Screening of Absorbents for Viscose Fiber CS₂ Waste Air and Absorption–Desorption Process

Abstract

Screening of absorbents is essential for improving the removal rate of carbon disulfide (CS₂) waste air by absorption. In this work, the UNIFAC model in Aspen Plus was utilized to calculate the excess Gibbs function and absorption potential of the binary system of CS₂ with various alcohols, ethers, esters, amines, and aromatic hydrocarbons. The results were used to quantitatively compare the efficiency of each solvent for CS₂ absorption. The theoretical predictions were then verified by absorption experiments in a packed tower. The results showed that the performance of various solvents to CS₂ roughly followed the order of esters < alcohols < amines < heavy aromatics < glycol ethers. Meanwhile, N-methyl-2-pyrrolidone (NMP) is the optimal absorbent for CS₂ waste air treatment. Additionally, the process parameters of absorption and desorption of NMP were optimized. The results illustrated that the average mass removal efficiency of CS₂ by NMP is 95.2% under following conditions: liquid–gas ratio of 3.75 L·m⁻³, a temperature of 20 °C, and inlet concentration lower than 10,000 mg·m⁻³. Under the conditions of 115 °C, 10 kPa, and a desorption time of 45 min, the average desorption rate of CS₂ is 99.6%, and the average water content after desorption is 0.39%. Furthermore, the recycled lean liquid can maintain an excellent CS₂ purification effect during the recycling process.

1. Introduction

Viscose fiber is classified as regenerated cellulose prepared using natural cellulose as raw material through the viscose process. With the merits of absorbing features and excellent performance, viscose fiber has been widely used. As the most critical solvent, carbon disulfide (CS₂) is often used to produce viscose fiber. Because of this, CS₂ is inevitably emitted into the atmosphere through exhaust gases [1,2]. Meanwhile, studies on the traceability of CS₂ in the atmosphere have also confirmed that human activity is the primary source of CS₂ [3,4,5]. Atmospheric CS₂ is rapidly oxidized to carbonyl sulfide and sulfur dioxide, which play an essential role in the global sulfur cycle [6]. The newly produced carbonyl sulfide and CS₂ affect stratospheric ozone levels and contribute to acid rain formation [4]. Furthermore, CS₂ is a dual odorous and harmful gas [7], which not only damages the human reproductive system [8], but also causes hearing loss [9,10]. Moreover, Schramm et al. [11] reported that long-term exposure to CS₂ increases the risk of cardiovascular disease in humans. Therefore, it is urgent to achieve efficient recovery and environmental treatment of CS₂ waste air in industrial processes.

Currently, the treatment technology of CS₂ waste air can be divided into two categories: elimination and recovery [2]. The elimination technologies mainly include catalytic hydrolysis [12,13,14,15], oxidation [16,17], and biodegradation [18,19,20]. Elimination technologies, as a promising strategy for CS₂ treatment, have attracted widespread scientific attention. However, the complex process and the issue of destroying the molecular structures of CS₂ considerably hamper its practical application in industrial processes, while the recovery technologies mainly include adsorption [21,22,23,24,25] and absorption. The adsorption method performs excellent removal, which is particularly applied for CS₂ waste air with large air volumes and low concentrations [26,27,28]. However, the adsorption method has poor applicability due to the adsorbent being easily deactivated and hardly regenerated under the conditions of actual CS₂ waste air in viscose fiber plants. In contrast, the absorption method comes to the fore in the CS₂ waste air recovery strategy on account of the advantages of a simple process, convenient operation, recyclable absorbent, and suitability for treating high-concentration waste air. Thus, it has received the attention of many scholars. For instance, Heldebrant et al. [29] studied the absorption and desorption of CS₂ in three ionic liquids, drawing on the experience of CO₂ absorption. They noted that CS₂ reacts similarly to CO₂ towards amidine/alcohol and guanidine/alcohol blends. Huo et al. [30] developed the UDS-F solvent to decline the simulated CS₂ waste air concentration from 400 mg·m⁻³ to 79 mg·m⁻³ (atmospheric pressure, 50 °C, and the liquid–gas ratio of 500). Although the mass removal efficiency of UDS-F solvent obtained from their experiments was only 80.3% for CS₂ exhaust, its removal effect was already significantly better than that of the conventional absorbent N-methyldiethanolamine (MDEA).

