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1、1,水的基本化学性质,BASIC WATER CHEMISTRY,2,目的,过程,获益,目的:了解水的基本化学性质过程:By studying basic elements of water chemistry such as units,equivalent weight,CaCO3 equivalents,acidity,alkalinity(P,M and BaCl2),pH,SiO2 control in boilers,corrosion and scaling indices,and by testing knowledge with a quiz,3,Purpose,Proces
2、s,Pay-off,contd,Pay-off:A knowledge of water chemistry is the basis of our business;without it we cannot claim to be a water treatment“on-site expert”,4,Water,the Universal Solvent,The polar charges on the water molecule make it an exceptional solvent:,O-2,H+,H+,d+,d-,5,HYDROLOGICAL CYCLE,Ca.Mg,SiO2
3、,Cl,SO4,SS,CO2,O2,organisms,colour(organics),Micro-,6,Expression of Results,%:used for concentrated solutionsppm:a weight relationship(one part/106 parts)ppb=ppm x 1000ppm as CaCO3:an equivalent weight relationshipmg/l=ppm in dilute waters epm=equivalents per million=mequiv/l=ppm/equivalent weight,7
4、,Equivalent Weight,Equivalent weight=weight required to combine with or replace 1 gm hydrogenor=gm molecular weight divided by the total charge on cations or anions when substance dissociates in solution,8,Example,Calcium phosphate:Ca3(PO4)2 3Ca2+2PO4-3 or.Ca3(PO4)2+6H+3Ca2+2H3PO4 Molecular Weight=(
5、3)(Ca)+(2)(PO4)=(3)(40)+(2)(95)=310Equivalent weight=310/6=51.7,9,Equivalent in Terms of CaCO3,CaCO3 selected as the“unit of exchange”because ithas a MW=100 and an equivalent weight=50.This allows direct subtraction and addition,e.g.,Total Hardness(TH)=20 ppm as CaCO3(by titrn)Calcium Hardness(CaH)=
6、8 ppm as CaCO3(by titrn),or 3.2 ppm as Ca(by AA,for example)Hence MgH=20-8=12 ppm as CaCO3(simple)but MgH=?!if Ca expressed as Ca,10,Equivalent in Terms of CaCO3,contd,CaCO3 equivalent=weight CaCO3 chemically equivalent to the amount of material present equiv.wt.CaCO3(=50)=(ppm as substance)x-equiv.
7、wt.substance,11,Example,Al2O3Al2O3+3H2O-2Al3+6OH-MW=(2)(27)+(3)(16)=102Equiv.wt.=102/6=17CaCO3 equiv.=ppm as Al2O3 x(50/17)=ppm as Al2O3 x 2.94so 100 ppm as Al2O3=294 ppm of Al2O3 as CaCO3,12,Workshop Exercise,A solution contains 50 ppm of Al2(SO4)3.14H2O What is the concentration expressed as CaCO3
8、?Molecular weights are in Appendix 1.(Groups of 2/Time=5 mins),13,Definition and Units of Conductivity,Conductivity or specific conductance 1=-mhos/m Resistivity in ohm.mwhere mho=(ohm)-1The SI unit for mho=Siemens(S),14,Commonly Used Units of Conductivity and Their Relationships,-mmhos/cm-mmhos(=mm
9、hos/cm)-mS/cm(=mmhos/cm)-mS/m(=100 x mS/cm)-mS/m(SI standard)=0.001 x mS/m=0.1 x mS/cm-TDS(ppm)(0.7)(conductivity in mmhos/cm),at 20oC and neutral pH for typical raw,boiler and cooling waters,15,Effect of Temperature on Conductivity,ctc25=-1+0.02(t-25)where c25=conductivity at 25oC ct=conductivity a
10、t toCTherefore,c20=0.9c25 and TDS 0.63c25,16,Definition of Acidity,Free Mineral Acidity(FMA):exists at pH 4.3(dueto strong acids like H2SO4,HNO3,HCl).Total Acidity:exists up to a pH=8.3,17,Definition of Alkalinity,Alkalinity is the capacity to neutralize acid and isequal to the total amount of HCO3-
11、,CO32-,and OH-Alkalinity exists above a pH=4.3,and total alkalinity=the alkalinity that exists down to themethyl orange or screened indicator change point,i.e.,=HCO3-+CO32-+OH-(see next slide).For thisreason,the total alkalinity is also called the“M”(for methyl orange)alkalinity.,18,Effect of pH on
