JP4594011B2 - Ethanol reforming method - Google Patents

Description

  The present invention relates to a method for reforming ethanol to obtain hydrogen. Specifically, the present invention relates to such a method with reduced carbon polymer production.

The development and use of fuel cells is considered by many to be the most promising way to reduce CO ₂ emissions and increase the efficiency of power production. The selection and distribution of feedstock for small fuel cell devices is a major problem in commercialization. The process of converting hydrocarbon feeds from natural gas to gasoline and diesel to hydrogen for fuel cells is an off-the-shelf technology. Methanol steam reforming is also well known, and a very compact apparatus can be constructed. The use of ethanol that can be obtained from biomass is gaining more and more attention for the purpose of both as a main feedstock or as an alternative to increase the flexibility of the feedstock in a given apparatus.

  Ethanol steam reforming is not a direct process. Ideally, the conversion of ethanol to hydrogen can proceed along the following reaction (1).

C ₂ H ₅ OH (g) + H ₂ O (g) ⇔ 2CO + 4H ₂ ΔH ₂₉₈ = 255 kJ / mol (1)
The equilibrium of reaction (1) shifts to the right even at low temperatures. However, in practice, ethanol is also converted to ethylene according to the following reaction (2).

C ₂ H ₅ OH ⇔ C ₂ H ₄ + H ₂ O ΔH ₂₉₈ = 45 kJ / mol (2)
Ethylene, even in small quantities, produces carbon in a short time on most steam reforming catalysts, thereby producing a carbon polymer. This carbon polymer causes significant deactivation. Therefore, a high ethylene content should be avoided.

  A dual catalyst system involved in the initial conversion of ethanol to ethylene and subsequent conversion to ethane can be used instead, followed by steam reforming of ethane. In the first catalytic zone, ethanol is converted to ethylene according to reaction (2). Further, ethylene reacts with hydrogen to produce ethane according to the following reaction (3).

C ₂ H ₄ + H ₂ ⇔ C ₂ H ₆ ΔH ₂₉₈ = -137 kJ / mol (3)
Excess hydrogen suppresses the equilibrium concentration of ethylene. FIG. 1 shows the equilibrium ethylene concentration at various temperatures and pressures when ethanol and hydrogen are fed in equimolar amounts. Low ethylene content is obtained under high pressure and low temperature.

  Therefore, hydrogen can be used for ethylene suppression as described above for reaction (3).

  An object of the present invention is to provide an ethanol reforming method in which the production of polymer carbon caused by an ethylene intermediate product is suppressed.

  Another object of the present invention is to provide a method in which the hydrogen required for the hydrogenation of ethylene to ethane is supplied by recirculation of the effluent obtained during the process of producing hydrogen from ethanol. It is.

Summary of the present invention

Therefore, the present invention is a method for selectively reforming ethanol in the presence of at least one catalyst comprising a catalytically active material supported on a carrier to obtain a hydrogen enriched product, The next stage in which at least one catalyst is active, i.e.
-The stream comprising ethanol and steam and the hydrogen-containing stream are transferred to a dehydration / hydrogenation reactor for dehydrating ethanol to ethylene and hydrogenating ethylene to an ethane-containing stream, wherein ethanol relative to hydrogen The molar ratio is 0.2-1
-Adiabatically pre-reforming the resulting ethane-containing stream to obtain a stream containing methane;
-Steam reforming the methane-containing stream to produce a mixture comprising hydrogen and carbon monoxide;
Converting the mixture comprising hydrogen and carbon monoxide from the steam reforming stage under water gas shift conditions to produce a hydrogen enriched product stream;
To the above method.

Detailed Description of the Invention

  FIG. 2 is a process layout for hydrogen production by ethanol reforming to produce hydrogen and illustrates two aspects of the present invention. This process is useful, for example, for producing hydrogen for fuel cells. Ethanol is steam reformed in a series of steps, each characterized by very high selectivity, and is continuously converted to hydrogen and carbon dioxide. This process is further described below.

In the first stage, the ethanol, water vapor and hydrogen-containing streams are transferred to a reactor where dehydration and hydrogenation reactions are performed according to the following reaction.
Dehydration of ethanol: C ₂ H ₅ OH ⇔ C ₂ H ₄ + H ₂ O (2)
Hydrogenation of ethylene: C ₂ H ₄ + H ₂ ⇔ C ₂ H ₆ (3)
Ethanol is dehydrated to ethylene according to reaction (2), and ethylene is hydrogenated to ethane according to reaction (3).

