technical literature droplet separation - · pdf fileabout 1 - 5 weight-% the disperse phase...

TechnicalLiterature

DROPLETSEPARATION

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1 Droplet Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2 Separation mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.2.1 Separation by Inertia . . . . . . . . . . . . . . . . . . . . . . 2 2.2.2 Separation by Interception . . . . . . . . . . . . . . . . . . . . 2

2.2.3 Separation by Diffusion . . . . . . . . . . . . . . . . . . . . . 2

3 Design of Knitted Wire Mesh Droplet Separators . . . . . . . . . . . . . 33.1 Gas Flow Velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . 33.2 Flooding Point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43.3 Pressure Drop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43.4 Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

4 Agglomeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

5 Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

6 Assembly instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

7 Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

8 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

9 Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

10 Materials and Sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

11 RHODIUS Standard Types . . . . . . . . . . . . . . . . . . . . . . . . . 9

12 Questionnaire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

13 Pressure Drop Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

14 Efficiency Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

15 RHODIUS Standard Designs . . . . . . . . . . . . . . . . . . . . . . . . 19

16 Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Technical Literature Droplet Separation

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1 Introduction

Gas and vapour streams are very important inchemical process technology. Differentprocess steps necessitate generation, cleaningand separation of this streams. An enrichmentof gas streams with liquids can be reachedboth through mechanical or thermal dropletgeneration - as e.g. in scrubbers andabsorption columns - as well as throughphysical-chemical reactions (condensation).Certain process runs then require a separationof liquid portions from the gas or vapourstream. Different systems are used dependingon liquid amount, droplet size and requiredpurity. Beneath cyclones and impact platesmainly droplet separators made of knittedwire mesh are employed. With littleexpenditure of energy (low pressure drop)finest droplets with diameters of nearly 1 µmcan be separated and efficiencies up to 99.9 %can be achieved.

Therefore separation problems with gas-liquid-separation are solved economically andcost-saving.

2 Fundamentals

A knitted wire mesh droplet separator is anindustrial instrumentation which retainsdroplets carried by a gas or vapour stream, i.e.which effects a phase separation between gasand liquid stream. Droplet separators arepredominantly used for exhaust airdecontamination. Besides liquid dropletscarried in process gas streams have to beseparated, too, as they could cause damage onthe instrumentation due to corrosion orerosion or due to depositing, caking andproduct contamination.The efficiency of a droplet separator can becharacterized with its fraction efficiencycurve. The required expenditure of theseparation is crucially governed by the feed

droplet spectrum and the specified size limitof the droplets.

2.1 Droplet Size

The size of the droplets decisively depends ontheir kind of origin and their prehistory. Twoprincipal mechanisms are responsible for theirformation: mechanical generation as well ascondensation.A rough distinction of the droplet size can bemade to the effect, that droplets bigger than10 µm are called spray and smaller ones arecalled mist or aerosoles. Spray is mainlyformed when liquids are atomized and thedroplet spectrum is the finer the more energyis put into the atomization process.

0,1 1,0 10 100 1000

Condensate of saturated steam

Sulphuric acid mist

Oil mist (sprayed)

Droplet entrainment in evaporators

Paper Filters

Cyclones under 1000 mm ø

Droplet size (µm)

RHODIUS-Droplet Separators

Applications for Separators

Size range of liquid droplets

Fig. 1: Applications for separators and characteristic particle sizes.

Aerosoles are mainly generated bycondensation of saturated steam and occur asmist in chemical reactions of gas mixtures,e.g. during the formation of liquid sulphuricacid through gaseous SO3 and H2O.Beneath the formation of the droplets also thephysical properties of the fluids are veryimportant. A low surface tension favours theformation of small droplets, a high viscosityon the other hand favours the formation oflarge droplets.


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The liquid droplets in a gas flow normallyhave different sizes. The distribution of thedroplet sizes is similar to the normaldistribution by Gauss.

2.2 Separation Mechanisms

The separation of droplets bigger than 30 µmnormally causes no problems, as an inertiaseparation can easily be carried out due to therelatively large mass of drops. Therefore theapplication of RHODIUS knitted wire meshdroplet separators predominantly yieldsdroplet size ranges smaller than 30 µm.

In this droplet size range and a liquid load ofabout 1 - 5 weight-% the disperse phasefollows the streamlines, so that the gas flowitself is not influenced.The separation of liquid droplets is based onthe effect, that the particles can not follow thestreamlines of the gas when they hit anobstacle and stick to a periphery.

In principle three separation mechanisms canbe distinguished, where the limits between thesingle mechanisms are not defined exactly.The separation always has to be seen as thesum of single effects.

2.2.1 Separation by Inertia

In Fig. 2 separation by inertia is shownschematically.