12、Alkalinity and Acidity,pH,MO ENDPOINT,PHENOLPHTH.END POINT,HCO3-,-,0 2 4 6 8 10 12 14,19,P Alkalinity,OH-+H+-H2OCO32-+H+-HCO3-(60)(1.67)+(1)(50)-(61)(0.82)expressed as CaCO3or 100+50-50 Hence,P=all OH-+1/2 CO32-,20,M Alkalinity or Total Alkalinity,As for P alkalinity plus.HCO3-+H+-H2O+CO2i.e.M=OH-+H
13、CO3-+CO32-and 2P-M=2(OH-+1/2CO32-)-(OH-+HCO3-+CO32-)=OH-HCO3-which becomes.OH-(2P-M 0)andHCO3-(2P-M 0),21,P(BaCl2)Alkalinity in Boiler Water,BaCl2+,3MgO.2SiO2.2H2O,(Serpentine),Ca3(PO4)2,CO3 Cl SO4,SiO3 PO4 OH,TP polymer,Ba(OH)2,BaCl2,TP polymer,Free causticalkalinity byacid-base titn,BaCO3BaSO4BaSi
14、O3Ba3(PO4)2SerpentineCa3(PO4)2,*,*NB:Do not filter,as CO2 pick-up will occur!,22,P(BaCl2),contd,TP polymer has COO-groups which will titrate asM alkalinity,reducing OH-alkalinity,but this is small enough to ignore in most cases(10%).P(BaCl2)=free caustic alkalinity(C.A.)=OHt-OHs-where OHt-=total C.A
15、.before serpentine production OHs-=C.A.consumed in producing serpentine(This concept is analogous to free and total chlorine),23,pH-Its Estimation in CW Systems,pH=-log10(H+concentration,moles/litre)(1)Yun(Pacific Corrosion 87)pH=0.094c+0.204pHmu+0.0018Mmu+6.78where c=cycles(1.3-4.9)M=alkalinity(CaC
16、O3)mu=make-up waterfor CaH=7-60,pH=6.3-8.3 in make-up water,24,Estimation of pH,contd,(2)Nalco DT Manual(1982)M=cycled up M-alk=(c)(Mmu)M 200 ppm:pH=7.5+log10(0.3M-60)*NB:30/300 Rule-CW:Palk=0.3M-60.for M 200,25,Estimation of pH,contd,(3)Boroughs et al:IWC Paper,1981:Discussion by D.J.Goldstein et a
17、lpH=4.4+1.6log10(M).Kunz,Yen&HesspH=4.5+1.43log10(M).Ionac Chemical Co.The Nalco T equations on the previous slide werean improvement on these equations.,26,Estimation of pH,contd,Adjustment of pH with AcidAcid addition actually drops the M alkalinity,as follows:1 ppm 100%H2SO4=1.02 ppm as CaCO3(ref
18、er to Appendix 1 in your notes)1 ppm 98%H2SO4=(0.98)(1.02)=1.0 ppm as CaCO3Hence 1 ppm 98%H2SO4 will drop M alk.by 1 ppm as CaCO3Then use the Yun or DT models to predict the new pH.,27,Estimation of pH,contd-Workshop Exercise,A make-up water for an open cooling system has thefollowing analysis:M=60
19、ppm as CaCO3 pH=7.8CaH=20 ppm as CaCO3Compare the cycled up pH at 4 cycles using theYun and DT methods.(Groups of 2/Time=5 mins),28,Alkalinity for Silica Control in Boilers,3Mg2+6OH-3Mg(OH)2(3)(24.3)+(6)(17)3Mg(OH)2+2SiO2-3MgO.2SiO2.2H2O+H2O(2)(60)Serpentine(insol.)2Na+2OH-2NaOH(2)(17)2NaOH+SiO2-Na2
20、O.SiO2+H2O*(60)Sodium Silicate(sol.)(*For softened or demin FW or for SiO2 left after serpentine formation),29,Alkalinity for Silica Control,contd,(1)Sodium Silicate FormationOH-(as OH)(2)(17)OH-(as CaCO3)(2)(17)(2.94)-=-or-=-SiO2(as SiO2)60 SiO2(as SiO2)60=1.67(2)Magnesium Silicate(Serpentine)Forma
21、tion OH-(as OH)(6)(17)OH-(as CaCO3)(6)(17)(2.94)-=-or-=-SiO2(as SiO2)(2)(60)SiO2(as SiO2)(2)(60)=2.50,30,P(BaCl2)for SiO2 Control in Boilers,(1)Serpentine FormationMg(as CaCO3)(3)(24.3)(4.10)-=-=2.5 SiO2(as SiO2)(2)(60)or SiO2 reacted to serpentine=Mg/2.5(2)Sodium Silicate FormationSiO2 to be contro
22、lled as sodium silicate=SiO2-(Mg/2.5)or OHt-OHs-=P(BaCl2)1.67SiO2-(Mg/2.5),31,P(BaCl2)for SiO2 Control,contd,(3)Design Caustic Alkalinity(Literature)=1.67n SiO2-(Mg/3)+150 where 3 is the“round-off”of 2.5?and n=cycles of concentrationDesign range=+50 on the above,i.e.+100-200,32,“Rule of Thumb”for P(
