What method of separation to separate methane from mixture of gases?

This separation is done at huge scale via distillation. Chilling as noted. Many engineering firms are skilled in the design of those systems with heat integration to limit the energy costs.

Adding more solid.

Explanation:

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Based on the concern, it is known that an unsaturated solution is made when one adds an amount of the solid that is smaller than its solubility for the given amount of solute at the specified temperature, thus, to make a saturated solution, more solute must be added in order for it to equal the solubility at the very same amount of the solvent and at the same specified temperature.

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