POLYMERISATION AND INHIBITION
Polymerisation can happen by :--
An initiator ( reaction partner )
Heat ( thermic polymerization )
Light ( photo polymerization )
Radiation ( radiation polymerization )
Chemicals which polymerise form complex aggregates called polymers by the combination of two or more identical molecules from the original chemical which is a monomer. Monomers are among the purest organic chemicals produced. Inhibitors converted to free radicals halt a second reaction chain. Inhibitors react with reactive intermediates thus removing them from the reaction.
When a single molecule reacts with another molecule of the same substance. A long chain molecule is formed having thousands of individual molecules. Can be with rapid evolution of heat to form sticky, resinous materials. Cargo then becomes viscous and unpumpable.
Some unsaturated unsaturated compounds with one or more double bondings are able to polymerise, like-- Vinyl compounds, Vinyliden compounds, Acryl compounds, Carbonyl compounds ( eg:aldehyde ), Styrene, Ethylene / propylene oxide, Ethylene, Propylene, Isobutylene, Butadiene, Isoprene.
Polymers are usually distinguished by a high molar mass (formula weight), often ranging into thousands or millions of grams per formula unit. Polymerization is a poly-reaction of a monomer compound whose molecule contains double bondings or rings . Ethylene is a highly flammable gaseous molecule with a formula weight (molar mass) of 32 grams per mol. When polymerized using a catalyst, it forms an insoluble solid called polyethylene. Polyethylene is a widely used commodity plastic.
Unstable chemicals decompose or polymerise. A decomposing chemical is stabilised and a polymerizing chemical is inhibited.
Chemicals like Acetone Cyanohydrin and Lacto Nitrile which decompose are stabilized by inorganic acid to prevent decomposition or poisonous gas will evolve.
Heat shortens life of the inhibitor. Inhibitors have a shelf life. Heat, light and radiation catalyses polymerization. Opposite of inhibitor is initiator. Add Sodium Naphtalene to Styrene and the tank gets polymerized. Hydrocarbons weaken inhibitors.
Inhibitors are not volatile. They are very difficult to remove by tank cleaning. PTT will be affected. An inhibitor killer can be used if next cargo is WWT.
A test for inhibitor content of Styrene consists of mixing one part of Styrene is 3 parts of Methanol. If the mixture remains clear or slightly bluish the product is intact. A slightly white turbidity indicates polymerization or that the inhibitor is slightly consumed. If there is an unexplained temperature rise consult shippers and operators immediately. The action might be as drastic as jettisoning the parcel.
Polymerization reactions are exothermic reactions. Reaction speed is strongly influenced by catalysts ( usually metals ). Many polymerization reactions are started by heating and by liquids after the formation of Peroxides.
Inhibitors are added in very low concentrations—10 to 40 ppm. Unlike monomers inhibitors prefer to react with Oxygen, and this way the formation of Peroxides is prevented. Inhibitors have a shelf life and they have to be replenished.
If the inhibitor is oxygen dependant NEVER nitrogen pad the tanks.
The list of chemicals required to be inhibited is attached to this manual. Generally all monomers, Acrylates, and chemicals like Acetone Cyanohydrin, Benzyl Chloride, Butadiene, Iso Propyl Ether, Isoprene, Pentadiene, and Vinyl chemicals require to be inhibited. Ethylene oxide , Propelene oxide and Iso Butylene can also polymerise.
No polymerising cargo can be loaded without a signed inhibitor certificate, a copy of which is attached to this manual.
It must contain the following minimum information—
Name and amount of inhibitor
Whether the additive is oxygen dependant
Date inhibitor was added and length of its effectiveness
Any temperature limitations qualifying the lifetime of the inhibitor
Action to be taken in case the voyage length exceeds the effective life time of inhibitor
Recommended method of determining additive level continued activity
You must remember that inhibitors added to liquid cargo may not inhibit reaction of cargo vapor in the ullage spaces. So it is imperative that after discharge of polymerizing cargoes , PV flame screens must be checked.
Polymerizing can take place if a Styrene Monomer tank is not cleaned for a few days.
Inhibitors are usually Amines, Phenols and Quinones.
Few examples are—
TBC/ p-Tert Butyl Catecol---slightly soluble in water
HQ/ Hydroquinone---soluble in water
MEHQ/ Hydroquinone Mono Ethyl Ether----slightly soluble in water.
24M6BP/ 2.4 Dimethyl 6 Tert Butyl Phenol---insoluble in water
Drying veg oils: even this is a sort of polymerization where molecules link together in a long chain making the oily residue more binding and tenacious.
Stabilizers prevent a compound from changing its chemical nature .
retard the chemical reaction
preserve chemical equilibrium
act as anti oxidants
keep pigments and other compounds in emulsion form
prevent colloids from precipitating
Heat of Polymerisation—The values is the heat liberated when the specified weight of the monomer polymerises to form the Polymer. In some cases the heat liberated is so huge that the tank can explode. The negative sign before the value indicates that heat is given off during the polymerization reaction.
Polymerization can be prevented by refrigeration or inhibition.. When temperature is too high the cargo will tend to evaporate. The initiator will be left behind. When the monomer condenses it can polymerise.
Ensure that extra inhibitor is supplied if the voyage is long from cold to warm climates. All cargo lines have to be drained and emptied back into tank. On warm days cool decks with sprinkler. Check cargo temperatures daily to ensure polymerization is not taking place.
Immediately after dischg , and tank dry cert is obtained , cold SW wash and leave water inside the tank, including pipelines—if terminal regulations permit. Ensure peep holes are cleaned properly. In case of polymerizing cargo fill cold water in common collector ASAP—check interior with a torch—clean out the drains physically. Water wash may be followed by warm ( Alkaline / Caustic solution). Water wash alone is insufficient to prevent PTT test failure .
If the common collector is used for polymerizing cargoes—open out the end covers and clean with water. Line blow to be done more than 4 times with polymerizing and solidifying cargoes.
Washing polymerizing chemicals or frying Veg oils with hot water will result in polymerized layer which is very difficult to remove. Polymerization may also take place when a tank is not cleaned and left dirty for a few days. Since the added inhibitor is not volatile , condensation of the vapor will allow inhibitor free chemical to be formed. Cleaning of these cargoes should always start with a cold pre-wash as soon as possible after discharge, due to loss of inhibitor protection.
The relationship between VP of the inhibitor and VP of the cargo to be inhibited has a important bearing on the effectiveness of the inhibitor. If the VP of inhibitor is greater that that of the cargo then the vapours are protected. For this reason dual phase inhibitors are used for volatile cargoes—one to protect the vapour space and the other to protect the liquid. Most inhibitors like Hydroquinone and Tertiary Butyl Catechol ( TBC ) are highly toxic .
Many inhibitors are more soluble in water than the cargo- so water must be kept out—or the concentration of the inhibitor will be reduced.
Certain cargoes do not have inhibitor –so these cargoes must be carried under inert gas. In which case the Oxygen content must not exceed 0.2%