EuroSun2008-2

Salt water experiments

For the salt water experiments, the same factorial design was intended to be implemented, but the results (a significant raise in conductivity, with values ranging from 40-60 pS/cm) when working with low fluxes on both cold and hot channels, and regarding the leakage problems we already had in the first stage, made us decided to evaluate the performance at the best conditions that were settled on the first stage, those are 20 l/min on both channels, and thus only the effect of temperatures were checked. Generally, smaller distillate fluxes and higher conductivities were observed when working with salt water, about 20% less distillate production and regarding conductivity, the values for fresh water experiments were never above 10 pS/cm (average value of 3.97 pS/cm), while for salt water ones were never below 12 pS/cm with an average value of 61.7 pS/cm.

Fig.3. a) Comparison between fresh and salt water experiments of distillate conductivity versus hot inlet temperature, working at 20 l/min of flow rate. b) Comparison between fresh and salt water experiments of distillate production versus hot inlet temperature, working at 20 l/min of flow rate.

Fig.4. AGMD modules at PSA.

5. Conclusions

Main conclusions of the experimental campaign were the expected ones:

• The variable with higher contribution to the distillate production is the hot feed temperature as reported in literature [6] (increasing feed temperature makes distillate production higher due to the exponential increase of vapour partial pressure), followed by the hot flow rate and their interaction.

• Rising hot feed flow rate increases the heat and mass transfer coefficients in the boundary layer on the membrane surface, thereby reducing the temperature and concentration polarization effects, and as a result increasing the distillate flow [9].

• Cold side temperature and flow rate have a lower effect on the production than the hot side, for the case of flow rate the effect is almost negligible.

• Although MD is claimed to be not affected by salt concentration of the feed inlet, the results of the experiments reveal that not only the conductivity of the distillate but the production is negatively influenced by salt concentration.

References

[1] J. Koschikowski, M. Wieghaus, M. Rommel, Solar thermal-driven desalination plants based on membrane distillation, Desalination, 156 (2003) 486-587.

[2] E. Tzen et al., Desing of a stand alone PV-desalination system for rural areas, Desalination, 119 (1998) 327­334.

[3] K. Kalidasa, Kn. K.S. K. Chockalingam, K. Srithar, Progresses in improving the effectiveness of he single basin passive solar still, Desalination, 220 (2008) 677-686.

[4] Z. Ding et al., Analysis of a solar-powered membrane distillation system, Desalination, 172 (2005) 27-40.

[5] M. S. El-Bourawi et al., A framework for better understanding membrane distillation separation process, Journal of membrane science, 285 (2006) 4 -29.

[6] A. M. Alklaibi, N. Lior, Membrane-distillation desalination: status and potential, Desalination, 171 (2004) 111-131.

[7] R. Chouikh, S. Bouguecha, M. Dhabbi, Modelling of a modified air gap distillation membrane for the desalination of seawater, Desalination, 181 (2005) 257-265.

[8] M. N. Chernyshov, G. W. Meindersma, A. B. De-Haan, Modelling of a temperature and salt concentration distribution in membrane distillation feed channel, Desalination, 157 (2003) 315-324.

[9] F. Banat, R. Jumah and M. Garaibeh, Exploitation of a solar energy collected by solar stills from desalination by membrane distillation, Renewable Energy, 25 (2002) 293-305.