Microsoft Word - PRES22_0064.docx DOI: 10.3303/CET2294066 Paper Received: 14 April 2022; Revised: 20 May 2022; Accepted: 02 June 2022 Please cite this article as: Galusnyak S.C., Dumbrava L.-D., Petrescu L., Dragan S., Cormos C.-C., 2022, Assessment of CO₂ Utilization Technologies Into Valuable C1 Organic Chemicals: a Modelling and Simulation Analysis, Chemical Engineering Transactions, 94, 397-402 DOI:10.3303/CET2294066 CHEMICAL ENGINEERING TRANSACTIONS VOL. 94, 2022 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Petar S. Varbanov, Yee Van Fan, Jiří J. Klemeš, Sandro Nižetić Copyright © 2022, AIDIC Servizi S.r.l. ISBN 978-88-95608-93-8; ISSN 2283-9216 Assessment of CO2 Utilization Technologies into Valuable C1 Organic Chemicals: a Modelling and Simulation Analysis Stefan C. Galusnyak*, Ionela D. Dumbrava, Letitia Petrescu, Simion Dragan, Calin C. Cormos Babes-Bolyai University, Faculty of Chemistry and Chemical Engineering, Arany Janos 11, Postal code: RO-400028, Cluj- Napoca, Romania stefan.galusnyak@ubbcluj.ro The concentration of carbon dioxide (CO2) within the atmosphere reached unprecedented levels mainly due to population and industrial growth, both of them requiring high energy consumption. Strategies such as the use of renewable energy sources, multiple actions aiming to improve the energy efficiency, or the integration of Carbon Capture Utilization and Storage (CCUS) technologies are currently investigated to mitigate the greenhouse emissions. The utilization of CO2 to produce value-added chemical compounds / energy carriers is of major importance to meet the emission targets set by the European Union. The current research is focused on the conversion and valorisation of captured CO2, through CO2 hydrogenation, to produce green C1 high- priced chemicals such as: i) substitute natural gas (SNG), ii) formic acid (FA), and iii) methanol (MeOH). Water electrolysis is considered for H2 production with the employ of renewable energy sources, as for example solar, wind, or hydro power, with the purpose of developing a green and sustainable technology. A thermal output of 100 MW was assumed in the case of SNG production, whilst considering an annual productivity of 10,000 t of formic acid and 50,000 t of methanol. Aspen Plus simulator software was used to model the SNG production, and ChemCAD process simulation software was used for the FA and MeOH production processes. The integrated mass and energy balance data were afterwards used to evaluate all considered cases from a technical perspective. The evaluated designs were validated based on data from the scientific literature. As the results show, the proposed CO2 utilization technologies are very promising in terms of high energy efficiency (50 – 60 % range) as well as high CO2 conversion yields (>90 %). 1. Introduction One of the key objectives of nowadays society consists in the greenhouse gas reduction and climate change mitigation by shifting the energy sector from fossil-based fuels towards sustainable alternatives (Cipoletta et al., 2020). Taking into account that the availability of renewable electric sources (RES) is heavily influenced by the geographic characteristics (Cormos et al., 2021), the employment of CCUS technologies is the most appropriate choice on a mid-term to meet the goals set within the Paris agreement (Chen et al., 2021). A lot of attention moved in the direction of Carbon Capture and Utilization (CCU) topic as the captured CO2 might be employed in the production of chemicals used either as green fuels or energy storage media (Abad et al., 2021), known as power-to-gas strategy (PtG). This concept is meant to eventually increase the competitivity of the CCUS technology from an economic perspective (Lin et al., 2022), while also exhibiting a positive social impact. Methanol is among the major CO2 hydrogenation products due to its large utilizations either as a feedstock in the production of valuable chemicals such as dimethyl ether, formaldehyde or dimethyl carbonate, or as a green fuel alternative (Yousaf et al., 2022). The CO2-to-MeOH through the hydrogenation process reached a Technology Readiness Level (TRL) of 8-9, with the “George Olah” plant located in Svartsengi, Iceland, approaching a commercial scale (CRI, 2022). SNG represents another PtG product that could possibly reduce and curb the CO2 emissions through either direct injection into the grid distribution or storage, achieving deep decarbonisation (Yin et al., 2022). Despite the advantages of the CO2 methanation over other conversion processes, for example higher selectivity and energy efficiency, as well lower cost (Sun et al., 2022), the TRL 397 is slightly lower compared to the MeOH production (Chauvy et al., 2019). Recently, numerous investigations were carried out towards FA production via CO2 hydrogenation (Chatterjee et al., 2021). Formic acid gained a lot of consideration as an energy carrier based on the FA catalytic decomposition to produce