Membrane evaporation for energy saving in CO2 chemical absorption

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Membrane evaporation for energy saving in CO2 chemical absorption process using a polybenzimidazole (PBI) film: mass and heat transfer Qinhui Ma, Mengxiang Fang, Tao Wang, Hai Yu, and Paul H. M. Feron Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01900 • Publication Date (Web): 31 Aug 2017 Downloaded from http://pubs.acs.org on September 5, 2017

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Energy & Fuels

Membrane evaporation for energy saving in CO2 chemical absorption process using a polybenzimidazole (PBI) film: mass and heat transfer Qinhui Ma a,b, Mengxiang Fang a, Tao Wang a,, Hai Yu b, Paul H.M. Feron b,

a

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Zheda Road 38,

Hangzhou, Zhejiang Province 310027, PR China b

CSIRO Energy, P.O. Box 330, Newcastle, NSW 2300, Australia

Corresponding author: [email protected] (T. Wang),

Corresponding author: [email protected] (P.H.M. Feron).

Abstract:

Intensive energy consumption remains a major challenge for the commercial application of CO2 chemical absorption. In this study, a membrane evaporation system based on PBI film was proposed for recovering latent heat from the hot lean CO2 solution in order to reduce the energy penalty during CO2 capture process. The effects of key operational parameters (i.e. evaporation temperature, sweeping gas flow rate and liquid flow rate) were systematically investigated. It was found that both vapor flux and recovered heat flux had exponential increases when the evaporation temperature increased. Sweeping gas flow rate and liquid flow rate had limited effects on both mass and heat transfer in membrane evaporation process. The PBI film showed 1

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good stability during a 14 days’ duration experiment. Reducing the membrane thickness can significantly improve the membrane evaporation performance. From the viewpoint of energy evaluation, when the evaporation mass reached 10 g/kg solution, the regeneration energy consumption could be reduced by 0.47 MJ/kg CO2, which demonstrates a great potential to save energy consumption in large-scale CO2 chemical absorption process.

Keywords:

CO2 capture, membrane evaporation, heat recovery, PBI dense membrane, mass transfer

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1. Introduction

Carbon Capture and Storage (CCS) is an emerging technology to relieve the global warming caused by excessive emission of Carbon Dioxide (CO2) mostly from the combustion of fossil fuel (e.g. coal, oil and natural gas), but it poses a deployment challenge due to its high capital cost and huge energy consumption.1-4 Among these various technologies under development, CO2 capture based on amine-scrubbing is the most mature and promising one.5, 6 The amine solutions absorb CO2 from a low CO2 concentration gas stream (~12%) in an absorber at low temperature (e.g. 40 oC) and release high purity CO2 (>99%) in a desorber at high temperature (e.g. 120 oC). The desorber requires a large amount of steam normally extracted from the turbines or the auxiliary boilers and condensed in the reboiler which generates the stripping steam in the column. A lean/rich solvent heat exchanger is required to exchange heat between the lean and rich solvent, resulting a hot rich solvent entering the column. In spite of the heat recovery, the CO2 regeneration energy consumption is still high (~4 GJ/t), which may reduce about 30% of the power plant efficiency.7, 8 In the distribution of regeneration energy, reaction heat, sensible heat and latent heat of water vaporization are the major three parts and occupy about 51%, 26% and 23% respectively.9

In order to reduce this penalty, research efforts have focused either on the improvement of process flow scheme to improve heat integration10-16 or on selection of new promising solvents to reduce the reaction heat.17-21 By adopting process modifications, there is a great potential to reduce the latent heat and sensible heat for each unit CO2 capture. In the desorber steam is 3


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produced at the bottom of the column by the reboiler with the dual function of generating a driving force for CO2-desorption and providing the heat required for CO2-desorption by condensation into the absorption liquid. The heat released by condensation will also provide the sensible heat that is needed to increase the temperature of the absorption liquid as it flows down the column. Recently, a concept of novel membrane assisted liquid absorbent regeneration (MALAR) was developed. This process employs membrane contactors as evaporators or condensers for heat transfer in the desorber with the aim of improving the energy efficiency.22-25 As Figure 1 shows, a membrane evaporator is proposed to be installed before the lean/rich heat exchanger. In the membrane evaporator, vapor can be produced from the hot lean solution coming from the bottom of the desorber with the temperature around 120 oC, and then permeate through the membrane and be fed back to the desorber as stripping steam. The MALAR process is distinct from the lean vapor compression process modification that would require an additional investment in a compressor to raise the pressure level.13, 26 It is essentially a passive operation in which the heat in the lean absorption liquid is recovered through membrane evaporation. In this way, the temperature of the solution decreases and the heat of the lean solution can be recovered at higher temperature level, leading to reduction of the lean/rich heat exchanger duty and a saving on the steam from the reboiler. The application of membranes for direct heat transfer via evaporation enables quite small approach temperatures, which would be unachievable and/or uneconomical with conventional heat exchangers. The detailed description of the process can also be referred from our previous work.22, 23

