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CHEMICAL PROBLEMS 2023 no. 2 (21) ISSN 2221-8688
123
UDC 66.074.332
CONTROL OF CO2 ABSORPTION BY NaOH SOLUTION USING pH, CONDUCTIVITY
AND TITRATION MEASUREMENTS
Sadig Kuliyev, Yunus Emre Tas and M. Selim Cogenli
Lentatek Space Aviation and Technology, Universiteler Mah. Ihsan Dogramaci Bul. Titanyum Blok, 17/B Teknokent ODTU, 06800 Ankara, Turkey;
e-mail: sadig.kuliyev@gmail.com
Received 23.01.2023 Accepted 11.04.2023
Abstract: The article deals with the issue of CO2 utilization by sodium hydroxide absorption. Sodium hydroxide (NaOH) is able to react with CO2 under atmospheric conditions to form carbonate or bicarbonate ions in solution. This study focuses on the absorption of CO2 by an alkaline solvent in a bubble column. Carbonate or bicarbonate ions were measured during carbonization using pH, conductivity and titration. Keywords: carbon dioxide, sodium hydroxide, conductivity, titration, absorption DOI: 10.32737/2221-8688-2023-2-123-131
Introduction
More than 81% of the world's energy needs are met by the consumption of fossil fuels (such as coal, oil, and natural gas), which result in emissions of pollutants such as nitrogen oxides, carbon monoxide, and hydrocarbons and greenhouse gases [1,2]. CO2 is the main greenhouse gas emitted as a result of human activity. Therefore, the problem of CO2 utilization is urgent. CO2 utilization is achieved through chemical, electrochemical,
photochemical, and biochemical methods. These studies are summarized in a comprehensive review of publication [3] and patent [4].
Photo-catalytic reduction provides for the use of light to convert CO2, while biochemical reduction is due to the use of enzymes and electrochemical reduction to the use of electrical energy. The reduction of CO2 products can include methanol, formic acid, CO, methane, ethylene, and gasoline. The review outlines recent advancements in the understanding and development of CO2 reduction through the above-mentioned methods [3].
The current status of CO2 capture patents and technologies was reviewed on the basis of the Espacenet patent database. Over 1000 patents were issued, with 60% published since
2000. There has been a sharp increase in the number of patents over the last 2 years. The top four sources of patents are Japan, the US, WIPO and China [4].
Despite the fact that the removal of CO2 from the atmosphere using NaOH is an energy-consuming process, research in this area is also ongoing [5-7].
In [6], it is proposed to use dissolved sodium hydroxide to remove CO2 from the air, followed by its regeneration and precipitation of calcite. The calcite then decomposes to form lime and CO2.
A prototype contactor was developed to measure the CO2 capture efficiency of NaOH spray and the energy requirements for full-size contactors. The contactor was designed to have a downward-flow, concurrent design for simple construction and maintenance of the particle trap system. The CO2 concentration was measured using an infrared gas analyzer, and carbonate concentration was measured in periodic liquid samples. The experiment also recorded temperature, relative humidity, and pressure drops [7].
Simulation and modeling studies of the absorption process of carbon dioxide with
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CHEMICAL PROBLEMS 2023 no. 2 (21)
sodium hydroxide are included in several studies [8,9].
In [9] study evaluated different models for the enhancement factor that consider the impact of chemical reactions on mass transfer. Four mass transfer rate models and two enhancement factor models were used to simulate gas-fluid mass transfer in a bubble column and then experimental data were compared. The Henket1 model was found to be the most accurate and the Hlawitschka model was better than a constant enhancement factor model. The mass transfer model had little effect on the final pH variation in the reaction.
By using a membrane system, studies were carried out to obtain Na2CO3 crystals by CO2 capture [10,11].
The process based on a membrane contactor for crystallizing Na2CO3x10H2O was proposed in the study [10] as the final step in capturing CO2. The performance of an osmotic membrane distillation-crystallization setup was evaluated by considering the effect of flow rates, concentration of the feed and osmotic solution, and feed temperature on mass and heat transfer coefficients.
