This chapter is from the book
This chapter is from the book
This chapter is from the book
1.4 Sustainability
A sustainable product is a product that meets the current needs without compromising the ability of future generations to meet their needs while protecting human health and the needs of society. Chemical engineers are ideally suited to evaluate the sustainability of a wide range of products and technologies, and therefore, we present an overview of sustainability and green engineering as an example of one of the ways that chemical engineering can contribute to society today and into the future.
Current estimates indicate that the societies of the world are consuming 50% more resources (e.g., energy, water, minerals, the ability to produce food) than the world can sustain (what the population of the world consumes in one year requires 18 months to replenish). The United States consumes natural resources at a rate that is more than 13 times the rate for the rest of the world. With the rapid growth in the economies of China and India, who make up almost 40% of the world’s population, the world resource consumption rate is expected to accelerate.
In addition to preserving resources for the future, sustainable engineering involves protecting human health and the needs of society. That is, the effects of pollution and the impact on global warming are factors that also should be considered in any sustainable design project. As a result, a number of engineering professional groups are concerned that sustainability should be an integral part of future designs. That is, sustainability, or at least improved sustainability, should be an objective for engineering design work as opposed to basing design solely on minimum cost or maximum profit without regard to the impact on sustainability. The problem is that a sustainable design will, in general, cost more to implement than its nonsustainable counterpart. Therefore, the challenge for you as a future engineer is to develop new approaches that make sustainability as economically viable as possible.
As an example, consider a sustainable design for a building. The following features are aspects related to a sustainable design of a building:
Nontoxic construction materials that can be produced from recycled materials using low-energy processing techniques
Energy efficient design (e.g., low heat transfer rates to or from the building) using materials that require low amounts of energy to produce
Renewable energy sources (solar panels, solar water heaters, etc.)
High durability for the building, yielding a long service life; materials that develop character as they age
Interior and exterior appearance as similar as possible to nature (i.e., producing a soothing environment for humans)
Designing for a low total carbon footprint (i.e., the total carbon dioxide liberated during the production of the materials used in the building and the process of constructing the building)
Using biomimicry (i.e., redesigning industrial processes along biological lines to produce building materials)
Transferring ownership from an individual to a group of people, similar to car sharing
Employing renewable materials that come from nearby sources
As you can see from this list, the design problem becomes more complicated when a more holistic approach to engineering is used, but on the other hand, this approach creates more opportunities for creative solutions.
1.4.1 Life-Cycle Analysis
A life-cycle analysis is a comprehensive method for developing a sustainable design (green engineering). A life-cycle analysis not only considers the effect of a product on the environment and on important resources but also considers all the steps used to produce a product and what happens to the product after its useful life has ended. Figure 1.2 shows a schematic example of a life-cycle analysis of a product.
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Figure 1.2 Schematic representation of a life-cycle analysis
As illustrated in Figure 1.2, raw materials are extracted from the earth, such as minerals and crude oil. These raw materials are refined into useable products, such as metals and chemical products, in a material processing operation. Next, these useable materials are used to manufacture parts of the final product and are assembled into the final product. Then, the product is used for its intended purpose for the life of the product. When the useful life of the product ends, the product must be disposed of and/or recycled. The recycling process recovers all or part of the product and returns the recovered material so that it can be used for other products in the future. Each of these steps, from raw material extraction to recycling materials, in general, requires the use of resources (e.g., energy) and has an environmental impact (e.g., generates pollutants), which is indicated by the two oppositely pointing arrows used in Figure 1.2. That is, each step in the life-cycle analysis generally requires resource consumption and results in pollution generation.
When a life-cycle analysis is used for a sustainable design, all the required resources and all the resulting loads on the environment are considered. Moreover, the effect of the design of the product on the ability of the product to be recycled at the end of life should be considered.
Now consider how a life-cycle analysis would be applied to the four types of engineering designs: a device, a process, software, and services. The elements of a life-cycle analysis for a typical device are shown in Figure 1.2: The materials that compose the device must be extracted, refined, and manufactured into parts for the device. Then the parts are assembled into the device. And after the useful life is complete, material recycle can be used.
