Harnessing Clean Water from Power Plant Emissions Using

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Harnessing clean water from power plant emissions using membrane condenser technology Jeong F. Kim, Ahrumi Park, Seong-Joong Kim, PyungSoo Lee, Young Hoon Cho, Hosik Park, S. E. Nam, and You In Park ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00204 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 30, 2018

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ACS Sustainable Chemistry & Engineering

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Harnessing clean water from power plant emissions using

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membrane condenser technology

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Jeong F. Kim1, Ahrumi Park1, Seong-Joong Kim1,2, PyungSoo Lee1, YoungHoon Cho1, HoSik

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Park1, SeungEun Nam1, YouIn Park1*

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1

Research Center for Membranes, Advanced Materials Division, Korea Research Institute of Chemical Technology, 141 Gajeongro, Yuseong, Daejeon 34114, Republic of Korea 2

University of Science & Technology (UST), 176 Gajeongro, Yuseong, Daejeon 34114, Republic of Korea

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*Corresponding Author Email Address: [email protected]

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Keywords: membrane condenser, flue gas dehydration, capillary condensation, white plume,

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ceramic membrane

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Abstract

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Power plants consume a major fraction of water to generate electricity, typically in the

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range between 30 – 50% of all fresh water sources. Most of the water from plants are lost

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with heat through stack and cooling towers. It has been reported that if 20% of these water

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can be recycled, power plants can be self-sustainable, allowing them to be located with higher

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flexibility. Membrane contactor can be an effective solution to harness this source of water,

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but most of the work have been focused on dense vapor separation membranes with limited

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success. In this work, we investigated potential application of membrane condenser

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technology to harness fresh water from power plants. It has been shown that membrane

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condenser configuration can be three orders of magnitude more effective in recovering water

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compared to dense vapor separation membranes, with a reasonable water/SOx selectivity of

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100. We have prepared suitable ceramic membranes as a proof-of-concept and achieved up to

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85% dehumidification efficiency in a single-pass flow. A thorough energy balance indicates

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that both heat and water flux must be carefully balanced to maximize the membrane

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condenser performance, and an effective module design must be developed.

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Introduction

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In the United States alone, power plants consume 40% of all available water sources (45%

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in EU) 1. It has been calculated that if 20% of the evaporated water can be recovered from a

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power plant, it can be self-sufficient from the process water point of view 2. The current

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power plants on average consume approximately 1.6 L of water to generate 1 kWh of

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electricity, which converts to 45,000 m3.hr-1 of water for a regular-sized 500 MW plant 3. As

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illustrated in Figure 1, two main sources of emissions in power plants are from the stack and

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the cooling towers. Streams emitting from a stack become saturated in the desulfurization

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step (FGD), and the streams from cooling towers are typically river or seawater evaporated to

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cool the steam cycle stream.

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Figure 1. Power Plant Operation Schematics. Significant amount of water and energy are lost through stack and cooling towers.

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The evaporated water (i.e. white plumes) also poses several downsides such as visual

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pollution, frost damage, and corrosion of chimneys and stacks. The current practice now is to

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intentionally heat up the emission stack to avoid corrosion 4, which consume additional

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energy. If the evaporated water can be effectively recovered, it can be a fruitful source of

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distilled water and latent energy, and it can relieve the exacerbating energy-water collisions,

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particularly during drought or hot weather. In addition, the technology can be valuable to

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other industries that employ water-cooling systems such as steel, semiconductors, and pulp

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industry.

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Several technologies to recover evaporated water exists such as heat exchangers,

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absorption, adsorption, and membranes. Implementation of heat exchangers to condense out

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the water vapor is an obvious option that has been considered for a long time 5, however, the

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recovered water is typically of low quality that requires further treatment, and the

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infrastructure is inherently prone to corrosion over time, appending additional maintenance

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cost. The use of liquid absorbent (glycol-based) or solid adsorbent has been partly

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implemented

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required to regenerate the absorbent makes the overall process uneconomical at the current

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level.

6-7

. The process yields high quality water that can be recycled, but the energy

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Membrane technology has been researched as a promising option to recover evaporated

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water. To reduce the water demand from the energy sector, US Department of Energy and

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Gas Technology Institute (GTI) have developed membrane-based system to recover

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evaporated water8. In Europe, CapWa2 and MATCHING grants aim to capture evaporated

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water from power plants are going through pilot trials.

