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.

 

 

 

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.

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. 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

 

 

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

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

? 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

? 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