DE4319973A1 - Catalysts for removing hydrogen from an atmosphere containing hydrogen, air and steam - Google Patents

Description

The invention relates to the application of a Catalyst for removing hydrogen from one Gas containing hydrogen, oxygen and steam mixture.

Hydrogen from a hydrogen and oxygen ent holding and therefore explosive gas mixture in particular in the case of nuclear power plant accidents Meaning. Such gas mixtures can be found especially at Kern melt accidents of light water reactors as well as at Heavy water moderated reactors occur.

In an already published German patent Application DE 37 25 290 was made with numerous designs Examples of the catalytic ability of different PdNiCu alloys. Both Studies of the effectiveness of evaporated and pollinated film or plate-shaped catalysts PdNiCu as well as PdAG and PdCu alloys became one greater potential for their application for disposal of hydrogen in meltdown accident situations found (see A.K. Chakraborty, et al. "Assessment the potential to prevent flammable H₂ mixtures  in accident conditions in LWR’s using catalytic acting metal foils ", GRS-A-1355, April 1987). This Alloys as well as palladium and platinum made of metallic and alloy base in the form of sponge, granules have a good ability to catalyze when available be of steam. These metals and their rich ones Alloys have the shortest response times up to catalytic reaction.

There are two during a so-called meltdown Phases in which larger amounts of hydrogen are formed and get into the safety container.

The first major release of hydrogen into the Safety container of an LWR system comes from the Zirconium water vapor reaction as a result of one serious core heating or the subsequent Melting of the fuel rods. Doing so until the beginning the residual water evaporation in the lower base plate of the Reactor pressure vessel of a reactor about 50% of the Zirconium of the core structure oxidized. This will approx. 600-700 kg of hydrogen in the safety container released. The remaining zirconium is only after one Melting of the reactor pressure vessel, especially in the Initial phase of a melt concrete  Interaction oxidized. In total, within a few hours 1350 kg of hydrogen released into the containment. The long-term water fabric production becomes the reac as the melt penetrates further into the concrete Tor pit through the oxidation of further metallic components in the melt and in the Be ton and determined by the reaction of gases with water vapor.

The released hydrogen can, depending on the local concentration, the turbulence and the geometrical conditions within the containment the De reach the flagration limit and u. May also lead to a local detonation. Des half is both local and global removal of hydrogen in one Accident situation with meltdowns of primary interest.

The time course and the location of the release of the hydrogen formed the primary circuit in the safety container depend on the accident process. At the LP case, the hydrogen is released through the leakage opening practically without delay released the containment. Only small masses are accumulated in the primary system H₂ saved (see German risk study nuclear power plants phase B: an investigation / society for reactor safety Cologne, publisher TÜV Rheinland, 1990). At the First of all, a large part of the hydrogen remains in the primary circuit. Of the Most of the hydrogen is released into the room through the open valves where the pressure relief valves are located. If the reactor pressure vessel fails the hydrogen still remaining in the primary system flows over the reactor pit in the security container.

Calculations for the spatial distribution of the gases (see German Risk Study Nuclear Power Plants Phase B: an investigation / Society for Nuclear Safety Cologne, publisher TÜV Rheinland, 1990) in the security container show that the energy and Mass entry in the containment, the spatial distribution of the gases in the Security container will affect. The convection is strongly influenced by local effective energy sources and sinks through the chimney effect of stairwells and similar affects.

The object of the invention is to provide a way to remove the hydrogen To design increased effectiveness at higher temperatures so that immediately after the inflow of hydrogen in the different rooms of the containment,  even in the presence of high vapor content, the hydrogen so it is quickly eliminated that an ignitable composition is not achieved.

This object is achieved by the characterizing features of Pa claim 1 solved.

Advantageous developments of the invention are characterized in the subclaims net.

The oxidation reaction of the two-breath hydrogen with the oxygen proceeds exothermic. It is completely irreversible up to 1300 K. In contrast to that This homogeneous reaction involves the catalytic oxidation with solid catalysts an equilibrium distribution of energy. However, a chain reaction takes place Radicals in the gas phase at higher temperatures and with certain mixture compositions with the catalyst surface instead.