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Many works exists that employ dense membranes that selectively permeate water vapor

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over other gases with excellent selectivity and permeability, reaching above 107 (H2O/N2) and

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105 barrer 9-10. However, the evaporated water through cooling tower and stack leaves at near

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atmospheric pressure, requiring additional compression or vacuum to apply the necessary

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driving force for separation. Hence, current vapor separation membranes are completely

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limited by pressure ratio11 and mass transfer resistance9, not by the membranes. There has

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been works that employ supported liquid membranes that combine advantages of absorbent

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and membranes

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analysis.

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but the work is largely in the research stage without firm economic

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Recent advances in membrane fabrication now allows more unique membrane

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configurations to be developed such as membrane distillation 13-14, membrane crystallization

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15

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actively participate in separation but provides an interface to facilitate chemical potential

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minimization (Figure 2). In the field of MC, a pioneering work was reported by GTI

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company

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Instead of permeating water vapor through a dense membrane, TMC exploits the capillary

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condensation phenomenon to selectively condense out water vapor within small membrane

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pores (vapor condenses at lower saturation pressures). With this innovative unit operation,

, membrane emulsifiers

8

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, and membrane condensers (MC)17. These membranes do not

by introducing an innovative concept of transport membrane condenser (TMC).

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Wang et al. 8 reported 19% reduction in greenhouse emission, 20% reduction in boiler water

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consumption, and 40% enhancement of process efficiency. More recent works by Zhao et

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al.18-19 and Chen et al.20-21 further investigated the efficacy of TMC configurations.

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Figure 2. Illustration of membrane-based dehydration configurations: (a) vapor permeation using a dense membrane, (b) transport membrane condenser using a microporous membrane to selectively condense water vapor within capillary pores, (c) conventional membrane condenser configuration using a hydrophobic microporous membrane to pass gas while condensing water vapor on the surface.

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Taking a slightly different approach, Drioli et al., 3, 17, 22-23 have reported conventional MC

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configuration to recover water vapor using polymeric membranes. However, as the water

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condenses outside of the pores and washes down, the quality of water was low. In addition,

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this configuration requires a new membrane module design to drain the condensed water. For

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capturing evaporated water from flue gas, many possible MC configurations exists22 but no

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clear optimizations in terms of membranes and process configurations have yet been carried

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out.

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In this work, we investigated key parameters to maximize the TMC configuration

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performance for capturing the evaporated water. We fabricated ceramic membranes and tested

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the effect of independent parameters on recovered water quality, as well as process conditions

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such as humidity, flowrates, and thermal gradients. Moreover, a full energy balance was

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carried out to reveal that TMC performance is highly dependent on the temperature gradient

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across the membrane, which can be tailored during membrane fabrication step. The obtained

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TMC performance was compared against the dense vapor separation membranes to show that

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TMC has a high potential to recover both water and energy from power plant emissions.

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Experimental

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Materials

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Ceramic alumina particle (α-Al2O3, d50=1.1 µm, 99.9%) for membrane fabrication was

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purchased from Sumimoto (Japan). Polysulfone (Ultrason S6010, BASF, Germany) was

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employed as polymer binder, DISPERBYK-190 (BYK, Altana, Korea) as dispersant,

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PEG200 (Sigma-Aldrich, Korea) as pore former, Mg(OH)2 (Sigma-Aldrich, Korea) was used

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as sintering aid. N-methlypyrrolidone(NMP, 99.9%) and ethanol (EtOH, 99.9%) were

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purchased from Samchun Chemicals (Korea). All the reagents were used without further

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purification. Hyflux20 ceramic membranes (mean pore size of 20 nm) were purchased from

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Hyflux Ltd (Singapore).

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Membrane Preparation

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Ceramic alumina membranes was prepared by extruding a ceramic dope solution (70%

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alumina particle, 6.5% polysulfone, 2% PEG200, 0.5% BYK-190, 0.3% Mg(OH)2, 20.7%

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NMP) using an extruder (FM-P20, Miyazaki Iron Works Co., Ltd., Japan) through a nozzle

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(OD: 3.2 mm, ID: 2.2 mm). Deionized water was employed as the bore solution. The

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extruded dope solution passed through an air gap of 10 cm then immersed in waterbath for

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phase inversion. The prepared pristine membrane was washed thoroughly in 20%

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EtOH/water for 24 hrs then sintered at 1450 oC for 2 hrs.

