CO2 removal in tray tower by using AAILs activated MDEA aqueous solution
Introduction
In recent decades, considerable attentions have been paid to climate change and carbon dioxide (CO2) emission and the reduction of CO2 has become a global issue [1,2]. Among various technologies for CO2 capture, the amine-based chemical absorption technology is considered to be the most robust and promising method for post-combustion CO2 capture due to their advantages of simple process, technological maturity and large absorption capacity [[3], [4], [5], [6]]. Conventional amines such as primary amine monoethanolamine (MEA), secondary amine diethanolamine (DEA) and tertiary amine N-methyldiethanolamine (MDEA) have been widely used for CO2 capture from a variety of industrial processes [[7], [8], [9], [10], [11], [12], [13]]. However, the drawbacks of aforementioned conventional amines aqueous solutions, including the high energy cost in regeneration or low rate in absorption, raise the urgent demand on new absorbents with low energy cost and high absorption rate for high efficient CO2 capture process. Reduction of water content in absorbent is a promising approach to decrease the regeneration energy cost. Recently, water lean solvent has been proposed to capture CO2. By replacing water with an organic cosolvent, water lean solvent takes the advantage of low water content [14]. Barzagli et al. [15] developed a water lean solvent consisting of 2-amino-2-methly-1-propanol (AMP), piperazine (PZ), ethyl glycol monomethyl ether (EGME) and approximately 15 wt% water. When applied to CO2 absorption, this water lean solvent showed high absorption efficiency and a relatively low regeneration temperature (353–363 K) at atmospheric pressure. Song et al. [16,17] developed a water lean solvent consisting of MEA, ethylene glycol (EG)/poly(ethylene glycol) (PEG) approximately 40 wt% water. This water lean solvent also demonstrated good CO2 absorption performance, e.g., at CO2 partial pressures lower than 10 kPa, the solubility of CO2 in MEA-EG aqueous solutions was higher than that in single MEA aqueous solution.
Application of amine blends as absorbent is also a commonly used approach to enhance the absorption performance and decrease the energy cost. It has been well documented that blending MDEA aqueous solution with primary, secondary or sterically hindered amines can overcome the drawbacks of single amine aqueous solution, because the blended amines preserve the high rate of the reaction of primary/secondary/sterically hindered amines with CO2 and the low enthalpy of the reaction of MDEA with CO2 [[18], [19], [20], [21], [22], [23]].
Another approach to improve the absorption performance of CO2 in MDEA aqueous solutions is to use the functionalized ionic liquids (FILs) as activators. The blends of FILs and MDEA preserve the desired property of FILs but without many of their inherent drawbacks such as high viscosity [[24], [25], [26], [27], [28], [29], [30], [31], [32], [33]]. Among FILs, amino acid ionic liquids (AAILs) are considered to be promising absorbents or activators for the removal of CO2 due to their unique characteristics like negligible vapor pressure, thermal stability and high CO2 solubility at atmospheric pressure [[34], [35], [36], [37], [38]]. AAILs are miscible with organic solvents and are able to dissolve native amino acids. They are composed of one cation and one amino acid anion which play important role in CO2 uptake and physical properties [39,40]. The cations of AAILs usually are groups like tetramethylammonium ([N1111]), 1-butyl-3-methylimidazolium ([Bmim]), 1-hexyl-3-methylimidazolium ([Hmim]). The amino acid anions usually include gylcinate ([Gly]), lysinate ([Lys]), valinate ([Val]), alanate ([Ala]) and so on. Previous experiments showed that the enthalpies of CO2 absorption in both AAILs aqueous solution and MDEA aqueous solution are lower than those in aqueous solution of traditional amine like MEA [[41], [42], [43]], hence the lower energy cost in regeneration process could be achieved when using the blends of AAILs and MDEA as absorbent. Moreover, it is reasonable to expect a lower temperature rise in the rich AAILs-MDEA solution due to the lower enthalpy of CO2 absorption. Compared with MDEA aqueous solution, AAILs-MDEA aqueous solution achieved higher CO2 absorption capacity because AAIL can not only act as a promoter but also absorb a certain amount of CO2 [44]. By far, there are many studies concerning the absorption of CO2 in AAILs activated MDEA aqueous solution, and all these studies show that AAILs activated MDEA aqueous solution has good application potential in CO2 capture process [[45], [46], [47], [48], [49], [50], [51]]. For example, Zhang et al. [45,46] showed that the addition of tetramethylammonium glycinate ([N1111][Gly]), tetraethylammonium glycinate ([N2222][Gly]), tetramethylammonium lysinate ([N1111][Lys]), tetraethylammonium lysinate ([N2222][Lys]) into MDEA aqueous solution significantly accelerated the absorption rate. Gao et al. [47] investigated the equilibrium absorption of CO2 in [N1111][Gly]-MDEA aqueous solutions under a wide range of pressure at temperatures ranging from 298 K to 318 K, indicating that satisfactory CO2 absorption capacity can be achieved. In our previous works [[48], [49], [50], [51]], we presented basic data of both absorption capacity and absorption rate for pure and low partial pressure CO2 absorption in 1-butyl-3-methylimidazolium glycinate ([Bmim][Gly]), [N1111][Gly] and 1-butyl-3-methylimidazolium lysinate ([Bmim][Lys]) activated MDEA aqueous solution. Our results demonstrated the effects of temperature, partial pressure, solution concentration and solution viscosity on absorption capacity and absorption rate. Moreover, we modeled the viscosities of carbonated AAILs-MDEA aqueous solutions and revealed the competitive effects of solution concentration and solution viscosity on CO2 absorption performance. Although the previous work demonstrated that MDEA aqueous solution has large CO2 absorption capacity and high CO2 absorption rate when activated by AAILs, the experiments have been rarely performed in industrial CO2 absorption equipment like packed tower or tray tower to verify the CO2 removal efficiency.
