Mixing under nitrogen or in a protective atmosphere

The demands placed on industrial mixing processes are constantly increasing. Product protection, process reliability, quality preservation and reproducibility play a crucial role, particularly when processing raw materials that are sensitive to oxidation. In the food industry, the manufacture of food supplements (nutra/nutraceuticals) and in the production of speciality chemical and functional products, even contact with atmospheric oxygen can trigger undesirable changes.

Oxidation reactions can impair the sensory, chemical and functional properties of a product. Possible consequences include changes in colour, loss of flavour, fat rancidity, degradation of active ingredients and a reduced shelf life. This is particularly critical in the case of high-quality powder blends, vitamin-containing formulations, plant extracts, unsaturated fatty acids and oxygen-sensitive special formulations.

Oxidation refers to chemical reactions in which electrons are released. When processing sensitive products, reactions involving molecular oxygen are particularly relevant. These can be accelerated by various factors, such as elevated temperatures, light, moisture, trace amounts of metals or mechanically enlarged surface areas.

Due to their large specific surface area, powdered products can be particularly sensitive to contact with oxygen. During mixing, particles are moved, dispersed and repeatedly brought into contact with the surrounding gas phase.

In the case of powders containing fat, for example, lipid oxidation may occur. This initially produces oxidation intermediates such as hydroperoxides, from which further compounds may form that impair odour, taste and product quality. Sensitive vitamins, natural colourings and plant-based ingredients may also lose their stability under unfavourable process conditions.

The most common oxidation-related changes in quality include:

• Lipid oxidation: Rancidity, off-flavours and loss of quality.
• Vitamin degradation: Reduction in the content of sensitive active ingredients.
• Pigment oxidation: Undesirable colour changes.
• Flavour degradation: Loss of characteristic sensory properties.
• Oxidation of plant extracts: Alteration of functional ingredients.
• Oxidation of reactive speciality substances: Reduced stability or processability.

The actual susceptibility to oxidation depends on the formulation, the quality of the raw materials, the temperature, the water content, exposure to oxygen and the duration of storage.

Nitrogen is a commonly used inert gas due to its comparatively low chemical reactivity under standard mixing conditions and its industrial availability. During inerting, the oxygen-containing atmosphere in the mixing chamber is displaced or diluted by nitrogen. This lowers the partial pressure of oxygen and reduces the likelihood of certain oxidative reactions.

The aim is to maintain an atmosphere that is as constant and low in oxygen as possible throughout the entire mixing process. To achieve this, the mixing chamber must be sufficiently airtight and the nitrogen supply must be tailored to the specific process conditions.

In the pressure cycling process, oxygen is reduced by repeatedly pressurising and depressurising the system. This requires a system that is suitable for such pressure operations.

Another option is continuous nitrogen blanketing. In this process, a defined nitrogen atmosphere is maintained during the mixing process to limit the ingress of ambient air.

The method used depends on the container geometry, the leak-tightness, the behaviour of the powder, the nitrogen consumption and the target residual oxygen content. There is no universally applicable oxygen limit. The permissible residual concentration must be determined on a product- and process-specific basis. In the case of combustible dusts, a distinction must also be made between quality-related oxidation protection and safety-related inerting.

Protection against oxygen requires a holistic plant design. An inertised mixing vessel alone is not sufficient if ambient air regularly enters the system during loading, dosing or emptying.

Closed mixing systems must be designed in such a way as to minimise uncontrolled gas ingress. Critical areas include, in particular, shaft seals, lid seals, product connections, maintenance access points and discharge devices.

Key design features include gas-tight connections – or connections that are sufficiently tight for the specific application – suitable sealing materials, defined pressure conditions and controlled gas management. It is important to bear in mind that even minor leaks can affect the quality of the protective gas. Appropriate leak testing and the monitoring of relevant process parameters are therefore essential components of the plant design.

Continuous or batch-based oxygen measurement can help to monitor and verify the inert gas conditions.

