Study of Dehumidification and Regeneration in a Starch Coated

Research Article pubs.acs.org/journal/ascecg

Study of Dehumidification and Regeneration in a Starch Coated Energy Wheel Leila Dehabadi,§ Farhad Fathieh,*,† Lee D. Wilson,§ Richard W. Evitts,‡ and Carey J. Simonson† §

Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, SK, Canada, S7N 5C9 Department of Mechanical Engineering and ‡Department of Chemical & Biological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, Canada, S7N 5A9

†

ABSTRACT: Energy wheels were designed using desiccantcoated materials to exchange energy between supply and exhaust air streams to reduce the energy required to condition ventilation air. Recent advances in energy wheel design that use biopolymer desiccant materials have provided the motivation in this paper to investigate the performance of starch coated energy exchangers. A small-scale high amylose starch (HAS) coated exchanger was built, and its transient response to a step change in inlet humidity was acquired and studied at conditions of variable air flow rate and temperature. The humidity response, dehumidification, regeneration capacity, and the rate of moisture removal were measured. The results demonstrate that greater regeneration flow rate and temperature can enhance the regeneration capacity. In spite of the increase in the regeneration capacity, the moisture recovery (latent) effectiveness decreased as the air flow rate and/or temperature increased. The effectiveness is reduced by 15% when the flow rate increases from 10 to 50 L/min at ω = 2 rpm; whereas, the difference in effectiveness is less than 4% at ω = 20 rpm. When the air flow temperature increased by 15 °C, insignificant changes in effectiveness were found below 4%. The results of this research are anticipated to contribute to future research and development on biopolymer coated energy exchangers. KEYWORDS: Starch, Water vapor, Energy wheel, Latent effectiveness, Regeneration, Transient

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typically occurs at elevated temperature and/or at higher flow rates.5 Latent effectiveness is generally used to quantify the moisture recovery performance in the wheels, and it is defined as the ratio of the actual moisture transfer rate to the maximum possible moisture transfer rate between the humid and dry air streams.6 Over the past decades, numerous analytical and experimental studies have been conducted to determine energy wheel performance at variable operating conditions.7−15 Moreover, several empirical correlations are proposed to predict the latent effectiveness based on the matrix structure and operating conditions.16−19 It was found that the desiccant sorption properties play an important role in the moisture recovery of energy wheels.20 The sorption capacity, sorption rate, and regeneration temperature are key factors which affect the latent effectiveness of desiccant wheels.21 In this regard, a desiccant with high rate of sorption and capacity with a low barrier of desorption activation energy is a preferred material for coating the wheel surface.20 Recently, biopolymers have been modified to yield materials with enhanced sorption capacity, regeneration ability, and

INTRODUCTION The need to develop energy exchangers with greater efficiency is a topic of great interest according to projected increases in energy demand based on trends over the past few decades. In developed countries, energy standards have established that heating/cooling, ventilating, and air conditioning (HVAC) units should be equipped with energy recovery exchangers.1−3 For this reason, energy (enthalpy) wheels have been extensively used as conventional air-to-air energy exchangers to regenerate energy (heat and moisture) from exhaust air and reduce overall energy consumption in HVAC systems. Energy wheels have a cylindrical matrix with a large number of honeycomb or corrugated flow channels. The wheel matrix is fabricated from metallic or plastic substrate coated with micrometer size desiccant material. Among various desiccants, silica gel, molecular sieves, activated alumina, and zeolites are widely used to coat the wheel matrix surface. The wheel matrix is divided by an impermeable plate into the supply and exhaust sections. While the humid air passes through the wheel channels, the desiccant particles adsorb water vapor (dehumidification process). Then, by turning the wheel, channels become exposed to the dry airstream and the accumulated moisture in the wheel matrix surface is released to the airstream (regeneration process). The continuous rotations of the wheel causes periodic moisture transfer between the supply and exhaust airstreams.4 The regeneration cycle © XXXX American Chemical Society

Received: June 10, 2016 Revised: October 24, 2016 Published: October 28, 2016 A

DOI: 10.1021/acssuschemeng.6b01296 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

The model parameters, kYN and τYN, are obtained by fitting the model to the breakthrough curves. The Yoon−Nelson model is relatively simple compared with other kinetic adsorption models that require extensive data on the adsorbate and adsorbent characteristics.2 Herein, the water vapor breakthrough curves on HAS particles were analyzed by applying the Yoon−Nelson model to experimental test data. Double Exponential Model. A double exponential model was also used to estimate the transient response of the exchanger. This model is based on the principle that the breakthrough curves are described by two exponential terms, as follows:

