A bridge to renewable energy
Rome wasn’t built in a day — and neither will the full shift to renewable energy occur that quickly. Our current economy has evolved on the basis of fossil resources; any abrupt transition will create havoc. Trillions of dollars of new investment will be needed, along with training people in new skills. Also, many technology aspects of the renewable economy await development.
To ensure a smooth transition, we need a “bridge” approach — one that will point us in the right direction, while still contributing to a reduction in the release of greenhouse gases.
One novel solution lies in shale gas. Over the last two decades, you’ve probably been reading about the wasteful flaring of natural gas at energy-producing sites. At atmospheric temperature and pressure, shale gas is a mixture of a number of gaseous hydrocarbons. Methane is the lightest gas, and is delivered through pipelines as natural gas.
Large amounts of shale gas often are flared off because it is not economical to separate out its heavier gases — ethane, propane and butane — to avoid their condensation before the methane can be put into the high-pressure pipelines that serve cold winter regions. Separating these heavier gases requires energy and capital-intensive cryogenic processes — an uneconomical equation at the small, distributed scale of shale gas plants.
Our novel process avoids this separation of methane, ethane, propane and butane, eliminating repeated or unnecessary operations and associated equipment. We discovered that the entire shale gas, without any separation, can be sent to a thermal dehydrogenation reactor to convert heavier hydrocarbons into molecules that are reactive under moderate reaction conditions. This not only avoids the front-end separation of methane from the shale gas — the methane also acts as an inert diluting agent, increasing the yield of the reactive molecules.
A second benefit stems from the fact that ethylene is the predominant reactive molecule formed in the dehydrogenation reactor. Thus, along with ethane, other heavier molecules, such as propane and butane, also are converted to ethylene at the operating conditions of the thermal dehydrogenation reactor. This has a strong impact on how the products from this reactor are treated downstream: rather than dealing with multiple reactive molecules, the downstream processes can be designed solely on the basis of ethylene.
Furthermore, the composition of shale gas — and hence the relative quantities of ethane, propane and butane — not only vary from location to location, but also with time spent onstream at a given location. By predominantly producing ethylene, the process becomes somewhat immune to the composition variations of the shale gas.
The ethylene can be reacted downstream to form much heavier gasoline and/or diesel types of molecules that are liquid under atmospheric conditions. This liquid can be separated easily from the methane, which becomes available for either pipeline or further conversion to other chemicals. The net outcome is a much simpler plant that can be built at a distributed scale.
We used state-of-the-art computational tools to design our process, which was developed in concert with various other research thrusts of the National Science Foundation (NSF) Center for Innovative and Strategic Transformation of Alkane Resources (CISTAR). The center’s vision is to create a transformative engineered system to convert light hydrocarbons from shale resources to chemicals and transportation fuels in smaller, modular, local, and highly networked processing plants.
Our process — which has been awarded three U.S. patents, all licensed for commercial use by the Purdue Office of Technology Commercialization — enables use of shale gas that has a much smaller carbon dioxide footprint than oil or coal. It is much simpler, has fewer reactors and separators, and can be built economically at a much smaller scale. For example, at 10 million standard cubic feet per day of the shale gas feed flowrate at a Bakken shale field (these fields underlie parts of Montana, North Dakota, Saskatchewan and Manitoba), the internal rate of return of the process is estimated to be about 26 percent.
As we transition to a renewable economy, we still are likely to need high-energy-density hydrocarbon liquids for airplanes, ships, and so on. Our process will allow the availability of this liquid, along with low-carbon-footprint methane for myriad other applications. In addition, our process is capable of making a number of useful chemicals in lieu of liquid fuels.
In the long term, the human race has to rely on energy and material resources that cannot be depleted and are available on a daily basis. Fossil resources have taken millions of years in the making, but their rate of use is much faster, courting depletion, and their carbon emissions have environmental consequences. Also, fossil resources are localized at certain very specific places, while renewable resources are distributed more equitably across the globe.
Therefore, for a clean environment, and guaranteed forever availability at almost all locations in the world, renewable resources are attractive as a long-term strategy.
We aim to build a bridge to that future.
Dr. Rakesh Agrawal, National Medal of Technology and Innovation Recipient, American Academy of Arts and Sciences Fellow, NAI Fellow, NAE Member
Winthrop E. Stone Distinguished Professor of Chemical Engineering,
Davidson School of Chemical Engineering
Professor, Environmental and Ecological Engineering (by courtesy)
Lead, Process Synthesis & Design, Life Cycle Analysis & Environmental Impact Thrust, CISTAR
Director, Sustainable Food, Energy and Water Systems NSF Traineeship (NRT)
College of Engineering