Such mixtures appear, e.g., in the recovery of natural resources from oil and gas wells. The products of such recoveries can include mixtures of gas, oil and water. These mixtures may comprise three phases. Depending on pressures and other physical circumstances the mixtures recovered may comprise two phases, a hydrocarbon phase containing oil and optionally dissolved gas, and water. It also occurs that hardly any gas is present so that there is a two-phase mixture.
It is desirable to separate the water phase from the hydrocarbon phase before transporting the valuable products further. Thereto devices have been developed to separate such mixtures into the respective phases, and removing the water phase.
Russian patent publication No. 2 077 364 discloses a device for separating a mixture of fluids into three phases, gas, liquid of lower density and a liquid of higher density, having a feed inlet, a gas outlet, and outlets for the liquid phases. During operation of this device, a mixture of gas, low-density liquid and high-density liquid is supplied to the feed inlet of the separation device. The mixture passes upwards through an upwardly inclined supply pipe towards the inlet of a downwardly inclined pipe. In the upper end of the downwardly inclined pipe, gas is separated from the liquids that fall downwards towards the lower end of the downwardly inclined pipe. Gas, lighter liquid and heavier liquid are separately removed from the device via the respective outlets.
A disadvantage of the known separation device is that turbulence is generated in the upwardly inclined feed pipe and that counter-current flow prevails in the downwardly inclined pipe, which phenomena adversely affect the separation efficiency of the device.
European Patent Application No. 1044711 describes a device for separating a mixture of fluids into three phases; gas, lower-density liquid (“lighter liquid”) and higher-density liquid (“heavier liquid”), having a feed inlet, a gas outlet, an outlet for the lighter liquid and an outlet for the heavier liquid. The device comprises:                a normally horizontal supply pipe with the feed inlet at its upstream end;        an inclined pipe having an inlet at its upper end that is connected to the outlet of the supply pipe and having a closed lower end;        a gas discharge system comprising a gas riser having an inlet that is located in the gas-filled space and an outlet that is in fluid communication with the gas outlet of the device;        a discharge system for lighter liquid having an inlet that is located in the lighter liquid-filled space and an outlet that is in fluid communication with the outlet for lighter liquid of the device; and        a discharge system for heavier liquid having an inlet arranged below the bottom level of the supply pipe and an outlet that is in fluid communication with the outlet for heavier liquid of the device,wherein the diameter of the supply pipe is selected such that during normal operation the velocities of the liquids are below a pre-determined value, wherein the ratio of the length of the supply pipe to its diameter is larger than 10, and wherein the slope of the inclined pipe is selected such that during normal operations a stratified flow is maintained in the inclined pipe.        
One of the advantages of this device over the device of the Russian patent is the maintenance of a stratified flow. However, the discharge systems, especially the discharge system for the heavier liquid is complicated. Although the design can operate at a wide range of operating conditions, it does not provide for any active flow control.