Patent Application
20240343576
The present disclosure relates to the field of nitric acid production in a mono pressure plant.
Pure nitric acid is a clear, colorless liquid with a strong odor. Nitric acid is produced in large quantities principally by catalytic oxidation of ammonia (Ostwald process). Ammonia is converted to nitric acid in several stages. The ammonia is first oxidized in an ammonia burner on platinum gauzes (commonly called ammonia converter) or cobalt balls, producing nitric oxide (in this disclosure also called nitrogen monoxide (NO)) and water:
The reaction product from (1), nitric oxide, following cooling, is then oxidized to nitrogen dioxide (NO2) and further to dinitrogen tetroxide N2O4 (g) in an oxidation section:
Cooling of nitrogen oxide gases is accomplished first through the use of a waste heat recovery system recovering the heat from the conversion of ammonia into nitric oxide, then through the use of a cooler condenser in which condensed nitric acid is separated from nitric oxide, nitrogen dioxide and dinitrogen tetroxide and nitric acid gases, collectively called NOx gases, and finally by heating the tail gas released at the outlet of the absorption tower in which the NOx gases are absorbed.
By absorption in water, nitrogen dioxide and dinitrogen tetroxide are converted to nitric acid and nitric oxide:
Weak nitric acid which is up to 68% (azeotrope) is obtained. Through a rectification process the concentration of nitric acid can be increased up to 99% concentrated nitric acid. The total reaction is given by the following formula:
The main process units in a nitric acid production plant, include an ammonia converter (conversion of ammonia into nitric oxides using oxygen over a suitable catalyst), an oxidation section (conversion of nitric oxide into nitrogen dioxide and nitrogen tetroxide), an absorber unit (for the absorption of NOx gases into water) and a bleacher unit (removal of unreacted dissolved gases, containing in particular NOx and gases, from the aqueous nitric acid solution, which give it its typical brownish color).
The process for the production of nitric acid can be differentiated into a mono-pressure (single-pressure) and dual pressure (split-pressure) process.
In a mono-pressure process, the converter and the absorber unit operate at roughly the same working pressure P1. Such mono-pressure process generally includes low-pressure (2 to 6 bara) and high-pressure (from above 6 to 16 bara, in particular 9 to 16 bara) processes.
The drive power for the air compressor typically originates from a tail-gas turbine and a steam turbine or electric motor. Accordingly, the compressor train of mono pressure nitric acid production plant typically comprises an air compressor, a tail-gas turbine, and a steam turbine or electric motor.
More in detail, referring to
The air used for the oxidation of ammonia is commonly denoted as primary air; the air used as stripping medium in the bleacher unit is commonly denoted as secondary air.
According to the prior art, the revamping of the nitric acid production plants to increase its capacity is commonly based on increasing the amount of primary air to the reactor, which leads to a proportional increase of the amount of nitric acid produced.
The increase of the amount of primary air in the reactor entails the installation of a new air compressor or the revamping of the existing one. This revamping has significant drawbacks. First of all, it entails elevated costs for the modification or replacement of the existing equipment, i.e. the air compressor and the corresponding turbines and electrical motor. In addition, the revamping of the equipment is also technically demanding, leading to long plant downtime.
Another problem related to nitric acid production plants is the high amount of energy required in order to operate the air compressor. Consequently, a high amount of energy is required to achieve the targeted nitric acid production throughput.
A goal of the present invention, therefore, is to provide a system and a method for operating the system which allows for the reduction or even suppression of power required to operate the air compressor in a mono nitric acid plant.
In CN110540178A (China Chengda Engineering Co Ltd, 2019), process for producing nitric acid is disclosed. Nitric acid is produced by a medium pressure method, which is characterized in that it comprises the following steps: the ammonia oxidation and absorption pressure is 0.5-0.6 MPa; enabling the tail gas leaving the absorption tower to pass through a carbon molecular sieve Temperature Swing Adsorption (TSA) treatment device to reduce the content of nitrogen oxides in the tail gas to be less than 100 mg/Nm3; the process air of the air compressor is used as the regeneration desorption gas of the carbon molecular sieve temperature swing adsorption treatment device, and the regeneration desorption gas containing the nitrogen oxide can be returned to the ammonia oxidation reactor for reuse; adding a layer of N2O decomposition catalyst in the oxidation reactor to reduce the content of N2O to 50-100 PPM through reaction; the nitric acid bleaching tower is arranged at the bottom of the absorption tower, and the two towers are integrated, so that the process flow is shortened, and the equipment investment is reduced. With regard to the amount of air being compressed by the air compressor, however, the same amount of air is to be compressed as would be in the absence of the TSA unit: in the presence of the TSA unit, the amount of air being compressed is initially split between the TSA unit and the ammonia oxidation reactor directly and, in the end, with the amount of compressed air leaving the TSA unit being directed also to the ammonia oxidation reactor, the total amount of air compressed by the air compressor ends up in the ammonia oxidation reactor.
In WO2018/162150A1 (Casale SA, 13 Sep. 2018) a solution is proposed to overcome the revamping drawbacks. WO2018162150A1 discloses a dual pressure plant for the production of nitric acid comprising a reactor providing a gaseous effluent containing nitrogen oxides, an absorber unit in which nitrogen oxides react with water providing raw nitric acid and, the absorber unit operating at a pressure greater than the pressure of the reactor, a compressor elevating the pressure of the reactor effluent to the absorber unit pressure, the plant also comprising a first HP bleacher unit and a second LP bleacher unit, the first HP bleacher unit stripping with air the NOx gas from the output stream of the absorber unit, thus providing a partially stripped nitric acid stream and a nitrogen oxides-loaded air stream, the former being fed to the second LP-bleacher unit and the latter being recycled to the oxidation section, upstream of the NOx gas compressor.
