Chapter 4 : Reactors - Chapter Notes, Chemical Engineering Chemical Engineering Notes | EduRev

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Chemical Engineering : Chapter 4 : Reactors - Chapter Notes, Chemical Engineering Chemical Engineering Notes | EduRev

 Page 1


4
REACTORS
4.1 INTRODUCTION
This chapter presents potential failure mechanisms for reactors and suggests
design alternatives for reducing the risks associated with such failures. The
types of reactors covered in this chapter include:
• Batch reactors
• Semi-batch reactors
• Continuous-flow stirred tank reactors (CSTR)
• Plug flow tubular reactors (PFR)
• Packed-bed reactors (continuous)
• Packed-tube reactors (continuous)
• Fluid-bed reactors
This chapter presents only those failure modes that are unique to
reaction systems. Some of the generic failure scenarios pertaining to vessels
and heat exchangers may also be applicable to reactors. Consequently, this
chapter should be used in conjunction with Chapter 3, Vessels, and Chapter 6,
Heat Transfer Equipment. Unless specifically noted, the failure scenarios
apply to more than one type of reactor.
4.2 PAST INCIDENTS
Reactors are a major source of serious process safety incidents. Several case
histories are presented to reinforce the need for safe design and operating prac-
tices for reactors.
Page 2


4
REACTORS
4.1 INTRODUCTION
This chapter presents potential failure mechanisms for reactors and suggests
design alternatives for reducing the risks associated with such failures. The
types of reactors covered in this chapter include:
• Batch reactors
• Semi-batch reactors
• Continuous-flow stirred tank reactors (CSTR)
• Plug flow tubular reactors (PFR)
• Packed-bed reactors (continuous)
• Packed-tube reactors (continuous)
• Fluid-bed reactors
This chapter presents only those failure modes that are unique to
reaction systems. Some of the generic failure scenarios pertaining to vessels
and heat exchangers may also be applicable to reactors. Consequently, this
chapter should be used in conjunction with Chapter 3, Vessels, and Chapter 6,
Heat Transfer Equipment. Unless specifically noted, the failure scenarios
apply to more than one type of reactor.
4.2 PAST INCIDENTS
Reactors are a major source of serious process safety incidents. Several case
histories are presented to reinforce the need for safe design and operating prac-
tices for reactors.
4.2. / Seveso Runaway Reaction
On July 10, 1976 an incident occurred at a chemical plant in Seveso, Italy,
which had far-reaching effects on the process safety regulations of many coun-
tries, especially in Europe. An atmospheric reactor containing an uncompleted
batch of 2,4,5-trichlorophenol (TCP) was left for the weekend. Its tempera-
ture was 158
0
C, well below the temperature at which a runaway reaction
could start (believed at the time to be 23O
0
C, but possibly as low as 185
0
C).
The reaction was carried out under vacuum, and the reactor was heated by
steam in an external jacket, supplied by exhaust steam from a turbine at 19O
0
C
and a pressure of 12 bar gauge. The turbine was on reduced load, as various
other plants were also shutting down for the weekend (as required by Italian
law), and the temperature of the steam rose to about 30O
0
C. There was a tem-
perature gradient through the walls of the reactor (30O
0
C on the outside and
16O
0
C on the inside) below the liquid level because the temperature of the
liquid in the reactor could not exceed its boiling point. Above the liquid level,
the walls were at a temperature of 30O
0
C throughout.
When the steam was shut off and, 15 minutes later, the agitator was
switched off, heat transferred from the hot wall above the liquid level to the
top part of the liquid, which became hot enough for a runaway reaction to
start. This resulted in a release of TCDD (dioxin), which killed a number of
nearby animals, caused dermatitis (chloracne) in about 250 people, damaged
vegetation near the site, and required the evacuation of about 600 people
(Kletz 1994).
Ed. Note: The lesson learned from this incident is that provision should have
been made to limit the vessel wall temperature from reaching the known onset tem-
perature at which a runaway could occur.
