Transcript of 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 manu- facture, 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, in- strumentation and
piping components, such as reactors, catalyst feed vessels, spent
catalyst filters, pumps, valves, pressure relief devices, pressure
regu- lators and check valves. Many such systems, particularly
those for hydro- genation of organic chemicals, are lo- cated
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 envi- ronment conditions
• Certified relief devices for process equipment and piping. The
relief devices vent discharges must be di- rected to safe
locations
• Adequate, dependable space venti- lation to prevent accumulation
of hydrogen gas pockets for systems located inside buildings
• Damage-limiting building construc- tion to protect personnel and
prop- erty.
Hydrogen’s hazards When gas is released or escapes from containers,
it presents detonation, def- lagration and fire hazards. The wide
flammability range, high burning rate, low ignition energy and
non-luminous flame of hydrogen accentuate the com- bustion
hazards.
The flammability range for hydro- gen 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 hydro- gen 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 igni- tion 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 combus- tion due to a release in air is
cannot be predicted. Some of the many possible hazardous outcomes
of hydrogen com- bustion are as follows: Fire: In this
case, a release of hydro- gen 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 pres- sure 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 hy- drogen 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 result- ing shock waves and hot product gases impinging upon
the surroundings out- side 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 — espe- cially in light
of hydrogen having the lowest molecular weight of an indus- trial
gas. Aside from being prone to leak easily through seals, the gas
read- ily permeates through various materi- als 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 in- clude 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 car- bon 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 com- pounds can significantly affect
materi- als’ suitability even when present only in trace
amounts.
Ordinary carbon steel, iron, low-alloy steels, chromium,
molybdenum, nio- bium, zinc, nickel, etc are not accept- able for
use at cryogenic temperatures. Cast iron is not acceptable for
hydro- gen 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 hy- drogen service
should be limited, be- cause 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, tempera- ture, 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
lo- cation of a hydrogenation or other hy- drogen-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 op- tion for location of a
hydrogenation system.
In cases where a reactor must be lo- cated 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 vent- ing. 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 reac- tions
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 prac- tice, the separation
distance should be checked by energy release calculations involving
the rupture of an overpres- surized 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
ser- vices must be designed to limit person- nel 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 build- ing 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 vol- ume. (Note, however, that this re- quirement
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 appro- priate 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 de- signed for at least 2 h of
fire resis- tance
• 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 suit-
able for handling hydrogen are sum- marized 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 impor- tant with hydrogen.
The piping to convey hydrogen gas from cylinders entails special
consid- erations. If the cylinders cannot be stored outside, then a
well ventilated storage shed is required. The tempera- ture 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 auto- matic 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 de- tection 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 rup- ture 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 hav- ing 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 ap- propriate locations to
prevent back- flow of process contents into the line
Control and monitoring Minimum instrumentation and con- trol
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 criti- cal safety instruments
installed with redundancy are those for hydrogena- tion-reactor
process temperature and pressure, and hydrogen detection in
confined spaces. Some of the common interlocks associated with
hydrogena- tion reactors are found in Table 3.
Hydrogen detection: Because hy- drogen 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 in- stalled in confined
spaces, current standard practice at hydrogenation plants calls for
alarm setpoint at 1% concentration by volume in the am- bient 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 hy- drogen 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 pos- sible
• Storage area, where hydrogen con- nections are routinely made and
separated
• Building-air intake ducts, if hydro- gen could be carried into
the build- ing
• Building-air exhaust ducts if hy- drogen could be released inside
the building
The appropriate response to a detec- tion of the presence of
hydrogen in ambient air varies with the likely degree of risk.
Examples of common responses include isolation of the hy- drogen
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 hydro- gen flame is low, the flame is
hard to see or feel. So, aside from the detection of the gas
itself, the design of a hydro- gen-using facility must provide
detec- tion of a hydrogen flame in all areas where leaks or
hazardous accumula- tions omay occur. Infrared (IR) and ultraviolet
(UV) are two technologies commonly used.
