Getting to Core of Safer Plants

Tuesday, May 31, 2011 @ 03:05 PM gHale

It is Inherently Safer to Develop Processes with Wide Operating Limits that are Less Sensitive to Burps and Upsets that Pose a Problem for Plants.

By Nicholas Sheble
Chemical engineer and process safety expert Trevor Kletz first proposed the concept of inherently safer (IS) chemical processes in 1977.

He placed emphasis on the inherent nature of the process. Related product design for safety and safer products, process and plant lifecycles have since advanced.

The basic concepts of inherently safer processes are:

• Substitution
• Minimization or intensification
• Moderation or attenuation
• Simplification
• Limitation of (hazardous) effects
• Avoiding knock-on effects
• Making incorrect assembly impossible
• Make status clear
• Tolerance of error
• Ease of control
• Administrative controls or procedures

In 2007, the Center for Chemical Process Safety (CCPS) of the American Institute of Chemical Engineers (AIChE) reduced these eleven concepts to four principles:
• Minimize
• Substitute
• Moderate
• Moderate and simplify

Victor Edwards, director of process safety at Aker Solutions in Houston, expands on IS in his article “Design SAFER Process Plants” in an issue of Chemical Engineering.

“A brief survey of successful case histories shows that most reported applications rely on only a few of the core IS principles,” Edwards said. He emphasized the opportunities presented by three particular, often-overlooked possibilities for inherently safer processes:
• Hybridization or transformation
• Create a robust process to stabilize or ensure dynamic stability
• Limit hazardous effects during conceptual and detailed engineering

Edwards explains the three principles this way:

Hybridization or transformation: One IS concept rests on an inherently safer process for the partial oxidation of cyclohexane. Partial oxidation processes often involve hazardous conditions, as illustrated by the Flixborough, England tragedy in 1974, which killed 28 people, destroyed a plant, led to new process safety regulations, and inspired Trevor Kletz to propose his inherently safer design concept.

The Flixborough plant carried out liquid phase oxidation of large inventories of hot cyclohexane in pressurized vessels. At some point a large flammable vapor cloud formed, ignited, and exploded with devastating effect.

The traditional cyclohexane-oxidation process to produce a mixture of cyclohexanone and cyclohexanol operated at low conversion rates to avoid formation of unwanted byproducts. The eventual product was nylon.

Oxidation of cyclohexane with air instead of oxygen is common practice to reduce risks of transition from a partial oxidation reaction to an uncontrolled deflagration in bubbles or in the vapor space in the reactor. Low conversions and reaction rates led to large inventories of liquid cyclohexane.

During systematic research on the flammability and deflagration hazards of cyclohexane, air and oxygen mixtures, Professor Jenq-Renn Chen, of National Kaohsiung First University of Science and Technology in Taiwan discovered the addition of a small amount of water, which is inert and does not participate in the reaction, helped also to make the flammable vapors inert.

Cyclohexane and water form minimum-boiling azeotropes (An azeotrope is a mix of two or more liquids in such a ratio that its composition cannot change by distillation. This is because if you boil azeotrope the resulting vapor has the same ratio of constituents as the original mixture).

The increase in the vapor pressure of the cyclohexane/water liquid results from the increased vapor pressure of the water. The water vapor makes the vapor mixture inert by lowering the upper flammable limit of the vapor.

Chen’s work suggests it will be safe and practical to use pure oxygen for cyclohexane oxidation. Benefits include IS operation and improved productivity. They also suggest this approach could extend to safer processes for partial oxidation of other liquid hydrocarbons using pure oxygen.

Chen’s approach is a first-order IS process innovation because it changes the chemistry of the gas phase in a gas-liquid reaction and prevents the unwanted side reaction of combustion from occurring in the gas phase.

Although not a claim to have demonstrated a new IS concept, Chen’s work is different from the classical definition of the “Substitute” principle because the same reactants, chemical reactions, and products are involved. If we changed the name “Substitute” to something like “Change in Chemistry” or “Hybridize,” then we could lump it in with the many successful applications that are possible when using the “Substitute” concept.

Chen’s innovation permits rapid cyclohexane oxidation at lower temperatures and pressures, and could thus be said to be an example of the inherently safer principle “Moderate.” However, Chen’s approach enables more moderate conditions by narrowing the flammability limits through the addition of a new component – water. It is thus an example of supplementation or hybridization.

Addition of an additional compound to a reaction mixture to minimize hazardous reactions may add complexity to the purification process, but it may be justified by the increased safety.

Create a robust process to stabilize or ensure dynamic stability: Not all process designs are inherently stable, and if the process design is to be safe, the process engineer must ensure dynamic stability as well as ensuring the steady-state mass and energy balances.

A number of processes exist that have narrow safe-operating limits but are stable because of the addition of control systems.

