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Gaseous oxygen measurement with amperometric sensors is the most direct solution for oxidation and explosion protection, according to Mettler Toledo.

Tank blanketing is the process of filling the headspace in storage vessels and reactors with an inert gas, usually to protect its contents from explosion, degradation or polymerisation due to the presence of oxidation but also to protect equipment from corrosion.

A blanketing system is normally designed to operate under higher-than-atmospheric pressures, therefore preventing outside air from entering the vessel.

As oxygen and moisture in the air can be undesired in numerous processes and applications, blanketing is done in a range of industries, including petrochemical, food and beverage, pharmaceutical and pure water.

Inerting is done for similar reasons but is not limited to storage tanks and reactors only.

Any confined space can be sparged with an inert gas to create the desired atmosphere.

This ranges from packing food under a protective atmosphere to increase shelf life to lowering the oxygen concentration in rooms where welding takes place to reduce the risk of fire.

Also typical is complete or partial inertisation in process equipment and unit operations such as centrifuges, mills, mixers, fluid-bed dryers, silos, pneumatic transport, incinerators and flare feedstock supply.

The most common type of inert gas used is nitrogen for economic reasons and because of its availability.

The simplest way to protect a vessel, for example a storage tank, against either overpressure or negative pressure, is to have an opening at the top of the tank.

In this way, any excess air or gas can freely leave the tank when product is pumped into the tank and, the other way around, air can flow into the tank when product is drained.

Such a system also enables the ‘breathing’ of the tank due to temperature fluctuations that can lead to significant volume changes.

This method is not suitable for all products.

Air entering the tank might contaminate the product and, especially when storing organic solvents and hydrocarbons, an explosive gas/air mixture will form above the product.

Undesired gases and vapours may be emitted into the atmosphere.

As these situations must be avoided, the tank needs to be sealed.

The tank does, however, need to be kept under constant pressure in order to avoid overpressure when it is filled or when temperature rises and to avoid vacuum when product is drained.

Especially large storage tanks do not handle low pressures very well.

The blanketing system is there to guarantee that the tank headspace is kept both under inert atmosphere and at constant pressure.

One way of achieving this is through continuous purging with nitrogen, which is a relatively easy and safe alternative.

Although this method requires a low capital investment, it involves high operating costs as it consumes nitrogen continuously.

Slightly more sophisticated is pressure-based blanketing.

In a traditional setup, such a blanketing system consists of: a blanketing valve or regulator, allowing the inert gas to enter the tank whenever required; a breather valve or vapour recovery valve to allow headspace gas to escape from the tank; a safety pressure/vacuum relief valve to prevent tank overpressure or vacuum, which could lead to the imploding of the tank, a risk that grows with tank size; and piping and inert-gas supply.

In this operation, the breathing valve opens when the headspace volume decreases and lets headspace gases leave the tank.

In case the product is pumped out of the tank or when temperature decreases, the blanketing regulator opens and fills the tank headspace with nitrogen, avoiding vacuum.

Maintaining a constant gauge pressure makes sure that air, and thus oxygen, does not enter the tank.

Wrongly engineered or poorly maintained blanketing systems may lead to serious incidents.

The theory that all blanketing systems leak one way or another is probably true, according to Mettler Toledo.

Its complexity, valves with moving parts, packings and sealings are prone to failure.

A malfunctioning pressure transmitter may register the wrong headspace pressure, causing high nitrogen consumption.

When a blanketing valve does not open far enough, the nitrogen flow is too low; this may result in a low headspace pressure, which may cause the tank to implode or the air to leak into the tank.

This can have consequences with regards to product quality and, depending on the stored product, can seriously increase the risk of explosion.

An increased focus on both safety and quality has led to a rise in the use of blanketing operations worldwide and consequently in the use of nitrogen.

For various reasons, the demand for nitrogen has globally increased to such an extent that nitrogen no longer has the status of a by-product of the oxygen production but is now considered a main product in itself.

The growing demand has had its impact on price, but as energy costs make up half the production costs of nitrogen, last year’s surge in energy prices only caused an average nitrogen price increase of more than four per cent.

Transport costs also play an important role here.

The way nitrogen ends up at the end user can differ substantially.

Smaller users get their nitrogen supplied compressed in cylinders or in bulk tank transport.

Larger ones may have nitrogen delivered by pipeline, but such an infrastructure is limited to heavier industrialised areas.

Others choose to have a nitrogen generation plant on site.

Therefore, the average price increase does not show the full picture.

All in all, smaller and mid-size users, especially in more remote locations, have reported price increases of 15 per cent or more since last year.

One way to increase safety and reduce the use of nitrogen simultaneously is to control inertisation as a function of oxygen concentration in the headspace.

