Patent Application
20250091869
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 two stages. The ammonia is first oxidized in an ammonia burner on platinum gauzes (commonly called ammonia converter), 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, nitrogen dioxide and dinitrogen tetroxide are converted to nitric acid and nitric oxide:
3NO2(g)+H2O(l)→2HNO3(aq)+NO(g) (4)
3N2O4(g)+2H2O(l)→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 mono-pressure process, the converter and the absorber unit operate at roughly the same working pressure. Such mono-pressure process generally includes low-pressure processes, LP, from 2 to 6 bar; and high pressure processes, HP, from 6 to 16 bar, in particular from 9 to 16 bar.
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 a 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 of power required to operate the air compressor in a mono nitric acid plant.
In CN110540178A (China Chengda Engineering Co Ltd, 2019), a 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 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 system for producing nitric acid at reduced power consumption is disclosed. The system comprises:
The system is further characterized in that it further comprises:
The inventors have realized that upon recirculating part of the tail gas to the compressed air stream at a pressure about the pressure P1 of the compressed air stream, downstream the air compressor and upstream the ammonia converter, while at the same time feeding pressurized oxygen to the compressed air stream, and maintaining the temperature in the ammonia converter in the range of 800 to 950° C. and the oxygen to ammonia molar ratio in the ammonia converter between 1.3 and 9, the net power consumption by the compressor train is reduced, considering the reduced power production from the tail gas expander, the compression of the tail gas to P1 and the reduction in power required by the air compressor. Therefore, with the system of the disclosure, power reduction is achieved at the same time as the size of the air compressor and the size of the tail gas expander are reduced, resulting in the reduction of the area footprint of the plant and simplification of the system. Also, 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 system of the disclosure, the system further comprises a tail gas heater, having an inlet in fluid communication with the absorption tail gas outlet and an outlet in fluid communication with the tail gas expander inlet, positioned upstream from the water cooler/condenser for heating the tail gas coming from the absorption tower to a temperature ranging between 200 to 650° C. with the heat from the NOx gas/steam mixture coming from the ammonia converter, and wherein means for splitting the tail gas is positioned upstream from the tail gas heater.
In one embodiment according to the system of the disclosure, the system further comprises a tail gas heater, having an inlet in fluid communication with the absorption tail gas outlet and an outlet in fluid communication with the tail gas expander inlet, positioned upstream from the water cooler/condenser for heating the tail gas coming from the absorption tower to a temperature ranging between 200 to 650° C. with the heat from the NOx gas/steam mixture coming from the ammonia converter, and wherein means for splitting the tail gas is positioned downstream from the tail gas heater.
In one embodiment according to the system of the disclosure, the source of pressurized oxygen-rich gas is supplied by a high pressure water electrolyzer.
In one embodiment according to the system of the disclosure, the system further comprises a source of oxygen-rich gas having a pressure at least equal to atmospheric pressure, in fluid communication with the inlet of the air compressor.
In one embodiment according to the system of the disclosure, the system further comprises an additional source of pressurized oxygen-rich gas in fluid communication with an area downstream the absorption tower and upstream the means for pressurizing.
In one embodiment according to the system of the disclosure, the system further comprises a bleacher for bleaching raw nitric acid-containing residual NOx gas, comprising an inlet for an oxygen-rich bleaching gas, an inlet for the nitric acid, an outlet for bleached nitric acid and an outlet for bleaching gases, wherein the outlet for the bleaching gases is in fluid communication with the gaseous NOx stream.
In another aspect of the disclosure, a method for producing nitric acid at reduced power consumption is disclosed. The method comprises the steps of:
The method is characterized in that it further comprises the steps of:
In one embodiment according to the method 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 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 step of:
In one embodiment according to the method of the disclosure, the method further comprises the steps of q) feeding an oxygen-rich bleaching gas to a bleacher for bleaching raw nitric acid-containing residual NOx gas; r) bleaching the nitric acid in the bleacher, thereby generating bleached nitric acid and bleaching gases; and s) mixing the bleaching gases with the gaseous NOx stream.
In another aspect of the disclosure, the use of the system the disclosure for performing the method of the disclosure is disclosed.
