Article pubs.acs.org/EF
Carbon Dioxide Fixation by Combined Method of Physical Absorption and Carbonation in NaOH-Dissolved Methanol Sang-Jun Han and Jung-Ho Wee* Department of Environmental Engineering, The Catholic University of Korea 43 Jibong-ro, Wonmi-gu, Bucheon-si, Gyeonggi-do 420-743, Republic of Korea S Supporting Information *
ABSTRACT: The present study investigates the potential of a process as one of the carbon capture and storage/utilization (CCSU) technologies. The process involves a technology that combines the CO2 physical absorption and mineral carbonation in NaOH-dissolved methanol and includes CO2 desorption and methanol regeneration. Sodium methyl carbonate (SMC) is precipitated as CO2-fixing material in relatively highly concentrated NaOH solution and the amount of CO2 physically absorbed is estimated to be approximately 4.36 g CO2/500 mL methanol at ambient condition. The CCSU capacity of the process based on the gas phase and cake weight measurement is calculated to be 11.02 g CO2/(6 g NaOH/500 mL methanol) and 10.17 g CO2/(6 g NaOH/500 mL methanol), respectively. The amounts of CO2 desorbed and CO2 fixed by SMC as well as Na2CO3 via the evaporation of solution that remains after carbonation are estimated to be 5.64 g CO2/(6 g NaOH/500 mL methanol) and 4.53 g CO2/(6 g NaOH/500 mL methanol), respectively. Therefore, the CO2 capture and fixation ratio of the process was calculated to be 55.46% and 45.54%, respectively. Although the overall methanol recovery ratio is calculated to be 90.1%, the loss can be further substantially reduced during the process by employing more advanced equipment and carrying out a more detailed operation. In addition, precipitated SMC separated methanol can be reused for the subsequent carbonation without any treatment.
1. INTRODUCTION Carbon dioxide (CO2) is the gas most influential in causing climate change. Although many alternative energy sources have been developed, some of which are now fully or partially commercialized, fossil fuels are currently primary energy sources and this trend will continue in the near future.1,2 Therefore, carbon capture and storage/utilization (CCSU) technology is regarded as one of the most important methods to reduce CO2 emission; it is also believed to be a promising interim approach until alternative energy sources that are economical and environmentally friendly have been sufficiently developed.3−5 CO2 capture using alkanolamine such as monoethanol-amine (MEA) and methyl-diethanol-amine (MDEA) solvents is known as a partially commercialized chemical absorption process.6,7 During the process, CO2 adheres to the amine to be converted into carbamate, which is a chemically stable substance, and desorbed from the solvent by stripping. Despite its commercialization, this technology has many drawbacks such as solvent degradation, corrosion of equipment, and high energy consumption for solvent regeneration.8−10 Physical absorption using the Rectisol, Selexsol, Purisol, Morphysorb, and Fluor as solvents is another commercialized process used to capture acid gases including CO2. Selexol is manufactured based on dimethyl ether and polyethylene glycol, while N-methyl-2-pyrrolidone (NMP) is used to Purisol as the main solvent.11,12 Morpholine and propylene carbonate are the primary components of Morpyhsorb and Fluor, respectively,13,14 while methanol is the key component of Rectisol solvent used as a physical absorbent to capture CO2.15−17 Essentially, because the physical absorption of CO2 is due to © XXXX American Chemical Society
the interaction between an unshared electron pair of an oxygen atom in solvent and carbon in carbon dioxide, the absorption capacity is low compared to that of a chemical absorbent. Therefore, absorption in the Rectisol process is operated at a low temperature of −40 °C and high average pressure of 5 MPa and the desorption is carried out by decompression.17,18 Therefore, the process consumed considerable electrical energy to maintain a low temperature and pressure swing. Mineral carbonation is a technology that can fix CO2 as carbonated precipitates produced by the reaction between CO2 and alkali metal ions and can address the problem of the absorption processes.19−21 The natural minerals and industrial residues including alkali metal oxide can be used as the source materials for mineral carbonation.22−24 However, the main problems in the process can be a very slow reaction and the considerable amount of energy required for crushing the source material to increase the reactivity. However, Na is one of the most abundant minerals and its reactivity with CO2 is very high compared to that of other alkali metals.25 Nevertheless, its mineral carbonation conducted in water based solution is ineffective due to the very high solubility of its carbonated product. Therefore, if the use of the carbonation process could obtain the carbonated Na substance as the precipitate, the carbonation process could be an option for mineral carbonation technology and could be achieved by combining the processes of physical absorption and mineral carbonation.26−28 Received: October 18, 2016 Revised: January 5, 2017 Published: January 5, 2017 A
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claim, when the amount of water generated resulting from the reaction is very small compared to methanol, NaOH can be used as the reactant. Instead, a very small amount of NaHCO3 can be simultaneously generated during the reaction according to the side reaction as expressed in eq 3.
