3 Diseño de solventes

56 Chapter 2 Molecular Structure Design

Note that the latent heat of vaporization of SF is sufficiently close to 18.4 kJ/mol to be acceptable, given the approximate estimation methods. Also, neither molecule contains chlorine, and hence, the ODPis zero.G


Solvent Design


Organic solvents play a key role in many aspects of chemical processing and in the delivery of chemicals to consumers. In chemical processing, solvents aroften used: (1) to mobilize solids, frequently dissolving them; (2) to clean equipment, as in re oving grease and grime; and (3) in cleaning clothing, as in dry cleaning. In contrast, in the delivery of chemicals toconsumers, solvents often convey particles onto surfaces in coatings, such as in paint and printing ink.
Until the past decade, the solvent market was dominated by a few principal products, sol- vents known for their ability to “dissolve most anything” (Kirschner, 1994). These included acetone, mixed xylenes, and 1,1,1-trichloroethane, which are manufactured in large-scale processes by the majorchemical companies. For environmental and health reasons, over tin past decade, there has been a gradual shift away from these solvents. The U.S. Environmental Protection Agency maintains a Toxic Release Inventory (TRI), which includes acetone, 1,1,1- trichloroethane and other common solvents, whose emissions into the air have been gradually reduced. Other solvents are included on the hazardousair pollutants (HAP) list of the 1990 Clean Air Act; for example, 1,1,1-trichloroethane which, like Freon refrigerants, accumulates in the stratosphere and destroys ozone. Furthermore, other solvents, like monomethylether, monoethylether, and their acetates, have been associated with high miscarriage rates.
Chemical companies are increasingly challenged to reduce the usage of these targeted sol-vents, and consequently, a host of solutions are being sought, including shifts toward: (1) aqueous solvents, where possible, (2) more concentrated paints and coatings, containing less solvent, and (3) hot-melt, ultraviolet-cured, and waterborne adhesives. For example, in the manufacture of cosmetics and personal care products, solvents are being dropped from some formulations due to their highcontent of volatile organic compounds (VOCs) and are being replaced, for example, by water-based hair sprays and solid deodorant sticks.
In meeting the challenge, chemical companies are designing a growing number of
environmentally-friendly solvents, that is, engineered or designer solvents, that satisfy the specifications for each application; and end users are altering theirusage patterns. As a re- sult, new solvents are appearing gradually, designed as specialty chemicals, to replace the use of commodity solvents; for example, diacetone alcohol cleaner is a replacement for ace- tone in the shipbuilding industry. In some cases, the cleaning methods themselves are chang- ing; for example, a one-step, vapor-degreasing process is replaced by a two-step process involving adip-tank rinse followed by drying. Another example includes the recycling and reuse of cleaning solvents. In the dry-cleaning business, because the principal solvent, per- chloroethylene, is suspected to be a carcinogen and appears on the list of HAPs, used solvent is being filtered and recycled, refrigerated condensers are being installed to recover vapor emissions, and new water andsteam-cleaning (wet) processes are being developed.
The search for each new specialty solvent can be viewed as a product design problem. For
this purpose, design strategies have been evolving, some of which have been computerized, at least partially. In this section, two examples are presented in which: (1) a new solvent is designed as a replacement for 1,1,1-trichloroethane, for...