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An experimental study of brine recirculation in humidification-dehumidification desalination of seawater

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Humidification-dehumidification (HDH) desalination systems can work with highly saline water feed. However, rejected brine volume from HDH systems is a critical issue of brine management these systems that can result in catastrophic environmental impacts. To minimize the rejected brine volume and increase the overall water recovery ratio in the HDH desalination system, a brine recirculation method is implemented by considering a bleeding flow of the concentrated brine stream. This enables the system to work at steady state high salinity feed. Experiments are conducted for feed salinity of range of 10-30% to investigate the method applicabilty. An open-air water-heated HDH desalination system with direct contact dehumidifier is experimentally examined under varied feed salinities to evaluate the viability and thermal performance of the proposed brine recirculation method. An analytical model is presented which can predict flow rates of bleeding and feed seawater at different water production rates. The results indicate that an overall recovery ratio of nearly 88% at a salinity of 30% can be achieved with the implementation of the proposed brine recirculation method. This method can significantly facilitate brine management related issues of HDH desalination systems by minimizing the rejected brine volume.
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Case Studies in Thermal Engineering
journal homepage: www.elsevier.com/locate/csite
An experimental study of brine recirculation in humidication-
dehumidication desalination of seawater
Saeed Dehghani
, Abhijit Date, Aliakbar Akbarzadeh
Energy Conversion and Renewable Energy Group, School of Mechanical and Automotive Engineering, RMIT University, Melbourne, 3083, VIC,
Australia
ARTICLE INFO
Keywords:
Humidication-dehumidication
Brine recirculation
Desalination
Brine management
Environmental sustainability
ABSTRACT
Humidication-dehumidication (HDH) desalination systems can work with highly saline water
feed. However, rejected brine volume from HDH systems is a critical issue of brine management
these systems that can result in catastrophic environmental impacts. To minimize the rejected
brine volume and increase the overall water recovery ratio in the HDH desalination system, a
brine recirculation method is implemented by considering a bleeding ow of the concentrated
brine stream. This enables the system to work at steady state high salinity feed. Experiments are
conducted for feed salinity of range of 1030% to investigate the method applicabilty. An open-
air water-heated HDH desalination system with direct contact dehumidier is experimentally
examined under varied feed salinities to evaluate the viability and thermal performance of the
proposed brine recirculation method. An analytical model is presented which can predict ow
rates of bleeding and feed seawater at dierent water production rates. The results indicate that
an overall recovery ratio of nearly 88% at a salinity of 30% can be achieved with the im-
plementation of the proposed brine recirculation method. This method can signicantly facilitate
brine management related issues of HDH desalination systems by minimizing the rejected brine
volume.
1. Introduction
Desalination plants produce concentrated brine as a waste product beside the potable water. The rejected brine from the desa-
lination plant is a critical environmental issue [1,2]. Having a desalination system with lower brine outlet reduces the cost of further
brine management and treatment, that can account between 5 to 33% of the total desalination cost [3]. On the Other hand, brine
disposal to the sea or to the water channels connected to the sea can cause critical issues to the environment [4]. which can cause a
series of problems to marine life and underground habitat [5]. Brine minimization strategy before rejecting the brine out of the
desalination system is a highly bifacial way of reducing the cost of brine management in further steps [2,6].
Desalination setup without any rejected brine or a zero-liquid discharge (ZLD) which avoids discharge is the most ecient way of
water desalination [7]. ZLD systems concentrate the brine close to saturation point, and father on crystalized it in the form of salts.
Crystallization of the concentrated brine can be reached by evaporation based methods such as solar evaporation ponds [8] or salt
crystallizers. Evaporation is used as the main technique for handling the rejected brine from desalination systems, either through
natural or forced convection of air such as wind-aided evaporation and brine concentrators [9].
A model is proposed by Akinaga et al. [10] to reduce the brine volume by applying evaporative coolers in seawater greenhouses to
https://doi.org/10.1016/j.csite.2019.100463
Received 14 December 2018; Received in revised form 26 March 2019; Accepted 7 May 2019
Corresponding author.
E-mail addresses: saedehghani@gmail.com,saeed.dehghani@rmit.edu.au (S. Dehghani).
Case Studies in Thermal Engineering 14 (2019) 100463
Available online 09 May 2019
2214-157X/ © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/BY-NC-ND/4.0/).
T
enable the cultivation of high-value crops and production of sea salt. However, using the proposed method there will be a need to use
solar evaporation ponds to achieve ZLD, but with a smaller area of the pond as the brine, the volume is reduced by a great extent.
Multi-eect distillation (MED), mechanical vapor compression (MVC) [11] and membrane-based multi-stage techniques [9,12]
have all shown the high capability of water ux improvement. Integrated FO processes with RO and MSF and MD [1,13,14] have also
been examined as a low energy process for brine volume minimization. However, membrane fouling still needs to be addressed as a
problematic issue [6].
Chung et al. [15] Thermodynamic analysis considering reversible, blackbox system components is performed with considering
two dierent brine management methods. The rst method is complete separation with salt production. The second brine man-
agement method considered is salinity-gradient power generation through pressure-retarded osmosis.
A Humidication-dehumidication(HDH) desalination system consists of two heat and mass exchanger devices(HME) known as
humidier and dehumidier. In the humidier, air is in direct contact with saline water while in the dehumidier can be either
directly or indirectly in contact with water. Mainly, there are three dierent HDH types introduced including HDH with an indirect
dehumidier, HDH with direct contact and HDH using bubble column [1618].
