The document discusses automation and safety considerations for the polymerization of vinyl chloride monomer (VCM) in polyvinyl chloride (PVC) production. VCM is flammable and toxic, and the exothermic polymerization reaction must be carefully controlled to prevent runaway reactions. Hazards include fires, explosions, and toxic emissions. A hazard assessment identifies scenarios like cooling failure, overfilling, and loss of agitation that could lead to runaway. Prevention strategies include alarms, addition of chemical inhibitors, and automatic depressurization of reactors in emergencies.

1. AUTOMATION AND SAFETY
The process considered is the polymerization of vinyl chloride monomer in the PVC. The case
study is based on a well - known process which is a substance the VCM that is flammable and
produces toxic combustion products also is a known carcinogen. The process is based a semi-
continuous plant consists of several reactors with times of 10 hours of polymerization. The heart of
the process is a cstr mechanically stirred reactor where the reaction heat is removed by cooling
water in jacket and where the reaction takes place in multiple reactors in parallel so that it operates
in a semi-continuous mode. If the reactor has undergone maintenance actions after the last batch,
this should be reclaimed from the air to minimize oxidation of the monomer which produces HCl
which can lead to corrosion of the vessel. In other way the first step is to treat the reactor with an
antifouling solution to prevent polymerization on the walls.
Then the liquid VCM is loaded to the reactor. An initiator (liquid peroxide) is dissolved in the
monomer. Because this compound can decompose is stored at low temperature in special
bunkers. Small amounts are taken for common use. The peroxide is introduced into a small
receiver to make sure that only the correct quantity is used.
After the initiator is added, the reactor is heated with hot water and brought to reaction
temperature. The agitation is necessary to suspend the monomer in the water used to control the
heat of reaction and for the purposes of product quality. Since the reaction is exothermic cooling
water circulating in the reactor jacket. The reaction is said finished when the pressure decreases,
indicating that most of the monomer has reacted. The polymer is discharged and sent downstream
of the reactor for further treatment (monomer recovery, stripping, drying).
HAZARDS IDENTIFICATION
The first step in developing the process is to identify the process parameters, define the risks to the
safety and the environmental impact and seek solutions for a more safer process .For this purpose
information is needed about the hazardous properties of the substances involved and products.
However the reaction conditions and the initiator must be carefully chosen to ensure that the
reaction rate is adequately monitored and avoiding phenomena of run-away while ensuring the
quality and improved production capacity choice is the polymerization in water but this requires the
use of small quantities of hazardous initiators whose safety must be assessed.
In this case the main dangers are associated with flammability and toxicity of the combustion
products VCM.
AS first step is useful an examination of incidents over the years:
in 1961 in a PVC plant in Japan an accident that killed 4 people was due to the discharge of the
batch from the wrong reactor so that the unreacted monomer was released in the plant which
contained reactors in parallel. The VCM vapors were triggered by a spark of some machinery
resulting in an explosion. In another incident a worker accidentally opened a manhole of a reactor
in service with leakage of large amounts of monomer that is burned and led to the death of
maintenance. people. In another incident an operator loaded the monomer reactor with the bottom
valve open. Other incidents occurred during the maintenance of a VCM pump due to the presence
of the peroxide contamination, or there was a release of VCM from a scrubber due to maintenance
problems to a clogged valve resulting in ignition and death of operators.

