On-site Nitrogen Generation For The Economical & Environmentally Friendly Production Of Food Grade Gas

Food production and preservation challenges

During the current global turmoil created by the Covid-19 pandemic, consumer expectations for high-quality, healthy and easily accessible food have stayed high, leading to ever-increasing demands on manufacturers and packagers.

Suppliers must comply with a number of strict regulatory requirements to deliver food that is safe for consumption. The challenge for many producers is that the food they supply must meet consumers' needs for quality and freshness whilst providing excellent value for money in the face of rising costs and supply chain issues.

Additionally, the public wants to understand more than ever about the food we consume with regards to concerns about source, safety, and environmental impact. The risks associated with the use of certain chemicals and additives in food production compel producers to seek more consumer-acceptable alternative methods of food preservation.

Throughout the manufacturing process, from bulk storage to processing and packaging, gases such as nitrogen and carbon dioxide are commonly used to protect food and beverage products from spoilage mechanisms.

Nitrogen can be regarded as “natural” and very safe for food use, making up over 78% of the earth’s atmosphere at mean sea level. It is a colourless, odourless, mainly inert gas, and as such, has zero effect on food products.

It does not impact visibility, appearance, or taint, and it leaves no harmful residues. We each inhale many thousands of litres of nitrogen every day in the air we breathe.

Nitrogen gas can retard spoilage mechanisms, including certain microbial species and enzymatic discolouration, when it is used to exclude ambient air, or more importantly, the oxygen content, together with correct temperature control and hygienic practices during the storage, processing and packaging of food products.

Nitrogen gas can extend food windows – the “use by” or “best before” labels – when used for blanketing and packaging. It reduces waste, saves money, helps the environment, and increases customer satisfaction.

What is food grade nitrogen?

Within the European Union, gases used for packaging food products, often called modified atmosphere packaging (MAP), are classed as food additives, allocated “E” numbers and come under very strict legislation with regards to specification. Nitrogen gas is designated E-941.

European Commission Regulation 231/2012 of March 9, 2012, lays down specific purity criteria on food additives (other than colours and sweeteners).

An extremely useful resource for guidance on food gas regulations is the European Industrial Gases Association (EIGA), that in conjunction with the Joint Expert Committee on Food Additives (JECFA), provides helpful, focused, and easy to understand information explaining the European legislature.

Within EIGA documents 125/18 and 194/15, details of the purity specification for nitrogen, E-941 are as follows:

Nitrogen* ≥ 99% v

Oxygen ≤ 1% v

Water ≤ 0.05% v (500ppmV)

*99% including other inert gases such as noble gases (argon mainly)

Impurities:

Carbon monoxide ≤10 ppmV

Methane and other hydrocarbons (such as methane) ≤100 ppmV

Nitrogen monoxide and nitrogen dioxide ≤ 10 ppmV

EIGA documents available from www.eiga.eu

Food grade nitrogen supply options

The vast majority of nitrogen supplied to industry is produced by the cryogenic separation of air within an air separation unit (ASU).

The development of the principle of ASU operation is credited to German engineer and entrepreneur, Karl von Linde, who focused originally in the early 1900s on oxygen production. In very simple terms, ambient air is compressed, expanded and cooled to lower its temperature to a point approaching nearly -200 degrees Celsius, whereby the main gas species of ambient air – nitrogen, oxygen and argon – can be extracted by their different liquefaction temperatures.

Liquid nitrogen produced by this method is high purity as pertains to the maximum remaining oxygen content, typically 5ppmV to 10ppmV. Generally, the nitrogen is either transported and delivered to end users as a liquid, stored in insulated bulk vessels, and either evaporated to a gas as required or evaporated to gas at the ASU site and compressed into high pressure cylinders.

An alternative to cryogenic distillation is for end users to make their own food grade nitrogen gas on-site. This process involves the use of a “nitrogen gas generator” that converts ambient compressed air into nitrogen by removing oxygen through molecular separation. There are two main technologies developed to separate compressed air into its two main component parts of nitrogen and oxygen, namely: hollow fibre membrane (HFM) and pressure swing adsorption (PSA).

Hollow fibre membrane uses thousands of small straw-like tubes, manufactured to be porous for specific molecular sizes of fast and medium speed gases, such as water vapour, CO₂ and O₂, whilst not permitting nitrogen to permeate, thus retaining it as a usable gas.

