Optimized process condensate preparation

Abstract

The present invention relates to a combined system for producing urea-forming material. The combined system comprises at least one urea synthesis device (1) and a urea formation device (2), the combined system comprises a process condensate cleaning device (10) designed to separate ammonia and urea from the process condensate of the urea synthesis device (1), and the combined system comprises a formation exhaust scrubbing device (4). The present invention is characterized in that the combined system further comprises, in addition to the process condensate cleaning device (10), a crude process condensate cleaning device (20). The urea synthesis device (1) is connected to the process condensate cleaning device (10) and the crude process condensate cleaning device (20) for transporting the process condensate, and the crude process condensate cleaning device (20) is connected to the formation exhaust scrubbing device (4).

Description

[Technical field]

[0001] The present invention relates to the treatment of process condensate produced during urea synthesis. [Background technology]

[0002] The process condensate produced in the urea synthesis contains both ammonia and urea as impurities. It is therefore purified in a three-stage process before further use. In the first step, ammonia is often removed at temperatures between 115°C and 140°C. Subsequently, hydrolysis, in which urea decomposes into ammonia and carbon dioxide, is carried out at temperatures between about 190°C and 200°C, typically above 15 bar, for a duration of 60 to 120 minutes. The resulting ammonia and carbon dioxide are subsequently removed in the third step. The separation processes for the first and third steps are identical, and therefore these two steps are typically carried out in one column that is simply split in the middle to sandwich the second step.

[0003] This three-stage process results in a very pure overall process condensate stream, especially with less than 3 ppm ammonia and less than 3 ppm urea. It is correspondingly expensive, complex, and energy intensive. The off-gas scrubbing required for urea formation, especially urea granulation or urea prilling, also represents a less stringent application purity requirement, especially since in this case urea is introduced as an impurity from the off-gas.

[0004] Therefore, it is energetically advantageous to use less purified process condensate for such off-gas scrubbing. To this end, US Patent Application Publication No. 2019 / 0177180 proposes separating a substream after the first stage and sending it to off-gas scrubbing. However, this has the consequence that the first and third stage streams in a common column contain very different amounts, which can have adverse effects on design and operation.

[0005] US Pat. No. 4,652,678 discloses a process for recovering valuable components from off-gas streams from urea synthesis.

[0006] US Pat. No. 4,410,503 discloses a process for removing urea, ammonia and carbon dioxide from dilute aqueous solutions. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] US Patent Application Publication No. 2019 / 0177180 [Patent Document 2] U.S. Pat. No. 4,652,678 [Patent Document 3] U.S. Pat. No. 4,410,503 Summary of the Invention [Problem to be solved by the invention]

[0008] It is an object of the present invention to provide a process condensate treatment that provides high purity for routine use as well as lower purity for off-gas scrubbing, thus allowing optimal process control with minimal energy consumption. [Means for solving the problem]

[0009] This object is achieved by an integrated plant having the features of claim 1. Advantageous developments are evident from the dependent claims, the following description and the drawings.

[0010] The integrated plant according to the invention is used to produce shaped urea material, in particular urea granules or urea prills. The term granules or prills is typically used for and refers to a particulate agglomerated material having a particle size suitable for use as a fertilizer and typically produced in a forming step, granulation or prilling, following synthesis. The integrated plant comprises at least a urea synthesis unit and a urea forming unit. The integrated plant typically also comprises an apparatus for the synthesis of ammonia, and often also a reformer for producing hydrogen. The apparatus for the synthesis of ammonia produces ammonia from hydrogen and nitrogen. This ammonia and carbon dioxide are used to produce urea in the urea synthesis unit, which is optionally made into granules in the urea forming unit with further components, such as ammonium nitrate, sulfur compounds, calcium carbonate, etc., which are widely used in particular as fertilizers. The integrated plant may also comprise a nitric acid synthesis unit and may also comprise an ammonium nitrate synthesis unit. In this case, ammonium nitrate may be mixed, for example, with urea as a further component, and forming, in particular granulation, may be carried out on the mixture. The integrated plant comprises a process condensate purifier, which is configured to separate ammonia and urea from the process condensate of the urea synthesis unit. This allows for further use of the water within the integrated plant or outside the integrated plant, or for its disposal. The process condensate purifier comprises one purified stream to a process condensate outlet. The purified stream typically passes through the process condensate in three stages to achieve high purity of the process condensate, which opens up a wide variety of uses for the purified process condensate. The integrated plant comprises a formed exhaust scrubbing unit connected to the urea forming unit and used to purify the off-gas from said unit. This formed exhaust scrubbing unit is used to remove urea entrained in the exhaust by scrubbing with water and to reduce nitrogen emissions. The formed exhaust scrubbing unit often comprises a second stage in which scrubbing with an acidic medium removes ammonia.

