103111countercurr-extrc

103111countercurr-extrc - Countercurrent extrac,on ...

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Unformatted text preview: Countercurrent extrac,on Absorp,on and stripping Wankat ch. 12 Natural gas liquids (C3+) recovery from natural gas CO2 capture from ﬂue gas Natural gas drying plant Absorp,on of water by ethylene glycol, followed by solvent regenera,on Objec,ves for this lecture •  Convert mole frac%ons into mole ra%os •  Derive opera,ng line equa,on for stripper column. •  Apply McCabe- Thiele analysis to mul,stage stripping using mole frac,ons •  Calculate frac,onal stage requirement •  Derive opera,ng line equa,on for absorber column •  Apply McCabe- Thiele analysis to mul,stage absorp,on using mole frac,ons McCabe- Thiele Analysis of Counter- current Stripper Column Given X0, XN, YN+1 and L/G ﬁnd N required 1. Convert VLE data to mole ra,os (unless x0 < 0.05) Note: y=x line has no use here. 3. Step oﬀ stages (use Murphree eﬃciencies if available). •(XN,YN+1) Y 2. Plot (XN, YN+1) and (X0, Y1) and draw opera,ng line. It will be below the VLE line. Note: calculate Y1 from X0 and opera,ng line equa,on (mass balance). 1 • 2 • 3 • • • •(X0,Y1) N = 3 To ﬁnd minimum stripping gas ﬂow rate (Gmin): X First ﬁnd (L/G)max, by plobng X0 on the VLE line (watch for other pinch points if VLE is curved). Lever- arm rule To es,mate frac,onal stage requirement in a packed column: Y VLE op. line (X4, Y4) • • , Y ) (X3 4 • XN X • 3 X4 • • XN X frac,onal stage = X N − X 3 requirement X 4 − X3 € McCabe- Thiele Analysis of Counter- current Absorber Column Given X0, Y1, YN+1 and L/G ﬁnd N required 1. Convert VLE data to mole ra,os (unless x0 < 0.05) Note: y=x line has no use here. 3. Step oﬀ stages (use Murphree eﬃciencies if available). • Y 2. Plot (X0, Y1) and (XN, YN+1) and draw opera,ng line. It will be above the VLE line. Note: calculate XN from YN+1 and opera,ng line equa,on (mass balance). (XN,YN+1)• • (X0,Y1)• • 3 • 1 • 2 N = 3 To ﬁnd minimum extrac,ng solvent ﬂow rate (Lmin): X First ﬁnd (L/G)min by plobng YN+1 on the VLE line (watch for other pinch points if VLE is curved). Recap •  The unit opera,ons absorp,on and stripping are oeen performed sequen,ally. •  When carrier gas is insoluble and solvent is non- vola,le, both gas and liquid streams are binary. •  McCabe- Thiele analysis needs to be done using mole ra,os instead of mole frac,ons, except when streams are very dilute. •  For absorp,on, the opera,ng line lies above the VLE line. •  Mass transfer eﬃciencies are low, and packed columns are common. Objec,ves for this lecture •  Describe McCabe- Thiele analysis for mul,ple dilute, non- interac,ng solutes •  Analysis of irreversible absorp,on •  Analysis of co- current extrac,on •  Introduc,on to Kremser equa,ons (analy,cal solu,on for dilute extrac,on) Homework: 12D7 due Wednesday, 12D20 due Friday Reading: Wankat, chapter 12 Prac,ce midterm exam (Friday) Mul,ple