Minimizing Fire Risk at CPI Facilities Volume 2

Focus on Plant Hazards
Process-unit layout, piping, instrumentation/control and materials of construction are but a few of the technical
issues requiring special attention during the design or retrofitting of process plants that use hydrogen as a
raw material
A Checklist for Safer Chemical Batch Reactions ................................................................................. 10
A good understanding of the reaction chemistry is needed for a safe process design
Designing Safer Process Plants ............................................................................................................ 12
Several often-overlooked strategies to increase inherent safety are discussed here
Preventing Self-Heating and Ignition in Drying Operations ............................................................... 17
Incident investigation reveals that the most common root cause is lack of understanding
Facts at Your Fingertips: Gas Hazard Definitions and Data ................................................................. 20
Facts at Your Fingertips: Static Electricity Discharge and Fire Prevention ........................................ 21
Designing for A Safe Process ............................................................................................................... 22
Incorporating safety considerations throughout process design lowers the risk of a hazardous event
Clearing the Air About Respiratory Protection .................................................................................... 26
Learn the basics about selection and regulatory compliance for these potentially life-saving devices
Eye-and-Face Personal Protective Equipment .................................................................................... 32
Protecting the eyes and face in the workplace is imperative to preventing the estimated 10–20% of work-related
eye injuries that result in temporary or permanent vision loss
Managing Compliant MSDSs and Labels ............................................................................................ 36
Compliant MSDS programs depend on inventory management, regulatory knowledge and consistent authoring
Design Safer Solids Processing Plants ................................................................................................ 39
This approach defines a systematic framework and points toward relevant sources in the public domain
Finding the Right Gloves To Fit the Application .................................................................................. 45
There is a wide range of gloves available for hand protection on the job. Matching gloves to their chemical-
resistance properties is one criterion for selection
Dust Control in the Chemical Processing Industries .......................................................................... 49
The prevention of dust hazards in the CPI is integral to process-safety management — widereaching mitigation
schemes must be implemented
Avoiding Static Sparks In Hazardous Atmospheres ........................................................................... 53
Relatively simple steps can be taken to mitigate risks associated with static electricity in process plants
Piping Design for Hazardous Fluid Service ......................................................................................... 57
Extra considerations and precautions are needed beyond the requirements of codes and standards
Chemical Protective Clothing ............................................................................................................... 64

While regulatory agencies outline requirements for communicating chemical hazards, the format of compli-
ance is up to the employer. Software tools can be helpful aids in meeting these requirements
Dust Explosions: Prevention & Protection ........................................................................................... 70
Understand what causes these disasters and then put these practical measures in place
Prevent Explosions During Transfer Of Powders Into Flammable Solvents ..................................... 75
Explanations of the development and characteristics of explosive atmospheres and the potential for an explo-
sion to occur are given, along with the advantages and disadvantages of preventative practices
Preventing Dust Explosions .................................................................................................................. 83
Risk management programs are critical for safe handling and processing of combustible dust as well as for
OSHA regulatory compliance
Compressed Gases: Managing Cylinders Safely ................................................................................ 86
Follow these recommendations to ensure the safe handling, storage and use of gas cylinders
Reduce Hazards in Process Vacuum Systems ..................................................................................... 93
Reduce explosion risks, and chemical and physical hazards, to ensure safer operation of vacuum pumps and
related systems
Tolerable Risk ......................................................................................................................................... 100
While determined risk is generally well understood, tolerable risk can be the missing link to complete
risk assessments
Vacuum Systems: Recommendations For Safe Operation ................................................................ 106
Follow this guidance to ensure that steam ejector systems, mechanical vacuum pumps and integrated vacuum

H ydrogen is widely employed for hydrogenation and other purposes in chemicals manufacture, petroleum refining and other chemical process
industries (Hydrogen: The Real Action Is Today, CE, February, pp. 28ff), and it holds promise as a fuel. However, this gas is highly explosive, prone to leakage and permeation, and dif- ficult to detect, so special care must be taken during process engineering design of equipment and systems that handle or contain hydrogen. This knowledge and attention to detail is all the more important inasmuch as the reactions involving hydrogen tend to be exothermic and in many cases are conducted under high pressures and temperatures.
Hydrogenation and other hydrogen- handling processes involve a consider- able amount of process equipment, instrumentation and piping components, such as reactors, catalyst feed vessels, spent catalyst filters, pumps, valves, pressure relief devices, pressure regulators and check valves. Many such systems, particularly those for hydrogenation of organic chemicals, are located inside a building. Such facilities must be designed with four levels of safeguards, namely:
• A high degree of automation, with remote operations, interlocks and alarms to monitor process and environment conditions
• Certified relief devices for process equipment and piping. The relief devices vent discharges must be di- rected to safe locations
• Adequate, dependable space ventilation to prevent accumulation of hydrogen gas pockets for systems located inside buildings
• Damage-limiting building construction to protect personnel and property.
Hydrogen’s hazards When gas is released or escapes from containers, it presents detonation, deflagration and fire hazards. The wide flammability range, high burning rate, low ignition energy and non-luminous flame of hydrogen accentuate the combustion hazards.
The flammability range for hydrogen in dry air at atmospheric pressure and ambient temperature is about 4 to 75%. With so wide a range, virtually any release of hydrogen has a great potential of igniting. The minimum energy required for ignition of hydrogen in air at atmospheric pressure is about 1.6 X 10–8 Btu, which is consid- erably less than the value for other fuels, such as methane (2.7 X 10–7 Btu at 14.7 psia). As a consequence of low ignition energy, even a small heat-producing source, such as friction and static charge, may result in ignition when hydrogen gas is released at high pressure. In fact, hydrogen is fre- quently thought of as self-igniting. All ignition sources in the hydrogenation system must be eliminated or safely isolated.
The outcome of hydrogen combustion due to a release in air is cannot be predicted. Some of the many possible hazardous outcomes of hydrogen combustion are as follows: Fire: In this case, a release of hydrogen gas in air ignites and burns like fuel at a burner. The size and type of flame will depend on the hydrogen re-
lease rate. In any case, the flame ra- diates very little heat, and is visually imperceptible under artificial light or daylight. Therefore, reliable methods of fire detection must be provided in facilities that handle hydrogen. Deflagration consists of a flame that propagates through a combustion zone at a velocity less than the speed of sound in the unreacted medium. The flammability range is the same as that for fire. The presence of con- fining surfaces such as piping, duct- ing or vessels can elevate the pressure and accelerate the flame speed. If the flame speed exceeds the speed of sound, the deflagration process can transition into a detonation. Detonations, propagating at a rate greater than the speed of sound within the unreacted media, generate high pressures. Detonation requires a richer hydrogen-oxidizer mixture and a more-energetic source of ignition to occur than does a deflagration. Very high pressures can be generated in a detonation when a pressure wave is reflected from wall to wall inside a building. Detonation is associated with shock waves and an accompany- ing blast wave that can severely injure personnel and damage property. BLEVEs (boiling-liquid expand- ing-vapor explosions): Theoretically, cryogenic containers with liquid hydrogen present are subject to BLEVE. Under rapid heating (for example, due to engulfment by fire), a vessel contain-
Feature Report
Process-unit layout, piping, instrumentation/control
and materials of construction are but a few of the
technical issues requiring special attention during
the design or retrofitting of process plants that use
hydrogen as a raw material
Part 1
Richard C.Hachoose
CH2M HILL
Lockwood Greene

