WO2019122297A1

WO2019122297A1 - Systems and methods for removing water from fertilizers

Info

Publication number: WO2019122297A1
Application number: PCT/EP2018/086496
Authority: WO
Inventors: Arne KLAVENESS
Original assignee: Yara International Asa
Priority date: 2017-12-21
Filing date: 2018-12-21
Publication date: 2019-06-27

Abstract

Systems and methods for removing water from fertilizer melts by means of a system comprising a stripper stage. The stripper stage comprises a sweep gas inlet. The pressure in the stripper stage is regulated by controlling the flow rate of the sweep 5 gas received at the sweep gas inlet.

Description

SYSTEMS AND METHODS FOR REMOVING WATER FROM FERTILIZERS

TECHNICAL FIELD

The present invention is in the field of fertilizer manufacture, in particular in the field of removing water from fertilizer melts.

BACKGROUND

Nitrogen-containing fertilizers are widely used in agriculture for increasing crop yields. In the production of granular fertilizers, fertilizer melts may be dried to facilitate the formation of a solid product that can be easily handled.

Known methods for removing water from aqueous solutions of nitrogen-containing fertilizers are for instance described in US3157466, US2089945, and FR2058551. US3157466 discloses a method for manufacturing ammonium nitrate which involves reacting aqueous nitric acid with gaseous ammonia in a neutralizer which is maintained within a predetermined alkalinity range by recycling the gaseous effluent from the neutralizer, then concentrating the resultant ammonium nitrate solution in multi-stage evaporators to obtain substantially anhydrous ammonium nitrate and subsequently prilling or granulating the molten ammonium nitrate substantially devoid of water.

US2089945 relates to a process for the production of substantially dry, molten compounds from solutions of said compounds in volatile solvents, with the removal of the solvent by evaporation, and more particularly to the production of ammonium nitrate of relatively low water content from its aqueous solutions.

FR2058551 relates to a method for preparing ammonium nitrate by treating liquid HNO3 with gaseous NH₃. The resulting product is concentrated first in vacuum and secondly by passing a NH₃ counter-current through it. This gives a 99.8% dry product which when granulated requires no further drying before use as a fertilizer. The current of NH₃ is subsequently used in the reaction itself.

EP0329014 discloses a process for concentrating a urea solution by evaporation of water from the urea solution. First, a urea synthesis solution is formed in a synthesis zone. Following a first and a further decomposition step that is operated at between 4 and 40 bar, a urea solution is obtained and evaporated in at least two steps. In the first evaporation step, use is being made of the heat released upon condensation of the gas mixture obtained during the decomposition step that is operated at between 4 and 40 bar. SUMMARY

It is an object of the present invention to provide systems and methods for removing water from aqueous fertilizers solutions which have improved energy efficiency compared to the prior art systems and methods.

Accordingly, provided herein is a system for removing water from a fertilizer melt comprising a stripper stage comprising one or more evaporators, the stripper stage having a first end and a second end, the first end comprising a fertilizer melt inlet and an sweep gas outlet, the second end comprising a fertilizer melt outlet and an sweep gas inlet, wherein the one or more evaporators comprise a heat source for heating the fertilizer melt; wherein the stripper stage is configured for operating at a pressure P_ds and a temperature T _s, wherein the stripper stage is configured for receiving a sweep gas through the sweep gas inlet (122) at a flow rate Q_sg, and wherein the system further comprises a controller configured to regulate the pressure P_ds by controlling the flow rate Q_sg of the sweep gas received at the sweep gas inlet (122).

In some embodiments, the system further comprises a sweep gas recycling unit in fluid connection with the gas outlet and the sweep gas inlet, the sweep gas recycling unit being configured for receiving a gaseous stream comprising two or more components from the gas outlet, the two or more components comprising sweep gas and vapour; at least partially separating the sweep gas from the other components in the gaseous stream; and providing at least a part of the gaseous stream at a recycling rate R_r to the sweep gas inlet; wherein the controller is further configured to regulate the pressure P_ds by controlling the recycling rate R_r.

In some embodiments, the sweep gas comprises an inert gas, optionally wherein between 10 wt% and 100 wt%, between 20 wt% and 99 wt%, between 30 wt% and 80 wt%, between 40 wt% and 70 wt%, or between 60 wt% and 60 wt% of the sweep gas is an inert gas.

In some embodiments, the recycling rate is between 10 and 90 vol %, optionally between 35 and 65 vol%, based on the volumetric flow rate of the sweep gas.

In some embodiments, the sweep gas recycling unit is further configured for selectively removing one or more further components from the sweep gas.

In some embodiments, the system further comprises an energy recuperation device comprising an inlet and an outlet, the inlet of the energy recuperation device being in fluid connection with the gas outlet of the stripper stage.

In some embodiments, the outlet of the energy recuperation device is in fluid connection with the inlet of the sweep gas recycling unit. In some embodiments, the energy recuperation device comprises a condenser configured to condense at least a part of the vapour comprised in the gas mixture leaving the stripping stage; and/or a turbine configured for extracting work from the gas mixture leaving the stripping stage.

In some embodiments, the one or more evaporators are co current evaporators; or the one or more evaporators are counter current evaporators.

In some embodiments, the one or more evaporators are shell-and-tube evaporators comprising a shell and one or more tubes disposed within the shell, the one or more tubes being vertically disposed within the shell; the one or more tubes extending from the first end to the second end of the evaporator.

