Optimal control of the distillation quality in a three-column methanol plant

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Volume 9, Issue 2, 2002 Chemical Production and Technology –39} Technical Measures and Improvements: Optimal Control of the Quality in the Three-Column Distillation Process for Methanol 1 Introduction to the Process Flow The 100 kt/a methanol plant in our factory utilizes the most advanced low-pressure methanol production method in China, along with an energy-efficient three-column distillation process. The distillation system is a key factor in controlling the quality of pure methanol; the effectiveness of the distillation process directly determines the product quality as well as the level of consumption. Since Bai took charge of operations in June 2000, in response to issues such as large fluctuations in product quality after the distillation unit was put into operation, frequent negative pressures in the atmospheric tower, and unstable pressures in the pre-distillation tower, we promptly organized relevant personnel to work on technical solutions, achieving significant results. We summarize our main technical improvements and operational experiences in production as follows. 2 Analysis of the factors affecting the quality of methanol products Figure 1 Process flow of the three-column distillation system 1. Pre-distillation column ； 2 Pressurized distillation tower ； 3. Atmospheric distillation tower ； 4. Return tank ； 5 Condenser ； 6. Reboiler ； 7 Condensing Reboiler ； 8 Recirculation Pump ； 9 Coolers ； 10. Preheater: In the three-column distillation process (see Figure 1), the main function of the preheating column is to remove light impurities such as methyl formate, dimethyl ether, and propanes. Such substances have low boiling points and are in gaseous state at room temperature; therefore, the level of the non-condensable gas temperature determines the effectiveness of removing light components, which in turn affects the quality of pure methanol – that is, it influences the acidity and alkalinity of the methanol product. Date of compilation: 11-07-2001; Date of revision: 12-14-2001. The KMnO4 value of the methanol product is also affected by this. Crude methanol contains heavy impurities such as higher alcohols, olefins, alkanes, and organic alcohols (also known as fusel oils or isobutyl oil fractions). These substances have high boiling points, and some of them, like ethanol, tend to form azeotropes with water. The boiling points of these azeotropes are close to that of methanol, so as methanol evaporates, the ethanol content in the product increases beyond acceptable levels. The function of the pressurized tower and the atmospheric pressure tower is to remove heavy components such as H2O and ethanol, in order to obtain high-quality methanol product. The coordinated operation of the pressurized tower and the atmospheric tower is key to controlling water content. The atmospheric pressure column is heated by methanol vapor coming from the pressurized column, which is used to heat the liquid at its bottom; in other words, the reboiler of the atmospheric pressure column functions as the condenser of the pressurized column. Therefore, the temperature at the bottom of the pressurized column determines the heat load of the atmospheric pressure column, and it is thus necessary to maintain proper material balance, heat balance, and vapor-liquid balance between the two columns. Otherwise, it is easy to cause negative pressure in the atmospheric pressure tower, resulting in an increase in the moisture content of the methanol product. The issue of negative pressure in atmospheric pressure towers is a key factor affecting system stability and product quality. The reasons for the formation of negative pressure in atmospheric pressure towers need to be analyzed from two aspects: process and equipment. 2.1 Equipment aspects: (1) There are leaks in the methanol vapor pipes at the top of the atmospheric pressure tower, or these pipes lack insulation; this makes it particularly easy for negative pressure to occur in the atmospheric pressure tower during winter ； (2) The installation location of the pressure control valve in the atmospheric pressure tower is inappropriate, preventing it from fulfilling the purpose of pressure regulation ； (3) The piping of the atmospheric tower reboiler is improperly designed, resulting in insufficient heat exchange area and consequently an excessively low heat load for the atmospheric tower ； (4) The vent piping for the non-condensable gases in the pre-tower is improperly designed, allowing the vent gases from methanol synthesis to affect the pressure in the pre-tower and thus causing instability throughout the system. 2.2 In terms of process technology, according to VIP Information http://www.cqvip.com, Zhao Shaomin and others discussed the optimization control of the distillation quality in methanol triple-column systems; Technical Measures and Improvements: (1) The heat load at the bottom of the pressure column is insufficient, which in turn results in an insufficient heat load in the atmospheric pressure column, that is, the heat balance cannot be properly controlled ； (2) The yield from the pressure tower is too high while the reflux rate is too low, resulting in a low methanol content in the components at the bottom of the pressure tower; in other words, the material balance is not well controlled ； 3) The production rate of the atmospheric pressure tower is too high while the reflux rate is too low, which disrupts the vapor-liquid equilibrium in the tower trays and results in a negative pressure. A sign that the vapor-liquid equilibrium in the atmospheric pressure column is disrupted is the rapid rise in temperature at several sensitive points within the column; the temperatures of plates 4, 6, 8, and 12 rise above 100°C (the feed plate of the atmospheric pressure column is located on plate 10). This indicates that the heavier components begin to move upward. If this situation persists for too long without timely intervention, it often results in the purified methanol obtained from the top of the column not meeting quality standards. 3 Treatment Measures and Optimization Procedures 3.1 Measures Taken Regarding the Equipment (1) The piping of the reboiler in the atmospheric pressure tower was improperly arranged, resulting in a 0.5 m liquid seal forming at the reboiler’s outlet. This reduced the heat exchange area of the reboiler. The issue was resolved in November 2000; the liquid seal at the outlet was eliminated, and the heat exchange area increased, causing the temperature at the bottom of the atmospheric pressure tower to rise by 3–5°C. (2) The pressure control valve of the atmospheric tower was moved from the first floor to a 7-meter platform on the second floor; the liquid seal was removed, thereby achieving pressure control with good results. (3) By changing the non-condensable gas pipeline of the pre-tower to a discharge tank, the issue of unstable pressure in the pre-tower was resolved, laying the foundation for the stable operation of the entire distillation system. 3.2 Measures taken in terms of the process: (1) Increase the methanol concentration at the bottom of the pressurized tower, raise the reflux volume in that tower, and reduce the output from it, so as to provide material for extraction in the atmospheric pressure tower (this is one of the main reasons for the negative pressure in the atmospheric pressure tower). This requires reasonable control of the product distribution between the pressurized column and the atmospheric pressure column, with pure methanol being extracted basically in a 4:6 ratio. (2) Increase the temperature of the reflux liquid in the atmospheric pressure column, from the designed 4°C to 50°C, in order to make use of the sensible heat of the reflux liquid as much as possible to accelerate the evaporation of methanol; for this purpose, the outlet pipe at the top of the atmospheric pressure column should be kept at a constant temperature, thereby temporarily increasing the pressure at the top of the column. (3) Raise the temperature at the bottom of the atmospheric pressure column by increasing the temperature at the bottom of the pressurized column, so as to ensure the heat required for distillation in the atmospheric pressure column and address the issue of insufficient thermal load in that column. (4) To stabilize the pressure of the atmospheric pressure column, operators should adjust the reflux rate in a timely manner based on the temperature at the sensitive point of the fourth tray in the column, ensuring that the temperature remains ≤90°C. c. Reduce the output from the atmospheric pressure column and increase the reflux rate appropriately to maintain the vapor-liquid equilibrium in the column. (5) Based on our operational experience, we have adjusted the non-condensable gas temperature from the designed value