Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to a method for treating
bitumen froth moving from a hot water process extraction plant
to a battery of centrifugal separators.
One of the world's largest reservoirs of hydro-
carbons is the Athabasca tar sand deposit in Northern Alberta.
The oil or bitumen from this deposit is presently being
extracted using the known hot water process.
In general terms, this process involves mixing
tar sand with water and steam in a rotating tumbler to
initially separate the bitumen from the water and solids of the
tar sand and to produce a slurry. The slurry is diluted
with additional water as it leaves the tumbler and is introduced
into a cylindrical primary settler vessel having a conical
bottom. The coarse portion of the solids settles out in this
vessel and is removed as an underflow and discarded. Most of
the bitumen and minor amounts of solids and water rise to the
surface of the vessel contents to form a froth. This froth
overflows the vessel wall and is received in a launder extending
around its rim. The froth is termed primary froth. A middlings
stream, comprising water, fine solids (-325 mesh), and a minor
amount of buoyant and non-buoyant bitumen, is withdrawn from
the mid-section of the vessel and is pumped to a sub-aeration
flotation cell. Here the middlings are relatively violently
agitated and aerated. The middlings bitumen becomes attached
to the air bubbles and rises through the cell contents to
form a froth. This froth, termed secondary froth, is
recovered in a launder and settled to reduce its water and
solids CQntent. The primary froth and settled secondary froth
are combined and preferably deaerated in a column with steam
to provide the feed stock for this invention. Typically the
feed stock froth comprises 62% bitumen, 29% water and 9%
solids. The temperature of froth after deaeration is typically
185F.
1, ~
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Following deaeration, the froth is pumped
through a feed conduit to a two-stage centrifugal separation
circuit. A hydrocarbon diluent is injected into the feed
conduit to mix with the froth. The diluent, usually naphtha,
is added to reduce the viscosity and specific gravity of the
froth bitumen phase to render it amenable to centrifugal
separation. In the first stage of the circuit, the diluted
froth is treated in one of a battery of scroll-type separators.
These separators remove most of the coarse solids from the
froth. The product is then passed through one of a battery
of disc-type separators to remove the remaining fine solids
and the water and produce a relatively clean diluted bitumen
stream.
In accordance with the prior art, the froth
has been pumped to the scroll-type separators through a feed
conduit terminating in a loop. The loop feeds a series of
separators individually connected thereto. A recycle pump
is included in the loop. This pump keeps the diluted froth
moving through the loop and prevents suspended solids from
settling out and plugging the line. However the pump induces
increased emulsification of the diluted froth and thereby
reduces the efficiency of the centrifugal separation process.
Another problem which affects the operation of
the scroll-type centrifuge battery has to do with surges in
the feed rate of the diluted froth fed to the individual
separators. If one of the scroll-type separators goes down,
there is a surge in feed rate to the remaining operating sep-
arators. This increase in feed rate is accompanied by an
increase in torque experienced by the machines. Frequently
a shear pin, protecting a separator's drive system against
excessive torque, will part, thereby rendering the machine
inoperative. If the change in feed rate conditions is not
sensed and corrected quickly, the entire battery of separators
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may be shut down.
Another problem arises from the operation of
the bitumen extraction plant. It is subject to variations in
tar sand feed composition and flow rate. For example, the
bitumen content of the mined tar sand can vary between 8~ and
13% by weight. In addition, the mine itself can be subject
to frequent shut-downs due to mechanical problems. These
sudden changes are reflected in changes in froth composition
and feed rates; these latter changes deleteriously affect the
operation of the dilution centrifuging circuit. This is
particularly the case with the disc-type separators, which
require steady feed conditions for best operation.
Still another problem associated with a dilution
centrifuging circuit arises from the lack of instrumentation,
applicable in a practical sense, available for accurately and
quickly determining the flow rate of bitumen through the feed
conduit. Because the bitumen content of the froth cannot
presently be easily determined, due to the presence of water
and solids, it is difficult to ensure a substantially constant
and desirable diluent/bitumen ratio in the diluted froth. More
particularly, it is found that a low diluent/bitumen ratio
tends to make operation of both sets of centrifuges difficult -
that is, the machines require a good deal of operator attention
in this circumstance. Further, a low diluent/bitumen ratio
reduces the separation efficiency of the machines. Due to these
problems, there is a tendency on the part of the operators to
seek to err on the side of too high a diluent/bitumen ratio,
thereby wasting diluent. In addition, a too high diluent/bitumen
ratio reduces the efficiency of the centrifugal separators.
