Patent Application
20240336481
The present disclosure relates to the field of nitric acid production in a dual 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:
4NH3(g)+5O2(g)→4NO(g)+6H2O(g) (1)
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:
2NO(g)+O2(g)→2NO2(g) (2)
2NO2(g)→N2O4(g) (3)
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, following compression through a NOx gas compressor, nitrogen dioxide and dinitrogen tetroxide are converted to nitric acid and nitric oxide:
3NO2(g)+H2O(I)+2HNO3(aq)+NO(g) (4)
3N2O4(g)+2H2O(I)+4HNO3(aq)+2NO(g) (5)
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:
NH3+2O2→HNO3+H2O (6)
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 dual pressure process, the absorber unit operates at a higher working pressure than the ammonia converter. Modern dual pressure processes feature a low-pressure ammonia converter operating typically at 2 to 6 bara, and a high-pressure absorber unit operating at 9 to 16 bara.
A dual pressure process requires an air compressor to feed low-pressure air (which comprises about 21 vol % of oxygen) to the converter, and a NOx gas compressor to feed high-pressure NOx gases to the absorber unit. The working pressure of an air compressor is from 2 to 6 bara, inclusive, and the working pressure of a NOx gas compressor is from 9 to 16 bara, inclusive.
The drive power for the air compressor typically originates from a tail gas turbine and a steam turbine or a power source such as an electric motor. Accordingly, the compressor train of a dual pressure nitric acid production plant typically comprises an air compressor, a NOx gas compressor, a tail gas turbine, and a steam turbine or a power source such as an 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. The increase of the primary air also causes a higher amount of gas to be processed subsequently into the NOx gas compressor. This entails the further revamping of the NOx gas compressor or the installation of a new one, and the modification or replacement of the tail gas and/or the steam-turbines and/or the electrical motor. Otherwise, the NOx gas compressor would easily achieve its process limit, thus becoming the bottleneck of the plant.
However, the 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, the NOx gas 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 dual 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 S A, 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 NOx gas compressor and, preferably, also 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 acid at reduced power consumption and reduced emissions, comprising, optionally, a source of pressurized air in fluid communication with the production plant, is disclosed. The system comprises:
The inventors have found that, instead of continuously supplying compressed air provided by an air compressor as primary air to the mixing unit, 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 at startup, but no longer after the production of the tail gas has started. An air compressor is thus not required for operating the nitric acid plant. In this context, an air compressor suitable for pressurizing the plant has a capacity of about 2,000 to 19,000 m3/h, which is much smaller than an air compressor for operating a prior art nitric acid plant, with a capacity of at least 300,000 m3/h. In a dual pressure nitric acid plant, the tail gas has a pressure higher than P1 and, therefore, a first pressure release means, such as the tail gas expander, can be used to expand a tail gas stream to a pressure P1. In addition, a first oxygen-rich gas having a pressure equal to or higher than P1 and up to P2 and a second oxygen-rich gas having a pressure higher than P2, 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 first tail gas stream being recirculated in the system, not only is the power demand of the system reduced, but 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 dual 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 system further comprises a bleacher for bleaching the stream of raw nitric acid-containing residual NOx gas, to provide a stream of bleached nitric acid, having an inlet for an oxygen-rich bleaching gas, and an outlet for off-gases in fluid communication with any gas stream downstream the ammonia converter and upstream the NOx gas compressor if the bleacher operates at a pressure equal to or higher than P1 and up to equal to P2, or in fluid communication with any stream downstream the NOx gas compressor and upstream the absorption tower if the bleacher operates at a pressure higher than P2, such that the supply for the second oxygen containing gas comes at least partly from the off-gases.
In one embodiment according to the production plant of the disclosure, part of the oxygen-rich gas or part of the first 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 is at least partly provided by part of the oxygen-rich gas by part of the first oxygen-containing gas or by part of a tail gas stream.
In one embodiment according to the production plant of the disclosure, the system further comprises a stream of a second oxygen-containing gas in direct fluid communication with any tail gas stream, particularly a stream of pressurized oxygen-rich gas in direct fluid communication with any stream of tail gas downstream the absorption tower or upstream the first pressure release means.
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 bleacher off-gases are all at least partly provided by a water electrolyzer, particularly a high pressure water electrolyzer.
