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Ethylene oxide – kinetics and mechanism
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Ethylene oxide – kinetics and mechanism Tapio Salmi, Mauricio Roche, Jose´ Herna´ndez Carucci, Kari Era¨nen and Dmitry Murzin Ethylene oxide is one of the most important intermediates in the chemical industry, being used for reactions with ethylene, water, alcohols and organic acids. No general agreement exists today, however, concerning its reaction mechanism and kinetics.Microreactors were proposed as a suitable technology for ethylene oxide production and as tools for rapid and precise investigations of epoxidation kinetics. An evaluation of previously proposed kinetic models was carried out, and rival kinetic models for the epoxidation reaction were screened with experimental data obtained with a laboratory-scale microreactor system. A new, more general kinetic model was proposed for ethylene oxide formation on silver catalysts. Address Laboratory of Industrial Chemistry and Reaction Engineering, Process Chemistry Centre, A˚bo Akademi, FI-20500 Turku/A˚bo, Finland Corresponding author: Salmi, Tapio ([email protected])
Current Opinion in Chemical Engineering 2012, 1:321–327 This review comes from a themed issue on Reaction engineering and catalysis Edited by Theodore T Tsotsis For a complete overview see the Issue and the Editorial Available online 6th July 2012 2211-3398/$ – see front matter, # 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.coche.2012.06.002
Introduction Ethylene oxide is an important industrial intermediate used in the production of many products. The major application of ethylene oxide is in the manufacture of ethylene glycol, which accounts for more than 70% of the total ethylene oxide consumption (in 2009). The production of ethoxylates consumes another 11%, and smaller amounts are used to make higher glycols, ethanolamines, glycol ether, and polyols [1]. Selectivity is a crucially important issue in ethylene epoxidation. An improvement of 1% in selectivity is estimated to produce a beneficial effect of several tens of million dollars annually [2]. Direct oxidation of ethylene in the presence of air or oxygen has today largely replaced the more complex routes to ethylene oxide production, as reviewed by Moulijn et al., 2004 [3]. In general, ethylene epoxidation is considered to follow a parallel reaction scheme, however, further oxidation of ethylene oxide can also take place [4]: C2 H4 þ ð1=2ÞO2 ! C2 H4 O www.sciencedirect.com
ðDH ¼ 105 kJ=molÞ
(I)
C2 H4 þ 3O2 ! 2CO2 þ 2H2 O
ðDH ¼ 1327 kJ=molÞ (II)
C2 H4 O þ ð5=2ÞO2 ! 2CO2 þ2H2 O ðDH¼ 1327 kJ=molÞ (III) Thermodynamics strongly favor the formation of the total oxidation products and the reason why ethylene oxide is not oxidized further is exclusively kinetic. Catalytic and kinetic aspects on the oxidation of ethylene to ethylene oxide are reviewed in this article. Microreactors are shown to be important tools in revealing the kinetics of ethylene oxide formation.
Epoxidation catalysts and kinetics – an overview Several metal catalysts have been investigated for ethylene epoxidation. Nakatsuji et al. [5] studied the activation of oxygen on copper, silver, and gold surfaces for this reaction. They analyzed theoretically the reactivity and the stability of oxygen on the surfaces. The reactivities of the superoxide species on Cu, Ag and Au surface towards ethylene were very similar, all producing ethylene oxide. Any differences observed are attributed to the different stabilities of the adsorbed oxygen species. Rojluechai et al. [6] studied the reaction over supported silver and gold catalysts. In the former case, it was found that the catalyst activity was favorable towards the production of ethylene oxide. Gold acts as a diluting agent on the silver surface creating new single silver sites, which favor molecular oxygen adsorption. They also studied an Au/TiO2 catalyst, reporting a higher selectivity to ethylene oxide, but a lower conversion compared with the silver catalyst. Amorim de Carvalho et al. [7] investigated the surface properties of Cs-promoted and non-promoted silver catalysts applied to the epoxidation reaction. It was found that cesium favored the formation of strongly adsorbed electrophilic oxygen, leading to epoxidation. The reactant mixture (ethylene and oxygen) modified the silver structure, favoring the formation of oxygen electrophilic species. The presence of cesium increased the yield of ethylene oxide, but the most important effects reported were on the stability and selectivity achieved. Lee et al. [8] investigated the effect of different support materials on the activity of silver catalysts. They concluded that supports materials such as g-Al2O3, SiO2, Current Opinion in Chemical Engineering 2012, 1:321–327
322 Reaction engineering and catalysis
MgO, SiC, TiO2, ZrO2 and V2O5, exhibited a pronounced activity in the ethylene oxide isomerization and oxidation. Therefore, these materials formed the products of the total oxidation (water and carbon dioxide) with a poor selectivity toward the epoxidation reaction. Nault et al. [9] first exposed a silver catalyst overnight to pure oxygen at 200 8C and then treated it with ethylene. After an initial increase, the catalyst activity decreased slowly, stabilizing after two hours. It was found that the catalyst treated with ethylene was more active than the one treated with oxygen. The catalyst treated with oxygen only showed an initial reduction in the activity followed by a gradual rise of activity, while the one treated with hydrogen only showed a decrease of the activity. The catalyst treated with ethylene oxide showed a very high activity, increasing for the first 30 min of the reaction. Kestenbaum et al. [10] used a microreactor with polycrystalline silver plates without any support. In order to increase the active surface, the catalyst was oxidized at 530 K under oxygen, at atmospheric pressures, and then reduced at 630 K under hydrogen, at atmospheric pressures, at the same flow rate. They showed an increase of the surface area by a factor of two or three, even with a low precision of the data. The experiments were carried out by varying the feed concentration of oxygen and ethylene, in addition to the total gas flow and pressure of the system. It was found that the selectivity in the microreactor depends on the partial pressure of oxygen; a selectivity of 50% was achieved. Thus, the yield increases with increasing oxygen pressure even at a slight increase of the conversion. However, an increase of the selectivity was noted unexpectedly up to an oxygen concentration of approximately 33%. In addition, it was found that an increase of the pressure up to 5 bar did not lead to an increase of the conversion. The catalyst was operated over 1000 h without any deactivation. Regarding the epoxidation of ethylene, various kinetic models have been proposed to describe the experimental data. Both Langmuir–Hinshelwood and Eley–Rideal mechanisms, assuming different active species and limiting step assumptions, have been proposed, without any definitive conclusions [18]. Klugherz and Harriot [11] suggested a kinetic model, in which ethylene and oxygen competitively adsorb on a partially oxygenated silver surface and bimolecular reaction between adsorbed ethylene and either atomic or molecular oxygen is the rate determining step. Metcalf and Harriot [12] considered molecular oxygen to be the active species for both reactions, and developed rate expressions to account for the inhibition by the reaction product. Petrov et al. [13], using a Ag/a-Al2O3 catalyst promoted by Ca additive in a circulation flow system, Current Opinion in Chemical Engineering 2012, 1:321–327
