Note: Descriptions are shown in the official language in which they were submitted.
1~;786~3
1 FIELD OF T~IE INVENTION
2 The invention pertains to a process involving treating diluted
3 bituminous froth, from the known hot water extraction process, by passing4 said diluted froth through an inclined plate settler to recover part of
the contained bitumen in a form pure enough to be fed directly to the
6 conventional downstream upgrading circuit.
7 BACKGROUND OF THE INVENTION
8 There are large surface deposits of tar sand in the Athabasca9 region of Alberta. These deposits are presently being produced by two
large commercial plants, one of which is owned by the assignee of this
11 application.
12 More particularly, these tar sand deposits are being mined
13 and the valuable contained component, bitumen, is extracted and recovered
14 by a process known as 'the hot water process'. The recovered bitumen is
then upgraded in a refinery-type facility to produce hydrocarbon products
16 hav;ng various commercial uses.
17 In this hot water process~ the as-mined tar sand is introduced
18 into a horizontal rotating drum, together with hot water and caustic.
19 The m;xture is retained in the drum for a short period while the ingredients
mix, the tar sand ;s heated, and the components of the tar sand are dispersed
21 in the water. The step is referred to as 'conditioning'. The slurry that22 emerges from the drum is diluted with additional hot water and screened
23 to remove rocks and oversize lumps of tar sand. The screened slurry is
2~ passed into a thickener-like vessel (referred to as the 'PSV', for 'primary
~3 separation vessel'). ~lere the slurry is retained for a period of time
~ under quiescent conditions. Bitumen globules, which have become aerated
27 in the conditioning step, rise to the surface of the PSV contents and fcrm
2~ a froth. This froth, called 'primary froth', is recovered. The primary
29 froth has a bitumen content of about 65% by weight, the balance being
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1 contaminants,in the form of water and solids. Most of the sand present in
2 the slurry drops downwardly into the conical lower end of the PSV and is
3 concentrated therein. The sand is removed through the bottom outlet.
4 This stream, referred to as 'primary tailings', is discarded. Some bitumen,
which has failed to ascend to the ~roth layer, and some solids remain in
6 the watery layer between the froth and the concentrated sand. This mixture
7 is referred to as 'middlings'. A stream of the middlings is continuously
8 withdrawn from the PSV and advanced to sub-aerated flotation cells. Here
9 the middlings are subjected to vigorous agitation and aeration. A froth
layer is produced by the cells as a result of this treatment - the froth
11 is referred to as Isecondary froth'. This secondary froth is 'dirtier'
12 than the primary froth - it has a relatively high content of water and
13 solids. The bitumen content is commonly only about 25%. The secondary froth
14 is passed into a tank and retained for a period of time, to allow some
of the solids and water to settle. The 'cleaned' secondary fro-th is de-
16 canted off and recombined with the primary froth to produce the 'combined
17 froth product'.
18 This combined froth produce is not acceptable yet For processing
19 in the upgrading circuit. The water and solids associated with the froth,
partially in an emulsified form, must be removed to produce a hydrocarbon
21 product which, after diluent removalJ is suitable for upgrading. Such a
22 product preferably is one containing at least 95% by weight hydrocarbon.
23 The cleaning of the combined froth product is conventionally
24 accomplished by a process referred to as 'dilution centrifuging'.
Dilution centrifuging involves first adding naphtha to the
26 combined froth product. This is done to give a less viscous hydrocarbon
27 phase and to increase the density difference between the hydrocarbon phase
28 and the water and solids phases. The resulting 'diluted froth' is now
29 amenable to treatment in centrifuges to e-ffect separation of ~he bitumen
from the water and solids.
6()
1 The commercially practised centrifuging process is carried
2 out in a two step operation. More particularly, the diluted froth is
3 first fed to a scroll-type centrifuge, which is adapted to remove the coarse
4 solids from the feedstock. The hydrocarban~rich product from the scroll
centrifuge is then passed to a disc-type centrifuge, to separate the
6 hydrocarbon from the remaining water and fine solids.
7 While the two stage centrifuging circuit has been used for
8 years in the two plants in operation, there are a good many problems
9 associated with it.
