Friday, December 17, 2010

TALL OIL ON CHEMICAL TANKERS-- CAPT AJIT VADAKAYIL

                             

CRUDE TALL OIL  AND UNDERSTANDING VISCOSITY --

Don’t rely on past practice ( of old timers ) , to find out if Tall oil requires a pre-wash ashore after discharge .

As you know since 1st Jan 2007—the Marpol prewash rules for high viscosity oils have changed. It is better to get the unloading temperature and the viscosity of Tall oil at that temperature declared in writing in a proper signed and sealed document .

Ideally the viscocity in mPas at 20 deg C should be declared in the B/ lading —as this figure varies GREATLY depending on the solution percentage of rosins , fatty oils , sterols, alcohols etc (or is deliberately fudged).

Tall oil is an oxidising , drying oil. Oxidation occurs in 3 phases, liquid, gum and solid.

Tall oil is a spent black liquid from the pulping process whose composition and properties can vary vastly making it imperative to get specs from shippers prior loading or planning stow.

It could be crude tall oil, distilled tall oil, tall oil fatty acid ( liquid rosin, talleol, tallol )----. It is usually used as paint base .

Chris code/ OTL
USCG group/ 34
Marpol cat/ Y
FOSFA banned
MP/ 15C
Viscocity at 20C/ 400 mPas ( for CTO 40% soln )
VP/ 3mm Hg at 200 C
Water solubility/ negligible
SG at 20C/ 0.95
FP/ 200 deg C
ST/ high
VH/ depends on MP—keep 20C above MP
AH/ 60C
DH/ 60C
Heating instruction for VH/ DH/ AH  and recirc to avoid sediments must be obtained in writing or the consignee may claim for off spec cargo.
If charterer wants NIL VH --from your side you just ensure that the cargo MUST remain sufficiently above MP during voyage.
Stable cargo
Bad previous cargo for WWT or UV/ PTT

Clean with cold water for 45 min as it is a drying oil , then at 75C hot wash with gracco injection of caustic/ alkaline cleaner.

Oxidation occurs in 3 phases—liquid , gum and solid. So clean tank asap or keep it wet. Drying oil –reacts with water hardness compounds.

If there is any leak on cargo transfer system, stop the transfer and empty the line system. Allow the product to solidify. Contain and sweep up, put the substance in an adequate container for later removal.

For cleaning purposes ensure to have on board sufficient caustic flakes (approximately 150 kg for a 1000 M3 tank).
As soon as loading is completed clear the lines to prevent solidification
Cargo heating  logs must be be maintained
When commencing tank cleaning, machine wash the tank(s) hot (80C-85C) for 6 hours at short drops.
On completion of washing fresh water rinse and strip.
Method : Recirc with strong caustic solution to saponify ( 10 kg in 100 litres of fresh water ratio) at 45 C one hour at each short drop— total 6 drops.—with heating coils running. Give a short FW rinse . SS tank can be refreshed with weak Metalbrite if next cargo is sensitive.
Then wash at 40C short drops one hour each 6 drops. –again rinse with FW
Toluene steam cargo lines or you will have to flush the lines for too long. Further cleaning after caustic can be done if required by Myrcene, Coaltar cleaner,


TALL OIL FATTY ACID –is NOT a drying oil like tall oil.  It is a yellow oily liquid with acrid adour. Used as cutting oils, soap, emulsifier, lubricants etc. very difficult to clean.
FOSFA— Banned
MP/ 1C—viscosity varies with heating
BP-232 C
FP/ 193 C
SG/ 0.9

Crude tall oil pitch is a semi-drying oil.

BEFORE HANDLING TALL OIL--YOU MUST UNDERSTAND WHAT IS VISCOSITY


                         VISCOSITY

The viscocity of a chemical cargo determines how easy it is to pump out the product and the amount of residue that will be left after discharge .
There are time charterers who will put your ship off hire when you can’t use your deep well centrifugal pump to discharge viscous cargoes which need a screw pump. After 1.1.07 with Cat Y/Z Marpol categories pre-wash depends on viscosity of cargo at DISCHARGE PORT temperature  --it becomes all the more imperative to know this branch of Physics—how increase in temperature reduces viscosity.

High-Viscosity Substance means a noxious liquid substance in Category X or Y with a viscosity equal to or greater than 50 mPa.s at the unloading temperature.

High viscosity chemical should be PRE-washed with hot water, with cold interface removed, and the washing discharged to shore reception facilities..

Viscosity is internal resistance of a fluid to flow or a measure of fluid friction---thus water is thin having low viscocity and honey is thick having high viscocity.

