Patent Application
20130284020
The present invention relates to a process for removing carbon dioxide from a gas stream, especially flue gases.
During the last decades there has been a substantial global increase in the amount of carbon dioxide emission to the atmosphere. Emissions of carbon dioxide into the atmosphere are thought to be harmful due to its “greenhouse gas” property, contributing to global warming. Following the Kyoto agreement, carbon dioxide emission has to be reduced in order to prevent or counteract unwanted changes in climate. The largest sources of carbon dioxide emission are combustion of fossil fuels, for example coal or natural gas, for electricity generation and the use of petroleum products as a transportation and heating fuel. These processes result in the production of gases comprising carbon dioxide. Thus, removal of at least part of the carbon dioxide prior to emission of these gases into the atmosphere is desirable.
Processes for removal of carbon dioxide from gases are known in the art. For example, in WO-A-95/21683 a process for removing and preventing emissions of carbon dioxide into the atmosphere is described. In the process described in WO-A-95/21683, carbon dioxide is transferred from exhaust gas exiting a gas turbine to an absorption column where it is contacted with an absorption liquid. Subsequently, the carbon dioxide enriched absorption liquid is regenerated by passing it to a stripper unit which is operated at a temperature of 120-150° C. where the carbon dioxide is released from the absorption liquid. A severe drawback of the process described in WO-A-95/21683 is the high energy consumption in the stripper unit, leading to a lower overall energy output. To improve the efficiency, the exhaust gas exiting the gas turbine is passed through a heat recovery steam generator, producing high pressure steam and low pressure steam. The low pressure steam is used to achieve the elevated temperatures needed for the regeneration of the absorbing liquid. Although this results in a better overall efficiency, additional equipment is needed.
Another example for removal of carbon dioxide from gases is described in US 2009/0199709. It describes a process using an absorbent that divides into two liquid fractions after acidic compounds have been absorbed, upon heating. The absorbent can be heated to a temperature ranging from 60° C. to 100° C. upon which phase separation takes place. In a next step the phase containing the acidic compounds is separated and regenerated using a distillation column, of which the reboiler is operated at a temperature above 100° C. A disadvantage of the process described in US 2009/0199709 is that regeneration still takes place in a distillation column at higher temperatures, resulting in high energy consumption, again leading to a lower overall energy output.
Thus, there is a need for a more energy-efficient and simple process for removal of carbon dioxide from gases.
It has now been found that carbon dioxide can effectively be removed from a gas stream in a very energy-efficient manner when a gas stream is first contacted with an absorption solvent, and the carbon dioxide enriched absorption solvent so obtained is separated into a mixture comprising three phases, which mixture is passed to a gas lift reactor wherein the absorption solvent is regenerated and carbon dioxide is released from the absorption solvent.
Accordingly, the present invention relates to a process for removing carbon dioxide from a gas stream comprising the steps of:
In accordance with the present invention the regeneration of the absorption solvent can be carried out in the gas lift reactor at a much lower temperature than that which is applied in conventional stripper units wherein adsorption liquids are regenerated, bringing about a substantial improvement in overall energy efficiency. Suitably, the process leads to an improvement in the overall energy efficiency of 5 to 35%, preferably of 10 to 35%, and more preferably of 15-35%.
The present invention relates to a process for removing carbon dioxide from a gas stream. The process is suitable for any gas comprising carbon dioxide. For example, the gases to be treated may be natural gas, synthesis gas, obtained for instance by (catalytic) partial oxidation and/or by steam methane reforming of hydrocarbons, e.g. methane, natural or associated gas, naphtha, diesel and liquid residual fractions, gases originating from coal gasification, coke oven gases, refinery gases, hydrogen and hydrogen containing gases, and flue gases. Suitably, the gas comprises in the range of from 0.25 to 70% (v/v) of carbon dioxide, preferably from 1 to 45% (v/v). In the event that the gas is a flue gas, the amount of carbon dioxide will generally be lower, suitably from 0.25 to 20% (v/v) and the gas will usually also comprise oxygen, preferably in the range of from 0.25 to 20% (v/v), more preferably from 0.5 to 15% (v/v), still more preferably from 1 to 10% (v/v). As the temperature of the gas stream to be treated will typically be relatively high, preferably the gas stream is cooled prior to being subjected to step (a). Cooling of the gas stream prior to being subjected to step (a) may be done by means known in the art, for example using a fan, a cooler or a gas-gas exchanger.
In accordance with the present invention the gas stream to be treated is contacted in step (a) with an absorption solvent. Suitably, the absorption solvent comprises one or more amines. The amines may be a single primary, secondary or tertiary amine, or may be mixtures of primary amines, secondary amines and/or tertiary amines.
