Patent Application
20230322569
Alkali bicarbonates and carbonates can be prepared by passing carbon dioxide through an aqueous solution of alkali hydroxide. A useful source of carbon dioxide is found in industrial CO2-bearing gas streams. For example, carbon dioxide emitted by fossil-fired power plants, or present in other industrial gas streams, can be captured by scrubbing the gas with an aqueous solution of alkali hydroxide. This could serve a twofold purpose: reduction of CO2 emission to the atmosphere and its direct, on-site conversion to commercially useful chemical reagents. For example, sodium bicarbonate (NaHCO3) is used in baking powders (it is also known as “baking soda”) and in the textile, paper and ceramics industries.
Various scrubber designs have been proposed to achieve the goal of CO2 capture from flue gases by alkali hydroxide solution, and the recovery of the carbonate/bicarbonate products. The conventional approach is based on a countercurrent packed column, i.e., an alkaline solution is sprayed from the top of a column filled with a packing material to become mixed with an incoming CO2-bearing gas flowing upward, as shown by Salmon et al. [Applied Science 2018, 8, 996], where the performance of a packed column was compared to that achieved by a fibrous membrane contactor. Other designs can be found in WO 2011/129707 and in U.S. Pat. No. 7,727,374; the latter specifically mentions CO2/NaOH bubble column designs. Shim et al. [Environmental Engineering Research, 2016; 21(3):297-303] reported that production of sodium bicarbonate in a bubble reactor had met with difficulties because the micro-sized holes of the sparger used to produce the bubbles were clogged by crystals of sodium carbonate, which precipitated out of the solution.
In co-assigned WO 2018/002710, it was proposed to remove target gases from air, mainly carbon monoxide arising in case of fire, with the aid of aqueous alkali hydroxide/hydrogen peroxide (MOH/H2O2 reagent; M=alkali metal, e.g., Na or K), using gas/liquid contact pattern based on creation of bubbles or microbubbles. The rationale behind this approach is that decomposition of hydrogen peroxide in a strongly alkaline solution, when controlled to adhere to certain stoichiometry, i.e., 2:3 molar ratio (MOH:H2O2) at appropriate pH, could result in very strong oxidation properties, owing to formation of the superoxide radical anion (O2—⋅). The generation of the superoxide was previously reported in a series of publications (WO 2013/093903; Stoin et al. ChemPhysChem, 2013, 14, 4158; and WO 2015/170317), where it was shown that the aqueous MOH/H2O2 reagent is useful in treating different pollutants, such as absorption of carbon dioxide from flue gases.
We have now studied the production of sodium carbonates/bicarbonates through the capture of CO2 by an alkaline solution, where the gas is brought into contact with the solution in the form of microbubbles. One factor which needs to be taken into consideration is the different solubility in water of the two salts: sodium carbonate is considerably less soluble in water than sodium bicarbonate (the same is true for the corresponding potassium salts). Because of this difference in solubility, predominance of sodium carbonate in the product may impair the efficiency of the process, due to clogging of the holes in the sparger generating the microbubbles, following premature crystallization of sodium carbonate. A similar problem was reported by Shim et al (supra) in their experimental set-up; the authors recommended to switch to a different design, incorporating a packed column.
Experimental work conducted in support of this invention indicates that controlled addition of hydrogen peroxide to an alkaline solution operating to capture CO2 in a microbubbles/liquid contact pattern, improves the conversion of CO2 into the salt products. In the presence of a suitable amount of added H2O2, high conversion rates of CO2 were measured over long time periods, suppressing the crystallization of sodium carbonate, presumably by shifting the chemistry in favor the formation of sodium bicarbonate.
