Heat and mass transfer characteristics in vacuum membrane distillation for water desalination

Abstract

The vacuum membrane distillation (VMD) is applied in desalination. The simultaneous heat and mass transfer and the operating condition are studied. The experiments were carried out on a laboratory VMD unit, using a pre-prepared flat sheet membrane. Feed solutions used were fresh and saline water ranged from 3000 to 40,000 ppm. The operating parameters studied was feed temperature (298–368K), absolute pressure at downstream of the membrane (80,000–20,000 Pa), feed velocity (0.001–0.004 ms–1). The permeability of the prepared membrane is measured by two methods; the results showed the feasibility and applicability of these methods. The operating conditions and membrane characteristics are proved crucial factors influencing the permeation through the membrane. The experimental data were used to estimate the individual liquid film transfer coefficients. The operating condition dependence is demonstrated. The results are expressed in an equation for each coefficient; temperature (T), pressure (P), feed velocity (v) and concentration (C). Also, the obtained empirical correlations at the liquid-vapor interface are expressed as a function in T, P, v, and C. Moreover, an empirical correlation for the Sherwood number is formulated. The developed empirical correlations are proved to be capable in predicting the VMD transport coefficients, and it may play a key in the process design and performance.