Patent Application
20240367976
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 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:
4 NH3 (g)+5 O2 (g)→4 NO (g)+6 H2O (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:
2 NO (g)+O2 (g)→2 NO2 (g) (2)
2 NO2 (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:
3 NO2 (g)+H2O (I)→2 HNO3(aq)+NO(g) (4)
3 N2O4 (g)+2 H2O (I)→4 HNO3 (aq)+2 NO (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+2 O2→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 dual pressure process generally includes low-pressure or LP processes, from 2 to 6 bar, and high pressure or HP processes, from 6 to 16 bar, in particular 9 to 16 bar.
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 (LP) ammonia converter operating typically at 2 to 6 bar, and a high pressure (HP) absorber unit operating at 9 to 16 bar.
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 pressured NOx gases to the absorber unit. The working pressure of an air compressor is from 2 to 6 bar, inclusive, and the working pressure of a NOx gas compressor is from 9 to 16 bar, inclusive.
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 dual pressure nitric acid production plant typically comprises an air compressor, a NOx gas 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. 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 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 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 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 system for producing nitric acid at reduced power consumption is disclosed. The system comprises:
The system is characterized in that it further comprises:
The inventors have realized that upon recirculating part of the tail gas to 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, a net reduction of the power consumption by the compressor train is gained, considering all the reduced power production from the tail gas expander, the increased demand on the NOx compressor, and the reduction of the 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 6 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:
In one embodiment according to the system of the disclosure, the system further comprises:
In one aspect of the disclosure, a method for producing nitric acid at reduced power consumption 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, in step j), 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:
In one embodiment according to the method of the disclosure, the method further comprises the steps of:
In one aspect of the disclosure, the use of the system of the disclosure for performing the method of the system is disclosed.
In one aspect of the disclosure, a method for revamping a system for producing nitric acid comprising:
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 drawings), 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 characterized in that it further comprises means for splitting 55 the tail gas 5 into a first tail gas stream 5 in fluid communication with the tail gas expander inlet 8 and a second tail gas stream 10, having a pressure P1 or adjusted to a pressure P1, in fluid communication with the compressed air stream 34; and means for adjusting the amount of tail gas 5 being split into the first tail gas stream 5 in fluid communication with the tail gas expander inlet 8 and the second tail gas stream 10 in fluid communication with the compressed air stream 34.
As defined herein, means for adjusting the oxygen to ammonia molar ratio 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 value 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 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 ranging 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 a first tail gas stream, i.e. the tail gas or the heated tail gas 5, another, second tail gas stream 10 of tail gas in fluid communication with the compressed air stream 34. 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 adjusting the amount of tail gas 5 being split into the second stream of tail gas 10 in fluid communication with the compressed air stream 34 and the first tail gas stream 5 in fluid communication with the tail gas expander inlet 8, 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.
The inventors have realized that upon recirculating part of the tail gas 5 to 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 air compressor is gained. Indeed, the tail gas 5 leaving the absorption tower 41 is more pressurized than the compressed air stream 34, and, whether upstream or downstream the tail gas heater 43, retains a pressure ranging from 9 to 16 bar. Hence, upon mixing the stream tail gas 10 in fluid communication with the compressed air stream 34 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 less power being produced from the tail gas expander 7 due to less tail gas 5 being expanded and the demand on the NOx gas compressor 40, 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 air compressor 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 is such that the heat exchange between the NOx gas 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 cooler 38 can be decreased.
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 recirculate to the ammonia converter 37 either one of the tail gas 5 or the heated tail gas 5 in fluid communication with the tail gas expander inlet 8.
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 there through. 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 of an anode producing oxygen gas according to the reaction
2H2O+2 e—=H2+2 OH—;
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 presence of an additional source of pressurized oxygen-rich gas 61 downstream the NOx gas compressor 40 presents benefits. Indeed, a reduction in the power demand by the NOx gas compressor 40 is achieved. Furthermore, when additional pressurized oxygen-rich gas 61 is supplied downstream the NOx gas compressor 40 but upstream the absorption tower 41, the absorption of NOx gases in the absorption tower 41 is improved which 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. When additional pressurized oxygen-rich gas is supplied 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. Further, additional power will be generated through the tail gas expander 7. As a result, the power demand on the compressor train 36 is reduced.
