carbon dioxide conversion could be achieved nearly around 523 K. In addition to this asserted that the formation of methanol prone to the temperature of reaction as compared to WGS reaction. The temperature dependence of carbon dioxide conversion and methanol yield is shown by Figure 2 [30].
Figure 2. Relationship between reaction temperature and CO 2 conversion and yield of methanol from experimental results and thermodynamic predictions [30].
The efficiency of production of methanol is limited by extreme temperatures due to thermodynamic limitations. Hence, Tsubaki and co-workers propounded a low temperature route for synthesis of methanol. They carried out the experiments at a temperature of 443 K on catalyst based on Cu using C2H5OH as solution in which the catalyst is suspended. It was demonstrated that low temperature based reaction mechanism followed: formate to methyl 15
formate to methanol steps in place of formate to methoxy to methanol route. It was also stated that the production at low temperature led to increased conversions (50-80%) and also reducing the cost of production without any thermodynamic equilibrium [9].
16
2.4.3 Space Velocity The Space velocity can have cumbersome effect on the produce of methanol. Xin et al. accounted that with increase in the space velocity both methanol yield and CO 2 conversion was decreased for an underlying value of carbon dioxide concentration. The results are illustrated by Figure 3 [30].
Figure 3. Relationship between space velocity and CO2 conversion and methanol yield [30]
Figure 4. Rates of methanol formation as a function of space velocity for methanol synthesis over Cu/ZnO/Al2O3catalyst with synthesis gas containing 10 vol% CO2 Reaction conditions: T=523 K, P=3.0 MPa, H2/COx=4
17
In the other study by Lee and co-workers, they founded that the yield of methanol increased at less space velocity along with limitation to specific carbon dioxide amounts on crossing which it starts to go down. It was stated that the topmost synthesis rate can be reached with an optimal value of space velocity, as illustrated in Figure 4 [31].
2.5 CATALYST A brief summary of the catalysts used in the formation of methanol is accounted in this section. First section discusses about the foundation concepts about the process of catalysis, then a short overview of the industrially utilized catalysts which are supported on Cu. In the remaining section, the studies aiming for finding more appropriate catalysts for the formation of methanol from carbon dioxide are discussed.
2.5.1 Catalysts Basics The catalysts alters the chemical reaction rates without being utilized in the reactions. They bring down the reaction initiation energy and thus the reactions are made to take place easily. They don’t have impact on the equilibrium position and do not allow the reactions to occur that are thermodynamically forbidden. Typically, in the chemical reactors several reactions occur. The selection of the catalysts are made on the basis of their influence on the reaction and thereby helping to increase the process selectivity. Thus the consumption of the feedstock materials is improved [32]. The heterogeneous catalysed reactions take place at liquid/solid or gas/solid interface. Hereby the chemical reactions followed by the diffusing and reacting species adsorption to the surface and also into the catalysts pores which is then followed by desorption and products diffusion to the bulk phase. The species diffusing diffuse by processes that can be categorized into internal and external diffusion. In case of external movement, reaction species are carried to the catalysts surface while in the case of internal diffusion, the reaction species are transferred into the catalyst pores where the species adhere to the catalyst active sites for the adsorption to take place. When the reaction is complete the products detach by desorption & in the next step they diffuse back into the bulk by internal and external diffusion [32].
18
The structural aspects of catalyst are of utmost importance regarding in concern with its usefulness. The fluid flow through the bed of the catalyst is influenced by the shape and size. Its mechanical strength helps to ensure its long enough lifetime. High selectivity and activity are ascertained by high enough surface area and correct chemical components. The addition of components helps to achieve stable operation of the catalysts. There are typically thee components of the catalysts: Support Promoters Active components The active sites holds the sole responsibility for the occurrence of chemical reactions. The active species are placed over the support which is the basis of the catalyst. To avoid undesired sintering, the catalyst must render an adequate surface area for the active species so that they can be evenly distributed. The support should have no active sites like the catalysts. The promoters enhance or inhibit the activity of the catalyst. To obtain the desired selectivity, activity and stability property only requires the addition of small quantity of promoters [32]. Catalyst preparation is to be paid great attention as final properties are affected by it. The procedure for preparing the catalyst is generally acquired through the experimental studies which are very time consuming. Impregnation and precipitation are the most widely methods which are used for the preparation of the heterogeneous catalysts. In the method of impregnation, the catalyst support is forged to little cylindrical shapes such as spheres, rings and pellets. Thereafter, the material of the support is bared to a solution with a suitable compound thereby providing the part that is active to a compound that can be easily be transformed into the active phase. The preparation of catalyst that can be precipitated out requires quick mixing of metal salts concentrated solutions which lead to precipitates in a form of high surface area. Filtering and washing is then followed after the precipitation step. Thereafter the conversion of precipitate takes place by drying and heating into appropriate oxides [33]. The reduction in the catalysts activity is deactivation. There may be chemical, thermal and mechanical reasons of the deactivation. Catalysts poisoning is due to the impurities adsorption on the surface of the catalyst and thereby blocking the passage to the active sites [34]. The catalyst can get fouled by unfavourable association of catalysts with reaction species [35]. This interaction is very usual cause of the catalysts deactivation. The sintering
19
process causes the cluster particles of active metal to form bigger particles which causes more reduction in the number of active sites and hence there is loss of active sites [36].
