A number of processes for recovery of bitumen from tar sands result in the formation of a hydrocarbon-water froth having an amount of finely divided solids dispersed therein. Typically, about 99 percent of the solids consists of quartz grains and clay minerals. The maximum sand grains size is about 1 mm diameter. About 99.9 percent of the mineral matter is finer than 100 microns (about -150 mesh).
One widespread tar sands treatment process is the so-called hot water extraction process. According to this process, a mined bitumen sand is sent to a conditioning drum. Caustic soda is added to adjust the pH to between about 7.5 to about 9.0. Steam is used to adjust the temperature to about 180.degree. to 190.degree. F. (82.degree. to 88.degree. C.) and make-up water is added to form a pulp having a solids content of about 70 percent. Oversized material is removed from this pulp by screening, and the screened pulp is sent to a flotation device. In the flotation device, the pulp is agitated to introduce air bubbles. Those components of the pulp which are least easily wetted are preferentially carried to the surface by the bubbles to form a froth. This froth is a fluid emulsion of water and hydrocarbons, such as bitumen. Non-hydrocarbon solids, such clay and sand, are typically dispersed in the fluid. The froth is separated from the bulk of the pulp. The so-called tar sands flotation froth which exits the flotation device typically contains about 40 to about 75 percent bitumen, about 10 to about 50 percent water and less than about 15 percent solids.
This froth is treated downstream by such processes as delayed or fluid coking, residual hydrocracking, or solvent deasphalting. In most cases, it is advantageous to decrease the water and/or solids content of the tar sands froth prior to such downstream processing.
A method for removing water and solids from tar sands froth which is commonly employed is centrifugation of the froth. Such methods are described in Evans et al. (Canadian Patent No. 918,091, 1973), Hall et al. (Canadian Patent No. 910,271, 1972) and Baillie (U.S. Pat. No. 3,900,389, 1975). Other hardware devices which have been proposed for solids removal include a hydrocyclone, as described by Given et al. (U.S. Pat. No. 3,338,814, 1967), an electrostatic desalter described by Anderson (U.S. Pat. No. 4,385,982, 1983) and an ultrasonic vibrator described by Jubenville (U.S. Pat. No. 4,358,373, 1982). One difficulty common to such hardware approaches is related to the fact that a solids-containing tar sands froth has a highly abrasive nature. Because of this, such hardware devices are relatively quickly rendered inoperable by attrition. Such devices are also relatively expensive to acquire, install and operate, particularly at field sites.
Other approaches to removal of water and solids from a tar sands froth have included chemical additions, ranging from a simple diluent addition such as that described by Nagey (U.S. Pat. No. 3,607,721, 1971), to more complicated chemical treatments such as those described by Canevari et al. (U.S. Pat. No. 3,331,765, 1967) and Moyer (U.S. Pat. No. 3,884,829, 1975). The cost of reagents has an effect on the economics of such processes. Even where some of the additives can be later recovered for recycle, a portion of the additives is typically degraded or otherwise lost to the recycle processes, particularly in processes which include treatment at elevated temperatures and/or pressure following addition of reagents. Furthermore, such processes involve a cost for transporting the additives to the treatment site which, in a tar sands froth application, is advantageously a field site.
A number of froth treatment processes involve the use of elevated temperatures or pressures during some portion of the treatment. Given et al. (U.S. Pat. No. 3,338,814, 1967) disclose a multi-step process for treating a bituminous emulsion, the first step of which involves a dehydration zone maintained at temperatures of from about 225.degree. F. to about 550.degree. F. (107.degree. C. to 288.degree. C.) and pressures of from about 4 psig to about 1000 psig in which vaporized water is removed from other constituents of the froth. Solids are separately removed downstream. May (U.S Pat. No. 2,864,502, 1958) discloses a multistage treatment for gas-oil-water emulsions including emulsion breaking under a pressure of 30 pounds.
Other heat/pressure treatment methods have been used to separate oil fractions in waste treatment processes. Cole et al. (U.S. Pat. No. 3,606,731, 1971) disclose that when the growth of algae in a water treatment facility or an API separator forms an algae-oil-water emulsion detrimental to water treatment processes, it is useful to coke the emulsion under autogenous pressure at elevated temperatures. In the feeds treated by Cole et al., the algae form an emulsifying agent. Cole et al. disclose heating the emulsion to coke the algae, thus substantially removing the emulsifying agent. Hess et al. (U.S. Pat. No. 3,716,474, 1973) disclose treating an oil-water sludge at a temperature of between about 750.degree. F. and 850.degree. F. (399.degree. C. to 454.degree. C.) at elevated pressures. In the examples disclosed in Hess et al., pressures of 3900 to 6150 psig were used. The Hess et al. process is directed to treatment of a sludge from a refinery disposal pit which typically contains emulsifying agents such as metallic salts and aromatic sulfonic acids. To remove metallic, particularly organometallic, contaminates, Hardy (U.S. Pat. No. 2,789,083, 1957) discloses treating a hydrocarbon oil, particularly gas oil or similar distillate oils, which involves subjecting an emulsion to a temperature above 500.degree. F. and a pressure of about 100 to 500 psig.
A common difficulty with previous froth treatment methods is the necessity for construction of elaborate and expensive apparatus for performing these processes. This necessity makes the processes particularly unattractive for application to tar sands recovery which is most economically conducted when sand and other solids are separated from bitumen before incurring the cost of transport to treatment facilities. Furthermore, in treating tar sands froths, such apparatus is susceptible to abrasion from solids. Methods which require addition of reagents have proven uneconomical for many applications and particularly where recycle of reagents is prevented because of thermal degradation.
Previous methods produce only slight, if any, increases in settling rates. These methods are accompanied by gravity settling which is typically extended in time, and often must be augmented with centrifugation.
Accordingly, it is an object of this invention to provide a process for separating hydrocarbons from a tar sands froth which can be practiced in the field.
It is also an object of this invention to provide a tar sands froth hydrocarbon separation process that involves minimal consumption of energy, reagents and equipment.
It is a further object of this invention to provide a process for treating a stream comprising a tar sands froth which results in a substantially instantaneous gravitation separation the hydrocarbon fraction from the treated stream.