4.2.2 3,4-DichloroanHine Autoclave Incident
In January 1976, a destructive runaway reaction occurred during the opera-
tion of a large batch hydrogenation reactor used in the production of 3,4-
dichloroaniline. The process involved the hydrogenation of 3,4-dichloronitro-
benzene (DCNB) under pressure in an agitated autoclave. The autoclave was
first charged with DCNB and a catalyst and then purged with nitrogen to
remove air. A hydrogen purge followed the nitrogen purge, after which steam
was applied to the reactor jacket and the temperature raised to within 2O
0
C of the
reaction temperature before additional hydrogen was admitted through a sparger.
The heat of reaction carried the temperature to the desired operating level.
During the early stages, the rate of reaction was limited by the heat
removal capacity of the autoclave cooling coil. This resulted in a relatively low
Page 3


4
REACTORS
4.1 INTRODUCTION
This chapter presents potential failure mechanisms for reactors and suggests
design alternatives for reducing the risks associated with such failures. The
types of reactors covered in this chapter include:
• Batch reactors
• Semi-batch reactors
• Continuous-flow stirred tank reactors (CSTR)
• Plug flow tubular reactors (PFR)
• Packed-bed reactors (continuous)
• Packed-tube reactors (continuous)
• Fluid-bed reactors
This chapter presents only those failure modes that are unique to
reaction systems. Some of the generic failure scenarios pertaining to vessels
and heat exchangers may also be applicable to reactors. Consequently, this
chapter should be used in conjunction with Chapter 3, Vessels, and Chapter 6,
Heat Transfer Equipment. Unless specifically noted, the failure scenarios
apply to more than one type of reactor.
4.2 PAST INCIDENTS
Reactors are a major source of serious process safety incidents. Several case
histories are presented to reinforce the need for safe design and operating prac-
tices for reactors.
4.2. / Seveso Runaway Reaction
On July 10, 1976 an incident occurred at a chemical plant in Seveso, Italy,
which had far-reaching effects on the process safety regulations of many coun-
tries, especially in Europe. An atmospheric reactor containing an uncompleted
batch of 2,4,5-trichlorophenol (TCP) was left for the weekend. Its tempera-
ture was 158
0
C, well below the temperature at which a runaway reaction
could start (believed at the time to be 23O
0
C, but possibly as low as 185
0
C).
The reaction was carried out under vacuum, and the reactor was heated by
steam in an external jacket, supplied by exhaust steam from a turbine at 19O
0
C
and a pressure of 12 bar gauge. The turbine was on reduced load, as various
other plants were also shutting down for the weekend (as required by Italian
law), and the temperature of the steam rose to about 30O
0
C. There was a tem-
perature gradient through the walls of the reactor (30O
0
C on the outside and
16O
0
C on the inside) below the liquid level because the temperature of the
liquid in the reactor could not exceed its boiling point. Above the liquid level,
the walls were at a temperature of 30O
0
C throughout.
When the steam was shut off and, 15 minutes later, the agitator was
switched off, heat transferred from the hot wall above the liquid level to the
top part of the liquid, which became hot enough for a runaway reaction to
start. This resulted in a release of TCDD (dioxin), which killed a number of
nearby animals, caused dermatitis (chloracne) in about 250 people, damaged
vegetation near the site, and required the evacuation of about 600 people
(Kletz 1994).
Ed. Note: The lesson learned from this incident is that provision should have
been made to limit the vessel wall temperature from reaching the known onset tem-
perature at which a runaway could occur.
4.2.2 3,4-DichloroanHine Autoclave Incident
In January 1976, a destructive runaway reaction occurred during the opera-
tion of a large batch hydrogenation reactor used in the production of 3,4-
dichloroaniline. The process involved the hydrogenation of 3,4-dichloronitro-
benzene (DCNB) under pressure in an agitated autoclave. The autoclave was
first charged with DCNB and a catalyst and then purged with nitrogen to
remove air. A hydrogen purge followed the nitrogen purge, after which steam
was applied to the reactor jacket and the temperature raised to within 2O
0
C of the
reaction temperature before additional hydrogen was admitted through a sparger.
