TECH SPECIAL .....Seven chemical separations to change the world

Seven chemical separations to change the world

The separation of mixtures of chemicals into their components is amongst the most significant processes carried out in the petroleum, refining and petrochemical industries. Distillation – the most widely used of separation technologies – is an energy-intensive operation making use of differences in the boiling points of the components of a mixture in its most simplistic version. As per some estimates, it accounts for anywhere between 10-15% of global energy consumption. It is obvious that improvements to energy efficiency of distillation and/or the use of alternative separation techniques that can do the job just as effectively, if not better, while using less energy will have major implications – both economic and environmental. In a world concerned about global warming and its poorly understood impacts, reducing the carbon footprint of separations has urgency like never before. A paper with the title ‘Seven chemical separations to change the world’ authored by two academicians – David Scholl and Ryan Lively from the Georgia Institute of Technology (Nature, Vol 532, April 28, 2017, pp 435-437) – provides a fascinating account of seven areas where alternate separation technologies could have significant impacts.   Hydrocarbons from crude oil The global refining industry processes around 9-mbpd (million barrels per day) of crude oil, and atmospheric distillation is by far the most widely used process in these plants. Globally, the industry consumes 230-GW of energy annually for distillation – equal to the total energy consumption of the UK in 2014. Crude oil is a complex mixture of hydrocarbons, compounds of sulphur & nitrogen, and even metals. Importantly, not all oils are the same, and vary in their physical and chemical properties. It is therefore for good reason that distillation is the most favoured separation technology for carving out crude oil into fractions that find use as fuel and chemical feedstock. Finding alternative technologies for separating crude oil into fractions is not easy. Membrane-based separations offer the lure of lower energy consumption, but little has so far been done to innovate materials that can separate many families of molecules at the same time and at the high temperatures needed to keep oil flowing. But the rewards of success can be huge!   Uranium from seawater Nuclear power is widely seen as an important component of any strategy to decarbonise energy production, despite legitimate concerns over its safety. While geological reserves of uranium – the main fuel for nuclear reactors – is limited to about 4.5-mt (enough to last about a century at current consumption levels), the oceans are known to have more than 4-bn tonnes, but at a concentration of a few parts per billion. The capture of uranium from seawater has been attempted by using porous polymers containing amidoxime groups, but these also capture other metals like vanadium, cobalt, nickel etc., and these need to be removed. The technologies that can do this separation have been proven at a small scale but need to be scaled up several times over to make an impact. Importantly, they can also be used to capture other strategic materials such as lithium (increasingly used in batteries).   Alkenes from alkanes The separation of alkenes (e.g. ethylene, propylene, butenes etc.), more commonly known as olefins, from alkanes – the saturated forms that are the main constituents of natural gas – assumes importance for the sheer size of their production. Global output of ethylene and propylene – the two simplest alkenes – exceeds 200-mt each and they are the fundamental building blocks on which a very large petrochemical industry is based. Alkenes are separated from alkanes by high-pressure cryogenic distillation at low temperatures (-160°C), and purification of just ethylene and propylene is estimated to account for 0.3% of global energy use – roughly the same as that consumed annually in Singapore. Alternatives being investigated include using porous carbon membranes that can carry out the separation in the gas phase – avoiding energy-intensive liquefaction – and at mild pressures (less than 10-bar). The challenge that still needs to be overcome is that the purity of the alkenes so produced does not exceed 99.9% – the level needed for making polyolefins. An intermediate approach could be a combo-system wherein membranes do the first stage of separation and cryogenic distillation does the ‘polishing’ to take purity of the alkene stream to the desired level. The impacts of even these will be very significant: a reduction in energy consumption by a factor of two or three. A major hurdle, however, is the scaling up of membranes – industrial requirements could be as high as 1-mn square metres of membrane surface area! This will call for whole new technologies to prepare these membranes.   Greenhouse gases from dilute emissions The anthropogenic emissions of carbon dioxide (CO2) and other greenhouse gases (such as methane) are significant contributors to climate change. CO2 is commercially captured from dilute gaseous streams by absorbing it in liquids such monoethanolamine, but the process of recovery of the pure CO2 requires heat and is therefore not economically viable for power plants – one of the major sources of CO2. If every power plant in the US, for example, were to use this technique, their incremental cost would be 30% of the growth in that country’s GDP – clearly impractical. What is needed is a simpler, less energy-intensive way of capturing CO2 and other hydrocarbons. But even if such a technology were to be developed there remains the challenge of gainful utilisation of the captured CO2. Existing applications – conversion to carbonates, urea, polyols etc. or use for enhanced oil recovery – can absorb just a tiny fraction and the only practical option seems to be to sequester CO2 in underground reservoirs, which raises its own set of issues.   Rare earth metals from ores The rare earths – the 15 elements that constitute the lanthanide series – have become commercially very important due their indispensible use in magnets, for renewable energy and as catalysts in petroleum refining. They are not rare – indeed they are more plentiful in the earth’s crust than silver, gold, platinum and mercury – but their isolation from ores is economically and environmentally challenging. The current technologies are a combination of mechanical (e.g. magnetic separation) and chemical (e.g. froth floatation) processes, but these are inefficient, and produce large amounts of waste (some of which is radioactive). Recycling techniques are needed to recover rare earths from products after their useful life, but the low dosage in which they are present poses technical challenges.   Benzene derivatives from each other Benzene and its derivatives – xylenes, toluene and ethylbenzene (in decreasing order of importance) – are the starting raw material for many polymers, fibres, solvents and fuel additives. These aromatics are today separated by distillation, and the combined energy consumption for this task is estimated at about 50-GW – enough to power 40-mn homes. Of particular importance is the separation of xylenes into its isomers – ortho, para and meta xylene. para-Xylene is the key raw material for making polyesters that finds use as fibre, film and bottle. The separation of the mixed xylenes into its constituents by distillation is challenging – the isomers have the same molecular weights and their boiling points are close. Membrane separation or use of sorbents have been touted as alternatives, but they will need to be demonstrated on a much larger scale to find broad industrial acceptance.   Trace contamination from water Supplying pure water for industrial or potable use is a multi-billion dollar business worldwide, but the technological options to desalinate water are limited to two: thermal desalination by distillation and membrane separation. Distillation uses 50 times more energy than dictated by thermodynamics. In contrast, reverse osmosis – that applies pressure across a membrane to salty water to produce pure water – uses just 25% more energy than the thermodynamic limit. But RO technologies are limited in the rate at which they can produce water, which means that a considerable flow of water can only be obtained by a significant scale-up (which adds to capex). Pre-treatment of the water is also needed, lest the impurities present damage the membranes. The key innovations needed are more resilient membranes – ones that do not foul and do not have to be replaced as often as is now the case. Change in training and emphasis The authors contend that tackling the above separation challenges will need changes in the training of chemists and chemical engineers. The current emphasis on distillation in academic training needs to be broadened to include other options such as adsorption, crystallisation and membranes to develop a more broadly skilled workforce. They also urge an early consideration of the scale at which eventual deployment of any new technology is likely to be and a comprehensive evaluation of economics and sustainability early in technology development.

Ravi Raghavan CHWKLY 3OCT17