Analysis of the treatment for salt deposition blockage in the RFCC fractionation column trays

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Analysis of the treatment for salt deposition and blockage in the trays of RFCC distillation columns ★ In recent years, due to factors such as variations in the types of raw materials purchased, the properties of the raw materials used in heavy oil catalytic units have changed frequently, which has led to a range of technical issues related to heavy oil processing. Salt deposition in the upper parts of the distillation columns is one of the problems that arise during production. ◆ The phenomenon of salt formation: Under normal operating conditions, the temperatures in various sections of the distillation tower should change accordingly as the reflux volume in each section varies; quality control should also be convenient and flexible. However, the following phenomena occur when salt forms in the distillation tower. 1. The top temperature changes slowly as the circulation flow rate varies. 2. The number of vacuum draws by the top circulation return pump increases significantly. 3. The gasoline and diesel processes overlap significantly. The flash point of the light diesel does not meet the requirements. The stripping steam at the bottom of the light diesel stripper cannot be adjusted, and using reflux for adjustment results in issues with the dry point not meeting the specifications. 4. When severe salt deposition occurs, light diesel cannot be extracted. 5. The pressure drop in the distillation column increases; the pressure at the top of the distillation column is low while the reaction pressure is high, resulting in a large pressure difference between the two vessels. In severe cases, only by reducing the feed rate can the pressure balance between the two vessels be maintained. The above phenomenon can be used as a basis for determining salt deposition and blockage in the upper part of the fractionation tower. ◆◆ Treatment of salt deposits  Washing process: Soft water is the best choice for treating salt deposits that form during distillation. Washing process:  Washing step 1: Reduce the reaction feed rate to 70% of the normal level, i.e., 40 tons per hour, in order to decrease the heat removed by the external heat exchanger and maintain the regeneration temperature. If necessary, burn oil can be sprayed to ensure proper charring. 2. Reduce the level of liquid in vessel 201 to 20%, decrease the amount of sulfur-containing wastewater discharged from the unit, and increase the water volume used for washing the rich gas before vessel 301 to 4–6 tons per hour. Keep the capacity level below 100%. 3. Stop using the lift pipe terminator (cut off at the collect pipe and feed nozzle). 4. Cut off the flow of crude gasoline to tower 301, increase the flow of absorbent supplied to tower 301, and maintain the three-tank circulation. 5. Adjust the top temperature of the fractionation tower to 95–98°C; control the amount of water using pump 202 and the cooling reflux control valve connected to tank 201 ； Wash the trays above the top circulation extractor and the light diesel extractor in stages according to the washing procedure. 6. After the water washing is complete, gradually increase the reaction feed and adjust the operation to normal.  Selection of operating parameters during water washing: 1. Top temperature of the distillation tower. The principle for controlling this temperature is to prevent water from evaporating inside the tower, while simultaneously causing the steam present in the tower to condense in the washing section in order to enhance the washing effect. If the top temperature is too high, a large amount of water will evaporate, causing the safety valve to activate and preventing the washing process from achieving its purpose. The boiling point of water at normal pressure is 100°C; therefore, for safety reasons, the top pressure is maintained at 0.10–0.12 MPa to keep the tower top temperature between 90–95°C. 2. Water supply volume. The water supply volume is indirectly controlled by the return tower temperature. For the cold return flow control valve, water supply is stopped when the return temperature gradually drops to 25–30°C; at this point the temperature in the tower rises, and once it reaches 90°C, water supply is resumed, and this process of washing is repeated. 3. The extraction temperature of light diesel shall be no less than 140°C. The purpose of controlling the extraction temperature of light diesel is to prevent water vapor from condensing below the top circulation extraction point. Floor 25 is equipped with a level gauge and a discharge outlet for draining water; pay attention to monitoring and sampling. Until the chloride ions reach the acceptable level. 4. If the pressure drop between layers 17 and 25 is too high, it is likely that the trays will also become clogged, resulting in no liquid level in the stripping tower. 5. Close the drain at layer 25 and continue cleaning the lower trays. The wash water is pumped from the stripping tower to the diesel water washing tank via a diesel pump for dehydration. The chloride ion test passed; washing should be stopped when the water at the diesel sampling port is clear and free of suspended particles.  Washing effect: The washing process lasted 3 hours, after which the operation of the distillation system gradually returned to normal. The product quality improved significantly, and control became more flexible. Operation parameters before and after washing, product quality (March 27, 2004)
Item: Before washing / After washing
Temperature at the top of the distillation tower, °C: 105 / 116
