Patent Description:
Calcined soda (sodium carbonate, Na<NUM>CO<NUM>) is used in many branches of industry, including glass-making, chemical (detergents), metallurgical, paper-making industry, and many more. Most soda is produced by the ammonia-soda process.

In the embodiments of the ammonia-soda process, many streams of process gases contain unreacted carbon dioxide. One of these is the gas leaving scrubbers, i.e., columns for washing gases from carbonation columns, being emitted to the atmosphere, and containing <NUM> - <NUM>% of CO<NUM>. Carbon dioxide may be absorbed in order to reduce the emitted stream. The carbon dioxide stream may be also reused in the soda production process in order to improve the process yield. A simplified plant flowchart is presented in <FIG>.

The source of carbon dioxide and burnt lime in the process for production of soda ash and baking soda are limestone and a fuel used for decomposition thereof. Heat necessary for limestone decomposition comes from combustion of solid, liquid, or gaseous fuels. Carbon dioxide and burnt lime form as a result of calcium carbonate decomposition reaction, according to the following reaction:.

The generated kiln gas containing about <NUM> vol. % of CO<NUM> is transported via a collector (<NUM>) using compressors (<NUM>) to carbonation columns (<NUM>). Precipitation of raw sodium hydrogen carbonate occurs during carbonation (saturation with carbon dioxide) of ammoniated brine. One can generally assume that the process for carbonation of ammoniated brine proceeds according to the following total reaction:.

NaCl + NH<NUM> + CO<NUM> + H<NUM>O = NaHCO<NUM> + NH<NUM>Cl.

This is an equilibrium reaction, where the conversion of sodium chloride into sodium hydrogen carbonate is not complete. The carbonation columns (<NUM>) are supplied with ammoniated brine (<NUM>) obtained in the ammonia absorption plant (<NUM>). The post-reaction gas stream (<NUM>) from carbonation columns (<NUM>) is subjected to purification from excessive amounts of unreacted CO<NUM> and NH<NUM>. The process occurs in serially co-operating scrubbers. The scrubber (<NUM>) for washing gases from carbonation columns is sprinkled with purified brine (<NUM>) in order to recover ammonia. Carbon dioxide is practically insoluble in purified brine. The gas (<NUM>) leaving the scrubber (<NUM>), being emitted to the atmosphere, contains <NUM> - <NUM>% of CO<NUM> and trace amounts of NH<NUM>.

Raw sodium hydrogen carbonate (raw bicarbonate) (<NUM>), being precipitated in carbonation columns (<NUM>) and separated from the mother liquor on filters (not shown in <FIG>), is subjected to thermal decomposition in calcining furnaces according to the reaction:.

<NUM> NaHCO<NUM> = Na<NUM>CO<NUM> + CO<NUM> + H<NUM>O,.

and the obtained sodium carbonate is a ready product - light soda ash. Light soda may be regarded as commercial product, or, following further processing by hydration and drying, or densification, may be converted into dense soda.

Carbonation of ammoniated brine is a multi-step process, and begins as early as in the NH<NUM> absorption plant (<NUM>). Process of saturating purified brine (<NUM>) with ammonia and carbon dioxide contained in gas (<NUM>) from distillation proceeds according to the following reactions:.

<NUM> NH<NUM> + CO<NUM> + H<NUM>O = (NH<NUM>)<NUM>CO<NUM>.

The obtained stream of ammoniated brine (<NUM>), after cooling down in the cooler (<NUM>) to the temperature of about <NUM>, is supplied to the carbonation plant.

After separating raw sodium hydrogen carbonate on filters, the remaining mother liquor is subjected to the ammonia recovery process, called distillation. Ammonium carbonate and ammonium hydrogen carbonate undergo thermal decomposition, to yield ammonia and carbon dioxide as products. The heat source is low-pressure steam. Ammonia contained in the form of ammonium chloride is decomposed using lime milk (not shown in <FIG>).

The decomposition processes of ammonium salts proceed according to the reactions:.

NH<NUM>HCO<NUM> + Q = NH<NUM> + CO<NUM> +H<NUM>O.

(NH<NUM>)<NUM>CO<NUM> + Q = <NUM> NH<NUM> + CO<NUM> + H<NUM>O.

<NUM> NH<NUM>Cl + Ca(OH)<NUM> = <NUM> NH<NUM>OH + CaCl<NUM>.

These reactions require heat Q, that is provided to the process using steam.

Soda ash may be a raw material for the production of baking soda. The main step of preparation of baking soda is the process for carbonation of the solution of sodium carbonate and sodium hydrogen carbonate. It is carried out in a custom-made carbonation column and consists in saturating the aqueous solution of sodium carbonates with gaseous carbon dioxide. Baking soda is formed according to the reaction:.

Na<NUM>CO<NUM> + CO<NUM> + H<NUM>O = NaHCO<NUM>.

Absorption of carbon dioxide in the column is not complete and it depends on a concentration of sodium carbonate and sodium hydrogen carbonate in the process operating conditions. As a result, the gas exiting carbonation columns contains up to <NUM> vol. % of CO<NUM>.

