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A new process for the capture of CO2 and reduction of water salinity
Desalination 411 (2017) 69–75
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A new process for the capture of CO2 and reduction of water salinity Muftah H. El-Naas a,⁎, Ameera F. Mohammad b, Mabruk I. Suleiman c, Mohamed Al Musharfy c, Ali H. Al-Marzouqi b a b c
Gas Processing Center, College of Engineering, Qatar University, Doha, Qatar Chemical and Petroleum Engineering Department, UAE University, Al Ain, United Arab Emirates Takreer Research Center, Abu Dhabi, United Arab Emirates
H I G H L I G H T S • • • • •
A new process for CO2 capture and water desalination has been developed. It is based on a Modified Solvay process without using ammonia. It is more efficient in CO2 capture and Sodium reduction than the Solvay process. The new process can achieve 99% CO2 capture efficiency and 35% Sodium removal. The energy requirements are much lower than that of the Solvay process.
a r t i c l e
i n f o
Article history: Received 4 July 2016 Received in revised form 3 February 2017 Accepted 12 February 2017 Available online xxxx Keyword: CO2 capture Water desalination Solvay process Sodium bicarbonate Reject brine
a b s t r a c t The present work evaluates a new process for the capture of CO2 and the reduction of water salinity. It is based on the Solvay process without the use of ammonia and involves the reaction of CO2 with saline water such as reject brine in the presence of calcium hydroxide. The effects of operating parameters such as reaction temperature, water pH and reaction stoichiometry on CO2 capture efficiency and sodium removal were examined for both traditional and modified Solvay process. The optimum conditions for maximum CO2 capture efficiency and sodium removal were determined using response surface methodology and were found to be at temperature of 20 °C and a pH of greater than 10 for both processes. At the optimum conditions, CO2 capture of 86% and 99% and sodium removal of 29% and 35% were achieved for the traditional Solvay and the Modified process, respectively. The water pH was found to be a key parameter in the effectiveness of the reaction process; higher pH leads to better process performance in both CO2 capture efficiency and sodium removal. The experimental results clearly illustrated that the Modified Solvay process is superior in terms of CO2 capture efficiency, sodium removal and energy consumption. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Climate change and variability is an evident phenomena around the world and can be attributed to global warming. Most of the observed increase in global average temperatures is very likely due to the observed increase in the concentrations of greenhouse gases such as CO2 in atmosphere. CO2 is a major contributor to global warming, and it is often emitted by various activities associated with industrial processes and the burning of various types of carbonaceous fuels [1]; it is believed to cause approximately 55% of global warming [2]. Relying on fossil fuels as the main source of energy in many parts of the world has contributed to the rise of CO2 emissions to unprecedented levels [3]. Many industries such as natural gas sweetening, hydrogen production for ammonia and ⁎ Corresponding author. E-mail address: [email protected] (M.H. El-Naas).
http://dx.doi.org/10.1016/j.desal.2017.02.005 0011-9164/© 2017 Elsevier B.V. All rights reserved.
ethylene oxide, oil refineries, iron and steel production facilities, desalination and power plants, cement and limestone manufacturing plants represent major sources of CO2 emissions. In recent years, there has been an increased interest in carbon capture and storage (CCS) as an option to reduce the CO2 emissions. CCS is based on the separation and capture of carbon dioxide produced by fossil fuel power plants or other sources either before or after combustion [1]. A number of CO2 capture technologies have been used in many processes such as oxyfuel combustion, post-combustion, pre-combustion and chemical looping combustion. Among the post-combustion capture techniques, the most promising and most effective are adsorption using solid sorbents [4], solvent absorption [5], cryogenic fractionation technology [6] and membrane separation [7]. CO2 capture using chemical solvent is one of the most promising technologies for decreasing the rising emissions of greenhouse gases [8]. Key parameters for selecting an effective solvent for CO2 absorption include high absorption, fast reaction
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kinetics, low degradation rate, and low regeneration energy [9] as well as the ability to handle large amounts of exhaust streams [10]. The most common solvents used for CO2 capture are amines (either in aqueous solution or immobilized on solid support) [11] and aqueous ammonia [12]. Aqueous ammonia is considered to be a competitive option for CO2 capture due to low chemical cost, corrosiveness and relatively low regeneration temperature [13]. The ammonia-based Solvay process has recently been considered for the capture of CO2 and the production of useful and reusable carbonate products, as well as the desalination of saline water [14]. The Solvay process is used for sodium carbonate (soda ash) manufacturing, where carbon dioxide and ammonia gas are passed through a saturated sodium chloride solution to form soluble ammonium chloride and a precipitate of sodium bicarbonate according to Reaction (1) below. The sodium bicarbonate is heated to form the washing soda; whereas the ammonium chloride solution is mixed with calcium hydroxide and heated to recover the ammonia according to Reactions (2) and (3), respectively. NaCl þ NH3 þ CO2 þ H2 O→NaHCO3 þ NH4 Cl
ð1Þ
2NaHCO3 →Na2 CO3 þ CO2 þ H2 O
ð2Þ
2NH4 Cl þ CaðOHÞ2 →CaCl2 þ 2NH3 þ 2H2 O
ð3Þ
Several studies and patent applications have described Solvay-based approaches for the capture of CO2 and desalination of high salinity water [14–17]. One of the major drawbacks of the Solvay process is the presence of ammonia, which is considered an environmental and health hazard. Exposure to high concentrations of ammonia can cause severe injuries such as skin, nose, throat and respiratory tract burning, which can be a reason of alveolar and bronchiolar edema and airway annihilation, leading to respiratory failure [18]. Ammonia is not involved in the overall Solvay reaction, but it plays a key role in buffering the solution at a basic pH; without ammonia, the acidic nature of the water solution will hinder the reaction and hence prevent the precipitation of sodium bicarbonate [14]. In addition to the safety concerns, the ammonia recovery process through Reaction (3) is an energy intensive step that adds to the overall cost of the process. The aim of the current study, therefore, is to modify the Solvay process to reduce or eliminate the drawbacks associated with the presence of ammonia and then use the modified process for the capture of CO2 and reduction of water salinity. In addition, carry out a comparative evaluation of the Solvay (S) and the Modified Solvay (MS) processes in terms of CO2 capture efficiency, sodium ion removal and energy demand. 2. Modification of Solvay process In the original Solvay process, concentrated brine is ammoniated through direct contact with ammonia gas and then contacted with CO2 in a bubble column reactor. The product which contains soluble ammonium chloride and precipitate of sodium bicarbonate is sent to a filter to remove the sodium bicarbonate. The remaining solution is reacted with calcium hydroxide to recover the ammonia, which is then sent again to the ammonization step. The proposed modification of the Solvay process does not involve the use of ammonia. Instead, calcium oxide is added directly to the bubble column reactor, which converts to calcium hydroxide as soon as it contacts the brine, raises the pH and captures CO2 by reacting with sodium chloride according to the reaction given below: 2NaCl þ 2CO2 þ CaðOHÞ2 →CaCl2 þ 2NaHCO3