2.5.2. Catalysts Used CO and CO2 hydrogenation favours the formation of higher alcohols. Dimethyl ether can even be formed in the formation of methanol. Thus there is requirement of an efficient and selective catalyst for producing methanol. Recently, the catalysts are there which allow the synthesis of pure methanol under low pressure (<100 atm) from synthesis gas. These catalysts consists of Cu and oxides mixture viz ZnO - Cr2O3 or ZnO-Al2O3. There have been also the use of various other oxides [14]. The preparation of catalyst is very essential concerning the usability of the catalyst. The catalyst preparation for the synthesis of methanol has been discussed. It has been reported that the manufacture methods and the reactions conditions (temperature, pressure & space velocity) also have a vast impact to the operation of the catalyst [37, 38, 39 and 40].
2.5.2.1 Cu/ZnO-Al2O3 catalysts ZnO-Cr2O3 was the foremost catalyst which was utilized for the synthesis of methanol from synthesis gas. BASF commercialized it by using it in the increased temperature and pressure formation. The enhancement in the refinement of feed for methanol formation (mainly sulphur separation) leads to a significant advancement in the catalysts enabling the use of Cu/ZnO catalyst. It can be operated at lower temperature and pressure and also has a high activity. The catalyst based on Cu/ZnO-Al2O3 is extensively studied in the literature. The analysis based on Cr2O3-based catalyst can also be found [41]. The kinetic models for synthesis of methanol are experimental based which are carried out using commercial catalyst which is based on Cu/ZnO-Al2O3. The mechanism of how species adsorb on the surface of catalyst has been analysed and hence a chain of studies are about it [42, 43 and 44]. This catalyst can easily move with high CO feed but deprives some of its activity with the feed rich in carbon dioxide [45]. Due to the environmental regulations, the use of carbon dioxide is very contemporary and thus there are many studies which have been carried out in finding
20
out an active catalyst for CO2 rich feed. Therefore catalysts based on Cu with metallic additives have been examined to accomplish this purpose. Saito et al. propounded that the catalysts based on Cu/ZnO operate well with the feed rich in monoxide but loses their activity when carbon dioxide amount is increased. It was observed that this loss in activity was because due to formation of water, formed during the hydrogenation of carbon dioxide along with the formation of methanol [46]. Novak et al. acquired results which proved that the activity loss of CuO/ZnO-Al2O3 catalyst is due to water formation [47]. Wu et al. proved the harmful effect of H2O to the Cu/ZnO catalyst by using various feed compositions [39]. They found that the yield of water increases and the yield of methanol decreases with increasing quantity of carbon dioxide in the feed. They came to the conclusion that reduction in activation of the Cu/ZnO based catalysts is accelerated in presence of water. They also assumed that on adding silica to the catalyst based on Cu/ZnO could hinder the harmful effect of H2O and thereby permitting the formation of methanol from the feed rich in carbon dioxide with the catalysts based on Cu/ZnO-based. According to Twigg & Spencer, poising due to chloride and sulphur, physical injury, thermal sintering and carbon deposition may bring down the activity of the catalyst based on Cu [48]. Quinn et al. propounded that arsine significantly deactivates the CuO/ZnO/Al2O3 based catalyst may be in the feed during three-phase methanol synthesis [49].
Yang
et
al.
propounded a newer method for the low-pressure methanol formation which uses alcohol promoted Cu/ZnO catalysts [2, 50]. Yang et al. and Reubroycharoen et al. utilized a Cu/ZnOAl2O3 catalyst while Zhang et al. utilized a Cu/ZnO catalyst. In all these researches propanol, butanol and ethanol were analysed as alcohol promoters. This newer method was carried out at a pressure of 3.0 MPa and a temperature of 443 K. This low reaction temperature led to high conversion of carbon monoxide (50-80%) [2]. Zn oxides have also been tested as an additives to the catalysts based on Cu. Huang et al. analysed the impact of oxide additives viz Cr, Co and Zn to the catalysts based on Cu. They perceived remarkable enhancements in the activity of the catalysts for the methanol formation and WGS reaction by adding Cr 2O3 to a Cu catalyst whereas by the addition of CoO addition no remarkable changes were perceived [51]. In addition, the influence of SiO2 on the activity of the catalysts which are based on Cu has also been analysed [52, 53]. They also noticed that the catalytic activity for the production of methanol from carbon dioxide increased with the addition of SiO2.
