EP4580784A1

EP4580784A1 - Ammonia scrubbing

Info

Publication number: EP4580784A1
Application number: EP23762497.8A
Authority: EP
Inventors: Johan Martens; Lander HOLLEVOET
Original assignee: Katholieke Universiteit Leuven
Current assignee: CIRCLAIR BV
Priority date: 2022-08-30
Filing date: 2023-08-30
Publication date: 2025-07-09
Also published as: KR20250057840A; CN120152778A; WO2024047118A1

Abstract

A method of purifying air comprising ammonia (C), by scrubbing the air with an acid containing aqueous washing liquid and consequently to contain a salt or combination of salts, wherein the Deliquescence Relative Humidity (DRH) of the salt or combination of salts dissolved in the aqueous solution is always kept lower than the Relative Humidity (RH) of the air.

Description

AMMONIA SCRUBBING

Background and Summary

BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention relates to ammonia scrubbing at reduced water usage. The present invention also relates to an ammonia scrubbing system that produces concentrated product without salt crystallization.

B. Description of the Related Art

In acid ammonia scrubbers, ammonia is absorbed in an aqueous solution where the pH of the aqueous solution is typically controlled to a value of 3-7 by the addition of sulfuric acid (H2SO4) or nitric acid (HNO3). Due to the absorption of ammonia from the gas phase, the pH increases, which must be compensated by adding sulfuric acid or nitric acid. In combination with ammonia, this forms a solution of ammonium sulfate ((NEU^SC ) or ammonium nitrate (NH4NO3). However, such acid scrubbers still face several problems.

These acid scrubbers typically consume large amounts of water, mainly due to evaporation, but also by purging the scrubbing water to avoid the ammonium sulfate or ammonium nitrate concentrations to become too high, which would result in the formation of salt crystals, which can block the washer bed and damage the pump of the scrubber. The solubility limit of ammonium sulfate and ammonium nitrate is relatively high (several 100 g/liter, the solubility limit of ammonium sulfate and ammonium nitrate in water are given in Figure 1 and Figure 2 as a function of temperature), but limiting the salt concentration to below the solubility level alone is not sufficient to avoid crystallization because some droplets may be isolated from the bulk inside the packed bed. The water of these droplets can then evaporate water, which increases the salt concentration up to a level where the salt concentration exceeds the maximum solubility and salt crystals form. To avoid the formation of salt crystals in the washer bed, the salt concentrations in the washing water are typically limited to only 1/5 to 1/3 of the solubility limit (Melse & Willers, 2004).

The ammonium sulfate or ammonium nitrate containing waste water is potentially valuable as fertilizer, but the fairly low nutrient concentration makes it less valuable and large storage tanks are required to store the waste water.

The Relative Humidity (RH) of the air in stables is typically not saturated with water. For pig stables for example, the RH can range between 50% and 80%. The recommended relative humidity (RH) range in pig stables is between 60% and 70%. This range has been shown to be the best for fighting respiratory infections. Too low humidity can cause respiratory problems in pigs, as it can dry out their nasal passages and make them more susceptible to infection. Too high humidity can also cause respiratory problems in pigs, as it can create a breeding ground for bacteria and viruses. Additionally, high humidity can lead to condensation, which can cause mold and mildew growth. The ideal RH range for pig stables can vary depending on the climate, the type of ventilation system, and the number of pigs in the stable. However, in general, the RH should be kept between 60% and 70% to promote pig health and welfare ^Simple ventilation tips as warm weather approaches” Ken Lamm and Casey Zangaro, Michigan State University Extension - March 10, 2020).

Although the RH inside the stable and inside the air scrubber can differ due to changes in temperature, the RH of the air is often still unsaturated and this, in combination with the low salt concentration in the scrubbing liquid, causes water to evaporate, and leads to the consumption of large amounts of water.

In state of the art scrubbers, acids like chloric acid, sulfuric acid or nitric acid can be used, and the choice of acid has little to no implications on how the air scrubber is operated. Table 1 provides an overview of the process control applied in state-of-the-art acidic ammonia scrubbers, compared to the method proposed in the present invention.

