Faculté Polytechnique

Transcription

1 Faculté Polytechnique Catalytic conversion of CO2 into methanol Master Thesis Summary Master 2 en Sciences de l Ingénieur Chimie Science des Matériaux Finalité procédés de l industrie chimique Service de Thermodynamique et de Physique Mathématique Timothée Bryans Sous la direction de : Promoteurs : Guy De Weireld, Diane Thomas Co-promoteur : Nicolas Meunier Juin 2017

2 Acknowledgement: This work was done on behalf of the ECRA Academic Chair. I acknowledge the European Cement Research Academy (ECRA) for the technical and financial supports accorded to the ECRA Academic Chair.

3 The reduction of CO 2 emissions is one of the greatest challenges that our generation must face. Several countries have decided to target lower CO 2 emissions to limit the effects upon global warming. Efforts are made in different sectors such as transportation, electricity, industry etc. Therefore, the cement industry, which is one of the most polluting industries in terms of carbon dioxide emissions (up to 7% of CO 2 global emissions), has to take action. The high amount of CO 2 produced by this industry comes from the synthesis of clinker (major component of cement) that involves the decarbonisation of limestone producing calcium oxide and carbon dioxide. This specific emission of CO 2 is in addition to that due to the combustion stages in the process of cement synthesis. Because of this mixed origin of carbon dioxide, the implantation of pre-combustion capture wouldn t be appropriate. That s why at the moment, post-combustion capture seems to be the best way to concentrate CO 2 from the flues gases in a secondary effluent which has then to be used in a responsible and clean way. There are two distinctive ways of using this CO 2 rich effluent. The first one called CCS (Carbon Capture and Storage) consists in geological storage of the effluent and is the most common. The second way, called CCU (Carbon Capture and Utilization) goes further as it uses CO 2 as a chemical feedstock for other industries. This second way of using CO 2 clearly seems more viable on a long-term basis. In this work, the focus is set on the re-use of CO 2 present in the cement industry s flue gases, concentrated in a CO 2-rich effluent by a post-combustion technique (amine absorption) and exploited as a source of carbon for the methanol production. The choice to investigate the specific use of CO 2 as a potential feedstock for methanol production was driven by economic, technology readiness and global CO 2 reduction criteria. Different study scales have been chosen around this problematic. First of all, the attention is centred on catalysts composition and on the kinetics of the reactions. The problematic of the catalyst s shaping is also considered. Then a general process for methanol production is considered and improved by integration loops reducing the ecological impact and the global cost of the overall re-use process. In conclusion, the financial potential of the process is overviewed. General process for methanol synthesis: Nowadays methanol production uses syngas as main reactant. Syngas is composed of H 2, CO and CO 2 and is mainly produced by steam methane reforming. The reactant gas is compressed at pressures between 50 and 100 bars and heated to temperatures reaching 400 C before entering the chemical reactor where it is catalytically converted to methanol (heterogeneous catalysis). The 3 major reactions taking part in the synthesis are: A) CO+2H2 CH3OH B) CO2+H2 CO+H2O C) CO2+3H2 CH3OH+H2O Where reaction A) is the hydrogenation of carbon monoxide into methanol, reaction B) is the reverse water-gas shift (RWGS) and reaction C) is the hydrogenation of carbon dioxide into methanol. For these reactions, the catalysts used generally consist of copper as the main active phase mixed with metallic oxides as promoters. When the methanol production using recycled CO 2 emitted by the cement industry is considered, the initial reactants used for the synthesis are only hydrogen and CO 2. Carbon monoxide

