Q18Engineering Chemistry

Question

(a) What do you understand by Priming and Foaming? (b) 0.5g of $C a C O_{3}$ was dissolved in HCl and the solution was made upto 500 mL with distilled water. 50 mL of this water sample required 48 mL of EDTA solution for titration. 50 mL of the sample water required 15 mL of EDTA and 50 mL of boiled water sample required 10 mL of EDTA solution for titration. Calculate the temporary, permanant and total hardness of the given water sample.

Answer 1

Part A: Boiler Troubles - Priming and Foaming

In industrial steam generation, boilers are expected to produce perfectly dry, saturated, or superheated steam. However, under certain adverse operational or chemical conditions, the steam produced is accompanied by liquid water droplets carrying dissolved and suspended impurities. This phenomenon of producing wet steam is collectively known as 'carry-over'. The two primary causes of carry-over are Priming and Foaming.

1. Priming: The Mechanical Carry-Over

Priming is defined as the violent, eruptive, and rapid boiling of water inside the boiler, which results in the mechanical projection of liquid water droplets into the steam space and subsequently into the steam delivery pipes. It is a physical phenomenon rather than a strictly chemical one.

Causes of Priming: - Improper Boiler Design: Insufficient steam space above the water level or poor arrangement of steam internal baffles. - Excessive Steam Velocity: Drawing steam from the boiler at a rate much faster than its design capacity. - Sudden Steam Demand: Rapid opening of steam valves causes a sudden drop in pressure, leading to explosive boiling. - High Water Level: Operating the boiler with the water level too high, leaving inadequate space for steam separation. - High Concentration of Dissolved Salts: Alkali metal salts increase the surface tension and boiling point, causing violent localized boiling.

Disadvantages of Priming: - The water droplets carry dissolved salts which deposit on superheater tubes, turbine blades, and valve seats as the water evaporates. This causes severe erosion and reduces turbine efficiency. - Wet steam carries much less thermal energy, reducing the overall thermal efficiency of the power plant. - Sudden slugs of water entering the steam turbine can cause catastrophic mechanical failure due to water hammering.

Prevention of Priming: - Maintaining the correct, design-specified water level. - Ensuring gradual and steady steam withdrawal rather than sudden demands. - Employing mechanical steam purifiers, baffles, and anti-priming pipes inside the boiler drum. - Regular 'blow-down' operations to reduce the concentration of dissolved sludge and salts.

2. Foaming: The Chemical Carry-Over

Foaming is the persistent and continuous formation of stable, non-collapsing bubbles or froth on the surface of the boiling water inside the boiler drum. Unlike the transient bubbles formed during normal boiling, these bubbles do not burst immediately upon reaching the surface.

Causes of Foaming: - Presence of Oils and Greases: These substances saponify (react) with the alkaline boiler water to form soaps. Soaps drastically lower the surface tension of water, strongly promoting bubble stability. - Organic Matter and Sewage: Finely divided organic matter acts as a stabilizing agent for the foam film. - High Concentration of Suspended Solids: Sludge and finely dispersed particles migrate to the bubble surface, mechanically reinforcing the bubble wall and preventing it from bursting.

Disadvantages of Foaming: - It makes it impossible to accurately read the true water level in the gauge glass, which is extremely dangerous. An operator might assume the boiler is full when it is dangerously empty, risking a boiler explosion. - It inevitably leads to massive carry-over of water and salts into the steam lines, causing the same severe problems as priming (erosion, scaling on turbines).

Prevention of Foaming: - Adding Anti-foaming Agents: Specific chemicals like castor oil or synthetic polyamides are added. These agents selectively lower the surface tension of the water even further, causing the stable bubble walls to thin out and rapidly collapse. - Adding Coagulants: Chemicals like sodium aluminate ( $N a A l O_{2}$ ) are added to coagulate finely suspended particles, causing them to settle at the bottom as sludge, which is then removed via blow-down. - Ensuring feed water is strictly free from any oil or grease contamination (using oil separators before the boiler).

Part B: EDTA Titration Numerical for Water Hardness

The EDTA (Ethylene Diamine Tetraacetic Acid) complexometric titration is the most accurate standard method for determining the total, permanent, and temporary hardness of a water sample. Hardness is primarily caused by dissolved $C a^{2 +}$ and $M g^{2 +}$ ions. EDTA forms highly stable, soluble, and colorless coordinate complexes with these metal ions at a strictly buffered pH of 10.