Pressure in the distillation tower, Mpa: 0.09 / 0.125
Temperature of the vapor drawn from the top of the tower, °C: 136 / 145
Temperature at which the vapor is returned to the tower, °C: 85 / 95
Temperature at which light diesel is drawn off, °C: 160–172 / 185–190
Flash point of light diesel, °C: 50–60 / 80–95
Freezing point of light diesel, °C: -9 to -12 / -12 to -15
Initial boiling point of light diesel, °C: 175–180 / 208–216
Dry point of crude gasoline, °C: 185–192 / 196–201
◆◆◆ Analysis and prevention of salt formation
Most of the raw materials purchased by our company are fuel oils. With the current level of electrodialysis technology in the petrochemical industry, it is not possible to remove all salts from these materials; rather, efforts are made to minimize the salt content as much as possible. In particular, organic nitrogen present in crude oil cannot be removed through electrodialysis or atmospheric/ vacuum distillation. The vacuum distillation desalination unit in our plant is not operational; therefore, there are inherent fundamental factors contributing to salt accumulation at the top of the distillation tower. The external factor causing this salt accumulation is the temperature at the top of the distillation tower, with a low extraction temperature at the 25th stage. There are many reasons for the low temperature at the top of the fractionation tower. The level of the distillation column top temperature is determined by the dry point of the crude gas. The reasons for the low top temperature are as follows: 1) The reaction pressure is low, resulting in a low vapor pressure of the oil and gas at the top of the distillation tower, which in turn leads to a low top temperature. 2) The degree of conversion is low; a lower load at the top of the distillation tower results in a lower temperature (as in the diesel production process). 3) The load is unevenly distributed across various sections of the distillation tower, and a lower load at the top leads to a lower temperature there. 4) The processing capacity is low, resulting in a low device load; a low load on the top of the fractionation tower leads to a low top temperature. Inside the distillation tower, as the oil and gas containing water vapor rise, condensation droplets are formed when the temperature reaches the dew point of the water vapor; in this process, NH3 and HCl combine to form an NH4Cl solution, whose boiling point is much higher than that of water. If this solution reaches the bottom of the downcomer, it will be concentrated through distillation. As the salinity increases to a certain level, the gap at the bottom of the downcomer decreases, and eventually this can disrupt the normal operation of the distillation tower. Ammonia and hydrogen chloride in the fractionation tower are generated from the nitrides and chlorides present in the feedstock under catalytic cracking conditions. 2RN+3H2→2R+2NH3↑ CI+H2O→OH+HCI↑ The nitrides and chlorides present in the raw materials are the factors that lead to salt formation; these are the internal factors, while the conditions in the distillation tower represent the external factors. Prevention of salt formation starts with two approaches: first, when purchasing raw materials, it is advisable to choose catalysts with low levels of salt and nitrogen if possible. Secondly, during the production process, the minimum temperature for condensation is determined by calculating the amount of water vapor entering the distillation tower and its vapor pressure within the tower; by keeping the temperature at the top of the tower above this value, salt formation can be avoided. Since the company was transferred to Lanxing Petrochemical, salt deposition caused blockages in the upper part of the distillation tower on March 27, 2004, and May 2, 2005. At that time, the oil primarily processed was from northern Shaanxi, which is high in salt content; low temperatures at the top of the tower led to blockages in the upper trays of the distillation tower. The pump used for circulating liquid at the top kept running out of fluid, and attempts to clear the blockages using steam were ineffective. In the most severe cases, there was no liquid level even in the stripping tower. Ultimately, the method of reducing water volume during washing was used to restore normal control of the operation. To date, the raw material procurement department has paid attention to controlling the salt content when purchasing raw materials, while the production workshop strictly monitors temperature parameters ； Thanks to joint efforts, no blockage of the distillation column trays occurred. ★★Conclusion 1. When temperature and pressure do not change significantly, the dry point of crude gasoline fails to meet the requirements; in light diesel, there are fluctuations in the liquid level of the stripping tower or even no liquid level at all, along with an increase in the pressure drop in the fractionation tower – this is generally attributed to salt formation that blocks the tower. 2. The salts that cause tower blockage are ammonium compounds and ferric chloride; the chloride ions come from organic chlorines and inorganic salts in the raw materials, while ammonia originates from nitrides in the raw materials. 3. One of the reasons for salt formation is the high salt content in the raw materials; another reason is that the temperature at the top of the distillation tower must be kept above 105°C, and the temperature at the 25th extraction stage must be above 140°C. 4. Ammonium chloride is readily soluble in water, and this issue can be resolved by using a scrubber; the scrubbing process takes about 4 hours. This avoids shutdowns and brings significant economic benefits.