ing pressurized liquid hydrogen may fail suddenly, producing this explosive effect. On the other hand, a cylinder containing compressed hydrogen gas is not subject to BLEVE if it fails.
Deflagrations and detonations alike are perceived as explosions. The resulting shock waves and hot product gases impinging upon the surroundings outside of the combustible region can also be referred to as blast waves. There is no combustion in a blast wave, but it physically displaces the surrounding gases and it propels shrapnel.
Most hydrogenation and hydrotreat- ing reactions are performed at high pressures (pressures as high as 100 atm or more are not unheard of) and over wide temperature ranges; for in- stance, for some types of reactions, –20 to 350°F. These conditions generate high stress in equipment and piping systems, so that achieving leak-tight design is not a trivial matter — especially in light of hydrogen having the lowest molecular weight of an industrial gas. Aside from being prone to leak easily through seals, the gas readily permeates through various materials that are impervious to other gases.
Materials selection Key considerations that must be taken into account when selecting materials of construction for hydrogenation and other hydrogen-handling system include the following: • Design temperature and pressure
• Hydrogen embrittlement • Permeability and porosity • Compatibility between dissimilar
metals In the design of low- and high-pressure hydrogen systems alike, stainless steel (Types 304, 316) is the most commonly used material for equipment, tubing, piping, fittings, and components. Other construction materials that are satisfac- tory in hydrogen service include Monel, Hastelloys, aluminum alloys, suitable grades of carbon steel, glass-lined carbon steel and copper alloys. However, the final selection should be based on full awareness of all the raw materials, catalysts and other substances that are used in the process. Some process compounds can significantly affect materials’ suitability even when present only in trace amounts.
Ordinary carbon steel, iron, low-alloy steels, chromium, molybdenum, nio- bium, zinc, nickel, etc are not acceptable for use at cryogenic temperatures. Cast iron is not acceptable for hydrogen service due to its porosity. Nickel should not be used because it is subject to severe hydrogen embrittlement.
Elastomers and plastics such as polytetrafluoroethylene are in many installations used for gasketing, O- rings, packing, seats and other sealing elements. Ideally, their usage in hydrogen service should be limited, because they can fail in the event of fire.
In a hydrogen environment, most welds are susceptible to hydrogen em-
brittlement. Therefore, post-weld an- nealing is recommended to restore the microstructure.
Outside is best Some important factors in determin- ing the location for a hydrogen facility include the following: • Climate condition (warm or cold) • Process condition of the hydrogen
reaction system (pressure, temperature, other properties)
• Quantity of hydrogen involved • Type of adjacent property • Nature and presence of other fuels or
oxidizers in the facility or vicinity • Protection afforded by shielding,
barricading, or other means. From a safety standpoint, the ideal location of a hydrogenation or other hydrogen-handling facility is outdoors. This allows any hydrogen leak to quickly disperse into the atmosphere, thus minimizing the potential for a deflagration or detonation. A separate, dedicated building, away from the rest of other buildings is the next best option for location of a hydrogenation system.
In cases where a reactor must be located indoors, the building should be designed to prevent leakage and mi- gration of hydrogen vapors into other parts of the building, as discussed below. The reactor should be installed on an outer building wall on the top floor; or at a building corner, where there are at least two walls for venting. A missile containment courtyard with limited access should be provided in front of the hydrogenation building, facing pressure relief panels.
The minimum distance requirements between properties constitutes a critical pa- rameter that must be evaluated early during the design phase. There are several useful
CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2006 55
HYDROGENATION
MANUFACTURE
Hydrogenation of complex molecules is a key reaction for organic synthesis of many products. Examples of hydrogenation reactions that are commonly encountered in the chemical industry include:
• Saturation of unsaturated aromatics, ole- fins, fatty acids and esters
• Reduction of nitro and nitroso compounds to amines
• Reduction of ketones, aldehydes, esters and carboxylic acids to alcohols

sources of information that relate hydrogen quantity to separation distance. Two such sources are the U.S. National Fire Protec- tion Assn.’s NFPA 59 (Table 5.4.1.2 Non-re- frigerated Container Installation Minimum Distances) and Data Sheet FM 1-44 of FM Global (Johnston, R.I.).
As a matter of good engineering practice, the separation distance should be checked by energy release calculations involving the rupture of an overpressurized vessel. The energy release is converted to equivalent pounds of TNT, a quantity which is related to shock and gas pressure waves. From the calculation, one can determine the resultant blast wave pressure at any distance from the source of explosion.
Damage-limiting construction All buildings used in hydrogen services must be designed to limit personnel injury and facility damage in the event of fire or explosion. The building should be constructed in accordance with the International Building Code, the NFPA (e.g. 55, 68), the Code of Federal Regulations 29CFR1910.103 (in the U.S.), as well as with any other codes and insurance regulations that have jurisdiction in the location. Dam- age-limiting construction for the building requires pressure resistant walls to contain explosion and pressure-re- lieving panels to vent an explosion.
Both NFPA 68 and FM Global’s datasheet FM 1-44 provide useful methods for sizing deflagration vent panels. Under the FM 1-44 guidelines, for example, the facility must meet the following design criteria: • The ratio of the enclosure surface
area (A s) to vent area (A v) should equal less than 7.25.
• A minimum of 1 ft2 of vent area is required for every 15 ft3 of room volume. (Note, however, that this requirement results in large facilities, which are costly in today’s economy. It is therefore recommended to con- sult with all stakeholders early in the design, to establish the appropriate codes, guides and standards for use in the design. This point is particularly important because other codes, such as NFPA 68, do not require this ratio criterion.)
• The pressure-resistant walls must be designed to a minimum pressure
of 100 and a maximum of 216 lb/ft 2. As good engineering practice, the pressure-resistant-wall rating should also be confirmed by calculations of the shock and gas pressure waves generated by the energy release of an overpressurized reactor (equivalent pounds of TNT).
Some features of a safely designed building in which hydrogen is stored or used are as follows: • Hinged doors swing outward in an
explosion • There are no pockets or space where
hydrogen gas could accumulate • Window panes (if installed) are shat-
terproof or plastic in frame • Floors, walls and ceilings are de-
signed and installed to limit the generation and accumulation of static electricity
• Floors, walls and ceilings are designed for at least 2 h of fire resistance
• Walls or partitions are continuous from floor to ceiling, and securely anchored
• The building is constructed of non- combustible materials, on a sub- stantial frame
• Restrained deflagration vent panels are present
• There is adequate ventilation, and any heating in rooms is limited to steam, hot water, or other indirect means
• Deflagration venting is provided in exterior walls or the roof
There are no national codes, standards or guides that cover design-limiting construction for detonation occurrence. Since detonation takes microseconds to
occur, per NFPA 68, it cannot be suc- cessfully vented. Also, there are cur- rently no known pressure relief devices that can react to a detonation speed.
Piping and supply lines The attributes of a piping system suitable for handling hydrogen are summarized in Table 1. Although some of those attributes, such as the ones relating to structural matters, are shared by other piping systems, the adherence to then is especially important with hydrogen.
The piping to convey hydrogen gas from cylinders entails special considerations. If the cylinders cannot be stored outside, then a well ventilated storage shed is required. The temperature in the shed must be kept below 50°C. Hydrogen cylinders must be segregated from cylinders, tanks, silos or other containers that store oxidant gases or other oxidants.
Hydrogen cylinders supplying the process must be connected through approved gas manifold. The following components and attributes are typical in such a hydrogen supply line: • The gas manifold usually consists
of pressure regulators, pressure gauges, relief valve(s), vent con- nections, and provision for automatic switchover between online and standby cylinders. An alarm for alerting operator to a switchover must be provided. A supply of inert gas, usually nitrogen, must be pro-
vided for purging the hydrogen lines before and after use. Hydrogen detection sensors with alarms are also required in the area
Feature Report
56 CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2006
TABLE 1. PIPING SYSTEMS FOR HYDROGEN
• Piping materials and designs should be in compliance with ANSI/ASME B31.1, ANSI/ASME B31.3, NFPA 55 and, if relevant, other industry standards and codes
• Critical piping systems should be protected from accidental damage. Install barriers to shield critical process piping, such as hydrogen supply lines
• All hydrogen lines should be labeled as “HYDROGEN”, with flow-direction arrows
• Hydrogen gas lines should not be buried. If underground installation cannot be avoided, the piping should be installed in an open trench with removable grating
• Avoid locating hydrogen lines beneath power transmission lines
• All piping lines in a hydrogen system should be electrically bonded per the NFPA 70 electrical code to prevent static charge accumulation in the system
• All piping and tubing joints should be made by welding or brazing. Flanges should only be used where absolutely necessary
• Assure adequate structural flexibility to limit forces and moments between fixed systems. Expansion joints and loops are acceptable in hydrogen service
• Design piping-support members to account for all concurrently acting loads transmitted into such members
• If polytetrafluoroethylene gaskets are used, install them in such a way that they are constrained on all sides (to prevent cold flow and subsequent leakage)

