نظريات تصميم وحدات فصل الزيوت عن مياه الصرف المتنوعة/Introduction to Separation of Oil and Water Introduction to Separation of Oil and Water

confine ourselves to the examination of oil droplets large enough that the quantity of oil
represented by them may cause environmental problems if discharged into surface or
subsurface waters.

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 on the
standard EPA tests.

Many jurisdictions, including King County, WA (Seattle)12 have
enacted standards allowing discharge oil levels considerably less than the EPA limit of 15
ppm oil and grease in the water discharged.

In order to minimize the possibility of such
discharge, it is wise to proceed carefully and cautiously in the design of oil-water separator
systems.

NON-DISSOLVED vs. DISSOLVED OILS

In general, all separators deal only with non-dissolved oils.

Most hydrocarbons have very
limited solubility in water, but certain aromatics such as Benzene have substantial
solubility and this must be taken into account in designing equipment where these
chemicals may be present.

If these are present, some other means of removing them
such as distillation may be considered.

WATER FROM OIL SEPARATORS

TWO AND THREE PHASE SEPARATORS

Two and three phase separators are often used in oil production and refining systems and
in chemical plants.

They are usually mostly empty vessels, sized based on empirical
relationships and often provided with rudimentary baffles and / or mesh pads for mist
elimination and heating arrangements to raise the temperature of the oil, thus decreasing
the viscosity and aiding the separation.

Two phase separators may be used where only oil and gas are present with no aqueous
phase or in situations where only small amounts of gas or no gas are present with the
aqueous and hydrocarbon phases.

They are also often known as “free water knockout
drums” and may be designed either as vertical or horizontal vessels. High pressure
systems may be designed as spheres because this is the most economical shape to
manufacture in a high pressure design.

Three phase separators are similar to two phase separators except that they are provided
with connections for water, oil and gas drawoff. Several general designs have been used;
the one shown below is typical of oil field practice.

Three phase separators are commonly
heated as well, with heat being derived from burning some of the gas in the incoming
stream.

ELECTROSTATIC TREATERS AND DESALTERS

The electrostatic desalter / treater process involves the creation of a high voltage electric
field through which the crude must flow from the entrance header below the electrodes to
the exit header in the top of the vessel.

The small water droplets in the crude are
coalesced in the electric field into large droplets which fall rapidly to the interface level
removing entrained salt and speeding up the settling rate of the water phase.

In the unit high voltage is applied to one of two sets of steel Electrode grids in the vessel.

These two sets of grids are parallel to the horizontal center line of the vessel.

The lower
grid (hot grid) is located near the center line of the vessel and is charged with the
secondary voltage or the transformer (high voltage).

This grid is suspended from an
insulated support frame.8
The upper grid is anchored to the vessel wall through the support beam and serves as a
ground grid.

The flow rate determines the required retention time in the electric field.

When this rate is
increased much beyond the capacity of the unit, the coalesced droplets cannot settle out
and some solid particles and/or water may carry over into the product.

Desalters and treaters differ in that desalters usually are provided with additional water
beyond what is naturally entrained in the oil flow and treaters are not.

This is because the
basic function of a treater is to remove the water that is present and the basic function of
the desalter is to remove the salts present by dissolving them in water and removing the
water.

The salts are removed because they cause corrosion problems downstream in the
refinery.

COALESCING CARTRIDGE SEPARATORS

Coalescing Cartridge separators are generally of two types:

• Packed separators, often called “hay packed”

• Filter cartridge separators

Packed Separators:

The packed type separators were first developed at the end of World War II for treatment
of aviation gasoline to remove water.

The term “hay pack” is really misleading because
the material often used is excelsior (shredded wood) and not hay.

The media is a densely
packed bale of fibrous material such as the excelsior, stainless shavings, Teflon shavings
or other fibrous material.

These systems generally include one or more bales of media in a horizontal pressure
vessel and operate by providing a surface for the aqueous phase to accumulate on and by
causing generally laminar flow to allow some settling time without channeling.

The
hydrocarbons flow horizontally through the vessel and as this is occurring, the aqueous
phase accumulates on and in the packs and forms large droplets which drain down to the
bottom of the vessel and are removed there.

Coalescing “Hay Pack” constructed of excelsior wood shavings.

(Note: Shown placed on end: support wheel on end would be vertical when installed)
While excelsior type products are effective to a degree in achieving the desired results
some problems and limitations are encountered.

