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Moderation Principle of Inherently Safer Design


Moderation means using materials under less hazardous conditions, also called attenuation. Moderation of conditions can be accomplished by strategies which are either physical (lower temperatures, dilution) or chemical (development of a reaction chemistry which operates at less severe conditions).

Dilution

Dilution reduces the hazards associated with the storage and use of a low boiling hazardous material in two ways - by reducing the storage pressure, and by reducing the initial atmospheric concentration if a release occurs. Materials which boil below normal ambient temperature are often stored in pressurized systems under their ambient temperature vapor pressure. The pressure in such a storage system can be lowered by diluting the material with a higher boiling solvent. This reduces the pressure difference between the storage system and the outside environment, reducing the rate of release in case of a leak in the system. If there is a loss of containment incident, the atmospheric concentration of the hazardous material at the spill location is reduced. The reduced atmospheric concentration at the source results in a smaller hazard zone downwind of the spill.

Some materials can be handled in a dilute form to reduce the risk of handling and storage:

  • Aqueous ammonia or methylamine in place of the anhydrous material
  • Muriatic acid in place of anhydrous HCI
  • Dilute nitric acid or sulfuric acid in place of concentrated fuming nitric acid or oleum (503 solution in sulfuric acid)

If a chemical process requires the concentrated form of the material, it may be feasible to store a more dilute form, and to concentrate the material by distillation or some other technique in the facility prior to introduction to the process. The inventory of material with greater intrinsic hazard (i.e. undiluted) is reduced to the minimum amount required to operate the process, but the distillation adds a new hazardous process.

Chemical reactions are sometimes conducted in a dilute solution to moderate reaction rates, to provide a heat sink for an exothermic reaction, or to limit maximum reaction temperature by "tempering" the reaction.

Refrigeration

Many hazardous materials, such as ammonia and chlorine, are stored at or below their atmospheric boiling points with refrigeration. Refrigerated storage reduces the magnitude of the consequences of a release from a hazardous material storage facility in three ways

  • reducing the storage pressure
  • reducing the immediate vaporization of leaking material and the subsequent evolution of vapors from the spilled pool of liquid
  • reducing or eliminating liquid aerosol formation from a leak

Refrigeration, like dilution, reduces the vapor pressure of the material being stored, reducing the driving force (pressure differential) for a leak to the outside environment. If possible, the hazardous material should be cooled to or below its atmospheric pressure boiling point. At this temperature, the rate of flow of a liquid leak will depend only on liquid head or pressure, with no contribution from vapor pressure. The flow through any hole in the vapor space will be small and will be limited to breathing and diffusion.

Material stored at or below its atmospheric pressure boiling point has no superheat. Therefore there will be no initial flash of liquid to vapor in case of a leak. Vaporization will be controlled by the evaporation rate from the pool formed by the leak. This rate can be minimized by the design of the containment dike, for example, by minimizing the surface area of the liquid spilled into the dike area, or by using insulating concrete dike sides and floors. Because the spilled material is cold, vaporization from the pool will be further reduced.

Many materials, when released from storage in a liquefied state under pressure, form a jet containing an extremely fine liquid aerosol. The fine aerosol droplets formed may not rain out onto the ground, but instead may be carried downwind as a dense cloud. The amount of material contained in the cloud may be significantly higher than would be predicted based on an equilibrium flash calculation assuming that all of the liquid phase rains out. This phenomenon has been observed experimentally for many materials, including propane, ammonia, hydrogen fluoride, and monomethylamine. Refrigeration of a liquefied gas to a temperature near its atmospheric pressure boiling point eliminates the two-phase flashing jet, and the liquid released will rain out onto the ground. Containment and remediation measures such as spill collection, secondary containment, neutralization, and absorption may then be effective in preventing further vaporization of the spilled liquid.

Figure 1 below is an example of a refrigerated storage facility for chlorine. This facility includes a covered spill collection sump which is covered to reduce evaporation to the atmosphere, both by containing the evaporating vapors and by reducing heat transfer from the surrounding atmosphere. The spill collection sump is vented to a scrubber which collects the chlorine vapor which evaporates from the sump.


