Embodiments disclosed herein relate to treatment of a bitumen- containing froth with a paraffinic solvent, and more particularly, to addition of froth to solvent in two or more additions for determining asphaltene precipitation therefrom. BACKGROUND

It is well-known in the oil sands industry to treat bitumen-containing froths, resulting from oil sands extraction processes, with paraffinic solvents. Bitumen typically comprises maltenes, which are of particular commercial interest, and asphaltenes. The asphaltenes are typically described as being pentane insoluble, toluene soluble components of the bitumen.

The froth is typically diluted in the paraffinic solvent, such as pentane, at particular solvent-to-bitumen ratios (S:B), for diluting the maltenes from the bitumen into the solvent and rejecting at least a portion of the asphaltenes therefrom. The solvent-diluted bitumen is then separated from the froth by gravity in a froth separation unit or vessel (FSU), the diluted bitumen product rising to the top and the precipitated asphaltenes, water and solids settling to the bottom. As the precipitated and agglomerated asphaltenes become associated with water and solids in the froth, the use of paraffinic solvent produces a relatively dry, diluted bitumen product which may be suitable for subsequent pipeline transport and downstream refining processes. As taught in Canadian Patent 2,588,043 to Shell Canada Energy, solvent is typically added to froth in an incremental manner to produce a froth/solvent mixture which can be separated, such as by gravity. A first volume of solvent, sufficient to result in an S:B ratio which is too low to precipitate the asphaltenes, but is sufficient to reduce the viscosity and density of the froth, is added to the froth, forming a froth/solvent mixture. Thereafter a second volume of solvent is added to the froth/solvent mixture, prior to a first FSU, to raise the S:B ratio to an S:B ratio sufficiently high that at least some of the ashaltenes precipitate therefrom. As one of skill will appreciate, the S:B ratio can be varied to result in a desired amount of asphaltene remaining in the product following settling in the FSU.

Overall, Applicant believes that in conventional paraffinic froth treatment operations using pentane as the diluent, the S:B ratio typically varies between about 2.2 to about 2.5 in order to produce a bitumen product having asphaltene remaining therein in the range of about 10-12%.

The large volumes of paraffinic solvent which are required in conventional paraffinic froth treatment contribute significantly to the cost of the operation. Not only is the cost of the solvent high, the cost for solvent recovery, on- site storage and safety considerations are not insignificant.

Clearly there is interest in the industry for systems and methods which reduce the amount of solvent required to produce a suitable product for downstream processes. Reductions in the amount of solvent permit significant savings both in capital costs and operational costs. Further there is interest in systems and methods that permit altering the amount of asphaltene in the product. SUMMARY

Embodiments taught herein split a froth stream into two or more volumes for addition to a paraffinic solvent which has a solubility parameter. In embodiments the froth stream is split into a first volume and a balance of the froth. The first volume is added to the solvent and results in a first volumetric solvent to bitumen ratio (S:B ratio) sufficient to precipitate asphaltenes therein and dissolve maltenes. The dissolved maltenes act to shift the solubility parameter of the paraffinic solvent which affects the S:B ratio at which asphaltenes are precipitated in the subsequent addition of the balance of the froth.

Embodiments taught herein allow varying objectives to be met. One such objective may be to reduce overall solvent requirements to produce a product having a target asphaltene content at about an asphaltene content produced in a conventional solvent-to-froth operation. Another objective may be to produce a product having a lower asphaltene content using the same amount of solvent as a conventional solvent-to-froth operation. Yet another objective may be to both reduce the asphaltene content in the product and reduce the amount of solvent required to achieve the reduction in asphaltene content in the product.

In one broad aspect, a method for treatment of a bitumen-containing froth using paraffinic solvent for forming a solvent-treated froth for separation to produce a diluted bitumen product, comprises: dividing an entirety of the froth into a first volume and a balance of the froth. The first volume of the froth is added to the paraffinic solvent at a first volumetric S:B ratio, the solvent having a first solubility parameter at which asphaltenes in the first volume of froth are precipitated and maltenes therein are dissolved in the paraffinic solvent forming a shifted solvent having a shifted solubility parameter. The balance of the froth is added to the shifted solvent for further precipitation of asphaltenes therefrom forming the solvent-treated froth having a second, overall volumetric S:B ratio, and a target asphaltene content remaining in the solvent-treated froth.

In embodiments, the balance of the froth can be divided into two or more subsequent volumes. The two or more subsequent volumes are added in sequence to the shifted solvent for forming a subsequent shifted solvent and a subsequent solvent-treated froth having a subsequent overall volumetric S:B ratio.