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Figure 1. Schematic illustration of the membrane evaporator system 23 The previous study22, 23 focused on the selection of porous membrane (PTFE membrane with a support layer) with a high porosity on the membrane surface, which resulted in a high mass transfer coefficient. The recovered heat flux can be up to 32MJ/(m2h) and heat recovery can be over 40%.22 Despite that, membrane wetting and vapor condensation occurred during the evaporation process and decreased both mass transfer and heat transfer across the membrane.23 In order to solve these two issues, novel membrane materials should be selected and tested for membrane evaporation process. Considering the practical working conditions, e.g. high temperatures (over 100 oC) and long term contacting with high concentration MEA solution, the selected membrane should have excellent thermal and chemical stability.

Polybenzimidazole (PBI) membranes received much attention in recent years for their outstanding characteristics, such as high thermal stability, chemical stability, excellent strength, 5


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flame retardance and low coefficient of thermal expansion.27 Due to the high glass transition temperature (427 oC) and good gas transport properties, PBI membranes are extremely suitable for gas separation at high temperature, e.g. H2/CO2 separation from synthesis gas.28,

29

The

selected PBI membranes have large gas permeability, high selectivity and long term durability. Meanwhile, PBI membranes also have been reported as suitable for nanofiltration,30 reverse-osmosis,31 forward-osmosis,32 pervaporation.33 According to the reports, PBI membranes are also hydrolytically stable after exposure to high pressure and temperature steam or water.34,

35

Thus, PBI membranes appear to be an excellent candidate for membrane

evaporation at high temperature and harsh conditions.

In this study, a dense PBI film is used for membrane evaporation with the model absorbent of MEA solution to evaluate the mass and heat transfer performance. The effect of key operational parameters (i.e. evaporation temperature, sweeping gas flow rate and liquid flow rate) on mass and heat transfer performance were systematically investigated. Long term durability of membrane evaporation test was conducted to evaluate the thermal and chemical stability of PBI membrane. In addition, the MEA permeability during the membrane evaporation was also measured to assess its potential influence on the evaporation performance.

2. Experiments and methods

2.1 Materials The PBI flat dense membrane was purchased from PBI Performance Products Inc. Company. Figure 2 shows the chemical structure of PBI membrane. The membrane is a solution cast 6


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polybenzimidazole (PBI) film produced from narrow range high molecular weight PBI polymer chains. It has a high glass transition temperature of 427 oC and a thickness of 55 microns. The tear strengths of film are 1.47 lb/mil at 23 oC and 0.88 lb/mil at 300 oC, respectively.

Pure water and 30 wt% MEA solution were selected as the model absorbents. The MEA solution was prepared by diluting pure MEA (Sigma-Aldrich, purity > 99%) into deionized water according to the mass fraction.

Figure 2. Chemical structure of PBI film: polybenzimidazole, poly (5, 5’-benzimidazole-2, 2’-diyl-1, 3-phenylene).

2.2 Experimental apparatuses Membrane evaporation experiments were conducted in a bench-scale setup as shown in Figure 3 (a). The PBI membrane was held between two chambers, with each chamber size of 92.1, 45.7 and 2.3 mm for length, width and height, respectively. The total gas/liquid contact area was 42 cm2. The solution was heated up to the target temperature by a thermo-regulated water bath and then pumped into the membrane module by a gear pump (Process Pump, Australia), with its flow rate monitored by a liquid flowmeter (Swagelok, Australia). Dry nitrogen was 7


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used as the sweeping gas on the other side of the membrane module, whose flow rate was controlled by a mass flowmeter (Swagelok, Australia). The inlet and outlet humidity of the sweeping gas was measured by humidity transmitters (Vaisala, Finland) and the detailed data were recorded by the software (Pico Technology, UK). The gas side pressure was at the atmospheric pressure while the liquid side pressure was controlled at 20 kPa above the atmospheric pressure by a pressure valve (Swagelok, Australia). Meanwhile, the inlet and outlet temperatures of the solution and sweeping gas were monitored by K-type thermal couples (RS, Australia) and recorded using a data logger (Pico technology, UK) with a typical instrument error of 0.2 oC.