considered a single-site Eley–Rideal mechanism and reported similar expressions for both the epoxidation and the total combustion reactions. Ghazali et al. [14], using a fixed bed semi-differential reactor at low temperature, and Park and Gau [15] found that the catalytic surface is partially covered by carbonaceous deposits under the reaction conditions, and used a dual-site Langmuir–Hinshelwood mechanism to represent the data. On the basis of the same mechanism, Borman and Westerterp [4] presented expressions accounting for the dependence of the reaction rates on the partial pressures of all components; their kinetic data were obtained using an industrial Ag/a-Al2O3 catalyst in an internal recycle reactor. The rate-determining step was assumed to be a bimolecular reaction between adsorbed ethylene and dissociatively adsorbed oxygen, with the competing reactions occurring on different catalytic sites. Stoukides and Pavlou [16] assumed that epoxidation and complete combustion occur on the same sites. Al-Saleh et al. [17] reported rate expressions, in which carbon dioxide was the only compound inhibiting both reactions. Lafarga et al. [18] studied the epoxidation of ethylene over a cesium-doped silver catalyst supported on a-Al2O3 pellets in a differential reactor. They considered several models found in the literature. They presented a kinetic model with simplifications based on a qualitative analysis, specifically a Langmuir–Hinshelwood mechanism, where the influence of products is negligible. In the studies of Borman and Westerterp [4] and Lafarga et al. [18], the Eley–Rideal type expressions were excluded owing to contradiction with their experimental data. In a recent study, Herna´ndez Carucci et al. [19] evaluated the ability of their data to fit two models. The first model assumes competitive adsorption of ethylene and oxygen on the catalyst surface, whereby oxygen is adsorbed dissociatively and the surface reaction is the rate-limiting step. The second model assumes competitive adsorption of ethylene and molecular oxygen on the surface and the surface reaction is the rate-limiting step. It was found that both models fitted the experimental data obtained in a microreactor, but the model corresponding to the molecular oxygen assumption was slightly better. Stegelmann et al. [2] summarized some of the conclusions found in the literature which they consider most important, such as that both epoxidation and complete oxidation display similar trends for the apparent activation energies and the reaction orders in oxygen and ethylene partial pressures. The reaction order in oxygen can exceed one at low oxygen pressures, and decrease to negative values for very high oxygen pressures. The reaction orders in ethylene have a value of approximately one at low ethylene pressures and become negative at high ethylene pressures. They concluded that the reaction orders in oxygen and ethylene of both epoxidation www.sciencedirect.com
Ethylene oxide – kinetics and mechanism Salmi et al. 323
Table 1 Some kinetic expressions for ethylene oxide formation found in the literature (silver catalysts) Kinetic expression
Reference i
ri ¼ ri ¼ ri ¼
KEi
pffiffiffiffiffiffi pffiffiffiffiffiffi Koi PE Po
Borman and Westerterp [4]
k pffiffiffiffiffiffiffiffiffiffiffi i P þ K i P Þ2 ð1 þ KEi PE þ Koi Po þ KCi PC þ KW W EO EO kK E K O PE PO
Ghazali et al. [14] Park et al. [15] Lafarga et al. [18]
ð1 þ K E PE þ K o Po þ K EO PEO Þ2 n ki PE POi 2
ð1 þ KEi PE Þ k1 P E P O k2 P E P O ; r2 ¼ r1 ¼ 1 þ K E PE K O PO 1 þ K E PE þ K O P O pffiffiffiffiffiffi k0 C E C O k0 C E C O ri ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ; r i ð1 þ K E C e þ K O C O Þ2 ð1 þ K E C e þ K O C O Þ a b
Petrov et al. [13] a Herna´ndez et al. [19] b
Models for epoxidation and total oxidation. Rival models for epoxidation.
and combustion suggest that adsorbed ethylene and some adsorbed oxygen species compete for the same active sites on the surface of the catalyst. The reaction rate orders indicate that ethylene competes more strongly for the active sites compared to the active oxygen species. However, surface science studies indicate that ethylene is only weakly adsorbed on silver, while atomic oxygen adsorbs strongly. Kinetic models are summarized in Table 1. As revealed by Table 1, there does not exist a general agreement about the rate equation for the ethylene epoxidation reaction.
Microreactors in ethylene epoxidation Microreactors have an excellent potential for carrying out epoxidation reactions safely by using gas mixtures that might be inside of the explosive region for a number of
reasons [20]. Since microreactors have only very small volumes, the energy liberated from an explosion in each of its channels would be less than 1 J, which would not be able to affect the integrity of the microreactor. A microreactor system allows for an excellent temperature control, which dramatically diminishes the risk of a runaway in the reactor. Experimental work with microreactors is highly efficient, since only small volumes of catalysts are needed; thus microreactors are excellent tools in the screening of new catalysts. Furthermore, kinetic experiments can be carried out very rapidly in microreactors, just by changing the temperature and pressure according to a planned operation scheme. The flow characteristics of gas-phase microreactors are well-defined, which enables a straightforward interpretation of the kinetic data.
Figure 1
Bypass of the reactor Thermocouple holes Outlet
Inlet 1
Cooling gas tube
Heating cartridges Pressure controller
Inlet 2
Thermocouple holes Current Opinion in Chemical Engineering
Microreactor for ethylene epoxidation. www.sciencedirect.com
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324 Reaction engineering and catalysis
Microreactor device for ethylene epoxidation
A typical microreactor device is shown in Figure 1. The microreactor elements are from the Institut fu¨r Mikrotechnik Mainz GmbH (IMM). The system consists of a gas-phase microreactor with internal heating/cooling; the microreactor was formed by two parts, which are the top plate and the housing. The housing part (Figure 1) has two housing stack cavities, where the plates (both mixing plates in the mixing zone and catalyst plates in the catalyst zone) are suitably fitted. The mixing plates consists of one stack of ten plates, the size of each plate is 7.4 mm 7.4 mm. Different catalyst plates can be used. Ten non-supported silver catalyst plates supplied by IMM (size of each plate, 9.5 mm 9.5 mm) were used in our study. Each plate had nine channels through which the flow passes. The channel volume of one mixer plate is approximately 1.5 mm3. For a catalyst plate, the channel volume is 2.5 mm3, in which the channels have a 460 mm width and an 80 mm height. The system was operated either under 1 bar or under 5 bar for various ethylene and oxygen concentrations. The temperature and the total flow were kept at the same values as those experiments conducted at 4 bar (523 K, 10 ml/min). An on line micro gas chromatograph was used for product analysis (Agilent Technologies 3000A Micro-Gas Chromatograph). The use of the micro GC coupled to the microreactor enables very short analysis times. Reactant concentration effect
The ethylene oxide selectivity at various concentrations is shown in Figure 2. In general, the ethylene oxide selectivity increases with increasing concentration of ethylene and oxygen. A closer examination of the curves in Figure 2 reveals that the selectivity is proportional to the square root of the oxygen concentration. Catalyst pretreatment effect
Herna´ndez Carucci et al. [19] described similar trends about the influence of the reactant concentrations on the
Selectivity towards ethyelne oxide[%]
Figure 2
epoxidation kinetics, but an improvement of the ethylene oxide yield was achieved in this work, even though the operating conditions were similar. The principal difference was the pretreatment applied to the pure silver microplates before each experiment. The kinetic experiments of Herna´ndez Carucci et al. [19] were performed according to a two-step pretreatment method, first with ethylene (20 vol%) and then with oxygen (20 vol%) for 20 min each. The pretreatment was done in order to achieve a stable and active catalyst by the end of pretreatment, even when the ethylene pretreatment led to a higher conversion and selectivity but a lower stability. A single pretreatment of ethylene was applied with a double function of: removing the oxygen adsorbed on the catalyst from a previous experiments and to prepare the catalyst surface for the reaction with sufficient active sites. To ensure stability, the time of the pretreatment was extended to one hour. It was expected that activity might increase with the time of pretreatment (even higher than with oxygen) as was observed by Herna´ndez Carucci et al. [19]. A prolonged pretreatment seems to have a higher favorable effect over the conversion and selectivity levels. An experiment involving 12 h of pretreatment with ethylene and 12 h of reaction was carried out. The catalyst remained active throughout the operation. The ethylene oxide selectivity seems to be linearly proportional to the ethylene conversion. It is feasible to compare the values of conversion and selectivity at the reaction time of 60 min from the long pretreatment over the pure silver microplates, with the conversion and the selectivity levels achieved for the same concentration from the short pretreatment (1 h). For the reaction of 20 vol% of oxygen and 20 vol% of ethylene for 1 h pretreatment, the conversion was 1.2% and the selectivity was 68%. On the contrary, for the pretreatment of 12 h, the conversion was 1.4% and the selectivity 73%. The longer pretreatment helped to slightly increase both conversion and selectivity. This was expected owing to the proportional effect of the pretreatment time with the activity and stability of the catalyst as reported by Herna´ndez Carucci et al. [19].
