You are designing a new glass factory near the ocean. Sand is to be mined wet from the beach. However, wet sand carries with it seawater entrained between sand grains. Salt must be removed by a washing process. A crossflow process will be employed with \(0.2 \mathrm{~kg}\) of wash water added to each stage for each \(1000.0 \mathrm{~cm}^{3}\) of wet sand fed. Wash water outlet from last stage will be used as wash water inlet for stage 3 . Wash water outlet from stage N-1 will be used as wash inlet for stage 2, and wash water outlet from stage \(\mathrm{N}-2\) as wash water inlet to stage 1 . All other stages have pure wash water inlet (see figure). Outlet concentration \(\leq 0.002\) weight fraction salt in the entrained liquid. How many stages are required? Data: Seawater is approximately \(3.5 \mathrm{wt} . \%\) salt, \(\varepsilon=0.40\) Note: This problem is not guess and check.

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Pure Water N |N-1| N-2 3 2 Sand + Entrained Seawater