Paraphrasing Essay Example | Topics and Well Written Essays - 1000 words - 1

Paraphrasing - Essay Example The heat transfer resistances are determined by the feed flow rate and feed concentration, feed temperature and other operating parameters. The system heat transfer is controlled by the heat transfer in the membrane the resistance of which is 27-46 times higher than that of feed stream. 1. Heat Transfer The total heat flux (Q), across the membrane is expressed by the following equation: (1) Where, U is the overall heat transfer coefficient is bulk temperature difference among the feed and permeate sides J is the trans-membrane mass flux is the latent heat of vaporization Under steady state conditions, derived from the heat balance, the heat transfer in the individual compartments of system is represented by the following equation: (2) On the basis of equation 8, and, or the temperatures on both sides of the membrane can be estimated using the following equations: (3) (4) Further the heat transfer coefficient of the membrane (hm) can be determined on the basis of thermal conductivitie s of the membraneand of the vapor that fills the pores, using the equation 5. (5) The heat transfer coefficients of the boundary layer and can next be estimated experimentally or can be calculated using empirical correlations of dimensionless groups, namely Nussselt number (Nu), Reynolds number (Re) and Prandtl number (Pr). These numbers can be calculated directly from the data available for aqueous NaCl solutions and water, using the equation 6. ... The primary set up included pure water operated at four selected feed temperatures viz. 40, 50, 60 and 70?C. 3. Membrane distillation coefficient and pure water flux The vapor pressure differencewas calculated at the membrane surface temperaturesand, and plotted against the steady state fluxes obtained at selected feed temperatures as illustrated in Fig 4. On the basis of equation (1), it can be concluded that the slope of the straight line of the plot thus obtained gives the value of membrane distillation coefficient, (C= 0.0004 kg/m2.h.Pa or C= 1.11E-7 kg/m2.s.Pa). The membrane distillation coefficient remains constant for a specific membrane and vapor properties. The experimentally determined value of C was equivalent to the value reported in literature (C= 8.5E-7 kg/m2.s.Pa). 4. Effect of feed temperature on permeate flux Permeate flux is largely dependent on fee temperature. Fluxes of pure water and aqueous NaCl solution is represented in Fig. 6. Elevation of feed temperature le ads to a rise in permeate flux as a consequence of rise in vapor pressure of gas-liquid interface on liquid feed side, which causes a simultaneous increase in the driving force of mass transfer. 5. Effect of feed flow rate on permeate flux At a constant temperature the mass flux is dependent on feed flow rate as depicted in Fig. 7 showing the flux time curve at four feed flow rate at a constant temperature of 40?C. The mass flux rate is directly proportional to feed flow rate since a rise in feed flow rate causes a rise in turbulence, reduction of heat transfer resistance in the boundary layers and consequential rise in mass transfer rates. Moreover, the increase of feed boundary heat transfer coefficientwith Reynolds number lead to further rise