ð4Þ
Calcium oxide or calcium hydroxide could be obtained from any alkaline solid waste products to avoid the need of calcination to produce
CaO according to the following reaction at high temperature [19]. CaCO3 →CaO þ CO2
ð5Þ
However, according to this reaction, the production of one mole of CaO results in the generation of one mole of CO2, while in the Modified Solvay process, each mole of Ca(OH)2 can be used to capture two moles of CO2. The process has the dual benefit of brine desalination and carbon dioxide capture. In addition, the process eliminates the need for ammonia recovery (Reaction (3)) which is an energy intensive step in the Solvay process. 3. Thermodynamic analysis HSC Chemistry 6.1 software [20] was used to perform a thermodynamic analysis based on sodium chloride as the main reactant in the reject brine. The software was used to define the equilibrium composition for Reactions (1) and (4) at different temperatures and to evaluate the heat of reaction as a function of temperature. Gibbs free energy minimization method was used to determine the number of moles present at equilibrium for any species at a fixed temperature and pressure [21]. Tables 1 and 2 present the calculated thermodynamic properties for Reaction (1) and (4), respectively. The analysis indicates that both reactions are spontaneous for the whole temperature range (0 to 90 °C) as indicated by the negative ΔG. The changes in the heat of reaction from (0 to 100 °C) for both reactions are negative, which indicates that the reactions are exothermic (at 20 °C, ΔH is −129 kJ/kmol for Reaction (1) and −208 kJ/kmol for Reaction (4)). Since a more negative ΔG indicates more spontaneous reactions [22], the Modified Solvay process is more spontaneous than the traditional Solvay process at 20 °C with ΔG of − 56 kJ/kmol and − 26 kJ/kmol, respectively. It is also worth noting that calcium carbonate formation (Reaction (6)) may take place and compete with the formation of sodium bicarbonate. CaðOHÞ2 þ CO2 →CaCO3 þ H2 O
ð6Þ
However, thermodynamic analysis indicated that Reaction (6) is only competitive when calcium oxide is present at amounts greater than or equal to the stoichiometric ratio. The formation of sodium bicarbonate can be favored by controlling the amount of Ca(OH)2 in the reaction [23]. 4. Experimental evaluation 4.1. Experimental apparatus Both Solvay and the Modified Solvay processes were evaluated experimentally using desalination reject brine in a bubble column reactor. Experiments were carried out in a stainless steel jacketed, bubble column reactor with an internal diameter of 78 mm and an overall height of 700 mm. The reactor was operated in a semi-batch mode, where the
Table 1 Thermodynamics data of Reaction (1). Temperature (°C)
ΔH (kJ/mol)
ΔS (J/mol·°C)
ΔG (kJ/mol)
0 10 20 30 40 50 60 70 80 90 100
−124 −129 −129 −129 −129 −128 −128 −128 −127 −127 −127
−332 −353 −352 −352 −351 −350 −349 −348 −347 −346 −346
−33 −29 −26 −22 −19 −15 −12 −8 −5 −1 2
M.H. El-Naas et al. / Desalination 411 (2017) 69–75 Table 2 Thermodynamic data for Reaction (4). Temperature (°C)
ΔH (kJ/mol)
ΔS (J/mol·°C)
ΔG (kJ/mol)
0 10 20 30 40 50 60 70 80 90 100
−212 −210 −208 −206 −205 −203 −202 −200 −199 −198 −197
−535 −527 −520 −514 −509 −504 −500 −496 −492 −488 −485
−66 −61 −56 −50 −45 −40 −35 −30 −25 −20 −16
brine mixture was exposed to a continuous flow of carbon dioxide mixture with air (10% CO2 and 90% air) at atmospheric pressure and 20 °C. The effluent gas line from the top of the reactor was connected to CO2 gas analyzer (Model 600 series of Non-Dispersive Infrared NDIR analyzers). A schematic diagram of the main units of a general Modified and traditional Solvay processes are shown in Fig. 1. 4.2. Brine samples and other reactants Reject brine samples were obtained from a local desalination plant operating MSF (Multi-Stage Flash) desalination process, with an average salinity of 71,700 mg/L. Ammonium hydroxide solution (25 wt%
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NH3 in H2O) and calcium oxide (99.9% trace metal basis) were acquired from Scientific Progress Medical and Scientific Equipment, UAE. A gas mixture of (10% CO2 and 90% Air) was purchased from Abu Dhabi Oxygen Company, UAE. 4.3. Experimental methods 4.3.1. Parametric study Screening of the Modified Solvay process parameters was performed using a parametric study (one factor at a time), and the variables with significant effects on CO2 capture and sodium removal were identified. One liter of the reject brine was mixed with calcium oxide concentration (20 g/L). The mixture was then fed to the reactor by piston pump; the reactor was operated in a semi-batch mode (batch for the liquid and continuous for the gas), and the reaction temperature was controlled at 20 °C by water circulation through the reactor jacket. The gas mixture was bubbled into the reactor at a controlled flow rate of 1 L/min for 240 min. Brine samples were collected every 60 min and analyzed for sodium removal. The effluent gas was continuously passed through a CO2 gas analyzer to determine the percentage of CO2 capture. The CO2 capture efficiency was calculated after each experiment by determining the ratio of the CO2 moles captured to the moles of CO2 loaded to the reactor. pH variation with time was also measured using a digital pH meter. The reactor contents, at the end of each experimental run, were drained into a volumetric flask and filtered. The solids were dried at room temperature and then examined by a scanning electron microscope (SEM) and X-ray diffractometer (XRD). The effects of the following parameters evaluated: calcium oxide concentration, reaction temperature and gas flow rate. In addition, the effect of ammonium bicarbonate on sodium removal in the Modified Solvay process at different temperatures have been studied by adding ammonium bicarbonate (13 w/w%) to the treated brine samples. Samples were then tested for sodium removal. 4.3.2. RSM optimization The CO2 capture and sodium removal were optimized for the Modified Solvay and the original Solvay processes using Response surface Methodology (RSM) in Minitab 17.0 application [24], and the experimental runs were designed in accordance with central composite design (CCD). One liter of reject brine was reacted with stoichiometric and optimum molar ratios of the Solvay and the Modified Solvay processes. The optimum temperature was found to be around 20 °C for both processes, but the other parameters were different. For the Solvay process, the reject brine was mixed with ammonium hydroxide solution in the molar ratio of 3NH3:1NaCl (6 wt% NH3 concentration) for optimum molar ratio and (1NH3:1NaCl) for stoichiometric molar ratio experiments. The optimum molar ratio for the Modified Solvay was determined to be (0.3 CaO:1NaCl) or 16 g CaO per liter; whereas the stoichiometric molar ratio is (0.5 CaO:1NaCl). The CO2 gas mixture was bubbled through the reactor content at a flow rate of 1 L/min for the stoichiometric ratio and at flow rates of 1.54 L/min and 0.76 L/min for the optimum conditions for the Solvay and the Modified Solvay processes, respectively. 5. Results and discussion 5.1. Effect of CaO concentration
Fig. 1. Schematic diagrams of the main units of (a) Modified and (b) traditional Solvay processes.
