NL1015827C2 - Extraction of pure CO2 from flue gases. - Google Patents

Description

Extraction of pure CO2 from flue gases CO2 has a multitude of industrial applications. In particular in the food industry, CO2 is used for, among other things, the carbonization of soft drinks, beers and mineral water and the rapid freezing of foodstuffs. The requirements that the food industry places on the purity of the CO2 to be used are high. CO2 product that meets these requirements is referred to below as food grade CO2. There are also various applications in which CO2 of (very) high purity (high purity) is required, such as various analytical and medical applications and application as a coolant. Due to the high demands placed on food grade and high purity CO2, despite the large quantities that are produced, there is a shortage.

When fossil fuels are burned, large amounts of CO2 are released. This emission is partly responsible for the greenhouse effect. The recovery of this CO2 is therefore extremely important. However, these flue gases also contain other components in addition to CO2. These components must be removed if the CO2 is to be classified as high purity or food grade. Examples of these components are SO2, NOx and CO.

20 The extraction of CO2 takes place on a large scale in ammonia processes. By further purifying this CO2 it can be made suitable for use in the food industry. Since ammonia plants are not always present within the numerous CO2 outlets, not all demand can be met in this way.

Another method used to produce CO2 is the controlled burning of fossil fuels. With these systems, a CO2-rich gas must first be generated in order to extract the CO2 here.

Various solvents are used for CO2 recovery from gases. The disadvantage of most solvents is that they degenerate or that the desorption requires a lot of energy. If more energy is needed for desorption, the total energy required for the process, per unit of CO2 produced, will be high.

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The sulfur and nitrogen compounds, which occur in addition to CO2 in flue gases, cause extra inefficiency and problems in extracting CO2 from flue gases. Namely, when these acid gases are absorbed, they form so-called heat stable salts with the solvent, so that less solvent is available for the absorption of CO2. As a result, the consumption of solvent will increase considerably, often in combination with an increase in corrosion in the system. This can be prevented by removing the SO2 and NOx from the flue gas in advance. However, this requires a separate removal installation which must be switched for the CO2 unit.

In addition to the (conventional) absorption / desorption systems described above, there is the possibility of extracting CO2 from gas flows by means of membrane filtration techniques. However, with the help of this technique it is not possible to meet the specifications that apply to food grade and high purity. The other disadvantages are high investment costs and operational costs. Another possibility is the extraction of CO2 by means of cryogenic separation. In order to achieve high purity, separation of the contaminants upstream is necessary.

US 4,477,419 describes a process for recovering CO2 from gas containing CO2, oxygen and / or sulfur compounds, wherein an aqueous alkanolamine solution in combination with copper is used as the absorber. The degradation products of the solvent can be removed with an active carbon bed, mechanical filter and an anion exchange resin.

US 4,537,753 describes a process for removing CO 2 and H 2 S from natural gas using an aqueous absorption liquid containing methyl diethanol amine. The solvent is regenerated by flashing.

Up to now, it has not been possible with any of the processes known in the art to obtain very pure CO2 from flue gases which can be referred to as food grade or high purity. The object of the present invention is therefore to provide a method for recovering CO2 from flue gases, wherein the recovered CO2 can be used as food grade or high purity CO2.

To this end, the invention provides a method for recovering highly pure CO2 from flue gases, which method comprises the following steps: a) absorption of CO2 from the flue gases in a solvent in an absorber, wherein a CO2 poor gas stream and a CO2 rich solvent stream leave the absorber; b) desorption of the absorbed CO2 in a desorber, a CO2-rich gas stream and a low-CO2 solvent stream leaving the desorber; wherein the solvent used is an aqueous solution containing an alkanolamine and an activator.

Preferably the alkanolamine is selected from monoethanolamine, diethanolamine triethanolamine, methyl diethanolamine monoisopropanolamine, diisopropanolamine, triisopropanolamine.

In particular, methyl diethanolamine (CH 3 N (CH 2 CH 2 OH) 2), hereinafter referred to as MDEA, is used. The combination of MDEA and activator is generally referred to as aMDEA.

The alkanolamine concentration in water is 5 to 80% by weight, preferably 40 to 60% by weight.

