EP4688225A1

EP4688225A1 - Production of sulfuric acid and carbon dioxide mineralization

Info

Publication number: EP4688225A1
Application number: EP24778459.8A
Authority: EP
Inventors: Zvi Elgat; Itamar HITE; Avi BEN MENASHE
Original assignee: Airovation Technologies Ltd
Current assignee: Airovation Technologies Ltd
Priority date: 2023-03-29
Filing date: 2024-03-26
Publication date: 2026-02-11
Also published as: WO2024201455A1; KR20260005894A; CN121038885A; IL323548A; JP2026511650A

Abstract

The invention provides a process for the preparation of sul furic acid and minerali zation of CO₂ from industrial gases, comprising : CO₂- absorption step, comprising continuously sparging C0₂- containing gas through an aqueous solution of ammonia in a first reactor, or continuously sparging C0₂-containing gas and ammonia gas through water, to form [ (NH₄ ) ₂CO₃/NH₄HCO₃ ] solution; continuously withdrawing [ (NH4 ) 2CO3/NH4HCO3 ] solution from the first reactor and providing said [ (NH₄ ) ₂CO₃/NH₄HCO₃ ] solution, after it has been optionally cooled, to a second reactor, wherein NH₃ and CO₂-bearing gas stream discharged from the first reactor is fed to the second reactor, thereby producing (NH₄ ) ₂CO₃ aqueous solution in the second reactor and releasing an essentially NH₃ and CO₂- free gas from the second reactor; CO₂-minerlai ztion step, comprising reacting the (NH4 ) 2CO3 in water with calcium sul fate [ CaSO₄ ], to form a solution of ammonium sul fate [ (NH₄ ) ₂SO₄ ] and precipitate solid calcium carbonate [ CaCO₃ ], separating CaCO₃ from said solution and recovering ammonium sulfate crystals from the solution; and H₂SO₄- format ion step, comprising thermally decomposing the (NH₄) ₂SO₄ to give sul furic acid and supplying a stream of ammonia released from the thermally decomposed (NH₄ ) ₂SO₄ to the CO₂-absorption step.

Description

Production of sulfuric acid and carbon dioxide mineralization

Sulfuric acid (H2SO4) is one of the most important chemicals. Sulfuric acid is largely manufactured by catalytic oxidation of sulfur dioxide, to give sulfur trioxide, which is then bubbled through sulfuric acid, whereby sulfur trioxide is absorbed in the solution. Water is then added at the proper rate, to obtain the ~98% acid.

H2SO4 can be recovered from ammonium sulfate [ (NH4)2SO4] by thermal decomposition, with concomitant release of ammonia, as described, for example, in DE 1151492, EP 83831 and WO

03/27018. In WO 96/09248, a multistep process was shown for preparing sulfuric acid, starting with the production of ammonium sulfate by reacting in water carbon dioxide, ammonia, and gypsum (calcium sulfate) , as follows:

CO2 + 2NH₃ + CaSO₄ + H₂O (NH₄)₂SO4 + CaCO₃

Fine grain calcium carbonate suitable for use in the paper industry was obtained. In the next stage described in WO 96/09248, ammonium sulfate was thermally decomposed to give sulfuric acid and ammonia. Figure 1 is a reproduction of the corresponding block diagram of the method of WO 96/09248. It is seen that a stream of gaseous ammonia released from the thermally decomposed (NH4)₃SO4 is supplied to the (NH4)₃SO4 / CaCO₃ formation reaction.

A process that enables CO2 mineralization into a stable, environmentally acceptable form, namely, calcium carbonate, alongside H2SO4 production, is highly advantageous. Huge environmental benefit can be gained from the proposed method of manufacturing of H2SO4, i.e., the capturing of atmospheric CO2 with the aid of ammonia and its conversion into calcium carbonate . Experimental results reported below indicate that the chemical reaction shown immediately above in accordance with WO 96/09248, in which NH3 and CO2 react with a slurry of CaSO4 in water, is difficult to advance to completion. We have found that the efficiency of the process is improved when the abovementioned chemical reaction is divided into steps, that are run in separate reactors. That is, a reaction between ammonia and carbon dioxide in water to form an aqueous ammonium carbonate solution first goes to completion, as follows :

2NH₃ + CO₂ + H₂O - (NH₄) ₂CO₃₍aq) (1)

Subsequently, the aqueous ammonium carbonate solution is reacted with gypsum to form a solution of ammonium sulfate and the water-insoluble calcium carbonate, as follows:

(NH₄) ₂CO₃(aq) + CaSO_{4 (}s) - (NH₄) ₂SO₄(aq) + CaCO₃(s) (2)

The last process step is the thermal decomposition of the ammonium sulfate, as follows:

(NH₄) ₂SO_{4 (S)} - H₂SO_{4 (}I) + 2NH_3(g) to obtain the sulfuric acid product. The gaseous ammonia released can then be supplied to the reaction of equation (1) .