80
Rate of ethylene oxide formation and apparent reaction orders
60
The dependence of the ethylene oxide formation rate on the reactant concentrations is displayed in Figure 3. Figure 3 shows that the curve corresponding to different oxygen concentrations, while maintaining the ethylene concentration constant (20 vol%), is rather linear. This means that the oxygen reaction order is close to one. This observation provides insights about the mechanism. In that case, the proportionality described above corresponds to molecular adsorption of oxygen on the catalyst. The curve corresponding to the different ethylene concentrations and constant oxygen (20 vol%) is non-linear; suggesting that the adsorption term of ethylene strongly influences the epoxidation rate.
40 20 0 0 10 20 30 Concentration of ethylene or oxygen in the feed [Vol-%] oxygen constant
ethylene constant
Current Opinion in Chemical Engineering
The effect of ethylene and oxygen concentrations on the ethylene oxide selectivity. Reaction conditions: silver microplates, total flow rate of 10 ml/min, 523 K and 4 bar. Current Opinion in Chemical Engineering 2012, 1:321–327
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Ethylene oxide – kinetics and mechanism Salmi et al. 325
Figure 4
Ln (Ethylene oxide formation rate)
Ethylene oxide formation rate [mol/s]
Figure 3
1,0E-07
5,0E-08
0,0E+00
-16
y = 0,4533x - 14,927 R2 = 0,949
-16,5 -17
y = 0,8922x - 13,66 R2 = 0,9689
-17,5 -18
0
0,02
0,04
0,06
Concentration of oxygen in feed [mol/:
-5
-4
Case of ethylene constant
Case of oxygen constant
Case of oxygen constant
Ethylene oxide formation rate at different reactant concentrations.
Lafarga et al. [18] have reported a favorable influence of the reactant concentrations on the reaction rate, whereby the increase of partial pressure of ethylene increases the reaction rate and exhibits saturation for higher pressures, indicating the presence of ethylene adsorption. Analogously, the effect of oxygen concentration was favorable to the reaction rate, but the saturation effect at higher pressures was not as apparent. Identifying whether the atomic or molecular oxygen is the main player acting in the reaction is one of the crucially important issues in ethylene epoxidation. A plot of the rate versus the square root of the oxygen concentration did not go through origin. This observation then excluded the possibility of atomic oxygen being active in epoxidation. The logarithm of the rate of formation of ethylene oxide as a function of the logarithms of the reactant concentrations is plotted in Figure 4. Figure 4 reveals the apparent reaction orders for the oxygen and ethylene, 0.89 and 0.45, respectively. The value of 0.89 indicates that the oxygen effect on the rate expression is close to first order. The deviation from a value of one could be caused by the adsorption term of oxygen, which has a mathematical influence on the rate. Oxygen adsorption seems to be weak or fast enough to result in a low order. The value of 0.45 indicates that ethylene adsorption strongly influences the epoxidation rate. Reaction mechanism and rate equations
The real reaction mechanism of ethylene oxide on the silver surface is still a controversial issue (Table 1). For example, the following reaction mechanisms have been considered: dissociative adsorption of oxygen followed by a surface reaction with adsorbed ethylene
-3
Ln ([Reactant])
Case of ethylene constant Current Opinion in Chemical Engineering
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-6
Current Opinion in Chemical Engineering
Determination of reaction orders: the parity of the logarithm of the epoxidation rate and the logarithm of the reactant concentrations.
(a Langmuir–Hinshelwood mechanism), non-dissociative adsorption of oxygen combined with its surface reaction with adsorbed ethylene (a Langmuir–Hinshelwood mechanism) as well as a reaction between adsorbed ethylene and oxygen from the gas phase (an Eley–Rideal mechanism). However, in the formation of ethylene oxide from ethylene and molecular oxygen, the O–O bond should be broken. The following three alternatives were considered here: a Langmuir–Hinshelwood mechanism with dissociative (atomic) adsorption of oxygen (a), a Langmuir–Hinshelwood mechanism with molecular adsorption of oxygen, combined with recombination of atomic oxygen species to molecularly adsorbed oxygen (b) and, an Eley–Rideal mechanism, in which molecular oxygen from the gas phase reacts with adsorbed ethylene forming ethylene oxide and adsorbed oxygen species (c). The adsorption of ethylene oxide and carbon dioxide was assumed to be negligible; similarly, the adsorbed amount of the intermediates for carbon dioxide formation (total oxidation) were assume to be negligible compared to the amounts of adsorbed ethylene, oxygen and water. The mechanisms and the corresponding rate expressions are presented in Table 2. The total oxidation rate yields equimolar amounts of carbon dioxide and water; thus the concentration of water appearing in the rate equation can be replaced by the concentration of carbon dioxide (K H2 O c H2 O ¼ K H2 O c CO2 in the rate equation). Further simplifications of the rate equations presented in Table 2 can be made by neglecting terms in the denominators; for instance, if the oxygen concentration is low, the oxygen terms in the denominators are discarded. If the reactor operates under differential conditions, that is, at low conversions, the Current Opinion in Chemical Engineering 2012, 1:321–327
326 Reaction engineering and catalysis
Table 2 Reaction mechanisms and rate equations 2a 1 2
E + * = E* O2 + 2* = 2O* E* + O*= EO + 2* 2E + O2 = 2EO
b
0
R1 ¼
k c E c O2 1=2 ð1 þ K E c E þ K O2 1=2 c O2 1=2 þ K H2 O H H2 O Þ
2
E + * = E* O2 + * = O2 * E* + O2* = EO + O* + * 2O* = O2* + *
2 1 2 1
2E + O2 = 2EO b
k0 c E c O2
R1 ¼
ð1 þ K E c E þ K O2 c O2 þ K O2
1=2
K O c O2 1=2 þ K H2 O H H2 O Þ
2
E + * = E* O2 + E* = EO + O* 2O* = O2 + 2*
2 2 1
2E + O2 = 2EO R1 ¼
b
k 0 c E c O2 ð1 þ K E c E þ K O2
1=2
c O2
1=2
þ K H2 O H H2 O Þ
a
The integers represent the stoichiometric numbers of the steps in the mechanism; bwater adsorption included since water is formed in the total oxidation. Component vector (aT): [E O2 EO CO2 H2O]. 1 1=2 1 0 0 Stoichiometric matrix (nT): . 1 3 0 2 2 Reaction stoichiometry: nTa = 0.