It is well-known that the screening of absorbents is critical to enhance the mass removal efficiency and reduce process energy consumption and cost. Previously, some scholars have proposed methods for screening absorbents or evaluating absorbents’ merits and demerits [31,32]. An absorbent screening method was established by Wang et al. [33] with SO₂ absorption capacity and desorption reaction heat values as indicators. Yet their method is inappropriate for the screening of absorbents for physical absorption processes. In addition, Lhuissier et al. [34] established a mass transfer model (i.e., SSR model) for predicting the total transfer coefficient of a packed tower. They successfully predicted the removal rate of VOCs, such as ethyl acetate, isopropanol, and toluene, from transformer oil and lubricant oil with the help of the SSR model. Rodriguez Castillo et al. [35] derived Henry’s law constants between different ionic liquids and toluene or dichloromethane by measuring the liquid–gas partition coefficients at equilibrium and then measured the diffusion coefficients of the gas molecules in the ionic liquid using a thermogravimetric microbalance (IGA-003). Finally, they used the above physical parameters obtained from these experiments to simulate the absorption process and evaluate the removal of toluene and methylene chloride from 23 different ionic liquids. Wang et al. [36] used IGA-003 to evaluate the absorption effect of dichloromethane in [Bmim][PF6] through a similar research approach. The difference is that they obtained data on the solubility of dichloromethane in [Bmim][PF6] at different pressures and then used a thermodynamic model fit to obtain Henry’s law constant. Although the scholars mentioned above provided three different methods for screening or evaluating absorbents, each requires extensive experiments to provide mass transfer data. This experimental process is costly and time-consuming; for example, each set of solubility data in the study of Wang et al. [36] took over 72 h. Meanwhile, IGA-003 requires a very low saturation vapor pressure of the solvent being measured, which limits its use in assessing the physical property data between organic solvents and exhaust gases. Furthermore, to our knowledge, no scholars have proposed a simple sorbent screening method in which CS₂ was selected as the primary treatment target.

In this paper, the CS₂ absorbers were effectively screened based on the theory of excess Gibbs function and absorption potential. Then, the feasibility of this theoretical prediction method was verified through the absorption experiment. Moreover, the absorption and desorption process parameters were experimentally optimized, laying the foundation for further industrial applications. The study results provide specific theoretical support for the resource recovery of CS₂ waste air from viscose fiber plants and also provide an academic reference for the design of absorber compound formulations.

2. Materials and Methods

2.1. Experimental Reagents

Table 1. Reagent specifications.

Experiment Reagent	Purity
N-Methyl-2-pyrrolidone (NMP)	99.7%
Ethylene glycol butyl ether acetate (BGA)	99.7%
Ethylene glycol (EG)	99.7%
N,N-Dimethylformamide (DMF)	99.7%
Ethylene glycol monobutyl ether (2-Butoxyethanol, BE)	99.7%
1,2,4-Trimethylbenzene (TMB)	99.5%
Ethanol absolute	99.7%
Triethanolamine (TEA)	99.7%
Diethylamine (DEA)	99.7%
Copper acetate	99.7%
Karl Fischer reagent	99.7%
CS2	99.5%

Note: The manufacturing company is Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

presents the reagents utilized in the experiments along with their respective specification parameters.

2.2. UNIFAC Model Simulation Calculation

The excess Gibbs function $(G^E)$ of the binary system is expressed as Equation (1).

$$\begin{array}{l}{G}^E=RT\left(n_1\ln \frac{{\xi }_{11}}{x_1}+n_2\ln \frac{{\xi }_{22}}{x_2}\right)\\ {\xi }_{11}=\frac{x_{11}{V}_{m1}}{x_{11}{V}_{m1}+x_{21}{V}_{m2}};{\xi }_{22}=\frac{x_{22}{V}_{m2}}{x_{12}{V}_{m1}+x_{22}{V}_{m2}}\end{array}$$

(1)

where $x_i$ is $i$-molecular macromolecular fraction, ${\xi }_{11}$ and ${\xi }_{22}$ are local volume fractions, $x_{ij}$ is the local molecular fraction of $j$-molecules around $i$-molecules, and $V_{m1}$ and $V_{m2}$ both represent the local molecular volume.

