Note: Descriptions are shown in the official language in which they were submitted.
:~2~34~5
FIELD OF THE INVENTION
2 This invention relates to a process for purifying
3 bitumen froth, to thereby obtain a diluted bitumen stream of good
4 enough quality to be fed to a downstream upgrading facility. By
'purifying' is meant that water and solids present in the froth
6 are separated from the bitumen.
7 BACKGROUND OF THE INVENTION
8 The oil sands of the Fort McMurray region of Alberta
9 are presently being exploited by two large commercial operations.
The process practised in these operations involves four broad
11 steps, namely:
12 - mining the oil sand;
13 - extracting the bitumen from the mined oil sand
14 using a process known as the 'hot water process',
to produce bitumen in the form of a froth
16 contaminated with water and solids;
17 - purifying the froth to separate the water and
18 solids from the bitumen; and
19 - upgrading the purified bitumen in a coking
facility to produce products which are suitable
21 for a conventional refinery.
22 The present invention has to do with the purifying
23 step. However, in order to understand the problems solved by
24 the invention, it is first necessary to review the steps of the
hot water process and the conventional froth purification
26 process.
27 As a beginning point, it needs to be understood that
28 oil sands comprises relatively lar~e quartz sand grains,
29 each grain being encapsulated in a thin sheath of connate
0 water. The water contains minute clay particles ~referred to
~ g
'~
~Z~39~65
1 as ~fines~). The bitumen is positioned in the interstices
2 between the water-sheathed grains of sand.
3 In the first step of the hot water process, the
4 mined oil sand is mi~ed in a rotating horizontal cylindrical
drum (or 'tumbler') with hot water t80~C) and a small amount
6 of NaOH freferred to as 'process aid~). Steam is sparged
7 into the slurry at intervals along the lengt~ of the drum, to
8 ensure that the exit temperature of the resultant slurry is
g a~out ~0~C.
The drum is slightly inclined along its length, so
11 that the mixture moves steadily therethrough. The retention
12 time is about 4 minutes.
13 This tumbling step is referred to as
14 'conditioning'. It involves heating of the bitumen and
displacement, by water addition, of the bitumen away from the
16 sand grains. Many of the released bitumen globules become
17 aerated by forming films around air bubbles entrained in the
18 tumbler slurry. Conditioning also involves reaction between
the ~aOH and bitumen to produce surfactants which facilitate
the bitumen-release and subsequent flotation/settling steps.
21 On leaving the tumbler, the conditioned slurry is
22 screened, to remove oversize rocks and lumps, and diluted
23 ~ith additional hot water. The resulting water/bitumen ratio
24 is about 6:1.
The diluted slurry is then introduced into a large
26 thickener~like vessel having a cylindrical upper portion and
27 a conical lower portion. The vessel is referred to as the
28 'primary separation vessel' or 'PSV'. Here the diluted
29 slurry is retained for about 45 minutes under quiescent
conditions. Under the influence of gravity, the sand grains
31 sink, are concentrated in the conical portion and are
~3~65
1 discharged as 'primary tailingsl through a valve and line
2 connected to the lower apex of the vessel. The bitumen
3 globules, rendered buoyant by air attachment, rise to the
4 surface of the PSV and form a froth. This froth is called
'primary froth' and typically comprises~
6 66.4~ by wt. bitumen
7 24.7% by wt. water
8 8.9% by wt. solids
9 The primary froth is skimmed off and recovered in a launder.
In between the layer of sand tailings in the base of the
11 vessel and the layer of froth at the top, there exists a
12 watery slurry referred to as 'middlings'. The middlings
13 contain fines and globules of bitumen which are
14 insufficiently buoyant to reach the froth layer.
A stream of middlings .is continuously withdrawn
16 from the PSV. These middlings are treated in a series of
17 sub-aerated flotation cells. In these cells, the middlings
18 are vigorously aerated and agitated, with the result that
19 contained bitumen is forced to float and form a dirty froth
20 referred to as 'secondary froth'. This secondary froth
21 typically comprises:
22 23.8% by wt. bitumen
23 58.7% by wt. water
24 l7.5% by wt. solids. ~,
To reduce the concentration of water and solids in
26 the secondary froth, it may be retained in a settling tank to
27 allow some of the contaminants to settle out. ~he 'cleaned~
28 secondary froth typically comprises:
29 4l.4~ by wt. bitumen
46.2~ by wt. water
3~ 12.4% by wt. solids.
