National Academies Press: OpenBook

A Research Agenda for Transforming Separation Science (2019)

Chapter: 6 Implementation of the Research Agenda

« Previous: 5 A Research Agenda: A Vision for the Future of Separation Science
Suggested Citation:"6 Implementation of the Research Agenda." National Academies of Sciences, Engineering, and Medicine. 2019. A Research Agenda for Transforming Separation Science. Washington, DC: The National Academies Press. doi: 10.17226/25421.
×

6

Implementation of the Research Agenda

Experimental, theoretical, and modeling breakthroughs can provide insights into longstanding separation problems and can address new challenges that have arisen in health care and environmental sustainability and stewardship. Many of the challenges were described in Chapters 14. Design and development of hierarchical separation materials and systems that were inconceivable just a few years ago are now within reach because of advances in materials synthesis, experimental characterization tools, molecular modeling and simulation, and harnessing of data science. However, implementation of the research agenda presented in Chapter 5 will require the reinvigoration of a vibrant separation-science and engineering community working to train a new generation of separation scientists and taking advantage of advanced experimental and computational tools. This chapter describes that vision and potential effects on industrial and commercial analytical-scale separations if the vision is realized.

The committee members had a strong impression that separation research has lost the prestige and appeal that it enjoyed for decades through the 1980s. Cutting-edge innovations—especially in chromatography, capillary electrophoresis, and ion exchange—had energized the community with excitement over how separation technology could contribute to improving human health and quality of life. The committee’s observation is supported by the data presented in Chapter 1—numbers of faculty members readily identifiable as separation researchers have decreased by 40% in the top chemistry departments and by 30%1 in the top chemical-engineering departments since the 1987 report Separation and Purification: Critical Needs and Opportunities (NRC, 1987). Moreover, many researchers who contribute or could contribute to advancing separation science do not identify themselves as separation scientists. For example, researchers investigating membranes or synthesizing solid adsorbents see themselves as material scientists and engineers rather than as separation researchers.

Graduate students view separations as old-fashioned. None of the top 20 chemistry or chemical-engineering programs list “separations” as one of their topical research areas. That is despite the continuing demand for separation scientists and engineers in the largest economic sectors of the United States, including mining, manufacturing, chemicals and petrochemicals, and agriculture. Within those sectors, the oil and gas, pharmaceutical, medical diagnostics, chemical, and agricultural industries all rely heavily on chemical separations for recovery and purification, quality control, and the detection of problems. Improvements in chemical separations, either through innovative separation methods or the availability of better materials, will have wide economic effects but will require a larger trained separations community.

GRADUATE AND UNDERGRADUATE EDUCATION IN SEPARATION SCIENCE

A key tool for rejuvenating the separations community is educational innovation. The chemistry and chemical-engineering fields share the philosophy that the combination of a fundamental understanding and an awareness of the current state of knowledge provides one with the intellectual tools to address a variety of applications. Undergraduate chemistry programs mainly include analytical-scale and preparative-scale separations, whereas undergraduate chemical-engineering programs focus on process-scale separations. Unfortunately, undergraduate courses usually do not convey the excitement of new advances in separation technology or the potential for alternative separation methods that might

___________________

1 These data were collected by committee members by identifying separation scientists and engineers in analytical-chemistry and chemical-engineering departments as listed in the 1987 American Chemical Society Directory of Graduate Research and current departmental Web sites. Separation scientists in all R1 (highest research activity) analytical-chemistry programs were counted. Chemical-engineering departments considered were those listed in 2018 by U.S. News & World Report as the top 20 chemical-engineering graduate programs.

Suggested Citation:"6 Implementation of the Research Agenda." National Academies of Sciences, Engineering, and Medicine. 2019. A Research Agenda for Transforming Separation Science. Washington, DC: The National Academies Press. doi: 10.17226/25421.
×

be less energy-intensive or solvent-intensive. For example, the survey of chemical-engineering departments found that in undergraduate chemical-engineering separation courses the majority of time is spent on thermal techniques, such as distillation, rather than techniques that use separation materials. Updating those courses could dramatically influence students’ perceptions of graduate research opportunities in separations. Some 90% of chemical-engineering departments use one of four classic separations textbooks; their original publication dates range from 1956 to 2005, as detailed below.

