Dr. Hatem Alsyouri Heat and Mass Transfer Operations

Transcription

1 Dr. Hatem Alsyouri Heat and Mass Transer Operations Chemical Engineering Department The University o Jordan Reerences 1. Wankat: and Coulson & Richardson Vol 6: Seader and Henley Vol 2: Chapter 6 1

2 Packed Columns Packed columns are used or distillation, gas absorption, and liquid-liquid extraction. The gas liquid contact in a packed bed column is continuous, not stage-wise, as in a plate column. The liquid lows down the column over the packing surace and the gas or vapor, counter-currently, up the column. Some gas-absorption columns are co-current The perormance o a packed column is very dependent on the maintenance o good liquid and gas distribution throughout the packed bed. 2

3 Representation o a Packed Column Packing material Packing Height Z 3

4 Components o a Packed Column 4

5 Advantages o Trayed Columns 1 Plate columns can handle a wider range o liquid and gas low-rates than packed columns. 2 Packed columns are not suitable or very low liquid rates. 3 The eiciency o a plate can be predicted with more certainty than the equivalent term or packing HETP or HTU. 4 Plate columns can be designed with more assurance than packed columns. There is always some doubt that good liquid distribution can be maintained throughout a packed column under all operating conditions, particularly in large columns. 5 It is easier to make cooling in a plate column; coils can be installed on the plates. 6 It is easier to have withdrawal o side-streams rom plate columns. 7 I the liquid causes ouling, or contains solids, it is easier to provide cleaning in a plate column; manways can be installed on the plates. With small diameter columns it may be cheaper to use packing and replace the packing when it becomes ouled. 5

6 Advantages o Packed Columns 1. For corrosive liquids, a packed column will usually be cheaper than the equivalent plate column. 2. The liquid hold-up is lower in a packed column than a plate column. This can be important when the inventory o toxic or lammable liquids needs to be kept as small as possible or saety reasons. 3. Packed columns are more suitable or handling oaming systems. 4. The pressure drop can be lower or packing than plates; and packing should be considered or vacuum columns. 5. Packing should always be considered or small diameter columns, say less than 0.6 m, where plates would be diicult to install, and expensive. 6

7 Design Procedure Speciy separation requirements Select type and size o packing Determine column height Z Select column internals support and distributor Determine column diameter 7

8 Packing Materials 1. Ceramic: superior wettability, corrosion resistance at elevated temperature, bad strength 2. Metal: superior strength & good wettability 3. Plastic: inexpensive, good strength but may have poor wettability at low liquid rate 8

9 Reerence: Seader and Henley 9

10 Structured packing materials Reerence: Seader and Henley 10

11 Characteristics o Packing Reerence: Seader and Henley 11

12 Reerence: Seader and Henley 12

13 Packing Height Z V out y out L in x in Transer Unit TU Height o Transer Unit HTU n TU TU TU TU Packing Height Z V in y in L out x out Packing Height Z = height o transer unit HTU number o transer units n 13

14 Methods or Packing Height Z 2 methods More common Equilibrium stage analysis HETP method Z = HETP N N = number o theoretical stages obtained rom McCabe-Thiele method HETP Height Equivalent to a Theoretical Plate Represents the height o packing that gives similar separation to as a theoretical stage. HETP values are provided or each type o packing Mass Transer analysis HTU method Z = HTU NTU HTU = Height o a Transer unit NTU = Number o Transer Units obtained by numerical integration Z Z V K a A K y x c L a A c y x y x Aout Ain Aout Ain y * A dy y d xa x x A A * A Z H OG N OG Z H OL N OL 14

15 Evaluating height based on HTU-NTU model Z H OG K V a y A c y y Aout Ain y * A dy y Substitute values to calculate H OG Integration = N OG N OG is evaluated graphically by numerical integration using the equilibrium and operating lines. Draw 1/y A* -y A on y-axis vs. y A on x-axis. Area under the curve is the value o integration. y 1 * y A y A A Evaluate area under the curve by numerical integration x Area = N 15

16 Two-Film Theory o Mass Transer At a speciic location in the column Local gas phase Local liq phase Overall gas phase or Liquid phase Gas phase Boundary layer Liq phase Boundary layer Re.: Seader and Henley 16

17 Alternative Mass Transer Grouping Phase LOCAL coeicient OVERALL coeicient Gas Phase Z = H G N G Z = H OG N OG M. Transer Coe.: k y a Driving orce: y y i M. Transer Coe.: K y a Driving orce: y y * Liquid Phase Z = H L N L M. Transer Coe.: k x a Driving orce: x x i Z = H OL N OL M. Transer Coe.: K x a Driving orce: x x * Note: Driving orce could be y yi or yi y is decided based on direction o low. This applies to gas and liquid phases, overall and local. 17

18 Graphical evaluation o N integral Assume we are evaluating y out N OG y y A out Ain dy y * A A y A y y A out Ain y * A 1 y A dy A y A y A * y A * -y A 1/y A * -y A y in y A in y A out Use Equilibrium data related to process e.g., x-y or absorption and stripping and the operating line rom mass balance. Obtain data o the integral in the given range and ill in the table Draw y A vs. 1/y A *- y A Then ind area under the curve graphically or numerically 18