In another aspect of the disclosure, a method for revamping a system or production plant for producing nitric acid is disclosed, wherein the existing system or production plant comprises
The method comprises 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 “in the ranges of” and “ranging from . . . 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.
As defined herein, a pressurized oxygen-rich gas is a gas having a pressure ranging from 9 to 30 bar, preferably 15 to 30 bar, 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.
As defined herein, air is ambient air, having a pressure about the atmospheric pressure.
Reference is made to
The system is further characterized in that it further comprises means for splitting 55 the tail gas 5 into a first tail gas stream 5 directed to the tail gas expander inlet 8 and a second tail gas stream 10 directed to means for pressurizing 53 for pressurizing the tail gas stream 10 to a pressure of about P1 in fluid communication with the compressed airstream 34, to provide in a pressurized tail gas stream 52 joining the compressed air stream 34; and means for adjusting the amount of tail gas 5 being split into the first tail gas stream 5 directed to the tail gas expander inlet 8 and the second tail gas stream 10 directed to means for pressurizing 53.
As defined herein, means for regulating the concentration of ammonia and oxygen in the ammonia converter 37 are any suitable means for assessing the amount of ammonia to be introduced in the system from a measure of the oxygen concentration, or the amount of oxygen to be introduced in the system from a measure of the ammonia concentration, such that the oxygen to ammonia molar ratio will range from 1.3 to 9. The oxygen or ammonia concentration can be determined, for example, from a measurement in the gas phase using a process gas analyzer. The oxygen or ammonia concentration can also be determined from computing using the concentration of the oxygen- or ammonia source being introduced in the system, the flow at which the source is introduced in the system, and the relative flow values of the gases at which the gases are mixed. Using the oxygen or ammonia concentration, the relevant flow of ammonia or oxygen respectively to be introduced in the system is, in turn, determined and is used in controlling the flow of ammonia or oxygen, from gaseous sources of ammonia or oxygen respectively at pre-determined concentrations. Controlling of the flow of gaseous ammonia or oxygen can, for example, be achieved through flow control valves. In particular, the means is an integrated process control system, in which the concentration of oxygen or ammonia respectively is measured, and the relevant flow of ammonia or oxygen respectively is thereby determined, thus controlling the flow of ammonia or oxygen, from gaseous sources of ammonia or oxygen respectively at pre-determined concentrations.
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 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 mean for achieving power from steam. In particular those means are a steam turbine connected to an electric generator.
As defined herein, means for splitting are any means suitable for splitting the tail gas 5 such as to generate, in addition to the tail gas 5, another gas stream 10 of tail gas directed to the means for pressurizing 53. 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, means for compressing are any suitable means for increasing the pressure of the tail gas 10 in fluid communication with the compressed air stream 34. In particular, the means for compressing is a gas compressor or a gas ejector through which a motive gas is flowing at a higher velocity than the tail gas 10 in fluid communication with the compressed air stream 34.
As defined herein, means for adjusting the amount of tail gas 5 being split into the first tail gas stream 5 directed to the tail gas expander inlet 8 and the second tail gas stream 10 directed to the means for pressurizing 53, 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 valve 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 to 950° C.
The inventors have realized that upon recirculating part of the tail gas 5 to the compressed air stream 34, at the pressure P1 of the compressed air stream 34, downstream the air compressor 36 and upstream the ammonia converter 37, while at the same time feeding pressurized oxygen 50, such as provided by a high-pressure electrolyzer, to the compressed air stream 34, and maintaining the temperature in the ammonia converter 37 in the range of 800 to 950° C. and the oxygen to ammonia molar ratio in the ammonia converter 37 between 1.3 and 9, net reduction of the power consumption by the compressor train can be achieved. Albeit the tail gas 5 leaving the absorption tower 41 is less pressurized than the compressed air stream 34 leaving the air compressor 36, the corresponding pressure drop only accounts for 0.5 to 1.5 bar. Consequently, the tail gas 5, whether upstream or downstream the tail gas heater 43, retains a pressure ranging from 1 to 16 bar. Hence, upon pressurizing the tail gas 10 to a pressure about the pressure of the compressed air stream 34 and mixing the pressurized tail gas 52 with the compressed air stream 34, less power is required to provide the necessary total amount of compressed gas to the mixing apparatus 35. By ensuring that the temperature in the burner is maintained in the range of 800 to 950° C., it is ensured that, despite the demand on the means for pressurizing 53 and less power being produced from the tail gas expander 7 due to less tail gas 5 being expanded, a net reduction of the power consumption by the compressor train is retained. In addition, in particular by providing a separate supply of high-pressure oxygen or oxygen-rich gas, it is ensured that the oxygen and ammonia concentrations in the ammonia converter 37 allow for the production of nitric acid of a commercial grade.
