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Membrane heat exchanger for novel heat recovery in carbon capture
Journal of Membrane Science 577 (2019) 60–68
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Membrane heat exchanger for novel heat recovery in carbon capture a,b,⁎
Shuiping Yan , Qiufang Cui ⁎⁎ Shuaifei Zhaod,e,
a,b
, Te Tu
a,b
, Liqiang Xu
a,b
, Qingyao He
a,b
T c
, Paul H.M. Feron ,
a
College of Engineering, Huazhong Agricultural University, Wuhan 430070, PR China Key Laboratory of Agricultural Equipment in Mid-lower Yangtze River, Ministry of Agriculture and Rural Affairs, Wuhan 430070, PR China CSIRO Energy, P.O. Box 330, Newcastle, NSW 2300, Australia d Department of Environmental Sciences, Macquarie University, Sydney, NSW 2109, Australia e State Environmental Protection Key Laboratory of Integrated Surface Water-Groundwater Pollution Control, School of Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Heat exchanger Membrane condenser Heat recovery Carbon capture Membrane heat exchanger
In this work, we experimentally demonstrate a commercial ceramic membrane heat exchanger (CMHE) with an average pore size of 4 nm for novel heat recovery in post-combustion carbon capture. The CMHE shows superior performance over a conventional stainless steel heat exchanger (SSHE) with the same dimensions in recovering heat from the stripped gas mixture (H2O(g)/CO2) on top of the stripper in a monoethanolamine-based rich-split carbon capture process. Due to the coupled mass and heat transfers of water vapor through the membrane, the CMHE has higher heat flux, heat recovery and overall heat transfer coefficient than the SSHE. Liquid water transfer dominates the mass transfer mechanism in the CMHE. Thermal conduction contributes to more than 80% of the total heat transfer, dominating the heat transfer through the membrane heat exchanger. Our study demonstrates that membrane heat exchangers can be excellent candidates for heat recovery in post-combustion carbon capture. In further research, more types of membranes with higher thermal conductivities (e.g., porous metal membranes with lower porosity and smaller thickness) should be fabricated and tested for further performance enhancement.
1. Introduction Because of its technical superiority and successful commercial implementation, post-combustion carbon capture (PCC) using aqueous alkanolamine solvents is considered as one of the most mature methods to reduce CO2 emissions [1–3]. However, CO2 chemical absorption still requires huge energy inputs for absorbent regeneration. Many strategies, such as screening new CO2 absorbents with low regeneration energy [4], inter-cooling [5], absorbent splitting [6], absorption-mineralization integration [3,7], and latent heat recovery from water vapor [8–10], have been studied to reduce the CO2 capture energy penalty. The rich-split process modification has been used to recover the waste heat by splitting a partial cold CO2 rich solvent before the leanrich heat exchanger to contact with the stripped gas mixture (i.e., H2O (g)/CO2) on top of the stripper in carbon capture pilot plants [6,11]. Metal heat exchangers have been demonstrated for such heat recovery from the H2O(g)/CO2 during regeneration [12]. However, metal heat
exchangers may not be efficient for a low-temperature gas mixture and face the issue of equipment corrosion caused by the gas mixture of CO2 and water vapor. Membrane heat exchangers can offer better performance in heat recovery since both mass and heat transfer occurs, whereas only heat transfer occurs in conventional heat exchangers. In our previous study, thermodynamic analysis of mass and heat transfer in a membrane heat exchanger with rich split was performed to estimate the heat recovery potential in post-combustion carbon capture [10]. In the concept (Fig. 1), water vapor from the H2O(g)/CO2 transfers through a membrane contactor into the bypassed cold rich solvent to enhance the heat recovery performance, and the gas-liquid membrane contactor acts as a novel membrane heat exchanger. Similar gasliquid membrane contactors have been used as membrane condensers for water and heat recovery from waste gas streams, such as power station flue gas [13–18]. However, membrane heat exchangers have not been experimentally demonstrated for heat recovery in carbon capture yet.
⁎
Corresponding author at: Key Laboratory of Agricultural Equipment in Mid-lower Yangtze River, Ministry of Agriculture and Rural Affairs, Wuhan 430070, PR China. ⁎⁎ Corresponding author at: Department of Environmental Sciences, Macquarie University, Sydney, NSW 2109, Australia. E-mail addresses: [email protected] (S. Yan), [email protected] (S. Zhao). https://doi.org/10.1016/j.memsci.2019.01.049 Received 7 December 2018; Received in revised form 27 January 2019; Accepted 28 January 2019 Available online 02 February 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.
Journal of Membrane Science 577 (2019) 60–68
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Fig. 1. Concept of permeable membrane heat exchanger for heat recovery from wet CO2 in post-combustion carbon capture (left: schematic diagram of the concept; right: experimental setup).
In this study, we for the first time experimentally investigated the heat recovery performance by a hydrophilic nanoporous ceramic membrane heat exchanger (CMHE) with rich split modification in postcombustion carbon capture (Fig. 1). CO2 loaded monoethanolamine (MEA) was selected as the rich split. A CMHE and a conventional stainless-steel heat exchanger (SSHE) were used and compared in terms of heat recovery performance and overall heat transfer coefficients. The superiority of the permeable CMHE to the impermeable SSHE was investigated. Furthermore, the heat and H2O transfer mechanisms were explored to provide perspectives on further tailoring membrane characteristics.
simulate the gas released from the stripper in PCC. As shown in Fig. 1 (right), both distilled water and CO2 with controlled mass flow rates were introduced into a spiral coil heater immerged in a thermostat oil bath (~130 °C) to achieve the desired temperature. Then the H2O(g)/ CO2 mixture was introduced into the tube side of the horizontallyplaced thermo-preserved heat exchanger. Meanwhile, CO2-rich MEA solvent was pumped into a heater to achieve the desired temperature (45–65 °C) and then passed through the shell side of the module countercurrently with the gas mixture. The inlet and outlet temperatures and pressures of the gas mixture and the MEA solvent were recorded. The relative humidity of the gas mixture was measured using a humidity sensor (HC2-IC102, Rotronic AG, Switzerland). The outlet gas mixture from the heat exchanger was condensed in a − 2 °C cold trap to determine the H2O flux across the membrane into the MEA solvent.
2. Materials and methods 2.1. Heat exchangers
2.3. Data analysis A monochannel nanoporous tubular ceramic membrane (Nanjing Aiyuqi Membrane Technology Co., Ltd., China) was used as the ceramic membrane heat exchanger (CMHE) and a custom-built 304 stainless steel tube was used as the conventional stainless-steel heat exchanger (SSHE) for heat recovery. Both heat exchangers had the same dimensions: inner diameter (ID) 8 mm, outer diameter (OD) 12 mm and total length 300 mm. The SSHE had an effective length of 300 mm, while the CMHE had an effective length of 270 mm. Thus, their effective membrane areas were 0.0075 m2 and 0.00675 m2, respectively. The CMHE had an inner separation layer (average pore size 4 nm; thickness ~ 16 µm), an intermediate layer (thickness ~ 20 µm) and an outer substrate. The porosity of the CMHE is 23% measured with mercury intrusion porosimetry (AutoPore Ⅳ 9500, Micromeritics Instrument Corporation, USA). For the heat exchanger module, the shell side was made of 304 SS (ID 15 mm and OD 19 mm).
The total heat flux can be calculated by:
Qrec = ≈
(mR + mH2O) hMEA (TLout )−mR hMEA (TLin ) 1000A C R ̅ m (T out − T in ) + m h (T out ) MEA
R
L
L
1000A
H2 O w
L
(1)
where Qrec is the total recovered heat flux from the gas mixture, MJ/(m2 h); mR is the mass flow rate of the rich MEA solvent on the shell side of the heat exchanger, kg/h; mH2O is the water flux across the membrane from the gas mixture into the cold rich solvent, kg/h. mH2O = 0 for the SSHE case; hMEA (T ) and h w (T ) are the specific enthalpy (kJ/kg) of the rich MEA solvent and water at temperature T, respectively; TLin and TLout are the inlet and outlet temperatures of the rich MEA solvent (°C), reR is the specific heat capacity of rich MEA solvent spectively; CMEA averaged over TLin and TLout in kJ/(kg K), which can be obtained from the reference [19]; A is the effective membrane area, m2. The error bars of the results in the following section indicate the uncertainties of the experimental measurements due to fluctuations of the experimental conditions.
