電導(dǎo)率評價防垢劑

1、TECHNICAL NOTENEW RAPID TEST FOR EVALUATION OF SCALEINHIBITORSI.DRELA*,P.FALEWICZ and S.KUCZKOWSKALaboratory of Technical Electrochemistry and Corrosion,Technical University of Wroclaw,Wyb.St.Wyspianskiego27,50-370Wroclaw,Poland(First received July1997;accepted in revised form February1998AbstractA

2、new method of evaluation of scale inhibitors e ciency has been developed and tested. This method is based upon solution conductivity measurement.It consists of determining a supersatura-tion level of any scale forming compound in a given water,at dened conditions,in the presence of a specied amount

3、of scale inhibitor and also without inhibitor.This method allows the stabilizing fea-tures of compounds under investigation to be di erentiated.With this method the e ciency of prevent-ing CaCO3precipitation from aqueous solutions was determined for three phosphonate compounds which are e ective as

4、corrosion inhibitors of mild steel in neutral water:2-phosphonobutane-1,2,4-tri-carboxylic acid(TBTC,ethylenediaminetetramethylenephosphonic acid(EDTMPand phosphono-N-methylenephosphonic derivative of polycaproamide(PMPP.It was ascertained that the above com-pounds are e ective in preventing CaCO3cr

5、ystallization from water.EDTMP showed the best stabiliz-ing properties and PBTC what poorer ones.#1998Elsevier Science Ltd.All rights reservedKey wordsscale inhibitors,water stabilization,phosphonate compounds,conductivityINTRODUCTIONPrecipitation and scale deposition present an im-portant problem i

6、n operating water cooling and heating installations.Deposits formed may cause severe crevice corrosion attack,growth of microor-ganisms and may deteriorate conditions of the heat exchange(Bond,1990.Enormous progress in science and technology, made in the area of new technics and apparatus,is often b

7、ound up with intensication of heat exchange processes in chemical industries,metal-lurgy,power industry,etc.Traditional methods of water treatment for industrial purposes(ltration, coagulation,decarbonization,outgassing,ion exchangehave become more and more expensive due to the general degradation o

8、f the environment and also are often of low e ectiveness.By prevent-ing precipitation and deposition on exchange sur-faces intensication of the heat exchange may be obtained.Any e ective means of water stabilization would allow the use of natural water of lower qual-ity and/or reused water and would

9、 lead to a re-duction of water consumption because of the possibility of concentrating it signicantly in circu-lation systems.At present,research concerning water stabiliz-ation which is to lower the tendency to evolve pre-cipitates,focuses on physical and chemical methods. Physical methods include

10、water treatment in mag-netic or electricelds and by using sonic waves (Limpert and Raber,1985.Chemical methods are methods consisting of adding small amounts(i.e.at a level of grams per1m3of agents into water. Those agents are usually composed of organic com-pounds.Chemical methods are more versatil

11、e,as they o er more possibilities for e cient water treat-ment over a wider variety such as water parameters and technical specications of operating systems. Crystallization results from three processes which interact mutually:supersaturation,nucleation and crystal growth.Water stabilizing chemicals

12、(e.g. complexonesmay inuence each of these processes. There is a lack of references as to the mechanism of crystallization inhibition of sparingly soluble salts by using complexones.In one of the few articles concerning these problems(Tomson and Matly, 1989nucleation of CaCO3,CaSO4and BaSO4 under in

13、uence of complexone types like phospho-nates and polyacrylates were presented.Based on analyses of the di usion rate into crystal being in the nucleation stage,two ways of a ecting crystal nucleation,i.e.increasing supersaturation and decreasing interfacial tension,were presented.A rel-Wat.Res.Vol.3

14、2,No.10,pp.31883191,1998#1998Elsevier Science Ltd.All rights reservedPrinted in Great Britain0043-1354/98$19.00+ 0.00PII:S0043-1354(9800066-9*Author to whom all correspondence should be addressed.3188evant equation describing this process was pro-posed.That equation was veried experimentally afterwa

15、rds.Complex forming of these compounds together with cations present in water impedes supersaturation and in this way reduces the rate of nucleation and crystal growth(Zielinski,1981. Even if a complete water stabilization does not appear at given conditions,the presence of these compounds causes su

16、spended solids to dispergate and to liquefy through adsorption,which changes the structure and properties of scale.Deposits which are being formed are better dispergated and of lower hardness as adhesion of individual par-ticles is weak.In reports from the last few years several organic compounds,wh

