hydroxylamine production: will a qra help you decide?
TRANSCRIPT
Hydroxylamine production: will a QRA help you decide?
Kiran Krishna, Yanjun Wang, Sanjeev R. Saraf, William J. Rogers, John T. Baldwin,Jai P. Gupta, M. Sam Mannan*
Mary Kay O’Connor Process Safety Center, Department of Chemical Engineering, Texas A&M University, College Station, TX 77843-3122, USA
Received 20 January 2003; accepted 22 April 2003
Abstract
The recent publication by the US Chemical Safety Board (CSB) concerning its findings on the Concept Sciences Inc. (CSI) incident
involving hydroxylamine (HA) has raised issues with regard to safe production of HA. This CSI incident was followed by another incident
that destroyed the Nissin Chemical HA plant in Japan, and today BASF is the sole commercial producer of HA. HA is an important solvent in
the pharmaceutical industry and is used as an etching agent in the semi-conductor industry.
This paper discusses a Quantitative Risk Assessment of a generic HA production plant, which integrates the findings of the CSB report and
the knowledge of potential HA reactivity hazards based on research at the Mary Kay O’Connor Process Safety Center. The intent is to
highlight safety concerns and major risk factors in the production and handling of HA and to provide risk assessment guidelines for potential
manufacturers. These guidelines are also applicable to the production strategies for other hazardous chemicals.
q 2003 Elsevier Science Ltd. All rights reserved.
Keywords: Hydroxylamine; Quantitative Risk Assessment; Reactivity; Run-away; Chemical process safety
1. Introduction
The chemical process industries handle, produce, and
store hazardous materials that are capable of potential
catastrophes. Often, changes during plant operations or lack
of accurate knowledge of reactive chemistry of the
components have been the cause of serious incidents in
the plant [1,2]. For example, the Concept Sciences Inc.
(CSI) hydroxylamine (HA:NH2OH) manufacturing unit in
Pennsylvania was destroyed in February 1999 [3] and was
followed by an explosion at the Nissin Chemical HA plant
in Japan in May 2000 [4].
Today BASF is the sole producer of HA, which is an
important solvent in the pharmaceutical industry and is used
as an etching agent in the vast semi-conductor industry. The
recent publication by the US Chemical Safety Board (CSB)
of its findings on the CSI incident [5] involving HA has
raised concerns with regard to the safe production of HA.
The CSB concluded that the process safety management
systems were incapable of addressing the hazards posed by
HA manufacture, but more importantly recognizes the fact
that the collection and analysis of process safety information
specific to the reactive and explosive hazards of HA were
inadequate. To prevent incidents in the future, designing a
safer HA production process requires an understanding of
the underlying hazards and risks due to the unstable nature
of the compound.
The aim of this paper is to perform a Quantitative Risk
Assessment (QRA) on a generic HA production plant based
on available data. Such an analysis should give an insight
into possible events that may lead to serious incidents and
help design a safer process. The risk involved in handling
other hazardous chemicals, can be evaluated using a similar
methodology.
2. HA production
The objective of the production unit is to manufacture
50 wt% (weight percent) HA (NH2OH) aqueous solution,
which is the maximum possible HA concentration permiss-
ible to be transported in the US. The process consists of the
following units: a continuous stirred tank reactor (CSTR),
multiple filtration units, and a packed distillation tower,
as shown in Fig. 1. The design of the plant is based on
0951-8320/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0951-8320(03)00115-7
Reliability Engineering and System Safety 81 (2003) 215–224
www.elsevier.com/locate/ress
* Corresponding author. Tel.: þ1-979-862-3985; fax: þ1-979-458-1493.
E-mail address: [email protected] (M.S. Mannan).
the information available in the CSB report [5]. The
properties utilized for elementary process design are
summarized in Table 1. Details of the calculations performed
for preliminary process design are available on request.
