Process Water Treatment

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Transcript of Process Water Treatment

CHAPTER 1 INTRODUCTION

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1.0 INTRODUCTION[1]Industry uses water to generate steam, to cool and heat, and to fill the varied voluminous needs of process and general plant services. Estimating the true economic impact of this water involves determining not only the required flow and available supply but also quality requirements and treatment costs.

In many industrial plants, the water once used in a particular application is still of higher quality than raw water. Therefore it can be cascaded or reused in other application requiring a lesser quality, here it is also important to define water quality criteria for any given usage.

In order to improve economy of industrial water use, it becomes imperative to reduce waste water volumes, recover and reuse wastewater, which can be done effectively by FEEDWATER TREATMENT.

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1.1 FEEDWATER DEFINITIONFeedwater can be defined as the water of certain standard that is recommended to be used in process equipment and general plant services. So that equipment performs at its best.

1.2 NEED FOR FEEDWATERIndustry uses water to generate steam, to cool to heat, and to fill the varied and voluminous needs of process and general plant services. During these process impurities present in water causes problems like corrosion, scale formation, embrittlement etc. To minimize such problems it is required that raw is treated first and then allowed to enter the industrial systems.

1.3 BASIC JOBS IN TREATMENT [2]Feedwater treatment is basically done to minimize following problems:-

1. SCALE. Calcium and magnesium salts, chiefly dissolved in water, deposite on tubesurfaces as scale when water evoporates.This reduces heat transfer, causes overheating of tube metal. Scale formation is avoided by (1) removing these hardness salt before water enters boiler or (2) adding chemicals to the water to cause these solids to form a soft sludge that can be removed by blowdown.

2. CORROSION Boilers, economizer, feedheaters, piping etc. become corroded when (1)water is acidic or (2) dissolved gases are present. Prescribed remedies are (1) neutralizing water acidity with an alkali (2) removing gases by dearation,followed by chemical scavenger that absorbs remaining traces and (3) feeding a corrosion inhibitor together with organic substances to protect surface of boiler and return lines.

3. CARRYOVER Slugs of water passing over with steam cause erratic superheat,mechanical troubles with turbines. Solids passing over deposit in super heaters on turbine blades may spoil materials in process. Alkalinity too high, oil in water, suspended solids are main cause of foaming,Bowdown and antifoam agents combat carry over.3

4. EMBRITTLEMENT.Metal failure from embrittlement is along drum seams, under rivetsand at tube ends where water flashing to steam through any small leaks in these stressed areas allows any sodium hydroxide in water to concentrate.Embrittlement can be checked by maintaining low hydroxide alkalinity, avoiding leaks at stressed metal, using special inhibiting agents.

1.4 APPLICATIONS OF FEEDWATERWater has wide range of application in industries, considering chemical industrial application, feedwater is used in: BOILER FEEDWATER COOLING WATER PROCESS WATER OR DEMINERALIZED WATER

In next chapters sections preparation of these waters from raw water is discussed. But before proceeding to treatment process of feedwater, it is required that raw water characteristics are understood.

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CHAPTER 2 RAW WATER

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2.1 RAW WATER [5]Pure water (H2O) is colourless, tasteless, and odourless. Because water becomes contaminated by the substances with which it comes into contact, it is not available for use in its pure state. To some degree, water can dissolve every naturally occurring substance on the earth. Because of this property, water has been termed a "universal solvent." Although beneficial to mankind, the solvency power of water can pose a major threat to industrial equipment. Corrosion reactions cause the slow dissolution of metals by water. Deposition reactions, which produce scale on heat transfer surfaces, represent a change in the solvency power of water as its temperature is varied. The control of corrosion and scale is a major focus of water treatment technology. Constituent Turbidity Chemical Formula/description NTU(nephlometric turbidity unit) Difficulties Caused imparts unsightly appearance to water; deposits in water lines, process equipment, etc.; interferes with most process uses chief source of scale in heat exchange equipment, boilers, pipe lines, etc.; forms curds with soap, interferes with dyeing, etc. Foam and carryover of solids with steam; embrittlement of boiler steel; bicarbonate and carbonate produce CO2 in steam, a source of corrosion in condensate lines. corrosion Means of Treatment coagulation, settling, and filtration

Hardness

calcium and magnesium salts Expressed in terms of CaCO3 conc.or in ppm

softening; demineralization; internal boiler water treatment; surface active agents

Free Mineral Acid H2SO4 , HCI. etc.

