1. 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics: Simple construction; low pressure drop; packings easily made of corrosion 腐蚀 resistant materials.Basic characteristics: Simple construction; low pressure drop; packings easily made of corrosion 腐蚀 resistant materials. Applications: (1)Distillation with low pressure drop and vacuum distillation; (2)Absorption and distillation with falling liquid flow rate far greater than upward vapor flow rate; (3)Corrosive materials; (4)Small diameter column; (5)Easily foaming materials; (6)Heat sensitive materials.Applications: (1)Distillation with low pressure drop and vacuum distillation; (2)Absorption and distillation with falling liquid flow rate far greater than upward vapor flow rate; (3)Corrosive materials; (4)Small diameter column; (5)Easily foaming materials; (6)Heat sensitive materials.

2. 2 Illustrational drawing of packed column

3. 3

4. 4 3-2-1 Packings 1. Characteristics of packings Function of packings: Supplying space of mass transfer.Function of packings: Supplying space of mass transfer. (1)Surface area per unit volume of packings 比表面 积 σ, m 2 /( m 3 packing layer) σ = n σ 0 σ = n σ 0 σ 0 =Surface area of a packing, m 2 /a packing σ 0 =Surface area of a packing, m 2 /a packing n=Number of packings, n/m 3 n=Number of packings, n/m 3

5. 5 (2)Packing void fraction 空隙率 ε: m 3 /(m 3 packing layer) V0 = Volume of a packing, m 3 /a packing (3)Packing factor 填料因子 : σ/ ε 3 = Dry packing factor, 1/m Φ=(σ/ ε 3 ) wet = Wet packing factor Φ=(σ/ ε 3 ) wet = Wet packing factor (4)Stacking density ρ p, kg/m 3 (Packing weight per unit volume of packings) (Packing weight per unit volume of packings) [Table 3-5][p.181] [Table 3-5][p.181]

6. 6 Basic requirement for packing: Large surface area per unit volume of packing σ; Large packing void fraction ε; Low Stacking density ρ p ; Good wetability; Low fabricating cost; Enough mechanical strength.Basic requirement for packing: Large surface area per unit volume of packing σ; Large packing void fraction ε; Low Stacking density ρ p ; Good wetability; Low fabricating cost; Enough mechanical strength. 2. Types of packing Classification according to the structureClassification according to the structure (1)Solid packing: Rings/Saddles/Sheets (2)Gauze packing: Net-like saddles / θ nets /Corrugated nets

7. 7 Classification according to the manner of installation:Classification according to the manner of installation: (1)Random packings ; (2)Structured packings. (2)Structured packings. Fluid mechanics and mass transfer performance comparison among several kinds of common used tower packings? (Rachig ring, Pall ring, Cascade ring [pp.178-181]Fluid mechanics and mass transfer performance comparison among several kinds of common used tower packings? (Rachig ring, Pall ring, Cascade ring [pp.178-181]

8. 8 鲍尔环 阶梯环

9. 9 金属环矩鞍填料（ Intalox Metal Tower Packing 球形填料

10. 10 波纹型规整填料 格栅型填料

11. 11 Wall flow phenomenon 壁流现象 and its decreasing.Wall flow phenomenon 壁流现象 and its decreasing. Liquid redistributor of packed column

12. 12 Main trends of new-type packing development:Main trends of new-type packing development: (1)Increasing flux; (2)Improving the uniform distributions of fluid flow; (3)Decreasing flow resistance.

13. 13 Two aspects of development:Two aspects of development: 1.Random packings: Rings and saddles. Advantage of ring-type packings: Large flux; Disadvantage of ring-type packings: Bad liquid redistribution.Advantage of ring-type packings: Large flux; Disadvantage of ring-type packings: Bad liquid redistribution. Advantage of saddle-type packings: Good liquid redistribution; Disadvantage of saddle-type packings: Small flux.Advantage of saddle-type packings: Good liquid redistribution; Disadvantage of saddle-type packings: Small flux. Combining the advantages of ring-type and saddle- type packings: Intalox saddle; Conjugate ring.Combining the advantages of ring-type and saddle- type packings: Intalox saddle; Conjugate ring.

14. 14 2.Structured packings: Performance is much better than random packings with very low pressure drop, good liquid redistribution, large flux. But the fabricating cost is expensive.Performance is much better than random packings with very low pressure drop, good liquid redistribution, large flux. But the fabricating cost is expensive.

15. 15 3-2-2 Fluid Mechanics of packed columns 填料塔流体力学性能 Including:Including: (1)Pressure drop of gas flowing through packed sections 气体压降 ; (2)Liquid flooding velocity 液泛气速 ; (3)Liquid and gas distributions 气两相流体分布 ; (4)Liquid holdup 持液量.