21
2.5.2.2 Catalysts with Zirconium Fisher et al. showed that Zirconium improves the catalysts activity for the formation of methanol from both carbon monoxide and carbon dioxide and hence it has been identified as a riveting support material to Cu-based catalysts [45, 52, 54, 55 and 56]. Jung et al. observed the use of Cu/ZrO2 catalyst use for the formation of methanol. The activity of catalyst was found to depend strongly on ZrO 2 phase. Liu et al. also reported the same on studying the catalysts based on Cu with the nanocrystalline Zr addition. In another study by Yang et al. they analogized the pure Cu/ZnO catalyst to the one supported with Zr. They propounded that Zr supported Cu/ZnO catalyst showed stronger selectivity and activity towards carbon monoxide and carbon dioxide, especially carbon dioxide. In a long period test, high activity was shown by a catalyst doped with ZrO 2 and is based on Cu/ZnO. They also found that the selectivity and the conversion were also greater with the catalyst supported by Zr [45]. Sloczynski et al. analogized the utilization of Ag, Au and Cu as catalysts for the formation of methanol. They were in the form of M/ (3ZnO.ZrO 2) where M is Ag, Au or Cu. The study reported highest activity was shown by the Cu containing catalyst in the synthesis of methanol. They inferred that there is apparent alliance between copper and the support which is beneficial for the production of methanol [57]. Fisher et al. studied the addition of Zr to Cu/SiO2 catalyst. They tested the catalysts the different Zr loadings and propounded that the synthesis increased with increase in the Zr loading. The effected resulted out to be more pronounced for the hydrogenation of CO than for the hydrogenation of CO2. It was also observed that with intermediate Zr loadings, the selectivity to methanol was maximum. Fisher and Bell analogized Cu/ZrO 2/SiO2 & Cu/SiO2 in the addition of hydrogen to carbon monoxide [54]. The mechanism for the hydrogenation of CO on the applied catalysts were also studied by them. It was acquired that the methanol synthesis rate was enhanced with Zr containing catalysts. The same catalyst was studied by Schilke et al. He also examined it with the addition of Ti. They acquired that Ti addition has the same influence as that of Zr addition [58]. Zhang Y et al. conducted experiments using 12Cu10Zr/γ-Al2O3 $ Cu/γ-Al2O3 catalysts. The effect of molar ratio, space velocity & reaction temperature on the synthesis of methanol were studied by them. They observed that the catalytic performance was improved by the presence if Zr in the catalyst. They also observed that the proper molar ratio of
22
H2/CO2, high space velocity and low temperature are advantageous for the synthesis of methanol [59]. The influence of Zr oxide and Ce addition to the catalysts was also studied was Pokrovski et al. They made and examined the Cu/CexZr1-xO2 with differing content of Ce and observed that the catalyst activity for the formation of methanol through the hydrogenation of carbon dioxide increase with the addition of Ce. The optimum amount of Ce needed in the catalyst was also determined. [56]. The effect of metal oxide additives viz Mn, Mg, Ga, Gd, B and In on Cr/ZnO/ZrO 2 catalyst was analysed by Sloczynski et al. They propounded that the catalyst activity was greatly influenced by the oxide additives. Highest yield of methanol was obtained with Ga addition whereas there was severe loss of activity on the addition of In. Toyir et al. accounted the persistency of Ga as an efficacious catalyst. The experiments were conducted using a catalyst based on Cu/ZnO/Ga2O3 in the formation of methanol by hydrogenation of carbon dioxide [60]. Liu et al. also analysed catalysts with oxide additive which are based on Cu but they mainly concentrated on the advantageous effect of the addition of nanocrystalline Zr on the activity of the catalyst [61]. Saito et al. observed the supporting action of the oxide of Ga to the catalyst in case when CO2was used to give methanol. They investigated with the additives of Zr, Cr, Al, and Ga. On the basis of their investigations, they manufactured two multicomponent catalysts (Cu/ZnO/ZrO2/Al2O3/Ga2O3 & Cu/ZnO/ZrO2/Al2O3) and observed that the catalyst activity was greater than that of the traditions Cu-based catalysts [20]. In the investigations of Saito et al. & Wu et al., they defined the optimal conditions for preparation of multicomponent catalysts. Saito et al. also accounted that the multicomponent catalyst which were developed were highly stable and effective for methanol production from CO 2 [39]. Omata et al. also put forward a similar type of multicomponent catalyst with various amount of substances measuring their activities. They prepared a neural network for predicting the activity on basis of the amount which had been then utilized for finding the optimum catalyst composition [62].