long-term stability. Research efforts on the development of new desiccants have resulted in materials with promising properties. However, comparatively few studies have evaluated starch biomaterials as alternative coatings for energy wheel/dehumidifiers.22,23 The field of adsorption science and technology has witnessed increasing attention toward the use of biomaterial adsorbents due to their unique adsorption properties. Starch and cellulose biopolymers are renewable resources of great interest due to their biodegradability, structural variability, relative, and synthetic versatility. Starch is a relatively low cost and environmentally benign biopolymer24 composed of amylose and amylopectin that results in linear and branched structures, respectively. Amylose is a linear polymer containing D-glucose units with α (1 → 4) linkages with average molecular weight of ranging from 1−5 × 104 amu with a degree of polymerization (DP) as high as 600.25 By contrast amylopectin is a branch biopolymer containing D-glucose units with α (1 → 6) linkages and variable molecular weight. Previous studies have reported on the affinity of starch, cellulose, and hemicellulose materials with water.26 The unique water adsorption properties of starch materials relate to the biopolymer morphology according to the relative amylose and amylopectin content.27 Recently, Dehabadi and Wilson reported that starch biopolymers with variable amylose and amylopectin content reveal unique adsorption properties toward water vs ethanol, according to the biopolymer structure.28 Fathieh et al.6 recently reported that the latent effectiveness of a wheel coated with high amylose starch (HAS) with a 15 μm particle size and 46 Å pore width is 2−13% higher than a wheel coated with the same amount of silica gel (55 μm particle size and 77 Å pore width). Accordingly, HAS was reported to be a promising biopolymer desiccant alternative for energy wheel/dehumidifiers. However, the reported results were limited to a relatively narrow range of operating conditions. In this research, we report a systematic investigation of the performance of an HAS coated wheel for a wider range of air flow rates and temperatures. The transient test was conducted using a small-scale HAS coated energy exchanger to determine the latent effectiveness of the full-scale wheel with the same matrix materials. The structure and sorption properties of HAS was specified by different characterization methods. A test facility described in our previous work6 was used to characterize the moisture uptake/removal properties within the exchanger during the transient testing. Furthermore, two analytical models were applied to the results to specify the transient sorption characteristics of the HAS coated exchanger.

⎧1 − γ e−t / τ1 − γ e−t / τ2 , 0 ≤ t ≤ ∞ step increase ⎪ 1 2 W (t ) = ⎨ ⎪ γ e−t / τ1 + γ e−t / τ2 , 0 ≤ t ≤ ∞ step decrease ⎩1 2 (2)

W(t) is the normalized humidity given by eq 3 where w is the air humidity ratio, w = gwater/gair. w − winit W (t ) = out wfinal − winit (3) In eq 2, γ and τ are the weighing coefficient and time constant of the wheel response to inlet humidity changes. The time constant and weighting factor were obtained by subjecting the exchanger to step changes in the inlet humidity, where the breakthrough curves were fit by a double exponential model (DEM). The two time constants and weighting factors for the double exponential model correspond to the fast sorption mode. The latter process likely occurs on the external surface and macropore domains, while slow moisture transfer occurs at the micropore sites of the desiccant material. The DEM can be also used to determine the latent effectiveness of energy wheels as indicated in a previous study.3 Based on the definition, the latent effectiveness of the wheel is calculated by4 εl =

(1 − e−π / ωτ1)2 1 ⎡ Ntu m = − ln⎢1 − 2ωγ1τ1 2 ⎣ 1 − e−2π / τ1 − 2ωγ2τ2

THEORY Yoon−Nelson Model. The Yoon−Nelson model1 was used to describe the adsorption behavior of breakthrough curves for transient adsorption processes. This concise model is based on the principle where a decrease in the adsorption probability of adsorbate is proportional to the probability of breakthrough. According to the Yoon−Nelson model, the normalized humidity during the transient period (breakthrough curves) can be expressed as

(1 − e−π / ωτ2)2 ⎤ ⎥ 1 − e−2π / ωτ2 ⎦

(5)

In eq 5, ω is the wheel angular speed. Herein, the small-scale exchanger was subjected to the sudden changes in the inlet humidity and the response of the exchanger (γ and τ) by monitoring the temperature and the relative humidity of the outlet stream. Therefore, the Ntum can be calculated by eq 5 which leads to determination of latent effectiveness by eq 4. The qualities of the fittings obtained from the DEM and Yoon−Nelson models were compared to demonstrate that the DEM model accounts for the transient behavior of the coated exchanger. Moisture Uptake/Removal Calculations. The moisture uptake (Ωads) and removal (Ωdes) per gram of desiccant during the moisture uptake (dehumidification) and removal (regeneration) steps were calculated by eqs 6 and 7.

e(kYNt − kYNτYN) 1 + e(kYNt − kYNτYN)

(4)

The number of transfer units, Ntum, was obtained from the analytical relation proposed by Fathieh et al.5 as follows:

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W (t ) =

Ntu m 1 + Ntu m

(1)

In eq 1, kYN is the Yoon−Nelson rate constant that corresponds to the adsorption rate and τYN is the time at which the breakthrough curve reaches 0.5 of the normalized changes. B


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ACS Sustainable Chemistry & Engineering Ωads =

ṁ a (wfinal − winit) Mdes

∫0

Ωdes =

ṁ a (winit − wfinal) Mdes

∫0

area and pore structure characterization using a Micromeritics ASAP 2020 (Norcross, GA). The surface morphology of the exchanger sheets coated with high amylose starch was obtained using a scanning electron microscope (SEM) with a Hitachi FEG-SEM SU6600. Small-Scale Exchanger. The moisture recovery performance of an HAS coated wheel was investigated by testing a small-scale exchanger coated with HAS particles, where an illustration is shown in Figure 2. The exchanger has 16 aluminum sheets with specific physical

t

(1 − W ) dt

(6)

t

W dt

(7)