In an HDH system with an indirect dehumidier, higher heat recovery can be achieved as air is used to preheat the seawater.
However, in direct-contact HDH systems, the saline water is only associated with the humidier. Therefore, corrosion-related pro-
blems in dehumidier are eliminated, and the capital investment cost of the system is lower in this case [16,18,19].
HDH technology has been proven to be an inexpensive and reliable desalination system regarding environmental friendliness. It
has been shown that HDH system inexpensive and reliable desalination system for applications using low-grade heat sources [17,19].
Narayan et al. [19] compared and evaluated water production capability of various HDH systems congurations. They reported that
multi-eect close-air open-water (CAOW) water-hated HDH system is more ecient in comparison with other congurations [17,20].
Performance evaluation of HDH systems with direct contact dehumidier has been subject of dierent studies. Performance of a
co-current and counter-current dehumidiers is studied and compared Klausner et al. [21]using a nite volume model. Yi Li et al.
[22] developed a heat and mass transfer model and parametrically investigated the performance features of an open-air DC HDH
system considering dierent operational conditions. Alnaimat et al. [23,24] performed a transient analysis for a solar HDH system
with direct contact dehumidier to examine the desalination process under dynamic operating conditions with transient variation
during the normal operation.
Swaminathan [25] compared dierent recirculation strategies including batch, semi-batch, continuous, and multistage, and
ranked them based on ux and energy eciency in a membrane distillation desalination system. Garg [26] developed a mathematical
model and studied the performance of multistage ash desalination with brine recirculation. The system coupled with nanouid-
based direct absorption solar collector as a heat source. Zubair [27] recirculated the dierent brine ow rates and evaluated per-
formance of the HDH working with indirect contact dehumidier. Jamil [28] implemented a thermo-economic analysis of a single
eect mechanical vapor compression(MVC) desalination system with and without brine recirculation consideration. They calculated
specic thermal energy consumption for the MVC desalination system with and without brine recirculation is to be 13 kWh/m
3
and
9.8 kWh/m
3
, respectively.
In this study, brine recirculation is a humidication-dehumidication system is utilized to experimentally examine the brine
recirculation technique for increasing the water recovery. Outlet brine stream form the HDH desalination system is recirculated at
dierent feed salinities and eect of the feed salinity on the system performance parameters such as water production rate, water
recovery ratio and gain output ratio are investigated.
Nomenclature
Symbol
cp
specic heat capacity at constant pressure (kJ/
kg.K)
GOR
gain output ratio ()
m
mass (kg)
Mass ow rate (kg/s)
Q
̇
Energy rate (kW)
RR
recovery ratio ()
S
Salinity (%)
T
Temperature (°C)
Greek letters
ω
absolute humidity of air (kg
w
/kg
a
)
φ
relative humidity of air (%)
Subscripts
a
air
dw
distilled water
b
bottom
t
top
sw
Seawater/saline water
w
water
max
maximum
s
salt
br
brine
bl
bleed-o
h
humidier
d
dehumidier
in
inlet
out
outlet
S. Dehghani, et al. Case Studies in Thermal Engineering 14 (2019) 100463
2
2. System description
Fig. 1 represents the schematic diagram of the HDH system with a direct-contact dehumidier. A fan blows air into the humidier
column, and at the same time, hot saline water is spraying from the top of the humidier column. As a result, air will gather moisture
while passing through packing lls inside the column. Next, hot moisturized air will be transferred to the dehumidier in order to
condense the water vapor carried by air. In dehumidier, air moisture condenses into distilled water and will be removed from the
sprayed water for further storage.
As shown in Fig. 1, the fresh water collected from the dehumidier will be further cooled with an external cooler and returned to
the dehumidier for spraying. In both humidier and dehumidier heat and mass directly transfers between air and water. Packing
lls are placed inside them for providing a contact surface for eective heat and mass transfer. To prevent water droplets carriage
with air moving out the humidier and dehumidier columns drift eliminators are placed at the outlet of these columns. After
humidication process, a fraction of the concentrated brine is bled owhile the rest of brine is collected in a tank to be sprayed again
into the humidier. At the same time seawater is added to the brine collection tank to maintain the salinity of the humidier saline
feed.
3. Governing equations
To extract the relationship between mass ow rates of the inlet and outlet streams with brine recirculation method, conversion of
mass is applied to the HDH system. It is assumed that the system is working in a steady state condition. When the HDH system is
operating at steady state condition and producing some distilled water, seawater is added, and some of the concentrated brine is bled
oto maintain the salinity of the feed saline water to humidier at a constant value as is depicted in Fig. 2. To satisfy this working
condition, mass conservation is applied to the HDH system for both water and salt. Salt mass balance is as:
Fig. 1. Schematic diagram of the HDH desalination system.
