Mass Transfer & Separation Processes

Transcription

1 Title Mass Transfer & Separation Processes Mass Transfer for 4 th Year Chemical Engineering Department Faculty of Engineering Cairo University Prepared by Dr. Ahmed Fayez Nassar 1 Contents of the Lecture Equipment for Gas (Vap) Liquid Contact 2. Packed Columns Packed Column Parts Packing Types / Materials / Properties Hydrodynamics of Packed Columns & Operation Checks Comparison between Packed and Tray Columns 3. Spray Columns 2

2 Columns Columns Tray Packed Spray 3 Packed Column Parts 4

3 3- Packed Column Accessories Demister (mist eliminator) Liquid Distributer (Liquid collector) Bed Limiter Packing Support 5 Demister It is a device fitted to vapor liquid separator vessels to enhance the removal of liquid droplets entrained in a vapor stream. Demisters may be a mesh type coalescer, vane pack or other structure intended to aggregate the mist into droplets that are heavy enough to separate from the vapor stream. 6

4 Demister 7 A distributor helps to optimize the performance of towers with packed beds. It distributes the liquid in an even pattern over the bed. Redistributors (Liquid collectors ) collect the liquid leaving an upper bed and redistributes it evenly over the next bed below (for H > 5 m). If the liquid distribution is bad this will result in dry locations which leads to drop in efficiency Distributer Types: o Tubular (pipes) o Orifice plate o Trough o Spray (problems with clogging) Distributor / Redistributors 8

5 Bed Limiter A bed-limiter is positioned inside the tower, directly above a randomly packed bed to confine any upward movement of packing. Maintaining a level top surface on the bed is important, especially if the vapor load is sufficient to fluidize the top of the bed under certain conditions. 9 Packing Support Packing supports should have the following properties o Strong to withstand the weight of the packing and liquid) o High free area (more than the packing) so that it don t form a bottle-neck. Support Types: o Grid-Support o Perforated plate o Bubble-Cap o Corrugated- Perforated plate o Mesh-Support 10

6 Design of Packed-Column Procedure 1. Select the type and size of packing. 2. Determine the column height required for the specified separation (H = HTU NTU or H = HETP NTP). 3. Determine the column diameter (capacity), to handle the liquid and vapor flow rates. 4. Select and design the column internal features: packing support, liquid distributor, redistributors.. 11 Type of Packing The principal requirements of a packing are: High surface area per unit volume (a) low HTU (G/K oy as) Large void fraction (high capacity + low pressure drop) Strong (don t break during loading or operation) Small density (low dead weight) Corrosion resistance Easily wetted by liquid Low cost 12

7 Type of Packing Packing Types Natural Fabricated Stones / Gravels Coal / Coke Unstructured (Random) Structured Rings Saddles Grids Wire Mesh 13 Random Packing Raschig Rings Pall Rings Hy-Pack Rings Partition Rings Lessing Rings Intalox Saddle Berl Saddle 14

8 Structured Packing The advantage of structured packing over random packing is their low HTU (typically less than 0.5 m) and low pressure drop (around 100 Pa/m) The cost of structured packing/m 3 will be significantly higher than that of random packing, but this is offset by their higher efficiency. The applications have mainly been in distillation, but structured packing can also be used in absorption 15 Structured Packing 16

9 Packing Material of Construction Packing Material Metallic Ceramic (unglazed) Wood Plastic Steel Aluminum Silica Alumina Adv: strong, small thickness with high void fraction Disadv: corrosion, high cost, bad wetting Adv: good wetting, no corrosion Disadv: fragile, large thickness with low void fraction Adv: good wetting, no corrosion Disadv: low void fraction, possibility of rottenness Adv: light, no corrosion Disadv: bad wetting 17 Installing Packing Methods of loading a packed column Dumping the packing in the tower after filling it with water. This is suitable for all columns and non-fragile packing materials. For large towers, a crane and/or workers can manually arrange the packing materials. 18

10 Size of Packing In general, the largest size of packing that is suitable for the size of column should be used, up to 50 mm. Small sizes are more expensive than the larger sizes. Use of too large packing size in a small column can cause poor liquid distribution. Column Diameter < 0.3 m (1 ft) m (1 3 ft) > 0.9 m (3 ft) Packing Size < 25 mm (1 in) mm (1 1.5 in) mm (2 3 in) 19 20