Q a is the air volumetric flow rate (m /s), ρa is air density (g/m3), and Mdes is mass of desiccant (g). 3

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MATERIALS AND EXPERIMENTAL METHODS

Material. In this study, the sorption of water vapor on a sample of high amylose starch (HAS) was investigated through dehumidification (adsorption) and regeneration (desorption) processes. HAS was chosen herein because of its high water vapor sorption capacity when compared against other types of starch materials.28 The molecular structure of high amylose starch (HAS) is shown Figure 1. Starch is a structurally complex biopolymer since it may

Figure 2. Schematic illustration of a small-scale energy exchanger.

Table 1. Physical Properties of the Substrate Aluminum Sheets Figure 1. Conceptual view of the molecular structure of high amylose “linear” starch, where the D-glucose monomer unit is shown in brackets and n represents the degree of polymerization.

material

thickness (mm) δm

width (mm) b

length (mm) L

weight sheet (g)

Al-3003

0.62 ± 0.02

80 ± 2

200 ± 2

30.78 ± 0.15

properties as outlined in Table 1, where the aluminum sheets are oriented in parallel to form 15 flow channels with a fixed dimension (4 mm in height and 8 mm in width). The resulting geometry of the exchanger has channels with a hydraulic diameter of 7.2 mm. A metallic/plastic foil, ca. 0.050 mm thick, is conventionally used for the manufacture of such typical industrial wheels. In this study, a thick aluminum substrate (0.62 mm) was used to build the small-scale exchanger since this configuration avoids nonuniform distribution in the flow channel and provides a rigid and smooth surface for coating of desiccant. The fine HAS powder was coated on both sides of the planar sheets using a powder coating method described elsewhere.6 Transient Test Facility. A test facility was developed to evaluate the response of the small-scale coated exchangers while a step change in the inlet humidity was introduced.37,38 The response of the smallscale exchanger was characterized by monitoring the temperature and humidity at the exchanger inlet and outlet during a single step test. A test section was prepared from extruded polystyrene insulation to place the exchanger during the dehumidification and regeneration test. A schematic illustration of the test section along with the small-scale exchanger are shown in Figure 3. Dry and humid air streams at specified different flow rate and temperatures are listed in Tables 2 and 3. The temperature profile at the inlet and outlet side was measured by an array of T-type thermocouples. Using a Hart Scientific 9107 (with a transfer standard bias uncertainty of ±0.1 °C), the thermocouples were calibrated within the range of 15−85 °C with 45 measurements at 5 °C increments. In addition, the response of the thermocouples to a step change in the temperature was thoroughly studied and reported elsewhere.5,7 The precision uncertainty for the measured temperature was obtained by repeating the test six times at fixed operating conditions, where a total uncertainty ±0.2 °C was determined. The humidity measurements were obtained by highly accurate humidity sensors (Honeywell type HIH-4021) at upstream and downstream sides of the exchanger with a total uncertainty (±2% RH) estimated over a 10−90% RH calibration range at 23 °C. Humidity sensors were calibrated by using Thunder Scientific 1200 mini two-pressure twotemperature humidity generator with bias uncertainty of 0.5% RH. At each equilibrium condition, 40 RH measurements with sampling

contain variable amounts of amylose and amylopectin.29 The fractional composition of amylose and amylopectin varies depending on the origin of the plant source. Variable amylose/amylopectin composition of starch materials can be achieved through plant breeding. For instance, tapioca starch is 17% amylose (and 83% amylopectin), while maize and wheat starch are 28% amylose (and 72% amylopectin).30 Starch derived from maize has amylose content around 70% (amylomaize) or ca. 0% (waxy-maize) .31 The presence of hydroxyl functional groups for the D-glucosyl monomer units of starch may contribute to its amphiphilic behavior, according to the conformational preference of the polysaccharide due to the exposure of apolar −CH groups vs polar −OH groups. Thus, the helical structure of amylose contains six glucose monomer units per turn in aqueous solution, where the helix interior is more apolar and the exterior is hydrophilic in nature.32,33 Additional evidence for the effect of molecular conformation on amphiphilic properties is shown by cycloamylose biopolymers (e.g., α-, β-, and γ-cyclodextrins).34 The macrocycle interior is apolar in nature, while the outer periphery is hydrophilic in nature due to the relative orientation of the −OH and −CH groups of the D-glucosyl mononomer units. The water affinity of starch relates to the accessibility of available −OH groups of the glucosyl units. It is important to note that hydrogen bonding along with capillary condensation is responsible for adsorption of water.32−36 The HAS sample selected for this investigation was characterized in a previous study6 (HAS, dp = 15 μm, Pw = 46 Å), where this biopolymer showed enhanced sorption properties of water vapor when compared with mesoporous silica gel. Characterization of the Samples. The structural characterization of HAS starch employed Fourier transform infrared (FT-IR) spectroscopy, particle size analyzer, nitrogen gas adsorption, and SEM, as described in a previous study.5 In brief, the FT-IR spectra of HAS was obtained with a BIO-RAD FTS-40 spectrophotometer operating in diffuse reflectance mode at a resolution of 4 cm−1 dispersed in KBr powder. The particle size of HAS was estimated using laser diffraction using a Mastersizer S Long Bench Particle Size Analyzer (PSA), Malvern Instruments. Nitrogen gas adsorption was used for surface C


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Figure 3. Test section, sliding and supporting plates, air ducts, and the small-scale exchanger within the test cell.