The details of the process are as follows:
1. The CSTR is fed with stoichiometric amounts of
potassium hydroxide (KOH) and hydroxylammonium
sulfate ((NH2)2SO4). The two streams combine in the
reactor according to the following stoichiometry:
2KOH þ ðNH2Þ2SO4 ! K2SO4 þ 2NH2OH
Preliminary studies performed in our laboratory indicate
that KOH catalyses the decomposition of HA.
2. The product stream from the reactor is fed to a filtration
unit to remove the solid potassium sulfate (K2SO4).
Fig. 1. Hydroxylamine production.
K. Krishna et al. / Reliability Engineering and System Safety 81 (2003) 215–224216
3. An accumulator is used to isolate the preceding process
(reaction and filtration) from the distillation. A heat
exchanger with an outlet temperature control is also
employed to achieve a suitable feed temperature for this
stream, which consists of approximately 30 wt% HA.
4. The stream is then fed to a vacuum distillation tower
to concentrate it to 50 wt% HA at the bottoms.
Preliminary calculations indicate that a packed dis-
tillation tower operated at 250 mmHg, and a reflux
ratio of 0.5 should enable the desired separation. Our
calculations also indicate that the distillation system is
sensitive to feed concentration and temperature.
Based on available values and calculations performed
on the various units, the production of 50 wt% HA
appears to be feasible. It is worth noting that the CSI
process involved concentrating HA to 80 wt% aqueous
solution and then lowering the concentration to 50 wt%
[5]. Studies performed at the Mary Kay O’Connor
Process Safety Center (MKOPSC) indicate that decompo-
sition of 50 wt% HA is initiated at ,120 8C (onset
temperature) and is extremely sensitive to metal con-
tamination [21] (e.g. iron, titanium). The onset tempera-
ture ðTONSETÞ is defined as the temperature at which a
detectable temperature rise is observed in the sample due
to exothermic reactions and is an indicator of incipience
of a reaction. Another study performed on HA indicates
that the decomposition onset temperature reduces with
increasing HA concentration [22]. The same paper also
reports that 80 wt% and higher concentrations of HA–
water solutions could detonate on mechanical impact.
The relationship between decomposition temperature HA
and concentration is shown in Fig. 2. This figure can be
used as a rough guide to estimate the hazards in the
process due to thermal instability of HA. The onset
temperature decreases with the addition of metal
contamination. If multiple filters are used in parallel,
deionized water is recommended for cleaning purposes to
avoid possible contamination.
Table 1
Summary of hydroxylamine properties
Property Value Reference
Physical appearance White-colorless [6]
Odor less
Solid crystals
Melting point (8C) 33.05 [6]
32.05 [7]
Boiling point 56–57 8C at 22 mmHg [7]
70 8C at 60 mmHg [8]
142 8C at 760 mmHg
(extrapolated)
[7]
Vapor pressure 0.27 mmHg at 0 8C [9]
5.3 mmHg at 32 8C [7]
10 mmHg at 47.2 8C [7]
40 mmHg at 64.6 8C [7]
100 mmHg at 77.5 8C [7]
400 mmHg at 99.2 8C [7]
Density of solid 1.2255 g/ml at 0 8C [10]
Density of liquid 1.204 g/ml at 33 8C [7]
1.2255 g/ml at 0 8C [11]
Specific gravity of vapor
(calculated)
1.14 [6]
Heat of formation, solid 225.5 kcal/mol at 25 8C [9]
Free energy of formation 25.6 kcal/mol at 25 8C [12]
Heat of fusion 3.94 kcal/mol at 32.05 8C [9]
Heat of sublimation 15.34 kcal/mol at 0
and 32 8C
[9]
Heat of solution (kcal/mol) 3.795 [6]
Heat of hydrolysis 1.96 kcal/mol at 20 8C [13]
Heat of vaporization
(kcal/mol)
11.4 [9]
