Smaller Is Safer—Simplifying Chemical Plant Safety

By Dennis C. Hendershot

Safety in the chemical industry has improved in recent years, thanks largely to new engineering and management controls. Now, a new safety philosophy that relies on eliminating or reducing the basic process hazards is extending the safety experience in chemical plants.

Under the traditional approach involving engineering and management controls, a company accepts the necessity of hazardous materials and processing operations and applies safety controls to reduce risk to tolerable levels. These safety controls may be expensive to install and maintain and their performance can never be perfect. Moreover, there is always the chance that they will fail, causing a serious accident.

A chemical process that relies instead on eliminating or reducing process hazards is being viewed as inherently safer. The process features that reduce risk are permanent and inseparable from the basic process design and technology. This strategy is receiving increased attention as a more reliable, robust, and potentially more cost-effective way of designing safety into a chemical plant.

There are four strategies for inherently safer chemical process design:

• Minimize—use smaller quantities of hazardous material or energy
• Substitute—use fewer hazardous materials or chemical reactions to produce a product
• Moderate—use less severe processing conditions, e.g., lower temperature or pressure, or use hazardous materials in a less hazardous form, e.g., diluted in water
• Simplify—eliminate unnecessary complexity from plant design, decreasing the potential for operating errors

Minimizing for Safety

Small is safe. By reducing the size of equipment in a chemical plant, the amount of hazardous material or energy that can be released in case of a leak or other loss of containment is also reduced. Ideally, the equipment should be reduced to a size small enough so that a total leak of the entire inventory from the equipment could not inflict personnel injury or environmental/property damage. Even if this goal is unattainable, control of the residual risk to people, the environment, and property from smaller equipment will be easier. An additional benefit is that smaller equipment is often less expensive, offering the potential for a plant design that is both safer and more economical.

Much can be accomplished to reduce the size of chemical plant equipment through good, traditional engineering practices. And development of new manufacturing technology—process intensification—offers even greater potential benefits. The design engineer should challenge the amount of inventory in every piece of chemical processing equipment, such as storage tanks, pipes, and processing equipment. The objective is to minimize inventory, while designing an operable and reliable plant.

The size of raw material storage tanks, for example, can be reduced using modern supply chain and inventory control techniques. Companies should also question all storage of hazardous in-process intermediates. There are many examples of the elimination of large intermediate storage tanks with no impact on plant operation.

Pipes should be designed to be large enough to do the required job, and no larger. A two-inch pipe contains four times as much material as a one-inch pipe of the same length. Locate the equipment and route pipes to minimize the length of hazardous material pipes.

Equipping for Safety

Fortunately, new and efficient processing technology offers great potential to make smaller, safer, and cheaper chemical plants. Some examples include:

• Use of innovative reactor designs, such as continuous stirred tank, tubular jet, loop, and static mixer reactors in place of large batch reactors. For a rapid chemical reaction, reactor size can be reduced by a factor of 100 or more.
• Compact heat exchanger designs that provide a large surface area for heat transfer with a small volume.
• Distillation column trays or packing designs to minimize liquid inventory.
• Distillation technology, such as thin film evaporators and centrifugal distillation equipment that is highly efficient and reduces inventory.
• Combined operations in a single piece of equipment, such as reactive distillation or mixer-settlers, reduces the number of vessels needed in a plant.
• Innovative ways of delivering energy to a chemical reaction, such as lasers, ultraviolet light, microwaves, and ultrasound, offer potential for reducing the size of reactors.

Process and equipment minimization are receiving more and more attention in the chemical engineering literature. For more information, contact Dennis_C_Hendershot@rohmhaas.com.