DE102004030513A1 - Absorbent for carbon dioxide, useful for promoting carbon monoxide shift conversion, comprises a lithium silicate that has been doped and/or thermally/chemically pretreated - Google Patents

Abstract

The invention relates to an absorbent for CO ₂ , in particular for carrying out equilibrium-limited processes in which CO ₂ is formed. The absorption medium for CO ₂ is based on a lithium-containing silicate, which can absorb and release CO ₂ particularly quickly by doping and thermal / chemical pretreatment or one of the two processes mentioned, the silicate structure consisting predominantly of SiO ₄ tetrahedra and LiOn polyhedra (n = 3, 4, 5, 6) and the lithium-containing silicate is preferably present as Li ₄ SiO ₄ . The lithium-containing silicate contains one or more of the doping elements Mg, Ca, Al, Cr, Fe, Co, Ni, Zn, Ti, Ge, Zr, V, Ta, P, Mo, W, Ga, In, Y, Cu, Mn , In this case, the doping element with Li and / or O or optionally with Si compounds, which form with Li ₄ SiO ₄ solid solutions and thereby have a molar Li / O ratio between 0.85 and 1.

Description

The invention relates to absorbents for CO ₂ , in particular for carrying out equilibrium-limited processes in which CO ₂ is formed.

In the chemical industry, a series of reactions is carried out in which the limitation by the thermodynamic equilibrium leads to an incomplete reaction of the reaction components and thus to low cost of the processes. For these reactions, overcoming equilibrium limitation is very desirable. In chemical equilibrium processes that generate CO ₂ , such as. As the steam reforming of hydrocarbons or the steam conversion of CO, this reaction product can be dissipated and thus the equilibrium shifted favorably towards Eduktverbrauch with a higher turnover. The separation of CO ₂ from product gas mixtures is also gaining in importance in connection with the reduction of emissions of climate-relevant gases (Göttlicher, G .: in: VGB Power Tech 2003, 96-101).

For the separation of CO ₂ from gases absorption processes are used, which work with liquid washing solutions. Due to the physico-chemical principles of action, these washes work satisfactorily only at low temperatures (<40 ° C.) and are therefore not available for most reactions to remove the CO ₂ from the reaction space. This eliminates the influence on the chemical equilibrium (see, for example: Kohl, A. and Nielsen, R .: Gas Purification, Gulf Publ. Comp., Houston 1997).

There are a number of processes in which CO ₂ can be adsorbed (physically bound) to porous solids. Disadvantage of all known adsorbents is the low CO ₂ uptake capacity at elevated temperatures. This is normally less than 1mmol / g as soon as the temperature exceeds 200 ° C (Sircar, S. et al .: in Carbon 34 (1996), 1-12). Absorption of CO ₂ (chemical bonding) to solids can be much higher CO ₂ loadings of the sorbent can be achieved, which minimizes sorbent consumption and can lead to higher process efficiency. Known absorbents for CO ₂ are calcium oxide and calcined dolomite. These can absorb CO ₂ over a wide temperature range, but must be regenerated at very high temperatures or with a considerable purge gas consumption (Silaban, A. et al .: in Chem. Eng. Comm.146 (1996), 149-162). Partly because of the high regeneration effort, processes with these sorbents could not prevail.

Lithium zirconate-based absorbents, such as. Eg in DE 198 42 228 A1 and EP 1145755 can be regenerated even at significantly lower temperatures, but absorb CO ₂ comparatively slowly. Absorbents of lithium silicate ( EP 1038576 ) absorb CO ₂ much faster and can be regenerated at temperatures slightly above those of lithium zirconate. Investigations have shown that the rate of CO ₂ uptake reaches satisfactory levels only above 600 ° C.

EP 1038576 discloses absorption materials containing lithium silicates of the general formula Li _x Si _y O _z with the condition x + 4y - 2z = 0 and x ≥ 4 and for the absorption of CO ₂ in the conversion of CO with H ₂ O to CO ₂ and H ₂ can be used. Furthermore, the absorption materials lithium zirconate and K ₂ CO ₃ may contain. Preferred sorbents contain lithium orthosilicate Li ₄ SiO ₄ and K ₂ CO ₃ , wherein a eutectic melt formed above 500 ° C should accelerate CO ₂ up to this temperature. Disadvantage of eutectic doping with potassium or sodium is an expected alkali discharge from the melt phase of, for example, K ₂ CO ₃ - Li ₂ CO ₃ , which can lead to considerable corrosion of system components.