In addition to the net saving in the power consumption by the compressor train mentioned above, the inventors have identified that, the recirculation of part of the tail gas 5 results in the temperature of the gaseous NOx stream 15 at the outlet of the ammonia converter 37 being such that the heat exchange between the gaseous NOx stream 15 and the tail gas 5 is more efficient: hence, in case the tail gas heater is present and the tail gas 5 is heated, the size of the heat exchanger 43 and of the coolers 38 can be decreased. Further, as less tail gas 5 is directed to the tail gas expander and, ultimately, to the atmosphere, the NOx emissions arising from the tail gas are reduced.
In one embodiment according to the system of the disclosure, the system further comprises a tail gas heater 43, having an inlet 46 in fluid communication with the absorption tail gas outlet 6 and an outlet 47 in fluid communication with the tail gas expander inlet 8, positioned upstream from the water cooler/condenser 38 for heating the tail gas 5 coming from the absorption tower 41 to a temperature ranging between 200 to 650° C. with the heat from the NOx gas/steam mixture 15 coming from the ammonia converter 37, and wherein means for splitting 55 the tail gas 5 is positioned upstream from the tail gas heater 43.
In one embodiment according to the system of the disclosure, the system further comprises a tail gas heater 43, having an inlet 46 in fluid communication with the absorption tail gas outlet 6 and an outlet 47 in fluid communication with the tail gas expander inlet 8, positioned upstream from the water cooler/condenser 38 for heating the tail gas 5 coming from the absorption tower 41 to a temperature ranging between 200 to 650° C. with the heat from the NOx gas/steam mixture 15 coming from the ammonia converter 37, and wherein means for splitting 55 the tail gas 5 is positioned downstream from the tail gas heater 43.
The person skilled in the art will understand that this is possible to compress either one of the tail gas 5 or the heated tail gas 5 in fluid communication with the tail expander inlet 8 or a mixture thereof.
The choice of the point from which the tail gas 5 is recirculated, that is upstream or downstream the tail gas heater 43, influences the temperature of the gas mixture 14, and thereby, the efficiency of the combustion in the burner. The system of the disclosure provides the necessary flexibility for the person skilled in the art to choose where to recirculate the tail gas 5 from. Thereby, he/she can achieve the optimal temperature for the gas mixture 14, depending on parameters including, for example, the gas volume flown to or the catalyst present in the converter 37, as well as the ratio of oxygen to ammonia in the gas mixture 14.
In one embodiment according to the system of the disclosure, the source of pressurized oxygen-rich gas 50 is supplied by a high pressure water electrolyzer. Stated differently, in particular embodiments, the system of the present disclosure comprises a high-pressure water electrolyzer, wherein the high-pressure water electrolyzer, in particular its anode, is in fluid communication with the compressed air stream, to provide an oxygen-rich gas/compressed air stream mixture.
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−;
a cathode, producing hydrogen gas according to the reaction
2H2O+2e−=H2+2OH−;
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 9 to 30 bar, preferably 15 to 30 bar.
A high pressure water electrolyzer hence results in the production of pressurized hydrogen at the cathode and pressurized oxygen at the anode. 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 50 at minimized power consumption.
Reference is made to
The presence of the source of oxygen-rich gas 54 implies that less air is to be pressurized in order to achieve the content of oxygen achieved in the prior art process at the outlet of the air compressor 36. As a result, the power demand on the air compressor 36 is reduced.
