Cryogenic condensation is an established technology for controlling emissions of organic solvent vapors classified as HAPs and VOCs. This pollution control technology can help companies meet emission standards while also bringing operational advantages to a production process.
Organic solvents are used in so many ways, in so many industries and in so many applications that it is difficult to imagine a future without them. Organic solvents are used commercially and industrially as cleaning, degreasing and sterilizing agents; in chemical synthesis; and in the production of surface coatings, inks and adhesives.
Although organic solvents have played an essential role in economic growth since the industrial revolution, exposure to a number of these solvents poses considerations to human health and environmental sustainability. According to the U.S. Environmental Protection Agency (EPA), organic solvent vapors that are known or suspected to be human carcinogens are classified as hazardous air pollutants (HAPs), and organic solvent vapors that are known or suspected to be highly photochemically reactive to form ground-level ozone are classified as volatile organic compounds (VOCs).
Organic solvents classified as VOCs and HAPs are regulated by the EPA through national air quality standards as part of the federal Clean Air Act. The EPA categorizes sources of air pollution (VOCs, HAPs, etc.) as:
• Mobile On-Road Sources. Cars and trucks.
• Mobile Nonroad Sources.Aircraft and agricultural field equipment.
• Nonpoint Sources. Stationary sources such as field burning, residential wood burning and small commercial sources such as local dry-cleaners.
• Point Sources. Large stationary sources of emissions like those of power plants, chemical plants, refineries and other heavy industrial facilities.
The federal criterion for point sources is the potential to emit more than 10 tons of a single air pollutant or more than 25 tons of a mixture of air pollutants a year. State and local air agencies regulate these point sources through construction and Title V operating air permits.
Specific emission requirements such as how much is allowed to be emitted per year or what pollution control technology needs to be implemented vary considerably depending upon the air quality of each area.
Areas classified as non-attainment zones, which have air quality that falls below the national standards, issue more stringent regulations to control air pollution than attainment zones, which are areas that are in compliance with the national air quality standards.
VOC emissions from all source types across the United States, as reported by the 2008 National Emissions Inventory, are illustrated in figure 2. Figure 3 shows a model of estimated lifetime cancer risks reported by the 2005 National-Scale Air Toxic Assessment based on HAP emissions from all sources. Figures 2 and 3 depict the general location of most non-attainment zones, which are found along the northeastern coast, the Gulf coast and the coast of southern California, all well-known heavy industry areas.
Several technologies have been implemented, and more continue to be developed, to reduce sources of air pollution and control the amount of air pollutants that are released into the environment. The latter are commonly known as air pollution control technologies and include flares, thermal or catalytic incineration, adsorption and volume concentrators, absorption, biofiltration, membrane technology, ultraviolet oxidation, plasma technology, and mechanical and cryogenic condensation.
Why Use Cryogenic Condensation?
Cryogenic condensation is an effective technology for controlling emissions of organic solvent vapors classified as HAPs and VOCs. This pollution control technology can help meet emission standards while also bringing operational advantages to a production process.
Developed in the 1980s and well-established across the European Union, cryogenic condensation is receiving considerable attention in the U.S. as a safe, clean and economic technology for controlling emissions of regulated organic solvents in the chemical, pharmaceutical and biotechnology industries.
Principles of Cryogenic Condensation
Cryogenic condensation uses liquid nitrogen to control the emissions of solvent vapors by taking advantage of the vapor-liquid equilibrium principle of multi-component mixtures. That is, as the temperature of a mixture is lowered, the saturation capacity of the carrier gas decreases, causing the concentration of components in the carrier gas to decrease as they condense into liquid droplets.
The refrigeration capacity and low boiling temperature (-320°F or -195°C) of liquid nitrogen gives these systems the flexibility to cool down gas streams from ambient to cryogenic temperatures, resulting in a wide range of control of the vapor-liquid equilibrium of any combination of solvent mixtures. Thus, it is possible to condense and recover practically every known organic solvent or mixture of solvents from gas streams at levels in excess of 99 percent.
Figure 4 shows the equilibrium gas-phase concentration of common solvents in gaseous nitrogen, the carrier gas, at various cryogenic temperatures. As an example, Figure 4 shows that a gas stream of nitrogen and dimethylformamide must be cooled down to -90°F (-68°C) to achieve an emission concentration of 10 parts per million by volume (PPMV) of dimethylformamide. The graph in figure 4 was generated using one company’s thermodynamic modeling software, which incorporates a proprietary chemical property database of components used in the chemical industry.
Although the principle of condensation is simple, the design and implementation of cryogenic condensation systems, with countless combinations of chemical components, concentrations and other process parameters, is not.
Careful design of heat exchangers is necessary to efficiently control the refrigeration value of liquid nitrogen, achieve the levels of emission control mandated by local air pollution agencies, and build the flexibility for systems to adjust to changes in regulation requirements or process conditions in the future.
Advantages of Cryogenic Condensation
When evaluating cryogenic condensation systems for emission control of regulated solvent vapors (VOCs or HAPs), decision makers should consider the following.
Flexibility. Emission concentration of solvent vapors can be readily maintained and controlled by adjusting the flow rate of liquid nitrogen into the system.
Nitrogen Recycling Opportunity. Liquid nitrogen used in indirect contact heat exchangers is uncontaminated and can be reused for applications such as blanketing, purging and pneumatic control. Additionally, existing liquid nitrogen storage, if normally vaporized before use, can be tied-in to the cryogenic condensation system to make use of its refrigeration value.
Solvent Recycling Opportunity. Depending on the composition and emission requirements of the solvent gas stream, the system can produce pure or nearly pure condensate streams, creating a recycle stream for improving process economics or a revenue stream that can be sold.
Compliance. Cryogenic condensation systems have achieved the lowest emissions limits mandated by the European Union. These emission limits are generally more stringent than those mandated by the EPA, though it is expected that U.S. environmental regulations will become tighter in the future.
Reliability. Systems are built with few moving parts, the bulk of which are the automated control valves. If correctly designed and built, these systems are known for low maintenance costs and high reliability.
Low Power Consumption. The majority of the power is consumed by the control system (control panel, instrumentation, etc.).
Although, in principle, nearly all organic solvents can be recovered via cryogenic condensation, implementation of systems worldwide suggests that the technology is most economic when gas stream flow rates are below 1,000 standard cubic feet per minute (SCFM), solvent vapor concentrations are above 1,000 PPMV, and the required temperature to meet the emission limit is below -40°F (-40°C). However, each individual case should be reviewed for technical and economic feasibility.
While, at times, purity requirements hinder the opportunity to do so, solvent recovery and reuse by means of cryogenic condensation systems remains a significant area of opportunity for many users of organic solvents today.
The application of liquid nitrogen in the world of cryogenics is far reaching. Cryogenic condensation technology for controlling emissions of regulated organic solvent vapors (VOCs or HAPs) is not confined to special applications. It is an established technology that can meet emission standards while bringing operational advantages to a production process.
It is certainly true that some companies have achieved significant commercial benefits by substituting heavily regulated substances, such as organic solvents, with alternatives, such as water-based solvents, freeing themselves from strict regulatory pressures. Clearly this practice should be encouraged; however, organic solvents continue to be an integral part both directly and indirectly of the manufacturing process that produces the wealth of consumer products and services we enjoy today. As such, innovative technologies such as cryogenic condensation for pollution control are paramount to continue promoting economic growth while maintaining environmental sustainability.