S. C.
Stouten
,
A.
Anastasopoulou
,
V.
Hessel
and
Q.
Wang
*
Laboratory of Chemical Reactor Engineering/Micro Flow Chemistry and Process Technology, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands. E-mail: q.wang0207@gmail.com
First published on 26th July 2017
The alkoxycarbonylation reaction can be realized in continuous flow under supercritical conditions by utilizing CO2 as a feedstock instead of CO. Conventionally, the synthesis of the methyl propionate is achieved in the first step of the Lucite Alpha process through the hydroesterification of ethylene with methanol and carbon monoxide. In this paper, synthesis of the methyl propionate process by replacing the carbon monoxide feedstock with CO2 and using a more robust and less expensive catalyst is simulated and evaluated from the perspective of environmental influence. A life cycle assessment was done of the methyl propionate production via the supercritical process utilizing CO2 as feedstock. For all nine impact categories – AP, GWP, EP, FAETP, HTP, Land use, MAETP, ODP and CED –, the novel process was compared to the performance of the existing state-of-the-art carbon monoxide-based process, the Lucite Alpha process. An 80% impact reduction was found for both the Global Warming Potential and the Ozone Depletion Potential. The major contribution to the impact reduction stems from the change from CO to CO2 as a feedstock, since the impact from CO as feedstock is strongly negative while the impact from CO2 as feedstock is strongly positive. Yet, also the supercritical conditions themselves show a notable environmental benefit, besides providing the enabling function for the new chemistry. A remarkable effect on steam, electricity, and cooling energy is given. The higher pressure required for the supercritical CO2 process was found to have minimal effect on the electricity use for compression.
Recently, the group of Beller20 successfully utilized CO2 as a feedstock for a ruthenium-catalyzed alkoxycarbonylation reaction, as shown in Scheme 1. This reaction uses CO2 gas, which is inexpensive compared to other, indirect, sources of CO2. In addition, the catalyst is based on the less expensive ruthenium, compared to more commonly used and more expensive palladium or rhodium based carbonylation catalysts. Finally, the catalyst system is also very robust, as it does not depend on the use of sophisticated ligands that are sensitive to deactivation by impurities in the feed.
Scheme 1 Methoxycarbonylation of alkenes with methanol and CO2 over a ruthenium catalyst, as performed by the group of Beller.20 |
The application of this novel alkoxycarbonylation reaction was further investigated at a laboratory scale by Stouten et al.,21 by performing the methoxycarbonylation of cyclohexene in continuous flow under supercritical conditions. The reaction rate was boosted more than five times by operating under flow conditions at a pressure of 120 bar and a temperature of 180 °C, obtaining a 77% yield with a 90 min residence time. Also investigated was the use of the catalyst in a heterogeneous manner, through immobilization on a solid support, as the robust nature of the catalyst makes it a promising candidate for such heterogenization. Although the immobilization was thus far unsuccessful, the advantages of a heterogeneous catalyst would be considerable.
In short, the alkoxycarbonylation reaction discovered by the group of Beller is very interesting as a potential alternative for industrial carbonylation reactions that utilize carbon monoxide as a feedstock. In the Lucite Alpha process, the monomer for poly(methyl methacrylate) (PMMA) is produced via the intermediate methyl propionate.22 Synthesis of the methyl propionate is achieved in the first step of the process through the hydroesterification of ethylene with methanol and carbon monoxide. As it was shown by the group of Beller,20 synthesis of methyl propionate is also possible via the novel alkoxycarbonylation reaction, simply by replacing the carbon monoxide feedstock with CO2 and using a more robust and less expensive catalyst.
An issue to be considered for practical application is that the ionic liquid phase is continually diluted with water produced in the reaction. Indeed, literature findings have shown strong reliance of the performance of the supported aqueous-phase catalysts employed in the hydroformylation on the water loading.23 The change in the selectivity of the aqueous system has been attributed to the water-mediated hydrogen bonding to the catalyst.
Yet, measures are known to reduce and handle the aforementioned challenges. For example, adjusting the substrate surface so as to advert accumulation of traces of water in the SILP (supported ionic liquid phase), has proved to be an important means of ensuring continuous activation of the species. Moreover, the employment of a water scavenger or a perfluoroalkyl-functionalized silica substrate has demonstrated an improvement in the stability of the employed catalyst (140000 TON).24 However, such engineering issues linked to product purification and catalyst recovery are likely to supersede the value of the final product.24 Continuous-flow systems could be a solution to that with both their steady-state homogeneous turnover in a stationary liquid phase and efficient integrated product separation. Also for this reason, this paper considers that kind of operation.
