RECOVERY OF PURE CO2 FROM FLUE GASES

Abstract: The present invention relates to a method for the recovery of very 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, a CO2-depleted 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-depleted solvent stream leaving the desorber; wherein the solvent used is an aqueous solution that contains an alkanolamine and an activator. Preferably a combination of methyldiethanolamine and piperazine is used. Using the method according to the invention it is possible to recover very pure CO2, that can be designated as high purity or food grade CO2.

Description: Recovery of pure CO 2 from flue gases CO 2 has a multiplicity of industrial applications. In the food industry in particular CO 2 is used for, inter alia, carbonating soft drinks, beers and mineral water and the rapid freezing of foods. The requirements imposed by the food industry on the purity of the CO2 to be used are high. CO 2 product that meets these requirements is hereinafter referred to as food grade CO 2 . Furthermore, there are various other applications in which CO 2 of (very) high purity is required; such as various analytical and medical applications and use as a coolant. Because of the stringent requirements imposed on food grade and high purity CO 2 , there is a shortage, despite the large quantities that are produced. During the combustion of fossil fuels in particular large quantities of CO 2 are liberated. These emissions are 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 CO 2 . These components have to be removed if the CO2 is to achieve the high purity or food grade qualification. Examples of these components are SO2, NOxand CO. CO2 recovery 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 there are not always ammonia plants in the vicinity of the numerous areas where there is a demand for CO2, it is not possible to meet the entire demand in this way. Another method that is used to produce CO2 is the controlled combustion of fossil fuels. With these systems a CO2-rich gas must first be generated in order to be able to recover the CO2 from this. Various solvents are used for the recovery of CO2 from gases. The disadvantage of the majority of solvents is that they degenerate or that the desorption requires a great deal of energy. If more energy is required for desorption, the total energy demand of the process, per unit CO2 produced, will be high. The sulphur and nitrogen compounds which are present in flue gases beside CO2 give rise to additional inefficiency and problems when recovering CO2 from flue gases. Because if these acid gases are absorbed, they form so-called heat stable salts with the solvent, as a result of which less solvent is available for the absorption of CO2. The consumption of solvent will increase appreciably as a result, frequently accompanied by an increase in corrosion in the system. This can be prevented by removing the SO2 and NO x from the flue gas beforehand. However, this requires a separate removal installation which has to be incorporated upstream of the CO2 unit. In addition to the (conventional) absorption/desorption systems described above, CO2 can be recovered from gas streams by means of membrane filtration techniques. However, it is not possible with this technology to meet the specifications which apply for food grade and high purity. The other disadvantages are high investment costs and operational costs. Another option is recovery of CO2 by means of cryogenic separation. With this technique upstream separation of the impurities is necessary in order to obtain high purity. US 4 477 419 describes a process for the recovery of CO2 from gas that contains CO2, oxygen and/or sulphur compounds, an aqueous alkanolamine solution in combination with copper as an absorbent. The degradation products of the solvent can be removed by an active carbon bed, mechanical filter and an anion exchange resin. US 4 537 753 describes a process for the removal of CO2 and H2S from natural gas with an aqueous absorption fluid that contains methyldiethanolamine. The solvent is regenerated by flashing. To date it has not been possible with any of the processes known from the prior art to obtain very pure CO2 that can be designated food grade or high purity using flue gases as the starting material. The aim of the present invention is therefore to provide a method for the recovery of 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 the recovery of very 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, a CO2-depleted 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- depleted solvent stream leaving the desorber; wherein the solvent used is an aqueous solution that contains an alkanolamine and an activator. Preferably, the alkanolamine is chosen from monoethanolamine, diethanolamine, triethanolamine, methyldiethanolamine, monoisopropanolamine, diisopropanolamine and triisopropanolamine. Methyldiethanolamine (CH3N(CH2CH2OH)2), hereinafter designated MDEA, is used in particular. The MDEA and activator combination is generally designated as aMDEA. The alkanolamine concentration in water is 5 to 80 wt.