Flow patterns in multiphase flows and designs

Flow patterns in multiphase flows

1. Multiphase Flow Flow Patterns in Multiphase Flows

2. Better predictions of D P and Holdup (volume fraction), if flow regime is known. Flow regime prediction is not only important for reliable design, but for pipeline operability. Phenomena like pipe corrosion and erosion depend on flow regimes. Distribution of corrosion, hydrate etc. inhibitors depend on flow regimes. Flow regime at pipe outlet affects gas-liquid separation efficiency. Importance of Flow Regime Predictions

3. Sketches of flow regimes for flow of air/water mixtures in a horizontal, 5 .1 cm diameter pipe. Adapted from Weisman (1983). Different Flow Regimes in a Horizontal Pipe for Gas-Liquid System

4. Flow regimes map for flow of air/water mixtures in a horizontal, 5 .1 cm diameter pipe. Adapted from Weisman (1983). Flow Regimes Map for a Horizontal Pipe (5.1 cm)

5. Flow Regimes Map for a Horizontal Pipe (2.5 cm) Flow regimes map for flow of air/water mixtures in a horizontal, 2.5 cm diameter pipe. Adapted from Weisman (1983).

6. Flow Regimes Map for a Horizontal Pipe of Different Diameter Changes in the flow regime boundaries for various pipe diameters: 1 .25 cm (dotted lines), 2.5 cm (solid lines), 5 cm (dash-dot lines) and 30 cm (dashed lines).

7. Bubbly Flow (Homogeneous Flow) Churn Turbulent (Heterogeneous Flow) Gas-Liquid Flow Regimes in a Vertical Pipe

8. Bubble flow Continuous liquid phase with dispersed bubbles of gas Slug flow Large gas bubbles Slugs of liquid (with small bubbles) in between Churn flow Bubbles start to coalesce Up and down motion of liquid Annular flow Gas becomes the continuous phase Droplets in the gas phase Different Flow Regimes in a Vertical Pipe for Gas-Liquid System

9. A flow regime map for the flow of an air/water mixture in a vertical, 2 .5 cm diameter pipe showing the experimentally observed transition regions hatched; Flow Regimes Map for a Vertical Pipe (2.5 cm) Generally in gas-liquid vertical flow system : Liquid volumetric flux << Gas volumetric flux

10. The vertical flow regime map for flow in a 3 .2 cm diameter tube Flow Regimes Map for a Vertical Pipe (3.2 cm) Generally in gas-liquid vertical flow system : Liquid momentum flux >> Gas momentum flux

11. Photograph of air water flow in a 10.2 cm diameter vertical pipe ~ 1% air ~ 5% air ~ 15% air Photographs of Air-Water flow in a Vertical Pipe (10.2 cm)

12. The evolution of the steam/water flow in a vertical boiler tube. Evolution of single component two phase flow

13. Pneumatic Conveying Transport of dry material through pipelines using air (gas) as the motive force when fluid is liquid, it is known as hydraulic conveying Advantages Material Can be picked up from multiple sources and delivered to many destinations No dust is emitted to environment Drawbacks High specific power consumption Potential particle breakage or degradation High wear rate on components Relatively short distances (typically less than 3000 ft) Unstable flow

14. Different Flow Regimes in a Horizontal Pipe for Liquid-Solid System Flow regimes for liquid-solid slurry flow in a horizontal pipe

15. Different Flow Regimes in a Horizontal Pipe for Gas-solid System (Pneumatic Conveying) Dilute phase Decreasing gas velocity Decreasing gas velocity For most of the material solid flow stop here

16. Flow Pattern depends on… Bulk Density Particle Size, Shape Particle Distribution Particle Density Cohesiveness Moisture Content Hardness Temperature sensitivity

17. Flow Pattern for Fine Powders High solid loading

18. Flow Pattern for Coarse Granular Particles

19. Classification of Solids Geldart ‘s boundaries Dixon’s boundaries Boundaries are ambiguous particularly for wide particle size distribution or for non spherical particle Group A: Fine powders Group B: Coarse granules Group C: Cohesive Group D: Big particles, highly permeable

20. Classification of Solids and Conveying Characteristics

21. Pneumatic Conveying Dilute Phase Characterized by high gas velocities (greater than 20 m/s), low solids concentration (less than 1% by volume), and low pressure drop per unit length of line Used for short route continuous transport of solids at a rate of less than 10 tons/hour Solid particles behave as individuals, fully suspended in the gas Can be pressure, vacuum, or pressure/vacuum systems Least economical method – high air volume required High velocities create pipe erosion Not suitable for fragile, abrasive, or large particle materials

22. Dilute Phase Pneumatic Conveying Keep air velocity above minimum entrainment velocity

23. Dense Type Pneumatic Conveying Keep air velocity above minimum entrainment velocity

24. Particles are not suspended in the gas More economical – low air volume required, smaller diameter pipe, and less maintenance Lower velocity so less pipe erosion Can handle high throughputs over long distances Drawback is that it requires a transporter for each input Batch versus continuous Solid: low velocity, pipe full of material. Good for fragile materials. Discontinuous: low velocity, material moves in plugs. Best for most applications. Continuous (Moving Bed): higher velocity, but much lower than dilute. Used for powders that can be fluidized. Dense Type Pneumatic Conveying

25. Dilute Phase Dense Phase Dilute vs Dense

26. Dilute Phase Dense Phase Dilute vs Dense

27. Classification of Conveying Pressure Type VacuumType

28. Phase Diagram

29. Performance Characteristic of Pneumatic Conveying for Coarse Materials ( Zenz Plot) Ws = solid flow rate This is a log-log plot Dilute Phase Dense Phase

30. Performance Characteristic of Pneumatic Conveying for Fine Materials ( Zenz Plot) Ws = solid flow rate This is a log-log plot Dilute Phase Dense Phase

31. Conveying rate v s Gas velocity

32. Important relationships Mass flow rate of particles Mass flow of fluid Solids loading = M p /M f

33. Pressure Drop in Pneumatic Transport Contributors to pressure drop Gas acceleration (gas acting on gas) Particle acceleration (gas acting on particles) Gas/pipe friction wall friction Solids/pipe friction wall friction Static head of solids fighting gravity Static head of gas fighting gravity Not considered: interparticle forces

34. Force Balance on Pipe Net force acting on pipe contents rate of increase in momentum of contents = F fw and F pw are gas to wall and solids to wall friction force respectively, L = pipe length,  = angle of pipe with horizontal Pressure Fluid to wall friction Solids to wall friction Gravity term for solids Gravity term for fluid Fluid momentum term Solid momentum term

35. Generally problematic. Solids that may be in suspension in vert / horiz transport may salt out as they go around bends. Worst case: vertical going to horizontal blinded tees recommended if bends are unavoidable No reliable correlations exist for bend pressure drops. Only a rough rule of thumb: Bend D P = D P for 7.5 m of vertical pipe under same flow conditions Bends