23、BaCl2)in Terms of TDS-SiO2 Control in Boilers,(1)Untreated FeedwaterTypical E.Aust.raw water:SiO2/TDS=0.10 Mg/TDS=0.15P(BaCl2)1.67n(0.10TDS)-n(0.15TDS/3)where n=cycles 1.670.10TDSBW-0.05TDSBW 0.084TDSBW,33,“Rule of Thumb”,contd,Require 0.025 ppm OH-/ppm TP polymeri.e.15 ppm max.or 0.006TDSBWHence P(
24、BaCl2)0.09TDSBW or as a rule of thumb:0.1TDSBW for SiO2 controland TP polymer reaction,34,Rule of Thumb,contd,(2)Softened FeedwaterAs before,SiO2/TDS=0.10P(BaCl2)1.67n(0.10TDS)1.670.10TDSBW 0.167TDSBWAllowing for TP polymer.P(BaCl2)0.17TDSBW,35,Example,An untreated boiler feedwater contains 5 ppm Mg
25、(asCaCO3)and 10 ppm SiO2.What min.caustic alk.will be required at 30 cycles?Answer:P(BaCl2)1.67300-(150/3)1.67(250)417 ppm,36,Workshop Exercise,The boiler feedwater in the above example is treated through a softener unit.What is the minimum caustic alkalinity required now?(Groups of 2/Time=2 mins),3
26、7,Indices to Predict Corrosion and Scaling Potential,1.Corrosion PotentialCorrosion Loading Index(CLI)=(Cl+SO4)/Mwhere all concentrations expressed in ppm as CaCO3The CLI is an indicator of the corrosivity of anygiven water.As a rough guide,a 10-fold increase in CLI may give a 4-fold increase in cor
27、rosivity,requiring a 4-fold increase in inhibitor concentration.,38,Indices,contd,Generally:CLI 10,corrosion control critical,water very aggressive,high potential for SCC of stainless steelFor chromate inhibitor:CrO4 demand(ppm)=2(CLI)0.6,39,Indices,contd,2.Scaling PotentialRyznar Stability Index(RS
28、I)Langelier Saturation Index(LSI)Puckorius calculation of RSI,LSIMiyamoto and Silbert LSI for pH 10.5*etc*especially applicable to ash handling systems,40,Indices,contd,RSI and LSIBoth based on difference between water pH and acalculated pH of saturation of CaCO3(pHs),and indicate whether or not the
29、 water is capable of dissolving more CaCO3.,RSI=2pHs-pH,41,Indices,contd,Table of Scaling Severity Related to IndicesLSI RSI Condition3.0 3.0 Extremely severe1.0 5.0 Severe0.2 5.8 Slight0.0 6.0 Stable water-1.0 8.0 Moderate tendency to dissolve scale-3.0 10.0 v.strong tendency to dissolve scale*note
30、:dissolve scale=corrosive,LSI=pH-pHs,42,Indices,contd,Practical Applications of LSI(1)H.Feitler,in a paper published in the Jan.1972 issueof MP,and discussed at length in a CanadianElectrical Association report in 1986,found that theLSI can reach values between 1.7 and 2.0 before scaling actually oc
31、curs,and I am recommending thatwe use this guideline as a quick check of the potential for a scaling problem.,43,Indices,contd,Practical Applications of LSI(2)The MicroCAPE program,“Mineral Solubility”uses an MIT model to more accurately predictscaling potentials.Make sure you are using Version2.3.,
32、44,Indices,contd,Puckorius Method to Calculate pHspHs=9.30+A+B-(C+D)where A,B,C,and D are factors to allow for TDS,temperature,calcium and alkalinity,respectively.Tables for these factors are in Appendix 2 of your notes.,45,Workshop Exercise,A cooling water at 38 oC has the following analysis:CaH=10
33、0 ppm as CaCO3 TDS=2500 ppmM-alk=75 ppm as CaCO3 pH=8.2What is the RSI and LSI,and will the water be scale forming or corrosive?Compare the predictions of the Puckorius Method and MicroCAPE“Min.Sol.”(Groups of 2/Time=10 mins),46,Quiz,22 questions worth 25 points-all worth 1 pt each,except(20)-(22)each worth 2 pts 16 points are required to pass the course(answers to be sent to K.Gehan for marking and issuing ofcertificates)Time allowed will be 60 mins.,47,Quiz,contd,48,Quiz,contd,49,Quiz,contd,