Due to a number of its features, methanol has relatively considerable potential for use as a solvent for such combined technology. First, methanol is commonly available, inexpensive, and easily handled compared to other solvents used for physical absorption. Second, it can dissolve a large amount of NaOH, which is the source of carbonated minerals. Finally, as in the Rectisol process, methanol can physically absorb a substantial amount of CO2. Therefore, the aim of the present study is to investigate the CCSU performance of the technology of combining the CO2 physical absorption and mineral carbonation in NaOHdissolved methanol solution. When CO2 is injected into the solution, the substances that are assumed to be sodium methyl carbonate (SMC) are synthesized, most of which are precipitated as CO2 fixing materials. Because SMC is not a commonly known material, it was characterized in this study via various analyses. At the same time, a constant amount of CO2 physically absorbed in the solution was quantified and compared to the pure methanol. In addition, the recovery ratio of methanol and the amount of CO2 desorbed were estimated from the precipitate- (or cake) based calculation. Therefore, the paper discusses the potential of the process as one of the CCSU technologies.
CH3OH(l) + NaOH(aq) + CO2 (aq) → CH3OH(l) + NaHCO3(s)
In fact, in the present work, a sufficiently constant amount of precipitates was synthesized via the carbonation in the NaOHdissolved methanol and the precipitates were confirmed to be SMC. These phenomena are detailed in the Results and Discussion section. The completion point of SMC synthesis resulting from the carbonation was assumed to be the moment when the temperature of the reaction solution reaches the highest value measured by thermocouple throughout the reaction because the carbonation is exothermic. 2.3. Reaction of Sodium Methyl Carbonate (SMC) with Excess Water. When SMC is dissolved in water as a limiting reactant, NaHCO3 is produced and CH3OH is regenerated according to eq 4. CH3OCOONa(s) + H 2O(l, excess)
2. THEORY 2.1. CO2 Physical Absorption into Methanol at Ambient Temperature. When CO2 is injected into the methanol solution, CO2 is physically absorbed by adhering to the unshared electron pair of oxygen atoms in the −OH groups in the methanol, as shown in Figure 1.29
→ NaHCO3(s) + CH3OH(l) + H 2O(l)
Therefore, the physical absorption in methanol can be expressed as given in eq 1.
CH3OCOONa → Na 2CO3 + CO2 + (CH3)2 O
(5)
The theoretical weight percentage of the thermally decomposed SMC residue, Na2CO3, is 54.04% because CO2 and DME are completely volatilized out as the gas phase. Therefore, the experimental composition of the residue was analyzed by thermogravimetric analysis (TGA; N-1000, SCINCO) and differential scanning calorimetry (DSC; DSC4000, PerkinElmer, Catholic University Center For Research Facilities), and the synthesized materials were therefore also identified by comparing their values to the theoretical values. 2.5. Methanol Regeneration. After carbonation, the precipitated SMC separated solution (i.e., the filtrates) consisted of methanol, dissolved SMC, physically absorbed CO2, and a very small amount of water. (Hereafter, the precipitated SMC separated solution is named “precipitated SMC separated methanol”.) For the recovery of methanol, SMC, and CO2, evaporation was conducted with the solution. In the evaporation, physically absorbed CO2 is first separated by stripping. Simultaneously, dried carbonated cake which fixes CO2 is generated at the bottom of the evaporator due to solution depletion. Finally, the pure methanol can be
(1)
In the present paper, the amount of CO2 physically absorbed in the pure methanol obtained from the experiment at the ambient and given conditions was 8.74 g CO2/L methanol. 2.2. Synthesis of Sodium Methyl Carbonate (SMC) via Carbonation in NaOH-Dissolved Methanol. When CO2 is injected into a constant amount of NaOH-dissolved methanol, sodium methoxide and water are generated by the reaction of pure methanol and NaOH. The sodium methoxide then reacts with the absorbed CO2 to generate sodium methyl carbonate (CH3OCOONa; SMC) according to eq 2.30 CH3OH(l) + NaOH(aq) + CO2 (aq) → CH3OCOONa(s) + H 2O(l)
(4)
Equation 4 can be used as one of the reactions to confirm that the synthesized material is SMC. If the reaction occurred, NaHCO3 is obtained as the precipitate to fix CO2 because its solubility in methanol is very small, 1.72 g NaHCO3/100 mL methanol,32 and the reproduced methanol can be reused as the reactant. The reproduced methanol concentration in the SMC dissolved solution can be controlled by varying both the reaction ratio of SMC and the water amount. 2.4. Thermal Decomposition of Sodium Methyl Carbonate (SMC). According to the research reported,33 which discussed the thermal decomposition of general alkali metal methyl carbonates, SMC can be thermally decomposed to sodium carbonate, CO2, and dimethyl ether (DME) as represented by eq 5.