Operation of the lime kiln plant and parameters reached by this plant are of key significance for the economics of the entire production process. One especially important parameter is the CO<NUM> concentration in the produced kiln gas, that is usually in the range of <NUM> - <NUM> vol. The CO<NUM> concentration has an important effect on the energy consumption for transport and compression of gaseous streams to carbonation columns in the production of soda ash and baking soda. This affects also the yield of the carbonation process, the size of crystals of the manufactured sodium bicarbonate and the humidity thereof, and, consequently, it is of key significance for consumption of energy and raw materials in the production process. The technological operation of lime kilns (including CO<NUM> concentration in the kiln gas) depends, inter alia, on the quality of raw materials used in the process, i.e. limestone and fuel. Among the potential possibilities of reducing the cost of used fundamental raw material, namely limestone, there is, inter alia, use of fine limestone having a granulation in the range of <NUM> - <NUM>. However, use of fine limestone for the lime burning process causes reduction of CO<NUM> concentration in the obtained kiln gas down to <NUM> - <NUM> vol. %, what is disadvantageous from the viewpoint of the carbonation process in production of soda ash and in production of baking soda, since this causes larger consumption of energy and consumption of raw materials.

Summing up, the sources of emission of carbon dioxide to the atmosphere comprise: (<NUM>) the excessive gas stream from the limestone decomposition, as well as unreacted carbon dioxidecontaining outlet gas streams from the processes (<NUM>) of carbonation of ammoniated brine, and (<NUM>) carbonation of the solution of sodium carbonate and sodium hydrogen carbonate in the production of soda ash and baking soda. Incomplete carbon dioxide consumption results from equilibrium conditions of the chemical reactions occurring in carbonation columns. The optimised operation of the production process results in emission of <NUM> - <NUM> tonnes of <NUM>% CO<NUM>/tonne of soda (source: <NPL>).

There are numerous methods aiming at limiting the emission of carbon dioxide into atmosphere, including those comprising uptake of carbon dioxide and returning to the production process. These are, in particular, chemical absorption methods, temperature swing absorption method (TSA), pressure swing absorption method (PSA), cryogenic distillation method, and membrane method.

The most frequently used methods for CO<NUM> uptake from industrial gases are the chemical absorption methods. The absorbents used are the solutions of amines, for example, of monoethanolamine (MEA), diethanolamine (DEA), diglycolamine (DGA), N-methyldiethanolamine (MDEA), <NUM>-amino-<NUM>-methylpropanol (AMP), or piperazine (PZ). Apart from amine solutions, one can use aqueous ammonia solutions.

The CO<NUM> absorption method using amine absorbents has been developed for removing acidic components, such as H<NUM>S and CO<NUM>, from the natural gas. Then it was adjusted for obtaining carbon dioxide from exhaust gases. The CO<NUM> absorption technology allows for removing about (<NUM> - <NUM>%) CO<NUM> and obtaining a very pure CO<NUM> stream (><NUM> vol.

The CO<NUM> absorption and desorption process in the amine method may be carried out depending on the parameters of the gas to be purified and on the destination of the obtained gas in the pressure range of from <NUM> bar(abs) to <NUM> bar(abs).

As illustrated in <FIG>, the process system for removing carbon dioxide with use the of amine solutions is composed principally of two parts, namely the absorption column, in which carbon dioxide is removed from exhausts, and the desorption column, where CO<NUM> desorption and absorbent regeneration occurs. The amine solutions absorb carbon dioxide to form labile chemical compounds. The CO<NUM> absorption process is carried out in the absorption column (<NUM>), to which the cooled gases containing CO<NUM> (<NUM>) are introduced from the bottom, and the cooled aqueous amine solution (<NUM>) having a low content of CO<NUM> is introduced from the top. The gases being purified in the CO<NUM> absorption column (<NUM>), before leaving it, are additionally washed with water (<NUM>) in the top section (<NUM>), what limits the emission of volatile compounds along with the purified gas (<NUM>). The carbon dioxide saturated absorbent leaves the column (<NUM>) and is warmed up in a heat exchanger (<NUM>) and then introduced into the desorption column (<NUM>). In the desorption column (<NUM>), thermal desorption of CO<NUM> occurs (at a temperature of <NUM> - <NUM>) from the saturated solution (<NUM>). Desorption along with regeneration of the solution in the desorption column (<NUM>) occurs by providing heat in the form of steam in the evaporator (<NUM>) located in the bottom part of the desorption column (<NUM>). The gaseous CO<NUM> along with small amount of steam (<NUM>) leaves the top of desorption column (<NUM>) via a condenser (<NUM>), in order to condensate the highest possible amount of steam.

Part of the regenerated solution stream (<NUM>) may be supplied in the form of stream (<NUM>) to the reclaimer (<NUM>), where the solution reclaiming process (<NUM>) occurs. Reclaiming consists in removing impurities of various origin from the amine absorbent solutions by supplying heat. The reclaiming process reduces amine consumption, filtration costs, and amount of precipitates in heat exchangers, what generally reduces operational costs of the amine plant.

The partial or total demand for reclaiming heat in the CO<NUM> concentrating process can be satisfied by using waste streams from other technological processes of the plant where the CO<NUM> absorption process is carried out.

The regenerated solution (<NUM>) prior to recycling into the CO<NUM> absorption column (<NUM>) is cooled down to the temperature of <NUM> - <NUM>, most often by a heat exchanger (<NUM>) (in contact with cool saturated solution) and a cooler (<NUM>). The obtained carbon dioxide (stream <NUM>) can be used in the soda-ammonia process (see <FIG>) for feeding kiln gas stream (<NUM>) or process gas into the baking soda carbonation plant, or may be stored in a tank (<NUM>).

<CIT>, having the filing date of <NUM>. <NUM>, discloses a method for production of soda by the ammonia-soda process from limestone having a high amount of impurities, where lean kiln gas from rotary lime kilns is subjected to carbon dioxide concentrating so that it reaches the concentration of <NUM> vol. Concentrating occurs by contact with a liquid selectively absorbing carbon dioxide, such as triethylene glycol, propylene carbonate, aqueous amine solution, in particular, aqueous monoethanolamine (MEA) solution, as well as aqueous potassium carbonate solution, followed by CO<NUM> desorption.

