July 23, 2017

Vacuum Swing Adsorption: Oxygen generation at a fraction of the cost

Given their advantages, it is not surprising that the niche of VSA plants in the mining sector is growing across Africa.   Oxygen production plants are in operation at Buzwagi and Bulyanhulu gold mines in Tanzania where oxygen is needed to enhance the recovery of gold in carbon-in-leach tanks.   

One cannot fail to notice the growth in the number of Vacuum Swing Adsorption (VSA) plant construction projects for mining clients in Africa.  Increasingly, conscious of the skyrocketing operating costs of their assets, mine operators are adopting VSA.

Though VSA oxygen generation technology is gaining currency, it has been around the gas separation block for quite some time. So, why is it that, all of a sudden, the industry is appreciating its significance more? What has elicited excitement in the industry coming from?

Resource-constrained mines now consider VSA as the most convenient option in industrial gas separation of all comparable technologies. The main feature that distinguishes VSA from cryogenic plant distillation (CPD) techniques of gas separation and pressure swing adsorption (PSA) techniques is that it operates effectively at near-ambient temperatures and pressures.

Typically, under minimal pressure, using special solids called zeolite adsorbents, VSA is able to liberate gases that have certain molecular characteristic and affinity for the absorbents from a gaseous mixture. This ease has made it a preferred option in oxygen generation.

A VSA plant at a gold mine in Tanzania

Why the industry is going VSA

Mainly, VSA has the following advantages over CPD and PSA: initial and lifecycle costs, energy efficiency and turnaround time.

  • Low initial and life cycle costs

Just to be set up, a PSA plant requires quite a cache of components, which include feeder air compressors, process valves and associated dryers and feed air filtering systems. This makes assembling a PSA plant convoluted and costly process. On the other hand, one adsorption vessel would be sufficient for a VSA plants, eliminating many of the design challenges associated with PSA plants.

Secondly, ensuring that PSA plants are functioning requires more resources. For instance, just the feed compressors – without mentioning process valves, dryers and feed air filtering systems – require both oil and water removal equipment hardware. In comparison, maintaining VSA plants, which have a single vessel, is significantly lower.

  • Energy-efficient and environmentally friendly

As they are operated on low pressure and less energy-intensive components, VSA have far lower power demands, which results in lower energy expenses. Energy expenses can be about 50% lower than PSA plants.

  • Shorter construction time

Operating in an environment in which time is of the essence and resources are overstretched, VSA plants should be the recommended option. Requiring fewer components, VSA plants take a shorter time to be assembled when compared with PSA plants. This enables a mine operator to cuts costs and utilise the facility when needed.

Trends

Obviously to address the fast-track demand for VSA plants, compact designs have become popular for their “plug and play” feature. Built and delivered as fully-packaged, the designs are incorporated into an existing industrial plant, eliminating the need for expensive brick-and-mortar construction.  With compact designs, time required for erection and start-up is significantly minimised.

Another fascinating trend is where a manufacturer owns the on-site plant over a specific contract term, operating and maintaining it to supply oxygen ‘over the fence’ as a utility. Training is provided to ensure that a mine’s personnel are acquainted with operating and maintaining plants to achieve high availability, or on-line time, and the most effective oxygen production.

Mining footprint

Given their advantages, it is not surprising that the niche of VSA plants in the mining sector is growing across Africa.   Oxygen production plants are in operation at Buzwagi and Bulyanhulu gold mines in Tanzania where oxygen is needed to enhance the recovery of gold in carbon-in-leach tanks.  The system enables high dissolved-oxygen levels to be maintained in the preconditioning and leach tanks to achieve higher gold recovery rates.

Over and above mining, VSA systems are already in application in steel making, glass, pulp and paper, and water and wastewater treatment.

Widening opportunities

 Considering the current trajectory that industry is on, one can safely predict that VSA is only going to grow bigger.  Incredible discoveries of natural gas reserves have opened up new opportunities regarding the use of natural gas in Africa as a significantly affordable alternative to ‘conventional’ sources. With respect to mines, replacing diesel with liquefied and locally manufactured gas is emerging as an environmentally friendly and energy efficient option.

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