Chapter1 - CIVL1005 - Environmental Engineering Mass and...

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1 CIVL1005 - Environmental Engineering Mass and Energy Transfer Mass and Energy Transfer Office: Rm. 6-30, Haking Wong Building Phone: 2859-1973 E-mail: kshih@hku.hk Dr. Kaimin Shih DEPARTMENT OF CIVIL ENGINEERING THE UNIVERSITY OF HONG KONG • Units of Measurement • Materials Balance • Energy Fundamentals - OUTLINES -
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2 Units of Measurement Environmental quantities were commonly measured and reported in both: U.S. Customary System ( USCS ). International System of Unit ( SI ). Units of Measurement
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3 Liquids are usually reported with “ concentrations ”, such as the weights of substances dissolved in water: 1 mg/L =1 g/m 3 = 1 ppm (parts per million) 1 g/L = 1mg/m 3 = 1ppb (parts per billion) 0.053 m M (millimole/liter) NaF = ? mg/L 0.053 10 -3 42 ( 42 = molecular weight of NaF) = 2.226 10 -3 g = 2.226 mg Therefore, 0.053 mM = 2.226 mg/L * 1 m 3 water = 1000 L = 1000 kg * m = 10 -3 , = 10 -6 Gases: 1 volume of gaseous pollutant / 10 6 volumes of air = ppm v(parts per million by volume) Idea gas law: PV = nRT P = absolute pressure (atm) V = volume (L) n = mass (mol) R = idea gas constant = 0.082056 L atm K -1 mol -1 T = absolute temperature (K), where K = o C+ 273.15 How much volume does 1 mole of idea gas occupy at 1am of pressure and 25 o C? 1 V = 1 0.082056 298.15 V = 22.465 L 1 mole of every ideal gas occupies the same volume , so: 1 ppmv = 1molecule of pollutant / 10 6 molecules of air
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4 Materials Balance Law of Conservation of Mass : Materials Balance Accumulation rate = Input rate Output rate + Reaction rate C s Q s + C w Q w = C m Q m
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5 Mass Balance/Metabolism of System Mass Balance/Metabolism of System s ±Δ s AC B D Systems boundary Santorio Santorio (1561-1636), an Italian physician and professor Reaction rate, r, in a completely mixed batch reactor ( CMBR ): dC/dt = r If r is positive Generation , negative Decay * Zero -order : r is a constant (k) Ex.: Rate of water evaporates from a bucket. * First -order : r = kC Ex.: Decay of radioactive materials. * Second -order : r = kC 2 Ex.: OH (hydroxyl radical) + OH = H 2 O 2 Nonconservative Pollutants
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6 Zero-order : r is a constant (k) r = dC/dt = k C – C 0 = -kt C = C 0 -k t First-order : r = kC r = dC/dt = kC C/C 0 = e -kt C = C 0 e -k t Second-order : r = kC 2 r = dC/dt = kC 2 C (1+ C 0 kt) = C 0 C = C 0 / (1 + C 0 k t ) Steady-state system, continuously stirred tank reactor ( CSTR ) : Reaction rate Input rate Output rate 0 = Input rate Output rate + Reaction rate Ex.: Ammonium ion (NH 4 + ) pollutant input concentration & flow rate into a CSTR (with volume V) are C s & Q s . Output concentration & flow rate are C m & Q m . Ammonium ion decay rate is a first-order reaction. The governing equation can be written as: 0 = C s Q s - C m Q m -kC V (Totally Mixed)
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7 Steady-state system, plug flow reactor ( PFR ) : Input rate Output rate Traveling time t = V / Q (= l / v , where v = velocity) Assuming ammonium ion decay rate is a first-order reaction. The output concentration ( C m ) can be calculated as: C m = C s e -k( V/Q ) Length = l Volume = V Q, C s Q, C m (Not Mixed) Step function response (not in steady-state): V dC/dt = QC i -Q C -V k d C + k g V
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This note was uploaded on 10/02/2010 for the course CIVIL ENGI 111 taught by Professor 222 during the Spring '10 term at SUNY Rockland.

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Chapter1 - CIVL1005 - Environmental Engineering Mass and...

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