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# L19 - Lecture 19 Gas Abs/Des in Packed Towers Va abs,dilute...

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Lecture 19: Gas Abs./Des. in Packed Towers V a L a L b V b L V Liquid Distributor abs,dilute op line y b y a eq line x a x b Material Balance Overall Unit: OMB: L a + V b = L b + V a CMB: L a x a + V b y b = L b x b + V a x a

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Control Volume: OMB: L a + V = L + V a CMB: L a x a + Vy = Lx+ V a y a V x L y V x V L y a a a a + = Operating Line Equation As z increases, L increases, V increases, L/V changes For a dilute mixture L/V = constant •Limiting Liquid-Gas Ratio Given: x a , y a , y b L/V (L/V)min y b y a x a
As L/V decreases, slope of op. line decreases As L/Vgoes to (L/V)min, driving force goes to zero, tower height goes to infinity Operate L/V greater than (L/V)min Usually 1.1 to 1.5 times (L/V)min As L/V goes up, tower height decreases, (capital cost) amount of fluid handled increases (operating cost) Calculation of Tower Height Rate of Mass transfer = r = K y a(y-y * ) a=interfacial area per unit volume x y y k k K 1 1 1 + =

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L V Z T dZ Amount of mass transfer from vapor to liquid in dZ = Vdy = K y a(y-y * )SdZ S=cross sectional area = T b a Z o y y y dZ y y dy a SK V * T y y y Z y y dy a SK
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L19 - Lecture 19 Gas Abs/Des in Packed Towers Va abs,dilute...

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