System and process for absorption cooling of a product ammonia synthesis gas from an ammonia converter in a dynamic ammonia plant operated at frequently changing loads of makeup gas

Abstract

A system and process for absorpon cooling a product ammonia synthesis gas from an ammonia converter in a dynamic ammonia plant operated at frequently changing loads of ammonia makeup gas.

Description

[0001] Title: System and process for absorption cooling of a product ammonia synthesis gas from an ammonia converter in a dynamic ammonia plant operated at frequently changing loads of makeup gas.

[0002] The present invention provides a system and process for absorption cooling a product ammonia synthesis gas from an ammonia converter in a dynamic ammonia plant operated at frequently changing loads of ammonia makeup gas.

[0003] A dynamic ammonia loop is a loop with a frequently changing supply of ammonia synthesis gas and needs a system that can adapt to variations in the amount of the gas mixture used for ammonia synthesis.

[0004] In an ammonia producing facility, it is a requirement to separate the ammonia product from the synthesis gas by cooling and condensing the product stream from the ammonia converter, in normal case by the use of a cooling compressor. Alternatively cooling water can be used, but the water temperature will be insufficient to ensure a low ammonia concentration of the gas returning to the converter, leading to inefficient production. In a dynamic plant, frequently changing load in the range 5 to 105 % a traditional centrifugal cooling compressor is not well suited, since to obtain high pressure it must operate at high speed. To ensure low load the anti-surge valve must be open leading to low efficiency at reduced load.

[0005] In a plant producing ammonia by electrolyzing water, a significant amount of waste heat is produced at low temperatures, too low to be utilized for steam production. Such heat is difficult to utilize and is more often than not simply discarded to the waste heat cooling system. The present invention employs a cooling system based on absorption. Systems using e.g. LiBr solution are well-known in the process industry and can utilize heat down to below 70 °C with reasonable efficiency. The cold stream of chilled water generated by absorption cooling exiting the system can most advantageous be utilized to cool the product steam to the ammonia separator. At high load, the system will be able to produce an amount of cold chilled water in sufficient capacity to enable an ammonia loop efficiency nearly comparable to the traditional process utilizing cooling compressors. At low plant capacity the efficiency is less, but as the plant is overdesigned at this operation condition, no adverse effect on the overall operation is expected. The cooling system design will operate fully automatic according to the overall plant operation and will not require the attention of the operating staff during load changes.

[0006] A typical ammonia chiller for liquefying at least a part of the ammonia in an ammonia product gas from an ammonia converter in an ammonia loop, cools and condenses the effluent gas from the converter on the primary side of the chiller by means of evaporating pure ammonia on the secondary side.

[0007] A typically refrigeration system employs a combination of compression and condensation to cool and condense ammonia vapor formed on the secondary side of the chiller into a liquid state.

[0008] The ammonia vapor, which is produced in the ammonia loop, is first compressed using a compressor. The compressor increases the pressure of the ammonia vapor, making it easier to cool and condense. The system is operated as explained in the following.

[0009] The compressed ammonia vapor then enters a condenser, which is a heat exchanger, and as the vapor cools, it releases heat to the cooling water and condenses into a liquid state.

[0010] The condensed liquid ammonia from the condenser then passes through an expansion valve or throttling device. The expansion valve reduces the pressure and temperature of the liquid ammonia, causing it to partially evaporate and further cool down.

[0011] The partially evaporated ammonia from the expansion valve enters the ammonia chiller on the secondary side and as the ammonia absorbs heat, it evaporates and returns to the suction side of the cooling compressor. The absorbed heat is taken from the ammonia product gas coming from the ammonia converter which is thereby being partly condensed on the primary side of the chiller.

[0012] By continuously circulating the ammonia through the compression, condensation, expansion, and evaporation processes, the chiller effectively cools and liquefies the produced ammonia in the ammonia loop. The compression and condensation stages increase the pressure of the vapor, while the expansion and evaporation stages decrease the pressure and temperature, allowing for the cooling and liquefaction of the product ammonia in the synthesis loop.

[0013] Absorption cooling systems can utilize different hydrophilic solutions, such as e.g. LiBr solution. The basic principles of operation remain the same for all hydrophilic solutions as the absorbent. The refrigerant used is typically water.

[0014] The absorption cooling systems with hydrophilic solutions requires a heat source to operate, and the efficiency of the system depends on the temperature difference between the heat source and the cooling load. As already mentioned hereinbefore in the present invention, the heat for the operation comes mainly from the front end of the ammonia plant, in particular heat from the electrolysis of water.

