Softening Flashcards
What is hard water?
When multivalent ions react with soap they cause the water be hard; Total Hardness = [Ca} + [Mg] = 1.5 mmol/L
Describe the principle of Pellet softening in a fluidized bed reactor?
The pH is increased by dosing a base (NaOH or Ca(OH)2) at the bottom of the reactor where it will be mixed with the raw water and seeding material. This will cause CaCO3 (calcium carbonate) to crystalise around the seeding grain and will grow until they are too big. Then they are removed and replaced or sold if reuse of pure CaCO3.
High flow rates 80m/h to 100 m/h
Calcium Carbonate (CaCO3) equilibrium is an important aspect for understanding softening chemical processes. Why is this?
Groundwater usually stays in the subsoil for many years before it is pumped up or flows out into surface water. Due to the long residence time underground, the CO2 present in the subsoil can mix with groundwater and can be in chemical equilibrium with the calcium carbonate in soil. Through the chemical equation of calcium carbonate equilibrium, limestone reacts with CO2 to form Calcium and Bicarbonate.
CaCO3 + CO2 + H2O <=> Ca2+ + 2HCO3-
When water that contains Ca2+ and HCO3- ions is heated the equilibrium changes and these ions will precipitate in the form of CaCO3.
Carbon Dioxide forms carbonic acid when dissolved in water. It is usually used for the stabilisation of water after chemical softening. Sketch a pH diagram for carbonic acid?
At low pH: 0-4 Only CO2 is present in the water
At Medium pH: 6-10 only HCO3 (Bicarbonate)- is present (Neutral 7-8 is mainly CO2 and HCO3-)
At high pH: 10-14 only CO3^2- (Carbonate) is present
The calcium carbonic acid equilibrium determines whether Calcium carbonate will precipitate.
- CO2 has an equilibrium with HCO3-
- HCO3- has an equilibrium with CO3^2-
- Calcium carbonate has an equilibrium
Write the equations for K1, K2 and Ka
(Hint: check the chemical formulas on the Formula page)
K1 = [H3O+] * [ HCO3-] / [CO2]
K2 = [H3O+]*[CO3^2-] / [HCO3-]
Ka = [Ca2+] * [HCO3^-]^2 / [CO2] = Ks * K1 * K2 ^-1
Ks = CaCO3 <=> Ca2+ + CO3^2-
Explain the concept of Tillman’s curve and how do you know if water is aggressive or will scale? How does temperature affect the Tillman’s curve?
Tillman’s curve: At a certain concentration of HCO3-, there is a certain concentration of CO2 where the water is in equilibrium.
[HCO3-] = 2 * Ka * [CO2]
When not in equilibrium we have a problem: Too much CO2 will attack the Calcium Carbonate causing it to dissolve and act aggressive - Above the line = Aggressive. When the water is oversaturated with HCO3 and low CO2 we have scaling: Below the line = Scaling
When you increase the temperature of water the equilibrium constant (Ka) changes and moves the curve up and to the left meaning more risk of scaling - CaCO3 precipitation.
What is the saturation pH (pHs) and the formula used to determine pHs?
The equilibrium pH for when Ca & HCO3- is in equilibrium on the Tillman’s curve.
[Ca2+] = 0.5*[HCO3-]
pHs = pk2 - pks - log([HCO3-] * [Ca2+])
What is the Saturation Index? Explain with a formula
The SI will tell you the composition of the water:
SI = pH - pHs
SI < 0 aggressive
SI > 0 scaling
When the SI = 0 this is when the water is in equilibrium
What is the purpose of adding a base for neutralisation?
Write 2 chemical formula’s for dosing Caustic Soda and Lime
To remove CO2 from the water
NaOH + CO2 => Na + HCO3-
Ca(OH)2 + 2*CO2 => Ca2+ + 2HCO3-
When choosing the best chemical to dose for neutralisation and softening it is important to consider the composition of your raw water. Why?
If you have water with a low HCO3- then you should not dose lime Ca(OH)2 bc it uses 2 mmol of HCO3-
If you have a water with a high [Na] then you should not dose Causic Soda (NaOH)
What is the advantages and disadvantages of dosing Lime Ca(OH)2 and Caustic Soda NaOH?
Lime Ca(OH)2:
Advantages: You do not add Na
Disadvantages: You produce 2 x more pellets & lime is more difficult to treat & removal of carry over
Caustic Soda NaOH:
Advantages: Low production of pellets
Disadvantages: You have to remove Na
Hydraulics: What is the formula for maximum resistance of a pellet softening bed?
Hmax = (1 - p) * L * (ρp -ρw / ρw)
Hmax = Maximum resistance (m)
ρw = density water (kg/m3)
ρp = density of pellets (kg/m3)
P = Porosity of the bed (-)
Hydraulics: What is the formula for the expansion of a fluidised bed?
E = Le/Lo = 1 - Pe / 1 - Po
Le = Height of expended bed (m) Lo = Heigh of fixed bed (m)
Pe = Porosity of expanded bed (-) Pe = Porosity of fixed bed (-)
What is the formula used to calculate the expanded bed porosity (Pe)?
Pe^3 / (1 - Pe) ^ 0.8 = 130 * ( ν ^ 0.8 /g ) * ( ρw / ρp - ρw ) * ( v^1.2 / d^1.8)
ν = Dynamic Viscosity v = velocity (m/s) d = dimeter of pellets (m)
What is split treatment and the advantages of using a split treatment for Softening over traditional pellet softening treatment? (4 points)
In what conditions can you use a split treatment?
A split treatment is where only part of the raw water will go undergo softening and reduce the hardness and part will bypass keeping the same hardness.
- You use fewer chemicals
- CO2 only has to be converted in one part of the flow
- Mixing the softened and raw water will give a lower super-saturation of CaCO3 which reduces acid dosing for pH neutralisation
- Investment costs are low because you need fewer reactors
Can only be used when you have a high Mg concentration - your effluent from the softening reactor cannot be lower than 0.5 mmol/L.