Figure 1. Polar interaction of a terminal hydroxyl group with a free carbon dioxide molecule.29
CH3OH(l) + CO2 (aq) → CH3OH − CO2 (aq)
(3)
(2)
31
Hirao et al. reported that the alkali hydroxide cannot be used as the reactant to obtain the precipitated alkali alkyl carbonates because they are decomposed by the water generated during the reaction. However, contrary to this B
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Figure 2. Experimental schematic diagram for CO2 physical absorption and mineral carbonation in NaOH-dissolved methanol. using various analyzing equipment including elemental analysis (EA; PerkinElmer 2400 series II, PerkinElmer), TGA, DSC, X-ray diffraction (XRD; Siemens Bruker AXS D-5000, Catholic University Center For Research Facilities), inductively coupled plasma-atomic emission spectroscopy (ICP-AES; Genesis Fee, SPECTRO), and scanning electron microscopy (SEM; S-4800, Hitachi). 3.2. Methanol Regeneration. Atmospheric evaporation for methanol regeneration was carried out with 100 mL of precipitated SMC separated methanol at 70 °C for 1 h. A Pyrex evaporator was employed and the temperature of the condenser was maintained at 4 °C. The volume of the distilled methanol was measured and its concentration was analyzed by high performance liquid chromatography (HPLC; Ultimate 3000, Dionex, The National Instrumentation Center for Environmental management). In addition, the cake remaining in the bottom of the evaporator was sampled and analyzed using XRD.
reproduced from the solution via condensation from the evaporation.
3. EXPERIMENTAL METHOD 3.1. CO2 Absorption and Synthesis of Sodium Methyl Carbonate (SMC). Thirteen solutions of NaOH (OCI Company Ltd., 99.99%) concentrations ranging from 0.5 to 10 g NaOH/500 mL methanol (OCI Company Ltd., 99.9%) were prepared as solutions for CO2 absorption and SMC synthesis. They were sufficiently stirred before CO2 injection for complete NaOH dissolution in methanol. The experimental schematic diagram for carbonation is shown in Figure 2. 500 mL of each solution was placed in the Pyrex cylindrical batch reactor (D, 110 mm; h, 80 mm) and stirred at 200 rpm using a magnetic stirrer bar for uniform reaction. The reactor was kept in a constant temperature water bath with the temperature maintained at about 25 °C. The feeding of the inlet gases was controlled by a mass flow controller (MPR-2000, MKP) at a flow rate of 1 L/min for CO2 and 2 L/min for N2. These gases were uniformly mixed in a gas mixer before injection. The outlet CO2 composition after absorption was measured at 1 s intervals using a nondispersive infrared (NDIR) gas analyzer (GA) (maMos-200, Madur Electronics). All of the gases flowing into the GA for analysis were passed through a condenser in advance to remove the moisture. Prior to the carbonation reaction, all of the experimental apparatus, including the gas line and the empty space in the reactor, was purged with pure N2. Thereafter, CO2 was mixed with pure N2 and the concentration of the gas mixture was confirmed to be stabilized. Subsequently, this gas mixture bypassed the reactor for about 3 min and, operating the 3-way valve equipped with an injection gas line, the mixture was injected into the reactor by a sparger made from a glass filter to uniformly disperse the gas in the solution. The pH and electrical conductivity (EC) variation during the reaction were measured every 5 s using a pH/EC meter (Orion 4 Star, Thermo Scientific). The carbonation reaction was assumed to be terminated when the CO2 composition of the outlet gas was equal to that of the feeding gas. All data obtained from the experiment including the pH, EC, and CO2 compositions were recorded on a computer. The total amount and the amount of CO2 captured according to the time were calculated using the methodology mentioned in our previous paper.34 All of the synthesized precipitates from the experiments were completely dried at 50 °C in a vacuum oven and were characterized
4. RESULTS AND DISCUSSION 4.1. CO2 Absorption and Carbonation in NaOHDissolved Methanol Solution. 4.1.1. In Low Concentration NaOH-Dissolved Methanol Solution (0.5−3 g NaOH/500 mL Methanol). SMC was not precipitated by the carbonation with solutions of which the dissolved NaOH concentration is less than 3.34 g/500 mL (6.68 g NaOH/L methanol; hereafter named “low concentrated solution”). From this result and considering eq 2, the solubility of synthesized substances, assumed to SMC, in the methanol solution is therefore calculated to be about 1.64 g SMC/100 mL methanol. Figure 3 shows variation of the CO2 outlet composition, temperature, pH, electrical conductivity, and the amount of CO2 absorbed during the carbonation conducted with 3 g of NaOH-dissolved methanol solution. As CO2 was injected into the solution, SMC began to synthesize according to eq 2 and the temperature was increased from 25 °C to the maximum value of 26.4 °C. The amount of CO2 absorbed had rapidly increased to the maximum temperature point where the synthesis was expected to be completed. The amount of CO2 absorbed was calculated to be 3.31 g. Thereafter, CO2 was physically absorbed into the C
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Figure 3. Variation of CO2 outlet composition, temperature, pH, electrical conductivity, and the amount of CO2 absorbed during the carbonation with 3 g of NaOH-dissolved methanol solution.