At present, there are many types of absorption facilities applicable for CO2 removal in pilot plants. Among these absorption facilities, packed tower and tray tower are the most popular ones. Packed tower has been widely used to separate acid gas, and possesses many advantages over tray tower, such as high efficiency, low pressure drop, small tower diameter, simple in structure and suitable for heat-sensitive, foaming and corrosive systems [52]. Compared with packed tower, tray tower takes the advantages of no packing consumption, lower operation cost, larger superficial gas velocity, larger interfacial area for gas-liquid dispersion, larger range of vapor-liquid ratio, accurate and reliable design, larger operating range and easy cleaning [52,53]. Moreover, when liquid goes downward packed tower, the total surface of the packing in the direction of the diameter cannot be wetted, hence the contact area gets less and CO2 may flow upward without contact with liquid [53].
Some studies concerning this issue have been focused on the removal efficiency and operation condition in tray tower [[54], [55], [56], [57]]. Pineda et al. [54] developed methanol-based Al2O3 and SiO2 nanofluids as CO2 absorbents and they observed good CO2 absorption performance in tray tower absorber. Jaber et al. [55] investigated the CO2 removal efficiency in a single perforated sieve tray tower by using MEA absorbent, and they found that in tray tower the efficiency increased with increasing MEA concentration and better removal efficiency can be achieved by increasing the liquid flow rate and the contact time between CO2 and the absorbent. Borhani et al. [56] developed equilibrium and non-equilibrium models for CO2 removal from natural gas steam by DEA-promoted potassium carbonate solution in tray tower. Putra et al. [57] showed that up to 99% CO2 can be absorbed by MEA solvent in valve tray tower. Although the previous studies suggested that the use of tray tower would improve the CO2 absorption in different absorbents, CO2 removal efficiency in tray tower using AAILs-MDEA aqueous solution has been rarely reported. The effects of AAIL/MDEA concentration, gas flow rate, liquid flow rate and plate number on CO2 removal efficiency of CO2 () and the overall volumetric mass transfer coefficient (KGav) have not been well documented so far.
The main purposes of this work were to (1) experimentally determine and KGav in tray tower; (2) illustrate the effects of absorbent concentration, absorbent flow rate, gas flow rate and plate number on and KGav; (3) determine a suitable absorbent composition to meet the commercially required CO2 removal efficiency (e.g., 85% in Huaneng Beijing Power Plant).
Section snippets
Materials
[Bmim][Gly], [N1111][Gly], [Bmim][Lys] and MDEA were used without further purification. The sample description is shown in Table 1. CO2 (purity in mole> 0.999) and N2 (purity in mole>0.9999) were purchased from the Jinglian Gas Supply Inc. Aqueous solutions of AAILs-MDEA were prepared by adding high purity water (Electrical resistivity >15 MΩ cm at 298 K) made from Heal Force ROE (Reverse Osmosis Electrodeionization)-100 apparatus.
Apparatus and procedure
The experimental apparatus is composed of one high-pressure CO2
Results and discussion
and KGav under different liquid flow rate(L), gas flow rate(G), AAIL/MDEA concentration(w) and plate number (Np) are shown in Table 3, Table 4, Table 5, Table 6, Table 7, Table 8.
Conclusions
In this work, [Bmim][Gly]-MDEA, [N1111][Gly]-MDEA, [Bmim][Lys]-MDEA aqueous solutions were used as absorbents for CO2 removal. and KGav in tray tower were measured. The effects of gas flow rate, absorbent flow rate, plate number of tray tower and mass fractions of AAILs and MDEA on the removal efficiency and transfer coefficient were demonstrated. Suitable absorbent compositions were determined to achieve both high and KGav. Our results show that:
- (1)
Addition of AAIL into MDEA aqueous
Acknowledgments
The authors appreciate the financial support by the National Natural Science Foundation of China (No. 51776072), the Fundamental Research Funds for the Central Universities (No. 2018MS116) and the Natural Science Foundation of Hebei Province (No. E2018502062).
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