4.2. Methodology
4.2.7. FO experimental setup for performance analysis of hydrogel
Chapter 04
The performance of the synthesised hydrogel was estimated in terms of hydrogel swelling and deswelling capacity. The swelling capacity of the hydrogels was estimated to evaluate water absorption capability. Before each swelling study, the hydrogels were placed in a hot-air oven at 70 °C for 4 h to ensure complete dehydrated products. A certain amount of dry gel (1 g) was immersed in DI water for a certain time 't' to absorb and completely swell. Over the period of time, the swelling ratio (Q, g g-1) of hydrogels was calculated using the following equation:
t d
d
W W
Q W
−
=
(4.4)
Wd and Wt represent the weight (in g) of the dried and swollen hydrogel at a time ‘t’, respectively.
The swelling ratio (Q, g g−1) can be defined as the fractional increase in the weight of the hydrogel due to water absorption. Further, to evaluate the deswelling capacity of the synthesised hydrogel under the thermal influence. A specified amount of swollen hydrogel was placed under a blower at room temperature for the dewatering process. The hydrogel dewatering continued until a weight change between the two readings was constant.
Material, methods, and experimental procedure
The FO performance for the given process was measured in water flux, RSF, and SRSF using estimated using equations (4.5), (4.2), and (4.3), respectively.
, w FS
m
Water flux J W
A t
=
(4.5)
where ∆WFS, Am, and ρ represent the weight change (in g) of FS over time '∆t' (in h), membrane contact area (m2), and density of the permeate water (assumed as 997 g L−1 at room temperature, at 25 oC) respectively.
Figure 4.7 Schematic representation of a) test set-up without heat, b) lab-scale experimental FO set-up with thermal dewatering section, c) and d) design used for 3D printing of integrated FO set-up
4.2.7.2. Experimental setup for FO batch process using dual draw solute
The integrated FO membrane module (Figure 4.7) suggests that the regeneration of swollen hydrogel is possible using thermal energy. However, it was observed that complete dewatering of hydrogel under the thermal influence is not practical for large-scale applications.
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Figure 4.8 a) Test scale set-up, b) schematic representation of a test-scale integrated membrane module
The regeneration of swollen hydrogel using high concentration RO rejects brine solution (63,500 mg L−1) was never explored, and according to our knowledge, and reportedly this is the first attempt of a concentration of liquid food (tea extract) using hydrogel as draw solute.
Due to swelling and osmotic pressure, the solvent from low-concentration FS permeates to the osmotic agent (semi-swollen hydrogel). Similarly, due to the osmotic gradient between the high-concentration brine solution and hydrogel, the solvent is expected to permeate from the hydrogel to the brine solution.
Figure 4.8 provides a schematic representation of an integrated membrane module. The intention behind this set-up was to minimise the chances of hydrogel degradation due to thermal treatment (blowing air at 39-50 oC) to improve the life-cycle of hydrogel for improved commercial feasibility.
4.2.7.3. Experimental setup for continuous FO using dual draw solute
To determine the practical feasibility of the given study, a series of experiments was conducted using the 3-tier membrane module (Figure 4.9) using DI as FS (on the top tier) and high concentration NaCl solution (38.4 g L−1,π =30.27 bar) on the bottom tier. The middle tier consists of a 1.8 g of dried hydrogel semi-swollen using 10 mL of DI. The 3-tier were separated using a semi-permeable membrane (area: 0.005 m2). The DI water and the highly concentrated NaCl solution were recirculated through the top and bottom tier of the membrane module at 45 L h−1 and 45 L h−1, respectively, in counter-current mode.
Material, methods, and experimental procedure
Figure 4.9 Schematic representation of the test scale experimental set-up using a 3-tier membrane module
The FO performance was measured in terms of permeate flux, RSF, and SRSF estimated using equations (4.1 to 4.3). In the first hour of the given process, a permeate flux and RSF of 27.81
± 0.09 L m−2 h−1 and 0.081 ± 0.025 g m−2 h−1, respectively. Over 6 h of the FO process, the permeate flux and RSF reduce 8.55 ± 1.22 L m−2 h−1 and 0.057 ± 0.026 g m−2 h−1. The given trial study suggests the feasibility of the given 3-tier design of the membrane module. However, further investigation needs to be performed to enhance the FO performance. The effect of the flow rate of the FS/ high concentration NaCl, the thickness of the middle tier, and other factors need further investigation to enhance FO performance.
The concentrated brine discharged by the desalination plants are denser than ambient seawater and therefore sinks and flows along the sea bottom, causing. Therefore, the highly concentrated brine must be diluted and treated before being released into the environment. This design suggests that the highly concentrated RO-reject brine can be diluted and passed to the aquatic body without causing a significant environmental impact.
4.2.7.4. Experimental setup for regeneration of hydrogel using solar radiation
Hydrogels are polymeric networks capable of swelling and shrinking reversibly in response to changes in the external environment. The water content of the swollen hydrogel is one of the critical parameters for assessing the dewatering performance. The water content (W) of the swollen hydrogel can be determined using the equation (4.5):
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( )
S i 100%
S
W W
W W
= − (4.5)
where 𝑊𝑠 and 𝑊𝑖 represent the weight (in g) of swollen hydrogel (after 24 h) and initial hydrogel.
Stimuli-responsive hydrogels can exhibit switchable sol-gel transition upon application of external triggers using external stimuli such as electric, magnetic, solar, and temperature. The swollen gel after the FO process was placed in a glass funnel separated by a Whatman filter (Grade 42). The temperature (oC) and relative humidity (% RH) was regularly measured using an infrared thermometer (HTC IRX-64 Digital Infrared Thermometer, Maker: HTC) and a humidity meter (HTC−1 high precision Humidity meter, Maker: HTC), respectively, at different exposure time. The dewatering efficiency (R) is calculated using:
1 0
W 100%
R W
=
(4.6)
where 𝑊1 is the weight of water loss during the solar dewatering test (in, g), i.e., the difference in the weight of swollen hydrogel before and after exposure to the sunlight over a certain period of time; 𝑊0 represents the weight of the water in the swollen gel before the dewatering test (in, g). In this study, the effect of solar stimuli was investigated for the effective dewatering of hydrogel used as a draw solute for the FO process (Figure 4.10).
Figure 4.10 Schematic representation of the test scale set-up of hydrogel regeneration under solar-influence