Note: Descriptions are shown in the official language in which they were submitted.
CA 02313492 2000-07-10
METHOD OF REMOVING WATER AND CONTAMINANTS
FROM CRUDE OIL CONTAINING SAME
The present invention is directed to an enhanced crude oil dehydration process
and
apparatus, and more particularly the present invention is directed to a crude
oil dehydration
process which can overcome the instability problems encountered with prior art
for treating high
water cut heavy oil streams, provide enhanced thermal energy input and
recovery methods and
remove suspended and dissolved compounds from inlet feed.
Throughout many regions of the world, heavy oil, a hydrocarbon material having
much
higher viscosity or lower API gravity (less than 20°API, typically
7° to 12°API) than
conventional petroleum crude, is being economically recovered for commercial
sale. During the
recovery process and prior to the transport to refineries for upgrading, the
heavy oil receives
preliminary treatment for water and solids removal to generally achieve basic
sediment and water
(BS & W) content less than 0.5% by volume and chloride content less than 30
ppm (wt). Water
content of the treated heavy oil typically is required to be 0.3% by volume or
less.
Conventional crude oil treatment methods were proven to be ineffective with
respect to
heavy oil until the advent of the technology set forth in US Patent Reissue
No. 33,999, Clare et
al., reissued July 21, 1992 and Canadian Patent 1,302,937, Clare et al.,
reissued on June 9, 1992.
These patents describe a simple apparatus which can be located in remote oil
producing areas for
dehydrating heavy oil with low risk of foaming and unstable operating, while
continuously
achieving dry oil which exceeds requisite specifications. These dehydrators
were found to be
restricted to feed oil water content of less than 5% water cuts and
susceptible to foaming and
process instability during high water feed rates. Throughout the operation of
several of these
dehydrators known from practicing the technology in patents Re33,999 and
1,302,937, areas for
CA 02313492 2000-07-10
improvement were discovered to overcome the limitations of feed oil water
content and unstable
operation caused by pretreatment upsets.
Further, additional problems have been experienced with the prior art in that
although
dehydrated heavy oil is achieved, high concentrations of suspended solids,
such as clay and silica
and dissolved compounds such as chlorides remain in the treated oil. These
undesirable
compounds continue to create many problems in pipeline transportation systems
and refinery
facilities to the extent that they depreciate the commercial valve of heavy
oil.
Accordingly, one object of the present invention is to provide advances to
overcome the
limitations encountered by the previous art.
One aspect of the present invention is to provide for a dry crude oil recycle
stream around
the dehydrator to mix with the feed to reduce the ratio of residual water to
oil contacting the oil
surface of the dehydrator and thereby allow for higher raw crude oil water
cuts, while
maintaining a stable dehydration operation.
A further aspect of one embodiment of the present invention is to provide a
method of
removing water from crude oil containing water, comprising of steps of
a) providing a source of crude oil containing water;
b) providing a dehydrator for dehydrating the crude oil containing water, the
dehydrator having an inlet and an outlet and a vaporizing surface of dry crude
oil at temperatures
sufficient to vaporize water contacting the surface;
c) exposing the source of crude oil to the dry crude;
d) vaporizing the water in the source; and
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e) re-circulating at least a portion of dehydrated crude oil for contact with
at least
one of the dehydrator or the source of crude oil for maintaining a
substantially uniform
temperature at the vaporizing surface.
Conveniently, when at least a portion of the dry crude oil recycle stream
around the
dehydrator enters the dehydrator and is distributed below the surface of the
hot crude oil in the
dehydrator a consistent temperature is maintained at or above the vaporization
temperature of
water and at or below the surface of the oil and throughout the contained oil,
thereby providing a
means to mitigate the risk of process upsets and instability due to foaming.
A further aspect of the present invention is to provide a dry crude recycle
stream around
the dehydrator to mix with the feed stream, to allow an input of supplemental
heat energy
(external or waste heat energy) or recovery of heat energy from the dehydrator
to result in an
energetically efficient and balanced process.
