Note: Descriptions are shown in the official language in which they were submitted.
L~ ~184
BACXG~OI~ND OF TEE- I~VENTION
This invention relates to a process for recover-
ing hydrocarbons from tailings produced by a dilution centri-
fuging circuit within an oil sand operation employing the
known hot water extraction process.
The hot water extraction process, used commer-
cially to recover bitumen from the Alberta oil sands, involves
the following series of steps:
tl) slurrying, heating and conditioning the oil
sand in a rotating drum in which it is mixed
with steam, hot water and caustic;
(2) diluting the slurry with more hot water and
introducing it into a primary separation
vessel where it is retained to permit buoyant
bitumen particles to rise to the surface and
form primary froth;
(3) withdrawing a stream of middlings from the
midpoint of the separation vessel and sub-
jecting it to induced air flotation in a
secondary recovery flotation cell to cause
contained bitumen to form secondary froth;
and
(4) combining the primary and secondary froths
to form a combined froth product.
The dilution centrifuging process used to remove
water and solids from the froth product of the hot water ex-
traction process involves the following steps:
(1) diluting the combined froth product with
naphtha to alter the viscosity and specific
gravity of the contained bitumen; and
(2) introducing the diluted froth into a two-
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4~8~
stage centrifugal separation circuit where
the coarse solids are separated from the
stream in a scroll-type centrifuge and the
water and fine solids are separated in a
disc-type centrifuge.
There is a significant loss of hydrocarbons in
the dilution centrifuging (D.C.) tailings produced by the
scroll and disc cen~rifuges. More particularly, in the plant
presently being constructed by the assignee of this invention,
there will be approximately 1.6 million pounds/hour of D.C.
tailings produced containing approximately 3.5% by weight bitu-
men, 2.1~ hydrocarbon diluent, 24.0~ solids and 70.4~ water.
The losses of bitumen and diluent per day will be approximately
3,900 barrels and 3,200 barrels, respectively. These figures
are only given by way of example, as it is expected that they
will vary significantly with plant operating conditions and
throughput.
The tailings composition shown above corresponds
to the predicted values for a commercial plant. The tailings
~ actually used in developing this invention were derived from
pilot plant operations and contained a significantly greater
proportion of water, as indicated in Example I. This extra
water is a direct result of the mechanical configuration of the
pilot plant centrifuges. However, the recovery of hydrocarbon
from the pilot plant tailings should be more difficult than it
will be from the commercial plant tailings, because the hydro-
carbon content of the pilot plant tailings is reduced by the
extra water.
A problem which has heretofore existed was how
to economically recover the hydrocarbons contained in the D.C.
tailings. If such a process could be developed, it would also
-` 10!~ 8~
be desirable that the diluent and bitumen be recovered in a
single step.
In this regard, it needs to be kept in mind that
the D.C. tailings is a unique and difficult material to
deal with. It contains a relatively small amount of bitumen
and diluent distributed throughout a large quantity of water
and solids. The bitumen in the D.C. tailings is originally
recovered as part of the ~roth produçed ~y the hot water ex-
traction process. Its rejection to the D.C. tailings together
with the proportionate amount of diluent suggests that these
hydrocarbons are physically associated with the froth solids
and are not recoverable by dilution centrifuging.
SUMM~R~ OF TH~ INVENTION
It has now been found that induced gas flotation
can successfully be applied to D.C. tailings to recover the
major portion of the bitumen and diluent. The froth which is
produced is found to be relatively high in solids and water
content, when compared with the froth produced by the primary
separation vessel.
It has also been discoyered that the major
portion of the hydrocarbons contained in the froth can be
recovered by mixing the froth with additional hydrocarbGn
diluent and passing the diluted froth through scroll-type and
disc-type centrifuge separators in series. 5atisfactory re-
covery of the original hydrocarbons and rejection of the
contained water and solids is best achieved when the added
diluent brings the diluent to bitumen weight ratio in the
mixture to a value above 1.0O
Broadly stated, the invention is a method for
treating tailings from the dilution centrifuging circuit
10~ 34
of a hot water extraction complex for the recovery of bitumen
from oil sands, said tailings containing bitumen, hydrocarbon
diluent, solids and water, comprising: introducing the
tailings into a flotation cell and retaining it therein
for a sufficient period of time, while subjecting it to
agitation and flotation using gas introduced into the
base of the body of tailings, to recover a significant
portion of the conta.~ned bitumen and diluent as froth and
reject a portion of the solids and water as underflow;
mixing t~e froth with a further portion of hydrocarbon ~ .
diluent; treating the diluted froth in a scroll-type
centrifugal separator to reject solids, water and a
minor part of the hydrocarbons in the diluted froth as
tailings-, and produce a first product stream comprising
hydrocarbons, water and a minor part of the solids in the
diluted froth; and treating the first product stream in
a disc-type centrifugal separator to reject water,
solids and a minor part of ~he hydrocarbons in the first
product stream as tailings, and produce a second product
stream comprising hydrocarbons and a minor part of the
water and solids in the first product stream~
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DESCRIPTION OF THE PR~FE~R~ ~MBODIME~IT
The in~ention is exemplified by the following
example:
A mixture of pilot plant D.C. tailings was
prepared by mixing 1 part of scroll centrifuge tailings with
12 parts of disc centrifuge tailings. Mixing was carried out
by placing 250 pounds of scroll tailings and 350 Imperial
gallons of disc tailings in a tank 1 and heating them to 170F
over a period of 1 hour while agitating them. The mixture
formed had a composition of 1.70~ by weight hydrocarbons,
6.85~ solids and 91.45% water.
