This invention relates to bitumen recovery from tar sands and more particularly to a treatment process for the reduction of entrained air in and conditioning of a froth product produced in a primary tar sand bitumen extraction process.
Tar sands are a geological formation, which are also known as oil sands or bituminous sands. The tar sands deposits provide aggregates of sand, mineral and water impregnated with bitumen. Significant deposits of tar sands are found in Northern Alberta in Canada and extend across an area of more than thirteen thousand square miles. The tar sands formation extends from the surface or a zero depth to depths of two thousand feet below overburden. These tar sand bitumen deposits are significant, measured in billions of barrels equivalent of oil (BOE) and represent a significant portion of the worldwide reserves of conventional oil reserves including the oil reserves of the Middle East.
The tar sands deposits are composed primarily of particulate silica. The bitumen content varies from about 5% to 21% with a typical content of about 12% by weight of the total material of the tar sands formation. Also included is a clay and silt component ranging from about 1% to 50% and more generally 10% to 30% by weight as well as a small amount of water in quantities ranging between 1% and 10% by weight. The bitumen is quite viscous and has an API gravity of about 6xc2x0 to 80xc2x0 and typically includes 4% to 5% sulfur and approximately 38% aromatics.
A process to extract the bitumen from the mineral material and water is required to produce a commercial petroleum product. In general terms this process involves mixing tar sand with water and steam or just hot water in a mixing vessel to separate the bitumen from the water and solids of the tar sand and produce an initial slurry. This initial slurry is diluted with additional water as it leaves the mixing vessel and is then introduced into a cylindrical primary settler vessel (PSV) having a conical bottom. The coarse portion of the solids settles out in this vessel and is removed as an underflow or tailings stream. Buoyant bitumen has sufficient air attachment to move upwards onto the surface of the fluid in the PSV. Consequently, most of the bitumen, some water and minor amounts of solids accumulate at the surface of the primary settler vessel to form a primary froth. This primary froth overflows the vessel wall and is received in a launder extending around its rim becoming the feed stock froth for downstream processing. A middlings stream comprising water, fine solids and a minor amount of buoyant and non-buoyant bitumen is withdrawn from the mid-section of the vessel and is directed to aeration flotation cell equipment.
In an aeration flotation cell, the middlings are agitated and aerated as described for example in Canadian patent 857,306 to Dobson. The bitumen component of the middlings becomes attached to air bubbles introduced into the aeration flotation cell and rises through the cell contents to form froth. This froth is termed secondary froth. The secondary froth is combined with the PSV feed stream, and nearly 100% of the bitumen in the secondary froth goes to primary froth on the next pass.
The sand and residual bitumen that settle to the bottom of the PSV are removed as a tailings stream. Typically, the tailings stream is further processed in a step called tertiary flotation, which produces another bitumen recovery stream that is combined with and becomes part of the PSV feed stream. Tertiary flotation is thus sometimes called bitumen recovery from tailings.
The primary froth discharging from the PSV is feed stock froth for downstream processing. Typically, the feed stock froth that is produced comprises 62% bitumen, 29% water and 9% solids by weight. The feed stock froth that is produced has a temperature generally in the range of 50 to 65 degrees Celsius. The bituminous feed stock froth that is produced has a very high viscosity even at temperatures as high as 65xc2x0 C. At this temperature the feed stock froth can have a viscosity of 16,000 cP and can be very difficult to pump because of this viscosity. The feed stock froth also contains a large volumetric percentage of air entrained as small bubbles. This air provided buoyancy to separate the bitumen from the sand material in the separation steps of the process. The air content of feed stock froth makes it difficult to pump as well. The supply of the bitumen to downstream processing at the processing plant requires a reduction in the air content of the froth to acceptable levels in order to promote adequate pumping efficiency. Also, air must be removed from the feed stock froth to maintain an air content that is outside the range of explosive limits when naphtha is added just prior to the process of froth cleaning, which is referred to as secondary extraction.
In the past, deaeration or the removal of air from the froth product has been accomplished by contacting the froth with low-pressure steam. Examples of froth deaeration using steam in various apparatus and steps in the bitumen recovery process are described in Canadian patents 630,710; 841,581; 1,072,474; 1,081,641; 1,137,906 and 1,144,098. The steam acts to release air from the froth through a combination of heating and reduction of the interfacial tension at the air-oil interface. The main disadvantage of steam deaeration is that it requires a supply of steam with the attendant capital and operating costs. Steam is also thought to be the cause of certain problems with emulsion formation in the feed stock froth, in that the water introduced in the form of steam tends to bind itself into the froth. The bound water requires additional work to remove it during the subsequent froth treatment steps.
Inclined plate separators have been heretofore proposed for use in separating solids from liquids, for example, as described in Canadian patent 1,097,574. Moreover, inclined plate separators have been proposed for bitumen extraction from tar sands as described in Canadian patents 1,201,412; 1,126,187; 1,254,171; 1,267,860 and published Canadian patent application 2,249,679. Heretofore, inclined plate separators have not been used for deaeration of bitumen froth.
