Thermodynamic analysis of chemical and phase equilibria in CO2

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Thermodynamic analysis of chemical and phase equilibria in CO hydrogenation to methanol, dimethyl ether and higher alcohols Kristian Stangeland, Hailong Li, and Zhixin Yu

Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04866 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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Industrial & Engineering Chemistry Research

Thermodynamic analysis of chemical and phase equilibria in CO2 hydrogenation to methanol, dimethyl ether and higher alcohols Kristian Stangeland1, Hailong Li2, Zhixin Yu1* 1

Department of Petroleum Engineering, University of Stavanger, 4036 Stavanger, Norway

2

Department of Energy, Building and Environment, Mälardalen University, 73123 Västerås, Sweden

*Corresponding author.

Tel.: +47-5183-2238;

Fax: +47-5183-2050;

Email address: [email protected]

1

ACS Paragon Plus Environment

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Abstract: CO2 hydrogenation can lead to the formation of various products, of which methanol, dimethyl ether (DME) and ethanol have received great attention. In this study, a comprehensive thermodynamic analysis of CO2 hydrogenation in binary (methanol/CO) and ternary product systems (methanol/CO with DME or ethanol) is conducted in Aspen Plus by the Gibbs free energy minimization method combined with phase equilibrium calculations. It is demonstrated that product condensation can be utilized to circumvent thermodynamic restrictions on product yield. Significant improvements in CO2 conversion can be achieved by operating at conditions favorable for product condensation, whereas the selectivity is mildly affected. The relevance of the results herein is discussed with regards to recent advances in catalysis and process design for CO2 hydrogenation. Our study highlights the importance of obtaining a thorough understanding of the thermodynamics of CO2 hydrogenation processes, which will be critical for developing potential breakthrough technology applicable at industrial scale. Keywords: CO2 hydrogenation; methanol; dimethyl ether; ethanol; thermodynamic analysis; phase equilibrium

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1. Introduction Despite the adoption of alternative energy sources and efficient energy systems to reduce carbon dioxide (CO2) emissions, the cumulative amount of CO2 in the atmosphere still needs to be reduced to limit the detrimental impact of climate change 1. Several strategies to mitigate CO2 emissions have been proposed over the last decades, among which carbon utilization by conversion to fuels and chemicals appears promising 2. Of the various products that can be directly synthesized from CO2, much effort is being put on the development of technologies for methanol production. Methanol is commonly used as solvent and feedstock to produce a variety of chemicals as well as a fuel additive 3-4. Today, methanol is commercially produced from synthesis gas (CO/CO2/H2) over CuO/ZnO/Al2O3 based catalysts, and the development of a few large-scale plants based on CO2/H2 is in progress 5. For instance, Carbon Recycling International (CRI) operates a methanol plant in Svartsengi (Iceland) and expanded its capacity to more than 5 million liters per year in 2015 6. Methanol synthesis from CO2 can be represented by three main reactions. The methanol synthesis reaction from CO2 (Eq. 1.1) and CO (Eq. 1.2) are exothermic, and are favored at low temperature and high pressure. The main side reaction during methanol synthesis from CO2 is the reverse water-gas shift (RWGS) reactions (Eq. 1.3). CO2 + 3H2 ↔ CH3OH + H2O

∆ = −49.5 kJ/mol

(1.1)

CO + 2H2 ↔ CH3OH

∆ = −90.5 kJ/mol

(1.2)

CO2 + H2 ↔ CO + H2O

∆ = 41.2 kJ/mol

(1.3)

Synthesis of dimethyl ether (DME) have also gained increasing interest lately

7-8

. Although DME can be

considered as a byproduct in CO2 hydrogenation to methanol, DME itself is a useful product and has potential in a wide range of applications. It can be used directly as a LPG replacement due to its similar physiochemical properties or as diesel fuel 9. Moreover, DME is an alternative to methanol in the production of light olefins and aromatics

10

as well as a viable feedstock in several other chemical

processes 11-13. DME is currently produced through methanol dehydration over solid acid catalysts, which 3



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is mildly exothermic (Eq. 1.4). The overall reaction to produce DME from CO2 and H2 is shown in Eq. (1.5). Optimism for such a scheme can be drawn from studies on reaction kinetics for one-step methanol and DME synthesis from CO2 and syngas, indicating that methanol dehydration is very fast and the watergas shift reactions are equilibrium controlled, rendering methanol synthesis to be the rate-determining step 14-15

.

2CH3OH ↔ CH3OCH3 + H2O

∆ = −23.4 kJ/mol

(1.4)

2CO2 + 6H2 ↔ CH3OCH3 + 2H2O

∆ = −123.0 kJ/mol

(1.5)

Furthermore, higher alcohols (C2+OH) synthesis is very attractive owing to their broad range of applications in chemical and polymer industries

16-17

. They can also be used as fuel additives in gasoline to

raise the octane number and the combustion efficiency in automobiles

18

. Three indirect routes from

methanol to higher alcohols have been reported, namely methanol and CO coupling followed by dimethyl oxalate hydrogenation

19

, methanol homologation

20

, and methanol carbonylation with subsequent

hydrogenation 21. However, direct conversion is again a promising route due to lower capital and operating costs, since it would be accomplished in a single step with fewer operation units. The direct conversion of syngas to higher alcohols have been widely studied over the years higher alcohols have received attention as well

23-24

22

, and recently, CO2 hydrogenation to

. The overall reaction to produce ethanol from CO2 and

H2 is shown in Eq. (1.6). 2CO2 + 6H2 ↔ CH3CH2OH + 3H2O

∆ = −86.7 kJ/mol

(1.6)

