Seven Chemical Separations to Change the World

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Refineries use huge amounts of thermal energy to process crude oil. Seven chemical separations to change the world Purifying mixtures without using heat would lower global energy use, emissions and pollution — and open up new routes to resources, say David S. Sholl and Ryan P. Lively.

ost industrial chemists spend their Other methods would enable new sources of separation processes that, if improved, days separating the components of materials to be exploited, by extracting metals would reap great global benefits. Our list large quantities of chemical mix from seawater, for example. is not exhaustive; almost all commercial Mtures into pure or purer forms. The processes Unfortunately, alternatives to distillation, chemicals arise from a separation process involved, such as distillation, account for such as separating molecules according to that could be improved. 10–15% of the world’s energy consumption1,2. their chemical properties or size, are under Methods to purify chemicals that are more developed or expensive to scale up. Engi SEVEN SEPARATIONS energy efficient could, if applied to the US neers in industry and academia need to Hydrocarbons from crude oil. The main petroleum, chemical and paper manufactur develop better and cheaper membranes and ingredients for manufacturing fossil fuels, ing sectors alone, save 100 million tonnes of other ways to separate mixtures of chemicals plastics and polymers are hydrocarbons. carbon dioxide emissions and US$4 billion in that do not rely on heat. Each day, refineries around the world pro energy costs annually3 (see ‘Cutting costs’). Here, we highlight seven chemical cess around 90 million barrels of crude

oil — roughly 2 litres for every person on dissolved in the oceans is ten times larger Greenhouse gases from dilute emissions. the planet. Most do so using atmospheric than that in known land-based resources; Anthropogenic emissions of CO2 and other distillation, which consumes about 230 giga the limited size of the latter may become a hydrocarbons, such as methane released watts (GW) globally3, equivalent to the total long-term barrier to energy storage. from refineries and wells, are key contribu energy consumption of the United Kingdom tors to global climate change. It is expensive in 2014 or about half that of Texas. In a Alkenes from alkanes. Manufacturing and technically difficult to capture these typical refinery, 200,000 barrels per day of plastics such as polyethene and poly gases from dilute sources such as power crude oil are heated in 50metretall columns propene requires alkenes — hydrocarbons plants, refinery exhausts and air. to liberate thousands of compounds such as ethene and propene, also known as Liquids such as monoethanolamine react according to their boiling points. Light gases olefins. Global annual production of ethene readily with CO2, but because heat must be emerge at the cool top (at around 20 °C); and propene exceeds 200 million tonnes, applied to remove CO2 from the resulting progressively heavier fluids leave at lower about 30 kilograms for each person on the liquid, the process is not economically viable and hotter points (up to 400 °C). planet. The industrial separation of ethene for power plants. If the approach was applied Finding an alternative to distillation from ethane typically relies on high-pressure to every power station in the United States, is difficult because crude oil contains cryogenic distillation at temperatures as CO2 capture could cost 30% of the coun many complex molecules, low as –160 °C. Purification of propene and try’s growth in gross domestic product each 7 some with high vis “A major ethene alone accounts for 0.3% of global year . Cheaper methods for capturing CO2 cosities, and myriad hurdle is energy use, roughly equivalent to Singapore’s and hydrocarbon emissions with minimal contaminants, includ scaling up annual energy consumption. energy costs need to be developed. ing sulfur compounds membranes.” As with crude oil, finding separation sys A complicating factor is deciding what and metals such as tems that do not require changes from one to do with the purified product. CO2 could mercury and nickel. It is feasible in princi phase to another could reduce by a factor of be used in a crude-oil production method ple to separate hydrocarbons according to ten the energy intensity of the process (energy known as enhanced oil recovery, or in verti their molecular properties, such as chemical used per unit volume or weight of product), cal farming and as chemical and biorefinery affinity or molecular size. Membranebased and offset carbon emissions by a similar feedstocks. But human activities emit so separation methods, or other nonthermal amount5. For example, porous carbon mem much of the gas8 that in practice much of it ones, can be an order of magnitude more branes are being developed that can separate will need to be stored long term in under energy efficient than heatdriven separa gaseous alkenes and alkanes (also called par ground reservoirs, raising other issues. tions that use distillation. But little research affins) at room temperature and at mild pres has been done. sures (less than 10 bar)6. But these cannot yet Rare-earth metals from ores. The 15 lan Researchers need to find materials that produce the more than 99.9% pure alkenes thanide metals, or rare-earth elements, are capable of separating many families of needed for chemicals manufacturing. are used in magnets, in renewable-energy molecules at the same time, and that work at In the short term, ‘hybrid’ separation technologies and as catalysts in petroleum the high temperatures needed to keep heavy techniques might help — membranes can refining. Compact fluorescent lamps use oils flowing without becoming blocked by be used for bulk separation and cryogenic europium and terbium, for example, and cat contaminants. distillation for ‘polishing’ the product. Such alytic convertors rely on cerium. Producing approaches would reduce the energy inten rare earths economically is a problem of Uranium from seawater. Nuclear power sity of alkene production by a factor of 2 or separation, not availability. Despite their will be crucial for future lowcarbon energy 3, until membranes become good enough to name, most of the elements are much more generation. Although the trajectory of the replace distillation entirely. A major hurdle plentiful in Earth’s crust than gold, silver, nuclear industry is uncertain, at current con is scaling up the membranes — industry platinum and mercury. Unfortunately, rare sumption rates, known geological reserves of might require surface areas of up to 1 million earths are found in trace quantities in ores uranium (4.5 million tonnes) may last a cen square metres. Deployment on this scale will and are often mixed together because they tury4. More than 4 billion tonnes of uranium require new manufacturing methods as well are chemically similar. exist in seawater at partperbillion levels. as advances in materials’ properties. Separation of rare earths from ores Scientists have sought ways to separate uranium from seawater4 for decades. There are materials capable of capturing uranium, ORNL such as porous polymers containing amid oxime groups. But these molecular ‘cages’ also capture other metals, including vana dium, cobalt and nickel. Chemists need to develop processes to remove these metals while purifying and concentrating uranium from seawater. In 1999–2001, Japanese teams captured around 350 grams of uranium using an adsorbent fabric4. Starting up a new nuclear power plant requires hundreds of tonnes of ura nium fuel, so the scale of these processes would need to be vastly increased. In par ticular, efforts to reduce costs for adsorbent materials are needed. Similar technologies could capture other valuable metals4, such as lithium, which is used in batteries. The quantity of lithium High-capacity (HiCap) polymers can separate metals such as uranium from solution.