In one embodiment according to the production plant of the disclosure, 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:
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, 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 one aspect of the disclosure, a method for revamping an (existing) production plant for producing nitric acid, wherein the (existing) production plant comprises:
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, an oxygen-rich gas is a gas comprising more than 21 vol % of oxygen, more in particular more than 30 vol %, more than 35 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 %, more than 98 vol % and more than 99 vol %, more in particular 100 vol % of oxygen. Such oxygen-rich gas can, for example, be provided by an air separation unit or a water electrolyzer.
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 35 vol %, 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 %, more than 98 vol %, and more than 99 vol %, more in particular 100 vol % of oxygen.
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 dual pressure methods and systems for producing nitric acid, which typically operates at two pressures, P1 and P2. Typically, P1 ranges from 2 to below 6 bara and P2 ranges from 6 to 16 bara.
The present disclosure generally relates to a dual 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 300,000 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 is characterized in that the system further comprises a means for splitting 55 a stream of tail gas (downstream the absorption tower 41) into a first tail gas stream 10 and a second tail gas stream 80, wherein the first tail gas stream 10 has a pressure equal to or higher than P1 and up to P2 and is in fluid communication with the oxygen-rich gas 50, and optionally the compressed air, wherein the mixing of the oxygen-rich gas 50 and the first tail gas stream 10 provides the first oxygen-containing gas 56. In particular, the production plant may further comprise a means for adjusting the amount of tail gas being split into the first tail gas stream 10 and the second tail gas stream 80.
Typically, the heat exchange system 43 comprises at least two heat exchangers 66, 67. The person skilled in the art will realize that it is possible to split a tail gas stream inside the heat exchange system, for example between the heat exchanger 66 and 67. In particular, the production plant may comprise further heat exchange systems, such that the gaseous NOx stream 22 or the NOx compressed gas stream 24 exchange heat with the tail gas 5.
As defined herein, a stream of tail gas, or a tail gas stream, is any gas stream downstream the absorption tower 41, between the absorption tower 41 and the point of communication between or the point of mixing of between the first tail gas stream 10 and the oxygen-rich gas 50.
As defined herein, a means for splitting is any means suitable for splitting a tail gas stream, such as to generate a first tail gas stream 10 and a second tail gas stream 80. 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 pressure release means is any suitable means for reducing the pressure of a gas stream, such as a tail gas stream. In particular, the pressure release means may be a gas expander or a gas ejector. A gas ejector provides the benefits of a simplified equipment, wherein mixing of different gas streams is combined with the reduction of the tail gas pressure. For instance, a tail gas stream, processed via the gas ejector, may act as the motive gas, and a second gas fed to the ejector can, for example, be ambient air at a pressure (e.g. atmospheric pressure) lower than the tail gas stream as motive gas. In particular, a tail gas stream may be fed as the motive gas to a gas ejector, and a second gas fed to the ejector is oxygen gas at a pressure lower than the tail gas stream as motive gas. In this context, both the feeding of air or oxygen through the gas ejector contribute to increasing the concentration in the first tail gas stream 10 and/or a further tail gas stream 83, 84 being recycled, thereby reducing the demand on the oxygen-rich gas 50. In particular, a tail gas stream is fed as the motive gas to the ejector and the second gas fed to the ejector is the NOx gas/steam mixture 15 or the gaseous NOx stream 22.
The person skilled in the art will realize that the means for splitting can be incorporated within the (first) pressure release means, provided that the (first) pressure release means includes at least two outlets for the gas stream being depressurized, in particular one outlet for the first tail gas stream 10 and another outlet for the second tail gas stream 80.
As defined herein, a means for adjusting the amount of tail gas being split into the first tail gas stream 10 and the second tail gas stream 80, 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 may be 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 may be an integrated process control system, in which the temperature in the ammonia converter 37 is determined through a 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 the tail gas stream, in order for the measured temperature to be maintained in the range 800-950° C.
As defined herein, a means for adjusting the oxygen concentration is any suitable means for regulating the amount of oxygen to be introduced in the system from a measurement of the oxygen concentration, such as by using a means for measuring the concentration of oxygen. 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, the flow at which the oxygen source, in particular the oxygen rich gas, is introduced in the system, particularly 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 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 oxygen is any means suitable for achieving a target concentration of ammonia and/or oxygen. In particular, such means are gas flow control means, in particular a flow control valve or an orifice or a guiding vane, for controlling the flow of the oxygen rich gas and/or the ammonia gas stream. In particular, the means is an integrated process control system, in which the concentration of oxygen is measured, and the target flow or 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.