The study [11] evaluated the potential of using membrane crystallization to recover Na2CO3 from aqueous streams for CO2 sequestration. The impact of various crystallization conditions (concentration and
flowrate of Na2CO3 solution and osmotic solution, and type of osmotic solution) on process performance was determined. Results showed that the concentration of the osmotic solution and the flowrate of the Na2CO3 solution were key parameters that influenced the process.
The paper [12] compares the performance of a traditional packed column and a novel membrane contactor for CO2 absorption. The results showed that the membrane contactor performed well as compared to the conventional column and even offered a higher intensification factor due to its compact and modular design.
The paper [13] examines the use of NaOH aqueous solution as an absorbent for capturing CO2 from flue gas. The CO2 absorption reaction occurs in consecutive steps, resulting in the production of Na2CO3 and NaHCO3. The reaction rate and capture efficiency are found to be highly dependent on the NaOH concentration. The mass ratio of absorbed CO2 that participates in the production of Na2CO3, NaHCO3 was calculated.
The goal of this research is to establish the conditions for the formation of compounds that may result from the reaction of CO2 and NaOH under low flow rates of CO2 and low concentrations of NaOH in a laboratory setting, and to identify the most effective methods for controlling the process.
Experimental
NaOH aqueous solution was prepared by dissolving NaOH powder (Chemical, 98%) in distilled water. Different concentrations of the absorbent solution (NaOH) were shown in the study. The pH and conductivity X (mS/cm) values of the concentrated solutions of the NaOH, Na2CO3 and NaHCO3 which had been prepared from commercial materials with a chemical purity of approximately (95-98%) were measured before the experiment.
The flow rates of the feeding gas (CO2) were 20-500 ml/min. The main purpose of the study was to determine the conditions for full capture of CO2 gas under laboratory conditions. This process included capturing CO2 in a scrubbing column with NaOH solution. CO2
capture experiments were conducted on laboratory scale setup. The reactor system was designed for this experiment. Reactors with the same diameter and 3 different volumes (1000cm3, 2550 cm3 and 7000 cm3) were analyzed in the study. Carbon dioxide was taken from a cylinder of compressed CO2,its flow rate is controlled by a mass flow controller. Finally, two mass flow controllers are present at the top and bottom of the column to measure the flow rate in the two sections of the column through the use of electronic pressure sensors placed inside the console. The degree of capture of the gas was established by measuring the inlet and outlet flow rates of the gas. The schematic diagram of this setup is shown in Fig. 1.
Fig. 1. Experimental setup for the chemisorption of CO2 into NaOH.
In order to analyse the liquid samples, the involves the use of phenolphthalein and methyl double indicators method used. This method orange.
Results and discussion
Initial pH of the absorbent solution plays a crucial role in establishing the mass transfer rate of gases into liquids. The pH of the solution varied from 12.7 to 13.7 by changing the NaOH
concentration at 25 °C (Fig 2). The pH of the solution slowly grew as NaOH concentration increased. Similar results were also typical for conductivity values (Fig 2).
Fig. 2. pH (line 1) and conductivity (line 2) values of NaOH solutions depending on percent
concentration at 25 °C
The effects of temperature on pH and conductivity were also studied by heating the scrubbing solution with immersion tank heater to vary the temperature of the Na2CO3 and NaHCO3 solution at different saturation ratios from 25°C to 30°C. The pH and conductivity values for Na2CO3 and NaHCO3 can be seen at
Fig. 3 and Fig. 4 respectively. The results shown in Fig. 4 indicate that, at less alkaline pH, there was an increase in the conductivity, characterized by the incorporation of the CO2 as HCO3- in the liquid phase, whereas, at a more alkaline pH, the conductivity decreased.
Fig. 3. pH and conductivity values of different Na2CO3 saturated solutions line 1 and 3 belongs at
25 °C, line 2 and 4 belongs at 30 °C
Fig. 4. pH and conductivity values of different NaHCO3 saturated solutions line 1 and 3 belongs at
25 °C, line 2 and 4 belongs at 30 °C
Fig 5. pH and specific conductivity change during absorption CO2 in NaOH solution. V (CO2) =50
ml/min, C (NaOH) =5%
Fig. 6. pH and specific conductivity change during absorption CO2 in NaOH solution. V (CO2)
=200 ml/min, C (NaOH) =5%
7 M I U « M I*
PH
Fig 7. Determination mixture of NaCO3 and NaHCO3 with different value of pH in absorption CO2 (V=200 ml/min) at 5% NaOH by titration. Vi -titration - present phenolphthalein,
V2-titration - present methyl orange.