2.2. Experimental setup In PCC, the stripped gas is wet CO2 containing a lot of water vapor and latent heat. Therefore, we used a mixture of water vapor and CO2 to 61
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Heat recovery ratio (HRR) is calculated by
HRR =
Qrec Qp
(2)
CCO2 mCO2 (TGin − TLin )+(mVin − mVout (TLin )) γV + Cw (mVin Qp =
− mVout (TLin ))·(TGin − TLin )+CCO2 mVout (TLin )·(TGin − TLin ) (3)
1000A
where QP is the theoretical maximum (ideal) heat flux that can be recovered when the gas mixture is cooled from the inlet gas temperature (TGin ) to an outlet gas temperature equal to the inlet rich MEA temperature (TLin ), MJ/(m2 h); mCO2 is the CO2 mass flow rate, kg/h; mVin is the mass flow rate of the inlet water vapor, kg/h; mVout (TLin ) is the mass flow rate of the outlet water vapor when the gas mixture is cooled to TLin , kg/h. mVout (TLin ) can be obtained by the VAISALA humidity calculator 3.1; CW is the specific heat capacity of water, kJ/(kg K). γV is the latent heat of water vapor, kJ/kg. The overall heat transfer coefficient of the heat exchanger (K OV , W/ (m2 K)) was determined by
Fig. 3. Relationship between water flux and transfer flux in the ceramic membrane heat exchanger. Experimental conditions: gas temperature 90 ± 0.5 °C, volumetric flow rate 7.16 m3/(m2 h), molar ratio of H2O(g) to CO2 = 1:1; CO2 rich MEA solvent inlet temperature 45 ± 0.5 °C, MEA concentration 30 wt%, solvent flow rate 28.29 m3/(m2 h), CO2 loading 0.45 mol/mol).
T in − T out
K OV
ln Gout L in TG − TL 1000Qrec 1000Qrec = = 3.6∆Tm 3.6 (TGin − TLout ) − (TGout − TLin )
(4)
dramatically from 5.7 to 14.4 kg/(m2 h) with the rise in gas mixture pressure. However, the ideal heat flux, representing the maximum recoverable latent and sensible heat within the H2O/CO2 gas mixture, does not increase much with the increase of the gas pressure, maintaining at about 51 MJ/(m2 h). The recovered heat flux through the membrane heat exchanger increases slightly from 27.0 to 30.5 MJ/(m2 h) with the rise in gas pressure. The heat recoveries are between 54.3% and 59.1% in the gas pressure range (6.5 – 54.2 kPa). Such heat recovery performance in post-combustion carbon capture is higher than the heat recovery performance from power station flue gas [13]. In a membrane heat exchanger, heat transfer and water transfer occur simultaneously [14,16,20]. Both conductive heat and convective heat fluxes contribute to the total heat flux. There is a strong linear relationship between the heat flux (Qrec ) and the water flux (Fig. 3). According to Fig. 3, we can determine the conductive heat flux of the CMHE (24.56 MJ/(m2 h)) when only conductive heat transfer occurs, namely, x = 0. For mass transfer in the CMHE, both H2O(g) and H2O(l) transfers occur simultaneously. The H2O(g) and H2O(l) fluxes can be estimated by the following equations:
where ΔTm is the logarithmic mean temperature difference between the gas mixture and rich MEA solvent, K; T is the gas or liquid temperature, °C; in and out in the superscript represent the inlet or outlet, respectively; G and L in the subscript represent the gas (water vapor and CO2) and liquid (MEA), respectively. 3. Results and discussion For heat recovery with a membrane heat exchanger, parameters of the gas phase and the liquid phase will affect the heat recovery performance. These parameters include the liquid phase (i.e., rich MEA solvent) inlet temperature and flow rate, and the gas phase (i.e., CO2 and water vapor) inlet pressure, flow rate, temperature and ratio of water vapor to CO2. Therefore, these parameters were systematically investigated. 3.1. Heat and mass transfer mechanisms in membrane heat exchanger Fig. 2 shows the membrane heat exchanger performance (i.e., heat flux, heat recovery and water flux) as a function of the gas mixture inlet pressure. The average water flux across the membrane increases
H2 O(g) H2 O(l) Qrec = Qcond + Qconv = Qcond + Qconv + Qconv
(5)
wherein H2 O(g) Qconv =
H2 O(l) Qconv =
ṁ H2O(g) Hg (TV̅ ) − ṁ H2O(g) Hl (TR̅ ) (6)
1000
ṁ H2O(l) Hl (TV̅ ) − ṁ H2O(l) Hl (TR̅ ) (7)
1000
And
ṁ H2O(g) +
ṁ H2O(l) = ṁ H2O
(8)
where Qcond is the conductive heat flux through the CMHE, MJ/(m h); Qconv is the convective heat flux associated with water transfer, MJ/(m2 H2 O (l) H2 O (g ) h); Qconv and Qconv are the convective heat fluxes associated with water vapor and liquid water transfer in MJ/(m2 h), respectively; ṁ H2O is the total flux of H2O including H2O(g) and H2O(l) from the gas mixture into the rich solvent in kg/(m2 h); ṁ H2O (g ) and ṁ H2O (l) is H2O(g) and H2O(l) transfer fluxes in kg/(m2 h), respectively; TV is the logarithmic mean temperature of the inlet and outlet gas mixture, °C; TR is the logarithmic mean temperature of the inlet and outlet bypassed rich solvent on the shell side of CMHE, °C; Hg (TV ) is the specific enthalpy of H2O(g) at TV (kJ/kg); Hl (TV ) is the specific enthalpy of liquid water at TV (kJ/kg) because that H2O(g) is firstly condensed on the tube side of 2
Fig. 2. Membrane heat exchanger performance as a function of inlet stripping gas pressure. Experimental conditions: gas temperature 90 ± 0.5 °C, volumetric flow rate 7.16 m3/(m2 h), molar ratio of H2O(g) to CO2 = 1:1; CO2 rich MEA solvent inlet temperature 45 ± 0.5 °C, MEA concentration 30 wt%, solvent flow rate 28.29 m3/(m2 h), CO2 loading 0.45 mol/mol). 62
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water vapor condensation flux and the total water flux decreases with the rise in gas mixture pressure. The theoretical water vapor condensation flux is defined as the difference in water vapor content between the inlet and outlet gas mixture per membrane area. Clearly, when the gas pressure is lower than 54.2 kPa, the total water transfer flux is less than the calculated water vapor condensation flux, suggesting the accumulation of the condensate on the inner membrane surface. In this case, the overall water transfer is limited by the rate of permeation (i.e., membrane performance). However, almost all of the condensate is transferred across the membrane into the rich MEA solvent at 54.2 kPa, suggesting the condensation-controlled mass transfer at a higher gas pressure. In practical PCC operation, condensate is unlikely to accumulate on the inner membrane surface since the gas mixture inlet pressure is often high. Fig. 4b demonstrates that the thermal conduction is the key heat transfer mechanism through the membrane heat exchanger, contributing to more than 80% of the total heat transfer. The heat transfer from the hot gas side occurs by three ways: (1) a small portion of water vapor transports into the rich solvent to release its latent heat directly; (2) most water vapor condenses first on the gas-membrane interface to release its latent heat. Then both latent heat and sensible heat of the H2O(g)/CO2 transfers through the membrane by conduction; (3) the sensible heat of condensate transfers by means of liquid water. Most of the heat recovery results from the latent heat of water vapor condensation and transfers through thermal conduction. These results suggest that increasing the thermal conductivity of the membrane is an effective way to enhance heat transfer, achieving high heat recovery. In the future, other types of membranes with high thermal conductivity can be used or membrane modification with high thermal conductivity materials can be done to realize better heat recovery performance. Reducing the porosity or thickness of the membrane can be another way to increase the membrane thermal conductivity [22,28]. Fig. 4. Analysis of (a) water transfer and (b) heat transfer in the ceramic membrane heat exchanger.