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Membrane Characterization

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Membrane water permeability was measured using a crossflow system at the transmembrane

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pressure of 1 bar. The membrane separation performance was tested against 1000 ppm

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polystyrene (PS) particle (30 nm and 100 nm particle size) colloid solution at 1 bar. The

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permeate and retentate stream was collected separately and their PS concentrations were

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measured using a UV spectrometer (UV-2401PC, Shimadzu, Japan) at 200 nm. The rejection

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of PS particle was calculated using Equation 1.

R (%) = 1 −

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(Eq. 1)

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where R stands for membrane rejection, and Cp and CR represent permeate and retentate

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concentrations, respectively.

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Pore size distribution was characterized using a capillary porometer (CFP-1200, PMI, USA).

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Each sample was first wetted with Galwick solution then tested up to 15 bar. Membrane

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morphology was observed using scanning electron microscopy (TM-3000, Hitachi, Japan).

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Flue gas dehydration apparatus & membrane modules

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Air dehydration experiments were carried out using the apparatus illustrated in Figure 3.

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Ceramic membrane (KRICT100 and Hyflux20) was first potted into a hollow fiber module

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and loaded onto the apparatus. The effective surface area of KRICT100 and Hyflux20

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module was 53.2 cm2 and 71.6 cm2, respectively.

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A humidified air stream was prepared by bubbling dry air through a heated waterbath. The

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relative humidity was controlled by mixing with a separate dry stream. The flowrate of each

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stream was controlled using a mass flow controller. The humidified stream was fed into the

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shell side of the membrane module. The water vapor condenses on the outer surface of the

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membrane and subsequently permeates through the membrane. The temperature and relative

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humidity of the retentate stream was then measured using a hygrometer. For the experiments

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to assess SO2 permselectivity, a separate SO2 stream (1000 µmol/mol N2) was mixed with the

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feed stream to give 100 ppm SO2 concentration. The feed stream gauge pressure was

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approximately 0.03 bar.

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A cooling water stream was circulated through the bore side of the membrane module. A

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slight negative pressure was applied (-0.3 bar) using a booster pump (LFP1150S, PS holding

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Ltd., South Korea) by placing the pump downstream of the module. The flowrate of the

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circulating water was controlled using a bypass line. The temperature of the water was

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maintained at 20 oC using a chiller. A reservoir tank was placed on a balance that is connected

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to a computer, and the increase in mass reading was continuously measured to calculate the

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water flux. A calibrated pH probe was immersed in the reservoir tank to measure the change

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of pH over time which was used to calculate the SO2 flux. A summary of experimental

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parameters are shown in Table I

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Table I. Summary of Dehydration Experimental Parameters Parameter

Set Point

Unit

Air Flowrate (Fair)

1000, 2000, 3000, 4000, 5000, 6000

sccm (STP)

Air Temperature (Tair)

60, 70, 80

Air Relative Humidity (Hair)

50, 80

%

Cooling Water Flowrate (Fwater)

0.9

L.min-1

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C

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Cooling Water Temperature (Twater)

20.0 ± 0.2

Differential Pressure (∆P)

-0.3

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o

C

bar

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Figure 3. Dehydration Experiment Test Apparatus. Black lines indicate hot gas stream flows, blue lines indicate cold liquid stream flows. MFC – Mass Flow Controller, F – flowmeter, T – thermometer, H – hygrometer, P – pressure gauge.

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Results and Discussion

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Transport Mechanisms

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In order to optimize the membrane condenser (MC) performance, it is important to

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clearly distinguish the difference between condensable vapors and non-condensable gases

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when discussing membrane transport mechanisms. Contrary to non-condensable gases (e.g.

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O2, N2), condensable vapors (e.g., H2O) can condense within small pores and block the

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transport of other gases. According to the Kelvin’s Equation (Eq. 2), the saturation pressure

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of condensable vapors decreases with decreasing pore size. Hence, the vapors condense more

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readily within pores. This phenomenon is commonly referred to as capillary condensation

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and many interesting works have applied this phenomenon to separate alcohol/water 25-26

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,

27-28

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olefin/paraffin

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phenomenon, it is possible to selectively condense vapors within the pores, and subsequently

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permeate the condensed liquid.

