الطرق المختلفة لفصل الزيوت والبترول عن المياه

Petroleum Wastewater
Crude Oil Basics
Crude oils are complex mixtures containing many different hydrocarbon compounds that vary in appearance and composition from one oil field to another.
Crude oils range in consistency from water to tar-like solids, and in color from clear to black. An "average" crude oil contains about 84% carbon, 14% hydrogen, 1%-3% sulfur, and less than 1% each of nitrogen, oxygen, metals, and salts.
Refinery crude base stocks usually consist of mixtures of two or more different crude oils.
Crude oils are also defined in terms of API (American Petroleum Institute) gravity. A crude with a high API gravity are usually rich in paraffins and tend to yield greater proportions of gasoline and light petroleum products.
Crude oils that contain appreciable quantities of hydrogen sulfide or other reactive sulfur compounds are called "sour." Those with less sulfur are called "sweet."
All crude oils are assayed and valued depending on their potential yield.
Crude Oil with low assay numbers is referred to as "Opportunity Crude".
This type of oil will be more difficult to process due to higher levels of contaminants and water.
This type of crude will typically give desalter equipment the most trouble and require the greatest skill of the operator.

Methods used for Oil Water Separation
Centrifuge
Rotary Drum Vacuum Filter
Dissolved Air Flotation (DAF)
Slope Plate Clarifiers
Biological Treatment
Evaporators
Gravity Separating Devices

Comparisons of Oil Water Separation Technologies
Centrifuge:
Uses large horsepower motors and because of
the number of moving parts is subject to high maintenance.
While centrifuges are effective at removing suspended
solids, they do not account for dissolved solids and heavy
metal species in solution.

The effluent from a centrifuge
would need further treatment prior to disposal.

Rotary Drum Vacuum Filter:
Quite effective at rejecting large
solids. sometimes filtrate must be sent back around to get
all of the smaller particles. Usually employs coarse filtration.
Vacuum filters require large floor areas and have high capital
Costs

Dissolved Air Flotation (DAF):
Large tanks where air is
bubbled into the bottom and with the use of flocculants,
solids are floated to the top and skimmed off. A very large
tank is required due to the residence time required. Also
chemical addition is a daily if not hourly process and is a
significant operating cost.

Slope Plate Clarifiers:
Cheap and easy to use. The process
relies on gravity to drop out heavy solids. Here again
colloidal materials with small mass and dissolved
constituents do not settle. Sometimes it is used in conjunction
with flocculation chemicals. These chemicals have limited
effect in dropping out heavy metals, BOD, and COD.

Biological Treatment:
This process relies on biological
activity to digest the solids in the wastewater. The problem
is that the system is extremely temperature and pH sensitive.
Also loading must be done at a set rate.

The operation of
this kind of system usually requires a very skilled operator.
It also can take up a lot of floor space due to the amount of
residence time required for the bugs to digest the materials.

Evaporators:
Can reduce wastewater to dry solids that can
be landfilled. Of course water re-use is not possible.
Evaporators have very high capital costs and consume huge
amounts of energy even for the most efficient models.

Membranes
Membrane separation technology has been around for many
years. Initially, the use of membranes was isolated to a
laboratory scale.

A membrane is simply a synthetic barrier,
which prevents the transport of certain components based
on various characteristics.
Membranes are very diverse in
their nature with the one unifying theme to separate.
Membranes can be liquid or solid, homogeneous or
heterogeneous and can range in thickness.
They can be
manufactured to be electrically neutral, positive, negative
or bipolar.
These different characteristics enable membranes
to perform many different separations from reverse osmosis
to micro-filtration. There are four main categories of
membrane filtration.
These are determined by the Pore size
or Molecular Weight Cut off
Filtration Type Particle Size Molecular Weight
Reverse Osmosis ≤0.001 m ≤100 Dalton
Nanofiltration 0.001-0.01 m 100 - 1000 Dalton
Ultrafiltration 0.01-0.1 m 1000-500,000 Dalton
Microfiltration ≥0.1 m ≥500,000 Dalton

Reverse Osmosis Membranes
The first category of membranes is for Reverse Osmosis.
These are the tightest membranes for separating materials.

They are generally rated on the % of salts that they can
remove from a feed stream.
However, they can also be
specified by Molecular Weight cutoff. Usually, the rejection
of NaCl will be greater than 95% in order to be classed as
an RO membrane.
The molecular weight cutoff is shown in
the table to the left. An example of their use would be for
filtering seawater in order to remove the salt.
They are also
used to remove color, fragrance and flavor from water
streams.
Reverse Osmosis membranes don’t have structural
pores. Filtration occurs as ionic species are able to diffuse
their way through the membrane itself.
Nanofiltration Membranes
A great deal of recent research has led to the improvement
of membranes in the range of Nanofiltration.
As the name
suggests, these membranes are used to separate materials
on the order of nanometers.
These membranes are not
usually rated based on their pore size because the pores are
very small and difficult to measure accurately.
Instead they
are rated based on the approximate molecular weight of the
components that they reject or the % of salts that they can
remove from a stream.
These membranes are used
predominately for wastewater treatment but they are also
used to concentrate material that has a wide range of particle
sizes.

Ultrafiltration Membranes
Conventional Ultrafiltration membranes are composed of
some type of polymer material with pores ranging from a
little less than 0.01 m to 0.1 m. These membranes are
used for many different separations including oily
wastewater treatment, protein concentration, colloidal silica
concentration and for the treatment of various wastewaters
in the Pulp & Paper industry.

Microfiltration Membranes
These membranes tend to be porous, with pores greater than
0.1m.
These types of membranes are used to separate larger
particulate matter from a liquid phase.
Some examples
would be coarse minerals or paint particles, which need to
be concentrated from an aqueous solution.

by
colonel.dr
bahaa badr
scientific advisor

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