Patent Publication Number: US-5290433-A

Title: Froth washer

Description:
FIELD OF THE INVENTION 
     This invention relates to a method for cleaning bitumen froth by removing some of its contained water and solids. It further relates to a froth cleaner for carrying out the method. 
     PRIOR ART 
     The present invention is an improvement of the froth leaner and process practiced therein, which are disclosed by V. P. Kaminsky in U.S. Pat. No. 3,784,464, owned by the present assignees. 
     Before commenting on the Kaminsky system, it is appropriate to provide some background on what post-primary bitumen froth is and how it is produced. 
     There are very large deposits of oil sands in the Athabasoa region of Alberta. These oil sands are strip mined and the valuable heavy oil (often referred to as &#34;bitumen&#34;) is extracted using a process referred to as the hot water process. Generally stated, the hot water process involves the following. 
     The as-mined oil sand is mixed with hot water and caustic in a horizontal rotating drum or tumbler for a few minutes. As a result of heating the oil sand slurry, generating surfactants in situ by reaction of caustic with components of the oil, and mechanically applying energy, the separation of the oil from the solids and its dispersion into the aqueous phase is facilitated. At the same time, air bubbles are entrained in the produced slurry as a result of cascading it in the tumbler. Some of these air bubbles become attached to liberated bitumen bubbles, thus rendering them floatable; 
     The slurry issuing from the tumbler outlet is screened, to remove rocks and oversized lumps, and is then diluted with additional hot water; 
     The diluted mixture is continuously settled for about 45 minutes in a very large, open -topped vessel having a cylindrical upper section and a conical bottom section. This vessel is referred to as the primary separation vessel (&#34;PSV&#34;). In the tumbler and PSV, more of the oil globules become aerated. As a result, the oil is floatable and rises by buoyancy to the top of the PSV to form an oily froth. This mechanism can be referred to as &#34;spontaneous flotation&#34;. The froth overflows the top rim of the PSV and is received in and led away by a channel or launder. At the same time, the coarse sand particles sink and are concentrated in the conical section of the PSV An underflow, referred to as &#34;primary tailings&#34;, is removed through a bottom outlet of the PSV. In the mid-section of the PSV, there exists a watery mixture comprising some non-buoyant oil and fine solids--this mixture is referred to as &#34;middlings&#34;. The primary tailings and middlings each contain a minor amount of oil which needs to be recovered; 
     the middlings are mixed with the primary tailings and the mixture is introduced into a cone settler referred to as the tailings oil recovery vessel (&#34;TORV&#34;). In the TORV, the feed mixture is deflected radially as it is fed in and is spread outwardly and horizontally. The out-moving mixture is contacted from below by an upwelling stream of aerated middlings (all of this is described in detail in U.S. Pat. No. 4,545,892). The result is that a second yield of froth is produced as the two streams mix. This middling recirculation/aeration recovers bitumen from the PSV tailings and middlings which would otherwise be lost. The froth overflows the rim of the TORV and is recovered. It is referred to as a form of &#34;post-primary froth&#34;; 
     A stream of middlings is withdrawn from the TORV and is processed in a bank of sub-aerated, impeller-agitated flotation cells. Under these conditions of intense aeration and agitation, a third oil froth is produced and recovered. This froth is also classed as &#34;post-primary froth&#34;. 
     The two streams of post-primary froth, either separately or in combination, provide the feedstock for the present invention. 
     It needs to be realized that the post-primary froth streams are highly contaminated with water and solids. A typical composition (which can vary widely) is: 
     bitumen--40% by weight 
     water--52% by weight 
     solids--8% by weight 
     It is necessary to remove these contaminants before delivering the oil stream to a refinery-like upgrading plant. This is conventionally done by de-aerating the froth, diluting the deaerated froth with naphtha and then treating the deaerated diluted stream in two stages of driven centrifugation, to firstly remove the coarse solids with scroll centrifuges and then remove the fine solids and water with disc centrifuges. 
     The centrifuging circuit is expensive and difficult to operate. The abrasive solids cause severe wear of the centrifuges. 
     It would be desirable to remove some of the water and solids prior to centrifugation. This would alleviate the production limitation imposed by the centrifugation plants. 
