Increase in the flow rate of gas released from the catalytic desorption tower and related analysis
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My unit is a catalytic device; recently, due to high temperatures, the gas production has been high, which has resulted in a heavy load on the absorption and stabilization system. The cooling capacity is insufficient, causing the pressure in the stabilization tower to be high. The opening degree of the non-condensable gas control valve in the reflux tank is between 30% and 70% (it should be closed under normal conditions), allowing large amounts of non-condensable gas to return to the compressor and then be sent to the absorption and stabilization system, creating a cycle of back-and-forth movement； To reduce the load on the absorption system, the plant switched from using dry gas for pre-heating to steam for this purpose. The effect was significant: the volume of rich gas entering the absorption tower and the volume of lean gas entering the same tower decreased by about 7,000 m3/h, and the flow rate of non-condensable gases also decreased ； But soon, it was observed that the flow rate of the desorbed gas from the desorption tower increased significantly, rising from 24,000–26,000 to 30,000–32,000 m3/h. This caused the desorbed gas to enter the absorption tower, leading to an increase in the pressure in the stabilizer reflux tank and an increase in the flow rate of non-condensable gas, thus initiating another vicious cycle. Suspected excessive absorption; reducing the flow rate of the absorbent had no effect ； The workshop analyzed the desorbed gas and non-condensable gas as follows: Desorbed gas – C2 (13.86%), C3+C4 (79.33%), C5 (4.31%); data before switching ； The desorbed gas contains C2 (0.68%), C3+C4 (80.34%), and C5 (18.97%). After the switch, the non-condensable gases consist of C2 (0%), C3+C4 (98.47%), and C5 (1.53%). Comparing the post-switch data with the previous data, it can be seen that the proportion of C2 has significantly decreased, while that of C3+C4 has increased slightly; the proportion of C5 has shown a notable increase ； What is the reason why the C1C2 components decrease while C5 increases in the desorbed gas?
Excessive resolution: reduce the temperature at the bottom of the resolution column or increase the pressure
The total amount of the C1C2 components in the evolved gas did not decrease; rather, the proportion of C1C2 decreased due to the increase in the C5 components.
The C3/C4 content in the desorbed gas is high, indicating significant over-desorption; the components in the desorbed gas should mainly be C1 and C2, rather than C3 and C4. Is the C3 content in the dry gas from your facility above the specified limit? Excessive separation leads to a large amount of C3 and C4 circulating repeatedly. The reason for the high pressure in the stabilizer column is that, although there is excessive separation, the C3 and C4 molecules, after being separated, are absorbed by the absorbent and return to the separation column, where they turn into deethanized gasoline that enters the stabilizer column. This deethanized gasoline contains a high amount of C3 and C4, and it evaporates in the stabilizer column, resulting in high pressure there. Operationally, reduce the temperature at the bottom of the distillation tower, find an appropriate temperature control, and increase the pressure at the top of the reabsorption tower
Since the stripping dry gas that was previously circulating between the reaction unit and the absorption/desorption unit was discontinued, the total amount of dry gas in the rich gas decreased as well. The partial pressure of dry gas in the absorption unit dropped, and the amount of dry gas entering the desorption unit further decreased. If the desorption gas volume remains unchanged, then the contents of C3 and C4 will naturally be high. When this problem occurs, first reduce the processing volume; then adjust the operation of the absorption stabilization unit to quickly restore stability. Finally, increase the processing volume gradually according to capacity
The separation tower is performing excessive separation; it might be necessary to reduce the heat input slightly. The temperature of the gas used for separation isn’t correct – an appropriate temperature needs to be set, as this is essential for ensuring proper absorption and separation processes. It’s also important to control the amount of absorbent oil used as well as the pressure in the tower, so that the dry gas meets the required standards. From a material balance perspective, C3 and C4 must reach a stable state; the underlying reason for this is surely the high stabilization pressure. There are several ways to control this stabilization pressure, with the goal of maintaining the proper vapor pressure of gasoline while also controlling the heat input at the bottom of the tower. If your non-condensable gas contains C5, it’s likely due to excessive heat input and insufficient reflux. The heat exchange capacity at the top of the stabilization tower can be adjusted through the stabilization pressure control system; in some installations, heat exchangers are used, while in others, air cooling is employed. Lowering the temperature of the liquid hydrocarbons helps to reduce the pressure
To absorb and stabilize excessive loads and address insufficient heat exchange, approaches at the reaction level can be considered, such as adjusting the properties of the raw materials, adding slag, and regulating the reaction temperature, in order to adjust the product distribution and reduce gas generation, thereby achieving the goal of lowering the load on the stabilization unit