A still further aspect of one embodiment of the present invention is to
provide a method
of removing water from crude oil containing water, comprising of steps of ;
a) providing a source of crude oil containing water;
b) providing a dehydrator for dehydrating the crude oil containing water, the
dehydrator having an inlet and an outlet and a vaporizing surface of dry crude
oil at a
temperature sufficient to vaporize water contacting the surface;
c) contacting the source and the dry crude oil to flash water from the source
to
thereby remove the water from the source; and
d) selectively heating the surface of the dry crude to return heat energy lost
from
flash evaporating water from the source.
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The dry crude oil surface may be selectively heated by reintroduction of dry
crude oil,
auxiliary heat addition, etc. The important aspect is that the heat used for
vaporization is
replaced so that a uniform or substantially uniform surface temperature is
maintained. This is
one important unit operation to maintain.
A further aspect of the present invention is to provide a means to remove
suspended
solids accumulated in the contained dry crude oil introduced with the source
oil or produced
during dehydration.
Suspended solids in the dry crude oil recycle stream, may be removed by a
separator
means on a continuous or batch basis to avoid buildup, plugging, and other
complications.
A further aspect of the present invention is to water wash the raw crude oil
source
combined with the treatment in the dehydrator to remove the soluble dissolved
solids or
contaminants introduced the source crude oil and generate a low conductivity
produced water
and clean dry crude oil.
Until the advent of the present invention, prior art methods related to heavy
oil
dehydration had been limited by the stability of operation and risk of
foaming, level of water cut
or emulsion level in the heavy oil feed and the level of chloride, clay and
silica compounds in the
dry sales crude oil.
A still further aspect of one embodiment of the present invention is to
provide a method
of removing water from a crude oil containing water, comprising of steps of
a) providing a source of crude oil containing water;
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b) providing a dehydrator for dehydrating the crude oil containing water, the
dehydrator having an inlet and an outlet and a vaporizing surface of dry crude
oil at a
temperature sufficient to vaporize water contacting the surface;
c) pre-washing the source of crude oil containing water to remove alkali
metals and
alkaline earth metals;
d) separating the alkali metals and the alkaline earth metals and oil in a
preliminary
separation step;
e) passing washed oil into the dehydrator to flash evaporate water from the
washed
oil to remove water therefrom;
f j selectively pre-heating crude oil to be treated in the dehydrator with
treated
dehydrated crude oil from the dehydrator to return heat energy lost from flash
evaporating water;
and
g) maintaining a uniform surface temperature.
Enhancements have been developed to eliminate the limits imposed by water cut
of the
source crude oil feed and to provide a very clean and dry heavy oil product
relatively free of
water, solids and chlorides.
The present invention relates to process enhancements to an apparatus used for
dehydrating crude oil containing water, comprising a casing, means for
admitting and
distributing the liquid crude oil into the casing and onto the host surface of
the dry crude oil,
means for controlling the level of crude oil and a means to transfer heat
energy sufficient to
maintain the liquid oil at or above the distillation temperature for
evaporating water and light
hydrocarbons.
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The light hydrocarbons and water exiting the casing are condensed by any
suitable means
known in the art, and collected and separated into water and light hydrocarbon
liquid phases.
Any non-condensible vapors are released from the apparatus for disposition by
any safe means.
Dry crude oil meeting pipeline BS & W specifications is pumped from the
dehydrator for
transport to refining and upgrading operations.
Typically, the dehydrator taught in the current art performed well to produce
dry crude
oil, however several problems have been encountered:
1. The dehydrator was limited to crude oil feed water cuts (wc) of less than
10
water to oil, and more specifically less than 5 % we to reduce the risk of
unstable operation with
foaming tendencies. This required the need for a conventional treater means
upstream of the
dehydrator to reduce raw crude oil water cuts from SO to 20 % we down to less
than S % we prior
to feeding the dehydrator.
2. The dry crude oil exiting the dehydrator contains high chloride content,
causing
metallurgy and corrosion problems with downstream refineries facilities and
transportation
pipelines.
