Separation of CO2 from Flue Gas via Absorption of Alcohol Amines

The separation of CO2 from flue gas by alcohol amine method was investigated in this paper. We conducted the absorption and desorption experiments using different alcohol amines: monoethanolamine (MEA), diethanolamine (DEA), methyl diethanolamine (MDEA), triethanolamine (TEA), piperazine (PZ), and their binary mixtures. Relevant absorption rate and desorption rate of CO2 were obtained and reported. Aspen Plus was applied to simulate the process of recovering CO2 from flue gas via absorption of PZ-MEA solution. The influences of the lean liquid CO2 load, lean liquid temperature and desorber pressure on the reboiler duty were investigated, and the optimal operating parameters were obtained on the basis of the minimum reboiler duty.


Introduction
The emission of greenhouse gases brings about the increasing sever greenhouse effect. The global average temperature has increased by 0.3 to 0.7 o C in the past century, and it may increase by about 3 o C at the end of 21 th century without effective actions to control the greenhouse effect. 1,2 Among all of the greenhouse emissions, CO 2 is the main component with a proportion more than 60 %, and the major source of the CO 2 in the atmosphere is the flue gas released from the combustion of fossil fuel. 3,4 Consequently, capture of CO 2 from flue gas could effectively mitigate the greenhouse effect, and make economic sense as well. 5 One of the most suitable and mature methods for capture of CO 2 from flue gas is the absorption using alcohol amines. [6][7][8][9][10] The most widely used amine is monoethanolamine (MEA), which is successfully applied in industry. [11][12][13] However, the limitation of MEA is high energy consumption in desorption process, which is reported as 4.2 to 5.2 kJ/(kgCO 2 ). 14,15 Consequently, we investigated the separation of CO 2 from the flue gas using different alcohol amines in this paper.

Materials and apparatus
The absorbents used in this work are MEA, diethanol amine (DEA), methyl diethanol amine (MDEA) and piperazine (PZ), which are supplied by Sinopharm Chemical Reagent Co., Ltd. The composition of the flue gas is CO 2 : 15 mol% and N 2 : 85 mol%. Figure 1 and 2 show the apparatus of the absorption and desorption experiments.

Experimental procedure
An appropriate amount of absorbent solution was first added into the reaction flask. The magnetic stirrer was turned on and the temperature was set to the experimental value. Then the flue gas was introduced into the flask with a constant flow rate of 4 ml/s. The gas flows of the two soap film flow meters were recorded with time. While the difference between the two meters was less than 5 ml/min, the absorption experiment was considered finished. Then the desorption experiment was conducted at atmospheric pressure. After the flow of the desorption gas, measured by the soap film meter, was less than 10 ml/min, the desorption process was considered as completed.

Calculation
The absorption rate (R a ) is defined as the number of moles of CO 2 absorbed in the solution per second. 16 It could be calculated by the following equation.
(1) (2) where n is the number of moles of the absorbed CO 2 in amine solution. V 1 and V 2 are the gas flows per minute measured by the soap film flowmeters, respectively. p, T are the given pressure and temperature of the soap film flowmeter. R is the gas constant with a value of 8.3145 J/(mol·K). The desorption rate (R d ) is defined as the number of moles of CO 2 desorbed from the absorption liquid per second. 17 It is obtained using the same calculation method.

Materials
The equilibrium stage model was used in the simulation. The compositions of flue gas are CO 2 : 9 mol%, N 2 : 88 mol% and O 2 : 3 mol%, and the flow rate is 54000 Nm 3 /h, which is given by Tarim Oilfield Petrochemical Company. The absorbent is PZ-MEA mixed solution with a total mass concentration of 30 wt%. The molar ratio of PZ and MEA in the absorption solution is 1:1. ELECNRTL equation is chosen as the property method for CO 2 -PZ-MEA-H 2 O system.