  As the catalyst for the dehydration reaction (2), there are many solid and liquid materials. As solid catalysts, essentially acidic catalysts such as alumina, silica alumina, zeolites, zirconia or other solid acids or related compounds have high activity in the dehydration of ethanol to ethylene. Previous industrial practices (see 1928 Ullmann ethylene section), aluminum oxide (alumina) are recommended (see German Imperial Patent (DR) No. 168291). Later silica-based materials, especially zeolites, proved to be very active in this reaction. Ullman, 2nd edition, 1928, Vol. 1, 754 pages or Kirkusmer, 3rd edition, 1980, Vol. As described on page 9,411, the use of these types of catalysts for the production of ethylene has been a well-known process dating back to 1797. These are also widely used industrially in recent years.

  Although many other catalysts are equally effective options, a generally preferred catalyst is based on alumina. Selectivity is a related problem for the production of acetaldehyde, which is suppressed by a high hydrogen partial pressure. Diethyl ether can also be produced, but its production can be suppressed by high temperatures.

In the hydrogenation reaction (3), a known catalyst is also useful. In the catalytic hydrogenation reaction, a transition metal supported on a carrier is used. Since this metal should be reduced under the reaction conditions, the catalyst is usually a group 8 metal (Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt) and Cu. Based. Catalysts based on several oxides and sulfides, such as MoS ₂ used in the oil industry, also have hydrogenation activity.

  The catalyst selected is usually a base metal. Considering the selectivity, Fe, Co and Ni catalyze some undesired reactions with ethylene, and it is difficult to keep them active under reaction conditions. That is, these metals catalyze the oxidation reaction or are embedded in polymer carbon. This is not the case for Cu, Ag and Au and noble metals such as Pd, Rh, Ru and Pt. All these metals have a very high activity in the above reaction. Usually, a catalyst based on Cu is the first choice.

  If the temperature is too high for the use of a catalyst based on Cu, a noble metal such as Pd or Pt can be used. Here, the carrier for Cu (or Pd / Pt) can conventionally be alumina or other acidic oxide as described for the dehydration reaction above, but this need not necessarily be the case. It is therefore possible to combine dehydration and hydrogenation reactions, and sometimes it is preferable to do so.

  FIG. 1 shows the ethylene equilibrium concentration as a function of temperature and pressure when supplying ethanol / hydrogen at 50/50 mol%. Again, it is observed that a high hydrogen partial pressure is advantageous for reducing the ethylene concentration.

  A preferred catalyst system for the first stage is a bifunctional catalyst system capable of catalyzing both dehydration and hydrogenation reactions. Such a catalyst system can be produced by supporting the above metal on a dehydration catalyst. Preferred systems are metals supported on solid acids such as alumina, silica alumina or zeolite, with preferred metals being Pt, Pd and Cu.

  Zirconia or alumina as a carrier is also preferable. Zirconia carriers can be used in a modified form, where the zirconia is tungstated or sulfated.

  It is expected that the hydrogenation / dehydrogenation reaction from ethane to ethylene (reverse reaction (3)) proceeds rapidly.

C ₂ H ₆ ⇔ C ₂ H ₄ + H ₂ (4)
A large hydrogen partial pressure limits the conversion of methane, but is beneficial to the life of the catalyst, because ethylene has a detrimental effect on carbon production on the pre-reforming catalyst.

  In the second and third stages, ethane in the effluent from the dehydration / hydrogenation reaction can be converted to hydrogen by steam reforming of the produced ethane. One option is to use an adiabatic prereformer and then heat to perform the main steam reforming. This embodiment is shown in FIG. FIG. 2 shows a process layout that can supply hydrogen to the PEM fuel cell device.

  In the second stage, ethane is preliminarily reformed adiabatically. This adiabatic pre-reforming step is conventionally performed using a nickel-based catalyst that is also used for hydrocarbon steam reforming. Other Group 8 metals, especially rhodium and ruthenium, are also very good steam reforming catalysts.