Figure 4. Maximum temperature, time reaching that point, total amount, and physical amount of CO2 absorbed, and the theoretical and experimental amounts of CO2 consumed for SMC synthesis in six low concentration solutions.
solution and its rate gradually decreased. In addition, the temperature of the solutions dropped and finally maintained with the state slightly less than ambient. The initial pH of 12.74 slightly decreased to about 10.06 during the SMC synthesis period and then very rapidly reduced to 8.9, which was almost maintained during the physical absorption. The reaction was completed at the pH of 8.8. EC steadily decreased until the SMC synthesis completion point and then slightly increased up to the final point. This phenomenon of EC variation is detailed in Section 4.1.4. The results of carbonation in six low concentration solutions including 3 g NaOH/500 mL are plotted in Figure 4, showing the maximum temperature during the reaction, the time reaching that point (SMC synthesizing time), and the total and physical amount of absorbed CO2 obtained from the experiment. The plot also shows the theoretical and experimental amounts of CO2 consumed for SMC synthesis. The total amount of CO2 absorbed linearly increased according to the NaOH concentration. The maximum temperature and the time taken to reach that point also increased at a very constant rate. As mentioned already, the amount of CO2 captured up to the maximum temperature point is equal to the amount of CO2 which directly participated in synthesizing SMC from eq 2. This amount is on average 16.78% higher than the theoretical value for the six solutions. This difference could be primarily due to the very small amount of physically absorbed CO2 that was included in the experimental data. Therefore, the amount of CO2 fixed in the SMC synthesis can be equal to or more than the calculated value, based on eq 2. The CO2 absorption capacity of pure methanol at 33 kPa of CO2 partial pressure and ambient temperature was measured to be 4.36 g CO2/500 mL methanol in the experiment. This value
is equal to 1.09 wt % of the solution, which is higher than that of Rectisol reported as 0.91 wt % at −20 °C under the same pressure.35 The amount of CO2 physically absorbed in the methanol, which is assumed to be the difference between the total amount of CO2 absorbed and the amount of CO2 participating in the SMC synthesis, decreased according to the NaOH concentration. This could be because the water generated when applying eq 2 hinders the physical absorption of CO2 in methanol and the amount of water is proportional to the NaOH concentration in the solution. 4.1.2. In High Concentration NaOH-Dissolved Methanol Solutions (4−10 g NaOH/500 mL Methanol). A constant amount of synthesized SMC was produced as powder via the carbonation in the high concentration NaOH solution (concentration over 3.34 g NaOH/500 mL); this therefore could be regarded as the supersaturated condition of SMC for methanol. The carbonation results of 6 g NaOH-dissolved methanol solutions including the CO2 outlet composition, temperature, pH, EC, and the amount of CO2 absorbed according to time are shown in Figure 5. The maximum temperature was measured to be 28.4 °C due to the high concentration of NaOH. The variations of pH, EC, and the reaction time were even larger than those of the low concentration system. In addition, the amount and rate of CO2 absorbed substantially increased during the SMC synthesis period and gradually decreased from the point where physical absorption dominantly began, as in the low concentration system. Figure 6 shows results from the carbonation in the high concentration solutions. The total and the amounts of CO2 consumed for SMC production linearly increased according to NaOH concentration, as in the low concentration system. The amount of CO2 absorbed for SMC production obtained from the experiment was almost consistent with the theoretical D
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concentration, as shown in this section on the graph (7−10 g NaOH/500 mL methanol). This could be because this section shows the point at which the amount of heat transferred to the circulating water in order to maintain the constant temperature of 25 °C is larger than that in the section showing the relatively lower concentration (4−6 g NaOH/500 mL methanol) due to their relatively higher temperature difference. On the other hand, compared to the lower concentration system, the amount of physically absorbed CO2 decreased less according to the NaOH concentration. This could be because the constant amount of generated water, which hinders CO2 physical absorption in methanol, is proportionately consumed via the reaction with some of the synthesized SMC precipitates, according to eq 4. Furthermore, the experimental amount of CO2 captured up to the maximum temperature point, which equates to the amount of CO2 consumed for SMC production, was eventually slightly lower than the theoretical value at the relatively higher concentration (over 7 g NaOH/500 mL methanol) as shown in this section in Figure 6. These phenomena possibly occurred because this section shows the stage at which the higher temperature influenced the amount of physically absorbed CO2, which differs from the main physical absorption occurring shown in the later section. 4.1.3. Sodium Methyl Carbonate (SMC) Production and Precipitation. The theoretical amount (weight) of SMC synthesized according to the NaOH concentration can be calculated based on eq 2, the values of which are shown in Figure 7.