  • Warren L. McCabe, Julian C. Smith, and Peter Harriott. Unit Operations of Chemical Engineering, 7th ed. McGraw-Hill, 2005 (originally published 1956).
  • Christie John Geankopolis, A. Allen Hersel, and Daniel H. Lepek. Transport Processes and Separation Process Principles, 5th ed. Pearson Education, Inc., 2018 (originally published 1978).
  • J. D. Seader, Ernest J. Henley, and D. Keith Roper. Separation Process Principles with Applications Using Process Simulators, 4th ed. John Wiley & Sons, Inc., 2016 (originally published 1997).
  • Philip C. Wankat. Separation Process Engineering, 4th ed. Prentice Hall, 2017 (originally published 2005).

Other useful texts include Harrison et al. (2015), Ladisch (2001), and Cussler (1997). A new textbook focusing more on materials-based separation process design could have an important effect on chemical-separation research and practice.

Graduate education provides another opportunity to develop the separations community. Graduate courses in separation science are offered only in chemistry departments in which analytical-chemistry faculty are doing research in this field. The syllabuses of the graduate separation-science courses in top analytical-chemistry programs show topics starting with an overview of equilibrium driving forces and the thermodynamic basis of separations, followed by flow and mass transport, and then specific applications, such as gas and liquid chromatography. Graduate courses in chemical engineering focus on single topics, such as membranes or bioseparations. One reason for that compartmentalization is that most chemical-engineering departments have only a single faculty member (if any) interested in separations, and that person might have insufficient expertise beyond his or her own topical area.

As the 1987 report stated, virtually no effort has been made to integrate or coordinate the analytical-chemistry and chemical-engineering courses; that remains the case today. As the present committee has found, chemists and chemical engineers use different language to describe similar concepts, in some cases the same words to describe different ideas. Thus, learning from each other is more challenging than just “looking it up”. The distinct course material on separations for chemists, chemical engineers, materials scientists, surface scientists, and people in other disciplines silos the communities, perpetuating a divergent separations community that rarely interacts.

Cross-fertilization across the diverse fields will create better communication, increase scientific collaboration, and advance research in separation science. An obvious place to start such cross-fertilization, as suggested many decades ago, is in coursework. The activation barriers are high, so departmental and college leadership, as well as funding agencies will need to play a role in the transformation. Information-technology resources could assist educators in developing multidisciplinary separation-science courses or modules that could be used in many universities.

Recommendation: Because separation science is integral to chemistry and chemical engineering, these academic departments should provide high-quality training in separations.

Finally, chemistry and chemical-engineering departments could benefit from consciously re-examining how they highlight and advertise the various research in their departments. For example, grouping experimental, theoretical, and modeling research on CO2 capture, membranes, metal organic frameworks, bioseparations, microfluidics and nanofluidics, and modeling of separation process intensification under “separations” could add coherence and draw appropriate attention to this important research field. Such a grouping could replace the current practice of embedding the topics in “materials”, “biotechnology”, “modeling and simulation”, and “process engineering”. Highlighting separation research in various departments of the same university together would be another way to develop a multidisciplinary separations community and attract separation researchers.

COLLABORATION OPPORTUNITIES

Researchers in chemistry and chemical engineering are the most likely to identify themselves as separation scientists. However, as discussed in the previous chapters the adoption of knowledge and incorporation of approaches from a wide array of other disciplines are necessary to support the research agenda described in Chapter 5. Those disciplines include different subfields of chemistry and chemical engineering (such as soft-matter and structural chemistry, process synthesis, fluid mechanics, and mass transfer), biochemistry, materials science and engineering, physics, data science, information and computer science, and bioengineering. The involvement of many disciplines has been critical for some of the key advances in separation technology, as described in Chapter 2. The first and crucial step, which will require conscious, concerted effort, is more extensive communication and collaboration between chemistry and chemical-engineering separation researchers. One way to enhance communication is through conferences and workshops, as discussed below.

Suggested Citation:"6 Implementation of the Research Agenda." National Academies of Sciences, Engineering, and Medicine. 2019. A Research Agenda for Transforming Separation Science. Washington, DC: The National Academies Press. doi: 10.17226/25421.
×

Recommendation: The chemistry and chemical-engineering separation communities should seek opportunities for substantive interactions that will inspire creativity and the integration of concepts in research and education.