Calcium oxide is a key reactant in the modified Solvay process and its concentration plays a key role sodium removal. Fig. 2 shows the effect of CaO concentration on sodium removal and CO2 capture efficiency. With increasing the CaO concentration, sodium removal increased rapidly; for example, sodium removal increased from 5% to 32% by increasing the CaO concentration from 5 to 20 g/L (0.5% to 2% CaO). As expected, a higher CaO concentration maintains higher pH level through the reaction time, as shown in Fig. 3. However, increasing CaO concentration
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Fig. 2. Sodium removal and CO2 capture efficiency versus CaO concentration at temperature of 20 °C and gas flow rate of 1 L/min.
more than 2% does not seem to increase the pH level any further, but may lead to hindering the mixing within the reactor and reduce sodium removal. This can be explained by increasing the mass transfer resistance between reactants [25]. Moreover, excess calcium oxide may also lead to the formation of calcium carbonate and thus competing with the formation of sodium bicarbonate. On the other hand, CO2 capture does not seem to be affected by increasing the CaO concentration beyond 20 g/L, which indicates that the CO2 capture is mainly related to pH level in contrast to sodium removal, which is related to both pH level and CaO concentration. When carbon dioxide dissolves in the brine, it forms carbonic acid (H2CO3), which dissociates at high pH into bicarbonate ions [26]. The precipitation of sodium bicarbonate is believed to be proportional to the sodium and bicarbonate ions concentration in the feed brine [27]. In addition, the increase in sodium removal is combined with an increase in CO2 capture percentage. For the specific reaction conditions, most of entering CO2 moles have been captured in the first 240 min for the CaO concentration 10–25 g/L, with maximum efficiency of 98.7% at CaO concentration of 20 g/L.
Fig. 4. Sodium removal and CO2 capture efficiency versus temperature at CaO concentration of 20 g/L and gas flow rate of 1 L/min.
two combining factors that are prevalent at higher temperature: lower CO2 solubility and higher sodium bicarbonate solubility. The first leads to lower formation of sodium bicarbonate, while the second leads to less precipitation of the formed bicarbonate. Table 3 shows the solubility of sodium bicarbonate in water at different temperatures [28]. As is the case for most solids, the solubility of sodium bicarbonate increases with increasing temperature, but this rise in the solubility could be reversed by the addition of ammonium bicarbonate [21]. Ammonium bicarbonate has a major effect on the possibility of using the Solvay process, since it is a significant intermediate in the formation of sodium bicarbonate and can enhance the efficiency of desalinating the reject brine [21]. Raising the concentration of ammonium bicarbonate and hence increasing the concentration of (HCO− 3 ) would drive Reaction (8) to the left and lower the solubility of sodium bicarbonate [21]. NH4 HCO3 ðaÞ↔NH4 þ þ HCO− 3
ð7Þ
NaHCO3 ðaÞ↔Naþ þ HCO− 3
ð8Þ
5.2. Effect of temperature The experimental results, as shown in Fig. 4, indicated that the sodium removal declined with increasing the reaction temperature, where less moles of CO2 were captured and hence less sodium bicarbonate was formed according to Reaction (4). This is believed to be caused by
Adding 13 wt% of ammonium bicarbonate to 8% NaCl solution, reduced sodium bicarbonate solubility to 0 g/100 g [21]. Fig. 5 shows the effect of ammonium bicarbonate in improving the sodium removal, which can have significant effect on the possibility of using the Modified Solvay process even at high temperatures. Previous studies proposed that best temperature condition for forward Solvay's reactions will be at room temperature [29], where reversible reactions can take place at a temperature range of 38–60 °C [30]. The reduction in the sodium removal at 40–50 °C can also be due to the low CO2 solubility and hence less reaction with sodium chloride [31]. The experimental results indicate that the solution pH decreases with increasing the reaction temperature. At temperature 50 °C the solution pH decreased from 11.39 to 9.67 compared with almost unchanged pH at temperature of 10 °C, where the pH sustain higher than 11, this can be explained by Le Châtelier's Principle. When increasing the temperature, the position of equilibrium moves to counter the temperature increase by absorbing the extra heat and forming more hydrogen ions and hydroxide ions.
Table 3 Sodium bicarbonate solubility in water at different temperatures [26].
Fig. 3. pH versus reaction time for different CaO concentrations at temperature of 20 °C and gas flow rate of 1 L/min.
Temperature (°C)
Solubility (g/L)
0 20 60 100
69 96 165 236
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gas flow rates, which can be attributed to the increase in CO2 moles loaded to the reactor per unit time. 5.4. Comparison between Solvay and modified Solvay process.
Fig. 5. Sodium removal with and without ammonium bicarbonate versus temperature at CaO concentration of 20 g/L and gas flow rate of 1 L/min.
This effect leads to increasing the value of Kw (The Ionic Product for water) and decreasing the pH [32]. This is in addition to the CO2 effect in reducing the pH during the reaction, where carbon dioxide reacts with water to give carbonic acid as discussed in Section 4.1. The solubility of CO2 gas plays a significant role in CO2 capture. Increasing the temperature reduces CO2 solubility and hence decreases the CO2 capture [31]. Maximum CO2 capture efficiency and sodium removal of 99.8% and 45.2% respectively, were achieved at temperature of 10 °C.
Both Solvay and the modified Solvay were compared in terms of CO2 capture efficiency, sodium removal and pH. It is worth noting here that the stoichiometric experiments were carried out at the same conditions for both processes (20 °C, 1 atm, a gas flow rate of 1 L/min); whereas, the optimum conditions experiments were carried out at the specific optimum conditions for each system. Fig. 7 (a) and (b) show plots of the percent CO2 capture efficiency and sodium removal, respectively. It is clearly illustrated that the Modified Solvay process is superior in terms of CO2 capture efficiency and sodium removal at both stoichiometric and optimum conditions. At the optimum conditions, CO2 capture of 86% and 99% and sodium removal of 29% and 35% were achieved for the traditional Solvay and the Modified process, respectively. It is also apparent that the Modified Solvay can sustain a higher pH of 12.1 and 11.8 compared to Solvay process with pH of 10.4 and 11.2 in both stoichiometric and optimum conditions, respectively, where pH is the main factor in the reaction process in terms of CO2 dissociation and sodium bicarbonate precipitation. The maximum CO2 capture capacity for calcium oxide or ammonia solution were found to be 1.44 and 1.15 mol CO2/L, respectively. It is worth noting here that even though the sodium removal in the Modified Solvay is better than that
5.3. Effect of gas flow rate The effect of gas flow rate on sodium removal and CO2 capture efficiency is shown in Fig. 6. Sodium removal increased about 30% with increasing the gas flow rate from 500 to 750 mL/min, which can be related to the increasing in reaction rate according to the increase in CO2 moles loaded to the reactor. However, flow rates higher than 750 mL/min seemed to have a slightly negative effect on sodium removal, which can be explained by decreasing the residence time of CO2 in the reactor and hence decreasing the reaction rate [33]. The effects of gas flow rate on pH and CO2 capture shows that increasing the gas flow rate decreases the CO2 capture efficiency according to the decrease in the gas residence time in the reactor [33]. Maximum CO2 capture efficiency of about 100% has been recorded at gas flow rate of 500 mL/min. The decline of pH during the reaction seemed to be more evident for high
Fig. 6. Sodium removal and CO2 capture efficiency versus gas flow rate at CaO concentration of 20 g/L and temperature of 20 °C.