%, preferably 40 to 60 wt.%. The activator used can be any activator suitable for such solvents. Examples of such activators are: polyalkylenepolyamines, in particular diethylenetriamine, triethylenetetramine, tetraethylenepentamine and dipropylenetriamine; alkylenediamines and cycloalkylenediamines, in particular hexamethylenediamine, aminoethylethanolamine, dimethylaminopropylamine and diamino-l,2-cyclohexane, heterocyclic aminoalkyl derivatives, in particular piperazine, piperidine, furan, tetrahydrofuran, thiophene and tetrahydrothiophene, aminoethylpiperazine, aminopropyl- piperazine, 2-methylpiperazine, 2,5-dimethylpiperazine, aminoethylpiperidine, aminopropylpiperidine, 2-piperidine-ethanol and furfurylamine; alkoxylalkylamines, in particular methoxypropylamine and ethoxypropylamine, and alkylmonoalkanolamines, in particular ethylmonoethanolamine and butylmono- ethanolamine, 2-methylaminoethanol, 2-ethylaminoethanol, 2-isopropylaminoethanol and 2-n-butylaminoethanol. Piperazine is preferably used. The activator concentration is 1 to 20 wt.%, preferably 2 - 8 wt.%, in the undiluted mother solution. This mother solution, which in general will also contain MDEA, is generally diluted 0 to 4 times, preferably 1.5 to 3 times, with water before it is fed to the absorber. The combination of methyldiethanolamine and piperazine has proved particularly suitable for obtaining CO2 of high purity from the relatively contaminated flue gases. CO2 of very high purity, also referred to in this text as high purity or food grade CO 2 , is defined here as CO 2 of a quality such that it can be used as such in the food industry. Food grade CO must contain at least 99.5 % (V/V) CO 2 . By means of the method according to the invention, together with CO2, sulphur and nitrogen compounds are removed. The sulphur and nitrogen compounds bind to the solvent. This bond is not broken by the 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. An upstream removal step for removal of the sulphur and nitrogen compounds is therefore not necessary in the new method. The method according to the invention has broad applicability for the recovery of food grade and high purity CO2 from flue gases. The homogeneous geographic spread of the locations of, for example, power stations now offers the possibility for producing food grade CO2 from the flue gases close to the outlet in any region where CO2 is sold. The construction of installations for the production of CO2 in the vicinity of, for example, the food industry is therefore not necessary. The requisite energy costs for absorbing and desorbing CO2 with the solvent used are low. The energy costs are consequently appreciably lower than the energy costs which are required when conventional solvents such as potassium carbonate are used. The operational costs are lowered by the prevention of accumulation of heat stable salts and other substances in the solvent circuit, by purification of the solvent using active carbon filtration and ion exchange, as will be described below. In addition, the degradation problems of the solvent are restricted by a good choice of solvent. According to the invention flue gases are understood to be the off-gases from the combustion of carbon-containing substances. Examples of these are the flue gases from power stations or flue gases originating from combined heat and power plants, gas turbines or utility centres. Step a) of the method according to the invention takes place in an absorber, a CO2- depleted gas stream and a CO2-rich solvent stream leaving the absorber. Preferably, contact between solvent and flue gas in the absorber takes place in counter-current. The absorber is in general 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 liberated from the solvent under the influence of heat. Absorbed impurities such as SO2 and NO x are not desorbed. A CO2-rich gas stream and a CO2-depleted solvent stream leave the desorber. The CO2- depleted solvent stream is returned to the absorber. The CO2-rich gas stream that leaves the desorber is preferably cooled, after which some condensed fluid is separated off and returned to the desorber. The desorber is a packed column in which the CO2 is removed from the solvent counter-currently. 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-depleted solvent stream that leaves the desorber and is returned to the absorber. An appreciable saving in the quantity of steam and cooling water required can be achieved by this means. According to an advantageous method for regeneration of the solvent, at least a portion of the CO2-depleted solvent stream is subjected to a filtration step before it is returned to the absorber. The filtration step comprises, successively, a first mechanical filter, an active carbon filter and a second mechanical filter. The first filter is preferably a cartridge filter that removes particles > approx. 