How can I invest most cost-effectively in nitrogen usage?
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Our plant currently has two sets of air separation units with a capacity of 4,500 M3 each, as well as one liquid nitrogen tank with a capacity of 450 M3. The normal daily demand for nitrogen is around 3,000–4,000 M3/h. Maintenance is carried out every four years; during the periods of shutdown and restart, the nitrogen consumption exceeds 9,000 M3/h. However, during the last few days before the shutdown, the nitrogen consumption is only around 1,000 M3/h. How can we optimize the existing setup? First, what would be the investment cost for another liquid nitrogen tank with a capacity of about 450 m3? Second, if another liquid nitrogen tank is invested, with two liquid nitrogen tanks available, is it possible to fill both tanks each time, then stop the air separation unit, and wait until the liquid nitrogen level is depleted before restarting the air separation unit? How are the costs of starting up, shutting down, and operating calculated in this way? 2 liquid nitrogen tanks of 450 m3 each are expected to operate for 6 days. Third, during the last 4 days of the shutdown, with 24,000 m3 of nitrogen required per day, is it more cost-effective to use the own air separation unit or to purchase nitrogen externally?
Regarding the situation you described, a storage tank with a capacity of 450 cubic meters should be sufficient. First of all, it’s definitely not possible to fill the tank completely with air, stop the oxygen separation process when it’s empty, and then start it again – not to mention that this approach will damage the equipment. Additionally, you can try to shorten the maintenance time by starting the oxygen separation process earlier; if necessary, add liquid nitrogen to accelerate startup. If tower heating is required, that’s another matter – in such cases, it’s possible to purchase an appropriate amount of liquid nitrogen externally. However, if you have an excess supply of air, it’s advisable to add another storage tank, as this could increase sales of liquid nitrogen and generate additional revenue
1. An investment of 150–200 thousand yuan should be sufficient for a 450 m3 liquid nitrogen tank. 2. I don’t think stopping the air separation unit repeatedly is a good solution; for mechanical equipment, both stopping and starting cause wear, and it’s permanent wear. Stopping the air separation equipment for cold standby is also not good ； You can calculate the startup and operating costs by yourself; for such a small air separation unit, electricity consumption is the main cost factor. 3. During the last four days of shutdown, and provided that the factory’s demand and safety are maintained (with 2 450m3 liquid nitrogen tanks expected to last for 6 days), it might be worth considering using a backup system for vaporization. Of course, the stability of the backup system must be ensured. These are just my humble opinions; I welcome any criticism and suggestions