Fig. 7. A comparison of (a) CO2 capture efficiency and (b) sodium removal for Solvay (S) and Modified Solvay (MS) at stoichiometric and Optimum conditions.
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Table 4 Minimum energy requirement and equivalent work for conventional ammonia based processa. Energy requirement Energy requirement for CO2 regeneration (MJ/kg CO2)
Energy requirement for NH3 recovery (MJ/kg CO2)
Equivalent work Steam consumption (MJ/kg CO2)
Pump work (MJ/kg CO2)
Compression work (MJ/kg CO2)
Cooling duty (MJ/kg CO2)
3.376
0.390
0.689
0.06
0.231
0.376
a
For ammonia concentration of 6 wt% [37].
in the conventional Solvay, the sodium removal is not significant and need to be improved through process optimization and using multistage treatment. Although both Solvay and the Modified Solvay processes involve replacement of some of the sodium chloride with calcium chloride, an overall salinity reduction of 19% was achieved in the tested one-stage Modified Solvay process. The treated brine, therefore, is still not suitable for drinking before further treatment. However, such treated water may be used for irrigation since calcium chloride is much less harmful to plants than sodium chloride. In fact, calcium chloride can be beneficial to many plants and may act as growth enhancer [34,35]. The optimized process may also be used for the treatment of high salinity produced water so that it can be reused for irrigation or low salinity flooding for Enhanced Oil Recovery (EOR). Although it is important to make a comparison of the operating cost between the conventional ammonia-based Solvay process and the proposed Modified Solvay, it is rather difficult to make such comparison based on the laboratory scale experimental data obtained in this study. It is possible, however, to make a rough estimation of the energy saving when considering the Modified Solvay process in comparison with the ammonia-based Solvay process. The Modified Solvay process eliminates all energy requirements associated with ammonia regeneration, which is estimated to be 16% of the total operating energy [36]. Moreover, the high volatility of ammonia makes it necessary to include a washing section to strip ammonia from the treated gas, which consumes about 10–15% of the total operating energy [37]. An estimation of the minimum energy requirement and equivalent work for the conventional ammonia-based process with 6 wt% NH3 concentration is presented in Table 4 [36]. In addition to the more than 30% reduction in operating energy, the equivalent work is significantly reduced in the Modified process by eliminating the work needed for steam consumption and compression work, which represent about 70% of the total equivalent work (Table 4). Even though most of the ammonia in the conventional Solvay process is recovered, it is estimated that about 10% is lost [38]. The cost of ammonia needed, based on the optimum condition of molar ratio of 3NH3:1NaCl, is estimated to be 70 US$/m3 treated gas. Assuming that only 90% of ammonia is recovered, the estimated cost of consumed ammonia is 7 US$/m3 treated gas. Whereas, the cost of the consumed CaO at optimum conditions (0.3CaO:1 NaCl) is about 1.28 US$/m3 treated gas, which indicates a saving of about 82% in the cost of consumable reactants. In addition, the conventional ammonia-based process consists of four main units: CO2 absorber; NH3 stripper; CO2 striper and NH3 absorber [39], while the only main unit required in the Modified process is the bubble contact reactor, which significantly reduces the operating and maintenance costs as well as the footprint of the process. 6. Conclusions A new method, based on a modified Solvay process, was developed for the capture of CO2 and desalination of high salinity water, wherein the high salinity water was mixed with 1 to 2% of calcium oxide to raise the saline water pH to above 10 and then contacted with CO2-containing gases. The CO2 reacted with sodium chloride and calcium hydroxide to form soluble calcium chloride and insoluble sodium bicarbonate. This process had the dual benefits of capturing CO2 and storing it in solid sodium bicarbonate and reducing the salinity of the effluent water. It was found that the Modified Solvay process is superior in
terms of CO2 capture efficiency and sodium removal at both stoichiometric and optimum conditions. It was also apparent that the Modified Solvay can sustain a higher pH than the traditional Solvay, without the use of ammonia, which makes the Modified Solvay process more economical and more environmental friendly. Acknowledgment The authors would like to acknowledge the financial support provided by Takreer research center as part of the TRC CO2 Project. They would also like to thank Dr. Hussain Awad and Eng. Ayat El Telib from Chemical and Petroleum Engineering Department at the UAE University for their help. References [1] K. Bennaceur, Chapter 26 - CO2 capture and sequestration, in: T.M. Letcher (Ed.), Future Energy, second ed.Elsevier, Boston 2014, pp. 583–611. [2] K. Hashimoto, M. Yamasaki, K. Fujimura, T. Matsui, K. Izumiya, M. Komori, A.A. ElMoneim, E. Akiyama, H. Habazaki, N. Kumagai, A. Kawashima, K. Asami, Global CO2 recycling-novel materials and prospect for prevention of global warming and abundant energy supply, Mater. Sci. Eng. 267 (1999) 200–206. [3] L.M. Calvo, R. Domingo, Influence of process operating parameters on CO2 emissions in continuous industrial plants, J. Clean. Prod. 96 (2015) 253–262. [4] A. Wahby, J. Silvestre-Albero, A. Sepúlveda-Escribano, F. Rodríguez-Reinoso, CO2 adsorption on carbon molecular sieves, Microporous Mesoporous Mater. 