Process venting In any hydrogen-using facility, provi- sion
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 hydro- gen (the definition of
“large” varying by industry), a flare system is gener- ally the
choice. The hydrogen-contain- ing exhaust process gas is piped to a
remote area, where it is burned with air in a multiple burner
arrangement.
CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2006 57
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
• Shut off the hydrogen supply and open vent line (block and bleed
design).
• Shut off the reactor heating system.
• Vent off the hydrogen to atmosphere.
3. On low speed of reactor agitator
• Shut off the hydrogen supply and open vent line (block and bleed
design).
• Shut off the reactor heating system.
4. On hydrogen detection in reactor room
• Shut off the hydrogen supply and open vent line (block and bleed
design).
• Shut off the reactor heating system.
• Start the room emergency exhaust fan.
• Shut off all rotating equipment motors in room.
5. On flame out of the flare unit
• Shut off the hydrogen supply and open vent line (block and bleed
design).
• Shut off the reactor heating system.
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 sys- tems is leaktight to avoid air
intrusion and possible detonation.
Although flare-system design tech- nology is mature, flares
continue to pose hazards of flame stability, flame blowoff, and
flame blowout. To mini- mize the malfunction of flare systems, it
is important to keep the stack dis- charge 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 con- ditions as the prevailing-wind direc- tion and
speed, proximity to adjacent buildings, vent stack height, and
local discharge limitations or other envi- ronmental
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 dis-
charges overpressurizing the low-pres- sure 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 de- signed with inert-gas (usually
nitro- gen) 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 pip- ing 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 op- eration, both nitrogen
and steam flow
should be commenced upon the intro- duction of hydrogen gas in the
process equipment.
Avoidance of flammable atmo- spheres 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 atmo- sphere. For additional safety, a certi- fied
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 de- signed 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 sys- tems as just discussed. For
emergency
venting, the plant must install safety valve or rupture
disks, as appropriate, on vessels, lines, and component sys- tems
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 run- away does exist, the emergency vent system
should be sized by a rigor- ous methodology such as that devel-
oped 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 rup- ture 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 de- vices 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
head- ers 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 ther- mal effects and slugs of liquid
during emergency relief.
Room ventilation In light of hydrogen’s wide flammabil- ity range
and low ignition energy, hy- drogen leaks or spills in a non-venti-
lated, 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-concen-
tration 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 concentra- tion 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 classifi-
cation 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 pres- sure 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 po- tential 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 pres- sure 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 environ- ment; therefore, purged enclosures are
employed routinely in this ap- plication
• 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 sur- face 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 mini- mum, 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 sprin-
kler 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 surround-
ings 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
conse- quences of a mishap. The people and facilities are separated
from the po- tential 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 un- used hydrogen gas from the process is safely disposed of by
flare system or
vented above other facilities. The in- strumentation 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: rhachoose@lg.com). He has
over 24 years of process engi- neering 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.
CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2006 59
Circle 40 on p. 93 or go to adlinks.che.com/5828-40
I t is a fact that safety-related in- cidents 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 check- list 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, ther- mal 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, in- cluding emergency venting
4. Ineffective plant operational proce- dures 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 con- trols and safeguards to
reduce risk. It is important to note that as a process undergoes
changes, the safety informa- tion also needs to be updated. The
final process-safety package should be at such a level that it can
be used for tech- nology-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 com- pany policies and procedures as well as country
and local regulations.
Process safety checklist The following items should be consid- ered
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 be- tween reagents and
solvents
• Identify potential reaction contam- inants 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 rela- tive 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 tem- perature range
• When optimizing the robustness of the process, consider other
reaction
variables, such as pH, concentra- tion, 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
reac- tivity, and communicate this infor- mation 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 re- moval capability of the
plant reactor system. Remember to include reac- tor 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 re- actor walls, fouling,
and so on, in plant-scale equipment
General chemistry and engineer- ing 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 exo- thermic 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 in- creases by the cube of the
vessel radius but the heat-transfer area in- creases by the square
of the radius A comprehensive hazard evalua-
tion should be conducted using appro- priate 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 ther- mal 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 gen- erated 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.
