Patent Application
20230356147
This application claims the benefit of priority under 35 U.S.C. § 119 (a) and (b) to French Patent Application No. 2204347, filed May 9, 2022, the entire contents of which are incorporated herein by reference.
The present invention relates to a process for the purification of a gas flow comprising at least one nitrogen oxide, for example nitrogen monoxide NO and/or NO2, by a purification unit with conversion of NO2 over a catalytic bed.
Nitrogen oxides (which comprise NOx compounds) are pollutants commonly emitted during the combustion of fossil fuels. NOx compounds in the atmosphere create tropospheric ozone, which is toxic when it is inhaled and contributes to the greenhouse effect. Furthermore, NOx compounds contribute to the formation of acid rain, which is harmful to plant and animal life, and also to goods. During treatment and purification stages in the presence of water, in particular compression (followed by refrigeration), NOx compounds will generate acid condensates, in particular nitric acid and nitrous acid.
It is thus preferable to employ a process for scrubbing flue gases which makes it possible to cool them before compressing them, and also to remove a portion of the dust which they contain. If the scrubbing is at high pressure, the conversion of NO to NO2 is accelerated.
The nitrogen oxides can be halted by using:
An SCR (Selective Catalytic Reduction) makes it possible to convert a portion of the NOx compounds (NO and NO2) into N2 and H2O by reacting them with ammonia or urea in contact with a catalyst at approximately 200-400° C. The passage section of the SCR reactor, the volume of the catalytic reactor and the amount of catalyst depend on the volume flow rate to be treated. Furthermore, the catalyst has a lifetime of the order of 3-5 years (requires replacement every 3-5 years). The cost (CapEx) of an SCR and also its size thus depend on the volume flow rate to be treated.
According to the SCR process, the gas containing NOx compounds mixed for example with ammonia subsequently passes through a multi-bed catalyst in a range of temperatures of between 250 and 380° C. The catalysts most often used are metal oxides on a TiO2 or Al2O3 support.
An example of gas flow to be treated is the gas generated by the heating furnace of a unit for the steam reforming of hydrocarbons, for example the reforming of methane in the presence of steam, known as Steam Methane Reforming or SMR. This reforming makes possible the production of hydrogen, an energy carrier which plays an increasing role in the decarbonization of various sectors, in particular transport and industry. In SMRs, hydrogen production is accompanied by significant CO2 production. A CO2 capture unit can be added to an SMR in order to reduce the carbon footprint of the production of hydrogen by SMR. CO2 capture (for example, purification of CO2 for food use or for sequestration) can be carried out cryogenically or non-cryogenically. CO2 is transported and sequestered, if need be, either under pressure or in liquid form.
On an SMR, the CO2 capture unit can be placed on the waste gases from a PSA which treats the product from the SMR or on the flue gases from the furnace, produced by the process for the production of heat necessary for the chemical reaction of the reforming. The advantage of CO2 capture on the flue gases is that this makes it possible to capture up to 100% (probably>80%) of the CO2 from the SMR. The CO2 originates from the reforming reaction of the methane if the waste gas from the PSA is recycled to the burners of the furnace and originates from the combustion of the gases in the burners of the SMR in order to maintain a high temperature in the furnace.
The SCRs are generally placed downstream of combustion units on low-pressure flue gases. For example, on some SMRs, SCRs are placed between the combustion furnace and the chimney for discharges of the flue gases to the atmosphere.
Alternatively, the supply of heat can also be ensured by thermal incorporation with the remainder of the process, such as, for example, with the hot flue gases from the combustion unit.
EP 2 176 165 A1 relates to the recycling of a stream enriched in NO2 upstream of a separation unit (and downstream of an existing SCR) which produces a stream enriched in CO2, a stream depleted in CO2 (non-condensables) and a stream enriched in NO2.