164 (2012) 280–287. [5] C. Nwaoha, C. Saiwan, P. Tontiwachwuthikul, T. Supap, W. Rongwong, R. Idem, M.J. Al-Marri, A. Benamor, Carbon dioxide (CO2) capture: absorption-desorption capabilities of 2-amino-2-methyl-1-propanol (AMP), piperazine (PZ) and monoethanolamine (MEA) tri-solvent blends, J. Nat. Gas Sci. Eng. 33 (2016) 742–750. [6] C. Song, Y. Kitamura, S. Li, Optimization of a novel cryogenic CO2 capture process by response surface methodology (RSM), J. Taiwan Inst. Chem. Eng. 45 (2014) 1666–1676. [7] A.M. Arias, M.C. Mussati, P.L. Mores, N.J. Scenna, J.A. Caballero, S.F. Mussati, Optimization of multi-stage membrane systems for CO2 capture from flue gas, Int. J. Greenhouse Gas Control 53 (2016) 371–390. [8] K. Goto, S. Kazama, A. Furukawa, M. Serizawa, S. Aramaki, K. Shoji, Effect of CO2 purity on energy requirement of CO2 capture processes, Energy Procedia 37 (2013) 806–812. [9] T. Sema, A. Naami, K. Fu, M. Edali, H. Liu, H. Shi, Z. Liang, R. Idem, P. Tontiwachwuthikul, Comprehensive mass transfer and reaction kinetics studies of CO2 absorption into aqueous solutions of blended MDEA-MEA, Chem. Eng. J. 209 (2012) 501–512. [10] A.L. Kohl, R.B. Nielsen, Chapter 15 - membrane permeation processes, in: A.L. Kohl, R.B. Nielsen (Eds.), Gas Purification, fifth ed.Gulf Professional Publishing, Houston 1997, pp. 1238–1295. [11] S. Stünkel, A. Drescher, J. Wind, T. Brinkmann, J.U. Repke, G. Wozny, Carbon dioxide capture for the oxidative coupling of methane process – a case study in mini-plant scale, Chem. Eng. Res. Des. 89 (2011) 1261–1270. [12] A. Y., H.L. Bai, Removal of CO2 greenhouse gas by ammonia scrubbing, Ind. Eng. Chem. Res. 36 (1997) (2490–1493). [13] A.C. Yeh, H. Bai, Comparison of ammonia and monoethanolamine solvents to reduce CO2 greenhouse gas emissions, Sci. Total Environ. 228 (1999) 121–133. [14] M.H. El-Naas, A.H. Al-Marzouqi, O. Chaalal, A combined approach for the management of desalination reject brine and capture of CO2, Desalination 251 (2010) 70–74. [15] K.E.T. Børseth, Method for Desalination of Water and Removal of Carbon Dioxide From Exhaust Gases, US/2007/7309440 B2, 2007. [16] P. Grauer, F.L. Krass, Method and Device for Binding Gaseous CO2 in Connection With Sea Water Desalination, US/2010/0196244 A1, 2010. [17] B.R. Constantz, Kasra Farsad, Miguel Fernandez, Desalination Methods and Systems That Include Carbonate Compound Precipitation, US/2010/7744761 B2, 2011. [18] C.M. Phillip Carson, Hazardous Chemicals Handbook, Library of Congress Cataloguing in Publication Data, 2002. [19] S. Heimala, in: J.D. Gilchrist (Ed.), Extraction Metallurgy, third ed. Pergamon Press, Oxford, 1989 (ISBN 0-08-036611-2). [20] A. Roine, HSC Chemistry 6 - Software Ver. 6.12 for Thermodynamic Calculations. Outotec Research Oy, 2007.
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Contents lists available at ScienceDirect
Desalination journal homepage: www.elsevier.com/locate/desal
A new process for the capture of CO2 and reduction of water salinity Muftah H. El-Naas a,⁎, Ameera F. Mohammad b, Mabruk I. Suleiman c, Mohamed Al Musharfy c, Ali H. Al-Marzouqi b a b c
Gas Processing Center, College of Engineering, Qatar University, Doha, Qatar Chemical and Petroleum Engineering Department, UAE University, Al Ain, United Arab Emirates Takreer Research Center, Abu Dhabi, United Arab Emirates
H I G H L I G H T S • • • • •
A new process for CO2 capture and water desalination has been developed. It is based on a Modified Solvay process without using ammonia. It is more efficient in CO2 capture and Sodium reduction than the Solvay process. The new process can achieve 99% CO2 capture efficiency and 35% Sodium removal. The energy requirements are much lower than that of the Solvay process.
a r t i c l e
i n f o
Article history: Received 4 July 2016 Received in revised form 3 February 2017 Accepted 12 February 2017 Available online xxxx Keyword: CO2 capture Water desalination Solvay process Sodium bicarbonate Reject brine
a b s t r a c t The present work evaluates a new process for the capture of CO2 and the reduction of water salinity. It is based on the Solvay process without the use of ammonia and involves the reaction of CO2 with saline water such as reject brine in the presence of calcium hydroxide. The effects of operating parameters such as reaction temperature, water pH and reaction stoichiometry on CO2 capture efficiency and sodium removal were examined for both traditional and modified Solvay process. The optimum conditions for maximum CO2 capture efficiency and sodium removal were determined using response surface methodology and were found to be at temperature of 20 °C and a pH of greater than 10 for both processes. At the optimum conditions, CO2 capture of 86% and 99% and sodium removal of 29% and 35% were achieved for the traditional Solvay and the Modified process, respectively. The water pH was found to be a key parameter in the effectiveness of the reaction process; higher pH leads to better process performance in both CO2 capture efficiency and sodium removal. The experimental results clearly illustrated that the Modified Solvay process is superior in terms of CO2 capture efficiency, sodium removal and energy consumption. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Climate change and variability is an evident phenomena around the world and can be attributed to global warming. Most of the observed increase in global average temperatures is very likely due to the observed increase in the concentrations of greenhouse gases such as CO2 in atmosphere. CO2 is a major contributor to global warming, and it is often emitted by various activities associated with industrial processes and the burning of various types of carbonaceous fuels [1]; it is believed to cause approximately 55% of global warming [2]. Relying on fossil fuels as the main source of energy in many parts of the world has contributed to the rise of CO2 emissions to unprecedented levels [3]. Many industries such as natural gas sweetening, hydrogen production for ammonia and ⁎ Corresponding author. E-mail address: [email protected] (M.H. El-Naas).
http://dx.doi.org/10.1016/j.desal.2017.02.005 0011-9164/© 2017 Elsevier B.V. All rights reserved.