regression. A comparison of the experimental reaction rate and the model reaction rate predictions by models (a), (b) and (c) revealed that model (c) (Table 2) gave the best description of the experimental data. Model (a) presented an unsuccessful prediction of the results, since it predicts the maximum reaction order of oxygen to be 0.5, which is not in a good agreement with the experimental observations. Models (b) and (c) described the experimental data almost equally well. These two models resemble each other quite a lot for the case that the concentration of adsorbed oxygen is low. In this case, the reaction order with respect to oxygen is close to one, whereas a difference appears in the reaction order of ethylene. For model (b), the reaction order with respect to ethylene can vary between 1 and 1 (i.e. a retarding effect of ethylene is possible); on the contrary, for model (c), the reaction order of ethylene is always between 1 and zero. Model (c) gives the reaction orders of oxygen between 1 and 0.5, whereas model (b) can predict reaction orders of oxygen between 1 and 1. In this work, the reaction order with respect to oxygen was rather high (about 0.9), but in the literature [2] it has been reported that oxygen at high concentrations can have a retarding effect on the epoxidation rate. Model (b) explains this phenomenon, namely that atomic and molecular oxygen co-exist on the silver surface, and at high oxygen concentrations they suppress the reaction order with respect to oxygen.
Conclusions amount of water formed might be omitted from the rate equation. A comparison of Tables 1 and 2 reveals that many of the previously proposed rate equations can be obtained as special cases of the rate equations proposed in Table 2. A closer examination of the rate equations reveals that the highest reaction orders with respect to oxygen are 1/2, 1 and 1 for equations (a), (b) and (c), respectively (Table 2). For the case that the adsorption term of oxygen is considerably smaller than that of ethylene and water, we get K O 1=2 c O2 1=2 K E c E . The microreactor elements operated under kinetic control, in the absence of external and internal mass transfer limitation – the degree of turbulence was high (a high Reynolds number) and the catalyst layer was thin. The ethylene conversion level was low (typically 3% or less). Thus it was possible to apply the plug flow concept on the experimental data. Moreover, the reactor and operation conditions fulfilled the common criteria of a differential reactor. Since the concentrations of the reactive gas-phase components were low compared to the concentrations of the inert gas, the change in the volumetric flow rate owing to chemical reactions was neglected. The estimation of the kinetic parameters was carried out by standard methods, by minimizing the sum of square errors by non-linear Current Opinion in Chemical Engineering 2012, 1:321–327
The ethylene oxide production processes, catalysts and kinetics were reviewed. Kinetic models proposed in the literature are controversial. A thorough analysis of the discrepancies was performed, new kinetic models were proposed and applied to data obtained from a laboratory-scale microreactor system. Microreactors were proposed as efficient tools for the kinetic analysis of ethylene epoxidation.
Acknowledgement
This work is part of the activities at the A˚bo Akademi Process Chemistry Centre within the Finnish Centre of Excellence Programme (2006–2011) appointed by the Academy of Finland.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest 1.
SRI Consulting: Ethylene Oxide (Abstract). World Petrochemistry. 2010.
2.
Stegelmann C, Schiodt NC, Campbell CT, Stolze P: Microkinetics modeling of ethylene oxidation over silver. J Catal 2004, 221:630-649. This is a fundamental work on the kinetic modeling of ethylene oxidation. 3.
Moulijn J, Makkee M, Van Diepen A: Chemical Process Technology. John Wiley & Sons, Ltd.; 2004:. p. 286–290.
4.
Borman PC, Westerterp KR: An experimental study of the kinetics of the selective oxidation of ethene over a silver on aAl2O3 catalyst. Ind Eng Chem Res 1995, 34:49. www.sciencedirect.com
Ethylene oxide – kinetics and mechanism Salmi et al. 327
The work demonstrates nicely the experimental techniques for kinetic studies of epoxidation. 5.
6.
Nakatsuji H, Hu Z, Nakai H, Ikeda K: Activation of O2 on Cu and Au surface for the epoxidation of ethylene: dipped adcluster model study. Surf Sci 1997, 387:328-341. Rojluechai S, Chavady S, Schwank J, Meeyyo V: Catalytic activity of ethylene oxidation over Au, Ag and Au-Ag catalysts: support effect. Catal Commun 2007, 8:57-64.
7.
Amorim de Carvalho M, Passo F, Schmal M: Study of the active phase of silver catalysts for ethylene epoxidation. J Catal 2007, 248:124-129. The work reveals the role of the active phase in ethylene epoxidation. 8.
9.
Lee JK, Verykios XE, Pitchai R: Support participation in chemistry of ethylene oxidation on silver catalysts. Appl Catal 1988, 44:223-237. Nault LG, Bolme DW, Johanen LN: Reaction rate studies of catalytic oxidation of ethylene. Ind Eng Chem Process Des Dev 1962, 1:170.
10. Kestenbaum H, Lange de Oliveira A, Schmidt W, Schu¨th F, Ehrfeld W, Gebauer K, Lo¨we H, Richter T, Lebiedz D, Untiedt I, Zu¨chner H: Silver-catalyzed oxidation of ethylene to ethylene oxide in a microreaction System. Ind Eng Chem Res 2002, 41:710-719. An important demonstration of the use of microreactors in ethylene epoxidation. 11. Klugherz PD, Harriot PK: Kinetics of ethylene oxidation on a supported silver catalyst. AlChE J 1971, 17:856. 12. Metcalf PL, Harriot P: Kinetics of silver-catalyzed ethylene oxidation. Ind Eng Chem Process Res Dev 1972, 11:478.
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13. Petrov L, Eliyas A, Maximov C, Shopov D: Ethylene oxide oxidation over a supported silver catalyst. I. Kinetics of uninhibited oxidation. Appl Catal 1988, 41:23-38. 14. Ghazali S, Park DW, Gau G: Kinetics of ethylene epoxidation on a silver catalyst. Appl Catal 1983, 6:195. 15. Park DW, Gau G: Ethylene epoxidation on a silver catalyst: unsteady and steady state kinetics. J Catal 1987, 105:81. 16. Stoukides M, Povlou S: Ethylene oxidation on silver catalysts: effect of ethylene oxide and external transfer limitations. Chem Eng Commun 1986, 44:53. 17. Al-Saleh MA, Al-Ahmadi MS, Shaladi MA: A kinetic study of the ethylene oxidation in a Berty reactor. Chem Eng J 1988, 73:37. 18. Lafarga D, Al-Joaied M, Bondy C, Varma A: Ethylene epoxidation on Ag-Cs/a-Al2O3 catalyst: experimental results and strategy for kinetic parameter determination. Ind Eng Chem Res 2000, 39:2148-2156. 19. Herna´ndez Carucci J, Halonen V, Era¨nen K, Wa¨rna˚ J, Ojala S, Huuhtanen M, Keiski R, Salmi T: Ethylene oxide formation in a microreactor; from qualitative kinetics to detailed modeling. Ind Eng Chem Res 2010, 49:10897-10907. The work demonstrates how kinetic parameters can be determined with the aid of microreactor technology. 20. Nijhuis TA, Chen J, Kriescher SMA, Schouten JC: The direct epoxidation of propene in the explosive regime in a microreactor – a study into the reaction kinetics. Ind Eng Chem Res 2010, 49:10479-10485. The big merit of the work is the coupling of kinetic and safety aspects.