Expressed in terms of CaCO3 conc.or in ppm

lime and lime-soda softening; acid treatment; hydrogen zeolite softening; demineralization by anion exchange

Free Mineral Acid

H2SO4 , HCI. etc., ppm

neutralization with alkalis

pH

pH=-log[H+]

pH varies according to pH can be increased by acidic or alkaline alkalis and decreased by solids in water; most acids6

natural waters have a pH of 6.0-8.0 SO42- , ppm

Sulphate

Chloride

Cl -,ppm

Nitrate

NO3- ,ppm

Fluoride

F- ,ppm

Sodium

Na+ ,ppm

Silica

SiO2 ,ppm

adds to solids content of water, but in itself is not usually significant, combines with calcium to form calcium sulphate scale adds to solids content and increases corrosive character of water adds to solids content, useful for control of boiler metal embrittlement cause of mottled enamel in teeth; also used for control of dental decay: not much significant industrially Adds to solids content of water: when combined with OH-, causes corrosion in boilers under certain conditions. scale in boilers and cooling water systems; insoluble turbine blade deposits due to silica vaporization

demineralization, reverse osmosis, electrodialysis, evaporation

demineralization, reverse osmosis, electrodialysis, evaporation demineralization, reverse osmosis, electrodialysis, evaporation adsorption with magnesium hydroxide, calcium phosphate, or bone black; alum coagulation demineralization, reverse osmosis, electrodialysis, evaporation

Iron

Fe2+ (ferrous) Fe3+ (ferric),ppm

Manganese

Mn2+,ppm

Discolours water on precipitation; source of deposits in water lines, boilers. etc.; interferes with dyeing, tanning, papermaking. Discolours water on aeration; coagulation and precipitation; source filtration; lime softening;7

hot and warm process removal by magnesium salts; adsorption by highly basic anion exchange resins, in conjunction with demineralization, reverse osmosis, evaporation aeration; coagulation and filtration; lime softening; cation exchange; contact filtration;

Aluminium

AI3+,ppm

Oxygen

O2,ppm

Hydrogen Sulphide Ammonia

H2S,ppm

of deposits in water lines, boilers. etc.; interferes with dyeing, tanning usually present as a result of floc carryover from clarifier; can cause deposits in cooling systems and contribute to complex boiler scales corrosion of water lines, heat exchange equipment, boilers, return lines, etc. cause of "rotten egg" odour; corrosion corrosion of copper and zinc alloys by formation of complex soluble ion high concentrations are objectionable because of process interference and as a cause of foaming in boiler deposits in heat exchange equipment, boilers, water lines, etc.

cation exchange; contact filtration;

improved clarifier and filter operation

deaeration; sodium sulphite

NH3,ppm

aeration; chlorination; highly basic anion exchange cation exchange with hydrogen zeolite; chlorination; deaeration lime softening and cation exchange by hydrogen zeolite; demineralization, reverse osmosis, electrodialysis, evaporation subsidence; filtration, usually preceded by coagulation and settling

Dissolved Solids

May contain salts in ppm

Suspended Solids

Undissolved matter in ppm

So, these are the contents in the raw water that are required to be removed to make it usable in industry.

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CHAPTER 3 BOILER FEEDWATER TREATMENT

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3.1 Boiler feedwater [4]Feedwater can be defined as the water of certain standard that is recommended to be used in process equipment and general plant services. Treatment of boiler feedwater aims at preventing scale and sludge depositions, corrosion and carryover. All natural waters contain varying amounts of suspended and dissolved matter as well as dissolved gases Impurities in water become an important consideration when water is to be used for steam generation. With the trend toward higher pressure boilers, pretreatment has become the key to successful operation of industrial power plants. Poor quality water gives poor quality steam.

3.2 Problems caused in boiler systems [3] 3.2.1 Boiler DepositsImpurities that form deposits Bicarbonates of calcium and magnesium dissolved in water break down under heat and give off car bon dioxide forming insoluble carbonates. These carbonates precipitate directly on the boiler metal and or form sludge in the water that deposits on boiler surfaces. Sulfate and silica generally precipitate directly on the boiler metal and are much harder to condition. Silica (