• An excess flow valve is required to shut down the hydrogen supply in the event of downstream line rupture or similar failures
• Automated block and bleed valves should be provided to isolate the cylinders from the reactor or other process equipment. A bleed line having a flame arrestor must be vented outside at a safe location
• Cylinders must be adequately earthed, and piping line bonded.
• Check valves must be installed in appropriate locations to prevent back- flow of process contents into the line
Control and monitoring Minimum instrumentation and control practices for hydrogen plants are summarized in Table 2. Note the im- portance of adopting a fail-safe design philosophy. Critical process-safety interlocks, soft- and/or hard-wired, should be actuated by redundant in-
strument sensors. Examples of critical safety instruments installed with redundancy are those for hydrogenation-reactor process temperature and pressure, and hydrogen detection in confined spaces. Some of the common interlocks associated with hydrogenation reactors are found in Table 3.
Hydrogen detection: Because hydrogen gas is colorless and odorless, means for its detection must be pro-
vided in all areas where leakage or hazardous accumulations may occur. For hydrogen-detection sensors installed in confined spaces, current standard practice at hydrogenation plants calls for alarm setpoint at 1% concentration by volume in the ambient air (which is 25% of the lower flammability limit, as given earlier). Furthermore, operators should not enter any confined space in which the ambient hydrogen concentration is greater than 0.25%.
Indoor process units that employ hydrogen should be supplied with fixed and portable hydrogen sensors. Por- table sensors are used by personnel when entering an area where a leak may have occurred. It is suggested that the fixed detectors be located in the following areas: • Reactor room, where hydrogen leak-
age, accumulation or spill is possible
• Storage area, where hydrogen con- nections are routinely made and separated
• Building-air intake ducts, if hydrogen could be carried into the building
• Building-air exhaust ducts if hydrogen could be released inside the building
The appropriate response to a detection of the presence of hydrogen in ambient air varies with the likely degree of risk. Examples of common responses include isolation of the hydrogen supply source, shutdown of the hydrogen-handling and process system, provision for issuing a visual and audible warning, and/or increased
ventilation of the enclosed space. In addition, remote television monitoring should be considered for systems not
visible from the control room. Fire detection: Because a hydrogen flame in air is usually almost invisible and because the emissivity of a hydrogen flame is low, the flame is hard to see or feel. So, aside from the detection of the gas itself, the design of a hydrogen-using facility must provide detection of a hydrogen flame in all areas where leaks or hazardous accumulations omay occur. Infrared (IR) and ultraviolet (UV) are two technologies commonly used.
Process venting In any hydrogen-using facility, provision must be made for safe disposal of unused hydrogen. The most-common methods employ burn-off flares or roof-venting.
For large quantities of unused hydrogen (the definition of “large” varying by industry), a flare system is generally the choice. The hydrogen-containing exhaust process gas is piped to a remote area, where it is burned with air in a multiple burner arrangement.
TABLE 3. INTERLOCKS OFTEN USED WITH REACTORS
Upset Process Condition Interlock Action
1. On high temperature in the reactor
• Shut off the hydrogen supply and open vent line (block and bleed design).
• Shut off the reactor heating system.
• Turn on the reactor cooling system.
2. On high pressure in the reactor
• Vent off the hydrogen to atmosphere.
3. On low speed of reactor agitator
4. On hydrogen detection in reactor room
• Start the room emergency exhaust fan.
• Shut off all rotating equipment motors in room.
5. On flame out of the flare unit
TABLE 2. INSTRUMENTATION AND CONTROL FOR HYDROGEN-HANDLING SYSTEMS
• All instrumentation and wiring designed to meet the electrical hazardous area classification of Class I, Division (I or II), Group B
• Instruments designed with appropriate range, accuracy, reliability and response time. The appropriate response times of the complete loop, from input device to final control element, should be consistent with the fast process kinetics of most reactions involving hydrogen
• Critical safety instruments provided with redundancy
• Computer control system provided with data acquisition capability, backup power, and fail-safe action on the loss of electrical power or instrument air

Flare systems are equipped with pilot ignition and a warning systems in case of flameout The exhaust-gas header to the flares is usually kept under slight negative pressure, so extra care must be taken to ensure that the vent systems is leaktight to avoid air intrusion and possible detonation.
Although flare-system design technology is mature, flares continue to pose hazards of flame stability, flame blowoff, and flame blowout. To minimize the malfunction of flare systems, it is important to keep the stack discharge velocity between 10 to 20% of the sonic velocity in hydrogen at the temperature prevailing in the exhaust line. Velocities above the recommended range may cause the flame to blow off or blow out.
For roof venting, the main variables to consider are such site-specific conditions as the prevailing-wind direction and speed, proximity to adjacent buildings, vent stack height, and local discharge limitations or other environmental restrictions.
In a roof-vent system for a process plant in which hydrogen is used, it is better to vent each major piece of equipment (such as each of several hydrogenation reactors) separately instead of using an interconnected collection header. The separate-vents approach avoids the possibility of any high-pressure, high-throughput discharges overpressurizing the low-pressure parts of the system. If, however, collection headers cannot be avoided, be sure to size the header to handle the flows from all discharges with only minimal back pressure developing at the lowest-pressure equipment.
The roof-vent lines should be designed with inert-gas (usually nitrogen) purging and steam snuffing at the end of each line. The purging takes air out of the system before introduction of hydrogen, and removes hydrogen at the end of the process. The design ac- tivity must include a review of all piping and equipment system to ensure that they can be adequately purged and leak tested prior to admission of hydrogen. The vent piping must be designed with care to prevent steam condensate from flowing back into the process equipment. During plant operation, both nitrogen and steam flow
should be commenced upon the introduction of hydrogen gas in the process equipment.
Avoidance of flammable atmospheres is a key aspect of preventing combustion hazards. If the exhaust
vent stream happens to be solvent- laden, the stream should be passed through the condenser, scrubber or ab- sorber prior to discharge to the atmosphere. For additional safety, a certified flame arrester should be installed in the vent line. Be aware, however, that flame arresters certified for use with hydrogen are still not common; the equipment suppliers are still in the process of development and cer- tification.
The venting system must be designed to handle both normal and emergency venting requirements. Normal venting usually involves, for example, exhaust gas streams from process reactions, distillation, and equipment and piping-systems purges, which are handled in the venting systems as just discussed. For emergency
venting, the plant must install safety valve or rupture disks, as appropriate, on vessels, lines, and component systems for emergency venting to prevent damage by overpressure. Each safety device must, of course, be sized for the complete system that it protects.
Although there are a few excep- tions, most hydrogenation reactions are not usually subject to runaway. In cases where a potential for runaway does exist, the emergency vent system should be sized by a rigorous methodology such as that developed by AIChE’s Design Institute for Emergency Relief Systems (DIERS).
As a rule, rupture disks are used for runaway conditions because their response time is faster than that of safety valves. In many cases, a rupture disk is installed in series with a safety relief valve, to protect that valve from corrosive chemicals or sticky solids such as catalyst particles. As a minimum, if there is no indication of a runaway-reaction potential, the relief devices should be sized for fire exposure conditions and set to relieve at the maximum allowable working pressure (MAWP) of the vessel.
The vent line from emergency devices should be routed to a catch tank.
This tank should be sized for at least one and half times the volume of the largest process vessel at the facility, to provide liquid containment and phase separation. Both horizontal and vertical catch tanks are common in practice. The tanks and vent headers are usually purged with nitrogen to reduce oxygen concentration to below 5% by volume. All nozzles, at- tachments, supports, and internals for the catch tank should be designed for shock loadings resulting from thermal effects and slugs of liquid during emergency relief.
Room ventilation In light of hydrogen’s wide flammability range and low ignition energy, hydrogen leaks or spills in a non-ventilated, confined space can readily form an ignitable gas mixture. Accordingly, all such spaces should be provided with room ventilation, in addition to the aforementioned hydrogen-concentration monitoring. Confined spaces for hydrogenation systems are usually designed for 15 to 20 air changes per hour during normal conditions, and 30 to 40 when high hydrogen concentration has been detected. To accommo- date the necessary ramped-up volu- metric flowrate for emergencies, the
ventilation exhaust fan should be a variable-speed or two-speed unit. Ex- haust fans must be fabricated of non- sparking materials, and their motors rated for the same electrical classification as that of the other motors in the room.
Rooms in which hydrogenations or other hydrogen-using operations take place must be kept at a negative pressure with respect to outside areas to prevent outward hydrogen flow. The room pressure should be monitored, with provision for an alarm to sound.
Electrical requirements Most fires in hydrogenation facilities are caused by electrical faults, so care- ful consideration needs to be given to the design of electrical equipment or wiring in such a facility. Electrical systems should be designed to comply with the Electrical Area Hazardous Classification. Per the NFPA 70 Code, an area where flammable hydrogen mixture is normally present is clas-
Feature Report