There is therefore required an accurate and practical system
for monitoring the ratio so that it can be accurately maintained
at the desired level.
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With the foregoing in mind, it is the principal
object of this invention to provide a method whereby the
stream of bitumen froth delivered to the separators varies
only slowly in composition.
It is a preferred object to provide a method
whereby the stream of bitumen froth is d~livered to the
separators at a generally constant flow rate.
It is another preferred object to provide
a method whereby the diluent/bitumen ratio of the diluted
froth is effectively monitored, so that the addition of diluent
may be controlled to ensure that the ratio is held substantially
constant.
It is another preferred object to provide a
method whereby a stream of diluted bitumen froth is supplied
to the separators, said stream having a composition which
varies only slowly and has a generally constant diluent/bitumen
ratio, said stream being supplied at a generally constant rate
and pressure.
In accordance with the broadest aspect of the
invention, at least the major portion of the bitumen froth
is retained and mixed in a surge tank for at least an hour.
It has been found that, when this is done, neither segregation
or coalescence of the froth components occurs to a seriously
deleterious degree, as would have been expected. As a result
of retaining and mixing the froth, variations in froth composi-
tion and flow rate are smoothed out, so that the separators
receive a stream whose composition and flow rate change only
slowly. As a result, the operation of the separators is
improved. Our work indicates that the degree of segregation
and coalescence, which occurs when the froth is retained and
mixed, diminishes if the temperature of the froth is higher
than about 150F. We prefer to use a froth having a temperature
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of about 185F.
In a preferred feature of the invention, the
viscosity of the diluted bitumen froth is measured in a novel
manner. It has been found that these measurements are indicative
of the relative viscosity of the continuous hydrocarbon phase
in the froth. Surprisi~gly, the presence of varying amounts of
solids and water in the froth does not seriously affect this
relationship. The addition of diluent to the undiluted froth
is controlled in response to these measurements. As a result,
the diluent/bitumen ratio in the diluted froth can be maintained
substantially constant at a desirable value.
In another preferred feature of the invention,
the feed conduit terminates in a distributor vessel, which is
connected by an upwardly extending connector pipe with each of
the scroll-type separators. In this manner, the need for a
circulating pump is eliminated. The pressure within the
vessel is monitored and the speed of the surge tank pump
varied in response thereto to further improve the control
over surging at the separators.
Broadly stated, the invention is an improvement
on the process wherein bitumen froth, comprising bitumen,
water and solids, is transferred through a feed conduit from
a hot water process extraction plant to a battery of centrifugal
separators and diluted with hydrocarbon diluent as it moves
between the plant and the battery. The improvement comprises
temporarily retaining at least the major portion of the
undiluted froth being transferred in a surge tank for at least
one hour and mixing the retained froth with incoming froth
without significant segregation or coalescence of froth components
within the tank, thereby producing a froth product stream whose
composition and flow rate change only slowly.
In the drawing:
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Figure 1 is a schematic diagram illustrating
the system; and
Figure 2a is a plot of the hydrocarbon phase
viscosity of diluted froth as a function of the naphtha-bitumen
ratio;
Figure 2b is a plot of tubing viscometer readings
taken on diluted froth as a function of naphtha-bitumen ratio.
With reference to Figure 1, deaerated bitumen
is continuously delivered from an extraction plant or source 1
to a froth surge tank 2. The surge tank 2 is preferably
only partially filled and is of sufficient capacity so that
the retention time therein is in the order of at least 1 hour,
preferably 2 - 3 hours. A plurality of side entry mixers 3
extend into the tank 2 for turning its contents over several
times while the froth passes therethrough.
On leaving the surge tank 2, the froth is pumped
A throu,gh the feed conduit 4 by a variable speed feed pump ~
Naphtha or like diluent is introduced into the
froth feed conduit 4 before it reaches the scroll centrifuge
battery 6. More particularly, naphtha is fed by a centrifugal
pump 7 from a storage tank 8 through a conduit 9 into feed
conduit 4.
The flow of naphtha through the conduit 9 is
controlled by a valve 10 on the discharge of a naphtha heater.
The rate of naphtha flow is measured by an orifice meter 12
ahead of the heater. The flow of combined naphtha and froth
(i.e. diluted froth) through the feed conduit 4 is also
measured with an orifice meter 13 downstream of the froth-
naphtha junction 20. The flow of naphtha is regulated as a
30 pre-set ratio of the diluted froth flow by a flow ratio controller
14, connected to the meters 12, 13. The flow ratio controller
14 operates to control the valve 10.