5 μm. The efficiency is higher than 99 %. The second filter is likewise preferably a cartridge filter that removes particles > approx. 10 μm and has an efficiency higher than 99 %. At least a portion of the CO2-depleted solvent stream which has been subjected to 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 regeneration of the solvent can be determined by a person skilled in the art on the basis of the desired efficiency of the separation step. So-called "heat stable salts" can be removed with this method. These heat stable salts are alkanolamine salts, in particular of MDEA, which cannot be regenerated by means of heat (such as steam stripping). These salts lower the efficiency of the treatment with MDEA. The MDEA salts lower the availability of MDEA for absorption of CO2. These salts can also give rise to corrosion in equipment made of carbon steel. Preferably, the residual amount of oxygen in the flue gas is combusted using a carbon-containing gas, such as natural gas, before the flue gas is fed to the absorber. Thus, the CO 2 concentration can be increased and the O2 concentration can be lowered, which is beneficial for the efficiency of the process. The present invention also provides an installation for carrying out the abovementioned process, which installation comprises: an absorber, which absorber is provided with a feed for flue gases, a feed for a solvent for CO2, a discharge for a CO2-depleted gas stream, a discharge for a CO2-rich solvent stream and a feed for make-up water; a desorber, provided with a feed for the CO2-rich solvent stream, a discharge for CO2-rich gas stream and a discharge for a CO 2 -depleted solvent stream. Preferably, the installation is provided with a heat exchanger, positioned between absorber and desorber, which heat exchanger is equipped to exchange heat between the CO 2 -depleted solvent stream originating from the desorber and the CO2-rich solvent stream originating from the absorber. If filtration and/or ion exchange is used for regeneration of the solvent, the installation is provided with a filtration unit to which at least a portion of the CO2-depleted solvent stream originating from the desorber is fed and optionally with an ion exchanger to which at least a portion of the CO2-depleted solvent stream originating from the desorber downstream of the filtration unit is fed. In order further to increase the CO2 concentration in the flue gas, the installation can be provided with a combustion installation provided with a feed for oxygen-rich flue gas, a feed for a carbon-containing gas and a discharge for oxygen-depleted flue gas. The present invention will now be further explained with reference to the appended 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 according to Example 1; Figure 3 is a process diagram of the method according to Example 2; Figure 4 is a process diagram of the method according to Example 3. In Figure 1 the incoming flue gas is, if necessary, cooled in the flue gas water cooler (G) and pumped through the flue gas compressor (I) to the absorber (A). If necessary, a knock-out drum (H) can also be positioned downstream of the cooler (G) in order to separate off mist droplets (that have formed). The CO 2 present is absorbed by the circulating solvent in the absorber (A). In addition to CO2, NO x and SO2 are also virtually completely absorbed. The two latter 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 CO 2 -depleted gas stream (3) issues from the absorber (A). This gas stream in general has a residual CO2 content of only 0.5 % (V/V). Because this stream contains more water (vapour) than the incoming gas stream, make-up water (2) is metered in. The CO2-rich solvent stream leaves the absorber via (4) and is fed via heat exchanger (J) to desorber (B). In the heat exchanger (J) the CO 2 -rich solvent stream (4) is heated by means of CO2-depleted solvent stream (5). The absorbed CO2 is stripped from the absorbent in the desorber or stripper (B). The requisite energy is supplied to the stripper (B) by means of reboiler (C). In addition to CO2, the gas that leaves the stripper (B) via the top contains some water vapour and solvent. These components are removed by cooling the gas stream in a condenser (D) and then separating off the condensed droplets in the distillate drum (E). The collected liquid is returned to desorber (B). The CO2 gas (7) now obtained is of such quality that it can be used as such as food grade and high purity CO2. By cooling to a lower temperature in the condenser (D) it is also possible to remove all of the water. The CO2-depleted bottom stream (8) from the desorber (B) is returned via heat exchanger (J) and cooler (F) to absorber (A). According to a second aspect of the invention, degradation products of the solvent and anions formed are removed from the CO2-depleted absorbent stream (5) that leaves the desorber, optionally after heat exchange has taken place in a heat exchanger (J) and the stream has been cooled in cooler (F). For this purpose the CO2-depleted solvent stream is filtered 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 