For a process, the elements of a life-cycle analysis closely follow the schematic in Figure 1.2. Moreover, the hardware elements used to implement a process (e.g., vessels, pumps, and processing equipment) are devices so that, with regard to the hardware of the process, the components of the life-cycle analysis are exactly the same as those used for a device. From an overall point of view, the pollution generated and the resources consumed during the useful life of a process would be expected to be the primary factors affecting resources and the environment far exceeding those associated with the hardware.
The life-cycle analysis of software and services is quite different from that of a device or process. In general, the impact of software and services on the resources and the environment is considerably less, although not always insignificant. For example, software that manages the operation of an automobile engine can have a significant effect on the resources and the environment during its useful life, while the development of the software itself would not have a significant effect.
A life-cycle analysis for a product can be simplified by using a database that provides the resource and environmental loads of a number of materials, including metals, plastics, and chemical feedstocks. Such a database eliminates the need to calculate the resource and environmental loads associated with the corresponding extraction and material processing and possibly manufacturing, as shown in Figure 1.2. The US Life Cycle Inventory Database (www.nrel.gov/lci) provides extensive information on metals, plastics, agricultural products, chemical feedstocks, services, and so on. A number of commercial databases are also available.
Example 1.1 Comparison of the Production of Ethanol and Gasoline Based on a Life-Cycle Analysis
Solution
Figure E1.1 shows schematics of the life-cycle analysis for the production of EtOH from corn and for the production of gasoline as a motor fuel. Note that for the production of EtOH, corn is produced by farming, which requires the use of fertilizers. The corn is used to produce EtOH using a fermentation and recovery process. The primary energy consumption and generation of GHG emissions occurs in the production of the fertilizer, growing the corn, and converting the corn to EtOH. In contrast, gasoline is produced by extracting crude oil from underground formations and refining the crude oil into gasoline and other products such as jet fuel, diesel, and lube oil. For gasoline, the primary energy consumption and generation of GHG emissions occurs during the crude oil production and refining, where refining is the largest contributor.
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Figure E1.1 Schematic of life-cycle analysis for EtOH from corn and gasoline
From Figure E1.1 you can see that a number of operations must be considered when performing this life-cycle analysis. Moreover, the expertise required to develop a life-cycle analysis for a case like this is expansive, requiring a multi-disciplinary team. For example, in order to develop a quantitative life-cycle analysis for EtOH from corn, expertise in agricultural operations, soil science, fermentation, separation science, and process systems is required. The US Department of Energy1 performed such a life-cycle analysis study and found that EtOH from corn used in motor fuel (i.e., gasohol) reduced GHG emissions by 20%.
1.4.2 Materials Sustainability
Materials are a natural part of most design projects. Therefore, it is important to use green, sustainable materials as much as possible. A life-cycle analysis is an excellent method to assess the sustainability of materials used in a project. Following the schematic of a life-cycle analysis shown in Figure 1.2, the initial impact on sustainability is the extraction of the material and its refining. The lower the concentration of a material that is extracted, in general, the more the energy required to refine it. The next key aspect is whether the material directly contributes to the environmental load during the useful life of the product of the design project. For example, certain pesticides can evaporate into the atmosphere and affect human health. And the final aspect is whether the materials can be reused or recycled after the useful life of the product. In summary, an ideal green material
Can be extracted and refined without undue use of resources and without significant environmental emissions.
Does not contribute to environmental releases during its useful life.
Can be reused or recycled to a significant degree.
Table 1.1 lists the abundance of selected elements in the earth’s crust. Even though gold makes up only 0.004 ppm of the total earth’s crust, highly concentrated deposits of gold have been found, thus simplifying its recovery. In contrast, rare earth metals, some of which make up more of the earth’s crust than gold, are found only in ores at relatively low concentrations. In terms of sustainability, abundance is only one factor. That is, the abundance and the net consumption from use together determine whether an adequate supply of a material is available. Listed in Table 1.2 are minerals that have been identified as having a limited supply based on their use in 1995.