Table 2. Test Conditions for the Transient Dehumidification/Regeneration Tests at Different Air Flow Rates Q a (L/min)

vf (m/s)

Redh

Tair (°C)

RHdry (%)

RHhumid (%)

ΔRHstep (%)

10 ± 1 25 ± 1 50 ± 1

0.050 ± 0.001 0.091 ± 0.001 0.172 ± 0.002

26 ± 2 43 ± 2 87 ± 2

22.5 ± 0.2

4±2

44 ± 2

40 ± 2

Table 3. Test Conditions for the Transient Dehumidification/Regeneration Tests at Different Air Temperatures Q a (L/min)

vf (m/s)

Redh

Tair (°C)

RHdry (%)

RHhumid (%)

ΔRHstep (%)

25 ± 1

0.091 ± 0.001

43 ± 2

22.5 ± 0.2 30.0 ± 0.2 37.5 ± 0.2

4±2

44 ± 2

40 ± 2

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RESULTS AND DISCUSSION Desiccant and Coated Sheets. The IR spectral results are given in Figure 4. The corresponding IR bands for high

frequency of 30 s were obtained. The hysteresis effects were studied by increasing and decreasing the reference RH values in one loop. The transient response of the humidity sensors was also studied where it was observed that the influence of the sensors on the humidity readings are trivial. An illustration of the test facility and detailed information on the humidifying/dehumidifying system, sensor calibration, and data acquisition system are provided elsewhere.5,7 The measurement of a single step test was carried out by the following procedure: 1. Preconditioning: • Dehumidification test (adsorption): the dry air, 4% RH and 23 °C, was passed through the exchanger for regenerating the desiccant particles to prepare the exchanger for the adsorption test. • Regeneration test (desorption): the preconditioning was similarly obtained by passing the humid air, 44% RH and 23 °C, before the regeneration process (desorption). The preconditioning was continued until the variations of temperature and humidity at the exchanger outlet remained stable within the uncertainty limits of the sensors for a period of at least 1 h. 2. A step change in the inlet humidity occurred after preconditioning, where the inlet humidity was altered by switching the humid and dry air ducts rapidly (within 1 s). The step change process in the inlet humidity for dehumidification and regeneration process is illustrated in Figure 3. 3. Transient measurements at the ducts were kept at their position until the humidity at the exchanger outlet reached the inlet humidity. By monitoring the humidity at the outlet, the response of the coated exchanger was recorded. The dehumidification and regeneration cycles were carried out by exposing the exchanger to the respective step change.

Figure 4. IR spectra of the high amylose starch.

amylose starch are as follows: the wide spectral band ca. 3400 cm−1 relate to the O−H stretching, while the band at 2900 cm−1 corresponds to C−H stretching, and C−C stretching at 1600 cm−1.8 The appearance of ring vibrations and anomeric C−H deformation bands are observed between 900 and 550 cm−1. Figure 5 presents the volume distribution of the HAS sample with respect to the particle diameter obtained from the laser particle size analyzer, where the particle size values for HAS indicate a narrow particle size distribution with a sharp peak between 10 and 20 μm. The average particle diameter (dp) for the HAS sample was 15 μm. D


