Whether used in the processes or given off as by-products, industrial gases are found in a variety of refinery applications, as Rob Cockerill finds out.
The subject of refining lends itself to a diverse range of industries, from oil refinery or metallurgy refinery, to natural gas processing plants and even coal-to-liquids projects.
As with most industrial gas applications, there exists a plethora of gas usages within the far-reaching refinery umbrella. An abundance of usages therefore opens up the application to a whole host of different and, at times, specialty gases.
Perhaps one of the first thoughts to spring to mind when thinking of gases in refining, is the utilisation for inerting and purging purposes.
Displacing oxygen either completely or to a reduced level requires the employment of inert gases such as nitrogen or carbon dioxide and ensures the safe and efficient process of refinery operations. Refinery plants experience improved product quality and the reduction of plant shutdown through the delivery of high quality inert gases, which clearly fulfil a fundamental role in the industry.
These crucial factors of consistency and optimum end product quality are effectively achieved through the bulk supply or, perhaps more economically, on-site generation of gases – though the selection of gas for each application is just as critical.
Gases for inerting
The process of inerting and purging covers a multitude of gases such as nitrogen, argon, helium and carbon dioxide, the use of each of which is largely application dependent. So which gases suit which operations?
Global supply and demand dilemmas for gases like helium and argon could ultimately affect trends of gas consumption within inerting and the refinery industry, though nitrogen is possibly the most widely used, versatile and economical of the inert gases deployed.
Nitrogen is the most cost-effective choice and accounting for around 78% of the air around us, is readily available in either bulk or on-site supply. It could also be seen as one of the more versatile solutions across the refinery sector, as it is put to good effect in a number of roles.
Preventing oxidation or combustion by atmospheric air and contamination by moisture, nitrogen can be used to provide protective atmospheres and may additionally be utilised for the flushing of unwanted or waste gases.
Similarly, the abundant, colourless gas delivers successful displacement or diluting of such unwanted gases courtesy of its oxygen-reducing properties.
While nitrogen is clearly functional and economical for inerting operations, in higher risk refinery applications argon is often the preferred choice due to its increased value inerting characteristics. Argon is the ideal alternative for use with highly reactive materials such as lithium and magnesium compounds, though murmurs of concern had arisen over the past twelve months regarding supply and demand for this noble gas.
Fellow noble gas helium, of far greater concern for supply and demand, finds use in applications where the other inert gases might potentially freeze – rendering itself valuable for the purging and pressurising of liquid hydrogen tanks and piping systems.
With boiling and melting points at the lowest among the elements, helium is also applicable in another area of refinery, that of creating advanced materials for the manufacture of superconductor wire for example.
Praxair notes through its website that ‘its high arc temperature and high heat transfer combined with helium’s inertness enable metallurgists to extract, smelt and refine such advanced materials as niobium and tantalum, which are used to manufacture superconductor wire, as well as zirconium and titanium’.
Gases in metal refinery
Refining is the process of purification of a substance and though generally thought of in terms of oil or fuel production or that of a natural resource, alternative variations include that of the metallurgy industry.
In the case of metal refining, it is often the way that the end product is chemically identical to the original material but in a much purer form. Predictably, gases have an assortment of roles in this technique.
Industrial gases such as oxygen, hydrogen, nitrogen, argon, chlorine and even sulphur hexafluoride (SF6) are involved in technologies for purification purposes in the non-ferrous metals industry, in applications such as stirring, as carrier gases and for the removal of contaminants.
Purifying or removing unwanted substances can be achieved through either oxygen, hydrogen, or sulphur hexafluoride injection, the benefits of which are readily extolled by Air Products South Africa through the company’s informative website.
Oxygen injection is used to remove sulphur and other contaminants from non-ferrous metals such as copper, lead, nickel or zinc, utilising the faster oxidation properties of oxygen to provide increased productivity. Offering productivity and efficiency gains in a differing manor however, the injection of hydrogen, in comparison, can be used to remove dissolved oxygen or to control the generation of metallic oxides in many copper and nickel baths.
Harnessing numerous benefits in one process, the injection of the more environmentally-friendly sulphur hexafluoride (SF6) in an inert gas (nitrogen or argon), into molten aluminium or copper removes hydrogen and other undesirable elements that cause porosity, inclusions or pinholes in the final product.
A more consistent, higher quality metal is produced by minimising hydrogen and alkali metal content, and promoting flotation of oxides and non-metallic inclusions.
In addition, SF6 degassing eliminates the maintenance associated with other degassing approaches that typically cause emission problems and corrosion of equipment and plant structures – therefore offering something of a complete refining solution.
Just as with other inerting operations previously discussed, nitrogen delivers the goods in the non-ferrous metals refining process too – accomplishing a similar role to that of hydrogen injection. Though hydrogen injection can control the generation of metallic oxides, nitrogen is employed to displace the oxygen above a molten bath and therefore reducing the formation of oxides at the melt surface. As a result, a higher yield and improved quality are obtained.