The setup shown in Figure 3(b) was designed for the MEA permeability measurement. Different from the evaporation experiment, pure water (100mL, 25 oC) was cycled in the permeate side and the pH value of the water was measured by a pH meter (Mettler-Toledo T50, Switzerland). We calculated the MEA concentration in the water according to the relationship between MEA concentrations and pH values and thus determined the MEA permeability.

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Figure 3(a). Experimental setup of membrane evaporation test

Figure 3(b). Experimental setup of MEA permeability measurement 9


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2.3 Theory 2.3.1 Vapor flux and overall mass transfer coefficient determination Vapor flux (‫ܬ‬௣ , kg/(m2h)) across the membrane is determined by the humidity change of inlet and outlet sweeping gas, shown as the following equation:

‫ܬ‬௣ =

൫ఠ೒,೚ೠ೟ ିఠ೒,೔೙ ൯௠ሶ೒ ஺

(1)

where ߱௚,௢௨௧ and ߱௚,௜௡ are the outlet and inlet gas humidity ratios (g/kg) respectively, ݉ሶ௚ is the mass flow rate of the sweeping gas (kg/h) and A is the liquid/gas contact area (m2). This equation is only valid for unsaturated gas stream. The overall mass transfer coefficient (‫ܭ‬௢௩ , mol/(m2sPa)) is defined as ௃

೛ ‫ܭ‬௢௩ = ∆௉

(2)

where ∆ܲ is the vapor partial pressure difference between the feed side and the permeate side and can be expressed as the logarithmic mean method,

∆ܲ =

൫௉೑,೔೙ ି௉೛,೚ೠ೟ ൯ି൫௉೑,೚ೠ೟ ି௉೛,೔೙ ൯ ୪୬൫(௉೑,೔೙ ି௉೛,೚ೠ೟ )/(௉೑,೚ೠ೟ ି௉೛,೔೙ )൯

(3)

ܲ௙,௜௡ , ܲ௙,௢௨௧ , ܲ௣,௜௡ and ܲ௣,௢௨௧ are the vapor pressures of the inlet liquid flow, outlet liquid flow, inlet gas flow and outlet gas flows, respectively. 2.3.2 Heat flux and overall heat transfer coefficient determination The recovered heat flux (q, kJ/m2h) is the heat flux transferred from the solution to the sweeping gas. It includes two parts, the latent heat for vaporization and the conductive heat through the membrane. It can be defined or expressed by the enthalpy change of the sweeping gas, ∆ா

‫∆ = ݍ‬௧஺ =

௛೚ೠ೟ ௠ሶ೚ೠ೟ ି௛೔೙ ௠ሶ೔೙ ஺

10


(4)

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where ∆‫ ܧ‬is the enthalpy change (kJ) of the gas during a time period ∆‫( ݐ‬h), ℎ௜௡ and ℎ௢௨௧ are the specific enthalpies (kJ/kg) of the inlet and outlet gas. ݉ሶ௜௡ and ݉ሶ௢௨௧ are the mass flow rates (kg/h) of the inlet and outlet gas. The latent heat flux for vaporization (‫ݍ‬௩ , kJ/(m2h)) can be calculated by the vapor flux across the membrane, and be expressed as

‫ݍ‬௩ = ‫ܬ‬௣ × ܳ

(5)

where Q is the specific latent heat of water (kJ/kg). 2.3.3 MEA permeability determination The MEA permeability (g/(m2h)) is calculated by the MEA concentration change of pure water, expressed as

‫ܬ‬ொ஺ =

(஼೟మ ି஼೟భ)௏ ஺∆௧

(6)

Where ‫ܥ‬௧ଶ and ‫ܥ‬௧ଵ are the MEA concentrations (g/L) in the pure water at time of t2 and t1, V is the total volume of the pure water (100 mL) and ∆‫ ݐ‬is the time interval between t2 and t1. The MEA concentration of the solution is determined by monitoring the pH value of the solution. The relationship between MEA concentration and pH value is showed in Figure S1 in Supporting Information.

