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Chemical Processes for a Sustainable Future
By Trevor M. Letcher, Janet L. Scott, Darrell A. Patterson
The Royal Society of ChemistryCopyright © 2015 The Royal Society of Chemistry
All rights reserved.
General Concepts in Sustainable Chemical Processes
DARRELL ALEC PATTERSON AND JANET L. SCOTT
1.1 WHAT IS A SUSTAINABLE CHEMICAL PROCESS?
The topics contained in this book represent the nexus between chemical engineering and chemistry. Covering many sustainable process technologies available, whilst outlining the chemistry that underpins them, this book differs from others which often focus on a subset of the processes or the chemistry, or on a specialist topic (e.g. process intensification). Process design equations and process modelling, scale up, process and energy integration, and process control are often neglected. Therefore, we aim to provide the first comprehensive coverage of sustainable chemical processes, the one-stop-shop for the area. This book therefore focuses on the chemical technologies (unit operations) from both a chemical engineering and chemistry perspective. In this chapter we discuss the overarching principles and concepts addressed in the individual chapters.
The subject of this book is sustainable chemical processes. But what are these? For us, they are processes (i.e. a set of linked unit operations that take in raw materials and energy, and ultimately convert these into chemicals, biochemicals, materials or products) in a resource efficient manner that allows preparation of the desired product with minimal production of waste. Sustainable chemical processes or clean technologies are those that help us meet the goals of sustainability and sustainable development in our process systems.
The essence of sustainability and sustainable development is to ensure that our use of the planet's limited material, energy and ecological resources will be such that we sustain the current standard of living for the increasing human population so that future generations are able live life at the current, or even better, standard of living. The latter is particularly applicable to the large proportion of the global human population that do not yet have access to clean water, adequate food, life-saving medication and other essentials for a happy, healthy life. Thus, sustainability and sustainable development has been defined as:
'... development which meets the needs of current generations while not compromising the ability of future generations to meet their own needs ...'.
It is always worth reminding oneself that this widely quoted definition of sustainability arose from a commission asked to formulate 'a global agenda for change' and to recall Gro Harlem Brundtland's words from her foreword to the report:
'... the 'environment' is where we all live; and 'development' is what we all do in attempting to improve our lot within that abode. The two are inseparable.'
The commission called for development and this has been reiterated in numerous reports since, for example:
'... economic development, social development and environmental stewardship are interdependent and mutually reinforcing components of sustainable development which is the framework for our efforts to achieve a higher quality of life for all people.'
A simple way of determining if a process or technology meets the goals of sustainability and sustainable development is to probe whether or not it meets all three components of the triple bottom line (also sometimes called the Three Pillars of Sustainability): people (social bottom line); planet (environmental bottom line); and profit (economic bottom line). There are various measures of these, which include:
People (social bottom line): worker happiness, industrial safety, benefits based on payroll expense, promotion rate, 'loss time accident frequency', 'expenditure on illness and accident prevention/payroll expense', 'number of complaints per unit value added', etc.
Planet (environmental bottom line): life cycle assessment (see Section 1.4); environmental impacts such as acidification, global warming, human health, ozone depletion, photochemical ozone, wastes – hazardous and non-hazardous, and ecological health; and resource usage such as energy use, material use, water use and land use.
Profit (economic bottom line): capital and operating costs, wealth created, value added per unit value of sales, value added per direct employee, and R&D expenditure as a percentage of sales.
When all three components of the bottom line are met (i.e. at the overlap of the three lobes, Figure 1.1), we have sustainability. The social and economic components are less directly related to the chemistry and chemical engineering emphasis of this book and therefore will not be our main focus (but they will be touched on where relevant). We will concentrate on ensuring that our sustainable chemical technologies meet the environmental bottom line. A systematic method for process choice that facilitates the selection of different technologies to meet this aim is to use the waste management hierarchy combined with the principles of green chemistry and green engineering.
1.2 THE PRINCIPLES OF GREEN CHEMISTRY AND GREEN ENGINEERING
1.2.1 The Twelve Principles of Green Chemistry
First set out by Paul Anastas and John Warner in their seminal work published in 1998, it is clear that these 12 principles were defined by chemists with synthesis in mind and, as with all sets of rules or guidelines, should always be implemented in combination with careful analysis and intelligent critical thought. Nonetheless, even in cases where the process developer might decide that a particular principle is not formulated in a manner that applies to their process, these provide an excellent checklist. For example, it is certainly not always best to conduct a synthetic chemical process at ambient temperature and pressure, as described in principle 6. There are a plethora of examples where an exothermic reaction is best conducted at elevated temperature, as the process is rapid and the evolved heat is used to maintain the reaction temperature (usually after an initial short heating stage to initiate reaction). If put in the context of energy efficiency, it is easy to determine what the optimum temperature should be: that at which the process proceeds at a reasonable rate and consumes the least energy (cooling costs energy too), while remaining safe.
For completeness, below we reproduce the 12 principles of green chemistry from Anastas and Warner,
THE TWELVE PRINCIPLES OF GREEN CHEMISTRY
1. Prevention: It is better to prevent waste than to treat or clean up waste after it has been created.
2. Atom Economy: Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product.
3. Less Hazardous Chemical Syntheses: Wherever practicable, synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment.
4. Designing Safer Chemicals: Chemical products should be designed to affect their desired function while minimizing their toxicity.
5. Safer Solvents and Auxiliaries: The use of auxiliary substances (e.g., solvents, separation agents, etc.) should be made unnecessary wherever possible and innocuous when used.
6. Design for Energy Efficiency: Energy requirements of chemical processes should be recognized for their environmental and economic impacts and should be minimized. If possible, synthetic methods should be conducted at ambient temperature and pressure.
