The petroleum industry has recently shown interest in the development of innovative alternatives to the conventional vessel-type separator. One such alternative is the gas-liquid cylindrical cyclone (GLCC) separator,,which is simple, compact, and low weight, and has low capital and operational costs. A new mechanistic model is proposed, for the first time, to predict the aspect ratio of the GLCC, based on its complex hydrodynamic multiphase flow behavior This model incorporates an analytical solution for the gas-liquid vortex interface shape, and a unified particle trajectory model for bubbles and droplets. A simplified GLCC design methodology, based on the foregoing mechanistic model, is developed and specific design criteria are proposed as user guidelines for GLCC design. A summary of four actual field application designs is provided to demonstrate the capability of the aspect ratio modeling and the impact that the GLCC technology may have on the petroleum industry.