(186f) PTFE Hollow Fiber-Based Contineous-Effect Membrane Distillation for Deep Concentration of Aqueous Solutions At High Temperatures | AIChE

(186f) PTFE Hollow Fiber-Based Contineous-Effect Membrane Distillation for Deep Concentration of Aqueous Solutions At High Temperatures

Authors 

Qin, Y. - Presenter, Chembrane Research & Engineering,Inc.
Wang, B., Tianjin University



Membrane distillation (MD) is a relatively new membrane separation process during which only vapor molecules from the feed stream (hot side) are transported through porous hydrophobic membranes and then re-condenses by strip stream (cold side). Membrane distillation usually can be classified broadly into four categories according to the nature of the cold side of the membrane: direct-contact membrane distillation (DCMD), vacuum membrane distillation (VMD), sweeping gas membrane distillation and air gap membrane distillation (AGMD). The advantage of membrane distillation over traditional separation processes include lower operating pressures, 100% (theoretical) of ions and other non-volatile constituents are rejected, and so on.
Membrane distillation was introduced in the 1960s and developed from 1980s onwards, but did not yet attain commercial status for limiting of membrane with the characteristics most suitable and thermal efficiency. Membrane distillation is usually operated at temperature of <90 oC. However, permeation flux (J) and thermal efficiency are much lower when dealing with concentrated salt solutions as a result of reduction of vapor pressure as well as concentration and temperature polarization effects. Even for low concentration solution, the performance ratio (PR) of conventional MD process typically ranged from 0.2 to 1, while the PR of MSF and MED usually ranged from 7 to 15. Another problem with MD is wetting or fouling of microporous membrane surface, which leads to the decrease of permeate flux and leakage; leakage further leads to the contamination of the permeate product by the impurities in the feed. Continuous-effect membrane distillation (CEMD) is based on the use of AGMD process, which has distinct characteristics of high PR value and internal recovery. Continuous-effect membrane distillation (CEMD) is based on the use of AGMD process, which has distinct characteristics with latent heat recovery and with a PR obviously more than 1.0. It has several advantages such as (1) completely eliminating evacuation operation, which implies that the AGMD-based CEMD module can minimize the leakage problem and thus has a long operation stability; (2) much higher PR than DCMD-based CEMD process since heat conductivity loss in an AGMD process is much less than that in a DCMD process; (3) dealing with feed streams containing acid, base, oxidant, toxic or volatile solutes, since the distillate will not directly contact with the cooling water stream.
In the present study paper, continuous-effect membrane distillation (CEMD) is explored in the range of 85-115 ℃ for deep concentrating feed of NaCl and MgCl2 solutions. The CEMD membrane was used polytetrafluoroethylene (PTFE), which could operate at high temperature. Under optimized operational conditions, The experimental result indicated that the J and PR were 3.1 L?m-2?h-1 and 15.2 for the 5% NaCl solution and 2.9 L?m-2?h-1 and 14.4 for the 5% MgCl2 when the value of T3 was 100 oC; the values of both J and PR decreased with the increase of feed concentration, however, their values could still achieved 1.88 L?m-2?h-1, 7.2 and 1.62 L?m-2?h-1, 6.4 respectively for the 25% NaCl and 30% MgCl2 solution when the value of T3 was 108 oC, while the salt retention rate could reach more than 99.95%. Also, a long-tern operational stability of the MEMD process was tested used 15% NaCl solution for 60 days. The experimental results indicated that CEMD process showed in good operation stability
In conclusion, the study showed that CEMD was feasible to deal with brine for the advantage of high PR and thermal efficiency at high temperature. It has a great potential value for development and application.