Art's Liquid-Vapor Separator

The Mott Souders - George Granger Brown Equation is:

Vmax = (K) [ (rL - rV) / rV ]0.5

where,

Vmax = the maximum allowable superficial vapor velocity through the vessel = 7.5 ft/sec

rL = the density of the liquid entering the vessel = 54.00 lb/ft3

rV = the density of the vapor entering the vessel = 0.06 lb/ft3

K = an empirical constant whose value depends on the type of vessel, the

use of internals, and the height of the disengaging space above any

mesh employed. = 0.25

Values of "K" are found in Ernest Ludwig's "Applied Process Design for Chemical and Petrochemical Plants",

Volume 1, chapter 4:

(Bottom of mesh shuld be at least 12 inches above the liquid surface)

Service Conditions K" Value

Clean fluids, moderate liquid load; fits 90% 0.36 Standard design

of process situations and applications. 0.35 High efficiency (?)

(the rating description is probably backwards…) 0.25 Very high efficiency (?)

High viscosity; dirty, suspended solids 0.40 Low density or Herringbone, high

through-put

Vacuum operations:

2" Hg absolute 0.20 Standard design

16" Hg absolute 0.27 High Efficiency

Corrosive chemical 0.21 Plastic coated wire, or plastic strand

3

4

5

6

7

8

9

10

11

12

13

14

This table is from Otto H. York data.

Values based on 12 lb/ft3 wire mesh. Design practice

normally does not exceed k of 0.4 even for higher

disengaging height.

Additionally, the GPSA Engineering Data Book (11th Edition, electronic) states that vertical separators

0.38

0.40

0.42

0.43

0.29

0.32

Calculation of a Gas-Liquid Separator or Flash Drum

K Values for Knitted Mesh

General type mesh

Variations of K with Disengaging Height

Disengaging height above mesh,

inches

0.35

Allowable "K" value

0.12

0.15

0.19

0.22

0.25

have a K factor (in ft/sec) value of 0.18 to 0.35. It recommends adjustment of the K factor due to system

pressure:

Pressure % of K

atm. 100

150 psi 90

300 psi 85

600 psi 80

1,150 psi 75

For Vertical Separator Design:

100 lb/hr = 0.463 ft3/sec

0.062 ft2

0.28 ft = 3.37 inches

The GPSA states that separators without mist extractors are sized using the above equation with a K

that is typically 1/2 of that used for vessel with mist extractors.

John M. Campbell, in his book "Gas Conditioning and Processing", states: "(Separator sizing).. Is

a semi-empirical approach since one cannot measure things like droplet size and other variables.

Furthermore, many of the assumptions in the derivation of the Souders-Brown equation are not satisfied in

actual practice.

"The value of K depends on all factors that affect separation other than density - such as vortex action,

foaming, pulsating flow, liquid flowing in heads, presence of solids, degree of separation needed, separation

length, varying gas to liquid ratios, and the like. It is not surprising that K varies widely in different

applications. How does one predict it? From experience!

"Do not be naïve. Most published capacity curves are for ideal conditions. Use them cautiously. Consider a

factor like pulsating flow from a well head or a two-phase line. The liquid enters in "heads". During this time

the gas rate is low. Between "heads" the gas rate is normal but little liquid enters. Using some average 24

hour gas and liquid rate to size the separator would be criminal, or at least unprofessional. There must be

sufficient capacity to handle properly the instantaneous rates expected.

"The value of K used in the Souders-Brown equation thus varies. The normal range of values is:

Separator cross-sectional area =

Separator inside diameter =

K = 0.06 to 0.35

Maximum vapor flow rate =

Art Montemayor September 30, 2005

Rev: 0

Project:

K Values

SGliquid 0.464 0.289 ft/sec

SGgas 0.6571 0.144 ft/sec

P 715 psig 0.231 ft/sec

T -76oF

Q 12,800 MScfd

Z 1.0000

rliquid = = 28.94 28.94 28.94 lbf / ft3

(P+14.7)(SGgas*28.9625) 3.37 3.37 3.37

Qrgas 7.43 7.43 7.43

(379.49)(24)(3600)