It is therefore an object of the invention to provide an absorbent, which has a high CO ₂ loading at high temperatures, allows regeneration at still reasonable temperatures without excessive Spülgaseinsatz, shows a high speed for CO ₂ absorption and a tolerable for plant components composition has. With the sorbent according to the invention, the process control of equilibrium-limited reactions should be substantially improved by sorption of the CO ₂ produced in statu nascendi to the sorbent according to the invention. In addition to going out to the coupled to the CO ₂ sorption of CO ₂ -forming reaction of the novel absorbent also targeted catalytic effect.

According to the invention, the object is achieved by an absorption medium for CO ₂ which is based on a lithium-containing silicate which can take up and release CO ₂ particularly quickly by doping and thermal / chemical pretreatment or one of the two processes mentioned, the silicate structure consisting predominantly of SiO ₄ - Tetrahedra and LiOn polyhedra (n = 3, 4, 5, 6) and the lithium-containing silicate is preferably present as Li ₄ SiO ₄ . The lithium-containing silicate contains one or more the doping elements Mg, Ca, Al, Cr, Fe, Co, Ni, Zn, Ti, Ge, Zr, V, Ta, P, Mo, W, Ga, In, Y, Cu, Mn .. The doping element goes with it Li and / or O or optionally with Si compounds which form solid solutions with Li ₄ SiO ₄ and have a molar Li / O ratio between 0.85 and 1 (0.85 <molar amount (Li) / molar amount (0) ≤ 1).

Advantageously, the lithium-containing silicate contains a solid solution of the general formula
Li _(4-3α) X _α SiO ₄ or (1-α) Li ₄ SiO ₄ + (α) LiXSiO ₄ , where "X" eg Al, Fe or Cr
or a solid solution of the general formula
Li _(4-2α) X _α SiO ₄ or (1-α) Li ₄ SiO ₄ + (α) Li ₂ XSiO ₄ , where "X" eg Mg, Ca, Zn, Ni
or a solid solution of the general formula
Li ₄ X _α Si _{(α 1)} O ₄ or (1-α) Li ₄ SiO ₄ + (α) Li ₄ XO _4, wherein "X" for example, Ti, Ge, Zr
or a solid solution of the general formula
Li _(4-α) X _α Si _(1-α) O ₄ or (1-α) Li ₄ SiO ₄ + (α) Li ₃ XO ₄ , where "X" eg V, Ta or P
or a solid solution of the general formula
Li _(4-2α) X _α Si _(1-α) O ₄ or (1-α) Li ₄ SiO ₄ + (α) Li ₂ XO ₄ , where "X" is Mo or W, for example
or a solid solution of the general formula
Li _(4-3α) X _α Si _(1-α) O (4-2α) and (1-α) Li ₄ SiO ₄ + (α) lixo _2, wherein "X" for example, Ni or Co
or mixtures of the aforementioned solid solutions and the parameter "α" assumes values of 0 to 0.2 (0 ≤ α ≤ 0.2).

In addition, the absorbent K ₂ CO ₃ and / or Na ₂ CO ₃ may contain.

Advantageous is the lithium-containing silicate on an inert carrier material fixed.

• 1) ein lithiumhaltiges Silikat bei Dotierung mit Fremdelementen (im Vergleich zum nicht dotierten Material) CO₂ im relevanten Temperaturbereich (ca. 400...600°C) mit wesentlich erhöhter Geschwindigkeit absorbiert werden kann. Insbesondere bei Feststofflösungen der Art (1-α)Li₄SiO₄+(α)LiAlSiO₄ (α = 0...0,2) bzw. (Li_(4–3α)Al_(α)SiO₄) stellen sich stark erhöhte CO₂-Aufnahmegeschwindigkeiten schon um die 500°C als auch etwas darunter ein. Gleiches gilt, wenn man Fe oder Cr anstelle von Al einsetzt, wobei es sich nicht zwangsweise (auch) um Feststofflösungen handeln muss.
• 2) ein lithiumhaltiges Silikat nach einer thermischen oder thermisch/chemischen Vorbehandlung CO₂ in einem engen Temperaturbereich besonders schnell aufnehmen und wieder abgeben kann. Der Bereich liegt etwa zwischen 670°C und 800°C. Bei atmosphärischem Druck wird CO₂ besonders schnell zwischen 690°C und 740°C aufgenommen (mit ca. 3 Ma.%/min) und zwischen 720°C und 770°C wieder abgegeben (mit ca. 6 Ma.%/min).
• 3) das lithiumhaltige Silikat katalytische Eigenschaften hinsichtlich einer CO-Umsetzung mit Wasserdampf zu CO₂ und H₂ (CO-Shift) besitzt. Der katalytische Effekt verstärkt sich wesentlich bei Verwendung des mit Fremdelementen dotierten Lithiumsilikates, besonders in Verbindung mit Eisen.