Reference is made to
The oxygen-rich gas can be fed either upstream or downstream the means for splitting 55. When oxygen-rich gas from such additional source is fed downstream the absorption tower 41, less air is to be pressurized in order to achieve the content of oxygen achieved in the prior art process at the outlet of the air compressor 36. As a result, the power demand on the air compressor 36 is reduced. Further, additional power will be generated through the tail gas expander 7. In addition, the power demand on the means for pressuring 53 is decreased, in particular if the oxygen-rich gas from such additional source is provided by a high pressure water electrolyzer, producing pressurized oxygen-rich gas at less power than through pressurizing oxygen gas, as pressurizing the water being electrolyzed in the electrolyzer consumes less power than pressurizing a gas.
In addition, additional pressurized oxygen-rich gas may be supplied upstream the absorption tower, which improves the absorption of NOx gases in the absorption tower, which, in its turn, results in additional nitric acid production and reduction of the emissions to the atmosphere. In addition, or alternatively, the size of the absorption tower 41 can be reduced.
Reference is made to
In this way, an efficient use of an oxygen-rich bleaching gas is made to increase the oxygen content in the absorption tower 41, thereby increasing the absorption of the NOx gases and reducing the corresponding emissions to air. In particular, the oxygen-rich bleaching gas 60 is provided by a high pressure water electrolyzer 66: as less energy is required to pressurize water than to pressurize oxygen gas, pressurized oxygen gas is obtained at minimum power consumption. The person skilled in the art will understand that the pressure of the bleaching gas 60 must be such that upon recirculating the bleaching gases 61 to the absorption tower 41, the pressure in the absorption tower 41 is about the pressure in the ammonia converter 37. Consequently, depending upon the pressure at which the water electrolyzer 66 is operated, the pressure can be adjusted and decreased accordingly with means conventional in the state-of-the art, for example pressure regulating valves (not shown). In addition, less secondary air must be compressed and supplied by the air compressor 36 to the bleacher 57, which results in savings in the power demand by the air compressor 36. Advantageously, the pressurized oxygen-rich gas produced by the high pressure water electrolyzer 66 can be the source of all streams 50, 54, and 66.
In another aspect of the disclosure, a method for producing nitric acid at reduced power consumption is disclosed. The method comprises the steps of a) compressing air in an air compressor 36, thereby producing a compressed air stream 34 having a pressure P1; b) mixing pressurized oxygen-rich gas 50 having a pressure higher than P1 with the compressed air stream 34, thereby obtaining an oxygen-rich gas/compressed air stream mixture 56; c) mixing the oxygen-rich gas/compressed air stream mixture 56 with an ammonia gas stream 32 in a mixing apparatus 35, thereby producing an ammonia/oxygen-enriched air mixture 14, such as to achieve an oxygen to ammonia molar ratio ranging from 1.3 to 9; d) oxidising ammonia in the ammonia/oxygen-enriched air mixture 14 in an ammonia converter 37 at a temperature ranging from 800 to 950° C., thereby producing a gaseous NOx gas/steam mixture 15, comprising water and nitric oxide; e) converting the steam generated in the ammonia converter 37 or from the gaseous NOx gas/steam mixture 15 into power; f) separating and condensing steam from NOx gas in the gaseous NOx gas/steam mixture 15, thereby generating an aqueous diluted nitric acid mixture 17 and a gaseous NOx stream 22, in a water cooler/condenser 38; g) absorbing the gaseous NOx stream 22 in an absorption tower 41, thereby producing a stream of raw nitric acid-containing residual NOx gas 27 and a tail gas 5 comprising NOx gases; and h) expanding the tail gas 5 or the heated tail gas 5 in a tail gas expander 7;
The method is characterized in that it further comprises the steps of i) pressurizing part of the tail gas 5 obtained from step g) obtained from the combination step g) and step h) to a pressure about P1, thereby generating a pressurized tail gas 52; j) mixing the pressurized tail gas 52 with the compressed air stream 34; k) measuring the temperature in the ammonia converter 37; and I) adjusting the amount of the total gas volume being pressurized in step i) if the temperature measured in step k) is outside the range of 800-950° C., such that the temperature in the ammonia converter is maintained in the range of 800 to 950° C.