3.2. Effect of rich MEA solvent flow rate on heat transfer CMHE to release its latent heat, and thus the temperature of condensate is equal to that of H2O(g); Hl (TR ) is the specific enthalpy of rich solvent at TR (kJ/kg); Qrec is the overall recovered heat by the rich solvent from the gas mixture, MJ/(m2 h). Fig. 4a shows the calculated liquid water and water vapor fluxes across the membrane. Interestingly, liquid water transfer dominates the mass transfer in the CMHE. The water transfer mechanism can be interpreted as following: (1) when the hot H2O(g)/CO2 mixture goes into the lumen of the CMHE, H2O(g) first transports through membrane pores into the rich solvent due to the gradient of water vapor partial pressure. As H2O(g) partial pressure in the gas phase is higher than the capillary condensation pressure determined by the Kelvin equation [21], H2O(g) is condensed in the nanopores through the capillary condensation mode [22]. No water vapor condensation in the tube of the CMHE occurs before H2O(g)/CO2 is cooled to its dew point temperature. In this case, water vapor may transport into the rich solvent mainly by the hopping behavior of H2O molecules in the adsorbed phase at steady state [23], suggesting H2O(g) transport always occurs no matter vapor condensation is present or not. However, vapor transfer without condensation may only account for a small portion and occur near the inlet of the gas mixture in the CMHE. (2) Because of the heat transfer (Figs. 2 and 3) and thus temperature drop of the H2O(g)/ CO2 gas mixture, most water vapor will condense at the gas-membrane interface [24–26]. Driven by the transmembrane pressure difference and the gradient of capillary force [23], the condensate at the gasmembrane interface is continuously sucked into the membrane pores and then the capillary condensate in the pores transports into the rich solvent through viscous flow [27]. Liquid water transfer occurs along most of the CMHE length, dominating the mass transfer (Fig. 4a). Fig. 4a also indicates that the difference between the theoretical
For energy saving in carbon capture, heat flux and heat recovery are two most important parameters. Fig. 5 displays the effects of the CO2 rich MEA solvent flow rate on the heat flux and heat recovery of both CMHE and SSHE. Both heat flux and heat recovery increase with the rise in MEA flow rate. This is caused by the fact that in a gas-liquid membrane contacting system the mass transfer resistance is principally
Fig. 5. Comparison of heat flux and heat recovery using ceramic membrane heat exchanger (CMHE) and stainless steel heat exchanger (SSHE) as a function of the rich MEA solvent flow rate. Experimental conditions: gas temperature 90 ± 0.5 °C, volumetric flow rate 7.16 m3/(m2 h), molar ratio of H2O(g) to CO2 = 1:1, gauge pressure 5 kPa; CO2-rich MEA solvent inlet temperature 45 ± 0.5 °C, MEA concentration 30 wt%, CO2 loading 0.45 mol/mol. 63
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from the liquid phase [29,30]. Thus, increasing the liquid (MEA) flow rate can effectively reduce the boundary layer effect and enhance heat and mass transfer. In practical application, however, the flow rate (i.e., split fraction) of the bypassed rich MEA solvent should not be too large. Otherwise, it may lead to a higher total regeneration energy input in the stripper due to the higher sensible heat requirement for increasing the CO2-rich solvent temperature to the regeneration temperature. Thus, the split fraction of the bypass rich MEA solvent should be optimized based on the total energy requirements of the whole PCC system. The rich split fraction is defined by the flow rate ratio of the bypassed cold rich MEA to the total MEA (mRTot in kg/h) in PCC. The total rich MEA flow rate is shown as following.
M mRTot = ⎛ MEA + W ⎝ ∆α
αR MCO2 VCO2 ⎞∙ ∆α ⎠ 22.4
3.3. Effect of rich MEA temperature on heat transfer Fig. 6 shows the effect of the rich MEA solvent temperature on heat transfer of the heat exchangers. Obviously, the membrane heat exchanger has higher heat flux and heat recovery than the conventional SSHE. Both heat flux and heat recovery drop dramatically with the rise in rich solvent temperature. The ideal flux also decreases mainly caused by the decrease of the latent heat released by the H2O(g)/CO2 with the increase of inlet rich solvent temperature as shown in Eq. (3). For example, when the MEA temperature increases from 45° to 65°C, the heat flux declines from 27.24 to 11.57 MJ/(m2 h) and the heat recovery drops from 54.7% to 31.0% for the membrane heat exchanger. These results indicate that the split solvent temperature plays an important role in heat recovery and a small change in solvent temperature can make a big difference on heat recovery of the system. Therefore, it is necessary to minimize the cold rich solvent temperature by adopting inter-cooling technology in the CO2 absorber [33,34].
(9)
where MMEA and MCO2 are the molecular weight of MEA and CO2 in kg/ mol, respectively; W is the mass fraction of the rich MEA solvent; VCO2 is the volumetric flow rate of CO2 regenerated in the stripper, m3/h; Δα is the net cyclic CO2 capacity of MEA during absorption and regeneration, mol/mol; αR is the CO2 loading of the rich MEA solvent. For common MEA-based PCC, αR = 0.45 mol/mol, and Δα = 0.25 mol/mol [31,32]. Fig. 5 also shows that the heat flux and heat recovery of the CMHE are higher than those of the SSHE. The superior heat recovery performance of the CMHE over the SSHE can be explained by two reasons. First, both conductive and convective heat transfers associated with water transfer occur from the inner membrane surface to the rich MEA solvent in the membrane heat exchanger, while only conductive heat transfer occurs in the conventional SSHE. Second, mass transfer at the gas-membrane and liquid-membrane interfaces of the CMHE can help to minimize the boundary layer effect, which further improves the heat transfer [28]. Therefore, the CMHE shows better heat recovery performance than the conventional SSHE. In practical long-term operation for heat recovery from gas mixtures, a membrane heat exchanger has better anti-corrosion performance than a conventional SSHE, particularly in the presence of acidic gases (e.g., CO2). Although membrane heat exchangers have several advantages than the conventional ones, membrane technology has not been widely studied for heat exchanging compared with its other applications. In the future, membrane-based heat exchangers are well worth investigating for broader heat recovery and energy savings.
3.4. Effect of gas flow rate on heat transfer Two modes were investigated in this study for exploring the effect of gas flow rate on heat transfer performance. In Mode І, the rich MEA solvent flow rate was fixed at 28.29 m3/(m2 h); in Mode ІІ, the gasliquid volume ratio was fixed at 200:1 and the split fraction is fixed at 0.25 with different gas flow rates, where the rich MEA flow rate varied from 18.26 to 47.15 m3/(m2 h). It should be noted that Mode І is just adopted to test the heat recovery performance of the heat exchangers without considering the real flow rate ratio of the rich MEA to the gas mixture. Mode ІІ is more suitable to the actual situation in which the split fraction of the bypassed rich MEA solvent is fixed. The relationship between the gas mixture flow rate and total rich MEA flow rate (mRTot in kg/h) is shown in Eq. (9). For mode І, with the increase of the gas flow rate, ideal heat flux increases linearly for both the CMHE and the SSHE due to the increased latent heat within the water vapor (Fig. 7a). As a result, the heat fluxes of the CMHE and the SSHE increase but not linearly with the increase of the gas flow rate. This is because that partial latent heat within the water vapor comes back to the stripper associated with vapor condensation on the gas side. For the SSHE without mass transfer, there is no convective heat transfer but conductive heat transfer, leading to a much lower heat flux across the exchanger compared with the CMHE. As expected, heat recoveries of the heat exchangers decline Fig. 6. Comparison of (a) heat flux and (b) heat recovery using ceramic membrane heat exchanger (CMHE) and stainless steel heat exchanger (SSHE) as a function of the inlet MEA temperature. Experimental conditions: gas temperature 90 ± 0.5 °C, volumetric flow rate 7.16 m3/ (m2 h), molar ratio of H2O(g) to CO2 = 1:1, gauge pressure 5 kPa; rich MEA solvent flow rate 28.29 m3/(m2 h), MEA concentration 30 wt%, CO2 loading 0.45 mol/mol.
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Fig. 7. Comparison of heat flux and heat recovery using ceramic membrane heat exchanger (CMHE) and stainless steel heat exchanger (SSHE) as a function of the gas flow rate at a fixed rich MEA solvent flow rate 28.29 m3/(m2 h) (mode І: a, b), and a fixed gas-liquid volume ratio 200:1(mode ІІ: c). Experimental conditions: gas temperature 90 ± 0.5 °C, molar ratio of H2O(g) to CO2 = 1:1, gauge pressure 2–16 kPa; MEA solvent inlet temperature 45 ± 0.5 °C, MEA concentration 30 wt%, CO2 loading 0.45 mol/mol.
dramatically with the increase of the gas flow rate due to the shorter contact time at high gas flow rates (Fig. 7b) [13,17]. Fig. 7 further demonstrates that the CMHE has better heat recovery performance than the conventional SSHE. Particularly, when the gas flow rate is 4.8 m3/ (m2 h), the heat recovery of the membrane heat changer is up to 81%, while the heat recovery of the SSHE is 70%. For mode ІІ, the heat transfer fluxes of the CMHE and the SSHE increase linearly but their heat recoveries reduce linearly with the increase of the gas flow rate. Such changes are different from those in mode І, mainly due to the change of the rich MEA flow rate. The rich MEA flow rates increase from 18.26 to 47.15 m3/(m2 h). Higher gas and liquid flow rates reduce the boundary layer resistance and thus enhance the heat transfer [18,28]. At the gas flow rates higher than 7.16 m3/(m2 h), the MEA flow rate in mode ІІ is higher than that in mode І. For example, the MEA flow rate is 47.15 m3/(m2 h) at the gas flow rate of 11.93 m3/(m2 h) in mode ІІ, while the MEA flow rate is 28.29 m3/(m2 h) in mode І. Therefore, more heat is recovered in mode ІІ compared with mode І. However, when the gas flow rate is 4.77 m3/(m2 h), the MEA flow rate is only 18.86 m3/(m2 h) in mode ІІ, leading to a lower heat flux in mode ІІ.
