Note: Descriptions are shown in the official language in which they were submitted.
Title: RECOVERY OF SOLVENTS FROM MIXED PRODUCTION FLUIDS
AND SYSTEM FOR DOING SAME
FIELD OF THE INVENTION
This invention relates to a hydrocarbon separation process and a
system to perform such a process. In particular, this invention relates to a
separation process in which one or more light hydrocarbon species, such as
solvents, which may be present in mixed recovered production fluids from an
in situ heavy oil or bitumen extraction process, can be separated from the
other
recovered fluids such as water, other light hydrocarbons and heavy
hydrocarbons, such as sales oil.
BACKGROUND OF THE INVENTION
Environmentally acceptable extraction of hydrocarbons from the oil
sands remains a challenge. In situ techniques, such as SAGD, are being used
but they come with a heavy carbon emission cost. As a result, other
technologies for in situ extraction are being explored. One area that shows
promise is the use of hydrocarbon based solvents in an in situ extraction
process. Solvents offer the potential to reduce the extraction temperature as
compared to SAGD. Such reduced temperatures reduce the energy required
to extract the hydrocarbons in place, resulting in much reduced carbon
emissions for the recovery of an equivalent amount of resource through a
solvent process as compared to, for example, SAGD.
An example of such an in situ solvent based process is the nsolv
gravity drainage process which uses condensing solvent within an
underground extraction chamber to warm and dissolve the bitumen in place
rather than using the high temperature and high pressure steam of SAGD to
mobilize the bitumen. The more modest temperatures and pressures can
result in significant potential energy savings as compared to conventional
1
CA 3073464 2020-02-21
SAGD with commensurate potential carbon emission reductions. Other
solvent based technologies have also been proposed including combining
solvent with steam and using solvent in conjunction with other sources of
energy, such as RF energy. In all of these cases solvent is being injected
into
the oil bearing formation to help mobilize the bitumen so the bitumen can
drain
and be recovered with the produced fluids.
The hydrocarbon based solvents referred to here are generally lighter
hydrocarbon species such as propane, butane and the like. Methane injection
has also been proposed in certain processes, such as VAPEX, where it
theoretically acts as a carrier gas or a displacement gas, but this process of
injecting a mixed gas has not worked in practice. Rather than helping, methane
may become a problem in the reservoir because it interferes with the mass
and heat transfer of the solvent onto the bitumen at the extraction interface.
However, some methane is usually naturally present in an underground gravity
drainage extraction chamber dissolved in the in situ bitumen or hydrocarbons.
Warming such hydrocarbons may result in methane as an off gas product from
the hydrocarbons and so methane is likely to be produced as part of the mixed
fluid production from a typical extraction. Any produced methane will need to
be separated out from the solvent to maintain solvent purity before the
solvent
may be re-injected into the ground.
An in situ extraction process typically requires that the bitumen
mobilizing material, whether it is solvent, steam, or even heat, be injected
into
or applied to the formation, generally into the oil bearing strata in an
effort to
make the bitumen mobile enough to drain through the formation. The oil rich
zone or pay zone may be comprised of porous sand, rock or the like and may
have heterogeneities such as clay lenses or the like. If the viscosity of the
bitumen can be reduced enough the bitumen may be able to drain by gravity
through the sand pack or porous parts of the formation. Then, the next step is
to recover the draining liquids from the oil rich zone, for example through
2
CA 3073464 2020-02-21
production tubing located within or below an extraction zone. Gases may also
be recovered either dissolved in the draining fluids or as a separate phase.
One typical form of recovery is through the use of horizontal wells, in which
an
upper well is an injection well and a lower well is the production well.
Water,
naturally present or injected as steam, may also be recovered as part of the
draining liquids. Artificial lift may be used to raise the production fluids
to the
surface.
The working fluid, such as solvent in the case of the nsolv , process,
may be injected in the upper injection well, interact with the bitumen so that
the bitumen and such fluids drain down to the lower production well through
an extraction chamber. While horizontal well pairs are common in these types
of extractions there are many other forms of well configurations that can be
used including one or more vertical wells alone or in combination with the one
or more horizontal wells. The common element of all of these extractions is
that a fluid mixture is recovered from the formation which includes some
combination of off gases such as methane and carbon dioxide, some
proportion of injected solvent gases such as methane, ethane, propane,
butane or the like, and some combination of water and heavy hydrocarbons
the latter of which may be separated from the other fluids to become sales
oil.
One or more of these fluids may be liquids or gases depending upon the
temperature and pressure at any given point in the process.
What is desired therefore is that when these mixed fluids (liquids and
gases) reach the surface there is an ability to separate and in some cases
purify these fluids into one or more species of these fluids. Typically the
separation and purification of hydrocarbons is done with a distillation column
and controlled cooling of the overhead gases to condense and concentrate the
same. U.S. Patent 2,775,103 teaches such a hydrocarbon separation which
includes a demethanizer column and an accumulator for removing gases such
as ethane from other gaseous species such as methane and hydrogen. It also
3
CA 3073464 2020-02-21
relates to the recovery of propane where the same is used as a solvent for
ethylene or ethane in the actual hydrocarbon separation process. In addition
to the demethanizer there is provided an accumulator followed by a knock out
drum.