It can be seen that hydrogenation of CO2 to methanol, DME or higher alcohols produces significant quantities of H2O as a byproduct. For example, a third of the H2 is converted to H2O when CO2 is reduced to methanol, which is considerably higher than that in the commercial methanol synthesis process from syngas

25

. The H2 requirement is therefore much higher, and the development of cost-efficient H2

production processes from renewable sources is necessary to make it economically viable in large scale 2627

. 4


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Over the last decades, several works have investigated the thermodynamics of CO2 and CO hydrogenation to methanol

24, 28-32

at different temperatures, pressures, feed gas compositions and other operating

parameters such as the recycle ratio. Some have also reported the simultaneous hydrogenation to methanol and DME 24, 28, 33, while little attention has been paid to the thermodynamics of higher alcohol synthesis 24, 34

. For methanol synthesis, one of the very interesting aspects is the role of product condensation and its

effect on the chemical equilibrium. Iyer et al. 35 investigated the effect of syngas composition on methanol synthesis in both single and two-phase regions by thermodynamic simulations. They found that product condensation enhanced the conversion to methanol and that the phase equilibrium had a dominant effect on the chemical equilibrium at relevant reaction conditions. Van Bennekom et al.

36

showed

experimentally that the conversion of syngas can exceed the predictions of gas-phase thermodynamics and visualized the in situ formation of a liquid phase in a view cell reactor. They claimed that phase separation is a function not only of operating conditions and feed compositions but also of the extent of reaction. Bros and Brilman 37 exploited a two-temperature zone reactor to induce condensation that led to an almost complete conversion of the H2/CO2 feed at 50 bar. Furthermore, methanol yield from CO2 hydrogenation beyond the gas-phase equilibrium has been achieved in an internally cooled fixed bed reactor. 38 It can be expected that product condensation should also have a profound effect on simultaneous synthesis of DME or higher alcohols along with methanol. The aforementioned thermodynamic studies were limited to a narrow range of operating conditions, and consequently, a more comprehensive thermodynamic analysis of CO2 hydrogenation to methanol, DME and higher alcohols is desirable. More importantly, only a few works have considered the formation of a liquid phase and a detailed study on the beneficial influence of condensation on the chemical equilibrium in CO2 hydrogenation reactions has not been reported, which warrant closer examination. Therefore, the objective of the present work is to comprehensively analyze the effect of operating conditions by utilizing both gas-phase and two-phase models on the thermodynamics of methanol synthesis as well as simultaneous synthesis of methanol/DME and methanol/higher alcohols (C2+OH). The simulations were

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carried out by the Gibbs free energy minimization method, which has been widely employed to investigate the thermodynamics of complicated reaction systems

24, 32, 39

. The Soave-Redlich-Kwong (SRK) equation

of state was used to calculate the phase equilibrium of the system. The relevance of the thermodynamic results herein is briefly discussed with regards to recent advances in catalysis and process design for CO2 hydrogenation.

2. Methodology The equilibrium composition of a reaction system can be accurately defined by the Gibbs free energy minimization method 40. At equilibrium, the total Gibbs free energy (G) of the system is at the minimum and its differential is zero. The total Gibbs free energy of the system at a certain temperature and pressure for a composition of compounds can be represented as Eq. 2.1. = ∑ = ∑ + ∑

(2.1)

where ni is the moles of species i and µi is the chemical potential of component i, R is the molar gas constant, T is the system temperature, is the fugacity of component i. Introducing Lagrange multipliers for species i, subject to mass balance constraints (Eq. 2.2):

+ ∑ = 0

(2.2)

where is the Lagrange multiplier, is the number of atoms of element j in species i. Eq. 2.3 is obtained by the combination of Eq. (2.1) and (2.2):

+ ∑

+ ∑ = 0

(2.3)

The basic relationship for every component i in the vapor and liquid phases of a system at phase equilibrium is (Eq. 2.4) 41: =

(2.4) 6


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where and is the fugacity of component i in the vapor phase and liquid phase, respectively. and can be calculated by Eq. (2.5) and Eq. (2.6) respectively. = φ " #

(2.5)

= φ $ #

(2.6)

where the fugacity coefficient φ% (& = v or l) is obtained from equation of state, p is the pressure, yi and xi is the mole fraction of component i in the vapor phase and liquid phase respectively. The equilibrium state at the specified T and p is determined by minimizing the Gibbs free energy for a given set of species without any specification of the possible reactions that might take place in the system. In this work, the minimization was accomplished using the RGibbs module available in the Aspen plus software. The SRK property model in Aspen plus was used to calculate the thermodynamic properties of the components, where the binary interaction parameters for CO2, CO, H2, H2O, and CH3OH were taken 36

from van Bennekom et al.

while the parameters for CH3OCH3 and C2H5OH were estimated by the

UNIFAC method in Aspen plus. The product composition at the reactor outlet was evaluated by calculating the COx conversion, product selectivity and product yield defined as follows (Eq. 2.7-2.10): Conversion of COx (%) =

'()*,, . '()*,/01 '()*,,

Selectivity of species i (%) =

Yield of species i (%) =

× 100

',/01 . ',, '()2,, . '()2,/01

',/01 . ',, '()2,,

× 100

× 100

Yield of species i (kmol/h) = 3 ,456

(2.7)

(2.8)

(2.9)

(2.10)

where Ni,in and Ni,out is the molar flowrate of species i at the inlet and outlet of the reactor, respectively, while ji is the number of carbon atoms in species i.