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, and water vapors

 

. Exploiting this capillary condensation





ln    = −2    



(Eq. 2)

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where ρ is the condensate density, R is the gas constant, T is the temperature, M is the

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molecular weight of the condensable compound, Pc is the capillary condensation pressure, Po

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is the vapor saturation pressure at a planar interface, σ is the interfacial tension, θ is the

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contact angle, and rp is the membrane pore radius.

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In addition to the transport mechanism, researchers have tried different MC

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configurations, as summarized in Figure 2(b), Wang et al.8 and Zhao et al.18 have exploited

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the condensation phenomenon to extract water using the transport membrane condenser

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(TMC) configuration. On the other hand, Drioli et al.3, 17, 23 have employed hydrophobic

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membrane to condense out water on the membrane surface while allowing dehydrated gas to

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permeate (Figure 2(c)). Compared to the TMC configuration, employing a hydrophobic

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membrane to induce surface condensation requires non-conventional module design to collect

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the liquid retentate, and typically results in low quality water, similar to using heat

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exchangers. Therefore, TMC configuration holds more potential to recover water in higher

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purity with a simpler module design.

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Before investigating the performance efficiency of membrane condensers, one must

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carefully consider the thermodynamic aspects of the overall process. As illustrated in Figure 1,

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the water vapor emitting from the cooling towers were intentionally evaporated to utilize the

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latent heat to cool the exothermic stream. Therefore, one must ask whether it is

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thermodynamically logical to re-condense the evaporated stream, which also requires

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considerable amount of cooling energy. One plausible explanation is that since the evaporated

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water has been distilled to some degree, the energy input can be justified if high quality water

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can be harnessed. In addition, as proposed by Wang et al.,8 the heat of condensation of

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evaporated water can be re-utilized to heat the boiler feed stream.

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It should be emphasized that capturing the evaporated water must be approached from

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the environmental perspectives, as minimizing water consumption is one of the top priorities

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for power plants. Therefore, it is crucial to develop an energy efficient process for capturing

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evaporated water to relieve the energy-water collisions.

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Membrane Characterization

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To avoid condensate-induced corrosion, the flue gas from stacks typically exit at an

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elevated temperature (70 ~ 90 oC) in a saturated state. Although polymeric membranes

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currently dominate the membrane market, the long-term thermal stability of polymer

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membranes in humid conditions is not guaranteed29. On the other hand, ceramic membranes

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hold distinct advantages such as excellent thermal and chemical stability over long term.

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Therefore, ceramic membranes have been tested in this work as a proof-of-concept.

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The in-house fabricated ceramic membrane (KRICT100) and commercial ceramic

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membrane (Hyflux20) was characterized using SEM, porosimetry, and rejection tests,

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summarized in Figure 4 and 5.

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Figure 4. SEM images of KRICT100 and Hyflux20 membranes. Hyflux20 membrane has a

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γ-alumina coating layer in the inner side.

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Figure 5. Pore size distribution data of KRICT100 and Hyflux20 membranes.

225 226

Table 2. Characterization of Ceramic Hollow Fiber Membranes

Memb.

PWP (Lm-2h-1bar-1)

Max pore size (nm)

Mean pore size (nm)

OD (mm)

ID (mm)

KRICT100

395

208

90

2.78

2.12

0.33

-

83.1%

Hyflux20

329

44

39

3.87

2.91

0.48

90.1%

-

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Thickness Rejection Rejection (mm) (PS 30 nm) (PS 100 nm)

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It can be seen that the ceramic hollow fiber membranes show isotropic morphology

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throughout the membrane thickness. In comparison to the KRICT100 membranes, Hyflux20

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membrane is coated with a γ-alumina layer on the inner surface of the fiber. The mean and

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maximum pore size of Hyflux20 was measured to be 40 nm, and 44 nm, respectively. And 30

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nm polystyrene particle rejection test gave 90.1% rejection.

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On the other hand, KRICT100 membrane exhibited the mean and maximum pore size of

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90 nm and 209 nm, respectively. And 100 nm polystyrene particle rejection test gave 83.1%

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rejection. Hence, the overall pore size of KRICT100 membranes is higher than that of

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Hyflux20 membranes. These two membranes have been tested for dehydration experiments.

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Flue gas dehydration

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Figure 6. Transport membrane condenser (TMC) water flux as a function of dry air flowrate

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for (a) KRICT100 and (b) Hyflux20 membranes.