The measure of catalytic performance is called specific catalytic activity (SCA) designated. It changes both with the temperatures and with the mixture compositions. This activity takes away both for the metals Ni, Co and Fe the increase in O₂ concentration as well as with the increase in temperature.

The effect is caused by the irreversible oxidation of the catalyst surface. So a similar change in this activity with temperatures and H₂ composition tongues indicates a change in the dissociative change in hydrogen the metal surface.

Studies with platinum show that the specific catalytic activity behaves inconsistently. In addition, the increase in temperature causes irreversible ler a decrease in the reaction rate and a change in the kinetic Ab dependency (see F.V. Hanson and M.J. Boudart, Joumal of Catalysts, 53, 56 (1978) was found by other studies that the platinum different ka had analytical features in oxidized and reduced states.

The surface structure of the metallic catalysts exerts a very strong influence the catalytic activity. In order to be able to carry out a detailed analysis, the exact knowledge of the topographical structure of the catalyst will be so  like the nature of the near-surface layers that are responsible for the Che mieadsorption are necessary. These layers become very much during the reaction strongly influenced by the reaction component oxygen.

There have been such studies of changes in the surface lying layers of palladium after some exposure to oxygen and hydrogen (see P. Legar, L. Hilaire et. al. "Interaction of Oxygen and Hydrogen with Palladium ", Surface Science 107 (1981) 533-546) Exposure to 1 bar O₂ at 550 ° C changes the binding energy in the Layers of palladium near the surface. It was both with Auger Spectroscopy (AES) and X-ray photoelectron microscopy (XPS) found that the formation of PdO is responsible for changing the surface structure was.

The catalytic reaction is associated with adsorption and desorption of the reactants on the surface of the catalyst. The results of such exchange attempts with absorption and desorption of hydrogen at different temperatures and atmospheric pressures have large tensions due to the concentration gradients (see K. Nakamura "On the Hardening and Topology Changes in Palladium Resulting from Hydrogen Absorption - Desorption Cycling Carried out above 588 K, Journal of Less common Metals, 84 (1982) 173-185). The problems associated with catalysis therefore call for longer and repeated use of the palladium such effects as embrittlement and corrosive Disintegration (see H. Yoshida et al. "Metallurgical Considerations on Pd, Pd-Alloy and their Metal-Hydrogen Systems "Fusion Technology, Vol. Sept 8 (1985) 2388-94). The problems were for the above. Purpose by adding palladium fixed with Ag, Au and Ru.

The oxidation of hydrogen on the catalyst plate is carried out by the following processes influenced:

• 1) Gas transport mechanism
  • - due to differences in buoyancy due to different densities
  • - Natural convection generated by heating the gas mixture
  • - molecular diffusion
  • - Turbulence generated due to the condensation of the steam
• 2) Mechanism of heat dissipation
  • - by natural convection from the catalyst plate to the gas mixture and from the mixture to the container wall
  • by radiation from the catalyst plate to the gas and to the container wall.
• 3) Kinetics of the reaction for simulating the reaction rate.

Taking into account the above The reaction rate for the oxidation was influenced hydrogenation using a modified Arrhenius equation as follows net:

where A and ΔE are the constants for the reaction rate, CH₃ and CO₂ the concentration NEN (moles / cm³) and N₁ and N₂ are the constants for the reaction.

Numerous measurement results were in good agreement with the model presentation calculation (see S.G. Markandeya and A.K. Chakraborty "Modeling of Ca talytic Recombiners for Removal of Hydrogen During Severe Accidents ", 12th Interna tional Conference on Structural Mechanics in Reactor Technology, Stuttgart, Aug, 15-20, 1993). With the calculations it was found that the values of the asset ration energy ΔE for the Pd-Ni-Cu alloy increases only moderately with temperature. The increase in the ΔE values causes a delay in the reaction rate for the Oxidation. According to the equation, the slight increase in Ak causes Activation energy with the temperature a strong increase in the reaction rate at high temperature.