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In all the simulations, the chemical equilibrium obtained by the gas-phase and two-phase thermodynamic model was calculated and compared. It should be emphasized that the results of these models were in perfect agreement in the gas-phase region.

3. Results 3.1. Effect of temperature and pressure Two main reactions occur during methanol synthesis from CO2, namely the methanol synthesis reaction (Eq. 1.1) and RWGS reaction (Eq. 1.3). It is well known that the optimal thermodynamic conditions for methanol synthesis are at low temperatures and high pressures. High product selectivity can also be achieved at these conditions as the RWGS reaction is suppressed at low temperatures and independent of pressure. However, the kinetics of methanol synthesis are greatly improved by increasing the temperature, although it results in reduced methanol selectivity as the RWGS is endothermic and becomes more prominent with increasing temperature. The components considered in the simulation of methanol synthesis include the main species, namely CO2, CO, H2, H2O and CH3OH. Methane and higher hydrocarbons have not been included as they are not favorable products over typically employed Cu based catalysts at methanol synthesis reaction conditions 5. Figure 1 presents the calculated CO2 conversion (a) and methanol selectivity (b) at different reaction temperatures and pressures for a H2/CO2 feed gas at the stoichiometric molar ratio of 3/1, which is the typical feed composition in methanol synthesis from CO2. Both temperature and pressure have a considerable effect on the equilibrium conversion of CO2. The CO2 conversion profiles resemble Ushaped curves, explained by the methanol synthesis reaction becoming less favored and the RWGS reaction more favored as the temperature is increased. Increasing the pressure strongly enhances the CO2 conversion at low temperatures. However, as the temperature is increased, the effect of pressure diminishes and eventually the CO2 conversion curves converge. This illustrates the expected effect of

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pressure on the reactions involved, specifically that the methanol synthesis reaction is enhanced by increased pressure whereas the RWGS is unaffected. It also explains the shift of the methanol selectivity curves to higher temperature as the pressure is increased. For example, the methanol selectivity at 160 oC and 10 bar is 90% whereas a pressure of 50 bar is required to reach the same selectivity at 240 oC. More importantly however, the methanol yield is higher in the latter case due to the significant effect of pressure on the CO2 conversion. An additional observation is that methanol production can hardly be effective at atmospheric pressure regardless of the temperature, because at these conditions both the CO2 conversion and methanol selectivity are greatly inhibited. An option to improve the CO2 conversion is to exploit the relatively low volatility of methanol. In other words, methanol will condense if its partial pressure exceeds its vapor pressure at the specific temperature and pressure in the reactor, and the same holds true for water. Consequently, a vapor-liquid region must exist at certain conditions if sufficient amounts of CO2 can be converted to methanol and water. This is of great relevance as the condensation of products results in their removal from the reacting gaseous mixture, thus driving the equilibrium towards product formation. In fact, this phenomenon has been confirmed experimentally by several groups based on both syngas and CO2 conversion

36-38, 42

. They observed that

gas-phase thermodynamics failed to predict the chemical equilibrium when the experiments were performed at reaction conditions where product condensation occurred. We calculated the chemical equilibrium by employing both gas-phase and two-phase thermodynamic models, which produced significant deviations in CO2 conversion at low temperatures. This can be seen in Figure 1 (a) where the gas-phase equilibrium is represented by dashed lines. To investigate the role of product condensation in more detail, the bubble- and dew point curves of the reacting mixture (H2/CO2 ratio = 3/1) were calculated and are presented in the Supporting information Figure S1(a). A narrow temperature and pressure region was acquired in which both a vapor phase and a liquid phase exist. Above the bubble point curve, the components of the product mixture are all in the liquid phase, and the incondensable gases typically make up less than 1%. Substantial differences in CO2

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conversion were obtained when gas-phase and two-phase models were employed in the liquid and twophase regions. This is presented in more detail in Figure 2 (a)-(c), which show the CO2 conversion, methanol selectivity and vapor fraction over the two-phase region at 100 bar. The CO2 conversion is strongly correlated with the vapor fraction, decreasing sharply with increasing vaporization of the products. The conversion drop over a 10 oC (197-207 oC) interval is remarkably from 99% to 55%. In contrast, the change in methanol selectivity is minimal due to the low activity of the RWGS reaction in this temperature range. Hence, from a thermodynamic point of view, only the methanol synthesis reaction is significantly affected by the condensation of methanol and water.

3.2. Effect of H2/CO2 ratio Processes employing direct conversion of CO2 and H2 to methanol are typically designed to operate with excess H2 43-44. This is used to limit the molar fraction of H2O in the reactor so as to limit the adsorption of H2O onto the catalyst and its subsequent adverse effects on the methanol production efficiency (i.e., blocking active sites, sintering, etc.). The effect of the initial H2/CO2 ratio on the equilibrium was examined in the range of 1/2 to 10/1, at temperatures from 150 to 350 oC and pressures of 10, 30, 50 and 100 bar. The CO2 conversion and the methanol selectivity at 100 and 50 bar are presented in Figure 3 (a)(d), while the results at 30 and 10 bar are shown in Figure S2 (a)-(d). Increasing the H2/CO2 ratio greatly enhances the CO2 conversion at all pressures. This can be explained by the fact that the methanol synthesis reaction (Eq. 1.1) is highly sensitive to the H2 partial pressure, where an increase in H2 partial pressure drives the reaction towards the products. As the temperature is increased, the methanol synthesis reaction becomes more limited and its contribution to the CO2 consumption decreases. Hence, the observed increase in conversion at higher temperatures indicates that the RWGS is also promoted by excess H2 24. The most significant change in methanol selectivity occurs when the H2/CO2 ratio is increased up to the stoichiometric proportion of 3/1, whereas the methanol selectivity is improved to a less extent upon

