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Foul Condensate Stripping

Ben Lin, P.Eng.A. H. Lundberg Systems Limited

Vancouver, B.C., Canada

2008 Kraft Recovery Short Course

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Agenda

1. Introduction2. Stripping Basics3. Stripping Equipment4. Stripper Integration5. Stripper Operation

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1. Introduction

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Introduction

Most Kraft mills have Foul Condensate Strippers in order to reduce Hazardous Air Pollutants under the Cluster Rule

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Stripping System Objectives

1. Meet environmental regulations

2. Reduce TRS emissions in effluent treatment systems

3. Produce a gas stream suitable for incineration

4. Produce relatively clean condensate suitable for reuse in the mill

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Pollution Control

1 to 22 to 4Odour(TRS)

1 to 22 to 4Toxicity(turpenes and red oils)

7 to 1014 to 20BOD(Kg/tonne)(Lb/ton)

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Cluster RuleReduce HAP by 92%Bleached mill– 10.2 lb HAP per ODTP– 330 ppm HAP in stripped condensateUnbleached mill– 6.6 lb HAP per ODTP– 210 ppm HAP in stripped condensateMethanol can be used as a surrogate for total HAPs

2. Stripping Basics

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Benefits of FC Stripping

Reduced effluent and air pollutants– Remove 98+% of TRS and 95+% of methanol from foul

condensate

Reduced mill water usage– Stripped condensate is relatively clean and can be reused

elsewhere in the mill

Reduced fossil fuel usage– Concentrate SOG to 50 wt% methanol and deliver to

incineration point for thermal destruction

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Alternatives

Hard piping to biological treatmentRe-using the condensates in a process where the vents are collected and incineratedAny other process that can meet the required removal efficiencies

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Typical Condensate Sources

Condensed batch blow steamCondensed batch digester relief steamCondensed continuous digester liquor flash steamTurpentine decanter underflowEvaporator combined condensatesNCG system condensates

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Foul condensate stripping

Contaminants are removed from foul condensate by the process of steam distillation

Most abundant and most difficult to remove is methanol, thus process is modelled as binary distillation process between methanol and water

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3. Stripping Equipment

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Stripping System Equipment

Foul condensate tankCondensate fiber filterCondensate pre-heatersStripping columnReboiler (optional)Reflux condenserSOG RD, ME, FA, firing nozzle

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Stripper Feed TankThe bigger the better30 minutes retention time minimumSeveral hours better300 series stainless steelMust be vented to CNCG

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Foul Condensate Pump

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Condensate Fiber Filter

Used to prevent plugging in downstream process equipment

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Condensate Preheater

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Condensate PreheaterStandard is shell and tube, with 300 series stainless steel tubesClean condensate on tube side, foul on the shell sideSpirals may be betterPlate type plugs quickly unless designed for white water serviceMust heat condensate to within 20 °F (10 °C) of column temperature

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Foul Condensate Preheater

Foul condensate is fed through preheaters counter-current to stripped condensate to recover heat

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Spiral Preheater

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Condensate Preheater

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Stripping Column

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Stripping ColumnStripping column has two sections of trays, the bottom “stripping” and top “enriching” section

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Stripping Column

Foul condensate is introduced between the enriching and stripping sectionsSteam is introduced below the bottom sectionSteam flows countercurrent to condensate flow down, thus “stripping”of methanol and other volatile contaminants

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Stripping Column

Steam passes up through “valves”in the trays, partially reboiling the liquid as it passes downBoiling the liquid will preferentially vapourize methanol and other volatiles

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Stripping Column

For 1000 tpd mill, (300 gpm, 1150 Lpm)

– Column 6’ diameter (2 m)– Column 60’ tall (18 m)– Operates at 10 psig (170 kPag)– Uses 20 valve trays– 5 trays in top section– 15 trays in bottom section– 300 series stainless steel

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Stripping Column

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Reboiler

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SOV line from stripper

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SOV line from stripper

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Reflux CondenserOverhead vapour is condensed in the reflux condenser and returned to the top of the column for “enriching” of the methanol contentA small amount of product, Stripper Off Gas, SOG, is bled off at the reflux condenserA reflux condensate temperature 30 °F less than the underflow temperature will give 50 wt%

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Reflux CondenserShell and tube heat exchangerAll 300 series stainless steelInstalled horizontally or verticallyCoolant through tubesUses water or weak black liquor for cooling

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Reflux Condenser

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Steam RequirementNormally use steam ratio of 0.2 pounds of steam per pound of condensateFor 300 gpm (1150 l/m),this is 30,000 lb/hr (13,600 kg/hr) steamRequire effective steam ratio of 0.16 to 0.18 for good efficiency

Steam Requirement

Some steam is condensed to bring the incoming condensate up to the column operating temperature

What is left is the EFFECTIVE STEAM

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Effective Steam

0 20 40 60 80 100Temperature Difference (°F)

.20

0

.04

.08

.12

.16%

Met

hano

l Rem

oval

Steam / Condensate Ratio(Effective)

.05 .10 .15 .20

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40

60

80

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Stripper Efficiency

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Compliance MonitoringMust show that the stripper is

operating with enough effective steam to ensure adequate methanol removal

For each stripper, develop a curve for desired efficiency at various steam ratios and delta T

Operate below the curve

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Compliance Monitoring

T

Steam / Condensate Ratio

010203040506070

.14 .16 .18 .20 .22 .24 .26

Must operate below the curve

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Compliance Monitoring

Simple alternate method

Always operate with enough steamthat you are in compliance

regardless of the heat exchanger

4. Stripper Integration

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Integrated Columns

AdvantagesAdvantages

DisadvantagesDisadvantages

Loss of EvaporatorEfficiency

Lower Capital CostLower Operating Cost

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Integrated Columns

For every K Cal lost due to integration, two or more K Cal are returned asusable fuel

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Fully Integrated Column

#1 #2

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Fully Integrated Column

#1 #2

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Fully Integrated Column

#1 #2

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Fully Integrated Column

#1 #2

SOG

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Evaporator Steam

Main Section

Reflux

Partially Integrated Column

Dedicated Section

Column

No. 1 Effect

SOG

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Operating ProblemsFoamingUnstable OperationSteam CollapseReflux ControlFiberPlugged Heat ExchangerTurpentine in Storage Tank

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5. Stripper Operation

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Stripper

Condenser

RefluxT

P

To Incineration

Cooling Water

Reflux ControlSimple

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Reflux ControlBetter

Stripper

To incineration

Cooling Water

T

P

Reflux

Reflux Tank

Liquid

Vapor

Condenser

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Stripper Gas System

PC

FCRefluxCondenser

Make upSteam

InsulatedLine

Vent

PurgeSteam

FA

ToIncineration

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Liquid Methanol

Cluster Rule now approvesMany AdvantagesHigher Stripper uptime– Store MeOH as a liquid fuel– Measure MeOH flow for

compliance monitoring

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Operating Procedures

Start up on steamAdd condensate slowlyMake small step changes

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Emergency Shutdown

Close pressure control valveStop condensate flowLeave steam on

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Conclusion

Stripping is a good method of meeting the

Cluster Rules