How are ammonium bicarbonate crystals formed and grown?

Reply to 1# Fresh and serene: It is based on the logic of crystallization; there is nothing else

The formation and growth of ammonium bicarbonate crystals: In the carbonization tower, as the carbonization reaction proceeds, ammonium bicarbonate is continuously formed in the solution. When the concentration of ammonium bicarbonate in the solution exceeds its solubility at that temperature, crystallization begins; at this point, the solution is known as a supersaturated solution. The supersaturation is defined as the difference between the concentration of ammonium bicarbonate in a supersaturated solution and its solubility at the same temperature. Saturability is the driving force for crystallization. Crystallization can occur only when there is supersaturation; as supersaturation is continuously generated and dissipated, crystallization can proceed continuously. The supersaturation of the ammonium bicarbonate solution is related to the concentration of ammonium bicarbonate in the solution and the temperature. When the temperature remains constant, as the carbonization reaction continues, the concentration of ammonium bicarbonate in the solution increases, raising its saturation level. When the concentration of ammonium bicarbonate in the solution remains constant, lowering the temperature reduces its solubility, causing the ammonium bicarbonate solution to become saturated or even supersaturated and leading to the precipitation of crystals. The formation of ammonium bicarbonate crystals consists of two stages, namely the nucleation stage and the crystal growth stage. If the rate of nucleus formation is much greater than the growth rate of crystallization, the crystals will be fine and numerous. If the rate of nucleus formation is much lower than the growth rate of crystallization, the crystals will be coarse and few in number. Therefore, to obtain large-grained crystals, as early as the beginning of crystallization as few crystal nuclei as possible should be formed in the solution, after which these nuclei are allowed to grow without new nuclei forming. If the crystals are too fine and in insufficient quantity, it is difficult for the supersaturation of the solution to be completely eliminated within the main column; as a result, large amounts of crystals tend to precipitate on the column walls, as well as in the water tank and mother liquor tank, causing scarring and blockages. The nucleation rate and crystal growth rate are related to the supersaturation and temperature of the solution. Increasing the supersaturation of the solution and lowering the temperature accelerates the formation of nuclei. By raising the temperature during the operation, the solubility of ammonium bicarbonate increases, the supersaturation of the solution decreases, and the rate of nucleation slows down. Increasing the supersaturation of the solution and raising the temperature can both increase the crystal growth rate. However, if the supersaturation of the solution is too high, or if the cooling is too rapid, the rate of crystal formation will exceed the rate of crystal growth, resulting in finer crystals. Therefore, it is necessary to control the supersaturation and temperature properly, as well as the nucleation rate and crystal growth rate, to ensure they are in balance. In production, in order to obtain crystals with large and uniform particles, the temperature is kept slightly higher during the nucleation stage in the carbonization main tower and at the initial stage of crystal growth, after which it is gradually reduced to continue the carbonization reaction. Lowering the temperature can increase supersaturation, promoting crystal growth. Inside the carbonization tower, driven by the airflow, the solutions in the upper and lower parts of the tower mix thoroughly; fine crystals are carried upward by the airflow, and as they grow larger, they sink to the lower part. The lower temperature in this area helps to make the crystals larger and more uniform in size. Therefore, in terms of operation, the residence time of the crystals within the tower should be increased slightly, and the temperature at the lower part of the tower should be reduced as much as possible, in order to achieve better crystallization and a higher conversion rate.

As mentioned in detail above, crystal growth is primarily driven by supersaturation, that is, the concentration difference. However, the final particle size of large numbers of crystals is also related to the nucleation rate.