I don't remember seeing 'empyreal' before,only 'empyrean'.
Webster 1913 seems to emphasize 'fire' in 'empyreal' and
seventh sphere in 'empyrean'.

Webster's 1913 Dictionary

Definition: \Em`py*re"an\ (?; 277), n. [See {Empyreal}.]
The highest heaven, where the pure element of fire was
supposed by the ancients to subsist.

The empyrean rung With hallelujahs. --Milton.

Webster's 1913 Dictionary

Definition: \Em*pyr"e*al\, a. [L. empyrius, empyreus, fiery, Gr. ?,
?, in fire, fiery; ? in + ? fire. See {In}, and {Fire}.]
Formed of pure fire or light; refined beyond a["e]rial
substance; pertaining to the highest and purest region of
heaven.

Go, soar with Plato to the empyreal sphere. --Pope.

{Empyreal air}, oxygen gas.


\Em*pyr"e*al\, n.
Empyrean. --Mrs. Browning.