The principle of acidic water stripping

Single-column pressurized side-stream extraction process: the top of the column is maintained at around 50 degrees, the side stream temperature is generally around 150 degrees, and the bottom of the column is kept at around 160 degrees

I’m asking about the principle: why is the temperature at the top of the tower kept around 50 degrees? A higher temperature can lead to the presence of ammonia, but why does an elevated temperature make it easier for ammonia to be carried away through extraction from the top? Why is the side line controlled at 150 degrees? Is it based on some property of ammonia? What happens if it’s too high? What will happen if it’s too low? Principle?

In the same field of study, I really want to find some relevant system materials to learn from, but there aren’t any.

Take a look at this post. http://bbs.hcbbs.com.cn/thread-1234590-1-1.html

This post was last edited by The_sky on 2013-9-9 at 16:53. The basic principle of ammonia extraction via a side stream on the pressurized side of a single tower is as follows: The acidic water fed into the acid vaporization unit contains mainly H2S and NH3; in addition, it contains small amounts of CO2 as well as trace amounts of oil, organic sulfur, phenols, and ammonium ions. In acidic water, hydrogen sulfide, ammonia, and carbon dioxide exist primarily in the form of ammonium hydrosulfide (NH4HS) and ammonium bicarbonate (NH4HCO3). These weak acid-weak base salts undergo hydrolysis in aqueous solution, resulting in the formation of free NH3, H2S, and CO2. The basic principle of this process is to take advantage of the significant difference in solubility of NH3 and H2S at different temperatures. Since hydrogen sulfide has a low solubility, when acidic water enters the stripping tower, most of the hydrogen sulfide in the liquid phase is first vaporized into the gas phase. The gas phase also contains some ammonia. As the liquid phase moves downward through the tower trays, under the action of the stripping steam, the remaining hydrogen sulfide and some ammonia in the liquid phase are transferred into the gas phase. The ammonia in the gas phase is then washed away again by the feed as it moves upward, while the hydrogen sulfide, after being washed of ammonia (i.e., through the action of the cold feed), is discharged from the top of the tower. The liquid phase continues to move downward; as the temperature at the lower part of the tower increases, ammonia in the liquid phase is once again vaporized into the gas phase due to the action of the stripping steam. At this point, extraction is carried out from the middle part of the tower, which reduces the partial pressure of ammonia in the gas phase and further promotes the volatilization of ammonia from the liquid phase. The ammonia-rich gas extracted via the side stream undergoes three-stage condensation – water separation at high temperatures and sulfur removal at low temperatures – before the crude ammonia gas is discharged from the third stage condenser ； At the bottom of the tower, steam at 1.0 MPa is used as a heat source for heating, and an inert medium is provided. By following the above steps, it is possible to separate gaseous NH3 and H2S from acidic water, thereby achieving the goal of purifying and reusing the acidic water. The top temperature is controlled at no more than 40°C, usually around 35°C; a lower temperature is favorable for ammonia absorption. The temperature at the side-line extraction point is maintained at no less than 150°C, usually around 153°C. This ensures, on the one hand, that there is sufficient stripping steam in the hydrogen sulfide stripping section (from the side extraction point of the stripper to the top tray), thereby ensuring an effective stripping of hydrogen sulfide from the liquid phase; on the other hand, it helps to reduce the amount of hydrogen sulfide carried in the ammonia-rich gas extracted via the side line. The key operational aspects of the stripping tower are as follows: 1. Temperature control in the packing section at the upper part of the tower is one of the critical factors; this temperature level reflects the emission rate of the gases at the tower top, that is, the amount of acidic gas exiting the tower compared to the amount of gas entering it. Theoretically, this ratio should be 100%. In practice, the temperature of the packing section is used as the main parameter, while the amount of acidic gas emitted at the tower top serves as a secondary parameter for coordinated control. 2. Heat supply control at the bottom of the acidic water stripping tower is the second key point; generally, 1.0 MPa steam is used as the heat source. The steam serves two purposes: one is to provide heat to raise the temperature of the feed water to the required level ； Secondly, it acts as an inert stripping medium to reduce the partial pressures of ammonia and sulfur in the gas phase, facilitating their separation from the liquid phase. For instrument control, a cascade control system is generally used, with the bottom temperature as the primary parameter and the steam flow rate to the reboiler as the secondary parameter. 3. The control of the side-stream extraction volume is the third key point. Generally, the extraction ratio, that is, the extraction volume divided by the feed volume, is used to determine whether the extraction level is appropriate. Only with an appropriate extraction ratio can it be ensured that the ammonia nitrogen content in the water at the bottom of the tower remains within acceptable limits, as well as that an appropriate amount of ammonia nitrogen is available for circulation. The side-stream extraction system is equipped with three-stage condensers; the liquid phase from these condensers is returned to the inlet of the feed pump or to the raw water tank. This amount of recycled fluid directly increases the concentration of the raw water fed into the tower, having a significant impact on the operation of the stripping tower. 4. The setting of the operating conditions for the third-stage condenser is the fourth key control point. By setting appropriate operating conditions for this condenser, it is possible to minimize the amount of ammonia nitrogen that circulates, while still producing a large quantity of crude ammonia gas. Otherwise, all the ammonia gas extracted via the side stream will be recycled, resulting in no ammonia gas being produced in the third-stage condenser, and thus preventing the separation of ammonia from the acidic raw water. The above are only the key points of the operations; in fact, there are many more, which can only be identified gradually through carrying out the operations, though the difficulty of these operations is not high.

This post was last edited by The_sky on 2013-9-9 17:01. The above are purely personal analyses and summaries; I hope we can all learn and improve together. If you have different opinions, feel free to discuss them in detail!