Control and operation of the stabilizer tower—what are the main tasks and influencing factors?

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I. The task of the stabilization tower is to further separate C3 and C4 compounds from the deethanized gasoline; liquefied gas is obtained at the top of the tower, while stabilized gasoline is produced at the bottom. To control product quality, it is necessary to ensure that the gasoline vapor pressure remains within acceptable levels ； To keep the C3 and C4 content in stable gasoline at no more than 1%, liquefied gas should be recovered as much as possible ； At the same time, to minimize the C5 content in liquefied gas, it is best to ensure that it contains no C5. In this way, the yield of stable gasoline is not reduced ； This eliminates the need for a C5 removal tower in the downstream gas fractionation unit ； It also ensures that no residue remains in household liquefied gas, contributing to energy savings. II. Main factors affecting the operation of the stabilizer tower (1) Reversion ratio. The reversal ratio is the ratio of the amount of reflux to the amount of product. The stabilized slurry refluxes as liquefied gas, and the product volume consists of liquefied gas plus non-condensable gas. Controlling the reflux volume at an appropriate reflux ratio is a characteristic of the operation of a stabilizer column. The stabilizer column must first ensure that the vapor pressure of gasoline at the bottom of the column is within acceptable limits, with all remaining light components being vaporized at the top of the column. The liquefied gas at the bottom of the tower is a multi-component mixture; small changes in the composition at the top of the tower do not result in a sufficiently sensitive change in temperature. Therefore, the stabilizer tower cannot adjust the reflux rate by controlling the temperature at the top of the tower; instead, it adjusts it according to a specific reflux ratio to ensure optimal distillation performance. Generally, the control reflux ratio for a stabilizer tower is 1.7 to 2.0. A device for deep stabilization is employed, and the reflux ratio is appropriately increased to 2.4–2.7 in order to improve the recovery rate of the C8 and C4 fractions. If the reflux ratio is too low, the distillation efficiency is poor, and a large amount of heavy components (such as CS, CO, etc.) will be carried along with the liquefied gas ； If the reflux ratio is too high, to achieve a satisfactory gasoline vapor pressure, it is necessary to increase the heat load on the reboiler at the bottom of the tower as well as the load on the condenser at the top of the tower; this reduces the efficiency of condensation and may even lead to an increase in the amount of non-condensable gas emitted, resulting in a decrease in the production of liquefied gas. (2) Stability of the tower top pressure: The tower pressure should be controlled such that the liquefied gases (C3, C4) are completely condensed, which means that the operating pressure must be higher than the saturated vapor pressure of these gases at their cooled temperature. Otherwise, it will be difficult to achieve complete condensation at the bubble point temperature of the liquefied gases, and the problem of uncondensed gas emissions cannot be resolved. The performance of the stabilizer tower is greatly influenced by the ethane removal rate in the desorption tower. If the ethane removal rate is low, the ethane content in the deethanized gasoline is high; when it becomes high enough that the liquefied gas at the top of the stabilizer cannot be completely condensed at the operating pressure, non-condensable gas must be discharged into the gas pipeline network. At this time, since the reflux tank operates under a once-equilibrium gasification process, it is inevitable that a significant amount of liquefied gas (C3, C4) is also carried into the gas pipeline network. Therefore, it is very important to control the outlet temperature of the reboiler at the bottom of the desorption tower based on its composition in order to ensure the liquefied gas recovery rate. The issue of non-condensable gas emissions from the stabilizer tower is also related to the condensation efficiency of the overhead condenser. After being cooled, liquefied gas has a high temperature, resulting in a larger amount of non-condensable gas. The temperature after cooling is mainly influenced by factors such as low air temperature and the cooling area of the water used for cooling. By appropriately increasing the operating pressure of the stabilizer tower, the bubble point temperature of the liquefied gas also increases. In this way, at the temperature after liquefied gas has cooled, condensation is facilitated, which helps to reduce non-condensable gases. After increasing the column pressure, the heat load on the stabilizer reboiler must be increased accordingly to ensure that the vapor pressure of the stabilized gasoline remains within acceptable levels. Increasing the heating capacity at the bottom of the tower is often limited by a shortage of heat sources. Generally, the pressure in the stabilizer tower ranges from 0.98 to 1.37 MPa (absolute) (10–14 Kg/cm2). For controlling this pressure, some systems use heat bypass around the top condenser; this method is commonly employed in cases of \"submerged condensers\" where the condenser is installed below the reflux oil tank ； Some adopt the method of directly controlling the top outlet valve. Used in cases such as air coolers at the top of a tower, where they are installed at a higher position than the reflux drum. (3) Feed position: The stabilizer tower is equipped with three feed inlets. Before entering the stabilizer tower, the feed must first exchange heat with the stabilized gasoline and be heated up, so that part of it vaporizes. The preheating temperature of the feed directly affects the distillation operation in the stabilizer; when the feed is preheated to a higher temperature, more vaporization occurs, and the proportion of heavier components in the gas phase increases. At this time, if the feed inlet is opened, it is easy for the heavier components to enter the lighter components at the top of the tower, thereby reducing the efficiency of distillation. Therefore, different feed inlets should be used depending on the feed temperature. The general principle is: select the feed location based on the degree of gasification of the feed gas ； Use the lower feed inlet when the feed temperature is high ； When the feed temperature is low, use the upper feed port ； Open at the bottom in summer, and open at the top in winter. (4) Bottom temperature of the tower: The bottom temperature of the tower should be such that it ensures a stable and acceptable gasoline vapor pressure. If the vapor pressure of gasoline is high, the bottom temperature of the tower should be increased; conversely, it should be decreased. The heating temperature of the bottom reboiler must be properly controlled. If the heat source for the reboiler at the bottom of the tower is insufficient and it is not possible to increase the feed preheating temperature any further, then it is necessary to reduce the operating pressure or the reflux ratio appropriately, thereby slightly diminishing the distillation efficiency of the stabilizing tower in order to ensure that the quality of the product at the bottom of the tower remains satisfactory.