The nitration of toluene process in a batch reactor is used to illustrate the application of qualitative tools in IISDET methodology. Nitration of aromatics is one of the oldest and most important industrial reactions for the formation of intermediates of many compounds, including pharmaceuticals, dyes, explosives, pesticides, etc. In spite of that, the nitration reaction is the second most hazardous reaction after polymerisation, which caused approximately 15 serious incidents in the UK involving thermal runaway chemical reactions in a batch/semi-batch reactor (Barton and Rogers, 1997).
Moreover, the US Chemical Safety and Hazard Investigation Board (USCSB) reported that 167 serious chemical incidents in the US, involved uncontrolled chemical reactions. For this reason, the nitration reaction was selected to illustrate the applicability of QAISD since the initial design of the nitration process is based on a batch system, where the reaction is generally fast and highly exothermic, involving flammable organics and a toxic mixture of acids. In addition, reactors represent a large portion of the chemical process, where most of the inherent dangers are present and with the thoughts that if inherent safety could be incorporated early in the reactor
process, preventab
Indus sulphuric aqueous of this re
Sulphuric ion, whic mononitr database reference design.
Table 4
Process Chemical Products Reactors Capacity Reaction Heat of re Reaction Reaction Decompo Heat of de Flammab Hazardou
then the r ble and an in strially, the p c acid in a tw acid phase a action is giv
c acid is use ch then reac ration proce for a batch es (Othmer,
4.3: Typical
s and Safety ls
time eaction
temperature pressure osition temper
ecomposition ility, reactivit us by-products toluene
remaining o nherently saf production o wo-phase liq
and the react ven by:
ed to donate cts with tolu ess (Halder h reactor pro 2004; Ullm
properties o Ullman Parameters
rature n
ty s
aci nitric +
110 of hazards fer process a of nitrotolue quid-liquid re
tion takes pl
e a proton to uene to form
et al., 2007 ocess to prod
mans, 2003
of toluene nit
’s, 2003; Br
Toluene, acid, 12-2 55-60wt%
40wt%p-n Liquid ph 6000-L 2-4h -216kJ/kg 35 – 40oC 1atm 160oC -162kJ/m Toluene, NOx, SOx
sulphu
id ⎯⎯⎯⎯
0
in the dow as a whole co enes is achie
eaction, in w lace in the a
o the nitric a m the three i 7). Table 4.3
duce mono-n 3; Bretherick
tration in a b etherick’s, 1 Ty 28-32wt% ni 20wt% water
% o-nitrotolue nitrotoluene hase Batch
g (Exothermic C
mol
Nitric acid, S
x, 4-nitrotolue n acid uric⎯⎯⎯
⎯ →
wnstream p ould be achi ved by mixi which toluen acid phase. T
acid, thus for isomers of n 3 is a summ nitrotoluene k’s, 1995)
batch reactor 1995) ypical Values
tric acid, 52-5 ene, 3-4wt% m
c)
Sulphuric acid ene-2-sulfonic ne nitrotolue
rocess wou eved.
ing nitric ac ne diffuses in
The stoichiom
rming a nitro nitrotoluene
mary of a t e based on s as the base
r (Othmer, 2
56wt% sulphu m-nitrotoluen
d c acid
water +
uld be
id and nto the metric
onium in the typical several e case
004;
uric ne,
35-111 4.3.1 Results analysis and discussions
The potential of inherent hazards in the toluene nitration process is explored in this study using the RIP tool. Table 4.4 shows the predicted hazards after applying the RIP tool in Stage I of QEISD. Based on the information given in Table 4.3, there are eleven potential inherent hazards i.e., the inherent hazards of H1 to H11, such as highly-reactive reagent, excessive heat of reaction, thermally decomposed chemical, large inventory, etc. After assessing the hazards with process safety references and nitration process literatures, nine out of the eleven inherent hazards have been screened as being prioritised hazards. These prioritised hazards have been identified as the inherent hazards that could lead to fire and explosion, due to thermal runaway.
The prioritised hazards for H1 and H3 are in agreement with several other publications, e.g., Chen and Wu (1996) and Chen et al., (1998). Their experiments showed that the desired reaction has a high potential to trigger a thermal explosion, which is caused by the decomposition of mononitrotoluene and nitric acid.
The prioritised hazards of H1, H2, H6, and H10, are selected in order to illustrate the application of the IDH tool. As described in Chapter 2, each hazard is required to go through ISD Heuristics i.e., Hazard Elimination, Consequence Reduction, and Likelihood Reduction, in order to generate all possible ISD options. Tables 4.5a and 4.5b provide the summary of the generated ISD options, to eliminate or minimise the above hazards. As described earlier, every single hazard will be examined through ISD heuristics and the related IS principles, in order to generate suitable ISD options.
