Black Liquor Evaporation3 5 Evaporation Technologies Task of the evaporator Take a waste stream (WBL) and turn it into fuel (SBL) for the recovery boiler Condense steam (or vapors)

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Black Liquor EvaporationBlack Liquor EvaporationDesign & OperationDesign & Operation

Jean-Claude PatelA.H. Lundberg Associates, Inc.

Naperville, IL

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TopicsTopics

Black Liquor PropertiesWhat did we just learn?What is critical for evaporator design?

Evaporation TechnologiesProcess Considerations at High SolidsConcentration TechnologiesMultiple Effect Evaporators

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WhatWhat’’s Black Liquor?s Black Liquor?

Complex mixtureSpent pulping chemicals (Inorganic salts, caustic, etc.)Organic matter (Lignin) dissolved from the woodNon-Process-Elements (NPE) such as K, Cl, etc.

Brought in with wood, water and fresh chemicalsNo purge points: Constantly recycled

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Black Liquor PropertiesBlack Liquor Properties

Chemical compositionMajor role on the performance of the evaporators

Na2SO4, Na2CO3 co-precipitate at high solidsRisk of scale formation

Critical physical propertiesBoiling Point Rise (BPR)Viscosity which impacts heat transfer

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Evaporation TechnologiesEvaporation Technologies

Task of the evaporatorTake a waste stream (WBL) and turn it into fuel (SBL) for the recovery boilerCondense steam (or vapors) on one side of a heating surface while boiling liquor on the other side

Process governed by the heat transfer law

Q = U x A x ΔT

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Evaporation TechnologiesEvaporation Technologies

“Q”: Heat exchange amount which can be accomplished

“A”: Heat transfer surface

“ΔT”: Temperature differentialΔT = Sat. Vapor T In – Liquor T Out

“U”: Heat transfer coefficient, a measure of the resistance to heat transfer

Depends on heating surface material & cleanlinessDepends on liquor properties & turbulence

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Rising Film Evaporator (LTV)Rising Film Evaporator (LTV)Vapor outlet

Liquor product

Condensate outlet

Vapor (steam) inlet

Liquor feed

NCG vent

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Rising Film Evaporator (LTV)Rising Film Evaporator (LTV)

Liquor film formed by generated vapors from boiling liquor at the bottom of the tubesPoor turndown, can’t handle high viscosities, minimum ΔT requirementWas the workhorse of the Industry, now found only in older mills

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Rising Film Evaporator (LTV)Rising Film Evaporator (LTV)

Low operating costLow propensity for foamingLow liquor viscosity and high flow-rate are ideal conditions

Only used today in WBL pre-evaporation where foaming is an issue:

Blow Heat Recovery

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Falling Film EvaporatorFalling Film Evaporator

Vapor (steam) inlet

Vapor outlet

Liquor feedLiquor product

Condensate outletNCG vent

Recirculatingliquor

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Film formed by mechanical means (Distribution plate)

High turndown, can handle higher viscosity (Gravity helps)

Primary technology worldwide for concentrations up to 50%TS

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PlatePlate--type FF Evaporatortype FF EvaporatorVapor outlet

Vapor inlet

Liquor feed

NCG ventLiquor product

Condensate

Distributor

Plate heating element

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Can operate at low ΔTFlexible (High turndown)Good resistance to scalingModerate HP consumptionEasily automatedFoams easily at low %TS

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Process Considerations at high solidsProcess Considerations at high solids

Precipitation of supersaturated components

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35 45 55 65 75

Dry solids content, wt-%

Wt-%

cry

stal

s in

BLS Total Na2SO4 + Na2CO3

= 8.2 wt-% of BLS

Burkeite

Dicarbonate

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Precipitation of supersaturated componentsUnits > ~ 50%TS must be designed as crystallizers

Control the precipitation processCrystals form and grow within the liquorNot as scale on the heat transfer surfaces

VS.VS.

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High liquor viscosityImpacts heat transfer due to low turbulence

Impediment to crystal growth

Pressurized storage or heat treatment needed?

Temperature becomes a critical design parameterHigh temperatures enhance hard scaling risk

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Increased corrosion tendenciesStress Corrosion Cracking in 300 series SS due to

High temperatures to control viscosityHigh alkalinity at high %TS

Duplex alloys required >75%TS

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High Solids TechnologyHigh Solids Technology

• Enhanced FC Crystallizer

• FFCrystallizer

• Switching FF Evaporator

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Falling Film ConcentratorsFalling Film Concentrators

FF heat transferEvaporation takes place at the heat transfer surface

High supersaturation developed within the liquor

Potential for excessive crystal nucleation

Risk of uncontrolled scale formation

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1st approach: FF CrystallizerKeep supersaturation low

Minimize evaporation/tubeLow Heat Flux (BTU/Sq.ft.)Large surface area High recirculation rate

High cost and HP usage

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2nd approach: Switching FF EvapsMultiple rotating bodies with one on wash at all times

As long as scale washes away faster than it forms, you stay ahead

Complex piping arrangement

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High turndown capability

Moderate HP consumption

Easily automated

On-line washing (switching type designs)

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Highly sensitive performanceLiquor chemistry changesSoap and fiber