3. Results and discussion

3.1 Membrane evaporation performance 3.1.1 Effect of evaporation temperature In the membrane evaporation process, temperature plays an important role as it determines the

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vapor partial pressure on the liquid side, which supplies the driving force for vapor flux trans-membrane behavior. Figure 4 shows the effect of evaporation temperature on the vapor flux across membrane for pure water and 30 wt% MEA solution. Pure water has higher transmembrane flux than a 30 wt% MEA solution at the same operation condition. The reason is that pure water has a higher vapor partial pressure than 30 wt% MEA at the same temperature (See Figure S2 in Supporting Information). As evaporation temperatures increase from 30 oC to 85 oC, the transmembrane vapor flux also exhibits an exponential growth for both pure water and 30 wt% MEA. The experimental results will be extrapolated to deduce values for a higher temperature (e.g. 120 oC), which is not achievable with the present experimental setup. Detailed results and discussion can be referred in section 3.4.

Figure 5 shows the effect of evaporation temperature on the overall mass transfer coefficient. The overall mass transfer coefficients were calculated according to Equation (2). The results show that the overall mass transfer coefficients decrease as the evaporation temperatures increase from 30 oC to 85 oC. Wang et al. obtained similar results when they conducted pervaporation dehydration of ethylene glycol via dual PBI/PEI membrane.36 This can be explained by the diffusion model in dense membrane, where the permeability (Pp) is controlled by diffusivity (D) and solubility (S) with the relationship of Pp = D · S.34 As temperature increases, the diffusion of gas in the dense membrane increases while the equilibrium solubility decreases. In this case, the decrease of mass transfer coefficient indicates that the reduction of solubility has greater effect on the vapor permeability than the increment of diffusivity of water vapor. Similar phenomena have also been reported by previous researchers in pervaporation 12


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process.37, 38

Figure 4. Effect of evaporation temperature on transmembrane vapor flux for pure water and 30 wt% MEA. Experimental conditions: sweeping gas flow rate 1L/min; liquid flow rate 10L/h.

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Figure 5. Effect of evaporation temperature on overall mass transfer coefficient for pure water and 30 wt% MEA. Experimental conditions: sweeping gas flow rate 1 L/min; liquid flow rate 10 L/h. 3.1.2 Effect of sweeping gas flow rate The flow rate of sweeping gas has a dual effect on the membrane evaporation performance. Increasing the sweeping gas flow rate can reduce gas boundary layer and thus reduce gas side mass transfer resistance. It also can help to reduce the humidity of the sweeping gas, which is in favor of lowering the vapor partial pressure in gas side and enlarging the driving force of mass transfer. The effects of sweeping gas flow rate on vapor flux and overall mass transfer coefficient are shown in Figure 6 and Figure 7. It was found that increasing the sweeping gas flow rate had limited effect on the vapor flux and overall mass transfer coefficient as the gas flow rate ranging from 0.5 L/min to 3 L/min. The overall mass transfer resistance (1/Kov) 14


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includes three parts: the liquid side (1/kl), the membrane side (1/km) and the gas side (1/kg), with the relationship of

ଵ ௄೚ೡ

ଵ

ଵ

ଵ

= ௞ + ௞ + ௞ . The gas side mass transfer coefficient (kg) is in ೗

೘

೒

proportion to the gas flow rate. This means that the resistance of gas side was small and negligible compared to the overall mass transfer resistance. In this study, the highest relative humidity in the outlet sweeping gas is about 90% (See Figure S3 in Supporting Information), so no vapor would be condensed and the change of humidity could reflect the vapor flux across the membrane.

The overall mass transfer coefficient has a similar tendency to the vapor flux as the sweeping gas flow rate increases. Increasing the sweeping gas flow rate from 0.5 L/min to 3 L/min slightly improved the overall mass transfer coefficient. However it does not mean gas flow rate can be very small. Gas flow rate largely affects the humidity of the sweeping gas. When the sweeping gas is saturated, vapor condensation will happen in the gas phase which may deteriorate the mass transfer process. In our previous study, vapor condensation happened in porous membrane especially for big pore size membrane with large vapor flux and low sweeping gas flow rate.23,

39

The condensation can largely reduce the vapor flux and heat

efficiency, due to the endothermic process of condensation.