Step 1: Preparation and Strength of Standard Hard Water (SHW)

Standard hard water is prepared by dissolving a known mass of pure, dry $C a C O_{3}$ in dilute HCl, evaporating it to dryness to form $C a C l_{2}$ , and then dissolving it in distilled water. Given data: - Mass of $C a C O_{3}$ dissolved = $0.5 g = 500 mg$ - Volume of solution prepared = $500 mL$

Therefore, the concentration of the SHW is: $500 mL$ of SHW contains $500 mg$ of $C a C O_{3}$ equivalent. So, $1 mL$ of SHW $\equiv$ $1 mg$ of $C a C O_{3}$ equivalent.

Step 2: Standardization of the EDTA Solution

Since EDTA is not a primary standard, its exact molarity must be determined by titrating it against the previously prepared Standard Hard Water. Given data: - Volume of SHW taken for titration = $50 mL$ - Volume of EDTA solution consumed from the burette = $48 mL$

From Step 1, we know that $50 mL$ of SHW contains exactly $50 mg$ of $C a C O_{3}$ equivalent. Therefore, $48 mL$ of EDTA is chemically equivalent to $50 mg$ of $C a C O_{3}$ equivalent. To find the equivalent strength of $1 mL$ of EDTA: $1 mL$ of EDTA $\equiv$ $\frac{50}{48} mg$ of $C a C O_{3}$ eq. $1 mL$ of EDTA $\equiv 1.04167 mg$ of $C a C O_{3}$ eq. (This is the standardization factor).

Step 3: Determination of Total Hardness (Raw Water)

Total hardness includes both temporary (bicarbonates) and permanent (chlorides, sulfates) hardness. The raw, untreated water sample is titrated directly. Given data: - Volume of raw water sample taken = $50 mL$ - Volume of EDTA solution consumed = $15 mL$

Using the standardization factor calculated in Step 2: Amount of $C a C O_{3}$ equivalent in $50 mL$ of the raw water sample: = (Volume of EDTA consumed) $\times$ (Strength of $1 mL$ EDTA) = $15 mL \times 1.04167 mg/mL = 15.625 mg$ of $C a C O_{3}$ eq.

To express this as Total Hardness in parts per million (ppm), which is equivalent to mg/L, we must calculate the amount present in $1000 mL$ (1 Liter): Total Hardness = $\frac{15.625 mg}{50 mL} \times 1000 mL/L$

Total Hardness = 312.5 ppm (or mg/L)

Step 4: Determination of Permanent Hardness (Boiled Water)

When the raw water is boiled thoroughly for an extended period, the temporary hardness-causing salts (calcium and magnesium bicarbonates) thermally decompose into highly insoluble precipitates ( $C a C O_{3}$ and $M g (O H)_{2}$ ). $C a (H C O_{3})_{2} Δ C a C O_{3} ↓ + H_{2} O + C O_{2} ↑$ $M g (H C O_{3})_{2} Δ M g (O H)_{2} ↓ + 2 C O_{2} ↑$

These precipitates are filtered out. The remaining clear filtrate contains ONLY the permanent hardness-causing salts (sulfates and chlorides), which are unaffected by boiling. This filtrate is then titrated with EDTA. Given data: - Volume of boiled and filtered water sample taken = $50 mL$ - Volume of EDTA solution consumed = $10 mL$

Using the standardization factor: Amount of $C a C O_{3}$ equivalent in $50 mL$ of the boiled water sample: = $10 mL \times 1.04167 mg/mL = 10.4167 mg$ of $C a C O_{3}$ eq.

To express this as Permanent Hardness in ppm: Permanent Hardness = $\frac{10.4167 mg}{50 mL} \times 1000 mL/L$

Permanent Hardness = 208.33 ppm (or mg/L)

Step 5: Determination of Temporary Hardness

Temporary hardness is simply the difference between the total hardness and the permanent hardness.

Temporary Hardness = Total Hardness - Permanent Hardness

Temporary Hardness = 312.5 ppm - 208.33 ppm = 104.17 ppm

Final Answer Summary: - Total Hardness = $312.5 ppm$ - Permanent Hardness = $208.33 ppm$ - Temporary Hardness = $104.17 ppm$

Q18Engineering Chemistry

Question

Answer

Part A: Boiler Troubles - Priming and Foaming

Part B: EDTA Titration Numerical for Water Hardness