Fig. 2: Separation by inertia

Every single wire in a knitted wire meshdroplet separator is an obstacle in the gasflow, therefore a deviation of the streamlinestakes place. Entrained droplets can not followthis deviation due to their inertia and hit theobstacle. This effect is mainly relevant fordroplets bigger than 10 µm.At the wires the single droplets grow together(coalesce) to bigger drops, then they form aliquid film on the wire surface and fall downdue to gravity where the liquid is drawn off .The efficiency of the droplet separationincreases with an increasing number ofdeviations.

2.2.2 Separation by Interception

The separation by interception is significantlyimportant, as soon as the diameter of a dropletis relatively large compared to the diameter ofa fibre (wire).With respect to the direction of the gas flowthe interception thus occurs in the boundaryzone of the fibre. The separation of dropletsby using knitted wire mesh is substantiallyaffected by this interception, in addition to theinertia.

2.2.3 Separation by Diffusion

For droplets of submicron size separation byinertia as well as the effect of interceptionbecome negligible. Droplets smaller than 1µm are separated by the Brownian movement.This is a continuous stochastic movement ofparticles which is caused by collisions withgas molecules. This particle movementincreases with decreasing particle size. Aparticle with a diameter of 0.1 µm is subjectto about 5 times the Brownian movement of aparticle with a diameter of 1 µm.The probabil ity of the particles to collide witha fibre and to be separated rises withincreasing Brownian movement. Thisseparation process is called diffusive


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separation which is shown in Fig. 3schematically.

Fig. 3: Diffusive separation

A presentation of diffusive and inertiaseparation dependant on gas velocity anddroplet size is shown in Fig. 4.

While separation by inertia increases withincreasing droplet size and gas velocity, theinfluence of diffusive separation decreaseswith respect to these parameters.There is a transitional region (0,2 - 0,7 µm) inwhich the separation efficiency reaches aminimum. In this range the effect ofinterception is predominant and therefore highefficiencies can be achieved with knitted wiremesh droplet separators.

Eff

icie

ncy

Droplet sizeGas velocity

Minimum efficiency

Separation by diffusion

Separation by inertia

Fig. 4: Effect of separation mechanisms on the efficiency.

3 Design of Droplet Separators

Knitted wire mesh droplet separators for gas-liquid-separation are designed computeraided. The optimum calculation yields acomplete separation of the liquid phase fromthe gas flow. Therefore the followingparameters have to be taken into account.• gas flow velocity• permissible pressure drop• required efficiency• liquid amount to be separated

RHODIUS produces the appropriate dropletseparator for every special application. Forthis purpose a large variety of materials isavailable to facilitate manufacturing to order.A selection of RHODIUS standard dropletseparators is shown on page 9.

3.1 Gas Flow Velocity

The maximum gas flow velocity refers to thatoperating point of the droplet separator wherethe knitted wire mesh package is flooded anda liquid entrainment of mostly agglomerateddrops occurs. Therefore the operating pointhas to be below the flooding point.For calculating the maximum gas flowvelocity umax a variety of parameters have tobe taken into account, e.g. gas and liquiddensity and the surface tension of the liquid tobe separated.The following simplified formula can beused:

u K Fl G

Gmax = ⋅

−ρ ρρ

where:

umax [m/s] maximum gas flow velocityρFl [kg/m3] liquid densityρG [kg/m3] gas densityK [-] constant: 0,04 - 0,15


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The maximum permissible velocity excludesthe formation of secondary drops due toimpingement on the fibres and avoidsflooding of the knitted wire mesh package.The design velocity is about 0,75 ⋅ umax.

3.2 Flooding Point

The determination of the flooding limit ofdroplet separators has to be taken into accountin the stage of design in order to ensurefaultless function of the droplet separator. InFig. 5 the design curve of knitted wire meshas well as its flooding limit is shown [1].

0,0001 0,001 0,01 0,1

X

0,01

0,1

1Y

Design curve forknitted wire mesh

Tower packing

Flooding limit ofknitted wiremesh

Fig. 5: Calculation diagram for designing knitted wire mesh droplet separators

where:

XL

GG

L

= ⋅ρρ

( )Y

u a

gG L

L

=⋅ ⋅ ⋅

⋅ ⋅

2 0 2

3

ρ µε ρ

,

L [kg/h] liquid flow rateG [kg/h] gas flow rateρL [kg/m3] li quid densityρG [kg/m3] gas densityu [m/s] velocity across an

unobstructed cross sectiona [m2/m3] surface area of the knitted

wire mesh packageµL [cP] viscosity of the liquidg [m/s2] acceleration due to gravityε [-] porosity

3.3 Pressure Drop

The pressure drop of knitted wire meshdroplet separators is very low due to the largefree volumes even at higher velocities. It risesalmost proportional with the thickness of thepackage and acts nearly proportional to itsdensity (with the same wire diameter andknitted wire mesh specification). Liquid load,viscosity, wetting behaviour of the liquid, aswell as the contamination level of the gasstream (solid particles) have a stronginfluence on the pressure drop.