Any activator suitable for such solvents can be used as an activator. Examples thereof are: polyalkylene polyamines, in particular diethylene triamine, triethylene tetramine, tetraethylene pentamine and dipropylene triamine; Alkylenediamines and cycloalkylene diamines, in particular hexamethylenediamine, aminoethylethanolamine, dimethylaminopropylamine and diamino-1,2-cyclohexane, heterocyclic aminoalkyl derivatives, in particular, piperazine, piperidine, furan, tetrahydrofuran, thiophene and tetrahydrothiophene, tetrahydrolidine pylpiperazine, 2-methylpiperazine, 2,5-dimethylpiperazine, aminoethylpiperidine, aminopropylpiperidine, 2-piperidineethanol and fufurylamine; alkoxyalkylamines, in particular methoxypropylamine and ethoxypropyl amine, and alkyl monoalkanolamines, in particular ethyl monoethanolamine and butyl monoethanolamine, 2-methylaminoethanol, 2-ethylaminoethanol, 2-isopropylaminoethanol and 2-n-butylaminoethanol.

Piperazine is preferably used.

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The activator concentration is 1 to 20% by weight, preferably 2 to 8% by weight in the undiluted parent solution. This parent solution, which will generally also contain MDEA, is generally diluted 0 to 4 times, preferably 2 to 3 times, with water before being added to the absorber.

The combination of methyl diethanolamine and piperazine has been found to be particularly suitable for obtaining high-purity CO2 from the relatively contaminated flue gases.

CO2 with very high purity, also referred to as high purity or food grade CO2 in this text, is defined here as CO2 of such quality that it can be used without further ado in the food industry. Food grade CO2 must contain at least 99.5% v / v CO2.

By means of the method according to the invention, sulfur and nitrogen compounds are removed simultaneously with the CO2. The sulfur and nitrogen compounds bind with the solvent. This bonding is not broken by heat supplied to the absorber. The CO2 produced contains only traces of these compounds. These levels are so low that the CO2 obtained meets the specifications for food grade and high purity CO2. A preliminary removal step to remove the sulfur and nitrogen compounds is therefore not necessary in the new process.

The method according to the invention is widely applicable for the extraction of food grade and high purity CO2 from flue gases. The homogeneous geographical spread of the locations of, for example, power plants now offers the possibility of producing food grade CO2 from the flue gases close to the point of sale within each CO2 outlet. The construction of installations for the production of CO2 in the vicinity of, for example, the food industry is therefore not necessary.

The energy costs required to absorb and desorb CO2 with the solvent used are low. The energy costs are therefore considerably lower than the energy costs required when conventional solvents such as potassium carbonate are used.

The operational costs are reduced by preventing the accumulation of heat stable salts and other substances in the solvent circuit, by purifying the solvent with active carbon filtration and ion exchange, as will be described below. 4 5 ven. In addition, the degradation problems of the solvent are limited by a good solvent choice.

According to the invention, flue gases are understood to mean the waste gases from the combustion of carbonaceous substances. Examples of this are the flue gases from 5 power plants or flue gases from cogeneration, gas turbines or utility centers.

Step a) of the process according to the invention takes place in an absorber, wherein a CO2-poor gas stream and a CO2-rich solvent stream leave the absorber. The contact between solvent and flue gas in the absorber preferably takes place in countercurrent. The absorber is generally a reactive absorber. The temperature in the absorber is 10 to 100 ° C, preferably 30 to 70 ° C.

Step b) takes place in a desorber. In the desorber, the CO2 is released from the solvent under the influence of heat. Absorbed contaminants such as SO2 and NOx are not desorbed. A CO2-rich gas stream and a CO2-poor solvent stream leave the desorber. The low CO2 solvent stream is returned to the absorber. The CO2-rich gas stream leaving the desorber is preferably cooled, after which some condensed liquid is separated and returned to the desorber. The desorber is a packed column in which the CO2 is removed from the solvent by countercurrent. The temperature in the desorber is generally 80 to 120 ° C, in particular 95 to 110 ° C.

According to a preferred embodiment of the invention, heat is exchanged between the CO2-rich solvent stream that leaves the absorber and the CO2-poor solvent stream that leaves the desorber and is recycled to the absorber. A considerable saving on the required amount of steam and cooling water can hereby be obtained.

