Method for upgrading bitumen froth

Abstract

The present invention provides a procedure for removing mineral matter and water from a primary bitumen froth recovered from tar sands which comprises diluting the raw bitumen froth with a hydrocarbon diluent boiling in the range of 350.degree.-750.degree.F. and thereafter settling the diluted bitumen froth in an autoclave settling zone to provide a froth product and a tailings product. The tailings product is thereafter subject to centrifugal force to provide a centrifugal tailings product and a centrifugal froth product. The centrifugal froth product is recycled to the settler zone.

Description

BACKGROUND OF THE INVENTION

The present invention relates to an improved process for upgrading bitumen froth such as that recovered by a hot water procedure for recovering bitumen from tar sands. The present invention, more particularly, relates to a method wherein the raw bituminous froth product containing mineral matter and water is diluted with a liquid hydrocarbon boiling in the range of 350.degree.-750.degree.F. and thereafter settled in an autoclave settling zone whereby mineral matter and water are removed from the bitumen froth.

Numerous deposits of bituminous tar sands exist throughout the world. The most extensive deposits are found in Northern Alberta, Canada. The sands are composed of a siliceous material, generally having a size greater than that passing a 325 mesh screen, saturated with a relatively heavy, viscous bitumen in quantities of from 5 to 21 weight percent of the total composition. More typically, the bitumen content of the sands is between 8 to 15 percent. This bitumen is quite viscous and contains typically 4.5 percent sulfur and 38 percent aromatics. Its specific gravity at 60.degree.F. ranges typically from about 1.00 to about 1.06. The tar sands also contain clay and silt. Silt is defined as material which will pass a 325 mesh screen but which is larger than 2 microns. Clay is material smaller than 2 microns including some siliceous material of that size.

There are several well-known procedures for effecting separation of bitumen from tar sands. One particularly effective method is that disclosed in Canadian Pat. No. 841,581 issued May 12, 1970 to Paul H. Floyd et al. The method of this disclosure provides that the bituminous sands are jetted with steam and mulled with a minor amount of hot water at a temperature in the range of 140.degree.-210.degree.F. in a conditioning drum.

An alkaline reagent can also be added to the conditioning drum usually in amounts of from 0.1 to 3.0 lbs. per ton of tar sands. The amount of such alkaline reagent preferably is regulated to maintain the pH of the middlings layer in a subsequent separator zone within the range of 7.5 to 9.0. Best results seem to be obtained at a pH value of 8.0 to 8.5. The amount of the alkaline reagent that needs to be added to maintain a pH value in the range of 7.5 to 9.0 may vary from time to time as the composition of the tar sands as obtained from the mine site varies. The best alkaline reagents to use for this purpose are caustic soda, sodium carbonate, or sodium silicate, although any of the other monovalent alkaline reagents can be used if desired.

Mulling of the tar sands produces a pulp which then passes from the conditioning drum to a screen. The purpose of the screen is to remove from the tar sand pulp any debris, rocks, or oversized lumps. The pulp then passes from the screen to a sump where it is diluted with additional water including middlings recycle stream from the separation cell. Recycling of the middlings is not essential in all cases, particularly when the clay content of the tar sands is high. In this event a relatively high rate of fresh feed water is employed to compensate for the high clay content while a correspondingly high rate of transfer of middlings layer to a secondary scavenger zone can be maintained.

The process as above described can also include sending a minor portion of the middlings recycle stream to the conditioning drum to supply all or a part of the water needed therein other than that supplied through condensation of the steam which is consumed. Also, a stream of the middlings recycle is sometimes introduced into the screen to flush the pulp therethrough and into the sump. As a general rule, the total amount of water added to the natural bituminous sands as liquid water and as steam prior to the separation step is in the range of 0.2 to 3.0 tons per ton of the bituminous sands. The amount of water needed within this range increases as the silt and clay content of the bituminous sand increases. The resulting pulp is thereafter carried into a separation cell maintained at a temperature of about 150.degree.-200.degree.F.

In the separation cell, sand settles to the bottom as tailings and bitumen rises to the top in the form of an oil froth which is the primary bitumen froth product. An aqueous middlings layer containing some mineral and bitumen is formed between these layers. A scavenging step is normally conducted on this middlings layer in a secondary flotation zone normally referred to as a scavenger zone. In the scavenger zone, the feed material is aerated so as to produce a scavenger froth product and a scavenger tailings product. The scavenger cell froth product normally contains a substantial amount of water and mineral matter and is thereafter treated in accordance with the procedure disclosed in Canadian Pat. No. 857,306 issued Dec. 1, 1970 to Ernest W. Dobson. This method comprises passing the scavenger froth to a settling zone wherein a froth product rises to the surface and the mineral matter within the froth product settles to the bottom of the settler zone. The froth product from this settling zone is thereafter combined with the froth product from the heretofore disclosed primary separation zone and subsequently combined with a diluent to provide a bituminous froth product suitable for further treatment prior to upgrading to synthetic petroleum crude.

