In this paper, we develop a simplified control oriented model of a compact flotation unit (CFU), which removes residual oil from produced water in oil and gas production systems. CFU is a class of separators that exploits the synergy between separation effects of a swirling flow and the effect of flotation, in which small gas bubbles attach to the oil droplets and float to the top of the separator, where they are removed. The purified water flows downward, and is removed from the bottom. Our CFU model consists of a simplified initial swirl separation part and a flotation part, in which populations of oil droplets, gas bubbles without oil droplet, and gas bubbles with oil droplet attached are tracked spatially. After analyzing the model, we use it as a basis for designing a control structure that operates the separation system optimally.