Minimize Wastes from Specialty Chemical Processes

Chemical Engineering Progress, Jan 2008 by Mueller, Jeffrey, Cipullo, Michael

Use this holistic approach as the basis for a process development strategy to identify environmentally sound, cost-effective, low-waste-generating processes.

Specialty chemical process development can encompass a wide range of chemistries, even for the same target molecule or application. As a result, the process development organization is often working with numerous raw materials, a wide range of solvents, and mixtures of molecules and/or isomers. Processes that make low-purity chemicals with many byproducts using a wide range of solvents can generate up to ten pounds of waste per pound of product. Although total volumes remain relatively small, in recent years, the U.S. Environmental Protection Agency (EPA) has been paying closer attention to this waste-to-product ratio.

The EPA requires companies to go through a pre-manufacture notification (PMN) process before it will allow the commercial production and sale of any chemical that is not on the Toxic Substances Control Act (TSCA) inventory. When production volumes are small - less than 10,000 kg/yr - the EPA may allow manufacturing under a low-volume exemption (LVE). If the EPA has concerns about employee chemical exposure or the environmental impact of the new chemical or process byproducts, it often limits production to significantly lower amounts. During its review of the application (90 days for a PMN, 30 days for a LVE), EPA may require additional testing, which will delay commercialization, or even deny the application, thereby blocking commercialization.

Clearly, it is in the manufacturer's best interest to identify, develop and commercialize the "best" process from both financial and environmental perspectives. This includes choosing the best solvents and raw materials (i.e., those with low cost, low employee health risk, recyclability, low flammability and minimal adverse environmental impact) and a route that is efficient (i.e., with fast kinetics, few steps, few byproducts and economical equipment).

Although processes that generate ten times more waste than product are becoming rarer, a typical specialty chemical operation may still average 1-2 lb of waste per pound of product. To effect a step change in the amount of waste generated per pound of product produced, specialty chemical manufacturers must formulate a comprehensive strategy that addresses both immediate needs and root causes. This article highlights the EPA's new emphasis on waste minimization in PMN and LVE applications, and presents a methodology known as "Best Process Design" for better synthesis-route identification and waste minimization during process development.

Immediate and short-term solutions. Short-term solutions to hazardous-waste minimization tend to focus on end-of-pipe programs or equipment. Typical minimization routes include treatment and closed-loop recycle of solvents back into the process using equipment such as batch or continuous distillation, extraction, or adsorption onto molecular sieves or ion-exchange resins. Cleaning solvent usage can be reduced by batch distillation and recycle or via installation of spray balls in process equipment. Solvents that cannot be reused in the process may be of high enough quality for use by other companies in such operations as furniture stripping and as paint thinners. Some materials may qualify as a "continued-use solvent" in very-low-grade solvent applications such as machine cleaning and drum washing, or as boiler feed/fuel under the EPA's commercial fuel-blending program. Finally, the solvent may be returned to a refinery feedstock tank if it is of high enough quality or octane level.

Root-cause and long-term solutions. Reference 1 provides a framework of 12 principles of green chemistry that chemists and engineers can use when designing new chemical processes. We believe the most important of those principles involves prevention. This means we must design the chemical process from the very beginning to prevent the generation of waste and byproducts.

The root-cause of high-waste-generating processes is often found in the chemistry or synthesis route chosen to produce a target molecule. To ultimately reduce waste, a process change or even a synthesis route change is needed. It can be very difficult and expensive to convert existing processes to green processes, because in many applications, this type of change must be approved by a customer and may involve significant expense for process-development work and product requalification. Unless there is a clear driver for change, such as regulatory requirements or significant cost reductions, process development groups invariably focus their time on new product/process development or on troubleshooting quality problems with existing products. Although not a quick solution for existing high-waste processes, implementation of a clear front-end process-development strategy that emphasizes waste minimization during new-product development can achieve the desired long-term results.

"Best Process Design"