FI108616B - Process for removing carbon dioxide CO2 from flue gas - Google Patents

Abstract

The invention relates to a process for removing carbon dioxide CO2 from flue gas, according to which process: - the flue gases entering the process are subjected to high pressure, and - are scrubbed 10.1 with solvent to transfer CO2 from the flue gas to the solvent, and - the flue gases are removed from the process after scrubbing, and - the solvent is led from the scrubbing process to an evaporation process 12, 13 at lower pressure, in which CO2 is separated from the solvent and led out of the process, and -the solvent is pumped back into the dissolving process, and - as scrubbing medium, a solvent such as methanol is used, which has a pressure gradient deviating strongly from Henry's law for the solubility of CO2 within a suitable temperature range. The liquefaction and evaporation are performed in the said temperature range close to the dew point of CO2 almost isothermally at the CO2 partial pressure used in the process, and - liquefaction and evaporation are performed thermodynamically in immediately reversible conditions. <IMAGE>

Description

108616

PROCESS FOR THE REMOVAL OF CARBON DIOXIDE, C02, FROM FLUE GASES

The invention relates to a process for separating carbon dioxide, CO 2, from a flue gas, comprising: - pressurizing the flue gases entering the process, and - washing them with a solvent to transfer CO 2 from flue gases to solvent I, and - flue gas removal from the process; and - the solvent is pumped back to the leaching process, and - the solvent, such as methanol, which has a strong non-Henry solubility pressure gradient at a suitable temperature range, is selected as the washing liquid.

There is a global desire to limit CO2 emissions due to their climatic effects. Large amounts of carbon dioxide-20 are released when burning fossil fuels. Carbon capture from flue gases has been technically difficult and economically unviable due to its high energy consumption.

25 Liquefaction of carbon dioxide is one way to separate it. However, the carbon dioxide triple point is at a relatively high pressure (5.1 bar), so that the exhaust gas always has at least this fractional pressure of carbon dioxide. The degree of separation thus remains modest.

30

The so-called rectisol process is presented e.g. U.S. Patent No. 2,863,527. In the example of the publication (Fig. 4), carbon dioxide is separated from the process gas at 20 bar in a concentration of 30%. The leaching ends at -30 ° C, which determines the concentration of CO 2 in the wash solution. The expansion evaporation occurs at -60 ° C, where only about 40% of CO 2 can be separated. Due to the low separation percentage, much of the expansion evaporation is 2 108616! The resulting CO2 solution must be distilled in a heat-consuming rectification column that does not produce the evaporative heat of CO 2 to cool the leaching column, but the corresponding cooling energy must be generated by a separate machine. All in all, the process presented is complicated and thermodynamically quite disadvantageous.

Energy consumption of commonly known CO2 separation processes is 600-900 kJ / kg CO2. In the oxygen combustion process, oxygen production 10 already requires 850 kJ / kg CO2 under ideal conditions.

The object of the present invention is to provide a more energy efficient and simpler method of CO2 separation. Characteristic features of the separation process according to the invention are set forth in claim 1. By performing the leaching and evaporation of CO 2 from the flue gas almost reversibly | in this method, the thermodynamic minimum value of the separation work, which is about 160 kJ / kg CO 2, is reached with 10-14% CO 2 in the flue gas and the extracted CO 2 is obtained at normal pressure.

20

The invention utilizes such a solvent, e.g.

• i t '' methanol, in which CO 2 is dissolved in a manner different from Henry's, so that its solubility increases sharply as the dew point of *. ·. This is illustrated in Figure 1, which shows the solubility of carbon dioxide in methanol at various CO 2 partial pressures as a function of temperature. For example, at 8 bar partial pressure, the C02 dew point is -46 ° C, and its solubility approaches infinite drop in temperature towards the dew point.

The figure shows that if the partial pressure of C02 in the flue gas is 8 bar, ·. and leaching is carried out at -45 ° C, so that the 85% saturation degree of the solution has 2000 kg CO 2 / tonne of methanol. Evaporation at -50 ° C at 4 bar leaves 485 kg CO 2 / tonne methanol, i.e. 75.7% CO 2 evaporated.

t 35 108616 3

An even closer process is achieved in the examples discussed below. They also provide more accurate removal of CO 2 from the flue gases using two-stage expansion evaporation or air flushing of the evaporation chamber.

5

The invention provides the following advantages: - when evaporation is carried out at a temperature slightly below the dissolution temperature and close to the part-10 pressure of CO2, the dissolution-evaporation cycle is almost reversible and its energy consumption is lower than before, - high thermal energy released in leaching can be absorbed by evaporation by bonding the leach and 15 evaporation rooms thermally to each other - these large heat fluxes are transferred from one fluid stream to another without the use of large size and low efficiency "dry" heat exchangers, 20 - evaporation can also produce cooling · · · · The possibility of an "integrated" process in which the same liquid mixture '· *' · 25 acts as both a CO 2 solvent and a heat transfer fluid, whereby the flue gas is cooled to the process temperature and heated after the process.