There is a need in the art to improve such ammonia scrubbers which remove ammonia from stable air via a chemical reaction. More in particular, a method is required which is able to lower water consumption and increase the product concentration when scrubbing ammonia out of an air stream which is not fully saturated with water.

Present invention solves such problems in the art for acid based ammonia scrubbing of air which is unsaturated with water (RH < 90% when entering the scrubber) by keeping the Relative humidity (RH) of the treated ammonia containing air higher than the Deliquescent Relative Humidity (DRH) of the salt or combination of salts dissolved in the aqueous washing liquid (the scrubbing solution). The required water is withdrawn from the treated air. The invention also includes measures to ensure the RH of the treated air remains above the DRH of the washing liquid, by chemical addition (e.g. urea) and/or by pre-humidification. The produced ammonium sulfate or ammonium nitrate and optionally urea solution has a high salt concentration of 25-75 wt%.

From patent application EP0377476, a method is known for removing ammonia from air originating from a bio-industrial process where the treated air is fully saturated with water (RH=100%) and water is extracted from the treated air, resulting in a RH of 95-98%. However, this invention does not include the treatment of air which is not saturated with water (RH<100 %). Furthermore, the approach presented generates a relatively low ammonium sulfate concentration between 5% and 20 %. Furthermore, the method proposed in EP0377476 does not include monitoring and controlling the humidity of the treated air.

SUMMARY OF THE INVENTION

The present invention solves the problems of the related art by ensuring the Deliquescence Relative Humidity of the dissolved salt or salts is lower than the relative humidity of the treated air, which allows a higher salt concentration in the product to be obtained while avoiding salt crystallization. This higher salt concentration is advantageous since it enables the system to match the equilibrium relative humidity of the product with the relative humidity of the treated air.

In accordance with the purpose of the invention, as embodied and broadly described herein, the invention is broadly drawn to reducing water usage and increasing product concentration in acidic ammonia scrubbers which treat air which enters the scrubber with RH <90%.

If air with a RH lower than the DRH of a salt or combination of salts in a scrubbing liquid is contacted with this liquid, it is thermodynamically possible for salt to crystallize in this liquid solution. However, it can be expected that in practice, the RH of the air needs to be sufficiently lower than the DRH to have a sufficient driving force for salt nucleation to occur and the lower limit for the RH of the air can be set somewhat lower than the DRH. However, it may also be of interest to maintain a safety margin when setting the lower limit for the RH. The lower limit for the RH of the scrubber can therefore be set between 10% above and 20% below the DRH, depending on the implemented safety margin.

According to this invention, in the air scrubber operated under adequate conditions the relative humidity of the incoming air is higher than the predetermined threshold (threshold is set between 10 % above the DRH and 20 % below the DRH of the ammonium salt of nitric acid or sulfuric acid and possibly in combination with other chemicals. However, the relative humidity of the incoming air can fluctuate, for example due to changing weather conditions or changing stable climate. In the exceptional case (< 30 % of the operation time on annual basis) that the RH of the incoming air decreases below the predetermined limit (between 10 % above and 20 % below the DRH), action is required to prevent the formation of salt crystals.

In one embodiment of the invention, when the relative humidity of the treated ammonia containing air decreases to below the predetermined threshold (threshold is set between 10 % above the DRH and 20 % below the DRH), the DRH of the salt solution is lowered by the addition of chemicals (e.g. urea) to the aqueous scrubbing solution.

In another embodiment of the invention, when the relative humidity of the treated ammonia containing air decreases to below the predetermined threshold (threshold is set between 10 % above the DRH and 20 % below the DRH), the humidity of the incoming air is increased by prehumidication before entering the scrubbing section, for example by spraying water into the gas stream, injecting steam into the gas stream or by bringing the gas into contact with water in a gas/liquid contactor.

