How to determine the capacity of liquid oxygen and liquid nitrogen storage tanks in methanol plants?

Given that air separation units sometimes come to a stop due to failures, many plants have installed backup systems for oxygen and nitrogen, using these emergency systems to keep the downstream processes running. The person in charge should also take into account any changes in the demand for instrument air and plant air. Of course, the larger the emergency oxygen and nitrogen backup systems, the better, as this helps reduce the pressure associated with the time required to restore normal operation of the air separation unit; even if repairs are necessary, it will not result in a shutdown of the downstream systems. Taking various factors into account, including the skill level of air separation operators, the oxygen and nitrogen pressures in the process of producing methanol from 100,000 tons of coke oven gas, and the hourly consumption of oxygen and nitrogen, it is advisable to opt for an 8-hour backup system for oxygen and nitrogen in case of accidents. Generally, liquid nitrogen and liquid oxygen storage tanks are subject to pressure constraints; the higher the pressure, the smaller the volume. It is best to consult the manufacturer.

For this issue, the time required for air separation must be taken into consideration; the storage amounts of liquid oxygen and liquid nitrogen should be sufficient to cover the time needed for emergency repairs related to air separation, in order to ensure the normal operation of the methanol production facility. Generally speaking, air separation units are less likely to experience failures that lead to shutdowns. However, if heating is required for thawing, it takes several days from shutdown and maintenance to resuming normal operation, and it becomes impractical to maintain such large storage levels; in such cases, the methanol plant should also be shut down.

My suggestion is to determine the amount of oxygen and nitrogen required based on the minimum production load of methanol in the subsequent system, and to determine the volume of the storage tanks according to the time needed to shut down or start up that air separation system. Generally, for a cryogenic air separation unit, the cold start takes less than 12 hours, while the initial startup requires around 72 hours; maintenance and similar activities are considered to take no more than 12 hours.

For your factory, a liquid oxygen and liquid nitrogen storage tank of 100 cubic meters should be sufficient. Liquid is filled in at night, and low-temperature products are sold during the day. An additional nitrogen storage tank should also be installed and kept full at all times; it can supply gas in case of emergencies, such as for cooling furnaces, as a protective gas, or when there isn’t enough instrument air, serving as such air. In the event of an emergency shutdown, the flow of low-temperature liquids will stop. Having a too large storage tank is unnecessary. Since low-temperature liquids are sold quickly, I think 100 cubic meters is enough.