Vapor-Liquid Separator Design

• Presented to CBE 497

• 15 Jan., 2002.

• By R. A. Hawrelak

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.

There are 3 Basic Design Zones in any Knock-out Pot

• The vapor-liquid inlet line.

• The vapor zone.

• The liquid zone.

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.

Baker Chart – Horizontal Flow

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.

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.

Design Basis – Vessel Economics

• Combine the three design zones with Pressure Vessel Economics to obtain the most cost effective KO Pot.

Types of KO Vessels

• Vertical – No Internals

Vertical KO – With Demister Mesh

Peerless KO Pots With Horizontal Flow Chevrons

FWG – Vertical Flow Chevron Vanes

Cyclone KO Pot With Tangential Entry

Porta-Test Centrifugal Separator

Horizontal KO Pots

• API-521 Horizontal KO Pot With No Internals

API-521 Horizontal KO Pot With Mesh Pad

Wu – Horizontal With Extended Inlet

Kettle Refrigeration Exchanger

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.

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.

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.

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.

Summary Of All Req’d Input

Step (1): Calc CFS Of Vapor

• CFS = Vapor cubic feet per second.

• CFS Vapor = Wv / 3600 / Dv.

• CFS Vapor = 16.29 cubic ft. per sec.

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*32.2/3/0.00067197^2)• CRe^2 = 1,411.49

Where C = Drag Coefficient

Re = Particle Reynolds Number

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(6.496-1.1478*LN(CRe^2)

+0.058065*LN(CRe^2)^2 -0.00097081*LN(CRe^2)^3)

• C = 2.35 for the example presented

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)

Step (5): Calc Drop Out Velocity

• Drop Out velocity, ut, from Perry VI - Eq 5-264.

• Ut = [Re / C*4*32.2 *cP* 0.00067197 *(DL-Dv) / 3 / Dv^2]^0.333333.

• Ut = 0.4659 ft./sec.

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.”

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. + 0.5(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.

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 = 1,629.02 cu. Ft.

Step (9): Calc Liq Vol for minimum of 2 ft. Liquid.• Liq Vol For 2 Ft. Minimum Liq Vol =

Vol L2 ft. = ()(2)(Dia)^2 / 4.

• Vol L2 ft. = 76.97 cu. Ft.

Step (10): Select Maximum of L3 Vol or L2 ft. Vol.• Vol L3 = 1,629.02 cu. Ft.

• Vol L2 = 76.97 ft. cu. Ft. = cu. Ft.

• Max Liquid Vol = 1,629.02 cu. Ft.

Step (11): calculate L3, ft.

• L3 = (Vol L3)(4) / ()(Vessel Dia)^2.

• L3 = 42.33 ft.

• This makes the vessel roughly 7 ft. in diam with an unusually high liquid level (L3).

Step (12): Document Liquid Retention Time

• Stated Liquid Retention Time Required from Max to Min Liquid Level = 8 minutes.

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.

Step (14): Calculate t-t Length• L total t-t = L1 + L2 + L3.• L total t-t = 3.83 + 11.42 + 42.33.• L total t-t = 57.58.• L/D = 57.58 / 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.

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.

Summary

1st pass 2nd pass final passDp, microns 100 50 54Ret Minutes 8 8 8.3

Step 6 Ves Dia ft. 6.67 10.51 9.95Step 7 L1, ft. 3.83 3.83 3.83Step 11 L3, ft. 42.33 17.14 21.52Step 13 L2, ft. 11.42 5.12 6.21Step 14 t-t length, ft. 57.58 26.09 31.57

L/D 8.63 2.48 3.17

Vertical KO Pot with Demister

Pad

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.

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.

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.

Future Demister Pad

• Make L1 a minimum of 3 ft. + 0.5(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.