Credit: Mr Thinktank, CC BY 2.0

Industrial and manufacturing firms commonly rely on solvents for a multitude of tasks: making products, cleaning and degreasing machinery and surfaces, working with materials such as coatings and paints and facilitating chemical reactions. Many corporate sustainability programs are seeking eco-friendly solutions around the employment of organic solvents, which are volatile organic compounds (VOCs) that have environmental and health effects.

Many organic solvents are carcinogenic or toxic. According to the Occupational Safety and Health Administration (OSHA) of the U.S. Department of Labor, millions of U.S. workers are exposed to solvents on a daily basis. Solvents present health hazards, the agency says, including “toxicity to the nervous system, reproductive damage, liver and kidney damage, respiratory impairment, cancer and dermatitis.”

In the United Kingdom, discussing solvents’ environmental effects, the UK government says:

Organic solvents react in the atmosphere in sunlight, producing an air pollutant known as “ground-level ozone.” High concentrations of ground-level ozone seriously affect human, animal and plant health. They also harm building materials, forests and crops.

The UK government says monitoring and managing solvent use and emissions not only help businesses meet regulatory requirements but also save them money.

Greening the Current Uses of Solvents

Doris Schulz, a communication consultant specializing in surface treatment, recently wrote in Sustainable Plant that a key to greener industrial cleaning is simply to determine whether a process in question can be done with water:

Before deciding to use solvents for cleaning, it’s advisable to run a series of cleaning tests to determine whether or not the required level of cleanliness can be achieved with a water-based cleaning process. It’s also a good idea to conduct an evaluation of economic efficiency.

Traditional solvents generally work better for heavily oiled parts or parts that are difficult to dry or have complex shapes, Schulz says. In certain industries, such as aviation and aerospace, the requirements might call for “flawless degreasing” that can only be achieved with traditional solvents. Some industries, such as electronics, might have requirements for “good material compatibility” or non-corrosive cleaning agents that point to conventional solvents.

In the absence of such requirements, an assessment might reveal opportunities for employing solvents with less damaging environmental and health impacts.

Another reason for greener solvent practices is energy conservation. Schulz writes:

Due to the fact that solvent regeneration is the most energy-intensive step in this type of cleaning process, modern solvent systems are equipped with heat recovery devices. These devices are being continuously improved. Today’s systems also are equipped with automatic distillation power adjustment to adapt to actual conditioning requirements, thus further reducing energy consumption and operating costs.

Automotive recycling. Gage Products Co., specialists in solvent recycling, discusses greener processes for automotive painting operations in a white paper, “How to Effectively Use Solvents While Controlling Environmental Impacts.” According to the report, paint application areas are, by far, the largest areas of solvent usage in automobile assembly plants.

Gage Products says a key target area for improving environmental performance is purging operations, when it’s time to switch from one color to another. The operation should aim for closed-loop recycling, in which used solvent is sent off-site for remanufacturing and brought back and reused for the same process. The amount wasted or emitted should be monitored, reported and kept minimally. The report stresses that “society can no longer tolerate one-time use and disposal” of solvents, although “obviously, the recycled product cannot affect finished product quality.”

Beside the environmental benefits, Gage Products stresses that such recycling processes increase the efficiency and reduce the costs of paint application systems.

Keys to Green Solvent Development

I asked Philip G. Jessop, Canada research chair in green chemistry at Queen’s University, in Kingston, Ontario, about green solvents. He tells me that “avoiding excessive energy consumption is one of the principles of green chemistry,” and that presents a key challenge in developing greener alternatives. “The problem is that solvents need to be removed after use,” he notes, and some important solvents require a great deal of energy use in their removal — for example, by distillation.

Jessop wrote in the journal Green Chemistry (“Searching for green solvents,” 2011, no. 13) that if researchers are interested in decreasing solvent-related environmental damage, so far they are looking in the wrong places. In reviewing the research that’s being done in the field of green chemistry, he discovered that most papers being published address solvents that are reaction media for synthesis.

He says, “Making such processes greener is not going to make much difference in the overall environmental impact of solvents because only a small fraction of the volume of solvents used in industrial activities is for chemical synthesis.” Focusing on the minor applications for solvents is only going to yield minor benefits.

Jessop says researchers who are interested in reducing the environmental damage of solvents should focus their efforts on four “grand challenges”:

1. Ensuring “that green solvents are available as replacements for non-green solvents of every kind.” Currently, this is not the case, he says, so “process chemists and engineers looking for solvents having certain desirable properties will often be unable to choose a green solvent.”
2. Learning to recognize “a green solvent when you see one.” Products are often promoted as “green solvents” without proof, says Jessop. Alternative products are proposed based just on one characteristic, such as being derived from biomass. To be truly green, says Jessop, a solvent has to be evaluated on all of its characteristics and on the environmental impact from its total life cycle, including its production.
3. Developing “easy-to-remove polar aprotic solvents.” This pillar is obviously important and makes a lot of sense to chemists but not so much to environmental columnists. Jessop explains to me that “polar aprotic solvents are, indeed, a very useful class of solvents; lots of processes in industry use them, but “it takes too much energy to remove these particular solvents.”
4. Finding ways to separate solvents from products without distillation. Solvents are frequently used in crucial manufacturing steps but then have to be removed. Distillation is commonly used, but it requires a lot of energy and involves the use of VOCs, which come with all of their attendant health and environmental problems.

Jessop believes meeting these four challenges requires that green chemistry researchers adjust their focus to areas that will have more positive effects. “Those of us who are motivated to reduce solvent-related environmental damage would do well to look at the most polluting applications of solvents, to determine the problems that currently make those applications less than green and to focus our research efforts on potential solutions to those problems,” he says.

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