[0011] According to the invention, the integrated plant comprises not only a process condensate purifier, but also a crude process condensate purifier. The crude process condensate purifier contains a crude stream. The crude stream undergoes a simpler purification compared to the process condensate purifier, so that the crude process condensate has a lower purity, but the cost and complexity of the crude purification are correspondingly reduced. The crude stream is separate and distinct from the purification stream. Thus, the crude process condensate purifier and the process condensate purifier are two separate devices, and the crude process condensate purifier is not precisely just the first stage of, for example, a process condensate purifier. The urea synthesis unit is connected to the process condensate purifier and the crude process condensate purifier for the transfer of the process condensate. The crude process condensate purifier is connected to the formed exhaust scrubbing unit. Thus, according to the invention, the stream of process condensate from the urea synthesis unit is split into two substreams. The first substream enters the process condensate purifier according to the prior art. Since this stream is fully purified, the first and third stages are configured and operated in this way for the same volume flow rate. The novel crude process condensate purifier, through which the second substream is passed, is arranged in parallel with the conventional process condensate purifier. Depending on the ambient conditions, the demands of the first and second substreams may, for example, be approximately equal. The crude process condensate purifier purifies the water stream required for the shaping exhaust scrubbing device, for example for granulation or prilling, only in an amount sufficient for this application and thus more energy-efficiently. Removal of urea from the process condensate is not necessary for the use of the process condensate for exhaust scrubbing. The crude process condensate purifier in particular includes only one stage and therefore does not include hydrolysis in the second step in particular. The amount of ammonia in the process condensate can be specifically adjusted via a separate crude process condensate purifier. According to the prior art, the ammonia released in the shaping, in particular in the granulation or prilling, is preferably removed from the exhaust air via acid scrubbing, for example with nitric acid or sulfuric acid.This forms the relevant ammonia salts depending on the acid used, which may be utilized as fertilizer or as input for further processes, for example to produce urea-ammonium nitrate (UAN). The ammonia remaining in the process condensate stream utilized for exhaust scrubbing downstream of the crude process condensate purification unit is scrubbed off in the acid scrubbing stage of the exhaust scrubbing. According to the prior art, the heat required for the process condensate purification unit is provided mainly in the form of steam that is directly fed to the process and thus becomes the process condensate itself. If the process condensate purification is performed more energy-efficiently, the amount of this fed steam can be reduced. This results in a reduction in the amount of process condensate produced, since the steam saved means that no additional water is introduced, which increases the amount of process condensate.

[0012] The strict separation of the process condensate purifier and the purified stream from the crude process condensate purifier and the crude purified stream results in two fundamental advantages. The first advantage is that the process condensate purifier in particular is configured for a constant purified stream passing through it. Thus, all stages are configured for the same volumetric flow rate, which is advantageous. The second advantage is that an upgrade in the context of a capacity expansion is therefore easily possible. For example, if a corresponding existing integrated plant is retrofitted to increase the production capacity, the existing process condensate purifier can continue to be operated, while a new additional separate crude process condensate purifier is installed, which purifies the additional process condensate stream resulting from the capacity expansion in a simple and reduced manner and makes it available for suitable applications, in particular for a formed exhaust scrubbing unit. Thus, a capacity expansion of the process condensate purifier can be advantageously avoided.

[0013] In a further embodiment of the invention, the crude process condensate purification unit is configured as a single stage for the removal of ammonia. The crude process condensate purification unit is configured in particular in the form of a column, in particular a tray column or a randomly packed column. It is therefore preferred if the second stage for hydrolyzing urea to ammonia and CO2 is not precisely a component of the crude process condensate purification unit.

[0014] In a further embodiment of the invention, the crude process condensate purification unit comprises a first heat exchanger configured to preheat the process condensate stream coming from the urea synthesis unit. This is achieved in that the heat exchanger is configured to cool the crude process condensate stream leaving the crude process condensate purification unit. Thus, process heat can be retained within the crude process condensate purification unit.

