Carbon Dioxide Capture And Conversion Methods And Systems

Abstract

The present invention provides a method of mineralisation of carbon dioxide. The method comprises forming an alkaline in aqueous solution containing carbonate anions by dissolving the carbon dioxide and an alkali such as ammonia in water. Next, the method comprises mixing the alkaline aqueous solution with a water source (such as a connate/formation brine or produced water or industrial waste waters or re-constituted mineral-bearing waters) containing magnesium and calcium cations. A first product (e.g. PCC) containing calcium cations and carbonate anions is precipitated in a first precipitation step at a first pH (e.g. around pH7.5) and then a second product (e.g. nesquehonite (NQ) a type of PMC) containing magnesium cations and carbonate anions is precipitated in a second precipitation step at a second, higher pH e.g. around pH 9.5.

Claims

1. A method of mineralisation of carbon dioxide, the method comprising: forming an alkaline aqueous solution containing carbonate anions by dissolving the carbon dioxide and an alkali in water; mixing the alkaline aqueous solution with a water source containing magnesium and calcium cations; selectively precipitating a first product containing calcium cations and carbonate anions in a first precipitation step at a first pH; and then selectively precipitating a second product containing magnesium cations and carbonate anions in a second precipitation step at a second pH, wherein the second pH is higher than the first pH.

2. A method according to claim 1 wherein the water source is a formation/connate brine, a produced water brine or another industrial waste brine.

3. A method according to claim 1 wherein the water source/brine contains a higher concentration of calcium ions than magnesium ions.

4. A method according to claim 1 wherein the carbon dioxide is derived from an effluent gas from an industrial plant.

5. A method according to claim 1 wherein the alkali for forming the alkaline aqueous solution is selected from sodium hydroxide, potassium hydroxide, calcium hydroxide, Clinker Kiln Dust (CKD), Lime Sludge (LS) or ammonia.

6. A method according to claim 1 wherein the first precipitation step is carried out at a first pH of 8.5.

7. A method according to claim 1 wherein the first product comprises precipitated calcium carbonate (PCC).

8. A method according to claim 7 wherein the first product is selected from calcite, aragonite or calcium carbonate monohydrate.

9. A method according to claim 8 wherein the first product is aragonite and the first precipitation step is carried out at a temperature greater than or equal to 40 C. and a pH of between 7 and 7.5.

10. A method according to claim 8 wherein the first product is calcite and the first precipitation step is carried out at ambient temperature and a pH of between 7 and 7.5.

11. A method according to claim 8 wherein the first product is monohydrocalcite and the first precipitation step is carried out at ambient temperature and at a pH of 8pH>7.5.

12. A method according to claim 1 wherein the second precipitation step is carried out at a second pH of >8.5.

13. A method according to claim 1 where the second product is nesquehonite.

14. A method according to claim 13 wherein the second precipitation step is carried out at a temperature equal to or less than 25 C.

15. A method according to claim 1 further comprising at least one intermediate precipitation step for precipitating at least one intermediate product comprising calcium and/or magnesium cations and carbonate anions at a pH between the first pH and the second pH.

16. A method according to claim 15 wherein the first product is aragonite or calcite, the second product is nesquehonite and the intermediate product is monohydrocalcite.

17. A method according to claim 1 wherein the alkali is ammonia and wherein the method further comprises an ammonia recovery step for recovering ammonia from a second supernatant liquid remaining after the second precipitation step.

18. A method according to claim 1 further comprising activation of the second product by heating.

19. A system for mineral sequestration of carbon dioxide, the system comprising: a carbon dioxide inlet; an absorbing stage connected to the carbon dioxide inlet for absorbing carbon dioxide in water containing an alkali to form an alkaline aqueous solution containing carbonate anions; a water source inlet for providing a water source containing magnesium and calcium ions; and a precipitation stage connected to the water source inlet for mixing the alkaline aqueous solution containing carbonate anions with the water source, selectively precipitating a first product containing calcium cations and carbonate anions in a first precipitation step at a first pH, and selectively precipitating a second product containing magnesium cations and carbonate anions in a second precipitation step at a second, higher pH.