4 will later appear due to the RWGS. Thus the chemical mechanisms appearing in this methanol synthesis will be the same that those in the synthesis from syngas. However the reactions speeds will be different because of the differences in concentrations of the reacting species and possible modifications in temperature and pressure conditions. Kinetic model: Several kinetic models have been proposed in scientific literature to describe the catalytic synthesis of methanol. Despite having the common hypothesis of Langmuir-Hinshelwood mechanism, the models presented in the literature differ on many points such as the origin of the carbon present in the methanol, the numbers of types of adsorption sites on the catalysts, the nature of the catalyst active phases, etc. Among the models presented, Graaf et al. s model 1 is considered the most suitable and reliable. In his model, Graaf has decomposed the 3 main reactions presented earlier in their supposed elementary steps. Reaction A) is now divided into four elementary steps, B) into two steps and reaction C) into six steps (without counting the adsorption/desorption reactions of the reactants and products on the catalyst). Knowing the elementary steps, the expression of the reactions speed for the three global reactions can be worked out: In which r CH3 OH,A3 = k ps,a3 r H2 O,B2 = k ps,b2 r CH3 OH,C3 = k ps,c3 f j is the fugacity of component j, ads [f CO f 2 H2 f CH3 OH/(f 2 H2 K A )] K CO 3 (1 + K ads CO f CO + K ads CO2 f CO2 )[f 1/2 H2 + ( K H 2 O ads 1/2 ) f H 2 O] K ads CO2 [f CO2 f H2 f H 2 Of CO K ] B 1 K H2 ads ads (1 + K ads CO f CO + K ads CO2 f CO2 )[f 1/2 H2 + ( K H 2 O ads 1/2 ) f H 2 O] K CO2 3 K H2 ads [f CO2 f 2 H2 f CH3 OHf H2 O/(f 2 H2 K C )] (1 + K ads CO f CO + K ads CO2 f CO2 )[f 1/2 H2 + ( K H 2 O ads 1/2 ) f H 2 O] k ps,ij is the pseudo kinetic constant of elementary step j in global reaction I, 3 K H2 ads K j ads is the adsorption equilibrium constant of component j, K i is the equilibrium constant of the reaction i. 1 G. H. Graaf, E. J. Stamhuis, and A. A. C. M. Beenackers, Kinetics of low-pressure methanol synthesis, Chem. Eng. Sci., vol. 43, no. 12, pp , 1988.

5 Graaf and his team have estimated all of the needed parameters appearing in these expressions by doing many experimentations of methanol synthesis on a CuO/ZnO/Al 2O 3 catalyst in pressure and temperature conditions varying in the following ranges: 210 C < T < 240 C and 15 bars < p < 50 bars. Recently these parameters have been re-estimated by Kobl 2 for CuO/ZnO/Al 2O 3 catalyst. Compared with Graaf s results, Kobl s seemed to be more reliable in relation with other experimental results presented in literature (particularly for high CO concentration feed). Therefore Kobl s parameters were used to simulate the reaction under Aspen Plus V8.8. At this point, two different aspects are studied. The first one consists in the experimental determination of the kinetic parameters of Graaf s model for slightly different catalysts (eg: catalyst containing zirconia instead of alumina). The second aspect consists in reducing the ecological footprint and the energy consumption of a CO 2 to methanol process already implemented in Aspen Plus V8.6 by N.Meunier during his thesis 3. Determination of the kinetic parameters: In order to estimate the needed parameters of Graaf s model, two different steps can be identified: -Obtaining experimental results with varying conditions (pressure, temperature, reactant s volume flow, reactants stoichiometry) with a specific catalyst. -Analysing the data to obtain the kinetic parameters for this catalyst in the range of conditions of experimentations. Experimental part: To obtain the experimental results, a pilot unit for methanol synthesis is currently being built at the Thermodynamic Department of the Faculté Polytechnique de Mons. Equipment delivery delays and other complications linked with the high pressure, high temperature and the use and production of dangerous products (H 2 and CO) have hindered the major experimental part. The experimental part that still could be investigated during this work was about the shaping of the catalysts. Two catalysts were investigated for the synthesis of methanol. The first one is CuO/ZnO/Al 2O 3 catalyst commonly used for the methanol synthesis from syngas. The second one is CuO/ZnO/ZrO 2 catalyst developed by the Strasbourg School of chemistry, polymers and materials. The two catalysts have copper as the main active phase and zinc oxide as a promoter. In the second one, zirconia, having a positive impact on the catalyst activity, replaces alumina. The CuO/ZnO/ZrO 2 catalyst is synthetized as powder but needs to be shaped (tablets or pellets) to be used in the pilot reactor. The shaping process will diminish the catalytic activity by reducing the surface area and the mass concentration of the active phase. This step has to be taken into account because it will also appear on the industrial scale. 2 K. Kobl, Aspects mécanistiques et cinétiques de la production catalytique de méthanol à partir de CO2/H2, Université de Strasbourg, N.Meunier, «CO2 capture in cement production and re-use: optimization of the overall process», Faculté Polytechnique de Mons.

