100 Most asked questions by Chemical Engineers

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Wednesday, June 15, 2011


            Mr. Lieberman – Every summer we run out of condenser capacity. My problem is hardness
            fouling deposits in the tubes. But I cannot shut-down. What do you recommend?

 ---Harry, England

Harry – Step 1 is to blow air or N2 into the cooling water inlet. Do this every day for ten minutes. It helps prevent accumulations of hardness deposits. Step 2 is to back-flush the exchanger. The bigger the back-flush valve the better. Step 3 is acid cleaning. This is done on-stream using inhibited HCl (10%). Carefully neutralize acidified water with NaOH as it leaves the condenser.



To Norman Lieberman – My alky DIB tower reboiler stalls-out. It just stops working. Meaning, it stops adding heat to the DIB. What is my problem?
      --Rob, pane operator, Texas
    Rob – The bottom tray of your tower is leaking. If you have a line connecting the bottom product line and the reboiler inlet line, raise the bottoms level to max and open the gate valve to this line. During your next turn-around, replace the bottom tray with a welded total trap-out chimney tray. The other, not so good way to restore thermosyphon liquid circulation, is to back liquid to the reboiler vapor return nozzle.


    Fired Heaters

    Mr. Norm Lieberman. At the seminar you taught us last year, you talked about adjusting excess O2. My boss says 3%-4% should always be our target. Is this right?

    ---James, Ohio

    Jim – I remember that you asked this same silly question at the seminar. Excess O2 must be adjusted without reference to an arbitrary target. Adjust your excess O2 (assuming you’re on TRC) to minimize fuel gas consumption to reach the desired heater outlet temperature. This is best done on the stack damper unless your draft in the fire box is less than 0.05”-0.10” H2O. If less than this range, pinch on your air registers.



    Norm, why is it normally best to suction throttle, rather than to spill-back, to control my FCU wet gas compressor suction pressure in the fractionator reflux drum?
      --Kumar, India

      Kumar, I assume you have a motor driven, fixed speed compressor. If you had a variable speed compressor, you would simply slow the compressor. When we suction throttle to hold back-pressure on the reflux drum it will start to force the spill-back valve to close. This does two things:
      1.      Total number of moles of wet gas compressed will drop.
      2.      Compression ratio (due to the suction throttle valve delta P) will increase.
      As long as you are on the flat portion of the flow vs. head compressor performance curve, the motor amps will decline.

      --Norm Lieberman


      Dear Norm – How can I tell for sure my debutanizer trays are flooding without running an expensive TRU-TEC radiation scan?
      --Fred, Texas

      Fred – A delta P survey is not always meaningful unless flooding is starting at the lower trays. Thus, with the reboiler on auto, increase the reflux. The tower bottom minus top temperature should increase. If this doesn’t happen, then the tower is losing tray efficiency due to entrainment. A loss of tray efficiency with increasing vapor-liquid traffic is due to jet flood.