It should be noted that the RH of the air is temperature-dependent and increases upon cooling, while the absolute humidity stays the same. In air scrubbers, the temperature inside the scrubber is equal to the temperature of the washing water, but can differ from the temperature of the incoming air. The RH of the incoming air which is referred to in this invention is always the RH of the incoming air at the temperature inside the scrubber, and not at the temperature of the air before it enters the scrubber. This RH can be determined by sensing the RH and the temperature of the air before it enters the scrubber, and sensing the temperature of the washing liquid in the scrubber, or by sensing the absolute humidity of the air before entering the scrubber, and sensing the temperature of the washing liquid in de scrubber.

The applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

Present invention concerns a method or system of purifying air comprising ammonia with a Relative Humidity (RH) < 90%, by scrubbing the air with an acid containing aqueous washing liquid consequently to contain a salt or combination of salts, characterised in that the formation of salt crystals is prevented by ensuring the RH of the treated air is always kept above a predetermined threshold, which is set between 10 % above the Deliquescent Relative Humidity (DRH) and 20 % below the DRH. Under normal operation (> 70% of the operation time on annual basis) the relative humidity of the treated air is above the DRH of the salt or combination of the aqueous solution. Present invention also concerns a method or system of taking up and purifying ammonia from an incoming air with Relative Humidity (RH) < 90%, by scrubbing the air by an acid containing aqueous washing liquid and containing a salt or combination of salts, whereby the formation of salt crystals is prevented by ensuring the RH of the treated air to be above the predetermined threshold which has a maximum acceptable value of 10 % above the Deliquescent Relative Humidity (DRH) of the salt or combination of salts and a minimum acceptable value of 20 % below the DRH of the salt or combination of salts and possibly urea with the exact threshold value chosen based on the safety margin which is chosen to be implemented, the method comprising increasing the RH of the incoming air and/or decreasing the DRH of the salt or combination of salts and possibly urea in the scrubbing liquid when the RH of the treated air falls outside of the threshold range. In a preferred embodiment the incoming air has a Relative Humidity (RH) of 45% to 90%, and preferably 50% to 80%.

In some embodiments, the RH of the air to be treated is increased by pre-humidification. The pre-humidification is done by spraying water in the treated air, injecting steam into the gas stream or by bringing the air in contact with water in a gas-liquid contactor and the DRH of the dissolved salt can be decreased by altering the composition of the scrubbing solution through addition of chemical compounds in the aqueous washing liquid and the acid can be nitric acid, sulphuric acid or a combination thereof. In some aspect or a combination of nitric acid and/or sulphuric acid and/or urea.

In another aspect, the present invention provides that the RH of the incoming air or air to be treated is sensed or monitored and whereby the DRH of the salts in the aqueous washing liquid is sensed and whereby if the relative humidity of the incoming air approaches the DRH of the salts in the aqueous washing liquid, the relative humidity of the air to be treated is increased through pre-humidification and/or the DRH in the aqueous washing liquid is decreased by chemical addition.

In yet another aspect, the present invention provides that the RH of the incoming air or air to be treated is sensed or monitored and whereby the DRH of the salts in the aqueous washing liquid is sensed and whereby if the relative humidity of the incoming air approaches the predetermined threshold (between 10% above and 20% below the DRH of the salts in the aqueous washing liquid), the relative humidity of the air to be treated is increased through pre-humidifi cation and/or the DRH in the aqueous washing liquid is decreased by adding a salt with high deliquescence relative humidity (above 70% at 20°C) and/or urea.

In yet another aspect, the present invention provides that the RH of the incoming air or air to be treated is sensed or monitored and the whereby the DRH of the salts in the aqueous washing liquid is sensed and whereby if the relative humidity of the incoming air approaches the DRH of the salts in the aqueous washing liquid, the relative humidity of the air to be treated is increased through pre-humidification and/or the DRH in the aqueous washing liquid is decreased by adding a chemical of the group consisting of urea, ammonium nitrate and ammonium phosphate or a combination thereof.

In an advantageous embodiment, the method according to the present invention further comprises that the RH of the incoming air or air to be treated is sensed with a humidity sensor and whereby the DRH of the salts in the aqueous washing liquid is sensed by a combination of a pH measurement with a pH sensor, a conductivity measurement with a conductivity sensor and a density measurement with a density sensor. The chemical added to the aqueous washing liquid can be pre-dissolved in water or the chemical is added to the aqueous washing liquid in solid form.