Mrate 2.20 2.20 2.20

rgas

K rliquid - rgas 0.79 0.40 0.64

rgas

Vrate 2.77 5.55 3.47

velmax

12 4A 22.55 31.89 25.21

p

Vertical Separator Sizing Calculations

rgas =

Mrate =

Vrate =

velmax =

Vertical

Separator

(w/ mist pad)

Vertical

Separator

(w/o mist

pad)

Vertical

Separator

(Compressor)

=

=A =

=

SGliquid*62.371

lbf / ft3

lbm / sec

inches

ft3 / sec

ft2

fID =

Separator (w/ mist pad)

Separator (w/o mist pad)

Compressor Separator

Inlet Conditions:

=

Units

ft / sec

=

=

Z(10.73)(T + 459.67)

of 13FileName: 228901629.xls.ms_office

WorkSheet: Vertical Nat. Gas Sep.


Rev: 0

The Specific Gravity of a liquid fluid is defined as the density of the given liquid (at a defined temperature) divided

by the density of water at the reference temperature of 60 oF ( 62.371 lb/ft

3)

The Specific Gravity of a gas is defined as the density of the given gas (at a defined temperature and pressure) divided

by the density of dry air at the reference temperature and pressure of 60 oF and 14.696 psia ( 0.076322 lb/ft

3 ). Usually

the Specific Gravity is given for a gas also at 60 oF.

The Molecular Weight of Dry Air is given as 28.9625 (GPSA Engineering Data Book; 1983)

One lb-mole of an Ideal gas yields a volume of 379.49 ft3 at 14.696 psia and 60

oF (GPSA Engineering Data Book.

Specific Gravity of a gas at 60 oF and 14.696 psia = (gas Molecular Weight) / (28.9625)

The equation used to calculate Natural Gas density is:

where,

rV = Natural Gas density at temperature T, lb/ft3

MW = Natural Gas Molecular Weight, lbs/lb-mol

P = Natural Gas pressure, psia

T = Natural Gas temperature, oR

Z = Natural Gas Compressibility Factor

rV = (MW) (P) / (10.73) (T) (Z)


WorkSheet: Vertical Nat. Gas Sep.

Vertical liquid-vapor separators are used primarily to disengage a liquid from a vapor when the volume of liquid is

small compared with the vapor volume. The maximum allowable vapor velocity in a vertical separator, to reduce

liquid carryover, is a function of liquid and vapor density and a constant, K, based on surface tension, droplet size,

and physical characteristics of the system. Usually these K values for vertical vessels vary between 0.1 to 0.35,

if mist eliminators are used to enhance disentrainment. Watkins(1)

developed a correlation between separation factor

(S Fac) and K value which has proved useful in sizing process liquid-vapor separators. Watkins' data have been fitted

to an equation, and this correlation is used in this program for calculation of K values for a range of separation factors

between 0.006 and 5.0. However, other K values can be entered if desired.

Once the K value and maximum vapor velocity are determined, the cross-sectional area of the separator is calculated

using the equation for Av. Vessel diameter is calculated from cross-sectional area by the equation for D, and

maximum liquid level is calculated from liquid flow rate and surge time using the equations for QL and LL.

It is normally recommended that 36 inches plus one-half the feed nozzle OD (48 inches minimum) be left above the

feed nozzle for vapor disengagement. Below the feed nozzle, leave 12 inches plus one-half the feed nozzle OD for

clearance between the maximum liquid level and the feed nozzle (minimum of 18 inches). At some value between

L/D ratios of 3 and 5 a minimum vessel weight will occur, resulting in minimum cost for the separator's steel material

of construction.

Equations employed:

L

V

V

L

W

WFacS

r

r=

)(exp 432 GXFXEXDXBKV =

V

VL

Max KVr

rr =

)3600(V

V

V

WQ

r=

Max

V

VV

QA =

pVA

D4

=

4

2DA

p=

)60(L

LL

WQ

r=

A

TQLL L )(

=

Nomenclature:

S Fac = Separation Factor

KV = Vapor velocity factor for vertical separators

VMax = Maximum allowable vapor velocity, ft/sec

WL = Liquid mass flow rate, lb/hr

WV = Vapor mass flow rate, lb/hr

rV = Vapor density, lb/ft3

rL = Liquid density, lb/ft3

QV = Vapor volumetric flow rate, ft3/sec

D = Separator's internal diameter, ft

A = Separator's cross-sectional flow area, ft2

AV = Separator's minimum cross-sectional flow area, ft2

References:

1. Watkins, R.N.: "Sizing Separators and accumulators", Hydrocarbon Processing; November, 1967;

pp. 253-256.

