High-pressure liquid ammonia purification and ammonia synthesis method

Abstract

The invention discloses a high-pressure liquid ammonia purification and ammonia synthesis method, which is characterized in that coarse high-pressure ammonia reaction gas at an outlet at the bottom of an ammonia synthesis tower is subjected to heat exchange and cooling to form coarse high-pressure liquid ammonia, the coarse high-pressure liquid ammonia and non-condensable gas are subjected to gas stripping by a high-pressure liquid ammonia purification gas stripping tower, and the non-condensable gas and gas stripping gas form circulating mixed gas; and qualified high-pressure liquid ammonia after gas stripping is directly sent to a downstream urea device. High-pressure liquid ammonia is subjected to high-pressure nitrogen gas stripping to obtain high-purity high-pressure liquid ammonia, the high-pressure liquid ammonia is directly sent to a downstream urea device, on one hand, energy of the high-pressure liquid ammonia can be effectively recycled, energy consumed by liquid ammonia pressurization of the downstream urea device and investment of a high-pressure liquid ammonia pump are reduced or eliminated, and the production cost is saved; after the high-pressure liquid ammonia purification gas stripping tower is adopted, medium-pressure and low-pressure flash evaporation systems are canceled, the pressure energy of a synthesis loop is fully utilized, and the investment of a synthesis device is greatly reduced; no medium-pressure and low-pressure flash evaporation system needs to be arranged, so that the compression power consumption of re-pressurization of medium-pressure and low-pressure flash evaporation steam is avoided, and the energy consumption is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of ammonia synthesis, and in particular relates to a method for purifying high-pressure liquid ammonia and synthesizing ammonia. Background Art

[0002] Ammonia is one of the important inorganic chemical products and plays an important role in the national economy. By 2023, the world's annual synthetic ammonia production will be close to 200 million tons, of which about 80% is used to produce chemical fertilizers and 20% is used as raw materials for other chemical products. In order to facilitate the storage and transportation of ammonia, the subsequent configuration of synthetic ammonia plants with urea is currently the most typical configuration in the industry.

[0003] As we all know, synthetic ammonia is directly synthesized from nitrogen and hydrogen under high temperature, high pressure and catalyst. German chemist Haber began to study the direct synthesis of ammonia from nitrogen and hydrogen in 1902. In 1908, he applied for a patent, namely the "circulation method". At present, commercial synthetic ammonia plants basically operate at a pressure of 130-180 barg in order to balance the energy consumption of the plant and the equipment investment. The matching urea plant is also usually operated at 180-200 barg. As the main process raw material for urea synthesis, liquid ammonia, after being separated from the high-pressure circuit, is usually flashed step by step to release the dissolved gas because it contains hydrogen, nitrogen and impurity gases such as argon and methane brought in by the fresh gas. At the same time, in order to recover the effective gas (H2, N2) released by the flash of high-pressure liquid ammonia, the medium and low pressure flash gas is usually further sent to the fresh section inlet of the synthesis gas compressor, and is used by the system after recompression.

[0004] Since hydrogen will accumulate in the urea unit, it will form explosive gas with the passivated oxygen in the urea unit, which brings great risks to the stable operation of the urea unit. Therefore, it is important for the synthetic ammonia process to ensure that the hydrogen dissolved in the liquid ammonia product is reduced to an acceptable range. At the same time, in the conventional process, liquid ammonia needs to be further pressurized from the low pressure after flash evaporation (usually 16 to 20 barg) to the urea synthesis pressure of 180 to 200 barg. This process of decompression and pressurization not only wastes precious energy, but also makes the selection and manufacturing of high-pressure ammonia pumps more difficult, increases the investment in downstream urea units, and reduces the reliability of unit operation. Summary of the invention

[0005] The purpose of the present invention is to solve the above technical problems and provide an energy-saving and highly safe high-pressure liquid ammonia purification and ammonia synthesis method.

