The desorption system is not working properly. What should I do?
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The gas flow rate of our unit has now reached 23,000, with the pressure in the low-pressure gas tank generally maintained at around 0.33 MPa. However, the low-pressure steam required by the desorption system cannot get in, as the pressure in that system is 0.28 MPa; the temperature in the second desorption tower reaches at most 140 degrees, and the waste liquid is often not up to standard. May I ask: 1. How can the waste liquid be made compliant? 2. Is it feasible to replace the low-pressure steam entering the desorption tower with medium-pressure steam (0.9 MPa)?
Personally, I think the temperature of the second distillation column in your unit is a bit low, resulting in incomplete distillation. That is what causes the waste liquid to fail the quality standards. What you mentioned, changing the low-pressure steam fed into the desorption tower to medium-pressure steam (0.9 MPa), should be feasible. However, it is still necessary to consider whether the demand from your medium-pressure steam users can be guaranteed.
Is the concentration of the ammonium carbonate solution too high? We’ve reached 148 degrees in the desorption tower; it would be better to use medium-pressure steam instead
Consider reducing the desorption operation pressure to ensure that low-pressure steam can be introduced. Since the higher the system load, the lower the pressure in the low-pressure drum, and consequently the saturation temperature of the steam decreases, the desorption temperature also becomes lower. The same is true for our desorption process, but it can still meet the requirements. If the steam pressure is to be increased, it is necessary to consider whether the design pressure of the desorption tower meets the requirements; this must be verified by a pressure vessel certification authority to determine feasibility, and the steam pressure cannot be increased arbitrarily.
The analysis pressure should be kept low, generally at 0.4 MPa; the steam pressure is usually around 1.0 MPa, while the temperature at the bottom of the tower is controlled at 145 degrees.
The temperature of the second distillation tower is low; doesn’t the low-pressure drum have a steam supply valve for medium-pressure steam? Also, check whether the wastewater met the standards at this temperature level in the past; if so, investigate the reasons for the high concentration of ammonium carbonate solution and consider diluting it as appropriate. Environmental protection is very important; resort to it only as a last resort.
Our load is also 23,000 Nm3/h; the temperature of the liquid during separation is between 138–140 degrees Celsius, and the pressure is around 0.27 MPa. We use low-pressure steam produced internally, with a pressure of 0.322 MPa. The volume of liquid separated per hour is 46 m3/h, and the amount of waste liquid generated never exceeds the specified limits. We have been running very stably. The possible reasons for the phenomenon observed by the poster are: 1. The pressure for analysis is too high, preventing steam from entering. 2. The amount of analysis is high, and the ammonia concentration is high. Is the low-pressure drum control set too high? This results in a low synthesis conversion rate, causing excess ammonia and carbon dioxide to end up in the ammonia solution. 3. Is the urea carried in the gas phase severe during your evaporation process? If urea levels are high, it will also increase the load on the decomposition process. 4. How is the hydrolysis system performing? The above are my personal opinions, for the original poster’s reference only
The advice given by this friend seems really good; it’s worth considering carefully. Thank you.
1: Please provide the components of your desorption feed； 2: How is the temperature of the material entering the desorption tower controlled? 3: Control of the desorption tower liquid level and the hydrolysis liquid level? Waiting for the next lesson*!
Our CO2 content is 7%, and the NH3 content is also 7%. The temperature of the material entering the desorption tower is maintained at around 91.7 degrees. The liquid level in the first desorption tower is around 40%, while it is kept at around 30% in the second desorption tower.
One more thing: Is the concentration in the return cold liquid level tank too high? Furthermore, you can lower the temperature of your soft water to facilitate better absorption of the gas phase. If the analysis shows that the waste liquid levels are still above the limit, check during maintenance to see if the packing inside the tower has been displaced
I think what the friend on the 7th floor said is absolutely correct – never increase the steam pressure arbitrarily, because the operating pressure of your distillation tower is 0.28 MPa, and it is this pressure that is used in their hydrolysis technology. Unqualified desorption liquid is closely related to hydrolysis; if the urea level is too high, 90% of the cases are due to inadequate hydrolysis, and it is best to increase the amount of steam used in the hydrolysis tower. If the levels of ammonia and carbon dioxide are too high, it is a desorption issue. The solutions are: first, increase the pressure of the vapor; second, slightly lower the temperature of the reflux condensate to prevent ammonia from escaping from the top of the tower; third, check whether the pressures are within acceptable limits. This post was last edited by lxq700918 on 2009-2-7 22:05.]
In response to the original poster’s question, the fellow sailors above have had a rather detailed discussion. Here, I would like to add the following for discussion with everyone: 1. If the desorption pressure is 0.28 MPa and the low-pressure steam pressure is 0.33 MPa, shouldn’t the low-pressure steam be able to be introduced into the desorption system? If it really cannot be added, it indicates that the resistance in the low-pressure steam pipeline from the low-pressure drum to the desorption tower is high. At this point, it is also possible to use 0.9 MPa medium-pressure steam, as this will not cause overpressure in the desorption tower. However, since the low-pressure steam is not utilized, it may lead to steam waste! 2. There are many reasons why the desorption waste liquid is not up to standard, and an accurate analysis cannot be conducted based on the information provided by the original poster. It would be more helpful for the analysis if the poster provided the specifications and dimensions of the desorption tower and hydrolysis tower, as well as the type of internal components and the current desorption load.
By the way, may I ask if your purification system also has a hydrolysis system? If so, then: 1. It is possible to increase the temperature of the material fed into the purification tower appropriately; you might try raising it to around 100°C, at which point the amount of low-pressure steam required will decrease somewhat； 2. Regarding the owner’s suggestion to keep the liquid level in the first distillation tower at around 40%, I believe this level is too low; it could be increased to around 60-70%. Similarly, the liquid level in the second distillation tower, which is currently around 30%, could also be raised to 40%. Due to the excessively low liquid level, it is very likely that cross-contamination of gases will occur; as a result, not all of the low-pressure steam enters the distillation tower in a short period of time, and instead it escapes through the liquid outlet pipeline. This will definitely result in the distilled waste liquid being of substandard quality! If the original poster can provide more detailed data, then more of our fellow sailors will be able to help you discuss it better. The two points mentioned above are just for your reference! ! !
Could considering the use of 3.8 MPa steam for hydrolysis be an option?
Is there any flooding phenomenon occurring in the desorption tower?
The question raised by the moderator is very accurate; an explanation of the entire system’s situation is needed in order to make a judgment
Please provide the components of your desorption feed； 2: How is the temperature of the material entering the desorption tower controlled? 3: Control of the desorption tower liquid level and the hydrolysis liquid level?
Based on the discussions of the above-mentioned individuals, I believe the key issue lies in the desorption tower. It might be that the resistance in the desorption tower is high, preventing steam from entering. I have encountered the aforementioned issue in multiple desorption towers. When the levels of ammonia and carbon dioxide in the feed are high, the load on the first desorption tower increases, leading to liquid flooding. This results in an excessive ammonia content in the output from the first desorption tower, which directly affects the hydrolysis of urea in the hydrolysis tower. It also increases the load on the second desorption tower, making it difficult for the entire hydrolysis-desorption system to operate properly. This post was last edited by lxq700918 on 2009-3-2 08:35.]