S. Dehghani, et al. Case Studies in Thermal Engineering 14 (2019) 100463
3
×=×mS mS
˙˙
win swin wbl bl,, , (1)
where S
sw,in
is the salinity of the inlet seawater to the system. And S
bl
is salinity of the bleed o. The salinity of a stream is dened as:
Sm
m
˙
˙100 (%
)
s
w(2)
The salinity of concentrated brine after the humication process in a closed-air HDH system is calculated as:
=
Sm
mm
˙
˙˙
bl
ssw
wsw dw
,
,(3)
The salinity of brine and bleed oare the same. And mass ow rate of the salt available in the saline water stream entering the
humidier is as:
mSm
˙˙
ssw sw wsw,, (4)
The mass ow rate of water in the stream feeding into the humidier is calculated as follow:
+=mmm
˙˙˙
w sw s sw sw,, (5)
Applying the mass conservation for the water streams of the HDH system results in:
=+mmm
˙˙˙
win wbl dw,,
(6)
To evaluate the performance of the proposed system performance parameter are considered. The overall recovery ratio of the
HDH system is the amount of water produced over the amount of seawater added into the system is dened as follow:
=
R
Rm
m
˙
˙
overall dw
sw in,(7)
The GOR of the system is representing heat recovery potential of the system and dened amount of heat released by condensation
over the amount of heat added to the seawater as:
=
G
OR mh
Q
˙
˙
dw fg
in (8)
The local recovery ratio is a ratio of the amount of produced water over the amount of saline water is fed into the humidier as:
=
R
Rm
m
˙
˙
local dw
sw (9)
The heat rate added to the saline water feeding into humidier is the summation of heat added due to the recirculated brine and
seawater inlet that can be calculated as:
=−+ −
Q
mc T T mc T T
˙˙()
˙(
)
in sw psw swt br sw pswin swt swin,, ,,, , (10)
Fig. 2. Mass ow rate balance of the system.
S. Dehghani, et al. Case Studies in Thermal Engineering 14 (2019) 100463
4
where c
p
is the specic heat capacity of saline water as follow [29]:
=+ ×+ ×cS
4.18 0.04396 0.000485
p(11)
The rate of cooling load is provided for dehumidication can be calculated as:
=−
Q
mcT T
˙˙(
)
out fw p fw b fw t,, (12)
The salinity of a solution can be determined by measuring the density of the saline water solution as [29]:
=−+ −
SρT998 0.4( 20)
6.5 (13)
where Tand ρare the temperature and density of the measured solution, respectively.
4. Experimental setup and procedure
Fig. 3 shows the photo of the experimental rig. Three tanks are considered for seawater inlet, bleed-obrine, and brine re-
circulation. Polyvinyl chloride (PVC) pipes are with a height of 3 m are used as a shell for humidier and dehumidier columns. The
diameter of the pipe is 1500 mm. Both humidier and dehumidier are lled with polypropylene made packing lls with a specic
surface area of 240 m
2
/m
3
and a channel size of 12 mm. The same packing lls are used in both humidier and dehumidier columns
with a total height of 2.25 m for each column. Two full cone spray nozzles are placed at the top of humidier and dehumidier
columns. Above each spray, a drift eliminator is considered in order to prevent droplets caring out of the column with blowing air.
A DC 12-V fan to blow the air at various ow rates and two DC 24-V pumps are utilized for spraying water in humidier and
dehumidier columns. Insulation sheets are wrapped around the columns to prevent heat transfer from the HME devices and
transferring pipeline. Two full cone nozzles are placed in the top of humidier and dehumidier columns. To provide a required heat
a gas burner heater along with plate heat exchanger is used. A chillier is employed for cooling down the freshwater after the
Fig. 3. Experimental setup of the HDH system with brine recirculation.
S. Dehghani, et al. Case Studies in Thermal Engineering 14 (2019) 100463
5
dehumidication process.
To measure the temperature of the air and water at the dierent point of the system T-type and K-type thermocouples are
employed. Two turbine ow meters with digital signal output are utilized to measure the feeding ow rates of freshwater and saline
water. Humidity sensor with analog is used to measure the inlet air humidity. To measure the air ow rate blowing through the HDH
system a hot-wire anemometer is employed. A data logger (DT80 Datataker) is used for collecting both analog and digital signals
outputs of the sensors over time. Accumulated water volume is measured over a time of 15 min using a graduated cylinder to
determine the water production rate. The uncertainties of the measurements are presented in Table 1.
4.1. Propagation of uncertainty of measurements
Considering a general case in which an experimental result ris a function of Jmeasured variables X
i
:
=rrXX X( , , .....,
)
J12 (14)
Eq. (14) is the data reduction equation used for determining rfrom the measured values of the variables X
i
. Then the propagated
uncertainty of ris given by:
⎜⎟
⎜⎟ ⎜⎟
=
+
++
Ur
XUr
XUr
XU.....
rXX JX
2
1
2
2
22
J11 (15)
where the U
Xi
is the uncertainty in the measured variable X
i
.
The uncertainty of the measured parameters is presented in Table 1. Applying the Eq. (15) to dierent parameters including
Recovery ratio, GOR, salinity, and pure water production rate the propagation of the uncertainty is determined. The higher values of
uncertainty are ± 6.1% for recovery ratio, ± 7.2% for GOR and ± 3.2% for salinity measurements.
5. Experimental results and discussion
Fig. 4 shows the predicted ow rate of bleed-oand inlet seawater to the fresh water production rate at 10, 20 and 30% salinities.
This provides values of seawater inlet and bleed-offflow rates to maintain humidier feed at the same salinity during the system
operation and prevent salt accumulation over time. These values are used for running the experimental tests. As it presented in Fig. 4,
for a specic value of water production rate of the HDH system, when the brine is recirculating at a higher salinity both the bleed-o
and the inlet seawater ow rates are decreased. By having rejected brine at a higher salinity and lower volume can facilitate brine
management. Binary lines with the same marker shape represent the corresponding recirculation salinity. Dashed line presents the
bleed-offflow rate and solid line presents the inlet seawater ow rate.
Prediction of the overall recovery ratio of the system at various salinities is presented in Fig. 5 using the mathematical model. The
overall recovery ratio is limited by the salinity value of each case. Increasing salinity from 10 to 30% increases the recovery ration
from 68% to 87%, respectively.