However to make the system work this patent teaches that a great deal
of refrigeration must be used. The feed temperature into the demethanizer
column is minus 56 F (-49 degrees C). The overhead product is mixed with a
propylene stream at minus 28 F (-33 degrees C) and the resulting mixture is
cooled to minus 56 F (-49 degrees C). Then the mixture is passed through a
first heat exchanger and cooled to minus 82 F (-65 degrees C) and then
through a second heat exchanger and cooled to minus 112 F (-80 degrees C)
in a knock out drum. The patent teaches that a temperature as low as possible
is preferred, to achieve a high quality separation.
While this may separate out the ethane species as desired, it is very
energy intensive. For separating ethane out from small volumes this may not
impose too significant an operating cost but for in situ solvent extractions
of
heavy oil or bitumen such as the nsolv technology that involve large volumes
of hydrocarbon solvent which is injected into the formation to condense, drain
and then be recovered, purified and then re-injected the energy costs for
refrigeration could be overwhelming. This is because large volumes of solvent
may be needed to mobilize the bitumen and will be co-produced with the
bitumen. The solvent may need to be recovered in large volumes and then re-
used to extract more bitumen. As such, a very refrigeration intensive
separation process that uses up a great deal of energy and contributes to the
carbon footprint or cost of the process is not desirable.
Canadian Patent application 2,777,966 describes a solvent injection
plant for enhanced oil recovery that uses a distillation column to recover and
purify the bulk of the solvent from produced bitumen. The invention also
comprehends the use of a free water knock out vessel to separate water and
4
CA 3073464 2020-02-21
solids from the bitumen and at least one flash vessel stage for removing
solvent as a vapour from the bitumen. In this configuration, additional flash
vessel stages are required to minimize solvent loss to the bitumen stream and
some solvent may be entrained in the water stream. For separating and
purifying large volumes of solvent, it is desirable to reduce solvent energy
requirements and capital costs.
SUMMARY OF THE INVENTION
A process and system is desired which may be suitable for high volume
solvent recovery, purification, reuse and recycling for the in situ
hydrocarbon
extraction process such as the nsolv condensing solvent extraction process.
What is further desired is an energy efficient process to separate the various
hydrocarbon solvent species out of a mixed production fluid in a surface
facility
so the solvent may be re-used and recycled back into the formation. Most
preferably such a separation will minimize solvent losses from the system and
also allow the solvent to be purified so as to prevent solvent impurities from
being injected into the formation and potentially interfering with any in situ
mass transfer or heat transfer process within the formation. Such a separation
process may preferably be tolerant to changes in feed composition over an
extraction period, may be able to handle large throughput volumes and may
be more energy efficient.
The present invention is directed to various embodiments which may
be able to efficiently separate the various hydrocarbon species to permit the
desired solvent to be recirculated and the other components of the mixed fluid
production from the in situ process to be either sold or used for some other
purpose such as for plant or other fuel. The present invention is also
directed
to various embodiments of exemplary systems or apparatuses to accomplish
this. The present invention may provide for substantial recovery of the
solvent
or working fluid from the mixed fluids production extracted from the reservoir
5
CA 3073464 2020-02-21
to limit working fluids losses to sales oil or to the fuel gas stream, which
losses
would otherwise have to be made up with make-up solvent or working fluid.
The present invention may further comprehend limiting a loss of working fluid
or solvent in any waste water stream, for example, a separated produced
formation water stream. The present invention may be incorporated into a
modular design which can be used in a variety of contexts and associated with
a variety of processes where a feed stream originates within the formation and
contains a mixture of hydrocarbons, including potentially large volumes of the
injected solvent working fluid. The present invention may provide an efficient
method for separating various species which minimizes the energy required to
do the separations and therefore to recover the working fluid in a cost
effective
manner. The present invention provides a separation process that may
operate at much higher separation temperatures than the prior art while still
achieving good separation results and thus may reduce the energy cooling
costs of the overall system. In part the present invention may provide that
the
cooling only be required at the end stage and even then that the coolest or
lowest separation temperature may be in the range of -25 to 10 C for a working
fluid consisting of propane for example. The present invention further
provides
that such cooling may only be applied to a small fraction of the recovered
fluids, as the majority of the separation and recovery of solvent can occur at
even higher temperatures.
In a further aspect the present invention may provide an energy efficient
method of recovering solvent that has been entrained in the separated bitumen
stream or sales oil. This may have an added benefit that the vapour pressure
of the sales oil is reduced to facilitate safe and practical storage. The
present
invention may also provide in a preferred embodiment that the energy required
for heating the streams for separation be recovered from other waste heat
available from other process streams within the system, to lower total energy
consumption.
