Reboilers

Question:

            Should we de-superheat our steam to our amine regeneration reboiler?

Answer:

            Yes. Use of saturated steam will retard the thermal degradation of the amine, and reduce the rate of reboiler tube failure. It’s a standard industry practice, especially critical if you are using MEA.

Reboilers

Question:

            Our steam supply is contaminated with non-condensibles (maybe CO₂?). We opened the vent on the top of the channel head to purge out the non-condensibles which we think are reducing steam condensation rates and our reboiler duty. This did not help. What do you suggest?

Answer:

            The non-condensibles will not accumulate in the top of the channel. They accumulate below the bottom pass partition baffle in the channel head. That is, the portion of the channel head that must be vented. Opening the vent on top of the channel head will vent out steam, not the non-condensible CO₂.

Reboilers

Question:

            I retubed a butane splitter (iso vs. normal) reboiler recently. Twenty percent of the tubes were plugged. Yet I lost reboiler capacity with the retubed bundle. Why did we lose capacity, rather than increase capacity by twenty percent?

Answer:

            The problem is smooth tubes that lack nucleate boiling sites. This happens all the time. The old tubes were rough and pitted. Increasing the condensing pressure of the reboiler steam will only make the problem worse. One possibility is to sand blast the tubes to roughen their surface. Alternately, you can use “Linde Hi-flux,” sintered metal coating. Whichever you find most cost effective.

Reboilers

Question:

            Does superheating steam aid or retard heat transfer?

Answer:

            I had a reboiler on a debutanizer reboiler in Aruba, supplied with 400 psig, 650°F superheated steam. There was a de-superheating station (i.e., a condensate injection point). As I reduced the steam supply temperature (at a constant pressure) with the condensate injection, my reboiler duty did not change. I believe the conventional answer is that superheat retards heat transfer. But, this was not the case in my only field experiment.

Reboilers

Question:

            Sometimes, when I OPEN the channel head condensate drain valve, my reboiler duty increases. But, sometimes when I partly CLOSE off the same valve, my reboiler duty decreases. What’s happening?

Answer:

            There are two problems with draining condensate from the channel head:

1.      Blowing the condensate seal.

2.      Condensate back-up.

If you open the drain valve too much, steam and water will blow-out together. The high velocity steam, blowing though the tubes, will reduce the condensing heat transfer coefficient. Hence, the observed loss in heat transfer and reboiler duty.

If you close the drain valve too much, water will back-up and submerge the lower few rows of tubes, which will reduce the surface area exposed to the condensing steam, which also diminishes the rate of heat transfer.

Without some method of observing the steam condensate level in the channel head, the optimum position of your condensate drain valve can only be found by trial and error.