[0006] To achieve the above-mentioned object, the present invention provides a method for purifying high-pressure liquid ammonia and synthesizing ammonia, specifically: the crude high-pressure ammonia reaction gas at the bottom outlet of the ammonia synthesis tower is subjected to heat exchange and cooling to form crude high-pressure liquid ammonia, the crude high-pressure liquid ammonia and non-condensable gas are stripped by a high-pressure liquid ammonia purification stripping tower, the non-condensable gas and the stripping gas form a circulating mixed gas, and the qualified high-pressure liquid ammonia after stripping is directly sent to the downstream urea device.

[0007] Furthermore, the high-pressure liquid ammonia purification stripping tower comprises an upper gas-liquid separation section, a middle heat exchange section and a lower stripping section. The crude high-pressure liquid ammonia and non-condensable gas enter the upper gas-liquid separation section of the high-pressure liquid ammonia purification stripping tower from the reactant inlet. The non-condensable gas is separated through the upper gas-liquid separation section as circulating gas. The crude high-pressure liquid ammonia exchanges heat with the hot ammonia entering from the hot ammonia inlet after passing through the middle heat exchange section. The cooled liquid ammonia is sent out from the cold ammonia outlet. The crude high-pressure liquid ammonia after further heat exchange further releases hydrogen, nitrogen, methane and argon dissolved in the crude high-pressure liquid ammonia. The crude high-pressure liquid ammonia after heat exchange and the high-pressure nitrogen entering from the stripping nitrogen inlet are stripped through the lower stripping section. The hydrogen dissolved in the crude high-pressure liquid ammonia is stripped out. The qualified high-pressure liquid ammonia is directly sent to the downstream urea device through the high-pressure liquid ammonia outlet.

[0008] Furthermore, the circulating mixed gas formed by the circulating gas and the stripping gas returns to the circulating gas compressor together after recovering cold energy through the circulating mixed gas outlet and the cold exchanger.

[0009] Furthermore, the circulating mixed gas is mixed with hydrogen and nitrogen compressed by a hydrogen and nitrogen compressor to form a mixed gas, and the mixed gas is pressurized together in the circulating gas compressor to form a high-pressure mixed gas; the crude high-pressure ammonia reaction gas at the bottom outlet of the ammonia synthesis tower is subjected to a primary heat exchanger, and the ammonia-containing reaction gas after the primary heat exchange is subjected to a secondary heat exchange with the high-pressure mixed gas in a secondary heat exchanger, and the high-pressure mixed gas enters the ammonia synthesis tower after being heated, and the hydrogen and nitrogen in the high-pressure mixed gas react with the catalyst in the ammonia synthesis tower to generate crude high-pressure ammonia.

[0010] Furthermore, after the secondary heat exchange, the crude high-pressure ammonia reaction gas is cooled step by step in a water cooler, a cold exchanger and an ammonia cooler, and then condensed to form crude high-pressure liquid ammonia. The crude high-pressure liquid ammonia and non-condensable gas enter the high-pressure liquid ammonia purification stripping tower for stripping.

[0011] Furthermore, a top gas-liquid separation component is arranged above the upper gas-liquid separation section, and the gas-liquid separation component is a demister or a blade-type separation component; the middle heat exchange section is a falling film heat exchanger, and the upper tube head of the falling film heat exchanger is slightly higher than the upper tube plate, and the upper tube head is provided with a rotating liquid dropper.

[0012] Furthermore, the pressure of the high-pressure mixed gas is 130-180 barg.

[0013] Furthermore, the temperature of the high-pressure mixed gas entering the ammonia synthesis tower is 170-230°C.

[0014] Furthermore, the pressure of the qualified high-pressure liquid ammonia is 130-180 barg.

[0015] Furthermore, the volume ratio of hydrogen to nitrogen in the hydrogen-nitrogen gas is 3.05 to 3.20. After nitrogen is extracted by supplementary gas, the stoichiometric ratio required by the synthesis loop is achieved.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] 1) The high-pressure liquid ammonia of the present invention adopts high-pressure nitrogen gas stripping to obtain high-purity high-pressure liquid ammonia. The high-purity high-pressure liquid ammonia has a pressure of 130-180 barg and is directly sent to the downstream urea device. On the one hand, the energy of the high-pressure liquid ammonia can be effectively recovered. At the same time, the energy consumed by the pressurization of the liquid ammonia of the downstream urea device and the investment in the high-pressure liquid ammonia pump are reduced or eliminated, thereby saving production costs. On the other hand, the requirements of the downstream urea device on the hydrogen content in the high-pressure liquid ammonia are met, thereby improving the safety and stability of the operation of the downstream urea device.