As shown by the experimental results reported below, in the absence of hydrogen peroxide, acceptable CO2 conversion rates (e.g., 80-90%) by the alkaline solution are developed quite slowly and are difficult to maintain over time. On the other hand, addition of a high amount of H2O2, to promote the stoichiometry recommended in WO 2013/093903, i.e., the 2:3 molar ratio (MOH:H2O2) r was shown to achieve high CO2 conversion very rapidly, but the chemical absorption of CO2 by the alkaline solution could not have been maintained over long time period. CO2 capture by the alkaline solution ceased abruptly, apparently due to sodium carbonate exceeding its solubility limit in the solution, leading to crystallization and clogging of the pores in the membrane which served to generate the microbubbles. That is, a 2:3 molar ratio (MOH:H2O2) advances the formation of Na2CO3—the less favorable product when a contact pattern based on microbubbles/liquid is used, as suggested by the chemical reaction represented by equation (1):
2NaOH+3H2O2+CO2→Na2CO3+4H2O+1.5O2 (1)
However, we have found that moderate supply of H2O2 to the alkaline solution, in close proximity to the orifices of the sparging device creating the bubbles, led to very good CO2 conversion rates which lasted over long test periods, with preferential formation of the bicarbonate salt. Without wishing to be bound by theory, it is believed that under slow addition of H2O2 to the alkaline solution, there is a sub-stoichiometric amount of H2O2 available to participate in the conversion of CO2 [by “sub-stoichiometric amount” is meant less than the 2:3 molar ratio (MOH:H2O2) of equation (1)]. Under these condition, the reaction which preferentially takes place in the vicinity of the orifices of the sparging device is a bicarbonate formation reaction, e.g.:
NaOH+nH2O2+CO2→NaHCO3+nH2O+½nO2(0.1≤n≤0.4) (2)
rather than the carbonate formation reaction, thereby minimizing, or delaying, blockage of the orifices of the sparging device by carbonate crystals.
Accordingly, the invention is primarily directed to a process for preparing alkali carbonate and/or bicarbonate salts, comprising:
The chemical absorption of CO2 by the alkaline solution to produce the bicarbonate/carbonate salts takes place in a reactor designed as a gas-liquid contactor (hereinafter the terms “reactor” and “gas-liquid contactor” are used interchangeably). Before it enters the gal-liquid contactor, the NOx and SOx-free flue gas is passed through a heat exchanger (to cool the gas stream to about 40-50° C.). The heat released by the flue gas can be recovered using a stream of fresh air, which may be guided to serve downstream operations, i.e., the separation of the bicarbonate/carbonate salts from the reaction mixture as described below. Water vapors present in the flue gas undergo condensation; water droplets can be removed from the two-phase fluid flow in a gas-liquid separator, e.g., horizontal or vertical knockout drum. The gas stream is now directed to the gas-liquid contactor, where the salts formation reactions occurs.
Regarding the aqueous MOH solution (e.g., NaOH or KOH), the concentration of the solution fed to the gas/liquid contactor is >10% by weight and up to saturation (48%), e.g. in the range of 25 to 35%, usually ˜30% by weight. The aqueous solution can be prepared on-site, either by dilution of commercial saturated solutions with water, or by dissolution of solid alkali hydroxide (available in the marketplace in the form of granules, pellets or flakes) in water. To enable continuous feed of the MOH solution to the reactor, one (or more) MOH production units may be provided. Each MOH production unit consists of a mixing tank in which solid MOH and water (i.e., fresh water and water recovered from downstream operations) are mixed and a pump to deliver the MOH solution to the gas-liquid contactor. Suitable pumps are made of stainless steel, and operate, for example, at capacity in the range of 10 to 120 m3/hour, or any other range, to meet the demand of the plant emitting the CO2 gas. Such a set-up, consisting of alternately operating MOH production and supply units, enables one unit to be active in supplying a pre-prepared MOH solution as a feed stream to the gas-liquid contactor (MOH charge phase), and the other to produce and store MOH solution (MOH production phase), with the bicarbonate/carbonate preparation process of the invention switching from one unit to another. Because the dissolution reaction of MOH in water is exothermic, it is beneficial to have each MOH production unit equipped with a recirculation line, such that MOH solution withdrawn from the mixing tank is passed through a cooler, whereby heat generated during the dissolution reaction is released from the MOH solution and absorbed by a chilled water stream, with the cooled MOH stream returning to the mixing tank.
