Technology selection for circulating water in urea plants in northern regions

There is a certain amount of evaporation of the urea cycle water return stream in the cooling tower, so it is necessary to top up water regularly; using soft water is too costly. In a closed-loop system, relying solely on the cold temperatures in the north to cool the supply and return pipes will not be very effective; moreover, the temperatures in the north during summer are not much lower than those in the south. There are no reports on the solution proposed by the original poster. This post was last edited by ygx2000 on 2008-1-3 12:47.]

Design Notes on the Cooling Scheme for Energy-Saving Water Film Evaporative Air Coolers: The soft water closed-loop cooling system uses soft water in place of traditional industrial circulating water; the soft water absorbs the heat from the cooling device, is then cooled by an evaporative air cooler, and returns to the cooling device, thus forming a closed loop within the system. The simple workflow of the system is as follows: chemical dosing unit → spray water ↓ cooling unit → evaporation air cooler → cold water circulation pump. Soft water. Technical description of the evaporation air cooler: The evaporation air cooler is the core equipment in a closed-loop cooling system for soft water. An evaporative air cooler is a new type of efficient and energy-saving cooling device that integrates water cooling with air cooling, as well as heat transfer and mass transfer processes. Our company’s evaporative air cooler, also known as the “energy-saving water film type air cooler”, is a **patented product (patent number: ZL03 2 41415.3). It has received funding from the Ministry of Science and Technology’s Technological Innovation Fund (funding code: 04C26214210704), and was designated as a key new product in 2005 (project number: 2005ED760026). The process of the evaporation air cooler is as follows: The main feature of the evaporation air cooler is that heat transfer is enhanced through the evaporation of a water film outside the tubes. In its operation, a circulation water pump is used to deliver circulating cooling water to a spraying system located above the tube bundle; this spraying system then sprays the cooling water downward onto the surface of the tubes, thereby forming a thin water film on the outer surface of those tubes ； At the same time, an axial flow fan is used to draw air in through the louver, causing the air to pass upward through the light tube bundle beneath the tube bundle. Evaporative air coolers enhance heat transfer outside the tubes by relying on the rapid evaporation of the water film on their outer surface, thereby achieving the cooling of the soft water inside the tubes. This is mainly because water has a relatively high latent heat of vaporization (the latent heat of vaporization of water at one atmosphere is about 570 kcal/kg), which leads to an improvement in the overall heat transfer efficiency. When the equipment is in operation, the evaporation of the water film on the outer surface of the tubes causes the humidity of the air passing through the tube bundle to **increase and approach saturation. The axial flow fan extracts this saturated wet air and sends it through a mist catcher located above the water spraying system; the water droplets contained in the saturated wet air are removed, after which the air is released into the atmosphere via the air outlet. Since the axial flow fan is located at the upper part of the equipment, it draws air upward, creating a negative pressure area below the fan; this accelerates the evaporation of the water film on the outer surface of the light tube, thereby enhancing the heat transfer outside the tube. During operation, the soft water flows inside the pipes, while air and spray water flow outside them. The air moves from bottom to top, and the spray water moves from top to bottom; the spray water, air, and soft water flow in a cross-flow pattern, whereas the cooling water and air flow in a counter-current pattern. In terms of the layout of the entire process, it also enhances the heat and mass transfer processes. The operation of evaporative air coolers is closely related to environmental and meteorological conditions. When designing the heat exchange area, the temperature conditions over a period of no more than 5 days are used as the basis for calculation, to ensure that the blast furnace cooling system can operate safely under the most severe conditions. Under normal climate conditions, some of the evaporative air coolers can be turned off, or the amount of water used for spraying can be reduced, as well as the wind volume. In winter, most of the water used for spraying can be discontinued, or even no water at all can be used, turning the system into a dry air cooler. Therefore, evaporative air coolers are also devices that help save water. The closed-loop system centered around the evaporative air cooler recovers all of the residual pressure of the cooling medium; the chilled water circulation pump only needs to generate a slight pressure increase to enable system circulation. In an open-type cooling tower system, all the residual pressure of the cooling medium is eliminated, and the chilled water circulation pump must build pressure starting from atmospheric pressure. The spray water needs to be circulated using a pump, but its volume is only one-third to one-fifth of that of the cooling medium water, and its head pressure is only one-third of that of the cold water circulation pump. Therefore, a closed-loop system centered on an evaporative air cooler is energy-efficient. Technical parameters table for a single evaporative air cooler
Item | Technical parameters and quantities
Equipment model | ZP12X6
Medium inside the tubes/medium outside the tubes | Soft water/air, spray water
Flow rate of the medium inside the tubes, m3/h | 1200
Inlet temperature/exit temperature of the medium inside the tubes, °C | 43/35
Design pressure of the equipment, Mpa | 1.0
Pressure drop of the equipment, Mpa | 0.05
Specifications of the heat exchange tubes | φ25 hot-dip galvanized seamless steel tubes, L=12m
Fan type | Axial flow
Number of fans | 8
Material of fan blades | Die-cast aluminum
Spray water pumps | 2 pipeline pumps
Water tank | Made of Q235 steel with fiberglass lining for corrosion protection
Demister | PVC

Energy-saving water film evaporation air coolers are widely used in power and metallurgical systems. In chemical plants, they are currently being applied in the Shanxi Lanhua facility, which produces 200,000 tons of methanol and 100,000 tons of dimethyl ether per year; unfortunately, no operational evaluations have yet been conducted. Based on the information provided on this forum, do you think it is feasible to use such coolers in urea production facilities?

Closed-loop recycling technology is too costly, and moreover, it isn’t necessary.

During the closed-loop circulation of soft water, as long as the pipelines and heat exchange equipment do not leak, only 3 to 5 tons of deionized water need to be added per week. Moreover, during heat exchange with the process equipment, the fouling coefficient of soft water is lower than that of conventional circulating water, resulting in significantly better heat exchange efficiency compared to conventional circulating water； In an open-loop circulating water system, the pressure at the suction inlet of the circulation pump is at atmospheric pressure, while in a closed-loop system it is around 0.15 MPa. Although evaporative air coolers are equipped with numerous fans and circulation cooling water pumps, some of these fans and pumps can be turned off depending on the atmospheric conditions. Overall, closed-loop circulation helps to save energy ； The water volume retained in the cooling water system of a soft water closed-loop circulation system is only 1/3 to 1/5 of that in an open-loop circulation system; as a result, evaporation, loss due to wind, and wastewater discharge are lower compared to open-loop systems, making it a water-saving solution ； By using desalinated water for heat exchange with cooling equipment, there is basically no risk of sludge clogging the heat exchange devices, thereby **extending the operational cycle of the process unit**. Therefore, it can be said that the soft water closed-loop system saves electricity and water, offers good heat exchange performance, and extends the operating cycle of chemical plants.