《air stripping》.doc

From thin-theory with water-side control: Dm/dt=-Dw =Dw/δw(C(z)-Ceq(z)) =Kl(C(z)-Ceq(z)) Kl=liquid-phase mass transfer coeff(piston velocity)[L/T] Ceq=water conc in equilibrium with gas conc=G/H Back to mass balance: QwC(z+z)-QwC(z)=Kl(C(z))-Ceq(z))aAtz Qw/AtKla =C(z)-Ceq(z) In limit as z→0 Qw/AtKla dc/dz=C(z)-Ceq(z) Qw/AtKla =dz Qw/AtKla==l reqd tower height To solve,need Ceq as function of C From Eq(z): G(z)=(C(z)-Cout) Ceq(z)=G(z)/H=(Qw/Qa)(C(z)-Cout)/H L=Qw/AtKla Skip in class L=Qw/AtKla[]In[] Since s=(Qa/Qw)H=stripping factor L=Qw/AtKla(s/s-1)In[] For design,stripper tower is represented as a stack of transfer units: L=HTU*NTU HTU=height of transfer unit=Qw/AtKla Generally Qw/At20gpm/ft2=0.014m/s Manufacturer can supply Kla values vs temperature and flow rate (but best to test in pilot studies before final design)Kla=0.01 to 0.05s-1 for VOCs Use Qw/At=20gpm/ft2 ,known Qw to find At Use Kla data ,Qw/At to find HTU NTU=number of transfer units =In[+1/s] Design graph (pg.10) gives fraction removed () vs. S and NTU Note marginal decrease in NTU for s3 插入图 Full design procedure is given by: Kavanaugh,M.C and Trussell,R.R,1980. Design of aeration tower to strip volatile contaminants from drinking water. Journal AWWA Vol.72 No12 pp684-692,December 1980. MWH,2005 gives same procedure Design procedure considers pressure drop for air flow through tower,a cost factor in that blower adds to power consumphon. Kuo,1999(handout) gives simplified procedure: Get H for contaminant of concern Select s between 2 and 10 - s=3.5 is good estimate Compute Qa/Qw=s/H From known Qw find A such that Qw/A20gpm/ft2=0.014m/s=14L/s/m2 Determine desired treated water conc,Cout From kown Cin ,desired Cout and estimated s Compute NTU=()In[