An Essence of Engineering

A small machine needs a continuous supply of oxygen. Chemistry is too complex. Why not pull it from the air all around us. No shortage of raw material.

Absorption (a) and adsorption (b) mechanism. Blue spheres are solute molecules.

Chemists tell engineers about adsorption [see diagram] Note though, that our process b) is gas to solid not liquid to solid as shown Nitrogen gas clings to some fine powders. Pass air over the powder; nitrogen is trapped and out comes pretty pure oxygen.

But there is a problem, so the engineer comes up with a clever solution that is not science but quintessential engineering.

Concentrated oxygen gas is needed for many uses in the modern world. Industrial processes such as steel making, engineering cutting, medicine and others need vast tonnages of the stuff. Modern steel production needs oxygen by the tonne; as do many large-scale chemical processes.

This school chemistry toy process of potassium chlorate or hydrogen peroxide is not going to give that. Engineers use cryogenic air separation to extract it from the air. For this, in simple terms, air is cooled down till it is a liquid and then very carefully allowed to warm up and at-196C [if it is at atmospheric pressure] the nitrogen boils off leaving, the liquid oxygen (which boils at -183C) behind

On a slightly smaller scale it is needed for oxy-acetylene cutting torches and supplies for whole hospitals. Here, although still produced cryogenically, it is distributed to customers in enormously heavy, forged steel bottles.

On a smaller scale still, many people with breathing problems need extra oxygen to live.  They need pure oxygen all the time in a form that they can breathe. Sometimes, for use by one person it comes in quite small bottles say only about 20cm long but, even so, it is still quite heavy to carry for any length of time.

Similarly, extra oxygen is needed to keep the transplant liver of the recent post alive throughout its journey, be that 2 hrs or 30 hrs.  Machine portability also meant low weight was very important. Doing it cryogenically would be too heavy and power hungry.

The answer for the engineer was an oxygen concentrator using adsorption.

The fine powders have an enormous surface area for the gas to cling. But air contains a staggeringly large number of nitrogen atoms. When the area is coated by nitrogen, more nitrogen passes through and contaminates the oxygen stream. It must be regenerated to free the old nitrogen.

But for an hour of oxygen, chemists tell us we will need an enormous heap of powder. This is possibly acceptable in a large plant with room for a big machine: but not for continuous oxygen for a liver transplant machine or an ill person to carry.

This brings us to the crux of the problem. Either we arrange a heavy container of a very large quantity of powder to give us long life before oxygen supply stops for regeneration. Or we have a small light container of powder that has a short life before oxygen supply stops for regeneration. Apparent stalemate.


The engineer’s insight was to use two very small containers at the same time. Air is pumped through the first container of zeolite powder until the powder is saturated and nitrogen comes through. So we stop using the first and start pushing air through the second. While the second is doing its job, the pressure in the first is dropped and the powder regenerates by releasing the nitrogen it has adsorbed. Then when the second is saturated, the first is now ready to take over and the second can regenerate.

So it is that small, light containers can produce a continuous supply of oxygen. Here is a good graphic. Now that’s engineering!

Without going into a lot of detail, absorption is that to absorb one substance within another. An example is of sugar being absorbed into a cup of coffee and being very difficult to separate again. Adsorption, on the other hand, is where one substance clings, fairly lightly, to the surfaces of another. Some say that, although not seen by The Lad, if a cube of pressed carbon powder is dropped into a glass of dyed water the colour will be removed as the dye clings to the surfaces of the carbon powder. But, crucially, this process can be relatively easily reversed: vigorously shake the glass of water and carbon and the colour will return to the water. This is regeneration.

Engineering is one of the three drivers advancing the human race. This blog describes professional engineering in the real world as it is not well served by the current media. An engineer is posting: not a ‘scientist’. Its target is the career seeker and also the general public.


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