Patent Application
20240115992
The present invention relates to a novel gas scrubber and to a novel process for scrubbing gas incorporating a particular connection of scrubber and evaporation.
In offgas scrubbers of chemical plants, chemical reactions and/or high offgas temperatures result in some cases in the release of large amounts of heat. For example, ammonia is scrubbed from the offgases of fertilizer plants, for example plants producing urea or ammonium nitrate-containing fertilizers, by an absorption process. To this end, a scrubbing agent which contains an acid, for example nitric acid or sulfuric acid, is generally used. The structure of a scrubber is known to those skilled in the art; for example a column having random packing may be used. Typically, scrubbing solution is circulated, regularly adding acid and discharging a part of the solution in order to limit the concentration. The discharged solution, e.g. ammonium sulfate or ammonium nitrate solution (salt solution) can be reused in a fertilizer complex. A high concentration is frequently sought here, which can be achieved for example by a subsequent evaporation. This requires additional consumption of a heat transfer medium (e.g. steam), which has disadvantageous effects on investment and operating costs of the plant.
High temperatures of the scrubbing solution in the scrubber are undesirable since they impair the absorption and, via excessive acid partial pressures, especially when using nitric acid, can contribute to the formation of aerosols which can be removed from the offgas again only with considerable effort. The circulating solution of the scrubber is therefore cooled, for example via a heat exchanger operated with cooling water. This is disadvantageous since the amount of heat removed is lost from the process and the additional cooling water consumption increases the investment and operating costs of the plant.
In order to optimize the economic viability and the environmental friendliness of a chemical plant, the consumption of an offgas scrubbing with subsequent evaporation should be minimized in terms of energy and operating media.
As prior art documents, CN 105126588 A, EP 0 172 660 A2 or U.S. Pat. No. 5,928,412 might be cited.
An object of the present invention was accordingly that of providing plants and processes which avoid the disadvantages of the prior art to date and/or further improve the plants and processes of the prior art to date.
The object is achieved in the context of the present invention by the subject matter of the appended claims, wherein the dependent claims constitute preferred embodiments.
Further configurations according to the invention and preferred subject matter emerge from the description which follows.
The present invention accordingly provides a gas scrubber comprising an absorption apparatus configured for receiving a gas and absorbing the gas in an absorption medium, wherein the absorption apparatus has an introduction apparatus for the gas, has an apparatus for circulating the absorption solution that has reacted with the gas, has a discharge apparatus for at least a part of the absorption solution that has reacted with the gas, has a cooler for the circulating absorption solution, an evaporation apparatus configured for evaporating at least a part of the liquid introduced, wherein the absorption apparatus and the evaporation apparatus are connected via a heat exchanger in such a way that the heat content of the absorption solution that has reacted with gas is used at least in part for the evaporation.
The present invention further provides a process for scrubbing gas, comprising the process steps: introducing a gas into an absorption apparatus, circulating the absorption solution, discharging a part of the circulating solution produced, concentrating the discharged solution by evaporating at least a part of the absorption medium, wherein the heat content of the circulating solution is used at least in part for the evaporation.
In the context of the present invention, the terms “salt solution” and “absorption solution” are used synonymously for the solution which is obtained in the neutralization of acid and ammonia-containing gas in the acid scrubber.
The invention therefore relates to a process for the cleaning of offgases from chemical plants with the simultaneous use of the offgas heat and heat of reaction, arising during the offgas cleaning, within the chemical plant, and to corresponding gas scrubbers.
A process has been developed which extracts heat from the circulating solution and uses it to increase the concentration of the discharged solution (so-called heat integration of both process steps). The consumption of the operating media steam or heat transfer medium and cooling water is reduced in the process, and at the same time the apparatus complexity is reduced since cooling and evaporation take place in one heat exchanger apparatus. One possible embodiment of such a heat exchanger is a falling-film evaporator; the structure of such a shell and tube heat exchanger is known to those skilled in the art. This is supplied at the bottom on the shell side with the warm circulating solution from the bottom of the scrubber as heating medium and on the tube side in counter-current with the solution to be evaporated that has been discharged from the scrubbing circuit. Using suitable apparatus measures, the solution is distributed over the upper tube sheet and runs down along the inner wall of the tubes. In the process, a proportion of the water is evaporated from the solution by the heat input and the solution is thus concentrated.
In order to ensure the necessary temperature difference between the hot and the cold side of the heat exchanger, the pressure on the cold side is selected such that the boiling temperature of the solution is always below the temperature of the circulating solution of the scrubber and hence heat transfer can take place.
The conditions for an integrated cooler-evaporator unit in the context of the present invention or for a process in which cooling and evaporating of the salt solution (absorption solution)—consisting essentially of water and ammonium nitrate—are the following: The temperature levels of the solution at the evaporator (TE,1 or TE,2) and those in the absorber-cooler/circulation cooler (TSS,1 or TSS,2) must overlap. The temperature TSS,1 (i.e. the temperature at which the salt solution enters the cooling unit) must in this case be greater than the temperature TE,2 (i.e. the temperature at which the salt solution exits the evaporator—then as a more concentrated solution).
