5.2. Simulation and Design Analysis of HFFO Membrane Module
5.2.2. Process flow-sheet simulation for the concentration of tea extract
Chapter 05
Results and discussions
in the module. Ideally, the DS should be operated at a higher flow rate. However, a higher flow rate also results in high-pressure build-up across the membrane, resulting in mechanical damage to the fiber. Therefore, the low or moderate DS flow rate is more suitable for extending the membrane lifetime, and as a result, in this study, the DS flow rate was not exceeded beyond 60 L h−1.
When the FS and DS flow rate is changed from 25/60 to 45/45, the SRSF was found to be significantly reduced by 26.81% (case I), 58.85% (case II), and 29.74% (for case III (a)). Using the given Modelica model, to determine the improved values of model parameters (FS and DS flowrates) by multi-criteria optimisation based on simulation runs were performed: (i) to minimize SRSF and (ii) maximize the net flux (𝐽𝑤). Thereby suggesting that further flow-sheet configuration would be performed using two FS/DS flow rate configurations (i.e., 25/60 and 45.14±0.08/ 45.21 ±0.29). Further, the function of the sweep-two parameter is to be used to study the dependence of response with respect to two parameters at the end of the integration interval (Figure 5.10).
Figure 5.10 Effect of feed and draw solution flow rate on the overall FO performance against different flow-sheet configuration
Results and discussions
5.2.2.2. Feed and draw solution in recycle mode
In this case, a single HFFO membrane (case IV) and two HFFO modules (case V) are placed in series. The FS and DS are re-circulated through the FO module in counter-current mode.
Maintaining other condition operating conditions same, it was observed that by implementing 2 FO membrane modules (case V) instead of 1 FO membrane module (case IV), the time required to reduce the FS (tea extract) by 93.542±0.822% could be significantly reduced by 7.5 times. In case VI, similar to case V, the FS and DS are re-circulated through two HFFO modules placed in series. However, instead of one HFFO module, two separate DS tank was recycled through the module separately. As discussed in the previous section, the DS and FS flow rate significantly impact the overall FO performance, and the same trend was observed in this study.
Therefore, case VI flow sheet configuration with FS and DS flowrate at 45.14 L h−1 and 45.21 L h−1, respectively, was most suitable for dewatering liquid food using the FO process.
5.2.2.3. Feed solution in recycled mode and draw solution mode in continuous mode
The best-performing process flow configuration (case VIII) is further modified in this section.
Here, the FS is recycled, whereas the DS is passed through the 2 HFFO modules in continuous mode. Due to the higher concentration gradient between the two solutions, the flux is much higher and, as a result, compared to other cases, where the time required to reduce volume by 93% was significantly lower compared to other cases. The DS can either be pumped continuously through the 2 HFFO membrane modules (case VII) or pumped separately through both HFFO modules using two pumps (case VIII). Similar to previous cases, the effect of FS and DS flow rates was investigated on the overall membrane performance.
The simulation data reveals that in both case VI and case VII, the time taken to reduce FS volume by 94.49 ±1.07% was almost the same for both flowrate configurations. In this section, it is observed that when the flow rate of FS/DS is changed from 25/60 to 45.14/45.21, the SRSF is reduced by 57.74% (case VII) and 66.87% (case VIII), respectively. Based on SRSF and SEC data, Case VIII was identified as the best-performing process flow sheet (Table 5.8).
Chapter 05
Table 5.8 Overview of the optimised FO process condition [FS flow rate: 45.14 L h−1 and DS flow rate: 45.21 L h-1]
I II III (a) III (b) IV V VI VII VIII
Time (min.) − − − − 10.5 6.5 3.75 2.39 3.01
FS volume reduced (%)
27.54 43.15 88.18 93.11 93.18 93.15 93.16 93.14 93.16
SRSF (g L−1)
0.17 0.13 0.14 0.597 1.57 1.57 3.06 0.22 0.21
SEC (kWh m−3)
0.50 0.61 0.88 4.14 1.57 1.57 3.06 3.11 1.59
5.2.2.4. Implementation of seawater and reject brine as potential draw solute for the concentration of tea extract
In this section, the effect of implementing fresh seawater and high-concentration reject brine as draw solute for the concentration of liquid food solution was investigated. Considering the best-performing process flow sheet (Case VIII) configuration, the FS is recycled, and the DS is circulated in continuous mode. The principal objective of this study is to utilize high- concentration seawater as DS for the preparation of concentrated tea extract (93.22±0.18%).
As discussed in section 3.9, the given objectives can be achieved when the FS and DS flow rate is maintained at 45.14±0.08 L h−1 and 45.21±0.08 L h−1, respectively. Due to higher water flux, the concentration process was rapid, along with simultaneous seawater dilution. Herein, Table 5.9 tabulates the effect of DS concentration on the overall process performance when seawater and red seawater are used as DS.
The study suggests that when the reject brine stream of red seawater is utilized as DS (105.175 g L−1), the solvent transport from DS to FS (tea extract) is significantly higher due to high concentration. Thus, the resulting FS volume to reduce by 93.10% in just 2.18 min, with minimal SRSF (0.19 g L−1). This study also suggests that the reject brine is diluted while concentrating the FS from 105.17 g L−1 to 32.18 g L−1.
Results and discussions
Table 5.9. Comparison of FO performance while using seawater and red seawater as draw solute [FS flow rate: 45.14 L h−1, and DS flow rate: 45.21 L h−1]
1.5 M NaCl
Seawater Red seawater
Fresh Brine reject Fresh Brine reject
Time (min.) 3 4.50 2.80 4.00 2.00
FS volume reduced (%) 94.05 93.68 93.26 91.98 93.35
SRSF (g L−1) 0.21 0.19 0.19 0.21 0.14
DS concentration (g L−1)
Initial 65.6 35 70 42.07 105.17
Final 25.92 17.16 25.03 19.50 81.38
Summary of simulation and design analysis of the HFFO membrane module
The developed one-dimensional module was validated using the batch experimental data and could predict DS and FS tank mass and concentration. The estimated model parameters, such as pure water permeability (Lp), solute permeability (B), and structural parameter (S), were found to be like the literature-reported values. Additionally, a series of simulation studies were performed to understand operating conditions' role (flow rate) on the overall FO performance.
The objective of this simulation study was to improve the overall FO performance (by maximising the water flux and minimising the SRSF). The feasibility of seawater and high- concentration reject brine as DS was also investigated. Due to high concentration, the reject brine (from the seawater desalination plant) can result in rapid dewatering with minimal SRSF (0.185 g L−1 to 0.207 g L−1) along with simultaneous seawater dilution.
5.3. Performance Analysis of Hydrogel as Draw Solute for Forward Osmosis Process