Patent ID: 12220689

FIG.1shows schematically a reactor plant for implementing a process of the invention. A reactor10is present, and comprises various catalyst beds10A. The iron oxides for reduction are located in the catalyst beds10A. The reactor10possesses a pipe system60with feed conduits60A, via which the at least one reducing gas is introduced into the reactor, and possesses takeoff conduits60B, via which the gaseous reaction mixture from the reduction is taken off from the reactor, allowing the gas to circulate in the reactor plant. The direction of the arrows here indicates the direction in which the gases circulate in the reactor plant. During the reduction of the iron oxides, the at least one reducing gas can be heated by means of a heating apparatus50. Also present is a NDIR sensor5, which is able to measure the water content and/or the ammonia content of the reaction gases from the reduction operation at the takeoff conduit60B. Present for this purpose on the takeoff conduit60B is an insulated and heatable conduit6. The conduit6here may have thermal insulation and/or a heating apparatus, which may serve to keep the temperature of the gas mixture for analysis high enough to prevent the water condensing and hence the result of the NDIR measurement being distorted. The pressure in the takeoff conduit6is reduced in stages to 1 to 2 barg and then to 0.01 to 0.5 barg for the NDIR measurement.

The text below illustrates the course of one embodiment of the process of the invention in this reactor plant.

Via a gas feed line60C, the at least one reducing gas, which preferably comprises hydrogen and nitrogen, can be compressed by means of a compressor25and then introduced into the interior of the reactor10via a feed conduit60A. At the start of the process, the gas mixture can be brought to the requisite temperature—for example, up to 450° C., preferably to 370° C. to 390° C.—via the heating apparatus50, and then passed through the catalyst beds10A. Control valves15are present throughout the system in order to set the gas flow. The further downstream the siting of the catalyst beds10A in the reactor10, the greater the size of the catalyst beds. During the reduction, the gas mixture resulting from the reduction, which comprises water and/or ammonia and also unreacted hydrogen and nitrogen, is taken from the reactor via the takeoff conduit60B. The water and/or ammonia content of this gas mixture is then determined by means of the NDIR sensor5, which is connected to the takeoff conduit60B via a conduit6at the reactor exit. The gas mixture resulting from the reduction can give up at least part of the heat still present subsequently, by way of a heat exchanger40. The gas mixture is then passed into a heat recovery boiler20, in which it gives up further heat to water which is passed through the heat recovery boiler. This water is introduced into the heat recovery boiler20by means of a connection21and is taken off in the form of steam from the heat recovery boiler via the connection22. The steam may be used, for example, for boosting the energy efficiency and thermal efficiency of the overall plant, for operating the compressors by means of steam turbines. Thereafter the gas mixture can give up further heat, via further heat exchangers40and/or via ammonia condensers40A, and so subsequently in the separator30there can be separation of a mixture of ammonia and/or water. The water/ammonia mixture separated off may then be removed from the system via the takeoff line30A. The reducing gas mixture comprising hydrogen and nitrogen may then be returned to the compressor25, for use in a further cycle for the reduction of the iron oxides.

The flow rate of the reducing gas and/or the heating rate may be set accordingly as a function of the concentrations of water and/or ammonia in the system that are determined by the NDIR sensor5; accordingly, during the reduction, the concentration of the water formed varies within a range around a certain limiting value. The NDIR sensor5may additionally be used to set the temperature in the separator30so that there is no freezing of ammonia, which would be detrimental to the operation of the separator.

FIG.2shows a flow scheme for the procedures and substeps which take place after the determination of water using an NDIR sensor. In this embodiment, the limiting value for the concentration of water in the gas phase may be, for example, 3000 ppmv, and the range around the limiting value may be ±200 ppmv. Beginning with the elements carrying reference80,81, the water content determined by means of NDIR is first read off on the analyzer of the reactor plant, and the value is compared with the limiting value. Depending on the outcome of the comparison, there are then three different substeps (C1), (C2) or (C3). If it is found that the measured value is more than 200 ppmv over the limiting value, in a process step (C2) the flow rate is increased and/or the heating rate is lowered (82A). Accordingly, the concentration of the water formed can either be reduced in the gas phase by the setting of a higher flow rate, leading to more rapid dilution of the water concentration in the gas phase, or the reduction is reduced by lowering of the heating rate. If it is found that the measured value is more than 200 ppmv below the limiting value, then the reduction status in the catalyst bed of the reactor is checked (82B), this being possible, for example, via the ascertainment of the temperature of the catalyst bed or, if the reducing gas comprises nitrogen and hydrogen, this is accomplished via the ammonia concentration in the gas phase. For the temperature measurement there may be temperature sensors at the entry and exit of the catalyst beds. The reduction of a catalyst bed may be considered to be complete when there is no further increase in the ammonia concentration at constant pressure and temperature. If it is found that the reduction in the catalyst bed is not yet at an end, the heating rate can then be increased (83B) in process step (C1). Should it be found that the concentration of water in the gas phase is within a range of ±200 ppmv around the limiting value, then the flow rate and/or the heating rate can be retained (82C) in a substep (C3).