10


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further increase of the H2/CO2 ratio. Utilizing greater than stoichiometric H2/CO2 ratios can be beneficial to enhance the methanol selectivity and limit byproduct formation (CO and H2O). Nevertheless, this will contribute negatively to the economics of the process because more H2 will be needed. According to our calculations, the highest yield of methanol at equilibrium can be obtained at the stoichiometric H2/CO2 ratio of 3/1 when the total flow rate of reactants is constant regardless of the reaction conditions. The disadvantage of altering the H2/CO2 ratio is more significant when product condensation is considered. The dashed lines in Figure 3 (a) and (c) represent the equilibrium conversion of CO2 predicted by gas-phase thermodynamics. Clearly, operating at non-stoichiometric H2/CO2 ratios reduces the effect of product condensation. This is due to the deficit in either CO2 or H2, which means that complete conversion of all reactants is not possible. For instance, the CO2 conversion is only mildly increased by product condensation at a H2/CO2 ratio of 2/1. In this case, H2 is the limiting reactant and the CO2 conversion to methanol is capped at 67%. Nonetheless, condensation of products still enhances the conversion and allows the reaction to proceed beyond that predicted by gas-phase thermodynamics. Figure 4 (a)-(c) show the CO2 conversion, methanol selectivity and vapor fraction at 100 bar over the twophase region for H2/CO2 ratios of 2/1, 3/1 and 5/1. The CO2 conversion follows the expected order of 5/1 > 3/1 > 2/1 for gas-phase thermodynamics. However, when product condensation is taken into consideration, the CO2 conversion is affected the most at a H2/CO2 ratio of 3/1. As a consequence, the CO2 conversion is higher at certain low temperatures at a ratio of 3/1 than 5/1. This illustrates the powerful influence of phase equilibria on the chemical equilibrium. It can be seen from the vapor fraction curves for ratios of 5/1 and 2/1 that the product mixture will consist of both vapor and liquid, and that the dew points are at slightly lower temperature with excess CO2 or H2 in the feed. Nevertheless, complete conversion of the limiting reactant is still possible at low temperatures at 5/1 and 2/1 due to product condensation.

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3.3. Effect of initial water concentration Water is by far the most important byproduct in methanol synthesis from CO2 and will always be present in equal or even larger quantities than methanol. The substantial amount of produced water has its significance in catalyst development, as both the activity and stability of the catalyst can be negatively affected 5. As water is a product of both the main reactions involved in methanol synthesis from CO2, it is expected that the CO2 conversion will decrease with the addition of water to the feed. The effect of the initial H2O concentration in the feed was examined by keeping the H2/CO2 ratio fixed at 3/1. The calculated CO2 conversion and methanol selectivity at different initial H2O concentrations at pressures of 100 and 50 bar are presented in Figure 5 (a)-(d), while the results at 30 and 10 bar are shown in Figure S3 (a)-(d). In general, the water content in the feed has a detrimental effect on the CO2 conversion regardless of the operating pressure, evidenced by the significant reduction in CO2 conversion as the H2O concentration increases. On the other hand, the product selectivity remains quite stable, where a slight drop in methanol selectivity with increasing H2O content is observed and is most prominent when the addition of water increases to 20 %. The small change in product selectivity with the addition of water can be attributed to H2O being a product in both the methanol synthesis reaction and the RWGS reaction, which inhibits the formation of both methanol and CO. The discrepancy in the results obtained from gas-phase thermodynamics and simultaneous modeling of phase and chemical equilibrium is more substantial when H2O is added to the feed. At certain reaction conditions where product condensation occurs, the addition of water to the feed stream improves the CO2 conversion. This can be observed in Figure 5 (c) at 150 oC where the highest conversion of CO2 is obtained with a feed gas with 20% water content. The effect of H2O in the two-phase region was studied more closely by calculating the CO2 conversion, methanol selectivity, and vapor fraction from 190-230 o

C, and the results are shown in Figure 6 (a)-(c). Two key observations can be made here. Firstly,

increasing the water concentration in the feed shifts the bubble and dew point to a higher temperature for the reacting mixture, thus improving the CO2 conversion by promoting product condensation in a wider

12


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temperature range. Secondly, the methanol selectivity remains high even with the addition of 20 % water, particularly at reaction conditions favorable for product condensation. Hence, product condensation is not only influenced by the pressure and temperature, but could also be enhanced by adding water or other condensable products to the reacting mixture.

3.4. Effect of CO2/CO ratio Handling increasingly larger concentrations of CO2 in the feed gas has become critical for some industrial methanol plants. In industrial methanol synthesis from syngas, the stoichiometric number (SN) can be defined as in Eq. (3.1). When the feed mixture has a SN of 2, all of its reactants can theoretically be converted to products through Eq. (1.1)-(1.3). 73 =

82 .9:2 9:2 ;9:

=2

(3.1)

Replacing some of the CO2 with CO during CO2 hydrogenation has its advantages and is of practical importance. In this work, the effect of the initial CO2/CO ratio was investigated by keeping the SN number at 2. The COx conversion to methanol at various CO2/CO ratio are presented in Figure 7 (a) and (b) for pressures of 100 and 50 bar, and in Figure S4 (a) and (b) for 30 and 10 bar. As expected, methanol synthesis is thermodynamically more favorable from CO than CO2 throughout the investigated reaction conditions. A gradual rise in COx conversion to methanol occurs as the CO fraction in the feed is increased. Moreover, the improved conversion can be maintained in a greater temperature range, particularly at high pressures. This is because more H2O is formed as a byproduct for CO2-rich feeds that generally reduces the equilibrium yield of methanol. The presence of CO is therefore advantageous for methanol synthesis as it moderates the H2O production, and thus, limits the inhibiting effect of H2O on the methanol yield.