     The Kaminsky patent disclosed a system, comprising a cleaner and its inherent method, for treating post-primary froth. The objective for the system was to reduce the water and solids contents of the froth, prior to centrifugation. The Kaminsky cleaner involved: 
     an upstanding open-topped outer vessel having a cylindrical upper section and a conical lower section, said vessel having an outlet at its base; 
     an upstanding, open-ended draft tube centrally positioned in the upper section of the outer vessel; and 
     a driven shaft carrying a pair of marine-type propellers, the upper propeller being adapted to compress or &#34;throw&#34; fluid downwardly and the lower propeller being adapted to throw it upwardly. The two opposed pitch impellors create an intense shear field which would function to break up the large bitumen globules into smaller dispersed droplets. The feed slurry would flow down the draft tube into the vessel by gravity. 
     In the operation of the washer: 
     water was added to the froth as it was being delivered to the upper inlet end of the draft tube; 
     the froth/wash water mixture was then mixed and sheared by the propellers; 
     the sheared mixture moved out of the open bottom end of the draft tube into the bottom section of the outer vessel chamber, which provided the quiescent environment of a settler. Water and solids, liberated by the shearing action, would be separated from the dispersed bitumen droplets. The droplets would rise through the annular upper section chamber by buoyancy and form a froth reduced in solids and water. This froth would overflow the outer vessel rim and be recovered; and 
     solids and water would move downwardly by gravity and be removed through the bottom outlet. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention a modified froth cleaner of the Kaminsky type is provided wherein the draft tube is closed at its bottom end by a transverse wall and upwardly extending slots are formed in the sidewall of the lower end of the tube, to provide the outlet from the tube. 
     It has been found that the oil loss with the underflow from the cleaner can be reduced from about 6% by weight, for a cleaner having an open-bottomed draft tube, to about 2% or less for the same cleaner modified in accordance with the present invention. 
     It is believed that the improvement in performance is explained by the following: 
     It appears necessary that quiescent conditions be maintained in the main body of fluid outside the draft tube, in order to promote flotation/settling separation of the froth components and to reduce oil losses by entrainment in the tailings; 
     When an open-bottomed draft tube is used, a vortex is generated in the fluid undergoing the propeller-driven mixing. This vortex extends down into the settling section of the vessel chamber and disturbs the fluid undergoing flotation/settling separation, thereby increasing the likelihood that oil will be lost through the bottom tailings outlet; 
     By providing transverse closure of the draft tube, the disturbing action of the vortex is eliminated; 
     Since the direction of flow is straight down with the open-bottomed prior art draft tube, there is a tendency for feed to short-circuit and oil to be lost with the tailings. By changing the direction of fluid flow as it leaves the draft tube, by blocking it with the transverse wall and causing it to move radially out through the slots, a stagnation point flow condition is induced. This change in turbulent structure is believed to be beneficial to the separation process. When the flow exits from the slots, the distribution is such that the oil globules are more concentrated at the top of the exit slots and the sand is more concentrated at the bottom of the slots. 
     In a preferred feature, middlings from the vessel chamber are recirculated and added to the froth feed incoming to the draft tube. Recirculation of the middlings has enabled elimination of fresh water addition to the incoming feed. 
     Broadly stated the invention is 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 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 propellers, the upper propeller being adapted to down throw the feed, the lower propeller 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 propellers 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. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 is a sectional side view of a froth cleaner in accordance with the invention; 
     FIG. 2 is a perspective partly broken away view of the froth cleaner of FIG. 1; and 
     FIG. 3 is a plot comparing washed froth quality of the prior art Kaminsky washer and the present washer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The froth cleaner comprises an upstanding outer vessel 2 having a tubular upper section 3 and a conical lower section 4. 
     A vertical tube 5 is centrally positioned and supported so as to extend coaxially into the chamber 6 of the vessel 2. The lower end of the tube 5 is landed at about the junction of the vessel sections 3, 4. 
     The vessel chamber 6 includes an annular passage 7 formed between the tube 5 and vessel section 3. 
     The tube 5 is open at its upper end and closed by a transverse wall 8 at its lower end. 
