DESCRIPTION

Field of application

The invention relates to the field of synthesis of nitric acid. Particularly the invention relates to a nitric acid process provided with a novel heat recovery system.

Prior art

The industrial production of nitric acid involves the catalytic oxidation of ammonia over a suitable catalyst, which is typically a platinum/rhodium catalytic gauze. The catalytic oxidation is performed in the presence of an oxidant, typically by mixing a gaseous stream of ammonia NH3 with a pre-heated current of air, to yield a gaseous effluent containing nitric oxides NO, NO2. The gaseous effluent is then absorbed in water to produce nitric acid and a tail gas.

The catalytic oxidation of ammonia is carried out in a suitable ammonia oxidation reactor, also known as ammonia burner, that is supplied with ammonia and air.

In a nitric acid plant, ammonia is stored as a pressurized liquid and according to the process demand is evaporated and conveyed to the ammonia burner without requiring a dedicated compressor. On the contrary, air must be compressed prior to be conveyed to the ammonia burner. Typically, an ammonia burner operates at a pressure comprised between 4 and 12 barg.

The air compressor is typically a multi-stage compressor provided with multiple compressor stages that are configured to progressively increase the pressure of air from one stage to another.

In a nitric acid plant, the multistage compressor can be part of a turboset unit that includes an expander used to recover energy from the tail gas extracted from the absorption step and with a driver e.g. a steam turbine or an electrical motor which provides the balance of power required to operate the turboset unit which is not provided by the expander.

It is known that the air compression in a nitric acid plant requires a significant amount of power to run and in some plant configurations, intermediate cooling can be provided between each compression stage to remove heat developed from the compression so to make more efficient the compression operation and to reduce the power demand of the multistage compressor.

During the intermediate cooling, the air discharged from a compression stage is cooled by indirect heat transfer with a suitable cooling medium e.g. air or water (such as seawater when available). Typically, the cooling medium after removing heat from the air effluent from a compression stage is discharged outside the process.

Unfortunately, notwithstanding the technical solutions adopted e.g. the use of intermediate cooling between compression stages, the compression of air in a nitric acid plant still require a significant amount of power to run thus significantly contributing to the overall energy consumption of the plant.

Currently, there is a demand for nitric acid processes characterized by low energetic consumption and high energy efficiency.

US 3 977 832 discloses a system for control of a nitric acid plant. A plant for the production of nitric acid is described among others in WO 2018 162150 and WO 2020 035521.

Summary of the invention

The invention aims to improve the energy efficiency and to reduce the energy consumption of the conventional nitric acid processes.

Accordingly, one aspect of the present invention is a process for the synthesis of nitric acid according to the claims.

The process for the synthesis of nitric acid comprises: a catalytic oxidation step of an ammonia feed in the presence of an oxidant to yield a nitrogen oxidecontaining gas, an absorption step of said nitrogen oxide-containing gas in water to produce nitric acid and a tail gas.

In the process said oxidant is subjected to multiple compression steps prior to be conveyed to said catalytic oxidation step with said ammonia feed, said multiples compression steps comprise at least a first compression step and a second compression step and an inter-cooling stage is carried out between said first compression step and said second compression step.

Said intercooling stage comprises a first cooling step in which an effluent of said first compression step is cooled by indirect heat transfer with a first cooling medium selected from any one of the following: said tail gas effluent of said absorption step; a demineralised water stream; a trim cooling water and at least a portion of said ammonia feed.

In the invention, the heat removed from the effluent of a compression stage can be recovered and exploited in the process to warm-up other process fluids and/or to produce useful steam. Accordingly, the energy efficiency of the process is increased and the energy consumption of the process is reduced. This is a significant advantage over the prior art where the heat removed from air compression interstage is generally lost being transferred to cooling water. Among others, an aspect of the invention is the finding that process streams available in the nitric acid process, like e.g. the tail gas of the absorber or the ammonia feed, can be judiciously used as cooling mediums for inter-stage cooling in the oxidant (e.g. air) compression process. By using these process streams, the heat removed in the compression process remains within the process, therefore the process is more efficient energetically. Another object of the invention is a nitric acid plant arranged to operate with the hereinabove disclosed process.

Detailed description of the invention

The process uses a burner for the catalytic oxidation of an ammonia feed. The ammonia feed can be mixed with the oxidant prior to be fed to the burner or alternatively the ammonia feed and the oxidant can be injected directly into the burner as separate streams.

According to the invention, the oxidant is subjected to multiple compression steps before being supplied to the burner. The multiple compressor steps can be in the number of 2 to 6, preferably from 2 to 4, more preferably 2 or 3.

The multiple compression steps comprise at least a first compression step and a second compression step wherein between said first compression step and said second compression step an inter-cooling stage is carried out. In the inter-cooling step, the effluent of the first compression step is cooled before being supplied to the second compression step.

According to the invention said inter-cooling stage comprises at least a first cooling step with one of the following cooling mediums: a tail gas effluent of said absorption step, a demineralised water stream, the ammonia feed or a portion thereof.

According to a preferred embodiment of the invention, the inter-cooling stage further includes a second cooling step in which an effluent of said first cooling step is cooled by indirect heat transfer with a second cooling medium selected from any one of the following: a tail gas effluent of the absorption step, a demineralised water stream, a trim cooling water, at least a portion of the ammonia feed. The cooling medium used in said first cooling step and the cooling medium used in said second cooling step may be the same or different. According to an embodiment of the invention, said second cooling stage can be followed by a third cooling step in which an effluent of said second cooling step is cooled by indirect heat transfer with a cooling medium which is preferably trim cooling water.

The first cooling step, the second cooling step and when applicable the third cooling step are preferably carried out in sequence in separate heat exchangers, for example shell and tube heat exchangers.