However, the reaction of equation (1) is not without difficulties. The NH₃-CO₂-H₂O system is temperature sensitive, and under varying ratios of reactants, can lead to the formation of different ammonium salts. A review paper entitled "amine versus ammonia absorption of CO₂ as a measure of reducing GHG emission: a critical analysis" [Bandyopadhyay, Clean Tech Environ Policy (2011) 13: 269-284] which discusses the reaction of NH₃ with CO₂, shows the multiple reactions occurring in the NH₃-CO₂-H₂O system. Equations (5) - (12) of the Bandyopadhyay paper show the di f ferent salts that are formed : (NH₄ ) 2CO₃, NH4HCO3 and NH4COONH2 . Low NH3/CO2 ratio favors the formation of ammonium bicarbonate [NH4HCO3 ] , which is far less soluble than ammonium carbonate , so solubility limitations need to be considered . On the other hand, working at high NH3/CO2 ratio , may result in loss of ammonia . The reason is that when a stack gas , or other C02-containing industrial gas , is passed through an ammonia solution in an absorption column, some ammonia would inevitably be carried of f by the outgoing gas stream vented from the reactor . In a large-scale facility, the loss of ammonia is very significant .

We have now found a process design that increases selectivity of the reaction of ammonia and carbon dioxide in water towards (NH₄ ) ₂CO₃ formation with minimal loss of unabsorbed ammonia . C02-containg gas stream ( from a stack gas ) is continuously introduced into an ammonium hydroxide solution charged in a first reactor, with the aid of a di f fuser, sparger or membrane ( these terms are used herein interchangeably) , to create many slow-moving CO2 bubbles or microbubbles in the f irst reactor . It should be noted that the ammonium hydroxide solution can be prepared beforehand in a concentrated form in a separate tank, and fed continuously to the reactor in the form of an aqueous stream, or alternatively, it can be formed in-situ, as ammonia gas released by the thermal decomposition of ammonium sul fate in accordance with reaction of equation ( 4 ) is continuously inj ected into water or dilute ammonia solution in the first reactor, concurrently with the flow of carbon dioxide in a similar fashion, i . e . , into a sparger immersed in the water . The reaction mixture formed in the first reactor, consisting of an aqueous solution of (NH ) 2CO3/NH4HCO3, is continuously withdrawn, optionally passed through a cooler, and supplied as a feed stream to a second reactor . The gas stream that exits the first reactor contains unreacted ammonia and/or unreacted CO2; the gas stream is fed to the second reactor, where it is absorbed by the (NH4 ) 2CO3/NH4HCO3 solution, as this solution is continuously provided to the second reactor. Experimental results shown below indicate that the process design of the invention makes it possible to minimize NH3 loss and achieve high conversion rate of CO2 to (NH4)2COs (>95%, >96% and >99%) . The outgoing air stream released from the second reactor to the atmosphere is essentially free of CO2 and residual NH3.

Accordingly, the invention is primarily directed to a process for the preparation of sulfuric acid and mineralization of CO2 from industrial gases (e.g., stack gases) , comprising:

C02-absorption step, comprising continuously sparging CO2- containing gas through an aqueous solution of ammonia in a first reactor, or continuously sparging C02-containing gas and ammonia gas through water, to form [ (NH4) 2CO3/NH4HCO3] solution; continuously withdrawing [ (NH4) 2CO3/NH4HCO3] solution from the first reactor and providing said [ (NH4) 2CO3/NH4HCO3] solution, after it has been optionally cooled, to a second reactor, wherein NH3 and C02-bearing gas stream discharged from the first reactor is fed to the second reactor, thereby producing (NH4)2CC>3 aqueous solution in the second reactor and releasing an essentially NH3 and C02-free gas from the second reactor;

C02-minerlaiztion step, comprising reacting the (NH4)2COs in water with calcium sulfate [CaSCh] , to form a solution of ammonium sulfate [ (NH4)2SO4] and precipitate solid calcium carbonate [CaCOs] , separating CaCCp from said solution and recovering ammonium sulfate crystals from the solution; and

H2S04-formation step, comprising thermally decomposing the (NH4)2SO4 to give sulfuric acid and supplying a stream of ammonia released from the thermally decomposed (NH4)2SO4 to the CC>2-absorption step. The effluent exiting the first reactor consists of mixed salts [ (NH4) 2CO3/NH4HCO3] solution at proportion of at least 3:1, e.g., 4:1 to 10:1 (by weight) . The effluent exiting from the second reactor consists essentially of (NH4)2CC>3 solution; the amount of the bicarbonate salt is, e.g., <1.0, <0.5, or <0.1 wt . % . By "essentially NHs-and CO2 free gas" it is usually meant not more 100 ppm NH3 and not more than 100 ppm CO2.

Figure 2 is a flow diagram showing one preferred process design, using two series-connected reactors, labeled REACTOR 1 and REACTOR 2, respectively, for carrying out the CO2- absorption step with ammonia. The subsequent steps, namely, the C02-minerlaiztion and the H2S04-formation steps which take place in a stirred batch reactor labeled REACTOR 3 and in REACTOR 4, respectively, are also shown in Figure 2.

The C02-absorption step is now described in detail with the aid of Figures 2 and 3 (the latter also describes the specific experimental set-up used for the tests reported below; elements identified by capital letters refer to Figure 2, elements identified by bold numeral font in parentheses refer to Figure 3) .

Starting with the C02-absorption step, the reactants that are supplied to REACTOR 1 consist of C02-containing gas (labeled "FLUE GAS INLET") , ammonia (5) (either aqueous NH₄OH or NH₃ gas) , and water. The effluent (6) of REACTOR 1, consisting of the [ (NH4) 2CO3/NH4HCO3] solution, is withdrawn from REACTOR 1 by a pump (Pl) , and enters REACTOR 2.

