Coalescing plate separators were designed to overcome some of the disadvantages of empty-tank type separators and provide separation of hydrocarbons from water streams in more compact and efficient systems. The empty tank type of separators are designed to remove 150 micron and larger droplets, and have effluent quality in the range of 150 mg/l, whereas coalescing plate separators may be designed to remove droplets down to 20 microns or less with effluent qualities down to 10 mg/l or less.
Since oil is usually present in a spectrum of small droplet sizes, the value of coalescing plate type separators may readily be seen as they produce relatively pure oil and do not require the use of absorbents which become waste when used. Separation of oil and water is different than the settling separation of solids in a clarifier. Oil droplets coalesce into larger, spherical droplets, while solids agglomerate into larger masses but do not coalesce into particles that have lower surface/volume ratios like oil.
When it is necessary to remove oil from water, coalescing plate module type oil-water separators are often a good solution because they remove the oil using only gravity for motive force, the separator modules are permanent and require little maintenance, no absorbents or other consumable items such as filter cartridges are required, and the oil that is separated is often recyclable.
No pumping or other utility costs are usually required (although pumped systems can be designed if this is required by the site conditions). They can be designed to operate under a great range of operating conditions and up to 100% oil. Separator systems can often be located underground, thus minimizing waste of valuable area on the surface.
Because oil-water separators operate using gravity as the operating principle, their design is more difficult and requires more expertise than design of filtration or other systems that operate under pressure, but the ongoing benefits of low operating and maintenance costs and the sale of recyclable oil usually outweigh the slight added expense of the initial designs. No absorbents are required, so disposal costs are limited only to the disposal of the recovered oil.
The following contains some general information on how oil-water separators operate and provides specific information on coalescing plate module type separators as well as constraints and design suggestions. Please contact us if we can offer further suggestions or information.
Coalescing plate module separator design
It is very difficult to be sure of maintaining laminar flow (as required by Stokes’s Law) in large empty-tank separators, because of the turbulence problem noted above. For this reason, coalescing modules are used to ensure laminar flow and therefore a system that behaves according to Stokes’s law.
The Stokes’s law calculation is accurate for oil droplet rise in the same way that it is accurate for solids settling – only if the particle size and continuous liquid viscosity are accurately known.
The problem with performing this calculation is therefore obtaining the following required data:
1. What are the respective specific gravities of the liquids?
2. What is the particle size?
3. What is viscosity of the water?
The design of separators usually requires design over a wide variety of temperatures (and therefore water viscosities) to account for summer and winter conditions as well as possible process upsets, so several water viscosities may be considered during design. The specific gravities are usually known or can be readily estimated. The viscosity of the water is readily obtained from literature data.
The oil droplet size, however, is much more difficult to determine.
Particle sizes of solid particles are fairly easy to determine in the laboratory, but oil droplet size information is much more difficult to obtain. One tedious way to determine oil droplet sizes is to take a microscopic photograph of droplets in water and count the various size droplets. Other methods have been used with varying success.
If the droplet size is not known, or a large range of droplet sizes is present (the normal situation), it is necessary to make some estimates of droplet sizes to determine the rise rates of the droplets and therefore the size separator required. These estimates are usually made based on previous experience with separation systems.
Because the volume of oil in a droplet is proportional to the cube of the diameter, it follows that very small droplets contain small quantities of oil. We may therefore confine ourselves to the examination of oil droplets large enough that, if quantities are present, they would contain enough oil to be troublesome. Oil should not be present in quantities great enough to cause oil sheens or even in the small quantities required to show more than 15 ppm.
A reasonable way to treat this problem of removal down to specific oil content is in a statistical manner. If we can show that fewer particles will pass through a separator than are required to cause 15 ppm in the effluent (or other requirement if a lower effluent is required), we can confidently predict that the separator will meet the effluent standards necessary under the law.
Coalescing plate separators were developed to remove the oil in an effective manner and still be resistant to plugging by solid particles. We normally offer PPS plate separator systems with mixed plates: vertical plates for the removal of solids and oil slugs and horizontal plates for the efficient removal of free oil droplets. The efficiency for the removal of free oil is very high due to the limited plate spacing of only 6 mm and the extended retention time in the plate packs, combined with the coalescing action provided by the shape of the plates. Since the coalescing plate packs mounted in the separator can (if necessary) be cleaned and re-used the spare parts requirement is very low.
These separator plates are corrugated so that two spacings (nominal 6 and 12 mm) can conveniently be made. Narrower spacings are more efficient and wider spacings are more resistant to plugging by any solids that might be present. A drawing of the separator modules is included in the appendix and a pictorial representation is provided below.
The flow in a module such as this is along the long axis of the module. Oil droplets rise up and meet the undersides of the plates where they are separated and solids particles fall onto the top surfaces of the plates and are directed to the bottom of the separator. A single module has limited capacity because the laminar flow requirement for Stokes’s Law applies, so systems are designed using multiple modules with modules placed side by side and stacked to allow for the flow rate and at the same time maintain laminarity.
a) The plates in the modules are designed to shed solids to the bottom of the separator, avoiding plugging and directing the solids to a solids collection area. In inclined plate systems, solids must slide down the entire length of the plates. The solids drop directly to the bottom of the separator.
b) The double corrugations provide surfaces that slope at a forty five (45) degree angle in all directions so that coalesced oil can migrate easily upward.
c) The coalescing plates are arranged in modules so that they can easily be stacked in a vault, tank, or above ground system.
Most large units are designed utilising plate modules installed in underground vaults. The primary advantages of vault installations are that the cost per unit flow is minimised and the below-grade installation is both convenient for gravity flow applications and does not waste valuable plant area.