The relevant process parameters include:

• Residual oxygen content (O₂): Verification of the quality of the protective gas.
• Nitrogen flow rate: Control of purging and gas supply.
• Container pressure: Monitoring of the specified pressure conditions.
• Product temperature: Limitation of thermal stresses.
• Mixing time and mixing intensity: Reproducibility of homogenisation.
• Leak-tightness: Detection of potential air ingress points.
• Oxygen content in the product: Additional assessment for critical formulations.

Measuring the oxygen content in the gas space is not necessarily the same as measuring the oxygen trapped or dissolved in the product.

Particularly in the food and nutraceutical industries, inert gas processes must be compatible with hygienic plant design concepts. These include easily accessible surfaces in contact with the product, suitable surface finishes, minimised dead spaces and verifiable cleaning procedures.

Depending on the product and plant design, CIP or WIP procedures, as well as dry cleaning in line with the hygiene concept, may be relevant. A well-thought-out hygienic design helps to reduce product deposits, prevent cross-contamination and create reproducible cleaning conditions.

Nitrogen is non-flammable; however, it can displace oxygen from the ambient air and thereby pose a risk of asphyxiation. Therefore, adequate room ventilation, oxygen warning systems where necessary, and a safely designed gas extraction system are required.

In the case of explosive dust atmospheres, the relevant explosion protection requirements must also be taken into account. A residual oxygen content sufficient to prevent oxidation must not automatically be regarded as safe from an explosion protection perspective. The relevant limit values must be determined on the basis of the specific substance and process conditions.

When processing combustible powders and bulk materials, in addition to protection against oxidation, the requirements for explosion protection in accordance with the European ATEX Directives 2014/34/EU and 1999/92/EC must also be taken into account. Explosive dust-air mixtures may form, particularly during feeding, homogenisation and discharge.

Targeted nitrogen inerting can reduce the oxygen content within the mixing system to such an extent that the formation of an explosive atmosphere can be prevented uander defined operating conditions. A prerequisite for this is ensuring that the substance-specific oxygen limit concentration (SGK) is reliably maintained below the threshold, whilst observing appropriate safety distances, as well as continuous or otherwise technically suitable monitoring of the process atmosphere. The necessary protective measures must be defined as part of a plant- and process-specific explosion risk assessment. In particular, potential air ingress, disruptions to the nitrogen supply, ignition sources, and start-up, shut-down and cleaning processes must be taken into account.

A nitrogen atmosphere does not, therefore, generally replace the ATEX-compliant design of the plant equipment used; however, it can be integrated into a holistic safety concept as an effective explosion protection measure. Particularly in the case of closed mixing systems, the combination of controlled inerting, suitable measurement technology and a plant design tailored to safety requirements offers the opportunity to combine product protection with explosion safety.

The development of industrial mixing processes is increasingly characterised by the combination of product protection, automation, energy efficiency, hygienic design and process data monitoring.

For GloMix, a holistic view of the mixing process is particularly important. The technical benefits of an inert gas application do not arise solely from the introduction of nitrogen, but from the coordination of all components throughout the process chain. In this context, the mixing tool, vessel geometry, sealing systems, gas supply, measurement technology and product discharge must be regarded as an integrated system.

Key areas of technological development include demand-based nitrogen dosing, the reduction of gas losses, the integration of smart sensor technology, and the digital monitoring of critical process parameters. The specific implementation depends on the mixing technology used, the product characteristics and the technical capabilities of the plant in question.

Mixing under nitrogen or in a protective atmosphere is an important process engineering method for protecting oxidation-sensitive products during industrial processing.

Particularly in the food, nutraceutical, and specialty products sectors, the controlled reduction of oxygen ingress can make a significant contribution to maintaining product quality, active ingredient stability, and sensory properties.

This requires a holistic approach to process management. In addition to the actual inerting process, the leak-tightness of the mixing system, closed-loop feeding and discharge, oxygen monitoring, and hygienic and safety-oriented design are of crucial importance.

For GloMix, exploring inert gas processes opens up the opportunity to systematically incorporate the requirements of demanding customer applications into the development and design of modern mixing solutions.

The key insight is this: Effective protection against oxidation does not begin with the introduction of nitrogen, but rather with a consistently closed and controlled design of the entire mixing process.

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