The issue of alkali amount used in distillation
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Our company currently has two sets of methanol production plants using 100,000 tons of coke oven gas per unit, with a methanol output of around 300 tons per unit. During technical analysis, it was found that there is a significant difference in the amount of alkali used for distillation between the two units; the alkali consumption in one unit is almost twice that of the other. Based on pH analysis of the distillation systems, the differences between the two units are not very large. Could some expert help analyze what could be the reason for this? There are differences in the gas volume and the synthesis pressure between the two sets; higher pressure may help to suppress side reactions. To date, little fusel alcohol has been recovered from the operation of these two sets, and no defects in the product have been detected so far. . .
Higher pressure, higher operating temperature, and more side reactions
Could you please provide the parameters of the synthesis systems for these two sets of equipment (temperature, pressure, space velocity), so that it can be determined which system requires more alkali?
The second set requires a higher amount of alkali, roughly twice as much; the gas flow rate for one set is 23,000–24,000, with a pressure of 42 MPA and a synthesis temperature of 235 at the outlet. The gas flow rate for the second set is 29,000, the pressure is 52, the temperature at the inlet is 236, and it is 231 at the outlet. . . I tested the crude methanol today, and the acidity of Unit 2 is higher than that of Unit 1. What’s the reason for this?
The pressures you use for methanol synthesis are 4.2 MPa and 5.2 MPa, right? If it’s 42 MPa and 52 MPa, it can be used for ammonia synthesis.
Is the bed temperature of 236 a bit high? The bed temperature can be appropriately reduced to see how it works.
Based on the data, there is no issue with the air velocity of the second unit; it’s just that the hotspot temperature is higher. My guess is as follows: 1. It’s possible that the synthetic catalyst in the second set has aged, resulting in poor selectivity, and therefore there is more acid in the crude methanol. To see this, look at the gas composition in the circulation loop. 2. Check the quality of the intake air to see if there are any substances that can produce acid.
The purpose of the alkali used in distillation is to protect the equipment; it has little to do with the product itself. Looking at the two production lines in your company, the pressure on the first line is a bit low; if it’s only 42 kilograms, then either the amount of material fed in is insufficient or there is a significant amount of leakage. The second line, on the other hand, seems to be operating normally.
There is no issue with the catalysts, as the catalysts in the two sets of equipment have been in use for less than half a year. In terms of temperature, they have not yet reached their optimal activity temperature. However, after analysis, it seems that the space velocity and production volume might affect the amount of alkali required. Still, the monthly production difference between the two sets is only around 1,000 tons, while the consumption doubles – which is hard to explain. With high pressure, side reactions also increase, but is the effect that significant? That brother’s situation is similar to mine; could you tell me about your consumption levels?
I think it is mainly the difference in pressure that leads to variations in the carbon dioxide content dissolved in crude methanol, which in turn results in differences in the amount of alkali required.
But on the distillation side, the pressure remains the same; no matter how much carbon dioxide is dissolved, it evaporates in the buffer tank and reservoirs!
In fact, the purpose of using alkali in distillation is simply to keep the pH of the methanol at above 7; whether the product is of acceptable quality or not is determined by the temperature and pressure used! :Victory: It’s just my humble opinion; as for the specific reasons, it’s better to consider more factors, have the testing team conduct more analyses, and also try various approaches on one’s own!
For different devices, with varying catalysts and operating temperatures and pressures, the by-products should also differ. It is important to maintain the catalyst’s good activity in order to produce plenty of methanol!
What are the differences between the fresh gas and recycled gas in the two sets?
Is your second set of equipment newly installed? The new synthesis system was not thoroughly purged, so there are more by-products in the product!