sified as Class I, Division 1, Group B, whereas an area where hydrogen is contained and only present under abnormal conditions is classified as Class I, Division 2, Group B. All potential sources of ignition should be prohibited in such areas.
Electrical installations in Class I, Division 1, should meet the following requirements: • Certified for use in hydrogen envi-
ronment • Intrinsically safe per NFPA 70 and
Underwriters Laboratories Specifi- cation UL 913
• Non-certified electrical equipment to be located in purged enclosures. In this case, the enclosure should be maintained under positive pressure and purged with an inert gas such as nitrogen. This is a meaning- ful requirement at present, because there are no commercially available motors that are suitable for use in Class 1 Division 1 Group B environment; therefore, purged enclosures are employed routinely in this application
• All piping joints electrically bonded • All portable equipment electrically
grounded prior to use For Class I, Division 2, Group B en-
vironments, explosionproof motors are not available. Standard totally enclosed, fan cooled (TEFC) motors can be used, provided that there are no arcing devices in the motor. Motors suitable for this area classification should include an approved thermal switch, which limits the external surface temperature of the motor housing for the Group B rating. Installations for explosionproof equipment should meet NFPA 70 and 496 guidelines as a minimum.
Fire protection A fire protection system is required for all hydrogenation facilities. At a minimum, the design of the system should include all the following features: • Automatic process shutdown system
on fire detection • Water sprinkler system • Water deluge system • Dry-chemical extinguishing system Dry-chemical extinguishers, carbon- dioxide extinguishers, nitrogen and steam are all acceptable for use to
extinguish small fires. As precaution to prevent major explosion hazards, it is recommended that a hydrogen fire should not be extinguished until the hydrogen source has been isolated, to prevent ignition of a large combustible cloud of the gas.
Hydrogenation equipment should be protected by water-deluge sprinkler systems designed for coverage of at least 0.35 gallons per minute per square foot. The fire-water supply system must be designed with enough capacity that when a hydrogen fire is detected, water can be applied on equipment in the nearby surroundings as well. The facility should be provided with a spill protection and drainage sized for the largest possible single spill to prevent the spread of fire to other areas.
Safety sum-up A hydrogenation or other hydrogen- using facility is designed safely if it minimizes the severity of the consequences of a mishap. The people and facilities are separated from the potential effects of fire, deflagration, or detonation originating from failure of hydrogen-handling equipment. All the confined spaces are adequately
ventilated to prevent accumulation of flammable mixtures and all unused hydrogen gas from the process is safely disposed of by flare system or
vented above other facilities. The instrumentation and control system is a “fail-safe design” with features such as redundant sensors for critical safety instruments, remote monitoring of critical information, remote operation with process safety interlocks.
Edited by Nicholas P. Chopey
Author Richard C Hachoose is a Senior Principal Technologist in the Somerset, NJ, office of CH2M HILL Lockwood Greene (Phone: (732) 868-2282; e-mail: [email protected]). He has over 24 years of process engineering design experience in the pharmaceutical, chemical and metallurgical processing industries. He is also an ad-
junct professor at Stevens In- stitute of Technology, where he teaches a course on Chemical Technology Processes in API Manu- facturing. He is a registered Chartered Engineer in the U.K. He holds a B.Sc. (Hons) in chemical engineering from the University of Manchester Institute of Science and Technology (UMIST), Manchester, England.
Circle 40 on p. 93 or go to adlinks.che.com/5828-40

I t is a fact that safety-related incidents do occur in the chemical process industries (CPI). These incidents have, in part, led to in-
creased attention to reactive-chemical issues by industry, government and other stakeholders. We know that good safety-management systems take the reactivity of chemicals as well as the energetics of both desired and adverse reactions into consideration.
This article provides a safety checklist that can be used as a guide for the design of a new process. Alternatively, it can be used to identify information gaps when existing processes undergo periodic review.
Runaway reactions It is known that in many cases, thermal runaway reactions occur due to the following factors: 1. Lack of understanding of the pro-
cess chemistry and energy for the desired reaction
2. Inadequate heat transfer capacity at the plant level
3. Insufficient control and inadequate plant-safety back-up systems, including emergency venting
4. Ineffective plant operational procedures and inadequate training The reader is encouraged to add
plant-specific items to this list, as needed. A hazard analysis can then be used to identify appropriate controls and safeguards to reduce risk. It is important to note that as a process undergoes changes, the safety information also needs to be updated. The final process-safety package should be at such a level that it can be used for technology-transfer purposes at the R&D or commercial-production stage by out-
sourcing contractors and by in-house personnel. When developing safety documentation, it is important to keep in mind that it must comply with company policies and procedures as well as country and local regulations.
Process safety checklist The following items should be considered in relation to process safety. Preliminary hazard assessment: • Determine the thermal stability of
all reaction components within the minimum and maximum process temperatures attainable under a worst-case scenario
• Identify unwanted interaction between reagents and solvents
• Identify potential reaction contaminants that may have an inhibitory or catalytic effect on the desired reaction
Quantification of desired reactions: • Determine the heat of reaction and off-
gas rates for the desired and quench reactions, including the heat resulting from accumulation of reagents or slow forming intermediates
• Determine the maximum adiabatic temperature for the reaction, and determine the basis of safety relative to the estimated boiling point of the reaction mixture
• Understand the relative rates of all chemical reactions
Quantification of adverse reactions: • Assess the thermal stability of the
reaction mixture over a wide temperature range
• When optimizing the robustness of the process, consider other reaction
variables, such as pH, concentration, conversion rate, off-gas rate, stability of starting and product sub-
strates in solution and as a slurry • Consider the potential and impact of
unwanted vapor-phase reactions • Develop a chemical-interaction ma-
trix for materials present in the reaction mixture, classify the reactivity, and communicate this information to operational personnel
Plant considerations: • Conduct a basic energy balance to
consider the heats during various additions, heat generated during the chemical reaction, and the heat removal capability of the plant reactor system. Remember to include reactor agitation as a source of energy (~2250 Btu/h/hp)
• Consider the impact of possible de- viations from the intended reactant charges and operating conditions
• Identify all heat sources connected to a reaction vessel and assume the maximum possible worst-case scenario
• Determine the effect of the lowest possible temperature to which the reactor heat-transfer fluid could cool the reaction mixture
• Consider the impact of temperature gradients and other issues, such as increased viscosity, freezing at reactor walls, fouling, and so on, in plant-scale equipment
General chemistry and engineering design concepts: • Design reactions that occur fairly
rapidly • If possible, avoid batch reactions
in which all the potential chemical energy is present at the onset of the reaction
• Use semi-batch processes for exothermic reactions in which the batch temperature and any off-gassing can
Solids Processing
ChemiCal engineering www.Che.Com april 2008 61

