desorption performance of aqueous monoethanolamine (MEA) solution during carbon dioxide capture process

Accepted Manuscript Effect of heat-stable salts on absorption/desorption performance of aqueous monoethanolamine (MEA) solution during carbon dioxide capture process Hao Ling, Sen Liu, Hongxia Gao, Zhiwu Liang PII: DOI: Reference:

S1383-5866(18)32962-9 https://doi.org/10.1016/j.seppur.2018.12.001 SEPPUR 15143

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

27 August 2018 27 November 2018 3 December 2018

Please cite this article as: H. Ling, S. Liu, H. Gao, Z. Liang, Effect of heat-stable salts on absorption/desorption performance of aqueous monoethanolamine (MEA) solution during carbon dioxide capture process, Separation and Purification Technology (2018), doi: https://doi.org/10.1016/j.seppur.2018.12.001

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Effect of heat-stable salts on absorption/desorption performance of aqueous monoethanolamine (MEA) solution during carbon dioxide capture process Hao Ling, Sen Liu, Hongxia Gao*, Zhiwu Liang* Joint International Center for CO2 Capture and Storage (iCCS), Provincial Hunan Key Laboratory for Cost-effective Utilization of Fossil Fuel Aimed at Reducing CO2 Emissions, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, P.R. China

*CORRESPONDING AUTHOR: 1. Tel: +86-15116365674, E-mail address: [email protected] (Hongxia Gao); 2. Tel: +86-13618481627, fax: +86-731-88573033; E-mail address: [email protected] (Z. Liang);

1

ABSTRACT

In order to study the effect of heat-stable salts (HSSs) on the CO2 absorption performance of amine solution, the CO2 absorption/desorption rates and cyclic CO2 capacity of 30 wt% (monoethanolamine) MEA solutions in the presence of various acidic degradation products were comprehensively investigated systematically by an improved rate-based screening method. In addition, the initial pH values and CO2 equilibrium solubility were also evaluated. These acidic degradation products considered were formic acid, acetic acid, propionic acid, butyric acid, glycolic acid, oxalic acid, lactic acid, malonic acid and bicine. The experimental results indicated that the carboxylic acids with different chemical structures can reduce the initial pH values, equilibrium solubility of CO2, absorption rate and cyclic CO2 solubility, but promote the CO2 desorption rate of aqueous MEA solution. Furthermore, a countermeasure was proposed to maintain absorption performance and reduce the energy requirement of solvent regeneration for an MEA amine-treating unit. The obtained results can provide a guidance for developing corresponding countermeasure for heat-stable salts control and removal in the CO2 absorption-stripping process using amine solutions.

Keywords: CO2 capture; Solvent degradation; Heat-stable salts; Solvent absorption/desorption; Energy reduction

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1. Introduction

CO2 capture, utilization and storage (CCUS) technology is a quite promising approach to control green gas emission, such as carbon dioxide (CO2) [1-3]. CO2 is the major contributor of the rise of the global mean temperature. In order to prevent the further deterioration of global warming, a few researchers have involved in the development of various CO2 capture technologies. Generally, CO2 capture technologies can be divided into three categories: pre-combustion capture, oxy-fuel combustion and post-combustion capture. Among them, post-combustion capture using alkanolamines as absorbent is currently considered as one of the most economical and wide technology to effectively remove CO2 and other acidic gases for a tradition coal-fired power plant [4-9]. CO2 and other acidic gases are absorbed into alkanolamines by an exothermic reversible reaction in the absorber. Whereafter, CO2 and acidic gases are released from alkanolamines by endothermic reversible reaction in regenerator before the lean CO2 solution re-enters the absorber. In the past decades, a great deal of experimental and theoretical researches have been carried out to study the reaction mechanism [10-12], reaction kinetics [9, 13, 14], solubility [15-17], mass transfer [18-20], solvent regeneration performance of CO2 absorption into alkanolamines. Based on the molecular structure, alkanolamines for CO2 removal and acidic gases can be categorized into three types: the primary amines (monoethanolamine (MEA)

and

diglycolamine

(DGA)),

secondary

amines

(diethanolamine

(DEA)

and

diisopropanolamine (DIPA)), and tertiary amines (methyldiethanolamine (MDEA)) [21]. Due to the high reaction kinetics with CO2, high solubility in water, low viscosity and cheap cost, aqueous MEA solution has been used to capture CO2 in many industrial processes for many years [10, 22]. However, post-combustion CO2 captures using aqueous solution of basic alkanolamines (i.e. MEA) still 3