A further air compressor is also provided, which supplies the first HP bleacher unit with air. Hence, energy is required in order to operate a first HP bleacher unit at a high-pressure and then recycle NOx gases to the delivery side of the NOx gas compressor.
Therefore, there remains a need for a process and a corresponding plant setup for minimizing or even suppressing the amount of energy required in order to operate the air compressor, in order to avoid bottle-necks in the nitric acid production throughput associated with those compressors.
In one aspect of the disclosure a production plant for producing nitric at reduced power consumption and reduced emissions, is disclosed. The production plant comprises, optionally, a source of pressurized air in fluid communication with the production plant comprising:
The inventors have found that, instead of continuously supplying pressurized air provided by an air compressor as primary air to the mixing unit at a pressure P1, it is possible to recirculate the first tail gas stream provided by the means for splitting at a pressure P1, particularly when combined with, at the same time, providing oxygen, particularly pressurized oxygen, to the system. Therefore, compressed or pressurized air only has to be supplied in order to start the process, in particular to pressurize the plant or system, but no longer after the production of the tail gas has started and an air compressor is not required for operating the nitric acid production plant. More in particular, an air compressor suitable for pressurizing the plant or system has a capacity of 2000 to 19000 m3/h, which is much smaller than an air compressor for operating a prior art nitric acid plant, with a capacity of at least 300000 m3/h. In addition, the first oxygen-rich gas having a pressure P1 and the supply of a second oxygen-rich gas provide oxygen to the ammonia converter and to the absorption tower, respectively, such that, in the absence of the primary and secondary air provided by the air compressor, the concentration of oxygen in the ammonia converter and in the absorption tower is similar to that in a standard dual pressure nitric acid plant.
In the absence of an air compressor and with the additional tail gas stream being recirculated in the system, not only is the power demand of the system reduced: the NOx emissions leaving the system are also reduced. Therefore, the size of the treatment unit for treating those NOx emissions is reduced with respect to the size in the corresponding standard mono pressure nitric acid plant. The system of the present disclosure thus achieves a significant power reduction together with a reduction of the area footprint of the plant and the simplification of the system, by the removal of the air compressor and reducing the size of the tail gas expander. In addition, the separate supply of the pressurized oxygen or oxygen-rich gas ensures an optimal conversion of ammonia to nitric oxide.
In one embodiment according to the production plant of the disclosure, the system further comprises one or more of:
In one embodiment according to the production plant of the disclosure, the production plant further comprises a bleacher comprising an inlet for an oxygen-rich bleaching gas, and an outlet for bleaching gases or off-gases, and wherein the bleaching gases or off-gases are in fluid communication with any stream between the ammonia converter and the absorption tower, such that the second oxygen-containing gas is at least partly provided by the bleaching gases or off-gases.
In one embodiment according to the production plant of the disclosure, part of the oxygen-rich gas or part of the oxygen-containing gas or part of the tail gas, is in fluid communication with the inlet of the bleacher, such that the oxygen-rich bleaching gas or stripping gas is at least partly provided by part of the oxygen-rich gas or part of the first oxygen-containing gas or part of the tail gas stream downstream the means for pressurizing.
In one embodiment according to the production plant of the disclosure, the oxygen-rich gas, the second oxygen-containing gas, the oxygen-rich bleaching gas and the oxygen-rich off-gases are all at least partly provided by a high pressure water electrolyzer.
In one embodiment according to the production plant of the disclosure, the production plant further comprises a tail gas expander for expanding the second tail gas stream to atmospheric pressure, to produce an expanded tail gas, wherein the means for pressurizing can be powered by the tail gas expander or by the steam turbine or by a power source.
In one embodiment according to the production plant of the disclosure, the plant further comprises a source of pressurized air in fluid communication with the system of the present disclosure, wherein the fluid communication between the source of pressurized air and the system is in direct fluid communication with the oxygen-rich gas.
In one aspect of the disclosure, a method for producing nitric acid at reduced power consumption and reduced emissions, in a production plant according to the production plant of the disclosure, is disclosed. The method comprises the steps of:
comprising the steps of:
In one embodiment according to the method of the disclosure, in step h), at least part of the tail gas obtained from step f) is pressurized in the means for pressurizing, thereby generating a pressurized tail gas, and wherein the method further comprises the steps of:
In one embodiment according to the method of the disclosure, the method further comprises the step of:
In one embodiment according to the method of the disclosure, the method further comprises the step of:
In one embodiment according to the method of the disclosure, the method further comprises the steps of:
In one embodiment according to the method of the disclosure, the method further comprises the step of:
In one embodiment according to the method of the disclosure, in step a), the pressurized air is supplied in the stream in direct fluid communication with the oxygen-rich gas.
In one aspect of the disclosure, the use of the production plant of the disclosure for performing the method of the disclosure, is disclosed.
In a further aspect of the disclosure, a method for revamping a production plant for producing nitric acid into a production plant according to the present disclosure is disclosed, wherein the production plant comprises:
The revamping method is characterized in that it comprising the steps of:
Throughout the description and claims of this specification, the words “comprise” and variations thereof mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this disclosure, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the disclosure is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.
