What should be noted when the urea plant is operating at low load? (Aqueous solution)

Points to note for low-load operation of urea are as follows: 1. It is necessary to pay attention to controlling various process parameters, which must be kept under control. 2. It is necessary to strictly control the product quality of urea, primarily to prevent the level of biuret from exceeding acceptable limits. 3. It is necessary to prevent corrosion in the system due to low load.

There are already many urea production plants in China that use carbon dioxide obtained through pressure swing adsorption decarbonization as a raw material, but the purity of that carbon dioxide is around 98.5%, which is not as low as what you mentioned. If the purity of carbon dioxide after pressure swing adsorption is only around 95%, and anti-corrosion air is added, the purity of CO2 entering the urea plant will be even lower. If that is the case, the conversion rate of the urine tower will surely be low, the amount of medium-pressure venting will be large, and there is even a possibility of overpressure in the urine tower; it’s uncertain whether the plant can operate properly. Therefore, it is recommended to identify the cause promptly and stop supplying urea.

We now have two sets of synthesis units: one uses MDEA for decarbonization, and the other uses pressure swing adsorption. We only use the carbon dioxide from MDEA in the urea production process, but these amounts are not sufficient to ensure full-capacity operation. I’m not very familiar with the pressure swing adsorption process, and I don’t know why the purity is so low.

Our plant uses gas produced by pressure swing adsorption. When there are operations such as switching between adsorption towers, the purity of the gas drops; at its lowest, it is only 94%. This gas is supplied to two urea production units (we plan to build a third unit, with provincial leaders attending the groundbreaking ceremony just a few days ago). One of these units was designed to produce 40,000 tons per year (this was a long time ago; after technical upgrades, its annual production capacity can now reach 200,000 tons). The other unit was designed to produce 300,000 tons per year; it came online at the end of 2005, and after technical upgrades its production capacity can now reach 400,000 tons. Both units use a full-circulation water solution process. When the demand is low, the production of old-style urea is reduced significantly in order to maintain the production of new urea. If nothing else works, stop one tower on each side (both sides have twin towers in operation), but everything should be done with the goal of maintaining the production of new urea, as it takes time to establish a stable process for its production. Low-load leadership only sets one requirement for us: the product quality must be of acceptable standard. This is what needs the most attention. I work in evaporation. Quality of products must be the lifeblood of a company, right? (A fine of 100 yuan is imposed for each quality issue that occurs during normal production, and the proportion of high-quality products must be over 96% per month.) This was assigned to us by the Quality Department. ）

First, it is necessary to pay attention to the control of various process parameters and maintain system water balance, in order to avoid increased system emissions resulting from imbalances； Second, attention should be paid to the quality of the finished products.

Our plant uses PC decarburization and PSA decarburization; the CO2 produced by both methods is sent for conversion into urea. The purity of the CO2 at the PSA outlet is higher than that obtained through PC decarburization. As far as I know, many manufacturers achieve higher purity of CO2 at the PSA outlet, so you should investigate the reason for this.

I’ll tell you a way to avoid fines: when the urea system operates at low load, the production volume is low, there is less material to evaporate and thus more heating is required; temperature control becomes difficult, which can lead to high levels of biuret. You should install a condensation valve at the inlet of the evaporation heater and keep it slightly open, adding some water to reduce the concentration of the urine-like substance. This increases the evaporation load, allowing better control of temperature and pressure, and the resulting product will be of satisfactory quality. The amount of water to add should be determined based on actual conditions, gradually. This post was last edited by lxq700918 on 2009-3-11 13:02.]

However, your approach will result in high steam consumption. However, it can be used for a short period of time as one of the emergency measures.

The method of using the 8th floor under low load is indeed feasible; we have had successful examples of this, only instead of adding condensate, we add surface cooling fluid.

Whether steam condensate or surface cooling fluid is added, the result is the same: it increases the steam consumption of the first and second stage evaporators.

Let’s discuss this in the context of the version for boating on the five lakes: if long-term operation at low load is caused by objective reasons (such as for more than half a year), I would suggest partially blocking the heat exchange tubes of the first and second stage evaporator heaters. This way, steam condensate or surface cooling fluid can be omitted to maintain the evaporation load, which not only reduces steam consumption but also ensures product quality – achieving two goals at once!

It’s better not to cause blockages; it’s ideal to control the amount of material added. The key is to find ways to improve heat recovery.