In Fig. 6 temperatures of air and water at dierent points of the HDH system are presented. As shown during each experiment the
temperatures are reasonably stable. The operational conditions are maintained the same for running the experiments at a dierent
salinity of humidier feed. In all experiments, top saline water, and top freshwater temperatures are xed at 71 °C and 10 °C,
respectively. In addition, both humidier and dehumidier feedwater ow rates adjusted to 1.75 kg/min. Comparing the humidier
outlet air temperature (T
a,t
)atdierent salinities shows that by increasing the salinity the temperature is slightly decreasing.
Therefore, increasing the feed salinity results in a less ecient humidication process. This aect the lower water production as
outcome of the desalination processes.
In Fig. 7 heating and cooling loads required for humidication and dehumidication processes for dierent salinity cases are
presented. Increasing the salinity increases the heating load of the humidication process. This is mainly due to the higher specic
heat capacity of saline water at higher salinity which requires more heat at a constant ow rate and temperature dierence. The
cooling load is slightly decreasing by 15% when salinity rises which is because air reaches a lower temperature after humidication at
higher salinity.
Fig. 8 shows the GOR and recovery ratio of the system with brine recirculation for dierent salinity cases. Brine recirculation in
higher salinity results in higher overall recovery ratio while it reduces GOR of the system. GOR reduction at a higher salinity is mainly
because providing higher heat load due to an increase in specic heat capacity of saline water as well as lower eciency in the
Table 1
Uncertainty of the measured data.
Measurement Uncertainty
K-type thermocouple ± 1.2 °C
T-type thermocouple ± 1 °C
Density meter ± 0.0005 g/cm³
Water ow meter ± 3% of the reading
Longitudinal dimensions ± 1 mm
Time ± 1sec
S. Dehghani, et al. Case Studies in Thermal Engineering 14 (2019) 100463
6
humidication process. It should be noted that; the experimental results of recovery ratio are in good agreement with the calculated
values presented in Fig. 5.
Fig. 9 shows the water production rate and local recovery ratio against dierent salinity. Increasing the salinity, the distilled water
production reduces from 4.9 kg/h to nearly 4 kg/h. The local recovery ratio which is dened in Eq. (9) is the ratio of produced water
over humidier feed saline water is decreasing at a higher salinity likewise water production. The local recovery ratio of HDH systems
are relatively small, and brine recirculation can be an implemented to increase the recovery ratio value.
Fig. 10 demonstrates the increase in bleed-o,S
bl
, salinity in comparison to the humidier feed salinity for dierent cases of 10,
20 and 30% salinities. When the brine is recirculated at a higher humidier feed salinity, a dierence of salinities between brine(S
br
)
and humidier feed(S
sw
) retains a larger dierence. Having a higher bleed-osalinity provides lower brine management cost.
Fig. 10: humidier saline water feed and bleed osalinities comparison.
6. Conclusion
An experimental study of brine recirculation in humidication-dehumidication with direct-contact dehumidier presented from
the results it is seen that brine recirculation can remarkably improve the heat recovery and overall recovery ratio of the desalination
system. Humidication process can occur at high salinity feed, therefore implementation of brine recirculation is a practical way in
the HDH system for minimizing the rejected brine volume. On the other hand, direct contact dehumidication process eliminates the
corrosion related issues of high saline feed water compared with indirect dehumidication. Increasing the salinity of recirculation
brine from 10 to 30% enhances the overall recovery ratio of the system from nearly 66 to 86%. However, GOR of the system slightly
decreases from about 0.65 to 0.45 by increasing the salinity from 10 to 30% as humidication process becomes less ecient at higher
salinity feed. Also, the experimental results show that higher salinity of the recirculated brine requires higher heating load and
reduces the eectiveness of the humidication process by decreasing the humidier outlet air temperature. Increasing the salinity
from 10% to 20% and 30% requires 8% and 16% higher heat load, respectively. The proposed method for brine recirculation provides
a valuable approach for thermal-based desalination applications which are capable of operating at high salinity feed.
Fig. 4. Flow rate of bleed-oand inlet seawater versus water production rate at 10%, 20% and 30% salinities of brine recirculation.
Fig. 5. The recovery ratio of the system versus salinity.
S. Dehghani, et al. Case Studies in Thermal Engineering 14 (2019) 100463
7
Fig. 6. Temperature proles air and water during the experiments.
Fig. 7. Heating and cooling loads at dierent salinities cases.
S. Dehghani, et al. Case Studies in Thermal Engineering 14 (2019) 100463
8
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S. Dehghani, et al. Case Studies in Thermal Engineering 14 (2019) 100463
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... Therefore, a promising desalination technique for small-scale potable water demand is HDH technology [7,8]. Unlike the thermal-and membrane-based desalination processes described above, HDH systems are relatively insensitive to high feed salinity and can operate efficiently at low operating pressures (~1 bar) and temperatures, making them suitable for decentralized production of potable water from high salinity seawater [9]. The HDH process mimics the rain cycle in which seawater evaporates into the air when exposed to radiation heat from solar energy; air carries water vapor at higher altitudes and condenses at very low pressures and temperatures. ...
... Step 2: The five unknown temperatures in the governing equations were T w2 , T w3 , T w4 , T a2 , and T a1 ; therefore, five independent equations were used to calculate the unknown value, including Eqs. (2), (5), (7), (9), and (11) that depend on the completion of Step 1. These nonlinear equations were solved using the Newton-Raphson method. ...