Suitably, the amine to be used in accordance with the present invention has the general formula R1R2R3N, wherein R1, R2 and R3 are independently selected from the group consisting of hydrogen, alkyl, cycloalkyl, alkenyl, alkoxy, alkenyl-O—X, wherein X is selected from the group consisting of hydrogen, alkyl, cycloalkyl, aryl, arylene, halogen alkyl. The alkyl may suitably be a primary, secondary or tertiary alkyl. The alkyl may be linear or branched. The alkyl may comprise 1-12 carbon atoms, in particular 1-6 carbon atoms. With more than 6 carbon atoms in the alkyl group, there is the potential that the amine solidifies upon reaction with carbon dioxide.
The alkenyl may be a primary, secondary or tertiary alkenyl. The alkenyl may be linear or branched. The alkenyl may comprise 1-12 carbon atoms, in particular 1-6 carbon atoms. The cycloalkyl may be C3-C9 cycloalkyl, in particular C4-C8 cycloalkyl and C6-C8 cycloalkyl.
The alkoxy may be a primary, secondary or tertiary alkoxy. The alkoxy may be linear or branched. The alkoxy may comprise 1-12 carbon atoms, in particular 1-6 carbon atoms. With more than 6 carbon atoms in the alkoxy group, there is the potential that the amine solidifies upon reaction with carbon dioxide.
Suitably, the absorption solvent comprises at least a primary amine selected from the group consisting of pentylamine, hexylamine, heptylamine, octylamine, cyclohexylamine, 2-methylcyclohexylamine, 2-methylbutylamine, 2-aminopentane, 2-aminoheptane, 2-aminohexane, 2-aminooctane, 2-aminononane, 3-methoxypropylamine, 2-methyl-1,5-diaminopentane, geranylamine, 2-ethyl-1-hexylamine, 6-methyl-2-heptylamine, cyclooctylamine, aniline, N-phenylethylendiamine, 2-phenylethylamine, N,N-dimethyl-4-cyclooctene-1-amine, and any mixture thereof.
Suitably, the absorption solvent comprises at least a secondary amine selected from the group consisting of dipropylamine, N-ethylbutylamine, dibutylamine, diisopropylamine, dicyclohexylamine, bis(2-ethylhexyl)amine, bis(alpha methylbenzyl)amine, bis(1.3-dimethylbutyl)amine, diallylamine, bis[(s)-1-phenylethyl]amine, bis-sec-butylamine, 2,2,6,6-tetramethylpiperidine, N-methylcyclohexylamine, benzyl-tert-butylamine, bis(2-ethylhexyl)amine, 4-tert-butylcyclohexylamine, and any mixture thereof.
Suitably, the absorption solvent comprises at least a tertiary amine selected from the group consisting of triethylamine, 2-(diethylamino)-ethanol, tripropylamine, tributylamine, N,N-dimethylcyclohexylamine, dimethyloctylamine, dimethyl(1-methyl-heptyl)-amine, dimethylallylamine, N-ethyldiisopropylamine, tris(2-ethylhexyl)amine, bis-(2-cyclohexyloxy-ethyl)-methyl-amine, bis(2-(2,4-diethyloctyloxy)-ethyl)-methyl-amine, (2-(2-dimethylamino-ethoxy)-ethyl)-dimethyl-amine, N-isopropylethylenediamine, N-methylenediamine, N,N-dimethylethylenediamine, N,N-dibutyltrimethylenediamine, tris[2-(isopropylamino)ethyl]amine, tris[2-(methylamino) ethyl]amine, and any mixture thereof.
Preferably, the absorption solvent comprises at least a secondary and/or at least a tertiary amine.
In accordance with the invention the absorption solvent has suitably a boiling point of at least 100° C., preferably at least 110° C., and more preferably at least 120° C. The absorption solvent has suitably a pKa value in the range of from 7.5-11, preferably in the range of from 8-10.5, and more preferably in the range of from 9-10. Suitably, the absorption solvent has a density at 25° C. in the range of from 0.7-1.1 g/mL.
Preferably, the absorption solvent comprises at least a secondary amine and at least a tertiary amine. The ratio secondary amine to tertiary amine is suitably in the range of from 0.5-30, preferably in the range of from 1-20, and more preferred in the range of from 2-10.
Suitably, the at least secondary amine has an absorption rate in the range of from 10−6-10−4 kmol/m2/s, preferably in the range of from 5·10−5-10−4 kmol/m2/s, and more preferably in the range of from 10−5-10−4 kmol/m2/s. Suitably, the at least tertiary amine has an absorption rate in the range of from 10−6-10−4 kmol/m2/s, preferably in the range of from 5·10−5-10−4 kmol/m2/s, and more preferably in the range of from 10−5-10−4 kmol/m2/s.
