Question 55 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is C ### Explanation for why Option C is Correct: The primary function of an oily-water separator (OWS) relies on gravity separation, which is governed by Stokes' Law. Elevating the temperature of the oily-water mixture impacts two critical properties that affect separation efficiency: viscosity and specific gravity differential. 1. **Effect on Viscosity:** Temperature is inversely proportional to viscosity for most liquids, including oil. When the temperature increases, the oil becomes thinner, meaning its **viscosity decreases**. Lower viscosity allows oil droplets to move (rise) more easily through the water phase. 2. **Effect on Specific Gravity Differential ($\Delta SG$):** Specific gravity (or density) decreases for both oil and water as temperature rises. However, the density of water changes less dramatically than the density of many oils. Furthermore, the goal in gravity separation is to maximize the difference between the density of the oil and the density of the water. Heating generally causes the oil (which is typically lighter than water) to expand and decrease its density more significantly than water does, thereby **increasing the specific gravity differential** ($\Delta SG$) between the two fluids. A larger $\Delta SG$ provides a greater buoyant force, causing the oil droplets to rise faster. Since heating **decreases the viscosity of the oil** and **increases the specific gravity differential** between the oil and water, the separation process is enhanced, making **Option C** the correct statement. *** ### Explanation for why the other options are incorrect: * **A) Heating the oily-water mixture increases the viscosity of the oil and decreases the specific gravity differential between the oil and water.** * This is incorrect because heating *decreases* viscosity (not increases it) and generally *increases* the specific gravity differential, which is necessary for efficient separation. * **B) Heating the oily-water mixture increases the viscosity of the oil and increases the specific gravity differential between the oil and water.** * This is incorrect because heating *decreases* viscosity (not increases it). While it correctly states the effect on the specific gravity differential, the statement on viscosity is fundamentally wrong. * **D) Heating the oily-water mixture decreases the viscosity of the oil and decreases the specific gravity differential between the oil and water.** * This is incorrect because while heating correctly *decreases* viscosity, it generally *increases* (or at least maintains) the specific gravity differential critical for effective gravity separation.