~2934~5
1 The primary and secondary froths are then combined
2 to provide the product of the hot water extraction process.
3 The ~combined froth~ typically comprises:
4 57.3% by wt. bitumen
34.2% by wt. water
6 8.4% by wt. solids.
7 This stream is too contaminated to be used as feed
8 to the downstream upgrading circuit. This latter circuit
g requires a feed typically comprising:
99.0% by wt. bitumen
11 - % by wt. water
12 1.0% by wt. solids.
13 So the combined froth product requires purification
14 (or water and solids removal) before it can be fed to the
upgrading circuit. Heretofore, this purification has been
16 obtained by using what is referred to as 'two stage dilution
17 centrifuging~. This operation involves:
18 1. Diluting the combined froth with naphtha.
19 This is done to reduce hydrocarbon phase
viscosity and increase the density
21 difference between the hydrocar~on phase
22 (bitumen dissolved in naphtha) and the
23 water and solids phase (referred to jointly
24 as 'sludge');
2. Passing the diluted froth through a low-
26 speed scroll centrifuge, to remove the
27 coarse solids and some of the water as a
28 cake , which is discarded; and
.
93~L65
1 3. Passing the scroll centrifuge product
2 through a high-speed disc centrifuge to
3 remove fine solids and most of the balance
of the water. The disc centrifuge product
S typically analyzes at:
6 59.4% by wt. bitumen
7 37.5% by wt. naphtha
8 4.5% by wt. water
0.4% by wt. solids
1~ The naphtha diluent and any contained water iB then
11 distilled out of the disc centr.ifuge product to produae the
12 purified bitumen product for advancing to the upgrading
process.
The described dilution centrifugin~ process has
been used because it is capable of producing a bitumen
16 product of the desired quality. But it is an operation that
17 is exceedingly expensive to maintain and operate due to the
18 erosive nature of the feed and the rotatlng character of the
19 centrifuges. For example, in use, the flights of the scroll
centrifuges wear badly, even though they are formed of
21 ceramic, and the hrittle ceramic flights commonly break and
22 put the machine out of balance. In the case of the disc
23 centrifuges, their sludge discharge no2zles are subject to
24 rapid wear and the separation interface between product and
reject in the stack of discs can easily be 'lost', with the
26 result that a significant amount of bitumen is lost with the
27 tailings. In addition, a large number of the machines must
28 be used, with attendant consumption of very large amounts of
29 electrical energy.
~ILZ93~6~;
1 Thus, there has long been a need for a viable
2 alternative to the dilution centri~uging circuit for
3 purifying bitumen froth.
4 The present invention involves a circuit of
interconnected known devices, namely mixers and inclined
,,,
6 plate settlers ('IPS').
7 An inclined plate settler comprises a stack of
8 parallel, spaced apart, solid plates, inclined downwardly
g from the horizontal and mounted within a containing vessel.
lo Each space between a pair of plates forms a discrete settling
11 zone. The feed mixture to be separated is distributed into
12 the spaces, at a point between their longitudinal ends. The
13 light components of the mixture rise to the underside surface
14 of the upper plate. These light components then travel up
said underside surface and are collected and recovered at the
16 upper ends of the plates. The heavy components of the
7 mixture sink towards the uppermost surface of the lower plate
18 and follow it downwardly, to be collected and recovered at
19 the lower ends of the plates.
A mixer can take any of various forms - the present
21 work involved simply a cylindrical container having a
22 submerged driven impellor positioned therein.