Figure 5. Variation of CO2 outlet composition, temperature, pH, electrical conductivity, and the amount of CO2 absorbed during the carbonation with 6 g of NaOH-dissolved methanol solution.
Figure 7. Theoretical amount (weight) of SMC synthesized and the amount of precipitated SMC obtained from the experiment according to NaOH concentration.
Figure 7 also shows the amount of precipitated SMC obtained from the experiment. The values were linearly correlated with the NaOH concentration and their individual correlation equation was estimated as in eqs 6 and 7.
Figure 6. Maximum temperature, time reaching that point, total amount and physical amount of CO2 absorbed, and the theoretical and experimental amounts of CO2 consumed for SMC synthesis in seven high concentration NaOH solutions.
value of 99.39%, and the time taken to reach the maximum temperature linearly increased sharply according to NaOH concentration. However, although the maximum temperature of the solutions increased according to NaOH concentration, the increasing rate substantially decreased at a relatively higher
Q SMC,T = 2.45[NaOH]
(6)
Q SMC,E = 2.28[NaOH] − 8.50
(7)
where QSMC, T = theoretical amount of SMC synthesized according to NaOH concentration (g SMC/500 mL methanol); QSMC, E = experimental amount of precipitated SMC E
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Energy & Fuels obtained from the experiment (g SMC/500 mL methanol); [NaOH] = NaOH concentration in the solution. In Figure 7, the slopes indicate the amount of SMC synthesized and the amount of SMC precipitated from the carbonation with 1 g of NaOH dissolved solution. The slope of the experimental value on precipitate was 0.17 (g SMC/g NaOH) smaller than that of the theoretical value on synthesis. This could be because the reactions of eqs 3 and 4 were actually carried out in the carbonation. That is, the portion of NaHCO3 (the lower molecular weight of 84.00 g/mol than 98.01 g/mol of SMC) is slightly increased in the precipitates obtained from the experiment according to eq 4 as the NaOH concentration increases. The solubility of SMC in the methanol solution can be estimated from the vertical difference between the two straight lines. This was quantitatively confirmed using ICP and is detailed in Section 4.2.1. 4.1.4. Electrical Conductivity at Three Points. Three electrical conductivities measured throughout the carbonation, at initial (ECi), final (ECf), and maximum temperature point (ECm), are depicted in Figure 8.
is measured to be the lowest value in the relatively low concentration solution system. Thereafter, EC slightly increases because H+ and HCO3− are generated according to the reaction as given in eq 8, where H2O is produced via eq 2 and CO2 is additionally physically absorbed. H 2O + CO2 → H+ + HCO3−
(8)
However, in high concentration solutions, a slightly constant amount of precipitated SMC reacts with water according to eq 4, which reduces water and thus the reaction of eq 8 is hindered during the reaction. Therefore, ECm and ECf were almost the same. In addition, these two values decreased as the NaOH concentration increased. This could be because the amount of precipitated SMC, which can reduce the measured EC value by physically floating in the space between the two electrodes of the EC meter, is linearly increased according to the NaOH concentration. Such variation in the features of EC confirms that SMC is synthesized by the reaction of ions in the solution. In addition, the result showing that ECf is higher than ECm at low concentration solutions verifies the occurrence of the reaction as given in eq 8. 4.2. Sodium Methyl Carbonate (SMC) Precipitate Confirmation and Characterization. 4.2.1. Elemental Analysis and Inductively Coupled Plasma (ICP) Results. The elements of C, H, O, and Na of the synthesized precipitate presumed to be SMC were quantitatively analyzed by EA and ICP. The results including their theoretical compositions are summarized in Table 1. Table 1. Theoretical Element Compositions of Sodium Methyl Carbonate (SMC) and Actually Synthesized Precipitates Presumed to be SMC Measured by EA and ICP Elements
C
H
O
Na
Theoretical (wt %) Experimental (wt %)
24.49 23.50
3.06 2.91
48.98 49.91
23.47 24.70
The compositions of the synthesized precipitates corresponded to the theoretical value of SMC. Their slight differences could be because a very small amount of NaHCO3 is present in the precipitates. The solubility of SMC in carbonated methanol solution at 25 °C calculated from the ICP result was about 1.67 g/100 mL. To verify the reaction occurrence as given in eq 4, a constant amount of synthesized precipitates was dissolved in water and filtered to obtain the reprecipitated cake. Subsequently, the cake was analyzed by XRD and the resulting peaks verify the cake to be NaHCO3 as shown in Figure S1 of the Supporting Information. In addition, based on eq 4, a constant amount of synthesized precipitates was dissolved in pure water to maintain the concentration of the regenerated methanol to be 1, 3, 5, and 7 wt % in solutions. NaHCO3 was very slowly precipitated in the solution and the distillation was carried out with the solution at 75 °C to separate the NaHCO3 powder and methanol aqueous solution. The distillates were collected to analyze the methanol concentration in the solution by HPLC and the results are shown in Table 2. The measured methanol concentrations in the synthesized material dissolved water solution were slightly lower than the theoretical values. This difference could be because a slight amount of NaHCO3 is produced during the carbonation and is
Figure 8. Three electrical conductivities measured, throughout the carbonation, at initial (ECi), final (ECf), and maximum temperature point (ECm) of all solutions.