Further provided is a method for removing water from a fertilizer melt by means of a system comprising a stripper stage comprising one or more evaporators, the stripping stage having a first end and a second end, the first end comprising a fertilizer melt inlet and a sweep gas outlet, the second end comprising a fertilizer melt outlet and an sweep gas inlet, wherein the evaporators comprise a heat source for heating the fertilizer melt, the method comprising the steps: a. providing a stream of a fertilizer melt to the fertilizer melt inlet of the stripper stage; b. providing a stream of a sweep gas to the gas inlet of the stripper stage; c. contacting the fertilizer melt and the sweep gas in the stripper stage; d. withdrawing a gaseous stream comprising two or more components at the gas outlet of the stripper stage; e. withdrawing a stream of dried fertilizer at the fertilizer melt outlet of the stripper stage; wherein the stripper stage operates at a pressure P_ds and a temperature T_ds, wherein the stripper stage receives the sweep gas through the sweep gas inlet at a flow rate Q_sg, wherein the system comprises a controller, and wherein the method further comprises the step: regulating the pressure P_ds, by means of the controller, by controlling the flow rate Q_sg of the sweep gas received at the sweep gas inlet.

In some embodiments, the system further comprises a sweep gas recycling unit in fluid connection with the gas outlet and the sweep gas inlet of the stripper stage, wherein the sweep gas recycling unit receives the gaseous stream comprising two or more components from the gas outlet, the two or more components comprising sweep gas and vapour; at least partially separates the sweep gas from the other components in the gaseous stream; and provides at least a part of the gaseous stream at a recycling rate R_r to the sweep gas inlet; wherein the controller further regulates the pressure P_ds by controlling the recycling rate R_r.

In some embodiments, the system used in the method is a system as described above. In some embodiments, the stripper stage comprises a cascade of evaporators, and wherein the controller is further configured for maintaining the pressure in each evaporator of the cascade of evaporators at a value which is equal to the pressure in the other evaporators (130), within a margin of error of 10%.

Further provided is the use of a system as described above for removing water from an aqueous solution of a nitrate-containing fertilizer such as ammonium nitrate, potassium nitrate, urea, an NP fertilizer, or an NPK fertilizer.

DESCRIPTION OF THE FIGURES

The following description of the figures of specific embodiments of the invention is only given by way of example and is not intended to limit the present explanation, its application or use. In the drawings, identical reference numerals refer to the same or similar parts and features.

Figs. 1 to 4 show embodiments of a system for drying an aqueous fertilizer as provided herein. The system comprises a stripper stage (100).

The following reference numerals are used in the description and figures:

100 - stripper stage; 1 10 - first end of stripper stage; 1 11 - fertilizer melt inlet; 1 12— gas outlet; 120 - second end of stripper stage; 121 - fertilizer melt outlet; 122 - sweep gas inlet; 130 - evaporator; 131 - evaporator first end; 132 - evaporator second end; 133 - evaporator fertilizer melt inlet; 134 - evaporator fertilizer melt outlet; 135 - evaporator sweep gas inlet; 136 - evaporator gas outlet; 200 - energy recuperation device; 300 - sweep gas recycling unit; 302 - sweep gas outlet of sweep gas recycling unit; 303 - off gas outlet of sweep gas recycling unit; 310 - mixer.

DESCRIPTION OF THE INVENTION

As used below in this text, the singular forms“a”,“an”,“the” include both the singular and the plural, unless the context clearly indicates otherwise.

The terms“comprise”,“comprises” as used below are synonymous with“including”, “include” or“contain”, “contains” and are inclusive or open and do not exclude additional unmentioned parts, elements or method steps. Where this description refers to a product or process which“comprises” specific features, parts or steps, this refers to the possibility that other features, parts or steps may also be present, but may also refer to embodiments which only contain the listed features, parts or steps. 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 term“approximately” as used when referring to a measurable value, such as a parameter, an amount, a time period, and the like, is intended to include variations of +/- 10% or less, preferably +/-5% or less, more preferably +/-1% or less, and still more preferably +/-0.1 % or less, of and from the specified value, in so far as the variations apply to the invention disclosed herein. It should be understood that the value to which the term“approximately” refers per se has also been disclosed.

All references cited in this description are hereby deemed to be incorporated in their entirety by way of reference.

Unless defined otherwise, all terms disclosed in the invention, including technical and scientific terms, have the meaning which a person skilled in the art usually gives them. For further guidance, definitions are included to further explain terms which are used in the description of the invention.

The terms“aqueous fertilizer solution” and“fertilizer melt” are used interchangeably herein.

Provided herein is a system for removing water from fertilizer melt. The system comprises a stripper stage. The stripper stage comprises one or more evaporators and has a first end and a second end. The first end comprises a fertilizer melt inlet and a gas outlet. The second end comprises a fertilizer melt outlet and a sweep gas inlet.

The evaporators each comprise a heat source for heating the fertilizer melt. For example, the heat source may be steam or microwave radiation.

The heat source of the evaporators may be, for example, a condensate or another liquid or gas. Using a liquid can be advantageous because it allows using temperature gradients. When the system is configured for maintaining a temperature gradient in the one or more evaporators, the system is preferably configured for maintaining the side of the stripper stage in which more water is in the liquid phase at a lower temperature than the side at which there is less water in the liquid phase. When a shell-and tube stripper is used as part of the stripping stage, the heat source may be, for example, steam flowing around the tubes of the shell-and tube-stripper. The stripper stage is configured for operating at a pressure P_ds and a temperature T_ds. The stripper stage is further configured for receiving a sweep gas through the sweep gas inlet at a flow rate Q_sg, and the system further comprises a controller configured to regulate the pressure P_ds by controlling the flow rate Q_sg of the sweep gas received at the sweep gas inlet.