In an advantageous manner for regenerating the solvent, at least a portion of the low CO2 solvent stream undergoes a filtration step before being returned to the absorber. The filtration step successively comprises a first mechanical filter, an active carbon filter and a second mechanical filter. The first filter is preferably a cartridge filter that removes particles> about 5 µm. The efficiency is greater than 99%. The second filter is also preferably a cartridge filter that removes particles> about 10 µm and has an efficiency greater than 99%.

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At least a portion of the low CO2 solvent stream that has undergone the filtration step is subjected to ion exchange before it is returned to the absorber. A cationic ion exchange resin is used.

The choice of equipment for regenerating the solvent can be determined by those skilled in the art on the basis of the desired efficiency of the separation step.

With the help of this method, so-called "heat stable salts" can be removed. These heat stable salts are salts of alkanolamine, in particular MDEA, which cannot be regenerated by heat (such as steam stripping). These salts reduce the effectiveness of treatment with MDEA. The salts of MDEA reduce the availability of the MDEA for absorption of CO2. These salts can also cause corrosion in equipment made from carbon steel.

Preferably, before supply to the absorber, the residual amount of oxygen in the flue gas is burned with the aid of a carbon-containing gas, such as natural gas. This allows the concentration of CO2 to be increased and the concentration of O2 to be lowered, which benefits the efficiency of the process.

The present invention also provides a device for carrying out the above process, which device comprises: an absorber, which absorber is provided with a supply for flue gases, a supply for a solvent for CO2, a discharge for a CO2 arnie gas stream, a discharge for a CO2-rich solvent stream and a make-up water supply; a desorber, provided with a feed for the CO2-rich solvent stream, a drain for a CO2-rich gas stream and a drain for a low-CO2 solvent stream.

The device is preferably provided with a heat exchanger, placed between absorber and desorber; which is adapted for heat exchange between the CO2-poor solvent stream from the desorber and the CO2-rich solvent stream from the absorber.

When filtration and / or ion exchange is used for regeneration of the solvent, the device is provided with a filtration unit to which at least a part of the CO2-poor solvent stream originating from the desorber is supplied and optionally with an ion exchanger to which at least at least part of 7 the CO2-poor solvent stream from the desorber is supplied after the filtration unit.

To further increase the CO2 concentration in the flue gas, the device can be provided with a combustion device provided with a supply for oxygen-rich flue gas, a supply for a carbon-containing gas, and a discharge for low-oxygen flue gas.

The present invention will now be further elucidated with reference to the accompanying figures, in which

Figure 1 is a process diagram of the method according to the present invention;

Figure 2 is a process diagram of the method of Example 1;

Figure 3 is a process diagram of the method of Example 2;

Figure 4 is a process diagram of the method according to Example 3.

In Figure 1 the incoming flue gas is cooled in the flue gas water cooler (G) if necessary and pumped through the flue gas compressor (I) to the absorber (A). If necessary, a knock-out drum (H) can be placed after the cooler (G) to separate (the resulting) droplets of mist. The CO2 present in the absorber (A) is absorbed by the circulating solvent. In addition to CO2, NOx and SO2 are also practically totally absorbed. The two last mentioned acid gases will not be desorbed in the stripper, but will form anions with the solvent. These anions lead to the formation of heat stable salts.

An outgoing CO2-poor gas stream (3) emerges from the absorber (A). This gas stream generally only contains 0.5 vol.% CO2. Because this stream contains more water vapor than the incoming gas stream, make-up water (2) is dosed. The CO2-rich solvent stream leaves the absorber via (4) and is passed via heat exchanger (J) to desorber (B). In the heat exchanger (J) the CO2-rich solvent stream (4) is heated by means of a CO2-poor solvent stream (5).

The absorbed CO2 is stripped from the absorbent in the desorber or stripper (B). The required energy is supplied to the strip-per (B) by means of reboiler (C). The gas leaving the stripper (B) through the top contains some water vapor and solvent in addition to CO2. These components are removed by cooling the gas stream in a condenser (D) and then separating the condensed droplets from 5 to 8 in the distillate drum (E). The collected liquid is returned to desorber (B). The CO2 gas (7) now obtained is of such a quality that it can be used as food grade and high purity CO2. By deeper cooling in the condenser (D), the water can also be completely removed.