Surprisingly, it has been shown that:

• 1) a lithium-containing silicate when doping with foreign elements (compared to the non-doped material) CO ₂ in the relevant temperature range (about 400 ... 600 ° C) can be absorbed at a significantly increased speed. Especially in solid solutions of the type (1-α) Li ₄ SiO ₄ + (α) LiAlSiO ₄ (α = 0 ... 0.2) or (Li _(4-3α ) Al _(α) SiO ₄ ) are strong increased CO ₂ uptake rates already around 500 ° C as well as slightly below. The same applies if one uses Fe or Cr instead of Al, which does not necessarily have to be (also) solid solutions.
• 2) a lithium-containing silicate after a thermal or thermal / chemical pretreatment CO ₂ in a narrow temperature range very quickly absorb and release. The range is approximately between 670 ° C and 800 ° C. At atmospheric pressure, CO _{2 is} absorbed very rapidly between 690 ° C and 740 ° C (at about 3 Ma.% / Min) and released again between 720 ° C and 770 ° C (at about 6 Ma.% / Min). ,
• 3) the lithium-containing silicate has catalytic properties with respect to CO conversion with water vapor to CO ₂ and H ₂ (CO shift). The catalytic effect increases significantly with the use of the impurity doped lithium silicate, especially in conjunction with iron.

With the absorbent according to the invention, the process control of equilibrium-limited reactions is substantially improved by sorption of the CO ₂ produced in statu nascendi with the sorbent according to the invention. This is achieved by the simultaneous elimination of CO ₂ sorption during a CO ₂ -forming reaction in the same process space (no spatial separation as in a laundry). The removal of CO ₂ from the reaction space ensures the shift of the chemical equilibrium in favor of the products (by constantly removing the product component CO ₂ from the reaction mixture). Equilibrium limitation is better overcome the faster CO ₂ can be absorbed by the sorbent.

A preferred application of the absorbents of the invention is a sorptionsunterstützte CO conversion (CO shift) to produce synthesis gas or hydrogen from gasification gases (e.g., coal, petroleum fractions, biomass, etc.) or Natural gas. A CO conversion is preferably supported catalytically, if the lithium-containing silicate of one or more of the elements Fe, Al, Contains Cr, Zn, Mg and / or Cu.

Through process coupling of sorption and reaction, in particular the CO conversion can be increased with simultaneously reduced water vapor consumption. When using doped Li ₄ SiO ₄ (eg with iron) may optionally be dispensed with the use of additional or conventional CO shift catalysts (eg Fe / Cr oxides).

The regeneration of the CO ₂ -loaded absorbent can take place either by increasing the temperature (temperature swing) or by lowering the pressure (pressure swing) or by combining both principles (optimization). The increase in temperature or the pressure reduction can be carried out under a technical vacuum or combined with a rinse of the sorbent (eg water vapor or other suitable gases).

For example, known lithium sources such as Li ₂ CO ₃ or LiOH or LiNO ₃ and silicon sources such as SiO ₂ or silica sol are used to produce the sorbent. The doped hetero elements can be in the form of oxides, nitrates or acetates in the synthesis, for example be introduced mixture. When Li ₂ CO ₃ , SiO ₂ and foreign elements in oxidic form are used, the powders are mixed-ground and, for example, calcined at 800-900 ° C. (in air or inert gas) for several hours. The molar ratio of the starting materials is chosen so that a mixture with the ratio of Li ₄ SiO ₄ to the foreign element compound (eg LiXSiO ₄ , Li _i XO _4, etc.) of at least 4: 1 results.

Lithium-containing silicate is thermally or thermally / chemically pretreated in gas atmospheres consisting of inert gas such as N ₂ , He, Ar, etc. and / or CO ₂ at temperatures between 650 ° C and 1300 ° C. In this case, a multiple gas exchange between inert gas and CO ₂ take place.

Advantageously, the absorbent according to the invention is used for the absorption of CO ₂ in the conversion of CO with H ₂ O to CO ₂ and H ₂ at a temperature between 450 ° C and 800 ° C. The advantage is that the balance is shifted to the product side despite the increased temperature, in contrast to the classic CO conversion, which must be carried out at the lowest possible temperatures and requires a catalyst.