Features, integers, characteristics, compounds, chemical moieties, or groups described in conjunction with a particular aspect, embodiment or example of the disclosure are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this disclosure (including the description, claims, abstract and drawing), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The disclosure is not restricted to the details of any foregoing embodiments. The disclosure extends to any novel one, or any novel combination, of the features disclosed in this disclosure (including the description, claims, abstract and drawing), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.
The enumeration of numeric values by means of ranges of figures comprises all values and fractions in these ranges, as well as the cited end points. The terms “ranging from . . . to . . . ” or “range from . . . to . . . ” or “up to” as used when referring to a range for a measurable value, such as a parameter, an amount, a time period, and the like, is intended to include the limits associated to the range that is disclosed.
Where the term “about” when applied to a particular value or to a range, the value or range is interpreted as being as accurate as the method used to measure it.
As defined herein, a pressurized oxygen-rich gas is a gas having a pressure ranging from 9 to 30 bara, preferably 15 to 30 bara, and comprising more than 21 vol % of oxygen, more in particular more than 30 vol %, more than 40 vol %, more than 50 vol %, more than 60 vol %, more than 70 vol %, more than 80 vol %, more than 90 vol %, more than 95 vol %, and more than 99 vol %, more in particular 100 vol % of oxygen.
As defined herein, an oxygen-rich gas is a gas comprising more than 21 vol % of oxygen, more in particular more than 30 vol %, more than 40 vol %, more than 50 vol %, more than 60 vol %, more than 70 vol %, more than 80 vol %, more than 90 vol %, more than 95 vol %, and more than 99 vol %, more in particular 100 vol % of oxygen. An oxygen-rich gas can, for example, be provided by an air separation unit or by a water electrolyzer.
As defined herein, air is ambient air having atmospheric pressure.
As defined herein, steam are water vapors.
As defined herein, the term “flow” refers to either a volumetric flow or a mass flow.
The present disclosure generally relates to a mono-pressure system and method for the production of nitric acid with important gains compared to conventional systems and methods, wherein the conventional primary air and/or secondary air consisting of pressurized air, provided by an air compressor with a typical capacity of at least 300000 m3/h, is replaced by the combination of (i) oxygen gas or an oxygen-rich gas, in particular a pressurized oxygen gas or oxygen-rich gas, such as produced by a high-pressure water electrolyzer as further discussed herein; and (ii) a recirculated tail gas stream, thus removing the need for an air compressor to generate compressed primary and/or secondary air. Stated differently, in the system and methods for the production of nitric acid according to the present disclosure:
Reference is made to
The production plant or system according to the present disclosure is characterized in that the system further comprises:
As defined herein a tail gas heater is a heat exchange system comprising one or more heat exchanger units. In the case the heat exchanger is made of multiple heat exchanger units, the person skilled in the art will realize that it is possible to split the stream of tail gas downstream the absorption tower 41 inside the tail gas heater 43.
As defined herein, a stream of tail gas, also referred to as a tail gas stream, is any gas stream downstream the absorption tower, such as located between the absorption tower 41 and the point of communication or mixing between the first tail gas stream 52 and the oxygen-rich gas 50.
As defined herein, a means for splitting is any means suitable for splitting a tail gas stream, particularly a tail gas stream downstream the absorption tower 41, such as to generate a first tail gas stream 52, and a second tail gas stream 74. In particular, the means for splitting is a T-connection having one inlet and two outlets, such that a gas flowing through the inlet of the T-connection is split into two gas streams of identical chemical composition.
As defined herein, a means for pressurizing is any suitable means for increasing the pressure of a gas stream. In particular, the means for pressurizing is a gas compressor or a gas ejector.
The person skilled in the art will realize that the means for splitting can be incorporated inside the means for pressurizing, provided that the means for pressurizing includes at least two outlets for the gas stream being pressurized.
As defined herein, a means for adjusting the amount of tail gas stream being split into the first tail gas stream 52 and the second tail gas stream 74 are any means for controlling the splitting in the means for splitting 55. In particular, the means for splitting 55 is a T-connection as described above and the means for adjusting is an orifice or a guide vane or a flow control valve at one or both of the outlets of the T-connection.
Even more in particular, the means is an integrated process control system, in which the temperature in the ammonia converter 37 is determined through the means for measuring the temperature. The temperature in the ammonia converter 37 is then used for controlling a flow control means in the means for splitting 55, thereby controlling the splitting of the tail gas 5, in order for the measured temperature to be maintained in the range of 800-950° C.
As defined herein, a means for adjusting the oxygen concentration are any suitable means for regulating the amount of oxygen to be introduced in the system from a measurement of the oxygen concentration. The oxygen concentration can be determined, for example, from a measurement in the gas phase using a process gas analyzer. The oxygen concentration can also be determined from computing using the concentration of the oxygen source being introduced in the system, in particular the oxygen concentration of the oxygen-rich gas 50, the flow at which the oxygen source, in particular the oxygen-rich gas, is introduced in the system, preferably the flow at which the ammonia gas stream is introduced in the system, and the relative flow values of the gases with which the oxygen source is mixed, in particular the relative flow values at which the oxygen-rich gas 50 and the ammonia gas stream are mixed. Using the oxygen concentration, the relevant flow of oxygen to be introduced in the system is, in turn, determined and is used in controlling the flow of oxygen, from a gaseous source of oxygen at a pre-determined concentration. Controlling of the flow of gaseous oxygen can, for example, be achieved through flow control valves. In this context, as defined herein, a means for regulating the concentration of ammonia and/or of oxygen is any means suitable for achieving a target concentration of ammonia and/or of oxygen. In particular, such means are gas flow control means, in particular a flow control valve or an orifice or a guide vane, for controlling the flow of the oxygen-rich gas 50 and/or of the ammonia gas stream 32. In particular, the means is an integrated process control system, in which the concentration of oxygen is measured, and the relevant flow of oxygen is thereby determined and achieved from controlling the flow of the first oxygen rich gas, from a gaseous source of oxygen at a pre-determined concentration.