[0015] In a further embodiment of the invention, the crude process condensate purification unit comprises a second heat exchanger. The second heat exchanger is configured to condense the gas stream leaving the crude process condensate purification unit. It may in particular condense a gas mixture containing water, ammonia and carbon dioxide. The process condensate may also be called a carbamate solution, since the ammonia and carbon dioxide present therein react partially with each other, especially in aqueous solution, to form carbamates. The second heat exchanger and the crude process condensate purification unit are preferably connected to a carbamate recycle conduit. This allows a substream of the carbamate solution to be recycled to the crude process condensate purification unit. This recycling allows the water content in the carbamate solution to be reduced as required, to increase the concentration of ammonia and carbon dioxide. This is advantageous, since this carbamate solution is fed to a urea synthesis unit, where a low water input is advantageous for reaction management.

[0016] In a further embodiment of the invention, the crude process condensate purification unit comprises a third heat exchanger. The crude process condensate purification unit further comprises a recirculation conduit for a substream of the crude process condensate stream. The third heat exchanger is arranged in the recirculation conduit and is configured to heat the recirculation substream. Steam can also be directly introduced to provide the required heat to the crude process condensate purification unit. However, this direct steam introduction increases the amount of water in the crude process condensate stream. In contrast, the indirect heat introduction via a heat exchanger leaves the total amount unchanged. The evaporation of the substream may in particular take place in the third heat exchanger or downstream thereof, for example downstream of a corresponding pressure valve.

[0017] In a further embodiment of the invention the integrated plant comprises a process condensate storage means arranged downstream of the urea synthesis unit and upstream of the process condensate purification unit and the crude process condensate purification unit.

[0018] In a further embodiment of the invention, the crude process condensate purifier and the urea synthesis unit are connected via a first conduit for recycling the carbamate solution formed in the crude process condensate purifier. The process condensate purifier and the urea synthesis unit are in particular connected via a second conduit for recycling the carbamate solution leaving the process condensate purifier. The first and second conduits are connected to each other. They can in particular be open to each other and enter the urea synthesis unit as a common conduit.

[0019] In a further embodiment of the present invention, the urea forming apparatus includes one or more fine exhaust scrubbing devices, for example droplet separation by demister. Since this is the last purification step before the discharge of exhaust air to the environment, the scrubbing solution used therein must meet certain purity requirements. If this requirement is not met by the crude process condensate from the crude process condensate purification device, another source of scrubbing solution must be selected for this last purification step. Typically, the clean process condensate from the process condensate purification device is then used as before. The amount of water in the fine exhaust scrubbing device is typically small compared to the total water requirement of scrubbing. Therefore, the above-mentioned positive effects of the crude process condensate purification device are maintained in this embodiment. [Brief description of the drawings]

[0020] [Figure 1] A cross section of the integrated plant is shown.

[0021] The integrated plant according to the invention will now be explained in more detail with reference to an exemplary embodiment shown in the drawing. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] FIG. 1. Exemplary embodiment The representation is purely schematic and has been greatly simplified to explain the invention. Conventional components, such as valves, heat exchangers, conveying devices, such as pumps, etc., have been omitted for simplicity and are well known to those skilled in the art of such plants. The representation is intended to explain the concept of the invention.

[0023] A cross section of the integrated plant is shown in FIG. 1. Ammonia and carbon dioxide are fed at the reactant inlet 30 of the urea synthesis unit 1 and are converted therein to urea. The urea is then transferred to the urea former 2. The exhaust from the urea former 2 is transferred to the exhaust scrubbing unit 4. The process condensate formed during the synthesis of urea from ammonia and carbon dioxide is transferred from the urea synthesis unit 1 to the process condensate storage means 3. This is where the process condensate stream is split according to the invention. As is known and customary for such integrated plants according to the prior art, a substream of, for example, 50% is transferred to the process condensate purification unit 10. The process condensate first passes through a fourth heat exchanger 14 into the first stage 11, where the ammonia is separated, for example at 140° C. and 3 bar. After passing through the first stage 11, the process condensate passes through a fifth heat exchanger 15 into the second stage 12, for example at 200° C. and 16 bar for 60 minutes. This causes the urea to react with water to obtain ammonia and carbon dioxide. Through the fifth heat exchanger 15, the process condensate enters the third stage 13 where the ammonia is separated (in a common column) under the same conditions as in the first stage 11. It then returns to the urea synthesis unit 1 via the second conduit 41. The purified process condensate passes from the third stage 13 through the fourth heat exchanger 14 to the process condensate outlet 32 ​​where it can be transferred to other processes.