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the particles may reduce the apparent number of sorption sites and reinforce the effects of the bonding agent on the water vapor sorption properties. The use of a powder coating method39 yields a monolayer coating which weakens the interaction of water vapor with the bonding agent while enhancing the desiccant particle−water vapor interactions. Details on the coating procedure have been reported previously.40 Thus, the sorption properties of the particles can be investigated with minimal interference effects of the bonding agent by the enhanced coating method. Some agglomerated particles were formed during the coating process which resulted in greater surface roughness and intraparticle voids. The agglomerated particles likely affect the water vapor sorption since the SEM results reveal that the surface of substrate was coated nonuniformly. The desiccant particles and regions of the uncovered surface may contribute to water vapor sorption via the polar groups of the resin surface. Fathieh et al.6 showed that the substrate and bonding agent do not significantly affect the sorption properties during the dehumidification and regeneration processes. It was reported that the aluminum substrate and acrylic adhesive do not participate significantly in the sorption of water vapor since the moisture transfer occurs mainly by the desiccant particles.6 The mass analysis for coated desiccant used in the exchanger is provided in Table 5, where the level of desiccant coating for industrial desiccant wheels is ca. 0.8− 1.6 mg/cm2.10 The amount of desiccant deposited by the powder coating method lies within the acceptable range. Transient Test Results. The effect of flow rate and temperature on dehumidification and regeneration processes were studied by performing single step transient tests with the conditions given in Tables 2 and 3. As the change in humidity at the outlet of the exchanger is a transient process, the response of the humidity sensors (Honeywell type HIH-4021) should be considered. For this reason, the characteristic response profile of the sensors was taken from the authors’ previous study,6 where the sensor response time (based on 90% of output) was estimated to at 22, 53, and 93 s for flow rates of 50, 25, and 10 (L/min), respectively. With the response of the sensors alone, the exchanger response can be decoupled from the combined response of the humidity sensors and the exchanger using the method reported previously.11 Moreover, the results were analyzed using the DEM and Yoon-Nelson models. Dehumidification. Figure 8 illustrates the response of the exchanger by monitoring the change in humidity with time during the dehumidification tests at different flow rates. In Figure 8a, the air normalized humidity ratio, obtained from eq 3, gradually increases until it approaches an equilibrium condition when the inlet and outlet humidity ratios are the same (W = 1). At this condition, the desiccant particles are saturated and the HAS particles are in equilibrium with the air flow. The trends for the breakthrough curve demonstrate that decreasing the flow rate results in an increase in the time required for saturation of the desiccants. For example, the data curves were analyzed by Yoon−Nelson and DEM models, eqs 1 and 2, to quantify the flow rate effects on the dehumidification process. The resultant parameters obtained by these models are given in Table 6. DEM shows only one mode for mass transfer (γ2 = 0) at the tested conditions. In addition, the DEM time constant reduces considerably as the flow rate increases. The same trend is seen for τ0.5 obtained through the Yoon−Nelson model, where the coefficient of determination (R2) indicates that the test data are well described at a flow rate of 10 L/min.

Figure 5. Particle diameter distribution of HAS estimated from laser particle size analysis.

The average pore width (PW) and available surface area (SA) for HAS, obtained through N2 gas adsorption are listed in Table 4. The adsorption isotherm is shown in Figure 6, where Table 4. Physical Properties of the Desiccant Samples Obtained by Laser Particle Size Analyzera and N2 Gas Adsorptionb sample ID HAS a

material

dpa (μm)

P Wb (Å)

accessible SAb (m2/g)

pore volumeb (cm3/g)

high amylose starch

15

46

0.56

0.0006

Laser particle size analyzer. bN2 gas adsorption at 77 K.

Figure 6. Sorption isotherms for HAS sample obtained through N2 gas sorption test at 77 K (solid lines serve as a guide the eye).

the uptake properties of the HAS are characteristic of a mesoporous material with relatively low accessible surface area (SA). In fact, the interactions between N2 and the surface of HAS are relatively weak and indicate that HAS a relatively low sorption capacity for N2. At P/P0 ∼ 0.8, higher adsorption for N2 occurs due to adsorption at the grain boundaries, while at P/P0 < 0.8, the hysteresis loop for sorption isotherms indicates that the adsorption branch is lower than the adsorption branch. This may result from the shrinkage of HAS polymer framework at low relative pressures and expansion of the framework at higher relative pressures, where similar behavior for such network polymers was previously reported.9 Figure 7 illustrates the SEM images of the exchanger surface coated with HAS particles where the HAS particles have uniform and approximately spherical shape. The SEM images for an industrial coating reported elsewhere reveal a multilayer particle coating in which numerous particles submerged into the bonding agent onto the wheel surface.3 Decreased accessibility of E


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Figure 7. SEM images of aluminum substrate coated with HAS at variable levels of magnification.

After the saturation, desiccant particles are unable to remove additional moisture from the airflow and regeneration is required. Table 6 lists the values of Ωsat obtained through DEM, Yoon− Nelson, and the experimental test data. Based on the results, the Yoon−Nelson model underestimates the moisture uptake, while the DEM estimate for the saturated moisture uptake agrees with the experimental data. In order to investigate the effect of air flow temperature on the dehumidification process, variable temperatures were employed to obtain the breakthrough curve (cf. Figure 9). The effects of air flow temperature on breakthrough curves (Figure 9a) and moisture uptake (Figure 9b) are noticeable when the dehumidification temperature increases from 22.5 to 30 °C. However, a further increase in air flow temperature does not considerably affect the water vapor adsorption. It should be noted that to achieve the same driving force for adsorption in

Table 5. Mass of Coated Particles on the Small-Scale Coated Exchangers desiccant

total mass coated (g)

mass coated per unit area (mg/cm2)

mass of desiccant to mass of the matrix (%)

HAS

3.20 ± 0.02

0.667 ± 0.004

0.6

At greater flow rates (50 L/min), the DEM model describes the breakthrough curves. Comparison between τ0.5 predicted by the Yoon−Nelson test data in Table 6 verifies that the difference between the times increased from 5% to 46% where the flow rates changed from 10 to 50 L/min. Moreover, KYN increases with flow rate, in agreement with literature data. Figure 8b shows the accumulative moisture uptake per unit mass of desiccant (Ωads) calculated from eq 6 during the dehumidification test. The desiccant moisture content increases with time until it becomes saturated.

