Process water quality factors

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1 CHEM-E6125 Environmental Management in Industry Water management in industry Quality and treatment of raw and process water prof. O. Dahl Learning objectives: 1. Understand why natural waters need treatment and what the treatment quality aims are. 2. Understand the relationship between water quality and corrosivity. 3. Explain the main steps used in the chemical, mechanical and other water treatments. 4. Understand the special quality requirements of boiler waters and steam, the special treatments used, boiler water circulation and steam generation systems used. Slide 1, Quality and treatment of raw and process water Process water quality factors Factors affecting process waters are: raw water quality raw water treatment system additives used in the process (when circulated) process water circulation closure (when circulated) In general, raw water quality is as received from the watercourse or supplier but it can be modified with different raw water treatment arrangements and can be: mechanically treated chemically treated treated via ion exchange (pre-step for steam generation or special purposes) used as boiler water Condensatetes from steam (primary and secondary ones) Slide 2, Quality and treatment of raw and process water 1

2 Process water quality factors Raw water treatment objectives: water purification treatments do not cause corrosion or sedimentation in pipe work waters do not adversely affect the processes waters do not adversely affect the products. Pipe work protection factors: water bicarbonate hardness not above 2,8 dh water does not contain aggressive CO 2 water oxygen concentration is at least 6 mg O 2 /dm 3 and water velocities are sufficient to maintain oxygenation (0,5 m/s as a safeguard) water does not contain large quantities of chlorides and sulphates. Slide 3, Quality and treatment of raw and process water Classification parameters Alkalinity; how good is the buffer capacity of the water under investigation? Total Alkalinity is sum of HCO 3-, CO 3 2- and OH - -ions Natural water: CaCO 3 + H CO 2 Ca HCO 3 - Picture a water with buffering capacities (bicarbonate ion), if alkalinity is: 0 or very small, then the water s ph decreases easily on addition of acidic substances. if larger, the ph will not change by the same proportion as the H + - concentration growth (smaller) lower alkalinity is easiest for water ph control changes. system corrosion risks grow (e.g. iron solubilization). Alkalinity classification generally recommended is: 10 mg/l - 60 mg/l (0,56 mmol/l 3,33 mmol/l): normal below 10 mg/l (0,56 mmol/l): water is acidified or susceptible to acidification. over 60 mg/l (3,33 mmol/l): water is calcium enriched or overdosed. A water s corrosive nature can be roughly estimated by the alkalinity and bicarbonate hardness as an additional measure to that of the significance of ph! Slide 4, Quality and treatment of raw and process water 2

3 Classification parameters A water s hardness means its concentration of the dissolved alkali earth metals, calcium and magnesium other alkali earth metals, iron, aluminium, zinc, and copper ions have an effect on water hardness. A water with too low a total hardness can give rise to system corrosion. Very hard waters can lead to the precipitation of calcium carbonate in system pipe work. the greater the hardness of the municipal tap water supply the lower the ph has to be. Water hardness can be divided in to two groups, namely temporary, and permanent hardness. If water is boiled Ca 2+ +2HCO 3- = CaCO 3 (s)+ CO 2 (g)+ H 2 0, water loose its bi-carbonate hardness (temporary) For example CaSO 4 still remain (permanent) Slide 5, Quality and treatment of raw and process water Classification parameters Grading or classification of waters Class/grade Hardness [mmol/l] [ dh] Very soft 0 0, Soft 0,378 0, Semi-hard 0,882 1, Hard 1,764 3, Very hard 3, dh = 10 mg/dm 3 (Mg 2+ and Ca 2+ ) Slide 6, Quality and treatment of raw and process water 3

4 Classification parameters Chloride, Cl - Chlorides in surface waters are usually below 10 mg/l from mg/l corrosive action increases. Sulphate, SO 4 2- Municipal tap water is given a quality limit of below 150 mg/l Readings taken in coastal areas are usually lower. Conductivity Finnish municipal tap water is given a target of < 40 ms/m (25 C). Illustrates the concentration of dissolved salts in waters (those promoting corrosive action). Slide 7, Quality and treatment of raw and process water Classification parameters Oxygen and carbon dioxide The addition of oxygen to raw waters is undertaken with the aim of: Fe and Mg ion removal (oxidation to insoluble form) corrosion inhibition, in which one applies specified oxygen concentrations to the water in municipal supplies - to improve taste and odour aspects. In Finland the large carbon dioxide concentrations in waters can be a problem (groundwater!). CO 2 removal can be done by aeration or via the addition of chemicals (for small concentrations.). In certain cases, CO 2 also needs to be added to waters as corrosion protection. Slide 8, Quality and treatment of raw and process water 4

