Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a method for treating oil
contaminated solids, particularly the residues formed as
a by-product from the treatment of oil contaminated water,
such as refinery or other industrial plant effluent, or
resulting from the long standing of crude petroleum or
petroleum products in tanks or other forms of storage.
As a first step in the treatment of oil contaminated
waters it is common practice to effect a rough separation in
a gravity separator to remove the bulk of the oil and suspended
solids. Suitable separators include parallel plate separators,
settlement tanks and API Separators as described in the "Manual
on Disposal of Refinery Wastes", published by the American
Petroleum Institute in 1969.
Oil recovered from such devices can be used as a
source of energy and the aqueous effluent is suitable for
further treatment to reduce its oil content and ~OD to very
low levels.
Such a further treatment is disclosed and claimed
in our Canadian Patent No. 1026021 which claims a method for
treating effluent water containing suspended oil which method
comprises passing the effluent water firstly through a sand
filter to remove the suspended oil and subsequently through a
biological percolating filter.
Returning to the primary separation devices, there
is a third product from them which builds up over a period of
time and the disposal of which gives rise to problems. This
is an oilysludge which can have a water content in the range
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20 - 95% by weight and an oil content in the range
5 - 70% by weight, the remainder being solid materials such
as silt, rust, carbon, calcium carbonate, etc.
Other sources of oily sludge are sand filter backwash
water from the effluent treatment process described in Canadian
Patent No. 1026021 and the top oily sludge layer from gas/air
flotation plants for the removal of dispersed oil and suspended
solids from water.
Oily sludge is a difficult material to process since
it tends to block filters and adhere to them.
The only environmentally accepted method of dealing
with these sludges is by burning, but this requires the provision
of special incinerators which are expensive to purchase, install,
operate and maintain and which result in the wasteful destruction
of oil.
A process has been disclosed in British Patent
Specification 1340931 in which oily solid particles suspended
in aqueous effluent are retained by a sand filter.
A sand filter operates by allowing the liquid to be
filtered to percolate slowly downwards through a thick bed of
filter sand. The sand is relatively coarse when compared with
the solid particles to be retained and the particles, especially
the finer ones, tend to pass through the upper reaches of the
sand. Eventually, however, the packing results in tortuosities
and restrictions which trap the particles, including the finer
ones.
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Because of their effectiveness in dealing with
materials which tend to block or pass through other
filters, sand filters are widely used in oil refineries
and similar installations
The same factors which result in the efficiency
of the sand bed when working as a filter give rise to
problems when regeneration is necessary, however. In
order to regenerate a filter, retained-solid particles
must be removed. Because particles are trapped at
all levels in the filter bed, regeneration, either by
backward or forward flushing, means that some particles
have again to traverse the bed and again are likely to
meet with further restrictions which tend to retain them
in the filter unless dislodged. In order to combat this,
it is necessary to wash either frequently or for long
periods and to use high flow rates and/or liquid under
high pressure with the ever present risk, unless great
care is exercised, of disturbing the structure of the
filter bed and rendering it inefficient for subsequent
filtration stages, unless restructured.
The residue resulting from the filtration of oily
sludge adheres strongly to a sand filter and the
difficulties of regeneration are such that sand filters
have been rejected for such operations.
There exists a need, therefore, for a simple
method of filtration which is capable of dealin( with
concentrated oily sludge, a more difficult feedstock t}-an
the dilute suspensions previously treated, and which
uses a filter which is easily regenerable
We have now discovered that a pre-coated surface
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filter is capable of meeting this need. It is indeed
surprising that a surface filter can be used since the
teachings of the art would suggest that an even thicker
depth filter would be necessary and that restructuring
the filter bed after regeneration could not be avoided.
Thus according to the present invention there is
provided a method for the treatment of an oily sludge
which method comprises the steps of filtering the sludge
undiluted with recovered oil in a pre-coated surface
filter, treating the filtration residue with a light
hydrocarbon solvent and/or steam stripping the extracted
residue.
Suitable pre-coat materials include diatomaceous
earth, fly ash and powdered polymers, e.g., polyurethanes.