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Figure 7. Transport membrane condenser (TMC) performance and dehumidification

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efficiency as a function of feed water vapor flowrate.

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The dehydration data are summarized in Figures 6 and 7. Several clear trends are

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visible from the experiments. First, the water flux increases proportionally with respect to the

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increasing feed flowrate, temperature, and humidity. Such trend is expected as increase in

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temperature and humidity increases vapor pressure gradient at the membrane outer surface.

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A better correlation can be obtained by plotting the membrane flux against the feed

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vapor flowrate. As shown in Figure 7, there is a strong linear relationship between the feed

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vapor flowrate and water flux through the membrane. On the other hand, there is no clear

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difference in performance between KRICT100 and Hyflux20 membranes, despite the

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difference in the pore size. Such trend clearly implies that the performance is limited by the

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rate of condensation, not by the rate of permeation (membrane performance). More

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specifically, the basic mechanism of TMC can be seen as condensation followed by

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permeation, which can be semi-qualitatively described using the resistance-in-series model

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(Eq. 3).

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J =

!  "#$ ' ()*+  ∆&#$

∆& )*+

=



,

-#∆.

!

'

,  - )*+ ∆(



(Eq. 3)

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where Rcond and Rperm are condensation and permeation resistances, respectively, kcon and

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kperm are rate constants for condensation and permeation, and ∆µ, ∆T, ∆P represent chemical

260

potential, temperature, and pressure gradients, respectively.

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The main driving forces for water condensation are the temperature and vapor

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pressure gradients between the membrane surface and the feed vapor stream, and the driving

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force for liquid water permeation through the membrane is the transmembrane pressure. As

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shown in Table 2, the permeability (kperm) of KRICT100 and Hyflux20 membranes are 329,

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395 L.m-2.hr-1.bar-1, respectively, indicating that these membranes are not permeation-limited.

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Hence, it can be deduced that the rate constant of condensation (kcon) is significantly lower

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than the permeability (kperm), making the overall process condensation limited, as observed in

268

Figure 7. Therefore, controlling the temperature and vapor pressure gradients at the

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membrane surface is crucial to maximize the TMC performance.

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Thirdly, the dehumidification efficiency plot exhibits a saturation trend, as shown in

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Figure 7(b). It can be seen that using the current membrane module, the maximum

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dehumidification efficiency approaches 85%. As the stream gets dehumidified within the

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membrane module, the driving force (vapor pressure gradient) for further dehumidification

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proportionally decreases, plateauing the dehumidification efficiency at 85%. This trend

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suggests the importance of module design and a delicate control of driving force (vapor

276

pressure gradient)

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The performance obtained in this work is compared against other membrane-based

278

dehydration processes in Table 3. It should be noted that performance comparison between

279

different processes can be subjective; however, the overall process productivity can be

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qualitatively compared in terms of dehumidification efficiency.

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Table 3. Literature Data for Gas Stream Dehydration Year

Material

Pore Size

Mode

2008

P(AA-AMPS)-PVA

Dense

Vapor diffusion, pressure

2008

sPEEK

Dense

Vapor diffusion, vacuum

2008

PVA

Dense

Vapor diffusion, sweep

2012

PEBAX

Dense

2005

Ethyl Cellulose

2005

Application Propylene dehyd. Flue gas dehyd.

Highest Flux (kg/m2/hr)

Selectivity

Ref 30

0.03 1

H2O/N2 > 107

4

Air dehyd.

0.2

N/A

31

Vapor diffusion, sweep

CH4 dehyd.

0.03

H2O/CH4 > 1500

Dense

Vapor diffusion, sweep

N2 dehyd.

0.25

9

Polysulfone

Dense

Vapor diffusion, sweep

N2 dehyd.

0.02

9

2005

PEO-PBT

Dense

Vapor diffusion, sweep

N2 dehyd.

0.17

9

2012

Ceramic membrane

6 - 8 nm

TMC, water cooling

Flue gas dehyd.

4.5

N/A

8

2015

Ceramic membrane

20 nm

TMC, water cooling

Flue gas dehyd

22

N/A

18

2017

Ceramic membrane

20 nm

TMC, water cooling

Flue gas dehyd

7.5

N/A

20

2017

α-Al2O3 ceramic

20 - 100 nm

TMC, water cooling

Flue gas dehyd.