The CO2 absorption in NaOH was measured using a pH and conductivity for 50 ml/min (Fig 5) and 200 ml/min (Fig 6) flow rates at long-term experimental studies. The presence of HCO3- ions in a wide pH range (4.5-12) CO2+NaOH environment is shown with the Bierrum graph [14-16].
In the study, pH and conductivity changes in the system were investigated in the absorption studies of CO2 gas (31.5% by mixing with a N2) at 1-5% range concentrations of NaOH solution. Experimental conditions do not permit full capture of the feed gas [13]. In our study, the range of the selected gas flow rate, the dimensions of the reactors we used, and the amount of solution used made it possible to ensure that the given CO2 gas was kept completely.
Fig. 7 presents the specification of Na2CO3 and NaHCO3 mixture in the function of the pH when phenolphthalein and methyl orange indicators are used. The sample taken from alkaline medium is titrated with HCl and two indicators (phenolphthalein and methyl orange) that change color and indicate what species is in the solution during titration. This method is used differently whether or not NaOH is still present in the solution. After reaching pH = 10.0, a sample was taken each time (5 ml) at a different pH from the reaction mixture starting from this value and titrated with 0.911 N HCl solution after the addition of phenolphthalein indicator (consumption V1). The same process was done with the addition of methyl orange indicator (consumption V2).
Fig. 8 presents the determination of Na2CO3 and NaHCO3 mixture via titrimetric analysis method. When the pH value is 10.67 as a result of the reaction between a CO2 flow rate of 100 ml/min and a 5% concentrated NaOH solution, a sample was taken each time (5 ml) at a different pH from the reaction mixture starting from this value and titrated with 0.0915 N HCl solution.
As can be seen from Fig. 8, V=48 ml and pH=7.41 corresponds to the equivalent point and the titration of the NaOH + Na2CO3 mixture is completed. At V=74 ml and pH=3.5, the formation of NaHCO3 is completed. According to the results of these analysis, the amount of NaHCO3 formed at each reached pH-value was calculated.
Fig 8. Titrimetric analysis mixture after absorption CO2 (V=100 ml/min) in %5 NaOH, at pH=10.7
The reaction of CO2 with water to form bicarbonate and hydrogen ions is important at low pH values. As the OH- ions are consumed, i.e. the pH is lowered, the CO2 converts into bicarbonate ions [17]. The results of the experiments conducted in the study are indicative that NaHCO3 formation is possible at high pH values.
Effects of different operating and design parameters, including concentration of NaOH solution, absorbent solution volume flow rate and total gas flow rate on CO2 removal efficiency were analyzed. The operating conditions of the all experiments conducted with the absorption column are reviewed in Table 1.
Table 1. Experimental conditions for absorption column
No Molarity (M) (NaOH) Total Volume (cm3) Flow Rate (ml/min) (CO2) Inlet Pressure (psi) (CO2) CO2 feed time (hour) pH (end of experiment) Conductivity (mS/cm) (end of experiment) Product
1 0.25 1000 50 13.56 3 7.64 16.68 NaHCO3
2 0.5 1000 50 13.56 6 7.80 24.4 NaHCO3
3 0.75 1000 100 13.56 6.5 9.16 36 N2CO3 + NaHCO3
4 1.25 1000 100 13.68 6 9.20 51 N2CO3 + NaHCO3
5 1.25 2550 50 14.16 6.5 9.43 48.6 N2CO3 + NaHCO3
6 1.25 2550 200 14.16 7 7.92 49 NaHCO3
7 1.25 2550 400 14.16 3.2 7.72 51.3 NaHCO3
8 2.5 7000 500 15.89 7 11.84 66.9 Undissolved CO2
When using the NaOH solution, it was found that there emerged different products when the total gas flow rate rose from 50 to 200 mL/min in the same total reactor volume. Experimental results showed that the total gas flow rate has a remarkable effect on the CO2 removal efficiency [18].