The presence of an additional tail gas expander 60 enables energy recovery from the stream of tail gas 10 in fluid communication with the compressed air stream 34, hence minimizing the loss of energy, due to part of the tail gas 5 being split upstream the tail gas expander 7. As a result, the net power consumption by the compressor train is reduced.
The presence of a gas ejector 56 using the stream of tail gas 10 in fluid communication with the compressed air stream 34 as the motive gas also presents benefits, including a reduction in the net power consumption by the compressor train. The mixing of air or oxygen with the stream of tail gas 10 in fluid communication with the compressed air stream 34 in 5 the gas ejector 56 also for a reduction of the amount of air that 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. The gaseous NOx stream 22, upstream the NOx gas compressor 40, has a pressure that is lower than P1. Therefore, the gaseous NOx stream 22 can be mixed with stream of tail gas 10 in the 10 gas ejector 56, the stream of tail gas 10 in fluid communication with the compressed air stream 34 being used as the motive gas. The mass flow of the compressed air stream 34 is thereby increased, which provides means for controlling the oxygen to ammonia ratio and the temperature in the ammonia converter 37.
Reference is made to
As defined herein, a high pressure bleacher is a bleacher operating with pressurized oxygen-rich gas as the stripping gas. The person skilled in the art will nonetheless understand that bleaching can be performed at any pressure, as long as the pressure of the stream resulting from mixing the bleaching gases leaving the bleacher 62 with the compressed NOx gas stream 24 results in a pressure ranging from 9 to 16 bar at the inlet of the absorption tower 41.
In a conventional dual pressure nitric acid plant, the bleacher 62 provides oxygen to the absorption tower 41. A first advantage is that 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. Moreover, when the bleacher 62 is supplied with an oxygen-rich gas 61, the absorption of the NOx gases in the absorption tower 41 is improved, which results in additional nitric production and reduction of the emissions to the atmosphere. In addition, or alternatively, the size of the absorption tower 41 can be reduced. Also, a reduction in the power demand by the NOx gas compressor 40 is achieved. Furthermore, if the oxygen-rich gas 61 is pressurized, the absorption of NOx gases in the absorption tower 41 is further increased through the increase of the partial pressure of oxygen in the absorption tower 41. Hence, if pressurized oxygen-rich gas 61 is supplied by a high pressure water electrolyzer 63, optimal absorption in the absorption tower 41 is achieved at minimum power demand for producing the pressurized oxygen-rich gas 61: the high pressure water electrolyzer 63 will result in the production of pressurized oxygen-rich gas 61 from pressurized water, which is less power consuming than pressurizing oxygen gas. Advantageously, the pressurized oxygen-rich gas produced by the high pressure water electrolyzer 63 can be the source of both streams 50 and 61, and also a source of pressurized oxygen-rich gas for being sent downstream the NOx compressor 40 and upstream the absorption tower 41.
Reference is made to
The method is characterized in that it further comprises the steps of j) mixing part of the tail gas 5 obtained from step h) at a pressure P1 with the compressed air stream 34, thereby generating a fluid communication between a stream of tail gas 10 and the compressed air stream (34); k) measuring the temperature in the ammonia converter 37; and l) adjusting the amount of the total gas volume mixed in step j) 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 and 950° C.
Reference is made to
In addition to the net saving in the power consumption by the air compressor 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 is such that the heat exchange between the NOx gas 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 cooler 38 can be decreased.
In one embodiment according to the method of the disclosure, the method further comprising the step of m) heating the tail gas 5 obtained in step h) 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 h) 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 h), part of the tail gas 5 obtained in step h) or in step m) is pressurized.
The person skilled in the art will understand that this is possible to recirculate to the ammonia converter 37 either one of the tail gas 5 or the heated tail gas 5 in fluid communication with the tail gas 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 50 used in the mixing step j).
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 and mixing the pressurized oxygen or oxygen-rich gas 50 with the compressed air stream 34.
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 presence of an additional source of pressurized oxygen-rich gas 61 downstream the NOx gas compressor 40 presents benefits. Indeed, a reduction in the power demand by the NOx gas compressor 40 is achieved. Furthermore, when additional pressurized oxygen-rich gas 61 is supplied downstream the NOx gas compressor 40 but upstream the absorption tower 41, the absorption of NOx gases in the absorption tower 41 is improved which results in additional nitric production and reduction of the emissions to the atmosphere. In addition, or alternatively, the size of the absorption tower 41 can be reduced. When additional pressurized oxygen-rich gas is supplied 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. Further, additional power will be generated through the tail gas expander 7. As a result, the power demand on the compressor train 36 is reduced.