European patent No. <CIT>, having the priority date of <NUM>. <NUM>, discloses the method for purifying exhaust gases and carbon dioxide absorption by the ammoniated solution or suspension (in particular, suspension of ammonium salts) at a temperature of <NUM> - <NUM>, preferably <NUM> - <NUM>, known as "chilled ammonia process" (CAP). Releasing of carbon dioxide occurs after compressing the said ammoniated solution or suspension with a high-pressure pump, preferably to the pressure of <NUM> - <NUM> bar, and warming to <NUM> - <NUM>. Carrying CO<NUM> absorption at low temperature using concentrated ammonia solution (<NUM> wt. %) improves the effectiveness of this process and reduces emission of NH<NUM>. Regeneration at a temperature of ><NUM> and under pressure of ><NUM> bar produces a compressed CO<NUM> stream having a low content of steam and ammonia. A disadvantage of this method is the necessity to use high pressures (what increases the energy consumption for compressing of media), and also the tendency for plant choking by the formed precipitates.

US patent No. <CIT>, discloses an integrated method for carbon dioxide capture and production of soda ash. Carbon dioxide is removed from the gas process stream by absorption in an ammoniated solution, and then desorbed in a regenerator, to obtain carbon dioxide-rich gas stream and an carbon dioxide-depleted ammoniated solution. Said gas stream, preferably directly, is sent from regenerator to carbonator, where carbonation of ammoniated brine with carbon dioxide occurs. The ammonia-containing fluid, being produced in the soda-ash process, is returned to carbon dioxide absorption from said stream of process gas.

The international patent application publication No. <CIT>, discloses the process for producing sodium carbonate/sodium hydrogen carbonate. This document describes concentration of carbon dioxide from lean kiln gas using the temperature swing concentrating module or using the amine method concentrating module. Utilisation of heat energy of steam under gauge pressure not exceeding <NUM> bar as well as energy of process streams at a temperature of <NUM> to <NUM> for concentrating CO<NUM> in the concentrating module is also described.

European patent No. <CIT>, discloses the device and method for integrated processing of outlet gases and production of soda ash. The technical solution consists in binding carbon dioxide from outlet gas in a carbonator using calcium oxide, to form calcium carbonate and releasing carbon dioxide from calcium carbonate in a calcining furnace. This process is integrated with a production process of soda ash by the ammonia-soda process.

The instant inventors, attempting to modernise production by integrating the existing plant for production of soda ash, baking soda and soil lime with the process for uptake carbon dioxide, unexpectedly found that such a close integration of processes makes it possible to simultaneously achieve many benefits, in particular, comprising: improved use of raw materials, reducing the environmental load of carbon dioxide emission, reducing the process streams, reducing the energy consumption for compression, pumping, and heating or cooling, possibility of using process streams at lower carbon dioxide concentrations, possibility of using limestone of inferior quality, greater flexibility of plant operation, and possibility of receiving external streams containing waste carbon dioxide from outside the plant.

A list of definitions of concepts useful for understanding the disclosure described in this specification is presented below. It should be emphasized that, in the case of conflict with the terms used in the art, the meanings indicated here take precedence. The conventionally used terms method and process are equivalent.

Absorbent - a solution used for uptake CO<NUM> from the process gases, used in the CO<NUM> separation/recovery process.

CO<NUM> absorption - unit process in the CO<NUM> separation/recovery process, consisting in absorbing carbon dioxide with an absorbent.

NH<NUM> absorption - unit process in the soda ash process, where purified brine is saturated with ammonia contained in an ammoniated gas from the distillation. Ammoniated brine is formed there.

Primary amine - amine having one substituent at an amine nitrogen atom.

Tertiary amine - amine having three substituents at an amine nitrogen atom.

Refined bicarbonate - an intermediate in preparation of baking soda.

Raw bicarbonate - an intermediate in preparation of soda ash. The main component is NaHCO<NUM> (ca. %), with admixture of ammonium salts and water.

Calcium chloride - the main component is CaCl<NUM>; by-product prepared from clear distiller effluents.

Absorption refrigerator (chemical compressor) - a device for lowering temperature of the cooled medium at a cost of provided energy. In absorption refrigerators, the energy is provided in heat form.

Ionic liquids - salts having melting points lower than <NUM>; many representatives of this class are liquids already at room temperature. Due to their very differentiated and unique physicochemical properties, these substances are employed as absorbents for the CO<NUM> absorption process.

Spray cooler - a gas-liquid heat exchanger, having one heat exchange surface sprayed with a liquid stream that lowers temperature to the temperature of wet-bulb thermometer (this is the lowest temperature accessible by evaporation at a given humidity and atmospheric pressure).

Cooling medium - thermodynamic medium that participates in heat exchange in a cooling device, having its temperature lower than the cooled medium. In particular, the useful cooling media comprise cooling water or other process streams used in soda production, such as: raw brine stream at a temperature of <NUM>-<NUM>, mother liquor stream obtained after separating sodium bicarbonate at a temperature of <NUM>-<NUM>, or also brine waste stream at a temperature of <NUM>-<NUM>.

Degradation of the absorbent solution - deterioration of absorption properties of the absorbent solution (the absorbent), with negative effect on CO<NUM> absorption.

Densification - increasing the bulk density of the obtained product.

