Chemical engineers have always been at the forefront of environmental protection. With a unique perspective that straddles both science and engineering, they work in teams with other professionals.
By designing complex solutions to our vexing environmental challenges, chemical engineers are striving to save the world we live in. One success is the conversion of the sulfur oxides in power plant gases into gypsum for use in wallboard. The removal of trace contaminants from drinking water by reverse osmosis is another.
Cars and the environment
We can breathe more easily thanks to the contributions of chemical engineers. By designing more efficient engines that produce fewer hazardous pollutants, they have helped reduce the environmental impact of gasoline- and diesel-powered cars, buses, and trucks.
Clearing the air
Compressed natural gas, sold at this station, is one product chemical engineers are working on to reduce the air pollution generated by fossil fuel-burning vehicles. Courtesy DOE/NREL.
Cars, trucks, and buses are essential for transportation and freight delivery around the world. However, the exhaust from the gasoline- and diesel-powered engines required to propel these vehicles has been a major cause of air pollution.
Chemical engineers, working with scientists and other engineers, have helped devise ways to cost effectively reduce the amount of pollution produced by petroleum-derived, fuel-burning engines. Key developments include:
- Improved engines with more efficient fuel- and air-management systems,
- Catalytic devices that destroy pollutants found in exhaust tailpipes, and
- Advanced petroleum-refining techniques that produce cleaner-burning fuels.
The catalytic converter is considered one of the most important contributions to the field of air-pollution control. It is now a standard feature on vehicles everywhere. It destroys the three main pollutants found in engine exhaust.
The converter consists of a porous honeycomb ceramic base material coated with a precious metal catalyst. The honeycomb structure provides high catalyst surface area, which maximizes the contact between the catalysts and the pollutants in the hot exhaust gases.
When this novel structure was first invented, it featured two distinct chemical engineering advantages:
- It maximized the amount of catalyst-coated surface area to which the engine exhaust may be exposed, and
- It minimized the amount of expensive precious-metal catalyst required.
To recognize how important catalytic converters are in environmental protection, President George W. Bush awarded the inventors of the catalytic converter, the chemical engineer John Mooney and the chemist Carl Keith, with the 2002 National Medal of Technology, the highest honor given for innovation in the United States.
Another way chemical engineers help reduce automotive air pollution is through advanced petroleum-refining techniques. One example is hydrotreatment, which uses hydrogen gas and a catalyst to produce gasoline and diesel fuel with significantly lower levels of sulfur and lead.
These techniques have made it possible to produce reformulated fuels that function as effectively as earlier leaded fuels, while releasing fewer pollutants.
Smokestacks, which were once the stereotypical image of factories and power plants, no longer belch black smoke into the air. Because of innovations by chemical engineers, industrial facilities now capture and neutralize air pollutants before they can be discharged into the environment.
Blue skies ahead
Pollution-control systems developed by chemical engineers generate clean stack gas containing steam instead of the smoky flue gases produced by power plant stacks, as shown in this artist’s rendering. Courtesy Constellation Energy.
Chemical engineers have helped provide new technologies to enable electric power plants and industrial facilities to significantly reduce such harmful airborne emissions as:
- Sulfur dioxide (SO2),
- Nitrogen oxides (collectively called NOx),
- Mercury, and
- Unburned hydrocarbons.
Reducing industrial air pollution
SO2 and NOx react with water to create acid gases, which in turn lead to acid rain. Acid rain damages cars and buildings, kills trees, destroys lakes and streams, and leads to respiratory and other health problems.
Chemical engineers developed flue gas desulfurization (FGD), now a widely used method of reducing acid gases in smokestacks. FGD works by using a wet scrubber spray tower in the flue or smokestack. During operation, acid gases are converted to neutral salts and other solid by-products, which are then removed.
Solutions to NOx emissions include selective catalytic reduction (SCR) systems that convert NOx emissions to harmless nitrogen gas and water.
Through the inventiveness of chemical engineers, wet scrubbers, SCR systems, and other pollution-control technologies have significantly reduced the amount of SO2, NOx, and other harmful emissions being released into the atmosphere. Specifically, the U.S. electric power industry has
- Reduced SO2 emissions in the United States by more than 5.5 million tons per year since 1990,
- Reduced NOx emissions by about 3 million tons per year since 1990, and
- Reduced acid rain deposition in the United States and Canada.
Cleaner coal use
Coal remains the cheapest and most plentiful of all the fossil fuels. However, it is also the most polluting. Chemical engineers have worked to perfect coal gasification, a method to generate electricity and produce fuels from coal with significantly less environmental impact. Now utilities can burn clean synthetic gas made from coal and have considerably fewer emissions than with traditional pulverized coal combustion.
Clean water, which is essential to human health, is also necessary for numerous manufacturing processes. Many innovative methods of treating raw water to make it suitable for drinking or for use in manufacturing have been developed by chemical engineers. They also work on wastewater treatment to enhance safety and enable reuse.
Purifying drinking water, treating wastewater
With explosive population and industry growth, the need for cost-effective water-purification and wastewater-treatment technologies has become more urgent than ever. Chemical-engineering principles are used to remove harmful pollutants from both raw source water and contaminated wastewater.
Specifically, chemical engineers have developed cost-effective methods to:
- Purify water from subsurface aquifers and surface sources, such as rivers and lakes, to produce potable drinking water;
- Produce purified water that meets the increasingly strict requirements for industrial use; and
- Treat contaminated industrial and municipal wastewater and sewage to make them suitable either for discharge to public waterways or for reuse.
Modern-day treatment of raw water sources or contaminated wastewater employs a wide array of physical, chemical, and biological techniques.