requires mechanical approaches (such as chemical blends, and ignores the role of magnetic and electrostatic separation) CUTTING COSTS trace contaminants. Academics and lead and chemical processing (such as froth Chemical separations account for about half ers in industrial research and development flotation). These are inefficient: they must of US industrial energy use and 10–15% of should establish proxy mixtures for common the nation’s total energy consumption. contend with the complex compositions of Developing alternatives that don’t use heat separations that include the main chemical mined ores, use large volumes of chemicals, could make 80% of these separations components and common contaminants. and produce lots of waste and radioactive by- 10 times more energy e cient. Second, the economics and sustainabil products. Improvements are sorely needed. ity of any separation technology need to be

SOURCE: DATA FROM REF. 1/US EIA FROM REF. 1/US SOURCE: DATA Commercial Transportation The recycling of rare earths from dis 19% 28% evaluated in the context of a whole chemical carded products is increasing. Bespoke process. Performance metrics such as cost processes could be designed because the per kilogram of product and energy use per chemical and physical compositions of the kilogram should be used. The lifetime and products are well defined. A variety of metal TOTAL replacement costs of components such as lurgical and gas-phase extraction methods US ENERGY membrane modules or sorbent materials have been explored, but recycled rare earths CONSUMPTION need to be factored in. are not yet part of most supply chains9,10. 98 Third, serious consideration must be Research is needed to reduce the ecological QUADS* given early in technology development to impact of key items containing rare earths the scale at which deployment is required. over their whole life cycle. Physical infrastructure such as academic and industrially operated test beds will be

