PROCESS AND APPARATUS FOR PRODUCING ALKALI BICARBONATES AND ALKALI CARBONATES

Abstract

The invention relates to a process for preparing alkali carbonate/bicarbonate salts, comprising continuously feeding aqueous alkali hydroxide solution into a gas-liquid contactor; forcing incoming CO.sub.2-containing gas stream through a sparging device submerged in the gas-liquid contactor below the surface level of the aqueous alkali hydroxide solution, to generate bubbles and/or microbubbles; adding hydrogen peroxide in proximity to orifices of the sparging device, from which the bubbles and/or microbubbles evolve, wherein the supply of hydrogen peroxide is adjusted to decrease alkali carbonate formation and increase alkali bicarbonate formation; and continuously discharging an effluent from the gas-liquid contactor and recovering therefrom carbonate and bicarbonate alkali salts predominated by the bicarbonate component. A gas liquid-contactor and an apparatus are also provided by the invention.

Claims

1. A process for preparing alkali carbonate/bicarbonate salts, comprising: continuously feeding aqueous alkali hydroxide solution into a gas-liquid contactor; forcing incoming CO.sub.2-containing gas stream through a sparging device submerged in the gas-liquid contactor below the surface level of the aqueous alkali hydroxide solution, to generate bubbles and/or microbubbles; adding hydrogen peroxide in proximity to orifices of the sparging device, from which the bubbles and/or microbubbles evolve, wherein the supply of hydrogen peroxide is adjusted to decrease alkali carbonate formation and increase alkali bicarbonate formation; and continuously discharging an effluent from the gas-liquid contactor and recovering therefrom carbonate and bicarbonate alkali salts predominated by the bicarbonate component.

2. A process according to claim 1, wherein the incoming CO.sub.2-containing gas stream flows 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.

3. A process according to claim 2, wherein the orifices of the sparging unit are distributed on its curved surface and the diameter of the orifices is from 50 and 700 μm.

4. A process according to claim 3, wherein the addition of H.sub.2O.sub.2 to the gas-liquid contactor takes place by injecting a plurality of individual H.sub.2O.sub.2 streams in proximity to the orifices.

5. A process according to claim 1, wherein the concentration of the alkali hydroxide solution is from 10% by weight and up to saturation, the flow rate of the alkali hydroxide solution is from 10 to 120 m.sup.3/hour, the concentration of the hydrogen peroxide solution is from 4% to 50% by weight and the flow rate of the H.sub.2O.sub.2 solution is adjusted within the range of 1 to 20 m.sup.3/hour.

6. A process according to claim 5, wherein the molar concentration of the alkali bicarbonate in the product recovered is not less than 90%.

7. A process according to claim 1, wherein the flow rate of aqueous H.sub.2O.sub.2 is increased or decreased in response to analysis of the distribution of bicarbonate/carbonate salts in the product mixture.

8. A process according to claim 1, wherein the CO.sub.2-containing gas stream is from a chimney of a power plant, incineration unit or steam methane reforming (SMR) plant.

9. A gas-liquid contactor, comprising: a longitudinal horizontal housing bounded by a bottom surface, a top section and lateral faces; an array of tunnel-shaped sparging units (28.sub.1), (28.sub.2), . . . , (28.sub.n), placed horizontally and parallel to each other in the interior of the housing, wherein a sparging unit is bounded by an upward facing curved surface, with orifices distributed on said curved surface; an array of tubes (26.sub.1, 26.sub.2, . . . , 26.sub.m; m≥n), placed horizontally and parallel to each other in the interior of the housing, each tube is provided with nozzle tips arranged along its length, with at least one tube being disposed in a space between a pair of adjacent tunnel-shaped sparging units; a gas inlet manifold coupled to said array of tunnel-shaped sparging units, suitable for introducing individual gas streams into said tunnel-shaped sparging units; a gas outlet opening located in the top section, connected to a gas discharge line; a first liquid feed line configured to provide a liquid flow of an aqueous alkali hydroxide into said housing via one or more liquid inlet openings; a second liquid feed line configured to provide liquid flow of aqueous hydrogen peroxide across said array of tubes; and a discharge opening, to which an effluent discharge line is connected, to remove reaction product from the gas-liquid contactor.