A. Kayode Coker, in his book "Fortran Programs for Chemical Process Design", gives the Watkins Curve which

relates KV to (WL/WV) (rV/rL)0.5

. He qualifies the data as based on five percent of the liquid being entrained with

the vapor and at 85 % of flooding.

The data on the curve are as follows:

KV

0.006 0.23

0.008 0.25

0.01 0.27

0.02 0.41

0.04 0.45

0.06 0.44

0.08 0.43

0.1 0.42

0.2 0.35

0.4 0.28

0.6 0.2

0.8 0.17

1.0 0.14

2.0 0.07

4.0 0.033

I regressed this data using the Datafit program and got a much better equation in:

y = x/(a+b*x-c*x2)

a = 0.01746538478

b = 1.620701278

c = -5.47525025

(WL/WV) (rV/rL)0.5

Vertical liquid-vapor separators are used primarily to disengage a liquid from a vapor when the volume of liquid is

small compared with the vapor volume. The maximum allowable vapor velocity in a vertical separator, to reduce

liquid carryover, is a function of liquid and vapor density and a constant, K, based on surface tension, droplet size,

and physical characteristics of the system. Usually these K values for vertical vessels vary between 0.1 to 0.35,

if mist eliminators are used to enhance disentrainment. Watkins(1)

developed a correlation between separation factor

(S Fac) and K value which has proved useful in sizing process liquid-vapor separators. Watkins' data have been fitted

to an equation, and this correlation is used in this program for calculation of K values for a range of separation factors

Once the K value and maximum vapor velocity are determined, the cross-sectional area of the separator is calculated

using the equation for Av. Vessel diameter is calculated from cross-sectional area by the equation for D, and

maximum liquid level is calculated from liquid flow rate and surge time using the equations for QL and LL.

It is normally recommended that 36 inches plus one-half the feed nozzle OD (48 inches minimum) be left above the

feed nozzle for vapor disengagement. Below the feed nozzle, leave 12 inches plus one-half the feed nozzle OD for

clearance between the maximum liquid level and the feed nozzle (minimum of 18 inches). At some value between

L/D ratios of 3 and 5 a minimum vessel weight will occur, resulting in minimum cost for the separator's steel material

Watkins, R.N.: "Sizing Separators and accumulators", Hydrocarbon Processing; November, 1967;

A. Kayode Coker, in his book "Fortran Programs for Chemical Process Design", gives the Watkins Curve which

relates KV to (WL/WV) (rV/rL)0.5

. He qualifies the data as based on five percent of the liquid being entrained with


Rev: 0

Project:

K Values

SGliquid 0.550 Separator (w/ mist pad) 0.310 ft/sec

SGgas 0.7000 Separator (w/o mist pad) 0.155 ft/sec

P 500 psig Filter Separator * 1.300 ft/sec

T 80oF

Q 50,000 M Scfd

L 30.0 Feet

rliquid = = 34.30 34.30 34.30 lbm / ft3

(P+14.7)(SGgas*28.9625) 1.75 1.75 1.75

Z(10.73)(T+460)

Qrgas 30.92 30.92 30.92

(379.49)(24)(3600)

Mrate 17.67 17.67 17.67

rgas

K rliquid - rgas 2.47 1.24 5.61

rgas

Vrate 7.14 14.29 3.15

velmax

12 4A 36.19 51.18 24.04 inches

p

Horizontal Separator Sizing Calculations

fID =

A =

velmax =

Vrate =

Mrate =

rgas =

=

=

=

LONE CAMP FILTER SEPARATOR-AT PLANT-HP TURBINES

Inlet Conditions:

* Filter Seps are Proprietary Designs. This is estimate only

SGliquid*62.367

Horizontal

Separator

(w/ mist pad)

Horizontal

Separator

(w/o mist pad)