The purpose of this continuous-flow based work is to study how the process based on the novel alkoxycarbonylation reaction compares with the industrial state-of-the-art, the Lucite Alpha process, from the perspective of environmental influence. To investigate this, a life cycle assessment (LCA) study was done to assess both processes. LCA studies assist the evaluation of processes and their ecological impact and, therefore, are often used as tools for decision making in the development of new processes.25–27 However, it is worth mentioning that there are two major approaches towards the sustainability evaluation of process intensified systems developed in R&D and project-funded work. In the case of industrial-lead explorations, such as H2020, lab-scale proof of concepts are transferred to pilot scale and these experimental data – at relevant industrial site – are used for sustainability analysis. The innovation degree of such process intensification (PI) is mediocre (TRL 3–5). On the contrary, in ground-breaking PI (TRL 1–2), where proof of concept at laboratory-scale is provided but no pilot plant demonstration is possible, sustainability assessment – at a preliminary level – is conducted based on the available, laboratory experimental data. In this study, the latter approach has been adopted given that the laboratory proof of concept for an analogous reaction using another substrate is demonstrated and an ex-ante environmental evaluation against the established industrial production route, the Lucite Alpha process, is being pursued.
The output of the reactor is a solution of methyl propionate and methanol in which the catalyst is dissolved. Subsequently, the solution is decompressed to atmospheric pressure and partially evaporated by a flash unit at 64 °C. The solution that remains is returned to the reactor to allow the catalyst to be recycled. The part of the solution that was evaporated, a mixture of methyl propionate and methanol, is then fed to a distillation column.
The distillation column was simulated in Aspen Plus using a RadFrac unit, with 80 equilibrium stages, a reflux ratio of 1.5 and operating at atmospheric pressure. The feed enters at stage 10. A 95% (m/m) solution of methyl propionate is obtained as bottom product, at 79 °C. As methyl propionate and methanol form an azeotrope at approximately 45% methanol and 55% methyl propionate, a mixture of both components is recovered as top product at 52 °C and recycled.
The output of the reactor is a supercritical mixture of methyl propionate, methanol and carbon dioxide in which the catalyst is dissolved, with most of the gaseous components remaining in the reactor. Subsequently, the solution is decompressed to 50 bar pressure at 50 °C. At these conditions, the solubility of the catalyst is poor, allowing it to be recovered and recycled. The product mixture is then passed through an absorption column, to remove water that was produced as part of the methoxycarbonylation reaction. The product mixture is then partially evaporated in a flash unit at 183 °C and 50 bar, to recover part of the CO2 before decompression to atmospheric pressure and purification in a distillation column.
The distillation column was simulated in Aspen Plus using a RadFrac unit, with 80 equilibrium stages, a reflux ratio of 1.3 and operating at atmospheric pressure. The feed enters at stage 10. Almost pure methyl propionate is obtained as bottom product, at 78 °C. As methyl propionate and methanol form an azeotrope at approximately 45% methanol and 55% methyl propionate, a mixture of both components and remaining gases is recovered as top product at 30 °C and recycled.
In addition to the supercritical CO2 process operating at 120 bar, the process was also simulated operating at 80 bar. In literature, experimental results show a decline in performance as going from 120 bar to 160 bar. Conversely, a further reduction in pressure from 120 bar to 80 bar may be feasible, possibly even improving performance. Reducing pressure below 80 bar was not considered beneficial, as the supercritical conditions, deemed essential for performance, would be lost. Compared to the process at 120 bar, only the initial pressurization of gas and liquid feed and the reactor pressure had to be lowered. After the reactor, the gas is depressurized to 50 bar in either case.
The functional unit has been defined as 1 ton of methyl propionate, and the inventory data required for the LCA modelling have been extracted from the Ecoinvent 3.0 embedded in the UMBERTO software. The values of the energy and material flows involved in the studied chemical processes have been acquired from the ASPEN simulations presented above. The LCIA method that has been selected is the CML2001 with the following impact categories being considered: acidification potential – average European (AP); global warming potential – 100 years (GWP); eutrophication potential – average European (EP); freshwater aquatic ecotoxicity potential – 100 years (FAETP); human toxicity potential – 100 years (HTP); land use; marine aquatic ecotoxicity potential – 100 years (MAETP); ozone depletion potential – 10 years (ODP); cumulative energy demand (CED).