The controller is preferably configured to regulate the pressure of the sweep gas to a total pressure which is higher than the vapour pressure of water in the fertilizer melt at its temperature in the stripper stage. Also, the controller is preferably configured for maintaining the partial pressure of water in the gas phase at a lower value than the vapour pressure of water in the fertilizer melt. Typically, the partial pressure of vapour at the sweep gas inlet is near 0 bar, for example lower than 0.1 bar, and the partial pressure of vapour at the gas outlet of the stripping stage is close to the total pressure, for example 99% of the total pressure. Preferably, the pressure regulator is configured for regulating the pressure in the stripper stage by means of the sweep gas flow rate and, when sweep gas is recycled, the recycling rate (see below).

Preferably, controller is configured to regulate the pressure by removing gas, e.g. by fully condensing the gas or pumping it away from the stripper stage.

These systems have excellent energy efficiency while still being able to effectively dry fertilizer melts. In particular, the controllers in the present systems allow controlling operating conditions (temperature, pressure) such that the gas and liquid phases are near equilibrium throughout the stripping stage. This in turn ensures that evaporation occurs as reversible as possible, which increases the system’s energy efficiency. In the present systems, heat can be recovered easily because the present systems allow the formation of high-pressure steam. In some embodiments, the present systems and processes allow producing 20 bar steam, compared to the production steam at a pressure of 2 bar or less with the prior art drying systems.

In some embodiments, the sweep gas comprises an inert gas. Optionally, between 10 wt% and 100 wt%, between 20 wt% and 99 wt%, between 30 wt% and 80 wt%, between 40 wt% and 70 wt%, or between 60 wt% and 60 wt% of the sweep gas is an inert gas.

In some embodiments, the sweep gas comprises reactive components such as NH₃. When nitrates are dried, NH₃ is preferably added to the sweep gas which allows controlling the pH of the fertilizer melt. In some embodiments, between 0.1 wt% and 99.9 wt%, between 1 wt% and 99 wt%, between 10 wt% and 90 wt%, between 20 wt% and 80 wt%, between 30 wt% and 70 wt%, or between 40 and 60 wt% of the sweep gas consists of a reactive component.

In some embodiments, the sweep gas comprises one or more further components that were evaporated from the melt. The nature of these components depends on the fertilizer stream which is treated.

In some embodiments, the system further comprises a sweep gas recycling unit in fluid connection with the gas outlet and the sweep gas inlet. The sweep gas recycling unit is configured for receiving a gaseous stream comprising two or more components from the gas outlet. The two or more components include sweep gas and vapour. The recycling unit is further configured for at least partially separating the sweep gas from the other components in the gaseous stream. In addition, the recycling unit is configured for providing at least a part of the gaseous stream at a recycling rate R_r to the sweep gas inlet. The controller is further configured to regulate the pressure P_ds by controlling the recycling rate R_r. The term“recycling rate” as used herein refers to the fraction of sweep gas which is recycled.

In some embodiments, part of the vapour exiting the stripper stage may be recycled along with the sweep gas. The dryer the sweep gas, the more energy intensive removing additional vapour from the sweep gas gets. Accordingly, it can be advantageous in terms of energy efficiency to recycle sweep gas which comprises some residual vapour, e.g. vapour in a concentration between 0.1 wt% to 1 wt%. When the recycled sweep gas comprises vapour, the controller is preferably further configured to regulate the pressure P_ds by controlling the amount of vapour in the recycled sweep gas.

Additionally, the gas exiting the stripper stage may comprise further components apart from sweep gas and water vapour. An example of such a further component is ammonia. The further components are advantageously recycled along with the sweep gas. Additionally, the further components may be removed from the sweep gas, for example by means of a scrubber positioned upstream from the sweep gas recycling unit, and downstream from the gas outlet of the stripper stage. When the recycled sweep gas comprises further components, the controller is preferably further configured to regulate the pressure P_ds by controlling the amount of further components in the recycled sweep gas.

In some embodiments, the system further comprises an energy recuperation device comprising an inlet and an outlet. The inlet of the energy recuperation device is in fluid connection with the gas outlet of the stripper stage. When the system also comprises a sweep gas recycling unit, the outlet of the energy recuperation device is in fluid connection with the inlet of the sweep gas recycling unit.

Preferably, the energy recuperation device is configured for recovering more than 80%, more preferably more than 90% of the enthalpy of evaporation comprised in the mixture comprising vapour and sweep gas which leaves the stripper stage.

The amount of energy which can be recovered from the gaseous stream leaving the stripper stage depends on the particular circumstances in which the present systems are operated. One trend is that when the fertilizer melts from which water is removed comprise more water, more energy can be recovered from the gaseous stream leaving the stripper stage.

In some embodiments, the energy recuperation device comprises a heat exchanger. The heat exchanger comprises a heat exchange fluid inlet and a heat exchange fluid outlet. During normal operation, heat exchange fluid flows into the heat exchanger via the heat exchange fluid inlet and leaves the heat exchanger via the heat exchange fluid outlet. Examples of suitable heat exchange fluids are liquid water and steam.

In some embodiments, the energy recuperation device may be based on an organic rankine cycle. Organic rankine cycle-based energy recuperation devices are particularly suitable for extracting work from the gas mixture exiting the stripper stage.

In some embodiments, the energy recuperation device comprises a condenser configured to condense at least a part of the vapour comprised in the gas mixture leaving the stripping stage. This type of heat exchanger allows for especially efficient recovery of the energy in the moisture-laden sweep gas which is collected at the outlet of the stripper stage.