CO<NUM> desorption - unit process in the process of CO<NUM> separation, consisting in separation carbon dioxide from amine solutions or aqueous ammonia solutions.

Distillation/ammonia recovery - part of the soda ash plant/process, where ammonia, in the form of ammonium salts contained in the mother liquor, is recovered and returned to absorption in the form of gas. In this phase of the process, wastes are generated in the form of suspension of calcium carbonate and other substances insoluble in an aqueous solution of calcium chloride and sodium chloride; these wastes are called distiller effluents/waste waters.

Carbon dioxide/CO<NUM> - a chemical compound; it is produced in the ammonia-soda process by lime burning and combustion of coke; other fuels, such as anthracite, gas, or fine coal, also can be used.

Filtration - part of the soda ash plant/process, where the raw bicarbonate precipitate is separated from the mother liquor.

Bottom gas/mixed gas - process gas having a carbon dioxide concentration most often in the range of <NUM> - <NUM> vol. %, following mixing the kiln gas and the calcination gas (><NUM> vol. % of CO<NUM>). It is supplied to the bottom part of a carbonation column (the ammonia-soda process).

Top gas - process gas, usually having a carbon dioxide concentration in the range of <NUM> - <NUM> vol. %; it is supplied to the middle part of a carbonation column (the ammonia-soda process).

Carbonation gas - process gas, usually having a carbon dioxide concentration in the range of <NUM> - <NUM> vol. %; it is supplied to the bottom part of a carbonation column in the washing phase (the ammonia-soda process).

Purified gas - CO<NUM> absorption process gas, leaving the CO<NUM> absorption column.

Outlet gas from soda ash - outlet gas from carbonation in the production of soda ash, containing <NUM> - <NUM> vol. % of CO<NUM>.

Outlet gas from baking soda - outlet gas from carbonation in the baking soda production process, containing <NUM> - <NUM> vol. % of CO<NUM>.

Kiln gas - gas produced in lime kilns, usually containing <NUM> - <NUM> vol. % of CO<NUM>.

Lean kiln gas - kiln gas having a CO<NUM> concentration of <NUM> - <NUM> vol.

Lean gases/diluted gases - outlet gases from the soda ash production processes, baking soda production, as well as lean kiln gas.

CO<NUM> separation/recovery plant - a plant for CO<NUM> separation from the gaseous stream.

Limestone - the fundamental mineral raw material for production of soda ash.

Carbonation (with relation to the soda ash plant) - part of the soda ash plant/process, where ammoniated brine is saturated with CO<NUM>-containing gases in a carbonation column. A raw bicarbonate suspension is formed.

Carbonation (with relation to the baking soda plant) - part of the baking soda plant/process, where saturation of the purified solution of sodium carbonate and sodium hydrogen carbonate with carbon dioxide occurs. A refined bicarbonate suspension is formed.

CO<NUM> absorption column - an apparatus in the CO<NUM> separation process, used for CO<NUM> uptake, where the binding of carbon dioxide in the absorbent solution occurs.

Desorption column - an apparatus in the CO<NUM> separation process, used for CO<NUM> desorption, where CO<NUM> is released from the absorbent solution (absorbent) using the provided heat energy.

Water washing column - an apparatus for limiting emission of ammonia and other unwanted components within outlet gas to be vented to atmosphere, and purifying further the obtained carbon dioxide stream.

Ammoniated condensate - a process liquid being the condensate resulting from cooling off-gas from the NH<NUM> distillation process.

Distilled condensate - a process liquid being the condensate after stripping ammonia.

Calcination process condensates - process solutions, being aqueous solutions of ammonium salts and of soda ash formed in the process of purification and cooling off-gas from calcining furnaces.

Crystallisation/drying - part of the soda ash plant/process, that consists in crystallisation and drying of sodium carbonate monohydrate. Soda ash/dense calcined soda is formed.

Cross-flow heat exchanger - an apparatus in the CO<NUM> separation process, being a heat exchanger, wherein a heat exchange occurs through a diaphragm between the saturated solution and the regenerated one.

Slaking of burnt lime - part of the soda ash process, consisting in quenching burnt lime with water, to yield lime milk required for ammonia recovery.

Condensate mixer/cooler - a device for making an appropriate solution of ammoniated condensates used in carbon dioxide absorption process.

Steam from expander of distiller effluents - off-gas formed by expansion of the fluid exiting from the bottom part of the distillation apparatus, under pressure of <NUM> - <NUM> bar.

Lime kilns - part of the soda ash plant/process, where burnt lime and CO<NUM> containing gas are produced. Vertical/shaft kilns are most often used in soda industry; horizontal kilns are more rarely used. The raw materials are limestone and a fuel such as coke, anthracite, gas, fine coal.

Scrubber of outlet gases - part of the plant responsible for ammonia absorption from outlet gases from carbonation column.

Gas separation scrubber - an apparatus in the soda ash process, comprising a column for ammonia absorption in order to obtain the purified stream of carbon dioxide.

Final scrubber - the scrubber for final purification of carbon dioxide from ammonia.

Absorption capacity - a measure of carbon dioxide binding capacity of the absorbent solution.

CO<NUM> recovery process - uptake of carbon dioxide from process gases.

Reclaimer - a device in the CO<NUM> separation process, wherein the amine solution taking part in CO<NUM> absorption is reclaimed.

Reclaiming - a process consisting in removal of impurities of various origin from the amine absorbent solutions by providing a heat stream. Reclaiming reduces consumption of amines, filtration costs, and amount of precipitates in heat exchangers, what generally reduces the operational costs of the amine plant.

