VPSA OXYGEN: Beautiful Oxygen Supply for EAF Steel Making

time 2018-09-06

Beijing Peking University Pioneer Technology Company Limited, Zhang Yi

Background

In July 2016, a piece of news was broadcast by CCTV that Jiangsu Huada Steel Company manufactures substandard steel, and after that, government focused to renovate this industry. Up to end of June 2017, more than 700 substandard steel company have been closed, and total capacity 0.14 billion were reduced therefore.

With going of Supply Reform and De-capacity Policy, especially under the background of banning of substandard steel company, the steel industry starts earning and the price of steel scrap touches the bottom and starts going up. In the long run, it is an important measure for environment conservation that expanding short process and reducing long process. It is one aim of sustainable development for steel industry that develop EAF steel making technology.

On basis of that, the EAF comes into re-production and new projects are increasing in China. At the same time, more and more new technology and process are applied in practice, which becomes more dependent on oxygen. In EAF steel making process, oxygen is used in many areas: oxygen oil adding, oxygen combustion nozzle, secondary combustion, liquid iron blending, etc. Oxygen plays roles of great significance, such as improving capacity, shortening period, maintaining refractory temperature, reducing fuel consumption. So economic and stable VPSA oxygen plant is the first option for EAF steel companies.

Oxygen needs status and pain spot

The traditional oxygen making method for EAF is cryogenic, also called low temperature fractionation, which was discovered by professor Linde. In reality, it is a liquefied gas technology with machines, which separates oxygen from air by different boiling points through processes, like gas compressing of throttling expansion or adiabatic expansion, cooling, etc.

There are several characteristics of cryogenic as following: high purity, high power consumption. With capacity increasing, the cost for one cubic meter oxygen is 0.3 to 1.0 CNY. At the same time, EAF steel making has an inconsistent oxygen needs, while cryogenic is consistent. So, in the cryogenic process, over produced oxygen will be discharged to air, and the rate is around 10%, even 20%.

With the booming of EAF steel making in recent two years, there still are many EAF companies that are in lack of oxygen. Those companies must buy liquid oxygen for long period to construct oxygen plant of their own. For the hot needs on oxygen in steel industry, the cost of oxygen is 1200 to 3000 CNY per ton, that is 1.7 to 4.3 CNY per cubic meter oxygen, which is much more expensive than average in steel industry.

For some certain steel companies, under the policy of environment conservation and restrict of electricity supply, their oxygen plant will stop operating unexpectedly, thus the production is in embarrassing situation. It takes at least 2 to 3 days to start and stop operating for cryogenic oxygen plant, so it is not a good option for company under such policy.

Compared with cryogenic, VPSA has the following advantages, low power consumption, short time to start and stop, flexible load control, which best meets the needs of EAF steel making.

Characteristics of VPSA Oxygen Technology

VPSA oxygen technology separates oxygen from nitrogen by zeolite adsorbent’s attraction of nitrogen due to strong reaction between zeolite and nitrogen molecule, during which oxygen passes through the adsorbent. This technology was invented in the 1950s by Praxair, and up to now, under the promotion of Beijing Peking University Pioneer Technology Company, Chinese bas become the biggest market of VPSA.

VPSA oxygen plant mains consists of power system (blower, vacuum pump, PLC system) and adsorption system. The raw material air will be input into a adsorption vessel after be dedusted by a filter. The vessels are installed with adsorbents. After pre-treatment, water, carbon dioxide and slight of other components in air will be attracted, and later nitrogen will be attracted by zeolite adsorbent. In this process, oxygen and argon will flow through the adsorbent and got enriched into the buffer tank. Oxygen purity can be adjusted among 50% to 93%.

When this process goes on to a certain degree, the adsorption reaches saturation point, at which vacuum pump will vacuumize the vessel (reverse process against adsorption). Water, carbon dioxide, nitrogen and other slight components will be desorbed into air, and adsorbent got re-generated. Such process is controlled automatically by a PLC system.

Advantage of VPSA oxygen technology:

1. Low power consumption, and per cubic meter oxygen cost 0.2 CNY.
2. Low maintenance cost on power system (blower and vacuum pump), for they are positive displacement equipment and no oil. It is easy to maintain.
3. High automatic control. Power system runs synchronously with adsorption system, and one button will start the plant. Two workers per shift, and no worker on duty.
4. Flexible turn down operation. The turn down ratio could be adjusted among 50% to 100% by time sequence.
5. Quick start and stop. The plant could produce oxygen 20 minutes after start.
6. Short construction periods. Vacuum pump and blower are conventional equipment, thus it usually costs 6 months from construction to producing oxygen.

Application of VPSA Oxygen Technology

Chinese EAF plants have started to replace cryogenic plants or liquid oxygen storage tanks by VPSA oxygen plants. Zhejiang Wantai Special Steel Company, Zunyi Changling Special Company, Luzhou Yixin Steel Company, Chizhou Guichi Guihang Metal Company replaced original oxygen plants by VPSA oxygen plants in recent years.

VPSA oxygen technology can produce 93% purity oxygen, so some EAF companies worry this purity is low and not suitable for special steel making. Actually, the beginning processes of ordinary steel making and special steel making are the same, during which some elements are added into the furnace to change the composition of special steel, and then oxygen is input into the furnace for de-carbon. Finally steel is produced, and there is no need of oxygen in fine-refining. So, the essences of special steel making and ordinary steel making are the same.

There are 3% nitrogen and 4% argon in the product oxygen gas, so some EAF companies worry that nitrogen will influence the specification of special steel. Actually, there is no certain requirement on nitrogen content in alloy steel, such as spring steel, bearing steel and seamless steel tube, especially no requirement in ordinary steel. Too much nitrogen can cause nitrogen content excessive to low-temperature impact steel (China has a specific requirement of 70ppm for nitrogen content in low-temperature impact steel, such as steel used in oil Wells for ships). However, so many EAF steel companies are not manufacturing such low-temperature impact steel, thus VPSA oxygen technology are of great potentials.

After sufficient data analysis, there are also rich experiences on EAF oxygen application. American company Praxair proved that VPSA oxygen technology won’t add the content of nitrogen in liquid steel in EAF process, after long time survey. 60% to 70% EAF plants use VPSA oxygen technology for steel making, and up to now, there are nearly 100 EAF plants in world using VPSA oxygen technology to steel making. It proves to be that VPSA oxygen technology can meet the requirements of EAF steel making.

Company Introduction
Beijing Peking University Pioneer Technology Co., Ltd. (here in after referred to as “PIONEER”) affiliated to Peking University, is mainly engaged in design and manufacture of PSA gas separation technology and equipment, also related adsorbent and catalyst. Pioneer can provide advanced and mature gas separation solutions for chemical industry, steel, non-ferrous metallurgical, glass fiber, paper making, electronic, refuse incineration and water treatment. Up to date, over 200 sets of medium/large-scale equipment have been built, with performance reaching the international advanced standard.


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