Coalcescing plate separators take advantage of the buoyancy of oil droplets in water to perform a separation.  The system consists of many corrugated plates stacked one above another, forming a set of narrow channels through which the oily water must flow.   As the water flows between the plates, the oil droplets rise and meet the underside of the plates where they are captured and eventually as more droplets are captured, a film and then large drops form.  It is possible to remove very small oil droplets in this way.  The large oil droplets rise to the surface of the separator through holes provided for that purpose and are removed by skimming devices.  Coalescing plates offer high efficiency and low operating and maintenance cost.  Following is a discussion of oil in water and how the equipment can function.  Mohr Separations Reseach will be happy to assist in the specifying of any separator of this type.

HYDROCARBONS IN WATER

Hydrocarbons in water can be present in a variety of forms. These are shown below, arranged generally in order of difficulty of removal ¹

1. Free oil - large droplets or sheets that rise freely to the surface. This oil is easily removed in simple gravity separators.
2. Mechanically dispersed oil - fine droplets ranging in size from a few microns up to a few millimeters. The oil found in droplets is usually the result of some mechanical mixing of oil and water such as is found in pumping or in turbulent flow through a pipe. The oil droplets can be found in a "bell curve" of droplet sizes with some small, some large and a predominance of average size droplets. The average size will vary dependent on the amount of mixing the two liquids have undergone as well as the presence or absence of emulsion causing surfactant chemicals. These dispersions may be removed by the use of an enhanced gravity system.
3. Chemically stabilized emulsions - droplet dispersions similar to mechanically dispersed oil, but with droplets stabilized by surface-active agents (surfactants). More surfactants or more mixing will cause a smaller average droplet size. The average droplet size is important because many separation devices are designed to capture droplets by gravity or enhanced gravity separation and if the average droplet size is smaller, the separator will have to be larger and consequently more expensive.
4. Oil adhering to solid particles. Can be removed by filtration or by enhanced gravity separation if the combined specific gravity is different from the water.
5. Dissolved oil - either truly dissolved oil or finely dispersed droplets so small (less than 5 microns) that removal by normal physical means is impossible. Dissolved oil must be removed by biological treatment, absorbents, distillation, or other non-gravity means.

In wastewater stream, the majority of the oil will be present as either free oil or mechanical dispersions of oil. These may be treated readily by enhanced gravity systems for removal of the hydrocarbons. Most hydrocarbon removal systems depend on gravity or enhanced gravity separation, taking advantage of the buoyancy of the droplets.

HYDROCARBON DROPLETS RISING

The rising of hydrocarbon droplets in a separator is governed by Stokes's Law ² This function, simply stated is shown in the following equation:

Where: V_p = droplet settling velocity, cm/sec

G = gravitational constant, 980 cm/sec²

µ = absolute viscosity of continuous fluid(water), poise

d_p = density of particle (droplet), gm/cm³

d_c = density of continuous fluid, gm/cm³

D = diameter of particle, cm

From the above equation, it may be seen that the important variables are the viscosity of the water, the difference in specific gravity of the water and hydrocarbons, and the hydrocarbon droplet size. After these are known, the droplet rise velocity and therefore the size of separator that is required may be calculated.

SEPARATOR DESIGN CONSIDERATIONS:

To calculate the required size of a separator, it is first necessary to calculate the rise velocity of the oil droplets. The size of the separator is then calculated by considering the path of a droplet entering at the bottom of one end of the separator and exiting from the other end of the separator. Sufficient volume must be provided in the separator so that the oil droplets entering the separator at the bottom have time to rise to the surface (and be captured there) before the water carrying the droplets exits the opposite end of the separator.

Conditions for the validity of Stokes’s Law are:

1) Particles are spherical,

2) Flow is laminar, both horizontally and vertically, and

3) Particles are the same size.

For separation of oil droplets from water, these conditions can be met because:

1) Oil droplets are spherical because surface tension (more properly interfacial tension between the water phase and the oil) minimizes the surface area, making the droplets spherical.

2) In an enhanced gravity coalescing plate separator, flow is laminar because the separator is designed to retain the Reynolds Numbers under the laminar limit. It should be noted that this is very difficult or impossible to attain in an API separator due to the large size of such separators.

3) The oil droplets will not be the same size, unless specifically made in a single size in a laboratory, so it is necessary to do numerous rise rate calculations for the various sizes expected to be present in the influent.

The viscosity of the water is readily obtained from literature data. The design of such separators often requires design over a wide variety of temperatures (and therefore viscosities) to account for summer and winter conditions. Flow rates and hydrocarbon content of the water must be determined or estimated for the particular system.

Coalescing plate media is provided in packs or modules that are composed of plates arranged horizontally, vertically, or at an angle from the horizontal of 45-60 degrees. The plates are spaced evenly on 0.25" to 3" center to center spacing. As the water flows between the plates, the droplets of oil rise up and meet the undersides of the plates where they are captured. The captured droplets form a film on the underside of the plates and eventually coalesce into large droplets that migrate upward through the plate pack to the surface of the separator. The oil layer that forms on the surface is skimmed off, either continuously by mechanical skimmers or periodically by vacuum trucks or pump.

One way of determining the amount of media required is to use a trial-and-error method. Using this method to determine the amount of media required, it is necessary to consider the rise rates of several sizes of droplets around the chosen mean droplet size. A quantity of media is chosen and the horizontal flow velocity through the media calculated. This allows calculation of the residence time within the media. Since the rise rates have been calculated, the residence time within the media can then be used to determine which droplets are captured. What amounts to a graphical integration of the volume of the total droplets that are not captured gives a total mg/L of oil escaping the media. If the effluent is not satisfactory, another amount of media must be estimated and the process repeated until a satisfactory answer is found. This very tedious calculation is best done by the use of a computer program.

After the amount of media is determined, it is then necessary to design the separator vault or other container for the media. It is necessary to consider the size of the inlet chamber so that solids may be disengaged and captured as much as possible before the coalescing plate packs. An oil dam must be provided downstream of the coalescing packs to ensure that the captured oil is not re-released to the environment. A water overflow weir must be provided downstream of the oil dam to positively set the water level in the separator.

It is necessary to take care in the design to ensure that the possible variation in operating conditions to ensure a design that will operate in a satisfactory manner over the range of requirements. Experience in design is the only means of ensuring a suitable separator for the applications.

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