contaminate the active carbon filter. The active carbon filter binds all organic degradation products. The second mechanical filter prevents carbon that has been flushed out from passing into downstream equipment. The filtered solvent is returned to the system. An ion exchanger (L) is installed to prevent accumulation of heat stable salts. A portion of the stream from the filtration step (K) is fed through the ion exchanger. The anions, formed from NO x and SO2, present in the flue gas are removed here. The regenerated solvent is returned to the absorber, optionally after passing through heat exchanger (J) and cooler (F). A small effluent stream (9) is discharged from the ion exchanger (L). Example 1 Food grade/high purity CO2 from flue gas from a power station Flue gas from a power station is fed at a rate of 225,147 Nm /hour to an installation according to the invention as shown in Figure 2. The designations in this figure correspond to those in Figure 1. The composition of this flue gas is (in % (V/V)): Component Wet Dry CO 2 4.15 4.51 Ar 0.93 1.01 O 2 11.96 13.00 N 2 74.92 81.48 H2O 8.04 ~ In addition the flue gas also contains the following components (in mg/Nm ): Component Concentration CO <100 NO x 50-77 SO 2 0.39 - 0.405 The temperature and the pressure of the flue gas are, respectively, 100 °C and 1.05 bara. The solvent used is methyldiethanolamine in combination with piperazine in water. The methyldiethanolamine concentration is 40 wt %. The following process conditions were used: Stream Temperature Pressure Capacity (°C) (bara) (kg/h) flue gas 100 1.05 280160 cooler outlet 50 0.98 Absorber inlet 66 1.11 outgoing gas 47 1.01 263880 rich solution (out) 57 1.75 816300 depleted solution (in) 47 1.01 799810 Desorber rich solution (in) 97 1.1 816300 depleted solution (out) 106 1.95 799810 outgoing gas 96 1.1 32610 Cooler solvent depleted solution (in) 104 10.0 depleted solution (out) 47 9.3 Overhead condenser condensate gas (in) 96 1.1 gas (out) 35 1.05 16490 16 tonnes of CO 2 per hour are obtained by this process. The pure CO2 gas obtained has the following composition: Component Unit Concentration CO 2 % (V/V) >99.5 CO ppm(V/V) <25 NO ppm(V/V) <2.5 NO 2 ppm(V/V) <2.5 S (total as SO 2 ) ppb(m/m) <50 BTX ppm(V7V) <10 This purity of the CO2 is such that it can be used without any problem in the food industry. A gas that contains 99.993 % (V/V) CO2 can be obtained by removing the water from this CO2. Example 2 CO2 gas having the same composition as in Example 1 is produced in the same way from flue gas, except that a process diagram according to Figure 3 is used. This means that, compared with Figure 2, a heat exchanger (J), a filter unit (K) and an ion exchanger (L) are additionally used. The following process conditions were used: Stream Temperature Pressure Capacity (°C) (bara) (kg/h) flue gas 100 1.05 280160 cooler outlet 50 0.98 Absorber inlet 66 1.11 outgoing gas 47 1.01 263880 rich solution (out) 57 1.75 816300 depleted solution (in) 47 1.01 799810 Desorber rich solution (in) 97 1.1 816300 depleted solution (out) 106 1.35 799810 outgoing gas 96 1.1 32610 Heat exchanger solvent rich solution (in) 57 5.7 816300 rich solution (out) 97 5.4 depleted solution (in) 104 10.0 799810 depleted solution (out) 67 9.6 Cooler solvent depleted solution (in) 67 9.6 799810 depleted solution (out) 47 9.3 Overhead condenser condensate gas (in) 96 1.1 gas (out) 35 1.05 16490 Regeneration filtration inlet 47 9.3 41060 filtered solution 47 cooler outlet 30 ion exchanger outlet 30 1.95 310 Compared with Example 1, this process offers the following advantages: • as a result of the heat recovery employed, the quantity of steam required is reduced to approximately 44 % and the quantity of cooling water required is reduced to approximately 33 %. • because the degradation products are collected in (K) it is not necessary to meter in any anti-foam agent. • as a result of the regeneration in (L) the quantity of solvent (MDEA) drained off is reduced to 0.02 %. Example 3 This example corresponds to Example 2, except that pre-combustion of the residual oxygen in the flue gas is carried out with the aid of natural gas in combustion unit (X) (see Figure 4). h order to produce the same quantity of CO2 as in Examples 1 and 2 97961 Nm /h flue gas is required instead of 225147 Nm /h. In addition, 4703 Nm /h natural gas is required for combustion of the residual oxygen in the flue gas. The other streams are comparable to those in Example 2. This embodiment offers the following advantages: • as a result of the higher CO2 concentration the process equipment can be of smaller construction. • as a result of the higher CO2 concentration the quantity of solvent required is reduced. • as a result of the higher CO2 concentration less activator is required. This reduces the desorption energy, as a result of which the total energy requirement of the process is reduced. • as a result of the lower O2 concentration, less solvent is degraded, whereby the consumption thereof decreases.