11 Height of Packing For the design of packed distillation columns, it is simpler to treat the separation as a staged process, and use the concept of the height of an equivalent theoretical plate (HETP) to convert the number of theoretical plates required to a height of packing. (H = HETP NTP) In distillation, for good liquid distribution and enough pressure drop (17 mm H 2 O/m for Pall rings 29 mm/m for saddle 42 mm/m for Raschig rings) Packing Size (mm) HETP (m) In absorption use H = HTU NTU 21 Column Diameter Normally, the column will be designed to operate at the highest economical pressure drop, to ensure good liquid and gas distribution. For random packing, the pressure drop will not normally exceed 80 mm H 2 O/m of packing height. At this value the gas velocity will be about 80% of the flooding velocity. For Absorbers and strippers work at 15 to 50 mm H 2 O/m of packing height For distillation, atmospheric and moderate pressure, work at 40 to 80 mm H 2 O/m of packing height Where the liquid is likely to foam, these values should be halved. For vacuum distillations the maximum allowable pressure drop will be determined by the process requirements, but for satisfactory liquid distribution the pressure drop should not be less than 8 mm H 2 O/m. The column cross-sectional area (diameter) for the selected pressure drop can be determined from the generalized pressure-drop correlation given in the Figure next slide. 22

12 0.1 * 2 l 13.1V w Fp l K4 v l v * Vw :gas mass flow rate per unit column cross 2 sectional area (kg/m s) -1 FP : packing factor (m ) : liquid viscosity (Pa s) l Column Diameter From graph get K 4 From equation get V w*, then calculate the diameter 23 For gas flow only Hydrodynamics of Packing (flooding) 2 H va Pdry f g d p 2 Pdry : dry pressure drop d p : equivalent diameter of packing va :actual velocity through packing f : friction factor vs Q va S vs : superficial velocity : void fraction f Re d v p a g, Re g 24

13 For gas liquid flow Hydrodynamics of Packing (flooding) P P wet wet c Pdry f ( L, type of packing, d p ) b Loading point is ill-defined, while flooding point is well-defined. a 25 With a dry packing, pressure drop increases as gas velocity increases according to the linear relationship. With liquid flowing in the column, the packing becomes wetted. Part of void volume in the packing is filled with liquid, thereby reducing the crosssectional area available for gas flow. At the same gas velocity, the pressure drop is higher for wetted packing compared to dry packing. But it increase with almost the same slope. As the gas velocity increases further, and reaches point a, the quantity of liquid retained in the packed bed increases significantly. There is a change in slope of the line at this point, which is known as the loading point, as liquid starts to accumulate (load) in the packing. Increasing the gas rate above this point results in sharp increase in pressure drop due to increased entrainment. This is accompanied by higher surface area and better mass transfer. At point b, there is another sharp change in the slope. At this point the liquid starts flowing out of the top of the column. It is known as the flooding point. Hydrodynamics of Packing (flooding) 26

14 Check for Wetting of Packing If very low liquid rates have to be used, below F LV = 0.01, the packing wetting rate should be checked to make sure it is above the minimum recommended by the packing manufacturer. volumetric liquid rate per unit area Wetting Rate packing surface area per unit volume Lw S a l Packing Type & Size Rings 3.0 in & Saddles (all sizes) Rings > 3 in Structured Packing Minimum Wetting Rate m 3 /sm m 3 /sm m 3 /sm 2 (a = m 2 /m 3 ) 27 To prevent channelling, use D d p 20 Check for Channelling Channeling often occurs in a packed tower. This phenomenon takes place when the fluid moving down the column moves towards the region of greatest void space; this occurs at the region near the wall where the packing is not tightly packed. Thus, liquid redistributors are used (every 3 5 m) to redirect the fluid flow towards the column center. 28

15 Tray Column Vs. Packed Column Choice of plates or packing To do this correctly, you need to compare the price of both columns for the same operation. But following points help in the choice: 1. Plate columns can handle wider range of liquid and gas flow-rates than packed columns. 2. Packed columns are not suitable for very low liquid rates. 3. Plate columns can be designed with more assurance than packed columns regarding good liquid distribution, particularly in large columns. 4. It is easier to make cooling in a plate column; coils can be put on the plates. 5. Withdrawal of side-streams from is easier from plate columns. 6. Tt is easier to clean plate columns; manways can be installed on the plates. With small diameter, it may be cheaper to use packing and replace the packing when it is fouled. 7. For corrosive liquids a packed column will usually be cheaper than the equivalent plate column. 8. The liquid hold-up is much lower in a packed column than a plate column. 9. Packed columns are more suitable for handling foaming systems. 10. The pressure drop per equilibrium stage (HETP) can be lower for packing than plates; and packing should be considered for vacuum columns. 11. Packing should always be considered for small diameter columns (< 0.6 m), where plates would be difficult to install, and expensive. 29 Tray Column Vs. Packed Column Cost Pressure Drop Cleaning Liquid holdup Corrosion Cooling Flexibility Packed Column Lower cost for small diameters (< 60 cm) Smaller Harder Low Better Harder Lower Tray Column Equal cost for large diameters Higher Easier (suitable for dirty fluids) High Worse Easier higher 30

16 Columns Columns Tray Packed Spray 31 Characteristics of Spray Columns 32

17 Characteristics of Spray Columns Advantages Disadvantages Uses Low cost Low efficiency (high HTU) For easy mass transfer operations Very low pressure drop Not suitable for small liquid loads Air humidification Suitable for corrosive materials Not suitable for liquids contaminated with solids Washing of flue gases 33