The gas handling requirements for three-phase separation are dealt with in a similar manner as discussed for two-phase separation. Traditionally, sizing for liquid-liquid separation has involved specification of liquid residence times.
Fig. 7-22 provides suggested residence times for various liquid-liquid separation applications. These figures generally assume equal residence times for both the light and heavy liquid phases.
While the residence time approach for liquid-liquid separation equipment design has been widely used in industry for years, it does have some limitations.
• the typical approach of assuming equal residence times for both liquid phases may not be optimum, e.g. It is generally much easier to separate oil droplets from water than vice-versa. Settling theory (Eq 7-1) explains this as being due to the lower viscosity of water compared to oil.
• Residence times do not take into account vessel geometry, i.e. 3 minutes residence time in the bottom of a tall, small diameter vertical vessel will not achieve the same separation performance as 3 minutes in a horizontal separator, again according to droplet settling theory.
• The residence time method does not provide any direct
indication as to the quality of the separated liquids, e.g. amount of water in the hydrocarbon or the amount of hydrocarbon in the water. Droplet settling theory can not do this either in most cases, but there is some empirical data available which allows for approximate predictions in specific applications.
Removal of very small droplets may require the use of specialized internals or the application of electrostatic fields to promote coalescence.
Liquid-liquid separation may be divided into two broad categories of operation. The first is defined as “gravity separation,” where the two immiscible liquid phases separate within the vessel by the differences in density of the liquids. Sufficient retention time must be provided in the separator to allow for the gravity separation to take place. The second category is defined as “coalescing separation.” This is where small particles of one liquid phase must be separated or removed from a large quantity of another liquid phase. Different types of internal construction of separators much be provided for each type of liquid-liquid separators. The following principles of design for liquid-liquid separation apply equally for horizontal or vertical separators. Horizontal vessels have some advantage over verticals for liquid-liquid separation, due to the larger interface area available in the horizontal style, and the shorter distance particles must travel to coalesce.
There are two factors that may prevent two liquid phases from separating due to differences in specific gravity:
• If droplet particles are so small that they may be suspended by Brownian movement. This is defined as a random motion that is greater than directed movement due to gravity for particles less than 0.1 micron in diameter.
• The droplets may carry electric charges due to dissolved ions. These charges can cause the droplets to repel each other rather than coalesce into larger particles and settle by gravity.
Effects due to Brownian movement are usually small and proper chemical treatment will usually neutralize any electric charges. Then settling becomes a function of gravity and viscosity in accordance with Stoke’s Law. The settling velocity of spheres through a fluid is directly proportional to the difference in densities of the sphere and the fluid, and inversely proportional to the viscosity of the fluid and the square of the diameter of the sphere (droplet), as noted in Eq 7-3. The liquid- liquid separation capacity of separators may be determined from Equations 7-13 and 7-14, which were derived from Equation 7-3.9 Values of C* are found in Fig. 7-23.
Since the droplet size of one liquid phase dispersed in another is usually unknown, it is simpler to size liquid-liquid separation based on retention time of the liquid within the separator vessel. For gravity separation of two liquid phases, a large retention or quiet settling section is required in the vessel. Good separation requires sufficient time to obtain an equilibrium condition between the two liquid phases at the temperature and pressure of separation. The liquid capacity of a separator or the settling volume required can be determined from Eq 7-12 using the retention time given in Fig. 7-22.