suffered three major challenges, i.e. high regeneration energy requirement, equipment corrosion and solvent degradation. The degradation of amine solutions have been inevitable in the CO2 capture process and can constitute a major expense for absorbent make up (by up to 10%). Therefore, it is essential to find an effective approach to reduce the products of solvent degradation. So far, a wide of work has been carried out to test solvent degradation products in order to understand and propose the degradation mechanism [23-30]. Amine degradation products are usually generated through three ways, namely carbamate polymerization, thermal degradation and oxidative degradation [26]. Carbamate polymerization generally occurs at the stripper under the higher temperature with existing CO2, generating the degradation products with high molecular weight; thermal degradation of MEA apparently occurs at high temperature more than 205 ℃ [26]; and oxidative degradation normally happens in the presence of O2 at the bottom of the absorber, producing oxidized fragments such as organic acids and NH3 [26]. The most common degradation products are generated by oxidative degradation, and play a significant role in corrosion and foaming due to they are easy to be formed and accumulated during CO2 absorption process [26]. Organic carboxylic acids are the common oxidative degradation products of amines, and usually reacted with the amine to form heat-stable salts (HSSs), because they are unstable in alkaline environment and will further react with amine absorbents to produce more complex compounds [31]. Some operational problems, such as excessive foaming, corrosion and capacity reduction, can usually be attributed to the accumulation of amine heat-stable salts which cannot be regenerated and absorb CO2 in the amine solution[32]. Thus, it indicated that the costs of operation and maintenance increase with increased amount of heat-stable salts during CO2 removal from flue gas. Therefore, it is

4

essential to evaluate the influence of heat-stable salts on the CO2 absorption/desorption performance, and then find methods to reduce or remove the heat-stable salts in the CO2 capture systems. On this basis, nine types of oxidative degradation acids, namely, formic acid, acetic acid, propionic acid, butyric acid, glycolic acid, lactic acid, malonic acid and oxalic acid, and bicine (N,N-bis(2-hydroxyethyl)glycine) which can react with amine to produce various heat-stable salts, were selected in order to evaluate their influence on the absorption and regeneration performances. In the present work, the effects of carboxylic acids on the five parameters, i.e. initial pH values, CO2 equilibrium solubility, CO2 absorption/desorption rate and cyclic CO2 capacity were investigated by using aqueous MEA solution. Furthermore, the influence of the chemical structures of tested carboxylic acids are comprehensively studied. Here, these carboxylic acids can be divided into three categories, monocarboxylic acids (formic acid, acetic acid, propionic acid, and butyric acid), dicarboxylic acids (oxalic acid and malonic acid) and hydroxy monocarboxylic acids (glycolic acid, lactic acid, and bicine). The molecular structure of these carboxylic acids are presented in Figure 1. In MEA solution, these acids will further react with MEA to generate corresponding heat-stable salts, as shown in Figure 2. Figure 1 Figure 2

5

2. Theory 2.1. Reactions of CO2 absorption into aqueous MEA solution Base on the zwitterion mechanism, primary amines and secondary amines firstly reacted with CO2 to form zwitterions as an intermediate, and then the intermediate was instantaneously undergoing deprotonation by the base (such as amine, OH−, or H2O) to form carbamate [9]:

where

is the zwitterion, an intermediate product, and

is the carbamate.

Subsequently, Lv et al. [10] reported that the CO2 absorption into aqueous MEA solution could be divided into two stages. In the first stage (at low CO2 loading), the main reaction was the formation reaction of carbamate between MEA and CO2 according to the zwitterion mechanism. In the second stage (at high CO2 loading), the major reaction was the hydration reaction of CO2. Meanwhile, the carbamate start to hydrolyze, in which the hydrolysis rate was faster than that of formation rate in the second stage of absorption. This is because that the existence of carbamate will be influenced by CO2 loading and it is rather unstable at high CO2 loading. –

6

(3)

2.2. Desorption of CO2-saturated MEA solution CO2 desorption from the CO2-saturated aqueous MEA solution was proved to be a reverse process of absorption and could not be completely regenerated under the normal conditions [10]. In the first stage (at high CO2 loading), some of the some of the

reacted with protonated MEA (

second stage (atlow CO2 loading), the carbamate start to react with

was heated to release CO2 and ) to form CO2 and carbamate. In the had been completely decomposed and the

to decompose into CO2 and MEA under the condition of

regeneration.

2.3. Formation of Heat-stable salts

Heat-stable salts were induced by the reaction of MEA and acids of MEA oxidation degradation. One of the first oxidative routes of MEA degradation was attributed to Jefferson Chemical [33]. Oxalic acid, which can form heat-stable salt with MEA, was produced by the reaction of MEA and O2 through a-amino acetaldehyde, glycine, glycolic acid, glyoxylic acid intermediate. Few years later, Rooney et al. [34] further completed the formation pathways on basis of the proposed by Jefferson Chemical [33] as shown in Figure 3. Afterwards, an increasing number of HSSs-induced species were founded with the development of detection method, such as propionic acid, butyric acid, lactic acid, malonic acid and bicine, etc. The formation process of these acids was considered as similar oxidative process according to radical-induced oxidation of MEA [27, 35]. The mechanisms 7

indicated that a peroxide radical was produced initially and it further reacted MEA and O 2 to form hydroperoxide, and then decomposed to form corresponding acids. Figure 3 These carboxylic acids have been reported to further form heat-stable salts with MEA after produced by MEA oxidation degradation [23, 31]. Based on the formation mechanism of heat-sable salts between carboxylic acids and MEA in literature, HSSs-induced species that involved present work were summarized and list as shown in Figure 2, including formates, acetates, propionates, butyrates, glycolates, lactates, malonates, oxalates, bicinate(N,N-bis(2-hydroxyethyl)glycinate) [23, 27, 31, 33-35].