Regeneration of the solution - a unit process in the CO<NUM> separation process, consisting in releasing of already absorbed carbon dioxide by providing heat energy to the solution.

Saturated solution - an absorbent liquid in the CO<NUM> separation process, being saturated with carbon dioxide downstream the CO<NUM> absorption column.

Multicomponent solution - mixtures of absorbent solutions in the CO<NUM> separation process, that comprise, for example, inorganic salts/amine, ionic liquids/amine.

Regenerated solution - an absorbent liquid in the CO<NUM> separation process, being an effluent from the desorption column and containing low contents of carbon dioxide.

Sequestration - a process consisting in separation and uptake carbon dioxide from process gases in order to limit its emission to atmosphere, by using it as a raw material or storage.

Condenser - an apparatus used in the CO<NUM> separation process, being a cooler for condensing moisture from a gaseous stream.

Ammoniated brine - product of saturation of purified brine with the ammoniated gas from distillation. Depending on the absorption stage, it differs in ammonia concentration.

Soda ash/calcined soda - the main component is Na<NUM>CO<NUM>; the product obtained by an ammonia-soda process. The fundamental raw materials are: purified brine and limestone.

Light soda, dense soda - grades of soda ash differing in granulation and bulk density.

Baking soda/refined bicarbonate/refined sodium hydrogen carbonate - the main component is NaHCO<NUM>; the raw materials are soda ash and a CO<NUM> containing gas.

Purified brine - the halite solution purified from calcium and magnesium salts.

Halite - the main component is NaCl; the fundamental mineral raw material for soda ash production.

CO<NUM> isolation efficiency - an index characterising the CO<NUM> isolation process, being the percentage amount of the removed carbon dioxide from the initial gas stream.

Liquid to gas ratio (L/G) - an index characterising the CO<NUM> isolation process, being the ratio of flow of the gas supplied to the column in relation to the absorbent stream [kg of liquid/kg of gas].

Reaction rate - the rate of forming or losing reagent as a result of the chemical reaction.

Brine wastes - distiller effluents/waste waters.

CO<NUM> utilisation - a waste management method for the captured carbon dioxide.

Burnt lime - product of thermal decomposition of limestone, used for preparation of lime milk.

Lime fertilizer - solids contained in distiller effluents converted into calcium fertilizer (soil lime) for use in agriculture.

Heat exchanger - an apparatus for heat exchanging between the solution streams.

Evaporator - a heating apparatus for solutions, used for isolating gaseous components from solutions and/or for concentrating solutions.

Steam generator - heat exchanger for generating steam without furnace.

Amino acid compounds or amino acid reaction products - free amino acids or inorganic salts of amino acids (for example, potassium or sodium salts, such as sodium glycinate or potassium glycinate).

Volatile compounds - gaseous compounds having high vapour pressure, that are emitted to the environment.

If not stated otherwise, all pressures specified in this specification in bar units are expressed in absolute values.

Now the invention will be discussed with reference to the accompanying drawings, where:.

The invention aims at development of low energy consuming and economically reasonable process for carbon oxide recovery using the absorption method for improving the quality of process gas that is directed as a raw material to the production of soda.

In the most general aspect of the invention, the process for carbon oxide recovery is defined in claim <NUM>.

Utilisation of carbon dioxide from the streams of process gases and outlet gases for enriching gaseous streams into carbon dioxide (CO<NUM>) up to <NUM> - <NUM> volume % on the dry gas basis leads to reduction in consumption of the raw material and/or allows to substitute it with a raw material having inferior parameters. Improving the economics of the soda production process depends not only on the quantity and quality of the raw material used, but also on the quantity of fuel used for processing it.

CO<NUM> enrichment of gaseous streams is carried out using a gaseous stream having a high CO<NUM> content (<NUM> - <NUM> vol. ), that is obtained from the streams of process gases and/or outlet gases, occurring in the soda production process, such as a kiln gas, lean kiln gas, outlet gas from the carbonation plant of raw soda, and/or outlet gas from the plant for carbonation of baking soda.

A kiln gas stream (<NUM>) (see <FIG>) is available under pressure in the range of <NUM> - <NUM> bar(abs) from the input side and/or output side of compressors (<NUM>) supplying the kiln gas to carbonation columns (<NUM>). The kiln gas may be drawn using an additional gas compressor (<NUM>). The CO<NUM> content in the gas (<NUM>) is from <NUM> to <NUM> vol. %, the gas temperature is in the range of <NUM> - <NUM>. In the case of obtaining kiln gas from a raw material of inferior quality, the CO<NUM> content in this lean kiln gas (<NUM>) is from <NUM> to <NUM> vol.

It is possible to divide the whole kiln gas stream (<NUM>) into a main stream and an auxiliary stream. The auxiliary stream, in an amount of <NUM> to <NUM>% of the whole stream, is supplied to the CO<NUM> obtaining system and is a CO<NUM> source for concentrating the main stream (<NUM>), in which the target CO<NUM> concentration value is <NUM> - <NUM> vol.

Another CO<NUM> source is the outlet gas (<NUM>) (see <FIG>) from the serial scrubbers (<NUM>), used for purifying the unreacted post-reaction gas (<NUM>) resulting from carbonation columns (<NUM>). The outlet gas (<NUM>) leaving the scrubbers (<NUM>), that formerly was emitted to the atmosphere, contains <NUM> - <NUM>% of CO<NUM> and trace amounts of ammonia. The gas temperature is in the range of <NUM> - <NUM>, the pressure is in the range of <NUM> - <NUM> bar(abs).

