Cooling Water. Watertreatment and chemical conditioning of open and closed cooling systems. Dr. K. Nachstedt, Weidner Wassertechnik, Herten

Transcription

1 Cooling Water Watertreatment and chemical conditioning of open and closed cooling systems Dr. K. Nachstedt, Weidner Wassertechnik, Herten

2 Recirculation of process water volume of utilization is increasing volume of delivery is nearly constant

3 Rate of cooling water at direct wastewater disposal

4 Cooling Systems 1 Flow-through system must be close to waters no recirculation of water very large volume of water consumption marginal expenses for water treatment ecological problem: temperature rise of waters rare used at the present state of technology

5 Cooling Systems 2 closed/semi-closed circulation system typical useage: hydraulics, engine, injection moulds neglectibal loss of water very strong corrosion inhibition possible connected to heat exchangers of secondary cooling systems (refrigerating machine / cooling tower

6 Cooling Systems 3 Open evaporating cooling systems mainly used system: cooling tower target: high rate of recirculation (cycles of conc.) consumption of water: evaporaton + blowdown watertreatment is always necessary

7 Cooling Systems 4 Cooling towers more than 80% of cooling capacity: evaporation very high cooling capacity: use of latent heat of evaporation consumption of water depends directly on water treatment

8 Problems to solve Closed circuits high temperatures may cause scaling and corrosion large copper surfaces in heat exchangers differences in electrical potentials (e.g. target cooling) Open circuits (cooling towers) high concentration of minerals in circulation water deposits of calcium carbonate (scale formation) biological growth (organic contamination, favorable enviroment for microorganism)

9 Priority tasks in cooling water treatment avoidance of inorganic deposits (scale inhibition) prevention / inhibition of corrosion control of biological growth (fouling control)

10 Steps to take Cooling equipment: Careful selection of materials and -combinations Flow velocity within the range 1,5 2 m/s low temperature of water and materials < 45 C (large surfaces of heat exchangers) Water: Outer water treatment (Units and equipment) filtration, softening, desalination Internal water treatment (chemical products) Inhibitors for corrosion and scaling, antimicrobials

11 Closed / semi-closed systems (1) Usually easy to treat: small systemvolume: typically < 200 m³ no sunlight: suppression of biol. growth only small entry of pollution need only small amounts of makeup water (small costs of treatment products) Makeup water: Town water / water from wells at T< 45 C softening / desalination at T> 45 C

12 Closed / semi-closed systems (2) Chemical treatment in the past: chromates, nitrites, zinc, polyphosphate today: combinationproducts dispergators (polycarbonic acids) corrosion inhibitors (molybdates, phosphonates, organic Nheterocyclic compounds) scaling inhibitors (phosphonates, polycarbonic acids) Typical products & equipment necessary: dosing system: metering pump + water meter with impulse output treatment product: ca. 3-4 kg/m³ water controling: testkits / online-analysis / corrosion tests biological contamination (well-water) : Biocides/ UV-burner

13 Closed / semi-closed systems (3) Most common mistake of closed circuit treatment Low concentration of corrosion inhibitor (Mo > 40 ppm) too large time intervals between the control measurements

14 Closed / semi-closed systems (4) Destructed surface of cast iron: 3 weeks after setting in operation with dangerous inhibitor concentration

15 Open evaporation cooling systems (1) Water treatment can be very complicated Systemvolume range m³ Makeup-water often above 100 m³/h frequently large pre-destructed pipesystems. historical material mix (iron, steel, zinc-coatings, brass, copper, aluminium) large amounts of dirt may enter the system water is sometimes in direct contact with the product seasonal large fluctuation of biological exposure.

16 Open evaporation cooling systems (2) First priority: Prevention of deposits Economical target: maximum of concentration cycles as possible! (saving of makeup water): Exceeding the limit of solubility of inorganic compounds: calciumcarbonate deposits (lime scale) concentration of chloride content in the circulation water: promoting of corrosion (pitting at passive metals) concentration of dirt in the circulation water: sludge-deposits can cause biological problems

17 Open evaporation cooling systems (3) Target: cycle no. CN > 2,5 Volume makeup (VM) = evaporation (VE) + blow down VM = VE x (CN/(VN-1)) (Hyperbel) CN = 2 CN = 3 CN = 4 = 100% VM = 75% = 64% -25% -11% Even at small cooling towers: enormous potential to save water and money!

18 Open evaporation cooling systems (4) Practical Limits of concentration cycels: carbonate-hardness < 20 dh (tot. alkal. < 7 mmol/l) (better, if available: SI < 2,8) Chlorid concentration < 200 mg/l (danger of pitting) Is it possible to operate an open cooling system without hardness-stabilizer? 15 mg/l = ca. 0,7 dh

19 Deposits (1) Effects of deposits Corrosion and microbiol. growth at and below deposits Loss of energy (flow resistance in pipes) Faulty flow (cavitation, erosion, promotion of corrosion) Poor heat transfer in heat exchangers (increas. tranf. coeff.)

