At the risk of excluding many key environmental issues, the following are singled out as being closely related to the design of chemical products and processes.
Burning of Fossil Fuels for Power Generation and Transportation
Because fossil fuels are the predominant sources of power worldwide, their combustion products are a primary source of several pollutants, especially in the urban centers of industrialized nations. More specifically, effluent gases from burners and fires contain sizable concentrations of SO2, the nitrogen oxides (NOx), CO, CO2, soot, ash, and
unburned hydrocarbons. These, in turn, result in many environmental problems, including acid rain (principally concentrated in H2SO4), smog and hazes (concentrated in
NOx), the accumulation of the so-called greenhouse gas
(CO2), volatile toxic compounds (e.g., formaldehyde, phe-
nol), and organic gases (e.g., CO), which react with NOx,
especially on hot summer days, altering the O3level. As the
adverse impacts of pollutants on animals, plant life, and humans are being discovered by scientists and engineers, methods are sought to reduce their levels significantly. In some cases, this is accomplished by one of several methods, such as separating the sources (e.g., sulfur compounds) from fuels; adjusting the combustion process (e.g., by reducing the temperature and residence time of the flame to produce less NOx); separating soot, ash, and noxious compounds from
effluent gases; reacting the effluent gases in catalytic con- verters; or through the use of algae to consume (through photosynthesis) large quantities of CO2 in flue gases (a
recently proposed technique now under study). As a rule of thumb, it should be noted that the cost of cleaning combustion products is approximately an order of magnitude less than the cost of removing contaminants from fuel. This is an important heuristic, especially when designing processes that are energy intensive, requiring large quantities of fuel.
Sustainability and Life-Cycle Design
By selecting sustainable raw materials and producing sus- tainable products, designers attempt to meet the needs of society today while respecting the anticipated needs of future generations. Such choices are also intended to avoid harming the environment and limiting the choices of future gener- ations. In some cases, this translates to the use of so-called green raw materials and the production of so-called green products. These often help to resolve health problems, provide environmental protection, preserve natural resour- ces, and prevent climate change.
As mentioned above, the growing emphasis on sustain- ability is closely related to the increasing recognition of global warming due to the greenhouse effect as well as political problems associated with the traditional suppliers of oil and natural gas. Historically, most chemical products have been derived from methane, ethane, propane, and aromatics, nor- mally obtained from oil and natural gas. Furthermore, a large percentage of energy for manufacturing (on the order of 80 percent) and wastes produced in manufacturing (also on the order of 80 percent) are associated with the chemical industries, including petroleum refining, chemicals produc- tion, forest products, steel, aluminum, glass, and cement. To achieve sustainability while producing high-quality products, it is desirable to use small amounts of raw materials and energy, and to produce small amounts of waste.
When planning for sustainability in the 21st century, the rapid growth of the large developing nations, especially China and India, is important. Some estimates project that the world population will stabilize at 9–10 billion people, with the consumption of commodities (steel, chemicals, lumber, . . . ) increasing by factors of 5–6 and energy by a factor of 3.5. Furthermore, the choices of resources are complicated by sustainability considerations. Decisions to
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take advantage of today’s cheap prices and easy accessibility may result in expensive or inaccessible raw materials for future generations.
With the price of oil having quadrupled in just three years, there has been a move toward the usage of renewable ‘‘green’’ resources. Following the lead in Brazil over the past few decades, biomass (e.g., sugars, corn, and cellulosic wastes) is being converted to ethanol, principally as a gas- oline substitute. In addition, biomass has been used to produce chemicals (e.g., 1,3-propanediol and tetrahydro- furan). However, such carbon sources, when burned as fuel or incinerated as waste, produce carbon dioxide, and consequently, it has become increasingly important to find practical ways to sequester carbon dioxide rather than release it into the atmosphere. For these reasons, alternate energy sources, such as hydrogen, nuclear, wind, solar, and geo- thermal, are gaining increased attention.
