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 urea synthesis contains both ammonia and urea as impurities. Therefore, it is purified in a three-stage process before further use. In the first step, ammonia is removed, often at temperatures between 115°C and 140°C. This is followed by hydrolysis, in which urea decomposes into ammonia and carbon dioxide, 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 then removed in the third step. The separation processes for the first and third steps are identical; therefore, these two steps are typically performed in a single column simply split in the middle to sandwich the second step.

[0003] This three-stage process results in a very pure overall process condensate stream, particularly containing less than 3 ppm ammonia and less than 3 ppm urea. This is correspondingly expensive, complex, and energy-intensive. The off-gas scrubbing required for urea formation, particularly urea granulation or urea prilling, also represents a less stringent application purity requirement, particularly 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, U.S. Patent Application Publication No. 2019 / 0177180 proposes separating a substream after the first stage and sending it to off-gas scrubbing. However, this results in the first-stage and third-stage streams in a common column containing very different amounts, which can have adverse effects on design and operation.

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

[0006] US Pat. No. 4,410,503 discloses a method 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. Patent No. 4,652,678 [Patent Document 3] U.S. Patent 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 and 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 present invention is used to produce shaped urea materials, particularly urea granules or urea prills. The terms granules or prills are typically used to refer to particulate agglomerated materials with particle sizes suitable for use as fertilizers, typically produced by synthesis followed by a shaping step, granulation, or prilling. The integrated plant includes at least a urea synthesis unit and a urea molding unit. The integrated plant typically also includes an ammonia synthesis unit and, in many cases, a hydrogen production reformer. The ammonia synthesis unit produces ammonia from hydrogen and nitrogen. The ammonia and carbon dioxide are used to produce urea in the urea synthesis unit, which is then granulated in the urea molding unit, possibly with additional components such as ammonium nitrate, sulfur compounds, calcium carbonate, etc., into granules, which are widely used, particularly as fertilizers. The integrated plant may also include a nitric acid synthesis unit or an ammonium nitrate synthesis unit. In this case, ammonium nitrate may be mixed with, for example, urea as an additional component, and the mixture may be shaped, particularly granulated. The integrated plant includes a process condensate purification unit configured to separate ammonia and urea from the process condensate of the urea synthesis unit. This allows for further use of the water within or outside the integrated plant, or for its disposal. The process condensate purification unit includes one purified stream leading to a process condensate outlet. The purified stream passes through the process condensate in three stages to achieve high purity of the process condensate. This opens up a wide variety of uses for the purified process condensate. The integrated plant also includes a formed exhaust scrubbing unit connected to the urea forming unit and used to purify the off-gas from the unit. This formed exhaust scrubbing unit is used to remove urea entrained in the exhaust by scrubbing with water and reduce nitrogen emissions. The formed exhaust scrubbing unit often includes a second stage in which scrubbing with an acidic medium removes ammonia.

[0011] According to the present invention, the integrated plant includes not only a process condensate purifier but also a crude process condensate purifier. The crude process condensate purifier contains a crude stream. Because the crude stream undergoes simpler purification than the process condensate purifier, 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 refinery stream. Therefore, the crude process condensate purifier and the process condensate purifier are two separate devices, and the crude process condensate purifier is not technically the first stage of the process condensate purifier. The urea synthesis unit is connected to the process condensate purifier and the crude process condensate purifier for process condensate transfer. The crude process condensate purifier is connected to a formed exhaust scrubbing unit. Thus, according to the present invention, the process condensate stream from the urea synthesis unit is split into two substreams. The first substream enters a conventional process condensate purifier. Because this stream is fully purified, the first and third stages are configured and operated at the same volumetric flow rate. The new crude process condensate purifier, through which the second substream is passed, is arranged in parallel with the conventional process condensate purifier. Depending on ambient conditions, the demands of the first and second substreams may be approximately equal. The crude process condensate purifier purifies only the water stream required for the forming exhaust scrubbing device, for example, for granulation or prilling, in an amount sufficient for this purpose, thus purifying more energy efficiently. Removal of urea from the process condensate is not required for use in exhaust scrubbing. The crude process condensate purifier specifically includes only one stage, and therefore does not include hydrolysis in the second step. The amount of ammonia in the process condensate can be specifically adjusted via a separate crude process condensate purifier. According to the prior art, ammonia released during forming, especially during granulation or prilling, is preferably removed from the exhaust gas via acid scrubbing, for example, with nitric acid or sulfuric acid.Depending on the acid used, this forms related ammonia salts, which may be utilized as fertilizers or as inputs for further processes, such as the production of urea-ammonium nitrate (UAN). Ammonia remaining in the process condensate stream utilized for exhaust scrubbing downstream of the crude process condensate purification unit is scrubbed away in the acid scrubbing stage of the exhaust scrubbing. According to the prior art, the heat required for the process condensate purification unit is primarily provided in the form of steam that is directly fed to the process and thus becomes the process condensate itself. If process condensate purification were performed more energy efficiently, the amount of steam supplied could be reduced. This results in a reduction in the amount of process condensate produced, since the steam savings mean that no additional water is introduced, which would increase 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 provides two fundamental advantages. The first advantage is that the process condensate purifier is specifically configured for a constant purified stream passing through it. Thus, all stages are advantageously configured for the same volumetric flow rate. The second advantage is that upgrades in the context of capacity expansion are therefore easily possible. For example, if a corresponding existing integrated plant is retrofitted to increase its production capacity, the existing process condensate purifier can continue to operate, while a new, additional, separate crude process condensate purifier is installed that purifies the additional process condensate stream resulting from the capacity expansion in a simple and reduced manner and makes it available for appropriate applications, particularly for shaped exhaust scrubbing equipment. Thus, 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 includes a first heat exchanger configured to preheat the process condensate stream coming from the urea synthesis unit. This is accomplished by the heat exchanger being 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 present invention, the crude process condensate purification unit includes a second heat exchanger. The second heat exchanger is configured to condense the gas stream exiting the crude process condensate purification unit. This may be particularly suitable for condensing a gas mixture containing water, ammonia, and carbon dioxide. The process condensate may also be referred to as a carbamate solution, since the ammonia and carbon dioxide present therein partially react with each other, particularly in aqueous solution, to form carbamates. The second heat exchanger and the crude process condensate purification unit are preferably connected to a carbamate recycle line. 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, if necessary, to increase the ammonia and carbon dioxide concentrations. This is advantageous because the carbamate solution is fed to a urea synthesis unit, where a low water input is beneficial for reaction management.