Figure 8. Effects of flow rate on dehumidification process in the HAS coated exchanger (Tair = 22.5 °C and ΔRH = 40%). F


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Table 6. Parameters for DEM and Yoon−Nelson Models Obtained through Curve Fitting to the Water Vapor Adsorption Breakthrough Curves at Tair = 22.5 °C and Different Flow Rates (ΔRH = 40%) double exponential model Q a (L/min)

τ1 (s)

γ1

10 25 50

93.2 53.1 22.5

1 1 1

τ2 (s)

Yoon−Nelson model

experiment

γ2

R2

Ωsat (gwv/gHAS)

k

τ0.5 (s)

R2

Ωsat (gwv/gHAS)

τ0.5 (s)

Ωsat (gwv/gHAS)

0 0 0

0.984 0.993 0.995

0.051 0.050 0.052

0.0295 0.0423 0.0656

81 45 19

0.988 0.989 0.947

0.048 0.046 0.044

77 42 13

0.052 0.051 0.052

Figure 9. Effects of air flow temperature on dehumidification process in the HAS coated exchanger (Qair = 25 L/min and ΔRH = 40%).

Table 7. Parameters for DEM and Yoon−Nelson Models Obtained through Curve Fitting to the Water Vapor Adsorption Breakthrough Curves at 25 L/min and Different Air Temperatures double exponential model Tair (°C)

τ1 (s)

γ1

22.5 30.0 37.5

53.1 35.9 29.0

1 1 1

τ2 (s)

Yoon−Nelson model

experiment

γ2

R2

Ωsat (gwv/gHAS)

k

τ0.5 (s)

R2

Ωsat (gwv/gHAS)

τ0.5 (s)

Ωsat (gwv/gHAS)

0 0 0

0.993 0.996 0.995

0.051 0.050 0.052

0.0423 0.0534 0.0617

45 27 25

0.989 0.976 0.966

0.046 0.044 0.045

42 24 19

0.051 0.049 0.049

Figure 10. Effects of flow rate on regeneration process in the HAS coated exchanger (Tair = 22.5 °C and ΔRH = 40%).

dehumidification tests, the air flow temperature was increased while the relative humidity (partial pressure) was fixed at 44%. This led to greater humidity ratios at elevated temperature (e.g. w = 7.4 g/kg at 22.5 °C and w = 11.5 g/kg at 30 °C). Therefore, the effect of temperature on sorption capacity can be determined with minor contributions related to relative humidity on the results. From the results in Figure 9 at saturated conditions (t > 200 s), it is concluded that the moisture content is nearly identical at each dehumidification temperature. This implies that increasing the dehumidification temperature by 15 °C at the same relative humidity does not affect the sorption capacity of HAS significantly; whereas, the kinetics of sorption vary such that the HAS particles become

saturated rapidly at higher temperature. However, above a dehumidification temperature of 30 °C, the moisture uptake curves are very similar to comparable sorption rate and capacity. The fitting parameters for the DEM and Yoon−Nelson models are given in Table 7. Increasing the dehumidification temperature at fixed relative humidity decreases the DEM and Yoon−Nelson time constants. Again, the Yoon−Nelson model appears to provide a better prediction of dehumidification results at lower temperature (Tair = 22.5 °C) based on the R2 values. On the other hand, DEM is a more accurate model compared to the Yoon−Nelson model when describing the breakthrough curves at the temperature and flow rate conditions. Similarly, the Yoon−Nelson model underestimates the G


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Table 8. Parameters for DEM Obtained through Curve Fitting to the Water Vapor Desorption Breakthrough Curves at Tair = 22.5 °C and Different Flow Rates (ΔRH = 40%) double exponential model

experiment

Q a (L/min)

τ1 (s)

γ1

τ2 (s)

γ2

R2

Ωsat (gwv/gHAS)

Ωsat (gwv/gHAS)

10 25 50

81.1 47.0 21.9

0.94 0.91 0.91

562 302 174

0.06 0.09 0.09

0.990 0.983 0.995

0.040 0.046 0.049

0.038 0.045 0.050

Figure 11. Effects of regeneration temperature on the transient response of HAS coated exchanger (Qa = 25 L/min and ΔRH = 40%).

of the regeneration temperature during the transient moisture removal period. The HAS particles are regenerated more rapidly when regeneration temperatures increase from 22.5 to 30.0 °C. However, a slight change was noted when the regeneration temperature increased to 37.5 °C. Similar to the adsorption process, an increase in temperature above 37.5 °C has no significant effect on the desorption of water vapor from the adsorbent surface. The accumulative moisture removal data in Figure 11b shows greater regeneration capacity with increasing temperature. However, a comparison of the results to dehumidification moisture uptake shown in Figure 9b at the highest regeneration temperature reveal that the moisture removal is about 18% lower than the moisture uptake at the same interval. This trend indicates that more energy is needed (higher temperature or flow rate) to overcome the adsorption affinity between the surface bound water and the adsorbate surface groups to fully regenerate the HAS coated particle. This could be attributed to the capillary condensation that occurs within the micropore domains of the starch coating. Compared to an adsorption process with one mode for mass transfer, a second mode with slower rate (higher time constant) was observed for regeneration. It was reported that in the case of capillary condensation, the bridging between the water vapor clusters may block access to the interpore sorption sites and increase the resistance to diffusion of adsorbate molecules.41 It was also observed that rate of sorption reduced drastically in the presence of capillary condensation within the pores.41 It is suggested that a second mode observed during desorption could be associated with the removal of micropore pore-bound water clusters; whereas, the first mode is due to the desorption form the outer surface and macropores of the adsorbent. The DEM model fitting parameters are listed in Table 9 and provide a better description of the regeneration process at elevated temperature. Comparing the results in Tables 8 and 9, it can be seen that by increasing the flow rates from 10 to 25 L/min, the first and second time constants are reduced to 41% and 46%, respectively. By comparison, a similar reduction

