CA2049178C - Froth washer - Google Patents

Abstract

The washer comprises an open-topped vessel having a tube mounted vertically and centrally therein. Post-primary bitumen froth is fed to the upper end of the tube. Pairs of down throw and up throw propellors mounted to rotate in the tube act to shear the froth as it passes down through the tube. At its lower end, the tube is closed off by a transverse wall. Slots are formed in the tube side wall to provide an exit. The sheared mixture thus changes direction as it leaves the tube. In the vessel chamber, solids and water sink downwardly and are removed as tailings through a bottom outlet. The sheared aerated bitumen rises and forms a new froth that is recovered in a launder.
Middlings are withdrawn from the chamber and recycled to join the feed. The product froth is reduced in water and solids relative to the feed froth.

Description

1 ~ FIELD OF THE INVENTION

2 This invention relates to a method for cleaning bitumen 3 froth by removing some of its contained water and solids. It 4 further relates to a froth cleaner for carrying out the method.

7The present invention is an improvement of the froth 8cleaner and process practised therein, which are disclosed by 9V.P. Kaminsky in U.S. Patent 3784464, owned by the present 10assignees.
11Before commenting on the Kaminsky system, it is 12appropriate to provide some background on what post-primary 13bitumen froth is and how it is produced.
14There are very large deposits of oil sands in the 15Athabasca region of Alberta. These oil sands are strip mined and 16the valuable heavy oil (often referred to as "bitumen") is 17extracted using a process referred to as the hot water process.
18Generally stated, the hot water process involves the following.
19 - The as-mined oil sand is mixed with hot water and caustic in a horizontal rotating drum or tumbler 21 for a few minutes. As a result of heating the oil 22 sand slurry, generating surfactants in situ by 23 reaction of caustic with components of the oil, 24 and mechanically applying energy, the separation of the oil from the solids and its dispersion into 26 the aqueous phase is facilitated. At the same 27 time, air bubbles are entrained in the produced 28 slurry as a result of cascading it in the tumbler.
29 Some of these air bubbles become attached to 1 ~ liberated bitumen bubbles, thus rendering them 2 floatable;
3 - The slurry issuing from the tumbler outlet is 4 screened, to remove rocks and oversized lumps, and is then diluted with additional hot water;
6 - The diluted mixture is continuously settled for 7 about 45 minutes in a very large, open -topped 8 vessel having a cylindrical upper section and a 9 conical bottom section. This vessel is referred to as the primary separation vessel ("PSV"). In 11 the tumbler and PSV, more of the oil globules 12 become aerated. As a result, the oil is floatable 13 and rises by buoyancy to the top of the PSV to 14 form an oily froth. This mechanism can be referred to as "spontaneous flotation". The froth 16 overflows the top rim of the PSV and is received 17 in and led away by a channel or launder. At the 18 same time, the coarse sand particles sink and are 19 concentrated in the conical section of the PSV.
An underflow, referred to as "primary tailings", 21 is removed through a bottom outlet of the PSV.
22 In the mid-section of the PSV, there exists a 23 watery mixture comprising some non-buoyant oil and 24 fine solids - this mixture is referred to as "middlings". The primary tailings and middlings 26 each contain a minor amount of oil which needs 27 to be recovered;
28 - the middlings are mixed with the primary tailings 29 and the mixture is introduced into a cone settler 1 ~ referred to as the tailings oil recovery vessel 2 ("TORV"). In the TORV, the feed mixture is 3 deflected radially as it is fed in and is spread 4 . outwardly and horizontally. The out-moving mixture is contacted from below by an upwelling 6 stream of aerated middlings (all of this is 7 described in detail in U.S. patent 4545892). The 8 result is that a second yield of froth is produced 9 as the two streams mix. This middling re-circulation/aeration recovers bitumen from the PSV
11 tailings and middlings which would otherwise be 12 lost. The froth overflows the rim of the TORV
13 and is recovered. It is referred to as a form of 14 "post-primary froth";
- A stream of middlings is withdrawn from the TORV
16 and is processed in a bank of sub-aerated, 17 impeller-agitated flotation cells. Under these 18 conditions of intense aeration and agitation, a 19 third oil froth is produced and recovered. This froth is also classed as "post-primary froth".