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As methanol synthesis from CO2 results in more condensable products than that from CO, product condensation should play a more significant role in methanol synthesis from CO2-rich feeds. This has been proven experimentally by van Bennekom et al. al.

35

36

and is also in agreement with the simulations of Iyer et

The dashed lines in Figure 7 represents the equilibrium conversion predicted by gas-phase

thermodynamics. The difference in COx conversion for the gas-phase and two-phase model becomes more significant with increasing CO2 concentration. Interestingly, the conversion for CO2-rich feeds exceeds that of more CO-rich feeds at certain reaction conditions. For example, at 200 oC and 100 bar, the twophase model predicts a higher COx conversion for CO2/CO ratios of 1/0, 10/1, and 2/1 than for ratios of 1/1 and 1/2. Figure 8 (a) and (b) presents the COx conversion to methanol and vapor fraction over the two-phase region for CO2/CO ratios of 1/0, 1/1 and 1/10. The bubble and dew points are dependent on the CO2/CO composition. Changing from pure CO2 to a feed with a CO2/CO ratio of 1/1 shift the two-phase region to lower temperature, whereas this region shifts to higher temperature upon further alteration to 1/10. This indicates that not only the conversion to methanol but also the quantities of water and methanol produced influence the condensation and in turn the COx conversion. When the CO2/CO ratio is 1/1, the additional molar yield of methanol fails to compensate for the reduction in water concentration. On the other hand, the shift of the two-phase region to a higher temperature for CO2/CO ratio of 1/10 can be attributed to the significant increase in molar yield of methanol for CO-rich feeds. Product condensation is thus most influential for CO2-rich feeds because it removes water in addition to methanol from the reacting gas mixture, thereby eliminating the strong inhibiting effect of H2O on the CO2-to-methanol reaction (Eq. 1.1).

3.5. Recycle ratio in thermodynamically controlled CO2 hydrogenation to methanol At typically employed reaction conditions, the restrictions imposed by the equilibrium on the methanol synthesis reactions limits the conversion per pass and therefore, necessitates recycle of the unconverted

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reactant gases to improve the economics of the process 45. These thermodynamic restrictions, as has been demonstrated in section 3.4, are even greater for CO2 rich feed streams. This section is aimed at investigating the recycle ratio needed to achieve greater than 99% conversion of CO2 in pure CO2 hydrogenation to methanol when the reactor is operated at thermodynamic equilibrium. The simulations were performed by adding an ideal separator and reflux of unreacted species (CO2, CO, H2). Recycle can be avoided, at least theoretically, if product condensation can be exploited. Hence, only gas-phase thermodynamics are used in these simulations. The contour plots of the recycle ratio at different temperatures and pressures are presented in Figure 9 (a) and (b) for H2/CO2 ratios of 3/1 and 5/1. The contours gradually become steeper as the temperature is increased, meaning a much higher pressure is needed to maintain the desired recycle ratio as the temperature is elevated. Most notably, the required recycle ratio increases considerably at 10 bar as the temperature is elevated and the recycle volumes are much larger than the volumetric feed rate. On the other hand, as the pressure is increased, the change in required recycle ratio is smaller in the typical temperature range employed for methanol synthesis from CO2 (200-275 oC). This can be explained by the significant improvement in thermodynamics with increasing pressure, as illustrated in section 3.1. When the H2/CO2 ratio is increased from 3/1 to 5/1, the contours shift to higher temperatures. This is expected as the recycle ratio is proportional to the CO2 conversion and the selectivity to methanol, which both improve as the H2/CO2 ratio is increased. Furthermore, the molar fraction of CO2 at the inlet is reduced when the H2/CO2 ratio is increased to 5/1 and hence, the required methanol yield to achieve greater than 99% conversion of CO2 is lower. In addition, the slope of the contours at 5/1 are less steep than those at 3/1, which is due to the superior CO2 conversion and methanol selectivity at higher H2 partial pressure. Sufficient temperature is needed to avoid severe kinetic limitations in CO2 hydrogenation to methanol. Therefore, one of the main ways of reducing the required recycle ratio of the process is to increase the operating pressure. A low operating pressure might be preferable for certain applications, for instance in 15



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small-scale delocalized methanol production sites where the use of high-energy demanding compression units is undesirable. In this case, to achieve more than 99% conversion of CO2 and minimize the required recycle ratio, a higher than stoichiometric H2/CO2 ratio could be beneficial.

3.6. Simultaneous synthesis of methanol and DME Although thermodynamic studies for simultaneous methanol and DME synthesis have been reported, the effect of product condensation has not been investigated. Even greater amounts of water are produced with the addition of methanol dehydration to the reaction scheme, and DME is easily condensed by applying mild pressure at room temperature. The simulations were carried out by adding DME to the components considered in methanol synthesis (CO2, CO, H2, H2O, CH3OH). Hydrocarbons can form as by products in methanol dehydration to DME, but can be minimized by shortening the residence time in the reactor 46 and optimizing the catalyst 8. Hence, hydrocarbons have not been included in the analysis. The results of the ternary product system consisting of methanol, DME and CO is shown in Figure 10 where the CO2 conversion and the selectivity of methanol and DME are presented in (a) and (b), respectively. When the formation of DME is taken into consideration, the equilibrium conversion of CO2 is increased over the majority of the investigated temperature range compared to the binary product system of methanol and CO. Similar to methanol synthesis, DME production is favored at low temperature and high pressure. The selectivity data indicates that achieving complete conversion to DME would be difficult or even impossible, and that a mixture of methanol and DME will always be present at equilibrium. The DME selectivity decreases monotonously as the temperature is increased, whereas the methanol selectivity curves show a volcano-shaped trend with respect to temperature. Nevertheless, DME is the major product and is affected the most when the operating conditions are changed. As in the previous binary system, the RWGS reaction eventually becomes the dominant reaction as the temperature is increased.