Typically, P1 ranges from 2 to 16 bara. The person skilled in the art will determine the optimal concentration of oxygen in the gases entering the ammonia converter 37 and the absorption tower 41, in order for the catalytic conversion of ammonia to nitric oxide to proceed optimally in the ammonia converter 37 and for the absorption of NOx gases in the absorption tower 41 to proceed optimally. Further, upon determining the oxygen content exiting the absorption tower 41, the skilled person will also weigh out the benefits of increasing the oxygen content in the absorption tower 41, in particular a reduced tower size due to improved absorption, against the drawback of a higher gas volume downstream the absorption tower 41 implying equipment, such as heat exchangers, of a larger size, for heating tail gas.
As defined herein, means for measuring the temperature are any means suitable for measuring and indicating the temperature in the ammonia oxidation burner. In particular, the means for measuring the temperature is a thermocouple or a thermocouple or a thermometer suitable for measuring and indicating a temperature as high as 1000° C. More in particular, the means for measuring the temperature is an infrared thermometer for measuring and indicating a temperature as high as 1000° C.
As defined herein, means for converting steam into power are any means for achieving power from steam. In particular, those means are a steam turbine connected to an electric generator.
Considering that a production cycle of a unit or plant comprises (a) a start-up phase, wherein the different processes are initiated, (b) a continuous and essentially constant phase or operation mode, wherein processes operate at a given working load that is usually kept constant during a production cycle; and (c) a shutdown phase, where processes are slowly and safely stopped, the term “during operation” or “during continuous operation” of a unit or plant, in particular of nitric acid plant, refers to the continuous operation mode wherein the unit or plant produces a product, in particular nitric acid.
The inventors have found that, instead of continuously supplying pressurized air 34 provided by an air compressor 36 to the mixing unit 35 at a pressure P1, in particular during continuous operation of the nitric acid plant, it is possible to recirculate the first tail gas stream 52 at a pressure P1. Therefore, compressed or pressurized air only has to be supplied in order to start the process, i.e. during the start-up phase of the nitric acid plant, in particular to pressurize the system, but no longer after the production of the tail gas has started, during the continuous operation phase or mode, and an air compressor 36 is not required. Due to pressure drop in the gas stream in its trajectory from the ammonia converter 37 to the outlet 6 of the absorption tower 41, the tail gas 5 has a pressure lower than P1 and, therefore, the means for pressurizing 53 provides a pressure P1 to the tail gas stream 10. In addition, the oxygen-rich gas 50 having a pressure P1 and the supply for an oxygen-rich gas 67 provide oxygen to the ammonia converter 37 and to the absorption tower 41, respectively, such that, in the absence of the primary and secondary air provided by the air compressor 36, the concentration of oxygen in the ammonia converter 37 and in the absorption tower 41 is similar to and can be controlled to that in a standard mono pressure nitric acid plant.
In the absence of an air compressor 36 and with the tail gas stream 52 being recirculated in the system, not only is the power demand of the system reduced: the NOx emissions leaving the system are also reduced. Therefore, the size of the treatment unit for treating those NOx emissions is reduced with respect to the size in the corresponding standard mono pressure nitric acid plant.
In one embodiment according to the production plant of the disclosure, the system further comprises one or more of:
Advantageously, the means for splitting 55 is located downstream the tail gas heater 43. Indeed, both the first tail gas stream 52 and the second tail gas stream 74 are then at an optimal temperature. In particular, this means that the first stream of tail gas 52 is at a temperature below 300° C., such that the first tail gas stream 52 can be fed to the ammonia converter 37 without the amount of ammonia fed through the stream 32, having to be adjusted in order to maintain the temperature ranging from 800 to 950° C. in the ammonia converter 37, the temperature at which the ammonia converter 37 is operable. In addition, the location of the means for splitting 55 at this location confers to the second tail gas stream 74 an optimal temperature for being expanded in the tail gas expander 7 such as to provide an optimal of energy which can be used to power the means for pressurizing 53. Further, the presence of a steam turbine 51 allows for the recovery of the heat of the steam produced in the ammonia converter 37 and this recovered heat can be used, at least partly, for powering the means for pressurizing 53. Finally, the use of the steam turbine 51 and the location of the means for pressurizing upstream the heat exchanger 73 contribute to operating the production plant in an energy-efficient manner. Indeed, less power is required for the means for pressurizing 53 to pressurize a stream of tail gas before it is heated by the heat exchanger 73.
In particular, the tail gas 5 is heated in the heat exchanger 73 from a temperature ranging from 20 to 250° C., to a temperature ranging from 100 to 450° C. Subsequently, the tail gas exiting the heat exchanger 73 is heated in the tail gas heater 43 to a temperature ranging from 200 to 550° C. The tail gas exiting the heat exchanger 73 then is at an optimal temperature for being treated in the De-NOx treatment unit 71 and, therefore, the De-NOx treatment unit 71 is located between the heat exchanger 73 and the tail gas heater 43. The person skilled in the art will, without any difficulty, select the proper location for the De-NOx treatment unit 71 such that the operating temperature of the De-NOx treatment unit 71 is in agreement with the temperature of the corresponding stream of tail gas.