If $V_{m1}\approx V_{m2}$, ${\xi }_{11}\approx x_{11},{\xi }_{22}\approx x_{22}$ can be obtained according to Equation (1). When the attraction between different molecules is stronger than that between the same molecules, then ${\xi }_{11}<x_1,{\xi }_{22}<x_2$. The further conclusion is that $G^E<0$, and the smaller $G^E$ illustrates the stronger attraction between dissimilar molecules. The maximum excess Gibbs function $(G_{\max }^E)$ is an extreme point of $G^E$ that exists under a certain temperature and pressure. The value of $G_{\max }^E$ is inversely proportional to the affinity between the absorbent molecules.

The absorption potential $({\psi }_i)$ of the binary system is calculated by Equation (2).

$${\psi }_i=\frac{1}{{\gamma }_i^{\infty }}$$

(2)

where ${\psi }_i$ is the absorption potential, and ${\gamma }_i^{\infty }$ is the infinite dilution activity coefficient of the $i$-component in the binary system. ${\psi }_i$ can reflect the solubility of a component in a solvent within a certain range; a larger value of ${\psi }_i$ shows a better compatibility of the corresponding solute with the solvent.

$G_{\max }^E$ and ${\psi }_i$ can be further calculated from the liquid–gas equilibrium data in the full concentration range of the binary system calculated by the UNIFAC model in Aspen Plus V11. Herein, $G_{\max }^E$ and ${\psi }_i$ of the binary system composed of CS₂ and 30 pure solvents (water, alcohols, glycol ethers, esters, amines, aromatic hydrocarbons, etc.) were calculated, respectively. In order to verify the feasibility of extending the absorbent screening theories of $G_{\max }^E$ and ${\psi }_i$ into the screening of CS₂ waste air absorbents, we selected the solvents from the abovementioned solvents for absorption experiments.

2.3. Experimental Method

2.3.1. Absorption Experiment Process

The specified concentration of CS₂ waste air was simulated by bubbling. The CS₂ waste air flows from bottom to top in the tower, and the absorbent contacts the gas countercurrent in the tower. After the absorption treatment, the tail gas enters the condensation system, and the absorption-rich liquid is extracted from the discharge port of the lower section of the absorption tower.

2.3.2. Experimental Analysis

The concentration of CS₂ gas was measured by DEA spectrophotometry (GB/T 14680-93) [37,38]. The specific measurement method is as follows:

(1)

Preparation of absorption solution: 0.0500 g of copper acetate was dissolved in anhydrous ethanol in a 100.0 mL volumetric flask, and the volume was fixed and stored in a refrigerator at low temperature. After adding 300.0 mL of absolute ethanol to the 500.0 mL volumetric flask, 10.0 mL of copper acetate ethanol solution, 2.5 mL of DEA, and 2.5 mL of triethanolamine were added to the solution in sequence. After the above operations, anhydrous ethanol was used to make up the volume to the mark to obtain the absorption solution. (The above solutions need to be used and prepared now.)

(2)

Preparation of CS₂ standard solution: 15.0 mL of absolute ethanol and 1 to 2 drops of CS₂ were sequentially added to a 25.0 mL volumetric flask, and the mass was recorded (accurate to 0.0001 g) with the stopper closed. Absolute ethanol was used to bring up to the mark and calculate the concentration of CS₂. The above solution was then diluted with absolute ethanol to contain about 10.00 μg of CS₂ standard solution per milliliter.

Draw the CS₂ standard curve: The standard solutions in

Table 2. CS₂ content corresponding to different standard solutions.

No.	0	1	2	3	4	5	6	7	8	9	10
Absorbent (mL)	10.0	9.9	9.8	9.7	9.6	9.5	9.4	9.3	9.2	9.1	9.0
Standard solution (mL)	0	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8	0.9	1.0
CS2 content (μg)	0	3.92	7.84	11.76	15.68	19.60	23.52	27.44	31.36	35.28	39.20

were prepared in 10 colorimetric tubes (10.0 mL) with stoppers. After each tube was mixed evenly, it was placed for 20 min. The absorbance of different samples was measured with absolute ethanol as a reference (435 nm, 3 cm cuvette). The CS₂ standard curve was plotted as shown in Figure 1.