16


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Figure 1S (b) shows the bubble point and dew point curves of the resulting product mixture (methanol/DME/CO/H2O). By comparing Figure 1S (b) and Figure 1S (a), it can be seen that product condensation occurs in a wider range of operating conditions in the ternary system than in the binary system. This could be ascribed to the different pressure-volume-temperature properties of DME, but also to the fact that more water is produced when methanol is dehydrated to DME. The influence of product condensation was examined more closely over the two-phase region by increasing the temperature at constant pressure (100 bar), and the resulting CO2 conversion, product selectivity, and vapor fraction are shown in Figure 11 (a)-(c). As was the case in the binary system, the CO2 conversion drops significantly over the two-phase region. Furthermore, the selectivity to DME and methanol is somewhat affected by the phase equilibrium and exhibit minor deviations from the predictions of gas-phase thermodynamics. As the vapor fraction increases in the reactor, the selectivity to DME slightly decreases whereas the selectivity to methanol increases. This is related to the phase equilibrium as the condensation of water allows further dehydration of methanol to DME.

3.7. Simultaneous synthesis of methanol and ethanol We briefly investigated a few different systems comprised of alcohol mixtures of C1-C2OH, C1-C3OH, and C1-C4OH. As for the previous reaction systems, CO2, CO, H2 and H2O were also included in the analysis. There is no industrial process available for higher alcohol synthesis even from CO, and hydrocarbons are produced in significant quantities along with alcohols over currently developed catalysts. For comparison and simplicity, we have neglected hydrocarbons in this analysis, because any effective catalyst system should have low selectivity to hydrocarbons for the process to be commercialized in the future. The CO2 conversion and product selectivity for CO2 hydrogenation to a product mixture of C1-C3OH/CO are shown in Figure S5, while that of C1-C4OH/CO is presented in Figure S6. As can be seen, the highest alcohol included in the analysis is the major product. Methanol is barely present at equilibrium for any of the systems and the selectivity to methanol is therefore not shown. Furthermore, the RWGS reaction is 17



Page 18 of 32

increasingly suppressed with the addition of a higher alcohol to the analysis. The product system consisting of C1-C2OH/CO will be discussed in more detail as the trends in CO2 conversion and product selectivity (viz. alcohols vs. CO) are quite similar for these systems. Besides, a detailed study of the thermodynamics of higher alcohol synthesis (C1-C4OH) from CO2 should include all possible isomers of propanol and butanol. Therefore, studying the effect of reaction conditions and phase equilibrium on CO2 conversion and product selectivity for such a reaction system would be quite comprehensive. The CO2 conversion and product selectivity for CO2 hydrogenation to a ternary system of methanol, ethanol, and CO are shown in Figure 12 (a) and (b), respectively. The CO2 conversion is greatly enhanced when ethanol is included as one of the products. Even higher temperature and lower pressure are less detrimental to the CO2 conversion and product selectivity compared to the methanol/CO and methanol/DME/CO product systems. The formation of ethanol is greatly favored over methanol regardless of the reaction conditions. In fact, methanol contributes to less than 0.2% of the products formed under the applied reaction conditions and is therefore not shown in Figure 12 (b). Moreover, the undesired byproduct CO is hardly produced even as the temperature is increased, particularly at the investigated pressures of 30, 50 and 100 bar. At the highest investigated temperature of 350 oC, the CO selectivity is 54% (10 bar), 14% (30 bar), 7% (50 bar), and 2% (100 bar), which is in stark contrast to the product composition in the methanol/CO and methanol/DME/CO systems where CO is comfortably the major product at 350 oC even at 100 bar. Methanol synthesis is typically conducted around 250 oC due to kinetic limitations at lower temperatures. At these conditions, the CO2 conversion is substantially limited by the thermodynamics, and significant amounts of CO is produced. Hence, a process that can produce higher alcohols in addition to methanol could theoretically be beneficial to the one-pass conversion of CO2 and reduce the yield of CO. The dashed lines in Figure 12 (a) represents the CO2 conversion obtained by gas-phase thermodynamics. Although product condensation plays a less significant role in boosting the CO2 conversion in this system, the bubble point and dew point curves at the investigated pressures are shifted to higher temperatures

18


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(Figure S1 (c)). The CO2 conversion, ethanol selectivity, and vapor fraction over the two-phase region at 100 bar is shown in Figure 13 (a)-(c). Product condensation is still beneficial for the CO2 conversion, as evidenced by the drop in CO2 conversion from 99% to 92% as the system goes from liquid to vapor in a narrow temperature range of 269-273 oC. The obtained selectivity to ethanol from the gas-phase and twophase models is rather similar throughout the two-phase region. In addition, the product selectivity is less affected by product condensation in this system compared to the other reaction schemes.

4. Discussion Experimental studies have unanimously demonstrated the advantage of high-pressure conditions in CO2 hydrogenation to methanol

47-48

. For instance, Gaikwad et al.