The basic pre-reforming reaction that occurs is the following reaction.
Ethane hydrocracking: C ₂ H ₆ + H ₂ ⇔ 2CH ₄ (5)
Ethane steam reforming: C ₂ H ₆ + 2H ₂ O ⇔2CO + 4H ₂ (6)
Methane steam reforming: CH ₄ + H ₂ O ⇔ CO + 3H ₂ (7)
Water gas shift reaction: CO + H ₂ O ⇔ H ₂ + CO ₂ (8)
At the outlet of the adiabatic prereformer, all C ₂ species are converted to C ₁ species such as CH ₄ , CO and CO ₂ . The effluent stream from the adiabatic prereformer may also contain H ₂ and H ₂ O.

The third stage is the main reforming stage, in which methane is steam reformed to carbon monoxide and hydrogen according to reaction (7).
Methane steam reforming: CH ₄ + H ₂ O ⇔ CO + 3H ₂ (7)
This stage takes place in the main reformer (also called steam reformer) and the majority of the residual CH ₄ is converted to carbon oxides, ie CO and CO ₂ , and hydrogen. The effluent stream from the main reformer may also contain some H ₂ O and CH ₄ . This is an endothermic process and the necessary heat is usually supplied by indirect heat exchange with the burned fuel. Alternatively, if a pressure swing absorber (PSA) is used for hydrogen purification, the waste stream from the PSA can be used for combustion. Under special circumstances at a biomass-based manufacturing site, if suitable construction materials are available, waste biomass can be burned and used as a heat source for the hydrogen generator.

In order to achieve high CH ₄ conversion, the temperature in the main reformer must be high, typically about 800 ° C. As a result, the CO content is relatively high due to the equilibrium in the water gas shift reaction established on the reforming catalyst.

Therefore, in the fourth stage, the CO is converted to H ₂ according to the water gas shift reaction (4).
Water gas shift (WGS) reaction: CO + H ₂ O ⇔ CO ₂ + H ₂ (8)
The WGS reaction can be performed using various catalysts depending on the temperature. At very high temperatures, such as 500-800 ° C., the catalyst from Haldol Topser A / S described in EP 1445235 can be used. It should be noted that the contents of the published European patent application are described in this specification. When the WGS reaction is carried out at high temperatures in a heat exchange reformer, this directly leads to the benefit of significantly improved thermal efficiency.

  Typically at temperatures as low as 150 ° C. to 400 ° C., catalysts named SK-201-2 and LK-841 manufactured by Haldol Topser A / S have CO below 0.1% by volume. Used to convert to level.

The WGS reaction (4) takes place at a much lower temperature than the reforming reaction (7), and WGS catalyst is characterized by its ability to convert CO without generating CH _4. The ethanol conversion stage, which produces hydrogen by dehydration, hydrogenation and reforming, can in principle be carried out using the same catalyst as long as the polymer carbon production reaction caused by ethylene is suppressed. For such operations, no catalyst having a sufficient life has been identified.

When ethylene production is suppressed by hydrogenation of ethylene, ethane is produced according to reaction (3).
Ethylene hydrogenation: C ₂ H ₄ + H ₂ ⇔ C ₂ H ₆ (3)
Excess hydrogen is required. As a result, the production of polymer carbon is reduced. Therefore, hydrogen is added to the initial dehydration / hydrogenation stage. This is done by recirculation. In one embodiment of the invention, the product obtained from stage 4, the shift stage, is purified and then separated into hydrogen and water (steam). This hydrogen can then be used as a hydrogen enriched anode feed to the fuel cell, as shown in FIG.

  The method of the present invention is particularly suitable for producing hydrogen for use in fuel cells.

  FIG. 2 shows that hydrogen recycle can be performed from several locations. 2 shows two recirculation possibilities representing two embodiments of the present invention. That is, a portion of the hydrogen enriched anode feed stream is recycled or a portion of the effluent from the adiabatic prereformer is recycled and used as a hydrogen source.

  A third aspect of the present invention relating to recirculation of a hydrogen-containing stream involves recirculating part of the effluent stream from the main reformer, ie the effluent stream from the steam reforming stage.

  A fourth aspect of the present invention relating to the recirculation of the hydrogen-containing stream involves the recirculation of part of the effluent stream from the shift stage.

