How does low-temperature methanol washing maintain the heat balance of the system at low load?

Maintain an appropriate and stable liquid level; ensure an adequate supply of liquid ammonia, use an appropriate amount of nitrogen for stripping, and minimize heat loss as much as possible when the pressures in all columns are normal

One additional point is to reduce the methanol circulation rate while ensuring that the purified gas meets the required standards. To ensure the saturation level of CO2 dissolved in methanol.

First, lower the liquid level in the quencher, reduce the methanol circulation rate appropriately, and increase the amount of nitrogen used for stripping within the allowable range. It is important to maintain equilibrium among the liquid levels in various towers; the key is to adjust the reflux flow at the bottom of the absorption tower in order to regulate the positions of the roughing section and the washing section

Does low-temperature methanol washing result in less heat released during vacuum flashing at low load conditions, due to less carbon dioxide being absorbed? If so, how is the cooling capacity of the system balanced?

Everything is interdependent and in a state of balance with opposing forces; the system maintains a cold balance in order to absorb acidic gases under certain system pressures, temperatures, and amounts of methanol used for washing. Low system load leads to a decrease in the amount of carbon dioxide per unit volume (even if the circulation volume is reduced, low load still results in less carbon dioxide to be absorbed per unit volume of methanol), and accordingly the amount of cooling energy required for flashing also decreases. On the other hand, as the system load drops, the amount of acid gas that needs to be absorbed also decreases, and the temperature rise resulting from absorption is reduced as well. Therefore, at low loads, it is possible to maintain effective absorption by appropriately reducing the circulation volume of methanol, thereby minimizing the cooling loss caused by circulation and increasing the saturation level of carbon dioxide in the methanol; there is no need to worry about the decrease in cooling energy for flashing. I think what needs to be given more consideration here is how to reduce the system’s energy consumption under low load conditions, that is, the power consumption of the ammonia compressor and the system’s methanol circulation pump!

At low load levels, keeping the system temperature a bit higher than usual is more convenient for operating the low-temperature methanol wash process. Let me give you a simple example: when the system’s load is low, in order to adjust the carbon dioxide level at the outlet of the scrubber, it is necessary to reduce the circulation rate. However, reducing the circulation rate helps to lower the system temperature, and a lower system temperature again forces one to reduce the circulation rate further (as the carbon dioxide level becomes lower). Of course, an excessively low circulation rate has negative effects on the liquid level and the operation of the pumps. Therefore, I recommend monitoring the ammonia reduction level simultaneously while reducing the circulation volume, to avoid keeping the temperature too low. Of course, when the system load is between 30% and 40%, no one is able to produce carbon dioxide; this is probably why the system is designed to have a flexibility range of 50% to 110%.

Your concerns are unfounded. You’re right – the load is low, so less carbon dioxide is produced during decomposition, and the system generates less cooling capacity. But with such a low load, it’s unnecessary to have too much cooling capacity! ! As long as the temperatures at all points in the system remain normal under any load, that’s sufficient!

The amount of CO2 desorbed is low, and at the same time the amount of acidic gases absorbed is also low; therefore, less heat is released! At low load, there is an excess of cooling capacity in the system at first; therefore, the load on the chiller should be reduced by lowering the valve controlling the flow into the subcooler, and at the same time, the circulation volume of lean methanol should also be decreased. [It is also possible to try not to reduce the circulation volume in order to utilize the excess cooling capacity.] Pay attention to the system pressure; the outlet syngas is of acceptable quality ； Various level control signals, surge in the recycle gas compressor.

1. Improve the system’s heat-insulation performance to eliminate severe heat loss in the tower, tanks, and pipelines located in the cold zone; the lower part of the tower should not feel cold to the touch, and the tower walls as well as the overhead pipelines should not develop ice or emit cold air. Heat losses are primarily concentrated around the tower in the cold zone, the inlet and outlet stop valves and flanges of the pumps, as well as the large automatic control valves and stop valves. The insulation layer is cracked; the nameplates of the cold equipment lack insulation and are covered in ice, resulting in significant heat loss. 2. Strengthen the cleaning of water coolers. 3. Proper use and cleaning of ammonia coolers: While ensuring normal operation of the process, try to increase the load on the ammonia compressor in order to reduce the ammonia evaporation pressure and thereby improve the cooling effect. If an oil seal is used for the ammonia compressor, the liquid ammonia in the system may contain oil. A large amount of ammonia oil adheres to the outer wall of the heat exchange tubes, increasing the thermal resistance of the heat exchanger and reducing the efficiency of ammonia cooling. An oil drainage device should be installed; oil needs to be drained each time the system is started or stopped, but this does not address the issue of liquid ammonia containing oil at its root. 4. Increase the CO2 desorption amount in the methanol washing cooling zone. CO2 desorption refrigeration is the main cooling source for methanol washing. The amount of desorption in the cold zone directly affects the heat balance. To improve the cooling effect of throttling flash evaporation, the following measures can be taken: reduce the pressure for desorption and flashing as much as possible. CO2 desorption in the cold zone mainly occurs in Tower 2 and Tower 3; a certain pressure difference is designed between these two towers. However, at full load, this pressure difference may decrease, which can impede the methanol solution in Tower 2 and disrupt the circulation process. Therefore, it is necessary to maintain a pressure difference between the two towers to ensure the circulation of methanol-rich fluid. Ensure an appropriate amount of nitrogen for stripping to provide sufficient desorption driving force. Reduce the methanol circulation rate while ensuring the outlet parameters of the purified gas are met. The larger the methanol circulation rate, the more favorable it is for absorption, but it reduces the saturation of CO2 in methanol, resulting in a lower desorption amount of CO2 in Tower 2. A large amount of gas is carried into Tower 3, affecting its operation. Conversely, reducing the circulation rate and increasing the saturation of CO2 in methanol can lower the temperature throughout the cold zone system and increase CO2 production. As the temperature decreases, the solubility of CO2 in methanol also increases, which is beneficial for absorption; however, the control of the circulation rate must be based on ensuring the purity of the purified gas. During operation, it is recommended to reduce the amount of methanol used for washing as well as the amount of washing fluid in the lower section of Tower 1, when the temperature of the lean methanol approaches the design value. When there is a slight increase in CO2 levels, increasing the washing volume slightly will help maintain production effectively. The regeneration temperature of the lean methanol can be lowered by appropriately reducing the regeneration pressure in the thermal regeneration tower. As the operating pressure is reduced, the boiling point of methanol also decreases. By appropriately lowering the operating pressure while maintaining the regeneration degree, heat input to the cold section of the system can be reduced, which helps to cool the methanol-poor stream.