In one embodiment according to the production plant of the disclosure, the production plant further comprises a bleacher 57 comprising an inlet 58 for an oxygen-rich bleaching gas 60 and an outlet for bleaching gases or off-gases 61. It is understood that the bleacher further comprises an inlet 64 for the stream of raw nitric acid containing residual NOx gas 27 and an outlet 59 for bleached nitric acid 63. The bleaching gases or off-gases 61 are in fluid communication with any gas stream between the ammonia converter 37 and the absorption tower 41, such that the supply for or source of an oxygen-rich gas, in particular the second oxygen-containing gas 60, 61, 67 is at least partly provided by the bleaching gases or off-gases 61.
The person skilled in the art will understand that the pressure of the bleaching gases or off-gases 61 is to be adjusted to about P1 before joining the gaseous NOx stream 22. When the stream of raw nitric acid containing residual NOx gas 27 is bleached, the amounts of NOx gases and nitrous acid HNO2 in the nitric acid solution are reduced. This in turn results in less brown fumes coming out of the nitric acid solution. In addition, the nitric acid solution provided by the bleacher is of a higher quality, that is purer.
Conveniently, when the stream of raw nitric acid containing residual NOx gas 27 is bleached, the bleaching gases or off-gases 61 is the oxygen-rich gas supplied by the corresponding source 67: the supply of the oxygen-rich gas 67, in particular the second oxygen-containing gas, is achieved through the bleacher 57 and no separate source of oxygen is required.
In one embodiment according to the production plant of the disclosure, part of the oxygen-rich gas 50 or part of the first oxygen-containing gas 56 or part of the tail gas 5, such as part of a tail gas stream 76, is in fluid communication with the inlet 58 of the bleacher 57, such that the oxygen-rich bleaching gas or stripping gas 60 is at least partly provided by part of the oxygen-rich gas 50 or part of the first oxygen-containing gas 56 or part of the tail gas 5, such as part of a tail gas stream 76, downstream the means for pressurizing 53.
If a bleacher 57 is present, as no secondary air is fed by an air compressor (36 in the standard nitric acid plant) to the bleacher 57, the bleacher can be conveniently fed by the oxygen-rich gas 50. Also, once tail gas 5 is produced and recirculated, part of the oxygen-containing gas 56 or part of the tail gas 5 can be fed to the bleacher 57: the concentration of NOx gases in the oxygen-containing gas 56 or in the tail gas 5 is sufficiently low that the bleaching in the bleacher 57 remains sufficiently efficient. As the means for pressurizing 53 is located in a tail gas stream, downstream the absorption tower 41, it is ensured that the oxygen-rich bleaching gas 60 has a pressure about P1.
In one embodiment according to the production plant of the disclosure, the oxygen-rich gas 50, the supply for the oxygen-rich gas 67, in particular the second oxygen-containing gas 60, 61, 67, the oxygen-rich bleaching gas 60, and the oxygen-rich off-gases 61 are all provided at least partly by a water electrolyzer 66.
A water electrolyzer is a device for the electrolysis of water, being the decomposition of water into oxygen and hydrogen gas, due to the passage of an electric current therethrough. This technique can be used to make hydrogen gas, a main component of hydrogen fuel, and oxygen gas. A suitable high-pressure water electrolyzer may comprise an anode producing oxygen gas according to the reaction
a cathode, producing hydrogen gas according to the reaction
an electrolyte consisting of an alkaline solution such as potassium hydroxide; and a porous diaphragm separating the anode and the cathode, in order to avoid the mixing of hydrogen gas and oxygen gas that together form an explosive mixture. Alternatively, the anode and the cathode may be separated by a solid polymer electrolyte such as the fluoropolymer Nafion, where the electrolyte provides the selective transport of protons from the anode to the cathode, as well as the electrical insulation between the anode and the cathode, and avoids the mixing of hydrogen gas and oxygen gas that together form an explosive mixture.
The anode and cathode can be made of nickel or steel, or mixtures thereof. Alternatively, for the purpose of enhancing the electrode reactions, the anode and cathode may contain catalysts that can be made of Iridium and Platinum, respectively. The diaphragm of an electrically insulating material is based on, for example, zirconia. The diaphragm has a porosity such that it forms a barrier against transport of hydrogen and oxygen gas bubbles, while containing a continuum of penetrated liquid electrolyte. An anode-diaphragm-cathode assembly constitutes an electrolysis cell. Electrolysis cells are piled in series in stacks that compose the core of an electrolyzer. The hydrogen and oxygen production for a given stack volume is proportional to the current density and inversely proportional to the stacking distance. Regardless of stack volume, the hydrogen and oxygen production is proportional to the total current. In addition to the stack, the electrolyzer comprises auxiliaries such as a current rectifier, a water demineralization unit, a water pump and a cooling system, a hydrogen purification unit, and instrumentation.
The electrolyzer is operated by applying a voltage corresponding to the standard potential plus the overpotential over each cell. The total voltage depends on the total number of cells of which the electrolyzer is comprised. OH— ions generated at the cathode migrate through the electrolyte in the diaphragm to the anode, where they are consumed by the anode reaction. Electrons travel the opposite direction in an external circuit. The electrolyzer may be operated at a temperature of 50 to 80° C., or 60 to 80° C., and a gas pressure of 2 bara, preferably, as a high-pressure water electrolyzer at a pressure of 9 to 30 bara, even more preferably 15 to 30 bara.