The need to leverage discoveries and tools from a wide variety of disciplines, beyond chemistry and chemical engineering, will continue to be critical in the new era of separation science. Those additions include the data-analytics and computational-chemistry techniques described in Chapter 5 that have advanced so dramatically in the last 30 years. They include numerous new or improved analytical-chemistry techniques that have come on line in the last few decades and are vital for fundamentally understanding and developing the ability to control molecules both in bulk and at interfaces. For example, the combination of x-ray surface scattering and sum-frequency generation spectroscopy (see Chapter 3) provides unprecedented insight into the orientation of molecules at interfaces (Rock et al., 2018).

Separation science now requires multidisciplinary, multi-investigator collaborations. Fortunately, there are many more federal research funding opportunities available for multi-investigator, multidisciplinary projects than there were when the 1987 report was published. Examples include the U.S. Department of Energy (DOE) Energy Frontiers Research Centers, the National Science Foundation (NSF) Science and Technology Centers, and NSF Engineering Research Centers programs. Nonetheless, multidisciplinary, multi-investigator collaborations will be critical in the new era of separation science, in which principal investigators in multiple disciplines and with diverse expertise will be needed to perform all the theoretical, computational, and experimental research needed to advance the frontiers. Intersecting disciplines include materials, physics, biochemistry, environmental engineering, information science, and molecular biology and such subfields as catalysis, surface science, nanoscience, spectroscopy, modeling and simulation, and transport phenomena.

Recommendation: Federal agencies should promote cross-fertilization of separation researchers in intersecting disciplines and encourage collaborative projects as a key priority.

A fruitful path to learning about technical advances and discoveries is by attending conferences. Conferences advance separation science by providing a forum for researchers to communicate ideas directly and present findings on basic research. They allow separation scientists and engineers to discuss limitations and needs regarding practical separation processes, and this can drive research and innovation. Conferences can be a major means of communication between different parts of the separations community that can result in collaboration and cross-fertilization. Many specialty conferences on separations occur in the United States and abroad.

It is interesting that conferences have evolved to focus on single classes of separation materials or techniques. Examples include the following:

  • HPLC is a conference on analytical-scale high-performance liquid chromatography and the monitoring of manufacturing. It has been held every year since 1973 and takes place in the United States about every 3 years.
  • Microscale Separations and Bioanalysis symposia have occurred annually since 1989, when they started as the International Symposium on High Performance Capillary Electrophoresis. They focus on microfluidic, capillary, and electrically driven analytical-scale separations and are hosted in the United States roughly every 3 years.
  • Gordon Research Conferences on Bioanalytical Sensors take place in alternate years; about half the programming is on chemical separations for clinical diagnostics, drug discovery, and drug development.
  • Gordon Research Conferences on Membranes: Materials and Processes take place in alternate years and focus on the development of membranes for a variety of important gas and liquid separations.
  • The International Conference on Fundamentals of Adsorption, organized by the International Adsorption Society, has occurred every 3 years since 1983 and has a primary focus on gas and liquid separations.
  • North American Membrane Society annual meetings are held each year, and a large part of the program focuses on gas and liquid separations.
  • The International Conference on Ionic Liquids in Separation and Purification Technology has taken place every 3 years since 2011 and includes a mix of process-focused and analytical separation-focused research.
  • Dedicated workshops and national user facilities meetings. Annual meetings at DOE-sponsored synchrotron and neutron facilities bring together domain scientists with beamline experts.

The specialized nature of the conferences hinders communication. Outside researchers generally do not attend specialty conferences, so no interchange occurs on the fundamentals that govern the performance of numerous types of separation materials and techniques. All categories of analytical chemists attend the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy (referred to as Pittcon), the largest analytical-chemistry conference in the world, which includes substantial programming on chemical separations. Similarly, all varieties of chemical engineers attend the annual American Institute of Chemical Engineers (AIChE) meeting, which includes sessions on chemical separations. The sessions at the AIChE meetings are divided by type of separation material or technique, and researchers generally attend sessions only on their subexpertise.

