Table 14.1 was proposed by Kletz in 199111and shows what inherent safety features can best be implemented at which point in the design phase. It is critically important to recall that inherent safety, as with everything in green chemistry and green engineering, should be implemented as early as possible in the design of a route or process.
Recently, Palaniappan et al. developed a material-centric methodology for integrating inherent safety and pollution prevention into process development.12Figure 14.5 shows a process for the systematic evaluation of different process alternatives. There are at least three very important points one should take from their approach. The first is that the methodology is based on a materials balance; that is, all the materials in a proposed process are listed and approximate volumes or masses are known. The second is that multiple alternatives are screened. It is too often the case that a single route is evaluated, (or perhaps two or so) before the route is tested. It should be understood that a green chemistry/green engineering approach is most successful if more time is spent in options generation. The third point is that options evaluation is carried out in a systematic and objective manner. Very frequently,
“conventional wisdom” or “chemists’ intuition” is used to justify a proposed approach, only to find that the route becomes unworkable on scale-up because of a processing, environ- mental, safety, or health problem. Bringing forward a systematic approach to process generation enables us to bring the scientific method into process design, limiting the possibility of making decisions based on erroneous intuition or personal bias.
Can I reduce the magnitude of the risk?
Will alternatives that I develop create any new risks?
Will technical or management systems be required to manage the risk?
Yes
No
Yes
No
No Yes
Technical System
Management System Reassess
risk
Reassess risk
Reassess risk
FIGURE 14.4 Flowchart for integrating IS into route strategy and process design.
TABLE 14.1 Inherent Safety Principles Considered in First Project Stages
Feature Conceptual Stage Flowsheet Stage PI Stage
Intensification
Substitution
Attenuation
Limitation of effects
By equipment design
By changing reaction conditions
Avoiding knock-on effects
By layout
In other ways
Making incorrect assembly impossible
Making status clear
Simplification
Tolerance
Ease of control
Software
In the materials-centric approach of Palaniappan et al., four aspects are considered:
process materials, material–process condition interactions, material–material interactions, and material–process unit interactions.
1. Process materials. All the materials in a process, including potential by-products, intermediates, and processing aids need to be identified. EHS issues may then be identified and assessed according to the key physical properties described in Chapter 3.
2. Material–process condition interactions. These interactions highlight the potential for materials to behave differently as process conditions change. For example, as the pH, temperature, or pressure is increased or decreased, a solvent may vaporize, a chemical may become unstable, or there may be a phase change—any of which might lead to an unsafe process condition. Knowledge of a material’s thermal decomposition temperature and their freezing or boiling points all need to be evaluated carefully under the process conditions expected. A good checklist to better understand material–process interactions has been drawn up by Hendershot13and is shown in the accompanying box. However, many tools and checklists are available to step you systematically through a review of a chemical route and process to determine the suspected interactions.
Chemical Reaction Hazard Identification
1. Know the heat of reaction for the intended and other potential chemical reactions.
2. Calculate the maximum adiabatic temperature for the reaction mixture.
3. Determine the stability of all individual components of the reaction mixture at the maximum adiabatic reaction temperature.
User input
Qualitative mass balance
Use knowledge or heuristics based on pollution prevention and waste minimization to
develop top-level alternatives
Trace hazardous material flow
Identify and ev aluate each input stream or process unit that contributes to that material flow Derive a set of detailed
alternatives Identify synergies, costs, benefits,
etc. of each alternative Assess alternatives using appropriate decision-making tools
Meet performance target including EHS/
green/sustainabiity criteria?
Stop
FIGURE 14.5 Material-centric methodology for integrating pollution prevention and inherent safety. (Adapted from Palaniappan et al.12)
temperature.
5. Determine the heat addition and heat removal capabilities of the pilot plant or production reactor.
6. Identify potential reaction contaminants.
7. Consider the impact of possible deviations from intended reactant charges and operating conditions.
8. Identify all heat sources connected to the reaction vessel and determine their maximum temperature.
9. Determine the minimum temperature to which the reactor cooling sources could cool the reaction mixture.
10. Consider the impact of higher temperature gradients in plant-scale equipment compared to a laboratory or pilot-plant reactor.