[0015] This cooling system is advantageous for dynamic loops compared to cooling with an ammonia refrigeration compressor used in traditional ammonia plants. Cooling is mainly required when the plant is operating close to maximum capacity. It is not required at low capacity where the efficiency of the cooling system is also lower.

[0016] In the following a step-by-step explanation is given of how absorption cooling works (Refer to Fig. 1):

[0017] 1. Absorption: The process starts with injection of refrigerant water into an evaporation chamber. In the bottom of the chamber a concentrated hydrophilic solution enables absorption of water vapor. The low pressure causes evaporation from refrigerant water, resulting in cooling of the water. The hydrophilic solution becomes diluted as it absorbs the water vapor.

[0018] 2. Heat Input: The diluted hydrophilic solution is then pumped to the generator, where it is heated. Heat can be supplied from various sources, such as natural gas, waste heat, or solar energy. The heat causes the water vapor to separate from the hydrophilic solution, forming water vapor and a concentrated hydrophilic solution.

[0019] 3. Condensation: The water vapor then moves to the condenser, where it is cooled and condensed back into a liquid state. This process releases heat to the cooling system

[0020] 4. Expansion: The liquid water then flows to the evaporator, where the partial pressure of water vapor is low due to the presence of the water absorbent. This causes the water partly to evaporate, resulting in a temperature drop and thereby absorbing heat from chilled water providing cooling.

[0021] 5. The water vapor is then absorbed by the absorbent to start the cycle again.

[0022] A system for absorption cooling using hydrophilic solution typically consists of several key components that work together to provide cooling. The general configuration of a hydrophilic solution absorption cooling system:

[0023] 1. Absorber: The absorber is where the absorption of water vapor (refrigerant) into a solution of hydrophilic solution takes place. The hydrophilic solution acts as the absorbent. The absorber is typically a vessel or column where the refrigerant vapor comes into contact with the hydrophilic solution, allowing the absorption process to occur.

[0024] 2. Generator: The generator is responsible for separating the refrigerant (water vapor) from the absorbent (hydrophilic solution). This is achieved by heating the hydrophilic solution in the generator. The heat source can be natural gas, waste heat, or other heat-generating methods. As the solution is heated, the water vapor separates from the hydrophilic solution, forming a concentrated hydrophilic solution and releasing the refrigerant vapor.

[0025] 3. Condenser: The refrigerant vapor produced in the generator is then condensed back into a liquid state in the condenser. The condenser typically consists of a heat exchanger where the refrigerant vapor is cooled, transferring heat to a cooling medium (such as air or water) and causing the refrigerant to condense. 4. After condensation, the liquid refrigerant flows through a shower head. This reduces the pressure of the refrigerant. At the same time producing droplets.

[0026] 5. Evaporator: The low-pressure refrigerant stream then enters the evaporator, which is the component responsible for cooling. Part of the refrigerant water injected evaporates due to low partial water pressure in the evaporator, thereby causing a temperature drop in the remaining water droplets. The refrigerant absorbs heat from the chilled water loop.

[0027] 6. Pump: The diluted hydrophilic solution from the absorber is pumped back to the generator to complete the cycle. The pump ensures the continuous circulation of the hydrophilic solution between the absorber and the generator.

[0028] The absorption cooling system using hydrophilic solution operates in a continuous cycle, with the refrigerant vapor being absorbed by the hydrophilic solution in the absorber, separated in the generator, condensed in the condenser, expanded in the expansion valve, and evaporated in the evaporator. This cycle allows for the continuous absorption from the chilled water system and release of heat to the cooling water (or cooling air) system.

[0029] The invention is a system and a process system for cooling of a product ammonia synthesis gas from an ammonia converter in a dynamic ammonia plant operated at frequently changing loads of makeup gas.

[0030] Preferred embodiments are the following.

[0031] 1. A system for absorption cooling of a product ammonia synthesis gas from an ammonia converter in a dynamic ammonia plant operated at frequently changing loads of makeup gas comprising: a) an ammonia chiller configured to remove heat from an ammonia product gas stream and to liquify part of produced ammonia in the product ammonia gas by means of chilled water; b) a water absorption cooling unit comprising an absorbent solution with high affinity towards a water refrigerant, configured to cool the chilled water; c) a heat exchanger to transfer heat from the chilled water to the water absorption cooling unit.

[0032] 2. The system of embodiment 1, further comprising: d) a regeneration step to heat and regenerate the absorbent solution and maintain its cooling capacity; and e) a circulation system to circulate diluted absorbent solution between the absorption cooling unit and a regeneration unit for the removal of the water refrigerant vapor from the absorbent solution.