The raw bitumen froth normally suitable for use in the process of this invention can be described as a raw bitumen froth which normally comprises 20 to 45 percent and more typically 30 percent water, 2.0 to 6.0 and more typically 3.0 to 5.0 percent mineral and the remainder being principally hydrocarbons. The raw bituminous froth containing these high concentrations of mineral matter and water is not suitable for typical refinery processing and, therefore, must be further upgraded to reduce mineral matter and water to substantially low concentrations. In the process of the present invention, this end is accomplished in an economic and efficient manner.

Heretofore this froth product recovered as a combination of the primary separation zone froth and the settled scavenger zone froth has been diluted with a hydrocarbon diluent and then upgraded by centrifuging to reduce the amount of mineral in the centrifuged bitumen product to about 1 to 2 weight percent. Generally, the weight ratio of diluent to bitumen (D/B) in the combined froths lies within the range of between 0.3 to 1.0. When the bitumen is later coked to produce synthetic crude, the minerals are concentrated to about 6 to 8 weight percent in the coke. This quantity of mineral matter is generally undesirable and renders the coke product somewhat difficult to recover. In commercial operation, this coke is burned to provide power. When the coke is burned in boilers, the contained minerals or ash must be removed to keep the boiler tubes from fouling. Excessive quantities of minerals reduce operational ability and require expensive maintenance and air pollution control equipment. The combined froth products also contain from 20 to 45 percent water which must be removed prior to the coking procedure.

One procedure for upgrading froth is disclosed in U.S. Pat. No. 3,607,721 issued Sept. 21, 1971. By this procedure, raw bituminous froth is diluted with a light hydrocarbon diluent such as coker naphtha and settled in a settling vessel. The sludge recovered as tailings from the settler is thereafter treated in a centrifuge to recover additional bitumen. Unfortunately, use of a light hydrocarbon diluent often leads to unreasonable losses of diluent to waste water streams which makes the process economically less attractive.

Heretofore, efficient and economical procedures which provide high volume production of substantially mineral and water-free bitumen froth from crude or raw bitumen froth have been overlooked. By way of the present invention, a new procedure for recovering a substantially water and mineral-free bitumen froth product from a raw bitumen froth product produced by a hot water separation procedure for recovering bitumen from tar sands is provided.

DESCRIPTION OF THE INVENTION

By the method of the present invention, raw bituminous froth is first diluted with a liquid hydrocarbon boiling in the range of 350.degree.-750.degree.F. and thereafter transferred to an autoclave settling zone wherein the diluted froth, in combination with a recycle centrifugal froth hereinafter described, is settled to provide an upper froth layer, a water layer, and a lower mineral tailings layer. The tailings layer is thereafter withdrawn from the autoclave settling zone, diluted with diluent similar to that added to the raw bitumen froth, and thereafter transferred to a centrifuge which is preferably a scroll centrifuge wherein a centrifugal froth product is recovered and recycled to the autoclave settling zone. By this process, a froth product substantially reduced in mineral matter and water and thereby suitable for further refining with well-known petroleum refining techniques is provided.

A distinct advantage of the present method lies in the dilution of the settler tailings with added hydrocarbon diluent. By this procedure, diluent can be added to the tailings prior to the centrifuge step in ratios of 1 to 1 to as high as 20 to 1 diluent to bitumen by weight. This high ratio of diluent to bitumen provides more efficient recovery of bitumen from this stream. The addition of diluent to the settler tailings stream provides a method whereby the diluent is most effective in recovery of bitumen from minerals. By this method, the amount of diluent added to the froth prior to the settling step can be reduced. The diluent not added to the froth fed to the settler can be added to the tailings from the settler where it is more effective in freeing bitumen from minerals. Thus, the overall quantity of bitumen losses in the tailings is appreciably reduced.