20. A system according to claim 19 wherein the carbon dioxide inlet is connected to a waste gas feed providing an effluent gas from an industrial plant.

21. A system according to claim 19 further comprising a gas treatment stage comprising a carbon dioxide scrubber and/or a particulate matter (PM) filter and/or a thermal heat exchanger.

22. A system according to claim 19 wherein the absorbing stage is adapted to bubble the carbon dioxide through the alkaline aqueous solution or to spray the alkaline aqueous solution into/across a flow path of the carbon dioxide.

23. A system according to claim 19 any nc wherein the water source inlet is connected to a water source comprising formation/connate brine or produced water brine.

24. A system according to claim 23 further comprising a produced water treatment stage comprising one or more of a deoiling stage, a soluble organics removal stage, a solids removal stage and/or a dissolved gas removal stage.

25. A system according to claim 19 wherein the precipitation stage comprises a first precipitation stage for selectively precipitating the first product in a first precipitation step at a first pH of 8.5.

26. A system according to claim 25 wherein the first precipitation stage is adapted to selectively precipitate the first product at a temperature between ambient temperature and 85 C.

27. A system according to claim 19 wherein the precipitation stage comprises a second precipitation stage for selectively precipitating the second (magnesium) product in a second precipitation step at a second pH of 9.

28. A system according to claim 27 wherein the second precipitation stage is adapted to selectively precipitate the second product at a temperature less than or equal to 40 C.

29. A system according to claim 19 wherein the alkali feed is connected to a supply of gaseous or aqueous ammonia.

30. A system according to claim 19 further comprising at least one intermediate precipitation stage for the precipitation of at least one intermediate product comprising calcium and/or magnesium cations and carbonate anions.

31. A system according to claim 19 further comprising an alkali recycling stage for recycling a calcium and magnesium ion-depleted second supernatant liquid to the precipitation stage or to the absorbing stage.

32. A system according to claim 31 wherein the alkali recycling stage is adapted to recover ammonia from the second supernatant liquid.

33. A system according to claim 19 further comprising an activation stage for activation of the second product by heating.

34. An oil/gas field comprising a system according to claim 19 wherein the water source is provided from a produced water separation and treatment plant connected to an oil/gas well.

35. An oil/gas field according to claim 34 wherein the waste gas feed is connected to a power generating plant, a cement factory, a steel factory, or an oil refinery.

36. (canceled)

37. (canceled)

38. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0101] Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

[0102] FIG. 1 is a graph showing selective precipitation of calcium carbonate and nesquehonite at increasing pH;

[0103] FIG. 2 is a schematic diagram showing a first embodiment of the present invention;

[0104] FIG. 3 is a schematic diagram showing a second embodiment of the system of the present invention; and

[0105] FIG. 4 shows a schematic diagram showing a third embodiment of the present invention.

DETAILED DESCRIPTION AND FURTHER OPTIONAL FEATURES OF THE INVENTION

[0106] FIG. 1 shows the theoretical yield of the precipitated first (calcium) product (PCC) and the precipitated second (magnesium) product (nesquehonite) at a temperature of 25 C. using a brine having a Ca.sup.2+Mg.sup.2+ ratio of 60,000:9,000 ppm and a NaCl concentration of 1.2 mol/L.

[0107] In a first precipitation step at a pH of <7.5, precipitation of substantially 100% of the PCC occurs with negligible precipitation of the nesquehonite. Accordingly, the PCC can be separated from the first (calcium ion-depleted) supernatant liquid.

[0108] In a second precipitation step carried out on the first supernatant liquid at a pH of >9.5, precipitation of 80% of the nesquehonite occurs. Any calcium salts have been previously precipitated in the first precipitation step resulting in the precipitation of substantially pure nesquehonite in the second step. The nesquehonite can be separated from the second supernatant liquid.