Expanding the stream of tail gas 10 in fluid communication with the compressed air stream 34 in an additional tail gas expander 60 enables energy recovery, hence minimizing the loss of energy, due to part of the tail gas 5 being split upstream the tail gas expander 7. As a result, the net power consumption by the compressor train is reduced.
Using a gas ejector 56 having the stream of tail gas 10 in fluid communication with the compressed air stream 34 as the motive gas, also presents benefits, including a reduction in the net power consumption by the compressor train. The mixing of air or oxygen with the stream of tail gas 10 in fluid communication with the compressed air stream 34 in the gas ejector 56 also allows for a reduction of the amount of air that 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. The gaseous NOx stream 22, upstream the NOx gas compressor 40, has a pressure that is lower than P1. Therefore, the gaseous NOx stream 22 can be mixed with the stream of tail gas 10 in 10 the gas ejector 56, the stream of tail gas 10 in fluid communication with the compressed air stream 34 being used as the motive gas. The mass flow of the compressed air stream 34 is thereby increased, which provides means for controlling the oxygen to ammonia ratio and the temperature in the ammonia converter 37.
Reference is made to
As defined herein, a high pressure bleacher is a bleacher operating with pressurized oxygen-rich gas as the stripping gas. The person skilled in the art will nonetheless understand that bleaching can be performed at any pressure, as long as the pressure of the stream resulting from mixing the bleaching gases leaving the bleacher 62 with the compressed NOx gas stream 24 results in a pressure ranging from 9 to 16 bar at the inlet of the absorption tower 41.
In a conventional dual pressure nitric acid plant, the bleacher 62 provides oxygen to the absorption tower 41. A first advantage is that 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. Moreover, when the bleacher 62 is supplied with an oxygen-rich gas 61, the absorption of the NOx gases in the absorption tower 41 is improved, which results in additional nitric production and reduction of the emissions to the atmosphere. In addition, or alternatively, the size of the absorption tower 41 can be reduced. Furthermore, if the oxygen-rich gas 61 is pressurized, the absorption of NOx gases in the absorption tower 41 is further increased through the increase of the partial pressure of oxygen in the absorption tower 41. Hence, if pressurized oxygen-rich gas 61 is supplied by a high pressure water electrolyzer 63, optimal absorption in the absorption tower 41 is achieved at minimum power demand for producing the pressurized oxygen-rich gas 61: the high pressure water electrolyzer 63 will result in the production of pressurized oxygen-rich gas 61 from pressurized water, which is less power consuming than pressurizing oxygen gas or air. Advantageously, the pressurized oxygen-rich gas produced by the high pressure water electrolyzer 63 can be the source of both streams 50 and 61, and also a source of pressurized oxygen-rich gas for being sent downstream the NOx compressor 40 and upstream the absorption tower 41.
In one aspect of the disclosure, the use of the system of the disclosure for performing the method of the disclosure is disclosed.
In one aspect of the disclosure, a method for revamping a system for producing nitric acid comprising an air compressor 36 for compressing air, 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 53; a mixing apparatus 35 for mixing the oxygen-rich gas/compressed air stream mixture 53 with an ammonia gas stream 32, 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 and, optionally, a cooler/separator 39, wherein the water cooler/condenser 38 is located upstream the cooler/separator 39, 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; a NOx gas compressor 40, for compressing the gaseous NOx stream 22, to provide in a compressed NOx gas stream 24; an absorption tower 41 downstream the NOx gas compressor 40 for absorbing the NOx gases from the compressed NOx gas stream 24 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; a tail gas expander 7 for expanding the tail gas, thereby generating an expanded tail gas 64, 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; and optionally, a tail gas heater 43 upstream the water cooler/condenser 38 and, optionally, the cooler/separator 39, for heating the tail gas 5 to a temperature ranging from 200 to 650° C., and comprising a tail gas heater inlet 46 and a tail gas heater outlet 47, wherein the tail gas heater inlet 46 is in fluid communication with the absorption tail gas outlet 6, and wherein the tail gas heater outlet 47 is in fluid communication with the tail gas expander inlet 8; into a system according to the system of 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 in fluid communication with the tail gas expander inlet 8 and a second tail gas stream 10 in fluid communication with 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 in fluid communication with the tail gas expander inlet 8 and the second tail gas stream 10 in fluid communication with the compressed air stream 34. 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 introducing a high pressure electrolyzer, said pressurized oxygen rich gas having a pressure higher than the pressure of the compressed air stream 34, and fluidly connecting the source of pressurized oxygen-rich gas, such as the high pressure electrolyzer, with the compressed air stream 34, to provide in an oxygen-rich gas/compressed air stream mixture.