[0033] 3. The system of embodiment 1 or 2, wherein the absorption solution is regenerated by an external heat source.

[0034] 4. The system of embodiment 3, wherein the external heat source is a low temperature waste heat from ammonia synthesis gas preparation.

[0035] 5. The system of embodiment 3, wherein the external heat source is waste heat generated by an electrolyzer.

[0036] 6. The system of any one of the preceding embodiments, wherein the absorbent solution is a hydrophilic solution.

[0037] 7. The system of embodiment 6, wherein the hydrophilic solution is a lithium bromide (LiBr) solution.

[0038] 8. The system of embodiment 6, wherein the hydrophilic solution is an NH4I / NH3 solution. 9. The system of embodiment 6, wherein the hydrophilic solution is an aqueous ammonia solution.

[0039] 10. A process for absorption cooling of product ammonia synthesis gas from an ammonia converter in a dynamic ammonia plant operated at frequently changing loads of makeup gas comprising the steps of a) withdrawing a stream of the product ammonia synthesis gas from the ammonia converter; b) cooling the withdrawn product ammonia synthesis gas in an ammonia chiller with a water refrigerant; c) removing heat from the withdrawn ammonia product gas stream by means of chilled water; and d) liquefying at least a part of the product ammonia gas, wherein the chilled water is prepared by means of an absorption cooling unit utilizing a water refrigerant.

[0040] Brief description of the drawings, in which

[0041] Fig.l schematically shows a unit (VAM) for absorption cooling of a product ammonia synthesis gas as further described hereinbefore; and

[0042] Fig.2 is simplified flow sheet showing a process for absorption cooling of product ammonia synthesis gas from an ammonia converter.

[0043] Referring to Fig.2. Product ammonia synthesis gas effluent is withdrawn from the ammonia converter and cooled with cooling water CW. The cooled product ammonia synthesis gas is subsequently further cooled in a chiller with chilled water produced in the vapor absorption unit (VAM) configured as shown in Fig.l. In the chiller a part of the ammonia in the ammonia product synthesis gas and liquified resulting in an ammonia synthesis product gas / liquid ammonia mixture by cooling with the chilled water, which after cooling the mixture is recycled to the VAM for cooling the heated chilled water in a continues cooling cycle. The liquid ammonia / ammonia synthesis product gas mixture is passed to an ammonia separator in which the liquid ammonia is separated from the ammonia product gas and withdrawn at bottom of the separator. The ammonia product synthesis gas withdrawn at top of the separator consists mainly of unconverted ammonia synthesis gas is recycled to the ammonia converter and mixed with makeup ammonia synthesis gas for further converting.

Claims

9Claims1. A system for absorption cooling of a product ammonia synthesis gas from an ammonia converter in a dynamic ammonia plant operated at frequently changing loads of makeup gas comprising: a) an ammonia chiller configured to remove heat from an ammonia product gas stream and to liquify part of produced ammonia in the product ammonia gas by means of chilled water; b) a water absorption cooling unit comprising an absorbent solution with high affinity towards a water refrigerant, configured to cool the chilled water; c) a heat exchanger to transfer heat from the chilled water to the water absorption cooling unit.

2. The system of claim 1, further comprising: d) a regeneration step to heat and regenerate the absorbent solution and maintain its cooling capacity; and e) a circulation system to circulate diluted absorbent solution between the absorption cooling unit and a regeneration unit for the removal of the water refrigerant vapor from the absorbent solution.

3. The system of claim 1 or 2, wherein the absorption solution is regenerated by an external heat source.

4. The system of claim 3, wherein the external heat source is a low temperature waste heat from ammonia synthesis gas preparation.

5. The system of claim 3, wherein the external heat source is waste heat generated by an electrolyzer.

6. The system of any one of the preceding claims, wherein the absorbent solution is a hydrophilic solution.

7. The system of claim 6, wherein the refrigerant is water and absorbent solution is a lithium bromide (LiBr) solution.

8. The system of claim 6, wherein the refrigerant is ammonia and absorbent solution is anNH4I solution.

9. The system of claim 6, wherein the refrigerant is an aqueous solution of ammonia .

10. A process for absorption cooling of product ammonia synthesis gas from an ammonia converter in a dynamic ammonia plant operated at frequently changing loads of makeup gas comprising the steps of a) withdrawing a stream of the product ammonia synthesis gas from the ammonia converter; b) cooling the withdrawn product ammonia synthesis gas in an ammonia chiller with a refrigerant; and c) liquefying at least a part of the ammonia in the product ammonia gas, wherein the refrigerant is prepared by means of an absorption cooling system.

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