Chemical and Energy Savings by Efficient Brownstock Washing – Mill Experiences
Chemical and Energy Savings by Efficient Brownstock Washing – Mill Experiences Carlos Alberto dos Santos, Mathiesen Group, csantos@grupomathiesen.com Hannu Hämäläinen: Nopco Paper Technology, Finland, hannu.hamalainen@nopco.fi Ariel Lamonato, Mathiesen do Brazil, alamonato@grupomathiesen.com Ramon S. Dorronsoro: Nopco Paper Technology, Spain, ramon.sdorronsoro@nopco.es 7 th International Colloquium on Eucalyptus Pulp May 26-29, 2015. Vitória, Espirito Santo, Brazil.
CONTENTS • Introduction – Brownstock washing chemistry principles – Defoamer chemistry aspects – How chemistry can improve washing efficiency • Experimental • Background mill results • Results and discussion • Conclusions • References
Importance of brownstock washing (BSW) • It’s a separation process of – Pulp fibre for papermaking – Washed black liquor with chemical & energy value • It’s a heterogenous process – Solid fibre, air and liquid media • It’s a dynamic process – Countercurrent wash flow • It’s a crossroad in fibreline operations – When BSW is well managed, usually mill is running well in other sections too – When BSW is doing bad, it is negatively seen in other pulp mill sections Pikka et all Kopra et all
BSW key variables • Wood (fibre) type – Surfactant chemistry, fibre mat behaviour • Equipment design (washer setup) – DD, press, drum filter, CB, diffusers, chemiwasher • Operational parameters – Dilutions, consistencies, tank & vat levels, pressures, temperatures, etc. and process control • Chemistry – Often neglected – Washing chemistry, surfactant chemistry • Application know-how – Engineers task to improve efficiency !
Air in system – foam! • Foam is a dispersion of gas in liquid that makes a complex network of interconnected films (lamellaes) • The lamellae are connected by three and radiate 120° outward from the connection points, known as plateau borders. Foam lamellae 200x Lamellae 1 Plateu 2 1 border 1 0 2 2 ° 0 0 ° °
Conditions to create foam • Mechanical energy: – Agitation W A – Pumping – Blowing of air, etc may increase the total surface area • Surface active substances • Speed of foam formation vs break down of the foam To create foam, work (W) is needed to increase the surface area (ΔA), where γ is the surface tension.
Forces affecting foam stability 1) Viscosity 2) Electrical double layer (EDL) repulsion 3) Marangoni effects 1 3 – – – – – – – + + + + + + Lamellae Higher – – – + + viscosity + + Surfactant 2 – – – Lower + + + + + + – – – – – – – viscosity Liquid Higher viscosity Surfactant F F Liquid 1 2 1
Foam break down • Gravitation – Drainage of liquid to the foam base • Plateu border pressure – Drainage of liquid from lamellae to plateu border • Gas bubbles – Diffusion of gas from small to larger bubbles due to pressure differences • Defoamer: surface active substances that spread on the foam lamellaes and break them down
Foam break down Lamellae break down by a Lamella break down by hydrophobic sphere hydrophobic liquid AW Film AW – contact angle Film thinnin * Oil lens g thinning Thinning Filmthinning due to Droplet 1 µm lamellae * film drainage and Streching due to streching AW drainage and capilary Film Particle must have pressure H thinnin * Bridging by oil Dewetting suficient size to o g and hole bridge lamellea and droplet l Hole Collaps formation be hydrophobe Unstable if e * > 90° Lamellae break down by competing surfactant Thinning gives rupture of lamellae
Value-adding defoamer approach Maximise production rate Increase BSW efficiency Differing dosing philosophy Not to minimize dosage Silicone emulsion defoamer should be a BSW booster Allows using less wash liquid Reduce evaporation cost and bottleneck Cleaner pulp Safe to use defoamer, not adhering to pulp / surfaces Defoamer formulated to follow filtrate
Value-adding defoamer approach A successful BSW improvement program starts with: Mill targets, demands and needs Review audit of process bottlenecks, layout and operation Tailor made planning and execution of a program: Program proposal Product selection based on lab test and previous experiences Trial proposal with technical details: dosing points and dosage level, instructions of measures to be taken Technical support and monitoring during trial, as well as readiness to adjust during trial Post-trial review and evaluation of added value