In some embodiments, the energy recuperation device comprises a turbine configured for extracting work from the gas mixture leaving the stripping stage.

In some embodiments, the energy recuperation device comprises a further reactor. This allows using the heat in the steam exiting the stripper stage to drive an endothermic chemical reaction.

In some embodiments, the system further comprises a scrubber. The scrubber comprises an inlet and an outlet. When the system comprises a scrubber, the outlet of the energy recuperation device is preferably in fluid connection with the inlet of the scrubber. Also, the outlet of the scrubber is preferably in fluid connection with the recycling unit.

The scrubber is highly effective at removing harmful components from the gas stream leaving the stripping stage.

When the present systems are used for drying nitrophosphate fertilizer melts, the gas stream leaving the stripping stage comprises ammonia. The scrubber can be advantageously used for removing ammonia from this gas stream before the sweep gas is recycled.

Depending on the aqueous solution from which water is removed, different evaporators are preferred. When the present systems and method are applied to removing water from aqueous solutions of urea, various types of strippers may be used, e.g. tray strippers or packed bed strippers. When the present systems and methods are used for removing water from aqueous solutions of NP or NPK fertilizer streams, shell-and-tube strippers are preferably used. NP and NPK fertilizer streams are particularly difficult to handle because of common problems related to shear thinning and fouling. Shell-and-tube strippers are particularly effective at dealing with such streams.

Shell-and-tube evaporators are evaporators that comprise a shell and one or more tubes disposed within the shell, the one or more tubes being vertically disposed within the shell; the one or more tubes extending from the first end to the second end of the evaporator. Preferably, the one or more shell-and-tube evaporators operate in counter current, and the one or more tubes are in fluid connection with the fertilizer melt inlet and the gas outlet at the first end; and the one or more tubes are in fluid connection with the fertilizer melt outlet and the gas inlet at the second end of the evaporator.

In some embodiments, the one or more shell-and-tube evaporate in co current, and in each evaporator, the one or more tubes are in fluid connection with the fertilizer melt inlet and the gas inlet at the first end and the one or more tubes are in fluid connection with the fertilizer melt inlet and the gas outlet at the second end.

The space between the tubes and the shell is typically arranged for allowing heating of the content of the tubes. In some embodiments, the space between the tubes and the shell is empty, which allows the passage of steam for heating the content of the tubes. Alternatively, the space between the shell and the tubes may comprise an electrical heater. Preferably, the tubes of the shell-and-tube evaporators are heated by means of steam.

In some embodiments, the stripper stage comprises one and not more than one evaporator. In these embodiments, the first end of the evaporator corresponds to the first end of the stripper stage, and the second end of the evaporator corresponds to the second end of the stripper stage. In these embodiments, the evaporator is preferably configured for operating in counter current. Preferably, the evaporator is a shell-and-tube stripper in these embodiments.

When a single evaporator is used, counter current inert gas flow is preferably employed. When multiple evaporators are used, co-or counter current may be used in the individual evaporators. When multiple co-current evaporators are used, the stripping stage as a whole preferably operates in counter flow by arranging the individual evaporators according to the stepwise counter-current principle. In the stepwise counter current principle, co-current flow occurs in the individual evaporators, but the evaporators are arranged in a counter current cascade (see example 4, and Fig. 4).

In some embodiments, the system comprises a cascade of evaporators. In the cascade the fertilizer melt inlet and outlet, and the gas inlet and gas outlet of subsequent evaporators are fluidly connected. The use of a cascade of evaporators can enhance the degree of water removal which can be obtained.

The cascade of evaporators is a sequence of evaporators being arranged along a path of aqueous fertilizer flow, and extending throughout the stripper stage. The first end of the stripper stage corresponds to the first end of the evaporator which is the most upstream in the fertilizer melt flow. The second end of the stripper stage corresponds to the second end of the evaporator which is the most downstream in the fertilizer melt flow.

In some embodiments, the sweep gas inlet of each evaporator is fluidly connected with a sweep gas supply. This allows providing fresh sweep gas to each evaporator. Preferably, the sweep gas inlet and outlet of subsequent evaporators in the cascade are fluidly connected. Accordingly, gas exiting from evaporator can be partially or wholly used as sweep gas for the next evaporator in the cascade, which increases the energy efficiency of the stripper stage.

In some embodiments, the system is configured for operating with a sweep gas that comprises a reactive component such as NH₃. In such embodiments, the reactive component of the sweep gas is preferably replenished between the gas outlet and gas inlet of subsequent evaporators.

When the stripper stage comprises a plurality of co-current evaporators, the evaporators are preferably bubble columns, and are arranged in a step-wise counter- current configuration. In other words, the stripper stage comprises a plurality of bubble columns. In each of the bubble columns, flow is co-current, but the bubble columns are arranged in a counter current cascade. Such stripper stages have an excellent mass transfer between the gas phase and the liquid phase. Bubble columns allow excellent removal of water from fertilizer melts. They allow obtaining increased heat and mass transfer, and thereby allow decreasing the size of the evaporators.

In some embodiment, the cascade comprises a plurality of counter current evaporators which are arranged in a counter current configuration.

In some embodiments, the stripper stage is configured to add NH₃ to the sweep gas after each evaporator. This allows raising the pH of the fertilizer melt, or maintaining the pH of the fertilizer melt at an (approximately) constant value throughout the stripper stage.

In some embodiments, the system further comprises a heater configured for preheating the fertilizer melt. The heater is in fluid connection with the fertilizer melt inlet of the stripper stage. Heating the fertilizer melt before it is brought to the stripper stage may enhance efficiency of removing water from the fertilizer melt.