3.5. Effect of gas temperature on heat transfer Fig. 8 compares the heat fluxes and recoveries of the two types of heat exchangers as a function of the gas inlet temperature. The ideal heat flux changes slightly with the increase of gas temperature mainly because the change of gas temperature only changes the sensible heat of the H2O(g)/CO2 at the fixed molar ratio of H2O(g) to CO2. Obviously, heat flux and heat recovery of the CMHE are higher than those of the SSHE. For example, when the gas mixture temperature is about 100 °C, the heat flux of the CMHE is 28.72 MJ/(m2 h), while the heat flux of the SSHE is 23.20 MJ/(m2 h) (Fig. 8a). The heat flux increases slightly with the rise in gas inlet temperature. However, the heat recovery shows interesting change trends with the increase of the gas inlet temperature. When the gas temperature is beyond 100 °C, the CMHE shows slightly decreased heat recovery performance. This is because that at higher temperatures above 100 °C the water vapor content within the gas mixture does not increase, thus water vapor transfer and its associated latent heat will not increase. At the same time, heat loss caused by vapor condensation and droplet refluxing to the stripper becomes severe. Therefore, the heat recovery shows a slight decrease at higher temperatures. The heat recovery of the CMHE is more than 55%, while 65
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Fig. 8. Comparison of (a) heat flux and (b) heat recovery using ceramic membrane heat exchanger (CMHE) and stainless steel heat exchanger (SSHE) as a function of the gas inlet temperature. Experimental conditions: volumetric flow rate of the stripped gas 7.16 m3/(m2 h), molar ratio of H2O(g) to CO2 = 1:1, gauge pressure 5 kPa; rich MEA solvent flow rate 28.29 m3/(m2 h), MEA solvent inlet temperature 45 ± 0.5 °C, MEA concentration 30 wt%, CO2 loading 0.45 mol/mol.
Fig. 9a also shows that the heat flux difference between the CMHE and the SSHE increases with the rise in molar ratio of the water vapor to CO2. This can be explained by the higher mass transfer flux at higher water vapor content for the CMHE [13]. High water vapor content with high latent heat leads to the heat flux enhancement with the increase of the molar ratio of water vapor to CO2. However, high heat flux also means temperature increase of the liquid MEA solution, leading to a decrease in the temperature difference (i.e., heat transfer driving force) between the liquid and gas streams. Therefore, when the molar ratio of water vapor to CO2 is over, the heat flux enhancement become less significant. The heat recovery is the ratio of the experimental heat flux to the ideal heat flux. When the molar ratio of water flux to CO2 is over 1.5, the experimental heat flux enhancement becomes less significant, while the ideal heat flux still increases linearly (Fig. 9a). As a result, the calculated heat recovery is relative low (< 40%) and does not follow a similar change trend with the heat flux enhancement (Fig. 9b). In practical PCC operation, the water vapor content in the gas mixture is often very high (saturated), and thus employing CMHE is superior to using conventional SSHE. Fig. 9b demonstrates almost linearly decreased heat recovery (e.g., from 69.4% to 35.1% for the
that of the SSHE is less than 51%. 3.6. Effect of water vapor content on heat transfer For heat exchanging, the recovered heat is mainly from the latent heat within the water vapor. Therefore, the water vapor content in the gas mixture has significant impacts on heat flux and heat recovery. Fig. 9 describes heat fluxes and recoveries of the two types of heat exchangers as a function of the molar ratio of water vapor to CO2. With the increase of the molar ratio of water vapor to CO2, the ideal heat flux increases linearly due to the great increase of the enthalpy of the H2O (g)/CO2 (mainly from water vapor). The heat fluxes of the heat exchangers also increase significantly, from 16.08 to 35.74 MJ/(m2 h) for the CMHE and from 13.66 to 28.19 MJ/(m2 h) for the SSHE over the water vapor/CO2 molar ratio range. When the molar ratio of water vapor to CO2 is over 1.5, the heat flux enhancement becomes less significant, particularly for the SSHE. This may be caused by the fact that much water vapor caused more severe condensation whereby heat goes back to the stripper through water droplets, rather than transfers through the contactor by conduction (Fig. 4b).
Fig. 9. Comparison of (a) heat flux and (b) heat recovery using ceramic membrane heat exchanger (CMHE) and stainless steel heat exchanger (SSHE) as a function of the molar ratio of water vapor to CO2. Experimental conditions: volumetric flow rate of the stripped gas 7.16 m3/(m2 h), gas temperature 90 ± 0.5 °C, gauge pressure 5 kPa; rich MEA solvent flow rate 28.29 m3/(m2 h), MEA solvent inlet temperature 45 ± 0.5 °C, MEA concentration 30 wt%, CO2 loading 0.45 mol/mol.
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Fig. 10. Comparison of overall heat transfer coefficients using ceramic membrane heat exchanger (CMHE) and stainless steel heat exchanger (SSHE) as a function of (a) rich MEA solvent flow rate, (b) MEA inlet temperature, (c) gas flow rate at the fixed rich MEA solvent flow rate, (d) gas flow rate at the fixed gas-liquid volume ratio, (e) gas inlet temperature, and (f) molar ratio of water vapor to CO2. The experimental conditions for each figure are the same with those for the previous figures where the corresponding parameters are the variations.
our new membrane heat exchanger (Fig. 10). In Fig. 10a, when the MEA flow rate is below 42.4 m3/(m2 h), the overall heat transfer coefficients increase with the rise in MEA flow rate, suggesting the severe boundary layer effect on the liquid side. With the increase of the MEA flow rate, the overall heat transfer coefficient of the CMHE is much higher than that of the SSHE because the mass transfer reduced heat transfer resistance at the boundary interfaces and the convective heat transfer from the mass transfer of the CMHE. With the increase of the gas flow rate, the convective heat transfer coefficient of gas flow also increases, which also contributes to the increased overall heat transfer coefficients at higher gas flow rates. Fig. 10b shows the
CMHE) with the increase of the molar ratio of water vapor to CO2. This suggests that more effective contact areas and contact time are required to increase the heat recovery when the water vapor content in the gas mixture is high. 3.7. Overall heat transfer coefficients The overall heat transfer coefficient is an important parameter in evaluating heat transfer performance of a heat exchanger. Comparison of the overall heat transfer coefficients of the two types of heat exchangers clearly demonstrates the superior heat transfer performance of 67
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significantly decreased overall heat transfer coefficients with the rise in MEA temperature, suggesting that temperature difference is the main driving force for both heat and mass transfer in the heat exchanging system. Compared the liquid flow rate (Fig. 10a), the gas flow rate at the fixed solvent flow rate has a much less effect on the overall heat transfer coefficient (Fig. 10c). At the fixed gas-liquid volume ratio, however, the overall heat transfer coefficient increases linearly with the increase of the gas flow rate (Fig. 10d). The CMHE always shows much higher overall heat transfer coefficients than the SSHE. The gas inlet temperature has a reverse effect on the overall heat transfer coefficient. With the increase of the gas inlet temperature, the overall heat transfer coefficients of both heat exchangers decline linearly (Fig. 10e). This result agrees with our previous finding in heat and water recovery from power station flue gas [13]. It can be explained by that higher gas inlet temperatures increase the heat fluxes across the contactors, leading to higher MEA temperatures on the liquid side, which will reduce the heat transfer efficiency. Fig. 10f displays that the overall heat transfer coefficient of the CMHE increases significantly with the rise in water vapor content within the gas mixture.
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14] [15]
4. Conclusion
[16]
This work, for the first time, experimentally demonstrates the heat recovery performance of a commercial hydrophilic ceramic membrane heat exchanger (CMHE) in post-combustion carbon capture through a rich-split process modification. In the CMHE, there is a strong linear relationship between heat flux and water flux. For the membrane heat exchanger, liquid water transfer dominates the mass transfer mechanism. Thermal conduction is the key heat transfer mechanism through the membrane heat exchanger, contributing to more than 80% of the total heat transfer. Due to the coupled mass and heat transfers of water vapor through the membrane, the CMHE is superior to the conventional SSHE in terms of heat flux, heat recovery and overall heat transfer coefficient. In the future, more types of membranes with higher thermal conductivities (e.g., porous metal membranes with lower porosity and smaller thickness) should be fabricated and tested for performance enhancement.
[17]
[18]
[19]
[20]
[21]
[22]
[23] [24]
Acknowledgements The project was supported by the National Key R & D Program of China (No. 2017YFB0603300), National Natural Science Foundation of China (No. 51676080), Guangdong Provincial Key Laboratory of Soil and Groundwater Pollution Control (No. 2017B030301012), and State Environmental Protection Key Laboratory of Integrated Surface WaterGroundwater Pollution Control.