In the case of a catalytic converter, the temperature of the catalytic converters is me of the gases to be reacted increased. The increase in temperature causes again a rapid implementation of the masses and thus a further intensification of the  Temperature increase instead. At the same time, the energy gained is the catalyst gates are lost due to the heat dissipation from the catalysts. The heat abs drove is made up of heat loss through conduction, heat dissipation lust due to convection and radiation. With increasing temperature wins the heat loss from the catalysts due to the convection and the loading radiation in importance.

The above theoretical considerations suggest that for a given inflow the masses to be reacted and after determining the heat loss the surface area of the catalysts, the choice of the catalysts to be used higher temperature of the catalyst plates for an effective removal of the Hydrogen can be determined.

The use of the catalysts at higher temperatures leads to a greatly reduced Hydrogen hazard. By the given at elevated temperature high hydrogen conversion rate, more hydrogen is eliminated in time and produces more steam. The steam produced in this way increases its inertizing Effect the energy requirement to achieve flammability. In the presence Sufficient amount of steam prevents the flammability of the mixture. The excess steam is after cooling on the colder Walls cause a rapid decrease in pressure and a rapid increase convection.

Test results with continuous supply of reacting gases showed that for a given surface size and with different inflow rate a temperature different plateau was reached. The higher temperature level, accelerated in connection with the larger supply of reactive gases the catalysis, the catalysis rate is essential at higher temperatures higher.

Follow-up examinations of the plasma or flame-sprayed catalyst plates with the PdNiCu alloy have shown that no damage to the catalysts after repeated catalysis at higher temperatures. A considerable one too Deviation in the reaction rate after repeated use could not be determined become. This is why palladium-rich alloys are particularly suitable in the mixed crystal area  with nickel and copper for use as catalysts for high temperatures raturation of hydrogen.

Also for the powerful catalytic conversions at higher temperatures The temperature resistances can also take the place of the coated alloys sponges or granules can be used. In this case, these piled up inside a box of coarse mesh. The network consists of Corrosion and temperature resistant stainless steel.

To ensure better heat conduction between the granules or sponges, different particle sizes are chosen so that the small particles occupy the gaps and improve the thermal conductivity between the particles become.

The invention and further details of the invention are described below of exemplary embodiments explained in more detail. The individual shows:

Fig. 1 temperature curve in a large-volume reaction chamber of 10 m³ in the catalytic oxidation of hydrogen on a Pd alloy with 95 wt .-% Pd, 4 wt .-% Ni, 1 wt .-% Cu, the layer on both sides on a carrier plate is applied. The temperature profile has a constant temperature for 30 minutes.

Fig. 2 course of the local concentration of hydrogen as a result of the H₂ inflow and a catalytic reaction, temperature course and other conditions for this catalytic reaction correspond to that of FIG. 1st

Fig. 3 temperature curve during a catalytic oxidation of hydrogen in a large-volume reaction chamber of 10 m². The course represents a slow decrease in temperature with a decreasing presence of hydrogen with a subsequent rapid decrease.

Fig. 4 course of the local concentration of hydrogen as a result of the inflow of hydrogen to the catalyst. The temperature profile on the catalytic plate due to the exothermic reaction corresponds to that of FIG. 3.

Fig. 5 course of the activation energy as a function of temperature change.

Fig. 6 change in the reaction rate for the oxidation of hydrogen on the Pd-Ni-Cu catalyst plate as a function of temperature.

Embodiment 1, Fig. 1st

It is the catalytic reaction of a Pd alloy, the 95 wt .-% Pd, 4 wt .-% Ni, 1 wt .-% Cu and that on a carrier plate made of austenitic steel is applied on both sides in a reaction chamber with a volume of 10 m³ examined. In the reaction chamber, the conditions for meltdown accidents can be seen Introduce adjusted gas mixtures. The gas mixtures are supplied once at the beginning of the experiment.

