Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FIELD OF INVENTION
The present invention rela-tes to a process for separa-ting
oil from oil-in-wa-ter emulsions by means of chcmical/physical
methods. More particularly, the invention relates to -the
breaking oE oil-in-water emulsions by the addition of paxticular
inorganic salts to the emulsions under conditions of controlled
agitation.
BACKGROUND OF THE INVENTION
lG In many areas of the world where crude oil is produced
from oil wells, the very high viscosity o:E some crude requires
that it be treated to reduce i-ts ~iscosity in order to render it
flowable so tha-t it may be recovered from the wells at economic
rates. A convenient method commonly employed to improve the
Elow rate of heavy crude oil is to inject steam at high pressure
into the oil-bearing strata. The steam effectively heats the
crude thereby reducing its viscosity and permi.ts recovery of the
oil by pumping or by pressure ejection from the well. During the
oil heating process, the steam is condensed to water which water
is dispersed in the oil in the form of a water-in-o.il emulsion.
~Eter being brought to the surface, the crude oil/water emulsion
is allowed to settle in settling tanlcs or basins where the water
phase containi~ dispersed, :Eine oil droplets separates from the
oil. Thi.s water :Eraction, commonly called produced water, has
a portion of crude oil, generally from about 500 ppm or more
emulsified with it as an oil-in-water emulsion. In arid regions
or the world, it is essential to conserve the water content of
produced water for reuse in the generation of Eurther steam.
However, before reuse in steam generation, the oil content of
the water must be reduced to acceptable concentrations in order
~o control frothing and to prevent damage to steam generating
apparatus. Also, in some jurisdictions, problems may arise with
respect to the discharge of untreated produced water onto terrain
or into water courses without reduction or removal of the oil
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Eraction. It has been the aim of -the industry -to reduce the oil
content of produced wa-ter to 20 ppm or less for reuse in steam
generation as well as for environmental considerations.
The nature of the oil-in-water ~produced water) emulsions
is such that the dispersed oil droplets are very small in size,
tending to range from 1-12 ,um, the greater proportion ranging
from 2-7 fum. Generally, oil droplets smaller than about 1 Jum
are not visible under microscopic examination and it is believed
that many droplets much smaller than 1 ~um are present in
produced water. These produced water emulsions demonstrate
particular s-tability and the oil droplets show little or no
evidence of coalescence even after agi-tation. These near micro-
emulsions are, thereEore, generally broken only with difficulty
and only after prolonged and sometimes costly treatment procedures.
The processes of -the prior ark, which generally deal wi-th
oil/water separation processes in general rather than with
produced water emulsions, tend to rely on emulsion breaking or
phase separation methods for oil-in-water emulsions which
involve a method wherein up to 500 ppm of an emulsion breaker or
separator, such as, or example, alum, ferrous sulpha-te or ferric
cilloride, is added to each one million parts of emulsion. These
emulsion breakers which comprise positively charged cations
uncti.on to neutralize -the negative charge normal:Ly carried on
the ~urface oE -the dispersed oil droplets thereby causing the
droplets to coagulate and agglomerate into globules or flocs.
By subjecting the water/floc mixture to mechanical agitation,
many of the flocs can be caused to further agglomerate in-to
larger flocs. These larger flocs moving in a random manner
through the water phase tend to entrain further droplets of oil
and any other particles of insoluble matter present in -the water.
After agitation, the flocs which comprise a coagulant mass, then
settle or float in the water phase depending on the density of
the mass. After settling, the coagulant and the water can then
be separated by means of filtra~on, centrifuge or other common
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methods. It has been found, however, -that these prior art
methods, in addition to employing large amounts of chemicals at
high cost, are not generally applicable to crude oil produced
water where the oil phase droplet size is less than 5 ~um and
where the oil content is relatively low. Consequently, less
costly but less effective physical methods are employed instead.
It has been the commonly held view that, use of a large excess oE
emulsion breaker relative to that re~uired to neutralize the oil
droplet surface charge, is essential to achieve flocculation of
the oil phase.
SUMMAR~ OF THE INVENTION
It has now been found that the clarification of crucle oil
produced water can be measurably improved by a process comprising
15 the ste~ps of: (a) add.irlg to the produced water an optimum
quan-tity o:E an emulsion breaker selected from the group of
cations A1~, Fe~3 and Fe~~2, (b) subjecting t.he cation-trea-ted
produced water to agitation by means of rotating paddles at a
paddle tip speed of from 0.15 to 0.~0 m/s, and (c) allowing the
treated and agitated mixture to separate by gravity. The rate of
separation may be expedited by addin~ to the a~i.tated mixture an
arnount of an anionic high molecular weight polymer and agita-ting
the mi~-ture for a further period~
The process of the invention can be practised to clar:i.fy
~5 oi].-in-water emulsions having a wide range oE oil con-tent but is
more particularly adapted to employment with produced water
emulsions having a crude oil content up to 25,000 ppm wherein the
oil droplet size is less than about 12 Jum. Practice of the
process achieves a reduction of oil in these emulsions typically
to less than 3 ppm and as low as 0~5 ppm. The resultant
clarified water is sufficiently low in contaminants to alleviate
concerns regarding ground disposal and may be employed in the
generation of steam without risk of fouling or damage of steam
generating apparatus.
The cation employed in the process of the present invention
is l.imi.ted to the ~roup AlLr+, Fe+~ and Fe+~~~l~ and us~fully
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includes any soluble salts comprising -these cations which foxm
an insoluble hydroxide in water. Particularly useful because of
ready availability and low cost are, for e~ample, commercial
alum (A12(SO4)3.xH2O), ferric chloride and ferrous sulphate.
