Sustainable Chemistry and Inherently Safer Design

Process safety, a discipline that focuses on the prevention of fires, explosions and accidental chemical releases at chemical process facilities, is a key element for a sustainable industrial chemistry, as indicated in the previous sections. There are three key elements for process safety: behavior, system and process.

Thorough and effective analyses of workplace incidents are critical components of a comprehensive safety management system. Yet, many incident analysis processes (i.e., accident investigations) fall short. They frequently fail to identify and resolve the real root causes of injuries, process incidents and near misses. Because the true root causes of incidents are within the system, the system must change to prevent the incident from happening again.

Process safety differs from the traditional approach to accident prevention [90]:

• there is greater concern with accidents that arise out of the technology;

• attention is given to foreseeing hazards and taking action before accidents occur;

• accidents that cause damage to plant and loss of profit but do not injure anyone should be considered in addition to those that do cause injury.

In general, there is greater emphasis on a systematic rather than a trial-and error approach, particularly on methods that identify hazards and estimate their probability and consequences. The term loss prevention can be applied in any industry, but is widely used in the process industries (particularly chemical industries), where it usually means the same as process safety.

Chemical plants, and other industrial facilities, may contain large quantities of hazardous materials. The materials may be hazardous due to toxicity, reactivity, flammability or explosivity. A chemical plant may also contain large amounts of energy - the energy is required either to process the materials or is contained in the materials themselves. An accident occurs when control of this material or energy is lost. An accident is defined as an unplanned event leading to undesired consequences. The consequences might include injury to people, damage to the environment, or loss of inventory and production, or damage to equipment.

The practices to process safety have been progressively changed with time, as briefly shown in Table 1.8. Process safety does not depend only on human errors or faults in equipments but on the whole system management. From the end of the 1970s there has thus been a large effort to develop risk assessment techniques and systematic approaches, as well as suitable strategies for successful process safety management (PSM). In the 1980s, the Bhopal accident pushed the chemical industry

Table 1.8 Process safety milestone practices.

Period

Type approach

Practice

Pre-1930s

Behavior

Identify who caused the loss and punish the guilty

Pre-1970s

Process

Find breakdown in, and fix man-machine interface

1970s, 1980s

Management

Development of risk assessment techniques

systems

and systematic approaches

1980s +

Comprehensive

Performance-, risk-based standards, regulations;

sustainable and inherent designs

towards a further step for a more comprehensive approach, for example, to an inherent safer design.

Inherently safer design of chemical processes involves the use of smaller quantities of hazardous materials, the use of less hazardous materials, the use of alternative reaction routes or process conditions to reduce the risk of runaway exothermic reactions, fires, explosions and/or the generation or release of toxic materials.

Notably, in some cases changes made to improve the environment have resulted in inherently less safe designs. For example, the collection of vent discharge gases for incineration or for absorption on carbon beds has resulted in explosions when the composition of the gases in the vent system has entered the flammable range.

Chemical process safety strategies can be grouped into four categories [91]:

1. Inherent: when the safety features are built into the process, not added on; for example, replacement of an oil-based paint in a combustible solvent with a latex paint in a water carrier.

2. Passive: for example, safety features that do not require action by any device - they perform their intended function simply because they exist; for example, a blast resistant concrete bunker for an explosives plant, or a containment dike around a hazardous material storage tank.

3. Active: for example, safety shutdown systems to prevent accidents (e.g., a high level alarm in a tank shuts automatic feed valves) or to mitigate the effects of accidents (e.g., a sprinkler system to extinguish a fire in a building). Active systems require detection of a hazardous condition and some kind of action to prevent or mitigate the accident. Multiple active elements involve typically a sensor (detect hazardous condition), a logic device (decide what to do) and a control element (implement action).

4. Procedural: or operating procedures, for example, operator response to alarms, emergency response procedures, safety rules and standard procedures, training. An example is a confined space entry procedure.

In general, inherent and passive strategies are the most robust and reliable, but elements of all strategies will be required for a comprehensive process safety management program when all hazards of a process and plant are considered.

1.5 Sustainable Chemistry and Inherently Safer Design | 49 Table 1.9 Examples of process risk management strategies. Source: adapted from Mannan [90].

