| Claims 1. A method for the purification of hydrogen sulfide-containing aqueous solutions, said method comprising passing the aqueous solution to be purified across at least two electrolytic cell elements (2, 3, 4), at least one of such cell elements employing a positive aluminum electrode and at least one other cell element employing a positive iron electrode, passing the aqueous solution to be purified, and reaction results obtained in the cell elements, into an extraction tower (7), and raising, by means of hydrogen released in elec- trolysis, the precipitate consisting of reaction results to a top end of the extraction tower (7) for a flock (15), and discharging purified water from the extraction tower, characterized in that the aqueous solution, which has been passed across the cell element equipped with an aluminum electrode, is passed across the cell element equipped with an iron electrode and only last- Iy into the extraction tower (7), that the cell elements' current is regulated, such that the cell element equipped with an aluminum electrode develops aluminum hydroxide and aluminum sulfate and the cell element equipped with an iron electrode develops iron hydroxide, and the precipitate consisting of aluminum hydroxide, aluminum sulfate and iron hydroxide is raised, by means of hydrogen released in electrolysis, to a top end of the extraction tower (7) for the flock (15), thus enabling the extraction of hydrogen sulfide in the form of aluminum sulfate from water into the flock (15). 2. A method as set forth in claim 1, characterized in that the aqueous solu- tion is passed across successively coupled cell elements directly from one cell element to the next. 3. A method as set forth in claim 1 or 2, characterized in that the electric current passing across each cell element (2, 3, 4) is regulated individually. 4. A method as set forth in claim 3, characterized in that the furthermost upstream and downstream cell elements are provided by cell elements equipped with an aluminum electrode, between which are provided one or more cell elements equipped with an iron electrode. 5. A method as set forth in any of claims 1-4, characterized in that the me- thod enables the removal of hydrogen sulfide from produced water created in oil or gas drilling. 6. An apparatus for the purification of hydrogen sulfide-containing aqueous solutions, said apparatus comprising an electrolytic cell which includes at least one cell element (2, 4) equipped with an aluminum electrode and at least one cell element (3) equipped with an iron electrode, as well as a water and electrolytically produced flock separating extraction tower (7) fitted with a purified water outlet pipe (9), characterized in that the aqueous solution to be purified has been passed from the cell element (2) equipped with an aluminum electrode directly into the cell element (3) equipped with an iron electrode, and that the extraction tower is in flow communication with the top end of a cell established by the successively coupled cell elements (2, 3). 7. An apparatus as set forth in claim 6, characterized in that the purified water outlet pipe (9) constitutes a communicating vessel with the extraction tower (7). 8. An apparatus as set forth in claim 7, characterized in that the purified water outlet pipe (9) is provided with a level adjustment (14), which enables regulating the moisture content of a flock (15) accumulating in a top end of the extraction tower (7). 9. An apparatus as set forth in claim 6, characterized in that the aluminum and iron electrodes are coupled to separate and individually controlled power sources. 10. An apparatus as set forth in any of claims 6-9, characterized in that the extraction tower is provided over a piece of its length with a mesh wall (8), which is surrounded by a purified water collecting pipe (16). |
Oil and gas drilling operations result in large quantities of produced water which contains considerable amounts of hydrogen sulfide dissolved in water. Given the toxicity of hydrogen sulfide, the treatment of produced water has become problematic. Hydrogen sulfide is a product of anaerobic microbial activity, resulting in the presence of hydrogen sulfide in many other water solutions as well, such as in landfill effluent.
US 2007/0029201 Al (Hannu L Suominen) discloses a method consistent with the preamble of claim 1, which enables providing oxidation-reduction reactions of desired types by regulating the current in cell elements. It is an object of the invention to further develop and enhance this prior known method for making it applicable to the purification of hydrogen sulfide-containing aqueous solutions. A specific application of the invention is the removal of hydrogen sulfide from produced water created in oil or gas drilling, while cleaning the produced water in other respects as well, thus enabling environmentally sound oil and gas drilling operations.
This object is achieved by the invention in the basis of method characterizing features presented in the appended claim 1. The object is also achieved by means of an apparatus presented in the appended claim 6. Preferred embodiments of the invention are presented in the dependent claims.
One working example of the invention will now be described more closely with reference to the accompanying drawings, in which Fig. 1 shows an apparatus according to one embodiment of the invention in a schematic vertical section, said apparatus enabling a method of the invention to be implemented, and Fig. 2 shows an apparatus according to a second embodiment of the invention.
Figs. 1 and 2 only differ from each other in terms of top sections thereof, i.e. in terms of an extraction tower for separating water from flock produced as a result of electrolysis. Reference is first made to features common to both figures in a special application of the invention.