The number o-E molecules of water in the alum may vary from about
10 to 24. The concentration of cation floccuIant employed is
from about 0.7 to 10.5 parts per million parts of produced water
containing not more than 25,000 ppm of dispersed oil and excellent
results are often obtained in the range of 1 to 3 ppm. In the
case of A1-~++ ion, the preferred range of concen-tration of
flocculan-t is from 1.5 to 2.6 parts per million par-ts of
produced water (25,000 ppm max. oil). In the case of Fe~ and
Fe~ , the preferred rangeof flocculant is from 2 to 6 parts per
million parts produced watsr (25,000 ppm max. oil). The cation
Elocculants of the invention are effective at the pH ran~e
normally found in crude oil produced water, that is, a pH range
from 7 to 8.5. Fe~3 and Fe~2 were found to be effective over a
wider pH range than Al-~ , namely from pH 7 to 10. Salt, mainly
sodium chloride, which is fre~uently present in crude oil
produced water at concentrations of from 0.3 to 1~, has the eEfec-t
of slightly increasing the amount of ~]~~ cation required to
produce flocculation. The interference of the sodium ion which
appears to compete with the ~ cation flocculant for the
char~e on the oil droplet may re~uire a doublin~ oE -the amount
o ~locculant used.
An optimum level of agitation is critical to the utility of
the process of the invention. Agitation of the cation-treated
emulsion provides a collision mechanism whereby the individual
3~ c~ispersed oil droplets are caused to come together and join to
produce flocs which are easily separable from the a~ueous medium.
However, at unduly high levels of agitation, the flocs are
prevented from forming and, hence, remain in suspension or
separate only slowly. For optimum flocculation and separation,
3~ it has been found that the turbulence level within the treated r
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paddle-agitated emulsion must be controlled to be equivalent
to a rotating paddle tip speed of from 0.15 to 0.4 m/s. By
"paddle tip speed" is meant the circumferential rate o-f travel
of the tip oE a horizontal blade rotated from a centre point
thereon. It has been surprisingly found that a level of
agitation of a metal cation-treated emulsion at a paddle tip
speed less than 0.15 m/s and greater than ~.~ m/s produces a
marked increase in the amount of oil retained in the water
fraction after separation.
DESCRIPTION OF PREFERRED EMBODIMENT
The process of the invention may be practised batc~wise by
introducing crude oil produced water into a tank or vessel
equipped with a paddle stirrer. ~ portion of cationic
flocculant chemical selected from Al~+, Fe-~+~ or Fe~ in an
amount of from 0.7 to 10.5 parts per million parts of emulsion
is added to the vessel and the mixture agitated by rotation of
the paddle stirrer. The level of agitation is controlled ~y
maintaining the speed of rotation of the paddle stirrer at a tip
speed of 0.15 to 0.4 m/s. Agitation is continued for a p?riod of
from 10 to 20 minutes after which time -the flocculant and water
phase àre allowed to settle for 30 minutes. The separa-ted water
phase is found to contain from 0.5 to 3 ppm of oil and insoluble
solid matter. The settling or separation time may be reduced
from 30 minutes to about 5 minutes if a small quantity, up to
about 1 ppm and, preferably, about 0.1 ppm, of an anionic hish
molecular weight polymer, such as polyacrylamides having various
charge densities is added to the produced water after primary
agitation. Fur-ther agitation at about half the initial speed for
an additional 5 to 10 minutes after polymer addition is required.
The polymer dose chosen, if used, will depend on the speed of
separation desired, with larger amounts of polymer generally
decreasing the separation time.
The process may also be practised in a substantially
continuous manner by employing an elongated vertical vessel or
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tower equipped with a central rotating shaft having a series
of paddles attached thereto. The oily emuIsion and flocculating
agent may be passed continuously into the base of the tower
where it rises through the agitated zone to a top outlet where
it is continuously removed to a separation vessel. The results
achieved in such a continuous process are less dramatic than
those achieved in a batch process.
E~AMPLE I
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A synthetic oil-in-water produced water emulsion was
prepared having an oil content oE 250 ppm and wherein -the oil
droplet size was 12 Jum or less. One litre of -the synthetic
~roduced water was placed in a polymethylpentene beak~r and
2.5 ml of ferric chloride solution containg 1 g/l as Fe~
~as added. The resultin~ concentration of Fe~ was 2.5 ppm of
the total composition. The emulsion was agitated by means of
paddle stirrer at a paddle tip speed of 0.4 m/s for 10 minutes
after which time 0.1 ppm of a solution of Betz 1160 polymer was
added. Stirring was continued for suc~essive 5 minute periods
20 at paddle tip speeds of 0.25, 0.2 and 0.15 m/s. The treated
emulsion was then allowed to stand for 15 minutes durin~ which
time tlle oil floc separated. Wa-ter from the separated aqueous
phase was clear in appearance and, upon analysis, was found to
contain 1 ppm of oil.
EXA~IPLE II
The method of Example 1 was repeated using the same
synthetic produced water (250 ppm oil) except tha-t an aluminum
potassium sulfate solution was used as the flocculant. The
concentration of Al~ was l.S ppm of the total composition.
The emulsion was agitated at a paddle tip speed of 0O4 m/s for
10 minutes after which 0.1 ppm of a solu-tion of Be-tz 1160 polymer
was added. Stirring was continued for successive one minute
periods at paddle tip speeds of 0.3, 0.25, 0.15 and 0.13 m/s.
The treated emulsion was allowed to stand for 15 minutes while
the oil floc separated. The separated water phase was clear in
appearance and contained 2.4 ppm of oil and unidentified solids.
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The emulsion breaking method of the invention, while
particularly applicable to the treatment of crude oil produced
water, is also effective for treatment of a wide range of oily
industrial waste waters having finely dispersed oil phases.