17、ich have the properties mentioned above,have been introduced.These compounds have been used to stabilize water.Compounds of the class of phosphonate-,aminoalkylphosphonate-, polycarboxypolyphosphonate-complexones and also light molecular weight acryl-and aminoacryl-poly-mers have been mentioned the

18、most frequently (Marshall,1987;Marshall and Germann,1987; Tomson and Matly,1989.Published data and our own previous research (Kubicki et al.,1989,1995;Falewicz and Kuczkowska,1992point out that chemical water treatment using phosphonate complexones o ers a good possibility of stabilizing water toget

19、her with reduction of its corrosivity.Laboratory research of water stabilization usually includes only qualitative evaluation of stabilizing properties of a given substance under dened con-ditions(Amjad,1989.This evaluation does not allow determination of the quantitative relation-ships between the

20、concentration of a stabilizing agent and the stabilization e ectiveness of the agent.That is why the authors of this paper devel-oped an original test method of complexone proper-ties to stabilize water.The new test is described in Section2.Nucleation is therst step of precipitation.The homogenous n

21、ucleation rate is a ected by the degree of supersaturation(Swhich is dened as a ratio of the crystallizing substance concentration to the concentration of saturation(solubility. Although nuclei formation is possible at each super-saturation(S1,its rate increases rapidly only when a specic value of t

22、he supersaturation is being exceeded,i.e.at a critical supersaturation.The nucleation rate is almost zero below this value.For most compounds which crystallize from aqueous solution the critical degree of supersaturation is very high and reaches a value of102103.The quantity of the critical degree o

23、f supersaturation reached is especially important in water systems where the problem of precipitation and deposit forming often exists.Calcium carbonate and sulfate are main scale compounds in most types of usable water.In this paper the inuence of chosen phos-phonate compounds on the critical degre

24、e of super-saturation for only calcium carbonate in water is presented.This new test uses conductivity measure-ments while calcium carbonate is being precipitated from calcium chloride solution by addition of sodium carbonate.Based on many tests,it was ascertained that using0.1M CaCl2as a titrated s

25、olution and0.1M Na2CO3as the titrating solution was the most con-venient set of solutions.MATERIALS AND METHODSThe test setup,presented in Fig.1,consists of a test cell thermostated and provided with conductivity sensor and magnetic stirrer.This setup includes also the CC-551 microcomputer conductiv

26、ity meter(Elmetronand the U-1 thermostat(MLW Pru fgera te-Werk.The tests consisted of putting60cm3of bi-distilled water,3cm3of0.1M CaCl2and a variable amount of phosphonate compounds in the thermostated vessel of 100cm3in volume.Then,the conductivity sensor was installed and after the temperature st

27、abilized at 2520.18C,the solution was titrated by using0.1M Na2CO3.The titrating solution was added in portions of 0.2cm3each.The conductivity value was read after1min of stabilizing.Solutions being tested were stirred at con-stant intensity with a magnetic stirrer during measure-ments.Before each t

28、itration the vessel,the sensor and the stirrer were washed with1M H2SO4and bi-distilled water to remove all traces of deposits which could become crys-tal nuclei while the measurements were being carried out. The results obtained are presented as a diagram of the sol-ution conductivity vs the volume

29、 of the titrating agent added,from which the initial point of the precipitation was determined.The following phosphonate compounds were selected to test their abilities to inhibit CaCO3precipitation process in aqueous solutions.These compounds are e ective in preventing the corrosion of steel in wat

30、er environment at neutral pH:.2-Phosphonobutane-1,2,4-tricarboxylic acid(PBTC: HOOCCH2CH2C COOH PO3H2 CH2COOH .Ethylenediaminetetramethylenephosphonic acid (EDTMP:H2PO3CH2 2N CH2 2N CH2PO3H2 2.Phosphono-N-methylenephosphonic derivative of polycaproamide (PMPP:Fig.1.Experimental setup for measurement

31、 of CaCO3supersaturation.Technical Note3189 CH 2 5N CH 2PO 3H 2 CO CH 2 5NHC PO 3H 2 2 n All agents used in the tests were of p.a.purity grade except PMPP which had been synthesized in the labora-tory from substrates of p.a.purity grade.RESULTS AND DISCUSSIONThe diagram in Fig.2is an example of a ti

32、tration of the solution containing scale inhibitor.Three lin-ear segments with increasing slope may be distin-guished there.At the point of intersection of the straight lines I and II,solution opalescence was usually noticed.However,even after waiting a long time within the second segment (1.4to 2.8

