2 replies to this topic

[dylant]

Does anyone know how does air separation plant get its cold energy? How much cold energy do they need to cool off 1 kg of feed?

Thanks,
Dylan

[Art Montemayor]

Dylan:

Carl von Linde revolutionized the concept of making compressed gases when he liquefied atmospheric air and subsequently distilled it into its basic components: Oxygen and Nitrogen.

He started from the idea of using the air itself as a refrigerant - using the cold generated when air moves from a higher pressure to a lower one for the needed cooling. This effect had already been described in 1862 by researchers William Thomson and James Prescott Joule - two individuals who have indelibly etched their names into the history of Chemical Engineering.

The higher air is compressed, the more cold is generated when it expands. This cooling effect (the famous Joule-Thomson Effect) increases exponentially when the air is pre-cooled. However, von Linde could not achieve the temperature needed to liquefy the air (about minus 190 ^oC) from expansion alone. That required a pre-cooling cycle in which the cold air generated by the expansion is used to further cool the compressed, pre-cooled feed air in a countercurrent manner. In a continuous process, the cold given off from each cycle was multiplied until the product air was liquefied and could be collected in a container.

While proven technology was available for compression and pre-cooling, the challenge for von Linde was to develop a suitable countercurrent apparatus. He decided on a 100-meter long double steel tube, which was wound into a spiral and well insulated, encased in wood. This was a rudimentary heat exchanger and it worked enough to allow Linde to successfully accomplish what he needed in 1895. The subsequent distillation was carried out in a "double" distillation column, where the overhead nitrogen gas from the high pressure (1st column) was used to furnish reboil heat to the low pressure column (2nd column) and liquefy itself and furnish automatic reflux to the 1st column. The low pressure column physically "sat" on the high pressure column. I consider this one (if not THE) of the greatest engineering ideas yet conceived. This is more than worth any student's time to research and learn more about. You will be fascinated by the ingenuity employed.

Linde’s original process of thermodynamic free expansion (at constant enthalpy) was vastly improved when Georges Claude applied the next thermodynamic process feature: free expansion of the compressed air through an expansion engine that not only increases the cooling effect, but also generates useful work (at constant entropy). The original Claude engine was nothing more than a metal piston using leather cups for sealing and working much like a steam engine in reverse. Today, turbo-expanders (which are much like centrifugal compressors working in reverse) are the machine of choice.

That’s how cold is produced to liquefy the feed air. Knowing the thermodynamic process, you should be able to calculate the theoretical amount of refrigeration required to liquefy 1 kg of air. That sounds like a neat Thermo problem to solve.

[Padmakar Katre]

Dear Dylan,
I have an article reagrding Air Seperation Unit of Linde's Double column method.I am uplaoding it for you.I hope it will help you.

Regards,
Padmakar

Attached Files

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