1. Vapor-Liquid Separator Design
Presented to CBE 497
15 Jan., 2002.
By R. A. Hawrelak

2. The Equation of State
Composition, temperature and pressure define the Equation Of State (EOS) for process streams in a chemical plant.
The EOS often shows a particular stream to be a two-phase mixture of vapor and liquid.
Chemical processes often require separation of the vapor stream from the liquid stream.
The separation usually takes place in a vapor-liquid separator called a knock-out pot.

3. There are 3 Basic Design Zones in any Knock-out Pot
The vapor-liquid inlet line.
The vapor zone.
The liquid zone.

4. Design Basis – Inlet Line
Inlet line: Baker Two Phase Flow in Perry VI, CEHB, Page 5-41.
Avoid high, two phase velocity which may atomize liquid into particles too small for fluid dynamic separation.
Avoid “Slug Flow” regime where vibrations may be damaging to inlet pipe.

5. Baker Chart – Horizontal Flow

6. Design Basis – Vapor Zone
The Vapor Zone: Perry VI, CEHB, Eq 5-263, page 5-66.
Establish a design basis for liquid entrainment in the vapor stream.
Select a design liquid particle diameter for liquid entrainment in the vapor stream.
Select a vessel diameter to establish a terminal velocity that will entrain particles smaller than the design particle diameter.

7. Design Basis – Liquid Zone
The Liquid Zone: Based on Liquid retention time.
Establish liquid residence times for normal liquid level variation.
Establish liquid residence times for alarming and shut-downs beyond normal liquid level variation.

8. Design Basis – Vessel Economics
Combine the three design zones with Pressure Vessel Economics to obtain the most cost effective KO Pot.

9. Types of KO Vessels
Vertical – No Internals

10. Vertical KO – With Demister Mesh

11. Peerless KO Pots With Horizontal Flow Chevrons

12. FWG – Vertical Flow Chevron Vanes

13. Cyclone KO Pot With Tangential Entry

14. Porta-Test Centrifugal Separator

15. Horizontal KO Pots
API-521 Horizontal KO Pot With No Internals

16. API-521 Horizontal KO Pot With Mesh Pad

17. Wu – Horizontal With Extended Inlet

18. Kettle Refrigeration Exchanger

19. This Presentation Considers
Vertical KO Vessel With No Internals
Vertical KO With Mesh Pad
As CBE 497 does not get to Phase III Engineering where line sizing is a factor, Inlet Line design is not part of this presentation.

20. Problem Statement
Design a KO Pot to separate 49,423 lb/hr of vapor from 382,290 lb/hr of liquid.
Working Range liquid level holdup shall be +/- 2 minutes on normal liquid level.
Provide 2 minutes liquid holdup from high opg LL to Max LL.
Provide 2 minutes liquid holdup from low opg LL to Min. LL.
Total Liquid Retention time = 8 minutes.

21. First Design Consideration
As the liquid rate is high (382,290 lb/hr), liquid volume will probably be the controlling design factor.
Consider using a Standard Vertical KO Pot with No Internals.

23. Problem Statement Cont’d
Vapor Destination – centrifugal compressor.
Liquid Destination – C2 Splitter reflux.
Compressor Spec – To prevent damage to the compressor, the liquid droplet size in the inlet vapor stream shall not exceed a particle diameter, Dp, of 150 to 300 microns.
Design Spec – To achieve a goal of 150 microns, design the KO Pot for a particle diameter, Dp = 100 microns.
Rate a 10 ft. dia. x 31 ft. t-t KO Pot.

24. Summary Of All Req’d Input

25. Step (1): Calc CFS Of Vapor
CFS = Vapor cubic feet per second.
CFS Vapor = Wv / 3600 / Dv.
CFS Vapor = cubic ft. per sec.