This invention accordingly provides the advantage that when fluctuations in the RH of the treated air cause the RH to drop below the predetermined threshold, the DRH is lowered or RH of the treated air is increased.

Some of the methods described above may be embodied as whereby when the de salt concentration of the aqueous washing liquid approaches the solubility limit, it is lowered by partly draining the aqueous solution and adding fresh water to the aqueous washing liquid or they may be embodied as operated on an air washer comprising 1) a scrubbing tower (A) with a humidity sensor and a temperature sensor at its air inlet and the scrubbing tower (A) operational connected with a liquid output guidance and a liquid input guidance with 2) a buffer tank (B) whereby the buffer tank is provided with a pH sensor, a conductivity sensor, a density sensor and a temperature sensor and an input guidance for supplying the tank with acid, water or salt.

These embodiment of the invention advantageously comprises to clean air emanating from a stable.

Detailed Description

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims and equivalents thereof.

Several documents are cited throughout the text of this specification. Each of the documents herein (including any manufacturer’s specifications, instructions etc.) are hereby incorporated by reference; however, there is no admission that any document cited is indeed prior art of the present invention.

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn to scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein. Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

It is to be noticed that the term “comprising”, used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression “a device comprising means A and B” should not be limited to the devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention. Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein.

It is intended that the specification and examples be considered as exemplary only.

Each and every claim is incorporated into the specification as an embodiment of the present invention. Thus, the claims are part of the description and are a further description and are in addition to the preferred embodiments of the present invention.

Each of the claims set out a particular embodiment of the invention.

The following terms are provided solely to aid in the understanding of the invention.

Inorganic salts are solid at very low RHs. As RH increases, a particle remains solid until it reaches a specific value at which the solid particle will abruptly take up large amounts of water vapour and be transformed to a saturated droplet. This solid-to-liquid phase transition is called deliquescence, and deliquescence relative humidity (DRH) is defined as the RH at which deliquescence takes place.

Salts with a low deliquescence relative humidity (DRH) suitable to decrease DRH in the aqueous washing liquid in present invention are for instance of the groups consisting of Calcium nitrate (DRH between 49 % and 56 % at 25 °C), Magnesium nitrate (DRH between 49 % and 54 % at 25 °C), calcium chloride (DRH < 31% at 25°C), magnesium chloride (DRH between 31 % and 15% 25 °C), Calcium Chlorate (DRH or any salt with a DRH < 70% at 25°C. When the relative humidity reaches the salt's DRH, the salt will absorb water vapor and dissolve, forming a solution. The emission of airborne ammonia is important contribution to particulate matter in the air, which can cause respiratory problems, and can put natural habitats under stress.

Intensive livestock farming, for instance, is a major source of airborne ammonia, but industrial emission sources can also have a significant contribution.

To mitigate ammonia emissions, wet scrubbers have been implemented as an end-of-pipe treatment technique.

Wet air scrubbers can be classified into two classes: biological scrubbers and acid scrubbers. In both types of scrubbers, ammonia is absorbed in an aqueous solution.

In a biological scrubber, absorbed ammonia (NH3) is digested by bacteria into nitrite (NO₂ ) and then into nitrate (NO3 ). Some biological scrubbers are equipped with an additional denitrification tank, which converts the nitrates into anaerobic conditions in nitrogen gas (N2). The aqueous solution has a neutral pH to maintain the bacterial culture responsible for the process.

In an acid scrubber, the pH of the aqueous solution is typically controlled to a value of 3- 7 by the addition of sulfuric acid (H2SO4) or nitric acid (HNO3). Due to the absorption of ammonia from the gas phase, the pH increases, which must be compensated by adding sulfuric acid or nitric acid. In combination with ammonia, this forms a solution of ammonium sulfate ((NH^SC ) or ammonium nitrate (NH4NO3).

The acid scrubbers have a superior ammonia removal rate (typically > 95 %), while biological scrubbers achieve typically only 70 %.