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Figure 6. Effect of sweeping gas flow rate on transmembrane vapor flux for pure water at different temperatures. Experimental conditions: liquid flow rate 10L/h.

z

Figure 7. Effect of sweeping gas flow rate on mass transfer coefficient for pure water at different temperatures. Experimental conditions: liquid flow rate 10L/h. 16


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3.1.3 Effect of liquid flow rate The effects of liquid flow rate on the vapor flux and overall mass transfer coefficient are shown in Figure 8 and Figure 9. Increasing the liquid flow rate can slightly improve the vapor flux due to the reduction of boundary layer and temperature polarization effect. When the liquid flow rate increased from 5 L/h to 30 L/h (the corresponding Reynolds number ranging from 300 to 1800), the transmembrane vapor flux and overall mass transfer coefficient improved by 9% and 5%, respectively. It indicates that liquid flow rate is not a dominant factor for PBI membrane’s evaporation performance, and the mass transfer resistance of liquid side is small and negligible compared to the overall mass transfer resistance. This result is in agreement with the previous study on the membrane distillation and evaporation.22,

23

Thus, we may ignore this part of

resistance when evaluating the overall mass transfer coefficient for PBI dense membrane.

Figure 8. Effect of liquid flow rate on the vapor flux for pure water and 30 wt% MEA. Experimental conditions: evaporation temperature 60 oC; sweeping gas flow rate 1L/min. 17


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Figure 9. Effect of liquid flow rate on the mass transfer coefficient for pure water and 30 wt% MEA. Experimental conditions: evaporation temperature 60 oC; sweeping gas flow rate 1L/min. 3.2 Long term durability of PBI membrane For industrial applications, the membrane evaporator should have a stable performance over an extended period of time. Long term membrane durability means reducing operation cost and improving economic feasibility of the membrane evaporation process. To demonstrate the long term durability, the PBI membrane evaporation experiment was operated continuously with the 30 wt% MEA solution at 80 oC for two weeks. During this period, the membrane evaporation performance was tested for 3 times a day to get a mean value. As shown in Figure 10, the vapor flux showed a small increase for the first 3 days and then almost maintained the same in the last 11 days of the experiment. The increase of vapor flux can be explained by the swelling of the 18


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PBI membrane when contacting with MEA and water. PBI membrane can be swollen by both MEA and water because of the hydrogen bonding interactions between their hydroxyl groups and the imidazole group of PBI.36 The molecules trapped between polymer chains widened the chain-chain distance and increased the free volume of the membrane, and thus to improve the mass transfer coefficient. The constant of the vapor flux in the following 11 days indicates that PBI membranes have good stability for the membrane evaporation process.

After the long term durability experiment, the membrane module was opened and no droplets were found on the gas side. This indicates that no vapor condensation occurred during the membrane evaporation process and the humidity change can reflect the actual transmembrane vapor flux. PBI membranes have been proved with high thermal and chemical stability in the area of gas separation at elevated temperatures according to the reports. Pesiri et al. successfully prepared m-PBI meniscus membranes with an approximate thickness of 4 microns and demonstrated H2/CO2 separations with the highest temperature of 340 oC.40 Yan et al. developed a dual-layer composite membrane (PBI as the outer selective layer and PEI as the inner supporting layer) for the pervaporation dehydration process and exhibited good long-term stability with the solution of ethylene glycol/water at 60 oC during a 33-days test.33, 36

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Figure 10. Long term durability of PBI membrane evaporation performance with 30 wt% MEA solution at 80 oC 3.3 MEA permeability measurement In the membrane evaporation process with MEA solution, both water vapor and MEA vapor can permeate through the membrane. However, the permeation of MEA might adversely affect the membrane evaporation performance. For instance, as MEA has much lower vapor pressure than water it could form a film on the membrane, blocking the transfer of water vapor. In any case, it is advantageous to have knowledge of the MEA permeability during the membrane evaporation experiments.