6 The extrusion technique was chosen, as it is the most common catalyst shaping technique used at an industrial level. For quantity reasons, the extrusion experiments had to be done on the CuO/ZnO/Al 2O 3 catalyst, available in large quantities and at a low price. Several experiments have been done to identify the right proportions of the different additives added to the catalyst powder. Among which, de-ionized water is used as solvent, methylcellulose as organic binder and alumina/bentonite as inorganic binder. After extrusion, the extrudates were dried at ambient temperature, cut, dried again in an oven at 110 C for 12h and then calcinated at 400 C during 4h (see figures 1 and 4). Their mechanical strength was then tested with equipment derived from the ASTM D norm. This mechanical test revealed that the use of bentonite as inorganic binder was better than alumina. The optimal composition of the pellets obtained is given in table 1 : Catalyst powder Methylcellulose Bentonite Water Mass (g) 5,6 0,15 1,4 7,5 Content (%) 78,3 2,1 19,6 - Table 1 Pellets composition Figure 1 Dried extrudates Figure 2 Cut and calcinated pellets Before applying the extrusion shaping method tested on the CuO/ZnO/Al 2O 3 catalyst to the CuO/ZnO/ZrO 2 powder, the catalytic activity loss due to the shaping process must clearly be assessed. Therefore experiments must be done on both on CuO/ZnO/Al 2O 3 industrial pellets and CuO/ZnO/Al 2O 3 home-made pellets before using this shaping process to the CuO/ZnO/ZrO 2 catalyst powder. Simulation model: About the analysis of the experimental data, our first thought was to use the Aspen Plus software. But in the case of Langmuir-Hinshelwood kinetic, the software is unable to determine the kinetic parameters from the experimental data. Thus a model had to be implemented by us to do so. The idea behind the Excel model developed for this occasion was to simulate the reactor for every experiment conducted and tuning the kinetic parameters of Graaf s model to minimize the difference between simulated and experimental results. The simulation model can be described as follows: -It needs all of the information about the reactor (volume, length, catalyst mass, etc.) and the entering conditions of the reactive flow (pressure, temperature, volume flow, CO 2/H 2 molar ratio). -A first estimation of the Graaf s model parameter must be given

7 -The reactor is considered as split in slices of equal volume and the residence time of the reactant flow in the first slice can be estimated. -With the hypothesis of constant characteristics (fugacity, temperature, pressure) of the inlet flow along the slice, the reaction s speed of the 3 global reactions can be calculated with Graaf s model and the estimated parameters. -Knowing the reactions speeds and the residence time of the reactant mixture in the slice, the number of moles of product produced and reactant consumed can be calculated and the composition at the beginning of the next slice can be determined. -The same reasoning can be applied to the following slices until the final slice of the reactor. All of the experimental results are simulated at the same time by the model. Each simulation having the same initial conditions as the experimentation it is linked to. The model try then to minimize the global difference between all of the experimental results and their simulation by tuning the kinetic parameters of the model. This allows us to obtain a set a kinetic parameters needed in the Graaf s model for specific interval of temperature, pressure and volume flow decided by the experimentations. And as long as we assume the elementary steps of the main reactions are identical, this can be applied to other catalysts. Improvements of the CO 2 to methanol process: Being now theoretically able to estimate the kinetic parameters needed in Graaf s model for our catalysts, the focus is switched to another aspect of the methanol synthesis: process optimization. A well-designed process is of major importance because in the end it s the financial balance of the process that will be decisive. A first work was done by N.Meunier in his PhD thesis. Based on a methanol synthesis process presented in a US patent (US ), he developed and implemented on Aspen Plus an upgraded version of the process (presented hereafter in figure 3). The objective of this section was to continue his work to make the CO 2 to methanol process less energy consuming and more economically attractive. In order to better understand the process, let s have a brief closer look at the methanol process presented in figure 3. The reactive mixture of H 2 and CO 2 (H 2/CO 2 molar ratio of 3) is compressed in COMP-1 to reach the reaction s pressure of 80 bars. And is heated in EX-1 to 250 C by the flux exiting the first reactor (REA-1). Then the reactants enter the first reactor (adiabatic reactor) where the conversion rate reaches 15%. A recycling flux is now added to the mixture before being heated again in EX-2 and entering the second reactor (isotherm at 250 C). The mixture is now cooled (COOLER) and de-pressurized to 75 bars in the flash unit. The gaseous phase containing unreacted H 2 and CO 2 but also CO is recycled towards reactor 2. The liquid fraction containing water and methanol is distilled at atmospheric pressure to obtain a 0.99 molar fraction methanol flow at the top of the column. The final flowsheet is shown on figure 4. The visual comparison with the initial flowsheet reveals the presence of 3 new exchangers which help reduce the global heat need of the process. The most important integration loop is the one going from reactor 2 to exchanger 3 and back again. In this loop, a heat-transport fluid transfers the excess heat produced by the globally exothermic reactions happening in reactor 2 that has to be kept at a constant temperature. This heat is then transported to the distillation column s reboiler through exchanger 3 thus reducing the amount of energy required.