22 The two streams of post-primary froth, either 23 separately or in combination, provide the feedstock for the 24 present invention.
It needs to ~e realized that the post-primary froth 26 streams are highly contaminated with water and solids. A typical 27 composition (which can vary widely) is:

Z04917~3 1 ~bitumen - 40% by weight 2water - 52% by weight 3solids - 8% by weight 4It is necessary to remove these contaminants before delivering the oil stream to a refinery-like upgrading plant.
6 This is conventionally done by de-aerating the froth, diluting 7 the deaerated froth with naphtha and then treating the deaerated 8 diluted stream in two stages of driven centrifugation, to firstly 9 remove the coarse solids with scroll centrifuges and then remove the fine solids and water with disc centrifuges.
11The centrifuging circuit is expensive and difficult to 12 operate. The abrasive solids cause severe wear of the 13 centrifuges.
14It would be desirable to remove some of the water and solids prior to centrifugation. This would alleviate the 16 production limitation imposed by the centrifugation plants.
17The Kaminsky patent disclosed a system, comprising a 18 cleaner and its inherent method, for treating post-primary froth.
19 The objective for the system was to reduce the water and solids contents of the froth, prior to centrifugation. The Kaminsky 21 cleaner involved:
22- an upstanding open-topped outer vessel having a 23cylindrical upper section and a conical lower 24section, said vessel having an outlet at its base;
25_ an upstanding, open-ended draft tube centrally 26positioned in the upper section of the outer vessel;
27 and 28- a driven shaft carrying a pair of marine-type 29propellers, the upper propeller being adapted to ~0~9178 -1 compress or "throw" fluid downwardly and the lower 2 propeller being adapted to throw it upwardly. The two 3 opposed pitch impellors create an intense shear field 4 which would function to break up the large bitumen globules into smaller dispersed droplets. The feed 6 slurry would flow down the draft tube into the vessel 7 by gravity.

9 In the operation of the washer:
- water was added to the froth as it was being 11 delivered to the upper inlet end of the draft 12 tube;
13 - the froth/wash water mixture was then mixed and 14 sheared by the propellers;
- the sheared mixture moved out of the open bottom 16 end of the draft tube into the bottom section of 17 the outer vessel chamber, which provided the 18 quiescent environment of a settler. Water and 19 solids, liberated by the shearing action, would be separated from the dispersed bitumen droplets.
21 The droplets would rise through the annular upper 22 section chamber by buoyancy and form a froth 23 reduced in solids and water. This froth would 24 overflow the outer vessel rim and be recovered;
and 26 - solids and water would move downwardly by gravity 27 and be removed through the bottom outlet.

S UMMARY OF TH E I NVE NT I ON
2 In accordance with the present invention a modified 3 froth cleaner of the Kaminsky type is provided wherein the draft 4 tube is closed at its bottom end by a transverse wall and upwardly extending slots are formed in the sidewall of the lower 6 end of the tube, to provide the outlet from the tube.
7 It has been found that the oil loss with the underflow 8 from the cleaner can be reduced from about 6% by weight, for a 9 cleaner having an open-bottomed draft tube, to about 2% or less 10 for the same cleaner modified in accordance with the present 11 invention.
12 It is believed that the improvement in performance is 13 explained by the following:
14 - It appears necessary that quiescent conditions be maintained in the main body of fluid outside the 16 draft tube, in order to promote flotation/settling 17 separation of the froth components and to reduce 18 oil losses by entrainment in the tailings;
19 - When an open-bottomed draft tube is used, a vortex is generated in the fluid undergoing the 21 propeller-driven mixing. This vortex extends down 22 into the settling section of the vessel chamber 23 and disturbs the fluid undergoing 24 flotation/settling separation, thereby increasing the likelihood that oil will be lost through the 26 bottom tailings outlet;