6
CA 3073464 2020-02-21
According to one embodiment, the present invention provides a
hydrocarbon separation system for separating produced fluids obtained from
an in situ hydrocarbon extraction process, said separation system comprising:
a separation chamber having a liquid inlet, a vapour inlet, a bottom
portion, and a top portion, said separation chamber being configured to
separate inputs into bottom liquids and top vapours, said separation chamber
having a bottom liquid outlet and a top vapour outlet,
a reboiler circuit to heat said bottom liquids to separate said liquids into
purified solvent and a reboiler vapour fraction, wherein said purified solvent
may be reused in said in situ extraction and said reboiler vapours are
recirculated back to said separation chamber; and
a multi-stage reflux circuit to receive said top vapours from said
separation chamber, said multi-stage reflux circuit comprising at least:
(i) a first stage to receive and cool said top vapours to a first
temperature
to condense a portion of said top vapours into a first reflux liquid, wherein
said
first reflux liquid is again passed through said separation chamber; and
(ii) a second stage to receive and cool said remaining top vapours from
said first stage to condense at least a portion of said remaining top vapours
into a recycle liquid at a second temperature, said second temperature being
lower than said first temperature, return said recycle liquid to said
separation
chamber and exhaust any non-condensing vapour as fuel;
and wherein the lowest temperature cooling is applied to the remaining
top vapours from said first stage reflux drum.
In a further embodiment of the invention, the separation chamber may
be a distillation column or the like, where to enhance separation mass
transfer
the column may contain trays and/or packing to enhance vapour-liquid contact
and may also rely on counter-current flow of the vapour and liquid streams to
enhance mass transfer driving force.
The present invention also comprehends a hydrocarbon separation
7
CA 3073464 2020-02-21
system for solvent recovery from separated hydrocarbons produced by means
of a solvent based in situ hydrocarbon extraction process, said separation
system comprising:
a separation chamber having a liquid inlet, a bottom portion, and a top
portion, said separation chamber being configured to separate inputs into
bottom liquids and top vapours, said separation chamber having at least one
bottom liquid outlet and a top vapour outlet,
a reboiler circuit to heat a portion of said bottom liquids to reboiler
vapours, wherein said reboiler vapours are recirculated back to heat said
separation chamber,
a reflux circuit to receive and condense said top vapours from said
separation chamber into recovered solvent, and non-condensable gas,
wherein a portion of said recovered solvent is returned to said separation
chamber, while the remaining recovered solvent is directed to said in situ
extraction, and said non-condensable gas may be used for fuel,
a vapourizer for heating the recovered solvent to gas phase for
injection; and
an oil cooler for final adjustment of the temperature of said purified oil
for safe storage. Such a system may optionally include a first cross heat
exchanger, wherein the liquid flowing into said separation chamber is
preheated while cooling the remaining bottoms liquid of said separation
chamber to a first temperature, and /or
a second cross heat exchanger, wherein said bottoms liquid at said first
temperature are cooled to a second temperature, which is lower than the first
temperature, producing purified sales oil, while preheating the recovered
solvent from said reflux circuit,
The present invention may also comprehend a water separation system
for further separating solvent from the water produced from a solvent based in
situ hydrocarbon extraction process, said water separation system comprising:
8
CA 3073464 2020-02-21
a bulk fluids separation vessel having a separation zone, and being
configured to separate mixed input fluids into mixed water and light/heavy
hydrocarbons;
a piping connection to permit said light/heavy hydrocarbons to be sent
to a downstream light/heavy separation;
a piping connection to permit the mixed water to be sent downstream
to a further water separation stage, the further water separation stage
including
a skim tank that receives said mixed water from said vessel, said skim tank
being sized and shaped to permit said mixed water to separate into a lower
density hydrocarbon rich stream and a higher density water stream;
a slop oil tank; and
a piping connection to permit a slop oil tank to receive said lower density
hydrocarbon rich stream and a further piping connection from the slop oil tank
to return at least a portion of said lower density hydrocarbon rich stream
back
to said bulk fluids separation chamber.
BRIEF DESCRIPTION OF THE DRAWINGS
Reference will now be made, by way of example only, to preferred
embodiments of the invention by reference to the following drawings in which:
Figure 1 is a schematic drawing of a first stage of a preferred
solvent/bitumen separation sequence for separating a mixed fluid production
into a bulk solvent stream, a product oil stream and a produced water stream
according to a preferred embodiment of the present invention,
Figure 2 is a schematic view of a solvent recovery and purification
sequence for the bulk solvent stream, produced from the sequence of Figure
1, according to a preferred embodiment of the present invention,
Figure 3 is a schematic view of a solvent recovery process for the
product oil stream, produced from the sequence of Figure 1, according to a
preferred embodiment of the present invention; and
9
CA 3073464 2020-02-21
Figure 4 is a schematic view of a solvent recovery system for the
produced water stream produced from the sequence of Figure 1, according to
a preferred embodiment of the present invention,
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Figure 1 shows a process flow schematic for processing a mixed fluid
stream recovered from an underground formation and for separating the mixed
fluid production into a bulk solvent stream, a product oil stream and a
produced
water stream. Thus Figure 1 provides context for the further separations
shown in Figures 2, 3 and 4, which show preferred embodiments of the
separation of working fluid or solvent from the three primary separation
streams produced by the primary separation process of Figure 1. The bulk
solvent stream may include lighter hydrocarbon species may be further
separated into a purified working fluid, such as a solvent, and the other
species
which may include various non-condensables species. Such further non-
condensable species may be used as fuel, for example, according to the
present invention. One of the aspects of the present invention may be to
recover a higher percentage of the working fluid at a lower energy cost from
these product streams as described in more detail below.