However, such acid scrubbers still face several problems. These acid scrubbers typically consume large amounts of water, mainly due to evaporation, but also by purging the scrubbing water to avoid the ammonium sulfate or ammonium nitrate concentrations to become too high, which would result in the formation of salt crystals, which can block the washer bed and damage the pump of the scrubber. The solubility limit of ammonium sulfate and ammonium nitrate is relatively high (several 100 g/liter), but to avoid the formation of salt crystals in the washer bed, the draining of the salt solution and addition of fresh water is steered by the salt concentration in the liquid. When this salt concentration exceeds a predetermined threshold (typically 1/3 - 1/5 of the solubility limit (Melse & Willers, 2004)), the salt solution is drained and/or fresh water is added. The equilibrium relative humidity of such a dilute salt solution is very high (> 95%) and this causes a lot of water to evaporate from the washing liquid, sulfate

For instance, air from livestock stables typically has a relative humidity between 50 and 80%, so when it comes into contact with the dilute salt solution, the water of the salt solution evaporates. This can lead to several hundreds of m³ of water consumption per year for a stable with 1000 fattening pigs (Melse & Willers, 2004).

The invention which will be described below is an improvement compared to current state of the art acid ammonia scrubbers in three ways: (1) the problem of salt formation in the packed bed is resolved, (2) the final product is more concentrated and (3) the amount of water lost due to evaporation is reduced significantly, or even eliminated.

In this invention, the crystallization of salts in the packed bed is avoided by closely monitoring the relative humidity of the treated air. As long as the relative humidity of the treated air is higher than the DRH of the salt solution, salt crystals will not be formed. When the salt concentration of the washing liquid is increased due to acid addition and ammonia absorption, this concentration increase is countered by absorbing additional water from the gas phase. This way, in principle, all the required water can be absorbed from the gas phase and no liquid water is required.

An aspect of the present invention is to ensure the RH of the treated ammonia containing air is higher than the DRH of the salt or combination of salts present in the washing liquid. The lower the RH of the treated air, the lower the DRH of the corresponding salt needs to be, with the DRH in decreasing order: (NHf^SC >(NH4)2SO4 + Urea > NH4NO3 > NH4NO3 + Urea. The DRH of ammonium nitrate, ammonium sulfate and combinations of ammonium nitrate, ammonium sulfate and urea are given in Figure 3.

If the RH of the air which needs to be treated is too low, the RH is increased by prehumidification of the incoming air and/or the DRH of the dissolved salt is decreased by altering the composition of the dissolved salt.

Figure 3: Deliquescence Relative Humidity (DRH) of Ammonium Nitrate, Ammonium Sulfate and combinations of Ammonium nitrate, ammonium sulfate and urea with given weight ratio’s for a temperature range of 0-30 °C.

For a sulfuric acid based scrubber, the DRH can be lowered by the addition of urea, ammonium nitrate or a mixture of urea and ammonium nitrate. Alternatively, the scrubber can make use of a combination of sulfuric acid (for its low cost) and nitric acid (to lower the DRH), which would also result in a mixture of ammonium nitrate and ammonium sulfate when ammonia is absorbed.

A nitric acid based scrubber, which produces an ammonium nitrate solution, is already characterized by a lower DRH than the sulfuric acid based scrubber, which produces an ammonium sulfate solution. The DRH of a nitric acid based scrubber and a sulphuric acid based scrubber can both be lowered by addition of Urea.

