METHOD FOR THE REMOVAL OF INERT GAS FROM LIQUID AMMONIA

Description

[0001] The present invention relates to a process for removing inert gases dissolved in liquid ammonia.

[0002] In ammonia production, the synthesis gas typically contains hydrogen and nitrogen, as well as inert gases such as methane and noble gases, which impair the ammonia yield. In these processes, fresh synthesis gas is typically first compressed to high pressure in several stages. The compressed, fresh synthesis gas is then fed into a cycle that passes through one or more catalyst-filled reactors where ammonia is produced. The cycle incorporates a separation system to remove the produced ammonia from the cycle in liquid form.

[0003] The inert gases are only soluble in ammonia in low concentrations and are therefore only removed in small quantities along with the ammonia. To prevent an accumulation of inert gases in the circuit, in these conventional processes, a portion of the gas circulated is continuously used as purge gas ( Purge ). Ammonia residues are then scrubbed from this purge gas. Hydrogen and, if necessary, nitrogen are separated and recovered, for example, via membranes or by decomposition at low temperatures. The remaining inert gases, particularly methane, argon, and helium, are discarded or used for other purposes, particularly for heating. The recovered hydrogen and, if necessary, the recovered nitrogen are mixed with the fresh synthesis gas before compression and thus utilized.

[0004] Separating higher-boiling gases from liquefied ammonia by evaporating the ammonia is known from WO 03 / 008333 A1 and US 2 050 511 A.

[0005] DE 100 57 863 A1 discloses a process for producing ammonia from fresh synthesis gas, which contains inert components other than hydrogen and nitrogen, in at least two reaction systems, wherein the synthesis of ammonia from synthesis gas takes place successively in different synthesis systems. Inert gas components are introduced via a purge gas stream ( Purge ) and discharged. Inert gases or "inerts" in ammonia production are those components that behave inertly during the conversion of hydrogen and nitrogen to ammonia. Examples of these inert gases include the noble gases and methane.

[0006] As far as the term "inert gases" is used in the present application, this term should be understood in the sense of the above definition.

[0007] However, it is energetically unfavorable to remove purge gas from the cycle, as this would subject large quantities of gas to pressure loss during separation and subsequently require laborious recompression. For this reason, an enrichment of inert gases from an original 1 to 2 vol.% in the fresh synthesis gas to 10 to 20 vol.% within the recirculated gas is often accepted, although this inevitably has the disadvantage that the partial pressures of hydrogen and nitrogen are considerably lower than in a synthesis cycle containing fewer or no inert gases. For this reason, the catalyst volumes and the reactors containing them must be significantly larger than would be necessary without inert gases in the synthesis cycle.

[0008] During ammonia synthesis, the synthesis gas in the reactor is converted into a gas mixture containing unreacted hydrogen and nitrogen, the ammonia formed, and the inert gases. At the reactor outlet, the ammonia produced is in gaseous form. To separate the ammonia from the reactant gas, it is condensed so that it can be withdrawn from the cycle in liquid form. Since the dew point of ammonia depends on its partial pressure, the condensation of ammonia is favored by high synthesis pressure, high ammonia concentration, and low temperature. A high ammonia concentration can be achieved with large catalyst volumes and low inert gas concentrations. A high synthesis pressure requires corresponding energy expenditure for the synthesis gas compression, and a low cooling temperature requires appropriate cooling devices for the gas circulating in the cycle.

[0009] It is known from DE 100 57 863 A1 that ammonia can be produced in several reaction systems connected in series. In this case, too, the ammonia is condensed and discharged from the synthesis cycle as liquid ammonia.

[0010] Another process for producing ammonia from a nitrogen-hydrogen mixture obtained from natural gas is known from DE 100 55 818 A1. Here, the raw product gas mixture undergoes multi-stage cooling and then enters a separator, from which crude ammonia is extracted in liquid form. The gaseous components separated in this process are generally returned to the reaction system as recycle gas. This publication also discloses the subsequent feeding of the crude ammonia to a urea synthesis process.