Suggested Citation:"6 Implementation of the Research Agenda." National Academies of Sciences, Engineering, and Medicine. 2019. A Research Agenda for Transforming Separation Science. Washington, DC: The National Academies Press. doi: 10.17226/25421.
×

There are opportunities to provide better communication between existing separations communities and to engage participants of diverse backgrounds and disciplines. For example, the Gordon Research Conferences have recently approved a new conference on chemical separations that will be held for the first time in 2020; it will bring together chemists, chemical engineers, and researchers in affiliated disciplines to focus on the common fundamentals of chemical separations, spanning a wide variety of materials and techniques. Interestingly, a Gordon Research Conference series on separation and purification ran from 1952 to 2000, but it was canceled as attendees opted for more specialized conferences. It is hoped that the new conference can reverse that trend.

Another option to bring together participants of diverse backgrounds and disciplines to communicate, learn from each other, and develop collaborations would be a chemical separation fundamentals summer school, akin to the American Chemical Society Green Chemistry and Sustainable Energy Summer School that is held every year. The school could be a 1- to 2-week intensive training, attended by young researchers, mostly graduate students and postdoctoral research associates, and taught by chemistry and chemical-engineering separation researchers (primarily faculty but with some industrial participation). Developing the curriculum and running the training each year would require federal or private funding, but such a school could immediately create the invigorated new community of young separation researchers that is needed.

A final opportunity to grow the separations community and allow members to learn from other disciplines is at the DOE facility user meetings. These generally include user-organized workshops that combine beamline scientists and specialized user groups. Separation scientists could interact directly with facility personnel to discuss opportunities for new tools to advance the field of separation science. Some of the tools are discussed in the next section.

ACCESS TO TOOLS AT THE NATIONAL LEVEL

National user facilities provide access to state-of-the-art experimental tools for sample characterization. Notable among those facilities are the Spallation Neutron Source at Oak Ridge National Laboratory and the three large synchrotron sources: the Advanced Light Source at Lawrence Berkeley National Laboratory, the Advanced Photon Source at Argonne National Laboratory, and the National Synchrotron Light Source at Brookhaven National Laboratory. Managed and operated by DOE, they provide a variety of opportunities for access to researchers who have demonstrated a need for their capabilities. The most common access is through a proposal system, specific to open-literature publishable studies and subject to peer review. Access to an agreed-on beamline is made available for a prescheduled timeslot.

The facilities are all interested in expanding their user bases and so give special considerations to new researchers. They make available beamline scientists who are knowledgeable about their beamline’s specific advantages and limitations. Those experts can provide advice and assistance through the entire process, from proposal writing to data acquisition. Several training programs are provided to familiarize potential users with the general technique principles, such as the National School on Neutron and X-ray Scattering, the School on Synchrotron X-ray Scattering Techniques in Materials and Environmental Sciences: Theory and Application, the Summer School on Synchrotron X-ray Absorption Spectroscopy, and the Ultrafast X-ray Summer School. Run by experts in the field, some of the schools even provide a hands-on opportunity for performing an experiment, including all aspects of data acquisition and analysis. With beamline experts available to advise and work with new users, the schools provide an underused method to access the opportunities provided by the national facilities.

Some routine experiments, such as those associated with materials synthesis and the evaluation of permeability coefficients, are performed by hand and by individual research groups using inhouse characterization equipment. There is a need to create automated and robotic systems for more precise synthesis and characterizing capabilities for separation media. Making instruments available at universities would meet an important need for the rapid screening of synthesis conditions and materials characterization and thereby provide higher-fidelity data for work routinely performed in university laboratories.

Just as the user-facility infrastructure has grown for experimentation, so has a national ecosystem for cyberinfrastructure that includes computation (for example, Leadership Class facilities), software development (for example, the NSF-funded Molecular Software Sciences Institute [MOLSSI] and the DOE-funded Scientific Discovery through Advanced Computing [SciDAC] Institutes), and visualization capabilities. Leadership Class computing user facilities enable massively parallel simulations to be performed on complex systems and realistic environments relevant to separation systems on various time and length scales. They also help to support migration of codes onto these environments. For example, calculations of heavy-element–containing systems must have accurate treatment of relativistic effects that is not possible currently in any routine way. The development of massive parallel graphics processing unit codes, such as DIRAC at the Oak Ridge Leadership Computing Facility, should make that possible.