The heat of reaction carried the temperature to the desired operating level.
During the early stages, the rate of reaction was limited by the heat
removal capacity of the autoclave cooling coil. This resulted in a relatively low
autoclave pressure. Later, when the hydrogenation rate fell off, the autoclave
pressure was allowed to increase. Based on field evidence and subsequent labo-
ratory work the following conclusions were reached as to the cause of the inci-
dent (Tong 1977):
• The primary cause was a sudden pressure increase due to runaway reac-
tion at about 26O
0
C.
• The reaction mass reached runaway temperature due to the buildup and
rapid exothermic disproportionation of an intermediate (3,4-dipheny-
hydroxylamine). The most likely trigger for this reaction was a 1O
0
C
increase in the reactor temperature set point (operator error).
Ed. Note: The lesson learned from this incident is that a, study should have been
made of exotherm potential and provision should have been made to limit tempera-
ture setpoint or an interlock provided to address this hazard. If possible a larger oper-
ating temperature margin should have been employed.
4.2.3 Continuous Sulfonation Reaction Explosion
During the startup phase of a continuous system (3 CSTRs in series) for the
sulfonation of an aromatic compound, a thermal explosion occurred in a
pump and recirculation line. Although the incident damaged the plant and
interrupted production, no personnel were injured.
Investigation revealed that, while recirculation of the reaction mass was
starting up, the pump and the line became plugged. This problem was cor-
rected and line recirculation was restarted. Four hours later the explosion
occurred, resulting in the blow-out of the pump seal, which was immediately
followed by rupture of the recirculation line.
Investigation further revealed that during pipe cleanout some insulation
had been removed, leaving a portion of the line exposed and untraced. This
condition apparently led to slow solidification of the reaction mass and a dead-
headed pump. Calculations based on pump data indicated that a temperature
of 6O
0
C above the processing temperature could be reached within 5 minutes
after dead-heading occurred. Previous studies had determined that the rate of
decomposition is considerable at this temperature and that the total heat of
decomposition (500 kcal/kg) is large (Quinn 1984).
4.3 FAILURE SCENARIOS AND DESIGN SOLUTIONS
Table 4 presents information on equipment failure scenarios and associated
design solutions specific to reactors. The table heading definitions are pro-
vided in Chapter 3, section 3.3.
Page 4


4
REACTORS
4.1 INTRODUCTION
This chapter presents potential failure mechanisms for reactors and suggests
design alternatives for reducing the risks associated with such failures. The
types of reactors covered in this chapter include:
• Batch reactors
• Semi-batch reactors
• Continuous-flow stirred tank reactors (CSTR)
• Plug flow tubular reactors (PFR)
• Packed-bed reactors (continuous)
• Packed-tube reactors (continuous)
• Fluid-bed reactors
This chapter presents only those failure modes that are unique to
reaction systems. Some of the generic failure scenarios pertaining to vessels
and heat exchangers may also be applicable to reactors. Consequently, this
chapter should be used in conjunction with Chapter 3, Vessels, and Chapter 6,
Heat Transfer Equipment. Unless specifically noted, the failure scenarios
apply to more than one type of reactor.
4.2 PAST INCIDENTS
Reactors are a major source of serious process safety incidents. Several case
histories are presented to reinforce the need for safe design and operating prac-
tices for reactors.
4.2. / Seveso Runaway Reaction
On July 10, 1976 an incident occurred at a chemical plant in Seveso, Italy,
which had far-reaching effects on the process safety regulations of many coun-
tries, especially in Europe. An atmospheric reactor containing an uncompleted
batch of 2,4,5-trichlorophenol (TCP) was left for the weekend. Its tempera-
ture was 158
0
C, well below the temperature at which a runaway reaction
could start (believed at the time to be 23O
0
C, but possibly as low as 185
0
C).