Article
Desalination, the process of obtaining potable water from seawater, has been regarded as an alternative solution to address the global issue of freshwater shortages. Considering the design, cost, and low-grade energy-driven advantages of the humidification-dehumidification (HDH) desalination process, this study aims to experimentally and theoretically investigate a laboratory-scale HDH system using different hydrophobic packing materials (i.e., hackettes, saddles, and snowflakes). We developed a mathematical model that simulates the HDH process by estimating the temperature, productivity, and gain output ratio (GOR) of each packing material and compared the results with the experimental data. The hackettes humidifier packing was found to exhibit the highest mass transfer coefficient (0.00331 kg/m²s), resulting in maximum productivity (0.64 kg/h at 80 ℃) and GOR (1.45 at 50 ℃). This was followed by saddle packing (0.00291 kg/m²s, 0.59 kg/h at 80 ℃, and 1.15 at 50 ℃, respectively), which contributed to a better HDH process than snowflakes (0.00227 kg/m²s, 0.56 kg/h at 80 ℃, and 1.13 at 50 ℃, respectively) due to improved heat and mass transfer efficiency in saddle packing. In addition, the effects of operating parameters (e.g., feed temperature, feed salinity, air, and water mass flow rates) on the thermal performance of HDH systems with different packings were investigated. It was confirmed that the results of the mathematical model were in good agreement with the experimental data.
... • Higher spraying seawater temperatures and pressure differences improve system performance. [42] OAOW-WH 4.9 kg/h 0.65 -• The cycle was driven by a gas burner. ...
Article
Drought has hit several water-stressed regions, with serious consequences for water and food security. In this regard, the low-cost and low-grade thermally driven humidification-dehumidification desalination cycle has good potential to provide freshwater in arid regions. From energy and economic point of view, different configurations of the humidification-dehumidification cycle have been proposed in the literature without a comprehensive comparison to identify the most suitable one. Thus, this paper investigates the thermal and economic performance of several multi-humidifier water- and air-heated humidification-dehumidification cycles. Thermodynamic models are structured to study the behavior of each cycle. The models rely on heat and mass transfer balances for each component. The effectiveness method is implemented for humidifiers and dehumidifiers to evaluate heat and mass exchangers. The models are validated using previously published data. The series seawater flow, triple humidifier, water-heated humidification-dehumidification cycle could deliver 151.2 kg/h of fresh water at a maximum temperature of 70 °C, with a gain output ratio of 1.995 and a specific cost of 2.73 $/m³. This cost rises by around 16% when a maximum temperature of 60 °C is employed. Results reveal that using a double-humidifier in an air-heated humidification-dehumidification cycle produces freshwater of 102.2 kg/h at the lowest price of $2.06/m³. It is concluded that the multi-humidifier water-heated humidification-dehumidification cycle produces more freshwater and less gain output ratio than the multi-humidifier air-heated humidification-dehumidification cycle.
... In some instances, miscarried brine droplets degrade the quality of the freshwater produced, preventing it from reaching its maximum potential (Arunkumar et al., 2013). The higher salt content in the humidified air due to the miscarried brine can corrode the components of the metal dehumidifier (Dehghani et al., 2019;He et al., 2018c). High saline seawater can damage the surface of the humidifier owing to the scaling effect resulting in material degradation and reduced operating efficiency (St. ...
Article
The solar and waste heat-powered humidification-dehumidification (HDH) desalination systems are gaining much significance owing to the alarming effects of global warming and water scarcity. Their performance depends on the efficiency of the individual humidification and dehumidification processes. To achieve maximum system efficiency, it is necessary to identify an effective combination of humidifiers and dehumidifiers under compatible operating conditions. Therefore, this study highlights the potential of various humidifiers and dehumidifiers in improving the efficiency of solar-powered and low-grade waste heat (HVAC system and PV panel)-powered HDH systems that effectively utilize sustainable energy sources (solar and waste heat) to enable cleaner production of decentralized freshwater. Each material exhibits intrinsic beneficial property and their influence on system and process effectiveness are elaborated. Among humidifiers, cellulose was found to be influential owing to its inherent water absorption capacity. Similarly, due to the efficient latent heat extraction, the finned-tube heat exchanger outperformed other dehumidifiers. By comparison, the combination of cellulose (34%) and finned-tube dehumidifier (56%) was effective in the HDH process due to the combination of efficient heat and mass transfer effects. Further, the higher heat capacity of water contributed to its dominant preheating (63.8%) compared to air (17.3%). It is inferred that waste heat-powered HDH offers the dual benefits of freshwater along with improved cooling effect (HVAC-HDH system) or electrical energy (PV-HDH system) while solar-powered systems produce only freshwater at a relatively high cost. Dual fluid preheating, closed circulation, enhanced wet area, optimum mass flow ratio across humidifier/dehumidifier, residual heat utilization, and adoption of thermal energy storage unit are identified to be significant factors in improving the HDH system performance. The scope of the potential system- and process-based improvements has been proposed to support increased efficiencies leading to cost-effective and continuous decentralized freshwater production.
... The large volume of brine produced has economic and environmental implications, especially when it is discharged into sensitive ecosystems. Humidification-dehumidification technology can work with very high salinity [54] allowing recirculation of brine in the dehumidifier. This recirculation reduces the rejected volume of brine. ...