10. A gas-liquid contactor according to claim 9, wherein the gas inlet manifold is installed externally to a first lateral face of the gas-liquid contactor; the aqueous alkali hydroxide inlet openings are located in the first lateral side of said housing and the second liquid feed line is installed in a second lateral side, opposite to said first lateral side.

11. A gas-liquid contactor according to claim 10, wherein the second feed line comprises a manifold splitting the main H.sub.2O.sub.2 feed solution into a plurality of individual stream which enter the gas-liquid contactor and flow across the array of tubes disposed in the gas/liquid contactor.

12. A gas-liquid contactor according to claim 1, wherein the orifices are distributed densely along the length of the curved surface of the tunnel-shaped sparging unit, creating a pattern consisting of orifices arranged in transverse arcs, wherein adjacent arcs are spaced 0.5 to 10 mm apart, each arc consisting of a plurality of orifices, with the center-to-center distance between adjacent orifices being from 0.5 mm to 5 mm.

13. A gas-liquid contactor according to claim 9, wherein the diameter of the orifice is between 50 and 700 μm.

14. An apparatus for producing alkali bicarbonate/carbonate, to be placed at the vicinity of a CO.sub.2-emitting plant such as a power plant, incineration plant and SMR plant, comprising: a gas-liquid contactor as defined in claim 9; a first blower forcing air/CO.sub.2 stream through said gas-liquid contactor; a set of pumps to deliver liquids and slurries; upstream processing units, including: one or more tanks accommodating an aqueous alkali hydroxide solution, connected to the first liquid feed line of said gas/liquid contactor; one or more tanks accommodating hydrogen peroxide, connected to the second liquid feed line of said gas/liquid contactor, a first heat exchanger using ambient air for heat transfer, said heat exchanger is provided with a feed line to receive air/CO.sub.2 stream from a chimney of said plant, wherein the outlet of said heat exchanger is connected to a first gas-liquid separator, equipped with a gas discharge line and a liquid discharge line, to withdraw air/CO.sub.2 and water streams, respectively, wherein said gas discharge line is connected to the gas inlet manifold of said gas-liquid contactor; downstream processing units, including: a gas-liquid separator fed by an effluent discharge line of said gas-liquid contactor; wherein a gas discharge line of said gas-liquid separator optionally joins the gas discharge line of said gas-liquid contactor, and a liquid discharge line of said gas-liquid separator splits into two lines, a first line is a recycle line connected to said gas-liquid contactor, said recycle line being equipped with a second heat exchanger using chilled water for heat transfer, and a second line which is a feed line of an evaporation unit; wherein the gas discharge line of said evaporation unit is provided with a condenser, and the liquid discharge line of said evaporation unit is the feed line of a liquid-solid separation unit, equipped with a conveyer supplying separated solids to a dryer.

15. A process for preparing alkali carbonate and bicarbonate salts in sequence, comprising: passing, in a first reactor, a CO.sub.2-bearing gas stream through aqueous alkali hydroxide to form alkali carbonate and an outgoing gas stream with reduced CO.sub.2 concentration; discharging from the first reactor an alkali carbonate-containing aqueous solution/slurry and feeding said aqueous solution/slurry to a second reactor, optionally after some alkali carbonate has been recovered from the aqueous solution/slurry; passing, in the second reactor, a CO.sub.2-bearing gas stream through the alkali carbonate-containing aqueous solution/slurry, to form alkali bicarbonate and an outgoing gas stream with reduced CO.sub.2 concentration; recovering alkali bicarbonate in a solid form from a liquid effluent of the second reactor; or thermally decomposing the alkali bicarbonate into alkali carbonate, carbon dioxide and water, to recover alkali carbonate and/or carbon dioxide in industrially usable forms.

16. A process according to claim 15, wherein the CO.sub.2-bearing gas stream which enters the first reactor is a flue gas, and the CO.sub.2-bearing gas stream which enters the second reactor is the outgoing gas stream with reduced CO.sub.2 concentration that left the first reactor.

17. A process according to claim 15, wherein the CO.sub.2-bearing gas stream which enters the second reactor is a flue gas, and the CO.sub.2-bearing gas stream which enters the first reactor is the outgoing gas stream with reduced CO.sub.2 concentration that left the second reactor.

18. A process according to claim 16, wherein the flue gas is emitted from a glass-producing industrial plant, to form sodium carbonate and sodium bicarbonate, with sodium carbonate being reused in the glass-producing industrial plant.