Preferably, the system further comprises a heater configured for preheating the sweep gas before it is fed to the stripper stage. This increases the system’s energy efficiency, and it allows reducing the size of the evaporators in the stripper stage.

In some embodiments, the stripping stage is configured to operate at a uniform temperature, the term“uniform temperature” indicating the presence of temperature variations of less than 10% throughout the stripping stage.

In some embodiments, the stripping stage is configured to operate at a non-uniform temperature. This can be accomplished, for example, by means of counter current and/or segmented heating. When a shell-and-tube stripper is used, a non-uniform temperature may be obtained by means of a heat exchange liquid flowing between the tubes and the shell.

When the fertilizer stream to be dried comprises nitrates, the temperature of the fertilizer stream is preferably between 170°C and 190°C, between 180°C and 190°C, or between 185°C and 190°C. At these temperatures, nitrate-based aqueous fertilizers are efficiently and safely dried.

When the fertilizer stream comprises at least 70 wt%, 80 wt%, 90 wt%, or 99 wt% of phosphate fertilizer based on the total fertilizer content of the fertilizer stream, the temperature of the fertilizer stream preferably is up to 900°C, up to 800°C, between 600 and 700°C, between 500 and 600°C, between 300 and 400°C, or between 200 and 300°C.

When the fertilizer stream comprises at least 70 wt%, 80 wt%, 90 wt%, or 99 wt% of urea based on the total fertilizer content of the fertilizer stream, the temperature of the fertilizer stream is preferably 150-190°C.

These temperature ranges ensure that drying occurs at a temperature below the decomposition temperature of the fertilizer being dried, such that the fertilizer can be dried efficiently and in a safe fashion.

Further provided is a fertilizer manufacturing system comprising a system for removal of water from a fertilizer melt as provided herein. The fertilizer manufacturing system is preferably configured for manufacturing an ammonium nitrate, urea, NP, potassium nitrate, or NPK fertilizer. Aqueous solutions of these compounds can be highly efficiently dried by means of the systems provided herein.

Preferably, the fertilizer manufacturing system comprises a prilling or granulating device for obtaining a plurality of fertilizer granules. Thus dried fertilizer melts can be converted to a form which can easily be applied to fields.

Preferably, the fertilizer manufacturing system further comprises a vacuum evaporation device configured for drying fertilizer granules below a water content of 5 wt%, 4, wt%, 2 wt%, 1wt%, 0.5 wt%, 0.2 wt%, or 0.1 wt%. Vacuum evaporation devices can be used for further reducing the water content of fertilizer granules in a very energy-efficient way.

Further provided is a method for removing water from a fertilizer melt by means of a system comprising a stripper stage comprising one or more evaporators. Preferably, the system is a system as described above. The stripper stage has a first end and a second end. The first end comprises a fertilizer melt inlet and a gas outlet. The second end comprises a fertilizer melt outlet and a sweep gas inlet. The evaporators comprise a heat source for heating the fertilizer melt. The method comprises the step of providing a stream of fertilizer melt to the fertilizer melt inlet of the stripper stage. Also, a stream of a sweep gas is provided to the gas inlet of the stripper stage. In the stripper stage, the fertilizer melt and the sweep gas are contacted, such that water is evaporated from the fertilizer melt, thus forming a gaseous mixture comprising sweep gas and water vapour.

This gaseous mixture is withdrawn as a gaseous stream at the gas outlet of the stripper stage. The gaseous stream comprises two or more components. The two or more components preferably include sweep gas and vapour. Preferably, the gaseous stream comprises at least 60% vapour, at least 70% vapour, at least 80% vapour, at least 90% vapour, or at least 95% vapour. In the stripper stage, the sweep gas takes up moisture from the fertilizer melt. Accordingly, a gaseous flow comprising vapour and sweep gas is gradually formed while the sweep gas flows counter current to the fertilizer melt. Also, this process results in the formation of a stream of dried fertilizer. Simultaneously, a stream of dried fertilizer is withdrawn at the fertilizer melt outlet of the stripper stage. The stripper stage operates at a pressure P_ds and a temperature T_ds. Also, the stripper stage receives the sweep gas through the sweep gas inlet at a flow rate Q_sg. The system further comprises a controller. The method further comprises the step of regulating the pressure P_ds, by means of the controller, by controlling the flow rate Q_sg of the sweep gas received at the sweep gas inlet. Preferably, the controller maintains the pressure of the sweep gas at a total pressure which is higher than the vapour pressure of water in the fertilizer melt at its temperature in the stripper stage. Preferably, the controller maintains the partial pressure of water in the gas phase lower than the vapour pressure of water in the fertilizer melt. In some embodiments, the partial pressure of vapour at the sweep gas inlet is near 0 bar, for example lower than 0.1 bar, and the partial pressure of vapour at the gas outlet of the stripping stage is close to the total pressure, for example at least 99% of the total pressure.

In some embodiments, the system further comprises a sweep gas recycling unit in fluid connection with the gas outlet and the sweep gas inlet of the stripper stage, wherein the sweep gas recycling unit receives the gaseous stream comprising two or more components from the gas outlet, the two or more components comprising sweep gas and vapour; at least partially separates the sweep gas from the other components in the gaseous stream; and provides at least a part of the gaseous stream at a recycling rate R_r to the sweep gas inlet; wherein the controller further regulates the pressure P_ds by controlling the recycling rate R_r. This further enhances the energy efficiency of the method, and it reduces the material cost, as less sweep gas needs to be provided.