2. Thereby the risk can be summarized as:
Jet fire: a leak from a pressurized system which burns and forms a jet of fire that impinge other
equipment (a jet from a 2 "hole produces approximately 10 meters) jet-fire
Flash -fire: a release of a liquid in pressure produces flammable vapors traveling toward a ignition
source.
Pool fire: a liquid release form a pool burning with flames which can be two high, three times the
width of the pool
Bleve: a pressurized container full of monomer exposed to external fire can yield due to
metallurgical weakness, such an event leads to the formation of a ball of fire. The safety valves do
not prevent the Bleve.
Explosion: the loss of gas in the environment confined port in the presence of primer to explosion
source.
Hydraulic Failure: Over filling a container with subsequent expansion of the liquid due to heating
can lead to the collapse of the vessel.
Stress corrosion failure: air in the system can lead to the presence of HCl that can lead to a loss of
mechanical integrity.
Toxic Combustion products: the combustion of the monomer leads to the presence of phosgene,
HCl, CO along with other toxic substances.
Runaway polymerization: polymerization if not well controlled can lead to excess pressure and
rupture the reactor.
A release of 27000 Kg of monomer (contents of the reactor) may produce a cloud of vapors with
aerosol which would have the form of a pancake and which can reach 450 meters affecting the
external areas of the establishment.
DEFINITION OF THE PROCESS
The operation steps are summarized as follows:
Pre-evacuation of air: if the reactor has been in maintenance it must be removed oxygen from the
air to the product quality problems and for the mechanical integrity of the reactor (Corrosion)
Preparation of the reactor: the empty rector is washed with water, tested for leaks if the manhole
was open and treated with antifouling.
demineralized water load: a controlled load of water is placed in excess to reactor .An excess
can lead to overload, a less quantity can lead to quality problems and problems of run-away. All
the other additives are added.
Charging the monomer: an accurate loading of the monomer is made.
Heating of the reactor: the initiator is added by its receiver, the reactor is heated to the
temperature at which begins the reaction (5 C below the operating temperature)
Reaction: the heating is removed, is passed through cooling water in the jacket is to check the
temperature of polymerization
Termination: When the pressure in the reactor is lowered means that there is no more monomer
to react, and the batch is discharged.
Discharge reactor: the reactor is discharged to the downstream unit, to prevent polymer deposit
on the bottom , the stirrer is held running The monomer is recovered for its reuse in the reaction.

3. There are two other systems that are used in emergency phase:
Shortstop chemical: an agent that terminates the polymerization of batch .However the agitation is
required for a good distribution of the shortstop to quickly stop the polymerization .In case of
failure of the agitator the shortstop must be added in 2 minutes, to use still the shaking motions dl
liquid in reaction. As back-up is used to lower the pressure in the reactor to generate the bubbles
that keep the reactor under stirring.
Automatic depressurization: in case of uncontrolled reaction, the system can be kept under control
with a depressurization of the reactor and discharge of vapors The heat of vaporization removes
the heat of reaction.
• HAZARD ASSESSMENT
The following is a table with a list of accidents and their prevention strategies
N Intial Event Process
Deviation
Process variable prevention
1 A failure in the cooling water
control
The loss of
control generates
runaway
Low-flow CW
High temp.react
High press.react.
Add.shortstop
in emergency
cooling water
Depressurizing
the reactor
(interlock)
PSV
2 Lack of batch agitation reduced cooling,
no reaction
uniformity leads
to runaway
Low motor
amperage
High temp React.
High pressure
reactor.
Add shortstop
Depress.react
psv
3 Lack of electrical energy Loss agitation,
runaway
stirrer motor off
low CW
high temp
high pressure
Add shortstop
Depress.react
psv
4 cooling pump stopped, Loss of cooling
runaway
Low-flow CW
high temp
high pres
steam turbine at
the pump
add shortstop
Depres react.
PSV
5 loading error in the recipe, too
initiator load
High
concentration of
initiator causes
runaway
high load
high weight
High press.
Add shortstop
Depress Reactor
(interlock)
PSV
6 Error control system overfilling
reactor
The reactor
becomes filled
with rising
temperature,
hydraulic
damage, release
of monomer
high weight
High level
High pressure
Compare weight
with the recipe
(interlock)
Depres react.
PSV

