How can gaseous ammonia in the liquid ammonia storage area be recovered effectively?
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This post was last edited by *aoye613 on 2009-9-6 at 14:12. In current ammonia synthesis plants with an annual production capacity of 300,000 tons, aside from the gaseous ammonia in the liquid ammonia storage area being sent back to the synthesis refrigeration system for recovery, some plants use a screw compressor in that area to compress the gaseous ammonia and then condense it before returning it to the storage tanks. Which of these two methods is better, and why?
A power-free ammonia recovery method can be used； The principle is as follows: This unpowered ammonia recovery device utilizes low-temperature technology to extract ammonia from the off-gases without requiring any additional energy input, thereby producing liquid ammonia as a finished product. This approach increases the value of ammonia in the off-gases and addresses the environmental pollution caused by dilute ammonia solutions. It is an energy-saving and efficient environmental protection solution. Process summary: The off-gas from the ammonia tank in the synthesis unit enters the first heat exchanger, where it exchanges heat with liquid ammonia separated in the first and second separation units, as well as with the cold gas coming from the energy converter. After this, the temperature drops to around -20°C, and the mixture enters the first separator, where some of the liquid ammonia is separated out. It then goes to the second heat exchanger, where it exchanges heat with the cold gas from the energy converter; the temperature drops to around -40°C, after which it enters the second separator to further separate the liquid ammonia. The liquid ammonia separated in both separators enters the first heat exchanger simultaneously, where it absorbs heat and vaporizes into ammonia gas, which is then sent to the booster pump and from there to the inlet of the ice maker. The gas released from the synthesis unit after passing through the tower is cooled in a condenser before entering the ammonia cooler, where its temperature is reduced to around -20°C. There, liquid ammonia is separated from this gas; after separation, the gas undergoes further heat exchange and then enters a buffer, from where it goes to an energy converter for expansion-based cooling, resulting in a temperature of around -65°C. It then enters a second heat exchanger, and after further heat exchange, it mixes with the gas released from ammonia separator 2 before entering the first heat exchanger. After exiting this exchanger, the gas enters an isobaric ammonia recovery tower to purify the ammonia, reducing its concentration to 5PPM, after which it proceeds to a pressure swing adsorption unit for H2 extraction. Design principle: The pressure energy of the off-gas from behind the towers in the ammonia synthesis system is utilized; through expansion via a specialized energy converter for ammonia recovery, external work is generated. This process consumes the internal energy of the off-gas itself, resulting in the greatest adiabatic enthalpy drop and allowing the temperature of the off-gas to drop rapidly to -40~60 degrees Celsius. C left and right. Due to the different boiling points of the components in the off-gas, gaseous ammonia is liquefied and separated out. Device features: 1. The entire device has a compact design, occupies little space, with equipment arranged reasonably for easy operation. 2. The energy converter uses a patented ammonia recovery product, featuring high efficiency, strong resistance to interference, low failure rate, durability, and reliable operation. 3. Choose high-efficiency separation equipment and heat exchange equipment. 4. The device features advanced technology, reliable operation, no pollution, and easy maintenance. Process parameters after ammonia recovery: 1. Pressure of 1.5 Mpa. 2. The ammonia content after air release is ≤1%. 3. The ammonia content in the vent gas ≤ 2%. 4. The blower pressure of the energy converter is 0.05 Mpa. 5. After air release and venting, the ammonia content in the water wash tower is ≤5PPM.
Gaseous ammonia is used in the refrigeration system, which eliminates the need for additional equipment investment and allows for the replenishment of the ammonia used in refrigeration; this makes it more economical. However, the amount of gaseous ammonia stored in the liquid ammonia tank is not clear, and since it is connected to the refrigeration system, factors such as the compressor load, the inlet pressure of the compressor, and the refrigeration load must be taken into account. Personally, I think this approach should not be applied with too much leniency. While using compressed gaseous ammonia for condensation and recovery increases costs, it is easier to control
I also agree to use passive ammonia recovery; although the investment is a bit higher, more ammonia can be recovered! Returning to freezing, using a small ice maker, etc., all increase consumption; in particular, using a small ice maker raises electricity usage. There is another method: if the requirements aren’t too high, you can use water to prepare dilute ammonia solution.
The liquid ammonia storage in the new plant is carried out at atmospheric pressure, which likely makes it unsuitable for ammonia recovery without external power. Additionally, there is a distance of around 800 meters between the tank area and the refrigeration system; due to this long distance, the ammonia reaching the refrigeration system is already under negative pressure. Therefore, I think it is appropriate to use a screw compressor tailored to the specific conditions.
If there is still capacity left in the existing synthetic refrigeration system, it is certainly preferable to use that system for recovery; it’s best to avoid a second option. If the load of the existing refrigeration system is already at its limit, the only option is to add another screw compressor to condense the compressed ammonia and return it to the storage tank, thereby achieving recovery.
Will the H2, N2, and other gases that were originally dissolved in liquid ammonia and have been released into the ammonia gas affect compression and the liquefaction of ammonia? And when these non-condensable gases are released, a large amount of ammonia is inevitably carried away, which is undesirable. Therefore, my suggestion is to first compress the gas to a pressure of over 1.7 MPa, and then use an isobaric recovery tower to recover ammonia; when used optimally, this isobaric recovery tower can reduce the ammonia concentration at the outlet to 10 PPm. The liquid phase can also include 20% concentrated ammonia solution. If not needed, concentrated ammonia can be distilled again to obtain liquid ammonia.
One more thing: there is a significant amount of hydrogen in ammonia tank gas and exhaust air; by recovering it completely and using it for the simplest process of reprocessing it into ammonia, the annual ammonia production can be increased by 3%. Therefore, hydrogen should be recovered further. To recover hydrogen again, it is not possible to directly compress it to produce ammonia.