One variant of the invention that is shown in Figure 3 includes circulation of the reaction mixture of reactor (1) , e.g., a major portion of the effluent (6) is circulated and returns to reactor (1) via process line (6ret) . A minor portion of the effluent (6) is diverted (7) from the circulation loop, and the diverted stream (7) enters reactor (2) . Optionally, stream (6ret) or stream (7) is passed through a heat exchanger (not shown) to remove heat. However, the circulation and cooling of the effluent of reactor 1 are not mandatory, and are not shown in Figure 2.

A gas discharge line (8) connects the gas outlet of REACTOR 1 (1) to a gas inlet in REACTOR 2 (2) , such that gas vented from REACTOR 1 (1) , with residual unreacted NH3 and CO2, flows to REACTOR 2 (2) . In REACTOR 2 (2) , the reaction goes to completion .

The effluent (9) of REACTOR 2 (2) , consisting of a highly concentrated [ (NH^COsJ solution (e.g., not less than 100 g/1 by weight, e.g., from 150 to 300 g/1) is delivered to a storage tank (labeled "HOLDING TANK" (10) ) . The treated gas is discharged from REACTOR 2 (2) through line (11) and passed through an absorption column (labeled "AMMONIA SCRUBBER") or a similar arrangement where residual NH3 is reacted with sulfuric acid to give (NH4)2SO4 that is treated as described below.

The reactors and the piping are made stainless steel or suitable plastic such PVC or HDPE . The wetted parts of the pumps delivering the liquid streams are made e.g., of stainless steel. As shown in Figure 3, an array of CO2 and NH3 sensors may be installed at different locations to monitor the progress of the reaction, i.e., CO2 sensors (12a, 12b, 12c) upstream to reactor (1) , downstream to reactor (1) and downstream to reactor (2) , respectively, and NH3 sensors (13a,

13b) , downstream to reactor (1) and downstream to reactor (2) , respectively. The sensors are part of an automated control system that regulates the feed rates of the reactants/product streams in the process. C02-containg gases from different sources can be used as a feedstock for the process. In addition to stack gases and industrial C02-containing gases in general, it is worth mentioning specifically streams produced at a steam methane reforming (SMR) plant, with relatively large proportion of CO2 (~17%) . In case of flue gases emitted by fossil-fired power plants, CO2 levels in the incoming gas stream is usually in the range from 10, 000 to 200, 000 ppm (from 1 to 20%, e.g., from 1 to 18%, e.g., from 1 to 15%, e.g., from 7 to 14% CO2) . Before it enters reactor (1) , the flue gas usually passes through a series of conventional treatments (not shown) , consisting of particulate removal (e.g., by filtration or electrostatic precipitation) , denitrification unit (conversion of nitrogen oxides) and wet scrubbing (for selective removal of SO2) , to generate C02-containing gas which is essentially free of other acidic components (the gas temperature at this stage is generally from 100-200° C) . The NO_X and SO_x-free flue gas is passed through a heat exchanger (labeled "INLET GAS H.EX") 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, to be guided to serve downstream operations. The C02-containing gas is drawn by a first blower/fan (not shown) , which generates gas flow of 50, 000-100, 000 m³/hour, and enters REACTOR 1 (1) , where the C02-absorption reaction with ammonia takes place (the CO2/air is bubbled in REACTOR 1 (1) at about 0.5 bar) .

REACTOR 1 (1) is a gas-liquid contactor. One feature of the process is that CO2 microbubbles are formed as the incoming air/CO2 gas stream (4) is injected into the liquid in REACTOR 1 (1) , through one or more sparging units (14) installed below the liquid level in REACTOR 1 (1) . Different geometries of sparging unit(s) can be used to create finely dispersed uniform CO2 microbubbles, depending on the configuration of REACTOR 1 (1) . For example, flat or tubular spargers (14) , made of chemically resistant plastics, such as HDPE, polypropylene, or stainless- steel (e.g., fine bubble air disc diffusers) , can be installed in REACTOR 1 (1) . The gas sparger orifice size is from 3 to 500 pm, e.g., 3 to 100 pm, e.g., 3 to 20 pm, for example, in the range from 3 to 10 pm. REACTOR 1 (1) may have a cylindrical shape, or may have a shape of rectangular parallelepiped, with one or more flat plate spargers, or one or more tubular spargers, fitted horizontally at the bottom of the reactor. Alternatively, some geometries enable the positioning of the spargers (s) in parallel to the longitudinal axis of REACTOR 1 (1) . Reactor configurations for absorption of CO2 with ammonia are described below, with specific sparger geometry .

There is more than one way to supply ammonia to REACTOR 1 (1) : A) a concentrated ammonium hydroxide (NH4OH) solution is continuously fed into REACTOR 1 (1) (such solution is prepared beforehand, by injecting the ammonia gas evolving in the thermal decomposition reaction of (NH4)2SO4 into water; the so- formed NH4OH solution is held in tank (5) , say, with ammonia concentration in the range from 10 to 30%. A centrifugal pump delivers the NH4OH solution to reactor (1) , through a lateral liquid inlet, or by injection into return line (6ret) . The injected ammonia is diluted by an appropriate volume of water that was previously charged to REACTOR 1 (1) .