3. Experimental section 3.1. Chemicals Reagent grade MEA (AR 99%), formic acid (AR 98%), acetic acid (AR 99.5%), propionic acid (AR 99%), butyric acid (AR 99%), glycolic acid (AR 98%), oxalic acid (AR 98%), lactic acid (AR 85-90%), malonic acid (AR 99.5%) and Bicine (AR 99%) were purchased from Aladdin Reagent (Aladdin Industrial Corporation, Shanghai, China). Commercial grade CO 2 and nitrogen (N2) with purity of 99.9 vol%, were supplied by Changsha Rizhen Gas Co. Ltd., China. The deionized water was prepared by a reverse osmosis ultra-pure water equipment (Model TS-RO-10L/H, ≤ 0.1µs/cm, Taoshi Water Equipment Engineering Co. Ltd.). 3.2. Experimental sample preparation To ensure the single factor effect of heat-stable salts, the mass concentration of aqueous MEA was prepared to 30 wt% for all MEA/acid solutions without regard to the chemical reaction of MEA 8

and acid. All desired solution samples were prepared to 100 g by weighing and mixing MEA, carboxylic acid and deionized water. Based on the equations shown in Figure 2, 1 mol MEA can generate 1 mol carboxylic acid. Then, 1 wt%, 2 wt%, 3 wt%, and 5 wt% MEA can produce 0.1637 mol, 0.3274mol, 0.4911 mol and 0.6548 mol carboxylic acid under oxidative degradation for 100 g solution. The detailed information for the tested MEA solutions with addition of acidic degradation product were presented in Table 1. Table 1 3.3. Absorption/desorption experiment The diagram of the absorption/desorption experimental apparatus is shown in Figure 4. The simulated flue gas was a mixture of CO2 and N2, which were controlled by mass flow controllers (model D07, ±1.5% accuracy, Seven Star, China), respectively. The absorption experiments were conducted at 40 ℃ and under atmospheric pressure. The gas mixture of 150 mL/min CO2 and 850 mL/min N2 with CO2 partial pressure of 15 kPa was adequately mixed and then bubbled into a reactor through a gas distributor. The reactor was a 250 mL flask filled with 100 g amine solution. The off-gas passed through a condenser and a drying tube, then flowed into an infrared sensor (COZIR-100, ± 0.01% accuracy, GSS Ltd., UK), while the constant temperature as well as steady stirring rate (1300 r/min) were provided by the water bath with a built-in magnetic stirrer (model DF101S, ±1.0% accuracy, Yuhua, China). The desorption process was similar to the absorption process, while the starting temperature of solution was set at 70 ℃ (the temperature of the water bath was controlled at 80 ℃), and the solution was then quickly heated to 80 ℃ in about 5 to 15 minutes. The carrier gas of 425 mL/min N2 was bubbled through the solution to strip CO2. The absorption process and desorption process were stopped when the CO2 concentration of outlet gas was close to 9

the CO2 concentration of the inlet gas, i.e the CO2 concentrations were close to 15% and zero for absorption and desorption, respectively. Figure 4. Additionally, a more appropriate gas distributor with bigger pore diameter was applied for the screening experiments, which is in order to minimize the impact of the mass transfer process caused by various viscosities. CO2 concentration of the inlet and outlet gas was measured by a CO 2 sensor connected to the computer, and data points were recorded automatically every 10 seconds. 3.4. Equilibrium solubility of CO2 and pH values The apparatus and experimental procedure for the determination of the CO2 equilibrium solubility and PH values of amine solutions were similar to our previous work and the detailed description can be found in Xiao et al., 2016 [2]. The diagram of the apparatus for CO2 equilibrium solubility measurement is showed in Figure 5. The CO2 loading of liquid phase was measured by titration using a Chittick apparatus [36]. The pH values was determined by a pH meter (model E-201-C, INESA Scientific Instruments, China), and the temperature of amine solution was controlled at 40℃ by the water bath. In addition, the standard buffer solution was applied to the calibration of the pH meter before each experiments. Figure 5 3.5. Calculations Since the flow rate of N2 (

, L/min) was constant during the absorption/desorption process,

the absorption/desorption rate of CO2 (

, g/(g·s)) can be then calculated by follows:

10

where

and

represent the CO2 mole fraction of inlet and outlet gas streams, respectively;

(g) and

(g/mol) are the mass of amine solution and molar mass, respectively.

The CO2 amount (

, g) into or out of the amine solution over time (t, min) then can be

expressed as in Eq. 14, while the CO2 loading change with time can be calculated by Eq. 15 and 16.

where

(g/g) is the CO2 loading at difference time nodes. Then the cyclic capacity (

be calculated by the difference of rich loading (

, g/g) and lean loading (

, g/g) can

, g/g) in Eq. 17.

According to Eqs. 13-16, the CO2 loading can be a function of time, and the CO2 loading of absorption process (

) and loading of desorption process (

) can be represented as

follows.

For loops, the initial loading and loading change with time are constant, and the lean loading (

,

) and rich loading (

,

) of absorption and desorption processes are

equal as follows：

11

where

and

are the initial and final time node of absorption process separately,

and

are the initial and final time node of desorption process respectively. Also, the reaction duration (

,

) of the absorption/desorption process is fixed, and can be represented as in the

following equations.

The time node and corresponding loading value can then be found by calculating Eqs. 20-23.