12

H2O/SO2 > 100

This work

32

282 283

As summarized in Table 3, most of the dehydration work have been performed using

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dense vapor separation membranes with outstanding permeability. However, as mentioned

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previously, emissions from power plants are high temperature streams (70 ~ 90 oC) near

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atmospheric pressure, requiring either compression or vacuum to provide the necessary

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driving force for vapor permeation. As expected, these high performance membranes with

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outstanding permeability are completely limited by pressure ratio, resulting in poor

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performance in real scales 4. On the other hand, TMC configuration can exploit the

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temperature gradient to selectively condense out water vapors in high efficiency (85% in this

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work). In addition, it can be seen in Table 3 that TMC membrane performance outperforms

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the dense vapor separation membranes, from one order of magnitude to as high as three

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orders of magnitude.

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Apart from the dehydration performances, emissions from power plant stacks contain

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small percentage of SOx and NOx compounds, even after the flue gas desulfurization (FGD)

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unit. According to Wang et al. 8 the condensate stream from TMC process is pure enough to

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be fed directly into the boiler feed water. It was claimed that capillary condensation prevents

298

permeation of SOX species; however, it should be noted that SOX species immediately get

299

oxidized under the presence of O2 and exists as a sulfurous acid (and sulfuric acid) by

300

accepting a proton from water vapor. Hence, it is yet questionable whether the mode of

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separation is by capillary condensation, or simply a competition between condensation (water)

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and absorption (SOx).

303 304

Figure 8 summarizes the water and SOx flux through Hyflux20 membranes in TMC configuration. The selectivity (α) is calculated using Eq. 4. 2345)* 76 α01 = 289 345)* :; 689:

305

(Eq. 4)

306

It can be seen that water to SOx selectivity is approximately 100 independent of the vapor

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flowrate, which is, interestingly, much higher than the simple Knudsen selectivity of 1.8

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(square root of molecular weight ratio). With the measured water/SO2 selectivity of 100, the

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overall process must be carefully designed to match the product stream quality. For instance,

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the residence time for the cooling water stream must not exceed the point at which the

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product quality is irreversibly compromised. It has been hypothesized that the membrane

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pore size can affect the membrane SO2 selectivity 8; however, it is likely that SO2 flux is a

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strong function of the liquid-gas interface area, hence it is necessary to induce vapor

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condensation within the pores to form a meniscus. Nevertheless, the TMC selectivity against

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SO2 still remains to be validated and the key parameters needs to be found to maximize the

316

selectivity.

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317 318

Figure 8. Water and SO2 flux through Hyflux20 membrane as a function of water vapor

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flowrate, selectivity (αTMC) value is written above each flux data.

320 321

Transport Membrane Condenser Heat Balance Profile

322

As shown in Section 3.3, the current membrane condenser technology is almost entirely

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limited by the rate of condensation, which is a strong function of temperature gradient

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between the membrane surface and the vapor feed stream. In order to design an effective

325

membrane and module, it is first necessary to understand the key parameters that determine

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the overall heat transfer coefficient.

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To model the temperature profile of the membrane and the system, a steady state energy

328

balance can be made around each fiber (See Supporting Information for detailed modeling

329

calculations). For analysis, the heat transfer coefficient at the air-membrane interface (outer

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fiber surface) was estimated using the Colburn correlation, and the heat transfer coefficient at

331

the membrane-water interface (inner fiber surface) was estimated using the Sieder-Tate

332

correlation.

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Figure 9. (a) Calculated membrane outer surface temperature as a function of feed air temperature for polymeric and ceramic membranes; (b) membrane temperature profile along the fiber thickness.

337

In order to maximize the driving force (temperature and vapor pressure gradient), it is

338

necessary to maintain a wide temperature gap between the feed stream and the membrane

339

outer surface temperature. Therefore, it is desired to keep the membrane outer surface

340

temperature as low as possible. Figure 9(a) clearly illustrates the effect of material thermal

341

conductivity on the membrane outer surface temperature. Assuming a membrane porosity of

342

70%, ceramic membranes (alumina) with high thermal conductivity (kalumina = 35 W.m-1.K-1)

343

can effectively maintain low surface temperature compared to typical polymeric membranes

344

(kPVDF = 0.19 W.m-1.K-1). Figure 9(b) illustrates the effect of feed temperature on the

345

temperature profile across the membrane cross-section. It can be seen that polymeric

346

membranes exhibit steeper temperature gradient along the thickness compared to ceramic

347

membranes, primarily due to the low thermal conductivity of the material itself.

