Lecture 2 - Phase Separation

8/11/2019 Lecture 2 - Phase Separation

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Phase separationPhase separationPhase separationPhase separation

(Ref: GPSA Eng Data Book; Biegler et al., 1997)

Copyright@Dominic Foo H82PLD - Plant Design Phase Sep - 2

Lecture outlineLecture outlineLecture outlineLecture outline

The Onion model

Common phase separation

processes

Uses of gas/liquid separators

Separation principles

Separator design & sizing

Linear mass balance for flash unit


TheTheTheThe Onion diagramOnion diagramOnion diagramOnion diagram

Reactor

Separation &

recycle

Heat exchange

network

Utilities (Linnhoff et al., 1982,Smith, 1995, 2005)

Leave it until H83RED(Reactor Design)

Current focus


IntroductionIntroductionIntroductionIntroduction The lecture handout (extracted from Gas Processors

Supplier Association GPSA Engineering Data Book) is acourtesy of Dr Murugan Selvan, WinSim Inc., Texas US.

5 types of phase separation problems:

Liquid droplets from vapour/gas (our focus) Liquid droplets from immiscible liquid

Solid particles from gas/vapour

Solid particles from liquid

Solid particles from other solid particles

Some common denominations: Flash tank: gas/liquid separation based on P

Knockout drum: to remove liquid droplets from gas

Liquid-liquid separator: 2 immiscible liquid phases Three phase separator: removal of gas from 2 immiscible liquids


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Uses of gas/liquid separatorsUses of gas/liquid separatorsUses of gas/liquid separatorsUses of gas/liquid separators After partial condenser on distillation column overheads

Serve 2 purposes:

Vapour/liquid separation Liquid hold-up for reflux purpose


Uses of gas/liquid separatorsUses of gas/liquid separatorsUses of gas/liquid separatorsUses of gas/liquid separators Product recovery via condensation

Effluent from reactor has 2 phases


Uses of gas/liquid separatorsUses of gas/liquid separatorsUses of gas/liquid separatorsUses of gas/liquid separators Refrigeration loop


Uses of gas/liquid separatorsUses of gas/liquid separatorsUses of gas/liquid separatorsUses of gas/liquid separators To protect high speed machinery e.g. compressors

Drum 1 protects machine from upstream operations (often known asdry drum)

Drum 2 & 3 are needed to prevent any liquid that may formed due tointer-cooling.


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Separator design & partsSeparator design & partsSeparator design & partsSeparator design & parts 3 types of separator designs:

Vertical

Horizontal Spherical

4 important parts of a separator: Primary separation section - to separate the

main portion of free liquid in the inletstream.

Gravity section designed to utilise gravityforce to enhance separation of entraineddroplets.

Coalescing section utilises coalescer / mist

extractor to remove the very small dropletsof liquid from the gas by impingement on asurface where they coalesce.

Liquid collection section acts as receiverfor all liquid removed from the gas


Factor for selection of separatorFactor for selection of separatorFactor for selection of separatorFactor for selection of separatorconfigurationconfigurationconfigurationconfiguration How well will extraneous material (e.g. sand, mud,

corrosion products) be handled?

How much plot space will be required?

Will the separator be too tall for transport if skidded?

Is there enough interface surface for 3-phase separation(e.g. gas/hydrocarbon/glycol liquid)?

Can heating coils or sand jets be incorporated if required?

How much surface area is available for degassing of

separated liquid? Must surges in liquid flow be handled without large

changes in level?

Is large liquid retention volume necessary?


Particle characteristicParticle characteristicParticle characteristicParticle characteristic


Vertical separatorVertical separatorVertical separatorVertical separator Selected when:

gas-liquid ratio is high; or

total gas volumes are low

Level control is not critical &

liquid level can fluctuateseveral inches withoutaffecting operating efficiency.

Mist extractors significantlyreduce the required diameter

Occupies a small amount ofplot space


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Horizontal separatorHorizontal separatorHorizontal separatorHorizontal separator

Most efficient when: large volumes of total fluids

large amounts of dissolved gas are present with the liquid.

Large liquid surge volume leading to longer retention time

More surface area per liquid volume better degassing

Handle a foaming liquid better than a vertical.

The liquid level responds slowly to liquid inventory


Spherical separatorSpherical separatorSpherical separatorSpherical separator

Highpressureservice

Compactsize isdesired

Liquidvolumesare small


Principles of separationPrinciples of separationPrinciples of separationPrinciples of separation 3 principles used to achieve physical separation:

Gravity settling Liquid droplets settle out fromgas phase when gravitational force > drag force ofgas flowing around the droplet

Momentum:

Separation occurs when a 2 phases (of differentdensities) stream changes direction sharply, greatermomentum prevent the heavier particles to turn asrapidly as the lighter fluid

Employed for 2 phases bulk separation

Coalescing: Very small droplets (fog or mist) cannot be separated by gravity coalesced

to form larger droplets that will settle by gravity.