ethylene oxide, oil refineries, iron and steel production facilities, desalination and power plants, cement and limestone manufacturing plants represent major sources of CO2 emissions. In recent years, there has been an increased interest in carbon capture and storage (CCS) as an option to reduce the CO2 emissions. CCS is based on the separation and capture of carbon dioxide produced by fossil fuel power plants or other sources either before or after combustion [1]. A number of CO2 capture technologies have been used in many processes such as oxyfuel combustion, post-combustion, pre-combustion and chemical looping combustion. Among the post-combustion capture techniques, the most promising and most effective are adsorption using solid sorbents [4], solvent absorption [5], cryogenic fractionation technology [6] and membrane separation [7]. CO2 capture using chemical solvent is one of the most promising technologies for decreasing the rising emissions of greenhouse gases [8]. Key parameters for selecting an effective solvent for CO2 absorption include high absorption, fast reaction
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kinetics, low degradation rate, and low regeneration energy [9] as well as the ability to handle large amounts of exhaust streams [10]. The most common solvents used for CO2 capture are amines (either in aqueous solution or immobilized on solid support) [11] and aqueous ammonia [12]. Aqueous ammonia is considered to be a competitive option for CO2 capture due to low chemical cost, corrosiveness and relatively low regeneration temperature [13]. The ammonia-based Solvay process has recently been considered for the capture of CO2 and the production of useful and reusable carbonate products, as well as the desalination of saline water [14]. The Solvay process is used for sodium carbonate (soda ash) manufacturing, where carbon dioxide and ammonia gas are passed through a saturated sodium chloride solution to form soluble ammonium chloride and a precipitate of sodium bicarbonate according to Reaction (1) below. The sodium bicarbonate is heated to form the washing soda; whereas the ammonium chloride solution is mixed with calcium hydroxide and heated to recover the ammonia according to Reactions (2) and (3), respectively. NaCl þ NH3 þ CO2 þ H2 O→NaHCO3 þ NH4 Cl
ð1Þ
2NaHCO3 →Na2 CO3 þ CO2 þ H2 O
ð2Þ
2NH4 Cl þ CaðOHÞ2 →CaCl2 þ 2NH3 þ 2H2 O
ð3Þ
Several studies and patent applications have described Solvay-based approaches for the capture of CO2 and desalination of high salinity water [14–17]. One of the major drawbacks of the Solvay process is the presence of ammonia, which is considered an environmental and health hazard. Exposure to high concentrations of ammonia can cause severe injuries such as skin, nose, throat and respiratory tract burning, which can be a reason of alveolar and bronchiolar edema and airway annihilation, leading to respiratory failure [18]. Ammonia is not involved in the overall Solvay reaction, but it plays a key role in buffering the solution at a basic pH; without ammonia, the acidic nature of the water solution will hinder the reaction and hence prevent the precipitation of sodium bicarbonate [14]. In addition to the safety concerns, the ammonia recovery process through Reaction (3) is an energy intensive step that adds to the overall cost of the process. The aim of the current study, therefore, is to modify the Solvay process to reduce or eliminate the drawbacks associated with the presence of ammonia and then use the modified process for the capture of CO2 and reduction of water salinity. In addition, carry out a comparative evaluation of the Solvay (S) and the Modified Solvay (MS) processes in terms of CO2 capture efficiency, sodium ion removal and energy demand. 2. Modification of Solvay process In the original Solvay process, concentrated brine is ammoniated through direct contact with ammonia gas and then contacted with CO2 in a bubble column reactor. The product which contains soluble ammonium chloride and precipitate of sodium bicarbonate is sent to a filter to remove the sodium bicarbonate. The remaining solution is reacted with calcium hydroxide to recover the ammonia, which is then sent again to the ammonization step. The proposed modification of the Solvay process does not involve the use of ammonia. Instead, calcium oxide is added directly to the bubble column reactor, which converts to calcium hydroxide as soon as it contacts the brine, raises the pH and captures CO2 by reacting with sodium chloride according to the reaction given below: 2NaCl þ 2CO2 þ CaðOHÞ2 →CaCl2 þ 2NaHCO3
ð4Þ
Calcium oxide or calcium hydroxide could be obtained from any alkaline solid waste products to avoid the need of calcination to produce
CaO according to the following reaction at high temperature [19]. CaCO3 →CaO þ CO2
ð5Þ
However, according to this reaction, the production of one mole of CaO results in the generation of one mole of CO2, while in the Modified Solvay process, each mole of Ca(OH)2 can be used to capture two moles of CO2. The process has the dual benefit of brine desalination and carbon dioxide capture. In addition, the process eliminates the need for ammonia recovery (Reaction (3)) which is an energy intensive step in the Solvay process. 3. Thermodynamic analysis HSC Chemistry 6.1 software [20] was used to perform a thermodynamic analysis based on sodium chloride as the main reactant in the reject brine. The software was used to define the equilibrium composition for Reactions (1) and (4) at different temperatures and to evaluate the heat of reaction as a function of temperature. Gibbs free energy minimization method was used to determine the number of moles present at equilibrium for any species at a fixed temperature and pressure [21]. Tables 1 and 2 present the calculated thermodynamic properties for Reaction (1) and (4), respectively. The analysis indicates that both reactions are spontaneous for the whole temperature range (0 to 90 °C) as indicated by the negative ΔG. The changes in the heat of reaction from (0 to 100 °C) for both reactions are negative, which indicates that the reactions are exothermic (at 20 °C, ΔH is −129 kJ/kmol for Reaction (1) and −208 kJ/kmol for Reaction (4)). Since a more negative ΔG indicates more spontaneous reactions [22], the Modified Solvay process is more spontaneous than the traditional Solvay process at 20 °C with ΔG of − 56 kJ/kmol and − 26 kJ/kmol, respectively. It is also worth noting that calcium carbonate formation (Reaction (6)) may take place and compete with the formation of sodium bicarbonate. CaðOHÞ2 þ CO2 →CaCO3 þ H2 O
ð6Þ
However, thermodynamic analysis indicated that Reaction (6) is only competitive when calcium oxide is present at amounts greater than or equal to the stoichiometric ratio. The formation of sodium bicarbonate can be favored by controlling the amount of Ca(OH)2 in the reaction [23]. 4. Experimental evaluation 4.1. Experimental apparatus Both Solvay and the Modified Solvay processes were evaluated experimentally using desalination reject brine in a bubble column reactor. Experiments were carried out in a stainless steel jacketed, bubble column reactor with an internal diameter of 78 mm and an overall height of 700 mm. The reactor was operated in a semi-batch mode, where the
Table 1 Thermodynamics data of Reaction (1). Temperature (°C)
ΔH (kJ/mol)
ΔS (J/mol·°C)
ΔG (kJ/mol)
0 10 20 30 40 50 60 70 80 90 100
−124 −129 −129 −129 −129 −128 −128 −128 −127 −127 −127
−332 −353 −352 −352 −351 −350 −349 −348 −347 −346 −346
−33 −29 −26 −22 −19 −15 −12 −8 −5 −1 2
M.H. El-Naas et al. / Desalination 411 (2017) 69–75 Table 2 Thermodynamic data for Reaction (4). Temperature (°C)
ΔH (kJ/mol)
ΔS (J/mol·°C)
ΔG (kJ/mol)
0 10 20 30 40 50 60 70 80 90 100
−212 −210 −208 −206 −205 −203 −202 −200 −199 −198 −197
−535 −527 −520 −514 −509 −504 −500 −496 −492 −488 −485
−66 −61 −56 −50 −45 −40 −35 −30 −25 −20 −16
brine mixture was exposed to a continuous flow of carbon dioxide mixture with air (10% CO2 and 90% air) at atmospheric pressure and 20 °C. The effluent gas line from the top of the reactor was connected to CO2 gas analyzer (Model 600 series of Non-Dispersive Infrared NDIR analyzers). A schematic diagram of the main units of a general Modified and traditional Solvay processes are shown in Fig. 1. 4.2. Brine samples and other reactants Reject brine samples were obtained from a local desalination plant operating MSF (Multi-Stage Flash) desalination process, with an average salinity of 71,700 mg/L. Ammonium hydroxide solution (25 wt%