The present invention uses an inclined plate separator to perform deaeration of a bituminous feed stock froth and operates without the need to inject steam into the froth to effect deaeration. The use of an inclined plate separator to effect bitumen froth deaeration is termed static deaeration herein. Static deaeration relies on the process known as reverse sedimentation to occur. The process of reverse sedimentation relies on the buoyancy of air bubbles in the froth to settle upwards due to the force of gravity given the differences in density between air and the other constituents of the froth.
An inclined plate separator arranged and configured in accordance with the invention provides satisfactory removal of the air entrained in the bitumen froth over a range of flow rates and range of temperatures that results in cost saving in comparison to processes that use steam to effect froth deaeration. The deaerated bitumen froth product will require further processing to become a marketable petroleum product; therefore, further processing of the deaerated bitumen froth requires transport of the deaerated bitumen froth product to other process equipment. A deaerated bitumen froth product produced by the inclined plate separator static deaerator of the present invention is more readily pumpable than conventionally deaerated froth, which facilitates transport of the static deaerated bitumen extract to other facilities in a bitumen extraction plant.
Variations and fluctuations in the feed froth air content do not adversely impact the air content of the output stream from the inclined plate separator froth deaerator. Tests reveal a low correlation between the product air content and the feed air content, provided process flow rates remain within approximately 20% of the nominal flow rate conditions prescribed for operation of the system. In accordance with the invention, static deaeration is preferably performed at temperatures above about 50xc2x0 C.
The apparatus of the invention provides a plurality of equidistantly spaced plates forming substantially parallel surfaces defining channels therebetween. The plates are arranged to provide a declination angle incline relative to horizontal. The angularly arranged plates provide a plurality of channels or pathways with at least one common dimension through which the froth to be deaerated flows. The angularly arranged plates are inclined at an angle to the horizontal that is between 15 degrees and 60 degrees and are preferably inclined at an angle of 30 to 35 degrees to the horizontal. As a result of the declination of the plates of the inclined plate separator, flow of the froth to be processed through the inclined plate separator is assisted by the force of gravity. Separation of the air from the froth, that is deaeration of the froth, occurs through settlement of the composition during passage through the inclined plate separator deaerator through the process known as reverse sedimentation. The air is settled out of and separated from the froth during passage of the froth through the inclined plate separator. The declination angle is chosen to provide sufficient hydraulic head to overcompensate for the estimated wall friction between the plates and to ensure that mineral does not accumulate in the channel bottoms. In one arrangement the declination angle is 30xc2x0 where the plate separation is 90 mm. In another arrangement, a declination angle of 35xc2x0 is used where the plate separation is 80 mm.
The deaerated froth output has a substantial dynamic viscosity generally in the order of 15,000 centipoise or more. Transport of the deaerated froth to other equipment for further processing is carried out by pipe transport. The viscosity characteristics of the froth output influence the pump equipment needed to pump the froth to downstage treatment. It is known that certain bituminous froth mixtures have been found to be amenable to favorable pipeline flow characteristics, known as core-annular flow, which significantly reduces pump power requirements from what would be expected for the measured viscosity of the bituminous froth fluid. Thus it is desirable to provide a froth deaeration treatment which does not adversely affect or indeed enhances the range of conditions or tendency of the deaerated bituminous froth to exhibit the reduced effective viscosity which is a principal beneficial characteristic of core-annular flow. The core-annular flow phenomenon is manifested as a drop in effective viscosity with resulting significantly reduced pressure drop during transport of the froth through a pipeline. It is believed that the core-annular flow effect results from the formation of free-water in the froth as it traverses the pipeline. In accordance with one theory of core-annular flow, it is believed that during transport through the pipe, free-water in the froth is propelled towards the walls of the pipe, which provides a core-annular-flow lubrication effect that facilitates transport of the froth through a pipe.
Core-annular flow may also be induced in a bituminous froth by adding water to the pipeline in which the froth is to be transported in a manner known as or referred to as water pushing. This process for inducing a core-annular flow phenomenon is described for example in Canadian patent 2,254,048 and published Canadian patent application 2,220,821 both to Neiman et al.
An expected benefit of static deaeration is thought to be enhancement of the range of conditions under which bituminous froth deaerated and conditioned in accordance with the invention will exhibit core-annular flow characteristics relative to the conditions obtained from steam deaeration. Moreover, it is believed that the core-annular flow characteristics exhibited by bituminous froth deaerated in accordance with the invention will be more favorable than the core annular flow characteristics exhibited by bituminous froth deaerated using conventional steam deaeration. One hypothesis is that static deaeration releases an additional quantity of water from the froth, due to shearing action within the froth as it passes through the static deaeration conditioner of the present invention. This additional water, together with the submicron-sized solids that are also released from the froth, results in an additional and substantial reduction in friction in the froth transfer process over the friction reduction achieved using steam deaeration. As a result, there is an anticipated enhanced lubricity exhibited by froth conditioned by static deaeration conditioning using the principles of this invention relative to the natural froth lubricity exhibited by froth deaerated by conventional steam deaeration. This is termed enhanced froth lubricity herein.
The preferred embodiments of the invention will now be described with reference to the attached drawings.