-
l - ll . Extendible whenever you want – to fit your concept and
meet your needs exactly. Don’t take any chances: only a Müller
original embodies the expertise gained over more than a
century.
f „ ll n in r t ” t d .nt n ystems” t y.
Müller GmbH, Industrieweg 5 D-79618 Rheinfelden, Germany Phone:
+49(0)7623/969-0 Fax: +49(0)7623/969-69 E-mail:
info@mueller-gmbh.com
A Company of the Müller Group
Innovation in Stainless Steel
www.mueller-gmbh.com
® ®
1. 1
L i d d e d D r u m s
D r u m s w i t h b u n g s / h o p p e
r ca n s
H o p p e r s
S i l o s
C o n ta i n e
r s
B u t t e r f l y
Va l v e s
D r u m T r u c k s
M ü l l e r D r u
m a n d
C o n t a i n
e r S y s t e
m s
C K 2 0 0 8
i n D ü s
s e l d
o r f
3 0. 4. 2 0 0 8
H a l l
7. 0, S
t a n d
7 0 B 2 0
Circle 20 on p. 76 or go to adlinks.che.com/7371-20
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 organi- zations have made impor- tant
contributions to the cre- ation 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 em-
phasizes the opportunities presented by three particular — and
often-over- looked — 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, sub-
stitution is best applied during prod- uct and process research,
while limita- tion of effects is most effective during plot plan
layout and equipment ar- rangement.
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 pro- duce 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 pro- cess plants and must
be applied to their associated raw materials and products. In
recent years, this has led to major efforts in green chemis- try
and engineering to develop prod- ucts, manufacturing processes, and
plants that are safer for both people and the environment.
Before green chemistry and engi- neering achieved prominence, there
were pioneering insights in the de- sign of safer process plants.
Early ap- proaches to safer processes often em- ployed additional
instrumentation and procedures. These measures were often helpful
and necessary, but instrumen- tation 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 em- phasis 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 indi- viduals and
organizations since Kletz’s path finding proposal, with many no-
table successes.
Complete coverage of the entire prod- uct/process/plant lifecycle
is needed to assure optimum health, safety and en-
vironmental performance of a chemi- cal enterprise.
This article focuses on how to en- sure maximum incorporation of IS
processes into the creation of a pro- cess plant by beginning at
the product and process research stages and con- cluding with the
detailed design. No effort is made to address the applica- tion of
inherently safer principles be- yond 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, explo- sion or toxic release is inherently safer than one
that could if one or more lay- ers 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 explo-
sion 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 repre- sent 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 op- erating 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 > proce- dural 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
under- stand 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 pro- cess
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 pro-
cesses often involve hazardous condi- tions, 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-oxida- tion 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 un- controlled 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
haz- ards 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
CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2011 45
46 CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2011
safe and practical to use pure oxygen for cyclohexane oxidation.
Benefits in- clude 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 con- cept, 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 suc- cessful applications
that are possible when using the Substitute concept.
Chen’s innovation permits rapid cy- clohexane oxidation at lower
tempera- tures and pressures, and could thus be said to be an
example of the inher- ently safer principle Moderate. How-
ever, Chen’s approach enables more moderate conditions by narrowing
the flammability limits through the addi- tion 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 reac- tions 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
chemi- cals 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 reac- tions (such as, certain
halocarbons).
Applications are not limited to partial oxidation with air or
oxygen;
other oxidations include chlorination and bromi- nation reactions,
for ex- ample. 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 ad- ditional compound to a reaction mixture to
min- imize hazardous reac- tions 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 ad- dition of another chemical component
transforms a potentially hazardous re- action 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 sys- tems. Dynamic stability and control of chemical
processes has been exten- sively 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 sys- tem that the operator can understand clearly and
manage effectively.
CCPS briefly mentions the advan- tages 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 intensifi- cation in a
reaction system that is in- tended 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 in- herently safer processes.