For example, one problem has to do with the fact that the
1l composition of the diluted froth can vary quite widely, as a result of
12 variations in the composition of the tar sand itself. The following
3 compositions are typical of froth compositions for 'high' and 'low'
4 quality froths that are produced in applicant's plant:
High quality Low quality
16 Hydrocarbon 80 55
17 Solids S 8
18 Water 15 37
19 Taking the case of the disc centrifuge, it has internal
components7 (such as discs and nozzles) which have operating parameters
21 (such as disc spacing and size of nozzles). Once these parameters are
22 set, they cannot be changed while the machine is in operation. So once
23 the parameter design is established for the machine, it has only a narrow
24 range of froth compositions that it can handleg at normal operating speed.
To provide disc centrifuge capacity adequate to cope with the
26 variable feed, it is therefore necessary to provide an excess of machines,
27 some of which are therefore standing idle much of the time.
28 Another significant problem characterizing the centrifuge
29 circuit is its very high maintenance cost. The wear on ~he machines,
gi~len the erosive nature of the mixture being processed~ is almost
31 prohibitive.
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1 The reason that the centrifuge circuit is in fact used is that
2 it leads to a final hydrocarbon product of the desired quality. Typically,
3 the disc centrifuge product comprises:
4 Hydrocarbon 95% by wt.
Solids 4.5%
6 Water 0.5%-
7 In summary then~ there has long been a need for a separating
8 means which could be inserted to supplement or partly substitute for the
9 centrifuges. The added separating means should be characterized by:
- the capacity to produce a hydrocarbon product suitable after
1l diluent removal for upgrading; and
12 - the capacity to improve the stream going on to the
13 centrifuge circuit to reduce its variations in
14 composition.
SUMMARY OF THE INVENTION
16 In accordance with the invention, diluted bituminous froth is
17 passed through an inclined plate settler, before it is treated in the
18 centrifuge circuit. The settler recovers a portion of the bitumen,
19 contained in the froth, in the form of a hydrocarbon stream which is
sufficiently pure to be acceptable, after diluent removal, as a produc-t
21 for feeding directly to the upgrading circuit.
22 The settler is controlled, to cope with variations in diluted
23 bituminous froth composition and feed rate, by varying the withdrawal
24 rate of underflow in response to the hydrocarbon content of the settler
overflow. More particularly, the underflow withdrawal rate is varied
26 to keep the overflow hydrocarbon content high enough that the overflow
27 is accpetable for feeding directly to the upgrading circuit. Stated
28 otherwise, the settler is operated at a cut intensity within the range of
29 product withdrawal rates that give product of the desired quality.
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1 The invention is characterized by a number of advantages:
2 (1) A machine fr~ee of moving parts and which works on the
3 principle of gravity separation has surprisinyly been
4 discovered to:yield a product comparable in q~ality
to that produced by machines operating with the mechanism
~ of powerful centrifugal separation;
7 (2) The settler yields a bitumen-containing underflow
8 product which surprisingly has only limited variations
9 in composition and which thus provides a much improved
feed for the centrifuge circuit;
11 (3) And part of the cleaning/separating circuit for the
12 diluted bituminous froth now takes the ~orm of a settler
3 which is substantially free of wear and maintenance
14 problems.
Broadly stated, the ;nvention is a process for treating diluted
16 bituminous froth from the hot water process, which comprises passing the
17 froth through an inclined plate settler to produce overflow and underflow
18 streams and varying the underflow withdrawal rate from the settler in
19 response to the hydrocarbon content of the overflow:from the settler to
maintain said overflow hydrocarbon content sufficiently high whereby it
21 may be fed directly to an upgrading circuit.
22 DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic flowsheet showing in operating
24 sequence a source of diluted bitumen froth, an inclined plate settler,
and a centrifuge circuiti
26 Figure 2 is a triangular plot showing the variation in
27 composition of diluted bitumen froth~ as experienced in applicant's
28 commercial plant; and
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1 Figure 3 is a plot showing ~ oil recovery versus product quality
2 for an inclined plate settler incorporated into a circuit in accordance
3 with the present invention.
