## 3 Results and Discussion  
### Effect of Membrane Materials on the Permeate Flux of the DCMD Process  
The experimental comparison of different membrane materials was conducted under the operations of 50 \({}^{\circ}\)C hot-side inlet temperature, 20 \({}^{\circ}\)C cold-side inlet temperature, 500 mL/min hot-side flow rate, 500 mL/min cold-side flow rate, and 3 wt% NaCl (300 g salt mixed with 10 L water) of the solution to be separated. During the experiment, the conductivity of the produced water was controlled below 10. Table 3 shows the experimental results. The experimental results indicate that both PTFE and PVDF membranes perform well, whereas pp membranes are not recommended. The permeate flux increases as the membrane pore size increases without considering any membrane fouling and scale-up issues. The optimal membrane pore size in this study is 1.0 \(\upmu\)m.  
Figure 5: Membrane module structure. As shown in Figure 6a, the total average permeate flux of a PTFE membrane with 1.0 \(\upmu\)m pore size is 12.87 kg/(m\({}^{2*}\)h), which fluctuates up and down from 15 to 60 min of the MD process and then stabilizes after 90 min. The average permeate flux for PTFE membranes with pore sizes of 0.45 \(\upmu\)m is 11.42 kg/(m\({}^{2*}\)h). The total average permeate flux for a PTFE membrane with a pore size of 0.22 \(\upmu\)m is 8.62 kg/(m\({}^{2*}\)h), which increases significantly from 15 to 60 min of the MD process and then fluctuates after the 60 min. The fluctuation of permeate flux is mainly due to the jittering of the experimental water pipe, which causes some experimental errors. For errors, the data use the mean value. The relative error between each set of data and the mean was then calculated. Finally, the error results were all calculated to be within 4%, which is within the allowable error range. As shown in Figure 6b, the total average permeate flux of a PVDF membrane with a pore size of 1.0 \(\upmu\)m is 11.64 kg/(m\({}^{2*}\)h), with an overall decreasing trend and a significant decrease from 60 to 90 min of the process, and it tends to be stable after 135 min. The total average permeate flux of PVDF membrane with a pore size of 0.45 \(\upmu\)m is 10.25 kg/(m\({}^{2*}\)h), and it fluctuates up and down during the process. The total average value of permeate flux for PVDF membrane with a pore size of 0.22 \(\upmu\)m is 8.40 kg/(m\({}^{2*}\)h), and the average value remains relatively stable. Figure 6c shows the performance of the PP membranes in the DCMD desalination process. The total average permeate flux of a PP membrane with a pore size of 1.0 \(\upmu\)m is 4.00 kg/(m\({}^{2*}\)h), with an overall increasing trend. The permeate flux increases when the process is running from 30 to 60 min, and it is stable after 135 min. The total average permeate flux of a PP membrane with a pore size of 0.45 \(\upmu\)m is 1.16 kg/(m\({}^{2*}\)h), with an overall decreasing trend. It decreases after 45 min and becomes stable gradually. The total average permeate flux of a PP membrane with a pore size of 0.22 \(\upmu\)m is 2.00 kg/(m\({}^{2*}\)h). The decline from 60 to 75 min is greater than that from 90 to 105 min, and it is stable after 135 min. Figure 7 illustrates how the performance of the DCMD desalination system is impacted by membrane thickness. Membrane thickness has an inverse relationship with the permeate fluxes of PVDF and PTFE membranes, while the permeate fluxes of PP membranes change insignificantly with the increase in thickness and remain at a low permeate flux. Figure 8 shows the effect of membrane pore size on the desalination performance of hydrophobic porous membranes with pore sizes of 0.22 \(\upmu\)m, 0.45 \(\upmu\)m, and 1.0 \(\upmu\)m. It can be seen that the differences between the permeate fluxes of PTFE and PVDF membranes are small, while PP membrane's permeate flux is significantly lower than that of the others. This situation arises because the PVDF membrane has moderate thermal stability and a surface energy of 3.03 \(\times\) 10\({}^{-2}\) N/m.