Liquefaction of Gases

Now suppose you put enough pressure on a gas to halve its volume — to make a pint of it into half a pint. The half-pint has as many molecules in it as the pint had. There are twice as many molecules in the gas, so it hits twice as many blows on a given area as it did when it was a pint. Accordingly, it is thrusting on its container twice as hard and it has twice the pressure.

If pressure is put upon any gas the molecules are crowded by the pressure towards each other, and when they get very near to each other they get within the range of each other's attraction. If the gas is one like carbon dioxide or sulphur dioxide, the crowded molecules may pull on each other so strongly that they hang together and the gas becomes a liquid. It is thus possible to turn many gases into liquids simply by compressing them. Ammonia, carbon dioxide, sulphur dioxide, chlorine and some other gases can easily be turned into liquid in this way. Any gas in fact can be turned into a liquid by compressing it — as long as it is not too hot. The jostling of the molecules, which we call heat, prevents the molecules clinging together and making a liquid; the attraction of the molecules pulls them together and causes them to make a liquid.

If there is a strong attraction, as with ammonia or carbon dioxide, pressure will liquefy the gas even if fairly warm; but gases like oxygen or hydrogen can only be liquefied by pressure if their molecules are calmed down by a great deal of cooling. So, if we try to see what happens if we compress a gas to the greatest extent possible, we find that it starts by halving its volume each time we double the pressure. Then we begin to find it more than halves its volume when we double the pressure on account of the molecules attracting each other. Then either the gas collapses into a liquid, or, if it is too hot to do this, increase of pressure drives the molecules still nearer and makes the volume smaller. Now the molecules get so close that they repel each other, and as their outer rings of electrons get nearer the repulsion between them gets huge and the gas becomes more and more difficult to compress and finally is incompressible as a liquid or a solid. The liquefying of gases is an important industry. A gas takes up several hundred times as much room as it does in the form of a liquid and so if we want to send it by train or ship it, it is best to send it as a liquid. Chlorine gas — the green-poison-gas — is used for many quite beneficent purposes such as bleaching, making dyes, medicines, etc. A ton of chlorine as gas would have a volume of 422 cubic yards. It would take about forty railway trucks to hold it.

If chlorine is compressed, it collapses to a greenish liquid, which is run into closed steel boilers mounted on railway wheels. A ton of chlorine as liquid occupies only one cubic yard. The chlorine under the pressure of some seven atmospheres (105 lbs. per 1 square inch) in the boiler remains liquid permanently. If the boiler were to be smashed up in a railway accident the effects would not be quite as disastrous as might be expected, for the evaporation of the liquid would cool it intensely and the gas would be but slowly evolved.

Gases like oxygen and hydrogen will remain liquid only at very low temperatures ( — 150 °C to — 250 °C) and so it is almost as difficult to keep them liquid at ordinary temperatures as it would be to keep water liquid if the world were red-hot! Accordingly, we transport oxygen and hydrogen compressed in cylinders to 120 times the pressure of the air. If the cylinder holds 1 cubic foot, we can accordingly pack 120 cubic feet of gas into it. Higher pressures would be too dangerous.

Regenerative Cooling, — Air, oxygen and such other gases as cannot be liquefied by simply compressing them at ordinary temperatures are now easily liquefied on the large scale by what is called “regenerativecooling”.

To liquefy air, we want a temperature of — 185 °C, compared to which the North Pole is a hot-house. Now cooling is just the slowing up of molecules: to liquefy air we want to slow up its molecules. How shall we do this? Well, if you want to slow up a stream of water you can make it push a water-wheel round; if you want to slow a horse, let it pull a cart; if you want to slow a molecule, let it do some work and so part with some of its energy. The method finally adopted is this. First compress your air and let it cool down to room temperature. Then make your cold compressed air push the piston of an air engine round. The piston is speeded up only by slowing the molecules down; in other words by cooling them. The air which comes out of the engine is at about — 50 °C. But this is not nearly cold enough; and this is where the clever trick comes in — we use this cold air to cool the compressed air before it reaches the engine. Our next lot of air reaches the cylinder at, say, — 40 °C, and by pushing the piston slows down its own molecules and comes out at, say, - 90 ºC. This very cold air cools the incoming air still more, so that ever colder air goes on coming into the cylinder and air much colder still leaves it, until quite soon — 180 °С is reached and the air liquefies. Liquid air boils at about — 185 °C, and therefore boiling liquid air is a very good means for making things extremely cold [2, С. 71 - 73].

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