11. Understand the rate of all chemical reactions.
12. Consider possible vapor-phase reactions.
13. Understand the hazards of the products of both intended and unintended reactions.
14. Consider preparing a chemical interaction matrix and/or a chemistry hazard analysis.
3. Material–Material interactions. Material incompatibilities are a key component of any process safety evaluation, as they can lead to uncontrolled exothermic excursions, rapid and uncontrolled gas evolution, rapid polymerizations, and/or combinations of such factors. Process parameters may also facilitate or retard these interactions, so the materials interactions need to be evaluated in light of these process conditions.
4. Material–process unit interaction. Unit operations common to chemical processing have been described throughout the book. As you have learned, these unit operations can profoundly affect such things as mixing rates, mass and energy transport phenomena, and other chemical equilibrium phenomena. In addition, materials can interact with the unit operation materials of construction and cause corrosion or other damaging effects to reactors, separators, and other equipment. Once again, these types of interactions need to be looked at very carefully and evaluated as to their potential for unsafe or environmentally damaging effects.
Many analytical tools and methodologies have been developed to identify potential areas of concern for both materials and chemical processes. Many of these are listed in Table 14.2.
A notable variation on the materials-centric approach is the hierarchical approach to evaluating chemical processes taken by Shah et al.15These authors take a layered approach to the assessment of hazards and risks associated with a given route or process alternative.
The first layer is known as the substance assessment layer, the second is the reactivity assessment layer, the third is the equipment assessment layer, and the fourth is the safety technology assessment layer. As you can see, once a particular route or process alternative has been chosen, this is very similar to the previous methodology. However, it is worth describing each layer in a bit more detail. The flowchart in Figure 14.6 illustrates the decision logic associated with this approach. As with the previous approach, the process begins with a chemical or process flowsheet where all the materials in the synthesis or process have been identified.
TABLE 14.2 Tools for Developing Better Process Safety Understanding
Theoretical and Computational Screening
Chemical Hazard Identification:
Experimental Screening for Thermal Stability
Process Hazard Identification:
Screening Tools for Reaction Rate and Kinetics
Emergency Relief Systems (ERS) Design, Screening,
and Direct Scale-up Process Design and Optimization
MSDS Blasting cap test Isothermal storage test (IST) RSST Reaction calorimetry (RCI)
Chemical compatibility matrices
Flame test Accelerating rate calorimetry
(ARC)
SuperChems Expert, for DIERS, QuickSize
Contalab Literature reactivity data, such
as Bretherick’s handbook, NFPA, hazard ratings
Gram-scale heating test Vent sizing package (closed test cell) (VSP)
Simple nomographs Atomic pressure-tracking adiabatic calorimetry (APTAC) Incident data Shock sensitivity test RC1 (Pressure vessel only,
after screening tests)
Computational fluid dynamics, SuperChems, Expert/DIERS
Chemical structure Drop weight test APTAC Specialized large-scale test
(mixing limited reactions, injection of reaction killers, chemical rollover, reactions at interface, etc.)
Formation energies Thermogravimetric analysis (TGA)
Heats of reaction, decomposition, solution
Differential thermal analysis (DTA)
Computed adiabatic reaction temperature at constant pressure and/or volume, (CART)
Reactive systems screening tool (RSST)
Oxygen balance Differential scanning calorimetry (DSC) Software tools such as the
ASTN CHETAH, NASA CET89, SuperChems, TIGER, etc.
Source:Adapted from ref. 14.
EHS Data for materials
Materials assessment layer
Are EHS risks acceptable?
Substitution or reduction in amounts of hazardous materials
Material reactivity or interaction
matrix
Reactivity assessment layer
Change of operating procedures or unit
operations Are reactivity
risks acceptable?
Equipment-related worst-case
scenario development
Equipment assessment layer
Are worst-case risks acceptable?
Investigation of other materials options or alternative chemical
synthetic routes
Do safety technologies mitigate or manage
the risk?
Is the overall risk acceptable?
Assessment finished!
Selection of other safety technologies Safety technology assessment layer
FIGURE 14.6 Flowchart for process design decision making. (Adapted from Shah et al.15)
403
1. Substance assessment layer. In the first layer, the substance assessment layer, the first objective is to substitute less hazardous or benign materials for hazardous materials. Next, we would seek to reduce the amounts of hazardous materials that are used in the process.
Substances are evaluated by looking at key physical and chemical properties of the materials, along with a variety of environmental, safety, and health parameters. Eleven categories are used to evaluate substances: mobility (i.e., where the compound ends up in the environment), fire, explosion, reaction and decomposition, acute and chronic toxicity, irritation, air- mediated effects (e.g., photochemical ozone creation potential), water-mediated effects (e.g., hydrolysis, biological oxygen demand), solid waste, persistence, and degradability. In a sense, the exact number of categories and the way in which they are scored or assessed is not as critical as having the right categories from the sustainability perspective of looking globally and acting locally. Consequently, there may be some variation in parameters, depending on whether or not you are considering local conditions and how these may be weighted.