The inventors have realized that upon recirculating part of the tail gas 5 to the compressed air stream 34, at the pressure P1 of the compressed air stream 34, downstream the air compressor 36 and upstream the ammonia converter 37, while at the same time feeding pressurized oxygen 50 to the compressed airstream 34, and maintaining the temperature in the ammonia converter 37 in the range of 800 to 950° C. and the oxygen to ammonia molar ratio in the ammonia converter 37 between 1.3 and 9, net reduction in the power consumption by the compressor train can be achieved. Albeit the tail gas 5 leaving the absorption tower 41 is less pressurized than the compressed air stream 34 leaving the air compressor 36, the corresponding pressure drop only accounts for 0.5 to 1.5 bar. Consequently, the tail gas 5, whether upstream or downstream the tail gas heater 43, retains a pressure ranging from 1 to 16 bar. Hence, upon pressurizing the tail gas 10 to a pressure about P1 and mixing the pressurized tail gas 52 with the compressed air stream 34, less power is required to provide the necessary total amount of compressed gas to the mixing apparatus 35. By ensuring that the temperature in the burner is maintained in the range of 800 to 950° C., it is ensured that, despite the demand on the means for pressurizing 53 and less power being produced from the tail gas expander 7 due to less tail gas 5 being expanded, a net power reduction consumption by the compressor train is retained. In addition, it is ensured that the oxygen and ammonia concentrations in the ammonia converter 37 allow for the production concentrated acid of commercial grade.
In addition to the net saving in the power consumption by the compressor train mentioned above, the inventors have identified that, the recirculation of part of the tail gas 5 results in the temperature of the gaseous NOx stream 15 at the outlet of the ammonia converter 37 being such that the heat exchange between the gaseous NOx stream 15 and the tail gas 5 is more efficient: hence, in case the tail gas heater is present and the tail gas 5 is heated, the size of the heat exchanger 43 and of the coolers 38 can be decreased. Further, as less tail gas 5 is directed to the tail gas expander and, ultimately, to the atmosphere, the NOx emissions arising from the tail gas are reduced.
In one embodiment according to the method of the disclosure, the method further comprises the step of m) heating the tail gas 5 obtained in step g) to a temperature ranging from 200 to 650° C. in a tail gas heater 43 positioned upstream from the water cooler/condenser 38 with the heat from the NOx gas/steam mixture 15 coming from the ammonia converter 37.
In one embodiment according to the method of the disclosure, the method further comprises the step of m′) heating the tail gas 5 obtained in step g) to a temperature ranging from 200 to 650° C. in a tail gas heater 43 positioned upstream from the water cooler/condenser 38 with the heat from the NOx gas/steam mixture 15 coming from the ammonia converter 37, wherein, in step i), part of the tail gas 5 obtained in step g) or in step m′) is pressurized.
The person skilled in the art will understand that this is possible to compress either one of the tail gas 5 or the heated tail gas 5 in fluid communication with the tail expander inlet 8 or a mixture thereof.
The choice of the point from which the tail gas 5 is recirculated, that is upstream or downstream the tail gas heater 43, influences the temperature of the gas mixture 14, and thereby, the efficiency of the combustion in the burner. The system of the disclosure provides the necessary flexibility for the person skilled in the art to choose where to recirculate the tail gas 5 from. Thereby, he/she can achieve the optimal temperature for the gas mixture 14, depending on parameters including, for example, the gas volume flown to or the catalyst present in the converter 37, as well as the ratio of oxygen to ammonia in the gas mixture 14.
In one embodiment according to the method of the disclosure, the method further comprises the step of n) operating a high pressure water electrolyzer in order to produce the oxygen gas or oxygen-rich gas 50 used in the mixing step b).
A high pressure water electrolyzer hence results in the production of pressurized hydrogen at the cathode and pressurized oxygen at the anode. 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 50 at minimized power consumption. Mixing step b) may thus comprise the steps of operating a high pressure water electrolyzer, such as at a temperature of 50 to 80° C., or 60 to 80° C., and a gas pressure of 9 to 30 bar, preferably 15 to 30 bar, thereby producing a pressurized oxygen or oxygen-rich gas 50 having a pressure higher than P1, and mixing the pressurized oxygen or oxygen-rich gas 50 with the compressed air stream 34.
