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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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Falling Film EvaporatorFalling Film Evaporator
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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Falling Film EvaporatorFalling Film Evaporator
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
0123
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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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Process Considerations at high solidsProcess Considerations at high solids
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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Process Considerations at high solidsProcess Considerations at high solids
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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Process Considerations at high solidsProcess Considerations at high solids
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
2020
Falling Film ConcentratorsFalling Film Concentrators
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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Falling Film ConcentratorsFalling Film Concentrators
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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Falling Film ConcentratorsFalling Film Concentrators
High turndown capability
Moderate HP consumption
Easily automated
On-line washing (switching type designs)
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Falling Film ConcentratorsFalling Film Concentrators
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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Forced Circulation CrystallizerForced Circulation Crystallizer
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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Forced Circulation CrystallizerForced Circulation Crystallizer
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
105116
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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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Multiple Effect EvaporationMultiple Effect Evaporation
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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Multiple Effect EvaporationMultiple Effect Evaporation
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)
1919
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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.
2020
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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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Multiple Effect EvaporationMultiple Effect Evaporation
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.