3. It was found that by flash evaporating off the water and by effectively
eliminating
all emulsions, solids such as clays and silica compounds, concentrated in the
dry oil phase, had a
tendency to buildup, plug and/or cause heat element damage.
4. It has been further experienced that the dehydrator is susceptible to
unstable
operations and foaming tendencies causing dehydration temperature swings and
wet oil
production.
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The present invention seeks to address these concerns by providing methodology
and
apparatus to exceed the performance of the dehydrator beyond the prior art.
In one embodiment of the invention, at least a portion of the dry crude oil
exiting the
dehydrator is recycled and mixed with the inlet crude oil feed prior to
entering the dehydrator
casing. By increasing recycle flow, a consistent and stable inlet water cut
composition can be
maintained at the entrance to the casing to control the tendency to foam and
create operational
complications. With greater recycle rates, the raw water cut levels can be
increased above the
10 % we stable level and continuous stable operation is maintained. This
eliminates the need for
conventional treatment ahead of the dehydrator and can avoid dehydrator
process upset if an
upstream treater is used and a treater upset occurs.
A further embodiment of the invention requires that at least a portion of the
recycled dry
crude oil be recycled and distributed immediately below the dry crude oil
evaporating surface.
This method ensures that the temperature of the surface of the dry oil in the
dehydrator is
maintained at or above the flash evaporating temperature of water. Water
droplets from the feed
are not permitted to penetrate the surface of the crude oil, thereby
preventing the cooling below
the surface and creating surface breakdown foaming and unstable dehydrator
operation.
Advantageously, external heat transfer means can be added to the recycle
circuit supra to
regulate the precise temperature of the feed stream to the dehydrator casing.
This method
enhancement will regulate the precise level of pre-flashing of water vapour in
the feed oil to
control the residual water level contacting the hot dry oil surface. This step
can be used to
prevent the overcooling of the bath and eliminate the foaming effects caused
by excessive
evaporation surface breakdown.
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The external heat transfer means can also be used to cool the feed stream.
This may be
used if the raw crude oil feed stream already contains sufficient or excessive
thermal energy
required to flash or distill the water vapor. Cooling the feed stream can
regulate the flashing
operation and prevent process complications.
As a further feature, a solid/liquid separation device, examples of which
include a filter,
hydro cyclone, centrifugal separators, gravity separators, centrifuge or any
combination thereof,
etc., may be employed in the circuit of the recycle stream continuously or on
a batch basis to
remove suspended solids from the hot dry oil.
Additionally, a clean water washing circuit may be added to the dehydrator
feed to reduce
undesirable dissolved compounds, such as chlorides, from the dry crude oil.
The entire
contaminated water stream, or a portion thereof, is treated by a suitable
treatment method to
create a clean water stream and a highly concentrated brine, slurry or solid
product. The
recovered clean water is recycled back to the raw crude oil for oil
pretreatment. Generally water
or any aqueous solution containing compounds for enhancing the extraction of
chloride is most
desirable, otherwise any regenerable fluid with a suitable aggressive
solubility for chlorides may
be considered.
It is preferable that in addition to achieving a dehydrated oil, having a BS&W
content of
less than 0.5% we by volume, the embodiments of the invention in combination,
or separately
applied, can produce a dry clean crude oil, substantially free of solids,
containing less than 30
ppm (wt) chlorides, in a continuous and stable operation, with low risk of
foaming and process
upsets. The oil produced by the present process is readily vendible and is
most desirable,
particularly in the case of heavy crude oils with gravities in the
7°API to 20°API range.
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Having thus described the invention, reference will now be made to the
accompanying
drawings illustrating preferred embodiments.
Figure 1 is a schematic flow diagram which illustrates the dry oil recycle to
the
dehydrator feed stream and dehydrator;
Figure 2 is an additional schematic flow diagram showing external heat
exchange on the
recycle for temperature adjustment of the feed or surface of the dehydrator or
both;
Figure 3 is a further schematic flow diagram showing a solid/liquid separator
for removal
of suspended solids; and
Figure 4 is a schematic flow diagram illustrating the addition of water
washing for
removal of dissolved compounds such as chlorides.