Article
Full-text available
We propose the use of green hydrogen as a fuel for a seawater heater in a humidification / dehumidification (HDH) desalination plant to increase its productivity, to allow scaling to large dimensions without negative environmental effects, and to guarantee continuous operation. We develop a mathematical model of the proposed HDH configuration. For operating conditions that guarantee very low NOX production, the fuel consumption is ~0.03 kg of H2 per kg of pure water produced. If the exhaust gases from the seawater heater are used for heat recovery, the GOR of the equipment may increase by up to 39 % in relation to the same equipment operating without heat recovery. The operation cost of freshwater is comparable to the costs obtained by other equipment in the literature. If the water produced in the combustion of hydrogen is condensed during the heat recovery process and then added to the freshwater produced, the production cost is reduced by 20 %. We found that an excess of air in the air+fuel mix beyond the minimum value appropriate for a low NOX generation does not provide significant benefits. The efficiency of the seawater heater has an impact on the production of pure water, but this impact is strongly mitigated by the heat recovery process. Fuel consumption increases proportionally with the decrease in the effectiveness of the heat recovery device, which is a key parameter for optimal performance. A hydrogen heater is also a good alternative as an auxiliary power source to guarantee continuous operation. On sunny hours a H2 heater may be used to increase productivity preheating the seawater, and at night the system could operate 100 % based on H2.
... where ρ is water density. The recovery ratio (RR) is defined as the ratio of the distillate flow rate to the water flow rate, as follows [27]: ...
Article
Full-text available
A hybrid capacitive deionization and humidification-dehumidification (CDI–HDH) desalination system is theoretically investigated for the desalination of brackish water. The CDI system works with two basic operations: adsorption and regeneration. During adsorption, water is desalted, and during the regeneration process the ions from electrodes are detached and flow out as wastewater, which is higher in salt concentration. This wastewater still contains water but cannot be treated again via the CDI unit because CDI cannot treat higher-salinity waters. The discarding of wastewater from CDI is not a good option, since every drop of water is precious. Therefore, CDI wastewater is treated using waste heat in a process that is less sensitive to high salt concentrations, such as humidification-dehumidification (HDH) desalination. Therefore, in this study, CDI wastewater was treated using the HDH system. Using the combined system (CDI–HDH), this study theoretically investigated brackish water of various salt concentrations and flow rates at the CDI inlet. A maximum distillate of 1079 L/day was achieved from the combined system and the highest recovery rate achieved was 24.90% from the HDH unit. Additionally, two renewable energy sources with novel ideas are recommended to power the CDI–HDH system.
... Abdelrahman et al. [24] made exergy and parametric study for freeze desalination with Reversed vapor compression cycle. Dehghani et al. [25] made an experimental analysis of brine recirculation in the humidification-dehumidification desalination of saltwater. Kariman et al. [26] performed an energy and exergy study of evaporation desalination system using a mechanical vapor compression system. ...
Article
A newly designed freezing desalination system was constructed. The system operates on the principle of reversed vapor compression. It is equipped with two identical heat exchangers, one of them works as an evaporator and the other as a condenser. A three-way valve was used to reverse the operation of heat exchangers, causing the ice in the evaporator to melt. The performance of the newly designed system was evaluated in both the forward and the reversed cycles. Electric current, power consumption, pressure drop, water productivity, and specific consumption were all investigated. The experiments were conducted with ice ratios of 0.27, 0.39, 0.45, 0.54, and 0.65 with cycle operating periods of 110, 135, 180, 240, and 300 min. The results showed that the electrical current, power consumption, and pressure drop were decreased with the reversed cycle. The maximum energy-saving percentage was 13% for cycle operation time 300 min. The amounts of freshwater reached 19 L for cycle operation times of 300 min from 30 L saline water. It is recommended to run our newly designed system at an ice ratio of 0.45 since these results save 1.5% in the reversed cycle, which is the most used cycle in the new design.
... The common methods of dehumidification include cooling dehumidification, liquid absorption dehumidification, electrochemical dehumidification, adsorption dehumidification, rotary desiccant dehumidification and membrane dehumidification [1][2][3][4]. Air conditioning is an air circulation control system to maintain temperature, humidity and cleanliness [5]. ...
Article
The present study pertains to an experimental measurement investigation of membrane physical properties for vacuum membrane dehumidifiers. The membrane material used in an air dehumidification system is a key role for the dehumidification efficiency. To find the most suitable membrane for dehumidification, the membrane physical characteristics are examined by using three critical indexes of air permeance (AP) and water vapor permeance (WP), and selectivity (SE). Two categories of dehumidification membranes, i.e. composite and dense membranes, are applied in the vacuum membrane dehumidifier with a serpentine flow channel. Firstly, the vapor transfer rates of both composite and dense membranes are determined and the experimental data discloses that the composite membranes, such as Sulfone and Metal–organic framework (Mof) membranes, provide higher vapor transport ability, which is not suitable for the vacuum membrane-based dehumidifier. Then, the measurement of AP, WP, and SE is further performed for the dense membranes of Nafion. The result shows that the order of either AP or WP is N.211, N.212, N.115, and N.117, but the SE defined as WP/AP has a different tendency and the order is N.117, N.115, N.212, and N.211. Finally, the results of AP, WP, and SE in this dehumidifier are also compared with the previous experiment.