In some embodiments, the sweep gas comprises one or more components which react with the fertilizer melt. Regardless of whether or not one or more components of the sweep gas react with the fertilizer melt, the stripper stage is preferably configured for maintaining a significant partial pressure of the sweep gas in the gas phase throughout the stripper stage, e.g. a partial pressure of 0.1 to 0.4 bar. This increases the efficiency by which the stripper stage removes water from the fertilizer melt. If the vapour pressure of water in the fertilizer melt exceeds the total pressure at one or more locations in the stripper stage, the stripper stage is preferably configured for maintaining a significant partial pressure of the sweep gas in the gas phase up to the point in the stripper stage where the vapour pressure of water in the aqueous fertilizer melt exceeds the absolute pressure.

In some embodiments, the pressure regulator regulates the pressure of the sweep gas to a pressure which is higher than, or equal to the vapour pressure of water in the fertilizer melt. The vapour pressure of water in the fertilizer melt decreases as the fertilizer melt flows through the stripper.

Preferably, the pressure is regulated by removing gas by fully condensing the gas or pumping it away from the stripper stage. One example of a gas which might be condensed is C0₂. In some embodiments, the absolute pressure in the stripper stage is between 1 and 100 bar, or between 3 and 50 bar, or between 6 and 10 bar, or between 2 and 8 bar. The absolute pressure in the stripper stage preferably corresponds to the vapour pressure of water in the melt which is stripped, at the temperature and water concentration at which the melt enters the stripping stage. Such high operating pressures allow converting vapour in the gas which is extracted from the stripping stage to high quality steam, which can be used as an energy source such that the overall energy efficiency of the process is enhanced.

Preferably, the flow rate of the gas phase in the stripping stage is higher at the gas outlet compared to the sweep gas inlet. For example, the flow rate at the gas outlet is 2 to 20, 4 to 16, or 10 times higher than the flow rate at the sweep gas inlet. The stripping stage preferably operates at a uniform pressure and temperature, with the term“uniform” indicating variations less than 10%. In order to accommodate the water which evaporates from the fertilizer melt, the flow rate increases towards the gas outlet.

In some embodiments, the fertilizer is ammonium nitrate, urea, an NP fertilizer, potassium nitrate, or an NPK fertilizer.

In some embodiments, the fertilizer melt is supplied at a flow rate between 100 and 200 tons per hour.

In some embodiments, the water content of the fertilizer melt is between 10 and 40 wt%, or between 15 wt% and 30 wt% at the fertilizer melt inlet of the stripper stage.

In some embodiments, the fertilizer melt is preheated by means of a heater before it is provided to the stripper stage. Preferably, heat from the energy recuperation device is used for preheating the fertilizer melt. This increases the stripping stage’s efficiency by increasing the vapour pressure of the aqueous fertilizer. Also, some fertilizer streams are solid at atmospheric conditions. Such fertilizer streams can be heated to turn them into liquids before being provided to the stripper stage.

In some embodiments, the sweep gas is chosen from the list consisting of C0₂, 0₂, N₂, He, Ar, air, flue gas, and mixtures thereof.

In some embodiments, the sweep gas comprises one or more reactive components such as NH₃, C0₂, and plant nutrient precursor gases.

In some embodiments, the sweep gas comprises an inert gas. In some embodiments, the sweep gas comprises 10 to 20 wt% inert gas, or 20 to 40 wt% inert gas, or 40 to 60 wt% inert gas, or 60 to 80 wt% inert gas, or 80 to 90 wt% inert gas, or 90 to 100 wt% inert gas. Preferably, the inert gas is chosen from the list consisting of C0₂, 0₂, N₂, He, and Ar. These sweep gases have in common that they do not react with the fertilizer melt or with the dried fertilizer stream.

Note that C0₂ is listed as both a reactive and inert gas. Depending on the aqueous fertilizer stream which is dried, C0₂ behaves as an inert gas or as a reactive gas.

In some embodiments, the gas mixture exiting the stripper stage is scrubbed by means of the scrubber. Preferably, the gas mixture is scrubbed after it has passed through an energy recuperation device, and before it is passed through a sweep gas recycling unit.

Preferably, the sweep gas is preheated before it is fed to the stripper stage. This increases the system’s energy efficiency, and it allows reducing the size of the evaporators in the stripper stage.

In some embodiments, the stripper stage comprises a cascade of evaporators. Preferably, the controller maintains the pressure in each evaporator of the cascade of evaporators at a value which is equal to the pressure in the other evaporators, within a margin of error of 10%.

In some embodiments, the water content of the dried fertilizer is between 0.1 wt% and 8 wt%, between 0.2 wt% and 0.4 wt%, or between 2 wt. % and 4 wt. % at the fertilizer melt outlet of the stripper stage. Preferably, gas mixture extracted from the stripper stage comprises less than 10% inert gas.

In some embodiments, the method further comprises the step of prilling or granulating the dried fertilizer, thereby obtaining a plurality of fertilizer granules. In some embodiments, the fertilizer granules are subjected to vacuum evaporation. This allows further reducing their water content.

In some embodiments, the fertilizer melt is processed in the system comprising the stripper stage until the water content of the fertilizer melt is in the range of 2 to 4 wt%. Then, the fertilizer melt is prilled or granulated, the resulting fertilizer granules are subsequently subjected to vacuum evaporation to further reduce their water content. This two-step process allows removing water from fertilizers in a very energy-efficient way. Further provided is the use of a system and/or fertilizer manufacturing plant as provided herein for removing water from melts of ammonium nitrate, urea, nitro phosphate, and/or potassium nitro phosphate. Aqueous solutions of these compounds can be dried very efficiently by means of the present systems and methods.