8 Figure 3 Initial flowsheet for the methanol process Figure 4 Final flowsheet for the methanol process Globally, the economic impact of the modifications done on the process is shown on table 2. We clearly see the economic benefit of the integrations done on the process, which are directly linked to energy savings and minimizing the ecological footprint. The cost reduction is particularly high in terms of operational costs, which are the yearly-imputed costs. These operational costs do not take into account the price of hydrogen needed for the process. In the first place, the hydrogen was supposed to be produced free by water electrolysis supplied by free green energy. The capital costs are also reduced but far less than the operational cost. The reduction of the operational should always be considered better because the economy is made every functioning year whereas a capital cost economy is only made once. Financial overview: Initial Process Final Process Variation (%) Opex (M$/year) 6,60 3,54-46,36 Capex (M$) 21,55 21,00-2,55 Table 2 Improvements in Operating and Capital costs Taking into account the latest process improvements, it is interesting to notice what would be the financial results of such a process. Having listed all the costs, including the operating costs for the hydrogen production (which were not taken into consideration in Table 2), and the benefit made by selling the methanol produced (as shown in table 3). We clearly see that the process cannot generate a financial profit yet. The negative balance is due to the high H 2 production costs that represent more than 90% of the overall costs. The hydrogen cost is only due to the electricityassociated cost for the water electrolysis that needs 54 kwh/kg H 2. The price of electricity which would enable a balanced financial statement would need to be 0.038$/kWh. At the moment this condition is only fulfilled for countries such as Iceland were the green energy production costs are very low but with technological improvements in green energy production and the rising of CO 2 quotas, the CO 2 to methanol process might become financially interesting in other countries.

9 Costs M$/year $/produced ton of methanol $/converted ton of CO 2 Capital (10 years amortisation) 2,10 9,96 7,24 Operating 3,54 16,79 12,21 H 2 Production 179,77 852,83 620,24 CO 2 Capture 12,65 60,00 43,64 Profits Total Methanol Sales 105, ,64-92,66-439,58-319,69 Table 3 Financial overview of the methanol process Conclusion and perspectives: In this work, the emphasis was set on the reuse of CO 2 emitted by the cement industry as potential feedstock for methanol synthesis. Different aspects of the subject have been studied. The catalytic reactions and kinetic models able to describe them were studied and compared. Graaf s model seemed to be the most suitable. An Excel model has been developed in order to calculate the needed kinetic parameters for Graaf s model based on experimentations. Nevertheless no experimentation has been done yet. A pilot unit is currently being built at the Polytechnic Faculty of Mons. The pilot experiments are thus part of the perspectives of this work as well as the comparison of the performances of the catalysts mentioned in this work (CuO/ZnO/Al 2O 3 and CuO/ZnO/ZrO 2 catalysts). Prior to any pilot experiments, the CuO/ZnO/ZrO 2 catalyst, synthetized as a powder, had to be shaped. For this reason, an extrusion protocol has been investigated. This protocol was tested on the CuO/ZnO/Al 2O 3 for convenient reasons and the mechanical strength of the pellets produced this way was assessed. However, before applying the shaping protocol to the CuO/ZnO/ZrO 2 powder, the reduction of the catalyst s activity due to the shaping process must be verified by experiments. The last part of this work was about the improvement of the CO 2 to methanol process. Several modifications done on the process flowsheet as well as the addition of integration loops have considerably reduced the operating costs of the process (about 50%) as a consequence of energy savings. However, other enhancements can still be implemented such as a closed water cooling system which would need the design of a cooling tower for example. A financial overview of the process shows that the process isn t profitable yet. A lower electricity price would be the most efficient way to reduce the expenses. The price of 0.038$/kWh has been identified as making the process profitable but other elements such as the rise of CO 2 quotas could also be beneficial.