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NH3 in H2O) and calcium oxide (99.9% trace metal basis) were acquired from Scientific Progress Medical and Scientific Equipment, UAE. A gas mixture of (10% CO2 and 90% Air) was purchased from Abu Dhabi Oxygen Company, UAE. 4.3. Experimental methods 4.3.1. Parametric study Screening of the Modified Solvay process parameters was performed using a parametric study (one factor at a time), and the variables with significant effects on CO2 capture and sodium removal were identified. One liter of the reject brine was mixed with calcium oxide concentration (20 g/L). The mixture was then fed to the reactor by piston pump; the reactor was operated in a semi-batch mode (batch for the liquid and continuous for the gas), and the reaction temperature was controlled at 20 °C by water circulation through the reactor jacket. The gas mixture was bubbled into the reactor at a controlled flow rate of 1 L/min for 240 min. Brine samples were collected every 60 min and analyzed for sodium removal. The effluent gas was continuously passed through a CO2 gas analyzer to determine the percentage of CO2 capture. The CO2 capture efficiency was calculated after each experiment by determining the ratio of the CO2 moles captured to the moles of CO2 loaded to the reactor. pH variation with time was also measured using a digital pH meter. The reactor contents, at the end of each experimental run, were drained into a volumetric flask and filtered. The solids were dried at room temperature and then examined by a scanning electron microscope (SEM) and X-ray diffractometer (XRD). The effects of the following parameters evaluated: calcium oxide concentration, reaction temperature and gas flow rate. In addition, the effect of ammonium bicarbonate on sodium removal in the Modified Solvay process at different temperatures have been studied by adding ammonium bicarbonate (13 w/w%) to the treated brine samples. Samples were then tested for sodium removal. 4.3.2. RSM optimization The CO2 capture and sodium removal were optimized for the Modified Solvay and the original Solvay processes using Response surface Methodology (RSM) in Minitab 17.0 application [24], and the experimental runs were designed in accordance with central composite design (CCD). One liter of reject brine was reacted with stoichiometric and optimum molar ratios of the Solvay and the Modified Solvay processes. The optimum temperature was found to be around 20 °C for both processes, but the other parameters were different. For the Solvay process, the reject brine was mixed with ammonium hydroxide solution in the molar ratio of 3NH3:1NaCl (6 wt% NH3 concentration) for optimum molar ratio and (1NH3:1NaCl) for stoichiometric molar ratio experiments. The optimum molar ratio for the Modified Solvay was determined to be (0.3 CaO:1NaCl) or 16 g CaO per liter; whereas the stoichiometric molar ratio is (0.5 CaO:1NaCl). The CO2 gas mixture was bubbled through the reactor content at a flow rate of 1 L/min for the stoichiometric ratio and at flow rates of 1.54 L/min and 0.76 L/min for the optimum conditions for the Solvay and the Modified Solvay processes, respectively. 5. Results and discussion 5.1. Effect of CaO concentration
Fig. 1. Schematic diagrams of the main units of (a) Modified and (b) traditional Solvay processes.
Calcium oxide is a key reactant in the modified Solvay process and its concentration plays a key role sodium removal. Fig. 2 shows the effect of CaO concentration on sodium removal and CO2 capture efficiency. With increasing the CaO concentration, sodium removal increased rapidly; for example, sodium removal increased from 5% to 32% by increasing the CaO concentration from 5 to 20 g/L (0.5% to 2% CaO). As expected, a higher CaO concentration maintains higher pH level through the reaction time, as shown in Fig. 3. However, increasing CaO concentration
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Fig. 2. Sodium removal and CO2 capture efficiency versus CaO concentration at temperature of 20 °C and gas flow rate of 1 L/min.
more than 2% does not seem to increase the pH level any further, but may lead to hindering the mixing within the reactor and reduce sodium removal. This can be explained by increasing the mass transfer resistance between reactants [25]. Moreover, excess calcium oxide may also lead to the formation of calcium carbonate and thus competing with the formation of sodium bicarbonate. On the other hand, CO2 capture does not seem to be affected by increasing the CaO concentration beyond 20 g/L, which indicates that the CO2 capture is mainly related to pH level in contrast to sodium removal, which is related to both pH level and CaO concentration. When carbon dioxide dissolves in the brine, it forms carbonic acid (H2CO3), which dissociates at high pH into bicarbonate ions [26]. The precipitation of sodium bicarbonate is believed to be proportional to the sodium and bicarbonate ions concentration in the feed brine [27]. In addition, the increase in sodium removal is combined with an increase in CO2 capture percentage. For the specific reaction conditions, most of entering CO2 moles have been captured in the first 240 min for the CaO concentration 10–25 g/L, with maximum efficiency of 98.7% at CaO concentration of 20 g/L.
Fig. 4. Sodium removal and CO2 capture efficiency versus temperature at CaO concentration of 20 g/L and gas flow rate of 1 L/min.
two combining factors that are prevalent at higher temperature: lower CO2 solubility and higher sodium bicarbonate solubility. The first leads to lower formation of sodium bicarbonate, while the second leads to less precipitation of the formed bicarbonate. Table 3 shows the solubility of sodium bicarbonate in water at different temperatures [28]. As is the case for most solids, the solubility of sodium bicarbonate increases with increasing temperature, but this rise in the solubility could be reversed by the addition of ammonium bicarbonate [21]. Ammonium bicarbonate has a major effect on the possibility of using the Solvay process, since it is a significant intermediate in the formation of sodium bicarbonate and can enhance the efficiency of desalinating the reject brine [21]. Raising the concentration of ammonium bicarbonate and hence increasing the concentration of (HCO− 3 ) would drive Reaction (8) to the left and lower the solubility of sodium bicarbonate [21]. NH4 HCO3 ðaÞ↔NH4 þ þ HCO− 3
ð7Þ
NaHCO3 ðaÞ↔Naþ þ HCO− 3
ð8Þ
5.2. Effect of temperature The experimental results, as shown in Fig. 4, indicated that the sodium removal declined with increasing the reaction temperature, where less moles of CO2 were captured and hence less sodium bicarbonate was formed according to Reaction (4). This is believed to be caused by
Adding 13 wt% of ammonium bicarbonate to 8% NaCl solution, reduced sodium bicarbonate solubility to 0 g/100 g [21]. Fig. 5 shows the effect of ammonium bicarbonate in improving the sodium removal, which can have significant effect on the possibility of using the Modified Solvay process even at high temperatures. Previous studies proposed that best temperature condition for forward Solvay's reactions will be at room temperature [29], where reversible reactions can take place at a temperature range of 38–60 °C [30]. The reduction in the sodium removal at 40–50 °C can also be due to the low CO2 solubility and hence less reaction with sodium chloride [31]. The experimental results indicate that the solution pH decreases with increasing the reaction temperature. At temperature 50 °C the solution pH decreased from 11.39 to 9.67 compared with almost unchanged pH at temperature of 10 °C, where the pH sustain higher than 11, this can be explained by Le Châtelier's Principle. When increasing the temperature, the position of equilibrium moves to counter the temperature increase by absorbing the extra heat and forming more hydrogen ions and hydroxide ions.
Table 3 Sodium bicarbonate solubility in water at different temperatures [26].