4. outside
7 Malfunction temperature
control, reactor overheating in
the heating phase
High
temperature,run-
away
High temp
High press
Add shortstop
in emergency
cooling water
(Interlocking)
Depress. react
PSV
8 Sealing the reactor out of
service
Possible leakage
of the monomer
from the seal
High pressure in
the seal
smoke detectors
on plant
Additional
ventilation around
the seal
Depres. High
pressure reactor
seal (interlock)
You can think of the following prevention strategy:
A) to treat the runaway scenarios where the agitator is running (N 1,4,5,7) can propose the
following sequence:
• High temperature or pressure the maximum flow rate of cooling water is activated (Interlock) and
alerts the operator with alarm
• If the temperature and pressure continue to grow the operator activates the addition of shortstop
• If even this method stops the reaction a "high-high" alarm on the temperature and pressure and
the interlock system depressurize the reactor
B) for runaway occurring stirrer for not running (N 2,3) other protections in addition to those of the
case A they are:
• The loss of agitation is indicated for low amperage to the operator by an alarm and after the
addition of a shortstop depressurization is required to mix the shortstop to the mass.
(Depressurization of the system is back-up to the runaway control)
C) Low or non-presence of cooling water: you use the same security of the case A, in addition if
the low flow rate is caused by the loss of electrical power, the operator is alerted by the low flow
and acts by starting the turbine steam on the pump.
D) water Overload or monomer: can lead to over-filling of the reactor with hydraulic damage. This
damage is avoided if there is an interlock between the weight of the reactor cells with high weight
alarm and the heating system of the reactor. A back-up is provided with an interlock "high-high"
level of pressure that activates the emergency depressurizing valves.
E) stirrer seal break: this can cause dangerous spills monomer. This is secured with interlock High
agitator sealing pressure and depressurization emergency.
F) Since the shortstop is so important to control the runaway an interlock is inserted to ensure the
availability of the shortstop, such interlocking will not allow the load of monomer if the shortstop
level into the container is low and if there is no nitrogen pressurization

5. ANALYSIS INCIDENTAL EVENTS
Event 1: Lack cooling water
This event starts a runaway which can become catastrophic .The protection is the shortstop and
the safety valves.
Event 2: outside agitator service
The event starts a runaway similar event 1 except that the depressurization is required to mix the
shortstop in the mass of the reactor, with agitation lack of the maximum flow rate of cooling is
insufficient to stop the runaway so the interlock depressurization is the only effective.
Event 3: Lack Electricity: same consideration as Event 2
Event 4: cooling pump out service
This event is similar to the event 1 except that the operator can stop the runaway only by operating
the turbine on the relevant pump or by adding the shortstop.
Event 5: double charge initiator
This event leads to an energetic runaway with high rate of reaction and evolution of heat even if
the cooling is functioning Both the PSV that the interlock depressurization system are indicated
for this event, as well as the addition of shortstop.
Event 6: Over-filling of the reactor
This event can lead to dangerous leakage of monomer. With the high number of batches per year
this event is very likely .The interlock and High weight alarm, level on weight cells are deemed
sufficient. The interlock depressurization is effective.
Event 7: Over-heating of the reactor
This event leads to runaway similar event 1.Effective prevention systems are interlocking with the
emergency cooling water and depressurization.
Event 8: sealing the reactor out of service
The special design of the seal reduces leakage of the monomer. The additional ventilation is
sufficient to minimize the risk and the low presence of operators on the system reduces the risk.
DESIGN OF A CONTROL SYSTEM
An electronic control system (PID control, PLC, DCS) is selected for the following reasons:
• The plant consists of several reactors
• The control room is at a remote location
• The valves have on off switches with the position indicated
• Operations from the control room reduce the presence of operators on the system
• Electronic Input are useful for recipe management
• It is possible to make a data analysis compared with those of the field

