Thus above, it found that there is a relation between PAc and PAI as the following.
PAc/PAI=2.069 (17)
3.1.2 Estimation of water quality
The change of pH, EC and PO43- concentration due to seawater mixing can be calculated by the following procedure.
The theoretical estimation of [OH-] consumption must be corrected by the factor 2.069 of eq. (17), since there is an adequate correlation between [OH-] and P-alkalinity. Eq. (14) indicates the PO43- is consumed by 1.422 mg/l (1.50*10-5 mol/l) and the OH- is consumed by 0.085 mg/l (5.00*10-6 mol/l) per 1 mg/l Ca2+ contamination, and also Eq. (15) indicates the PO43- is consumed by 1.954 mg/l (2.06*10-5 mol/l) and the OH- is consumed by 0.350 mg/l (2.06*10-5 mol/l) per 1 mg/l Mg2+ contamination.
The electric conductivity K25 of each substance at 25℃ is expressed by the following equation.
K25=Σ (ni*λni25) (18)
where, ni; concentration of each substance ppm, λ ni25; electric conductivity corresponding to unit concentration of each substance at 25℃, that is, Ca2+; 2.98μs/cm, Mg2+; 4.39μs/cm, PO43-; 21.8μs/cm, OH-, 11.66μs/cm.
(1) Chloride ion concentration
CL=Cls*a=17500*a (19)
where, CL; chloride ion concentration of boiler water, mg/l, Cls; chloride ion concentration of seawater, mg/l, a; mixing rate of seawater to boiler water contents.
(2) Hydroxyl molarity and pH
[OH]=[OH]0-([OH](14)+[OH](15))*PAc/PAI
=[OH]0- (5.00*10-6*358+2.06*10-5*1260) *2.069*a=[OH]0-0.0574*a (20)
pH=log10[OH]-log10Kw (21)
where, [OH]0; initial hydroxyl molarity, [OH](14) ; hydroxyl molarity consumed by the precipitating reaction of eq. (14), [OH](15); Hydroxyl molarity consumed by precipitating reaction of eq.(15), Kw; ion product of water.
(3) Phosphate ion concentration
PO=PO0- (PO(14)+PO(15))
=94970*K1*[OH]0- (358*1.422+1260*1.954)*a
= 94970*K1*[OH]0-2971*a (22)
where, PO0 ; initial concentration of PO43- mg/l, PO(14); PO43- concentration consumed by the reaction of eq. (14), mg/l, PO(15); P043- concentration consumed by the reaction of eq. (15), mg/l, K1; mol ratio of PO4/OH of boiler treatment chemicals.
(4) Electric conductivity
EC=EC0+ECs-(EC(14)+EC(15))
=EC0+ECs-(ECCa+ECMg+ECPO+ECOH)*a
=EC0+48700*a-{2.98*Cas+4.39*Mgs+2.18*(1.422*Cas+1.954*Mgs) +11.66* (0.085*Cas+0.350*Mgs)} *a
=EC0+48700*a- (7.07*Cas+12.73*Mgs)*a (23)
where, EC0; initial electric conductivity, ECs; electric conductivity increased due to mixing of seawater EC(14); electric conductivity reduced by precipitating in eq. (14), EC(15); electric conductivity reduced by the reaction of eq. (15), ECCa, ECMg ,ECPO, ECOH; electric conductivity of each substance reduced by the reactions of eq.(14) and (15), Cas; Ca2+ concentration of seawater, mg/l, Mgs; Mg2+concentration of seawater, mg/l.
3.2 Theoretical estimation when SiO2 coexists
If SiO2 is mixed together with Mg in the feed water, MgSiO3 is formed in boiler water as shown by eqs. (10) and (12), that is, Mg reacts at first with SiO2 to form MgSiO3 then its quantity increases as the SiO2 content increases. When SiO2 has been consumed, it is conceivable that the rest of Mg will react with other substances to form [Mg3(PO4)2]n・Mg(OH)2, and, as a result, that both of pH and P043- concentration will decrease. Accordingly, the mole ratio of MgSiO3 to [Mg3(PO4)2]n・Mg(OH)2 will be determined by the ratio of Mg and SiO2 content in the feed water. The boiler water quality contains SiO2 can be estimated by the same manner with the conventional model.
4. RESULTS AND DISCUSSION
4.1 Change of boiler water quality induced by seawater mixing
Table 2 compares new theoretical estimation with experimental results of boiler water quality change induced by seawater mixing.
As can be seen in Table 2, the measured and the theoretical values change with a similar tendency, and the new theoretical values are nearer to the measured one than the conventional one. The predicted values, however, are a little different from the measured values. This difference might be remedied if a less value of n of eqs. (11) and (13) is adopted. Anyway, these results make sure that Mg ion reacts not only with OH ion but also with PO4 ion.
4.2 Change of boiler water quality by SiO2 coexisting
When OH- and/or PO43- coexists, and when silica is mixed into the feed water, MgSiO3 is first formed in boiler water as shown by eqs. (10) and (12). If all of SiO2 has been consumed, it is conceivable that the rest of Mg will react to form [Mg3(PO4)2]n・Mg(OH)2 and that both of pH and PO43- concentration are resulted in decrease. Accordingly, the mole ratio of MgSiO3 to [Mg3(PO4)2n・Mg(OH)2 will be determined by the ratios of Mg and SiO2 content, in the feed water.
