Liquid-Vapor Separation Efficiency [envp0102].doc

8/10/2019 Liquid-Vapor Separation Efficiency [envp0102].doc

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ANALYTICAL STUDY OFLIQUID/VAPOUR SEPARATION EFFICIENCY

BY

Dr. W.D. Monnery

ChemPe! Pro"e## Te"hno$o%y L!&.''( R)n"hr*&%e B)y NW

C)$%)ry+ AB

Dr. W.Y. S,r"e-De)r!men! o Chem*")$ 0 Pe!ro$e1m En%*neer*n%

Un*,er#*!y o C)$%)ryC)$%)ry+ AB T2N 3N4

Se!em5er (+ 2666


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SUMMARY

The purpose of this work was to establish the separation efficiency of flare knock"outdrums and determine the e!pected entrained liquid droplet diameter that is carried over tothe flare. This was accomplished by using a field pilot plant skid at the Prime #est $astCrossfield gas plant. The skid consisted of gas and liquid inlets test separators andentrained liquid collection in a filter%coalescer. The raw test data was entrained liquidcarryover amount as a function of gas velocity data.

$!perimental results provide incipient entrained liquid carryover velocities. The data showthat carryover rises sharply after the incipient carryover velocity and separation efficiencydrops below &&.&'. $!perimental results indicate that entrained liquid carryover averagedroplet diameters are ()) to *)) microns for flare knock"out drums at +) to +)) psig.Calculations show that the ma!imum stable droplet size can be very large at low velocitiesand 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. n order to verifythe estimated droplet sizes and distributions, further e!perimental work must include theaddition of online droplet size and distribution measurement equipment.

$!perimental results provide quantitative data for the relationship between horizontal andvertical - factors and allowable velocities, which has to date been empirical andsubective. These results show that the factor between horizontal and vertical - factorsand allowable velocities vary from about +.// to +.*0 as 1%2 varies from /.3 to *.3.

4odelling results based on using the e!perimental data give entrained liquid averagedroplet diameters that are consistent with AP 3(+ for flare knock"out drums 5/))"*))

microns6 as well as other open literature. To avoid carryover, flare knock"out drumsshould be designed using a droplet size of /)) microns.

/


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3.6 INTRODUCTION/BAC78ROUND

This study is part of the Alternative 7laring Technologies program sponsored by$nvironment Canada, CAPP and PTAC. This study focuses on the efficiency of gravityseparation as it relates to flare knockout drum design and operation.

8ne of the critical issues in facilities process design and operation is vapour%liquidseparation. This is also an important issue for the improvement of e!isting flaring systems.The problem for flaring systems is that with the uncertainty of design and operatingconditions, liquid carryover droplets may be of such a size and composition that they areincompletely combusted. This results in the emission of many undesirable compounds tothe atmosphere, as has been outlined in previous studies and of the current 9overnmentand ndustry study aimed at mitigating emissions in flares.

There is an abundance of literature available on vapour%liquid separation and equipmentdesign, yet there has never been a systematic, comprehensive study to verify the accepteddesign methodology. 1iquid vapour separator design is described in several engineeringand operating company guidelines, the 9P:A $ngineering 2ata ;ook and recentpublications such as :vrcek and 4onnery 5+&&/6 and 4onnery and :vrcek 5+&&6. Although generaldesign methodology is well accepted, it is the subectivity of some of the separationparameters 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 theliquid particle size going to flare. This data can also be used to check current designcriteria and estimate liquid carryover at operating conditions.


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2.6 T9EORY OF 8RAVITY SEPARATION AND SIMPLIFIEDCOALESCENCE MODELLIN8

n a liquid"vapour separation vessel, there are typically three stages of separation. Thefirst stage, primary separation, uses an inlet diverter to cause the largest droplets toimpinge by momentum and then drop by gravity. The ne!t stage is gravity separation ofsmaller droplets as the gas flows through the vapour disengagement section of theseparator. The final stage is mist elimination, where the smallest droplets are coalesced onan impingement device, such as a mist pad or vane pack, followed by gravity settling ofthe larger formed droplets. n vessels like flare knockout drums, we are primarilyconcerned with gravity separation since they typically have no coalescing internals, such asmist pads.

7or gravity separation, the allowable velocity is determined so that the requireddisengagement area can be determined. 7or a vertical vessel, performing a force balanceon the liquid droplet settling out provides the necessary relationship. #hen the net gravityforce, given by $q.+,

cL

L!"

g

g#$

65 = 5+6

;alances the drag force, given by $q. (,

c

!%%

g

&%'$

((6?%5

= 5(6

the liquid droplets will settle at a constant terminal velocity, = T. $quating $qs. + and (results in@

%

L!T

'

%g&

/

65< = 5/6

ence, as long as the vapour velocity, =B, is less than =T, the liquid droplets will settleout. $q. / can be rewritten as $q.