[0109] FIGS. 2 and 3 shows a schematic representation of systems according to a first and second embodiment of the present invention.

[0110] The systems comprise a carbon dioxide inlet 1 which may be connected to a waste gas feed providing a waste gas such as an effluent gas from an industrial plant e.g. a cement factory.

[0111] The carbon dioxide inlet 1 may feed the effluent gas to a gas treatment stage 20 (shown in FIG. 3) comprising a carbon dioxide scrubber 17 and a particulate matter filter 18 to separate the carbon dioxide from the waste gas and to remove any particulates. Any oxygen and/or nitrogen from the waste gas feed are expelled from the systems via a gas outlet 3. A water recovering unit (not shown) may also be included in the gas treatment stage 20 for recovering water vapour from the waste gas. The recovered water may be used in the washing stage 13 described below.

[0112] The systems further comprise an alkali feed 2 connected to a source of gaseous or aqueous ammonia.

[0113] The systems further comprise an absorbing stage 7 connected to the carbon dioxide inlet 1 and the alkali feed 2. In the absorbing stage 7, ammonia from the alkali feed 2 is dissolved in water to provide an alkaline aqueous solution. Ammonia dissociates into ammonium ions (NH.sub.4.sup.+) and hydroxide anions (OH.sup.) upon dissolution in water.

[0114] The alkaline aqueous solution is then sprayed across a stream of the carbon dioxide at a temperature between 0-30 C. in order to dissolve the carbon dioxide to form an alkaline aqueous solution containing carbonate anions.

[0115] The carbon dioxide reacts with the hydroxide anions in the alkaline aqueous solution to form the alkaline aqueous solution containing carbonate (and bicarbonate) ions:

CO.sub.2(g)+2NH.sub.3(aq)+H2O(I).fwdarw.(NH.sub.4).sub.2CO.sub.3(aq) (5)

CO.sub.2(g)+NH.sub.3(aq)+H.sub.2O(I).fwdarw.(NH.sub.4)HCO.sub.3(aq) (6)

CO.sub.2(g)+(NH.sub.4).sub.2CO.sub.3+H.sub.2O(I).fwdarw.2NH.sub.4HCO.sub.3(aq) (7)

[0116] The absorbing stage 7 comprises a pH meter/controller 19 (shown in FIG. 3) for monitoring/controlling the pH of the alkaline aqueous solution and the alkaline aqueous solution containing carbonate anions.

[0117] The systems further comprise a water source (brine) inlet 4 for providing a brine containing magnesium and calcium ions. The water source (brine) inlet 4 is connected to a water source comprising formation/connate brine or produced water brine (obtained as a by-product of gas/oil extraction).

[0118] The brine contains a higher concentration of calcium ions than magnesium ions. The ratio of calcium ions to magnesium ions in the brine may be around 7:1.

[0119] The brine contains calcium ions (e.g. as calcium chloride) in a concentration range of between 2.5 to 100 g/L and magnesium ions (e.g. as magnesium chloride) in a concentration range of 2.5 to 50 g/L e.g. around 9 g/L.

[0120] The systems comprise a brine filtration and treatment stage 21 (shown in FIG. 3) for filtration/treatment of the brine. The filtration and treatment stage comprises: a de-oiling stage comprising a de-oiling cyclone; a soluble organics removal stage; a solids removal stage comprising a wellhead de-sanding cyclone; and a dissolved gas removal stage comprising a dissolved air flotation unit. The dissolved gases removed by the dissolved gas removal stage may be supplied to the carbon dioxide scrubber to extract carbon dioxide for introduction into the carbon dioxide inlet 1.

[0121] The systems further comprise a precipitation stage 8 which is connected to the water source inlet 4 and is also connected to an alkaline aqueous solution feed 10 extending between the absorbing stage 7 and the precipitation stage 8 for transferring the alkaline aqueous solution containing carbonate ions from the absorbing stage 7 to the precipitation stage 8 where it is mixed with the brine at a temperature of around 5 C. and ambient pressure.