sorption capacity, while the DEM model overestimates the sorption capacity at higher dehumidification temperature. Regeneration. The effect of flow rate on the regeneration process is shown in Figure 10. The breakthrough curves for regeneration tests with DEM fitted curves are illustrated in Figure 10a. The HAS particles were regenerated more rapidly by increasing the flow rate. This effect is beneficial for industrial processes where quick regeneration is required. In addition, the DEM curves provide a good description of the regeneration process at different flow rates. In Figure 10b, the accumulative moisture removal data determined by eq 7 indicate that higher levels of water vapor are desorbed as the regeneration flow rates increase during the 600 s regeneration interval. In fact, the desorption process occurs more rapidly at higher flow rates. Comparison of the accumulative moisture removal with accumulative moisture uptake in Figures 10b and 8b indicate that the amount of moisture adsorbed in the same period is about 24% higher than the regenerated amount at 10 L/min. The HAS particles can be fully regenerated in 600 s if the regeneration flow rate increases to 50 L/min. This trend relates to different adsorption and desorption performance that occurs in the dehumidification and regeneration steps. Using the DEM model results (Table 8), regeneration curves were analyzed to test the experimental data with a high degree of confidence (maximum R2), where the fitting parameters were obtained at variable flow rates. In contrast to dehumidification, two modes of desorption were observed in regeneration tests. The first mode is relatively fast (τ1 < τ2) and dominant (γ1 > γ2). The second mode of desorption is more significant at higher flow rates. Similar to the dehumidification test, the DEM time constant is reduced as the flow rate increases. The fast rate of desorption occurring at the initial phase relates to the presence of abundant water vapor molecules on the surface. The effect of temperature on desorption was investigated by performing the regeneration test at variable temperature (22.5, 30.0, and 37.5 °C). The breakthrough curves for regeneration are shown in Figure 11a and reveal the important role H


Research Article


gels and the HAS coated exchanger showed that the more water (∼25 wt % more) can be regenerated per unit mass of HAS compared to the silica gel at the same regeneration conditions (T = 22.5 °C, RHdry = 4%, RHhumdi = 44%, Vf = 0.05 m/s).6 This fact implies the opportunity of replacing the commercial materials with starch to coat the wheels and reduce the regeneration energy with comparable or enhanced moisture transfer capacity. Latent Effectiveness. The use of the time constants and weighting factors obtained from transient testing, eq 4 provided estimates used to calculate latent effectiveness, where Ntum was obtained from eq 5. The calculated latent effectiveness values at different flow rates and temperatures during the dehumidification and regeneration process are shown in Figures 12 and 13. With the known uncertainty bounds of humidity and temperature measurements, the uncertainty bounds in the results were calculated by the method of the propagation of error which was described in detail previously.5,43 Comparing to the latent effectiveness of the silica gel coated wheel in the literature,6 the effectiveness for both the dehumidification and regeneration is 10% and 4% higher at angular speeds of 1 and 20 rpm, respectively, at conditions when Tair = 22.5 and Q = 10 L/min. These results provide support that starch has great potential to coat energy wheels and enhance their performance. Based on the results in Figure 12, the latent effectiveness is reduced as the flow rate increased, where a reduction is more apparent at lower rpm values. For instance, the effectiveness is reduced by 15% when the flow rate increased from 10 to 50 L/min at ω = 2 rpm in Figure 12a. Based on these results, the influence of flow rate on the wheel performance at low speed for a starch coated wheel (drying wheel) is more significant, while the

Table 9. Parameters for DEM Obtained through Curve Fitting to the Water Vapor Desorption Breakthrough Curves at 25 L/min and Different Air Temperatures double exponential model

experiment

Tair (°C)

τ1 (s)

γ1

τ2 (s)

γ2

R2

Ωsat (gwv/gHAS)

Ωsat (gwv/gHAS)