The reaction was carried out under vacuum, and the reactor was heated by
steam in an external jacket, supplied by exhaust steam from a turbine at 19O
0
C
and a pressure of 12 bar gauge. The turbine was on reduced load, as various
other plants were also shutting down for the weekend (as required by Italian
law), and the temperature of the steam rose to about 30O
0
C. There was a tem-
perature gradient through the walls of the reactor (30O
0
C on the outside and
16O
0
C on the inside) below the liquid level because the temperature of the
liquid in the reactor could not exceed its boiling point. Above the liquid level,
the walls were at a temperature of 30O
0
C throughout.
When the steam was shut off and, 15 minutes later, the agitator was
switched off, heat transferred from the hot wall above the liquid level to the
top part of the liquid, which became hot enough for a runaway reaction to
start. This resulted in a release of TCDD (dioxin), which killed a number of
nearby animals, caused dermatitis (chloracne) in about 250 people, damaged
vegetation near the site, and required the evacuation of about 600 people
(Kletz 1994).
Ed. Note: The lesson learned from this incident is that provision should have
been made to limit the vessel wall temperature from reaching the known onset tem-
perature at which a runaway could occur.
4.2.2 3,4-DichloroanHine Autoclave Incident
In January 1976, a destructive runaway reaction occurred during the opera-
tion of a large batch hydrogenation reactor used in the production of 3,4-
dichloroaniline. The process involved the hydrogenation of 3,4-dichloronitro-
benzene (DCNB) under pressure in an agitated autoclave. The autoclave was
first charged with DCNB and a catalyst and then purged with nitrogen to
remove air. A hydrogen purge followed the nitrogen purge, after which steam
was applied to the reactor jacket and the temperature raised to within 2O
0
C of the
reaction temperature before additional hydrogen was admitted through a sparger.
The heat of reaction carried the temperature to the desired operating level.
During the early stages, the rate of reaction was limited by the heat
removal capacity of the autoclave cooling coil. This resulted in a relatively low
autoclave pressure. Later, when the hydrogenation rate fell off, the autoclave
pressure was allowed to increase. Based on field evidence and subsequent labo-
ratory work the following conclusions were reached as to the cause of the inci-
dent (Tong 1977):
• The primary cause was a sudden pressure increase due to runaway reac-
tion at about 26O
0
C.
• The reaction mass reached runaway temperature due to the buildup and
rapid exothermic disproportionation of an intermediate (3,4-dipheny-
hydroxylamine). The most likely trigger for this reaction was a 1O
0
C
increase in the reactor temperature set point (operator error).
Ed. Note: The lesson learned from this incident is that a, study should have been
made of exotherm potential and provision should have been made to limit tempera-
ture setpoint or an interlock provided to address this hazard. If possible a larger oper-
ating temperature margin should have been employed.
4.2.3 Continuous Sulfonation Reaction Explosion
During the startup phase of a continuous system (3 CSTRs in series) for the
sulfonation of an aromatic compound, a thermal explosion occurred in a
pump and recirculation line. Although the incident damaged the plant and
interrupted production, no personnel were injured.
Investigation revealed that, while recirculation of the reaction mass was
starting up, the pump and the line became plugged. This problem was cor-
rected and line recirculation was restarted. Four hours later the explosion
occurred, resulting in the blow-out of the pump seal, which was immediately
followed by rupture of the recirculation line.
Investigation further revealed that during pipe cleanout some insulation
had been removed, leaving a portion of the line exposed and untraced. This
condition apparently led to slow solidification of the reaction mass and a dead-
headed pump. Calculations based on pump data indicated that a temperature
of 6O
0
C above the processing temperature could be reached within 5 minutes
after dead-heading occurred. Previous studies had determined that the rate of
decomposition is considerable at this temperature and that the total heat of
decomposition (500 kcal/kg) is large (Quinn 1984).