Entropy of sublimation 40.4 cal/K/mol at 0
and 32 8C
[9]
Entropy for gas, calculated 56.33 cal/K/mol at 25 8C [14]
Heat capacity of gas 11.17 cal/K/mol at 25 8C [14]
Molecular volume (cm3) 27.4 [7]
Dissociation constant 1.07 £ 1028 at 20 8C [7]
Dielectric constant 77.63–77.85 [7]
Proton affinity (kcal/mol) 211 [7]
pKa (NH3OH)þ 6.04 at 20 8C [15]
pKb 8.13 at 20 8C [15]
pH (50% aq.) 11 [15]
N–O bond distance (A) 1.46 [14]
Bond dissociation energy (kcal/mol)
H2N–OH 61.3 [9]
HO–OH 51.0 [9]
H2N–NH2 60.0 [9]
Flash point Explodes at 129 8C [16]
NFP classification Health 2 [17]
Fire 0
Stability 3
Heat of formation solid
(kcal/mol)
227.3 [7]
Heat of formation liquid
(kcal/mol)
225.5 [18]
221.7 [19]
Heat of formation gas (kcal/mol) 210.2 [9]
Vapor pressure data A: 73.552 [20]
lnðPÞ ¼ A þ B=T þ C lnðTÞ þ DTE ;
where P—pressure (atm), T—
temperature (K)
B: 21.0434 £ 104
C: 25.8582
D: 1.7605 £ 10217
E: 6.0Fig. 2. The effect of HA concentration on the onset temperature.
K. Krishna et al. / Reliability Engineering and System Safety 81 (2003) 215–224 217
In addition, KOH catalyses the decomposition of HA and
may lead to potential runaway behavior. Therefore,
operation of the reactor in a semi-batch mode, where
KOH is added to the bulk (NH2)2SO4 in the reactor, with
good mixing, is suggested. However, the use of semi-batch
mode complicates the reactor operation and as a result
increases the risk due to human error. Further investigation
by the MKOPSC involves design of a safer operation for the
reactor.
3. Development of a reactivity risk index
The decomposition of HA is sensitive to excursions of
temperature, HA concentration, metal contamination, and
KOH concentration (as indicated by recent studies in our
laboratory). In this section, we discuss development of an
algebraic equation called a reactivity risk index (RRI) that
will take account of the reactivity hazards due to
temperature, concentration and metal contamination. At
present we do not have sufficient information to quantify the
effect of pH on the decomposition.
The RRI was defined earlier in the context of reactive
chemicals [23] and in this case is defined as follows:
RRI¼TPROCESS
TONSET
CHA;MAX
CHA;CRITICAL
expCCONTAMINANTS
CCONTAMINANTS;CRITICAL
!
where
TPROCESS maximum process temperature
TONSET onset temperature indicating the onset of a
significant reaction
CHA;MAX maximum HA concentration
CHA;CRITICAL critical HA concentration
CCONTAMINANTS concentration of metal ions
CCONTAMINANTS;CRITICAL critical concentration of metal
ions
From Fig. 2 we obtain the following relationship between
onset temperature and HA concentration:
TONSETð8CÞ ¼ 415:6 expð22:8CHA;MAXÞ
where
CHA;MAX maximum HA concentration (wt%).
A value of 0.8 can be assigned to CHA;CRITICAL since HA
is reported to decompose spontaneously at 80% and higher
concentrations [22]. An exponential functional dependence
is assigned for the effect of contamination so that for the
limiting case, CCONTAMINANTS ¼ 0; the exponential term
is unity. Therefore, in the absence of contaminants and
substituting the TONSET with the earlier relationship, the RRI
becomes:
RRI ¼TPROCESS
415:6 e22:8CHA;MAX
CHA;MAX
0:8
We obtain the following values of RRI for the different
units:
1. Reactor: TPROCESS ¼ 50 8C; CHA;MAX ¼ 0:3; RRI ¼ 0:11
2. Filter: TPROCESS ¼ 25 8C; CHA;MAX ¼ 0:3; RRI ¼ 0:06
3. Distillation: TPROCESS ¼ 75 8C; CHA;MAX ¼ 0:5; RRI ¼
0:46
The above results indicate that the distillation tower
poses the maximum risk due to thermal and concentration
excursions and without any safeguards.
