Enhanced Study of CO₂ Hydrate Formation in Marine Oil–Gas Based on Additive Effect

Abstract

During marine oil–gas extraction, significant amounts of carbon dioxide (CO₂) gas are often produced. Effectively separating these associated CO₂ gases during extraction has become a critical technical challenge. Therefore, this paper aims to enhance the efficiency of CO₂ hydrate-based capture technology and conduct relevant research. The goal is to increase the driving force for hydrate formation by combining the traditional thermodynamic additive TBAB with pressure modulation and to improve the hydrate formation rate through the use of multiple kinetic promoters. This paper presents the initial investigation into the effect of the thermodynamic accelerator tetrabutylammonium bromide (TBAB) on the characteristics of CO₂ hydrate formation. The promotion effects of TBAB solutions with varying mass concentrations (3%, 5%, 7.5%, 10%) and reaction pressures (3 MPa, 4 MPa) were subjected to a systematic analysis, and the optimal conditions were identified as 4 MPa and a 5 wt% TBAB concentration. Subsequently, the impact of combining TBAB with kinetic promoters (SDS, nano Al₂O₃, L-methionine, L-leucine) on CO₂ hydrate generation characteristics was further investigated. In this paper, the effect of a single promoter on the generation characteristics of CO₂ hydrate was investigated, and the efficient carbon trapping ability of the complex promoter was verified, which provides theoretical support for the application of CO₂ trapping technology using the hydrate method.

1. Introduction

At present, global climate change has become a major challenge for the international community. In 2023, global CO₂ emissions reached 35.8 billion tons [1]. Among them, about 78% of CO₂ from flue gas emitted by fossil fuel combustion will become the maximum emissions source in 2022 [2]. The operation of internal combustion engines is also dependent on the combustion of fossil fuels, resulting in the emission of considerable quantities of CO₂ into the atmosphere [3,4]. Therefore, carbon capture technology has received increasing attention [5,6].

Conventional CO₂ capture technologies cover a variety of means [7,8,9,10,11,12], such as chemical absorption, physical adsorption, and low-temperature condensation separation. However, there are problems such as a large amount of absorbent used, high energy consumption, easy leakage and the corrosion of equipment, high investment cost of the device, residual impurities left over, as well as degradation and foaming of the absorbing medium, etc., so it is necessary to continue to push forward the research and development of new technologies that are more efficient, economical and environmentally friendly. Among the emerging technologies, the hydrate method shows great potential due to its low energy consumption, simple process, and good environmental compatibility, and is considered to be the frontier direction in the field of CO₂ capture [13,14,15]. Hydrate carbon capture and storage is a technology that converts CO₂ into hydrates and subsequently sequesters it. This method exploits the fact that CO₂ forms hydrates with water under high pressure and low temperature, thereby achieving gas separation and long-term storage of CO₂ [16]. Hydrates are solid cage-like structures formed by gas molecules and water molecules under specific conditions, which offer high CO₂ storage efficiency and low risk of leakage [17].

Gas hydrates, also known as cage hydrates, are solid crystals that form under specific high-pressure, low-temperature conditions [18,19,20,21,22,23]. They construct diverse cage-like structures through hydrogen bonding between water molecules and embed gas molecules in them to form ice-like substances [24,25,26,27]. As an emerging means of CO₂ capture_, hydrate-based technology still faces challenges in terms of application and promotion [15,28]. For example, its formation and stabilization require specific high-pressure and low-temperature conditions, slow generation rates, and low gas storage density [25,29,30]. Currently, hydrate formation can be enhanced by adding hydrate promoters, which can be categorized into two main groups: thermodynamic promoters and kinetic promoters, according to their mechanism of action [16,17,18,19].

Thermodynamic promoters simplify the process of hydrate formation and regulate phase equilibrium by reducing the required energy threshold required for the reaction to occur. Among the common thermodynamic promoters are tetrahydrofuran (THF), cyclopentane (CP), tetrabutylammonium bromide (TBAB), tetrabutylammonium fluoride (TBAF), and tetrahydropyran (THP). TBAB and THF are effective and commonly used thermodynamic accelerators, which are able to generate hydrates by themselves and occupy only large cage-like structures, providing small cage sites for CO₂, thus enhancing CO₂ capture [31,32]. During the hydrate generation process, the presence of TBAB reduces the equilibrium pressure threshold of hydrate formation and enhances the separation efficiency of CO₂. Li et al. [33] verified the thermodynamic properties of TBAB by adding 5 wt% TBAB in a mixture of 19.9% CO₂ + 80% N₂ at a temperature of 277.5 K and a pressure of 4.3–7.3 MPa. Jean et al. [34] demonstrated that the addition of 0.29 mol% TBAB drastically reduced the pressure during CO₂/N₂ hydrate formation. Furthermore, TBAB is an environmentally friendly catalyst that is non-volatile, non-flammable, non-corrosive, and exhibits low cost and high thermochemical stability. Together, these findings demonstrate the practical application value of TBAB in the field of flue gas carbon capture. Tang [35] used 0.5 to 3.0 mol% aqueous THF solutions to capture CO₂ from a gas mixture of 59% CO₂ + 41% N₂, and CO₂ recoveries (0.71–0.85) were obtained that were significantly greater than those of the pure water system (0.54). Tetrabutylammonium fluoride (TBAF) had the same effect of lowering the equilibrium pressure and elevating the reaction temperature. Kim [36] et al. tested the generation characteristics of CO₂ hydrate by adding TBAF to a gas mixture (20% CO₂ + 80% N₂). TBAF significantly changed the phase equilibrium of the hydrate, and the phase equilibrium pressure was reduced to 1.04 MPa at a temperature of 300 K. However, the CO₂ capture ability was poor.