The MEA permeability was measured in the set-up shown in Figure 3(b) using a 30 wt% MEA solution, based on the pH changes in the permeate. The relationships between total MEA 20


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permeance and evaporation time at the temperatures of 60 oC and 80 oC are shown in Figure 11. The resulting MEA flux is given in Table 1. The MEA permeability fluxes were 0.249 and 0.355 g/(m2h) at the evaporation temperatures of 60 oC and 80 oC, respectively. Compared to the water vapor flux at the same evaporation temperature, the MEA flux was almost three orders of magnitude lower than the vapor flux. It indicates that the MEA flux can be ignored when evaluating the membrane evaporation performance, especially for heat flux calculation. For the porous membrane in our previous study, the MEA flux was two orders of magnitude lower than water vapor flux.23 This suggests that the PBI dense membrane has a better selectivity for the water vapor and MEA vapor than porous membrane.

Figure 11. Total MEA permeability in cycled pure water with evaporation time

Table 1. Comparison of MEA flux and water flux at evaporation temperature of 60 oC and 80 o

C 21


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Evaporation

MEA flux

Water flux

MEA/Water

temperature

g/(m2h)

g/(m2h)

/

60 ℃

0.249

73.4

0.34%

80 ℃

0.355

161.1

0.22%

3.4 Insight of practical application In the above sections, membrane evaporation experiments based on PBI dense membrane with a thickness of 55 microns were conducted to obtain the mass transfer coefficient with the working temperature ranging from 30 to 85 oC. The results showed the obtained vapor flux and recovered heat flux increased exponentially with evaporation temperature. In practical operation, the temperature of the lean solvent from the desorber is around 110 oC - 120 oC.41, 42 Thus we can estimate the simulated vapor flux and recovered heat flux at higher temperature using the obtained relationship. Figure 12 shows the simulated vapor flux and recovered heat flux along with the evaporation temperature for 30 wt% MEA solution. We can extrapolate that at the evaporation temperature of 120 oC, the vapor flux will be 650 g/(m2h) and the recovered heat flux will be 2000 kJ/(m2h).

Reducing the membrane thickness can be an effective way to reduce membrane side resistance and improve membrane evaporation performance. Due to the outstanding mechanical stability, the thickness of PBI membrane can be greatly reduced.33, 40 According to the above analysis, the mass transfer resistances of gas side and liquid side are negligible compared to membrane 22


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side. So we assumed the resistance of these two parts can be ignored when the membrane thickness ranging from 5 to 80 microns, and the overall mass transfer coefficient has an inversely proportional relationship to the membrane thickness (shown in Figure S4 in Supporting information). According to the calculation, when the membrane thickness reduces to 5 microns, the mass transfer coefficient can be as high as 50×10-8 mol/(m2sPa), which is close to the result obtained from previous porous membrane.23 In this case, the vapor flux and recovered heat flux can be up to 7.15 kg/(m2h) and 22 MJ/(m2h) for practical application.

Figure 12. Simulated vapor flux and heat flux with evaporation temperature for 30wt% MEA solution. Experimental conditions: sweeping gas flow rate 1L/min; liquid flow rate 10L/h. 3.5 Evaluation of energy saving for membrane evaporation system We evaluated the potential energy saving of the membrane evaporation system for the traditional MEA-based CO2 capture system using the following assumptions: 1) The heat for water evaporation was derived from the CO2 lean solution giving rise to a reduction of solution 23


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temperature; 2) No heat losses to the surroundings; 3) The heat of conduction was ignored compared to the heat of evaporation; 4) The mass transfer coefficient was consistent, hence ignoring the effect of temperature reduction. Basic data used for the evaluation (e.g. physical property, operational parameters) was listed in Table 2. Figure 13 illustrates the relationships between evaporation mass and the recovered heat, reduction of solution temperature and energy saving for per kg CO2 capture. In the membrane evaporation system, the evaporation mass of vapor could be controlled by adjusting the contacting membrane area and residence time of solution in the membrane module. When the evaporation mass reached 10 g/kg solution, the recovered latent heat was 22.6 kJ, and the corresponding regeneration energy consumption could be reduced by 0.47 MJ/kg CO2. Due to the endothermic process of evaporation, the reduction of solution temperature was about 6 oC. Compared to the total CO2 regeneration energy consumption of 4 GJ/t, the application of membrane evaporation system can reduce part of energy consumption potentially. Table 2. Basic information of physical property and operational parameters Parameter

Value

MEA concentration

30%

temperature of lean solution, oC

120

specific heat capacity Cp, kJ/(kg·K)

3.78

latent heat of vaporization, kJ/kg

2256.8

CO2 cyclic loading, mol/mol

0.22

24


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Energy & Fuels

Figure 13. Effect of evaporation mass per kg solution on the recovered heat, reduction of solution temperature and energy saving for per kg CO2 capture