Suitably, in step (a) the gas stream is contacted with an aqueous solution that comprises the absorption solvent in a concentration in the range of from 1-7M, preferably in the range of from 1-5M.
Step (a) is suitably carried out at a pressure in the range of from 1-150, preferably between 25 and 90 bar, and at a temperature in the range of from 0-60° C., preferably in the range of from 35-55° C. Step (a) results in a gas stream with a reduced carbon dioxide content and absorbing solvent rich in carbon dioxide. Suitably, more than 80%, preferably more than 90% and more preferably more than 95% of the carbon dioxide is removed.
In step (b) at least part of the gas stream with reduced carbon dioxide content obtained in step (a) is withdrawn. As this purified gas stream comprises low concentrations of carbon dioxide, it may now be vented into the atmosphere, or used elsewhere.
In step (c) at least part of the carbon dioxide-containing absorption solvent obtained in step (a) is heated to obtain a mixture comprising a gas phase, an aqueous liquid phase and a non-aqueous liquid phase, wherein carbon dioxide is predominantly present in the gas phase. Preferably, the entire carbon dioxide-containing absorption solvent as obtained in step (a) is heated in step (c) to obtain the mixture comprising a gas phase, an aqueous liquid phase and a non-aqueous liquid phase. In step (c) carbon dioxide is predominantly present in the gas phase. The small amount of the carbon dioxide still absorbed is predominantly present in the aqueous liquid phase. Suitably, at least 70%, preferably at least 80%, and more preferably at least 90% of the carbon dioxide is present in the gas phase. More preferably, at least 95%, and most preferably at least 99% of the carbon dioxide is present in the gas phase. Suitably, at least 60%, preferably at least 70%, of the amount of the carbon dioxide still absorbed is predominantly present in the aqueous liquid phase.
In step (c), the separation into a gas phase, an aqueous liquid phase and a non-aqueous liquid phase is established by heating the absorption solvent. Preferably, the absorption solvent is heated to a temperature in the range of from 60-100° C., more preferably 65-95° C., and most preferably 65-85° C. Suitably, the heating is performed via heat exchange of the carbon dioxide-containing absorption solvent with a hot gas or liquid. This heating step effects that the mixture is obtained having a temperature close to or in the range of the temperature at which regeneration of carbon dioxide-containing absorption solvent will take place in step (d). At least part of the heat requirements of step (c) can suitably be fulfilled using heat from at least part of the gas stream which comprises carbon dioxide as withdrawn from step (e) generated in the regeneration performed in step (d).
In step (d) at least part of the mixture obtained in step (c) is introduced into a gas lift reactor to further release carbon dioxide absorbed in the aqueous liquid phase and the non-aqueous liquid phase, whereby the gas phase establishes circulation and mixing of the aqueous liquid phase and the non-aqueous liquid phase between a lower section of the gas lift reactor and an upper section of the gas lift reactor. Preferably, the entire mixture as obtained in step (c) is introduced into a gas lift reactor. Preferably, in step (d) the carbon dioxide present in the gas phase has a superficial velocity in the range of from 0.01-1 m/s. In step (d), the aqueous liquid phase and the non-aqueous liquid phase have a superficial velocity in the range of from 0-1.5 m/s. In addition, one or more inert gases may also be introduced together with the mixture into the bottom part of the gas lift reactor to ensure proper circulation of the mixture inside the gas lift reactor. The advantage of using such gas lift reactor is that a very good circulation and mixing of the three phases, being the gas phase, the aqueous liquid phase and the non-aqueous liquid phase, is established. As an effect of this circulation, the biphasic solvents are maintained in a finely dispersed turbid state and thereby extensive phase separation of the aqueous liquid phase and the non-aqueous liquid phase (or biphasic solvent) is avoided. If one would use a stirred tank reactor, which is the common practise for regenerating such biphasic solvent systems, the mixture obtained from step (c) of the process needs to be mixed in the stirred tank reactor by mechanical stirring. The energy saving established by using the gas lift reactor is significant.
In the process according to the present invention any known type of gas lift reactor can suitably be used. In this respect reference can for instance be made to Ullmann's Encyclopedia of Industrial Chemistry, Vol. B4, pages 290-291, 1992, VCH Publishers, Inc., wherein a number of gas lift reactors has been described.
Suitably, the gas lift reactor comprises a riser connected to an upper and lower part of the reactor, a downcomer arranged around the riser, inlet means for introducing the absorption solvent which comprises the three phases, outlet means for withdrawing the gas stream, and outlet means for withdrawing the mixed aqueous liquid phase and the non-aqueous liquid phase or regenerated absorption solvent. Preferably, the height-to diameter ratio of the riser is in the range of from 1-15.
