By Andritz |
By drying in a closed-loop system, the fluid bed dryer presents a breaking new ground in ammonium chloride drying and cooling
As a technology leader in the development of efficient drying processes for bulk solids, ANDRITZ was one of the first companies to introduce fluid bed technology with internal heat exchangers for numerous applications in the chemical and minerals industries. This also applies to the soda ash industry. One special application is the drying of ammonium chloride generated with Hou’s process. ANDRITZ has developed a patented solution with a closed gas loop system. The design of the fluid bed dryer system using HDC Technology in a closed loop has been developed to meet increasing environmental standards with a focus on the reduction of ammonia emissions taso the atmosphere.
Recovery of emissions and re-integration into the soda ash process
Due to its good solubility in water, ammonium chloride is rarely found as a mineralogical salt. It is usually a by-product in chemical processes, mainly from Hou’s process for the chemical synthesis of soda ash (sodium carbonate).
Ammonium chloride is usually removed from the production cycle via crystallization and centrifugation and requires drying for proper bagging, storage, and transport. The main application for ammonium chloride is fertilizer production, where it is used as a valuable raw material for nitrogen supply.
The raw ammonium chloride from crystallization also contains other side products such as a considerable amount of ammonium carbonate. The ammonium carbonate decomposes in any thermal drying stage forming carbon dioxide and, more importantly, odor-intensive ammonia gas, which is toxic in high concentrations.
The process developed by ANDRITZ removes the ammonia from the closed-loop system without using any additional acid. The waste stream from the system can be re-integrated into the upstream processes of the soda ash plant.
Drying and cooling in a closed-loop system
The illustration below demonstrates the process: The wet ammonium chloride is dewatered in a pusher centrifuge and then conveyed to a HDC type fluidized bed dryer with internal heating exchangers for optimized heat transfer.
Fluidization in the closed-loop system is achieved with a mix of water vapor, carbon dioxide, ammonia and oxygen depleted air. Due to the drying and thus decomposition of ammonium carbonate, the off-gas from the dryer is enriched with evaporated water and with a considerable amount of ammonia.
A bag filter is used for separating the fines, followed by a combined condenser/absorber column to condense the evaporated water from the drying process and to absorb the ammonia from the gas stream.
With this setup, large amounts of ammonia can be separated from the gas stream while only a small amount of fresh water is required. That way, effluent quantities can be kept small and the absorber can be operated without additional acid dosing.
A second, smaller absorber column treating a bleed gas stream can be used to reduce the ammonia content to close to zero.