The overall volumetric S:B ratio can remain constant while the first volume of froth is increased for reducing the amount of asphaltene in the product.

In embodiment the constant volumetric S:B ratio is a volumetric S:B ratio equivalent to a volumetric S:B ratio for an addition of a volume of the paraffinic solvent to the entirety of the froth, for maintaining the volume of the paraffinic solvent at a constant volume.

In embodiments the overall volumetric S:B ratio is decreased to below the constant volumetric S:B ratio and the first volume of froth is increased for decreasing the asphaltene content in the diluted bitumen product and descreasing the volume of the paraffinic solvent BRIEF DESCRIPTION OF DRAWINGS

Figure 1A illustrates a conventional scheme for introduction of solvent to froth for forming a solvent-treated froth stream as a feed for at least a primary FSU, according to the prior art;

Figure 1 B illustrates a conventional scheme for introduction of first and second volumes of solvent to froth for forming a solvent-treated froth stream as a feed for at least a primary FSU, according to the prior art;

Figure 2 is a graph illustrating the asphaltene content in a solvent- treated feed stream at various volumetric solvent-to-bitumen ratios (S:B) for solvent streams having different solubility parameters;

Figure 3A illustrates an embodiment disclosed herein wherein froth is introduced to solvent in two separate additions;

Figure 3B illustrates an embodiment disclosed herein wherein froth is introduced to solvent in three or more separate additions;

Figure 4 represents specific examples according to embodiments disclosed herein wherein a constant S:B ratio is maintained for a first volume of 30% or 15% of the froth added to solvent having a solubility parameter of 14.4, the balance being 70% or 85% of the froth added to the solvent in a second addition thereto;

Figure 5 represents a specific example according to embodiments disclosed herein wherein a constant asphaltene content in the product is maintained at 12% with a reduced overall S:B ratio; Figure 6A illustrates an alternate embodiment, wherein a portion of the solvent-treated froth stream of Fig. 3 is recycled to the solvent stream prior to the addition of at least the first volume of froth thereto; and

Figure 6B illustrates the embodiment shown in Fig. 6A wherein the recycled solvent-treated froth is split, a first portion added prior to the first addition of froth and a remainder added prior to the second addition of the froth. DETAILED DESCRIPTION OF EMBODIMENTS

Prior Art - solvent addition to froth

As discussed in the Background and shown in Figs. 1A and 1 B, prior art systems for paraffinic froth treatment typically add solvent S to a froth feed stream F for forming a solvent-treated froth stream FT which is thereafter fed to a gravity separation vessel, such as a froth separation vessel (FSU) 10.

In one prior art embodiment, as shown in Fig. 1A, solvent S is added to the froth F in a single addition at an S:B ratio, greater than 1 .5 by volume, and is mixed therein using an in-line mixer 12 for precipitating at least a portion of the asphaltenes therefrom.

In an alternate prior art embodiment, as shown in Fig. 1 B, the solvent S is added to the froth F in two separate additions. An in-line mixer 12 is used following each addition of solvent S to ensure adequate mixing of the solvent S with the froth F. A first volume of solvent S1 is added to result in an S:B ratio which does not precipitate asphaltenes, but which lowers the viscosity of the feed stream for enhancing separation in an FSU vessel 10 to which the froth treated feed stream FT is fed. A second volume of solvent S2 is added to increase the S:B ratio to greater than 1.5 at which asphaltenes precipitate in the solvent for precipitating at least a portion of the asphaltenes and forming agglomerates therefrom. Embodiments utilizing froth addition to solvent

For simplicity of the discussion which follows, Application refers to and shows only a first or primary froth separation vessel (FSU) which is referred to herein as "the FSU 10". In operation, one or more FSU 10, including one or more countercurrent FSU's or cyclones may be utilized, alone or in series, according to conventional or other operational schemes.

As is understood by those of skill in the art, different asphaltenes precipitate under different solvating conditions. Precipitation is induced by a balance in the solubility parameters δ of the oil components in the solvents S, paraffins being poor solvents and aromatics being good solvents. Paraffinic froth treatment is intended to precipitate the asphaltenes that exist on water surfaces and to precipitate sufficient asphaltene material to agglomerate fine solids in froth into large agglomerates that readily sink during separation processes. In the case of high temperature paraffinic froth treatment, heating also reduces the asphaltene solubility, providing an additional benefit with respect to enhanced precipitation of asphaltene at any given volumetric S:B ratio.