B) by continuously sparging NH3 gas through the aqueous medium in REACTOR 1 (1) (i.e., the ammonia gas that was released from the thermal decomposition of (NH4)2SO4, as described below, is cooled down/compressed and stored in a liquid form or cooled down and stored as a gas under pressure in an ammonia cylinder, labeled in "AMMONIA TANK" in Figure 2) . Regarding option B) , the gas streams of CO2 and NH3 can be sparged separately through the liquid (i.e., separate streams enter REACTOR 1 (1) via inlets at different locations through different sparging units, or the streams are mixed upstream to REACTOR 1 (1) , and the joined (CO2+NH3) stream is sparged through the liquid. There are several techniques to create a mixed stream, e.g., by injecting pressurized ammonia to the CO2 gas stream, or by venturi injection.

For example, assuming a cylindrical-shaped reactor, CO2 microbubbles are formed with the aid of spargers as previously described whereas an L-shaped sparge tube is used to disperse NH3 microbubbles in the water. The short section of the L- shaped sparger receives the NH3 stream from a lateral inlet of the reactor. The long section is immersed in the liquid, in parallel to the longitudinal axis of the cylindrical-shaped reactor. The long section is perforated, so that ammonia escapes through the orifices and dissolves well in the solution .

An alternative design of a gas-liquid contactor accommodating tubular spargers, that can be adapted for absorption of CO2 with ammonia to form ammonium carbonate, is described in Figures 1 to 5 of co-assigned WO 2022/130380. The gas-liquid contactor comprises a longitudinal horizontal housing bounded by a bottom surface, a top section and lateral faces; an array of tunnel-shaped sparging units, 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; one or more gas inlet manifold (s) 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; the gas discharge line is connected to a second gas-liquid contactor; a first liquid feed line configured to provide a liquid flow of an aqueous ammonium hydroxide into said housing via one or more liquid inlet openings; and a discharge opening, to which an effluent discharge line is connected, to remove reaction product from the gas-liquid contactor; the effluent discharge line is connected to a second gas-liquid contactor.

Regarding the conditions in REACTOR 1 (1) , the amount of ammonia supplied to REACTOR 1 (1) is regulated to match the stoichiometric demand (nearly 2:1 mole ratio NH3/CO2) by adjusting the feed rate of either the aqueous NH4OH stream or ammonia gas. For example, CO2 is measured in the incoming gas stream and unabsorbed NH3 is determined in the outgoing gas streams (by NH3 sensors 13a, 13b positioned at the outlets of the first and second reactors) . Based on the measured level of CO2 entering REACTOR 1, the automated control system can alter the flow rate of aqueous ammonia by controlling the pump delivering aqueous ammonia from tank (5) , or with the aid of mass flow controllers/pressure regulators located at the lines supplying ammonia gas from the AMMONIA TANK. Reaction temperature ranging from about 10 to 40°C are preferred, with temperature of 10 to 30°C being especially preferred. The pH in REACTOR 1 (1) is slightly alkaline.

Gas (8) is discharged from REACTOR 1 (1) , drawn to REACTOR 2 (2) , usually with the aid of a second blower/fan shown in Figure 2, enters reactor (2) via the bottom of the reactor or a lateral inlet, preferably through a sparging unit (14) , and is bubbled in reactor (2) under about 0.5 bar. The liquid effluent exiting REACTOR 1 (1) , consisting of the [ (NH₄) 2CO3/NH4HCO3] solution, is fed to REACTOR 2 (2) , optionally after it was passed through a heat exchanger (not shown) , which cools the solution down to 20 to 30°C, and then the cooled solution enters REACTOR 2 (2) . For example, the heat exchanger is based on a cooling coil, through which a coolant travels or any other type of a heat exchanger. Residual NH3/CO2 in gas (8) mixes with the [ (NH4) 2CO3/NH4HCO3] solution in REACTOR 2 (2) .

Usually, REACTOR 2 (2) is a duplicate of REACTOR (1) . However, other configurations are possible, i.e., the sparging of gas (8) through REACTOR 2 (2) is beneficial but not essential; alternative gas/liquid contact patterns may be considered for REACTOR 2 (2) .

Experimental results reported show that the level of unabsorbed ammonia measured downstream to REACTOR 2 (2) was at least twofold, at least threefold, and even at least fivefold lower than the level measured downstream to REACTOR 1 (1) , with CO2 conversion exceeding 90% and even 95% (99%) .

The C02-minerlaiztion step occurs when calcium sulfate is suspended in the previously formed ammonium carbonate solution. The two inorganic salts exchange ions to form an insoluble precipitate, calcium carbonate, and ammonium sulfate solubilized in the solution. As shown in Figure 2, The reaction takes place in a batch reactor (labeled REACTOR 3) under agitation. Any source of calcium sulfate may be used, chiefly gypsum (i.e., CaSO4'2H2O, the naturally occurring mineral form of calcium sulfate; gypsum is also readily available as a by-product obtained in the manufacture of phosphoric acid, known by the name phosphogypsum) . But the hemihydrate CaSO4'UH2O and anhydrous CaSO4 can also serve as starting material for the reaction. For example, gypsum is added to a stirred batch REACTOR 3 (e.g., with external jacket) that was previously charged with the aqueous solution of (NH4)2CO3 received from HOLDING TANK. The concentration of the (NH4)2CO3 in the solution is preferably at least 150 g/1. The gypsum supplied to the reaction may be milled beforehand; say, to provide finely divided gypsum with particle size of 50 mesh (up to 300 pm) or 60 mesh (up to 250 pm) . The finer the particle size of the gypsum, the higher the efficiency of the reaction. The pulverized calcium sulfate is added in a portionwise manner, either as a solid or a slurry in water. The progress of the reaction in REACTOR 3 can be monitored by sampling the reaction mixture to determine the concentration of solubilized carbonate, as CO3²Raq) is progressively consumed by the reaction and is separated from the solution in the form of the water-insoluble calcium carbonate. The reaction advances effectively at temperature in the range of 15 to 40°C.