Benzene derivatives from each other. The Residential Industrial needed to take new technologies from the supply chains of many polymers, plastics, 21% 32% lab to pilot scales so that any perceived risk fibres, solvents and fuel additives depend on can be reduced. Managing this will require 45–55% benzene, a cyclic hydrocarbon, as well as on Energy consumed by academia, government agencies and indus its derivatives such as toluene, ethylbenzene separation processes try partners to collaborate. and the xylene isomers. These molecules are Fourth, current training of chemical

separated in distillation columns, with com 49% engineers and chemists in separations often bined global energy costs of about 50 GW, 20% places heavy emphasis on distillation. Expo enough to power roughly 40 million homes. 20% 11% sure to other operations — such as adsorp The isomers of xylene are molecules tion, crystallization and membranes — is with slight structural differences from each Membrane-based Thermal crucial to develop a work force that is able to other that lead to different chemical proper separation would use separations implement the full spectrum of separations Distillation ■ ties. One isomer, para-xylene (or p-xylene), Drying technologies that the future will require. is most desirable for producing polymers 90% Evaporation less energy such as polyethylene terephthalate (PET) Non-thermal David S. Sholl and Ryan P. Lively are than distillation and polyester; more than 8 kilograms of separations professors in the School of Chemical &

p-xylene is produced per capita each year *A quad is a unit of energy equal to 1015 British Thermal Units Biomolecular Engineering, Georgia Institute in the United States. The similar size and (1 BTU is about 0.0003 kilowatt-hours). of Technology, Atlanta, Georgia, USA. boiling points of the various xylene isomers e-mail: [email protected] make them difficult to separate by conven 25% more energy than the thermodynamic 5 1. Oak Ridge National Laboratory. Materials for tional methods such as distillation. limit . But reverse-osmosis membranes pro Separation Technologies: Energy and Emission Advances in membranes or sorbents cess water at limited rates, requiring large, Reduction Opportunities (2005). could reduce the energy intensity of these costly plants to produce a sufficient flow. 2. Humphrey, J. & Keller, G. E. Separation Process Technology (McGraw-Hill, 1997). processes. As for other industrial-scale Reverse osmosis of seawater is already done 3. US Dept. Energy Advanced Manufacturing Office. chemical processes, implementing alterna on commercial scales in the Middle East Bandwidth Study on Energy Use and Potential tive technologies for separating benzene and Australia. But the practical difficulties Energy Saving Opportunities in U.S. Petroleum Refining (US Dept. Energy, 2015). derivatives will require that their viability be of handling more-polluted water — includ 4. Kim, J. et al. Sep. Sci. Technol. 48, 367–387 (2013). proved on successively larger scales before ing corrosion, biofilm formation, scaling and 5. Koros W. J. & Lively, R. P. AIChE J. 58, 2624–2633 commercial implementation. Construct particulate deposition — mean that expen (2012). ing a chemical plant can cost US$1 billion sive pretreatment systems are also needed. 6. Xu, L. et al. J. Membr. Sci. 423–424, 314–323 (2012). or more, so investors want to be sure that Developing membranes that are more 7. Interagency Working Group on Social Cost of a technology will function before building productive and resistant to fouling would Carbon (US Govt.). Social Cost of Carbon for new infrastructure. drive down the operating and capital costs Regulatory Impact Analysis (2013). 8. Song, C. Catal. Today 115, 2–32 (2006). of desalination systems to the point that the 9. Jordens, A., Cheng, Y. P. & Waters, K. E. Miner. Eng. Trace contaminants from water. Desali technique is commercially viable for even 41, 97–114 (2013). nation — whether through distillation or highly polluted water sources. 10. Massari, S. & Ruberti, M. Resour. Policy 38, 36–43 membrane filtration — is energy and capital (2013). intensive, making it unfeasible in many dry NEXT STEPS areas. Distillation is not the answer: ther Academic researchers and policymakers CORRECTION modynamics defines the minimum amount should focus on the following issues. The graphic ‘The dirty ten’ in the of energy needed to generate potable water First, researchers and engineers must Comment ‘Three steps to a green from seawater, and distillation uses 50 times consider realistic chemical mixtures. Most shipping industry’ (Z. Wan et al. Nature more energy than this fundamental limit. academic studies focus on single chemicals 530, 275–277; 2016) gave the wrong Reverse-osmosis filtration, a process that and infer the behaviour of mixtures using unit for PM2.5 concentrations. It should applies pressure across a membrane to salty this information. This approach risks have been µg per m3, not mg per m3. water to produce pure water, requires only missing phenomena that occur only in

CORRECTION The Comment ‘Seven chemical separations to change the world’ (D. S. Sholl and R. P. Lively Nature 532, 435–437; 2016) gave the incorrect units for atmospheric distillation. It should have read 230 GW globally.