Why does the final cooling tower use condensate to cool the gas? What are the benefits of using condensate?

The poster is referring to the horizontal tube final cooling tower, right! The cooling of gas is achieved through circulating water and refrigeration water; the condensate merely serves to wash away naphthalene adhering to the pipes.

What the original poster should have meant is not the horizontal-tube type

I’m not referring to the type of horizontal tube initial cooler; I’m talking about the final cooling tower in the crude benzene washing unit. This tower is divided into two sections – one section where circulating condensate is sprayed using low-temperature water, and another section where circulating condensate is sprayed using regular circulating water.

The temperature of the gas coming out of the saturator is 55 degrees, while the optimal temperature for benzene removal is 25–28 degrees; therefore, the gas needs to be cooled further. In the lower section of the final cooling tower, condensate cooled by circulating water is used for spraying, with the temperature controlled at 35–40 degrees. In the upper section, condensate cooled by refrigerated water is used for spraying, with the temperature maintained at 25–28 degrees. When the condensate contains a high amount of naphthalene, the resistance in the final cooling tower increases significantly. In such cases, it is necessary to flush the tower and remove the waste substances.

The packing material inside the tower can be wooden grids, steel mesh, wreaths, and lightweight porcelain, etc. The resistance per meter of height for various fillers, as well as the surface area per cubic meter of filler (specific surface area), vary. These two parameters affect the processing capacity of the tower, operating costs, and capital investment. Adjust the temperatures of the circulating fluids in the upper and lower sections of the final cooling tower based on the temperature of the gas exiting the tower, to ensure that this gas temperature remains within the specified range ； Adjust the temperature of the oil-poor gas fed into the benzene washing tower based on the temperature of the gas after the final cooling tower, to ensure that water vapor in the gas does not condense into the circulating washing oil. (If it is a horizontal tube type, then it is the same as the primary cooler.)

The final cooling tower uses condensate to cool the gas; it can be reused or partially renewed. You are referring to a two-stage direct-cooling final cooling tower (with packing); the circulating fluid in the upper stage is cooled by low-temperature water, while that in the lower stage is cooled by circulating water. Additionally, an alkali is added to the upper stage in order to further absorb hydrogen sulfide from the gas. Excess condensate is discharged to the ammonium sulfide or phenol-cyanide wastewater treatment section.

The process flow for this final cooling can be described as follows: The gas at approximately 55°C, coming from the ammonium sulfide unit, first enters the final cooling tower at its lower section, where it is cooled in two stages. In the lower stage, circulating cooling water at around 37°C is used, while in the upper stage, circulating cooling water at around 24°C is employed to cool the gas to around –25°C. After that, the gas proceeds to the benzene washing tower. There, the crude benzene is removed from the gas through oil-poor washing; part of this gas is sent back to the coke oven and the crude benzene tubular furnace for use as heat, while the remaining gas is delivered to end-users. The circulating cooling water in the lower section of the final cooling tower enters this section from the middle of the tower; it comes into contact with the gas in the opposite direction to cool it, and is then pumped out. It passes through the lower section’s circulating spray cooler, where it is cooled to 37°C using circulating water before being sent back to the middle section of the final cooling tower for reuse. The circulating cooling water in the upper section of the final cooling tower enters this section from the top of the tower to cool the gas; after that, it is pumped out and cooled to 24°C using low-temperature water in the upper section circulation spray cooler, before being returned to the top of the final cooling tower for reuse. At the same time, a certain amount of alkaline solution is added to the upper section of the final cooling tower to further remove H2S from the gas, ensuring that its concentration remains at ≤200 mg/m3. The condensate discharged from the lower section is sent for treatment in the phenol-cyanide wastewater treatment process, while the alkaline-containing condensate discharged from the upper section is sent to the top of the ammonia evaporation tower in the ammonium sulfate production unit to decompose the fixed ammonium in the remaining ammonia water.

If the original poster knows why a primary cooler uses condensate for circulation, they should understand why this is the case. It’s all based on the same principle

Reading everyone’s replies has taught me a lot... At first I thought that the alkali was added directly to the remaining ammonia solution using a metering pump, and then it went to the ammonia vaporization tower... I also gained a better understanding of crude benzene. On the 9th floor, I understand that the horizontal tube primary cooler uses circulating cold liquid to wash away the crystalline naphthalene between the tubes; similarly, isn’t the condensate from the final cooling stage used to remove any remaining naphthalene in the gas that wasn’t removed during the drum cooling stage?

In the case of a horizontal tube type, it is used to wash crystalline substances, colloids such as naphthalene and amines, etc. In the case of a direct cooling type, it is used to remove ammonia and lower the temperature.