Regarding the interpretation of the generated LCA results, the graphs presented in the discussion section below have been designed in such a way to reflect the contribution – in both absolute and normalized value representation – of the material and energy exchanged flows of each examined process to the aforementioned impact categories. In the case of the CO2 feedstock, the “credit” related to its use has been expressed with a negative value in those environmental impact categories which a relevant contribution has been observed.
Aside from the effect from switching to methoxycarbonylation with CO2, the absolute contribution from the energy (cooling energy), utility (steam), as well as the common raw materials (ethylene) for these two reaction systems are much lower for the supercritical CO2 process when compared to the Lucite Alpha process. As such, the impact from these three factors for the supercritical CO2 process is also lower (Fig. 4). Furthermore, since steam consumption reflects the heating energy (see the process simulation section), we can also conclude that the total energy consumption of the newly designed supercritical CO2 process is much lower than that of the existing process. The main reason for this reduction in energy consumption is the switch to supercritical conditions, creating a single phase system and, thus, eliminating mass transfer limitations inherent to a gas–liquid reaction. For the Lucite Alpha process, the biphasic system requires the reactor design to be aimed at maximizing the interfacial area between gas and liquid phases to boost mass transfer. However, this places considerable operational limits on the reactor, lowering the maximum conversion at which the process can be operated. Lower conversion is translated to less effective downstream recovery of the product. The low energy consumption of the supercritical CO2 process is the second obvious advantage compared to the existing Lucite Alpha process.
The absolute contribution from electricity and methanol consumption is quite similar in all processes. So the high pressure system of the supercritical CO2 process doesn't necessarily increase the demands of power consumption caused by pumps/compressors. According to the stoichiometry of these two reaction systems, the methanol consumption of the Lucite Alpha process is 75% of that occurring in the supercritical CO2 process. As such, the Lucite Alpha process consumes less methanol per functional unit of methyl propionate produced.
Surprisingly, the difference between high pressure and low pressure supercritical CO2 process is not that obvious, even though a higher energy consumption would be expected from operating at higher pressure. However, it can be calculated that very little additional work needs to be done by the compressor when increasing pressure from 80 to 120 bar. The required work can be found to scale roughly with ln(V2/V1), meaning it scales logarithmically with the compression ratio. And while the compression ratio from 1 to 80 bar is 80, the compression ratio from 80 to 120 bar is only 1.5.
Finally, the absolute CO2-eq emission for either supercritical CO2 process is only around 12% of that for the Lucite Alpha process.
Fig. 5 Normalized life cycle impact factors of Lucite Alpha process: 1. AP, 2. GWP, 3. EP, 4. FAETP, 5. HTP, 6. Land use, 7. MAETP, 8. ODP, 9. CED. |
Fig. 6 Normalized life cycle impact factors of the supercritical CO2 process at high pressure: 1. AP, 2. GWP, 3. EP, 4. FAETP, 5. HTP, 6. Land use, 7. MAETP, 8. ODP, 9. CED. |
Overall, changing from the Lucite Alpha process to the supercritical CO2 process results in a sharp decrease for all the life cycle impact categories studied, as depicted in Fig. 9. The GWP and ODP especially, decrease to 20% of that of the Lucite Alpha process. All other impact categories except for land use decrease to around 40%. The substitution of CO by CO2 accounts for most of these changes and is the main reason that the supercritical CO2 process performs such better. Because, not only is the CO feedstock a strong contributor to many impact categories, but also the use of CO2 can actually reduce the impact in some cases.
Fig. 9 Comparison of all three processes. 1. AP, 2. GWP, 3. EP, 4. FAETP, 5. HTP, 6. Land use, 7. MAETP, 8. ODP, 9. CED. |
The major conclusions deduced from the given LCA study can be summarized as follows:
– Consumption of CO2 as feedstock results in a considerable reduction of the GWP – 80% as to the conventional one –.
– Operation under supercritical conditions shows a remarkable impact on steam, electricity, and cooling energy consumption.
– The process-related effect is also notable and stronger in other impact categories than GWP.
– Finally, the use of higher pressure in the supercritical process seems to have a minimal effect on the electricity use (for compression).
In short, the supercritical CO2 process is a very promising alternative to the existing Lucite Alpha process. However, with respect to the industrial application of the novel process, considerable effort needs still to be directed towards a reliable, stable operation and flawless working plant, the scale-up and economy of the supercritical operation, the life-time and regeneration/recycle of the smart, immobilized catalyst, and much more.
This journal is © The Royal Society of Chemistry 2017 |