SS: High Pressure Gas-Liquid Separation: CO2 Droplets: One-Component Two-Phase System as Model Fluid for High-Pressure Hydrocarbon
Authors: Dupuy Pablo Matas , Fernandino Maria , Svendsen Hallvard , Westra Remko 
Published: July 2010

Abstract
In the process of performing either scientific experiments or research and development related to the design and optimization of high pressure separator units, both laboratory experiments and tests in prototypes are needed. In order to emulate the low interfacial tensions often experienced in high pressure hydrocarbon systems, the utilization of carbon dioxide (CO2) as model fluid is studied. The present work describes how the CO2 system behaves at saturation conditions. It describes this system and compares it with traditional laboratory systems and real fluids (from the field). CO2 at saturation pressure under normal temperatures presents an interesting system with low interfacial tension, below 3 mN/m, while the liquid/gas density ratio is around 3. The availability of the fluid (CO2) in research centers and academia is high. When planning a matrix of experiments as part of a data base of reproducible laboratory fluids, the present system is an independent base vector ideal for studying the high Weber/low Reynolds number regime.
Results show that it is possible to obtain streams of droplets for droplet experiments. The mean diameter in the studied regime with the utilized nozzle was of the order of 100 microns, while the smallest droplets possible to track with the presented technique were around 40 microns. Droplet/wall collision experiments were focused in this work. Both coalescence and bouncing were observed on both dry and wet walls.
The absence of real fluid experiments at laboratory conditions generates a lack of basic knowledge about what is happening in real scrubbers. This system is proposed to be representative for a part of the flow/property region of interest in real scrubbers. This basic knowledge is fundamental when designing separation units at high pressures for gas processing stages such as sub sea gas separation concepts.

Introduction
Separation in high-pressure gas-liquid hydrocarbon systems represents a topic of interest for many engineering fields. E.g. in liquid-in-gas flows, where all the liquid is to be removed through processes involving droplet collisions with droplets, walls and liquid surfaces. Presently, there is no perfect solution for liquid removal at high pressures since the internal physics and mechanisms are not yet fully understood. Computational models are not reliable either and need experimental validation.
Proper experimental investigations in these systems are very difficult as the following challenges have to be taken into account: explosion risk, properties dependent on equilibrium conditions between the two phases, mixtures showing near critical-point behaviour, and increased density of the gas phase, among others. If a good experiment design is obtained for a given gas-condensate composition, there is no scaling rule for a different composition, since physical properties are hardly known.
When an experiment with real scales and fluids cannot be run, laboratory models and test facilities are used instead. The idea relies on the fact that the system of equations that rules both the model and the real cases are the same but with different parameters representing physical and geometrical magnitudes, but if similitude is conserved scaling rules apply. Hoffmann and Stein (2008) justify the use of similitude for two reasons: " Certain simplified scaling rules and dimensionless quantities allow the designer or practitioner to make quick, ‘back-of-the-envelope’ type calculations and decisions pertaining to cyclone design and performance.?? And, " the scaling formulae allow one to better ‘see’ the effects of changes in one variable upon another - both qualitatively and quantitatively.??