A high-pressure water electrolyzer hence results in the production of pressurized hydrogen at the cathode and pressurized oxygen at the anode, such as having a pressure of 9 to 30 bara, even more preferably 15 to 30 bara. What is required to perform high-pressure electrolysis is to pressurize the water used in the electrolysis process. As pressurizing water requires less power than pressuring a gas, the use of a high-pressure water electrolyzer results in the production of pressurized oxygen-rich gas at minimized power consumption.
Conveniently, the water electrolyzer 66 provides oxygen to all the various points where oxygen needs to be fed. In particular, the supply of oxygen from the electrolyzer 63 is sufficient to provide all of the oxygen of the first oxygen-rich gas 50, the second oxygen-containing gas 60, 61, 67, the oxygen-rich bleaching gas 60 and the off-gases 61. In this manner, the system is simplified and can comprise a single source of oxygen from which oxygen-containing gas streams can be produced. In particular, oxygen-containing streams can be produced at the desired pressure, using standard pressure adjustment means. The use of a high-pressure electrolyzer operable at 9 to 30 bara, even more preferably 15 to 30 bara, is particularly useful as a source of oxygen gas to be supplied to a bleacher operating at a pressure higher than P1. In addition, as less energy is required to pressurize water than to pressurize oxygen gas, pressurized oxygen gas is obtained at minimum power consumption.
Another advantage of the presence of a high-pressure water electrolyzer lies in the potential to, in parallel to producing oxygen gas that can be used in nitric acid production, also produce hydrogen gas. Such hydrogen gas is produced in a green manner, that is without the conventional use of natural gas which results in the production of the green-house gas carbon dioxide, CO2. The hydrogen gas can then be used in the production of ammonia in a Haber-Bosch or also conventionally named synthesis gas unit. The high-pressure water electrolyzer then enables the integration of the ammonia and nitric acid production processes.
In one embodiment according to the production plant of the disclosure, the production plant further comprises a tail gas expander 7 for expanding the second tail gas stream 74 to atmospheric pressure, to produce an expanded tail gas 70, wherein the means for pressurizing 53 can be powered at least partly by the tail gas expander 7 or by the steam turbine 51 or by a power source.
In the presence of such tail gas expander 7, the second tail gas stream 74 can be expanded, thereby providing power that can be used for powering the means for pressurizing 53.
In one embodiment according to the production plant of the disclosure, the fluid communication between the source of pressurized air 34, for pressurizing the system during the startup phase, and the system is in direct fluid communication with the oxygen-rich gas 50. It is preferred to introduce pressurized air 34 through a direct fluid communication with the oxygen-rich gas 50. In this manner, upon operating the means for pressurizing during the start-up of the system, it is ensured that air flows through the ammonia converter 37 when ammonia 32 is being fed to ammonia converter 37, such that there is a sufficient concentration of oxygen to convert ammonia into nitric oxide. Subsequently, the nitric acid process being induced, tail gas 5 and the first tail gas stream 52 can be produced and recirculated to the mixing unit 35, upon feeding the oxygen-rich gas 50.
Reference is made to
The method is characterized in that it further comprises the steps of
Typically, P1 ranges from 2 to 16 bara. The person skilled in the art will determine the optimal percentages of oxygen in the gases entering the ammonia converter 37 and the absorption tower 41, in order for the catalytic conversion of ammonia to nitric oxide to proceed optimally in the ammonia converter 37 and for the absorption of NOx gases in the absorption tower 41 to proceed optimally. Further, upon determining the oxygen content exiting the absorption tower 41, he will establish weigh out the benefits of increasing the oxygen content in the absorption tower 41, against the drawback of a higher gas volume downstream the absorption tower 41 implying equipment, such as heat exchangers, of a larger size, for heating tail gas.
The inventors have found that, instead of continuously supplying pressurized air 34 provided by an air compressor 36 to the mixing unit 35 at a pressure P1, in particular during continuous operation of the nitric acid plant, it is possible to recirculate the first tail gas stream 52 at a pressure P1. Therefore, compressed or pressurized air only has to be supplied in order to start the process, i.e. during the start-up phase of the nitric acid plant, for pressurizing the system, but no longer after the production of the tail gas has started, during the continuous operation phase or mode, and an air compressor 36 is not required. Due to pressure drop in the gas stream in its trajectory from the ammonia converter 37 to the outlet 6 of the absorption tower 41, the tail gas 5 has a pressure lower than P1 and, therefore, the means for pressurizing 53 provides a pressure P1 to a tail gas stream 10, and thus to the first tail gas stream 52. In addition, the oxygen-rich gas 50 having a pressure P1 and the supply for an oxygen-rich gas 67 respectively provide oxygen to the ammonia converter 37 and to the absorption tower 41, such that, in the absence of the primary and secondary air provided by the air compressor 36, the concentration of oxygen in the ammonia converter 37 and in the absorption tower 41 is similar to and can be controlled to that in a standard mono pressure nitric acid plant.
In the absence of an air compressor 36 and with the tail gas stream 52 being recirculated in the system, not only is the power demand of the system reduced: the NOx emissions leaving the system are also reduced. Therefore, the size of the treatment unit for treating those NOx emissions is reduced with respect to the size in the corresponding standard mono pressure nitric acid plant.
