For the water quality characteristics in Table 1.0, soften the water using caust
ID: 815743 • Letter: F
Question
For the water quality characteristics in Table 1.0, soften the water using caustic soda (NaOH) and calculate;
(i) Chemicals required per day
(ii) Solids produced per day
Depth
Table 1.0 Water quality characteristics Parameter
Concentration
CO2
20 mg/L
Ca(HCO3)2
200 mg/L
MgSO4
100 mg/L
Flowrate
25,000 m3/day
NaOH equations
CO2+2(NaOH)-->Na2CO3+H2O
Ca(HCO3)2+2(NaOH)-->Na2CO3+H2O
Mg(SO4)+2(NaOH)-->Mg(OH)2+Na2SO4
Table 1.0 Water quality characteristics Parameter
Concentration
CO2
20 mg/L
Ca(HCO3)2
200 mg/L
MgSO4
100 mg/L
Flowrate
25,000 m3/day
Explanation / Answer
Lets calculate how much CO2 is in that 25,000 m3/day stream
20 mg/L is the same as 20 g/m3
so Amount of CO2 needed = 20 g/m3 * 25,000 m3/day = 500,000 g/ day or 500 kg/day of CO2
Do the same for Ca(HCO3)2
amount of Ca(HCO3)2 needed = 200 g/m3 * 25,000 m3/day = 5,000,000 g/day or 5 ton of Ca(HCO3)2 per day
now MgSO4 = 100 g/m3 * 25,000 m3/day = 2,500,000 g/day = 2.5 ton MgSO4 per day
B) Solids produced per day
CO2+2(NaOH)-->Na2CO3+H2O
Ca(HCO3)2+2(NaOH)-->Na2CO3+H2O
Mg(SO4)+2(NaOH)-->Mg(OH)2+Na2SO4
Solids are Na2CO3 and Mg(OH)2
Check out stoichimetrics
1 mol of CO2 : 1 mol of Na2CO3
1 mol of Ca(HCO3)2 : 1 mol of Na2CO3
1 mol of Mg(SO4) : 1 mol of Mg(OH)2
Calculate the amount of moles per unit... we need MW of each substance
1 mol of CO2 : 1 mol of Na2CO3
44 kg/kmol : 105.98 kg/kmol
or 2.4x
1 mol of Ca(HCO3)2 : 1 mol of Na2CO3
162 kg/kmol : 105.98 kg/kmol
or 0.65x
1 mol of Mg(SO4) : 1 mol of Mg(OH)2
120 kg/kmol : 58.3 kg/gmol
2.05x
Calculate now each substance produced
2.4x where x = 500 kg CO2/day
2.4*500 = 1200 kg of Na2CO3 per day
0.65x where x = 5 ton of Ca(HCO3)2 per day
0.65*5 = 3.25 ton of Na2CO3 per day
Total Na2CO3 = 1.2 ton + 3.25 ton = 4.45 ton of Na2CO3 per day
2.05x where x =2.5 ton MgSO4 per day
2.05*2.5 = 5.12 ton Mg(OH)2 per day