A good understanding of the reaction
chemistry is needed for a safe process design

be maintained through controlled addition of the reagent
• For highly exothermic reactions, avoid using temperature control of the reaction mixture as the only means for limiting the reaction rate
• When scaling up a reaction, account for the impact of vessel size on heat generation and heat removal: The
volume of the reaction mixture increases by the cube of the vessel radius but the heat-transfer area increases by the square of the radius A comprehensive hazard evalua-
tion should be conducted using appropriate estimation and experimental techniques to identify potential reac-
tion hazards in materials, as well as the desired and adverse reactions. We use estimation techniques, differential scanning calorimetry (DSC), Carius Tube, and reaction calorimetry as needed. Identify any adverse or thermal runaway reactions and, if needed, characterize them using adiabatic calorimetry, such as ARC (accelerat- ing rate calorimetry) or an Adiabatic Dewar Calorimeter. If required, the
vent size can be determined using Design Institute for Emergency Relief Systems (DIERS, an AIChE industry alliance) methodology with data generated using an adiabatic dewar or
Vent Sizing Package (VSP; a special-
ized adiabatic calorimeter that uses temperature, pressure, and rate data to allow for sizing emergency vents). The references given below were used to develop this article and are an ex- cellent source of information. n
Edited by Dorothy Lozowski
Author Richard Kwasny is the as- sociate director of the Pro- cess Safety Laboratory at Chilworth Technology Inc. (250 Plainsboro Road, Build- ing 7, Plainsboro, N.J. 08536; Email: rkwasny@chilworth. com; Phone: 609–799–4449), where he has responsibility for the quality, safety and productivity of the thermal hazard laboratory. His exper-
tise encompasses all areas of chemical reaction hazards and flammability of dusts, vapors and gases. Kwasny completed his Ph.D. at London South Bank University. His research developed, in part, an assessment strategy to allow for the safe scale up of reduction reactions. Before his current position, Kwasny headed the Process Safety & Hazard Identification group for Scher- ing-Plough’s Chemical and Physical Services Dept. Previously, he managed the Dow Chemical Reactive Chemicals testing function for Canada. Kwasny is a member of the ASTM Committee E27 on hazard potential of chemicals.
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References 1. Hendershot, Dennis C., A Checklist for In-
herently Safer Chemical Reaction Process Design and Operation, Center for Chemical Process Safety International Conference and Workshop on Risk and Reliability, 2002.
2. Kwasny, Richard S., “Hazard Assessment Strategies for Reduction Reactions,” South- bank University, London, 1999.
3. Barton, J. and Rogers, R., “Chemical Reaction Hazards,” 2nd ed., Gulf Publishing, 1997.
4. “Bretherick’s Handbook of Reactive Chemical Hazards,” 6th ed., Butterworth Heinemann, 1999.
5. “Guidelines for Chemical Reaction Hazard Evaluation and Application to Process De- sign,” Center for Chemical Process Safety of the AIChE, 1995.
62 ChemiCal engineering www.Che.Com april 2008
Environmental Manager

M any individuals and organizations have made important contributions to the creation of inherently safer (IS)
products, processes and process plants [1–3]. A brief survey of successful case histories shows that most reported applications relied on only a few of the core IS principles. This paper emphasizes the opportunities presented by three particular — and often-overlooked — possibilities for inherently safer processes.
The methods proposed here ensure integration of IS methods beginning with process conception and continu- ing through process plant engineering design. Particular emphasis is given to matching the IS principles with the state of the project. For example, substitution is best applied during product and process research, while limitation of effects is most effective during plot plan layout and equipment arrangement.
The chemical process industries (CPI) face the challenge of working with processes and products that present many hazards, such as the following: • The manufacture of fuels uses and
produces products that burn with significant energy release
• Certain basic chemicals, such as mineral acids and halogens are toxic and/or corrosive
• Many manufacturing processes ei- ther release or require significant
energy transfer to achieve chemical transformation
• Some manufacturing processes produce benign products but require hazardous chemical intermediates in their manufacture
For these reasons, rigorous process and product safety practices must be used throughout the lifecycle of process plants and must be applied to their associated raw materials and products. In recent years, this has led to major efforts in green chemistry and engineering to develop products, manufacturing processes, and plants that are safer for both people and the environment.
Before green chemistry and engineering achieved prominence, there were pioneering insights in the design of safer process plants. Early ap- proaches to safer processes often employed additional instrumentation and procedures. These measures were often helpful and necessary, but instrumentation and operators can fail, especially when faced with complexity.
Trevor Kletz [1] recognized that “What you don’t have can’t leak”, when he first proposed the concept of the inherently safer chemical processes in 1977. His approach placed an emphasis on the inherent nature of the process. Since then, important related concepts such as product design for safety and safer products, process and plant lifecycles have also advanced.
Creation of IS processes has been the
objectives of a number of creative individuals and organizations since Kletz’s path finding proposal, with many no- table successes.
Complete coverage of the entire product/process/plant lifecycle is needed to assure optimum health, safety and en-
vironmental performance of a chemical enterprise.
This article focuses on how to ensure maximum incorporation of IS processes into the creation of a process plant by beginning at the product and process research stages and con- cluding with the detailed design. No effort is made to address the application of inherently safer principles beyond plant design, although these are also important.
Layers of protection The classical onion diagram (Figure 1) illustrates the safety layers that technical professionals throughout
Feature Report