For example, when the identified hazard, such as the highly exothermic reaction of toluene nitration in liquid phase (H6) is assessed using the IDH work-flow diagram, five potential ISD options are generated. For example, the substitution to vapour phase reaction (OP8), minimisation of volume by replacing the batch reactor with a continuous or intensified reactor (OP3), moderation of current reaction energy by using diluted nitric acid (OP5), etc. Several options are also repeated in other hazards during this process,in order to meet the objective of this stage, which is to create all possible ISD options and not to conclude at the first identified solution. In addition, this IDH tool also allows the identification of potential conflicts, due to the implementation of IS principles, as shown in both tables.
112
Some of the identified ISD options are found to be feasible to eliminate the hazard, such as the option to nitrate toluene via a vapour phase reaction (OP8) (Dagade et al., 2002; Sawant et al., 2007; Pirngruber et al.; 2007). The vapour phase nitration of toluene is found to be a very fast reaction of less than 1-hour reaction time that will not allow the accumulation of reactive reagents. The possibility of decomposition, due to excessive heat of the reaction when there is a failure in the cooling system, could be minimised or eliminated through this ISD option. This option could also minimise the inventory of reaction mixture, as it is repeated as an option to minimise the large liquid phase inventory (H10).
In addition, OP3 proposed to reduce the volume by replacing the batch reactor with a continuous mode or intensified reactor, such as a micro reactor, which is also possible to minimise the consequence of thermal runaway, due to the high heat of the reaction as the minimum volume of the reaction mixture available in the vessel during the process. This was proven possible through recent findings by Halder et al., (2007) that micro reactors have been shown to have a very high heat transfer rate, due to their high surface area to volume ratio, which enables the micro reactors to control highly exothermic reactions efficiently. One of the identified new hazards, if the design is modified according to OP3, is the potential of complexity in controlling the intensified reactor, which is in agreement with Luyben and Hendershot’s (2004) findings, that the fast dynamics of the reactor could endanger the stability of the process against disturbances, and hence, could lead to a thermal explosion.
Based on the above results, the designer would have many ISD options to consider during the early design stage, and thus, be able to conduct necessary investigations and experiments prior to choosing the best design option that is inherently safer. The identified solutions are also in contrast with the conventional safety measures to manage hazards of the toluene nitration process, which commonly focuses more on controlling the cooling system, installation of pressure relief devices, and classification of explosion zones (Shah, 2004). The identified ISD options (in Stage II) may need to proceed to the evaluation stage for selection of the most appropriate design solution; which is not only inherently safer, but also could reduce the lifecycle cost of the process. Thus, the evaluation stage could be done using the
113
Table 4.4: Results from the application of RIP to identify inherent hazards in toluene nitration QUALITATIVE EVALUATION OF INHERENTLY SAFER DESIGN (QEISD)
Stage I: Identification of Inherent Hazards using the RIP method Process: Production of Mononitrotoluene using mixed acid
Process Unit: Batch reactor Materials in Process Unit: Toluene, Nitric acid, and Sulphuric acid
Register Investigate Prioritise
Design Factor
Process Attributed with Base
Case data Hazard Indicator Predicted Hazard Prioritised Hazard
Chemicals
reactant: toluene, nitric acid,
sulphuric acid reactive,
incompatibility, flammability,
toxic, stability,
etc.
H1: use a highly reactive reagent H1: highly reactive reagent (nitric acid) H2: use a highly concentrated reagent H2: high concentration reagent (sulphuric
acid)
end-product: mononitrotoluene H3: use a chemical that easily decomposes H3: decompose chemical (mononitrotoluene) H4: create incompatibility of reagent and
products
H4: incompatible reaction (nitric/sulphuric acid with H2O)
by-product: NO2, SOx, H2O
H5: use a high energy molecular group H5: high energy molecular group (nitro compounds)
Reaction conditions
heat of reaction: -216kJ/kg
exothermic, hazardous inventory, elevated
temperature or pressure, high concentration,
liquid or vapour phase,
etc.
H6: use reaction route that produces high heat of
reaction H6: high heat of reaction route (mixed acid)
volume: 6000L liquid inventory H7: operate at a high temperature to activate
decomposition
temperature: max 25degC, due to
highly exothermic
H8: operate at low temperature to accumulate high reagent
pressure: 1atm H9: create high pressure, due to gas evolution H9: create high pressure, due to gas evolution (by-products, such as NO2)
concentration: 98% sulphuric acid, 60% nitric acid
H10: accumulate large inventory of liquid-phase
mixture H10: large inventory (liquid phase)
reaction phase: liquid-phase
114
QUALITATIVE EVALUATION OF INHERENTLY SAFER DESIGN (QEISD) Stage I: Identification of Inherent Hazards using the RIP method Process: Production of Mononitrotoluene using mixed acid
Process Unit: Batch reactor Materials in Process Unit: Toluene, Nitric acid, and Sulphuric acid
Register Investigate Prioritise
Design Factor
Process Attributed with Base
Case data Hazard Indicator Predicted Hazard Prioritised Hazard
Type of reactor
batch: large inventory high inventory, agitator speed,
hot spot, etc.