H2 and CO2, with the possibility of converting back the CO2 into FA, in the presence of H2 (Verma et al., 2021). The European Green Deal strategy identifies great opportunities in lowering the industrial greenhouse gas emissions and generating sustainable products, strengthening the level of interest, over short to medium-term, towards the CCUS systems (European Commission, 2019). The present research is focused on the technical evaluation of green C1 chemicals production in the framework of the CO2 utilization topic. The key novelty aspects brought by the current study relate to the in-depth technical evaluation of the CO2 conversion into added-value green C1 chemicals through the CO2 hydrogenation technology. The assessment methodology considers relevant evaluation tools such as modelling and simulation, validation, mass and energy integration, quantification of overall performances etc. The C1 molecules selection was based on high industrial interest exhibited towards SNG and MeOH, as the first could be injected in the gas grid as a natural gas substitute, whereas the latter is an intermediate key component or may be used as a green fuel (Chauvy et al., 2019). Formic acid was considered based on its promising results obtained when used as a fuel in the direct formic acid fuel cells. 2. Plant configuration and models assumptions The following cases were investigated in the current study: Case 1: Green SNG production from renewable H2 and CO2 through the CO2 hydrogenation process; Case 2: Green formic acid production starting from electrolytic H2 and CO2 by means of CO2 hydrogenation technology; Case 3: Green methanol production using renewable H2 and CO2 through the CO2 hydrogenation technology. As presented before, Cases 1-3 convert the captured CO2 and renewable H2 through the CO2 hydrogenation technology into valuable chemicals. The CO2 reach stream is considered as being acquired from a CCS system coming from a carbon intensive process such as cement, iron and steel or ammonia production. Hydrogen is typically produced starting from fossil fuels, mostly natural gas, through the steam methane reforming process. However, to achieve the environmental targets set by various regulations, to approach near-zero CO2 emissions, alternative processes such as biomass gasification or water electrolysis have to be considered (Pérez-Fortes et al., 2016). The current research assumed H2 production through water electrolysis. The employment of RES as for example photovoltaics, wind or hydropower is of high importance in order to achieve a proper environmental performance. The process flow diagrams for the above-described processes are further presented in Figures 1- 3. Figure 1 depicts the CO2 methanation process. Peng-Robinson was chosen as thermodynamic package due to the employment of light hydrocarbons. The raw-materials, captured CO2 and electrolytic H2, are brought to the working conditions. The feed-streams are compressed in a single-step compression unit to 10 bar, after which being preheated to 350 °C before entering the first reactor. The reaction section consists of four adiabatic reactors to maximize the CO2 conversion. The reactions occurring in the CO2 methanation process are: CO2 + 4H2 ⇆ CH4 + 2H2O (R1) CO2 + H2 ⇆ CO + H2O (R2) CO + 3H2 ⇆ CH4 + H2O (R3) H2 Electricity + - O2 Water CO2 rich flow SNG Water CW CW Steam Figure 1: Process flow diagram for the SNG production by means of the CO2 methanation process 398 The outlet of the first reactor is divided into two streams, the first one is recycled to the reactor inlet to keep the temperature below 650 °C and avoid catalyst deactivation. To achieve high CO2 yield, the reaction mixture of each unit is fed to a heat exchanger to cool down the temperature to 350 °C. The outlet of the fourth reactor is cooled down to 25 °C, condensing and removing the water vapour from the main product to obtain the desired outlet specifications. Process’ configuration and operational parameters was such established to prevent the formation of CO and carbon deposition. Figure 2 illustrates the FA production through the CO2 hydrogenation technology. Based on the operational conditions (i.e., high pressures) and employed chemical compounds, Predictive Soave-Redlich-Kwong (PSRK) thermodynamic model was used. As observed, the process can be divided into three different sections. To assume the worst possible case, it is presumed that the CO2 reach flow enters the system at both ambient temperature and pressure, for this reason the CO2 stream follows a compression stage with intermediate cooling to reach the reaction conditions. Hydrogen is produced through electrolysis. Besides the H2 stream obtained at high pressure, oxygen (O2) is also produced and may be further used within other sections or processes. Even if H2 comes at a high pressure, 30 bar, a further compression with intermediate cooling is needed to achieve the reaction conditions. Formic acid is