Depending on the product and the reason for blanketing or inerting, there are tolerances when it comes down to the maximum allowable oxygen concentration.

Certain monomers require zero per cent oxygen to prevent polymerisation.

Others require a small amount of oxygen for the same reason.

In the case of explosion protection, the oxygen concentration does not necessarily need to be zero.

For all solvents, a so-called limiting oxygen concentration (LOC) exists.

Below this concentration, there is no risk of explosion.

Controlling the nitrogen purge based on the concentration will cut nitrogen costs.

The measurement of oxygen concentration in the tank headspace provides an important safety parameter.

The goal is to prevent the LOC, which is specific for each product that is blanketed and is also referred to as the maximum oxygen concentration (MOC).

These specific values can be found in databases such as Chemsafe.

For safety reasons, two threshold values are established below the LOC: the lower intervention level and the upper intervention level.

These two threshold values are the set points that control the purging of nitrogen.

The moment the oxygen concentration reaches the lower intervention limit, the nitrogen flow is intermitted.

The purging of the vessel is resumed when the upper intervention limit is exceeded.

This means that the inerting system keeps the consumption of nitrogen minimal while safe operation is ensured.

Depending on individual process conditions and solvent, the oxygen level is usually maintained somewhere between two per cent and 12 per cent volume.

The system consists of an oxygen analyser system, a PLC, a blanketing regulator, a breathing valve and a safety relief valve.

This method is suitable for most inerting operations and processes that require an exact oxygen level.

For tank blanketing, however, the situation is more complicated.

The main objective is still to allow the tank to breathe, which still depends solely on headspace volume changes.

It is not possible to stop the nitrogen flow just because the oxygen level has reached the set point while product continues to be pumped from the tank.

As discussed, the created vacuum could seriously damage the tank.

This problem can be overcome by replacing some of the nitrogen by air.

Once again, the aim is not zero oxygen.

Instead of purging with pure nitrogen, a mix of nitrogen and air can be used.

Technological developments in onsite nitrogen generation make this even easier.

With membrane filters or molecular sieve-type technologies, it is possible to produce any purity of nitrogen up to 99.9 per cent on demand.

The oxygen analyser enables a reliable and cost-efficient nitrogen supply and a safe blanketing process.

Often, however, the use of oxygen analysers encounters resistance, the reason being, typically, the oxygen analyser system with its complicated installation and the high capital and operation costs.

A conventional system usually consists of an extractive-type oxygen analyser, typically based on paramagnetic or, less frequently, zirconium-oxide technology.

Both types of analyser require extensive sample conditioning, a pump to drag the sample, tubing, valves, filters, coolers, heaters and dryers.

It requires a huge amount of maintenance and potential for failure due to the complexity of the system and the fragility of the analysers.

Apart from all the peripherals that contribute to the investment, the analyser itself is costly.

The reluctance to install such a system to control a blanketing operation is understandable.

The solution can, however, be much simpler and more reliable.

With amperometric oxygen measurement, Mettler Toledo offers a cost-efficient and safe solution in a 12mm probe that eliminates the need for a sample conditioning system.

The amperometric or polarographic oxygen electrode can be mounted directly into the vessel or nitrogen discharge piping and is not sensitive to dust, moisture or solvents.

A retractable sensor mounting assembly enables the retrieval of the sensor without interrupting the process, such as for calibration.

No special calibration gases are needed, as a one-point calibration with air is all it takes to maintain probe accuracy.

In critical applications, the system can be equipped with redundant oxygen probes for additional safety and self-check purposes.

Compared with traditional extractive analyser technology, the Mettler Toledo solution is available at a lower cost.

Users will also benefit from maintenance savings; the only maintenance needed is the occasional refilling of electrolyte.

The inertisation and nitrogen blanketing of vessels and process equipment is gaining ground globally thanks to safety and product quality awareness.

Price increases on the world nitrogen market, however, cause users to look for more economical technologies with lower nitrogen consumption.

One of the most efficient ways of minimising the use of nitrogen and increasing safety at the same time is controlling the inerting or blanketing based on oxygen concentration.

Drawbacks are the high-maintenance demands and the high installation costs.

Advances in sensor technology and the intelligent automation of the measurement point enable reliable oxygen measurement in a simplified way, eliminating the need for extensive sample conditioning.

Nitrogen savings and the lowest possible maintenance needs allow for the fast payback of the oxygen analyser.

Although the principle of polarographic oxygen measurement is more than 50 years old, the technology has not ceased to develop.

Improvements have been made to the cell itself and high-performance membranes now enable fast and accurate measurement in a range of applications.

One of the most recently added features is the integration of the intelligent sensor management platform.

Using intelligent sensor technology improves reliability and reduces maintenance efforts.

Continuous self diagnosis provides real-time status information and predicts maintenance requirements in detail.

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