1. Process design
5. Physical protection (relief devices)
6. Physical protection (dikes)
7. Plant emergency response
8. Community emergency response
1
2
3
4
5
6
7
8
to increase inherent safety are discussed here

the CPI use to prevent process plant incidents. This diagram helps to ex- plain the following four basic process risk-management strategies: Inher- ent, passive, active, and procedural or administrative Inherent safety is at the core of the onion — the process design. A process that cannot have a major fire, explosion or toxic release is inherently safer than one that could if one or more layers of protection were to fail. Passive safety layers represent the addition of such safety features as a
dike or a blast wall. Because passive layers of protection require no active intervention by a human or by a ma- chine, they are deemed more reliable than active layers of protection or procedural layers of protection. None- theless, the ability to make an explosion impossible — when possible — is clearly better than trying to mitigate the effects of a potential explosion by adding a blast wall. Active layers of protection represent such features as the basic process control system, a safety-instrumented system, and mechanical interlocks. Procedural or administrative safety layers are generally considered to be the least reliable and include operating procedures and operator inter-
vention. Depending on the site-specific hazard, procedural or administrative controls may be entirely appropriate.
In general, the preferred ranking of methods to control process risks is shown below:
Inherent > passive > active > procedural or administrative
Basic concepts Inherently safer process concepts are summarized below [1]: • 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 proce-
dures In 2007, the Center for Chemical Process Safety (CCPS) of the Ameri- can Institute of Chemical Engineers (AIChE) concluded that these eleven basic concepts could be reduced to the following four principles [ 2]: • Minimize
• Substitute
• Moderate
• Moderate and simplify
This more concise set of principles makes IS practices simpler to understand and easier to apply. The excel- lent new CCPS book (2009) goes on to distinguish between first-order and second-order IS: • First-order IS efforts change the
chemistry of a process • Second-order IS efforts change the
process variables As can be seen by a survey of the process safety literature, most published work has applied one or more of the first four concepts of the eleven cited by Kletz and Amyotte [1] For this reason, this article emphasizes three other promising concepts.
Often-overlooked IS concepts Three underutilized IS concepts are presented here and illustrated with examples: 1. Hybridization or transforma-
tion. One relatively new IS concept is based on the recent innovative work by Chen [5] who reports 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 large pressurized
vessels. When containment was lost, a large flammable vapor cloud formed, ignited, and exploded with devastating effect (Figure 2, from Mannan [6]).
The traditional cyclohexane-oxidation process to produce a mixture of cy- clohexanone and cyclohexanol (K/A oil or ketone/alcohol oil) was operated at low conversion rates (typically 3–5%) to avoid formation of unwanted byprod- ucts. The K/A oil was subsequently con-
verted into adipic acid and caprolactam for the production of 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, Chen [5] discovered that the addition of a small amount of water — which is inert and does not par- ticipate in the reaction — helped to inert the otherwise flammable vapors. Cyclohexane and water are known to form minimum-boiling azeotropes. The increase in the vapor pressure of the cyclohexane/water liquid results from the increased vapor pressure of the water. The water vapor inerts the
vapor mixture by lowering the upper flammable limit of the vapor [5]. Chen’s work suggests that it will be

safe and practical to use pure oxygen for cyclohexane oxidation. Benefits include both IS operation and improved productivity. They also suggest that this approach could be extended 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 reference [5] did not 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 the name Substitute were broadened to names such as Change in Chemistry or Hybridize, then it could be lumped 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. How- ever, 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.
Although not proposed by Chen [5] himself, his work suggests that there may be many other opportunities for transformation or hybridization of other potentially hazardous reactions to make them inherently safer.
Although water would be high on any- one’s list as a potentially transform- ing additive, it probably will not help many potentially hazardous reactions. However, there are many other chemicals that may be inert to the reaction and thus also be capable of inerting the
vapor phase involved in an otherwise reactive liquid-vapor reaction. For in- stance, there are many examples of azeotropic mixtures in the literature and there are many compounds that could prove inert to oxidation reactions (such as, certain halocarbons).
Applications are not limited to partial oxidation with air or oxygen;
other oxidations include chlorination and bromi- nation reactions, for example. And there may be other examples of vapor- liquid reactions, such as hydrogenation reactions, where addition of a new chemical could improve the safety of the process.
Addition of an additional compound to a reaction mixture to minimize hazardous reactions may add complexity to the puri- fication process, but it may be justified by the increased safety.
Chen’s [5] paper on cyclohexane oxidation illustrates transformation or hybridization, in which the basic chemistry is maintained, but the addition of another chemical component transforms a potentially hazardous reaction process into a much safer one. 2. Create a robust process to sta-
bilize 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 that the steady-state mass and energy balances are achieved. A number of processes exist that have narrow safe- operating limits but have been made stable by the addition of control systems. Dynamic stability and control of chemical processes has been extensively studied [7 ].
Designing the process to be more inherently stable to process upsets with and without control systems is clearly inherently safer, although this principle is not addressed in most dis- cussions of IS. 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.
CCPS briefly mentions the advantages of designing processes that are inherently more stable or robust [ 2]:
“It is inherently safer to develop processes with wide operating limits that are less sensitive to variations in the operating parameters...Sometimes this type of process is referred to as a forgiving or robust process.”
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.
CCPS [ 2] also cites the work of Luyben and Hendershot [8] that high- lights how minimization or intensification in a reaction system that is intended to improve process safety may lead to less robust processes with the opposite effect.
I propose here that Stabilize or En- sure 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 ( Mini-
mize) 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 un- scheduled process shutdown [ 2].
Chemical reactors carrying out exothermic chemical reactions are perhaps the best known examples of processes that can be dynamically unstable. Harriott [9] provides the il- lustration of an irreversible first-order chemical reaction being conducted in a continuous-flow, stirred-tank reactor (CSTR). Figure 3 shows the heat-generation rate by the chemical reaction as a function of reactor temperature. Heat-generation rates are low at low
Reactor temperature
/ h

temperatures, but as temperature increases, the reaction rate increases rapidly because of the exponential dependence of the reaction rate co- efficient on temperature. At higher reactor temperatures, the shrinking concentration of reactant (due to con-
version to product) reduces the reaction rate and partially overcomes the still-increasing reaction-rate coeffi- cient. The heat-generation rate eventually reaches a constant maximum
value when the reaction has reached complete conversion.
Figure 3 also shows three different straight lines for the heat-removal rate from the reactor for three different reactor-cooling-system designs. To achieve a steady-state energy balance, the rate of heat generation (Qheat gen-
erated) by the chemical reaction must equal the rate of heat removal ( Qout) by the reactor cooling system. That energy balance occurs when the heat generation curve intersects the heat removal curve (where Qheat generated
= Qout). In Figure 3, the three different heat-removal-rate lines intersect the reactor heat generation rate curve at five points. At four of these points ( A, B, C, E), the steady-state energy balance solution is stable. At each of these points, if there is an increase in temperature, the rate of heat removal increases more rapidly than the rate of heat generation by the reaction and the reactor temperature tends to re- turn to the desired operating point. Similarly, if the temperature drops slightly at one of these four operating conditions, the rate of heat removal decreases more than the rate of heat generation by the reactor and the temperature trends back up to the desired operating condition.
In contrast, point D in Figure 3 is an inherently unstable operating condition even though the steady state rate of heat generation by the reactor equals the rate of heat removal by the reactor cooling system. At point D, an increase in reactor temperature increases the rate of heat generation by the reactor
more than it increases the rate of heat removal by the reactor cooling system, so the reactor temperature increases more instead of cooling back to the desired operating point.
This further increase in reactor temperature then leads to an even larger rate of heat generation rate by the reactor and additional heating of the reactor. Without any effective control actions, the reactor temperature will tend to increase to point E in Figure 3 before it stabilizes.
Similarly, in Figure 3 a decrease in reactor temperature at point D could eventually lead to the reactor temperature and conversion dropping back to point C.
Clearly, of the three reactor cooling- system designs represented by the three straight lines in Figure 3, the reactor cooling system represented by line CDE is the least desirable from a dynamic-stability perspective. Ad- dition of 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 effi- ciencies in distillation trains through heat integration. In these cases, the feed to a column may be preheated by the bottoms product of a second downstream 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 Stabi- lize as tools of the process engineer. It should be considered when consider- ing other means to assure inherently safer processes during process design. 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. 3. Limit hazardous effects during conceptual and detailed engineer-
ing. David Clark published a seminal paper [10] on the limitation of effects when siting and designing process plants. He reminds us that 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, such as the Dow Fire and Explosion Index [11] and the Dow Chemical Exposure Index [12, 13], pro-
vide quantitative screening estimates of the hazards from various parts of a chemical process. Other indices have been developed and evaluated to per- form a similar objective to the Dow indices [1, 2, 14]. These screening tools can identify those parts of a process where increased separation distances are needed 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 non- linear 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 ben-
efits in crane and other maintenance access, ergonomic advantages and decreased risk of incident escalation. Future plant expansions or process improvements are also facilitated, although expansions that decrease spacing may increase hazardous effects.
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
• 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