Article
An innovative humidification – dehumidification (H-D) system, consisting of two primary components, a humidifier, and a condenser, was designed and built, together with its various accessories, to generate desalinated water from seawater simply and cost-effectively. The experimental work was split into two parts. The first was concerned with the humidification of ambient air in a specifically designed and developed humidifier. The influence of significant factors on the % relative humidity of the humidified air, as well as the temperatures of the exiting air and water, was examined using the natural draft and air-blown forced convection. The second part pertained to combined H – D of air to produce desalinated water. The findings revealed the presence of a hydrophilic plant (Loofa Egyptiaca) as packing that has never been utilized previously, is branching and mimics structured packing to a large extent. The presence of four stages, each with just a shallow Loofa bed height, was enough to create exit air with 100 % relative humidity. The unit's daily productivity was highest (133.72 kg water per total volume of packed section) at high water flow rates and temperatures (60 °C), as well as when using cold water in the condenser, with a condensation efficiency of 93.5 % without the need for more complicated and expensive coil-type or finned tube-copper condensers. Furthermore, completely desalinated water was produced, which was superior to potable water.
Article
There is an opportunity to save energy and reduce operational expenses when choosing a suitable desalination method aided by computational modeling. Existing models are not conducive to generalized comparisons be- tween different desalination methods. Therefore, this study developed metamodels for six desalination methods, grouped them into thermal and molecular transport families, and validated their predictive performance within 9% difference from published data. This validated framework allowed comparisons of desalination methods at their prescribed ranges of operational conditions that they were designed for. These conditions specify feed salinity ranges of 1.6 to 2.4 g/kg for Capacitive Deionization and Reverse Osmosis (RO), 2.8 to 4.2 g/kg for Electrodialysis, 28 to 42 g/kg for Thermovapor Compression and Humidification-Dehumidification, and 37 to 55 g/kg for Multi-Effect Distillation (MED). Despite different operational conditions, all models exhibit non-linear, positive correlation between energy consumption and system size in response to feed salinity and production rate. The framework is also employed in a cross-comparative analysis between MED and RO whose results suggest that energy intensity for MED is an order of magnitude greater than RO for the same operational conditions, but actual operational costs are comparable. Overall, the framework is ready for deployment in case studies of actual desalination plants.
Article
Rapid expansion of the unconventional oil and gas extraction has increased American energy independence, but also led to increased production of large amounts of contaminated water. Wastewater from the oil and gas industry contains a wide range of contaminants. Injecting such contaminated water into disposal wells or discharging it to the environment jeopardizes freshwater resources. Conventional and membrane-based wastewater treatment techniques are often not effective choices to treat highly contaminated wastewater; however, a suitable way to remove contaminants from wastewater is to selectively separate water in a process analogous to humidification-dehumidification (HDH). Only a few studies have investigated the use of HDH for wastewater treatment. In this study, a novel HDH system is introduced for treating highly contaminated water, such as oil and gas flowback and produced water. In this HDH process, a non-condensable gas, such as air, mixes with wastewater vapor to facilitate the separation of contaminants. As a result, clean water condenses from the multicomponent gaseous mixture while air carries organic contaminants out of the dehumidification section. A laboratory apparatus was constructed and experiments were performed to investigate the HDH process for wastewater treatment and to study the effects of flow dynamics including air flow rate on the composition of treated water. Different contaminants including benzene, toluene, 2-propanol and 2-butoxyethanol were tested. Experimental results showed that the system can be successfully applied for removing volatile and/or toxic organic contaminants from wastewater. A representative multicomponent mixture of fracking wastewater was successfully treated using the experimental setup and clean water with quality of 98.3% was obtained. It was revealed that increasing the air-to-vapor mass ratio improves purity of treated water. Quantitative analysis showed that by increasing the air-to-vapor mass ratio from 0.6 to 5.9, the fraction of separated 2-propanol through the air was improved from 43% to more than 96% of the initial amount. ASPEN software was employed to simulate equilibrium conditions. Experimental results were observed to have lower mass fraction of residual contaminant in the treated water compared to equilibrium state.
Article
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A mathematical model for multistage flash (MSF) desalination system with brine recirculation (BR) configuration is developed in this study. The heat source for BR-MSF is chosen to be a nanofluid-based direct absorption solar collector (DASC) for which a numerical model is developed. Both these systems, BR-MSF and DASC are coupled via a counter-flow heat exchanger. The overall performance of the combined system is quantified in terms of gained output ratio (GOR). Moreover, the variation in GOR caused by various influencing parameters such as height (H) and length (L) of solar collector, nanoparticle volume fraction (fv) and incident flux on the collector (q) is studied in detail. The effect of these parameters on the top brine temperature (To) is also discussed. The study shows that DASC can be used as a heat source for BR-MSF system and gives high GOR ranging between 11 to 14 depending on the various operating conditions. This system is also compared with a parabolic trough collector (PTC) based BR-MSF system and it is found that DASC-based BR-MSF system gives higher GOR under identical conditions (relatively 11% higher). The exergy analysis is also presented for this system which shows the irreversibilities associated with various physical processes and components of the overall system and in addition to that exergy efficiency is also calculated for the overall system.
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Brine disposal is a major challenge facing the desalination industry. Discharged brines pollute the oceans and aquifers. Here is it proposed to reduce the volume of brines by means of evaporative coolers in seawater greenhouses, thus enabling the cultivation of high-value crops and production of sea salt. Unlike in typical greenhouses, only natural wind is used for ventilation, without electric fans. We present a model to predict the water evaporation, salt production, internal temperature and humidity according to ambient conditions. Predictions are presented for three case studies: (a) the Horn of Africa (Berbera) where a seawater desalination plant will be coupled to salt production; (b) Iran (Ahwaz) for management of hypersaline water from the Gotvand dam; (c) Gujarat (Ahmedabad) where natural seawater is fed to the cooling process, enhancing salt production in solar salt works. Water evaporation per face area of evaporator pad is predicted in the range 33 to 83 m 3 /m 2 ·yr, and salt production up to 5.8 tonnes/m 2 ·yr. Temperature is lowest close to the evaporator pad, increasing downwind, such that the cooling effect mostly dissipates within 15 m of the cooling pad. Depending on location, peak temperatures reduce by 8-16 °C at the hottest time of year.
