VaporLiquid Separator Design Presented to CBE 497 15

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 vaporliquid 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 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 – C 2 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) / c. P^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 *c. P* 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, L 1 • L 1 Vapor ht. Referenced to C. L. Of inlet nozzle. • L 1 Vapor ht. = 3 ft. + 0. 5(Noz Diam. ). • L 1 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, L 3, For Specified Retention Time • Cubic Ft. Of Liquid = Vol L 3. • Vol L 3 = (WL)(Ø min. ) / DL / 60 cu. ft. • Vol L 3 = 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 L 2 ft. = (p)(2)(Dia)^2 / 4. • Vol L 2 ft. = 76. 97 cu. Ft.

Step (10): Select Maximum of L 3 Vol or L 2 ft. Vol. • Vol L 3 = 1, 629. 02 cu. Ft. • Vol L 2 = 76. 97 ft. cu. Ft. = cu. Ft. • Max Liquid Vol = 1, 629. 02 cu. Ft.

Step (11): calculate L 3, ft. • L 3 = (Vol L 3)(4) / (p)(Vessel Dia)^2. • L 3 = 42. 33 ft. • This makes the vessel roughly 7 ft. in diam with an unusually high liquid level (L 3).

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

Step (13): Calculate L 2 • L 2 is the height from the C. L. of the inlet nozzle to the max Liquid level. • L 2 = 0. 25(L 3) + 0. 5(Inlet Nozzle dia. ). • L 2 = (0. 25)(42. 33) + (0. 5)(20/12) =11. 42 ft.

Step (14): Calculate t-t Length • • • L total t-t = L 1 + L 2 + L 3. 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 L 1 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.