23
2.5.2.3 Pd- based catalysts A Cu/ZnO catalysts deactivates quickly at high temperature in the synthesis of methanol [63]. The supported catalysts based on Pd have been analysed and found to be sturdier [63]. The impact of preparation methods to Pd/ZnO catalysts was studied by Kim et al. They propounded the best structure of catalyst and also put forwarded the method to obtain such structure. The effect of various oxide additives to supported Pd catalysts was also studied by Tsubaki and Fujimoto [64]. They showed by their experiments that the Pd supported catalyst supported by CeO2 has long lifetime and high activity for the formation of methanol from carbon dioxide. They also observed that similar results were shown by TiO 2 and La2O3. Zhang et al. carried out experiments using a Pd supported ZnO catalyst and showed that the performance of the catalyst improved by the addition of Pd [63]. A Cu-based Ce catalyst was developed by Shen et al. They compared the above with a Ce supported catalyst based on Pd and demonstrated that the catalyst based on Cu functioned equally well as the catalyst which was based on Pd. A Pd/Ga2O3 catalyst was developed by Fujitani et al. whose activity was similar to Cu/ZnO catalyst. There was a good effect of Ga 2O3 to the based on Pd catalyst [65]. The experiment gave good results when performed using a catalyst based on Pd/SiO 2 with Ga as an additive. Chiavassa et al. used a Pd/SiO2 catalyst with Ga as a promoter and studied the effect of the products of reaction to its activity. They propounded that H 2O and CO has harmful effects on the catalyst selectivity and activity [66]
2.5.2.4 Other Catalysts Maack et al. analysed the formation of methanol from carbon monoxide. Cu-based catalyst was prepared by them with the addition of potassium. They observed by adding potassium as a promoter in the synthesis and the catalyst which was prepared possesses a good selectivity for the synthesis of methanol. They propounded that the performance of the catalyst was not affected by CO2 with the used regime of pressure [67]. Shao et al. analysed catalysts based on Pt and developed a couple of catalysts, first: Pt, Cr and SiO3, second: with Pt, W and SiO 3. It was seen that they were highly selective & for the formation of methanol from carbon dioxide. But, it is not feasible to use the noble metals as catalysts [68].
24
Table 2. Various catalysts used along with the reactions, reactor used and the reaction conditions
Author
Catalysts
Reaction
Feed
T
[mol%]
[K]
Klier
CuO-ZnO
CO +2H2 ↔ CH3OH
CO: 0-30
et al.
30-70 metal
CO2 + 3H2 ↔ CH3OH +
CO2: 0-30
(1982)
Atomic %
H2O
H2: 70
498523
P [bar ]
Type of Reactor
Tubular 75
Integral Fixed bed
Ared + CO2 ↔ AOX + CO CuO-ZnOGraaf
Al2O3
et al.
Haldor
(1988)
Topsoe Mk 101
CO +2H2 ↔ CH3OH
CO: 0-22
CO2 + 3H2 ↔ CH3OH +
CO2: 1-26
483-
H2O
H2: 67, 4-
518
CO + H2O ↔ CO2 + H2
90
Gradientless 15-50
spinning basket
CO: 0-30 Bussche And
CuO-ZnO-
Froment
Al2O3
(1996) Wedel et al. (1988) Ledako wicz et al. (1992)
BASF S3-85 < 60µm
CO2 + 3H2 ↔ CH3OH + H2O CO2 + H2 ↔ CO + H2O
CO2: 0-30 H2: 70
453-
pCO2/pCO
553
Tubular
: 0-4 CO: 17-50
CO +2H2 ↔ CH3OH
15-51
CO2: 1-5 H2: 31-65
493523
20-60
Grdientless Autoclave Stirred
BASF S3-85 And BT-d
CO: 17-50 CO +2H2 ↔ CH3OH
< 63µm
CO2: 1-5 H2: 31-65
490533
autoclave and 20-60
bubble column slurry reactor
25
Author
Catalysts
Reaction
Feed
T
[mol%]
[K]
P [bar ]
Type of Reactor
CuO-ZnOAl2O3 Commercial Skrzypek
Blasiak’s
CO2 + 3H2 ↔ CH3OH +
CO: 0-20
et al
catalyst
H2O
CO2: 5-35
(1991)
CuO: 62,
CO2 + H2 ↔ CO + H2O
H2: 10-80
460550
Integral fixed 30-90
bed and also differential
ZnO: 30 and Al2O3 : 7 wt% CO +2H2 ↔ CH3OH
Coteron and
Cu70Zn30 and
Hayhurst
Cu70Zr30
(1994)
CO + CO2 + H2 ↔ CH3OH
CO: 10-40
+ H2O
CO2: 0-10
In accordance with the
H2: 40-70
473523
Continuous 10
tubular, differential
mechanism propounded by Chinchen et al.
Setinc
Commercial
and
Cu/ZnO/
Levec (2001)
CO: 10-20
473-
CO +2H2 ↔ CH3OH
CO2: 10-
513
Al2O3
CO2 + 3H2 ↔ CH3OH +
20
(C79 – 5GL;
H2O
H2: 60-80
26
34-41
Slurry reactor
Sud Chemie AG)
CO + H2O ↔ CO2 + H2
CHAPTER 3. FIXED BED REACTOR MODELING
The expanding potential of computers have headed to vast investigation of reactor performance and design, both in the steady and unsteady state. The design and analysis of the reactor classes are continually challenged with the extent of refinement. The extent of sophistication depends firstly on the process i.e. the scheme of the reaction and on its sensitivity to the disturbances in the operating conditions. The extent of accuracy with which the transport and the kinetic parameters is of equal importance. The problems which have to be faced in the modeling and design of fixed bed reactor is illustrated in Figure 5.