In a laymans language it is a measure of the ability of a liquid to flow through a hole. (For the highest common denominator-- The force per unit area needed to shear a fluid is proportional to the velocity gradient. This constant of proportionality is the viscosity.)

This resistance acts against the motion of any solid object through the fluid and also against motion of the fluid itself past stationary obstacles. Viscosity also acts internally on the fluid between slower and faster moving adjacent layers.

All fluids, exhibit viscosity to some degree. Viscosity may be thought of as fluid friction.

Viscosity diminishes as temperature rises, often by about 2% per degree C.

The Poiseuille and the Poise are units of dynamic viscosity sometimes called absolute viscosity. 
1 Poiseuille (Pl) = 10 poise (P) = 1000 cP
Phonetically it is spelled pwaz.  Poise (P) named after Jean Loius Marie Poisueuille. It is more commonly expressed, particularly in ASTM standards, as centipoise (cP). The centipoise is commonly used because water has a viscosity of 1.0020 cP (at 20 °C)  1 poise = 100 centipoise = 1 g/( cm.s) = 0.1 Pa·s.

1 centipoise = 1 mPa·s.

Screw pumps can handle 1 million mPas or cP.

The "thicker" the fluid, the greater its resistance to shear stress and the more rapid the decay of its flow.

Kinematic viscosity

The SI physical unit of kinematic viscosity is the (m2/s). The CGS physical unit for kinematic viscosity is the stokes (abbreviated S or St), named after George Gabriel Stokes . It is sometimes expressed in terms of centistokes (cS or cSt).
1 stokes = 100 centistokes = 1 cm2/s = 0.0001 m2/s.

Viscosity can be a major factor that affects the value of the Reynolds number.

The Reynolds number is the ratio of inertial forces to viscous (drag) forces  and is used for determining whether a flow will be laminar or turbulent. Laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion, while turbulent, on the other hand, occurs at high Reynolds numbers and is dominated by inertial forces, producing random eddies, vortices and other flow fluctuations.

The transition between laminar and turbulent flow is often indicated by a critical Reynolds number. Within circular pipes the critical Reynolds number is generally accepted to be 2300
If we want to calculate the Reynolds number , we can use the following equation.

R = 3160 x Q x Gt D x µ
where: R = Reynolds number
Q = liquid's
flow rate, gpm
Gt = liquid's specific gravity
D = inside pipe diameter, in.
µ = liquid's viscosity, cp

Critical velocity= Visc coefficient X  RN/ SG  X  dia
When the Reynolds number is less than 2000, flow will be described as laminar
When the Reynolds number is greater than 4000, flow will be described as turbulent
When the Reynolds number is in the range of 2000 to 4000 the flow is considered transitional.
For e.g You may find highly viscous hydraulic oils may exhibit laminar flow in most conditions while things like water will be turbulent

NOW LET US DIGRESS BACK TO CHEMISTRY

High Viscosity Substance
A substance meets the criteria of a high viscosity substance, if the viscosity exceeds a certain limit, that depends on the Product Group.

In the old system ( before 31st Dec 2006 ) if a product was in Category A or B, or Category C and within a special area and the viscosity at discharge conditions is greater than or equal to 25 mPa*s, the substance has to be treated as high viscosity substance.  If the product is Category C and outside of a special area and the viscosity at discharge conditions is greater than or equal to 60 mPa*s, the substance has to be treated as high viscosity substance.
In the new system after 1st Jan 2007  a high-Viscosity Substance means a noxious liquid substance in Category X or Y with a viscosity equal to or greater than 50 mPa.s at the unloading temperature.

Products with a high viscosity
These products should be washed at higher temperatures. In general the viscosity is closely related to the temperature and will decrease at higher temperatures. During washing there should be no ballast water or cold cargoes adjacent to the tank to be cleaned. Washing as soon as possible after discharge is recommended. The respective product characteristic of the individual product must be observed. 

Cargoes with  high viscosity require a higher temperature wash water. The wash water then has to be heated to bring the viscocity of the chemical down. Remember the HO bunkers taken by chief engineer is reduced in viscocity from 380 cst to 15 cst prior to injection into the ME—the heating to be done, say upto 135 deg C is known to him.

Some chemicals like LO additives, veg/ animal oils, polyols etc do not have a defined MP, but a range. For such cargoes viscocity is used as a measurement of the products liquidity of handling characteristics, and the term pour point is used instead.



CAPT AJIT VADAKAYIL ( 28 years in command )
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