In oil and gas production, large gravity based separators are used to separate the different phases; water, oil, and gas, into clean phases. The produced water is cleaned in a number of treatment stages to reduce the oil content to an acceptable level. This water is either reinjected into the reservoir to increase the hydrocarbon recovery, injected into a disposal well, or discharged over board. A typical North Sea offshore oil production system consists of two to four stages of bulk separation and two or three stages of produced water treatment, as illustrated in Figure 1.
Upstream and in-between the different separators and produced water treatment stages, valves and pumps are installed to control flow and pressure. These valves and pumps are regarded as a necessary evil with regards to the separation efficiency. Shear forces acting on the fluids passing through the valves and pumps cause emulsification and droplet break-up which have a detrimental impact on downstream separator efficiency.
A number of measures can be employed to counteract the negative impact on separation efficiency from these valves and pumps. The residence time of a separator can be increased, more treatment stages can be utilised, heat can be applied, separation enhancing production chemicals can utilised, etc. These measures can, however, have a significant negative impact on the CAPEX; increased footprint and weight, and OPEX; power and chemical consumption. A more constructive way is therefore to deal with the cause of the issue/problem, which is to reduce the negative effect of the valves and pumps on the separation efficiency.
Oil water separation in gravity separators occurs by droplets from the dispersed phases traveling through the continuous phases to the phase interphase. The maximum vertical velocity (terminal velocity) of a droplet in fluid can be described by the equilibrium of the forces acting on the droplet, being buoyancy, gravity and drag. This results in Stokes’ Law:
Where:
v
t Terminal velocity [m/s]
g Gravitational constant [m/s²]
d Droplet diameter [m]
ρ
c Density continuous phase [kg/m³]
ρ
d Density dispersed phase [kg/m³]
µ
c Dynamic Viscosity continuous phase [Pa·s]
The diameter of the droplet has the largest impact on the settling velocity. Doubling the droplet diameter will result in a quadrupling of the velocity. Droplet size therefore has a large impact on the separation efficiency, and maximizing the droplet size is vital for efficient separation.
Flow in pipes, valves and pumps of a petroleum process system, is normally turbulent. Hinze has formulated the maximum droplet size that can exist in a turbulent flow regime:
Where:
We
crit Critical Weber number [-]
σ Interfacial tension [N/m]
ε Mean energy dissipation rate per unit mass [W/kg]
The critical Weber number, density and interfacial tension are all dependent on the fluid properties and composition, and can therefore be regarded as constant. The only variable left to influence the maximum droplet size is therefore the mean energy dissipation rate per unit mass, ε. In order to increase the maximum droplet size, ε has to be decreased.
For the flow through a valve ε can be defined as:
Where:
ΔP
perm The permanent pressure drop [Pa]
Q Flow rate [m³/s]
V
dis Volume used for energy dissipation [m³]
Hence, in a given flow system, the only parameter that can be used to reduce the mean energy dissipation is the volume involved in dissipating the required energy, V
dis. By controlling this volume the mean energy dissipation rate, and thereby the droplet size, can be controlled, which in turn has a large impact on the separation efficiency.