Current Opinion in Chemical Engineering 2012, 1:321–327
Ethylene oxide – kinetics and mechanism Tapio Salmi, Mauricio Roche, Jose´ Herna´ndez Carucci, Kari Era¨nen and Dmitry Murzin Ethylene oxide is one of the most important intermediates in the chemical industry, being used for reactions with ethylene, water, alcohols and organic acids. No general agreement exists today, however, concerning its reaction mechanism and kinetics.Microreactors were proposed as a suitable technology for ethylene oxide production and as tools for rapid and precise investigations of epoxidation kinetics. An evaluation of previously proposed kinetic models was carried out, and rival kinetic models for the epoxidation reaction were screened with experimental data obtained with a laboratory-scale microreactor system. A new, more general kinetic model was proposed for ethylene oxide formation on silver catalysts. Address Laboratory of Industrial Chemistry and Reaction Engineering, Process Chemistry Centre, A˚bo Akademi, FI-20500 Turku/A˚bo, Finland Corresponding author: Salmi, Tapio ([email protected])
Current Opinion in Chemical Engineering 2012, 1:321–327 This review comes from a themed issue on Reaction engineering and catalysis Edited by Theodore T Tsotsis For a complete overview see the Issue and the Editorial Available online 6th July 2012 2211-3398/$ – see front matter, # 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.coche.2012.06.002
Introduction Ethylene oxide is an important industrial intermediate used in the production of many products. The major application of ethylene oxide is in the manufacture of ethylene glycol, which accounts for more than 70% of the total ethylene oxide consumption (in 2009). The production of ethoxylates consumes another 11%, and smaller amounts are used to make higher glycols, ethanolamines, glycol ether, and polyols [1]. Selectivity is a crucially important issue in ethylene epoxidation. An improvement of 1% in selectivity is estimated to produce a beneficial effect of several tens of million dollars annually [2]. Direct oxidation of ethylene in the presence of air or oxygen has today largely replaced the more complex routes to ethylene oxide production, as reviewed by Moulijn et al., 2004 [3]. In general, ethylene epoxidation is considered to follow a parallel reaction scheme, however, further oxidation of ethylene oxide can also take place [4]: C2 H4 þ ð1=2ÞO2 ! C2 H4 O www.sciencedirect.com
ðDH ¼ 105 kJ=molÞ
(I)
C2 H4 þ 3O2 ! 2CO2 þ 2H2 O
ðDH ¼ 1327 kJ=molÞ (II)
C2 H4 O þ ð5=2ÞO2 ! 2CO2 þ2H2 O ðDH¼ 1327 kJ=molÞ (III) Thermodynamics strongly favor the formation of the total oxidation products and the reason why ethylene oxide is not oxidized further is exclusively kinetic. Catalytic and kinetic aspects on the oxidation of ethylene to ethylene oxide are reviewed in this article. Microreactors are shown to be important tools in revealing the kinetics of ethylene oxide formation.
Epoxidation catalysts and kinetics – an overview Several metal catalysts have been investigated for ethylene epoxidation. Nakatsuji et al. [5] studied the activation of oxygen on copper, silver, and gold surfaces for this reaction. They analyzed theoretically the reactivity and the stability of oxygen on the surfaces. The reactivities of the superoxide species on Cu, Ag and Au surface towards ethylene were very similar, all producing ethylene oxide. Any differences observed are attributed to the different stabilities of the adsorbed oxygen species. Rojluechai et al. [6] studied the reaction over supported silver and gold catalysts. In the former case, it was found that the catalyst activity was favorable towards the production of ethylene oxide. Gold acts as a diluting agent on the silver surface creating new single silver sites, which favor molecular oxygen adsorption. They also studied an Au/TiO2 catalyst, reporting a higher selectivity to ethylene oxide, but a lower conversion compared with the silver catalyst. Amorim de Carvalho et al. [7] investigated the surface properties of Cs-promoted and non-promoted silver catalysts applied to the epoxidation reaction. It was found that cesium favored the formation of strongly adsorbed electrophilic oxygen, leading to epoxidation. The reactant mixture (ethylene and oxygen) modified the silver structure, favoring the formation of oxygen electrophilic species. The presence of cesium increased the yield of ethylene oxide, but the most important effects reported were on the stability and selectivity achieved. Lee et al. [8] investigated the effect of different support materials on the activity of silver catalysts. They concluded that supports materials such as g-Al2O3, SiO2, Current Opinion in Chemical Engineering 2012, 1:321–327
322 Reaction engineering and catalysis
MgO, SiC, TiO2, ZrO2 and V2O5, exhibited a pronounced activity in the ethylene oxide isomerization and oxidation. Therefore, these materials formed the products of the total oxidation (water and carbon dioxide) with a poor selectivity toward the epoxidation reaction. Nault et al. [9] first exposed a silver catalyst overnight to pure oxygen at 200 8C and then treated it with ethylene. After an initial increase, the catalyst activity decreased slowly, stabilizing after two hours. It was found that the catalyst treated with ethylene was more active than the one treated with oxygen. The catalyst treated with oxygen only showed an initial reduction in the activity followed by a gradual rise of activity, while the one treated with hydrogen only showed a decrease of the activity. The catalyst treated with ethylene oxide showed a very high activity, increasing for the first 30 min of the reaction. Kestenbaum et al. [10] used a microreactor with polycrystalline silver plates without any support. In order to increase the active surface, the catalyst was oxidized at 530 K under oxygen, at atmospheric pressures, and then reduced at 630 K under hydrogen, at atmospheric pressures, at the same flow rate. They showed an increase of the surface area by a factor of two or three, even with a low precision of the data. The experiments were carried out by varying the feed concentration of oxygen and ethylene, in addition to the total gas flow and pressure of the system. It was found that the selectivity in the microreactor depends on the partial pressure of oxygen; a selectivity of 50% was achieved. Thus, the yield increases with increasing oxygen pressure even at a slight increase of the conversion. However, an increase of the selectivity was noted unexpectedly up to an oxygen concentration of approximately 33%. In addition, it was found that an increase of the pressure up to 5 bar did not lead to an increase of the conversion. The catalyst was operated over 1000 h without any deactivation. Regarding the epoxidation of ethylene, various kinetic models have been proposed to describe the experimental data. Both Langmuir–Hinshelwood and Eley–Rideal mechanisms, assuming different active species and limiting step assumptions, have been proposed, without any definitive conclusions [18]. Klugherz and Harriot [11] suggested a kinetic model, in which ethylene and oxygen competitively adsorb on a partially oxygenated silver surface and bimolecular reaction between adsorbed ethylene and either atomic or molecular oxygen is the rate determining step. Metcalf and Harriot [12] considered molecular oxygen to be the active species for both reactions, and developed rate expressions to account for the inhibition by the reaction product. Petrov et al. [13], using a Ag/a-Al2O3 catalyst promoted by Ca additive in a circulation flow system, Current Opinion in Chemical Engineering 2012, 1:321–327