Kinetic promoters accelerate the rate of hydrate generation mainly by changing the kinetic properties of the solution, such as increasing the diffusion coefficient of the solution and decreasing the gas–liquid surface tension [37]. Such promoters cover several classes of compounds, mainly including nanoparticles [38,39] (alumina Al₂O₃, graphite powder, titanium dioxide), porous media [29] (e.g., quartz sand, silica gel), ionic liquids, as well as [40] bio-promoters (e.g., amino acids [41], lignosulfonates), surfactants [42], etc., and the common surfactants are SDS (sodium dodecyl sulfate), CTAB (brominated cetyltrimethylammonium), and STS (sodium tetradecyl sulfate) and so on. Said et al. [43] found that nano-Al₂O₃ showed significant enhancement effects at concentrations of 0.1 to 0.3 wt%, with gas consumption increasing by 30% to 65%. Liu et al. [40] investigated the significant effect of 0.1 wt% L-methionine on the absorption rate of CO₂ gas and the promotion of hydrate formation in 99.8% CO₂ gas. Li et al. [44] systematically investigated the role of amino acids, with a special focus on the effect of L-tryptophan as a kinetic enhancer of carbon dioxide hydrate generation. Yan Li et al. [45] found that 2 wt% L-leucine had a significant effect on the absorption rate of CO₂ gas as well as on the promotion of hydrate formation rate, with an optimal CO₂ normalized absorption rate of 62.17 ± 0.78 mmol/mol and a CO₂ occupancy of 66% in the hydrate phase.

As the pro-generation effect of hydrate promoters was verified, scholars began to investigate the effect of thermo–kinetic hybrid compounding additives on CO₂ hydrate generation, and Kumar [46] found that the process of CO₂ hydrate generation in a gas mixture of CO₂ and N₂ was positively affected in a solvent mixture of THF and SDS. Tang [35] also used SDS and THF as additives to capture CO₂ from a gas mixture of 59% CO₂ + 41% N₂ by the hydrate method and obtained that the most effective concentration range for SDS was 100 to 300 ppm.

Hydraulic fracturing is a technique employed in the extraction of gas hydrates, whereby fractures are formed at the base of the deposit, and a coolant with a temperature exceeding that of the deposit is pumped into these fractures. The principal objective of the method is the release of natural gas from the hydrate and the subsequent transportation of the decomposition products and the coolant to the surface. By applying high pressure inside the wellbore, the fracturing fluid can create cracks in the rock, increasing the flow path for the fluid and facilitating the release of natural gas.

The results of the above scholars show that single additives have a certain promotion effect on CO₂ hydrate generation, but there are fewer studies on the effect of thermo–kinetic hybrid additives on CO₂ hydrate generation, especially on changing the CO₂ hydrate generation characteristics to improve the capture effect in the associated gas and industrial waste gas. Therefore, in this paper, single additives that have been proven to have significant promotion effects were first screened for experiments, and evaluation criteria were set based on the experimental data. Then, the effects of additive combinations on CO₂ capture by the hydrate method are analyzed, and the optimal additive ratios under specific experimental conditions are determined. The most effective promoter combinations applicable to associated gas and industrial waste gas were subjected to analysis and determination through synthesis. As the quantity of quasi-liquid water represents the limiting factor for the formation of hydrates and the diffusion of gases within the system, the rate of hydrate formation may be reduced at lower temperatures. Furthermore, the rate of gas diffusion is also reduced at lower temperatures, which in turn increases the time required for gas molecules to come into contact with quasi-liquid water. Consequently, we selected higher temperatures and corresponding pressure conditions as the experimental conditions [47]. The characteristics and kinetic parameters of CO₂ hydrate generation were optimized.