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, he will also 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, such 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 thermometer suitable for measuring and indicating a temperature ranging as high as 1,000° C. More in particular, the means for measuring the temperature is an infrared thermometer for measuring and indicating a temperature ranging as high as 1,000° 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 start-up phase, wherein the different processes are initiated; 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 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 a 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 compressed air 34 provided by an air compressor 36 to the mixing unit 35, in particular during continuous operation of the nitric acid plant, it is possible to recirculate the first tail gas stream 10 at a pressure P1. Therefore, 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 5 has started and an air compressor 36 is not required. The tail gas 5 has a pressure higher than P1 and, therefore, a first pressure release means, such as the tail gas expander 7, can be used to provide reduce the pressure of the tail gas stream and provide a tail gas stream with pressure P1. In addition, the oxygen-rich gas 50 and the supply of a second oxygen-containing gas 68, which typically have a pressure equal to or higher than P1 and up to P2 to 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 dual pressure nitric acid plant.
In the absence of an air compressor 36 and with the tail gas stream 10 being recirculated in the system, not only is the power demand of the system reduced, but 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 dual 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 are located downstream the heat exchange system 43. Indeed, both the first tail gas stream 10 and the second tail gas stream 80 are then at an optimal temperature. In particular, this means that the first stream of tail gas 10 is at a temperature below 300° C., such that the first tail gas stream 10 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 80 an optimal temperature for being expanded in the pressure release means 60, in particular a tail gas expander, such as to provide an optimal of energy which can be used to power the NOx gas compressor 40. 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 tail NOx gas compressor 40. Finally, the use of the steam turbine 51 contributes to operating the production plant in an energy-efficient manner.
In particular, the tail gas 5 is heated in a heat exchanger 67 of the heat exchange system 43 and then in the heat exchanger 79 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 79 is heated in the heat exchange system 43 to a temperature ranging from 200 to 550° C. The tail gas exiting the heat exchanger 79 then is at an optimal temperature for being treated in the De-NOx treatment unit 70 and, therefore, the De-NOx treatment unit 70 is located between the heat exchanger 79 and the heat exchange system 43. The person skilled in the art will, without any difficulty, select the proper location for the De-NOx treatment unit 70 such that the operating temperature of the De-NOx treatment unit 70 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 system further comprises a bleacher 62 for bleaching the stream of raw nitric acid-containing residual NOx gas 27, to provide a stream of bleached nitric acid 75, wherein the bleacher has an inlet 81 for an oxygen-rich bleaching gas 72, in particular wherein the inlet 81 is in fluid communication with a high-pressure water electrolyzer 63, and an outlet 73 for the bleacher's off-gases 77. It is understood that the bleacher 62 further comprises an inlet for the stream of raw nitric acid containing residual NOx gas and an outlet for bleached nitric acid. The bleaching gases or off-gases 77 are in fluid communication with any gas stream downstream the ammonia converter 37 and upstream the NOx gas compressor 40 if the bleacher 62 operates at a pressure higher than P1 and up to equal to P2 (
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 77 correspond to the second oxygen containing gas (or the oxygen-rich gas supplied by the supply 68) having a pressure higher than P1 and up to P2 (
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 83, 84, is in fluid communication with the inlet 81 of the bleacher 62, such that the oxygen rich-bleaching gas or stripping gas 72 is at least partly provided by part of the oxygen-rich gas 50 or part of the oxygen-containing gas 56 or part of the tail gas 5, such as part of a tail gas stream 83, 84.
If a bleacher 62 is present, as no secondary air is fed by an air compressor (36 in the standard nitric acid plant) to the bleacher 62, the bleacher 62 can be conveniently fed by the oxygen-rich gas 50. Also, once tail gas 5 is produced and recirculated, the first oxygen-containing gas 56 or part of a tail gas stream can be fed to the bleacher 62: 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 62 remains sufficiently efficient.
In particular, the system further comprises means for pressurizing 78 the oxygen-rich gas 50 or the first oxygen-containing gas 56 or the tail gas 5, when used as a stripping gas in the bleacher, to a pressure equal to or higher than P2, such that the bleacher 62 is a high-pressure bleacher, that is a bleacher operating at a pressure ranging from above 6 bara and up to 16 bara (
In one embodiment according to the production plant of the disclosure, the system further comprises a stream of a second oxygen-rich gas 74 in direct fluid communication with any tail gas stream downstream the absorption tower 41. More in particular, the system further comprises a stream of a pressurized oxygen-rich gas in direct fluid communication with any tail gas stream upstream the first pressure release means 7.