Ensuring the DRH of the dissolved solutions is lower than the RH of the incoming air enables the possibility to operate the scrubber at higher salt concentrations, namely the salt concentration with which the solution is in equilibrium with the RH of the treated air. By allowing these higher salt concentrations, it becomes possible to strongly reduce or even eliminate water losses due to evaporation. In Figure 4, the relative humidity with which an ammonium nitrate solution is in equilibrium (meaning water evaporation and condensation are equal) is shown as a function of ammonium nitrate concentration for different temperatures. In Figure 5, the same relationship is shown for an ammonium nitrate/urea (weight ratio 1/0.8) solution. A similar relationship exists for other combinations of urea, ammonium nitrate and/or ammonium sulfate. Figure 4: Equilibrium relative humidity as a function of ammonium nitrate concentration for different temperatures for an aqueous solution of ammonium nitrate

Figure 5: Equilibrium relative humidity as a function of ammonium nitrate concentration for different temperatures for an aqueous solution of Ammonium nitrate/urea with a weight ratio of 1/0.8

State of the art process control

A state of the art acid ammonia scrubber comprises a packed bed (A) in which the ammonia containing air (C) is brought into contact with the aqueous solution. The treated air (D) is vented or sent to other downstream processing steps. The aqueous solution is sent to a buffer tank (B) from which it is recirculated to the packed bed (A). The pH of the aqueous solution is measured with a pH sensor (E) and controlled by adding acid (K). The salt concentration is measured with a conductivity sensor (F) and the conductivity is set to a fixed maximum, typically set in such a way that the salt concentration does not exceed 1/5 to 1/3 of its solubility limit. If the maximum is exceeded, part of the aqueous solution is drained (M). Water (L) is added to compensate for the drained product (M) and for evaporation taking place in the packed bed (A). An overview of state of the art process control is given in Table 1.

Table 1 : process control method for state of the art ammonia scrubbing with acids, compared to the method of the present invention, with two different embodiments of the present invention.

Process control in the present invention

In this invention, the relationships between solubility and temperature, DRH and salt composition and equilibrium relative humidity and salt concentration form the basis for the process control, rather than a predetermined limit for salt concentration.

Similar to the control of pH in state of the art scrubbers, the pH of the washing liquid is monitored to remain below a predetermined value (typically between 3 and 6). When the pH increases due to absorption of ammonia and conversion to ammonium, concentrated acid (nitric acid or sulfuric acid) is added. The relative humidity of the incoming air is measured with a humidity sensor (J). Under normal operation (> 70 % of the operation time on annual basis), the RH of the incoming air is higher than a predetermined limit (between 10 % above and 20 % below the DRH). In this situation, salt crystallisation due to exceeding the solubility limit cannot occur, as concentration increases are compensated by the absorption of additional water from the treated air. Therefore, no fresh water addition is necessary to avoid exceeding the solubility limit and a concentrated product is achieved.

Due to the absorption of ammonia and water from the air and the addition of concentrated acid, the volume of the washing water increases. To prevent overflowing, the product is drained and stored when a predetermined level is exceeded. The concentration of the product can vary due to changing temperature and RH of the air, which changes the equilibrium salt concentration (see Figure 4 and 5), but the annual average concentration of dissolved salt(s) and possibly Urea is high: between 25 wt% and 75 wt%. This concentration is significantly higher than what can be achieved by the state of the art process control.

In one embodiment of the invention, the temperature of the washing liquid is monitored to ensure no rapid temperature drops take place, as this can lower the solubility limit of the dissolved salts and cause salt crystallisation before the washing liquid has the time to absorb sufficient additional water from the treated air. Rapid temperature decreases can be countered by supplying additional heat to the liquid and or the air.

In the exceptional case that the relative humidity of the incoming air approaches the DRH of the salts in the aqueous solution (<30 % of the operation time on annual basis), action is taken to increase the RH of the treated air and/or decrease the DRH of the washing liquid.

In one embodiment of the invention, the relative humidity of the incoming air (C) is increased through pre-humidifi cation by spraying water (N) through the air to be treated, injecting steam, or by bringing the air to be treated into contact with water in a gas-liquid contactor. This water (N) has no salts, urea or acid added to it, so the equilibrium RH of the this water will be close to 100%. In this pre-humidification step, water is evaporated, which will increase the RH of the air to be treated. Furthermore, the heat of evaporation of the water will cause the temperature of the air to be treated to drop, which further increases the RH. The combination of these effects increases the RH of the air to be treated to above the predetermined threshold (between 10% above and 20% below the DRH). The amount of water which is added to the gas stream can be controlled to limit the amount of evaporated water to what is required to increase the RH of the incoming air only to the required level.