22.5 30.0 37.5

47 33 25

0.91 0.91 0.90

302 203 200

0.09 0.09 0.10

0.983 0.987 0.995

0.046 0.050 0.050

0.045 0.047 0.049

in time constants was achieved by increasing the temperature from 22.5 to 37.5 °C. This fact denotes that the change in flow rates or temperature cause a similar effect on the regeneration cycle while proper flow conditions lead to a more energyefficient wheel. The regeneration data denotes that HAS can be effectively regenerated using dry airflow at moderate flow rates and temperatures (Treg < 40 °C), which typically occurs in such energy wheels. In a drying wheel, where moisture transfer is the main application, lower regeneration temperature can substantially reduce the regeneration energy. For this application, the HAS where the Treg < 40 °C, this would result in a reduced energy demand for regeneration compared to conventional desiccants such as mesoporous silica gels and microporous molecular sieves. Although molecular sieves display large uptake capacity with steep isotherms at low humidity values (P/P0 ∼ 0.2), a higher regeneration temperature (90−120 °C) is required due to the strong bonding between the adsorbed water molecules and the adsorbent surface.42 In addition, a previous study on mesoporous silica

Figure 12. Effects of flow rate on the latent effectiveness for (a) dehumidification and (b) regeneration processes.

Figure 13. Effects of temperature on the latent effectiveness for (a) dehumidification and (b) regeneration. I



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latent effectiveness of an energy wheel is less affected by changing the flow rate from 10 to 50 L/min. Comparison of the effectiveness in Figure 12a and b, the difference in dehumidification and regeneration effectiveness is within the uncertainty bounds of the experiment for the range of flow rates studied. At a similar flow rate, an increase in temperature results in reduced effectiveness, Figure 13. Similar to the effect of flow rate, a reduced temperature effect occurs at greater wheel speeds, a difference of less than 2% effectiveness was observed at ω = 20 rpm. The latent effectiveness values obtained from dehumidification and regeneration are similar for all three temperatures in Figure 13. A general observation from Figures 12 and 13 denotes that that the latent effectiveness is sensitive to both the air flow rate and the inlet temperature especially at low angular speeds. Although starch showed good performance during the dehumidification and regeneration process over the tested conditions, more studies are required to gain a greater understanding of the durability and regeneration of the materials. Moreover, during the ultimate cold conditions, frosting in the exchanger may substantially reduce the exchanger effectiveness and life cycle44,45 which has not been considered in this study.

Research Article

AUTHOR INFORMATION

Corresponding Author

*Phone: +1 (306) 966 5440. Fax: +1 (306) 966 5427. E-mail: [email protected] (F.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Government of Saskatchewan, Ministry of Agriculture (Agriculture Development Fund).

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ABBREVIATIONS AND NOMENCLATURE b, width of exchanger channels (m); dh, hydrodynamic diameter of channels (m); dp, particle diameter (μm); HAS, high amylose starch; L, length of exchanger channels (m); Mdes, mass of coated desiccant (kg); ṁ a, air mass flow rate (kg/s); Ntum, number of moisture transfer; P, pressure (Pa); Pw, pore width (Å); Qa, volumetric air flow rate (cm3/s or L/min); Re, Reynolds number; SG, silica gel; t, time; W, normalized humidity ratio; w, air humidity ratio (kgwatervapor/kgair); γ, response weighting factor; δm, thickness of the exchanger sheets (m); εL, latent effectiveness; ρa, air density (kg/m3); τ, time constant; ω, wheel angular speed (1/s or rpm)

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CONCLUSIONS This study reports on the regeneration process for a starch coated energy wheel. A small-scale starch coated exchanger was built to predict the full-scale wheel effectiveness of high amylose starch (HAS) coatings as a suitable alternative desiccant for such energy wheel systems. The powder coating was used to deposit micron-sized starch particles on the aluminum sheets of the exchanger. The SEM results illustrate that the HAS coatings reported herein ensure that the monolayer deposition of particles onto the exchanger surface with the acrylic bonding agent affords optimal adsorption−desorption properties with water vapor at variable conditions. The effects of air flow on the dehumidification and the regeneration processes were investigated by performing single step transient tests on the coated exchanger at variable air flow rates and temperatures. The double exponential and the Yoon− Nelson (YN) models were used to correlate the breakthrough curves. The YN model provided a good description at lower flow rates (10 L/min); whereas, the DEM model was more accurate at higher flow rates (50 L/min). An increase in the flow rate did not affect the accumulative moisture uptake/ removal (sorption capacity), but it gave a faster exchanger response. As the air flow rate increased from 10 to 50 L/min, the exchanger time constant increased 4-fold. Moreover, the latent effectiveness decreased with the flow rate (15% reduction at ω = 2 rpm) but the change in effectiveness was less significant at higher rpm values (less than 4% at ω = 2 rpm). Similar to flow rate, the exchanger response time was faster when air flow temperature increased from 22.5 to 37.5 °C. Slight changes in the latent effectiveness was observed with a rise in temperature (below 4% at ω > 10 rpm, 6% at ω = 2 rpm). Comparison of the regeneration and dehumidification results indicated favorable regeneration ability for starch over the range of conditions. Acceptable exchanger performance for the starch coated device over the range of conditions (22.5−37.5 °C and 10−50 L/min) indicate the promising potential of biodesiccant coated wheels. Further studies are underway to characterize the performance of starch coated wheels at conditions relevant to temperate climate zones with freezing conditions.

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Study of Dehumidification and Regeneration in a Starch Coated

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