In other domains, the SciDAC Institutes help to find solutions to emerging technological difficulties associated with massive-scale computation. For example, Quantification of Uncertainty in Extreme Scale Computations (QUEST) and Scalable Data Management, Analysis and Visualization (SDAV) both have close ties to computational user facili-

Suggested Citation:"6 Implementation of the Research Agenda." National Academies of Sciences, Engineering, and Medicine. 2019. A Research Agenda for Transforming Separation Science. Washington, DC: The National Academies Press. doi: 10.17226/25421.
×

ties and work with application teams to provide technical solutions that are broadly applicable in the computational-science community. MOLSSI has a mission of improving software in the computational molecular sciences. It supports community-led workshops that target software-related challenges relevant to the computational molecular sciences and numerous Software Summer Schools. It also provides access to software tools and educational materials relevant to the computational molecular sciences.

In combination, the national user facilities and the cyberinfrastructure community that has developed over the last decade constitute a wealth of opportunity and expertise for modeling, visualization, and analyses, all of great importance to the separations community.

Unfortunately, many separation researchers are not aware of those experimental and computational facilities or do not understand the process for gaining access to them. Better outreach to the separations community is encouraged.

HOW FULFILLING THE RESEARCH AGENDA WILL AFFECT INDUSTRIAL PRACTICE

Only a few forces drive the adoption of new technologies by industry. A primary cause of change is reduction in manufacturing costs due to reductions in energy consumption or better use of capital or operating resources. Financial resources are also used better when the recovery of high-value products is improved. Practical technology that allows cost-effective and improved throughput and product recovery encourages adoption by industry. Where government regulation establishes product purity or composition requirements, compliance becomes a primary driving force behind the implementation of new technology.

Fundamental research in selectivity and capacity is critical for developing technologies that industry can practically implement. The presence of stable and well-described synthetic methods allows reproducible and low-cost implementation of new materials. Understanding physical properties and having detailed models of new systems lay the proper foundation for design-engineering concepts. The study of multicomponent streams in novel separation applications is necessary if the new technology is to be considered for practical use. More important, the development of fundamental models of the separation process itself will allow engineering simulations of the process, which is necessary for eventual application of new separation technology.

The use of new materials in industrial applications is enhanced by understanding of the long-term behavior of separation systems. For industrial practice, three elements are important: cycling, stability, and aging. Proper process design requires full knowledge of the reversibility of adsorption or absorption processes, including the empty- and full-state properties. The stability of a system with respect to continuing operation (cycle reproducibility) is critical for process simulation and modeling. Although materials that have extreme lifetimes are useful, it is important from a practice perspective that aging and long-term performance be understood on a fundamental basis. Understanding of those aspects will lead to the development of practical process designs that include appropriate redundancy or replacement capability.

The fundamental study of the interface in a separation system is important if the physical process of separation, including enhancements of the physical properties and knowledge of materials, is to be understood. The study of the interface is particularly relevant to membrane separations and extraction systems, where the interface governs separation behavior. Modeling and simulation in the presence of complex process streams is important if industrial application of these techniques is to grow. In addition, a more thorough understanding of the molecular-scale behavior of a system at the interface allows researchers to model and improve mass transport at the microscopic level. The fundamental characterization (analytical, modeling, and simulation) of the interface during separation based on the complex intermolecular chemistry and local physical phenomena will provide opportunities to address the challenges of separations that involve dilute solutions.

Improvements in data modeling and simulation lead to practical understanding and the ability to use robust process models that sustain industry confidence. The presence of well-modeled separation options based on fundamental study will also allow greater success in process synthesis for separation operations while reducing design-engineering time and costs.

In summary, implementation of the research agenda described in Chapter 5 is critical for reducing risks associated with the adoption of next-generation separation technologies by industry. Replacing familiar, established separation systems with new technology requires a high level of confidence and reliability. The fundamental phenomena of initial use and long-term use must be well understood so that performance models can be trusted and used for reliable process and economic decision-making. Implementing the research agenda will provide such reliability and enable industry to commercialize new separation technologies to achieve sustainability, enhance human health, and improve quality of life around the world.