Considerations for the selection and design of oil-water separator systems
General design considerations:
Numerous factors must be considered in the selection and design of oil-water separation systems. Among these are:
1. Flow rate and conditions.
2. Degree of separation required – effluent quality.
3. Amount of oil in the inlet water.
4. Existing equipment – such as concrete vaults or pumps
5. Emulsification of the oil.
6. Treated water facilities.
7. Recovered oil disposal method.
For industrial and some municipal applications, flow rate, amount of oil, and conditions may be easily determined. For storm water applications, it may be necessary to estimate water flow quantities.
The degree of separation required is usually a matter of statutory or regulatory requirements, but if the water is discharged to a sanitary sewer or industrial treatment plant effluent oil content may be negotiable.
The amount of oil in the inlet water may be known, especially in industrial applications, but it will often be necessary to estimate the quantity in storm water applications.
The degree of emulsification of the oil is difficult to assess, but many steps can be taken to discourage the formation of emulsions and encourage the break-up of emulsions that are inadvertently created. Sometimes it may be necessary to replace existing equipment such as centrifugal pumps (which produce difficult to separate emulsions) with equipment more suitable for use with oil water separators. Many times it may be necessary to substitute quick-break detergents for conventional detergents that also can cause emulsions. If it is at all possible, detergents and soaps should be avoided and cleaning using only hot water or steam used.
It is necessary to ensure that adequate size piping is provided for downstream treated water removal to avoid flooding the separator and perhaps filling the oil reservoir with water.
A downstream sample point should be provided to allow for effluent testing.
Adequate storage facilities for the removed oil should be provided and means for recycling the oil included. Careful records of removed and recycled oil should be kept to avoid possible future regulatory problems.
The following is a discussion of several of the points touched briefly on above concerning design of oil-water separation systems.
Much of the performance of an oil-water separator depends on the influent conditions, because equipment or conditions that cause small droplet sizes in the influent to the separator will cause requirements for a larger separator to accommodate the additional time required for the smaller droplets to coalesce.
Conditions that cause small droplets are any conditions that cause shear in the incoming water. The following are (more or less in order of severity) some factors that can cause small droplet sizes:
1. Pumps, especially centrifugal pumps.
2. Valves, especially globe valves.
3. Other restrictions in flow such as elbows, tees, other fittings, or simply unduly small line sizes.
4. Vertical piping (horizontal is better)
Emulsifying agents such as soaps and detergents greatly contribute to small droplet sizes in addition to disarming coalescing plates and discouraging coalescing.
Ideal inlet conditions for an oil-water separator are:
1. Gravity flow (not pumped) in the inlet piping.
2. Inlet piping sized for minimum pressure drop.
3. Inlet piping straight for at least ten pipe diameters upstream of the separator (directly into nozzle)
4. Inlet piping containing a minimum of elbows, tees, valves, and other fittings.
Note: Our separators are provided with an inlet elbow or tee inside the separator pointing down. This is an exception to the above rules and is intended to introduce the influent water below the oil layer on the surface, thus avoiding disturbances of the surface oil and possible re-entrainment of some of the already separated oil.
While gravity flow conditions are not often obtained except in sanitary sewer systems, storm water or some process water applications, a positive displacement pump such as a progressive cavity type pump, may be used because they provide minimum disturbance of the fluid. The best choice if gravity flow is not available is a progressive cavity type pump.
Inlet piping should be as smooth as possible to avoid turbulence caused by pipe roughness. Smooth PVC is preferable to rough concrete.
Sometimes anti-emulsificant chemicals are utilised, but extreme care must be exercised in the use of these chemicals to ensure that they do not make the emulsion worse instead of improving it. Plant operators have a tendency to believe that if a pint of anti-emulsificant is good, that five gallons is better. It is necessary to provide sufficient education to avoid this problem and best to avoid use of such chemicals.
If large quantities of solid particles are expected, it is wise to provide a grit removal chamber before the stream enters the separator. These chambers should be designed according to normal design parameters for grit removal.
Effluent designs are also important in the operation of oil-water separators. Downstream piping and other facilities must be adequately sized to process the quantity of water (and oil) from any likely event. Manholes overflowing during a heavy rainstorm will surely cause any oil caught in them to be re-released into the environment.
Effluent piping must be designed with siphon breaks so that it is not possible to siphon oil and water out of the separator during low flow conditions.
Oil must be removed from the separator a regular basis, preferably continuously. If not removed in a timely manner, this oil may fill the separator, blinding the media and causing high effluent oil contents. It may eventually become re-entrained at the next rainfall event and reintroduced into the environment.
Removing the oil from the separators is not enough to protect the environment; it must also be recycled to ensure that it is not merely treated as a waste and avoid possible problems elsewhere from improper disposal. Current law can hold the owner of the oil-water separator responsible if the oil is not properly disposed of, even if the owner had paid for proper disposal.
Removal of unwanted oil from water, especially storm water, continues to be a problem, both environmentally and economically. The use of coalescing plate module type oil-water separators has a number of benefits from both economic and environmental standpoints:
• Economical and reliable operations.
• Low operating and maintenance costs.
• Gravity operated, so no utilities required.
• Recyclable oil recovered.
• Can take little above ground space.
• High efficiency and ability to meet environmental regulations.
• Predictable performance.
• Ability to handle surges of oil or water.
If it is necessary to remove oil from water, either from storm water or industrial streams, coalescing plate module type separators are often a very good long-term choice. Please contact us if we can provide suggestions or additional information.