19. A process according to claim 1, wherein one or more steps of absorbing CO.sub.2 molecules from CO.sub.2-bearing gas stream into an alkaline solution is aided by addition of hydrogen peroxide to the alkaline solution.

Description

[0078] In the drawings:

[0079] FIGS. 1 and 2 show a perspective view of a gas-liquid contactor which is generally parallelepiped in shape, bounded by a bottom surface, a top section and four lateral faces.

[0080] FIGS. 3 to 5 show a perspective view of the interior of the gas-liquid contactor, with sparging units installed and an array of tubes deployed inside the gas-liquid contactor.

[0081] FIG. 6 schematically shows a gas-liquid separation unit downstream of the gas-liquid contactor.

[0082] FIG. 7 schematically shows a vacuum flash separation unit.

[0083] FIG. 8 schematically shows a centrifuge and a drying unit.

[0084] FIG. 9 shows the experimental set-up used in Examples 1 to 4.

[0085] FIG. 10 is CO.sub.2 levels versus time plot, measured in the experiments reported in Examples 1 to 4.

[0086] FIG. 11 is a flowchart of a process according to the invention.

[0087] FIG. 12 is a flowchart of a process according to the invention.

EXAMPLES

Examples 1 (Comparative) 2-3 (Invention) and 4 (Comparative)

[0088] A set of experiments was conducted to study the effect of addition of H.sub.2O.sub.2 on the chemical absorption of CO.sub.2 from CO.sub.2/air stream bubbled through sodium hydroxide solution.

[0089] The experimental set-up is shown in FIG. 9. Reactor (100) is tubular in shape (inner diameter: 9 cm; height: 40 cm). 5 mm thick stainless steel membrane (101) is mounted horizontally inside the reactor, about 2.5 cm from the bottom the reactor. The pore size of the membrane was 147 μm; center to center distance between adjacent pores is ˜50 μm.

[0090] 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.

[0091] The CO.sub.2 source was a commercial 100% CO.sub.2 held in a gas cylinder. Pumps made CO.sub.2 and air to flow into, and mix in, gas mixer (102) to create a mixed stream of 1000 ppm-CO.sub.2 bearing air, which was directed by pump (103) to reactor (101) at a flow rate of 13 L/min (gas inlet rotameter (104)).

[0092] 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”. H.sub.2O.sub.2 stream is fed below the surface level of the sodium hydroxide solution in the reactor, in proximity to membrane (101).

[0093] A pair of CO.sub.2 detectors (105in and 105out—BGA-EDG-MA, Emproco Ltd., Israel) connected to the incoming (1000 ppm-CO.sub.2 bearing air and outgoing (purified) streams (106 and 107, respectively) were used to measure the concentration of CO.sub.2, respectively.

[0094] During operation, the incoming air/CO.sub.2 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. CO.sub.2 was chemically absorbed by the sodium hydroxide medium. CO.sub.2 levels in the incoming and outgoing gas streams were recorded continuously over the test period.

[0095] The results are presented graphically in FIG. 10. The constant CO.sub.2 level in the incoming air/CO.sub.2 gas stream measured throughout the time of the experiment is shown (1000 ppm=0.1%). CO.sub.2 concentration plots versus time (measured in the outgoing stream) for each experiment are also shown. The end of the experiment is indicated by reaching equal CO.sub.2 levels measured in the inlet and outlet streams (i.e., 1000 ppm).

[0096] It is seen that absent added H.sub.2O.sub.2, chemical absorption of CO.sub.2 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 H.sub.2O.sub.2, conversion rates of CO.sub.2 were improved significantly. Under addition of H.sub.2O.sub.2 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 H.sub.2O.sub.2 to the scrubbing system (at 1 ml/h and 2 ml/h) enable high conversion rates of CO.sub.2 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.

TABLE-US-00001 H2O2 flow rate CO2 absorption CO2 absorption Example (ml/hour) lasted over % 1 (comparative) 0 36 hours 80% (measured over 5 hours) 2 (invention) 1 42 hours 80-90% (measured over 37 hours) 3 (invention) 2 53 hours 80%-90% (measured over 52 hours) 4 (comparative) 10 28 hours 80-90% (measured over 26 hours)