In one embodiment according to the method of the disclosure, the method further comprises the step of o) sending an oxygen-rich gas having a pressure at least equal to atmospheric pressure 54 to the inlet 48 of the air compressor 36.
The presence of the source of oxygen-rich gas 54 implies that less air is to be pressurized in order to achieve the content of oxygen achieved in the prior art process at the outlet of the air compressor 36. As a result, the power demand on the air compressor 36 is reduced.
In one embodiment according to the method of the disclosure, the method further comprises the step of p) feeding a source of pressurized oxygen-rich gas downstream the absorption tower 41 and upstream the means for compressing 53. In particular, the pressurized oxygen or oxygen-rich gas may be produced by operating a high pressure electrolyzer.
The oxygen-rich gas can be fed either upstream or downstream the means for splitting 55. When oxygen-rich gas from such additional source is fed downstream the absorption tower 41, less air is to be pressurized in order to achieve the content of oxygen achieved in the prior art process at the outlet of the air compressor 36. As a result, the power demand on the air compressor 36 is reduced. Further, additional power will be generated through the tail gas expander 7. In addition, the power demand on the means for pressuring 53 is decreased, in particular if the oxygen-rich gas from such additional source is provided by a high pressure water electrolyzer, producing pressurized oxygen-rich gas at less power than through pressurizing oxygen gas, as pressurizing the water being electrolyzed in the electrolyzer consumes less power than pressurizing a gas.
Reference is made to
In this way, an efficient use of an oxygen-rich bleaching gas is made to increase the oxygen content in the absorption tower 41, thereby increasing the absorption of the NOx gases in step g) and reducing the corresponding emissions to air. In particular, the oxygen-rich bleaching gas 60 is provided by a high pressure water electrolyzer 66: as less energy is required to pressurize water than to pressurize oxygen gas, pressurized oxygen gas is obtained at minimum power consumption. The person skilled in the art will understand that the pressure of the bleaching gas 60 must be such that upon recirculating the bleaching gases 61 to the absorption tower 41, the pressure in the absorption tower 41 is about the pressure in the ammonia converter 37. Consequently, depending upon the operating pressure of the water electrolyzer 66, the pressure can be adjusted and decreased accordingly with means conventional in the state-of-the art, for example pressure regulating valves (not shown). In addition, less secondary air must be compressed and supplied by the air compressor 36 to the bleacher 57, which results in savings in the power demand by the air compressor 36. Advantageously, the pressurized oxygen-rich gas produced by the high pressure water electrolyzer 66 can be the source of all streams 50, 54, and 66.
In another aspect of the disclosure, the use of the system the disclosure for performing the method of the disclosure is disclosed.
In another aspect of the disclosure, a method for revamping a system for producing nitric acid comprising an air compressor 36 for compressing air to a pressure P1, comprising an inlet 48 and an outlet 49, to provide in a compressed air stream 34; optionally a source of pressurized oxygen-rich gas 50 having a pressure higher than the pressure of the compressed air stream 34, in fluid communication with the compressed air stream 34, to provide in an oxygen-rich gas/compressed air stream mixture 56; a mixing apparatus 35 for mixing the oxygen-rich gas/compressed air stream mixture 56 with an ammonia gas stream 32, optionally pre-heated in a pre-heater, to provide in an ammonia/oxygen-enriched air mixture 14; an ammonia converter 37 for oxidising ammonia in the ammonia/oxygen-enriched air mixture 14, to provide in a NOx gas/steam mixture 15, comprising water and nitric oxide; means for measuring (not shown) the temperature in the ammonia converter 37; means for regulating the concentration of ammonia and of oxygen in the ammonia converter 37; a steam turbine 51 or an electric motor and means for converting steam into electricity, for converting steam into power, in fluid communication with the ammonia converter 37 or the NOx/gas steam mixture 15; a water cooler/condenser 38, for separating and condensing steam from NOx gas in the gaseous NOx gas/steam mixture 15, thereby generating an aqueous diluted nitric acid mixture 17 and a gaseous NOx stream 22; an absorption tower 41 downstream the water cooler/condenser 38, for absorbing NOx gases in water, to provide in a stream of raw nitric acid-containing residual NOx gas 27 and a tail gas 5 comprising NOx gases, comprising an absorption tower tail gas outlet 6 for evacuating the tail gas 5; and a tail gas expander 7 for expanding the tail gas downstream of the absorption tower comprising a tail gas expander inlet 8 in fluid communication with the absorption tower tail gas outlet 6, and a tail gas expander outlet 9;
into a system according to the disclosure is disclosed.
