High Pressure Gas-Liquid Separation: Diffusion Coefficient, the Silent Coalescence Inhibitor
Authors: Dupuy P.M. , Fernandino M. , Jakobsen H.A. , Svendsen H. 
Published: July 2010

Abstract
Liquid from gas separation is beneficed from the coalescence of droplets with other droplets or with liquid films formed in separator internals. The low liquid droplet coalescence efficiency observed at high pressures is a technical problem which is under research. Achieving higher deposition efficiencies at high pressures may allow for separators to be operated at higher pressures in the process chain. In order to maximize the rewards of subsea separation and processing, high pressure separation must be mastered.
This paper contains a theoretical study of the reasons that could lead to a low deposition efficiency. From all the phenomena involved only one is selected to study, the coalescence of liquid when a droplet approaches close enough to another liquid surface. From this phenomenon, special attention is presented to the diffusion coefficient, the parameter that changes most from ambient conditions to high pressures.

Introduction
High pressure separation in a process chain may offer several advantages if a reasonable understanding of the separation performance at these conditions is available. For subsea separation installations, separation at high pressures may be a necessity for deepwaters if series pump boosting is not desirable. To elaborate, if the operating pressure of the subsea separator must be in a typical range (&lt;75 bar), then the boosting requirements for current commercially available pumps may require pumps in series. Thus significantly enhancing the subsea infrastructure and deteriorating the incentives of subsea processing. Furthermore, separation at high pressures may enable sizable reductions in compressor requirements therefore allowing for CAPEX savings and operation power savings. Nonetheless, there still exists a gap in the industry's ability to accurately predict the gas/liquid separation at high pressure.
High pressure research and experimentation into separation phenomenon are being performed at the Norwegian University of Science and Technology. This research is providing intriguing results that will act to provide a key understanding into the design and performance prediction of high pressure separators.
Efficient gas/liquid separation requires that any liquid droplets suspended in the gas phase must settle or be collected from the gas phase before exiting the separator. The ideal method to achieve this collection of small liquid droplets is for the droplets to coalesce with one another, or to the wall, thus forming larger volumes of liquid that are more easily separated. However, at higher pressures the effectiveness of liquid coalescence differs from that at ambient conditions. The overall deposition efficiency is altered first, by the possibility of a droplet of reaching a wall. Higher pressures reduce the interfacial tension, reducing the mean droplet size of a droplet population. Smaller the droplet size, smaller the Stokes number. The gas density is increased as well, reducing the density difference between gas and liquid. This reduces even more the Stokes number. This means that the particle will, most likely, follow the gas fluid more easily, since its own inertia becomes less important compared with drag forces. Since the gas velocity normal to the wall is zero, thus the droplets are not carried into the wall. External forces can be used to force them to reach the wall or surface such as electrical fields or acceleration fields. Acceleration fields such as gravity or centrifugal produced by rotations have the disadvantage that generates a force which is proportional to the density. Thus the separation force ratio is again related to the density ratio. On the other hand, the gas turbulent structure will affect more dramatically the droplet trajectory, but keeping the velocity normal to the wall equal to zero.
Once the droplet reaches the wall by either a strong acceleration field or chance, it interacts with the liquid surface, i.e. it collides. The outcomes of the collision can be of several different types, Yarin (2006) and Rein (2002).