5 Classification parameters ph Municipal tap waters are given ph limit values: ph 6,5 9,5; and a quality objective/target of: ph 7,0 8,8. natural groundwaters and surface water ph values in Finland are generally mildly acidic, in the range ph 6 7. acidity can be due to aggressive carbonic acid. utilised materials such as cast iron, zinc steel, copper, concrete and asbestos are mostly corroded, unless waters are made alkaline. Suitable ph is in the range ph 7,0 8,8 (depending on the calcium concentration and alkalinity). in view of corrosion, the ph level should be as constant as possible Aluminium and iron hydroxide are used for electrically conditioning and precipitating out solids to obtain optimum water ph s depends on the raw water s quality, temperature and availability of precipitation chemicals for iron (Fe(III)-salts) optimum ranges are ph 4,5 5,5 and ph 8,0 8,5 Slide 9, Quality and treatment of raw and process water Corrosion inhibition Finnish groundwaters are generally soft and acidic when handling surface water precipitation, ph is always on the acid side. Whether the issue is ground water or surface water, inhibiting corrosion via water treatment methods means: raising the ph increasing alkalinity in the case of groundwaters, the Fe concentration means that aeration is also advantageous in view of corrosion. A water s quality and it s corrosivity have a clear relationship. For example iron pipe: Fe + 2H + Fe H (H will stay surface of ironpipe) This balance is greatly affected by CO 2 and O 2 concentration in water Slide 10, Quality and treatment of raw and process water 5

6 Water supply aquisition The options for the location of a treatment plant need take into account to access to fresh water. process waters can be taken from rivers and lakes. in special or emergency situations water can be taken from the local municipal supply system. The classification criteria of river and lake waters for particulate and chemical components is as follows: coarse/large impurities (>10-4 mm): soil inorganics, organic material, algae, micro organisms, wastes colloidal impurities ( mm): humus, (oil, waste leachate, rust) dissolved impurities ( mm): salts, dissolved gases and silicic acid (SIO 2 ) (various salts are ionised such as Na +, Ca 2+, K +, Mg 2+, Fe 3+, Al 3+, Cl -, SO 4 2-, CO 3 2-, HCO 3-, NO 3-, and NO 3- ) Slide 11, Quality and treatment of raw and process water Mechanically treated water Water for mechanical treatment is usually taken from rivers and lakes Utilisation aims: cooling systems use as rinse waters, for floors etc some for the actual process needs de-barking plant (far away from final products) Preparation (gross impurities): removal of the coarsest impurity fraction sieving or screening treatment large fractions can also be removed in sand filters and clarifiers Slide 12, Quality and treatment of raw and process water 6

7 Chemically treated water Chemically treated waters taken from rivers mechanical treatment is used as a first stage the central aim is to adjust the ph and hardness and remove humus Humus removal raw water is warmed to 40 C chemicals are added (rapid striring) humus forms a flaky floc mixing tank clarifier humus is blown off sand filtration to water storage reservoir and then to the hardness removal stage Slide 13, Quality and treatment of raw and process water Chemically treated water Principles Diagram Slide 14, Quality and treatment of raw and process water 7

8 Chemically treated water Flocculating chemicals: Aluminium sulphate Al 2 (SO 4 ) 3 Ferric chloride FeCl 2 ph adjustment chemicals Sodium hydroxide NaOH Sodium aluminate NaAlO 2 Slaked Lime Ca(OH) 2 Ferric chloride or aluminium sulphate react with sodium hydroxide, or lime to form a floc Chemically treated water is the most generally used type of process water in the paper industry if chemically is disinfected (O 3, Cl 2 or others) water is potable) Slide 15, Quality and treatment of raw and process water Ion exchange treated water Total salt removal or ion exchange In one or more usually several steps or sequences depends on the use quality requirements and the raw water s initial quality/impurity level Ion exchange filters: K1, K2 = weak and strong cation exchange A1, A2 = weak and strong anion exchange MB = hybrid exchangers (strong cation and anion exchangers) Slide 16, Quality and treatment of raw and process water 8

9 Ion exchange treated water Cation exchangers exchanges metal ions (Na +, Mg 2+, Ca 2+ ) H + polystyrene in resin attracts positive ions activation with dilute H 2 SO 4 - or HCl in solution, counter current or with the current Anion exchangers exchange non metal ions (Cl -, SO 4 2- ) OH - cation exchange H + and anion exchange OH - H 2 O (clean) activation via dilute NaOH in solution CO 2 -removal to reduce the strong anion exchange load Plentiful carbonates and bicarbonates in the water Slide 17, Quality and treatment of raw and process water Boiler water Boiler water is made chemically by treating waters via ion exchange. Treatment includes chemical addition as well as gas removal. Gas removal/scavenging: occurs on the tower on top of the feed water reservoir/storage. gas solubility depends on temperature and pressure (determined by the raw water s own characteristics). removal operation is based on boiling (solubility at boiling point is 0). oxygen is removed but other gases can also be removed (e.g. CO 2 ). Slide 18, Quality and treatment of raw and process water 9