Poor operation at high viscosityDistribution and heat transferOperation at high temperatures (Calcium scaling)Liquor Heat Treatment (Expensive)

Product %TS swings (Switching type designs)High risk of plugging (Non-switching designs)

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Forced Circulation CrystallizerForced Circulation Crystallizer

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Reynolds Enhanced Crystallizer (REX)Reynolds Enhanced Crystallizer (REX)

Spiral tube inserts disrupts the boundary layer at the tube wall, highest resistance to heat transfer

Apparent Reynolds number in the turbulent region even at high liquor viscosities

High U coefficient

Lower tube velocities

Lower HP

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Boiling suppressionNo evaporation during heat transfer very low supersaturation levels developedCrystallization point is never exceeded within the heater Eliminates uncontrolled scale formation

Liquor viscosityNot as much an issue: No film, no distribution deviceCan be operated at lower temperatures Lower risk for liquor decomposition and hard scale formation

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Excellent resistance to scalingVery infrequent washing needed, if anyHigh tolerance to liquor chemistry swingsHigh turndown capabilityModerate HP consumptionEasily automatedSimple and robust

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Multiple Effect EvaporationMultiple Effect Evaporation

100,000 lbs of steam will evaporate only 100,000 lbs of water from the liquor, if heat content is used only once

Economic operation dictates the multiple effective use of the heat content of the steam used for evaporation

Venting, radiation and other losses prevent from ever attaining full theoretical efficiency

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Single Effect OperationSingle Effect Operation116k lb/hr steam

150k lb/hr WBL 15% TS

45k lb/hr SBL 50% TS

105k lb/hr vapor

116k lb/hr condensate

Steam Economy = = = 0.9water evaporatedsteam supplied

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TwoTwo--Effect OperationEffect Operation57k lb/hrsteam

150k lb/hr15% BL

45k lb/hr50% BL

51k lb/hr vapor

111k lb/hr condensate

54k lb/hrvapor

99k lb/hr23% BL

Steam Economy = 105/57 = 1.8

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Simple SixSimple Six--Effect Evaporator SetEffect Evaporator SetSteam

WeakblackliquorProduct

liquor

Condenser

Steam Economy ~ 5

1 2 3 4 5 6

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Typical Six Effect SetTypical Six Effect Set35 psig35 psig

FeedFeedliquorliquor

ProductProduct

2525”” vacuumvacuum

To soap skimmingTo soap skimmingFrom soap skimmingFrom soap skimming

1 2 3 4 5 6

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Operating conditions in the MEE are setAvailable live steam pressure (typ. 60 psig or less)Vacuum in last effect and SC (typ. around 25” Hg)

Defines overall ΔT available Actual ΔT much lower due to BPR

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Cumulative BPR Impact on Cumulative BPR Impact on ΔT

Temp., °F Temp., °C

Steam temperature 274 134

Condenser temp. 132 56

ΔT maximum 142 79

Sum of BPR's 43.7 24.3

ΔT actual 98.3 54.6

Capacity loss 31% 31%

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Available ΔT per effectSix effect train: 98.3/6 = 16.4º FSeven effect train: 98.3/7 = 14.0º F Eight effect train: 98.3/8 = 12.3º F

Minimum ΔT required with LTV EvaporatorsBelow 13-15ºF, LTV effects “stall” and behave poorly

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Why the Why the ΔΔT/T/effect limit with effect limit with LTVsLTVs??

Need to go back to Q = U x A x ΔT

Low ΔT implies large area A (i.e. many tubes)Low evaporation per tube

Vapor generation per tube becomes too low for film formation & for making it rise to the top

Some tubes flood and “burp” at randomOthers percolate returning liquor to the bottom liquor box (i.e. Mr. Coffee)

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Rising Film (LTV) Multiple Effect EvaporationRising Film (LTV) Multiple Effect Evaporation

ΔT/effect limitSets number of LTV bodies which fit within overall ΔTSets steam economy

LTV trainsSix effects maximumFew 7 effect trains around

Unstable & very “twitchy”

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Rising Film (LTV) Multiple Effect EvaporationRising Film (LTV) Multiple Effect Evaporation

Little turndown capabilityAt low capacities, ΔT dropsbelow ΔT/effect limitMinimum operation

~ 70% of design rate

Older mills rely on several LTV sets for flexibility.

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Falling Film (FF) Multiple Effect EvaporationFalling Film (FF) Multiple Effect Evaporation

Film created by mechanical meansLiquor recirculation and distribution deviceNo minimum ΔT issues

Higher efficiencies achievable

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Falling Film (FF) Multiple Effect EvaporationFalling Film (FF) Multiple Effect Evaporation

Several 7 and 8 effect FF trains in USA, Asia, Brazil and Europe

High turndown~ 20% of design rate

Modern mills can rely on a single line of FF evaporators

Easily matches evaporation demand from production

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Integration of several evaporators (“effects”)Vapors from one effect drive the next one

Entire train performance depends on the operation of each single effect and surface condenser

“The weak link governs”

Optimizing performance will require working on one weak link and then the next one, etc.

Documents

Black Liquor Evaporation3 5 Evaporation Technologies Task of the evaporator Take a waste stream (WBL) and turn it into fuel (SBL) for the recovery boiler Condense steam (or vapors)