Oxygen enrichment in the petrochemicals industry

Adding oxygen enrichment to oxidation reactions in the petrochemicals industry offers a reduced carbon footprint, productivity benefits, and cost efficiencies.

Heinz-Dieter Obermeyer
Linde Gas

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Article Summary

Growing climate change pressures are focusing the spotlight on the need to decarbonise many sectors of industry, particularly those that are energy intensive and/or rely on fossil fuels. The petrochemical industry is a case in point. All petrochemical value chains rely on crude oil. In addition, energy-intensive oxidation processes are often required.

Many operators would thus welcome an opportunity to improve their carbon footprint overall, especially if this came with the added bonus of greater capacity, improved hydrocarbon conversion efficiency, and a lower energy bill. In many instances, this can be achieved without heavy investments in new equipment. The answer often lies in something as simple as oxygen enrichment. Adding oxygen can also give operators the flexibility they need to respond to fluctuations in demand and overcome seasonal bottlenecks, resulting, for instance, from diminished air blower performance and greater off-gas abatement needs in warmer months.

Oxygen enrichment
Oxidation processes in refining and petrochemistry typically rely on the oxygen in ambient air. However, this is not particularly efficient as ambient air contains 78.8% nitrogen. This inert nitrogen ‘ballast’ has to be routed through the various process steps, thus increasing the compressing, heating, cooling, and blowing effort.

Replacing or enriching this air with oxygen increases the partial pressure of oxygen and reduces the process gas flow. This has multiple knock-on effects including gains in capacity and energy efficiency. This technology is widely established and successfully deployed for sulphur processing in Claus units, for instance, and for regenerating in fluid catalytic crackers (FCC). However, the benefits of oxygen enrichment are not restricted to refining and base chemicals. Increasingly, it is also being seen as a way for petrochemical companies to safely decarbonise their operations while boosting throughput.

Intensifying petrochemical processes with oxygen
In the petrochemical industry, catalytic air oxidation in the gas/liquid phase is used in the manufacture of intermediates and commodities such as terephthalic acid (PTA), acetaldehyde, phenol/acetone, cyclohexanone, and benzoic acid. They are also used in the production of fine and specialty chemicals and to heat feed streams, condition catalysts, and treat waste streams.

Experience in industrial scale plants has shown that the ambient air used for these processes can be enriched with oxygen — generally up to a maximum oxygen concentration (MOC) of 28 vol% — at minimal expense and without compromising on safety. This has the effect of intensifying the oxidation process and reducing unnecessary ballast in the process air.

Typical petrochemical reactions
In the following, we will take a closer look at how oxygen enrichment can benefit two gas/liquid oxidation reactions, namely toluene to benzoic acid and p-xylene to PTA. Benzoic acid is a commercial as well as an intermediate product used in the food and pharmaceutical industries. The oxidation of toluene with air in the liquid phase is the most important step in the benzoic acid manufacturing process. It typically takes place at temperatures between 120°C and 180°C, at pressures of up to 10 bar and with a catalyst such as cobalt naphthenate.

Field trials have shown that by enriching the oxygen concentration in the process air from 21 vol% to 24 vol% or 27 vol%, the reaction speed and rate of conversion can be significantly accelerated at temperatures between 140°C and 160°C and a pressure of 9 bar. Selectivity can be increased and the oxygen content in the off-gas quickly drops to a very low level (see Figures 1 and 2).

PTA is a PET (polyethylene terephthalate) intermediate in huge demand worldwide. Here, oxygen enrichment is widely used to increase the oxygen concentration in the process air to between 23 vol% and 28 vol%. Experience has shown that this can boost capacity by anything from 12% to 30% while also reducing flue gas volumes considerably (see Figure 3).

Similar benefits have been observed with oxidations in fluidised beds, typically for reactions such as converting propylene and ammonia to acrylonitrile, ethylene dichloride (EDC) to vinyl chloride monomer (VCM)/polyvinyl chloride (PVC), and butane to maleic anhydride (MA).

Advantages of oxygen enrichment
Oxygen enrichment in petrochemistry offers a number of advantages beyond mere process intensification. These can typically be divided into decarbonisation, capacity, and cost gains.

Smaller carbon footprint
- Reduction in carbon emissions and in off-gas streams thanks to lower process gas flow
- Lower energy bill for heating and cooling steps, resulting in a better carbon balance for emissions trading
- Option of using off-gas — often with a higher energy content — as a fuel for waste abatement

Productivity benefits
- Fast, flexible way to increase plant throughput, also enabling seasonal debottlenecking during hot weather periods where air density is lower and air blowers/compressors may be struggling to provide sufficient process air
- Higher plant availability due to reduced strain on process equipment
- Increased conversion and selectivity, so more product can be extracted from the feed, thus also reducing waste and low value products

Cost efficiencies
- Cost savings for feedstock thanks to higher hydrocarbon yield and lower losses
- Energy savings for heating as the accelerated reaction speed means a lower temperature is required
- Reduced waste stream abatement effort with less fuel needed for off-gas incineration
- Flexible way to circumvent the need for capex-heavy investments in new equipment such as air blowers to increase capacity
- Low capex, easy retrofit option for existing plants
- Reduction in capex for new plants due to smaller equipment footprint

Safety matters
Despite the compelling advantages of oxygen enrichment, many petrochemical players are reticent to choose this avenue due to perceptions and reservations around oxygen safety. Like all oxidation reactions, safety is of course paramount in oxygen-enriched reactions.

Issues to be avoided include uncontrolled and runaway reactions due to local temperature excursions, flammable gas phases of a sizeable volume, gas bubbles (which explode) with oxygen concentrations above the conversion threshold, potentially dangerous operating conditions when oxidation reactors are started up and powered down, and oxygen-enriched air temperatures above those of the reaction when the air enters the reactor.

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