2.3.3. CS₂ Gas Mass Concentration

Gas sampling was carried out using an absorbing liquid in an ice-water bath, the sample was placed at room temperature, and the sample liquid was prepared. Take an appropriate amount of sample into a 10.0 mL colorimetric tube with a stopper, add absorbing solution to the mark, and shake well. Then, follow the steps of drawing a standard curve, and check the content of CS₂ on the standard curve.

The formula for calculating the mass concentration of CS₂ gas is shown in Equation (3).

$$C=\frac{m{V}_1}{V_2{V}_n}$$

(3)

where $C$ is the concentration of CS₂ in the measured gas, mg·m⁻³; $m$ is the amount of CS₂ in the sample liquid taken during the sample measurement, μg; $V_1$ is the constant volume of the sample solution, mL; $V_2$ is the volume of the sample solution taken during the measurement, mL; and $V_n$ is the gas production volume in the standard state, L.

2.3.4. Mass Removal Efficiency for CS₂

The calculation formula of the CS₂ mass removal efficiency is shown in Equation (4) [34].

$${\eta }_1=\frac{C_{in}-C_{out}}{C_{in}}\times 100\%$$

(4)

where ${\eta }_1$ is the mass removal efficiency for CS₂; $C_{in}$ is the intake concentration for CS₂, mg·m⁻³; and $C_{out}$ is the CS₂ waste air concentration, mg·m⁻³.

2.4. Absorption and Desorption Process

2.4.1. Absorption Process

The absorption experiment device shown in Figure 2 is used in the study of the absorption process. The influence of process parameters such as liquid–gas ratio, absorbent temperature, and intake air concentration on the mass removal efficiency, is considered, and the optimal absorption process conditions are determined through multiple sets of experiments.

• The effect of the liquid–gas ratio on mass removal efficiency

Under the condition that other factors remain unchanged, the liquid–gas ratio is adjusted by fixing the gas flow rate and adjusting the liquid flow rate. The other relevant experimental conditions are as follows: the intake air concentration is 8000 mg·m⁻³, the gas flow rate is 0.24 m³·h⁻¹, and the absorption temperature is 20 °C.

2.

The effect of temperature on the mass removal efficiency

When investigating the effect of different temperatures on the mass removal efficiency of CS₂, the remaining relevant experimental conditions are: the intake air concentration is 8000 mg·m⁻³ and the liquid–gas ratio is 3.75 L·m⁻³.

3.

The effect of intake air concentration on the mass removal efficiency

When investigating the effect of different intake air concentrations on the CS₂ mass removal efficiency, the remaining relevant experimental conditions are: temperature 20 °C, liquid–gas ratio 3.75 L·m⁻³.

2.4.2. Factors Affecting the Resorption Performance of Regeneration Lean Liquid

Generally speaking, the water content and CS₂ content in the absorbent are the main factors affecting the absorption performance of the regeneration lean liquid. Using the absorption apparatus depicted in Figure 2, this study investigated the influence of water and CS₂ contents on the mass removal efficiency of the regenerated lean liquid. Considering that the water and CS₂ contents in the absorbent have a synergistic effect on the mass removal efficiency, 0.5‰ of CS₂ was added to the absorbent with different water contents to investigate the synergistic effect on the mass removal efficiency (the intake air concentration was 8000 mg·m⁻³; the liquid–gas ratio was 3.75 L·m⁻³; the temperature was 20 °C).

The moisture content was measured using a Karl Fischer method trace moisture analyzer (BYES-8, Bangyi Precision Measuring Instrument Co., Ltd., Shanghai, China), and the formula for calculating the moisture content is (Equation (5)):

$$W=\frac{T\times V}{m}\times 100\%$$

(5)

where $W$ is the moisture content, %; $T$ is the titer of Karl Fischer reagent in water, g/mL; $V$ is the injection volume, mL; and $m$ is the quality of the sample, g.

The content of CS₂ in the liquid before and after desorption was measured by UV spectrophotometer. The calculation formula of CS₂ desorption rate is (Equation (6)):

$${\eta }_2=\frac{m_1-m_2}{m_1}\times 100\%$$

(6)

where ${\eta }_2$ is the CS₂ desorption rate, %; $m_1$ is the CS₂ content in the rich liquid before desorption, g; and $m_2$ is the residual CS₂ content in the lean liquid after desorption, g.