[25]
[26]
[27]
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Membrane heat exchanger for novel heat recovery in carbon capture a,b,⁎
Shuiping Yan , Qiufang Cui ⁎⁎ Shuaifei Zhaod,e,
a,b
, Te Tu
a,b
, Liqiang Xu
a,b
, Qingyao He
a,b
T c
, Paul H.M. Feron ,
a
College of Engineering, Huazhong Agricultural University, Wuhan 430070, PR China Key Laboratory of Agricultural Equipment in Mid-lower Yangtze River, Ministry of Agriculture and Rural Affairs, Wuhan 430070, PR China CSIRO Energy, P.O. Box 330, Newcastle, NSW 2300, Australia d Department of Environmental Sciences, Macquarie University, Sydney, NSW 2109, Australia e State Environmental Protection Key Laboratory of Integrated Surface Water-Groundwater Pollution Control, School of Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Heat exchanger Membrane condenser Heat recovery Carbon capture Membrane heat exchanger
In this work, we experimentally demonstrate a commercial ceramic membrane heat exchanger (CMHE) with an average pore size of 4 nm for novel heat recovery in post-combustion carbon capture. The CMHE shows superior performance over a conventional stainless steel heat exchanger (SSHE) with the same dimensions in recovering heat from the stripped gas mixture (H2O(g)/CO2) on top of the stripper in a monoethanolamine-based rich-split carbon capture process. Due to the coupled mass and heat transfers of water vapor through the membrane, the CMHE has higher heat flux, heat recovery and overall heat transfer coefficient than the SSHE. Liquid water transfer dominates the mass transfer mechanism in the CMHE. Thermal conduction contributes to more than 80% of the total heat transfer, dominating the heat transfer through the membrane heat exchanger. Our study demonstrates that membrane heat exchangers can be excellent candidates for heat recovery in post-combustion carbon capture. In further research, more types of membranes with higher thermal conductivities (e.g., porous metal membranes with lower porosity and smaller thickness) should be fabricated and tested for further performance enhancement.
1. Introduction Because of its technical superiority and successful commercial implementation, post-combustion carbon capture (PCC) using aqueous alkanolamine solvents is considered as one of the most mature methods to reduce CO2 emissions [1–3]. However, CO2 chemical absorption still requires huge energy inputs for absorbent regeneration. Many strategies, such as screening new CO2 absorbents with low regeneration energy [4], inter-cooling [5], absorbent splitting [6], absorption-mineralization integration [3,7], and latent heat recovery from water vapor [8–10], have been studied to reduce the CO2 capture energy penalty. The rich-split process modification has been used to recover the waste heat by splitting a partial cold CO2 rich solvent before the leanrich heat exchanger to contact with the stripped gas mixture (i.e., H2O (g)/CO2) on top of the stripper in carbon capture pilot plants [6,11]. Metal heat exchangers have been demonstrated for such heat recovery from the H2O(g)/CO2 during regeneration [12]. However, metal heat
exchangers may not be efficient for a low-temperature gas mixture and face the issue of equipment corrosion caused by the gas mixture of CO2 and water vapor. Membrane heat exchangers can offer better performance in heat recovery since both mass and heat transfer occurs, whereas only heat transfer occurs in conventional heat exchangers. In our previous study, thermodynamic analysis of mass and heat transfer in a membrane heat exchanger with rich split was performed to estimate the heat recovery potential in post-combustion carbon capture [10]. In the concept (Fig. 1), water vapor from the H2O(g)/CO2 transfers through a membrane contactor into the bypassed cold rich solvent to enhance the heat recovery performance, and the gas-liquid membrane contactor acts as a novel membrane heat exchanger. Similar gasliquid membrane contactors have been used as membrane condensers for water and heat recovery from waste gas streams, such as power station flue gas [13–18]. However, membrane heat exchangers have not been experimentally demonstrated for heat recovery in carbon capture yet.
⁎
Corresponding author at: Key Laboratory of Agricultural Equipment in Mid-lower Yangtze River, Ministry of Agriculture and Rural Affairs, Wuhan 430070, PR China. ⁎⁎ Corresponding author at: Department of Environmental Sciences, Macquarie University, Sydney, NSW 2109, Australia. E-mail addresses: [email protected] (S. Yan), [email protected] (S. Zhao). https://doi.org/10.1016/j.memsci.2019.01.049 Received 7 December 2018; Received in revised form 27 January 2019; Accepted 28 January 2019 Available online 02 February 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Concept of permeable membrane heat exchanger for heat recovery from wet CO2 in post-combustion carbon capture (left: schematic diagram of the concept; right: experimental setup).
In this study, we for the first time experimentally investigated the heat recovery performance by a hydrophilic nanoporous ceramic membrane heat exchanger (CMHE) with rich split modification in postcombustion carbon capture (Fig. 1). CO2 loaded monoethanolamine (MEA) was selected as the rich split. A CMHE and a conventional stainless-steel heat exchanger (SSHE) were used and compared in terms of heat recovery performance and overall heat transfer coefficients. The superiority of the permeable CMHE to the impermeable SSHE was investigated. Furthermore, the heat and H2O transfer mechanisms were explored to provide perspectives on further tailoring membrane characteristics.
simulate the gas released from the stripper in PCC. As shown in Fig. 1 (right), both distilled water and CO2 with controlled mass flow rates were introduced into a spiral coil heater immerged in a thermostat oil bath (~130 °C) to achieve the desired temperature. Then the H2O(g)/ CO2 mixture was introduced into the tube side of the horizontallyplaced thermo-preserved heat exchanger. Meanwhile, CO2-rich MEA solvent was pumped into a heater to achieve the desired temperature (45–65 °C) and then passed through the shell side of the module countercurrently with the gas mixture. The inlet and outlet temperatures and pressures of the gas mixture and the MEA solvent were recorded. The relative humidity of the gas mixture was measured using a humidity sensor (HC2-IC102, Rotronic AG, Switzerland). The outlet gas mixture from the heat exchanger was condensed in a − 2 °C cold trap to determine the H2O flux across the membrane into the MEA solvent.
2. Materials and methods 2.1. Heat exchangers
2.3. Data analysis A monochannel nanoporous tubular ceramic membrane (Nanjing Aiyuqi Membrane Technology Co., Ltd., China) was used as the ceramic membrane heat exchanger (CMHE) and a custom-built 304 stainless steel tube was used as the conventional stainless-steel heat exchanger (SSHE) for heat recovery. Both heat exchangers had the same dimensions: inner diameter (ID) 8 mm, outer diameter (OD) 12 mm and total length 300 mm. The SSHE had an effective length of 300 mm, while the CMHE had an effective length of 270 mm. Thus, their effective membrane areas were 0.0075 m2 and 0.00675 m2, respectively. The CMHE had an inner separation layer (average pore size 4 nm; thickness ~ 16 µm), an intermediate layer (thickness ~ 20 µm) and an outer substrate. The porosity of the CMHE is 23% measured with mercury intrusion porosimetry (AutoPore Ⅳ 9500, Micromeritics Instrument Corporation, USA). For the heat exchanger module, the shell side was made of 304 SS (ID 15 mm and OD 19 mm).
The total heat flux can be calculated by:
Qrec = ≈
(mR + mH2O) hMEA (TLout )−mR hMEA (TLin ) 1000A C R ̅ m (T out − T in ) + m h (T out ) MEA
R
L
L
1000A
H2 O w
L
(1)
where Qrec is the total recovered heat flux from the gas mixture, MJ/(m2 h); mR is the mass flow rate of the rich MEA solvent on the shell side of the heat exchanger, kg/h; mH2O is the water flux across the membrane from the gas mixture into the cold rich solvent, kg/h. mH2O = 0 for the SSHE case; hMEA (T ) and h w (T ) are the specific enthalpy (kJ/kg) of the rich MEA solvent and water at temperature T, respectively; TLin and TLout are the inlet and outlet temperatures of the rich MEA solvent (°C), reR is the specific heat capacity of rich MEA solvent spectively; CMEA averaged over TLin and TLout in kJ/(kg K), which can be obtained from the reference [19]; A is the effective membrane area, m2. The error bars of the results in the following section indicate the uncertainties of the experimental measurements due to fluctuations of the experimental conditions.