Another CO<NUM> source is the outlet gas from the baking soda plant (not shown). This gas is a product of the process for saturation (carbonation) of a calcined soda solution using CO<NUM>. Outlet gas contains significant amounts of CO<NUM>, due to the limitations resulting from the equilibrium conditions of the carbon dioxide absorption process and the occurring chemical reactions. The CO<NUM> content in this gas is from <NUM> to <NUM> vol. %, the gas temperature is in the range of <NUM> - <NUM>, and the pressure is in the range of <NUM> - <NUM> bar(abs).

Another CO<NUM> source can be the flue gases and/or other CO<NUM> containing gases, being produced on the premises of the factory, originating from combustion of liquid, solid, or gaseous fuel in order to generate heat and/or electrical energy for the production requirements of the ammonia-soda process. The CO<NUM> content in the gas is from <NUM> to <NUM> vol. %, the gas temperature is in the range of <NUM> - <NUM>. The pressure is in the range of <NUM> - <NUM> bar(abs).

Another CO<NUM> source may be the gas stream intended for sequestration (not shown) from an external supplier, and obtained in an industrial factory belonging to energetic, metallurgic, chemical, cement-making, and/or food industry. Utilisation of such a CO<NUM> stream is justified by current European Union politics in the area of limiting emission of greenhouse gases (CO<NUM> utilisation is profitable because of the introduced emissions trading system).

CO<NUM> can be obtained from the above-mentioned gaseous streams by chemical absorption.

According to one embodiment of the invention, the CO<NUM> absorption solution may contain potassium salts, for example, K<NUM>CO<NUM> in an amount of <NUM> - <NUM> wt. %, preferably <NUM> wt. %, and/or other inorganic salts that activate the reaction of CO<NUM> to carbamate, and an addition of <NUM> - <NUM> wt. % of an amine having a high rate constant of the reaction with carbon dioxide (having a rate constant greater or equal to <NUM> dm<NUM>/mol·s at a temperature of <NUM>).

According to another embodiment of the invention, the CO<NUM> absorption solution may contain ionic liquids in an amount of <NUM> - <NUM> wt. % the CO<NUM> absorption solution may also contain amino acid compounds, such as potassium diglycinate, and/or aqueous or anhydrous multicomponent solutions comprising mixtures: inorganic salts/primary amine (excluding MEA), ionic liquids/primary amine (excluding MEA) and/or inorganic salts/amino acid compounds.

Use of solutions of inorganic salts, ionic liquids, or amino acid compounds brings some advantages compared to conventional amine absorption methods. The proposed absorption systems are less volatile, what results in a significant drop in size of the node for purification of outlet gases from the CO<NUM> separation plant (both of outlet gas as well as pure CO<NUM> returned back to the soda production plant). Additionally, degradation of the solutions based on the above-mentioned absorbents is much less both in the CO<NUM> absorption as well as the desorption stage. This affects the purity of obtained CO<NUM> as well as reducing the running costs. The solutions of such a type should also exhibit lower energy consumption than, for example, amine absorption methods.

The above-mentioned absorption methods require cooling the regenerated absorbent solution stream down to the temperatures of <NUM> - <NUM>, preferably <NUM> - <NUM>, more preferably <NUM> - <NUM>, and cooling the obtained CO<NUM> stream down <NUM> - <NUM> in order to cause condensation of the moisture contained therein. The following media can be utilised for cooling the absorbent solution and/or for condensing the moisture contained in the CO<NUM> outlet gas stream:.

Besides, the above-mentioned absorption methods for obtaining CO<NUM> from gaseous mixtures require significant amounts of heat in order to regenerate the absorbent solution. The heating medium may be steam or process condensates and other streams having a high temperature above <NUM>. Internal process streams from soda production or streams of external origin may be the source of heat for regeneration.

One of the heat sources is the kiln gas stream, resulting from the lime burning process (at a temperature of about <NUM>), that is cooled prior to directing to the compressor and/or carbonation columns (<NUM> - <NUM>). Using the heat recovered from the kiln gas for the CO<NUM> concentration properly, one can provide about <NUM> - <NUM>% of the total heat requirement of the CO<NUM> absorption process. This improves significantly the economics of the CO<NUM> absorption process.

To reduce costs of the heating medium, the regeneration heat requirement may be partially covered by using one or more of the following streams:.

In the case of using absorption methods for obtaining CO<NUM>, it is possible to supply to the absorption column, at the same time and in any ratio, the streams:.

These streams are simultaneously directed to the CO<NUM> absorption column (or the battery of columns) (<NUM>). Here, the gas having a higher CO<NUM> content (the kiln gas and/or outlet gas from the baking soda plant) is supplied to the bottom part of the column (or the battery of columns) (<NUM>), whereas one or both gaseous stream having lower CO<NUM> content are supplied to the middle part of the absorption column (or the battery of columns) (<NUM>). The CO<NUM> recovery system is operated in the continuous absorptiondesorption process, and the stream of CO<NUM> saturated stream may be directed to one desorption column or to a battery of desorption columns (<NUM>). Such a technical design features improvement of the general driving module of the absorption process in the whole absorption column with relation to the standard design, where the gas streams are mixed before supplying to the absorption column. This results in higher efficiency of the CO<NUM> absorption process. Such a design allows also to make use of two additional CO<NUM> streams occurring in the factory, and to significantly reduce the amount of fuel needed for soda production process in the form of limestone.

