Problems with the pipeline between the primary evaporation separator and the secondary evaporation heater in the urea production process using carbon dioxide stripping
Thread Content
Urea is produced by the carbon dioxide stripping method. There is a U-tube between the first-stage evaporation separator and the second-stage evaporation heater. This pipe has special requirements: the bottom of the two-stage evaporation heater must be at least 6 meters away from the bottom of the pipe. The angle between the straight pipe coming out of the evaporation separator and the U-tube is less than 45 degrees; in other words, the minimum angle at which the U-tube can be raised above the horizontal level is 45 degrees. The thermometer installed on the straight pipe downstream of the evaporation separator is at a maximum height difference of 2 meters from the bottom of the pipe. The pressure in the first-stage evaporation separator is 33.3 KPa, while the pressure in the second-stage evaporation heater is 3.33 KPa. Moreover, the equipment height of the first-stage evaporation separator is higher than that of the second-stage evaporation heater. Excuse me, everyone, why is this? How can we explain the requirements for this pipe?
The U-tube serves as a liquid seal. Since there is no control valve in the vaporization liquid line, if a U-tube is not used to maintain the liquid level, gas from the first stage will flow into the second stage, preventing the vacuum level in the second stage from reaching the required value. Based on the normal pressure difference between the first and second stages of production, a U-tube height of around 3 meters is sufficient. Considering the increase in the pressure difference between stage 1 and stage 2 during startup/shutdown of evaporation or in accident conditions, the actual height is even greater.
However, the pressure in stage 1 is higher, while the pressure in stage 2 is lower. The height of section 1 is even higher. Went all that liquid from section 1 to section 2, didn’t it? How does it serve as a liquid seal?
There is a pressure difference between section 1 and section 2, of about 0.3 kilograms. When there is a disruption in the supply to section 1 or when the feed rate is insufficient, this pressure will cause the fluid to flow over the U-shaped tube. However, this pressure difference is only sufficient to push the urine to a height of 2.5 meters; a urine column of 2.5 meters high counteracts this pressure difference. In actual design, the height difference between the bottom of the U-shaped tube and the inlet of section 2 is approximately 5 meters, which is much higher than the required 2.5 meters
It actually serves to create a balance pressure difference and provide a liquid seal!
This is mainly to prevent gas from one section from entering the other section; therefore, a U-shaped tube is used to serve as a liquid seal. The formula is h=p/ρg ； Note: (p = p1 – p2), where P1 represents the pressure in stage 1 and P2 represents the pressure in stage 2; ρ is the density of urine. Theoretical calculations suggest around 2.5 meters, but for safety reasons, it is generally kept between 5 and 6 meters.
Reply to 1# gestapo_123 regarding the issue of the horizontal angle of the U-tube: the main consideration should be to ensure that the pipeline is completely emptied during discharge, in order to prevent urine from accumulating and crystallizing in case of an accident. Additionally, having a certain angle helps to reduce the amount of pipe needed.
Is it better to have straight pipes from the evaporation separator to the U-tube, or should there be an angle? Why?
Considering: 1. liquid seal, 2. smooth urine flow, 3. equipment installation cost, 4. urine retention time