4.3 FAILURE SCENARIOS AND DESIGN SOLUTIONS
Table 4 presents information on equipment failure scenarios and associated
design solutions specific to reactors. The table heading definitions are pro-
vided in Chapter 3, section 3.3.
4.4 DISCUSSION
4.4.1 Use of Potential Design Solutions Table
To arrive at the optimal design solution for a given application, use Table 4 in
conjunction with the design basis selection methodology presented in Chapter
2. Use of the design solutions presented in the table should be combined with
sound engineering judgment and consideration of all relevant factors.
4.4.2 General Discussion
Reactors may be grouped into three main types: batch, semi-batch, and con-
tinuous.
In a batch reactor, all the reactants and catalyst (if one is used) are charged
to the reactor first and agitated, and the reaction is initiated, with heat being
added or removed as needed. In a semi-batch reactor, one of the reactants is
first charged to the reactor, catalyst is also charged and the reactor contents are
agitated, after which the other reactants and possibly additional catalyst are
added at a controlled feed rate, with heat being added or removed as needed.
In a continuous reactor all the reactants and catalyst (if one is used) are fed
simultaneously to the reactor, and the products, side products, unconverted
reactants, and catalyst leave the reactor simultaneously. In some continuous
reactors, the catalyst is held stationary, either in tubes or occupying the entire
cross-section of the vessel.
Batch and semi-batch reactors are used primarily where reaction rates are
slow and require long residence times to achieve a reasonable conversion and
yield. This often means large inventories and, if the contents are flammable,
there is a potential for serious fires should a leak develop. Many of these reac-
tors have agitators, and if there is an agitator failure (stoppage or loss of the
impeller), some reactions can run away (Ventrone 1969; Lees 1996).
Heat removal is also a concern for batch or semi-batch reactors conduct-
ing exothermic reactions. Since the external jacket may not be adequate to
remove the heat of reaction, it may be necessary to install an internal cooling
coil as well, or an external heat exchanger with recirculation of the reactor con-
tents. These additional items of heat transfer equipment increase the potential
for leakage problems and may lead to a runaway if the coolant leaks into the
reactants.
Continuous reactors are considered to be inherently safer than batch or
semi-batch reactors as they usually have smaller inventories of flammable
and/or toxic materials. Tubular reactors are generally used for gaseous reac-
tions, but are also suitable for some liquid-phase reactions. Gas phase reactors
generally have lower inventories than liquid-phase continuous reactors of
Page 5


4
REACTORS
4.1 INTRODUCTION
This chapter presents potential failure mechanisms for reactors and suggests
design alternatives for reducing the risks associated with such failures. The
types of reactors covered in this chapter include:
• Batch reactors
• Semi-batch reactors
• Continuous-flow stirred tank reactors (CSTR)
• Plug flow tubular reactors (PFR)
• Packed-bed reactors (continuous)
• Packed-tube reactors (continuous)
• Fluid-bed reactors
This chapter presents only those failure modes that are unique to
reaction systems. Some of the generic failure scenarios pertaining to vessels
and heat exchangers may also be applicable to reactors. Consequently, this
chapter should be used in conjunction with Chapter 3, Vessels, and Chapter 6,
Heat Transfer Equipment. Unless specifically noted, the failure scenarios
apply to more than one type of reactor.
4.2 PAST INCIDENTS
Reactors are a major source of serious process safety incidents. Several case
histories are presented to reinforce the need for safe design and operating prac-
tices for reactors.
4.2. / Seveso Runaway Reaction
On July 10, 1976 an incident occurred at a chemical plant in Seveso, Italy,
which had far-reaching effects on the process safety regulations of many coun-
tries, especially in Europe. An atmospheric reactor containing an uncompleted
batch of 2,4,5-trichlorophenol (TCP) was left for the weekend. Its tempera-
ture was 158
0
C, well below the temperature at which a runaway reaction
could start (believed at the time to be 23O
0
C, but possibly as low as 185
0
C).