Upon completion of the reaction (on industrial scale, the reaction is expected to last from 0.5 to 5 hours) , the effluent exiting REACTOR 3 (in the form of a suspension) is separated into an aqueous and solid phase, e.g., by filtration, centrifugation, or any other acceptable technique (in Figure 2, separation by a centrifuge is shown) .

The calcium carbonate that was separated from the suspension in the first centrifuge is stored in a tank (labeled "LIMESTONE TANK") . In fact, the calcium carbonate may still contain some residual ammonium sulfate, such that a second separation step may be desired, to collect a second crop of ammonium sulfate. To this end, the calcium carbonate is conveyed to a reactor (labeled "LIMESTONE WASH REACTOR") , to which water is supplied, whereby ammonium sulfate is extracted into the aqueous phase. Solid/liquid separation, e.g., centrifugation or any other suitable filtration equipment, provides the solid calcium carbonate that is essentially free of ammonium sulfate. The calcium carbonate may be disposed in a landfill site (through "LIMESTONE outlet"] , as this mineral form of carbon dioxide is environmentally acceptable. Alternatively, industrially acceptable grades of CaCOs may be recovered by further treatment steps, e.g., by drying the reaction product.

The clear supernatant/ filtrate streams obtained after the removal of the insoluble calcium carbonate in the first and second centrifuges (and optionally, also the solution generated in the ammonium scrubber positioned downstream to REACTOR 2, by the reaction of unabsorbed ammonia with sulfuric acid) are delivered and held in a storage tank (labeled "AMMONIUM SULFATE TANK" in Figure 2) . The stored solution consists of ammonium sulfate in water, e.g., at concentration of at least 150 g/1.

Ammonium sulfate is recovered from the solution by conventional methods, namely, concentration by evaporation, e.g., by a thin film evaporation (the corresponding unit is labeled "THIN FILM DRYER" in Figure 2) or in a spray drier, whereby ammonium sulfate crystallizes out. The water content of the wet crystals is usually from 5 to 10 % by weight; the residual water can be removed in a fluidized bed dryer. The so-formed dried (NH4)2SO4 crystals are used to produce sulfuric acid with concomitant release of ammonia.

The H2S04-formation step, i.e., the thermal decomposition of the previously formed ammonium sulfate crystals, is carried out by methods known in the art, e.g., as described in DE 1151492, EP 83831 and WO 03/27018. Perhaps the most elegant approach is shown in the latter publication, based on the melting of ammonium sulfate added to a concentrated sulfuric acid (95-97%) . Ammonium sulfate undergoes melting at 235°C and decomposition at >250°C. The boiling point of concentrated sulfuric acid is above 300°C. So, there is a temperature "window" permitting the melting/decomposition of ammonium sulfate yet kipping the acid in a liquid state.

As shown in Figure 2, the thermal decomposition takes place in REACTOR 4, which is charged with sulfuric acid heated to ~275- 285°C, e.g., 280°C. Next, ammonium sulfate crystals are added to the hot sulfuric acid. The weight ratio of (NH4 ) 2SO4/H2SO4 is in the range of 1:10 to 1:20. As the ammonium sulfate reacts with the hot acid, it decomposes to gaseous ammonia and sulfuric acid. The NH3 gas formed is vented, cooled down (the corresponding heat exchanger is labeled "AMMONIA H.EX" in Figure 2) and stored in a gaseous form under pressure or compressed and stored as liquid ammonia (labeled "AMMONIA TANK" in Figure 2) for use in the C02-absorption reaction. The indigenously formed H2SO4, i.e., by the decomposition of ammonia sulfate, readily mixes with the previously charged concentrated sulfuric acid. To keep a constant liquid level in the reactor, H2SO4 is continuously withdrawn from the reactor. In this way, a concentrated (~98%) sulfuric acid is produced, which may undergo dilution in water as appropriate.

Figure 2 shows a preferred scheme of recycling of process heat and process streams, to effectively meet the needs of the process with minimal energy costs. As the hot sulfuric acid product is continuously withdrawn from REACTOR 4, it is passed through a heat exchanger (labeled "STEAM H.EX"; also referred to herein as 'the third heat exchanger' ) , where the acid transfers heat to the water vapors discharged from the evaporator (i.e., from the "THIN FILM DRYER") . The vapors are used again to heat the evaporator, following recompression by an electrically operated compressor (the mechanical vapor recompression unit is labeled "MVR") . The acid departing from the third heat exchanger is still very hot; its temperature may be in the range from 150 to 200°C. In the first heat exchanger (labeled "SULFURIC ACID H.EX") , heat is transferred from the sulfuric acid to the ammonium sulfate solution, as it flows from its storage tank to the evaporator (i.e., through the process line connecting the "AMMONIUM SULFATE TANK" to the "THIN FILM DRYER") . Then the cooled sulfuric acid is stored and diluted as appropriate (labeled "SULFURIC ACID") . Before the stream of ammonium sulfate enters the evaporator, it is further heated as it is passed through a second heat exchanger supplied by vapors discharged from the evaporator (labeled "CONDENSATE ACID H.EX") . Water formed by condensation can be stored in a reservoir (labeled "CONDESATE TANK") and supplied by process lines to REACTOR 1 and for treating the calcium carbonate, e.g., at the stage of extracting a second crop of ammonium sulfate in the "LIMESTONE WASH REACTOR".