In the reaction chamber 10 catalyst plates with a size of 20 × 20 cm hung asymmetrically with the help of frames. The plates are arranged so that continuous heat conduction from one plate to the other is not possible is. The increase and decrease in temperature due to the catalytic reaction or the heat dissipation are determined by the size of the individual plate. The history The catalytic reaction is carried out by thermocouples attached to the plate and in the immediate vicinity of the plate z. B. 10 cm from the plate concentration meter pursued for hydrogen.

The catalytic reaction started after the introduction of 50% by volume Steam, 40 vol .-% air and 10 vol .-% H₂ in a gas mixture in the reaction chamber has contained. Only after the introduction of hydrogen from near the ground at the time of 92 min the temperature of the catalyst plate rose from 105 ° C 410 ° C. The temperature remained almost constant at 408 ° C from 92 min to 125 min. The constant temperature was due to the balance of heat generation due to the catalytic reaction of the available hydrogen and the heat removal from the Plate determined.  

Embodiment 2, Fig. 2nd

In this picture the decrease in the concentration of hydrogen during the Embodiment 1 described catalytic reactions of the same plate shown.

Therefore, the other parameter conditions, such as concentration of the Gas mixture as well as the initial temperature, the selected conditions of the above Try. A concentration meter was used to determine the H₂ concentration in the immediate vicinity, e.g. B. 10 cm from the plate.

As in the exemplary embodiment according to FIG. 1, the catalytic reaction also takes place immediately after the introduction of the hydrogen. Into the gas atmosphere. Since this plate was the top of the plate arrangement and the hydrogen from the gas mixture had flowed in as the lightest element in the direction of the buoyancy, there was a local increase in the H₂ concentration to 17.5%. Due to the flow, the course of the H₂ concentration is marked with three peaks. Although hydrogen was broken down by the catalytic reaction, the inflow provided an additional supply of the hydrogen.

After the first flow of hydrogen at 90 minutes, the concentration at the top plate increased to 10% after 96 minutes. Corresponding to the H₂ concentration at this time, as can be seen in FIG. 1, the temperature also reached its highest value of 410 ° C. The H₂ concentration decreased to 4% after 104 min. The temperature of the plate should drop due to the decaying catalytic reaction. The further increase in the H₂ concentration to 17.5% after 106 min ensured that the temperature level of the plate was maintained at 410 ° C. up to 21 min. Due to the higher temperature, more hydrogen was converted in this temperature range at a constant temperature. Due to the increased reaction with abundant hydrogen, the temperature of the plate should increase. Since the heat loss from the plate was much greater at higher temperatures, it was compensated for by the heat of formation on the plate. After 178 minutes, the concentration curve initially showed a steep decrease and later a much shorter increase. Due to the renewed concentration increase, the temperature increased slightly and remained constant at the level of 410 ° C to 126 min. Then the temperature dropped due to the consumption of hydrogen in FIG. 1.

Embodiment 3, Fig. 3rd

In Fig. 3, the temperature profile of a catalytic oxidation of hydrogen for a Palladiumiegierung is shown having the same composition as in Example 1. In this figure, the temperature profile of the catalytic converter from the upper position of the plate assembly is shown in the same way as in FIG. 1. In a departure from the first exemplary embodiment, the concentration of the gas atmosphere was changed for this experiment. In this embodiment, the reaction chamber contains a gas atmosphere with the following proportions; 70 vol.% Steam, 23 vol.% Air. Then 7 vol .-% hydrogen was introduced in the gas atmosphere from the vicinity of the spherical bottom.

After the introduction of hydrogen after 105 min, the temperature rose from 110 ° C to 380 ° C. The temperature slowly increased almost linearly from 107 min to 150 min from. After 150 min an accelerated decrease in temperature set in and reached after 165 min the low plateau of the initial temperature of 110 ° C. The accelerated Decrease in temperature was caused by decrease in hydrogen.

Embodiment 4, Fig. 4th

FIG. 4 shows the decrease in the concentration of hydrogen during a catalytic oxidation of hydrogen. The composition of the gas atmosphere and the temperature profile during the catalysis correspond to the relationships shown in exemplary embodiment 3.