In embodiments disclosed herein, Applicant relies upon known solubility characteristics of asphaltenes in paraffinic solvents, where for example the solubility parameter δ for asphaltenes in pentane is 14.4 and for maltene is 14.9, such as taught in "Simultaneous removal of asphaltenes and water from water-in- bitumen emulsion" by Zhao et al; Fuel Processing Technology 89; 2008; pp. 933- 940, to design a process which may meet one or more process objectives. The objectives may include, but are not limited to, reducing the amount of solvent S required to produce an acceptable product stream, producing product streams having lower asphaltene content than conventional systems using the same amount of solvent; obtaining a product having a designed asphaltene content using paraffinic solvents which comprise contaminants such as aromatics; reducing or minimizing fouling in downstream heating systems by producing a product which has an asphaltene content below the heating systems threshold for soluble asphaltenes; and both reducing the amount of solvent required and reducing the amount of asphaltene remaining in the product.

An acceptable product stream is generally understood to have a water and solids content of less than about 0.5% and to contain largely maltenes. The diluted bitumen product typically comprises some asphaltene, for example between about 10 to 12% asphaltene however, as one of skill in the art will understand, both the amount of water and the amount of asphaltene in the product can be designed to vary according to the concepts disclosed herein.

Having reference to Fig. 2, as one of skill will appreciate, at any given temperature, as the amount of maltene which is dissolved in the paraffinic solvent S increases, the solubility parameter δ for the solvent S is increased. As the solubility parameter δ increases, a greater volume of the solvent S, and hence a higher volumetric S:B ratio is required to obtain a target amount of asphaltene in the diluted bitumen product. Temperatures typically range between about 70°C to about 160°C. The example shown in Fig. 2 was generated at 80°C. With increasing volumes of froth F, and thus maltene added to the solvent S, the shift in the solubility parameter δ can be calculated. In the case of a solvent S having a solubility parameter of 14.4, the shift as a result of varying amounts of maltene dissolved therein can be calculated so as to interpolate a series of curves between the curve for the original solubility parameter δ of 14.4 and, for example, the curve for a shifted solubility parameter δ of 14.9 shown in Fig. 2, using the following formula: δ =∑ δί Θ where: δί = solubility parameter of each component

Θ = volumetric amount

Unlike conventional paraffinic froth treatment processes, wherein solvent S is added to the froth F to result in a solvent-treated froth FT, embodiments disclosed herein add froth F to solvent S in two or more additions thereto. More particularly, the entirety of the froth F is divided into at least a first volume F1 and a second volume or balance F2 of the froth F, wherein the first volume F1 is less than the volume of the balance F2. While benefits are derived when the first volume F1 comprises any volume of the froth F less than the volume of the balance F2, Applicant believes that embodiments wherein the first volume F1 is from about 5% to about 40% of the total volume of the froth F, are most advantageous with respect to the economics of the system. Generally, with respect to the two or more additions of the froth F to the solvent S, embodiments comprise a mixing train having two or more points of entry for froth addition control, such as through hydraulic balance, pumping or valving.

In an embodiment, as shown in Figs. 3A and 3B, the first volume of the froth F1 is added to the solvent S and is mixed therein, such as by using an in- line mixer 12, to result in a first volumetric S:B ratio which is sufficiently large to precipitate the asphaltene from the bitumen therein and to dissolve the maltenes. In embodiments, the first volumetric S:B ratio is greater than 5. At the first volumetric S:B ratio, Applicant believes substantially all of the asphaltenes are precipitated from the first volume of froth F1 and substantially all of the maltenes are dissolved in the solvent S.

The solvent S has a baseline, first solubility parameter 51 . As a result of the maltenes dissolved into the first volume F1 of solvent S, the first solubility parameter 51 of the solvent S increases. Thus, the solvent S, together with the maltene, acts as a shifted solvent Ss having a higher, second solubility parameter 52. Thereafter, the balance of the froth F2 is added to the shifted solvent Ss in one or more additions thereto, with asphaltene being precipitated from the bitumen therein according to Fig. 2. In embodiments, the overall volumetric S:B ratio following the second addition of the balance of the froth F2 is less than about 5.

Referring again to Fig. 2, an example is provided to illustrate a comparison between prior art solvent-to-froth treatment and froth-to-solvent embodiments disclosed herein. In the example provided, the solvent S has the first solubility parameter 51 of 14.4. According to the prior art, if a target asphaltene content of 12% is desired to remain in the final diluted bitumen product, the overall volumetric S:B ratio required for a solvent S having a solubility parameter δ of 14.4, as determined from the 51 curve in Fig. 2, is 2.2. Solvent S is added to the froth F to achieve the volumetric S:B ratio of 2.2.