5 The CO2-poor bottom stream (8) from the desorber (B) is returned to the absorber (A) via heat exchanger (J) and cooler (F). According to a second aspect of the invention, degradation products of the solvent and formed anions are removed from the low CO2 absorber stream (5) leaving the desorber, optionally after heat exchange has taken place in a heat exchanger (J) and the stream is cooled in cooler (F ).

The low CO2 solvent stream is filtered for this purpose in filtration unit (K). Filtration unit (K) consists successively of a first mechanical filter, an active carbon filter and a second mechanical filter. The first mechanical filter removes the components that could cause contamination of the active carbon filter. The activated carbon filter binds all organic breakdown products. The second mechanical filter prevents leached carbon from ending up in subsequent devices. The filtered solvent is returned to the system.

An ion exchanger (L) is installed to prevent heat stable salts from accumulating. Part of the stream from the filtration step (K) is passed through the ion exchanger 20. Here the anions present in the flue gas, formed from NOx and SO2, are removed. The regenerated solvent is returned to the absorber, optionally after passing through heat exchanger (J) and cooler (F). A small waste stream (9) is discharged from the ion exchanger (L).

Example 1 Food grade / high purity CO2 from flue gas from a power plant

Flue gas from a power plant is supplied with 225,147 NnrVuur to a device according to the invention as shown in Figure 2. The designations in this figure are similar to those in Figure 1. The composition of this smoke gas is (in% vol / vol): 0 1 S 8? 7 9

Component wet dry "CCb .................................... 415 ........ ........ 451 .............

Ar 0.93 1.01 02 11.96 13.00 N2 74.92 81.48 H2Q_8.04

In addition, the flue gas also contains the following components (in mg / Nm3): component concentration _ <100 ΝΟχ 50-77 SO2_0.39-0.405 The temperature and pressure of the flue gas are 100 ° C and 1.05 bara, respectively. Methyl diethanolamine in combination with aqueous piperazine is used as the solvent. The methyldiethanolamine concentration is 40% by weight.

The following process conditions were applied: flow temperature pressure capacity _fC) _ (bara) _ (kg / hour) flue gas 100 1.05 280160 outlet cooler_50_0.98_ absorber inlet 66 1.11 outgoing gas 47 1.01 263880 rich solution (off) 57 1.75 816300 poor solution ( in) _47_1.01_799810 desorber rich solution (in) 97 1.1 816300 poor solution (off) 106 1.95 799810 outgoing gas_96_LI_32610 solvent cooler poor solution (in) 104 10.0 poor solution (out) _47_93_

Condensate overhead condenser gas (in) 96 1.1 gas (out) 35 1.05 16490 10

With this process, 16 tons of CO2 per hour are obtained. The pure CO2 gas obtained has the following composition: \ ·; 7 10 component unit concentration CO2% v / v> 99.5 CO ppmv <25 NO ppmv <2.5 NO2 ppmv <2.5 S (total as SO2) ppbwt <50 BTX _ppmv_ <10_

This purity of the CO2 is such that it can be used without any doubt in the food industry. By removing the water from this CO2, a gas can be obtained that consists of 99.993% (v / v) of CO2.

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Example 2

From flue gas with the same composition as in Example 1, CO 2 gas produced with corresponding composition is produced in the same manner, with the proviso that a process scheme according to Figure 3 is applied. This means that a heat exchanger (J), a filter unit (K) and an ion exchanger (L) are used additionally with respect to figure 2.

The following process conditions were applied: flow temperature pressure capacity (° C) (bara) (kg / hour) flue gas 100 1.05 280160 outlet cooler_50_0.98_ absorber inlet 66 1.11 outgoing gas 47 1.01 263880 rich solution (off) 57 1.75 816300 poor solution ( in) _47_L01_799810 desorber rich solution (in) 97 1.1 816300 poor solution (off) 106 1.35 799810 outgoing gas_96_LI_32610 solvent heat exchanger rich solution (in) 57 5.7 816300 rich solution (off) 97 5.4 poor solution (in) 104 10.0 799810 poor solution (off) _ 67 9.6 solvent cooler poor solution (in) 67 9.6 799810 poor solution (off) 47 9.3 11

Condensate overhead condenser gas (in) 96 1.1 16490 gas (out) _35_L05_

Regeneration inlet filtration 47 9.3 41060 filtered solution 47 outlet cooler 30 outlet ion exchanger_30_1.95_310

Compared to example 1, this process offers the following advantages: • the required amount of steam has been reduced to approx. 44% and the required amount of cooling water has been reduced to approx.