H10: accumulate large inventory of liquid-phase
mixture H10: large inventory (batch reactor)
controls in agitation H11: generate hot spot in a reactor H11: hot spot generated in a reactor (speed of mixer)
controls of cooling system
115
Table 4.5a: Results of ISD options from the application of IDH (Stage II) to generate ISD options for toluene nitration process QUALITATIVE EVALUATION OF INHERENTLY SAFER DESIGN (QEISD)
Stage II: Generation of Inherently Safer Design Options using the IDH method Process: Production of Mononitrotoluene using mixed acid
Process Unit: Batch reactor Materials in Process Unit: Toluene, Nitric acid, and Sulphuric acid Prioritised Hazard ISD Heuristic IS
Guideword
ISD
Indicator ISD Variable ISD Option Prompts on potential of other hazards
H1: highly reactive reagent (nitric acid)
Hazard Elimination
Substitute hazardous
substance new or safer
substances OP1: substitute with less energetic
nitrating reagent decomposition due to batch reaction time
Substitute process route
new or safer process chemistry
OP2: substitute with energetic nitrating reagent such as acetyl nitrate
incompatibility with other reactants
Consequence Reduction
Minimise inventory
volume, process phase, new equipment
OP3: minimise volume use with CSTR/smaller reactor/intensified reactor
elevated operating conditions, increase complexity in control Minimise energy volume , reaction
phase, new equipment
OP4:minimise volume use by changing reaction phase to gas
elevated operating conditions, increase complexity in control Moderate reaction
condition
temperature, pressure, dilution, catalyst
OP5: moderate reaction condition
with dilute nitric acid increase inventory of reactant
H2: high concentration reagent (sulphuric acid)
Hazard Elimination
Eliminate hazardous
substance new or safer
substances OP6: eliminate with solid acid
catalyst toxic release
Eliminate process route new or safer process chemistry
OP7: eliminate with green ionic
liquid toxic release
Consequence Reduction
Minimise inventory
volume, process phase, new equipment
OP3: minimise volume use with CSTR/smaller reactor/intensified reactor
elevated operating conditions, increase complexity in control Minimise energy volume , reaction
phase, new equipment
Moderate reaction condition
temperature, pressure, dilution, catalyst
OP5: moderate reaction condition with dilute sulphuric acid
116
Table 4.5b: ISD options for inherent hazards H6 and H10 after application of IDH (Stage II) for toluene nitration process QUALITATIVE EVALUATION OF INHERENTLY SAFER DESIGN (QEISD)
Stage II: Generation of Inherently Safer Design Option using the IDH method Process: Production of Mononitrotoluene using mixed acid
Process Unit: Batch reactor Materials in Process Unit: Toluene, Nitric acid, Sulphuric acid Prioritised Hazard ISD Heuristic IS Guideword ISD Indicator ISD Variable ISD Option Prompts on potential of other
hazards
H6: highly heat of reaction route (mixed
acid)
Hazard Elimination
Substitute hazardous
substance new or safer substances OP8: substitute with vapour phase reaction route
elevated operating conditions, increase complexity in control process route new or safer process
chemistry
Consequence Reduction
Minimise inventory volume, process phase, new equipment
OP3: minimise volume use with CSTR/smaller reactor/intensified reactor
elevated operating conditions, increase complexity in control Minimise energy volume, reaction phase,
new equipment
OP9: minimise volume with increase
in mixing speed
Moderate reaction condition temperature, pressure,
dilution, catalyst OP5: moderate reaction energy with
dilute nitric acid toxic release
Moderate OP10: moderate reaction energy with
catalyst autocatalysis
Likelihood Reduction
Simplify complexity
strength of equipment, min no of units/utilities,
resistant materials, process layout
OP11: simplify vessel by designing withstand high pressure vessel
loss of containment
strength of equipment,
resistant materials
117
QUALITATIVE EVALUATION OF INHERENTLY SAFER DESIGN (QEISD) Stage II: Generation of Inherently Safer Design Option using the IDH method Process: Production of Mononitrotoluene using mixed acid
Process Unit: Batch reactor Materials in Process Unit: Toluene, Nitric acid, Sulphuric acid Prioritised Hazard ISD Heuristic IS Guideword ISD Indicator ISD Variable ISD Option Prompts on potential of other
hazards
H10: large liquid phase inventory (batch
reactor)
Hazard Elimination
Substitute hazardous
substance new or safer substances OP8: substitute with vapour phase reaction route
elevated operating conditions, increase complexity in control process route new or safer process
chemistry
Consequence Reduction
Minimise inventory volume, process phase, new equipment
OP3: minimise volume use with CSTR/smaller reactor/intensified reactor
elevated operating conditions, increase complexity in control Moderate energy volume , reaction phase,
new equipment
OP9: moderate energy with increase
in mixing speed autocatalysis
reaction condition temperature, pressure,
dilution, catalyst
Likelihood Reduction
Simplify complexity
strength of equipment, min no of units/utilities,
resistant materials, process layout
OP12: simplify vessel with gravity liquid transfer to avoid leakage
loss of containment
strength of equipment,
resistant materials
118
end-user’s expert judgement. Selection could be based on the feasibility of the design option, preliminary design costs, safety impacts, etc.
4.4 Case III: Qualitative evaluation of ammonia for selective catalytic reactor in