[0024] A further substream of the process condensate, for example 50%, is transferred from the process condensate storage means 3 to the crude process condensate purification unit 20. There, the process condensate first enters a first heat exchanger 22 where it is preheated and then transferred to a column 21, for example a random packed column. The gas stream leaving the top of the column 21 passes through a second heat exchanger 23 where it is condensed. That substream is returned to the column 21 and the remaining substream passes through a first conduit 40 into the urea synthesis unit 1. The crude process condensate leaving the bottom of the column is split and a substream is heated and at least partially evaporated via a process condensate recycle conduit and a third heat exchanger 24 and returned to the column 21. The remaining substream of the crude process condensate is sent for heat recovery via the first heat exchanger 22 and subsequently enters a formed exhaust scrubbing unit 4 to absorb further urea from the exhaust. [Explanation of symbols]

[0025] 1 Urea synthesis equipment 2 Urea forming equipment 3. Process condensate storage means 4. Molded exhaust scrubbing equipment 10 Process condensate purification equipment 11 First Stage 12 Second Stage 13 Third Stage 14 Fourth Heat Exchanger 15 Fifth Heat Exchanger 20 Crude Process Condensate Purification Unit 21 Column 22 First heat exchanger 23 Second Heat Exchanger 24 Third Heat Exchanger 30 Reactant inlet 31 Product outlet 32 Process condensate outlet 40 First Conduit 41 Second Conduit

Claims

1. An integrated plant for producing a formed urea material, said integrated plant comprising at least one urea synthesis unit (1) and a urea formation unit (2), said integrated plant comprising a process condensate purification unit (10) configured to separate ammonia and urea from a process condensate from said urea synthesis unit (1), said process condensate purification unit (10) comprising one purified stream to a process condensate outlet (32), said integrated plant comprising a formed exhaust scrubbing unit (4).

4. The integrated plant of claim 3, wherein in addition to the process condensate purification unit (10), the integrated plant comprises a crude process condensate purification unit (20), the crude process condensate purification unit (20) containing a crude purification stream, the crude purification stream being separate and distinct from the purification stream, the urea synthesis unit (1) being connected to the process condensate purification unit (10) and to the crude process condensate purification unit (20) for the transfer of process condensate, and the crude process condensate purification unit (20) being connected to the formed exhaust scrubbing unit (4).

2. 2. An integrated plant according to claim 1, characterized in that the crude process condensate purification unit (20) is configured as a single stage for the removal of ammonia.

3. 3. An integrated plant according to claim 1 or 2, characterized in that the crude process condensate purification unit (20) comprises a first heat exchanger (22), which is configured to preheat the process condensate stream coming from the urea synthesis unit (1), and which is configured to cool the crude process condensate stream leaving the crude process condensate purification unit (20).

4. 4. An integrated plant according to any one of claims 1 to 3, characterized in that the crude process condensate purification unit (20) comprises a second heat exchanger (23), the second heat exchanger (23) being configured to condense the gas stream leaving the crude process condensate purification unit (20).

5. 5. An integrated plant according to claim 4, characterized in that the second heat exchanger (23) and the crude process condensate purification unit (20) are connected to a carbamate recycle line.

6. 6. The integrated plant according to any one of claims 1 to 5, characterized in that the crude process condensate purification unit (20) comprises a third heat exchanger (24), the crude process condensate purification unit (20) comprising a recycle conduit for a substream of the crude process condensate stream, the third heat exchanger (24) being arranged in the recycle conduit for heating the recycle substream.

7. 7. Integrated plant according to any one of claims 1 to 6, characterized in that the integrated plant comprises a process condensate storage means (3), which is arranged downstream of the urea synthesis unit (1) and upstream of the process condensate purification unit (10) and the crude process condensate purification unit (20).

8. 8. Integrated plant according to any one of claims 1 to 7, characterized in that the crude process condensate purification unit (20) and the urea synthesis unit (1) are connected via a first conduit for recycling the carbamate stream leaving the crude process condensate purification unit (20).

9. 9. The integrated plant according to claim 8, characterized in that the process condensate purification unit (10) and the urea synthesis unit (1) are connected via a second conduit for recycling the carbamate stream leaving the process condensate purification unit (10), the first conduit and the second conduit being connected to each other.

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