4. Results and discussion 4.1. Initial pH values To clear study the effects of carboxylic acids, the initial pH values of the simulated degraded MEA solutions were measured by a pH meter at 40℃. As shown in Figure 6, the result showed that i) the addition of individual carboxylic acid plays obvious role in the decrease of pH value compared with 30 wt% MEA; ii) the pH value of MEA/formic acid solution decreases with the increasing concentration of formic acid; iii) the different carboxylic acids provide different decrease in pH value of 30 wt% MEA/2% carboxylic acid solutions, and the order of the pH values for the tested solutions is 30 wt% MEA/5% formic acid solution < 30 wt% MEA/2% oxalic acid solution < 30 wt% MEA/3% formic acid solution < 30 wt% MEA/2% malonic acid solution < 30 wt% MEA/2% formic acid solution < 30 wt% MEA/2% glycolic acid solution < 30 wt% MEA/2% lactic acid solution < 30 wt% MEA/2% acetic acid solution < 30 wt% MEA/2% butyric acid solution < 30 wt% MEA/2% propionic acid solution < 30 wt% MEA/2% bicine solution < 30 wt% MEA/1% formic acid solution. Then, it can be concluded that 30 wt% MEA/2% dicarboxylic acid solutions < 30 wt% MEA/2% 12

monocarboxylic acid solutions and 30 wt MEA/2% hydroxy monocarboxylic acid solutions, and 30 wt% MEA/2% hydroxy monocarboxylic acid solutions < 30 wt%/2% monocarboxylic acid solution with the same number of carbon atoms, especially except for 30 wt MEA/2% bcine system. The obvious decrement of the pH value for tested solution with the addition of carboxylic acid could be attributed to the fact that carboxylic acid could react with MEA, leading to the reduction of the amount of free active MEA molecules and the total CO2 capacity of amine solution. Additionally, the carboxylic acids are typically weak acids, meaning that they only partially dissociate into cations and

anions in neutral aqueous solution, and the acidity of them relies on the whole

molecular structure as well as the number of carboxyl group. Furthermore, according to inductive effect of molecular structure, the acidity of dicarboxylic acid is generally greater than those of monocarboxylic acid and hydroxy monocarboxylic acid under the same number of carbon atoms; the acidity of hydroxy monocarboxylic acid is larger than monocarboxylic acid under the same carbon chain lengths; and the acidity of carboxylic acid decreases with the increase in the number of carbon atoms. Thus, it can be concluded that the experimental initial pH values of MEA solutions with the addition of different acids are consistent with the acidity order of added acids. Figure 6. 4.2. Equilibrium solubility of CO2 The CO2 equilibrium solubility of amine solution is an important parameter in the CO2 absorption process. Figure 7 presents the CO2-saturated solubility values of 30 wt% MEA solutions with the addtion of different individual carboxylic acids measured by VLE setup at 40℃ with the CO2 partial pressures of 15kPa and 100kPa, respectively. It can be found that i) the addition of carboxylic aicd into 30 wt% MEA solution can reduce the equilibrium solubility of CO2 compared 13

with that of fresh 30 wt% MEA solution; ii) the CO2 equilibrium solubility of MEA/formic acid solution decreases with the increasing concentration of formic acid for both the CO2 partial pressure of 15kPa and 100kPa, respectively; iii) and, the CO2 equlibrium solubility of 30 wt% MEA/2% carboxylic acid solutions are different for the additon of different individual carboxylic acids. This phenomenon is consistent with the trend of experimental pH values, which can be attribuated to the reduction in the amoun of free active MEA molecules by the reaction of MEA with acids. In addition, the loss ratio of CO2 capacity for all tested solutions were also calculated to evaluate the effect of individual carboxylic acids on the degraded MEA solution at 40℃ with the CO2 partial pressure of 15kPa and 100kPa, as presented in Table 2. The available loss ratio of CO2 capacity

can obviously display the extent of the loss of CO2

equilibrium solubility, and the loss ratio also can reflected the extent of the loss of free active MEA molecules. The two highest loss ratios of CO2 capacity were obtained by the aqueous 30 wt% MEA/2% oxalic acid and 30 wt% MEA/2% malonic acid solutions, and the lowest loss ratios of CO2 capacity was provided by the aqueous 30 wt% MEA/2% bicine system. Additionally, it can be found that the loss ratio of CO2 capacity for 30 wt% MEA/2% monocarboxylic acid solutions and 30 wt% MEA/2% hydroxy monocarboxylic acid solutions are less than that of 30 wt% MEA/2% dicarboxylic acid solutions, and 30 wt% MEA/2% monocarboxylic acid solutions is less than 30 wt% MEA/2% hydroxy monocarboxylic acid solutions with the same carbon chain length, especially except for 30 wt% MEA/2% bcine. Then, it can be concluded that the effect of the addition of carboxylic acid into MEA solution is in accord with the experimental results of initial pH values. This phenomenon can be explained by the fact that the dicarboxylic acid has two carboxylic groups (–

) which can

consume more free active MEA molecules than those of monocarboxylic acids and hydroxy 14

monocarboxylic acids attached with one carboxylic group. However, the hydroxy monocarboxylic acids (e.g. glycolic acid and lactic acid) have hydroxyl group (hydrolysis of –

) which could affect the

to reduce the amount of active MEA molecules compared with those of

monocarboxylic acids (e.g. acetic acid and propionic acid) under the same number of carbon atoms. Table 2 Figure 7 4.3. Absorption rate Several phenomenon has been observed during the CO2 absorption process and was essential to be described here: 

Heat was released by mixing carboxylic acid and aqueous MEA solution.



