Mineral oils: processes and products – Tar sand treatment with liquid
Reexamination Certificate
2001-06-11
2004-06-08
Griffin, Walter D. (Department: 1764)
Mineral oils: processes and products
Tar sand treatment with liquid
C208S187000, C208S391000
Reexamination Certificate
active
06746599
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a staged gravity settling process for removing contaminants, namely water and particulate solids, from light hydrocarbon diluent-diluted bitumen froth derived from water-based extraction of bitumen from oil sand.
BACKGROUND OF THE INVENTION
Oil sand, as known in the Fort McMurray region of Alberta, Canada, comprises water-wet, coarse sand grains having flecks of a viscous hydrocarbon, known as bitumen, trapped between the sand grains. The water sheaths surrounding the sand grains contain very fine clay particles. In summary then, oil sand comprises: bitumen; particulate solids (coarse sand and clay “fines”); and water. A sample of oil sand, for example, might comprise 70% by weight sand, 14% fines, 5% water and 11% bitumen. (All % values stated in this specification are to be understood to be % by weight.)
When mixed with hot water, the bitumen will separate from the sand grains and be dispersed into the water phase.
For the past 25 years, the bitumen in McMurray oil sand has been commercially recovered using a water-based process. In the first step of this process, the oil sand is slurried with hot water, steam, usually some caustic and naturally entrained air. The slurry is mixed, for example in a tumbler or pipeline, for a prescribed retention time, to initiate a preliminary separation or dispersal of the bitumen and solids and to induce air bubbles to contact and aerate the bitumen. This step is referred to as “conditioning”. The conditioned slurry is then further diluted with hot water and introduced into a large, open-topped, conical-bottomed, cylindrical vessel (termed a primary separation vessel or “PSV”). The diluted slurry is retained in the PSV under quiescent conditions for a prescribed retention period. During this period, aerated bitumen rises and forms a froth layer, which overflows the top lip of the vessel and is conveyed away in a launder. Sand grains sink and are concentrated in the conical bottom. They leave the bottom of the vessel as a wet tailings stream containing a small amount of bitumen. Middlings, a watery mixture containing solids and bitumen, extend between the froth and sand layers.
The wet tailings and middlings are separately withdrawn, combined and sent to a secondary flotation process. This secondary flotation process is commonly carried out in a deep cone vessel wherein air is sparged into the vessel to assist with flotation. This vessel is referred to as the TOR vessel. The bitumen recovered by flotation in the TOR vessel is recycled to the PSV. The middlings from the deep cone vessel are further processed in induced air flotation cells to recover contained bitumen.
The hot froths (80-85° C.) produced by the PSV and flotation cells are combined and subjected to cleaning, to reduce water and solids contents.
More particularly, it has been conventional to dilute this bitumen froth with a light hydrocarbon diluent, specifically naphtha, to increase the difference in specific gravity between the bitumen and water and to reduce the bitumen viscosity, to thereby aid in the separation of the water and solids from the bitumen. By way of example, the composition of naphtha-diluted bitumen froth typically might have a naphtha/bitumen ratio of 0.65 and contain 20% water and 7% solids.
This diluent-diluted bitumen froth, derived from water-based extraction of bitumen from oil sand, is commonly referred to as “dilfroth”.
Separation of the bitumen from water and solids is then carried out. This may be done by treating the dilfroth in a sequence of scroll and disc centrifuges. Alternatively, the dilfroth may be subjected to gravity separation in a series of inclined plate separators (“IPS”) in conjunction with countercurrent solvent extraction using added light hydrocarbon diluent.
These prior art centrifuge and IPS techniques for removing water and particulate solids from dilfroth have not been entirely satisfactory. Typically the “cleaned” froth, (commonly referred to as “dilbit”), may still contain at least 1.5% water and 0.5% solids. These contaminants cause problems in the downstream refinery-type processes used to upgrade the dilbit to produce useful end products. More particularly, the water contains chlorides, which cause corrosion in heat exchangers. The solids plug catalysts. For these reasons, the upgrading sector of these plants have specified that the dilbit should contain <1.0% water and <0.3% solids.
Researchers have long sought to develop a practical and viable alternative process which would reliably produce dilbit having the specified smaller concentrations of water and solids. It would be even more desirable to reduce the contamination to levels in the order of <0.5% water and <0.2% solids. In addition, it would be desirable to achieve this using a system which eliminates the centrifuges, as these are expensive to operate and cause emulsification. However, solutions have been constrained by the following realities:
the clays and asphaltenes in the bitumen have an affinity for each other. They tend to concentrate at water/hydrocarbon interfaces and act to limit coalescence of water droplets into larger globules that would settle rapidly to enable further reduction of water content in the dilbit product;
the loss of bitumen with tails must be minimal, as this is environmentally undesirable and of course reduces oil recovery;
NIB ratio in dilbit should not exceed 0.8; and
given the huge volumes processed in these operations, the equipment used should be simple and reasonably inexpensive to operate and additives, such as demulsifiers, should be used only sparingly.
SUMMARY OF THE INVENTION
In accordance with one embodiment of the invention, the following steps are practised in combination:
Dilfroth, preferably having a naphtha/bitumen ratio of 0.5:1 to 0.8:1, is fed into a gravity settler vessel, referred to as the “splitter”. The splitter has outlet means for withdrawal of solids and aqueous phase from the bottom and outlet means for overflow of the hydrocarbon phase at the top. The vessel should be enclosed at the top and vapor-tight to prevent escape of diluent. The splitter has a feed inlet, preferably intermediate its ends. The vessel may have a sand rake for moving sand to the central bottom outlet. The dilfroth is temporarily retained (for example 15 to 30 minutes) in the splitter chamber so that the froth settles to form a bottom layer of sand and aqueous middlings, a rag layer and a top layer of hydrocarbons (referred to as “dilbit”). Middlings is a mixture comprising mainly water containing some fines and bitumen. An underflow stream of middlings and settled sand, containing some hydrocarbon, (collectively referred to as “splitter tails”), is removed through the bottom outlet. An overflow stream of splitter dilbit is removed through the top outlet. The splitter dilbit comprises hydrocarbons contaminated with, typically 3-5% water and 0.5-2.5% solids. The solids are almost entirely fines. The splitter tails comprise mostly water, typically containing 10-25% solids and 8-20% hydrocarbons;
In an optional or preferred feature, the dilfroth is directly introduced into the splitter middlings layer, beneath the rag layer and above the settled sand. The reason for this is explained below;
In another preferred feature, the feed rate of dilfroth to the splitter, per square meter of horizontal cross-sectional rag or vessel chamber area, is maintained below 6 m
3
/h of dilfroth for each m2 of rag area. More preferably, the feed rate is maintained at about 4 m
3
/h or less. Otherwise stated, the hydrocarbons/water interface area loading rate or flux is maintained below 6 m/h, preferably below 4 in/h. It is found that the thickness of the rag layer begins to increase if the flux is high, for example at 8 in/h. As a result, oil loss with the tails increases and/or contamination of the dilbit also increases. The reason for this is explained below;
In another preferred feature, the elevation of the hydrocarbon/middlings interface in the splitter chamber is monitored, for example
Cymerman George
Dougan Pat
Lorenz Jim
Mayr Corey
Tran Tom
AEC Oil Sands Limited Partnership
Arnold Jr. James
Griffin Walter D.
Millen White Zelano & Branigan P.C.
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