Fig. 3. pH versus reaction time for different CaO concentrations at temperature of 20 °C and gas flow rate of 1 L/min.
Temperature (°C)
Solubility (g/L)
0 20 60 100
69 96 165 236
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gas flow rates, which can be attributed to the increase in CO2 moles loaded to the reactor per unit time. 5.4. Comparison between Solvay and modified Solvay process.
Fig. 5. Sodium removal with and without ammonium bicarbonate versus temperature at CaO concentration of 20 g/L and gas flow rate of 1 L/min.
This effect leads to increasing the value of Kw (The Ionic Product for water) and decreasing the pH [32]. This is in addition to the CO2 effect in reducing the pH during the reaction, where carbon dioxide reacts with water to give carbonic acid as discussed in Section 4.1. The solubility of CO2 gas plays a significant role in CO2 capture. Increasing the temperature reduces CO2 solubility and hence decreases the CO2 capture [31]. Maximum CO2 capture efficiency and sodium removal of 99.8% and 45.2% respectively, were achieved at temperature of 10 °C.
Both Solvay and the modified Solvay were compared in terms of CO2 capture efficiency, sodium removal and pH. It is worth noting here that the stoichiometric experiments were carried out at the same conditions for both processes (20 °C, 1 atm, a gas flow rate of 1 L/min); whereas, the optimum conditions experiments were carried out at the specific optimum conditions for each system. Fig. 7 (a) and (b) show plots of the percent CO2 capture efficiency and sodium removal, respectively. It is clearly illustrated that the Modified Solvay process is superior in terms of CO2 capture efficiency and sodium removal at both stoichiometric and optimum conditions. At the optimum conditions, CO2 capture of 86% and 99% and sodium removal of 29% and 35% were achieved for the traditional Solvay and the Modified process, respectively. It is also apparent that the Modified Solvay can sustain a higher pH of 12.1 and 11.8 compared to Solvay process with pH of 10.4 and 11.2 in both stoichiometric and optimum conditions, respectively, where pH is the main factor in the reaction process in terms of CO2 dissociation and sodium bicarbonate precipitation. The maximum CO2 capture capacity for calcium oxide or ammonia solution were found to be 1.44 and 1.15 mol CO2/L, respectively. It is worth noting here that even though the sodium removal in the Modified Solvay is better than that
5.3. Effect of gas flow rate The effect of gas flow rate on sodium removal and CO2 capture efficiency is shown in Fig. 6. Sodium removal increased about 30% with increasing the gas flow rate from 500 to 750 mL/min, which can be related to the increasing in reaction rate according to the increase in CO2 moles loaded to the reactor. However, flow rates higher than 750 mL/min seemed to have a slightly negative effect on sodium removal, which can be explained by decreasing the residence time of CO2 in the reactor and hence decreasing the reaction rate [33]. The effects of gas flow rate on pH and CO2 capture shows that increasing the gas flow rate decreases the CO2 capture efficiency according to the decrease in the gas residence time in the reactor [33]. Maximum CO2 capture efficiency of about 100% has been recorded at gas flow rate of 500 mL/min. The decline of pH during the reaction seemed to be more evident for high
Fig. 6. Sodium removal and CO2 capture efficiency versus gas flow rate at CaO concentration of 20 g/L and temperature of 20 °C.
Fig. 7. A comparison of (a) CO2 capture efficiency and (b) sodium removal for Solvay (S) and Modified Solvay (MS) at stoichiometric and Optimum conditions.
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Table 4 Minimum energy requirement and equivalent work for conventional ammonia based processa. Energy requirement Energy requirement for CO2 regeneration (MJ/kg CO2)
Energy requirement for NH3 recovery (MJ/kg CO2)
Equivalent work Steam consumption (MJ/kg CO2)
Pump work (MJ/kg CO2)
Compression work (MJ/kg CO2)
Cooling duty (MJ/kg CO2)
3.376
0.390
0.689
0.06
0.231
0.376
a
For ammonia concentration of 6 wt% [37].
in the conventional Solvay, the sodium removal is not significant and need to be improved through process optimization and using multistage treatment. Although both Solvay and the Modified Solvay processes involve replacement of some of the sodium chloride with calcium chloride, an overall salinity reduction of 19% was achieved in the tested one-stage Modified Solvay process. The treated brine, therefore, is still not suitable for drinking before further treatment. However, such treated water may be used for irrigation since calcium chloride is much less harmful to plants than sodium chloride. In fact, calcium chloride can be beneficial to many plants and may act as growth enhancer [34,35]. The optimized process may also be used for the treatment of high salinity produced water so that it can be reused for irrigation or low salinity flooding for Enhanced Oil Recovery (EOR). Although it is important to make a comparison of the operating cost between the conventional ammonia-based Solvay process and the proposed Modified Solvay, it is rather difficult to make such comparison based on the laboratory scale experimental data obtained in this study. It is possible, however, to make a rough estimation of the energy saving when considering the Modified Solvay process in comparison with the ammonia-based Solvay process. The Modified Solvay process eliminates all energy requirements associated with ammonia regeneration, which is estimated to be 16% of the total operating energy [36]. Moreover, the high volatility of ammonia makes it necessary to include a washing section to strip ammonia from the treated gas, which consumes about 10–15% of the total operating energy [37]. An estimation of the minimum energy requirement and equivalent work for the conventional ammonia-based process with 6 wt% NH3 concentration is presented in Table 4 [36]. In addition to the more than 30% reduction in operating energy, the equivalent work is significantly reduced in the Modified process by eliminating the work needed for steam consumption and compression work, which represent about 70% of the total equivalent work (Table 4). Even though most of the ammonia in the conventional Solvay process is recovered, it is estimated that about 10% is lost [38]. The cost of ammonia needed, based on the optimum condition of molar ratio of 3NH3:1NaCl, is estimated to be 70 US$/m3 treated gas. Assuming that only 90% of ammonia is recovered, the estimated cost of consumed ammonia is 7 US$/m3 treated gas. Whereas, the cost of the consumed CaO at optimum conditions (0.3CaO:1 NaCl) is about 1.28 US$/m3 treated gas, which indicates a saving of about 82% in the cost of consumable reactants. In addition, the conventional ammonia-based process consists of four main units: CO2 absorber; NH3 stripper; CO2 striper and NH3 absorber [39], while the only main unit required in the Modified process is the bubble contact reactor, which significantly reduces the operating and maintenance costs as well as the footprint of the process. 6. Conclusions A new method, based on a modified Solvay process, was developed for the capture of CO2 and desalination of high salinity water, wherein the high salinity water was mixed with 1 to 2% of calcium oxide to raise the saline water pH to above 10 and then contacted with CO2-containing gases. The CO2 reacted with sodium chloride and calcium hydroxide to form soluble calcium chloride and insoluble sodium bicarbonate. This process had the dual benefits of capturing CO2 and storing it in solid sodium bicarbonate and reducing the salinity of the effluent water. It was found that the Modified Solvay process is superior in
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