Similar numerals employed in the Figures denote similar elements.
With reference to Figure 1, heavy oil with a viscosity of between 7°API
and 20°API
denoted by numeral 10, typically includes a mixture of crude oil, water,
oil/water emulsion,
dissolved compounds such as chlorides and solid particles such as clay, metals
and silicas. The
crude oil is generally received in a gravity separator, heated or non heated
treater 12, under
pressure from between atmospheric pressure to 100 psig. Heated treaters
typically operate from
170°F to 285°F (77°C to 141 °C). In the treaters,
solid particles and bulk brackish water is
separated and removed from the raw crude oil at 14. Water cuts of less than
10%, to more
typically 5% by volume can be achieved in the raw crude feed 18 exiting the
primary treatment
through a valve member 20. The water stream 22 generally contains dissolved
compounds such
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as sodium chloride, (5,000 to 50,000 ppm (wt)) and silica, and suspended
compounds such as
clay and sand.
The raw crude oil at approximately 5% water cut in the emulsion form,
containing no free
water, enters the dehydrator 24 where the crude oil and emulsions are evenly
distributed onto the
hot surface of dry crude oil (not shown), operating at or abave the
evaporation temperatures of
the water. Water is flashed off the oil or separated by distillation, with
water and low boiling
temperature hydrocarbon components from the oil exiting through the column 26
and passing
through line 28. If desired, the water and lower boiling components may be
sent to a condenser
30 and subsequently to a vapor liquid separator 32. Dehydrated higher boiling
point crude oil is
discharged from the dehydrator 24 through line 34.
In the separator 32, water and light hydrocarbons are separated by differences
in specific
gravity. The water is discharged through line 36 and pump 38. The light
hydrocarbons are
transferred from the separator 32 using pump 40 via line 42, and can be
removed for disposal at
line 44 or at least a portion recycled and mixed at 11 with the inlet crude
oil 10 via line 46, to
dilute the incoming crude oil and thereby facilitate its further treatment.
Non condensable, i.e.
light hydrocarbons, inert gases (nitrogen, carbon dioxides, hydrogen sulphide)
are vented from
separator 32 and disposed of or recavered by any suitable safe means.
As shown by Figure 1, dry oil can be recycled from 48 and recycled as stream
50 to mix
with the inlet feed 18, at 19, prior to being distributed onto the hot oil
surface in the dehydrator 24.
In order to maintain the temperature of the hot oil surface, at least a
portion of the recycle
stream 50 can be recycled directly to the dehydrator 24 and be distributed at
or immediately
below the surface of the hot dry crude oil. It has been found that by
recycling the dry crude oil to
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inlet stream 18, and separately or in combination with recycling dry crude oil
to the surface of
the hot bath by using stream 52 (dashed lines), the following significant
benefits can be realized:
a) The water cut of the raw crude oil at stream 18 can be increased to greater
than
10%, and even greater than 20% by volume. This enhancement means that the
requirement for
conventional treatment denoted as 12 can be eliminated, without risk of
process instability and
foaming of the dehydrator.
b) If a conventional primary treatment 12 is used, the recycle stream can be
used to
isolate the dehydrator from unstable or operational complications if the
pretreatment becomes
unstable. This means that the dry crude oil sales specification is not at
risk, and rerun of off spec
sales oil from sales oil storage tanks and pipelines is avoided.
The ratio of recycle at 50 to inlet feed can vary depending on the actual
temperature and
rate of the recycle 52 and the level of feed conditioning and water cut
reduction required at the
inlet to the dehydrator. Similarly, the ratio of recycle 52 to recycle 50 will
vary for each
application in order to establish a balance between dehydrator feed
conditioning and dehydrator
surface temperature. Depending on the relative size of oil recycle 50 to dry
sales oil 34, common
pumps or separate pumps may be used, as known to those skilled in the art.