2. Experimental Part

2.1. Experimental Set Up

The experimental system used to study the effect of accelerators on CO₂ capture by the hydrate method was built in this paper, and its structure is shown in Figure 1. The system mainly consists of a visible reactor (inner diameter of 70 mm, inner cavity height of 65 mm, volume of 250 mL), a magnetic stirrer, a constant temperature water bath, a Matsushima thermocouple (accuracy of 0.01 K), a Baoji Hengtong BP93420 pressure sensor (range 0~15 MPa, accuracy of 0.075%), and a data acquisition system from Advantech. In addition to the experimental setup, there were measuring instruments such as an electronic balance (OHAUS International Trading Co., Ltd., Shanghai, China, model CP513, maximum weighing capacity 510 g, dividing value 0.1 g) and a gas chromatograph (Tianmei Technology Co., Ltd., Beijing, China, model GC7900). In this experiment, 3 mm stainless steel tubing supplied by the Kumagawa manufacturer was used as the piping connecting the reactor, gas source, and detection sensor, and the valves were mainly ball valves and needle valves.

2.2. Experimental Materials and Methods

2.2.1. Experimental Materials

The specifications of the accelerator and solvent materials used in this paper are shown in

Table 1. Experimental material list.

Materials	Abbreviation/ Molecular Formula	Parameter	Source
Deionized water	H2O		Lab-created
simulation Flue gas	CO2 + N2	17.02% CO2 + 82.98% N2	Dalian Dart Gas Co., Dalain, China
Tetrabutylammonium bromide	TBAB	99% N2	Shanghai McLean Biochemical Technology Co., Shanghai, China
Sodium dodecyl sulfate	SDS	92.5~99%	Shanghai McLean Biochemical Technology Co., Shanghai, China
Nano-aluminum oxide	Al2O3	20 wt% aqueous solution	Shanghai Aladdin Biochemical Technology Co., Shanghai, China
L-methionine	C5H11NO2S	99%	Shanghai Titan Technology Co., Shanghai, China
L-leucine	C6H13NO2	99%	Shanghai McLean Biochemical Technology Co., Shanghai, China

below:

2.2.2. Experimental Methods

The specific experimental procedures are as follows:

(1)

Water bath preparation: check and turn on the low-temperature thermostatic water bath, set the experimental temperature, and turn on the experimental data acquisition system and display terminal;

(2)

Cleaning the reaction reactor: clean the reaction reactor with deionized water several times to avoid the residual experimental reagents affecting the accuracy of the experiment;

(3)

Preparation of solution: use an electronic balance to accurately weigh the reagents, dissolve them in 150 mL deionized water, and make sure that they are fully stirred until they are evenly mixed;

(4)

Solution injection: put the prepared solution as well as the rotor into the reactor, lock the reactor, and check the sealing;

(5)

Pre-cooling: place the reaction reactor in a constant temperature water bath device, start the magnetic stirring system, and wait for the temperature in the reaction reactor to drop to a preset value and remain stable;

(6)

Exhausting miscellaneous gases: connect the gas cylinder, control the pressure 1 MPa, purge the gas 3–4 times, and empty the original gas in the reactor;

(7)

Injection of reaction gas: open the inlet road valves and the valve in front of the pressure gauge and start inflating the speed as far as possible to maintain uniformity to ensure that each group of experiments has the same injection time and ensure pressure stabilization and then close the inlet road valves and cylinders. Then, the reaction begins;

(8)

Collect residual gas: after the reaction is finished, close the magnetic stirrer and collect residual gas;

(9)

Collect decomposition gas: after collecting residual gas, release the gas in the reaction reactor and exhaust the original gas with a vacuum pump. Then, put it to room temperature in a stirring table to decompose, and collect decomposition gas when the room temperature is restored;

(10)

Repeat operation steps (5)–(9), and perform two groups of the same experimental conditions;

(11)

The end of the experiment: release the gas in the reaction reactor, dispose of the waste solution, and clean the reactor to prepare it for the next set of experiments;

(12)

Use a gas chromatograph to detect the experimental gas.

The flowchart is presented in the accompanying Figure 2.