Chemical engineers refer to separating dangerous materials from good water as a treatment train. At various stages in the multistage treatment process, unwanted constituents are separated using:
- Vacuum or pressure filtration,
- Membrane-based separation,
- Carbon-based and zeolite-based adsorption, and
- Advanced oxidation treatments.
Activated carbon is a highly adsorbent form of carbon that is produced when charcoal is heated. Its extremely intricate internal-pore structure provides exceptionally high internal surface area: just 5 grams of activated carbon has the surface area of a football field. Activated carbon removes impurities via adsorption from both aqueous and gaseous waste.
Membranes allow materials of a certain size or smaller to pass through but block the passage of larger materials. Imaginative arrays of membrane materials in innovative physical configurations are used to separate unwanted solids and dissolved chemicals from tainted water. During operation, purified water diffuses through the microporous membranes and collects on one side of the membrane, while impurities are captured and concentrated on the other side. In places like Key West and Saudi Arabia potable water is produced from seawater using membrane processes.
Today, membranes made from cellulose acetate, ceramics, and polymers are widely used. The applications come in a variety of innovative designs, including tubular, hollow-fiber, plate-and-frame, and spiral-wound configurations. The goals of membrane design are to:
- Maximize the available surface area,
- Reduce membrane pore size (to allow for the more precise removal of smaller contaminants),
- Minimize the pressure drop the fluid will experience when flowing through the unit, and
- Identify more cost-effective system designs.
Worldwide, about 85% of childhood sickness and 65% of adult diseases are thought to be produced by waterborne viruses, bacteria, and intestinal protozoa that cause diarrhea and other potentially life-threatening diseases. The addition of oxidizing agents—chemical ions that accept electrons—has proven effective against these microorganisms. Today, a variety of advanced oxidation techniques kill such disease agents and disinfect water, thanks to ongoing developments pioneered by the chemical-engineering community.
Historically, chlorine-based oxidation has been the most widely used, and it is very effective. However, the transportation, storage, and use of chlorine (which is highly toxic) present significant potential health and safety risks during water-treatment operations. To address these concerns chemical engineers and others have developed a variety of alternative oxidation treatments that are inherently safer, and in many cases more effective, than chlorination. These include:
- Ultraviolet light,
- Hydrogen peroxide, and
Each of these powerful oxidizing agents destroys unwanted organic contaminants and disinfects the treated water without the risks associated with chlorine use.
Recycle and reuse
One person's waste can become another person's treasure. Recycling post-consumer paper, metal, and plastic reduces the environmental impact of acquiring more raw materials. Chemical engineers help make recycling possible. In manufacturing, reusing industrial waste also offsets raw-material and energy requirements.
Turning waste into gold
In the 1970s, increasing environmental awareness renewed interest in recycling. Today, about 32% of the average 1,643 pounds of waste produced annually per person in the United States is recycled. Significantly less municipal waste is being dumped in our dwindling landfills. Chemical engineers have played a key role in building the post-consumer and industrial waste recycling industry.
Any successful recycling program must have three basic qualities:
- A suitable collection infrastructure,
- Appropriate reprocessing techniques to convert the waste into suitable end products, and
- A need or a market for the recycled products.
Chemical engineers and metallurgists have worked together for decades to perfect metal recycling techniques. Sometimes it is easy. For instance, stainless-steel cans can often be recycled directly back into the steel mill feed stream with little or no prior processing. Recycling aluminum, however, is more challenging.
The process for recycling aluminum was developed by chemical engineers in the 1960s, and aluminum is now one of the most widely recycled materials. Almost two-thirds of the aluminum cans in the United States are recycled, and 85% to 90% of the aluminum in cars is recycled.
Before aluminum is reused, all lacquer, paint, and labels are removed in a heated oven. Cans are then chopped into small pieces and added to a molten aluminum bath along with chemicals to remove any impurities. The remaining aluminum is formed into ingots for reuse by fabricators.
The widespread use of recycled aluminum saves energy and reduces pollution, because mining and processing raw bauxite ore to extract the aluminum it contains is very energy and waste intensive. Specifically,
- Each one ton of aluminum cans produced from recycled cans saves five tons of bauxite;
- The reuse of aluminum cans reduces air pollution by 99% and energy consumption by 95% compared with the production of virgin aluminum from bauxite; and
- The 54 billion cans that the United States recycled in 2006 saved the equivalent of 15 billion barrels of crude oil.
Paper is another post-consumer product that is now routinely recycled. Because paper mills cannot use recycled paper as a direct substitute for virgin tree pulp, chemical engineers have devised and optimized processes that involve:
- Blending recycled paper and water to produce a pulp slurry,
- Removing all inks and other performance chemicals in the paper, and
- Filtering the slurry to remove solid impurities.
One of the biggest technical hurdles chemical engineers had to overcome was the fact that recycled pulp has shorter fibers than virgin pulp. This characteristic makes the finished paper weaker and less attractive. By combining virgin pulp (typically from wood chips) with recycled pulp, chemical engineers solved the problem with a processing technique that produces newsprint and other recycled-paper products that meet all strength and aesthetic requirements.
Today, more than 70% of the newsprint in the United States is collected for reuse, significantly reducing both the disposal burden on landfills and the environmental costs of harvesting virgin wood.
Because plastics are used in so many aspects of our daily lives, they represent an ever-growing part of the nation's waste stream. In landfills they present a particular problem, as they do not degrade readily. In addition, significant amounts of crude oil and energy are used in producing plastics.
We can now recycle most plastics into useful products. Because of chemical-engineering innovations, plastics are separated by machine and reprocessed without significant material breakdown, enabling reuse of many such plastic products as pipe, toys, and decorations. This process not only protects our environment from plastic litter but also helps the United States become more energy independent.