4. Conclusions

In this study, a dense PBI film is used for membrane evaporation to recover latent heat from the hot lean CO2 solution coming from the desorber of CO2 chemical absorption system. The effect of key operational parameters (i.e. evaporation temperature, sweeping gas flow rate and liquid flow rate) on vapor flux, mass transfer coefficient and heat transfer performance were investigated. We found that both vapor flux and recovered heat flux increased exponentially with an evaporation temperature increase. The effect of sweeping gas flow rate and liquid flow rate had limited effect on both mass and heat transfer in membrane evaporation process. The MEA permeability was also determined and was negligible compared to the vapor flux. The

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PBI membrane showed good stability during a 14 days’ duration experiment. Reducing the membrane thickness can significantly improve the membrane evaporation performance. When the PBI thickness reduces to 5 microns, the vapor flux and recovered heat flux can be up to 7.15 kg/(m2h) and 22 MJ/(m2h). For the membrane evaporation system, when the evaporation mass ranging from 1-10 g/kg solution, the regeneration energy consumption could potentially be reduced by 0.047-0.47 MJ/kg CO2, which demonstrates a great potential to save energy consumption in large-scale CO2 chemical absorption process.

Acknowledgments

This work was financially supported by the National key R&D Program of China (No. 2017YFB0603300), CSIRO Energy, the National Natural Science Foundation of China (No. 51276161) and Fundamental Research Funds for the Central Universities of China. Qinhui Ma is grateful for financial support from the China Scholarship Council which enabled him to carry out this work at CSIRO.

References: (1) Wang, M.; Lawal, A.; Stephenson, P.; Sidders, J.; Ramshaw, C. Chemical Engineering Research and Design 2011, 89, 1609-1624. (2) Zhao, M.; Minett, A. I.; Harris, A. T. Energy & Environmental Science 2012, 6, 25. (3) Davison, J. Energy 2007, 32, 1163-1176. (4) Leung, D. Y. C.; Caramanna, G.; Maroto-Valer, M. M. Renewable and Sustainable Energy Reviews 2014, 39, 426-443. (5) Ahn, H.; Luberti, M.; Liu, Z.; Brandani, S. International Journal of Greenhouse Gas Control 2013, 16, 29-40. (6) Rochelle, G. T. Science 2009, 325, 1652-1654. (7) Pellegrini, L. A.; Moioli, S.; Gamba, S. Chemical Engineering Research and Design 2011, 89, 1676-1683. (8) Goto, K.; Yogo, K.; Higashii, T. Appl Energ 2013, 111, 710-720. (9) Dash, S. K.; Samanta, A. N.; Bandyopadhyay, S. S. International Journal of Greenhouse Gas Control 2014, 21, 130-139. 26