Before filtering sludges with a high solids content,
water is preferably added as a diluent. Alternatively
a light hydrocarbon solvent maybeemployed.
The filtration may be carried out at ambient or
elevated temperature. If elevated temperature is used,
the oily sludqe may be directly heated by conventional
means, e.g., steam coils.
Suitable surface filters include plate, leaf and
tube or candle filters. The filters are preferably
operated under pressure as opposed to vacuum.
In such filters, the pre-coat is effectively the
filter medium and the function of the plate, leaf and
tube or candle, etc., is to act as a support.
The preferred hydrocarbon washing solvent is kerosine,
but other solvents such asnaphtha are also suitable.
Treatment may be at ambient temperature but higher
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temperatures may be advantageous in certain cases.
Solvent washing may be carried out in two stages,
with the relatively clean solvent from the second stage
of one cycle being used in the first stage of the next
cycle to wash heavily contaminated solids.
Filter aids similar to or identical with the pre-
coat medium may be added to the sludge before filtration
to ensure longer and improved filtration by increasing
the porosity of the filter cake, reducing the differential
pressure per unit cake thickness and preventing the cake
from blinding,
For a low concentration of up to 1% by weight solids
in the feed sludge, the amount of filter aid used is
preferably in the range 2 - 4 times the weight of solids.
For a medium concentration of 1 - 4% solids, the amount
of filter aid used is preferably in the range 1 - 2 times
the weight of solids. For a high concentration, e.g.,
4% or higher solids, the amount of filter aid used is
preferably in the weight range 0.5 - 1.5 times the weight
of solids. In general, the finer the particle si~e of
the solids, the more filter aid is required.
It is advantageous to use the minimum quantity of
filter aid consistent with satisfactory filtration,
since the less filter aid employed, the slower the build
up of filter cake and the longer the filtration run.
As an additional feature, it may be advantageous
to interpose a drying stage after the filtration and
before the solvent extraction. Drying may be achieved
by treatment with hot or cold air
The filtration flowrate is suitably in the range
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1 - 2Q0 gallons per square foot of filtration area per hour
(0.Q5 - 10 m3/m /hr), preferably in the range 10 to 50 gal/ft /
hr (Q.5 - 2.5 m3/m2/hr).
Solvent consumption and flowrate are dependent on the
; oil content of the filtered solids.
Steam consump~on and flowr-ate are dependent on-tlle
boiling point of the solvent.
The filtered solids are easily removed from the filter,
e.g., by scraping or centrifugal action, thus leaving the filter
clean to resume the next cycle.
It is believed that the solid particles present in the ~-
sludge stabilise emulsification of the oil and water also present
and that, with their removal, the oil and water can subsequently
separate more easily. Some oil, however, is retained by the
solids a~d is removed by the solvent extraction and/or steam
stripping.
By the method of the present invention it is possible
to convert an oily sludge to a dry solid containing less than
1~ by weight oil. Such material is suitable for land fill
operations.
The invention is illustrated by the following example.
Example
Sludge used : Refinery API Separator Bottom Sludge
Analysis : Oil 40%
Water 52.5
Solid 7.5~
Filter : Stainless Steel Candle Filter.
The filter tube (100 micron aperture) was pre-coated
with diatomaceous earth to a thickness of 3mm. rrne....................... .;
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sludge was slurried with 9 parts water to 1 part sludge
and 0.4% w/w diatomaceous earth was added to the slurry
as a filter aid. The slurry was then filtered at ambient
temperature at an average flowrate of 24 gallons/hr/ft
of filtration area (1,25 m3/m2/hr). The filtration was
terminated at a maximum differential pressure of 4 bar
when 3.4 gallons/ft2 of filtration area (150 mitre/m2)
had been filtered. The filter body was drained and
filled with kerosine at ambient temperature. The
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filtered solids were then washed in situ with 3 gallons
kerosine/ft2 of filtration area. The filtration body
was drained and the filtered solids steam stripped in
situ for 15 minutes. The steam pressure was 25 psig
(1,7 bar (ga)),
Analysis of filter cake : Oil 0.8%
Water 0.6%
Solid 98,6%