  The use of manufactured hydrogen allows favorable process integration with the fuel cell. In a fifth aspect of the invention relating to recirculation of hydrogen-containing streams when PEM and SOFC type fuel cells are used, the effluent anode stream can be recycled to the dehydrogenation / hydrogenation stage, In the case of the SOFC type, waste heat from the fuel cell can be used for ethanol reforming.

  The hydrogen recycle ratio is related to carbon production. By recycle ratio is meant the percentage or percentage of hydrogen recycled from one of the five possibilities described above. At a particular value of the recycle ratio, the hydrogen concentration is at a level such that the production of carbon polymer is not a significant factor in the performance of the reforming catalyst.

  A recycle ratio of 15-50% is preferred in the process of the present invention. Most preferred is a recycle ratio of 15-25%.

  The recycle ratio required to obtain the above molar ratio between ethanol and hydrogen depends on where the recycle is withdrawn. The amount of hydrogen present in the effluent stream from each process of the present invention increases as the process proceeds. Therefore, the most hydrogen is present in the hydrogen enriched anode feed stream and relatively little hydrogen is present in the effluent from the pre-reforming stage. Thus, for example, when recirculating the effluent from the shift stage, the necessary recirculation to the dehydration / hydrogenation stage is required compared to recirculating the effluent stream with a relatively low hydrogen content from the pre-reforming stage. The circulation ratio is small. Therefore, recycling from the pre-reforming stage requires a larger recycle ratio to provide sufficient hydrogen to meet the ethanol: hydrogen molar ratio required in the dehydration / hydrogenation stage.

  The molar ratio of ethanol to hydrogen (ethanol: hydrogen) in the dehydration / hydrogenation stage is preferably 0.2 to 1, most preferably 0.5 to 1.

  In one embodiment of the invention, the ethanol: hydrogen ratio is 1: 1 and the recycle ratio is 15-25% recycled from the hydrogen enriched anode feed stream to the dehydration / hydrogenation stage.

  In another embodiment of the invention, the ethanol: hydrogen ratio is 1: 1 and the recycle ratio is 15-25% by recycling the effluent from the shift stage to the dehydration / hydrogenation stage. is there.

  In yet another embodiment of the invention, the ethanol: hydrogen ratio is 1: 1 and the recycle ratio is recycled from the main reforming stage (steam reforming stage) to the dehydration / hydrogenation stage. 15 to 50%.

  The hydrogenation / dehydration reaction is preferably carried out under a temperature of at least 200 ° C. and a gauge pressure of at least 1 bar.

  One particularly preferred method for performing recirculation is by using an ejector. The reason why ejector recirculation is preferred is that the inlet stream, water and ethanol are all in liquid form, so that they are pumped at high pressure and then evaporated to run the stream to operate the ejector recirculation. This is because it can be generated.

  The method of the present invention is illustrated in the following examples.

  This will be described based on the process layout shown in FIG. Table 1 shows the equilibrium ethylene concentration at the inlet of the reforming zone, the steam: ethanol ratio at various recycle rates, and the operating pressure for the recycle of hydrogen enriched anode feed.

  Table 1 shows that at least 25% recycle of the hydrogen product reduces the ethylene concentration before being applied to the reformer catalyst to levels as low as 30 and 10 ppm at 5 and 15 bar gauge pressures, respectively. It shows that.

  It is clear that the ethylene content increases significantly when the recycle ratio is small. The higher the pressure, the milder the ethylene reduction.

The efficiency of hydrogen production (the concentration of H ₂ produced relative to the concentration of C ₂ H ₅ OH in the feed) changes at a rate of less than 5% when the process parameters are changed. The optimal process layout is a compromise between the hydrogen recycle ratio and the water vapor feed rate that leads to a moderate ethylene reduction and economical process equipment utilization.

  Catalysts for ethanol dehydration and ethylene hydrogenation were tested in the following procedure.

200 mg of the 150-300 μm fraction of the sieved catalyst was placed in a U-shaped quartz reactor having an inner diameter of 4 mm. The catalyst was reduced by flowing 100 ml / min 10% H ₂ in He (STP) over the catalyst at a temperature of 300 ° C. Thereafter, 107.4 ml / min (STP) of a gas mixture containing 6.7% by mole of ethanol, 13.3% of H ₂ , 40% of He and 40% of H ₂ O are flowed over the catalyst and the reaction temperature is set to 2 The temperature was increased from 150 ° C to 400 ° C with a gradient of ° C / min.

