Urgent reminder: Exchange of operational experience regarding stabilizer towers
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Dear colleagues, we would like to provide an update on the recent operation of our catalytic unit’s stabilizer. Regarding the current issues and challenges, we hope you can offer some guidance. Thank you! Recently, due to the high processing load on our units, the absorption and stabilization unit at the back has been overloaded. To resolve this issue, management proposed a solution: to directly feed the 30 T/H of crude gasoline into the output of the absorption and stabilization unit, where it will merge with the stabilized gasoline and then be sent to the vapor removal system at the back end. (It should be emphasized here that the components coming out of the top of our distillation tower are cooled first and then sent to the first-stage crude gasoline tank; after further cooling, they are sent to the second-stage crude gasoline tank. The 30 T/H of crude gasoline that we have redirected comes from the first-stage tank.) The temperature of the component coming out of the fractionation tower and entering the first stage of processing is controlled at around 60 degrees Celsius. Tests show that the vapor pressure of the crude steam in this stage is approximately 67 KP. Before switching to the crude steam route, the feed to the stabilizer ranged from 290–340 T/H, with a top temperature of 57–59 degrees Celsius and a bottom temperature of 156–160 degrees Celsius. The tower pressure was maintained between 0.92–0.98 MP. At this time, tests indicated that the vapor pressure of the steam coming out of the stabilizer was between 70–83 KP. After the switch, abnormalities occurred in the stabilizer: although the feed rate dropped to around 310 T/H, the top temperature rose to 59–63 degrees Celsius and wouldn’t go down. The reflux rate increased from around 170 T/H before the switch to around 220 T/H, yet the top temperature still didn’t decrease. This led to significant amounts of C5 being carried along with the liquefied gas. Another issue was that the vapor pressure of the gasoline exiting the stabilizer remained high. Since the vapor pressure of the gasoline was too high, management requested that it be reduced to below 70 KP. The top pressure remained roughly the same as before. To lower the vapor pressure, the bottom temperature was increased to around 164 degrees Celsius. However, this resulted in even more C5 being carried along with the liquefied gas, and the vapor pressure continued to be high. Even by increasing the reflux rate, the top temperature didn’t drop. I hope colleagues can offer some suggestions on how to reduce the vapor pressure while minimizing the amount of C5 in the liquefied gas, keeping the reflux rate at around 170 T/H, as before. (Before switching to the crude steam route, we added 125 T/H of absorbent; after the switch, it was around 100 T/H.) In my opinion, after the switch, the amount of gasoline flowing into the absorption and stabilization system decreased by 30 T/H. Combined with the reduced amount of absorbent used after the switch, the total decrease in gasoline flow in this system was 55 T/H. Relatively speaking, the proportion of liquefied gas in the feed to the stabilizer became much higher after the switch, as the amount of liquefied gas remained almost unchanged while the amount of gasoline decreased significantly. As a result, some of the light components like C5 in the gasoline were carried away by the larger amount of liquefied gas (compared to before the switch), causing the top temperature to rise. The change in the liquid-to-vapor ratio affected the mass and heat transfer processes in the stabilizer’s trays, potentially leading to some mixing. Due to the high top temperature, there was a lot of C5 in the liquefied gas, and the vapor pressure also didn’t decrease. I think it would be necessary to restore the liquid-to-vapor ratio in the feed to the stabilizer to its previous level, which might help stabilize the operation of the stabilizer. What are everyone’s thoughts? I am humble and ignorant; I earnestly ask for your guidance! This post was last edited by zhenghemqx on 2009-3-7 21:40]
I believe the following adjustments should be made: first, the bottom temperature of the stabilizer should be adjusted; it should not be increased just because the steam pressure does not meet the requirements. Lower it to around 150 degrees; secondly, the reflux should be increased. The reason is that when the heavier components in the crude gasoline are removed from the unit, the feed composition to the stabilization system becomes lighter. To maintain the original bottom temperature, the gas-phase load increases, resulting in flooding of the stabilization tower; the separation efficiency deteriorates significantly, and the product quality fails to meet standards. By reducing the bottom temperature, the gas-phase load is decreased; increasing the reflux ratio improves the separation efficiency, and thus the product quality meets the standards. Personal opinions are for reference only.
We have increased the reflux rate, but the temperature at the bottom of the tower has not decreased; on the contrary, it has increased. As mentioned earlier, if the temperature at the bottom of the tower is reduced, wouldn’t it be easier for the light component C4 to end up in the gasoline? Won’t this result in a high vapor pressure of gasoline, thereby causing the product to fail quality standards?
The poster can make the following adjustments: 1. Increase the pressure at the top of the stabilizer to 10–10.5 kilograms. 2. Gradually reduce the reflux flow at the top of the tower. 3. While reducing the reflux flow, maintain the temperature at the bottom of the tower at its current level; further adjustments can then be made based on analytical results. 4. Ensure optimal desorption by taking into account the carbon dioxide content in the liquefied gas. When both the vapor pressure of carbon pentane in the liquefied gas and that of the stabilized gasoline are above normal levels, it is usually an indication that the tower is overloaded (other than due to equipment issues).
There is little room for adjustment in the stabilizer tower; either the vapor pressure of gasoline is too high or there is C5 present at the top of the tower. 1. One approach is to increase the amount of absorbent used, thereby enhancing the absorption capacity and reducing the amount of light components that reach the stabilizer tower. 2. Increase the temperature at the bottom of the distillation tower. 3. If the stabilizer tower is selectable, change the feed position of the stabilizer tower. 4. If the dry gas meets the requirements and there is room for adjustment, reduce the pressure in the reabsorption tower.
The facts show that the vapor-liquid ratio in the stabilizer is indeed too low. Moreover, the gasoline with a capacity of 30 T/H consists of components that are part of the first stage of purification; these are relatively heavy components within the gasoline, which results in a high vapor load in the stabilizer and leads to incomplete separation. These are the reasons for the above issues. After improvements were made to the gasoline with a capacity of 30 T/H, absorption and stabilization processes proceeded normally.
When the gasoline becomes lighter, the bottom temperature of the desorption tower should be reduced appropriately to prevent excessive desorption, which could further increase the load on the desorption tower and lead to a poorer desorption effect, thereby affecting the operation of the stabilizer tower.