Value-adding defoamer approach Value added Usual defoamer Savings $$ Chemicals Steam Chemicals Steam Lower Other Investment Other Cost Expenses Expenses
Washing Efficiency – Typically measured by COD carryover to next stage – Mills use some correlations, like COD carryover, soda loss , conductivity in filtrate leaving BSW, it depends case by case. • Lignin, sulphur compounds, carbohydrates, methanol and other materials that doesn’t relate well with washing performance and subsequent bleachability of pulp. Ala-Kaila et all – Individual washing efficiencies can be calculated – Using E10 can be compared with different consistencies (not practical) – Then, operation remain on: • COD carryover • Soda Loss carryover • Conductivity
Experimental • Industrial full-scale mill trials: – Provided state-of-art silicone defoamer and washing aid chemicals in same product – Provided application service – Each case designed to follow Clean Pulp philosophy: • Auditing mill process design and ways of operation • Technical performance • Process limitations • Laboratory screening of defoamer / washing aid • Selection of best product
Objectives • Process approach needs to take into account equipment design, operating parameters, chemistry and application know-how. The concept is centered on maximizing productivity through following measures: – Maximise production rate (through higher drainage ability and stability/robustness) – Increase BSW efficiency – Reduce bleaching chemical cost and total operation cost – Formulate defoamer NOT to produce deposits, safe operation instead – In the light of the concept, the least and last consideration is minimization of defoamer dosage. Other benefits are priorized, like: • Higher solids in black liquor to evaporation ( $$$ energy savings) • COD Carryover reduction ($$$ chlorine dioxide reduction)
Background Mill Results Mill case A • Typical kraft pulp mill, earlier optimized for lowest possible defoamer unit cost. • Periodical issues with kappa variation, • Production disturbances with abrupt stops of varying periods. • We identified the BSW was “too optimized” chemistry -wise, • We switched current silicone defoamer to another one with more robust chemistry, • Applied new dosing points alongside the existing old ones, and increased the total dosage with higher defoamer unit cost as such. • In turn, the impact on total cost and productivity was remarkable.
Mill case A Conductivity Reduction
BL dry solids increase
Less stops on BSW and higher net production
Background Mill Results Mill case B • Case B highlights another brown stock washing plant. • Improvement done by increasing drainage rate of filtrates through the washer by using proper silicone defoamer with drainage effects. • Regular defoamer product without said effect was replaced. • “Mill B” bleaching operation was influenced positively by cleaner pulp entering first bleaching plant stage. • Higher BSW efficiency was possible by higher drainage rate through the washer and presses. • Evidence of this effect was showed by higher vacuum rates through deckers, with improved discharge. • When trial stopped decker vacuum reduced instead. • Wash press operations were also benefited by higher drainage effects through BSW.
Background Mill Results Mill case B ( cont.) • Overall BSW efficiency improvement resulted in 10 to 15% lower conductivity in pulp carried out to bleaching plant. • Savings in first month operation achieved an average of 1,0 kg ClO2 / ADT, measured as chlorine dioxide. • These savings would pay out several times cost of silicone defoamer as washing aids.
Background Mill Results Mill case B ( cont.) Vacuum increase and conductivity reduction Decker Vacuum Conductivity Press #1 25000 Data 0 20000 0 50 100 150 200 250 300 350 400 -5 µS/cm Post -10 15000 Trial Post -15 bar Trial 10000 -20 -25 5000 -30 0 50 100 150 200 250 300 350 400 -35 Data Reference Transition Trial Post Trial Reference Transition Trial Post Trial
Background Mill Results Mill case B Chlorined dioxide reduction Total Chlorine Dioxide 24 Post Trial 22 20 ClO2, kg/ADT 18 16 14 12 10 0 50 100 150 200 250 300 350 400 Data Reference Transition Trial Post Trial
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