The hydrate generation experiments were carried out under different working conditions with the application of promoters to study the effects of promoter type, promoter concentration, and initial pressure on the CO₂ hydrate generation characteristics and carbon capture capacity. In order to reduce the bias of experimental data and enhance the credibility of the results, it was ensured that each set of experiments was repeated at least twice. For Case A, the effects of a single thermodynamic promoter, TBAB concentration, and experimental pressure on the CO₂ hydrate generation process were investigated by fixing the temperature condition at 3 °C, and the TBAB concentration and experimental pressure were used as the conditioned variables for the experimental study. The experimental reagents were categorized into four concentrations of 3 wt%, 5 wt%, 7.5 wt%, and 10 wt%, and the pressures were 3 MPa and 4 MPa, respectively. For Case B, the effects of a single kinetic promoter concentration, as well as the experimental pressure on the CO₂ hydrate generation process, were explored due to the low CO₂ percentage of 17.02% in the simulated flue gas used for the experiments in this paper. Drawing on the experimental conditions studied by Linga et al. [48], the temperature of this experiment was set to two groups of 0 °C and 1 °C, and the initial pressure of the experiment was set to 10 MPa. The agents chosen were 0.1 wt% SDS [49], 0.3 wt% nano-Al₂O₃ [43], 0.1 wt% L-methionine [40] and 1 wt% L-leucine [50]. For Case C, the effect of the use of TBAB combined with the kinetic promoter on the CO₂ hydrate generation process was investigated; firstly, the optimal reaction temperature and pressure conditions and TBAB concentration were confirmed according to Case A, based on which the kinetic promoter was added to form a thermo–kinetic compound.

A total of 20 groups of experiments were carried out in the three cases, and the specific experimental conditions included experimental serial number, experimental temperature, experimental pressure, and experimental concentration, as shown in

Table 2. Experimental conditions of CO₂ hydrate generation.

Experiment Number	Experimental Concentration/wt%	Initial Pressure/MPa	Experimental Temperature/°C
A1	5 wt% TBAB	3	3
A2	5 wt% TBAB	3	3
A3	5 wt% TBAB	4	3
A4	5 wt% TBAB	4	3
A5	7.5 wt% TBAB	3	3
A6	7.5 wt% TBAB	3	3
A7	7.5 wt% TBAB	4	3
A8	7.5 wt% TBAB	4	3
A9	10 wt% TBAB	3	3
A10	10 wt% TBAB	3	3
A11	10 wt% TBAB	4	3
A12	10 wt% TBAB	4	3
B1	0.1 wt% SDS	10	3
B2	0.3 wt% Nano Al2O3	10	3
B3	0.1 wt% L-methionine	10	3
B4	1 wt% L-leucine	10	3
C1	AS wt% TBAB + 0.1 wt% SDS	PAS	TAS
C2	AS wt% TBAB + 0.3 wt% Nano-Al2O3	PAS	TAS
C3	AS wt% TBAB + 0.1 wt% L-methionine	PAS	TAS
C4	AS wt% TBAB + 1 wt% L-leucine	PAS	TAS

. One noteworthy aspect is the selection of temperature and pressure conditions throughout the phase equilibrium range for hydrate formation. In this study, we have chosen a temperature that is not exceedingly low while maintaining relatively mild pressure conditions. These findings have implications for subsequent industrial applications.

2.3. Experimental Data Processing

2.3.1. Induction Time

As shown in Figure 3, the hydrate formation process involves three key steps: first, the dissolution of gas, then nucleation, and finally, the growth of crystal nuclei [51]. The induction period is a period of time before the hydrate begins to form, and with the gradual formation of the hydrate, a large amount of heat will be released, leading to a sharp rise in temperature and the formation of an obvious exothermic peak, which is used to determine the generation and induction time of the hydrate.

2.3.2. Calculation of Gas Separation Coefficient

The gas separation coefficient is a physical quantity that describes the relative degree of separation of gases under specific conditions at a certain temperature and pressure. The CO₂ gas separation coefficient χ in this experimental system can be calculated from the molar ratio of N₂ and CO₂ in the gas phase before and after the reaction.

$$\chi ={\displaystyle \frac{y_{N_2,g}/y_{C{O}_2,g}}{y_{N_2,f}/y_{C{O}_2,f}}}$$

(1)

$y_{C{O}_2,f}$ and $y_{C{O}_2,g}$ are the molar ratios of CO₂ in the gas phase at the beginning and the end of the reaction, respectively; $y_{N_2,f}$ and $y_{N_2,g}$ are the molar ratios of N₂ in the gas phase at the beginning and the end of the reaction.

2.3.3. Calculation of Gas Consumption

The ideal Equation of state for CO₂ gas is as follows:

$$P_{i}V=nZR{T}_i$$

(2)

Pi and Ti are the pressure and temperature of the gas in the reaction reactor fed back by the temperature and pressure acquisition equipment at i time, respectively;

V is the volume of the gas phase in the reaction reactor, V = 2 × 10⁻⁷ m³;

R represents the ideal gas constant, R = 8.314 kJ/(kmol∙K);

Zi represents the gas compression factor at moment i, which can be calculated from Pitzer’s Equation (3):

$$z_i=1+\left[0.083-{\displaystyle \frac{0.422}{T_r^{1.6}}}\right]\times {\displaystyle \frac{P_r}{T_r}}+w\times \left[0.139-{\displaystyle \frac{0.172}{T_r^{4.2}}}\right]\times {\displaystyle \frac{P_r}{T_r}}$$

(3)

$w$ represents the eccentricity factor of CO₂, $w$ = 0.022394;

Pr and Tr represent the contrast pressure and contrast temperature of the gas, respectively, which can be calculated according to Equations (4) and (5):

$$P_r={\displaystyle \frac{P_i}{P_c}}$$

(4)

$$T_r={\displaystyle \frac{T_i}{T_c}}$$

(5)

In Equations (4) and (5), Pc represents the critical pressure of CO₂ Pc = 7.45 MPa;

Tc represents the critical temperature of CO₂ Tc = 304.25K.