Risk management strategy category

Example

Comments

Inherent

Passive

Active

Procedural

An atmospheric pressure reaction using nonvolatile solvents that is incapable ofgenerating any pressure in the event of a runaway reaction

A reaction capable of generating 22 kPa pressure in case of a runaway; carried out in reactor which may operate up to 36 kPa

A reaction capable of generating 22 kPa, realized in a reactor with a 1 kPa high-pressure interlock to stop reactant feeds and a properly sized 3 kPa rupture disc discharging to an effluent treatment system

The same reactor described in example 3 above, but without the 1 kPa high-pressure interlock. Instead, the operator is instructed to monitor the reactor pressure and stop the reactant feeds if the pressure exceeds 3 kPa.

No potential for overpressure

The reactor can contain the runaway reaction, but 2 kPa pressure is risky and reactor could fail due to a defect, corrosion, physical damage or other cause

The interlock could fail to stop the reaction in time, and the rupture disk could be plugged or improperly installed, resulting in reactor failure in case of a runaway reaction. The effluent treatment system could fail to prevent a hazardous release

There is a potential for human error, the operator failing to monitor the reactor pressure, or failing to stop the reactant feeds in time to prevent a runaway reaction

Table 1.9 gives some examples of process risk management strategies. Note, however, that these examples refer only to the categorization of the risk management strategy with respect to the hazard of high pressure due to a runaway reaction. The processes described may involve trade-offs with other risks arising from other hazards. For example, the non-volatile solvent in the first example may be extremely toxic, and the solvent in the remaining examples may be water. Decisions on process design must be based on a thorough evaluation of all the hazards involved.

Table 1.9 refers to a batch chemical reactor as an example. The hazard of concern is a runaway reaction causing high temperature and pressure and potential reactor rupture. The preferable (inherent) approach is to develop a chemistry that is not exothermic, or mildly exothermic, for example, where the maximum adiabatic exothermic temperature is lower than the boiling point of all ingredients and onset temperature of any decomposition or other reactions. In a passive approach, the maximum adiabatic pressure of a reaction is lower than the maximum reactor pressure design. The hazard (pressure) still exists, but is passively contained by the pressure vessel. In an active strategy the maximum adiabatic pressure for 100% reaction is higher than the reactor design pressure, but an active control is present, for example, progressive introduction of the limiting reactant with temperature control to limit potential energy from reaction. In addition, high temperature and pressure interlocks to stop feed and apply emergency cooling are used and emergency relief systems are provided. In the procedural approach, the automatic devices are substituted by a trained operator to observe temperature, stop feeds and apply cooling, if the temperature exceeds critical operating limit.

There are various techniques to achieve classical risk reduction, but generally these approaches to safety are mostly an afterthought in the design. They may use a safety review or process hazards analysis (PHA), such as a hazard and operability study (HAZOP) or a "what if?/checklist study", merely as a project check instead of a preemptive hazards reduction tool. If these studies are carried out at the latter stages of engineering or during construction there is a natural tendency to avoid expensive redesign or rework. In the inherently safer design, elimination or significant reduction of the process hazards occurs during the design by adopting suitable approaches, which fall into the following categories:

• Minimize: Significantly reduce the quantity of hazardous material or energy in the system, or eliminate the hazard entirely if possible. It is necessary to use small quantities of hazardous substances or energy in (i) storage, (ii) intermediate storage, (iii) piping and (iv) process equipment, as discussed in the previous sections. The benefits are to reduce the consequence of incident (explosion, fire, toxic material release), and improve the effectiveness and feasibility of other protective systems (e.g. secondary containment, reactor dump or quench systems). Process intensification (see below) is also a way to reach this objective.

• Substitute: Replace a hazardous material with a less hazardous substance, or a hazardous chemistry with a less hazardous chemistry. Examples are water-based coatings and paints in place of solvent-based alternatives. They reduce fire hazard, are less toxic, have a better smell and lower VOC (volatile organic compound) emissions, and reduce hazards for end user and also for the manufacturer. Safer use and better sustainability thus go in the same direction. Another example is substitution of chemicals used for refrigeration. Initially, ammonia, light hydrocarbons and sulfur dioxide were used. They were later substituted by inherently safer alternatives, for example, CFCs (chloro-fluoro-carbons). However, in around the 1980s, CFCs were discovered to be active in stratospheric ozone destruction and thus were later banned (Montreal Protocol entered into force in 1989). CFCs were initially substituted by HCFCs, where not all the C-H bonds in alkanes were substituted by C-X bonds (X is an halogen group), but the phasing out of also these chemicals is programmed. New substitutes for hydrofluorocarbons (HCFC) should be thus developed, but their impact should be also minimized both by severe regulations on their disposal and by re-design of refrigerators to minimize the quantity of flammable hydrocarbons. Currently, in home refrigerators as little as 120 grams of hydrocarbon refrigerant is used. This example shows that substitution of chemicals sometimes is not a simple problem. In fact, this is one of the critical points in REACH legislation discussed in the previous section.