7. Use of Renewable Feedstocks: A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable.
8. Reduce Derivatives: Unnecessary derivatization (use of blocking groups, protection/deprotection, temporary modification of physical/chemical processes) should be minimized or avoided if possible, because such steps require additional reagents and can generate waste.
9. Catalysis: Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.
10. Design for Degradation: Chemical products should be designed so that at the end of their function they break down into innocuous degradation products and do not persist in the environment.
11. Real-time Analysis for Pollution Prevention: Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.
12. Inherently Safer Chemistry for Accident Prevention – Substances and the form of a substance used in a chemical process should be chosen to minimize the potential for chemical accidents, including releases, explosions, and fires.
Reproduced from: P. T. Anastas and J. C. Warner, Green Chemistry: Theory and Practice, Oxford University Press, New York, 1998, p. 30 with kind permission from Oxford University Press.
1.2.2 The Twelve Principles of Green Engineering
In some cases the 12 principles of green chemistry are difficult to apply directly in engineering applications, in particular those that do not involve chemicals or reactions. Consequently a further set of 12 principles were developed by Paul Anastas and Julie Zimmerman and published in 2003 as a tool to systematically apply the principles of sustainability and to achieve sustainable goals within engineering and in particular as a way of identifying and applying sustainability within engineering design (and as such these can be used as performance criteria). The principles are very general since they are intended to apply at all scales and apply to all engineering disciplines (chemical, electrical, civil, environmental, mechanical, systems, etc.). For engineers, these principles can be used as additional criteria that, in a complex system, can be optimised in addition to the normal parameters used in design to define and optimise a system.
Since the goals of achieving sustainability are shared, the 12 principles have some commonality with the 12 principles of green chemistry. For completeness, the 12 principles are reproduced below.
THE TWELVE PRINCIPLES OF GREEN ENGINEERING
PRINCIPLE 1: Inherent Rather Than Circumstantial
Designers need to strive to ensure that all material and energy inputs and outputs are as inherently non-hazardous as possible.
PRINCIPLE 2: Prevention Instead of Treatment
It is better to prevent waste than to treat or clean up waste after it is formed.
PRINCIPLE 3: Design for Separation
Separation and purification operations should be a component of the design framework and designed to minimize energy consumption and materials use.
PRINCIPLE 4: Maximize Efficiency
Products, processes, and systems should be designed to maximize mass, energy, space, and time efficiency.
PRINCIPLE 5: Output-Pulled Versus Input-Pushed
Products, processes, and systems should be 'output pulled' rather than 'input pushed' through the use of energy and materials.
PRINCIPLE 6: Conserve Complexity
Embedded entropy and complexity must be viewed as an investment when making design choices on recycle, reuse, or beneficial disposition.
PRINCIPLE 7: Durability Rather Than Immortality
Targeted durability, not immortality, should be a design goal.
PRINCIPLE 8: Meet Need, Minimize Excess
Design for unnecessary capacity or capability (e.g. 'one size fits all') solutions should be considered a design flaw.
PRINCIPLE 9: Minimize Material Diversity
Material diversity in multicomponent products should be minimized to promote disassembly and value retention.
PRINCIPLE 10: Integrate Material and Energy Flows
Design of products, processes and systems must include integration and interconnectivity with available energy and materials flows.
PRINCIPLE 11: Design for Commercial 'Afterlife'
Products, processes, and systems should be designed for performance in a commercial 'afterlife.'
PRINCIPLE 12: Renewable Rather Than Depleting
Material and energy inputs should be renewable rather than depleting.
Reprinted with permission from: J. B. Zimmerman and P. T. Anastas, Environmental Science and Technology, 2003, 37, 94A–101A, copyright American Chemical Society, 2003.
There are also other sets of green engineering principles including the Sandestin Green Engineering Principles and the Hannover Principles.
For examples of how to apply the 12 principles of green engineering see 'Design Through the Twelve Principles of Green Chemistry' and 'EcoWorx, Green Engineering principles in Practice'.
1.3 THE WASTE MANAGEMENT HIERARCHY FOR PROCESS SELECTION
Waste minimisation is essentially embodied in the second principle of green engineering: 'It is better to prevent waste than to treat or clean up waste after it is formed'. In order to put this principle into practice, either to choose a method/technology/process for dealing with a waste stream, or to choose between alternative technologies when designing a new process or process option, so that the minimal amount of waste/environmental impact is produced, the hierarchy of waste management should be followed (Table 1.1).
Best practice is to start at the top of the hierarchy (i.e. eliminating the waste) and if this is not possible, to systematically test options working downwards towards the bottom (least preferable). That is, try to eliminate, reduce or recycle the emission streams from a process first by looking at the source of the emissions. Failing that, try to recover energy from the waste stream, then treat the waste stream to lessen the risk and environmental impact, and finally discharge the resulting stream to the environment. Disposal with no treatment is the least preferable option. This hierarchy has been made law in many countries and regions, including the European Union.
The nearer a chemical process and/or chemical technology is to the top of the hierarchy of waste management practice, the more sustainable it is (especially in terms of the environmental bottom line).
1.4 TAKING A LIFE CYCLE APPROACH
To determine the environmental impacts of a particular set of processes, new material or product, or to compare processes and products, one requires rigorous metrics that can be objectively applied to assist decision making. Life cycle assessment (LCA) is widely accepted as an appropriate technique to provide the data for evaluation of environmental impacts, through all stages of the lifetime of a product from raw materials extraction to final disposal or recycling (cradle to grave). LCA is defined by a set of international standards: ISO 14040: LCA – Principles and Framework and ISO 14044: LCA – Requirements and Guidelines (see ref. 11 for a review).
Excerpted from Chemical Processes for a Sustainable Future by Trevor M. Letcher, Janet L. Scott, Darrell A. Patterson. Copyright © 2015 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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