4.3 Induction period and surface tension in MACC operation.
Based on the classic nucleation theory, the surface tension was determined by the induction period under different supersaturations36,60,61. In the low supersaturation zone, heterogeneous nucleation was dominant, and in the high supersaturation zone, homogeneous nucleation played the controlling role. The measured nucleation induction period with different membrane package density is as follows in the Figure 8.
Comparing the nucleation induction period with membrane module involved, it can be seen that the lower coolant temperature can provide the greater supercooling degree and correspondingly shorten the nucleation induction period under the tested temperature range. With increasing membrane module package density, the nucleation induction period almost inversely shortened (when PTFE membrane module package density increased from 120 m2/m3 to 240.6 m2/m3 then to 361.0 m2/m3, correspondingt ind at 8 ℃ declined from 240s to 160s then to 80s). This result indicated that the nucleation induction period when the membrane module involved was highly impacted by the heat transfer property of the membrane module, which would also inevitably influence the membrane interface supercooling degree.
In addition, owing to roughness, hydrophobicity, and the thickness of membrane determines heterogeneous nuclear energy barrier of PES was smaller than that of PTFE, the induction time was also shorten when PES membrane module was introduced; The nucleation induction period with the same PTFE and PES membrane area were listed in Table 3. At the same terminal temperature, plotting lnt ind versus (lns )-2 yields two straight lines with different slopes, which represented the homogeneous nucleation and heterogeneous nucleation, respectively, which can be obtained by linear fitting (show in Figure S2 and S3 as supporting materials).