Regarding the hydrogen peroxide, it is supplied to the reactor in an aqueous form, at a concentration of—for example—from 4 to 50%, e.g., 10 to 50%, e.g., 30 to 40% by weight, e.g., by using a commercial 35% by weight grade, or by diluting H2O2 grades of higher strengths (50-70% by weight).
Turning now to the design of the gas-liquid contactor, it should be noted that the sparging device mounted in the gas-liquid contactor, to convert the incoming CO2-containing streams into microbubbles, may be configured in different geometries, e.g., flat geometry (perforated plates, horizontally positioned membranes) and tubular configuration.
However, one preferred process design is based on forcing incoming CO2-containing gas stream through an array of tunnel-shaped sparging units submerged in the gas-liquid contactor below the surface level of the aqueous alkali hydroxide solution, wherein a sparging unit is bounded, at least in part, by a curved surface. For example, a sparging unit operative in the process of the invention may be in the shape of a semi-cylinder. The orifices of the sparging unit are distributed on its curved surface, as explained below. An important property of the sparging device is that the size of orifices, through which the air/CO2 gas mixture is forced to create the microbubbles, is less than 700 μm, e.g., less than 500 μm, e.g., diameter ranging from 50 to 300 μm, e.g., 100 to 200 μm. H2O2 addition to the gas-liquid contactor preferably takes place by injecting a plurality of individual H2O2 streams in proximity to the orifices provided on the curved surface of the tunnel-shaped sparging units. This can be achieved with the aid of pipes positioned adjacent to, and parallel with, the sparging units, as described below.
An illustrative gas-liquid contactor, with the characteristics set forth above, suitable for use in the preparation of bicarbonate/carbonate salts by the process of the invention, is described in detail in reference to
A blower/fun (not shown), e.g., operating to generate gas flow of 1000-7000 m3/hour, draws the air/CO2 stream into the reactor. A gas manifold (14, 141, 142, . . . 14n; e.g., 1≤n≤8) is installed in a lateral face (20a) of the gas-liquid contactor (10), dividing the incoming gas stream flowing via conduit of 30-100 cm in diameter (14) into subsidiary streams which enter reactor (10) via pipes (141, 142, . . . 14n). A purified (CO2-free) air stream is released through gas outlet opening (18), which is centrally positioned in the top section (16) of the reactor (the design illustrated for top section (16), consisting of inclined trapezoidal plates 16a and 16b and inclined triangular plates (16c, 16d), creating a sloping roof design, is not mandatory)).
The aqueous alkali hydroxide solution is fed to the gas-liquid contactor (10) by pipe (24) through one of the lateral sides of the gas-liquid contactor (e.g., the same side (20a) of the reactor, through which the incoming air/CO2 gas stream enters). Pipe (24) is installed in parallel to lateral side (20a) such that the aqueous alkali hydroxide solution flows into the reactor via evenly spaced openings shown in the corresponding internal wall in
The aqueous hydrogen peroxide is fed to the reactor (10) from the opposite lateral side (20c), with the aid of a manifold (26) splitting the main H2O2 feed solution into a plurality of individual stream which enter the reactor and flow across an array of pipes disposed in the gas-liquid contactor (shown in
Numeral (21) indicates a discharge pipe, through which the reaction mixture is continuously discharged (and treated to recover the crystalline bicarbonate/carbonate salts, as shown in
The interior of the gas/liquid contactor, with the unique arrays of sparging units and H2O2 pipes installed therein, is now described in detail in reference to
Tunnel-shaped sparging units (281), (282), . . . , (28n) are placed horizontally and parallel to each other at the bottom of gas/liquid contactor (10), below the surface level of the alkaline solution (marked by a dashed line (32). The distance between adjacent sparging units is from 5 to 50 mm, e.g., 10 30 mm. For example, the cross-section of a tunnel-shaped sparging unit corresponds to a circle or a segment of a circle with radius in the range from 10 mm to 300 mm i.e., a part of a circle bounded by an arc and its chord, wherein the corresponding central angle α that lies on the chord is preferably in the range from 90° a 360°. For Example, the tunnel-shaped sparging unit has the shape of a semi-cylinder (α=1800). In another example, the tunnel-shaped sparging unit has a tubular shape (full circle, α=360°).