In the following, the invention will be described by means of the following examples; 30 shown in the accompanying figures. In the examples, the partial CO2 of the flue gas. the pressure is assumed to be 8 bar, ie if the flue gas contains 10% CO 2,, ···, leaching occurs at 80 bar.

t

Fig. 1 shows the solubility of carbon dioxide in methanol at various pressures of ί 108616 4

Figure 2 shows a two-stage expansion evaporator column for removal of CO 2 Figure 3 shows a column system with a separate heat transfer cycle 5 Figure 4 shows a system with air flush

Figure 5 shows a "wet" heat exchanger for cooling / heating the flue gas before / after the CO2 separation Figure 6 shows an integrated system comprising a wet heat exchanger and a CO 2 separation section in the same column 10

In the example of Figure 2, the dissolving and evaporating spaces 10.1, 12, 13 are built on the same column 10. They form a heat exchanger so that the flue gas flows upwards in one or more | ! in a tube in which the solvent flows downward and the envelope 15 serves as an evaporation space. Also, the reverse structure, where evaporation occurs in the tubes and dissolution in the surrounding jacket, is possible.

The flue gas connections are designated by reference numerals 11.1 and 11.2. The high pressure pump 15 pumps the CO2 poor methanol into the dissolution space 10.1, where it is enriched as it flows downstream of the flue gas stream. Liquid engine 16 can utilize some of the energy of depressurization of the resulting CO 2 - *** solution when the solution stream is passed through, i .: through line 17, my first evaporator is 12 (4 bar) to a liquid coil cooling coil 14 from which it is sprayed: ti>! to the solvent in the evaporation chamber. The purpose of this arrangement is to prevent the solution stream from cooling strongly as its pressure drops. Most of the carbon dioxide separates under this pressure and this portion is sucked into the tap of the compressor 20. Subsequently, the solution stream is passed through pipeline 18 to another evaporation chamber 13 (2 bar), where additional CO 2 is separated, which is sucked into compressor 20. Finally, the impoverished methanol stream is pumped back to 80 bar and a new cycle begins.

·· »» 35 As in the other examples, fillers, bottoms, or other structures may be placed in the dissolution and evaporation rooms to control the flow of gas and liquid and to enhance heat and mass transfer. In a carefully designed leaching state, leaching can be performed approximately reversibly, since the CO 2 pressure of the rising flue gas stream decreases while the CO 2 concentration of the solvent flowing against it increases.

The evaporation process here is a two-step expansion evaporation. When the process is carried out under conditions where the solubility of CO 2 decreases rapidly as the pressure drops, evaporation also provides a near-reversible process. This is evident from the fact that, if CO 2 were bubbled into chambers 12 and 13, the process would transfer CO 2 to the pure flue gas, where its partial pressure would rise close to the partial pressure of the supplied CO 2 (4 bar). The reversibility in the example of Figure 2 is due to the fact that the majority of CO 2 is obtained at 4 baris-15 with a partial pressure of 8 bar in the flue gas. In the rectisol process, the loss is significantly greater.

In the solution of Figure 3, the leaching and evaporation are carried out in separate columns 10 'and 10 ". Functionally otherwise, the same reference numerals as above are used for the same sections. The heat flux is transferred from leaching to evaporation by separate liquid circulation (pump 21). The desired temperature difference between the bottom and the bottom end is of the order of 5..10 ° C. At a higher final evaporation temperature, CO 2 is more precisely separated from the solvent, leaving less CO 2 in the flue gas.

«I t

An example of an air rinsing process is shown; Figure 4.

30; ·] The flue gas is introduced into the dissolution section 10.1 of column 10 where it flows upward as its CO 2 dissolves into the downstream solvent. The CO 2 solution then flows through the column enrichment section through the turbine or liquid engine 16 and through the heat transfer coil 23 35 to the evaporation tubes 26 in the leaching column at a pressure lower than the original partial pressure of CO 2 and 108616 6 where the CO 2 evaporates under heat. Evaporation tubes 26 continue through the enrichment column to a chamber at its lower end, where flushing air (air connection 27) is bubbled through the solvent and from which the impoverished solvent is pumped to the upper end of the leaching column by pump 15 via line 19.

The gas flow from the evaporation tubes to a suitable pressure and the compressed gas are cooled until most of its CO 2 liquefies.

10

The gaseous mixture of CO 2 and air from the liquefaction is conducted cold to the column concentrate (Compound 28) where it increases the concentration of CO 2 in the solution from the leaching section and from which the leaching gas remains. the alloy is associated with the flue gas flow to the leaching portion.

The gas phase C02 is thus returned to the separation circuit without increasing the solvent flow.

By air rinsing, the evaporation process is reversed with leaching, i.e., the CO 2 pressure of the gas in the evaporation space increases upwards while decreasing its concentration in the downstream solution. This will get you very close to the recovery process.