The CO2 was bubbled into 30 wt% MEA/carboxylic acid solutions, resulting in by different foaming degrees except for 30 wt% MEA/formic acid, 30 wt% MEA/2% acetic acid and 30 wt% MEA/2% bicine systems.

Figure 8 and Figure 9 demonstrate the effects of concentration of formic acid and individual carboxylic acids on the absorption rates of tested solutions, respectively. It can be found that the absorption rate firstly decreases slowly and then quickly with an inflection point with the increase in CO2 loading in CO2 absorption process. The reason for this result could be due to the absorption process completed through two stages, in which the reaction rate of MEA and CO2 in period of the first stage was faster than that hydration reaction rate of CO2 in period of the second stage (as shown in Eqs.1-2 and 3-7). It can be pointed that the addition of acids just can affect the position of the inflection point, but doesn't affect the trend in the behavior of CO2 absorption performance for MEA systems. 15

In addition, it can be seen obviously that MEA/carboxylic acid solution provided lower absorption rate than that of fresh 30 wt% MEA solution. This is due to the decrease in the pH values and the amount of free active amine molecules with the introduction of carboxylic acid into MEA solution on basis of mechanism shown in Figure 2, causing the reduction of intermolecular effective collision between CO2 and MEA. However, the negative effect should be double in practical condition, because the formation process of heat-stable salts is first to generate acids by MEA oxidative degradation and then these acids react with MEA to produce heat-stable salts. Thus, these carboxylic acids play an important role in inhibiting the absorption rate of MEA aqueous solution. The formic acid is the most abundant acid in the process of MEA oxidation degradation [37], because formic acid is more easily formed and accumulated in solution. As presented in Figure 8, the absorption rate decreases with the increasing concentration of formic acid for MEA/formic acid solution. This can be explained that the initial pH values or the amount of free active amine molecules decrease with the increasing concentration of formic acid, leading to the increment in the concentration of

or the reduction of effective collision between CO2 and MEA.

The effect of different individual carboxylic acids on the CO2 absorption rates for 30 wt% MEA/ 2% carboxylic acid solutions compared with that of fresh 30 wt% MEA solution are presented in Figure 9. The experimental results showed that 30 wt% MEA/ 2% formic acid and 30 wt% MEA/ 2% acetic acid solutions exhibit the lowest absorption rates among all tested amine solutions. A possible explanation for this phenomenon could be described as the mass transfer behavior can be enhanced by the turbulence of gas and liquid phases caused by the foaming during the CO 2 absorption into MEA/acid solution. It has been mentioned that the addition of 2% formic acid, acetic acid and bicine into 30 wt% MEA solution did not form stable or obvious foaming phenomenon. 16

Then, the CO2 absorption rates of these three tested solution are lower compared with other solution systems. In addition, the order of CO2 absorption rate for these three systems is: 30 wt% MEA/2% formic acid ≈ 30 wt% MEA/2% acetic acid < 30 wt% MEA/2% bicine acid, which can be explained by the initial pH values as shown in Figure 6. Furthermore, it is worth to note that the absorption rate of the 30 wt% MEA/2% dicarboxylic acid solutions (e.g. 30 wt% MEA/2% oxalic acid solution and 30 wt% MEA/2% malonic acid solution) are usually greater than both 30 wt% MEA/2% monocarboxylic acid solutions (e.g. 30 wt% MEA/2% formic acid solution, 30 wt% MEA/2% acetic acid solution and 30 wt% MEA/2% propionic acid solution) and some 30 wt% MEA/2% hydroxy monocarboxylic acids (e.g. 30 wt% MEA/2% bicine solution and 30 wt% MEA/2% glycolic acid solution). This phenomenon indicated that the foaming properties play a greater role in CO2 absorption rate than the decease of initial pH value or the decreasing amount of free active amine molecules. Additionally, the chemical structures present a significant influence on the absorption performance, as shown in Figure 9. By comparing 30 wt% MEA solution/2% monocarboxylic acid solutions, it can be found that the inhibiting effect of 2% formic acid ≈ 2% acetic acid > 2% propionic acid > 2% butyric acid (carbon chain length from C1 to C4) on the CO2 absorption rate of 30 wt% MEA solution. For 30 wt% MEA solution/2% dicarboxylic acid solutions, it can be concluded the inhibiting effect on the absorption performance decreases with the length of carbon chain (i.e. 2% oxalic acid > 2% malonic acid). In addition, the negative influence on the CO2 absorption rate with the addition of hydroxy monocarboxylic acid for 30 wt% MEA/2% hydroxy monocarboxylic acid solutions gives the order of 2% glycolic acid > 2% lactic acid (carbon chain length from C2 to C3). A possible conclusion for this phenomenon could be that the absorption rate of 17