According to various embodiments of the invention, an inter-cooling stage may comprise one or more cooling steps with the same or different cooling fluid. Preferably, the cooling steps of an inter-cooling stage may use different cooling fluids wherein the sequence of cooling fluids is in accordance with the temperature of the fluids, the hotter fluids being used first and the colder fluids being used subsequently. According to an interesting embodiment, the first cooling medium is the tail gas effluent of the absorption step.

Preferably when the tail gas is used as a cooling medium after the inter-cooling stage the so obtained heated tail gas can be subjected to an expansion step to recover power. The recovered power can be exploited in the nitric acid synthesis process. This is a particularly advantageous example of a valuable use of the heat removed in the inter-stage cooling of compression.

Preferably, when the intercooling stage comprises a first cooling step and a second cooling step, the first cooling medium is a demineralised water and the second cooling medium is a trim cooling water.

Preferably, after heat transfer, said demineralised water is supplied to the deaerator or to the steam drum where steam is generated avoiding or limiting a low pressure steam consumption for the demineralised water pre-heating.

According to an alternative embodiment, said first cooling medium is a tail gas and said second cooling medium is a trim cooling water. The trim cooling water is a particularly effective cooling because it is not dependent on the process demand e.g. it does not depend on the amount of ammonia that is oxidised in the burner (and tail gas withdrawn from the absorption step) and it does not depend on the amount of demineralized water flow. Therefore, it can be effectively used to compensate or to adjust the temperature of the effluent of a compression step prior to a subsequent compression step or prior to a catalytic oxidation step.

Furthermore, the trim cooling water can be advantageously used during the startup operation especially when the amount of cooling medium such as the demineralized water flow is not sufficient to obtain an effective cooling of the effluent of a compression step.

According to another embodiment, said first cooling medium is a demineralised water stream and said second cooling medium is at least a portion of said ammonia feed.

Preferably the indirect heat transfer between the effluent of a compression step with one of the above-mentioned cooling mediums can be carried out in a gasgas heat exchanger provided with a tube side and a shell side. Preferably, the cooling medium circulating in the shell-side of the heat exchanger while the effluent of a compression stage i.e. the oxidant, that is heated as a result of the compression operation, circulates in the tubes side of the heat exchanger.

Preferably, the temperature of said first cooling medium and when applicable of said second cooling medium and of said third cooling medium after the intercooling stage is increased of about 30 °C to 80 °C.

Preferably, the process for the synthesis of nitric acid is a mono-pressure nitric acid synthesis process. Preferably, the ammonia oxidation and the nitric acid absorption are carried out within the same pressure range. Preferably said nitric acid process is operated at a medium pressure comprised between 4 and 8 barg or at high-pressure comprised between 9 and 12 barg. The term barg denotes bar relative.

Preferably the oxidant used for the oxidation of ammonia is air or an oxygen- enriched air. More preferably the oxidant used for the oxidation of ammonia is air.

Preferably, the flow rate of air that is conveyed to the multiple compression steps is in the range of 10’000 Nm ³ /h to 165’000 Nm ³ /h.

Preferably, the temperature of said oxidant at the outlet of said multiple compression steps is in the range of 110 °C to 220 °C and the pressure of said oxidant at the outlet of said multiple compression steps is in the range of 4 to 12 barg.

According to a particular interesting application of the invention, said second cooling step and said third cooling step are carried out only during start-up operations.

The plant of the invention includes an ammonia burner arranged to perform a catalytic oxidation step of an ammonia feed in the presence of an oxidant, such as air, to yield a nitrogen oxide-containing gas, and an absorption step of said nitrogen oxide-containing gas in water to produce nitric acid and a tail gas.

The plant further includes a multiple-stage compressor arranged to raise the pressure of said oxidant prior to feed the oxidant to said catalytic oxidation step, the multiple-stage compressor includes at least a first compression stage and a second compression stage.

An inter-cooling stage is provided between said first compression stage and said second compression stage, wherein said inter-cooling stage comprises one or more heat exchanger arranged to cool the effluent of the first compression stage by indirect heat transfer with one or more of the following: a tail gas effluent of said absorption step, a demineralised water stream, a trim cooling water and at least a portion of said ammonia feed.

Accordingly, the inter-cooling heat exchanger (or each of the heat exchangers) has a hot side traversed by the effluent of the first compression stage and a cold side traversed by the respective cooling medium. Accordingly, the cold side has suitable connections with process lines of the plant. For example, when the tail gas is used, the cold side of the heat exchanger is suitably connected with the tail gas line.

A nitric acid plant according to the invention may be configured to implement the various embodiments which have been described above with reference to the process. For example, the inter-cooling section may include two or more heat exchangers arranged to use the same or different cooling medium.

In an embodiment, the inter-cooling section comprises: a first intercooling heat exchanger arranged between a first compression stage and a second compression stage and using a stream of tail gas from the absorption tower as a cooling medium; one or more heat exchanger(s) between subsequent pairs of compression stages, said one or more heat exchangers using, as a cooling medium, any of a demineralised water stream, trim cooling water, at least a portion of said ammonia feed.

In various embodiments of the invention, the demineralized water can be imported or can be taken, at least in part, from a condensate outlet of a steam turbine. Said water taken from a steam turbine condensate may have a temperature of 30 to 40 °C.

The tail gas is typically taken from top of an absorption column.

Transferring heat to the ammonia feed, or at least to a portion thereof, has the advantage of pre-heating the ammonia feed, which is generally in a liquid state and needs to be evaporated prior to feeding the ammonia burner.

The trim cooling water may be taken from a chilled water package of the nitric acid plant, for example a package arranged to cool the absorber. In some embodiments said chilled water package includes an absorption machine, such as Li-Br absorption machine.