33、cm 30.1M Na 2CO 3no precipitation occurred.The value of the last point on the straight line II (arrow-marked one,which relates to the value of 2.8cm 3of 0.1M Na 2CO 3was taken as the bound-ary concentration of the titrating agent,which caused the initiation of CaCO 3precipitation.At the volume of th

34、e titrating agent,represented by the value of 3.0cm 3on the diagram,a deviation from the linear relationship appears and a rapid precipi-tation begins,from which a rapid decrease of the conductivity results within a few minutes.Further titration shows a linear relationship to the concen-tration agai

35、n but with an increased slope (line III in Fig.2.The relationship between the solution con-ductivity and the concentration of the precipitating agent described above only occurs with the presence of complexones.In the case where the solution con-tains only CaCl 2with no complexones,there is no 2nd s

36、egment,which means solution opalescence and precipitation appear simultaneously.Values for the degree of supersaturation measured for solutions containing phosphonate compounds were compared to the supersaturation obtained at similar conditions but without the ad-dition of the phosphonates.So the va

37、lues for the degree of supersaturation in the following are rela-tive ones (S r expressed as a ratio of the supersa-turation of CaCO 3in the presence of aphosphonate compound (S p ,to the value of the supersaturation under the same conditions for water alone (S 0:S rS p S 0In calculations,both the d

38、ilution of phosphonate compounds and deposit forming ion concentrations caused by variable critical values of the titrating agent were taken into account.In the case of the water solution containing 60cm 3of distilled water and 3cm 3of 0.1M CaCl 2and without any ad-ditions,0.6cm 3of 0.1M Na 2CO 3was

39、 consumed before opalescence occurred.So the ionic product of the CaCO 3precipitation at these conditions may be calculated as follows:Volume of the solution when turbidity rst occurs,6.36102dm 3.Calcium ion concentration (Ca 2+, 4.71103M.Carbonate ion concentration (CO 32,9.43104M.Ca 2 CO 23 4X 441

40、06From the handbook of physical chemistry the ionic product of CaCO 3at 258C is 4.8109,so the value of its supersaturation under test con-ditions (S 0is 925.Dependence of the relative degree of supersatura-tion of CaCO 3in the water solution (S r on concen-trations of the three phosphonate compounds

41、 selected is presented in Fig.3.It appears from Fig.3that in the case of EDTMP and PBTC,the relative degree of supersaturation of CaCO 3,equal to 3.7,is possible to achieve even at small concentrations of these agents in water (up to 5m mol/dm 3.A further increase in concentration,however,causes a s

42、maller increase in the relative supersaturation.Better results are obtained using EDTMP than PBTC.The highest supersaturation,7times higher than for water alone,was obtained for EDTMP but at a relatively high concentration in water (47m mol/Fig. 2.Titration diagram for the solution containing2m mol/

43、dm 3 PBTC.Fig.3.Dependence of degree of water supersaturation with CaCO 3(S r on concentration of phosphonate com-pounds.Technical Note3190dm3.It is apparent that increasing the EDTMP concentration10times,i.e.from5to50m mol/dm3, only allows the degree of supersaturation to double. As for PMPP,which

44、is the most e ective in pre-venting corrosion of the mild steel in neutral water, its anti-scale properties are much worse than the other two.So,at the concentration of10m mol/dm3, the supersaturation obtained for PMPP is over2 times lower than for EDTMP and PBTC.Taking the supersaturation acquired

45、into account and also the costs of using anti-scale compounds, the values of5to10m mol/dm3for EDTMP and PBTC,which correspond to 2.24.4and 1.42.7mg/dm3,respectively,are optimal concentrations for gaining a satisfactory stabilizing e ect.CONCLUSIONSThe following conclusions can be drawn from the cond

46、ucted investigations:.A new method for evaluation of the e ective-ness of scale inhibitors was presented,which allows their stabilizing properties to be distinguished quickly and selectively.With this method one may determine a quanti-tative relationship between scale inhibitor concen-tration and re

47、lative supersaturation of scale forming compounds.Small concentrations of EDTMP(2.2mg/dm3 and of PBTC(1.4mg/dm3give a considerable rela-tive supersaturation of CaCO3in water,3.7times, and at higher concentrations of EDTMP(22mg/ dm3relative supersaturation may exceed7times. .Results obtained with this method have con-rmed known and very good anti-scaling properties of EDTMP and PBTC.AcknowledgementThe authors thank the State Committee for Scientic Research(grant3T09B09208for supporting this research.REFERENCESAmjad Z.(1989Calcium sulfate dihydrate scale,for-mati