Wednesday, May 5, 2010
HYDROLYZABLE CARGOES , CHEMICAL TANKERS -- CAPT AJIT VADAKAYIL
HYDROLYSIS , CHLORINATED SOLVENTS , FFA
Cargoes which react with water to form acid are called hydrolysable cargoes.
People get squeamish about this because zinc silicate cannot take pH less than 6 and most organic epoxies cannot take pH less than 3.5 on the acid scale.
They are carried in stainless steel tanks. Examples:-- Esters ( Acetates and Acrylates ) Pthalates and certain halogenated compounds like Methylene chloride.
Hydrolysable cargoes must be carried in perfectly dry tanks with moisture content less than 100 ppm ( 200 ppm for zinc ). It is important to vent to eject aBsorbed/ aDsorbed cargo in coating. This must be done prior doing tankcleaning with water.
However certain viscous non-volatile products like DOP and Texanol cannot be evaporated by venting and must be cleaned off by warm water immediately after discharge.
Hydrolysis :-- salt + water = acid + base
Since epoxy coating absorbs cargo like a sponge , it means that two successive cargoes may require to be compatible with each other. For example after EDC if you load a cargo containing water, Hydrochloric acid will be formed which will destroy the epoxy coating.
Vinyl acetate monomer plus water gives Acetic acid. Chloroform plus water gives Formic acid.
Esters can saponify in contact with water at higher temperatures resulting in increased short chain type FFA which can be aggressive to tank coatings.
Hydrolysis is a chemical process in which a molecule is cleaved into two parts by the addition of a molecule of water. This is distinct from a hydration reaction, in which water molecules are added to a substance, but no cleavage occurs.
Hydrolysis is an oxidation reaction between Chlorinated HC and residual water. Oxidation occurs due to exposure to radiation or the presence of certain catalysts and can be prevented by inhibitors. Hydrolysis is accelerated at higher temperatures.
Chlorinated carboxylic acids are very corrosive and not carried in SS tanks.
After carriage of Hydrolyzable cargoes such as Acetates, the tanks should be ventilated to remove all traces of cargo before any water is introduced.
The breakdown of vegetable and animal oil fats to fatty acids is promoted by the presence of water particularly at higher temperatures. Hydrolysis is also promoted by the action of certain micro-organisms. Tanks in which the oil is being stored or shipped should always be clean and dry before use.
Chlorinated/ Brominated solvents hydrolyse in presence of water to form Organic and Inorganic acids which spoil the paint coating. Shippers must give the moisture content in PPM. Water content should not excced 100 ppm or 0.01% especially for epoxy. Nitrogen pad is a must to exclude moisture.
Chlorinated solvents are slowly hydrolyzed to acids, SO ENSURE water tank cleaning is NOT stopped in an early stage.
FREE FATTY ACIDS (IN VEG OILS AND ANIMAL FATS) --FFA
FFA of certain heating veg and animal oil cargoes will increase if the moisture content is too high.
Veg and animal oils have different SG at different levels after long storage. FFA of the layer closest to the coils increases faster and cause damage to the bottom— so recirc Vegetable oil and Animal oil in coated cargo tanks.
Advantages are less manual labour for squeezing after discharge and not allowing a difference in the FFA content between the lower and upper levels in the tanks.
Fat is combined with glycerine and fatty acid. When the fat is hot , it get decomposed to FFA and glycerine ( hydrolysis ). Increase of FFA means the cargo will become worse in quality. The quality of vegetable oils and animal fats is largely determined by their AV acid value. A fresh oil or fat has low AV. Parcels having different FFA shall not be commingled . Rapid heating results in increase in FFA content or discolouration of the cargo.
Vegetable oil and Animal oils have a FREE FATTY ACID content (FFA). The hydrolysis of edible fats and oils is promoted by the presence of water, particularly at higher temperatures. If hot prewashed FFA will increase rapidly to a level, which will coagulate the albumen in the Vegetable/Animal oil. This damages the surface of the zinc silicate lining.
Changes in quality of the veg fats and oils occur during shipment due to hydrolysis and oxidation , and this loss in quality is taken into account when edible fats and oils are processed further into food during refining.
Previous cargoes that are able to accelerate hydrolysis of edible fats and oils are:
acids and bases e.g. acetic acid, acetic anhydride, citric acid, fatty acids, phosphoric acid, sulphuric acid, and sodium hydroxide solution.
The general loss of quality due to oxidation (increase in peroxide values) or hydrolysis (increase in free fatty acid [FFA] content) during a voyage can be eliminated by , nitrogen-blanketing the tank, especially if the cargo is refined oil to be used directly in the food-chain without re-refining.
Normal values for refined oils in the Codex Standards indicative for oxidative or hydrolytic rancidity status are:
Peroxide value: up to 10 meq active oxygen/ Kg oil
Acid value/saponification value: up to 0.6 mg KOH/g oil.
Oxidation affects the colour and flavour (taste, odour) of the oil, producing oxidative rancidity.
Hydrolysis affects the flavour (taste, odour) and acidity of the oil, producing hydrolytical rancidity.
Analytical methods for hydrolysis are based on determination of the hydrolysis products. Hydrolysis tests are based on the determination of the FFA content, humidity, acid value (or saponification value) or hydroxyl value. The acid value is the most widely used test method. Refined oils for domestic consumption are most susceptible to hydrolysis .
The Chief officer on a chemical tanker must smell the tanks before they are loaded or unloaded and look into them during loading and unloading. Smell is a very sensitive sense. The purpose of deodorization is to reduce the level of free fatty acids and to remove odours, off-flavours and other volatile components, such as pesticides and light polycyclic aromatic hydrocarbons. Deodorization of edible fats and oils is done by distillation.
Neutralization ( alkali refining) is performed on edible fats and oils to reduce the content of free fatty acids and oxidation products of free fatty acid.
FFA of veg/ animal oils attack zinc silicate paint which can cause zinc to shed contaminating cargo-- Zinc Silicate paint cannot take pH < 6 . Recirc of veg/ animal oil in Zinc coated tank is a must to equalize the FFA content between the upper and lower layers.
-------CAPT AJIT VADAKAYIL ( 28 YEARS IN COMMAND )