[0011] However, the liquid ammonia produced during ammonia synthesis using conventional processes usually still contains dissolved gases, which can be disruptive, for example, during subsequent urea synthesis, as they can lead to the formation of explosive exhaust gases in the urea plant. To avoid this, it is advantageous to have as few dissolved gases as possible in the liquid ammonia used as a feedstock in urea synthesis.

[0012] The object of the present invention is therefore to provide an effective process for removing dissolved inert gases from the liquid ammonia which is produced during ammonia synthesis.

[0013] The solution to the above-mentioned problem provides a process for removing dissolved inert gases from liquid ammonia of the type mentioned at the outset with the features of claim 1.

[0014] According to the invention, the liquid ammonia with the inert gases dissolved therein is first evaporated in at least one first process step, then compressed in at least one second process step and subsequently condensed again in at least one third process step.

[0015] The gases dissolved in the liquid ammonia fed into the system are primarily inert gases as defined above, i.e., gases that behave neutrally during the synthesis of ammonia, such as, in particular, the noble gases helium, argon, and methane. However, small amounts of hydrogen and nitrogen, the reactant gases of ammonia synthesis, may also be dissolved in the ammonia after synthesis. These gases do not fall under the term "inert gases" (and thus under claim 1), but can also be removed from the liquid ammonia using the procedure according to the invention. The inert gases, as well as hydrogen and nitrogen, are generally present in the liquid ammonia in dissolved quantities that are, for example, in the range of no more than approximately a few hundred ppm.

[0016] A particular advantage of the process according to the invention is that a product stream of "warm" liquid ammonia is obtained, freed from inert gases (and other gases), which is at a higher pressure than normal pressure and is thus suitable for immediate use in processes requiring pure liquid pressurized ammonia, for example, for urea synthesis or the like. However, if the ammonia were first cooled, for example, below the boiling point, which for ammonia is -33 °C, and expanded to normal pressure, in order to then store it as liquid ammonia at low temperatures in tanks, it would first have to be reheated and compressed for further processing of the type mentioned. This would require energy, so the process according to the invention leads to significant energy savings.In addition, the process according to the invention provides ammonia in a higher degree of purity than is usually obtained in ammonia synthesis, so that further purification steps for removing disturbing residual amounts of inert gases dissolved in the liquid ammonia are not required when the ammonia originating from ammonia synthesis is processed directly.

[0017] A preferred development of the invention provides that the condensation of the ammonia in the third process step takes place at an elevated pressure and at a temperature above +10°C, preferably at a temperature in the range of +10°C to +30°C. When reference is made herein to a product stream of "warm" liquid ammonia, this means that it is liquid ammonia at a temperature above the boiling point of ammonia under atmospheric pressure (which is -33°C), i.e., the liquid ammonia is present, for example, at approximately room temperature and thus under elevated pressure. The exact temperature of the warm liquid ammonia purified of dissolved gases depends on the temperature of the cooling water by means of which the condensation of the ammonia takes place in the third process step.

[0018] "Cold" liquid ammonia, on the other hand, is understood to be liquid ammonia that is either below the boiling point of -33 °C and thus liquid at atmospheric pressure, or at slightly higher temperatures, for example, between -33 °C and +10 °C and correspondingly increased pressure. This "cold" liquid ammonia, within the meaning of the present invention, thus exists at a lower temperature than the aforementioned "warm" liquid ammonia.

[0019] According to a preferred embodiment of the invention, the heat absorbed by the ammonia in the evaporation process step is used to cool a process stream. The evaporation process step, in which heat is extracted from the environment, can thus be used, for example, to cool process streams outside the system, for which no separate coolant needs to be provided.