produced through CO2 hydrogenation at 123 °C and 60 bar. The reaction occurring in the hydrogenation reactor is as follow: CO2 + H2 ⇆ HCOOH (R4) Further, a flash unit is used to separate the FA from the CO2 and H2 mixture, considering a 95 % efficiency and recycling the gaseous phase at the inlet of the reactor. H2 Electricity CO2 and H2 + - O2 Water CO2 rich flow Formic acid CW CW Figure 2: Process flow diagram for the FA production by means of the CO2 hydrogenation process The process flow diagram for MeOH production starting from CO2 and green H2 as raw-materials is shown in Figure 3. The Universal Functional Group Activity Coefficient (UNIFAC) was chosen as thermodynamic model for the MeOH production process, based on the involved chemical substances and operating conditions. As in the previously presented cases, both CO2 and H2 feed-streams are compressed up to 78 bar in a four-stage, and one-stage compression unit to reach the working pressure. The compression section is performed with intermediate cooling by using cooling water (CW) at 15 °C. After being compressed, the raw materials are mixed with a recycle stream, fed to a heat exchanger where the mixture is preheated to reach the reaction temperature, 210 °C and further sent to the reactor. The reactor is modelled as a plug flow reactor with the following two main reactions taking place, R5 and R6. CO2 + 3H2 ⇆ CH3OH + H2O (R5) CO2 + H2 ⇆ CO + H2O (R6) The reaction mixture is used to perform heat integration, a fraction of the reactor outlet preheats the reactor inlet, while the rest is used to preheat the column feed. The reaction mixture is sent to a heat exchanger to lower the temperature to around 30 °C, followed by a gas-liquid separation to almost completely remove the gaseous 399 phase from the water-MeOH mixture. A distillation column is used to separate the water from the liquid MeOH, obtained at the top of the column with a purity higher than 99 %. H2 Electricity CO2 and H2 + - O2 Water CO2 rich flow CW CW CW CW Methanol Water Figure 3: Process flow diagram for MeOH production through CO2 hydrogenation technology The main assumptions considered in the process modelling section performed within the current study are summarized in Table 1. Table 1: Main design assumptions for the considered case studies Case name Process modelling and simulation design assumptions Case 1 Raw-materials: CO2, H2, water; Main product: SNG; Thermodynamic package used: PENG-ROBINSON; Reactor: Number: 4; Adiabatic thermal mode; Inlet temperature: 350 °C; Pressure: 10 bar; 52 % CO2 conversion rate per reactor; Cooling water temperature: 15 °C; Heat exchanger: ΔTmin.: 10 °C; Pressure drop: 2-5 %; Pump: 85 % efficiency; Case 2 Raw-materials: CO2, H2, water; Main product: FA; Thermodynamic package used: PSRK; Reactor: Isothermal mode: 123 °C; Pressure: 60 bar; 19 % CO2 conversion rate per reactor; Flash separator; Top product pressure: 15 bar; Bottom product temperature: 180 °C; Cooling water temperature: 15 °C; Heat exchanger: ΔTmin.: 10 °C; Pressure drop: 2-5 %; Pump: 85 % efficiency; Case 3 Raw-materials: CO2, H2, air and water; Main product: MeOH; Thermodynamic package used: UNIFAC; Reactor: Isothermal mode: 215 °C; Pressure: 78 bar; 22 % CO2 conversion rate per reactor; Distillation column: 58 stages; Reflux ratio: 1.2; Bottom component recovery: 0.25 % MeOH; Cooling water temperature: 15 °C; Heat exchanger: ΔTmin.: 10 °C; Pressure drop: 2-5 %; Pump: 85 % efficiency; 3. Results and discussion The evaluated scenarios include water electrolysis, to obtain the required amount of H2, together with the CO2 conversion processes through the CO2 hydrogenation technology. A program built in MATLAB software was used to perform the simulation for the water electrolysis process. The results were validated based on the research published by Bolat and Thiel (2014). The CO2 conversion processes were performed using ChemCAD, version 7, and Aspen Plus process simulation software, version 11. The CO2 methanation process was performed as according to the study made by Chauvy et al. (2021). The results for the CO2-to-SNG production process are in line with those obtained in the scientific literature, being further scaled up to the desired productivity. The results for the FA production process were validated based on the study performed by Mardini and Bicer (2021). The results for the MeOH production process are in good agreement with those obtained by 400 Pérez-Fortes et al. (2016). The mass and energy balance data acquired from the process modelling and simulation section were then used to estimate the key performance indicators shown in Table 2. Table 2: Technical key performance indicators for the evaluated scenarios Parameter Units Case 1 Case 2 Case 3 Water flowrate kg/t 4,860.00 2,132.28 2,030.64 H2 flowrate kg/t 540.00 236.92 225.63 O2 flowrate kg/t 4,320.00 1,895.36 1,805.01 CO2 flowrate kg/t 2,903.75 5,172.87 1,650.90 CO2 conversion per process % 94.51 95.23 