23 SUMNARY OF THE INVENTION
24 The present invention is based on the following
experimen~ally determined observations:
26 - That bitumen froth is amenable to high quality
27 separation in a first IPS, but in that first
28 stage of separation only part of the bitumen
29 in the feed reports as overhead product;
a34~
1 - That the underflow from tAe first IPS, containing
2 a significant proportion of the bitumen in the
3 original feed, is not amenable to high quality
separation in a second IPS. It appears that the
first stage underflow contains stable emulsions
6 that will not readily resolve in tAe second IPS
7 or that much of the hydrocarbons that did not
8 report to the overflow in the first stage will
9 also not report to the overflow in the second
stage; and
11 - That if light hydroc~rbon diluent (e.g. naphtha)
12 is mixed with the first stage underflow, then
this mixture is amenable to good quality
~ 4 separation in the second IPS.
Having conceived and tried the underlying
16 experimental work that resulted in these observations,
applicants conceived a purification circuit for bitumen froth
18 that would incorporate the following features:
19 - the use of a plurality of serially connected
^ inclined plate settlers, with a subsequent
21 settler being fed the underflow from a
22 preceding settler;
23 - the addition of light hydrocarbon
24 diluent or solvent, ln a progressively
richer concentration, to the bitumen- -
26 containing stream moving through the
27 series of settlers, said bitumen-containing
28 stream becoming progressively leaner in
29 bitumen as it moves through the circuit; and
~93~6S
1 - the use of mixers before each settler to mix
2 the added diluent with the bitumen.
3 A circuit or line consisting of three pairs of alternating mixers
4 and settlers was tested. ~he overflow stream from the first
settler provided the only bitumen product stream produced from
6 the circuit. The bitumen/diluent overflow stream from the second
7 settler was recycled to the first mixer to be combined with the
8 froth feed. The low-bitumen/high-diluent overflow stream from
9 the third settler was recycled to the second mixer. Thus more
diluent was supplied to the relatively bitumen-lean underflow
11 stream being supplied to the second mixer. And finally, fresh
12 diluent was supplied to the third mixer to dissolve the small
13 amount of bitumen in the underflow stream of the second settler.
14 When applied to typical combined bitumen froth this
circuit demonstrated:
16 - that the bitumen product stream from the first
17 IPS was of the same order of purity as that
18 derived from a conventional dilution centrifuging
19 circuit; and
- that the recovery of bitumen by the test circuit
21 was of the same order as that obtained by
22 dilution centrifuging.
23 Stated otherwise, we have made the surprising discovery
24 that a process using three mixingjIPS separation steps in
series, combined with a counter flow of solvent, gives product
26 of as good quality as that obtained from the centrifuge process
27 (said qual.ity being referred to as "upgrading quality"),
28 together with comparable hydrocarbon recovery and a sludge
29 tailings that is substantially hydrocarbon-free. And the
1 components of the present circuit are without moving parts
2 (except for the pumps and impellors) and thus are
3 characterized by comparatively low maintenance costs.
4 DESCRIPTION OF T~lB DRAWINGS
Figure 1 is a block diagram showin~ the steps of
6 the process in accordance with the preferred embodiment; and
7 Figure 2 is a schematic showing the circuit of
8 processing components or units and their pipe
9 interconnections.
DESCRIPTION OF THE PREFERRED EMBODIMENT
11 The test work underlying the present invention was
12 carried out in 3-stage mixer/IPS circuit. The invention will
now be described with respect to that circuit, although it
4 could also be conducted in 2, 4 or even more stages.
15 ~ More particularly, combined bitumen froth was fed
16 to a circuit A comprising: a first mixer l; a first IPS 2; a
17 second mixer 3; a second IPS 4; a third mixer 5; a third IPS
18 6; and appropriate connecting lines.
19 The combined froth was introduced into and mixed in
the first mixer 1 ~ith a first recycled overhead stream from
21 the second IPS 4. This first recycled overhead stream was
22~ depleted in bitumen but enriched in naphtha, relative to the
23 combined froth f~ed.
24 ~he first mixer 1 comprised a cylindrical body la
having a flat bottom lb. An impellor lc was positioned to
26 stir the contents of the mixer.
~3~5
l The mixture from the first mixer 1 was fed to the
2 inlet of the first IPS 2. The first IPS 2 was simply a box
3 2a having an inlet 2b, an overhead outlet 2c, and an
4 underflow outlet 2d. The box contained a pair of inclined
spaced-apart plates 2e.