In one embodiment according to the method of the disclosure, in step h), at least part of the tail gas obtained from step f) is pressurized in the means for pressurizing 53, in particular thereby generating a pressurized tail gas; more in particular wherein step h) is performed before step i), and wherein the method further comprises the steps of
Advantageously, the means for splitting 55 is located downstream the tail gas heater 43. Indeed, both the first tail gas stream 52 and the second tail gas stream 52 are then at an optimal temperature. This means that the first tail gas stream 52 is at a temperature below 300° C., such that the first tail gas stream 52 can be fed to the ammonia converter 37 without the amount of ammonia fed through the stream 32, having to be adjusted in order to maintain the temperature ranging from 800 to 950° C. and at which the ammonia 37 is operable, in the ammonia converter 37. In addition, the location of the means for splitting 55 at this location confers to the second tail gas stream 74 an optimal temperature for being expanded in the tail gas expander 7 such as to provide an optimal of energy which can be used to power the means for pressurizing 53. Further, the presence of a steam turbine 51 allows for the recovery of the heat of the steam produced in the ammonia converter 37 and this recovered heat can be used, at least partly, for powering the means for pressurizing 53. Finally, the use of the steam turbine 51 and the location of the means for pressurizing upstream the heat exchanger 73 contribute to operating the production plant in an energy-efficient manner. Indeed, less power is required for the means for pressurizing 53 to pressurize a tail gas stream before it is heated by the heat exchanger 73.
In particular, the tail gas 5 is heated in the heat exchanger 73 from a temperature ranging from 20 to 250° C., to a temperature ranging from 100 to 450° C. Subsequently, the tail gas exiting the heat exchanger 73 is heated in the tail gas heater 43 to a temperature ranging from 200 to 550° C. The tail gas exiting the heat exchanger 73 then is at an optimal temperature for being treated in the De-NOx treatment unit 71 and, therefore, the De-NOx treatment unit 71 is located between the heat exchanger 73 and the tail gas heater 43. The person skilled in the art will, without any difficulty, select the proper location for the De-NOx treatment unit 71 such that the operating temperature of the De-NOx treatment unit 71 is in agreement with the temperature of the corresponding tail gas stream.
In one embodiment according to the method of the disclosure, the method further comprises the step of t) bleaching the stream of raw nitric acid 27 containing residual NOx gas in the bleacher 57, thereby producing the bleached nitric acid 63.
The person skilled in the art will understand that the pressure of the bleaching gases or off-gases 61 is to be adjusted to about P1 before joining the gaseous NOx stream 22. When the stream of raw nitric acid containing residual NOx gas 27 is bleached, the amounts of NOx gases and nitrous acid HNO2 in the nitric acid solution are reduced. This in turn results in less brown fumes coming out of the nitric acid solution. In addition, the nitric acid solution provided by the bleacher is of a higher quality, that is purer.
Conveniently, when the stream of raw nitric acid containing residual NOx gas 27 is bleached, the bleaching gases or off-gases 61 is the oxygen-rich gas supplied by the corresponding source 67: the supply of the oxygen-rich gas 67 is achieved through the bleacher 57 and no separate source of oxygen is required. In one embodiment according to the method of the disclosure, the method further comprises the step of u) supplying part of the oxygen-rich gas 50 or part of the first oxygen-containing gas 56 obtained in step i) or part of a tail gas stream obtained in step g), to the inlet 58 of the bleacher 57 in step t).
If such bleaching step t) is performed, as no secondary air is fed by an air compressor (36 in the standard nitric acid plant) to the bleacher 57, the bleacher can be conveniently fed by the oxygen-rich gas 50. Also, once the tail gas 5 is produced and recirculated, the oxygen-containing gas 56 or a tail gas stream can also be fed to the bleacher 57: the concentration of NOx gases in the oxygen-containing gas 56 or the tail gas 5 is sufficiently low that the bleaching in the bleacher 57 remains sufficiently efficient.
In one embodiment according to the method of the disclosure, the method further comprises the steps of
Conveniently, the water electrolyzer 66 provides oxygen to all the various points where oxygen needs to be fed. In particular, the supply of oxygen from the electrolyzer 63 is sufficient to provide all of the oxygen of the first oxygen-rich gas 50, the second oxygen-containing gas 60, 61, 67, the oxygen-rich bleaching gas 60 and the off-gases 61. In this manner, the system is simplified and can comprise a single source of oxygen from which oxygen-containing gas streams can be produced. In particular, oxygen-containing streams can be produced at the desired pressure, using standard pressure adjustment means. The use of a high-pressure electrolyzer operable at 9 to 30 bara, preferably 15 to 30 bara, is particularly useful as a source of oxygen gas to be supplied to a bleacher operating at a pressure higher than P1.
Another advantage of the presence of a high-pressure water electrolyzer lies in the potential to, in parallel to producing oxygen gas that can be used in nitric acid production, also produce hydrogen gas. Such hydrogen gas is produced in a green manner, that is without the conventional use of natural gas which results in the production of the green-house gas carbon dioxide, CO2. The hydrogen gas can then be used in the production of ammonia in a Haber-Bosch or also conventionally named synthesis gas unit. The high-pressure water electrolyzer then enables the integration of the ammonia and nitric acid production processes.
In one embodiment according to the method of the disclosure, the method further comprises the step of x) expanding the second tail gas stream 74 in the tail gas expander 7 to atmospheric pressure, thereby producing the expanded tail gas 70.
In the presence of such tail gas expander 7, the second tail gas stream 74 can be expanded, thereby providing power that can be used for powering the means for pressurizing 53.
In one embodiment according to the method of the disclosure, in step a), the pressurized air 34 is supplied in the stream in direct fluid communication with the oxygen-rich gas 50.