This mixture was fed at a rate of 23.5 lbs./min.
into a ban~ of flotation cells 2 consisting of three 0.8 cubic
foot capacity cells. These cells were connected in series
such that the under~low from one cell became the feed for the
next cell. Each cell was equipped with a Den~er* Sub-A type
aeration impellor and air was introduced into the tailings
through the impellor mechanism as the mixture was agitated.
The nominal operation con~itions for each cell in the bank
were as follows:
retention time ~ 2.26
impeIlor speed - 750 r~p.m.
air rate - 0.175 SC~M/cu. ft~ of cell volume
power input - 0.09 HP/cu. ft. of cell volume.
The froth produced from the three cells was
combined to yield 2.25 lb~min~ of total froth while the final
(Cell 3) tailings were produced at a rate of 21.25 lbs./min.
Total froth and final tailings had the following compositions:
4~4
TAsLE I
% by we~ightTot;al Froth Final Taillngs
Hydrocarbons16.2~ 0.19
Solids 31.73 4.66
Water 51.99 95.15
Since the throughput requirements of the experi-
mental flotation bank were much less than that of the centrifu-
gation equipment, it was necessary to accumulate the froth
produced from a number of flotation tests. The froth com-
position shown above is representative of one flotation test.Froth was accumulated for centrifu~ation test work in barrels
which stood for several days prior to testing. A small
portion of clear water was drained from the barrels leaving a
total accumulated froth having the following composition:
TABLE II
% by weightA u~ulated Froth
Hydrocarbon20.25
Solids 35.32
Water 44-43
600 lbs. of the froth, having a temperature of
about 120F, were introduced into a mixer 3 and mixed therein
with 119 lbs. of hydrocarbon diluent having a temperature of
60F, said diluent in this case being a naphtha fraction
having a boiling point within the range 150F to 450F. The
mixture was stirred and heated to a final temperature of
170F.
The diluted ~roth has a composition as
follows:
~3~18~
TABI,E III
% by weight Diluted Froth
EIydrocarbon 33.48
Solids 29.~5
Water 37.07
Work to date indicates that in the order of 1
to 3 parts of diluent per part of hydrocarbon in the froth is
suitable for rendering the bitumen amenable to separation by
centrifugal separation.
The diluted froth was fed at 30 I.G.p~m.
into a 12 inch diameter x 30 inch long Bird* scroll-type
centrifugal separator 4 operated at the following conditions:
Bowl Speed- 1350 r.p.m.
Pool Depth- 0.43 inches
The compositions and rates of the products
obtained from the scroll separator are as follows:
TABLE IV
O by weightScroll Ta-ilings Scroll Product
Hydrocarbons5.46 56.51
Solids 78.01 5.66
Water 16.53 37.83
Feed Split30.5 69.5
The material balance closures on hydrocarbon,
water and solids are 81.8%, 118.3% and 106.1% respectively
and in light of the difficulties encountered with such a test,
this closure is deemed adequate~ Despite the difficulties
with material balance closure, it can be concluded that:
(1) only a minor part of the hydrocarbons originally in the
diluted froth reports in the tailings o~ the scroll separator;
and (23 most of the solids are removed by the separator so that
the product stream only contains a minor part of the solids
originally in th.e dlluted froth.
10~ 34
The product from the scroll centri~uge test
was accumulated in a tank and used to run a number of tests
in a De Laval*SX 204-T disc type centrifugal separator.
Since the tankage was poorly agitated the composition of disc
centrifuge feed tended to vary from test to test. One particu-
lar test utilized a feed having the following composition:
TABL~ V
~'by'wè`i'ght'Disc`Feed
Hydrocarbons68.12
Solias 3.67
Water 28~22
The product was passed into a De Laval* SX 204-T
disc-type centrifugal separator at a xate of 9 I.G.P.M. This
separator was operated at the following conditions:
Bowl Speed5650 r.p.m~
Disc Spacing0.5 mm
Capacitance Tank Pres-
sure 10 psig
Product Back Pressure 10 psig
The compositions and rates of the products
obtained from the disc separator are as follows:
T~BLE VI
'%'by wei~ht'Disc Tailings Disc Product
Hydrocar~ons0.36 84.39
Solids 0.71 4.20
Water 98.93 11.42
It is to be noted that: (1) the disc separator
rejects most of the solids and water but only a minor part
of the hydrocarbons in the feed from the scroll separator; and
(2~ the product stream from the disc separator is essentially
hydrocarbons and contains only a minor part of the solids and
water in the original feed.
* - indicates registered trade mark
