A mixing device for mixing concurrently-flowing streams of vapor and liquid in a vessel contains flow baffles forming a tangential inlet orifice through which the liquid and vapor streams flow at high flow velocity and in a tangential direction into a swirl box. The high momentum of the streams in the inlet orifice, the tangential direction of the streams, and the large distance between the inlet orifice and the vessel centerline, result in a violent swirling flow and a large number of fluid rotations in the swirl box, thereby thoroughly mixing the streams entering through the inlet orifice. A mixed stream of vapor and liquid exits the mixing device through an outlet opening in the bottom of the mixing device. An impingement plate located below the outlet opening spreads the liquid and decreases the velocity of the mixed stream. The temperature and chemical composition of the mixed stream are equilibrated.