Another aspect of the invention is an apparatus for the preparation of sulfuric acid and mineralization of CO2 from industrial gases, comprising: at least a first reactor and a second reactor connected in series, with one or more sparger (s) installed in the first reactor, and preferably also in the second reactor, wherein a feed line supplied by a source of C02-containing gas is connected to the sparger (s) in the first reactor; a first piping line connecting the effluent outlet of said first reactor to a liquid inlet in the second reactor; a first gas discharge line connecting the gas outlet of said first reactor to the gas inlet in the second reactor; a second piping line connecting the effluent outlet of said second reactor to one or more storage tanks; a second gas discharge line exiting the second reactor to release gases to the atmosphere ; with CO2 sensor positioned upstream to the first reactor, and NH3 sensor positioned downstream to the first reactor ;

A third reactor ( e . g . , a stirred batch reactor ) supplied by said one or more storage tank ( s ) , wherein a third piping line connects the ef fluent outlet of said third reactor to a first solid/ liquid separation unit , said first solid/ liquid separation unit discharging to a liquid storage tank and a solid storage tank, wherein the liquid storage tank is connected by a process line to an evaporation unit , with a first heat exchanger and a second heat exchanger located on said process line entering the evaporation unit , said evaporation unit being provided with a solid discharge line and vapors recirculation loop, with a third heat exchanger and optionally a mechanical evaporation recompression means located along said vapors recirculation loop ;

A fourth reactor, connected to said evaporation unit to receive solid material therefrom, with a gas discharge line exiting said fourth reactor and entering an ammonia tank, with a cooler and/or compressor positioned along said gas discharge line , wherein the ammonia tank is connected by a process line to the first reactor or to a vessel supplying said first reactor ; and wherein a fourth piping line connects the ef fluent outlet of said fourth reactor to the third heat exchanger and to the first heat exchanger ; and

Blowers and pumps for feeding and withdrawing gaseous and liquid process streams .

In the drawings

Figure 1 shows a prior art process design (WO 96/ 09248 ) , with the bubbling of NH3 and CO2 through a slurry of CaSO4 in water . Figure 2 is a preferred process flow diagram of the invention, consisting of three maj or parts : CCd-absorption step with ammonia to give ammonium carbonate , taking place in two series-connected reactors ; C02-minerali zation step, in which the ammonium carbonate reacts with gypsum to give ammonium sul fate and calcium carbonate ; and thermal decomposition of the ammonium sul fate to give sul furic acid and ammonia .

Figure 3 shows the experimental set-up used in the Working Examples .

Figure 4 shows CO2 conversion versus time plot for the experiment of Example 3 .

Figure 5 shows CO2 conversion versus time plot for the experiment of Example 6 .

Examples

Examples 1 and 2

Preparation of ammonium carbonate from ammonia and carbon dioxide: effect of two reactors connected in series

A series of experiments was performed to study the reaction between ammonia and carbon dioxide in water, to form ammonium carbonate .

Experimental set-up

The experimental set-up is shown in Figure 3. Reactors (1) and (2) have the same configuration. Each reactor consisted of a rectangular tank with the following dimensions: length=50 cm; width=25 cm; height=50 cm) . A tubular sparger (length=~50 cm, diameter=60 mm, made of EPDM or stainless steel) was installed horizontally in each reactor. The diameter of the orifices in the sparger was ~10 pm.

The liquid discharge line of reactor (1) was connected to a lateral inlet in reactor (2) . Centrifugal dosing pump Pl was installed to withdraw the liquid effluent from reactor (1) . A major portion of the effluent (6) is circulated and returns to reactor (1) via pipe (6ret) . A minor portion of the effluent (6) is diverted (7) from the circulation loop, and the diverted stream (7) enters reactor (2) . The product solution was withdrawn (9) from reactor (2) via a conduit to a storage tank (10) where it was collected.

CO2 source was a commercial 100% CO2 held in a gas cylinder. CO2 and air streams were made to flow into, and mix in, a gas mixer to create a mixed CCh/air (6%) stream (4) which was directed by a blower to reactor (1) at varying flow rates as set out in Table 1 (gas flow meter was positioned on the line) . A second blower was installed to draw the outgoing gas stream (8) leaving reactor (1) into reactor (2) , through the tubular sparger mounted at the bottom of reactor (2) . The gas stream (11) that was discharged from the top of reactor (2) was passed through an acid trap (consisting of 6% H2SO4 solution) .

Aqueous NH4OH (28% by weight solution) was held in tank (5) connected by a polypropylene pipe to reactor (1) . The flow of the NH4OH solution from tank (5) to reactor (1) was driven by peristaltic pump.

An array of sensors was deployed in the system. Two CO2 sensors, and three NH3 sensors. One CO2 sensor was placed at the entrance of reactor (1) (to measure CO2 concentration in the incoming air/C02 mixed stream) and the other was placed downstream to the acid trap (to measure CO2 levels in the purified gas) . The three NH3 sensors were placed downstream to reactor (1) , reactor (2) and the acid trap, respectively, to determine the proportion of unreacted ammonia escaping from the first and second reactors and the trap. pH electrode was immersed in reactor (1) .