According to an embodiment taught herein, wherein the objective is to maintain a constant volumetric S:B ratio and to lower the asphaltene content in the product, the overall volumetric S:B ratio is maintained at the constant volumetric S:B ratio, such as at the S:B ratio of 2.2 as in the prior art. Unlike the prior art however, the froth F is added to the solvent S in the at least two separate additions F1 , F2, as described above. While the overall volume of solvent S remains constant, as in the prior art, the amount of asphaltene which remains in the diluted bitumen product is decreased with increasing volumes of froth F added in the first froth F1 addition. The split ratio of the first and second volumes of froth F1 ,F2 can thus be varied to produce a diluted bitumen product which has the target asphaltene content.

Embodiments wherein product has a lower asphaltene content may be particularly advantageous where downstream heating apparatus has a fouling threshold for the amount of asphaltene in the product stream, above which fouling occurs. The volumetric S:B ratio can be maintained at the conventional prior art ratio, such as 2.2, while the first volume of the froth F1 and the balance F2 are varied to drop the asphaltene content to below the fouling threshold.

Having reference to Fig. 2 and Fig. 4, in a specific example where the constant overall volumetric S:B ratio is maintained at 2.2, the first volume of froth F1 added is 30% of the froth F and the balance of froth F2 added is the remaining 70% of the froth F. The froth F contains about 60% bitumen. The first solubility parameter 51 of the solvent S is 14.4. After the first addition of the 30% of froth F1 , the first volumetric S:B ratio is calculated to be about 7.3 (S:B/%F1 = 2.2/0.3) which results in precipitation of substantially all of the asphaltene from the bitumen in the first volume of froth F1 as read from the 14.4 curve in Fig. 2. Further, substantially all of the maltenes from the bitumen in the first volume of froth F1 are dissolved in the solvent S.

To be conservative, it is assumed that all of the maltenes are dissolved in the solvent S for forming the shifted solvent Ss. The first solubility parameter 51 is shifted to the second solubility parameter 52, being about 14.9.

As one of skill will appreciate, asphaltene precipitation in paraffinic froth F is substantially irreversible at less than about 10 minutes after the addition of the solvent S to the bitumen B in the froth F. Thus, any asphaltene which precipitates as a result of the addition of the solvent S will not redissolve during the remainder of the treatment process or in the diluted bitumen product produced therefrom.

With respect to the shifted solvent Ss, it is assumed that the dissolved maltene acts only as a contaminant in the shifted solvent Ss which increases the first solubility parameter 51 to the second solubility parameter 52 of 14.9. It does not however act to increase the volume of the solvent S.

Following the addition of the balance of the froth F2, being 70% of the froth F, the overall volumetric S:B ratio in the shifted solvent Ss decreases to 3.14 (S:B/%F2 = 2.2/0.7). According to Fig. 2, the calculated volumetric S:B ratio of 3.14 as read from the curve for the second solubility parameter 52 of 14.9 results in an asphaltene content remaining in the second addition of froth F2 of about 10.5%. If it is conservatively assumed that the bitumen from the first volume of froth F1 has about 1 % asphaltene remaining therein after precipitation in the solvent S, and the asphaltene remaining in the bitumen after the treatment of the second volume of froth F2 with shifted solvent Ss is about 10.5%, the asphaltene in the final treated froth FT is about 7.6% [30% of 1 % + 70% of 10.5% = 7.6%], which is significantly lower than the prior art case at an S:B of 2.2 which resulted in a target asphaltene content of 12%.

As shown in Table A below, while the overall volumetric S:B ratio is maintained at a constant throughout at 2.2, the split ratio of addition volumes of first and second froth F1 ,F2 can be varied, resulting in different amounts of asphaltene remaining in the final diluted bitumen (dilbit) product as compared to prior art, where the volume of solvent S and the volumetric S:B ratio must be increased to achieve the lowered asphaltene content.

Table A

In another embodiment, wherein the objective is to produce a product having a desired target asphaltene content, one can back-calculate, at each of the split ratios of froth F1 ,F2, to determine the volumetric S:B ratio required to achieve the desired or target asphaltene content.

As shown in Table B below and in Fig. 5, in an example where the target asphaltene content is 12%, the first volume of froth F1 added to the solvent S can be varied, such as between about 5% to about 25%, to result in overall volumetric S:B ratios of between about 2.15 to about 1 .75, which are lower than the prior art volumetric S:B of 2.2 required to achieve an asphaltene content of 12% in the final product.