MEA/acid solution increases with the increase in carbon chain length or molar mass of same type carboxylic acid. Because the inductive effect of election absorption decreases with the increase of carbon chain length, leading to the reduction in the acidity of carboxylic acid. Figure 7 Figure 8 4.3. Desorption rate It can be found that 30 wt% MEA/carboxylic acid solution provided better desorption rate than that of pure 30 wt% MEA solution with the absence of carboxylic acid from Figure 10 and Figure 11. The experimental result indicted that the presence of acid (i.e. low pH) could facilitate the desorption rate and reduce the energy requirement for CO 2 desorption from rich solutions. A possible explanation could be that a lot of

were introduced with the addition of carboxylic acid into

aqueous MEA solution, and these

can promote the desorption reaction (Eqs. 9,10 and 12) to

push toward a direction of desorption, especially the decomposition reaction of carbamate (Eq. 12) which is the key step of desorption process [38]. Therefore, the formation of acids in CO2 absorption process of MEA solution can accelerate the desorption rate and reduce desorption energy consumption for CO2 desorption of MEA solution. The Figure 10 shows that the desorption rate increases with the increasing concentration of formic acid for 30 wt% MEA/formic acid solution. According to the explanation mentioned above, the reason for this phenomenon could be that the amount of

increase with the increasing

concentration of formic acid for simulated degraded MEA solution, leading to the enhancement of the desorption process.

18

As depicted in Figure 11, the results indicated that different individual carboxylic acid of 30 wt% MEA/2% carboxylic acid solution play a different role in desorption processes of rich solutions. It was observed that the three greatest desorption rates were successively provided by the aqueous 30 wt% MEA/2% malonic acid, 30 wt% MEA/2% oxalic acid and 30 wt%/2% formic acid solutions, and the lowest desorption rate was obtained by aqueous 30 wt% MEA/2% bicine, respectively. This can be explained by these initial pH values as shown in Figure 6. Figure 10 Figure 11 4.4. Cyclic CO2 capacity The cyclic CO2 capacity of amine solution is an important parameter in the continuous process of CO2 capture and it directly related to the absorption rate and energy requirement of solvent regeneration during practical absorption/desorption process [39, 40]. In the present work, the value of the cyclic CO2 capacity is calculated by the difference of the rich CO2 loading and lean CO2 loading at a certain period of regeneration. The cyclic CO2 capacity by a whole quasi-cycle is defined as the absolute values of the difference of CO2 loadings of absorption and desorption experiments. The cyclic CO2 capacities of 30 wt% MEA solution without carboxylic acid and 30 wt% MEA/carboxylic acid solutions containing nine individual carboxylic acid were determined and compared for two periods of 10 minutes and 20 minutes, respectively. As presented in Table 3 and Figure 12, the cyclic CO2 capacities of the simulated degraded MEA solutions are generally lower than that of MEA solution without carboxylic acid except for 30 wt% MEA/2% butyric acid, 30 wt% MEA/2% lactic acid and 30 wt% MEA/2% malonic acid solutions for a certain period of 10 minutes, indicating to the addition of different acids can generally reduce the cyclic CO 2 capacity of MEA 19

solution. The reason for this result could be explained by the fact that the inhibiting effect of these acids on the absorption performance is greater than that of the accelerating effect on the desorption process for MEA solution for two periods of 10 minutes and 20 minutes, while the effect is contrary for 30 wt% MEA/ 2% butyric acid solution, 30 wt% MEA/2% lactic acid solution and 30 wt% MEA/2% malonic acid solution for a certain period of 10minutes. The available loss ratio of cyclic CO2 capacity

is shown in

Table 3，it can more visually be observed that the effect of acids on cyclic CO2 capacity of the degraded MEA solution for two periods of 10 minutes and 20 minutes, reflecting to the level of the energy consumption of the degraded MEA solution compared with clean 30 wt% MEA solution. It can observed that the order of the loss ratio of cyclic CO2 capacity is 30 wt% MEA/2% formic acid solution > 30 wt% MEA/3% formic acid solution > 30 wt% MEA/1% formic acid solution > 30 wt% MEA/5% formic acid solution and 30 wt% MEA/2% formic acid solution > 30 wt% MEA/1% formic acid solution > 30 wt% MEA/3% formic acid solution > 30 wt% MEA/5% formic acid solution for two period of 10 minutes and 20 minutes, with the change concentration of formic acid, respectively. For different individual acids, the order of the loss ratio of CO 2 capacity for simulated degraded MEA solution is 30 wt% MEA/2% acetic acid solution > 30 wt% MEA/2% glycolic acid solution > 30 wt% MEA/2% propionic acid solution> 30 wt% MEA/2% bicine solution > 30 wt% MEA/formic acid solution > 30 wt% MEA/2% oxalic acid solution for a certain period of 10 minutes and 30 wt% MEA/2% acetic acid solution > 30 wt% MEA/2% glycolic acid solution > 30 wt% MEA/2% propionic acid solution> 30 wt% MEA/2% bicine solution > 30 wt% MEA/formic acid solution > 30 wt% MEA/2% oxalic acid solution > 30 wt% MEA/2% butyric acid solution > 30 wt% MEA/2% lactic acid solution > 30 wt% MEA/2% malonic acid solution for a certain period of 20 20