The feeding of a stream of a second oxygen-rich gas 74 allows to reduce the amount of the first oxygen-rich gas 50 having to be provided to the mixing unit 35. In particular, the stream of the second oxygen-rich gas 74 can be fed downstream the heat exchange 43 and upstream the first pressure release means 7, which allows more power to be exported from the first pressure release means, e.g. from the tail gas expander 7.
In one embodiment according to the production plant of the disclosure, the first oxygen-rich gas 50, the second oxygen containing gas 68, 72, 77, the stream of the second oxygen-rich gas 74, which is particularly in fluid communication with any stream of tail gas downstream the absorption tower 41, and the oxygen-rich bleaching gas 72 and the off-gases 77 are all provided at least partly by a water electrolyzer 63, in particular a high-pressure water electrolyzer 63.
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
2OH—=H2O+½O2+2e−;
2H2O+2e−=H2+2OH−;
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 9 to 30 bara—as a high pressure water electrolyzer, 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 a pressure of 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 pressurised oxygen-rich gas at minimized power consumption.
Conveniently, the water electrolyzer 63 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, the second oxygen-rich gas 74, the second oxygen-containing gas 68, the oxygen-rich bleaching gas 72, and the off-gases 77. In this manner, the system is simplified and comprises a single source of oxygen from which different oxygen-containing gas streams can be produced. In particular, oxygen-containing gas 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 is particularly useful as a source of oxygen gas to be supplied to a bleacher operating at a pressure higher than P2.
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 the nitric acid production, also produce hydrogen gas. Such hydrogen gas is produced in a green manner, i.e. without the conventional use of natural gas, which results in the production of the greenhouse gas carbon dioxide. The hydrogen gas can then be used in the production of ammonia in a Haber-Bosch process, also conventionally named synthesis gas unit. The high pressure water electrolyzer thus enables the integration of the ammonia and nitric acid production processes.
In one embodiment according to the production plant of the disclosure, the fluid communication between the source of pressurized air 65, for pressurizing the system during the startup phase, and the system is in direct fluid communication with the first oxygen-rich gas 50, particularly having a pressure P1.
It is preferred to introduce pressurized air 65 at the start-up in the tail gas stream in direct fluid communication with the oxygen-rich gas 50. In this manner, upon operating the NOx compressor 40 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, the tail gas 5 is produced and the first tail gas stream 10 can be 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 6 bara and P2 ranges from 9 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, he will also weigh out the benefits of increasing the oxygen content in the absorption tower 41, such as a reduced tower size due to improved absorption, against the drawback of a higher gas volume downstream the absorption tower 41, which requires equipment, such as heat exchangers, of a larger size, for heating tail gas.
In particular, the gaseous NOx stream 22 or the NOx compressed gas stream 24 exchange heat with the tail gas 5. In particular, the gaseous NOx stream 22 or the NOx compressed gas stream 24 exchange heat with the tail gas 5.
The inventors have found that, instead of continuously supplying compressed air 34 provided by an air compressor 36 to the mixing unit 35, it is possible to recirculate the first tail gas stream 10 at a pressure P1, in particular during continuous operation of the nitric acid plant. 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 5 has started, during the continuous operation phase or mode of the nitric acid plant, and an air compressor 36 is not required. The tail gas 5 has a pressure higher than P1 and, therefore, a first pressure release means 7, such as a tail gas expander, can be used to provide a stream of tail gas of pressure P1. In addition, the first oxygen-rich gas 50 and the second oxygen-containing gas 68, having either a pressure equal to or higher than P1 and up to P2, 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 that in a standard dual pressure nitric acid plant.
In the absence of an air compressor 36 and with the first tail gas stream 10 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 dual pressure nitric acid plant.
In one embodiment according to the method of the disclosure, the first tail gas stream 10 mixed in step k) is obtained after step j), in particular the first expanded tail gas 64 is split in step k), and the method further comprises the steps of
Advantageously, the means for splitting 55 are located downstream the heat exchange system 43. Indeed, both the first tail gas stream 10 and the second tail gas stream 80 are then at an optimal temperature. This means that the first stream of tail gas 10 is at a temperature below 300° C., such that the first tail gas stream 10 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 in the ammonia converter, ranging from 800 to 950° C., 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 80 an optimal temperature for being expanded in the pressure release means 60 such as to provide an optimal of energy which can be used to power the NOx gas compressor 40. 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 tail NOx gas compressor 40. Finally, the use of the steam turbine 51 contributes to operating the production plant in an energy-efficient manner.