non- interac,ng solutes Consider mul,ple soluble components (B, D, E…) in solvent C, to be stripped using gas A OR Consider mul,ple gas components (B, D, E…) in carrier gas A, to be absorbed using solvent C If streams are dilute and component do not interact with each other, assume VLE for each component is independent Treat each one as a single- component problem and solve sequen,ally. Example: 2- component absorp,on xB,0 xD,0 yB,1 yD,1 (xD,N,yD,N+1)• 1 Separa,on of B requires N = 3. Y (xB,N,yB,N+1)• • • • 1 •1 N yB,N+1 yD,N+1 xB,N xD,N (xB,0,yB,1)• (xD,0,yD,1)• Specify yB,N+1, yD,N+1, xB,0, xD,0 Specify L/V and yB,1. Find N and yD,1 • • •3 •3 •2 • 2 Separa,on of D must also use N = 3 and same L/V. Trial- and- error: guess yD,1 X A good idea to use a diﬀerent graph for each component… Irreversible absorp,on Solvent contains a reagent R which reacts irreversibly with solute B to form non- vola,le product RB R + B(g) → RB e.g., OH- + H2S(g) → S2- + H2O Since equilibrium lies far to the right, xB ≈ 0 yB ≈ 0 Equa,on of the equilibrium line: yB = 0 Irreversible absorp,on of B A + R x0 = 0 C y1 = 0 1 Only one theore,cal equilibrium stage required … Y yN+1 • N B + C yN+1 A + RB xN = 0 Specify yN+1, x0, L/V. Required: xN = y1 = 0 (x0,y1)• (x1,y1) VLE • X (B + RB) With low Murphree eﬃciency A + R x0 = 0 C y1 = 0 More than one actual equilibrium stage required … 1 yN+1 • Y • • N B + C yN+1 A + RB xN = 0 Specify yN+1, x0, L/V, y1 ≠ 0 • • • 3 • •2 (x0,y1)• •1 •6 EMV = 0.25 • 5 • 4 VLE X (B + RB) Co- current absorp,on • Use higher vapor velocity to increase mass transfer rate • Use smaller diameter column without risk of ﬂooding • Gives higher eﬃciency than countercurrent column L x0 V y0 (x0,y0)• 1 V yj L xj y0V + x0L = y jV + x j L N V yN € Specify y0, x0 = 0, xN, yN = 0 Y j Only one theore,cal equilibrium stage required if reac,on is irreversible and mass transfer is fast … L xN ⎛ྎ y V + x L ⎞ྏ L 0 y = − x + ⎜ྎ 0 ⎟ྏ V V ⎝ྎ ⎠ྏ (x1,y1) VLE • X (B + RB) Analy,cal solu,on 1 When streams are dilute, we can approximate VLE data by a straight line. 0.9 0.8 0.7 0.2 y(MeOH) 0.6 0.5 0.15 y(MeOH) 0.4 0.3 0.2 0.1 y = mx 0.1 0.05 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 x(MeOH) Obtain the slope, m, from Henry’s Law: 0 0 0.01 0.02 0.03 0.04 x(MeOH) PB = HB xB where PB is the par,al pressure of B, and HB is the Henry’s Law constant Note: HB = HB(T), like an equilibrium constant. 0.05 0.06 Kremser equa,ons Change in vapor composi,on between adjacent stages: (Δy ) j = y j +1 − y j 0.2 (x2,y3) • (Δy)2 y(MeOH) 0.15 (x1,y2) • •(x2,y2) 0.1 € (Δy)1 = y2- y1 (x0,y0)• 0.05 •(x1,y1) € ⎛ྎ L L ⎞ྏ y j +1 = x j + ⎜ྎ y1 − x0 ⎟ྏ V V ⎠ྏ ⎝ྎ y j = mx j (Δy ) j ⎛ྎ L ⎞ྏ ⎛ྎ L ⎞ྏ = ⎜ྎ − m⎟ྏ x j + ⎜ྎ y1 − x0 ⎟ྏ V ⎠ྏ ⎝ྎ V ⎠ྏ ⎝ྎ € 0 0 0.01 0.02 x(MeOH) Special case: L/V = m; (Δy)j = Δy NΔy = Δy1 + Δy2 + Δy3 + … € = yN+1 – y1 0.03 Kremser equa,on: N = yN+1 − y1 y −y = N+1 1 Δy y1 − L x0 V ...
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