minutes, respectively. Among them, the loss ratio of cyclic CO2 capacity of 30 wt% MEA/2% malonic acid solution < 30 wt% MEA/2% butyric acid solution < 30 wt% MEA/2% lactic acid solution is negative values for a certain period of 10 minutes, indicating the cyclic CO2 capacities of the three simulated degraded MEA solution are greater than 30 wt% MEA without carboxylic acid. Furthermore, the highest loss ratio of cyclic CO2 capacity was provided by the aqueous 30 wt% MEA/2% acetic acid solution for two periods of 10 and 20 minutes; and the lowest loss ratios of cyclic CO2 capacity was obtained by the aqueous 30 wt% MEA/2% oxalic acid and 30 wt%MEA/2% malonic acid for two periods of 10 minutes and 20 minutes, respectively. Then, it can be concluded that the effect of carboxylic acid on cyclic CO2 capacity of MEA solution is more complicated compared with CO2 equilibrium solubility and initial pH values, and the addition of carboxylic acid can reduce the CO2 capture performance. Table 3 Figure 12 4.5. Gas-liquid material balance Gas-liquid material balances plays an important factor to assess the reliability and accuracy of experimental method and device. The CO2 loading can be calculated using the CO2 concentration of inlet gas and outlet gas which was record by the infrared detector during absorption/desorption processes, while the CO2 loading of solution can be determined by titration using a Chittick apparatus. All the data points of the CO2 loading are drawn in Figure 13, it is obvious seen that the CO2 loading calculated by gas phase showed good agreement with the CO2 loading calculated by solutions, with the absolute average relative deviation (AARD) of 2.35%. Figure 13. 21

4.6. Countermeasures As the experimental result mentioned above, the presence of acids will reduce the absorption performance and promote the desorption performance for MEA solution. A typical MEA absorption process is that the flue gas fed from the bottom of the absorber column reacts with MEA introduced from the top of the absorber column, and then the rich solution is transported into the stripper to release CO2 by a higher temperature. Finally, the lean solution is then returned into the absorber. Because the oxidative degradation of MEA aqueous solution is unavoidable in the absorption process, the acidic degradation products will be accumulated as the increasing number of absorption cycles. In order to maintain the absorption performance and reduce the energy requirement of solvent regeneration, the responding approaches should be required and proposed. Due to these acidic degradation products can reduce the absorption rate and promote the desorption rate for the CO2 capture process by using aqueous MEA solution, the degraded MEA solution can be introduced into the top of the stripper to reduce the energy consumption of solvent regeneration, and then can be better to be removed from MEA solution before returned to the absorption column. The simplified process of absorption/desorption demonstrated in Figure 14. In addition, the cyclic CO2 loading will also be enhanced. It should be noted the methods to remove these acidic degradation products from the amine solution is essential investigated in the future.

5. Conclusions

The simulated degraded MEA solutions with individual addition of nine carboxylic acids (i.e. formic acid, acetic acid, propionic acid, butyric acid, glycolic acid, lactic acid, malonic acid and oxalic acid, and bicine) have been investigated for comprehensively understanding the effect of 22

heat-stable salts on the CO2 capture performances with experimental methods. The initial pH values of all tested solutions were determined at 40 ℃, the equilibrium solubility of CO2 in tested solutions was measured at 40 ℃ with the CO2 partial pressures of 15kPa and 100kPa, and the absorption/desorption rates and cyclic CO2 capacities of all tested solutions were also tested by using an improved rate-based screening method. The experimental results concluded that: 1. The both initial pH values and equilibrium solubility of CO2 decrease with the addition of carboxylic acid into MEA solution compared with fresh MEA solution, and increase with the increment concentration of carboxylic acids. The orders of the initial pH value and equilibrium solubility of CO2 both are 30 wt% MEA/2% oxalic acid solution < 30 wt% MEA/2% malonic acid solution < 30 wt% MEA/2% formic acid solution < 30 wt% MEA/2% glycolic acid solution < 30 wt% MEA/2% lactic acid solution < 30 wt% MEA/2% acetic acid solution < 30 wt% MEA/2% butyric acid solution < 30 wt% MEA/2% propionic acid solution < 30 wt% MEA/2% bicine solution. It should be noted that the orders of initial pH value and equilibrium solubility of CO2 are consistent with the acid strength of carboxylic acid, i.e. the order of the acidity of carboxylic acid is oxalic acid > malonic acid > formic acid > glycolic acid > lactic acid > acetic acid > butyric acid > propionic acid > bicine. 2. The the addition of carboxylic acid into aqueous MEA solution as well as the increasing concentration of carboxylic acid can reduce the absorption rate. By comparing all simulated degraded MEA solution, the 30 wt% MEA/ 2% formic acid and 30 wt% MEA/ 2% acetic acid solutions exhibit the lowest absorption rates, which can be attributed to the fact that the foaming properties play a greater role in CO2 absorption rate than the decease of initial pH value or the decreasing amount of free active amine molecules. In addition, the chemical structures also present a 23