As has been noted in this book and elsewhere, EHS and sometimes physical and chemical property data are often unavailable for new materials in early development or recent commercialization. In these cases, one needs to rely on quantitative structure–activity relationship (QSAR) models, nearest-neighbor approaches, or other means to arrive at best- guess estimates of physical, chemical, and/or EHS properties.
2. Reactivity assessment layer. This is essentially the same as the material–material assessment level in the material-centric methodology we discussed earlier. As noted then, compound–compound interactions are frequently the cause of many process upsets and safety accidents and incidents, so the main objective of this step is to make certain that we know of any potential for process upsets. As chemists, we frequently make use of energetic materials because we value their reactivity; faster reactions that are kinetically and thermo- dynamically more favorable are less likely to result in unwanted side reactions and by- product formation. However, using these materials at scale is often difficult at best and they are better avoided.
Mosley et al.16and others have described procedures for the systematic assessment of reactive chemical hazards. Figure 14.7 is a fictitious example of an interaction matrix.
The intention of tools such as this matrix is to help in visualizing potential interactions between materials and process conditions. While there are many databases containing reactivity data, it takes some time and effort to compile accurate data from scratch. It is also necessary through experimentation to determine potential reactivity concerns under the proposed process conditions so that there are no surprises due to changes in pH, temperature, or mixing.
3. Equipment assessment layer. Once substances are evaluated and their potential for uncontrolled reactivity has been assessed, it is important to look at how equipment and collections of equipment used in unit operations will perform under the process conditions proposed. The objective here is to determine what might occur in the event of equipment failure. This is an extremely important step, as many industrial accidents occur as a result of some sort of equipment failure. Remember that risk is a function of hazard, the potential or probability for an event to occur (i.e., exposure potential in the case of occupational hygiene, or the potential for a pressure relief valve to stick shut in the case of process safety), the severity of the event (i.e., a temperature rise of a few degrees all the way to an explosive event), and the frequency of occurrence. By substituting materials or changing the chemical reactions, we reduce the severity of the event and the inherent hazard of the process.
The equipment assessment layer seeks to reduce the frequency of occurrence of an adverse event. Through the use of worst-case scenariosthat are related to equipment failures in different unit operations used for a process, risk can be characterized and prevention and protection measures modeled using unit operation models. Once again, there are systematicways of doing this for any given process scenario. For example, Stoessel17has developed a systematic approach to assessing exothermic runaway reactions that simplifies the analysis of risk.
The equipment assessment layer is important, as it enables the consideration of vent sizing, vent locations, different reactor configurations, reactant feeds, mixing rates, and other factors. Hendershot13has also drawn up a list of potential options for process design considerations that involve process and equipment:
. Rapid reactions are desirable.
. Avoid batch processes in which all of the potential chemical energy is present in the system at the beginning of the reaction step.
. Use gradual addition or semi batch processes for exothermic reactions.
. Avoid using control of the reaction mixture temperature as the only means of limiting the reaction rate.
. Account for the impact of vessel size on the heat generation and heat removal capabilities of a reactor.
. Use multiple temperature sensors in different locations in the reactor for rapid exothermic reactions.
. Avoid feeding a material to a reactor at a higher temperature than the boiling point of the reactor contents.
Given the history of process safety catastrophes and the critical part that equipment failure has played in these events, equipment assessment is critical in the design of a safe process and plant.
4. Safety Technology Assessment Layer. Despite our best efforts to remove process hazards associated with materials, to reduce the probability of reactive chemical upsets and prevent equipment malfunctions or limit the frequency of their occurrence, every process will always have a certain amount of associated risk. This means that there will invariably be a need for certain bolt-on pieces of safety equipment, control operations, or control technologies to help mitigate or manage the residual risk. Once again, there are a variety of recommendations that can be made, based on the particular scenario being evaluated for each unit operation in the overall process configuration.
Reactant Reactant
Solvent Sol
ven t B
Reac tant Solvent A M
150° C st Operator
Etc .
Reactant A Reactant B Solvent A Solvent B Reactant A / Solvent A Mixture
FIGURE 14.7 Interaction matrix: example of all interactions safe.