An increased yield and enhanced product quality are also achieved through the refinery ‘stirring’ process. ‘Gas injection to stir molten metal improves homogeneity of both alloy composition and temperature,’ claims the Air Products South Africa website.
Refining for the future
An avenue of refining that may take on increased significance in future is that of natural gas processing, a process that both uses and produces various forms of industrial gases.
Natural gas such as LNG for example, could prove to be a source of energy security through diversity – as recently explored by gasworld’s Focus Feature covering LNG in the UK.
It is also though, another example of gases in refinery, yet from a different perspective. Natural gas processing plants, or fractionators as they are also known, are used to purify natural gas extracted from underground gas fields with the processed gas very different to that of the raw product.
While primarily composed of methane as the lightest and shortest hydrocarbon molecule, raw natural gas also consists of carbon dioxide, hydrogen sulphide, nitrogen, helium, and hydrocarbons like ethane, propane and isobutene.
All of which feature in varying quantities and are either emitted or recovered in the natural gas processing operation.
Raw natural gas is commonly collected from a well and following an initial processing at collection point, the natural gas condensate is transported to an oil refinery, after which the raw gas is pipelined to a gas processing plant for purification.
At this stage the removal of acid gases takes place and the first evidence of industrial gases is seen, with both hydrogen sulphide and carbon dioxide separated either by amine treating or the use of polymeric membranes to dehydrate and divide.
With water vapour then removed through glycol dehydration or Pressure Swing Adsorption (PSA), the natural gas process moves on to the elimination of nitrogen, so prevalent in the application of refinery.
This is achieved through a number of methods, notably either a cryogenic process involving low temperature distillation, an adsorption process using lean oil or special solvent, or another adsorption process utilising activated carbon or molecular sieves.
From here, the recovery of natural gas liquids and residue gas takes place, with the residue gas as the final, purified sales gas which is pipelined to end-users.
Casting our minds back to the more conventional means of gases in refinery once again, still on the subject of natural gas and the focus of gas-to-liquids (GTL) becomes ever prominent in modern climates – just as coal-to-liquids (CTL) does.
GTL technology, which is in constant development and ever-evolving, offers feasibility, design optimisation and application to smaller gas deposits. Also affording competitive capital costs, operational costs, feedstock costs and high utilisation rates, GTL is increasing in significance and is believed to be an economically viable alternative to oil in the future.
So how does GTL work and where are gases involved?
Natural gas utilised through GTL processes often involves the modification to syngas and the later production of petrochemicals or liquid fuels via the Fischer-Tropsch technique.
A direct conversion from gas is achievable through GTL operations though can be difficult and economically unattractive, while indirect conversion by synthesis gas (syngas) is the preferred method.
Syngas involves the conversion of natural gas to hydrogen and carbon monoxide in a desired 2:1 ratio, which is best achieved by partial oxidation and involves the use of industrial gases. Using air for oxidation creates a further diluted syngas, but the use of oxygen produces a purer syngas and ensures no nitrogen contamination. A slightly increased level of investment however, the oxygen route requires the industrial gas to be provided via an (ASU).
The Fischer-Tropsch process, as widely employed by Sasol, Shell and StatoilHydro among others, then follows and essentially creates a synthetic petroleum substitute through a catalysed chemical reaction in which the syngas mix is converted in liquid hydrocarbons.
The final stage in a successful chain that could see this branch of refinery, and the industrial gases involved, become an integral part of industry in future.
Additional refinery inerting gases
While nitrogen, helium and argon are perhaps the most common gases of choice for inerting applications, the door is not closed to other gases and gaseous chemicals.
Carbon dioxide is used for spot inerting operations and applications where the gas’ acidic properties are particularly suitable, according to BOC.
Other gaseous chemicals such as sulphur hexachloride are deployed for aluminium degassing, for example.
Refining and gases go hand-in-hand, whether employed in the refining process or given-off as a by-product along the way.
Just as a variety of gases are released through natural gas processing, it seems relevant to note that a huge source of CO2 used for merchant activity is via recovery from industrial activity.
The CO2 is recovered as a by-product of other processes such as chemical, synthesis gas or from industrial combustion plants, including from fertiliser production like ammonia, the purification of synthesis gases, and also from ethylene oxide production.
As an example, CO2 is removed from exhaust gas in post combustion scrubbing using an amine scrubbing process and the technology to do so can be retrofitted to a plant or equipment, thereby enhancing its functionality.
While CO2 can be recovered from refinery operations, questions over its quality and purity surface and provoke a further purification process – which could be regarded as a further refining application.
To meet the required standards of food & beverages for example, greater purification is necessary and involves a chain of pre-concentration or pre-cleaning of the raw gas, followed by compression and additional refining to remove impurities such as hydrocarbons, sulphur compounds, oxygenates and water.