There are a few questions; please help analyze them! !
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I am an operator at Shanjiao. Our distillation system is a three-plus-one tower design with an annual production capacity of 200,000 tons of methanol; the main heat source is the converted gas from the purification process. When there isn’t enough heat, an additional 30 kilograms of medium-pressure steam at 300°C is used as a supplement. The equipment configuration consists of a pre-tower (50 trays), a pressurized tower (90 trays), and a atmospheric tower (90 trays), all of which are of the floating valve type. The recovery tower (currently out of use) has 20 trays in its lower section and a structure with two layers of packing in its upper section. In this process, crude alcohol with a synthesis temperature of 40°C, a pressure of 0.4 MPa, and a purity of around 85% is combined with an alkaline solution and fed into the pre-tower (the current bottom temperature of the tower is around 76°C, and the bottom pressure is 0.04 MPa) ； The top of the tower has a temperature of 65°C and a pressure of 0.008 MPa; the gas at the tower top passes through a pre-tower condenser and an expansion gas cooler to separate the light components (with an exhaust gas temperature of 40°C), after which it is sent to the pre-tower reflux tank. From there, it is pumped back to the top of the tower via a reflux pump for full reflux operation. The pre-tower bottom liquid (with a current specific gravity of 0.85–0.87 and a pH value of 7–9) is pumped to 1.1 MPa using the feed pump for the pressure tower, and then sent into that tower (the current bottom temperature in the tower is 130°C, with a pressure of 0.4–0.5 MPa) ； The top temperature is 110°C. Distillation is carried out with the pressure difference across the entire tower maintained above 0.043 MPa; the vapor at the top of the tower serves as the heat source for the atmospheric-pressure tower, while the condensate enters the reflux tank, where it functions as either reflux liquid or product. The liquid at the bottom of the tower is fed into the atmospheric pressure tower for distillation; the gas at the top of the tower is cooled to below 50°C before being sent to the reflux tank. After being pressurized to 0.7 MPa by a reflux pump, it is used either as reflux liquid or as product. The bottom liquid from the atmospheric pressure tower is pumped to 0.6 MPa by the feed pump of the recovery tower and sent to the recovery tower or to the gas generation section for use. The system is currently experiencing some issues that have not been resolved yet: 1. Is it normal for the pressure at the top of the atmospheric pressure tower, in the reflux tank, and on the 44th tray to be negative? ； 2. The bottom temperature of the atmospheric pressure column is currently 102°C, but the top temperature remains between 50–55°C. Is it normal that the reflux ratio of the atmospheric pressure column (currently 8–10) is so high? Currently, the yield from the atmospheric pressure column is very low, generally below 10 tons. Even a slight increase in production leads to a rapid rise in the temperature of the sensitive plate. Full reflux operation occurs frequently, and it’s difficult to lower the temperature. What is the problem? The atmospheric pressure tower has a fusel oil collection port on the 14th floor; in this case, is it necessary to collect fusel oil?
A top temperature of 50–55 degrees in an atmospheric pressure tower is definitely incorrect. Unless it is a negative-pressure tower, or the material is not methanol. First, you need to rule out issues with the instrument. The pre-tower cannot be under negative pressure either; if it were, how could your light components be discharged?
This post was last edited by Qingfeng Liushui on 2009-12-14 at 19:53. Hello, I have one suggestion: 1. Increase the temperature of the return liquid slightly; a temperature above 50 degrees is also fine. In this way, the negative pressure problem will improve, and it may even be resolved if things go well. 2. It is also possible to apply a slight positive nitrogen pressure to the reflux tank or the pre-top tower. 3. The reflux rate in the atmospheric pressure tower is too high; the operating conditions of this tower are poor, and the methanol concentration in the kettle liquid is certainly high. By controlling the reflux ratio at 2.5, the reflux rate can be gradually reduced. 4. The pot temperature is also a bit low; it’s mainly due to excessive backflow – 103-106 is sufficient.