According to Equation (6), the molar amount of gas consumed ∆n at t time for CO₂ hydrate generation can be calculated:

$$\Delta n=n_0-n_t={\displaystyle \frac{V}{R}}\left({\displaystyle \frac{P_0}{Z_0{T}_0}}-{\displaystyle \frac{P_t}{Z_t{T}_t}}\right)$$

(6)

∆n denotes the gas consumption, mol; V denotes the volume of the gas phase in the reactor, m³;

P₀, T₀ and P_t, T_t are the pressure and temperature of the initial and final states, respectively;

R denotes the molar gas constant, R = 8.3145 J/(mol∙K);

Z denotes the gas compression factor.

3. Results and Discussion

3.1. Influence of Thermodynamic Additives on CO₂ Hydrate Generation Characteristics

Excessively high phase equilibrium conditions for CO₂ hydrate generation within associated gas and industrial waste gas is a key factor limiting the commercial application of CO₂ capture by the hydrate method. Thermodynamic promoters have shown significant effects in reducing the hydrate phase equilibrium. Therefore, in this paper, the most representative thermodynamic additive TBAB was selected to carry out a series of experiments and coupled with the effect of pressure to investigate the influence of TBAB concentration and pressure on the characteristics of CO₂ hydrate generation in the flue gas, which provides theoretical guidance for efficient CO₂ capture.

3.1.1. Influence of TBAB on CO₂ Hydrate Generation Characteristics

As shown in Figure 4, under the experimental conditions of Case A, when 5 wt% TBAB was added to the flue gas system, the induction times of CO₂ hydrate generation were 75 and 41 min, while when the TBAB concentration was increased to 7.5 wt%, the induction time are 51 and 37 min. When the TBAB concentration was further increased to 10 wt%, the hydrate generation induction time decreased to 4 and 23 min. A negative correlation was observed between the experimental concentration and the induction time (Figure 5). This phenomenon suggests that the cationic TBA+ is able to promote the formation of hydrate nucleosomes with the increase of TBAB concentration, and thus, the hydrate generation induction time is gradually shortened with the increase of TBAB concentration [52]. In addition, the differences in the hydrate generation induction time obtained in two sets of replicate experiments with the same concentration of TBAB suggest a high degree of stochasticity in the hydrate reaction. At the microscopic level, this is mainly due to the instability of the hydrate nucleation.

After the formation of CO₂ hydrate, the average gas consumption in the three concentration systems of 5 wt%, 7.5 wt%, and 10 wt% is summarized in

Table 3. Gas consumption in hydrate formation experiments in the system with different concentrations of TBAB at an initial pressure of 4 MPa.

Initial Conditions	TBAB Concentration	Final Gas Consumption
3 °C 4 MPa	5 wt%	0.0225 mol
	7.5 wt%	0.0222 mol
	10 wt% 0.0119 mol	0.0119 mol

. The gas consumption values of the 5.0 wt% and 7.5 wt% systems are close to 0.0225 mol and 0.0222 mol, respectively, while the 10.0 wt% system has the smallest gas consumption of 0.0119 mol, which is only half of the values of the 5.0 wt% and 7.5 wt% systems. Considering the observations of the experimental process, it is hypothesized that at some point in the experiment, a magnetic stirrer obstruction event may occur in the 10.0 wt% experimental group, and due to the fast rate of hydrate generation and high concentration in the early stage of the experiment, the magnetic stirrer was subjected to increase resistance, which resulted in a slowing down of the rate of hydrate generation and a decrease in gas consumption in the later stage of the experiment. Combining the induction time, gas consumption, and economic cost, 5 wt% TBAB concentration is the best choice for promoting CO₂ capture based on the no-hydrate method in this study system.