1 - By providing transverse closure of the draft tube, 2 the disturbing action of the vortex is 3 eliminated;
4 - Since the direction of flow is straight down with the open-bottomed prior art draft tube, there is 6 a tendency for feed to short-circuit and oil to 7 be lost with the tailings. By changing the 8 direction of fluid flow as it leaves the draft 9 tube, by blocking it with the transverse wall and causing it to move radially out through the slots, 11 a stagnation point flow condition is induced.
12 This change in turbulent structure is believed to 13 be beneficial to the separation process. When 14 the flow exits from the slots, the distribution is such that the oil globules are more 16 concentrated at the top of the exit slots and the 17 sand is more concentrated at the bottom of the 18 slots.
19 In a preferred feature, middlings from the vessel chamber are recirculated and added to the froth feed incoming to 21 the draft tube. Recirculation of the middlings has enabled 22 elimination of fresh water addition to the incoming feed.
23 Broadly stated the invention is a method for cleaning 24 post-primary bitumen froth feed containing bitumen associated with water and solids contaminants, comprising: providing 26 apparatus comprising an upstanding open-topped vessel having a 27 tubular upper section and a conical lower section, said vessel 28 forming a chamber communicating at its base with means for 29 removing solids-rich underflow, said vessel having a ;;~04917~3 1 substantially vertical tube positioned in its upper section 2 whereby said vessel and tube form an annular passage between 3 them, as part of the chamber, said tube having an upper portion, 4 in which shearing of the froth takes place, and a lower outlet portion through which the sheared froth leaves the tube, said 6 tube forming a bore, said outlet portion having a transverse wall 7 closing the lower end of the tube bore, said outlet portion 8 forming slots in its side wall, which slots extend upwardly from 9 the transverse wall and provide an outlet for the sheared froth, said tube having a rotatable shaft extending downwardly into the 11 tube bore, said shaft carrying at least one pair of vertically 12 spaced apart propellors, the upper propellor being adapted to 13 down throw the feed, the lower propellor being adapted to up 14 throw the feed; introducing the froth feed into the upper end of the tube, whereby it moves down through the tube bore; shearing 16 the froth in the tube bore with the propellors as it moves 17 downwardly therethrough; causing the sheared froth to change 18 direction from downward flow to outward radial flow as it exits 19 through the slots; temporarily retaining the sheared froth under quiescent conditions in the vessel chamber, whereby buoyant 21 bitumen in the sheared froth rises through the annular space to 22 form froth product reduced in water and solids content, relative 23 to the froth feed; recovering the newly-formed froth product from 24 the vessel; and continuously withdrawing fluidized solids from the base of the vessel.

~:0~9178 2Figure 1 is a sectional side view of a froth cleaner 3 in accordance with the invention;
4Figure 2 is a perspective partly broken away view of the froth cleaner of Figure 1.; and 6Figure 3 is a plot comparing washed froth quality of 7 the prior art Kaminsky washer and the present washer.

9The froth cleaner 1 comprises an upstanding outer vessel 2 having a tubular upper section 3 and a conical lower 11 section 4.
12A vertical tube 5 is centrally positioned and supported 13 so as to extend coaxially into the chamber 6 of the vessel 2.
14 The lower end of the tube 5 is landed at about the junction of the vessel sections 3, 4.
16The vessel chamber 6 includes an annular passage 7 17 formed between the tube 5 and vessel section 3.
18The tube 5 is open at its upper end and closed by a 19 transverse wall 8 at its lower end.
20Circumferentially spaced apart, vertical slots 9 are 21 formed by the side wall 10 of the lower outlet portion 11 of the 22 tube 5.
23A rotatable driven shaft 12 extends downwardly into 24the bore 13 of the tube 5. The shaft 12 carries a plurality of pairs of propellers. Each pair comprises an upper propeller 14, 26 operative to throw or compress fluid downwardly, and a lower 27 propeller 15, operative to throw fluid upwardly. The propellers 28 are designed so that there is a net downward flow.
29The vessel's lower section 4 has a bottom outlet 16 connected by a line 17 with a withdrawal pump 18.

2049~78 1 Means, such as a line 19, are provided to feed post-2 primary froth to the open end 20 of the tube 5.
3 A launder 21 extends around the vessel 2 at its upper 4 end, to receive and remove produced froth.
A vertically slidable sleeve 22 is mounted around the 6 tube 5. The sleeve 22 may be lowered or raised by cables (not 7 shown) to adjust the open area of the slots 9.
8 A middlings recycle line 24 connects the vessel chamber 9 6 with the feed line 19. A pump 25 is positioned in the line 24 to recycle middlings to the line 19 and tube 5.
11 In operation, post-primary froth is fed continuously 12 into the open upper end 20 of the tube 5. It may be diluted by 13 recycle of middlings from the vessel chamber 6. The froth is 14 mixed and sheared by the opposed pairs of propellers 14, 15 as it moves down through the tube 5. In the course of this action, 16 the globules of oil are sheared and broken up into much smaller 17 globules, with a concomitant liberation of some of the water and 18 solids associated with the original globules. Some attachment 19 of air bubbles to the dispersed bitumen droplets also occurs in the turbulent mixing zone inside the bore 13 of the tube 5, due 21 to the formation of fine air bubbles. This process increases the 22 bitumen/air bubble attachment efficiency.
23 As the stream of froth exits the tube 5, it changes 24 direction, as it is blocked from continuing downwards by the wall 8 and must leave through the slots 9.
26 In the course of changing direction, the buoyant oil 27 globules and sinking solids particles tend to stratify due to 28 gravity.