2.4.3. Desorption Process

The main factors that influence the desorption effect are temperature, pressure, and time. To simulate the absorption of rich liquid, 1% water and 1.5‰ CS₂ are added to the absorbent. The effect of temperature on the analytical effect is analyzed (10 kPa, 20 min). The effect of pressure on the analytical effect is analyzed (115 °C, 20 min). The effect of time on the analytical effect is analyzed (115 °C, 15 kPa).

3. Results and Discussion

3.1. Analysis of Absorbent Screening Results

$G_{\max }^E$ and ${\psi }_i$ of various solvent and CS₂ composition systems calculated by Aspen Plus V11 are shown in

Table 3. CS₂-solvent system maximum excess Gibbs function and absorption potential (20 °C, atmospheric pressure).

No.	Solvent	Molecular Formula	${\mathit{\gamma }}_{\mathit{i}}^{\mathit{\infty }}$	${\mathit{\psi }}_{\mathit{i}}$	${\mathit{G}}_{\mathbf{max}}^{\mathit{E}}$/J·mol−1
1	Water	H2O	3979.458	2.513 × 10−4	3157.89
2	EG	C2H6O2	1.092	0.916	59.37
3	1-Pentanol	C5H12O	2.470	0.405	764.03
4	1-Octanol	C8H18O	1.530	0.654	428.75
5	Tripropylene glycol	C6H14O4	1.876	0.533	788.37
6	Hexylene glycol	C6H14O2	3.276	0.305	1175.25
7	Dibutyl ether	C8H18O	1.021	0.980	28.21
8	BE	C6H14O2	0.511	1.959	−518.43
9	Diethylene glycol butyl ether	C8H18O3	0.548	1.824	−514.26
10	Triethylene glycol monobutyl ether	C10H22O4	1.205	0.830	301.70
11	Triethylene glycol dimethyl ether	C8H18O4	1.259	0.794	283.23
12	Ethylene glycol monomethyl ether	C3H8O2	0.769	1.300	−180.79
13	Ethylene glycol propyl ether	C5H12O2	0.568	1.762	−423.45
14	Ethylene glycol monoethyl ether	C4H10O2	0.648	1.544	−312.85
15	Propylene glycol monoethyl ether	C5H12O2	2.420	0.413	784.49
16	Hexyl acetate	C8H16O2	1.341	0.746	305.36
17	Propylene carbonate	C4H6O3	4.467	0.224	1110.05
18	Ethylene glycol diacetate	C6H10O4	3.017	0.331	1071.45
19	BGA	C8H16O3	1.403	0.713	370.59
20	Ethylene glycol monoethyl ether acetate	C6H12O3	1.904	0.525	602.54
21	Ethyl acetoacetate	C6H10O3	2.952	0.339	995.69
22	Methyl salicylate	C8H8O3	3.386	0.295	1167.32
23	DMF	C3H7NO	0.971	1.030	−18.01
24	MDEA	C5H13NO2	3.821	0.262	852.26
25	Diisopropanolamine	C6H15NO2	2.272	0.440	945.29
26	Diethanolamine	C4H11NO2	1.902	0.526	645.66
27	TEA	C6H15NO3	4.263	0.235	1636.44
28	NMP	C5H9NO	0.838	1.194	−132.00
29	Morpholine	C4H9NO	0.890	1.123	−83.30
30	TMB	C9H12	0.892	1.121	−64.61

. As can be seen from Table 3, the absorption capability follows the order of esters < alcohols < amines < heavy aromatics < glycol ethers. Generally, according to the principle of “similarity-solubility”, the order of the above should be glycol ethers > heavy aromatics > esters > alcohols > amines. However, CS₂ is a nonpolar reagent with a linear geometry similar to CO₂. It interacts with polar molecules via van der Waals forces and dipole–dipole interactions, which may be responsible for the superior affinity of CS₂ even in partially polar solvents. In fact, reports of the increased physical uptake of CO₂ resulting from van der Waals and dipole–dipole forces are commonplace [39,40].

The average mass removal efficiency results obtained after multiple groups of absorption experiments for NMP, BE, BGA, TMB, DMF, and EG are shown in Figure 3. The order of the mass removal efficiency of each solvent on CS₂ waste air is as follows: NMP > TMB > DMF > BE > BGA > EG. Combining with the data in Table 3, it can be seen that in addition to BE and EG, the mass removal efficiencies of NMP, TMB, DMF, and BGA on CS₂ waste air are consistent with the trends predicted by $G_{\max }^E$ and ${\psi }_i$. Combining the viscosity data of the six absorbents given in

Table 4. Physical properties of absorbent.