4. Risk assessment
Quantitative risk analysis helps the chemical process
industry in two ways: it identifies the dominant contributors
to the total risk, and it quantifies the benefits of possible
changes. The first step is to analyze the total risk associated
with the base case and to calculate the contributions. These
findings lead naturally to the specification of possible
measures to improve reliability or reduce the damage
potential. Fig. 3 depicts the procedure involved in
quantitative risk analysis.
The first step in evaluating the risk associated with a
chemical process is to identify potential hazards. As stated
earlier, our concern here is an exothermic decomposition
reaction occurring in the process. Based on our knowledge,
Fig. 3. Quantitative risk analysis scheme.
K. Krishna et al. / Reliability Engineering and System Safety 81 (2003) 215–224218
the major risk lies in the CSTR and distillation tower
besides the design fault, construction error, and other
external factors. The Top Event, ‘Significant decomposition
of HA occurs in the process’, consists of four sub-events:
corrosion, external heat, decomposition in the CSTR, and
decomposition in the distillation tower. Fault tree tech-
niques are used to estimate the probability of these events
with the existing safeguards. The results of the fault tree are
analyzed and conclusions and recommendations are
determined.
4.1. Fault tree analysis
Fault tree analysis (FTA) is a deductive technique to
analyze systematically and logically how equipment
failures, operator errors, and external factors can cause an
incident. A fault tree can be generated by asking ‘What can
cause this event?’ until primary failures or faults are
achieved.
As described in the above section, an auto-oxidation can
occur under any of the following conditions:
1. High temperature: Under clean conditions, the onset
temperature of 50 wt% HA is around 120 8C, according
to MKOPSC research [21]. This temperature is not
normally present in the process. The neutralization
reaction in the reactor is only mildly exothermic, local
hot spot may not achieve this temperature even if the
agitator is not working properly. However, high
temperature can be caused by external heat such as a fire.
Fig. 4. The fault tree with the Top Event: ‘Significant decomposition of HA occurs in the process’.
K. Krishna et al. / Reliability Engineering and System Safety 81 (2003) 215–224 219
2. High concentration (70 wt% or above): Under normal
operation conditions, the process cannot achieve this
concentration. But local high concentration can be
caused by water evaporation as a result of external heat
or distillation malfunction. Our calculation shows that
decreased feed temperature, lower pressure, condenser
failure and sub-cooled feed, and increased re-boiler heat
load of the distillation tower can cause higher
concentration.
3. Contamination: Contaminants including heavy metals
like iron, copper, chromium, nickel, their alloys, and
their ions, dust, oxidizing agents, and bases must be
avoided. Experiments show that HA decomposition is
significantly enhanced by metal/bases even in trace
quantities. The CSTR is close-topped. Contaminants
can enter the process via a feed line or construction
material corrosion.
