Air separation unit – How we suggest Sudan customer to choose?
Customers often can’t understand what they want for air separation unit, so we should strengthen our communication and understanding with them in order to give right suggestion.
Background
We receive an inquiry from a steel mill in Sudan. They ask for cryogenic air separation unit to produce liquid oxygen for their own use. Their oxygen gas consumption is about 1700Nm3/hr. Thus, we give them our suggestion after knowing their basic information.
As we know, in developing countries, people use the scrap metal or steel scrape for steel making. People use electric furnace. Oxygen gas is injected in the electric furnace as well as be used to cut billet.
The pressure of oxygen gas for steel making is 12-16 bar, and it depends on the electric furnace.
Analysis
Generally speaking, they should not build an air separation unit in order to get liquid oxygen by storing them in the cryogenic liquid storage tanks first, then vaporizing liquid oxygen to gas, and then provide oxygen gas to the electric furnace.
Why should not? Reasons are 1) the price for equipment is quite high, 2) the electricity cost for operating will be 80%-120% higher. It’s very uneconomical. Of course, to those who don’t own an air separation unit, they have to provide oxygen gas for their electric furnaces by vaporizing liquid oxygen, because they don’t have an air separation plant to produce oxygen gas for their use. So, if people own an air separation unit but do the way like those who don’t have, it will be not reasonable. That’s why some people are not sure about their final need. As the manufacturer, we hope to know every client’s need clearly so that we suggest and provide them most suitable proposal.
As for air separation units, energy consumption mainly consists of two parts. One is for oxygen separation or say for producing oxygen gas. The other is for oxygen liquefying. For oxygen separation, the energy consumption to this 1700Nm3/Hr plant is about 0.7~0.75kwh/Nm3O2 (including oxygen compressor power) . And for oxygen liquefying, it’s about 0.9kwh/Nm3O2. If we produce the liquid oxygen and then vaporizing, the energy consumption is equals to 0.7+0.9, but actually we waste the 0.9 part finally and only get 0.7 part useful.
Suggestion
We prepare a technical proposal for client’s reference.
In this technical proposal, the capacity of oxygen gas is 1700 Nm3/h. The pressure get out from fractionating tower is atmospheric pressure (0.3 barg). After oxygen gas through buffer vessel , it will be condensed to 18 bar by oxygen compressor. Then through middle pressure buffer vessel, and then through oxygen pressure reducing device to 12 bar and then sent to in steel furnace finally.
Besides, this plant in this technical proposal also produces some part of liquid oxygen 220Nm3/h, which will be stored in 160M3 sotrage tank. When this plant shuts down for maintenance, we can transfer the liquid oxygen stored in 160M3 tank to 60 M3 high pressure storage tank by liquid oxygen pump, and then by using vaporizing, pressure reducing device, and then sent to steel furnace. When there is extra liquid oxygen, client can sell it to others. (As we know the market in the Sudan has great need for liquid oxygen, which can bring good economic benefit.)
If the customer needs liquid nitrogen, they can switch the operating condition to condition II (only to operate one adjusting valve), then they can get liquid nitrogen 220 Nm3/h and at the same time it won’t affect the production of oxygen gas.
The further optimization measures:
- When 60M3 storage tank is filled full, it equals to 60×800＝48000Nm3 oxygen gas. It can use time 48000/1700=28 hours. Similarly, when 160M3 storage tank is filled full, it can use for time 75 hours. So, when both storage tanks are filled full, it can use for 4.3 days. Therefore, only one set of 60M3 storage tank is enough.
- If the client plans to production expansion of his steel furnace, he can reduce the air expansion quantity of expander so that the oxygen gas capacity will reach as high as 2250Nm3/h. However, there will be no output for liquid oxygen and liquid nitrogen in this condition. ”
If you have any idea, welcome to share with us. Thanks.
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