In the specific design shown in
Orifices (29) are distributed densely along the length of the curved surface of the tunnel-shaped sparging unit, preferably in a uniform manner, e.g., creating a pattern consisting of orifices arranged in transverse rows along the length of the curved surface, or more precisely, in arcs (36) (owing to the curvatures of the surface). Adjacent arcs are spaced 0.5 to 10 mm apart. Each arc consists of a plurality of orifices; the center-to-center distance between adjacent orifices is from 0.5 mm to 5 mm. As pointed out above, the diameter of the orifice is between 50 and 700 μm, to ensure the creation of microbubbles of a desired size within the aqueous solution.
The semi-cylindrical or cylindrical sparging units are optionally rotatable along their axial axis, to create shear forces acting on the bubbles evolving through the orifices, to reduce their size, and expel salt crystals from the vicinity of the orifices to the bulk solution.
Manifold (26) comprises a plurality of H2O2 tubes (261, 262, . . . , 26m; m≥n). At least one peroxide tube (26i) is disposed in a space between two adjacent sparging units, that is, parallel to the sparging units. The H2O2 tubes and the sparging units are approximately equal in length, but the H2O2 tubes are smaller in diameter, e.g., H2O2 tube typically has a diameter of between 3 mm and 30 mm. Nozzles (23) are disposed along the length of the H2O2 tube, having a diameter between 0.1 mm and 2 mm, with the nozzle tips being directed towards one of the two adjacent sparging units. The nozzles are preferably evenly spaced along the H2O2 tube, the distance between adjacent nozzles being, for example, between 1 cm and 10 cm. To ensure sufficient flushing of the surface (35), each of the nozzles injects a pressurized stream at a small inclined angle of 1 deg to 90 deg relative to the horizontal. It has been found that a supply of continuous pressurized H2O2 streams directed towards each of the sparging units (28) minimizes the accumulation of sodium carbonate crystals, which over time clog the orifices (29) of the sparging units. The sparging units (28) and H2O2 tubes (26) are made of stainless steel.
Another aspect of the invention is a gas-liquid contactor, comprising:
On industrial scale, the flow rate of the alkali hydroxide solution fed continuously to the gas-liquid contactor is from 10 to 120 m3/hour. As pointed out above, the flow rate of the H2O2 supplied to the gas-liquid contactor is adjusted, e.g., within the range of 1 to 20 m3/hour such that carbonate formation is minimized and bicarbonate formation is maximized, e.g., the molar concentration of the bicarbonate in the product recovered is not less than 70%, e.g. >90%, >95%, >99%.
The feed rate of aqueous H2O2 may be determined by trial and error, based on the load of CO2 in the incoming gas stream, the concentration and purity of the alkali hydroxide solution and the specific design of the sparging units and H2O2 tubes, reaction and outside temperature, to achieve a steady flow of a pumpable slurry discharged from the gas-liquid contactor, with CO2 conversion rates, on industrial scale, >80%-90%. Experimental work conducted in support of this invention in the lab suggests that an appropriate feed rate of aqueous H2O2 to achieve prolonged salt formation reaction is one that reaches high CO2 conversion neither too slowly nor too rapidly. The flow rate of aqueous H2O2 may also be increased or decreased in response to analysis of the distribution of bicarbonate/carbonate salts in the product mixture.
The effluent which is continuously discharged from the reactor is an aqueous solution or more likely, a pumpable slurry. Isolation of the salts from the effluent consists of three major steps:
Another aspect of the invention is an apparatus for producing alkali bicarbonate/carbonate, to be placed at the vicinity of CO2-emitting plant (e.g., a power plant, an incineration plant and SMR plant), comprising:
The process of the invention utilizes different CO2-containing gases as a feedstock. For example, it may be used to convert CO2 produced by steam methane reforming (SMR) into carbonate/bicarbonate salts. SMR generates hydrogen and CO2 as by-product; so CO2 capture/removal technologies are integrated into SMR. The invention is well suited for this purpose, i.e., to receive CO2-containing gas from a steam methane reforming plant, to benefit from the relatively large concentration (˜17%) of CO2 and absence of impurities in streams emitted by SMR. With such a feedstock, high purity grades of carbonates/bicarbonate salts can be obtained.