Table 2
Failure rate data
Rate Units Description Sources
1.36 £ 1026 H Agitator failure [24]
2.40 £ 1024 H Analyzer system did not
catch the high concen-
tration event
[24]
1.00 £ 1026 H Condenser failure Engineering
estimate
4.00 £ 1027 H Controller fails high [24]
1.36 £ 1026 H Controller stuck [24]
0.0001 0 Construction fault [25]
0.0001 0 Sufficient conta-
minants exists in
the KOH feed
Engineering
estimate
0.0001 0 Sufficient contaminants
exists in the (NH2)2SO4 feed
Engineering
estimate
3.00 £ 1027 H Control valve fails open [24]
3.00 £ 1027 H Control valve fails to
open on demand
[24]
3.00 £ 1027 H Control valve stuck [24]
0.0001 0 Design fault [25]
1.14 £ 1029 H External fire exposure [26]
0.001 0 Feed too hot Engineering
estimate
1.10 £ 1026 H Fire detector fails to
detect external fire
[24]
6.60 £ 1027 H Flow meter fails high [24]
6.60 £ 1027 H Flow meter fails low [24]
1.14 £ 1028 H Flow meter on (NH2)2SO4
feed line leaks externally
[27]
1.14 £ 1028 H Flow meter stuck [27]
0.1 0 (NH2)2SO4 feed pressure
decreases
Engineering
estimate
0.1 0 KOH feed pressure increases Engineering
estimate
0.0001 0 Loss of purge air Engineering
estimate
0.1 0 Liquid level too high Engineering
estimate
0.001 0 Operator failure [25]
4.57 £ 10210 H Manual valve on (NH2)2SO4
feed line
leaks externally
[28]
3.00 £ 1027 H Manual valve stuck open [24]
0.001 0 Operator add concentrated
HA to accumulator by mistake
[25]
0.03 0 Operator fails to respond
to the flame alarm
[24]
0.001 0 Operator failure [25]
0.07 0 Alarm failure [24]
6.60 £ 1027 H Pressure indicator fails high [24]
6.60 £ 1027 H Pressure indicator fails low [24]
0.04 0 Vacuum pump fails to stop
on demand
[24,27]
0.001 0 Re-boiler design error [25]
0.0001 0 Control set point too high [25]
0.0001 0 Pressure set point too low [25]
0.07 0 Alarm failure [24]
9.70 £ 1025 H Temperature sensor failure [24]
6.60 £ 1027 H Temperature sensor fails low [24]
6.60 £ 1027 H Temperature sensor stuck [24]
Note: H represents failure rate per hour and 0 represents probability of
failure.
Table 3
QRA results
Event Probability
Corrosion occurs in the process 2.00 £ 1024
High temperature/high concentration due to external heat 3.93 £ 1027
Significant decomposition occurs in the CSTR 3.21 £ 1026
Significant decomposition occurs in the distillation tower 9.50 £ 1026
Table 4
High-risk contributors
Cut set Probability Contribution (%)
Design fault 1.00 £ 1024 46.9
Construction material error 1.00 £ 1024 46.9
Table 5
Results of the cut set analysis
Cut set Probability Contribution to
the remaining
6.2% (%)
Condenser fails, vacuum pump
fails to stop on demand, operator
fails to respond to alarm
2.59 £ 1026 19.5
Distillation re-boiler heat load too high,
vacuum pump fails to stop on demand,
operator fails to respond to alarm
1.20 £ 1026 9.0
CSTR agitator failure, control valve of
temperature interlock system fails to
open, operator fails to respond to alarm
9.39 £ 1027 7.1
Pressure indicator on distillation tower
fails high, vacuum pump fails to stop
on demand, operator fails to respond
to alarm
5.7 £ 1027 4.3
K. Krishna et al. / Reliability Engineering and System Safety 81 (2003) 215–224220
The fault tree with the Top Event designated as
significant decomposition of HA occurs in the process is
shown in Fig. 4.
4.2. Data and data sources
The failure rate and probability data have been provided
in Table 2. The primary reference is CCPS [24]. Other
references include Moss [28], Rasmussen et al. [27], and
Lees [25]. Contamination and corrosion data are based on
engineering judgment.
4.3. Results and discussion
With the failure rate data above, the calculated results are
summarized in Table 3. The four sub-events, listed in
Fig. 5. Partial sub-tree for the event: ‘Significant decomposition occurs in the CSTR’.
K. Krishna et al. / Reliability Engineering and System Safety 81 (2003) 215–224 221
Table 3, lead to a probability of 2.13 £ 1024 to the TOP
event—significant decomposition of HA occurs in the
process. From the cut set analysis, the two cut sets that
contribute the most (93.8% together) are shown in Table 4.
Since HA decomposition is significantly catalyzed by
metal/dust contaminants, design fault and construction
errors become dominant. To eliminate the human errors in
the design and construction materials, multiple independent
inspections are recommended.
