Heating a liquid until it evaporates is one of the most fundamental ways to separate chemical mixtures. Just turn on the heat and wait. On the industrial scale, the problem is not so much the wait, but the heat. The chemical industry requires a lot of it to separate chemical compounds using distillations—as much as 15% of the U.S.’s energy consumption, according to an Oak Ridge National Laboratory report.
Chemical manufacturers are more cognizant than ever of the need to cut energy use and prevent pollution. It’s one part green chemistry and sustainability. But another part is keeping an eye on the company bottom line, because with commodity chemicals, price is important and saving every penny in production counts. Even so, chemical companies find that it’s tough to adopt alternatives to large-scale distillation that don’t rely on heating. That’s because distillation is a dependable, well-understood method that works, and replacing it can’t happen overnight.
In fact, developing nonthermal separation processes to refine crude oil and purify drinking water is an XPrize-type challenge on par with sequencing the human genome or developing the first atomic bomb in the Manhattan Project. What’s needed is a coordinated effort to pool knowledge and resources.
To that end, the American Chemical Society and the American Institute of Chemical Engineers are leading a new alternative separation initiative, called AltSep, to advance low-energy separation technologies. AltSep is coordinated by the Chemical Manufacturers Roundtable, an industry group established by the ACS Green Chemistry Institute (GCI) and led by Robert J. Giraud of Chemours and Amit Sehgal of Solvay.
AltSep aims to address common problems and identify needs in separation science and technology and then translate promising ideas into implementable, cost-effective, and low-business-risk technologies. Designing for energy efficiency, which is one of the 12 Principles of Green Chemistry, requires chemical companies to operate differently than they have in the past. Giraud and Sehgal want chemical companies to face these challenges together, before their new products go head-to-head in the marketplace, similar to the way the semiconductor industry works together with its technology road map to ensure uniform cost-effective advancements in chip manufacturing.
GCI roundtable members began exploring the idea for AltSep in 2013, Giraud explains. At first, the group gathered company insights on what it would take to replace distillation when a manufacturer is considering refurbishing an existing plant, adding a new unit, or building a complete facility from scratch (ACS Sustainable Chem. Eng. 2014, DOI: 10.1021/sc500427d).
Giraud and Sehgal have also spent time describing the sustainable separation challenge at chemical conferences and roundtable-led workshops. Their aim is to garner support from large and small companies, universities, national laboratories, and private research institutes for developing new processes and industry standards. Chemical companies don’t typically design and build their own equipment, such as distillation towers, but rely on equipment vendors and engineering firms instead. So those firms are participating in AltSep as well.
To help get things started, the AltSep team received a $500,000 competitive planning grant from the National Institute of Standards & Technology’s Advanced Manufacturing Technology Consortia program. The roundtable members have been using the grant to hammer out the details for an innovation road map, slated to be completed by the end of this year, for achieving their vision of a chemical process industry that operates with more energy-efficient separation processes.
“AltSep has tapped into the power of collaboration,” Giraud says. “The progress we’ve made thus far is based on the collective wisdom and effort of hundreds of scientists and engineers who have participated in our workshops and conference sessions. The more sustainable, more competitive road ahead will be paved by these innovators and many more working together.”
Defining the problem
One of AltSep’s starting points was taking a look at the types of chemical separations that could yield the greatest sustainability benefits from cuts in energy use. Chemical engineers David S. Sholl and Ryan P. Lively of Georgia Institute of Technology identified these top targets, which they have dubbed the “seven chemical separations to change the world” (Nature 2016, DOI: 10.1038/532435a).
“We wanted to highlight how much of the world’s energy is used for commodity chemical separations and point to some areas where large advances could potentially be made by expanding research,” Sholl says. “These processes are largely invisible to most people, but there are large potential rewards—to both energy and the environment.”
Isolating hydrocarbons from crude oil is one of the seven. Every day, 95 million barrels of crude oil get processed around the world, which is roughly 2 L for each person on the planet. Sholl and Lively explain that in a typical refinery some 200,000 barrels of crude oil per day are heated up to hundreds of degrees to separate compounds according to their boiling points. This takes place in the iconic 50-m-tall distillation towers that rise from petroleum refineries and petrochemical plants.
Another important petrochemical process among the seven is high-pressure cryogenic distillation of ethylene and propylene from alkanes. Run at temperatures as low as –160 °C, the process yields feedstock to make plastics. Some 200 million tons of ethylene and propylene are produced globally each year, Sholl and Lively note, about 30 kg per person. Another refinery process among the seven is separating benzene derivatives such as toluene, xylenes, and ethylbenzene from each other.