3.1.2. Effect of Pressure on CO₂ Hydrate Generation Characteristics

Pressure is a key factor affecting hydrate generation and determining production safety. Therefore, in order to reveal the specific effect of pressure on the hydrate generation rate, the induction time and gas consumption of hydrate generation were experimentally investigated under two pressure conditions, 3 MPa, and 4 MPa. Figure 6 shows the relationship between the hydrate reaction pressure and the average induction time, the induction time shortening as the pressure increases. In the 5 wt% TBAB system, the induction time of hydrate generation was 78 min for the experimental pressure of 3 MPa, which was reduced to 53 min when the experimental pressure was increased to 4 MPa, a decrease of 32.05%; in the 7.5 wt% TBAB system, the induction time of hydrate generation was 75 min for the experimental pressure of 3 MPa, which was reduced to 53 min when the experimental pressure was increased to 4 MPa. the induction time was reduced to 44 min, with a decrease of 41.33%; this phenomenon indicates that under the condition of the same reaction temperature, the increase of the hydrate reaction pressure will promote the gas escape, increase the gas–liquid contact area, and then accelerate the hydrate nucleation rate, which ultimately leads to the shortening of the induction period of hydrate [53].

As shown in Figure 7, in the process of gas hydrate formation, for the experimental system with the same TBAB concentration, the gas consumption rate and final gas consumption increase with the reaction pressure increase. Among them, in the system with a pressure of 4 MPa and a TBAB concentration of 7.5 wt%, the phenomenon of secondary hydrate formation also occurred. This is because, as the initial experimental pressure increases, the gas supersaturation in the system increases, and the hydrate growth drive force increases, which in turn leads to an increase in the hydrate growth rate and final gas consumption [53]. This suggests that high pressure can accelerate the hydrate formation process and effectively capture CO₂ from the industrial waste gas within the safe production range.

3.1.3. Synergistic Effect of TBAB Concentration and Pressure on CO₂ Hydrate Formation Characteristics

Table 4. Experimental data of CO₂ hydrate generation at 3 °C.

Experiment No.	Initial Pressure (MPa)	TBAB Concentration (wt%)	Molar Proportion of CO2 in the Gas Phase at the End of the Reaction (%)	Molar Proportion of N2 in the Gas Phase at the End of the Reaction (%)	CO2 Gas Separation Coefficient	Average Value of Separation Coefficient
1	3	5	6.22	93.78	3.10	3.03
2	3	5	6.51	93.49	2.96
3	3	7.5	11.08	88.92	1.65	1.63
4	3	7.5	11.34	88.66	1.61
5	3	10	14.1	85.9	1.25	1.28
6	3	10	13.6	86.4	1.31
7	4	5	3.39	94.33	3.41	3.81
8	4	5	4.64	95.36	4.23
9	4	7.5	7.01	92.99	2.73	2.67
10	4	7.5	7.32	92.68	2.61
11	4	10	15.36	84.63	1.13	1.19
12	4	10	14.24	85.75	1.24

shows the effects of TBAB concentration and reaction pressure on the CO₂ gas separation coefficient: with the increase of TBAB concentration_, the CO₂ gas separation coefficient decreases; with the increase of reaction pressure, the CO₂ gas separation coefficient increases. This is consistent with the experimental data on the effect of gas consumption in Section 3.3.2, and the CO₂ gas separation coefficients of the [12] groups of experiments are all greater than 1, which indicates that TBAB can separate CO₂ by the hydrate method. The experimental results of control experimental groups with different pressures corresponding to each concentration are summarized in Figure 8, from which it can be seen that the average induction time, gas consumption, and separation coefficient of the 4 MPa experimental group are better than those of the 3 MPa experimental group, except that the separation coefficient of the 4 MPa experimental group is slightly smaller than that of the 3 MPa experimental group in the 10 wt% experimental group, which is presumed due to the weakening effect of the concentration on the separation coefficient than the promoting effect of the pressure when the concentration of TBAB is too high. The weakening effect of concentration is greater than the promoting effect of pressure.

Overall consideration, when the temperature is 3 °C, the 5 wt% TBAB concentration is more suitable for the separation of CO₂ within the industrial waste gas by using the hydrate method, and the best CO₂ separation effect is achieved when the pressure is 4 MPa, with a separation factor of 3.81.

3.2. Effect of Kinetics Additives on the Characteristics of CO₂ Hydrate Formation in the Industrial Waste Gas System

Unlike thermodynamic catalysts, kinetic promoters mainly increase the hydrate generation rate without participating in the hydrate [54] formation process [54]. Under the condition of Case C, four single kinetic promoters were selected for testing experiments, mainly analyzing the induction period of CO₂ hydrate formation and CO₂ separation coefficients with the 0.1 wt% SDS addition; the results are shown in Figure 8.

