The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.
Aspects of the present invention pertains to a method for manufacturing granules from a liquid through a granulation process. Granulation processes are known in the art, and are often applied for the manufacture of, e.g., fertilizers. A problem in the manufacture of granules from a liquid of a source material, in particular a fertilizer, is that all fertilizer granulations produce an amount of dust in the granulator and in the coolers. The dust is entrained by the air that the exhaust fan is sucking through the granulator and coolers. That entrained dust has to be removed from the air before the air can be vented back into the atmosphere. The recovered dust will be recycled back into the process.
Many types of scrubbers have been developed for removing entrained dust from the air, depending on the particle size of the generated dust and on the requested efficiency of the dust removal. There are basically two types of scrubbers, namely dry scrubbers and wet scrubbers. Dry scrubbers include dust removers like cyclones or filters. They work well for coarse dust particles and have the advantage that they recover the dust as a solid product that can be recycled at low energy cost. Their disadvantages are sensitivity to plugging up (especially when humid air has to be scrubbed) and the low efficiency in removing fine dust particles (unless extreme pressure drops over the dust removers and consequently high power consumptions are used). Wet scrubbers dissolve or suspend the dust in water and recover the dust as a low concentration solution or suspension in water. The wet scrubbers come in different types, they are mostly very reliable and are able to remove fine dust particles without increasing tremendously the pressure drop over the scrubber.
An issue with wet scrubbers is that most of the water or even all of the water must be removed from the solution or suspension before the recovered dust can be recycled again into the process. The water is mostly removed in a vacuum concentrator; this process consumes at least the same amount of steam as the amount of water that is removed. For this reason it is important that the water content of the recovered solution or suspension is as low as possible.
The dust generated in a granulator is a mixture of coarse particles (0.1 mm or even more produced by the crushers) and fine particles (in the 10-50 micron range produced by the sprayers). In some cases like urea or ammonium nitrate granulation, the granulators can also produce an amount of sub-micron dust, i.e. dust with a particle size below 1 micron, more specifically below 0.5 micron. Such extremely fine dust is not generated by mechanical items like crushers or sprayers but is produced by reactions between gases present in the air.
For example, in the case of granulation of ammonium nitrate, the ammonium nitrate melt contains an amount of dissolved free ammonia and free nitric acid. When the hot melt comes in contact with air ammonia and acid evaporate from the melt and are entrained by the air, they will recombine later into extremely fine particles when the air stream is cooled. For urea, extremely fine particles may be formed as follows: at high temperature, e.g., in the range of 120-140° C., urea will partially convert into ammonium cyanate (equilibrium concentration, depending on the temperature in the range of 0-1 wt. %). At the stated temperature, ammonium cyanate may split into ammonium and cyanuric acid, which may evaporate. Upon a decrease in temperature, the ammonium and cyanuric acid will combine to form very fine particles of ammonium cyanate, which will in turn be converted to urea.
The sub-micron particles are visible in the exhaust air stream as turbidity (after dissipation of the water vapor in case of a wet scrubber). They are too fine to be caught on any mechanical filter or droplet catcher. Some are so fine that they even escape the standard dust sampling methods because those methods do not consider that particle size as “particulate matter” any more. Nevertheless, submicron dust is a pollutant and a loss of product for the process. There is therefore need for a granulation process wherein submicron dust is removed at least in part. Such process should be flexible and energy-efficient.