The present invention relates to an SCR placed not downstream of a combustion unit on low-pressure flue gases but on a stream preconcentrated in NO2 by virtue of one or more separation processes upstream of the SCR which are placed in series or in parallel so that the flow to be treated is lower. The advantage of such a solution is that of significantly reducing the CapEx of the SCR, it being possible for the flow of the flue gases to be treated to be 100 times greater than the flow entering the SCR. One of the disadvantages is that the temperature of the flue gases (˜200-400° C. necessary for the SCR) is then no longer inevitably available at the inlet of the SCR. Furthermore, the fact of concentrating in NO2 the stream to be treated in the SCR can also result in this stream being concentrated in certain impurities at the inlet of the SCR (for example SO2).
More specifically, the unit for concentrating in NO2 upstream of the SCR might advantageously be a concentrator, such as a PSA unit or membranes, or a distillation column operated at a temperature below ambient temperature (for example advantageously over a temperature range between [−40; 10]° C. and a fluid at the inlet of the distillation column comprising >50 mol % of CO2 and <50 mol % of N2).
Furthermore, the stream preconcentrated in NO2 at the outlet of the unit for concentrating in NO can predominantly comprise CO2 (concentration range [50; 99.5] mol %) and nitrogen (concentration range [0.5; 50] mol %).
The fact of having to heat the inlet fluid in the SCR also has the advantage of being able to choose and regulate the reaction temperature of the SCR, which is an important parameter which influences the chemical reactions taking place in the SCR. In the normal application of SCRs, the temperature of the flue gases is endured and not regulated.
Furthermore, in order to limit the emissions of CO2 and of NH3 (originating from the NH3 which passes into the SCR without reacting) to the atmosphere, the products of the SCR can be recycled in the process (recycle fluidically connected to the unit for concentrating in NO2 upstream of the SCR with, between the two, other possible unit operations, such as a means for compressing and a unit for drying the fluid).
As mentioned, the unit for concentrating in NO2 can also concentrate in other impurities, such as SOx compounds (in particular SO2). If such is the case, there is a risk of the SOx compounds reacting with the NH3 to form in particular ammonium bisulfate (ABS) in the SCR, which risks fouling and corroding the catalyst and the economizer.
In order to limit the risks of formation of ABS, it is proposed, according to the invention, to dilute the inlet stream of the SCR with another fluid (such as residual nitrogen) (even if this brings about an increase in the inlet volume flow rate in the SCR).
This dilution flow can also make it possible to ensure a constant flow at the inlet of the SCR despite a potential variation in the flow exiting from the unit for concentrating in NO2 and/or to contribute necessary constituents to the SCR, such as molecular oxygen (for example: distillation column, the liquid outlet flow of which depends on the liquid reflux at the column top).
It thus becomes possible to reduce or to prevent the flow of air which has to be sent to the SCR with the ammonia.
It is also advantageous to use the dilution flow even in the absence of SOx in order to be able to adjust the concentration and/or the flow and/or the temperature of the gas feeding the SCR.
According to a subject-matter of the invention, provision is made for a process for the purification of a gas flow containing NO and/or NO2, carbon dioxide and nitrogen, in which:
According to other optional aspects of the invention:
According to another subject-matter of the invention, provision is made for an appliance for the purification of a gas flow containing NO2, carbon dioxide and nitrogen comprising a unit for purification by adsorption, a treatment unit, a unit for the catalytic conversion of NO2, means for sending the gas flow to the unit for purification by adsorption in order to be separated therein into a flow enriched in carbon dioxide and in NO2 and depleted in nitrogen and into a fluid depleted in carbon dioxide and in NO2 and enriched in nitrogen, means for sending the flow enriched in carbon dioxide and in NO2 and depleted in nitrogen to the treatment unit in order to form a fluid enriched in NO2 with respect to the treated flow, means for sending the fluid enriched in NO2 to the catalytic conversion unit making possible the conversion of at least a portion of the NO2 in the presence of ammonia and of oxygen to give nitrogen and water in order to produce a gas depleted in NO2 with respect to the fluid enriched in NO2 and means for sending at least from time to time a fluid having nitrogen as main component, for example at least a portion of the fluid depleted in carbon dioxide and in NO2 and enriched in nitrogen, to the catalytic conversion unit.
The treatment unit can comprise a distillation column for producing the fluid enriched in NO2 with respect to the treated flow and a gas depleted in NO2 and means for separating the gas depleted in NO2 in order to form a fluid rich in carbon dioxide.