2.2. Experimental setup In PCC, the stripped gas is wet CO2 containing a lot of water vapor and latent heat. Therefore, we used a mixture of water vapor and CO2 to 61
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Heat recovery ratio (HRR) is calculated by
HRR =
Qrec Qp
(2)
CCO2 mCO2 (TGin − TLin )+(mVin − mVout (TLin )) γV + Cw (mVin Qp =
− mVout (TLin ))·(TGin − TLin )+CCO2 mVout (TLin )·(TGin − TLin ) (3)
1000A
where QP is the theoretical maximum (ideal) heat flux that can be recovered when the gas mixture is cooled from the inlet gas temperature (TGin ) to an outlet gas temperature equal to the inlet rich MEA temperature (TLin ), MJ/(m2 h); mCO2 is the CO2 mass flow rate, kg/h; mVin is the mass flow rate of the inlet water vapor, kg/h; mVout (TLin ) is the mass flow rate of the outlet water vapor when the gas mixture is cooled to TLin , kg/h. mVout (TLin ) can be obtained by the VAISALA humidity calculator 3.1; CW is the specific heat capacity of water, kJ/(kg K). γV is the latent heat of water vapor, kJ/kg. The overall heat transfer coefficient of the heat exchanger (K OV , W/ (m2 K)) was determined by
Fig. 3. Relationship between water flux and transfer flux in the ceramic membrane heat exchanger. Experimental conditions: gas temperature 90 ± 0.5 °C, volumetric flow rate 7.16 m3/(m2 h), molar ratio of H2O(g) to CO2 = 1:1; CO2 rich MEA solvent inlet temperature 45 ± 0.5 °C, MEA concentration 30 wt%, solvent flow rate 28.29 m3/(m2 h), CO2 loading 0.45 mol/mol).
T in − T out
K OV
ln Gout L in TG − TL 1000Qrec 1000Qrec = = 3.6∆Tm 3.6 (TGin − TLout ) − (TGout − TLin )
(4)
dramatically from 5.7 to 14.4 kg/(m2 h) with the rise in gas mixture pressure. However, the ideal heat flux, representing the maximum recoverable latent and sensible heat within the H2O/CO2 gas mixture, does not increase much with the increase of the gas pressure, maintaining at about 51 MJ/(m2 h). The recovered heat flux through the membrane heat exchanger increases slightly from 27.0 to 30.5 MJ/(m2 h) with the rise in gas pressure. The heat recoveries are between 54.3% and 59.1% in the gas pressure range (6.5 – 54.2 kPa). Such heat recovery performance in post-combustion carbon capture is higher than the heat recovery performance from power station flue gas [13]. In a membrane heat exchanger, heat transfer and water transfer occur simultaneously [14,16,20]. Both conductive heat and convective heat fluxes contribute to the total heat flux. There is a strong linear relationship between the heat flux (Qrec ) and the water flux (Fig. 3). According to Fig. 3, we can determine the conductive heat flux of the CMHE (24.56 MJ/(m2 h)) when only conductive heat transfer occurs, namely, x = 0. For mass transfer in the CMHE, both H2O(g) and H2O(l) transfers occur simultaneously. The H2O(g) and H2O(l) fluxes can be estimated by the following equations:
where ΔTm is the logarithmic mean temperature difference between the gas mixture and rich MEA solvent, K; T is the gas or liquid temperature, °C; in and out in the superscript represent the inlet or outlet, respectively; G and L in the subscript represent the gas (water vapor and CO2) and liquid (MEA), respectively. 3. Results and discussion For heat recovery with a membrane heat exchanger, parameters of the gas phase and the liquid phase will affect the heat recovery performance. These parameters include the liquid phase (i.e., rich MEA solvent) inlet temperature and flow rate, and the gas phase (i.e., CO2 and water vapor) inlet pressure, flow rate, temperature and ratio of water vapor to CO2. Therefore, these parameters were systematically investigated. 3.1. Heat and mass transfer mechanisms in membrane heat exchanger Fig. 2 shows the membrane heat exchanger performance (i.e., heat flux, heat recovery and water flux) as a function of the gas mixture inlet pressure. The average water flux across the membrane increases
H2 O(g) H2 O(l) Qrec = Qcond + Qconv = Qcond + Qconv + Qconv
(5)
wherein H2 O(g) Qconv =
H2 O(l) Qconv =
ṁ H2O(g) Hg (TV̅ ) − ṁ H2O(g) Hl (TR̅ ) (6)
1000
ṁ H2O(l) Hl (TV̅ ) − ṁ H2O(l) Hl (TR̅ ) (7)
1000
And
ṁ H2O(g) +
ṁ H2O(l) = ṁ H2O
(8)
where Qcond is the conductive heat flux through the CMHE, MJ/(m h); Qconv is the convective heat flux associated with water transfer, MJ/(m2 H2 O (l) H2 O (g ) h); Qconv and Qconv are the convective heat fluxes associated with water vapor and liquid water transfer in MJ/(m2 h), respectively; ṁ H2O is the total flux of H2O including H2O(g) and H2O(l) from the gas mixture into the rich solvent in kg/(m2 h); ṁ H2O (g ) and ṁ H2O (l) is H2O(g) and H2O(l) transfer fluxes in kg/(m2 h), respectively; TV is the logarithmic mean temperature of the inlet and outlet gas mixture, °C; TR is the logarithmic mean temperature of the inlet and outlet bypassed rich solvent on the shell side of CMHE, °C; Hg (TV ) is the specific enthalpy of H2O(g) at TV (kJ/kg); Hl (TV ) is the specific enthalpy of liquid water at TV (kJ/kg) because that H2O(g) is firstly condensed on the tube side of 2
Fig. 2. Membrane heat exchanger performance as a function of inlet stripping gas pressure. Experimental conditions: gas temperature 90 ± 0.5 °C, volumetric flow rate 7.16 m3/(m2 h), molar ratio of H2O(g) to CO2 = 1:1; CO2 rich MEA solvent inlet temperature 45 ± 0.5 °C, MEA concentration 30 wt%, solvent flow rate 28.29 m3/(m2 h), CO2 loading 0.45 mol/mol). 62
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water vapor condensation flux and the total water flux decreases with the rise in gas mixture pressure. The theoretical water vapor condensation flux is defined as the difference in water vapor content between the inlet and outlet gas mixture per membrane area. Clearly, when the gas pressure is lower than 54.2 kPa, the total water transfer flux is less than the calculated water vapor condensation flux, suggesting the accumulation of the condensate on the inner membrane surface. In this case, the overall water transfer is limited by the rate of permeation (i.e., membrane performance). However, almost all of the condensate is transferred across the membrane into the rich MEA solvent at 54.2 kPa, suggesting the condensation-controlled mass transfer at a higher gas pressure. In practical PCC operation, condensate is unlikely to accumulate on the inner membrane surface since the gas mixture inlet pressure is often high. Fig. 4b demonstrates that the thermal conduction is the key heat transfer mechanism through the membrane heat exchanger, contributing to more than 80% of the total heat transfer. The heat transfer from the hot gas side occurs by three ways: (1) a small portion of water vapor transports into the rich solvent to release its latent heat directly; (2) most water vapor condenses first on the gas-membrane interface to release its latent heat. Then both latent heat and sensible heat of the H2O(g)/CO2 transfers through the membrane by conduction; (3) the sensible heat of condensate transfers by means of liquid water. Most of the heat recovery results from the latent heat of water vapor condensation and transfers through thermal conduction. These results suggest that increasing the thermal conductivity of the membrane is an effective way to enhance heat transfer, achieving high heat recovery. In the future, other types of membranes with high thermal conductivity can be used or membrane modification with high thermal conductivity materials can be done to realize better heat recovery performance. Reducing the porosity or thickness of the membrane can be another way to increase the membrane thermal conductivity [22,28]. Fig. 4. Analysis of (a) water transfer and (b) heat transfer in the ceramic membrane heat exchanger.