Figure 1, which depicts a flow sheet which may be suitable, for example,
for an nsolv facility, shows a production well 10, which produces a mixed
fluid stream 12. The primary constituents of the mixed fluids stream 12 are
bitumen, formation water and solvent, but many other materials may also be
present including light hydrocarbon species other than an identified working
solvent species, solids and the like. The precise nature of the mixed fluid
will
vary, both as to the reservoir and over time depending upon what stage the
extraction is at and how large the in situ extraction chamber has grown. The
preferred solvent used for recovery may be selected according to the reservoir
characteristics, such as native pressure, temperatures, depth, porosity and
the
CA 3073464 2020-02-21
like, but for a preferred condensing solvent process such as the nsolv
technology the solvent may be propane, butane, pentane, ethane, COS, H2S
or the like. It will be understood that all of these solvents, and any others
such
as will be known to those skilled in the art and which may be useful in an in
situ solvent extraction process for the extraction of hydrocarbons, are
comprehended by the present invention. The species selected for having the
correct condensing characteristics for the reservoir (see Canadian Patent
2,591,354) may be called the working fluid. The term fluid as used herein
comprehends both a gas or a liquid or both.
The primary production stream of mixed fluids 12 may be submitted to
a primary separation stage identified by dashed box 14 which includes a
primary water separation step, such as a free water knock out vessel or the
like, to remove water and solid particulate shown at 13 from a light/heavy
blend
shown at 15, as will be understood by those skilled in the art.
The light/heavy blend 15 is forwarded to a light/heavy separator system
16, which in general has at least two output lines. By the term light/heavy,
the
hydrocarbons to be recovered are considered to be the heavy portion, while
the solvent and any non-condensable species are considered to be the light
portion. The light/heavy separator 16 may heat the mixed fluid light/heavy
blend 15 such that most of the working fluid and non-condensable species are
vapourized to separate them from the remaining heavy oil or bitumen
components (which collectively may be referred to as "sales oil").
The separator 16 has some output lines as shown. The first line 24 from
the separator 16 is a sales oil line 26 through which the separated heavy
fluids
(which may also be called bitumen, product oil, or sales oil) may be sent for
storage or for sales all through line 26, or for additional solvent recovery.
It
will be understood that in a preferred embodiment as described below, the
heavy hydrocarbon fraction may be sent for further working fluid or solvent
recovery. The mixed light fluids that are separated may comprise a high
11
CA 3073464 2020-02-21
pressure stream shown at 17, and a low pressure stream shown at 18. A
compressor 20 may be provided to compress the low pressure stream 18
before it is added to the high pressure stream at 22 which together form a
bulk
solvent stream. The bulk solvent stream is then sent for further processing as
shown at A. It is referred to as a bulk solvent stream since it is comprised
mainly of solvent, but also includes noncondensables such as methane,
carbon dioxide and the like. Additional output lines of intermediate pressure
mixed solvent streams may also be generated depending on the number of
separation stages employed and are thus comprehended by the present
invention.
The light/heavy separator system 16 may also include a number of
pumping, heat exchanger, and vessel stages as will be understood by those
skilled in the art.
The liquid phase of the production stream 15 may be further pressurized
with a booster pump and/or heated with a heat exchanger (not shown) to
recover a portion of the working fluid or solvent as vapour at a pressure that
is
sufficiently high to feed subsequent purification stages without
recompression.
Furthermore, a temperature and pressure of the first stage light/heavy
separator may be selected, taking advantage of relative volatilities of
dissolved
impurities and the working fluid, so as to concentrate the impurities in the
vapour output of the first stage, i.e. 17, thereby avoiding the purification
requirement of working fluid liberated from heavy hydrocarbons in the
downstream light/heavy separation stages, i.e. 18. Therefore, in one
embodiment of the present invention, if the low pressure mixed solvent stream
18 is sufficiently low in impurities concentration, it may be combined
directly
with the downstream purified working fluid, 30.
Also shown on Figure 1 is a return line 30 which is returning purified
solvent to the injection well 32.
Figure 2 shows an energy efficient solvent recovery system 110
12
CA 3073464 2020-02-21
according to the present invention. The system 110 includes a primary
separation chamber 112, which may be for example, a deethanizer, and a
source feed 114 for receiving the high pressure mixed light fluids line 22. As
noted above a source for such a stream may be a primary separation stage
for an in situ extraction process using a condensing solvent as the working
fluid, but the present invention comprehends that the energy efficient process
may also be useful in other large volume through-put separation situations.
The mixed lighter fluids or recovered fluids in the source feed 114 may
contain
a mixture of solvent and non-condensable (at reservoir extraction conditions)
species. Most preferably, any heavy hydrocarbons, such as sales oil, water
and any other heavy species have already been removed. Prior to being fed
into the chamber 112 there is shown a preliminary deethanizer feed drum 132
for separating the source feed 114 into liquids at 113 and gases at 115 as
explained in more detail below.