The amount of heat QH required for the evaporation of the salt solution in the evaporator must be at least predominantly covered by the amount of heat QC that is released in the acid scrubber/circulation cooler.
If the above two conditions are not fully met, the integrated process according to the present invention can be modified by the following options 1 and/or 2.
Option 1: A heat exchanger or a heating apparatus may be introduced into the course of the salt solution discharged from the acid scrubber, via which the temperature of the salt solution (e.g. ammonium nitrate solution) can be increased so that the temperature on entry into the cooling unit (temperature TSS,1) is sufficiently high. This measure also increases the overall amount of heat available.
Option 2: A second evaporator stage may be introduced into the course of the concentrated salt solution downstream of the integrated cooler-evaporator unit according to the present invention. In this manner, the first, integrated evaporation stage may be operated at relatively low exit concentrations, which reduces the required temperature at which the concentrated salt solution exits the integrated cooler-evaporator unit (temperature TE,2).
In the context of the present invention, the pressure in the integrated cooler-evaporator unit should always be set to be lower than the pressure in the acid scrubber.
The achievable concentration of the concentrated salt solution is determined in the context of the present invention by the amount of usable heat.
If the temperature TE,2 at which the concentrated salt solution leaves the integrated cooler-evaporator is higher or at least just as high as the temperature TSS,1 at which the not yet concentrated salt solution enters the integrated cooler-evaporator, then the pressure in the evaporator can be reduced and the concentration in the absorber increased and/or options 1 and/or 2 can be additionally employed.
If the amount of heat in the absorber (acid scrubber) is lower than the available amount of heat in the evaporation, then it is possible to increase the concentration in the absorber and/or to additionally use option 1 and/or 2.
In a variant of the present invention, in particular for the case of ammonia scrubbing with nitric acid and subsequent evaporation, the following operating parameters are preferred:
Within the context of the present invention, a conventional process for offgas scrubbing and subsequent evaporation was used as a starting point. A high consumption of cooling water and steam became evident with this. Since the intention was for this process to be able to be carried out in existing plants with a limited availability of utilities, this would impede implementation. The complete, conventional process (scrubber+evaporation) was subjected to a test (inter alia calculation of the mass balances including the utility consumptions (cooling water and steam) and determination of the suitable operating parameters for the conventional process).
In the course of this testing, it was then surprisingly found that the configurations according to the invention can avoid the disadvantages of such conventional processes.
In the context of the present invention, the device unit formed of heat exchanger and evaporation apparatus is also referred to as integrated cooler-evaporator.
In the context of the present invention, the absorption apparatus is also referred to as acid scrubber.
In the context of the present invention, the evaporation apparatus is also referred to as evaporator.
The present invention inter alia also provides the following embodiments designated with Roman numerals:
Embodiment I. A gas scrubber comprising
Embodiment II. The gas scrubber according to embodiment I, characterized in that the gas scrubber is configured for the treatment of an offgas, in particular an ammonia-containing offgas, and for absorption with an acid-containing scrubbing medium, in particular an aqueous solution containing nitric acid or sulfuric acid.
Embodiment III. The gas scrubber according to either of embodiments I and II, characterized in that the heat exchanger is a recuperator, preferably a falling-film evaporator, particularly preferably a shell and tube heat exchanger.
Embodiment IV. The gas scrubber according to any of the preceding embodiments, characterized in that the absorption apparatus is a column comprising internals (random packing, structured packing, separating trays) or a venturi scrubber.
Embodiment V. The gas scrubber according to any of the preceding embodiments, characterized in that the heat exchanger is configured such that the circulated absorption solution that has reacted with the gas is guided as heating medium into the heat exchanger, and the portion diverted from the absorption solution that has reacted with the gas is guided in counter-current into the heat exchanger as a medium to be evaporated, wherein the gas scrubber is preferably configured to set the temperature of the circulated absorption solution that has reacted with the gas and is entering the heat exchanger to be higher than the temperature of the portion diverted from the absorption solution that has reacted with the gas and entering the heat exchanger.
Embodiment VI. The gas scrubber according to any of the preceding embodiments, additionally comprising an apparatus for reducing the pressure on the cooler side of the heat exchanger. As a matter of principle, this leads to flash evaporation, since the pressure on the cooler side is necessarily reduced to boiling condition in order to achieve the required lower temperature level.
Embodiment VII. The gas scrubber according to any of the preceding embodiments, additionally comprising an apparatus for the feeding in of fresh absorption medium.
Embodiment VIII. The gas scrubber according to any of the preceding embodiments, additionally comprising
Embodiment IX. A process for scrubbing gas, comprising the following process steps:
Embodiment X. The process according to embodiment IX, characterized in that the circulating solution and the discharged solution are both conducted separately through a common heat exchanger, the circulating solution being fed to the heat exchanger as heating medium and the discharged solution being fed to the heat exchanger in counter-current as medium to be evaporated.