[0020] According to a preferred embodiment of the invention, for example, the heat absorbed by the ammonia in the evaporation process step is used to condense ammonia in an ammonia synthesis plant. In this variant, the evaporation process step according to the invention allows for coupling with the condensation step of ammonia in an ammonia synthesis plant. Both process steps can be linked in an energetically advantageous manner.

[0021] According to a preferred embodiment of the present invention, the condensation of the ammonia in the third process step takes place at an elevated pressure in the range of, for example, approximately 10 bar to approximately 30 bar, preferably in the range of approximately 15 bar to approximately 25 bar. The pressure required here depends, as mentioned above, on the respective temperature at which the ammonia is condensed.

[0022] According to a preferred embodiment of the invention, the evaporated ammonia is compressed after evaporation together with the gases previously dissolved in the ammonia. This means that the dissolved gases are removed only afterward in the ammonia condenser.

[0023] According to a preferred embodiment of the invention, the compressed ammonia is condensed with the aid of cooling water after compression. The condensation temperature therefore depends on the temperature of the cooling water used.

[0024] A preferred development of the invention provides that the gases insoluble in the ammonia during the condensation of the ammonia are removed from the device used for condensation. During the condensation, the temperature and pressure are preferably selected such that the ammonia is just condensing. At or near the boiling point, the solubility of the gases in the liquid ammonia is lowest, so that the gases are then no longer dissolved in the ammonia and can therefore be separated from the liquid ammonia as gases and removed.

[0025] According to a possible development of the invention, it can be provided that the gases extracted in the previously described process are recycled. This is useful, for example, if these gases still contain portions of the reactant gases hydrogen and / or nitrogen, which can then be recycled to the ammonia production plant and used there to produce further ammonia.

[0026] According to a preferred development of the invention, only a first substream of a reactant stream of liquid ammonia is evaporated in the first process step, then compressed, subsequently condensed, and discharged from the system as ammonia freed from gases, particularly inert gases, in a liquid, pressurized warm stream. A second substream is branched off from the reactant stream and initially branched off from the system as a liquid cold stream without further processing. Regarding the terms "warm" stream and "cold" stream used here, reference is made to the definitions given above. The "warm" substream is generally fed to a plant in which further processing of the ammonia is planned, for example, a urea synthesis plant.This has the advantage that the purified ammonia, which is present at an elevated temperature and correspondingly increased pressure, can be processed directly, eliminating the need for heating and compression. This also avoids the energy loss that would occur if the ammonia were first cooled below its boiling point (-33 °C) and then temporarily stored at ambient pressure.

[0027] The second partial stream, which is branched off from the reactant stream, can, however, initially be temporarily stored as a cold stream of liquid ammonia and therefore does not need to undergo the purification steps of evaporation, compression, and condensation provided by the invention. This second partial stream can be temporarily stored cold, for example in a tank. For this purpose, this partial stream is expanded and cooled below the condensation temperature of the ammonia of -33 °C. During this expansion, the gases dissolved in the ammonia outgas, so that the purification process according to the invention for removing gases dissolved in the ammonia is not required for this partial stream. In other words, a portion of the ammonia for which it is known from the outset that it will not be further processed warm and under increased pressure, but for which cold interim storage is provided, can be branched off as a cold partial stream.

[0028] According to a preferred development of the invention, the first warm partial stream of ammonia, which is condensed and freed from the inert gases, represents a main stream of the ammonia discharged from the system. Depending on the intended use for the ammonia and how the process according to the invention is linked to subsequent processes for further processing of the ammonia, the warm partial stream, which constitutes the main stream, can, for example, constitute 70% to 100%, preferably up to about 90%, of the ammonia processed in the system, so that the partial stream which is not further processed and is discharged as a cold partial stream, for example, represents a smaller partial stream of 0% to 30%, preferably of at least about 10%, of the total reactant stream fed to the plant.