For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:
The flow F contains carbon dioxide, nitrogen and NO and/or NO2, and also optionally at least one of the following components: N2O, SON, oxygen, argon. Typically, it does not contain hydrogen or methane, indeed even only possibly traces. The oxidation of NO, when present, to NO2 can take place little by little during all the parts of the process where oxygen and NO are present in the gas phase. The rate of oxidation is higher at high pressures and low temperatures. The oxidation is catalysed by adsorbents, such as those present in the dryer and the PSA.
This gas F is produced at high temperature and thus is cooled by scrubbing with water in a scrubbing tower Q to produce a cooled gas 1. The cooled gas 1 is compressed by a compressor C to between 5 and 15 bar abs and subsequently is dried in a dryer S, for example by adsorption, to produce a dry gas 5. The dry gas 5 is sent to a pressure swing adsorption PSA unit comprising several adsorbers operating in offset fashion in a known way. The PSA produces a flow 6 enriched in carbon dioxide and in NO2 and depleted in nitrogen and a fluid 17, 19 depleted in carbon dioxide and in NO2 and enriched in nitrogen; the fluid 17, 19 possibly contains oxygen.
The flow 6 is cooled in a heat exchanger E1 to a temperature which makes possible the liquefaction of the NO2 in the flow 6, producing a cooled fluid 7 which is separated by distillation and/or partial condensation. There is seen here a distillation column K producing a flow 9 depleted in NO2 and a bottom liquid 11 enriched in NO2. The liquid 11 is vaporized (not illustrated) to produce a gas which is expanded in a valve V1 and sent as gas 13 to be treated in the selective catalytic reduction SCR unit after heating in the heat exchanger E3.
The SCR reduction unit is fed with ammonia and/or with urea and also by a source of oxygen, for example air, if the gas 13 does not comprise enough oxygen. An injection of air may, however, be necessary to atomize the ammonia or the urea. The SCR unit produces a gas 15 in which the NO2 has been partially converted into nitrogen and into water. This gas 15 is sent to the scrubbing tower to recover the carbon dioxide which it contains. This also makes it possible to prevent sending ammonia to the atmosphere.
At least a portion 17 of the gas depleted in carbon dioxide and in NO2 and enriched in nitrogen can be mixed with the gas 11 to form the gas 13. The valve V2 regulates the amount of gas 17 mixed with the gas 11, this valve being controlled by an FIC, in order to detect the flow rate of the fluid 13, and/or by an AIC, in order to detect the content of a component of the fluid 13.
Another portion 19 of the gas depleted in carbon dioxide and in NO2 and enriched in nitrogen can be sent to the atmosphere.
The gas 17 is richer in nitrogen than the vaporized liquid 11 and thus makes it possible to enrich the vaporized liquid 11 in nitrogen. The gas 17 is also richer in oxygen than the vaporized liquid 11 and makes it possible to enrich the gas 11 in oxygen in order to reduce the amount of oxygen to be sent to the SCR unit from another source, if need be.
Nitrogen has the advantage of being a neutral gas which does not influence the reaction mechanisms in the reaction chamber of the SCR (unlike air, which contains 02).
If the gas 5 contains at least one SON, there is a risk of the SOx being present in the gas 13, indeed even of being enriched by the upstream treatments. There is thus a danger of at least one SOx (in particular SO2) reacting with the NH3 to form in particular ammonium bisulfate (NH4)HSO4 (ABS), which risks fouling and corroding the catalyst of the SCR unit. In order to limit the risks of formation of ABS, the inlet stream 13 of the SCR unit is diluted with the fluid 17 rich in nitrogen, preferably containing at least 90 mol %, indeed even at least 95 mol %, of nitrogen and preferably at least 1 mol % of oxygen, indeed even at least 2 mol % of oxygen. This can result in an increase in the inlet volume flow rate in the SCR unit.
This dilution flow 17 can also make it possible to ensure a constant flow at the inlet of the SCR despite a potential variation in the flow 11 exiting from the unit for concentrating in NOx and/or to contribute necessary constituents to the SCR unit, such as molecular oxygen and/or water. For example, the distillation column K has a liquid outlet flow 11 which depends on the liquid reflux at the column top and the flow 11 is thus variable.