93.38 Energy consumption MWe/t 33.42 16.32 15.07 Product rate kg/h 1,000.00 1,000.00 1,000.00 Main product purity wt. % 82.69 98.61 99.96 As can be noticed from Table 2, the results are expressed as specific consumptions per 1 t of product to allow an easier comparison between the alternative cases, even though the annual production is different depending on the conversion scenario. By comparing the amount of raw materials used to produce 1 t of desired product, it can be observed that a higher amount of H2 (e.g., 540.00 kg/h) is required in the first scenario, SNG production. Similar values are needed for the FA production (e.g., 236.92 kg/h) and MeOH production processes (e.g., 225.63 kg/h). As considering that water electrolysis is employed for H2 generation, the highest amount of water is required in Case 1 (e.g., 4,860.00 kg/h) since the H2 flowrate is at least 2.3 times higher in Case 1 as compared to Case 2 (i.e., FA production) and Case 3 (i.e., MeOH production). Oxygen is released as a by-product of the electrolysis process. The high quantities released in Case 1 might be seen as an advantage over the other conversion scenarios when considering either a technical perspective (O2 is ready to be used within other section of the process) or economic point of view (the amount of O2 produced can be sold as a by-product). The amount of CO2 needed could be brought from a CCS system integrated within a carbon intensive process as for example cement, iron and steel, or ammonia industry. The highest quantity of CO2 is needed for the FA production process, Case 2, 5,172.87 kg/h, which also relates to the lowest CO2 conversion rate per reactor, 19 %. As in contrast to Case 2, an approximately 1.8 times lower CO2 flowrate is required in Case 1 (e.g., 2,903.75 kg/h) and a roughly 3 times lower value for the MeOH production process (e.g., 1,650.90 kg/h). The highest CO2 conversion rate per reactor is achieved in Case 1, around 52 %, being followed by the CO2-to-MeOH process with 22 %. However, as presented in Table 2, each hydrogenation scenario displays CO2 conversion rate higher than 90 % (for the whole process). The energy requirements are strongly related to the amount of H2 used due to the fact that H2 is produced through water electrolysis process. As shown in Table 2, the largest energy consumption is registered in Case 1 (e.g., 33.42 MWe), which, as already mentioned, requires the highest quantities of H2. In comparison with Case 1, FA production process (i.e., Case 2) demands a 2 times lower amount of energy per 1 t of product (e.g., 16.32 MWe), while a 2.2 times lower value (e.g., 15.07 MWe) is needed for MeOH production. The highest purity is achieved in the CO2-to-MeOH scenario, 99.96 wt. % purity, followed by the FA production with 98.61 wt. %, and 82.69 wt. % achieved in the SNG case. 4. Conclusions The atmospheric CO2 concentration have reached its highest levels in history. The current study evaluates the CO2 conversion and valorisation through the CO2 hydrogenation technology to produce green C1 valuable chemicals as for example SNG, FA, and MeOH. The technical results show that the CO2-to-MeOH process requires the lowest amount of raw materials, CO2 and H2, whilst leading to the highest purity for the main product (e.g., 99.96 wt. %). In terms of purities, the second highest is achieved in the CO2 hydrogenation to FA production, whilst SNG production records the lowest, 82.69 wt. %. The much-needed H2 is produced through water electrolysis. The energy consumption is strongly related to the H2 flowrate. Carbon dioxide to SNG presents the highest energy requirements among the three conversion scenarios, followed by the FA, and MeOH production process. Consequently, as the technical results suggest, the CO2-to-MeOH conversion scenario represents the best alternative in regard to the CO2 utilization technologies, that being validated by the high TRL, the process slowly approaching a commercial scale. Nomenclature CCUS – Carbon Capture Utilization and Storage SNG – Substitute Natural Gas CCU – Carbon Capture and Utilization 401 TRL – Technology Readiness Level RES – Renewable Electric Sources PtG – Power-to-Gas MeOH – Methanol FA – Formic acid CW – Cooling water PSRK – Predictive Soave-Redlich-Kwong equation of Gmehling UNIFAC – Universal Functional Group Activity Coefficient Acknowledgments The research leading to these results has received funding from a grant of the Romanian Ministry of Education and Research, project number PN-III-P4-ID-PCE-2020-0032, within PNCDI III. References Abad D., Vega F., Navarrete B., Delgado A., Nieto E., 2021. Modeling and simulation of an integrated power- to-methanol approach via high temperature electrolysis and partial oxy-combustion technology. International Journal of Hydrogen Energy, 46, 34128–34147. Bolat P., Thiel C., 2014. Hydrogen supply chain architecture for bottom-up energy systems models. 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