[0029] The present invention further relates to a plant for removing inert gases dissolved in liquid ammonia, comprising at least one line for supplying liquid ammonia to at least one ammonia evaporator, at least one ammonia compressor operatively connected to this ammonia evaporator, at least one ammonia condenser operatively connected to the ammonia compressor with means for discharging a stream of inert gases separated in the ammonia condenser, and at least one collecting container operatively connected to the ammonia condenser for receiving liquid ammonia freed from inert gases.

[0030] According to a further preferred embodiment of the plant according to the invention, the line for supplying liquid ammonia is connected to an outlet line of a plant for producing ammonia or is such an outlet line. In this preferred variant, ammonia can be freed from the gases dissolved in the ammonia and thus further purified in the plant according to the invention directly from the production process without intermediate cooling, at the pressure and temperature at which it is produced in this production process.

[0031] According to a further preferred development of the invention, the plant comprises a container for holding ammonia contaminated with inert gases, from which a first line leads to the ammonia evaporator, with at least one branch line branching off from this first line, by means of which a partial flow of liquid ammonia is discharged from the plant. In this way, it is possible to only partially, usually predominantly, free the ammonia from the gases dissolved in the ammonia in the plant according to the invention and discharge it as a warm, purified partial flow, while a smaller partial flow is passed on via the branch line without the purification process as a cold stream of liquid ammonia, in order to then store it, usually in a cold state.

[0032] The present invention will be explained in more detail below using an exemplary embodiment with reference to the accompanying drawings. Figure 1a schematically simplified flow diagram of an exemplary system according to the invention.

[0033] The following is based on reference to Figure 1 A possible embodiment of the present invention is explained in more detail. The illustration shows a simplified flow diagram of an exemplary plant according to the invention for removing inert gases dissolved in liquid ammonia from the ammonia. The illustration is highly simplified schematically, and only the plant components essential to the present invention are shown.

[0034] A process stream 10 of liquid ammonia, which contains dissolved gases such as inert gases and optionally other gases and which originates from a plant for the production of ammonia (this ammonia plant is not shown here), is fed via line 11 to a tank 12 in which liquid ammonia can be temporarily stored if necessary prior to the processing according to the invention. Liquid ammonia is discharged from this tank 12 via line 13, which splits downstream at a branch 14, so that two partial streams of liquid ammonia are created. A first, smaller partial stream flows via line 15 and is fed as a cold partial stream of liquid ammonia to another tank 16. There, the ammonia can be expanded to atmospheric pressure, whereby gases dissolved in the ammonia can outgas and be discharged via line 17. The cold liquid ammonia can be discharged via line 18 and e.g.exported or, after heating and compression, fed to a urea plant via line 17.

[0035] Starting at branch 14, however, the majority of the partial stream is processed according to the process according to the invention and fed to an ammonia evaporator 20 via line 19. After evaporation, the process stream is fed to an ammonia compressor 22 via line 21. After compression, the process stream passes via line 23 to an ammonia condenser 24. There, the compressed ammonia can be condensed with the aid of cooling water. The temperature of the ammonia after condensation is, for example, approximately 30 °C, and the condensed liquid ammonia is under correspondingly increased pressure. During this condensation, the gases to be separated from the ammonia escape and can therefore be withdrawn from the condenser in gaseous form via line 25 and discharged from the plant via outlet line 26.

[0036] The purified liquid ammonia can, if necessary, be temporarily stored under pressure at the appropriate temperature in a tank 27. A warm stream of pressurized liquid ammonia can then be fed from this tank via line 28 to another plant, such as a urea plant. Intermediate storage in tank 27 can also be omitted if, for example, there is an immediate need for a corresponding amount of "warm" liquid ammonia for another process in which the ammonia is required as a warm pressurized stream and can thus be directly processed without intermediate cooling.