16. 16 1.Pressure drop of gas flowing through packing depth (Relationship between ΔP/Z~u) Superficial gas velocity based on empty tower 空塔气速 u: Ⅰ Ⅱ Gas velocity based on empty tower u Ⅲ

17. 17 (1)L=0 ： Straight line (Dry packing) (2)L  0: Curve Two turning points: A(loading point 载点 ) and B(flooding point 泛 点 ).Two turning points: A(loading point 载点 ) and B(flooding point 泛 点 ). Three zones: Ⅰ.Constant liquid holdup zone;Three zones: Ⅰ.Constant liquid holdup zone; Ⅱ.Liquid loading zone; Ⅲ.Flooding zone. Logarithmic coordinates Ⅰ Ⅱ Ⅲ Gas velocity based on empty tower u

18. 18 Question: Why does the curve move left when L  ？ Under the same u, the larger the L is, the more left the curve is, that is, L  （  P/Z ） Under the same u, the larger the L is, the more left the curve is, that is, L  （  P/Z ）  Under the same V and L  0 ，Under the same V and L  0 ， Actual velocity u actual > Gas velocity of empty tower u Logarithmic coordinates Ⅰ Ⅱ Ⅲ Gas velocity based on empty tower u

19. 19 u actual > u, therefore, under the same u, (  P/Z) with L  0 is greater than (  P/Z) with L=0. Logarithmic coordinates Ⅰ Ⅱ Ⅲ Gas velocity based on empty tower u

20. 20 Before point A, when u , the liquid holdup of the column does not change, this is constant liquid holdup zone Ⅰ.Before point A, when u , the liquid holdup of the column does not change, this is constant liquid holdup zone Ⅰ. After point A, when After point A, when u , the liquid holdup of the column increased, this is liquid loading zone Ⅱ. At point B and afterwards, flooding happens, this is flooding zone Ⅲ.At point B and afterwards, flooding happens, this is flooding zone Ⅲ. Logarithmic coordinates Ⅰ Ⅱ Ⅲ Gas velocity based on empty tower u

21. 21 Logarithmic coordinates Ⅰ Ⅱ Ⅲ Gas velocity based on empty tower u Phenomena of flooding: The packed sections fluctuate violently. Pressure drop of packed sections rises more rapidly (  P   ). Liquid rapidly accumulates, and the entire column may fill with liquid. Flooding is detrimental operation.Phenomena of flooding: The packed sections fluctuate violently. Pressure drop of packed sections rises more rapidly (  P   ). Liquid rapidly accumulates, and the entire column may fill with liquid. Flooding is detrimental operation.

22. 22 2. Flooding Normally, liquid is the scattered phase and gas the continuous phase. When flooding, liquid becomes the continuous phase and gas the scattered phase. At flooding point B, the corresponding gas velocity is u F 泛点气速At flooding point B, the corresponding gas velocity is u F 泛点气速

23. 23 (1)Factors influencing (velocity at flooding) 泛点气速 u F: u F: 1) Characteristics of packings:  填料因子   u F  (Flooding will not happen easily). For example, if void fraction  is large enough, the permissible gas flux will be large enough too, and u F .  is a comprehensive factorcombining the influences of ,  and packing structures on column performance.For example, if void fraction  is large enough, the permissible gas flux will be large enough too, and u F .  is a comprehensive factorcombining the influences of ,  and packing structures on column performance. 2)Fluid physical properties:  V  and    u F  ；  L   u F . 3)Ratio of liquid and gas flow rates 液气比 : (w L /w V )   u F 

24. 24 (2)Determining the velocity at flooding u F (Eckert generalized correlation: Figure 3-24, p.184)

25. 25 (3)Determining （  P/Z ）： After determining u F, take u =(0.5~0.85)u F ， calculate A and B ， then determine  P/Z from Eckert generalized correlation.After determining u F, take u =(0.5~0.85)u F ， calculate A and B ， then determine  P/Z from Eckert generalized correlation.

26. 26 3. Wettability Wetted surfaces of packings are effective mass transfer area.Wetted surfaces of packings are effective mass transfer area. Definition of spray density U [p.185]:Definition of spray density U [p.185]: Liquid volume sprayed per unit time per unit cross section of the column, [m 3 /m 2.s] Minimum spray density U min : the lowest value of U that ensures the packings wetted.Minimum spray density U min : the lowest value of U that ensures the packings wetted.

27. 27 (L W ) min ——Minimum wetting rate, [m 3 /m.s]  —— 填料的比表面积 wetting rate: Liquid volume flow rate per unit length of packing periphery. (3-37)

28. 28 3-2-3 Calculations of Packed Columns 1.Column diameter: V S =gas volume flow rate, m 3 /s Attention: (1) D T should be rounded up; (2)Checking:Attention: (1) D T should be rounded up; (2)Checking: (Avoiding wall flow)

29. 29 2.Column height (1)Calculation from the number of transfer units: Z=H Oy *N Oy ; etc. (2)Calculation from HETP: Z=HETP*N T, etc.

30. 30 3-2-4 Accessory Structures of Packed columns 1.Packing supports (a)Grid plate

31. 31 Function of packing support:Function of packing support: Supporting the packings and the liquid holdup, ensure fluent flow of gas and liquid. Basic requirement:Basic requirement: Enough mechanical strength, and free cross sectional area. (b)Riser 升气管式

32. 32 2.Liquid distributors Function: Uniformly distribute the liquid on the surfaces of packings.Function: Uniformly distribute the liquid on the surfaces of packings. Shower nozzle type Annular tubes with multi-holes Overflow pipes

33. 33 3.Liquid redistributor Function: Reducing the non-uniform distribution of liquid, and reducing the wall flow. 4.Mist catcher (Entrainment eliminator) Function: Eliminating the entrained liquid drops in the gas stream at the outlet. 5.Gas inlet Function: Ensuring the uniform distribution of gas flow on the cross sectional area of the column. [Comparison between packed and plate towers?]