It will be understood by those skilled in the art that each geological
formation or reservoir will have its own unique characteristics such as
temperature, pressure, porosity, water content and the like and so the
preferred operating conditions for any given extraction can vary. Further,
there
are a number of different processes which can be used, one of which is the
nsolv extraction process. Various working fluids can be used and the choice
of working fluid will depend on the in situ conditions and the process. Two
preferred working fluids for nsolv are propane and butane, but other fluids
may also be chosen depending upon the conditions. The term working fluid in
this disclosure is intended to comprehend any fluid which can be used in an in
situ extraction process and which may be recovered in a mixed fluid production
from the reservoir, and then needs to be separated from the other non-working
fluids so it may be recovered, for example, for reuse. Thus, according to the
present invention, there will be mixed production fluids recovered to surface
which have the desired hydrocarbons as well as many other compounds and
13
CA 3073464 2020-02-21
the present invention is designed to separate and purify the same as may be
desired for being able to utilize the recovered working fluids again. For
example in an in situ solvent stimulation it is important to separate out the
solvent and to obtain the desired purity specifications so that in some cases
the solvent can be reused in the formation to encourage more hydrocarbon
recovery. Further, a more complete separation is desired to reduce solvent
loss or waste in sales oil, in produced water or in the fuel gas stream ¨ in
other
words a more complete separation leads to higher recovery and re-use of the
working fluid with less waste and may be preferred.
In Figure 2 various terms are used by way of example only. For
example, where the re-circulating solvent or working fluid is propane the
system that is being operated may be a deethanizer. The column recovers a
liquid propane solvent stream at the bottoms 120 ¨ the lighter components
(ethane, methane & lighter) are being removed off of the top 142 as a fuel gas
stream and this is defined as a deethanizer. If the column was designed to
make a purified butane solvent product instead, components lighter than
butane would be boiled off and the system would normally be referred to as a
depropanizer. It will be understood that the present invention applies to many
forms of solvent and the precise term for each part of the apparatus may vary
depending upon the nature of the species being separated that step.
Therefore, the terms demethanizer, deethanizer and depropanizer are used
herein by way of example only as the present invention comprehends the
separation of various species depending upon the choice of the working fluid.
More specifically, where the working fluid is propane or butane, a
demethanizer may be used.
In Figure 2, assuming the working fluid or solvent is propane, the
deethanizer 112 includes a top tray 116 and a bottom tray 118. Bottom liquids
are taken off below the bottom weir at 120 and the bottom liquids are sent to
a reboiler 122. The reboiler 122 includes a weir 124 and the light fractions
or
14
CA 3073464 2020-02-21
reboiler vapours are removed at 126 and recycled to the deethanizer 112
through inlet 128 and the reboiler liquids are removed at 130.
According to one embodiment of the invention, the reboiler liquids are
a purified working fluid that may be vapourized and reused in the in situ
recovery process and may be, for example, nearly pure propane. The present
invention comprehends other reboiler configurations can be used to provide
heat input into the column but the one shown may provide reasonable results.
Coming off the top of the deethanizer is a line 142 which takes the top
vapours to a deethanizer condenser 144. This condenser 144 may, for
example, be an air cooled condenser. Then the condensed material, which is
referred to as a first reflux liquid, is fed into a reflux drum 146, through
line 145.
The liquids are drained through line 148 to a pump 150 and then back into the
deethanizer at 152. The remaining vapours (or top gases) are taken off at 154
through line 156. At this point, 157, monoethylene glycol (MEG) may be added
as a desiccant to remove any residual water and prevent the formation of
hydrates as the gases from the first reflux drum are cooled. The next step is
to pass the first reflux fluids through a cross heat exchanger at 158, whereby
the fluid stream is cooled through heat exchange contact with the recycle line
160 and the recycle line 160 is warmed by the first reflux fluids coming off
the
reflux drum through line 156. MEG may be added as a desiccant to remove
residual water and prevent the formation of hydrates as the gases from the
cross heat exchanger 158 are cooled.
Next the remaining top vapours are passed through a trim condenser
or overhead chiller 170 before entering a recycle drum 172. The temperature
in the recycle drum in the example of the working fluid or solvent being
propane, may be in the range of about -25 to 10 degrees C and may be for
example about 0 degrees C or even colder. A drain is provided from the
recycle drum 172 at 174 to drain off the MEG desiccant. Then it can be passed
through a regenerator 178 and reused by being re-injected into the process
CA 3073464 2020-02-21
either before or after the cross exchanger at 157, 180 or both. The MEG may
be injected by means of an injection spray nozzle to atomize the MEG (for
good mixing with the gas) and may be sprayed onto the inlet tube sheet of the
cross exchanger and chiller. The MEG may be a liquid at the temperature of
the recycle drum. The MEG may remain in the liquid phase during the chilling
process. As the water is condensed from the vapour stream during the chilling
process, it may be absorbed by the MEG. The MEG and water may form a
homogeneous liquid phase that is heavier than the hydrocarbon liquid solvent
to facilitate separation. The combination therefore may form a second liquid
phase that settles out below the hydrocarbon phase by gravity. The MEG and
water may then be withdrawn from a pipe 174 located at the bottom of the
second reflux vessel where the MEG and water may be collected. Since the
volume of MEG & water may be relatively small in comparison to the
hydrocarbon liquid phase, an enlarged piece of pipe may be used at the bottom
of this vessel to collect the MEG and water phase. An interface measurement
device may be used to detect the hydrocarbon and MEG and water interface
which accordingly allows the MEG and water to be preferentially removed.