[0037] The advantage of the process according to the invention is that it can be used to produce a purified "warm" partial stream of liquid ammonia on the one hand and, if necessary, also a "cold" partial stream of liquid ammonia, which does not require any separate treatment. List of reference symbols

[0038] 10Process stream 11Line 12Vessel 13Line 14Branch 15Line 16Tank 17Line 18Line 19Line / first partial stream 20Ammonia evaporator 21Line 22Ammonia compressor 23Line 24Ammonia condenser 25Line 26Outlet line 27Tank 28Line

Claims

1. A method of removing inert gases, such as the noble gases helium and argon, and methane, dissolved in liquid ammonia, characterized in that the liquid ammonia together with the gases dissolved therein is first evaporated in at least one first method step, then compressed in at least one second method step, and subsequently condensed further in at least one third method step.

2. The method as claimed in claim 1, characterized in that the ammonia is condensed in the third method step at an elevated pressure and at a temperature above +10°C, preferably at a temperature in the range from +10°C to +30°C.

3. The method as claimed in claim 1 or 2, characterized in that the ammonia is condensed in the third method step at an elevated pressure in the range from 10 bar to 30 bar, preferably in the range from 15 bar to 25 bar.

4. The method as claimed in any of claims 1 to 3, characterized in that the heat absorbed by the ammonia in the method step of evaporation is used for cooling of a process stream.

5. The method as claimed in claim 4, characterized in that the heat absorbed by the ammonia in the method step of evaporation is used for condensation of ammonia in an ammonia synthesis plant.

6. The method as claimed in any of claims 1 to 5, characterized in that the evaporated ammonia is compressed together with the gases dissolved in the ammonia.

7. The method as claimed in any of claims 1 to 6, characterized in that the compressed ammonia is then condensed with the aid of cooling water.

8. The method as claimed in any of claims 1 to 7, characterized in that the gases that are insoluble in ammonia on condensation of the ammonia are drawn off from the apparatus (24) used for condensation.

9. The method as claimed in claim 8, characterized in that the gases drawn off are recycled into a plant for preparation of ammonia.

10. The method as claimed in any of claims 1 to 9, characterized in that only a first substream (19) of a reactant stream of liquid ammonia is evaporated, then compressed, subsequently condensed and removed from the system as ammonia that has been freed of the gases, especially inert gases, in a liquid pressurized warm stream (28), while a second substream (15) is branched off from the reactant stream and branched off from the system as a cold liquid stream without further workup at first.

11. The method as claimed in claim 10, characterized in that the first substream (19) of the ammonia which is condensed and freed of the inert gases constitutes a main stream of the ammonia removed from the system.

12. The method as claimed in claim 10 or 11, characterized in that the second cold substream (15) is fed to a tank (16) and is expanded to standard pressure in said tank, in the course of which the gases, especially inert gases, that were dissolved in the ammonia beforehand outgas and then are drawn off from the tank (16).

13. A plant for removal of inert gases, such as the noble gases helium and argon, and methane, dissolved in liquid ammonia, comprising at least one conduit (11, 13, 19) for the supply of liquid ammonia to at least one ammonia evaporator (20), at least one ammonia compressor (22) functionally connected to said ammonia evaporator (20), at least one ammonia condenser (24) functionally connected to said ammonia compressor (22) and having means (25) of removing a stream of inert gases separated off in said ammonia condenser (24), and at least one collecting vessel (27) which is functionally connected to said ammonia condenser (24) and is for accommodating liquid ammonia that has been freed of inert gases.

14. The plant as claimed in claim 13, characterized in that the conduit (11) for the supply of liquid ammonia is connected to an output conduit of a plant for preparation of ammonia or is such an output conduit.

15. The plant as claimed in either of claims 13 and 14, characterized in that it comprises a vessel (12) for the accommodation of ammonia contaminated with inert gases, from which a first conduit (13) leads to the ammonia evaporator (20), wherein at least one branch conduit (15) that branches off from said first conduit is used to remove a substream of cold liquid ammonia from the plant and preferably to supply it to a further tank (16) in which the ammonia is expanded to standard pressure.