Absorbent	Viscosity/mPa·s	Saturated Vapor Pressure/Pa	ORL-RAT LD50/mg·kg−1
NMP	2.08	31.5	3915
BE	3.35	77.5	1480
TMB	0.96	206.5	5000
DMF	0.87	394.3	2800
BGA	1.80	80.9	2400
EG	21.05	7.3	4700

Remark: The temperature is 20 °C; the pressure is atmospheric pressure; the LD50 test method is oral administration by rodents.

, it can be found that compared with NMP, TMB, and DMF, BE has good affinity, but its viscosity is large. Similarly, the viscosity of EG is greater than that of BGA. Overall, the greater viscosity leads to greater interfacial mass transfer resistance between the liquid–gas two phases, which in turn leads to a decrease in mass transfer rate and a poorer absorption effect [41]. Therefore, the mass removal efficiency of BE and EG on CS₂ waste air has an opposite trend to the predicted effect of $G_{\max }^E$ and ${\psi }_i$.

In conclusion, in the low viscosity system, it is feasible to use $G_{\max }^E$ and ${\psi }_i$ to screen CS₂ absorbents, and it can greatly reduce the complicated screening process and expensive screening costs in previous studies. At the same time, it can be seen that NMP is a CS₂ waste air absorbent with good compatibility with CS₂ waste air, low volatility, and slight toxicity.

3.2. NMP Solution Absorption Process Conditions

The NMP absorption process study using experiments is shown in Figure 4. It can be seen from Figure 4a that when the liquid–gas ratio does not reach 3.75 L·m⁻³, the CS₂ mass removal efficiency increases significantly, and the change tends to be gentle after reaching 3.75 L·m⁻³. The improvement of the liquid–gas ratio increases the effective contact area between the CS₂ gas and the NMP liquid, thereby improving the mass transfer efficiency of the liquid–gas interface. However, when the liquid–gas ratio is increased to a certain level, the effective contact area is already large enough. The effect of further improving the liquid–gas ratio on the mass removal efficiency is not obvious, while the energy consumption continues to increase.

It can be seen from Figure 4b that increasing the temperature will lead to a decrease in the mass removal efficiency, and the decrease in the mass removal efficiency is particularly significant when the temperature exceeds 20 °C. The main reason for this phenomenon is that the increase of temperature will increase the kinetic energy of each molecule in the absorbent, and it will both weaken the interaction between CS₂ and NMP molecules and increase the desorption rate, resulting in a decrease in the absorption of CS₂ molecules by NMP.

Figure 4c shows the experimental results of the absorption of CS₂ waste air with six different intake concentrations. It can be seen from Figure 4c that the mass removal efficiency of NMP to CS₂ first increases and then decreases sharply with the increase of the concentration. According to double film theory, the increasing intake air concentration of CS₂ will increase the CS₂ partial pressure in the gas film and enhance the driving force of the mass transfer process, promoting the CS₂ waste air absorption process. However, when the intake air concentration of CS₂ is more significant than 10,000 mg·m⁻³, the dissolution capacity of the absorbent for CS₂ is limited by the solubility, i.e., the absorber reaches saturation, resulting in a sharp decline in the mass removal efficiency. Meanwhile, the similar conclusion was reached by Qing et al. [42] when they studied the effect of CO₂ inlet concentration on the CO₂ removal rate.

Figure 4d shows the results of three groups of absorption experiments carried out under the above process conditions, and the average mass removal efficiency is up to 95.2%. To sum up, the optimal process conditions for NMP to absorb CS₂ waste air are the liquid–gas ratio of 3.75 L·m⁻³, a temperature of 20 °C, and an intake air concentration lower than 10,000 mg·m⁻³.

3.3. Analysis of Desorption Experiment Results

3.3.1. Regeneration of Lean Liquid Resorption Performance

Figure 5a,b show the effects of water content and CS₂ content in NMP on the mass removal efficiency, respectively. Figure 5c shows the results of the synergistic effect of water content and CS₂ content in NMP on the mass removal efficiency. In Figure 5a,b, the mass removal efficiency decreased sharply with the increase of water content and CS₂ content in NMP. When the moisture content exceeds 1.00%, or the CS₂ content exceeds 0.5‰, the mass removal efficiency will drop to less than 90.0%.

