Tertiary amine refers to the amine with trivalent group connected with three alkyl groups in the molecule.Tertiary amine has a wide range of uses, which can be used as a component of prepared products, as well as an intermediate product of various special chemical derivatives, and is an important raw material for the production of quaternary ammonium salts.
carbon dioxideCapture is to achieve carbon emission reductionimportantOne way.CO2 capture technology mainly includes physical absorption method, membrane separation method, chemical absorption method, etc. Among them, chemical solvent absorption method is the oldest, mature and widely used decarbonization method.Chemical absorption methodIt takes advantage of the acidic characteristics of CO2, adopts alkaline solution for acid base chemical reaction absorption, and then uses reverse reaction to realize solvent regeneration.Strong alkali, such as K2CO3, can also be used as solvent and can be heated for regeneration, but this solvent has serious corrosion on the system.In industry, the chemical absorption method mainly uses the aqueous solution of organic alcohol amine as the absorbent, and uses the system consisting of absorption tower and regeneration tower to capture CO2.Several common alcoholamines in industry are primary amine MEA (monoethanolamine) and secondary amine DEA（Diethanolamine）And tertiary amine MDEA（N-Methyldiethanolamine) and TEA（Triethanolamine）Etc.CO2 capture by various amines has its own advantages and disadvantages. The absorption rate of primary amine and secondary amine is fast, but it is easy to generate stable carbamate, and the regeneration degree is low;The tertiary amine has good regeneration performance, but its absorption rate is slow.Mixed amine absorption solution can take advantage of the advantages of single component organic amine and make up for the disadvantages. High efficiency and low consumption composite absorbent is the development trend of absorbent.
MEA is the earliest and most widely used absorbent for industrial production, and optimization of MEA absorbent is one of the current concerns.For absorbent performance only, "high efficiency and low consumption" mainly refers to high absorption and low energy consumption (i.e. high regeneration performance).In view of the high regeneration performance of tertiary amine, which can make up for the shortcomings of MEA in this respect, the mixed amine (MEA/tertiary amine) absorbent composed of MEA and tertiary amine has received extensive attention. These studies mainly focus on the mass transfer process (kinetics) of CO2 absorption.For the study of capture performance, MDEA and TEA are mainly studied for tertiary amine, and the remaining tertiary amine is less studied[1]。
Experimental materials and methods
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1 Reagent
ExperimentalChemical Reagents。Prepare with pure amine and distilled water: ① single organic amine aqueous solution with a concentration of 2 mol/L, including MEA, DEAE, DMAE and MDEA aqueous solutions; ②The total concentration of MEA/DEAE, MEA/DMAE, MEA/MDEA mixed amine aqueous solution is 2 mol/L. The molar ratio of MEA to tertiary amine in the mixed amine solution is 4:1.
2 Experimental Equipment and Methods
(1) Experimental device
The absorption and regeneration rate of CO2 by absorption solution is measured by an experimental device.Constant temperature oil bath is measured by DF-101XP collectorA magnetic stirrer(Jintan Zhengji Instrument Co., Ltd.) and oil composition.175-T2Digital thermometer(Detu Instrument Co., Ltd.) and D-600CDMass flowmeter(Dongguan Dexin Electronic Technology Co., Ltd.) is controlled by a computer, and temperature and flow data are collected every 2 seconds.
(2) Experimental methods
Put the absorption bottle with 100 mL absorption solution into the constant temperature oil bath, and set the stirring speed to 180 r/min.When the temperature of the absorption solution reaches 40 ℃, the pure CO2 gas is introduced into the absorption bottle.Inlet gas flow is determined byGas mass flowmeterSet the control, this experiment is 400mL/min, and the CO2 flow at the outlet is recorded by the flowmeter. When it is close to the inlet flow, stop the experiment, and this is the saturated absorption point for absorbing CO2.The absorption amount is obtained by integrating the absorption rate with the absorption time.
Conduct regeneration experiment with CO2 saturated rich solution at 110 ℃, and stop the experiment when the outlet flow rate is lower than 5 mL/min[2]。
Results and discussion
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1 Absorption/regeneration of organic monoamine
(1) Absorption comparison of organic monoamine
CO2 absorption rate, adsorption amount, absorption temperature and absorption time of 2 mol/L MEA, DEAE, DMAE and MDEA aqueous solutions.The absorption rate and amount of MEA are much higher than those of tertiary amine in about 8 min at the beginning of absorption, and the order of magnitude is MEA>DEAE>DMAE>MDEA. Then the order of absorption rate and amount of MEA and tertiary amine is MEA<MDEA<DMAE<DEAE.
Temperature change is the reflection of the degree of reaction, and a large temperature change indicates a large degree of reaction.The temperature rise degree of the reaction process of MEA absorption solution is far greater than that of tertiary amine, and the reaction degree of MEA and CO2 is large, so the absorption rate is far greater than that of tertiary amine.In addition, MEA has only one temperature peak (heating), indicating that its absorption of CO2 is a chemical reaction, that is, it generates relatively stable carbamate, so the absorption rate is fast.H2O acts as a base catalyst.
The absorption of CO2 by tertiary amine is composed of multiple absorption processes, and the "reaction" degree of each absorption process is only slightly weakened, so the absorption rate is basically unchanged.The pH values of 2 mol/L MDEA, DMAE and DEAE absorption solutions were lower than 13.0.The research shows that the absorption of CO2 by tertiary amine solution with pH lower than 13.0 is mainly to use a pair of lone pair electrons of N atom on amine molecule to form a hydrogen bond between free amine and water, which enhances the reaction activity of H2O and CO2. Amine acts as an alkali catalyst to promote CO2 hydration, and produces unstable HCO3, so the heating is low, the reaction degree is low, and the rate is slow.Since H2O is always sufficient in the absorption solution, the reaction continues until the tertiary amine is exhausted, showing that the absorption rate of tertiary amine has not changed much, that is, it exceeds MEA in the later stage.