The initial point indicates the state of the NaOH-dissolved solution before the carbonation. ECm and ECf are the EC value at the point where SMC synthesis and the overall absorption is completed, respectively. ECi was the highest among the three ECs at a given constant NaOH concentration solution because NaOH is sufficiently ionized to independent Na+ and OH− in the solution. Although they increased according to the NaOH concentration, the increasing rates slightly reduced in the highly concentrated solution section. This could be because the individual effect of Na+ and OH− was reduced due to their high concentration. As CO2 was injected into the solution, SMC began to be synthesized. Although the SMC is present as the dissolved form in the relatively low concentration solutions, it reduces the EC of the solution because OH− is consumed and CH3OCOO− is generated in the solution (here, the equivalent electrical conductivity of the dissolved CH3OCOO− can be substantially less than that of OH− due to its larger ion size). Therefore, EC decreased until the maximum temperature point, while the ECm F
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value, intensity, and relative intensity are summarized in Table S1 of the Supporting Information. 4.2.5. Scanning Electron Microscopy (SEM) of SMC. The synthesized SMC particles were observed by SEM and are shown in Figure S4 of the Supporting Information. Although the crystalline structure of SMC needs to be investigated in detail, the physical appearance of the SMC particles is plate-shaped and their size is about 14.2 × 19.5 μm2. In addition, the SMC material is very brittle and weak. 4.3. Methanol Regeneration and Na2CO3 Recovery via Evaporation. For 6 g NaOH dissolved in 500 mL methanol solution, the methanol concentration and volume (or weight) variation throughout the process, from before carbonation to regenerated completed state, are summarized in Table 3.
Table 2. Methanol Concentration in a Constant Amount of SMC and Synthesized Precipitates Dissolved Aqueous Solution Purposed composition of methanol in SMC dissolved water solution (wt %) Experimental composition of methanol in synthesized material dissolved water solution (wt %)
1
3
5
7
0.96
2.91
4.54
6.09
mixed in the precipitated SMC, while a small amount of methanol was volatilized during the distillation process. Therefore, considering all these results, the synthesized precipitate is confirmed to be SMC as the main component. 4.2.2. Thermal Gravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) Results. The TGA and DSC results of synthesized SMC are shown in Figure S2 of the Supporting Information. The material began to thermally decompose at around 150 °C; then, from about 300 °C, its weight rapidly decreased to reach a minimum of 58.8% at 317.24 °C. In addition, the heat of the reaction was measured to be 303.75 J/g by DSC analysis. Considering the decomposed temperature, a sufficient amount of synthesized SMC was thermally decomposed according to eq 5 using a high temperature tube furnace. The remaining decomposed substances were analyzed by XRD, the results of which are shown in Figure S3 of the Supporting Information. The decomposed substances were verified to be Na2CO3 and the weight loss value was very similar to the theoretical value based on eq 5. In addition, the CO2 emission from the cake was confirmed. However, DME could not be identified in the study. 4.2.3. Heat of Formation of SMC. The heat of the formation of the SMC synthesis was estimated to be 208.37 kcal/mol by calculating the heat of the formation of each substance from eq 5, as reported in the references as well as the heat of the reaction previously measured by DSC.32 4.2.4. X-ray Diffractometer (XRD) Analysis of SMC. As mentioned already, the synthesized material was confirmed to be SMC by various analyses. Therefore, the standard XRD patterns of SMC can be reported as shown in Figure 9. While the structure of SMC had been discussed by X-ray crystallography,36 the XRD patterns of SMC have not been previously reported in the related literature. Their numerical d