EXAMPLES

Example 1

In a first example, reference is made to Fig. 1 which shows an embodiment of a system for removing water from an aqueous fertilizer as provided herein. It comprises a stripper stage (100) which has a first end (1 10) and a second end (120). The stripper stage (100) comprises one or more evaporators. The first end (110) comprises a fertilizer melt inlet (1 11 ) and a gas outlet (1 12). The second end of the stripper stage (100) comprises a sweep gas inlet (122) and a fertilizer melt outlet (121 ). Sweep gas is provided to the sweep gas inlet (122) and dried fertilizer is extracted from the fertilizer melt outlet (121 ).

A fertilizer melt is provided to the fertilizer melt inlet (1 11 ) and a gaseous mixture comprising mostly vapour (e.g. 90-95 wt%) and sweep gas (e.g. 5-100 wt%) is extracted from the gas outlet (1 12). Depending on the aqueous fertilizer stream being treated, the gaseous mixture may comprise additional components such ammonia or carbon dioxide. The gas outlet (1 12) is fluidly connected to an energy recuperation device (200). In the energy recuperation device (200), heat of evaporation in the sweep gas is recovered. Suitable energy recuperation devices include heat exchangers and turbines.

The energy recuperation device (200) is further fluidly connected with a sweep gas recycling unit (300). In the sweep gas recycling unit (300), sweep gas is recovered from the gas mixture that was extracted from the first end (1 10) of the stripper stage. The recovered sweep gas is mixed in a mixer (310) with fresh sweep gas which is supplied via a sweep gas supply line (123). The mixer (310) is configured for providing an amount of fresh sweep gas via the sweep gas supply line (123) that corresponds to the amount of sweep gas that was not recoverable by means of the sweep gas recycling unit (300). The sweep gas exiting the mixer (310) is fed to the sweep gas inlet (122) of the stripper stage (100). Example 2

In a second example, reference is made to Fig. 2. Fig. 2 shows an embodiment of a system for removing water from an aqueous fertilizer according to example 1. In the system of Fig. 2, the stripper stage (100) comprises one and only one evaporator (130). The first end (131 ) of the evaporator (130) corresponds to the first end (110) of the stripper stage (100). The second end (132) of the evaporator corresponds to the second end (120) of the stripper stage (100).

Example 3

In a third example, reference is made to Fig. 3. Fig. 3 shows an embodiment of a system for removing water from an aqueous fertilizer according to example 1. In the system of Fig. 3, the stripper stage comprises three evaporators (130). The three evaporators (130) are arranged in a cascade through which sweep gas and fertilizer melt flow counter current. Each evaporator (130) comprises a first end (131 ) and a second end (132). The sweep gas inlets (135) and gas outlets (136) of adjacent evaporators (130) in the cascade are in fluid connection. Similarly, the fertilizer melt inlets (133) and fertilizer melt outlets (134) of adjacent evaporators (130) in the cascade are in fluid connection. The first end (131 ) of the evaporator (130) which is most upstream in the fertilizer flow corresponds to the first end (1 10) of the stripper stage. The second end (132) of the evaporator (130) which is most downstream in the fertilizer flow corresponds to the second end (120) of the stripper stage. Accordingly, the fertilizer melt outlet (134) and sweep gas inlet (135) of the evaporator (130) which is most downstream in the fertilizer flow corresponds to the fertilizer melt outlet (121 ) and the sweep gas inlet (122) of the of the stripper stage. Also, the fertilizer melt inlet (133) and the gas outlet (136) of the evaporator (130) which is most upstream in the fertilizer flow respectively correspond to the fertilizer melt inlet (1 11 ) and the gas outlet (112) of the stripper stage.

The gas outlet (112) of the stripper stage is in fluid connection with an energy recuperation device (not shown). The energy recuperation device is arranged to extract the enthalpy of evaporation from the gas mixture exiting the stripper stage. After this energy has been recovered from the sweep gas, the sweep gas is recycled.

Example 4

In a fourth example, reference is made to Fig. 4. Fig. 4 shows an embodiment of a stripper stage for use in the present systems for drying aqueous fertilizers. In particular, the stripping stage comprises a stepwise counter current cascade of evaporators (130). In other words, co-current flow of gas and liquid phases occurs within each evaporator (130), but the evaporators (130) themselves are arranged in a way that ensures counter current flow at the level of the cascade. This is accomplished as follows:

Each evaporator (130) comprises a first end (131 ) and a second end (132). The sweep gas inlets (135) and gas outlets (136) of adjacent evaporators (130) in the cascade are in fluid connection. Similarly, the fertilizer melt inlets (133) and fertilizer melt outlets (134) of adjacent evaporators (130) in the cascade are in fluid connection. For each evaporator (130), the first end (131 ) comprises a fertilizer melt inlet (133) and a sweep gas inlet (135), and the second end (132) comprises a fertilizer melt outlet (134) and a gas outlet (136).

The first end (131 ) of the evaporator (130) which is most upstream in the fertilizer flow corresponds to the first end (1 10) of the stripper stage. The second end (132) of the evaporator (130) which is most downstream in the fertilizer flow corresponds to the second end (120) of the stripper stage. Accordingly, the fertilizer melt outlet (134) and sweep gas inlet (135) of the evaporator (130) which is most downstream in the fertilizer flow respectively correspond to the fertilizer melt outlet (121 ) and the sweep gas inlet (122) of the of the stripper stage. Also, the fertilizer melt inlet (133) and the gas outlet (136) of the evaporator (130) which is most upstream in the fertilizer flow respectively correspond to the fertilizer melt inlet (1 11 ) and the gas outlet (112) of the stripper stage.