significant influence on the absorption performance, and the absorption rate of MEA/acid solution increases with the increase in carbon chain length as well as the molar mass for the same type carboxylic acid. 3. The formation of carboxylic acids in the CO2 absorption process of MEA solution can promote the desorption rate and reduce desorption energy consumption, and the desorption rate increases with the increase concentration of carboxylic acid. The three greatest desorption rates were successively obtained by the aqueous 30 wt% MEA/2% malonic acid, 30 wt% MEA/2% oxalic acid and 30 wt%/2% formic acid solutions, and the lowest desorption rate was obtained by aqueous 30 wt% MEA/2% bicine, respectively. 4. The cyclic CO2 capacities of the simulated degraded MEA solutions are generally lower than that of fresh MEA solution except for 30 wt% MEA/2% butyric acid, 30 wt% MEA/2% lactic acid and 30 wt% MEA/2% malonic acid solutions for a certain period of 10 minutes, indicating the addition of different acids can generally reduce the cyclic CO 2 capacity of MEA solution. 5. In order to maintain the absorption performance and reduce the energy requirement of solvent regeneration, these acidic degradation products should be removed from MEA solution before returning to the absorption column and can be remained in the stripper. All these data can provide a basis for the developed approaches to control and remove the degradation products of amine solution for the CO2 capture process.

24

Acknowledgements

The financial support from the National Natural Science Foundation of China (NSFC-Nos. 21536003, 21706057, 21606078 and 51521006), the Natural Science Foundation of Hunan Province in China (No. 2018JJ3033), the China Postdoctoral Science Foundation (Grant No. 2018M630899), and the China Outstanding Engineer Training Plan for Students of Chemical Engineering & Technology in Hunan University (MOE-No. 2011-40).

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29

Tab

Table 1 List of tested solution Number

Concentration

Number

Concentration

1 2-5 6 7 8

30 wt% MEA 30 wt% MEA/1, 2, 3, 5% formic acid 30 wt% MEA/2% acetic acid 30 wt% MEA/2% propionic acid 30 wt% MEA/2% butyric acid

9 10 11 12 13

30 wt% MEA/2% glycolic acid 30 wt% MEA/2% oxalic acid 30 wt% MEA/2% lactic acid 30 wt% MEA/2% malonic acid 30 wt% MEA/2% bicine

Table 2. Impact of acids on the capacity loss ratio of tested solutions Addition of acid to 30 wt% MEA Fresh solvent formic acid Acetic acid Propionic acid Butyric acid Glycolic acid Lactic acid Oxalic acid Malonic acid Bicine

0 1% 2% 3% 5% 2% 2% 2% 2% 2% 2% 2% 2%

30

Capacity loss ratio, % PCO2=15kPa PCO2=100kPa 7.860 8.070 10.70 16.47 12.78 18.47 17.23 19.88 10.42 16.06 9.28 14.48 9.47 15.06 10.61 16.31 10.51 16.14 14.39 19.22 11.93 17.80 6.060 5.990

Table 3. Impact of acids on cyclic CO2 capacity of tested solutions Addition of acid to MEA Clean solvent 1% formic acid 2% formic acid 3% formic acid 5% formic acid 2% Acetic acid 2% Propionic acid 2% Butyric acid 2% Glycolic acid 2% Lactic acid 2% Oxalic acid 2% Malonic acid 2% Bicine

10min cyclic CO2 capacity (g/g) Loss ratio (%) 0.0122 0.0110 9.86 0.0108 11.51 0.0109 10.45 0.0111 9.23 0.0100 17.87 0.0105 14.36 0.0129 -5.73 0.0103 15.79 0.0126 -2.94 0.0121 1.28 0.0134 -9.47 0.0105 13.82

31

20min cyclic CO2 capacity (g/g) Loss ratio (%) 0.0209 0.0192 7.97 0.0191 8.34 0.0193 7.63 0.0196 6.21 0.0180 13.80 0.0183 12.37 0.0205 1.90 0.0180 13.62 0.0205 1.77 0.0198 4.92 0.0209 0.02 0.0191 8.45

Figure

Figure 1. Categories of carboxylic acids according to chemical structures.

32

Figure 2. Reaction of MEA with its acidic degradation products.

33

Figure 3. Mechanism of the oxidative degradation of MEA proposed by Rooney et al [34].

34

Figure 4. Schematic of the absorption/desorption apparatus.

Figure 5. Schematic diagram of VLE setup.

35

Figure 6. Initial pH for experimental solution.

Figure 7. CO2 equilibrium solubility and initial pH for experimental solutions

36

Figure 8. Effects of the addition of different degradation formic acid on CO2 absorption rate (40℃) for MEA solution

Figure 9. Effects of the addition of different degradation acids on CO2 absorption rate (40℃) for MEA solution

37

Figure 10. Effects of the addition of different degradation formic acid on desorption rate (80℃) for MEA solution

Figure 11. Effects of the addition of different degradation acids on desorption rate (80℃) for MEA solution

38

Figure 12. Cyclic CO2 capacities of 10min and 20 min comparison of the addition of different degradation acids on MEA solution and clean MEA solution.

Figure 13. Gas−liquid material balance for all tested systems.

39

Figure 14. Simplified process improvement of conventional solvent absorption/desorption system.

40

Highlights



Heat-stable salts can reduce the absorption performance of MEA solution.



Heat-stable salts can promote the desorption performance of MEA solution.



Structure of acid have a significant effect on performance of MEA solution.



Countermeasures was proposed to maintain absorption/desorption performance.

41