[0018] 2) The present invention adopts a high-pressure liquid ammonia purification stripping tower, thereby eliminating the medium-pressure and low-pressure flash evaporation systems, making full use of the pressure energy of the synthesis loop, and greatly reducing the investment in the synthesis device; at the same time, because there is no need to set up medium- and low-pressure flash evaporation systems, the compression power consumption of re-pressurizing the medium- and low-pressure flash vapor is avoided, saving energy consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic flow chart of the high-pressure liquid ammonia purification and ammonia synthesis method of the present invention;

[0020] Figure 2 for Figure 1 Schematic diagram of the structure of the medium and high pressure liquid ammonia purification stripping tower;

[0021] Figure 3 for Figure 2 Schematic diagram of the upper tube head structure of the falling film heat exchanger. DETAILED DESCRIPTION

[0022] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0023] like Figure 1In the method for purifying high-pressure liquid ammonia and synthesizing ammonia, the hydrogen and nitrogen gas (the molar ratio of which is slightly higher than 3, preferably 3.05-3.20, and the stoichiometric ratio required by the synthesis loop is reached after supplementary gas stripping nitrogen) from the upstream device is compressed by a hydrogen and nitrogen compressor, and then mixed with a circulating mixed gas (gas stripping gas + circulating gas) to form a mixed gas, which is pressurized to 130-180 barg in the circulating gas compressor to form a high-pressure mixed gas; the crude high-pressure ammonia reaction gas at the bottom outlet of the ammonia synthesis tower is subjected to a primary heat exchanger, and the primary heat exchanger is used to heat the crude high-pressure ammonia reaction gas at the bottom outlet of the ammonia synthesis tower. The heated ammonia-containing reaction gas undergoes a secondary heat exchange with the high-pressure mixed gas in the secondary heat exchanger, and the high-pressure mixed gas is heated to 170-230°C and enters the ammonia synthesis tower; the hydrogen and nitrogen in the high-pressure mixed gas react with the catalyst in the ammonia synthesis tower to generate crude high-pressure ammonia. Since the ammonia synthesis reaction is an exothermic reaction, the temperature of the crude high-pressure ammonia reaction gas leaving the ammonia synthesis tower rises to 420-460°C. The high-temperature crude high-pressure ammonia reaction gas undergoes a primary heat exchange to recover the reaction heat (usually by-product steam), and then exchanges heat with the high-pressure mixed gas at the outlet of the circulating gas compressor.

[0024] The crude high-pressure ammonia reaction gas after the secondary heat exchange is cooled step by step through a water cooler, a cold exchanger and an ammonia cooler, and then condensed to form crude high-pressure liquid ammonia. The crude high-pressure liquid ammonia and non-condensable gas (hydrogen and nitrogen) enter the high-pressure liquid ammonia purification stripping tower for stripping. After stripping, the non-condensable gas and the stripping gas used as circulating gas form a circulating mixed gas, which is returned to the circulating gas compressor together after the cold exchanger recovers the cold. The qualified high-pressure liquid ammonia after stripping is directly sent to the downstream urea device.

[0025] like Figure 2 The high-pressure liquid ammonia purification stripping tower shown comprises an upper gas-liquid separation section 1, a middle heat exchange section 2 and a lower gas stripping section 3. A high-pressure liquid ammonia purification stripping tower is provided with a high-pressure liquid ammonia outlet 7 at the bottom and a circulating mixed gas outlet 6 at the top. A gas-liquid separation component 8 is arranged above the upper gas-liquid separation section 1, a reactant inlet 4 is provided at the lower part of the upper gas-liquid separation section, a cold ammonia outlet 10 is provided at the upper part of the middle heat exchange section 2, and a hot ammonia inlet 9 is provided at the lower part, and a gas stripping nitrogen inlet 5 is provided at the lower part of the lower gas stripping section 3.