3.2. Effect of rich MEA solvent flow rate on heat transfer CMHE to release its latent heat, and thus the temperature of condensate is equal to that of H2O(g); Hl (TR ) is the specific enthalpy of rich solvent at TR (kJ/kg); Qrec is the overall recovered heat by the rich solvent from the gas mixture, MJ/(m2 h). Fig. 4a shows the calculated liquid water and water vapor fluxes across the membrane. Interestingly, liquid water transfer dominates the mass transfer in the CMHE. The water transfer mechanism can be interpreted as following: (1) when the hot H2O(g)/CO2 mixture goes into the lumen of the CMHE, H2O(g) first transports through membrane pores into the rich solvent due to the gradient of water vapor partial pressure. As H2O(g) partial pressure in the gas phase is higher than the capillary condensation pressure determined by the Kelvin equation [21], H2O(g) is condensed in the nanopores through the capillary condensation mode [22]. No water vapor condensation in the tube of the CMHE occurs before H2O(g)/CO2 is cooled to its dew point temperature. In this case, water vapor may transport into the rich solvent mainly by the hopping behavior of H2O molecules in the adsorbed phase at steady state [23], suggesting H2O(g) transport always occurs no matter vapor condensation is present or not. However, vapor transfer without condensation may only account for a small portion and occur near the inlet of the gas mixture in the CMHE. (2) Because of the heat transfer (Figs. 2 and 3) and thus temperature drop of the H2O(g)/ CO2 gas mixture, most water vapor will condense at the gas-membrane interface [24–26]. Driven by the transmembrane pressure difference and the gradient of capillary force [23], the condensate at the gasmembrane interface is continuously sucked into the membrane pores and then the capillary condensate in the pores transports into the rich solvent through viscous flow [27]. Liquid water transfer occurs along most of the CMHE length, dominating the mass transfer (Fig. 4a). Fig. 4a also indicates that the difference between the theoretical
For energy saving in carbon capture, heat flux and heat recovery are two most important parameters. Fig. 5 displays the effects of the CO2 rich MEA solvent flow rate on the heat flux and heat recovery of both CMHE and SSHE. Both heat flux and heat recovery increase with the rise in MEA flow rate. This is caused by the fact that in a gas-liquid membrane contacting system the mass transfer resistance is principally
Fig. 5. Comparison of heat flux and heat recovery using ceramic membrane heat exchanger (CMHE) and stainless steel heat exchanger (SSHE) as a function of the rich MEA solvent flow rate. Experimental conditions: gas temperature 90 ± 0.5 °C, volumetric flow rate 7.16 m3/(m2 h), molar ratio of H2O(g) to CO2 = 1:1, gauge pressure 5 kPa; CO2-rich MEA solvent inlet temperature 45 ± 0.5 °C, MEA concentration 30 wt%, CO2 loading 0.45 mol/mol. 63
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from the liquid phase [29,30]. Thus, increasing the liquid (MEA) flow rate can effectively reduce the boundary layer effect and enhance heat and mass transfer. In practical application, however, the flow rate (i.e., split fraction) of the bypassed rich MEA solvent should not be too large. Otherwise, it may lead to a higher total regeneration energy input in the stripper due to the higher sensible heat requirement for increasing the CO2-rich solvent temperature to the regeneration temperature. Thus, the split fraction of the bypass rich MEA solvent should be optimized based on the total energy requirements of the whole PCC system. The rich split fraction is defined by the flow rate ratio of the bypassed cold rich MEA to the total MEA (mRTot in kg/h) in PCC. The total rich MEA flow rate is shown as following.
M mRTot = ⎛ MEA + W ⎝ ∆α
αR MCO2 VCO2 ⎞∙ ∆α ⎠ 22.4
3.3. Effect of rich MEA temperature on heat transfer Fig. 6 shows the effect of the rich MEA solvent temperature on heat transfer of the heat exchangers. Obviously, the membrane heat exchanger has higher heat flux and heat recovery than the conventional SSHE. Both heat flux and heat recovery drop dramatically with the rise in rich solvent temperature. The ideal flux also decreases mainly caused by the decrease of the latent heat released by the H2O(g)/CO2 with the increase of inlet rich solvent temperature as shown in Eq. (3). For example, when the MEA temperature increases from 45° to 65°C, the heat flux declines from 27.24 to 11.57 MJ/(m2 h) and the heat recovery drops from 54.7% to 31.0% for the membrane heat exchanger. These results indicate that the split solvent temperature plays an important role in heat recovery and a small change in solvent temperature can make a big difference on heat recovery of the system. Therefore, it is necessary to minimize the cold rich solvent temperature by adopting inter-cooling technology in the CO2 absorber [33,34].
(9)
where MMEA and MCO2 are the molecular weight of MEA and CO2 in kg/ mol, respectively; W is the mass fraction of the rich MEA solvent; VCO2 is the volumetric flow rate of CO2 regenerated in the stripper, m3/h; Δα is the net cyclic CO2 capacity of MEA during absorption and regeneration, mol/mol; αR is the CO2 loading of the rich MEA solvent. For common MEA-based PCC, αR = 0.45 mol/mol, and Δα = 0.25 mol/mol [31,32]. Fig. 5 also shows that the heat flux and heat recovery of the CMHE are higher than those of the SSHE. The superior heat recovery performance of the CMHE over the SSHE can be explained by two reasons. First, both conductive and convective heat transfers associated with water transfer occur from the inner membrane surface to the rich MEA solvent in the membrane heat exchanger, while only conductive heat transfer occurs in the conventional SSHE. Second, mass transfer at the gas-membrane and liquid-membrane interfaces of the CMHE can help to minimize the boundary layer effect, which further improves the heat transfer [28]. Therefore, the CMHE shows better heat recovery performance than the conventional SSHE. In practical long-term operation for heat recovery from gas mixtures, a membrane heat exchanger has better anti-corrosion performance than a conventional SSHE, particularly in the presence of acidic gases (e.g., CO2). Although membrane heat exchangers have several advantages than the conventional ones, membrane technology has not been widely studied for heat exchanging compared with its other applications. In the future, membrane-based heat exchangers are well worth investigating for broader heat recovery and energy savings.
3.4. Effect of gas flow rate on heat transfer Two modes were investigated in this study for exploring the effect of gas flow rate on heat transfer performance. In Mode І, the rich MEA solvent flow rate was fixed at 28.29 m3/(m2 h); in Mode ІІ, the gasliquid volume ratio was fixed at 200:1 and the split fraction is fixed at 0.25 with different gas flow rates, where the rich MEA flow rate varied from 18.26 to 47.15 m3/(m2 h). It should be noted that Mode І is just adopted to test the heat recovery performance of the heat exchangers without considering the real flow rate ratio of the rich MEA to the gas mixture. Mode ІІ is more suitable to the actual situation in which the split fraction of the bypassed rich MEA solvent is fixed. The relationship between the gas mixture flow rate and total rich MEA flow rate (mRTot in kg/h) is shown in Eq. (9). For mode І, with the increase of the gas flow rate, ideal heat flux increases linearly for both the CMHE and the SSHE due to the increased latent heat within the water vapor (Fig. 7a). As a result, the heat fluxes of the CMHE and the SSHE increase but not linearly with the increase of the gas flow rate. This is because that partial latent heat within the water vapor comes back to the stripper associated with vapor condensation on the gas side. For the SSHE without mass transfer, there is no convective heat transfer but conductive heat transfer, leading to a much lower heat flux across the exchanger compared with the CMHE. As expected, heat recoveries of the heat exchangers decline Fig. 6. Comparison of (a) heat flux and (b) heat recovery using ceramic membrane heat exchanger (CMHE) and stainless steel heat exchanger (SSHE) as a function of the inlet MEA temperature. Experimental conditions: gas temperature 90 ± 0.5 °C, volumetric flow rate 7.16 m3/ (m2 h), molar ratio of H2O(g) to CO2 = 1:1, gauge pressure 5 kPa; rich MEA solvent flow rate 28.29 m3/(m2 h), MEA concentration 30 wt%, CO2 loading 0.45 mol/mol.
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Fig. 7. Comparison of heat flux and heat recovery using ceramic membrane heat exchanger (CMHE) and stainless steel heat exchanger (SSHE) as a function of the gas flow rate at a fixed rich MEA solvent flow rate 28.29 m3/(m2 h) (mode І: a, b), and a fixed gas-liquid volume ratio 200:1(mode ІІ: c). Experimental conditions: gas temperature 90 ± 0.5 °C, molar ratio of H2O(g) to CO2 = 1:1, gauge pressure 2–16 kPa; MEA solvent inlet temperature 45 ± 0.5 °C, MEA concentration 30 wt%, CO2 loading 0.45 mol/mol.
dramatically with the increase of the gas flow rate due to the shorter contact time at high gas flow rates (Fig. 7b) [13,17]. Fig. 7 further demonstrates that the CMHE has better heat recovery performance than the conventional SSHE. Particularly, when the gas flow rate is 4.8 m3/ (m2 h), the heat recovery of the membrane heat changer is up to 81%, while the heat recovery of the SSHE is 70%. For mode ІІ, the heat transfer fluxes of the CMHE and the SSHE increase linearly but their heat recoveries reduce linearly with the increase of the gas flow rate. Such changes are different from those in mode І, mainly due to the change of the rich MEA flow rate. The rich MEA flow rates increase from 18.26 to 47.15 m3/(m2 h). Higher gas and liquid flow rates reduce the boundary layer resistance and thus enhance the heat transfer [18,28]. At the gas flow rates higher than 7.16 m3/(m2 h), the MEA flow rate in mode ІІ is higher than that in mode І. For example, the MEA flow rate is 47.15 m3/(m2 h) at the gas flow rate of 11.93 m3/(m2 h) in mode ІІ, while the MEA flow rate is 28.29 m3/(m2 h) in mode І. Therefore, more heat is recovered in mode ІІ compared with mode І. However, when the gas flow rate is 4.77 m3/(m2 h), the MEA flow rate is only 18.86 m3/(m2 h) in mode ІІ, leading to a lower heat flux in mode ІІ.