Except for the design fault and construction errors, the
five major contributors to the remaining 6.2% are listed in
Table 5, where the last column is the contribution to the
remaining 6.2%. While cut set analysis quantitatively
describes each cut set and identifies the major risk
contributors, it obscures the observation without identify-
ing the intermediate gates. If we examine the fault tree
directly, part of which is shown in Figs. 4 and 5, some
conclusions can be easily reached. For the sub-event
‘Significant decomposition occurs in the CSTR’, a large
portion of the risk comes from the ‘excessive KOH fed or
local KOH concentration too high’ gate. From Fig. 5, the
probability of ‘decomposition in the CSTR’ without the
protection system is 1.41 £ 1022, of which 1.39 £ 1022
comes from ‘excessive KOH feed or local KOH
concentration too high’. This is reasonable since OH2
catalyzes HA decomposition. Therefore, the performance
of the flow ratio control and agitator becomes a major
safety concern. To remedy this problem, an alternative
reaction mechanism or a semi-batch/batch reactor design
is preferred.
5. Benefit of design changes and safeguards
Decomposition reaction of HA is exothermic, and a large
heat and volume of toxic gases can be generated and
released. Different designs or safeguards will either reduce
the probability of significant decomposition or lessen the
severity level. The benefit of design changes and safeguards
can be easily verified and compared by the fault tree results.
For illustration, we present here the benefits of a
temperature interlock system on the reactor and quench
valve protection at the bottom of the distillation tower.
Application to other design changes and safeguards can be
done similarly.
Without the temperature interlock on the CSTR, the
protection against temperature rise, upon occurrence of HA
decomposition in the reactor, is solely provided by alarms
and operator, which is obviously less reliable than an
automatic interlock. In the absence of the ‘temperature
interlock fails to shutdown the CSTR’ scenario, a
probability of 5.6 £ 1024 of ‘significant decomposition in
the CSTR’ is obtained, which is much higher than original
3.21 £ 1026 with the temperature interlock, as can been
seen from Figs. 5 and 6. Therefore, the temperature
interlock can improve process safety by two orders of
magnitude in this process.
Likewise, without the quench valve protection, the safety
operation of the distillation tower depends on the alarms and
workers only. Re-evaluating the fault tree, with the
‘automatic protection system fails’ in Fig. 7, we obtain a
probability of 1.81 £ 1024 for sub-event ‘significant
Fig. 6. Partial sub-tree for the event: ‘Temperature interlock fails and operator does not respond to alarm’.
K. Krishna et al. / Reliability Engineering and System Safety 81 (2003) 215–224222
decomposition occurs in the distillation tower’, which again
is higher than the previous value of 9.5 £ 1026 with quench
valve protection.
6. Conclusions
This paper discusses QRA of a generic HA production
plant, integrating the findings of the CSB incident report and
the knowledge of potential HA reactivity hazards from
research at the MKOPSC. Our work shows that HA
production process is inherently highly hazardous. A
layered, highly reliable protection system is required to
ensure a safe operation [29 – 31]. The benefit of a
temperature interlock on the CSTR and automatic quench
valve protection on the distillation tower are quantified and
verified by FTA. Due to the high sensitivity of HA
decomposition to the hydroxyl (OH2) ion, a semi-batch
Fig. 7. Partial sub-tree for the event: ‘Significant decomposition occurs in the distillation tower’.
K. Krishna et al. / Reliability Engineering and System Safety 81 (2003) 215–224 223
reactor design, or the development of alternate reaction
mechanisms for the production of HA, is suggested.
Development of a RRI will indicate relative risk
associated with the various parts of the process and help
identify high-risk contributors. This information will help in
developing layers of protection to ensure a safer process.
This paper demonstrates that a very thorough study of the
reactive hazards is required. Existing assessment methods
would have identified the high-risk hazards in the
production process and adopting safeguards would have
helped to reduce the risk in the process.
A better understanding of the chemicals and processes
involved in a chemical plant will ultimately help in
improving chemical process safety.
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