Excelsior has a limited life span in that,
being organic, it will decompose.

Excelsior, or any equivalent natural or synthetic fibrous
type material, has a tendency to clog easily with dirt, debris, and other contamination so
that, in many instances, frequent replacement is required.

The results of clogging due to
collection of debris and contaminants tends to cause the fluid to channel, that is, the fluid
flows through the filter media in small channels so that only a relatively small percent of
the filter medium is actually effectively utilized.

Another difficulty with the use of excelsior
and similar configured natural and synthetic materials is that such materials are, by their
nature, randomly oriented, having no preferential direction of inclination of elongated
components.

For this reason water droplets coalescing on such material are not
preferentially downwardly oriented in their flow path, and therefore droplets can detach
from horizontal portions of the filter medium and become resuspended in the fluid stream.

FILTER CARTRIDGE SEPARATORS:

Filter cartridge type separators are often used in treating refined petroleum products to
remove water generated as part of the processing or acquired during time in pipelines or
tankage.

The cartridges used are generally composed of a dense fiberglass mat held
together with a phenolic plastic binder and are used in conjunction with a screen separator

cartridge of a hydrophobic nature.

The flow is commonly from inside to outside in the
coalescing cartridge and outside to inside in the separator cartridge.

The water drops are
coalesced into large drops in the coalescer cartridge and are barred from exiting the filter
vessel with the hydrocarbons by the separator cartridge.

Because the coalescer
cartridges are very dense they are also fine filter cartridges and tend to plug easily (much
more easily than the packing described above or other methods of separation).

For this
reason they are only used where excellent water removal is necessary and few or no solid
particles are present.

Common applications are treating jet fuel, gasoline, or kerosene for


water removal. Figure 4 below shows a schematic of the operation of a typical coalescer
/ separator system.

ABSORBENT SEPARATORS

Absorbents are often used as a final treatment stage in the use of hydrocarbons such as
jet fuel or gasoline.

Referring to them as “separators” is a misnomer because they do not
separate and remove the water they only sequester it so that it cannot pass downstream
to the automobile or airplane.

They are compact and easily mounted on a fueling truck,

but are a very expensive way to remove water from hydrocarbons and are only used
where absolutely necessary.

Legal Aspects

In general, water is only removed from oil and other hydrocarbons in an industrial setting
and there is limited or no discharge to the environment, so the only legal aspects of
treating oil to remove water are the safety aspects (OSHA, etc.) and possible legal
aspects of discharge of the water.

For this reason, the following only deals with the legal
aspects of removing oil from water.

Oil in water discharges from industrial and other facilities are governed by a variety of
federal, state and local laws.

Included are the Clean Water Act (CWA) and its
amendments, the Oil Pollution Act of 1990, the Coastal Zone Management Act and others
5.

Most hydrocarbon wastes are not covered by the Resource Conservation and Recovery
Act of 1976 and its amendments (RCRA) or the Comprehensive Environmental Response,
Compensation, and Liability Act (CERCLA) also known as the Superfund Act5.

These
wastes, produced by the extraction, transportation, refining, or processing of oil and
natural gas, are specifically exempted from being regulated as "hazardous wastes" under
any other laws.

The basic law covering discharges is the Clean Water Act.

It was originally enacted as the
Federal Water Pollution Control Act of 1972, but was amended extensively in 1977.

The
1977 amendments, in conjunction with the earlier legislation, became known as the Clean
Water Act. Under the terms of this Act, amended Section 402 created the National
Pollutant Discharge Elimination System (NPDES) permit system.

Permits for point sources
under this system are granted by the Environmental Protection Agency (EPA) or by states
with EPA approved programs.

After enactment of this law, any discharges other than
those covered by the permit are illegal.

Although the Clean Water Act was enacted
primarily to control discharges from Publicly Owned Treatment Works (Sanitary Sewer
Plants) and toxic discharges from industrial plants, it also controls discharges of petroleum
and other hydrocarbons into the waters of the United States.

Most states and localities require discharges to contain 15 ppm or less oil and grease,
based on a 24 hour composite sample.

Oil and grease may include petroleum
hydrocarbons as well as animal and vegetable oils. Some localities have established
lower discharge limits. King County, Washington, which includes the Seattle area, requires
discharges to be less than 10 ppm12.

