Chemical Tanker Guide Online
Home || Chemical hazards || Various chemicals || Cargo Stowage || Care ||Tank cleaning ||Handling equipments ||



Presence of various contaminations in chemical cargo - How to determine?

To maintain product quality onboard modern chemical tankers any contaminants in the cargo need to be observed. Following guideline may be useful determining various product contaminants in noxious liquid chemicals.

Odour

For methyl ethyl ketone (MEK) the presence of water can be proven by adding a drop of oil. If the sample turns a brownish colour there is water in it.

An unusual smell is an indication of cargo contamination and should be reported when it is noticed upon loading. For normally odour-free products such as glycol, odour may be a severe contamination in itself. In order to check products which have a strong "natural" odour one can moisten a filter paper with the product and let the liquid evaporate. If a strange odour should then arise from the filter paper it is an indication of a contamination (by a product with a higher boiling point than the cargo in question).

Sulphur contamination (sulphides)

Certain products, in particular "virgin naphta feedstock'' (petroleum naphta) are severely contaminated by minor amounts of sulphides (and also lead compounds), which poison catalysts in further processing. Previous heavy oils or dirty harbour ballast water may have left traces of sulphides in the cargo tanks.

Usually sulphides are detected by means of a "copper strip test", according to ASTM D 130-65. A polished copper strip is immersed in the product during a period of 2 3 hours at 100 deg C resp 50 deg C. If sulphides are present the copper will become darkened and can be compared with ASTM standard shades where 0 means no discolouration, I - 3 increasing discolouration and 4 corrosion. Usually, however, the test is used as a "yes or no"-test without mentioning figures. The method is easily applicable on board.

Another less frequently used method uses mercury. A small amount of the product is mixed together with mercury in a test tube. If sulphides are present the mercury will become dull in appearance and the product sample turn a brownish colour. The use of mercury should, however, be avoided.


Flash point

The flash point is a measure to show at what temperature flammable vapours are released in appreciable amounts, The flash point refers to a defined test equipment and is only a relative measure: the fact that a cargo is below its flash point does not necessarily mean that the tank is safe against ignition The flash point for a certain product is defined in its specification. A lowering of the flash point means that the product has been contaminated with a more volatile product, which is a severe contamination. A redistillation of the parcel may become necessary. Flash point lowering is of particular concern with cargoes of type jet fuel, gas oil, marine diesel oil.

Flash point determination is carried out by means of the Pensky-Martens apparatus described in ASTM D 93-66 and well known in all petro-chemical laboratories. A product sample is succesively heated and in the vapour space above the liquid a standard ignition flame is introduced at regular intervals until an ignition occurs. Two methods are used: open cup (o c) respectively closed cup (c c). The latter gives a lower flash point with a difference of 5 - 7 degr.C. Closed cup is the most commonly used method.


Modern Chemical tanker hellespont credo underway

Distillation range

Any pure liquid has a defined boiling point. Commercial products often consist of mixtures with also other compounds in accepted amounts. Boiling of a commercial product therefore means that the more volatile components will boil off first and the heavier components later and at a higher temperature. This results in a range in temperature between commencement of boiling and finished boiling (dry retort), called "distillation range". The distillation range is defined in the specification for a certain product parcel. Deviations indicate that the product has been contaminated.





The test equipment is defined in ASTM D 1078, D 1437 and E 133-58. 100 cc of the product is heated to boiling in a retort and the vapours condensed in a water-cooled condensor. A thermometer is inserted in the vapour stream.

The initial boiling point (IBP) is defined as the temperature at which the first drop is condensed in the condensor. The final boiling point is read when the retort just becomes dry, dry point (DP). (For certain products, particularly gasolines, the final boiling point is read when the last drop leaves the condensor, end point (EP).)

The distillation range then becomes: t = IBP - DP (in general) degr.C or degr.F
t = IBP - EP (gasolines)
This test is used on all hydrocarbons: alcohols, aromatics, esters, ketones, glycols etc.
The method is best suited for laboratories ashore.


Refractive index

This method is based on the property of light to be refracted at different angles-when it passes a prism into different liquids. This test is realized in an instrument called a refractometer where only a drop of the liquid to be tested is needed to cover the surface of the prism. The refractive angle can be, me4Lsured with good accuracy. The angle of refraction is a typical property for every pure product. The method is sensitive to temperature. Usually a standard temperature of 20 0 C is used, but temperature corrections can also be made.

Standards are defined in ASTM D 1747-62. Refractometers are available in laboratory and portable models. The latter can easily be used on board.

There are tables available against which refractometer readings can be interpreted in terms of concentrations. The refractive index of a mixture of two products is proportional to the relative contents of the products. Therefore the instrument is suitable to determine strength of solutions, e g sugar in water (molasses), glycol in water. It is also used for the determination of hydrocarbon mixtures (aromatics) and vegetable oils


Solidifying point

The solidifying point (freezing point) is well defined for pure products. Deviations indicate that contaminants are present.