From Figure 9, it can be observed that during the whole reaction process, the temperature in the reactor only fluctuated within a certain small range, the temperature curve did not appear to have obvious exothermic peaks, and through the visual window of the reactor, we did not observe the formation of hydrate during the experiment, and we subsequently repeated the experiment many times under the same conditions, which also did not generate hydrate. The residual gas was collected after the pressure in the reactor no longer changed, and the residual gas components were analyzed by gas chromatography. The CO₂ gas separation coefficient was 2.19 when the reaction temperature was 1 °C and 2.80 when the reaction temperature was 0 °C. Although the value of the gas separation coefficient was greater than 1, indicating a certain carbon trapping ability in this set of experiments, the pressure (10 MPa) in this set of experiments was much greater than the reaction pressures (3 MPa and 4 MPa) in Case A, thus increasing the CO₂ solubility to a certain extent, but it was still small compared with the CO₂ separation coefficient in Case A. The CO₂ separation coefficient in this set of experiments was also greater than that in Case A. The CO₂ separation coefficient in this set of experiments was greater than that in Case A. Subsequent test experiments with the addition of single nano-Al₂O₃, L-methionine, and L-leucine were also conducted, all of which also failed to produce hydrates and had poor separation factors. The above results show that the formation of CO₂ hydrate can enhance the CO₂ separation and capture effect to a greater extent; pressurization and the addition of surfactants can only increase the solubility of CO₂ in water to a certain extent, which not only makes the reaction conditions more severe but also makes the CO₂ capture effect not very satisfactory. Therefore, this paper concludes that the hydrate method of capturing low-concentration CO₂ in industrial waste gas needs to be coupled with thermal-kinetic promoters to achieve a better CO₂ capture effect.

3.3. Experimental Study of CO₂ Hydrate Formation Characteristics in Thermal/Kinetic Combination Promoter System

In order to investigate the effect of TBAB coupled with the kinetic promoter on the CO₂ hydrate formation characteristics, firstly, according to the experimental results in Section 3.1, the concentration of TBAB with optimal CO₂ capture effect under the conditions (4 MPa, 3 °C) was determined to be 5 wt%, and with reference to the concentration of kinetic promoter in other pieces of literature, four groups of compound promoter concentrations were set up as 5 wt%, respectively. TBAB + 0.1 wt% SDS, 5 wt% TBAB + 0.3 wt% nano-Al₂O₃, 5 wt% TBAB + 0.1 wt% L-methionine, 5 wt% TBAB + 1 wt% L-leucine, to carry out experimental studies on the induction time, gas consumption, and separation factor of the generation of CO₂ hydrate.

3.3.1. Effect of Different Compounding Accelerators on the Induction Time of CO₂ Hydrate Formation

The induction time of CO₂ hydrate formation in five groups is shown in Figure 10. The addition of 0.1 wt% SDS, 0.3 wt% nano-Al₂O₃, 0.1 wt% L-methionine, and 1 wt% L-leucine to 5 wt% TBAB reduced the induction time of CO₂ hydrate formation to 37 min, 21 min, 4 min, and 3 min, which were 31.5%, 61.1%, 92.6%, and 94.6%, respectively, compared with the addition of a single 5 wt% TBAB system. In particular, the reduction in the induction time of the experimental system with SDS and nano-Al₂O₃ was smaller and less facilitated than that of the experimental systems with L-methionine and L-leucine. In addition, the error columns of the induction time of the three experimental groups of a single 5 wt% TBAB and the addition of 0.1 wt% SDS and 0.3 wt% nano-Al₂O₃ are large, which indicate that the reaction stability of these three complex combinations was poor. Therefore, from the analysis of the results of this experiment, it can be concluded that the combination of TBAB with different kinetic promoters can effectively reduce the induction period of hydrate generation, and the system with the addition of 0.1 wt% L-methionine and 1 wt% L-leucine has the shortest induction time and high reaction stability. It is noteworthy that TBAB is a quaternary ammonium salt, wherein the cation is capable of forming a half-cage structure with water molecules. Additionally, L-methionine and leucine, as amino acids, contain amino and carboxyl groups that can form hydrogen bonds with water molecules, thereby enhancing the interactions between water molecules and thus contributing to the formation of hydrates.

3.3.2. Effect of Different Compounding Promoters on Gas Consumption for CO₂ Hydrate Formation

The total gas consumption data of CO₂ hydrate formation in different complexed promoter systems are presented in Figure 11, and it was found that the addition of 0.1 wt% SDS was effective in increasing the final gas consumption, while the addition of 0.3 wt% nano-Al₂O₃, 0.1 wt% L-methionine, and 1 wt% L-leucine enhanced the final gas consumption by 42~45%. Zhang [55] et al. proposed that amino acids can be used to increase CO₂ hydrate gas storage capacity by capillary action surface adsorption behavior, and the CO₂ hydrate generated in the L-methionine system has a more porous structure, which can provide more space for gas–liquid contact. Al₂O₃ nanoparticles have a large specific surface area, which can effectively enhance the mass transfer capacity of the solution and thus promote CO₂ dissolution, significantly shorten the induction period of hydrate, and increase the gas consumption; in addition, Al₂O₃ also improves the thermal conductivity of the solution, which helps to transfer the heat released from the hydrate formation, and further promotes the hydrate generation [56]. Therefore, from the analysis of the experimental results, it can be concluded that the combined kinetic promoter of TBAB can effectively improve the gas consumption of CO₂ capture by the hydrate method, among which the system with the addition of 0.3 wt% nano-Al₂O₃, 0.1 wt% L-methionine, and 1 wt% L-leucine showed the best CO₂ capture effect and high reaction stability.