Beyond the refinery, the listed separations include purifying gas streams such as natural gas and synthesis gas and removing carbon dioxide, methane, and other greenhouse gases from the emissions of power plants, oil and natural gas wells, and cement production facilities. The remaining three processes on the list are the desalination and removal of trace contaminants from water, the isolation of rare-earth elements from each other in mining ores and during product recycling, and the recovery of uranium from seawater to use it for generating nuclear power.
When it comes to distillation alternatives, solvent extraction, chromatography, and crystallization are tried-and-true methods to separate molecules. The techniques work well in a lab or for small-scale processes, such as pharmaceutical and fine chemical production. But they’re resource intensive and often not practical on a larger scale.
The Georgia Tech engineers estimate that nonthermal approaches such as membranes and adsorbent materials are more realistic options. Replacing distillation towers with sets of membrane-based cartridges, for example, can be 10 times as energy efficient as heat-driven distillation, Sholl and Lively note. Although membrane separations are used in some refinery and pharmaceutical processing already, they remain underdeveloped or are too expensive to scale up to replace distillation.
Besides organic polymers, a range of other high-performance porous organic, inorganic, and ceramic materials can be fabricated into membrane films or used as adsorbent materials, Sholl and Lively point out. Possible new classes of membrane materials could include carbon nanotubes, metal-organic frameworks, graphene oxide, and membrane proteins such as aquaporins.
But all these materials face large barriers to practical application, Sholl and Lively say (Nat. Mater. 2017, DOI: 10.1038/nmat4860). The materials must selectively and quickly isolate molecules and ions that have similar properties, do so with minimal applied pressure to keep energy use down, and avoid fouling to reduce cleaning maintenance and extend lifetime.
Researchers must also consider manufacturing issues when designing new materials, Sholl and Lively say. For example, an industrial-scale separation operation might require membrane modules with 10,000 m2 of surface area, which is a tall order to manufacture.
But the effort will be worth it, Lively believes, not only to improve chemical processing but also to open up new commercial opportunities. In particular, he sees advantages for nonthermal separation technologies as more biomass is used as a feedstock to make commodity chemicals. Separation processes that don’t rely on heat, he says, could help prevent complex heat-sensitive molecules from undergoing side reactions.
“One special thing in the U.S. at the moment is that there is a real renaissance in the chemical process industry, with new plants being built and more on the way,” Sholl adds. “That means there is an opportunity to change the way we practice chemical manufacturing.”
Identifying a pathway to solutions
To satisfy one of the first needs in developing AltSep’s road map, the Chemical Manufacturers Roundtable partnered with chemical informatics specialists Vincent K. Shen, Nathan A. Mahynski, and their colleagues at NIST’s Material Measurement Labortatory to obtain a $300,000 Department of Energy High Performance Computing for Manufacturing grant. The funding is providing computing time at Lawrence Berkeley National Laboratory in conjunction with big data architect Debbie Bard to better understand the properties needed in porous materials to achieve energy-efficient separations of multicomponent fluids. The yearlong study, just now getting under way, will systematically explore parameters, including pore geometry and intra- and intermolecular fluid dynamics, that describe molecular-material interactions.
“The key to low-energy separations lies in the molecules themselves—their properties and their interactions,” Solvay’s Sehgal says. “We should let the chemistry guide the engineering for low-energy separation pathways.”
AltSep is also connected with the American Institute of Chemical Engineers’ Rapid Advancement in Process Intensification Deployment (RAPID), a consortium of companies, universities, federal agencies, and nonprofit groups put together to broadly address manufacturing challenges. Officially launched in March, RAPID is funded with $70 million from the Manufacturing USA program and $70 million from the consortium’s members. AltSep academic collaborator James A. Ritter of the University of South Carolina serves as a coleader of one of RAPID’s six focus areas, Intensified Process Fundamentals, for which he is coordinating industry-academic-government collaboration on separation technologies.
As AltSep planning continues apace, complementary research efforts with dotted-line relationships to the initiative are springing up. In late 2015, chemical engineers at Newcastle University launched a U.K.-based academic-industry research partnership aimed at developing the next generation of long-lasting membranes for batch and continuous industrial processing. Funded by the U.K.’s Engineering & Physical Sciences Research Council and led by Newcastle chemical engineer Ian Metcalfe, the project is called SynFabFun, a derivation of “from membrane material synthesis to fabrication and function.”
“Understandably, industry is often reluctant to adopt new systems because the long-term reliability of membranes can’t be guaranteed,” says Metcalfe, whose research focuses on inorganic-based membrane materials such as metal oxides. His group tests its membrane materials by subjecting them to the equivalent of 30 years of use over a short period. “Our aim is to develop the ‘immortal’ membrane, or at least one that will outlive the lifetime of the industrial plant or particular piece of equipment where it is being used.”