Table 1.1 Mass Abundance of Selected Elemen ts in Earth’s Crust2
Element |
Mass Fr. |
Element |
Mass Fr. |
Element |
Mass Fr. |
||
|---|---|---|---|---|---|---|---|
O |
46.4% |
S |
0.03% |
Pb |
12 ppm |
||
Si |
28.2% |
C |
0.02% |
U |
2.7 ppm |
||
Al |
8.2% |
V |
0.01% |
Sn |
2.0 ppm |
||
Fe |
5.6% |
Cl |
0.01% |
As |
1.8 ppm |
||
Ca |
4.1% |
Cr |
0.01% |
Mo |
1.5 ppm |
||
Na |
2.4% |
Ni |
75 ppm |
W |
1.5 ppm |
||
Mg |
2.3% |
Zn |
70 ppm |
Bi |
0.17 ppm |
||
K |
2.1% |
Cu |
55 ppm |
Pd |
0.15 ppm |
||
Ti |
0.6% |
Co |
25 ppm |
Hg |
0.08 ppm |
||
P |
0.1% |
Li |
20 ppm |
Ag |
0.07 ppm |
||
Mn |
0.1% |
N |
20 ppm |
Pt |
0.005 ppm |
||
Fl |
0.06% |
Ga |
15 ppm |
Au |
0.004 ppm |
Table 1.2 Elements with a Limited Supply3
Degree of Supply |
Elements |
|---|---|
Potentially highly limited |
Ag, Au, Cu, As, Se, Te, Zn, Cd |
Potentially limited |
Co, Cr, Mo, Ni, Pb, Pt, Ir |
Besides elements, the sustainability of materials that result from plant growth, such as crude oil and lumber, should be considered. For example, the production of lumber results in a removal of GHGs from the atmosphere even though the milling and transportation will generate some GHGs. Of course, the consumption of crude oil, in general, results in significant GHG emissions.
With regard to the availability and supply of a material, as the supply of a material decreases, the market price for that material tends to increase. For example, during the mid-1970s, a shortage of crude oil dramatically increased the price of crude oil and, as a result, the price of gasoline. This price increase for crude stimulated exploration for crude oil as well as conservation efforts. Therefore, by the early 1980s, there was an excess of crude on the market and the price of crude oil dropped dramatically.
Another important natural resource is phosphorus. Before the advent of modern farming practices, farmers used wastes (e.g., compost) to return phosphorus to their soil after their crops consumed it during their growth cycle. Today, most farmers use inorganic phosphate to fertilize their crops. Estimates predict that currently known reserves of phosphate (i.e., a source of phosphorus) will be exhausted in 80 years at the current consumption rate. What this means is that the current phosphate reserves that are easy to extract and refine will be exhausted. Even if new high-quality phosphate reserves are not identified, an expected increased price of phosphate should drive the processing of lower-quality phosphate reserves and the extraction of phosphorus from waste material. In addition, an increase in the cost of phosphate should also encourage farmers to use their fertilizer more efficiently.
With regard to the elements in Table 1.1 that have been identified as having a potentially limited supply, if the increased consumption of one of these elements begins to reduce its supply, the price of that element would be expected to increase. This increase in price would stimulate increased exploration for it and could possibly make ores that were previously uneconomical to refine financially viable for refining. In addition, the increased price of the material would be expected to increase the recovery from end-of-life products and recycling. During the design phase of a project, the use of a material with a potentially limited supply should be viewed as increasing the overall risk of the project. That is, a material with a potentially limited supply can be susceptible to significant price increases in the future, which could affect the economic viability of a project.
1.4.3 Environmental Releases and Toxicity
The release of chemicals during extraction, refining, use, or end of life can result in a significant environmental load affecting human health or the health of ecosystems.
