Comparison of horizontal and vertical three-phase separators
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With the development of our country’s economy, energy consumption is increasing year by year, while the available reserves of oil and natural gas are decreasing. Increasing the recovery rate of oil and natural gas and making the most of existing resources to produce more energy is one of the main tasks at present. The three-phase separator is usually the first device on an oil production platform that processes well fluid; subsequent to it are devices such as heat exchangers, exchangers, and distillation towers. Therefore, the efficiency of the three-phase separator is crucial for the economic and stable extraction of oil and gas, and it also has a significant impact on the efficiency and service life of the subsequent devices. Comparison between horizontal and vertical three-phase separators: Currently, three-phase separators are mainly divided into horizontal three-phase separators and vertical three-phase separators. When performing the same separation task, vertical separators take up less space than horizontal separators ； However, under the same static flow conditions, the horizontal separator has a better liquid buffering capacity ； Vertical separators are more conducive to the sedimentation of sand in oil and water, and allow for easy removal from the bottom. Vertical separators are more effective in handling crude oil production fluids with a low oil-to-gas ratio and solid particles; therefore, vertical three-phase separators are primarily used in practical applications on oilfield platforms where the amount of crude oil processed is small and the sand content is high. In a horizontal separator, the gravity settling velocity is perpendicular to the flow velocity, while in a vertical container it moves in the opposite direction to the flow. Therefore, when using the principle of gravity settlement for separation, horizontal separators are more effective than vertical ones. Additionally, horizontal separators have a larger surface area, which helps to increase the stability of phase equilibrium; hence they are more effective when dealing with crude oil production fluids that contain bubbles or are emulsified. Vertical separators are generally quite tall; therefore, without specialized ladders and platforms, it is difficult to operate the control devices inside the separator. Moreover, if there are height restrictions during transportation, the containers on the skid-mounted unit sometimes need to be removed from the separator. In summary, the cost of a horizontal separator of the same volume is lower than that of a vertical separator ； The outer wall of a vertical separator with the same volume and flow rate must be thicker than that of a horizontal separator. If the vertical separator is exposed to strong winds, the load acting on its sides must be taken into account, which also increases the wall thickness and thus raises the cost of the container. As can be seen from the above comparison, horizontal separators are the most economical and practical choice for conventional oil-gas separation, especially for treating emulsions, foams, and crude oil production fluids with a high oil-to-gas ratio. China generally has a high oil production level. In recent years, the areas where oil has been discovered in China have poor environmental conditions, which make extraction there difficult; as a result, the oil is transported to downstream facilities for processing. This leads to a high flow rate of crude oil entering the separator, thereby causing significant disruption to the oil-water interface within it. Horizontal three-phase separators, however, have a larger oil-water interface, which helps to prevent disruptions to this interface caused by high inlet flow rates. Therefore, horizontal three-phase separators are generally used on oil production platforms in our country.
Internal components of the separator: The inlet component is primarily used to separate the gas phase from the liquid phase. It achieves gas-liquid separation by rapidly changing the momentum of the incoming fluid. Although the designs of the inlet components vary in practical applications, they can be broadly classified into three types based on their primary mechanism of action: baffles, swirl elements, and elbow pipes, as shown in Figures 2-1, 2-2, and 2-3 respectively. The gas and liquid components in the feed fluid have the same velocity; since the density of the liquid phase is higher than that of the gas phase, the liquid phase possesses higher energy and is less prone to a change in momentum direction compared to the gas phase. Therefore, when the feed fluid hits the baffle, the liquid phase will settle downward, while the gas phase will flow around the baffle. Using hemispherical or conical baffles results in less disturbance to the flow compared to angle irons or flat plates, which helps to effectively reduce the problems of mist entrainment or emulsification.
2.1 Cyclone element: The cyclone element uses centrifugal force to separate the gas phase from the liquid phase. The use of swirl elements can effectively separate gas-liquid two-phase fluids, thereby minimizing the likelihood of crude oil foaming. However, the use of swirl elements generally requires a tangential velocity of at least 6 m/s; as a result, the effective range for adjustment is limited. When the flow rate decreases, the reduction in tangential velocity significantly affects the separation performance of the swirl elements. When the particle size distribution of the liquid phase in the fluid is between 1 and 50 μm³, the liquid phase exhibits good flowability and is not easily separated through vortex motion; separation can be achieved using a collision-based separation mechanism. A bent tube utilizes collision to separate the gas and liquid phases. Inside the bent tube, the liquid phase slows down along with the gas phase and collides with the wall surface; the liquid phase is then adsorbed to the wall surface, thereby completing the separation of the liquid phase from the gas phase. 2.2 Rectifying elements: When the feed fluid enters the container at a high velocity, it causes significant fluctuations in the fluid within the container, which affects the liquid phase separation in the gravity separation zone and increases the time required for gravity separation. Therefore, it is necessary to install flow rectifying elements at the front section of the gravity separation zone in the separator in order to reduce the impact of feed fluctuations and stabilize the flow field, so that the liquid level control instruments and the overflow outlet can function properly. Generally speaking, a rectifying element is a perforated baffle installed in the gravity sedimentation area of the separator, perpendicular to the flow direction. 2.3 Coalescing elements The coalescing elements provide a surface area for water droplets to coalesce, and serve as baffles and vortex suppressors. Generally, the coalescing elements are installed at positions about 1/3 and 2/3 from the ends of the container. There is space above and below the coalescing element, allowing solids and water to pass through from below, while gas passes through from above. In the gravitational sedimentation zone, if the droplet size is small, its sedimentation speed will be very slow, resulting in a longer time for liquid phase separation. By installing coalescing elements, small droplets can be combined into larger ones, thereby reducing the liquid phase separation time. As the droplets flow past the coalescing element, they are captured by its surface due to surface forces, resulting in the formation of a film on the surface of the coalescing element. Subsequently, the captured droplet collides with the film; the droplet deforms and a liquid interlayer is formed between it and the film. Due to the pressure exerted by the droplet, this interlayer gradually spreads outward and becomes thinner. When the thickness of the interlayer reaches the critical value δc, it ruptures and the droplet merges into the film. This process is shown in Figure 2-4, thereby completing the coalescence of the droplets on the coalescing element. When the droplets coalesce to a sufficiently large particle size, they settle and separate from the coalescing element.
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