What is to be jointly optimised is the method of drawing the outlet gas and lean kiln gas into the CO<NUM> absorption column, the place for introducing the concentrated gas (<NUM>) into the production process, the pressure conditions of the CO<NUM> absorption process and the construction of the absorption column.

When kiln gas is the CO<NUM> source (not according to the invention) then kiln gas may be drawn from the kiln gas compressor (<NUM>). In the soda production process, before entering the carbonation reactor, the kiln gas becomes compressed to <NUM> - <NUM> bar(abs). Many studies show that the CO<NUM> chemical absorption process performs better under higher pressure; moreover, ammonia (formed during amine degradation) emission along with the purified gas is easier to control. This design additionally reduces capital costs due to removal of a gas compressor for pumping kiln gas through the CO<NUM> separation system, as well as due to the smaller size of the absorption column and the remaining equipment. The absorption process under higher pressure makes it also possible to use absorbents that absorb CO<NUM> physically. The intermediate designs are also possible, in the case of multistage compression, kiln gas may be drawn from between the compression sections (<NUM>). The recovered CO<NUM> can be also directed to a place between compressors.

Waste heat from the soda production by the ammonia-soda process can be also used as a heat source for supplying absorption refrigerators (chemical compressor) that cool the streams in the CO<NUM> recovery process. Lowering the process temperatures in the CO<NUM> absorption methods, such as of the absorption solution and/or intermediate streams used in the column cooling systems, and/or systems for washing out volatile compounds, and/or condensing in the CO<NUM> recovery system. These are: increasing the efficiency of the CO<NUM> absorption process, decreasing the level of emission of ammonia vapours formed as a result of absorbent degradation, decreasing the humidity contained in outlet gases, etc. The literature and modelling analyses proved also that cooling down the above-mentioned streams even by several degrees results in reducing the amount of absorbent, and washing water most of all, reduces the energy consumption of the process, and, therefore, reduced the running costs.

Waste heat from the production of soda by the ammonia-soda process can be also used for reclaiming the absorption solutions in the methods for obtaining CO<NUM>. A suitable source of the total heat for the process and/or partial and/or initial heating of the regenerated solution is:.

The recovered CO<NUM> can be returned to the production process in two locations:.

A person skilled in the art will appreciate that the abovediscussed technical solutions may be used in any combinations most suited to needs and conditions of the specific soda production system by the ammonia-soda process.

The advantages of such a technical solution comprise, inter alia, improved use of raw materials, reducing the environmental load of carbon dioxide emission, reducing the process streams, and thus reducing the energy consumption for compression, pumping, and heating or cooling, possibility of using process streams at lower carbon dioxide concentrations, possibility of using limestone of inferior quality, greater flexibility of plant operation, and possibility of receiving external streams containing waste carbon dioxide from outside the plant, for example, from the cement plants, thermal power stations, metallurgy, or waste incineration plants.

Besides obvious advantages for the natural environment, such process improvements have a significant economical aspect.

The invention will be better comprehensible for the person skilled in the art after learning the embodiment examples. These examples are only illustrative and not intended to limit the scope of the invention in any manner.

If not stated to the contrary, all percentage values are expressed in volume %, and pressures are absolute, i.e., measured against vacuum and not the atmosphere.

Lean kiln gas (<NUM>), having the following parameters: CO<NUM> content of <NUM> - <NUM> vol. %, temperature of <NUM> - <NUM>, pressure of <NUM> - <NUM> bar, flow rate of <NUM> - <NUM>/h, being drawn as one of the products of lime burning process in shaft lime kilns with air blow, is directed to the initial purification column (not shown in <FIG>). In this column, the gas is cooled down and purified from sulphur oxides down to the level of <<NUM> ppm SO<NUM>. Then the purified kiln gas is directed to the CO<NUM> absorption column (<NUM>) operating under pressure of <NUM> - <NUM> kPa(abs), where it is contacted in a counter-current with the regenerated absorbent solution (<NUM>) at a temperature of <NUM> - <NUM>, whereby the CO<NUM> content in the kiln gas (<NUM>) drops down to <NUM> - <NUM> vol. % of CO<NUM>. The gas purified in the absorption column (<NUM>), before leaving it, is washed with a part of the solution drawn from the column (not shown in the Figure) and with fresh water, what occurs in the top part of the absorption column (<NUM>), in the water washing section (<NUM>). The CO<NUM> absorption solution contains <NUM> wt. % of <NUM>-amino-<NUM>-methylpropanol, <NUM> wt. % of piperazine, and water as absorbent.

The carbon-dioxide saturated absorbent (<NUM>) (<NUM> - <NUM> mol CO<NUM>/mol of amine), leaving the absorption column, is pumped into the desorption column (<NUM>) operating under pressure of <NUM> - <NUM> kPa(abs). In the meantime, the saturated solution (<NUM>) is warmed up in a heat exchanger (<NUM>) (where heat is exchanged between the regenerated solution (<NUM>) and the saturated solution (<NUM>)). The stream (<NUM>), in the evaporator of the desorption column (<NUM>), is warmed up to <NUM> - <NUM> using low-pressure steam, what results in regeneration of absorbent (<NUM> - <NUM> mol CO<NUM>/mol of amine). Then the solution (stream <NUM>) is directed to the column (<NUM>), where CO<NUM> is released. The mean energy necessary to regenerate the absorbent is <NUM> - <NUM> MJ/kg CO<NUM>. The released stream of CO<NUM>, containing <NUM> - <NUM> vol. % of CO<NUM> and <NUM> - <NUM> vol. % of moisture, is cooled down in an end cooler (<NUM>). Cooling of gas results in condensation of water that accumulates in the condensate tank (not shown in the drawings), from which it is pumped to the CO<NUM> absorption (<NUM>) and desorption column (<NUM>) (stream <NUM>) at any ratio. After cooling, carbon dioxide (stream <NUM>) may be subjected to additional purification or storage, depending on further application. Then the stream of carbon dioxide is directed to the stream of lean kiln gas (<NUM>) in order to enrich it into CO<NUM>.