3.5. Effect of gas temperature on heat transfer Fig. 8 compares the heat fluxes and recoveries of the two types of heat exchangers as a function of the gas inlet temperature. The ideal heat flux changes slightly with the increase of gas temperature mainly because the change of gas temperature only changes the sensible heat of the H2O(g)/CO2 at the fixed molar ratio of H2O(g) to CO2. Obviously, heat flux and heat recovery of the CMHE are higher than those of the SSHE. For example, when the gas mixture temperature is about 100 °C, the heat flux of the CMHE is 28.72 MJ/(m2 h), while the heat flux of the SSHE is 23.20 MJ/(m2 h) (Fig. 8a). The heat flux increases slightly with the rise in gas inlet temperature. However, the heat recovery shows interesting change trends with the increase of the gas inlet temperature. When the gas temperature is beyond 100 °C, the CMHE shows slightly decreased heat recovery performance. This is because that at higher temperatures above 100 °C the water vapor content within the gas mixture does not increase, thus water vapor transfer and its associated latent heat will not increase. At the same time, heat loss caused by vapor condensation and droplet refluxing to the stripper becomes severe. Therefore, the heat recovery shows a slight decrease at higher temperatures. The heat recovery of the CMHE is more than 55%, while 65
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Fig. 8. Comparison of (a) heat flux and (b) heat recovery using ceramic membrane heat exchanger (CMHE) and stainless steel heat exchanger (SSHE) as a function of the gas inlet temperature. Experimental conditions: volumetric flow rate of the stripped gas 7.16 m3/(m2 h), molar ratio of H2O(g) to CO2 = 1:1, gauge pressure 5 kPa; rich MEA solvent flow rate 28.29 m3/(m2 h), MEA solvent inlet temperature 45 ± 0.5 °C, MEA concentration 30 wt%, CO2 loading 0.45 mol/mol.
Fig. 9a also shows that the heat flux difference between the CMHE and the SSHE increases with the rise in molar ratio of the water vapor to CO2. This can be explained by the higher mass transfer flux at higher water vapor content for the CMHE [13]. High water vapor content with high latent heat leads to the heat flux enhancement with the increase of the molar ratio of water vapor to CO2. However, high heat flux also means temperature increase of the liquid MEA solution, leading to a decrease in the temperature difference (i.e., heat transfer driving force) between the liquid and gas streams. Therefore, when the molar ratio of water vapor to CO2 is over, the heat flux enhancement become less significant. The heat recovery is the ratio of the experimental heat flux to the ideal heat flux. When the molar ratio of water flux to CO2 is over 1.5, the experimental heat flux enhancement becomes less significant, while the ideal heat flux still increases linearly (Fig. 9a). As a result, the calculated heat recovery is relative low (< 40%) and does not follow a similar change trend with the heat flux enhancement (Fig. 9b). In practical PCC operation, the water vapor content in the gas mixture is often very high (saturated), and thus employing CMHE is superior to using conventional SSHE. Fig. 9b demonstrates almost linearly decreased heat recovery (e.g., from 69.4% to 35.1% for the
that of the SSHE is less than 51%. 3.6. Effect of water vapor content on heat transfer For heat exchanging, the recovered heat is mainly from the latent heat within the water vapor. Therefore, the water vapor content in the gas mixture has significant impacts on heat flux and heat recovery. Fig. 9 describes heat fluxes and recoveries of the two types of heat exchangers as a function of the molar ratio of water vapor to CO2. With the increase of the molar ratio of water vapor to CO2, the ideal heat flux increases linearly due to the great increase of the enthalpy of the H2O (g)/CO2 (mainly from water vapor). The heat fluxes of the heat exchangers also increase significantly, from 16.08 to 35.74 MJ/(m2 h) for the CMHE and from 13.66 to 28.19 MJ/(m2 h) for the SSHE over the water vapor/CO2 molar ratio range. When the molar ratio of water vapor to CO2 is over 1.5, the heat flux enhancement becomes less significant, particularly for the SSHE. This may be caused by the fact that much water vapor caused more severe condensation whereby heat goes back to the stripper through water droplets, rather than transfers through the contactor by conduction (Fig. 4b).
Fig. 9. Comparison of (a) heat flux and (b) heat recovery using ceramic membrane heat exchanger (CMHE) and stainless steel heat exchanger (SSHE) as a function of the molar ratio of water vapor to CO2. Experimental conditions: volumetric flow rate of the stripped gas 7.16 m3/(m2 h), gas temperature 90 ± 0.5 °C, gauge pressure 5 kPa; rich MEA solvent flow rate 28.29 m3/(m2 h), MEA solvent inlet temperature 45 ± 0.5 °C, MEA concentration 30 wt%, CO2 loading 0.45 mol/mol.
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Fig. 10. Comparison of overall heat transfer coefficients using ceramic membrane heat exchanger (CMHE) and stainless steel heat exchanger (SSHE) as a function of (a) rich MEA solvent flow rate, (b) MEA inlet temperature, (c) gas flow rate at the fixed rich MEA solvent flow rate, (d) gas flow rate at the fixed gas-liquid volume ratio, (e) gas inlet temperature, and (f) molar ratio of water vapor to CO2. The experimental conditions for each figure are the same with those for the previous figures where the corresponding parameters are the variations.
our new membrane heat exchanger (Fig. 10). In Fig. 10a, when the MEA flow rate is below 42.4 m3/(m2 h), the overall heat transfer coefficients increase with the rise in MEA flow rate, suggesting the severe boundary layer effect on the liquid side. With the increase of the MEA flow rate, the overall heat transfer coefficient of the CMHE is much higher than that of the SSHE because the mass transfer reduced heat transfer resistance at the boundary interfaces and the convective heat transfer from the mass transfer of the CMHE. With the increase of the gas flow rate, the convective heat transfer coefficient of gas flow also increases, which also contributes to the increased overall heat transfer coefficients at higher gas flow rates. Fig. 10b shows the
CMHE) with the increase of the molar ratio of water vapor to CO2. This suggests that more effective contact areas and contact time are required to increase the heat recovery when the water vapor content in the gas mixture is high. 3.7. Overall heat transfer coefficients The overall heat transfer coefficient is an important parameter in evaluating heat transfer performance of a heat exchanger. Comparison of the overall heat transfer coefficients of the two types of heat exchangers clearly demonstrates the superior heat transfer performance of 67
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significantly decreased overall heat transfer coefficients with the rise in MEA temperature, suggesting that temperature difference is the main driving force for both heat and mass transfer in the heat exchanging system. Compared the liquid flow rate (Fig. 10a), the gas flow rate at the fixed solvent flow rate has a much less effect on the overall heat transfer coefficient (Fig. 10c). At the fixed gas-liquid volume ratio, however, the overall heat transfer coefficient increases linearly with the increase of the gas flow rate (Fig. 10d). The CMHE always shows much higher overall heat transfer coefficients than the SSHE. The gas inlet temperature has a reverse effect on the overall heat transfer coefficient. With the increase of the gas inlet temperature, the overall heat transfer coefficients of both heat exchangers decline linearly (Fig. 10e). This result agrees with our previous finding in heat and water recovery from power station flue gas [13]. It can be explained by that higher gas inlet temperatures increase the heat fluxes across the contactors, leading to higher MEA temperatures on the liquid side, which will reduce the heat transfer efficiency. Fig. 10f displays that the overall heat transfer coefficient of the CMHE increases significantly with the rise in water vapor content within the gas mixture.
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14] [15]
4. Conclusion
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This work, for the first time, experimentally demonstrates the heat recovery performance of a commercial hydrophilic ceramic membrane heat exchanger (CMHE) in post-combustion carbon capture through a rich-split process modification. In the CMHE, there is a strong linear relationship between heat flux and water flux. For the membrane heat exchanger, liquid water transfer dominates the mass transfer mechanism. Thermal conduction is the key heat transfer mechanism through the membrane heat exchanger, contributing to more than 80% of the total heat transfer. Due to the coupled mass and heat transfers of water vapor through the membrane, the CMHE is superior to the conventional SSHE in terms of heat flux, heat recovery and overall heat transfer coefficient. In the future, more types of membranes with higher thermal conductivities (e.g., porous metal membranes with lower porosity and smaller thickness) should be fabricated and tested for performance enhancement.
[17]
[18]
[19]
[20]
[21]
[22]
[23] [24]
Acknowledgements The project was supported by the National Key R & D Program of China (No. 2017YFB0603300), National Natural Science Foundation of China (No. 51676080), Guangdong Provincial Key Laboratory of Soil and Groundwater Pollution Control (No. 2017B030301012), and State Environmental Protection Key Laboratory of Integrated Surface WaterGroundwater Pollution Control.
[25]
[26]
[27]
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