The maximum theoretical load of MEA on CO2 is 0.5 mol.CO2/molMEA，The experiment shows that the load of MEA is greater than the theoretical load, because H2O not only catalyzes MEA, but also absorbs CO2 itself;The maximum load of tertiary amine on CO2 is 1 mol CO2/mol tertiary amine, but the measured value is less than this value, one of the reasons is that the absorption reaction speed is too slow and the reaction is not complete.Although the absorption load of tertiary amine is higher than that of MEA, the initial absorption rate is too low to be used as industrial absorbent alone.
(2) Comparison of organic monoamine regeneration
The regeneration rate gradually increased at first, reached the highest value and then gradually decreased until the end of the reaction.It can be seen from the regeneration rate that the tertiary amine is significantly higher than MEA, because regeneration is the reverse reaction of absorption, and the salt formed by the tertiary amine absorbing CO2 is unstable and easy to decompose.Tertiary amine is completely regenerated in the shortest time, while MEA requires a long time to completely regenerate, and the regeneration degree is the lowest.The final regeneration amount is DEAE>DMAE>MDEA>MEA;The regeneration amount of tertiary amine is in the same order as the absorption amount, because when the same type of amine is absorbed and regenerated, the more the absorption amount is, the more the regeneration amount will be in sufficient time.The regeneration rate reflects the regeneration degree, which is different from the regeneration amount in the order of MDEA>DEAE>DMAE>MEA. MDEA has become a research hotspot in recent years because of its high regeneration rate and regeneration rate.
Absorption and regeneration performance of 2 MEA/tertiary amine mixed amine
(1) Absorption properties of MEA/tertiary amine
The absorption rate and amount of mixed MEA/tertiary amine and the temperature change in the absorption process.The mixed solution of MEA/tertiary amine has the absorption characteristics of both MEA and tertiary amine.Since the chemical reaction rate of MEA and CO2 is greater than that of tertiary amine, the absorption rate of mixed amine in the initial absorption stage is mainly determined by MEA, while in the later stage, it is mainly determined by tertiary amine with slow reaction rate, and the absorption amount is the same.Therefore, the initial absorption rate of MEA/tertiary amine is less than MEA, and the later absorption rate is opposite.Since the reaction degree between MEA and CO2 is much higher than that of tertiary amine, the temperature change of mixed amine is similar to that of MEA.
At the initial stage of absorption, the absorption amount of MEA/tertiary amine at the same concentration is less than MEA, but higher than MEA at the later stage. The total absorption amount is shown in Table 4. The absorption amount of MEA/tertiary amine is greater than MEA, of which MEA/MDEA is the highest, 1.13 times that of MEA.It was also found that monoamine DEAE and DMAE had obvious foaming phenomenon when absorbing CO2;Their mixed amine with MEA also has low foaming property, while the mixed amine of MEA and MDEA does not foam.
(2) Regeneration performance of MEA/tertiary amine
It can be seen from the regeneration of mixed MEA/tertiary amine that the desorption rate of MEA/tertiary amine is significantly higher than that of single component MEA;In the same desorption time, the desorption load MEA/MDEA>MEA/DMAE>MEA/DEAE>MEA, MEA/MDEA desorption effect is better.,The regeneration rate of MEA/tertiary amine mixed system is much higher than that of single component MEA, indicating that the energy consumption of mixed amine regeneration is significantly lower than that of MEA.
3 Interaction between MEA and tertiary amine
The mixed amine absorption solution not only has the absorption characteristics of its constituent amine, but also has interaction.As a base, tertiary amine can catalyze the absorption of MEA, so there is interaction between them.The interaction coefficient is calculated by the enhancement factor, which is complex.
There is a strong interaction between MDEA and tertiary amine, that is, MEA and tertiary amine mutually promote absorption, and the interaction between MEA and MDEA is the strongest.The interaction is also related to the absorption time, which gradually increases and then decreases with the absorption.The interaction between mixed amine regeneration process and absorption process in the regeneration process is completely different, and the interaction decreases quickly with the initial regeneration time, and then increases.The synergy between MDEA and MEA is the strongest in both absorption and regeneration processes.
From the perspective of industrial application, the appropriate absorption solution should be selected. The solvent that can absorb as much CO2 as possible at the initial stage of absorption and regeneration should be selected, and its regeneration rate and regeneration amount should be large enough.MEA and tertiary amine meet the requirements of the former and the latter respectively, and are not suitable for industrial application if used alone.When mixed, the absorption capacity and regeneration capacity are increased, but the absorption rate and initial absorption capacity are low, which is not conducive to industrialization.The regeneration synergy between MEA and tertiary amine is strong, which can give play to the advantages of MEA absorption and improve the regeneration amount and regeneration rate.From the selected tertiary amine, MDEA can better optimize MEA[1]。
summary
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(1) The absorption of CO2 by single component organic amine shows that the absorption rate of MEA is fast, but regeneration is difficult;Although the absorption rate of tertiary amine is low and the absorption saturation time is long, the absorption amount of tertiary amine is high and it is easy to regenerate.
(2) The absorption capacity and regeneration effect of three mixed MEA/tertiary amine amines with MEA as the main absorption body are better than that of MEA, but the absorption rate is lower.
(3) There is interaction between tertiary amine and MEA in the absorption and regeneration process, and the interaction is related to the type of tertiary amine and absorption time;Among them, the synergy between MDEA and MEA is the strongest.MEA/MDEA has good absorption and regeneration performance, and the absorption process is not foaming, so it is a good decarbonization absorbent[3]。
