Technical Scenario

Question 1

You’re working at a refinery, and the distillation column is not achieving the desired product purity. The temperature profile is fluctuating, and the reflux ratio seems off. How would you approach this situation?

Answer 1

1. Clarify the Details:
   o I would start by confirming the key parameters:
    What is the target purity?
    Has there been any change in feed composition, flow rate, or operating conditions?
    Are there any known issues with the control system?
2. Break Down the Problem:
   o First, I would focus on temperature fluctuations. This could indicate:
    A malfunction in the temperature sensors or control loops.
    A problem with the reboiler or condenser that’s impacting heat duty.
    Potential feed disturbances or variations in composition.
3. Identify Key Principles:
   o This problem ties back to distillation principles. I’d assess:
    The reflux ratio: if it’s too low, it might not provide enough liquid back to the column for proper separation.
    The temperature profile: check if each stage has the correct vapor-liquid equilibrium.
4. Propose a Solution Strategy:
   o Check the control loops: Verify whether the sensors, controllers, or actuators are malfunctioning. I would also review the historical data to identify when the fluctuations started.
   o Assess the reflux system: Increase the reflux ratio to see if it stabilizes the purity. If there are issues with the reboiler or condenser, I would inspect those for fouling, leaks, or mechanical problems.
   o Look into feed composition: Analyze if any changes in feed quality have impacted the column’s performance. If the feed is richer in heavier components, the column may need adjustments in pressure or heat input.
5. Safety and Efficiency:
   o Throughout, I’d ensure the safety valves and relief systems are functioning in case of overpressure or other hazards.
   o My approach would aim to minimize disruption to the process and avoid energy wastage.
6. Further Considerations:
   o If adjusting the reflux or repairing equipment doesn’t solve the issue, we may need to recalibrate the column or conduct a detailed HAZOP study to prevent long-term performance degradation.

Question 2

Question: You notice that the heat exchanger in one of the refinery units is underperforming. The temperature of the outlet stream is lower than expected, and heat transfer efficiency seems to have dropped. How would you approach diagnosing and solving this issue?

Answer 2

1. Clarify the Details:
   o What type of heat exchanger is it (e.g., shell-and-tube, plate)?
   o Are both the hot and cold fluid flow rates and inlet temperatures as expected?
   o Has there been a noticeable increase in pressure drop across the heat exchanger?
2. Break Down the Problem:
   o Fouling: A common cause for reduced heat transfer is fouling on the heat exchanger surfaces. I would suggest inspecting for any build-up (e.g., scaling, organic deposits).
   o Leaks: Check for leaks in the tubes or plates, which could result in mixing of the streams and lower heat exchange.
   o Flow rates: Reduced flow rates or blockages could reduce the overall heat transfer.
3. Solution Strategy:
   o Inspect the heat exchanger for fouling and suggest cleaning if necessary.
   o Monitor flow rates and ensure the pumps or flow controllers are working properly.
   o Check for any air pockets in the system, which could reduce the effective heat transfer area.
4. Safety and Efficiency:
   o Ensure all valves and safety controls are functioning during inspection, and maintain safe operating pressure.
   o Focus on energy efficiency, suggesting an upgrade to a more efficient heat exchanger design if fouling is frequent.

Question 3

Question: A centrifugal pump in the refinery is showing signs of cavitation (noisy operation, reduced flow rate, and possible damage to the impeller). How would you address the cavitation issue?

Answer 3

1. Clarify the Details:
   o What fluid is being pumped, and what are its temperature and vapor pressure?
   o What are the suction and discharge pressures of the pump?
   o Has there been any recent change in flow rates or system design?
2. Break Down the Problem:
   o Cavitation typically occurs when the pressure at the pump inlet drops below the vapor pressure of the liquid, causing vapor bubbles to form.
   o Causes could include:
    Low net positive suction head (NPSH).
    Blockage in the suction line.
    Excessive pump speed.
3. Solution Strategy:
   o Increase NPSH by:
    Lowering the pump inlet temperature (to prevent the liquid from reaching its vapor pressure).
    Increasing the suction head by raising the tank level or lowering the pump.
    Reducing flow restrictions in the suction line by checking for any obstructions or using a larger diameter pipe.
   o Lower the pump speed to reduce cavitation.
   o Consider using a different pump impeller or a pump more suited for high NPSH conditions.
4. Safety and Efficiency:
   o Make sure the pump is operating within safe limits to prevent impeller damage.
   o Minimize downtime by suggesting regular maintenance checks for cavitation.