The method comprises the steps of introducing means for splitting 55 the tail gas 5 into a first tail gas stream 5 directed to the tail gas expander inlet 8 and a second tail gas stream 10 directed to means for pressurizing 53 for pressurizing the tail gas stream 10 to a pressure of about P1 in fluid communication with the compressed air stream 34, to provide in a pressurized tail gas stream 52 joining the compressed air stream 34; and introducing means for adjusting the amount of tail gas 5 being split into the first tail gas stream 5 directed to the tail gas expander inlet 8 and a second tail gas stream 10 directed to means for pressurizing 53. In certain embodiments, in case the existing system does not comprise a source of pressurized oxygen-rich gas, the revamping method further comprises the step of introducing a source of pressurizing oxygen-rich gas 50, such as a high pressure electrolyzer, said pressurized oxygen rich gas having a pressure higher than the pressure of the compressed airstream 34, and fluidly connecting the source of pressurized oxygen-rich gas with the compressed air stream 34, to provide in an oxygen-rich gas/compressed air stream mixture 56.
As defined herein, means for adjusting the concentration of ammonia and oxygen in the ammonia converter 37 are any suitable means for assessing the amount of ammonia to be introduced in the system from a measure of the oxygen concentration, or the amount of oxygen to be introduced in the system from a measure of the ammonia concentration, such that the oxygen to ammonia molar ratio will range from 1.3 to 9. The oxygen or ammonia concentration can be determined, for example, from a measurement in the gas phase using a process gas analyzer. The oxygen or ammonia concentration can also be determined from computing using the concentration of the oxygen- or ammonia source being introduced in the system, the flow at which the source is introduced in the system, and the relative flow values at which the gases is mixed. Using the oxygen or ammonia concentration, the relevant flow of ammonia or oxygen respectively to be introduced in the system is, in turn, determined and is used in controlling the flow of ammonia or oxygen, from gaseous sources of ammonia or oxygen respectively at pre-determined concentrations. Controlling of the flow of gaseous ammonia or oxygen can, for example, be achieved through flow control valves. In particular, the means is an integrated process control system, in which the concentration of oxygen or ammonia respectively is measured, and the relevant flow of ammonia or oxygen respectively is thereby determined, thus controlling the flow of ammonia or oxygen, from gaseous sources of ammonia or oxygen respectively at pre-determined concentrations.
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 mean for achieving power from steam. In particular those means are a steam turbine connected to an electric generator.
As defined herein, means for splitting are any means suitable for splitting the tail gas 5 such as to generate, in addition to the tail gas 5, another gas stream 10 of tail gas directed to the means for pressurizing 53. 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, means for compressing are any suitable means for increasing the pressure of the tail gas 10 in fluid communication with the compressed air stream 34. In particular, the means for compressing is a gas compressor or a gas ejector through which a motive gas is flowing at a higher velocity than the tail gas 10.
As defined herein, means for adjusting the amount of tail gas 5 being split into the first tail gas stream 5 directed to the tail gas expander inlet 8 and the second tail gas stream 10 directed to the means for pressurizing 53, 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 valve 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.
Reference is made to
Reference is made to
Reference is made to
Therefore, when compared to the example 1, a net power of 76 kWh/t 100% HNO3 (50%) was saved upon recirculating 40% of the tail gas.
Therefore, when compared to the example 2, a net power of 85 kWh/t 100% HNO3 (55%) was saved upon recirculating 65% of the tail gas.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21193034.2 | Aug 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/073641 | 8/25/2022 | WO |