Based The following examples illustrate the invention:

Example 1 (negative example):

Doping with foreign elements:

A mixture is prepared with a molar ratio of Li / Al / Si / O = 4.1 / 0.1 / 0.9 / 4. For this purpose, 6.17 g of Li ₂ CO ₃ , 0.21 g of Al ₂ O ₃ and 2.20 g of SiO _{2 are} ground for 1 minute in the vibrating mill for the purpose of homogenization. Subsequently, the mixture is calcined at 850 ° C for 8 hours in air. The calcined sample is reamed in a mortar to remove any sintering. As a result, a solid solution consisting of (1-α) Li ₄ SiO ₄ and αLi ₅ AlO ₄ having a mole fraction α = 0.1 and a Li / O molar ratio of 1.025 has been formed. A subsample was tested in a thermobalance with respect to its loading rate for carbon dioxide (100 vol .-% CO ₂ , 1 bar, heating 1K / min linear). It was found that only an insufficient increase in the loading rate (0.05% by mass / min) can be achieved compared to pure Li ₄ SiO ₄ .

Example 2 (negative example):

Li ₄ SiO ₄ without thermal pretreatment:

10 g of Li ₂ CO ₃ and 4.1 g of SiO ₂ are ground for 1 minute in the vibrating mill and calcined at 850 ° C for 14 hours in air. The calcined material is rubbed in a mortar to break up sintering. After heating in inert gas (helium), the loading was carried out with CO ₂ at a constant 700 ° C in the thermobalance. As a result, CO _{2 was} only accelerated after a holding time of more than 90 minutes. (700 ° C)

Example 3:

Doping with foreign elements:

A mixture is prepared with a molar ratio of Li / Al / Si / O = 3.7 / 0.1 / 1/4. For this purpose, 4.42 g of Li ₂ CO ₃ , 0.16 g of Al ₂ O ₃ and 1.94 g of SiO _{2 are} ground for 1 minute in the vibrating mill and then calcined at 850 ° C in air for at least 8 hours. The calcined material is rubbed in a mortar. The result is a solid solution with (1-α) Li ₄ SiO ₄ and αLiAlSiO ₄ with a mole fraction α = 0.1 and a Li / O molar ratio of 0.925. The result was an accelerated CO ₂ uptake with linear heating at 1 K / min already from 500 ° C. (0.11% by weight / min) in comparison to pure Li ₄ SiO ₄ (0.01% by mass ₎ . / min).

Example 4:

Doping with foreign elements:

A mixture is prepared with a molar ratio of Li / Cr / Si / O = 3.7 / 0.1 / 1/4. For this purpose, 4.53 g of Li ₂ CO ₃ , 0.25 g of Cr ₂ O ₃ and 2.0 g of SiO ₂ are mixed together. The mixture is ground in the vibratory mill for 1 minute and calcined in air at 850 ° C for at least 8 hours. The calcined material is rubbed in a mortar. The result is a solid solution with (1-α) Li ₄ SiO ₄ and αLiCrSiO ₄ with a mole fraction α = 0.1 and a Li / O ratio of 0.925. The loading rate of 0.13 mass% / min at 500 ° C is significantly above the corresponding value for Li ₄ SiO ₄ (0.01 mass% / min).

Example 5:

Doping with foreign elements:

It is a mixture with a molar ratio of Li / Fe / Si / O = 3.7 / 0.1 / 1/4, eg from 4.4 g Li ₂ CO ₃ , 0.26 g Fe ₂ O ₃ and 1, 93 g of SiO ₂ produced. The mixture is ground for 1 minute in the vibratory mill and then calcined at 850 ° C in air for at least 8 hours. The calcined mate rial is rubbed in a mortar. The result is a solid solution with (1-α) Li ₄ SiO ₄ and αLiFeSiO ₄ with a mole fraction α = 0.1 and a Li / O ratio of 0.925. The sample had a loading rate of 0.14 mass% / min at 500 ° C.

Example 6:

Li ₄ SiO ₄ with thermal pretreatment:

A mixture of 28.9 g of Li ₂ CO ₃ and 11.7 g of SiO _{2 is} produced. The mixture is ground for 1 minute in the vibratory mill and then calcined in air at 850 ° C for 14 hours. The calcined material is rubbed in a mortar and treated alternately at 700 ° C for at least three cycles with an atmosphere of CO ₂ and helium. (Cycle: 1 hour loaded with 100 vol .-% CO ₂ , then desorb for 1 hour in helium stream, pressure 1 bar) As a result, an accelerated CO ₂ uptake (3-5 Ma .-% / min) immediately set at 700 ° C.

Example 7:

Li ₄ SiO ₄ with thermal pretreatment:

A mixture of 27.6 g of Li ₂ CO ₃ and 11.2 g of SiO _{2 is} produced. The mixture is ground for 1 minute in the vibratory mill and then calcined in air at 850 ° C for 14 hours. The calcined material is rubbed in a mortar and pretreated at 1000 ° C for 5 hours in a helium stream. The result was an accelerated CO ₂ uptake of between 700 ° C and 730 ° C.