As will be understood by those skilled in the art the MEG that is first
injected into the exchangers may be referred to as a lean MEG and may be
typically made of 80% MEG and 20% water. When the lean MEG comes in
contact with the gas stream that is being cooled, it absorbs more water that
is
being condensed and may therefore contain more water. This may form a rich
MEG stream which for example may now be 70% MEG and 30% water. The
rich MEG must now be regenerated (or re-concentrated) back to the lean MEG
conditions, so the water that was absorbed during chilling phase is to be
removed. This may be done by a unit referred to as a MEG Regeneration unit
178, which for example may heat up the MEG solution to boil the desired
amount of water off. As will be understood by those skilled in the art the
exact
proportions of water in the lean and rich MEG streams will vary from
separation
16
CA 3073464 2020-02-21
to separation and that the foregoing percentages are by way of example only.
It can now be understood that the MEG is used to absorb any water that
might be present in the remaining top gases from the reflux drum 146 to
prevent such water from freezing or forming hydrates during the following
cooling stages and thus impairing the free flow of fluids through the system
110.
It will also be appreciated by those skilled in the art that the MEG
injection is just one way to inhibit the freezing of water in this gas stream
or
the formation of hydrates. Other methods include TEG Dehydration, molecular
sieve dehydration, methanol injection, and the like which are also
comprehended by the present invention, however MEG is preferred as it is
believed to be more economic.
The non-condensing fractions from the recycle drum 172 are removed
at 182 and may be used as fuel. These gases will primarily consist of
nonworking fluids such as methane and ethane when for example the working
solvent is propane. The recycle fluids can then be drained at 184, passed
through recycle pump 186 and through line 160 to cross-heat exchanger 158.
Eventually, line 160 joins mixed source feed line 114 and is thus sent back
into
the demethanizer feed drum 132 for recycling. In this way the present
invention
is able to capture significantly more of the in situ working fluid at higher
separation temperatures thereby reducing the operating cost of the in situ
stimulation due to refrigeration requirements and by avoiding losing working
fluid from the gases being sent to be used a fuel or the like. For example,
when the working fluid is propane, the present invention may recover at least
90% of the solvent and up to approximately 97.5% of the propane produced in
the production fluids. In the case where the working fluid is butane the
system
and process may be able to recover at least 90% and up to approximately 99%
of the butane working fluid for recycling back into the formation. In general,
the present invention seeks to recover at least 90% of the solvent, preferably
17
CA 3073464 2020-02-21
at least 95% and most preferably over 97% of the solvent working fluid. As
noted before, the higher the solvent recovery the lower the demand for make-
up solvent.
As previously discussed the recycle fluids are passed through a cross
exchanger 158, to cool the incoming liquids and thus recover some of the
energy used to cool them in the recycle drum 172.
By way of example consider a 30,000 BPD (barrels per day) nsolv
facility in which propane is being used as a solvent or working fluid. In a
prior
art process configuration, namely without the secondary reflux system of the
present invention, approximately 5,000hp of propane refrigeration is required.
However with a two stage reflux system as illustrated in Figure 2 and
described
above, the propane refrigeration requirement may be reduced to about 300hp
while at the same time achieving potentially about 10% more solvent recovery.
In this sense the present invention may be considered to be more efficient
than
the prior art one stage separation processes. Further the prior art one stage
separation process would require so much additional energy, to achieve the
very low temperatures required to achieve the further degree of separation
that
the energy cost of any such further cooling would make a comparable degree
of separation uneconomic.
The present invention comprehends that a more (relative to a one stage
separation) energy efficient and more complete separation may be achieved
at 8 degrees C, however, lower temperatures are also comprehended such as
0 degrees C and even colder. A temperature range of between 10 degrees C
and minus 25 degrees C is comprehended for the second condensation stage,
and the choice of temperature may be determined for example by a cost
calculation having regard as well to the choice of working fluid. As will be
understood by those skilled in the art the exact system configuration may be
based on an estimate of the additional cost of the cooling energy required and
the capital cost of the cooling equipment as compared to the value of the
extra
18
CA 3073464 2020-02-21
solvent recovered. Of course the most preferred temperature will depend on
the solvent or working fluid chosen. However, the present invention of a multi-
stage separation system and method may provide a more complete separation
at a higher temperature than the prior art one stage reflux systems.