27
Figure 5. Aspects to be dealt with in the modelling of fixed bed reactors The modelling of fixed bed reactor can be grouped in two categories: Pseudohomogeneous Heterogeneous Here we will only discuss about the Pseudohomogeneous Models.
3.1 PSEDOHOMODENEOUS MODELS (Basic 1-D Model) 3.1.1 Model Equations The ideal or basic model presumes that the temperature and concentration gradients are only considered in the axial direction. The overall flow itself is the single transport which operates in this direction and hence accounted plug flow type. The equation of conservation for a reaction and at steady state taking place in a cylindrical tube may be written as: −d ( U s C A ) =r A ρB 3.1.1 .1 dz us ρ g C p
dT U =(−ΔH ) r A ρB −4 ( T −T r ) 3.1.1 .2 dz dt
28
−d pt ρ g u 2s =f 3.1.1 .3 dz dp
With the initial conditions: at z=0, CA=CA0, T=T0, pt = pt0. In (3.1.1.3), dp is the equivalent particle diameter. In the specific cases ρg, cp, and cp may be considered to be constant. This system of ordinary differential equations is integrated by means of Runge-Kutta method. The integration purpose can be the design of a new reactor or the simulation of an existing reactor. Equation (3.1.1.1) can be obtained from material balance on a reference component (say A) over an elementary cross section of a tubular reactor containing dW amount of catalyst. To eliminate the bed density, the rate equations of homogeneously catalysed reactions are based on unit catalyst weight, rather than the basis on reactor volume. Over an elementary weight of catalyst, the material balance for A on the conversion basis may be written as: r A dW =F A 0 dx A 3.1 .1 .4 Where FA0 is the molar feed rate of A, or r A ρ B Ωdz=F A 0 dx A 3.1 .1.5 From which (3.1.1.1) can be easily obtained. U in (3.1.1.2) is overall heat transfer coefficient. It is defined by 1 1 d Ab 1 = + + 3.1.1 .6 U α j λ Am αu where
αi = bed side heat transfer coefficient (kJ/m2soC) λ = heat conductivity of the wall (kJ/m2soC)
αu = medium side heat transfer coefficient (kJ/m2soC) Au = heat exchanging surface, heat transfer medium side (m2)
29
Ab = heat exchanging surface, bed side (m2) Am = log mean of Ab and Au (m2) For keeping the surfaces ratio close to one, the thickness of the wall d is small.
αi, for heating up the reaction mixture may be based on Leva’s correlation (1949): α jd p d G =0.813 p λg µ
0.9
( )
e−6 d
p
/dt
For cooling
3.1.1.7
α jd p dp G =3.50 λg µ
0.7
( )
−4.6 d p / dt
e
where dp = equivalent particle diameter (m) dt = tube diameter (m) However, a linear relation between the Reynolds numbers and Nusselt numbers (1972): α j d p α 0j d p c µ d pG = +0.033 p 3.1.1 .8 λg λg λg µ
( )
The impact of the catalyst properties and the tube diameter enter the correlation through α j0 called static contribution: 0
α 0j =
where
10.21 λ e 4/ 3 dt λ0e
is the static contribution to the effective thermal conductivity.
The parameter which now remain to be specified is friction factor f. Special attention is required by the pressure drop equation (3.1.1.3). It is different from the from the Fanning equation. Special care has to be taken for checking the pressure drop relation from which f was derived. Ergun equation (1952) is the most popular equation for determining the pressure 30
drop through packed beds. He considered that the packed bed consist of a bundle of nonconnecting parallel channel with the hydraulic radius Rh (ratio of void fraction ε, to surface of solid per m3 of bed av): R h=
ε av
Equivalent particle diameter dp is the sphere diameter having the same surface area per unit volume as that of the actual particle, Sv: S v=
av 1−ε
So that d p=
6 ( 1−ε ) 3.1 .1.9 av
The pressure drop for laminar flow in an empty conduit given by Hagen Poiseuille when written in the form of (3.1.1.3) gives a friction factor
(1−ε )2 36 f= 3.1.1 .10 ε 3 ( d p G/ µ ) Since the packed bed reactor channels are not straight, a correlation factor of 25/6 has to be introduced by Ergun to fit the experimental data, so that (3.1.1.11) becomes f=
(1−ε )2 150 3.1.1 .11 ε 3 ( d p G/ µ )
For highly turbulent flow, the Burke & Plummer equation gives a friction factor f =1.75
1−ε 3.1 .1.12 ε3
Adding both f=
b ( 1−ε ) 1−ε a+ 3.1.1 .13 3 ℜ ε
31
where a=1.75 and b=150. Handley and Heggs (1968) propounded a value of 1.24 for a and 368 for b. McDonald derived a = 1.8 for smooth particles & 4.0 for rough particles and b = 180. Hicks (1970) concluded that Ergun equation is restricted to Re/ (1-ε) < 500 & Handley and Hegg’s equation to 1000 < Re/ (1-ε) < 5000. He propounded an equation for spheres which gives friction factor:
( 1−ε )1.2 −0.2 f =6.8 ℜ 3.1 .1 .14 ε3 The values of a and b drastically change at dt/dp below 5. Reichelt and Blasz (1957) proposed an equation of equivalent particle diameter for hollow rings: d p=6
V (cylinder ) Em S(external cylinder )
where E=
m=
V ( particle ) S(external cylinder ) 3.1 .115 S ( particle ) V (external cylinder )
d e /d i d 2e
0.4
( ) ε
d 2i
d 2e
0.75
( )
+ 0.010 ε
d 2i
where de and di are the external diameters of the hollow cylinders respectively.