Application of some of the other IS principles can adversely
affect the dy- namic stability of a process. For exam- ple, reduced
liquid inventories ( Mini-
mize) in a distillation train make the process inherently safer
from one per- spective because the smaller process inventory
decreases the consequences of loss of containment. However, the
smaller inventory also shortens the response time of the
distillation sys- tem to process upsets, increasing the risk that
the basic control system will not be able to restore the
distillation system to the desired operating condi- tions 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-gen- eration rate by the chemical reaction as a function of
reactor temperature. Heat-generation rates are low at low
Reactor temperature
/ h
CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2011 47
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 reac- tion rate and partially
overcomes the still-increasing reaction-rate coeffi- cient. The
heat-generation rate even- tually 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 differ- ent
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 differ- ent 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 tem- perature trends back up to the desired
operating condition.
In contrast, point D in Figure 3 is an inherently
unstable operating condi- tion 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 de- sired operating point.
This further increase in reactor tem- perature 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 temper- ature 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 sta- bility — but at the
cost of installation and maintenance of the control sys- tem 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 in- crease 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 engi- neer to address the dynamic
stability
of both the uncontrolled process and the controlled process to
ensure a ro- bust 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, ex-
plosion, and toxic effects with separa- tion distance.
Comparatively small de- creases 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
in- dices [1, 2, 14]. These screening tools can identify those
parts of a process where increased separation distances are needed
to limit potential escala- tion of an incident.
In one typical plant design, a 10% increase in separation distances
for all units increases total plant invest- ment cost by only 3%.
Similarly, dou- bling 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
sepa- ration 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, al- though expansions
that decrease spac- ing 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
48 CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2011
Applying different IS principles As discussed, the
different IS prin- ciples are best applied at different stages of
the process plant timeline.
Although IS checklists are often used at the screening
process hazards anal- ysis (PHA) level, much more is needed
throughout the development and de- sign 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 ap- plied during the
process development, conceptual design and detail design phases.
Stabilize or Ensure Dynamic
Stability is also best done during de- sign
development.
Limitation of effects, which is closely related to passive
protection, has its greatest impact during development of the plot
plan and equipment ar- rangement.
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 vari- ables. Examples include Minimize,
Simplify and Stabilize the opera-
tions.
It is also helpful to distinguish be- tween IS processes and IS
plants. Even when hazards cannot be eliminated from the chemistry
of the process, the plant using the po- tentially 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 opportu- nities 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 in- sights from my colleagues at Aker Solutions and at the
leading operating companies whose facili- ties 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 technolo- gies. 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 pro- fessor 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 ef- fects’
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 in- herently 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 chemi- cal 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 man- ufacturing 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 ap- proach, 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 af- fect the output of the process, in terms of product
quality degradation, for ex- ample. 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 understand- ing of self-heating
phenomena. This article provides an introduction to self-heating
phenomena and suggests measures to control this type of igni- tion
source.
What is self-heating? Not all particulate solids that are clas-
sified 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 sig- nificant 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 condi- tions
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 ex-
ceeds rate of heat loss. If a material undergoes an exothermic
chemical reaction (or multiple reactions) or de- composes
exothermically, the tempera- ture of the material will rise due to
the heat released from the exothermic re- action 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
tempera- ture, 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 tempera- ture 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 sam- ple
(left), even though the self-heating has not led to smoldering or
burning of the material
Solids Processing
heat generation rate, increases expo- nentially 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
mate- rial on fire or cause dust explosions, particularly when the
smoldering material is disturbed and exposed to air (Figure 1).
Many plants that ex- perience self-heating incidents have a history
of “near misses” where some self-heating occurs but does not prog-
ress 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 im-
portant 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 chemi- cal 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 usu-
ally 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 avail- ability 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 differ- ent 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 dur- ing the self-heating process,
all testing equipment needs to be fitted with ex- plosion
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
differ- ent scales, allow extrapolation to large-scale storage
condi- tions. 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 exother- mic 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 con- ducted for
materials whose thermal stability characteris