considered a single-site Eley–Rideal mechanism and reported similar expressions for both the epoxidation and the total combustion reactions. Ghazali et al. [14], using a fixed bed semi-differential reactor at low temperature, and Park and Gau [15] found that the catalytic surface is partially covered by carbonaceous deposits under the reaction conditions, and used a dual-site Langmuir–Hinshelwood mechanism to represent the data. On the basis of the same mechanism, Borman and Westerterp [4] presented expressions accounting for the dependence of the reaction rates on the partial pressures of all components; their kinetic data were obtained using an industrial Ag/a-Al2O3 catalyst in an internal recycle reactor. The rate-determining step was assumed to be a bimolecular reaction between adsorbed ethylene and dissociatively adsorbed oxygen, with the competing reactions occurring on different catalytic sites. Stoukides and Pavlou [16] assumed that epoxidation and complete combustion occur on the same sites. Al-Saleh et al. [17] reported rate expressions, in which carbon dioxide was the only compound inhibiting both reactions. Lafarga et al. [18] studied the epoxidation of ethylene over a cesium-doped silver catalyst supported on a-Al2O3 pellets in a differential reactor. They considered several models found in the literature. They presented a kinetic model with simplifications based on a qualitative analysis, specifically a Langmuir–Hinshelwood mechanism, where the influence of products is negligible. In the studies of Borman and Westerterp [4] and Lafarga et al. [18], the Eley–Rideal type expressions were excluded owing to contradiction with their experimental data. In a recent study, Herna´ndez Carucci et al. [19] evaluated the ability of their data to fit two models. The first model assumes competitive adsorption of ethylene and oxygen on the catalyst surface, whereby oxygen is adsorbed dissociatively and the surface reaction is the rate-limiting step. The second model assumes competitive adsorption of ethylene and molecular oxygen on the surface and the surface reaction is the rate-limiting step. It was found that both models fitted the experimental data obtained in a microreactor, but the model corresponding to the molecular oxygen assumption was slightly better. Stegelmann et al. [2] summarized some of the conclusions found in the literature which they consider most important, such as that both epoxidation and complete oxidation display similar trends for the apparent activation energies and the reaction orders in oxygen and ethylene partial pressures. The reaction order in oxygen can exceed one at low oxygen pressures, and decrease to negative values for very high oxygen pressures. The reaction orders in ethylene have a value of approximately one at low ethylene pressures and become negative at high ethylene pressures. They concluded that the reaction orders in oxygen and ethylene of both epoxidation www.sciencedirect.com
Ethylene oxide – kinetics and mechanism Salmi et al. 323
Table 1 Some kinetic expressions for ethylene oxide formation found in the literature (silver catalysts) Kinetic expression
Reference i
ri ¼ ri ¼ ri ¼
KEi
pffiffiffiffiffiffi pffiffiffiffiffiffi Koi PE Po
Borman and Westerterp [4]
k pffiffiffiffiffiffiffiffiffiffiffi i P þ K i P Þ2 ð1 þ KEi PE þ Koi Po þ KCi PC þ KW W EO EO kK E K O PE PO
Ghazali et al. [14] Park et al. [15] Lafarga et al. [18]
ð1 þ K E PE þ K o Po þ K EO PEO Þ2 n ki PE POi 2
ð1 þ KEi PE Þ k1 P E P O k2 P E P O ; r2 ¼ r1 ¼ 1 þ K E PE K O PO 1 þ K E PE þ K O P O pffiffiffiffiffiffi k0 C E C O k0 C E C O ri ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ; r i ð1 þ K E C e þ K O C O Þ2 ð1 þ K E C e þ K O C O Þ a b
Petrov et al. [13] a Herna´ndez et al. [19] b
Models for epoxidation and total oxidation. Rival models for epoxidation.
and combustion suggest that adsorbed ethylene and some adsorbed oxygen species compete for the same active sites on the surface of the catalyst. The reaction rate orders indicate that ethylene competes more strongly for the active sites compared to the active oxygen species. However, surface science studies indicate that ethylene is only weakly adsorbed on silver, while atomic oxygen adsorbs strongly. Kinetic models are summarized in Table 1. As revealed by Table 1, there does not exist a general agreement about the rate equation for the ethylene epoxidation reaction.
Microreactors in ethylene epoxidation Microreactors have an excellent potential for carrying out epoxidation reactions safely by using gas mixtures that might be inside of the explosive region for a number of
reasons [20]. Since microreactors have only very small volumes, the energy liberated from an explosion in each of its channels would be less than 1 J, which would not be able to affect the integrity of the microreactor. A microreactor system allows for an excellent temperature control, which dramatically diminishes the risk of a runaway in the reactor. Experimental work with microreactors is highly efficient, since only small volumes of catalysts are needed; thus microreactors are excellent tools in the screening of new catalysts. Furthermore, kinetic experiments can be carried out very rapidly in microreactors, just by changing the temperature and pressure according to a planned operation scheme. The flow characteristics of gas-phase microreactors are well-defined, which enables a straightforward interpretation of the kinetic data.
Figure 1
Bypass of the reactor Thermocouple holes Outlet
Inlet 1
Cooling gas tube
Heating cartridges Pressure controller
Inlet 2
Thermocouple holes Current Opinion in Chemical Engineering
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324 Reaction engineering and catalysis
Microreactor device for ethylene epoxidation
A typical microreactor device is shown in Figure 1. The microreactor elements are from the Institut fu¨r Mikrotechnik Mainz GmbH (IMM). The system consists of a gas-phase microreactor with internal heating/cooling; the microreactor was formed by two parts, which are the top plate and the housing. The housing part (Figure 1) has two housing stack cavities, where the plates (both mixing plates in the mixing zone and catalyst plates in the catalyst zone) are suitably fitted. The mixing plates consists of one stack of ten plates, the size of each plate is 7.4 mm 7.4 mm. Different catalyst plates can be used. Ten non-supported silver catalyst plates supplied by IMM (size of each plate, 9.5 mm 9.5 mm) were used in our study. Each plate had nine channels through which the flow passes. The channel volume of one mixer plate is approximately 1.5 mm3. For a catalyst plate, the channel volume is 2.5 mm3, in which the channels have a 460 mm width and an 80 mm height. The system was operated either under 1 bar or under 5 bar for various ethylene and oxygen concentrations. The temperature and the total flow were kept at the same values as those experiments conducted at 4 bar (523 K, 10 ml/min). An on line micro gas chromatograph was used for product analysis (Agilent Technologies 3000A Micro-Gas Chromatograph). The use of the micro GC coupled to the microreactor enables very short analysis times. Reactant concentration effect
The ethylene oxide selectivity at various concentrations is shown in Figure 2. In general, the ethylene oxide selectivity increases with increasing concentration of ethylene and oxygen. A closer examination of the curves in Figure 2 reveals that the selectivity is proportional to the square root of the oxygen concentration. Catalyst pretreatment effect
Herna´ndez Carucci et al. [19] described similar trends about the influence of the reactant concentrations on the
Selectivity towards ethyelne oxide[%]
Figure 2
epoxidation kinetics, but an improvement of the ethylene oxide yield was achieved in this work, even though the operating conditions were similar. The principal difference was the pretreatment applied to the pure silver microplates before each experiment. The kinetic experiments of Herna´ndez Carucci et al. [19] were performed according to a two-step pretreatment method, first with ethylene (20 vol%) and then with oxygen (20 vol%) for 20 min each. The pretreatment was done in order to achieve a stable and active catalyst by the end of pretreatment, even when the ethylene pretreatment led to a higher conversion and selectivity but a lower stability. A single pretreatment of ethylene was applied with a double function of: removing the oxygen adsorbed on the catalyst from a previous experiments and to prepare the catalyst surface for the reaction with sufficient active sites. To ensure stability, the time of the pretreatment was extended to one hour. It was expected that activity might increase with the time of pretreatment (even higher than with oxygen) as was observed by Herna´ndez Carucci et al. [19]. A prolonged pretreatment seems to have a higher favorable effect over the conversion and selectivity levels. An experiment involving 12 h of pretreatment with ethylene and 12 h of reaction was carried out. The catalyst remained active throughout the operation. The ethylene oxide selectivity seems to be linearly proportional to the ethylene conversion. It is feasible to compare the values of conversion and selectivity at the reaction time of 60 min from the long pretreatment over the pure silver microplates, with the conversion and the selectivity levels achieved for the same concentration from the short pretreatment (1 h). For the reaction of 20 vol% of oxygen and 20 vol% of ethylene for 1 h pretreatment, the conversion was 1.2% and the selectivity was 68%. On the contrary, for the pretreatment of 12 h, the conversion was 1.4% and the selectivity 73%. The longer pretreatment helped to slightly increase both conversion and selectivity. This was expected owing to the proportional effect of the pretreatment time with the activity and stability of the catalyst as reported by Herna´ndez Carucci et al. [19].
