The heel, and why we publish it

A sorbent bed does not come back empty. Heat it at 95 °C under 80 millibar for forty minutes and most of the carbon dioxide leaves, but a fraction stays bound to the strongest sites on the amine. On BR-4 at the duty we run, that fraction settles at 0.06 millimoles per gram. We call it the heel and we print it on the front page.

It is a commercial number, not a chemical one

There is no temperature at which the heel is zero. There is only a temperature and a dwell time at which stripping the next tenth of a millimole costs more in heat than the tonne it releases is worth. That crossover is where a plant stops, and different plants stop in different places for entirely defensible reasons.

Ours is at 0.06. Pushing to 0.03 is possible on this bed and takes roughly a third more thermal energy per cycle for about four per cent more working capacity. We ran that comparison for six weeks in the spring and stopped, because the arithmetic did not survive contact with our heat contract.

What it changes for a buyer

Two things, both worth knowing before you sign anything.

• Working capacity, not equilibrium capacity, is what you are buying. The bed holds 1.25 millimoles per gram at equilibrium. It delivers 1.126 per cycle. Anyone quoting the first number is quoting a laboratory result.
• The heel is stable, and a rising heel is a warning. It creeps up as the amine oxidises. We measure it every cycle, and a bed whose heel has climbed past 0.09 is a bed on its way out of service. It is the earliest number we have that says a column needs changing, which is why it goes in the run log rather than in a report at the end of the quarter.

Why it is on the front page

Because the alternative was a page that only showed the bed filling. A bed that fills and empties cleanly, cycle after cycle, is not what we operate. The last heading on our front page is heavier than the first by exactly the weight of the heel, and that is the most honest thing the page does.