4 DESCRIPTION OF THE P~EFERRED EMBODIMENT
A circuit in accordance with the invention is shown in
6 Figure 1. This circuit was used in developing the present invention.
7 The circuit comprised a conventional source 1 of diluted
8 bituminous froth. More particularly, the source 1 included a froth storage
9 tank 2 which received bituminous froth from a hot water process plant (not
shown). A source 3 of light hydrocarbon diluent, preferably naphtha,
11 was also provided. The froth from the storage tank 2 was fed via line
12 4 and pump 5 to a diluent froth mixer 6. Naphtha was also fed, via line
13 7, into the mixer 6. A stream of diluted bitumlnous froth issued from
14 mixer 6 and was fed via line 8 into an inclined plate settler 9. Over-
flow and underflow streams from the settler 9 issued through lines 10, 11
16 respectively. A pump 12 controlled the withdrawal rate through underflow
17 line 1l.
18 Typically, the diluted bituminous froth produced by the source 1
19 has a naphtha/bitumen ratio of about 0.6 to 0.8. The froth composition
commonly falls within the enclosed area shown in Figure 2.
21 The inclined plate settler 9 used by applicant in the course
22 of developing the present invention was a Model LGS 2500/45 unit available
23 from Axel Johnson Inc. of Montreal. The settler parameters were as
24 folloWS:
Dimensions: height 4.6 m
26 length 6.0 m
27 width 3.7 m
28 Type of plates: flat
29 Effective plate
surface area: 350 m2
31 Plate angle: 45G.
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1 The plates ~ere adjustable to a limited extent - their spacing
2 could be varied between 3.2 and 5.0 cms.
3 Applicant inserted the test settler 9 in its plant circuit
4 immediately before its conventional centrifuge circuit (not shown), fed
diluted froth to the settler, and passed the settler underflow on to the
6 plant~s centrifuge circuit.
7 In the course of testing the inclined plate settler 9 in this8 operation, applicants found that, i~ the settler was controlled by monitoring
9 the settler overhead stream hydrocarbon content and varying the settler
underflow withdrawal rate in response thereto, one could maintain the
1l purity of the overflow stream at a value in the order of about 95%. More
12 particularly, the underflow withdrawal rate was controlled by varying the3 speed of underflow pump 12~ in response to periodic composition analyses
4 of the overflow stream. Table I sets forth a comparison of the composition
of a typical settler overflow product with a typical disc centrifuge pro-
16 duct previously obtained with no settler in the line:
17 TABLE I
18 Component (wt%) Disc Centrifuge IPS
19 Hydrocarbon 94.7 95.3
Solids 0.8 0.8
21 Water 4.5 3.9
22 When the settler 9 was operated to produce an overhead product
23 with a purity in accordance with Table I, it w~s found that about 75%
24 to 85% of the hydrocarbon contained in the diluted froth could be recovered
as overhead product.
26 In summary then, about 85% of the contained hydrocarbon in the
27 diluted froth can be recovered as settler product having a purity in the
28 order of about 95% hydrocarbon. This is supported by the data displayed
29 in Figure 3.
7~
I
1 It was also shown, when the settler 9 was tested, that the
2 underflow product had a relatively constant composition. This provided
3 a feed for the centrifuge circuit which was much easier to cope with than
4 diluted froth. This is supported by the following typical data obtained
during testing of the settler in the circuit:
6 _un A
7 feed 74% hydrocarbon
8 20% water
9 6% solids
hydrocarbon re-
11 covered from
12 settler 85%
13 ratio of hydro-
14 carbon/solids +
water in the under-
16 flow 0.50
17 Run B
18 feed 66% hydrocarbon
19 28% water
6% solids
21 hydrocarbon re
22 covered from
23 settler 85%
24 ratio of hydro-
carbon/solids +
26 water in the under-
27 flow ~
g
~l2~:i7~360
SUPPL_N NTARY DISCLOSURE
It can be advantageous to operate the separation
process with the feed froth at an elevated temperature. By
operating at a higher temperature, the viscosity o-f the
hydrocarbon is reduced. This allows the solid particles to
settle more rapidly. In addition, at higher temperature, the
water droplets coalesce more readily, which facilitates their
separation from the hydrocarbon. A higher purity produ~ct can
be produced ~ith lower residence time.
At higher temperatures, fractions of the diluent
can approach or exceed their atmospheric boiling point. To
prevent flashing of the diluent and to contain the pressures
generated, it is then necessary to operate the circuit at
elevated pressure, using a controlled back pressure valve in
the vent line from the settler.
In accordance with this aspect of the invention
then, the cixcuit is made pressure~retaining using
conventional means and the process is operated at elevated
temperature and pressure.
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