Haughey and Beveridge (1969) correlated the void fraction in packed beds of spheres as:
[
2
]
( d t /d p −2 ) ε =0.38+0.073 1+ 3.1 .1.16 2 ( d t /d p )
32
CHAPTER 4. KINETIC AND REACTOR MODELING 4.1 REACTION KINETICS Synthesis of methanol using syngas is an exothermic equilibrium reaction. The catalyst which is mainly used is an oxide catalyst based on the copper/zinc. Used oxide additives include, viz Cr2O3, ZrO2, and Al2O3 The main reactions involved in the production of methanol are Carbon monoxide Hydrogenation (ΔG = -25.34 kJ.mol-1; ΔH°298= - 90.55 kJ.mol-1)
CO +2H2 ↔CH3OH
(1)
Carbon dioxide Hydrogenation CO2 + 3H2 ↔CH3OH + H2O
(ΔG = 3.30 kJ.mol-1; ΔH°298= - 49.43 kJ.mol-1)
(2)
Reverse Water-gas shift reaction (ΔG = -28.60 kJ.mol-1; ΔH°298= 41.12 kJ.mol-1)
CO + H2O ↔ CO2 + H2
(3)
The model uses the kinetic equation propounded by Vanden Bussche and Froment (1996) [13].
(
k 1 pCO p H 1− 2
r
CH 3 OH
=
(
2
=
(
K p CO p
2
O
3
2
H2
2
2
2
2
2
RWGS
3
eq 2
) )
k3 pH O 1+ + √ k 4 pH +k 5 p H O PH
[
k 2 pCO 1−K eq 3
r
pCH OH p H
(
p H O pCO p CO pH 2
2
2
)]
k3 pH O 1+ + √ k 4 pH +k 5 p H O pH 2
2
2
33
2
3
)
3
In the above equation all the constants (kj) follow Arrhenius equation and equilibrium constants were acquired from the studies that are listed in Table 1 [69].
( RTB )
k j= A j exp
j
Table 3. Kinetic Equation Frequency Factors k1 k1 k1 k1 k1
A B A B A B A B A B
eq
K2
1.07 36696 3453.38 0.499 17197 6.62*10-11 124119 1.22*1010 -94765 10
3066 −10.592 T
−2073 +2.029 T
eq
K2
10
4.2 DEVELOPMENT OF MODEL 4.2.1 Model Assumptions The assumptions considered for developing the mathematical model for the simulation of the pipe shell fixed bed reactor are: a) A plug flow reactor model is assumed. b) Mass and heat transfer along with the diffusion in the catalyst pellet were combined in rate constants. c) The mixture of gas is considered as an ideal gas. d) The reactor is simulated in the steady state condition and 1-D mathematical model is considered. e) Since the ratio of bed length of catalyst (10m) to the particle size of catalyst (0.04m), the radial variations of the parameters can be neglected [70]. f) The radial diffusion in the particles of the catalyst is not considered in the shell and tube fixed bed reactor. g) The reactor is considered single phase.
34
4.2.2 Model Equations A plug flow reactor model was assumed for modelling of methanol reactor. The conditions are considered to be of steady state. In this model, single phase is considered in the reactor. The mole flow is taken as the base of mass balance because in the case of a multiphase-reactor, the molar flow of components is used for writing the balance equations. Component balance equation are illustrated as follows: dFi =ρ c ( r i ) A dl Where i is the molar flow of component i,
ρc
density of catalyst, r i the rate of reaction i
and A is cross-sectional area of reactor. Energy balance equation is given by 2
(−∆ H j) ρ c ( r i ) A dFi ∑ i = dl ∑ Fi∗C p i
Where ΔHi is heat of reaction, ri
ρc
density of catalyst, A cross-sectional area of reactor,
is the ith component reaction rate of the,
component &
Fi
i
Cp
i
is molar heat capacity of the ith
is the ith component molar flow rate. The relationship between the partial
pressure and the molar flow of components assuming the gas to be ideal is illustrated by Pi=P
Fi F =∑ F i Ft t i
Where Pi is ith component partial pressure, P is the total pressure, Fi the ith component molar flow rate and Ft is total molar flow rate.