Figure 1.3 illustrates the connection between certain emissions (i.e., pollution) and human health and the health of ecosystems. Pollution results in global warming, ozone depletion, smog formation, acid rain, and so on. These in turn affect human health, cause damage to ecosystems, and disrupt human activities (e.g., increased damage from natural disasters). For example, consider the emission of chlorofluorocarbons (CFCs). CFCs damage ozone in the stratosphere, resulting in an increase in UVB radiation on the surface of the earth, which in turn increases skin cancer and the occurrence of cataracts and causes human immune system suppression, crop damage, and damage to marine life.4
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Figure 1.3 Overall effect of the emission of pollutants
Roughly 2000 new chemicals are introduced each year, and companies, government agencies, and the public need a method to evaluate the potential risks of these new chemicals. Evaluation is done by using screening tests, and in certain cases, extensive testing is required. That is, screening is used to identify which chemicals have the potential to be environmentally risky, and then extensive testing is used to evaluate those chemical to determine if they, in fact, represent a significant risk to human health and the health of ecosystems.
Table 1.3 outlines an approach used to screen chemicals for environmental risks. The first entry (dispersion and fate) has to do with the tendency of the chemical to accumulate in water, air, soil, and living organisms. The second entry (degradation rates) relates to how quickly the chemical is degraded in water, air, soil, and living organisms. The category “uptake by organisms” is related to specific factors that affect uptake and degradation in organisms while “uptake by humans” considers the rates at which the chemical is able to enter the human body and the rates at which the human body is able to expel it or degrade it into a nontoxic form. “Toxicity and other health effects” has to do with how the level of exposure to the chemical affects the health of organisms and humans.
Table 1.3 Chemical Properties Needed to Perform Environmental Risk Screenings5
Aspects Affecting the Environment |
Relevant Properties |
|---|---|
Dispersion and fate |
Volatility, density, melting point, water solubility, soil sorption coefficient, effectiveness of wastewater treatment |
Degradation rates in the environment |
Atmospheric oxidation rate, aqueous hydrolysis rate, rate of degradation by sunlight, rate of microbial degradation, adsorption |
Uptake by organisms |
Volatility, the ability to dissolve in fat, molecular size, degradation rate in an organism |
Uptake by humans |
Transport through the skin, transport rates across lung membrane, degradation rates within the human body |
Toxicity and other health effects |
Dose–response relationships |
If the screening process determines that a chemical could pose a significant risk to human health or to ecosystems, a detailed assessment of the impact of the chemical on the environment would be required. This assessment would involve testing with a range of laboratory animals with a range of exposure scenarios (e.g., exposure to a single large dose, exposure to multiple smaller doses, and continuous exposure to a low-dose level). The results of these studies may include the degree of reduction in life expectancy as well as the characteristics of the offspring of the laboratory animal.
1.4.4 Principles of Green Engineering
To effectively address the full range of design cases, it is necessary to have a general set of guidelines that, when applied, ensure a sustainable design. A number of guidelines for sustainable design have been developed.6
Following is an overview of the key factors identified here as the principles of green engineering, which are based on previous work in this area:
Use energy and material inputs that are as inherently nonhazardous as possible. Because material and energy input have such an important effect on the sustainability of a product, it is important to ensure that they are as nonhazardous as possible. When hazardous materials are used, special controls and planning for adverse conditions are required, which increases the cost and complexity of a design.
Minimize wastes. The generation of wastes creates special problems due to the difficulty and cost associated with dealing with wastes, especially hazardous wastes. Therefore, during the design phase, it is important to minimize waste generation. In certain cases, it is possible to find a use for a “waste product,” such as using it as a feedstock for another process. For example, during the early days of crude oil refining, natural gas was considered a waste product and was burned into the atmosphere until it was determined that natural gas could be used for heating homes and businesses. In other cases, it may be possible to modify the chemistry of a reaction so that waste products are eliminated or at least significantly reduced.
Minimize energy consumption. In certain industries (e.g., the refining, petrochemical, and mineral purification industries), the energy usage for purification is a primary operating expense. In these cases, energy usage can be significantly reduced using heat integration, that is, using waste heat (i.e., cooling that is required in the process) from one part of the process to provide heat to another part of the process.
Minimize material usage. Material usage can be minimized by designing the system or process so that the greatest possible conversion of the feed material to the product is obtained. Material use can also be minimized by extracting the feed components from waste streams so that they can be recycled to the process or using components from waste streams as a feedstock for other processes.