In a particular attractive embodiment of the present invention the gas lift reactor comprises a reactor having the form of a loop, which loop comprises inlet means for introducing the mixture as obtained in step (c), outlet means for withdrawing the gas stream, and outlet means for withdrawing the mixed aqueous liquid phase and the non-aqueous liquid phase or regenerated absorption solvent. Preferably, the loop comprises a pump for pumping the liquid phases in the reactor, which pump is arranged downstream of the outlet means for withdrawing the gas stream and upstream of the inlet means for introducing the mixture as obtained in step (c).
Suitably, the gas lift reactor comprises a demister arranged upstream of the outlet means for withdrawing the gas stream.
Suitably, the regeneration of the absorption solvent in step (d) is carried out at a temperature in the range of from 60-100° C., which brings about a significant improvement in overall energy efficiency when compared to known process such as described in WO-A-95/21683, and in US 2009/0199709. Preferably, the regeneration in step (d) is carried out at a temperature in the range of from 65-95° C., more preferably at a temperature in the range of from 70-90° C. Step (d) is suitably carried out at a pressure in the range of from 1-30 bar, preferably in the range of from 1-20 bar.
In step (e), a gas stream which comprises carbon dioxide is withdrawn from the gas lift reactor. Suitably, at least part of the carbon dioxide gas stream as withdrawn in step (e) is recycled to step (d).
Steam that is present in the gas stream withdrawn in step (e) can at least partly be condensed to provide heat that can be used at some stage of the process. At least part of the carbon dioxide-containing absorption solvent as obtained in step (a) is heated through heat exchange with the hot gas stream enriched in carbon dioxide as withdrawn from step (e).
The gas stream withdrawn in step (e) containing almost pure carbon dioxide, can for example be used in enhanced oil recovery. The carbon dioxide enriched gas stream that is withdrawn from step (e) has an elevated pressure. Suitably, the pressure of said carbon dioxide enriched gas stream is in the range of from 5 to 30 bar, preferably from 8 to 30 bar. In applications where the carbon dioxide enriched gas stream needs to be at a high pressure, for example when it will be used for injection into a subterranean formation, it is already at an elevated pressure. In a suitable embodiment, the pressurised carbon dioxide enriched gas stream as withdrawn from step (e) is used for enhanced oil recovery, suitably by injecting it into an oil reservoir where it tends to dissolve into the oil in place, thereby reducing its viscosity and thus making it more mobile for movement towards the producing well. In another suitable embodiment, the pressurised carbon dioxide gas stream as withdrawn from step (e) is pumped into an aquifer reservoir for storage. In yet another suitable embodiment, the pressurised carbon dioxide-enriched gas stream is pumped into an empty oil reservoir for storage. For all the above options, a series of compressors can suitably be used to pressurise the carbon dioxide-enriched gas stream to the desired high pressures.
In step (f) a mixture of the aqueous liquid phase and the non-aqueous liquid phase is withdrawn from the gas lift reactor, which mixture has released carbon dioxide. In a subsequent step (g) cooling of at least part of the mixture of the aqueous liquid phase and the non-aqueous liquid phase takes place to obtain regenerated absorption solvent.
Suitably, at least part of the stream of the regenerated absorption solvent as obtained in step (g) is recycled to step (a). Preferably, the entire liquid phase as obtained in step (f) is cooled in step (g) and recycled to step (a). Cooling in step (g) can suitably take place by passing the mixture to a heat exchanger, where it is cooled against the carbon dioxide-containing absorption solvent as obtained in step (a). Cooled regenerated absorption solvent can subsequently be passed to step (a).
One embodiment of the present invention is illustrated in
In
In
The invention is illustrated using the following non-limiting Examples.
In a process as described in
A similar process as described in Example 1 was carried out, except that instead of gas lift reactor 8 a conventional stirred tank reactor was used which operated at a pressure of 1 bar and a temperature of 85° C., whereby carbon dioxide-containing absorption solvent is passed via line 6 to the upper part of the stirred tank reactor.
In accordance with the present process (Example 1) an energy saving was established in the range of from 0.1-0.22 kWatt when compared with Example 2 wherein the mixture obtained from the heat exchanger 5 needed to be mixed in the stirred tank reactor by mechanical stirring. The energy saving established depended on the actual stirring speed applied in the stirred tank reactor. In view of the above, it will be clear that the process according to the present invention brings about a significant energy saving. Additionally, the use of the gas lift reactor in accordance with the present invention provides an improvement in stability of operation.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10194082.3 | Dec 2010 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/072062 | 12/7/2011 | WO | 00 | 7/16/2013 |
