An Even Better Desalinator

Inserting = 0 yields Eq. (3.26), the result when nothing was retained by the water stream, which was the case in the evaporator. Check. 

Let s use operating equations (3.50) and (3.51) to study the effect of the retained brine. Is the output rate sensitive to Is the purity sensitive to That is, is it worthwhile to reduce the amount of brine retained by the ice The previous analysis revealed that the maximum production of potable water is obtained at the maximum allowable salt content, 0.0005 wt fraction salt. Substituting into Eq. (3.50) yields 

Because the salt content of the waste brine must be at least that of seawater and at most that of pure salt, a is limited to the range 0.035 o 1.0. From the limits on a and Eq. (3.52), the limits on the weight fraction of retained brine are 0.0005 0.0143, assuming that the potable water has 0.05 wt% salt. This tells us that the process will not function if the ice retains more than 1.4 wt% brine, P 0.014. This also tells us that for p 0.0005, the salt content of the potable water is less than 0.05 wt%. 

Substituting Eq. (3.52) into Eq. (3.50) gives the operating equation for a desalinator that produces potable water with 0.05 wt% salt, the maximum allowable concentration  

At )8 = 0.01, the original specification on the freezer-desalinator, Eq. (3.53) yields Fvy 4 = 30 kg/min, as we calculated before. At )8 = 0.0005, Eq. (3.53) yields Fwa = 96.5 kg/min - all the water exits stream 5 (but with 0.05 wt% salt). Although the mass balance permits P = 0.0005, it would be impractical to freeze seawater to produce solid ice and solid salt, and then separate the solids. Some water must leave with the brine. What if we reduce the retained brine by a factor of 2, to )8 = 0.005 Equation (3.53) predicts we will double our output to Fwa = kg/min. Clearly, it is worth investigating designs to reduce retained brine. 

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