[0026] The crude high-pressure liquid ammonia and the non-condensable gas enter the upper gas-liquid separation section 1 of the high-pressure liquid ammonia purification stripping tower from the reactant inlet 4. The non-condensable gas is separated as circulating gas through the upper gas-liquid separation section 1. The crude high-pressure liquid ammonia is heat-exchanged with the hot ammonia entering from the hot ammonia inlet 9 after passing through the middle heat exchange section 2. The cooled liquid ammonia is sent out from the cold ammonia outlet 10. The crude high-pressure liquid ammonia after further heat exchange further releases the hydrogen, nitrogen, methane, argon and other gases dissolved therein. The crude high-pressure liquid ammonia after heat exchange and the high-pressure nitrogen entering from the stripping nitrogen inlet 5 are passed through the lower gas exchange section 2. The stripping section 3 is gas stripping, and the trace hydrogen dissolved in the crude high-pressure liquid ammonia is stripped out. The qualified high-pressure liquid ammonia is directly sent to the downstream urea device through the high-pressure liquid ammonia outlet 7; the stripping gas after gas stripping contains H2, N2 and NH3, as well as some inert gases such as Ar and CH4. The circulating gas and the stripping gas are returned to the circulating gas compressor together after recovering the cold energy through the top gas-liquid separation component 8, the circulating mixed gas outlet 6 and the cold exchanger, and the hydrogen and nitrogen in the stripping gas and the circulating gas are reused to maintain the hydrogen and nitrogen ratio in the whole ammonia synthesis loop.

[0027] The gas-liquid separation component 8 is a demister or a blade-type separation component; the middle heat exchange section 1 is a falling film heat exchanger, the upper tube head of the falling film heat exchanger is slightly higher than the upper tube sheet, and the upper tube head is provided with a rotating liquid dropper 11, such as Figure 3 shown.

[0028] Crude high-pressure liquid ammonia is stripped with high-pressure nitrogen to obtain high-purity high-pressure liquid ammonia. The pressure of high-purity high-pressure liquid ammonia is 130-180 barg (approximately equal to the pressure during high-pressure synthesis), which is directly sent to the downstream urea unit. On the one hand, it can effectively recover the energy of high-pressure liquid ammonia. At the same time, it reduces or eliminates the energy consumed by the pressurization of liquid ammonia in the downstream urea unit and the investment in the high-pressure liquid ammonia pump, saving production costs; on the other hand, it meets the requirements of the downstream urea unit for the hydrogen content in the high-pressure liquid ammonia, improving the safety and stability of the operation of the downstream urea unit.

[0029] After adopting the high-pressure liquid ammonia purification stripping tower, the medium-pressure and low-pressure flash evaporation systems are eliminated, the pressure energy of the synthesis loop is fully utilized, and the investment in the ammonia synthesis unit is greatly reduced; at the same time, because there is no need to set up medium- and low-pressure flash evaporation systems, the compression power consumption of re-pressurizing the medium- and low-pressure flash vapor is avoided, saving energy.

[0030] In this embodiment, the high-pressure nitrogen can come from an upstream air separation unit and be provided after being pressurized and gasified by a liquid nitrogen pump; or the medium-pressure nitrogen from the upstream air separation unit can be provided after being pressurized by a compressor.

Claims

1. A method for purifying high-pressure liquid ammonia and synthesizing ammonia, characterized in that: The method specifically comprises the following steps: the crude high-pressure ammonia reaction gas at the bottom outlet of the ammonia synthesis tower is subjected to heat exchange and cooling to form crude high-pressure liquid ammonia; the crude high-pressure liquid ammonia and non-condensable gas are stripped by a high-pressure liquid ammonia purification stripping tower; the non-condensable gas and stripping gas form a circulating mixed gas; and the qualified high-pressure liquid ammonia after stripping is directly sent to a downstream urea device.