     Circumferentially spaced apart, vertical slots 9 are formed by the side wall 10 of the lower outlet portion 11 of the tube 5. 
     A rotatable driven shaft 12 extends downwardly into the bore 13 of the tube 5. The shaft 12 carries a plurality of pairs of propellers. Each pair comprises an upper propeller 14, operative to throw or compress fluid downwardly, and a lower propeller 15, operative to throw fluid upwardly. The propellers are designed so that there is a net downward flow. 
     The vessel&#39;s lower section 4 has a bottom outlet 16 connected by a line 17 with a withdrawal pump 18. 
     Means, such as a line 19, are provided to feed post-primary froth to the open end 20 of the tube 5. 
     A launder 21 extends around the vessel 2 at its upper end, to receive and remove produced froth. 
     A vertically slidable sleeve 22 is mounted around the tube 5. The sleeve 22 may be lowered or raised by cables (not shown) to adjust the open area of the slots 9. 
     A middlings recycle line 24 connects the vessel chamber 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. 
     In operation, post-primary froth is fed continuously into the open upper end 20 of the tube 5. It may be diluted by recycle of middlings from the vessel chamber 6. The froth is 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, the globules of oil are sheared and broken up into much smaller globules, with a concomitant liberation of some of the water and solids associated with the original globules. Some attachment of air bubbles to the dispersed bitumen droplets also occurs in the turbulent mixing zone inside the bore 13 of the tube 5, due to the formation of fine air bubbles. This process increases the bitumen/air bubble attachment efficiency. 
     As the stream of froth exits the tube 5, it changes direction, as it is blocked from continuing downwards by the wall 8 and must leave through the slots 9. 
     In the course of changing direction, the buoyant oil globules and sinking solids particles tend to stratify due to gravity. 
     On exiting the slots 9, the sheared mixture enters the chamber 6, wherein it is temporarily retained under quiescent conditions. The aerated oil globules rise through the fluid in the annular passage 7 and form froth which overflows into the launder 21 and is recovered. The solids and water tend to sink, are concentrated in the conical section and are withdrawn by the pump 18 through the line 19. 
     The efficacy of the modified cleaner is demonstrated by the following example. 
     EXAMPLE 1 
     Pilot data from froth cleaning tests using the present washer/settler is provided in Table 2. Data from the Kaminsky froth washer is provided in Table 1 for comparison. 
     A comparison of these two data sets revealed similar feed compositions. Kaminsky&#39;s data averaged: 40.3% oil, 49.2% water and 10.5% solids. The feed composition for the test involving the present washer averaged 36.9% oil, 37.4% water and 25.3% solids. 
     The processing conditions for the present tests were more severe than in Kaminsky&#39;s case, with a feed loading rate averaged at 1927 versus the rate of 953 kg/m 2 .min in Kaminsky&#39;s experiments. This loading is defined by mass flow rate of the feed per cross section area of the mixing zone. Kaminsky&#39;s washer was characterized by having a larger settling area, with a loading of 244 kg/m 2 .min versus 273 kg/m 2 .min in the present case. Wash water was used to enhance the washing efficiency in Kaminsky&#39;s tests, at a rate of 27 lb. of fresh water per 100 lb. of bitumen, which is equivalent to 16 lb. of fresh water per 100  lb. of froth, assuming washed froth to contain 60% bitumen. With the present mixer/settler, fresh water addition was not required. 
     The data of Tables 1 and 2 are plotted in FIG. 3. The data shows that the present washer design out-performed the prior art design. At equivalent washed froth quality, the oil loss with tails in the runs using the present washer averaged 1%. The oil loss with tails in the prior art washer runs averaged 7%. 