26. Step (2): Calc ( C )( Re^2 ) CRe^2 from Perry VI - Eq 5-263
CRe^2 = (A)( Constant)
A = (Dp/304800)^3 (DL - Dv)(Dv) / cP^2
Constant = (4*32.2/3/ ^2)
CRe^2 = 1,411.49
Where C = Drag Coefficient
Re = Particle Reynolds Number

27. Step (3): Calc Drag Coefficient, C
Table 5-22, Perry VI, Page 5-67, gives C values versus CRe^2. These values have been curve fitted to a polynomial for the Re range 0.1 to 2,000 as follows:
C = EXP( *LN(CRe^2)
*LN(CRe^2)^ *LN(CRe^2)^3)
C = 2.35 for the example presented

28. Step (4): Calc Particle Reynolds Number, Re
Re = (CRe^2 / C)^0.5
Re = 24.5
Re falls within range 0.1 < Re < 2,000
OK to proceed to Step (5)

29. Step (5): Calc Drop Out Velocity
Drop Out velocity, ut, from Perry VI Eq
Ut = [Re / C*4*32.2 *cP* *(DL-Dv) / 3 / Dv^2]^
Ut = ft./sec.

30. Step (6): Calc Vessel Diameter
Area = (CFS / ut) = (3.14 / 4 )(D)^2.
KO Dia = (CFS / ut /0.785)^0.5.
KO Dia = 6.67 ft.
Round Diameter to Nearest 3.”
Rounded Diameter = 7’ 0.”

31. Step (7): Calc Ht. Above C.L. Of Inlet Nozzle, L1
L1 Vapor ht. Referenced to C.L. Of inlet nozzle.
L1 Vapor ht. = 3 ft (Noz Diam.).
L1 Vapor ht. = 3.83 ft. (C.L. to top t-L).
See Design Uncertainty at end of this report for future addition of a demister pad, if required.

32. Step (8): Calc Liquid Vol, L3, For Specified Retention Time
Cubic Ft. Of Liquid = Vol L3.
Vol L3 = (WL)(Ø min.) / DL / 60 cu. ft.
Vol L3 = 1, cu. Ft.

33. Step (9): Calc Liq Vol for minimum of 2 ft. Liquid.
Liq Vol For 2 Ft. Minimum Liq Vol = Vol L2 ft. = (p)(2)(Dia)^2 / 4.
Vol L2 ft. = cu. Ft.

34. Step (10): Select Maximum of L3 Vol or L2 ft. Vol.
Vol L3 = 1, cu. Ft.
Vol L2 = ft. cu. Ft. = cu. Ft.
Max Liquid Vol = 1, cu. Ft.

35. Step (11): calculate L3, ft. L3 = (Vol L3)(4) / (p)(Vessel Dia)^2.
This makes the vessel roughly 7 ft. in diam with an unusually high liquid level (L3).

36. Step (12): Document Liquid Retention Time
Stated Liquid Retention Time Required from Max to Min Liquid Level = 8 minutes.

37. Step (13): Calculate L2
L2 is the height from the C.L. of the inlet nozzle to the max Liquid level.
L2 = 0.25(L3) + 0.5(Inlet Nozzle dia.).
L2 = (0.25)(42.33) + (0.5)(20/12) =11.42 ft.

38. Step (14): Calculate t-t Length
L total t-t = L1 + L2 + L3.
L total t-t =
L total t-t =
L/D = / 6.67 = 8.63.
Economic L/D range between 3 to 4.
Repeat Process with lower Dp to increase dia and lower t-t length.
Second Pass. Try Dp = 50 microns.

39. Other Design Steps Step (15): Check L/D ratio (Goal 4-6)
Step 16: Old Schieman Sizing Method.
Step (17):Calculate Liquid Entrainment (HTRI).
Step (18): Determine Flow Regime for Inlet Pipe using Baker Chart for Horizontal Flow.

40. Summary

41. Vertical KO Pot with Demister Pad

42. Design Basis Design is vapor liquid systems with lower liquid rates.
The particl size is usually set at a default value of 500 microns, which is rain drop sized particles.
The wire mesh demister pad is usually 6 to 12 inches thick.
The vapor stream will exit with liquid drops no greater than 3 microns.

43. Design Procedure
The design procedure is exactly the same as for KO Pots without internals.
Set the particle size at 500 microns and proceed as before till an economic vessel with and L/D range of 3 to 4 is found.

44. Design Uncertainty
If the design is based on a vertical vessel with no internals and there is some uncertainty that the KO Pot will achieve the desired liquid particle size, provision can be made to add a wire mesh demister pad at a later date.

45. Future Demister Pad
Make L1 a minimum of 3 ft (inlet nozzle dia.) for vessel diameters 4 ft. and smaller.
For vessels larger than 4 ft. in dia., make L 1 = 0.75(Vessel dia.).
This will allow room to add a demister at a later date, if needed.