Table 3. Methanol Concentration, Volume, and Weight Variation of 6 g NaOH/500 mL Methanol Solutions Throughout the Process (Methanol Density; 0.792 kg/cm3) volume and weight in solution
step of process (state of solution) Before carbonation (Fresh methanol) After carbonation (Precipitated SMC separated methanol) Evaporation and condensation (Regenerated methanol) Overall
concentration in solution (wt %)
volume (mL) ⟨reduced ratio; %⟩
weight (g)
methanol regeneration ratio based on weight
99.5
500 ⟨−⟩
394
99.3
455.5 ⟨91.1⟩
358.2
90.9
99.4
451 ⟨90.2⟩
355
99.1
90.1
While the volume of methanol was reduced by approximately 9% during the carbonation and its concentration was very slightly reduced due to water generation, the precipitated SMC separated methanol in which substantial SMC is still dissolved was regenerated to almost pure methanol with the ratio of 99.1% by desorbing CO2 and leaving cakes of carbonated Na compound on the bottom of the evaporator. Therefore, the overall recovery ratio of methanol based on the weight was calculated to be 90.1% throughout the process. However, the methanol loss could be further reduced during the process by using more sophisticated equipment, including an advanced condenser and a solid−liquid separation unit, as well as a more detailed operation. For example, about 20 mL of methanol is lost by volatilizing to the air during the filtration which separates SMC from the solution. Considering the XRD peaks of the carbonated Na compound generated from evaporation, as shown in Figure S5 of the Supporting Information, the substance was verified to be a mixture of Na2CO3 and SMC. The results indicated that some of the SMC dissolved in the solution was converted to Na2CO3 during the evaporation due to its instability and methanol depletion. However, CO2 was still fixed in the cakes after evaporation. The weight of these cakes was measured to be 5.28 g/(6 g NaOH·500 mL methanol) and their thermal decomposition characteristics obtained by TGA analysis are shown in Figure S6 of the Supporting Information. From these results and from the decomposition characteristics of the SMC and Na2CO3, the compositions of SMC and
Figure 9. XRD patterns of the synthesized material, sodium methyl carbonate (SMC). G
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Table 4. Total and Individual CO2 Capture and Storage/Utilization Capacity (CCSU) of 6 g of NaOH Dissolved in 500 mL Methanol and Amount of CO2 Desorbed and Fixed in the Process Experimental data CCSU capacity (g CO2/(6 g NaOH/500 mL methanol)) Process step Overall Physical absorption Carbonation for SMC precipitation Evaporation of solution after carbonation (Cake weight basis)
As SMC cake As Na2CO3 cake
Based on gas phase analysis
Based on precipitate (or cake) weight measurement
Amount of CO2 desorbed and fixed (g CO2/(6 g NaOH/500 mL methanol))
11.02 4.36 -
10.17 4.36a 2.26 0.99
10.17 (5.64; desorbed, 4.53; fixed) 4.36 (All desorbed via evaporation) 2.26 (All fixed by SMC powder) 0.99 (All fixed by SMC powder)
-
2.56
6.66b
3.55 5.81c
1.28 g; desorbed 1.28 g; fixed by Na2CO3 powder 3.55
Sub total Carbonation for SMC synthesis a
Based on gas phase analysis. bTheoretical value based on eq 2. c2.26 + 3.55 (g CO2/(6 g NaOH/500 mL methanol)).
of the NaOH based on gas phase analysis and cake weight measurement was estimated to be 1.11 g CO2/g NaOH and 0.97 g CO2/g NaOH, respectively. However, if the technically unnecessary loss of methanol in the process such as carbonation and evaporation is reduced, the performance would be substantially increased.