In Figure 5c, the CS₂ content in the fixed NMP was kept constant at 0.5%. The water content was controlled within 0.5%, and the mass removal efficiency can be maintained above 89.0%. Therefore, to ensure that the desorbed absorbent retains excellent absorption, the CS₂ content in the regenerated absorbent must be controlled within 0.5% and the water content within 0.50%. Comparing Figure 5b,c, it can be seen that NMP containing CS₂ and water at the same time will make the mass removal efficiency drop more obviously, that is, the effect of the two on the mass removal efficiency has a synergistic effect. The effect of water content on CS₂ uptake by NMP is similar to the findings of Fu et al. [43] on the effect of water content on CO₂ uptake.

3.3.2. Desorption Process Conditions

Figure 5d,e show the effects of temperature and pressure on the desorption effect, respectively. When the constant pressure is 10 kPa, the CS₂ desorption rate increases with the increase of temperature. When the temperature reaches 115 °C, the desorption rate is close to 99.0%. At this time, the CS₂ desorption is relatively complete. The water is continuously vaporized, and the water content in the rich liquid is continuously reduced. At a constant temperature of 115 °C, the lower the pressure, the better the desorption of the rich liquid, the higher the CS₂ desorption rate, and the lower the water content in the rich liquid. When the pressure is 10 kPa, the CS₂ desorption rate is 99.1%, and the water content is 0.55%. At this time, the regeneration lean liquid has good absorption performance after desorption. Figure 5f shows the effect of desorption time on the desorption effect. When the desorption time was 45 min, the CS₂ desorption rate reached 99.9%, and the water content in the regenerated lean liquid after desorption was 0.38%. At this time, the desorbed absorbent had good absorption performance, and the mass removal efficiency could be maintained above 90%.

Altogether, the optimal desorption process conditions for absorbing rich liquid are: temperature 115 °C, pressure 10 kPa, and desorption time 45 min. Figure 6a shows three groups of desorption experiments carried out under the abovementioned process conditions; the average desorption rate reaches 99.6%, and the average water content after desorption is 0.39%.

3.3.3. NMP Absorbent Cyclic Absorption Performance

Figure 6b shows the experimental results of multiple absorptions of CS₂ exhaust gas with the regenerated and unregenerated absorber. As shown in Figure 6b, when the NMP-rich liquid without desorption is subjected to the cyclic absorption experiment, it no longer has the absorption capacity after four absorption operations. However, the mass removal efficiency of the desorbed NMP lean liquid can reach more than 90.0% in the first three times, and the mass removal efficiency of the regenerated lean liquid after seven times of desorption can still be maintained at 80.6%. Due to incomplete regeneration and continuous loss of absorbent during regeneration, the mass removal efficiency of the regeneration lean liquid shows a trend of gradual deterioration. In engineering, the mass removal efficiency of regeneration lean liquid can be restored by intermittently replenishing fresh NMP solvent.

4. Conclusions

In this paper, the theory of the maximum excess Gibbs function and the absorption potential for screening absorbents were successfully introduced to the process of CS₂ waste air absorbent screening. Based on the absorption experiments, we concluded that it is feasible in low-viscosity systems. At the same time, NMP with slight toxicity and easy regeneration was selected as a highly efficient absorbent to recycle CS₂ (95.2%). Our method of screening absorbents is simpler and more efficient than previous methods of evaluating absorbents and has a broader range of applications [34,35,36]. Moreover, the optimal process conditions for NMP to absorb CS₂ waste air are: liquid–gas ratio 3.75 L·m⁻³, temperature 20 °C, and intake air concentration lower than 10,000 mg·m⁻³. The regenerated lean liquid has excellent resorption performance and can be recycled.

In summary, we simplify the screening steps of CS₂ waste air absorbent in a low-viscosity system, contributing to efficiently screening absorbents with low operating cost. Meanwhile, we believe that further design of low-cost composite absorbers should also prioritize NMP as the primary component. This paper has significant application value for recycling CS₂ waste air in viscose fiber plants and reducing CS₂ pollution and carbon emissions from the source.