The solidifying Point is measured by placing a test tube with a thermometer in a cooling bath. The product is kept stirred and the temperature is read at short intervals. It will fall to a minimum and afterwards rise slightly and remain constant for a while. This temperature is defined as the solidifying point.

Examples on solidifying points:

Benzene + 5,4 degr. C
Cyclohexane + 4,6 degr. C
Soyabean oil + 22,2 degr.C
Sulphur + 117,0 degr.C
Phenol + 40-41 degr.C
Paraxylene + 13,2 degr. C
Observe the high solidifying point of benzene and paraxylene.


Permanganate-time test

This is a test to determine contaminants in alcohols and ketones (e g in acetone). The method is described in ASTM D 1363-67.

2 cc potassium permanganate solution (conc: 0,2 g KMnO dissolved in 1 litre of distilled water) is added to a 50 cc sample in a test tube. Contaminants in the sample chemically reduce the potassium permanganate which changes colour from red to yellowish. The time is noted for the sample to assume the same shade as a standard reference solution of coboltchloride-uranylnitrate. A shorter time for this colour change means a greater amount of contaminants. Acetone is tested at 25 0 C, methanol at 15 0 C. The test tube should be kept dark during the test. After some practice this method can be carried out on board, and is in fact already in use in some ships.


Gas chromatography

In a gas cbromatograph a small product sample is injected and made to pass througn a pipe filled with a very fine mineral powder by means of a carrier gas stream (N., H 2$ He, argon). Temperature is kept constant. The lighter components will pass through the tube easily but the heavier ones will take more time. In this way the components are separated at the outlet where an ion-cell picks up an electric signal which is recorded. Each product has a typical "print". Deviations indicate contaminants.

The method is only suitable for laboratory work ashore. A gas chromatograph test from product samples taken prior to loading and upon discharge can tell if contamination has occurred while the cargo was on board. This method is some times used taking "fingerprints" from an oil spill and from a suspected source-in order to establish the origin of a pollution.


pH scale

A scale which indicates the acidity or alkalinity of a solution. Its range is 0 to 14. pH 7 represents absolute neutrality. A value of 0 represents high acidity (e.g. concentrated acids) and 14 represents high alkalinity (e.g. a caustic soda solution).

Water soluble products are readily tested by means of litmus paper where a colour change directly indicates the pH-value roughly. There are also electrode -meters available where the pH-value can be read directly from a scale. The pH-value can also be established by means of neutralisation with a sodium hydroxide (caustic soda) solution from which the acidity can be calculated from the amount of alkaline matter added. A colour reagent shows when the solution changes from acidous to alkaline.

Vegetable and animal oils require the determination of pH value, which indicates the contents of free fatty acids. This is, in turn, a measure of the quality of the oil, or the degradation of the oil during transport. The oil is first dissolved in alcohol and afterwards neutralized with sodium hydroxide as indicated above.


Viscosity

Viscosity is measured by means of capillary tubes (ASTM D 445-IP and ASTM D 2515) or by an apparatus measuring the friction of liquid between two cylinders of which one is revolving. Measurements on board will rarely be needed. Viscosity is interesting as regards the pumpability of a product. Usually the upper limit for centrifugal pumps is considered to lie around 400 centistoke (cSt), corresponding to about 1500 Seconds Redwood (sec R) or about 50 degrees Engler ( degr.E). Screw pumps may pump liquids with viscosities as high as 10. 000-20. 000 cSt.


Spectro-photometry

A spectro photometer is used to trace minute contaminants (1 - 2 ppm). One common application is the determination of degree of polymerisation in styrene monomer.

The principle of the instrument is that a product sample is transluced with a light of a known wave length (ultra violet, visible or infra red light). A certain part of the light is absorbed and does not reach a detector cell. The amount of absorption is known for pure products. For common contaminants there are calibration curves available for comparison with readings.






Related Info:

Determining water contamination in chemical cargo

Chloride contamination in chemical cargo - how to resolve?

Varoius product contamination in chemical cargo - how to resolve?

Using acid wash method for a cargo of aromatics contaminated by a previous oil cargo

APHA (Hazen) method for determining color of very light chemical products

Use of Oxygen analysers

Cargo vapour detection equipment


Main Info pages!

Home page ||| Chemical hazards ||| Cargo planning & Stowage ||| Cargo loading ||| Cargo documents ||| Safe stability ||| Cargo care ||| Preparation for unloading ||| Inert gas systems |||Gas freeing ||| Nitrogen handling ||| Chemical handling Safe practice |||Handling equipments ||| Cargo & Ballast pumps ||| Cargo tanks |||Tank cleaning |||Special cargoes |||Spills emergencies |||Fire protection




Chemicaltankerguide.com is merely an informational site about various aspects of chemical tankers and safety tips that may be particular value to those working in: Chemical Handling, Chemical Storage, Liquefied Chemical Suppliers, Chemical Shipping, Chemical Transportation, Chemical Terminals, Bulk Chemical Services and Chemical Processing. If you are interested in finding out more about chemical tanker guideline please visit IMO official website. For any comment please Contact us

Copyright © 2011 Chemical Tanker Guide.com All rights reserved.