Calculation
The recovery of CO 2 (R) is calculated using the following equation: where F r and F f are CO 2 molar flow in the gas product of the desorber and feed gas, respectively. CO 2 load is defined as the number of moles of CO 2 absorbed by 1 mole alcohol amine. 18 There are rich liquid CO 2 load and lean liquid CO 2 load in the simulation. Rich liquid, i.e., high CO 2 content liquid, is the liquid out from the bottom of absorber. Lean liquid is the liquid entering into the absorber to absorb CO 2 from flue gas. The absorbent is PZ-MEA binary solution. One PZ molecule has two amidogens, and one MEA molecule has one amidogen. Consequently, one mole PZ could absorb two moles of CO 2 in theory, and one mole CO 2 for one mole MEA in the same way. CO 2 load ( ) is calculated as follows:

Results and Discussion
1. Absorption by single alcohol amine  First, we measured the absorption rates of flue gas in 0.5 mol/L MEA solutions at 30 o C, 40 o C and 50 o C, respectively. As shown in Figure 3, the absorption rate at 40 o C is the highest. The absorption rate increases with the temperature. However, higher temperature will inhibit the absorption as the reaction between MEA and CO 2 is exothermic. Consequently, we chose 40 o C as the optimal reaction temperature in this work. Figure 4 shows the absorption rates of flue gas in different MEA solutions at 40 o C. We can find that the absorption rate increases with the MEA concentration. However, higher concentration of amine brings about higher cost and faster erosion of equipment.
Consequently, the concentration of amine with more than 1 mol/L was not investigated in this work. Then, we measured the absorption rates of flue gas using various absorbents with the same concentration of 1 mol/L at 40 o C Figures 5 and 6 show R a and R d of flue gas using different single amines, respectively.  We can find that PZ and MEA have much higher adsorption rate of CO 2 than the other three amines. The initial adsorption rate of MEA is slightly higher than that of PZ, but the situation reverses after 60 min. It shows that the overall absorption effect of PZ is the best among the five amines. The adsorption rate of TEA and MDEA are the slowest, because they are tertiary amines and they have different reaction mechanism from the other three amines. 19 Due to the absence of hydrogen atoms around the nitrogen atom, TEA and MDEA can not form NCOOwith CO 2 . The reaction between tertiary amines and CO 2 could be expressed using equation (3). 20 It indicates that tertiary amines only behaves as the catalyst of CO 2 hydrolysis. Consequently, the 28 Separation of CO2 from Flue Gas via Absorption of Alcohol Amines reaction rate is much slower than that between primary amine, secondary amine and CO 2 at the same conditions. 21 (3) Figure 6 shows that the maximum R d appears at about 22 minutes after the desorption reaction starts. The desorption time of TEA/CO 2 and MDEA/CO 2 are relatively short, i.e., the desorption of TEA/CO 2 and MDEA/CO 2 are relatively easy. 1 mol/L. Based on the experiments in this work, we found that the binary amines with equal concentration, i.e., 0.5 mol/L-0.5 mol/L, has the best performance in consideration of both absorption and desorption processes. Figures 7 and 8 show the adsorption rate and desorption rate of mixed amines. Table 1 presents the desorption temperature of mixed amines.

R N+CO +H O R NH +HCO  → ← 
It shows that PZ-MEA has the largest absorption capacity, and it has the lowest azeotropic temperature as well, resulting in low energy consumption in CO 2 desorption process. It also indicates that there is interaction between different alcohol amines because the absorption rate of mixed alcohol amines is not a simple adding of two single amines. PZ-MEA has the best performance in both CO 2 absorption and desorption processes in this work.

Simulation of Aspen Plus
The flow diagram of simulation is shown in Figure 9. The concentration of CO 2 in the product is defined as 98 mol%, and the recovery of CO 2 is 85 %. The influences of lean liquid CO 2 load, lean liquid temperature, and the desorber pressure on the reboiler duty are investigated and shown in Figure 10 to 12. The optimal operating parameters based on the minimum reboiler duty are presented in Table 2.     Table 2 shows that at the optimum operating condition, the duty of reboiler is 3.45 MW, accounting for about 45 % of the energy consumption of the whole system, i.e., the highest energy consumption part of the CO 2 recovery.

Conclusions
In this paper, we systematically studied the separation of CO 2 from flue gas using different alcohol amines. The absorption and desorption rates of CO 2 were obtained and reported. The results demonstrate that PZ -MEA binary mixed amines have the best performance in all of the amines studied in this work in consideration of both absorption and desorption effects. Aspen Plus was also applied to simulate the process of recovering CO 2 from flue gas. We investigated the influences of lean liquid CO 2 load, lean liquid temperature and desorber pressure on reboiler duty, and optimized the operating parameters on the basis of the minimum reboiler duty.