Apply just-in-time manufacturing. Just-in-time manufacturing is manufacturing that meets the demand for a product precisely when the product is needed, thus eliminating wastes and reducing the need for inventory. In terms of design, this means designing a system or process precisely for the expected demand and completing the project so that the product of the design is available only when the demand is present.
Design for proper durability. Proper durability means that a product is designed to last for the designed useful life and afterwards can be easily transformed into materials that can be recycled or reused or that can degrade into environmentally benign products.
Design for recycling or reuse after end of life. Materials can be recycled after end-of-life use by designing the product so that material recycling is simple and easy to implement. For example, scarce materials can be used in a way that facilitates their recovery for recycle. Components of a product can be designed so that they can be used in future generation devices (e.g., parts of a cell phone). Also, products can be designed for reuse (e.g., soft drink bottles).
Use a life-cycle analysis to minimize the environmental impact of the project. A life-cycle analysis is the most complete way to evaluate the impact of a project on the environment and natural resources. In this manner, the key areas of environmental load and resource depletion can be identified and addressed. For example, if the areas of material extraction and refining are the primary contributors to environmental load and resource depletion for a project, recycling would clearly be the best approach to improve the sustainability of the project.
Use renewable sources of energy and materials. The use of renewable sources of energy and materials reduces environmental impact and resource depletion. Solar panels are an example of a renewable energy source, and lumber is an example of a renewable material.
Engage both communities and stakeholders in the project. It is critically important to involve local communities from the conceptual stages through the completion of a design project when the project has any real or perceived impact on the local community. In addition, sustainable goals can sometimes be met by affecting a change in social behavior (e.g., the development of autonomous vehicles so that the use of vehicles is shared).
The principles of green engineering are really a checklist of factors that should be considered during the design process; otherwise, the design can be less sustainable than it could have been.
1.4.5 Optimization and Sustainable Engineering
Each of the principles of green engineering presented here should be applied in a balanced fashion. For example, strictly minimizing the energy consumption may result in an excess use of materials. That is, all the relevant factors, including the impact on the environmental load and ecosystems as well as the material and energy cost, should be considered when optimizing a sustainable design project (i.e., finding the optimal sustainable design).
The conventional approach to optimization of a design project neglecting the impact on the environment and society is to consider the costs associated with the end product of the design along with the expected income generation to identify the optimum design over the life of the product considering the time value of money.
When the impact on the environment and society are considered during the design process, the problem arises that the environmental and the societal impact are not easily represented on a monetary basis. Nevertheless, several different approaches are available for including the impact on the environment and society in the design process.
The most direct means of including the impact on resources and the environment is to follow government regulations. For this case, the government regulations would represent constraints on the design process that have to be satisfied for any valid design. For example, maximum SO2 emissions are set by EPA regulations for coal-fired electric utilities. While there are certain cases where this approach is valid (e.g., the maximum safe chemical concentrations), it is not feasible to develop government regulations for the full range of factors that affect sustainability. For example, GHG emissions do not lend themselves to explicit limits.
Another approach is to rate products on the basis of their total impact on resources and the environment. For example, new home construction can be rated according to the total resource depletion and total pollution generation. Ratings such as a silver, gold, or platinum can be assigned to a new house based on sustainability and, of course, a platinum rating will command a higher price than a gold rating, which will command a higher price than a silver rating. The success of this approach depends on the judicious selection of the criterion to quality for each classification, taking into account the costs necessary to qualify for each classification and the resulting benefit to resources and the environment. However, not all design projects fit into this approach.
Another approach is to estimate the total cost to society of specific emissions. Total cost to society includes increases in medical costs, lost productivity, and a reduction in life expectancy. For example, the EPA has estimated the total cost to society for CO2, CH4, and NO2 emissions. In this manner, the economic cost of GHG emissions can be considered directly during the optimization of a design project using a life-cycle analysis combined with values for the cost to society for the pollutants under consideration. This approach has generated considerable controversy and was removed from the EPA website in January 2017.