[0122] The pH of the mixture is adjusted to a pH of around 7.5 using alkali from the alkali feed 2 and PCC (calcite) is precipitated at a temperature between 0-30 C. and ambient pressure in a first precipitation stage 8a according to the following reactions:

(NH.sub.4).sub.2CO.sub.3(aq)+CaCl.sub.2(aq).fwdarw.CaCO.sub.3(s)+2NH.sub.4Cl(aq) (12)

(NH.sub.4).sub.2CO.sub.3(aq)+CaCl.sub.2(aq)+xH.sub.2O(I).fwdarw.CaCO.sub.3.xH.sub.2O(s)+2NH.sub.4Cl aq) (13)

[0123] If aragonite precipitation is required, the temperature can be increased to 40 C. e.g. up to 85 C. or up to 60 C. The systems comprise a first product outlet line 9a connected between the first precipitation stage 8a and a first product separation/filtering stage 12a.

[0124] A supernatant liquid feed 11 is provided to allow the passage of calcium ion-depleted first supernatant liquid from the first precipitation stage 8a to the second precipitation stage 8b.

[0125] The pH of the first supernatant liquid is adjusted to a second, higher pH of around 9 in the second precipitation stage 8b (by the addition of alkali from the alkali feed 2) and nesquehonite is precipitated at 25 C. and ambient pressure in a second precipitation step according to the following reaction:

(NH.sub.4).sub.2CO.sub.3(aq)+MgCl.sub.2(aq)+3H.sub.2O(I).fwdarw.MgCO.sub.3.3H.sub.2O(s)+2NH.sub.4Cl(aq) (19a)

[0126] The systems comprise a second product outlet line 9b connected between the second precipitation stage 8b and a second product separation/filtering stage 12b.

[0127] The first and second product separation/filtering stages 12a, 12b are for separating/filtering the PCC and nesquehonite from the first and second supernatant liquids respectively.

[0128] The product separation/filtering stages 12a /12b each comprise a respective hydrocyclone separator (not shown) and a respective membrane filter (not shown) to separate the precipitated product(s) from the first/second supernatant liquids.

[0129] The systems further comprise a washing stage 13 for washing the PCC and nesquehonite (to remove chloride salts and alkali) after their separation/filtration from the first and second supernatant liquids respectively. The washing stage comprises a first washing stage 13a for washing the PCC after its separation/filtration from the first supernatant liquid and a second washing stage 13b for washing the nesquehonite after its separation/filtration from the second supernatant liquid.

[0130] Dilute brine resulting from the washing stage 13 is drained from the system at a brine outlet 6. The dilute brine can be safely re-injected into the ground.

[0131] The systems further comprise a drying stage 14 for drying the precipitated (and washed) PCC and nesquehonite. The drying stage comprises a first drying stage 14a for drying the PCC and a second drying stage 14b for drying the nesquehonite. Each drying stage includes a vacuum unit 22a, 22b and at least one steam drum 23a, 23b for drying the products.

[0132] The washed and dried products may be removed from the systems at product ports 5a, 5b.

[0133] The systems may further comprise an activation stage 15 for activation of the nesquehonite by heating.

[0134] The systems further comprise an alkali recycling stage 16 for recovering ammonia from the ammonium chloride-containing second supernatant liquid. The recovered ammonia can be directed to the alkali feed 2 and re-used in the absorbing stage 7. The ammonia recovery can comprise the following reactions:

(NH.sub.4).sub.2CO.sub.3(aq)+MgCl.sub.2(aq).fwdarw.MgCO.sub.3(s)+2NH.sub.3(g)2HCl(aq) (20)

(NH.sub.4).sub.2CO.sub.3(aq)+MgCl.sub.2(aq)+xH.sub.2O(I).fwdarw.MgCO.sub.3.xH.sub.2O(s)+2NH.sub.3+2HCl aq) (21)

(NH.sub.4)HCO.sub.3(aq)+MgCl.sub.2(aq)MgCO.sub.3(s)+NH.sub.3(g)+2HCl(aq) (22)

[0135] These reactions can be driven by heat e.g. by heat obtained from the waste gas. Therefore, the system further comprises a heat exchanger (not shown) for transferring heat from the waste gas to the alkali recovery stage 16.

