Saturday, December 4, 2010

MOLASSES ON CHEMICAL TANKERS-- CAPT AJIT VADAKAYIL

                       MOLASSES


Molasses is not Newtonian fluid and pumpability does not only depend on viscosity, but on brix value.

Molasses is a water soluble dark brown syrupy liquid left after sucrose has been removed from the mother liquor in sugar manufacture.

Fluids are broadly classified a Newtonian and non Newtonian depending upon their obedience to the laws of classical mechanics. For a Newtonian liquid rate of flow is proportional to the force applied. Some fluids will not flow till the force exerted is greater than the yield point.

Pumpability depends on brix value, its place of origin, storage time, fermentation process, and last but not least, temperature during period the cargo is on board the ship. BRIX indicates pumpability of product.

SG=400/400-brix in deg

Molasses exhibits the phenomenon called critical viscosity which means that above a certain dry matter content the viscosity increases at a greater rate than might be expected from the increased dry matter content. The critical viscosity for cane molasses lies between 81 and 85º hydrometer brix. The viscosity of molasses is affected both by dry matter and temperature, for example a rise in temperature of 10 C may reduce the viscosity to half or less and a reduction in the dry matter content will also decrease viscosity.

Check heating coils and ensure that steam traps are in perfect condition. Only water must be there in the coils not steam. Heating is controlled by throttling the return valve. The inlet valve must be 100% open. If steam is present in the steam coils instead of hot water the molasses will caramalise on the coils.

The Brix number is a measure of the sugar concentration and equals the percentage by weight of sucrose in solution. Brix number usually in the range of 80 – 90. Coatzacoalcos 86.16 , Karachi 84,  Red Sea 84, Buenaventura 85. Low Brix molasses are easily discharged by Framo deepwell pumps. However those with Brix number 89/93 are considered very thick like honey and impossible to discharge with a centrifugal pump. A screw portable framo pump has to be arranged , and a good booster pump ashore.

Brix is a term originally used for pure sucrose solutions to indicate the percentage of sucrose in the solution on a weight basis. Molasses contains, glucose, fructose, raffinose and numerous non-sugar organic materials, as well as sucrose. Therefore the Brix value for molasses will often differ dramatically from actual sugar or total solid content. It really represents specific gravity

It is non flammable and non toxic.

Composition is typically 20% sucrose + 20% reducing sugar + 20% organic material + ash and water.
There are two type of molasses – cane molasses and beet molasses. Blackstrap molasses is the syrup from which no more sugar can be obtained economically.

Synonym of molasses  – Treacle.

It is used as animal feed and raw material for alcohols.

WWT is not required for loading this cargo rather white water standard is enough.

Marpol category/ OS
SG/ 1.45
BP/ 204C
FP/ 60C
VH/ 40C
AH/ 60C
DH/ 45

Reducing sugars decrease the solubility of sucrose and inorganic ash increases it. The viscosity of both beet and cane molasses is influenced by constituents other than sugar. Beet molasses is usually of lower viscosity, particularly at ambient temperatures, than cane molasses, but in both cases it is affected by the dry matter content.

Molasses may have a viscosity of several hundred centistokes whereas a very heavy lubricating oils may be in the region of 100 centistokes.

The visual effect of fermentation in molasses are an increase of volume and the creation of a layer of foam in, and especially on top of the molasses. Besides the development of foam, the temperature sharply increases a couple of ºC per 24 hours, and at all times an alcoholic odour can be observed, sometimes in combination with the odour of Acetic acid. Steam and molasses form acetic acid.

Fermentation can be stopped by blowing in air into the molasses, the available oxygen will destroy the anaerobic bacteria and the entire fermentation stops.

Molasses fermentation occurs when molasses is diluted with salt or fresh water and is accelerated by heat. During fermentation CO2 (with possible traces of ethanol and higher alcohol vapour ) is given off, which will produce inhalation hazards in compartment containing molasses residue. Reacts with concentrated nitric and concentrated sulphuric acid

Zinc coatings have restrictions regarding the pH , fermentation acidity will be less tha 4 and will destroy the zinc coating, which cannot take less than 6 pH. White patches are always an indication of acid attack on zinc.

 Some charterers will advise you to take in lake water to save on FW charges . you must issue a protest letter in such cases as the bacteria in lake water accelerated the fermentation. The acidity will become less than 4 pH which cannot be handled by normal epoxy coating on a long voyage.

Purge all Framo cargo pump cofferdams and ensure no leakage in seals. Fill cofferdams with fresh water.

Loading to be started carefully via drop line ( avoid free fall loading ). Molasses is never loaded through the impeller unless a molasses filter must be used.

Obtain heating instruction and full specification from shipper, otherwise a letter of protest to be issued.

At a temperature 40ºC molasses is relatively stable but as the temperature is raised sugar may be lost by thermal decomposition.  At temperature over 60ºC  there is always the possibility of thermal decomposition and complete destruction of cargo.

Heating instruction to be strictly followed and heating log completed. It is important to maintain accurate temperature. Note that temperature should be measured at several places and levels within the tank as different temperature can be experienced.

When ballasting if possible keep tanks slack  to avoid tank top cooling down too quickly.

It is important to check p/v valves daily for good operation to avoid tank overpresuure.

During discharging maintain the highest allowed temperature for best discharge rate.

Use as big as a discharge hose available, avoid too many pipe bends and pressure losses on discharge line.

It is better to run many cargo pumps at reduced speed and reduced hydraulic pressure due to foaming and cavitation.  Reduce pump further when tank is about ¾ empty.

Arrange small water flow through cofferdam during discharge. This water flow will wash away any molasses and have good cooling effect on seal arrangement.

If there is too much foam in the molasses during last part of discharge there are several   methods with agreement with surveyors/receivers:
Adding of non-toxic chemicals
        Recirculation through drop line
        Injection of live steam 

If live steaming has been carried out during discharge it reduces clingage than a good hot water wash is all that is required.

If carmelization has occurred on the coils then after tank clean and dry crack coils and burn the caramel to black cinder . Then hammer coils vigorously with wooden mallets . Remove balance powder with scotch brite pads, especially underneath. Don’t try to remove with Kew Machine—the caramel will become like sticky chocolate.

Give a good hot wash to tankclean . This cargo is completely water soluble. Don’t allow the molasses to dry in the tank. Use Caustic soda to de odourise in case the next cargo is odour sensitive.

It is possible to load MEG FG after molasses.  Though it is classified as high HC cargo, it is water soluble.

Molasses is an excellent chelating agent. An object with iron rust placed for two weeks in a mixture of one part molasses to nine parts water will lose its rust due to the chelating action of the molasses.

CAPT AJIT VADAKAYIL ( 28 years in command )
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7 comments:

  1. This is an amazing compilation of data.
    I am seriously impressed.
    Good work. I read the molasses section, and I wish I had seen it 2 years sooner.

    ReplyDelete
  2. Good Day Capt..
    Im Chief Officer on one of the Product carriers..although this ship has carried Molasses several time but Im new to it ...Gr8 to see ur work reagrding Molasses...Hope it will work for me too

    Pradeep

    ReplyDelete
  3. Sir, thanks a lot for the information. I am a chief engr on tankers. this is the first time i am carrying molasses.

    ReplyDelete
  4. Very informative and very useful for all Chemical Tanker hand who are going to carry this cargo.

    ReplyDelete
  5. Can you load Molasses on a vessel with deck exchangers instead of coils?

    ReplyDelete