20 Ablagerungen (2) Plate of heat exchanger Pipe of cooling system

21 Products and methods of treatment (1) Basics of treatment products (state of the art) phosphonic acids and phosphonates scale inhibition corrosion protection Polycarbonic acids / polycarbonates dispersing agent / mobilisation of sludge scale inhibition organic N-heterocyclic compounds (Triazoles) corrosion inhibition of yellow metals additives for stabilisation of the formulation

22 Products and methods of treatment (2) Phosphonic acids: most effektive scale inhibitors Threshold-inbitors (nonstoichiometric application) Example: stoichiometric usage of known chelates

23 Products and methods of treatment (3) Phosphonic acids / Phosphonates Threshold-inhibition (far below stoichiometric ratios) principle of operation: blocking of crystal growth under-stoichoimetric ratio 1:<1000 underdosing: no hard deposits (easy to purge away) synergistic effect in combination with polycarbonic acids usual concentration 5-10 g/m³ makeup water Inhibition / prevention of corrosion amplification effect for other inhibitors forming of protective membranes: anodic and cathodic corrosion inhibition

24 Products and methods of treatment (4) Phosphonic acids / phosphonates Important difference: no polyphosphates! (From the past!) Easy hydrolysis! Degradation to ortho-phosphate! Phosphonic acid are stable to hydrolysis in cooling water

25 Products and methods of treatment (5) Most effective phosphonic acid: PBTC

26 Products and methods of treatment (6) Dipersing of sludges: anionic polymeres geringe Molmassen ( ) water soluble (highly negative charged) electrostatic effects mobilise sludge and slurry

27 R & D of formulations Tube-Blocking-Test / Scale inhibition test (NACE)

28 Practical application Scale inhibitors / corrosion inhibitors Dosing proportional to addition of makeup water typical dosing: 6-20 g/m³ makeup water control of product concentration: total phosphate or special polymertest (if polymers are detectable) limit of scale stabilisation: 20 dkh / 7mmol/l TA above limit of stabilisation: dosing of mineral-acid (internal decarbonisation) Ion-exchange / reverse osmosis: decarbonisation / softening / desalination

29 Biological Control Antimicrobials / biocides Not oxidizing biocides Isothiazolinones (CIT/MIT, deliv. 1,5%) DiBromNitriloPropionamid (DBNPA, deliv. 20%) Oxidizing biozides Chlorine (sodium hypochlorite) Chlor/Brom-Hydantoin (halogen donor; tablettes) Bromine (Flow through reactor with chlorine) Chlordioxid ( one component, production system) Ozon (AOX / CSB-reduction possible; expensive)

30 Applikation of antimicrobials Nonoxidizing antimicrobials always shockdosing (avoiding of resistence!) interval time after dosing: 7-14 days necessary concentration 0,1 kg / m³ circuit volume use with surfactant to combat biofilms expensive for large circuit volumes Oxidizing antimicobials continuous dosing possible may promote corrosion chlordioxide excellent for biofilms Control of treatment success: microbiological examination in regular intervals

31 Treatment of open circuits

32 Automatic system for open cooling circuits Dosing of scale inhibitors Control of concentration cycles via conductivity PH-Control and ph-regulation Treatment with microbials nonoxidizing: dosing at selectable times oxidizing: dosing depends on continuously measurement (sensor, on-line analysis) Meeting WHG Anhang 31 (luminicence test) Blocking of blow down / pre-blow-down

33 Automatic management system for cooling water Cooling tower Analytical Parameter conductivity concentration cycles total alkalinity concentration cycles ph acid dosing / decarb. total- PO4 ortho - PO4 carboxylate fluorescence water circuit Information phosphonate x F = conc. treatment prod. conc. treatment prod. T cooling capicity sensor for halogen Oxid. biozide content

34 KWMS 1: Coolingwater-Managementsystem

35 KWMS 1: maximum of information for maximum of reliability

36 Keywords: External Water Treatment If no economical internal treatment is possible: Softening of water works continuously large consumption of regeneration salt (often more than Kg/year in medium circuits) Decarbonisation (partial desalination) internal decarbonisation (decreasing ph, acid dosing) external decarbonisation (weak acid ion exchanger) Desalination Full desalination (Ion-exchanger, anionic + cathionic) Reverse Osmosis (Costs of energy! 75% yield )

37 Summary Basic schematics of open and closed circuits have been introduced. Targets of cooling water treatment in open and closed circuits were pointed out. Internal (chemical) and external (equipment based) treatment possibilities and limitations have been discussed. Problems of cooling systems and possible solutions of suitable water treatment have been shown. Modern treatment substances and their properties in formulated products have been listed and discussed. Automatic management systems or cooling water and projects of R & D have been introduced.

38 Further questions + informations Dr. K. Nachstedt Tel. 0178/ k.nachstedt@wasserchemie.de Weidner Wassertechnik GmbH Zechenstrasse Herten