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The problem associated with the reverse osmosis (RO) system is its brine disposal. Experimental salinity gradient solar pond (SGSP) with surface area 4.65 m² was established to extract and apply heat on laboratory scale direct contact membrane distillation (DCMD). In this study, the performance of the SGSP and DCMD system under Pakistan's climatic condition of Islamabad was evaluated. The heat extraction was carried out using internal heat exchanger by passing fresh water through it at different flow rates in summer and winter. Maximum temperature of 37 °C in summer and 28.5 °C in winter was extracted. The extracted heat from SGSP can be used to pre-heat the brine for temperature driven desalination processes. Least drop in lower convective zone (LCZ) temperature of SGSP was observed at flow rate of 7.5 L/min. In DCMD, two temperatures obtained from SGSP (28.5 and 37 °C) at feed side were maintained to investigate the permeate flux, percentage salt rejection and total dissolve solids (TDS). Two further temperatures 50 and 60 °C were maintained to investigate the DCMD performance. Flux increased as temperature difference between feed and permeate increased. 28.5 °C in terms of SGSP temperature was also feasible for DCMD process as flux was maintained over time.
Article
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Growing desalination capacity worldwide has made management of discharge brines an increasingly urgent environmental challenge. An important step in understanding how to choose between different brine management processes is to study the energetics of these processes. In this paper, we analyze two different ways of managing highly saline brines. The first method is complete separation with production of salts (i.e., zero-discharge desalination or ZDD). Thermodynamic limits of the ZDD process were calculated. This result was applied to the state-of-the-art industrial ZDD process to quantify how close these systems are to the thermodynamic limit, and to compare the energy consumption of the brine concentration step to the crystallization step. We conclude that the brine concentration step has more potential for improvement compared to the crystallization step. The second brine management method considered is salinity-gradient power generation through pressure-retarded osmosis (PRO), which utilizes the brine's high concentration to produce useful work while reducing its concentration by mixing the brine with a lower salinity stream in a controlled manner. We model the PRO system coupled with a desalination system using a detailed numerical optimization, which resulted in about 0.42 kW h/m³ of energy saving.
Article
Thermal-energy-driven desalination processes such as membrane distillation (MD), humidification dehumidification (HDH), and multi-stage flash (MSF) can be used to concentrate water up to saturation, but are restricted to low per-pass recovery values. High recovery can be achieved in MD through feed recirculation. In this study, several recirculation strategies, namely batch, semibatch, continuous, and multistage, are compared and ranked based on flux and energy efficiency, which together influence overall cost. Batch has higher energy efficiency at a given flux than semibatch and continuous recirculation because it spends more operating time treating lower salinity water for the same value of overall recovery ratio. Multi-stage recirculation is a steady-state process that can approach batch-like performance, but only with a large number of stages. Feed salinity rises during the batch operating cycle, and as a result feed velocity may have to be increased to avoid operating above the critical specific area wherein both GOR and flux are low due to significant heat conduction loss through the membrane. Finally, the choice of optimal membrane thickness for batch operation is compared to that of continuous recirculation MD.
Article
The current study is focused on thermoeconomic analysis of a single effect mechanical vapor compression (MVC) desalination system operating with and without brine recirculation. For this purpose, first- and second-law analyses are carried out to estimate the energy consumption and second-law efficiency of the plant. A single stage seawater reverse osmosis plant is also presented for the sake of comparison. For thermal systems, a detailed heat exchanger design is provided to calculate an overall heat transfer coefficients, heat transfer areas, and the pressure drops on the cold- and hot-sides of the heat exchangers. Besides, the product cost is calculated and compared by using two different cost estimation methods. Moreover, it is demonstrated that the cost-flow method of economic analysis is more elaborative and useful because it enables the component level cost optimization. The calculations reported the values of specific energy consumption (SEC), second law efficiency and product cost to be 10 to 13 kWh/m³, 8 to 9% and 1.7 to 2.3 $/m³, respectively. Furthermore, it is shown that the input parameters like cost index factor, electricity cost, compressor efficiency and the heat transfer areas have a remarkable influence on the product cost and must be selected carefully for accurate cost estimation.
Article
The main purpose of this study is to develop a sustainable zero liquid discharge desalination system. Direct contact membrane distillation unit is connected directly to salinity gradient solar pond (SGSP) to achieve zero liquid discharge desalination. The used system contains a hydrophobic microporous membrane module and a plastic pipe circulating all over the pond water surface to be used as a cooling system. The pipe also used as a wave suppression system as it is floating over the top of the pond water surface. The system is sourced by the hot and high concentrated saline water that is extracted from non-convective zone as a feed solution, then, the brine discharges at the lower convective zone of the solar pond. Therefore, if the saturated brine is used to produce salts, there will not be any brine left over which may lead to zero liquid discharge desalination. The system is modelled theoretically and solved by Matlab simulation program. It has been found that the system has the ability to deliver 52 l/day of fresh water for m2 of membrane coupled with SGSP, consuming almost 11 kW/m2 of thermal energy. Also, the trans-membrane coefficient of the used membrane is proved to be 0.001 kg/m2/Pa/h. The results are analysed and the system performance is evaluated and presented in this paper.