In another embodiment of the invention, the DRH of the washing liquid can be increased by altering the composition of the washing water through the addition chemicals (O) (e.g. urea, ammonium nitrate, ammonium phosphate or a combination of these chemicals). The chemicals can be pre-dissolved in water or can be added in solid form and dissolve upon addition to the aqueous solution of the scrubber. This way, the DRH is increased until the RH is above the threshold predetermined relative to the DRH of the washing liquid (between 10 % above and 20 % below the DRH).

The DRH is dependent on the composition of the aqueous solution, which can be determined by a combination of a pH measurement with a pH sensor (E) and/or a conductivity measurement with a conductivity sensor (F) and/or a density measurement with a density sensor (G) and/or by monitoring the acid addition (K) and/or by monitoring the chemical addition (O).

The solubility limit is dependent on the temperature and the ratio of different salts and urea present in the aqueous solution. However, as long as the RH is higher than the predetermined threshold (set relative to the DRH between 10 % above and 20% below the DRH) and rapid decreases in temperature are avoided, the solubility limit is not reached, as increases in salt concentration are countered by absorption of water from the air. Therefore, in this invention, in contrast with the state of the art, the washing water is drained based on the level of the liquid B in the buffer tank, which can be regulated by a level sensor (P) or by an overflowing system. The temperature is determined by a temperature sensor (H).

If the system is equipped with a urea or salt addition (O), the DRH of the washing liquid can be determined by a combination of a temperature measurement with a temperature sensor (H), a pH measurement with a pH sensor (E) and/or a conductivity measurement with a conductivity sensor (F) and/or a density measurement with a density sensor (G) and/or by monitoring the acid addition (K) and/or by monitoring the salt addition (O). If the system is not equipped with a salt or urea addition (O), a temperature sensor (H) is sufficient to determine the DRH, as the DRH is equal to the DRH of the ammonium salt corresponding to the used acid (NH4NO3 for nitric acid, (NH4)2SO4 for sulfuric acid, a combination of NH4NO3 and urea when using a mixture of urea and HN03 as acid, or a combination of (NH4)2SO4 and urea when using a mixture of urea and H2SO4 as acid). The equilibrium relative humidity of the solution will automatically evolve to match the relative humidity of the treated air. If the equilibrium relative humidity of the solution is higher than the RH of the treated air, the concentration automatically increases due to continuous ammonia absorption, acid addition to compensate for the ammonia absorption, and due to water evaporation. If the equilibrium relative humidity is lower than the RH of the treated air, it is increased automatically without taking action due to water absorption.

Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

Drawing Description

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

FIG. l is a graphic showing the solubility of ammonium sulfate in water as a function of temperature

FIG. 2: is a graphic showing the solubility of ammonium nitrate in water as a function of temperature

FIG. 3: is a graphic showing the deliquescence Relative Humidity (DRH) of Ammonium Nitrate, Ammonium Sulfate and combinations of Ammonium nitrate, ammonium sulfate and urea with given weight ratio’s for a temperature range of 0-30 °C. FIG. 4: is a graphic showing the equilibrium relative humidity as a function of ammonium nitrate concentration for different temperatures for an aqueous solution of ammonium nitrate

FIG. 5: is a graphic showing the equilibrium relative humidity as a function of ammonium nitrate concentration for different temperatures for an aqueous solution of

Ammonium nitrate/urea with a weight ratio of 1/0.8

FIG. 6: is a schematic overview of the acid ammonia scrubber

References to this application

Melse, R., & Willers, H. C. (2004). Toepassing van luchtbehandelings- technieken binnen de intensieve veehouderij .

Claims

AMMONIA SCRUBBING Claims What is claimed is:

1. A method of taking up and purifying ammonia from incoming air with Relative Humidity (RH) < 90%, by scrubbing the air by an acid containing aqueous washing liquid and containing a salt or combination of salts, whereby the formation of salt crystals is prevented by ensuring the RH of the treated air to be above the predetermined threshold which has a maximum acceptable value of 10 % above the Deliquescent Relative Humidity (DRH) of the salt or combination of salts and a minimum acceptable value of 20 % below the DRH of the salt or combination of salts and possibly urea with the exact threshold value chosen based on the safety margin which is chosen to be implemented, the method comprising increasing the RH of the incoming air and/or decreasing the DRH of the salt or combination of salts and possibly urea in the scrubbing liquid when the RH of the treated air is below the predetermined threshold.