By way of further non-limiting example, for a propane based extraction
as noted above the approximately 97.5% of propane solvent may be recovered
in the recycle liquid stream leaving the second reflux drum 160 from the first
stage reflux drum vapour stream 156. For the butane based extraction it may
be even higher at over 99%. Again, in the case where the working fluid or
solvent is propane, the main components exiting the deethanizer may be
methane (about 2.5%), ethane (about 14.8%), propane (about 81.5%) & other
(about 1.2%); the main components exiting the first reflux drum vapour line
may be methane (about 9.9%), ethane (about 25.2%), propane (about 62.3%)
& other (about 2.6%); as the gas is cooled the same components exist, but the
gas stream becomes lighter as the heavier components condense out first.
The pressure in the first reflux drum may be, for example, about 1950 kPa(g).)
The present invention comprehends that the form of the first cooling
means could be air cooling, which is economic. Other suitable means could be
water cooling or even high level refrigeration cooling, although both of these
alternatives would likely be less economic than the preferred air cooling.
In this example the fluid components entering the cooling system prior
to the second reflux drum may be methane (about 9.9%), ethane (about
25.2%), propane (about 62.3%) & other (about 2.6%); the main components
of the liquid stream leaving the second reflux drum being sent back to the
deethanizer may be methane (about 7.2%), ethane (about 24.9%), propane
(about 65.8%) & other (about 2.1%); the main components of gas stream
leaving second reflux drum and being sent to fuel may be methane (about
43.9%), ethane (about 28.8%), propane (about 22.2%) & other(about 5.1%).
Again, as the gas stream is cooled, it becomes progressively lighter; the
19
CA 3073464 2020-02-21
proportion of propane in the final gas stream being sent to fuel may therefore
be lower ¨ the flow rate of this stream may also be quite low so the mass of
propane being sent to fuel is reduced or minimal. The pressure in the second
reflux drum may be about 1881 kPa(g).)
The preferred external refrigeration system which may be utilized for
the second reflux drum cooling application may be propane refrigeration,
which may include a propane compressor, air cooled condenser and
accumulator vessel to cool and recirculate the propane refrigerant.
Where the working solvent is also propane, the make-up propane
solvent may be used to supplement or be added in place of an external
refrigeration system. The make-up propane may be added as the underground
extraction chamber grows. The make-up propane may be stored as a
pressurized liquid (for example 875 kPa at 20 C for 100% propane) and may
be also passed through the deethanizer to ensure its purity before being
injected. By first passing the make-up propane through the cross-exchanger
170, vapours entering the secondary reflux drum can be cooled to 0 C or even
colder. Since propane has a high vapour pressure, as the pressure decreases,
the make-up propane vapourizes, absorbing heat from the vapours entering
the secondary reflux drum in the cross-exchanger. After the cross-exchanger,
the heated make-up propane may be sent to the solvent compressor 20, so
that it can enter the deethanizer system with the flashed solvent vapour
stream
at 22 for purification. By using the make-up propane solvent in the overhead
chiller, the load on the external refrigeration system is reduced and may even
be eliminated, leading to equipment cost savings for the surface facility.
The present invention also comprehends other commercially available
refrigeration systems could be used to achieve the same result provided that
an external means for cooling the vapours is provided. In this regard, and
again by way of example only, the vapour from the first reflux drum may be
cooled from about 38 C to about 27 C; and the recycle liquid stream may be
CA 3073464 2020-02-21
heated from about 0 C to about 33 C.
It will be further understood that the present invention can be used for
solvents, or working fluids, other than propane, which need to be recovered
from a mixed produced fluid stream, such as butane, in which case the
operating conditions (temperature and pressure) would be adjusted
accordingly and would differ from the values identified in the above non-
limiting
examples.
The two step process of the present invention may therefore provide for
an energy efficient way of recovering the large volumes of recirculating
solvent
from the fluids produced from the reservoir by an in situ extraction
process. The deethanizer 110 can be operated at a fairly high temperature,
as compared to the prior art, but one that may be low enough to cause the
solvent to condense. Then the top gases can be further cooled in the reflux
drum, for example for propane to about 38 C to cause a further condensation
to occur. The last stage, again for propane, includes a further temperature
reduction, for example down to 0 degrees, but is only required for the
remaining top gases from the reflux drum which are relatively small in volume
as compared to the total working fluid throughput through the deethanizer. As
a result, according to the present invention, the maximum cooling is only
applied to the smallest volume portion of the fluids separation. Further by
means of the recycle loop with the cross current heat exchange as shown, the
energy that was used to cool the fluids may be recovered to a certain extent.
In a further embodiment the second reflux stage may be replaced with
an absorption column packed with a molecular sieve or the like, or the
absorption column may be added onto the end of the two stage separation
according to the present invention to recover any even greater percentage of
the working fluid as outlined above. In either case a column having
hydrocarbon absorbing materials may be used to preferentially absorb certain
hydrocarbon species and will be selected according to the working fluid being
21
CA 3073464 2020-02-21
used in the separation process. Just before or at the point where the
absorbent has been fully charged, i.e. it cannot absorb any more working
fluid,
then it is necessary to isolate the unit from the process flow and remove the
absorbed hydrocarbons, such as for example by heating the absorbent
materials. Since this may involve a process interruption the present invention
comprehends that two absorption units may be used in a side by side manner
so one can be absorbing while the other is being heated and cleansed of any
absorbed hydrocarbon species. Alternatively, a selective membrane module
system may be used to separate remnants of the working fluid from one or
more impurity species such as carbon dioxide. Thus, the present invention
contemplates that the second separation step may not require a second reflux
drum with cooling as described above. Thus in broad scope the present
invention comprehends a solvent recovery which includes a two or more step
separation process, where each subsequent step is applied to a smaller
amount of fluid and can be done more efficiently than trying to achieve a one
step separation on a much large volume of fluid.