48

showed that 90% CO2 conversion and

more than 95% methanol selectivity could be achieved at 442 bar. It was also demonstrated that, under such high-pressure conditions, a dense phase was formed due to product condensation and that internal mass transfer, related to the catalyst pellet size, limited the overall reaction rate. Furthermore, the twophase region can be shifted to even higher temperatures by increasing the pressure. According to our calculations, at a pressure of 200 bar, the change from liquid to vapor occurs at about 240 oC, which is close to the values reported by van Bennekom et al.

36

Nevertheless, the majority of studies in the

literature is conducted at low pressures (99.9% and >99.9% respectively when phase condensation is considered. On the other hand, the phase equilibrium has a minor effect on the selectivity to the desired products in these reaction systems. At 200 o

C, the selectivity to desirable products is 99.0% for methanol, 99.6% for methanol/DME and >99.9% for

methanol/ethanol, while the selectivity is increased to approximately 100% for all systems when product condensation is taken into account. Substantial improvement in CO2 conversion and selectivity above the dew point can be observed when DME or ethanol is produced along with methanol, which is related to the thermodynamically more favorable formation of DME and ethanol compared to methanol. At 300 oC for instance, the selectivity to desirable products is 58.3 for methanol synthesis, 83.1 for methanol/DME and 99.8 for methanol/ethanol. A few experimental studies have reported methanol synthesis utilizing product condensation to achieve CO2 conversions beyond the equilibrium predicted by gas-phase thermodynamics. In 1990, Hansen et al. 42

reported a once-through process for syngas conversion to methanol in a pilot study. They demonstrated

that space-time yields, carbon conversion and byproduct levels comparable to industrial practice were possible in long-term experiments by exploiting product condensation. A different approach is the use of membrane reactors to increase the methanol yield, and several groups have employed such reactors to investigate CO2 hydrogenation to methanol 49-51. Despite the successful partial separation of methanol and water from the reaction mixture, to the best of our knowledge, conversions beyond the equilibrium predicted by gas-phase thermodynamics has not yet been accomplished in membrane reactors. By utilizing product condensation however, Wu et al.

38

recently reported methanol yields far greater than that

predicted by the gas-phase equilibrium. These experiments were conducted in an internally cooled fixed bed reactor over a Cu/ZnO/Al2O3 catalyst. In contrast to the behavior in a conventional fixed bed reactor, the methanol yield increased with temperature in the range of 220-260 oC. For example, at reaction conditions of 240 oC and 30 bar, the yield of methanol was 9.9% in the conventional reactor while it was

20


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remarkably 36.6% in the internally cooled reactor. According to our calculations, the methanol yield at equilibrium without condensation is merely 12.6% at the reaction conditions employed. Only product condensation can explain the substantial increase in methanol yield. Bos and Brilman 37 developed a novel reactor design for the conversion of CO2 and H2 to methanol. The reactor was based on natural convection, consisting of a high-temperature reaction zone and a low-temperature condensation zone. This allowed significant reaction rates and at the same time greater than 99.5% conversion of reactants to methanol even at a relatively low pressure of 50 bar. The enhanced CO2 conversion to methanol due to the beneficial effect of product condensation, as illustrated by experimental works highlighted and the thermodynamic modelling herein, is of practical importance and can aid in the realization of efficient CO2 to methanol processes. Conventional Cu based catalyst appears the most promising candidate for methanol synthesis from CO2 5 and have shown promise in condensation enhanced CO2 hydrogenation to methanol 37, 38

.

One of the main drawbacks of modern low-temperature methanol synthesis loop designs employed in methanol synthesis from syngas is the high recycle rate of unconverted gases with the accompanying high energy demand and investment costs 42. The high recycle ratios employed are dictated by the need for high feedstock utilization and the thermodynamic limitation imposed by the gas-phase equilibrium. Methanol synthesis from CO2 is even more restricted by the thermodynamics. Consequently, even greater recycle ratios are required to achieve sufficient utilization of the feedstock. This would put further economic pressure on the successful industrial implementation of CO2 hydrogenation to methanol in large scale. Taking into account the high cost of CO2

52

and hydrogen

26

, wasteful use of the feedstock can hardly be

cost-effective. It should be noted that the CRI’s methanol plant design is based on recycling unreacted gases44, and they also claim that production of methanol from CO2 hydrogenation could be profitable in certain locations around the world 53. In contrast to CO2 hydrogenation to methanol, DME synthesis by dehydration of methanol is a mature process

52

. This reaction is often coupled with the syngas-to-methanol process on an industrial scale to

21



Page 22 of 32

produce DME or olefins as the final product. Compared to the syngas-methanol-DME process, using CO2 as the starting material would yield considerably more water. Unless product condensation can be utilized to remove parts or most of the water, the ability of the catalyst to handle the water-rich reaction environment while maintaining high activity becomes critical

54

. The major cause of deactivation in

dehydration of methanol is the formation of coke on the catalyst, especially in long-term operations 55. In a one-step process for the simultaneous synthesis of methanol and DME, high-temperature treatment to burn off the coke could have a detrimental effect on the methanol synthesis catalyst. Furthermore, the catalytic stability might be affected by deactivation mechanisms corresponding to each reaction step or by specific deactivation modes originating from interactions between the two catalysts or the distinct reaction environment created by combining the two reaction steps in a single unit 54. These catalytic and technical challenges must be addressed for the successful development of one-step CO2 to methanol and DME processes. However, our thermodynamic analysis demonstrated that the CO2 conversion can be enhanced by simultaneous methanol and DME synthesis and that product condensation could improve the product yield. Direct CO2 hydrogenation to methanol and DME is normally performed over either a hybrid catalyst or mechanically mixed methanol synthesis catalyst with acidic catalyst. γ-Al2O3 is the most investigated solid acid catalyst for methanol dehydration. This catalyst appear unsuitable for one-pot synthesis application as it tends to adsorb water strongly which causes deactivation, and it also requires high temperature to achieve sufficient activity