In particular, the tail gas 5 is heated in the heat exchanger 79, in particular in a heat exchanger 67 of the heat exchange system 43 and then in the heat exchanger 79, 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 79 is heated in the heat exchange system 43, particularly in a heat exchanger 66 of the heat exchange system 43, to a temperature ranging from 200 to 550° C. The tail gas exiting the heat exchanger 79 then is at an optimal temperature for being treated in the De-NOx treatment unit 70 and, therefore, the De-NOx treatment unit 70 is located between the heat exchanger 79 and the heat exchange system 43. The person skilled in the art will, without any difficulty, select the proper location for the De-NOx treatment unit 70 such that the operating temperature of the De-NOx treatment unit 70 is in agreement with the temperature of the corresponding stream of tail gas.
In one embodiment according to the method of the disclosure, the method further comprises the step of w) bleaching the stream of raw nitric acid-containing residual NOx gas 27 in the bleacher 62, thereby producing the stream of bleached nitric acid 75.
When the stream of raw nitric acid containing residual NOx gas 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 77 correspond to the second oxygen-containing gas supplied by the corresponding supply or source 68 for an oxygen-rich gas having a pressure higher than P1 and up to P2 (
In one embodiment according to the method of the disclosure, the method further comprises the step of w1) supplying part of the first oxygen-rich 50 or part of the first oxygen-containing gas 56 obtained in step k) or part of the tail gas stream 5, 83, 84, such as obtained in step g), to the bleacher 62 in step w).
If a bleacher 62 is present, i.e. if such a bleaching step is performed, as no secondary air is fed by an air compressor (36 in the standard nitric acid plant) to the bleacher 62, the bleacher 62 can be conveniently fed by the first oxygen-rich gas 50. Also, once tail gas 5 is produced and recirculated, the first oxygen-containing gas 56 or part of the tail gas stream can be fed to the bleacher 62: the concentration of NOx gases in the first oxygen-containing gas 56 or the tail gas 5 is sufficiently low that the bleaching in the bleacher 62 remains sufficiently efficient.
In particular, the system further comprises the step of w2) pressurizing the first oxygen-rich gas 50, part of a tail gas stream, and/or the first oxygen-containing gas 56 to be supplied to the bleacher in the means for pressurizing 78, to a pressure higher than P2, such that the bleacher 62 is a high-pressure bleacher, that is a bleacher operating at a pressure ranging from above 6 or 9 bara and up to 16 bara (
In one embodiment according to the method of the disclosure, the method further comprises the step of x) supplying a stream of a second oxygen-rich or oxygen-containing gas 68, 72, 77, particularly as a stream of a pressurized oxygen-rich gas, to a tail gas stream, particularly to a tail gas stream upstream the first pressure release means 7. The feeding of a stream of a second oxygen-rich gas 74 allows to reduce the amount of the first oxygen-rich gas 50 having to be provided to the mixing unit 35. In particular, the stream of a second oxygen-rich gas 74 can be fed downstream the heat exchanger 43 and upstream the first pressure release means 7, such as a tail gas expander, which allows more power to be exported from the first pressure release means 7, such as a tail gas expander.
In one embodiment according to the method of the disclosure, the method further comprises the steps of
Conveniently, the water electrolyzer 63 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-rich gas 74, the second oxygen containing gas 68, the oxygen-rich bleaching gas 72 and the off-gases 77. In this manner, the system is simplified and can comprise a single source of oxygen from which different oxygen-containing gas streams can be produced. In particular, different oxygen containing streams can be produced at the desired pressures, using standard pressure adjustment means. The use of a high-pressure electrolyzer operable at a pressure of 9 to 30 bara, preferably at a pressure of 15 to 30 bara, is particularly useful as a source of oxygen gas that can be supplied to a bleacher operating at a pressure higher than P2.
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, i.e. without the conventional use of natural gas, which results in the production of the greenhouse gas carbon dioxide. The hydrogen gas can then be used in the production of ammonia in a Haber-Bosch process, also conventionally named synthesis gas unit. The high pressure water electrolyzer thus enables the integration of the ammonia and nitric acid production processes. Furthermore, as pressurizing water requires less energy than pressurizing a gas, the use of a high-pressure water electrolyzer results in the production of pressurized oxygen-rich gas at minimized power consumption.