The regenerated hot absorbent solution from the regeneration column (<NUM>) is pumped towards the CO<NUM> absorption column (<NUM>), meanwhile exchanging heat in a heat exchanger (<NUM>) with the saturated solution (<NUM>), and then it is cooled down to the temperature of <NUM> - <NUM> by the use of a diaphragm cooler (<NUM>). The regenerated solution (<NUM>) is introduced into the CO<NUM> absorption column (<NUM>) at the liquid to gas (L/G) ratio of <NUM> - <NUM>/kg.

Using the same equipment configuration as in Example <NUM>, the CO<NUM> the recovery process is carried out with a primary amine and for <NUM> - <NUM>/h of the outlet gas (<NUM>) from the raw soda carbonation plant. The outlet gas contains <NUM> - <NUM> volume % of CO<NUM>, has a temperature of <NUM> - <NUM> and a pressure of <NUM> - <NUM> kPa(abs). The CO<NUM> absorption process is carried out in the absorption column (<NUM>), at a temperature of <NUM> - <NUM> and under pressure of <NUM> - <NUM> kPa(abs). The desorption process is carried out in the desorption column (<NUM>), at a temperature of <NUM> - <NUM> and under pressure of <NUM> - <NUM> kPa(abs). The regenerated solution (<NUM>) is introduced into the absorption column (<NUM>) at the liquid to gas (L/G) ratio of <NUM> - <NUM>/kg. The carbon oxide content in the saturated solution (<NUM>) is <NUM> - <NUM> mol CO<NUM>/mol of amine, and the carbon oxide content in the regenerated solution (<NUM>) is <NUM> - <NUM> mol CO<NUM>/mol of amine. The CO<NUM> isolation efficiency is <NUM> - <NUM>%, and the heat requirement is <NUM> - <NUM> MJ/kg CO<NUM>.

Using the same equipment configuration as in Examples <NUM> and <NUM>, the recovery process is carried out with a primary amine and for <NUM> - <NUM>/h of the outlet gas from the plant for carbonation of baking soda. The outlet gas contains <NUM> - <NUM> volume % of CO<NUM>, has a temperature of <NUM> - <NUM> and a pressure of <NUM> - <NUM> kPa(abs). The CO<NUM> absorption process is carried out in the absorption column (<NUM>), at a temperature of <NUM> - <NUM> and under pressure of <NUM> - <NUM> kPa(abs). The CO<NUM> desorption process is carried out in the desorption column (<NUM>), at a temperature of <NUM> - <NUM> and under pressure of <NUM> - <NUM> kPa(abs). The regenerated solution (<NUM>) is introduced into the absorption column (<NUM>) at the liquid to gas (L/G) ratio of <NUM> - <NUM>/kg. The carbon oxide content in the saturated solution (<NUM>) is <NUM> - <NUM> mol CO<NUM>/mol of amine, and the carbon oxide content in the regenerated solution (<NUM>) is <NUM> - <NUM> mol CO<NUM>/mol of amine. The CO<NUM> isolation efficiency is <NUM> - <NUM>%, and the heat requirement is <NUM> - <NUM> MJ/kg CO<NUM>.

Claim 1:
A process for recovering carbon dioxide for its reuse in the production of sodium carbonate and sodium hydrogen carbonate by the ammonia-soda process, comprising:
contacting, in a CO<NUM> absorption column, the streams of process gases and/or outlet gases occurring in the process for producing sodium carbonate and sodium hydrogen carbonate by the ammonia-soda process, comprising:
- the outlet gas stream from the plant for carbonation of soda ash having a carbon dioxide content of <NUM> - <NUM> vol.%, temperature in the range of <NUM> - <NUM>, and pressure of <NUM> - <NUM> bar (abs), and/or
- the outlet gas stream from the plant for carbonation of baking soda having a carbon dioxide content of <NUM> - <NUM> vol.%, temperature in the range of <NUM> - <NUM>, and pressure of <NUM> - <NUM> bar (abs),
- and optionally one or more of streams of flue gases or other gases containing carbon dioxide, resulting from combustion of solid, liquid, or gaseous fuels in order to produce heat or electrical energy to meet the production requirements by the ammonia-soda process,
- and optionally one or more of streams of flue gases or other gases containing carbon dioxide, originating from an external supplier
with a stream of aqueous absorbent solution,
to form aqueous absorbent solution enriched in carbon dioxide, heating aqueous absorbent solution enriched in carbon dioxide in evaporator,
desorption of gaseous carbon dioxide with regeneration of aqueous absorbent solution in a desorption column,
cooling regenerated aqueous absorbent solution and returning it to said CO<NUM> absorption column, and
removing from the desorption column and cooling the stream having a high carbon dioxide content for using in the process for producing sodium carbonate and sodium hydrogen carbonate by the ammonia-soda process,
characterised in that the aqueous absorbent solution contains:
- ionic liquids in an amount of <NUM> - <NUM> wt.%; or
- amino acid compounds, such as potassium glycinate; or
- the mixtures: inorganic salts/primary amine other than MEA, ionic liquids/primary amine other than MEA and/or inorganic salts/amino acid compounds.