80
Rate of ethylene oxide formation and apparent reaction orders
60
The dependence of the ethylene oxide formation rate on the reactant concentrations is displayed in Figure 3. Figure 3 shows that the curve corresponding to different oxygen concentrations, while maintaining the ethylene concentration constant (20 vol%), is rather linear. This means that the oxygen reaction order is close to one. This observation provides insights about the mechanism. In that case, the proportionality described above corresponds to molecular adsorption of oxygen on the catalyst. The curve corresponding to the different ethylene concentrations and constant oxygen (20 vol%) is non-linear; suggesting that the adsorption term of ethylene strongly influences the epoxidation rate.
40 20 0 0 10 20 30 Concentration of ethylene or oxygen in the feed [Vol-%] oxygen constant
ethylene constant
Current Opinion in Chemical Engineering
The effect of ethylene and oxygen concentrations on the ethylene oxide selectivity. Reaction conditions: silver microplates, total flow rate of 10 ml/min, 523 K and 4 bar. Current Opinion in Chemical Engineering 2012, 1:321–327
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Ethylene oxide – kinetics and mechanism Salmi et al. 325
Figure 4
Ln (Ethylene oxide formation rate)
Ethylene oxide formation rate [mol/s]
Figure 3
1,0E-07
5,0E-08
0,0E+00
-16
y = 0,4533x - 14,927 R2 = 0,949
-16,5 -17
y = 0,8922x - 13,66 R2 = 0,9689
-17,5 -18
0
0,02
0,04
0,06
Concentration of oxygen in feed [mol/:
-5
-4
Case of ethylene constant
Case of oxygen constant
Case of oxygen constant
Ethylene oxide formation rate at different reactant concentrations.
Lafarga et al. [18] have reported a favorable influence of the reactant concentrations on the reaction rate, whereby the increase of partial pressure of ethylene increases the reaction rate and exhibits saturation for higher pressures, indicating the presence of ethylene adsorption. Analogously, the effect of oxygen concentration was favorable to the reaction rate, but the saturation effect at higher pressures was not as apparent. Identifying whether the atomic or molecular oxygen is the main player acting in the reaction is one of the crucially important issues in ethylene epoxidation. A plot of the rate versus the square root of the oxygen concentration did not go through origin. This observation then excluded the possibility of atomic oxygen being active in epoxidation. The logarithm of the rate of formation of ethylene oxide as a function of the logarithms of the reactant concentrations is plotted in Figure 4. Figure 4 reveals the apparent reaction orders for the oxygen and ethylene, 0.89 and 0.45, respectively. The value of 0.89 indicates that the oxygen effect on the rate expression is close to first order. The deviation from a value of one could be caused by the adsorption term of oxygen, which has a mathematical influence on the rate. Oxygen adsorption seems to be weak or fast enough to result in a low order. The value of 0.45 indicates that ethylene adsorption strongly influences the epoxidation rate. Reaction mechanism and rate equations
The real reaction mechanism of ethylene oxide on the silver surface is still a controversial issue (Table 1). For example, the following reaction mechanisms have been considered: dissociative adsorption of oxygen followed by a surface reaction with adsorbed ethylene
-3
Ln ([Reactant])
Case of ethylene constant Current Opinion in Chemical Engineering
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-6
Current Opinion in Chemical Engineering
Determination of reaction orders: the parity of the logarithm of the epoxidation rate and the logarithm of the reactant concentrations.
(a Langmuir–Hinshelwood mechanism), non-dissociative adsorption of oxygen combined with its surface reaction with adsorbed ethylene (a Langmuir–Hinshelwood mechanism) as well as a reaction between adsorbed ethylene and oxygen from the gas phase (an Eley–Rideal mechanism). However, in the formation of ethylene oxide from ethylene and molecular oxygen, the O–O bond should be broken. The following three alternatives were considered here: a Langmuir–Hinshelwood mechanism with dissociative (atomic) adsorption of oxygen (a), a Langmuir–Hinshelwood mechanism with molecular adsorption of oxygen, combined with recombination of atomic oxygen species to molecularly adsorbed oxygen (b) and, an Eley–Rideal mechanism, in which molecular oxygen from the gas phase reacts with adsorbed ethylene forming ethylene oxide and adsorbed oxygen species (c). The adsorption of ethylene oxide and carbon dioxide was assumed to be negligible; similarly, the adsorbed amount of the intermediates for carbon dioxide formation (total oxidation) were assume to be negligible compared to the amounts of adsorbed ethylene, oxygen and water. The mechanisms and the corresponding rate expressions are presented in Table 2. The total oxidation rate yields equimolar amounts of carbon dioxide and water; thus the concentration of water appearing in the rate equation can be replaced by the concentration of carbon dioxide (K H2 O c H2 O ¼ K H2 O c CO2 in the rate equation). Further simplifications of the rate equations presented in Table 2 can be made by neglecting terms in the denominators; for instance, if the oxygen concentration is low, the oxygen terms in the denominators are discarded. If the reactor operates under differential conditions, that is, at low conversions, the Current Opinion in Chemical Engineering 2012, 1:321–327
326 Reaction engineering and catalysis
Table 2 Reaction mechanisms and rate equations 2a 1 2
E + * = E* O2 + 2* = 2O* E* + O*= EO + 2* 2E + O2 = 2EO
b
0
R1 ¼
k c E c O2 1=2 ð1 þ K E c E þ K O2 1=2 c O2 1=2 þ K H2 O H H2 O Þ
2
E + * = E* O2 + * = O2 * E* + O2* = EO + O* + * 2O* = O2* + *
2 1 2 1
2E + O2 = 2EO b
k0 c E c O2
R1 ¼
ð1 þ K E c E þ K O2 c O2 þ K O2
1=2
K O c O2 1=2 þ K H2 O H H2 O Þ
2
E + * = E* O2 + E* = EO + O* 2O* = O2 + 2*
2 2 1
2E + O2 = 2EO R1 ¼
b
k 0 c E c O2 ð1 þ K E c E þ K O2
1=2
c O2
1=2
þ K H2 O H H2 O Þ
a
The integers represent the stoichiometric numbers of the steps in the mechanism; bwater adsorption included since water is formed in the total oxidation. Component vector (aT): [E O2 EO CO2 H2O]. 1 1=2 1 0 0 Stoichiometric matrix (nT): . 1 3 0 2 2 Reaction stoichiometry: nTa = 0.