2. The method according to claim 1, whereby the RH of the air to be treated is increased by pre-humidification.

3. The method according to claim 2, whereby pre-humidification is done by spraying water in the treated air, injecting steam into the gas stream or by bringing the air in contact with water in a gas-liquid contactor.

4. The method according to anyone of the claims 1 to 3, whereby the DRH of the dissolved salt or combination of salts is decreased by altering the composition of the scrubbing solution through addition of chemical compounds in the aqueous washing liquid.

5. The method according to anyone of the claims 1 to 4, whereby the acid is nitric acid, sulphuric acid or a combination thereof, or a combination of nitric acid and/or sulphuric acid and/or urea

6. The method according to anyone of the claims 1 to 5, whereby the RH of the incoming air or air to be treated is sensed or monitored and whereby the DRH of the salts in the aqueous washing liquid is sensed and whereby if the relative humidity of the incoming air approaches the DRH of the salts in the aqueous washing liquid, the relative humidity of the air to be treated is increased through pre-humidification and/or the DRH in the aqueous washing liquid is decreased by chemical addition.

7. The method according to anyone of the claims 1 to 5, whereby the RH of the incoming air or air to be treated is sensed or monitored and whereby the DRH of the salts in the aqueous washing liquid is sensed and whereby if the relative humidity of the incoming air approaches the predetermined threshold (between 10% above and 20% below the DRH of the salts in the aqueous washing liquid), the relative humidity of the air to be treated is increased through pre- humidification and/or the DRH in the aqueous washing liquid is decreased by adding a salt with high deliquescence relative humidity (above 70% at 20°C) and/or urea.

8. The method according to anyone of the claims 1 to 5, whereby the RH of the incoming air or air to be treated is sensed or monitored and the whereby the DRH of the salts in the aqueous washing liquid is sensed and whereby if the relative humidity of the incoming air approaches the threshold between 10 % above and 20% below the DRH of the salts in the aqueous washing liquid, the relative humidity of the air to be treated is increased through pre-humidification and/or the DRH in the aqueous washing liquid is decreased by adding a chemical of the group consisting of urea, ammonium nitrate and ammonium phosphate or a combination thereof.

9. The method according to anyone of the claims 6 to 8, whereby the RH of the incoming air or air to be treated is sensed with a humidity sensor and whereby the DRH of the salts in the aqueous washing liquid is sensed by a combination of a pH measurement with a pH sensor, a conductivity measurement with a conductivity sensor and a density measurement with a density sensor.

10. The method according to any one of the claims 6 to 8, whereby the chemical added to the aqueous washing liquid is pre-dissolved in water.

11. The method according to any one of the claims 6 to 8, whereby the chemical is added to the aqueous washing liquid in solid form.

12. The method according to any one of the claims 1 to 11, whereby when fluctuations in the RH of the treated air cause the RH to drop below the predetermined threshold, the DRH is lowered or RH of the treated air is increased.

13. The method according to any one of the claims 1 to 11, whereby when the de salt concentration of the aqueous washing liquid approaches the solubility limit, it is lowered by partly draining the aqueous solution and adding fresh water to the aqueous washing liquid.

14. The method according to any one of the claims 1 to 11, operated on an air washer comprising 1) a scrubbing tower (A) with a humidity sensor and a temperature sensor at its air inlet and the scrubbing tower (A) operational connected with a liquid output guidance and a liquid input guidance with 2) a buffer tank (B) whereby the buffer tank is provided with a pH sensor, a conductivity sensor, a density sensor and a temperature sensor and an input guidance for supplying the tank with acid, water or salt.

15. The method according to anyone of the previous claims to clean air emanating from a stable.