• t «* <t * · *, ·. · The flue gas can be cooled to process temperature and heated '· *'? After separation of CO 2, the "wet" heat exchanger of Figure 5 circulates a low vapor pressure and freezing point liquid, e.g., a mixture of 1,2-propylene glycol and water, between two columns 30 and 31. These are interconnected by pipelines 32 and 34 with transfer pumps 31 and 33. Flue gas 30 is supplied to conduit 30.1 and purified and heated i · ·; [flue gas exits conduit 31.1. Connections 30.2 and 30.1 are connected, ··· _ to joint 11.1 and 11.2 in the above figures. The heat exchange can also be combined with the separation of the S compounds, e.g. using the Wellman-Lord-M M process. As with any of the columns discussed herein,

Mill 35 also wet column heat exchanger columns can be equipped with fillers, 108616, bottoms, or other structures to control gas and liquid flow and enhance heat and material transfer. The wet heat exchanger reduces the loss of solvent 5 used to separate CO 2 into the flue gas by scrubbing its incoming flue gas, which thus arrives at the CO 2 separation impregnated with a solvent.

By combining the wet heat exchanger and the CO 2 separation, the "integrated" process of Figure 10 6, where all the sub-processes are performed in column 10, is functionally the same reference numerals as above.

The flue gas is cooled to the process temperature in the cooling section 10.2 with a cold solvent from the evaporation chamber 13. It goes up then to the dissolution section 10.1 where CO2 is dissolved in the solvent flowing against it. The solvent is collected in the intermediate bottom 24, from where it is passed to two-step evaporation (chambers 12 and 13) as in the examples above.

20

When the solvent also acts as a heat transfer fluid, a fluid having a sufficiently low vapor pressure and a low freezing point, in addition to the solvent properties described above, must be selected. These include aqueous mixtures of, for example, propylene carbonate and polyhydric alcohols.

The heat exchange efficiency reaches its maximum when the heat capacity flow rate of the solvent / heat exchange fluid is approximately the same as that of the flue gas.

»* * ': 30 i»

Most preferably, the pressurized fumes exiting the separation process are used to produce mechanical or electrical energy by heating it and expanding it in a turbine or other power source such that the heating and expansion steps can be one of a kind. '35 or more.

Claims (10)

8 108616 A process for separating carbon dioxide, CO 2, from a flue gas, comprising: 5. pressurizing the flue gases entering the process, and - scrubbing (10.1) with a solvent to transfer CO 2 to the flue gas, and - flue gas removal from the process; leading from the scrubbing process to a lower pressure evaporation process (12, 13) where CO2 is removed from the solvent and removed from the process, and - the solvent is pumped back to the leaching process, and - a solvent such as methanol having a strong non-Henry solubility in CO2 pressure gradient in a suitable temperature range, characterized in that - leaching and evaporation is carried out at said temperature range close to the dew point of CO 2 at a partial pressure of CO 2 used in the process, and leaching and evaporation are carried out under thermodynamically near reversible conditions. Process according to Claim 1, characterized in that the leaching and evaporation are carried out in the same column (10) in the separating chambers (10.1, 12, 13) to transfer heat from leaching to evaporation. A process according to claim 1, characterized in that the dissolution and evaporation are carried out in two or more columns (10 ', 10 ") interconnected by a separate heat transfer circuit (21). Process 35 according to any one of claims 1 to 3, characterized in that the difference between the dissolution and evaporation temperatures is in the order of 4 to 8 ° C for dissolution of heat to the evaporation. Process according to one of claims 1 to 4, characterized in that the evaporation (12, 13) is carried out in two or more stages and the CO2 is removed by a tap compressor (20) to a common outlet pressure. A process according to any one of claims 1 to 5, characterized in that the solvent is passed from a higher pressure leach through a liquid engine (16) or a turbine to a lower pressure evaporation to utilize the pressure energy. | A process according to any one of claims 1 to 6, characterized in that the incoming flue gas stream is cooled by the outgoing flue gas stream using a wet heat exchanger (30, 31) with a separate liquid circulation, which simultaneously traps the solvent residue from the flue gas stream. 20th A · · · · * · according to any one of claims 1 to 6. a process characterized in that the incoming flue gas stream is cooled by an exhaust flue gas stream using a wet heat exchanger 25 (10.2, 10.3, 32) integrated in a column (10) used for leaching CO 2, wherein the cold solvent is obtained from the evaporator. cools (10.2) the incoming flue gas, whereupon the heated · · '···' solvent is led to the top of the column (10) (10.3), where it heats the flue gas and simultaneously cools itself before settling into the leaching portion. 30 The process according to any one of claims 1 to 8, characterized in that »» »t> |>; - at least part of the evaporation of the CO 2 solution obtained in the leaching process is carried out in a state which (27) is supplied with air, which is removed from the resulting CO 2 air mixture by pressurizing to 10 108616 and cooling until most of the CO 2 liquefies, - the liquefied CO 2 is added, either in liquid or gaseous form, to the CO 2 stream leaving the separation process, 5. the gaseous CO 2 air mixture remaining from the liquefaction is returned (28) to the CO 2 concentration of the solvent used for flue gas scrubbing. Process according to one of claims 1 to 9, characterized in that the pressurized flue gas leaving the separation process is used to produce mechanical or electrical energy by heating it and expanding it in a turbine or other power plant so that one or more heating and expansion steps can take place. 15