Coalescing devices force gas to follow a tortuous path. The momentum forcesthe droplets to collide & form larger droplets settle out by gravity.

Examples: wire mesh screens, vane elements, and filter cartridges

Any separator may employ 1 or > principles, but fluid phases must beimmiscible & of different densities.


Design of separator without mistDesign of separator without mistDesign of separator without mistDesign of separator without mistextractorsextractorsextractorsextractors To design a separator without a mist extractor, the

minimum diameter droplet to be removed must be settypically in the range of 150 - 2,000 microns.

Gravity settlingis utilised as the sole mechanism for

separating the liquid and gas phases. Calculation of vessel length:

Assumption: the time for the gas to flow from inlet to outlet = timefor the liquid droplet of size Dp to fall from the top of the vesselliquid surface.

Then relate the length (L) of the separator to its diameter (Dv) as afunction of this settling velocity (Vt):

Ratio ofL/Dv is in the range of 2:1 4:1vt

A4

DV

Q

L =


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Gravity settlingGravity settlingGravity settlingGravity settling Forces acting on the particle can be described

mathematically using the terminal or free settling velocity:

Combining:

Drag coefficient C is a function of: Particle shape (considered as solid & rigid sphere)

Reynolds number, given as:

( )CA

gMVt

=

pgl

glp2

gtp1488Re

VD=

2

p

p

3

p

p

l

p

2and

23

4

=

==

DA

DV

m

( )C

gDV

=

g

glp

t3

4


CCCC of Rigid Spheresof Rigid Spheresof Rigid Spheresof Rigid Spheres

( )2

gl

3

pg

8

210x95.0

(Re)

=

DC


Gravity settlingGravity settlingGravity settlingGravity settling limited caselimited caselimited caselimited case


Example 1Example 1Example 1Example 1 ---- Separator designSeparator designSeparator designSeparator design(without mist extractors)(without mist extractors)(without mist extractors)(without mist extractors) A horizontal gravity separator (without mist

extractor) is required to remove all entrainmentwith Dp > 150 microns from a 60 million standardcubic feet per day (MMscfd) of gas.

Gas property:

MW = 21.72

Pressure = 500 psig

Temperature = 100F

Viscosity = 0.012 cp

Ideal gas compressibility (Z) = 0.9 Liquid specific gravity = 0.50


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Example 1Example 1Example 1Example 1 SolutionSolutionSolutionSolution

H2O (American unit)

1 ft = 12 in

lb-mole379 ft33600 s24 hrday21.72 lb1 lb-mole1 hr1 day60 x 106 ft3

1 micron = 0.00003937 in

100 + 460R



( )2

gl

3

pg

8

210x95.0

(Re)

=

D

C



L

Dv



Vessels up through 24 in. diameter have nominal pipe dimensions

Larger vessels are rolled from plate with 6 in. internal increments in diameter.

Seider et al. (2003)


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Example 2Example 2Example 2Example 2 ---- Separator designSeparator designSeparator designSeparator design(without mist extractors)(without mist extractors)(without mist extractors)(without mist extractors) What size vertical separator

without mist extractor is

required to meet the conditions

used in Example 1?

Solution:

Area:

Diameter:

Hence, ID of 90 in is needed

2

t

A ft7.4146.0

2.19===

V

QA

in87ft92.74

4

2

===

AD

DA

Dv


Separator with mist extractorSeparator with mist extractorSeparator with mist extractorSeparator with mist extractor Wire mesh pads are frequently used as entrainment separators for the removal

of very small liquid droplets (< 10 microns) higher overall percentageremoval of liquid.

The pressure drop across a wire mesh pad is negligible. The effect of the

Pbecomes significant only in the design of vacuum services & for equipmentwhere the prime mover is a blower or a fan


Separator with mist extractorSeparator with mist extractorSeparator with mist extractorSeparator with mist extractor Wire mesh pads can be used in horizontal vessels.

The orientation of the mesh pad is preferred in the horizontal plane.