Substitution of an hazardous chemical is often an even more complex problem, in particular regarding the trade-off between inherently safer design and sustainable chemistry. Several examples are discussed in subsequent chapters. We thus limit our discussion here to a few aspects. Up until around the 1960s the Reppe process was employed for of synthesis of acrylic esters:

It was substituted by the new process of oxidation of propylene to acrylic acid via acrolein using heterogeneous Bi-molybdate based catalysts followed by acid-catalyzed reaction of acrolein with the alcohol:

Although substitution was motivated by the availability at that time of propylene and lower cost of the process, it was also a significant improvement in terms of safety, because acetylene is flammable and extremely reactive, carbon monoxide is also toxic and flammable, nickel carbonyl catalysts are toxic, environmentally hazardous (heavy metals), and carcinogenic, and anhydrous HCl (used in the reaction) is toxic and corrosive. However, the new process from propylene cannot be considered inherently safer. Hazards are primarily due to the flammability of reactants, corrosivity of the sulfuric acid catalyst for the esterification step (new solid acids have eliminated this hazard, as discussed in subsequent chapters), small amounts of acrolein as a transient intermediate in the oxidation step, and reactivity hazard for the monomer product.

• Moderate: Reduce the hazards of a process by handling materials in a less hazardous form, or under less hazardous conditions, for example at lower temperatures and pressures. Dilution is one of the key words. Aqueous ammonia should be used instead of anhydrous NH3. Aqueous HCl in place of anhydrous HCl. Sulfuric acid in place of oleum. Figure 1.15 shows an example of the relevant effects observed for the concentration ofammonia measured in air as a function of distance from the place of rupture of a tank containing anhydrous or diluted ammonia solution. Less severe processing conditions are also another keyword. The use of improved catalysts is a critical element in reaching this objective and will be discussed extensively in the following chapters.

• Simplify: Eliminating unnecessary complexity to make plants more "user friendly" and less prone to human error and incorrect operation. In the previous section we emphasized how an objective of sustainable chemistry is the development of novel solutions to reduce complexity of chemical processes, which also allows a better control and an improvement of safety.

One way to simplify processes is to eliminate equipment, by combining reaction and separation. The use of membranes is discussed in Chapter 4. Another relevant example is reactive distillation. Figure 1.16 compares the traditional methyl acetate process with that based on reactive distillation (Eastman Chemical) [97-99]. Eastman

0 Distance, km

Figure 1.15 Concentration of ammonia measured in air as a function of the distance from the place of the rupture of a tank containing anhydrous or diluted ammonia solution.

Chemical Co.'s methyl acetate reactive distillation process and processes for the synthesis of fuel ethers are classic success stories in reactive distillation. Improvements for the Eastman process are very high: five-times lower investment and five-times lower energy use than the traditional process. However, combining reaction and distillation is not always advantageous and in some cases it may not even be feasible. The methyl acetate process based on reactive distillation has fewer vessels, pumps, flanges, valves, piping and instruments. This is an advantage also in terms of safety and maintenance. However, a reactive distillation column itself is more complex (multiple unit operations occur within one vessel) and thus more difficult to control and operate. It is thus not possible to make unique conclusions.

The concept of inherently safer design was first proposed by Kletz, who developed a set of specific design principles for the chemical industry [92] (see also [50]), but it has been publicized and promoted later by many technologists from petrochemical and chemical companies such as Dow, Rohm and Haas, ExxonMobil, and many others. A relevant source of information is the book Inherently Safer Chemical Processes: A Life Cycle Approach [49].

For inherent safety, while prevention, detection and mitigation are all considered, the emphasis should be on prevention. For example, moving the proposed location of a flammable liquid storage tank away from a public fence line may greatly reduce the consequences of a release and may reduce or eliminate the costs of providing the added protection system required if it is not. Inherent safety includes the consideration of more than just design features of a process. Inherent safety principles include human factors, in particular those related to the design and operating conditions. Finding an error-likely situation, such as controls being too difficult to access or too complicated, and working to reduce the clutter and confusion or to improve the accessibility to reduce the chance of a human error is an example of inherent safety in action.

Acetic Acid

Acetic Acid

Inherently Safer Design Examples
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