Vapor-Liquid Separator Design

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.

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

• The vapor-liquid inlet line.
• The vapor zone.
• The liquid zone.

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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.

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Baker Chart Horizontal Flow
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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.

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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.

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Design Basis Vessel Economics

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

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Types of KO Vessels

• Vertical No Internals

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Vertical KO With Demister Mesh
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Peerless KO Pots With Horizontal Flow Chevrons
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FWG Vertical Flow Chevron Vanes
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Cyclone KO Pot With Tangential Entry
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Porta-Test Centrifugal Separator
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Horizontal KO Pots

• API-521 Horizontal KO Pot With No Internals

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API-521 Horizontal KO Pot With Mesh Pad
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Wu Horizontal With Extended Inlet
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Kettle Refrigeration Exchanger
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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.

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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.

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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.

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Problem Statement Contd

• 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.

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Summary Of All Reqd Input
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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.

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Step (2) Calc ( C )( Re2 )

• CRe2 from Perry VI - Eq 5-263
• CRe2 (A)( Constant)
• A (Dp/304800)3 (DL - Dv)(Dv) / cP2
• Constant (432.2/3/0.000671972)
• CRe2 1,411.49
• Where C Drag Coefficient
• Re Particle Reynolds Number

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Step (3) Calc Drag Coefficient, C

• Table 5-22, Perry VI, Page 5-67, gives C values
  versus CRe2. These values have been curve fitted
  to a polynomial for the Re range 0.1 to 2,000 as
  follows
• C EXP(6.496-1.1478LN(CRe2)
• 0.058065LN(CRe2)2 -0.00097081LN(CRe2)3
  )
• C 2.35 for the example presented

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Step (4) Calc Particle Reynolds Number, Re

• Re (CRe2 / C)0.5
• Re 24.5
• Re falls within range 0.1 lt Re lt 2,000
• OK to proceed to Step (5)

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Step (5) Calc Drop Out Velocity

• Drop Out velocity, ut, from Perry VI - Eq
  5-264.
• Ut Re / C432.2 cP 0.00067197 (DL-Dv) / 3
  / Dv20.333333.
• Ut 0.4659 ft./sec.

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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.

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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.

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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.

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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.

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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.

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Step (11) calculate L3, ft.

• L3 (Vol L3)(4) / (p)(Vessel Dia)2.
• L3 42.33 ft.
• This makes the vessel roughly 7 ft. in diam with
  an unusually high liquid level (L3).

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Step (12) Document Liquid Retention Time

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

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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.

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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.

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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.

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Summary
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Vertical KO Pot with Demister Pad
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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.

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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.

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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.

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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.