56-57

. On the other hand, zeolites appear promising as they offer high activity for

methanol conversion at relatively low reaction temperatures and show high resistance towards water adsorption 58-59. Ethanol and higher alcohols are generally more desirable than methanol, as clean fuels, fuel additives and chemicals

60

. Synthesis of higher alcohols from CO2 and H2 has however been proven much more difficult

than methanol synthesis. Besides, no catalytic system reported to date has performed sufficiently well to justify industrial implementation even in its synthesis from syngas

22

. Although more hydrogen is

22


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necessary to reduce CO2, hydrogenation of CO2 to higher alcohols can be of practical interest because of environmental considerations. The number of experimental work on direct CO2 hydrogenation to higher alcohols is limited compared to that of CO hydrogenation. A recent study of CO2 hydrogenation over a Pt/Co3O4 catalyst suspended in water/1,3-dimethyl-2-imidazolidinone showed promising selectivity to ethanol, accompanied by significant quantities of methanol, propanol and butanol 61. However, methane was formed with at least twice the yield of alcohols. Interestingly, isotopic labeling experiments using D2O and 13CH3OH showed that water was the hydrogen source and that methanol homologation to ethanol was possible. Similar results were reported in isotopic labeling experiments over a Ru-Rh based homogeneous catalytic system 62. Heterogeneous catalysts reported for this reaction can typically be classified into four categories: Rhbased, Mo-based, modified methanol synthesis and modified Fisher-Tropsch systems. Although a mixture of methanol and ethanol have been reported over these catalysts, the obtained selectivity to alcohols compared to byproducts (hydrocarbons, undesired C2+ oxygenates and CO) has generally been poor

63-65

.

Further catalyst development is therefore critical for any potential production of methanol, ethanol or higher alcohols from CO2 simultaneously. Nevertheless, our thermodynamic calculations and those of others demonstrate that CO2 hydrogenation could benefit significantly if higher alcohols could be synthesized effectively either as the major product or in a single-step along with methanol.

5. Conclusion We have demonstrated that product condensation has profound beneficial effects in CO2 hydrogenation to methanol processes. A higher methanol yield than that predicted by gas-phase thermodynamics can be achieved by exploiting product condensation. For product condensation to occur in methanol synthesis, a certain pressure and low temperature must be employed in the reactor. Operating at higher or lower than the H2/CO2 stoichiometric ratio reduces the influence of product condensation on the chemical

23



Page 24 of 32

equilibrium. Addition of water can enhance product condensation and thus, improve the CO2 conversion to methanol at certain temperatures. However, caution must be taken to avoid excess water vapor in the gas phase, which will have negative effects on the overall thermodynamics and deactivate the catalyst. Introducing CO in the feed mixture and adjusting the SN number to the optimal value is advantageous for the yield of methanol. Gas-phase thermodynamics is also greatly enhanced with increasing CO concentration, whereas product condensation is generally less influential at relevant reaction conditions on the overall conversion of carbon oxides to methanol when the CO fraction is increased. If product condensation is not utilized, recycling of unconverted gases must inevitably be included in the process design. In this case, a higher pressure reduced the required recycle ratio and operating at higher than stoichiometric H2/CO2 ratios can further limit the recycle volumes. The effect of product condensation was also found advantageous for simultaneous production of methanol and DME or ethanol. Product condensation occurs over a wider range of pressure and temperature for the ternary systems than in the binary system, particularly when ethanol is included. In simultaneous methanol and DME synthesis, DME is the major product at equilibrium with a methanol selectivity typically lower than 20%. Moreover, the RWGS reaction eventually becomes the dominant reaction at higher temperatures as in methanol synthesis. On the other hand, the product composition at equilibrium in simultaneous methanol and ethanol synthesis consists almost entirely of ethanol. CO is barely formed when ethanol is included in the analysis even at a temperature of 350 oC and moderate pressure. In addition, product condensation is the least influential on the chemical equilibrium for this product system. We conclude that product condensation can be utilized to improve the equilibrium yield in various CO2 hydrogenation reactions. In general, this leads to significant enhancement in CO2 conversion whereas the product selectivity is either only mildly or not at all affected. As a consequence, recycle streams and associated bleed/purge streams might be unnecessary. Further exploration of reactor and process designs that can exploit product condensation might lead to viable concepts for the commercial development of CO2 hydrogenation to methanol, DME and ethanol processes.

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Acknowledgements The authors would like to thank the financial support from the Norwegian Ministry of Education and Research and Department of Petroleum Engineering, University of Stavanger for this project.

Supporting Information Figures presenting: bubble point and dew point curves for CO2 hydrogenation to methanol, methanol/DME and methanol/ethanol; effect of H2/CO2 ratio on CO2 conversion and methanol selectivity at 30 and 10 bar; effect of initial H2O concentration on CO2 conversion and methanol selectivity at 30 and 10 bar; COx conversion at different CO2/CO ratios at 30 and 10 bar; CO2 conversion and selectivity for C1-C3OH/CO product system; CO2 conversion and selectivity for C1-C4OH/CO product system.

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Thermodynamic analysis of chemical and phase equilibria in CO2

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