Should it be found that the reduction of the iron oxides in a catalyst bed has already been completed, verification may be carried out as to whether there are still catalyst beds in the reactor with iron oxides requiring reduction (83A). If this is not the case, the reduction is at an end (84) or, if there are further catalyst beds, the entry temperature of the subsequent bed can be increased by the closing of a valve in order for the reduction of the next catalyst bed to commence (85). The heating rate can be set by adjustment of the control valve.

FIG.3shows, in a flow scheme, the procedures and substeps which arise after the determination of the ammonia. For these purposes, in a first step (90), the concentration of ammonia in the gas phase is ascertained and then the respective freezing point of the aqueous ammonia solution is calculated (91) as a function of the process parameters, such as pressure, water content in the gas phase and temperature. The temperature of the condensing apparatus is then compared with the calculated freezing point of the aqueous ammonia solution (92) and different substeps are performed depending on the outcome. The process substeps may be, in particular, a process step (C4), in which the temperature of the condensing apparatus is increased if the temperature of the condensing apparatus is less than 5 K over the calculated freezing point of the aqueous ammonia solution (92B). A further possibility is for the temperature of the condensing apparatus to be lowered further in a process step (C5) if the condenser temperature is more than 10 K above the calculated freezing point of the aqueous ammonia solution. A determination is first made here as to whether the plant allows the temperature of the condenser to be lowered further (92C). Many plants do not allow any further drop in the condenser temperature, particularly if the condenser temperature is already −20° C. Depending on whether a further lowering is possible, then either the condenser temperature is lowered further (92D) or the operating parameters are retained (92E). A process of this kind represented inFIG.3with the corresponding substeps makes it possible for water and/or ammonia to be condensed from the gas phase in a particularly reliable way, and at the same time prevents freezing of the condensing apparatus. If the condenser temperature is in a range from 5 to 10° C. above the freezing point of the aqueous ammonia solution, the condenser temperature can be retained (92A).

FIG.4shows, in a diagram, the experimentally determined temperature profile and the experimentally determined water content in volume % in the gas phase for a reduction of iron oxides with a synthesis gas containing 76.5% hydrogen, 22.5% nitrogen and 10% argon (volume % in each case). The iron oxides were reduced for different times at different flow rates of the synthesis gas, with a reduction in the water content in the gas phase being possible through an increase in the flow rate. The curve denoted100shows here the course of the water content as a function of the temperature for reduction over 25 hours with a flow rate of 250 l/h. The curve denoted101shows the course of the water concentration in the gas phase for a 30-hour reduction with a flow rate of 400 l/h. The course of the water concentration for a 30-hour reduction with a flow rate of 400 l/h is identified by the curve denoted102. A further reduction over a period of 40 hours with a flow rate of 1200 l/h was likewise carried out (curve denoted103). It is clearly apparent that the concentration of the water decreases with increasing flow rate, owing to the dilution effect of newly arriving gas, but on the other hand the time which is needed for the reduction goes up.FIG.4also shows that as a result of the increase in the flow rate, the maximum concentration of the water formed can be lowered successively from very high values of about 12000 ppmv at a flow rate of 250 l/h, so that lastly, at a flow rate of 1200 l/h, concentrations of water formed of under 1500 ppmv are attained, which are no longer detrimental to the catalytic activity of the catalysts or catalyst precursors formed.

FIG.5shows the experimentally determined catalytic activity of six different catalysts based on wüstite, which were exposed during the reduction to different water contents of under 2000 ppmv to 8000 ppmv. It is clearly apparent that the catalytic activity drops by about 5% to 10% as the water content goes up, especially beyond a limiting value of about 4000 ppmv.

FIG.6shows the course of the water concentration (curve denoted110) in the gas phase at the reactor exit in ppmv, and the temperature profile (curve denoted120), over a certain period t. The water concentration was measured by means of an NDIR sensor. Iron oxides based on wüstite were reduced at a pressure of 90 bar in a hydrogen-containing atmosphere at a flow rate of 1200 NL/h (NL=normal liter (volume at 1013.25 mbar and 0° C.)). It is clearly apparent that at the start of the reduction, the temperatures and the water content in the gas phase are still relatively low and the reduction rate rises as time goes on, owing to the increasing temperature, leading to a higher water content at the reactor exit. Eventually a maximum water content is reached, at which point the water content at the reactor exit drops again because of the decreasing reduction rate. It is clearly apparent that in the reduction of wüstites, water contents in the gas phase of more than 3000 ppmv or more than 3500 ppmv can be measured. Such high water contents can be effectively prevented by a process of the invention for activating iron oxides.

The invention is not limited by the description with reference to the working examples. The invention instead embraces every new feature and also every combination of features, including in particular every combination of features in the claims, even if that feature or that combination is not itself explicitly indicated in the claims or working examples.