The reaction was carried out under vacuum, and the reactor was heated by
steam in an external jacket, supplied by exhaust steam from a turbine at 19O
0
C
and a pressure of 12 bar gauge. The turbine was on reduced load, as various
other plants were also shutting down for the weekend (as required by Italian
law), and the temperature of the steam rose to about 30O
0
C. There was a tem-
perature gradient through the walls of the reactor (30O
0
C on the outside and
16O
0
C on the inside) below the liquid level because the temperature of the
liquid in the reactor could not exceed its boiling point. Above the liquid level,
the walls were at a temperature of 30O
0
C throughout.
When the steam was shut off and, 15 minutes later, the agitator was
switched off, heat transferred from the hot wall above the liquid level to the
top part of the liquid, which became hot enough for a runaway reaction to
start. This resulted in a release of TCDD (dioxin), which killed a number of
nearby animals, caused dermatitis (chloracne) in about 250 people, damaged
vegetation near the site, and required the evacuation of about 600 people
(Kletz 1994).
Ed. Note: The lesson learned from this incident is that provision should have
been made to limit the vessel wall temperature from reaching the known onset tem-
perature at which a runaway could occur.
4.2.2 3,4-DichloroanHine Autoclave Incident
In January 1976, a destructive runaway reaction occurred during the opera-
tion of a large batch hydrogenation reactor used in the production of 3,4-
dichloroaniline. The process involved the hydrogenation of 3,4-dichloronitro-
benzene (DCNB) under pressure in an agitated autoclave. The autoclave was
first charged with DCNB and a catalyst and then purged with nitrogen to
remove air. A hydrogen purge followed the nitrogen purge, after which steam
was applied to the reactor jacket and the temperature raised to within 2O
0
C of the
reaction temperature before additional hydrogen was admitted through a sparger.
The heat of reaction carried the temperature to the desired operating level.
During the early stages, the rate of reaction was limited by the heat
removal capacity of the autoclave cooling coil. This resulted in a relatively low
autoclave pressure. Later, when the hydrogenation rate fell off, the autoclave
pressure was allowed to increase. Based on field evidence and subsequent labo-
ratory work the following conclusions were reached as to the cause of the inci-
dent (Tong 1977):
• The primary cause was a sudden pressure increase due to runaway reac-
tion at about 26O
0
C.
• The reaction mass reached runaway temperature due to the buildup and
rapid exothermic disproportionation of an intermediate (3,4-dipheny-
hydroxylamine). The most likely trigger for this reaction was a 1O
0
C
increase in the reactor temperature set point (operator error).
Ed. Note: The lesson learned from this incident is that a, study should have been
made of exotherm potential and provision should have been made to limit tempera-
ture setpoint or an interlock provided to address this hazard. If possible a larger oper-
ating temperature margin should have been employed.
4.2.3 Continuous Sulfonation Reaction Explosion
During the startup phase of a continuous system (3 CSTRs in series) for the
sulfonation of an aromatic compound, a thermal explosion occurred in a
pump and recirculation line. Although the incident damaged the plant and
interrupted production, no personnel were injured.
Investigation revealed that, while recirculation of the reaction mass was
starting up, the pump and the line became plugged. This problem was cor-
rected and line recirculation was restarted. Four hours later the explosion
occurred, resulting in the blow-out of the pump seal, which was immediately
followed by rupture of the recirculation line.
Investigation further revealed that during pipe cleanout some insulation
had been removed, leaving a portion of the line exposed and untraced. This
condition apparently led to slow solidification of the reaction mass and a dead-
headed pump. Calculations based on pump data indicated that a temperature
of 6O
0
C above the processing temperature could be reached within 5 minutes
after dead-heading occurred. Previous studies had determined that the rate of
decomposition is considerable at this temperature and that the total heat of
decomposition (500 kcal/kg) is large (Quinn 1984).
4.3 FAILURE SCENARIOS AND DESIGN SOLUTIONS
Table 4 presents information on equipment failure scenarios and associated
design solutions specific to reactors. The table heading definitions are pro-
vided in Chapter 3, section 3.3.