Hollow fibres Membrane module operation

Pressure swing adsorption uses vessels filled with a material called carbon molecular sieve (CMS), designed to adsorb the slightly smaller oxygen molecules under pressure, whilst the pore is too small for the larger nitrogen molecules to enter. After a defined time period, the pressure is released within the vessels and the oxygen is desorbed from the CMS and flushed to atmosphere.

Hence, the process is named pressure swing adsorption. Generally, at least two vessels filled with CMS are utilised, with one being “on-line”, generating nitrogen by adsorbing oxygen, and the other “regenerating” by de-pressurising and desorbing oxygen. This provides a continuous output of nitrogen gas.

In general, PSA technology produces nitrogen gas at food grade purities more economically than hollow fibre membrane, quite simply because it uses less compressed air per cubic metre of gas generated. For most food grade gas applications, PSA technology is the preferred choice.

Environmental impact of nitrogen supply methods

According to data published by EIGA within position paper PP33/2019, the specific energy consumption for an ASU to produce 1 tonne of liquid nitrogen is 549kW.

One tonne of liquid nitrogen evaporates to produce 861 cubic metres of gas at atmospheric temperature and pressure (ATP).

By comparison, to produce the equivalent 861m³ of nitrogen gas by PSA technology at food grade 0.5% maximum remaining oxygen content using a 7 barg standard industrial air compressor, Parker estimates that approximately 240kW of electrical energy is required.

This is less than half of the energy consumed by an ASU. Assuming the same mix of renewable, fossil fuel and nuclear electricity production methods for both the ASU and PSA generated gas, this then equates to less half the CO₂ emissions.

It is also important to consider that the ASU power consumption is only for the cryogenic process to produce liquid nitrogen. It does not include evaporative and filling loses, or truck-based deliveries, whereby some vehicles produce in excess of 1kg of CO2e/km travelled.

Nitrogen gas purity versus what’s in the pack?

So far, we have considered food grade nitrogen legislation and specification, along with the different supply methods.

The criteria that most food manufacturers are really concerned with is: “What is the maximum remaining oxygen content (MROC) in the finished package goods?”

To achieve the desired shelf life, an MROC of 1-4% is generally acceptable for food products, which are suitable for packaging using vertical form fill and seal machinery (VFFS), such as potato crisps, nuts, coffee, grated cheese and the like. The precise number will depend on the specific product. This is normally confirmed by batch testing the pillow packs using hand-held or benchtop bag gas oxygen analysers.

A commonly expressed concern is that, if cryogenically produced nitrogen has only 10 parts per million remaining oxygen content, then surely the packaged product would have far less oxygen in it when compared to using on-site nitrogen generation, at say 0.5% MROC?

In practice, this is not the case because it is economically and technically impossible to scavenge all of the ambient air, and hence oxygen, from a bag as it is formed and filled on a VFFS machine. Some ambient air remains and is also dragged into the pack as the product is dropped from the multi-head weigher. The flushing gas helps to dilute the remaining ambient oxygen but does not eliminate it.

Compared to the use of evaporated liquid nitrogen, gas generated on-site can achieve the same result with regards to MROC in the bag, easily and economically.

A few months ago, Parker was approached by a European coffee producer to investigate changing from a long-standing liquid nitrogen system to on-site generated gas. The aim was to reduce costs and reliance on an outsourced supply.

Paramount to the potential customer was that any change, especially the purity, must under no circumstances alter their signature coffee aroma and taste.

Logistically and operationally, setting up a loan nitrogen generator on a pilot trial basis was not possible. In response, Parker developed a device that could dilute the high purity liquid nitrogen on one packaging line gas feed by introducing food grade compressed air, increasing the oxygen content to the desired trial set points.  

Subsequently packaging on one line was commenced at 10ppm (control), 0.1% and 0.5% MROC in the nitrogen flushing gas. The trial coffee packs were analysed by the quality assurance department for pack MROC confirmation against a target of less than 2% oxygen in the bag.

The test results indicated in the table, demonstrate that the difference between using high purity evaporated liquid nitrogen and on-site generated gas is negligible and provides an MROC level well within the upper limit.

Further, shelf life, taste and aroma test comparisons were performed on the trial batch set aside, and this confirmed that there was absolutely no degradation in product quality using on-site generated gas.