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2ata ;ook as follows@

(

/? 65+)&3.)

L!%)

= 5*6

where 2Pis in ft 5microns /.(?)?+)"*6, densities are in lb%ft/and viscosity is in cP.

)))'% %)0.


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*($( = 5?6

where subscript indicates horizontal and subscript B indicates vertical. The factors are

either empirical or based on the fact that the time the liquid droplet takes to drop verticallythrough the vapour flow area must be less than the time it takes to travel horizontallybetween the inlet and outlet nozzles. This results in the correction factor 7 stated asfollows, $q. &@

+ *L$ %= 5&6

where 1$ is the effective horizontal length of travel of the liquid droplet and B is thevertical distance from the inlet to the liquid surface.

As such, there is considerable subectivity in determining horizontal - factors.

'.6 MET9ODOLO8Y

The research program, as outlined originally by $nvironment Canada, CAPP and PTACwas as follows@

+. dentify current liquid removal technologies and practices and develop astandardized testing methodology for knockout systems.

(. dentify acceptable knockout performance@-nockout efficiency will be the measure of performance. The definition of

FacceptableG knockout efficiency must be based upon what is attainable with thetechnology under field conditions. Hs &&' efficiency attainable>I

/. 1ab testing of the current technology@The identified liquid removal system5s6 must be tested to confirm that they willmeet proposed regulations and under what operating conditions. The effect ofsuch parameters as pressure, flow rate, compositions, ambient temperature, watercontent and hydrocarbon liquids content on knockout efficiency must bedetermined.


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3.1 EXPERIMENTAL APPARATUS AND PROCEDURE

To make the results as realistic as possible, all testing was done in the field at the Prime#est 5previously Amoco6 $ast Crossfield 9as Plant, with no lab testing done. Theapparatus used for the e!periments was a pilot plant scale skid, which is shown in 7igures+ " &. The apparatus consists of gas inlet piping, liquid pumping and inection, testseparators and a high efficiency filter%coalescer to collect entrained liquid from the testseparators. There are three horizontal test separators, each a +) inch nominal outsidediameter 5&.+/G26 and lengths of (J*G, /J*G and 3J*G, used in order to study the effect ofvessel length on allowable velocity. The test separators each had e!ternal cage throttlinglevel control with the float custom made to work for fine control within the separatordimensions and for the butane liquid. A manual globe valve located on the gas outletpiping controls the skid pressure. The gas flow is monitored downstream using aaliburton flow indicator and controlled by a manual globe valve on the gas inlet piping.

9as inlet temperature was that delivered to the skid from the gas plant. The liquidinection pump is a KAC metering pump with a ma!imum flow of /0.3 gph at 03) psi.

$!periments proceeded as follows 5refer to 7igure +6. 7or each horizontal vessele!periment at a given pressure, a gas flow rate was calculated, a priori, to give the desiredvelocity at the set liquid level for that particular e!periment. At the beginning of thee!periment, the gas pressure was set using the outlet manual globe valve and flow wasadusted using the inlet globe valve thus providing the desired flow at the desired pressure.8nce the gas flow stabilized, the liquid flow was started in an amount known to over"saturate the gas. At this time, the skid was left until the flows and temperatures reached asteady operating condition. 2uring the e!periment, the gas"liquid mi!ture flowed to the

selected test separator and the overhead vapour stream leaving the selected test separatoralong with any entrained liquid flowed to the filter%coalescer vessel, where the entrainedliquid was collected in the boot. The liquid level in the boot was recorded at the beginningand at the end of the test period, with the difference being the collected entrained liquid.The gas leaving the filter%coalescer was metered such that this value along with theamount of liquid collected provided a Fbucket and stopwatchG type e!periment. n orderto ensure that entrained liquid was not in the gas downstream of the filter%coalescer,composition and hydrocarbon dewpoint measurements of the inlet and outlet gases weretaken. n addition, collected liquid was analyzed. The e!perimental data 5liquid level, gasflow, liquid 5carryover6 collected6 was then used to determine the gas velocity,corresponding separation efficiency and entrained liquid droplet diameter.

?


