Stripper Info [PDF]

Zitiervorschau

Foul Condensate Stripping 2008 Kraft Recovery Short Course

Ben Lin, P.Eng. A. H. Lundberg Systems Limited Vancouver, B.C., Canada

Agenda 1. 2. 3. 4. 5.

Introduction Stripping Basics Stripping Equipment Stripper Integration Stripper Operation 2

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1. Introduction

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 5

Pollution Control

BOD Toxicity (turpenes and red oils) Odour (TRS)

(Lb/ton) 14 to 20

(Kg/tonne) 7 to 10

2 to 4

1 to 2

2 to 4

1 to 2 6

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Cluster Rule ƒ Reduce HAP by 92% ƒ Bleached mill – 10.2 lb HAP per ODTP – 330 ppm HAP in stripped condensate

ƒ Unbleached mill – 6.6 lb HAP per ODTP – 210 ppm HAP in stripped condensate

ƒ Methanol can be used as a surrogate for total HAPs 7

2. Stripping Basics

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Benefits of FC Stripping ƒ Reduced effluent and air pollutants – Remove 98+% of TRS and 95+% of methanol from foul condensate

ƒ Reduced mill water usage – Stripped condensate is relatively clean and can be reused elsewhere in the mill

ƒ Reduced fossil fuel usage – Concentrate SOG to 50 wt% methanol and deliver to incineration point for thermal destruction 9

Alternatives ƒ Hard piping to biological treatment ƒ Re-using the condensates in a process where the vents are collected and incinerated ƒ Any other process that can meet the required removal efficiencies

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Typical Condensate Sources ƒ Condensed batch blow steam ƒ Condensed batch digester relief steam ƒ Condensed continuous digester liquor flash steam ƒ Turpentine decanter underflow ƒ Evaporator combined condensates ƒ NCG system condensates 11

Foul condensate stripping ƒ Contaminants are removed from foul condensate by the process of steam distillation ƒ Most abundant and most difficult to remove is methanol, thus process is modelled as binary distillation process between methanol and water 12

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3. Stripping Equipment

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Stripping System Equipment ƒ Foul condensate tank ƒ Condensate fiber filter ƒ Condensate pre-heaters ƒ Stripping column ƒ Reboiler (optional) ƒ Reflux condenser ƒ SOG RD, ME, FA, firing nozzle 15

Stripper Feed Tank ƒ The bigger the better ƒ 30 minutes retention time minimum ƒ Several hours better ƒ 300 series stainless steel ƒ Must 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 Preheater ƒ Standard is shell and tube, with 300 series stainless steel tubes ƒ Clean condensate on tube side, foul on the shell side ƒ Spirals may be better ƒ Plate type plugs quickly unless designed for white water service ƒ Must heat condensate to within 20 °F (10 °C) of column temperature 20

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

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Stripping Column ƒ Foul condensate is introduced between the enriching and stripping sections ƒ Steam is introduced below the bottom section ƒ Steam flows countercurrent to condensate flow down, thus “stripping” of methanol and other volatile contaminants 26

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Stripping Column ƒ Steam passes up through “valves” in the trays, partially reboiling the liquid as it passes down ƒ Boiling the liquid will preferentially vapourize methanol and other volatiles 27

Stripping Column For 1000 tpd mill, – – – – – – –

Column 6’ diameter Column 60’ tall Operates at 10 psig Uses 20 valve trays 5 trays in top section 15 trays in bottom section 300 series stainless steel

(300 gpm, 1150 Lpm) (2 m) (18 m) (170 kPag)

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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 Condenser ƒ Overhead vapour is condensed in the reflux condenser and returned to the top of the column for “enriching” of the methanol content ƒ A small amount of product, Stripper Off Gas, SOG, is bled off at the reflux condenser ƒ A reflux condensate temperature 30 °F less than the underflow temperature will give 50 wt% 34

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Reflux Condenser ƒ Shell and tube heat exchanger ƒ All 300 series stainless steel ƒ Installed horizontally or vertically ƒ Coolant through tubes ƒ Uses water or weak black liquor for cooling

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Reflux Condenser

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Steam Requirement ƒ Normally use steam ratio of 0.2 pounds of steam per pound of condensate ƒ For 300 gpm (1150 l/m), this is 30,000 lb/hr (13,600 kg/hr) steam ƒ Require effective steam ratio of 0.16 to 0.18 for good efficiency 37

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 .20 .16 .12 .08 .04 0

0

20

40

60

80

Temperature Difference

100

(°F)

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Stripper Efficiency % Methanol Removal

100 80 60 40 20 0

.05

.10

.15

.20

Steam / Condensate Ratio (Effective)

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Compliance Monitoring ¾Must show that the stripper is operating with enough effective steam to ensure adequate methanol removal ¾For each stripper, develop a curve for desired efficiency at various steam ratios and delta T ¾Operate below the curve 41

Compliance Monitoring 70 60

T

50 40 30 20 10 0 .14

Must operate below the curve

.16

.18

.20

.22

.24

Steam / Condensate Ratio

.26 42

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Compliance Monitoring Simple alternate method Always operate with enough steam that you are in compliance regardless of the heat exchanger

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4. Stripper Integration

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Integrated Columns Advantages Lower Capital Cost Lower Operating Cost

Disadvantages Loss of Evaporator Efficiency 45

Integrated Columns For every K Cal lost due to integration, two or more K Cal are returned as usable 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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Partially Integrated Column Evaporator Steam

SOG

Main Section Dedicated Section Reflux No. 1 Effect Column

Operating Problems ƒ ƒ ƒ ƒ ƒ ƒ ƒ

Foaming Unstable Operation Steam Collapse Reflux Control Fiber Plugged Heat Exchanger Turpentine in Storage Tank 52

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5. Stripper Operation

Reflux Control Simple To Incineration

P

Condenser

T

Reflux

Cooling Water

Stripper

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Reflux Control Better Condenser

Cooling Water

Vapor

T

To incineration

P

Liquid

Reflux Stripper

Reflux Tank 55

Stripper Gas System Vent

PC FA Insulated Line Reflux Condenser

FC

Make up Steam

To Incineration

Purge Steam 56

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Liquid Methanol ƒ Cluster Rule now approves ƒ Many Advantages ƒ Higher Stripper uptime – Store MeOH as a liquid fuel – Measure MeOH flow for compliance monitoring

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Operating Procedures ƒ Start up on steam ƒ Add condensate slowly ƒ Make small step changes

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Emergency Shutdown ƒ Close pressure control valve ƒ Stop condensate flow ƒ Leave steam on

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Conclusion Stripping is a good method of meeting the Cluster Rules 60

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