Filter

Separator *Units

ft2

ft / sec(L/10)0.56

lbm /ft3

=

lbm /sec

ft3 / sec

=

=

Wet Gas in Gas out

Liquid out


WorkSheet: Horizontal Sep

Art Montemayor August 21, 2005

Rev: 0

ANALYTICAL STUDY OF 'LIQUID/VAPOR SEPARATION EFFICIENCYBY

Dr. W.D. Monnery Dr. W.Y. Svrcek

Chem-Pet Process Technology Ltd. Department of Chemical & Petroleum Engineering

335 Ranchridge Bay NW University of Calgary

Calgary, AB Calgary, AB T2N 1N4

5-Sep-00

SUMMARY

The purpose of this work was to establish the separation efficiency of flare knock-out drums and determine the

expected entrained liquid droplet diameter that is carried over to the flare. This was accomplished by using a

field pilot plant skid at the Prime West East Crossfield gas plant. The skid consisted of gas and liquid inlets test

separators and entrained liquid collection in a filter/coalescer. The raw test data was entrained liquid carryover

amount as a function of gas velocity data.

Experimental results provide incipient entrained liquid carryover velocities. The data show that carryover rises

sharply after the incipient carryover velocity and separation efficiency drops below 99.9%. Experimental results

indicate that entrained liquid carryover average droplet diameters are 200 to 600 microns for flare knock-out

drums at 10 to 100 psig. Calculations show that the maximum stable droplet size can be very large at low

velocities and the calculated liquid droplet size distribution indicates that there can be substantial variance in the

droplet size and that the latter may not be very uniform. In order to verify the estimated droplet sizes and

distributions, further experimental work must include the addition of online droplet size and distribution

measurement equipment.

Experimental results provide quantitative data for the relationship between horizontal and vertical K factors and

allowable velocities, which has to date been empirical and subjective. These results show that the factor between

horizontal and vertical K factors and allowable velocities vary from about 1.33 to 1.67 as L/D varies from

3.5 to 6.5.

Modeling results based on using the experimental data give entrained liquid average droplet diameters that are

consistent with API 521 for flare knock-out drums (300-600 microns) as well as other open literature. To

avoid carryover, flare knock-out drums should be designed using a droplet size of 300 microns.

1.0 INTRODUCTION/BACKGROUND

This study is part of the Alternative Flaring Technologies program sponsored by Environment Canada, CAPP

and PTAC. This study focuses on the efficiency of gravity separation as it relates to flare knockout drum

design and operation.

One of the critical issues in facilities process design and operation is vapor/liquid separation. This is also an

important issue for the improvement of existing flaring systems. The problem for flaring systems is that with the

uncertainty of design and operating conditions, liquid carryover droplets may be of such a size and composition

that they are incompletely combusted. This results in the emission of many undesirable compounds to the

atmosphere, as has been outlined in previous studies and of the current Government and Industry study

10 of 13FileName: 228901629.xls.ms_office

WorkSheet: V-L Separator Analysis


Rev: 0aimed at mitigating emissions in flares.

There is an abundance of literature available on vapor/liquid separation and equipment design, yet there has never

been a systematic, comprehensive study to verify the accepted design methodology. Liquid vapor separator

design is described in several engineering and operating company guidelines, the GPSA Engineering Data Book

and recent publications such as Svrcek and Monnery (1993) and Monnery and Svrcek (1994). Other

publications of note are Watkins (1967) and Talavera (1990).

The present design philosophy is to simply attempt to be conservative enough so that separation equipment will

work. Unfortunately, the definition of how conservative these designs are remains in question. Furthermore,

equipment that does function properly at design rates may need to be re-rated for increased rates or at off-design

operating conditions and the above mentioned problem appears again (how conservative?). Although general

design methodology is well accepted, it is the subjectivity of some of the separation parameters used in the

models that are in question.

As such, the purpose of the research is to determine the efficiency of gravity separation. Specifically, it is to

determine the velocity at which carryover occurs and to estimate the liquid particle size going to flare. This data

can also be used to check current design criteria and estimate liquid carryover at operating conditions.

1.0 THEORY OF GRAVITY SEPARATION AND SIMPLIFIED COALESCENCE MODELLING

In a liquid-vapor separation vessel, there are typically three stages of separation. The first stage, primary

separation, uses an inlet diverter to cause the largest droplets to impinge by momentum and then drop by gravity.

