Feasibility of feeding the liquid from the flash tank directly to the first-stage evaporator

I think the flash vapor should be directly fed into Stage 1 vaporization, that is, what is known as vacuum feeding. It’s sufficient to create a pressure difference between the flash vapor and Stage 1 vaporization. The advantages are: 1) a lower Bi value, 2) savings in electricity consumption for a urine pump; of course, the urine pump is still needed for starting up the system and for pumping urine from the tank in abnormal situations. It’s a bit hard to understand why so many factories have to put the flashed urine into a urine tank first before pumping it up.

The advantage of venting the urine to a urine tank before pumping it back up is that it prevents the previous process from affecting evaporation.

There must be a reason for it; surely not a single person in the global community of those involved in CO2-based urea production would fail to understand this.

This post was last edited by 1025199692 on 2010-1-10 at 17:55. *It has become a common practice; no one in the industry is willing to be the first to try this. From a process perspective, all aqueous solutions go directly into the evaporation stage, as there is a medium-pressure system that acts as a buffer, minimizing the impact of the previous system on the evaporation process ； In the CO2 stripping method, when the liquid level in the stripping tower fluctuates, both the pressure of the low-pressure gas phase and the evaporation vacuum change simultaneously, with no margin for adjustment. The ammonia stripping method should be considered. A few thoughts.

Both water urination and air urination are practices practiced by the Nordics and Dutch; water urination came first, followed by air urination – think about it.

Doing this helps to maintain a stable evaporation load. In addition to ensuring product quality, as mentioned earlier, another important factor is steam balance; fluctuations in the load will inevitably cause the steam pressure to rise and fall, which has a significant impact on the previous aspects. Another important point is that if there is a failure in the evaporation granulation process or the belt system, it is possible to stop only the evaporation process. When a fault occurs in the subsequent system, the urine tank operates at a low liquid level (10%; we usually use 7%) and the system’s load is reduced to 70%, allowing for approximately 8 hours of time for emergency repairs before the liquid level in the urine tank reaches 90%

There is a difference between the full aqueous solution circulation method and the CO2 stripping method. The aqueous solution full-circulation method features a medium-pressure system, which enables a smooth transition of the materials. In the CO2 stripping method, the material passes directly from a high-pressure system to a low-pressure system; if the liquid resulting from flashing goes straight to the first stage of evaporation, it will hinder the stable operation of the entire system, leading to large fluctuations in its performance.

From a feasibility analysis, it is entirely achievable as long as there is a sufficient head difference between the flash tank and the first-stage evaporator. However, for large urea plants, once the system experiences fluctuations, it takes a long time to make adjustments and restore normal operation, and the impact on production is significant as well. Therefore, stability is sought during the production process to minimize external influencing factors. Perhaps for this reason, in large urea production plants, a direct feeding method is not used; instead, the material flows from the flash tank to the urine tank and then is pumped to the evaporation section. This feeding method ensures the stable operation of the evaporation system.

Theoretically, it is feasible to feed the liquid from flash evaporation directly into the first stage of evaporation. However, in practical operation, there are too many uncertainties. As the colleague above mentioned, a medium-pressure system provides buffering for the complete circulation of aqueous solutions, but such a system is not available for gases. If fluctuations in the preceding system have a significant impact on the material fed into the evaporation process, it will inevitably lead to a series of problems in the evaporation system and affect the quality of the final product. By feeding the liquid through a urine tank and urine pump into the first stage of evaporation, the stability of the feed to the evaporation system can be ensured, the impact of fluctuations in the preceding system on evaporation is reduced, and the quality of the final product is maintained.

Personally, I think it would be possible to use both processes – just add bypass pipes to the pipeline leading to the urine tank and to the pipeline leading to the evaporator. Turn them on when the situation is stable and turn them off when it’s not; wouldn’t that work?