[0016] In a further embodiment of the present invention, the crude process condensate purification system includes a third heat exchanger. The crude process condensate purification system further includes a recycle conduit for a substream of the crude process condensate stream. The third heat exchanger is disposed within the recycle conduit and configured to heat the recycle substream. Steam can also be directly introduced to provide the heat required for the crude process condensate purification system. However, this direct steam introduction increases the amount of water in the crude process condensate stream. In contrast, indirect heat introduction via a heat exchanger leaves the total amount unchanged. Evaporation of the substream may occur, in particular, 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, which is 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 particularly 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 particularly 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, such as a demister for droplet separation. Because this is the final purification step before exhaust gas is released into the environment, the scrubbing solution used therein must meet certain purity requirements. If these requirements cannot be met by the crude process condensate from the crude process condensate purification device, a separate source of scrubbing solution must be selected for this final 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 for scrubbing. Therefore, the aforementioned positive effects of the crude process condensate purification device are maintained in this embodiment. [Brief explanation 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 INVENTION

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

[0024] A further substream of the process condensate, e.g., 50%, is transferred from process condensate storage means 3 to crude process condensate purification unit 20. There, the process condensate first enters first heat exchanger 22 where it is preheated and then transferred to column 21, e.g., a randomly packed column. The gas stream exiting the top of column 21 passes through second heat exchanger 23 where it is condensed. That substream is returned to column 21, and the remaining substream passes through first conduit 40 to urea synthesis unit 1. The crude process condensate exiting the bottom of the column is split, and a substream is heated and at least partially vaporized via a process condensate recycle conduit and third heat exchanger 24 and returned to column 21. The remaining substream of the crude process condensate is sent for heat recovery via first heat exchanger 22 and subsequently enters formed exhaust scrubbing unit 4 to absorb additional 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 unit 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, the integrated plant comprising at least one urea synthesis unit (1) and a urea formation unit (2), the integrated plant comprising a process condensate purification unit (10) configured to separate ammonia and urea from a process condensate from the urea synthesis unit (1), the process condensate purification unit (10) comprising one purified stream to a process condensate outlet (32), the process condensate purification unit (10) configured as an apparatus for carrying out a three-stage process, and the integrated plant comprising a formed exhaust scrubbing unit (4), In addition to the apparatus (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 the crude process condensate purification unit (20) for the transfer of process condensate, the crude process condensate purification unit (20) being connected to the formed exhaust scrubbing unit (4), the crude process condensate purification unit and the process condensate purification unit being therefore two separate units, the crude process condensate purification unit being not the first stage of the process condensate purification unit, 1. An integrated plant, characterized in that the crude process condensate purification unit (20) comprises a first heat exchanger (22), the first heat exchanger (22) configured to preheat the process condensate stream coming from the urea synthesis unit (1), and the first heat exchanger (22) configured to cool the crude process condensate stream leaving the crude process condensate purification unit (20).

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. 2. The integrated plant of claim 1, wherein the crude process condensate purification unit (20) comprises a second heat exchanger (23), the second heat exchanger (23) configured to condense the gas stream exiting the crude process condensate purification unit (20).

4. 4. An integrated plant according to claim 3, characterized in that the second heat exchanger (23) and the crude process condensate purification unit (20) are connected to a carbamate recycle line.

5. 2. The integrated plant of claim 1, wherein the crude process condensate purification unit (20) comprises a third heat exchanger (24), the crude process condensate purification unit (20) comprises a recycle conduit for a substream of the crude process condensate stream, the third heat exchanger (24) configured in the recycle conduit for heating the recycle substream.

6. 2. The integrated plant according to claim 1, 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).

7. 2. The integrated plant of claim 1, wherein 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).

8. 8. The integrated plant of claim 7, wherein 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), and the first conduit and the second conduit are connected to each other.

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