Saemundsson gives a theoretical pressuredrop calculation for pouring knitted wiremesh [2]. This relation is valid for drypackages and takes all relevant parameters ofdifferent knitted wire mesh specifications intoaccount (e.g. wire diameter and porosity). Thecalculation of the pressure loss coeff icientwas modified through the empiricalformulations g and f. This new equationconfirms the measured pressure drops (drymeasurements) of stainless steel typesrelatively exact shown on page 11/12.

This relation is:

( )∆ p

H

R

u

h

L= ⋅ ⋅ ⋅−

ζρ

α2 1

2

2

with:

RD

hF= − ⋅1

4

αα

hydraulic radius

ζ = +g f

Re Re ,0 2pressure loss coefficient

Re =−

⋅ ⋅ ⋅u

RhL

L14

αρµ

Reynolds’ number

where are: g = -1,56 * p + 771,2 f = -0,0038 * p + 2,72

for p ≤ 300.


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D [m] wire diameterH [m] thickness of the packingα [-] packing density (1-ε)µ [Pas] dynamic viscosityρ [kg/m3] gas density

Index: L referring to gas (air)F referring to wire (fibre)

3.4 Efficiency

Below the flooding limit efficiency increaseswith increasing gas flow velocity. At the sametime the pressure drop rises square whatcauses higher investments and essentiallyhigher operating costs. Therefore each plantoperator has to find out the optimum pointbetween high separation efficiency andeconomic efficiency. The evaluation will turnout in favour of eff iciency or low operatingcosts depending on the kind of application.

Proper design of the RHODIUS dropletseparators yields separation eff iciencies up to99.9 %. Separation eff iciencies always haveto be seen in connection with the size limit ofthe droplets.Therefore RHODIUS always specifiesseparation efficiencies with the correspondingsize limit, e.g. efficiency 99.9 % for droplets≥ 5 µm. The efficiency curves shown on page15 ff. are based on the theoretical calculationof Bürkholz [1] and depend on the measuredpressure drops.

4 Agglomeration

In order to achieve high eff iciencies fordroplet spectrums in the range of a fewmicrons either an increase of the gas flowvelocity or the use of a two-stage separator isnecessary. This construction ensures highoperational flexibil ity at a comparatively lowpressure drop.

The first stage acts as an agglomerator.Increasing the gas flow velocity (e.g. byreducing the gas flow area) as well as chosingappropriate packages ensures that the packingwill be flooded. In this process it has to betaken into account that the stage ofcoalescence is run with sufficient liquid. Ifnecessary, a part of the separated liquid canbe sprayed before the first stage.In the first stage an agglomeration of verysmall droplets into larger ones takes place,and these are subsequently separated withoutany problems in a second stage that is runwith lower velocity.An additional - often desirable - effect comeswith the use of an agglomerator, namely theliquid column forming in the packagefacil itates a post-absorption of gaseoushazardous substances.

5 Construction

The design of the droplet separator accordingto the required operational conditions leads toan exact definition of the knitted wire meshtype. However, additional manufacturingdetails must be determined. Depending on thespecific requirement, the knitted wire meshmay be finished in the form of a roll (wrappedroll) or as a combination of layers (knittedwire mesh mats). The cutting edges stemmingfrom the fabrication of a package are providedwith a framing. In order to ensure an optimumpress fit with the installation, the dropletseparator is manufactured in oversize relativeto the inside dimensions of the vessel.The data sheets in the appendix show furtherdetails with regard to installation, grids andsupport constructions as well as subdivisionof the droplet separators into differentsegments.

A great variety of materials and an ownengine and tool construction enablesRHODIUS to match her knitted wire meshdroplet separators to almost every application.


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The orders of magnitude for dropletseparators range from small pressed parts of afew centimeters in diameter and thickness toconstructions with diameters up to 10 metres.

Please notice, however, that RHODIUS doesnot produce vessels or complete solutions.

6 Assembly instructions

The single segments of the droplet separatorshould be arranged according to thepresentation on page 19 or the attacheddetailed drawing, respectively.Before installing the segments you shouldcheck which manner of fastening isprescribed. Some possible fastening methodsare shown on page 27.In case of fastening with stud bolts (see page27 figure C) these have to be fixed to thesupporting structure according to the drawingbefore putting these segments in place.Installation of the single segments is carriedout crosswise to the support bars. The outersegments are installed first then the inner onesfollow, i.e. the middle segment is the one tobe installed at last. As for sealing purposes thesingle segments are oversized they have to belaid tightly together. For pushing in the lastsegment metal plates are used which are laidleft and right to the segments alreadyinstalled. Then you have to remove the metalplates and check the sealing effect of thesegments.Now the segments can be fastened to thesupporting structure as prescribed.