As defined herein, means for adjusting the oxygen to ammonia molar ratio 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 value 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 with which the source 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 thermometer suitable for measuring and indicating a temperature ranging as high as 1000° C. More in particular, the means for measuring the temperature is an infrared thermometer for measuring and indicating a temperature ranging 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. 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 adjusting the amount of tail gas 5 being split into the second stream of tail gas 10 in fluid communication with the compressed air stream 34 and the first tail gas 5 in fluid communication with the tail gas expander inlet 8, 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 800-950° C.
Reference is made to
The residual 76% of tail gas 5 was sent to the tail gas expander 7. The residual NOx gas in the raw nitric acid stream 27 was then stripped out with a gaseous medium (not shown) such as an oxygen-containing gas or air, inside the bleacher unit (not shown), operating at about the same pressure as the ammonia converter of 5.2 bar. The drive power for both the air compressor 36 and the NOx compressor 40 originated from the tail gas expander 7, the additional tail gas expander 60 and the steam turbine 51. The net power associated to the air compressors 36, the NOx gas compressor 40, the tail gas expander 7 and the additional tail gas expander 60 was 37 kW/h/t 100% HNO3. This power was produced by the steam turbine 51.
Reference is made to
Ambient air 4 was compressed in an air compressor 36, generating the compressed air stream 34. Ammonia 32 was mixed with the oxygen-rich gas/compressed air stream mixture 53, in a mixing apparatus 35, and the resulting ammonia/oxygen-enriched air mixture 14 was fed to an ammonia converter 37, operating at a pressure of 5.2 bar. In the ammonia converter 37, ammonia was oxidized over a mixed platinum/rhodium catalyst, thus obtaining a low-pressure NOx gas/steam mixture 15, comprising water and nitric oxide (NO). The heat of the mixture coming out of the ammonia converter was recovered using the steam turbine 51. The NOx gas/stream mixture was subsequently cooled down in a water cooler/condenser 38 to temperature where the water condenses, and an aqueous diluted nitric acid mixture 17 was separated from a gaseous NOx stream 18. Subsequently, the LP gaseous NOx stream was further oxidized to further convert the NO to NO2 and N2O4, and cooled down again in a cooler/separator 39 to separate out another aqueous diluted nitric acid mixture 17 which was directed to an absorption tower 41. On the other end, the gaseous NOx stream 22 was compressed in the NOx gas compressor 40 to a pressure of 12 bar, thereby producing the pressurized NOx gaseous stream 24. The pressurized NOx gaseous stream 24 was sent to the absorber unit 6 too. Inside the absorber unit 6, the high pressure NOx gas reacted with water to produce the tail gas 5 and a stream of raw nitric acid also containing residual NOx gas, which was fed to a bleacher (not shown). The heat from the gaseous NOx stream 24 was used for heating the tail gas 5 in the tail gas heater 43 to 450° C. The entire tail gas stream 5 was sent to the tail gas expander 7. The residual NOx gas in the raw nitric acid stream 27 was then stripped out with a gaseous medium (not shown) such as an oxygen-containing gas or air, inside the bleacher unit (not shown), operating at low pressure; the bleacher unit was generally operated at about the same pressure as the ammonia converter, of 5.2 bar. The drive power for the air compressor 36 and the NOx compressor 40 originated from the tail gas expander 7 and the steam turbine 51. The net power associated to the air compressor 36, the NOx compressor 40 and the tail gas expander 7 was 75.5 kW/h/t 100% HNO3. This power was produced by the steam turbine 51.
Therefore, when compared to the example 1, a net power of 39 kWh/t 100% HNO3(50%) was saved upon recirculating 24% of the tail gas.
Therefore, when compared to the example 2, a net power of 12 kWh/t 100% HNO3 (16%) was saved upon recirculating 42% of the tail gas.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21193029.2 | Aug 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/073637 | 8/25/2022 | WO |
