Chemical Process
The purpose of tanning in leather industry is to

Stiffen the leather
Smoothen the leather
Impart water resistance
Make it flexible

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Chemical Process
Liquefaction of gases can not be done by

Adiabatic expansion through a throttle valve (Joule-Thomson expansion)
Exchange of heat with colder stream
Adiabatic expansion against a piston or in a turbine
Merely compressing it beyond critical pressure

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