It is also becoming common to consider the full life cycle when designing chemical products. A growing class of products, formed from biomass, are biodegradable. For example, biodegradable microcapsules carrying pharma- ceuticals are injected into the bloodstream for delayed drug delivery over extended periods on the order of one month. In these cases, the raw materials are renewable and there are no waste-disposal issues.
Handling of Toxic Wastes
In the chemical and nuclear power industries, large quantities of toxic wastes are produced annually, largely in wastewater streams, which in 1988 amounted to 97 percent of the wastes produced, as shown in the pie chart of Figure 1.8. While a small portion is incinerated (on the order of 3 percent in the late 1980s), the bulk is disposed of in or on the land, with a variety of methods having been introduced over the past century to bury these wastes. Since the late 1960s, many of the burial sites (e.g., Love Canal, Times Beach) have threat- ened the health of nearby residents and, more broadly, have threatened to contaminate the underground water supply throughout entire states and countries. In this regard, studies by the state of California have shown that aqueous waste streams from the processing of electronic materials are posing widespread threats to the groundwater in California’s Silicon Valley. In fact, this area has a leading number of sites on the U.S. National Priority List of toxic waste dumps (which is comprised of approximately 10,000 sites through- out the United States). In process design, it is essential that facilities be included to remove pollutants from wastewater streams. The design of mass-exchange networks (MENs) for this and other purposes is the subject of Chapter 10.
Bioaccumulated Chemicals
Probably the most well-known cases of chemicals that have been discovered to bioaccumulate in the soil and plant life are the insecticide DDT (1,1-bis(4-chlorophenyl)-2,2,2- trichloroethane; C14H9Cl5) and the solvent PCBs (poly-
chlorinated biphenyls). DDT was sprayed in large quantities by low-flying airplanes to kill insects and pests throughout the 1950s. Unfortunately, although effective for protecting crops, forests, and plant life, toxic effects in birds, animals, and humans were strongly suspected, as discussed in Section 1.6. Consequently, DDT was banned by the U.S. EPA in 1972. Its effect, however, will remain for some time due to its having bioaccumulated in the soil and plant life.
Toxic Metals and Minerals
In this category, major changes have taken place since the late 1960s in response to the discoveries of the toxic effects of lead, mercury, cadmium, and asbestos on animals and humans. After lead poisoning (accompanied by brain damage, disfigurement, and paralysis) was related to the ingestion of lead-based paints by children (especially in older buildings that are not well maintained), the EPA banned lead from paints as well as from fuels. In fuel, tetraelhyl lead had been used as an octane enhancer throughout the world. It was subsequently replaced by methyl tertiary-butyl ether (MTBE), which is also being replaced due to reports that it can contaminate ground water. Mercury, which has been the mainstay of manometers in chemistry laboratories, has similarly been found to be extremely toxic, with disastrous effects of accidental exposure and ingestion reported periodically. In the case of asbestos, its toxic effects have been known since the late 1940s, yet it remains a concern in all buildings built before then. Gradually, as these buildings are being renovated, sheets of asbestos insulation and asbestos ceiling tiles are being removed and replaced by nontoxic materials. Here, also, the incidents of asbestos poisoning are associated most often with older buildings that have not been well maintained.
Summary
As the adverse effects of these and other chemicals becomes better understood, chemical engineers are being called on to satisfy far stricter environmental regulations. In many cases, these regulations are imposed to be safe even before suffi- cient data are available to confirm toxic effects. For these reasons, chemical companies are carefully reexamining their existing products and processes, and evaluating all proposed
582 plants reporting 3.0% Solid Waste
6.7 million tons
97.0% Wastewater 213.2 million tons
Figure 1.8Hazardous waste generation in the United States in 1988 (Eisenhauer and McQueen, 1993)
plants to confirm that they are environmentally sound, at least insofar as meeting the regulations imposed, or anticipated to be imposed, by the environmental regulation agencies.