The next stage is gravity separation of smaller droplets as the gas flows through the vapor disengagement section

of the separator. The final stage is mist elimination, where the smallest droplets are coalesced on an impingement

device, such as a mist pad or vane pack, followed by gravity settling of the larger formed droplets. In vessels

like flare knockout drums, we are primarily concerned with gravity separation since they typically have no

coalescing internals, such as mist pads.

For gravity separation, the allowable velocity is determined so that the required disengagement area can be

determined. For a vertical vessel, performing a force balance on the liquid droplet settling out provides the

necessary relationship. When the net gravity force, given by Eq.1,

……….(1)

balances the drag force, given by Eq. 2,

……….(2)

the liquid droplets will settle at a constant terminal velocity, UT. Equating Eqs. 1 and 2 results in:

cL

VLP

Gg

gMF

r

rr )( =

c

VVPD

Dg

UDCF

rp 22)8/(=

c

VVPD

Dg

UDCF

rp 22)8/(=




Rev: 0

The purpose of this work was to establish the separation efficiency of flare knock-out drums and determine the

expected entrained liquid droplet diameter that is carried over to the flare. This was accomplished by using a

field pilot plant skid at the Prime West East Crossfield gas plant. The skid consisted of gas and liquid inlets test

separators and entrained liquid collection in a filter/coalescer. The raw test data was entrained liquid carryover

Experimental results provide incipient entrained liquid carryover velocities. The data show that carryover rises

sharply after the incipient carryover velocity and separation efficiency drops below 99.9%. Experimental results

indicate that entrained liquid carryover average droplet diameters are 200 to 600 microns for flare knock-out

drums at 10 to 100 psig. Calculations show that the maximum stable droplet size can be very large at low

velocities and the calculated liquid droplet size distribution indicates that there can be substantial variance in the

droplet size and that the latter may not be very uniform. In order to verify the estimated droplet sizes and

distributions, further experimental work must include the addition of online droplet size and distribution

Experimental results provide quantitative data for the relationship between horizontal and vertical K factors and

allowable velocities, which has to date been empirical and subjective. These results show that the factor between

horizontal and vertical K factors and allowable velocities vary from about 1.33 to 1.67 as L/D varies from

Modeling results based on using the experimental data give entrained liquid average droplet diameters that are

consistent with API 521 for flare knock-out drums (300-600 microns) as well as other open literature. To

This study is part of the Alternative Flaring Technologies program sponsored by Environment Canada, CAPP

and PTAC. This study focuses on the efficiency of gravity separation as it relates to flare knockout drum

One of the critical issues in facilities process design and operation is vapor/liquid separation. This is also an

important issue for the improvement of existing flaring systems. The problem for flaring systems is that with the

uncertainty of design and operating conditions, liquid carryover droplets may be of such a size and composition

that they are incompletely combusted. This results in the emission of many undesirable compounds to the




Rev: 0

There is an abundance of literature available on vapor/liquid separation and equipment design, yet there has never

been a systematic, comprehensive study to verify the accepted design methodology. Liquid vapor separator

design is described in several engineering and operating company guidelines, the GPSA Engineering Data Book

The present design philosophy is to simply attempt to be conservative enough so that separation equipment will

work. Unfortunately, the definition of how conservative these designs are remains in question. Furthermore,

equipment that does function properly at design rates may need to be re-rated for increased rates or at off-design

operating conditions and the above mentioned problem appears again (how conservative?). Although general

design methodology is well accepted, it is the subjectivity of some of the separation parameters used in the

As such, the purpose of the research is to determine the efficiency of gravity separation. Specifically, it is to

determine the velocity at which carryover occurs and to estimate the liquid particle size going to flare. This data

In a liquid-vapor separation vessel, there are typically three stages of separation. The first stage, primary

separation, uses an inlet diverter to cause the largest droplets to impinge by momentum and then drop by gravity.

The next stage is gravity separation of smaller droplets as the gas flows through the vapor disengagement section

of the separator. The final stage is mist elimination, where the smallest droplets are coalesced on an impingement

device, such as a mist pad or vane pack, followed by gravity settling of the larger formed droplets. In vessels

like flare knockout drums, we are primarily concerned with gravity separation since they typically have no

For gravity separation, the allowable velocity is determined so that the required disengagement area can be

determined. For a vertical vessel, performing a force balance on the liquid droplet settling out provides the



Art's Liquid-Vapor Separator

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Transcript of Art's Liquid-Vapor Separator