Our 100,000-ton system’s atmospheric pressure tower is currently under negative pressure, but the temperature at the bottom of the tower is as high as 108 degrees. The reflux volume is also considerable, though not as high as yours. I can now achieve positive pressure by introducing some nitrogen, which turns the pressure positive.
In your case, it’s normal for the atmospheric pressure tower to be under negative pressure! Firstly, the boiling point of methanol at normal pressure is 64 degrees, while the temperature at the top of your tower is only around 55 degrees. What is the saturated pressure of methanol at 55 degrees? It is definitely less than 1 atm, so the pressure in the atmospheric tower and the reflux system is negative. Your reflux ratio is way too high. From your description, it seems that you pay close attention to changes in temperature at sensitive points. If your requirements for ethanol are not very high, you can gradually reduce the reflux ratio, open the water supply valve on the reflux cooler, and slowly increase the amount of product obtained, while monitoring the range at which the sensitive points remain unaffected, so as to ensure that the water content in the product does not change significantly. Then gradually raise the top temperature to around 64 degrees. Enabling the extraction of fusel oils can remove some heat, thereby reducing the temperature of the sensitivity point.
The production conditions in your facility aren’t properly adjusted. It’s incorrect to say that the production load of the atmospheric pressure tower is less than 10 tons; it should be around 15 tons. The products are mainly obtained from the atmospheric pressure tower. When adjusting the operating conditions, first ensure that the atmospheric pressure tower operates at a stable level, and only then adjust the pressure in the pressurized tower. Once stability is achieved, the pressures will be appropriate. However, the top temperatures of both towers remain stable at all times, as this affects the quality of the products. 1. The temperature of the atmospheric pressure tower in your plant is incorrect; the temperature at the top of the tower is only in the 50s degrees. This is due to an excessive amount of reflux, which results in negative pressure. It is necessary to reduce the heat supply to the reboiler of the atmospheric pressure tower, as well as to lower the reflux ratio to between 2.2 and 3.4, so that the temperature at the top of the tower remains between 62 and 66 degrees. This process also helps to reduce the load on the pressurized tower. 2. When working at stage 1, the pressure tower will be affected, and both pressure and temperature will change. It’s not a problem; once the atmospheric pressure tower reaches equilibrium, the materials and heat in the pressure tower can be balanced again. The temperature and pressure in the pressure tower will then be as they should be (pressure and temperature are related to each other, that is, to the saturated vapor pressure). But the most important thing is that the reflux ratio must remain within the normal range (2.2–3.0). 3. Fusel oil extraction is carried out in accordance with the standards for the grade of your methanol product; some manufacturers have high requirements regarding methanol quality, and extraction is done only in such cases. The reflux ratio is more or less like this. I have been working with distillation for over 5 years, but I haven’t been involved in it for the past year or so as I’ve been working on project construction; therefore, some of the figures aren’t entirely accurate, and I’ve lost a bit of familiarity with the process.
The heat load on the three towers is too low; there is definitely negative pressure at the top of the towers. This is mainly because the flow rate of your circulating water coolers is too high. It might be necessary to increase the reflux and steam supply to the second tower. The temperature at the bottom of the towers is 102 degrees, and it’s not certain that the methanol level in the reactor will exceed the allowable limits – the main issue is still the low pressure. It is necessary to increase the load on the three towers, as the amount used by those towers at present is too low; that’s why you think the reflux ratio is high. If the backflow to Tower 2 is increased, the load on the three towers will rise a bit. The product quality is good in all cases.
Your normal pressure reflux is too high; reduce it to around 2-2.5, and increase the amount of fusel oils to address the issue.
1. The reflux ratio is too high, resulting in a too low top temperature of the tower； 2. The temperature at the bottom of the atmospheric pressure tower should also be increased slightly, to around 108
:)Thank you all for your analysis. But for some reason, whenever the reflux in the atmospheric pressure column is reduced, the moisture content of the product may exceed the specified limits. More strangely, lately, when full reflux is used in the atmospheric pressure column, the liquid level in the reflux tank rises, and it’s difficult to lower this level again; once a large amount of product is taken out, the temperature in the column increases rapidly