Claims

1. A system for removing water from a fertilizer melt comprising a stripper stage (100) comprising one or more evaporators (130), the stripper stage (100) having a first end

(1 10) and a second end (120), the first end (110) comprising a fertilizer melt inlet

(1 1 1 ) and an sweep gas outlet (1 12), the second (120) end comprising a fertilizer melt outlet (121 ) and an sweep gas inlet (122), wherein the one or more evaporators (130) comprise a heat source for heating the fertilizer melt; wherein the stripper stage is configured for operating at a pressure P_ds and a temperature T_ds, wherein the stripper stage is configured for receiving a sweep gas through the sweep gas inlet (122) at a flow rate Q_sg, and wherein the system further comprises a controller configured to regulate the pressure P_ds by controlling the flow rate Q_sg of the sweep gas received at the sweep gas inlet (122).

2. The system according to claim 1 further comprising a sweep gas recycling unit in fluid connection with the gas outlet and the sweep gas inlet, the sweep gas recycling unit being configured for

receiving a gaseous stream comprising two or more components from the gas outlet, the two or more components comprising sweep gas and vapour;

at least partially separating the sweep gas from the other components in the gaseous stream; and

providing at least a part of the gaseous stream at a recycling rate R_r to the sweep gas inlet;

wherein the controller is further configured to regulate the pressure P_ds by controlling the recycling rate R_r.

3. The system according to claim 1 or 2 wherein the sweep gas comprises an inert gas, optionally wherein between 10 wt% and 100 wt%, between 20 wt% and 99 wt%, between 30 wt% and 80 wt%, between 40 wt% and 70 wt%, or between 60 wt% and 60 wt% of the sweep gas is an inert gas.

4. The system according to claim 2 or 3 wherein the recycling rate is between 10 and 90 vol %, optionally between 35 and 65 vol%, based on the volumetric flow rate of the sweep gas.

5. The system according to any one of claims 2 to 4 wherein the sweep gas recycling unit is further configured for selectively removing one or more further components from the sweep gas.

6. The system according to any one of claims 1 to 5 further comprising an energy recuperation device comprising an inlet and an outlet, the inlet of the energy recuperation device being in fluid connection with the gas outlet of the stripper stage (100).

7. The system according to claim 6 wherein the outlet of the energy recuperation device is in fluid connection with the inlet of the sweep gas recycling unit.

8. The system according to claim 6 or 7 wherein the energy recuperation device comprises a condenser configured to condense at least a part of the vapour comprised in the gas mixture leaving the stripping stage; and/or a turbine configured for extracting work from the gas mixture leaving the stripping stage.

9. The system according to any one of claims 1 to 8 wherein the one or more evaporators are co current evaporators; or wherein the one or more evaporators are counter current evaporators.

10. The system according to any one of claims 1 to 8 wherein the one or more evaporators (130) are shell-and-tube evaporators comprising a shell and one or more tubes disposed within the shell, the one or more tubes being vertically disposed within the shell; the one or more tubes extending from the first end (131 ) to the second end (132) of the evaporator (130).

1 1. A method for removing water from a fertilizer melt by means of a system comprising a stripper stage (100) comprising one or more evaporators (130), the stripping stage (100) having a first end (1 10) and a second end (120), the first end (1 10) comprising a fertilizer melt inlet (1 1 1 ) and a sweep gas outlet (112), the second (120) end comprising a fertilizer melt outlet (121 ) and an sweep gas inlet (122), wherein the evaporators (130) comprise a heat source for heating the fertilizer melt, the method comprising the steps:

a. providing a stream of a fertilizer melt to the fertilizer melt inlet (1 11 ) of the stripper stage (100); b. providing a stream of a sweep gas to the gas inlet (122) of the stripper stage (100);

c. contacting the fertilizer melt and the sweep gas in the stripper stage (100); d. withdrawing a gaseous stream comprising two or more components at the gas outlet (1 12) of the stripper stage (100);

e. withdrawing a stream of dried fertilizer at the fertilizer melt outlet (121 ) of the stripper stage (100);

wherein the stripper stage operates at a pressure P_ds and a temperature T_ds, wherein the stripper stage receives the sweep gas through the sweep gas inlet (122) at a flow rate Q_sg, wherein the system comprises a controller, and wherein the method further comprises the step: regulating the pressure P_ds, by means of the controller, by controlling the flow rate Q_sg of the sweep gas received at the sweep gas inlet (122).

12. The method according to claim 1 1 wherein the system further comprises a sweep gas recycling unit in fluid connection with the gas outlet and the sweep gas inlet of the stripper stage, wherein the sweep gas recycling unit

receives the gaseous stream comprising two or more components from the gas outlet, the two or more components comprising sweep gas and vapour;

at least partially separates the sweep gas from the other components in the gaseous stream; and

provides at least a part of the gaseous stream at a recycling rate R_r to the sweep gas inlet;

wherein the controller further regulates the pressure P_ds by controlling the recycling rate R_r.

13. Method according to claim 1 1 or 12 wherein the system is a system according to any one of claims 1 to 10.

14. The method according to any one of claims 1 1 to 13 wherein the stripper stage comprises a cascade of evaporators, and wherein the controller is further configured for maintaining the pressure in each evaporator (130) of the cascade of evaporators at a value which is equal to the pressure in the other evaporators (130), within a margin of error of 10%.

15. Use of a system according to any one of claims 1 to 10 for removing water from an aqueous solution of a nitrate-containing fertilizer such as ammonium nitrate, potassium nitrate, urea, an NP fertilizer, or an NPK fertilizer.