In order to more completely define the method of the present invention, the following description is presented. Referring to the drawing, raw bituminous froth of the type normally recovered from hot water extraction techniques, hereinabove disclosed and described, is transferred via line 11 and combined in line 13 with hydrocarbon diluent boiling in the range of 350.degree.-750.degree.F. received from line 12. The diluent added to the froth at this step can be in the range of 0.3 to 1.0 volume of diluent per volume of froth. In this example, 0.5 volume of diluent per volume of bitumen is used. The diluted bitumen froth is thereafter heated to a temperature in the range of 300.degree.-1000.degree.F. by way of heat exchanger 14 and thereafter transferred to line 15 where it is combined with a centrifugal froth product hereinafter described. From line 15, the combined froth is transferred to autoclave settling zone 16 which can be defined as an autoclave-type pressure vessel having heat exchange means such as a steam jacket or other means. The froth product is settled in this zone at a temperature in the range of 300.degree.-1000.degree.F. and a pressure in the range of 0 to 1000 psig to provide an upper froth layer comprised substantially of bituminous froth, a middle layer comprised substantially of water containing minor amounts of mineral matter and bitumen, and a lower tailings layer comprised substantially of mineral matter and containing minor quantities of bitumen. The refined froth product is recovered via line 17 from autoclave 16 which product is a froth substantially reduced in mineral matter and water and suitable for further petroleum refining. Water is removed from autoclave settling zone 16 via line 18. This water can thereafter be recycled to other areas where fresh water is required or can be discarded. The tailings from autoclave settling zone 16 are withdrawn via line 19 and combined in line 21 with five volumes of liquid hydrocarbon diluent boiling within the range of 350.degree.-750.degree.F. per volume of bitumen in the tailings from line line 20. The diluted tailings are thereafter transferred to scroll centrifuge 22 wherein a centrifugal froth product is recovered via line 24 and recycled to line 15 where it is combined with the raw diluted bitumen froth. A tailings stream is withdrawn from scroll centrifuge 22 via line 23 and discarded.

Typically, the bituminous froth product recovered from autoclave settling zone 16 can be characterized as containing at least 96 percent hydrocarbon and no more than 0.5 percent mineral matter and 3.5 percent water. The advantages of this froth refining technique are particularly realized by greater volume throughput of froth as well as improved quality of the bitumen froth product as compared to other known techniques. Equally important is the fact that substantial benefits and economics are realized by low maintenance and low capital investment resulting from use of the bitumen froth process of the herein disclosed invention. A particularly suitable advantage of the present invention is the use of the higher boiling liquid hydrocarbon diluent which provides greater viscosity reduction to the froth and further provides efficient settling of the froth at elevated temperatures, an advantage heretofore not disclosed in the art. This greater viscosity reduction comes about because at constant pressure, a higher temperature can be employed with a higher boiling diluent than with a lower boiling diluent. The higher the temperature to which the diluent-bitumen mixture is heated, the lower its viscosity.

Thus, one embodiment of the present invention comprises a method wherein a bituminous froth product is substantially improved by removal of mineral matter and water comprising the steps of:

a. combining a raw bitumen froth product containing substantial quantities of mineral matter and water with a hydrocarbon diluent boiling in the range of 350.degree.-750.degree.F.;

b. thereafter heating said diluted froth to a temperature in the range of 300.degree.-1000.degree.F.;

c. combining said heated diluted froth product with centrifugal froth herinafter recovered via step f;

d. settling said combined diluted froths in an autoclave settling zone at a temperature in the range of 300.degree.-1000.degree.F. at a pressure in the range of 0- 1000 psig to provide an upper bitumen froth layer, a middle water layer containing minor quantities of bitumen and mineral matter, and a lower tailings layer comprised substantially of mineral matter with minor quantities of bitumen;

e. transferring the tailings layer to centrifuging step;

f. centrifuging said tailings in a centrifuge to provide a centrifugal froth product which is thereafter recycled and combined with said diluted froth feed; and

g. recovering from the settling zone a froth product having substantially reduced mineral matter and water.

The liquid hydrocarbon diluent suitable for use in the method of the present invention can generally be defined as a petroleum naphtha containing aromatics, saturates, and olefins boiling in the range of 350.degree.-750.degree.F.

Claims

The invention claimed is:

1. A method for removing mineral matter and water from bituminous froth containing bitumen, mineral matter, and water which comprises:

a. combining a raw bituminous froth containing substantial quantities of mineral matter and water with a hydrocarbon diluent boiling in the range of 350.degree.-750.degree.F.;

b. heating said diluted froth from step (a) to a temperature in the range of 300.degree.-1000.degree.F.;

c. combining said heated diluted froth with centrifugal froth hereinafter recovered via step (f);

d. settling said combined diluted froth from step (c) in an autoclave settling zone at a temperature in the range of 300.degree.-1000.degree.F. at a pressure in the range of 0- 1000 psig to provide an upper bitumen froth layer, a middle water layer containing minor quantities of bitumen and mineral matter, and a lower tailings layer comprised substantially of mineral matter with a minor quantity of bitumen;

e. combining said lower tailings layer with a liquid hydrocarbon diluent boiling in the range of 350.degree.-750.degree.F. and transferring the diluted tailings to a centrifuging step;

f. centrifuging said tailings to provide a centrifugal froth which is thereafter recycled and combined with said heated diluted froth in step (c); and

g. recovering from the settling zone of step (d) said bitumen froth as product having substantially reduced mineral matter and water.

2. A method according to claim 1 wherein the ratio of said diluent to bitumen in step (a) is in the range of 0.3:1 to 1:1.

3. A method according to claim 1 wherein the diluent to tailings ratio of step (e) is in the range of 1 to 20 volumes of said diluent per volume of bitumen in the tailings.