Z043~7~3 1 On exiting the slots 9, the sheared mixture enters the 2 chamber 6, wherein it is temporarily retained under quiescent 3 conditions. The aerated oil globules rise through the fluid in 4 the annular passage 7 and form froth which overflows into the launder 21 and is recovered. The solids and water tend to sink, 6 are concentrated in the conical section and are withdrawn by the 7 pump 18 through the line 19.
8 The efficacy of the modified cleaner is demonstrated 9 by the following example.

12 Pilot data from froth cleaning tests using the present 13 washer/settler is provided in Table 2. Data from the Kaminsky 14 froth washer is provided in Table 1 for comparison.
A comparison of these two data sets revealed similar 16 feed compositions. Kaminsky's data averaged: 40.3% oil, 49.2%
17 water and 10.5% solids. The feed composition for the test 18 involving the present washer averaged 36.9% oil, 37.4% water and 19 25.3% solids.
The processing conditions for the present tests were 21 more severe than in Kaminsky's case, with a feed loading rate 22 averaged at 1927 versus the rate of 953 kg/m2.min in Kaminsky's 23 experiments. This loading is defined by mass flow rate of the 24 feed per cross section area of the mixing zone. Kaminsky's washer was characterized by having a larger settling area, with 26 a loading of 244 kg/m2.min versus 273 kg/m2.min in the present 27 case. Wash water was used to enhance the washing efficiency in 28 Kaminsky's tests, at a rate of 27 lb. of fresh water per 100 lb.
29 of bitumen, which is equivalent to 16 lb. of fresh water per 100 ~:049178 1 lb. of froth, assuming washed froth to contain 60% bitumen. With 2 the present mixer/settler, fresh water addition was not required.
3 The data of Tables 1 and 2 are plotted in Figure 3.
4 The data shows that the present washer design out-performed the prior art design. At equivalent washed froth quality, the oil 6 loss with tails in the runs using the present washer averaged 1%.
7 The oil loss with tails in the prior art washer runs averaged 8 7%.

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Claims (4)

1. Apparatus for cleaning post-primary froth feed containing bitumen, water and solids, comprising:
an open-topped vessel having a tubular upper section and a conical bottom section communicating at its base with means for underflow removal, said vessel forming a chamber and having means at its upper end for recovering and removing froth overflowing the vessel's rim;
an upstanding open-topped tube forming a bore and being positioned in the upper section of the vessel in spaced relation with the vessel side wall to cooperate therewith to form an annular passage, said tube being associated with means for introducing froth feed into the upper end of the tube bore, said tube having an upper portion, in which shearing and aeration of the froth takes place, and a lower outlet portion having a transverse wall closing its lower end, the side wall of the outlet portion forming a plurality of slots extending up from the transverse wall, said slots providing outlet means through which sheared froth may leave the tube bore; and a driven rotatable shaft extending down into the bore of the upper portion of the tube, said shaft carrying one or more pairs of opposed throw propellers in vertically spaced arrangement, each pair consisting of a downthrow propeller positioned above an up throw propeller.

2. The apparatus as sets forth in claim 1 comprising:
means for recycling middlings from the vessel chamber to the means for introducing froth feed.

3. A method for cleaning post-primary bitumen froth feed containing bitumen associated with water and solids contaminants, comprising:
providing apparatus comprising an upstanding open-topped vessel having a tubular upper section and a conical lower section, said vessel forming a chamber communicating at its base with means for removing solids-rich underflow, said vessel having a substantially vertical, open-topped tube positioned in its upper section whereby said vessel and tube form an annular passage between them, as part of the chamber, said tube having an upper portion, in which shearing of the froth takes place, and a lower outlet portion through which the sheared froth leaves the tube, said tube forming a bore, said outlet portion having a transverse wall closing the lower end of the tube bore, said outlet portion forming slots in its side wall, which slots extend upwardly from the transverse wall and provide an outlet for the sheared froth, said tube having a rotatable shaft extending downwardly into the tube bore, said shaft carrying at least one pair of vertically spaced apart propellors, the upper propellor being adapted to down throw the feed, the lower propellor being adapted to up throw the feed;
introducing the froth feed into the upper end of the tube, whereby it moves down through the tube bore;
shearing the froth in the tube bore with the propellors as it moves downwardly therethrough;

causing the sheared froth to change direction from downward flow to outward radial flow as it exits through the slots;
temporarily retaining the sheared froth under quiescent conditions in the vessel chamber, whereby buoyant bitumen in the sheared froth rises through the annular space to form froth product reduced in water and solids content, relative to the froth feed;
recovering the newly-formed froth product from the vessel; and continuously withdrawing fluidized solids from the base of the vessel.

4. The method as set forth in claim 3 comprising:
withdrawing fluid from the chamber and recycling it and adding it to the froth feed being introduced into the tube.