Applying different IS principles As discussed, the different IS principles are best applied at different stages of the process plant timeline.
Although IS checklists are often used at the screening process hazards analysis (PHA) level, much more is needed throughout the development and design of a process plant.
For example, Substitute is best done during the product and process research phases before significant investments of time and resources in a particular product and process are made. Hybridize or Transform is best done during process research and de-
velopment, as is Moderate. Minimize, Simplify, and Error tol-
erance have the best result when applied during the process development, conceptual design and detail design phases. Stabilize or Ensure Dynamic
Stability is also best done during design development.
Limitation of effects, which is closely related to passive protection, has its greatest impact during development of the plot plan and equipment arrangement.
IS processes and plants As mentioned previously, the CCPS [2] defines two levels of inherent safety: • First-order inherent safety results
from changes in the chemistry of a process that reduces the hazards of the chemicals used or produced.
Substitute or Hybridize efforts lead to first-order inherent safety
• Second-order inherent safety results from changes in the process variables. Examples include Minimize,
Simplify and Stabilize the operations.
It is also helpful to distinguish between IS processes and IS plants. Even when hazards cannot be eliminated from the chemistry of the process, the plant using the potentially hazardous process can be made inherently safer through ju- dicious design.
Note also that even with IS process chemistry, it is essential to employ IS principles during the process and plant design to ensure an IS plant.
Tools for IS plant design There are a number of tools available to aid in designing process plants that are inherently safer (Box, p. 18). Al- though inherently safer reviews are a
valuable tool for identifying opportunities for improvement, it is important to keep the principles of inherently safer in mind throughout the design process. n
Edited by Suzanne Shelley
Acknowledgments I gratefully acknowledge the process safety insights from my colleagues at Aker Solutions and at the leading operating companies whose facilities we have helped to design, from Professors Sam Mannan, Trevor Kletz, Ron Darby, Harry West and the Mary Kay O’Connor Process Safety Center at Texas A & M University, and from many others in the community of process safety professionals. The financial support of Aker So- lutions is also appreciated.
Author Victor H. Edwards, P.E., is director of process safety for Aker Solutions Ameri- cas Inc., (3010 Briarpark Drive, Houston, TX 77042; Phone: 713-270-2817; Fax: 713-270-3195; Émail:
vic.edwards@akersolutions. com). In his 28 years with
Aker, Edwards’ experience includes process engineering, safety management and pro-
cess, biochemical and environmental technologies. He has received numerous accolades in the areas of safety and environmental engineering, including five DuPont awards, and has contrib- uted extensively to the engineering literature. His earlier experience includes assistant professor of chemical engineering at Cornell Uni - versity, an assignment at the National Science Foundation, pharmaceutical research at Merck, alternate energy research at United Energy Resources, visiting professor at Rice University and process engineering at Fluor Corp. Edwards earned his B.A.Ch.E from Rice University and his Ph.D. in chemical engineering from the Uni-
versity of California at Berkeley. A registered professional engineer in Texas, he is an AIChE Fellow, and a member of ACS, AAAS, NFPA, NSPE, and the N.Y. Academy of Sciences.
References 1. Kletz, Trevor A., and Amyotte, Paul, “Process
Plants – a Handbook of Inherently Safer De- sign,” 2nd Ed., Taylor and Francis, Philadel- phia, PA, 2010.
2. Center for Chemical Process Safety (CCPS), “Inherently Safer Chemical Processes – A Life Cycle Approach,” 2nd Ed., AIChE, New
York, NY, 2009. 3. Hendershot, Dennis C., An overview of inher-
ently safer design, Process Safety Progress, Vol. 25, No. 2, 98–107, June 2006.
4. Dowell, III, Arthur M., Layer of protection analysis and inherently safer processes, Pro- cess Safety Progress, Vol. 18, No. 4, 214–220, Winter 1999.
5. Chen, Jenq-Renn, An inherently safer process of cyclohexane oxidation using pure oxygen –
An example of how better process safety leads to better productivity, Process Safety Progress, Vol. 23, No. 1, 72–81, March 2004.
6. Mannan, Sam, Ed., “Lee’s Loss Prevention in the Process Industries,” 3rd Ed., Elsevier But- terworth Heinemann, Oxford, U.K., 2005.
7. Edgar, Thomas F., and others, Process Control, Section 8 in “Perry’s Chemical Engineers Hand- book,” 8th Edition, Don W. Green, Editor-in-Chief, McGraw-Hill Book, New York, NY, 2008.
8. Luyben, W.L., and Hendershot, D.C., “Dy- namic disadvantages of intensification in inherently safer process design,” Industrial
Engineering Chemistry Research, Vol. 43, No. 2 (2004) cited in CCPS, 2009.
9. Harriott, Peter, “Process Control,” McGraw- Hill, New York, NY, 1964.
10. Clark, David G., Applying the ‘limitation of effects’ inherently safer processing strategy when siting and designing facilities, Process Safety
Progress, Vol. 27, No. 2, 121–130, June 2008. 11. “Dow’s Fire and Explosion Index Hazard Clas-
sification Guide”, 7th Ed., American Institute of Chemical Engineers, New York, NY, 1994.
12. “Dow’s Chemical Exposure Index Guide”, American Institute of Chemical Engineers, New York, NY, 1994.
13. Suardin, Jaffee, Mannan, M. Sam, and El- Halwagi, Mahmoud, The integration of Dow’s Fire and Explosion Index (F&EI) into process design and optimization to achieve inherently safer design, Journal of Loss Prevention in the
Process Industries, Vol. 20, pp. 79–90, 2007. 14. Khan, Faisal I., and Amyotte, Paul R., How
to make inherent safety practice a reality, Canadian Journal of Chemical Engineering,
Vol. 81, No. 2, 2–16, February 2003.
Additional suggested reading 1. Edwards, David, Editorial – Special Topic
Issue – Inherent safety – Are we too safe for inherent safety?, “Process Safety and Envi- ronmental Protection – Transactions of the Institution of Chemical Engineers Part B,”
Vol. 81, No. B6, 399–400, November 2003. 2. Englund, Stanley M., Inherently safer plants:
Practical applications, Process Safety Prog- ress, Vol. 14, No. 1, 63–70, January 1995.
3. French, Raymond W., Williams, Donald D., and Wixom, Everett D., Inherent safety, health, and environmental (SHE) reviews, Process Safety
Progress, Vol. 15, No. 1, 48–51, Spring 1996.
4. Gupta, J.R., and Edwards, D.W., Inherently safer design — Present and future, “Process Safety and Environmental Protection — Trans- actions of the Institution of Chemical Engineers Part B,” Vol. 80, 115–125, May 2002.
5. Gupta, J.R., Hendershot, D.C., and Mannan, M.S., The real cost of process safety — A clear case for inherent safety, “Process Safety and Environmental Protection – Transactions of the Institution of Chemical Engineers Part B,” Vol. 81, No. B6, 406–413, November 2003.
6. Hendershot, Dennis C., et al., Implementing inherently safer design in an existing plant, Process
Safety Progress, Vol. 25, No. 1, 52–57, March 2006. 7. Kletz, Trevor A., Inherently safer design: The
growth of an idea, Process Safety Progress, Vol. 15, No. 1, 5–8, Spring 1996.
8. Lutz, William K., Take chemistry and phys- ics into consideration in all phases of chemical plant design, Process Safety Progress, Vol. 14, No. 3, 153–160, July 1995.
9. Lutz, William K., Advancing inherent safety into methodology, Process Safety Progress,
Vol. 16, No. 2, 86–88, Summer 1997. 10. Maxwell, Gary R. Edwards, Victor H., Robert-
son, Mark, and Shah, Kamal, Assuring process safety in the transfer of hydrogen cyanide manufacturing technology, Journal of Hazardous
Materials, Vol. 142, pp. 677–684, June 2007. 11. Overton, Tim and King, George M., Inher-
ently safer technology: An evolutionary approach, Process Safety Progress, Vol. 25, No. 2, 116–119, June 2006.
12. Study, Karen, A real-llife example of choosing an inherently safer process option, Process Safety
Progress, Vol. 25, No. 4, 274–279, December 2006.