Another aspect of the invention is a multistep-step process that incorporates the MOH/H2O2 chemistry into the preparation of carbonate (e.g., Na2CO3) and alkali bicarbonate (e.g., NaHCO3) in succession, i.e., producing Na2CO3 and NaHCO3 in two separate reactors, designated in the description that follows Reactors A and B, respectively. The new process design offers high degree of flexibility as it can be manipulated to afford multiple valuable products, each of which is useful in its own right—Na2CO3, NaHCO3 and CO2—by exploiting their interconvertibility.
For example, the glass industry—a significant user of sodium carbonate—could benefit a lot from the proposed process design, as the gases emitted from the chimney stack of a glass-making plant could be treated to supply the Na2CO3 raw material consumed by the plant. That is, the invention enables the implementation of a “local” circular economy model in manufacturing sites of industries which use sodium carbonate in fuel-powered plants, such as the glass industry. Owing to the interconvertibility of the materials, each industry could decide on the desired proportions between the Na2CO3, NaHCO3 and CO2 products according to its needs, as shown in the flow charts of
Accordingly, the invention provides a process for preparing alkali carbonate and bicarbonate salts in sequence, comprising:
Each step involving CO2 capture from a gas stream by an aqueous solution of NaOH and/or Na2CO3 can be achieved with the aid of hydrogen peroxide, as previously described. It should be noted that the CO2-bearing gas supplied to the Na2CO3 formation reaction in Reactor A could be the high CO2 content gas emitted from the chimney, with the outgoing gas stream from Reactor A, with reduced CO2 content, moving on to Reactor B, as a feed in the NaHCO3 formation reaction. But reversing the order of gas feed is possible as well, namely, the high CO2 content gas emitted from the chimney is delivered to Reactor B, to be absorbed by alkali carbonate-containing aqueous solution/slurry and give the bicarbonate. In that case, the gas stream exiting Reactor B, with relatively low CO2 level, is supplied to Reactor A, where it is absorbed in NaOH solution.
One useful flow chart of the process is seen in
The outgoing gas stream which leaves Reactor A still carries significant amount of CO2, for example, from 2 to 5%. So the excess flue gas that has not been treated in Reactor A is fed to reactor B, to complete the removal of CO2 by transformation into bicarbonate, releasing CO2-free air to the atmosphere.
The liquid effluent discharged by Reactor A can be divided into two portions. One portion is treated to recover sodium carbonate in a dry form, to meet the demand in the production plant. Na2CO3 is separated, by water evaporation in a drier. As shown in
Reactor B is a gas-liquid contactor designed to advance conversion of the CO2 molecules into sodium bicarbonate in a selective manner over sodium carbonate. For this reason, Reactor B can be configured as previously described, scrubbing CO2 with fresh aqueous NaOH solution or even better, with the aqueous stream discharged from Reactor A, in the presence of hydrogen peroxide, leading to formation of sodium bicarbonate according to reaction equation (2) that was discussed in great detail above:
NaOH+nH2O2+CO2→NaHCO3+nH2O+½nO2(0.1≤n≤0.4) (2)
The effluent discharged from Reactor B is an aqueous solution or more likely, a pumpable slurry which contains sodium bicarbonate in a dissolved/suspended form. Having separated the sodium bicarbonate by the techniques discussed above in reference to
2NaHCO3(a)→Na2CO3(s)+CO2(g)+H2O(g) (3)
It is seen that the thermal decomposition of sodium bicarbonate produces pure CO2 alongside sodium carbonate. So, following the bicarbonate generation step in Reactor B, the bicarbonate is passed through a thermal regeneration unit where it undergoes desorption at 100-200° C. to give back sodium carbonate and release high-purity CO2. Na2CO3 that is obtained is transferred to a dryer positioned downstream to Reactor A, combined with the Na2CO3 directly produced in Reactor A, to be recycled and reused in the glass production plant. As to the CO2, it may be compressed and utilized, for example by the food or refrigeration industries; returned to the gaseous feed of the process to minimize the amounts of unreacted sodium hydroxide; or disposed by injection into deep geological formations.