10 Boiler water Chemicals added to boiler water are: N 2 H 4 (Hydrazine) chemical oxygen removal/scavenger (removes free oxygen) N 2 H 4 + O 2 2 H 2 O + N 2 (input below 200 C) N 2 H 4 4 NH 3 + N 2 (input above 200 C) Na 3 PO 4 (tri sodium phosphate) hardness removal (30 50 bar range) (to reduce calcium and magnesium content) Sodium hydroxide (caustic soda) / ammonia / cyclohexylamine / morpholine (morfoliini - tetrahydro-1,4-oxazine diethylene oximide) / butylamine / aminomethylpropanol alkalising agents (corrosion inhibitors) Slide 19, Quality and treatment of raw and process water Steam and its generation The pulp and paper industry s need for steam is mainly for 3 and 10 bar steam (basic boiler waters), which are used in: paper machine drying sections chemical pulping/digestion, mechanical pulp manufacture other processes for heating purposes In pulp mills, steam is produced mainly in recovery boiler the aim is the robust and smooth operation of the network via pressure adjustments. steam production needs to react quickly to changes in consumption demands. backup boilers are used to compensate during high load periods. steam storage and backup condensers need to equalise changes quickly. bigger boilers can also produce commercial electricity as well! Excluding the pulp industry, steam production is powered by peat, natural gas or coal (less often). Bark boilers etc. Small boilers may not be sufficient to meet the processes steam requirements. Slide 20, Quality and treatment of raw and process water 10

11 Condensate Condensate treatment objectives: to speed up purging after process changes. to remove corrosion products from the circuit. to remove condensate and heat exchanger discharge impurity effects. Condensate treatment is necessary in all industrial power plant departments. Treatments: mechanical filters (sand filtration, activated carbon filtration, candle filters, film filters) magnetic filters (separators) ion exchange Slide 21, Quality and treatment of raw and process water Municipal water supply and mechanically treated process water quality requirements Water quality parameter Chemical treatment Mechanical treatment Quality requirement Municipal supply Process water ph 6,5-9,5 - Temperature C <25 - Turbidity, FTU or NTU <4 (FTU) <100 (NTU) Colour, Pt/l <15 - Odour / taste, mg/l <2/12 C / <3/25 C - Hardness, mg/l <60 <100 Alkalinity, mg/l <30 <150 Conductivity, ms/m <40 - Active chlorine, mg/l <1,0 - Aluminium, mg/l <0,2 - Calcium, mg/l <100 - Chlorides, mg/l <100 <200 Copper, mg/l <1,0 - Manganese, mg/l <0.05 <0,5 Iron, mg/l <0,2 <1,0 Zinc, mg/l <3,0 - Sodium, mg/l <150 - Potassium, mg/l <12 - Magnesium, mg/l <50 - Slide 22, Quality and treatment of raw and process water 11

12 Municipal water supply and mechanically treated process water quality requirements Water quality parameter Chemical treatment Mechanical treatment Quality requirement Municipal supply Process water Silicates, mg/l - <100 Silver, mg/l <0,01 - Dry solids, mg/l 1500 / 180 C - Dissolved solids, mg/l - <500 TOC, mg/l <20 - COD, Cr or Mn, mg/l <3,0 (Mn) - KMnO 4 reading <12 - Ammonium, NH 4+, mg/l <0,5 - Ammonium, NH 4 -N, mg/l <0,4 - Sulphate, mg/l <150 - Phosphate, PO 4 -P, mg/l <0,1 - Chlorite, mg/l <0,2 - EDTA, μg/l <200 - Nitrogen, mg/l <150 - Dissolved hydrocarbons, mg/l <0,01 - Phenol, mg/l <0, Surface active substances mg/l <0,2 - Slide 23, Quality and treatment of raw and process water Pressure boiler (20 bar) boiler and feed water quality requirements Water quality parameter Hot water Steam (full) Boiler water: Boiler pressure, bar Boiler water: ph 25 upper limit <11 <10,5 Boiler water: ph 25 lower limit >9 >9,5 Boiler water: P value, mval/kg 8 8 Boiler water: Conductivity, γ 25, ms/m <200 <450 Boiler water: Sodium and Potassium, Na mg/kg <390 <880 Boiler water: Phosphate, PO 4, mg/kg <10 20 <10 20 Boiler water: Silicates, SiO 2, mg/kg <150 <60+6* p-value Boiler water: KMnO4, mg/kg <300 <300 Feed water: ph 25 8,5 9,5 8,5 9,5 Feed water: Oxygen, O 2, mg/kg <0,05 <0,02 Feed water: Hardness, mval/kg <0,03 <0,02 Feed water: Total Iron, mg/kg <0,05 <0,05 Feed water: Total Copper, mg/kg <0,01 <0,01 Feed water: KMnO4, mg/kg <300 <300 Feed water: Silicates, SiO 2, mg/kg <150 <60+6* p-value Feed water: Sodium and Potassium, Na mg/kg <390 <880 Feed water: Conductivity, γ 25, ms/m <200 <450 Feed water: Oil, sludge, foam, mg/l <1 <1 Slide 24, Quality and treatment of raw and process water 12