In each of the split ratios shown in Table B, the final product has a target asphaltene content of 12%. As one can appreciate, the reduction in solvent usage is significant, particularly where larger volumes of froth F are added in the first addition F1 . Table B

In yet another embodiment, a plant can be designed to meet the objective of producing a product having a target asphaltene content of between about 10% and about 12% in the final product using an overall volumetric S:B ratio which is lower than the prior art volumetric S:B of 2.2 for a solvent having a δ of 14.4 as taught in the prior art.

In an example of this embodiment, the plant is designed to handle maximum production at a volumetric S:B ratio of 1.9. The first volume of froth F1 added to the solvent S1 is varied between about 17% to about 29% which results in a final dilbit product having from about 12% to about 10% asphaltene content therein.

As described above, the less maltene which is dissolved in the solvent S, the less the solubility parameter δ is shifted. For example, in the case where the first volume of froth F1 is 17%, the first solubility parameter 51 is shifted to the second solubility parameter δ2 which in this case is about 14.75. Thus, one must interpolate between the 14.4 curve and the 14.9 curve shown in Fig. 2 for each of the volumes of froth F added at the first addition F1 to determine the second solubility parameter 52 for the shifted solvent Ss.

In the prior art case, a baseline S:B ratio of between 2.2 and 2.5 is required to achieve an asphaltene content of 12% to 10% in the final product. In the prior art, the plant, solvent recovery units, solvent storage, pumps, overhead solvent condensers, solvent heaters, flaring and the like would be sized for maximum production at the maximum S:B of 2.5.

As can be seen, use of a volumetric S:B ratio of 1 .9 according to embodiments taught herein, would result in a 24% reduction in flow handling, solvent evaporation, condensation, emergency flaring as well as the size of solvent storage and de-inventorying units compared to the prior art case. Applicant believes, for a plant producing about 200 kBB/d reducing the volumetric S:B to 1 .9 would result in a capital cost saving alone of over $100 million. Further, cost savings are realized in the operating costs as can be appreciated by those of skill in the art.

Having reference again to Figs. 3A and 3B, as one of skill will appreciate, while embodiments have been described above in the context of two additions of froth F1 ,F2 to the solvent S (Fig. 3A), three or more additions can be used (Fig. 3B). The balance of the froth F2 is split into two or more subsequent volumes, each of which is added in sequence to the shifted solvent Ss. Each addition of froth F to shifted solvent Ss results in additional asphaltene precipitation and forms a subsequent shifted solvent Ss. The volumetric S:B ratio after the addition of the first volume F1 is generally greater than 5, while the subsequent volumetric S:B ratio of a last of the additions of the subsequent volumes of the froth F3... , is less than 5.

As is understood by those of skill in the art, asphaltene agglomerates act to encourage the formation of additional asphaltene agglomerates. The solvent- treated froth FT formed following the addition of the greater amount of the froth F2 to the shifted solvent Ss comprises asphaltene agglomerates therein.

Thus, having reference to Figs. 6A and 6B and in another embodiment, at least a portion of the solvent-treated froth FT is recycled upstream for addition to the froth F, prior to the additions of the froth F to the solvent S. The portion of solvent-treated froth FT which is recycled comprises about 5% to about 30% of the solvent-treated froth FT. All of the portion of the solvent treated froth FT can be recycled to a main froth line as shown in Fig. 6A. Alternatively, the portion of the solvent-treated froth FT can be split into at least first and second portions and added before each addition of the first volume and balance of the froth F1 ,F2, as shown in Fig. 6B.

In the case where the recycle of the solvent-treated froth FT is split, a first portion FT1 of the recycled solvent-treated froth FT is added to the froth F and is mixed therewith using an in-line mixer 12 prior to the addition of the first, lesser volume of froth F1 to the solvent S. The remaining portion FT2 of the recycled solvent-treated froth FT is thereafter added to the froth F and mixed therewith using an in-line mixer 12 prior to the addition of the balance of the froth F2 to the shifted solvent Ss. As one of skill in the art will appreciate, in embodiments taught herein, products can be produced having a designed asphaltene content using paraffinic solvents S which comprise contaminants, such as aromatics. As will be appreciated, the effect on the solubility parameter δ of the aromatic contaminants, which typically have a higher solubility parameter δ, can be calculated using the formula: δ =∑ δί Θ where: δί = solubility parameter of each component

Θ = volumetric amount