Na2CO3 in the cakes were calculated as 41.63% (2.20 g) and 58.37% (3.08 g), respectively. Because 3.08 g of Na2CO3 corresponds to 5.70 g of unconverted SMC dissolved in solution, before evaporation, according to eq 5, total amount of dissolved SMC in the precipitated SMC separated methanol solution was 7.9 g/500 mL methanol. This value is about 4% lower than the solubility of SMC in the solution previously described in Section 4.2.1. This difference could be because the composition of Na2CO3 and SMC in the cakes was calculated solely based on the TGA analysis. Finally, the amount of CO2 fixed in the cake was calculated to be 3.55 g CO2/(6 g NaOH· 500 mL methanol). 4.4. CO2 Capture and Storage/Utilization Capacity of NaOH-Dissolved Methanol. Considering the experimental data such as physical absorption, reaction of CO2 with NaOH in methanol, and the regeneration of the solution, the total and individual CCSU capacities of 6 g NaOH dissolved in 500 mL methanol can be calculated in detail and are summarized in Table 4. The results were calculated based on the gas phase analysis (i.e., based only on the data measured by the gas analyzer) and the precipitate (or cake) weight measurement. The total CCSU capacity based on cake weight measurement was 10.17 g CO2/(6 g NaOH/500 mL methanol), which is 8.3% lower than that based on the gas phase analysis. This could be because a very slight amount of all components, including the precipitate as well as the CO2 capture, is necessarily lost in filtration, drying, and evaporation during the process. The amount of CO2 desorbed and CO2 fixed in the cakes of SMC and Na2CO3 resulting from evaporation could be calculated based on the cake weight measurement analysis. The desorbed amount was 5.64 g CO2/(6 g NaOH/500 mL methanol), in which the amount of CO2 physically absorbed in methanol and amount of CO2 consumed to generate the dissolved SMC during carbonation is 4.36 and 1.28 g CO2/(6 g NaOH/500 mL methanol), respectively. On the other hand, the fixed amount was calculated to be 4.53 g CO2/(6 g NaOH/ 500 mL methanol), which is stored CO2 in the precipitated SMC and Na2CO3. Therefore, the CO2 capture and fixation ratio of the process was estimated to be 55.46% and 45.54%, respectively. Finally, considering the amount of CO2 physically absorbed by methanol (4.36 g CO2/500 mL methanol) and the recovery ratio of methanol, the performance of CO2 capture and fixation
5. CONCLUSION In this study, the carbon capture and storage/utilization (CCSU) performance of a technology that combines CO2 physical absorption and mineral carbonation in NaOHdissolved methanol were investigated and the recovery ratio of the methanol used as solvents was discussed. The potential of the process as a CCSU technology was thus examined, with some conclusions. Sodium methyl carbonate (SMC) was synthesized and most SMC were precipitated as CO2-fixing material in a solution where the dissolved NaOH concentration is higher than 6.68 g NaOH/L methanol. The SMC solubility was then calculated to be approximately 1.64 g SMC/100 mL methanol at 25 °C. The amount of synthesized and precipitated SMC very linearly increased according to the NaOH concentration in the solution and the slope of the equation obtained from the experiments on precipitate was very similar to the theoretical value on synthesis. In addition, SMC was identified and characterized via various analysis methods using EA, XRD, TGA, DSC, and HPLC. Although CO2 was physically absorbed in methanol with approximately 4.36 g CO2/500 mL methanol on average, it slightly varied according to the dissolved NaOH concentration in the solution. This is presumably because the composition of water generated is slightly different in each solution. The CCSU capacities of this process based on the gas phase and obtained precipitate analysis were estimated to be 11.02 g CO2/(6 g NaOH/500 mL methanol), (or 1.11 g CO2/ g NaOH) and 10.17 g CO2/(6 g NaOH/500 mL methanol), (or 0.97 g CO2/g NaOH), respectively. The amount of CO2 desorbed via the evaporation of solution remaining after carbonation was 5.64 g CO2/(6 g NaOH/500 mL methanol). In addition, the amount of CO2 fixed by SMC and Na2CO3 calculated based on cake weight measurement analysis was 4.53 g CO2/(6 g NaOH/500 mL methanol). Therefore, the CO2 capture and the fixation ratio of the process were calculated to be 55.46% and 45.54%, respectively. While the overall methanol recovery ratio based on the weight was calculated to be 90.1%, the loss can be substantially reduced further during the process H
DOI: 10.1021/acs.energyfuels.6b02709 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
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by using more sophisticated equipment such as an advanced condenser and a solid−liquid separation unit, and by carrying out a more detailed operation. Although the methanol was regenerated by evaporation in this study, we investigated the possibility of precipitated SMC separated methanol to reuse for the next carbonation cycle by adding NaOH to the solution without any other treatment. It is expected that the results of this research will contribute to substantially decreasing the amount of methanol regeneration energy, thus increasing the potential of the process as a CCSU technology to be reported in the near future.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b02709. XRD patterns, TGA and DSC results, SEM images of the synthesized material, and d values and intensities in the XRD pattern of the synthesized material (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] or
[email protected]. Tel.: +822-2164-4866. Fax: +82-2-2164-4765. ORCID
Jung-Ho Wee: 0000-0001-5142-2391 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by Basic Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2013R1A1A2A10010414) as well as supported by the Catholic University of Korea, Research Fund, 2016.
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DOI: 10.1021/acs.energyfuels.6b02709 Energy Fuels XXXX, XXX, XXX−XXX