The components molar heat capacity in the reactor is propounded from the following equation along with the information provided in Table 2
35
Cp = Ai + Bi T +C i T 2+ D i T −2 R where, R is the gas constant, Cp is the molar heat capacity and Ai, Bi, Ci and Di are constants
Enthalpy change of reactions is calculated and used in modeling of the reactor. T o T
∆ H =∆ H
o 298
+ ∫ ∆ C p dT 298
Also ∆ C p=∑ ∆C p ( products )−∑ ∆ C p ( reactants )
Table 4. Frequency Factors of Enthalpy Equation Chemical Species CH3OH H2O H2 CO2 CO
A
103B
106C
10-5D
2.211 3.47 3.249 5.457 3.376
12.216 1.45 0.472 1.045 0.557
-3.450 -
0.121 0.081 -1.157 -0.031
4.3 SOLUTION TECHNIQUE The set of ordinary differential equations (FIVE components participating in the reaction; H2, CO2, CO, H2O, CH3OH and Temperature) are solved using MATLAB 2013a professional software. This set of SIX differential equations are solved at each step interval of the tube of the pipe-shell fixed bed reactor. The step size is given as 0.02, i.e. the length of the tube is divided into 500 different sections for the tube length of 10 m. The related set of ordinary differential equations are solved simultaneously using the ode solver tool ode45 in MATLAB. Based on the initial values and boundary conditions, solver computes all the parameters and create the molar flow rate profile at each step interval. The reactor operating conditions are listed in Table 3 that have been used in this simulation.
36
With the inbuilt subroutines, the solver (ode45) checks for required tolerance, relative error and absolute error in the calculations. Thus, it keeps on solving for every step interval, until it reaches the final length of the reactor. Finally, the whole of reactor data is saved in Microsoft Excel file for further analysis. The reactor data is compared and analysed for best reactor configuration. Table 5. Catalyst Properties, Feed Conditions and Industrial Reactor Specification
Parameter
Value
Unit
H2
80
-
CO
4.76
-
CO2
2.95
-
CH3OH
0.3
-
H2O
0.06
-
N2
0.01
-
CH4
11.92
-
Inlet temperature
498
K
Inlet pressure
50
Bar
Number of tubes
5947
-
Diameter of tube
0.04
m
47400
Kmol hr -1
10
m
1063
kg m-3
Feed composition (mole %)
Flow rate of feed gas Length of tube Typical properties of catalyst Density of catalyst bed
37
CHAPTER 5. RESULTS & DISCUSSIONS
The results of modelling and simulation done with the help of MATLAB code is showed here. The effects of various parameters like the inlet temperature of the reactor, reactor pressure and H2/CO2 mole ratio on the synthesis of methanol are studied. Molar flow rate profile of the components hydrogen, carbon monoxide, carbon dioxide, methanol and steam against the tube length has been tabulated and graphed. In addition to the molar flow rate profiles of the components, graphs showing the yield of methanol, conversion of hydrogen and conversion of carbon dioxide along the tube length are also provided. Four definitions are commenced to study the conversion of CO, conversion of hydrogen and yield of DME through the reactor length:
X CO
XH
2
=
2
=
F CO
2¿
− FCO out 2
F CO FH
2¿
−F H out 2
FH
YieldCH OH (H )= 3
2
YieldCH OH (C )= 3
∗100
2¿
∗100
2¿
FCH
3
FH
F CH
OH
* 100
2
3
OH
( FCO + F CO ) * 100 2
Where FCO
2¿
,
FH
2¿
are molar flow rates at the inlet of the reactor,
are molar flow rates at the outlet of the reactor,
YieldCH
2
X CO conversion of carbon dioxide, 2
F H out 2
XH
2
YieldCH OH (H ) methanol yield against hydrogen present in the
conversion of hydrogen , synthesis gas and
FCO out and
3
3
OH (C )
2
is methanol yield against carbon present in the synthesis 38
gas. The reactor conditions have been tried to be optimized for the maximum conversion of carbon dioxide.
39
5.1 VALIDATION OF MODEL Industrial results for synthesis of methanol ether are obtained from the work of Panahi et al. [71]. The simulated results for the shell and tube fixed bed reactor model has been validated against the results of Panahi et al. indicating that the model used in this study can be utilized for simulation to find the optimum operating conditions and maximum conversion of carbon dioxide and yield of methanol. Table 6. Comparison of results with simulated results given by Panahi et al. Panahi et. al.