Produced water from oil or gas drilling is delivered by a pump 12 through a pipe 6 and a valve into a cell, the latter including a first upstream cell ele- ment, wherein an inner electrode 1 is surrounded by a tubular aluminum electrode 2. The next is a tubular iron electrode 3 and the last one is a tubular aluminum electrode 4. The electrodes are separated from each other by a dielectric, and the electric current passing across each cell element 2, 3, 4 is regulated individually, i.e. the electrodes are connected to separate and indi- vidually controlled power sources 11. The inner electrode 1 can be common to all cell elements, or it may be provided with a different coating at different cell elements 2, 3, 4. What is essential is that the negative inner electrode 1 is more electronegative than the positive outer electrode, which can be for example an aluminum and iron electrode. The inner electrode can be made of stainless steel, nickel, chromium, platinum, or precious metal alloys with a major difference in electronegativity with respect to the outer electrode. The power sources 11 have their positive poles coupled to the outer electrodes and their negative pole to the inner electrode. The aqueous solution to be cleaned is passed across an annular electrolysis space 5 of the successive electrolytic cell elements 2, 3, 4, and currents in the cell elements are regulated in such a way that the cell element 2 equipped with an aluminum electrode produces aluminum hydroxide and aluminum sulfate and the cell ele- merit 3 equipped with an iron electrode produces iron hydroxide. Since the aluminum electrode 2 precedes the iron electrode 3, the sulfur of hydrogen sulfide is obtained in aluminum sulfate, thus avoiding the problem of the cell element with an iron electrode producing iron sulfide, which upon precipita- tion does not turn into a solid matter that would discharge along with the flock. The aluminum hydroxide, evolving in the cell element 2 equipped with an aluminum electrode, works as a molecular sieve, and the iron hydroxide, evolving in the cell element 3 equipped with an iron electrode, works also as a molecular sieve which is nevertheless denser than the molecular sieve pro- vided by aluminum hydroxide. The precipitate produced jointly by aluminum hydroxide and iron hydroxide is well capable of binding those very small hydrogen bubbles created in the reactions, and at the same time it also bonds the aluminum sulfate to the developing flock, which is raised upward by hydrogen bubbles evolving on the negative electrode. The flock, which is gen- erated jointly by the developing precipitate and the hydrogen bubbles, has a specific gravity which is slightly less than that of water, whereby the flock rises within a water and flock separating tower 7 at a faster rate than the flow of water proceeding therein. Prior to the rise of precipitate or flock up into the extraction tower 7, it is preferred that the final cell element be provided with one more aluminum electrode 4 capable of removing dissolved residual iron from the water. The iron ion Fβ 3 , evolving in the iron cell element 3, is sufficiently large for becoming trapped in an aluminum hydroxide mesh.
The aqueous solution to be purified, and the reaction results which had evolved as described in electrolytic cell elements and precipitated into a solid matter, are passed into the extraction tower 7 and rise therein by the action of hydrogen released in electrolysis, accumulating for a flock 15 in a top end of the extraction tower 7, the flock going out by way of an outlet pipe 10. In the working example of fig. 1, a purified water outlet pipe 9 connects by way of an upright pipe branch 9a to a bottom part of the extraction tower 7 below the top end of a supply pipe 7a. The level of the pipe branch 9 is adjustable, typically within a formation region of the flock 15, to a position not higher than the level of the pipe branch 10. This adjustment can be used as a means of having an impact on the moisture content of outgoing flock.
Reference numerals 17, 18 and 19 stand for washing lines, by way of which the apparatus is cleaned at prescribed intervals.
In fig. 2, the extraction tower 7 is provided over a portion of its length with a mesh wall 8, which is surrounded by a purified water collecting pipe 16. Connected to the pipe 16, by way of a pipe branch 9a, is a purified water outlet pipe 9, which is provided with a level adjusting means useful for regu- lating the moisture content of a flock 15 accumulating in a top end of the extraction tower 7. Hence, the extraction tower 7 and the pipes 16 and 9 establish a communicating vessel by way of the mesh 8, whereby, as the level of the pipe 9 is lowered, the water level in the tower 7 is also lowered and the flock 15 present at a flock outlet pipe 10 becomes drier. The mois- ture content of flock 15 can be adjusted so as to enable its flow by itself out of the outlet pipe 10 without requiring a separate scraper. The mesh wall 8 only functions as a flow guide, not as a sieve.
The proposed method and apparatus enable the extraction of hydrogen sul- fide from water in the form of aluminum sulfate evolving in the flock 15. Along with hydrogen sulfide, many other impurities can be extracted from produced water into the flock 15, making the purified water returnable to nature or reusable in the process of oil or gas production. Described above is just one cell, along with an extraction tower therefor. A desired number of such cells, along with extraction towers therefor, can be set in a parallel relationship, whereby a valve 13 can be used for closing or opening the number of side-by-side purification units consistent with the amount of produced water. The positive electrodes in the cells of side-by- side purification units can be connected in parallel from a single power source 11.
The annular aluminum and iron electrodes 2, 3, 4 may have an inner diameter varying typically within the range of 50-100 mm, a length which can be about 250 mm, and a wall thickness of about 10 mm. The extraction tower 7 has a length of even several tenfold with respect to its inner diameter.
Hence, the evolving flock 15 and the purified water have enough time to separate from each other.
The current and voltage values for cell elements, capable of providing desired reactions, depend on multiple aspects such as the conductance of a solution, its flow rate across the cell, and the electronegativity difference between solution components and anode. The conductance of a solution can be regulated with additives and the flow rate can be adjusted as desired. Because of its participation in the reaction, the solution has its conductance changing during the reaction. These reasons necessitate that optimal current and voltage values be found experimentally for each application. Typically, the operation proceeds within a current range of 10-100 A and a voltage range of 100-10 V. In one application, the voltage can be 10 V and the current 40-50 A. In another application, the voltage can be 50 V and the current 20-30 A. In addition, each cell element is different in terms of its voltage and current in order to enable each cell element to have an optimal input in its own reaction. Consequently, this requires plenty of trial runs with various current and voltage values before finding proper current and voltage values for each application. This notwithstanding, a skilled artisan will be able to employ the invention, because the practice of trial runs and tabulation of values and measuring results is routine work. The extraction tower 7 is preferably made transparent at least over a piece of its length in order to enable conducting a rough adjustment of current and voltage values by monitoring the formation of flock. An objective is the formation of sharply defined flock, with clear liquid remaining therebelow.