The ratio of ethanol to hydrogen was 0.50.
Example 1
Pt supported on H-ZSM5 zeolite was used as a catalyst. The metal content was 1% by weight. When the outlet gas was analyzed with a mass spectrometer, the following facts were observed. The reaction did not occur below 225 ° C. Above 225 ° C, all ethanol was converted to ethane. The ethylene concentration was determined to be less than 200 ppm under all temperature conditions, mainly determined by the equilibrium of reaction (3).
Ethylene hydrogenation: C ₂ H ₄ + H ₂ ⇔ C ₂ H ₆ (3)
Example 2
Pd supported on tungstated porous zirconia was used as a catalyst. The metal content was 0.5% by weight. When the outlet gas was analyzed with a mass spectrometer, the following facts were observed. The reaction did not occur below 250 ° C. Above 250 ° C, all ethanol was converted to ethane. The ethylene concentration was determined to be less than 200 ppm under all temperature conditions, mainly determined from the equilibrium of reaction (3).
Ethylene hydrogenation: C ₂ H ₄ + H ₂ ⇔ C ₂ H ₆ (3)
Example 3
Pt supported on porous gamma alumina γ-Al ₂ O ₃ stabilized with phosphorus was used as a catalyst. The metal content was 1% by weight. When the outlet gas was analyzed with a mass spectrometer, the following facts were observed. The reaction did not occur below 360 ° C. Above 360 ° C, all ethanol was converted to ethane. The ethylene concentration was determined to be less than 200 ppm under all temperature conditions, mainly determined by the equilibrium of reaction (3).
Ethylene hydrogenation: C ₂ H ₄ + H ₂ ⇔ C ₂ H ₆ (3)
In all examples, carbon production was so low that it did not affect the activity of the catalyst.

FIG. 1 shows the ethylene equilibrium concentrations at various temperatures and pressures when ethanol / hydrogen is fed at 50/50 mol%. FIG. 2 shows a process layout for performing hydrogen production by ethanol reforming according to two embodiments of the present invention.

Claims (12)

A method for producing a hydrogen enriched product by selectively reforming ethanol in the presence of at least one catalyst comprising a catalytically active material supported on a carrier, wherein said at least one catalyst exhibits activity In a dehydration / hydrogenation reactor for the following stages: a stream comprising ethanol and steam and a stream comprising hydrogen to dehydrate ethanol to produce ethylene and hydrogenate ethylene to produce an ethane-containing stream. The stage of introduction, wherein the molar ratio of ethanol to hydrogen is 0.2-1;
-Adiabatically pre-reforming the ethane-containing stream to produce a stream containing methane;
-Steam reforming the methane-containing stream to produce a mixture containing hydrogen and carbon monoxide;
Converting the mixture comprising hydrogen and carbon monoxide from the steam reforming stage under water gas shift conditions to produce a hydrogen enriched product stream;
Including the above method. The process of claim 1 wherein the molar ratio of ethanol to hydrogen is 0.5-1. The method of claim 2, wherein the hydrogen enriched stream is provided by recirculating a portion of the effluent from the adiabatic pre-reforming stage, steam reforming stage or water gas shift stage. The method of claim 3, wherein the hydrogen enriched stream is provided by recirculating the effluent stream from the steam reforming stage at a recycle ratio of 15-50%. 4. The method of claim 3, wherein the hydrogen enriched stream is provided by recycling the effluent stream from the water gas shift stage at a recycle ratio of 15-25%. The method of claim 1, wherein the hydrogen enriched stream is provided by recycling a portion of the hydrogen enriched product stream at a recycle ratio of 15-25%. The method of claim 1, wherein the catalytically active material for the dehydration / hydrogenation stage is selected from the group of Pt, Pd and Cu. 8. The method of claim 7, wherein the carrier is selected from the group of alumina, alumina silica, zirconia and zeolite. 9. The method of claim 8, wherein the zeolite is ZSM5, zirconia is tungstated or sulfated, and the alumina is gamma alumina. The process of claim 1, wherein the dehydration / hydrogenation reaction is carried out under a temperature of at least 200 ° C and a gauge pressure of at least 1 bar. 4. The method of claim 3, wherein recirculation is performed using an ejector. The method of claim 1, wherein the hydrogen enriched product stream is supplied to the fuel cell as an anode feed stream.

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