Matlab Model
Percentage Error
CO2 Conversion
49.15
47.855
2.634 %
Temperature
528.2
515.25
2.452 %
40
(a)
(b)
(c)
41
(d)
(e)
Figure 6. Model Results of various parameters at T=498 K & P=50 bar (a) Production of methanol (b) Yield of methanol w.r.t. C (c) Conversion of carbon monoxide (d) Conversion of hydrogen, and (e) Temperature along the length of the reactor
42
5.2 EFFECT OF PRESSURE
(a)
(b)
(c)
Figure 7. Effect of Pressure on the molar flow rates of methanol (a) T=478 K (b) T=498 K (c) T=518K 43
(a)
(b)
(c)
Figure 8. Effect of Pressure on carbon monoxide conversion (a) T=478 K (b) T=498 K (c) T=518 K 44
(a)
(b)
(c)
Figure 9. Effect of Pressure on the yield of methanol w.r.t. C (a) T=478 K (b) T=498 K (c) T=518 K 45
(a)
(b)
(c)
Figure 10. Effect of Pressure on the yield of methanol w.r.t. H2 (a) T=478 K (b) T=498 K (c) T=518 K 46
(a)
(b)
(c)
Figure 11. Effect of Effect of Pressure on Carbon Dioxide conversion (a) T=478 K (b) T=498 K (c) T=518 K 47
(a)
(b)
(c)
Figure 12. Effect of Pressure on the conversion of Hydrogen (a) T=478 K (b) T=498 K (c) T=518 K 48
The effect of pressure on the production of methanol, conversion of hydrogen, conversion of carbon dioxide and yield of methanol at the outlet of the reactor is shown in the above figures. In the above figures, H2:CO2 molar feed ratio is 27.11:1 at different constant temperatures. As it can be concluded from the graph of methanol flowrate shown by Figure 6 that molar flowrate of methanol increases with the increase in pressure. This happens because, high pressure favours the production of methanol. On increasing the pressure the thermodynamic equilibrium shifts forward due to the Le-Chatilier’s principle and hence it supports the forward reaction leading to the production of the desired product i.e. Methanol. High pressure also favours the conversion of carbon dioxide, conversion of hydrogen and the yield of methanol.
5.3 EFFECT OF TEMPERATURE The temperature impact on the production of methanol, conversion of hydrogen, conversion of carbon dioxide and yield of methanol at the outlet of the reactor is shown in the following figures. In these figures, H2:CO2 molar feed ratio is 27.11:1 at different constant pressures. As it can be concluded from the graph of methanol flowrate that molar flowrate of methanol, conversion of carbon dioxide, conversion of hydrogen and the yield of methanol does not have considerable change in their values on increasing the temperature i.e. these changes are very less as compares to that of changing pressures. So it can be concluded that the methanol synthesis process is greatly affected by changes in pressures as compared to that of change in the temperatures. The temperature should be kept as low as possible as with the increase in the temperature the thermodynamic equilibrium shifts backward due to the Le-Chatilier’s principle and hence the conversion of carbon dioxide decreases with increase in the temperature According to this principle for an exothermic reaction, if the temperature increases, the backward reaction is more favoured due to backward shift of the thermodynamic equilibrium and thereby the conversion of the main reactant decreases with the increase in the temperature.
49
(a)
(b)
Figure 13. Effect of Temperature on the molar flow rates of methanol (a) P=38 bar (b) P=50 bar
50
(a)
(b)
Figure 14. Effect of Temperature on the yield of methanol w.r.t. C (a) P=38 bar (b) P=50 bar
51
(a)
(b)
Figure 15. Effect of Temperature on the yield of methanol w.r.t. H2 (a) P=38 bar (b) P=50 bar
52
(a)
(b)
Figure 16. Effect of Temperature on carbon dioxide conversion (a) P=38 bar (b) P=50 bar
53
Table 7. Comparison of Temperature Effect in Methanol Synthesis
54
55
The above table shows the effect of variation in temperature during the production of methanol. It is developed using the methanol studies performed by various researchers using different catalysts and kinetics. It is propounded that the kinetics used by various researchers for methanol production shows a very negligible effect on the conversion of carbon monoxide, carbon dioxide and the yield of methanol. So from the above table it can be concluded that the temperature does not have a very significant effect in the production of methanol.
56
5.4 EFFECT OF H2/CO2 MOLE RATIO
(a)
(b)
Figure 17. Effect of H2/CO2 Mole Ratio on (a) Yield of Methanol (b) Molar flow rates of methanol 57
CONCLUSION
A pseudo-homogeneous model of methanol reactor was developed and solved numerically using Runge-Kutta-Verner fourth and fifth order method in MATLAB. The variation profile of important parameters in the reactor was found by the pseudohomogeneous mathematical model. The propounded mathematical model was used for calculating the production of the primary product i.e. methanol against the variation of pressure, temperature and H2/CO2 mole ratio in the feed. It can be concluded from the results that high pressure and low temperature are most favourable for the production of methanol for the kinetics proposed by Vanden Bussche and Froment (1996). It can also be founded from the results of propounded model that Yield of Methanol w.r.t. hydrogen has an optimum value in H2/CO2 = 3.
58
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