FINAL THOUGHTS

Through the implementation of the research agenda, separation science will have the opportunity to address many questions that sit at the forefront of chemistry and chemical engineering. The advances will require improvements in separation-science education, collaborations, community-building, and use of experimental and computational resources. If it is successfully implemented, the transformation of separation science could affect analytical-scale and process-scale industries and lead to greater U.S. economic

Suggested Citation:"6 Implementation of the Research Agenda." National Academies of Sciences, Engineering, and Medicine. 2019. A Research Agenda for Transforming Separation Science. Washington, DC: The National Academies Press. doi: 10.17226/25421.
×

competitiveness, a more sustainable chemical-manufacturing ecosystem, and improved human and environmental health. Leadership in this arena will directly address issues of critical national importance including energy, conservation, and resource management.

REFERENCES

Cussler, E.L. 1997. Diffusion, Mass Transfer in Fluid Systems, Cambridge University Press.

Harrison, R.G., P.W. Todd, S.R. Rudge, and D.P. Petrides. 2015. Bioseparations Science and Engineering (Topics in Chemical Engineering) 2nd Edition, Oxford University Press.

Ladisch, M. 2001. Bioseparations Engineering, Principles, Practice and Engineering, Wiley.

NRC (National Research Council). 1987. Separation Purification: Critical Needs and Opportunities. Washington, DC: The National Academies Press.

Rock, W., Qiao, B., Zhou, T., Clark, A. E., Uysal, A., 2018. “Heavy Anionic Complex Creates a Unique Water Structure at a Soft Charged Interface.” Journal of Physical Chemistry C 122(51):29228-29236.

Suggested Citation:"6 Implementation of the Research Agenda." National Academies of Sciences, Engineering, and Medicine. 2019. A Research Agenda for Transforming Separation Science. Washington, DC: The National Academies Press. doi: 10.17226/25421.
×
Page 76
Suggested Citation:"6 Implementation of the Research Agenda." National Academies of Sciences, Engineering, and Medicine. 2019. A Research Agenda for Transforming Separation Science. Washington, DC: The National Academies Press. doi: 10.17226/25421.
×
Page 77
Suggested Citation:"6 Implementation of the Research Agenda." National Academies of Sciences, Engineering, and Medicine. 2019. A Research Agenda for Transforming Separation Science. Washington, DC: The National Academies Press. doi: 10.17226/25421.
×
Page 78
Suggested Citation:"6 Implementation of the Research Agenda." National Academies of Sciences, Engineering, and Medicine. 2019. A Research Agenda for Transforming Separation Science. Washington, DC: The National Academies Press. doi: 10.17226/25421.
×
Page 79
Suggested Citation:"6 Implementation of the Research Agenda." National Academies of Sciences, Engineering, and Medicine. 2019. A Research Agenda for Transforming Separation Science. Washington, DC: The National Academies Press. doi: 10.17226/25421.
×
Page 80
Suggested Citation:"6 Implementation of the Research Agenda." National Academies of Sciences, Engineering, and Medicine. 2019. A Research Agenda for Transforming Separation Science. Washington, DC: The National Academies Press. doi: 10.17226/25421.
×
Page 81
Next: Appendix A: Committee and Staff Biographical Sketches »
A Research Agenda for Transforming Separation Science Get This Book
×
Buy Paperback | $58.00 Buy Ebook | $46.99
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

Separation science plays a critical role in maintaining our standard of living and quality of life. Many industrial processes and general necessities such as chemicals, medicines, clean water, safe food, and energy sources rely on chemical separations. However, the process of chemical separations is often overlooked during product development and this has led to inefficiency, unnecessary waste, and lack of consensus among chemists and engineers. A reevaluation of system design, establishment of standards, and an increased focus on the advancement of separation science are imperative in supporting increased efficiency, continued U.S. manufacturing competitiveness, and public welfare.

A Research Agenda for Transforming Separation Science explores developments in the industry since the 1987 National Academies report, Separation and Purification: Critical Needs and Opportunities. Many needs stated in the original report remain today, in addition to a variety of new challenges due to improved detection limits, advances in medicine, and a recent emphasis on sustainability and environmental stewardship. This report examines emerging chemical separation technologies, relevant developments in intersecting disciplines, and gaps in existing research, and provides recommendations for the application of improved separation science technologies and processes. This research serves as a foundation for transforming separation science, which could reduce global energy use, improve human and environmental health, and advance more efficient practices in various industries.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!