Experimental

The membrane acts as a barrier between gas and liquid phases and does not add selectivity for one gas species over the other. The main reasons for gas dissolution into the liquid phase are the magnitude of gas solubility and whether a reaction takes place. In the case of physical absorption (CO2 is dissolved into water) there is no reaction case, which otherwise would enhance mass transfer of the gas species. It may well be assumed that dissolution of nitrogen into water does not happen or it is negligible compared to that of CO2, since the solubility of the latter is at least 30 times higher than that of N2 at ambient conditions (For CO2:29.6 mol/m3, for N2:0.7 mol/m3).

Table 18.2 Contactor characteristics.

Cartridge configuration

Parallel flow

Maximum flow rates

500 ml/min

Overall length (with adapters)

22 cm

Membrane material

Polypropylene

Membrane porosity

40%

OD/ID

300 pm OD/220 pm ID

Number of fibers

2300

Active surface area (Am)

0.18 m2

In this study a Liqui-Cel Mini-Module 1 x 5.5 was used as a gas-liquid contactor. Mini-modules are increasingly gaining attention and were used in a number of cases (Rzechowicz and Pashley, 2006; Zhang et al., 2006; Jaenicke, 2005), but they were not extensively studied. The characteristics of the membrane contactor used are listed in Table 18.2.

The flow diagram of the unit designed and constructed for the gas-liquid absorption is shown in Fig. 18.1. The feed gas, namely a gas mixture containing 15% CO2-85% N2 (simulating a typical composition in major components of flue gases from a coal combustion), is introduced directly from a gas cylinder to the fiber lumen of the module for minimum pressure drop, while the liquid (pure water or aqueous solutions of NaOH or KOH of 0.5 M, both) flows through the shell side at a pressure slightly higher than that of the gas. The feed gas flow is controlled and measured with a gas flow meter, while the liquid absorbent is pumped from the liquid storage tank by a gear pump (ISM446 BVP-Z Standard Gear Pump), so that pulseless flow is ensured. The composition of the gas streams is monitored by sampling a slip stream from the feed or exit line and diverting it into a Perkin-Elmer 8500 gas chromatograph with a thermal conductivity detector. The module was kept in an isothermal condition and experiments were carried out at 21-24oC. After each experiment the membrane was dried by passing nitrogen through the module overnight. This was done in order the liquid to be removed and evaporated from the contactor. Details for the drying procedure can be found in Liqui-Cel web site.

CHROMATOGRAPHY ANALYZER

CHROMATOGRAPHY ANALYZER

18.3 Results and Discussion

For the case of physical absorption (pure water flowing in the shell side, gas mixture flowing in the lumen counter-currently to liquid flow) the membrane module reaches steady state in the first minutes of operation. In a period of 35 min the efficiency of the membrane module is kept constant to that of the first 5 min. However, the removal of CO2 is very limited, due to the small ratio of liquid to gas flow rate (Ql/Qg), reaching 17% for Q\/Q% equaling 0.67. Increasing the liquid flow rate results in enhanced CO2 removal: for a ratio of 30.74 the recovery of

CO2 is almost complete when using pure water as the absorbent, as depicted in Fig. 18.2. However, the removal capacity of the membrane module does not vary linearly with the liquid to gas flow rate ratio, but rather there is retardation to absorption of CO2 from the physical solvent (Fig. 18.3). The declining slope of CO2 removal vs. Ql/Qg ratio indicates that the curve will reach a complete removal of CO2 for very high ratios, thus leading to high amounts of liquid consumption. This is also the result of the absence of the chemical reaction enhancement, since the value of the equilibrium constant of the reaction between H2O and CO2 is very small.

Y

Y

Q|/Qg

■ 0.67

▼ 7.72

a 17.23

• 30.74

Time (min)

16 24

Time (min)

Fig. 18.2 CO2 removal vs. time for various liquid to gas flow rate ratios using water as an absorbent (ambient temperature and pressure).

Using basic substances can enhance CO2 absorption since the latter is a mild acid gas. Aqueous solutions of NaOH or KOH with a concentration of both being 0.5 M were prepared and used as solvents instead of pure water. Now the absorption is complete even when a very low Q/Qg ratio was used. It is noted that for the same liquid to gas flow rate ratio of either NaOH or KOH (0.5 M) CO2 removal remains the same as depicted in Fig. 18.4. For a ratio of 0.12 CO2 removal reaches 95%, when using NaOH or KOH as the absorbent. For higher values, as 0.20, removal is complete, which is a big advancement for CO2 removal, since the minimum liquid consumption is achieved. Compared to the corresponding ratio for physical absorption of CO2, the consumption of the chemical solvent is 150 times lower.

A series of experiments with aqueous solutions of NaOH or KOH of 0.25 M was conducted, so that the most effective basic substance is identified. Figures 18.5 and 18.6 show the percentage removal of CO2 using mild concentrations of NaOH or KOH. First, it is evident that the removal efficiency of the membrane contactor remains the same for identical ratios, but for different gas or liquid flow rates. This feature of the membrane device indicates that the removal efficiency is only dependent on the liquid to gas flow rate ratios regardless of the gas flow rate chosen. That is, it is advantageous to use the highest possible gas flow rate without major decline in the removal efficiency. They also show that NaOH is a better reactant than KOH in treating CO2. For the same liquid to gas flow rate ratios NaOH removes larger quantities of CO2 than that with the use of KOH.

Fig. 18.3 CO2 removal vs. Q\IQg using water as absorbent (ambient temperature and pressure).

■ 0.20 (NaOH/KOH 0.5 M) • 0.126 (NaOH 0.5 M) T 0.122 (KOH 0.5 M)

Time (min)

Fig. 18.4 CO2 removal vs. time for different Q/Qg ratios using NaOH or KOH (both of 0.5 M) as absorbents (ambient temperature and pressure).

In all cases considered CO2 removal increases with the increase of the liquid to gas flow rate ratio. Keeping the gas flow rate constant the increase of the liquid flow rate leads to higher quantity of liquid to absorb the gas species. The increase of the gas flow rate (for constant liquid flow rate) leads likewise to higher CO2 removal, since the concentration difference between the lumen and the shell side increases. Another aspect of the chemical absorption is the slight increase of the removing capacity of CO2 in the case of constant liquid flow rate compared to the constant gas flow rate case. This means that the increase of the liquid flow rate has higher impact to mass transfer than the increase of the gas flow rate: There is higher resistance to mass transfer in the bulk gas phase than that of the liquid one. The effect of the chemical reaction enhancement undermines the liquid phase resistance resulting to augmented CO2 when the liquid flow rate increases.

70 0.15

0.25

0.20

Liquid to gas flow rate

Fig. 18.5 CO2 removal vs. liquid to gas flow rate ratio for constant gas flow rate.

ra 85

70 0.18

0.20

Liquid to gas flow rate

Fig. 18.5 CO2 removal vs. liquid to gas flow rate ratio for constant gas flow rate.

ra 85

0.21

Liquid to gas flow rate

Fig. 18.6 CO2 removal vs. liquid to gas flow rate ratio for constant liquid flow rate.

0.24

0.21

Liquid to gas flow rate

Fig. 18.6 CO2 removal vs. liquid to gas flow rate ratio for constant liquid flow rate.

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