Designing the process to be more inherently stable to process upsets with and without control systems is clearly inherently safer, although most discussions of IS do not address this principle. The IS principle “Ease of Control” has usually been interpreted to mean a process with a control system that the operator can understand clearly and manage effectively.

The CCPS briefly mentions the advantages of designing processes inherently more stable or robust: “It is inherently safer to develop processes with wide operating limits that are less sensitive to variations in the operating parameters. Some call this type of process forgiving or robust.”

Designing a robust process increases inherent safety by imposing a change in the process variables and is a form of “Moderate,” a second-order inherently safer design. The CCPS cites the work of Luyben and Hendershot that highlights how minimization or intensification in a reaction system intended to improve process safety may lead to less robust processes with the opposite effect.

Edwards proposes in his article that “Stabilize or Ensure Dynamic Stability” be added to the list of IS concepts to be sure that it is not overlooked in the quest for inherently safer processes.

Application of some of the other IS principles can adversely affect the dynamic stability of a process. For example, reduced liquid inventories (Minimize) in a distillation train make the process inherently safer from one perspective because the smaller process inventory decreases the consequences of loss of containment.

However, the smaller inventory also shortens the response time of the distillation system to process upsets, increasing the risk that the basic control system will not be able to restore the distillation system to the desired operating conditions and avoid a potentially unsafe operating condition and/or an unscheduled process shutdown.

Chemical reactors carrying out exothermic chemical reactions are perhaps the best-known examples of processes that can be dynamically unstable. Using an effective control system might be able to provide dynamic stability — but at the cost of installation and maintenance of the control system and at the cost of residual risk if the control system fails.

Another example of potential sources of process instability results from efforts to improve energy efficiencies in distillation trains through heat integration. In these cases, the bottom product of a second downstream column may preheat the feed to a column. This may increase the risk of process upsets due to increased interactions between the two columns.

While avoidance of add-on controls has always been a goal of inherently safer design, achievement of that goal has seldom mentioned the concepts of “Ensure dynamic stability” or “Stabilize” as tools of the process engineer.

The process engineer should work closely with the control systems engineer to address the dynamic stability of both the uncontrolled process and the controlled process to ensure a robust process.

Limit hazardous effects during conceptual and detailed engineering: Edwards cites David Clark’s seminal paper on the limitation of effects when siting and designing process plants. Clark reminds us there is a strong, non-linear decrease of fire, explosion, and toxic effects with separation distance. Comparatively small decreases in separation distance have a major effect, while larger increases in separation offer diminishing returns.

Methods from the Dow Fire and Explosion Index and the Dow Chemical Exposure Index provide quantitative screening estimates of the hazards from various parts of a chemical process. Other indices have been developed and evaluated to perform a similar objective to the Dow indices. These screening tools can identify those parts of a process where increased separation distances are necessary to limit potential escalation of an incident.

In one typical plant design, a 10% increase in separation distances for all units increases total plant investment cost by only 3%. Similarly, doubling the separation distance for a hazardous unit representing 10% of the investment cost of the plant would cost only 3% more. Because of the nonlinear effect of separation distance, doubling the separation distance for a hazardous unit could reduce explosion overpressures on the adjacent units by a factor of four or more.

The strong decrease in hazardous effects with modest increases in separation distances will often more than justify increased capital cost.

Spacing also offers important benefits in crane and other maintenance access, ergonomic advantages and decreased risk of incident escalation. Future plant expansions or process improvements are easier although expansions that decrease spacing may increase hazardous effects.

Here are Edwards’ recommended tools for inherently safer process plant design.
• Process hazards reviews
• Chemical interaction matrices
• Dow Fire and Explosion Index and Chemical Exposure Index
• Fire, explosion and toxic-release consequence modeling and risk assessments
• Layer of protection analysis
• Spacing tables for units and for process equipment
• Dynamic process simulation
• Inherent safety analysis
• Periodic design reviews during product and process research, development and design
• Reviews of plant siting, plot plan, equipment arrangement and 3-D computer models
• Occupied building evaluation and design
• Area electrical classification
• Safety integrity level assessments and safety instrumented systems
• Human factors reviews
• Ergonomics reviews
• Safety case development
• The design process itself

There will be a cost, but to read Edwards’ entire article, please click here.

The article is from Edwards’ paper presented at Mary Kay O’Connor Process Safety Center’s 2009 International Symposium: “Beyond Regulatory Compliance, Making Safety Second Nature.” Click here to purchase the original paper from Texas A&M University.

Nicholas Sheble ( is an engineering writer and technical editor in Raleigh, NC.

On 1 June 1974, a vapor cloud explosion destroyed the Nypro cyclohexane oxidation plant at Flixborough.

On 1 June 1974, a vapor cloud explosion destroyed the Nypro cyclohexane oxidation plant at Flixborough.

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