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. = (p)(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) / (p)(Vessel Dia)^2. 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

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.