5 33%.

• anti-foaming agent does not have to be dosed by catching the degradation products in (K).

• the regeneration in (L) reduces the amount of discharged solvent (MDEA) to 0.02%.

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Example 3

This example corresponds to example 2, with the proviso that pre-combustion of the residual oxygen in the flue gas is carried out with the aid of natural gas in the combustion unit (X) (see Figure 4).

15 For producing as much CO2 as in examples 1 and 2 there is 97961

Nm3 / hour flue gas instead of 225147 Nm3 / hour required. In addition, 4703 Nm3 / hour of natural gas is required for burning the residual oxygen in the flue gas. The other flows are similar to those in example 2.

This embodiment offers the following advantages: • due to the higher CO2 concentration, the process devices can be built smaller.

• the required amount of solvent is reduced due to the higher CO2 concentration.

· Due to the higher CO2 concentration, less activator is required. This lowers the disposal energy, thereby reducing the total energy requirement of the process.

? il 1 R Q o 7 12 • due to the lower O2 concentration, less solvent is broken down, reducing consumption.

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Claims (16)

A method for recovering highly pure CO2 from flue gases, the method comprising the steps of: a) absorption of CO2 from the flue gases in a solvent in an absorber, a CO2-poor gas stream and a CO2-rich solvent stream leaving the absorber; b) desorption of the absorbed CO2 in a desorber, a CO2-rich gas stream and a CO2-poor solvent stream leaving the desorber; Wherein the solvent used is an aqueous solution containing an alkanolamine and an activator. 2. A method according to claim 1, wherein the alkanolamine is selected from monoethanolamine, diethanolamine triethanolamine, methyl diethanolamine monoisopropanolamine, diisopropanolamine, triisopropanolamine. The method of claim 2, wherein the alkanolamine is methyl diethanolamine. The method according to any of claims 1 to 3, wherein pipera zine is used as the activator. The method of any one of claims 1 to 4, wherein the CO2 poor solvent stream is recycled to the absorber. 25 The method of claim 5, wherein heat is exchanged between the CO2 rich solvent stream leaving the absorber and the CO2 poor solvent stream leaving the desorber and returned to the absorber. A method according to any one of the preceding claims, wherein the CO2 rich gas stream leaving the desorber is cooled, after which some condensed liquid is separated and returned to the desorber. 1015827 The method of claim 5, wherein at least a portion of the low CO2 solvent stream from the desorber for recycling to the absorber undergoes a filtration step. 5 The method of claim 8, wherein the filtration step successively comprises a first mechanical filter, an active carbon filter, and a second mechanical filter. 10. Method according to claim 8 or 9, wherein at least a part of the CO2-poor solvent stream which has undergone the filtration step is subjected to ion exchange before it is returned to the absorber. A method according to any one of the preceding claims, wherein prior to introduction to the absorber the residual amount of oxygen in the flue gas is burned. 15 A device for recovering CO2 comprising: an absorber, which absorber is provided with a feed for flue gases, a feed for solvent for CO2, a drain for a low CO2 gas stream, a drain for a CO2 rich solvent stream and a feed for make -up water; 20 a desorber, provided with a feed for the CO2-rich solvent stream, a drain for a CO2-rich gas stream and a drain for a CO2-poor solvent stream. 13. Device as claimed in claim 12, which is provided with a heat exchanger, placed between absorber and desorber, arranged for heat exchange between the CO2-poor solvent stream from the desorber and the CO2-rich solvent stream from the absorber. 14. Device as claimed in claim 12 or 13, which is provided with a filtration unit 30 to which at least a part of the CO2-poor solvent stream from the desorber is supplied. T '* 7 15. Device as claimed in claim 12, 13 or 14, which is provided with an ion exchanger to which at least a part of the CO2-poor solvent stream from the desorber is supplied. Device as claimed in any of the claims 12 to 15, which is provided with a combustion device provided with a supply for oxygen-rich flue gas, a supply for a carbon-containing gas and a discharge for low-oxygen flue gas. 1915827