     
                       TABLE 1                                                     
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Typical Feed                                                              
composition Average Feed Rate                                             
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40.3% Oil   953 kg/m2 · min (based on mixer area)                
49.2% Water 244 kg/m2 · min (based on settler area)              
10.5% Solids                                                              
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              Impellor      Washed F                                      
     Feed     Tip           Compo-         Oil                            
Run  T        Speed    (%   sition         Losses                         
#    (deg. C.)                                                            
              (rpm)    Oil) (% Water)                                     
                                    (Solids)                              
                                           (%)                            
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477  54.4     300      37.6 57.2    5.2    4.8                            
477  54.4     600      41.8 52.8    5.4    6.5                            
477  54.4     900      60.9 33.3    5.8    7.4                            
477  54.4     1200     63.3 31.7    5.0    7.3                            
478  82.2     800      54.6 38.0    7.4    7.4                            
478  82.2     800      57.5 35.9    6.6    7.1                            
478  82.2     800      70.2 26.3    3.5    11.6                           
479  71.1     800      66.0 28.3    5.7    8.1                            
479  71.1     800      64.9 30.3    4.8    15.9                           
479  71.1     800      61.2 32.9    5.9    7.4                            
479  71.1     1200     68.5 26.2    5.3    9.5                            
479  71.1     1200     62.9 29.8    7.3    6.8                            
480  71.1     800      49.5 44.9    5.6    4.5                            
481  54.4     800      48.2 45.0    6.8    2.2                            
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 *washed water = (lb. of water)/(lb. of washed froth                      
 
    
     
                                           TABLE 2                                 
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       Averaged Feed composition                                          
                           Average Feed Rate                              
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       36.9% Oil           1927 kg/m2 · min (based on mixer      
                           area)                                          
       37.4% Water          273 kg/m2 · min (based on settler    
                           area)                                          
       25.3% Solids        Wash Water: none                               
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         Middling                                                         
    Feed Recycle   Feed             Washed Froth Oil                      
    T    Rate      Composition      Composition  Losses                   
Run #                                                                     
    (deg. C.)                                                             
         (kg/min)                                                         
              (% Oil)                                                     
                   (% Water)                                              
                          (Solids)                                        
                               (% Oil)                                    
                                    (% Water)                             
                                            (Solids)                      
                                                 (%)                      
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 1  74   32   38.4 39.0   22.6 51.8 37.6    10.6 0.13                     
 2  81   32   40.7 41.0   18.4 54.3 35.4    10.3 0.45                     
 3  81   32   38.4 40.6   20.9 59.1 29.9    10.9 0.72                     
 4  71   32   31.1 49.6   19.3 60.8 28.8    10.3 0.87                     
 5  86   N/A  37.7 38.0   24.3 53.9 33.4    12.8 0.37                     
 6  83   N/A  38.5 34.1   27.4 46.4 41.4    12.2 4.12                     
 7  85   N/A  33.9 37.7   28.4 58.0 30.4    11.6 2.39                     
 8  83   20   31.8 45.3   22.9 60.9 28.0    11.2 1.54                     
 9  68   32   31.0 42.4   26.6 58.3 32.7    8.9  0.75                     
10  70   32   40.4 34.9   24.7 58.3 32.2    9.5  0.30                     
11  83   32   41.6 39.1   19.3 60.5 30.3    9.3  2.86                     
12  83   N/A  42.7 39.0   18.3 62.2 29.0    8.8  0.76                     
13  83   12   41.7 28.8   19.5 61.4 29.4    9.2  0.63                     
14  76   N/A  33.6 42.6   23.8 50.5 38.5    11.1 0.65                     
15  89   N/A  28.6 34.6   36.8 64.2 28.6    7.2  0.55                     
16  86   N/A  39.0 35.0   26.0 68.1 25.3    6.6  0.44                     
17  67   N/A  27.4 35.4   37.2 60.5 26.9    12.6 0.86                     
18  68   N/A  43.1 32.4   24.5 50.0 29.4    24.5 0.09                     
19  69   N/A  49.0 29.9   21.0 50.3 31.3    18.4 0.20                     
20  63   N/A  38.5 37.2   24.3 49.8 40.4    9.9  0.23                     
21  72   N/A  39.3 36.1   24.6 55.4 35.3    9.3  0.49                     
22  82   32   26.1 34.1   39.8 64.3 26.6    9.1  6.36                     
23  ?    32   25.5 33.0   41.5 62.6 28.7    8.7  2.53                     
24  ?    32   42.4 36.7   20.9 58.8 30.9    10.4 0.41                     
25  75   32   33.7 35.9   30.5 53.6 34.9    11.5 0.41                     
26  ?    32   38.6 38.9   22.5 51.6 36.7    11.7 0.51                     
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