Experimental protocol

Each experiment began by charging reactors (1) and (2) with 20 liters of deionized water. The tubular sparger was submerged about 10-20 cm below the surface level of the solution.

The air/CO2 mixed gas stream was drawn by the first blower (0.5 bar) into reactor (1) , through a lateral opening, and was forced to flow through the sparger to create microbubbles, at the flow rates tabulated in Table 1 below. The second blower operated at 0.5 bar to push the outgoing gas stream from reactor (1) into the sparger installed in reactor (2) . Pump supplied an aqueous NH4OH at a flow rate adjusted to meet ~ the 2 : 1 stoichiometry demand; the temperature at the reactor was 15°C-25°C and pH was slightly basic. The flow rate of the liquid stream pumped from the first reactor, and supplied to the second reactor was 4 L/min; residence time was a few minutes .

The experiments were performed according to two programs: only reactor (1) was active; both reactors (1) and (2) were active. CO2 and NH3 levels in the incoming and outgoing gas streams were recorded continuously over the test period by the sensors deployed at various points. Each run lasted 8-12 hours.

Results

The results are shown in Table 1. The CO2 level in the incoming air/C02 gas stream measured throughout the time of the experiment was 6%.

Table 1

Examples 1A and 2A correspond to the experiments performed with a single reactor and Examples IB and 2B are the parallel experiments, run under the same conditions, but with a pair of reactors. The results indicate the benefit gained by running the reaction using two reactors in series: loss of ammonia is minimized, and conversion rate is increased by ~10% upon switching to the experimental design utilizing two reactors. Examples 3 and 4

Preparation of ammonium carbonate from ammonia and carbon dioxide in two reactors connected in series

Additional experiments were performed to study the reaction between ammonia and carbon dioxide in water, to form ammonium carbonate , using the same experimental set-up and protocol described in Examples 1 and 2 . This time all experiments were run using a pair of reactors . Reaction conditions and results are tabulated in Table 2 . The experiment lasted five hours .

Table 2

The results tabulated in Table 2 show that high conversion rates , e . g . , 99% , can be achieved with the process design of the invention . The proportion of ammonia escaping from the second reactor was appreciably lower compared to unabsorbed ammonia that was carried of f by the discharge gas from the first reactor . The results are also shown graphically as CO2 conversion versus time plot in Figure 4 for Example 3 . Average CO2 conversion measured over the five hours period was ~ 99% .

Example 5

Preparation of calcium carbonate and ammonium sulfate from ammonium carbonate and calcium sulfate

1 liter of ammonium carbonate solution at concentration of 150 g/ 1 was charged to a reaction vessel , followed by addition of gypsum ( 210 g; particle si ze 60 mesh) . The mixture was stirred for two hours by a standard agitator . The progress of the reaction was monitored by collecting samples at intervals of thirty minutes , to determine the change in the concentration of solubili zed carbonate anion ( CO3²- ) . The dissolved carbonate anion "disappears" from the solution due to the precipitation of CaCO₃.

For determination of the concentration of CO3²- in the aqueous phase , a sample of the reaction mixture was filtrated through a Whatman 41 filter paper to separate the suspended solid from the aqueous phase . A clear filtrate was collected, from which a sample was taken and added to a closed vessel with one outlet connected to a measuring tube . The vessel was charged with 6% sul furic acid solution . Gaseous carbon dioxide that evolved due to the reaction of sul furic acid with solubili zed carbonate was collected in the measuring tube :

The results are tabulated in Table 3 below . The changes measured over the 120 minutes test period are expressed in percentage relative to the initial concentration of CO3²- at t=0 .

Table 3

Example 6 (comparative ; WO 96/09248)

Single step reaction : CO₂ + 2NH₃ + CaSO₄ + H₂O (NH₄) ₂SO₄ + CaCO₃

A stirred reactor was used to test the ef ficiency of the conversion of CO₂ , ammonia, and aqueous gypsum slurry into ammonium sul fate and calcium carbonate in a single step . The reactor was charged with 30 liter of NH4OH 25%. Air/CCh (6%) mixed gas stream continuously flowed into the reactor, via a sparger, at a feed rate of 8 L/min, with concurrent feed of 25% NH4OH solution at a flow rate supplying the ~2 : 1 molar ratio demand throughout the experiment. After a few minutes, gypsum (250 g; particle size 60 mesh) was added to the reactor, resulting in intense bubbling. The experiment lasted 6 hours; the reactor effluent was continuously discharged to a storage tank (the reactor was ~ 60% full by volume during the reaction) . CO2 level was continuously recorded in the gas that was vented from the reactor. The results are given in the form of CO2 conversion versus time plot in Figure 5. Average CO2 conversion measured over the six hours period was ~95%.

At the end of the experiment, the solid was removed from the reaction mixture and the filtrate was collected. The concentration of solubilized carbonate anion (CO₃ ²~) in the filtrate was measured by the technique described above, i.e., with the aid of sulfuric acid, determining the volume of CO2 that bubbles off because of the reaction of unreacted ammonium carbonate with sulfuric acid. It was found that the concentration of unreacted ammonium carbonate was 61 g/L. The result indicates that a process design based on supplying aqueous ammonia into a reactor charged with ground gypsum suspended in water, with continuous flow of CO2 microbubbles through the mixture, is inefficient. Although CO2 is absorbed by ammonia in the system, and ammonium carbonate that is formed reacts in turn with calcium sulfate, to give ammonium sulfate and calcium carbonate, the overall single step reaction (CO₂ + 2NH₃ + CaSO₄ + H₂O (NH₄)₂SO4 + CaCO₃) does not go to completion as a fairly large proportion of ammonium carbonate remained unreacted in the solution.