Although 7% of hydrogen was introduced into the lower area of the container, the buoyancy in the upper area resulted in an accumulation of more than 20%. In addition, the lower plate has caused a continuous buoyancy of the hydrogen. Due to the continuous inflow at the upper catalyst plate, the hydrogen concentration, in contrast to an exponential decrease, would only show linear degradation. This resulted in an almost linear decrease in temperature in FIG. 3. After 135 minutes, the hydrogen came up again from below. As a result, more heat was generated on the upper plate and the exponential decrease in temperature was hindered for a short time.

Embodiment 5, Fig. 5th

Fig. 5 illustrates the change in the activation energy as a function of temperature for the oxidation reaction of hydrogen is. Were in all embodiments for the catalytic promotion of the reaction coated stainless steel plates as carrier material with a coating of 95 wt .-% Pd, 4 wt. -% Ni and 1 wt .-% Cu applied. In order to be able to react, the molecules have to absorb a certain amount of energy and this amount of energy is called activation energy (E).

The increase in activation energy with temperature indicates that, in spite of the increased temperature, more energy has to be supplied in order to maintain the reaction. Therefore, major changes in activation energy reflect a delay in the rate of the reaction. From Fig. 5 shows that / mol to 42 KJ / mol increases for the catalytic oxidation of hydrogen, the demand for activation energy of 26 KJ at 373 K at 873 K. This change in activation energy was determined from the model simulation of the numerous experimental values. This temperature-dependent change is considered small and shows the good catalytic properties of the selected alloy at higher temperatures.

Exemplary embodiments 6, FIG. 6

. Fig. 6 illustrates the exponential factor of the change of activation energy through the temperature as a function of the temperature of the catalyst is Da represented the constant values of a certain reaction in the Arrhenius equation other factors, the relationship e presses ^{- Δ E / RT} from the reaction rate . If the relationship is multiplied by the constant A, the product Ae ^{- Δ E / RT} gives the number of effective collisions of the molecules to be reacted, which lead to a chemical reaction.

. From Figure 6 it is apparent that the reaction rate e ^{- Δ E / RT} comprises first large changes with temperature. This dependence becomes flatter over 673 K and indicates that the maximum reaction rate has been reached. Since the heat transfer constantly increases even at higher temperatures and can practically equilibrate with the generation of heat, it becomes more advantageous for the rapid removal of the hydrogen to achieve a faster reaction rate at higher temperatures. Depending on the spatial flow of hydrogen, the powerful higher temperature for the higher conversion can therefore be selected by determining the catalyst size.

To use the catalysts, the surface size is included the goal of dimensioning the desired high hydrogen conversion. Increased steam convection leads to rapid homogeneity Nization of the gas atmosphere surrounding the catalyst. The structure of the catalysts, their surface properties and the catalytic material should be chosen so that the optimal catalytic properties at elevated temperature are not at risk.

Claims (7)

1. Use of a catalyst which is used to remove hydrogen from a gas mixture containing hydrogen, oxygen and steam, characterized in that the catalyst is operated with inflowing hydrogen at an elevated temperature which is kept below the deflagration limit by heat dissipation. 2. Application according to claim 1, characterized, that a PdNiCu alloy is used as a catalyst , in particular an alloy with 95 wt.% Pd, 4th % By weight Ni, 1% by weight Cu. 3. Application according to claim 1 or 2, characterized, that the catalyst operated at a temperature in which steam forms on the catalyst, the with the formation of an inertizing atmosphere Deflagration limit to higher temperatures shifts. 4. Application according to claim 1, 2 or 3, characterized, that convection of the steam to shift the Deflagration limit is generated. 5. Application according to one of the preceding claims, characterized, that the steam generated on colder walls condensed.   6. Application according to one of the preceding claims, characterized, that the catalyst as a catalyst layer a heat-conducting carrier material is applied. 7. Application according to one of the preceding claims 1 to 6, characterized, that catalysts in the form of sponges or Granules in bulk within a network frame dissipating heat from the inner area of the Fill are used outside.