Question 4

Question: A distillation column in a crude oil distillation unit is experiencing flooding, leading to poor separation and decreased efficiency. What steps would you take to identify and fix the issue?

Answer 4

1. Clarify the Details:
   o What is the feed composition and flow rate?
   o Has the operating pressure or temperature changed recently?
   o Are there any issues with the reboiler or condenser?
2. Break Down the Problem:
   o Flooding can occur when the liquid flow down the column overwhelms the vapor flow going up, causing liquid to accumulate on the trays.
   o Causes could include:
    Excessive feed flow rate or a change in feed composition.
    Problems with vapor/liquid flow balance (improper reflux ratio).
    Faulty reboiler or condenser operation.
3. Solution Strategy:
   o Reduce feed rate to decrease liquid flow down the column.
   o Adjust the reflux ratio to find the right balance between vapor and liquid flows.
   o Check for fouling in the reboiler or condenser, which could be reducing heat transfer efficiency.
   o Inspect the trays or packing inside the column for any blockage or damage.
4. Safety and Efficiency:
   o Ensure that the distillation column isn’t operating under conditions that could lead to overpressure or hazardous releases.
   o Recommend ongoing monitoring of flow rates and pressure to avoid flooding in the future.

Question 5

Question: You’re assigned to troubleshoot a reactor that’s not reaching its expected output because of insufficient heating. There is a clear heat loss in the system. How would you diagnose and address the issue?

Answer 5

1. Clarify the Details:
   o What type of reactor is it (e.g., batch, continuous)?
   o What is the operating temperature and pressure?
   o How is the heat being supplied (e.g., through a jacket, internal coils)?
2. Break Down the Problem:
   o Heat loss could be due to poor insulation or equipment malfunction.
   o Issues could be with:
    The insulation around the reactor or pipes.
    Heat transfer fluid flow.
    Leaks in the heating system.
3. Solution Strategy:
   o Inspect the insulation around the reactor and pipelines for any gaps or damage. Repair or replace as needed.
   o Check the heat transfer fluid (if applicable) for flow rate, temperature, and any signs of leaks.
   o Analyze the heat input system for proper function (e.g., the heater or steam supply).
4. Safety and Efficiency:
   o Ensure the reactor operates within the correct temperature range to avoid any unsafe reaction conditions.
   o Suggest heat recovery systems if applicable to improve overall energy efficiency.

Question 6

Question: You’re working with a process vessel that is experiencing an unexpected pressure build-up. The pressure relief valve is set correctly, but the pressure continues to rise. How would you approach solving this?

Answer 6

to Approach:
1. Clarify the Details:
o What is the normal operating pressure of the vessel?
o Has the vessel experienced any changes in feed composition, temperature, or reaction rates?
o Are there any known issues with the control system?
2. Break Down the Problem:
o Pressure build-up could result from:
 Blockages in the outlet.
 Increased reaction rates leading to excess gas production.
 Malfunctioning pressure relief system.
3. Solution Strategy:
o Inspect the outlet line for any blockages or valve malfunctions, ensuring that gases or liquids can leave the vessel properly.
o Analyze the feed composition to ensure there hasn’t been a change that’s increasing gas production or vapor pressure.
o Check for any control system issues that might be improperly regulating pressure.
4. Safety and Efficiency:
o Ensure the vessel’s pressure relief system is working as a backup safety measure.
o Recommend additional sensors or alarms to prevent future overpressure events.