Claims

1) A process for the preparation of sulfuric acid and mineralization of CO2 from industrial gases, comprising:

C02-absorption step, comprising continuously sparging CO2- containing gas through an aqueous solution of ammonia in a first reactor, or continuously sparging C02-containing gas and ammonia gas through water, to form [ (NH4) 2CO3/NH4HCO3] solution; continuously withdrawing [ (NH4) 2CO3/NH4HCO3] solution from the first reactor and providing said [ (NH4) 2CO3/NH4HCO3] solution, after it has been optionally cooled, to a second reactor, wherein NH3 and C02-bearing gas stream discharged from the first reactor is fed to the second reactor, thereby producing (NH4)2CC>3 aqueous solution in the second reactor and releasing an essentially NH3 and CO2-free gas from the second reactor;

H2S04-formation step, comprising thermally decomposing the (NH4)2SO4 to give sulfuric acid and supplying a stream of ammonia released from the thermally decomposed (NH4)2SO4 to the CC>2-absorption step.

2) A process according to claim 1, wherein the NH3 and CO2- bearing gas stream discharged from the first reactor is sparged through the solution in the second reactor.

3) A process according to claim 1 or 2, wherein the sparging is through a gas sparger with orifice size is the range from 3 to 500 pm. 4) A process according to claim 3, wherein the orifice size of the sparger is the range from 3 to 20 pm.

5) A process according to any one of claim 1 or 4, wherein the level of unabsorbed ammonia measured downstream to the second reactor is at least threefold lower than the level measured downstream to the first reactor.

6) A process according to any one of claims 1 to 5, wherein the (NH₄) ₂CO₃ solution produced in the second reactor contains not more than 0.5 wt . % ammonium bicarbonate.

7) A process according to any one of the preceding claims, wherein the C02-minerlaiztion step takes place in a third reactor and the ammonium sulfate solution formed undergoes evaporation to recover ammonium sulfate crystals.

8) A process according to claim 7, wherein upstream to the evaporation, the solution of ammonium sulfate is heated in a firth heat exchanger by heat transferred from the hot sulfuric acid formed by the thermal decomposition of the ammonium sulfate in a fourth reactor.

9) A process according to claim 8, wherein before the stream of ammonium sulfate enters an evaporator, it is further heated in a second heat exchanger by vapors discharged from the evaporator, wherein water formed by condensation of said vapors is supplied by process lines to the first reactor in the C02-absorption step and/or for treating the calcium carbonate formed in the CCh-minerlaiztion step.

10) A process according to claim 8 or 9, comprising evaporating the ammonium sulfate solution in a thin film evaporator, conveying ammonium sulfate crystals to the fourth reactor, continuously withdrawing hot sulfuric acid from the fourth reactor, passing the hot sulfuric acid through a third heat exchanger to transfer heat to water vapors discharged from the evaporator, recompressing the vapors, and directing them to heat the evaporator.

11) A process according to claim 10, wherein the sulfuric acid departing from the third heat exchanger is passed through the first heat exchanger to transfer heat to the ammonium sulfate solution upstream to evaporation, as the ammonium sulfate solution flows from a storage tank to the evaporator.

12) An apparatus for the preparation of sulfuric acid and mineralization of CO2 from industrial gases, comprising: at least a first reactor and a second reactor connected in series, with one or more sparger (s) installed in the first reactor, and preferably also in the second reactor, wherein a feed line supplied by a source of CCh-containing gas is connected to the sparger (s) in the first reactor; a first piping line connecting the effluent outlet of said first reactor to a liquid inlet in the second reactor; a first gas discharge line connecting the gas outlet of said first reactor to the gas inlet in the second reactor; a second piping line connecting the effluent outlet of said second reactor to one or more storage tanks; a second gas discharge line exiting the second reactor to release gases to the atmosphere; with CO2 sensor positioned upstream to the first reactor, and NH3 sensor positioned downstream to the first reactor; a third reactor supplied by said one or more storage tank(s) , wherein a third piping line connects the effluent outlet of said third reactor to a first solid/liquid separation unit, said first solid/liquid separation unit discharging to a liquid storage tank and a solid storage tank, wherein the liquid storage tank is connected by a process line to an evaporation unit , with a first heat exchanger and a second heat exchanger located on said process line entering the evaporation unit , said evaporation unit being provided with a solid discharge line and vapors recirculation loop, with a third heat exchanger and optionally a mechanical evaporation recompression means located along said vapors recirculation loop ; a fourth reactor, connected to said evaporation unit to receive solid material therefrom, with a gas discharge line exiting said fourth reactor and entering an ammonia tank, with a cooler and/or compressor positioned along said gas discharge line , wherein the ammonia tank is connected by a process line to the first reactor or to a vessel supplying said first reactor ; and wherein a fourth piping line connects the ef fluent outlet of said fourth reactor to the third heat exchanger and to the first heat exchanger ; and blowers and pumps for feeding and withdrawing gaseous and liquid process streams .