Figure 1 - Chlorine storage system with collection sump with vapor containment

Less Severe Process Conditions

Processing under less severe conditions, close to ambient temperature and pressure, increases the inherent safety of a chemical process. Some examples include:

  • Improvements in ammonia manufacturing processes have reduced operating pressures. In the 1930s ammonia plants operated at pressures as high as 600 bar. In the 1950s, process improvements had reduced operating pressures to 300-350 bar. By the 1980s, ammonia processes operating in the 100-150 bar range were being built. Besides being safer, the lower pressure plants are also cheaper and more efficient.
  • Catalyst improvements allow methanol plants and plants using the Oxo process for aldehyde production to operate at lower pressures. The process also has a higher yield and produces a better quality product.
  • Improvements in polyolefin manufacturing technology have resulted in lower operating pressures.
  • Use of a higher boiling solvent may reduce the normal operating pressure of a process, and will also reduce the maximum pressure resulting from an uncontrolled or runaway reaction.
  • Semi-batch or gradual addition batch processes limit the supply of one or more reactants, and increase safety when compared to batch processes in which all reactants are included in the initial batch charges. For an exothermic reaction, the total energy of reaction available in the reactor at any time is minimized. However, the inherent safety benefits of semi-batch operation are only realized if the limiting reactant is actually consumed as it is fed, and there is no buildup of unreacted material. A number of process upsets, such as contamination with a reaction inhibitor, operating at too low a temperature, forgetting to charge a catalyst to the reactor, or forgetting to start the agitator, could result in buildup of unreacted material. If any of these upsets causing loss of reaction can occur, it is important to be able to ensure that the reactants are indeed being consumed as they are fed in order to realize the inherent safety benefits of a semi-batch process. The reactor could be monitored to provide confirmation that the limiting reactant is being consumed, by on-line analysis or by monitoring some physical property of the batch that is reliably correlated to reaction progress.
  • Advances in catalysis will result in the development of high yield, low waste manufacturing processes. Catalysts frequently allow the use of less reactive raw materials and intermediates, and less severe processing conditions. High yields and improved selectivity reduce the size of the reactor for a specified production volume. High selectivity for the desired product also reduces the size and complexity of the product purification equipment. It may be possible to develop a catalyst that is sufficiently selective that it becomes unnecessary to purify the product at all, as in a process for HCFC-141b (CHsCFCh).

Secondary Containment - Dikes and Containment Buildings

Secondary containment systems are best described as passive protective systems. They do not eliminate or prevent a spill or leak, but they can significantly moderate the impact without the need for any active device. Also, containment systems can be defeated by manual or active design features. For example, a dike may have a drain valve to remove rain water, and the valve could be left open. A door in a containment building could be left open.

Guidelines for the design of storage facilities for liquefied gases to minimize the potential for vapor clouds:

  • Minimize substrate surface wetted area
  • Minimize pool surface open to atmosphere
  • Reduce heat capacity and/or thermal conductivity of substrate
  • Prevent "slosh over" of containment walls and dikes
  • Avoid rainwater accumulation
  • Keep liquid spills out of sewers
  • Shield the pool surface from the wind
  • Provide vapor removal system to a scrubber or other emission control device
  • Provide liquid recovery system to storage where possible
  • Avoid direct sunshine on containment surfaces in hot climates
  • Direct spills of flammable materials away from pressurized storage vessels to reduce the risk of a boiling liquid expanding vapor explosion (BLEVE)

Containment buildings have been used to limit the impact of loss of containment incidents for many toxic materials, including chlorine and phosgene. Containment buildings can cover a wide range of structures, from a simple, light structure to reduce evaporation of a spill of a relatively nonvolatile toxic material, to a very strong pressure vessel designed to withstand an internal explosion. Evolution in the design of a phosgene handling facility from an open air plant through various stages of increasing containment, culminating in the design of Figure 2. The process is totally enclosed in a large pressure vessel capable of withstanding the overpressure in case of a flammable vapor deflagration inside the containment vessel.

Figure 2 - A chemical process totally contained in a large pressure vessel

Containment buildings are an example of inherent safety conflicts and tradeoffs. A containment building provides protection outside the building, but it can also trap and concentrate material from small leaks inside the building, increasing the risk to personnel entering the building.

Provisions must be made to ensure worker protection for a process located in a containment building. For example, the atmosphere in the containment structure should be monitored for hazardous vapors, operations should be remotely controlled from outside the containment structures, access should be restricted, and proper personal protective equipment (PPE) should be used when entry into the containment structure becomes necessary.

In particular, great care must be taken when evaluating tradeoffs for a containment building for a flammable and toxic material such as hydrogen cyanide. A leak or fire inside the building could cause a confined vapor cloud explosion which destroys the building. The total risk may actually increase.



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