Embodiment X. The process according to either of embodiments IX and X, characterized in that the gas is an offgas, in particular an ammonia-containing offgas.
Embodiment XII. The process according to any of embodiments IX to XI, characterized in that the absorption medium is an acid-containing scrubbing medium, in particular an aqueous solution containing nitric acid or sulfuric acid.
Embodiment XII. The process according to any of embodiments IX to XII, characterized in that the temperature of the circulating solution entering the heat exchanger is higher than the solution present in the evaporator.
Embodiment XIV. The process according to any of embodiments IX to XIII, characterized in that the pressure on the side of the heat exchanger at which the evaporation takes place is adjusted or regulated such that it is lower than the pressure on the side of the heat exchanger through which the circulating solution is conducted, preferably such that the temperature of the solution to be evaporated is below the temperature of the circulating solution, especially such that the boiling temperature of the solution to be evaporated is below the temperature of the circulating solution.
Embodiment XV. The process according to any of embodiments IX to XIV, characterized in that in addition either the salt solution discharged from the acid scrubber is additionally heated prior to entering the heat exchanger, preferably by means of a further heat exchanger or a heating apparatus, or the concentrated salt solution after leaving the evaporator is further evaporated, preferably by at least one further evaporator stage, or both the salt solution discharged from the acid scrubber is additionally heated prior to entering the heat exchanger, preferably by means of a further heat exchanger or a heating apparatus, and the concentrated salt solution after leaving the evaporator is further evaporated, preferably by at least one further evaporator stage.
Embodiment XVI. The process according to any of embodiments IX to XV, characterized in that it is carried out using a gas scrubber according to any of embodiments I to VII.
The present invention is elucidated in more detail hereinbelow with reference to the drawings. The drawings are not to be interpreted as limiting and are not to scale. The drawings moreover do not contain all of the features comprised by customary plants but rather are reduced to the features essential to the present invention and for the understanding thereof. Identical reference signs in the various figures each have the same meaning, as given in the list of reference signs.
To this end, acid 1 and water 2 are introduced into an acid scrubber 4 and additionally ammonia-containing offgas 3; cleaned offgas 3a then exits the acid scrubber. A salt solution (ammonium nitrate solution) is formed from the acid and the gas in the acid scrubber and exits downwards as salt solution 5. A part of this salt solution 5 is circulated and fed back to the acid scrubber 4 as a scrubbing solution. As part of this circulation of the salt solution 5, the latter is cooled by a circulation cooler 6. The temperature TSS,1 is the temperature at which the salt solution 5 enters the circulation cooler 6 and the temperature TSS,2 is the temperature at which the salt solution 5 exits the circulation cooler 6 again. The acid scrubber 4 is operated here with a pressure pSCR. The portion of the salt solution 5 that is not circulated is then conducted to an evaporator 7. This salt solution 5 has an entry temperature into the evaporator 7 TE,1. The salt solution 5 is concentrated in the evaporator 7 in that a part of the liquid is evaporated and leaves the evaporator 7 as steam 9. The concentrated salt solution 5a leaves the evaporator 7 at a lower temperature TE,2. The evaporator 7 is operated with a pressure pEvap. The heat of reaction QR is used in the acid scrubber 4. In the circulation cooler 6, the temperature of the salt solution 5 is lowered and heat is removed as cooling power QC. In the evaporator 7 heat energy is input via the heating power QH.
According to this conventional prior art process, two different apparatuses are needed in order firstly to cool the salt solution (the circulation cooler) and in order secondly to concentrate the salt solution by evaporation, namely the evaporator.
Here, too, acid and water or makeup water 2 are reacted with offgas 3 in an acid scrubber 4 and cleaned offgas 3a is obtained. Salt solution 5 is discharged from the acid scrubber 4 and sent to further processing. In contrast to the conventional prior art process, however, an integrated cooler-evaporator 8 is used in the process according to the invention, in which the cooling of the salt solution 5 and the evaporation of the salt solution 5 take place together. Here too, the salt solution 5 has an entry temperature TSS,1 which is higher than the exit temperature TSS,2, before a part of the salt solution 5 is then recycled back into the acid scrubber 4. In contrast to the processes and/or apparatuses of the prior art, however, in the integrated cooler-evaporator 8 according to the present invention, the absorption apparatus 4 and the evaporation apparatus 7 are connected via a heat exchanger in such a way that the heat content of the salt solution that has reacted with gas is not extracted via a circulation cooler and then lost, but is instead also used directly for evaporation of the salt solution 5 to give a concentrated salt solution 5a with the release of steam 9. For this,
In
It is pointed out that, in the legend to the figures and the description thereof, the “Q”, illustrated scientifically correctly, would have to be a “Q” with a dot on top, but for typesetting and printing reasons in the context of the present description is illustrated as a simple “Q”; in the figures it is correctly represented as “Q with dot”.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2019/5678 | Oct 2019 | BE | national |
| 10 2019 215 611.8 | Oct 2019 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/077742 | 10/2/2020 | WO |