2. The method for purifying high-pressure liquid ammonia and synthesizing ammonia according to claim 1, characterized in that: The high-pressure liquid ammonia purification stripping tower comprises an upper gas-liquid separation section (1), a middle heat exchange section (2) and a lower stripping section (3). Crude high-pressure liquid ammonia and non-condensable gas enter the upper gas-liquid separation section (1) of the high-pressure liquid ammonia purification stripping tower from a reactant inlet (4). The non-condensable gas is separated through the upper gas-liquid separation section (1) and used as circulating gas. The crude high-pressure liquid ammonia is heat exchanged with hot ammonia entering from a hot ammonia inlet (9) after passing through the middle heat exchange section (2). The cooled liquid ammonia is sent out from a cold ammonia outlet (10). The crude high-pressure liquid ammonia after further heat exchange further releases hydrogen, nitrogen, methane and argon dissolved in the crude high-pressure liquid ammonia. The crude high-pressure liquid ammonia after heat exchange and the high-pressure nitrogen entering from the stripping nitrogen inlet (5) are stripped through the lower stripping section (3). The hydrogen dissolved in the crude high-pressure liquid ammonia is stripped out. The qualified high-pressure liquid ammonia is directly sent to a downstream urea device through a high-pressure liquid ammonia outlet (7).

3. The method for purifying high-pressure liquid ammonia and synthesizing ammonia according to claim 1, characterized in that: The circulating mixed gas formed by the circulating gas and the stripping gas returns to the circulating gas compressor together after passing through the circulating mixed gas outlet (6) and the cold exchanger to recover the cold.

4. The method for purifying high-pressure liquid ammonia and synthesizing ammonia according to claim 3, characterized in that: The circulating mixed gas is mixed with hydrogen and nitrogen compressed by a hydrogen and nitrogen compressor to form a mixed gas, and the mixed gas is pressurized together in the circulating gas compressor to form a high-pressure mixed gas; the crude high-pressure ammonia reaction gas at the bottom outlet of the ammonia synthesis tower is subjected to a primary heat exchanger, and the ammonia-containing reaction gas after the primary heat exchange is subjected to a secondary heat exchange with the high-pressure mixed gas in a secondary heat exchanger, and the high-pressure mixed gas enters the ammonia synthesis tower after being heated, and the hydrogen and nitrogen in the high-pressure mixed gas react under the action of a catalyst in the ammonia synthesis tower to generate crude high-pressure ammonia.

5. The method for purifying high-pressure liquid ammonia and synthesizing ammonia according to claim 3, characterized in that: The crude high-pressure ammonia reaction gas after the secondary heat exchange is cooled step by step in a water cooler, a cold exchanger and an ammonia cooler, and then condensed to form crude high-pressure liquid ammonia. The crude high-pressure liquid ammonia and non-condensable gas enter the high-pressure liquid ammonia purification stripping tower for stripping.

6. The method for purifying high-pressure liquid ammonia and synthesizing ammonia according to claim 2, characterized in that: A top gas-liquid separation component (8) is arranged above the upper gas-liquid separation section (1), and the gas-liquid separation component (8) is a demister or a blade-type separation component; the middle heat exchange section (1) is a falling film heat exchanger, and the upper tube head of the falling film heat exchanger is slightly higher than the upper tube plate, and the upper tube head is provided with a rotating liquid dropper (11).

7. The method for purifying high-pressure liquid ammonia and synthesizing ammonia according to claim 4, characterized in that: The pressure of the high-pressure mixed gas is 130-180 barg.

8. The method for purifying high-pressure liquid ammonia and synthesizing ammonia according to claim 4, characterized in that: The temperature of the high-pressure mixed gas entering the ammonia synthesis tower is 170-230°C.

9. The method for purifying high-pressure liquid ammonia and synthesizing ammonia according to claim 1, characterized in that: The pressure of the qualified high-pressure liquid ammonia is 130-180 barg.

10. The method for purifying high-pressure liquid ammonia and synthesizing ammonia according to claim 4, characterized in that: The volume ratio of hydrogen to nitrogen in the hydrogen-nitrogen gas is 3.05-3.20.

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