regression. A comparison of the experimental reaction rate and the model reaction rate predictions by models (a), (b) and (c) revealed that model (c) (Table 2) gave the best description of the experimental data. Model (a) presented an unsuccessful prediction of the results, since it predicts the maximum reaction order of oxygen to be 0.5, which is not in a good agreement with the experimental observations. Models (b) and (c) described the experimental data almost equally well. These two models resemble each other quite a lot for the case that the concentration of adsorbed oxygen is low. In this case, the reaction order with respect to oxygen is close to one, whereas a difference appears in the reaction order of ethylene. For model (b), the reaction order with respect to ethylene can vary between 1 and 1 (i.e. a retarding effect of ethylene is possible); on the contrary, for model (c), the reaction order of ethylene is always between 1 and zero. Model (c) gives the reaction orders of oxygen between 1 and 0.5, whereas model (b) can predict reaction orders of oxygen between 1 and 1. In this work, the reaction order with respect to oxygen was rather high (about 0.9), but in the literature [2] it has been reported that oxygen at high concentrations can have a retarding effect on the epoxidation rate. Model (b) explains this phenomenon, namely that atomic and molecular oxygen co-exist on the silver surface, and at high oxygen concentrations they suppress the reaction order with respect to oxygen.
Conclusions amount of water formed might be omitted from the rate equation. A comparison of Tables 1 and 2 reveals that many of the previously proposed rate equations can be obtained as special cases of the rate equations proposed in Table 2. A closer examination of the rate equations reveals that the highest reaction orders with respect to oxygen are 1/2, 1 and 1 for equations (a), (b) and (c), respectively (Table 2). For the case that the adsorption term of oxygen is considerably smaller than that of ethylene and water, we get K O 1=2 c O2 1=2 K E c E . The microreactor elements operated under kinetic control, in the absence of external and internal mass transfer limitation – the degree of turbulence was high (a high Reynolds number) and the catalyst layer was thin. The ethylene conversion level was low (typically 3% or less). Thus it was possible to apply the plug flow concept on the experimental data. Moreover, the reactor and operation conditions fulfilled the common criteria of a differential reactor. Since the concentrations of the reactive gas-phase components were low compared to the concentrations of the inert gas, the change in the volumetric flow rate owing to chemical reactions was neglected. The estimation of the kinetic parameters was carried out by standard methods, by minimizing the sum of square errors by non-linear Current Opinion in Chemical Engineering 2012, 1:321–327
The ethylene oxide production processes, catalysts and kinetics were reviewed. Kinetic models proposed in the literature are controversial. A thorough analysis of the discrepancies was performed, new kinetic models were proposed and applied to data obtained from a laboratory-scale microreactor system. Microreactors were proposed as efficient tools for the kinetic analysis of ethylene epoxidation.
Acknowledgement
This work is part of the activities at the A˚bo Akademi Process Chemistry Centre within the Finnish Centre of Excellence Programme (2006–2011) appointed by the Academy of Finland.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest 1.
SRI Consulting: Ethylene Oxide (Abstract). World Petrochemistry. 2010.
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Stegelmann C, Schiodt NC, Campbell CT, Stolze P: Microkinetics modeling of ethylene oxidation over silver. J Catal 2004, 221:630-649. This is a fundamental work on the kinetic modeling of ethylene oxidation. 3.
Moulijn J, Makkee M, Van Diepen A: Chemical Process Technology. John Wiley & Sons, Ltd.; 2004:. p. 286–290.
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Borman PC, Westerterp KR: An experimental study of the kinetics of the selective oxidation of ethene over a silver on aAl2O3 catalyst. Ind Eng Chem Res 1995, 34:49. www.sciencedirect.com
Ethylene oxide – kinetics and mechanism Salmi et al. 327
The work demonstrates nicely the experimental techniques for kinetic studies of epoxidation. 5.
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Nakatsuji H, Hu Z, Nakai H, Ikeda K: Activation of O2 on Cu and Au surface for the epoxidation of ethylene: dipped adcluster model study. Surf Sci 1997, 387:328-341. Rojluechai S, Chavady S, Schwank J, Meeyyo V: Catalytic activity of ethylene oxidation over Au, Ag and Au-Ag catalysts: support effect. Catal Commun 2007, 8:57-64.
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Lee JK, Verykios XE, Pitchai R: Support participation in chemistry of ethylene oxidation on silver catalysts. Appl Catal 1988, 44:223-237. Nault LG, Bolme DW, Johanen LN: Reaction rate studies of catalytic oxidation of ethylene. Ind Eng Chem Process Des Dev 1962, 1:170.
10. Kestenbaum H, Lange de Oliveira A, Schmidt W, Schu¨th F, Ehrfeld W, Gebauer K, Lo¨we H, Richter T, Lebiedz D, Untiedt I, Zu¨chner H: Silver-catalyzed oxidation of ethylene to ethylene oxide in a microreaction System. Ind Eng Chem Res 2002, 41:710-719. An important demonstration of the use of microreactors in ethylene epoxidation. 11. Klugherz PD, Harriot PK: Kinetics of ethylene oxidation on a supported silver catalyst. AlChE J 1971, 17:856. 12. Metcalf PL, Harriot P: Kinetics of silver-catalyzed ethylene oxidation. Ind Eng Chem Process Res Dev 1972, 11:478.
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13. Petrov L, Eliyas A, Maximov C, Shopov D: Ethylene oxide oxidation over a supported silver catalyst. I. Kinetics of uninhibited oxidation. Appl Catal 1988, 41:23-38. 14. Ghazali S, Park DW, Gau G: Kinetics of ethylene epoxidation on a silver catalyst. Appl Catal 1983, 6:195. 15. Park DW, Gau G: Ethylene epoxidation on a silver catalyst: unsteady and steady state kinetics. J Catal 1987, 105:81. 16. Stoukides M, Povlou S: Ethylene oxidation on silver catalysts: effect of ethylene oxide and external transfer limitations. Chem Eng Commun 1986, 44:53. 17. Al-Saleh MA, Al-Ahmadi MS, Shaladi MA: A kinetic study of the ethylene oxidation in a Berty reactor. Chem Eng J 1988, 73:37. 18. Lafarga D, Al-Joaied M, Bondy C, Varma A: Ethylene epoxidation on Ag-Cs/a-Al2O3 catalyst: experimental results and strategy for kinetic parameter determination. Ind Eng Chem Res 2000, 39:2148-2156. 19. Herna´ndez Carucci J, Halonen V, Era¨nen K, Wa¨rna˚ J, Ojala S, Huuhtanen M, Keiski R, Salmi T: Ethylene oxide formation in a microreactor; from qualitative kinetics to detailed modeling. Ind Eng Chem Res 2010, 49:10897-10907. The work demonstrates how kinetic parameters can be determined with the aid of microreactor technology. 20. Nijhuis TA, Chen J, Kriescher SMA, Schouten JC: The direct epoxidation of propene in the explosive regime in a microreactor – a study into the reaction kinetics. Ind Eng Chem Res 2010, 49:10479-10485. The big merit of the work is the coupling of kinetic and safety aspects.
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