1

Effects of viscosity on CO2 absorption in aqueous piperazine/2methylpiperazine

Ye Yuan, Brent Sherman, Gary T. Rochelle*

The University of Texas at Austin, Department of Chemical Engineering.

200 E Dean Keeton St. Stop C0400, Austin, TX 78712-1589

Abstract:

Piperazine (PZ) has high absorption rate, good stability, low viscosity and high capacity, while a narrow solid solubility window limits its application. Aqueous 2-methylpiperazine (2MPZ) and 2MPZ/PZ blend are attractive as they preserve most of the benefits of PZ and overcome its solubility issue. 8 m 2-methylpiperazine (2MPZ) and 4 m/4 m 2MPZ/PZ were previously studied by Chen. At lower amine concentration, viscosity decreased and the liquid film mass transfer coefficient (kg’) also increased. Based on kg’avg and viscosity normalized capacity, defined as capacity/(µμ/µμ#$%&)0.175, the optimal amine concentration is 4 m for 2MPZ and 2.5 m/2.5 m for PZ/2MPZ. The kg’avg is 8.3*10-7 mol/s*Pa*m2 and 8.0*10-7 mol/s*Pa*m2, respectively. Normalized capacity is 0.87 and 0.89 mol/kg solvent, respectively. In short, 4 m 2MPZ and 2.5 m/2.5 m 2MPZ/PZ are competitive solvents for amine scrubbing. Compared to 8 m PZ, the solid solubility of 8 m 2MPZ and 4 m/4 m 2MPZ/PZ is improved; however, the kg’ was 70% and 85% less than that of 8 m PZ, respectively. Equation 1 is the pseudo-first-order rate expression of kg’. The diffusivity of 2MPZ at higher concentration is lower because of the higher viscosity. Assuming the chemical reaction rate (k2) and physical solubility HCO2 do not vary much as concentration changes, an optimal concentration for kg’ should exist.

 𝑘*’ =-./0∗20∗[45678]

:./0 (1)

The absorption rate (kg’) and CO2 solubility of 2 m, 4 m, 6 m, and 8 m 2MPZ were measured at four CO2 loadings across the lean and rich operating range at 40 ºC using the wetted wall column (WWC). The results show increasing P* as loading increases. Given the CO2 solubility, capacity was calculated using Equation 2

∆C = $=>?>@AB  ?>C∗(E=?F@GHIJKLME=?F@GHN.IJKL)PCHQ$=>?>@AB  RS%&∗ST

(2)

The average liquid-film mass transfer coefficient (kg

’avg) with P*CO2 between 0.5kPa and 5kPa

was calculated by dividing the log-mean average of the flux over the log-mean average of the driving force. Viscosity, kg

’avg, capacity, and normalized capacity are listed in Table 1 and

plotted in Figure 1. In the operating range of P* from 0.5kPa to 5kPa, 4 m 2MPZ shows the highest kg

’avg, followed by 6 m and 2 m 2MPZ, with 8 m 2MPZ the lowest. As shown in Figure

1, when concentration goes up, viscosity and capacity increase, and normalized capacity and

2

kg’avg reach maximum between 4 m and 6 m. The rate and capacity of 4 m 2MPZ are

comparable to 8 m PZ. Known the optimal concentration for 2MPZ is between 4 m and 6 m, 2.5 m/2.5 m 2MPZ/PZ was measured, and the kg

’avg and capacity are comparable to 4 m 2MPZ

and 8 m PZ. Smoothed and extrapolated result will be presented using a rigorous thermodynamic and kinetic model in Aspen Plus® based on Electrolyte Nonrandom Two-liquid (e-NRTL) for 4 m 2MPZ and 2.5 m/2.5 m 2MPZ/PZ.

Table 1: kg’,avg, viscosity, capacity, and normalized capacity of 2MPZ and PZ at 40 °C

Figure 1: kg’, viscosity, capacity, and normalized capacity of 2MPZ at 40 oC

00.10.20.30.40.50.60.70.80.91

1

2

4

8

16

32

2 4 6 8

Capcity

/normalize

d  Ca

pacity,    mol  CO2/kg  so

lven

t

kg',  10

-­‐7mol/Pa*m

2 *s;          V

iscosity

,  cP

molality,  m

Performance of 2MPZ at 40 oC

kg'

viscosity

capacity

normal  capacity

Conc. Amine µ Capacity Normal. Capacity kg’,avg *107

m cP mol CO2/kg solvent capacity/(𝜇/𝜇#5VW)0.15 mol/s*Pa*m2

2 2MPZ 2 0.38 0.52 7.3 4 2MPZ 3.8 0.71 0.87 8.3 6 2MPZ 7.2 0.75 0.81 7.9 8 2MPZ 16 0.84 0.81 6

2.5/2.5 2MPZ/PZ 5.1 0.76 0.89 8.0 4/4 2MPZ/PZ 15 0.89 0.84 7.1 5 PZ 4 0.63 0.73 11.3 8 PZ 12 0.79 0.79 8.5