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A viscometer 15 operates to provide a viscosity controller
14a with a signal indicative of the viscosity of the diluted
froth. The viscosity signal is utilized by the viscosity
controller 14a to re-set the naphtha/froth ratio setpoint on
the flow ratio controller 14 in order to maintain the diluted
froth at a more or less constant viscosity.
The viscometer 15 comprises a positive displacement
pump 16 which withdraws a sample at a constant rate from the
diluted froth conduit 4 and pumps it through a conduit loop
17 and back into the conduit 4. A differential pressure cell
18 measures the pressure drop across the loop 17 and transmits
the required proportional signal to the viscosity controller 14a.
A suitable viscometer is obtained by using a 3L2 Moyno pump and
a 25 foot long loop of 3/8 inch inside diameter tubing.
It has been found that variations in the viscosity
of the diluted froth arise largely from variations in the
bitumen content of the froth. So little variation in the
froth viscosity arises from changes in the solids and water
contents thereof that they can be ignored. The froth viscosity
measurement can therefore be used as an indicator of the
relative viscosity of the hydrocarbon phase in the diluted froth.
The diluted froth conduit 4 terminates in a
distributor vessel 19. This vessel 19 has upwardly inclined
connector lines 20, each leading to a scroll centrifuge 21.
A pressure sensor 22 monitors the pressure within the distributor
vessel 19 and transmits a signal proportional thereto to a
controller 23 which regulates the speed of the froth feed pump
5 to reduce pressure surges at the vessel 19.
The present system is characterized by several
advantages. By providing a large froth surge tank and blending
its contents, wide fluctuations in the froth composition and
flow rate are evened out so that changes occur only gradually.
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This improves the operation of the separator circuit. In
addition, as a result of providing surge capacity, it is
possible to use a fast-acting pressure responsive system to
control the froth feed pump and minimize surge conditions at
the scroll separators. By monitoring the viscosity of the
diluted froth, it is possible to maintain the diluent/bitumen
ratio generally constant at a pre-determined value. This
improves the efficiency of the separators and conserves diluent.
Finally, by using a distributor vessel, emulsification of the
dilu~ed froth is reduced. Not all of these advantages need be
incorporated in a system. As stated, the broadest aspect of
the invention is the concept of retaining and mixing the
undiluted froth. However it is preferable to incorporate the
other features of the invention as well.
Certain aspects of the invention are exemplified
by the following data:
Example I
The usefulness of agitating froth to improve its
homogeneity can be illustrated by comparing the standard deviation
of the water content (or solids content) of unagitated diluted
froth with the standard deviation of the water content (or solids
content) of agitated diluted froth. Typically, the mean o the
water and solids contents of diluted froth from average tar sand
as well as their standard deviations, both in the agitated and
unagitated states are as follows.
UNAGITATED FROTH AGITATED FROTH
Component Mean Std. Dev. Mean Std. Dev.
Water 26.27% 5.46% 27.79% 3.37%
Solids 7.56% 5.72% 6.06% 0.93%
As shown in the tables above, the expected solids content of
a random sample of unagitated diluted froth could vary from
1.84% to 13.28% while the expected solids contents of a random
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sample of agitated diluted froth could vary only from 5.13%
to 6.99%.
Example II
The viscometer data shown in Figures 2a and 2b
show that the tubing viscometer, when applied to bitumen froth,
does, in fact, give readings indicative of the hydrocarbon
phase viscosity. Figure 2a shows the hydrocarbon phase viscosity
of naphtha-bitumen samples (which were taken during tubing
viscometer tests) as a function of the naphtha/bitumen ratio.
The data illustrated in Figure 2b shows the actual tubing
viscometer readings as a function of naphtha/bitumen ratio.
It will be noted that the curves are similar in shape. Comparison
of the low water froth data with the high water froth data in
Figure 2b shows that the viscosity readings (between high and
low water froth) vary by a maximum of 10% while the water content
of the froth varied from 27% to 47%. Figure 2b also illustrates
that the tubing viscometer can be made more insensitive to the
froth water content by providing the viscometer with a long
residence time suction device (30 sec.) thereby allowing some
of the water to separate from the hydrocarbon before the
mixture enters the tubing downstream of the pump. By "suction
device" is meant an upwardly inclined (45) suction pipe
extending from the centre of the froth line to the centrifugal
pump supplying the viscometer loop. A 2 1/2"diameter suction
pipe was used to provide a "long residence time", thereby
allowing some solids and water to settle out of the sample and
slide back to the froth line. A 1/4" diameter suction pipe
was used to provide a "short suction time".
