On the selection of liquid ammonia loading solutions
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Currently, a project is underway to produce 100,000 tons of synthetic ammonia via methanol co-production. During the preliminary design review, regarding the current loading scheme – where the operating pressure of the liquid ammonia storage tank is 1.6 MPa and loading is carried out using pumps with the exhaust gases returning to the liquid ammonia storage tank – experts have proposed two suggestions for energy savings in the system: 1) Increase the design pressure of the spherical tank; eliminate pressure reduction in the liquid ammonia system, use pressure differences for loading, and send the exhaust gases to a washing tower; 2) Increase the design pressure of the spherical tank, eliminate pressure reduction in the liquid ammonia system, use pressure differences for loading, and compress the exhaust gases with a small compressor before sending them to the ammonia synthesis system. I’m not sure which approach is more reasonable, nor how it can be implemented specifically. I would appreciate your guidance on this matter; thank you. Additionally, we are looking to hire a synthetic ammonia engineer; those who are interested can add me on WeChat at WYS1341
Increasing the design pressure of spherical tanks is certainly unreasonable. The exhaust gas is pressurized by a compressor before entering the scrubber tower
Is it necessary to have a non-powered ammonia recovery system? Is a compressor still required to feed the scrubber tower?
The recovery rate in the washing tank is slightly lower, but energy consumption is low, investment requirements are minimal, and it is easy to operate； Using a small compressor results in a high recovery rate, but it has high energy consumption, large investment costs, and is relatively difficult to operate.
Check the pressure of the scrubber tower; in some units, the scrubber tower is at the same pressure as the liquid ammonia storage tank. Using water absorption will suffice; ammonia itself is also a product, especially nowadays in ammonia-based flue gas desulfurization
Now, sliding vane pumps are used for unloading liquid ammonia and liquefied gas
Personal opinion: 1. From a safety perspective, the operating pressure of the spherical tank should be reduced； 2. If methanol is produced as a by-product, is the vent gas used to produce ammonia? In that case, the amount of inert gas is very low, so emissions are minimal; the compressor can only operate intermittently, which is not economical ； 3. It’s better to install a vehicle pump; when the pressure in the storage tank is low, loading is slower ； 4. Can the vapor be recovered and sent directly to coking without washing (it’s not clear whether coking is involved); the amount is very small, so adding washing equipment would be uneconomical in terms of investment and operation costs.
Ammonia synthesis is carried out using vent gas; coking occurs as well. For the exhaust gases from the system, we consider recovering them through coking processes. I’m not sure what the exhaust gas emission level is during loading; I’m assuming a initial loading pressure of 1.0 MPa. In the two companies I’ve worked with recently, compressors are used to send the exhaust gas from the tank trucks back into the storage tanks, thereby keeping the pressure in the tank trucks at around 1.0 MPa, while the pressure in the storage tanks is maintained at 1.7 MPa. The loading time for each tank truck is about 1 hour (for a capacity of 24 tons)
My idea is that we shouldn’t increase the pressure in the spherical tank; once the pressure goes up, the overall power consumption rises, so how can we save energy? From the perspective of safe operation, it’s better to keep the pressure low. The idea of installing a vehicle pump is great; I wonder why experts want to change it. Is it due to concerns about leaks? Magnetic pumps, shielded pumps, or other pumps with better sealing capabilities could be used instead
That’s right; the pressure of liquid ammonia leaving the system is 2.3 MPa. This allows the pressure to remain at a higher level, eliminating the need for pressure reduction. The ammonia can be loaded using its own pressure, so there’s no need to use pumps. Moreover, pumps are prone to cavitation, which can lead to more failures and leakage points during loading