Reducing Solvent Emissions in the Laboratory

Processes such as concentration and evaporation generate tons of solvent vapors annually that are released into the atmosphere. In analytical chemistry laboratories, growing concern about the use and emission of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) centers on the environmental impact of solvents vented through fume hoods and the safety of laboratory personnel. Improperly vented solvent vapors can cause eye, nose and throat irritation; headaches; loss of coordination; and damage to the liver, kidney and central nervous system. HAPs like benzene, methylene chloride and formaldehyde are known to cause cancer and to have other serious health effects (see Table 1).

Table 1 – Effects of methylene chloride exposure on central nervous system

OSHA limits for allowable indoor DCM vapor levels are 25 ppm per eight-hour time weighted average and 125 ppm for short-term exposure. As per the U.S. EPA’s regulations for outdoor air quality, companies are required to apply for permits if they emit over 10 tons of specific solvents. Several firms have been heavily fined for failing to meet the guidelines set by state EPA branches.

Concern over DCM is not limited to the United States. Some countries in the European Union have implemented an environmental tax on chlorinated solvents to discourage their use. The use of DCM in other areas, for instance, as a component in paint strippers, has been banned.

Volatile organic compounds outdoors

VOCs are organic chemical compounds that evaporate readily under standard temperature and pressure. These include hexane, acetone, methylene chloride, benzene and formaldehyde. Since VOCs encompass such a wide range of compounds, health effects vary from relatively harmless to humans to acutely toxic. Once solvent vapors are safely contained in fume hoods for release to the outside atmosphere, the greatest concern is the formation of ground-level ozone. Although ozone is not directly emitted from any human-made sources, it is formed as nitrogen oxides (NOx) react with organic compounds in the presence of sunlight. The EPA defines VOCs based specifically around this as:

Volatile organic compounds (VOC) means [sic] any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates [sic] in atmospheric photochemical reactions.1

The definition specifically excludes compounds that have been designated by the EPA as having negligible photochemical reactivity, including methane, ethane and methylene chloride:

Although methylene chloride is exempted as a VOC for the purpose of outdoor ozone formation, it is important to note that it is still considered a hazardous air pollutant for other reasons, and is subject to strict regulations both indoors and out.

Ozone

While ozone is beneficial in the upper atmosphere, at ground level it contributes to smog formation and causes a host of health issues, including respiratory tract irritation, asthma attacks and damage to the lining of the lungs. Elevated levels of ozone in the air hinder plant growth as well, reducing crop yields by up to 50%. Ozone levels in the United States are highest in regions of California, Texas, Maryland, New York, New Jersey, Connecticut and Pennsylvania.

The EPA has set standards for maximum ground-level ozone concentrations. To be in compliance, locations cannot exceed 0.070 ppb ozone over an eight-hour average more than three times per year. States classified as “nonattainment areas” that have not met this limit are required to submit plans for reducing VOC, NOx and ozone within their boundaries. These plans take into account VOC and NOx sources throughout the state, including automobile exhaust, coating processes and industry use. Most of the regulations regarding VOCs originate at the state level and are dependent on the State Implementation Plan.

Reducing solvent emissions

There are several ways to lessen harmful solvent emissions. Some methods have been developed to reduce the amount of harmful substances used throughout the course of a reaction, either by performing the reaction under milder conditions (for example, in water), or without any liquid solvent at all. The idea is to eliminate emissions by completely eliminating the source of the pollution.

The challenge in this approach is the upfront investment in selecting and often developing new reactions. A solvent can be integral to the path of a reaction, even though it does not end up in the final product. Aqueous solutions have different properties than organic solutions, with different polarity. Additionally, reactions in solid state have access to a different range of intermediate products than reactions in solution due to the crystal lattice structure, and can end up with completely different final products. Once new methods have been developed, solvent-free reactions tend to have high yields, require little purification after synthesis and generate less waste. This means less money spent on chemicals, waste disposal and sample handling.

Another way to address laboratory solvent emissions is to reduce sample starting volumes. This often requires a change in instrumentation and to a lesser degree a change in method; however, new reactions do not need to be developed. Most solvent used during the procedure may still be emitted but, overall, less is emitted for the same number of tests or amount of product.

Figure 1 – Organomation S-EVAP Solvent evaporator for eight Kuderna-Danish flasks with centralized collection. Setups for individual collection at each position are available as well.

A third option for reducing emissions is to treat solvent vapor with methods such as adsorption, thermal destruction or condensation before the vapor can be released into the atmosphere. This is the easiest change to implement, requiring only equipment to capture and handle vapor downstream of evaporation. Up until the evaporation step, the procedure remains the same, which eliminates the need to validate a new method.

Adsorption works by passing vapor-laden gas across an adsorbent, for example, activated carbon or zeolite. VOC vapors transfer from the gas to the adsorbent and are trapped within its pores, scrubbing the gas stream of solvent. Adsorption can work well for relatively low concentrations of solvent vapor and would generally be installed as a facility-wide system.

Like adsorption, thermal destruction is typically installed as a facility-wide system. Solvent vapor is incinerated at high temperatures. This method is most effective at high vapor concentrations; at low concentrations, additional fuel must be added to maintain the temperature of the incineration chamber.

Condensation works by decreasing temperature or increasing pressure, forcing solvent vapor out of the air and into liquid form. It is most effective for high concentrations of solvent with low boiling points. Solvents reclaimed through condensation can either be recycled for use in less sensitive applications or safely disposed of as hazardous waste. The S-EVAP solvent evaporator (Organomation, Berlin, Mass.) recaptures up to 97% of the solvent it evaporates (see Figure 1).

Reference

  1. http://www.ecfr.gov/cgi-bin/text-idx?SID=3c8e1745d039017a99e95 4ee03c61206&mc=true&node=pt40.2.51&rgn=div5#se40.2.51_1100; Title 40, Chapter 1, Subchapter C, Part 51, Subpart F 51.100 (s).

Amy Valladares is a sales technician at Organomation Associates, 266 River Rd. W., Berlin, Mass. 01503-1699, U.S.A.; tel.: 888-838-7300; e-mail: sales@ organomation.com; www.organomation.com

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