7 Maintenance

Due to their high porosity of 89 to 99 %droplet separators are relatively insensitive tosoiling. Under normal operating conditionswith sufficient high liquid flow the dropletseparator cleans automatically by itself.

Solids are washed out by the liquid flow. Incase of less liquid flow and high solidconcentration it is advisable to install ascrubber in front of the knitted wire meshdroplet separator.However, in the event of deposits or caking inthe knitted wire mesh package, it can becleaned by jets of water, vapour, or dilutedbases or acids. This treatment must bechemically compatible with the materialsinvolved.

The cleaning may be done within the vesselwith an equipment already installed (counterstream rinsing equipment) or externally.For designing the cleaning equipment thekind and quantity of the pollution has to betaken into account.

RHODIUS suggests the following standardvalues:

• quantity of water: 20-80 l/m2min• jetting time: 5-10 min• distance of the nozzles 300-500 mm• distance nozzles - wire mesh: 300-500 mm• jetting admission pressure: approx. 3 bar

8 Applications

From the many possible applications ofRHODIUS droplet separators a few may belisted here:

• Evaporators To avoid entrainment and to improve product purity

• Absorption- and destillation columnsIncrease of flow rates and product purity at the same time

• Vacuum- and compressed air systemsSeparation of the condensate generated

• Oil mist separatorWaste air abatement and recovery of oils and lubricants


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• Fat filters / fatty acid systems Separation of fatty acids

• Paint shopsSeparation of lacquer particles

• Sulphuric acid plantsSeparation of sulphuric acid mist

• Air conditioning and waste air systemsSeparation of liquid and solid particles

• Cooling towersRetaining aerosoles

• Seawater desalination plantssee evaporators

9 Literature

[1] Bürkholz, ArminDroplet SeparationVCH-Verlag, Weinheim, 1989

[2] Saemundsson, Helgi B.Abscheidung von Öltropfen ausströmender Luft mit Draht-

gestrickpaketenVerfahrenstechnik 2, Nr. 11(1968), S. 480-486

[3] Bulag, S.Hochwirksame Tropfenabscheiderbei der RauchgasreinigungChemische Industrie, Jan. 1983

[4] Fritz, W.; Kern, H.Reinigung von AbgasenVogel Verlag, Würzburg, 1990

[5] Riemer, H.Abscheidung von Nebeln, Spraysund löslichen Feststoffen ausGasströmenCAV, Mai 1979


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10 Materials and Sizes

Metals Synthetics Glas

• All usual stainless and acid resistant steels (SS)• Special materials: - Monel * - Inconel * - Incoloy * - Titanium - Copper - Aluminium - Brass - Galvanized steel * Trade Mark

• PE (Polyethylen)• PP (Polypropylen)• PVC (Polyvinylchlorid)• PVDF (Polyvinylidenfluorid)• ETFE (modif. PTFE)*• PES (Polyester)

* Trade Mark: Hostaflon (HM)

• Glas staple fibre (GSF)• Glas silk

Wire:∅ 0,05 - 0,50 mmStandard:∅ 0,12 / 0,28 mm

Monofilament:∅ 0,22 - 0,60 mmStandard:∅ 0,27 / 0,40 mmMultifilaments of PP and PES

Single fibre:∅ ca. 0,01 mm

Droplet separator sizes:

• Diameter: any size in the range of [mm] bis [m].• Thickness: any thickness at all. Standard: 25 mm, 50 mm, 100 mm, 150 mm, 200 mm

The contents of this documentation are based on the present knowledge of RHODIUS.They do not imply any guaranty of properties. (December 1995, rev. 2001)


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Problem Description

Application

Flow direction vertical horizontal

Existing dimensions [mm]

Required efficiency [%] Droplet size limit [µm]

Max. permissible pressure drop [mbar, Pa]

Calculation to existing dimensions max. eff iciency

Operational Data

Operational pressure [bar absolute] Temperature [°C]

Gas / vapour density [kg/m3] or molecular weight

Flow rate [kg/h, Nm3/h, m3/h]

Kind and quantity of the liquid to be separated [kg/h]

Liquid density [kg/m3] Liquid viscosity [m Pa s/ c Poise]

Construction

Required design square rectangular circular

knitted wire mesh pad with grids

with U-frame with welded mesh

with expanded metal

Material

Rhodius GmbH, Treuchtlinger Straße 23D-91781 Weißenburg i. Bayern,

Tel. 0049-9141/919-0, Fax 0049-9141/919-45

Company:

Name:

Tel.:Fax:


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Rhodius GmbHTreuchtlinger Straße 23

D-91781 Weißenburg/Bay.Tel.: + 49 (0) 9141/919-0Fax: +49 (0) 9141/919-45

[email protected]

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