M any solid materials can exhibit self-heating, which — if unchecked — can progress to a fire or even
explosion. And even if the situation does not get that far, it is likely to affect the output of the process, in terms of product quality degradation, for example. Recognizing that your product in powder or granular form can self- heat is the first step in controlling the risks associated with self-heating.
Whenever self-heating incidents are investigated, we find that a common root cause is a lack of understanding of self-heating phenomena. This article provides an introduction to self-heating phenomena and suggests measures to control this type of ignition source.
What is self-heating? Not all particulate solids that are classified as combustible dust (in other words, pose a dust explosion hazard) will self-heat at normal processing temperatures, and conversely, some of the materials that do self-heat react too slowly to pose a dust explosion hazard. Some materials can self-heat at ambient temperatures, especially in large-scale storage, but for most materials the hazards arise when they are heated.
Self-heating can arise by one of two different mechanisms: by exothermic (heat releasing) chemical reactions and by exothermic decomposition. The chemical reactions are often the same as what occurs during a fire or explosion: an oxidation reaction with the oxygen in the air. At the start of the self-heating process, the reaction is very slow, like steel that oxidizes
with atmospheric oxygen to form rust. Decomposition happens in a material that is unstable, and the material will fall apart while releasing heat. A significant difference between the two mechanisms is that decomposition does not require additional reactants and is therefore largely independent of the environment, while an oxidation reaction only happens if certain conditions are present, making it more dif- ficult to predict its occurrence without detailed experimental studies.
What happens in self-heating? Step 1. Rate of heat generation exceeds rate of heat loss. If a material undergoes an exothermic chemical reaction (or multiple reactions) or de- composes exothermically, the temperature of the material will rise due to the heat released from the exothermic reaction or decomposition. In the mean- time, some of the heat is lost to the environment. If the rate of heat loss exceeds the rate of heat generation, the temperature of the material will be the same as the ambient temperature, otherwise, it will increase. Due to the poor thermal conductivities of many solids, a large portion of the re-
action heat is retained in the powder. Step 2. Resulting temperature
rise further increases chemical reaction rate exponentially. The temperature rise of the material due to the exothermic reaction will further increase the chemical reaction rate, which in turn will cause the temperature to increase further. The increase of material temperature also results in an increase in the rate of heat loss. However, the rate of heat loss increases linearly with temperature, while the chemical reaction rate, and thus the
Solids Processing
most common root cause is lack of understanding
Pieter Zeeuwen and Vahid Ebadat
Chilworth Global

Solids Processing
FIGURE 1. After completion of a test, in which self-heating of the product took place, the product was completely burnt. The charred and partly molten remains no longer fit inside the sample holder
Preventing Self-Heating and Ignition in Drying Operations
FIGURE 2. Product in the test cell (right) is discolored significantly after the test compared to the original sample (left), even though the self-heating has not led to smoldering or burning of the material

Solids Processing
heat generation rate, increases exponentially with temperature. Conse- quently, the heat generation rate will exceed the rate of heat loss and the temperature of the material will rise higher. This process is referred to as self-heating. Self-heating begins at a temperature at which the rate of heat generation is greater than the rate of heat loss and this temperature is called the exothermic onset temperature.
Subsequent effects Potential smoldering. Self-heating of solid materials usually results in smoldering, which can set the material on fire or cause dust explosions, particularly when the smoldering material is disturbed and exposed to air (Figure 1). Many plants that experience self-heating incidents have a history of “near misses” where some self-heating occurs but does not progress to full-blown ignition. In such cases there may be “black spots” in an otherwise light-colored product, or a lump of charred product may be found, a so-called “smoldering nest”. It is important to recognize such occurrences as indications of a potentially serious problem, rather than to learn to live with it.
Potential release of flammable
gases. Self-heating reactions may also produce flammable gases, which may lead to gas explosions in process
vessels or compromise product quality (Figure 2).
Testing self-heating behavior The exothermic onset temperature is influenced not only by the chemical and physical properties, such as chemical reaction kinetics and heat of reaction, but also by other factors, including the following: • Dimension and geometry of the solid
bulk • Ambient airflow • Availability of oxygen in the bulk
• Additives and contaminants
Usually, the material has to be exposed to an elevated temperature for a period of time before it self-heats. This time is referred to as the induction time, which is dependent on temperature; and usually the higher the temperature, the shorter the induction time will be.
Because of the influencing factors mentioned, a single test is usually un- able to predict self-heating behavior for all different drying and storage conditions. Instead, separate tests have been developed to simulate the conditions where the powder is in bulk form (Figure 3), layer form (with air
flowing over the powder; Figure 4) and aerated form (Figure 4), where air is passing through the bulk of the product, increasing the oxygen availability for the reaction and also help- ing to remove heat from the reacting material. For large-scale storage situ- ations tests are carried out at different scales so that the effect of the size of the bulk material can be assessed (Figure 5). All tests are carried out in temperature-controlled ovens (Figure 6) that allow screening tests (with the temperature ramped up at a defined rate) and isothermal testing (with a constant temperature controlled within narrow margins). Because of the potential for violent reactions during the self-heating process, all testing equipment needs to be fitted with explosion protection.
Learning from a real incident In one incident, the powder in a fluid- ized bed dryer caught fire when the powder conveyer in the dryer was turned off in order to fix clogging in an upstream wet-product conveyer. During this period, the hot air supply was continued.
A screening test was conducted to determine whether this powder could self-heat under the conditions that ex-
FIGURE 4. (Top left) In the test for “aerated” conditions, air flows through the sample from top to bottom, which are both closed by sintered glass discs. The cylindrical section has a 50 mm dia. and a height of 80 mm. (Bottom) For "air over layer" testing, warm air flows over the powder layer in the sample tray. (Top right) The wire basket for “basket testing” is illustrated more clearly in Figure 5
FIGURE 5. “Basket test” sample holders, for testing at different scales, allow extrapolation to large-scale storage conditions. The baskets typically have sides of 25, 50 and 100 mm

CHEMICAL ENGINEERING WWW.CHE.COM AUGUST 2011 47
isted in the dryer before the incident. An typical powder sample was placed in a temperature-programmed oven and the temperature of the oven was increased from 20 to 400°C at a rate of 0.5°C/min (Figure 7). The exothermic onset temperature was identified to be 166°C, which was lower than the hot air temperature of the dryer. The result indicated that self-heating was the most probable ignition source for this incident.
This test provides a useful tool for a quick identification of the self-ignition hazard of materials and should be conducted for materials whose thermal stability characteris

Minimizing Fire Risk at CPI Facilities Volume 2

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