Another approach to benefit from the interconversion of Na2CO3, NaHCO3 and CO2 is seen in
As shown in detail
The outgoing gas stream is directed from Reactor B to Reactor A, carrying up to about 3% CO2 (namely, more than half of the CO2 loading was reduced as CO2 molecules were absorbed by the carbonate in Reactor B to form the bicarbonate). Reactor A operates akin to the description pertaining to
2NaOH+3H2O2+CO2→Na2CO3+4H2O+1.5O2 (1)
So, in Reactor A, CO2 molecules are absorbed in, and react with, the NaOH/H2O2 aqueous phase. The CO2-depleted gas stream (up to about −0.3% CO2 content) that leaves Reactor A could therefore be released to the atmosphere or recycled to the chimney, whereas the carbonate-containing liquid effluent discharged from Reactor A flows to Reactor B as previously described, to give the bicarbonate. The bicarbonate solution can be circulated through a heat exchanger at a temperature below 50° C. to minimize release of CO2. The bicarbonate is useful in its own right, or it can be thermally decomposed as previously described in reference to
In the drawings:
A set of experiments was conducted to study the effect of addition of H2O2 on the chemical absorption of CO2 from CO2/air stream bubbled through sodium hydroxide solution.
The experimental set-up is shown in
In each of the experiments, the reactor was charged with 250 ml aqueous NaOH (30% by weight solution), such that the liquid level in the reactor was 7 cm, i.e., the membrane (101) was submerged about 4.5 cm below the surface level of the solution.
The CO2 source was a commercial 100% CO2 held in a gas cylinder. Pumps made CO2 and air to flow into, and mix in, gas mixer (102) to create a mixed stream of 1000 ppm-CO2 bearing air, which was directed by pump (103) to reactor (101) at a flow rate of 13 L/min (gas inlet rotameter (104)).
Hydrogen peroxide solution (10% solution) is continuously added to reactor (100) at different flow rates (1 ml/h, 2 ml/h and 10 ml/h; reference experiment with no addition at all) using peristaltic pump “B”. H2O2 stream is fed below the surface level of the sodium hydroxide solution in the reactor, in proximity to membrane (101).
A pair of CO2 detectors (105in and 105out—BGA-EDG-MA, Emproco Ltd., Israel) connected to the incoming (1000 ppm-CO2 bearing air and outgoing (purified) streams (106 and 107, respectively) were used to measure the concentration of CO2, respectively.
During operation, the incoming air/CO2 mixed gas stream (106) entered reactor (100) through the bottom of the reactor and was forced to flow through the membrane (101) to create bubbles. CO2 was chemically absorbed by the sodium hydroxide medium. CO2 levels in the incoming and outgoing gas streams were recorded continuously over the test period.
The results are presented graphically in
It is seen that absent added H2O2, chemical absorption of CO2 occurs, at least to some extent, over time period of 36 hours. However, during the test period, conversion rates measured were not satisfactory. With the supply of H2O2, conversion rates of CO2 were improved significantly. Under addition of H2O2 at high flow rate (10 ml/h), a very efficient absorption (90%) is rapidly achieved, but cannot be maintained over long time periods (up to ˜27-28 hours). Moderate addition rates of H2O2 to the scrubbing system (at 1 ml/h and 2 ml/h) enable high conversion rates of CO2 by the sodium hydroxide medium over prolonged time periods (˜40 hours and 53 hours, respectively). The conditions and results of the experiments also presented in tabular form below.
| Number | Date | Country | |
|---|---|---|---|
| 63125553 | Dec 2020 | US | |
| 63354393 | Jun 2022 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/IL2021/051488 | Dec 2021 | US |
| Child | 18335092 | US |