6 The dimensions of th2 mixer and IPS units used are
7 set forth in Table 1. The several mixers and IPS's in the
8 circuit were identical to the described units.
9 TABLE I
lo Length of IPS - 5
11 Spacing between plates - 1-1/2
12 Dimensions of plates - 5~ x l~
l3 Mixer vessel - 12" diameter
l4 12" to 16" of liquid
in the vessel during
l6 operation
17 Type of impellor - 6~ diameter marine
18 propeller
19 Impellor rpm - ~20 - 680
Separation of the bitumen, water, and solids,
21 present in the mixture fed from the first mixer 1, took place
22 in the first IPS 2. A first overhead product stream, which
23 was the only bitumen-rich product from the circuit, was
24 obtained. This stream was enriched in bitumen relative to
the original froth feed. (The compositions of these streams
26 are set forth in Table II below.)
27 The underflow stream from the first IPS 2 was fed
28 to the second mixer 3. ~ere it was mixed with a second
29 recycled stream from the third IPS 6. This second recycled
stream was very depleted in bitumen but relatively rich in
31 naphtha.
11
lZ~3~S
1 The mixture from the second mixer 3 was fed to the
2 inlet of the second IPS 4. Separation occurred therein and
3 overflow and underflow streams were produced. T,he overflow
4 stream was the stream recycled to~ t'he first mixer, as
previously stated.
6 The second underflow'stream, produced by the second
7 IPS 4, was fed to the third mixer 6. This second underflow
S stream was quite lean in bitumen - more particularly~ it was
9 depleted in bitumen relative to the first underflow stream.
10 In the third mixer 6, the second underflow stream
11 was mixed with fresh pure naphtha. The mixture was fed to the
12 inlet of the third IPS 6 and underwent separation therein.
13 The overflow stream from the third IPS 6 was recycled to the
14 second mixer 3, as previously stated. ~he underflow stream,
virtually free of bitumen, was discarded as tails.
16 The stream compositions and separation results are
17 set forth in Table II.
~293~6~;
TABLE II
2 COMPOSITION (% BY WT. ~
3 STREAN BITUMEN~ATER SOLIr)5 NAPHTHA RATE
4 k~
Combined froth
6 feed 57.334.2 8.5 - - 1.96
7 First recycled
8 ovexflow (from
9 2nd. IPS) 19.714.1 1.9 63.4 1.59
Overflow product
11 (from 1st IPS) 55.7a~.7 0.7 39.0 2.02
12 1st IPS
13 underflow 20.752.7 12.1 14.5 1.52
Second recycled
l~ overflow (from
16 3rd IPSJ 2.8053.8 8.3 35.1 3.01
17 2nd IPS
18 underflow 2.913.7 74.6 9.3 2.95
19 Fresh diluent 99.5 0.81
3rd IPS
21 underf7Ow 0.2077.320.3 2.4 0.75
'~
13
1 SUPPLEMENTARY DISCLOSURE
2 This supplementary disclosure relates to a
3 variation of the circuit described in the principal
4 disclosure.
It can be advantageous to operate the separation
6 process at elevated temperature because the viscosity of the
7 hydrocarbon is thereby reduced. ~his allows the solid
8 particles to settle more rapidly. In addition, at hiyher
9 temperature the water droplets coalesce more readily, which
facilitates their separation from the hydrocarbon. A high
11 purity product is thereby produced at lower residence time,
12 with the consequence that the capacity of the equipment is,
13 in effect, increased.
14 At such higher temperatures, fractions of the
diluent can approach or exceed their atmospheric boiling
16 point. To prevent flashing of the diluent, and to contain
the pressures generated, it is necessary to surround the
18 functioning units of the equipment with pressure-retaining
lg houslngs.
This may be effected in conventional fashion by
21 closing in the components of the circuit, as indicated
22 diagrammatically in Figure 3, and operating the process at
elevated temperature and pressure.
~;:
~:
:`
.
; - 13~ -