Claims: Claims 1. Method for the recovery of very 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, a CO 2 -depleted 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- depleted solvent stream leaving the desorber; wherein the solvent used is an aqueous solution that contains an alkanolamine and an activator. 2. Method according to Claim 1, wherein the alkanolamine is selected from monoethanolamine, diethanolamine, triethanolamine, methyldiethanolamine, monoisopropanolamine, diisopropanolamine, and triisopropanolamine. 3. Method according to Claim 2, wherein the alkanolamine is methyldiethanolamine. 4. Method according to any of Claims 1 to 3, wherein piperazine is used as the activator. 5. Method according to any of Claim 1 to 4, wherein the CO2-depleted solvent stream is returned to the absorber. 6. Method according to Claim 5, wherein heat is exchanged between the CO2-rich solvent stream that leaves the absorber and the CO2-depleted solvent stream that leaves the desorber and is returned to the absorber. 7. Method according to one of the preceding claims, wherein the CO2-rich gas stream that leaves the desorber is cooled, whereafter some condensed fluid is separated off and returned to the desorber. 8. Method according to Claim 5, wherein at least a portion of the CO2-depleted solvent stream from the desorber is subjected to a filtration step before it is returned to the absorber. 9. Method according to Claim 8, wherein the filtration step comprises, successively, 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 portion of the CO2-depleted solvent stream that has been subjected to the filtration step is subjected to ion exchange before it is returned to the absorber. 11. Method according to one of the preceding claims, wherein the residual amount of oxygen in the flue gas is combusted before the flue gas is fed to the absorber. 12. Installation for the recovery of CO2, comprising: an absorber, which absorber is provided with a feed for flue gases, a feed for a solvent for CO2, a discharge for a CO2-depleted gas stream, a discharge for a CO 2 -rich solvent stream and a feed for make-up water; a desorber, provided with a feed for the CO2-rich solvent stream, a discharge for CO2-rich gas stream and a discharge for a CO2-depleted solvent stream. 13. Installation according to Claim 12, which is provided with a heat exchanger, positioned between absorber and desorber, equipped for exchanging heat between the CO2- depleted solvent stream originating from the desorber and the CU2-rich solvent stream originating from the absorber. 14. Installation according to Claim 12 or 13 which is provided with a filtration unit to which at least a portion of the CO2 solvent stream originating from the desorber is fed. 15. Installation according to Claim 12, 13 or 14 which is provided with an ion exchanger to which at least a portion of the CO2-depleted solvent stream originating from the desorber is fed. 16. Installation according to any of Claims 12 to 15, which is provided with a combustion installation provided with a feed for oxygen-rich flue gas, a feed for a carbon- containing gas and a discharge for oxygen-depleted flue gas.

Inventors: DE WIT JACOBUS JOHANNES, DE WIT, JACOBUS, JOHANNES
Applicants: CONTINENTAL ENGINEERING B V, WIT JACOBUS JOHANNES DE, CONTINENTAL ENGINEERING B.V., DE WIT, JACOBUS, JOHANNES
Application year: 2001
Main classification: B01D53
IPC codes: B01D53, C01B31