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F*%1re 3 Pro"e## F$o; S"hem)!*" For Se)r)!or S-*&

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HC(Liquid)-1-80

HC-2-80

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HC(Liquid)-1-80

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HC-2-80

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Separator C

By Pass Valve By Pass Valve

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2.** /, & 1** S!S

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c!6 .2** C#

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117** /, & 182** S!S,P .0 3Pa$ 4 .85C

c!6 .2** C#

Filter / Coalescer Separator

2.** /, & 2** S!S,P .0 3Pa$ 4 .85C

c!6 .2** C#

VAJ Metering Pump

%a& . $PH 4 12 3Pa$0** 9PSH#


10/24

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11/24

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12/24

7or vertical separator, tests were performed using a


13/24

Table ( :aturated 9as To Test :eparators

Component 4ole 7raction

+) psig +)) psig


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3.3 DATA PROCESSING

7or a given e!periment, the e!perimental data consisted of pressure, temperature, actual

gas flow and the amount of liquid collected%carried over. 7rom the temperature andpressure and the gas and liquid compositions, density and viscosity values were obtainedfrom the process simulator 5M:M:T46.

The following describes how data was processed to determine parameters@

:eparation $fficiency 5'6 D H1iquid n 5m/%d6 1iquid Collected 516%$!perimental Time5hr6N (< hr%d % +))) 1%m/I % 1iquid n 5m/%d6 N +))

Belocity 5ft%s6 D Actual 9as 7low 5ft/%hr6 % 7low Area At :et 1iquid 1evel 5ft(6 %/*))5hr%sec6

$!perimental - 7actor 5ft%s6 D Belocity 5ft%s6 % H51" B6%BI).3

2roplet diameter was determined by iterative calculations as follows@+. $stimate droplet diameter(. Calculate L from $q. */. Calculate C2from $q. 0


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L

L+

L

!,!+

#A)!

&

"

-e%

%


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Table / $!perimental 2ata at ncipient 1iquid Carryover

:eparator Press 5psig6 Belocity5ft%s6

- 7actor 5ft%s6 2roplet 2iameter5microns6

oriz 51%2@ /.) "


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These adustments compare to those previously stated in the literature. 7or e!ample,#atkins 5+&*06 stated that the horizontal adustment should be about +.(3 for horizontalvessels, commonly designed with an 1%2 of /.). Adusting the horizontal - factors tovertical ones using $qs. ? and +< and then applying the force balance equations gives theentrained liquid droplet diameter results shown in Table


18/24

n addition to the droplet size and distribution, the separation efficiency was calculated asa function of the gas velocity. The results are given below in Appendi! . t can be seenthat carryover rises sharply after incipient carryover velocity is reached and the separationefficiency drops below &&.&'. Capps 5+&&


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t should be noted that although some e!periments were run with mist eliminator pads,because they are not used in flare knock"out drums, these results are not discussed indetail. owever, the data show that for the same velocities, mist eliminator padsdecreased the entrained liquid carryover by () //' of the values for gravity separationalone for the test separators with an 1%2 of /


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5.2 Recoen!"#ions

+. To avoid carryover, flare knock out drums should be designed using a droplet size of

/)) microns. This is an allowable vapour velocity below the ncipient Carryovervelocity determined in this study.(. A continuous online droplet size and distribution measurement system must be

installed before any further e!perimental data is collected./. Testing of other separation and coalescing devices should be undertaken.

?.6 Nomen"$)!1re

C2, 2rag Coefficient2, 2iameter, in or ft

2P, 2roplet 2iameter, microns2Pipe, Pipe 2iameter, in7, 7actor in $q. ?72, 2rag 7orce,79, 9ravity 7orceg, 9ravity Acceleration, ft%s(

gc, 2imension Proportionality Constant, 5lbf%lbm65ft%s(691$, $ntrained 1iquid 4ass 7lu!, lb%s"ft(

B, Bertical eight, ft-, - 7actor 5$q. 36, ft%s-, orizontal - 7actor, ft%s-B, Bertical - 7actor, ft%s1, 1ength, ft1$, $ffective 1ength, ft4P, 2roplet 4ass, lbe, eynolds umber=T, Terminal Belocity, ft%s=B, Bertical Belocity, ft%s#e, #eber umberL, Parameter 2efined by $q. *O, Translation Bariable in ormal 2istribution

, Biscosity, cP

, Pi umber

1, 1iquid 2ensity, lb%ft/

B, Bapor 2ensity, lb%ft/

, :urface Tension, dyne%cm

, :tandard 2eviation 5$q. +)6

()


21/24

.6 REFERENCES

Arnold, -.$. and C.T. :ikes, F2roplet settling theory key to understanding separator"

sizing correlationsG, 8il Q 9as K., Kuly (+, +&?*, p. *).

Capps, .#., FProperly :pecify #ire"4esh 4ist $liminatorsG, Chem $ng Prog, 2ecember+&&


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Aen&*< I

E


23/24

Aen&*< II

Dro$e! D*#!r*51!*on C)$"1$)!*on#


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Aen&*< III

Mo&e$$*n% C)$"1$)!*on#

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