As can be seen in the experiment number 8 (Table 1), CO2 gas inlet flow rate is 500 mL/min but the gas flow rate at the reactor exit is approximately 250 mL/min after 7 hours due to the gas flow rate added to the reactor and the high concentration of NaOH in the reactor. 50% of the gas entering the system leaves the reactor without reacting. Since the temperature formed in the reaction reaches high values (80-90 0C), it causes the gas to leave the reactor without reacting. In other experiments, no gas is released from the reactor at the end of the experiment.
In order to understand the presence of compounds formed in the reaction, phenolphthalein and methylorange indicators were placed in the reaction reactor. The color change observed during the reaction of the indicators added into the reactor is given in Fig. 8. Since the NaOH concentration is high in the initial state of the reaction, the phenolphthalein indicator is colored dark red. Then, as CO2 continued to be fed into the reactor, a light red color was observed with the formation of CO32-ions. At that, CO2 gas converts existing CO32-ions to HCO3- ions starting from the upper part of the reactor (color of both indicators is visible in the reactor). After complete conversion, only the color of the methyl orange indicator is visible in the reactor. Color fuel changes into yellow which indicates that all carbonate has reacted to form bicarbonate.
Fig. 8. (a) Color in high concentration of NaOH; (b) Color change phenolphthalein after forming Na2CO3; (c) Forming mixture of Na2CO3 + NaHCO3 color phenolphthalein and methylorange (d) Color fuel change to yellow after NaHCO3 is completely obtained.
Conclusion
We followed the formation of carbonate and bicarbonate ions depending on pH value of the NaOH + CO2 system. Control over product
formation can be measured not only by titrimetric analysis, but also by pH and conductivity measurement of solutions.
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CO2-nin NaOH MOHLULU iLO UDULMASI PROSESiNO pH, XÜSUSi KECiRiCiLiK VO TiTRLOMO METODLARI iLO NOZAROT EDiLMOSi
Sadiq Quliyev, Yunus Omra Ta§, M. Salim Cogenli
Lentatek Kosmik Aviasiya vd Texnologiya, Universiteler Mah. ihsan Dogramagi Blvd. Titan Blok, 17/B Teknokent ODTU, 06800 Ankara, Türkiyd e-mail: sadig.kuliyev@gmail.com
Xülasa: Maqalada natrium hidroksidla karbon qazinin udulmasi tadqiq edilmi§dir. Natrium hidroksid normal atmosfer §araitinda CO2 ila reaksiya gira bilir. 9sas xüsusiyyat CO2-nin mahlulda karbonat va ya bikarbonat ionlarinin amala galmasi reaksiyasidir. Bu tadqiqat CO2-nin bir reaktorda (qabarciq sütununda) qalavi halledici tarafindan udulmasina yönalmi§dir. Karbonat va ya bikarbonat ionlari, CO2 udulmasi zamani, pH, kegiricilik va titrlama ila müayyen edilmi§dir. A?ar sözlar: karbon qazi, natrium hidroksid, kegiricilik, titrlama
КОНТРОЛЬ ПОГЛОЩЕНИЯ CO2 РАСТВОРОМ NAOH С ПОМОЩЬЮ ИЗМЕРЕНИЙ PH, ЭЛЕКТРОПРОВОДНОСТИ И ТИТРОВАНИЯ
Садиг Кулиев, Юнус Эмре Таш и М. Селим Чёгенли
Университет Мах. Бульвар Ихсана Дограмачи Титанюм, Блок № 17/10 06800 Анкара, Турция
e-mail: sadig.kuliyev@gmail.com
Аннотация: В статье рассмотрен вопрос утилизации СО2 путем абсорбции едким натром. Гидроксид натрия NaOH способен реагировать с CO2 в атмосферных условиях с образованием ионов карбоната или бикарбоната в растворе. Это исследование фокусируется на поглощении CO2 щелочным растворителем в барботажной колонке. Ионы карбоната или бикарбоната измеряли во время карбонизации с использованием pH, электропроводности и титрования.
Ключевые слова: углекислый газ, гидроксид натрия, электропроводность, титрование