348

Therefore, from the performance perspective, it certainly is more effective to utilize

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ceramic membranes for membrane condenser applications. However, ceramic membranes are

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brittle, rendering them difficult to handle in large scale. On the other hand, polymeric

351

membranes exhibit relatively low thermal stability but can be more cost effective. Since

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membrane condenser process is a relatively new technology, it is yet unclear which

353

membrane material can give economic and process advantage in large scale, and both options

354

are being explored. Nevertheless, the main goal is to maintain low surface temperature to

355

maximize the driving force.

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Figure 10. Calculated membrane surface temperature as a function of thickness and porosity,

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for (a) polymeric membrane; and (b) ceramic membrane.

359

When fabricating membranes, apart from tailoring the pore size, independent parameters

360

under control include membrane dimensions (diameter and thickness) and porosity. Figure 10

361

illustrates the effect of these parameters on the membrane condenser performance (outer

362

surface temperature) for polymeric and ceramic membranes. As expected, it can be seen that

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the membrane temperature increases with increasing thickness for both materials.

364

Interestingly, it was found that the two studied materials give opposite trends as a

365

function of porosity. For polymeric materials, membranes with higher porosity exhibit lower

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membrane temperature. On the other hand, for ceramic membranes, low porosity display

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lower membrane temperature. Such opposite trends results from the assumption that the open

368

pores are filled with water during membrane condenser operation, and water has a thermal

369

conductivity (k = 0.67 W.m-1.K-1) between that of ceramic and polymeric material.

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The trends observed in Figures 9 and 10 can give an important direction to tailor the

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membrane characteristics to improve the membrane condenser productivity. For polymeric

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membranes, it is desirable to maximize the membrane porosity while reducing the thickness.

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It should be noted that as membrane condenser operates at relatively low transmembrane

374

pressures, the mechanical integrity is not expected to be a significant issue. For ceramic

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membranes, lower porosity is preferred yet has negligible effect on the membrane

376

temperature due to its high thermal conductivity. Instead, more focus can be placed on

377

controlling the membrane pore size to improve the condensed water quality.

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With the escalating water shortage and exacerbating energy-water collisions, reducing the

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water consumption from power plants is becoming an important topic from both

380

environmental and economic perspectives. Based on this work, it is envisaged that membrane

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condenser technology can play a unique role to harness clean water from flue gas emissions,

382

and further research on large scale process feasibility will be the topic of our subsequent work.

383 384

Conclusions

385

In this work, an effective method to harness clean water from power plant emissions using

386

membrane condenser technology is proposed. Compared to dense vapor separation

387

membranes that suffer from low driving force, the proposed transport membrane condenser

388

(TMC) configuration exhibited water flux up to 12 kg.m-2.hr-1, as high as three orders of

389

magnitude higher compared to the vapor separation membranes. In addition, the TMC

390

process gave a reasonable water/SOX selectivity of 100, which is much higher than the

391

Knudsen selectivity of 1.8. It was determined that the current TMC process is completely

392

limited by the rate of condensation, and a better membrane and more effective module design

393

must be developed that enhances the vapor pressure gradient. The current limit of

394

dehumidification efficiency was determined to be approximately 85%, after which the driving

395

force cannot be maintained to induce water condensation. It is envisaged that further

396

dehydration can be achieved by controlling the membrane pore size to induce capillary

397

condensation. The exact nature of water/SOx selectivity has not been fully elucidated, but it is

398

speculated the SOX permeation is a strong function of contact surface area at which SOX can

399

absorb. In order to utilize the condensate product for boiler feed water, the process design

400

must minimize the cooling water residence time while maximizing the condensation rate.

401

This work showed a potential to apply TMC process to harness clean water form power plant

402

emissions, but further membrane development, preferably with polymeric materials, is

403

necessary to suit the TMC requirements.

404

Supporting Information

405

Supporting Information includes 7 equations with detailed heat balance simulations.

406

Acknowledgement

407

This project was supported by the R&D program of the institutional research program of

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Korea Research Institute of Chemical Technology (KK1802-B00 and BS.K18M503)

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503 504

The currently unsustainable water and energy loss from power plants can be recovered using

505

membrane condenser technology

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