Recycle 52 can also
be provided by separate pumping means.
Referring to Figure 2; shown is an enhancement to the recycle variation of
Figure 1,
where a heat exchanger means 54 is added to the recycle circuit to condition
the temperature for
streams 50 and 52. The streams, 50 and 52 can be heated or cooled to the same
temperature or
independently to separate temperatures in order to seek the thermal balance of
the feed stream
and hot crude oil bath surface. Any form of suitable heat source, such as
direct fired heaters,
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indirect fired heaters, heat exchangers or heat recovery or cooling apparatus
may be selected. A
further consideration for temperature at the streams 50 and 52 is whether the
feed is from a
heated primary treatment means at 170°F to 285°F (77°C to
141 °C) or from a raw crude storage
tank at 60°F to 100°F (16°C to 38°C).
Figure 3 illustrates an additional enhancement to include a solid/liquid
separator means
62, used to remove suspended solids such as clay, sand, and precipitated salts
from the
dehydrated crude oil. The solid/liquid separator 62 may be selected from any
suitable separator
device known to those skilled in the art, such as gravity separators,
clarifiers, filter, screens,
cyclones and centrifuges. The recycle stream from 50, is sized to satisfy the
range of operation
of the solid/liquid separator device 62 and specifically sized to accommodate
a solids removal
rate at 64 greater or equal to the solids content entering the dehydrator 24
at 18 and being
produced in the dehydration process.
The removal of the solids can be performed on a continuous or batch basis and
primarily
allow for the ongoing removal of solids from the dehydrator 24 to prevent
buildup and plugging.
Buildup of solids on the heating elements contained in 24 or external to 24 is
detrimental to the
elements performance and can become a safety issue.
Turning to Figure 4, shown is a further variation of the invention showing the
addition of
a water wash means to the dehydrator to remove dissolved solids. The raw crude
oil can contain
high concentrations of sodium, calcium, magnesium, chlorides, sulfur,
carbonates, silica, etc. All
these compounds, especially the chloride are currently undesirable in the dry
crude sales product
and may have significant commercial impact on the price for the crude oil, or
even restrict sales.
Typically, refineries are currently requiring less than 30 ppm(wt) chlorides
in the sales crude oil.
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Using the enhancement shown by Figure 4, clean water 66 is injected and
intimately
mixed with the raw crude oil 10 at 68. The feed mixture l0 is passed through
separator 12. The
bulk of the brine contaminated water is separated from the oil and discharged
through line 22 to
a water treatment unit 70.
The washed crude oil is discharged at 18 and becomes the feed stream to the
dehydrator. The feed can be conditioned either in the separator 12 or by using
the
recycle stream SO and 52 to ensure stable dehydrator 24 operation. The washed
crude at 18
contains significantly reduced levels of dissolved compounds, meeting or
exceeding the sales
oil specification requirements.
The water treatment scheme selected for each application must ensure that the
undesirable compounds in stream 22 are sufficiently removed to satisfy the
process removal
requirements at 18. Typical water treatment practices, are microfiltration,
reverse osmosis,
distillation, flocculation, clarification and coagulation.
Treated water enters the treated water vessel 74 and is transferred by pump 76
for
reinjection at 68 using line 66.
As an option, condensed water from the separator 32 can be transferred
directly by
pump 78 to either the treated water vessel 74 by line 80 or to a water
treatment unit 70 by
line 82 if water treatment is required. The net water production would
discharge from the
separator 32 at stream 84, or from the water treatment unit 70 by means of
stream 88. Fresh
water makeup can be introduced to the treated water vessel 74 at 90 if a water
balance
deficit is encountered.
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By following the enhancements independently or in combination, the process
methods
as described by this invention, will result with dry clean crude oil meeting
or exceeding new
sales specifications for commercial sale.
Although embodiments of the invention have been described above, it is not
limited
thereto and it will be apparent to those skilled in the art that numerous
modifications form
part of the present invention insofar as they do not depart from the spirit,
nature and scope of
the claimed and described invention.
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