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(10) Neveux T., L. M. Y. C. Chemical Engineering Transactions 2013, 35, 337-342. (11) Oyenekan, B. A.; Rochelle, G. T. Aiche J 2007, 53, 3144-3154. (12) Kamijo, T. I. M. M. Apparatus and method for CO2 recovery. 2006. (13) Le Moullec, Y.; Neveux, T.; Al Azki, A.; Chikukwa, A.; Hoff, K. A. International Journal of Greenhouse Gas Control 2014, 31, 96-112. (14) Li, K.; Yu, H.; Feron, P.; Tade, M.; Wardhaugh, L. Environ Sci Technol 2015, 49, 10243-10252. (15) Li, K.; Leigh, W.; Feron, P.; Yu, H.; Tade, M. Appl Energ 2016, 165, 648-659. (16) Li, K.; Cousins, A.; Yu, H.; Feron, P.; Tade, M.; Luo, W.; Chen, J. Energy Science & Engineering 2016, 4, 23-39. (17) Li, H.; Moullec, Y. L.; Lu, J.; Chen, J.; Marcos, J. C. V.; Chen, G. International Journal of Greenhouse Gas Control 2014, 31, 25-32. (18) Liu, J.; Wang, S.; Svendsen, H. F.; Idrees, M. U.; Kim, I.; Chen, C. International Journal of Greenhouse Gas Control 2012, 9, 148-159. (19) Zhang, J.; Agar, D. W.; Zhang, X.; Geuzebroek, F. Energy Procedia 2011, 4, 67-74. (20) He, F.; Wang, T.; Fang, M.; Wang, Z.; Yu, H.; Ma, Q. Energ Fuel 2017, 31, 770-777. (21) Wang, T.; He, H.; Yu, W.; Sharif, Z.; Fang, M. Energ Fuel 2017, 31, 4255-4262. (22) Zhao, S.; Cao, C.; Wardhaugh, L.; Feron, P. H. M. J Membrane Sci 2015, 473, 274-282. (23) Zhao, S.; Feron, P. H. M.; Cao, C.; Wardhaugh, L.; Yan, S.; Gray, S. Sep Purif Technol 2015, 146, 60-67. (24) Yan, S.; Zhao, S.; Wardhaugh, L.; Feron, P. H. M. Environ Sci Technol 2015, 49, 2532-2540. (25) Zhao, S.; Wardhaugh, L.; Zhang, J.; Feron, P. H. M. J Membrane Sci 2015, 475, 445-454. (26) Sanchez Fernandez, E.; Bergsma, E. J.; de Miguel Mercader, F.; Goetheer, E. L. V.; Vlugt, T. J. H. International Journal of Greenhouse Gas Control 2012, 11, S114-S121. (27) Borjigin, H.; Stevens, K. A.; Liu, R.; Moon, J. D.; Shaver, A. T.; Swinnea, S.; Freeman, B. D.; Riffle, J. S.; McGrath, J. E. Polymer 2015, 71, 135-142. (28) Kumbharkar, S. C.; Liu, Y.; Li, K. J Membrane Sci 2011, 375, 231-240. (29) Singh, R. P.; Dahe, G. J.; Dudeck, K. W.; Welch, C. F.; Berchtold, K. A. Energy Procedia 2014, 63, 153-159. (30) Wang, K. Y.; Yang, Q.; Chung, T.; Rajagopalan, R. Chem Eng Sci 2009, 64, 1577-1584. (31) Sawyer, L. C.; Jones, R. S. J Membrane Sci 1984, 20, 147-166. (32) Wang, K. Y.; Chung, T.; Qin, J. J Membrane Sci 2007, 300, 6-12. (33) Wang, Y.; Gruender, M.; Chung, T. S. J Membrane Sci 2010, 363, 149-159. (34) Berchtold, K. A.; Singh, R. P.; Young, J. S.; Dudeck, K. W. J Membrane Sci 2012, 415-416, 265-270. (35) Yang, T.; Chung, T. Int J Hydrogen Energ 2013, 38, 229-239. (36) Wang, Y.; Chung, T. S.; Neo, B. W.; Gruender, M. J Membrane Sci 2011, 378, 339-350. (37) Chung, T.; Guo, W. F.; Liu, Y. J Membrane Sci 2006, 271, 221-231. (38) Guo, W. F.; Chung, T.; Matsuura, T. J Membrane Sci 2004, 245, 199-210. (39) Zhao, S.; Feron, P. H. M.; Xie, Z.; Zhang, J.; Hoang, M. J Membrane Sci 2014, 462, 9-16. (40) Pesiri, D. R.; Jorgensen, B.; Dye, R. C. J Membrane Sci 2003, 218, 11-18. (41) Oyenekan, B. A.; Rochelle, G. T. Ind Eng Chem Res 2006, 45, 2457-2464. (42) Cottrell, A. J.; McGregor, J. M.; Jansen, J.; Artanto, Y.; Dave, N.; Morgan, S.; Pearson, P.; Attalla, M. I.; Wardhaugh, L.; Yu, H.; Allport, A.; Feron, P. H. M. Energy Procedia 2009, 1, 1003-1010. 27


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Table 1. Comparison of MEA flux and water flux at evaporation temperature of 60 oC and 80 o

C Evaporation

MEA flux

Water flux

MEA/Water

temperature

g/(m2h)

g/(m2h)

/

60 ℃

0.249

73.4

0.34%

80 ℃

0.355

161.1

0.22%

Table 2. Basic information of physical property and operational parameters Parameter

Value

MEA concentration

30%

temperature of lean solution, oC

115

specific heat capacity Cp, kJ/(kg·K)

3.78

latent heat of vaporization, kJ/kg

2256.8

CO2 cyclic loading, mol/mol

0.22

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Membrane evaporation for energy saving in CO2 chemical absorption

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