6. operating station
The operators in the control room have access to a lot of equipment through the console to make
an analysis of the process, to make problem solving analysis, the variable status monitoring, trend
analysis, alarm analysis
sensor selection
Level in the reactors: choosing a radar level that can be mounted outside the reactor. The system
is mounted to monitor the loading and unloading stages of reactor. to the systems of washing with
high pressure water. Avoiding an entry into the reactor is important to the carcinogenic nature of
the monomer.
Temperature: The temperature is measured from RDT and It is inside sheaths in order to facilitate
the slipping of the thermocouple. The cockpit is equipped with a pressure indicator to indicate any
losses in the cockpit.
Pressure: The primary system consists of a pressure transmitter with diaphragm seal
Flowrate : to load the monomer using turbine flowmeters that have appropriate characteristics of
reliability and also allow integration of the past volume.
Weight: load cells are supplied to each reactor to provide an indirect indication of the level and
indication of the amount loaded.
stirrer current: a current sensor is provided for indication stirrer marching
final elements selection
The valves are picking according to their characteristics for minimal losses in the environment and
in the second place to minimize polymer buildup. Ball valves or butterfly with high closing seal are
selected.
Controller Selection
The more the system in use is a DCS because transients are relatively low and a normal DCS is
sufficient for the application.
administrative procedures to maintain integrity
It may be necessary to conduct a FAT (factory acceptance test) on the DCS.A control system to
validate the procedure of the system control logic is required including the analysis of the control
sequence of the reaction to batch. Some SOP (standard operating procedures) will be provided to
operators which describes all process steps, how the process control (set-point, process alarms,
temperature and pressure limits, range during the reaction, which actions to take in case of
deviation)
An operator confirms that the action was taken should be required before moving on to a next
process ..Procedure step that describes what to do when critical parameters are in alarm .Another
procedure must be issued so that if the software is updated safety is not compromised. This
procedure shall include:
• Basic review of the decision to upgrade

7. • Make available back-up copies of the software current system
• Process Operations are stops during the validation period
• All the graphics pages are validated as correct
• The control logic are tested to the design criteria
• The temperature and pressure control limits are always operating
• All changes are communicated to the operating staff must understand the extent of change
A formal procedure should be in all cases in which they are required reviews, approvals,
documentation Master copy of changes of the entire system configuration must be kept and placed
in a safe place for use in the event of total failure of the system.
A review schedule must be provided.
on interlock procedures
A procedure must state that no interlock can be bypassed during the time required for the reaction.
No alarm should be bypassed at any time of the process. Any calibration of equipment must be
done with the process not operating. .The procedure must provide that if there are abnormalities in
the interlock the operator must proceed with the plant shutdown. The procedure should not allow
any changes to the process parameters and interlocks when the first has not been sufficiently
endorsed and except there was conducted a HAZOP analysis or FMEA.
The procedure must allow access to interlocks systems by authorized persons only who knows the
password system and its operation. All maintenance work on the control and interlock systems
should be documented, indicating the initial problem, identify the causes, and the implementation
of the solution, provided the person responsible .The procedure must provide that a workstation is
configured for your system control and another for interlocks. A functional test should be conducted
on the interlocks before putting them into service and at regular intervals. The test system must
validate the following points:
• The operation and range of inputs including the primary devices and the input modules of
interlock
• The logic of the operations associated with each input device
• The set points of all inputs and the contact position of the switch• Alarms with their duties
• The function of all output or final control elements
• The correct action of the final control elements (valves, actuators)
• Any variable or output that indicates the status of the installation process
• The current software version
• If the action in the absence of the energy system (EE, instrument air) is correct
other procedures
The training of staff that use the software must be conducted before putting the system into
service and should be repeated in case of changes.
The documentation on the current software system must be updated, any changes must be
documented.
An audit should be done for control and follow-up of the system should be implemented The
audit must include:
• Review of all changes made since the last audit or verification
• Review of all the problems that occurred with the software
• Verification of the functional checks of the system annually facts
• Check that all official documentation is in order

8. • Verify that the person know how to use the software correctly
• Check that the planned has been realized
• Check emergency procedures including simulation periods
Reference
guidelines for safe automation of chemical processes –CCPS-Aiche
ALFREDO RUGGIERO