In another effort, this past October chemical engineers at Imperial College London launched the Barrer Centre, a research institute for membrane and adsorption science and technology. Barrer Centre Director Andrew Livingston says he and his colleagues, who are SynFabFun participants, aim “to make breakthroughs in the way industry carries out molecular separations.”
Beyond its research activities, the Barrer Centre will encourage technology transfer and entrepreneurship to spur commercial developments. Livingston and his team are also providing training and technical information for engineers in research and industry. “There is so much work to do, and the prize is large,” Livingston says.
Growing a bigger list of options
As researchers explore new separation strategies, they’ve focused on how to exert more accurate control over the molecular architecture of membranes to improve their performance. Georgia Tech’s Lively and his group are working to develop membranes to separate hydrocarbons and industrial chemicals based on concepts of aqueous reverse osmosis used in water desalination.
In one example, working in collaboration with researchers at ExxonMobil, the team created porous hollow fibers using cross-linked poly(vinylidene fluoride) as precursors. Carbon molecular sieve membranes made by pyrolyzing the fibers are capable of separating o-xylene and p-xylene, which are structurally very similar, just by pressurizing the liquid at room temperature (Science 2016, DOI: 10.1126/science.aaf1343). Normally this type of separation isn’t possible by distillation and requires complex, resource-intensive chromatography or crystallization, Lively says.
Meanwhile, Imperial College London’s Livingston and coworkers are developing a new approach to a method called “interfacial polymerization” for producing ultrathin, superstrong porous polymer membranes. The goal, Livingston says, is to improve selectivity and at the same time improve the membrane’s resistance to degradation by the organic compounds being separated.
The researchers fabricate polymer nanofilms on cross-linked polyimide substrates using twisted aryl monomers that leave gaps in the film’s structure. The cavities are large enough for the membrane to be very permeable, with the substrate polymer acting as a scaffold that ensures the nanofilm remains stable (Nat. Mater. 2016, DOI: 10.1038/nmat4638).
“We have managed for the first time to create interconnected three-dimensional polymer network membranes in which we can control the size of pores and their connectivity,” Livingston says. “This means we can have a more accurate separation of molecules at a higher processing rate with lower consumption of energy.”
Although membranes dominate the conversation, researchers are devising multiple options to tackle nonthermal separations. For example, Taiwan’s nonprofit Industrial Technology Research Institute (ITRI) has created “green glass adsorbent technology.” The technology, designed specifically for removing heavy-metal ions such as arsenic, cadmium, chromium, and lead from industrial and mining wastewater without distilling or evaporating water, was a finalist in the 2016 R&D 100 Awards given by R&D Magazine.
Rather than bury tons of waste from liquid-crystal display panels in landfills, ITRI researchers identified a crushing and extraction process that separates the liquid crystals and the indium coating on the glass for recycling while creating a porous adsorbent material from the glass itself. When added to water, the adsorbent extracts heavy metals. Then the collected metal ions can be desorbed, concentrated, and recycled, and the glass adsorbent gets regenerated and reused. The green glass adsorbent is less costly and has a wider range of applications than ion-exchange resins or chemical processes such as precipitation and coagulation that are currently used, according to ITRI deputy director Huan-I Hung.
In another case of membraneless separation, Martin Z. Bazant of Massachusetts Institute of Technology and coworkers are working on a system that uses an electrically driven shock wave to purify a flowing stream of salty water. In this process, called shock electrodialysis, water flows through a porous glass frit. When an electric current is applied to the frit, the water divides into regions where the salt concentration is either depleted or enriched. When the current becomes strong enough, it generates a shock wave between these two zones, dividing the streams and allowing the fresh and salty water to be separated.
“AltSep is directly aligned with our goals of making industrial separations more energy and capital efficient,” says Hannah Murnen, vice president of business development at AltSep industrial collaborator Compact Membrane Systems. In particular, the program is providing valuable networking for her company and other organizations interested in advancing separation technologies, Murnen adds.
Compact Membrane Systems has developed a fluoropolymer membrane system containing silver ions, called Optiperm, for gas-phase separation of light olefins and alkanes. As a gas stream under pressure passes down the center of a spiral-wound membrane cartridge, silver ions in the membrane interact with the olefins, forming complexes that shuttle the olefins through the membrane as the alkanes proceed onward.
Metal-mediated separations aren’t new, but before now researchers haven’t been able to find a material and processing system that can both embed silver and perform over a long time without fouling. Optiperm was recognized for its benefit over conventional column distillation by being selected as the overall winner in the 2016 ICIS Innovation Awards presented by chemical information services firm ICIS. Murnen says a plant-scale propylene-propane Optiperm demonstration project is due to start up at Delaware City Refining Co. this summer.
“Membranes are one of the best ways that we as a chemical community can make separations more efficient,” Murnen says. “And AltSep is focused on making the use of them a reality rather than an interesting laboratory project.”