Figure 3 shows an energy efficient solvent recovery unit (SRU) applied
to the bitumen stream 26 according to a further aspect of the present
invention.
The unit consists of a multistage distillation column 210, reflux drum 212,
reboiler 214 and condenser 216 to further purify the separated heavy fluids or
product oil 26 while recovering solvent that has been entrained. As will be
understood by those skilled in the art, the distillation column may have
stages
established by internal trays or packing. The top stage vapour outlet 218 of
the
distillation column 210 is condensed at 216 and sent through the reflux drum
212. The vapour from the reflux drum 212 is non-condensable gas 220 which
may be used as a fuel gas in the plant, for example. The liquid 221 from the
reflux drum 212 is recovered solvent, which is preheated in a solvent
preheater
224, which may be a cross heat exchanger, and vapourized at 226 for solvent
injection 30. A portion 228 of the bottoms liquid of the distillation column
210
22
CA 3073464 2020-02-21
is sent to the reboiler 214, which heats the distillation column by
recirculating
vapourized bottoms liquid. The remaining bottoms liquid is purified product
oil
which is sent to a cross heat exchanger or separated oil preheater 230 to
preheat the incoming bitumen 26, while pre-cooling the purified product oil to
a first temperature. The purified product oil may then be cooled to a second
temperature, which is lower than the first temperature in the solvent
preheater
224. A final adjustment of the purified product oil temperature may be done in
a product oil cooler 230 for tighter control of the vapour pressure and
solvent
content in the final sales oil. As previously stated, depending upon the
nature
of the working fluid, the present invention comprehends that the separation
facility may include a number of separation stages, at appropriate interstage
fluid pressure and/or temperature, it may employ parallel streams in some or
all of the stages, and may adjust the number of cross heat exchangers to
maximize the energy efficiency and cost efficiency of the process as shown in
Figure 3.
By way of example, consider a 10,000 BPD oil nsolv facility in which
butane is being used as a solvent or working fluid. In a prior art process
configuration, three stages of parallel flash vessels are used for light/heavy
separation, without the solvent recovery unit of the present invention. The
total
energy requirement of the system is about 30 MW, accounting for both heating
duty and electrical power. To achieve the same total solvent recovery
according to the present invention, by using the solvent recovery unit in
place
of the two downstream flash vessels, the total energy requirement is also
about
MW, however it requires about 30% less electrical power than the prior art
25 configuration. This results in overall energy cost savings since
electrical power
is more expensive than heating duty, which may be provided by waste heat or
fuel recovered at the plant..
Figure 4 shows a mixed fluid separation system, according to a further
aspect of the present invention, which minimizes the hydrocarbon loss to the
23
CA 3073464 2020-02-21
produced water 13 of the circuit described by Figure 1. For ease of
understanding the free water knock out vessel (FWKO) 12 is shown together
with the mixed water stream 13 and the light heavy hydrocarbon stream 15.
The FWKO may include a water/hydrocarbon interface level control 202, using
a nucleonic based density profiler along the separation zone of the FWKO,
which may also have the appropriate vessel baffles and siphon breaking with
piping 294 and flow control valve 296 to discourage hydrocarbon entrainment
in the water outlet stream, as will be understood by those skilled in the art.
The
mixed water 13 may be collected in a skim tank 300, which is sized and shaped
to permit the mixed water to separate including providing for a sufficient
height
and sufficient residence time for separation of the high density phase from
the
low density phase for the design flow rate. The high density phase is
separated
water, which may be sent to disposal or further treatment as needed. The low
density phase contains recovered floatable hydrocarbons 306, which are
skimmed off the top of the skim tank and may be sent to a slop oil tank 308.
From the slop oil tank 308, at least a portion of the recovered hydrocarbons
are sent back to the FWKO at 310. The present invention comprehends the
operator may select the skim tank size and hydrocarbon recycle rates 310 to
maximize separation efficiency.
In one embodiment of the present invention, the skim tank 300 may be
replaced by one or more hydrocyclones. As will now be understood this
requires the fluids in the FWKO to have enough residence time to be able to
provide a steady feed rate for the hydrocyclones.
While the foregoing description describes preferred embodiments of the
invention it will be understood that variations are comprehended by the
present
invention. Some of these variations have been discussed above and others
will be apparent to those skilled in the art as falling within the broad scope
of
the claims appended hereto. For example, various working fluids can be
recovered according to the present invention, and may require different
24
CA 3073464 2020-02-21
temperatures and different concentrations than provided in the non-limiting
example above, depending upon the nature of the recovered fluids.
CA 3073464 2020-02-21