In one embodiment according to the method of the disclosure, in step a), the pressurized air 65, particularly having a pressure P1, is supplied in the stream in direct fluid communication with the oxygen-rich gas 50, particularly having a pressure P2.
It is preferred to introduce pressurized air 65 at the start-up in the tail gas stream in direct fluid communication with the oxygen-rich gas 50. In this manner, upon operating the means for pressurizing and the NOx compressor 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, the tail gas 5 is produced and the first tail gas stream 10 and recirculated to the mixing unit 35, and mixed with the first 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 production plant for producing nitric acid, in particular for revamping an existing production plant into a production plant according to the present disclosure is disclosed, wherein the existing system or production plant for producing nitric acid comprises:
The revamping method comprises the steps of
The term “oxygen-rich gas” is used as defined elsewhere herein.
Typically, the heat exchange system 43 comprises at least two heat exchangers 66, 67. The person skilled in the art will realize that it is possible to split a stream of tail gas inside the heat exchange system, for example between the heat exchanger 66 and 67. In particular, the production plant both prior to and after revamping comprises further heat exchange systems, such that the gaseous NOx stream 22 or the NOx compressed gas stream 24 exchange heat with the tail gas 5.
As defined herein, a stream of tail gas/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 between or mixing of the first tail gas stream 10 and the (first) oxygen-rich gas 50.
As defined herein, a means for splitting is any means suitable for splitting a tail gas stream, such as to generate a first tail gas stream 10 and a second tail gas stream 80. 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 pressure release means is any suitable means for reducing the pressure of a gas stream, such as a tail gas stream. In particular, the pressure release means may be a gas expander or a gas ejector. A gas ejector provides the benefits of a simplified equipment, wherein mixing of different gas streams is combined with the reduction of the tail gas pressure. For instance, a tail gas stream, processed via the gas ejector, may act as the motive gas, and a second gas fed to the ejector can, for example, be ambient air at a pressure (e.g. atmospheric pressure) lower than the tail gas stream as motive gas. In particular, a tail gas stream may be fed as the motive gas to a gas ejector, and a second gas fed to the ejector is oxygen gas at a pressure lower than the tail gas stream as motive gas. In this context, both the feeding of air or oxygen through the gas ejector contribute to increasing the concentration in the first tail gas stream 10 and/or a further tail gas stream 83, 84 being recycled, thereby reducing the demand on the oxygen-rich gas 50. In particular, a tail gas stream is fed as the motive gas to the ejector and the second gas fed to the ejector is the NOx gas/steam mixture 15 or the gaseous NOx stream 22.
The person skilled in the art will realize that the means for splitting can be incorporated within the (first) pressure release means, provided that the (first) pressure release means includes at least two outlets for the gas stream being depressurized, in particular one outlet for the first tail gas stream 10 and another outlet for the second tail gas stream 80.
As defined herein, a means for adjusting the amount of tail gas being split into the first tail gas stream 10 and the second tail gas stream 80, 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 may be 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 may be an integrated process control system, in which the temperature in the ammonia converter 37 is determined through a 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 the tail gas stream, in order for the measured temperature to be maintained in the range 800-950° C.
As defined herein, a means for adjusting the oxygen concentration is any suitable means for regulating the amount of oxygen to be introduced in the system from a measurement of the oxygen concentration, such as by using a means for measuring the concentration of oxygen. 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, the flow at which the oxygen source, in particular the oxygen rich gas, is introduced in the system, particularly 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 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 oxygen is any means suitable for achieving a target concentration of ammonia and/or oxygen. In particular, such means are gas flow control means, in particular a flow control valve or an orifice or a guiding vane, for controlling the flow of the oxygen rich gas and/or the ammonia gas stream. In particular, the means is an integrated process control system, in which the concentration of oxygen is measured, and the target flow or 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.
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, he will also 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, which implies 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 thermometer suitable for measuring and indicating a temperature ranging as high as 1,000° C. More in particular, the means for measuring the temperature is an infrared thermometer for measuring and indicating a temperature ranging as high as 1,000° C.
Reference is made to
Reference is made to
| Number | Date | Country | Kind |
|---|---|---|---|
| 21193029.2 | Aug 2021 | EP | regional |
| 22150904.5 | Jan 2022 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/073638 | 8/25/2022 | WO |