It should be noted, however, that nano-Al₂O₃ can be prepared by a variety of methods, including laser-induced vapor deposition, plasma vapor synthesis, chemical vapor deposition (CVD), and solid-phase synthesis (e.g., these include mechanical comminution, amorphous crystallization, pyrolysis, combustion, and liquid-phase methods such as precipitation, sol–gel, hydrothermal synthesis, microemulsion, and electrochemistry). The utilization of nano-Al₂O₃ may potentially give rise to adverse effects, including the generation of harmful gases during the preparation process, the transportation, and transformation of nanoparticles with a high specific surface area and activity in the environment, and the inhalation of nano-Al₂O₃ dust, which could have detrimental effects on human health.

3.3.3. Effect of Different Compounding Promoters on the Separation Factor of CO₂ Hydrate Formation

Figure 12 shows the values and errors of the CO₂ gas separation coefficient in different kinetic promoters combined with the TBAB system. When different kinetic promoters were added to the 5 wt% TBAB solution, the separation coefficient of CO₂ decreased slightly. It was hypothesized that it might be possible that the surfactant SDS could reduce the surface tension of water, the nano-Al₂O₃ had a high specific surface area, and both L-methionine and L-leucine belonged to nonpolar amino acids, and the nature of these solutions enhanced the solubility of CO₂ [57], and some of the CO₂ gases were left in the solution at the end of the hydrate decomposition process, which resulted in a slight decrease in the CO₂ separation coefficient.

4. Conclusions and Prospect

4.1. Conclusions

In this paper, we take the CO₂ capture and efficiency enhancement in the industrial waste gas of hydrate method as the research objective and carry out experimental research around the optimization of hydrate generation promoter. First, we optimized the concentration of the thermodynamic additive TBAB and the reaction pressure, and then we carried out the rapid generation of CO₂ hydrate under the conditions of different TBAB-kinetic promoter combinations and studied the effect of the combinations of promoter on the characteristics of the generation of CO₂ hydrate in the industrial waste gas. The effect of the compounding promoters on the characteristics of CO₂ hydrate generation in the industrial waste gas was investigated. The specific conclusions are as follows:

(1)

The TBAB concentration of 5 wt%, with the largest gas consumption (0.0225 mol) and separation coefficient (3.81), and the relatively short average induction time, is the optimal choice for the industrial waste gas to promote CO₂ hydrate formation in this paper. It provides a theoretical basis for the concentration of high efficiency for subsequent industrial exhaust gas treatment;

(2)

The average induction time, gas consumption, and separation coefficient of the 4 MPa experimental group are better than those of the 3 MPa experimental group, but it should be specified that the higher reaction pressure would increase the trapping cost and decrease the safety. The complementary effect of the higher-pressure environment is better than that of the lower-pressure environment, and the ensuing safety issue is worth considering for the subsequent industrial waste gas treatment;

(3)

In this paper, the TBAB system is studied in combination with a kinetic promoter. Compared with the pure TBAB system, the introduction of kinetic promoters significantly shortened the induction time and increased the gas consumption of CO₂ hydrate. Among them, the shortest induction time was 3.5 min in the L-methionine-containing experimental group with the strongest stability of hydrate formation, and the highest gas consumption was observed in the L-methionine-containing nano group and the L-leucine-containing experimental group with the gas consumption of 0.0412 mol and 0.04015 mol, respectively; however, the incorporation of kinetic promoters slightly decreased the gas separation coefficient of CO₂ compared with that of the pure TBAB system. In summary, 5 wt% TBAB + 0.1 wt% L-methionine is the best promoter compounding scheme in this paper, which has the shortest induction time, the largest gas consumption, and the best separation coefficient.

4.2. Outlook

In order to promote the further practical application of hydrate technology, it is necessary to carry out more in-depth research work, which can be carried out in the following aspects:

(1)

The additives used in the experiments in this paper are conventional types in the laboratory, and the carbon capture effect has the prospect of further improvement, which should continue to carry out the screening, research, and development of new additives;

(2)

Follow-up experiments can also be changed by using the solution spray method, bubble method, and other ways to increase the gas–liquid contact area to improve the effect of gas separation further;

(3)

Further study of the effect of concentration ratios and multivariate combinations of thermodynamic compounding promoters on the CO₂ hydrate formation characteristics of industrial waste gas.