Salt formation at the top of the distillation tower

Do the temperature and pressure recording curves at the top of the tower show regular wave-like variations? The temperature at the top of the tower is difficult to control. This is a symptom of salt deposition on the tray. Online tower washing is carried out without shutting down the system; the flow rate is reduced by 30%, and water is injected at the top of the tower to keep the temperature below 100 degrees. The side stream closest to the salt-formation tray (determined based on changes in pressure and temperature along the tower) is identified, and the temperature of the fluid extracted from this side stream is maintained between 103 and 105 degrees – a higher temperature promotes vaporization while a lower temperature facilitates flow to the lower trays. The condition of salt washing is assessed by monitoring changes in CI- levels resulting from the drainage via the side stream pump. When the CI content stops decreasing, the effect is essentially achieved. Gradually resume operations, return the reflux to normal operation, and restore the processing volume.

Salt deposition can cause significant fluctuations in the temperature of the sensitive trays, as well as instability in the pressures at the top and bottom of the tower with an increasing pressure difference. The most direct solution is to inject water at the outlet of the reflux pump; washing with water is indeed the simplest approach

Materials prepared for reference: Measures for online water washing of salts in the 3-stage distillation tower 3. Online water washing of the air-cooling tube banks at the top of the distillation tower 3.1.1 Process for online water washing of the distillation tower’s air coolers As shown in Figure 1, the oil and gas at the top of the distillation tower pass through six parallel air coolers; after being cooled by water, they enter the oil and gas separator. The crude gasoline goes to the absorption tower, while the rich gas is sent to the compressor. The sulfur-containing wastewater is discharged from the plant to the wastewater stripping unit. As shown in Figure 2, to enable online water washing of the air-cooled tube bundles, two pressure gauges are installed behind the inlet valve of each air cooler, while one pressure gauge and one thermometer are installed at the outlet. A DN50 deaerated water line is connected to the inlet of each air cooler. The severity of salt deposition can be determined based on the outlet temperatures of each air cooler. 3.1.2 Precautions for operation: The water washing should be carried out when production is stable; the optimal time is during the preheating of the coke tower, as the distillation tower has lower heat levels, making it easier to control the top temperature. 1) Based on the outlet temperatures of each air cooler, wash first those with severe salt deposition, cleaning one set at a time. When it is time to close the two inlet valves of that set of air coolers, shut down the air coolers in that set. The personnel on site should maintain close communication with the operators inside the plant; if the pressure at the top of the distillation tower increases significantly, the inlet valve should be opened fully immediately to prevent any damage to the distillation tower. 2) When closing the deoxygenated water valve, pay attention to the load on the sulfur-containing wastewater pump; the water supply volume should not exceed the pump’s maximum flow rate. If the oil-water interface level remains above 80% for an extended period, the water supply amount should be reduced, while also checking whether the crude gasoline contains water. 3) The air cooling time for each group during washing should be maintained at over 4 hours. Start flushing with deoxygenated water for 1 hour, then close the outlet valve to build up pressure. Each group should be pressurized with air cooling for at least 2–3 times. If, when the outlet valve is fully closed, the reading on the outlet pressure gauge is roughly the same as that on the inlet pressure gauge, then fewer pressurization cycles are necessary; if the difference between the two readings is >0.03 MPa, then continued pressurization and washing are required. Until the outlet pressure > 0.1 MPa, and the pressure difference between inlet and outlet

Phenomenon: The temperature at the top of the first distillation column rises rapidly. 2. The top pressure drop increases, and the column pressure fluctuates in a sawtooth pattern. The circulation volume of the 3 units has declined, and light diesel cannot be drawn out. The pressure drop across the 4 trays increases, resulting in unstable flow in the top circulation. Treatment method: Reduce the feed rate for tower washing, or cut off the feed.