It is preferred to introduce pressurized air 34 through a direct fluid communication with the oxygen-rich gas 50. In this manner, upon operating the means for pressurizing during the start-up of the system, it is ensured that air flows through the converter 37 when ammonia 32 is being fed to converter 37, such that there is a sufficient concentration of oxygen to convert ammonia into nitric oxide. Subsequently, the nitric acid process being induced, tail gas 5 and the first tail gas stream 52 can be produced and recirculated to the mixing unit 35, upon feeding the oxygen-rich gas 50.
In one aspect of the disclosure, the use of the production plant of the disclosure for performing the method of the disclosure, is disclosed.
In one aspect of the disclosure, a method for revamping a system, i.e. an existing system, for producing nitric acid into a production plant according to the production plant of the disclosure is disclosed, wherein the system, i.e. the existing system for producing nitric acid, comprises:
The revamping method comprises the steps of
The term “oxygen-rich gas” is used as defined elsewhere herein.
As defined herein a tail gas heater is a heat exchange system comprising one or more heat exchanger units. In the case the heat exchanger is made of multiple heat exchanger units, the person skilled in the art will realize that it is possible to split the stream of tail gas downstream the absorption tower 41 inside the tail gas heater 43.
As defined herein, a stream of tail gas, also referred to as a tail gas stream, is any gas stream downstream the absorption tower, such as located between the absorption tower 41 and the point of communication or mixing between the first tail gas stream 52 and the oxygen-rich gas 50.
As defined herein, a means for splitting is any means suitable for splitting a tail gas stream, particularly a tail gas stream downstream the absorption tower 41, such as to generate a first tail gas stream 52, and a second tail gas stream 74. In particular, the means for splitting is a T-connection having one inlet and two outlets, such that a gas flowing through the inlet of the T-connection is split into two gas streams of identical chemical composition.
As defined herein, a means for pressurizing is any suitable means for increasing the pressure of a gas stream. In particular, the means for pressurizing is a gas compressor or a gas ejector.
The person skilled in the art will realize that the means for splitting can be incorporated inside the means for pressurizing, provided that the means for pressurizing includes at least two outlets for the gas stream being pressurized.
As defined herein, a means for adjusting the amount of tail gas stream being split into the first tail gas stream 52 and the second tail gas stream 74 are any means for controlling the splitting in the means for splitting 55. In particular, the means for splitting 55 is a T-connection as described above and the means for adjusting is an orifice or a guide vane or a flow control valve at one or both of the outlets of the T-connection.
Even more in particular, the means is an integrated process control system, in which the temperature in the ammonia converter 37 is determined through the means for measuring the temperature. The temperature in the ammonia converter 37 is then used for controlling a flow control means in the means for splitting 55, thereby controlling the splitting of the tail gas 5, in order for the measured temperature to be maintained in the range of 800-950° C.
As defined herein, means for adjusting the oxygen concentration are any suitable means for assessing the amount of oxygen to be introduced in the system from a measurement of the oxygen concentration. The oxygen concentration can be determined, for example, from a measurement in the gas phase using a process gas analyzer. The oxygen concentration can also be determined from computing using the concentration of the oxygen source being introduced in the system, in particular the oxygen concentration of the oxygen-rich gas 50, the flow at which the oxygen source, in particular the oxygen-rich gas, is introduced in the system, preferably the flow at which the ammonia gas stream is introduced in the system, and the relative flow values of the gases with which the oxygen source is mixed, in particular the relative flow values at which the oxygen-rich gas 50 and the ammonia gas stream are mixed. Using the oxygen concentration, the relevant flow of oxygen to be introduced in the system is, in turn, determined and is used in controlling the flow of oxygen, from a gaseous source of oxygen at a pre-determined concentration. Controlling of the flow of gaseous oxygen can, for example, be achieved through flow control valves. In this context, as defined herein, a means for regulating the concentration of ammonia and/or of oxygen is any means suitable for achieving a target concentration of ammonia and/or of oxygen. In particular, such means are gas flow control means, in particular a flow control valve or an orifice or a guide vane, for controlling the flow of the oxygen-rich gas 50 and/or of the ammonia gas stream 32. In particular, the means is an integrated process control system, in which the concentration of oxygen is measured, and the relevant flow of oxygen is thereby determined, thus controlling the flow of oxygen, from a gaseous source of oxygen at a pre-determined concentration.
Typically, P1 ranges from 2 to 16 bara. The person skilled in the art will determine the optimal concentration of oxygen in the gases entering the ammonia converter 37 and the absorption tower 41, in order for the catalytic conversion of ammonia to nitric oxide to proceed optimally in the ammonia converter 37 and for the absorption of NOx gases in the absorption tower 41 to proceed optimally. Further, upon determining the oxygen content exiting the absorption tower 41, the skilled person will also weigh out the benefits of increasing the oxygen content in the absorption tower 41, in particular a reduced tower size due to improved absorption, against the drawback of a higher gas volume downstream the absorption tower 41 implying equipment, such as heat exchangers, of a larger size, for heating tail gas.
As defined herein, means for measuring the temperature are any means suitable for measuring and indicating the temperature in the ammonia oxidation burner. In particular, the means for measuring the temperature is a thermocouple or a thermocouple or a thermometer suitable for measuring and indicating a temperature as high as 1000° C. More in particular, the means for measuring the temperature is an infrared thermometer for measuring and indicating a temperature as high as 1000° C.
Reference is made to
Reference is made to
| Number | Date | Country | Kind |
|---|---|---|---|
| 21193034.2 | Aug 2021 | EP | regional |
| 22150921.9 | Jan 2022 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/073643 | 8/25/2022 | WO |