19 Distillation random case Equilibrium line operating lines 19

20 20 7 point Simpson s rule: h d h Simpson s Rule or approximating the integral h d h 5 points Simpson s rule: h d h 3 points Simpson s rule:

21 ABSORPTION/STRIPPING IN PACKED COLUMNS Y L' V ' Re.: Seader and Henley Y L' V ' n1 n 1 o 21

22 22

23 23

24 Counter-current Absorption local gas phase Y Y in Y 1 Y out in Y 2 Y 3 Y 3 i Y 4 Y out Y 5 Y in out in slope kxa k a y out 24

25 Counter-current Absorption overall gas phase Y Y in Y 1 Y out in Y 2 Y 3 vertical Y 4 Y 3 * Y out Y 5 Y in out in out 25

26 Counter-current Absorption local liquid phase Y Y in slope kxa k a y Y out in Y out Y in out i 4 in out 26

27 Counter-current Absorption overall liquid phase Y Y in Y out in horizontal Y out Y in out 3 * in out 27

28 Note: This exercise rom Seader and Henley was solved using an equation based on a certain approximation. You need to re-solve it graphically using Simspon s rule and compare the results. 28

29 29

30 30

31 Stripping Exercise Wankat 15D8 We wish to strip SO 2 rom water using air at 20C. The inlet air is pure. The outlet water contains mole raction SO 2, while the inlet water contains mole raction SO 2. Operation is at 855 mmhg and L/V = 0.9 L/V max. Assume H OL = 2.76 eet and that the Henry s law constant is 22,500 mmhg/mole rac SO 2. Calculate the packing height required. 31

32 Solution T = 20C P = 855 mmhg Air solvent V y in = 0 Water L x in = = x out = = P tot = 855 mmhg H = 22,500 mmhg SO 2 /mole rac SO 2 p SO2 = H x SO2 y SO2 P tot = H x SO2 y SO2 = H/ P tot x SO2 or y SO2 = m x SO2 where m = H/ P tot = 22,500/855 = 26.3 used to draw equilibrium data Draw over the range o interest, i.e., rom x=0 to x= at x= 0 y = 0 at x= y = 26.3 * = =

33 y SO x out x SO x in

34 Vy out y out = L x in -x out Vy out 0 = L 11x x10-4 y out = 10x10-4 L/V L/V = 0.9 L/V max From pinch point and darwing, L/V max = slope= L/V = = y out = 10x10-4 L/V = 10x y out = = Draw actual operating line 34

35 y SO x out x SO x in

36 y SO x SO

37 y SO x SO

38 38 x x* 1/x-x* 1.0E , E E-04 10, E E-04 10, E E-04 10, E E-04 10, E E-03 10,000 Apply a graphical or numerical method or evaluating N OL For example, we can use Simpson s rule. The 7 point Simpson s rule deined as ollows: * Ain A out x x A x A x dx d h d h

39 39 Substituting values rom Table gives N OL = 9.5. Z = H OL given N OL calculated = Z = t d * Ain A out x x A x A x dx 1 * x x

40 40 5-point method h d h Pay attention to accuracy o drawing and obtaining data. Grades will be subtracted in case o hand drawing!

41 Distillation in a Packed Column Read Section 15.2 Wankat 2 nd Ed. Or Section 16.1 Wankat 3 rd Ed. 41

42 1. Feed 2. Distillate 3. Bottom 4. Relux 5. Boilup 6. Rectiying section 7. Striping section 8. Condenser 9. Re-boiler 10. Tray plate or stage 11. Number o Trays 12. Feed tray 42

43 V L Rectiying V L Stripping 43

44 Equilibrium and Operating Lines Graphical Design Method Binary mixtures 44

45 Rectiying section Stripping section H G = H L = HTU V k y a A c L k x a A c H OG = V K y a A c H OL = L K x a A c Slope o tie line N G = N L = N OG = N OL = y Ai y A x Ai x A y out y in x out NTU x in y out y in x out x in dy A y Ai y A dx A x A x Ai dy A y A y A dx A x A x A = k xa = L H G k y a V H L HTU H G = V k y a A c H L = L k a A c H OG = V K y a A c H OL = L K x a A c Slope o tie line N G = N L = N OG = N OL = y Ai y A x Ai x A y out y in x out NTU x in y out y in x out x in dy A y Ai y A dx A x A x Ai dy A y A y A dx A x A x A = k xa = L H G k y a V H L 45

46 y A i x A i x A y x Ai Ai y x A A kxa k a y L V H H G L y A 46

47 Example 15-1 Wankat pages 109 and

48 Distillation Exercise 15D4 Wankat 48

49 49

50 50

51 ya yai yai-ya 1/yai-ya

52 Diameter calculation o Packed Columns Read sections 10.7 to 10.9 Wankat 2nd or 3 rd Ed. 52