Example 8:

Catalytically active sorbent material (Fe-doped Li ₄ SiO ₄ ):

A mixture of 11.1 g of Li ₂ CO ₃ , 0.65 g of Fe ₂ O ₃ and 4.9 g of SiO _{2 is} prepared. The mixture is ground for 1 minute in the vibratory mill and then calcined at 850 ° C in air for at least 8 hours. The calcined material is ground in the vibratory mill (1 min). The result is a solid solution of (1-α) Li ₄ SiO ₄ and αLiFeSiO ₄ with a mole fraction α = 0.1 and a Li / O molar ratio of 0.925. The material was introduced as a bed in a flow reactor (cross section 5 cm ² ) and at 500 ° C by a gas mixture consisting of 800 ppmv CO in N ₂ at 500 ° C flows through. The CO conversion was twice as high for the Fe-doped Li ₄ SiO ₄ at gas volume flows of 30 l / h and 15 l / h as for the undoped Li ₄ SiO ₄ and was ten times higher than in the empty reactor.

Claims (10)

Absorbent for CO ₂ , based on a lithium-containing silicate, which can take up and release CO ₂ particularly quickly by doping and thermal / chemical pretreatment or one of the two methods mentioned, wherein the silicate structure predominantly of SiO ₄ tetrahedra and LiOn polyhedra (n = 3, 4, 5, 6) and the lithium-containing silicate is preferably present as Li ₄ SiO ₄ . Absorbent according to claim 1, characterized in that the lithium-containing silicate one or more of the doping Mg, Ca, Al, Cr, Fe, Co, Ni, Zn, Ti, Ge, Zr, V, Ta, P, Mo, W, Ga , In, Y, Cu, Mn, wherein the doping element with Li and / or O or Si forms compounds which form with Li ₄ SiO ₄ solid solutions and thereby have a molar Li / O ratio between 0.85 and 1. Absorbent according to claim 2, characterized in that the lithium-containing silicate is a solid _{solution of} the general formula Li _(4-3α) X _α SiO ₄ or (1-α) Li ₄ SiO ₄ + (α) LiXSiO ₄ where "X" = Al , Fe or Cr or a solid _{solution of} the general formula Li _(4-2α) X _α SiO ₄ or (1-α) Li ₄ SiO ₄ + (α) Li ₂ XSiO ₄ , where "X" = Mg, Ca, Zn or Ni or a solid solution of the general formula Li ₄ X _α Si _(1-α) O ₄ or (1-α) Li ₄ SiO ₄ + (α) Li ₄ XO _4, wherein "X" = Ti, Ge or Zr or a solid solution of the general formula Li _(4-α) X _α Si _(1-α) O ₄ or (1-α) Li ₄ SiO ₄ + (α) Li ₃ XO _4, wherein "X" = V, Ta or P or a solid solution of the general formula Li _(4-2α) X _α Si _(1-α) or O ₄ (1-α) Li ₄ SiO ₄ + (α) Li ₂ XO _4, wherein "X" = Mo or W or solid solution of the general formula Li _(4-3α) X _α Si _(1-α) O _(4-2α) and (1-α) Li ₄ SiO ₄ + (α) lixo _2, wherein "X" = Ni or Co or mixtures of vo contains mentioned solid solutions and the parameter "α" assumes values from 0 to 0.2. Absorbent according to any one of claims 1 to 3, characterized in that the lithium-containing silicate additionally contains K ₂ CO ₃ and / or Na ₂ CO ₃ . Absorbent according to any one of claims 1 to 4, characterized in that on the lithium-containing silicate applied to a substrate is. Absorbent according to any one of claims 1 to 5, characterized in that the lithium-containing silicate thermally or thermally / chemically in gas atmospheres consisting of inert gas such as N ₂ , He, Ar, etc. and / or CO ₂ at temperatures between 650 ° C and Pretreated at 1300 ° C becomes. Absorbent according to any one of claims 1 to 6, characterized in that the lithium-containing silicate a Catalytically supported CO conversion. Absorbent according to claim 7, characterized that the lithium-containing silicate of one or more of the elements Fe, Al, Cr, Zn, Mg and / or Cu. Use of the absorbent according to any one of claims 1 to 6 for the absorption of CO ₂ to support equilibrium-limited, CO ₂ -generating chemical reactions. Use of the absorbent according to one of claims 1 to 6 for the absorption of CO ₂ in the conversion of CO with H ₂ O to H ₂ and CO ₂ at a temperature between 400 ° C and 800 ° C.