[0136] The third embodiment shown in FIG. 4 is essentially the same as the first embodiment described above but an intermediate precipitation stage 8c is included to precipitate an intermediate product.

[0137] As discussed above, PCC (calcite) is precipitated in a first precipitation step within the first precipitation stage 8a at a pH of around 7.5.

[0138] The system comprises a first product outlet line 9a connected between the first precipitation stage 8a and the first product separation/filtering stage 12a.

[0139] A first supernatant liquid feed 11a is provided to allow the passage of first supernatant liquid from the first precipitation stage 8a to the intermediate precipitation stage 8c.

[0140] The pH of the first supernatant liquid is adjusted to the intermediate pH of around 8.5 in the intermediate precipitation stage 8c (by the addition of alkali from the alkali feed 2) and monohydrocalcite is precipitated at 0-30 C. and ambient pressure in an intermediate precipitation step.

[0141] The system comprises an intermediate product outlet line 9c connected between the intermediate precipitation stage 8c and an intermediate product separation/filtering stage 12c.

[0142] An intermediate supernatant liquid feed 11b is provided to allow the passage of intermediate supernatant liquid from the intermediate precipitation stage 8c to the second precipitation stage 8b.

[0143] The pH of the intermediate supernatant liquid is adjusted to the second pH of around 9 in the second precipitation stage 8b (by the addition of alkali from the alkali feed 2) and nesquehonite is precipitated at 25 C. and ambient pressure in a second precipitation step according to the following reaction:

(NH.sub.4).sub.2CO.sub.3(aq)+MgCl.sub.2(aq)+3H.sub.2O(I).fwdarw.MgCO.sub.3.3H.sub.2O(s)+2NH.sub.4Cl(aq) (19a)

[0144] The system comprises a second product outlet line 9b connected between the second precipitation stage 8b and the second product separation/filtering stage 12b.

[0145] The product separation/filtering stages 12a, 12b, 12c are for separating/filtering the PCC, monohydrocalcite and nesquehonite from the first, intermediate and second supernatant liquids respectively.

[0146] The product separation/filtering stages 12a /12b /12c each comprise a respective hydrocyclone separator (not shown) and a respective membrane filter (not shown) to separate the precipitated product(s) from the first/second supernatant liquids.

[0147] The system further comprises a washing stage 13 for washing the PCC and nesquehonite (to remove chloride salts and alkali) after their separation/filtration from the first and second supernatant liquids respectively. The washing stage comprises a first washing stage 13a for washing the PCC after its separation/filtration from the first supernatant liquid, an intermediate washing stage 13c for washing the monohydrocalcite after its separation/filtration from the intermediate supernatant liquid and a second washing stage 13b for washing the nesquehonite after its separation/filtration from the second supernatant liquid.

[0148] Dilute brine resulting from the washing stage 13 is drained from the system at a brine outlet 6. The dilute brine can be safely re-injected into the ground.

[0149] The systems further comprises a drying stage 14 for drying the precipitated (and washed) PCC, monohydrocalcite and nesquehonite. The drying stage comprises a first drying stage 14a for drying the PCC, an intermediate drying stage for drying the monohydrocalcite and a second drying stage 14b for drying the nesquehonite. Each drying stage includes a vacuum unit 22a, 22b and at least one steam drum 23a, 23b for drying the products.

[0150] The washed and dried products may be removed from the system at product ports 5.

[0151] The system may further comprise an activation stage 15 for activation of the nesquehonite by heating.

[0152] The system further comprises an alkali recycling stage 16 as described above.

[0153] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the scope of the invention.