Less efficient in vertical orientation


Design of separator (with mistDesign of separator (with mistDesign of separator (with mistDesign of separator (with mistextractors)extractors)extractors)extractors) Vertical separator with mist

extractors: Critical velocity equation:

Souders & Brown correlation:

Horizontal separator > 10 ft withmist extractors: Critical velocity equation:

Souders & Brown correlation:

Ratio ofL/Dv is in the range of2:1 4:1

56.0

g

gl

10

=

LKV

t

g

gl

=KV

t

( )56.0

10glgm

=

LCG

( )glgm = CG


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Example 3Example 3Example 3Example 3 ---- Separator designSeparator designSeparator designSeparator design(with mist extractors)(with mist extractors)(with mist extractors)(with mist extractors) What size vertical separator

equipped with a wire mesh mistextractor is required for theconditions used in Example 1?

Solution: K= 0.28 ft/s

Critical velocity:

Area:

Diameter:


ft/s05.107.2

07.22.3128.0t =

=V

2

t

A ft3.18

05.1

2.19===

V

QA

in85ft8.44

4v

2v

===

AD

DA


Separator with vane type mistSeparator with vane type mistSeparator with vane type mistSeparator with vane type mistextractorsextractorsextractorsextractors When fouling or hydrate formation is

possible/expected, mesh pads are not used

vane or centrifugal type separators aremore appropriate.

Vane type separator designs are proprietary& not easily designed with standardequations consult manufacturers.


Separators with centrifugalSeparators with centrifugalSeparators with centrifugalSeparators with centrifugalelementselementselementselements Advantage of a centrifugal separator

over a filter separator: much lessmaintenance

Disadvantages of centrifugalseparators:

Some designs do not handle slugswell

Efficiency is not as good as othertypes of separators

P tends to be higher than vaneor clean knitted mesh mistextractors

Narrow operating flow rangefor highest efficiency

Designs are proprietary consultmanufacturers


Filter separatorFilter separatorFilter separatorFilter separator


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Filter separator designFilter separator designFilter separator designFilter separator design Higher separation efficiency than the centrifugal separator,

but it uses filter elements, which must periodically be

replaced. The approximate filter surface area for gas filters can be

estimated from based on applications such as molecularsieve dehydrator outlet gas filters. For dirty gas service theestimated area should be increased by a factor of 2 or 3.

Removal efficiency: Most dry solid particles 10 microns are removable

Particles of < 10 microns: ~99%

For heavy liquid loads, or where free liquids are contained in theinlet stream, a horizontal filter separator with a liquid sump, whichcollects and dumps the inlet free-liquids separately from coalescedliquids, is preferred.


Filter separator designFilter separator designFilter separator designFilter separator design Pressure drop:

Efficiency depends on the proper design of the filter pack, i.e., a

minimum P while retaining an acceptable extraction efficiency. P ~ 1-2 psi is normal in a clean filter separator.

If excessive solid particles are present, it may be necessary to cleanor replace the filters at regular intervals when a P > 10 psi isobserved.

Rule of thumb to change filters: maximum P of 25 psi, otherwisecartridge units might collapse.

Removal of the filter pack is easily achieved by using aquick-opening closure.

Critical velocity equation:

g

gl

t 3.1

=V


Approximate gas filter capacityApproximate gas filter capacityApproximate gas filter capacityApproximate gas filter capacity


Example 4Example 4Example 4Example 4 Filter separatorFilter separatorFilter separatorFilter separator A filter separator is required to handle a flow of 60 MMscfd

at conditions presented in Example 1. Estimate the diameterof a filter separator.

Solution: Critical velocity equation:

Area:

Diameter:


ft/s88.407.2

07.22.313.1

t =

=V

2

t

A ft93.388.4

2.19===

V

QA

in26.9ft2.24

4v

2v

===

AD

DA


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Linear mass balance (MB)Linear mass balance (MB)Linear mass balance (MB)Linear mass balance (MB) A flowsheet has many non-linear equations

describing: Connectivity of units through process streams

Equations for each unit, e.g. internal M/E balances,equilibrium relationships

Physical properties, e.g. enthalpy, equilibrium constant,thermo properties, etc.

It will be useful (e.g. manual calculation duringpreliminary process design) to linearise theseequations subject to the following approximations: Ideal solution in all calculations

Most streams are available as saturated vapour/liquid(valid for equilibrium staged operations)


Linear MB for flash unitLinear MB for flash unitLinear MB for flash unitLinear MB for flash unit Most fundamental & important in a flowsheet building

block for equilibrium staged operations, e.g. distillation,

absorption. Consider a flash model to separate ncomp components, the

overhead split fraction (recovery) of component k (k) isgiven as:

V, ykvk = V yk

L, xklk = L xk

F, zkfk = F zk

k

kk

f

v=


VLE approximationsVLE approximationsVLE approximationsVLE approximations Approximation for VLE:

Antoine equation for vapour pressure:

Raoults law: yk P =xk Po

k , or defined asK-value as:

With respect to key component n, relative volatility of componentk is defined as:

Rearrange the approximations give the relation betweenrecovery of component k in terms of key component n:

k

kkk

CT

BAP

+=

oln

kk

k

k KP

P

x

y==

o

o

o

n

k

n

k

P

P

K

Kn

k ==

( ) nn

k

nk

nk

11 +=


VLE approximationsVLE approximationsVLE approximationsVLE approximations At bubble point (saturated liquid effluent stream):

i yi = i Kixi = 1

Given the average relative volatility = ii/n xi ,

K-value is redefined as a simplified bubble pointequation:

For T fixed & P unknown, P can be calculated from:

For P fixed & T unknown, T can be calculated from:

a~

o

nk

P

PK kk

==

)(a~ o

TPP kn

k=

PTP nk

ka~

)(o =


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Calculation steps for fixed T , PCalculation steps for fixed T , PCalculation steps for fixed T , PCalculation steps for fixed T , P

1. Pick a key component n and guess n2. Calculate K

k,

k/n@ specified T

3. Evaluate k for each component k by:

4. Reconstruct mass balance & calculate molefractions: v

k

= k

fk

= yk

k

vk

lk = (1 k) fk = xk k lk5. If the bubble point equation is satisfied, calculation

is stop. Otherwise, re-guess n and go to Step 3.

( ) n

n

k

nk

nk

11 +=


Example 5Example 5Example 5Example 5 Flash calculationFlash calculationFlash calculationFlash calculation Calculate the flowrate and mole fractions of the top &

bottom product streams of a flash drum that operated @ 1

bar & 390K. Component flowrates of the feed stream, as well as their

boiling points (BP) & Antoine coefficients are given in thetable.

59.443395.5716.115641840O-xylene (X)

53.673096.5216.013738350Toluene (T)

52.342788.5115.900835330Benzene (B)

CkBkAkBP (K)fk (kmol/h)Comp, k






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Steps forSteps forSteps forSteps for kkkk& fixed T (or P)& fixed T (or P)& fixed T (or P)& fixed T (or P)1. Guess T (or P)

2. Follow Steps 2 for fixed T& P case3. Follow Steps 3 for fixed T& P case

4. Follow Steps 4 for fixed T& P case

5. 2 cases:

For Tfixed:

For P fixed, solve for T from PTP nk

ka~

)(o

=

)(a~ o

TPP kn

k=


Problem for revision 1Problem for revision 1Problem for revision 1Problem for revision 1 Feed to a flash tank consists of 25 moles pentane, 40

moles cis-2-butene & 35 moles n-butane.

Find the recovery of n-butane @ P = 200 kPa & T = 300K

If the flash tank operates @ 100 kPa, at whattemperature could you recover 60% cis-2-butene in thevapour?

Remark: Make your own effort to obtain Antoinecoefficients!


Problem for revision 2Problem for revision 2Problem for revision 2Problem for revision 2 A flash vessel is used to separate a mixture that consists of

propane, n-pentane and n-hexane. Component flowratesand Antoine coefficients are shown in the following table.

Tasks: Find the components mole flowrates and fractions in the overhead

stream when the flash vessel is operated at 5 atm and 90C (1 atm= 760 mmHg). If the flash tank operates @ 8 atm, at what temperature could you

recover 60% n-pentane in the vapour?(Final exam 2007, Q5)

224.2101168.7206.87050n-Hexane (C6H14)233.2051075.786.87630n-Pentane (C5H12)

246.990803.8106.80420Propane (C3H8)

CBA(kmol/h)

Antoine coefficientsFlowrateComponent


Problem for revision 3Problem for revision 3Problem for revision 3Problem for revision 3 A flash vessel is used to separate a mixture that consists of ethane,

propane, butane, n-pentane and n-hexane. Component flowrates andAntoine coefficients are shown in the following table.

Tasks: Find the components molar flowrates and fractions in the overhead stream

when the flash vessel is operated at 7 atm and 363K (1 atm = 760 mmHg).

If the flash tank operates at 358K, at what pressure could we recover 60%of n-butane in the vapour stream?

(Final exam 2008, Q1)

-48.801171.176.876020n-Hexane (C6H14)-40.191064.846.852930n-Pentane (C5H12)

-34.42935.866.808920n-Butane (C4H10)-26.16804.006.803420Propane (C3H8)-16.42663.726.829110Ethane (C2H6)

Ck

Bk

Ak

(kmol/h)

Antoine coefficientsFlowrateComponent k

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Lecture 2 - Phase Separation