4.4 DISCUSSION
4.4.1 Use of Potential Design Solutions Table
To arrive at the optimal design solution for a given application, use Table 4 in
conjunction with the design basis selection methodology presented in Chapter
2. Use of the design solutions presented in the table should be combined with
sound engineering judgment and consideration of all relevant factors.
4.4.2 General Discussion
Reactors may be grouped into three main types: batch, semi-batch, and con-
tinuous.
In a batch reactor, all the reactants and catalyst (if one is used) are charged
to the reactor first and agitated, and the reaction is initiated, with heat being
added or removed as needed. In a semi-batch reactor, one of the reactants is
first charged to the reactor, catalyst is also charged and the reactor contents are
agitated, after which the other reactants and possibly additional catalyst are
added at a controlled feed rate, with heat being added or removed as needed.
In a continuous reactor all the reactants and catalyst (if one is used) are fed
simultaneously to the reactor, and the products, side products, unconverted
reactants, and catalyst leave the reactor simultaneously. In some continuous
reactors, the catalyst is held stationary, either in tubes or occupying the entire
cross-section of the vessel.
Batch and semi-batch reactors are used primarily where reaction rates are
slow and require long residence times to achieve a reasonable conversion and
yield. This often means large inventories and, if the contents are flammable,
there is a potential for serious fires should a leak develop. Many of these reac-
tors have agitators, and if there is an agitator failure (stoppage or loss of the
impeller), some reactions can run away (Ventrone 1969; Lees 1996).
Heat removal is also a concern for batch or semi-batch reactors conduct-
ing exothermic reactions. Since the external jacket may not be adequate to
remove the heat of reaction, it may be necessary to install an internal cooling
coil as well, or an external heat exchanger with recirculation of the reactor con-
tents. These additional items of heat transfer equipment increase the potential
for leakage problems and may lead to a runaway if the coolant leaks into the
reactants.
Continuous reactors are considered to be inherently safer than batch or
semi-batch reactors as they usually have smaller inventories of flammable
and/or toxic materials. Tubular reactors are generally used for gaseous reac-
tions, but are also suitable for some liquid-phase reactions. Gas phase reactors
generally have lower inventories than liquid-phase continuous reactors of
equal volumes, and thus are usually inherently safer. Long, thin tubular reac-
tors are safer than large batch reactors as the leak rate (should a leak occur) is
limited by the cross-section area of the tube, and can be stopped by closing a
remotely operated emergency isolation valve in the line (Kletz 1990).
Continuous-flow stirred tank reactors (CSTR) are also considered to be inher-
ently safer than batch reactors as they contain smaller amounts of flammable
or toxic liquids. Since they are agitated, however, they have the same agitator
failure hazard as batch reactors, and can experience runaways if this occurs.
Exhibit 4.1 is a comparison of different types of reactors from the safety per-
spective (CCPS 1995).
EXHI BIT 4.1
Comparison of Different Reactor Types from the Safety Perspective
Plug Flow Reactor
(PFR)
Continuous-Flow
Stirred Tank
Reactor (CSTR) Batch Semi-Batch
ADVANTAGES
• Low inventory
• Stationary
condition (steady
state operation)
• Stationary
condition (steady
state operation)
• Agitation provides
safety tool
• Streams may be
diluted to slow
reaction
• Agitation provides
safety tool
• Controllable
addition rate
• Agitation provides
safety tool
• Large exotherm
controllable
DISADVANTAGES
• Process
dependency
• Potential for hot
spots
• Agitation present
only if in-line
mixers are
available
• Difficult to design
• Large inventory
• Difficult to cool
large mass
• Difficult start-up
and shutdown
aspects
• Precipitation
problems
• Low throughput
rate
• Large exotherm
difficult to control
• Large inventory
• All materials
present
• Starting
temperature is
critical (if too low,
reactants will
accumulate)
• Precipitation
problems
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