Also important are the new stormwater management rules published by the EPA in 1990
(NPDES Permit Application Regulations for Storm Water Discharges; Final Rule, 1990).
The reasoning behind stringent regulation of stormwater is included in the "National Water

Quality Inventory, 1988 Report to Congress", as discussed in the Federal Register,
November 16, 1990.

This report concludes that "pollution from diffuse sources, such as
runoff from agricultural, urban areas, construction sites, land disposal, and resource
extraction is cited by the States as the leading cause of water quality impairment.

" These
sources appear to gain in importance as discharges of industrial process wastewaters and
municipal sewage plants come under increased control.

Stormwater discharges were covered under the CWA but not required to have permits
under the NPDES system until the final rules were published in the Federal Register,
November 16, 1990. "Stormwater discharges" refer to discharges consisting entirely of
rainwater runoff, snowmelt runoff, or surface runoff and drainage. Waters that do not meet
this definition are not covered by these regulations.

The new rules specify that facilities
with stormwater discharges from "areas containing raw materials, intermediate products,
finished products, by-product, or waste product located on site" will require a NPDES
permit.

Several categories of facility are specifically exempt from these regulations,
notably stormwater runoff from mining operations, oil and gas exploration, production,
processing, or treatment operations, and parking lots whose rainwater sewers are not
interconnected with manufacturing facility sewers.

It should be noted that the above concerns mostly legal aspects of discharging waters
directly to rivers and streams.

If it is desired to discharge the water to a sanitary sewer,
different rules apply:

DISCHARGE TO SANITARY SEWER (PRETREATMENT)

Sanitary sewer authorities are under the same requirements as other dischargers to the
waters of the US:

They must meet the requirements of the Clean Water Act. In addition,
these authorities must also meet the requirement of the Clean Air Act (industrial
dischargers do too, but their discharges to air are not directly tied to the water quality as
are the sanitary sewer plants).

To meet their air discharge permit requirements, most
sanitary sewer authorities require industrial discharge to the sanitary sewer to have a
maximum oil content.

The allowable amounts vary, but are often about 100 mg/l oil in the
water.

Many industrial plants choose to discharge their wastewater to sanitary sewer because
the regulatory paperwork requirements are less.

If it is desired to make such a discharge,
the local authorities must be consulted to determine their requirements for discharge
quality and monitoring. Technical solutions to problems with discharge to sanitary sewer
are much the same as solutions to discharge to surface water, but may be easier to
implement because of (usually) higher discharge limits.


SYSTEMS AVAILABLE FOR REMOVING OIL FROM WATER

Systems for removing oil from water range from very simple holding ponds with or without
skimming arrangements to very elaborate membrane technology-based systems.

For
most applications in removing oil, the simplest systems are often inadequate (although
often used) and the most complicated are either too expensive or too maintenance intensive.

Most of the following discussion, therefore, will concentrate on methods of
separation intended to meet regulatory requirements with minimum cost and maintenance.

Gravity Separation

The simplest possible separator is an empty chamber with enough volume to contain
spills. A typical spill control separator i.

A spill control separator is too
small to intercept small droplets and is only suitable for intercepting spills of oil or grease.
Spill control separators are only effective if any accumulated oil is removed regularly.

If the
oil is not removed regularly, a storm may flush the accumulated oil out of the separator
into the downstream sewer.


API separators are gravity type separators similar to spill control separators, but are
generally larger, more sophisticated, more effective, and are usually equipped with oil
removal facilities.

The American Petroleum Institute (API) provides design criteria for oilwater
separators. A design method is provided in the API Manual on Disposal of Refinery
Wastes,

1.

API separators are extensively used in oil refineries and chemical processing facilities
where waters containing relatively large amounts of oil are present and need to be
processed to meet the requirements of NPDES permits.

A diagram of a typical API
separator is shown in Figure 6 (Adapted from API Publication 421, 1990).

It should be
noted that this same API publication contains a survey of refinery API separators that
indicate most of them do not meet the requirements of the Clean Water Act.

The API separator has successfully been used in refineries for many years.

It is much
more effective than simple holding ponds or spill control separators.

Advantages of the
spill control separator and API separator are simplicity of design, low cost, low
maintenance, and resistance to plugging with solids.

The primary disadvantage of these
simple gravity separators is the poor quality of separation that they provide.
It should also be noted that the design method for API separators mentioned above
requires approximately a 45 minute residence time and many separators which are
utilized as “API type” do not include enough volume to provide the 45 minutes residence
time required.