ABS cannot be prevented from forming if the SCR unit is not operated at a sufficiently high temperature. Thus, to remove the ABS formed, the temperature has to be increased up to 300-350° C., the reaction for the formation of ABS being reversible.
The flow 17 can be varied in order to target a set flow (over a certain range of variation) entering the SCR unit. Thus, if the flow 11 falls, the flow 17 increases, and vice versa.
It will also be understood that, according to alternative forms of the invention, the flow 17 added to the flow 11 can be a gas having, as main component, nitrogen originating from a source other than the PSA unit. It can originate from another unit treating the cooled gas 1 and/or from a network, for example a pipeline transporting nitrogen and/or an appliance for air separation, for example by cryogenic distillation. Alternatively, the flow 17 can be varied in order to target a given composition.
For example, it is possible to target a given ratio between the CO2 content and the nitrogen content of the flow 13. It is possible to target a given oxygen content of the flow 13 or a given content of impurities, such as SO2. The addition of water to the flow 11 makes it possible to reduce the formation of compounds. This is because water acts as inhibitor for some undesirable chemical reactions taking place in the SCR unit. In practice, air is often added to the inlet flow 13 if there is a need to increase the 02 concentration or to more easily atomize the ammonia in the injector. The process comprises the addition of ammonia or of urea to the SCR unit upstream of the reaction chamber (the concentration of aqueous phase of which can potentially be adjusted as a function of the need for water).
The dilution flow can be characterized in the following way:
To recycle the product 15 of the SCR unit in the process is counter-intuitive for a person skilled in the art with regard to the management of the NOx compounds. Generally, the product of the SCR is directly sent to the atmosphere (SCR placed immediately before the chimney/silencer to reduce the NOx compounds sent to the atmosphere).
The valves V3 on the flow 21, from which are divided the flows 23 and 25, and V4 on the flow 25 make it possible to regulate the amounts of gas sent to the air or downstream of the SCR unit.
In this Figure, the flows 21, 23 are not necessarily present.
If the SCR unit operates under pressure (typically at a pressure slightly greater than that of the dryers), this makes it possible to reduce the size of the item of equipment and to improve the specific energy by directly recycling, under pressure, the gas 15 produced by this SCR unit upstream of the dryers S.
The flow 15 can be recycled downstream of the compressor C. In this case, the flow 17 has to be compressed upstream of the inlet of the SCR unit.
Otherwise, the flow 15 can be recycled in an inter-stage of the compressor C and, in this case, the flow 17 can be sent to the inlet of the SCR unit without compressing it.
In all the cases mentioned, the SCR unit can be incorporated in a unit for the production of a flow rich in CO2, for example by partial condensation and/or distillation. The flow 9 depleted in NO2 can be treated by partial condensation and/or distillation in a system of columns for producing at least one fluid rich in CO2, for example containing at least 90 mol % of CO2. Preferably, the flow 9 is not heated but is sent directly to the partial condensation and/or to the distillation. Downstream of this cold separation, the majority of the NO, if present, will have been converted to NO2, which coexists with N2O4.
In order to concentrate the flow to be treated in NO2, a bottom reboiler of the distillation column K can be added (optionally) upstream of the SCR unit or the inlet temperature of the fluid 7 in the distillation column K can be adjusted so as to obtain a certain flow or a certain concentration at the outlet of the distillation column K.
In order to prevent problems of corrosion in the economizer, the materials (for example stainless steels, and the like) will be carefully chosen.
Use may also alternatively be made of less noble materials for this economizer by regulating the outlet temperature of the fluid which has reacted and by making sure that it remains above its dew point. For this, a heater will also be provided upstream of the economizer on the fluid to be treated.
Preferably, the low-temperature separation of the NO2 in the units K or N takes place in the same thermally insulated chamber as the separation of the fluid depleted in NO2 produced by the separation of the NO2 to produce a fluid containing at least 90% of CO2.
It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2204347 | May 2022 | FR | national |
